Page 26 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 9

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Dissolution of Au chloride (AuCl 2 ) requires highly acid, saline and
oxidising conditions as provided by deep continental brines; precipitation occurs
under reducing conditions for example, in the presence of ferrous iron, which
readily reduces Au:
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AuCl 2 ‡ Fe 2‡ ‡ 3H 2 O ˆ Au (s) ‡ Fe (OH) 3 ‡ 3H ‡ 1.2
With continued evaporation and increasing salinity, calcium is generally the first
ion to be precipitated, either as calcite under neutral to oxidising conditions, or
gypsum where there is an excess of Ca to CO 3 . Halide precipitates under high
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saline conditions as observed in the saline playas.

Organic complexes

Organic/biologically based complexes important for the mobility of Au in soils
include cyanide complexes, organic complexes, colloidal gold and biological
effects.

Cyanide complexes

Organic complexes capable of mobility of Au in soil profiles include cyanide
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complex Au (CN) 2 . A highly organic horizon can contain high levels of cyanide
and produce Au mobility. Gray (1997b) lists several authors including Watterson
(1985), Korobushkina et al. (1974) and Rogers and Knowles (1978) whose studies
of the influence of microorganisms on Au solubility, release or decomposition of
cyanide and release of amino acid ligands may have value in Au exploration.
Au cyanide solubility is limited by the availability of cyanide. However
certain plants and micro-organisms are known to release cyanide (Sneath, 1972)
and can accumulate appreciable gold. Cyanogenic bacteria are frequently
associated with plants, soil and organic matter and highly organic horizons or
the immediate surroundings of an active cyanogenic plant root system could
contain sufficient cyanide to promote Au mobility.

Colloidal gold
Gold readily forms molecular aggregations up to 5 m in size (colloids or sols)
and such chemical species have been known for centuries. Where stabilised by
organic matter, colloidal gold has been observed in the laboratory by such
workers as Goni et al. (1967), Ong and Swanson (1969) and Fedoseyeva et al.
(1983) and has been postulated as an important mechanism for the mobilisation
of gold. Being negatively charged, these colloids could be mobile in negatively
charged soils, precipitating in contact with a soil horizon containing positively
charged minerals such as Fe oxides. However, attempts to demonstrate the
natural occurrence of colloidal Au have been unsuccessful (Boyle, 1979;