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.
Summary:
As Az Zabirah bauxite deposit is almost flat dipping approximately 5° to the north ease, a conventional open cut strip mining is proposed utilizing O&K loading units and Caterpillar haulage and ancillary mining equipments.
The key mining activities include the following as proposed by GHD Pty Ltd (subcontractor with
the EPCM):
• Land clearing, topsoil removal and storage for later use.
• Dune sand removal. Dozed to form buttress support for blasting.
• Drilling of hard bands in the overburden (OVB), in the Iron Cap (ICP) and in the economic Bauxite horizon (BXZ).
• Geophysical logging of drill holes, to obtain better understanding of deposit.
• Blasting of hard bands in the overburden (OVB), in the Iron Cap (ICP) and in the economic Bauxite horizon (BXZ), using discrete deck charges in the different materials. Material will only be blasted where the increase in costs (drill and blast) would be offset by increase loading productivity, otherwise material will be free dug.
• Overburden stripping and formation of mining benches.
• Formation of initial external waste dumps, progressive in pit dumping of waste, filling mined out voids and dump rehabilitation in stages.
• Establishment of access ramps to various mine benches.
• Bauxite mining and transport to the Bauxite processing facility at the mine.
• Establishment of ROM Bauxite stockpiles for blending.
• Operational control of mining with regard to equipment deployment, grade control and mine safety.
Dune Sand Removal.
• It is proposed that a Caterpillar D11T dozer be used to rip and doze the duricrust, then push the Aeolian surface sand and underlying sandstone/ siltstone over edge of the highwall into the existing pit void to form buttress support for blasting of overburden and the ore body.
• Overburden will be used to confine the Bauxite ore and allow fragmentation without excessive movement or dilution.
Blasting.
It is proposed to discretely blast the remnant overburden, Ironstone capping and Bauxite ore with discretely placed explosive charges, to provide suitable fracture of the overlying material while only slightly displacing the ore body. The Bauxite ore will be confined with buttress support along the exposed free face and above its insitu location, by overburden material, to keep it in place during blasting.
This provides adequate fragmentation of overburden, Ironstone capping and Bauxite ore while minimizing orebody movement and supporting high productivity high capacity excavation equipment.
Overburden Removal.
It is proposed to remove the competent overburden material above the Bauxite ore (PBZ) layer by an O&K RH 120 or RH 200 excavator.
Overburden will be removed by 100 t to 190 t haul trucks (i.e. Cat 777 to Cat 789) to outof-pit waste dumps or used as buttress support in-pit against the ore face as required.
Overburden will be placed on in-pit waste dumps where possible in preference to hauling to out of pit dumps. Most overburden material, apart from that in the first two strips of each pit, will be hauled to in-pit dumps. This is possible due to the strip mining layout of the operation.
It is proposed that the removal of Bauxite ore (PBZ) will be undertaken by an O&K RH 120 Hydraulic Shovel.
The triangular arrangement between boom and stick of this machine will enable it to cut to a defined horizontal floor, as designed by geophysics, spatial control systems and mine planning tools.
It is proposed that ore be transported by the same size trucks used on overburden removal (Cat 777 to Cat 789) to the ROM pad, then to the blending stockpile or to the ROM dump hopper for crushing.
Roads and Ramps.
Haul roads is constructed from the suitable type of materials excavated from the pit during the pre-strip operation, it will maintained by single 16m motor grader. In additional to maintaining the roads the grader is also used for cleaning truck spillage from he roads, for general bench cleaning after blasting operation and for maintaining the truck turning areas on waste dump. Haul roads widths are to be up to 27m, depending on truck class selected (3.5 times the width of truck), with ramp gradients no greater than 10%.
Contract Mining.
At this study, it is planned that all mining activities are to be undertaken by mining contractors. These contract mining activities include:
- Overburden removal
- Bauxite mining
- Drilling
- Blasting and Explosive Supply.
Mine Design.
Geotechnical parameters for 50m deep open pit.
Height of bench - 5.0m,
Total pit slop height - 50.0m,
Berm width - 2.5m,
Batter Angle - 85 degrees,
Catch berm width - 8.0m,
Height between catch berms - 25.0m,
Overall slop angle - 56 degrees,
Angle of Repose - 34 degrees,
Swell Factor - 40 %.
Dump Design.
The specifications for the haul dump design are as following:
1. Offset 50m from haul road alignment.
2. 20m degrees overall.
3. 100m maximum from face.
4. Average 850m single distance.
5. Swell factor of 1.2.