Mercury Isotope Signatures as Tracers for Hg Cycling at the New Idria Hg Mine

Mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) of Hg isotopes provides a new tool for tracing Hg in contaminated environments such as mining sites, which represent major point sources of Hg pollution into surrounding ecosystems. Here, we present Hg isotope ratios of unroasted ore waste, calcine (roasted ore), and poplar leaves collected at a closed Hg mine (New Idria, CA, U.S.A.). Unroasted ore waste was isotopically uniform with delta Hg-202 values from -0.09 to 0.16 parts per thousand (+/- 0.10 parts per thousand, 2 SD), close to the estimated initial composition of the HgS ore (-0.26 parts per thousand). In contrast, calcine samples exhibited variable delta Hg-202 values ranging from -1.91 parts per thousand to +2.10 parts per thousand. Small MIF signatures in the calcine were consistent with nuclear volume fractionation of Hg isotopes during or after the roasting process. The poplar leaves exhibited negative MDF (-3.18 to -1.22 parts per thousand) and small positive MIF values (Delta Hg-199 of 0.02 to 0.21 parts per thousand). Sequential extractions combined with Hg isotope analysis revealed higher delta Hg-202 values for the more soluble Hg pools in calcines compared with residual HgS phases. Our data provide novel insights into possible in situ transformations of Hg phases and suggest that isotopically heavy secondary Hg phases were formed in the calcine, which will influence the isotope composition of Hg leached from the site.