Tonalite-hosted early Proterozoic gold deposit at Osikonmaki, southeastern Finland

KONTONIEMI, OLAVI and EKDAHL, ELIAS, 1990. Tonalite-hosted early Proterozoic gold deposit at Osikonmäki, southeastern Finland. Bull. Geol. Soc. Finland 62, Part 1, 61—70. The Osikonmäki gold deposit is hosted by an early Proterozoic tonalite close to the Archean-Proterozoic boundary along the Raahe-Ladoga suture zone. The structure, petrology and chemistry of the Osikonmäki intrusion are typical of synkinematic I-type granitoids having calc-alkaline affinity. The deposit was discovered by boulder tracing and is situated in a deformed shear zone. Gold is concentrated in open F3 synforms plunging 20 degrees to the ESE. The dominant ore minerals are pyrrhotite, arsenopyrite, löllingite and chalcopyrite, occurring as disseminations or in places as compact veins and stringers. Gold occurs as native gold and electrum in both sulphide and silicate phases and is invariably accompanied by Bi-bearing minerals with variable Se and Te contents. The measured mineral resources in the eastern part of the shear zone are 2 Mt at 3 g/t Au and 0.77 % As. Mineralogical and geochemical alteration is limited to the sheared and mineralized zone itself, manifested by biotitization, weak silicification and potassic alteration.