Pd-Ag-Au Minerals in Clinopyroxenites of the Kachkanar Ural-Alaskan-Type Complex (Middle Urals, Russia)

The study of noble metal minerals of the Ural-Alaskan-type (UA-type) complexes has been traditionally focused on their platinum-bearing dunites and chromitites, while clinopyroxenites have been poorly considered. In this study, we report the first detailed data on the noble metal mineral assemblage in clinopyroxenites of the Kachkanar intrusion, which is a part of a UA-type complex and is renowned for its huge Ti-magnetite deposits. High concentrations of Pd, Au and Ag are closely linked to Cu-sulfide mineralization in amphibole clinopyroxenites, in which they form Pd-Ag-Au minerals: keithconnite Pd3-xTe, sopcheite Ag4Pd3Te4, stutzite Ag5-xTe3, hessite Ag2Te, merenskyite PdTe, kotulskite Pd(Te,Bi), temagamite Pd3HgTe, atheneite (Pd,Hg)3As, potarite PdHg, electrum AuAg and Hg-bearing native silver. Among those, six mineral phases are first reported for clinopyroxenites of the Ural platinum belt. Our evidence supports a petrological model, suggesting that during fractionation of high-Ca primitive magmas at high oxygen fugacity, Pt, Os, Ir, Ru and Rh accumulate in early olivine-chromite cumulates, while Pd, Au and Ag reside in the melt until sulfide saturation occurs and then concentrate in sulfide mineralization. Subsequently, this sulfide mineralization is likely affected by cumulate degassing, which results in a partial resorption of the sulfides and Pd, Au and Ag remobilization by fluid. Second-stage concentration of the sulfides and the chalcophile noble metals in the amphibole-rich rocks may occur when H2O from the fluid reacts with pyroxenes to form amphiboles, and the fluid becomes oversaturated with sulfides and chalcophile elements.