Abstract
Gold and silver contents in Noril’sk ore are controlled by the amount of sulphides and bulk Cu grade. Relative concentrations, re-calculated to 100 % sulphide, depend on type of ore: they are higher for disseminated ore than for massive ore and are the highest for low-sulphide platinum ore. Gold occurs mainly as high-fineness Au–Ag alloy in pyrrhotite-rich ore, whereas silver enters chalcopyrite mainly as solid solution. Increase in Cu grade correlates with an increase in the concentration of silver in chalcopyrite. Gold and silver form discrete minerals such as Au–Cu alloys, Au–Ag alloys, tellurides, sulphides, selenides, sulphobismuthides, Ag and Ag–Pd chlorides in Cu-rich ores; they also enter the structures of complex platinum-group minerals. The Au–Ag mineralisation is related to the post-magmatic hydrothermal stage under temperature conditions of 350–50 °C. Silver entered crystallizing chalcopyrite in solid solution in the late-magmatic stage, while all of the gold and the remainder of the silver and some platinum-group elements were transported predominantly as chloride and hydrosulphide complexes in hydrothermal fluids.
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