Summary:
The Chirano mines and associated mineralised deposits lie within the Proterozoic terrain of southwest Ghana along a major structure separating the Sefwi Belt to the west from the Kumasi Basin to the east known as the Bibiani Shear Zone (“BSZ”). The belt and basin architecture comprises rocks of Birimian age, with the belts being dominated by mafic volcanics and the basins typified by fine-grained, deep marine sediments. Both are intruded by granites. The Chirano deposits lie close to a splay off the BSZ known as the Chirano Shear Zone (“CSZ”). The deposits that are integral in the operations of Chirano occur along a mineralised zone over 11km long. However, regional exploration continues to identify interesting, mineralised targets as extensions both to the north and south of the CSZ. The mineralised zone is characterised by intense folding and subsequent foliation, ubiquitous quartz-carbonate veining and tectonic transpression induced brecciation.
The deposits are hosted by fractured and altered mafic volcanics and granite and include stacked arrays of parallel quartz-filled veinlets, veinlet stockworks and mineralised cataclasites. The geometry and shape of the deposits range from tabular (Obra), or pipe-like (Tano) to multiple parallel lodes (Paboase). The mineralised zone thickness ranges from a few metres to over 70m. Most deposits dip very steeply towards the west or southwest and plunge steeply. Generally, the tenor of gold mineralisation is related to intensity of hydrothermal alteration (silica, ankerite, albite, sericite, pyrite), veining and brecciation. The gold is fine-grained and is commonly associated with 1% - 5% pyrite.
Akwaaba
Mineralization is hosted within dominantly quartz porphyry, hydrothermally altered quartz rich dolerite that is variably foliated and Tarkwaian sediments with strong brecciation characterised by wide and consistent carbonate alteration.
The deposit contains high-grade mineralised shoots which occur at subtle dip-changes in the fault-shear zone that plunge towards the north and contain hydrothermally brecciated basalt and cataclasis. Locally, the shear zone is intruded by tonalite, which is strained, altered, and mineralized. The mineral deposit, which is 20-50m wide, has gold grades which correlate well with the intensity of alteration, veining, brecciation and 1 to 5% pyrite.
Suraw
The mineralisation is hosted in a shear-breccia zone of quartz dolerite and quartz porphyry where a strong, relatively early brittle-ductile fabric has been overprinted by multi-phase brecciation and a quartz-sulphide fracture mesh. A grey smoky quartz vein that varies in width from few centimetres up to 5 metres is considered a marker located immediately east of the high-grade fault zone which serves as a guide to development and exploration. The mineralization is bounded on the hanging wall side, especially at depth, by a large mass of highly competent albite altered tonalite.
Akoti
The Akoti North and extended deposits are sub-vertical tabular zones of mineralization hosted within two differently striking portions of the mineralised horizon. The fault zone strikes about 035º at the south and about 005º at the north of the deposit and is hosted within quartz dolerite. At both ends of the deposit, the main fault surface within the lode horizon is sub-vertical and extremely planar. A minor volume of the fault zone is intruded by tonalite that locally forms an intrusive breccia with dolerite. Mineralization is hosted within hydrothermally altered basalt and tonalite, which are commonly foliated. High grade zones contain hydrothermally brecciated rocks and cataclasis.
The Akoti South deposit is about 500m south from the Akoti North pit. Mineralization within the Akoti South pit dips steeply westwards in multiple small lodes. The main zone of mineralization is associated with a grey/black smoky pyritised quartz vein which sits in a zone of strong shearing, brecciation and alteration – the Akoti South Main Shear.
A second zone of mineralization is associated with the quartz replaced shear that contacts the western mafics with the slightly more tonalite rich felsic and mafic package which makes up the central portion of the Akoti South pit – The Western shear. This shear can be traced striking northwest towards the western side of Akoti North pit.
The western part of the south wall of the Akoti South pit is predominantly made up of strongly weathered fine to medium grained mafics. The bulk of the lithologies found in the Akoti South pit consist of a typical mixture of predominantly strongly altered and deformed mafics, which are distinct from the sequence in the western part of the pit, intruded by lesser strongly altered tonalites. The altered tonalite is a blocky, medium-grained grey/green granitoid that has partially re-silicified quartz/feldspar. The mafics are strongly altered to quartz – sericite – ankerite – pyrite generally with occasional albite alteration especially immediately to the west of the Akoti South Main shear.
Paboase
The Paboase mineral deposit lies along a planar segment of the main mineralized shear zone between Akoti and Tano; this is intersected at a high angle by numerous faults mapped in the open pit. The geology within the open pit is a mixture of mafics (basalts and dolerites) intruded by a more significant quantity of tonalite, with quartz vein stockworking, than is seen at all other Chirano deposits apart from Tano. Where tonalite intrudes the Paboase main fault it seems to be more mineralized than intrusions further away from the fault.
The mineralization is hosted to the western side of the steeply Northeast dipping main mineralized shear zone locally referred to as the Paboase fault. Within the pit confines the mineralization is mainly hosted in a strongly silica– albite ± hematite altered quartz stock worked tonalite but at depth the best mineralised zones occur within a wide zone of strongly sheared, tectonically and hydrothermally brecciated, predominantly mafic (quartz dolerite) package. Gold grades at depth are highest where carbonate – sericite - silica ± albite ± chlorite alteration has been most intense, bleaching the rocks to a pale greyish-brown, although tonalite rocks also carry high grades locally where intensely sheared and altered.
Tano
The Tarkwaian sediments, visible in the upper pit walls, are a strongly foliated, folded, deformed and altered sequence of arkosic/argillaceous sediments with polymictic conglomerates. Contacting the Tarkwaian to the west and separating it from the main mineralised shear zone is a sequence of unmineralized Birimian mafic volcanics and felsic – intermediate intrusives. Mineralisation is mostly hosted within a domain of unfoliated, hydrothermally altered and crushed tonalite that locally has a high density of quartz ± carbonate veins. The high-grade mineralisation at Tano occurs within a hydrothermally quartz – albite – sericite altered hematite dusted pyritic tonalite with a high density of quartz veining.
Obra
The Obra deposit is a tabular zone of mineralization hosted within a northeast striking fault (Obra fault-OF) and sub vertical dipping of the Chirano shear zone (CSZ). Mineralization occurs in strongly hydrothermally altered, brecciated qtz-co3 and stockwork veined rocks of mafic(dolerite) + tonalite mixed fabric comprising of about 60% mafic + 40% tonalite combined host. Locally the Obra mineralization has a grid N-S trend following the regional structural trend of the Sefwi Belt.
The volume of tonalite rocks highly influences the dilatancy-fracturing, brecciation, fluid transport and deposition, hydrothermal alteration and most importantly the distribution of grades within the mineral deposit.