Climate-induced geochemical and morphological evolution of placer gold deposits at Rich Hill, Arizona, USA

Abstract

Placer gold deposits can serve as a wet/dry paleoclimate indicator because they preserve a record of physical and chemical modification. We hypothesized that such a record would be present across the four main placer units at Rich Hill, Arizona, USA. The oldest placer unit, a paleo-erosional surface on a modern topographic high, records moderate precipitation and erosion with modest transport distance following 25–22 Ma unroofing of the lode gold source. By 17–5 Ma, high-angle Basin and Range faulting produced a shallow basin that preserved three additional placer units. The oldest is a thin gold-rich gravel within bedrock gravity traps, suggestive of a steep gradient and abundant nonseasonal precipitation. The middle unit has well-rounded gold nuggets with deep chemical weathering that record abundant nonseasonal precipitation, but with a less steep gradient and significant fluvial sediment deposition. The uppermost unit is a pulse placer unit deposited by a series of landslides and debris flows during a period of lower, seasonal precipitation. During this dry period, and continuing to the present, microbial communities may have been established within the seasonally wet bedrock traps of the lowermost placer unit. This resulted in biological modification of placer gold chemistry and production of Mn-Ba–oxide biomats, which have coated and cemented both placer gold and sediments. This record of physical and chemical change within a sequence of placer gold units is consistent with known Arizona paleoclimate conditions, and it demonstrates the potential utility of this technique for paleoenvironmental reconstructions.