Evaluating a Mineralogical Control on Arsenic and Lead Concentrations in California Gold Mine Tailings

Neptune, Chad K. and DeGraff, Jerome
[email protected]

Abandoned gold mining operations in California often host tailings piles, which are a source of various heavy metal contaminants including arsenic (As) and lead (Pb). Based on internal USDA Forest Service studies, it has been determined that some tailings are a concern due to high As and Pb while others are only a concern for high As. The research hypothesis is that this difference reflects a mineralogical control on the presence and concentration of As and Pb. This information would be valuable in the prioritization of mining sites for mitigation, as identifying whether both As and Pb are a concern or only As is key in determining the level of risk posed by the tailings. Ore from two mines (Bright Star and May‐Lundy) in the Sierra Nevada provided a preliminary test of this hypothesis. Samples were collected from presumed ore found in proximity to mine adits or milling sites. A biased sampling method, based on the presence of clearly visible concentrations of metal sulfide minerals, served as a selection approach. Prior to lab processing, the samples were evaluated for their proportion of metal sulfide minerals to non‐metallic minerals, to establish the range of variability at each mine site. A Gyral grinder was used to reduce samples to particles of less than 149 microns in size. The samples were then analyzed with a Niton XL3t model X‐ray fluorescence (XRF) device for a one‐minute interval. Based on this initial sampling, it is suggestive that the ratio of Pb/As, in the ore material reflects the concentration ratios within the tailings at the respective mine sites. This method assumes that a whole rock analysis is indicative of the proportion of As to Pb bearing minerals present.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013