Origin and depositional paleoenvironment of Triassic polyhalite in the Jialingjiang Formation, Sichuan Basin.

The Sichuan Basin contains the most prolific marine potash salt deposits in China. Although a vast amount of polyhalite has been found in the Triassic Jialingjiang Formation, the production of soluble potash deposits has yet to begin. The origin and sedimentary environment of polyhalite has remained unclear, and this information could have a significant economic impact. The recent discovery of protogenetic polyhalite co-existing with halite crystals in cores from the early Triassic Jialingjiang Formation in eastern Sichuan Basin provides vital clues to its origin and depositional environment. Two types of polyhalite have been identified, through the examination of sedimentary textures and fabrics, mineral composition and contents, and major and trace elements from cores. The first type is protogenetic polyhalite, which co-exists and is commingled with halite crystals. The second type is secondary in origin with a radiating texture and was formed by the metasomatism of gypsum. Triassic polyhalite and halite was deposited during periods of freshwater and sylvite-saturated, evaporated seawater mixing. Further dilution of the freshwater resulted in gypsum precipitation among the halite crystals. After periods of desalination, increasing evaporation leads to the formation of highly concentrated and dense brines that are rich in potassium and magnesium. These heavy brines sink downward through intercrystalline pores and fractures, replacing gypsum with polyhalite through metasomatism. These sequential phenomena occur because the Triassic marine basins were relatively small and tectonically active. Although these shallow, small salt basins can have a fast concentration rate, they can be easily desalinated by the peripheral freshwater input in an overall arid but fluctuating arid and wet climate. The alternating periods of evaporation and desalination are responsible for the formation of soluble potassium salt deposits. Our proposed mechanisms provide useful information for understanding the distribution of polyhalite. These mechanisms can be used to guide future exploration and production of Triassic marine potash salts in the periphery of the Tethys Ocean. [ABSTRACT FROM AUTHOR]