Abstract

The method of quasi-equilibrium directed crystallization was used for experimental modeling of the behavior of Pt, Pd, and Au in the presence of As, Te, Bi, and Sn during the fractional solidification of Cu-rich sulfide magma. Our experimental melt contained (in mol.%): Fe 33.20, Cu 16.55, S 50.03, Pt 0.03, Pd 0.02, Au 0.02, As 0.02, Bi 0.03, Te 0.02, and Sn 0.08, which is similar in composition to the massive cubanite ores from the platinum-copper-nickel deposits of the Noril'sk group. During crystallization, base metal sulfides [pyrrhotite solid solution (Po ss) and high-temperature cubanite (isocubanite, Icb)] crystallized successively from the melt. The constructed curves show variations in the concentrations of Fe, Cu, and S along the ingot. Based on these data, we calculated melt composition during crystallization and determined the distribution coefficients of components between Po ss or Icb and the melt. The behavior of minor elements in this process was studied. The data show that the content of minor elements in Po ss is below the detection limit of electron microprobe analysis. Tin dissolves in isocubanite, whereas Pt, Pd, Au, As, Bi, and Te are present in the matrix as microinclusions. These elements form their own microphases: kotulskite–sobolevskite (PdBixTe1–x) and moncheite–insizwaite Pt(TexBi1–x)2 solid solutions, cooperite (Pt,Pd)S, sperrylite Pt(As,S)2, froodite PdBi2–x, bismuthinite Bi2S3–x, Au, lisiguangite CuPtBiS3, and unnamed PtBiS3–x. Also present are drop-shaped eutectic-like inclusions in which PdBixTe1–x is the matrix containing micron-sized inclusions of Au and PdBi2–x. The constructed crystallization history of the experimental sample demonstrates the behavior of both major and minor elements in each of the zones. A probable mechanism of formation of monophase and polyphase inclusions is proposed. The mineral forms of Pt, Pd, and Au phases in the samples and their morphology are comparable to the typical natural types in massive Cu-rich sulfide ores from the Noril'sk deposits.

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