Early proterozoic AuCoU mineralization in the Kuusamo district, northeastern Finland

Abstract

Abstract The Kuusamo volcano-sedimentary belt is part of an Early Proterozoic rift system in the Fennoscandian Shield extending from the Norwegian Sea to Lake Onega. The rocks range in age from 2.5 to 2.0 Ga. Sulphide deposits are confined to the two lowermost sedimentary units, the Sericite Quartzite and Siltstone Formations. These are intruded by albite diabase and syenite dikes and sills. The mineralization is associated with pervasive hydrothermal alteration. Spilitization and albitization predate mineralization and appear to be premetamorphic. The later FeMg-silicate and K-silicate alteration is associated with mineralization in ductile shear zones, whereas carbonatization and silicification dominate in brittle zones. Ore deposits and occurrences can be divided into three groups: uranium deposits, CoAuU-bearing sulphide deposits, and Fesulphide deposits. Uranium deposits are of the sulphide-poor, stratiform sandstone type often associated with an albitization front. The U deposits are also related to albite diabase dikes and alkaline granitic intrusions. The CoAuU deposits can be subdivided into two end members: replacement ores in a ductile deformation zone and breccia ores in a brittle deformation zone. At Juomasuo the sulphide mineralization is located in a FeMg-silicate and K-silicate alteration zone within a ductile shear zone surrounded by a wide albitized zone. The ore reserves are 1 Mt grading 5–6 g/t Au and 0.2% Co. Other enriched elements are Cu, U, W, Mo, Se, Te and Bi. The Konttiaho deposit represents a breccia ore in a brittle deformation zone where the CoAuU mineralization is associated with carbonate and quartz within albitized country rocks. Typical ore minerals in both types are pyrite, pyrrhotite, cobaltite, Co-pentlandite, chalcopyrite, uraninite, molybdenite, scheelite, gold and tellurides. The Fe-sulphide deposits are similar to the CoAuU deposits but lack precious metals. Sulphide mineralization is epigenetic, post peak-metamorphic and related to the Svecokarelian orogeny at 1.8–1.9 Ga. The mineralization is controlled by stratigraphy, antiform structures and shear zones in the central part of the volcano-sedimentary belt. The broad elemental spectrum suggests a complex origin and probably a deep-seated source for the metals.