PHASE RATIOS IN THE SYSTEM PT-FE-S-O

Abstract

We thought that it would be interesting to consider how platinum-iron-sulfur phases behave at lower temperatures (300° to 600°C) in an oxygen-bearing system, i.e., in an environment that is more natural, judging by recent data. We studied the system Pt-Fe-S-O at 300° to 600°C under hydrothermal conditions (0.1n to 1n weight percent HCl). The main aim of our experiments was to determine the fS(2) and fO(2) ranges within which exchange reactions between ferroplatinum, isoferroplatinum and tetraferroplatinum would occur. Our data to a considerable extent confirm the view that the composition of platinum-metal minerals changes during the late stage of copper-nickel ore formation, and that such minerals could include 'primary' and 'secondary' varieties. The spatial separation of cooperite and ferroplatinum alloys most likely reflects the order of their generation during ore deposition. Platinum that originally contains little iron, usually deposited from high-temperature melt, could be converted by exchange reactions with iron-bearing minerals like pyrrhotite and troilite to isoferroplatinum and tetraferroplatinum. Such conversions would undoubtedly be promoted by fluids and by the high activity of iron (aFe) in the system.