Summary:
Property Geology and Mineralization
The AGM deposits are hosted along the NE-SW Asankrangwa structural shear corridor, which is defined by NE-SW trending lineaments and magnetic lows and is about 7 km wide and over 50 km long.
Nkran
Nkran occurs on a 20° trending jog on the Nkran Shear Corridor. The Nkran Shear is characterized by sheared siltstones (phyllites) dominant on the western side of the shear and sandstone dominant on the east. The central part of the Nkran deposit consists of a series of wacke and sandstone-dominated stratigraphy that has been intruded by several felsic porphyry intrusions (see Davis, 2016). Consistent mappable lithologies are the western sandstone, the central sandstone, the eastern sandstone, the felsic granitic porphyry intrusive unit, and the skinny breccia unit which is located within the eastern sandstones and runs along the strike of the deposit.
Esaase
Broadly speaking, the Esaase deposit area can be referred to as a ‘system of gold-bearing quartz veins hosted by tightly folded Birimian-age sedimentary rocks arranged along an NNE-SSW trending strike’. Since the maiden resource release in October 2007, various simplified geological models have been used to constrain the resource estimation.
Akwasiso
The Akwasiso deposit lies some 4 km NE of the main Nkran deposit and geologically bears many similarities to Nkran. A granite intrusion surrounds a 080° dipping cross structure and mineralization hosted in bounding 035°N sub-vertical shear structures transgressing a sandstone/siltstone sequence.
Abore
The Abore deposit is located on the Abore-Esaase shear corridor which also hosts the Esaase deposit. The main rock types observed within the Abore pit consist of carbonaceous shale, siltstone (phyllite), thinly bedded wacke, and thickly bedded sandstone. The sedimentary sequence has been intruded by a granitic (tonalitic) intrusion.
Asuadai
The Asuadai deposit is located on the regional NE trending Nkran shear zone, approximately 10 km long strike from Nkran. The prospect features a massive intermediate (tonalite) granitoid hosting a quartz stockwork system.
Adubiaso
The Adubiaso geology comprises a sub-vertical stratigraphy of interbedded greywacke and phyllite, with three sub-vertical granite (porphyry) dykes obliquely cross-cutting the stratigraphy. A steep dipping (65° E) quartz vein system cuts across Birimian metasedimentary rocks, which dip steeply at 75° to the west. The vein system appears to be related to a NE fracture system (distinct from the Nkran structure) along the contact zone between dominantly phyllitic units on the east and coarser greywackes on the west, which host most of the gold-bearing veins. The central part of the vein system is 15 m to 20 m wide, but it tapers to about 10 m at both ends; the vein system has a strike length of about 700 m although the main area of economic significance is the central 300 m of the zone.
Miradani North
The Miradani North deposit was mined between 1996 and 2016 by previous operators. The pit is 250 m long by about 120 m wide with a depth of about 60 m. Most of the oxide ore is depleted but the fresh rock remains untouched.
Dynamite Hill
The Dynamite Hill deposit is located on the Nkran shear trend about 7 km north of the Nkran pit and 4 km north of the Akwasiso pit where it offsets a regional north-south mafic dyke and a localized east-west crosscutting structure. The area is underlain by fine to medium-grained greywackes (intermittent strong alterations) intercalated with argillites (phyllites), and intrusions of altered felsic rock (feldspar quartz porphyry/granitoid), quartz veins, and stockworks. The initial depth of oxidation was between 20 to 50 m below the surface on rugged terrain but a portion of the oxidized rock has been mined out. The deposit was mined in 2018 from an RL of about 330 m to 180 m.
DEPOSIT TYPES
Two broad styles of gold deposits are present in southwest Ghana:
• Structurally controlled lode or orogenic gold deposits
• Paleoplacer disseminated gold deposits in Tarkwaian conglomerates.
The primary controls on mineralization in the Asankrangwa Belt are structural in origin. Certain sandstone units within the Birimian metasedimentary package provided favourable rheological conditions that optimized gold deposition often close to major lithological contacts with either Birimian metavolcanic rocks or Tarkwaian metasedimentary rocks (Griffis et al, 2002). The deposit type targeted by the AGM is this structurally controlled mesothermal quartz vein style mineralization (orogenic gold type deposits). This is the most important type of gold occurrence in West Africa and is commonly referred to as the Ashanti-type. Milesi et al. (1992) recognized that mesothermal quartz vein style deposits are largely confined to tectonic corridors that are often over 50 km long and up to several kilometres wide and usually display complex, multi-phase structural features, which control the mineralization.
There are at least two separate gold mineralizing events that are linked to the structural evolution of the area. Mineralization is linked to:
• Early isoclinal folding, shearing and/or duplexing of stratigraphy controlling the location of deformation zones and fluid flow
• A late approximate east-west compressional event that generated shallow dipping to flat orientated conjugate vein sets that crosscut the earlier rock fabric and gold mineralization.
This brittle style deformation postdates the emplacement of granitic intrusive units into the core of the existing deformed and sheared sedimentary rocks. Orogenic gold deposits formed between 2.2 and 2.0 Ga, intrusionrelated (and skarn) between 2.2 and 2.1 Ga, and paleoplacer types between 2.06 and 1.8 Ga.
Gold mineralization is associated with major NE striking, 5 m to 40 m wide graphite-chlorite-sericite bearing fault zones. In particular, gold mineralization is developed where the NE fault zones intersect major ENE striking fault zones, and especially where they are recognized to have influenced granite emplacement, alteration and gold geochemical trends.
Left stepping flexures (10 km to 30 km scale) in the NE striking fault zones (which produce more northerly striking fault sections), are important for the localization of gold mineralization. Other local complexities in stratigraphy and fault geometry, associated with major NE striking faults, are also important for example, folds in stratigraphy that may produce saddle reef style mineralization, or fault duplexes.
The most common host rock is usually fine-grained metasedimentary units, often in close proximity to graphitic, siliceous, or manganiferous chemical sediments. However, in some areas, mafic volcanic rocks and belt intrusions are also known to host significant gold occurrences. Refractory type deposits feature early-stage disseminated sulphides in which pyrite and arsenopyrite host important amounts of gold overprinted by extensive late stage quartz veining in which visible gold is fairly common and accessory polymetallic sulphides are frequently observed. This type includes important lode/vein deposits in Ghana such as at the Obotan and Esaase area. A second non-refractory style of gold mineralization occurs in which gold is not hosted within sulphide minerals either in early, or late stage mineralization. These deposit types have lower sulphide content in general and often lack the needle-like arsenopyrite that is common in the refractory type deposits.
The Asanko Gold deposits demonstrate a late (second) phase of gold mineralization hosted in granitoids (Nkran basin type granite), emplaced in regional shear corridors. The deposits are situated within the Birimian metasedimentary units, but the granitoid intrusions and mineralization both occur at contacts between greywacke and carbonaceous phyllite units.