ORIGIN AND SIGNIFICANCE OF SULFATE MINERAL CRUSTS IN THE CAVERNS OF SONORA, SUTTON COUNTY, TEXAS

Caverns of Sonora is a popular tourist cave that is world renown for its spectacular carbonate speleothems. The cave is located in the central portion of Edwards Plateau and has approximately 10 km of explored passage. The cave is thought to have a hypogenic origin because of its maze pattern and characteristic morphological features, including a large fault that is exposed in a substantial portion of the cave and may represent a possible source for hypogene fluids. The cave also contains some rare and compelling deposits of metatyuyamunite and gypsum, which are commonly associated with hypogene and even sulfuric acid speleogenesis. Here we evaluated this possible mineral evidence for hypogene speleogenesis by characterizing white mineral crusts found throughout the cave. White crusts were sampled from three different cave levels. Powder XRD showed that white crusts were composed of nearly pure gypsum and, in some cases, calcite. The gypsum crusts were located in areas where calcite speleothems are absent and appear to have been protected from dripping water, although in one case, a gypsum crust was overlaying calcite speleothems. Stable isotope ratios (δ³⁴S_sulfate) of the gypsum crusts range from +6.2 to +11.5‰, enriched in ³⁴S compared to gypsum from sulfuric acid caves like the Frasassi Caves and Carlsbad Cavern, but lighter than dissolved sulfates in the Edwards Aquifer. The isotopic data does not clearly support but also does not entirely exclude a sulfuric acid origin, and future work will put these isotopic values in a regional and stratigraphic context to determine possible sources for the sulfur. We will also present data on microbial communities associated with gypsum and microcrystalline calcite deposits in the cave and discuss the implications for microbial contributions to calcite speleothem deposition.