San Agustin — 2025 Technical Report

This report describes the crushing, agglomeration, heap leaching, and carbon adsorption process facilities at the San Agustin operation in Durango State, Mexico, as of the January 2025 report date.

Report context

The San Agustin Technical Report, dated January 2025, covers the open pit mine and heap leach operation located in Durango State, Mexico. The report presents both the original plant design criteria and the actual operating history through late 2024, including the suspension of crushing and stacking in September 2024. The process plant has been in operation since 2017.

Processing route

Crushing and conveying

The crushing circuit at San Agustin consists of two separate plants. Plant 1 includes a primary jaw crusher (4460 jaw, 250 HP), two secondary cone crushers (K500, 500 HP each), and two double-deck vibrating screens (2.4 m width x 6.1 m length, 75 HP). Plant 2 is nearly identical but uses slightly smaller equipment, with one vibrating screen and one cone crusher in the secondary circuit. The design crushing production rate is 30,000 t/d, with an annual design tonnage of 10,095,000 tonnes. Crushed product size is P 80 22 mm.

Ore is fed from the dump hopper across a vibrating grizzly, with oversize directed to the jaw crusher and undersize to the primary crushed ore stockpile. The secondary crushing circuit operates in open circuit. Crushed product from both plants joins at a common stockpile and is conveyed 0.5 km to the heap leach pad on two overland conveyors, followed by 47 grasshopper transfer conveyors and two radial stackers with extendable stingers. Ore is stacked in 8 m high lifts.

Agglomeration

Prior to April 2021, ore was belt-agglomerated with 2 kg/t to 3 kg/t cement (with the exception of 2019) and 5.6 kg/t lime as required for improved percolation. Since April 2021, a synthetic polymer agglomeration aid (DustTreat) has been used in place of cement. The polymer addition rate started at 0.05 kg/t and was at 0.0065 kg/t as of September 2024. The lime addition during 2024 was estimated at approximately 7.5 kg/t. No percolation problems have been reported with polymer use.

Heap leaching

The heap leach pad is a multiple-lift, single-use type with a total design capacity of 64 Mt of ore. As of the end of September 2024, 61 Mt had been stacked, leaving 3 Mt of remaining capacity. The report proposes two small leach pad extensions: one to the northwest (currently 75% complete) and a southeast pad planned for 2026, adding 15.5 Mt of capacity.

The pad is constructed on a prepared surface with a compacted subgrade, 0.3 m of low permeability soil, a single 1.5 mm linear low-density polyethylene (LLDPE) liner, and 0.6 m of drainage gravel with perforated drainage pipes. Leach solution containing 350 ppm to 450 ppm NaCN is applied at a rate of 8 L/hr/m² over a 75-day leach cycle. The design solution application rate is 8 L/h/m². Pregnant leach solution (PLS) flows by gravity to the PLS pond, which is double lined with 1.5 mm HDPE and has a leak detection system.

Carbon adsorption

PLS is pumped from the PLS pond to the carbon-in-column (CIC) adsorption circuit at a rate of 1,200 m³/hr. The circuit consists of two trains of five cascade-style adsorption columns each. The larger train has columns with a holding capacity of 14 t of activated carbon per column; the smaller train has columns with a holding capacity of 1.5 t per column. Carbon is pumped counter-current to solution flow using recessed impeller pumps. Loaded carbon is dewatered and placed into 500 kg super-sacks for transport to La Colorada Mine for carbon stripping, acid washing, and thermal regeneration. Following adsorption, barren solution flows by gravity to the barren solution tank where cyanide concentration is adjusted before being pumped back to the leach pad.

Merrill-Crowe

A small stand-alone Merrill-Crowe plant was installed in November 2020 to treat a 250 m³/hr stream of PLS by zinc precipitation, with the resulting precipitate shipped to La Colorada for smelting. The plant was shut down in April 2024 due to low silver values in solution.

Key reported parameters

Technical qualifications

The report notes that the actual processing rate has averaged approximately 20,000 t/d since 2023, below the design rate of 30,000 t/d. Crushing and stacking ended in late September 2024. Realized gold recovery of 59.5% is below the assumed endpoint recovery of 61.3% from testwork on oxide, transition, and sulphide samples. Realized silver recovery of 8.9% is lower than the 15.5% endpoint recovery from testwork, attributed to poor silver recovery in the carbon adsorption plant, intentionally lower sodium cyanide levels in leach solutions to favour gold recovery, and insufficient leach times. The report states that realized silver recovery will ultimately not be in line with testwork expectations despite the addition of the Merrill-Crowe plant in November 2020. Reagent usage is reported as in line with metallurgical testwork expectations and within industry norms.

The report also notes that the land use change permit does not include an area to the southwest where the LOM plan proposes to expand the pit. The application for this permit was submitted in September 2024, and the report states the legal timeframes for review and approval can be extended by the Mexican environmental authority.

Source: San Agustin , 2025 Technical Report, January 2025, sections 1.15, 1.16, 1.17, 17.1, 17.3, 17.4, 17.5, and 17.4.3.