Obuasi gold mine is owned and operated by AngloGold Ashanti (Ghana) Limited (AAGL), a company registered in Ghana. AAGL through AngloGold Ashanti Holdings plc is wholly owned by AngloGold Ashanti plc.
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Summary:
Geology
The mine is located within the Obuasi concession area in southwestern Ghana along the northeasterly striking Ashanti volcanic belt. The deposit is one of the most significant Proterozoic gold belts discovered to date. The Ashanti belt predominantly comprises sedimentary and mafic volcanic rocks, and is the most prominent of the five Birimian Supergroup gold belts found in Ghana.
Deposit type
The Birimian was deformed, metamorphosed, and intruded by syn- and post-tectonic granitoids during the Eburnean tectonothermal event around two billion years ago. Folding trends are dominantly north-northeast to northeast. Elongate syn-Birimian basins developed between the ridges of the Birimian system, and these were filled with the Tarkwaian molasse sediments made up primarily of conglomerates, quartzose and arkosic sandstones and minor shale units. Major faulting has taken place along the same trends. The Lower Birimian metasediments and metavolcanics are characterised and defined by argillaceous and fine to intermediate arenaceous rocks. These rocks are represented by phyllites, metasiltstones, metagreywackes, tuffaceous sediments, ash tuffs and hornstones in order of decreasing importance. Adjacent to the shear zones, these rocks are replaced by sericitic, chloritic and carbonaceous schists, which may be graphitic in places. Multiple lodes are a common feature. Mineralised shears are found in close proximity to the contact with harder metamorphosed and metasomatically altered intermediate to basic Upper Birimian volcanics. The competency contrast between the harder metavolcanic rocks to the east and the more argillaceous rock to the west is thought to have formed a plane of weakness. During crustal movement, this plane became a zone of shearing and thrusting coeval with the compressional phases.
Mineralisation style
Gold mineralisation is associated with shear zones and pervasive silica, carbonate and sulphide hydrothermal alteration which occur in tightly folded Lower Birimian schists, phyllites, meta- greywackes, and tuffs, along the eastern limb of the Kumasi anticlinorium. They are found near the contact with harder metamorphosed and metasomatically altered intermediate to basic upper Birimian volcanics. There are two broad styles of gold mineralisation including free milling quartz vein gold and sulphiderich, disseminated and refractory gold which form alteration haloes around the quartz vein lodes. Sulphide mineralisation is dominated by arsenopyrite and quartz mineralisation, which is associated with spatially variable, but exceptionally high-grade visible gold in quartz veins.
Mineralisation characteristics
Two main ore types are present, namely quartz vein and sulphide ore. The quartz vein type consists mainly of quartz with free gold in association with lesser amounts of various metal sulphides containing iron, zinc, lead and copper. This ore type is generally non-refractory. The sulphide ore type is characterised by the inclusion of gold in the crystal structure of arsenopyrite minerals. Higher gold grades tend to be associated with finer grained arsenopyrite crystals. The sulphide ore is generally refractory.