Distribution of Uranium as Function of Sediment Particle Size

Uranium concentrations as a function of size were determined for 100 samples from the Estancia Valley, New Mexico, 100 samples from northeast of the Grants mineral belt, and 46 samples from the Black Hawk mining district, southwestern New Mexico. For the Estancia and Grants samples, statistical analysis shows uranium concentrations in stream sediments size fractions greater than 0.125 mm to be nearly independent of particle size whereas uranium concentration increases with decreasing particle size less than 0.125 mm. The standard deviations of uranium concentrations from both areas vary only slightly for all size fractions. For the Black Hawk mining district the uranium concentrations are strongly dependent on sediment particle size with uranium increasing rapidly with decreasing particle size. A correlation coefficient of 90% is calculated for increasing uranium content with surface area of sedimentary particles. In contrast, a much lower correlation coefficient for uranium-to-surface area is observed for the Estancia and Grants samples. We propose that the observed statistical differences are related to the fact that the stream sediments of the Black Hawk mining district represent control by mechanical disintegration of igneous and metamorphic rocks with little effect of chemical weathering, whereas the sediments of the Estancia Valley and Grants area are more chemically mature. The data imply that the nature (maturity) of the sediments sampled must be considered when sampling on a large grid, such as one sample/10 km², or else risk false anomalies (positive or negative) or masking any anomalies which may be present.