The tectonic contacts between the Birimian metasedimentary and adjacent metavolcanic rocks host orogenic gold deposits which are being exploited by the major gold mines in Ghana. These hydrothermal gold deposits have a spatial relations with and are controlled by geological structures. At the Chirano mine, reactivated pre-existing faults served as conduits for gold-bearing hydrothermal fluids which were deposited in a subparallel fracture within altered mafic volcanic rocks approximately 200 m west of the Chirano shear zone. Furthermore, gold-bearing sulphide mineralization at the Obuasi mine was precipitated during a D2Ob deformational event characterised by NW–SE compression. Conversely, gold bearing quartz veins at the Edikan mine formed during a D3Edk deformational event characterised by WNW–ESE shortening. Similarly, the unique orogenic gold mineralization in palaeoplacers at the Damang mine occurs within a D3D low displacement fault-fracture mesh. Additionally, mineralized quartz stockworks and disseminated auriferous sulphides at the Bogoso mine were formed in the vicinity of right-hand bends and splays in steeply dipping faults. Several N- to NNE-striking sinistral crosscut faults oblique to the belt-basin boundary are considered to be the defining geological structures controlling gold mineralization at the Ahafo mine. On the other hand, the defining structure at the Wassa mine is a kilometric-scale asymmetric synformal F3W fold. These findings show that subsequent ground selection for orogenic gold exploration in the Birimian should focus on identifying areas with discordant secondary and tertiary deformational structures along or close to the belt-basin contact.

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Structural Controls on Orogenic Gold Mineralization: Evidence from Major Operating Mines in Ghana

  • Samuel Kwame Atta

摘要

The tectonic contacts between the Birimian metasedimentary and adjacent metavolcanic rocks host orogenic gold deposits which are being exploited by the major gold mines in Ghana. These hydrothermal gold deposits have a spatial relations with and are controlled by geological structures. At the Chirano mine, reactivated pre-existing faults served as conduits for gold-bearing hydrothermal fluids which were deposited in a subparallel fracture within altered mafic volcanic rocks approximately 200 m west of the Chirano shear zone. Furthermore, gold-bearing sulphide mineralization at the Obuasi mine was precipitated during a D2Ob deformational event characterised by NW–SE compression. Conversely, gold bearing quartz veins at the Edikan mine formed during a D3Edk deformational event characterised by WNW–ESE shortening. Similarly, the unique orogenic gold mineralization in palaeoplacers at the Damang mine occurs within a D3D low displacement fault-fracture mesh. Additionally, mineralized quartz stockworks and disseminated auriferous sulphides at the Bogoso mine were formed in the vicinity of right-hand bends and splays in steeply dipping faults. Several N- to NNE-striking sinistral crosscut faults oblique to the belt-basin boundary are considered to be the defining geological structures controlling gold mineralization at the Ahafo mine. On the other hand, the defining structure at the Wassa mine is a kilometric-scale asymmetric synformal F3W fold. These findings show that subsequent ground selection for orogenic gold exploration in the Birimian should focus on identifying areas with discordant secondary and tertiary deformational structures along or close to the belt-basin contact.