The Cozamin mine in Mexico has two main mineralised vein systems, namely the Mala Noche Vein (MNV) and the Mala Noche Footwall Zone (MNFWZ). Image courtesy of Capstone Mining Corp.
The Cozamin mine lies in the Zacatecas Mining District, approximately 3km north of the city of Zacatecas. Image courtesy of Capstone Mining Corp.
Fully owned by Capstone Mining, the underground mine primarily produces copper. Image courtesy of I Made Pasek/Shutterstock.

The Cozamin mine in Zacatecas, Mexico, is an operating underground mine that primarily produces copper, with zinc, silver and lead as byproducts.

The mine area has been active since pre-Hispanic times and commercial operations at the Cozamin mine started in August 2006.

Capstone Mining, the owner of the Cozamin operation, completed a life-of-mine plan (LOMP) in January 2021, which forecasted the mining of ore until early 2031. A technical report on the mine for the updated LOMP was published in March 2021. The production capacity of the mine is being increased to 3,780tpd through a ramp-up, which is scheduled for completion in the first quarter of 2021.

Cozamin project location and geology

The Mexican mining operation is located in the municipality of Morelos, approximately 3km north of Zacatecas, Mexico. The Cozamin copper-zinc and silver mining property encompasses 93 mining concessions covering a total area of approximately 4,260ha. Capstone directly holds 45 of the mining concessions while the other 45 concessions are held by Mining Opco, the company’s wholly owned subsidiary.

The Cozamin mine lies in the Zacatecas Mining District which comprises a belt of epithermal and mesothermal vein deposits containing precious metals such as silver and gold and base metals such as copper, lead and zinc. The mining district features siliceous volcanic and sub-volcanic rocks underlain by sedimentary and meta-sedimentary rocks. The area is dominated by Triassic metamorphic rocks of the Zacatecas Formation overlain by volcanic rocks of the Upper Jurassic or Lower Cretaceous Chilitos Formation.

Cozamin mine mineralisation and reserves

Mineralisation at Cozamin is found in two main mineralised vein systems known as the Mala Noche Vein (MNV) and the Mala Noche Footwall Zone (MNFWZ). The MNV is an east-west striking zone, dipping at 60° to the north. The majority of the strike at MNFWZ occurs beneath the tailings pond with a strike length of more than 2.2km and an average dip of 54° to the north-east.

The proven and probable mineral reserves of the Cozamin mine were estimated at 14.1 million tonnes (Mt) grading 1.77% copper (Cu), 44g/t silver (Ag) 0.54% Zinc (Zn) and 0.21% lead (Pb), as of October 2020.

Mining operations at Cozamin mine

The current life of mine plan forecasts mining 14.2Mt of ore beginning from November 2020 to early 2031.

The underground mining operations at the Cozamin mine use the longitudinal long-hole open stoping (LHOS) method for bulk ore extraction, which is supported by horizontal drifting to develop the access for stoping. Filtration and paste plants are planned to be commissioned in January 2023 after which, approximately 72% of the reserves volume will be backfilled with paste.

The underground access is via two service and haulage ramps that form a connected, one-way haulage loop and a hoisting shaft.

The LHOS operations involve bottom-up extraction on retreat within stoping panels along or parallel to the strike of the vein. The vein’s hanging wall and footwall will form the sidewalls of the stope. Each stoping sub-level is backfilled before mining the sub-level above. Stoping continues for up to 72m along strike to a vertical rib pillar left in-situ and resumes post the rib pillar up to the central access point.

Cozamin mine mineral processing

The run-of-mine (ROM) ore undergoes crushing in a primary jaw crusher and a secondary standard head cone crusher. The screen oversize is conveyed to a tertiary Metso HP4 crusher and on to two ball mill circuits operating in parallel and the original ball mill circuit. The original ball mill was recommissioned to provide additional grinding capacity in 2013 and 2014, and again in 2018 to accommodate the increased mine throughput over the years.

The cycle overflow from the grinding circuit reports to a common flotation conditioning tank and is mixed with lime, zinc depressant and modifiers prior to flotation. The slurry from the grinding circuit undergoes initial copper flotation, which reports directly to the copper cleaning circuit. Tailings are transported to the rougher and scavenger flotation cells to enable further copper recovery.

The copper rougher concentrates are sent to a two-stage cleaning system while the rougher tailings report to the zinc rougher flotation for zinc mineralisation. The copper concentrate is thickened while the underflow is filtered in a Larox pressure filter to produce the final concentrate.

The zinc concentrate is thickened and filter pressed to a product moisture level of approximately 10%. The lead concentrate is thickened and pumped into a drum filter to produce a final product with a moisture content of 8%.

Offtake agreement

Capstone entered a streaming agreement with Wheaton Precious Metals in December 2020 for the offtake of 50% of the silver production from Cozamin until the delivery milestone of 10Moz is achieved, following which the stream will drop to 33% for the remaining mine life.

Cozamin mine infrastructure

The Cozamin mine is accessible via paved roads to the mine area boundary and then all-weather roads that provide access to the mine.

The power supply requirement is met from the national power grid with approval to draw 7.5MW. A permit to raise the approved power supply to 9.5MW is pending approval, which is anticipated by the end of Q1 2021. The mine is supported by on-site generators, with a capacity of 1MW.

The process and mine water supply is secured from various sources, including rainfall, permitted wells, a municipal water treatment plant, and groundwater inflow from abandoned mines in the region.