Abstract in English
| Suomenkielinen tiivistelmäThe field trip will start in the town of Skellefteå which is the main municipality in the northern part of the County of Västerbotten (Fig. 1). Skellefteå is located close to the coast by the Skellefte river and approximately 700 km north of Stockholm. The Rönnskär smelter of Boliden AB, which treats all concentrates from the Skellefte District, is located at the coast east of Skellefteå. From Skellefteå the field trip will make day trips to various parts of the Skellefte district where base-metal deposits, owned by Boliden AB and gold-deposits owned by Terra Mining AB, will be examined. The field trip will continue across the Bothnian bay in Finland from Vaasa to central Ostrobothnia. The first stop will be the Hitura Ni-mine, which is the only active Ni-mine in Finland today. Near the Hitura mine the Kopsa Au-Cu-prospect is located. Investigations in Kopsa were renewed in 1996 by a Canadian junior company Glenmore Highland Ltd. During the trip from Kopsa to Pyhäsalmi some outcrops will be seen. After staying overnight in Pyhäsalmi the field trip continues to the Pyhäsalmi Cu-Zn-S-mine and the Mullikkoräme Zn-Cu-Pb-mine.
Geology of the Fennoscandian (Baltic) Shield
Introduction
The Fennoscandian or Baltic Shield (both names occur in the
literature) occupies the northern part of Europe. Precambrian
areas are exposed in Norway, Sweden, Finland and Russia and their
continuation beneath the platform cover sequences to the east and
south (Fig. 2) is becoming better understood
through studies within the European "Europrobe" project. The
craton of which the exposed Fennoscandian Shield forms a part is
bordered to the west by the Caledonian orogenic belt. Precambrian
rocks of the same craton again outcrop in the Ukrainian Shield
and the Voronezh Massif (cf. Gee & Zeyen 1996).
The Fennoscandian Shield is composed of Archaean to Neoproterozoic rocks. It is beyond the scope of this guide book to describe all different settings in detail, but adjoining areas that both pre- and post-date the Svecofennian rocks that are of interest in this field trip will be briefly described. The term Svecokarelian is used for the orogeny that occurred between 1900 and 1800 Ma (i.e. emphasising deformation and metamorphism as defining the orogeny), while the term Svecofennian is used for the supracrustal rocks that were emplaced during c. 1.95 Ga to 1.85 Ga. To the unfamiliar reader of literature on the Fennoscandian Shield it is important to remember that these terms are not used consistently in the literature.
Pre-Svecokarelian crustal growth
The pre-Svecokarelian crustal growth can be subdivided into
Archaean and Palaeoproterozoic. During the Archaean greenstone
belts and TTG-terranes formed, while crustal growth during the
Palaeoproterozoic involved rifting of the Archaean basement with
the formation of riftfill sequences of sedimentary and igneous
rocks, and addition of juvenile Palaeo-proterozoic crust by
accretionary processes along the margin of the Archaean
continent.
Scattered occurrences of granitoid intrusions with an age of 2.0 to 1.9 Ga occur both in Finland and Sweden. Their tectonic significance is not fully understood and they are not described further here.
The Svecokarelian orogeny and Svecofennian crustal growth
As mentioned above Svecofennian rocks are defined as the
supracrustal rocks deposited between c. 1.95 and 1.85 Ga and
related intrusive rocks. The late orogenic S-type granites
emplaced around 1.83–1.80 Ga in Sweden and the late to
post-Svecokarelian, c. 1.80 Ga, A- to I-type alkaline Revsund and
TIB granitoids in Sweden are also included in the Svecofennian
rocks. In Finland there is strong evidence for a peak-metamorphic
event at c. 1.88 Ga (Korsman 1988) and this age of metamorphism
is also described from northernmost Sweden (cf. Skiöld
1988). However, in the Skellefte district and southwards in
Sweden and in southern Finland the age of peak metamorphism is
between 1.84 and 1.80 Ga (cf. Billström & Weihed 1996 and
refs. therein).
A number of Palaeoproterozoic volcanic provinces such as the Vihanti–Pyhäsalmi area and the Tampere Schist Belt in Finland and the Bergslagen area and Skellefte District in Sweden, are bordering the Karelian craton. These areas are probably remnants of volcanic arc systems that were accreted to the Karelian continent shortly after the emplacement of the rocks. All these areas are extensively mineralised and the different character of the metallogeny in each area reflect variations in the tectonic setting. It is likely that the earliest volcanic arc to be accreted onto the Karelian craton was the Vihanti–Pyhäsalmi area which is 1.93–1.90 Ga in age. The rocks of the coeval Tampere Schist Belt and the Skellefte District have recently been proposed to be part of the same volcanic arc system (Ekdahl 1993). All rocks in the Skellefte district have primitive e(=epsilon)Nd values which indicate only minor incorporation of older material into the source for these rocks. In contrast the Tampere Schist Belt greywackes contains a marked component of older partly Archaean material (Huhma, 1987). The existence of a felsic basement only slightly older than the volcanic rocks is possible in the Skellefte District and the tectonic environment in which the rocks were emplaced may have resembled the present day Okinawa through. In contrast, Allen et al. (1996b) interpreted the extremely felsic Bergslagen area as part of a continental arc system with shallow marine basins between land areas. The rocks are also more alkaline in character than the Skellefte rocks and the metallogeny is characterised by different types of iron ores which do not exist in the Skellefte district. The so called Leptite belt of south-western Finland which contains several small iron deposits and Zn-Cu-Pb deposits (the Orijärvi–Aijala deposits) has long been considered as an eastern continuation for the Bergslagen region in Sweden.
At the same time as the areas discussed above formed, the magmatism seems to have continued in the rifted craton to the north, but the opened oceans were probably partly closed after 1.95 Ga as indicated by the Jormua and Outokumpu ophiolites. In the Kiruna area the greenstones are unconformably overlain by c. 1.89 to 1.87 Ga old felsic volcanic rocks which may manifest a time of convergent plate movements and closing of the oceans and rift basins.
Post-Svecokarelian crustal growth
All supracrustal rocks were intruded by granitoids at c.
1.89–1.85 Ga, both in Finland and Sweden. For example, the
Central Finland Granitoid Complex is dominated by intrusions of
this age. These rocks are often granodiorites to tonalites and
appear to be comagmatic with at least some of the coeval volcanic
rocks. The granitoids have also been the subject for exploration
since they are known to contain porphyry type Cu and Au deposits
and shear zone hosted Au deposits both in Finland and Sweden (cf.
Weihed 1992b and references therein). One such occurrence, Kopsa,
will be shown on the field trip. Subsequent magmatism is closely
related to peak metamorphism at 1.84–1.80 in Sweden when the
Svecokarelian metamorphism peaked in the Skellefte district and
areas to the south. The area between the Skellefte and Bergslagen
districts is characterised by high-grade ortho- and
para-gneisses. Minimum melt granites and pegmatites are common
among these migmatites and in some areas they form homogeneous
granitoids like the Skellefte- and Härnö granites.
At c. 1.80–1.78 Ga major calc-alkaline batholiths referred to as the Revsund granitoids and the Trans-Scandinavian Igneous Belt (TIB), were emplaced at the western border of the craton. This magmatism seems to have continued intermittently to c. 1.65 Ga. Scarce supracrustal rocks are associated with these batholiths, e.g. the Duobblon and early Dala volcanic rocks, and the Småland porphyries in Sweden.
Younger anorogenic magmatism in the Fennoscandian shield includes the Mesoproterozoic (1.65–1.55 Ga) Rapakivi granites in both Finland and Sweden as well as mafic dyke swarms which were emplaced at different times after the Rapakivi intrusions. All of these rocks intruded a cratonized crust.
Metallogeny
The Fennoscandian Shield hosts several world class ore deposits.
Unlike other shield areas the Proterozoic, and not the Archaean,
has so far been the main contributor to metals in the
Fennoscandian Shield. In Finland the Outokumpu copper mine was
the base for the Outokumpu mining company. Other famous deposits
include the Vihanti and Pyhäsalmi massive sulphide deposits
and the Kemi chromite deposit. In Sweden, world class deposits
include the Kiruna iron ore, the Boliden and Falun massive
sulphide ores, and the low-grade copper ore at Aitik. A good
recent review on Finnish metallogeny is found in Nurmi and
Sorjonen-Ward (1996).
Six base metal and gold mines are currently in operation in Finland (Fig. 3): Kemi, Pyhäsalmi, Mullikkoräme, Hitura, Orivesi, and Pahtavaara. This field trip will examine the Pyhäsalmi and Mullikkoräme massive sulphide deposits (see below). The Hitura mine is the only operating Ni-mine and it is one of several early Proterozoic Ni-deposits found in mafic-ultramafic rocks, most of which are situated in central and eastern Finland. The old term Raahe–Ladoga zone is widely used to describe the NW-SE trending zone where most of the Finnish sulphide deposits are located. The Orivesi gold deposit is situated in south-western Finland in the Tampere schist belt (Fig. 3).
Traditionally Sweden has been divided into three main mining areas, namely the Bergslagen area in the south, the Skellefte district in northern Sweden, and the Norrbotten region in northern-most Sweden (Fig. 4). Today, only two mines are in operation in Bergslagen: the zinc-lead deposit Zinkgruvan near Åmmeberg and the zinc-lead-copper deposit Garpenberg. A good review on metallogeny and volcanology of the Bergslagen district is found in Allen et al. (1996b). In the Norrbotten region the iron mines at Kiruna and Malmberget are still big producers of iron from underground operations, while the Aitik open pit mine is a low-grade disseminated copper-gold deposit. In the Skellefte district massive sulphide ore is extracted from six mines (Långdal, Kankberg, Åkulla östra, Renström, Petiknäs, and Kristineberg) and gold ore is mined in one open pit operations at Björkdal and underground at Åkerberg. The closed Enåsen gold deposit in Sweden which is situated in the Bothnian basin south of the Skellefte district has been interpreted as a metamorphosed and deformed high-sulphidation epithermal ore body (Hallberg 1994). In the Caledonian orogen the only mine still operating is the Laisvall lead-zinc mine.
The genesis of most of the deposits mentioned above, apart from the deposits in the Skellefte District and the Pyhäsalmi-Mullikkoräme area, will be the themes for other field trips and will not be discussed further here.
The Skellefte District
The Skellefte district (Fig. 5) is a loosely
defined area, approximately 150 x 50 km in size, in northern
Västerbotten and southern Norrbotten in the northern part
of Sweden. It is extremely rich in mineral occurrences and today
5 underground mines and one open pit are in operation on
VHMS-ores, and the Björkdal gold-lode deposit is mined in
an open pit operation while the Åkerberg gold-lode deposits is
mined underground.
History of mining
The name "Skellefte district" stems from Högbom (1899) who,
in a note for the Geological Society, described his trips to the
interior of northern Västerbotten and southern Norrbotten.
However, the record of mineral exploration dates back to the
early eighteenth century. The brief historical outline of
pre-modern mining below is based largely on the summary presented
in Tegengren (1924).
The first record of mining is from 1704 when copper ore was mined in the north-eastern part of the Skellefte district at Storkåge- träsk (Hermelin 1804). Scattered short-lived trial mining seems to have prevailed throughout the eighteenth century. Around 1850 attempts were made to mine silver east of the town of Skellefteå (Moröns silvergruva). I early 1900 gold was discovered at Krångfors and this resulted in intensified exploration in the area. Several minor gold-arsenic mineralisations were discovered as well as Fe-Cu sulphide mineralisations. However, none of the mineralisations were found to be economic and in 1908, only the Krångfors claim was still held.
The modern history of the Skellefte district began after the first world war when the Kristineberg ore (Fig. 5) was found in 1918. At this time, exploration in the area was carried out by Centralgruppens Emmissionsbolag (predecessor to Boliden AB) and the Geological Survey of Sweden. The improvements of electrical and electromagnetic geophysical exploration methods by Lundberg and Nathorst were a big success and led to the discovery of several base metal sulphide ores during 1920–1922. Mensträsk, Näsliden, Rakkejaur, and Bjurfors were the major discoveries. In 1921 Centralgruppens Emmissionsbolag investigated an area where ore boulders had been found and trenches dug near the village of Svanfors. They used the new electrical methods and found some interesting anomalies 2.5 km north of the village of Bjurliden (misnamed Boliden on topographic maps). They decided to drill these anomalies and intersected auriferous arsenic-copper ore on the 10th of December 1924. They had discovered the Boliden ore. Because the name Boliden occurred on the topographic maps the name remained and the Centralgruppens Emmissionsbolag became the Boliden Mining Company now Boliden AB.
Another discovery of auriferous base-metal ore in Holmtjärn was made during the 1920 s in the central part of the district, but the Boliden ore was the only to be mined continuously until the Rävliden mine in the west opened in 1936.
Of the mines operating today, the Långdal ore was discovered in the 1930 s and put in production 1967, the Renström ore was discovered in the mid 1920 s and put in to production in 1952, the Kankberg ore was discovered in the mid 1940 s and production started 1991, the Petiknäs ore was discovered in 1986 and production started in 1992, and finally the Kristineberg mine started production in 1940. The gold deposits Björkdal and Åkerberg were discovered in 1985 and 1988 and put nto production in 1988 and 1989, respectively.
Regional geology
The Skellefte district forms part of the Svecofennian c.
1.90–1.85 Ga supracrustal sequence and associated intrusive
rocks in the northern part of Sweden. The area is dominated by
a thick pile of felsic volcanic units which host over 85 pyritic
base metal deposits (cf. Allen et al. 1996). The geology of the
district has been the subject of numerous papers since
Högbom (1899) first gave the area its present name. In
thischapter, an overview of stratigraphy, rock types, volcanic
facies, geochemistry, ages and provenance of rocks, and the
structural and tectonic evolution is presented.
Many stratigraphic schemes have been proposed for the Skellefte district and the interpretation of the relationship between the supracrustal rocks and different intrusive suites have also changed through time. The main debate has been the relationship between supracrustal rocks and different intrusive suites as well as the relationship between different sedimentary and volcanic rocks. Already early this century, geologists (cf. Eklund 1923, Tegengren 1924, Högbom 1937, Gavelin 1939) recognised a lower, mainly felsic volcanic unit which was called the Skellefte volcanic rocks. These were thought to be overlain by an early sedimentary sequence, often called the "Skellefte sediments" and a later coarse clastic unit called the Vargfors Group. It was generally accepted that the Skellefte volcanic rocks were intruded by an older granitoid suite, the Jörn granitoids. The relationship between the Jörn granitoid and younger supracrustal units as well as the relationship between the Revsund granitoids and these youngest units was a matter of debate. Kautsky (1957) proposed that there were at least two stratigraphically different schist units within the so called Maurliden and Elvaberg series and hence indicated that a more dynamic view on the lateral extent of different rocks units was necessary. The first dynamic approach was made by Helfrich (1971) who also recognised lateral as well as vertical facies changes. In 1980 Lundberg published a paper where the relative ages of rocks for the first time was in agreement with the present view. In all these stratigraphic schemes a major unconformity between the Skellefte Group and Vargfors Group was proposed with the Vargfors Group resting undeformed on top of the steeply dipping strta of the deformed Skellefte Group. This has been challenged in the last years by Weihed et al. (1992) and Allen et al (1995, 1996). In their view both the Skellefte and Vargfors Groups contain the same tectonic fabrics and there is no major time difference between the two units (see below).
The exact stratigraphic position of the Arvidsjaur Group has long been a matter of controversy. The relationship to different generations of granitoids have also been a matter of argument. Högbom (1937) for example placed the Arvidsjaur Group (Arvidsjaur porphyries) below the Vargfors Group which was also considered younger than the Revsund granite. This view was shared by Gavelin (1955), while Kautsky (1957) for the first time considered the Revsund granitoids younger than the Vargfors Group, but still regarded the Arvidsjaur Group as older than the Vargfors Group. Allen et al. (1996) regarded, on new age evidence and facies analysis of clastic rocks, the Arvidsjaur Group as a lateral facies variety to the Vargfors Group in the Skelleftedistrict. They concluded that at least the top of the Arvidsjaur Group is coeval with Vargfors sedimentary rocks in the Skellefte district.
In this guide book we adopt the scheme of Allen et al. (1996) and the different units are described below according to the generalised stratigraphy presented in Figure 6 from Billström & Weihed (1996) after Allen et al. (1996).
The internal stratigraphy of the Skellefte Group is extremely variable (Fig. 6), and the main rock types are volcaniclastic rocks, coherent subvolcanic porphyritic intrusions, and lavas. Allen et al. (1996) also include all intercalated sedimentary rocks which range in composition from mudstone to breccia-conglomerate in the Skellefte Group. Some coarser sedimentary rocks are lime-cemented and only very rare, minor limestones occur within the Skellefte Group.
he composition of the volcanic rocks is mainly felsic in character with rhyolites and rhyodacites dominating. However, the composition of rocks vary considerably between areas as can be seen in Figure 7 where the relative abundance of rock compositions are estimated from different domains (Allen et al. 1996).
Allen et al. (1996) divide the Vargfors Group into the Mensträsk conglomerate which is a volcanic breccia-conglomerate consisting mainly of Skellefte Group clasts (including the Mensträsk breccia of Kautsky 1957), the Elvaberg Formation which is composed of argillitic sedimentary rocks with varying abundance of sandstone and breccia interbeds, (corresponding to the Elvaberg slates and Phyllite Series of previous workers), the Abborrtjärn conglomerate (Kautsky 1957) which is a polymict conglomerate and sandstone with abundant Jörn GI-type granitoid clasts and Skellefte Group volcanic clasts, the Dödmanberg conglomerate which is a conglomerate (and sandstone) with a range of clast types including Arvidsjaur Group volcanic and sedimentary rocks, mafic volcanic rocks, granitoids, vein quartz and jasper, and the Gallejaur Volcanic rocks consisting of moderate- to high-Mg basalt lavas, intrusions, and volcaniclastic rocks (the Vargfors Andesite of Kautsky 1957), and other subordinate basalt, andesite, dacite, rhyolite, and interbedded sedimentary rocks.
Deformed bodies of Jörn granitoids occur in several places in the Skellefte district and the Jörn intrusives have long been regarded as comagmatic with the Skellefte Group volcanic rocks. This relationship is, however, not confirmed more than as an overlap in age of the two rock types. Intrusive porphyries within the Jörn batholith texturally resemble the coarser varieties of subvolcanic intrusion within the volcanic pile. The intrusive porphyries have been dated at 1886±15/9 Ma (Weihed & Schöberg 1991) which is within error the same age as the Skellefte Group volcanic rocks.
The Gallejaur intrusion is situated immediately west of the Jörn Granitoid Complex. The rocks range from gabbro to monzonite in composition and are intimately related to the mafic volcanic rocks of the Vargfors Group. It is often difficult to distinguish the intrusive and extrusive phases. Age determinations on both the Gallejaur intrusion and the Vargfors Group are identical within error limits c. 1875 Ma. The Gallejaur intrusion may be related to the so-called perthite-monzonite suite to the north (cf. Skiöld 1988) which to the north post-dates the main deformation.
To the south and east of the Skellefte district the high grade gneissic and migmatitic equivalents to the supracrustal rocks in the Skellefte District are intruded by minimum melt granitoids. The migmatites are often invaded by leucocratic neosome which grades into pegmatites and granitic material. The granites associated in space with these high grade rocks are referred to as Skellefte granites and have ages around 1.80 Ga (e.g. Weihed & Persson in prep, Romer & Smeds 1994). Similar granites to the south are referred to as Härnö granites (e.g. Lundqvist 1991, Claesson & Lundqvist 1995).
The Revsund intrusive rocks form part of a large granitoid batholith belt which borders the Svecokarelian orogeny to the west. If the TIB rocks are included it means that the batholith belt is over 1500 km long and in places over 200 km wide (Fig. 2). The Revsund batholith is composed of many discrete, often rounded plutons. The coarse feldspar porphyritic, granitoids are calc-alkaline and display a monzonitic trend from diorite to granite. Rare occurrences of charnockitic black granites are probably early plutons which have been heated by subsequent magmas. The age of the Revsund granitoids vary between 1.80 and 1.78 Ga and they are considered to have risen from a lower crustal level as hot dry melts (cf. Claesson & Lundqvist 1995).
In a small study of the geochemical composition of host rocks to ores made by Allen et al. (1995) no obvious bimodality in the composition of volcanic rocks could be found. They suggested that almost all sampled rocks in the study were calc-alkaline volcanic arc-type rocks. Allen et al. (1996) presented histograms of the relative abundance of different rock types in different domains illustrating a fundamental difference in evolution between different parts of the Skellefte district from extremely felsic centres in the Petiknäs areas to andesite dominated domains like Hälträsk (Fig. 7). In Figure 8, rocks from different domains are plotted in common classification and discrimination diagrams.
In contrast to the Na-rich submarine volcanic rocks of the Skellefte Group, the Arvidsjaur Group volcanic rocks are subalkaline and have a higher potassium content at similar SiO2 values (cf. Lundberg 1980).
The early Jörn granitoids which are coeval with the Skellefte Group volcanic rocks are tonalitic to granodioritic in composition and calcic to calc-alkaline in character (cf. Wilson et al. 1987, Weihed 1992a). The relationship between the subvolcanic intrusions in the Skellefte Group and the high-level porphyritic intrusions within the Jörn suite is still an open question. The subvolcanic intrusions are generally more felsic in character and unpublished preliminary results indicate that they are distinctly different in REE-composition from the high-level intrusions of the Jörn suite, and thus are not genetically related.
The age of volcanic rocks of the Skellefte Group is constrained from only three U-Pb zircon ages. In the eastern part of the district, the Bjurvattnet sample represents a subvolcanic quartz-phyric intrusion emplaced into the lower part of the Skellefe Group volcanic rocks. The intrusion is interpreted to be within error limits the same age as the intruded volcanic rock. The age is 1884±6 Ma (Billström & Weihed 1996). In the central part of the district, two U-Pb zircon dates represent the upper part of the Skellefte Group. The age 1882±8 Ma from Nicknoret (Welin 1987) is poorly described in the literature, but is apparently from the uppermost part of the Skellefte Group north of the Skellefte river in the central part of the district. A new age determination from the south-central part at Melestjärn gave an age of 1889±4 Ma for the upper-most part of the Skellefte Group (Billström & Weihed 1996). There are no age determinations on volcanic rocks of the Skellefte Group from the western part of the district. Although there are no good age determination on the lower part of the stratigraphy of the Skellefte Group, it seems plausible that the entire group span the time interval between 1890 and 1880 Ma.
So far only one age determination has been carried out on the Vargfors Group. The sample was taken from a felsic ignimbrite intercalated with mafic volcanic and sedimentary rocks in the middle part of the Vargfors Group north-west of Nicknoret. The age of this ignimbrite is 1875±4 Ma (Billström & Weihed 1996). This age is c. 5 million years younger than the ages of the Skellefte Group and confirms that the Vargfors Group is only slightly younger.
The oldest intrusive rocks in the district are the Jörn granitoid rocks which have been dated in several localities. Wilson et al. (1987) dated the oldest outermost GI phase of the Jörn batholith at 1888±20/14 Ma, the GII at 1874±48/26 Ma, and the GIII at 1873±18/14 Ma. The ages are poorly constrained but indicate that at least the GI is coeval with the volcanic rocks of the Skellefte Group. The question remains whether the GII-GIV may be related to Vargfors and Arvidsjaur type rocks rather than to Skellefte Group volcanic rocks as indicated by the ages. The age of the Antak granite, 1879±15/12 Ma (Kathol & Persson, in press), which is comagmatic with parts of the Hej volcanic rocks, belonging to the subaerial Arvidsjaur Group, also indicates that the younger Jörn type intrusions may be related to the Arvidsjaur magmatism.
Weihed & Schöberg (1991) dated intrusive porphyries in the Tallberg deposit in the Jörn GI granitoid at 1886±15/9 Ma. Recent age determinations on deformed Jörn type granitoids in the southern part of the district indicate that the calc-alkaline magmatism continued to about 1.85 Ga as indicated by the age determinations on the Sikträsk intrusion at 1859±3 Ma (Weihed & Vaasjoki 1993) and 1854±9 Ma (Billström& Weihed 1996). This opens the possibility for volcanism at this age and a single zircon Pb-Pb evaporation age on a coherent feldspar porphyritic dacite from the Boliden area in the eastern part of the district yielded an age of 1869±15 Ma (Bergman Weihed et al. 1996). Bergman Weihed et al. (1996) also report a monazite age of 1852±4 Ma for the alteration system surrounding the Boliden deposit.
The minimum mel granites of the Skellefte and Härnö type have been sparsely dated, partly because of problems with inherited zircons. A monazite age for the Härnö granite at 1822±5 Ma is presented by Claesson & Lundqvist (1995). Unpublished data for the Skellefte granite by Weihed & Persson (in prep.) indicates that titanites in this rocks are 1798±4 Ma. Recently, Romer and Nisca (1995) published a titanite age of a skarn, 1825±4/3 Ma, from the Burträsk area south of the Skellefte district which they regarded as part of a hornfels aureole surrounding a calc-alkaline granitoid of the Jörn type and hence indicating the age of intrusion of the granitoid. However, this area is in middle to upper amphibolite facies, and the skarn related to peak regional metamorphism. In addition an unpublished age of c. 1.89 Ga for the intrusion makes this interpretation questionable. A more likely interpretation is that the published age represents the age of peak-metamorphism in the area which fits well with the ages of minimum melt granites at 1.82 to 1.80 Ga.
The alkaline to calc-alkaline Revsund type granites all have ages that fall within the age interval 1.801.78 Ga (Patchett et al. 1987, Skiöld 1988, Claesson & Lundqvist 1995). This means that they overlap in time with the minimum melt granitoids, but are derived from a deeper source, have risen and crystallised at shallower levels, while the minimum-melt granites like Härnö and Skellefte have been more static in the crust.
Scattered occurrences of Archaean gneisses have recently been dated (cf. Lundqvist et al. 1996) less than 100 km north of the Skellefte District. These occurrences are situated along what is described by öhlander et al. (1993) as the Archaean-Proterozoic border which was delineated by a shift from positive e(=epsilon)Nd values south of a line going from Piteå to Jokkmokk to progressively more negative values towards the north for 1.89 Ga intrusive rocks. This area also seems to correspond to a magnetic low on aeromagnetic maps.
Billström & Weihed (1996) showed that the Skellefte Group rocks hve very primitive e(=epsilon)Nd values and thus do not contain any Archaean crustal material. The possibility of a slightly older basement, c. 2.0 to 1.9 Ga in age, could not be ruled out on the basis of e(=epsilon)Nd values. Allen et al. (1996) argued on the basis of the overwhelmingly felsic character of the volcanism that the Skellefte Group must have been emplaced on a rifted continental basement during arc extension. The possibility is thus still open that the primitive, early Proterozoic, c. 1.95 Ga old mafic to felsic Knaften rocks may constitute a basement to the Skellefte Group which was totally reworked during the magmatism at 1.90–1.80 Ga. The existence of 2.0 to 2.1 Ga old rocks in the area is also indicated by the presence of detrital zircons of this age in the Bothnian basin metasedimentary rocks (Claesson et al. 1993).
Two major phases of folding have been proposed for most of the area. The early folds are tight to isoclinal, with upright axial surfaces and variably plunging fold axes (cf. Weihed et al. 1992). Axial surfaces strike north-east in the eastern and western parts of the Skellefte district and west-northwest in the central part of the district. In the strongly altered volcanic rocks and the sedimentary units in the western Skellefte district, fold axes plunge 45° towards SW (Edelman 1963, Du Rietz 1953) and a crenulation cleavage developed along the axial surfaces. In the central Skellefte district, fold axes are variable. Most fold axes plunge 45° SE, but a range of plunges between SE and SW has been observed. An axial planar foliation developed as a penetrative grain shape cleavage in coarser volcanic rocks and as a crenulation cleavage in laminated tuffs and schists. Penecontemporaneous slip along the cleavage is common in this area and shear zones occur in the same general orientation as the axial surfaces. Most of these shear zones have a reverse oblique-slip movement where the southern side has moved upwards over the northern side (Bergman Weihed 1997). In the vicinity of Boliden in the eastern Skellefte district Smith (1986) and Slade (1986) report fold axes and lineations that plunge 75° E and a related axial planar grain shape cleavage. Similar observations were made in the Boliden mine (Bergman Weihed et al. 1996).
The presence of a crenulation cleavage as an axial planar cleavage to the first major observed folds indicates that an earlier foliation is present. Such a foliation is reported by Allen et al. (1996) who interpret it as a bedding parallel foliation. Folds related to this early foliation have ben observed in thin sections from the sedimentaryunits west of Kristineberg (Bergman Weihed 1997) and similar observations were also made in mudstones in the central part of the Skellefte district (Bergman Weihed unpublished data). The early foliation may have formed during an early recumbent phase of folding and this has been suggested for the Långdal area by Talbot (1988) and Assefa (1990).
The second major phase of folding produced open folds with steep north- to northeast-striking axial surfaces and fold axes coaxial with the early folds. A spaced axial planar S2 crenulation cleavage developed locally and, in the central part of the Skellefte district, the intensity of the second folding increases towards the south with larger amplitude folds and a more penetrative cleavage. A number of north-striking shear zones are present in the Skellefte district and surroundings. These have a dominating reverse dip-slip movement and they are interpreted to have formed during or after the second major phase of folding (Fig. 9).
The massive sulphide deposits in the Skellefte district were deformed together with the supracrustal rocks, but some differences in structural expression can be observed between the sulphides and the surrounding volcanic pile. In general the massive sulphide ores show stronger deformation than the adjacent volcanic rocks, with shorter wavelength and higher amplitude folds, locally even with the development of mullions. Shearing is also in some instances localised to the massive sulphide ores (Talbot 1988, Bergman Weihed et al. 1996). The structures containing gold probably formed during the later stages of the second deformation. These structures involve e.g. semi-brittle shear zones in Tallberg (Weihed 1992) and en échelon quartz veins in Grundfors (Bergman 1992).
The timing of deformation is poorly constrained. No differences in structural style can be observed between the deepest exposed level in the volcanic pile and the uppermost Vargfors sedimentary units and andesitic lavas, and many syn-volcanic intrusions (Jörn-type) show the same structures as the supracrustal rocks. This implies that the first major phase of folding was not completed by 1854 Ma, the youngest intrusive age of a syn-volcanic intrusion (Weihed & Vaasjoki 1993). The post-volcanic Revsund granite, however, cuts the first major folds and thus limits this folding event to before 1.78 Ga (Skiöld 1988). The second major phase of folding at least locally affects the Revsund granite and must thus have occurred after 1.78 Ga.
Tectonic interpretations of the area have generally focused on the Skellefte district. Hietanen (1975) proposed a subduction zone dipping north beneath the Skellefte district and after that, many similar models have been proposed (e.g. Rickard & Zweifel 1975, Lundberg 1980, Pharaoh & Pearce 1984, Berthelsen & Marker 1986, Gaál 1986, Weihed et al. 1992). A northward subduction is supported by a magnetotelluric survey (Rasmussen et al. 1987) which found a low-resistivity slab dipping north under the Skellefte district and by a seismic reflection profile in the Bothnian Bay (BABEL goup 1990) which reports a north-dipping reflector east of the Skellefte district. The volcanic rocks in the Skellefte district (Skellefte Group) are thusinterpreted to represent some kind of volcanic arc. To the north, subaerial volcanic rocks (Arvidsjaur Group) represent a continental environment coeval with the volcanic arc and, to the south, the large area of metamorphosed greywackes may be interpreted as a fore-arc environment (Weihed et al. 1992). An Archaean component is detected in e(=epsilon)Nd values and as detrital zircons in the greywackes indicating that Archaean crust was present somewhere in the area. Towards the north-east around Luleå, however, deformed Archaean granitoid intrusions have recently been discovered (Lundqvist et al. 1996a). It is possible that tectonic contacts are most common between Archaean and younger rocks.
Metallogeny of the VHMS deposits
The Skellefte district has long been regarded as a massive
sulphide district with mining from complex base metal ores. To
date about 15 different VHMS deposits (of which some contain
several ore bodies) have been, or are presently being mined, all
by Boliden AB. During the last decade intense gold exploration
in the area has resulted in the mining of two gold lode deposits:
Björkdal (Terra Mining AB) and Åkerberg (Boliden AB). Apart
from these gold discoveries, other gold occurrences are presently
being evaluated by different companies. From the VHMS mining and
exploration a total of over 160 million tonnes of pre-mining ore
reserves with an average of 1.9 g/t Au, 47 g/t Ag, 0.7% Cu, 3.0%
Zn, 0.4% Pb, 0.8% As, and 25.6% S have been outlined (Allen et
al. 1996). The Boliden AB concentrator in Boliden today treats
1.4 million tonnes of complex ore producing 8 000 tonnes of Cu,
5 000 tonnes of Pb, 60 000 tonnes of Zn, 76 tonnes Ag and 2
tonnes of Au annually. The Björkdal mine produces c. 3
tonnes of Au per year, which makes the total Skellefte district
Au production c. 5 tonnes/year. Including the contribution of Au
from Aitik (Norrbotten) and Garpenberg (Bergslagen) the annual
production of Au in Sweden is approximately 7 tonnes.
Beside the economic deposits of VHMS and gold lode type, mining has been carried out on a Ni-deposit in the NW-part of the district, Lainejaur, and on a Li-bearing pegmatitic deposit, Varuträsk, in the eastern-most part (cf. Weihed et al. 1992). Occurrences of porphyry type Cu-Au deposits have also been described (Tallberg). All these different ore types are described briefly below.
Detailed description of individual deposits is beyond the scope of this guidebook. Interested readers are referred to the following papers for individual deposit descriptions: Boliden — Ödman (1941), Grip & Wirstam (1970), and Bergman Weihed et al. (1996); Långdal — Martinsson (1987), Talbot (1988), and Assefa (1990); Renström — Duckworth (1991); Petiknäs — Jonsson et al. (in prep.), Holmtjärn — Nicholson (1993); Mensträsk ores — Grip (1951); Rakkejaur — Trepka-Bloch (1985, 1989); Näsliden — Svenson (1982); Kristineberg — Du Rietz (1953). General information on the massive sulphide deposits can be found in Rickard (1986) and Allen et al. (1996).
In ternary metal ratio plots (Fig. 10), deposits like Boliden and Holmtjärn (type 3 above) are clearly different to most other deposits. In Boliden and parts of the Holmtjärn massive arsenopyrite ore, quartz and tourmaline veins together with extremely high gold grade make these deposits different from other massive sulphide deposits in the Skellefte district. An epithermal origin for the Boliden deposit has been proposed by Bergman Weihed et al. (1996).
The alteration envelopes are generally asymmetric with most intense alteration in the stratigraphic footwall (Allen et al. 1996). The main alteration type is quartz-sericite-pyrite alteration which may extend from 100 m to 1 km along strike and up to 2 km into the footwall like in Långdal and Långsele (Allen et al. 1996). An inner chloritic zone exists at Långsele and Rävlidmyran, while calc-silicate rocks are present in Rävlidmyran, Rävliden, Rakkejaur, Renström, Långdal, and Näsliden, which according to Allen et al. (1996) are of hydrothermal origin, except the Näsliden occurrence and part of the Rävlidmyran calc-silicates. Allen et al. (1996) also describe alteration zones which extend into the hangingwall at Holmtjärn.
A special case is the Boliden deposit with a central zone of extremely leached rocks consisting of a silica-alumina residue with sericite-quartz-andalusite and corundum (Bergman Weihed et al. 1996). This zone is enveloped by a sericite-rich zone and an outer chlorite-rich zone. Bergman Weihed et al. (1996) attribute this to strong cation leaching in an environment similar to modern high sulphidation epithermal systems. A similar alteration type is present in the Mångfallberget area immediately north of the Boliden deposit.
The massive sulphide ores have been extensively investigated by lead isotope studies (cf. Rickard 978, Rickard & Svenson 1984, Vaasjoki & Vivallo 1990, Billström & Vivallo 1994, and Bergman Weihed et al. 1996). Billström & Vivallo (1994) proposed a two stage mixing model for the source of lead in the massive sulphides, where Archaean crustal material was recycled to the mantle and also formed a crystalline felsic basement at 2.0 Ga. The metasomatised mantle and the felsic basement was partially melted at 1.89 Ga and local mixing between mafic and felsic volcanic sources are indicated by short linear trends (see Fig. 11), within individual domains in the district. This model of Billström & Vivallo (1994) fits well with the tectonic setting proposed by Allen et al. (1996) where they suggested that the massive sulphides were related to a continental arc under extension. Possible remnants of an older basement to the Skellefte volcanism may be the rocks of the Knaften area which have an age exceeding 1.95 Ga (Wasström 1993, 1996).
Between 1944 and 1948 a shaft was sunk down to the 470 m level and the ore was investigated by drilling on every 50 m level. Full scale production started in 1953. In 1959 the existing shaft was deepened to the 870 m level followed by drifting and diamond drilling in order to investigate the ore at deeper levels. The hoisting system from 1959 is still in use, but ore from lower mining levels is trucked up in a ramp to the lowest raising level.
At present, the deepest mining level is at 970 m, but preparations to deepen the mine to 1050 m have already begun. A drilling program to test the possible continuation of the ore at greater depths was completed in 1996. Several good ore intersections were obtained between the 1260 and 1550 m levels and further drilling in order to confirm grades and ore continuity will start in 1997.
Pre-production ore reserves down to c. 1000 m were 9 Mt with 2.8 g/t Au, 155 gt Ag, 0.8% Cu, 6.5% Zn, 1.5% Pb, and 15% S. During 1996 about 140,000 tonnes were mined by the cut-and-fill method. In 1995, construction of a 2.5 km long drift on the 800 m level from Renström to the Petiknäs mine was started. It was completed by the end of 1996 and when it is taken into operation in 1997, ore from the Petiknäs mine will be hauled by train to Renström and hoisted to surface there.
The ore bearing unit, which also contains the Renström ore, consists of rhyolitic, pumice-bearing, siltstone and sandstone. The pumice occurs as diffuse, irregular, strongly chloritic clasts with 1–3 mm large quartz phenocrysts occasionally as big as 7 mm. Due to alteration, feldspar phenocrysts are rarely preserved. Where pumice is abundant, the rock looks very much like a coherent porphyry. The pumice is interpreted as pyroclastic fallout or as blocks that floated off a nearby submarine rhyolite dome and then became waterlogged and sank (Allen et al. 1996). Inconnection with the ore deposition, hydrothermal solutions altered the rocks of the ore-bearing unit. Chloritization, silicification, dolomitization, and sericitization have thus largely altered the original composition and appearance beyond recognition. For example, in strongly chloritized parts, pumice may be recognised only as quartz crystals. In addition to the alterations mentioned, there is also widespread sulphide impregnation.
The ore-bearing pumiceous unit is overlain by laminated siltstone and sandstone with minor intercalations of black phyllite with traces of pyrrhotite and rare graphite. Graded bedding implies that the sediments were partly deposited from turbidity currents. The siltstone-sandstone also contains more or less calcareous parts. A syn-volcanic, often calcareous and calcite brecciated andesitic sill has intruded the silty-sandy unit (Fig. 13).
Rhyolitic mass flows overlie the siltstones-sandstones and consist of several up to 35 m thick graded beds. They are rhyolitic with up to 0.4 m large lithic, mainly rhyolite clasts, and pumice in the form of flattened, chlorite-rich, feldspar porphyritic lenses (fiamme). The tops of the beds are sandy-silty. The matrix of the mass flows is characterised by 1–2 mm large feldspar crystals and about 0.5 mm large quartz crystals. In the southern part of the mine at depths below c. 1 km therhyolitic mass flows have suffered strong carbonatization.
Quartz-feldspar porphyritic rhyolite domes intrude both the ore-bearing unit (below c. 1100 m level) and the overlying siltstones-sandstones and rhyolitic mass flows in the mine. The porphyries are early intrusions and to judge from diffuse contacts and hyaloclastite breccias (peperite), they intruded while the sediments were still unlithified. Peperitic parts of the porphyries are texturally similar to, and difficult to distinguish from, the pumiceous ore-bearing unit. Phenocrysts in the porphyries are usually 2–3 mm large, but quartz may be up to 7 mm, set in a fine-grained more or less sericitized, chloritized and silicified matrix.
A major dextral fault striking about N-S and dipping steeply west occurs about 300 m west of Renström. Stratigraphic correlations across the fault are uncertain. The rocks in the area are represented by strongly sheared mafic intrusions. West of the main fault the rocks strike west-northwest, but between the fault and Renström they strike roughly N-S, possibly as a result of reorientation in connection with the faulting. East of the mine the strike of the stratigraphy is again oriented more E-W.
The structure presented in Figure 13 is the result of detailed core logging where stratigraphic facings and bedding structures have been measured and plotted in order to give a clue as to the spatial orientation and degree of folding. Rhyolitic mass flows west of the ore show normal grading facing west and identical mass flows SE of the ore show younging to the east. The stratigraphically underlying siltstones-sandstones were partly deposited from turbidity currents and show the same younging pattern as the rhyolitic mass flows. Bedding structures are partly sub-parallel to drill cores and varying orientations indicate a fold closure to the north. The pumice bearing sediments that host the ore are thus the oldest, occupying the core of an antiform. The structure on the 700 m level also implies that the ore is in the core of a refolded fold, which is now isoclinal, and partly oriented about ESE-WNW. That part of the structure increases strongly in length with depth and is at least 300 m long on the 1500 m level. The rocks dip steeply and th F2 fold axes plunge about 75_ to SSE.
The ore bodies consist of several lenses, which is probably due partly to original deposition and partly to later deformation. The maximum length of the ore is about 400 m and the maximum width about 20 m. Ore reserves are calculated down to the 1050 m level, but mineralisation with good grades is known down to 1550 m. At higher levels the ore strikes approximately N-S and dips about 80 E, but judging from deep drill holes, the strike on the 1500 m level is near E-W. The information below 1050 m is still sparse, but the reorientation of the ore is interpreted to be due to folding around a fold axis plunging c. 75 SSE.
The geology of the Renström mine is structurally complex and a first compressional phase gave rise to folds oriented approximately E-W. A second phase, possibly in connection with shearing, folded the ore around F2 axes plunging 75 SSE. Younger, steep shear zones, which have partly affected the ore, are common. This shearing produced a variety of textures like lenticular elongate pyrite orientation, pyrite banding, and durchbewegung structures (Duckworth & Rickard 1993).
As mentioned above, ore deposition was accompanied by alteration like chloritization, silicification, dolomitization, and sericitization. Quartz crystals in dolomite matrix are interpreted as relics of pumice, indicating that the dolomite was not deposited as a chemical sediment but is part of the alteration system. Within the alteration there is also widespread sulphide dissemination, which partly forms a chalcopyrite-pyrite-pyrrhotite stringer zone (Jonsson & Larsson 1986).
Shallow-marine to fluvial, andesitic volcaniclastic rocks occur less than 50 m below the Renström-Kyrkvägen ore-bearing unit whereas the ore-bearing unit is overlain by turbiditic siltstone-sandstone with intercalations of black phyllite, which were probably deposited in water depths of 100 m r more (Allen et al. 1996).
Renström is interpreted to be a replacement ore since it occurs within a pumiceous, pyroclastic fallout unit regarded to have been rapidly emplaced (Allen et al. 1996). Besides, there is no consistent metal zoning pattern within and between different ore lenses. The rhyolitic rocks associated with the Renström ore are interpreted as part of a rhyolite cryptodome-tuff cone volcano (Allen et al. 1996).
Construction of a 2 km long decline to access the Petiknäs South ore-bodies started in March 1989, and the first ore was produced from the 290 m level in Petiknäs South in March 1992. In 1995, construction of a 2.5 km long ore haulage drift on the 800 m level from Renström to the Petiknäs mine was started. It was completed by the end of 1996 and when it is taken into operation in 1997, ore from Petiknäs will be hauled by train to Renström and hoisted to surface there. The decline now extends down to the 600 m level, and will be deepened down to the 750 m level (corresponding to the 800 m level in the Renström system) where it is to be connected to the ore haulage drift to Renström.
At present, cut and fill mining is only taking place in the Petiknäs South ore body, from the 220 m level down to the 600 m level. During 1996 about 393 000 tonnes of ore were mined.
Pre-production ore reserves in Petiknäs South down to c. 750 m level were 6.5 Mt with 2.3 g/t Au, 108 g/t Ag, 1.1% Cu, 4.8% Zn, 0.9% Pb and 37% S. For Petiknäs North the corresponding figures were 1.3 Mt with 5.6 g/t Au, 103 g/t Ag, 1.3% Cu, 5.6% Zn, 0.9% Pb, and 14% S.
Eruptive material shed from these domes consists of altered pumiceous siltstones and volcanic sandstones. This unit is dominated by finely laminated grey-black siltstones, with minor quartz-fragment rich, sandstone units. Chloritized pumice clasts ranging from 1 to 7 cm in size, and quartz crystal-rich layers are distributed throughout the siltstone and indicate a pyroclastic origin for this unit. Graded bedding indicates a stratigraphic facing direction towards the south. This unit is the host rock to the massive ores.
A polymict mafic mass flow breccia dominates the hanging wall of the Petiknäs South area. The breccia unit contains 10–20% angular to subrounded clasts in a mafic matrix composed of silt and sand. The three main lithic clast types that occur within this unit are: bluish coloured feldspar porphyry clasts, white rhyolitic quartz-feldspar porphyry, and cream coloured altered basaltic clasts. The source rocks to these clast types are not represented in the rocks immediately adjacent to the mine. Graded bedding observed within the unit indicates a stratigraphic facing direction towards the south. Several faulted contacts are present indicating that this unit was partly tectonically emplaced into its present position. The unit is approximately 100–150 m thick. A pumiceous quartz-feldspar rich rhyolitic tuff with a sandy matrix occurs stratigraphically above the polymict mass flow breccias. This 50–100 m thick unit also contains some weak sulphide dissemination. A weakly feldspar-quartz porphyritic intrusion of rhyolitic composition is emplaced into the polymict mass-flow breccia unit and the pumiceous felsic unit. Stratigraphically above these rocks, a weakly feldspar porphyritic dacitic intrusions with a greenish colour occurs. The unaltered, or weakly saussuritized feldspar crystals are 2–7 mm in size and make up 1–5% of the rock. The unit also contains xenoliths of the pumiceous sandy rhyolitic unit mentioned above. An andesite sill-like intrusion has displaced parts of the B-C and A-D lenses. It is a homogeneous, dark green, fine-grained, magnetic unit, composed of plagioclase, amphibole, some biotite and chlorite. The chlorite occurs along contacts and along small shears. In addition to this, euhedral pyrite crystals are common, as well as caronate veinlets.
A high-level, Jörn-type granitoid occurs to the south of the mine as a large intrusion, with several apophyses intruding into the upper stratigraphic levels of the mine lithology.
The ore system shows a progression from massive pyrite-Zn-Cu-Pb-Au-Ag ores in the basal B- and C-lenses, to Zn-Pb-Ag ores in the stratigraphically higher A- and D-lenses. The maximum length of the B-C lens occurs at the 400 m level where the ore-bodies are roughly 300 m long, and the maximum width is 24 m. The B-C lens also shows a strong metal zonation with sphalerite-pyrite rich margins and a more chalcopyrite-pyrite dominated central part. The B-C lens is underlain y patchy to pervasive silicification often associated with sulphide stringer zones. The ores are situated within strongly silicified, sericite-chlorite altered felsic pumiceous volcanic sandstone/tuff material, which is often strongly pyrite-sphalerite mineralised. The alteration is manifested in breakdown of feldspar to the typical greenish sericite-silica assemblages, which overprint the original textures of the rock.
The A- and D-lenses are situated within strongly garnet-chlorite-magnetite altered volcanic rocks, which probably were pumiceous rhyolitic volcanic sandstones. They are smaller, massive to semi-massive ZnS-PbS-FeS2 ores, which grade into low-grade disseminated pyrite-sphalerite mineralisation. The average Zn-grade of the A-lens is around 10%. Maximum strike length of the A and D-lenses is 60 m with a maximum width of 8 m at the 350 m level.
A late andesitic sill-like intrusion, occurs within the ore-bearing unit, cross-cutting both the B-C lens and the Zn-Pb-rich A-D-lenses. This andesite exhibits both chilled margins and faulted contacts, and abruptly truncates the individual ore-lenses, creating an apparent fold pattern. It is magnetic and composed of plagioclase, biotite, and amphibole, with chlorite preferably along faulted contacts.
The preferred mode of formation for the whole Petiknäs system is an exhalative sea-floor type massive sulphide deposit associated with a cryptodome-hyaloclastite-tuff cone rhyolitic volcano (Allen et al. 1996, Jonsson et al. in prep.). A minor thin subeconomic mineralisation situated stratigraphically below the main B-C lens is interpreted as having formed as a sub-sea floor replacement type of ore. The total thickness of the ore-bearing unit is about 200 m.
As mentioned above gold deposits have been known in the district since the last century. Most of the early workings were on quartz-arsenopyrite veins hosted by metasedimentary rocks. One of these is the Grundfors deposit, situated c. 10 km south of Boliden. This subeconomic deposit consists of 1 to 50 cm wide quartz veins locally with abundant arsenopyrite and gold (cf. Bergman 1992). Other subeconomic gold lode deposits described in the literature include Storklinten (Bergström & Lundqvist 1989), Vinliden (Broman et al. 1995, Bergström 1996), and Middagsberget/Fäbodliden (öhlander & Markkula 1994, Sundblad et al. 1994). Shear zone hosted remobilised gold in the Tallberg porphyry type deposit is also described by Weihed (1992b).
The Boliden deposit, from which 123 tonnes of gold were recovered at an average grade of 15.5 g/t, is also discussed in the massive sulphide section above. This deposit is interpreted by Bergman Weihed et al. (1996) as an equivalent of modern high sulphidation epithermal type of deposits.
No thorough description has been published on the Åkerberg deposit so only a brief summary of the geology of this gold deposit can be given here. The deposit is situated in the north-eastern part of the Skellefte district c. 15 km NNE of the Björkdal deposit (Fig. 5). The gold is confined to a c. 20 m wide zone of cm wide parallel quartz veins within a gabbroic intrusion. The host gabbro probably belongs to the c. 1.89–1.87 Ga old Jörn suite and the ore is cut by late pegmatites related to the 1.80 Ga Skellefte type intrusive rocks. Published figures on grade and tonnage are scarce but, the grade is reported at c. 3 g/t and the proven tonnage is >1 million tonnes (cf. Weihed et al. 1992).
The gold occurs as electrum, amalgam, or tellurides. In Björkdal both electrum and tellurides are found (incl. tsumoite and bismuth-tellurides). The silver content in electrum in Björkdal varies between 5 to 10% (Broman et al. 1994). The gold in Middagsberget occurs as droplets in arsenopyrite or at interfaces between arsenopyrite grains, as inclusions together with pyrrhotite in arsenopyrite, as free grains in silicates, or in fractures in carbonates (öhlander & Markkula 1994). In Tallberg the gold is most commonly electrum (av. Au75-Ag25) with minor coloradoite, petzite, hessite, and sylvanite. In Boliden the gold is mainly found in strongly brecciated ore as free grains in quartz or at the contact between quartz and arsenopyrite, in fine-grained arsenopyrite as small inclusions together with Bi-Se minerals, in pyrite ore together with sulfosalts and galena in veins cutting the pyrite, and in tourmaline ore in fractures in tourmaline and quartz. All gold in Boliden is present as an Au-Ag-Hg alloy. The gold in Vinliden is associated with tourmaline and Sb-bearing minerals (Bergström 1996).
The structural control on the Björkdal deposits is similar in th sense that tensional quartz veins in the footwall to a major thrust are mainly found in the more competent host intrusive. According to Billström (1996) the early quartz veins were formed in association with the emplacement of the quartz-monzodiorite at c. 1905 Ma. Billström (1996) proposed three subsequent thermal events which all caused remobilization of gold. Fluid inclusion and stable isotope data from Björkdal (Broman et al. 1994, Billström 1996) suggest a complex fluid evolution involving magmatic, marine, and meteoric fluids.
Billström & Weihed (1996) suggested that gold was introduced at. c 1.9 Ga as mesothermal gold in Björkdal. High gold grades are found in VHMS deposits with an age of 1.89–1.88 Ga, epithermal style gold in Boliden was tentatively emplaced as late as 1.85 Ga and finally syn-peak metamorphic gold deposits were formed during crustal thickening during high heat flow in the middle crust in deposits like Grundsfors, Vinliden and Storklinten at c. 1.84–1.80 Ga.
The Björkdal ore body (Fig. 17) was first indicated by geochemical sampling in 1983, then in solid rock in 1985. Mining began in 1988, with the development of the Björkdal ore body by open pit mining. The ore reserve 1997 is estimated to 14 million tonnes at 2.2 ppm.
The first ore body, now the central open pit, was followed in 1991 by the eastern open pit. These two pits are now connected. In 1992 larger scale mining with bigger blasts started. In 1996 the new western pit was opened, where mining is presently on a higher level than in the other pit. In 1995 underground mining was commenced with two drifts at the 108 m and 130 m levels. Ore was taken out between the drifts beneath the limestone (Fig. 17). The production was 46 000 tonnes of ore with a grade of 2.8 ppm gold.
The whole Björkdal area is strongly deformed or tectonized
and several shear zones cut the area in different directions.
There are examples of both ductile and brittle deformation zones.
The area around the mine can be characterised tectonically as an
area with internally well preserved megalithons, which are
bordered by shear zones. Mapped displacements are in the order
of a few meters in the mine area.
The timing of the thrusting is poorly known and late reactivation
associated with the intrusions of Revsund granitoids can not be
ruled out.
The gold bearing quartz veins are concentrated close to the
marble contact and often the veins strike parallel to the contact
in the western ore body. The quartz veins are up to meter wide
and can be followed for several hundred meters. The ore dips
about 25° to the north beneath the limestone. Several brittle
deformation zones cut the quartz veins and the gold seems to be
concentrated in areas where the brittle zones cross cut the
quartz veins, possibly indicating late mobilisation of gold.
Shear zones are striking east-west, nearly perpendicular to the
wide quartz veins, and have subhorizontal dips. A generalised
vertical profile can be seen in Figure 18.
The central ore body is characterised by quartz veins up to one
meter wide and up to 200 m long. The mineralised quartz veins can
be followed down to 50–70 meterdepth where they get thinner
or pinch out. The frequency of quartz veins in the main direction
is highest close to the contact to the volcanic rocks, the main
thrust and the contact to a folded marble tongue. These quartz
veins are rich in gold and bulk mining is done in this part of
the mine. Second order shear zones with quartz veins are also
present in this ore body and can contain high values of gold.
These quartz veins are characterised by the existence of
tourmaline and chlorite.
The main thrust in the contact zone strikes E-W and is 30–40
m wide. Tensional quartz veins were preferentially emplaced
beneath the thrust within the more competent intrusive, but
smaller veins can be seen for instance in boudinaged amphibolites
within the deformation zone between the marble and the volcanic
rocks. The mineralisation in this deformation zone consists of
small amounts of pyrite, pyrrhotite, and chalcopyrite with gold
grades of maximum 1–2 ppm.
In the eastern ore body quartz veins with extremely high values
of gold have been found. This ore body consists of less deformed
rocks between transverse shear zones. The main quartz veins are
more or less vertical, and they may deflect close to the
deformation zones. A marked albitization can be noticed close to
the gold mineralisation. At the intersection between the shear
zones and major quartz veins an area of extremely high gold
grades exist. The grade in this ore body varies between 3 and 20
ppm gold.
The main quartz veins are very distinct with sharp contacts. The
host rock alteration is insignificant only 2–3 dm away from
the quartz veins. A weak sericitization is found close to the
veins. The gold seems to be concentrated in the rim of the veins,
which are rich in pyrite and pyrrhotite. Tsumoite,
bismuth-telluride and scheelite are associated with gold.
Bornite, covellite, and cuprite exist as secondary copper
minerals as well as pure copper.
The Tallberg porphyry deposit is situated in the outer and oldest
GI phase of the Jörn granitoid and it is associated with
high-level quartz-feldspar porphyritic dykes and intrusions. The
age of these porphyritic intrusions is 1886±15/12 Ma (Weihed &
Schöberg 1991) which is, within errors, the same as the GI
host rock and the age of the host rocks to most massive sulphide
deposits in the district. The deposit is of a lowgrade (0.3% Cu) and high tonnage (>50 Mt), disseminated
type. A typical alteration zoning exists with a proximal zone of
phyllic alteration with quartz, sericite, and pyrite grading out
into a distal propylitic alteration with abundant chlorite. Ore
minerals occur both disseminated and in a sulphide-quartz vein
stockwork which is most intense in the central part of the
deposit. Metals are also zoned with Cu concentrated to the
central parts while Zn and Pb are concentrated to the marginal
parts. Magnetite appears to be an early phase of the alteration
system. Gold grades are low (< 1 gt), but in strongly
sheared sericite alteration zones cutting the deposit the gold
grades can reach >10 g/t. Stable isotope data and fluid inclusion
studies indicate that the sulphides precipitated when magmatic
fluids mixed with sea water at elevated temperatures of 450 to
500° C (Weihed & Fallick 1994, Weihed et al. 1992).
The Lainejaur deposit has been described by Grip (1961) and
Martinsson (1987). The total production was only 100 526 tonnes
of ore with a content of 2.20% Ni, 0.93% Cu, and 0.1% Co (Grip
1961). The ore is associated with a gabbroic intrusion composed
of a feeder (?) dyke with apophyses and an overlying gabbro
laccolith. Three types of ore were identified: disseminated
nickel-pyrrhotite, massive nickel-pyrrhotite-chalcopyrite at the
contact between the dyke, laccolith and metasedimentary wall
rocks, and late stage nickel-arsenic veins.
The älgliden deposit is associated with a 3.5 km long and
up to 100 m wide mafic dyke in the Jörn batholith (Fig. 3).
The dyke is a multiple intrusion with c. 90 000 tonnes of copper
and 26 000 tonnes of nickel at grades of 0.7% and 0.2%
respectively (Nilsson 1985).
The Lappvattnet deposit is one of several Ni-occurrences in the
so called Nickel zone or Burträsk Shear Zone which is a
prominent NE-striking shear zone c. 30 km south of the Skellefte
District (see cover illustration). The ultramafic intrusions with
associated Ni-Cu ore are small, <150 m long and <40 m wide
(Nilsson 1985). The Lappvattnet deposit was the subject of trial
mining and a shaft was sunk to the 120 m level in the period
1978–1982, but the ore was considered subeconomic. A total
of 11 000 tonnes of Ni at 1% and 2 300 tonnes of Cu at 0.2% was
outlined.
The idea, first described by Hietanen (1975), that rocks of the
Svecofennian Domain are closely related to Palaeoproterozoic
island arcs which formed 1.8–2.0 Ga ago (Gaál 1985,
Kähkönen 1989, Park 1991, Ekdahl 1993, Lahtinen 1994,
Kousa et al. 1994) is now widely accepted. Most of the
supracrustal rocks of the SD are turbiditic metasedimentary
rocks. Graphite bearing schists, black schists, and carbonate
beds are common intercalations in many areas. The minimum age of
deposition of the turbidites can roughly be estimated by the age
of detrital zircons from the metasedimentary rocks. The zircons
show a bimodal age distribution with the majority of tqazhe
zircons having ages between 1.91–2.1Ga while some are
Archaean in age (Huhma et al. 1991, Claesson et al. 1993).
Volcanic rocks exist as narrow discontinuous belts or limited
occurrences among both metasedimentary rocks and intrusive
complexes. Metavolcanic rocks in central Ostrobothnia represent
two different age groups which also have distinct chemical
characteristics.
Among the plutonic rocks in the SD, quartzdioritic to
granodioritic and granitic intrusions dominate over mafic and
ultramafic intrusions (Simonen 1980). Granite-pegmatites,
probably of S-type, are common n the Bothnian Belt. Intrusive
rocks of the central and eastern part of central Ostrobothnia are
mantle derived, Palaeoproterozoic rocks with no sign of Archaean
components (Huhma 1986, Lahtinen 1994).
Geographically the supracrustal rocks of central Ostrobothnia can
be divided into three different areas or blocks, separated from
each other by shear zones, thrusts, or intrusions (Fig. 21). The south-western-most part of central
Ostrobothnia can be included in the Bothnian Belt, dminated by
greywackes and pelites metamorphosed to biotite-plagioclase
schists and gneisses. These are intercalated with thin elongated
formations of intermediate to mafic metavolcanic rocks of N-type
MORB to WPB characteristics (Vaarma & Kähkönen 1994).
Well preserved primary sedimentary and volcanicstructures
indicate a turbiditic origin for the schists and a subaqueous
depositional environment for the metavolcanic rock. The
metamorphic grade in this part of the BB increases towards the
Bothnian Bay.
The Nivala gneiss complex (Ngc) which is dominated by migmatitic
mica gneisses, is situated between a N-trending belt of granitoid
batholiths (Toholampi-Rautio-Kalajoki) in the west and the
SE-trending Ruhaperä Fault Zone. These gneisses locally have
amphibolitic, or graphite and sulphide bearing intercalations
(Isohanni et al. 1985). Migmatites of the Ngc are host rocks to
the Ni-bearing ultramafic intrusions in the Nivala area. Around
the Ngc several formations of metavolcanic rocks (Kuusaa Fm,
Kangas Fm, Sievi Fm, Antinneva Fm) occur, probably emplaced
stratigraphically above the Ngc. The volcanic formations are
separated from each other by more ore less continuous
volcaniclastic rocks composed of polymict conglomerates and
finer-grained silt to sandstones, often crossbedded. Most of the
pebbles in the conglomerates are of volcanic origin. Granitoid
pebbles are also observed in conglomerates in the
Haapajärvi, Ylivieska and Raahe areas. The U-Pb zircon age
of one granitoid pebble from the Settijärvi conglomerate
(Haapajärvi area) is about 1888 Ma (Vaasjoki & Sakko 1988).
Well preserved primary structures indicate subaerial or shallow
water depositional environment for the calc-alkaline metavolcanic
rocks. The metavolcanic rocks vary from basalts to potassium
rhyolites in composition and have a mature island arc affinity.
This volcanism in the northern part of the SD is closely related
to early, syntectonic magmatism of the CFGC, which is c.
1890–1875 Ma in age. This magmatism is also related to the
peak of regional metamorphism in this area. The Ngc area is
interpreted as a gently dipping domal structure surrounded by
younger, locally flat lying volcanic and sedimentary formations
(i.e. Kuusaa, Kangas, Sievi and Lestijärvi Fm).
The third major area in central Ostrobothnia is the
Vihanti–Pyhäsalmi ore belt, which constitutes the
north-western part of the Savo Belt (SB), bordered by the
Ruhaperä Fault Zone and Haapavesi igneous complex in the
south-west and the Revonneva Shear Zone and the Archaean Basement
Complex in the north and east. The Vihanti–Pyhäsalmi
belt is also divided into two major blocks by the SW-trending
Oulujärvi shear zone (Kärki et al. 1993), indicating
that the Vihanti and Pyhäsalmi belts are at least
tectonically separated (Fig. 20). This moderately to highly
metamorphosed area is dominated by migmatitic mica gneisses. The
turbiditic origin of these metasedimentary rocks can still be
recognised in some places. The mica gneisses are intercalated
with minor quartz feldspar gneisses, black schists, skarn beds,
and some amphibolites of volcanic origin. Massive sulphide
deposits of the SB are closely related to these volcanic
environments of which the most important are the Pyhäsalmi
and Vihanti ore belts. The lithologically similar Pielavesi and
Kiuruvesi fields south-east and east of Pyhäsalmi are
correlated with the latter (Ekdahl 1993).
The Pyhäsalmi volcanic complex (Pvc) includes two volcanic
formations, the Ruotanen Fm and Mullikkoräme Fm, and the
Kettuperä gneiss (Kg). These rocks are intruded by numerous
syntectonic intrusions. The lowest part of the Pyhäsalmi
volcanic complex consists of silicic tuffaceous and pyroclastic
rocks with minor mafic intercalations. Towards the top mafic
pyroclastic rocks, pillow lavas, and pillow breccias become more
abundant. The metavolcanic rocks in the Pyhäsalmi area are
locally well preserved, but most of the rocks have been strongly
altered by hydrothermal processes and deformed by later tectonic
events. The volcanic formations of the Pvc are bimodal (Mäki
1986). Most of the felsic rocks can be classified as
calc-alkaline, low-K rhyolites, derived from melting of an
unknown Palaeoproterozoic sialic crust. The felsic volcanic rocks
are considered cogenetic (Kousa 1990, Lahtinen 1994) with the
oldest known rock, the Kettuperä gneiss dated at 1930±15 Ma
(Helovuori 1979). A quartz porphyritic variety of these rhyolites
from the Riitavuori Mb yields an U-Pb zircon age of 1921±2 Ma.
The mafic metavolcanic rocks are sub-alkalic, low-K tholeiitic,
basalts to basaltic andesites, with primitive IAT affinity (Kousa
et al. 1994).
The stratigraphy of this, in part, poorly exposed area is not
well known. General descriptions of some supracrustal formations
have been made (Luukkonen & Lukkarinen 1986), and nowhere a
basement has been identified. At the moment only very general
conclusions can be made, indicating that the BB, Ngc, and SB
including the Pyc, may be older than 1910 Ma. Calc-alkaline
metavolcanic rocks and related volcaniclastic metasediments in
the central part of the area may represent the youngest
Palaeoproterozoic supracustal rocks in the north-western end of
the Raahe–Ladoga Zone.
In central Ostrobothnia the earliest compressional structures are
isoclinal to tight F1 folds (Luukas 1991) which have been
identified in well preserved mica gneisses near the border of the
Archaean craton. It is plausible that this folding thickened the
originally thin ore horizons in the Ruotanen Formation making the
ores economic. This rarely identified structural stage has been
considered to be coeval with the nappe emplacements and recumbent
folds described from eastern Finland by Koistinen (1981). During
the D2 stage, the D1 structures were openly refolded reflecting
progressive deformation in the same compressional regime.
The D1-D2 tectono-metamorphic stage caused nappe emplacement
towards east or north-east with considerable crustal thickening
as a result. The abundant intrusions associated with the D2 stage
were emplaced within a very narrow time interval between 1890 and
1880 Ma. Ni-bearing mafic magmas are possibly related to deep
faults along the Raahe–Ladoga Zone at this time (Ekdahl et
al. in prep.).
The D3 phase caused intense F3 folding, which refolded the
earlier flat lying structures into an upright position along the
Raahe–Ladoga Zone. Contemporaneous magmatism produced a
large volume of tonalites and tonalitic migmatites, related to
high temperature, low pressure metamorphism at 670–800 °C
and 5 kb. (Korja et al. 1994). The emplacement of pyroxene
bearing granitoids at 1884 Ma, which are characteristic of the
Raahe–Ladoga Zone is related to this stage. In the later
stages of D3 the deformation style changed gradually from folding
to ductile shearing causing large scale dextral SE-trending
strike slip faults along the Raahe–Ladoga Zone initiating
major fragmentation of the crust. At the north-western end of the
RLZ the dextral Ruhaperä Fault Zone (RuFZ) and the Revonneva
Shear Zone (ReSZ) are examples of these shear zones (Fig. 20).
West of the Raahe–Ladoga Zone the effect of the D3 was not
penetrative and more open and flat lying F3 structures were
generated. SE- and N-trending F3 axial planes are the most
conspicuous mesoscopic and macroscopic structures in central
Ostrobothnia. The SE-trending structures in the RLZ and in the
northern Ostrobothnia Schist Belt, east of Oulu, indicate that
the compressional field was directed in SW-NE during D3
(Kärki et al. 1993).
After the D3 event, folds and ductile shear zones were formed
during the D4 stage. The E-trending axial planes of F4 folds can
be seen in many places in Ostrobothnia indicating that the
compressional field was shifted from SW-NE to N-S at this stage.
The Vihanti mine is situated at the south-eastern limb of a large
D4 antiform. The most conspicuous structural feature of the D4
stage is the crustal scale sinistral SW-trending Oulujärvi
Shear Zone (OSZ), which transects the Kainuu Schist Belt and the
Archaean craton (Fig. 20). As earlier mentioned the sinistral
movement on the OSZ caused a separation of the
Vihanti–Pyhäsalmi belt into two separate blocks, the
Vihanti and Pyhäsalmi belts, respectively. At this stage the
Revonneva and Ruhaperä shear zones formed during the D3
stage were reactivated with significant movements for example
around the Vihanti block. This block which is characterised by
a high gravity anomaly and a high metamorphic grade is surrounded
by major D4 shear zones and nicely displays the fault controlled
nature of metamorphic blocks in the RLZ (Korsman et al. 1988).
Other significant D4 features are the faults and shear zones that
surround the large intrusive bodies at the western and southern
edge of the Nivala gneiss complex. These faults are important for
the localisation of the Au-mineralisations in Ostrobothnia. The
south-western-most faults of the OSZ that extend to the
Pyhäsalmi area have a strong structural influence on the
Pyhäsalmi and Mullikkoräme deposits. Granitic partial
melts seen as potassium-rich neosomes and abundant pegmatites
along the shear zone were generated during this deformation stage
in the Oulujärvi region. Age data preented from the OSZ by
Kärki et al. (1995) show that granitic intrusions were
emplaced 1860–1800 Ma ago. This magmatism can be related to
the 1830–1810 Ma tectonometamorphic event in the
south-eastern part of the RLZ (Korsman et al. 1984, Korsman et
al. 1988). Kärki & Laajoki (1995) pointed out that the D4
event is more important than yet realised in the crustal
deformation history in Finland.
The Vihanti zinc deposit was discovered in 1947 by the GSF and
the mine was opened 1954. After producing 26 Mt in 38 years the
famous mine was closed in 1992. The Pyhäsalmi deposit was
discovered in 1958 when a local farmer was digging a well in his
back yard. Production started in 1962 and with the present
reserves mining will continue into the next century. The
Pyhäsalmi mine has had three satellite mines, the
Kangasjärvi mine which closed in 1985, the Ruostesuo mine
which closed in 1989, and the Mullikkoräme mine, which is
still in production. Also the Vihanti and Pyhäsalmi mines
are owned by the Outokumpu Oy.
The grade and tonnage of the deposits and important
mineralisations can be seen in Table 4.
Because of strained economy the annual production was increased
from originally 300 000 tonnes, first to 450 000 tonnes and to
500 000 tonnes in 1981. When the Ni- prices collapsed in 1982 the
mine was closed but left in care and maintenance. Because of the
high price of Ni, the mine was opened during a short period in
1984–1985. A third start up was made in 1988 and since then
the mine has been in production.
The total amount of hoisted ore is 9.3 Mt with 0.58% Ni,
amounting to 38 000 tonnes of nickel metal.
Outokumpu acquired the mining licences from the state in 1951.
The reserves were estimated to 0.7 Mt of ore with 6% Zn and 0.4
Mt of pyrite mineralisation, but it was understood that the total
reserves were higher. At the end of the same year development
work started. The reserves were increased to more than 6 Mt with
12.5% Zn, 0.8% Cu, and 0.5% Pb.
Annual production was planned to 400 000 tonnes. In 1968 the
capacity was doubled. In the 1970 s the plant was rebuilt and the
annual capacity was increased to 1 Mt. The mine was closed in
1992 after a total production of 28.1 Mt with 5.12% Zn, 0.48% Cu,
0.36% Pb, 25 g/t Ag, and 0.49 g/t Au.
In 1967, after extension and automatization of the processing
plant, the annual production was increased from 600 000 to 800
000 tonnes. The current annual production from the underground
mine is 1.08 Mt.
During the intensive exploration period after the discovery of
the Pyhäsalmi deposit, several small massive sulphide
deposits were found in the Pyhäsalmi region. None of these
were economic by themselves. Using a separate crushing station
and increasing the capacity of the processing plant, it has been
possibly to mine the best parts of the different smaller deposits
in satellite mines. The first satellite mine was the
Kangasjärvi mine in Keitele, c. 35 km south of
Pyhäsalmi. The Kangasjärvi Zn-pyrite ore was discovered
by the GSF in 1964. The second satellite mine to be opened was
the Ruostesuo mine in the Kiuruvesi commune c. 30 km east of
Pyhäsalmi. The ore deosit was found already in 1959 by
airborne geophysical measurements. The Ruostesuo mine was in
production in 1988–1989. The third deposit, the
Mullikkoräme deposit, was discovered by the Outokumpu Oy in
1987. The Kangasjärvi and Ruostesuo mines were small open
pits while the Mullikkoräme mine was the first underground
satellite mine. Production started in period in 1989–1990
from levels +50 to +140. A deep ore body called Siperia was found
in 1990, and production from the +525 level started 1996.
The total production from the satellite deposits has been c. 800
000 tonnes. Production figures are found in Table 4.
Altogether seven base metal mines have been in operation in
central Ostrobothnia of these three are still in production. The
total metal production from the area up to the end of 1996 was:
39 000 tonnes of nickel, 1 800 000 tonnes of zinc, 300 000 tonnes
of copper and 70 000 tonnes of lead. The Pyhäsalmi mine has
produced 16 000 000 tonnes of pyrite concentrate and
Pyhäsalmi is today the largest pyrite producer in the world.
The Vihanti mine in addition produced 7000 kg of gold and 420 000
kg of silver. The amount of gold and silver produced in
Pyhäsalmi is 5400 kg and 246 000 kg, respectively. All of
several discovered gold mineralisations are so far
subeconomic.
The Pyhäsalmi island arc (PIA) zone, which is the volcanic
component of the Savo Belt, is about 10–30 km wide and at
least 300 km in length. Hydrothermal activity has been confined
to areas with volcanic rocks which define a discontinuous,
deformed belt. According to Vaasjoki (1981) the Pb-Pb age of the
Pyhäsalmi-Vihanti ores is c. 1.9–2.0 Ga and the ata
suggest that the island arc volcanism was generated from a
homogenised crust, influenced by mantle derived magmas.
The PIA zone is known to contain about 55 mineralisations in
addition to the economic ore deposits like the Lampinsaari
(Vihanti), Ruotanen (Pyhäsalmi), Mullikkoräme, Kallio-
kylä and Kangasjärvi deposits. The total in situ metal
content of the PIA zone is estimated to 2 400 000 tonnes Zn, 500
000 tonnes Cu, 100 000 tonnes Pb, and 15 tonnes Au (Gaál
1990).
According to Ekdahl (1993) the ore deposits in the PIA zone have
the following characteristics:
The whole complex is strongly deformed in several stages, and the
lithology is overturned (Fig. 22). The original
volcanic vent(s) has not been detected (the hanging wall contact
is tectonic and the direction or magnitude of the displacement
is not known.). Different stock like plutonic rocks are common
in the region (the Haapajärvi igneouscomplex), but in the
Lampinsaari area they occur as dykes and veins, often associated
with faults.
Two generations of sulphide mineralisations have been identified,
both of which started with pyrite precipitation. The Hautakangas
and Hautaräme pyrite bodies have been partially mined out,
especially the Cu- and Zn-bearing parts. Some skarn lenses
contain enough Cu to form separate chalcopyrite-pyrite ores. Of
these some have been mined for Cu, although the Cu-grade was
commonly as low as 0.5%. Pyrite and pyrrhotite were treated as
gangue in these ores.
By far the most important ore type is the zinc ore. It forms
several ore bodies of different size and grade and practically
all mineable bodies have been mined out. The most common host
rock is dolomite or dolomite skarn. Zn content varies from 3% to
over 20%. Besides Zn the ore contains Cu, Pb, Ag, and S. The
largest ores have been the Ristonaho, Välisaari,
Lampinsaari, Isoaho, and Hautaräme ores. The youngest
sulphide mineralisation is a Pb-Ag-Sb mineralisation with diffuse
"clouds" of galena. This type of mineralisation contains
noticeable amounts of Ag in sulphosalts together with Au. The
typical host rock is a coarse diopside skarn.
More detailed descriptions of the Vihanti deposit can be found
in Rouhunkoski (1968), Rauhamäki et al. (1978), and
Rehtijärvi et al. (1979).
The metavolcanic rocks in Pyhäjärvi can be divided into
two groups on the basis of field observations and chemical
composition (Kousa et al. 1994). The western part of the
Pyhäjärvi area belongs to the Nivala gneiss complex
(Ngc) and the metavolcanic rocks there can be compared to the
Kuusaa formation (KuF) (see above). The Pyhäsalmi and
Mullikkoräme ores are situated in the eastern part of the
volcanic belt (the Pyhäsalmi volcanic complex). The
Pyhäsalmi volcanic complex (Ruotanen schist belt in
Helovuori, 1979) can be divided into the Ruotanen Formation (RuF)
and the Mullikkoräme Formation (MuF) (Fig.
24). A large gneiss area (the Kettuperä gneiss), which
is intruded by syntectonic plutons is situated between the
Ruotanen and Mullikkoräme formations. Both formations
contain large areas of mafic volcanic rocks with pillow lavas
(the Mukurinperä Mb and Tetrinmäki Mb), sodium-rich
rhyolitic volcanic rocks (the Lippikylä Mb and Riitavuori
Mb), and volcaniclastic rocks (the Pellonpää Mb and
Purola Mb). The stratigraphy of the area is still unsolved.
According to Lahtinen (1994) the Kettuperä gneiss and the
rhyolites in the Riitavuori and Lippikylä Mb:s are
chemically similar and they may be genetically related. The age
of the Kettuperä gneiss and the rhyolites, 1.93 and 1.92 Ga
respectively, also suggests a common history (Kousa 1990). Age
determinations for the plagioclase porphyrites which have been
found as inclusions in the massive ore indicate distinctly
younger ages at 1875 Ma (Helovuori 1979). These plagioclase
porphyritic inclusions as well as the quartz-porphyry and
amphibolite inclusions, are unaltered and represent younger
intrusive dykes. Similar dykes have been found in outcrops and
drill cores outside the massive ore.
Field observations indicate that most of the mafic volcanic rocks
seem to be younger than the main part of the felsic volcanic
rocks. The volcanism started with felsicvolcanism in an
extensional continental margin, followed by mafic volcanism in
a rifted marine environment. Large scale hydrothermal alteration
is associated with this stage causing strong sodium enrichment
in the rhyolites. Ore formation near the centres of mafic
volcanism seems to be related to this period. Without longer
hiatus, the volcanism continued with more calc-alkaline volcanism
south and west of the Pyhäsalmi volcanic complex.
The stratigraphy is unclear. Polyphase deformation together with
amphibolite facies metamorphism make the interpretation
difficult. Both felsic and mafic volcanic rocks are strongly
altered near the ore. Geology of the mine area is shown in Figure 25 and the structural model of the ore
indicating different deformation phases, is explained in Figure 26.
The ore body has a complex shape due to the polyphase deformation
and is surrounded by a large alteration zone. The contact between
unaltered and altered volcanic rocks can be sharp or gradational.
The thickness of the altered rock sequence at the surface is
about 100 m in the west and 300 m in the east (Fig.
27). At the deeper levels the alteration zone is only a few
meters thick. Sericite-quartzites with a high pyrite content are
present adjacent to the ore. Pennitised cordierite porphyroblasts
are common. Two zones of cordierite-anthophyllite rocks occur in
the footwall, partly within the sericite quartzites. A zone of
cordierite mica gneisses that contain portions of a
cordierite-anthophyllite rock occurs outside the hanging wall
sericite quartzites. The folded alteration zone is over 5 km long
at the surface, i.e. the Lepikko Mb in Figure 25. A 3–15 m
thick talc-schist occurs near the footwall contact, partly inside
the ore (Fig. 27). This schist is 100–200 m long and
200–300 m deep and pinch out with depth. This talc-schist
often contains high grade Zn-mineralisation.
The composition of the Pyhäsalmi ore varies both
horizontally and vertically. As a rule sphalerite is concentrated
in the central part of the ore and chalcopyrite near the contact.
The highest barite contents are generally encountered in the
sphalerite-rich areas. The Pyhäsalmi ore is a massive pyrite
ore with 70% sulphides. The sphalerite-rich ore is in some places
finely banded and thin porphyritic bands are common. Round pyrite
phenocrysts occur in the fine-grained sphalerite matrix of the
porphyry ore. A pyrite dissemination which in some places has a
breccia structure exists around the massive ore. Pyrrhotite has
replaced pyrite in the southern end of the ore. Pyrrhotite
replacement seems to be related to the intrusion of pegmatites
and is common in strongly deformed (D4) areas.
Underground percussion sample drilling is done by a Tamrock Solo
505 electro-hydraulic percussion drilling machine. The percussion
holes are 20–50 m long. The sludge is collected with a
Thompson sludge cutter at 0.9 m intervals (half of a rod). Sludge
samples are logged and assayed. Sampling is done in horizontal
holes and holes drilled upwards. Down hole geophysical
measurements are done with a OMS-log system. Gamma-gamma
(density) measurements is used for determinations of the massive
ore contacts and inductive conductivity measurements for
estimations of the contact between pyrite- and pyrrhotite ore.
Percussion sample drilling is scheduled at a yearly rate of 8 000
m. Bore hole logging with the OMS-logg is also made in production
holes, when it is necessary to identify large wall rock inclusion
in the ore or pyrrhotite- from pyrite ore. Increased information
(sample grid from each 20 m to 10 m) with accurate surveying and
better positioning of diamond drill and percussion holes has
helped the mine to decrease waste rock dilution from 30% in 1980
to 8% today.
During the mining period geophysical investigations (especially
TEM-37) and core drilling continued at deeper levels. The deep
ore zone was discovered by drilling in 1991. Drifting of the
decline to the +540 level was ready in 1995 and inventory
drilling of the Sipeia ore (the southern part of the "Deep Ore")
was made in 1994–1995. The total ore reserves of the Siperia
ore was initially 715 000 tonnes with the grade 8,28% Zn, 0,43%
Cu, 1,2 g/t Au and 81 g/t Ag. Production began in May 1996. The
ore is transported to the Pyhäsalmi mill for treating.
The other ore bodies in the Mullikoräme deposit are smaller
and have lower metal contents. A typical zoning of metals inside
individual ore bodies and also between different ore bodies is
recognisable.
The Säviä Cu-Zn deposit occurs within hydrothermally
altered, felsic, pyroclastic rocks in the upper parts of the
lowermost volcanic sequence. Hydrothermal alteration resulted in
the transformation of primary minerals into an assemblage
consisting of chlorite and montmorillonite, which was further
recrystallized during metamorphism to a
garnet-cordierite-anthophyllite assemblage. Ore deposition took
place during the same hydrothermal processes, during which metals
were liberated from silicates and precipitated as sulphides in
proximity to volcanic vents (Makkonen 1981).
The Säviä ore body was originally a stratiform massive
sulphide sheet, 3–5 m thick, with underlying disseminated
and veined stinger ores which were folded and tectonically
thickened during the isoclinal F1 phase and also affected by
tight F2 folding, so that it is now c. 20–30 m thick.
Underlying garnet-cordierite-anthophyllite rocks are located on
either side of the tightly folded ore body (Fig. 29). During the
D3 deformation the Säviä deposit was split into
separate lenses and overthrust towards the north-west.
The ore consists of pyrite, chalcopyrite and pyrrhotite, with
occasional sphalerite. Angular fragments of wall rock suggest
remobilization of ore while the country rocks were deformed in
a brittle manner. Chalcopyrite increases in abundance downwards,
and the highest grades are found in the massive ore. Chalcopyrite
is also distinctly enriched where the ore has been breccited.
Moderately elevated Au and Ag contents have been recorded in the
main ore body and small amounts of barite, carbonate and
anhydrite occur in association with copper ore.
The Säviä deposit is estimated to contain around 4.0
Mt of ore, with 1.1% Cu and 28% S and a separate sulphur-zinc ore
body contains an additional 1 Mt, with 2.0% Zn and 33.0% S
(Laitakari 1968) (total mineral resources). The Au and Ag grade
of the Säviä deposit is in average 0.2 ppm and
5–10 ppm respectively.
The Kangasjärvi deposit has also been overthrust towards the
north-west and has a highly elongate, flattened profile, plunging
towards the south-east at about 45°. The ore is clearly
stratabound and pyrite and sphalerite are present as layered
bands and massive aggregates, with rare chalcopyrite and
pyrrhotite. Gangue minerals include quartz, barite, muscovite,
chlorite and diopside. Galena, gold and silver has been found at
the margins of the ore body, while the abundance of zinc and the
Zn/Cu ratio decreases in the lower part of the deposit
(Rehtijärvi 1984). Hydrothermal alteration increases in
intensity towards the footwall of the deposit and is
characterised by increases in Fe, Mn, Mg, P, Ba, Au, S, Zn and
Co and decreases in Ca, K and Sr (Rasilainen 1991). The
Kangasjärvi deposit is analogous to the Pyhäsalmi ore
body both in terms of its metal abundance and its structural
position. The deposit was mined in 1986 by Outokumpu, who
extracted c. 86 000 tonnes of ore averaging 5.4% Zn, 0.06% Cu and
38% S, which was concentrated at their Pyhäsalmi plant.
The ores occur in the altered portions of a rock suite that
contains felsic, intermediate, and mafic volcanic rocks. The host
formation forms part of a schist area surrounded by gneissose,
migmatitic oligoclase-dominated granites and granodiorites rich
in inclusions (Huhtala 1979). The strongly folded formation is
metamorphosed in granulite facies (Fig. 30).
Mining and ore reserve estimation of the Ruostesuo deposit was
problematic because of the strong mobilisation of the ore.
The ore is situated in the transition zone between the felsic
volcanic rocks and the overlying mafic volcanic rocks. The rocks
around the ore are chemically altered, resulting in almost total
depletion of calcium and sodium. Felsic rocks are now cordierite-
and sillimanite-bearing schists, which contain biotite and
garnet. Coarse grained, massive or foliated
cordierite-anthophyllite rocks, cordierite-anthophyllite-garnet
rocks, anthophyllite amphibolite, and cummingtonite gneisses
occur as alteration products of mafic tuffs and tuffites. The
formation contains separate ore bodies located in a ribbon-like
folded zone.
The ores are brecciated or massive pyrite-pyrrhotite occurrences
with some chalcopyrite and sphalerite. The ores in
Kalliokylä differ from each other geochemically,
particularly in their zinc content. In the south-east, ores in
cordierite-anthophyllite rocks are poor in zinc, assaying 0.2 to
0.4% Zn and 0.4 to 0.6% Cu. In the north-east (the Ruostesuo
deposit) the ores are hosted by felsic tuffites, amphibolites and
cummingtonite gneisses. These ores assay 2.4% Zn and 0.4% Cu. In
Table 4 the Kalliokylä figures represent all mineralisations
except Ruostesuo.
The total production from the Ruostesuo deposit was 238 000
tonnes with 0.3% Cu, 2.63% Zn and 31.1% S.
Perusteksti UK>The sulphide ore form a flat body, about 2 km
long, which is situated in altered gneisses close to the contact
with the oligoclase granite. The alteration zone around the ore
is composed of fine-grained cordierite gneisses and
coarse-grained garnet-cordierite and cordierite-anthophyllite
gneisses (Huhtala 1979). The ore is either disseminated or
massive. Pyrrhotite, pyrite, sphalerite, and chalcopyrite are the
main minerals. Magnetite is abundant and is locally intergrown
with pyrite. The best parts contain c. 1% Cu and 0.5% Zn (Huhtala
1979). The total tonnage of the mineralisation is c. 2 Mt.
According to Pajunen (1987) there is a clear Mg increase in rocks
near the deposit and at least part of the alteration halo is of
metamorphic origin. Due to the high grade metamorphism the
genesis of the mineralisation is an open question. The deposit
is controlled by prominent NE-trending deformation structures
(Pajunen 1987).
Pyrite-pyrrhotite mineralisations occur in alteration zones
containing quartz, cordierite, biotite, anthophyllite,
sillimanite, garnet, andalusite, and muscovite. The
mineralisation is composed of several small lenses in a 2 km long
zone. Total reserves are estimated at 0.7 Mt of 3.2% Zn and 0.3%
Cu.
The characteristics of the Rauhala deposi are very similar to
other Precambrian massive sulphide deposits formed on the
seafloor by hydrothermal exhalative processes. The galena lead
of the Rauhala deposit differs from the lead in the
Vihanti–Pyhäsalmi deposits and belong, according to
Vaasjoki (1989), to the younger lead group of the Central Finland
Granitoid Complex.
The Rauhala deposit has later been investigated by the Outokumpu
company and new ore reserve estimates have lowered the earlier
figures markedly. Original figures were 1.2 Mt of ore with an
average grade of 6.66% Zn, 1.59% Cu, 1.24% Pb, 67 ppm Ag, and 0.5
ppm Au. The deposit has not been mined because of the shallow dip
(30°), thin ore horizons, and problematic mineralogy.
In the future the Hitura concentrate will be smelted at
Harjavalta. The ore hoist is 600 000 tonnes per year and the mine
annually produces about 3000 tonnes of nickel in concentrate. In
June 1996 the total hoist of ore had reached 9 million tonnes and
the ore reserves are estimated to warrant production for at least
five more years. Production of Finnish Ni-mines can be seen in
Table 6.
Description of ore types:
Outokumpu Oy delineated a low grade Cu-Au deposit with
considerable tonnage at Kopsa, which was regarded as a porphyry
type ore hosted by a tonalitic stock (Gaál & Isohanni 1979).
Recent drilling by Glenmore Highlands Inc. has revealed
encouraging gold grades in several drill cores and trenches. The
Laivakangas deposit was studied by Outokumpu in the mid-1980 s
after the discovery of a mineralised boulder. Mineralisation
occurs as discrete quartz vein systems at the contact of a
tonalitestock and metabasalt. Mineral resource estimates indicate
c. 400 000 tonnes of ore grading 4 g/t (Sandberg 1985,
Mäkela et al. 1988). The Geological Survey of Finland
discovered the Kiimala deposit after drilling till geochemical
anomalies (Kojonen et al. 1991). Disseminated mineralisation is
hosted by a sheared and altered gabbroic intrusion, and is
estimated to contain about 1.2 Mt of ore grading 1.5 g/t.
The main common features of the gold showings in the area appear
to be an association with shear zones and a close spatial
relationship with synorogenic (1.89–1.87 Ga) tonalitic
stocks or subvolcanic intermediate intrusions. Most deposits are
either hosted by the intrusions or occur adjacent to intrusive
contacts. However, there is no convincing evidence of a genetic
relationship between granitic fluids and gold mineralisation,
rather the competent intrusive rocks may have provided structural
control of the mineralising fluids.
Gold typically occurs in discrete quartz vein systems with
limited alteration halos and less commonly as disseminations in
shear zones. Alteration minerals include quartz, sericite,
biotite, K-feldspar, and epidote, but carbonates are notably
lacking. Sulphides are fairly abundant usually consisting of
Fe-sulphides, chalcopyrite, arsenopyrite, and loellingite. Gold
typically occurs as free gold which is commonly associated with
tellurides and Bi minerals. Geochemically, the occurrences are
enriched in As, Te, Bi, Se, Ag, Sb and W (Nurmi et al. 1991).
a) Diffusely stratified, andesitic, coarse-grained sandstone and
conglomerate with light grey and reddish andesite(?) clasts in
a green chlorite-rich matrix. These are overlain by finer-grained
sandstone, partly with cross bedding.
b) Traction current bedding in pebbly, andesitic sandstone. Pink
to brown, oxidised, angular clasts of different composition. The
outcrops are part of the shallow-marine to fluvial succession of
andesites and basaltic andesites constituting the footwall of the
ore-bearing unit at Renström.
The basaltic pillow lavas are well preserved with individual
pillows up to one meter in size. Well developed radial gas
escapement trains can be seen in the pillow interior. The lavas
are overlain by pillow breccias consisting of broken pillows and
fluidal bombs which are interlayered with deposits of quench
fragmented spalled bomb rinds. The whole sequence is interpreted
as a subaqueous, basaltic fire-fountain deposit which is a
subaqueous equivalent to Hawaiian-style fire fountaining. The
environment is interpreted by Allen et al. (1996) as a high
discharge fissure eruption which jetted rapidly quenched bombs
above the sea-floor which then were deposited as quench breccias
(Fig. 35c). Similar deposits have also been found in the
enström area.
The traverse starts in a probably slightly reworked, monomict,
clast-rich felsic volcanogenic mass flow deposit/conglomerate.
It continues into successively more reworked and polymict
conglomerates which, at the base, are dominated by volcanic
clasts. The traverse ends in a polymict conglomerates dominated
by volcanic, microgranite and granitoid clasts intercalated with
sand- and siltstones.
The Hej volcanic rocks comprise rhyolitic to dacitic, quartz-
and/or feldspar-porphyritic lavas and volcaniclastic rocks,
including a rhyolitic crystal tuff, intermediate plagioclase
porphyries and volcanic breccias (Stop 2:7), rhyolitic ash-fall
tuffs, non-welded and welded, typically purple ignimbrites (Stops
2:9 and 2:10). The ash-fall tuffs and the ignimbrites are
considered to form a 400–500 m thick compound cooling unit,
emplaced in an intra-caldera setting (Rapp 1996).
Intercalations of epiclastic sedimentary rocks, mainly polymict
conglomerates and subordinate impure sandstones, occur at several
stratigraphic levels within the volcanic succession. The most
extensive of these conglomerate lenses, here referred to as the
Urikberg conglomerate (Stop 2:8), has a north-south extension of
more than 4 km and a thickness exceeding 100 m. The clasts range
from 1 to 50 cm in size and consist of lavas, volcaniclastic
rocks, and granodiorites of the Jörn GI type (Ehrenborg
1987, Kathol 1995, Rapp 1996), all these rocks being derived from
the east. Due to the lack of argillaceous material in the matrix
and the occurrence of thick massive beds, this conglomerate is
considered to represent theresult of mass flows initiated by
slope wasting in areas with relatively high relief and
unweathered volcanic rocks.
Emplacement of the Hej subgroup in a terrestrial environment is
indicated by the occurrence of eutaxitic textures, lithophysae,
fiamme, and welding within the rhyolitic ignimbrites, as well as
by the red colours of most of the felsic rocks. Further evidence
for the terrestrial nature of the Hej subgroup is given by the
occurrence of drying cracks within an intercalated sandstone lens
in the Urikberg conglomerate (Ildikó Antal, Ingmar
Lundström, pers. comm. 1996).
The metamorphic grade within the Hej subgroup does not exceed
lower greenschist facies and therefore primary textures are well
preserved, apart from epidote alteration of feldspar phenocrysts
and epidote growth within lithophysae.
In the eastern part of the field trip area, the rocks dip at
moderate angles to the west and north-west, and minor overturned
limbs indicate a deformation style with tight to isoclinal folds
with vergence to the east and south-east. However, no foliation
related to these folds has been observed. The western part of the
Hej volcanic rocks dips steeply to the east and south-east. In
general, the rocks of the Hej subgroup lie right way up with the
purple, welded ignimbrite at the top of the succession, occupying
the core of the major synform.
The Hej subgroup is intruded by the even grained, fine to medium
grained, light red or reddish Antak granite in the south-central
parts (Fig. 38). Through geophysical modelling,
the thickness of the elliptic Antak intrusion (c. 5 x 8 km) is
estimated to be between 200 and 500 metres at the present
erosional level. The intrusive contact relationship between the
Antak granite and the volcanic rocks has only been observed in
one locality, but at a map scale, the granite contact cuts the
bedding within the Hej subgroup confirming the intrusive
relationship. A comagmatic origin of the Antak granite, and the
ignimbrites and rhyolites of the Hej subgroup is indicated by REE
and trace element patterns of these rocks (Kathol & Rapp 1996).
A coeval origin of these rocks is also indicated by a zircon U-Pb
dating of the Antak granite which yielded an age of 1879 +15/-12
Ma (Kathol & Persson in press) which is comparable to the 1878±2
Ma age of the rhyolitic ignimbrites at Skyberget (Skiöld et
al. 1993). The Antak granite is thus considered to have
crystallised at a shallow level within comagmatic extrusive
phases. The granite itself probably represents the position of
the eruptive centre of the rhyolitic parts of the Hej volcanic
rocks.
The origin of the breccia is a subject for discussion. Rapp
(1996) suggested deposition and emplacement from syn-volcanic,
gravity-driven volcaniclastic mass flows which were
contemporaneous with the emplacement of the grey dacitic feldspar
porphyritic lava flow. Jig-saw fit texture is suggested to have
formed by quench fragmentation of the hot lava fragments after
deposition.
The core of the Skellefte Group anticline is mostly altered to
low grade, but the regional metamorphic alteration increases to
high grade in the north-east (Fig. 5). Björkdal is situated
in an area characterised by lower middle grade rocks.
Mapping by the geological survey of Sweden in the Björkdal
area has revealed an unexpected number of complications. This
presentation is therefore highly provisional, because many
significant features of the geology of the Björkdal area are
still poorly known. However, the main structural feature of the
Björkdal mine area is a set of ENE to E-W striking, gently
north-dipping shear zones. They both obscure the contact
relationships between the pluton, the metavolcanic and
metasedimentary rocks and destroy almost every primary structure
and texture. Kinematic indicators are few, but indicate thrusting
towards the south. Movement was probably oblique, because
sinistral movements occur on top surfaces. The shear zones are
subhorizontal in the central part of the pluton and gently
south-dipping in its southern part. The meta-supracrustal rocks
therefore seem to have been thrust towards the south above the
pluton (Fig. 40). The gold bearing quartz veins of the mine seem
to have formed simultaneously wit this thrusting, because they
both cross-cut shear zones and are deformed thereof.
Occasionally, garnet is found in the shear zones, which also seem
to be related to a peculiar actinolite blastesis, particularly
in the metavolcanic rocks.
The contact relations between the pluton and the overlying
supracrustal rocks are obscured by deformation. The pluton is
covered by a thin marble layer, which seems to follow the base
of the overlying metavolcanic rocks rather closely. As the pluton
has been found to be older than any known rocks of the Skellefte
Group, it is tempting to consider the marble as deposited on the
plutonite rather than as intruded by it.
A fine-grained, quartz-mica mylonite or gneiss with occasional
relics of quartz phenocrysts, set in a very fine-grained matrix,
has been found directly above the marble. Its composition and
structure is very similar to that of the tonalite and is thus
very difficultto map out in a consistent way. However, its
position above the marble layer as well as minor textural
differences make a distinction from the tonalite gneisses
justified.
North of the Björkdal mine, a peculiar, poorly understood
rock forms an almost km-wide unit. It is affected by numerous,
gently north-dipping shear zones, probably related to the zones
in the mine. The rock has a planar fabric and the characteristic
feature is a penetrative to porphyroblastic, post-deformative
actinolite blastesis which gives the rock a rather mafic
appearance. As it frequently displays reasonably recognisable,
lath-formed plagioclase phenocrysts and probable quartz-calcite
amygdules, the rock has previously been interpreted as some kind
of a mafic, coherent volcanic rock. However, the rock cannot have
been originally mafic because it has a fairly quartz-rich matrix.
Furthermore, the actinolite blastesis forms a jig-saw fit pattern
and actnolite porphyroblasts very frequently straddle the
boundaries of clast-like, more felsic parts, which may in fact
be alteration relics. As the same actinolite blastesis also
occurs in other rock types, it is most probably due to some kind
of poorly understood alteration, here possibly related to the
shear zones.
Metasedimentary rocks form the structurally and stratigraphically
highest unit in the Björkdal area (Fig. 39 and 40). The
contact with the underlying metavolcanic rocks is not exposed,
but seem to be situated slightly below a thin layer of a polymict
conglomerate. At higher stratigraphic levels, rusty black schists
with some sulphide prospects occur as well as turbiditic
arenites, indicating rapidly changing depositional environments.
Despite medium grade alteration, these rocks frequently display
beautifully preserved sedimentary structures and clastic
textures.
The following is based on exploration and research carried out
since the discovery at Kopsan Kallio (the only outcrop on the
property) in 1939 and on the ongoing exploration program. The
main mineralisation at Kopsa is within a stockwork of quartz
veins, fracture fillings, and disseminations in the granodiorite
intrusion. However, some mineralisation has also been found in
the contact zone (e.g. Sorola). The predominant sulphide minerals
re chalcopyrite, arsenopyrite, and pyrrhotite, with lesser
amounts of pyrite. Gold occurs in native form, in tellurides, and
closely associated with arsenopyrite, while silver is mainly
contained in chalcopyrite. Copper is predominantly contained in
chalcopyrite and in rare cases, in cubanite (Ervamaa 1952).
The sulphides occur as massive vein filling fractures, generally
a few mm wide, as semi-massive segregations within quartz veins
and silicified zones, and in disseminated form. The entire Kopsa
intrusion is mineralised. The main alterations accompanying
mineralisation in the granodiorite are feldspathization
(potassium enrichment, sodium and calcium depletion) and
silicification (Gaál & Isohanni 1979). Other elements enriched
in the mineralisation are Bi, Te, Sb, W, and Mo.
Mineralisation in the contact zone metasedimentary rocks, as
exemplified by the Sorola prospect, is similar to that described
above. However, quartz veins are generally absent, the main
potassic alteration is biotite and sericite, and the silver to
gold ratios are much higher. Chalcopyrite, pyrrhotite, and
arsenopyrite occur as massive to semi-massive lenses, fracture
fillings, and in disseminated form in irregular zones, up to tens
of meters wide.
Glenmore Highlands´ current exploration program has involved
shallow percussion drilling, geophysical investigations,
trenching, and reverse-circulation and diamond drilling, mainly
in the granodiorite. Results to date indicate that gold and
copper mineralisation mostly is enriched in steeply-dipping,
quartz-rich zones (a few meters to over 10 m wide), concentrated
over a large portion of the intrusion, and consistent to depths
> 175 m.
During the visit different types of mineralisations will be seen
in trenches.
Altogether 17,5 km2 were covered by geophysical
measurements, and in all about 900 till samples were collected
with a hydraulic percussion drill. Diamond drilling, totally
2 079 m, was carried out during 1986–1988. The ore reserves,
which are based on 9 drill hole intersections, indicate about 0,3
Mt of ore with a grade of 2,5 ppm Au and 0,8% As.
There is no doubt that the mineralisation is related to the
NW-striking shear zone, which crosscuts metasedimentary rocks,
gabbros, and granitoids. The deposit consists of several ore
bearing lenses, most of which strike in the direction of the main
shear zone. A few lenses, however, are oblique. Within the shear
zone the hydrothermal solutions associated with the gold
mineralisations have caused silicification, sericitization and
chloritization. In places the amount of potassium feldspar
increases considerably.
The Vesiperä gold mineralisation is hosted by a
coarse-grained hypabyssal gabbro, which shows ophitic texture in
the fine-grained matrix. There are two types of mineralisation:
1) narrow quartz and sulphide rich veins and 2) disseminated ore
with only a weak sulphide content. In the quartz veins the main
ore minerals are arsenopyrite, pyrite, marcasite, chalcopyrite,
pyrrhotite, ilmenite, rutile, and graphite. Arsenopyrite often
has a core of loellingite. Native gold, bismuth, and electrum
occur as small inclusions in arsenopyrite and loellingite. In the
disseminated ore type the most common ore minerals are
pyrrhotite, pyrite, arsenopyrite, and chalcopyrite, with minor
ilmenite, rutile, and graphite. In addition to the inclusions in
arsenopyrite native gold occurs as inclusions in silicates
(Sipilä 1988).
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& Zeyen 1996).
Fig. 3. Active mines in Finland. 1=Pahtavaara, 2=Kemi, 3=Hitura,
4=Pyhäsalmi, 5=Mullikkoräme, and 6=Orivesi.
Fig. 4. Active mines in Sweden. 1=Kiruna, 2=Malmberget, 3=Aitik,
4=Laisvall, 5=Åkerberg, 6=Björkdal, 7=Kristineberg,
8=Petiknäs, 9=Renström, 10=Kankberg, 11=östra
Åkulla, 12=Långdal, 13=Garpenberg, 14=Zinkgruvan.
Fig. 5. Geology of the Skellefte district. Modified from Allen
et al. (1996).
Fig. 6. Stratigraphy of the Skellefte district. Modified from
Billström & Weihed (1996) after Allen et al. (1996).
Fig. 7. Abundance of different rock types in different areas of
the Skellefte district, compared to abundance in modern areas.
From Allen et al. (1996).
Fig. 8. Geochemical diagrams of rocks in the Skellefte district.
In diagrams a), d), and e) 75 analyses from the central and
eastern Skellefte district are plotted in conventional
classification diagrams. In diagrams b), c) and f–i) basalts
only from the same areas are plotted. For discussions see
text.
Fig. 9. Major shear zones, thrusts and form lines of foliations
from the Skellefte District. From Bergman Weihed (1997).
Fig. 10. Ternary diagrams of metal composition of the major VHMS
deposits in the Skellefte District. Deposits are listed in Table
2. Larger circles are deposits with a tonnage exceeding 1.1 Mt.
Stars refer to tuff cone-cryptodome-hyaloclastite associated
deposits. From Allen et al. (1996)
Fig. 11. Lead isotope diagrams of the Skellefte district. Data
from Billström & Vivallo (1994) and Billström
unpublished.
Fig. 12. Geology of the Renström area (published with
permission from Boliden AB)
Fig. 13. Geology of the Renström deposit (published with
permission from Boliden AB)
Fig. 14. Geology of the Renström Petiknäs area.
Loctions of Figures 12, 13, and 15 indicated. Modified from Allen
et al. (1996)
Fig. 15. 300m level of Petiknäs south. Data from Jonsson
unpublished.
Fig. 16. Profile 2030Y Petiknäs south. Data from Jonsson
unpublished.
Fig. 17. The Björkdal mine. (published with permission from
Terra Mining AB).
Fig. 18 Generalised cross section Björkdal mine (Albino
unpublished).
Fig. 19. Generalised lithological map of Ostrobothnia with the
ore deposit locations. The maps in the Figs. 19 and 20 are based
on the Bedrock Map of Central Fennoscandia (Lundqvist et al.
1996b).
Fig. 20. Major tectonic features and nomenclature of the
lithological units. OSZ=Oulujärvi Shear Zone, RLZ=Raahe
Ladoga Zone, ReSZ=Revonneva Shear Zone, RuFZ=Ruhaperä Fault
Zone, ABC=Archaean basement complex, BB=Bothnian Belt, SB=Savo
Belt, CFGC=Central Finland Granitoid Complex, Kal=Kalajoki
Batholith, Rau=Rautio Batholith, Toh=Toholampi Batholith,
Vää=Vääti granodiorite, Ngc=Nivala gneiss
complex, Pvc=Pyhäsalmi volcanic complex, KuF=Kuusaa
Formation, SiF=Sievi Formation, AnF=Antinneva Formation,
LeF=Lestijärvi Formation.
Fig. 21. E-W trending schematic cross section of the bedrock of
central Ostrobothnia. Abbreviations as in Fig. 20.
Fig. 22. Cross section 130, Vihanti Mine.
Figure 23. Lay out of the Pyhäsalmi Mine.
Fig. 24. Lithostratigraphic map of the Pyhäsalmi volcanic
complex.
Fig. 25. Geological map of the Ruotanen formation.
Fig. 26. Structural model of the Pyhäsalmi ore deposit.
Fig. 27. Profile x=2600, Pyhäsalmi Mine.
Fig. 28. Geological map of the Mullikkoräme formation.
Fig. 29. The Pyhäsalmi-Pielavesi ore zone with a typical
section over the Säviä deposit.
Fig. 30. Geology of the Kalliokylä deposit (after Huhtala
1979).
Fig. 31. Location of the Svecofennian nickel province in
Fennoscandia.
Fig. 32. General geology of the Hitura area.
Fig. 33. Geological map of the +200 m level and vertical section
x 3650 of the Hitura ultramafic complex.
Fig. 34. Variation of major and ore-forming elements in the drill
core Hi-328. Location of this horizontal drill hole is indicated
in Fig. 33.
Fig. 35. Photographs of the main lithological units in the
Granbergsliden area. a) Pillow lava, b) pillow breccia, and c)
spalled bomb rinds of the fire fountaining unit.
Fig. 36 Map of central Skellefte District (from Allen et al.
(1996) with the different stops in Granbergsliden, Nicknoret and
Maurliden indicated.
Fig. 37. Simplified geological map of the Hej area, modified from
Kathol & Rapp (1996).
Fig. 38. Geological map of the field trip area west of the
village Hej. The map is based on Rapp (1996) and ongoing mapping
by the Geological Survey of Sweden (SGU).
Fig. 39. Geology of the Björkdal area. Based on mapping by
I Lundström, Geol. Surv. Sweden, unpublished.
Fig. 40. Profile through the Björkdal pluton.
Fig. 41. Geological map of the vicinity of the Kopsa Cu-Au
deposit (after Gaál & Isohanni 1979).
Table 2. Grade and tonnage of major VMS deposits in the Skellefte
district (from Allen et al. 1996).
Table 3. A nongenetic classification of gold deposits in the
Skellefte district modified from Weihed et al. (1992)
Table 4. Cu-Zn mines and main mineralisations in Central
Ostrobothnia.
Table 5. Chemical composition of the rocks in Pyhäsalmi
mine. From Zr to Ba in ppm, except in sample 6.
Table 6. Nickel ore hoist in Finland 1939-1996.
Table 7. Average chemical compositions of the disseminated ore
types in the Hitura mine in wt.% (Hautala and Sotka 1976).
Host rocks
The host rocks are composed of intrusive and volcanic rocks. The
host intrusive is a quartz-monzodiorite, and probably belongs to
the Jörn intrusive suite. The quartz-monzodiorite is
relatively undeformed in its central parts, but is more deformed
near the contact and is sometimes recrystallized to a
granoblastic texture. The monzodiorite is cross-cut by a system
of quartz veins (Fig. 17 and 18). The quartz
veins have at least three different trends of which two are
mineralised. A marble is situated between the intrusive and the
overlying supracrustal rocks in the western and the eastern open
pits, but is not present in the central open pit.Structures
One of the dominant structural trends in the Skellefte district
is N60°W, which is also the strike of foliations in the
Björkdal mine area. The Kågedals deformation zone to the
south of the Björkdal pluton is parallel to the structural
trend of the Skellefte district. The Björkdal mine is
situated on the northern contact of the Björkdal pluton.The gold ore
The gold in the mine is associated with the brittle quartz veins
which were introduced in tension fractures associated with low
angle thrusting towards south. Gold has probably been transported
by magmatic fluids as Au(HS)2- complexes. Late
hydrothermal fluids, redistributed the gold under high pressure
and temerature in the range of 200–300 °C (Broman et al.
1994).Other non-economic deposits
Apart from gold lode and massive sulphide ores, the Skellefte
District also hosts porphyry type deposits (e.g. Tallberg), minor
mafic- to ultramafic-hosted Ni-deposits (Lainejaur, Lappvattnet,
Mjödvattnet, Njuggträskliden, and älgliden), and
Li-pegmatites (Varuträsk). Although these deposits will not
be studied during the field trip a short description of each type
is given below as they are important for understanding the
metallogenetic evolution and the tectonic setting of the
Skellefte District.Porphyry type deposits
Porphyry type deposits have now been described rld, and the
Tallberg deposit in the Skellefte district is one of the best
described (e.g. Weihed et al. 1987, Weihed 1992a, Weihed 1992b,
Weihed & Fallick 1994).Ni-deposits
Only minor Ni-deposits occur within the central part of the
Skellefte District (e.g. älgliden). In the north-western
part of the district, the Lainejaur deposit was mined during the
secondworld war and to the south, several Ni-deposits have been
investigated, including the Lappvattnet, and Njuggträskliden
prospects. A review of these Ni-deposits can be found in Nilsson
(1985).Li-occurrences
Lithium-bearing pegmatites are found in the Varuträsk area
in the eastern-most part of the Skellefte district (cf. Quensel
1960). The pegmatite is of a lithium-cesium-tantalum-type
(LCT-type) according to Romer & Smeds (1994) and it is associated
with the Skellefte-type granitoids. Romer & Smeds (1994) dated
columbite from this deposit which yielded an age of 1775±11 Ma.
Other similar pegmatites are known in the eastern-most part of
the Skellefte District, but they have not been studied in any
detail.Other deposits
To the west and north of the Skellefte district, Mo, W, and U
mineralisations have been identified. None of these have been
mined, but a large number of reports on theoccurrences exist. The
interested reader is referred to öhlander (1986 and
references therein) for information regarding these deposits.Geology and mineral deposits of the Central Ostrobothnia
Regional geology
The bedrock of central Ostrobothnia is a part of the Svecofennian
Domain (SD) bordered by the Archaean Basement Complex (ABC) in
the east and the Central Finland Granitoid Complex (CFGC) and the
Bothnian Belt (BB) in the south and south-west. The eastern part
of central Ostrobothnia belongs lithologically to the Savo Belt
(SB) and structurally to the Raahe–Ladoga Zone (RLZ) (Fig. 19 and 20). This area is well known for its
VHMS deposits.Deformation history
The Palaeoproterozoic formations of Ostrobothnia have been
variably affected by multiphase deformation which now is
manifested in fault bounded blocks, with internally different
structural and metamorphic history. The deformation history can
be divided into an early phase of thrusting towards the craton
in the north, with D1 and D2 folds developed, and a younger phase
of shearing which produced the major shear zones of the central
Fennoscandian Shield (Kärki et al. 1993).History of mining
Introduction
Mining in central Ostrobothnia has been continuous since the
second world war. The first mine, the Makola nickel mine in
Nivala, was opened in 1941. The Makola mine was in production
between 1941–1946 and 1951–1954. The first indications
of nickel in the Hitura area were obtained during the
investigations in Makola. Both deposits were found by the
Geological Expedition (to day the Geological Survey of Finland,
GSF). The Kopsa gold-copper mineralisation was also found during
this exploration period. Production in Hitura started in 1970.
Both the Makola and Hitura mines are operated by the Outokumpu
Oy. The location of the mines can be seen in Figure 19.Ni-mines
Makola
The first ore reserve estimation made by the Geological
Expedition was 1.5 Mt of Ni-ore with 0.81–1.03% Ni and 0.42%
Cu. Outokumpu Oy calculated the reserves to 0.7 Mt. The demand
for Ni was high due to the war and the decision to start mining
was fast. Makola was the first mine which was wholly planned and
built by the Outokumpu Company. The mine was small and annual
production was planned to be 50 000 tonnes (3 000–5 000
tonnes of Ni-Cu-concentrate). In 1941–1942 the
Ni-Cu-concentrates were sold to Germany and after 1942 all
concentrates went to Outokumpu s smelters, first to Imatra and
later to Harjavalta. After the second world war the price of the
nickel collapsed and Outo kumpu closed the mine in 1946. The mine
was left in care and maintenance and when nickel prices went up
again during the Korean crisis, Outokumpu decided to reopen the
mine in 1951. The capacity of the mill was increased to 80 000
tonnes per year. In 1954 all reserves were exhausted and the mine
was closed. The total production from the Makola mine was 410 000
tonnes of ore with 0.86% Ni, equivalent to 1250 tonnes of nickel
metal.Hitura
The first holes on the Hitura deposit were drilled in 1939, but
because of poor results the investigations were concentrated on
the Makola deposit. The GSF restarted exploration in Nivala in
1961 and when Outokumpu obtained the mining licences of the
deposit in 1964 the company started underground exploration. The
results were negative, as the mineralisation, although large, was
too low grade and also difficult to concentrate. However, with
investment support from the Finnish state the decision to open
the mine was made in 1969 and the production started in 1970.Massive Sulphide Mines
The Vihanti Mine
The Pyhäsalmi and satellite mines
The Pyhäsalmi ore deposit was found in 1958 after a very
intense drilling period. The first estimation of reserves
indicating 12.2 Mt of ore with 0.81% Cu, 2.93% Zn, and 36.8% S,
was done in February 1959. Construction works started in August
1959 and production commenced in March 1962, only three and a
half years after the discovery. The mine started as an open pit
and the final depth of 125 m of the open pit wasreached in 1967.
Underground mining started the same year. The total production
from the open pit was 6.8 Mt of ore and 5.6 Mt of waste rock.VHMS-deposits
Introduction
The Raahe–Ladoga Zone (RLZ) has been interpreted as a
collisional boundary zone between the Archaean continent and
Palaeoproterozoic, Svecofennian lithosphere. Several epochs of
mineralisation and different metallogenic provinces are
associated with of this zone (Ekdahl 1993). In the early orogenic
stage intracratonic rifting in the east produced the
Jormua-Outokumpu ophiolites, 1960 Ma in age, and the associated
Cu-Zn-Co-Ni ores. Volcanic complexes and associated
Vihanti–Pyhäsalmi type ore deposits, 1920 Ma in age,
are related to immature island arc volcanism on the margin of the
Archaean craton. Subsequent synorogenic basic and ultrabasic
intrusions, 1890–1880 Ma in age, host Ni-Cu deposits.
Synorogenic calc-alkaline intrusions hostnumerous porphyry type
occurrences and epigenetic Au-As mineralisation. These represent
the last significant metallogenic stage in the RLZ.
The Vihanti deposit
One of the few large ore deposits in the Raahe–Ladoga Zone
is the Vihanti ore deposit (the whole complex including the ores
is also known as the Lampinsaari Complex), which is situated near
the north-western end of the Raahe–Ladoga Zone.General geology
The host rock complex is Palaeoproterozoic in age and is composed
of different gneissic rocks of volcanic and sedimentary origin.
The Lampinsaari complex mainly constitutes felsic volcanic and
calcareous rocks (dolomites and skarn). Cordierite bearing
metasomatic rocks and black schists are also typical. The host
rock to the zinc ores is commonly dolomite or skarn, although the
ores are related to felsic volcanism.Ores
The ore bodies often show a clear zoning of metals. Apart from
sulphides, the oldest mineralisation seems to be the
uranium-phosphorous horizon (UP), which is situated near the
bottom of the formation in felsic volcanic rocks, but also in
skarn and dolomites. Uranium is found either in uraninite or in
apatite and phosphorous in apatite. The total tonnage is more
than 1 million tonnes, but the grade of uranium and phosphorous
is too low for exploitation.Remarks
Few attempts have been made to "unfold" the deformed rock
sequence. The host rock formation has originally been a single
volcanic horizon with two main cycles of sulphide precipitation.
No major extensions can be expected in any directions. The Isoaho
Zn-ore pinch out under the 1000 m level, and is intruded by
granite veins. The Kuuhkamo, Nevasaari, and Vilminko showings,
a few kilometres to the south on the other hand are
mineralisations where possible ore is still undiscovered.The Pyhäsalmi Zn-Cu-S deposit
Introduction
Production started in 1962. Underground development was
coincident with open pit mining until 1976 when the open pit was
closed. Underground mining has since continued and the
Pyhäsalmi mine celebrated its 35th anniversary in march
1997. The average grade is 0.85% Cu, 2.85% Zn, 37% S, 0.4 g/t Au,
and 14 g/t Ag. The ore reserves are c. 4 Mt and the total
production has been 30 Mt. The ore from the three satellite
deposit hasbeen processed in the Pyhäsalmi mill. The mill
has produced a pyrite concentrate from the old tailings together
with ore from the satellite mines. The total production in 1997
will be 1.08 Mt from the Pyhäsalmi mine, 0.24 Mt from the
Mullikkoräme mine and 0.3 Mt from the D-pond. Altogether 40
000 tonnes of Cu-concentrate (25% Cu), 65 000 tonnes
Zn-concentrate (52% Zn), 3 000 tonnes of Pb-concentrate, and 870
000 tonnes pyrite-concentrate will be produced. Some gold and
silver is recovered as byproducts with the Cu- and
Pb-concentrates.Mining
The mine layout is shown in Figure 23. The main
shaft, 500 m deep, is used for ore hoisting and personnel
transport. The auxiliary shaft is mainly used for ventilation.
There is also a blind shaft between the +730 and +400 levels for
ore hoisting. Crushing stations are situated on levels +400 and
+660. The decline starts from surface and continues down to the
+1050 level. The deepest production level is planned to +1090.
At the present, sub level stoping and bench cut and fill, are the
main mining methods. Sub levels are spaced in 30–40 m
intervals. The stopes are relative small, 20 000–40 000
tonnes. A high horizontal stress field (up to 120 MPa in pillars)
perpendicular to the ore, together with weak wall rocks cause
serious rock mechanical problems.Geology
The supracrustal rocks in Pyhäjärvi are a part of a
Palaeoproterozoic, Svecofennian schist belt. The NW-trending
schist belt is situated between the Archaean craton in the east
and the Central Finland Granitoid Complex in the west.Geology of the ore deposit
The Pyhäsalmi ore deposit is a typical volcanic hosted
massive sulphide deposit. The volcanic rocks are mainly felsic
pyroclastic rocks and coherent quartz-porphyries. Mafic volcanic
rocks are coarse-grained tuff breccias and lavas, including
pillow lavas. Mafic and felsic dykes are common.Rock chemistry
The chemical composition f the different rock types in the
Pyhäsalmi area is presented in Table 5.
The felsic volcanic rocks are sodium-rich rhyolites with high a
SiO2 content and the mafic volcanic rocks are basaltic low
K-tholeiites of primitive island arc type. The volcanism was thus
bimodal in character. Sericite-quartzites and cordierite gneisses
were originally felsic volcanic rocks which can be deduced from
the TiO2/Zr ratio while the cordierite-anthophyllite rocks were
originally mafic volcanic rocks.Alteration
The rhyolites in the Ruotanen formation are enriched in sodium.
Near the ore the rhyolites are altered to sericite-rich schists
with disseminated pyrite (sodium depletion and enrichment of
potassium). Cordierite bearing sericite schists and quartzites,
with high MgO content exist in the hanging wall. The basalts are
altered to cordierite-anthophyllite rocks that have clearly
higher MgO content and lower CaO content than the originalbasalts
(Table 5). The talc schists are strongly deformed and altered and
are originally probably dolomitic limestones.Ore types and structures
The ore body is 650 m long and 80 m wide in the centre. The shape
of the ore body resembles an open "S" which tails cut off at both
ends. Four different deformation events have affected the ore
(Luukas 1992) and have also caused mobilisation. The ore is
intruded by pegmatites. The ore continues down to at least 1300
m below the surface and is in the deepest part only 5–15m
thick. The contacts of the massive ore are sharp and conformable,
but remobilised sulphide veins cut the country rocks at high
angles. Fragmented felsic dykes and possibly hydrothermal
limestone inclusions are common in the ore.Mining technique
Ore delineation and assessment is based on diamond drilling,
percussion drilling, bore hole geophysics and geological mapping.
Diamond drilling is carried out by two drilling rigs. Underground
diamond drilling is scheduled at a yearly rate of 10 000–11
000 m. Drilling is done in two shifts with one man per shift. 50%
of the drilling is for ore delineation and 50% for exploration.
Investigation drillings are normally performed in a two stage
procedure. Initial exploration holes are made by diamond drilling
from the decline. The length of the holes varies from 100 m to
300 m. The holes are drilled in fixed profiles with 25 m
intervals. Secondary diamond drilling is done after development
drifts are ready in the footwall. The profiles are drilled after
the initial stope plans. Drilling profiles are situated in the
middle of the stope if it is 10–15 m wide, but if the stope
is over 15 m wide two or more profiles are drilled about 10 m
from each other. Drilling profiles are always perpendicular to
the ore.The Mullikkoräme mine
The Mullikkoräme ore deposit was first transected by
drilling in 1987, drilling geophysical anomalies. An underground
mineralisation close to the surface was discovered. Exploration
geophysics and intense core drilling continued and in 1989
Outokumpu Finnmines Oy took the decision to exploit the
A-mineralisation, which consisted of three different ore bodies
with a total tonnage of 247 000 tonnes with the grade of 7,75%
Zn (in situ). Mining of those ores took place in two periods
between 1990–1993, and the total amount of ore mined was 310
000 tonnes.General geology
The Mullikkoräme Formation (Fig. 28) is
about 5 times 2 km and is situated adjacent to a large north
trending tectonic zone. Although the volcanic complex has much
in common with the Ruotanen Formation in the Southwest, it is a
separate formation with different ores types. The Pb-isotope
composition of Mullikkoräme is comparable to other deposits
in the Pyhäsalmi-Vihanti area (Mati Vaasjoki pers. comm.).
Also here the volcanism has been bimodal and the mafic rocks
typically consist of pillow lavas while the felsic rocks are
rhyolitic lavas and pyroclastic rocks. Mafic volcanic rocks
predominate in the west while felsic rocks are more common in the
east. Outside the ore zone a mylonitized contact of a gneissic
granite appears abruptly. Folds are seldom seen, probably because
of the faulting and shearing which controls both the
mineralisation and alteration zones in the volcanic rocks. The
dip of the strongest schistosity (S2) is subvertical and the
plunge of the fold axis is towards south-southwest. The
metamorphic grade is much more lower than at the Pyhäsalmi,
mostly in greenschist facies. Alteration minerals include:
chlorite, sericite, phlogopite, talc, quartz, cordierite, and
pyrite. Some alteration types are hydrothermal, and some are
related to regional metamorphism (talc schist derived from
dolomite).Ore formations
The Mullikkoräme deposit is a typical small size
polymetallic massive sulphide deposit. The largest ore body is
the Siperia ore body which contains 700 000 tonnes of ore with
zinc as the economically most important metal. The Siperia ore
body has an average Zn content of over 8%, but the margins of the
ore bodies, which commonly are vein-like, may contain over 30%
Zn. The copper content averages 0.4% but the western wall often
has a higher content. The average lead content is about 1.2% and
the sulphur content is about 18%. In addition, the ore contains
approximately 1 ppm Au. Pyrite is the most abundantsulphide
followed by sphalerite, chalcopyrite, galena (commonly
accompanied by Ag), and pyrrhotite. Magnetite, and barite also
occur. The immediate host rock is often dolomite. In the southern
part of the Siperia ore body the host rock is dominated by a
felsic volcanic rock with talc-chlorite schist intercalations.Other VHMS-type mineralisations in central Ostrobothnia
Apart from the Vihanti and Pyhäsalmi deposits several
smaller mineralisations have been found in this region. The
Säviä, Kalliokylä, Kaskela and Kangasjärvi
mineralisations are hosted by volcanic rocks, and the
Hallaperä and Vuohtojoki mineralisations by tuffaceous meta-
sedimentary rocks. High metamorphic grade and strong deformation
make the interpretation of the genesis for these mineralisations
difficult.The Säviä Cu-Zn deposit
The Pielavesi-Pyhäsalmi ore zone is oval shaped and c. 40
km wide and 100 km in length. It is characterised by bimodal
volcanism, hydrothermal alteration and stratabound Zn-Cu
mineralisation (Fig. 29). Smaller
mineralisations tend to occur in close proximity to known major
ores, such as the Ruotanen, Kangasjärvi and Säviä
deposits.The Kangasjärvi Zn-Cu-S deposit
The Kangasjärvi deposit occurs within the southern extension
of the volcanic schist belt in which the Pyhäsalmi deposits
is situated in the northern end (Fig. 20). The deposit is hosted
by a succession of mafic, intermediate, and felsic volcanogenic
rocks and their altered derivatives. Rasilainen (1991)
interpreted the sequence as formed in a convergent plate margin
setting. Country rocks include altered
cordierite-sillimanite-garnet-anthophyllite gneisses,
cummingtonite-bearing gneisses and calc-silicate rocks. The
volcanic sequence is characterised by magnesium enrichment and
by the absence of carbonate minerals, indicating a proximal
Kuroko-type environment of deposition (Rehtijärvi 1984).The Ruostesuo Zn-Cu deposit
The Ruostesuo Zn-Cu deposit, which was mined in 1988–1989,
is part of a large mineralisation system in the Kalliokylä
volcanic complex c. 30 km east of Pyhäsalmi. The
mineralisations were found in 1959 by airborne geophysicl
measurements by the Outokumpu Company.The Hallaperä Zn-Cu mineralisation
The Hallaperä Zn-Cu mineralisation was found in 1967 by
Outokumpu Oy. This pyrite-pyrrhotite mineralisation was detected
by airborne geophysical methods (Huhtala 1979). The country rocks
are dominated by mica gneisses of sedimentary origin that often
contain garnet, sillimanite and muscovite. The mica gneiss
contains hornblende- and cummingtonite-bearing bands and
quartz-oligoclase schists, which indicate a tuffaceous origin
(Huhtala 1979).The Vuohtojoki Zn-Cu mineralisation
The investigation in the Vuohtojoki area started in 1958 when a
local farmer found a zinc rich boulder. Ground geophysical
measurements and subsequent diamond drillings led to the
discovery of the mineralisation in 1959 by the GSF. The area is
completely devoid of outcrops and all geological interpretations
are made from drill cores. The predominant rock types are a
biotite-plagioclase gneiss with hornblende bearing portions and
a quartz-feldspar schists consisting of a variety of volcanic
rocks and intercalated sediments. Thin volcanogenic amphibolites,
some of which contain cummingtonite are also present in the area
(Huhtala 1979).The Rauhala Zn-Cu-Pb deposit
A stratiform Zn-Cu-Pb deposit, the Rauhala deposit, is situated
in central Pohjanmaa c. 10 km east of the town of Ylivieska. A
thin and folded ore sheet is hosted by metasedimentary rocks
which are intruded by quartz-diorites (Västi 1989). The
major ore minerals are pyrrhotite, sphalerite, chalcopyrite,
galena, and pyrite. The deposit is geochemically zoned,
resembling other volcanic- or sediment-hosted massive sulphide
deposit with a vertical zoning of Cu+As+Au-Zn-Pb+Ag+Sb+Ba from
bottom to top and a lateral zoning of As+Au-Cu-Zn-Pb+Ag+Sb from
distal to proximal (Rasilainen & Västi 1989).The Hitura Ni-Cu deposit
Introduction
After the third start up of the Hitura mine in 1988 the mine has
been in operation first as an open pit and after 1990 production
gradually went underground. This was feasible due to a sales
contract for concentrate concluded with Kokkola Chemicals Oy for
the period 1991–1996. At the Kokkola Chemical Plant, the Hi-
tura concentrate was dissolved in an autoclave to produce nickel
salts including sulphate, carbonate, chloride, acetate, and
nickel powder.Regional geology
The Svecofennian nickel province in southern Finland (Fig. 31) is characterised by a number of
mafic-ultramafic intrusive bodies in a roughly circular area of
supracrustal rocks around the Central Finland Granitoid Complex
(Papunen & Vorma 1986). The Hitura ultramafic body lies in the
northern segment of the supracrustal ring-structure around the
Central Finland Granitoid Complex. The stratigraphy of the
supracrustal rocks in the surroundings of the Hitura ore (Fig. 32), as reported by Isohanni et al. (1985),
is from oldest to youngest: 1) gneissose granitoid; probably a
remobilised basement, 2) migmatitic mica gneisses with
intercalations of graphite and sulphide-bearing gneisses and
minor amphibolites,3) metagreywacke with intercalations of
intermediate and felsic tuffites, mafic, intermediate, and felsic
metavolcanic rocks, and 4) intermediate and felsic metavolcanic
rocks. The ultramafic and mafic intrusions are located in the
second stratigraphic unit. The Hitura area includes a number of
ultramafic bodies, and the Makola deposit lies only about 3.5 km
from Hitura. In addition to migmatitic gneisses, the immediate
surroundings are characterised by a belt of sulphide and
graphite-bearing schists not far from the Hitura body. The area
is one of poor outcrop, and geophysics, geochemical sampling, and
diamond drilling have all contributed to the geological
understanding of the area. The U-Pb zircon age of a felsic dyke
intersecting the Hitura body, 1877±2 Ma, is the minimum age of
the complex (Isohanni et al. 1985).The Hitura ultramafic complex
The Hitura ultramafic complex, composed of three closely spaced
serpentinite bodies (Fig. 33), has horizontal
dimensions of 0.3 x 1.5 km. The vertical extension has not been
established although the deepest drill section is at the 550 m
level and geophysical surveys indicate a continuation to at least
1000 m. As the bodies roughly conform with the wall rocks and are
structurally continuous in a vertical dimension they can be
described as vertical plugs. Due to its mineralisation, the
northernmost body is the best studied. It displays compositional
zoning with a homogeneous serpentinite (metadunite) core grading
to a zone of amphibole-bearing serpentinite (metaperidotite) and
further to a discontinuous chlorite serpentinite and amphibole
rock at the margin of the body (Fig. 33). The contact against
gneissic wall-rocks is tectonic, sharp, undulating and filled
with talcose joint mortar. The contact zone is characterised by
dislocated mafic blocks, erratic wall-rock inclusions and massive
sulphide lumps in soft talcose matrix, indicating late-tectonic
movements. The western contact zone contains large tongues of
partly mineralised ultramafic rocks in the gneiss. Locally, the
contact amphibole rock is absent and the serpentinite is in
direct contact with the wall-rock. The serpentinite core is
fine-grained, porous and of low-density
(2.4–2.6 g/cm3). The main minerals are antigorite
(80–90%), chlorite, calcite, and magnetite (7–9%), with
local talc, amphiboles, graphite, and phlogopite, and also
chromite and arsenides. The proportion of sulphides varies. Talc
is abundant close to the contacts, but small specks of talc are
common throughout. The rock was originally an olivine adcumulate,
but the primary igneous texture is visible only in the small
intercumulus specks of talc, chlorite and altered sulphides. The
texture is strikingly similar to that of the serpentinized
olivine adcumulate of the Mt. Keith Ultramafic Complex, Western
Australia (Hill et al. 1990). The primary igneous mineralogy has
been totally obliterated and antigorite is a fine-grained
felt-like mass with stringers of magnetite and small blebs of
altered sulphides. Amphibole-bearing serpentinite is an
alteration product of olivine orthocumulate and olivine-ortho-
pyroxene cumulate, and the primary ol-opx cumulus textures are
locally preserved as serpentine and amphibole pseudomorphs. The
marginal variety of lithology is an amphibole rock that is either
a metapyroxenite or a metasomatically derived rock in contact
with the felsic gneiss. The TiO2 values, which are higher here
than in the other varieties of the ultramafic rocks, reflect a
pyroxenitic origin. The Cr2O3 values (Fig. 34)
of the core are lower (0.65% Cr2O3) than those in the margin (1%
Cr2O3), indicating that chromite was an abundant liquidus phae
in the crystallisation of olivine orthocumulates and Ol-Opx
cumulates. Zn displays an interesting enrichment in the core of
the body, with peak values of up to 500 ppm, whereas the marginal
rocks contain only 150 ppm Zn (Fig. 34). The peaks do not
coincide with those of S, indicating that Zn was incorporated in
spinel phases during early crystallisation of the magma
ormetamorphically derived magnetite. Papunen & Penttilä
(1996) indicated on the basis of the Pearce (1968) molecular
proportion diagrams that the average Mg# (molar FeO/(FeO+MgO))
of the core is 0.79 and that olivine and orthopyroxene were the
fractionating phases (Si/FM=1.86). The calculated average Mg# of
the core is 0.81. It decrease gradually towards the marginal rock
varieties, and in the contact amphibole rock it is about 0.76.
As olivine is totally altered in the core of the body, the
composition can only be determined indirectly from the TiO2 vs.
MgO (anhydrous) variation diagram, which indicates a maximum
value of 44% MgO for olivine, corresponding to Fo83. According
to Roeder & Emslie (1974), Fo83 olivine has been in equilibrium
with a basaltic magma with about 11% MgO, which refers to a
high-Mg basaltic composition of the primary magma. The
fractionated basaltic magma type is in accordance with the
relatively high average Cu/(Ni+Cu) ratio of sulphides. The Ni vs.
MgO variation diagram of barren ol-adcumulate samples gives an
approximation of 2200 ppm Ni for primary olivine. Since the
fractionation trend is from olivine to orthopyroxene, the magma
type can be referred to as the "Kotalahti type" (Mäkinen
1987), although the fractionation did not reach the liquidus of
plagioclase. The body is intensely sheared, with shear zones
containing chlorite and local graphite, and random intersecting
pegmatite veins ranging in width from a few cm to 10 m. Zonal
reaction rims comprising almost monomineralic layers in the
succession chlorite, anthophyllite, actinolite, and talc,
surround pegmatite zones within serpentinite (Papunen 1971). K,
Al, Fe, and Si occur in greater abundances in the alteration
zones than in serpentinite. The reaction zones correspond to
those described by Marshall & Mancini (1994) from the Vammala
deposit. Pervasive random shearing and intersecting felsic
pegmatitic veins were accompanied by thorough alteration,
chloritization, and an increase in Al content throughout the
serpentinite body, thus changing the primary Al/Ti ratios, which
are anomalously high in the samples analysed.Ore types
Sulphides occur throughout the northern part of the Hitura
complex. The ore has been divided into three textural types: 1)
scattered fine-grained sulphides disseminated in the serpentinite
core, 2) medium- to coarse-grained moderate dissemination in the
marginal serpentine-amphibole rock, and 3) high-grade
interstitial disseminated sulphides and massive accumulations in
the amphibole rock of the contact zone, locally extending to the
gneissic wall-rock. The average compositions of different ore
types are presented in Table 7.
The sulphide mineralogy reflects the polyphase evolution of the
complex. The primary magmatic immiscibility and crystallisation
of sulphide melt, control the abundances of sulphides in the
body. The silicate magma was already sulphide saturated during
the crystallisation of olivine adcumulates in the core of the
body and gave rise to the sulphide dissemination existing in
small interstices of olivine crystals. Residual silicate magma
and associated sulphide melt was more abundant in
olivine±pyroxene orthocumulates in the marginal part of the body,
which now hosts most of the sulphides. The shape of the complex
implies that the melt was fractionated in a tube, probably in a
feeder channel of overlying volcanic rocks. The early
crystallised olivines accumulated and were pressed in the centre,
and the residual melt was enriched along walls of the tube,
forming the ol-px orthocumulates and pyroxenites. Secondary
hydration and carbonation brought about serpentinization and
talc-carbonate alteration combined with the deposition of
secondary sulphides, minor graphite, and magnetite. Deformation
in the eastern and western contact zones fragmented the ore
deposits and the alteration was intensive but heterogeneous. The
abundance of sulphides was controlled by primary igneous
processes but the mineralogy, texture, and beneficiation
properties of the ore types are due to secondary processes.Au-deposits
Gold mineralisation in central Ostrobothnia
The existence of a number of gold prospects in western Finland
demonstrates the gold potential of the area although so far no
mineable deposits have been located. However, the Björkdal
and Åkerberg gold mines in the Skellefte District in Sweden share
common characteristics with gold prospects in western Finland and
may represent the same gold province. Gold was first discovered
in the Ostrobothnia area in an outcrop at Kopsa in 1939, but
prospecting did not lead to any economically interesting
discoveries at that time. It was not until the late 1970 s and
early 1980 s that modern gold exploration commenced in the area,
mainly by Outokumpu Oy and the Geological Survey of Finland.
Impetus for gold exploration in most targets arouse by
identification of mineralised glacial boulders and outcrops or
by geochemical anomalies in till (Nurmi 1991).Stop descriptions
Day 1 — The Petiknäs and Renström mines
The Stops at the Petiknäs and Renström mines will be
at available underground exposures. Separate handouts will be
distributed at the mine explaining the different underground
stops and drill hole PEK207 which cross-cuts the mine
stratigraphy.Outcrops of the mine stratigraphy
Stop 1:1
Sandstone and conglomerateStop 1:2
Drill core 1239 from Renström showing the ore-bearing unit
including the ore, the stratigraphically overlying
sandstones-siltstones, and rhyolitic mass flows.Granbergsliden
Stop 2:1
The Granbergsliden mafic volcanic centre constitutes one of the
rare possibilities to examine basaltic volcanic rocks of the
Skellefte Group. The Granbergsliden mafic volcano was at least
400 m thick and 3 km wide. In this stop the participants will be
able to study: basaltic pillow lavas (Fig.
35a), related pillow breccias, in situ and resedimented
hyaloclastites (Fig. 35b), and quench fragmented spalled bomb
rind deposits (Fig. 35c).Stop 2:2
Along the road to the Granbergsliden mafic volcanic centre,
outcrops of coherent quartz-feldspar porphyries associated with
the Holmtjärn massive sulphide deposit can be found. The
Holmtjärn area is characterised by subvolcanic
quartz-feldspar intrusions and lavas which also form the main
part of the subaqueous rhyolite cryptodome-tuff volcanoes which
seem to be intimately associated with several massive sulphide
ores in the Skellefte district (cf. Allen et al. 1996).Nicknoret
Stop 2:3
The uppermost Skellefte Group and the overlying Vargfors Group
can be studied in some exposures near the Skellefte river at
Nicknoret (Fig. 36). In an approximately 200
m long section we will see volcanogenic conglomerates of the
uppermost Skellefte Group grading into Vargfors Group polymict
conglomerates with clasts from both Jörn type granitoids and
the subaerial Arvidsjaur volcanic rocks. Erratic clasts of mafic
volcanic rocks of the Vargfors Group as well as intercalated
mafic lava flows of the same composition confirm that all rock
units at this locality formed close in time. Approximately 1 km
upstream from this stop, a welded ignimbrite yielded an age of
1875±4 Ma (Billström & Weihed 1996) which is considered to
represent the middle part of the Vargfors Group. Outcrops c. 3
km downstream was dated by Welin (1987) at 1882±8 Ma which is
considered to represent the age of the top of the Skellefte Group
in this area (cf. Billström and Weihed 1996).Maurliden
In the Maurliden area situated c. 5 km ESE of the previous stop
(Fig. 36), the complex transition from the Skellefte Group to the
Vargfors Group can be studied. Several subeconomic massive
sulphide deposits exist in the area (e.g. the Maurliden
deposits). These deposits were interpreted by Allen et al. (1996)
to be related to subaqueous rhyolite cryptodome-tuff volcanoes
of which some may have been emergent above sea level. The
complexity of the transition from Skellefte- to Vargfors Group
rocks in the Maurliden area is illustrated by subvolcanic
quartz-feldspar porphyries (Skellefte Group) which are intrusive
into lime-cemented clastic rocks of the Vargfors Group (Stop
2:6). Sandstone and mudstone turbidites (Stop 2:5), which are
part of the Vargfors group, show evidence of very active
tectonism during deposition. Frequent rip up clasts, ruptured
beds, internal sliding of beds, soft sediment deformation and
small scale growth faults are common, as well as dewatering
structures like load casts. Complex contact relationships between
volcanic rocks and turbidites are indicated by the fact that
younging directions in the turbidites often indicates younging
towards the older volcanic rocks, which is probably explained by
both syn- and post-sedimentation faulting. The presence of pebble
conglomerates with well rounded clast in a carbonate dominated
matrix(Stop 2:4) indicates that the environment may have been
shallow marine, probably with a pronounced relief.Stop 2:4
Lime-cemented conglomerates are best exposed at the shore of the
Skellefte River. The clast material in the conglomerate is
polymict with clasts from Skellefte volcanic rocks and Arvidsjaur
volcanic rocks. All clasts are well rounded and the pebbles are
flattened in the main cleavage and elongated parallel to the fold
axis of the main deformation.Stop 2:5
A few hundred meters from the lime-cemented conglomerates, good
exposures of the turbiditic metagreywackes will be examined.
These rocks show abundant evidence of intense syn-depositional
deformation indicating active tectonism at the time of
deposition.Stop 2:6
A quartz-feldspar porphyry which intruded the lime-cemented
conglomerates will be shown at a finalstop in the Maurliden area.
Contact relationships are not entirely clear, but the existence
of typical Skellefte Group volcanic rocks within the
conglomerates illustrates the complex relationship between the
Skellefte- and Vargfors Groups.Terrestrial volcanic rocks with epiclastic intercalations in
the Hej area, northern Västerbotten County
The bedrock west of the village Hej in northern Västerbotten
County mainly consists of felsic and subordinate intermediate to
mafic volcanic rocks, here called the Hej volcanic rocks (Fig. 37). Together with some epiclastic
intercalations the entire succession is called the Hej subgroup
(Ehrenborg 1987). This subgroup belongs to the Arvidsjaur Group
and occurs in a north-northeast trending synform which is
situated close to the transition zone between the subaqueous and
the subaerial volcanic successions of the Skellefte and the
Arvidsjaur Groups, respectively.Stop 2:7 — Volcanic breccia south-west of
Urikberget
A volcanic breccia is exposed in two north-striking and
west-dipping strips in the eastern and western part of
Urikberget. East of the breccia a grey dacitic feldspar porphyry
lava occurs, which forms the matrix to the volcanic breccias. No
distinct contact is exposed between the volcanic breccia and the
lava and in the transition zone, irregular flow-banded fragments
occur in the lava. The volcanic breccia is both clast- and
matrix-supported and the margins between fragments and the
feldspar porphyritic matrix vary from diffuse to sharp. The
fragments are angular to subrounded and mainly consist of
elongated flow-banded fragments, vesicular mafic fragments, and
vesicular grey-greenishdacite fragments with or without feldspar
phenocrysts. Jigsaw fit texture occurs in parts where the red
feldspar porphyritic matrix dominates.Stop 2:8 — Urikberg conglomerate west of Urikberget
The Urikberg conglomerate has a north-south extension of more
than 4 km and a thickness exceeding 100 m. The conglomerate is
poorly sorted and matrix supported. It is characterised by
granitic and volcanic clasts with sharp contacts towards the
gritty matrix. The granitic clasts are derived from the GI
granodiorite phase of the Jörn batholith. The tectonic
contact between the Urikberg conglomerate and the volcanic
breccia is exposed in a 4 m wide NW-striking shear zone.Stop 2:9 — Rhyolitic ash-fall tuffs south of
Boljerberget
Ash-fall tuffs are exposed south and east of the Boljerberget
hill. These tuffs are characterised by very fine-grained, gently
west dipping layers alternating with a coarser-grained,
vesicular, quartz-feldspar porphyry. Stratigraphic younging is
indicated by impact-sags in fine-grained layers and the
truncation of underlying layers. The air fall emplacement is
indicated by good sorting, lamination, stratification, truncation
of underlying layers, impact sags of ballistic clasts, and the
lack of traction structures such as cross-stratification.
Stop 2:10 — Welded ignimbrite at Boljerberget
Welded ignimbrites are exposed at and around the Boljerberget
hill. These are lithophysae-bearing, quartz-feldspar porphyritic
rhyolites with fiamme and eutaxitic textures. The matrix varies
in colour from blue-grey to violet which is due to different
oxidation states of iron. A steeply dipping bedding parallel
foliation is defined by fiamme and the eutaxitic textures. In
thi-section the rock contains broken crystals, eutaxitic, and
granophyric textures. The degree of welding is moderate to dense.
The poor stratification, variable degree of welding, and the
absence of reworked horizons of the welded ignimbrites are
consistent with rapid emplacement. either from sustained flows
or from a rapid succession of flows.The Björkdal area
Regional geology
The Björkdal area is situated on the hinge-line of a major
anticlinal structure, the core of which, outside the
Björkdal area, is occupied by metavolcanic rocks belonging
to the Skellefte Group of Allen et al. (1996), cf. Figure 5. For
the Boliden area, Allen et al. (1996) state that the metavolcanic
rocks are overlain by metasedimentary rocks via adisconformity.
In the Björkdal area, metasedimentary rocks overlie
metavolcanic rocks, but the contact relations are obscured by
deformation (Fig. 39 and 40). Furthermore, the
core of the anticline is here occupied by a
tonalitic-trondhjemitic pluton, which appears to be older than
any of the metavolcanic rocks dated so far. Billström &
Weihed (1996) presented a preliminary age for this rock of c.
1905 Ma. A tectonic contact between this pluton and the overlying
metavolcanic rocks will be seen in the Björkdal mine.Rock types
The Björkdal pluton consists of a greyish, fine- to
medium-grained, sometimes trondhjemitic tonalite-granodiorite to
quartz-monzodiorite. The trondhjemitic to tonalitic varieties
dominate the northern parts, the granodiorites the southern
areas. The rock is mostly foliated to a somewhat gneissic
structure, particularly in the strongly sheared area around and
south of the mine, whereas the south-eastern part of the pluton
is almost isotropic. In these better preserved rocks,
characteristically glomerophyric plagioclases occur. Relics of
these are diagnostic for the recognition of the tonalite as the
precursor of the strongly deformed and altered gneisses in and
to the south of the mine, where the tonalite at places is altered
to very feldspar-poor quartz-mica gneisses and mylonites with a
microscopic banding. Post-deformative tourmaline indicates late
alteration in this area.Stop 3:1 — The Björkdal open pit
The stop will be devoted to available exposures in the
Björkdal open pit.Stop 3:2 — Small outcrop N of the road 1 km ESE of
Björkdal village
A greyish, medium-grained, even-grained, isotropic,
recrystallized and metamorphic tonalite in the centre of the
Björkdal pluton. This outcrop was sampled for radiometric
age determination and Billström & Weihed (1996) report a
preliminary U-Pb zircon age of 1905 Ma.
Stop 3:3 — Outcrops in the forest, 3 km NNE of
Björkdal village
Turbiditic metasediment with various sedimentary structures and
post-deformative cordierite porphyroblasts.
Stop 3:4 — Outcrops around the road W of L
Mörttjärn
Actinolite altered metavolcanic rocks. Reddish, felsic,
plagioclase-phyric, amygdaloidal metavolcanic rocks occur as
clasts or alteration relics in a greenish, actinolite-rich
matrix. Jig-saw fit relations and diffuse borders towards the
matrix suggest that the felsic parts are in fact alteration
relics rather than clasts. Furthermore, relics of the
plagioclase-phyric and amygdaloidal structure can occasionally
be seen in the matrix and indicate that the matrix formed by
alteration of the material in the felsic parts.
Metavolcanic rocks of the Skellefte Group in the Boliden
area
The significant features of the Skellefte Group metavolcanic
rocks are described in the introductory text and by Allen et. al.
(1996). The stops described below aim to demonstrate a few of the
most significant facies types of the group.Stop 3:5 — Outcrop N of the road, 2 km NE of
Svanström
Thick-bedded, matrix supported, polymict volcanic breccia to
sandstone, rich in volcanic clasts. Current bedding and traction
deposition structures indicate shallow water deposition. Debris
shed off from nearby volcanic centre?
Stop 3:6 — Outcrops on Mångfallberget ski slope
Quartz-feldspar porphyritic, coherent, rhyolitic, metavolcanic
rock with relics of a polygonal fracture pattern suggesting
polygonal cooling joints. Lava or subvolcanic intrusion.Stop 3:7 — Outcrop N of small road, 3 km SE of
Kankberg
Volcanic siltstones with fine planar bedding with syn-sdimentary
deformation structures. The stop demonstrates the subaqeous
nature of much of the Skellefte Group metavolcanic sequence.
Stop 3:8 — Torrberget hill, 8,5 km SE of
öster-Jörn
Thick-bedded to massive, matrix-supported, poorly sorted,
somewhat polymict volcanic breccia with numerous pumice and other
volcanic fragments set in a felsic matrix. Rotated pumice clasts
and wavy, planar structure indicate synvolcanic/depositional
deformations (welding or compaction) leading to fiamme
structures. The rock is interpreted as a juvenile, pyroclastic
flow deposit.
Stop 4:1 — The Hitura Mine
The Hitura mine is located c. 80 km east from the coast of the
Bothnian bay along the river Kalajoki. During the visit different
types of ultramafic rocks and surrounding mica gneisses will be
seen in the open pit and in the underground mine. Also drill
cores through the ultramafic body with different ore types can
be examined.Stop 4:2 — The Kopsa Gold-Copper Prospect
The Au-Cu-Ag mineralisation at Kopsa occurs within and close by
a late-orogenic, granodiorite/tonalite stock which intrudes
Palaeoproterozoic metasedimentary and metavolcanic rocks near the
southern edge of the Main Sulphide Belt in western Finland (Fig. 41). The Kopsa stock, which is about 1200 m
long and 600 m wide, is one of at least two mineralised
intrusions in a 10 km long, ENE-trending structure. Information
on other intrusions and this regional structure is limited
because of extensive glacial overburden and little previous
exploration.Stop 4:3 — The Vesiperä gold mineralisation in
Haapavesi
The Vesiperä gold mineralisation is located about 20 km east
of the town of Ylivieska. The mineralisation was found in an
outcrop in 1984. An arsenopyrite-rich sample contained 75 ppm Au
and 15% As. In the course of the next two years the GSF
carriedout regional bedrock mapping, geophysical ground surveys
(magnetic, electromagnetic, and IP) and geochemical till
samplings with the percussion drill.Stop 4:4:1 — The Kuusaa formation
The Kuusaa formation, 35 km NW of Pyhäjärvi, mainly
consists of highly porphyritic, felsic to mafic metavolcanic
rocks with plagioclase and/or uralite phenocrysts. In the
southern parts of the Kuusaa formation primary volcanic
structures are extremely well preserved giving some information
on the physical conditions at the time of eruption. In some
metamorphosed basic lava flows, amygdaloidal and flow-top breccia
structures can be recognised as well as pyroclastic breccias and
ash-flow tuffs in intermediate metavolcanic rocks. Most of the
silicic members are composed of tuff and lapilli tuff beds with
some pyroclastic breccia and volcanic conglomerate
intercalations. Locally there are also ash-flow tuffs with
possible ignimbritic textures. Chemically the metavolcanic rocks
of the Kuusa formation form a continuous calc-alkaline trend from
basaltic andesites to rhyolites with high-K affinity. The U-Pb
zircon age of a quartz-feldspar porphyritic rhyolite of the
Kuusaa metavolcanic formation is 1887±2 Ma (Kousa unpublished)
which is roughly the same age as the 1880 Ma Pihtipudas
metavolcanic rocks (Aho 1979).Stop 4:4:2 — The Settijärvi conglomerate
At the stop situated in the south-western margin of the Kuusaa
formation a polymict, volcaniclastic conglomerate with volcanic
pebbles derived from the Kuusaa formation and plutonic pebbles
of quartz-dioritic composition in mainly intermediate tuffaceous
matrix can be seen. The U-Pb zircon age of these plutonic pebbles
is 1888±7 Ma (Vaasjoki & Sakko 1988). The quartz-dioritic pebbles
in the Settijärvi conglomerate may be derived from the
Vääti intrusion described below. This conglomerate and
the volcanic rocks of the Kuusaa formation are intersected by a
roughly E-W trending diabase dyke swarm.Stop 4:4:3 — The Vääti granodiorite
The main constituents of the bedrock between the Kuusaa formation
and the Pyhäsalmi volcanic complex are large syn- or
late-tectonic granitic and granodioritic to dioritic intrusions.
The so called Vääti granodiorite south of the Kuusaa
formation is one example. The Vääti granodiorite also
includes more mafic varieties and is usually slightly foliated
indicating a syntectonic origin. The Kuusaa volcanic rocks are
probably genetically related to this intrusion. The youngest
intrusive phase is represented by coarse-grained potassium
feldspar porphyritic granites, about 1880–1865 Ma in age
(Kousa et al. 1994).The Pyhäsalmi Mine
Stop 5:1:1–3 — Bimodal volcanism in the Ruotanen
formation (Lippikylä Mb.)
This stop consists of different outcrops west and south of the
mine with felsic porphyries, pyroclastic rocks with sedimentary
interbeds, mafic tuffs and tuff breccias with felsic fragments,
and possibly also pillow lavas of the Mukurinperä member.
The rather weak deformation in most of the outcrops of unaltered
volcanic rocks is typical of the more distal parts of the
stratigraphy compared to altered rocks near the ore.
Stop 5:1:4 — Altered rocks east of the mine behind old
open pit (Lepikko Mb.)
In this stop several outcrops representing different type of
alteration will be seen. Sericitic alteration with cordierite and
sillimanite, cordierite-quartz rock fragments in pyrite rich
matrix, massive pyrite ore, younger pegmatite dykes, and
unaltered mafic dykes with zeolites cutting the altered rocks can
be studied, as well as cordierite biotite gneisses with
lapilli-like texture, massive cordierite-anthophyllite rocks,
gneisses representing very strong Mg-alteration, felsic looking,
light grey cummingtonite-quartz-magnetite-anthophyllite rocks
with clear mafic composition, and younger unaltered
quartz-feldspar porphyries.
Stop 5:1:5 — Drill core exhibition
Representative drill cores from different ore types, volcanic
rocks and altered rocks will be shown.Mullikkoräme mine
Stop 5:2:1 — Drill core exhibition
Different ore types, volcanic rocks, and their altered
equivalents will be exhibited on this stop.Stop 5:2:2 — The Tetrinmäki pillow lavas
This stop consists of a large outcrop area with well preserved
pillow lava and other volcanic rocks. Mafic dykes
(Tetrinmäki Mb) can also be seen.Stop 5:2:3 — The Riitavuori felsic volcanic rocks
A typical outcrop of felsic volcanic rocks of the Riitavuori Mb.
Among other rocks quartz porphyries (1.92 Ga) can be seen. In
some places the rocks are more coarse-grained and intrusive
like.Stop 5:2:4 — Outcrops near the decline
Pillow lavas and lava breccias with strong epidotization and
local strong chloritization are exposed near the decline. In the
mine it will also be possible to study the ore and waste rock
stock piles.Acknowledgements
The editors would like to express their sincere thanks to all the
contributing authors and to all involved in preparing and running
the field trip. We would also like to thank Boliden AB, Outokumpu
Mining OY, Terra Mining AB, and the Geological Surveys of Finland
and Sweden for their support of this guidebook and the field
trip.
References
Aho, L. 1979. Petrogenetic and geochronological studies
of metavolcanic rocks and associated granitoids in the Pihtipudas
area, Central Finland. Geological Survey of Finland, Bulletin
300. 22 p.
Figure captions:
Fig. 1. Route Map. Stops indicated.
Table titles:
Table 1. Age constraints on different events within and around
the Skellefte District.
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