A geochemical study of the Ashanti gold deposit at Obuasi, Ghana

The Ashanti gold deposit at Obuasi occurs in the Early Proterozoic Birimian formation. The orebodies consist of gold-sulphides disseminated in metasediments and metavolcanics, and gold-bearing quartz veins. Gold occurs mainly as native gold normally with arsenopyrite, pyrrhotite and pyrite in the metasediments and metavolcanics. In the quartz veins, however, it is associated with tetrahedrite and sphalerite. Microprobe analysis of gold grains gave an Ag content of about 8 wt.%. Textural relationships among the various mineral associations suggest that pyrrhotite, pyrite, arse-nopyrite and possibly gold may have been formed esrlier in the paragenetic sequence. The precipitation of some amounts of gold may have occurred during the later stages of the mineralisation sequence. Average arsenopyrite composition determined from electron microprobe study is: Fe: 33.6 ± 0.5 at.%, As: 29.3 ± 1.2 at.%, S: 37.1 ± 1.1 at.%, This suggests an average temperature of formation of less than 300℃ for the sulphides and possibly the gold assuming equilibrium was attained between pyrite, pyrrhotite and arsenpopyrite. The δ34S values for sulphide mineral separates are in the range between -4.1 and -10.8%. The values were, however, constrained between -4.1 and -7.0%. The narrow range and the depleted nature of the δ34S values may suggest a sedimentary sourcc of sulphur which may have undergone high temperature homegenisation before deposition. Sulphides from quartz vein samples had the most depleted δ34S values. This may indicate a different mineralisation history of the sulohides in the quartz veins from those dissemi-nated in metasediments and metavolcanics

ガーナ,南部アシャンティ帯の原生代プリンスタウン・グラニトイド岩体に関する主成分地球化学

The Pateoproterozoic metavolcanic rocks of the southern Ashanti greenstone belt of Ghana are intruded by three major suites of granitoids, locally called Prince's Town, Dixcove and Ketan plutons. The Prince's Town pluton is the largest intrusive body in the Axim area, and tends to separate the Axim volcanic branch from the Cape Three Points branch. The Pluton consists of granitic to dioritic rocks, which are generally massive but occasionally display alignment of ferromagnesian minerals. The rocks contain mainly plagioclase, K-feldspar, quartz, amphibole, biotite and opaques. The feldspars are mostly sericitized and saussuritized, and alteration of amphibole and biotite to epidote and chlorite is common. Accessory minerals include apatite, sphene and zircon. The geochemical data indicate that the rocks are tonalitic to granodioritic in composoition, metaluminous (ASI<1) and have I-type characteristics. The granitoids have the SiO2 content of 63-70% ; total iron, as Fe2 O3 of 3.10-5.80% ; (Na2O+K2O) content of 5.01-6.96% and Na2O/K2O ratios from 1.34 to 2.70 ; and are characterized by Mg# ranging from 53 to 48. The Fe*(=FeOtot/FeOtot+MgO) and modified alkali-lime index (MALI) of the rocks indicate that the Prince's Town pluton is dominantly magnesian and calcic in nature. Higher values in molar CaO/(MgO+FeOtot) coupled with low molar AI2O3/(MgO+FeOtot) may suggest their derivation from partial melting of metabasaltic to metatonalitic sourcc, with a possible contribution from metagreywacke, but preclude any contribution from metapelitic sources. The Birimaian metavolcanic rocks are the likely source material candidate for the rocks. CIPW norm calculations yielded a crystallization temperature of～650-685℃ and a pressure of 4-7kb for the rocks, suggesting a lower crustal souree. The Prince's Town plutonic rocks also show characteristice of plutons emplaced in a volcanic are tectonic setting environment. This observation is largely consistent with previous studies conducted on granitoids from other parts of the southern Ashanti greenstone belt c and the belt-type granitoids of Ghana as a whole

Physico-Chemical Analysis of Surface and Groundwater in the Ayensu River Basin in the Central Region of Ghana

The hydrochemistry of the Ayensu river basin in the central Region of Ghana has been established. The methodology consisted of physicochemical sampling and laboratory analysis of both groundwater and surface water resources in the Basin and basic statistical analysis of the laboratory results.. Generally, the groundwater is weakly acidic with a mean pH value of 6.33±0.01, had high electrical conductivity and TDS values in the range, 297.65 to 6011.0 and 100.85 to 2746.0 respectively. Three main hydrochemical facies have been identified in the basin. These are Na- Cl, Ca- Mg - Cl and Ca - Mg - SO4 water types. Groundwater is to a large extent potable. However, approximately 24% and 22% respectively of groundwater samples had chloride and sulphate concentrations slightly exceeding the respective WHO maximum acceptable limits for drinking water. The concentrations of aluminium, iron and manganese were the only minor ions that significantly exceeded their respective detection limits. Nearly 43% of the groundwater samples had the Al3+ concentrations exceeding the WHO acceptable limit of 0.2 mg/l for drinking water, which reflects the acidic nature of the groundwater. The main geochemical process influencing the hydrochemistry of the Ayensu river basin is mineral dissolution. Keywords Groundwater quality, hydrochemistry, Mineral dissolution, Central Region Ghana. 

Contamination impact and human health risk assessment of heavy metals in surface soils from selected major mining areas in Ghana

Analysis of soil samples around pristine and major gold-mining areas in Ghana was carried out for heavy metals as part of a larger soil contamination and metal background study. The surface soil samples were digested using microwave digester (aqua regia) and analyzed with ICP-MS for As, Cd, Hg, Zn, Co, Cu, Mn, Fe, Al, V, Cr, and Pb. The average concentrations (mg/L) for the metals ranged from 0.01 ± 0.01 (Cd) to 86,859.36 ± 47.07 (Fe) for the pristine sites, and 0.01 ± 0.01 (Cd) to 59,006.95 ± 79.06 (Fe) for the mining sites. Mercury was below the detection limit of the analytical instrument (0.029). The concentrations of heavy metals from this study were used to assess their contamination levels, and health risks. The results showed that, the metals ranked by severity of health risks as As > Pb > Cr > Cd. Principal component analysis (PCA) and cluster analysis showed two groupings with the PCA showing metals variability explained by 79.02%. Results from the PCA and Cluster analysis indicate anthropogenic sources of the metals which may be emanating from gold-mining activities. Results from multi-criteria ranking and pattern recognition employing PROMETHEE and GAIA revealed major contribution of the metals from the mining sites with metal variability explained by 72.83%. This is the first time a multi-criteria approach is employed to characterize heavy metal contamination in Ghana, and the study nevertheless brought to light the impact of mining on human health and the environment with implications for other mineral areas around the globe

Groundwater residence time in basement aquifers of the Ochi-Narkwa Basin in the Central Region of Ghana

International audienceGroundwaters from basement aquifers in the Ochi-Narkwa basin of the Central Region together with rain and surface waters have been analysed for stable isotopes (delta O-18, delta H-2 and delta C-13) and radioisotopes (H-3 and C-14) to determine sources of recharge, groundwater residence time and flow path. The mechanism of recharge to the groundwaters is by direct infiltration of past local rainfall of mean isotopic composition delta O-18 = -3.8 parts per thousand V-SMOW and delta H-2 = -18 parts per thousand V-SMOW. Tritium in the groundwaters ranged from 0.05 +/- 0.07 to 4.75 +/- 0.16 TU. Tritium data revealed that 85% of the groundwater samples were of modern recharge or young waters. The C-14 content of the groundwaters ranged between 9.50 pMC in borehole CR2-50 at Ekumfi Asokwa to 113.56 pMC in borehole CR3-26 at Onyaadze. Evaluation of H-3 and C-14 data distinguished three groups of water namely (1) waters characterised by high H-3 and high C-14 depicting modern recharge, (2) waters showing a mixture of young and old water due to fractures and (3) waters showing low H-3 and low C-14 contents referred to as very old waters and include borehole CR2-50 at Ekumfi Asokwa. The estimated age or residence time of this older water is 19,459 years BRbased on uncorrected age. The major flow direction is northwest-southeast. The dominant months contributing to recharge in the study area were February, March, April, May, June, August, September and October. Groundwater residence times in the basement aquifers of the Ochi-Narkwa basin showed that groundwater abstraction is sustainable and requires that the recharge areas are protected from contamination