The Sadiola Hill Gold Mine operated by AngloGold Limited is located in Mali, some 510 km to the north west of the capital Bamako. The deposit falls within the Kenieba-Kedougou window, which comprises an outlier of the early Proterozoic 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 sediments are cut by the major Senegalo-Malian shear zone which has a number of associated gold deposits located in its splays.
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. The diorite is discontinuously developed within the fault and is strongly altered in the mineralised zone.
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.
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 sulphides which are dominated by arsenopyrite (that contains fine grained gold), pyrite and pyrrhotite occurring as fine veins and disseminations. 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.
Exploration commenced in 1990 when IAMGold, a Canadian junior secured he mineral rights and entered into a joint venture in 1992 with Anglo American, whose interest later passed to AngloGold. A regional geochemical survey in 1987 had rediscovered the deposit when a clear 7000x3000 m anomaly was delineated, centred on Sadiola Hill where artisanal workings in saprolitic oxide ore could be followed for 1200 m along trend. 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.
The project is owned (2000) by AngloGold (38%), IAMGold (38%), the Mali Government (18%) and the International Finance Corp (6%).
(Source: Porter GeoConsultancy, www.portergeo.com.au , 2001)