Tectono-thermal evolution and fluid history recorded in multigeneration calcite veins of the early Paleozoic Wufeng-Longmaxi shales, South China

The Late Ordovician−early Silurian transition witnessed intense tectono-thermal activity that generated widespread fracture systems in sedimentary basins worldwide. Those fractures are not only structural features but also preserve critical records of fluid flow and tectonic evolution. Despite extensive research on the organic-rich shales of the Wufeng-Longmaxi Formation and their importance as a major unconventional reservoir, the timing of fracture development, the sources of fracture-filling fluids, the nature of fluid-rock interactions, and their linkage to tectono-thermal events remain poorly constrained. To address this gap, we investigated vein-filling fractures in the Wufeng-Longmaxi shales from a weakly deformed zone using a multipronged approach, including petrographic observation, fluid inclusion microthermometry, micro-laser Raman spectroscopy, and isotopic analysis. The results show a decrease in fracture vein fill along the vertical section, with four distinct fracture types (types I−IV) identified, including two subtypes within type III (type III1 and type III2). Most fractures are filled with calcite, followed by quartz, or a mixture thereof. Type I and type II fractures, which are nearly horizontal and filled with calcite and pyrite, respectively, represent early-stage mineralization associated with synsedimentary processes and initial basin subsidence. Nearly vertical type III1 fractures, filled with δ13C-depleted fibrous calcite and often solid bitumen, reflect organic fluid migration driven by tectonic compression and fluid overpressure during the Indosinian orogeny. Oblique type III2 fractures contain twin-crystal and δ18O-depleted calcite, along with occasional siliceous veins and mafic bands, suggesting hydrothermal fluid activity linked to volcanic processes. The dominance of CH4 vapor inclusions, with minor CO2, further supports a volcanic degassing origin. These features indicate that type III2 fractures formed within the Wufeng-Longmaxi shales during postdepositional tectonic uplift along the Yangtze margin associated with the Himalayan orogeny. Ongoing postdepositional tectonic uplift related to the Himalayan orogeny resulted in the development of low-angle type IV fractures, characterized by late-stage mineral precipitation. These findings offer refined constraints on the diagenetic evolution of the shales and provide new insights into fluid-rock interaction driven by multistage postdepositional tectono-thermal events from the Late Ordovician to the Cenozoic in South China.