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

Abstract

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 in the layered intrusion, which would allow more time for exsolution of the PGE than in the case of the BMS from subvolcanic sills. In the PGE-reefs from the metamorphosed layered intrusion (Penikat, Finland) the percentage of siderophile elements present in BMS covers a larger range, of ∼ 8 to 70%. There are many more PGM present and there has been extensive recrystallization of the BMS. Most of the PGM are associated with BMS, but in many cases the Pd-bismuthotellurides and in a few cases the Pt-sulfarsenides and -arsenides are not found associated with BMS. Three processes could possibly have led to this. The BMS, which originally contained the PGM as exsolutions, dissolved during metamorphism, leaving behind insoluble PGM. Alternatively the bismuthotellurides, arsenides and sulfarsenides could have been locally remobilized from the BMS into the surrounding silicates during metamorphism. The third possibility is that Pd and to a lesser extent Pt could have been introduced to the PGE reef by fluids and precipitated as PGM from the fluid.