Goldgroup's Cerro Prieto mine in northwestern Mexico has produced over 120,000 oz of gold since 2013 from a three-stage crushing plant, a lined heap leach pad and an on-site ADR plant and refinery.
Cerro Prieto is an operating open pit gold mine in the municipality of Cucurpe, Sonora, about 50 km southeast of Magdalena de Kino and 160 km south of Tucson, Arizona. Minas de Oroco Resources, a wholly owned subsidiary of Goldgroup Mining Inc., holds 100% of the 6,980 hectares of concessions and runs the mine. It has produced since 2013 and had made more than 120,000 oz of gold in 11 years to March 31, 2024. Output was about 11,500 oz a year. The technical report, effective 4 April 2025, updates the mineral resource for the Esperanzas deposit inside the mine, along with metallurgy and costs from the current operation.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Target crushing rate | 216,000 | tons per month | Three-stage plant |
| Nominal crushing rate | 250 | tph | Equivalent to 67% availability |
| Pregnant solution to ADR | About 263 | m3/h | 61% daily utilization |
| Carbon adsorption capacity | 15 | t | Activated carbon |
| Atmospheric desorption | 2.5 | t per 30 hours | Carbon |
| Pressure desorption | 1.0 | t per 12 hours | Carbon |
| Carbon regeneration | 1.5 | t per day | 50% utilization |
| Lift height | 6 | m | Radial stacker |
| Solution application rate | 8 | L/h/m2 | Drip irrigation |
| Barren solution cyanide | 350 | ppm | pH adjusted to 10.4 |
| Gold recovery for cut-off | 72 | % | Resource estimate assumption |
| Site water use | About 1,000 | m3/day | Process and mine |
| Annual gold production | About 11,500 | oz | At the report date |
Overview
Run-of-mine ore is crushed in three stages and limed. An overland conveyor and portable conveyors carry it to the pad, where a radial stacker places it in 6 m lifts. Drip lines apply dilute cyanide solution. Pregnant solution drains over the geotextile at the pad base and goes through ponds to the adsorption-desorption-recovery (ADR) plant. There, gold and silver load onto activated carbon in up-flow columns. The carbon is stripped, the solution is electrowon, and the sludge is smelted to doré on site. Metalor USA Refining Corporation in North Attleborough, Massachusetts refines the doré.
Key Process Stages
Crushing
Haul trucks feed the primary jaw crusher directly when possible. Otherwise they stockpile next to it, and a front-end loader feeds the crusher when mining is not under way. The jaw product goes to a secondary cone crusher in closed circuit with a screen and then to a tertiary cone crusher, also in closed circuit. The plant targets 216,000 tons a month at a nominal 250 tph. An automatic sampler takes a sample of crushed ore every hour for the metallurgical staff.
Conveying and stacking
Hydrated lime is added to the crushed ore at a rate set by a valve linked to a weightometer. The overland conveyor discharges through a tripper to portable ramp conveyors. On top of the pad, a second tripper feeds grasshopper conveyors, a horizontal feed system and the radial stacker. The stacker builds 6 m lifts about 90 m wide, working in retreat. A dozer rips the surface before stacking, smooths the stacked ore and lays the drip tube.
Heap leach pad and solution management
The pad has been built in stages. Its liner is a compacted soil layer under a linear low-density polyethylene geomembrane, with a textured liner at the toe for stability. Perforated pipes collect pregnant solution and feed a lined channel to either the pregnant pond or an operating pond that recycles solution to the pad.
Solution goes on at 8 L/h/m2 to the top and sides. Historical flows are about 5,500 m3/day of pregnant solution and 6,000 m3/day of barren. Barren solution, the weakest, is best used for secondary leaching of ore already under leach. Intermediate solution from secondary cycles is best used on fresh ore. Eight-inch HDPE headers feed 2 inch Aquamine sub-headers tapped for drip tube. Welded steel fittings link the two without proprietary parts. A standard leach panel is about 37 m by 85 m.
ADR plant
Pregnant solution first passes a mesh filter and then flows upward through a series of fluidized bed carbon columns. The first column carries the most gold and silver, and each later one less. After a second filter, the solution goes to the barren pond, where cyanide is brought to 350 ppm and pH to 10.4 before it returns to the heap.
Loaded carbon is acid washed and stripped in an atmospheric circuit and a pressure circuit. The plant regenerates up to 1.5 t of carbon a day. The strong eluate goes to electrowinning. The cathode sludge is filtered, dried, fluxed with silica, borax, fluorspar and nitre, and melted in a crucible furnace. Doré is poured, weighed, stamped and stored for shipment, and the slag is kept for further treatment. A 625 kVA diesel generator keeps the ADR plant running through power cuts.
Additional Interesting Data and Summary
The ADR plant ran well below capacity. The carbon columns treat about 160 m3/h of solution at 61% daily utilization. Electrowinning and stripping handle 12.5 m3/h at about 58%.
Recovered gold ranged from 11,410 oz in 2020 to 15,776 oz in 2018 between 2017 and 2023. Overall gold recovery in the ADR plant ran from 45.86% to 69.45% over the same period, and silver recovery stayed below 5%. Cyanide use ranged from 0.441 to 0.787 kg/t, hydrated lime from 1.00 to 2.56 kg/t, and annual power use from about 3.2 to 4.3 million kWh.
Column tests on Esperanza and Nueva Esperanza composites led the report to recommend stage crushing to 80% passing 3/8 inch and holding barren cyanide at 600 ppm at pH 10.5 to 11.0 to raise recovery. The resource estimate assumes 72% gold recovery.
The crushing and ADR crews work 12 hour shifts on an eight days on, four days off rotation. The process staff includes 11 crushing operators, 5 heap leach operators and 4 refinery operators.
Key Processes
- Three-stage crushing with jaw and cone crushers in closed circuit
- Lime addition on the conveyor
- Overland conveying and radial stacking in 6 m lifts
- Drip irrigation with cyanide solution on a composite-lined pad
- Carbon adsorption in up-flow columns
- Atmospheric and pressure carbon stripping with kiln regeneration
- Electrowinning and doré smelting on site
Source: NI 43-101 Technical Report, Cerro Prieto Project, Heap Leach Project, Magdalena de Kino, State of Sonora, Mexico, 4 April 2025.
Project website: Cerro Prieto Gold Mine
