The role of Te, As, Bi, Sn and Sb during the formation of platinum-group-element reef deposits: Examples from the Bushveld and Stillwater Complexes

Abstract

The distribution of platinum-group element (PGE) and Te, As, Bi, Sb and Sn (TABS) in whole-rock samples, and in disseminated base metal sulfides (BMS) pentlandite, pyrrhotite and chalcopyrite from the Bushveld and Stillwater Complexes are reported. The samples are from: the Merensky Reef (Bushveld), the J-M Reef (Stillwater), Picket Pin deposit (Stillwater), and also barren sulfide-bearing samples, from outside the reef intervals from both intrusions. The objective of the study was to document the distribution of PGE and TABS in PGE-reef deposits, and to investigate whether TABS play a significant role during the formation of PGE-reef deposits.The whole-rock concentrations of PGE and TABS (except for As) correlate with S and PGE, and thus their distribution appear to be controlled by BMS. The distribution of As, and to a lesser extent Sb, correlate with incompatible elements and with changes in K-phologopite compositions, suggesting that these elements are controlled both by the amount of trapped liquid in cumulate rocks, and the amount of sulfides. The possible role of TABS in forming pre-nucleation clusters (nanonuggets) to enrich the reefs in PGE is considered and discarded, because the ratio of TABS/PGE < 0.3 is too low for TABS to form pre-nucleation clusters with PGE. A model where both the PGE and TABS are initially collected by a base metal sulfide liquid is favoured. During equilibrium crystallization of the sulfide liquid, Pd, Pt and the TABS were incompatible with the crystallizing BMS, and concentrated in the fractionated liquid eventually crystallizing as TABS-rich PGM. However, a portion of Pd, Pt and TABS also partitioned into the crystallizing BMS. During cooling and S-loss they exsolved from the BMS as PGM included in the BMS. In the reefs, the ratio of PGE/TABS is very high, and thus during exsolution all of the TABS could be accommodated in PGM, and consequently the BMS of the reefs have very low TABS concentrations. In contrast, outside the reefs the PGE/TABS ratios are low, and thus some TABS remained in the BMS, resulting in the BMS outside the reefs having higher TABS concentrations than those from within the reefs.