The location of the chalcophile and siderophile elements in platinum-group element ore deposits (a textural, microbeam and whole rock geochemical study): Implications for the formation of the deposits

Recent analytical developments now make it possible to determine chalcophile and siderophile elements in situ in base metal sulfide minerals (BMS). Three points can be considered using these analyses: a) are the different elements preferentially concentrated in any particular BMS; b) what percentages of the siderophile and chalcophile elements are present in the BMS; c) what processes affect the distribution of the siderophile and chalcophile elements among the BMS. We have compared siderophile and chalcophile element distributions in pentlandite, chalcopyrite and pyrrhotite from platinum-rich ore deposits that have undergone different cooling rates and degrees of metamorphism to address these questions. We found that Re, Os, Ir, Ru and Rh are concentrated in both pentlandite and pyrrhotite. In addition to these elements pentlandite concentrates Ni, Co and Pd. Copper, Zn, Cd and Ag are concentrated in chalcopyrite or cubanite. Gold and Pt do not preferentially concentrate in any particular BMS, with very little of these elements located in BMS. The BMS from sulfide droplets of the Noril'sk (Russia) host almost all of the siderophile elements (except Pt and Au) and much of the Co and Ag. Platinum occurs as Pt-bearing mineral inclusions within the BMS. The droplets occur in unmetamorphosed subvolcanic sills, which would have cooled relatively quickly. The high percentage of PGE in the BMS and the close association of the Pt-minerals with the BMS suggest that the model whereby a base metal sulfide liquid collected the siderophile and chalcophile elements to form the deposit is correct. We suggest that the Pt partitioned into the sulfide liquid and could have partitioned into the BMS at high temperatures, but that a lower temperatures the BMS structure would not accommodate the Pt and Pt-minerals exsolved during cooling. Alternatively, if Pt could not partition into the BMS then Pt would have concentrated in the fractionated sulfide liquid and crystallized as Pt-minerals from the final liquid. In the platinum-group element (PGE) reefs of unmetamorphosed layered intrusions (Busveld Complex, South Africa and Great Dyke, Zimbabwe) ∼ 30 to 60% of the siderophile elements (except Pt and Au) are present in pentlandite, pyrrhotite and chalcopyrite. The balance is found in platinum-group minerals (PGM), which occur associated with the BMS. We suggest that the reason that a larger percentage of PGE are in the form of PGM is the result of the slower cooling of the BMS i