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The company has now completed its retrofit and wellfield upgrades work; officials said that will help it quickly and efficiently recommence mining once it is safe to do so.

"The board of directors has determined [it is] the most responsible decision in terms of prioritising employee health and safety," Excelsior said of the closure.

"During this period, Excelsior will continue to maintain the wellfield in accordance with all state and federal permit requirements, while ensuring that the wellfield and production facilities are ready for the inevitable re-start."

President and CEO Stephen Twyerould added that employee health and safety was a priority in its decision. It also confirmed it will be taking advantage of "the inherent flexibility of the in-situ mining method" and saving cash while it waits for the pandemic to abate.

The Gunnison copper project, located in Cochise County, had been targeting June quarter production. The operation was permitted for 125 million pounds per year of copper cathode production.