Abstract

The origin of gold nuggets in placers is enigmatic as they can be mechanically separated from the primary vein deposit or can be of secondary origin, 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 and unmodified by weathering.

Reconnaissance elemental analyses revealed that vein and placer gold from the Rich Hill district, Arizona, contain up to thousands of ppm Pb and therefore both are suitable for Pb isotope analyses. We measured Pb isotopes in placer gold nuggets and compare the results to primary vein gold and sulfides and also gangue minerals and host rocks in the Rich Hill area. Primary vein gold and galena show similar Pb isotope ratios with 206Pb/204Pb ranging from 18.364 to 18.486, 207Pb/204Pb from 15.574 to 15.593, and 208Pb/204Pb from 37.150 to 37.359. This indicates that the ore metals were derived from a similar source during a hypogene hydrothermal event in the geologic past. Present-day Pb isotope compositions of local rocks in the area show distinct values compared to the primary vein mineralization. Age-corrected and modeled Pb isotope compositions for local magmatic rocks do not provide a clear picture for a possible hydrothermal ore metal source due to possible alteration processes and the long time elapsed since the mineralization event. The distinct present-day Pb isotopes in local lithological units, however, provide an excellent tracer for deciphering the origin of the placer gold.

All the placer gold nuggets show very distinct Pb isotope compositions when compared to their respective hydrothermal vein source represented by the vein gold and galena. The placer gold Pb isotopes show very wide variations, with 206Pb/204Pb ranging from 17.985 to 20.057, 207Pb/204Pb from 15.494 to 15.749, and 208Pb/204Pb from 36.863 to 38.606. The placer nuggets follow the present-day Pb isotope trend observed in rocks and sediments in the area. In addition, the Pb isotopes in majority of the placer gold nuggets are far more radiogenic compared to the vein gold, sulfides, and even gangue minerals. This is indicating that a significant part, if not all, of the Pb in the placer gold was relatively recently acquired from the placer sediments. The observed distinct Pb isotope compositions between primary and placer samples suggest that gold undergoes significant chemical changes during the supergene stage. Considering a number of possible scenarios, the most plausible explanation for the observed present-day Pb isotope similarity between placer gold and local rocks is formation of a significant part of the total mass of these nuggets within the placer environment.

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