Volatile transport of precious metals at 1000°C: Speciation, fractionation, and effect of base-metal sulfide

Abstract

The volatile transport of precious metals in sealed silica-glass tubes at 1000°C has been investigated further by varying thermal gradient and using charges of FeNiPGEAu monosulfide ± S ± FeCl 2 ± NaCl ± C. Addition of FeCl 2 permits F Cl 2 to be increased to 10 −4.8 at F S 2 = 0 0.1. The transport of PGE-Au is controlled and reproducible when the thermal gradient along the glass tubes is low and essentially restricted to the cool end. Although appreciable transport of precious metals requires the presence of metal chlorides, in addition to a high F S 2 and adequate permeability, transport does not appear to depend on F Cl 3 , further supporting our earlier suggestion of complex speciation of PGE in the vapor phase. In combination with added S and NaCl, FeCl 2 does not markedly enhance the transport of precious metals over the amounts associated with NaCl alone and, in the absence of NaCl, FeCl 2 results in reduced transport of Au. Transported Pd correlates with Pt ( r = 0.89) and Rh with Ir ( r = 0.86), but Au is only weakly correlated with Pt ( r = 0.67) and Pd ( r = 0.43), as is Os with Ir ( r = 0.57). The AuPt and AuPd correlations improve to r = 0.87 and r = 0.66, respectively, when experiments without NaCl are excluded. Transport of PGEAu varies with composition of the base-metal sulfide matrix: FeS results in fractionation of PGE-Au with greater transport of Pt, Pd, and Au, reproducing the positive PGE-Au pattern of the Fe-rich magmatic PGE-sulfide ores of mafic rocks, whereas NiSFeS results in little fractionation, reproducing the relatively flat PGEAu pattern of the Ni-rich sulfide ores of ultramafic rocks. Late-magmatic S- and Cl-bearing fluids could selectively mobilize base and precious metals in mafic/ultramafic complexes and concentrate them in permeable pathways with the characteristic geochemical signatures observed in nature.