Highly siderophile elements in platinum-group element ore deposits

摘要

We have studied the distribution of siderophile elements among base metal sulphides (BMS) and platinum-group minerals (PGM) found in platinum-group element (PGE) ore deposits. In order to address the question of which processes affect the formation of these deposits we have selected deposits that have undergone different cooling rates and degrees of metamorphism.ududIn sulfide droplets from the unmetamorphosed Noril’sk sills, which have undergone rapid cooling, almost all of the siderophile elements (except Pt and Au), occur in BMS. This suggests the model whereby a base metal sulfide liquid collects the PGE to form these deposits is correct. Platinum occurs as PGM exsolutions within the BMS. Possibly the Pt partitioned into the sulfide liquid, but it exsolved from the BMS during cooling. ududIn the PGE-reefs of unmetamorphosed layered intrusions (Busveld Complex and Great Dyke) ~30 to 60 % of the siderophile elements (except Pt and Au) are present in BMS. The balance is found in PGM, which occur as exsolutions in the BMS or as grains at the contact with the BMS. The reason that a larger percentage of PGE are in the form of PGM is 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. ududIn the PGE-reefs from the metamorphosed layered intrusion (Penikat) the percentage of siderophile elements present in BMS covers a larger range, ~ 8 to 70 percent. There are many more PGM present and there has been extensive recrystallization of the BMS. Possibly the recrystallization of BMS during metamorphism facilitated the formation of a large number of PGM. The Pd-PGM are not always found associated with BMS. Three processes could have led to this: a) the BMS, which originally contained Pd, dissolved during metamorphism, leaving an insoluble PdPGM; b) the Pd could have been introduced to the PGE reef by metamorphic fluids; c) the Pd could have been locally remobilized into the silicates adjacent to the BMS.