Abstract

Platinum, palladium, iridium, and gold were determined by radiochemical neutron activation analysis in coexisting pyrrhotite, pentlandite, chalcopyrite, and magnetite from the deep ore zone of Strathcona mine, Sudbury, Ontario. Platinum and palladium are the most abundant noble metals and their average concentrations reach ppm levels in chalcopyrite and pentlandite, whereas average gold and iridium values do not exceed 15 ppb, except for the gold content of pentlandite which averages 118 ppb.Individual noble metals are strongly associated with specific sulfide minerals. Palladium and gold are concentrated with pentlandite, and platinum and iridium are associated with chalcopyrite. Relative to the most abundant sulfide, pyrrhotite, palladium and gold are concentrated in pentlandite by a factor of approximately 14, and platinum and iridium are concentrated in chalcopyrite by a factor of approximately 6. Except for iridium, magnetite is depleted in noble metals relative to all sulfide minerals.Using phase relations in the Cu-Fe-Ni-S system as a guide to the paragenesis of the principal sulfide minerals, it is suggested that the palladium-pentlandite association is best explained by a solid solution of much of the palladium in pyrrhotite at magmatic temperatures with a subsequent exsolution with pentlandite below 300 degrees C. The same process was presumably significant in controlling the gold distribution.Some chalcopyrite probably exsolved from the pyrrhotite solid soluton at subsolidus temperatures, but the direct precipitation of chalcopyrite from a copper-rich magmatic liquid is also feasible. The platinum-chalcopyrite association therefore suggests that the platinum distribution is controlled in part by a solid solution in pyrrhotite and in part by fractionation into a copper-rich magmatic liquid. The distribution of platinum values in this Strathcona deep ore zone suite indicates that both processes were operative.

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