STABLE ISOTOPE DISCRIMINATION OF HYPOGENE AND SUPERGENE SULFATE MINERALS IN ROCK PILES AT THE QUESTA MOLYBDENUM MINE, NEW MEXICO

Chemical and mineralogical changes due to pyrite weathering are of interest with respect to understanding long-term geotechnical stability of mine rock piles at the Questa Moly mine. The ability to discriminate between ancient and modern processes is important for establishing the extent of weathering within the piles. An initial inventory of sulfur minerals was established for end member isotope compositions in rocks from ore, the hydrothermal alteration envelope, hydrothermal alteration scars, and mine rock piles. Ore body sulfides vary by 1 permil about an average value of 0 permil δ³⁴S ,typical for magmatic processes in stockwork molybdenum systems. Pyrite from alteration scars tends to be slightly more negative (0.0 to –4.0 permil) and may reflect a different mode of hydrothermal origin. Sulfate from the ore body has markedly positive δ³⁴S (5-10 permil) accompanied by positive δ¹⁸0_SO4 values (6-15 permil) reflecting magmatic values. Sulfates from alteration scars have δ³⁴ values from –5 to 0%o and δ¹⁸0_SO4 of 0±3 permil. Sulfates with specific mineral habits that suggest formation within the pile have δ34S values similar to the ore body pyrite and more negative δ¹⁸0_SO4 values (-3 to -10 permil). Sulfates found in the piles fall into all three categories; hypogene, supergene, and recent weathering, thus demonstrating that stable isotope analysis is a useful tool in differentiating the environment of formation of sulfates. Correlating the isotopic compositions with mineral textures allows for petrographic assessment of the origins of sulfate minerals in the rock pile.