The Cerro del Gallo copper-gold-silver deposit can be considered to be a member of a distinct subclass of “reduced” porphyry-style copper-gold mineralization as first proposed by Rowins (2000c). These reduced porphyry copper-gold deposits lack primary hematite, magnetite and sulfate minerals, but contain abundant hypogene pyrrhotite, commonly have carbonic ore fluids, and are associated with ilmenite-bearing, reduced I-type granitoids (Rowins 2000c). Cerro del Gallo displays all these features. In addition, there is a consistent pattern of higher temperature potassic alteration overprinted by lower temperature propylitic-style mineral alteration. Propylitic alteration boundaries are gradational and irregular, and more widespread than potassic alteration.

The Cerro del Gallo copper-gold-silver deposit also has characteristics supporting an intrusion related gold system (IRGS) model as proposed by Thompson et al 1999; Rowins 2000c; Champion 2005; Hart 2005. IRGS deposits are typically found in continental tectonic settings inboard of convergent plate boundaries, often where the regional metallogeny is characterized by tungsten-tin magmatic provinces. Felsic intrusives have an intermediate oxidation state between ilmenite and magnetite series with a gold-enriched metal assemblage derived from igneous fractionation, and a distinctive metallogenic signature of gold, bismuth, tin and tungsten. Hydrothermal fluids are carbonic, and pyrrhotite is common.

The Cerro del Gallo Project has several epithermal veining systems, one of which, the Ave de Gracia, transects Cerro del Gallo. None of the vein systems has had chemical determination for classification of sulfidation type, although there are several geological characteristics similar to low sulfidation epithermal deposits, all of them determined by geological mapping and geological description. Some of these characteristics are: a) sericite or illite ± adularia, chlorite alteration minerals; b) filling cavities/porosities vein type, banding and hydrothermal breccias; c) carbonate replacement textures; d) low content of bulk sulfur, low presence base metals (Pb, Zn).

In and around the Cerro del Gallo deposit there is also massive sulfide mineralization, however, these occurrences need further investigation. These rocks are composed of more than 60% sulfides, with variable quantities of pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite and galena and are normally strata bound unless remobilized. In the earlier logging programs these occurrences of massive sulfides were described as skarns.

The main copper-gold mineralizing event is considered to be intrusive related and referred to as a copper-gold porphyry system. Within the district there are also younger epithermal vein systems high in silver and lead.

At Cerro del Gallo mineralization is hosted in both felsic intrusive and volcanic tuff wall-rock. Mineralization is disseminated and vein or fracture controlled and extends from 200 meters to 400 meters outward from the mineralizing intrusive complex.

The strongest gold mineralization at Cerro del Gallo is associated with intense quartz stockwork veining and silicification within a wall-rock annulus forming the outer limits of a felsic stock with the system losing intensity outward with a decrease in stockwork and quartz veining density.

Sulfide make up less than 2% by volume of the mineralized rocks. Gold-copper mineralization is zoned concentrically around the felsic intrusive with higher grade gold mineralization proximal to and within an outer annulus of the intrusion. The highest copper grades are found outward from the gold zone. Zinc mineralization is locally anomalous outside the copper zone, metal zonation boundaries are gradational and there is an overlap in the gold-copper zone and the copper-zinc zone.

Pyrite is the dominant iron sulfide mineral with accessory pyrrhotite and marcasite dispersed throughout the mineralized system. Pyrite occurs in two forms; as euhedral to subhedral cubic crystals and crystal aggregates in veins and disseminations of primary origin, and also as secondary fine-grained patches after pyrrhotite (Mason 2005a). Pyrrhotite is most common in the outer copper zone. It is magnetic and in high enough concentration to form a donut shaped magnetic anomaly around the Cerro del Gallo mineral system.

Native gold occurs within vein quartz and inclusions within pyrite, chalcopyrite and bismuthinite. Gold grains range in size from generally 0.5-4 microns in diameter to rarely 10-20 microns (Mason 2005a; Mason 2006a; Townend 2006). Electron-probe microanalysis on a limited number of native gold grains indicates gold has a fineness of 860-880 with silver making up the rest (Mason 2006b, Mason 2007). In terms of geochemistry and all elements assayed, gold has the strongest correlation with bismuth.

The majority of silver at Cerro del Gallo is related to late structurally controlled epithermal veins that overprint the intrusive related copper-gold system. Within the veins, Perez (2018) identified tetrahedrite and electrum as the main silver minerals associated with galena and sphalerite. Silver can also occur as small grains of native silver or ruby silver as either pyrargyrite or proustite.

Chalcopyrite is the most common base metal sulfide phase and occurs in fracture veinlets often associated with intense silicification and as disseminations. Chalcopyrite is commonly closely associated with pyrite and marcasite, and rarely as inclusions in coarse arsenopyrite (Townend 2006). Traces of bornite have also been reported (Mason 2006b; Townend 2006), however this mineral is not volumetrically significant.

Minor secondary copper minerals including malachite and azurite have formed through weathering processes and are locally present in surface outcrops. Native copper, covellite and chalcocite are found deeper in the regolith profile, and their formation is attributed to supergene weathering processes.

Arsenopyrite is relatively common, occurring as coarse discrete grains often associated with chalcopyrite.

The Cerro del Gallo project has been planned as an open-pit truck and shovel operation. The truck and shovel method provides reasonable cost benefits and selectivity for this type of deposit. Only open-pit mining methods are considered for mining at Cerro del Gallo.
The open pit mining operation will require the stripping and disposal of waste rock. The total waste quantity is approximately 57.8 million tonnes with an overall mining waste-to-ore ratio of approximately 0.63:1. The bulk of the waste rock from the pit will be placed into a single external waste dump located to the east of the pit and north of the HLF and is named as the Waste Rock Dump (WRD).

For the PFS and based on the waste rock quantities and maximum footprint area provided by others, Golder developed a WRD design in its 2019 Study. Due to the potential for acid waste rock drainage three geomembrane lined contact water ponds are designed at the base of the three natural drainages within the area of the WRD.

The initial waste that is extracted from the pit will be placed below the Phase 1 and 4 leach pad footprints as structural fill and select waste rock, thereafter the waste will be placed in the western drainage of the WRD. After the western drainage of the WRD is full to capacity, the WRD surface will be regraded to an overall slope of 3H:1V that will cover the contact water pond and the surface will receive a cover system. The same process will be conducted for the remaining two drainages that the WRD is sited in. Therefore, typically only one contact water pond will be active at one time during operations. The contact water ponds have been sized to contain (below the spillway) the 1 in 2-year, 24- hour precipitation event falling on the contributing WRD and undisturbed ground areas.

The waste dump is designed with a capacity of approximately 55.0 million tonnes using a dry waste rock density of 1.9 t/m3. Although the estimated waste rock generated during the life of mine is approximately 2.8 million tonnes more than the WRD capacity, approximately 3.4 million tonnes of waste rock will be used for leach pad construction.

The WRD will be built in lifts to ensure overall geotechnical stability.

Proven and Probable ore is to be sent from the pit directly to the crusher prior to heap leaching. A small stockpile may be maintained near the crusher so that trucks may dump and return to mining operations in the event of unexpected crusher down time.

While it is assumed that the Cerro del Gallo property will be mined using a mining contractor, it has been planned as an open-pit mine using CAT 777 haul trucks and front-end loading equipment. Mine production assumes the use of a 13 cubic meter front-end loader and 91-tonne haul trucks.

Drilling requirements were based on pattern size, drilling penetration rates, and non-drilling time. Blast patterns were assumed for production, trim-row, and pioneer drilling. Production drilling is used for both waste and ore. The production drilling assumes 5.4-meter spacing and 5.4-meter burdens with 5.0-meter bench heights and an additional 1.5 meters of sub-drill. The pattern size is based on allowing for 3 meters of stemming and achieving a powder factor of approximately 0.25 kg of explosives for each tonne of rock. Trim rows are used to reduce the powder factor near high-walls and to reduce the damage caused by blasting. This drilling will be done using the production drills and will be accomplished by reducing the pattern size to 4.5 meters by 4.5 meters, and eliminating the sub-drilling. Pioneer mining will be done to gain initial access to the top of the hill.

Crushing for the CdG project is accomplished by a three-stage crushing system with an open primary crushing circuit, closed secondary, and tertiary crushing circuits operating seven days per week, 24 hours per day. Run of mine (ROM) material will be delivered and direct dumped, as much as possible, by haul truck from the mine into the 200-tonne ROM feed bin. A permanent rock breaker will be installed to break any oversized material. ROM material from the coarse ore bin will be delivered by a vibrating grizzly feeder.

Oversize material is crushed using a primary jaw crusher while the undersize material is combined with the primary jaw product on a primary crusher discharge conveyor. The primary jaw crusher is operated in open circuit and is designed to crush the vibrating grizzly oversize to 80% passing 145 mm. The primary crushing products is stockpiled by a coarse ore stockpile feed stacker conveyor.

Material from the primary crushed stockpile is reclaimed using reclaim feeders and is conveyed to the secondary screen feed conveyor. The secondary crushing circuit includes two double deck vibrating screens and two cone crushers. The secondary crushing circuit is operated in closed circuit with a product size of 80% passing 36 mm.

Primary crushed ore is combined with the secondary cone product and is fed to the secondary screen. The secondary screen oversize is transferred to the secondary cone crusher surge bin by conveyors and is fed to the secondary cone by a belt feeder. The secondary cone crusher discharge recycles back to the secondary screen.

Secondary screen undersize material is conveyed to the secondary product storage bin, reclaimed using belt feeders and is transferred to the tertiary crushing circuit. The tertiary crushing circuit consists of an HPGR crusher operated in closed circuit with a fine screening plant. The design for the final crushed product is 80% passing 4 to 6 mm. Material from the secondary product storage bin is transferred to the HPGR crusher. The product from the HPGR is conveyed to a bin. The bin has multiple discharge points onto three triple-deck vibrating tertiary screens. The tertiary screen oversize is transferred to the HPGR recycle conveyor and recirculated to the HPGR feed conveyor. The tertiary screen undersize is stockpiled by the fine ore stacking conveyor.

Material from the fine ore stockpile is reclaimed using reclaim feeders and conveyed to the agglomeration system by the fine ore reclaim conveyor. The fine ore reclaim conveyor discharges to a splitting chute to feed two parallel agglomeration feed conveyors. Cement is added to the ore on the agglomeration feed conveyors from the cement silos, depending on the material type. The cement addition rate is controlled by a weightometer mounted on the individual agglomeration feed conveyors. The agglomeration feed conveyors feed two parallel agglomeration drums where barren process solution is added and cement is blended in. The agglomeration drums discharge onto the agglomeration discharge conveyor and the material is transported to the stacking system.

The Cerro del Gallo ore contains cyanide soluble copper that results in high cyanide consumption during the heap leaching process. A SART plant is included that releases cyanide associated with the copper-cyanide complex, allowing a significant portion of it to be recycled back to the leach process as free cyanide, and produces a copper-silver precipitate that can be sold.

The barren solution from the SART plant will be processed in a carbon adsorption-desorption-recovery (ADR) plant to recover gold.

Heap Leaching
A single heap leach facility has been designed for the site by Golder. The Heap Leach Facility (HLF) has an ore capacity of approximately 92 million tonnes (Mt) using a dry ore density of 1.6 tonne/m3 (t/m3). The HLF design is described in detail in the Golder's report “Cerro Del Gallo PFS – Heap Leach Facility Design” (Golder 2019).

The 139.8-hectare HLF has a maximum heap height of 80 meters. Ore is designed to be stacked at a rate of 1 ........