Summary:
The Wassa property lies within the southern portion of the Ashanti Greenstone Belt along the eastern margin of the belt within a volcano-sedimentary assemblage located at proximity to the Tarkwaian basin contact. The eastern contact between the Tarkwaian basin and the volcanosedimentary rocks of the Sefwi group is faulted, but the fault is discrete as opposed to the western contact of the Ashanti belt where the Ashanti fault zone can be several hundred meters wide. Deposition of the Tarkwaian sediments was followed by a period of dilation and the intrusion of late mafic dykes and sills.
The Wassa deposit is located on the eastern flank of the northeast trending Ashanti Belt, a Paleoproterozoic greenstone belt which was formed and deformed, along with the dividing Birimian and Tarkwaian sedimentary basins during the Eoeburnean and Eburnean orogeny. Most deposits found within the Ashanti belt can be classified as lode gold deposits or orogenic mesothermal gold deposits, with the exception of the Tarkwaian paleoplacer deposits which have a sedimentary origin.
The Wassa lithological sequence is characterized by lithologies belonging to the Sefwi Group and consisting of intercalated meta-mafic volcanic and meta-diorite dykes with altered meta-mafic volcanic and meta-sediments which are locally characterized as magnetite rich, banded iron formation like horizons
The Wassa mineralization is subdivided into a number of domains, namely; F Shoot, B Shoot, 242, South East, Starter, 419, Mid East and Dead Man’s Hill. Each of these represents discontinuous segments of the main mineralized system which extends for approximately 3.5 km along strike from surface and is still open at depth. The SAK deposits are located approximately 2 km to the southwest of the Wassa Main deposit on the northern end of a well-defined mineralized trend parallel to the Wassa Main trend. The mineralization is hosted in highly altered multi-phased greenstone-hosted quartz-carbonate veins interlaced with sedimentary pelitic units.
Mineralization within the Wassa Mine is structurally controlled and related to vein densities and sulphide contents. In detail, the mineralization generally consists of broadly tabular zones containing dismembered and folded ribbon-like bodies of narrow quartz vein material, zones are typically 10 m to 50 m wide within a 900 m mineralized corridor.
Three vein generations have been distinguished on the basis of structural evidence, vein mineralogy, textures and associated gold grades. Evidence further relates the majority of gold mineralization to the earliest recognized vein generation which is believed to be syn-Eoeburnean. Gold grades broadly correlate with the presence of quartz-dolomite/ankerite-tourmaline bearing quartz veins and the presence of sulphide minerals (predominantly pyrite) within and around the quartz veins. Gold grades appear to be spatially restricted to the quartz veins, vein selvages and the immediate wall rocks. The alteration haloes developed around the veins and pervasively developed within the core of the Wassa Fold contain lower grade mineralization.
HWINI-BUTRE
The Hwini-Butre deposits can be characterized as mafic intrusive hosted, orogenic shear zones. The deposits are hosted within diorite and granodiorite intrusive rocks of the Mpohor complex.
The Hwini-Butre concession is underlain by three deposits: Adoikrom, Dabokrom and Father Brown, which are all characterized by different styles of mineralization. The Hwini-Butre deposits are hosted within the Mpohor mafic complex, which consists mainly of gabbroic and gabbrodioritic intrusive horizons. . At Father Brown and Dabokrom, mineralization is associated with quartz vein systems which are locally surrounded by extensive, lower grade, disseminated quartz stockwork bodies, especially at Dabokrom. The Father Brown deposit is characterized by well-developed fault-filled quartz veins which are, as is the case for Dabokrom, light grey with carbonate and mica accessory minerals and minor tourmaline and feldspar.
BENSO
The Benso deposits can also be characterized as mafic intrusive hosted, orogenic shear zones deposits, which are hosted by Birimian metavolcanics into which coarse plagioclase porphyry units have intruded and are generally conformable with the volcaniclastic units.
The Benso concession is underlain by four main deposits: Subriso East, Subriso West, G Zone and I Zone. All the deposits are characterized by similar style of mineralization. As with Hwini-Butre, the Benso deposits are hosted within mafic intrusive rocks of gabbroic to dioritic composition, which intrude a thick volcano-sedimentary sequence mainly composed of mafic volcanic flows. Mineralization at Benso is associated with late deformational stages of the Eburnean orogeny and deposits are shear hosted along subsidiary structures.
CHICHIWELLI
The Chichiwelli deposits can also be characterized as mafic intrusive hosted, orogenic shear zones, the deposits are hosted within diorite and granodiorite intrusive rocks
The Chichiwelli deposit consists of two sub-parallel mineralized trends which hosts two distinct types of mineralization. The Chichiwelli West trend is a shear zone hosted deposit with a quartz, carbonate, sericite and potassic alteration assemblage, the mineralization is associated with pyrite. The Chichiwelli East trend is a quartz vein associated deposit with an ankerite and sericite alteration assemblage. Mineralization is also associated with pyrite along vein selvages and in the wall rocks.