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Abstract

Near-Surface deposits obviously lend themselves to open pit mining, while deep-seated ones must be mined by underground methods. In addition to the constraints imposed by location with reference to the ground surface, the size and shape of a deposit largely determine the mining layout, the stripping ratio, the amount of waste rock that must be treated together with the ore, and the percentage of the geologic resource that can be recovered. Although it would appear to be self-evident, errors in the size, shape, and specific location of the contacts of the deposit remain one of the principal sources of gross error in ore reserve estimation.

The position of the deposit with regard to the characteristics of the surrounding formations may limit the selection of applicable mining methods. Soft, friable wall rocks may limit the size of underground workings, or may dictate lowangle pit slopes. At the Lakeshore mine in Arizona, an extremely weak hanging wall overlying a relatively competent sulfide-bearing tactite flowed downward between the large fragments of broken tactite during block caving, significantly diluting the grade of the in situ reserve. At the Tenneco operation in Virginia City, Nevada, a pit slope excavated in fractured and altered footwall rock failed, thereby threatening the city water supply, and effectively curtailed the operation.

Especially where underground operations are a possibility, all drill holes should be surveyed—it is extremely costly to have to redevelop a mine if the initial workings do not coincide with the position of the orebody. Figure VII-1

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