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.
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Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the