Use of Placer Gold Characteristics to Locate Bedrock Gold Mineralizatio

Exploration & Mining Geology, Vol. 4, No. 4, 1995

JEFFREY S. LOEN, P.O. Box 4702, Butte, Montana 59702, US

The size, morphology, surface texture, per cent mineral inclusions, and bulk chemistry (Ag and trace elements) of placer gold particles vary systematically according to transport distance from source rocks, as indicated by a case study from the Pioneer district in Montana, and data from the literature. Placer gold samples show a rapid decrease in mean and maximum particle length, and a gradual increase in ?atness index (F.I. = length + breadth/2 thickness) from about F.I. = 2 in bedrock deposits, to as much as F.I. = 45 over transport distances of several tens of kilometers. Primary mineral inclusions (mainly quartz, sul?de minerals, and iron oxides) are mostly lost within 10 km to 20 km of ?uvial transport; this loss of quartz and trace metals causes a major change in bulk chemistry of placer gold samples. Following the loss of inclusions and the development of ?akes, edges of ?akes are folded and the mean number of folded edges in a sample increases with distance. When viewed under the electron microscope, surface textures of placer gold particles also become progressively more pitted with distance. Analysis of placer gold particles can be used to help evaluate placer source-area problems (placers with no known hard-rock gold deposits). For example, coarse placer gold containing abundant inclusions indicates a proximal source. On the other hand, an abundance of thin ?akes is evidence of a long transport distance and possible reworking from older gravels. Placer gold characteristics can therefore provide information on distance to mineralized source rocks
Mots Clés: Gold, Gold Characteristics, Placer Gold, Mineralization