Introduction
Banso is located approximately 180km northwest of Accra, in the Ashanti gold belt. It is ~20km west of Newmont's 8Moz Akyem gold mine and ~10km east of AngloGold Ashanti's 50Moz Obuasi gold mine.
Castle has completed detailed soil sampling, pitting and two drilling programs over the Banso prospect and intersected gold mineralisation over a 3km east west trending zone.
A soil sampling program has been completed on the Anweaso and Fereso Licences testing the south western area of the project close to the Ashanti belt margin. A total of 1,202 soil samples were collected and submitted for analysis. Results are expected in early June 2007.
Project Geology
The Obuom (Bosumtwi) Range is underlain by a thick sequence of Birimian mafic and intermediate volcanic rocks, which extend from southwest of Lake Bosumtwi. The Obuom Range includes Birimian meta-volcanic rocks, with subordinate argillaceous and arenaceous meta-sedimentary rocks, and a variety of mafic and granitoid intrusive rocks. The western margin of the belt features a transition from interbedded Birimian meta-volcanic and meta-sedimentary rocks that grade laterally into meta-sedimentary clastic rocks of the Kumasi Basin.
The rolling hills between the Obuom Range in the west and the Anum River in the east, are comprised of Tarkwaian sedimentary rocks, locally intruded by mafic sills. The trend of the hills also reflects the northeast strike of the underlying rocks. The Tarkwaian rocks are represented by conglomerates, quartzites, and micaceous and feldspathic quartzites of the Banket Series and Kawere Formation respectively.
Aeromagnetic images indicate a number of mafic sills within the Tarkwaian and Birimian successions, as well as a number of Mesozoic dolerite dykes that strike between north and north-northwest. Quartz porphyry and aplite intrusions occur in Birimian rocks close to the eastern contact with the Tarkwaian rocks to the west of Amamanso.
To the east of the Anum River and east of Achease is the large Banso granitoid complex, phases of which intrude the Tarkwaian rocks, with radiometric dating giving an age of about 2,081 Ma. This intrusion is therefore most likely of the Cape Coast suite.
Exploration Potential
Although the Fereso and Anweaso licences have been assessed by a number of companies in the 1990’s, there has been no drilling or even systematic mapping and trenching. A limited number of trenches over the old workings at Sikaso were excavated, with one trench returning 4m at 8.0g/t Au. Mineralisation is associated with quartz veins in minor shear zones hosted by Birimian mafic volcanic rocks, which coincide with east-northeast trending soil anomalies. No follow up drilling has been completed.
The western side of the Fereso licence follows the northeast trending structural contact between the Birimian metavolcanic rocks to the west and the Tarkwaian sedimentary rocks to the east. Historic workings are scattered over approximately 10km of strike length along the northeast structural contact that is also coincident with numerous small felsic porphyry intrusions. Along this trend to the northeast, historic workings and porphyry intrusions have not been noted.
The historic Imbatiem (or Mitiam) mine and the Kwameng, Amormoso, Sikaso and Hwere (Fwere) prospects are all located within the Fereso Licence area or immediately west inside the Bosumtwi Range Forest Reserve. The prospects occur along a zone that strikes northeast, being associated with the same regional structure. Mining is concentrated on quartz veins hosted in Birimian chlorite schists and fine-grained granitoids.
Imbatiem is located 3.5km north-northwest of Bodwesango. It comprises six adits, an open cut and four shallow shafts. A mineralised quartz vein (0.3m to 1m wide) that strikes north-northeast and is sub-vertical was reportedly driven on for a length of 100m in one of the adits. The vein contains tourmaline and accessory pyrite, and occurs in sheared Birimian mafic meta-volcanic rocks and quartz porphyry intrusions. AuVista NL sampled the workings and concluded that the deposit did not have sufficient size to support a mining operation.
The Sikaso Prospect is about 1km south-southwest of Imbatiem. Trenching by AGC returned 4m at 8.0g/t and 4m at 1.1g/t Au associated with quartz veins along shears within Birimian mafic volcanics coinciding with east-northeast trending soil anomalies.
Mineralisation at the Kwameng workings is limited to a thin horizon (1m to 1.5m) of paleao-placer style gold mineralisation within Banket Series conglomerate. Sampling of Tarkwaian quartzites at the Amormoso workings returned no significant gold values.
A regional soil geochemistry program identified one major gold anomaly at Danso. Previously named the Nsuaem soil anomaly, the Danso target comprises three narrow east-northeast striking soil anomalies that were identified from AGC regional sampling (3,600m x 50m, 2,000m x 50m and 800m x 50m). These are consistent with an east-northeast striking major regional shear zone. Quartz veins occur in proximity to the intersection of this major shear zone and the contact between Birimian meta-volcanic and Tarkwaian sedimentary rocks. Gold soil results have a maximum value of 2,220ppb, with the majority >100ppb Au lying close to known quartz veins.
The Dwendwenase and Banso licences include the Banso and Banso East (partly covered be Newmont licences) soil anomalies. These anomalies are separated by the Mirasa Hills Forest Reserve. The majority of work was completed on the licences by BGH, discovering the Akyem deposit with the same geochemical program. The company’s subsequent focus was on the Akyem deposit area.
The Banso grid lies on the Banso and Dwendwenase licences to the west of the Mirasa Hills Forest Reserve. Two anomalies (+100ppb), termed the Eastern and the Western anomalies, were outlined on the Banso grid. The western anomaly has a strike length of 2km with widths between 100m and 450m, and gold values up to 795ppb (Figure 7). The eastern anomaly extends for 4km, and varies between 100m and 500m in width with gold values up to 595ppb.
Trenching was completed across the defined soil anomalies. Results from the Eastern Anomaly include channel sample intercepts of 9m at 1.29g/t and 8m at 2.12g/t Au. Mineralisation is associated with quartz veins in sericite schists and altered phyllites. Trench intercepts from the Western Anomaly include 9.85m at 9.19g/t, 5.4m at 1.57g/t, 13.25m at 1.56g/t, 23m at 2.33g/t and 8.10m at 4.34g/t Au. This mineralisation is associated with quartz veins in gabbro, felsic intrusive and meta-sedimentary rocks.
The Banso East grid lies to the east of the Mirasa Hills Forest Reserve. Part of this soil sampling grid occurs on the Dwendwenase Licence, with the remainder occurring on Newmont’s licence to the east. A soil anomaly with a strike of 1.5km and values up to 2,495ppb Au was outlined. Trenching generated mineralised intercepts of 20.5m at 1.66g/t and 14.95m at 1.10g/t Au. The gold mineralisation is related to quartz veins in felsic and mafic intrusive rocks, and meta-sedimentary rocks.
There are three untested gold soil anomalies within the Dwendwenase and Banso licences. The regional exploration program that defined these anomalies on the Banso and Banso East grids was the same one that led to the discovery of Newmont’s Akyem deposit. This program was originally designed to explore for Banket Series palaeo-placer style mineralisation within the east-northeast trending Tarkwaian syncline. The trenching and soil anomaly results generated are considered extremely encouraging.
The Banso Project occurs near the intersection of two major shear zones within the Ashanti Belt. Significant gold occurs in potentially economic grades and widths in surface trenches at the Banso Prospect, in addition to numerous other untested gold soil anomalies throughout the project area. The Banso Project has excellent potential to host near-surface, economic gold deposits.