Genetic Pd, Pt, Au, Ag, and Rh mineralogy in Noril’sk sulfide ores

Abstract

The undeformed ore-bearing intrusions of the Noril’sk ore field (NOF) cut through volcanic rocks of the Late Permian–Early Triassic trap association folded in brachysynclines. Due to the nonuniform load on the roof of intrusive bodies, most sulfide melts were squeezed, up to the tops of ore-bearing intrusions; readily fusible Ni–Fe–Cu sulfide melts were almost completely squeezed. In our opinion, not only one but two stages of mineralization developed at the Noril’sk deposits: (i) syntrap magmatic and (ii) epigenetic post-trap metamorphic–hydrothermal. All platinum-group minerals (PGM) and minerals of gold are metasomatic in the Noril’sk ores. They replaced sulfide solid solutions and exsolution structures. All types of PGM and Au minerals occur in the ores, varying in composition from pyrrhotite to chalcopyrite, talnakhite, mooihoekite, and rich in galena; they are localized in the inner and outer contact zones and differ only in the quantitative proportions of ore minerals. The aureoles of PGM and Au–Ag minerals are wider than the contours of sulfide bodies and coincide with halos of fluid impact on orebodies and adjacent host rocks. The pneumatolytic PGM and Au–Ag minerals are correlated in abundance with the dimensions of sulfide bodies. Their amounts are maximal in veins of late fusible ore composed of eutectic PbSss and iss intergrowths, as well as at their contacts. The Pd and Pt contents in eutectic sulfide ores of NOF are the world’s highest. In the process of noble-metal mineral formation, the fluids supply Pd, Pt, Au, As, Sb, Sn, Bi, and a part of Te, whereas Fe, Ni, Cu, Pb, Ag, Rh, a part of Te and Pd are leached from the replaced sulfide minerals. The pneumatolytic PGM of the early stage comprises Pd and Pt intermetallic compounds enriched in Au along with Pd–Pt–Fe–Ni–Cu–Sn–Pb(As) and (Pd,Pt,Au)(Sn,Sb,Bi,Te,As) solid solutions. Pneumatolytic PGM and Au minerals of the middle stage are products of solid-phase transformation and recrystallization of early PGM in combination with the newly formed mineral species Sb-paolovite–insizwaite–geversite–maslovite, niggliite, tetraferroplatinum, rustenburgite–atokite–zvyagintsevite, moncheite, majakite, plumbopalladinite, polarite in association with altaite. The late minerals of the middle stage include stannopalladinite, tatianaite–taimyrite, Ag–Pd–Pt tetraauricupride, and cuproauride. PGM and Au–Ag minerals of the late stage are represented by sobolevskite–sudburyite–kotulskite, maslovite–michenerite, low-Sb paolovite, hessite, cabriite, Au–Ag minerals with fineness of 870–003, froodite, Sb-free insizwaite, Bi-free geversite, and Sb-free niggliite. Electrum and kustelite in PGM aggregates are not zoned. Crystals of Au–Ag minerals that grow over PGM minerals are smoothly zoned. Their zoning may be direct (crystal margins are enriched in Ag), inverse, oscillatory, and complex. Despite favorable annealing conditions, exsolution structures are not identified in Au–Ag minerals from the Noril’sk ores. Sperrylite—the latest of pneumatolytic PGM—occurs as metacrysts up to 14 cm in size. Sperrylite, which replaces high-Sb minerals, contains up to 11 wt % Sb. Pneumatolytic noble-metal minerals originated under the effect of the fluids released during crystallization of sulfide melts in an extremely reductive setting and at extremely low fS2; temperature drops from ~450 to ~350°C. Metamorphic–hydrothermal Ag mineralization (native silver, Hg-silver, sulfides and selenides, chalcopyrite–lenaite solid solutions, argentopentlandite), Pd mineralization (vysotskite, palladoarsenide, vincentite, Sb-free Ag-paolovite, malyshevite, native palladium), and Pt mineralization (kharaelakhite, cooperite, native platinum) develop in those parts of orebodies that are affected by low-grade metamorphism.