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Gold-rich rim formation on electrum grains in placers.
Groen-JC; Craig-JR; Rimstidt-JD
Can Mineral 1990 Jun; 28(2):207-228
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.
OP; Journal Article
Issue of Publication
The Canadian Mineralogist
Virginia Polytechnic Institute
Page last reviewed: September 2, 2020
Content source: National Institute for Occupational Safety and Health Education and Information Division