Geochemical evolution of PGE-sulfide mineralization of the Khudolaz differentiated complex in the South urals: The role of R-factor and hydrothermal alteration

Abstract

The article highlights the results of mineralogical and geochemical studies of disseminated Cu-Ni-PGE sulfide mineralization from three intrusions of the Khudolaz Differentiated Complex (KDC) in the South Urals. Sulfide mineral assemblages are shown to have formed across a wide temperature range (about 1100–150 °C) during the cooling of an immiscible sulfide melt and subsequent hydrothermal dissolution and redeposition of sulfides. During the solidification process, a crystallization of sulfide solid solutions (mss, iss, etc.) occurred, from which pyrrhotite, pentlandite, and chalcopyrite were subsequently exsoluted. The oxidation, substitution and dissolution of primary sulfides at the hydrothermal stage led to the formation of mineral aggregates of complex morphology, containing complete or partial pseudomorphs of secondary sulfides, silicates and oxides, as well as redeposited sulfide aggregates. According to LA ICP MS data for sulfides and the bulk trace element composition of ores, the depletion of sulfide minerals by some chalcophile and platinum-group elements (PGE) took place during their hydrothermal alteration (partial dissolution and replacement). Secondary sulfides such as pyrite and violarite largely inherited the geochemical features of the primary minerals during pseudomorphic replacement of pyrrhotite and pentlandite. The redeposited sulfides (pyrite and chalcopyrite) are primarily enriched by chalcophile trace elements. Leaching of PGEs from pyrrhotite and pentlandite took place during hydrothermal alteration. Here, the most significant loss was determined for Te, resulting in the depletion of sulfide minerals and ores in Te by about 5 times. The identified decrease of Te/Sb values in altered sulfides by a factor of 3–20 indicates an enrichment in Sb and a high Dhydrothermal fluid/sulfide partition coefficient (≫ 1). It is shown that the hydrothermal fluid was enriched in Sb. On the other hand, the S/Se value increases by a factor of ~ 1.5, demonstrating an Se mobility during metasomatic processes comparable to other chalcogenides.Based on the study of assemblages and traced evolution of the chemical composition of PGE minerals, three stages of mineralization are proposed: 1) crystallization of immiscible chalcogenide and highly fractionated sulfide melts; 2) segregation of isomorphic impurities of chalcogenides and PGE during the sulfides exsolution from sulfide solid solutions; 3) interaction of Pd leached from primary sulfide and PGE minerals with Sb-rich hydrothermal fluid.The calculated value of the R-factor (150–502) indicates a moderate productivity of the KDC intrusions for Cu-Ni sulfide mineralization, but a rather low potential for PGE. Although the degree of primary sulfide melt PGE-enrichment is not inferior to that of many Cu-Ni-PGE deposits in the world, the economic potential of the Khudolaz Complex intrusions is significantly reduced by their small sizes. At the same time, the strong depletion of primary sulfides in Ni, Cu, Pd, and Pt during hydrothermal alteration led to the enrichment of host rocks in the external contact.