Page 18 - Handbook of Gold Exploration and Evaluation

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Nature and history of gold

Gold (Au) is a transition metal between Ag and Rg in the chemical series of the
Periodic Table. Its atomic number is 79, and atomic mass 196.96655 (2) g/mol,
and has only one stable isotope number 197. The gold isotype 198 Au (half-life
2.7 days) is used in some cancer treatments. The metal has been known and
prized as an object of beauty and for its unique properties of chemical stability,
electrical conductivity, malleability and ductility (trivalent and univalent) since
mankind's earliest awakenings. As a standard of value against which to appraise
the costs of labour, goods, currency and national economy, it has been the
standard of many currencies since the world's first coinage in Lydia between
643 and 630 BC. The name for gold is derived from the historic English word
`geolo', for yellow and the chemical symbol for gold Au, from the Latin name
for gold `aurum' (glowing dawn).

1.1 Gold mineralogy
Natural resources of elemental gold are mainly contained in the mineral gold
(plus 85%Au) and in seawater. The oceans contain a major resource of gold in
solution but individual estimates are variable, depending upon the location of
samples, which appear to range in gold content from as low as 0.1 to as high
as 2.0 ppb by weight. Emery and Schlee (1963) note gold grades in the top
10 m of sediments in the Atlantis 2 Deep between 5 and 10 ppm. However,
attempts to recover gold from seawater on a commercial scale have so far
failed, mainly because of the large quantities of water involved; ion exchange
appears to offer the present best avenue for research. Salt, bromine and
magnesia are recovered from seawater on a large scale hence the oceans must
be regarded as a potential gold source of major proportions. Element
associations are broadly classified on the basis of their affinities for metals,
sulphides, silicates or gas phases, and are referred to in Table 1.1 as sidero-
phile, chalcophile, lithophile and atmophile respectively (from Goldschmidt,
1922). Basically a siderophile element, gold has some characteristics that
relate it to chalcophile group elements. The general ubiquity of gold is