The Buffalo Reef and Selinsing gold deposits are thought to be mesothermal lode gold deposits, with auriferous quartz-pyrite- arsenopyrite±stibnite veining, with associated hydrothermal alteration. The two deposits are structurally controlled and thought to be part of the same structural trend, with the Buffalo Reef deposit occurring along strike and to the north of the Selinsing deposit.

Selinsing.
The Selinsing gold deposit is hosted by a 20 m to 100 m thick shear zone that dips 55° to 75° towards mine grid east (082° true grid). This zone or “envelope” of sheared rocks has been variably mineralised and intruded by gold- bearing quartz veins and stockworks. The quartz veins are likely to have been emplaced along fault surfaces, which are thought to be reverse thrusts caused by compression from the east. Strike-slip movement is not thought to be significant; however, a north-westerly trending structure, which post-dates the gold mineralisation, has been identified and may have a strike-slip component. The host rocks for the Selinsing shear zone consist of a series of finely interbedded argillites and very fine-grained arenites, along with sequences of quartz-rich, variably silicified sediments of likely tuffaceous origin, which are referred to as “felsic tuff” and a few thin beds of quartzite conglomerate. These host rocks are collectively known as “the mine sequence series”.

The true thickness of the mine sequence is not well understood. One interpretation is that the mine sequence has a true thickness of about 200 m; however, the footwall contact is not well defined, and it is difficult to distinguish between the mine sequences in the field due to the fine-grained nature of the host rocks. A second interpretation is that within the shear zone, repetition of these units by shearing creates a structural thickening of the sequence.

The hangingwall rocks at Selinsing are a distinctive sequence of predominantly competent, wellbedded, dark-coloured limestones. Towards the base of the limestones is a narrow unit of black, well-bedded carbonaceous shales, which may be calcareous in places. The contact of these hangingwall units with the mine sequence below is thought to be a tectonic or faulted contact due to the unconformable nature of the bedding on either side of the contact. The contact itself is characterised by large water-filled clay-lined cavities. The footwall contact of the mine sequence is poorly understood as the base of the mine sequence has not been extensively explored. However, the footwall rocks consist of the same type of grey-black limestone as found in the hangingwall and it is thought that the footwall rocks are the same as the hangingwall, repeated due to faulting. This would mean that the less competent mine sequence rocks were more deformed by shearing due to rheological contrasts between the limestones and the argillites and arenites. The hangingwall limestones have locallydeveloped folding resulting from easterly compression.

Gold and sulphide mineralisation are associated with these rock types as well as intensive replacement by quartz and calcite gangue minerals.

The gold mineralisation at Selinsing is hosted within a shear zone that strikes at 350° and dips 60–70° to the east, with the higher-grade mineralised shoots within the main mineralised shear plunging to the southeast. The main shear zone is hosted within a sequence of felsic tuff and very fine clastic argillite with calcareous material and limestone in the hangingwall. High grade mineralisation is often associated with quartz stockworks and quartz-carbonate veins within highly deformed sedimentary rocks. Pressure and temperature studies on fluid inclusions in quartz from veins suggest that the mineralisation formed at a temperature in the order of 200–350°C and at a depth of between 2 km and 5 km.

The gold at Selinsing is generally in the form of fine-grained gold particles (<20 µm) commonly associated with pyrite and arsenopyrite and rarely with chalcopyrite. Visible (mm-scale) gold, although not common, occurs in quartz veins within the shear zone. The higher-grade quartz veins can be over a metre in true thickness and have been traced up to 300 m along strike and 200 m down dip. Lower grade gold mineralisation occurs as finely disseminated gold within intensely deformed envelopes around the quartz veins within the shear zone. Disseminated pyritisation also occurs within the deformed country rock within the shear zone, with the presence of euhedral arsenopyrite as a good indicator of elevated gold grades.

Buffalo Reef.
The Buffalo Reef deposit occurs approximately 1 km to the east of the Raub-Bentong suture. The area is dominated by argillite and limestone of Permian age to the east, with conglomerates and sandstones of Devonian age to the west. Low grade regional metamorphism up to Greenschist facies (locally up to Amphibolite facies) occurs throughout the area. The sediments have subsequently been intruded by granitic bodies of approximately Jurassic age. These intrusive bodies occur to the east of Buffalo Reef and generally form elevation highs.

The dominant structural feature present is a 200 m wide, north-south striking shear zone, with an apparent sinistral sense of displacement, which parallels the tectonic Raub-Bentong suture to the west. The shear zone is composed of graphitic shale with minor interbedded fine-grained sandstone and tuffaceous rock. Bedding within the sediments typically dips 65–75° to the east and strikes towards a bearing of 330–360°.

Gold mineralisation at Buffalo Reef is structurally controlled and associated with Permian sediments within a 200 m wide shear zone that parallels the north-south trending Raub-Bentong suture. Mineralisation occurs over a total strike length of approximately 2.6 km. Rocks within the Buffalo Reef shear zone have typically undergone silica-sericite-pyrite alteration to varying degrees.

The gold occurs within moderately to steeply east-dipping veins and fracture zones, which range in thickness from 1 m up to 15 m in thickness (average thickness is approximately 10 m in the main mineralised veins), although local flexures in the veins can host mineralisation up to 25 m in thickness. Veins, which are boudinaged in some areas, are generally composed of massive quartz with 1–5% (by volume) sulphide minerals, namely pyrite and arsenopyrite, along with varying amounts of stibnite. The stibnite generally occurs in association with elevated gold grades; however, the presence of gold does not necessarily indicate high stibnite levels (i.e. the stibnite tends to be associated with gold, rather than the gold being associated with stibnite).

The Selinsing and Buffalo Reef deposits have been mined using open pit methods for nine years and the mining contractor for the mining activities is Minetech Construction Sdn Bhd (Minetech). The contract is current, and Minetech is responsible for the mining of ore and waste including:
- Drill and blast activities
- Load and haul activities
- Rehabilitation activities.

All mining functions relating to health and safety, mine planning and mining technical support are provided by Monument. Currently, there is no plan for any underground operations.

Geotechnical studies were undertaken by Snowden as a geotechnical review of the Selinsing and Buffalo Reef mines in 2016 (Geotechnical Review of Selinsing and Buffalo Reef). In 2018, another review was carried out by Peter OBryan ans Associates (POB (2018)) confirming much of the findings by Snowden for Buffalo Reef but revised the design guidelines for the Selinsing pit area.

The Selinsing and Buffalo Reef pits have the following design considerations:
- Maximum recommended batter height: 10 m
- Minimum berm width: 4 m (increased where necessary to suit maximum inter-ramp angle).

The mining method is conventional open pit drill and blast, load and haul on a 2.5 m mining flitch with a 10 m high blasting bench, reflective of semi-selective mining. The maximum excavator bucket size of 2.3 m3 is matched to this selectivity.

In order to achieve a target of 955 kt/a of ore, as well as up to an additional 9 Mt/a of waste, Snowden (Dec 2016) proposed the following fleet requirements:
- Drilling, based on LOM ore release:
- Selinsing pits: From one to two drills
- Buffalo Reef pits: From one to three drills
- Individual units move from Selinsing to Buffalo Reef as required.
- Loading, based on LOM ore release:
- Selinsing pits: From one to three excavators
- Buffalo Reef pits: From one to three excavators
- Individual units move from Selinsing to Buffalo Reef as required.
- Hauling, based on LOM ore release:
- Selinsing and Buffalo Reef pits: From six to 18 x 30-t trucks.

The existing Selinsing Gold Plant is currently treating oxide gold ore. The process includes primary, secondary and tertiary crushing followed by ball milling, gravity concentration and CIL. Future plant feed will contain a sulphide component, which renders the ore refractory, meaning the gold that cannot be effectively recovered by conventional CIL processing. A new processing route has been investigated which incorporates flotation and bacterial oxidation (BIOX®) ahead of the CIL circuit to achieve the target gold recovery.

Based on the selected mining rate of 950,000 t/a at an assumed 91.32% plant utilisation, together with the known required residence time capacity of the rougher and cleaner flotation banks, the size and number of tanks could then be selected based on the standard capacities of cylindrical flotation tanks as offered by the major equipment suppliers including Outotec, Metso and FLSmidth. A flotation tank design incorporating external concentrate launders was preferred by the on-site team, as it allowed for a slightly smaller gross tank volume to be selected. To minimise the effects of possible slurry short circuiting, a minimum number of six flotation tanks was selected for the rougher-scavenger bank, in accordance with industry practice.

BIOX® feed.
Rougher concentrate if of sufficient sulphur grade (6–8% S) and cleaner flotation concentrate will be pumped to a high rate thickener of 9 m diameter. Thickener underflow will be pumped to the BIOX® surge tank at 50% solids; the BIOX® surge tank will have 1,004 m3 live volume that will provide 60 hours capacity. Flotation concentrate stored in the stirred BIOX® surge tank will be pumped to the primary BIOX® reactors with automatic dilution to achieve a 20% solids concentration in the BIOX® feed.

BIOX® reactors.
The BIOX® feed slurry will be pumped to the feed splitter ahead of the BIOX® reactors. Dosed quantities of nutrient will be added at the feed splitter from which the slurry will gravitate to the BIOX® reactors. The six BIOX® reactor tanks will be configured as three primary reactors in parallel followed by three secondary reactors in series. At average feed rates the total residence time in the BIOX® reactors will be 5.5 days, split equally between the primary and secondary reactors at 2.75 days each. The BIOX® culture in the reactor tanks will be maintained in a healthy condition by controlling the temperature, slurry pH and dissolved oxygen levels. The temperature will be controlled at 41°C by pumping cooling water through the cooling cools; the slurry pH will be controlled by adding either sulphuric acid or milled limestone to maintain the range of pH 1.3–1.6; dissolved oxygen levels will be maintained by pumping low pressure air through spargers installed at the base of each BIOX® reactor tanks. If the correct conditions are maintained, approximately 70% of the sulphide sulphur in the BIOX® feed will be oxidised in the primary reactors. Slurry in the primary reactors will flow via riser pipes to the secondary BIOX® reactors where further oxidation will take place. After the first secondary reactor it is expected that the sulphide oxidation will have reached 85%; the second reactor is expected to achieve 91% and the final secondary reactor 95% oxidation.

BIOX® CIL.
Slurry from the pH adjustment tanks will be pumped to the new BIOX® CIL circuit, comprising six tanks for a total residence time of 24 hours. Defoamer will be added to control froth formation in the CIL. Cyanide solution will be dosed into the first CIL tank to leach the gold in the BIOX® residue. Oxygen will be added to the CIL tanks to maintain dissolved oxygen levels. Activated carbon for the adsorption of gold will be transferred counter current from tank to tank using airlifts. Loaded carbon will be harvested from the first CIL tank and pumped to the existing carbon recovery screen above the elution column. Batches of 1.5 tonnes of loaded carbon will first be soaked in hydrochloric acid to dissolve any carbonate residues that may have coated the carbon. The activated carbon will then be transferred to the elution column where caustic soda and cyanide solution will be pumped at temperature and pressure to initiate the elution process. Pregnant eluate will be pumped to the two electrowinning cells where the gold be deposited onto the cathodes. The cathodes will be cleaned periodically to recover the deposited gold which will then filtered, dried and smelted into gold bars.

BIOX® CIL detox.
The BIOX® CIL slurry will discharge from the last CIL tank into the CIL detoxification tank. A solution of sodium metabisulphite (SMBS) will be added to the slurry along with some copper sulphate solution which will act as a catalyst and low-pressure air providing oxygen for the process; caustic soda or hydrated lime may be added to maintain pH in the range of 8–10. Cyanide and WAD species are converted to cyanate which is further oxidised to ammonia and carbon dioxide. Oxidation of thiocyanate will be limited to 10–20%. The detox slurry will be pumped up to the existing leach tanks which will be converted for use as ASTER™ reactor tanks.

ASTER process.
ASTER™ is an acronym for Activated Sludge Tailings Effluent Remediation and is a process that uses certain microorganisms to catalyse the metabolism of cyanide and thiocyanate. In a process similar to the BIOX® process, the micro-organisms are maintained at specific slurry conditions and fed with suitable nutrients. The process works at a pH range of 6.5–8.0, and a temperature of 20–40°C.

Water recovery thickener.
The water recovery thickener will receive slurry from the ASTER™ secondary reactor, the neutralisation tanks and the flotation tailings. Thickened tailings will be pumped to the existing tailings storage facility; thickener overflow will be pumped to the existing process water tank.