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.