Isotopic and textural discrimination between hypogene, ancient supergene, and modern sulfates at the Questa mine, New Mexico

Chemical and mineralogical changes due to pyrite weathering are of interest with respect to understanding long-term physical stability of mine rock piles at the Questa mine, New Mexico. The ability to discriminate between ancient and modern processes is important for establishing the extent of modern weathering within the piles. Initial inventories of sulfur minerals and representative isotope compositions in rocks from orebodies, the hydrothermal alteration zones associated with orebodies, hydrothermal alteration scars, and mine rock piles were determined. Ore body sulfides have δ34SCDT of 0 ± 4‰, typical for sulfides formed by magmatic processes in stockwork Mo systems. Pyrite from alteration scars has a wide range of δ34S values from 0.0‰ to −13.6‰. Sulfate from the ore body has markedly positive δ34S (5–10‰) accompanied by positive δ18OSO4 values (6–15‰) reflecting equilibrium formation from magmatic fluids. Sulfates from alteration scars have δ34S values over a broad range, similar to alteration scar pyrites, from −10.6‰ to 0‰ and δ18OSO4 of 0 ± 3‰. Sulfates with fine grained, delicate, and euhedral mineral habits suggesting recent formation within the mine rock piles, have δ34S values similar to orebody pyrite and alteration scars but more negative δ18OSO4 values (−3‰ to −10‰). Sulfates from all three sources occur in these piles, and their stable isotope values have proven useful in differentiating them and their environments of formation (i.e., hypogene, ancient supergene, and recent weathering). Correlating the isotopic compositions with textures allows petrographic assessment for the origins of sulfate minerals in the rock piles, but this must be applied with caution because some sulfate mineral recycling has occurred.