Page 45 - Handbook of Gold Exploration and Evaluation

26 Handbook of gold exploration and evaluation

1.8 Holtermann Nugget photograph from Mineral Industry of NSW (1928).

Lawrence (1984) remarks on the finely divided state of most supergene gold in
the Pacific region of the Southern Hemisphere, stating that the gold is deposited
characteristically as tiny, discrete, cubo-octahedral crystals scattered throughout
the weathered rock.
The possibility that large gold nuggets may be formed by supergene accretion
in the oxidation zone of a primary gold orebody cannot be entirely discounted
because mechanisms for chemical accretion do exist. However, no examples are
known of nuggets comprising central cores of primary gold encased in thick
layers of high-fineness gold of sufficient proportions to materially increase the
average grade of the nugget. Indeed, the greater abundance of nuggets found in
residual rather than in lode material may simply be due to the enormous
quantities of primary gold-bearing material from which the regolith has derived,
compared with the very small quantities of hard rock mined in nugget producing
areas. Whiting and Bowen (in Smyth, 1869) are noted as saying, in 1876, that 80
of 118 slugs of gold (each weighing more than 0.6 kg) recorded from Victorian
quartz reefs were found within 50 cm of an intersecting quartz reef and indicator.
The location of the others had not been recorded, but the authors had little doubt
of a similar genetic origin. Figure 1.9 is a photograph of miners at Hill End, NSW
displaying their gold nuggets.