Partitioning of Platinum-Group elements (PGE) and chalcogens (Se, As, Te, Sb, Bi) at controlled fo2-fs2 conditions
Keywords: chalcogen
intermediate solid solution
magmatic
monosulfide solid solution
partitioning
PGE

Abstract A more quantitative understanding of how highly siderophile elements (HSEs: Os, Ir, Ru, Rh, Pt, Pd, Au, Re), Ag, Pb and chalcogens (As, Se, Sb, Te and Bi) behave during magmatic sulfide solidification in nature builds on several important aspects: 1) accurate measurements of their concentrations in natural samples; 2) quantitative constraints on their partitioning behaviors; 3) a crystallization model that can reasonably simulate the ore deposits of interest. In order to better measure the concentrations of these elements during the solidiﬁcation of sulﬁde melts, we have developed a new Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) standard (Ge-Sb-S glass; Ge6) for improving the trace level measurements of these elements in both sulfide and silicate glass matrices. The Ge6 composition has excellent glass forming capability and can incorporate a variety of dopants up to the 100 ppm level, except for Ru, Os, Re, W and Mo, which can be doped to 5-10 ppm without sacrificing homogeneity. Reference materials (JBSulfide, NiS4, JGb-1, JG-1a, JB-2, BIR-1, BHVO-1) were tested using Ge6 as the standard, and the results agree with accepted values. To better understand and simulate the partitioning behaviors of these elements, a series of experiments to measure partition coeﬃcients (D values) between monosulﬁde solid solution (MSS) and sulﬁde melt, as well as MSS and intermediate solid solution (ISS) were also conducted, at 0.1 MPa and 860–926 °C (where these phases can be stabilized), log fS2 -3.0 to -2.2 (similar to the Pt–PtS buﬀer), with fO2 controlled at the fayalite–magnetite–quartz (FMQ) buﬀer. The IPGEs (Os, Ir, Ru), Rh and Re are found to be compatible in MSS relative to sulﬁde melt with D values ranging from ~20 to ~5, and DRe/DOs of ~0.5. Pd, Pt, Au, Ag, Zn, Pb, as well as the chalcogens, are incompatible in MSS, with D values ranging from ~0.1 to ~1 x 10-3. For the same metal/sulfur ratio, D values for the IPGEs, Rh and Re are systematically larger than most previous studies and correlate with higher oxygen content in the sulﬁde liquid, reﬂecting the signiﬁcant eﬀect of oxygen on increasing the activity coeﬃcients for these elements in the melt phase. MSS/ISS partitioning experiments reveal that Ru, Os, Ir, Rh and Re are partitioned into MSS by a factor of >50, whereas Pd, Pt, Ag, Au and the chalcogens partition from weakly (Se, As) to strongly (Ag, Au) into ISS. Uniformly low MSS- and ISS- melt partition coeﬃcients for the chalcogens, Pt, Pd, Ag and Au will lead to enrichment in the residual sulﬁde liquid, but D values are generally too large to reach early saturation in Pt–Pd-chalcogen-rich accessory minerals, based on current solubility estimates. Instead, these phases likely precipitate at the last dregs of crystallization. Modeled evolution curves for the PGEs and chalcogens are in reasonably good agreement with whole-rock sulﬁde compositions for the McCreedy East deposit (Sudbury, Ontario), consistent with an origin by crystallization of MSS, then MSS + ISS from sulﬁde magma.