The Copperstone gold project is estimated to contain 802,048t of gold reserves. Image courtesy of Alchemist-hp.
Kerr Mines is currently undertaking a two-phase resource expansion program to increase the resources and extend the mine life of the Copperstone project. Image courtesy of Parolan Harahap.
The underground Copperstone gold project will be accessed using two portals. Image courtesy of Parolan Harahap.

Copperstone gold project is a high-grade underground gold project being developed along the Walker Lane Belt in western Arizona, US. Kerr Mines is the owner and will operate the historic gold project.

Formerly an open-pit mine operated by Cyprus Minerals, Copperstone produced more than 500,000 ounces (oz) of gold from 1987 to 1993. American Bonanza acquired the project, constructed a 450tpd flotation mill in 2011, and commenced underground mining from a decline located at the bottom of the open-pit in 2012.

Kerr Mines completed the phase one resource expansion program at the underground mine in 2017 and commenced the phase two program in February 2019 with the aim of producing between 38,000oz and 40,000oz gold a year.

The phase two will extend the mine life to eight years as against the initially proposed 4.4 years.

Pre-feasibility study of the expansion project was completed in April 2018.

Copperstone gold project location, geology, and mineralization

Situated in the ‘Basin and Range’ province of La Paz County, the Copperstone project forms a part of the Walker Lane mineral belt. The project comprises tertiary age detachment faults and high angle normal faults, which form majority of the geology in the region.

Majority of gold mineralization at Copperstone is identified between the low-angle and moderate Copperstone fault.


The proven and probable ore reserves of the Copperstone gold mine as of 2018 were 884,100 tons (t) of gold, grading 0.198oz per ton (oz/t), while the contained gold was estimated at 175,100oz.

Mining and processing of ore at the Copperstone gold project

Mechanized cut-and-fill mining method is proposed for the underground mine, which will be accessed through two portals. The project is expected to produce 600 tons of ore per day (tpd), with 840tpd to be processed per mill working day.

The underground ore will be processed using whole ore leach method, at an extraction rate from 88% to 97%.

The ore will be crushed at the existing crushing circuit and conveyed to the ball mill, where it will be ground to P80 minus 200 mesh, in a closed-circuit arrangement. The cyclone overflow will be pumped to the new leach plant, where it will undergo cyanide leaching, carbon loading, stripping, and electro-winning to produce gold doré bars.

Infrastructure facilities

The project can be accessed through the Arizona Highway 95, a two-lane paved highway connecting the Parker and Quartzsite towns in Arizona.

Electrical power supply for the project will be provided by Arizona Public Service Company (APS). The existing 69kV power line at the site will be used, which has sufficient line capacity for the new operation.

Water will be sourced from either dewatering of the underground mine and/or from three existing patch living trust permitted wells located near the project.

Financing of the Copperstone gold project

Kerr Mines signed a letter of intent (LoI) with Pandion in July 2018, under which the latter agreed to provide financing of $27.6m in two tranches. Kerr Mines has also agreed to sell 95,000oz of gold to Pandion over a period of 40 months, at a discounted price.

Sprott Private Resource Lending, in November 2018, agreed to provide up to $25m of senior secured project financing for the Copperstone project development.

Contractors involved

Hardrock Consulting was chosen to prepare and submit the pre-feasibility report for the Copperstone gold mine, while Resource Development was engaged for conducting metallurgical testing of the deposit.

Kerr Mines engaged Godbe Drilling and American Drilling, in August 2017, to conduct core and reverse circulation (RC) drilling.

DMC Mining Services was engaged as the mine development contractor, while Major Drilling Group International was engaged for the phase two resource expansion program.