The Gunnison copper project is a copper mine located in Arizona, southwestern US. It achieved the first copper cathode production in December 2020.
The project is 100% owned by Excelsior through its wholly-owned subsidiaries Excelsior Arizona and Excelsior Mining JCM. Construction for the project began in December 2018, following the closure of a $75m financing package provided by Triple Flag.
The project is expected to produce 35.2 billion ounces of pure copper cathode during its mine life of 24 years.
Gunnison copper project location
The Gunnison copper project is located on 3,869ha of land, approximately 99km from the city of Tucson, southeast Arizona. It is situated on the Little Dragoon Mountains in the Cochise Mining District of Cochise County.
The property is located 2.4km from Johnson Camp mining district and is accessible via the US Interstate 10 (I-10) from Tucson, Benson, and Willcox.
Gunnison project geology and mineralisation
The copper project is situated within Arizona’s copper porphyry belt and contains copper oxide and sulphide mineralisation mixed with molybdenum.
The north star deposit represents the material deposit within the project area. It is situated in the Mexican highland part of the Basin and Range physiographic province. The province is defined by fault-bounded mountains with large igneous intrusives at the cores and separated by deep basins covered by quaternary and tertiary gravels.
The rocks within the Little Dragoon Mountains range from pinal group schists, aged 1.4 billion years, to recent holocene sediments. The southern part of the mountains mainly comprises tertiary Texas canyon quartz monzonite, while the northern part is dominated by pinal group schists and paleozoic sediments.
The copper sulphide mineralisation occurs in the neighbouring skarn facies, especially along the stratigraphic units such as the Martin and Abrigo formations. Primary mineralisation exists in the form of veinlets and stringers of bornite and chalcopyrite.
Gunnison copper project reserves
Gunnison copper project is estimated to contain 782 million short tonnes of proven and probable reserves graded at 0.29% of copper containing 72 billion ounces of copper.
Gunnison copper project is being developed in three stages to minimise the capital at risk. It includes six major ponds for pregnant leach solution (PLS), evaporation, raffinate, utility and drain-down purposes. While some of the ponds are retrofits of existing ponds, others are newly constructed ones.
The PLS ponds along with the raffinate ponds enable a closed-loop system for the circulation of mining fluids between the neighbouring Johnson camp mine (JCM) production facilities and the wellfield. The JCM will be used for production at a rated capacity of 11,339t a year.
A new utility pond is being developed to accept the raffinate or PLS solutions to provide operational flexibility. A newly constructed pipeline drain-down pond can accommodate any type of solution in case of maintenance work on the pipeline.
The annual copper production will range from 192 million ounces (Moz) to 880Moz during the first four years. Production will reach 1,200Moz in the subsequent two years, before achieving a rate of 2,000Moz in the seventh year. Production will drop to below 1,600Moz in year 20 and decline to 140Moz in year 24.
Mining and processing at the copper mine
The project employs in-situ recovery (ISR) method to recover copper from the North Star deposit. It includes the injection of a low pH raffinate solution into the ore body through multiple injection wells. The lixiviant travels through the joints and fractures of the mineralised bedrock to dissolve the copper. The resulting PLS is recovered using a series of recovery wells enclosing the respective injection well.
The PLS is then be pumped to the surface to strip the copper from the solution by the solvent extraction/electrowinning (SX-EW) process, which includes the extraction and concentration of the dissolved copper from the PLS by the SX, followed by the reduction of the concentrated copper to copper cathode by the EW plant. The copper is then recovered by SX and the barren solution undergoes re-acidification with sulphuric acid to form a new lixiviant that is sent back to the well field and re-injected. ISR is performed as a continuous mining operation by recycling the solution to the wellfield to continue the leaching cycle.
Infrastructure at Gunnison copper project
The project utilises the existing plant facilities, ponds and infrastructure at the Johnson camp mine. Future works will include the construction of the Gunnison SX-EW plant and the Gunnison tank farm.
The site receives power from a 69kV power line owned by the Sulfur Springs Valley Electric Cooperative.
Water from the existing wells and mine shafts on JCM is pumped to a water storage tank at the JCM site. The project site also includes a potable water tank and chlorination system.
M3 was contracted to prepare the feasibility study for the project. Schmueser and Associates was selected to serve as the general contractor.
Hydro Resources performed drilling works required for the project.