CONCENTRATIONS OF PLATINUM-GROUP ELEMENTS AND GOLD IN SULFIDES FROM THE STRATHCONA DEPOSIT, SUDBURY, ONTARIO

Sulfide minerals and magnetite from the Strathcona deposit, Sudbury, O ntario, have been analyzed for the platinum-group elements (PGE) and A u with in situ, ultra-sensitive accelerator mass spectrometry (AMS). P yrrhotite contains Ir, Rh and Ru at levels close to those of the bulk sulfide ores (recalculated to 100% sulfides), ranging from tens to mor e than 100 ppb; Pt, Pd and Au are present in the pyrrhotite at much lo wer levels than in the bulk sulfide ores. Among all the other minerals , including pentlandite, chalcopyrite, cubanite, bornite and magnetite , pentlandite is the only phase that accommodates significant amounts of Pd, with concentrations in the range from hundreds to thousands of ppb. The concentrations of the other PGE and Au are extremely low in a ll of the sulfides and magnetite in comparison with the values of the ores. They range from several ppb to sub-ppb levels. Although pentland ite contains significant amounts of Pd, its overall contribution to th e ore of the Deep Copper Zone is not great because the ore contains on ly small amounts of this mineral. Mass-balance calculations indicate t hat most of the Pt, Pd and Au in the ore is not present in the sulfide s and magnetite, but must occur in discrete accessory minerals. The pa rtition coefficients of PGE between Mss and sulfide liquid have been e stimated from the compositions of minerals and ore to be 6.5 x 10(-3) to 8.6 10(-2) for Pt, 6.8 x 10(-3) to 3.9 x 10(-2) for Pd, 4.7 x 10(-3 ) to 8.0 X 10(-2) for Au, and 5.9 to 6.7 for Ir. The variations in the concentrations of Pt and Ir in the deposit can be explained by a proc ess involving fractional crystallization of monosulfide solid solution (Mss) from a parental sulfide liquid and mixing of varying proportion s of variably fractionated liquid with Mss, if the partition coefficie nts between Mss and sulfide liquid are less than 0.1 for Pt and from 6 .7 to 3.5 for Ir.