From Penecontemporaneous Seawater to Deep Hydrothermal Fluids: Records of Multi-Stage Superimposed Fluid Evolution in Ediacaran Dolomites, Tarim Basin

The Ediacaran dolomites of the Tarim Basin constitute a strategic frontier in global ultra-deep hydrocarbon exploration, yet their complex diagenetic evolution and porosity preservation mechanisms remain pivotal challenges for predicting reservoir "sweet spots." To decipher this history, our study employs an integrated approach—combining detailed petrography with in-situ U-Pb geochronology, trace element analysis, and  in-situ C-O isotopic data-to reconstruct a high-precision, multi-stage diagenetic fluid sequence for the Qigebrak Formation dolomites.

This work not only clarifies the primary origin of the Ediacaran microbial dolomites (Md1) but also delineates six key diagenetic phases: four dolomite cement generations (Cd1-Cd4), one episode of hydrothermal saddle dolomite (Sd), and late-stage calcite veins (Cd5). The evolutionary trajectory is defined as follows: (1) Penecontemporaneous Stage (~583-538 Ma): The microbial matrix (Md1) yields U-Pb ages of 583–559 Ma, consistent with deposition. Its seawater-like REE signatures (high Y/Ho, LREE depletion) and C-O isotopes confirm penecontemporaneous dolomitization in an evaporative setting. Subsequent fibrous/bladed cements (Cd1, Cd2), dated to ~541–538 Ma, display high Mg and inherited seawater chemistry, marking the end of early marine cementation. (2) Shallow-to-Intermediate Burial Stage (~466–409 Ma): Cement Cd3 (~466 Ma) shows negative Ce anomalies and elevated BSI, reflecting mildly reducing modified seawater. A significant fluid shift is recorded by Cd4 (~409 Ma), which exhibits marked MREE enrichment ("bell-shaped" REE patterns) and sharply increased BSI, indicating influence from deep, reducing connate brines during the Late Caledonian to Hercynian. (3) Deep Burial and Tectonic-Hydrothermal Stage (~215 Ma): Saddle dolomite (Sd) is dated to ~215 Ma (Indosinian). Coupled with strong positive Eu anomalies and hydrothermal mineralogy, it unequivocally records tectonically driven, fault-focused hydrothermal fluid influx. Late calcite veins (Cd5) represent final fracture-fill during deep burial.

By establishing an absolute geochronological diagenetic framework, this study precisely pins the timings of fundamental fluid-property shifts. Our results demonstrate that the early rigid framework of penecontemporaneous dolomite (Md1) and marine cements (Cd1/Cd2) was essential for preserving primary porosity against deep burial compaction. In contrast, mid-to-late diagenetic fluids were governed by the basin's tectonic rhythm; the Indosinian hydrothermal event (Sd) underscores the critical role of deep-seated faults in superimposing reservoir modification. These findings deliver a temporally calibrated evolutionary model for ancient cratonic dolomites and provide seminal geological evidence to guide the prediction of ultra-deep hydrocarbon "sweet spots."