# Diagnosis of the Permian Fengcheng Formation in the Mahu Sag, Junggar Basin, China
## 5 Discussion
### Types of Alkaline Lake Diagenesis and Their Causes
#### 5.1.4. Formation of Reedmergnerite  
The appearance of reedmergnerite marks the middle-high temperature alkaline diagenesis of the Fengcheng Formation, and its formation is closely related to the hydrothermal fluid. The main evidence is as follows: (1) During the period of the Fengcheng Formation, different rare earth element distribution models showed that the fluids mainly came from endogenous fluids in different parts, including subduction zones, crustal, and mantle sources, and the boron required for reedmergnerite mainly came from deep alkaline hydrothermal fluids [56; 66]. (2) The measurement of boron isotope \(\delta^{11}\)B showed that the fluids of different forms of reedmergnerite in the Fengcheng Formation were from the same source, and the fluid may come from deep hydrothermal sources [56]. (3) Reedmergnerite is widely developed near the fault, and during the Fengcheng Formation, regarding the volcanic activity [67], boron-rich hydrothermal fluids affected by volcanic activity could migrate along developed fault networks. (4) Reedmergnerite was formed in a closed alkaline lake environment [41], and it was not easy for enthetic materials to enter the lake basin through relatively normal and gentle migration. The hydrothermal fluid in the deep is more capable of entering this closed environment under the external force of the huge energy [56], bringing the required substances. (5) The hydrothermal fluid can provide a sufficient reaction temperature, which can reach the experimental temperature required for the synthesis of reedmergnerite [46; 47], while the reedmergnerite in the Fengcheng Formation was developed in the Feng 2, with a burial depth greater than 3000 m [56], and in 2018, [PERSON] studied the thermal history recovery characteristics of the Junggar Basin by using paleo-temperature scale methods, such as vitritic reflectance and fission track [60], and found that the geothermal gradient of the Fengcheng Formation was 5-3 \({}^{\circ}\)C/100 m. The formation temperature of the reedmergnerite development layer was about 140 \({}^{\circ}\)C (the average geothermal gradient is 4 \({}^{\circ}\)C, and the annual average temperature is about 20 \({}^{\circ}\)C), which cannot reach the temperature threshold required for the formation of reedmergnerite, while the hydrothermal fluid becomes capable of providing the greatest possible chance of such a high temperature. According to the phenomenon actually observed in the thin slices, it was found that reedmergnerite had a metasomatism relationship with alkaline minerals, such as sodium-calcium carbonate (Figure 4f), trona (Figure 4g), and carbonate minerals (Figures 3h and 4a,b). Alkaline minerals and carbonate minerals have certain commonalities. Regarding chemical composition, alkaline minerals are carbonate minerals with different Na\({}^{+}\) contents. Because the composition of carbonate minerals and alkaline minerals is different from that of reedmergnerite, two processes of dissolution and recrystallization were involved in the metasomatism [57]. Different dissolution degrees, different crystal habits, and different order degrees during recrystallization may be important reasons for the formation of different forms of reedmergnerite [49]. At present, there are different opinions on the division of the formation period of reedmergnerite, including the buried diagenetic period and quasi-contemporaneousperiod [24; 39; 46; 56; 63]. Temperature and pressure are the key factors in the formation of reedmergnerite [63]. The burial was shallow in the quasi-contemporaneous period. The temperature and pressure were not up to the formation conditions of reedmergnerite, and the carbonate minerals and alkaline minerals metasomatized by reedmergnerite were mainly developed in the Feng 2 Member, with a burial depth greater than 3000 m [56]. Therefore, it was speculated that the reedmergnerite was mainly formed by the metasomatism of carbonate minerals and alkaline minerals formed earlier under the action of an abnormal hydrothermal factor during burial diagenesis.