AN UNUSUAL ASSOCIATION OF HYDROTHERMAL PLATINUM-GROUP MINERALS FROM THE IMANDRA LAYERED COMPLEX, KOLA PENINSULA, NORTHWESTERN RUSSIA

Abstract

An unusual association of platinum-group minerals (PGM) is observed in a chromitite layer of the Mount Bol' shaya Varaka deposit, Imandra layered complex, Kola Peninsula, Russia. The PGM typically occur as cluster-like. or chain-like micro-aggreates. and as separate minute g-rams located in contact with chromite along grain boundaries. The PGM are enclosed in magnesiohornblende, edenite, talc, or phlogopite, and may transect the host hydrous silicates; they include sperrylite, laurite-erlichmanite. unnamed Cu-2(Ni,Co)Pt3S8. hollingworthite, pyrite rich in the platinum-group elements (Ru 10.96, Os 0.98, and Ir 0.54 wt.%), nickelian platarsite(?), (Pt0.71Ni0.26Co0.05)(Sigma1.02)As1.10S0.88, daomanite, and cooperite. Concentrations of Rh and S in the rhodian sulfurian sperrylite vary front 1.0 and 0.8 to 6.6 and 4.9 wt.%, respectively, and the composition most enriched in Rh and S is (Pt0.78Rh0.20Ir0.03)(Sigma1.01)(As1.53S0.47)(Sigma2.00). The correlations Pt-Rh (correlation coefficient R = -0.99), Pt-As (R = 0.98), PtS (R = -0.99). Rh-As (R = -0.97), Rh-S (R = 0.97), and As-S (R = -1.00) are important and consistent with the existence of a new series of solid solution, extending from PtAs2 toward the pyrite-type Rh1-xS2 ("Rh2S5"). Most of the PGM precipitated during deuteric alteration at a postmagmatic-hydrothermal stage of crystallization of the chromitite. The majority of the laurite-erlichmanite grains display a close textural relationship with the hydrous silicates and form part of the hydrothermal mineralization at Imandra.