Page 28 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 11

Gold in which silver is greater than 15% and less than 50% is classified as a
separate mineral, electrum. It ranges in fineness units from about 800 to plus
990.

Electrum
Electrum, from the Greek `elektron' (a substance that develops electricity under
friction) is a common name given to all intermediate varieties in the isomorphous
Au-Ag series. The physico-chemical properties of electrum vary with the silver
content. Physically, with increasing silver the colour changes from yellow to near-
white, the metal becomes less dense and decreases from about 800 to around 550
fine. Chemically with increasing silver, the lower fineness mixture becomes less
stable than higher fineness gold and is thus more prone to alteration by weathering.
Electrum is sometimes coated with halogens and sulphide compounds, which yield
a thin film of native silver under suitably reducing conditions.

Amalgum
Mercury has a strong affinity for gold and may occur in nature as amalgum
(Au 2 Hg 3 ). Naturally occurring amalgum is much more widely distributed in
gold-mining districts than previously thought. Fricker (1980) notes that 2% Hg
has been identified in some South African gold, previously unrecognised
because of its use in the gold recovery process. Dredgers in the Ampulit gold
placer operation, Kalimantan, Indonesia recovered more mercury than was used
in the treatment plant. However, this may have been due to the recovery of
mercury lost in previous panning operations.
Cinnabar (HgS 2 ) belongs to the same type of mineralisation as most low-
temperature, hydrothermal gold. It is generally more stable in oxidising
conditions than most other sulphide minerals and due to its high density
( ˆ 8:09) is often found in gold placer heavy mineral concentrates. Black films
of metacinnabarite, native mercury and occasionally mercury chlorides may
occur as secondary minerals of minor economic importance.

Tellurides
Gold tellurides occur widely in both epithermal and Archaean greenstone belt
type deposits. The most common varieties are petzite (AgAu) 2 Te, silvanite
(AuAgTe 4 ) and calaverite (AuTe 2 ). Compounds such as aurobismuthite
5
(BiAuAg) S 6 are rare.

1.1.4 Measures of purity (fineness)
The purity of gold is measured in terms of fineness. Gold is commercially
available with a purity of 99.999%. Gold of fineness 1,000 is pure gold, i.e.,