Deep-seated cave inception and shallow sulfuric acid maze cave genesis in Southern Irecê Basin, São Francisco Craton (Brazil)

Abstract

Ioiô Cave is a 4.7 km long maze cave in the southern tip of the Irecê Basin (Bahìa, Brazil), and although still actively forming today, it hosts signs of a long speleogenetic history. Deep rising hydrothermal fluids weathered the carbonates, creating dark ghost-rocks and quartz and dolomite veins, mainly in the anticlinal hinges and below the siliciclastic seals. This silicification, although not directly dated, is probably associated with the end of the Brasiliano-age tectono-thermal activity (Lower Cambrian) based on isotopic and trace element data and regional tectonic correlations. Since the Plio-Quaternary, the progressive exhumation of the carbonate reservoir increasingly favored the introduction of meteoric oxygen-rich water from the surface, causing sulfide oxidation at shallow aquifer depth. The CO₂ produced by Sulfuric Acid Speleogenesis (SAS) rose along fractures and degassed at shallow depth, producing carbonic speleogenesis close to the water table. This carbonic speleogenesis, probably still active, produced a maze network, by horizontal diffusion of aggressive fluids from the feeders. Surface breaching increased air flow activating degassing and supersaturation of the basins, with deposition of subaqueous calcite shelves, carved with bubble trails resulting from CO₂ degassing related to still-ongoing pyrite oxidation (localized SAS).