Formation time and fluid source of calcite veins and geological significance: An example from the Triassic Qinglong Formation carbonate reservoirs in the Huangqiao area, Subei Basin

Objective Calcite veins are the products of tectonic diagenesis. Information about fracture opening and fluid activity was recorded by calcite veins. Methods Multiple approaches, consisting of thin section observation, cathodoluminescence, U-Pb isotope dating, and REE, C, O and Sr isotope analyses of calcite veins from the Triassic Qinglong carbonate reservoirs in the Huangqiao area, Subei Basin, are used to analyse the origin of vein-forming fluid. Results Four stages of calcite veins were identified in the Qinglong Formation and successively formed at (115.30±0.42), (97.03±0.43), (85.29±0.25), (45.5±19.0) Ma. In the first stage, the calcite veins were derived from deep hydrothermal fluids.And in the second stage, the calcite veins were derived from mixed fluids of atmospheric fresh water and seawater. Formation water and deep shell-source hydrothermal fluid mixed in the vein-forming fluid of the third-stage calcite veins. In the fourth stage of calcite veins formation, deep mantle-source hydrothermal fluid