The Giant Jinchuan Ni-Cu-(PGE) Deposit: Tectonic Setting, Magma Evolution, Ore Genesis, and Exploration Implications
Chusi Li, Edward M. Ripley, 2011. "The Giant Jinchuan Ni-Cu-(PGE) Deposit: Tectonic Setting, Magma Evolution, Ore Genesis, and Exploration Implications", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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The Jinchuan Ni-Cu-(PGE) deposit is the largest single magmatic Ni deposit in the world. It is hosted by a small ultramafic intrusion in the Longshoushan terrane located in the western part of the North China craton. Phase equilibrium analysis using available whole-rock and mineral chemical data confirms that the parental magma of the intrusion is of high Mg basaltic composition, but the primary magma may have contained up to 18.5 wt percent MgO. The involvement of a long-term enriched subcontinental lithosphere mantle (SCLM) source is inferred from high, negative εNd values (–6 to –12). Small amounts (mostly 5–15%) of crustal contamination are suggested by Sr-Nd isotopes. Positive γOs values (20–150) are consistent with selective assimilation of crustal sulfide, but evidence from sulfur isotopes is inconclusive. The δ34S values of most samples (∼80%) from the Jinchuan deposit vary between –2 and +2 per mil, which is within the range that is characteristic of mantle-derived sulfur. Analytic modeling suggests that both fractional crystallization and crustal contamination played a role in triggering sulfide saturation in the Jinchuan magmatic system. Lower olivine/sulfide ratios in the orebodies than the cotectic ratio suggest that material sorting by flow differentiation and gravitational settling was important during ore formation. A low amount of trapped silicate liquid in the intrusion (<30%) can be best explained by loss of liquid to the peripheral sills or dikes of the Jinchuan magma plumbing system. Lower PGE tenors in the Jinchuan deposit relative to those of sulfide liquids expected to segregate from PGE undepleted high Mg basaltic magma are consistent with previous sulfide segregation at depth. Recent U-Pb zircon-baddeleyite dating has shown a crystallization age of ∼830 Ma for the Jinchuan intrusion. The new age indicates that the emplacement of the Jinchuan intrusion was contemporaneous with the initial stage of Rodinia breakup. Some researchers have suggested that the breakup of the Rodinia supercontinent was triggered by a hypothetical super mantle plume located beneath the Yangtze craton and that the Jinchuan intrusion and country rocks are from the Yangtze craton. However, regional stratigraphic correlation indicates that the Longshoushan terrane was part of the North China craton prior to Rodinia breakup. This suggests that the Jinchuan mafic-ultramafic magmatism took place in the North China craton, not in the Yangtze craton, and that regional exploration for the Jinchuan-type deposits should focus on the western part of the North China craton instead of the Yangtze craton.
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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