Hydrothermal silicification and hypogene dissolution of an exhumed Neoproterozoic carbonate sequence in Brazil: Insights from fluid inclusion microthermometry and silicon-oxygen isotopes

Hypogene dissolution-precipitation processes strongly affect the petrophysical properties of carbonate rocks and fluid migration pathways in sedimentary basins. In many deep carbonate reservoirs, hypogene cavernous voids are often associated with silicified horizons. The diagenesis of silica in carbonate sequences is still a poorly-investigated research topic. Studies exploring the complexity of silica dissolution-precipitation patterns in hypogene cave analogues are therefore fundamental to unravel the diagenetic and speleogenetic processes that may affect this kind of reservoir. In this work, we investigated an exhumed and silicified Neoproterozoic carbonate sequence in Brazil hosting a 1.4 km-long cave. Quartz mineralization and silicified textures were analyzed with a multidisciplinary approach combining petrography, fluid inclusion microthermometry, silicon-oxygen stable isotope analyses and U-Th-Pb dating of monazite crystals. We found that an early silicification event caused the replacement of the dolostone layers with micro-crystalline quartz forming chert nodules. This event was likely associated with mixing fluids (ancient Neoproterozoic seawater and hydrothermal solutions sourced from the underlying Mesoproterozoic basement) at relatively low temperatures (ca. 50-100 degrees C) and shallow depth. After the tectonic deformation produced by the Brasiliano orogeny, silica dissolution was promoted by high temperature and alkaline hydrothermal solutions rising from the quartzite basement along deep-rooted structures. Hypogene hydrothermal alteration promoted the dissolution of the cherty layers and the precipitation of chalcedony and megaquartz. Homogenization temperatures from primary fluid inclusions in megaquartz cement indicate minimum formation temperatures of 165-210 degrees C. Similar temperature estimates (110-200 degrees C) were obtained from the delta Si-30 and delta O-18 isotope systematics of quartz precipitated from hydrothermal solutions. The dissolved salts in the fluid inclusions were evaluated as NaCl + CaCl2 from microthermometric data combined with cryogenic Raman spectroscopy, corresponding to salinity ranging between 17 and 25 wt.%. No reliable age constraints for hydrothermal silica dissolution-precipitation phases were obtained from monazite U-Th-Pb dating. However, our results, interpreted in the regional context of the Sao Francisco Craton, suggest that the Cambrian tectono-thermal events could have been amongst the possible drivers for this hypogene process in the basin.