Early genesis of hydrothermal dolostone: New insights from U–Pb geochronology and geochemistry

ABSTRACT

While the conventional ‘deep burial–late hydrothermal’ model has commonly been applied to explain hydrothermal dolostone (HTD), its validity is continuously questioned by the emerging ‘shallow burial–early hydrothermal’ model. This study presents a classic example of HTD from the Permian Qixia–Maokou Formation in the Sichuan Basin. By integrating petrography, U–Pb geochronology, elemental mapping and geochemical analyses, the authors constrained the fluid sources, absolute timing and tectonic controls on hydrothermal dolomitisation, interpreting this HTD as being of the ‘shallow burial–early hydrothermal’ type. The two main types of dolostones are classified as matrix dolostone (MD) and HTD. HTD occurs preferentially along strike‐slip faults, associated with hydraulic breccias and hydrothermal minerals (e.g. fluorite, quartz), indicating strong tectonic–hydrothermal linkage. HTD exhibits δ ¹³ C values (2.8 to 4.9‰) similar to those of MD and coeval seawater carbonate, but shows markedly negative δ ¹⁸ O values, positive δEu anomalies, Sr depletion, elevated high Mn/Sr ratios and high fluid‐inclusion homogenisation temperatures (100.1°C to 176°C)—all consistent with mixing between seawater and deep‐sourced hydrothermal brines. The Triassic U–Pb ages of HTD (244.8 ± 1.5 to 236.5 ± 2.2 Ma) overlap with the MD ages (244.6 ± 2.9 to 242.8 ± 1.7 Ma) within uncertainty. Cross‐cutting relationships of sub‐horizontal stylolites further confirm a shallow‐burial, early diagenetic dolostone origin. Collectively, the data in this study support a fault‐controlled model in which strike‐slip fault systems, activated during regional extension, facilitated downward infiltration of seawater and upward flow of hydrothermal fluids. Thermal convection drove the fluid mixing and generated high‐salinity diagenetic fluids that precipitated HTD upon pressure release in shallow strata. These findings are not consistent with earlier research that considered analogous HTD as a product of deep burial‐late genesis and highlight that fault‐controlled early‐stage hydrothermal dolomitisation may be more common in extensional settings than previously recognised. This study provides an absolute chronology‐based framework for understanding HTD formation and offers a valuable analogue for similar tectonic settings worldwide.