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Yamfo Sefwi / Ahafo Project - Ahafo South, Subika, Apensu, Awonsu, Amoma - Ahafo North, Yamfo, Susuan, Teekyere, Subenso |
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Ghana |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Ahafo South and Ahafo North deposit clusters, previously the Yamfo-Sefwi Project, are located in western Ghana near the towns of Kenyasi and Ntotroso in the Brong-Ahafo Region, ~290 km NW of Accra. The operations are 107 km NW of Kumasi, and 40 km south of the regional capital Sunyani.
The Ahafo South operation comprises, from SW to NE, the
Subika,
Apensu,
Awonsu and
Amoma,
deposits which are distributed along an ~15 km strike length of the Kenyase-Yamfo shear zone. The Ahafo North deposits are, from SW to NE, Yamfo, Susuan, Teekyere and Subenso over a 12 km strike length. Yamfo is ~12 km NE of Amoma.
Go to: Regional Setting, Sefwi Belt, Ahafo South, Ahafo North and Reserves & Resources
A Ghanaian-German cooperative mineral exploration program conducted between 1989 and 1991 identified a 1.2 km long gold soil anomaly. Follow-up 50 x 400 m grid soil sampling by the Ghanaian company Minconsult in 1992 confirmed and better defined this gold anomaly. In 1993, a joint venture (JV) agreement was signed between the French government's Bureau Recherché Geologiques et Minieres (BRGM) and Gencor Ltd to explore in Ghana and neighbouring Cote d'Ivoire. In 1994, that JV completed an option agreement over the Yamfo area and formed Centenary Gold Mining Company (41% BRGM, 41% Gencor, 8% Minconsult and 10% Ghanaian Government). In the same year, La Source Compagnie Miniere SAS was formed, in which Normandy Mining Limited held 60% and BRGM 40%, to take over BRGM’s West African exploration and mining assets. Over this period, stream sediment and soil sampling, trenching, rotary air blast, reverse circulation (RC) and core drilling was undertaken in the Yamfo (now Ahafo North) area and what is now Ahafo South, culminating in an initial mineral resource estimate. In 1996, a scoping study evaluated the Teekyere West, Yamfo Central, Subenso and Line 10 deposits (now within the Ahafo North area), followed by an updated resource estimate and feasibility study. In 1998, La Source acquired the former Gencor and Minconsult interests in Yamfo. In 1998, BRGM exited the La Source JV, which was dissolved, and Normandy, through its 100% ownership of La Source and thereby Centenary Gold, took control of the operation. In 2000, Centenary Gold Mining Company Limited changed it's name to Normandy Ghana Gold Limited. A major drilling program had been initiated in September 1998 to test new targets on both the Yamfo (now Ahafo North) and Sefwi (now Ahafo South) licenses and a broader pre-feasibility study was completed in November 1999, followed by a feasibility study in 2000.
Meanwhile, from 1996, work on the separate Rank Mining Concession (now the Ntotroso license area), located within the north-central core of the Sefwi (Ahafo South) licenses, had identified eight major gold anomalies. These were designated Zones A to G inclusive, and Sika Aminaso. This concession was held by the Rank JV (40% Rank Mining Company Limited, 60% Moydow Mines International Inc). RC drilling continued testing these anomalies into 1997, targeting zones A to C, while identifying a Zone E (now Subika), with a resource estimate being published later that year for these four zones. Work between 1996 and 1999 outlined the deposits that are now known as Amoma, Apensu, Awonsu and Subika in the Sefwi area (Ahafo South). Between 1997 and 2001, La Source progressively acquired an interest in Rank Mining by funding exploration and development, to achieve complete ownership by the end of that period. In 2000, La Source and Moydow had signed a Development and Production agreement, whereby treatment of mineralisation from Rank Concession deposits would be treated through the proposed Sefwi plant. A feasibility study, managed by La Source, and based on the Subika and Zone A deposits was commissioned in 2000 (Doe, 2019). The initial resource outlined in the Yamfo-Sefwi project area by 2000 amounted to ~215 tonnes of contained gold (Masurel et al., 2021).
In early 2002, Newmont acquired Normandy and thereby it's interests in the Yamfo, Sefwi and Ntotroso licenses. The combined project was renamed 'Ahafo'. A feasibility study which evaluated the deposits of the Ahafo South and Ahafo North areas was completed in 2003. In December of that year,
Newmont acquired the remaining interest in Rank from Moydow Mines, and the area was incorporated into the Ahafo South area. As a result of the 2003 feasibility study, a decision was taken to construct a process plant in the Ahafo South area and commence mining from the Apensu deposit, deferring mining activity in Ahafo North. Open pit mining commenced in 2006. The Apensu and Amoma deposits have since been paused, and by 2018, mining was underway at the Subika open pit and from late 2018 at the Subika underground. Open pit mining resumed at Awonsu and Amoma in late 2019.
Exploration has continued in the Ahafo South area, outlining the Subika underground mineralisation in 2006 to 2008, whilst the Apensu South deposit was discovered ~1 km south of the Apensu open pit in November 2013, and the Apensu Deeps area was located in 2014. An expansion of the Apensu mineralisation to
the north was identified in 2016. The Ahafo South and North areas are separated by the Shelterbelt Forest Reserve.
Production from Ahafo South in the year ending 31 December, 2022 amounted to 17.85 tonnes of gold (Newmont website, 2023).
Regional Setting
Ghana is located in southern West Africa. It largely lies within the West African Craton, which is sub-divided into the Archaean Reguibat Shield to the north in Mauritania, separated by the Neoproterozoic to Phanerozoic rocks of the 1000 km wide Taoudenni Basin from the Palaeoproterozoic Man Shield in the southernmost third of the craton, between Ghana and Senegal. The Man Shield is divided into two sectors, a western third composed of Meso- to Neoarchaean, 3.0 to 2.5 Ga, 'Liberian' age rocks, and an eastern terrain of Palaeoproterozoic 2.25 to 2.05 Ga Birimian rocks. The Birimian is composed of five, NE-SW trending, evenly-spaced tholeiitic to felsic composition volcanic belts, separated by basins filled by predominantly turbiditic sedimentary rocks. The transition zones between the volcanic rocks and the sedimentary rocks are occupied by chemical sedimentary rocks. All the units are regarded as contemporaneous, probably representing laterally equivalent facies. The Birimian units are overlain by, and interbedded with, the Tarkwaian detrital sedimentary sequences composed of conglomerates (with Birimian clasts), sandstones and phyllites, which appear to have accumulated in restricted basins within the volcanic belts of the Birimian towards the end of the volcanic cycle.
The Birimian sequence is intruded by three Eburnean age granitoid suites, the:
i). Cape Coast type granitoids, dated at 2.1 to 1.9 Ga, that are generally intruded into the sedimentary basins between volcanic belts, and represent a multiphase intrusion involving four separate magmatic pulses. These are locally well foliated, often migmatitic and potash rich, dominantly two mica granitoids, occurring as muscovite-biotite granite, granodiorite, porphyroblastic biotite gneiss, aplites and pegmatites. The granites are characterised by numerous enclaves of schists and gneisses;
ii). Dixcove type granitoids, which are dated at ~2.18 to 2.15 Ga, are associated with the volcanic belts and are dominated by hornblende-bearing granite or granodiorite, locally grading into quartz diorite and hornblende diorite. They occur as non-foliated discordant and semi-discordant bodies
within the enclosing country rocks, which are generally Upper Birimian meta volcanics. They are intruded along deep seated faults in three, successive, distinct phases which progress from mafic to felsic, i.e., gabbro → diorite → granodiorite. Overall the complex has lower SiO2 and Al2O3, but marginally higher CaO contents than the Cape Coast granite, as well as a higher Na2O/K2O ratio. However, unlike the Cape Coast granite, it is devoid of lithophile
elements such as Li, Be, Sn;
iii). Bongo, Tongo and Banso post-Tarkwaian K-rich granitoids, mainly porphyritic, hornblende-microcline plutonic granites that are locally found in north-eastern Ghana within the volcanic belts, and are estimated to have been emplaced at ~1.8 Ga.
The volcanic rocks within the greenstone belts are mainly basaltic and are metamorphosed to varying degrees from lower greenschist to lower amphibolite facies, and encompass elongate plutons of the Dixcove granitoid suite. These NE-SW trending granite-greenstone belts range from 25 to 50 km in width, and are separated by 50 to 75 km wide basins containing a sedimentary succession that mainly consists of fine to medium-grained lithologies (argillites and wackes) with variable amounts of volcaniclastic material. Cape Coast-type granitoids intrude these metasedimentary rocks.
The strong NE-SW structural grain controls i). the axial planes of the broad anticlinoria that expose the sedimentary basins; ii). synclinoria that characterise the volcanic belts; iii). regional-scale faults that separate these domains.
These faults and associated structures have undergone a complex history, including thrusting and shearing, with both normal and strike-slip displacement, and have exerted a major role in the emplacement of gold mineralisation. The regional scale, NE-SW trending Kenyasi Thrust Fault that separates the Sefwi Greenstone Belt to the SE from the Sunyani Basin to the NW, has controlled the regional structure. Numerous ENE to NE splays have been mapped from this thrust. Other north to NW-trending structures are occupied by mafic dykes.
The Birimian granite-greenstone belts represent the world’s premier Paleoproterozoic gold province, including the Ashanti Greenstone Belt with an endowment of >3000 tonnes of contained gold (Thébaud et al., 2020; Goldfarb et al., 2017), whilst the neighboring Sefwi granite-greenstone belt hosts the Ahafo South and North gold systems containing an estimated ~465 and ~100 tonnes of gold respectively (Masurel et al., 2021).
Sefwi Granite-Greenstone Belt
Low-grade meta-sedimentary successions of the Sunyani-Comoé and Kumasi basins flank the Sefwi Granite-greenstone Belt to the NW and SE respectively. The Sefwi Belt itself, has been subjected to regional peak upper greenschist to lower amphibolite facies metamorphism in it's central and northwestern sections, in contrast to low- to mid-greenschist facies to the SE (Hirdes et al., 1992, 1993; Galipp et al., 2003). The mafic and felsic volcanic and volcaniclastic rocks of the Sefwi Group that principally constitutes the greenstone belt, have been dated at from 2.20 to 2.16 Ga, attributed to a period of juvenile magmatic crust accretion (Hirdes et al., 1992, 1996; Hirdes and Davis, 1998; Loh and Hirdes, 1999).
These sequences were intruded by voluminous regional monzogranitic plutons between ~2.16 and 2.15 Ga (Feybesse et al., 2006; McFarlane et al., 2019), interpreted to have been generated by the partial melting of mafic rocks of the Sefwi Group at deep crustal levels (Pouclet et al., 1996; McFarlane et al., 2019). Locally, these monzogranites are overlain across an erosional unconformity by sedimentary rocks of the Sunyani-Comoé Basin which contain detrital zircons derived from these plutons (e.g., Pouclet et al., 1996; Vidal et al., 1996; Oberthür et al., 1998). Volcaniclastic rocks containing clasts of mafic to intermediate basement with maximum deposition ages of ~2.15 Ga have been reported (e.g., Grenholm et al., 2019), whilst detrital zircons across SW Ghana indicate the widespread deposition of the thick flysch series of greywackes, sandstones, siltstones and mudstones in the basin occurred after ~2135 Ma (e.g., Davis et al., 1994; Oberthür et al., 1998; Hirdes et al., 2007; Adadey et al., 2009; Masurel et al., 2022).
Inversion of the Sunyani-Como&ecute; basin is interpreted to have coincided with emplacement of the 2116 ±2 Ma Wenchi granite, which intruded folded and metamorphosed sedimentary rocks (Hirdes et al., 1993). The deposition of synorogenic molasse, similar to the Tarkwaian of the Ashanti granite-greenstone belt to the SE, was restricted to a narrow segment of the Bibiani Fault system. These immature polymict conglomerates contain reworked crustal clasts from both the Kumasi flysch series and Sefwi Group volcano-plutonic rocks (Agyei Duodu et al., 2009). In contrast, the inferred equivalent synorogenic molasse Tarkwaian cover sequence of the Ashanti Granite-greenstone Belt is areally much more extensive. This is inferred to suggest differing i). tectonic histories and/or ii). rates of exhumation, erosion and preservation.
Sedimentary rocks within a narrow ~50 km wide zone along the southeastern margin of the Sunyani-Comoé Basin have been intruded by numerous 2.09 to 2.08 Ga two-mica granites and leucogranites (Hirdes et al., 1992, 2007; Hirdes and Davis, 1998). Deformation features and elongation of these granitoids are interpreted to suggest they were emplaced under an oblique dip-slip deformation regime linked to the reactivation of the inherited late Eburnean tectonothermal architecture (Feybesse et al., 2006; Jessell et al., 2012).
The Ahafo South and North deposit clusters lie along the structurally complex northwestern margin of the Sefwi Granite-greenstone Belt which is characterised by a number of long-lived, N and NE to ENE-striking faults and shear zones. The most significant of these, the Kenyase-Yamfo shear zone, represents a major, broad, tectonostratigraphic boundary juxtaposing deep crustal volcano-plutonic rocks of the Sefwi Granite-greenstone Belt over younger, lower greenschist facies sedimentary rocks of the adjacent Sunyani-Comoé Basin to the NW. This is interpreted to represent both crustal thickening and exhumation during the 2115 to 2060 Ma Eburnean Orogeny. These relationships suggest an initial D1 tectonometamorphic event that buried the volcanic and volcaniclastic rocks of the Sefwi Granite-greenstone Belt to maximum depths of as much as 30 to 40 km, at 600 to 700°C, between ~2135 and 2095 Ma (Feybesse et al., 2006; McFarlane et al., 2019). Deformation subsequently switched to a transpressional regime, with east-west directed shortening under regional greenschist facies metamorphic conditions, coincident with the 2090 to 2080 Ma emplacement of peraluminous leucogranites in the adjacent Sunyani-Comoé Basin (Jessell et al., 2012). Exhumation related to sinistral-normal transtension (D2; McFarlane et al., 2019) has been dated at 2073 ±2 Ma (metamorphic monazite; e.g., Masurel et al., 2022). Alternatively, uplift and exhumation of high-grade rocks may have occurred coeval with transpression, involving >10 km of dextral lateral displacement on the Ketesso high-strain zone that defines the SE margin of the greenstone belt (McFarlane et al., 2019), with a slip vector plunging 5 to 10°SW (Hirdes et al., 2007). A late Eburnean transpressional deformation regime in the Ahafo area also resulted in the sinistral reactivation of the Bibiani Shear Zone in the core of the greenstone belt (transgressing to the SE margin in the south), and sinistral-reverse displacement along the Kenyase-Yamfo shear zone on the NW margin of the greenstone belt (Feybesse et al., 2006; McFarlane et al., 2019; Masurel et al., 2022).
All of these structural elements are truncated and/or dissected by discontinuous N- to NNE-striking, transpressional, sinistral faults along the northwestern margin of the Sefwi Belt (McFarlane et al., 2019b). This pulse is inferred to represent the same geodynamic trigger that initiated the bulk of gold mineralisation at Bibiani and Chirano located along the south-central margin of the Sefwi Granite-greenstone Belt (Allibone et al., 2004; Goldfarb et al., 2017; Thébaud et al., 2020).
The latest deformation pulse in the Ahafo area coincides with local, discontinuous, conjugate ENE-striking dextral and NW-striking sinistral faults (D5 of McFarlane et al., 2019), that are associated with low-displacement offsets of earlier structures and fabrics (Masurel et al., 2022).
Geology of the Ahafo South Cluster of Deposits
The majority of the gold deposits of the Ahafo South cluster, i.e., Apensu, Awonsu and Amoma, straddle the anastomosing, NE (40°) striking and 60°SE dipping Kenyase-Yamfo shear zone, which corresponds to the rheological contact between i). competent, upper greenschist to lower amphibolite facies volcanic rocks with elongate composite plutons of the Sefwi belt, andii). rheologically weaker, lower greenschist facies sedimentary and volcano-sedimentary rocks of the Sunyani-Comoé Basin. However, in contrast, the Subika deposit, is found ~2 km east of the Kenyase-Yamfo shear zone, internal to the Sefwi Belt, and is entirely hosted by intrusive rocks.
The intrusive rocks in the hanging wall of the Kenyase-Yamfo shear zone have a variable grain size and composition and are characterised by locally abundant xenoliths and rafts of mafic igneous rocks such as microgabbro, dolerite and amphibolite. The least altered intrusions range from diorite at Subika to granodiorite and monzogranite at Apensu, Awonsu and Amoma, occurring as ~1 to>5 km wide bodies, with sharp but structurally overprinted contacts with supracrustal country rocks. The pre- to synkinematic plutons are crosscut by abundant pre-mineral mafic dykes and chonoliths prior to gold mineralisation. These mafic bodies are massive, dark green, fine grained and predominantly composed up of plagioclase feldspars, actinolite, chlorite, quartz and calcite.
Subika
The Subika deposit is hosted by a composite, predominantly diorite, pluton of Dixcove Suite granitoids that also includes gabbro, microdiorite and mixed diorite-gabbro. Together these form a complex that also includes lesser crosscutting mafic, microgranodiorite, aplite and granitic pegmatite dykes. In the deposit area, the host diorite contains prominent and ubiquitous 5 to 25 cm thick pink veins that are essentially composed of quartz and K feldspar. These veins consistently strike ENE and dip steeply to the south. The mafic dykes, which are abundant, occur as two sets that are 0.1 to 1.5 m thick, composed of fine- to medium-grained, greenish mafic rock that respectively strike ENE and dip steeply to the south, and ENE- to NE- with a shallow southerly dip, accompanied by rarer north dipping conjugates.
The earliest observed structural texture, is a sub-horizontal magmatic foliation in the host diorite, defined by flattened mafic clasts (e.g., microgabbro and amphibolite xenoliths) and hornblende, cut by all dykes and veins. No cross-cutting relationship between the steep mafic dyke set and sub-parallel feldspar veins has been readily observed, although the shallow-dipping mafic dykes are seen to postdate the felsic veins (Masurel et al., 2021).
The granitoids are cut by multiple NE- to ENE-striking shallow-dipping mylonitic shear zones that are interpreted to occur as imbricate thrusts varying from <1 to as much as 10 m in thickness, and can be traced for over 1 km across benches in the open pit. Zones of brittle fracturing and dilatant breccias are commonly developed above the mylonite zones (Masurel et al., 2021). These imbricate thrusts include four main structural zones, namely (after Doe, 2019):
• The Victor Fault on the southern end of the Subika deposit, occurring as a major 2 to 6 m wide shear zone that strikes at 60° and dips ~20 to 30°SE. Locally it anastomoses into three branches, Victor, Victor A and Victor Lower Faults and is cut by dilatant breccias and brittle shears throughout. It has apparently been reactivated, and displaces the Subika mineralisation by as much as 40 m in an inferred sinistral sense;
• The Kaalbas Fault, which is just oblique to the Victor fault and higher in the thrust pile, with a slightly more easterly trend and shallower dip;
• The Hatch Zone, which appears to be an anastomosing, almost east-west trending structure, with two to three individual planes, each of which is 1 to 3 m in thickness, developed within an overall 6 to 25 m wide structural zone. Mineralisation appears to be displaced by ~50 m across this zone;
• The Deep One Shear, which is apparently confined to the northern end of the deposit.
There is a spatial association between these shallow-dipping mylonitic shear zones and shallow-dipping mafic dykes in the open pit, with kinematic indicators such as obliquity of foliation to shear zone boundaries, drag of the early felsic veins into shear planes, S-C fabrics, and striation lineations with associated stepped fibres implying reverse displacement.
A poorly defined structure, known as the Subika (or 'Magic') Fracture Zone (Baah-Danso, 2011), strikes at 50° and dips 60°SE, and controls the bulk of the ore. In the field it is correlated with a steep, 5 to 30 m wide, beige alteration zone that has sharp contacts with the least altered diorite wall rocks. The gold-related hydrothermal alteration associated with the 'Magic Fracture Zone' at Subika comprises a continuous zone of quartz-albite-sericite-carbonate-pyrite alteration that can be subdivided into three distinct zones (after Doe, 2019 and Masurel et al., 2021):
i). proximal strong alteration, with the mineral assemblage silica-Fe carbonate-albite-pyrite and abundant quartz-carbonate veining that carries >3 g/t Au, and is very closely associated with intensely-altered cataclasites, with sharp contacts along the edges of the stockwork/veined fracture zone;
ii). intermediate moderate alteration, with ~1 to 3 g/t Au, characterised by a chlorite-sericite-pyrite-albite-Fe carbonate-silica assemblage. This zone forms a narrow halo varying from 1 to 30 m in width surrounding the core of the 'Magic Fracture Zone', and is consistently wider in the hanging wall where it is controlled by widely spaced quartz veinlets. Moderate alteration locally extends outward into crosscutting reactivated mylonite zones;
iii). distal weak alteration with lower grade ~0.1 to 1 g/t Au, primarily characterised by chlorite bleaching, with the mineral assemblage chlorite-sericite-pyrite, that occurs as a broader halo around the fracture zone and extends out into re-activated mylonite zones.
The steep Subika/Magic Fracture Zone and the shallow-dipping mylonitic shear zones are oblique to each other, but have a common deformation style and hydrothermal alteration. Narrow cryptic slip planes have also been mapped adjacent to the Subika/Magic Fracture Zone. These comprise steep, discontinuous, 10 to 15° striking, sinistral, parasitic, centimetre scale faults which cut across the low-angle shear zones but locally have striation lineations and slickenfibres made of a hydrothermal mineral assemblage equivalent to that of the ore (Masurel et al., 2021).
The Subika deposit is located ~2 km SE of the Apensu Main deposit (see below), in the hanging wall of the Kenyasi Thrust Fault, but lies on the separate and parallel Subika/Magic Fracture Zone. It has horizontal dimensions of ~2200 x 400 m, and has been tested to ~1000 m below surface, where it remains open at depth and along strike. In drill core, the Subika/Magic Fracture Zone is 5 to 50 m thick and comprises multiple, laterally stacked, high-grade (>3 g/t Au) lenses, typically separated by 2 to 5 m thick intervals of waste. These waste intervals correspond to shallow-dipping mafic dykes, with economic-grade mineralisation essentially focused on their contact margins and in fractured zones in diorite wall rocks. Some ~70 to 80% of that gold mineralisation occurs as stacked arrays of shallow dipping, 5 to 20 mm thick quartz-carbonate shear veins with associated zoned alteration in the vein selvages. The remaining 20 to 30% is in extensional to stockwork vein arrays in fractured zones adjacent to the shear zones. Most of the veins are impregnated with pyrite, with ~3 to 10% disseminated pyrite closely associated with gold in wall rocks to these veins, and in some cases with sparse visible gold at their contact with the host rock. The intensity of veining and extent of hydrothermal alteration in vein selvages is directly proportional to the grade of mineralisation.
The shallow dipping mylonitic shear zones are also mineralised with ~0.5 to 3 g/t Au where in close proximity to the Subika/Magic Fracture Zone but become subeconomic with increasing separation. This mineralisation is preferentially localised along strong rheological gradients such as dyke contacts.
Better gold grade mineralisation occurs in dilatant zones within the Subika/Magic Fracture Zone, ranging from 1 to 60 m in width, and plunging at ~20 to 70°. These high grade shoots include those that plunge at 10 to 25°SW, and appear to be controlled by sinistral jogs, created by offsets across the intersection between the Magic Fracture Zone and the shallow dipping mylonite zones. In addition, 40 to 50°NE plunging ore shoots are well defined and correlate with the structural intersection between cryptic NNE-striking faults and the Kenyase-Yamfo Shear zone. Higher-grade mineralisation also correlates with steeper-dipping segments (i.e., restraining bends) of the Kenyase-Yamfo shear zone, which are bound by low-angle thrusts and mafic dykes. The intensity of proximal alteration and gold grades is also stronger immediately beneath these low-angle thrusts and mafic dykes (Masurel et al., 2021). Lower grade mineralisation in the hanging wall tends to only persist for ~30 m from the dilatant zones (Doe, 2019).
Weathering has produced a thin saprolite zone that varies from 1 to 15 m in thickness, within which the primary sulphides are completely oxidised. Partial oxidation of these sulphides persists for a further 2 to 15 m below the level of complete oxidation. The base of oxidation is extremely irregular with fingers extending downwards along joints and fractures into the unweathered rock (Masurel et al., 2021).
Apensu
The Apensu deposit is hosted within a dextral jog in the main Kenyase-Yamfo Thrust/Shear fault zone at the southern extremity of the known Ahafo mineralised trend. It is divided into the Apensu Main and Apensu Deeps sections, representing the sections of the deposit that are amenable to open pit and underground extraction respectively. Apensu Main represents the shallower section of the northern half of the overall deposit, whilst Apensu Deeps encompasses a series of steeply-dipping, structurally-controlled, high-grade shoots beneath the Apensu open pit that share similar structural relationships and controls to the Apensu Main mineralisation. Apensu Deeps has plan dimensions of 3900 x 600 m, and by 2018, had been tested to vertical depths of ~800 m below surface. It has been subdivided along strike into four zones, Apensu South, Apensu Gap, Apensu Main, and Apensu North. As of the end of 2018, mineralisation remained open at depth in all zones, and to the north in Apensu North.
All sections of the deposit are hosted within, and aligned parallel to sub-parallel to the NE-SW trending Kenyase-Yamfo Shear and secondary splays of that same structure, and typically have moderate to steep dips towards the SE. The plunge of high-grade mineralisation varies from sub-vertical, as at Apensu South → moderate southwesterly as at Apensu Main and the lower sections of Apensu North → shallow southwesterly in the upper sections of Apensu North.
Mineralisation is developed in mylonitic to cataclasite hosts along the sheared contact between footwall Birimian volcano-sedimentary rocks and hanging wall metamorphosed granodiorite, which the Kenyase-Yamfo Shear separates. The footwall succession includes phyllonite, meta-volcanosedimentary and mixed mylonitic volcanosedimentary rocks after protoliths of siltstones, argillites, volcaniclastic and mafic to intermediate volcanic rocks. The lithological contact between these two suites has been obscured by protomylonites and mylonites that mark the Kenyase-Yamfo Shear zone which has a 10 to 75 m true width. This boundary is irregular along strike, varying from a strike of 35 to 45°, and dip ranging from ~40 to 70°SE. In addition, abundant, low-angle, 1 to 5 m thick mafic dykes have intruded the hanging-wall granodiorite forming two dominant sets: i). NNE striking, with shallow east and west dips; and ii). ENE striking, and shallowly S dipping. Steeper mafic dykes also intruded subparallel to the Kenyase-Yamfo Shear zone and locally coalesce into thicker mafic bodies. The latter includes a ~3 to 90 m thick mafic chonolith, which has been encountered below the open pit levels throughout the Apensu South, Main, North, and neighbouring Awonsu deposit, intruding along the Kenyase-Yamfo Shear zone, and containing partially assimilated granodiorite (Masurel et al., 2021).
The footwall meta-volcanosedimentary rocks exhibit a steep mineral stretching lineation within foliation planes, interpreted to confirm the suggested reverse/thrust displacement along the Kenyase-Yamfo Shear structure (Masurel et al., 2021).
In the hanging wall granodiorite to the east, there are numerous stacked, low-angle shear zones, similar to those described above at Subika. These shear zones, which strike NNE and are shallow-dipping, are intruded by and are intimately associated with the parallel set i). mafic dykes described above, and can be traced across the open pit, to intersect the 40° striking and 60°SE dipping Kenyase-Yamfo Thrust Fault, where they appear to have been dragged into it or terminated in its immediate footwall. These low-angle shear zones project across strike into the Subika deposit. Kinematic indicators, e.g., obliquity of foliation to shear zone boundaries; S-C fabrics; and striation lineations along shear vein boundaries and slip planes, all indicate reverse displacement along these low-angle thrust faults, which are more commonly developed along the lower contact margins of shallow-dipping mafic dykes (Masurel et al., 2021).
In addition, the footwall rocks to the Kenyase-Yamfo Shear fault zone display oblique striation lineations and slicken-fibres that locally overprint the steep to downdip lineations within mylonitic foliation planes. These are taken to indicate late sinistral-reverse displacement (Masurel et al., 2021).
The shear zone fabrics and fault geometries described above are inherited from early compressional deformation and include a strong cataclastic deformation of the hanging wall granitoids. Mineralised hanging wall splay faults that are mineralised in the Apensu Main pit, can be readily traced in drill core into the Apensu Deeps. The intersection of these splays with the Kenyase-Yamfo Shear/thrust are apparently the primary control of the higher-grade ore-shoot.
The Apensu Gap zone differs from the Apensu South and Apensu Main zones, as it lacks the mafic unit that is associated with Apensu South, and cataclasis is only very weakly developed. Instead, the low-angle faults apparently control and limit the extent of better grade gold mineralisation. Apensu North is developed in a structural jog repetition on the Kenyase-Yamfo Shear fault zone beneath the Apensu Main deposit. Doe (2019) identified six specific, east-dipping structures straddling the Kenyase-Yamfo Shear fault zone within the structural framework of the Apensu Main deposit described above. These are, from oldest to youngest (after Doe, 2019): i). a zone of plastic deformation in footwall mylonitic and graphitic meta-volcanosedimentary units; ii). to iv). three hanging wall splays splitting from the Kenyase-Yamfo Shear, designated S1, S2 and S3, forming zones of mylonite that have subsequently undergone brittle reactivation; v). a graphitic splay fault in the footwall that is interpreted as a plastically-deformed, locally anastomosing shear zone; and vi). a cataclasite composed of finely-crushed rock with local tectonic breccias that was the result of structural late re-activation as brittle deformation in the rigid hanging wall granitoid.
Significant gold mineralisation at Apensu is hosted by protocataclasite (Woodcock and Mort, 2008) within both the hanging wall granodiorite batholith and mafic chonolith. The main ore zone at Apensu South is hosted by rocks of the mafic chonolith, with higher grades in the steeper segments of the Kenyase-Yamfo Shear zone, particularly within the chonolith, near its contact with granodiorite. In the lower reaches of Apensu Main, the chonolith is apparently bounded on its hanging-wall side by a steep mafic dyke which appears to partly control the localisation of gold mineralisation. The main ore zone at Apensu North, is found ~300 m below the open pit where a thick mafic chonolith was intruded along a steeper section of the Kenyase-Yamfo Shear zone, beneath a low-angle shear zone occupied by a mafic dyke. The main ore zone at Apensu North thins where the mafic chonolith thickens and terminates where the Kenyase-Yamfo Shear zone 'flattens out'. The plunge of the overall southeasterly dipping high-grade mineralisation also varies, from sub-vertical at Apensu South → moderate southwesterly at both Apensu Main and the lower sections of Apensu North → shallow southwesterly in the upper sections of Apensu North.
Hydrothermal alteration and associated structural 'damage zones' related to mineralisation become more extensive heading northeastward from Apensu South, through Apensu North, towards the adjacent Awonsu deposit <1 km to the NE. This trend corresponds to the thickening of the mafic chonolith, and it's progression from gently SW-plunging, to subhorizontal towards the NE. Gold mineralisation at Apensu is directly related to a brittle overprint of older ductile mineral fabrics that grades from incipient brecciation and local fault-hydrothermal breccia, to protocataclasite. The identification of alteration zoning is rendered difficult by the brittle ore textures, although high-grade gold mineralisation is consistently associated with a texturally destructive proximal silica-sericite-Fe carbonate-pyrite-albite mineral assemblage (Masurel et al., 2021). However, Doe (2019) described 4 types of alteration assemblages associated with mineralisation, as recognised in mine mapping, from the most to least altered, as follows:
• pervasive silicification with strong sericite, pyrite, rare iron carbonate veinlets, local albite as disseminated crystals, and the complete destruction of chlorite;
• grey to yellow massive silica and sericite patches that are 1 to 10 cm thick and are controlled by small brittle shears or mylonitic zones;
• mild bleaching of some chlorite to 'paler' micas, accompanied by the formation of ankerite and rare siderite, plus calcite veinlets and patches of <1% pyrite, and rare, thin, 1 to 3 cm milky quartz veins with occasional associated visible gold;
• un-altered greenschist facies assemblage, including chlorite, calcite and rare pyrite with no evidence of hydrothermal alteration.
A wider hanging-wall alteration envelope is marked by amphibole-epidote destruction for up to 250 m distal to the principal fluid pathway, i.e., the Kenyase-Yamfo Shear zone, and closely coincides with the modelled 0.2 g/t Au grade shell (Masurel et al., 2021).
Mineralisation is centred on the Kenyase-Yamfo Shear zone, which varies from ~10 to 75 m in true width, straddled by >5 g/t Au mineralisation over widths of from 30 to 150 m. Higher grades of >5 g/t Au are hosted in, or immediately adjacent to, strongly-altered quartz-calcite veined cataclasite. Veining ranges from 0.1 to 3.0 cm veinlets → 2 to 10 cm silica-rich veins. Grades of ~0.5 to 1.5 g/t Au are more commonly developed in the fractured, moderately altered hanging wall granodiorite. Lower-grade mineralisation typically forms a halo 20 to 80 m in width (Doe 2019). Mineralisation is characterised by an assemblage of silica-albite-carbonate-sericite-pyrite alteration, as described above, associated with <1 cm thick quartz veining and brittle chlorite-filled fractures. The better gold mineralisation occurs in quartz-calcite veins associated with pyrite grains that can vary from fine disseminations to 1.5 mm in size. Gold occurs as single 1 to 20 µm diameter grains, but also commonly occurs in clusters of grains from 5 to 10 µm. Gold does not appear to be associated with either arsenopyrite or rutile, and is generally silver-poor, with <5 ppm Ag in the gold ore. Visible gold is found in the veined cataclasite. Locally, 0.2 to 2.0 cm wide quartz veins can return assays with >32 g/t Au due to coarse gold. In the oxide zone, gold is associated with coarse goethite pseudomorphs after euhedral pyrite. Gold grains in the oxidised zone range from 5 to 10 µm. Manganese oxides are also observed in oxide mineralisation (Doe, 2019).
Awonsu
The Awonsu deposit is separated from Apensu North by a gap of <1 km of lower-grade, sub-economic mineralisation within the Kenyase-Yamfo Shear fault zone. As such it is interpreted to represent a re-development of the Apensu deposit. A noticeable dextral bend of ~6° occurs along that structure between the two deposits (Masurel et al., 2021). The mined Awonsu deposit had plan dimensions of ~1800 x 150 m, and was drill tested to 450 m vertical depth, although in 2019, mineralisation remained open at depth and along strike to the north. The deposit was concealed by 1 to 7 m of transported duricrust that locally comprised a cemented ferruginous cap of eluvial/alluvial fragments and iron oxide pisolites. The the duricrust was underlain by a zone of intense weathering that persisted to a depth of ~40 m, to a saprolite zone that was intensely oxidised, leached, and mottled, containing saprolittic clay and quartz fragments.
As at Apensu, the footwall to hypogene mineralisation, below the Kenyase-Yamfo Shear fault zone, is a suite of mixed mafic volcanic rocks and pelitic to turbiditic sedimentary units that have been metamorphosed, whilst the hanging wall is composed of granodiorite. The sheared contact between the footwall and hanging wall suites is occupied by mixed mylonitic and cataclasite rocks and dilatant breccias, developed during plastic and ductile deformation. Rocks mapped as late-stage, fine-grained aplite dykes that are sub-parallel to the Kenyase-Yamfo Shear zone have been logged, although it is suspected these may instead represent fine-grained mylonites (Doe, 2019).
The shear zone varies from 5 to 100 m true width, whilst associated >0.5 g/t Au mineralisation straddling the structure ranges from 5 to 150 m in width. Higher gold grades of >1.5 g/t Au are hosted within, or immediately adjacent to, strongly-altered cataclasite, as zones that are from 5 to 60 m in width. Grades of >5 g/t Au are rare, but can be as much as 30 m wide. Gold grades of 0.5 to 1.5 g/t Au are more commonly developed in fractured, moderately-altered, hanging wall granodiorite. Locally, particularly on the northern side of the deposit, higher-grade intervals within the hanging wall alteration zone are found in discontinuous mylonite zones, and as fine stringer quartz veins.
Lower grades of <1.5 g/t Au typically form a 2 to 50 m wide halo in the hanging wall, whilst a narrower low-grade fringe ranging from 5 to 30 m in width, occurs in the footwall. Higher-grade shoots have a southward plunge, typically averaging ~2 to 5 g/t Au, in contrast to >5 g/t Au at Apensu. Awonsu is the only Ahafo South cluster deposit with multiple generations of cross-cutting milky to opaque quartz veinlets with open-space filling of minor pyrite and gold mineralisation.
The ore at Awonsu is associated with shallow- to moderately dipping shear zones, whilst the principal ore zone is associated directly with the brecciated upper margin of a thicker (relative to Apensu) deep mafic chonolith that was emplaced within the Kenyase-Yamfo Shear, beneath a shallow dipping mafic dyke (Masurel et al., 2021). Five main structures have been recognised at Awonsu, namely, from oldest to youngest (after Doe, 2019): i). the main Kenyase-Yamfo Shear thrust/fault zone; ii). and iii). two hanging wall duplex splays off the former structure, designated Kenyasi Splay 1 and 2, which are characterised by locally anastomosing zones of mylonite in granodiorite; iv). Kenyasi Footwall Splay which is a plastically deformed, graphitic, locally anastomosing shear zone; and v). a cataclasite unit, which may be a late, sinistral, brittle re-activation of the Kenyase-Yamfo Shear thrust/fault zone.
Doe (2019) described the same 4 alteration assemblages associated with mineralisation as recognised at Apensu (see above) but with two additions which differentiate areas of stockwork veining and sheeted milky quartz veins. Alteration at Awonsu, however, is less intense than at Apensu.
In the hypogene zone, gold was predominantly associated with pyrite, mostly occurring as inclusions or as sub-micron size veinlets. Gold particles are usually from 2 to 30 µm across, and occur as both euhedral and subhedral crystals as large as 1.5 mm in diameter. In the oxide zone, gold grains, range from 5 to 10 µm across and occur as inclusions within, or marginal to, goethite derived from pyrite.
Multiple generations of cross-cutting milky to opaque quartz veinlets occur within the mineralised zone containing minor open-space fill pyrite and gold mineralisation. These stockwork vein networks commonly contain visible 'coarse gold', and generally occur in highly-altered granitoid or cataclasite zones. In addition, sub-parallel, sheeted, 0.1 to 2 cm milky quartz veins with minor pyrite and occasional coarse gold, cross-cut fresh to weakly-altered hanging wall granodiorite. These milky quartz veins generally occur in sets of 2 to 10 veinlets that are each separated by 0.1 to 1 m.
Ore Shoots at Apensu and Awonsu after Masurel et al. (2021)
The bulk of the high grade mineralised shoots at Apensu and Awonsu plunge at 20 to 30°SW, which follows the lineation formed by the intersection of the Kenyase-Yamfo Shear and the low-angle thrust faults and associated mafic dykes. At Apensu, higher-grade mineralisation correlates with steeper-dipping segments (i.e., restraining bends) of the Kenyase-Yamfo shear zone (e.g., Apensu North, Apensu Main). An exception is at Apensu South, where a high-grade ore shoot has a steeper plunge, possibly correlated with a discreet sinistral flexure/dilational bend in the Kenyase-Yamfo Shear zone.
Mafic chonoliths were preferentially developed below the intersection of the low-angle mafic dykes and the main Kenyase-Yamfo Shear zone and are closely related to the thicker and higher-grade zones of mineralisation. These chonoliths commonly only have a vertical extent of ~100 to 250 m, and are developed in zones of local steepening of the main structure. There is an apparent periodicity in chonolith development, which have at ~250 to 300 m vertical spacing, which corresponds to the relative spacing of the Apensu Main, Apensu North, and Awonsu shallow SW-plunging ore shoots. The chonolith geometry appears to have also influenced the alteration zonation, as at Awonsu, where the mafic chonolith intruding along the Kenyase-Yamfo Shear zone is bounded on its upper margin by shallow-dipping mafic dykes. The contact zone is interpreted to have induced anisotropy, with strain preferentially partitioned along the chonolith's upper contact.
Amoma
The Amoma deposit is located ~11 km NE of the Apensu deposit, on the southern margin of the Bosumkese Forest Reserve that divides the Ahafo South and North deposit clusters. It is separated from Awonsu by a sparsely drilled interval of apparently lower-grade and thinner mineralisation along the re-activated Kenyase-Yamfo Shear. It is considered to represent 'Kenyasi-style' mineralisation, developed on the sheared contact between footwall rocks which comprise a mixture of mafic volcanic, and pelitic to turbiditic sedimentary units of the Birimian succession, and the hanging wall metamorphosed Dixcove granitoids, as is also the case at both Apensu and Awonsu. The hanging wall granitoids, which are of dioritic to tonalitic composition, were overthrust onto the footwall volcano-sedimentary units, producing a number of different plastically-deformed and mixed granitoid and volcano–sedimentary units, forming mixed mylonites. Subsequent cataclasite and associated dilatant breccias were formed during later brittle faulting. Late-stage aplite dykes that parallel the Kenyasi Thrust Fault cross-cut all earlier lithologies. A notable difference to Apensu and Awonsu is the absence of mafic dykes within the Kenyase-Yamfo Shear zone or in the hanging wall granitoids.
The deposit is overlain by an up to 8 m thick duricrust that comprises iron pisolites and transported alluvial cobbles, underlain by 20 to 50 m of saprolite.
In the Amoma open pit, the Kenyase-Yamfo Shear zone deviates from its regional orientation of 40° and 60°SE dip, with a clockwise rotational dextral bend to trend at 45° and dip 65°SE. A tight, upright, axial-planar, ductile penetrative fabric is apparent within the footwall sequences, with a superimposed NNE to NE striking, and gently NE and SW plunging asymmetrical parasitic fold set. Within the hanging-wall granitoids, a pronounced NE striking, steep mylonitic foliation is evident, marked by the plastic flow of both feldspar and quartz, indicating dynamothermal conditions of more than ~450°C (Passchier and Trouw, 2005). High-angle, NE striking shear zones are also abundant in the hanging-wall granitoid. Several steep NNW to NNE striking, 0.1 to 1 m thick, discontinuous, low-displacement faults are observed in the central part of the open pit, with striation lineations and stepped fault plane fibres indicating sinistral displacement. These faults crosscut the ductile fabrics and earlier structures in both footwall and hanging-wall rocks, and spatially coincides with the local reorientation of the Kenyase-Yamfo Shear zone relative to its regional strike.
Doe (2019) recognises four structural events at Amoma. These are i). the earliest Kenyasi Thrust Shear faulting; ii). the Footwall Fault, which may be coeval with, or part of, the Kenyasi Shear fault, and is a locally anastomosing ductile shear zone marked by graphite in the footwall of the Kenyasi Shear fault; iii). and iv). later hanging wall duplex splay faults, Splay 1 and Splay 2 respectively, which are sub-parallel to the Kenyasi Thrust Fault, and locally marked by anastomosing zones of mylonite and dilatant breccia; v). the latest event, the development of the cataclasite unit, which represents late sinistral brittle reactivation of the Kenyasi Shear fault.
Amoma is more heavily-deformed relative to the other Kenyasi deposits at Ahafo South, and is interpreted to possibly represent a transition to deposit styles more similar to those found in the Ahafo North area (Masurel et al., 2021).
As of 2018, the Amoma deposit had horizontal dimensions of 1500 x 170 m, and been tested to ~300 m vertical depth. The bulk of the gold mineralisation is developed in the cataclasite unit crackle and mosaic fault breccias, as well as quartz-carbonate stockwork veins and bleached wall rocks, and, to a lesser extent, protocataclasite after granodiorite. In general, the stronger the brecciation, the higher the grade, whilst the presence of thin 1 to 5 cm thick quartz-carbonate veins is a strong indicator of economic mineralisation (Masurel et al., 2021).
Zones containing grades of >0.5 g/t Au vary from 10 to 110 m in width. Higher grade cores with >1.5 g/t Au are hosted within the cataclasite, whilst lower grades of 0.5 to 1.5 g/t Au form a halo that is locally 20 to 50 m wide within the hanging wall granitoids, and a narrower 0 to 30 m in width in the footwall mixed mylonite units (Doe, 2019).
Wallrock alteration is characterised by strong silicification, zones of albitisation, and up to 5% fine to medium-grained pyrite. The more intense proximal alteration is an assemblage of silica-Fe carbonate-pyrite-albite-sericite. This pervasive alteration is overprinted by brittle deformation, and cross-cutting chlorite. Gold is indicated to predominantly occur in association with pyrite, mostly as inclusions with a particle size that is usually 2 to 30 µm. Pyrite grains are both euhedral and subhedral crystals that are as large as 1.5 mm. Where oxidised, gold predominantly occurs as 5 to 10 µm grains which occur as inclusions within, or as discrete grains marginal to goethite/limonite after pyrite.
Ahafo North Cluster of Deposits
The Ahafo North project incorporates four proposed open pit mines, which are from SW to NE, Yamfo, Susuan, Teekyere and Subenso, distributed over a strike length of ~12 km, and a standalone mill located ~30 km NE of the Ahafo South operation. These will eventually involve mining from six different pits, the Subenso North, Subenso South, Yamfo Northeast, Yamfo South, Susuan and Teekyere West.
Commercial production for the project is expected in the second half of 2025 (Newmont, December 2022).
As at Ahafo South, the gold deposits at Ahafo North are developed close to the sheared contact zone between the Birimian Sefwi volcanic belt, which is dominated by tholeiitic metavolcanics and metasediments, from the mainly volcaniclastic shales, greywackes and siltstones of the Sunyani Basin to the NW. This sheared contact is occupied by the major SW-NE trending Kenyase-Yamfo Shear zone. Ahafo North differs from Ahafo South in the absence of significant Dixcove granitoids in the immediate deposit area.
Three Eburnian deformation events are in evidence (after Bird and Sylvain, 1999), namely:
• an early D1, characterised by an S1 foliation which produces both the main parting in the rocks, and ductile shear zones that define many of the important lithological contacts in the region;
• a regional D2, responsible for F2 folding with an S2 crenulation cleavage, and ductile transcurrent faulting/shearing reworking D1 structures. The mineralisation at Yamfo is hosted within shear zones of this phase;
• a final brittle D3 deformation, which has reworked D2 structures, and is associated with hydraulic brecciation.
The Ahafo North deposits are located within or close to hydrothermally altered, NE-SW trending, brittle-ductile shear zones that are parallel to sub-parallel to the regional scale Kenyase-Yamfo Shear, but 2 to 4 km to its SE. Mineralisation is hosted within Birimian metamorphosed sedimentary, volcaniclastic and pyroclastic rocks. In the Ahafo North area, two parallel gold bearing shears have been identified, the Yamfo and Teekyeere shears, with the latter ~500 m to the SE of the former, but converging to the SW. Both strike NE-SW and dip at ~60°SE. Discontinuous mineralisation has been intersected between the two structures, in the hanging wall of the Teekyeere Shear. To the SW, the Ahafo North trend is truncated and offset by east west faulting, presumably related to D3 (Bird and Sylvain, 1999).
Gold is generally associated with sheared, altered and brecciated greywacke/siltstone in which the main alteration products are quartz-carbonate and feldspar-chlorite-pyrite. Gold particles are usually fine grained, <25 µm, and associated preferentially with carbonate and quartz-carbonate veinlets and stockworks, together with >5% fine grained pyrite (Bird and Sylvain, 1999 after Pinte, 1997).
The mineralised shears average 20 m, but may be up to 70 m in thickness. Narrow high grade shoots of limited extent are indicated within the main ore zones. An example from Teekyere West has a grade of ~14 g/t Au over a mean width of 2.8 m (Bird and Sylvain, 1999).
Little further geological information has been encountered on the Ahafo North deposits, but as more becomes available it will be included herein.
Reserves and Resources
Estimated total resource at the Yamfo-Sefwi Project (Normandy Mining Ltd., 1999) in December 1999 at a 0.5 g/t Au cut-off:
64.56 Mt @ 3 g/t Au for 190 tonnes of contained gold.
Pre-mining Ore Reserves (Mineral Resources not published) at Ahafo South, as of 31 December 2005 were (Newmont Form 10-K Annual Report, 2007):
Ore Reserves
Probable Reserve - 142.34 Mt @ 2.44 g/t Au for 375 tonnes of contained gold, with an 88% recovery
Ore Reserves and Mineral Resources at Ahafo South, as of 31 December 2012 were (Newmont Reserves and Resources Report, 2012):
Ore Reserves
Proved + Probable Reserve
Open pit - 166.1 Mt @ 1.90 g/t Au for 316 tonnes of contained gold;
Underground - 4.4 Mt @ 4.43 g/t Au for 19.6 tonnes of contained gold;
Stockpiles - 24.7 Mt @ 1.01 g/t Au for 24.9 tonnes of contained gold;
TOTAL - 195.2 Mt @ 1.85 g/t Au for 360 tonnes of contained gold.
Mineral Resources
Measured + Indicated Resource
Open pit - 75.50 Mt @ 1.26 g/t Au for 95 tonnes of contained gold;
Subika Underground - Nil;
TOTAL - 75.50 Mt @ 1.26 g/t Au for 95 tonnes of contained gold.
Inferred Resource
Open pit - 38.50 Mt @ 1.43 g/t Au for 55 tonnes of contained gold;
Subika Underground - 8.50 Mt @ 4.66 g/t Au for 40 tonnes of contained gold;
TOTAL - 47.00 Mt @ 2.02 g/t Au for 95 tonnes of contained gold.
NOTE: Reserves are additional to Resources. Total gold endowment - 550 tonnes
Ore Reserves and Mineral Resources at Ahafo South and North, as of 31 December 2022 were (Newmont Reserves and Resources Report, 2022):
Ahafo South
Ore Reserves
Proved + Probable Reserve
Open pit - 47.60 Mt @ 1.81 g/t Au for 86 tonnes of contained gold;
Underground - 22.60 Mt @ 3.06 g/t Au for 69 tonnes of contained gold;
Stockpiles - 22.10 Mt @ 0.91 g/t Au for 20 tonnes of contained gold;
TOTAL - 92.30 Mt @ 1.90 g/t Au for 175 tonnes of contained gold.
Mineral Resources
Measured + Indicated Resource
Open pit - 20.20 Mt @ 1.09 g/t Au for 22 tonnes of contained gold;
Underground - 24.7 Mt @ 3.53 g/t Au for 87 tonnes of contained gold;
TOTAL - 44.90 Mt @ 2.43 g/t Au for 109 tonnes of contained gold.
Inferred Resource
Open pit - 10.20 Mt @ 1.29 g/t Au for 13 tonnes of contained gold;
Subika Underground - 11.00 Mt @ 3.44 g/t Au for 38 tonnes of contained gold;
TOTAL - 21.20 Mt @ 2.41 g/t Au for 51 tonnes of contained gold.
Ahafo North
Ore Reserves
Proved + Probable Reserve
Open pit - 50.10 Mt @ 2.37 g/t Au for 119 tonnes of contained gold;
Mineral Resources
Measured + Indicated Resource
Open pit - 15.70 Mt @ 1.81 g/t Au for 28 tonnes of contained gold;
Inferred Resource
Open pit - 10.00 Mt @ 1.50 g/t Au for 15 tonnes of contained gold;
NOTE: Reserves are additional to Resources. Total gold endowment at both Ahafo South and North - 497 tonnes.
In addition to the references listed below, the description above has been drawn in part from Doe, 2019 - Ahafo South Operations, Republic of Ghana; an NI 43-101 Technical Report prepared for Newmont Mining Corporation by Donald Doe, 237p.
The most recent source geological information used to prepare this decription was dated: 2021.
Record last updated: 22/11/2023
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Yamfo-Sefwi - Ahafo South Complex
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Anonymous 1998 - Normandy Mining Limited: in Extracts from the Normandy Mining Limited 1997 Annual Report pp 12-14, 17, 26-28.
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Anonymous 1999 - Yamfo-Sefwi: in Register of African Gold 1999/2000 p108
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Bird, D., Sylvain, J.-P., 1999 - Discovery Case History for Yamfo-Sefwi Gold Mineralisation (Ghana): in New Generation Gold Mines 99, Case Histories of Discovery, Perth, November 1999, Australian Mineral Foundation, Adelaide, Conference Proceedings, pp. 151-161.
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Masurel, Q., Morley, P., Thebaud, N. and McFarlane, H., 2021 - Gold Deposits of the ~15 Moz Ahafo South Camp, Sefwi Granite-Greenstone Belt, Ghana: Insights into the Anatomy of an Orogenic Gold Plumbing System: in Econ. Geol. v.116, pp. 1329-1354.
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McFarlane, H.B., Ailleres, L., Betts, P., Ganne, J., Baratoux, L., Jessell, M.W. and Block, S., 2019 - Episodic collisional orogenesis and lower crust exhumation during the Palaeoproterozoic Eburnean Orogeny: Evidence from the Sefwi Greenstone Belt, West African Craton: in Precambrian Research v.325, pp. 88-110.
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Takyi-Kyeremeh, K., Wemegah, D.D., Preko, K. and Menyeh, A., 2019 - Integrated geophysical study of the Subika Gold Deposit in the Sefwi Belt, Ghana: in Cogent Geoscience, v.5, 16p., https://doi.org/10.1080/23312041.2019.1585406
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