Sulphide Segregation in Ferropicrites from the Pechenga Complex, Kola Peninsula, Russia

The Palaeoproterozoic Ni–Cu sulphide deposits of the Pechenga Complex, Kola Peninsula, occur in the lower parts of ferropicritic intrusions emplaced into the phyllitic and tuffaceous sedimentary unit of the Pilgujärvi Zone. The intrusive rocks are comagmatic with extrusive ferropicrites of the overlying volcanic formation. Massive lavas and chilled margins from layered flows and intrusions contain Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphide mass ratio) are high and of the order of 104–105, and they indicate the segregation of only small amounts of sulphide liquid in the parental ferropicritic magma. In differentiated flows and intrusions the sulphide liquids segregated and accumulated at the base of these bodies, but because of a low silicate–sulphide mass ratio the sulphide liquids had a low PGE tenor and Pt/Ir and Cu/Ir ratios similar to the parental silicate melts. During cooling the sulphide liquid crystallized 40–50% of monosulphide solid solution (mss) and the residual sulphide liquid became enriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. The Cu-rich sulphide liquid locally assimilated components of the surrounding S-rich sediments as suggested by the radiogenic Os isotopic composition of some sulphide ores (γOs > 100).