Company proposes to mine Arizona copper deposit using in-situ method

Vancouver-based Excelsior Mining (CVE:MIN) is proposing a different method of mining its North Star copper deposit in southern Arizona. Excelsior’s Gunnison Copper Project contains over 4 billion pounds of oxide copper, and Excelsior plans to mine it using the in-situ recovery (ISR) method.

In ISR, the ore is left in the ground rather than broken and treated. An acid solution is pumped underground using a series of injection and recovery wells. The solution moves through cracks and fissures in the orebody, thus dissolving the minerals, and the “pregnant” solution is then pumped back to surface, where it is recovered in a solvent extraction and electrowinning (SXEW) facility.

While ISR has been used as in the uranium mining industry for decades, it has not typically been applied to copper mining. For ISR to work with copper, the copper oxide needs to be readily dissolved by a weak acid solution; it needs to be highly fractured and broken up so that the solution can move through the rock underground; and the oxide orebody needs to be entirely below the water table.

Excelsior CEO and President Stephen Twyerould said all three conditions apply to the North Star deposit:

“The geology of Arizona is somewhat unique and many of these copper oxide orebodies that would have been at the surface once, have subsided down and been covered over by overburden or gravel, and now sit deep enough to be below the water table, and that means that you can do this insitu recovery.”

In Excelsior’s recently released preliminary economic assessment for the Gunnison project, capital costs are estimated at $324 million, including construction of an SXEW facility. That capex is roughly half of what the company would pay if it went with a traditional open-pit operation, says Twyerould. The same goes for the operating expenses.

“We don’t have to pay the truck driver and the fuel and the maintenance and the tires, to move waste that has got no copper in it.”

Skeptics of the technology may point to the risk of groundwater contamination, but Twyerould says the same technique is used to remediate contaminated sites anywhere in the world.

“We create a depression in the water table over where we’re mining, and that creates a low-pressure area. What that means is the surrounding water is moving inwards towards where we’re mining, and therefore none of the acid can move outwards against that flow, or up that pressure gradient.”

“The neat thing about our operation is the standard remediation practice is actually built into our mine design.”

The risk is further alleviated in the case of the Gunnison project because the minesite is in a remote area where the nearest town is “upstream” of the project and no water users are downstream, noted Twyerould.

Excelsior plans to do a prefeasibility study in the new year and possibly fast-track the project through a new round of capital raisings, Twyerould said.

If approved, the mine would produce 85.6 million pounds of copper per year. The company is targeting 2015 for commercial production and the minelife is estimated at 20 years.

Link to Excelsior’s video on in-situ recovery here

Photo of exploration at the Gunnison Copper Project is courtesy of Excelsior Mining Corp.