Location in Space (Dilution)
Published:January 01, 1995
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
Figures & Tables
Ore Reserve Estimates in the Real World
In Simplest Terms, the work of the mineral industry may be defined as the search for and production of some naturally occurring mineral substance useful for some specific purpose, and as such, that substance is both a defining element of and the underlying basis for our existence as a species.
The fundamental requirement for any venture designed to extract a mineral substance from the ground is the presence of a reserve of that substance, and the estimation of the quantity and quality of the available reserve is the single most important step in the development of a mineral discovery. In fact, the basic purpose of ore reserve estimation is to provide the first step in the evaluation of a business opportunity:
… all financial calculations can be no more than the transposition of the ore reserve estimate into other terms.
(King et al., 1982, p. 65)
Prior to roughly 1970, there was little or no standardization of nomenclature regarding the various levels of reliability attached to estimates of mineral resources or reserves, and quite frequently the same term was applied to estimates of widely differing reliabilities, made for widely different purposes. It was generally accepted that the term “ore” should be restricted to material having at least a remote possibility of economic viability, and that the terms “proven” or “measured,” “probable” or “indicated,” and “possible” or “inferred” should be used to denote estimates based on progressively less reliable data. In 1980, based on modifications of a standardized system proposed in 1976, the U.S. Bureau of Mines (USBM) and U.S. Geological Survey (USGS) published a revised classification system (Fig. I-1), in which the term “resources” was applied to an overall “concentration of [a] naturally occurring solid, liquid or gaseous material in or on the earth’s crust in such form and amount that economic extraction from the commodity is currently or potentially feasible.” The term “reserves” was restricted to “that part of an identified resource that meets specified minimum physical and chemical criteria ...[and] may encompass those parts of the resources that have a reasonable potential for becoming economically available within planning horizons beyond those that assume proven technology and current economics” (USBM … USGS, 1980, p. 2).
In 1996, the United Nations proposed a three dimensional scheme for the classification of resources and reserves (Fig. I-2) that assigns a numerical degree of reliability to each of three axes, representing economic, feasibility, and geologic elements of the evaluation (United Nations, 1996; Kirk, 1998).