ION MICROPROBE DETERMINATION OF SULFUR ISOTOPE VARIATIONS IN IRON SULFIDES FROM THE POST BETZE SEDIMENT-HOSTED DISSEMINATED GOLD DEPOSIT, NEVADA, USA

Secondary ion mass spectrometric analyses of ore samples from the Post/Betze sediment-hosted disseminated gold deposit were utilized to constrain elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Although Au and As are covariant in arsenian pyrite, Au apparently was depleted much more rapidly from the hydrothermal solutions than was As. Sensitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyses of iron sulfides from the Post/Betze deposit vary widely from deltaS-34 = -29 to 23 parts per thousand and provide important information on the origin of sulfur and constraints on depositional mechanisms. Ore solutions had high deltaS-34 values and were most likely derived, at least in part, from thermochemical reduction of lower Paleozoic seawater-derived sulfate, possibly bedded barite. Late-stage ore fluids (deltaS-34(sulfide) = - 12 to -29 parts per thousand) are extremely depleted in S-34 relative to main-stage ore fluids (deltaS-34(sulfide) = 16-23 parts per thousand). Although such low deltaS(34) values can be generated hypothetically from the original ore fluids by oxidation (possibly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having low isotopic values.