INSIGHTS FROM PB ISOTOPES FOR THE ORIGIN OF PLACER GOLD IN RICH HILL, ARIZONA

The origin of native gold nuggets in placers is enigmatic as they can be mechanically separated from the primary vein deposit or can be formed in the placer environment. Pb isotopes can be a valuable tracer as a match in the isotopic ratios between vein and placer gold will indicate that the placer nuggets are simply mechanically separated from the hydrothermal vein. In this work we measured Pb isotopes in placer gold nuggets and compare the results to primary vein gold, sulfides, gangue minerals, and host rocks in the Rich Hill area, Arizona. Primary vein gold shows very similar Pb isotopic ratios to vein galena indicating that the ore metals were derived from the same source during an ancient hydrothermal event. All of the placer gold samples, however, show distinct lead isotopic compositions when compared to the hydrothermal vein source represented by the vein gold and galena. Apparently gold undergoes significant chemical evolution in the placer and is not simply mechanically separated from the hydrothermal vein. We envision three possible scenarios for explaining the Pb isotopic compositions of the nuggets, including diffusion, inclusions of placer minerals or coatings, and formation within the placer environment. Diffusion of sedimentary Pb in the nuggets is unlikely to play a major role as there is no Pb isotopic equilibrium between vein gold and gangue minerals. Pb isotope signal in the nuggets is not likely to be associated with surficial coatings or inclusions as before analyses all gold samples were cleaned overnight in hot nitric acid. Presence of placer minerals enclosed completely in the gold matrix will be supportive of growth in the placer environment. Furthermore, the placer gold Pb isotopes are overall far more radiogenic compared to the primary vein gold, sulfides, and even vein gangue minerals, suggesting that Pb in the nuggets was acquired relatively recently from the local sediments. The most likely scenario explaining the overall Pb isotopic systematics of the placer gold is a model supporting nugget growth in the placer by chemical and/or biological processes.