Abstract: The “two-storey” potash deposit model has led to significant progress in the exploration of Jurassic potassium resources in the Mengyejing area, Jiangcheng salt belt, Simao Basin. However, studies of other salt belts in the basin are lacking. In this paper, salt rock samples from the Lower Cretaceous Mengyejing Formation (K₁m) in well L2 in the Mohei area, Zhengdong salt belt were analyzed through microscopic observation, scanning electron microscope and energy dispersive X-ray spectroscopic analysis (SEM-EDS), as well as in situ elemental analysis of rock salt by electron probe microanalysis (EPMA) to study their petrography, mineralogy and elemental content in detail. Meanwhile, the rock salt samples from the Middle Jurassic Huakaizuo Formation (J₂h) in well MK-1 in the Mengyejing area, Jiangcheng salt belt were also studied. The K and Br trace elemental contents and 10³Br/Cl (mass fraction) ratios in these samples were compared between the two wells in the context of geological evolution of the study area. In the K₁m formation in well L2, there are two types of rock salt in the mud-gravel salt rocks. The main type is cement-like detrital halite particles with plastic rheological characteristics, the other is clastic halite precipitate wrapped in detritus of salt or clay minerals. The characteristics of both rock salt types indicate secondary origins. The EPMA results show that the maximum K contents were 0.09% in the main and 0.18% in the inclusion types of rock salt from the K₁m formation in well L2 and 0.13% in the potassium-bearing rock salt in the J₂h formation in well MK-1; while the Br contents were 60×10^-6 at most, (70-410)×10^-6 and (70-500)×10^-6, respectively, and the 10³Br/Cl (mass fraction) ratios were 0.10 at most, 0.12-0.71 and 0.12-0.85, respectively, in the three types of rock salt. The main type, with significantly lower mineral contents compared to the other two types, corresponds to terrestrial source, sea-continent mixed source or recrystallized halite. The other two types, similar in mineral contents, correspond mostly to marine halite, with minor potassium and carnallite crystallized from seawater. It is inferred that the detritus-wrapped halite is an evidence of salt diapir associated with ancient saline sediment. A metallogenic model is proposed on the basis of the above analyses as follows: After Middle Jurassic deep marine sediment migrated to the K₁m formation via salt diapirism, one part—modified via leaching, melting and mixing by a combination of Middle Jurassic residual seawater migrated from the inner basin, continental source water flew from the basin margin and deep hydrothermal fluid—formed new brine, while the other part remained as ancient salt gravel. In the late Early Cretaceous during evaporation of the new brine, ancient salt gravel became enwrapped by newly crystallized salt and terrigenous clastic minerals. In the later period, cement-like halite deposit was formed via brine evaporation. This metallogenic model epitomizes the application of the “two-storey” metallogenic model in the Zhengdong salt belt.

Key words: halite, EPMA, in situ elemental content, material source, metallogenic model, Zhengdong salt belt, Simao Basin