Mine Grade Control
Published:January 01, 1995
Several standard texts discuss the role of geology at an operating mine (e.g., McKinstry, 1953; Peters, 1978), and the role of geology within the organizational structure of a mining company. However defined, and whatever the reporting responsibilities, most geologic work at an operating mine usually falls under the general category of “grade control,” and serves as a central source of predictive information to other areas of the operation.
At most mines, the grade control program has three principal goals:
To forecast the grade or physical characteristics of the material to be mined during the next month or year. This is a cooperative effort involving all of the mine staff, and is the first step in preparing the financial projection for the period in question. This function is obviously simply an extension of the estimates prepared in the feasibility stage of the project.
To ensure that the material mined as “ore” is acceptable to the mill and purchaser, and that it is separated during mining from nonprofitable material to the extent possible. Again, this effort requires close cooperation between geological, engineering and operations personnel, and often involves trade-offs from the ideal for any or all of the parties involved.
To supply the operating staff with reasonably accurate outlines of the orebody, so that development work can be laid out in such a way as to make available the maximum amount of ore at the least cost.
Note that the role of the geological staff at an operating mine
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).