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Open File Report 38

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:


Gold Species

Sulphide Weathering


Laterite and Soil

gold-organic matter
gold colloids

Capillary Zone

AuCl4- and AuCl2-

Arid-Acid Conditions


Arid-Alkaline Conditions


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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