ORIGINAL PAPER Fractionation of platinum, palladium, nickel, and copper in sulfide–arsenide systems at magmatic temperature H. M. Helmy • C. Ballhaus • R. O. C. Fonseca • T. J. Nagel Received: 16 May 2013 / Accepted: 17 October 2013 / Published online: 5 November 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract Experimentally derived phase relations of arsenide in sulfide melt are presented to quantify the fractionation paths of As-bearing sulfide melts. When a natural sulfide melt reaches arsenide saturation, a separate Ni–PGE-rich arsenide melt ex- solves. The arsenic saturation concentration in an Fe–Ni–Cu sulfide melt is between 0.5 and 1.5 wt%. The affinities of the chalcophile metals for an immiscible arsenide melt follow the order Pt [ Pd [ Ni  Fe & Cu. In natural systems, arsenide exsolution will be triggered by the activity of the nickel arsenide components dissolved in sulfide melt, Ni being the most com- mon base metal with strong affinity to the As n- anionic species. Arsenic may have a major effect on the fractionation paths of sulfide melts even if no separate arsenide phase forms. Arsenic, and probably many other chalcogens and metalloids in mag- matic melts, may undergo associations with Pt and Pd well before discrete PGE minerals become stable phases. Keywords Platinum and palladium fractionation  Arsenides  Sulfides  Magmatic melts  Experimental study Introduction It is a common observation in Fe–Ni–Cu magmatic sulfide deposits that the platinum-group elements (PGE) occur as discrete minerals. In many ore deposits, the PGE show a great affinity toward rare ligands such as As, Sb, Te, Bi, and Sn (e.g., Vermaak and Hendriks 1976; Helmy et al. 1995; Helmy 2004; Power et al. 2004; Holwell and McDonald 2007; Dare et al. 2010; Godel et al. 2012; Pin ˜a et al. 2013). This is somewhat surprising. Many of the noble metals, notably Ir, Ru, Rh, and Pd, fit rather well in the lattices of one or more of the common base metal sulfides that usually coexist with PGE phases. Since the PGE are mere trace quantities in magmatic sulfide melts, one would intuitively expect that they can be accommo- dated in solid solution in crystalline base metal sulfides. Szentpe ´teri et al. (2002) speculated that many of the discrete PGE phases could be subsolidus exsolutions from base metal sulfides. This may be viable for PGE sulfides (Ballhaus and Ulmer 1995; Ballhaus and Ryan 1995). Arsenic, Sb, Te, Bi, and Sn, however, are so poorly soluble in monosulfide solid solution (mss) (Helmy et al. 2010) that it is difficult to envision how these elements could exsolve simultaneously with the PGE from mss in the quantities and proportions required to form discrete stoichiometric PGE phases. A more likely scenario is that the PGE become enriched together with their ligands in late-stage sulfide melts (cf. Fleet et al. 1993), then precipitate as PGE arsenides, antimonides, tellurides, bismuthotellurides, and stannites when phase saturation is reached. Textural observations on undeformed PGE-rich magmatic sulfide mineralizations support this scenario: Discrete PGE min- erals are often found concentrated along grain-boundary triple junctions of base metal sulfides (Godel et al. 2010), i.e., the locations where derivative sulfide melts that pref- erentially wet the surfaces of crystalline sulfides are expected to accumulate. To test the role of chalcogens and metalloids in PGE fractionation, it is necessary to understand the phase Communicated by J. Hoefs. H. M. Helmy  C. Ballhaus  R. O. C. Fonseca  T. J. Nagel Steinmann Institut, Universita ¨t Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany H. M. Helmy (&) Department of Geology, Minia University, Minia 61519, Egypt e-mail: hmhelmy@yahoo.com 123 Contrib Mineral Petrol (2013) 166:1725–1737 DOI 10.1007/s00410-013-0951-9