A Hamid Mumin

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Mesothermal gold ores of the Bogosu and Prestea concessions of southwestern Ghana are hosted by Birimian (2.1 Ga) carbonaceous sedimentary rocks. Gold mineralization is post-peak metamorphism, and occurs within dilatant zones of the Ashanti Structural Belt. Host sedimentary and mafic lithologies have been altered to an assemblage containing abundant ankerite, siderite and sericite.;Deeper portions of the gold system are exposed on the Prestea concession, and higher-level ores at Bogosu. Crack-seal quartz veins and carbonaceous shear-zones are the common ore types in the brittle-ductile rocks at Prestea. Tectonically-disrupted sediments with smaller zones of carbonate-altered mafic dikes are the common ore types at Bogosu.;Pyrrhotite was the first sulphide to precipitate; however, increase in pH and fO{dollar}\sb2{dollar} resulted in its alteration to marcasite and pyrite. This was followed by precipitation of arsenian pyrite and arsenopyrite. In-situ analysis and ion mapping by SIMS suggest that most of the gold may have precipitated in solid-solution with the arsenian pyrite and arsenopyrite. However, gold was subsequently redistributed, concentrated and crystallized as colloidal and microscopic particles within voids, microfractures and at grain boundaries of the arsenian pyrite, arsenopyrite and trace amounts of other sulphides.;Ankerite-siderite and arsenopyrite geothermometers indicate that mineral deposition occurred from about 400 to 145{dollar}\sp\circ{dollar}C. The lower limit of carbonate formation temperatures suggest that the ambient country-rock temperatures at the time of mineralization were about 145 and 215{dollar}\sp\circ{dollar}C for Bogosu and Prestea ores, respectively. Peak metamorphic temperatures are estimated at {dollar}{lcub}\approx{rcub}375\sp\circ{dollar}C.;Calculated oxygen and carbon-isotope values for the hydrothermal fluid ({dollar}\rm\delta\sp{lcub}18{rcub}O \approx 11.8\pm 1.5\perthous,\ \delta\sp{lcub}13{rcub}C \approx -12.7\ \pm\ 2.2\perthous{dollar}) are consistent with the generation of a metamorphic ore fluid in equilibrium with the Birimian sedimentary rocks. However, lower calculated fluid {dollar}\delta\sp{lcub}18{rcub}{dollar}O and {dollar}\delta\sp{lcub}13{rcub}{dollar} values of 8.1 {dollar}\pm{dollar} 2.0{dollar}\perthous{dollar} and {dollar}-13.8\ \pm\ 3.3\perthous{dollar}, respectively, for quartz and carbonates in mineralized samples suggest that phase separation of the ore fluid to a CO{dollar}\sb2{dollar}-rich gas phase and a residual ore fluid occurred prior to mineralization.;Geological and geochemical evidence suggest that the Ashanti Gold Belt formed in a sediment-dominated accretionary prism similar to the southern coast of Alaska.



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