Mineralogic and Stable Isotope Studies of Hydrothermal Alteration at the Jinchuan Ni-Cu Deposit, China

The Jinchuan intrusion hosts one of the largest magmatic Ni-Cu sulfide deposits in the world. Net-textured and disseminated pyrrhotite, pentlandite, and chalcopyrite are found in olivine-rich rocks (60–80 vol %) that occur as a northwest-southeast–striking dike. Olivine in the intrusion has been variably converted to mixtures of serpentine and fine-grained magnetite. Interstitial pyroxene and minor plagioclase have been altered to mixtures of chlorite, amphibole, epidote, clinozoisite, Na-rich feldspar, and lesser amounts of calcite. Although the interstitial net texture of sulfide minerals surrounding olivine grains is preserved, up to 30 vol percent of sulfide minerals are replaced by magnetite, serpentine, and chlorite. An external origin for Mg and Si needed for the isovolumetric replacement of sulfides cannot be discounted, but a local origin related to olivine and pyroxene alteration is favored. Serpentine and chlorite may be enriched in Cu and Ni, but the enrichment can account for less than 1 percent of the metals lost during the replacement of sulfide minerals by silicates and oxides. Some of the metals and sulfur that were removed from the net-textured sulfide assemblages have been redeposited on a centimeter scale in fine ribbons and veins within serpentine, chlorite, and amphibole. Sulfur isotope values of the pyrrhotite-pentlandite-chalcopyrite assemblage at Jinchuan fall in the range of −2 to 8 per mil, with over 80 percent of the values between −2 and 2 per mil. Immediate country rocks in the Jinchuan area are Precambrian granitoids, schists, and marbles with sulfur concentrations less than 100 ppm. Although the δ 34 S values of the main sulfide assemblage at Jinchuan are in the range that is characteristic of mantle-derived sulfur, assimilation of sulfur from unexposed country rocks cannot yet be discounted. Secondary pyrite occurs in veins and stringers, and rarely as a replacement of pyrrhotite, and is characterized by δ 34 S values between −7 and −27 per mil. The strongly negative δ 34 S values of secondary pyrite are thought to be a result of oxidation of primary magmatic sulfide assemblages and partial reduction of aqueous sulfate. Oxygen and hydrogen isotope values of silicate minerals suggest that at least two fluids have been involved in the hydrothermal alteration at Jinchuan. The δ 18 O values of plagioclase, amphibole, and serpentine all suggest the involvement of a fluid with a δ 18 O value between ~2 and 5 per mil at 300° to 400°C. The δD values of amphibole and one population of serpentine suggest a δD value for the fluid in the range of −45 to −63 per mil. These values are consistent with the involvement of evolved seawater or a metamorphic fluid in the early stages of alteration. The possible involvement of evolved seawater in hydrothermal alteration is consistent with a rift setting for the emplacement of the Jinchuan intrusion. However, the wide range in serpentine δD values (computed δD of the fluid = −45 to −103‰) suggest either that serpentinization involved a mixture of evolved seawater and/or metamorphic water and meteoric water or, more likely, that serpentine continued to exchange hydrogen isotopes with late-stage meteoric waters after it formed.