UNRAVELING AQUEOUS SULFATE STABLE ISOTOPES FROM PYRITE OXIDATION AND GYPSUM DISSOLUTION IN THE UPPER ANIMAS DRAINAGE BASIN, SAN JUAN MOUNTAINS, COLORADO

Combined results from aqueous major-ion chemistry and sulfur isotopes in the upper Animas River basin show three dominant processes: 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 (δ³⁴S in pyrite and δ³⁴S and δ¹⁸O 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 (δ³⁴S(SO₄)= 15 to 18 per mil and δ¹⁸O(SO₄)= -3 to 5 per mil). Pyrite is significantly lighter (δ³⁴S= –7 to 2.5 per mil). Gypsum and anhydrite dissolution and pyrite oxidation are the dominant sources of dissolved sulfate. Distinct trends of mixing between water dominated by gypsum/anhydrite dissolution and water dominated by pyrite oxidation are observed. An indistinct tendency for aqueous sulfate in water samples from unmined areas to have slightly higher δ¹⁸O relative to aqueous sulfate in water from mined areas is also apparent. A distinct relationship between the δ¹⁸O(SO₄)values for pyrite oxidation-dominated waters and the δ¹⁸O in water is not apparent, although the 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 can confound the interpretation of oxygen isotopic data from aqueous sulfate in mineralized and mined areas.