The Philippines Archipelago constitutes one of the world's premier porphyry copper provinces and is a typical area for the study of island arc porphyry systems (predominantly calc alkaline porphyry deposits).

While the Didipio gold-copper deposit has many broad similarities to the predominant Philippines calcalkaline porphyry deposits, it is not a classic, large porphyry-style deposit. Rather, it is a smaller alkaline mineralised stock containing disseminated and fracture vein-controlled gold-copper mineralisation that has been overprinted by late stage, structurally controlled, higher-grade, gold copper mineralisation.

The Didipio porphyry Au-Cu deposit exhibits features that are common to other alkaline porphyries found in Eastern Australia and British Columbia, Canada.

The Didipio gold-copper deposit is hosted within the multiphase Didipio Stock, which is in turn part of a larger alkalic intrusive body, the Didipio Igneous Complex. The deposit has been identified as an alkalic gold-copper porphyry system, roughly elliptical in shape at surface (480m long by 180m wide) and with a vertical pipe-like geometry that extends to at least 800m below the surface. The local geology comprises north-northwest trending, steeply (80° to 85°) east-dipping composite monzodiorite intrusive, in contact with volcaniclastics of the Mamparang Formation. The monzodiorite lies in a circular topographic depression that is coincident with a circular IP anomaly.

Porphyry-style mineralisation is closely associated with a zone of K-feldspar alteration within a small composite porphyritic monzonite stock intruded into the main body of diorite (Dark Diorite). The extent of alteration is marked by a prominent topographic feature (the Didipio Ridge) some 400m long and rising steeply to about 100m above an area of river flats and undulating ground.

Chalcopyrite, gold and silver (electrum) are the main economic minerals in the deposit. Chalcopyrite occurs as fine-grained disseminations, aggregates, fracture fillings and veins.

Fine grained gold occurs as micro-inclusions in sulphides, as well as free gold, electrum and telluride. Visible gold is rare. Chalcopyrite can replace magnetite and is, in turn, replaced by bornite. Bornite occurs as alteration rims around and along fractures within chalcopyrite grains.

The deposit is oxidised from the surface to a depth of between 15m and 60m, averaging 30m. The oxide zone forms a blanket over the top of the deposit and largely comprises silicification, clay and carbonate minerals, accompanied by secondary copper minerals including malachite and chrysocolla.

Based on the optimisation study conducted in the fourth quarter of 2014, the Didipio open pit mine is expected to be completed in 2017. Following approximately five years of open pit mining starting in 2012 and twelve years of underground mining commencing production in 2018, mining operations are planned to cease in 2032.

Open pit pre-strip operations commenced in January 2012. Ore mining commenced in mid-2012, and first ore was fed to the mill in mid December 2012. Material mined during the pre strip phase was used for the run-of-mine (“ROM”) stockpile base, haul roads and the first TSF embankment construction.

The open pit has over-achieved the mining schedule in each successive year of operation to-date. The open pit mining contractor has sufficient mining equipment on site. The open pit operation has been scheduled to deliver to the mill, ore of sufficient grade to achieve 100,000 gold ounces and 14,000 tonnes of copper per year over the life of the mine. Medium grade ore stockpiled during open pit operations will be milled at the end of the open pit and will be blended with the ore coming out of the underground operation. Waste rock mined during 2016 supplied the TSF embankment and waste rock dump construction. The TSF embankment has largely been constructed during the operational stage of the open pit, using the waste for the bulk, compacted and engineered fill. The crest of the embankment is now at the 2810mRL (2m remaining to be constructed). The filter and clay areas are at the 2785mRL. These will be raised approximately 5m per annum until the final elevation is reached at the 2812mRL.

Based on the current design, an open pit mine will generate cash flow until the underground mine comes into production early 2018. An optimisation study was completed during the second half of 2014, which raised the crown pillar elevation and increased the production rate of the underground mine from 1.2Mtpa to 1.6Mtpa. The present underground mine plan is based on sub-level open stoping (“SLOS”) with paste backfill. The stopes are generally 20 x 20 metres and have a height of 30 metres. They are arranged in a “checker board” pattern to extract the broad ore zones. Cemented paste backfill is placed as each stope is completed to allow extraction of the adjacent stopes without creating unstable spans. The stoping sequence occurs in three phases (primary, secondary and tertiary) so that the current production stope is always surrounded on four sides by either unmined rock or filled voids. Production from the underground mine is scheduled to ramp up to approximately 1.6 Mtpa by 2020, with mining scheduled to be completed by 2030.

Recovery of copper and gold at Didipio is achieved from the use of froth flotation following a conventional SAG Mill-Ball Mill grinding circuit.

The process flowsheet utilises a conventional process for recovery of a gold-copper concentrate and doré.

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 single toggle crusher, selected to handle 900mm maximum lump size, crushes the ROM ore to a typical P80 product size of 100mm.

The ore from the crusher is transported via conveyor one (CV-001) and CV-006 to a transfer bin. The transfer bin has a live capacity of approximately 15 minutes of mill feed. An apron feeder located beneath the bin transfers the crushed ore onto the mill feed conveyor CV-003, if CV-003 (or the SAG mill) is offline a diverter gate at the top of the bin directs the ore onto CV-002 the Extra Fine Ore (“EFO”) conveyor, CV-002 discharges ore onto a 5,000 tonne emergency stockpile.

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.

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.

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.

The purpose of the gravity circuit is to recover free gold from the Flash Flotation concentrate. 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.

Cyclone overflow reports by 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 cleaner cells, the rougher tailings report to the flotation tailings hopper for pumping to the tailings thickener.

Concentrate from the cleaner cells feeds the bank of re-cleaner cells. Tailings from the re cleaner cells re-join the rougher concentrate as feed to the cleaner cells. Concentrate from the re-cleaner cells will be directed to the final concentrate pumpbox and then transferred to the concentrate thickener. The tails from the cleaner cells feed into the cleaner scavenger cells with the tailing from these cells reporting to the cleaner-scavenger cells. The tails from the cleaner-scavenger cells reports to the flotation tails pumpbox.

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

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.

The filter cake discharges to a concentrate stockpile of about 15 days capacity located within the concentrate storage shed. The concentrate is loaded into rigid trucks using a FEL with a nominal payload of 20 wet tonnes per load. Composite samples are prepared from trucks as they are loaded, for moisture and metal content. A weighbridge weighs all trucks leaving site to account for movement, inventory control of material and tracking for permit requirements.

Concentrate is trucked by road to a storage shed located at Poro Point, La Union with the capacity to hold up to 15,000t of material. Ships are loaded periodically in 5,500 or 11,000t shipments.

Combined flotation tailings will gravitate to a combined flotation tailings hopper and are pumped to a 20m diameter high rate thickener with a vane feed well. Flocculent, Magnafloc 919, is dosed to the thickener feed box by variable speed helical rotor pumps to aid in the settling of concentrate and to provide necessary clarity in thickener overflow.

The concentrates from the Falcon concentrator are screened with a Kason screen and the products individually retreated using a Gemini shaking table. Fine free gold is separated from the predominantly chalcopyrite/bornite primary gravity concentrate with screening utilised to improve the processing efficiency of the table. Concentrates from the table are filtered and dried prior to smelting in a standard diesel fired barring furnace. The tailings and middlings product from the table are retreated in a small Falcon concentrator, with the concentrates joining the table concentrates for smelting. The tailings from the secondary concentrator are returned to the final concentrate pumpbox to minimise any gold losses from the gravity cleaning circuit.

The dried gravity concentrates are mixed in batches with fluxes designed to allow the best separation of the gold and silver into doré. These batches are smelted and poured into moulds to produce the gold/silver doré bars, which assay 85% gold and 15% silver. Iron and base metal levels in the bars are typically less than 3%.