Prosper M. Nude

Tectonic significance of carbonatite and ultrahigh-pressure rocks in the Pan-African Dahomeyide suture zone, southeastern Ghana

Abstract The association of carbonatite and ultrahigh-pressure (UHP) metamorphic rocks in the Dahomeyide suture zone of southeastern Ghana is unique among the Neoproterozoic orogens that surround the West African craton (WAC). Carbonatite occurs in an alkaline complex that decorates the sole thrust of the suture zone and is characterized by high concentrations of incompatible trace elements such as light rare earth elements (LREE), Sr and Ba. Within the suture zone deformed alkaline rocks, including carbonatite, together with mafic granulites form an imbricate stack of thrust panels that involve 2.1 Ga rocks of the WAC basement. The dominant rock unit of the suture zone is composed of mafic granulites in which garnet megacrysts preserve a diagnostic microstructure of UHP metamorphism; it consists of a crystallographically controlled array of exsolved rutile rods in garnet. Metamorphic pressures estimated from Ti concentrations in the inferred precursor garnet indicate P>3 GPa, which requires subduction (and exhumation) of the suture zone rocks to and from mantle depths during collisional orogeny on the WAC margin. Available age constraints on carbonatite magmatism suggest that continental rifting, leading to the formation of the passive WAC margin c. 700 Ma, occurred c. 100 Ma before intrusion of carbonatite, which was preceded by HP and UHP metamorphism at 610±5 Ma.

Significance of regolith mapping and its implication for gold exploration in northern Ghana: a case study at Tinga and Kunche

ABSTRACT The complex regolith in the savanna belts of northern Ghana is believed to be a major cause of the failure of gold exploration in the area. The present study highlights the integration of field regolith mapping, pitting and trenching in the classification of regolith and its significance to gold exploration at Tinga and Kunche in northern Ghana. These two areas are underlain by Birimian greenstones but have not seen any gold production apart from the reported gold occurrences known to be associated with the metavolcanic and metasedimentary rocks in the area. The importance of understanding the regolith, the landscape, and the sampling medium has not been seriously considered in past exploration programmes. The classification of the regolith regimes into ferruginous, relict, erosional and depositional regimes in the two study areas aided in determining the differences in the soil profiles, the source of the regolith materials and nature of weathering. For relict and erosional regimes, optimum depth of sampling was established to be between 0.2 and 0.4 m. However, for depositional regimes, because of the variable overburden thickness, the base of the transported materials was sampled. The pits and trenches dug also contributed in identifying the in-situ and transported regolith. The study shows that, in savanna areas of northern Ghana, relict regimes generally have preserved laterite profiles whereas the ferruginous, erosional and depositional regimes are associated with landscape modifications.

Use of termitaria in surficial geochemical surveys: evidence for >125-μm size fractions as the appropriate media for gold exploration in northern Ghana

ABSTRACT Extensive lateritization and widespread sheet wash and alluvial deposits characterize the thick regolith in the savannah regions of northern Ghana. As often is the case in these areas, the presence of these cover materials mask geochemical gold (Au) response in soils during surficial gold exploration. Anomaly detection thus becomes very difficult perhaps due to gold grain encrustation during lateritization and anomaly dilution by sheet wash deposits. Termite mound samples collected from areas of thick regolith, transported overburden and laterite cap in gold bearing areas of northern Ghana which were analyzed for gold defined anomalous zones. Gold contents were determined from size fractions consisting of −125 μm, +125–250 μm, +250–500 μm and +500 μm. The gold contents show relatively insignificant changes in concentration and in repeat samples in the −125 μm and +125–250 μm size fractions, but there were significant differences when sub-samples were re-analysed in the coarser samples. Gold content repeatability was relatively better in the fine size fractions (−125 μm) and decreased in the coarser size fractions. The study showed that termite mounds can be used as a geochemical sample medium to support conventional soil surveys especially in areas under thick regolith and transported cover, and the −125 μm size fraction appears the most appropriate.

Hydrogeological Conditions of a Crystalline Aquifer: Simulation of Optimal Abstraction Rates under Scenarios of Reduced Recharge

A steady state numerical groundwater flow model has been calibrated to characterize the spatial distribution of a key hydraulic parameter in a crystalline aquifer in southwestern Ghana. This was to provide an initial basis for characterizing the hydrogeology of the terrain with a view to assisting in the large scale development of groundwater resources for various uses. The results suggest that the structural entities that control groundwater occurrence in the area are quite heterogeneous in their nature and orientation, ascribing hydraulic conductivity values in the range of 4.5 m/d to over 70 m/d to the simulated aquifer. Aquifer heterogeneities, coupled possibly with topographical trends, have led to the development of five prominent groundwater flowpaths in the area. Estimated groundwater recharge at calibration ranges between 0.25% and 9.13% of the total annual rainfall and appears to hold significant promise for large-scale groundwater development to support irrigation schemes. However, the model suggests that with reduced recharge by up to 30% of the current rates, the system can only sustain increased groundwater abstraction by up to 150% of the current abstraction rates. Prudent management of the resource will require a much more detailed hydrogeological study that identifies all the aquifers in the basin for the assessment of sustainable basin yield.

Petrological and Geochemical Characteristics of Mafic Granulites Associated with Alkaline Rocks in the Pan-African Dahomeyide Suture Zone, Southeastern Ghana

Prosper M. Nude1, Kodjopa Attoh2, John W. Shervais3 and Gordon Foli4 1Department of Earth Science, University of Ghana, P.O. Box LG 58, Legon-Accra, 2Department of Earth &Atmospheric Sciences, Cornell University, Ithaca, NY 14853, 3Department of Geology, Utah State University, Logan UT 84322, 4Department of Earth and Environmental sciences, University for Development studies, Navrongo Campus 1,4Ghana 2,3USA