Understanding the Sadiola Hill oxide zone (Mali), its geochemical complexities and alternation signatures

Daouda, Traore
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The Sadiola goldfield is located in the eastern Kédougou Kéniéba Inlier of the West African Craton. The study area is located in the western region of the Republic of Mali close to the international border between Mali and Senegal, and approximately 75 km south of the regional capital of Kayes in the sub-Sahelian region of West Africa. The goals of this research are to study the different geological relationships: oxide and unaltered rock (calc-silicate, siltstone-shale-greywacke); oxide and structure; oxide and mineralisation; supergene alteration and mineralisation; supergene alteration and structure. To achieve the Sadiola Hill oxide zone geochemical complexities and alteration signatures study, many research studies were conducted such as lithological and oxide 2D mapping, lithological and oxide 3D modelling. The oxide profiles were sampled for gold analysis, XRF, and XRD analysis. The found minerals were separated into light and heavy. The key minerals identified by XRD are alunite (K2Al6 (SO4)4(OH)12) and jarosite (K2Fe63+ (SO4)4(OH)12). The mineral association suggests a highly acidic environment and acidic supergene conditions. The examination of oxides also allowed distinguishing the different ferricrete horizons in the Sadiola goldfield. Significant gold mineralisation is hosted in the oxide profile (weathered calc-silicate) in the Sadiola Hill opencast pit. Sadiola gold plant at Sadiola gold mine is fed oxide ore mined from the oxidation profile in the weathered rock. The oxide ore fed into the Sadiola gold plant includes laterite, transitional oxide and saprolitic oxide. The age of oxide saprolite and oxide transitional zones is assumed to have formed during recent weathering by deeply penetrating meteoric waters. In general, the geology of the oxide feed is assumed to be derived from weathering and decomposition of rocks of the Birimian Supergroup. The need of oxide-feed to sustain Life of Mine of the Sadiola goldfield mean that exploration for oxide ore is critical but understanding the geology and geochemistry of the oxide is also vital, particularly in terms of ore optimization. Five main lithologies are identified from mapping the Sadiola Hill area and these include calc-silicate, siltstone-shale-greywacke, metasandstone, diorite and quartz-feldspar porphyry. Furthermore, six calc-silicate sub-facies are identified as (1) thick-bedded (more than 0.5-1 cm) marble with an alternation of white and black layers, (2) thin-bedded (3-5 mm), (3) massive marble, (4) slump-folded marble, (5) carbonaceous siltstone and (6) pure marble. Oxide mapping in the Sadiola Hill opencast pit identified three oxide profiles and three distinct alteration types as follows (Type 1) decarbonated calc-silicate alteration, (Type 2) Fe alteration (oxide-jarosite-siderite), and (Type 3) potassic clay alteration. The Fe-oxide and potassic clay alteration profile are related to the gold mineralisation at the Sadiola goldfield. 3D lithological modelling conducted in the Sadiola goldfield has established a significant relationship between gold mineralisation and the calc-silicate unit. The 3D oxide profiles and hard rock modelling defined three types of gold mineralisation according to the age of the host. Young gold is found in the ferricrete of Eocene-Miocene age, the second type of gold is found in the oxide profile (oxide saprolite and transitional) of unknown age and the third type is found in the hard rocks of Birimian age. Gold mineralisation is associated with the (1) lithological contacts, (2) north-south trending structure (Sadiola Fracture Zone), (3) northeast trending faults, while it is proportionally with iron (Fe) and potassium (K) alteration. The three-oxide development has been sampled individually to characterise the oxide geochemistry and alteration signature. The geochemistry study of the oxide zone of the opencast mine identified the following minerals (1) Silica or quartz (SiO2), (2) Goethite (Fe+ 3O.OH), (3) Muscovite (KAl3Si3O10 (OH) 2), (4) Siderite (FeCO3), (5) Biotite KFeMg2 (AlSi3O10) (OH) 2, (6) Bernalite (Fe (OH) 3), (7) Orthoclase (KAlSi3O8), (8) Alunite (K2Al6 (SO4)4(OH)12) and (9) Jarosite (K2Fe63+ (SO4)4(OH)12). The presence of jarosite and alunite are an indicator of an acidic pH condition. The XRF analysis results established the relationship between the gold grade and silica, potassium, and iron. The presence of alunite and jarosite highlight a highly acidic environment and acid supergene conditions. The oxidation of the calc-silicate rock caused the diminution of volume and subsequent concentration of the gold mineralisation in the decarbonated calc-silicate mainly along the structures. This diminution of volume also caused the collapse of the contact between the siltstone-shale-greywacke in the west of the Sadiola Hill opencast pit and the decarbonated calc-silicate rock in the east. This contact is known as Sadiola Fracture Zone. The collapses of the decarbonated calc-silicate is associated with the flexure of the Sadiola Fracture Zone, a number of discontinuous bodies of diorites and quartz-feldspar porphyry units, and possibly produced the soft sediment deformation in the Sadiola Hill opencast pit. Sadiola goldfield can be classified as an oxide gold enrichment deposit.
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfilment of the requirements for the degree of Master of Science, November 2019