OXYGEN AND HYDROGEN ISOTOPIC 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%) that occur as a northwest-southeast striking dike. Olivine in the intrusion was variably altered to serpentine and magnetite. Interstitial pyroxene and minor plagioclase were altered to 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, sulfide minerals are replaced by serpentine and chlorite in amounts up to 30%. Oxygen and hydrogen isotopic values of silicate minerals suggest that at least two fluids were involved in the hydrothermal alteration. δ ¹⁸O values of plagioclase (1.1 to 3.1 per mil), amphibole (2.4 to 4.1 per mil, and serpentine (2.1 to 3.6 per mil) all suggest the involvement of a fluid with a δ ¹⁸O value between ~2 and 5 per mil at 300°-400°C. Olivine δ ¹⁸O values indicate that mantle signatures of 5 to 6 per mil are preserved in samples where the degree of serpentinization is less than 40%. As the intensity of serpentinization increases, olivine δ ¹⁸O values decrease to as low as 3.4 per mil. Amphibole δD values (-75 to -86 per mil) suggest a fluid δD value in the range of -10 to -45 per mil. These values are consistent with the involvement of evolved seawater that exchanged with mafic rocks during water-rock interaction . This hypothesis is consistent with a rift setting for the Jinchuan Intrusion. The wide range in serpentine δD values (-75 to -123 per mil and computed fluid δD -61 to -112 per mil) suggest either that serpentinization involved a mixture of evolved seawater and meteoric water, or more likely that serpentine preferentially continued to exchange hydrogen isotopes with late stage meteoric waters. The strong evidence for the importance of fluids in the subsolidus evolution of the Jinchuan deposit points out the need to understand secondary processes in magmatic systems. Mineral compositions, metal ratios, and radiogenic isotopic signatures may be perturbed by hydrothermal processes that may often be overlooked.