The primary deposit has been identified as an alkalic gold-copper porphyry system, roughly elliptical in shape at surface (450 metres long by 150 metres wide) and with a vertical pipe-like geometry that extends to at least 800 metres below the surface. The porphyry-style mineralisation is closely associated with a zone of K-feldspar alteration, the extent of which is marked by the Didipio ridge, which is approximately 400 metres long and rising steeply to about 100 metres above an area of river flats and undulating ground.

Chalcopyrite and gold, along with pyrite and magnetite, are the main metallic minerals in the deposit. Higher grade gold and copper mineralisation is closely associated with the Quan Porphyry and Bugoy Breccia, both of which are elongate in plan-view along the north-south trending, steeply north-east dipping Tatts Fault Zone.

Mineralisation
Porphyry style gold-copper mineralisation has been recorded over a strike length of approximately 450 metres, a width of up to 150 metres, and to a vertical depth of greater than 800 metres. The tabular composite intrusive and associated alteration and mineralisation strike in a northwest – southeast direction and dip steeply (80 to 85 degrees) north east. Higher grade gold and copper mineralisation is closely associated with the Quan Porphyry and Bugoy Breccia, both of which elongate in plain view along the Tatts Fault Zone. This mineralisation is surrounded by stockwork mineralisation that extends as a steeply east-dipping ellipsoidal shaped body, 110 metres to 140 metres wide, from the surface to a depth of 650 metres. Below a depth of 650 metres, the mineralisation is more tightly constrained forming a carapace around the Bufu Syenite, with extensions of higher-grade mineralisation continuing southwards along discrete structures. Higher gold-copper grades are also localised within the footwall (west) skarn, which is 5 metres to 15 metres wide, subvertical, open at depth and contains vein-type mineralisation over a strike length of 150 metres.

The deposit is oxidised from the surface to a depth of between 15 metres and 60 metres, averaging 30 metres. The oxide zone forms a blanket over the top of the deposit. A 5 metre to 15 metre thick transition zone is present over most of the deposit.

Brecciation of the QFC at the top of the Leached Zone (Bugoy Breccia) is characterised by high gold-copper grades. The gold and copper may have been remobilised and concentrated within the breccia matrix. Within the QFC Zone, highest grade mineralisation is generally coincident with an overlap of Mixed Zone alteration. Grades are typically low where the Mixed Zone does not coincide with the QFC Zone at depth. The Mixed Zone is also notable in that it includes significant disseminated chalcopyrite-bornite-pyrite mineralisation, a feature not common in other alteration zones. Very high-grade gold-copper mineralisation is also a feature of the Skarn Zone where it occurs typically as coarse (2 mm to 4 mm) disseminations of chalcopyrite-bornite-magnetite overprinting the calc-silicate matrix. Outside the QFC Zone, chalcopyrite and gold mineralisation are generally lower-grade. Minor disseminated chalcopyrite may also occur with magnetite and chlorite as retrograde alteration of mafic grains. Locally, there is strong development of disseminated mineralisation.

Since commencement of the project mining has transitioned from open pit mining to underground mining. A small amount of mining remains as part of the Crown Stabilisation Project (“CSP”), which will be completed in early 2022. The remaining LoM mining schedule consists of underground material (not including surface stockpiles which also contribute to mill feed).

The mining method for the removal of the CSP material via the open pit is conventional drill, blast, load and haul with standard mid-sized mining equipment comprising 90 tonne class off-road haul trucks and 200 tonne excavators.

Crown Pillar
In 2018 the Breccia Pit project was successfully completed. The low-strength crown pillar within the Breccia Zone was removed via open pit methods, and was replaced with approximately 69,000m3 of engineered CRF comprising waste rock, tailings, cement and water. CRF was utilised for backfilling for several reasons including its ability to be completed independently of underground paste requirements, and an overall stronger final product. Stripping of the pit floor and backfilling with CRF eliminates the need for lateral development to access the top of crown pillars stopes at the topmost level, allowing for extraction from the lower level in a geotechnically sound environment. Studies showed that this method resulted in no large-scale impacts on pit wall stability whilst delivering favourable economic returns due to early access to high grade ore and increased underground stope recoveries. Stoping has commenced in the upper levels adjacent to the CRF material in the Breccia Pit with excellent results (little to no overbreak).

Following successful completion of the Breccia pit, a project was initiated in early 2019 called the Crown Strengthening Project (“CSP”) where similar principles from the Breccia Pit were to be applied to the more competent Monzonite rock mass on the eastern side of the crown pillar as without strengthening would also be subject to high stresses. The CSP mining via open pit methods is due to be completed in early 2022, with CRF backfilling is to be undertaken through to 2024. Crown pillar stopes in the monzonite zone are up to 40m high to maximise ore recovery. This is higher than the Breccia Zone and possible due to more favourable stoping conditions.

Underground mining
Production from the underground mine is planned to ramp-up to 1.6 Mt of ore per annum. A total of 2 stopes have been completed and paste filled as of the end of 2021 since the commencement of the underground mine.

The long hole open stoping method (‘LHOS’) is employed at Didipio for the extraction of underground stopes. Stope dimensions vary depending on their location within the orebody. On the eastern side of the orebody in the monzonite zone, stopes are up to 60m high whereas in the breccia zone on the western side of the orebody, more conservative stope dimensions are adopted due to poorer ground conditions. These include, where required, significant stope crown support to prevent unravelling. Paste backfill is utilised for backfilling of all stope voids.

Underground development commenced in April 2015 via a portal within the open pit. Stoping commenced in December 2017 with throughput ramping up to 1.6Mtpa rates in 2018. Underground operations were suspended in July 2019 due to expiration of the FTAA. During the FTAA renewal process, the underground mine and associated infrastructure were kept in a state of operational readiness with underground production resuming in November 2021. An initial ramp up period during 2022 will result in a production rate of 1.4Mtpa and is scheduled to increase to a maximum of 1.7Mtpa over the course of the life of mine before underground production tails off in 2032 and 2033.

The current decline face has advanced to the 2180mRL. Approximately 28km of lateral development remains in the mining schedule which includes capital development in the lower part of the mine to enable establishment of active dewatering and pumping infrastructure (CPS 1).

Stopes are mined via the long hole open stoping method (‘LHOS’) mining method allowing for a high degree of mechanisation and good mining selectivity, high mining recovery and scheduling flexibility. A primary/secondary stoping sequence is utilised, where primary stopes are separated by a secondary stope. Extraction of the secondary stope can only occur after the two immediately adjacent primary stopes have been mined, backfilled, and have had time to cure. All stopes are backfilled with paste fill.

Stope Design
Several different stope designs are utilised at Didipio. All methods rely on the development of a slot drive to provide initial void for subsequent stope firings. The standard stope design is based on a 30m high level interval and is nominally 20mW x 20mL x 30mH. This stope design is utilised mainly in the Breccia Zone for stopes beneath paste (top-down sequence) and minimises overbreak associated with the weaker host rock. In the Monzonite zone on the eastern side of the orebody, more competent ground conditions are encountered. Double lift stopes in the Monzonite Zone up to 60m are designed increasing stope productivity and reducing ore drive development requirements.

Once the slot drive has been developed, drilling in the slot drive and main production rings can commence. The recently purchased Rhino Raisebore Rig is utilised to ream out a 750mm diameter hole to assist with establishing the void for the initial slot firing. 89mm infill blast holes are drilled around the Rhino hole to create a 3.5m x 3.5m excavation.

Following the initial 3.5m x 3.5m slot firing, slot extensions rings are fired with an additional ring to create a safe brow for future firings. Following creation of the slot void, firing of the main production rings can take place which is where the bulk of the ore tonnes for each stope are located. The firing process for single lift and dual lift stopes is very similar.

Primary Crushing
The crushing circuit is situated next to the ROM pad. Mining trucks haul ore from the open pit to the ROM pad. ROM ore is fed by a front end loader (“FEL”) through an 800mm square aperture static grizzly into a 100-tonne live capacity ROM bin. The FEL is required to remove oversize material retained by the static grizzly.

The ROM ore is reclaimed from the ROM bin by an apron feeder and is discharged on to a static grizzly into a single toggle crusher. Fines will bypass the crusher. Static grizzly bars are set at nominally 100mm clearance.

The single toggle crusher, selected to handle 900mm maximum lump size, crushes the ROM ore to a typical P80 product size of 100mm. An overhead travelling crane is provided for changing out crusher jaw plates and for maintenance on other adjacent equipment. Dust suppression water sprays are provided at the ROM bin and at the head of the transfer bin feed conveyor, emergency stockpile feed conveyor and SAG mill feed conveyor. The sprays will be automatically turned on/off from the plant control system.

Primary and Secondary Grinding
The 7.3m diameter by 4.57m effective grinding length (“EGL”) grate discharge SAG mill is fitted with steel liners and pulp discharges and initially processed 2.5 Mtpa of ore. The SAG mill is equipped with a 4,300 kW wound rotor induction motor and Liquid Resistance Starter (“LRS”) and has capability to provide speed variation through a Slip Energy Recovery (“SER”) unit.

Media charging is from 900kg drums of 125mm grinding balls via a kibble to the mill feed chute. A target ball charge of 12% is maintained with a media addition rate of 0.28kg/tonne of feed. Mill load is determined from monitoring the hydrostatic pressure in the trunnion mill lube system and controls the mill feed rate. A microphone is used to monitor the mill for low load conditions to allow the mill speed to be reduced to minimise liner damage.

Discharge from the SAG mill flows through a rubber-lined trommel and into a common mill discharge hopper. Oversize from the trommel screen (scats) is directed to the scats recycle conveyor for return on to the SAG mill feed conveyor. A Sandvik CH-440 pebble crusher will be commissioned in Q4 2014 to reduce the scats size to -12mm. The current scats recycle conveyor discharge will be redirected into the crusher feed bin and a new transfer conveyor will transfer the crushed material back to the mill feed conveyor allowing the current system to be used as a crusher bypass.

The 5.5m diameter by 8.38m EGL rubber lined ball mill is fitted with a 4,300 kW wound rotor induction motor, LRS, trommel screen and retractable feed spout/chute. Discharge from the ball mill flows through a rubber-lined trommel into the common mill discharge hopper. The combined SAG and ball mill discharge is pumped to a nest of eight Krebs 20” hydrocyclones. The hydrocyclone underflow is split, with approximately 40% reporting to ball mill feed. The other 60% reports to an Outotec SK-500 Flash Flotation Rougher cell for recovery of the coarse liberated gold and copper particles. The concentrate from the Flash Flotation Rougher reports to a gravity circuit and the hydrocyclone overflow gravitates on to the flotation rougher circuit.

Recovery of copper and gold at Didipio is achieved from the use of a combination of flotation following a conventional SAG mill/ball mill grinding circuit and gravity gold recovery. The plant has been successfully running and exceeding 3.5Mtpa nameplate since the 2014 processing plant upgrade, with a well-established workforce and management team in place until June 2019 when operations were suspended. Following renegotiation of the FTAA in July 2021 the plant was restarted in November 2021 with full production expected in Q2 2022.

First ore was introduced to the plant on December 14, 2012, and the plant commenced commercial production on April 1, 2013.

Since commissioning, a ramp-up project to de-bottleneck the plant with the aim of achieving 40% above plant design to 3.5Mtpa, was achieved during Q4 2014. With further improvements and fine-tuning over 2015 & 2016 the plant is now capable of processing up to 4.0Mtpa and is potentially able to achieve 4.3Mtpa with further minor improvements with a minor capital outlay.

Ore is processed using a conventional SAG/Ball mill/Pebble Crusher (SABC) grinding circuit with a secondary pebble crusher circuit followed by froth flotation for recovery of gold/copper concentrate. A gravity circuit is incorporated within the grinding and flotation circuits to produce gold bullion on site. Copper concentrate is transported by road to the San Fernando port facilities for export.

Gravity Circuit
The gravity circuit utilises a Falcon SB2500 concentrator batch concentrator. A bypass option allows the Flash Flotation Rougher concentrate to bypass the concentrator and report directly to the Flash Flotation Cleaner when the concentrator is in a rinse cycle or is offline. The other gravity circuit components consist of a surge bin for the concentrate, a Gemini table treating all the concentrate and a further Falcon model SB250 concentrator on the table tails, all of which is located in the secured area gold room.

The concentrate from the SB2500 concentrator unit gravitates to the gold room for further processing. The tailings from the concentrator reports to the Flash Flotation Cleaner TC-10 flotation cell where the coarse copper and gold particles are recovered with the concentrate, then report to the combined final concentrate hopper with the Re-cleaner concentrate and pumped to the concentrate thickener. The tailings from the Flash Flotation Cleaner report to a hopper and are then pumped back to the combined mills discharge hopper to be pumped back to the cyclones.

Flotation Circuit
Cyclone overflow reports by a gravity line to the first of six rougher flotation cells. Outotec TC-40 tank cells are used for the roughers with progressively increasing froth crowders installed down the train. Rougher concentrates are pumped to the Falcon SB750 fine gravity concentrator (GC003), while rougher tailings report to the flotation tailings hopper for pumping to the tailing’s thickener. Tails of the GC003 feed the cleaner bank, and its concentrate is discharged to the gold room.

The concentrate from the cleaner/cleaner-scavenger cleaner cells can be fed to either the feed of the recleaner cells or the cleaner cells dependent on concentrate grade. The concentrate from the cleanerscavenger cells report back to the feed of the cleaner cells.

Concentrate Handling
Final copper concentrate is thickened in a 12m diameter high rate thickener fitted with a van feedwell and deaeration tank. The underflow is pumped at about 60-70% solids to a 400m3 storage tank. A Outotec PF-930 horizontal plate pressure filter press produces a concentrate filter cake at about 8% moisture, which will be suitable for transport and sea freight to smelter customers.