Small-scale studies of roasted ore waste reveal extreme ranges of stable mercury isotope signatures
Résumé
Active and closed Hg mines are significant sources of Hg contamination to the environment, mainly due to large volumes of mine waste material disposed of on-site. The application of Hg isotopes as source tracer from such contaminated sites requires knowledge of the Hg isotope signatures of different materials potentially released to the environment. Previous work has shown that calcine, the waste residue of the on-site ore roasting process, can exhibit distinct Hg isotope signatures compared with the primary ore. Here, we report results from a detailed small-scale study of Hg isotope variations in calcine collected from the closed New Idria Hg mine, San Benito County, CA, USA. The calcine samples exhibited different internal layering features which were investigated using optical microscopy, micro X-ray fluorescence, micro X-ray absorption spectroscopy (mu-XAS), and stable Hg isotope analysis. Significant Fe, S, and Hg concentration gradients were found across the different internal layers. Isotopic analyses revealed an extreme variation with pronounced isotopic gradients across the internal layered features. Overall, delta Hg-202(+/- 0.10 parts per thousand, 2 SD) describing mass-dependent fractionation (MDF) ranged from -5.96 to 14.49 parts per thousand, which is by far the largest range of delta Hg-202 values reported for any environmental sample. In addition, Delta Hg-199 (+/- 0.06 parts per thousand, 2 SD) describing mass-independent fractionation (MIF) ranged from -0.17 to 0.21 parts per thousand. The mu-XAS analyses suggested that cinnabar and metacinnabar are the dominant Hg-bearing phases in the calcine. Our results demonstrate that the incomplete roasting of HgS ores in Hg mines can cause extreme mass-dependent Hg isotope fractionations at the scale of individual calcine pieces with enrichments in both light and heavy Hg isotopes relative to the primary ore signatures. This finding has important implications for the application of Hg isotopes as potential source tracers for Hg released to the environment from closed Hg mines and highlights the need for detailed source signature identification. (C) 2014 Elsevier Ltd. All rights reserved.