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University of Toronto, Department of Geology, Toronto, ON, Canada
The Ni-Cu-PGE ore deposits of the Noril'sk region have developed in feeder channels for Permo-Triassic continental flood basalt volcanism, particularly within 100- to 300-m-thick, elongate, stratified intrusions that merge with an extensive system of peripheral sills, through which the magma is thought to have continued to surface. The mineralization is centered within two ore junctions, the Noril'sk and Talnakh ore junctions. Principal ore types include massive ore, which occurs typically below or at the lowest margin of the intrusions, disseminated ore in picritic and taxitic gabbrodolerite, which constitute the lower part of the intrusions, and copper ore which either forms an impregnation in hornfelsic country rock adjacent to massive ore or occupies the matrix of breccia zones that have developed at the front and upper contact of the intrusions. Although showing a close spatial relationship to the intrusions, the massive ore, particularly that underlying the Northwest Talnakh intrusion, appears to have been injected separately. Over 500 samples have been analyzed for Ni, Cu, S, Pt, Pd, Rh, Ru, Ir, Os, and Au by NiS fire assay, acid dissolution, and neutron activation. The massive ore of the Kharayelakh orebody beneath the Northwest Talnakh intrusion has fractionated as injection proceeded from east to west, with the fractionated liquid moving progressively farther west. This liquid has also soaked into the surrounding hornfels to produce the Cu-rich mantle, and above the western part of the Kharayelakh orebody, where the Cu enrichment is most pronounced, has also affected the lower part of disseminated ore zones within the intrusion. Study of the disseminated ore has revealed that this has not fractionated to a great degree and that adequate sampling gives an average composition close to that of the primary sulfide liquid. Disseminated ore from different parts of the Talnakh ore junction are characterized by very different metal tenors, with Rh ranging from 4,000 to 10,000 ppb. The composition of the disseminated ore is modeled as the result of initial low tenor sulfides becoming trapped in the ore-bearing intrusions, and then interacting with successive surges of new magma passing along the feeder channels represented by their host intrusions on the way to the surface. The different tenors of the different ore junctions and parts of ore junctions are the consequence of the effective number of times the magma passing through a particular channel exceeds the amount of sulfide within the channel, higher ratios leading to higher tenors.
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