The Gunnison Copper Project is a copper mine owned by Excelsior Mining Arizona located in Cochise County in south-eastern Arizona, the US.
Excelsior Arizona is a wholly owned subsidiary of Excelsior Mining.
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All mineral interests of the mine were acquired by Excelsior from the James L. Sullivan Trust in January 2015.
The interests represent mineral rights to the North Star and South Star Copper deposits of the project.
The project is to be developed in three production stages.
Stage 1 has capacity of 25 million pounds per annum (mppa), Stage 2 will have 75mppa, and Stage 3 will have 125mppa capacity.
The construction of Stage 1 was completed in Q4 2020 and went into production by JCM SX-EW plant.
Stage 2 will start production in 4th year of the mine plan, while Stage 3 will commence in 7th year of the mine life.
The first copper cathode production was achieved at Gunnison in December 2020.
Location and Site details
Gunnison Mine is located approximately east of Tucson and 1.5 miles southeast of the Johnson Camp Mining District in Arizona, USA.
The project covers an area of approximately 3,869 hectares (9,560 acres).
Gunnison can be accessed by the I-10 freeway from Tucson and Benson to the west or Wilcox (east).
The project lies at elevations from 4,600 to 4,900 feet above mean sea level in the eastern Basin and Range physiographic province of southeastern Arizona.
Ownership History of Gunnison Copper Project
The North Star deposit was discovered in the 1960s.
There was no direct mining history of the North Star deposit before Excelsior’s involvement.
Several companies have explored the deposit since its discovery.
In the late 1960s, North Star was partially controlled by Cyprus and partly by other private companies.
A division of Superior Oil Company went into a joint venture with Cyprus and private companies for the northern half of the deposit in 1970. At the same time, North Star’s southern part was controlled by Quintana Minerals Corporation.
By the late 1970s, Superior relinquished its rights to North Star but Cyprus maintained its interest.
In 1977, Cyprus handed over the deposit back to the private owners.
No work was conducted from the early 1980s through 1992.
In 1983, Mr Sullivan optioned the deposit to Magma Copper Company. Magma relinquished the project in 1997 after the project was acquired by BHP-Billiton (BHP).
BHP then, relinquished its option in 1997.
Phelps Dodge Mining Company optioned the deposit and drilled several holes with Mr Sullivan. Phelps Dodge then returned the project to Mr Sullivan in 1998.
From 1999 to 2006, no work was done at the project. In May 2007, AzTech Minerals Inc. acquired an option for the project.
In June 2010, AzTech and Excelsior Arizona decided for merger which was completed in October 2010.
The Excelsior Arizona is the present owner of the project.
Geology, Mineralisation, and Mineral Reserves
The Gunnison Copper Project is situated on the eastern edge of the Little Dragoon Mountains.
The rocks in the mountains have ages ranging from 1.4 billion years old (Pinal Group Schists) to recent Holocene sediments.
Predominately, the southern part of the mountains consists of the tertiary Texas Canyon Quartz Monzonite.
The northern part consists of the Pinal Group schists and the Palaeozoic sediments with regional copper mineralisation.
The mineralisation consists of copper sulphide, formed in the proximal (higher metamorphic grade) skarn facies.
It lies within stratigraphic units like Abrigo and Martin Formations and structurally complex zones.
The sulphide mineralisation is of three types.
These are fracture coatings and vein fillings, distinct quartz-orthoclase-carbonate ± magnetite and chalcopyrite veins 0.2-10cm wide, and disseminations.
The veins include retrogressive haloes of actinolite, chlorite, and epidote.
The primary mineralisation occurs in the form of stringers and veinlets of bornite and chalcopyrite.
The copper-oxide mineralisation is hosted in calc-silicate skarns as fracture coatings and vein fillings in chrysocolla form.
The rest of oxide mineralisation occurs as replacement patches and disseminations.
The total mineral resource estimate (measured+ indicated) according to PFS is 911.6 million tons containing 5,315 million pounds of copper at a grading of 0.29%.
Mining and Ore processing at Gunnison
The extraction of copper from oxide mineralisation is proposed to be done by using ISR method.
The method is based on the fractured nature of the host rock, presence of water-saturated joints and fractures within the ore, copper mineralisation occurring along fracture surfaces, to operate near to Interstate 10, and to avoid challenges of open pit mining in alluvium area with approximately 300-800 feet thickness.
The method involves injection of a barren leach solution (lixiviant) with weak sulphuric acid into ore body by injection wells.
The acidified solution dissolves oxide copper minerals.
Surrounding injection wells, the recovery wells extract copper-bearing pregnant leach solution (PLS) and combine to form the feed solution for the SX-EW process.
The processing is to be done in the SX-EW facility.
The facility can recover copper from PLS at a feed grade of 1.63 gram per litre (gpl) to produce cathode-quality copper (99.99%) purity.
The depleted portions of the mineralised zone are rinsed by injecting non-acidic water.
The rising process (3 stages) separates and dilutes the PLS remaining in the formation.
The third stage of rising washes out the neutralised solution until all regulated components are below given concentrations.
Infrastructure and Power Supply
The infrastructure includes access roads, processing plant SX-EW, tank farm, and associated facilities like warehouses, buildings etc.
A water treatment plant and sulphur burning sulphuric acid plant are also a part of the project’s infrastructure.
The other facilities include ponds, impoundments, railroads, water supply and distribution, sanitary waste disposal, waste management, transportation, shipping, and communications.
The power to mine is to be supplied by an existing 69kV power line. It approaches project site along the eastern boundary of Section 31.
The power line is owned by the Sulfur Springs Valley Electric Cooperative Inc.
Contractors of the project
The PFS of the Gunnison Copper Project was prepared by M3 Engineering & Technology Corporation.
Schmueser and Associates was selected as the general contractor of construction, while Hydro Resources conducted drilling of production wellfield and compliance wells.
The market making service was provided by Venture Liquidity Providers Inc. for help in maintaining an orderly trading market for common shares of Excelsior.
Excelsior engaged Liolios Group to support investor relations activities.
IMC provided supported in preparing geology, mineral resource, and reserve of the PFS.
Haley & Aldrich helped in preparing hydrology, extraction methods, and production schedule of the PFS.
Leach prepared metallurgy and leaching recovery of the PFS.
