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Sadiola |
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Mali |
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Au
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Super Porphyry Cu and Au
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The Sadiola Hill Gold Mine is located in Mali, some 510 km to the north west of the capital Bamako.
(#Location: 13° 54' 15"N, 11° 41' 0"W).
The deposit falls in the northern section of the Keniéba-Kedougou window, which comprises an outlier of the Palaeoproterozoic Birimian Series on the north-east margin of the Kenema (or Man) Shield. The Birimian Series is composed of a collage of volcanics to the west, overlain by the younger, dominantly sedimentary domain of the Kofi Formation to the east. These younger sedimentary rocks are cut by the major Senegalo-Malian shear zone which has a number of associated gold deposits located in its splays. The deposit is located just east of the terrane boundary defining Senegalo-Malian Shear Zone.
The gold mineralisation at Sadiola Hill is associated with the near north-south trending Sadiola Fracture Zone, a diorite filled fault zone that separates a greywacke and meta-pelite sequence to the west from an impure marble to the east of the Kofi Formation. The diorite is discontinuously developed within the fault and is strongly altered in the mineralised zone. Regional metamorphism is to greenschist facies with amphibolite facies metamorphism observed in the contact aureoles around major intrusions.
The structural history following the Birimian volcanics, sediments and intrusives comprised F1 folding, F2 shearing and faulting (including the major regional Senegalo-Malian Lineament/Shear Zone which generally separates the volcanic and sedimentary terranes), then the emplacement of the diorite, followed by the mineralisation accompanying F3. The area is characterised by north-south ductile and NE-SW brittle fracturing. Structure controls the mineralisation which occurs in a major flexure in the Sadiola Fault zone, trapped below a roll in the diorite (dyke) which plunges at 15 to 20° N and acts as a cap.
Within the Sadiola Main Pit the bulk of the ore is hosted within the brittle-ductile Sadiola Fracture Zone and impure footwall carbonates. Mineralisation also occurs along the array of NNE-trending shears although gold grade decreases with increasing distance from the Sadiola Fracture Zone. Mineralisation is shear-hosted and associated with a polyphase hydrothermal alteration history comprising an early calc-silicate phase followed by a potassic alteration stage. The metal associations of the ore typically comprise As-Au-Sb and minor to trace amounts of Cu-W-Mo-Ag-Bi-Zn-Pb-Te-Fe-bearing mineral species.
Structural controls on primary mineralisation in the FE satellite pits 6 km to the SE are similar to that of Sadiola but later karstification and protracted
weathering resulted in the formation of a gold residuum. Lithostratigraphic contacts also appear to have been an efficient interface for channeling fluids (AngloGold Ashanti Reserve and Resource Report, 2016).
Protracted lateritic weathering has produced a deep kaolin rich clay zone to depths of 200 to 300 m (influenced by fracturing), which has decarbonated (decalcified) the marble at and near the surface. Gold has been enhanced in the kaolinitic oxide zone, particularly in the decarbonated marble (formed from limestones and limey sediments) which hosts the bulk of the mineralisation. This saprolite zone contains the bulk of the resource, and all of the current reserve as a flat lying body which is mostly free digging. It passes down into a soft sulphidic/saprolitic zone below the redox boundary, and then to untested hard sulphide mineralisation which contains 3 to 4 g/t Au.
In the primary zone the gold, with a fineness of 850 to 970, is associated with both arsenic and antimony sulphides which are dominated by arsenopyrite (that contains fine grained gold), stibnite, pyrite, pyrrhotite and gudmuntite occurring as fine veins and disseminations. Primary gold is fine grained, dominantly <15 µm, with rare grains approaching 50 µm. Visible gold is rare. The primary mineralisation is accompanied by calc-silicate, potassic (biotite) and carbonate alteration and by silicification. The carbonates are Fe rich (actinolitic) regionally and Mg rich (scapolitic) in the mineralised zones. The sequence of alteration commences with early calc-silicates, followed by potassic (biotite-sericite) associated with the mineralisation, then retrograde propylitisation. The best grades are associated with biotite.
The gold mineralisation in the Sadiola main pit is related to the interaction of the north-striking Sadiola Fracture Zone and a NNE-striking fault array. The Sadiola Fracture Zone follows the competency contrast between the brittle hangingwall greywacke and the ductile footwall marbles and is mineralised over a drilled strike length of ~2500 m. The stratigraphy is intruded by discontinuous diorite and quartz-feldspar porphyry dykes. Mineralisation occurs in all main rock types although the bulk of the mineralisation is hosted in the footwall carbonates adjacent to the Sadiola Fracture Zone (AngloGold Ashanti Reserve and Resource Report, 2019).
At the FE satellite pits, 6 km to the SE of the Sadiola Main pit, mineralisation is hosted in marbles adjacent to the upper contact with carbon-rich pelites. Gold is associated with NNE-striking faults and lens-shaped breccia zones that are broadly parallel to the NW-trending stratigraphy. The FE4 deposit is located in an interbedded sandstone and pelite sequence with mineralisation predominantly hosted in breccia along a NE-striking regional shear and several subsidiary NNE-rending faults (AngloGold Ashanti Reserve and Resource Report, 2019).
At Tambali, located 2 km to the south of the Sadiola main pit, the mineralisation is associated with two sets of structures, orientated NNE (dipping steeply SE) and NW (dipping SW). These structures are often related to thin tourmaline-quartz-rich shears/veins or zones of (mostly NNE trending) quartz-feldspar porphyry intrusions that have undergone later shearing. A NW trending graphite-rich brecciated boundary between SW-dipping sandstones in the east, and meta-pelites in the west, is also evident. Bedding parallel shearing is also indicated in some areas, possibly accounting for some of the west -dipping mineralised structures. The Tambali mineralisation is a subset of the one observed at Sadiola and was subjected to similar structural controls (AngloGold Ashanti Reserve and Resource Report, 2019).
Sadiola has a history of alluvial gold workings dating back to the 11 th century. Exploration commenced in 1990 when IAMGold, a Canadian company secured the mineral rights and entered into an earn-in option joint venture in 1993 with Anglo American, whose interest later passed to AngloGold. A regional geochemical survey in 1987 had rediscovered the deposit when a clear 7000 x 3000 m anomaly was delineated, centred on Sadiola Hill where artisanal workings in saprolitic oxide ore could be followed for 1200 m along trend. In 1994, a feasibility study was completed on the property and accepted by the Malian government. Construction started in 1995, and after a capital expenditure of USD 80 million, the first gold was poured in December 1996, with full production being achieved in 1997. By 1999 the throughput had reached 5.2 Mt of ore and by September 1999 the operation had produced 40 t (1.28 Moz) of gold.
In late 1999 the resource totalled 106 Mt of ore @ 2.39 g/t Au, using a 1.0 g/t Au cut-off. This corresponds to 254 t (8.2 Moz) of contained gold. In 2000 the total resource was quoted at 171 Mt @ 1.78 g/t Au for 304 tonnes of contained gold.
As of 31 December, 2019, the remaining Measured + Indicated + Inferred Mineral Resource was 53.86 Mt @ 1.83 g/t Au for 96.82 tonnes of contained gold. The estimated total Proved + Probable Ore Reserve at the same date was 24.5 Mt @ 2.01 g/t Au for 49.23 tonnes of contained gold. The exclusive Mineral Resource (i.e., that part of the Mineral Resource that was not converted to an Ore Reserve, defined as the Mineral Resource that is outside the current Ore Reserve designs, but inside the Mineral Resource shells and includes the Inferred Mineral Resource within the Ore Reserve design) was 28.32 Mt @ 1.71 g/t Au for 48.4 tonnes of contained gold.
The operating company Société d'Exploitation des Mines d'Or de Sadiola S.A. (SEMOS) was owned (in 2000) by AngloGold (38%), IAMGold (38%), the Mali Government (18%) and the International Finance Corp (6%). AngloGold subsequently merged with Ashanti Goldfields Limited to become AngloGold Ashanti in 2004, retaining the original AngloGold share. In November 2009, IAMGOLD and AngloGold Ashanti announced that they were acquiring the International Finance Corporations 6% interest. Sadiola ceased mining during 2018 and transitioned to a stockpile treatment operation. In December 2020, IAMGOLD and AngloGold Ashanti, who then held a 41% interest each, sold their interest in SEMOS to Alllied Gold Corp., a private company. After adjustments ownership became Allied Gold (40%) and the Government of Mali (20%).
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Sadiola
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Selected References:
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Anglo American Corp 1997 - Socio-economic Impacts at Sadiola: in Mining Environment Management, March, 1997 pp 7-9
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Anonymous 1999 - Sadiola Hill: in Register of African Gold 1999/2000 pp 128-129
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Cantello G 1999 - Sadiola Gold Mine - Mali, West Africa: in Notes for a presentation at the AMF Pacific Gold 99 International Study Tour 1p
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Foster R P 1999 - Destination Africa: New Frontiers, New Mineral Exploration Opportunities: in African Mining: Exploration and Investment Opportunities Conference, Perth, Australia: 16-17 November, 1999 AJM, Sydney pp 1-11
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Masurel, Q., Thebaud, N., Miller, J., Ulrich, S., Hein, K.A.A., Cameron, G., Beziat, D., Bruguier, O. and Davis, J.A., 2017 - Sadiola Hill: A World-Class Carbonate-Hosted Gold Deposit in Mali, West Africa: in Econ. Geol. v.112, pp. 23-47.
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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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