REDOX CHANGES CAUSED BY EVAPORITE AND CARBON ASSIMILATION AT NORIL'SK AND THEIR ROLE IN SULFIDE PRECIPITATION

Despite the worldwide importance of the Noril'sk district (Russia) as a source of Ni, Cu, Pd, and Pt, the processes involved in the formation of the massive sulfides in the district remain poorly understood. The heavy S isotopic compositions of the ores have been used to suggest that significant amounts of S were derived from anhydrite in the country rocks, but assimilation of anhydrite alone would oxidize the magma beyond the stability of sulfides. It has been suggested that carbon assimilation (present in the host rocks as coal seams), in addition to anhydrite, would keep the system reduced and provide a viable mechanism for sulfide saturation. To test this hypothesis, we analyzed three hypothetical scenarios for magma contamination: (a) sequential assimilation of anhydrite and carbon; (b) simultaneous assimilation of anhydrite and carbon; (c) sequential assimilation of carbon and anhydrite. In all three scenarios, carbon-mediated sulfate reduction produces significant amounts of CO₂, likely resulting in exsolution of a CO₂-rich volatile phase. Experiments simulating assimilation of anhydrite and carbon formed gas-rich vesicles with nucleation of sulfides on their surfaces. Some Noril'sk massive ores contain vesicles (now filled with secondary phases) analogous to those produced in the experiments. These textures support the hypothesis of anhydrite and coal assimilation and suggest that sulfate saturation occurred at or near the final level of magma emplacement because transport from deeper locations would cause separation of sulfides and vesicles due to their density contrast. The high Fe³⁺ contents of spinels in the host rocks are also consistent with significant oxidation of the magmas at a relatively early stage in their evolution. Based on these lines of evidence, we suggest that sulfide saturation occurred as a two-stage process involving assimilation of anhydrite, which oxidized the magma (significantly increasing S-carrying capacity), followed by assimilation of carbon, which reduced the magma (significantly reducing the S-carrying capacity and triggering sulfide saturation). This model explains the heavy S in Noril'sk ores, their close association with evaporites and coal seams, and the anomalously large amount of sulfide in the mineralized intrusions.