The Cangrejos and Gran Bestia deposits are gold-copper, silica-saturated, alkalic porphyry-style deposits. This type of deposit is found along paleo-subduction margins (Carter, 1981; Cox et al., 1987).

Gold-copper mineralization is associated with sulfides that occur as open-space breccia fill or as disseminations in former mafic phenocrysts. Total sulfide content is generally less than 5% and consists of chalcopyrite and pyrite with minor bornite, molybdenite and pyrrhotite. In drill hole C17-65, native copper is found on fracture surfaces at depths ranging from 142 m to 186 m. This is not very common and is probably due to strong oxidation along a fracture zone.

The highest gold grades occur in calcic-sodic altered breccias at contacts between the porphyritic quartz diorite (PQD) and the metamorphic complex. The best gold grades occur in breccias where the dominant clast type is equigranular quartz diorite (EQD). The EQD is more mafic in composition, and gold is preferentially deposited in reducing (mafic) environments.

Cangrejos Zone
Current understanding of the surface geology of the Cangrejos Zone is limited because of a lack of outcrop exposures. An east-northeasterly trending porphyritic quartz diorite (PQD) intrudes quartz diorites (EQD) and schists (SCH) of the metamorphic complex. The southeastern contact is steep, but the northwestern contact dips approximately 50° to the southeast and is defined by porphyry dikes. The edges of the porphyry are brecciated with dominant PQD clasts. These breccias extend at depth and may relate to breccia pipes that are associated with the intrusion of the porphyry. Breccias with dominant metamorphic clasts (EQD, SCH) occur at the edge of the porphyry.

The mineralized zone extends for approximately 1,000 m in a northeasterly direction, has widths ranging from 70 m to 600 m, and has been defined to a depth of at least 600 m below surface. The zone remains open to expansion with further exploration to the west and at depth.

Gran Bestia Zone
As with Cangrejos, current understanding of the surface geology of the Gran Bestia Zone is limited because of a lack of outcrop exposures.

The southern portion of the Gran Bestia deposit is similar to the Cangrejos deposit in that porphyritic quartz diorite (PQD) intrusions are present. Gold-copper mineralization occurs at the contact between the porphyries and the surrounding metamorphic country rock, with higher grades associated with breccias and quartz vein stockworks. The southern breccia has dimensions of 400 m (northwest-southeast) by 200 m (northeast-southwest).

The northern part of the Gran Bestia deposit is underlain by a polymictic intrusive breccia. Metamorphic complex clasts are hosted in a fine-grained diorite porphyry matrix. Several massive diorite porphyry dikes also occur within this breccia. The northern breccia has dimensions of 600 m by 400 m.

The gold-copper mineralized zone at Gran Bestia extends for approximately 700 m (north-south) by 600 m (east-west) and has been defined to depths ranging from 200 m in the south to 700 m in the north. The zone remains open to the north, west, and at depth.

The Cangrejos Project consists of two adjacent open pit mines referred to as 1) Cangrejos (eastern pit) and 2) Gran Bestia (western pit). Both pits will be mined with conventional hard rock open pit mining methods. The terrain is very steep with evergreen montane secondary forest, saprolite, and saprock overlying both pit areas. Dealing with the saprolite and saprock presents challenges for mine operations. This was anticipated during the development of the mine production schedule and in selecting the mine equipment.

The mine delivers 40,000 tpd of feed to the plant beginning in mid-year one through year five. The production ramps up during year six so the mine delivers 80,000 tpd of mill feed in years 7 to 25. IMC developed the production schedules by rock types of saprolite, saprock, partially oxidized, and fresh rock.

The preproduction period begins after the road between the site facilities and the crusher site is completed. The initial five months are for access pioneering only. Additional pioneering continues throughout the period of preproduction mining.

Waste material was reported by the rock types of saprolite, saprock, partially oxidized, and fresh rock. During the first five years of the mine life, the saprolite and saprock are delivered to the saprolite storage facility (SSF) located south-southeast of the Cangrejos pit. The SSF is constructed in four stages.

The fresh rock waste is delivered to the waste rock storage facility (WRSF) located south of the Cangrejos pit and the primary crusher. A portion of the saprock is delivered to the WRSF during the first five years. After year five, all of the saprolite and saprock are stored in the main WRSF. The WRSF is built in five stages.

Cangrejos is unique in that the waste haulage distances begin to slowly reduce over much of the mine life from year seven until just a few years before the end of the mine life in year 25. This is because the waste dump rises in elevation to meet the exit elevation from the pit.

Run-of-mine (ROM) material is hauled from the mine by truck and either stockpiled or directly dumped into the primary crusher. The primary crushed material is transported by an apron feeder and short conveyor to the overland conveyor system for transport to the process plant site. The primary crusher and the structure for the overland conveyor are sized for 80,000 tpd. The drives on the overland conveyor are sized to convey 40,000 tpd. They are designed to be changed quickly to support the 80,000 tpd operation during the plant expansion.

The overland conveyor system transports the crushed material to the coarse material stockpile. Apron feeders draw material from the coarse material stockpile and transport it to a double-deck banana screen. Underflow from the screen discharges to the coarse material stockpile and the screen oversize is crushed in two secondary cone crushers that operate in parallel. Discharged material from the crushers drops onto a transfer conveyor that returns it to feed the banana screen.

Apron feeders withdraw material from the coarse material stockpile and a conveyor belt transfers it to a log washer. The log washer is included in the conceptual design in order to remove clay that may hamper subsequent operations. Fines from the log washer are pumped to the ball mill hydro-cyclone feed. Oversize from the log washer is fed to the washing screen. Oversize from the washing screen is conveyed to the single HPGR with 3,200 kW dual drives (i.e., 6,400 kw total). Discharge from the HPGR is conveyed to the washing screen that is operated in closed circuit with the HPGR. Oversize from the washing screen is returned to the HPGR for further size reduction and undersize from the screen is the product from the HPGR that is split and fed to two ball milling circuits that each operate in closed circuit with hydrocyclones. The cyclone underflow is fed to the ball mills. Discharge from the ball mills is pumped to the cyclone feed. Overflow from the cyclones has a particle size distribution of 80% passing (P80) 90 µm.

The ball mill cyclone overflow is pumped to the bulk rougher flotation circuit that consists of two sets of tank cells that operate in parallel to provide approximately 30 minutes of retention time.

Concentrate from the rougher flotation circuit is reground in a Vertimill to reduce the particle size of the bulk rougher concentrate to P80 35 µm. The cleaner flotation circuit consists of three stages of cleaner flotation plus cleaner-scavenger flotation. Concentrate from the third cleaner flotation circuit is fed to the bulk concentrate thickener.

Underflow from the bulk concentrate thickener is pumped to an agitated conditioning tank where sodium hydrosulfide (NaHS) is added to depress copper minerals to prohibit them from floating. From the conditioning tank, the bulk concentrate is pumped to the molybdenum flotation circuit that consists of rougher flotation, five stages of cleaner flotation, and a cleaner scavenger flotation circuit.

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