CRC LEME
Open File Report 38
ABSTRACT
The aqueous chemistry of gold in the weathering environment
Gray, D.J.
Literature on the chemistry of gold in the weathering environment
was critically examined. Additional thermodynamic data were calculated
by the author to test and clarify conclusions drawn by other workers.
New hypotheses on some of the mechanisms of gold mobility and alteration
were also advanced.
Section 2 gives a brief summary of gold primary mineralisation
and associated minerals. Section 3 gives relevant information on
weathering processes and methods of analysis of thermodynamic data.
The weathering environments under which the thermodynamically unstable
thiosulphate and sulphite anions (which are effective ligands for
gold) can be generated are discussed. An explanation of the importance
of solution characteristics such as Eh, pH, and the presence of
ligands such as thiosulphate or chloride for the mobilisation of
gold is also given. It is demonstrated in Section 4 that gold can
be mobile in particular environments: either ligated by inorganic
anions such as chloride, thiosulphate or cyanide; as a colloid;
or under biological influence. These mobilized forms of gold may
be precipitated from solution by a number of mechanisms: changes
in solution chemistry; reactions with metal ions in solution; adsorption
onto the solid phase; or decomposition of ligand molecules. This
is dealt with in Section 5.
The later sections of this report present a more general overview
of gold chemistry in the weathering environment. Section 6 discusses
the observed increases in gold fineness in the weathering zone and
how this will be influenced by the form of the aqueous gold species:
thus, for example, equilibration of gold and silver with a chloride
rich solution will precipitate very high fineness gold; while the
presence of thiosulphate in solution will result in electrum of
lower fineness. Section 7 discusses alteration of primary gold grains:
in particular a theory of galvanic silver loss whereby gold and
silver are leached from electrum grains and gold is redeposited
on the same grain, is suggested for the widespread occurrence of
low silver rims in the weathering zone. Section 8 discusses the
hypothesised environments of gold redistribution and the gold species
that are considered to be important in each environment, as below:
Environment |
Gold Species |
Sulphide Weathering |
Au(HS)2-
Au(S2O3)23- |
Laterite and Soil |
Au(CN)2-
gold-organic matter
gold colloids |
Capillary Zone |
AuCl4- and AuCl2- |
Arid-Acid Conditions |
|
Arid-Alkaline Conditions |
Au(OH)2- |
Further work on gold dissolution and re-precipitation, that may
lead to a better understanding of the geochemistry of gold during
weathering and in developing exploration strategies, is suggested
in Section 9. Much of this work will be initiated by the author
shortly.
Last updated: Thursday, January 06, 2000 08:27 AM
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