Page 205 - Handbook of Gold Exploration and Evaluation

180 Handbook of gold exploration and evaluation

brium potential. A description is given by Hostettler (1984) of Eh theory, and of
the problems involved in measurement and interpretation.

3.6.1 Solution processes
Factors such as pH, Eh and salinity have major effects on the speciation and
solubility of many elements, and how they are controlled in natural systems
(Gray, 1997a). Salinity can arise from various processes:
· rock weathering
· evaporation
· dissolution of previously deposited halite (NaCl)
· sea water incursion
· aerosol deposition of sea water.
One or more of these processes could be important at a particular site. For
example, the ground waters in the Southern Yilgarn district of Western Australia
are hypersaline, with salinities greater than seawater. This appears most likely to
be due to long-term deposition of salt from aerosols, coupled with low rainfall,
high temperature and flat topography, with evaporation as a major source of
water removal in a virtually closed system.
Various salts precipitate as salinity increases with evaporation. The first
major ion to be precipitated is calcium, either as calcite (CaCO 3 ) under neutral
2ÿ
to alkaline conditions (i.e. where CO 3 is present) or as gypsum where there is
2ÿ
an excess of Ca to CO 3 (Gray, 1997a). Halite precipitates are observed in
saline playas under highly saline conditions (about 30% solids, or 300 mg/L).
Saline playas are common across most of the Yilgarn Basin as the result of
these processes (Mann, 1982). The pH of the ground waters is largely controlled
by the underlying geology with acid lithologies giving rise to neutral to acid
ground waters and basic lithologies giving rise to more alkaline waters. The
upper limit to the pH of ground waters is about pH 10, due to carbonate
deposition.
This lithological control however, is complicated by oxidation during
weathering. Gray (1997a) describes the effects of the oxidation of pyrite and
other sulphides. Pyrite oxidation occurs in two stages, both of which generate
hydrogen ions (acidity). The generation of sulphate, which generally occurs at
depth, is the first reaction:
2FeS 2 + 7O 2 + 2H 2 O , 2Fe 2+ + 4SO 4 2ÿ + 4H + 3.7
(Pyrite)
Diffusion of oxidising species such as O 2 limits this step and acid conditions
may be neutralised by other reactions, such as carbonate or feldspar weathering.
Closer to the surface, the second process `Ferrolysis', which also generates
acidity results in the oxidation and hydrolysis of Fe (Brinkman, 1997).