"After only three months of acid injection, we have collected enough copper to start initial production; this will allow us to commission and evaluate SX-EW [solvent extraction-electrowinning] capabilities," president and CEO Stephen Twyerould said.

"Optimisation programmes are being rolled out to additional wells and we are confident that production will be expanded over the coming months."

Excelsior had started initial mining operations in January at Gunnison but the project was suspended in March due to the COVID-19 pandemic.

The company began preparing for a restart in August, to target an initial 25 million pounds of copper a year.

Excelsior said smaller-scale operations in response to COVID-19 considerations had focused on well optimisation and confirmed the company could establish operations which minimised health and safety risks regarding the virus.

Only one COVID-19 case had been reported at Gunnison and the company said successful contact tracing and isolation measures were implemented without any need to shut down operations again.

It said the optimisation programme had solved the issue of copper and other precipitates blocking wells and the company was aiming to ramp up to full, nameplate capacity through 2021.

"A variety of issues still need to be worked through, such as how to minimise the impact of carbon-dioxide on fluid flows and how best to maximise flow rates in general; however, we believe that all of these challenges are temporary and will not impact long-term copper production," Twyerould said.

The extraction project is permitted to produce up to 125Mlb of copper cathode a year.

A 2016 feasibility study projected an after-tax NPV7.5 of US$807 million and an IRR of 40% for the 24-year project, using a US$2.75/lb copper price.

Excelsior shares (TSX: MIN) have ranged from C42-$1.23 over the past year.

They closed up 15.9% to 80c to capitalise it at $191.7 million (US$147 million).