Platinum-group elements and gold in sulfide melts from modern arc basalt (Tolbachik volcano, Kamchatka)
Abstract Sulfide melt inclusions entrapped in primitive olivine phenocrysts can be used to understand the compositions of early sulfide melts that may ultimately contribute to magmatic sulfide ore deposits. Sulfide globules hosted in olivine (86–92mol% Fo) from the Tolbachik basalt (the 1941 eruption) are characterized in terms of their major and trace element abundances using electron microscopy and LA–ICP–MS analysis. Distribution of major elements within individual sulfide globules varies from homogeneous to heterogeneous. Phases include monosulfide solid solution (MSS) and intermediate solid solution (ISS) intergrowths and exsolved low-temperature minerals such as pyrrhotite, pentlandite, chalcopyrite and cubanite. Trace elements (platinum-group elements — PGE, Ag, Te, Au, Pb and Bi) are also present in solid solution in sulfide phases and as micron-sized particles (“nuggets”). Such nuggets of dominantly Au, Pt, Au–Pd and Pd–Te are contained randomly within sulfide matrices or, more commonly, at phase boundaries. Nuggets are also attached to outer surfaces of sulfide globules. Concentrations of PGE in sulfides follow a log normal distribution over four orders of magnitude. The highest measured noble metal concentrations in the analyzed globules (436ppm Au+PGE) are 13.3ppm Au, 115ppm Pt and 299ppm Pd, whereas 40% of globules have Highlights Primitive arc basalt magma represents the 1941 eruption of Tolbachik volcano, Kamchatka. Silicate–sulfide melt immiscibility is recorded as melt inclusions in olivine Fo 86–92 . Sulfide melt inclusions are highly variable in Ni/Cu and PGE, Ag, Te, Au, Pb and Bi. Micron-sized particles of Au, Pt, Au–Pd and Pd–Te are abundant in sulfide globules. Tolbachik sulfide melts are compositionally similar to sulfides in magmatic deposits.
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