In-situ copper mining at the Santa Cruz deposit, Casa Grande, Arizona: a cooperative research project between the Santa Cruz Joint Venture and the United States Bureau of Mines.
The U.S. Bureau of Mines has started a production-scale, in situ mining field research project in cooperation with the Santa Cruz joint venture, which is owned 50 pct by Asarco Santa Cruz, Inc., a subsidiary of Asarco Incorporated, and 50 pct by Freeport Copper Company, a subsidiary of Freeport-McMoran Gold Company. This research is being conducted at the Santa Cruz Project, located 7 miles west of Casa Grande, Arizona. Santa Cruz is a large oxidized porphyry copper deposit, concealed beneath post-mineral alluvium at depths of 1,500 to 3,000 ft. The research objective is to evaluate in situ mining as a method for extracting copper from a previously unmined copper oxide deposit. Funding is provided through a cost-sharing program, with the U.S. Bureau of Mines contributing 75 pct and the Santa Cruz joint venture 25 pct over the life of the project. Important project milestones reached to date include site selection, well field orientation, and demonstration that significant amounts of water can be injected into the mineralized zone at pressures below the threshold for hydraulic fracturing. Current work will characterize the hydrologic regime and gather flow data from communication tests to support an application for an environmental permit to inject a dilute acid solution. Future steps will involve completion of a five-spot well pattern, construction of a small solvent extraction-electrowinning plant, operation of the well field for 12 to 18 months, and final technical and economic evaluation.