Reseach On Element Geochemical Behaviors During Weathering Of Granodiorite In Zhou Koudian

Stream sediment is the main medium on sampling in the Regional Geochemistry-National Reconnaissance(RGNR) project in China, and soil is the main source of stream sediments. Element behaviors during the weathering from soil to stream sediment have played an important role in delineating and interpreting the geochemical anomalies.This paper takes granodiorite weathering profile in southwest of Zhoukoudian, Beijing as the research object, and reseachs the change behavior of the trace elements in the weathering process. The soil developed over granodiorite was selected and 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 saline and acid solution solution was extracted sequentially to simulate the geochemical process from soil to stream sediment. After analyzing, weathering index WIG is not only suitable for the quantitative description of weathering process of granite, but also for the quantitative description from soils to stream sediments developed over granodiorite, and is more sensitive and wide application scope than other weathering indexs. With the grain size from soil to stream sediment samples developed over granodiorite decreases(increasing of the sample particle level),the weathering index WIG gradually reduces and shows that the soil and stream sediment samples developed over granodiorite have the characteristics of the weathering degree enhancement along with grain size fine. In our country, natural landscape areas, climatic conditions, the physical and chemical conditions, the mechanism of rock weathering are great different, so in different regions based on stream sediment survey and soil survey of the best sampling size are not the same. On the basis of changing of the content of trace elements from coarse level to fine level, samples with particle size of 80~100 mesh are suggested as the best sampling medium for the geochemical exploration in this region. With respect to the difference of element contents in samples with different particle size, the 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 sampling medium(soil or stream sediment) with same size fraction to recognize element anomalies in geochemical exploration. Accroding to element behaviors of weathering profile and different particle size samples in simulation experiment, the empirical equation is suitable for the quantitative study of 27 trace elements in the process from the rocks to soil to the stream sediments, and may have an important potential application on determining geochemical backgrounds of trace elements.