Sources of Cuprous Sulfide Mineralization and High-Ni Olivine of the Rudnaya Dyke (Imangda Cluster, Norilsk region): Based on Compositional, Isotope and Model Data

Abstract

The Rudnaya dyke of the Imangda ore cluster is composed of weakly differentiated olivine-bearing to olivine gabbrodolerites with globular and interstitial sulfides of a chalcopyrite–cubanite- (pentlandite-pyrrhotite) composition. Along with sulfide mineralization, gabbrodolerites of the dyke contain xenoliths of hornfelsed basalts, abundant amygdales and rare grains of zoned olivine-1 Fo_90–47 that coexists with subhedral olivine Fo_74–36 of the second generation. Modeling with the COMAGMAT and alphaMELTS programs showed that high-Mg Ol-1 with Cr-spinel inclusions cannot be crystallized from a Fe-enriched tholeiitic melt with 4.8–7.3 wt % MgO and 11.6–16.7 wt % total Fe₂O₃ that is parental for the dyke. The variation trend and high Ni up to 0.5 wt % NiO in the cores of xenocrystic olivine Fo_90–76 in contrast to maximum Fo₈₃ and 0.4 wt % NiO in olivine from the ore-bearing intrusions and picritic basalts of the Norilsk region point toward the presence of deep-seated cumulates of picritic magmas, which had not exchanged with sulfide liquid. Platinum group element (PGE) abundances increase (up to 2.2 ppm) with increasing Cu/Ni in the whole rocks and proportions of pentlandite in a sulfide association. A specific pattern of chalcophile metal distribution with Ni, Os and Ir minima, elevated Cu/Ni (5–15) and Cu/Pd (3200–10 900) along with lower PGE tenor (PGE content in 100% sulfide) of sulfides (2–65 ppm) and Pd content in pentlandite (<175 ppm) compared to those of the ore-bearing intrusions suggest that cuprous sulfide mineralization was not physically captured from highly fractionated sulfide fractions of ore-bearing magmas but is cogenetic with a magma of the dyke. Sulfide saturation, near-simultaneous with fluid saturation and degassing, was achieved in the dyke conduit due to assimilation of sedimentary sulfur and volatiles from Devonian evaporites that is supported by the heavy S isotope composition of dyke’s sulfides with the average δ³⁴S = 14.7 ±1.1‰ (n = 31) close to the values for sulfides from the endocontact zones of the Imangda ore-bearing intrusions hosted by the Devonian sequences. The initial isotopic characteristics of dyke rocks (Sr_i 0.70517–0.70532, ɛNd from –0.4 to 0.8) imply its comagmatic origin with the Norilsk-type intrusions whereas the overall data do not exclude its spatial connection with an upper crustal conduit network of the ore-bearing magmas.