Page 37 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 19

1.3 Tracings of polished sections of gold grains from Master Mine, Gold Creek,
Granite County, Montana (after Loen, 1994).

24 and 27 contain several elements that are separated as a result of polishing.
Loen (1994) infers, from his examination of gold grains from sediments in Gold
Creek, Granite County, Montana, that a vein of gold crystal develops initially
rounded edges having high-fineness. Any weaknesses in the rim are penetrated
and low-fineness gold-silver alloys within the grain are dissolved, possibly in
multiple episodes of gold dissolution. Development of deep cavities is then
followed by precipitation of nearly pure gold on the interiors of the cavities.
Local penetration of the gold enriched border leads to dissolving of the low
fineness Au-Ag interior causing development of cavities followed by
precipitation of nearly pure gold layers in the interiors (Fig. 1.4c).
Giusti (1986) observed three major types of gold in his studies of the
morphology of fine-grained gold in sediment of the North Saskatchewan and
Athabasca Rivers, Alberta Canada:

· primary gold, often hidden in folds in the grain
· secondary gold related to re-disposition of `new gold' in the grains
· secondary gold related to plastic deformation and internal recrystallisation.
Although none of the grains appeared to be of entirely secondary gold, all of the
gold grains collected by the author showed the presence of gold-rich rims
ranging in thickness between 1 and 30 m. Giusti (1986) proposed a number of
simple criteria (Table 1.9) as general aids to deciphering the geological history
of gold placers recognising, however, that the assumption of a single cycle of
erosion is an oversimplification.
Sketches by Giusti (1986) illustrate the nature of high-grade gold precipita-
tion on fine-grained gold under the influence of mechanical and electrochemical