Geology, mineralogy and geochemistry of the Fe-Co-Au-(U) deposits in the Paleoproterozoic Kuusamo Schist Belt, northeastern Finland.

Abstract

The volcano-sedimentary sequence of the KSB was deposited discordantly on an Archaean granite gneiss complex in Paleoproterozoic rift environment. The sediment-dominated sequence, which starts and ends with a volcanogenic formation, is divided into eleven lithostratigraphic formations. Four of them are volcanogenic, mainly from andesitic to basaltic in composition, but possibly with minor acid and alkalic units. Sedimentation of the lower part of the sequence took place in arid, at first slightly transgressive and then regressive conditions, probably yielding evaporitic units with halite beds which also formed salt diapirs within the clastic sediments. Extensive hydrothermal activity causing pervasive and wide-spread alteration was evidently generated by mafic magmatism ca. 2.2 Ga ago, when the main heat sources were the slowly crystallising dikes and sills. Hydrothermal circulations in the unconsolidated sediments dissolved the salt accumulations, causing extensive release of sodium. This led to widespread brecciation and albitization of sediments and spilitization of volcanic rocks. Furthermore, it is suggested that the liberated chlorine reacted with pyrrhotite in the sediments, producing hibbingite [g Fe2(OH)3Cl] which is very reactive and stable only under reducing conditions and, therefore, the sites of abundant hibbingite are deeply disintegrated near the present surface of the bedrock. The formation of hibbingite and albitization of country rocks took place prior to the main sulphide-gold mineralization stage, which took place in the albitized sediments close to the contact of the Greenstone formation II.

Pervasive hydrothermal alteration also prevailed during the mineralization, resulting in the formation of Fe-Co-Au deposits at Kouvervaara and Meurastuksenaho and Fe-Co-Au-U deposits at Konttiaho, Sivakkaharju, Hangaslampi and Juomasuo. Temperature zonation of the hydrothermal systems were well evolved in and around every deposit. Hydrothermal processes (mainly Fe-Mg metasomatism) at Kouvervaara includes the formation of garnet, amphibole, biotite, chlorite, second-stage albite, ferroan dolomite and minor sericite in the order of decreasing temperature outwards from the centre of the deposit. At Meurastuksenaho calcite, epidote and talc were also generated along with the above-mentioned silicates. In the Fe-Co-Au-U deposits the silicate alteration (Fe-Mg and K metasomatism) took place with decreasing temperature inwards of the deposits and included scattered amphibole and abundant biotite within the outer zones and increased chloritization toward the centre of the deposits which were occupied by intense sericitization. Both deposit types are enveloped by variously carbonatized rocks, and within each deposit second-stage albitization was also generated. With the exception of the calcite formation at the centre of the Meurastuksenaho deposit, carbonatization developed prior to the main silicate alteration phases, which took place at higher temperatures (280-600 OC) in the Fe-Co-Au deposits compared to the Fe-Co-Au-U deposits where the alteration temperature varied from 270 to 400 OC. The pressure was inevitably below 2 kbar. In both deposit types, the Fe content of silicates decreases and the Mg content increases outwards from the intense alteration zones. The host rock alteration reactions also led to the impregnation of rutile as a result of the replacement of ilmenite.

The main ore minerals are quite similar in the different deposits. The most common sulphide is pyrrhotite, which is encountered in all deposits. Pyrite is uncommon at Kouvervaara but it is the main sulphide in sericitized rocks. The only detected copper mineral is chalcopyrite, and cobaltite is the only sulfarsenide despite locally high As-contents. Cobaltpentlandite, mostly as lamellae in pyrrhotite, is the other common cobalt mineral. Some deposits contain molybdenite, and nearly all the galena is radiogenic as a result of uraninite decay. Magnetite is more common in the Fe-Co-Au deposits, rare at Juomasuo and is absent at Konttiaho and Sivakkaharju. Magnetite mineralization took place at higher temperatures than and prior to the main sulphide mineralization which re-equilibrated above 250 OC in all deposits. The crystallisation of sulphides occurred in alkaline reducing conditions in the Fe-Co-Au deposits, but the conditions were more oxidizing and acidic in the Fe-Co-Au-U deposits. Native gold is closely related to tellurides and bismuth minerals, but gold had a long crystallisation time and a wide temperature range, from the first sulphides to the later stages of tellurium and selenium mineralization. Bismuth mineralization, including native bismuth, maldonite, bismuthinite, hedleyite, tellurobismuthite and tsumoite, favoured the Fe-Co-Au deposits in which the tellurium mineralization was weak but extremely strong in the Fe-Co-Au-U deposits. Altaite, melonite, frohbergite, calaverite and mattagamite are the most common tellurium minerals. Only at Hangaslampi native tellurium has been recognised. The only detected selenide is clausthalite.

The ore-forming element associations vary between individual deposits, but in general the deposits were enriched in the following elements: Fe, S, Co, As, Cu, U, Mo, W, Pb, B, Au, Te, Se, Bi and LREE. Abundances of Ni, Zn, Ag and Sb are very low. Pb is completely radiogenic. The geological surroundings, hydrothermal alteration types and major and trace element characteristics of the mineralization suggest that the deposits in the KSB belong to the extensive, recently defined Fe-oxide-Cu-Au class.

Key words (GeoRef Thesaurus, AGI): schist belts, gold ores, cobalt ores, iron ores, uranium ores, metamorphic rocks, geochemistry, geochemistry, mineral composition, hibbingite, ore minerals, mineral deposits, genesis, hydrothermal alteration, albitization, metasomatism, Proterozoic, Paleoproterozoic, Proterozoic, Paleoproterozoic, Kuusamo, Finland

Erkki Vanhanen
Geological Survey of Finland
P.O. Box 77
FIN-96101 ROVANIEMI, FINLAND

E-mail: Erkki.Vanhanen@gsf.fi

ISBN 951-690-803-9
ISSN 0367-522

Price: FIM 188 (incl. VAT 8%)

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