Biogeochemical methods have been widely applied for uranium mineral exploration, but there have been few applications for low-grade uranium deposits. This paper describes a biogeochemical method of exploration for U in black shale areas. Sampling was performed in October 2010 and the following May along three survey lines at 15 sampling points 30 m apart. Daimyo oak (Quercus dentata) leaves and twigs, Japanese red pine (Pinus densiflora) twigs, and soils were collected at each sampling site, and the same suite of samples from five sites in a control area. Soil samples were analysed by INAA and four-acid digestion ICP-AES, and plant samples by ICP-MS after ashing to calculate values levelled to a dry-weight basis.
The range of U in soils is from 2.7 to 694 ppm (mean 91.8 ppm; median 34.5 ppm); in plant samples, the ranges in Quercus dentata leaves and twigs, and Pinus densiflora twigs are from 0.007 to 2.687 ppm (mean 0.160 ppm; median 0.063 ppm), from 0.002 to 0.53 ppm (mean 0.037 ppm; median 0.012 ppm), and from 0.002 to 0.2 ppm (mean 0.014 ppm; median 0.009 ppm), respectively.
In general, profiles of U concentrations in soil and plant samples coincide relatively well with the mineralized zone, although an inferior relationship at some sites may be due to down-slope displacement of soil. Quercus dentata twigs are considered to be useful as a biogeochemical medium to explore for low-grade U deposits in the study area.