La Cumbre Mineral Resource Update and PEA — 2022

The 2022 mineral resource update and preliminary economic assessment for the La Cumbre project describes a two-stage processing scheme, with heap leaching for oxide and transition ores and a flotation plus tank leaching route for primary sulfides, supported by a common SART, Merrill Crowe, and carbon adsorption recovery circuit.

Report context

The August 2022 technical report presents a mineral resource update and preliminary economic assessment for the La Cumbre deposit. The processing design covers two ore types: oxide and transition ores treated via heap leaching, and primary sulfides treated via grinding, flotation, and carbon-in-leach. The report includes detailed process descriptions for both stages, with metallurgical balances for the primary sulfide route.

Processing route

Ore handling and crushing

The mine-to-mill concept is applied at La Cumbre to produce optimal granulometry. Ore and waste are deposited separately in a grass hopper feeding a conveyor belt system. All crushed material is transported by an overland conveyor, with ore sent to La Perla and waste to Matecaña.

Material is stockpiled according to destination: oxides to process or stockpile at La Perla, transition ore to process or heaps at La Perla, sulfides to heaps at Matecaña, and waste at Matecaña. Only primary zone material in stockpiles is crushed separately in a semi-mobile crushing plant, with a 20 m³ front-end loader feeding the transfer hopper. The semi-mobile crusher and stockpile locations move as excavation fronts advance.

At La Perla, ore from the reception yard is extracted by two plate feeders onto a conveyor feeding an 8' x 20' double deck vibrating screen generating coarse (+17 mm), intermediate (-17 to +12.7 mm), and fines (-½"). Coarse product goes to cone crushers, intermediates to grass hopper belts, and fines to the agglomerating drum.

Agglomeration

Fines from the screens (-½") are agglomerated by dosing cement at 3 to 7 kg/t ore, lime, and barren solution in a 1,000 tph agglomerating drum. The agglomerated material discharges onto a conveyor joining non-agglomerated ore. Curing occurs on the platform during loading and irrigation system assembly. After curing, material is extracted from Stock Pile 2 by feeder belts onto a main belt feeding a continuous conveying and stacking system. Intermediate-sized stockpile material (>½") is also discharged onto this collecting belt to mix with agglomerated material for better permeability.

Heap leaching

The heap leach pad is waterproofed with a polyethylene geomembrane layer, equipped with corrugated and perforated pipes for collection of gold and silver enriched solutions. The pad is covered with overliner material of high permeability to prevent geomembrane deterioration.

The pad system consists of an infeed conveyor with a full tripper transferring ore to a mobile stacking system of 5 grasshoppers and 2 in-line stackers. Cells measure 90 m x 18 m (1,620 m²) with flow lines and drop irrigation systems using SCH 40 iron pipes. Flat hoses with self-compensating systems, pressure gauges, flow meters, and purge valves distribute solution.

The irrigation cycle is 17.5 days with 1,000 ppm NaCN leach solution. The first 5 days are humectation at 5 l/h/m² to condition permeability, followed by 12.5 days production irrigation at 10 l/h/m². Leached material is moved by conveyor belts to the tailings deposit.

Solution percolating during the first 5 days has the highest gold concentrations and is stored in the PLS pond. Solution from the remaining 12.5 days contains lower gold content and is stored in the Intermediate Leach Solution (ILS) pond. Rich solution is collected by perforated pipes under the dynamic pile, protected by a drainage layer of 2½" to 3" rock fragments, flowing to PLS and ILS settling ponds.

SART process

The SART (sulfurization, acidification, recycling and thickening) circuit treats rich solution to recover copper as copper sulfide and convert weak acid dissociable cyanide to free cyanide for recovery and recirculation in the AVR circuit. The process includes precipitation, thickening, and filtration of copper sulfides, plus neutralization of rich solution with gypsum precipitation and thickening.

Sodium hydrosulfide and sulfuric acid are added to an agitated precipitation tank to react with dissolved copper cyanide complexes, forming copper sulfide precipitates and dissolved hydrocyanic gas. The precipitate slurry is thickened, with most precipitates pumped to a filter press for filtered copper sulfide. The acidified rich solution overflow is pumped to neutralization tanks with lime addition in a series of three agitated tanks. Gypsum precipitates from the thickener are recirculated to the leach pad.

AVR circuit

The Acidification, Volatilization and Reneutralization circuit recovers dissolved hydrogen cyanide gas from acidified rich solution, converting it to sodium cyanide for reuse. This circuit maintains cyanide balance between Merrill Crowe, column carbon, and heap leach processes when treating transition ores with high copper content requiring high cyanide concentration.

Dissolved hydrocyanic gas is released by blowing process air through acidified rich solution in a packed bed desorption column. The gas mixture passes through a packed bed absorption column where alkaline caustic soda converts hydrocyanic gas to NaCN. Process air circulates in a closed loop between columns, with a small bleed of clean air maintaining vacuum conditions. The AVR process operates in closed vessels under vacuum to prevent hydrocyanic gas escape. The NaCN solution recovered from the absorption column is recirculated to the dilution water tank for reuse.

Merrill Crowe circuit

The rich solution from the PLS pond is pumped to the SART process and then to the Merrill Crowe plant. Solution enters a hopper tank and passes to clarifying filters using diatomite to reduce turbidity to less than one nephelometric turbidity unit. Two preparation tanks are required for diatomite: a precoat mix tank and a body feed tank. Feed flow to the Merrill Crowe circuit is 64 m³/h.

Filtered rich solution passes to a deoxygenation tower removing dissolved oxygen mechanically under high vacuum, reducing oxygen concentration to less than 0.5 ppm. Deoxygenated solution is fed through steel pipes to precipitation filter presses with zinc powder and lead nitrate added by peristaltic pump. Gold precipitation occurs instantaneously, with precipitates retained in filter chambers.

Barren solution containing less than 0.02 ppm Au and Ag is stored in a steel tank, recharged with cyanide, and pumped to the dynamic stack for irrigation. Merrill Crowe circuit efficiency is reported at 98%.

Carbon adsorption (CIC)

The ILS solution passes through adsorption circuits in cascade configuration after copper precipitation in the SART circuit. Carbon is loaded to 2.4 kg Au per ton of carbon before moving to the next process. Two CIC banks operate in parallel with electromagnetic flowmeters on inlet lines.

Barren solution from each adsorption circuit passes through two DSM stationary curved screens to separate carbon particles, then to barren tanks. Antiscalant and 25% leach solution are added at each tank outlet to readjust solution strength before pumping to heap leach pads, forming a permanently closed circuit.

Carbon loaded with precious metals passes through a sieve separating coarse carbon from fine particles in barren solution.

Carbon desorption and electrowinning

Carbon from columns is transported to a desorption reactor, a batch process receiving rich carbon. Desorption uses hot stripping solution at 130°C to 140°C, heated by a heat exchanger with hot oil from a heater. Solution from the desorption reactor passes through a duplex filter for fine carbon retention, then through a cooler reducing temperature to 70°C before entering desorption cells. Electrolytic cells recover gold as precipitates at the cathode, with 98% recovery of dissolved gold and silver.

The electrolytic precipitate undergoes acid washing in two reactors: first with sulfuric acid, second with nitric acid, under a fume hood with neutralization tower. The product is the final process product, passing to smelting.

Carbon reactivation

Carbon reactivation begins with chemical washing using 3% diluted hydrochloric acid at 90°C to remove carbonates and sulfates that affect load-carrying capacity. Thermal reactivation follows in a furnace, heating carbon gradually and indirectly to 700°C with short holding time. Reactivated carbon passes through size classification to discard fines before reuse in carbon columns.

Smelting

Precipitates with gold and silver contents are dried and mercury is extracted in retort furnaces at approximately 600°C with 24-hour cycle time under vacuum at 180 mm Hg. Mercury vapor is collected by water-cooled condensers and stored in special containers. The vacuum stream passes through activated carbon recovering more than 99.5% of mercury before discharge.

Dry precipitate is mixed with fluxes (sodium borate, silicon dioxide, sodium nitrate, sodium carbonate, calcium fluoride) and charged to induction furnaces at around 1,300°C for slagging. Doré bars are produced by cascade casting, then cleaned, coded, and stored in a vault. Slag passes through a closed crushing circuit with jaw crusher and screen (0.8 mm mesh, Tyler Series 20 mesh), with fine product sent to a gravimetric concentrator. Concentrate is smelted with the next batch.

Contact water detoxification

Contact water from La Cumbre mine and Matecaña deposits is analyzed daily for dissolved metals and acidity. Drainage from ROM ore piles at La Perla is sent to the plant process. Contact water is channeled to a collecting pool, pumped to an agitator tank where lime is added to reach pH 9 to 10, neutralizing acidity and forming heavy metal hydroxides. The flow enters an aerator tank where atmospheric oxygen oxidizes iron and sulfates, then a second tank where reducing agent precipitates metals such as Cu and Pb as sulfides, and a third tank where coagulant is added.

Flocculant is added in the overflow channel of the aeration tank before a clarifier where precipitates settle. Overflow passes to a sedimentation pond, while sludge from the clarifier goes to a filter press. Sulfuric acid is added at filter outlet to counteract alkalinity if necessary. Produced muds are sent to the tailings deposit.

Solution detoxification plant at La Perla

Water drainage from the tailings tank at La Perla is collected and sent to the barren tank for water balance in the process plant. Excess water is treated at the La Perla solution detoxification plant. Barren solution is directed to a flow pool supplying reactor tanks: three reactor tanks in series discharging by overflow. Hydrogen peroxide, copper sulfate, and caustic soda (10%) are added to the first tank, with the same reagents to the second tank. The third tank neutralizes charges with coagulant and anionic flocculant to bind suspended solids. Effluent enters a clarifier for solid-liquid separation, with overflowing water passing through a carbon column system to trap undetoxified ions. Treated water complies with maximum permissible limits for environmental discharge.

Reagents used include hydrogen peroxide (50%), copper sulfate pentahydrate, lime, coagulant, and flocculant.

Primary sulfide processing

The metallurgical treatment of primary sulfides consists of crushing, haulage and stockpiling, milling, gravimetry, flotation, tank leaching (CIL), SART-AVR, Merrill Crowe, ADR plant, smelting, contact water detoxification, and solution detoxification. In this stage, agglomeration, heap loading, and heap leaching from the oxide/transition route are replaced by grinding, gravimetric, flotation, and carbon tank leaching sections.

Milling

Crushed ore from the La Perla crushing plant is stored in a fine ore hopper, extracted by feeder belts to two 15'Ø×25' primary mills. Discharge from each primary mill falls by gravity onto a high-frequency vibrating screen. Retained fraction enters a pump box driving ore to a cyclone nest. Cyclones classify by size: fine ore goes to flotation, coarse ore fraction feeds three 18'Ø×26' secondary mills, each with high-frequency vibrating screens. Fine fractions from both primary and secondary mill screens enter the gravimetric section, distributed equally to 8 centrifugal gravimetric concentrators. Gravimetric concentrate is sent to tank leaching; gravimetric tailings are sent to the cyclone nest pump box.

Gravimetry

High-frequency vibrating screens at each mill discharge classify ore to optimum size for centrifugal gravity concentrators. Concentrators operate intermittently, stopping for concentrate harvest. Concentrate is stored in a bin until transferred to the leaching section. Gravity concentrator tailings recirculate to the mill through the cyclone nest pump box.

Flotation

Milled ore is gravity fed to the flotation section. Pulp enters two in-series conditioners with 14 minutes residence time where flotation reagents are added. The conditioned pulp enters the rougher stage consisting of 6 flotation cells, followed by the scavenger stage with 6 flotation cells. Total residence time for both stages is 50 minutes. Scavenger discharge is the final flotation tailing sent to the tailings dam.

Rougher concentrate is diverted to cleaning stage 2, consisting of 5 banks of 4 cells each. Scavenger concentrate is diverted to cleaning stage 1, consisting of 10 banks of 4 cells each. Cleaning stage 2 tailings recirculate to cleaning stage 1; cleaning stage 1 tailings recirculate to the flotation head. Cleaning stage 2 concentrate is the final flotation concentrate sent to the concentrate thickener, with thickened concentrate sent to tank leaching.

Tank leaching (CIL)

Gravimetric and flotation concentrates are combined and enter the leaching section. First stage is concentrate regrinding in cyanide medium in an 8'Ø×13' ball mill where cyanide solution is applied. Mill discharge enters a pump box feeding a cyclone nest. Cyclones classify by size: coarse fraction recirculates to the ball mill, fine fraction enters thickener 1 for solid-liquid separation. Rich solution from thickened slurry is pumped to the Merrill Crowe plant; thickened slurry is diverted to the activated carbon tank leaching section.

First leaching and adsorption stage consists of 3 agitator tanks in series, each with 12 hours residence time. Discharge enters thickener 2 for solid-liquid separation. Overflow with low gold content recirculates to the first leach tank. Thickened pulp enters the second leaching and adsorption stage of 3 agitator tanks with 12 hours retention each. Discharge from the second bank enters thickener 3. Overflow solution recirculates to the fourth leach tank; thickened slurry is diverted to the detox plant.

Carbon harvesting occurs when gold content in liquid streams overflowing from thickeners increases to 0.05 ppm. Loaded carbon is transferred to the carbon desorption and reactivation plant.

Pulp tailings

The slurry tailings dam receives flotation tailings and leaching tailings. Flotation tailings are sent directly to the tailings pond. Leach tailings must be detoxified before entering the tailings pond. Solids settle to form a beach and clarified water mirror, with clarified water pumped to the process water storage tank and recirculated.

Merrill Crowe operation parameters

Barren solution is recomposed in the barren tank with caustic soda to control pH at 10.5 to 11, NaCN strength at 1,000 ppm, and antifouling at 4 ppm. Recomposed solution is recirculated to heap leach pads using a vertical turbine pump. Inlet lines to carbon columns and the Merrill Crowe hopper tank are dosed according to carbonate and sulfate content. Pump motors operate with variable speed drives for operational flexibility. Process solution flow is maintained with industrial water or barren solution from the well of major events, with a submersible pump installed for this purpose.

Key reported parameters

The metallurgical balance for primary sulfide treatment reports 80.6% global gold recovery, with 84.9% recovery in concentrate, 95.0% leaching extraction, and 99.9% ADR recovery.

Technical qualifications

The report presents a preliminary economic assessment, and the process design is based on a combination of design criteria and testwork, with no full-scale operating data available for the La Cumbre project. Specific limitations identified in the report include:

• The metallurgical process description designates a Merrill Crowe circuit, with the report also referencing carbon adsorption in columns, indicating a dual recovery approach dependent on solution characteristics and process stage.

• The heap leach design assumes a 17.5-day irrigation cycle with 5 days humectation and 12.5 days production irrigation, but the report notes that actual heap performance will depend on ore permeability and solution percolation characteristics that require site-specific verification.

• The SART and AVR circuits are designed to treat transition ores with high copper content, but the report does not provide full metallurgical characterization of the copper content or its variability across the deposit.

• Recovery figures for the primary sulfide route are presented as a metallurgical balance with 80.6% global recovery, 95% leaching extraction, and 99.9% ADR recovery, however the report notes these are based on design criteria and laboratory or pilot testwork, with no demonstration-scale or full-scale operating history.

• The report states that the detoxification plant treated water complies with maximum permissible limits for discharge, but does not provide the specific regulatory standards or permit limits against which compliance is measured.

• The report does not provide details of the flotation feed grade, particle size distribution, reagent consumption rates, or mass pull percentages beyond the metallurgical balance table presented.

• The report describes a gravity concentration stage with 8 centrifugal concentrators, but does not specify the equipment model, size, or manufacturer.

• The carbon adsorption circuit is described as operating with 5 columns in series, with the loaded carbon transferred to desorption when loaded to 2.4 kg Au/t carbon, but the report does not provide the carbon transfer rate, carbon inventory, or carbon loss rates.

• The report does not quantify water balance, solution losses, or reagent consumption rates beyond the cyanide concentration, pH, and antifouling dosages stated.

• The report references a dynamic pad and stockpile, but does not provide heap height, stacking rate, or pad capacity details.

• The flotation circuit design indicates a specific cell arrangement, but the residence time is only given for the combined rougher and scavenger stages.

• The report does not disclose the copper content of the ore or concentrate, despite the SART circuit being designed specifically for copper-bearing solutions.

• Cyanide consumption and destruction efficiency are not provided, despite the detailed description of the detoxification process.

• The report does not identify the ore throughput rate for the primary sulfide flotation route, only the nameplate ADR capacity of 15,000 t/day for the heap leach.

• Gold and silver feed grades are presented in the metallurgical balance for the flotation concentrate leaching circuit, but reconciliation with resource grade estimates is not provided.

Source: *La Cumbre Mineral Resource Update and PEA*, August 2022, Sections 17.0 Recovery Methods, 17.1 Process Description and Flowsheet of the Heap Leach Process, 17.1.9.2 Desorption and Electrowinning, 17.1.9.3 Carbon Reactivation, 17.1.10 Smelting, 17.1.11 Detoxification of Contact Waters, 17.1.12 Solution Detoxification Plant at La Perla, 17.2 Process Description and Flowsheet of Treatment of Primary Sulfides, 17.2.2 Milling, 17.2.3 Gravimetry, 17.2.4 Flotation, 17.2.5 Leaching, 17.2.6 Pulp Tailings, 17.2.7 Metallurgical Balance.