Abstract

A key function of many quantitative mineral resource assessments is estimation of the number of undiscovered deposits. Numerous techniques can be used directly or as guidelines to make these estimates. Most robust of these methods is a form of mineral deposit model wherein for a deposit type, the numbers of deposits per unit area from well-explored regions are counted and the resulting frequency distribution is used either directly for an estimate or indirectly as a guideline in some other method. Ratios of the number of deposits per unit area can be used in histograms to show variation of deposit densities by type. In three-part quantitative resource assessments, prevention of bias requires that deposits of each type be defined the same way as in delineation of permissive tracts and as in construction of grade and tonnage models; these same definitions and rules must also be applied to deposit density models. This paper provides mineral deposit densities for 13 selected deposit types from previous publications and this study. Many of the specially selected areas reported here provide standards to identify what should be high estimates of the number of undiscovered deposits in most situations. Previously reported densities follow: 1. Low-sulfide gold quartz vein deposits in four regions have deposit densities ranging from 0.0043 to 0.0054 deposit per square kilometer. 2. Bedded barite deposits in Nevada have a deposit density of 0.074 deposit per square kilometer. 3. Diamond kimberlite pipe deposits in southern Africa have a deposit density of 0.000012 deposit per square kilometer. 4. Podiform chromite deposits in California have a deposit density of 0.115 deposit per square kilometer. 5. Franciscan volcanogenic manganese deposits in California and Japan have deposit densities of 0.019 and 0.03 deposit per square kilometer, respectively. 6. Cuban volcanogenic manganese deposits in Cuba and Fiji have deposit densities of 0.1 and 0.0066 deposit per square kilometer, respectively. 7. Cyprus volcanogenic manganese deposits in Cyprus have a deposit density of 0.046 deposit per square kilometer. Densities from this study follow: 1. Placer gold deposits in two locations in Alaska have deposit densities of 0.0054 and 0.0061 deposit per square kilometer. 2. Cyprus massive sulfide deposits in the Troodos ophiolite have a deposit density of 0.0082 deposit per square kilometer. 3. Kuroko massive sulfide deposits in five localities have deposit densities ranging from 0.0033 to 0.03 deposit per square kilometer. 4. Porphyry copper deposits in Nevada and Arizona have deposit densities of 0.00015 and 0.0012 deposit per square kilometer, respectively. 5. Climax porphyry molybdenum deposits in New Mexico and Colorado have deposit densities of 0.00031 and 0.00033, respectively. 6. Wolframite (tungsten) quartz vein deposits in Xihuashan, China, have a deposit density of 0.04 deposit per square kilometer. Frequency analysis of 39 podiform chromite deposit densities demonstrates a highly skewed distribution, suggesting that mean estimates can be misleading; however, probabilistic estimates are possible using log-transformed data and the normal distribution. The same data show deposit density varying with size of permissive area-a regression equation is required to provide unbiased estimates of deposit densities for all but median sized areas.

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