Page 46 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 27

1.9 Miners at Hill End, New South Wales, proudly displaying their gold-quartz
nuggets (Holtermann Collection, Mitchell Library, Sydney Australia) (after
Nolan, 1980).

Discovery and exploitation of supergene gold deposits in the lateritic regolith
of the Yilgarn Block, Western Australia (Butt and Anand, 1997) provides further
evidence of secondary gold enrichment in the regolith. A review by Wilson
(1984) suggests that while many of the nuggets are clearly weathering resistates,
primary in origin with Ag content >5%, others of higher fineness (Ag < 5%)
have features that indicate secondary accretion. Mann (1984a) postulates nugget
distribution similar to that for gold enrichment in general. Nuggets have also
been found in laterite of the Cloncurry region of Queensland, often where little
or no basement gold mineralisation is known.
However, while mechanisms for secondary gold enrichments in weathering
profiles are well known, the origin of large gold nuggets within these profiles is
still uncertain. In partly dissected terrain the nuggets may be residual from now
eroded overlying horizons; even the apparent association of nuggets with the
base of the pisolitic laterite horizon and with calcrete may originate in this
manner (Butt, 1988).

The fine gold problem
The question of what constitutes fine gold has never been satisfactorily resolved
because of the many different standards applied to finely divided particles by
chemical, mining and civil engineers. To the early miners, fine gold was gold
that could not be recovered easily in the prospecting dish, and little notice was
taken of the loss of a few fine colours in samples dressed by panning. Good
recoveries were not expected below 200 m and generally the term fine gold
referred to grains smaller than about 150 m in size. In modern jigging practice,