TRANSPORT OF PLATINUM-GROUP ELEMENTS IN A S-DOMINATED VAPOR AT MAGMATIC TEMPERATURES

One of the existing models in vogue for the formation of platinum-group elements (PGE) - deposits suggests that the PGE partition into a fluid or a vapor phase and precipitate when they encounter vapor-undersaturated magma. Previously, the possibility of PGE transport was confirmed experimentally for fluids dominated by H2S (Baker et al., 2001) and by Fe-chloride complexes (Fleet and Wu, 1993, 1995). The aim of the present research is to test by means of experiments the possibility of PGE transport by a S-dominated vapor. The experiments were performed at 1000oC in the system Fe-Ni-Cu-S doped with 3500 – 1500 ppm of Pt, Ir, Pd, Rh and Ru by the method of evacuated silica-glass tubes. The results obtained suggest that a S-dominated vapor could play an important role in Pd transport; at 1000oC at least 30 ppm of Pd were transported through the vapor phase during S transfer from a S-rich pyrrhotite bearing 3500 ppm Pd to a S-poor pyrrhotite receiver. In the contrast to Pd, only 2 ppm of Ru and 1.4 ppm of Rh were transported, and no transport of Pt and Ir was detected (minimum detection limits=0.014 ppm and 0.009 ppm respectively). In addition, after vapor transport experiments the PGE-content of the initial pyrrhotite was reduced significantly at the end of experiments: from 3500 ppm to 3200 ppm for Pd, from 1500 ppm to 1400 ppm for Ru, from 2483 ppm to 2016 ppm for Rh, from 2600 to 2300 for Ir, and from 1800 to 1600 for Pt. We interpret this depletion as the result of PGE redistribution in the course of S removal from the pyrrhotite to the vapor phase including both PGE transfer to the vapor phase and exsolution of individual PGE minerals from the latice of a S-depleted pyrrotite. Similar experiments were conducted for various base-metal sulfide associations including [Po], [Mss], [Mss+liquid], [Liquid], and [Mss+liquid+PGE alloys] with the aim of revealing PGE-species responsible for the PGE transport in a S-dominated vapor.