Geochemical and Mineralogical Controls on Metal(loid) Mobility in the Oxide Zone of the Prairie Creek Deposit, NWT

Prairie Creek is an unmined high grade Zn-Pb-Ag deposit in the southern Mackenzie Mountains of the Northwest Territories, located in a 320 km² enclave surrounded by the Nahanni National Park reserve. The upper portion of the quartzcarbonate-sulphide vein mineralization has undergone extensive oxidation, forming high grade zones, rich in smithsonite (ZnCO₃) and cerussite (PbCO₃). This weathered zone represents a significant resource and a potential component of minewaste material. This study is focused on characterizing the geochemical and mineralogical controls on metal(loid) mobility under mine waste conditions, with particular attention to the metal carbonates as a potential source of trace elements to the environment. Analyses were conducted using a combination of microanalytical techniques (electron microprobe, scanning electron microscopy with automated mineralogy, laser-ablation inductively-coupled mass spectrometry, and synchrotron-based element mapping, micro-X-ray diffraction and micro-X-ray absorbance) and the elements of interest included Zn, Pb, Ag, As, Cd, Cu, Hg, Sb and Se. Results include the identification of minor phases previously unknown at Prairie Creek, including cinnabar (HgS), acanthite (Ag₂S), metal arsenates, and Pb-Sb-oxide. Anglesite (PbSO₄) may also be present in greater proportions than recognized by previous work, composing up to 39 weight percent of some samples. Smithsonite is the major host for Zn but this mineral also contains elevated concentrations of Pb, Cd and Cu, while cerussite hosts Zn, Cu and Cd, with concentrations ranging from 6 ppm to upwards of 5.3 weight percent in the two minerals. Variable concentrations of As, Sb, Hg, Ag, and Se are also present in smithsonite and cerussite (listed in approximately decreasing order with concentrations ranging from <0.02 to 17 000 ppm). A significant proportion of the trace metal(loid)s may be hosted by other secondary minerals associated with mineralization. Processing will remove significant mineral hosts for these elements from the final tailings, although some may remain depending on whether the smithsonite fraction is left as tailings. Significant Hg and Ag could remain in tailings from cinnabar and acanthite that is trapped within smithsonite grains, which were found to act as a host for up to 53% of the Hg and 79% of the Ag contained in some samples. In a mine waste setting, near-neutral pH will encourage retention of trace metal(loid)s in solids. Regardless, oxidation, dissolution and mobilization is expected to continue in the long term, which may be slowed by saturated conditions, or accelerated by localized flow paths and acidification of isolated, sulphide-rich pore spaces.