Biological and Geochemical Development of Placer Gold Deposits at Rich Hill, Arizona, USA

Erik Melchiorre,Paul Orwin,Robert Allison,Maria Rea,Frank Reith,Jeff Yahn

doi:10.3390/min8020056

Abstract

Placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, was studied using a range of micro-analytical and microbiological techniques to assess if differences in (paleo)-environmental conditions of three stratigraphically-adjacent placer units are recorded by the gold particles themselves. High-angle basin and range faulting at 5–17 Ma produced a shallow basin that preserved three placer units. The stratigraphically-oldest unit is thin gold-rich gravel within bedrock gravity traps, hosting elongated and flattened placer gold particles coated with manganese-, iron-, barium- (Mn-Fe-Ba) oxide crusts. These crusts host abundant nano-particulate and microcrystalline secondary gold, as well as thick biomats. Gold surfaces display unusual plumate-dendritic structures of putative secondary gold. A new micro-aerophilic Betaproteobacterium, identified as a strain of Comamonas testosteroni, was isolated from these biomats. Significantly, this ‘black’ placer gold is the radiogenically youngest of the gold from the three placer units. The middle unit has well-rounded gold nuggets with deep chemical weathering rims, which likely recorded chemical weathering during a wetter period in Arizona’s history. Biomats, nano-particulate gold and secondary gold growths were not observed here. The uppermost unit is a pulse placer deposited by debris flows during a recent drier period. Deep cracks and pits in the rough and angular gold from this unit host biomats and nano-particulate gold. During this late arid period, and continuing to the present, microbial communities established within the wet, oxygen-poor bedrock traps of the lowermost placer unit, which resulted in biological modification of placer gold chemistry, and production of Mn-Fe-Ba oxide biomats, which have coated and cemented both gold and sediments. Similarly, deep cracks and pits in gold from the uppermost unit provided a moist and sheltered micro-environment for additional gold-tolerant biological communities. In conclusion, placer gold from the Devils Nest deposits at Rich Hill, Arizona, USA, preserves a detailed record of physical, chemical and biological modifications.

Highlights

IntroductionThe occurrence of natural gold alloys of Au + Ag ± Cu (hereafter referred to as native primary gold) within lode ore deposits is well understood and has long been interpreted as having a shallow hydrothermal origin ([1,2] and the references there in)
The occurrence of natural gold alloys of Au + Ag ± Cu within lode ore deposits is well understood and has long been interpreted as having a shallow hydrothermal origin ([1,2] and the references there in)
Following 5–17 Ma of high-angle basin and range faulting, a shallow basin was produced on the shoulder of Rich Hill that preserves the three placer gold deposits of the Devils Nest

Summary

Introduction

The occurrence of natural gold alloys of Au + Ag ± Cu (hereafter referred to as native primary gold) within lode ore deposits is well understood and has long been interpreted as having a shallow hydrothermal origin ([1,2] and the references there in). Placer gold occurrence has been attributed to primary, secondary and a combination of primary and secondary formation [3]. In the most established theory of primary placer formation, a gold mass forms as a lode deposit during a hydrothermal event and survives physical and chemical weathering as native gold grains due to the low chemical reactivity and malleability of gold [1,4]. A secondary, accretionary origin for some placer gold has been suggested by observations of bacterioform gold, high-purity gold overgrowths and nanoparticulate gold in biomats [3,5,6,7,8,9]. The recent consensus of many workers is that the processes of both primary and secondary models likely play a role in placer gold formation, with the specific contribution of each end member varying from location to location [3,11,12,13]

Methods

Discussion

Conclusion