Is accessory baryte in carbonate speleothems more common than previously thought? Insights from caves in Germany

The discovery of non-hydrothermal baryte (BaSO4) speleothems in Lechuguilla Cave (New Mexico, USA) has raised the suspicion that speleothemic baryte in caves might be more common than previously recognized. This is primarily expected in caves with a similar geological setting in Paleozoic carbonate host rocks with tectonic fractures or paleokarst features that allowed hydrothermal fluids to mineralize veins and ore deposits that include baryte as a source of barium sulfate to form secondary cave minerals. To test this hypothesis, subaqueous and subaerial speleothems from caves in the Variscan Basement of Germany that match this geological context were examined. In all the caves investigated, calcite pool spar was found containing euhedral, tabular baryte crystals 5 to 300 μm in size. These were most abundant near the inflow of water and at the bottom of the pools, particularly on up-facing surfaces and in small cavities or intercrystalline voids. This distribution suggests a minerogenesis driven by a combination of episodic evaporation and density-driven processes, possibly involving meromixis. Water samples show elevated concentrations of barium as well as sulfate and saturation indices for barium sulfate are mostly positive (supersaturation with respect to baryte), indicating conditions favoring precipitation of baryte. Accessory baryte was also found in subaerial stalactites, helictites, coralloids, and wall crusts, with capillary seepage, surface diffusion, and evaporation considered the main processes of ion transport and supersaturation leading to mineral formation. Finally, baryte spherulites up to 400 μm in diameter were found in a deposit of cryogenic calcite formed during a Pleistocene glacial period. This extends the temperature range of speleothemic baryte to freezing conditions, when solute rejection during the formation of cave ice led to supersaturation of the residual water and triggered the minerogenesis of calcite and baryte, representing a climatically controlled endmember of baryte formation. Together, these findings confirm that accessory baryte in carbonate speleothems is common in non-hydrothermal, epigenic karst settings where baryte occurs in the host rock.