A New Geochemical Perspective on Hydrochemical Evolution of The Tibetan Geothermal System

作者:Zhang, Y. F.*; Tan, H. B.; Zhang, W. J.; Huang, J. Z.; Zhang, Q.
来源:Geochemistry International, 2015, 53(12): 1090-1106.
DOI:10.1134/S0016702915120125

摘要

The uplift of the Tibetan Plateau has caused the development of many high-enthalpy geothermal fields that are rich in exploitable rare and dispersed resource elements. However, the mechanism of the unusual enrichment of these resource elements is still unclear. From a geochemical viewpoint, including major chemical compositions, some rare and dispersed resource elements and trace elements in geothermal water and some river samples from the northern Lhasa block (saline lake area), the southern Lhasa block (Gangdise volcanic belt) and Tethyan Himalaya in Tibet, this study provides new insights into the mechanism of the hydrochemical evolution of the Tibetan geothermal system. The Cl-type geothermal waters in the Gangdise volcanic belt and Tethyan sedimentary area show similar chemical characteristics that are apparently different from that of surface cold waters. The concentrations of Sb, Tl, As, K, Cs, Li, Rb, Ga, B, Cl, Th, Sc, Mn, V, and Ti in Cl-type geothermal waters are at least one order of magnitude higher than those in surface cold waters, but the concentrations of Ca, Mg, Ni, Cr, Zn, Fe, Co, Cu, Pb, and U in Cl-type geothermal waters are slightly higher or even lower than those in surface cold waters. Some rivers and streams in Tibet also show high concentrations of toxic elements. These rivers and streams are mainly polluted by geothermal spring discharge and are unsuitable for drinking. Some ions and elements (such as Ca, Mg, Ba, Sr, SO42- and Mn) in HCO3--type geothermal waters from sedimentary rocks are affected by the availability of soluble minerals such as calcite, dolomite and gypsum. However, the other dissolved elements in HCO3--type geothermal waters show the characteristic of mixing Cl-type geothermal waters with surface cold waters. The origin of deep fluids in Cl- and HCO3--type springs is related, and this origin probably involves the contribution of crustal partial melting rather than single rock leaching. Thus, deep circulating groundwater mixes with residual magmatic fluids and evolves into the unusual enrichment of geothermal mineral resources.