The Background Values and Distribution Characteristics of Stream Sediments in the Yangbajain-Qinglong Region, Tibet

The background values of chemical elements in stream sediments are an indispensable tool in geochemical prospecting work, and they are of important significance for regional geochemical anomalies assessment. However, there is a lack of poly-elements background values in Tibet, which are based on the surface material from the micro landscape area thereby limiting understanding of the existing stream sediment anomalies to a certain extent. Analysis of the chemical elements background values and distribution characteristics of 69 kinds of elements in 2800 stream sediment combination samples covering about 12000 km2 in Yangbajain-Qinglong area have been conducted and are reported in this paper. From north to south, the area is divided into three geochemical divisions of Bangor-Basu, Longer-Nanmulin and Lhasa-Zayu. The results indicate that content of SiO2 (68.6%-77.0%) in the study area is higher compared to the average value of stream sediments nationwide. The high content of SiO2 is related to strong weathering and occurrence of large-scale intermediate-acid intrusive rocks in the study area. While other elements are lower and differentiated in three geochemical divisions. Bangor-Basu division is characterized by enrichment of Li, Sr, B, N and Cl which are related to high-temperature hydrothermal or gas-liquid. Longer-Nanmulin division is enriched with iron group elements, rare element (Zr) and rare earth element (Sc). Lhasa-Zayu division is enriched with petrogenetic elements, light rare earth elements, rare elements, dispersed elements and chalcophile metallogenic elements. The differences in element content distribution are closely related with the stratum, rock mass, deposits and mineralization in the geochemical divisions. This study gives the background values for 69 kinds of elements in the Yangbajin-Qinglong area, such as the chalcophile metallogenic elements of Au, Ag, Cu, Pb and Zn with 0.26 ng/g, 41 ng/g, 4.04 μg/g, 25.2 μg/g and 25.6 μg/g, respectively. These data fill a vacancy in the chemical elements background values on the micro-landscape areas of Tibet, and provide references for regional exploration study and geological prospecting work in Yangbajain-Qinglong and adjacent areas.