Geological characteristics and implications for potassium prospecting of Triassic deep polyhalite, Guang'an structure, Sichuan Basin

The Guang'an Anticline is one of the important hidden potassium bearing structures in Sichuan Basin. Polyhalites occurs widespread in the Leikoupo and Jialingjiang Formations of lower-middle Triassic. There are few previous studies on implications of potassium prospecting of polyhalite, and whether polyhalite is a prospecting indicator of soluble sylvite is still a debated topic. In this work, based on field investigation and previous well review, combined with the data of logging and core testing of ZK1 well in Guang'an structure, the geological conditions and implications of potassium prospecting of polyhalite are studied, in order to provide reference for the next sylvite exploration and development in Guang’an area. Six mineralized polyhalite layers are intersected by ZK1, with a maximum thickness of 7.03 m and a total thickness of 11.21 m. Polyhalite is mainly massive, disseminated with a small amount of banded, clustered and star shaped, and the paragenous mineral is anhydrite. Under the microscope, the polyhalite occurs as plate, fibrous, anhedral granular, radial and crumby, etc. The potassium bearing sequence is interbedding of anhydrite and polyhalite with the highest grade of KCl 22.9% (average grade 14.57%), and the halite layer is not developed. The results suggest that the polyhalite in ZK1 is mainly derived of epigenetic metasomatism with little primary origin. In the areas close to the central facies of the salt lake, the depression with relatively thin anhydrite layer is key target of soluble sylvite.