TRACE ELEMENT GEOCHEMISTRY AND METAL MOBILITY OF OXIDE MINERALIZATION AT THE PRAIRIE CREEK ZINC-LEAD-SILVER DEPOSIT, NWT
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The Prairie Creek Deposit is an unmined high grade Zn-Pb-Ag deposit located in the southern Mackenzie Mountains of the Northwest Territories. The site is confined within the boundaries of the Nahanni National Park Reserve and is upstream of a UNESCO World Heritage Site. The upper portion of the primary quartz-carbonate-sulphide vein mineralization has undergone extensive oxidation, forming zones rich in smithsonite (ZnCO3) and cerussite (PbCO3). This weathered zone represents a significant resource and a potential component of mine waste material. This research is focused on the characterization of the geochemical and mineralogical controls on metal mobility at Prairie Creek, with particular attention to the metal carbonates as a host for trace elements and their role in sequestering those elements in mine waste conditions. Detailed mineralogical analyses were conducted using a combination of SEM, EMP, MLA, LA-ICP-MS, and synchrotron-based µXRD, µXRF and µXANES techniques. Smithsonite was found to contain concentrations of Ca, Mg, Fe, Cu, Pb, Cd, Sb, Hg, As, Mn, Ag and Se in approximately decreasing order, whereas cerussite (expected to be removed as Pb concentrate) hosts Zn, Cu, Sb, Ag, Hg, Fe, Ca, Cd, As, Mn and Se. A significant proportion of the trace metals may also be attenuated by other secondary minerals. This study has identified previously unreported minor phases, including cinnabar (HgS), acanthite (Ag2S), bindheimite (Pb2Sb2O6(O,OH)), and multiple metal arsenates. Anglesite (PbSO4) may be present in greater proportions than is indicated by previous work. Ore processing will remove most Pb and Zn sulphides as well as cerussite; however, smithsonite will remain in tailings as a major source for most metals. Mobilized Pb may be released primarily from anglesite, with As coming from arsenates, Ag from acanthite and anglesite, Sb from bindheimite, Fe from smithsonite and goethite, Cu from smithsonite, anglesite, malachite, azurite and covellite, and Hg and Se from a combination of smithsonite, anglesite, cinnabar and arsenates. In a mine waste setting, near-neutral pH will favour precipitation and attenuation of trace metals. Regardless, oxidation, dissolution and mobilization is expected to continue at a slow rate, and changes to more acidic or reducing environmental conditions could significantly increase metal release.