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Gold mineralization in As-rich mesothermal gold ores of the Bogosu-Prestea mining district of the Ashanti Gold Belt, Ghana: remobilization of “invisible” gold

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Abstract

The Bogosu-Prestea mining district of southwestern Ghana is a 33 km section of the Early Proterozoic Ashanti Gold Belt. Greenschist facies carbonaceous and carbonate-bearing turbidites and greywackes, and mafic dikes host numerous economic mesothermal gold deposits. Structurally higher ores in the Bogosu concession have brittle deformation and consist of disseminated-sulphide lodes in tectonically-disrupted sedimentary rocks and carbonate-altered mafic dikes. Most gold occurs as micrometre-size particles in arsenian pyrite, and as “invisible” gold in arsenian pyrite and arsenopyrite. The structurally deeper ores of the adjoining Prestea concession are associated with brittle-ductile deformation and consist of extensive crack-seal quartz-veins and graphitic shear zones. Only minor amounts of “invisible” gold were detected; in these deeper lodes, gold occurs dominantly as abundant microscopic and larger particles in sulphide/arsenide minerals and in gangue. The gold distribution patterns revealed by SIMS microprobe analysis and ion maps, EMP and colour staining suggest that most of the primary gold in the Bogosu-Prestea system precipitated in solid-solution with sulphide/arsenide minerals. However, post-depositional concentration and redistribution occurred, in increasing degree with: 1) increase in metamorphic/hydrothermal gradients in the gold system (depth), 2) decrease in the refractory properties of the host mineral, and 3) increase in the amount of post-depositional, host-mineral recrystallization and deformation. Gold evolved from primary solid-solution within sulphide/arsenide minerals, to colloidal and micrometre-size particles concentrated in voids, fractures and internal grain boundaries, and finally to microscopic and larger particles at sulphide/arsenide grain margins and in the gangue assemblage. The general conclusions presented here are applicable to As-rich gold deposits of all ages, worldwide. The presence of gold in late fractures is insufficient evidence for late-stage introduction of gold in mesothermal gold systems.

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Authors and Affiliations

  1. Department of Earth Sciences, University of Western Ontario, N6A 5B7, London, Ontario, Canada

    A. H. Mumin & M. E. Fleet

  2. Amtel, University Research Park, University of Western Ontario, N6A 5B7, London, Ontario, Canada

    S. L. Chryssoulis

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  2. M. E. Fleet
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  3. S. L. Chryssoulis
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Mumin, A.H., Fleet, M.E. & Chryssoulis, S.L. Gold mineralization in As-rich mesothermal gold ores of the Bogosu-Prestea mining district of the Ashanti Gold Belt, Ghana: remobilization of “invisible” gold. Mineral. Deposita 29, 445–460 (1994). https://doi.org/10.1007/BF00193506

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