IN-SITU QUANTITATIVE-ANALYSES FOR PGE AND AU IN SULFIDE MINERALS OF THE JINCHUAN NI-CU DEPOSIT BY ACCELERATOR MASS-SPECTROMETRY

Pyrrhotite, pentlandite and chalcopyrite from the Jinchuan Ni-Cu depos it, China, have been analyzed for the first time for all PGE and Au, u sing in situ accelerator mass spectrometry (AMS). Trace amounts of Pt (0.07-2.2 ppb) were detected in all three minerals. These contents are much lower than the whole-rock concentrations of Pt, which attain sev eral ppm. Iridium content of the minerals falls between 1.7 and 37 ppb , which is generally less than half of the whole-rock concentrations. Os, Ru and Rh contents in these sulfide minerals increase in the order : chalcopyrite, pyrrhotite, pentlandite. Pentlandite can normally acco unt for up to 46% of the Os. Ru and Rh of the sulfide ores, and the ab undances of these elements in pentlandite increase with those of the w hole rock. Pentlandite is enriched in Pd (122-6362 ppb) by up to 20 ti mes the whole-rock concentrations. A good linear relationship is obser ved between the Pd content of pentlandite and that in 100% sulfide, su ggesting that equilibrium was reached during the exsolution of pentlan dite from monosulfide solid-solution (Mss), and that Pd is structurall y bound. Pyrrhotite and chalcopyrite contain much less Pd than pentlan dite and the whole rock, indicating that they are not important Pd car riers. Au invariably is low in the sulfide minerals, probably reflecti ng its preference for the liquid phase during crystallization of the s ulfide melt. Enrichment of Pd, Os, Ru and Rh, and depletion of Pt, Au and Ir, in the sulfide minerals suggest that the first four PGE probab ly existed as metal sulfides, which tend to enter the Mss, whereas the other three elements remained as stable metals that preferred the liq uid during the crystallization of sulfide melt. The data have also dem onstrated the great potential of accelerator mass spectrometry in the study of the geochemistry of PGE and the metallogenesis of Ni-Cu-PGE s ulfide deposits.