Multistage Dolomitization of Deeply Buried Dolomite in the Lower Cambrian Canglangpu Formation, Central and Northern Sichuan Basin

The dolomite from the Cambrian Canglangpu Formation in the Sichuan Basin has demonstrated substantial potential for natural gas exploration. However, a systematic understanding of the dolomite types and the formation history of this horizon is lacking. Therefore, in this study, we combined rock mineralogy with isotopic geochemistry (C, O and Sr), micro thermometry of fluid inclusions, and in situ REE analyses of core and field section samples to explore the properties and evolution of dolomite in the Cambrian Canglangpu Formation in the central-north Sichuan Basin. Six types dolomite are identified: silt crystalline dolomite (D1), fabric-retentive dolomite (D2), fabric-destructive dolomite (D3), fine crystalline dolomite (D4), medium to coarse crystalline dolomite cement (C1a and C1b) and saddle dolomite cement (C2). Among them, the earliest dolomite is found in tidal flats and led to the predominant formation of D1, which is characterized by a similar δ18O to Cambrian seawater composition. D2 and D3 are found in grain shoal facies and D4 represented reflux dolomitization during the shallow burial stage. Specifically, D4 with a dirty-core and clean-rim may have recrystallized from pore seawater with secondary dolomitization. D3 showed depleted δ13C values, which is the result of secondary dolomitization of light carbon brines that are derived from organic matter degradation in medium to deep buried environment. The fluid inclusion data demonstrates that the C1a and C1b dolomites formed in brine under a high temperature and salinity. The intrusion of sulfur- and metal-rich (Zn and Fe) mesothermal hydrothermal fluids led to C1b formation and sphalerite mineralization. Precipitation of the C2 in the macroscopic vugs was promoted by sulfate-rich reducing hydrothermal fluid, and thermal sulfate reduction led to δ13C depletion, an extremely low REE content, and a positive Eu anomaly.