Page 223 - Handbook of Gold Exploration and Evaluation

194 Handbook of gold exploration and evaluation

Gold mobility in arid environments

During the gradual change from a wet savanna climate in mid-Miocene to an
arid climate, general changes to and slowing of chemical reactions in the drier
(post-Miocene) climates resulted from lowering water tables coupled with minor
uplift as shown in Fig. 3.26. Several reversals to more humid climatic conditions
favouring deep weathering probably occurred in many places. This would have
caused fluctuations in the water table with temporary rises and stillstands. In the
Yilgarn Block of Western Australia, erosion, partly from the loss of protective
vegetal cover, reduced the already low relief so that the upper horizons became
exposed to highly oxidising conditions.
Concomitant with such climatic and environmental changes, the principal
mechanisms of gold dispersion may also have changed, in particular increased
salinity will have encouraged the formation of soluble gold and silver halide
complexes. Lawrence (1984) suggests that because of the solubility and reaction
rates of gold in saline solutions any earlier formed enrichment, whether lateritic
or saprolitic would be redissolved during later events. Salinity can arise from
various processes, e.g. rock weathering, evaporation, dissolution of previously
deposited halite, seawater incursion and aerosol deposition of seawater. Saline
playas are common features in lateritic gold placer settings across most of the
southern Yilgarn Block (Mann, 1984b).
In redeveloped humid conditions of the Holocene in the Darling Ranges of
the Yilgarn Block, leaching of precipitated salts by increased rainfall and
recreation of redox conditions suitable for ferrolysis would have produced acid,
saline and oxidising ground waters capable of dissolving gold (Mann, 1984a).
Although of very short duration in geological terms, chemical reactions during
these humid periods would have been very rapid so that a significant redistri-
bution of gold is possible. In their study of the reduction of gold halides by
organic matter, Gatellier and Disnar (1988) calculated rate constants of the order
s
of 10 ÿ5 ÿ1 in the temperature range 40±100 ëC. During these relatively short
humid periods existing gold grains could be dissolved and mobilised as chloride
complexes and reprecipitated either by organic matter or following reduction by
ferrous iron. A wide variety of organic/biologically based complexes include
cyanide complexes, organic complexes and colloidal gold where stabilised by
organic matter (Gray, 1997b).
Similar type lateritic concentrations are known from deeply weathered
regoliths in South Africa, Brazil and the Guyanas of South America. In South
Africa the possible hydrothermal origin of Witwatersrand gold is still being
discussed after more than 100 years of mining (see Section 5.4.4). The fact that
the great Rand deposits gave rise to neither extensive eluvial nor alluvial
deposits is difficult to explain (Boyle, 1979).