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Fluid-flow mapping at a copper leaching operation in Arizona

Zonge Engineering & Research Organization, Tucson, Arizona, U.S.

George Ring and Martin Rex

BHP Copper, San Manuel, Arizona, U.S.

Corresponding author: norm@zonge.com

The first 20% of the full text of this article appears below.

At the San Manuel copper mine in southeastern Arizona, recovery of copper from the oxidized portion of this porphyry mineral resource is being achieved through a large in situ leaching operation using weak sulfuric acid solution. In the past, this activity was coordinated with open pit and underground mining, but in today's economic climate only the in situ operation continues. The acid solution (20 grams/liter) is injected into wells (unpressurized at varying depths up to several hundred meters), usually at rates of only a few tens of gallons per minute. The copper-bearing pregnant leach solution (PLS) is recovered either in nearby recovery wells or in collection areas in the underground workings 350–500 m below the surface.

Because of the economic efficiency of this method, the in situ operation at San Manuel has expanded from two test wells in the mid-1980s to 900-plus wells covering more than 650 000 m² of the open-pit mine. Over the past 12 years, geophysical surveys have been useful in both planning and monitoring the expansion of the in situ field.

At this site, geophysical surveys served economic as well as environmental needs. Economically tracking and monitoring the leachate is important to ensure against the loss of valuable (copper-bearing) solution. In addition, if the fluid flow is understood, then the most efficient recovery system can be used (recovery well versus underground collection). Environmentally, injecting sulfuric acid into the subsurface doesn't conjure up an attractive image. The ability to provide government entities with geophysical results to support monitoring well information and hydrologic models has been important in meeting state government requirements with respect to protection of the local aquifer.

The geophysical method used was controlled-source audio-frequency magnetotellurics (CSAMT), a well-established resistivity tool. Briefly, this method is an electromagnetic, far-field (plane-wave) resistivity mapping tool that provides very good . . . [Full Text of this Article]