Published:January 01, 1995
The tonnage factor or average in-place bulk density of the orebody is one of the most important variables in the calculation of the weight of product to be expected from a proposed mining operation. Errors in the determination of the in-place dry bulk density upon which the tonnage calculations of a deposit are based may completely overshadow minor arithmetic errors in the calculation of average grades. Although a tonnage factor is required if the reserve report is to form the basis of a bankable document, many reserve studies fail to include any sort of data substantiating the figure chosen. Even if the figure used is simply an industry average for a given deposit type, this fact should be clearly specified.
The Buckhorn gold mine in Nevada is an example of a situation where an error in the specific gravity value used for the evaluation of the deposit resulted in a major change in profitability. The initial evaluation used a tonnage factor of 12–13 ft3 to the ton (equivalent to a bulk specific gravity of 2.5–2.7), which would correspond to a typical figure for solid rock. Three separate mining groups had reported this value, and it was not questioned (G. Cole, 1993, pers. commun.). Unfortunately, due to the high clay content, the correct tonnage factor for the Buckhorn ore is closer to 18, equivalent to a specific gravity of about 1.75. Even if the grade had been estimated correctly, the number of tons of ore within the fixed limits of
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).