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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

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