Fluid inclusions in the salt minerals from the Permian Fengcheng Formation in the Urho region, Junggar Basin, Xinjiang

The Permian Fengcheng Formation as the important hydrocarbon exploration target in the Urho region, Junggar Basin, Xinjiang is built up of the genetically special deep-lake salt-bearing volcanic-sedimentary formations. The detailed study of characteristics and genesis of the fluid inclusions in the salt minerals from the Permian Fengcheng Formation is based on the techniques such as core observation, thin-section examination, scanning electron microscopy, X-ray diffraction analysis, microprobe analysis and microthermometry of the inclusions. The ore-forming fluids of the salt minerals are assigned to the H₂O-NaCl systems with the moderate to low temperature (51.2 to 125.7℃), moderate to high salinity (13.72% to 24.93% NaCl), moderate to high density (1.06 to 1.16 g/cm³), and high-alkaline thermal brines with CH₄ gases. The deep-lake salt-bearing volcanic-sedimentary formations are thought to be associated with the sublacustrine hydrothermal effusion in the Fengcheng Formation. It is speculated that the genetic mechanisms for the salt minerals are involved into the disequilibrium and inversion of the deep-seated high-alkaline thermal brines caused by the mixing of sinking lake water and mantle-derived magmatic water, extraction of soluble salts during the upward migration of the mixed hydrothermal brines, and the eruption of the hydrothermal brines from the sublacustrine vents and then mixing with the low-concentration lacustrine water at the bottom of the lakes, and the formation of the compositionally complicated salt minerals at last.