Cozamin mine is an underground copper-zinc-lead-silver mine located in the Zacatecas Mining District, Mexico. Fully owned by Capstone Gold, a subsidiary of Capstone Mining, the mine has been in operation since August 2006.

Minera Cozamin that held most of the concessions in the Mala Noche Vein (MNV) which hosts the Cozamin mine was established in 1982. The Cozamin mine was sold to Minera Argenta, a subsidiary of Bacis, in October 1996 and the mine was later closed due to low metal prices in 1998.

Capstone Gold acquired a 90% interest in Cozamin in December 2005 with Bacis retaining the remaining 10% interest which was converted to a 1.5% NSR royalty in June 2006. Commercial production commenced in August the same year.

Capstone filed a technical report with updated mineral reserves for the mine in October 2020 and entered into a definitive purchase agreement with Wheaton Precious Metals in December 2020.

Under the terms of the agreement, Wheaton will pay an upfront cash consideration of approximately £110m ($150m) for the purchase of 50% of the silver production at Cozamin, until 10 million ounces (Moz), after which the company will buy 33% of the total silver production over the remaining mine life.

Project location and geology

The copper and zinc-lead-silver mine is located approximately 3km north of the city of Zacatecas in the Morelos Municipality of Mexico. The Cozamin Mine encompasses 93 mining concessions covering a total area of approximately 4,260 hectares.

The Zacatecas Mining District is located at the Southern Sierra Madre Occidental Physiographic Province, close to the Mesa Central Physiographic Province in north-central Mexico. The project is based on an epithermal belt and mesothermal vein deposits containing silver, gold and base metals like copper, lead and zinc.

Mineralisation and reserves

The copper mineralization at the Cozamin mine is characterised by massive pyrrhotite-pyrite-chalcopyrite intermediate sulphidation deposits hosted in veins, and fracture-controlled systems of veinlets, while the Zinc sulphidation mineralisation either envelope, overprint or brecciate the intermediate copper sulphidation zones.

The proven and probable mineral reserves at the Cozamin mine were estimated to be approximately 10.18 million tonnes (Mt), grading 1.79% copper, 0.36% zinc, 0.05% lead, and 41g/t silver, as of April 2020.

Mining methods

The construction of a one-way ramp is currently underway to unlock over 50 million pounds of copper and 1.5 million ounces (Moz) of silver. The ore production rate is scheduled to ramp up to 1.35 million tonnes per annum (Mtpa) in Q2 2021.

The Cozamin Mine employs the longitudinal long hole open stoping (LHOS) underground mining methods supported by horizontal drifting.

The underground mine is accessed via two ramp declines. The 430m-long Guadulapana ramp decline is used solely for ore hoisting, while the smaller San Ernesto ramp is used primarily for light vehicle passage.

Mineral processing

The run-of-the-mine (ROM) ore undergoes crushing in a primary jaw crusher from where the ore is conveyed to the secondary standard head cone crusher followed by tertiary crushing in a Metso HP4 cone crusher.

The grinding circuit comprises two parallel, primary ball mills to reduce the size of approximately 80% of the ore material to 230 microns. The ball mills operate in closed circuit with 0.66 m diameter cyclones.

The finely ground ore material is transferred to the flotation cells to produce a concentrate slurry which reports directly to the copper cleaning circuit. The tailings are reported by gravity to the banks of rougher and scavenger flotation cells for additional copper recovery and the copper rougher concentrates are cleaned in a two-stage cleaning system.

The copper rougher flotation tailings are transferred to the zinc rougher flotation to produce zinc rougher concentrate which passes to a regrind circuit for additional zinc recovery and is transferred to a two-stage zinc concentrate cleaning circuit.

The copper, zinc and lead concentrates are thickened and filtered in a Larox pressure filter, disc filter, and a drum filter respectively to produce the final concentrate products.

Infrastructure facilities

The power supply for the Cozamin mining operations is supplied from the national power grid via an 115kV line, while the additional power and the backup power are available from onsite generators of 1MW capacity.

The fresh water supply is received from three primary sources that include the permitted wells, the groundwater from nearby underground mines, as well as the discharge water from a local municipal water treatment facility.

Contractors involved

Capstone engaged Sandvik Mining and Construction as the mining equipment contractor in August 2018. The contract is valid till July 2021.

Transportes Mineros del Cobre (TMC) was engaged as the primary contractor for the transport of concentrates.