Page 44 - Handbook of Gold Exploration and Evaluation

Nature and history of gold 25

formation and in the amount of gold drawn out of the rock below'. In the late
1850s, Selwyn and Ulrich (in Smyth, 1869) suggested cutting nuggets in half
and determining, by appropriate analyses, differences between gold in central
portions of reefs and those of surrounding masses, formed by deposition from
meteoric waters. Another proponent of the growth theory, Dr Landsweert (1869)
experimented with very dilute solutions of gold chloride using carbonaceous
material (brown iron ore) as a decomposing agent. In his opinion, `the
occurrence of larger nuggets in gravel deposits that have been found in quartz
ledges, with the fact that alluvial gold almost universally has a higher standard
of fineness, would seem to imply a different origin for the two'.
There have also been many sceptics. Smyth (1869) refers to a paper by a Mr
George Foord, reputedly the foremost authority of the day on all questions
relating to the chemistry of gold. In this paper Foord writes, `A good deal has
been said and written concerning the formation of nuggets by the coalescence of
grains in the alluvial state. No one can say that this has never taken place but if
my opinion was asked, it would lean very little to the acceptance of this view.
The physiognomy of nuggets points almost invariably to their position in the
lode.'
Nor did Emmons (1940) find much in support of the growth theory, although
it was much favoured by miners in the goldfields of California and Australia. He
pointed to the size of the Holtermann nugget (Fig. 1.8) from a lode in Hill End,
NSW, Australia, which weighed almost 2,800 oz. and to a piece of gold from
Carson Hill, California, which weighed 2,300 oz. Both of these nuggets were
heavier than the two largest alluvial nuggets from the Victorian goldfields
`Welcome' (2,218 oz.) and `Welcome Stranger' (2,268 oz.).
In his review of arguments for and against the formation of nuggets by
chemical accretion Boyle (1979) concluded that the gold in placers is of both
detrital and chemical origin. In his opinion `one should view the formation of
nuggets in a dynamic sense, the agents forming them being both chemical and
mechanical and their action being concomitant'.
In 1926, Johnson and Ugloo (1926) proposed a chemical explanation to
account for the appearance of coarse gold in the Pleistocene alluvial deposits of
the Cariboo Mining District, British Columbia, Canada. In their view, the gold
deposits could have derived from deep weathering and supergene enrichment of
quartz veins containing arsenopyrite and pyrite in Tertiary times. In support of
this interpretation, Eyles and Kocsis (1989) compared the fineness of the lode
gold (500±911) with that of the placer gold (775±950). They suggested that the
increased fineness of the placer gold together with the crystal shape of the placer
gold (dodecahedrons, cubes and octahedrons) is indicative of deposition from
solution and the possible gradual accretion of gold particles by supergene
processes. Supergene gold enrichment is reported from Bingham and Ok Tedi
where a portion of the gold is substantially coarser than in the subjacent sulphide
zones (Rush and Seegers, 1990). Under different environmental conditions,