VOLATILE TRANSPORT OF PLATINUM-GROUP ELEMENTS IN SULFIDE-CHLORIDE ASSEMBLAGES AT 1000°C

Abstract

The platinum-group elements (PGE) are appreciably volatile in dry systems in the presence of both Cl and S at 1000°C. The present experiments were conducted in sealed silica-glass tubes, with charges of either pure metals or PGE-bearing Fe-Ni-Cu sulfides and alloys; Na halide (principally NaCl); ± S, Te, As, Bi, C, and basalt powder. Metal transport was detected by visual inspection, XRD, SEM, and neutron activation analysis (NAA). Equivalent partial pressures were estimated from the transport of pure NaCl. For single and binary metal charges, transport occurs in the following sequences: Fe > Cu > Ni > Pd; and Cl > I >>> F. In a series of controlled experiments with polymetallic charges and analysis by NAA, the maximum amounts of PGE transported in two days at 1000°C are as follows: Os, 70 ng; Ir, 56 ng; Rh, 150 ng; Pt, 2550 ng; and Pd, 2500 ng on an absolute basis; and Os, 1100 ppm; Ir, 610 ppm; Rh, 860 ppm; Pt, 12,400 ppm; and Pd, 5900 ppm as a proportion of Fe sulfide transported. Platinum and Pd are the dominant PGE transported in experiments with high | s 2 and the chondritenormalized amounts exhibit a positively fractionated PGE pattern. The greater volatility of Pt and Pd in S-Cl-bearing systems may point to a general explanation for the positive PGE patterns of mafic rocks and magmatic sulfide and PGE deposits.