GOLD

Oxidation and Mobility of gold

General principles

8. Precipitation of gold by H₂S. Hydrogen sulphide precipitates native gold from solution in the absence of solubilizing agents such as the alkali carbonates. Such a mechanism may operate where pockets of H₂S occur in the oxidized zones due to the oxidation of various sulphides at low Eh, due to the action of H₂SO₄ on sphalerite and other sulphides, and possibly due to the bacterial reduction of sulphates. In the zones of reduction (supergene sulphides) the precipitating effect of H₂S may also be partly responsible for the enrichment of gold in chalcocite, covellite and other supergene sulphides.

9. Precipitation of gold by reaction with supergene and hypogene sulphides. Gold is commonly enriched in supergene sulphide zones and also in the upper parts of sulphide ore bodies just below the zone of oxidation, even where the latter are lacking in supergene sulphides. As mentioned above part of the precipitation of gold in these zones of reduction may be due to the effect of H₂S. On the other hand electrochemical and replacement effects are probably also involved. Gold is the lowest metal in the electro-chemical series and its sulphide is probably less soluble than the sulphides of copper, zinc and iron (pyrite). Hence gold should replace these elements in their sulphide combinations. Whether this is actually what takes place or not is a question. However it is a fact that many chalcocite zones in copper sulphide deposits are enriched in gold (and silver). Much of the gold is submicroscopic or extremely finely divided. Only occasionally does one see visible gold in these zones that is obviously of supergene origin.

Selected Bibliography

Al'bov, M.N., 1952: Forms of the migration of gold in the oxidized zone of mineral deposits; Akad. Nauk SSSR, Izv., Ser. Geol., no. 4, p. 41-52.

Boyle, R.W. et al., 1975: Some observations on solubility of gold; Geol. Surv. Can., Pap. 63-23, p. 6.

Eddingfield, F.T. l913a: Alteration and enrichment in calcite quartz-manganese gold deposits in the Philippine islands; Philipp. J. Sci., v. 8, p. 125-134.

Lakin, H.W., 1969b: Solution of gold by porphyry copper leach solutions; U.S. Geol. Surv., Circ. 622, p. 4.

Listova, L.P. et al. 1966: Dissolution of gold in media forming during oxidation of some sulphides; Metallogen. Osad. Metamorf. Porod. Akad. nauk SSSR, Lab. Osad. Polez. Iskop., p. 189-199. (Chem. Abstr., v. 68, 88967h.)

Machairas, G., 1967: Dissolution et recristallisation de l'or primaire pendant l'oxydo-reduction des sulfures auriferes; Bur. Rech. Geol. Min. (Fr.), Bull. (2^e Ser.), Sec. 2, no. 3, p. 1-109.

Morris, H.T. and Lovering, T.S., 1952: Supergene and hydrothermal dispersion of heavy metals in wall rocks near ore bodies, Tintic district, Utah; Econ. Geol., v.47, p. 687-716.

Roslyakov, N.A., 1976: Gold in the zone of hypergenesis of gold ore deposits of East Trans-Baikal, and gold behaviour in the hypergenic processes of an oxidizing series, Yu. G. Scherbakov, ed., "Nauka" Publ. House, Siberian Br., Novosibirsk, p. 113-140.

Roslyakov, N.A. et al., 1972: Form of deposition and migration of gold in the weathering crust of gold-sulphide deposits; Tr. Inst. Geol. Geofiz., Akad. Nauk SSSR, Sib. Otd., v. 149, p. 125-138.

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This document is in the public domain.
March, 2011