Reserve Calculations
While the estimate of resources can give us an idea of the quantities of mineralized material in the earth, this does not imply anything in particular about the practical question of mining the deposit. A reserve estimate, on the contrary, refines the estimate of resources by placing economic constraints on the grade and size of the material which is brought into the calculation.
The cutoff grade is the grade below which the rock is assumed to be non-economic to mine. This grade will change according to such factors as costs of mining, metallurgical recoveries, and possible credits from minerals which are associated that can be recovered as byproducts. Any parts of a mineralized zone that have grades which are lower than the cutoff are not counted into the estimate in a reserve calculation.
If a professional in mining is calculating a reserve estimate for a potential underground mine, it is essential to include only the mineralized areas that have enough width to do the mining. Small areas of waste along and between the sides of the mineralized areas will be included as dilutive material in the estimates.
A pit must be designed to show the limits of the mineralization, plus the waste rock that has to be removed if the estimate is being made for an open-pit mine. The ratio of stripping is the mass of waste rock that has to be removed to mine a unit mass of ore – for instance, if an ore body which is of about 20 million tonnes is mined from a pit with a total of 80 million tonnes of rock, the 60 million tonnes of waste give the pit a stripping ratio of 3 to 1.
It is very common nowadays to make a few reserve calculations, having each one based on a different cutoff grade and are showing different designs of pits or mines. The company can afterwards choose from a variety of different possibilities when a final feasibility study on the project is developed.
In the same manner as is used for resource estimates, reserve estimates are labelled to show how reliable they are – possible, probable and proven. The only reserves that are considered in a study of feasibility are the proven and probable reserves.
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