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Insights from Pb isotopes for native gold formation during hypogene and supergene processes at Rich Hill, Arizona

George D. Kamenov, Erik B. Melchiorre, Faye N. Ricker and Ed DeWitt
Insights from Pb isotopes for native gold formation during hypogene and supergene processes at Rich Hill, Arizona
Economic Geology and the Bulletin of the Society of Economic Geologists (November 2013) 108 (7): 1577-1589

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 (super 206) Pb/ (super 204) Pb ranging from 18.364 to 18.486, (super 207) Pb/ (super 204) Pb from 15.574 to 15.593, and (super 208) Pb/ (super 204) Pb 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 (super 206) Pb/ (super 204) Pb ranging from 17.985 to 20.057, (super 207) Pb/ (super 204) Pb from 15.494 to 15.749, and (super 208) Pb/ (super 204) Pb 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.


ISSN: 0361-0128
EISSN: 1554-0774
Coden: ECGLAL
Serial Title: Economic Geology and the Bulletin of the Society of Economic Geologists
Serial Volume: 108
Serial Issue: 7
Title: Insights from Pb isotopes for native gold formation during hypogene and supergene processes at Rich Hill, Arizona
Affiliation: University of Florida, Department of Geological Sciences, Gainesville, FL, United States
Pages: 1577-1589
Published: 201311
Text Language: English
Publisher: Economic Geology Publishing Company, Lancaster, PA, United States
References: 37
Accession Number: 2013-088730
Categories: Economic geology, geology of ore depositsIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch map
N33°45'00" - N34°00'00", W111°34'60" - W111°19'60"
Secondary Affiliation: California State University at San Bernardino, USA, United StatesU. S. Geological Survey, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Society of Economic Geologists. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201352
Program Name: USGSOPNon-USGS publications with USGS authors
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