Temporal constraints on hydrothermal circulation associated with strike-slip faulting in the Permian Maokou carbonates, central Sichuan Basin (SW China)

Abstract

Despite the ubiquity of hydrothermal fluid circulation in the crust, its timing is difficult to precisely determine using traditional thermometric dating due to strongly disturbed geothermal fields. We present a case study from the mid-Permian hydrocarbon-bearing dolostone reservoirs, situated proximally to strike-slip faults in the central Sichuan Basin (China), to highlight the utility of carbonate in-situ U-Pb geochronology in delineating the timing of hydrothermal fluid flow. Matrix and void-filling cement dolomite phases were identified in the porous Maokou Formation carbonates. Predominantly manifesting as saddle dolomites along fractures, these cements yielded statistically homogeneous U-Pb ages (242.5 ± 3.1 Ma, 238.9 ± 8.2 Ma, 244 ± 15 Ma, and 239.2 ± 6.5 Ma), slightly postdating the established stratigraphic timeline (ca. 273 to 259 Ma). The narrow temporal span suggests early emplacement following a short burial, compatible with the petrographic observation of burial stylolites crosscutting saddle dolomites. Integrating shallow emplacement depths inferred from a comparison between the dolomite U-Pb ages and a published burial history, with fluid-inclusion microthermometry, elucidates the hydrothermal system that was once present in this field. The pervasive occurrence of hydrofracturing, along with dolomite geochemical indicators represented by bell-shaped (REE + Y) PAAS patterns and elevated Y/Ho ratios, provide additional evidence for the hydrothermal fluid injection and resulting dolomitization. The dolomites' positive Eu anomalies and the parent fluids’ 18O enrichment indicate a source from deeper basinal pore waters that have interacted significantly with ambient rocks. The overlying Triassic evaporites may have provided Mg-rich brines, mixing into the circulating hot fluids responsible for hydrothermal dolomitization. Our dolomite U-Pb ages coincide with the seismically estimated timing of transtensional strike-slip fault activation. This temporal correspondence, combined with spatial associations and lateral slickensides along fracture planes, suggests that the hydrothermal fluid movement is highly relevant to the active strike-slip faults. This observation reveals a previously unrecognized early-middle Triassic, structurally controlled hydrothermal activity in the region. We propose that this episode of hydrothermal circulation was driven by extensional tectonism in response to the spreading of the Proto-Tethys Ocean. This finding is not in line with the earlier research that considered late Permian Emeishan volcanism to have been the main trigger for hydrothermal fluid influx. The long-standing view regarding hydrothermal dolomitization in this region may need to be re-evaluated.