A consistent understanding of regional geology and structure is required to ensure that gold exploration targets are efficiently selected and prioritized. Although successive geological maps of the Akorade area represent advances in understanding and provide a great deal of useful information, all are known to have significant deficiencies in detail when compared with objective data that has become available during exploration.
Castle Peak therefore commissioned a review of regional geological, geophysical and geochemical datasets of the Akorade area in October 2012. This report presented the results of the review as a series of maps and diagrams with explanatory text, and included a 1:100,000 scale provisional re-interpretation of the regional geology (see sidebar figure by J.W. Arthurs).
In 2011 Castle Peak Mining commissioned Geotech Airborne Ltd to carry out a high resolution airborne geophysical survey covering 3,425 km of the Akorade area. The dataset used in this study consists of reduced-to-pole magnetic, digital terrain model, gamma-ray spectrometry and VTEM images. These images were re-interpreted for this review at a basic visual level without re-processing.
As well, a dataset of outcrops in the area was collated from unpublished maps in the archives of the Ghana Geological Survey, Castle Peak Mining's own mapping in two concessions, contract mapping in three concessions, and a PhD research study over a large part of the Ashanti Belt. The latter formed part of AMIRA International's West African Exploration Initiative (WAXI Project; student: S. Perrouty). The geophysical interpretations and the outcrop datasets together represented objective ground-truth against which to test regional geological interpretations.
Geotech Airborne's interpretation of their own data (2011) lacks fit in detail when compared with outcrop data. However, the general outlines are agreed and one very significant advance for exploration is the identification of major N-S shear zones on the east side of the property.
Structural Interpretation by Stéphane Perrouty
The PhD research study referenced in J.W. Arthurs commissioned report was authored by Stéphane Perrouty from the Université de Toulouse (France) and Monash University, School of Geosciences (Australia) dated June 8, 2011. The article was entitled "Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations." The abstract from the article is pasted below (see corresponding sidebar figure by Stéphane Perrouty):
The 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".
The 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 2187 Ma and 2158 Ma 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--2125 Ma) 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--2097 Ma) 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.