GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 255-1
Presentation Time: 9:00 AM-6:30 PM


EMERMAN, Steven H., Malach Consulting, 785 N 200 W, Spanish Fork, UT 84660

This financial analysis of the Resolution Copper Mine proposed by Rio Tinto within the Tonto National Forest, Arizona, has focused on four aspects: electricity consumption, water consumption, land subsidence, and design criteria for tailings dams. The discovery of geothermal water while drilling the primary access shaft could result in additional electricity consumption of 24 MW solely for mine dewatering and refrigeration under the best-case scenario and 1650 MW under the worst-case scenario, corresponding to total electricity consumption of 260 MW and 1900 MW, or 3% and 22%, respectively, of the peak power capacity of the Salt River Project. Rio Tinto has estimated water consumption of 15,700 acre-feet per year from the Central Arizona Project. However, based on the grade and production rate, water consumption of 50,000 acre-feet per year would be more typical, and the export of water with the tailings alone would result in water consumption of 25,600 acre-feet per year. The proposal includes block caving, a type of underground mining that involves controlled cave-ins of overlying rock with land subsidence as a typical consequence. Rio Tinto has predicted that the maximum depth of the crater will be 984 feet, but that the subsidence zone will reach only 1500 feet from the culturally sensitive escarpment of Apache Leap. However, the West Boundary Fault, which connects the footprint of the ore body with Apache Leap, was mapped by Rio Tinto in the wrong location with an offset of 2000 feet. Based upon the uncertainty in the prediction of the maximum crater depth, the probability that the subsidence zone will reach Apache Leap is 5.3%. The five alternatives for the tailings storage facilities would all be designed to withstand the 5000-year earthquake, although a dam for which the failure would result in the loss of human life should be designed to withstand the Maximum Credible Earthquake. Although one of the alternatives includes the storage of filtered tailings, the tailings could travel at least 10,400 feet from the dam by slumping alone to impact Superior (population 2837) at a distance of 2500 feet. Failures of the other alternatives would impact Queen Valley (population 820) at a distance of 19,000 feet, Florence (population 26,074) at a distance of 10.3 miles, or Dripping Springs (population 235) at a distance of 17,000 feet.