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

摘要

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 delta S-34(CDT) of 0 +/- 47 parts per thousand, typical for sulfides formed by magmatic processes in stockwork Mo systems. Pyrite from alteration scars has a wide range of delta S-34 values from 0.0 parts per thousand to -13.6 parts per thousand. Sulfate from the ore body has markedly positive delta S-34 (5-10 parts per thousand) accompanied by positive delta O-18(SO4) values (6-15 parts per thousand) reflecting equilibrium formation from magmatic fluids. Sulfates from alteration scars have delta S-34 values over a broad range, similar to alteration scar pyrites, from -10.6 parts per thousand to 0 parts per thousand and delta O-18(SO4) of 0 +/- 3 parts per thousand. Sulfates with fine grained, delicate, and euhedral mineral habits suggesting recent formation within the mine rock piles, have delta S-34 values similar to orebody pyrite and alteration scars but more negative delta O-18(SO4) values (-3 parts per thousand to -10 parts per thousand). 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. (C) 2007 Elsevier Ltd. All rights reserved.