Graphical representation of production results versus estimates in placer mining

Abstract

Placer deposits are renowned for the difficulties inherent in the estimation of their mineral reserves. Mining and treatment often produce a recovered grade very different from that originally estimated. A measure traditionally used in the placer mining industry to report such technical performance is the ratio of recovered grade to estimated grade. The ratio, referred to as a factor, is determined and reported for a specified duration of continuous mining or for a mining block size. Over the life of mine numerous data pairs are generated for the estimated grade and the corresponding factor. The frequently inverse relationship between the two measures is expressed graphically on a scatter plot. Logarithmic scales are used for both the independent variable (estimated grade) and the dependent variable (factor). The factor may vary widely, and best-fit trend lines are derived by using either power or quadratic polynomial formulae. The relative estimated grade (each estimate divided by the average of all estimates of grade within the mined extent of a placer) may replace estimated grade as the independent variable. The graphical representation allows performance to be compared and contrasted for different mineral placers and mining methods employing diverse units of grade measurement.

Model trend lines result from the direction of mining in the longitudinal and transverse dimensions of the placer. The mining site, whether in the proximal or distal zone of the placer, also influences the form of the model. Other parameters, apart from estimated grade, affect the factor. They include sample support, sample spacing and method of mining. Sampling density and the duration of the reporting period determine the degree of slope and the curvature of the trend line. An important model results from attempting to exploit marginal grade outliers. Uses of the plot include monitoring the effects of changes in operational policy, mine planning and revenue forecasting. The average, historic factor achieved in a deposit, in places used as a correction factor, is not always a reliable indicator of future performance in the same placer. Care should be applied when using such a factor to substantiate reserve estimates. The simple graphical technique continues to be employed, and its application is not limited to placers.

Keywords

Placer Estimated grade Recovered grade Recovery factor Recovery factor plot Regression Trend line

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