The Pechenga Ni-Cu Sulfide Deposits, Northwestern Russia: A Review with New Constraints from the Feeder Dikes
Eero J. Hanski, Zhen-Yu Luo, Harry Oduro, Richard J. Walker, 2011. "The Pechenga Ni-Cu Sulfide Deposits, Northwestern Russia: A Review with New Constraints from the Feeder Dikes", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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The Paleoproterozoic, synvolcanic Ni-Cu sulfide deposits at Pechenga are hosted by conformable, sill-like ferropicritic differentiated intrusions, injected into carbonaceous and sulfidic graywackes and shales of the Productive Formation. Due to the presence of abundant sulfides in the country rocks, assimilation of S-rich sedimentary material has commonly been attributed as the most significant factor that triggered sulfide immiscibility and led to the formation of the Pechenga ores. Several Ni-Cu sulfide prospects are associated with a ferropicritic dike system that transects the thick pillow lava succession of the Kolosjoki Volcanic Formation underlying the mentioned sedimentary unit, showing that the magma was saturated in sulfide prior to reaching the stratigraphic level where pyritic black shales occur. Our new rhenium-osmium isotope data from the Pahtajärvi prospect (yOs in the range of +52 to +69) reveal that a significant component of radiogenic Os was present in the magma. This together with new S isotope data is compatible with the Pahtajärvi ultramafic dike acting as a feeder conduit to ore-producing magma chambers in the upper part of the Productive Formation. Our results and other evidence, indicating potential nonradiogenic osmium of seawater in the basin where sediments of the Productive Formation were deposited, requires that the current model invoking country rocks as the main source of sulfur and radiogenic osmium in the Ni-Cu deposits needs to be reevaluated. Exogenic sulfur from Archean supracrustal rocks is not supported by the absence of mass-independent fractionation of sulfur isotopes in the Pahtajärvi sulfides.