The Role of Te, As, Bi, and Sb in the Noble Metals (Pt, Pd, Au, Ag) and Microphases during Crystallization of a Cu-Fe-S Melt

• Published in: Minerals
• Main Authors: Elena Fedorovna Sinyakova, Nikolay Anatolievich Goryachev, Konstantin Aleksandrovich Kokh, Nikolay Semenovich Karmanov, Viktor Aleksandrovich Gusev
• Format: Article in Journal/Newspaper
• Language: English
• Published: MDPI AG 2023
• Subjects: Cu-Fe-S system; noble metals; TABS; directional crystallization; immiscible sulfide and semimetal liquids; Mineralogy; QE351-399.2; norilsk
• Online Access: https://doi.org/10.3390/min13091150; https://doaj.org/article/cc5b7afb4b7548ee9ae42de173ba4d31

Quasi-equilibrium directional crystallization was performed on a melt composition (at. %): 18.50 Cu, 32.50 Fe, 48.73 S, 0.03 Pt, Pd, Ag, Au, Te, As, Bi, Sb, and Sn, which closely resembles the Cu-rich massive ores found in the platinum-copper-nickel deposits of Norilsk. Base metal sulfides (BMS) such as pyrrhotite solid solution (Fe,Cu)S 1±δ ( Poss ), non-stoichiometric cubanite Cu 1.1 Fe 1.9 S 3 ( Cbn *), and intermediate solid solution Cu 1.0 Fe 1.2 S 2.0 ( Iss ) are progressively precipitated from the melt during the crystallization process. The content of noble metals and semimetals in the structure of BMS is below the detection limit of SEM-EDS analysis. Only tin exhibits significant solubility in Cbn * and Iss , meanwhile Pt, Pd, Au, Ag, As, Bi, Sb, and Te are present as discrete composite inclusions, comprising up to 11 individual phases, within their matrices. These microphases correspond to native Au, native Bi, hessite Ag 2 Te, sperrylite Pt(As,S) 2 , hedleyite Bi 2 Te, michenerite PdTeBi, froodite PdBi 2 , a solid solution of sudburite-sobolevskite-kotulskite Pd(Sb, Bi) x Te 1−x , geversite PtSb 2 , and a multicomponent solid solution based on geversite Me(TABS) 2 , where Me = Σ(Pt, Pd, Fe, Cu) and TABS = Σ(Te, As, Bi, Sb, Sn). Most of the inclusions occur as thin layers between BMS grain boundaries or appear drop-shaped and subhedral to isometric grains within the sulfide matrix. Only a small fraction of the trace elements form mineral inclusions of sizes ≤ 0.5 μm in Poss , most likely including PtAs 2 and (Pt,Pd)S. It is likely that the simultaneous presence of noble metals (Pt, Pd, Au, Ag) and semimetals (As, Te, Bi, Sb) in the sulfide melt leads to the appearance of liquid droplets in the parent sulfide melt after pyrrhotite crystallization. The solidification of droplets during the early stages of Cbn * crystallization may occur simultaneously with the cooling of later fractions of the sulfide melt, resulting in the formation of Iss . In addition, abundant gas voids containing micro-inclusions ...