Genesis of Gypsum/Anhydrite in the World-Class Jinding Zn-Pb Deposit, SW China: Constraints from Field Mapping, Petrography, and S-O-Sr Isotope Geochemistry.

The world-class Jinding deposit in SW China has ~15 Mt of Zn and Pb metals combined, in an evaporite dome containing amounts of gypsum/anhydrite. These gypsum and anhydrite are mainly located in limestone breccias (Member I), gypsum-bearing complexes (Member III), and red mélange, with some occurring as veins in clast-free sandstone (Member IV) and as fractures/vugs of host rock. The gypsum/anhydrite and dome genesis remain equivocal. The gypsum in limestone breccias and in red mélange with flow texture contains numerous Late Triassic Sanhedong limestone fragments. The δ³⁴S (14.1%–17%), δ¹⁸O (9.7%–14.6%), and ⁸⁷Sr/⁸⁶Sr ratios (0.706913–0.708711) of these gypsum are close to the S-O-Sr isotopes of the Upper Triassic Sanhedong Formation anhydrite in the Lanping Basin (δ³⁴S = 15.2%–15.9%, δ¹⁸O = 10.9%–13.1%, ⁸⁷Sr/⁸⁶Sr = 0.707541–0.707967), and are inconsistent with the Paleocene Yunlong Formation gypsum in the Lanping Basin (⁸⁷Sr/⁸⁶Sr = 0.709406–0.709845), indicating that these gypsum were derived from the Upper Triassic Sanhedong Formation evaporite but not from the Paleocene Yunlong Formation, and formed as a result of evaporite diapirism. The δ³⁴S (14.3%–14.5%), δ¹⁸O (10.1%–10.3%), and ⁸⁷Sr/⁸⁶Sr ratios (0.709503–0.709725) of gypsum as gypsum–sand mixtures in gypsum-bearing complexes are similar to the ⁸⁷Sr/⁸⁶Sr ratios of gypsum in the Yunlong Formation of the Lanping Basin and Cenozoic basins in the northern part of the Himalayan–Tibetan orogen, suggesting that the material source of this gypsum was derived from the Yunlong Formation, and formed as a result of gypsum–sand diapirism. The gypsum veins in clast-free pillow-shaped mineralized sandstone and the gypsum in host rock fractures and vugs formed after the supergene minerals such as smithsonite. The δ³⁴S (−16.3%~−12.7%) and δ¹⁸O (−9.8%~−4.7%) of this gypsum indicate that the gypsum is of supergene origin with sulfate derived from the reoxidation of reduced sulfur. We confirmed that the Jinding dome is genetically related to diapir of the Late-Triassic Sanhedong Formation evaporite. Clast-free sandstone and gypsum-bearing complexes in the dome were produced by diapir of the Paleocene Yunlong Formation unconsolidated gypsum–sand mixtures. [ABSTRACT FROM AUTHOR]