COPPER ISOTOPIC COMPOSITION OF STREAM WATER DOCUMENTS SULFIDE MINERAL DISSOLUTION DURING ACID MINE DRAINAGE

Important fluxes and mechanisms during environmental cycling may be documented by stable metal isotope ratios in natural and contaminated systems. New measurements of the Cu isotope composition of acid mine drainage reveal that this water is enriched relative to Cu sulfide minerals near Mineral Creek (CO, USA). We measured the δ⁶⁵Cu values of Cu sulfide minerals and stream waters in this drainage using a Finnigan Neptune multi-collector ICPMS. Local enargite and chalcopyrite display δ⁶⁵Cu values of 0.34 ± 0.04‰ (2σ) and 0.27 ± 0.04‰, respectively, which fall within the typical range of δ⁶⁵Cu values reported for primary continental Cu sulfides (about -1.5 to 1.75‰). The most acidic, metal-rich water in the study area flows from a mine adit, and is also the most isotopically enriched (1.71 ± 0.04‰). Assuming isotopic equilibrium, the calculated isotopic fractionation factor [αaq-min = (δ⁶⁵Cuaq+ 1000)/(δ⁶⁵Cu_min + 1000)] between Cu in the adit water and that in enargite (the most visibly abundant primary Cu sulfide in the area) is 1.001, which is equivalent to the αaq-min value estimated in our previous leaching experiments for chalcopyrite. A spring emanating from a mine tailings pile in the downstream study reach displays the lowest isotopic composition (1.34 ± 0.04‰). This relatively depleted signature may result from leaching of more weathered Cu mineral sources. Using the αaq-min value of 1.003 determined between aqueous Cu and chalcocite (a secondary Cu sulfide mineral) from our previous experiment, we predict that the isotopically depleted spring waters may have equilibrated with secondary Cu minerals with δ⁶⁵Cu values near -1.66‰. This value lies within the range of δ⁶⁵Cu values reported for secondary Cu minerals (about -2.0 to 3.5‰). Alternatively, the depleted signature of the spring may result from other mechanisms for preferential loss of ⁶⁵Cu. Given that the pH of the stream rises from about 3.0 near the adit to about 5.0 in the downstream reach, this fractionating process may be sorption of aqueous ⁶⁵Cu to surfaces. Experiments aimed to test these hypotheses are ongoing.