Accuracy of ore-reserve estimates for uranium-vanadium deposits on the Colorado Plateau

Abstract

The U. S. Geological Survey has made estimates of the reserves of uranium and vanadium in the carnotite deposits explored by Geological Survey drilling on the Colorado Plateau. This report presents an appraisal of the of the reserve estimates for carnotite deposits in the Uravan mineral belt, the causes of inaccuracy, and the significance of the estimates in terms of the total known reserves of the region. The appraisal was confined to carnotite deposits that contain ore reserves of 1,000 short tons or more. These reserves are classed as indicated and inferred reserves. Reestimates of the amount of material in the ground at the time of the original estimate were made for more than 30 deposits. This was done where information on size and grade, additional to that used at the time of the original estimate, was available. The reestimated reserves (revised reserves) include production since the original reserve estimates, plus the estimate of reserves remaining in the ground, and, therefore, can be compared with the original estimates, to furnish a measure of the of the original estimates. The reappraisal indicates that for individual deposits the original reserve estimates range from large overestimates to very large underestimates (for both tonnage and grade). However, the original reserve estimate for the total reserves of all the carnotite deposits studied in the Uravan mineral belt is a very small underestimate. For the carnotite deposits studied in the mineral belt, the revised tonnage of ore reserves is about 15 percent greater than originally estimated. In terms of grade, the original and revised estimates are the same for UjOa and differ by less than 2 pounds per ton for V2 Os. Primarily as a function of the increased tonnage of reserves, the amount of contained UaOg is about 15 percent greater, and the amount of contained V2 Os is about 20 percent greater than originally estimated. All of the individual deposits contained more than half the amount of ore originally estimated. A little more than 25 percent of the deposits contain over twice as much ore as originally estimated. The errors in estimation for individual deposits are mutually compensating; as more deposits are grouped into a single estimate, the amount of error decreases both in tonnage and grade. 131 132 CONTRIBUTIONS TO THE GEOLOGY OF URANIUM The general source of error in the original estimates is the restricted number of points of observation (mostly drill holes) on which the estimates are based. This is complicated by the deviation of the ore deposits from an idealized uniformly tapering layer both in plan and in section, and by the erratic distribution of metal values within the deposit. No mathematical correction can be applied to the reserve estimate to compensate for this sampling, error. As the deposits studied in the mineral belt represent a fair sample of all the uranium-vanadium deposits that the Geological Survey is finding in the Morrison formation, the realization ratios (the accuracy of prediction figures) given above can be applied with reasonable confidence, to the total of the estimates of known reserves for all of the deposits found by the Geological Survey. This total estimate is one of the major objectives .in the exploration work. As the error in the original estimate of total reserves for the deposits studied in the mineral belt is small, the methods of exploration and estimation being used are adequate to achieve a realistic appraisal of the total uranium-vanadium reserves of the region. '