Projection of Data
Published:January 01, 1995
The Samples on which all ore reserve estimates are based are but a minute portion of the total deposit. Closely spaced drilling of the small Copper Flat porphyry copper deposit in New Mexico produced about 200 tons of core, yielding 3 tons of assay pulps (only a small portion of which was actually analyzed) with which to define 60 million tons of ore and roughly 150 million tons of waste (Dunn, 1992). The values obtained from these tiny portions of the deposit must necessarily be projected into the vast unknown volume surrounding the sample points.
In most discussions of reserve estimation, the various methods of projecting sample data focus on projections of grade or tenor. A proper reserve estimate, however, requires the estimation of numerous other parameters necessary to define the true worth of the deposit. Such parameters might include specific gravity, vein thickness, ore type, metallurgical recovery, moisture content, rock hardness or grindability, proportion of deleterious components, and geotechnical parameters (e.g., rock quality designation).
Obviously, any or all of these parameters may vary from place to place within the deposit, but with few exceptions, (e.g., vein thickness, mineralogy or ore type, and sometimes specific gravity), they are more often taken as overall averages rather than adequately evaluated in even the most elegant reserve estimates and reports.
In addition to the characteristics of the ore itself, it is sometimes necessary to estimate various values within the waste rock or material that must be left in place for environmental or
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).