SULFIDE MINERAL SOLUTION AND REDEPOSITION AT THE JINCHUAN CU-NI DEPOSIT: CONSTRAINTS FROM STABLE ISOTOPIC STUDIES

    The Jinchuan Ni-Cu sulfide deposit occurs within an olivine-rich ultramafic body (~6000 x 300 x 1000 m) that intrudes Proterozoic metasedimentary (primarily marbles) and granitic igneous rocks.  Olivine has been extensively serpentinized and disseminated sulfide assemblages (pyrrhotite-chalcopyrite-pentlandite) have been partially replaced by silicate and oxide minerals.  Our preliminary sulfur isotopic measurements indicate the presence of three populations of sulfide: 1) the largest number of samples fall between -2 and 2 per mil VCDT, values that are similar to those that are expected for mantle-derived sulfur, 2) a second population shows a wide range of values from -7.2 to -21.7 per mil, and 3) a small number of samples show values from 3 to 7.3 per mil.  Petrographic analyses clearly show that the anomalously low and high d³⁴S values are associated with vein-forming pyrite, and not with the interstitial pyrrhotite-chalcopyrite mineralization.       Although we cannot rule out three distinct sulfur sources for the sulfide populations, we offer an alternative that is related to the hydrothermal alteration of the rocks.  Oxygen and hydrogen isotopic analyses of olivine and serpentine suggest that a low-¹⁸O, low-D fluid of probable meteoric water origin was involved in hydrothermal alteration.  Olivine and serpentine d¹⁸O values range from 2.1 to 5.5 per mil, and serpentine dD values vary from -90 to -124 per mil.  Dissolution of magmatic sulfide minerals by a fluid of intermediate oxidation state may cause the production of both aqueous sulfate and sulfide.  Our calculations indicate that at temperatures between 300° and 400°C (Δ sulfate-sulfide=19-30 per mil), the generation of fluids with sulfate/sulfide values between 0.5 and 0.9 may produce sulfate and sulfide d³⁴S values that range from 3 to 16 per mil, and -26 to -8 per mil, respectively.  Redeposition of this sulfur as pyritic vein material requires the reduction of sulfate (potentially by reaction with ferrous iron or carbonic material), but can lead to the anomalously positive and negative d³⁴S populations observed in the Jinchuan deposit.  The results confirm the importance of secondary processes in the interpretation of isotopic systematics in mafic-rock-related ore systems.