CHEMICAL AND BORON ISOTOPE COMPOSITIONS OF NONMARINE BRINES FROM THE QAIDAM BASIN, QINGHAI, CHINA

The general chemistry and boron isotope composition were investigated in fresh waters and hypersaline brines from the Qaidam Basin, northern Qinghai-Xizang (Tibet) Plateau, China. This basin is a large, tectoni cally active system, isolated from the ocean and composed of thick ela stic and evaporite sediments. The modern playas are subject to intense evaporation and are characterized by hypersaline brines as well as po tash and berate evaporites. The chemical composition of the dissolved solutes in the modern brines and waters reveals three main sources: (1 ) inflow of hot springs enriched in sodium, sulphate and boron. Evapor ation of these waters leads to a high Na/Cl ratio (>1), a Na-Cl-SO4 br ine and an evaporite mineral assemblage of halite-mirabilite-borate (L akes Daqaidam and Xiaoqaidam); (2) inflow surface river waters which a re modified by preferential dissolution of halite and potassium and ma gnesium salts characterized by a Na-(Mg)-Cl solute type with low Na/Cl (<1), Br/Cl, Li/Cl and B/Cl ratios; and (3) Ca-chloridic subsurface b rines which are controlled by both salt dissolution and dolomitization processes. Evaporation and salt crystallization of mixtures of the la tter two types leads to a ''marine-like'' brine (e.g., a Na-Mg-Cl type , Na/Cl ratio <<1) and mineral assemblages similar to that predicted f or progressive evaporation of seawater (e.g., Qarhan playa: halite-syl vite-carnallite-bischofite). The delta(11)B-values of the input waters to the Qaidam Basin (range of -0.7 to +10.9 parts per thousand vs. NB S-951) and brines from salt lakes (+0.5 to +15.0 parts per thousand) a re similar to those of associated granitic rocks (delta(11)B = -2.3 to +3.7 parts per thousand; n = 3) and hence indicate the non-marine ori gin of these fluids. The highest delta(11)B-values are associated with fluids with low B/Li ratios, indicating selective removal of elementa l boron and B-10 by adsorption onto clay minerals. The magnitude of B- 11 enrichment due to adsorption is similar to 15-20 parts per thousand , and thus non-marine brines are well distinguished from marine-derive d brines (delta(11)B of +39 to +59 parts per thousand) preserving the large isotopic difference (similar to 40 parts per thousand) of their source waters. It is proposed to use this distinctive isotopic signatu re for tracing the origin of ancient evaporite environments.