花岗闪长岩风化层土壤及其盐水与酸不溶物元素变化行为研究:以热液成矿指示元素为例

Behavior of Major and Trace Elements in Soil and Its Insoluble Matter from Saline and Acid Solution Developed over Granodiorite in the Southwest of Beijing,China:A Case Study on Indicator Elements of Hydrothermal Mineralization

从土壤到水系沉积物风化过程中微量元素含量的变化行为对化探异常的圈定和解释具有重要参考价值。以北京西南房山花岗闪长岩体发育的土壤为研究对象,通过盐水及盐酸溶液去除土壤中的活动态微量元素,以模拟含盐水系及酸性水系,实验查明从土壤到水系沉积物风化过程中微量元素的变化行为。实验结果发现从土壤到盐水不溶物再到酸不溶物,花岗岩风化指数（WIG值）逐渐降低,表明WIG是表征其风化程度的良好地球化学指标。样品粒度越细其风化程度越高,热液成矿元素含量从粗粒级到细粒级含量逐渐升高,建议采样粒度为80-100目。

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 ele-ments during the weathering from soil to stream sediment have played an important role in delineating and inter-preting 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 se-quentially 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 weath-ering with finer particle size.Contents of most indicator elements of hydrothermal mineralization are also increas-ing with finer particle size.Samples with particle size of 80 -100 mesh （or 0.2 mm to 0.154 mm）are sugges-ted 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 minerali-zation 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.