USE OF PLACER GOLD CHARACTERISTICS TO LOCATE BEDROCK GOLD MINERALIZATION

The size, morphology, surface texture, per cent mineral inclusions, an d 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 da ta from the literature. Placer gold samples show a rapid decrease in m ean and maximum particle length, and a gradual increase in flatness in dex (F.I. = length + breadth/2 X thickness) from about F.I. = 2 in bed rock deposits, to as much as F.I. = 45 over transport distances of sev eral tens of kilometers. Primary mineral inclusions (mainly quartz, su lfide minerals, and iron oxides) are mostly lost within 10 km to 20 km of fluvial transport; this loss of quartz and trace metals causes a m ajor change in bulk chemistry of placer gold samples. Following the lo ss of inclusions and the development of flakes, edges of flakes are fo lded and the mean number of folded edges in a sample increases with di stance. When viewed under the electron microscope, surface textures of placer gold particles also become progressively more pitted with dist ance. Analysis of placer gold particles can be used to help evaluate p lacer source-area problems (placers with no known hard-rock gold depos its). For example, coarse placer gold containing abundant inclusions i ndicates a proximal source. On the other hand, an abundance of thin fl akes is evidence of a long transport distance and possible reworking f rom older gravels. Placer gold characteristics can therefore provide i nformation on distance to mineralized source rocks.