ABSTRACT
The 2.44 Ga Kuusijärvi layered intrusion in northeastern Finland hosts a 15-33 m thick platinum-group mineral (PGM)-Ni-Cu sulfide-bearing gabbro layer in the Marginal Series, which is mineralized by disseminated chalcopyrite, pyrrhotite, pentlandite and pyrite and platinum-group minerals merenskyite (62% of the PGM), michenerite (1.3%), kotulskite (5%), Pd-rich bismuthian melonite (25.3%), sperrylite (6.6.%) and platinum group element (PGE)-rich cobaltite. The PGE- Cu-Ni sulfide-bearing gabbro has Cu/(Cu+Ni) of 0.53 and Pt/(Pt+Pd) of 0.29. The PGM have grain sizes of <40 µn, mostly in the range 5-15 p.m. The PGM occur primarily in connection with hydrosilicates between amphibole and pyroxene grains in the hosting gabbro. Partly the PGM occur at sulfide grain boundaries. On a textural basis it is concluded that the PGE-mineralization took place at a late stage in connection with the crystallization of hydrosilicates, possibly during metamorphic retrograde recrystallization at temperatures below 500° C.
