Estimates of ore reserves at major Japanese metal mines seem, at first glance, to be readily understandable to American engineers and geologists. Close scrutiny, however, generally reveals unexpected features at each mine.
For example, the Hitachi copper mine formerly segregated its developed ore reserve into positive and probable on the basis of the possibility of high-grade and low-grade production from individual stopes.
Tests were made by the writer at Hitachi on the accuracy of the original reserve estimates of a few ore blocks by comparing those figures with the final production data. The results suggesthat the average reserve estimate of positive ore may be accurate within 20 percent, plus or minus.
A ten-year supply of proved ore at the big mines is not unusual; hence Hitachi is not much concerned with the additional categories of probable and possible ore, unless the positive ore has become inaccessible because of caving of the workings. For the sake of comprehensiveness, however, the writer studied the assay maps and stope maps in collaboration with the mine management and estimated tonnages of probable and possible ore, qualified by lower reliability ratings. Exploration prospects also were described in actual figures, subject to a variation of plus-or-minus 75 percent.
