Sources of Cu-Rich Sulfide Mineralization and high-Ni Olivine of the Rudnaya Dyke (Imangda Junction, Norilsk Region): Based on Compositional, Isotope and Model Data

Abstract

The Rudnaya dyke of the Imangda ore junction is composed of the weakly differentiated olivine-bearing to olivine gabbrodolerites with sulfide globules and disseminated sulfides of (pentlandite-pyrrhotite)-chalcopyrite-cubanite composition. Along with cogenetic sulfide mineralization, dyke’s gabbrodolerites contain xenoliths of hornfelsed basalts, abundant amygdales and rare grains of zoned Ol-1 Fo90-47 with 0.5–0.06 wt % NiO that coexist with subhedral olivine Fo74-36 of the second generation. Modeling in the COMAGMAT and alphaMELTS programs showed that high-Mg olivine 1 with Cr-spinel inclusions could not be crystallized from a Fe-enriched tholeiitic magma that is parental for the dyke with 4.8–7.3 wt % MgO and 11.6–16.7 wt % total Fe2O3. The trend of variations and high Ni up to 0.5 wt. % in the cores of xenocrystic olivine Fo90-76 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 picritic cumulates, which magma had not exchanged with sulfide liquid. Platinum group element (PGE) abundances increase (up to 2.2 ppm) with Cu/Ni in the whole rocks as well as with proportions of pentlandite in a sulfide association. A specific chalcophile metal distribution, which is characterized by Ni, Os and Ir minima, elevated Cu/Ni (5–15) and Cu/Pd (3200–10 900) as well as lower both PGE tenor of sulfides (2–65 ppm) and Pd content in pentlandite (175 ppm) compared to typical of ore-bearing intrusions, suggests that Cu-rich sulfide mineralization was not mechanically 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 due to assimilation of sedimentary sulfur and volatiles from Devonian evaporites in the dyke conduit 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 in sulfides from the endocontact zones of the ore-bearing Imangda intrusions hosted by Devonian strata. The initial isotopic characteristics of dyke’s rocks (Sri 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 even its spatial connection with an upper crustal conduit system of the ore-bearing magmas.