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Supergene gold mineralogy at Ashanti, Ghana: Implications for the supergene behaviour of gold

Published online by Cambridge University Press:  05 July 2018

R. J. Bowell
R. J. Bowell*
Affiliation:
Department of Geology, The University, Southampton SO9 5NH
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Abstract

At the Ashanti concession, Ghana, gold-bearing quartz veins and disseminated sulphide lodes occur in narrow (1-3 m) shear zones with altered argillites and metatholeiite host rocks. The mineralisation is concealed by up to 10 m of kaolinite-mica forest ochrosol soils, beneath which is a saprolitic zone of leached rock extending down 60-70 m to the hypogene ore zone. In the unweathered hypogene orebody, gold occurs as free grains in quartz, as sub-microscopic inclusions in the disseminated arsenopyrite, as gold tellurides and as aurostibite. The gold is released from the hypogene orebody by physical dissaggregation and chemical dissolution, the latter involving hydroxyl, thiosulphate, cyanide, and fulvate complexing. Dissolution and reprecipitation of the gold appears to have taken place largely in situ with little evidence of supergene enrichment. Consequently, the gold mineralogy of the soils is complex with residual and secondary gold grains exhibiting widely different textural and compositional characteristics. Residually enriched grains display pitted, rounded surfaces and have silver-depleted rims, while supergene gold grains are compositionally homogenous and have unpitted surfaces. The supergene grains display platelet, dendritic, irregular and octahedral habit. A fine grained spongy form of gold has also been observed from weathered telluride-bearing quartz veins. Much of the secondary gold is intergrown with iron oxides and hydroxides. The gold mineralogy of the Ashanti soils appears to be controlled by physico-chemical processes active during the lateritic pedogenesis producing residual and supergene enrichment of gold.

Keywords

goldsupergene mineralisationAshantiGhana
Type
Research Article
Information
Mineralogical Magazine , Volume 56 , Issue 385 , December 1992 , pp. 545 - 560
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

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Footnotes

*

Present address: Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD.

References

Amanor, J. (1979) The geology of Ashanti gold mines and implications for exploration. MSc. thesis (unpubl.), University of London.Google Scholar
Baes, C. F. J. and Mesmer, R. E. (1976) The hydrolysis of cations. J. Wiley and Sons.Google Scholar
Baker, W. E. (1978) The role of humic acids in the transport of gold. Geochim. Cosmochim. Acta., 42, 645–9.CrossRefGoogle Scholar
Barranova, N. N. and Rhyzenko, B. N. (1981) Computer simulation of the Au-CI-S-Na-H20 systems. Geokhimiya, 18, 989–1001. [in Russian].Google Scholar
Benedetti, M. and Boulegue, J. (1990) Transfer and deposition of gold in the Congo watershed. Earth Planet. Sci. Lett., 100, 108–17.CrossRefGoogle Scholar
Bowdish, F. W. (1983) Fire assay for gold and silver. In Proc. Int. Conf. on Gold and Silver (Y. S. Kim, ed.) 7-10. Reno, Nevada.Google Scholar
Bowell, R. J. (1991) The mobility of gold in tropical rain forest soils. Unpubl. PhD thesis. Univ. of Southampton.Google Scholar
Bowell, R. J. Foster, R. P., and Stanley, C. J. (1990) Precious and base metal telluride mineralisation, Ashanti, Ghana. Mineral Mag., 54, 617–27.CrossRefGoogle Scholar
Bowell, R. J. Foster, R. P., and Stanley, C. J. Gize, A. P., Hoppis, H. A., Laffoley, N. A., and Rex, A. J. (1991) The mineralogical and chemical characteristics of tropical rain forest weathering profiles: implications for gold exploration. In Proc. of Brazil Gold'91: The Economics, Geology, Geochemistry, and Genesis of Gold deposits. (E. A. Laderia, ed.), 713-9. A. A. Balkema.Google Scholar
Bowles, J. F. W. (1988) Mechanical and chemical modification of alluvial gold. Aus. I.M.M. Bull. Proc., 293, 9–11.Google Scholar
Bowles, J. F. W. Cameron, N. R., Beddoe-Stephens, B., and Young, R. D. (1984) Alluvial gold, platinum, osmium-iridium, copper-zinc and copper-tin alloys from Sumatra-their composition and genesis. Trans. Inst. Min. Metall. (Sect. B), 93, 823–30.Google Scholar
Boyle, R. W. and Jonasson, I. R. (1973) The geo-chemistry of arsenic and it's use as an indicator in geochemical prospecting. J. Geochem. Explor., 2, 251–96.CrossRefGoogle Scholar
Boyle, R. W. and Jonasson, I. R. Alexander, W. M., and Ashin, G. E. M. (1975) Some observations on the solubility of gold. Geol. Surv. Canada, 75-24, 8 p.CrossRefGoogle Scholar
Brooks, R. B., Chatterjee, A. K., and Ryan, D. E. (1981) Determination of gold at the ppt level in natural waters. Chem. Geol., 33, 163–9.CrossRefGoogle Scholar
Cloke, P. L. and Kelly, W. C. (1964) Solubility of gold under inorganic supergene conditions. Econ. Geol., 59, 259–70.CrossRefGoogle Scholar
Colin, F. and Viellard, P. (1991) Behaviour of gold in lateritic equatorial environment: weathering and surface dispersion of residual gold particles, at Dondo Mobi, Gabon. Appl. Geochem., 6, 279–90.CrossRefGoogle Scholar
Conn, E. E. (1969) Cyanogenic glycosides. Agr. and Food Chemistry, 17, 838–43.CrossRefGoogle Scholar
Desborough, G. A. (1970) Silver depletion indicated by microanalysis of gold from placer occurrences, western United States. Econ. Geol., 65, 304–11.CrossRefGoogle Scholar
Dove, P. M. and Rimstidt, J. D. (1985) The solubility of scorodite FeAsO4.2H20. Am. Mineral., 70, 838–44.Google Scholar
Feth, J. H. (1966) Nitrogen compounds in natural water-a review. Water Resources Res., 2, 41–58.CrossRefGoogle Scholar
Fontana, M. G. (1986) Corrosion Engineering. (Third Edition). McGraw-Hill, New York, 353 p.Google Scholar
Freise, F. W. (1931) The transportation of gold by organic underground solutions. Econ. Geol., 26, 421–31.CrossRefGoogle Scholar
Freyssinet, P., Zeegers, H., and Tardy, Y. (1987) Formation de l'or dans les cuirasses latèritiques: dissolution et précipitation. C.R. Acad. Sci. Paris, 305, S6rie II, 867-74.Google Scholar
Gadet, M. and Pouradier, J. (1972) Hydrolyse des complexes de For (I). 275, 1061-4.Google Scholar
Garrels, R. M. and Christ, C. L. (1965) Solutions, Minerals and Equilibria. Harper and Row, 450 p.Google Scholar
Goldberg, E. D. (1987) Comparative chemistry of platinium and other heavy metals in the marine environment. Pure Appl. Chem., 59, 576–91.CrossRefGoogle Scholar
Groen, J. C., Craig, J. R., and Rinstidt, J. D. (1990) Gold-rich rim formation on electrum grains in placers. Can. Mineral., 28, 207–28.Google Scholar
Johnson, P. R., Pratt, J. M., and Tilley, R. I. (1978) Experimental determination of the standard redox potentials of gold (l) ion. J. Chem. Soc. Chem. Comm., 1978, 606–7.CrossRefGoogle Scholar
Jorgensen, C. K. and Pouradier, J. (1970) Un noveau type de stabilisation des ‘champides ligands’ dans les complexes linéaines du cuivre (I), de l'argent (I) et de l'or (I). J. Chem. Phys., 67, 124–7.Google Scholar
Junner, N. R. (1932) The geology of the Obuasi goldfield. Memoir Gold Coast Geological Survey, 2, 65 p.Google Scholar
Kelly, W. C. and Goddard, E. N. (1969) Telluride ores of Boulder County, Colorado. Geol. Soc. Amer. Mem., 109, 237 p.Google Scholar
Koide, M., Hodge, V., Goldberg, E. D., and Bertine, K. (1988) Gold in seawater: a conservative view. Appl. Geochem., 3, 237–42.CrossRefGoogle Scholar
Krauskopf, K. B. (1951) The solubility of gold. Econ. Geol., 46, 858–70.CrossRefGoogle Scholar
Kronberg, B. I., Fyfe, W. S., Leonardos, O. H., and Satos, A. M. (1979) The chemistry of some Brazilian soils: element mobility during intense weathering. Chem. Geol., 24, 211–29.CrossRefGoogle Scholar
Lakin, H., Curtin, G., and Hubert, A. (1974) Geochemistry of gold in the weathering cycle. U.S.G.S. Bull. 1330, 80 p.Google Scholar
Lecomte, P. and Colin, F. (1989) Gold dispersion in a tropical rain forest weathering profile at Dondo Mobi, Gabon. J. Geochem. Explor., 34, 285–301.CrossRefGoogle Scholar
Levinson, A. A. (1980) Introduction to Exploration Geochemistry. (2nd Ed.). Applied Publishing.Google Scholar
Mann, A. W. (1984) Mobility of gold and silver in lateritic weathering profiles: some observations from Western Australia. Econ. Geol., 79, 38–49.CrossRefGoogle Scholar
McHugh, J. B. (1988) Concentration of gold in natural waters. J. Geochem. Explor., 30, 85–94.CrossRefGoogle Scholar
Ong, H. L. and Swanson, V. E. (1969) Natural organic acids in the transportation, deposition, and concentration of gold. Quart. Colo. School. Mines, 64, 395–425.Google Scholar
Pouribaix, M. (1966) Atlas of electrothermal equilibria. Pergamon Press, Oxford. 645 p.Google Scholar
Puddephatt, R. J. (1978) The Chemistry of Gold. Elsevier, Amsterdam. 490 p.Google Scholar
Puddephatt, R. J. (1987) Gold. In Comprehensive Coordination Chemistry in the synthesis, reactions, properties, and application of coordination compounds (G. Wilkenson, R. Gillard, and J. McCreverly). 5, Pergamon Press. 869-923.Google Scholar
Renders, P. J. and Seward, T. M. (1989) The stability of hydrosulphido- and sulphido-complexes of Au(I) and Ag(I) at 25°C Geochim. Cosmochim. Acta, 53, 245–53.CrossRefGoogle Scholar
Richardson, S. and Vaughan, D. J. (1989) Surface alteration of arsenopyrite. Mineral. Mag., 53, 201–12.Google Scholar
Schmidaur, H., Graf, W., and Mtiller, G. (1988) Weak intramolecular bonding relationships: The confirmation of attractive interaction between gold (I) centres. Angew. Chem. Inst. Ed. Engel., 27, 417–9.CrossRefGoogle Scholar
Seilger, D. S. (1981) Cyanogenetic glycosides and lipids: structural types and distribution. In Cyanide in Biology, 233-67.Google Scholar
Skibsted, L. H. and Bjerum, J. (1974) Studies on gold complexes II. The equilibrium between gold (I) and gold (III) in ammonia system and the standard potentials of the couples involving gold, diaminegold (I), and tetraaminegold (III). Acta Chem. Scand., A28, 764-70.Google Scholar
Stallard, R. F. and Edmond, J. M. (1981) Geochemistry of the Amazon: 1. Precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge. J. Geophys. Res., 86, 9844–58.CrossRefGoogle Scholar
Strickland, P. H. and Lawson, F. (1973) The measurement and interpretation of cementation rate data. In Int. Symp. Hydrometallurgy (D.J.I. Evans and R. S. Shoemaker, eds.). Am. Inst. Min. Metall. and Petroleum Engineers, Inc., New York.Google Scholar
Varshal, G. M., Velyukhanova, T. K., and Baranova, N. N. (1984) The geochemical role of gold (III) fulvate complexes. Geochem. Int., 21, 139–46.Google Scholar
Vlassopoulos, D. and Wood, S. A. (1990) Gold speciation in natural waters: I solubility and hydrolysis of gold in aqueous solution. Geochim. Cosmo-chim. Acta, 54, 3–12.CrossRefGoogle Scholar
Wagman, D. D., Evans, H. H., Parker, V. B., Schumm, R. H., Harlow, I., Bailey, S. M., Churney, K. L., and Butall, R. L. (1982) The NBS tables of chemical thermodynamic properties. Selected values for inorganic and organic substances in SI units. J. Phys. Chem. Ref. Data I1, supp. 2: 392.Google Scholar
Webster, J. G. (1986) The solubility of Au and Ag in the system Au-Ag-S-O2-H20 at 25°C and 1 atm. Geochim. Cosmochim. Acta, 50, 1837–45.CrossRefGoogle Scholar
Webster, J. G. and Mann, A. W. (1984) The influence of climate, geomorphology, and primary geology on the super-gene migration of gold and silver. J. Geochem. Explor., 22, 21–42.CrossRefGoogle Scholar
Wilson, A. F. (1984) Origin of quartz-free gold nuggets and supergene gold found in soils and laterites—a review and some new observations. Austral. J. Earth Sci., 31, 303–16.Google Scholar
Xue, T. and Ossco-Asare, K. (1985) Heterogeneous equilibria in the Au-Ag-CN-H20 system. Metal. Trans., 16B, 455-63.CrossRefGoogle Scholar