"There has not been a new copper mine permitted within the state of Arizona in over a decade; I congratulate the Excelsior management team for achieving this important milestone that keeps Excelsior Mining on schedule to become the next new copper producer in the United States," chairman Mark Morabito said.

The Environmental Protection Agency has issued a Class III underground injection control area permit which allows production of up to 125 million pounds per annum, Excelsior said.

President and CEO Stephen Twyerould said it was an important milestone to initiating low-cost, in-situ copper production "at one of the most environmentally friendly and economically compelling mine sites in the United States".

"We look forward to continuing our work with all regulatory agencies and stakeholders as we head into the construction phase this summer," he said.

A 2016 feasibility study put initial capex at US$46.9 million, with an after-tax NPV of $807 million and an IRR of 40%, using a $2.75/lb copper price and a 7.5% discount rate.

The project is expected to produce about 2.2 billion pounds of copper cathode over a 24-year mine life.

Excelsior raised about $30 million in a private placement that closed in January, which was backed by a Greenstone Resources affiliate, taking Greenstone's stake to 48.49% of Excelsior.

"This financing secures a significant amount of funding that allows us to continue advancing our Gunnison Copper Project towards production," Twyerould said at the time.

The company said in May it would need additional financing to commence construction at Gunnison.

The project's metrics are enhanced by the company's 2015, $8.4 million acquisition of the neighbouring Johnson Camp Mine which had ceased copper operations in 2010.

Excelsior's shares, which were trading at C86c 12 months ago, closed down 11c to $1.20 yesterday as the copper price weakened on US-China trade tensions.