Abstract

The wetting behavior of As-rich sulfosalt melts against monosulfide solid solution (MSS) is investigated using experiments to elucidate late-stage fractionation processes in magmatic sulfide systems, which may control the distribution of platinum group elements. A range of As-rich melt compositions is found to wet MSS, including those that contain significant proportions (1%–45%) of precious metals (Pt, Pd, and Au). However, extremely Au rich or Pt rich sulfosalt melts (>∼40% Au; >∼50% Pt + 5% Au) do not wet MSS. These results imply that if magma contamination and/or fractionation processes were able to produce a late-stage As-rich melt (sulfosalt melts crystallize cooler than MSS) that exceeds ∼0.2% of the rock volume, then an interconnected melt drainage network would be able to form along MSS crystal triple junctions. The dense sulfosalt melt could thereby drain downward, progressively sequestering incompatible Bi, Sb, Te, Pt, Pd, and Au to form sulfosalt melt accumulations that continue to fractionate to form the platinum group minerals. This late-stage fractionation model is consistent with the observed mineral distribution in numerous magmatic sulfide deposits.

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