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

Minerals

Paul M. Orwin¹  Erik B. Melchiorre²  Maria Angelica D. Rea³  Frank Reith³  Jeff Yahn⁴  Robert Allison⁵ 

[1] Department of Biology, California State University, 5500 University Parkway, San Bernardino, CA 92407, USA;Department of Geology, California State University, 5500 University Parkway, San Bernardino, CA 92407, USA;Department of Molecular and Cellular Biology, School of Biological Sciences, The University of Adelaide, Adelaide, South Australia 5005, Australia;J.W. Mining, Wickenburg, AZ 85390, USA;Rob’s Detectors, Surprise, AZ 85374, USA;

关键词: gold;    placer;    nano-particulate;    biomat;    Comamonas testosteroni;    Arizona;   

DOI  :  10.3390/min8020056

来源: DOAJ

【 摘 要 】

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.

【 授权许可】

Unknown