A ubiquitous hydrothermal episode recorded in the sheet-crack cements of a Marinoan cap dolostone of South China: Implication for the origin of the extremely 13C-depleted calcite cement

Abstract

Sheet-crack – primarily filled with chalcedony, quartz and calcite – is a type of significant sedimentary structure in almost all documented cap carbonates in platform-to-slope settings, which sharply overlie the global Marinoan (∼635Ma) glacial tillite. The Jiulongwan section, located in the Yangtze Gorges areas, South China, is significant for the first discovery of extremely 13C-depleted (δ13C down to −41‰) calcite cements in sheet-crack structure (Jiang et al., 2003a), which provides direct evidence for the methane seep hypothesis (Kennedy et al., 2001). Focusing on the calcite cements in the same section, Bristow et al. (2011) proposed a hydrothermal origin based on high temperatures (378°C) determined by carbonate clumped isotope thermometry (CCIT). In this study, we provide evidence for the hydrothermal model using fluid inclusion technology (FIT) to quartz cements in the Jiulongwan section. The widespread sheet-crack structures in South China have uniformmineralparageneticassemblages, which apparently reflect three stages of a hydrothermal fluid activity, including dolomitization at the early stage, silicification comprising chalcedony and quartz at the middle stage, and calcitization characterized by carbonaceous calcite filling preexisting voids at the late stage. Primary aqueous fluid inclusions from quartz crystals have homogenization temperatures of approximately 160–220°C (mean, 192°C, n=31), while the salinity histogram contains two peaks at low salinity (6.3–8.3wt.% NaCl equiv.) and high salinity (18.0–20.8wt.% NaCl equiv.), reflecting precipitation caused by mixing of high- and low-salinity fluids. A modified and detailed hydrothermal model is proposed indicating that the sheet-crack structure resulted from successive thermal fluid activity after karstic dissolution due to postglacial isostatic rebound. This model is compatible with the unified sequence of glacio-eustatic events after the termination of Marinoan glaciation (Zhou et al., 2010) and the temperature results determined by CCIT (Bristow et al., 2011).