Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations

Abstract

Integration of regional geophysical datasets and detailed field observations provide new insights into the paleoproterozoic structural evolution of southwestern Ghana. The study area is dominated by three metavolcanic and metasedimentary packages known as the Sefwi Group, the Kumasi Group (Birimian) and the Tarkwa Group (Tarkwaian) that were intruded by abundant TTG granitoids during the Eoeburnean and Eburnean phases of an event termed the “Eburnean Orogeny”. This study identifies an Eoeburnean (pre-Tarkwaian) deformation event (D1) that produced significant deformation in the Sefwi Group metavolcanics. D1 is associated with N-S shortening manifested as regional scale folding in the southern Ashanti Belt. D1 synorogenic granitoids were intruded between 2187Ma and 2158Ma under greenschist metamorphic condition. Syn-D1 gold mineralisation associated with quartz veining could be the original source of Tarkwaian paleo-placers and/or remobilised gold concentrations along major shear zones.D2 represents an extensional phase associated with the Kumasi Group sedimentation (2154–2125Ma) which could be related to activation of major structures such as the Ashanti Fault as low angle detachments that controlled the deposition of the Kumasi Group and the opening of the Kumasi and Akyem Basin. The Tarkwa Group (2107–2097Ma) unconformably overlies the Birimian Supergroups and was deposited in response to D3 shortening. D3 resulted in the inversion of syn-D2 detachments faults within the Ashanti Belt. NW-SE D3 shortening produced regional scale folding within the Birimian and the Tarkwaian metasediments. D4 deformation corresponds with sinistral reactivation of D3 thrust faults, and is locally associated with macro-scale folding at Obuasi and Wassa gold mines. By the end of D4, the regional scale architecture was built and was only slightly modified by the two last events. D5 postdates the Eburnean metamorphic peak and corresponds to open recumbent folds associated with a subhorizontal crenulation cleavage. D6 is present as a subvertical crenulation cleavage and reverse faults associated with NE-SW shortening.