Page 34 - Handbook of Gold Exploration and Evaluation

16 Handbook of gold exploration and evaluation

pure gold over gold grain surfaces. Such patinas may either afford a measure of
protection against wear by providing a more or less continuous cover over the
exterior of the grain or, by gaining access along cracks, slip planes, etc., cause
internal corrosion within the grain itself. A patina, once formed, is subject to
removal or modification during recycling when exposed to physical wear or
chemical weathering in an alkaline environment.
The nature of surface corrosion is, nevertheless, far from clear. Two con-
flicting schools of thought favour either variations of direct chemical plating of
gold on grain surfaces or around nuclei or preferential leaching of silver from
grain surfaces leaving behind a gold-enriched rim. Fisher (1935) argued for the
removal of silver and the deposition of pure gold on the surface of gold grains by
electrochemical corrosion. Based upon observations of gold grades in ancient
terraces and conglomerate beds in the Bulolo deposits of New Guinea he
concluded that average gold grades were enhanced as a result of this process. It
was established later that while average grades were generally a few points
higher in these deposits than in adjacent streambeds and flats, the gold was a
mixture of both low and high fineness gold in various proportions and was
derived from two different provenances.
Desborough (1970) analysed gold grains from 24 placers in the western
United States and Alaska by micro-probe analysis. He concluded: `the low silver
(high gold) rims of most of the gold grains examined, could best be explained by
the apparent greater solubility and ease of oxidation of the metal silver in gold-
silver alloy grains at low temperatures found in the placer environment'.
Deposition of low-silver gold on the surfaces of gold grains was considered less
likely, although chemically possible, because of the substantial quantities of
gold that would be required from predominantly gold-deficient aqueous solu-
tions. Chao (1969) measured the gold concentration in natural stream waters,
including some of the streams from which Desborough's samples were taken.
These showed that, while the soluble gold content was generally less than
0.20 ppb, the silver content of the same solutions was two to 100 times that of
the gold.
Boyle (1979) noted that `the weight of chemical evidence with respect to the
relatively pure rinds of gold on nuggets points to the chemical precipitation of
gold with some silver on the outer surfaces rather than that the silver was
leached from the surface layer. All of the chemical evidence points to the
probability that gold will be precipitated on any available gold nucleus provided
that reducing agents are present'.
McDonald et al. (1990) examined mechanisms for the formation of gold rims
on a number of gold particles from New Zealand, Australia and Alaska in Table
1.7. The tests suggested that formation of gold-rich rims on gold grains is a
function of the silver content of the native gold, the chemical modification of
low fineness gold (e.g., 20% Ag) being due to the preferential leaching of silver
from grain surface. High fineness gold could have a rim of almost pure gold