EVALUATING THE RESOURCE POTENTIAL AND ENVIRONMENTAL CHARACTERISTICS OF MILL TAILINGS AT THE ABANDONED KATHERINE GOLD MINE, LAKE MEAD NATIONAL RECREATION AREA, ARIZONA (Invited Presentation)

Abandoned mine remediation is a formidable challenge for land managers, but the potential to recover resources by reprocessing mine waste during remediation offers the opportunity to offset some of the cost. The U.S. Geological Survey is conducting a multi-disciplinary study with the National Park Service (NPS) to better understand both the resource potential of mill tailings at the abandoned Katherine Au-Ag mine and to evaluate the environmental risks associated with remediation. The mine and mill, which operated from 1903 to 1943, are located within the Lake Mead National Recreation Area. A reconnaissance study by the U.S. Bureau of Mines in the early 1990s estimated a total mass of tailings of approximately 640,000 t averaging 0.344 g/t Au and 15.2 g/t Ag. Our evaluation of the tailings includes composite sampling for bulk geochemical, mineralogical, geometallurgical, and environmental characterization, grade-tonnage modeling, hyperspectral spectroscopy, and soil, streambed and surface water sampling. In addition, drone-based lidar is being used to refine tailings volume estimates. Ecosystem services valuation will consider the impacts of potential reprocessing and remediation on services, such as cultural resources, recreation, endangered species, and water supply. Preliminary results indicate that the tailings mineralogy, in order of decreasing abundance, is dominated by quartz, K-feldspar, albite, calcite, muscovite, and chlorite. Acid-base accounting results document alkaline paste pH (7.5-8.7), low acid-generating potential (< 0.9 kg CaCO₃/t), moderate acid-neutralizing potential (83.3-123.0 kg CaCO₃/t). Thus, acid-drainage risks are low. Despite the potential value of contained Au and Ag at the site, current regulations prohibit reprocessing mine waste on NPS land. The costs and logistics of potentially removing the material to an off-site location highlight the challenges faced by land managers and policy makers related to abandoned mine remediation.