Asante Gold Corp. acquired the Bibiani Gold Project in Ghana from Resolute Mining Limited (“Resolute”) through the purchase of all the issued and outstanding common shares of Mensin Bibiani Pty. Ltd, with the Ghanaian Government retaining 10% free carried interest in the mining operations.
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Summary:
The gold deposits at Bibiani are structurally controlled mesothermal lode-type deposits. The mineralisation is associated with quartz veins and quartz stockworks which are hosted within a sequence of Lower Birimian fine to medium grained turbiditic sandstones. The sedimentary turbidite sequence is tightly folded, with west-dipping axial planes and localised development of steep west-northwest dipping shear zones which have acted as conduits for the initial gold mineralisation. Gold is closely associated with arsenopyrite and pyrite. Sericite alteration is also commonly observed, both along mineralised structures, and associated with the felsic intrusives. High-grade veins often occur within graphitic shales which generally contain significant sedimentary/metamorphic pyrite.
The Bibiani Project consists of a multi-deposit complex with essentially three mineralised deposits. The MGBL assets are primarily made up of the Bibiani Main Pit, historically a surface and underground mine, with numerous satellite pits along strike to the northeast and southwest along the Bibiani Shear Zone.
Satellite mineralised deposits situated on the same geological structure include South Hill, Russell (south of Main Pit), Big Mug, Little Mug, Pamunu South, Pamunu North and on identified geological splays off the main structure to the northeast includes Walsh, Strauss South, Strauss, Ahiman, and Grasshopper. The adjacent Walsh and Strauss Pits have been exploited previously as open pit operations and were further exploited by Asante within the last two years.
The license area is interpreted to be underlain by volcanic/volcaniclastic/sedimentary rocks related to the transition zone between the lower and upper Birimian Supergroup. In the southern part of the license the rocks strike about 20º to 30º E and dip steeply to the southeast. Further to the north the strike changes to between 40ºE and 50ºE.
Underground and surface mapping has shown that the mineralisation is bounded by a series of faults-vein breccia lodes of varying size and gold mineralisation grades. The mapping indicates that there are several cross faults that offset the stratigraphy, but there is no clear indication that there is a relationship between these faults and the gold mineralisation. The trend of the Bibiani mineralised deposit appears to continue northwards to the Pamunu River, some 2km to the north of the Bibiani Mine. A parallel splay from the Bibiani trend continues up to the Bibiani North mineralised deposit, which centres about 1km to the north of the Bibiani Mining Lease.
Bibiani Main, Big Mug & South Hill
Most of the geological study has been focused on the Bibiani Main mineralisation, which is hosted by Kumasi Basin turbidites several hundred metres east of the Sefwi Shear. Sediments are metamorphosed to lower greenschist facies, with primary textures well preserved away from altered and high strain zones. Turbidites are generally monotonous, and no marker horizons have yet been discovered, which has limited stratigraphic understanding. Common overturned fining-up sequences and fold vergences together indicate that the sequence exposed in the pit and surrounding drillholes is located on the eastern limb of a district-scale antiformal syncline. At least three generations of cleavage are developed but limited detailed geological mapping means the overall kinematics are not yet fully understood.
The Bibiani Main Zone within the open pit and the underground zone is mineralised over a strike length of approximately 2km. At the centre of the mine the deposit strikes 030° to 035° which changes to around 020° at the northern end of the mine. In general, the mineral deposit dips east at 60° to 80°, crossing the regional structural fabric at acute angles.
Wall rocks adjacent to the quartz veining demonstrate fine-grained disseminated iron-carbonate and sericite alteration with associated sulphide mineralisation. These alteration haloes can also contain gold values up to 2g/t Au.
Some prominent structures measured in the pit walls and berms of the Main Pit include faults, quartz veins, intrusives, dykes, cleavage planes and bedding planes. These structures have significant control on gold mineralization in Bibiani especially extensional faults that serve as conduit for fluid flow and subsequent mineral deposition. Mapping data shows very strong evidence of folding in the metasediments. The metasediments have been pushed up against a competent metavolcanic rock suite resulting in tight and upright folding.
At Main Pit north, gold mineralization occurs within zones of quartz-ankerite-dolomite veining and gold is strongly associated with arsenopyrite and pyrite. Sericite alteration also commonly observed both along mineralized structures, and in and around the felsic intrusive. High-grade veins often occur within carbon rich shales with significant pyrite mineralization. Most of the gold mineralisation at Bibiani is syn-orogenic and is associated with quartz and quartzankerite (Fe-dolomite) veins and quartz stockworks. Both vein types are associated with pyrite ± arsenopyrite. There is a positive relationship between the presence of gold and the presence of arsenopyrite. The maximum arsenopyrite content has been observed to be around 2% to 3%. Microscopic examination confirms that much of the gold occurs along edges or cracks within the sulphide grains. The size of gold grains is typically less than 50 microns, generally observed to be between 1 to 10 microns in size.
Big Mug mineralized deposit is located directly north of the Main Pit, underlain by Birimian metasedimentary rocks in the eastern part and by intercalated metasedimentary and metavolcanic rocks in the western part of the Mining Lease area. Big Mug however does not have the tonalite intrusive at the east of the main pit but rather a less continuous dacite dyke that pre-dates the main deformation (as evidenced by a pervasive cleavage absent from the tonalite).
South Hill mineralization, located directly south of Main pit, is part of the main Bibiani mineralization trend striking around 450 NE and dipping moderately to the SE. The geology includes the Birimian metasediment intruded by granitoids located east of the main lode. Both lithologies appear to be intruded by quartz veins. High grade gold mineralization is concentrated in strongly foliated and silicified metasediment (breccia) located west of the granitoid. Low grade mineralization, on the other hand, is concentrated locally at zones where quartz veins intercept the granitoid thereby creating dilation zones for gold mineralization. Mineral alterations within these rocks include carbon, sericite, silica and sulphides (mainly pyrite).
The mineralized zone exhibits strong shearing and brecciation with abundant quartz veining and increased carbon concentrations which may have structurally contributed to the ore deposition by aiding deformation within the fluid corridor.