All known uranium occurrences and deposits in Niger are located in sandstones and conglomerates within the Tim Mersoï Basin. They are all classified to belong to the sedimentary tabular, paleo channel and roll-front or sandstone types.

Sandstone-hosted uranium deposits are marked by epigenetic concentrations of uranium in fluvial/lacustrine or deltaic sandstones deposited in fluviatile continental environments frequently in the transition areas of higher to lower flow regimes such as along paleo ridges or domes. Roll-front type deposits contain impermeable shale or mudstones often capping or underlying or separating the mineralized sandstones and ensure that fluids move along and within the sandstone bodies.

In the sandstone-type deposits, uranium was typically precipitated from oxidizing fluids by reducing agents such as plant matter, amorphous humate, sulphides, iron minerals and hydrocarbons. The oxidation and reducing facies display typical colours and can assist in exploration target selection. The fluid migrations and deposition of uranium leaves behind a distinct colour change from red hematitic (oxidized) to grey-green (reduced). The primary uranium minerals in most sandstone-type deposits are uraninite, pitchblende, coffinite and some secondaries.

The rocks present within the Dasa Project area range in age from Precambrian to lower Cretaceous age.

The rocks are mostly clastic sediments with minor carbonates. They originated from the Aïr Massif which has been continuously eroded since at least the Mesozoic. The sediments were laid down in a continental setting and are generally comprised of fluvial and deltaic settings. In this environment, large shallow rivers meander across flat topography and create complex flow patterns where the coarse-grained sands and gravel are concentrated in the actual channels with the highest flow energies, while low energy flow regimes on the floodplains and tidal areas create silt and mudstone-type sediments.

Thin section work and petrographic studies by Activation Lab (2007) on Dasa samples has revealed that the uranium host rocks are sandstone and wacke which are variably oxidized. The main component is angular quartz, some plagioclase and lesser orthoclase. They are cemented by goethite, amorphous iron-hydroxides and various secondary uranium-rich minerals.

The original cement between the grains of quartz and feldspar consisted of sericite and carbonate which were replaced during later stages by goethite and the amorphous iron-hydroxides. The quartz and the feldspar contain micro fractures partly filled with uranium-rich oxide. The latter also rim some of the silicates. Uranophane in form of radiating aggregates forms cement between the silicates and partly replaces them.

GAC initiated a mineralogical study of the uranium mineralization on its property (Molebale, 2012) with five drill samples and five residue samples submitted for analysis. The samples were from drillholes ASDH 351, 353, 354(1), 354(2) and one DADH sample. The samples were split into representative portions and polished sections were prepared. Subsamples were pulverized for x-ray diffraction (XRD).

Five uranium-bearing minerals have been identified in Dasa samples (Molebale, 2012):
• Carnotite K2(UO2)2(VO4)2 x 3H2O
• Uranophane Ca(UO2)2SiO3(OH)2 x 5H2O
• U-rich titanite (U,Ca,Ce)(Ti,Fe)2O6
• Coffinite U(SiO4)1-x(OH)4x
• Torbernite Cu(UO2)2(PO4)2 x 11H2O
• Autunite Ca(UO2)2(PO4)2 x 12H2O.

Majority of the mineralization is comprised of carnotite, uranophane and uranium-rich titanite and contribute to most of the uranium in the ASDH samples in terms of mass %, while torbernite is dominant in the DADH sample. The average grain size for the observed uranium-bearing minerals is -38 µm.

The source of the uranium is very likely leaching of the frequent volcanic tuff and ash blankets and intercalations now altered to analcimolite within the Wagadi and Dabla sediment packages. This has occurred over time in the geological history of the area and probably began as pre-uranium concentrations during the early sedimentation in favourable reducing environments such as organic matter-rich lower flow regimes and in favourable lithologies. The first stratiform mineralized bodies would have been formed during the early digenesis. Later, structural deformation and ground water movement within coarser grained organic-rich sediments aided by fluid movements and influenced by faults and tectonic activity, initiated roll front like redistribution of the uranium thus giving the mineralized bodies their present shape.

The PEA proposes the development of an underground mine using a sublevel blast-hole retreat with cemented paste backfill as a mining method. The mining method proposed includes the mechanized short-hole development of the main decline, access ramp, level development and crosscut drives as primary and secondary accesses to the mineral deposit on a 20 m sublevel spacing and a 15 m collection drive spacing.

Primary access to the underground workings will be via a dedicated mechanized decline ramp developed from surface. Following the excavation of a boxcut, the mechanized decline ramp will be developed as a 5 m-wide x 5 m-high excavation at a maximum gradient of 14.3% (1:7) with a minimum radius of curvature of 25 m for efficient haulage. The mechanized decline ramp has been positioned to minimize development required to access the high-grade stoping initially and provide shortest distances to centre of mass of each of the major stoping areas. The main mechanized decline ramp cross-sectional area has been selected to allow for hauling with up to 50-tonne trucks; however, installation of conveying equipment (continuous belt or rail bound conveyance) would not be possible whilst simultaneously allowing tramming of mechanized vehicles in the decline.

Ramps will be developed off the main decline ramp at appropriate positions to allow for access to he sublevel development. The mechanized ramps (5 m wide x 5 m high) developed at a maximum gradient of 14.3% (1:7) with a minimum radius of curvature of 25 m will allow for truck haulage from loading points on each sublevel to the main decline for transport to surface.

Secondary development consists of sublevel ramp access development (5 m wide x 5 m high) developed horizontally from the ramp to connect with the strike horizontal drive (5 m wide x 5 m high) on each sublevel with a minimum of 15 m stand-off from the mineralized envelope. Sublevels will be spaced at 20 m vertical intervals. Horizontal mineralized material collection crosscut drives (5 m wide x 5 m high) developed at right angles to the mineral deposit strike, spaced at 15 m offsets, will traverse the deposit’s mineralized envelope. It is proposed that the remote loading of blasted mineralized material will be loaded at the intersection point of the mineralized material collection drive and the strike drive for transport via the ramp to surface for discharge at either ROM pad, stockpile (high grade (HG), medium grade (MG), low grade (LG)), very low grade (VLG) stockpile or extra low grade (XLG)/pure waste storage area.

Once the mineralized material collection drive is driven to final position, a slot will be developed using a longhole production drilling rig to create a free breaking face for subsequent ring blasting (3 m rings).

The mine utilizes a mechanized sublevel blast-hole retreat stoping method for mineralized material extraction and mechanized transport. The mining cycle consists of the following key production areas:
• Mechanized development
• Mechanized long-hole stoping.

The mining cycle consists of the following activities:
1. Drilling.
2. Blasting.
3. Support drilling and bolt/cable anchor installation.
4. Loading.
5. Hauling.
6. Backfilling of waste into open stopes.

The drilling activity is separated into short shot-hole, long shot-hole and support drilling. Different mechanized drilling machines are required for each of these sub-activities.

Support drilling is performed by up to two support drilling rigs capable of drilling long anchor holes for installation of cable anchors.

Short shot-hole drilling is performed by double-boom drill rigs. Primary support of split set may be performed using the double-boom drill rigs. CSA Global calculates that up to four double-boom drill rigs will be required for the planned development.

Long shot-hole drilling is performed by a top hammer long-hole drilling machine capable of drilling up to 32 m long-holes (64–89 mm diameter).

The blasting activity is supported by charge-up crews and utility vehicles modified for the purposes of transporting explosives and blasting accessories and charging of shot-holes. Modified utility vehicles will be loaded at the surface explosives storage magazine where emulsion will be sensitised and loaded into the special purpose explosives kettle located on the charge-up vehicle. It is planned that water-resistant emulsion explosives will be used in conjunction with booster initiators and shock tube detonators. Blasting will be initiated at fixed intervals (at the end of the shift) from a central control room once shift clearance procedures are complete. CSA Global calculates that up to four charge-up vehicles will be required for the planned stoping and development activities.

The loading of blasted mineralized material and waste as well as the backfilling of open stopes with waste material is achieved using a single type of LHD in order to minimize equipment type numbers and spare holding requirements. Mucking of waste development, mineralized material drive development and stoping blasted material will utilize a 10-tonne class LHD. Cross-sectional dimensions of primary and secondary development will allow for access of a 10-tonne class LHD to all areas of the mine whilst maintaining adequate running clearances.

Backfilling of waste (when available) into the open stopes will utilize the same class 10-tonne LHD.

Transport of mineralized material and waste to surface is achieved by the loading of broken mineralized material/waste into 33-tonne class diesel haul trucks and hauling via the main transport drives and ramp systems to surface. Cross-sectional dimensions of primary and secondary development will allow for access of a 33-tonne class truck up to the intersection of the mineralized material drive and strike drive where LHD will load blasted stoping mineralized material into awaiting haul trucks for transport to surface.

The planned ancillary equipment fleet at Dasa will consist of various utility vehicle modified for transport of equipment, consumables and stores in and out of the mine. In addition to the utility vehicle fleet, a roadway maintenance vehicle, integrated tool handler and LDVs are planned as part of the ancillary equipment fleet. CSA Global calculates the number of support vehicles to be nine including a surface crane, excluding an additional four LDVs.

The Dasa underground ventilation system has been designed with two parallel fans on surface at the return air raise. Fresh air to the underground workings is provided through the 5.0 m diameter fresh air raise and ramp. Return air from underground workings is exhausted by one of two 4.5 m diameter internal return air raises located at either end of the footwall access drifts. On level 416 (higher overcut), the footwall access drift connects the two internal return air raises to the return air raise to surface.

The mineralized material is crushed to -250 mm and delivered to the coarse mineralized material stockpile and is reclaimed from the stockpile and fed into a bin by a front-end loader. A controlled mill feed is achieved by an apron feeder which feeds a conveyor belt to the SAG mill, where the mineralized material is dry ground in closed circuit with dry screens to produce a P100 of 0.6 mm screen undersize which feeds the pug drum feed bin.

The SAG mill is air swept with hot air to dry the mineralized material. The air from the mill is passed through a bag filter system to maximize recovery of particulate material before releasing the air to the atmosphere.

The mill discharge is lifted to the classification screens by a bucket elevator system with subsequent dedicated screen module feeders to distribute mineralized material to the three screens.

The Dasa Project process plant has been designed to process 1,000 tpd fresh mineralized material (365 Ktpa) which is calculated to be 48.5 tonnes per hour (tph) (at a utilization of 86%). The processing facility consists of:
• Dry SAG and classification
• Pug leaching and curing
• Uranium extraction circuit (re-pulping and solid liquid separation)
• Uranium purification and precipitation circuit
• Drying and packaging
• Backfill paste plant.

Pug Leach
The milled mineralized material is fed to a pugging drum where it is mixed with sulphuric acid, nitric acid, sodium nitrate and water to maintain at least 85% m/m solids. The mixture is treated in the pugging rotating drum for about 20 minutes followed by curing on slow moving horizontal conveyor belts operating in series providing a three-hour residence time. The drum is gas sealed to ensure that the NOx gases produced during pugging are collected with minimum dilution and used to regenerate ........