|
Chirano - Akwaaba, Kolua, Suraw, Akoti, Paboase, Tano, Obra, Sariehu, Mamnao |
|
|
Ghana |
| Main commodities:
Au
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Chirano Project comprises an ~10 km long string of semi-connected, steeply plunging deposits/shoots developed within the NE-SW trending Bibiani/Chirano Shear Zone. This structure separates the Palaeoproterozoic Sefwi Greenstone Belt to the west from the from the Kumasi Basin to the east. It is located in southwestern Ghana, ~250 km WNW of the capital Accra and 80 km WSW of the regional capital of Kumasi.
(#Location: 6° 18' 40"N, 2° 23' 0"W).
The individual deposits, from south to north include:
Akwaaba,
Kolua,
Suraw,
Akoti,
Paboase,
Tano,
Obra,
Sariehu and
Mamnao, although some are further divided into individual shoots. Go to: Resources and Reserves.
During the mid 1930s, the area in which the Chirano string of deposits were later found straddled concessions separately held by Gold Coast Selection Trust and Anglo-African Goldfields Ltd. Billiton International Metals BV subsequently held the area for a period in the late 1980s, probably as part of their regional reconnaissance for gold-bearing laterite deposits. During the early 1990s, two prospecting concessions covering the Chirano area were taken out by related Ghanaian companies, Johnsons Limited and Chirano Goldfields Company Limited who, in November 1993, signed an agreement with Placer Outokumpu Exploration Ltd for the latter to undertake exploration in those titles. However, after several unsuccessful exploration campaigns, the project was joint ventured to a British company, Reunion Mining plc in May 1995. In 1996 work was halted by a government imposed moratorium on exploration work in Forest Reserves. In 1997, exploration resumed after Reunion was given permission to do so under strict environmental constraints and temporary permits. Reunion carried out the first detailed exploration, extending and in-filling the soil sampling grid and excavating trenches across the better soil anomalies. Many of the trenches defined wide-zones of potentially ore grade gold mineralisation and several prospects worthy of drilling were defined. Reunion was focused on shallow oxide mineralisation.
However, due to the various environmental restrictions, Reunion chose to concentrate efforts elsewhere and in mid 1998 negotiated an option agreement with the Canadian based Red Back Mining Inc. By April 1999, Red Back's 100% owned subsidiary Chirano Gold Mines Limited (CGML) had acquired 95% interest in the Project, with the remaining 5% being purchased by Red Back in November 2005. Red Back undertook extensive drill programmes over the upper 100 to 200 m of the string of gold prospects that had been previously outlined, with the view of developing a series of open pit mines. A prefeasibility study for establishing a mining operation was completed by CGML in 2000, and a Bankable Feasibility Study was completed in early 2003. By mid 2004, all of the deposits had been drilled sufficiently to define their gold resources. In October 2004, Red Back Mining decided to develop the Chirano deposits, where Proved + Probable Ore Reserves were estimated at 17.8 Mt @ 1.9 g/t Au for ~34 t of contained gold. Mine construction was completed in September 2005. The gold plant was commissioned in the same month, and the first gold poured on 10 October 2005. The plant was expanded by 2009 to enable treatment of 3.5 Mtpa to accommodate the Akwaaba underground deposit. Exploration had earlier begun below the open pits, initially resulting in the discoveries of three high grade mineralised shoots below the Akwaaba and Paboase pits, with the first underground mine, Akwaaba, coming into production during the 2nd half of 2008. This was followed by underground operations at Paboase in 2012, Akoti in 2016, Tano in 2020, Suraw in 2021 and Obra in 2022.
In September 2010 Kinross Gold Corporation had acquired Redback and it's interest in CGML, with the Ghanaian Government exercising its right to take a 10% interest. In April 2022 the Canadian headquartered Asante Gold Corporation entered into a share purchase agreement with Kinross Gold Corporation to acquire Kinross' 90% interest in the Chirano Gold Mine.
Regional Setting
Ghana is located on the the eastern margin of the Man Shield, the southern half of the exposed West African Craton. The Man Shield is, in turn, 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 volcanic rocks, accompanied by hypabyssal intrusions and minor related volcaniclastic rocks, were erupted and emplaced between ~2250 and 2150 Ma during the Eburnean 1 Orogeny, which also involved granitoid plutonism between ~2205 and 2130 Ma, and greenschist-amphibolite facies metamorphism at ~2150 Ma (e.g., Allibone et al., 2004). The transition zones between these volcanic rocks and the sedimentary successions 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. Both the igneous, sedimentary and Tarkwaian rocks were affected by the Eburnean 2 Orogeny, which involved further metamorphism, granitoid
magmatism, and complex multiphase deformation between ~2116 and 2088 Ma (Hirdes et al., 1992; Davis et al., 1995; Oberthür et al., 1998; Allibone et al., 2002).
The pattern of prominent NE trending mafic volcanic/intrusive belts, separated by coeval sedimentary basins with late granitic magmatism is characteristic of the Birimian. This would suggest initial deposition under a broad transtensional regime of crustal attenuation and opening of fractures during the Eburnian 1 Orogeny, transitioning to later transpression and uplift during the Tarkwaian as the stress field was rotated. The overprinting Eburnian 2 fabric is defined by numerous major thrusts that have imbricated slices of Birimian mafic igneous basement and interleaved them with broadly coeval turbiditic and younger fluvial sedimentary rocks at scales ranging from a few hundred metres to hundreds of kilometres. This is interpreted to have formed as the stress field continued to migrate to NW-SE directed compression.
See also the Regional Setting paragraph of the Yamfo Sefwi - Ahafo Project record for a more detail.
The Chirano deposits straddle the faulted boundary between the Sefwi Volcanic Belt to the west and the sedimentary Kumasi Basin to the east, defined by the Bibiani Shear Zone. The Kumasi Basin separates the Sefwi and Ashanti belts, the latter located some 70 km to the east. In the centre of the Chirano section, the faulted margin between the Sefwi Volcanic Belt and Kumasi Basin has a northerly trend, in contrast to the rest of southwest Ghana where this boundary generally trends northeasterly. It is uncertain as the whether this deviation in orientation reflects a flexure/jog in the contact, or is a north-south faulted offset. In the Chirano segment of the contact zone, the Sefwi Volcanic Belt and Kumasi Basin are separated by an inlier of younger Tarkwaian sedimentary rocks up to 2 km wide. As with most exposures of Tarkwaian stratigraphy throughout the Birimian of West Africa, the inlier at Chirano is interpreted to have been preserved adjacent to a major transcurrent shear structure by crustal extension before the major compressive deformational phase of the Eburnean Orogeny. At Chirano, this sedimentary inlier dips steeply and strikes between north and NE, parallel to the regional structural trend, although reversals in younging direction imply the presence of tight internal folds within the Tarkwaian rocks, although none have been mapped. Immediately to the west of the contact between the Tarkwaian sedimentary inlier and the Sefwi belt, there is an elongate, >10 km x <150 m granitic intrusion, the 'Chirano Granite', which is found within, or near, most of the Chirano gold deposits. This granite, however, locally has a maximum width of as much as 600 m.
The 'Chirano Granite' is a massive, medium-grained (2 to 4 mm) homogenous, equigranular biotite tonalite. It is flanked to the west of the Chirano Shear Zone/fault by numerous, narrower, parallel dykes of the same composition cutting and isolating slivers of dioritic, gabbroic and Birimian basaltic volcanic rocks. Plagioclase and quartz are the dominant primary minerals with 1 to 2 vol.% biotite, whilst trace amounts of K feldspar accompany accessory zircon and apatite.
The fault between the Sefwi Volcanic Belt and the Tarkwaian is known as the Chirano Shear, whilst the fault between the Tarkwaian and the Birimian sedimentary rocks of the Kumasi Basin is referred to as the Bibiani Shear.
Geology and Mineralisation
The Birimian sequence, which is intruded by and flanks the western margin of the Chirano Granite, is characterised by mafic rocks, ranging from fine grained basalts to medium grained dolerite. The Tarkwaian sedimentary rocks to the east of both the granite, and the Chirano Shear, comprise arkosic sandstone, siltstone and minor conglomerate. The granite itself is a composite body, incorporating numerous thin, but extensive, sheets of mafic rock. Although the granite is generally thin, and strikes NNE, it includes a thicker, ~1.5 km long middle section with a northerly trend known as the Paboase Bulge, which has been interpreted to represent a major sinistral jog affecting both the granite and the Chirano Shear.
The fourteen known gold deposits all occur along a single mineralised 'horizon' that extends for several kilometres to the north and south, but is laterally centred on the Paboase Bulge which extends from the Akoti to the Tano deposits. This horizon incorporates fractured, veined and altered mafic volcanic rocks and granite which host the mineralisation.
The Chirano gold deposits range from stacked parallel veinlet systems to vein stockworks, breccias and cataclasites. Most are within 200 m of the Chirano Shear zone. The individual deposits have strike lengths of from 150 to 700 m, and vary from a few metres to over 70 m in thickness. Most deposits dip steeply to the west, although lesser shallow west and vertical dips are locally evident, whilst two dip steeply to the east. Anomalously flat dips are found in short sections of the lode horizon at Mamnao Central, Obra South, Sariehu and Akoti South, although these areas do not have any spatial relationship with thicker or higher-grade mineralised intervals. In some of the deposits, thicker zones of gold mineralisation are seen to have developed where nearby parallel lodes coalesce. Such deposits have a single thick core, which splits into two or three thinner peripheral zones along strike, e.g., Tano.
Mineralisation either plunges steeply to the north, or directly down dip with very good continuity, and no known gaps caused by oblique faults or dykes. Individual deposits range from generally tabular (e.g., Obra, Sariehu and Suraw) to more pipe-like (e.g., Tano and Akoti North) morphologies. The longer strike length, more tabular bodies generally comprise at least two shorter lenticular shoots, such as the Obra Main and North lenses. These lenses may also be separated by a small dextral jog such as those at Obra and Sariehu. Within the Paboase Bulge there are several parallel lodes, whereas elsewhere along the mineralised horizon, there is commonly only a single zone evident.
Many of the deposits/lodes exhibit a consistent asymmetry, characterised by:
• an abrupt change from elevated gold grades to barren hosts on the eastern margin of the lode, whilst a more gradual transition occurs on the western side, enclosing patches of low-grade material or thin mineralised veins;
• Unusually high gold grades and strong quartz veining are often concentrated near the eastern margin of the deposit;
• quartz veins on the eastern margin dip steeply west, in contrast to those on the west with more shallow dips to the west, discordant with the lode envelope.
The Chirano Mining Lease area is interpreted to have undergone prolonged structural deformation particularly along the Chirano Shear Zone corridor where sinistral strike syn-mineral slip movement is indicated, with a strong positive correlation between the shear geometry and mineralisation characteristics. Within individual deposits, there is generally a positive correlation between the intensity of fracturing/brecciation, and strength of gold mineralisation, although the degree of fracturing varies greatly between deposits. For example, at Obra, mineralisation is generally hosted by severely deformed and brecciated granite cataclasite, whilst much of the Tano lode is less intensely fractured and takes the form of a stockwork or vein swarm. It is evident at Obra that brecciation, veining and alteration have been prolonged and/or the result of repeated episodes of deformation, with some clasts in the cataclasite containing veins that predate the brecciation and later veins cutting through the breccia. Fragments of altered and unaltered rock are juxtaposed in some parts of the breccia, implying alteration before deformation, whilst adjacent fragments indicate alteration that overprints the brecciation. Elsewhere, stylolites crosscut the breccia and early veins but are cut by later veining.
According to Allibone et al. (2002) mineralisation of economic significance at Chirano is, in a number of deposits, predominantly hosted by the albite- and carbonate-altered tonalite intrusive 'Chirano Granite' (as distinct from the Birimian sedimentary rocks and graphitic shear zones in mafic volcanics that host most of the other major deposits in Ghana), while weaker alterered zones containing <1 ppm gold grades extend into adjacent Birimian mafic igneous rocks and Tarkwaian Group sediments. Pink albite alteration (albite-quartz-pyrite-hematite-rutile ± leucoxene-chalcopyrite ± gold) and later pale carbonate alteration (ankerite-sericite-quartz-pyrite-hematite-gold-Ag-Au-telluride) are characteristic of the gold deposits at Chirano.
Individual Deposits
The key attributes of the individual deposits are as follows, from SW to NE (after Begg et al., 2022):
Akwaaba
Akwaaba is the southernmost deposit in the Chirano mine leases, and the first underground resources exploited. It constitutes a NNE striking, steeply west dipping tabular zone of mineralisation hosted within and straddling the Chirano Shear. At Akwaaba, the shear is a 20 to 50 cm thick zone of grey to black fault gouge that marks the contact between the Sefwi-Bibiani belt volcanics and the younger Tarkwaian sedimentary rocks.
The hanging wall Birimian mafic sequence is ~150 m thick, mostly composed of fine-grained, dark greenish grey, chloritic basalts and fine to medium grained doleritic intrusions. It also includes lesser tuffs, namely a basal, fine grained, grey green, massive to well bedded fragmented tuff; a poorly sorted, clast poor, matrix supported, lithic and quartz crystal rich tuff; a possibly felsic, massive, quartz rich crystal tuff containing blue-grey quartz crystals up to 3 mm in diameter in a fine-grained groundmass. Barren, lithologically controlled, early epidote veining is found throughout the hanging wall mafics. Un-mineralised calcite/quartz and pure quartz veins are also extremely common especially within hanging wall dolerites.
The deposit is 20 to 50 m wide and strongly altered, with gold grades that correlate with the intensity of alteration, veining, brecciation and the presence of 1 to 5% pyrite. The western part of the mineralised zone is mainly hosted by hydrothermally altered dolerite that is variably foliated, and contains high-grade mineralised shoots. These shoots occur at subtle dip-changes in the Chirano Shear Zone that plunge towards the north and contain hydrothermally brecciated basalt and cataclasites. The shear zone is locally intruded by strained, altered and mineralised tonalite of the 'Chirano Granite'. The intensity of albite-ankerite veining increases towards the main mineralised interval until their selvages coalesce to pervasively alter the host rock, whilst conversely the calcite and epidote vein intensity dramatically decreases. Proximal to the shear zone, mineralised brecciation increases substantially, firstly forming what is locally referred to as the 'Black Breccia' and then immediately adjacent to the shear, the 'Brown Breccia'. The eastern section of the mineralised zone, on the other side of the shear is hosted by the Tarkwaian sequence which is mainly a sub-arkosic arenite with lesser interspersed polymictic conglomerate.
Suraw
The Suraw deposit is ~1800 m north of Akwaaba and ~300 m south of the Akoti South deposit, hosted by the strike extension of the sequence at the latter. A fine to medium grained dolerite with variable leucoxene after titano-magnetite occurs in both the stratigraphic footwall to the west, and to a lesser extent in the hanging wall to the east of the Suraw mineralised zone. Typical equigranular, granitic textured tonalite of the 'Chirano Granite' dominates in the immediate hanging wall to the mineralised zone, and is also significant in the footwall sequence. Locally, this intrusion has a magmatic breccia texture with angular fragments of dolerite near intrusive contacts, and is more abundant than at either Paboase or Akoti. Numerous small veins and dykes of glassy felsic rock and locally quartz porphyritic rocks are abundant in and around the mineralised zone.
The Suraw deposit is ~400 m long and is a steeply ENE dipping, thin tabular deposit that persists for at least 1500 m down plunge. It is hosted by strongly sheared, quartz veined, replaced, and altered mafic rocks containing significant leucoxene. The mineralisation occurs within a shear-breccia zone where a strong, relatively early brittle-ductile fabric has been overprinted by multi-phase brecciation and a quartz-sulphide fracture mesh. Textures suggest switching between brittle and ductile regimes, possibly reflecting cyclic switching between fluid overpressure and ductile creep during ore-stage deformation. The mineralisation hanging wall is bounded, by a large mass of highly competent albite altered tonalite, especially at depth. In the most strongly deformed areas, veining and brecciation form grey siliceous breccia zones. Ankerite-albite-pyrite alteration is characteristic of proximal, ore-related alteration, with pyrite content and brecciation/veining intensity correlating with gold grade. As the dip of the shear-breccia structure steepens the grade and width of the mineralised zone appears to improve.
Tarkwaian sedimentary rocks, a mixture of poorly bedded lithic-arkosic wacke intermixed with polymictic pebble to boulder conglomerate with an arkosic matrix dominate the eastern sequence. The Tarkwaian and volcanic sequence contact is not noticeably deformed, with most of the strain instead being accommodated within the mineralised zone.
Native gold is the main gold phase at Suraw, with most of the gold grains occurring within pyrite and being between 1 and 10 µm in size. Larger gold grains between 15 and 25 µm in size can account for up to 50% of the total gold mass.
Akoti
Mineralisation at the Akoti South pit dips steeply westwards, occurring as multiple small lodes. The main mineralised zone is associated with a grey to black smoky pyritic quartz vein which lies within the Akoti South Main Shear in a zone of strong shearing, brecciation and alteration. A second mineralised zone in the central part of the Akoti South pit is associated with a quartz replaced shear, the Western Shear, that occupies the contact between the western mafic volcanic sequence and the slightly more tonalite rich mixed felsic and mafic package in that part of the pit. This shear strikes NW to the western wall of the Akoti North pit. The bulk of the Akoti South pit comprises a typical mixture of predominantly strongly altered and deformed mafic igneous rocks, whilst the sequence in the western part of the pit is intruded by lesser strongly altered, blocky, medium-grained grey/green tonalite that has been partially re-silicified quartz/feldspar. The mafic igneous rocks are strongly altered to quartz-sericite-ankerite-pyrite, generally with occasional albite alteration, especially immediately to the west of the Akoti South Main shear. The southern wall of the Akoti South pit is a strongly altered and deformed Tarkwaian boulder conglomerate and similarly strongly altered, fine grained Tarkwaian sediments, enclosed within Birimian volcanic rocks. The boulders appear to be boudinaged, and closer to the Akoti Main shear, they are very strongly deformed in a high strain zone proximal to the contact with the Birimian mafic and tonalite mineralised package.
Akoti North is ~500 m north of Akoti South, whilst Akoti Extended is hosted by differently trending sections of the same Lode horizon, which strikes at 35 and ~5° at the south and north end of the pit respectively. Both deposits are sub-vertical, tabular, mineralised zones, hosted within quartz dolerite. The main fault surface remains sub-vertical and very much planar throughout. A minor proportion of the fault zone is intruded by tonalite of the 'Chirano Granite' that locally forms an intrusive breccia with dolerite clasts. Mineralisation is hosted within hydrothermally altered, commonly foliated, basalt and tonalite, with high grade zones within hydrothermally altered and brecciated rocks and cataclasites.
Paboase
This deposit occupies a planar section associated with the main mineralised Chirano Shear Zone, between Akoti and Tano. Within the open pit, a mixture of mafic rocks, mainly basalt and dolerite, are intruded by a significant width of tonalite of the 'Chirano Granite', and are cut by quartz stockwork veining. Where the tonalite intrudes the main fault, it appears to be more strongly mineralised than when distal to the fault. Mineralisation is hosted within and to the west of the steep, NE dipping, main mineralised shear zone, where it is locally referred to as the Paboase Fault. It predominantly occurs as a strongly silica-albite ±hematite altered quartz stockwork within tonalite, although at depth, the more strongly mineralised intervals are found within a wide zone of strongly sheared, tectonically and hydrothermally brecciated, predominantly mafic quartz-dolerite package. Gold grades at depth are strongest where carbonate-sericite-silica ±albite ±chlorite alteration is most intense, bleaching the rocks to a pale greyish-brown. However, tonalite rocks also carry high grades locally where intensely sheared and altered.
The Paboase deposit exhibits a high level of structural complexity throughout, with at least three main fault orientations: i). NW striking Paboase main and Western faults; ii). large east-west striking structures; and iii). a number of oblique faults. The latter two appear to intersect, but not cross, the main mineralised shear zone. The main Paboase mineralised zone appears to be sandwiched between two NW striking shear zones and has cataclastic textures similar to the Obra deposit. Grade tends to increase west rapidly from the Paboase fault into the most strongly altered and deformed host rocks, before decreasing gradually towards a western bounding fault.
Tano
Tano is situated between the Paboase and Obra deposits, towards the northern end of the Paboase Jog/Bulge and is hosted within a hydrothermally altered and brecciated tonalite. This intrusion is located in the hanging wall of a steeply west-dipping segment of the mineralised Tano Shear zone, the strike of which varies from NW in the south, to north-south in the north. Strongly foliated, folded, deformed and altered Tarkwaian arkosic/argillaceous sedimentary rocks with polymictic conglomerates are exposed in the upper pit walls. To the west of the Tarkwaian sequence, separating it from the main mineralised shear zone, is a sequence of un-mineralised Birimian mafic volcanic rocks and felsic to intermediate intrusives. Mineralisation is predominantly hosted within un-foliated, hydrothermally altered and crushed tonalite, locally containing dense quartz ±carbonate veins. The high grade mineralisation occurs within hydrothermally quartz-albite-sericite altered and hematite-dusted pyritic tonalite with strong quartz veining. The main Chirano Shear passes to the east of the deposit and the pit, with the mineralised shear observed in the pit being sub-parallel to and possibly a splay from it. The Tano Shear is characterised by a ductile chloritic fabric, in contrast to the brittle character the bulk of the mineralised structure. Where the ductile shearing and mineralisation are in contact, they are separated by a brittle fault. Locally, brittle faults separate commonly ductile lithologies with gold grades below detection limits, from high grade >10 g/t Au brittle fabric zones. The absence of incipient mineralisation adjacent to such high grade zones implies post-mineral juxtaposition.
Obra
Obra is a tabular mineralised zone associated with both the NE striking Obra fault zone and the sub-vertically dipping Chirano Shear zone. Mineralisation is hosted by strongly hydrothermally altered, brecciated and quartz-carbonate stockwork veined rocks, comprising mixed dolerite (~60%) and tonalite (40%). The amount of tonalite strongly influences the competence and consequently the fracturing and brecciation, and as a result, the fluid ingress, hydrothermal alteration and distribution of gold grades. The Obra mineralisation parallels the regional structural trend of the Sefwi Greenstone Belt, and is bounded by the Obra Fault to the west and the Chirano Shear zone to the east, both of which are characterised by shears and zones of intense foliation. Bedding within folded Tarkwaian sedimentary rocks to the east of the Chirano Shear zone steepens and is drag folded into the Chirano Shear zone. The Obra and Chirano Shear zone are closely spaced, and were intruded by tonalite at an early stage of deformation relating to those structures. An ~10 to 15 m wide, very fine grained, strongly re-silicified, foliated, and yellow to grey or green altered mylonitised zone is evident on both ends of the pit. Alteration within the mylonite primarily comprises silica-ankerite-sericite-chlorite and is generally pervasive, overprinting any remnant textures.
Sariehu
Sariehu is broadly a massive, but rather sinuous body with two local zones of thickening, a 500 m strike length and a vertical plunge. It is predominantly hosted by altered Birimian mafic rocks. The mineralised zone is characterised by a stockwork of veins, mainly cutting fine-grained quartz-dolerite, intruded by diorite and dolerite. Massive silica flooding has obliterated the original rock fabric, and rendered the zone medium to coarse grained. The dolerite is partially overprinted by carbonate-rich alteration, although some relict textures and mineralogy are preserved. The rocks of the ore zone are crushed and veined. Quartz veining predominates, including thick massive quartz veins, not all of which carry good gold grades. Assays decline sharply from mineralisation to barren wallrock on the eastern/footwall side of the mineralised zone where quartz veining is more intense, in contrast to the more gradual opposite margin of the lode. Other thin lodes, parallel to the main mineralised zone, are locally developed on the western/hanging wall side, but not on the eastern margin. A major shear has been mapped close to the western/hanging wall margin of the lode, but little evidence of shearing is seen on the other side. Mafic rocks predominate to both the west and east of the mineralised zone. To the west, these mafic rocks include minor pegmatite, and a couple of thin gold lodes. To the east, there is generally a thin sliver, up to 15 m thick, of mafic rock, and then Tarkwaian arkose. Mineralisation tends to 'pinch-out' to the north and south. Sariehu is the most pyrite rich deposit at Chirano. The paragenesis has been interpreted as follows:
• An early pulse of hydrothermal alteration over intervals of several hundred metres, produced an assemblage of albite and ankerite with the Birimian mafic rocks during prograde greenschist metamorphism and regional deformation;
• Silica flooding and carbonate veining followed as hydrothermal activity increased;
• The silica flooding led to an increase in competence of the rock, promoting brittle deformation and brecciation, allowing the ingress of fluids producing gold mineralisation, sulphide deposition and the infilling of fissures by chlorite, carbonate and silica;
• During the waning stages of hydrothermal activity, narrow quartz veins cut the alteration packages, with some gold along their margins and in associated pyrite, but insufficient to appreciably complement the grade.
Resources and Reserves
The following are pre-mining Mineral Resources within the main deposits of the Chirano Project, listed from SW to NE (after Allibone et al., 2002):
Akwaaba - 0.560 Mt @ 2.7 g/t Au
Suraw - 1.180 Mt @ 2.4 g/t Au
Akoti - South 0.830 Mt @ 1.5 g/t Au
Akoti - North 1.290 Mt @ 3.3 g/t Au
Akoti - Extended 1.030 Mt @ 3.2 g/t Au
Paboase - 4.810 Mt @ 1.9 g/t Au
Tano - 3.180 Mt @ 2.9 g/t Au
Obra South 0.670 Mt @ 2.0 g/t Au
Obra - 10.910 Mt @ 1.8 g/t Au
Sariehu - 3.490 Mt @ 1.8 g/t Au
Mamnao South - 0.670 Mt @ 2.5 g/t Au
Mamnao Central - 1.500 Mt @ 1.8 g/t Au
Mamnao North - 0.280 Mt @ 1.5 g/t Au
TOTAL 30.400 Mt @ 2.1 g/t Au for 63.8 tonnes (2.05 Moz) of gold.
Proved + Probable Ore Reserves, October 2004 - 17.8 Mt @ 1.9 g/t Au for ~34 t of contained gold (Begg et al., 2022).
Remaining Mineral Resources, Exclusive of Ore Reserves, as of 31 December 2021 were as follows (Begg et al., 2022):
Underground
Akwaaba - Measured + Indicated Resource - 2.296 Mt @ 1.99 g/t Au
Inferred Resource - 0.223 Mt @ 2.48 g/t Au
Suraw - Measured + Indicated Resource - 0.922 Mt @ 2.32 g/t Au
Inferred Resource - 1.558 Mt @ 2.71 g/t Au
Akoti - Measured + Indicated Resource - 2.121 Mt @ 2.04 g/t Au
Inferred Resource - 0.514 Mt @ 1.89 g/t Au
Paboase - Measured + Indicated Resource - 0.164 Mt @ 2.13 g/t Au
Inferred Resource - 0.063 Mt @ 1.89 g/t Au
Tano - Measured + Indicated Resource - 2.072 Mt @ 1.77 g/t Au
Inferred Resource - 0.646 Mt @ 2.24 g/t Au
Obra - Measured + Indicated Resource - 3.476 Mt @ 1.66 g/t Au
Inferred Resource - 1.788 Mt @ 1.87 g/t Au
TOTAL Underground Measured + Indicated Resource - 11.050 Mt @ 1.88 g/t Au, for 20.8 tonnes of contained gold, comprising
Measured Resource - 4.751 Mt @ 2.01 g/t Au and
Indicated Resource - 6.299 Mt @ 1.79 g/t Au plus
TOTAL Underground Inferred Resource - 4.791 Mt @ 2.22 g/t Au, for 10.6 tonnes of contained gold.
Open Pit
Kolua - Measured + Indicated Resource - 2.296 Mt @ 1.99 g/t Au
Inferred Resource - <0.01 Mt @ 1.26 g/t Au
Akoti South - Measured + Indicated Resource - 3.17 Mt @ 0.88 g/t Au
Inferred Resource - 0.01 Mt @ 1.32 g/t Au
Obra - Measured + Indicated Resource - 6.70 Mt @ 0.79 g/t Au
Inferred Resource - 0.90 Mt @ 0.67 g/t Au
Sariehu - Measured + Indicated Resource - 2.18 Mt @ 0.80 g/t Au
Inferred Resource - 0.03 Mt @ 0.89 g/t Au
Mamnao - Measured + Indicated Resource - 4.83 Mt @ 0.90 g/t Au
Inferred Resource - 0.32 Mt @ 0.86 g/t Au
TOTAL Open Pit Measured + Indicated Resource - 17.05 Mt @ 0.85 g/t Au, for 14.5 tonnes of contained gold, comprising
Measured Resource - 4.45 Mt @ 0.80 g/t Au and
Indicated Resource - 12.6 Mt @ 0.86 g/t Au plus
TOTAL Open Pit Inferred Resource - 1.26 Mt @ 0.73 g/t Au, for 0.9 tonnes of contained gold.
NOTE: Akoti South, Obra, Mamnao, Kolua, Sariehu open pits were evaluated at cut-off grades of 0.24, 0.20, 0.31, 0.45 and 0.22 g/t Au respectively.
NOTE: Mineral Resources are exclusive of Ore Reserves.
Combined Underground + Open Pit Ore Reserves, as of 31 December 2021 were as follows (Begg et al., 2022):
Proved Open Pit - 2.4558 Mt @ 1.94 g/t Au;
Proved Underground - 2.3214 Mt @ 2.37 g/t Au;
Proved Stockpiles - 0.8227 Mt @ 0.79 g/t Au;
SUB-TOTAL Proved Reserves - 5.6000 Mt @ 1.51 g/t Au for 8.5 tonnes of contained gold;
Probable Open Pit - 2.6593 Mt @ 1.87 g/t Au;
Probable Underground - 7.5006 Mt @ 2.67 g/t Au;
SUB-TOTAL Probable Reserves - 10.1599 Mt @ 2.20 g/t Au for 22.3 tonnes of contained gold;
TOTAL Probable + Probable Reserves - 15.7598 Mt @ 1.95 g/t Au for 30.8 tonnes of contained gold;
NOTE: Mineral Resources are exclusive of Ore Reserves.
NOTE: While Begg et al. (2022) specifically state that Mineral Resources are inclusive of Ore Reserves, a release by Asante Gold Corporation dated 15 May, 2023, states that this was in error, and that in fact, Mineral Resources are exclusive of Ore Reserves. The figures quoted in both reports for each classification are the same.
TOTAL Production between 2005 and December 2021 - 50.52 Mt @ 2.29 g/t Au for 115.77 tonnes of included gold. Metallurgical recovery ranged from 93.7% in 2005, to 87% in 2021 as grades varied from 1.84 g/t in 2005, to a maximum of 3.57 g/t in 2011 and declined to 1.59 g/t in 2021 (after Begg et al., 2022).
The information in description was drawn from: Begg, D.M., Claridge, D., Mandava, S,M. and Bezuidenhout, G., 2022 - Chirano Gold Mine, Ghana, West Africa; an NI 43-101 Technical Report, prepared for Asante Gold Corporation; 251p. and
Stuart, H., 2009 - Chirano Gold Mine, Republic of Ghana; an NI 43-101 Technical Report, prepared for Red Back Mining Inc.; 136p. and Allibone et al. (2002) as cited below.
The most recent source geological information used to prepare this decription was dated: 2022.
Record last updated: 29/11/2023
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.
Chirana central plant
|
|
|
|
Allibone, A., Hayden, P., Cameron, G. and Duku, F., 2004 - Paleoproterozoic Gold Deposits Hosted by Albite- and Carbonate-Altered Tonalite in the Chirano District, Ghana, West Africa: in Econ. Geol. v.99, pp. 479-497.
|
|
Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
