Page 27 - Handbook of Gold Exploration and Evaluation

10 Handbook of gold exploration and evaluation

Kolotov et al., 1980) due possibly to experimental difficulties at low Au
concentrations. On this basis, Gray (1997b) suggests that Au soils may only be
important for Au mobility in the presence of organic matter.

Biological effects
Some plants can absorb and accumulate gold but may also affect the re-
distribution of gold by taking it up at depth and releasing it at the surface as litter
(Erdman and Olson, 1985). In some cases it is believed that this hypothesis may
account for a significant scale of gold depletion in sub-surface horizons of the
regolith. However, while certain species of bacteria may either release or
decompose cyanide (Smith and Hunt, 1985) thus potentially affecting Au
solubility, the release of amino acid ligands by other bacteria could cause
significant Au solubility (Korobushkina et al., 1974).

1.1.3 Alloys of gold

Naturally occurring gold is never pure. Hydrothermal solutions leach other
elements as well as gold from the rocks through which the solutions pass. Some
of these elements are present in trace quantities only; the proportions of others
such as silver and tellurium may be significant and materially effect fineness.
Gold forms natural alloys with silver, copper, mercury and tellurium; less
commonly with titanium, bismuth, palladium, lead and zinc. Varieties in
primary ores include cuproaurite (copper gold), porpezite (palladium gold) and
bismuthaurite (bismuth gold). Whilst these minerals are seldom found in alluvial
detritus, their presence in a weathering zone may help unravel the geological
history of an area under review.
Gold has a characteristic metallic yellow colour, but can be black or ruby
when finely divided. In commercial operations mixing gold with other metals
also changes its colour:
· blue ± gold with iron
· green ± gold with a higher silver content than copper
· pink (or rose) ± 50% gold, 45% copper, and 5% silver
· white ± gold with nickel, zinc, copper, and tin; manganese-nickel is often
used because it bleaches gold
· yellow ± 50% gold, 25% silver, and 25% copper.
The principal natural alloys of gold are native gold, electrum, amalgum and
tellurides.

Native gold
Native gold is in itself an alloy, normally containing an isomorphous mixture of
gold and silver in the proportions of 4 to 15% by weight, rarely less than 1%.