Laser Ablation ICP-MS Trace Element Fingerprinting of Placer Gold from Rich Hill, AZ: Geochemical Identification of at Least Three Generations of Distinct Placers.

Event Center A&B

Placer gold at Rich Hill, Arizona contains trace element distributions which provide a geochemical “fingerprint” that suggests the possible lode origins for the placer gold. There are three main placer units at Rich Hill: The lower-most “black placers,” the middle “white placers,” and the uppermost “red placers.” In addition, there are remnants of a special fourth unit, called the “potato patch,” which is much smaller in volume. The most prominent geochemical feature of the placer gold from the three main units is that it all exhibits leaching of silver, and enrichment of copper in the outermost rims, suggesting long transport or long exposure at the surface. Trends of geochemistry within the cores of individual placer gold grains show changing trace element chemistry over time. These trends suggest placer formation from the downward erosion of a single hydrothermal vein source with typical chemical zonation. The potato patch gold typically has its own unique geochemistry, suggesting origins from a separate lode source, or a more complex weathering history.