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Siguiri - Bidini, Eureka East and North, Foulata, Kalamagna, Kami, Kosise, Kounkoun, Kozan North and South, Seguelen, Saraya, Sintroko South, Silakoro, Sokunu, Soloni, Sorofe |
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Guinea |
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Au
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Super Porphyry Cu and Au
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The Siguiri gold deposit cluster is located ~520 km NNE of Conakry, the capital of Guinea and 25 km NW of the town of Siguiri in northeastern Guinea, near the Mali border, and ~190 km southeast of Bamako, the capital of Mali.
Historical artisinal gold mining in the surrounding district has been undertaken from as early as the 3rd century, although no reliable records have been maintained. French interests became involved in the area in the late 19th and early 20th centuries, with records of 1 to 3.8 t of gold being produced annually between 1931 and 1951, although little exploration work was completed. A phase of Russian backed exploration took place between 1960 and 1963, focussing on placer deposits along the major river channels in the area. In 1980, SOMIQ (Société Miniére Internationale du Quebec) secured exploration rights over parts of the area, including Siguiri. SOMIQ focused its work on the Koron and Didi placer areas, with Chevaning Mining Company Ltd. being formed to undertake a detailed economic evaluation of the prospect, with more intensive work beginning in the late 1980s. Société Aurifére de Guinea took over from these predecessors and continued work on the placer deposits. Operations on the Koron placer reached a peak in 1992 with 1.1 t gold being produced, although operational difficulties led to the mine being shut down later that year. Golden Shamrock commenced a feasibility study in 1995, after which Ashanti Goldfields invested in the deposit and Siguiri mine started production in 1998 as Société Ashanti Goldfields de Guinea (SAG). In 2004, the merger of AngloGold and Ashanti resulted in the operation being absorbed into AngloGold Ashanti. AngloGold Ashanti completed the design and construction of an 8.5 Mtpa
saprolite soft rock treatment plant which was commissioned it in 2005 and later increased to a capacity of 12 Mtpa. Construction of a combination plant to process sulphide and transitional material as well as the existing oxide capacity, was commenced in 2017 and was completed in early 2021.
Geology, Structure and Mineralisation
The Siguiri mineralisation occurs as Palaeoproterozoic orogenic quartz-vein hosted deposits located in the Birimian Siguiri Basin of West Africa.
The Siguiri basin covers about 40 000 km2 in Guinea and adjacent Mali. It contains upper Birimian sedimentary, volcaniclastic and volcanic and intrusive rocks. The sedimentary rocks of this succession were derived from Paleoproterozoic volcanic rocks and felsic intrusions that were emplaced during the early stages of the Eburnean orogeny (Milési et al., 1989; Feybesse and Milési 1994; Begg et al., 2009). The ~2095 Ma (Feybesse et al., 1999) Niandan komatiite suite and the mafic to felsic volcanic rocks of the Kéniéro Range form the southwestern margin of the basin. To the south, the Sassandra fault juxtaposes the rocks of the Siguiri basin with those of the Kénéma-Man Archaean domain (Egal et al., 2002), whilst to the east, the Siguiri basin abuts the Yanfolila granite-greenstone belt, which comprises volcaniclastic sedimentary and mafic to intermediate volcanic rocks. To the north, the flat-lying sandstones of the Neoproterozoic Taoudeni basin unconformably overlie the Siguiri basin. Palaeoproterozoic intrusive rocks emplaced into the Siguiri basin sediments, include the Maléa monzogranite (Parra-Avila, 2015; Parra-Avila et al., 2017) which outcrops north of the Siguiri district (Lebrun et al., 2017).
Exposure within the Siguiri Basin is generally poor due to a thick lateritic duricrust which covers large portions of it. The predominantly sedimentary rocks of the basin have undergone greenschist facies metamorphism and comprise a well-bedded turbiditic sedimentary sequence, with some brecciated and possibly volcanic members. Related secondary gold occurs as alluvial and colluvial gravels in laterite cover.
The deposits of the Siguiri district are hosted in fine-grained organic-rich shale, siltstone, greywacke interbeds, graded greywacke beds and rare conglomerate, that has been differentiated into three main sedimentary suites, the:
• Balato Formation, which is dominated by centimetre scale alternations of dark grey siltstone shale-siltstone and greywacke; overlain by the
• Fatoya Formation, is dominated by metre scale beds of medium- to coarse-grained greywacke fining upward and towards the west to siltstone and shale; and the
• Kintinian Formation, a thick package of shale and sandstone with a basal polymict clast-supported conglomerate.
Mineralisation is structurally controlled within a suite that has undergone at least three distinct deformation phases: i). an initial north-south D1 compression which developed minor folds with W- to WNW-gently plunging fold axes without a clear axial planar cleavage; ii). a second and more substantial main deformation D2 event associated with east-west to ENE-WSW directed compression producing to north-south dominated structural grain of the district and creating interference patterns between F1 and F2 folds; iii). a third, D3 event that evolved progressively from D2 compression into an early-D3 east-west to ENE-WSW directed transpression and a late-D3 NNW-SSE directed-transtension responsible for most of the gold mineralisation in the Siguiri district; and iv). the fourth and last event, D4, which was an NW-SE oriented compressional event responsible for the localised overprinting of veining by a steep to shallowly dipping NNE-SSW ductile cleavage.
A deep weathering oxidation profile is developed in the region, varying from depths of between 50 to 150 m. Mineralised saprolite has provided the main oxide feedstock for the Carbon in Pulp (CIP) processing plant, while a separate treatment option is required to mine the fresh rock extensions of the ore deposits.
Primary gold is hosted in all three of the lithostratigraphic units of the Siguiri region as listed above. However, the bulk of known mineralisation is found in the central and more competent Fatoya Formation. In some of the orebodies, mineralisation exhibits a strong lithological control and is preferentially developed in coarser-grained sandstone/greywacke units that have higher fracture/vein densities relative to fine-grained rocks.
Mineralisation dominantly follows sub-vertical north-south thrusts, NE-SW dextral shear zones and WNW-ESE sinistral faults, all of which are associated
with the main D2 deformation event. In most of the open pits, the mineralised veins are a relatively consistent sub-vertical NE-trending conjugate quartz vein set, despite the high variation in orientation of bedding and other major structures. Mineralised veins are more intensely developed along major structural trends, with quartz-carbonate-sulphide veins following structures. Some of these structures occur as incipient fracture patterns (or 'damage zones') producing discrete stockworks of mineralised quartz-carbonate veins along a trend, rather than clearly defined continuous structures. These 'damage zones' may be 10 to 15 m wide zones of higher vein density compared to the surrounding rocks. Individual veins in these 'damage zones' and other structures are often 4 to 5 cm thick (Lebrun et al., 2017).
Two primary mineralisation styles have been differentiated at Siguiri, namely: i). precipitation of gold-bearing pyrite associated with proximal albite and distal carbon alteration, and development of carbonate (ankerite)-pyrite veins with rare albite, and ii). ENE-WSW trending native gold bearing quartz veins with carbonate (ankerite) selvages which crosscut carbonate-pyrite veins and have arsenopyrite ±pyrite halos (AngloGold Ashanti). These veins appear to vary from 0.5 to 10 cm in thickness.
Lebrun et al. (2017) summarises the mineralisation as follows: Orogenic gold mineralisation of the Siguiri district occurs as a dominant vein-hosted and a minor disseminated style. The earlier veins includes gold-rich, pyrite- and ankerite-bearing brecciated veins proximal to NW-SE trending structures, and crosscutting quartz-carbonate-arsenopyrite conjugate veins. The second, and most common vein type, occurs preferentially along four orientations of discrete faults or incipient structures, including north-south to NNE-SSW reverse faults, NE-SW to ENE-WSW dextral shear zones, east-west normal faults, and WNW-ESE sinistral shear zones. The disseminated mineralisation is also developed along these same structures, where they intersect favorable lithologies. The authors suggest these structures may reflect the local expression of more fundamental deeper structures that control the location of the Siguiri district within the Siguiri basin. Based on finite strain and conjugate vein analysis, Lebrun et al. (2017) interpreted the veining and gold mineralisation to have culminated late during the D3 progressive deformation event, as the local stress field was switching in response to the waning regional maximum stress. These stress switches have been widely recognised to have occurred at 2080 to 2070 Ma across the whole West African craton and are not restricted to the Siguiri area. These switches are interpreted to be related to the lowering of both the deviatoric stress and of the differential stress, and are viewed as a precursor or trigger for rapid fluid flow and orogenic gold mineralisation (Lebrun et al., 2017).
The Siguiri, deposit comprises a series of orebodies located over a generally east-west zone within a 50 km radius of the Siguiri processing plant. The main cluster lies within a NNW-SSE elongated zone of ~15 x 3 km in Block 1, while Block 2 to the WNW includes Saraya and Foulata, and Block 3 which contains the Kounkoun deposit is to the NE. The orebodies in the three blocks that contribute to the Siguiri operation include: Bidini sulphide, transitional and oxide, Eureka East and North, Foulata, Kalamagna, Kami sulphide, transitional and oxide, Kosise, Kounkoun, Kozan North and South, Seguélén sulphide, transitional and oxide, Saraya sulphide, oxide and transitional, Sintroko South, Silakoro, Sokunu, Soloni and Sorofe sulphide, transitional and oxide.
Reserves and Resources
Measured + Indicated + Inferred Mineral Resources (including 62.9 Mt in stockpiles) for this string of deposits as at 31 December, 2020 totals:
224.71 Mt @ 0.96 g/t Au, for 216.61 tonnes of contained gold;
Proved + Probable Ore Reserves (including 49.5 Mt in stockpiles) for those with calculated reserves, at the same date, totals:
73.53 Mt @ 0.80 g/t Au for 58.84 tonnes of contained gold.
Total cumulative production from the Siguiri District to the end of 2016 is estimated to be ~180 tonnes of gold (Lebrun et al., 2017).
This summary and resource/reserve estimates are drawn from the AngloGold Ashanti Mineral Resource and Ore Reserve Report, as at 31 December, 2020, as well as Lebrun et al. (2017) which is cited below and describes a selection of the main orebodies in more detail.
The most recent source geological information used to prepare this decription was dated: 2020.
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.
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Selected References:
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Lebrun, E., Miller, J., Thebaud, N., Ulrich, S. and McCuaig, T.C., 2017 - Structural Controls on an Orogenic Gold System: The World-Class Siguiri Gold District, Siguiri Basin, Guinea, West Africa: in Econ. Geol. v.112, pp. 73-98.
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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.
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