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A major gold province of the world exists in the Proterozoic Birimian and Tarkwaian supracrustal rocks of West Africa. The bulk of the gold comes from the primary lode occurrences of the Birimian rocks of Ghana (formerly The Gold Coast). Birimian lithofacies is characterised by subaqueous fine-grained sediments with bimodal volcanic material. Metasedimentary rocks include phyllites and metawackes. Metavolcanic rocks are predominantly tholeiitic basalts. Komatiites and banded iron formations (BIF) are absent.Gold is in 5 parallel, evenly spaced, more than 300 km long, northeast-trending volcanic belts separated by basins containing pyroclastic and meta-sedimentary units. The most prominent is the Ashanti volcanic “greenstone” belt, which hosts the Ashanti Goldfields Corporation mines at Obuasi (more than 800,000 kg Au since 1896), the Billiton Bogosu Gold mine at Bogosu, and the State Gold Mining Corporation mines at Prestea, Bibiani and Konongo.Gold, ranging from 2 to 30 ppm, is in quartz veins of laterally extensive major orebodies which deeply penetrate fissures and shear zones at contacts between metasedimentary and metavolcanic rocks. The veins consists mainly of quartz with carbonate minerals, green sericite, carbonaceous partings and metallic sulfides and arsenides of Fe, As, Zn, Au, Cu, Sb, and Pb. Gold occurs in carbonate fillings in fractured quartz veins. Country rocks, which contain rutile, anatase and granular masses of leucoxene, along ore channels, have been hydrothermally altered to carbonates, sericite, silica and sulfide minerals. Fluid inclusion evidences suggest that mineral deposition took place at about 350°C and 140 bar from dilute aqueous solutions. Timing deduced from ore textures, however, show complex multi-stage mineralization events, with higher temperature minerals commonly having formed later than lower temperature ones. Geochemical studies of materials produced by tropical processes, especially soils, are essential in prospecting poorly exposed terranes of west Africa. Trace and major element distributions at mines and mineral occurrences can indicate mineralization otherwise difficult to detect.This paper highlights the features of the Ghanaian gold deposits that may aid the current search for new deposits along the gold belts. Exploration based on geochemistry is highly important, but should be integrated with data from accompanying geological, lithologic, mineralogical, and structural studies. 相似文献
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Benony K. Kortatsi Collins K. Tay Geophrey Anornu Ebenezer Hayford Grace A. Dartey 《Environmental Geology》2008,53(8):1651-1662
Alumino-silicate mineral dissolution, cation exchange, reductive dissolution of hematite and goethite, oxidation of pyrite
and arsenopyrite are processes that influence groundwater quality in the Offin Basin. The main aim of this study was to characterise
groundwater and delineate relevant water–rock interactions that control the evolution of water quality in Offin Basin, a major
gold mining area in Ghana. Boreholes, dug wells, springs and mine drainage samples were analysed for major ions, minor and
trace elements. Major ion study results show that the groundwater is, principally, Ca–Mg–HCO3 or Na–Mg–Ca–HCO3 in character, mildly acidic and low in conductivity. Groundwater acidification is principally due to natural biogeochemical
processes. Though acidic, the groundwater has positive acid neutralising potential provided by the dissolution of alumino-silicates
and mafic rocks. Trace elements’ loading (except arsenic and iron) of groundwater is generally low. Reductive dissolution
of iron minerals in the presence of organic matter is responsible for high-iron concentration in areas underlain by granitoids.
Elsewhere pyrite and arsenopyrite oxidation is the plausible process for iron and arsenic mobilisation. Approximately 19 and
46% of the boreholes have arsenic and iron concentrations exceeding the WHO’s (Guidelines for drinking water quality. Final
task group meeting. WHO Press, World Health Organization, Geneva, 2004) maximum acceptable limits of 10 μg l−1 and 0.3 mg l−1, for drinking water. 相似文献
3.
Benony K. Kortatsi 《Aquatic Geochemistry》2007,13(1):41-74
Hydrochemical and stable isotope (18O and 2H) analyses of groundwater samples were used to establish the hydrochemistry of groundwater in the Ankobra Basin. The groundwater
was generally mildly acidic, low in conductivity and undersaturated with respect to carbonate phases. Major ions except bicarbonate
were low and dissolved silica was moderately high. Silicate minerals weathering is probably the main process through which
major ions enter the groundwater. Groundwater samples clustered tightly along the Global Meteoric Water Line suggesting integrative,
smooth and rapid recharge from meteoric origin. The majority of the boreholes and a few hand dug wells cluster towards the
Ca–Mg–HCO3 dominant section of the phase diagram, in conformity with the active recharge and short residence time shown by the isotope
data. Aluminium, arsenic, manganese, iron and mercury were the only trace metals analysed with concentrations significantly
above their respective detection limits. Approximately 20%, 5%, 40% and 25% respectively of boreholes had aluminium, arsenic,
iron and manganese concentrations exceeding the respective WHO maximum acceptable limits for drinking water. The relatively
large percentage of boreholes with high concentration of aluminium reflects the acidic nature of the groundwater. 相似文献
4.
Collins K. Tay Benony K. Kortatsi Ebenezer Hayford Isaac O. Hodgson 《Environmental Earth Sciences》2014,71(12):5079-5097
Hydrochemical and stable isotopes (18O and 2H) analyses of groundwater samples were employed to establish the origin of major dissolved ions in groundwater within the Lower Pra Basin. Results showed that, the major processes responsible for chemical evolution of groundwater include: silicate (SiO4)4? dissolutions, ion exchange reactions, sea aerosol spray and pyrite (FeS2) and arsenopyrite (FeAsS) oxidations. The groundwater is strongly acidic to neutral, with pH generally range from 3.5 to 7.0 pH units and mean 5.9 (±0.5). Approximately 89 % of boreholes had pH values outside the World Heath Organization (WHO, Guidelines for drinking water quality, 2004) guideline value for drinking water due principally to natural biogeochemical processes and therefore, not suitable for potable purposes. Electrical conductivity (EC) range from 57.6 to 1,201 μS/cm with mean 279.3 (±198.8) μS/cm. Total dissolved solids (TDS) range from 32 to 661 mg/L with mean 151.7 (±106.8) mg/L, with 98.6 % of groundwater as fresh (TDS < 500 mg/L). The chemical constituents generally have low concentrations and are within the WHO (Guidelines for drinking water quality, 2004) guideline value for drinking water. The relative abundance of cations and anions is in the order: Na+ > Ca2+> Mg2+ > K+ and HCO3 ? > Cl? > SO4 2?, respectively. A plot of ?18O ‰ against 2H ‰ showed that, ground and surface waters clustered on or closely along the Global Meteoric Water Line, suggesting that, the waters emanated principally from meteoric source with evaporation playing an insignificant role on the infiltrating water. 相似文献
5.
Characterization and appraisal of facets influencing geochemistry of groundwater in the Kulpawn sub-basin of the White Volta Basin, Ghana 总被引：1，自引：0，他引：1
Groundwater composition in the Kulpawn basin is largely controlled by aluminosilicates dissolution and cation exchange resulting
in mainly Ca-Mg-HCO3 and NaHCO3 water types. Principal component analysis, Piper graphical classification, and stable isotope (18O and 2H) of groundwater and surface-water samples were used to delineate geochemical processes and groundwater facies. The groundwater
is mildly acid to neutral and low in conductivity. Chemical constituents except HCO3
− and SiO2 have low concentration. No cation shows clear majority, however, the order of relative abundance is Na+ > Ca2+ > Mg2+ > K+. HCO3
− is the predominant anion and the order of abundance is HCO3
− > NO3
− > SO4
2− > Cl−. SiO2 concentration is high compared with the major cations. Dissolution of plagioclase, pyroxene and biotite and cation exchange
are responsible for groundwater composition. Isotopic data suggest integrative, smooth and rapid recharge from meteoric origin.
The groundwater quality is generally good for domestic usage; however, 18 and 47% of boreholes respectively have NO3
− and F− levels outside WHO recommended limits suggesting potential physiological problems in some localities. The groundwater has
low sodium absorption ratio and low to moderate salinity hazard but significant magnesium hazard partially limiting its use
for irrigation. 相似文献
6.
Benony K. Kortatsi 《Environmental Geology》2006,50(3):299-311
Chemical and isotopic analyses are used to characterize and identify the relevant water-rock interactions, which are responsible for the poor groundwater quality in the Accra Plains. Four main water types are identified. Processes that singly or in combination influence the chemical composition of each water type include halite dissolution carbonate dissolution and precipitation, seawater intrusion, cation exchange, evaporative concentration of solutes and aluminosilicates dissolution. These processes contribute considerably to the concentration of major ions in the groundwater. Stable isotope contents of the groundwater suggest mainly direct integrative recharge. A few samples plot along the meteoric-seawater mix line which is coincidentally the evaporative line. The Cl−/Br− ratios of some of these are close to 300 confirming marine origin, others probably concentrated by evaporation have their Cl−/Br− ratios significantly lower than 300. Groundwater is qualitatively good for drinking purposes only along the foothills of the Akwapim Togoland ranges. 相似文献
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