Figure 2.

Plots of stable isotopes of aqueous sulfate. A) δ³⁴S_SO4 vs. δ¹⁸O_SO4; δ³⁴S of sulfide minerals shown as vertical bars; B) δ³⁴S_SO4 vs. dissolved copper concentration. Data points with error bars indicate non-detects for copper, for which symbols plotted at one-half of the method detection limit (MDL) and error bars extend upward to the MDL (0.003 mg L^-1). Enrichment of δ³⁴S_SO4 in residual mine pool is likely caused by preferential reduction of ³²SO₄ by sulfate-reducing bacteria, producing H₂S that scavenges aqueous copper from solution.