Mineralogy, Genesis and Diagenesis Environment of Alkaline Lacustrine Deposits in the Lower Permian Fengcheng Formation of Mahu Sag, northwestern Junggar Basin

Lower Permian high-quality lacustrine source rocks of the Fengcheng Formation in the Mahu Sag are considered to be the oldest alkaline lacustrine deposits. Abundant alkaline minerals were identified based on core observation, optical microscopy, scanning electron microscopy backscattering electron imaging and energy-dispersive spectrum analysis. According to the elementary composition, distribution and genesis, alkaline minerals are divided into four categories: (1): Sodium carbonate minerals: wegscheiderite, trona and nahcolite; (2): Sodium-rich carbonate minerals: shortite, northupite and eitelite; (3): Sodium silicate minerals: reedmergnerite and searlesite; (4): Characteristic minerals: catapleiite, elpidite, bonshtedtite, loughlinite, leucosphenite and janhnsite.

As the main mineral of sodium carbonate minerals, wegscheiderite precipitated from high concentration and alkalinity sodium carbonate-bicarbonate brines at least above 60 ℃, trona and nahcolite are produced by the reaction of wegscheiderite with water vapor in air. Thermodynamic simulation suggests that sodium-rich carbonate minerals northupite and eitelite precipitated diagenetically from the alkaline groundwater systems Na⁺, Ca²⁺, Mg²⁺/Cl^-, CO₃^2-//H₂O. The presence of reedmergnerite and searlesite affirms that the Fengcheng Formation is deposited in an alkaline environment and highly affected by hydrothermal fluids, and the characteristic minerals are associated with the sediments of weathering products of sodium-rich volcanic rocks, both of which indicate alkaline lacustrine deposits are strongly associated with post-orogenic volcanic activities. The occurrence of alkaline minerals indicates that high alkalinity saline pore fluids existed in the buried lacustrine sediments during the diagenesis stage, which suggests alkaline sodium carbonate-bicarbonate lakes or groundwaters existed in the Mahu Sag.