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Gold-rich rim formation on electrum grains in placers.

Authors
Groen-JC; Craig-JR; Rimstidt-JD
Source
Can Mineral 1990 Jun; 28(2):207-228
NIOSHTIC No.
10008677
Abstract
Grains of placer gold from several localities in the southeastern United States exhibit a high-purity gold rim. The individual rims on the various grains range from <1 to 60 um in thickness and have silver contents of 3.3 to 0 wt pct (average of 0.9 Wt pct), even though the core compositions range from 45.1 to 2.8 Wt pct silver. This gold rimming most likely is responsible for the commonly cited cases of gold from placer deposits assaying at higher values of fineness than the gold in the corresponding source iode. A gold- rich rim apparently forms by precipitation of gold from the surrounding solution, because simple leaching of silver from the electrum surface is an ineffective mechanism for the enrichment of gold. Diffusion of silver to the surface of a placer gold grain to expose it to oxidizing meteoric waters, and thus create a diffusion- enhanced leaching process, proceeds far too slowly to produce the observed natural rim thickness; furthermore, this mechanism fails to produce the sharp gradients in concentration observed in natural grains. Comparison of the complexation efficacies of 49 different ligands indicates cn-, oh-, nh3, cl-, i-, br-, and hs- to be the ligands most capable (in decreasing order) of transporting gold in ordinary stream environments. Self-electrorefining of placer electrum grains is a likely process of forming gold-rich rims and probably operates in tandem with dissolution-precipitation (cementation) to produce the observed phenomena.
CODEN
CAMIA6
Publication Date
19900601
Document Type
OP; Journal Article
Funding Type
Contract
Fiscal Year
1990
NTIS Accession No.
NTIS Price
Identifying No.
MIR 27-90
Issue of Publication
2
ISSN
0008-4476
NIOSH Division
WO
Source Name
The Canadian Mineralogist
State
VA; DC
Performing Organization
Virginia Polytechnic Institute
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