Geochemical Overprinting and Secondary Placer Crystal Formation in the La Cholla District, Quartzsite, Arizona, USA: Evidence from Copper Isotopes, Morphology, and Trace Elements

Abstract

Geochemical overprinting on placer gold deposits produces complex deposits with apparently paradoxical geochemical and morphological attributes. The La Cholla placer gold deposits near Quartzsite, Arizona, USA, contain sharp and often undamaged gold crystals within high-energy gravels as a result of postdepositional recrystallization by localized hot springs. Geothermal fluids emanated from a basin-bounding fault and left a distinct signature that includes recrystallized gold with low Pb and δ65Cu > 1.14‰, secondary uraninite and carnotite, travertine deposits, and mounds of highly silicified placer gravels. Surrounding these deposits are placer gravels with angular gold morphology and geochemistry indicative of flash-flood pulse placers with a lode gold source < 5 km and gold particles with low Pb and δ65Cu < 0.77‰. The multitude of small local lode gold source veins is likely related to specialized fluid migration along mid-Tertiary detachment faults. The unroofing of these veins at 5 to 17 Ma by high-angle Basin and Range faulting led to extensive placer development, with subsequent localized modification by geothermal fluid migration along the same faults. This study documents geochemical overprinting and modification of placer deposits and bears upon the study of other placers containing crystalline gold and uranium mineralization. Similar deposits may document past geothermal activity and identify structural fluid conduits and steep paleo-topographic gradients.