Hydrochemistry Characteristics and Origin of New Brine Sandy Gravel inEarly Pleistocene of Heibei Concave in Qaidam Basin

Confined water in deep sandy gravel layer is newly-discovered large-scale pore brines in Heibei concave in the western Qaidam basin, which poses as an alternative area for potash development due to the following features: medium to strong water yield property, closeness to the surface with water level ranging from 8 to 24 m, high degree of mineralization, high KCl content fit for commercial development, and small salt buildup when mined. The statistical results show that concentration changes of TDS, Cl^-, Na⁺, Cs⁺, B₂O₃, Ca²⁺, Mg²⁺, Sr⁺, NO₃^-, and Rb⁺ in the brine are small with even distribution. On the contrary, concentration changes of SO₄^2- are big with uneven distribution; concentration changes of Br^-, I^-, Li⁺, K⁺ and uniform degree fall in the range in-between those of the two above-mentioned groups of elements; Na⁺, Cl^-, Ca²⁺ and Sr²⁺, TDS show normal negatively skewed peak distribution, K⁺, SO₄^2-, Li⁺ show non-normal positively skewed peak distribution. In component clustering analysis diagram, K⁺, SO₄^2-, Li⁺ fall into a sub-class, whereas Cl^-, TDS and Na⁺ fall into another sub-class The ion content change curve shows that content of Na⁺ and Cl^- share the same change trend with that of mineralization degree and B₂O₃ in the pore brine from east to west. Compositions of K⁺, SO₄^2-, Mg²⁺ are consistent, with the same change trend with that of Li⁺. Ratio of C_Na/C_Cl in pore brine is 0.85-0.96, and bromine chloride coefficient is 0.01-0.34, similar to those of salt karst filter brine. It finds two laws of salting, namely, magnesium sulfate and chloride in the metastable phase diagram (25 ℃) of the quaternary system of Na⁺, K⁺, Mg²⁺//Cl^--H₂O and quinary system of Na⁺, K⁺, Mg²⁺//Cl^-, SO₄^2--H₂O, which might have formed due to interaction of intercrystalline brine in the sedimentary formation and pore water in the sandy gravel.