Summary:
The deposit has an overall strike length of approximately 105 km, trending in occurs a Northwest- Southeast direction, and an identified width of 5 km. It comprises of three main zones, South zone, Central zone and the North zone, with each zone being approximately 30 km long.
The Biyadh Sandstone (Lower Cretaceous) represents the oldest rock formation, outcropping south west of the bauxite outcrops. The unevenly eroded bauxite assemblage (Lower Clay Zone, Bauxite Zone, Upper Clay Zone) is overlain by the Wasia Formation (Upper Cretaceous) and Aruma Limestone on the north eastern limb of the deposit.
• Biyadh Sandstone (Bi).
The Biyadh Sandstone outcrops along the south western portion of the south zone area, striking NW-SE over a distance of approximately 3km to 11km North West of the village of Al Bi'ithah. The sandstone outcrops are frequently associated with rough, rocky desert, hammadah, chaotic heaps of flat, platy blocks, or steep, rocky hills. The exposed thickness of the Biyadh Sandstone is approximately 5m.
The Biyadh Formation comprises primarily fine- grained, well-bedded sandstone (15cm 20cm thick) with either a kaolinitic, hematitic or siliceous matrix.
• Lower Clay Zone (LCZ).
The lower clay is characterized by by the gradational reduction with the depth in the intensity of bauxitisation. It is essentially the transition zone between Bxz and the Parent rock Sequence. The upper part of the LCZ exhibits similar pisolitic textures to the BXZ, which change with increasing depth to bauxitic clays, Kaolinitic clays, Kaolinised parent rock and finally unaltered parent rock.
Surface exposures extend discontinuously over a distance of approximately 25km a long south western margin of the bauxite outcrops in the south zone. The LCZ is comprised predominantly of variegated clay, which may be white, violet, blue, reddish brown, grey or green-grey in colour. Iron-rich hematitc beds, hematitic pisolites and thick kaolin strata frequentely occur within the upper portion of the LCZ. The LCZ is approximately 2m to 5m thick with a gradational contact to the overlying Bauxite Zone, frequently making the delimitation between the respective zones is difficult.
• Bauxite Zone (BXZ).
The BXZ lies directly under the UCZ and constitutes the potentially economic portion of the Bauxite Zone. The contact with UCZ is thought to be represent a solution front and is usually identified by a sharp reduction in silica from the UCZ to the BXZ. The BXZ exhibits a variety of textures that are generally described in terms of the pisolite size. The BXZ thickness is variable between 0 to 10m, but it is on average approximately 3m.
The generalized stratigraphic sequence of the bauxite zone is as follows:
- A 20cm to 40 cm thick layer of typically slightly pisolitic hematite which overlies the bauxite (this hematite layer was founded in only the trial mine area and the not exceed 1 km north of the trial mine). The hematite has largely been eroded, its remnants forming the black proluvial deposits of the sebkhas basin.
- Underlying the iron cap is an approximately 1m thick zone of bauxitic conglomerate, formed from clasts of pisolitic bauxite and hematite, which have been cemented by a bauxitic or kaolinitic matrix.
- The bauxitic conglomerate is, in turn, underlain by a 1,0m -1,5m thick pisolitic bauxite horizon. Typically, the pisolitic bauxite is hard, reddish brown in colour and is comprised of approximately 50% -70% of fine, medium to coarse pisolites embedded in bauxitic matrix.
- The pisolitic bauxite grades into a kaolinitic bauxite, which has a gradational contact with the re- silicated pisolitic kaolin or variegated clay, of the Lower Clay Zone. The pisolitic bauxite horizon may also grade laterally into either clayey pisolitic bauxite or bauxitic clay respectively, or alternatively, it may be truncated by erosion.
• The Upper Clay Zone (UCZ).
The Upper Clay (UCZ) comprises the top of the BXZ and usually exhibits a sharp, reasonable flat contact with the Overburden sequence. The UCZ generally consists if kaolinitic claystone, often exhibiting indistinct pisoliths and concretions. Riofinex concluded that the UCZ formed as a result of the resilication of the top of the BXZ. The UCZ is, on average, approximately 2m thick but this may vary from several centimeters to more than 4m.
• Wasia Formation.
The Wasia Formation has been sub-divided into three informal units and several horizons, namely:
- The Lower Unit (W1).
The lower Wasia Formation comprises predominantly loose, almost matrix-free sandstone and sand, with occasional lenses of, or interbedded, clay up to a few meters thick.
- The Middle Unit ( W2a, W2b).
The Middle Unit of the Wasia Formation may be sub- divided into lower clay (W2a) and an upper, siltstone horizon (W2b) respectively.
The lower horizon (W2a) comprises variegated clay in a variety of colors, namely: grey, white, yellow, bluish grey, violet and reddish brown colors. The lower horizon is typically friable and disintegrates into angular or platy fragments on the surface. The contact between the lower horizon and the upper siltstone horizon (W2b) is a gradational, through a 4m to 6m thick yellow-brown clay bed.
The upper siltstone horizon (W2b) represents one of the most characteristic marker beds in the entire area. In some areas, it forms the lowest, relatively narrow step of the north eastern escarpment, while in other areas it forms flat, rocky outcrops comprising beds up to 20cm -30 cm thick. The upper siltstone horizon is yellow-brown in color and generally has a high calcium carbonate content; sometimes grading into a silty limestone or marl. The thickness is varied between 1,0m to 3m.
- The upper Unit (W3a, W3b, W3c).
The Upper Unit of the Wasia Formation forms the central, and very prominent, morphological step of the north eastern escarpment above the Middle Wasia siltstone and below the vast, flat rocky plateau of the Aruma Limestone. It is divided into lower clay (W3a), a middle limestone (W3b) and an upper variegated clay (W3c) horizon respectively.
The lower clay horizon (W3a), which attains a thickness of between 4,0m and 5,0m, is comprised predominantly of green-grey, relatively homogenous clay that may have been deposited in a shallow basin environment. Mottled clay may, however, also be present over the lower portion of the unit, representing continental conditions. The transition to the upper, limestone horizon (W3b) is gradual, through approximately 1 m thick yellow marl bed.
The limestone horizon (W3b) may has a thickness of some 3,0m to 4,0m, while in some places and 1,0m or may be replaced by a 0,5m to1,5m thick yellow marl horizon in other places. The limestone is well-bedded (30cm to 40cm thick) and has a characteristic yellow- brown color and cryptocystalline texture. Over the south eastern portion of the central morphological plateau, it is covered by a 10cm to 20 cm thick, black hematite crust, representing the onset of the next continental cycle.
The upper horizon of the Upper Unit of the Wasia Formation (W3c) is comprised predominantly of organic ich, dark grey to black clay, and white, brown, yellow or violet variegated clay having a combined thickness of 4m to 20m.
• Aurma Formation.
The Aruma Limestone Formation varied in thickness from 2,0m to 3, 0 m over in some places, and may attain a thickness of up to 4,0m to 5,0m in other places.
In some places of the plateau the lower part of the formation comprises a 1,0m to 1,5 m thick, white, nodular limestone bed, which may grade over the lower half meter into a marly limestone or marl bed.
The upper portion of the plateau comprises a massive, red-brown to brown, microcrystalline limestone, approximately 1,5m to 3,0m thick, which forms the main morphological feature, namely the rocky plateau.