PRIMARY PLATINUM-BEARING COPPER FROM THE LESNAYA-VARAKA ULTRAMAFIC ALKALINE COMPLEX, KOLA-PENINSULA, NORTHWESTERN RUSSIA

The Lesnaya Varaka ultramafic alkaline complex in the northeastern Fen noscandian Shield (Kola Peninsula, NW Russia) is a concentrically-zone d intrusion with a dominantly dunitic core (Fo(85-92)) surrounded by c linopyroxenites. The complex resembles an Alaskan-type intrusion, but differs in its strong alh aline affinity. Native copper occurs as smal l (5 to 15 mu m), subhedral to euhedral crystals, isolated within tita nomagnetite, in a dunite containing abundant titanomagnetite-perovskit e mineralization (up to similar to 30 modal %). Nickel-rich (4.1-4.5 w t.% Ni) tetraferroplatinum is also present as minute (up to similar to 5 mu m) subhedral crystals, enclosed by titanomagnetite. They are typ ically partially rimmed by rhodian pentlandite (similar to 6 wt.% Rh). The copper crystals contain 0.6 to 10.1 wt.% Pt, 2.1 to 3.0 wt.% Ni, and essential Fe (approximately 2 to 3 wt.%). There is a wide variatio n in the Pt content between individual crystals, but its distribution within single crystals is fairly constant. Compared with Cu-Pt alloys from other localities, solid solution of Cu with Pt in the Lesnaya Var aka native copper is low. Unlike most occurrences in ultramafic rocks, the crystalline copper at Lesnaya Varaka appears to be a primary phas e, which formed under moderately oxidizing conditions and at very low sulphur activities.