Gravity and magnetic surveys have been used to delineate potential gold mineralization on the Sefwi belt of Ghana. The study area, which is intrusive dominated hosting pockets of small scale mining operations locally referred to as Galamsey, is without any scientific trend of gold mineralization. The study aims at mapping lithological units, structural setting and relating Galamsey sites to delineate potential zones of gold mineralization. Scintrex CG5 gravimeter and GEM‘s Overhauser magnetometer were used for gravity and magnetic data acquisition respectively. The magnetic data were corrected and enhancing filters such as RTP, analytical signal and 1VD were applied using oasis montaj 7.1. Gravity data were also reduced to a common datum (geoid) using the oasis montaj software to produce a Bouguer anomaly map. Regional/residual separation technique produced a residual gravity map. The RTP and analytical signal filters from the magnetic data and residual gravity anomaly map from the gravity data helped in mapping belt type (Dixcove) Birimian granitoids and mafic intrusive unit, interpreted as gabbro. The first vertical derivative filter was useful in mapping NE/SW minor faults and crosscutting dykes largely concentrated in the belt type Birimian granitoids. All the three mapped Galamsey sites fell on a minor fault and are associated with the belt type granitoids which have been a lead in delineating potential zones of gold mineralization.

1.1       Background
Ghana falls in the West African Craton with most of its primary gold lodes occurring in deep-seated shear zones that have been controlled by local unconformities between the Birimian metavolcanics and metasediments (Kesse, 1985). Among the five northeast trending volcanic belts in Ghana is the Sefwi belt with large gold deposits on the southeast and northwest margins. Primary gold occurs along the structures that form the sheared margin of the belt. The gold is probably of syngenetic volcanic origin related to greenstone volcanism and associated sedimentation, remobilized during the Eburnean orogeny to become concentrated and localized along major shear (Griffis et al., 2002).

Gravity and magnetic geophysical survey have been extensively used for mineral exploration to aid in delineating metaliferous ores (Airo & Mertanen, 2008; Hinson et al., 2015). In order to have an observable response, the geophysical method applied requires a difference in the target of interest and the surrounding rock in terms of their physical properties. Potential field methods (gravity and magnetic) are capable of delineating the subsurface geology in terms of lithology, geological structures and hydrothermal altered zones especially in structurally guided mineral zones where contrast in density and magnetic susceptibility help yield better results.

In order to enhance exploration techniques and reduce cost of drilling, geophysical tools including potential field methods which are comparatively cheaper and faster in operation are resorted to.

Magnetic data was used to characterize the structural control on gold at the Central Lapland greenstone belt , Finland (Airo & Mertanen, 2008). Gravity survey was used to investigate the structurally guided W-Sn mineral resource related to granites in Nanling Range of South China (Chen et al., 2015). Again aeromagnetic data was interpreted to detect a possible gold mineralization in Kyerano on the Sefwi belt of Ghana. This was achieved by determining lithological boundaries, geological structures and hydrothermal altered zones linking to mineralization (Wemegah et al. 2015). Aeromagnetic and aerogravity survey data were integrated to investigate the mineral potential of a section of the Volta Basin of (Hinson et al., 2015).

1.2       Problem statement
The Hwediem concession in the Ahafo district of Ghana in the Sefwi belt has prominent structurally controlled gold mineralization. The area is characterized by pockets of small scale gold mining operations (Galamsey) which have no concrete mapped trend of gold mineralization. Previous geological exploration surveys have detected anomalous zones of gold mineralization. Geophysical tools are relied upon to enhance the exploration program and reduce the cost of drilling. Due to the detection of sulphides (chargeable) and quartz veins (electrically high resistive) from previous survey, induced polarization (IP) and pole dipole geophysical methods were carried out respectively. The results of the IP and the resistivity survey could not clearly define the anomalous pattern due to the complex nature of the mineralization in the area.

Subsequently, due to the unsatisfactory results from the IP and resistivity, this research resorted to potential field geophysical survey (gravity and magnetics) with the view that contrasts in magnetic susceptibility and density will be helpful in mapping out the different lithological units, define geological structures and delineate any possible gold mineralization zones.

1.3       Justification
There have been instances in Ghana suggesting that exploration activities are discontinued because of the inability to detect feasible gold mineralization for further studies especially where conventional methods of gold exploration were used (Griffis et al., 2002). As exploration for mineral deposits becomes hard, geophysical methods are growingly depended on to help locate mineralization areas (Bishop & Emerson, 1999). Magnetic and gravity geophysical survey work best for lithological mapping, locating hydrothermal altered zones and geological structures linked with mineral deposit. With regards to the study area in the Sefwi belt, contrast in rock density and magnetic susceptibility could help to delineate structurally controlled mineralization.

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Item Type: Ghanaian Topic  |  Size: 72 pages  |  Chapters: 1-5
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