Gold deposits within the Imbo Project are associated with the globally important Neo-Archean orogenic gold deposits. Gold mineralisation is associated with the epigenetic mesothermal style of mineralisation. This style of mineralisation is typical of gold deposits in Neo-Archean greenstone terranes and is generally associated with regionally metamorphosed rocks that have experienced a long history of thermal and deformational events. These deposits are invariably structurally controlled.

The Adumbi gold deposit is found within the Ngayu Archean greenstone belt, one of a number of Archean-aged, granite-greenstone belts that extend from northern Tanzania into northeastern DRC and then into the Central African Republic. The greenstone belt terrain in northeastern DRC has a number of major gold belts including Moto (Kibali), Kilo, Mambasa, Ngayu and Isiro.

The Adumbi deposit displays five distinct geological domains with the BIF unit attaining a thickness of up to 130 m in the central part. There is a higher-grade zone of gold mineralisation termed the “replaced rock zone” (RP zone) associated with alteration and structural deformation that has completely destroyed the primary host lithological fabric. The RP zone occurs in the lower part of the Upper BIF package and in the Lower BIF package, and transgresses the Carbonaceous Marker, located between the Upper and Lower BIF packages, both along strike and down dip. The geological interpretation from the Loncor drill intersections demonstrates that the mineralised BIF increases in thickness with depth and thus confirms the existence of significant underground potential at Adumbi below the mineral resources within the open-pit shell.

The detailed logging of the mineralised cores indicated a direct relationship between gold values and the percentage of sulphide mineralisation and intensity of silicification. In general, pyrite is the dominant sulphide followed by pyrrhotite, then arsenopyrite. When pyrite and pyrrhotite are associated with arsenopyrite, the gold values are very significant, compared to when pyrite is associated with pyrrhotite only. Silica is associated with the highest degree of hydrothermal alteration within the zones and serves as a marker of mineralisation; however, without sulphides, the gold values are insignificant. Specks of visible gold are occasionally found, generally within fractures and are present in white to grey, glassy, weak to moderately brecciated quartz veins.

Mineralisation in this environment is commonly of the fracture and vein type in brittle fracture to ductile dislocation zones. At the Adumbi deposit, the gold mineralisation is generally associated with quartz and quartz-carbonate-pyrite ± pyrrhotite ± arsenopyrite veins in a BIF horizon.

Most of the existing structural data for Adumbi is from underground mapping with some additional data from regional mapping.

Stereonet plots for bedding show two major planes oriented 315°/81° and 137°/84° defining a shallow northwesterly plunging fold (316°/07°).

It is observed that foliations are generally parallel to bedding, with average orientations of 314°/79° and 315°/81°, respectively, while the quartz veins have a general relatively less northerly orientation of 309°/79°.

Stereonet plot for the Adumbi quartz veins have two major planes oriented 309°/79° and 125°/83° defining a linear structure that is shallowly plunging to the southeast. It is not known if the intersection of these quartz vein major planes is associated with the mineralising event, but it is doubtful as it is known from previous interpretation that mineralisation at Adumbi is characterised by steep plunging shoots.

Gold mineralisation at Adumbi is generally associated with quartz and quartz-carbonate pyrite ± pyrrhotite ± arsenopyrite veins in a BIF horizon.

In the central part of the Adumbi deposit, three main zones of gold mineralisation are present:
1. Within the Lower BIF Sequence;
2. In the lower part of the Upper BIF Sequence (Zones 1 and 2 are separated by the Carbonaceous Marker, which is essentially unmineralised);
3. A weaker zone in the upper part of the Upper BIF Sequence.

Over the life of mine (LOM) of Adumbi, a total of 49.77 Mt grading 2.17 g/t Au is expected to be mined from the open pit and delivered to the processing plant.

The scheduling process consists of developing a mine plan that is economically optimal using the inventory included in the practical pit designs and pit shell. The production schedule was based on a methodology of block selections from the block model inside the designed pits.

The mining production schedule sequence is planned to mine the Cut 1 and Cut 2 pit designs from the beginning of Year 1 to get rapid access to the ore and manage the strip ratio. Mining of the Cut 3 pit shell starts from the beginning of Year 2. The Adumbi deposit is planned to be to provide a throughput of 5.0 Mt/a of ore to the processing plant. Adumbi has an average stripping ratio of 11:1.

A conventional open-pit shovel-and-truck method will be used for the mining of sufficient ore to supply a 5.0 Mt/a of oxide, transition and fresh ore throughput. The mining functions of the operation will be carried out by a mining contractor.

The practical pit design incorporates the ramps together with the appropriate inter-ramp slope angles.

The open-pit design criteria were as follows:
• A nominal bench height of 8 m;
• The inclusion of the ramping systems, resulting in the recommended overall slope angles of approximately 40° to 45° being achieved;
• Haul road widths of 25 m, including safety berms, providing sufficient room for twoway traffic for the planned 140 t capacity truck fleet;
• Haul road grades at a 10 % continuous gradient.

Principal haul roads shall be designed and constructed to connect the pit working areas to the primary crusher and the waste dumps.

Primary Crushing
The ROM ore will be transported to the plant by dump trucks, which will tip into the ROM bin. The ROM bin (with a capacity of at least one and a half times the truck size) will be equipped with a static grizzly to ensure that the oversize material does not report to the primary crusher. The crushing circuit will consist of the single-stage jaw crusher.

An apron feeder located under the ROM bin will be used to withdraw the ore from the bin at a controlled rate and will discharge onto a vibrating grizzly feeder to scalp off the fines while the oversize from the feeder reports to the crusher. The feeder undersize (fines) will discharge onto the sacrificial conveyor. The jaw crusher will reduce the oversize material to the required product size and discharge onto the sacrificial conveyor. The feed to the crushing circuit is measured using the crushing feed weightometer located on the bin feed conveyor.

The scalped feeder undersize and the jaw crusher product will be combined and will be transferred from the sacrificial conveyor onto the bin feed conveyor, where it will be conveyed to the mill feed bin.

The crushed ore will be withdrawn from the mill feed bin at a controlled rate using the apron feeder onto the mill feed conveyor feeding the SAG mill.

Pebble Crushing
The SAG mill scalped oversized scats will discharge onto the scats discharge conveyor. The scats will then be conveyed into the pebble crusher via the pebble crusher feed bin. A weightometer will measure the amount of scats for process control.

An overband electromagnet on the scats discharge conveyor will remove the steel balls or steel discharged together with the scats ahead of the pebble crusher. The pebble crusher product will be discharged onto the SAG mill feed conveyor and fed back to the mill.

Milling
The milling circuit will consist of the primary SAG mill, secondary ball mill and a pebble crusher to reduce the size of the pebbles generated by the SAG mill.

The trommel screen undersize from both the SAG and ball mills will gravitate to the mill discharge sump where sump dilution water will be added at a controlled rate to achieve the required solids feed density to the cyclone cluster. The diluted slurry is pumped to the cyclone cluster using the variable speed cyclone feed pumps.

The underflow from the cyclone cluster will gravitate back to the ball mill through the splitter box with an option of gravitating back a portion to the SAG mill. A portion of the cyclone underflow is directed to feed the gravity recovery circuit.

The cyclone overflow slurry will gravitate directly to the trash screen. The trash screen removes oversize material (such as misplaced oversize particles, vegetal debris, plastic fragments, blast fuses, and wires) from the cyclone overflow stream.

Grinding media (steel balls) are added to the SAG mill using the ball addition hopper that discharges onto the mill feed conveyor. The drums of steel balls are opened, and the balls are emptied into the hopper. Then the balls roll onto the mill feed conveyor.

Steel balls will also be added to the ball mill using the ball loading hopper kibble. The kibble is lifted by the crane (hoist) into the grinding media feed chute. Steel balls will be discharged into the mill by lowering the ball loading hopper to its rest position located directly above the mill feed chute(s).

The plant design has been based on a nominal capacity of 5 Mt/a for all ore types (oxide, transition and sulphide). The plant design is based on the initial separate treatment of the oxide ore, followed by the introduction of a pebble crushing facility into the comminution circuit as the transition and sulphide ores are introduced into the plant feed.

The process flowsheet includes the following:
• Single-stage crushing facility, utilising a primary crusher to produce a crushed product size suitable for SAG milling;
• A crushed ore surge bin which, when full, allows crushed ore to be diverted to an emergency stockpile from which ore can be reclaimed by front-end loader (FEL) to feed the mill during the periods when the crushing circuit is offline;
• Two-stage SAG and ball mill combination, operating in closed circuit with the cyclone cluster to produce the required size;
• Additional pebble crushing system to crush pebbles from the SAG mill when treating ........