Aqueous-Sulfate Stable Isotopes-A Study of Mining-Affected and Undisturbed Acidic Drainage
2007
The combined results from major-ion chemistry and sulfur isotopes show three dominant processes affecting water quality: gypsum and anhydrite dissolution, pyrite oxidation, and calcite dissolution. This conclusion is based on more than one hundred samples including water, pyrite, gypsum, and anhydrite, collected in the Animas River watershed study area and analyzed for stable sulfur and oxygen isotopes (δ 34 S in pyrite and δ 34 S and δ 18 O in aqueous sulfate and sulfate minerals). Gypsum and anhydrite are shown to be dominantly hypogene (hydrothermal) in origin with heavy sulfur and oxygen isotopic compositions (δ 34 S SO4 =15 to 18 per mil and δ 18 O SO4 = -3 to 5 per mil). Pyrite is significantly lighter (δ 34 S= -7 to 2.5 per mil). Gypsum and anhydrite dissolution and pyrite oxidation are the dominant sources of dissolved sulfate in these waters. Distinct trends of mixing between water dominated by gypsum/anhydrite dissolution and water dominated by pyrite oxidation are inferred from the data. A tendency for aqueous sulfate in water samples from unmined areas to have slightly higher δ 18 O relative to aqueous sulfate in water from mined areas is also apparent in the data, although there is considerable dispersion in the values, with several overlapping data points. Part of this enrichment is caused by differences in isotopic composition originating from aqueous sulfate derived from different locations and alteration types. Another possible reason for this small enrichment in δ 18 O in aqueous sulfate in pyrite oxidation-dominated waters might be the greater production of ferric iron in mined environments that leads to greater utilization of oxygen from the water molecule rather than molecular oxygen from air in the formation of aqueous sulfate. A distinct relationship between the δ 18 O SO4. values for pyrite oxidation-dominated waters and the δ 18 O 4 in water is not apparent, although the δ 18 O SO4 values are generally in a range that agrees with laboratory experiments. Evaporation, mixing of pyrite oxidation-dominated water with gypsum/anhydrite-dominated water, and dilution or mixing of water subsequent to pyrite oxidation are all important processes that can confound the interpretation of oxygen isotopic data from aqueous sulfate in mineralized and mined areas.
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