Nevertheless, a weak negative correlation (R2=−0.32R2=−0.32; Fig. 5) is observed locally between δ34S and δ66Zn in the streams of the Silverton area, in southern Colorado (Locations 1 through 10, Fig. 1). This may reflect the initial variation of δ66Zn in ore mineralization (e.g., sulfides) present in this area (e.g., Borrok et al., 2009). A similar relationship between δ34S and δ66Zn was previously observed for the hydrothermal sulfide deposits in Ireland (Wilkinson et al., 2005). Generally, early precipitating sulfide minerals incorporate lighter Zn PMPA leading to increases of δ66Zn in hydrothermal fluids (John et al., 2008). Consequently, sulfides precipitating in later stages are characterized by higher δ66Zn. It should be noted that in our water samples from the Silverton area, the δ34S of stream SO4 is slightly higher (−2.5 to +6.0‰+6.0‰) compared to the local sulfide mineralization (−6.9 to +2.5‰+2.5‰), suggesting some contribution of SO4 from dissolution of hydrothermal sulfate minerals (gypsum, anhydrite) with higher δ34S of +14.6 to +18.0‰+18.0‰ (Nordstrom et al., 2007). Therefore, our results imply that, in addition to potential kinetic Zn isotope fractionations during sulfide ore formation, there might be another process involving preferential uptake of lighter Zn isotopes into sulfate minerals and heavier Zn isotopes into sulfides. Based on our results, however, these fractionations would be relatively small (within 0.2‰) and would require further focused investigations to properly quantify.
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