Deciphering temperatures and sources of hydrothermal and geothermal fluids in the formation of saddle dolomites, upper Ediacaran, South China

While a complete understanding of subsurface fluid circulation is contingent upon the determination of the temperature and the source of fluids, it is still a challenge to distinguish between hydrothermal and geothermal origins in deep-time successions. Here we used clumped isotopes (Δ47) and fluid inclusion analysis to determine the formation temperature of dolomites and combined this with the analysis of in situ rare earth elements (REEs) and strontium isotopes (87Sr/86Sr) to constrain the source of fluids in the formation of the dolomites in the upper Ediacaran successions of South China. Three generations of dolomite cements (D1–D3) were identified, including the equant (D1) and two types of saddle dolomite cements (D2 and D3). The D1 and D2 dolomites show significantly elevated Δ47-temperatures, reaching up to 310 °C, which is higher than the maximum formation temperature (~230 °C), thus providing definitive evidence for the presence of hydrothermal activities. In contrast, the temperature derived from both Δ47 values and fluid inclusions in the D3 dolomite is lower, with an average of 203 ± 30 °C. Because this does not exceed the maximum formation temperature, a geothermal origin is suggested. The distinctive characteristics of 87Sr/86Sr ratios and shale-normalized REE and yttrium patterns of D2 and D3 reveal different sources between hydrothermal and geothermal fluids, predominantly originating from the metamorphic basement and shale successions, respectively. This study reveals the existence of two distinct hot fluids (hydrothermal versus geothermal) for the formation of saddle dolomites, emphasizing the need for caution in (re)interpreting structure-controlled hydrothermal dolomitization.