Introduction
The Sunyani Basin Project is located approximately 400km northwest of Accra and is situated 40km west-southwest of Sunyani, the capital of the Brong Ahafo Region. The project consists of one granted Reconnaissance Licence and one Prospecting Licence covering a total of 238km².
Project Geology
The geology of the Sunyani Basin is somewhat analogous to the Kumasi Basin, however the area is characterised by fewer volcanoclastics and Dixcove suite intrusive rocks. Thick sequences of Birimian meta-sedimentary rocks are bounded by the Sefwi Belt on the southeast and the Bui Belt to the northwest. At its narrowest point, near Sunyani, the Basin is 65km wide, but broadens considerably to the south.
The area is dominated by meta-sedimentary rocks, mainly argillite and greywacke, but also includes minor chert and volcanoclastic rocks. These units have been deformed into broad, northeast trending folds with sub-horizontal axes and steeply dipping axial planes. The rocks display mineral assemblages consistent with greenschist facies regional metamorphism.
Several large, north-easterly elongate, granitoid batholiths of unknown affinity are exposed in the basin. Many smaller intrusions have been identified in several areas. The large intrusions usually display considerable contact metamorphic effects that extend well into the metasedimentary rocks. Airborne magnetics indicate the presence of numerous, fairly narrow, Mesozoic dolerite dykes. These generally strike either north-south or east-west. The principle tectonic fabric and main regional fault systems strike northeast.
The licence areas straddle a contact zone between Birimian volcanoclastic and meta-sedimentary rocks. Most of the licences are characterised by intense laterite development and very limited outcrop.
Exploration History
The Sunyani Basin Project has seen very limited, mostly colonial and ancient artisanal mining activity. Limited modern exploration (soil sampling) was completed in the late 1990’s.
The southern portion of the Sunyani Basin has received only minor exploration attention. There are a number of historical references to alluvial gold occurrences in recent stream sediments where minor artisanal working has taken place. Fairly extensive alluvial workings have been noted around Berekum and Pamu, however the source of this mineralisation has never been identified. The area contains widespread quartz veining, but historic sampling has revealed no indications of primary gold. One exception was the Awiam Prospect, described as a narrow quartz vein ( <0.5m wide) which yielded considerable, though erratic gold grades.
Close to the village of Koraso, quartz veins underlying auriferous alluvial gravels were found to include minor gold. Indications of similar occurrences have been located at Botifire and Achochoboso, about 8km south and southeast of Wamanafo respectively.
The Botifire and Berekum licences include the prospects of Koraso, Biadani, Achochoboso and Botifire. The licences have had limited exploration and the prospects are based on ancient artisanal pits and historic documentation. The Koraso soil anomaly represents the only prospect defined by modern exploration techniques.
The Koraso Prospect is located 8km northwest of Berekum. It lies close to the northern boundary of the Berekum Licence and consists of numerous artisanal pits on terraced and recent alluvial gravels. A short adit and a small open pit have been excavated on quartz veins that contain low gold values and underlie the terraced gravels
Since listing Castle has completed soil sampling over part of the Berekum tenement near the village of Koraso where previous exploration work by another Company in the 1990s had reported highly anomalous soil values up to 43g/t gold. The sampling by Castle did not reproduce these very high grade values but did report a strong anomaly north of Koraso village including two soil values above 1g/t gold.
Field inspection of this area revealed extensive widths (+20m) of quartz veining in this area. A program of infill sampling is planned.
A soil sampling program is currently underway on the Bofitire Reconnaissance Licence.
Koraso - pit with sheeted quartz veining
The Biadani, Achochoboso and Botifire prospects are described as a field of artisanal pits in alluvial quartz gravels. No significant exploration has been undertaken on these prospects.
Exploration Potential
The Berekum and Botifire licences have only been the subject of preliminary exploration and no drilling has been completed on either licence.
Potential exists on both licences for the discovery of the following deposit types:-
- Recent unconsolidated placers; although widespread gold anomalism in the streams is noted, these appear to be of low grade and limited size, and are considered unlikely to be of economic interest.
- Orogenic vein-dominant deposits hosted primarily in Birimian meta-volcanic and volcanoclastic rocks and associated with major regional structures. All bedrock gold mineralisation defined to date in the Sunyani Basin appears to be related to quartz veins in Birimian meta-sedimentary and volcanoclastic rocks.
- Orogenic deposits hosted in granitoid and felsic porphyry intrusions. The Sunyani Basin is broadly analogous to the Kumasi Basin and could be expected to host similar styles of gold mineralisation. Much of the more significant mineralisation in the Kumasi Basin is associated with late stage granitoid/felsic porphyry intrusions which occur along major intrabasinal shears or “failed belts” (Ayanfuri, Mpesetia, and Abore).
