Predicting the True Reserve of a Steeply Dipping Deposit in a Multi-Deviation Angle Exploration Operation.

Borehole deviation is the most precise method of delineating ore bodies in hard rock exploration. In order to achieve a precise delineation and consequently a reliable reserve estimate, borehole trajectories should ideally intercept the ore bodies at 90°. However, because of rock mechanical properties and imperfectness in orientation of borehole trajectories and other factors, borehole trajectories hardly intercept the ore bodies at 90°. Consequently, the ore reserve estimated from borehole data are liable to differ from true values. This in-turn affects the value of the mineral projects and the entire investment profile. In this paper, we have studied the impact of borehole deviation on ore reserve for a range of deviation angles from 10°, 15°, 20°, 25°, 30°; using geometrical modelling. Most importantly, we have been able to develop a mathematical model relating the true reserve to the false reserve through coefficient of variation of the false reserves from their true values. This coefficient has been estimated for various ranges of deviation angles. We have also shown that this coefficient of variation depends on the angle of deviation only and not on ore body thickness or reserve. Consequently, they can be applied to estimate the true reserve from the false reserve for any deposit once the angles of deviation are known.

When a vertical hole is being drilled and it drifts away from vertical trajectory and becomes inclined, the hole may be said to have deviated from the vertical.

In many circumstances this gives a negative result. However the deviation of hole from the vertical due to varying mechanical properties of rocks and other factors is made use of in mineral exploration (Marjoribanks 1997). Indeed the drill hole is given some inclination so that it deviates to such an extent as to intercept the ore body at 90° and thus depict the true thickness of the ore
body. In most circumstances, the hole does not intercept the ore body at 90°. This is also a deviation since the exploratory borehole drifts away from the designed trajectory. This deviation negatively affects the exploration process as it gives a false impression of the ore body thickness and consequently reserve estimate. Dominy et’al (2004) emphasizes that deviation of exploratory holes from the designed trajectory is one of the factors causing uncertainties in reserve estimation which in turn affects the accuracy of mine feasibility. Arsentiev A.I (1972) opines that uncertainties about the reserve of a deposit can lead to underestimation or overestimation of the value of a mineral project depending whether the mine planner in a risk averter or a risk taker.

This research studies how true reserve can be predicted from false reserve caused by hole deviation.

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