Barite replacement boxwork in the Frasassi caves (Italy)

Abstract

The Frasassi caves, located in the Sentino River Gorge in Ancona Province (Marche, Italy), contains boxwork in a small zone in the inner part of the cave system where it is closely associated with subaqueous corrosion produced by sulfuric acid speleogenesis. The boxwork consists of barite that replaces calcite spar-filled veins and limestone porosity and projects from the corroded cave walls. The replacement involved only the calcite that protrudes from the rock surfaces, indicating that the process took place in the cave environment, and therefore the boxwork is not simply a product of differential corrosion. I hypothesize that the boxwork formed in nearly stagnant bodies of sulfidic water as a consequence of lateral recharge of O2-rich bicarbonate water moving upward along the walls because of its lower density. The acidity produced by the oxidation of H2S enhanced limestone corrosion and incision of rills and channels, while also releasing sulfate ions that induced the precipitation of barite. This sulfuric acid process was sufficiently slow to dissolve the micritic bedrock and replace spar with barite, preserving the shape of the network of fissures and pores formerly filled by calcite.