Abstract:

Stream sediment is the main medium sampled in the Regional Geochemistry-National Reconnaissance (RGNR) project in China, and soil is the main source of stream sediments. Behaviors of major and trace elements during the weathering from soil to stream sediment have played an important role in delineating and interpreting the geochemical anomalies. The soil developed over granodiorite in the southwest of Beijing was selected and its insoluble matter from 0.5 mol/L NaCl solution and then from 0.1 mol/L HCl solution was removed sequentially to simulate the geochemical process from soil to stream sediment. According to the particle size of soil, seven size fractions were meshed from 5 mm to less than 0.154 mm for two soil samples, then its insoluble matter from each size fraction was extracted sequentially from saline to acid solution. The results indicate that the sample mass ratio of soil insoluble matter from saline and acid solution respectively to its soil are all less than 13% and their values of WIG (weathering index of granite) are decreasing clearly from soil to saline insoluble matter and then to acid insoluble matter. This shows WIG is a good geochemical index to describe the sample’s weathering degree during the process from soil to stream sediment (represented by the insoluble matter). Soil and its insoluble matter (or stream sediment) developed over granodiorite have more extensive degrees of weathering with finer particle size. Contents of most indicator elements of hydrothermal mineralization are also increasing with finer particle size. Samples with particle size of 80-100 mesh (or 0.2 mm to 0.154 mm) are suggested as the best medium sampled for the geochemical exploration in this region. With respect to the difference of element contents in samples with different particle size, contents of indicator elements of hydrothermal mineralization in each size fraction vary unclearly in soil and its insoluble matter. This provides an example to verify the feasibility of collecting different medium (soil or stream sediment) with same size fraction to recognize element anomalies in geochemical exploration.

Key words: granodiorite, acid insoluble matter, particle size, WIG