Abstract in English
| Suomenkielinen tiivistelmäThe plate tectonic paradigm has been widely applied in interpreting crustal growth, deformation and metallogenesis in the Svecofennian domain, beginning with a synthesis by Hietanen (1975). Northeast-vergent emplacement of the Jormua ophiolite (Kontinen, 1987) and Outokumpu assemblage onto the Karelian craton foreland is inferred to record the initial collision with a Svecofennian oceanic island arc (Ward, 1987), generating primitive tonalites from a low-K tholeiitic source (Lahtinen, 1994). Continued volcanism within this arc at 1.92–1.90 Ga led to the formation of VHMS deposits, including the Pyhäsalmi Zn-Cu deposit (Ekdahl, 1993). Reversal of subduction polarity following collision (Gaal, 1990), or a further arc-arc collision (Lahtinen, 1994) is invoked to explain the most extensive phase of volcanism, magmatism and deformation in southern and western Finland, and also Sweden, including the Skellefte district (Allen et al., 1996) between 1.89–1.86 Ga.
Although archeological evidence exists for at least transient human occupation in eastern Finland as far back as 7000 BP, it is only within the last hundred years or so that the region has seen any large scale commercial exploitation of mineral resources. The first extensive operations involved the quarrying of soapstone at Nunnanlahti towards the end of the nineteenth century, although production declined steadily and eventually ceased for a time. The industry has however been revived during the last decade with spectacular success, such that the two companies operating at Nunnanlahti now represent the largest soapstone quarrying enterprises globally; a visit to one of the quarries has therefore been included in the excursion itinerary.
Undoubtedly the most important mineral discovery in eastern Finland was the finding of the distinctive serpentinite-associated Cu-Co-Zn-Au massive sulfide deposits at Outokumpu during the early part of this century. The Outokumpu mine was at one time the largest Cu mine in Europe and together with two other deposits, Keretti and Vuonos, yielded a total of 1.1 Mt copper between 1913 and their closure in 1989. Even though no deposits are currently being mined in the area, the discovery provided a firm basis for the development and expansion of the mining and metals processing industry in Finland, an area which still ranks very highly in terms of export earnings.
Only one other Outokumpu-type massive sulfide deposit has been exploited to date, namely the Luikonlahti mine, which operated from 1968–1983 and was situated some tens of kilometers to the north of Outokumpu. This deposit will be visited during the excursion in conjunction with an examination of a nearby kimberlite pipe. The only one other Proterozoic base metal deposit to have been mined in the region is the Hammaslahti Cu-Zn-Au deposit, which represents a different style of mineralization, having been hosted by rift phase turbiditic greywackes and distally associated with mafic volcanics; time constraints, and the poor quality of exposure sice cessation of mining activities in 1987, nevertheless precludes us from visiting this area, which lies some 70 km to the southeast of Outokumpu (Appendix 1). The Talvivaara prospect will however, present an opportunity for examining an extensive and rather unusual sediment-hosted Ni-Cu-Zn deposit.
Mention should also be made of the late Archean Siilinjärvi carbonatite, some 20 kilometers northeast of Kuopio, which in terms of annual tonnage, is currently the largest mining operation in Finland. Extensive reserves of kyanite, resulting from Svecofennian metamorphism of a thick paleoregolith developed across the Archean-Proterozoic unconformity, have also been investigated in recent years (Kohonen and Marmo, 1992). Otherwise, until recently the Archean of eastern Finland has received less attention from an exploration viewpoint. The Otravaara massive pyrite deposit, hosted by a submarine felsic volcanic sequence was worked briefly early this century, but recent years have seen a growing interest in the gold potential of greenstone belts in eastern Finland and a number of prospective areas have been delineated, principally on the basis of till geochemical anomalies. At the risk of making our timetable rather tight, and with the additional motive of showing examples of some of the oldest rocks in Europe, we have decided to include one of the most promising prospects in the Ilomantsi greenstone belt within the excursion programme.
We hope that you find the excursion interesting and rewarding!
The Kola-Lapland Province, to the NE of the Karelian craton, records the amalgamation at around 1.9 Ga of several distinct crustal units of both Proterozoic and Archean age, and is more characteristic of collisional tectonic processes. In contrast, the Svecofennian Province, to the SW of the Karelian craton, and covering more than half a million square kilometers, is entirely early Proterozoic in age, and indicates relatively rapid formation nd accretion of new crust between about 1.97–1.86 Ga.
For a general review of the Shield the reader is referred to Gaál and Gorbatschev (1987) and Gaál (1990), while recent reviews of the Russian parts of the Kola and Karelian provinces can be found in Turchenko (1992) and Rundkvist & Mitrofanov (1993). The tectonic evolution of the northernmost part of the Shield has also been summarized by Berthelsen and Marker (1986) and Gaál et al.,(1989). Recent syntheses of Svecofennian metallogeny and petrogenesis have been published by Ekdahl (1993) and Lahtinen (1994) respectively.
During the Paleoproterozoic Svecofennian Orogeny, the Karelian craton was subjected to several tectonic events directed from both east and west. The transition zone between the Kola and Karelian Provinces, known as the Belomorian domain, is considered to represent late Archean high grade gneiss terrain that was extensively reworked, if not exhumed, during thrusting southwestwards over the eastern margin of the Karelian Province at 1.9–1.8 Ga. Conversely, the southwestern margin of the Karelian Province was strongly deformed and imbricated during NE-directed thrusting associated with the Svecofennian Orogeny and emplacement of the Outokumpu assemblage and associated sedimentary and basement nappes at about 1.9 Ga. As stated above, the Svecofennian Province is isotopically distinctive and represents rapid crustal growth and deformation around 1.9 Ga, with no compelling evidence for the large-scale involvement of pre-existing crust. Evidence is thus accumulating to support interpretations invoking collisional shortening involving an island arc and a passive continental margin at around 1.9 Ga, followed by the development and successive accretion of further arc terranes over the next hundred million years or so (Hietanen, 1975, Kontinen, 1987, Ward, 1987, Gaál, 1990, Lahtinen, 1994).
Recent exploration has also shown that Archean greenstone belts in eastern Finland contain gold mineralization similar to that in other shield areas. The first discoveries were made in the Hattu schist belt, near the southwestern margin of the Karelian craton, where the Geological Survey of Finland has documented and drilled more than ten prospects within a structurally defined gold anomaly zone over 40 km in length. The Hattu schist belt represents a distinctive sediment-dominated supracrustal sequence that records rapid crustal growth and deformation between 2.75–2.72 Ga. Felsic volcaniclastic sediments in this belt, and lithofacies, as well as geochemistry of granitoids and some basalts are consistent with a collisional arc setting. Gold is hosted by extensive structurally controlled alteration systems and in contrast to many classical greenstone terrains, there is evidence for mineralization preceeding or accompanying the peak of regional metamorphism.
Some potential for Zn and Ag mineralization within the felsic supracrustal sequences of eastern Finland has also been recognized, with the Taivaljärvi prospect being the best documented (Kopperoinen & Tuokko, 1988). Soapstone deposits developed in ultramafic rocks represent a volumetrically minor but economically significant resource in several greenstone belts.
A major plume event has been implicated in the genesis of the 2.45 Ga mafic layered intrusions, especially since the recognition that an extensive suite of komatiitic basaltic volcanics in the eastern part of the Karelian Republic is of similar age and may have had similar source characteristics as well (Puchtel et al., 1996). Moreover, Heaman (1997) has attempted to correlate these rocks with the Matachewan and Hearst mafic dykes swarms and Huronian flood basalts of the Superior province of Canada, and suggested that this was a global event, equivalent to younger large igneous provinces.
Terrigenous clastic sediments discordantly overlie these layered intrusions, with field relationships commonly recording angular discordances, suggesting tilting of the layered intrusions during ongoing extensional deformation (Ward et al., 1989).
Fe-tholeiite dikes intruded the Archean basement throughout eastern Finland and some of these, which cut the basement unconformity and terminate within the Proterozoic cover sequence, have been interpreted as feeder channels for lava flows (Pekkarinen and Lukkarinen, 1991). The Tohmajärvi mafic volcanic complex has been dated to 2.1 Ga and is believed to correspond to the initial phase of rifting and basin subsidence, being closely associated with coarse-clastic turbidites that record an Archean basement provenance. Nykänen et al. (1994) attributed geochemical differences, particularly with respect to REE patterns, between the dyke swarms and the Tohmajärvi complex to melting at different depths during progressive extension, with the latter being derived from more attenuated continental margin lithosphere.
Coarse clastic lithofacies occuring in association with the Tohmajärvi volcanic complex are interpreted as proximal, partly channelized prograding fan sequences characterized by fining and thinning upward sequences which pass upwards and laterally into quartzose metapsammites assigned to a suprafan — middle fan setting (Ward, 1988). Stratigraphical relationships with associated chemogenic and pelitic lithologies are obscure, due to complex deformation, but the coarser lithologies are considered to represent an influx of coarse detritus associated with basin formation and rapid subsidence after 2.1 Ga (Fig. 2). Sporadic calc-turbidites and pink dolomite clasts in thick-bedded debris flows are of particular interest as they may be the only surviving evidence for the existence of a penecontemporaneous carbonate shelf environment at the basin margin.
Finer grained sequences are then considered to overlie the coarser and volcanic-related sediments, recording a more prolonged phase of starved intracratonic basin or passive margin deposition that was eventually terminated, or rejuvenated by a renewed phase of rifting between 1.97–1.95 Ga, which culminated in the formation of oceanic crust, now best represented by the Jormua ophiolite (Kontinen, 1987).
Fine-grained facies commonly contain fine-grained diseminated pyrrhotite, as well as some lithologies containing abundant carbonate and graphite. Some thin and weakly graded sedimentary couplets consist of impure calcareous and graphitic laminae that could have formed from tractional currents in a shallow marine environment or else from dilute turbidity currents that transported terrigenous or authigenic carbonate shelf detritus into a deeper basin environment. Presumed chemogenic or hydrothermal deposits are also present, lacking textural evidence for the presence of detrital quartz and clay minerals and characterized by a rather distinctive metamorphic paragenesis comprising calcite — microcline — Mg-tremolite — pyrrhotite — graphite — albite — sphene; a satisfactory explanation has not yet been found for the widespread precipitation of authigenic or diagenetic feldspar or a hydrous precursor to feldspar, and it is not clear whether the depositional environment was a deep restricted basin or might have been shallow enough to produce evaporites. Nevertheless, the generally high sulfide and graphite contents of these sediments point to the possibility of microbial activity in an anoxic environment — be it shelf or basin floor.
Carbon isotope studies by Karhu (1993) and Melezhik and Fallick (1996) show that the Jatulian carbonates deposited between 2.2 Ga and 2.0 Ga in the Fennoscandian Shield record a signicant isotope excursion, that corroborate less comprehensive results from elsewhere. This change, from more normal d(=delta)13C values of around 0 per mil at 2.4 Ga to +12 per mil by 2.2 Ga, and back to 0 per mil by 2.0 Ga, is the largest such excursion so far documented, and requires the burial of a large amount of carbon, in the form of organic matter, in marine sediments at that time. This is consistent with the presence of extensive black schists within the Kalevian sedimentary sequences of this age in eastern Finland. Since evidence has recently been found for the development of the earliest eukaryotes during this time interval (Han and Runnegar, 1992), it is interesting to speculate whether the isotope excursion reflects this development, or whether changes in the chemistry of the hydrosphere and atmosphere enabled evolutionary advances to take place (cf. Karhu and Holland, 1996).
This time period also broadly overlaps with other events such as the first appearance of redbeds in the Fennoscandian Shield, widespread development of stromatolites in carbonate platforms, and the earliest record of carbonate concretions in pelitic sediments (Melezhik and Fallick, 1996). This in turn may have implications for the age and distribution of stratiform sediment-hosted sulfide deposits, since the relative proportions of carbonates and carbonaceous sediments within intracratonic basins and rifted margin sequences may be of great significance in controlling the redox behaviour of basinal fluids.
The intercalation of turbiditic greywackes of predominantly Archean basement provenance with more mafic mass flow deposits therefore led Ward (1988) to propose a model involving either distal hydrothermal acivity related to the Tohmajärvi mafic volcanic complex, or diagenetic leaching of metals from deeper basin sediments after the cessation of volcanism, with more permeable coarse clastic deposits acting as preferred pathways for fluid migration, while finer grained clay-rich and locally graphitic units would have acted as barriers to fluid flow. This model may encounter difficulty in explaining the migration and deposition of copper, which should require higher temperatures, and more recent lithogeochemical evidence for a distinct alteration halo around the deposit (Loukola-Ruskeeniemi et al., 1993) favors a more local and direct rather than distal and diagenetic source for hydrothermal activity. The predominance of Cu and Zn with respect to Pb is in either case consistent with metal leaching from predominantly basaltic rather than felsic volcanics (cf. Large 1992), while the Archean aspect to the Pb model ages (Vaasjoki, 1981) is in accord with the rifted cratonic margin setting and granitic basement provenance of the coarse clastic sediments hosting the deposit.
Structural and lithofacies mapping, in combination with recently acquried geophysical data, are continuing in the area, in order to establish the potential for discovery of further deposits within the sequence.
The Outokumpu assemblage was first described as an ophiolite by Wegmann in 1928, an interpretation quite consistent with the concept as understood in Alpine ters at that time. However, whether the assemblage contains all or some of the elements explicit in more recent definitions is still debated (Park, 1988a). Interpretation of primary lithologies and geochemical characteristics has been difficult, partly due to intense deformation but also as a result of extensive hydrothermal processes associated with ore formation. In studying some of the serpentinite bodies of the Outokumpu assemblage near Luikonlahti, Park (1983) also identified a relict high temperature aureole enclosing the serpentinite and interpreted it to represent their emplacement as high level sills. However, more recent studies, still in progress (Asko Kontinen, personal communication), have shown that relict olivine and enstatite is unlikely to be of primary magmatic or contact metamorphic origin, since primary mineral assemblages were already converted to hydrothermally altered serpentinite and talc-carbonate lithologies prior to the onset of regional metamorphism. Olivine is therefore reinterpreted as belonging to a peak regional metamorphic assemblage that was in turn extensively serpentinized during late stage retrogression. Park (1988a) favoured a back-arc setting for volcanism and exhalative mineralization, deduced from the presence of deformed vesicular pillow lavas, as well as immobile trace element geochemistry.
At the scale of the whole Fennoscandian Shield, the full significance of mafic and ultramafic magmatism and rifting at this time has not been properly appreciated. In addition to the Jormua ophiolite and Outokumpu assemblage, a major province of essentially undeformed picritic flood basalts is present around Lake Onega, in the eastern part of the Karelian craton, while a suite of picrites, hosting major nickel deposits at Pechenga, in the Kola Peninsula, also have an age of 1.97 Ga (Hanski et al., 1990). Hanski (1989, 1992) found that the chemistry and isotopic characteristics of the Pechenga ferropicrites were consistent with a plume-related origin, but advocated caution in advancing this interpretation, since the regional setting and duration of subsequent events need to be well-constrained. Moreover, it is not entirely clear that the Kola picrites formed in their present position with respect to the Karelian Province, due to the presence of several intervening allochthonous terranes (Berthelsen & Marker, 1986).
It is also apparent that the main part of the volcanics within the Lapland greenstone belt structurally, as well as stratigraphically overlie units correlated with the 2.1–2.0 Ga rift phase (Ward et al., 1989), while similar rocks, including komatiites, in the Karasjok greenstone belt in adjacent northern Norway have yielded Sm-Nd isochron ages between 2.0–2.1 Ga (Krill et al., 1985, Bergh & Torske, 1988, Barnes & Often, 1990). The recent recognition of a suite of serpentinites with depleted harzburgitic mantle affinities within the greenstone sequence in central Finnish Lapland (Hanski et al., 1996) indicates that a large part of this sequence must be allochthonous, presumably emplaced onto the Karelian craton at some time prior to 1.89 Ga (Sorjonen-Ward, 1996). It is hence conceivable that the Lapland greenstones, the Jormua ophiolite and the Outokumpu assemblage each represent the deformed remnants of a formerly contiguous segment of oceanic crust being consumed towards the southwest and emplaced across the Karelian province foreland, which would then imply and allochthon of dimensions 50–100 km in width and greater at least 500 km in length, which for example, is an area similar in magnitude to that of the Oman ophiolite.
It is tempting therefore to speculate as to whether a plume was responsible for extensive mafic and ultramafic magmatism at the scale of the Fennoscandian Shield in the period preceding the 1.97–1.95 Ga rifting process. Moreover, is it possible that the impingement of a plume at that time might have influenced the geometry of plate boundaries and motions, leading to sea-floor spreading, in much the same way as plumes have been implicated with initiation of sea-floor spreading, or reorganization of plate-motions in the Phanerozoic?
Although most of this monotonous sequence is allochthonous, its widely transgressive nature is indicated by local preservation of basal depositional contacts in several places; these include the 2.1–2.0 rift phase deposits, the Jormua ophiolite, the Outokumpu assemblage and, in the southern parts of the region, they locally rest directly on Archean basement retaining paleoregoliths. The absence of the 2.1–2.0 Ga rift phase sediments in the latter areas is not simply a result of lateral facies changes during basin transgression. Instead, it may relate to the incipient stages of attempted subduction of the passive margin, with the development of flexural bulge and consequent erosion of the earlier rift-phase passive margin deposits, prior to general subsidence and deposition of the main part of the monotonous sequence. Systematic changes in clast composition in basal coarse-clastic deposits are also consistent with progressive denudation of firstly cover sediments and then Archean basement in the source area, with intrabasinal detritus dominating at later stages (Ward, 1988). So far, attempts to establish the provenance of these deposits using petrography, chemistry, Sm-Nd model ages and detrital zircon have yielded ambiguous results, neither confirming or precluding derivation from the Svecofennian arc terranes (Ward, 1987). However, the textural immaturity and chemical homogeneity of the sediments led Kontinen and Sorjonen-Ward (1991) to favor an origin as a major submarine fan system prograding over the subsiding passive margin, just prior to collision, with sediments being derived from the craton, or a more distant orogenic source (configurations c and d in Fig. 3).
Such a tectonic setting also appears to be in accord with the geometry of compressive deformation, which is most readily interpreted in terms of the Outokumpu assemblage and overlying sediments having been thrust over the Karelian foreland during imbrication and attempted subduction of the craton margin. Moreover, there is no evidence for any contemporaneous arc activity within the craton itself, as would be expected if subduction took place beneath it, nor are there any intrusive or volcanic units within the Svecofennian fold belt known to be coeval with the Outokumpu assemblage or the Jormua ophiolite; the oldest dated Svecofennian arc-related lithologies are some 30 Ma younger than the Outokumpu assemblage, (Huhma, 1986; Vaasjoki & Sakko, 1988, Lahtinen, 1994). A minimum age constraint for deposition of the sediments (and hence a maximum age for the onset of deformation) is however given by 1.94–1.92 Ga U-Pb SHRIMP ages for detrital zircons — a result which does indeed approach that of the earliest Svecofennian intrusions and volcanics (Huhma et al., 1991). Although considered less likely than the passive margin scenario, if the Svecofennian arcs were a major source of sediment, the situation could then resemble that depicted in configurations a and b in Figure 3.
Likewise, it is reasonable to consider that the Jormua ophiolite was similarly emplaced from the southwest across the Archaean basement and subsequently imbricated and refolded into a tight synform, thus representing an allochthonous klippe (Sorjonen-Ward, 1991, Peltonen et al., 1996). A corollary of this interpretation is that the Jormua and Outokumpu sequences belong to the same large tectonic unit, with a total preserved area of at least 20 000 km2.
The overall structural and metamorphic evolution of the Outokumpu region has been well documented as a result of several detailed studies (Koistinen 1981, Park et al., 1984). The earliest, D1, D2 and D3 structures relate to the formation of thrust nappes and recumbent folds that are considered to record a combination of progressive deformation, diachronism and changing structural style across the region, with the locus of most intense deformation migrating towards the east and northeast (Ward, 1987, Ward and Kohonen, 1989). Progressive migration of deformation towards the foreland, with thick-skinned imbrication of the Archean craton margin following earlier thin-skinned deformation (including the emplacement of the ophiolite nappes) seems to be broadly consistent with available metamorphic data showing a thermal peak following the main thrusting event (Ward, 1987).
Of the three D1 nappes shown in Appendix 1, the Outokumpu nappe is the highest structurally and is well defined due to the strongly magnetic character of the Outokumpu assemblage; the boundaries between the Kompakka and Savonranta nappes are not so clear, except for the eastern margin of the Kompakka nappe above the Suhmura Thrust Zone. Downward facing F3 and F5 folds are also found in this region and indicate that the Kompakka nappe may be a recumbent fold nappe, rather than a simple thrust nappe. Elsewhere it has been more difficult to recognize inverted D1 or D2 fold limbs, although most determinations of younging directions with respect to early foliations seem to indicate that the bulk of the allochthonous sequence is upward facing.
In general however, an interpretation involving sustained NNE-SSW compression from about 1.90 Ga — 1.86 Ga is consistent with the observed structural sequence and its overall geometry, namely the transition from N-NE-directed thrusting to the development of NW-trending dextral transcurrent shearzones, such as the Suvasvesi Shear Zone (Väyrynen, 1939, Halden, 1982), which clearly shows truncation and reorientation of the Outokumpu nappe (Appendices 1 and 2). Reorientation and reactivation of early, thrust-related structures within the NW-dextral shear zones was intimately associated with extensive granite and gabbro emplacement at 1.89–1.86 Ga, which must also made a major contribution overall crustal strength, in turn influencing responses to further compression.
If crust is mechanically and thermally unstable, isostatic equilibrium can be restored in several ways, including rapid erosion, tectonic expulsion or escape, extensional collapse and generation of intracrustal granites, or by convective thinning of the lithosphere involving delamination at the base of the crust and underplating by hotter asthenospheric material, of which the Tibetan plateau is regarded as a type example (Platt & England, 1994). Lahtinen (1994), in a comprehensive synthesis of collisional and post-collisional Svecofennian magmatism in central and southern Finland, invoked post-collisional underplating by mafic magma whose enriched mantle geochemical characteristics differed from the more depleted mantle signatures of the Karelian Province mafic volcanics, as well as from the earliest Svecofennian, pre-collisional arc magmas.
On the other hand, deep seismic surveys have revealed the existence of an unusually thick crust in the region (Korja et al., 1993), which tends to suggest that widespread crustal collapse was inhibited (assuming that the features present in seismic images are truly inherited from Paleoproterozoic time). Understanding the implications of these different lines of evidence are of both academic and economic interest in eastern Finland; in particular, the deep thermal structure and history of the region is highly relevant because of recent diamond exploration activity.
Archean basement between Kuopio and Nunnanlahti — and including the Luikonlahti area — has undergone extensive ductile reworking during the Svecofennian, usually such that feldspar porphyroclasts are recognizable in a highly strained quartz-mica matrix (Kohonen et al., 1991). This suggests plastic behaviour by quartz and semi-plastic behaviour by feldspar, which as the dominant minerals, would control the overall strength of rock at least at this crustal level.
Moreover, some granitoids, post-dating thrusting and crustal thickening, and closely associated with the NW-dextral dextral transcurrent shear zones, have ages of 1.87–1.86 Ga, and clearly show isotopic and chemical evidence for melting of Archean crust at depth (Huhma, 1986, Lahtinen, 1994).
This combined evidence for crustal thickening and widespread rheological weakening of the crust can be linked kinematically to the formation oblique sinistral normal sense shear znes. A scenario involving sustained N-directed compression, for which evidence exists in southern Finland at both 1.88 Ga (Nironen, 1989) and 1.84 Ga (Ehlers et al., 1993), would allow for flow of weakened middle crust outwards, at a high-angle to the direction of convergence, which is an appealing way of explaining the orientation of linear fabrics throughout the region. Such an interpretation would also allow extensional and compressional structures to form simultaneously at different crustal levels, explaining such the observed successive overprinting of earlier low-angle structures by sinistral oblique normal shear zones as they propagated towards the foreland, coeval with crustal shortening elsewhere in the fold belt (Kohonen, et al., 1991).
Interaction with strike-slip displacements along the NW-dextral shear zones could be responsible for the broad regional refolding of the Outokumpu nappe, which has given rise to the current pattern of distribution of the Outokumpu assemblage (Appendix 1). Alternatively, this later folding could relate to a regional reorientation of stresses such that the NW-trending dextral shear zones record deformation essentially parallel to the continental margin, as favoured in the Cordilleran analog applied by Park et al. (1984). It should be noted however, that lithological units can be traced across individual shear zones (Appendices 1 and 2), suggesting that they record displacements of the order of tens of kilometers, rather than representing terrane boundaries between disparate tectonic units.
Perhaps these boundary conditions, namely sustained convergence, with a combination of tectonic uplift and extensional orogen-parallel transfer of middle crustal material, enabled a dynamic equilibrium to exist between crustal thickening and crustal extension and allowing the Svecofennian lithosphere to eventually attain thermal and gravitational equilibrium in spite of the crust remaining nearly 60 km thick.
When drilling resumed, this time with better techniques, it was found that the main orebody continued at depth, while two additional smaller ore bodies were also discovered (Figs. 5, 6). Ore reserves were estimated at eight million tons of copper ore containing: Cu 1.1% (in chalcopyrite); Zn 0,6% (in iron sphalerite); Co 0.1% (in Co-pentlandite and in the lattice of pyrite and pyrrhotite) and S 18% (mainly in pyrrhotite). Dimensions of the main ore were: maximum width 80 m, depth 480 m and length 700 m. The dip of the ore body was nearly vertical, allowing the use of low cost mining methods, while the ore mineralogy of the ore was favorable for producing high grade mineral concentrates. Production commenced in 1968 after mine development, construction of railroads, power lines, roads and other infrastructure and continued until 1983. Annual production was of the order of half a million tons of ore and after three years of open pit mining, operations continued underground using a sublevel caving method. About 220 people were employed in various aspects of mining operations and the mine had a major impact on the local economy. Five different mineral concentrates were transported by rail to domestic smelters: copper, cobalt and zinc to several Outokumpu smelters, pyrrhotite to the sulfuric acid plant built by Kemira Oy at Siilinjärvi and roasted iron oxide from Siilinjärvi to the Rautaruukki iron furnaces at Raahe. Mining at Luikonlahti was carried out by the Finnish paper company, Myllykoski Oy, which was at that time the owner of Malmikaivos Oy.
Prospecting continued throughout the duration of mining operations and as the cpper ore became progressively depleted, Malmikaivos delineated and evaluated reserves of high quality talc raw material (soapstone) at Luikonlahti. Talc production commenced after modification of the sulfide concentration plant for talc flotation and the application of new jet milling technology for micronizing the products. Talc is an important industrial mineral for the forest products industry. Production of talc started in 1979, at first alternating in shifts with copper ore processing. After closure of the copper mine, mining of a variety of talc products continued at a rate of 80 000 tonnes per year at full capacity.
In 1908 Otto Trustedt, a geologist employed by the Finnish Geological Commission (the precursor to the Geological Survey of Finland), was sent to Kivisalmi, some 60 km SE of Outokumpu, to investigate a large glacial erratic dredged up during widening of a channel between some of the main waterways in Karelia; the boulder proved on analysis to assay 3.74 % copper. Funded by the National Board of Industry Trustedt then commenced tracing the source of the boulder. He had two main clues to follow; although the main trend of the glacial striations at Kivisalmi was to the west, there was another set of striations trending nearly north-south. Since the boulder conisted largely of quartzite, and the only known exposures of quartzite were to the north and northeast of Kivisalmi, Trustedt started examining reported quartzite localities, not realizing that the trail was in principle misleading, since the quartzite in the erratic was quite different in character from the detrital orthoquartzites that he was seeking. During the course of the search, however, another geologist, W. W. Wilkman recognized that he had encountered banded sulfide-bearing quarzite schist, resembling that of boulder, on the northern slope of a peculiar hill named Outokumpu.
At Outokumpu Trustedt started a systematic search for boulders, finding several interesting floats. He was able to delineate a boulder train, he mapped the focus of the train with a magnetometer, and he had exploration pits and trenches dug. He decided that the ore deposit might be located in the quartzite between the Outokumpu hill and the younger mica gneiss 300 m north of it. The first two drill holes failed to verify his hypothesis but the third hole intersected the ore between 28.85 and 38.18 m on 17 March 1910.
Another major deposit, the Vuonos ore body, was located by deep drilling in 1965. As such it is one of the few blind ore bodies lacking surface manifestations to have been found in Finland, its discovery being based on scientific principles. Geologists had been able to recognize the structural and lithological continuation of Outokumpu type features northeastwards from the existing mine. Some of these features, such as the coexistence of graphite bearing quartz rocks and black schists and of serpentinite bodies could be followed by means of geophysics. Some of the diagnostic characteristics of the Outokumpu assemblage, such as pervasive enrichment in nickel, chrome and cobalt, could be identified due to dense drilling in the critical zone. However, the principal clue was to recognize the increase in the Co/Ni ratio of country rock when approaching an ore deposit. This encouraged geologists to drill a few holes a little deeper, the first of which encountered the ore.
The above scientific exploration cocepts were further developed within the so-called Outokumpu Project in the early 1980's. The Project was able to identify the structural control of the main mineralized (and then deformed) zone and to locate new ore occurrences within that zone. Unfortunately, none of these new occurrences have proven to be economic so far, but one of them, the Kylylahti deposit, is still under investigation, and will be discussed further below.
From a regional exploration point of view it is important to note that in the Vuonos area, the Outokumpu assemblage and the enclosing metasedimentary sequence form a gently dipping layer overlying Archean basement and the Proterozoic Maarianvaara Granite at relatively shallow depth, whereas the basement-cover contact plunges to greater depths along strike both to the northeast and southwest, as illustrated in Figure 7. The results of magnetic, gravity and AMT surveys all lend support to this interpretation (see Appendix 2). It is therefore concluded that the ore-critical part of the Outokumpu assemblage has been removed by erosion from the area between Vuonos and Keretti, and that the sequence may have been tectonically thinned between the axial culmination at Vuonos and Kokonvaara, becoming thicker again further to the northeast, towards the axial depression between Perttilahti and Kylylahti (Fig. 7).
However, this interpretation implies that the ore-critical horizon would be expected to occur at depth possibly in excess of 1000 m, and the expense of drilling to such depths required the development of a well-defined structural model, based on careful interpretation of magnetic, gravity and AMT data. The resultant structural model, depicted in Figure 8, was tested by drilling a profile four kilometers to the northeast of the Vuonos ore (Fig. 9).
Drill hole 733 (Fig. 9) intersected a sequence of black schists nearly 100 m thick, and was assumed to envelope Outokumpu assemblage lithologies — this conclusion was then verified when the drill hole 737, structurally below 733, penetrated around 150 m of Outokumpu assemblage lithologies, including a six meter thick mineralized layer that proved on analysis to show the same element enrichment patterns found in the Ni-bearing mineralization above and around the Vuonos ore body. Finally, drill hole 740 successfully penetrated a sizeable Outokumpu-type ore body.
Petrophysical measurements were carried out on recovered drill core and it was found that while the Outokumpu assemblage lithologies present on the inferred lower limb of the fold had a mean density of 2.93 gcm-3, the overlying metasediments had mean densities of 2.75 gcm-3 and were moreover of sufficient thickness that the two units could potentially be successfully discriminated using gravimetric modelling. It was found that the modelled prisms and calculated values agreed very well with observed densities, thus providing support for the overall model. On this basis, a further exploratory hole (741 in Fig. 9) was then drilled to a record depth for Finland, of 1297.50 m. Using the results obtained from the drilled profile, a series of gravimetric models were then developed at kilometer intervals between Vuonos and Kylylahti, resulting in the block diagram shown in Figure 10.
Subsequent drilling showed that actual intersection deviated from the modelled horizons by a relatively small amount, effectively of the order of several per cent with respect to each meter drilled. Drilling also intersected an Outokumpu-type ore body at a distance of 3.5 km northeast of Vuonos. The total length of the ore-prospective synformal structure in this area, before it intersects and extrapolates above the present erosion level near Kylylahti, is some 13 km.
When considering the potential for finding more Outokumpu-type ore deposits beyond this particular zone, it is important to precisely define the position of the ore-critical horizon within the overall tectonostratigraphical sequence. The use of aeromagnetic pixel images has greatly enhanced the ability to follow key horizons throughout the area, enabling a more detailed interpretation of stratigraphical relationships than was previously possible (see Appendix 2). The concentric patterns of magnetically responsive and subdued units evident in aeromagnetic pixel images clearly delineate structural domes and basins and also demonstrate that many lithological units are laterally extensive. This in turn indicates that the Outokumpu assemblage occurs as a coherent package at a consistent stratigraphical level on a regional scale, over an area ofsome 4500 km2. This support the hypothesis that the assemblage and associated ores represent products of intense hydrothermal interaction between seawater and submarine — presumably basaltic — volcanics; Br, Cl and S isotope data are also consistent with this model.
During the second exploration phase, between 1994 and 1995, the surface extension of the deep ore body was identified, as well as further extensions to the disseminated sulfide envelope. This was undertaken on the basis of the results of reconnaissance gas geochemical, geophysical and drilling surveys carried out during the years 1991–1993. The second phase involved the use of avariety of geophysical techniques, notably Gefinex 400S, magnetic, Slingram and gravity surveys and IP measurements, as well as drilling. As a result of these investigations, estimated reserves were increased to some extent.
On the basis of these investigations, the Outokumpu assemblage at Kylylahti is interpreted as defining a synformal structure plunging some 30° towards the southwest. The main lithologies present, from the center outwards, are serpentinite, gabbro lenses, talc-carbonate rocks, Cr-bearing skarns and quartz rocks, with an envelope of black schists separating them from the enclosing micaceous metasediments, just as at Keretti.
The host rock to the Cu-Co mineralization is a mixed Cr-skarn and quartz rock lithology immediately adjacent to black schists at the eastern edge of the fold (Fig. 11). The main Cu-Co bearing horizon has been traced from a depth of 500–700 m almost to the present erosion surface. The mineralization appears to have a step-like geometry approaching the surface, due to disruption by numerous transverse faults. The main ore zone is flanked by a somewhat irregular and less continuous zone of disseminated sulfides, which is also anomalous with respect to Cu, Co and Ni. It is also of interest that podiform chromite has been encountered in the Kylylahti area.
Resource estimates carried out for the Kylylahti Cu-Co deposit to date indicate probable reserves of around 2.5 Mt containing 2.5% Cu, 0.4% Co and 0.8 g/t Au, and possible additional reserves of a further 2.5 Mt, averaging 0.6% Cu, 0.2% Co and 1.2 g/t Au (Outokumpu Mining Annual Report, 1995).
During the Black Schist Project more than one thousand samples were selected from unweathered drill cores representing Outokum- pu-type ores and prospects from throughout the Outokumpu-Kainuu region (Fig. 12). For comparison, samples were also taken from Early Proterozoic black schists from southern, western and northern Finland.
Several genetic models have been developed to explain the origin of the Outokumpu rock assemblage, with debate having generally centered around two contrasting hypotheses:
It is probable that both of the contrasting hypotheses contain some valid points: part of the carbonate rocks are of sedimentary origin, while others are derived from alteration and metamorphism of the present serpentinites. Black schist formations from 20 to 120 m thick encountered in the immediate vicinity of the calc-silicate rocks of the Outokumpu rock assemblage certainly are of sedimentary origin, as also evidenced in their geochemical characteristics (Loukola-Ruskeeniemi, 1991, 1992).
The original positions and age relations of the various rock types comprising the Outokumpu assemblage are difficult to establish however, due to the effects of complex, polyphase deformation and amphibolite facies metamorphism (Koistinen, 1981).
Sulfur isotope d(=delta)34S values for the sulfides of the Outokumpu ores and sulfides within black schists and quartz rocks resemble each other. The average value for 258 sulfide specimens in the Outokumpu deposit is -3.5 per mil. (Mäkelä, 1974) Sulfur isotope results for pyrite in black schists occurring next to the Kylylahti ore (Fig. 16) provide comparable values to those of the pyritic ore (from -5 to 16 in ore compared with values from -5 to ~10.5 in black schist). Furthermore, layered pyritic ore shows a slight compositional layering overprinted by a tectonic layering, foliation (Koistinen, 1981). It is therefore probable that at least part of the Outokumpu ore formed primarily on or close to the seafloor.
Black schist layers can be easily located by geophysical measurements, and on the aeromagnetic anomaly map (Appendix 2), it is possible to trace continuous pyrrhotite-bearing black schist formations for great distances. In addition to the anomalous black schists, which typically are found near serpentinites or altered remnants of the serpentinites, other black schists containing < 3% graphitic carbon and sulfur are also encountered.
Throughout the Kainuu region, the serpentinite-associated black schists contain anomalous concentrations of nickel, copper, zinc and manganese. The largest known Ni-Cu-Zn deposit found so far is at Talvivaara, in the rural municipality of Sotkamo (Fig. 12), although it is of relatively low grade. Ni-Cu-Zn- and Mn-rich black schists are also encountered near the Jormua ophiolite to the north of Talvivaara.
Proximity to an Outokumpu type ore is seen in the black schists of the Vuonos mine by increasing Cu (>2000 ppm) and Co (>200 ppm) concentrations. At the Outokumpu Keretti mine however, the black schists lack such high Cu and Co concentrations, the difference being due to the fact that at Vuonos, part of the ore was actually hosted by black schists.
In the Outokumpu district, graphite-rich rocks have certain textures that have not been seen in any other Early Proterozoic black schists studied in Finland: black schists with 3-mm-thick greenish grey tremolite-rich layers (see figures in Loukola-Ruskeeniemi, 1992), a textural feature which is common in quartz rocks and calc-silicate rocks. Graphite-rich fine grained quartz rocks are also specific to the Outokumpu rock assemblage.
Evidence of hydrothermal processes is clearly recorded in the chemical composition of the Outokumpu-Kainuu rift-related black schists (Loukola-Ruskeeniemi, 1991), but it is not easy to distinguish black schists associated with ore of economic grade from black schists associated with subeconomic mineralization.
The contact between the soapstone and serpentinite is typically almost vertical except in the southwestern part of the deposit, where the soapstone dips at a rather low angle beneath the serpentinite body. Contacts are generally gradational over several meters rather than abrupt, corresponding to a progressive decrease in the proportion of talc and magnesite and a concomitant increase in the amount of serpentine minerals. The contact between the soapstone and quartz rock is however much more clearly defined, although here too there is a distinct transition zone usually at least several meters thick.
The general sequence of lithologies, proceeding from serpentinite to regional wall rock is as follows:
serpentinite — soapstone — talc schist — skarn — quartz rock — black schist — micaceous schist. The skarn is usually tremolitic but in some places contains diopsie. The quartz rocks closely resemble the typical Outokumpu-type quartzitic sulfide ore and locally may consist almost entirely of silica, although generally they contain small amounts of sulfides, fuchsite, phlogopite, chlorite and tremolite and typically are black in color.
The distribution of the deposit is controlled, apart from its position at the margin of the serpentinite lense, by a lineation plunging towards the southwest at about 30 degrees; both the ore body and the adjacent skarn and quartz rocks are elongated downwards in this direction. A N-S trending fault truncates the ore body just beyond the present southwestern end of the quarry accompanied by numerous chloritic veins that are believed to be related to the faulting or shearing process.
The abundance of chlorite varies from 5–10% and sulfides from 1–2%. Nickel arsenides are present in variable amounts, but never exceeding 0.3% of the rock; chromite is more abundant than magnetite, but together they comprise less than 1% of the rock.
Talc typically occurs as discrete flaky grains with grain sizes generally from 0.05–0.2 mm. In highly sheared zones however, talc may occur as homogeneous foliated masses in which individual grains are difficult to distinguish.
So far, none of the Proterozoic dykes have been shown to demonstrably cut the Nunnanlahti greenstones in proximity to the soapstone deposits, so that we are still not sure to what extent the alteration represements a Proterozoic retrogression and fluid influx, as opposed to an entirely Archean phenomenon. Thus, even if an Archean age for the Nunnanlahti greenstones were vindicated by isotopic dating, the question of the timing of the hydrothermal processes that produced the soapstone alteration remains unsolved. Hopefully, evidence will be forthcoming in the form of Proterozoic dykes that either truncate alteration assemblages as well as Archean structures, or else show direct evidence of Svecofennian hydrothermal alteration themselves. Throughout the nearby region, similar metadolerite dykes intruding basement gneisses and Paleoproterozoic quartzites typically display quartz-albite-carbonate-biotite-chlorite-sulfide vein and shear alteration assemblages and stable isotope investigations are currently in progress to assess whether or not different stages of fluid flow can be linked to alteration and deformation events, and hopefully providing some insights into the history of alteration at Nunnanlahti.
The geological history of the Nunnanlahti area is further complicated by the intensity of Proterozoic deformation, during which the Archean basement and Proterozoic cover have been tectonically imbricated, a feature already apparently to the earliest geologists to work in the region (Frosterus & Wilkman, 1920). In effect, the Nunnanlahti soapstones can be said to lie within an allochthonous Archean greenstone belt sandwiched between Paleoproterozoic. The main Nunnanlahti Shear Zone, with the soapstone quarries lying in the hanging wall, has a complex history, being interpreted as an early thrust, or steep frontal ramp within a thrust system (to explain the fact that Archean rocks are structually on top of Proterozoic turbidites), which was reactivated as an oblique sinistral normal sense shear zone (to explain kinematic indicators such as rotated porphyroclasts, cleavage duplexes, minor folds and truncations of lithological units and magneticanomalies at map scale). In the vicinity of the shear zone regional structural trends in both basement and cover rocks are progressively transposed into a NW-SE orientations with an intense moderately dipping foliation and S-plunging lineation; in the most highly strained domains the foliations in the protomylonitic Archean granitoids, as well as the Nunnanlahti greenstones and soapstones are essentially congruent with those in the Proterozoic sediments (Kohonen et al., 1991). It therefore seems reasonable to assume that in both units, these structures represent the same Svecofennian deformation.
The western margin of the transposed zone at Nunnanlahti also coincides with an abrupt increase in crustal thickness (Luosto et al., 1990). The greater thickness is most readily attributed to thrust stacking although an alternative explanation involves crustal thinning to the east during the early stages of the 2.1 Ga rifting and extensional event; this in turn may have enhanced the potential for fault reactivation in this area during subsequent basin inversion and thrusting (Kohonen et al., 1991, Kohonen, 1995).
By the turn of the century, soapstone from Nunnanlahti was being widely employed in the ornate facades of many of the Art Nouveau period buildings being constructed at the time, the most notable examples in Helsinki being the the Finnish National Museum and the National Theater. The principal market for soapstone products during these early years was Czarist Russia, but after early success, the fall in value of the Russian rouble caused by the Russian-Japanese war led to the company being passed into receivership in 1907. The following year production resumed under the name "Osakeyhtiö Vuolukivi — Aktiebolaget", with steam generation of electricity to supply power to more mechanized processing operations. Profitability was only marginal and ownership changed hands several times throughout the period of the First World War and the turbulence of the Finnish civil war and independence but with the subsequent rapid expansion the paper industry, there was an increasing demand for soapstone as a lining in furnaces and kilns, a purpose for which thermal and chemical properties were ideally suited.
The ownership base was broadened, with a rapid icrese in investment and in 1925 the company was again renamed as "Suomen Vuolukivi Osakeyhtiö". The success of the enterprise led to the establishment of additional quarrying operations by three junior companies, although none of these survived for long, due to the combined effects of increasing competition and limited markets, especially during the Depression of the 1930's. Production was then interrupted by the Second World War and operations never fully recovered, eventually ceasing entirely during the 1960's.
In 1980 however, the Suomen Vuolukivi Oy shares were purchased by the chairman of the company now known as Tulikivi Oy ("tuli- kivi" translating roughly into English as "firestone"), and quarrying operations were revived after more than a decade of inactivity. By the middle of the 1980's, export markets for Tulikivi soapstone fireplaces and ovens and dimension stones were growing rapidly throughout central Europe, and production lines have been modifed and expanded several times, using technology developed at Nunnanlahti, making the operations a market leader in the field. In 1987 a separate company, Mittakivi Oy was established, also with its own production line at Nunnanlahti and specializing in the manufacture of individual ovens and fireplaces according customer wishes and requirements; Mittakivi Oy also became a wholly-owned subsidiary of Tulikivi Oy in 1996.
Tulikivi now exports finished products, principally soapstone ovens and dimension and tiling stones, to more than ten countries and has an annual turnover of nearly 170 million Finnish marks and directly employs about 300 people. Quarrying operations at Nunnanlahti presently include the sawing of some 70 000 m3 of soapstone annually, of which about 18 000 m3 is suitable for dressing and oven manufacture. It is also necessary to quarry some 35 000 m3 of waste rock, and to remove around 60 000 m3 of overburden prior to quarrying.
Koli is the highest point (347 m above sea level and about 250 m above Lake Pielinen) on a series of prominent ridges of quartzite that can be traced through much of North Karelia. In fact, without these early Proterozoic quartzites, which are widespread throughout eastern Finland and Lapland the topography of the country would be very subdued indeed, and winter sports enthusiasts would have trouble in finding suitable locations for downhill skiing. It is therefore somewhat ironic that these highly mature sediments presumably deposited in broad river systems flowing across a very flat and mature Paleoproterozoic landscape, have ultimately proven to be the most resistant to erosion!
The Koli Formation obviously takes its name from this locality (Kohonen & Marmo, 1992) and is well exposed here as a massive orthoquartzite which has been intensely recrystallized, so that primary features, such as cross bedding and layers contiang coarser quartz detritus can only be discerned on close inspection. Asymmetric pressure solution foliations are well developed in some exposures, with orientations reflecting the geometry of Svecofennian thrusting. More impressive are the ice-blue fibrous kyanite and quartz veins with orientations consistent with formation within dilatational fractures associated with thrusting.
Looking towards the northwest from the summit of Koli, it is evident that the line of hills changes trend, forming a broad headland jutting out into Lake Pielinen. This is one of the few areas in Finland where structural features are visibly expressed in the topography, since the break in slope at the foot of the hills effectively coincides with the unconformity at the base of the Proterozoic sequence. Exactly how the basement-cover interface could deform in this way, forming a major syncline, is still unclear. However, rheological contrasts and inhomogeneities, such as between greenstone belts, and granitoid basement may have had an important effect in localization of strain, just as at Nunnanlahti, and particularly where contacts were in favourable orientations (Kohonen et al., 1991). The Kolinniemi syncline appears to be a good example of this, with large-scale ductile deformation of the basement-cover interface being best developed adjacent to the greenstone sequence that can be traced around the base of the syncline. Looking towards the southeast from Koli, and beyond the line of sight to the northwest, where the Proterozoic rocks overlie granitoids instead of greenstones, the basement-cover unconformity reverts to a more typical orientation, with moderate dips to the southwest.
In spite of locally intense and complex deformation, detailed structural mapping in areas of good exposure, combined with an abundance of well-preserved depositional younging criteria and high-resolution geophysical data, has enabled local stratigraphical sequences and facies relationships to be established with some confidence. The schist belt is distinctive in consisting principally of felsic volcanogenic and commonly coarse-grained epiclastic deposits, typically of turbiditic aspect. Sporadic thin mafic and ultramafic flows occur throughout the sequence, particularly at higher stratigraphical levels in the northern part of the schist belt. Although the primary goal of regional mapping and structural studies were to understand controls on gold mineralization, a secondary aim was to provide a basis for assessing the potential of the supracrustal sequence from the point of view of sedimentary or volcanic-hosted massive sulfide deposits. Accordingly, two partially overlapping felsic volcanic complexes have been delineated in the northern part of the schist belt, with the southern part of the schist belt being less diverse lithologically and characterized by presumably more distal finer-grained and thinner-bedded sedimentary facies.
Isotopic data indicate that deposition, deformation and granitoid intrusion were very closely related in time, and almost indistinguishable statistically, the ages of the earliest supracrustal units, at 2754±6 Ma, effectively overlapping with those of syntectonic granitoids. All exposed contacts between the Hattu schist belt and these granitoids are intrusive, or else tectonically modified, and hence the granitoids cannot represent depositional basement to the greenstone belt. No other depositional basement to the Hattu schist belt has been identified, nor have any unconformities been recognized within the mapped sequence. Therefore, there is no direct evidence that the Hattu schist belt developed on an ensialic substrate. The involvement of older continental crust is also evident from some highly evolved granitoids, including tourmaline-muscovite leucogranites, which appear to be analogous to those in Phanerozoic collisional belts (Sorjonen-Ward, 1993, O'Brien et al., 1993). Isotopic data from both detrital and magmatic zircons indicate the presence of older crustal material both in sedimentary detritus and in the source regions of some granitoids, with SHRIMP zircon studies having revealed inheritance from a protolith up to 3.1–3.2 Ga in age. (Sorjonen-Ward & Claoué-Long, 1993). Most of the Hattu schist belt sediments are nevertheless considered to represent reworking of penecontemporaneous volcanogenic deposits, whose age is relatively precisely constrained at 2.76! 5 Ga (O'Brien et al., 1993).
Since no field evidence exists to suggest that any of the granitoid intrusions were pretectonic, rapid and extensive crustal generation and deformation between 2.76–2.73 Ga is implied. There is accordingly, a close relationship between regional folds and shear zones, and much of the deformation may record accommodation to the emplacement of syntectonic granitoids. For example, a distinctive suite of biotite-tonalites intruded the sequence, initially as tabular semiconcordant sheets during the early stages of fold propagation, and were subsequently deformed along with their host rocks.
The overall structural geometry of the Hattu schist belt is characterized by upward-facing, generally steeply dipping structures; depositional younging determinations are almost invariably upward facing and hence militate against the existence of major recumbent structures. Refold interference patterns observed at outcrop and map scale are therefore more likely to represent progressive deformation of initially upright structures with strain becoming more partitioned within discrete narrow zones. The kinematic histories of these zones suggest the importance of regionally coaxial and vertical constrictional strains, although evidence for more localized local strike-slip deformation is certainly present. Rather than invoking separate deformation episodes, a progressive continuum interpretation is preferred, in which younger structures record the partitioning of deformation into discrete, high-strain zones, with an increasing component of constrictional strain related to continuing granitoid intrusion (Sorjonen-Ward, 1993). It has been possible to establish a close, probably sequential relationship between tightening of folds, attenuation of fold limbs, development of shear zones with strike-slip displacements, and the propagation of new folds due to strain incompatibilities. This resulted in either distinct overprinting fabrics, or transposition and recrystallization of early fabrics. It has thus been possible to delineate distinct D1 domains based on F1 fold and S1 foliation relationships to bedding and S2 (Fig. 18). Depending on the orientation and F2 fold wavelength, D1 structures have been deformed either by coaxial refolding, or by attenuation and extension during oblique D2 shear. The mechanical behaviour of various lithologies has clearly been significant, with major displacements taking place adjacent to banded iron formations, while shortening of mafic units has in some instances been accommodated by the development of strike-slip duplexes, the most significant of which, the oblique sinistral contractional Juttuhuuhta Duplex, is indicated in Figure 18. The resultant regional geometry suggests a transpressional regime with plutons being emplaced into dilatant sites within a N-NE trending dextral shear system. Although there is as yet no evidence for the overall duration of deformation and metamorphism, structures are readily interpreted as representing progressive deformation, rather than a sequence of separate tectonic events. Microstructural evidence clearly indicates dynamic recrystallization during deformation under upper-greenschist to lower-amphibolite conditions and that the thermal metamorphic peak was synchronous with, or outlasted deformation (Sorjonen-Ward, 1993). The growth of mica, actinolite, garnet and locally, staurolite and kynaite porphyroblasts, was associated with later stages of deformation, and in some instances outlasted it. This metamorphic mineral growth, and accompanying dynamic recrystallization is also observed to overprint alteration assemblages, particularly in the southern part of the schist belt. Hence, the timing of gold mineralization with respect to the peak of metamorphism differs from that in classic late Archean low-grade greenstone terrains in that metamorphic textures indicate recrystallization after gold deposition. On the other hand, while the presence of mineralization in syntectonic tonalites provide direct evidence for gold introduction during deformation, the progressive nature of deformation and prograde dynamic recrystallization of mineral assemblages doe not support the concept of a separate younger metamorphic event being superimposed on lower grade deposits. The U-Pb zircon age obtained from the Kuittila Tonalite (2745±10 Ma) places an upper age limit on the timing of gold mineralization, while concordant monazite and titanite ages around 2700 Ma are considered to constrain the onset of cooling following post-mineralization metamorphic recrystallization.
It has been difficult to characterize the gold deposits or assess the nature, genesis and timing of fluids using stable isotope and fluid inclusion data or thermochronology since the Hattu schist belt was situated within the foreland to the Svecofennian Orogeny and was reheated to greenschist facies by burial beneath a sequence of nappes around 1.9 Ga. Thus for example, muscovite and biotite have K-Ar mineral ages between 1811–1707 Ma, and yield a poorly constrained Rb-Sr isochron of 1655+/-290 Ma. Hornblende K-Ar data tend to retain Archean ages, as does the U-Pb system in titanite and zircon. Although this indicates P-T conditions during the Svecofennian overprint comparable with those at which Archean lode gold deposits characteristically form, there is no compelling reason for advocating a Proterozoic origin for the gold. Several generations of Early Proterozoic mafic dikes contain Svecofennian greenschist assemblages but are essentially unstrained and transect the alteration zones and some gold prospects without being themselves altered or mineralized.
Mineralisation does not appear to occur preferentially in any particular lithology, although lithological transitions may be favoured, due to associated chemical and rheological gradients. Pervasive fluid flow is however also indicated by broad, highly deformed alteration zones and disseminated rather than vein style mineralisation at some prospects. The albite-carbonate alteration mineralogy so characteristic gold deposits in Archean mafic and ultramafic provinces is rare in the Hattu schist belt, although carbonate is locally present in mineralized tonalites. Instead, chlorite, muscovite and tourmaline dominate alteration parageneses in mineralized metasediments. Native gold is fine grained (mostly <15 m) and occurs largely in free-milling form between silicate grains, associated with pyrite, pyrrhotite and minor arsenopyrite and rutile; it is typically intergrown with Bi, Pb, Ag, Fe and Au tellurides and native bismuth. Gold mineralization at most prospects is geochemically distinctive, with notable enrichments of Te, B, Bi (and locally Mo and W) compared to many other Archean gold provinces, reflected in the abundance of tourmaline, tellurides and locally scheelite, while conversely As, Ag, W and S are rather low, which is manifest in the generally low abundances of sulfides. The syntectonic Kuittila Tonalite contains an early molybdenite-scheelite vein system overprinted by sheared biotite-muscovite-calcite-scheelite-pyrite-gold veins and demonstrates the syntectonic timing of mineralization. Extensive alteration of granites and country rocks in the northern part of the schist belt is typified by microcline-muscovite-pyrite alteration and is perhaps more reminiscent of epithermal systems. Therefore, it is possible that gold mineralization in the Hattu schist belt does record some kind of interaction between magmatic and metamorphic influences during active tectonic processes.
Outokumpu Mining Oy purchased the exploration rights to Pampalo prospect, which is the largest deposit in the area, in 1994 and is currently carrying out feasibility studies; During 1995, some 26 000 tonnes of ore were quarried and processed during trial mining, yielding about 330 kg of gold (Outokumpu Quarterly and Annual Reports, 1997), while further drilling intersected mineralization to a depth of 280 m. A further exploration phase is currently planned, involving a decline with 900 m of tunnel, to a depth of 100 m, from which more detailed drilling will take place. (Outokumpu Mining press release, 23/06/1997).
The mineralogy of the deposit was studied in detail by Kojonen et al., (1993) who found that gold typically occurs in free milling form, as inclusions along fractures and alteration zones in silicate grains, or at dynaically recrystallized grain boundaries. Gold is also common as inclusions within pyrite, intergrown with tellurides of Pb, Bi, Ag, Au and Fe, while the Ag content of native gold varies from 1.0–24.8%. Associated sulfides include, in addition to pyrite, which can occur as large porphyroblasts, chalcopyrite (with inclusions of mackinawite and cubanite), pyrrhotite, galena and spahalerite. Magnetite, ilmenite (after rutile) and goethite have also been found in the gold-bearing mylonitic seams in the discovery outcrop.
Gold and associated sulfide minerals are principally disseminated rather than vein-hosted, occurring within several moderately plunging lodes within a steeply dipping highly sheared and heterogenous lithology whose origin has been difficult to infer. By analogy with the lithic units identified outside the mineralized zone, the lithology probably correlates with the coarse-grained, conglomeratic felsic to intermediate volcaniclastic unit overlying the basal mafic volcanic unit of the Pampalo Formation. This is consistent with fact that the main ore zone is bounded to the west by a mafic volcanic unit, which is reasonably interpreted as the basal member of the Pampalo Formation, since it is immediately underlain by a banded iron fomration and in turn by the conglomerates and greywackes of the Tiittalanvaara Formation. The eastern boundary of the main ore zone is structurally more complex, being in the refolded toe of the Juttuhuuhta duplex, so that stratigraphical contacts are no longer easy to determine. However, it but locally coincides with a highly sheared talc-chlorite-biotite schist which represents the hydrothermally altered equivalent of an ultramafic volcaniclastic unit that may originally have been an eruptive breccia. The diversity of lithologies in the Pampalo area is regarded as having had important influence on the localization of mineralization, in providing chemical and mechanical contrasts during deformation. It should be noted that the ultramafic unit itself is seldom mineralized, except in proximity to intrusive porphyry dykes, some of which contain misoriented but foliated enclaves of tonalite, which again strongly supports a syntectonic timing for the mineralization.
The Juttuhuuhta contractional fault duplex is thought to have formed due to incompatabilities related to tightening of a major synform to the south of Pampalo, resulting in imbrication of the mafic units of the Pampalo Formation nd their translation with a sinistral sense out of the core of the synform (Fig. 18). The western, or basal bounding shear (analogous to a floor thrust in low-angle duplexes) is taken to coincide with the thin banded iron formations associated with the transition from the turbiditic greywackes of the Tiittalanvaara Formation to the more massive basaltic units at the base of the Pampalo Formation (Sorjonen-Ward, 1993). The introduction of gold at Pampalo is believed to post-date the formation and main stage of deformation within the Juttuhuuhta Duplex. Instead, it is likely to be associated with deformation of the toe of the duplex, attributed to rotational back-folding as relatively greater amounts of sinistral shear strain were transferred from the Pampalo Shear System to the Kelokorpi Shear Zone somewhat further west (Fig. 18).
Pyrite and pyrrhotite are the dominant sulfide minerals in the Talvivaara deposit. Pyrite, pyrrhoite, chalcopyrite, sphalerite, alabandite, and pentlandite occur both as fine-grained disseminations (<0.01 mm) and as coarser grains in quartz-sulfide veins. During recrystallization and remobilization processes, part of the nickel that was bound in the fine-grained pyrite, became incorporated into pyrrhotite and pentlandite (Loukola-Ruskeeniemi, 1995).
The occurrence is in part syngenetic in origin because nickel is uniformly distributed throughout the extensive nickel-rich horizon (recall that the size of the deposit is 300 Mt). The sulfur isotope d(=delta)34S compositions are also comparatively uniform and the occurrence of pentlandite, chalcopyrite, and sphalerite in the fine-grained (<0.01 mm) sulfide material also leads to the same conclusion.
The elevated concentrations of nickel, copper, and zinc in the occurrence have their origin in hydrothermal processes, since the rare earth element patterns of the Talvivaara black schists exhibit positive europium anomalies. Sulfur isotope compositions are also comparable to those of sulfides in polymetallic sulfide deposits in recent Red Sea deeps and the Guaymas basin. Furthermore, the median S/Se ratio (4100�) in the Talvivaara black schists is comparable with that of average Outokumpu ore (5400).
Black calc-silicate rocks with calcium concentrations exceeding 3.5% are encountered as intercalations ranging from 10 cm to 3 m thick within the black schists. They probably represent hydrothermal precipitates, because they have elevated Ca, Mg, F, P, and Ag concentrations, similar to solutions found in the recent Galapagos mounds hydrothermal field. They also exhibit rare earth element values typical of manganese-rich seawater near recent hydrothermal sites, and heavy sulfur isotope values (from -3.8 to +20.8 for pyrrhotite and from ~7.1 to +20.8 for pyrite).
The Jormua Ophiolite was emplaced across the foreland, probably as a relatively thin thurst sheet, and has been preserved by subsequent thick-skinned deformation which imbricated and folded the basement-cover contact, including the cratonic pre-rift and rifted and passive margin sedimentary sequences, resulting in the present synformal, klippe-like geometry. The early stages of deformation must also have involved tectonic disruption of the pseudostratigraphy within the ophiolite assemblage itself, so that the Jormua Ophiolite Complex now comprises several fault-bounded "blocks" representing different sections through Paleoproterozoic oceanic crust and mantle lithosphere (Peltonen et al., 1996).
The middle, or Lehmivaara Block resemble the Antinmäki Block in many respects, with the mantle lherzolites and harzburgites at Lehmivaara having been intruded by mafic dykes whose abundance progressively increases to the extent that they form a spectacular sheeted dyke complex with only rarely prserved septa of mantle rocks and gabbro. Gabbros have never been observed to truncate sheeted dykes, although the gabbros, sheeted dykes and basalts of the Antinmäki and Lehmivaara Blocks, all share an E-MORB type chemical affinity and have e(=epsilon)Nd values at 1.95 Ga of around +2 (Peltonen et al., 1996). The basalt and gabbros alike are relatively primitive, with Mg-numbers of 73–59, while REE distributions are generally flat or show slight LREE enrichments; the negative Nb anomalies commonly associated with arc/backarc basalts are nowhere apparent. The gabbros range in composition from high-Mg gabbros to ilmenite-rich ferrogabbros and locally show dioritic to leucotonalitic segregations. The gabbros have also yielded the most precise U-Pb zircon age data obtained so far from the Jormua Ophiolite Complex, namely 1953 ± 2 Ma (Peltonen et al., submitted).
The mantle peridotites of the Lehmivaara Block contain an additional generation of ultramafic-mafic dykes not present in the Antinmäki Block; these are older than the E-MORB sheeted dykes and have OIB basaltic characteristics, having e(=epsilon)Nd values of around 0 at 1.95 Ga and exhibiting considerable LREE enrichments with respect to chondritic values and high (1.5–6.0%) TiO2 abundances (Peltonen et al., 1996).
The westernmost, Hannusranta Block differs significantly from the Antinmäki and Lehmivaara Blocks in that the E-MORB type gabbros, basaltic dykes and pillow lavas are entirely absent. Instead, serpentinized lherzolitic-harzburgitic mantle peridotites are extensively intruded by dykes consisting of clinopyroxenite, garnet (pseudomorphs) — amphibole — clinopyroxenite and hornblendite. The clinopyroxen- ite-hornblendite dykes are without doubt igneous cumulates that crystallized in melt channels and pathways in the upper mantle. The chemistry of the preserved primary pyroxenes and amphiboles indicates that the melts percolating through these pathways and precipitating the cumulates were of alkali basalt composition. Isotopic data suggest that these were broadly coeval and closely related to the OIB dykes in the Lehtivaara Block, with a combined whole rock — clinopyroxene Sm-Nd isochron yielding an age that overlaps with the U-Pb zircon data obtained from the Hannusranta dykes. The stability field of the inferred primary mineral parageneses suggest that the dykes crystallized at depths of 25–50 km and at temperatures of 900–1000° C (Peltonen et al., submitted).
In general, the mantle peridotites of the Jormua Complex have been thorougly serpentinized and extensively recrystallized during lower amphibolite facies regional metamorphism following their emplacement. The only primary mineral phase encountered is chromite, though even this commonly shows the effects of zonal alteration and recrystallization. Otherwise, the most widely expressed equlibrium metamorphic mineral paragenesis consists of antigorite + Cr-magnetite + tremolite (with sporadic olivine in the Hannusranta Block). Most of the serpentinized peridotites represent variably depleted lherzolites and harzburgites, though dunites are also present. The peridotites show a preferred alignment defined by strained chromite grains and bands of pseudomorphed pyroxene; the fact that sheeted dykes and gabbros truncate this foliation attest to it being of mantle origin and hence indicate that the peridotites are in fact mantle tectonites. Some of the dunites must nevertheless represent magmatic cumulates, particularly in view of their association with podiform chromitite bodies.
The Cr/Al ratios of accessory chromite within the serpentinites varies from 42–68 and Mg/Fe2+ values vary considerably, reflecting various episodes of metasomatic and metamorphic re-equilibration, a phenomenon which is also evident from their unusually high (0.5–4.0%) Zn concentrations.
The Hannusranta Block, with its evidence for deep-level crystallization of pyroxene cumulates from an alkali basaltic melt, is considered to represent a remnant of subcontinental mantle lithosphere, comparable to that present below modern rifted continental margins. The similar ages of these rocks and the Jormua E-MORB gabbros suggests a relatively rapid transition from the early alkaline stage of magmatism, which is again characteristic of the evolution of incipient ocean basins, such as the present Red Sea (Coleman and McGuire, 1988). The absence of a thick layered mafic and ultramafic cumulate sequence, such as those found in many other ophiolites, suggests that the closest analogues for the Jormua Ophiolite Complex are to be found in slow-spreading small ocean basins formed during the early stages of continental break-up and separation (Peltonen et al., 1996). A true oceanic, rather than intracratonic rift setting is also implied by the absence of terrigenous deposits intercalated amongst the pillow basalt sequence.
The initial serpentinization process, under relatively low-T conditions caused alteration of olivine and pyroxene to pseudomorphic mesh textured serpentine (chrysotile + lizardite + magnetite) and bastite (lizardite); during subsequent regional metamorphism, these were replaced by a static growth of antigorite. In other words, the Jormua serpentinites are effectively metaserpentinites.
Pyrrhotite-bearing lithologies can be separated from magnetite-bearing units geophysically by virtue of their having high conductivities and high remanent magnetization. The dominance of the remanence component over induced magnetization becomes obvious from petrophysical analysis of a large number of samples; magnetite- and pyrrhotite-bearing lithologies from eastern Finland plot in totally different fields with respect to their relative induced and remanent magnetizations (Fig. 21). Lithologies that generate significant magnetic anomalies have susceptibilities above 2000 10-6 SI, which is here regarded as the upper limit of paramagnetic behaviour. In that field the dominance of remanence in pyrrhotite-bearing sediments is indicated by high Q-values (Königsberger ratios), which cluster between 10–100. In contrast, metadiabases, which have magnetite as their main magnetic mineral, are dominated by induced magnetization and typically have Q-values below 1–2. When petrophysical analysis based on a large number of measurements is combined with aeromagnetic interpretations, general conclusions can be made concerning the distinctive magnetic anomaly patterns due to magnetite- or pyrrhotite-bearing lithologies. These results can also be applied to other Precambrian shield areas.
The dominance of remanence is responsible for the irregularity in magnetic anomaly patterns since the remanence vector often lies in the plane of the structure that contains the pyrrhotite. In a fold, for example, this direction is changed with respect to the geomagnetic field vector. In pyrrhotite-bearing rocks the direction of remanence has been strongly influenced by the form and nature of the magnetic bodies and the orientation of pyrrhotite grains relative to the present and ancient geomagnetic fields. This contrasts with the situation in magnetite-bearing rocks, where the magnetization vector is aligned within the present Earth's magnetic field, particularly in cases where the magnetization is carried by coarse grained magnetite but commonly also when it is fine grained. However, very fine grained magnetite of pseudo single domain or single domain size also retains a strong remanence. In banded iron ores the fine magnetite grains are oriented parallel to the layering, resulting in anomalies similar to those produced by black schists, with Q-values >100, and with much higher amplitudes.
Many of the most prominent aeromagnetic anomalies in the Kainuu schist belt are caused by mafic to ultramafic igneous rocks having magnetite as their main magnetic mineral, and pyrrhotite-bearing metamorphosed black shales. In the vicinity of the Jormua ophiolite complex the black schists are associated with strong anomalies beacause of the enrichment of pyrrhotite due to tectonic and metamorphic processes (Appendix 3, upper map). The combination of the apparent resistivity and magnetic maps illustrates the discrimination of magnetite- and pyrrhotite-related anomalies (Appendix 3, lower map). The magnetite-bearing serpentinites and differentiated mafic intrusions are resistive units between the highly conductive black schist formations. The weakly magnetic black schists representing a lower stratigraphic level can also be separated from the overlying mica schists as better conductors.
The variable appearances of black schist anomalies depend on the geological environment. According to a thermomagnetic and mineralogic study (Airo & Loukola-Ruskeeniemi 1991) on black schists from Kainuu Schist Belt, their magnetization was due to the ferrimagnetic monoclinic pyrrhotite. Black schists from different geological settings, characterized by variation in their sulfide- and graphite-contents, were distinguished on the basis of their typical magnetic properties. The geophysical properties correlated closely with the geochemistry and petrography of black schists. Quiet deposition of pelitic laminae resulted initially in a weak magnetic banding within the graphite-bearing schists. Where a small amount of pyrrhotite is present the induced magnetization is dominated by the paramagnetic matrix which does not contribute to the remanence. Combined sedimentary and tectonic fabrics produced more complex patterns with higher amplitudes due to the increased amount and larger grain size of pyrrhotite, particularly in small fractures and on cleavage planes coincident with high strain. In reduced environments in the Precambrian basement, crustal scale fracture or shear zones are often associated with a magnetic lineaments related to increased pyrrhotite content. In contrast, in magnetite-bearing lithologies the fracture and shear zones are weakly magnetic, as magnetite is commonly destroyed during retrograde metamorphic processes.
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Fig. 2. Schematic depiction of lithofacies relationships during the early stages of 2.1–2.0 Ga rifting and subsidence of the southwestern margin of the Karelian craton (from Ward, 1988).
Fig. 3. Alternative scenarios for tectonic and depositional environment of the <1.95 Ga monotonous clastic sequences overlying the Jormua ophiolite and Outokumpu assemblage, from Ward (1987); a: highly implausible situation with sediments representing post-arc flysch incorporated into accretionary prism with putative subduction beneath Karelian craton, for which no coeval continental arc magmatism exists; b: a more reasonable scenario in which the Outokumpu assemblage is interpreted as a back-arc basin, so that sediments could represent a mixture of passive margin and arc-derived detritus. However, this does not provide a simple mechanism to account for the relatively late thermal metamorphic peak following tectonically thickening and nappe emplacement; c: attempted subduction of passive margin towards the southwest, with oceanic island arc supplying most of the detritus, due to restricted sediment supply and low relief on the cratonic foreland; same tectonic setting as in c, but with sediments deposited on the passive margin shelf and slope dominating supply — this is the preferred scenario to satisfy current tectonic and provenance considerations.
Fig. 4. Progressive deformation continuum reflecting uniform boundary conditions but varying internal responses to explain the geometry and sequence of structures in eastern Finland, after Ward (1987) and Ward & Kohonen (1989). (a) Emplacement of cover nappes towards the N and NE and eventual imbrication of deep levels, involving thrusting of basement. (b) Relative increase in vertical stresses, due to buoyancy of buried crust and abundant partial melting (due to post-collisional magmatic underplating?) and/or relaxation of far-field collisional stresses, resulting in the generation of dextral NW-trending strike-slip shear zones. (c) Continued response to gravitational and thermal instability leads to the development of normal sense shear zones. (d) Alternative possibility in which external boundary conditions change, such as rotation of convergence vector, leading to progressive overprinting and reorientation of structures; in this case, the later structures have a stronger strike-slip rather than normal sense displacement.
Fig. 5. Geology of the Luikonlahti area.
Fig. 6. Cross-section NW-SE looking SW (down plunge). Across the mine site the section is constrained by borehole and stope records to a depth of 500 m below the surface. The rest of the section is constructed by projection of information from the surface and other serial sections along paraboloid trajectories reflecting the x-y profile of the F2 folds, controlled by the L2 intersection lineations (from Park, 1988b).
Fig. 7. Vertical profile along NE-SW trend showing location of Keretti and Vuonos ore bodies with respect to culminations and depressions of the basement-cover interface.
Fig. 8. Structural model used in the dipping prism gravimetric modelling. Inferred position of the ore-critical horizon, trending obliquely across the fold hinge, is also indicated.
Fig. 9. Gravimetric interpretation of the test profile at the Kylylahti prospect, 4km northeast of Vuonos. Dipping prisms are assigned densities according to inferred distributions of principal lithologies. Broken lines indicate the inferred trace of the eclined synformal structure.
Fig. 10. Block diagram viewed towards northeast, showing a sequence of interpreted gravimetric profiles, including the drilled test profile of Figure 3, at northing co-ordinate y =200.
Fig. 11. Simplified geology of the Kylylahti fold structure.
Fig. 12. Geologic setting of the Kainuu and Outokumpu areas within the Fennoscandian Shield. Massive sulfide districts: I = Outokumpu; II = Skellefte; III = Vihanti-Pyhäsalmi (Vih, Vihanti Zn ore deposit); IV = Aijala-Orijärvi; V = Bergslagen. Prospects and closed mines sampled during the Black Schist Project: 1 = Puolanka; 2 = Melalahti; 3 = Jormua; 4 = Talvivaara; 5 = Alanen; 6 = Pappilanmäki and Korpimäki; 7 = Ruukinsalo; 8 = Losomäki; 9 = Miihkali; 10 = Viurusuo; 11 = Keretti mine; 12 = Kaasila; 13 = Kalaton; 14 = Vuonos mine; 15 = Sukkulansalo; 16 = Kylylahti; 17 = Sola. Poi = Poikkijärvi iron formation; Lah = Lahnaslampi talc deposit.
Fig. 13. Geologic setting of the Outokumpu district, modified from Huhma (1975). Prospect numbers refer to the same localities as in Fig. 12.
Fig. 14. Outokumpu Keretti Cu-Co mine, cross section Y=186.63 (modified after Koistinen, 1981).
Fig. 15. Vuonos Cu-Co-Zn mine, cross section 194150 (modified from Peltola, 1978).
Fig. 16. Kylylahti Cu-Co occurrence, cross section x=72.700±50 m (modified after data from the Outokumpu Mining Oy).
Fig. 17. Geological map of the Horsmanaho talc deposit.
Fig. 18. Simplified geological and structural element diagram of the Hattu schist belt (from Sorjonen-Ward et al., 1997).
Fig. 19. Outcrop diagram showing lithologies in the Poikopää section (Excursion Stop 6.1), representing the stratigraphically lowermost exposures outcropping in the northern part of the Hattu schist belt (from Sorjonen-Ward, 1993).
Fig. 20. Generalized geological map of the Talvivaara area. Numbers 305, 308, and 329 refer to drill hole numbers.
Fig. 21. Magnetic properties of magnetite- and pyrrhotite-bearing lithologies in eastern Finland. Magnetite-bearing rocks are represented by metadiabases (crosses) and banded iron ores (squares). Pyrrhotite-bearing rocks are black schists (open triangles) and sulfide-ores (black triangles). Magnetic volume susceptibilities and Q-values (Königsberger-ratios) from 536 samples, are taken from the GSF petrophysical database.