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

Abstract

The Lesnaya Varaka ultramafic alkaline complex in the northeastern Fennoscandian Shield (Kola Peninsula, NW Russia) is a concentrically-zoned intrusion with a dominantly dunitic core (Fo85-92) surrounded by clinopyroxenites. The complex resembles an Alaskan-type intrusion, but differs in its strong alkaline affinity. Native copper occurs as small (5 to 15 μm), subhedral to euhedral crystals, isolated within titanomagnetite, in a dunite containing abundant titanomagnetite-perovskite mineralization (up to ~ 30 modal %). Nickel-rich (4.1-4.5 wt.% Ni) tetraferroplatinum is also present as minute (up to ~ 5 μm) subhedral crystals, enclosed by titanomagnetite. They are typically partially rimmed by rhodian pentlandite (~ 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 variation 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 Varaka native copper is low. Unlike most occurrences in ultramafic rocks, the crystalline copper at Lesnaya Varaka appears to be a primary phase, which formed under moderately oxidizing conditions and at very low sulphur activities.