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 cons train elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Altho ugh Au and As are covariant in arsenian pyrite, Au apparently was depl eted much more rapidly from the hydrothermal solutions than was As. Se nsitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyse s of iron sulfides from the Post/Betze deposit vary widely from deltaS -34 = -29 to 23 parts per thousand and provide important information o n 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 seawat er-derived sulfate, possibly bedded barite. Late-stage ore fluids (del taS-34(sulfide) = - 12 to -29 parts per thousand) are extremely deplet ed 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 gen erated hypothetically from the original ore fluids by oxidation (possi bly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having l ow isotopic values.