The permit was awarded by the Arizona Department of Environmental Quality (ADEQ), and will be active after completion of an appeal period of 30 days.
Excelsior is currently working to obtain an underground injection control permit from the Environmental Protection Agency.
Excelsior Mining president and CEO Stephen Twyerould said: “With the receipt, earlier this year, of an amended aquifer protection permit for our Johnson Camp mine, we can now focus exclusively on the timely completion of the federal permit, which is the final operating permit required for production.”
Commercial production from the Gunnison project is an advanced staged in-situ recovery copper extraction project expected to start next year.
Covering 9,560 acres, the project is located 65 miles south-east of Tucson Arizona within the copper porphyry belt.
Based on a feasibility study conducted last year, the project has a net present value (NPV) of $1.17bn pre-tax and $807m post-tax, and internal rate of return (IRR) of 48% pre-tax and 40% post-tax at 7.5% discount rate.
Initially, the project is expected to produce 25 million pounds of copper cathode per annum, with further plans to expand production capacity in phases to 125 million pounds per annum.
The project has probable mineral reserves of 4.5 billion pounds contained within a measured and indicated resource of 4.9 billion pounds.
The final expansion stage of the project includes construction of an acid plant at full production.