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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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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
3.
The manganese deposit of Nsuta, in the Ashanti Belt of Southern Ghana, is sandwiched between Birimian metasedimentary rocks.
The metasedimentary rocks contain interbedded carbonate-rich layers, which exhibit a characteristic banded appearance near
the contact with the orebody. The orebody is a carbonate-type manganese-formation and in terms of origin is considered here
as a Mn-analogue of the volcanogenic-exhalative Algoma type iron-formation. The protolith of the orebody (chemical sediment
including Fe-bearing rhodochrosite and alabandite) is envisioned to have been formed in a marine basin with relatively high
CO2 activity and Eh-pH conditions were extremely low (Eh 1 to −0.6 Volt and pH 8 to 11) during Birimian times (2170–2180 Ma).
These conditions occurred immediately below the shelf break in a shallow-marine environment. Subsequent submarine weathering
(halmyrolysis) followed later by metamorphism of Eburnian age (2100 Ma) led to the formation of Mg-Ca-Fe-bearing rhodochrosite,
the dominant mineral in the orebody. Other minerals of the orebody are: sulfides (e.g. two generations of alabandite sphalerite,
pyrite, millerite, niccolite, gersdorffite, and molybdenite), oxides and hydroxides (vanadium-bearing jacobsite, galaxite;
brucite, Mn2+-todorokite), Mn-silicates and an unknown boron mineral. Pyrochroite, possibly preceded by manganosite, occurs as a retrograde
mineral. This mineral assemblage forms the protore of the Nsuta deposit. Opaque Mn4+-todorokite replacing Mn2+-todorokite, manganite, manganomelane, pyrolusite and nsutite which formed at the expense of rhodochrosite, are of supergene
origin and represent the economic part of the deposit. The orebody is interleaved between the associated pelitic-psammitic
metasedimentary rocks suggesting that its protoliths was deposited over a time interval during the sedimentation of the latter.
Both units underwent subsequent processes (submarine weathering and metamorphism) together. The compositional differences
between the orebody with high Mn and CO2 and low Si and Al contents relative to the metasedimentary rocks are explained by a model involving the continuous sedimentation
of continent-derived materials (protolith of the metasedimentary rocks). During this time a pulsatory phase of submarine volcanism
and consequent precipitation of materials of essentially volcanogenic-exhalative origin occurred (protolith of the orebody).
From the exhalations, the carbonate minerals in both the manganese-rich sediments and the metasedimentary host-rocks (in the
latter in the form of layers and disseminations leading to relatively high concentrations of Mn, Ca and CO2) were precipitated.
Received: 18 April 1997 / Accepted: 16 July 1998 相似文献
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