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The Kharaelakh intrusion is one of several sill-like, multi-phase gabbroic intrusions that host world-class Ni-Cu-PGE deposits in the Noril'sk-Talnakh region. The sulfide ores in the Kharaelakh intrusion are characterized by high δ34S values (10-12‰) and high PGE concentrations (∼7 ppm Pt, ∼25 ppm Pd). The high δ34S values require addition of external S with elevated isotope ratios such as anhydrite (δ34S∼20‰) in the footwall strata and a low R-factor (magma/sulfide mass ratio <500) during sulfide segregation, whereas the high PGE concentrations require extremely high R-factors (>2000) if the concentrations of Pt and Pd in the magma are similar to the levels in the un-depleted basalts (<10 ppb) that are spatially and temporarily associated with the ore-bearing intrusions. Such a dilemma can be explained by complete sulfide resorption by a new flux of mantle-derived magma into a deep staging chamber to form a PGE-enriched, sulfide under-saturated magma. Interaction of the PGE-enriched, sulfide under-saturated magma with anhydrite at higher crustal levels could have produced sulfide liquids with high PGE concentrations as well as high δ34S values. This new model is supported by the occurrence of anhydrite-sulfide assemblages in the Kharaelakh intrusion. In situ analysis by ion probe reveals that both anhydrite and sulfide have elevated S isotopic compositions. The δ34S values of the anhydrite crystals and co-existing sulfides range from 18 to 22‰ and from 9 to 11‰, respectively. The sulfate-sulfide Δ values are from 9 to 12. These values correspond to equilibrium temperatures of 1100-800oC, overlapping the temperatures of magmatic sulfide liquids. In addition to the anhydrite-sulfide assemblages, isolated anhydrite crystals with δ34S values ranging from 16 to 19‰ are also present in the samples. The isolated anhydrite crystals occur with interstitial silicates such as biotite. Some of the anhydrite crystals contain clinopyroxene inclusions. The textural relations suggest that the magma became saturated with both sulfide and sulfate after assimilating small amounts of anhydrite-bearing evaporites by chemical dissolution. Mass balance calculations indicate that 1-1.5 wt% of anhydrite assimilation is required to explained the S isotopic compositions of the sulfide ores in the Kharaelakh intrusion. Available experimental results from the literature indicate that the oxidation states at which both sulfide and sulfate can coexist are between FMQ+1.5 and FMQ+2. If FeO in the magma was the only reducing agent, the initial magma should not be more oxidized than FMQ-2 in order for both sulfide and sulfate to coexist in the contaminated magma. The required initial oxidation states of the magma are at the lower end of submarine basalts. However, other reducing agents such as graphite in black shales and CH4 in sour gas that are common in the footwall strata may have also been involved, hereby relaxing the required oxidation state for the initial magma.
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