New Mexico Geological Society Annual Spring Meeting — Abstracts

Caves associated with fluorite-jarosite mineralization in the Hansonburg and North Franklin Mountains districts may have formed from different solution types. These caves display spongiform and irregular walls and contain significant amounts of halloysite clay.

The caves at Hansonburg are mineralized and large masses of gypsum contain mineralized limestone rafts and textures indicating limestone replacement. The low δ³⁴S values of gypsum (-7.2%) and jarosite (-25 %) are similar to those for gypsum and alunite, respectively, at Carlsbad Caverns, New Mexico and also to those of hydrothermal pyrite (-23%), consistent with cave formation during ore deposition. The aqueous sulfate that formed jarosite and some of the primary gypsum was likely derived from the oxidation of isotopically light H₂S. Some of the later cave fill gypsum at Hansonburg has high δ³⁴S values (6.5 to 13.9%) indicating aqueous sulfate derivation from the overlying Permian formations.

The cave at the Copiapo deposit in the North Franklin district is unmineralized and is located below the level of ore, a feature commonly observed in the Illinois-Kentucky fluorspar district. The presence of prosopite, CaAl₂(F,OH)₈, and fluorite in the halloysite envelope around the deposit attest to the high F activity in the earliest mineralizing solutions. Exceptionally low δ³⁴S values for some of the jarosite (~-25%) and gypsum (-11%) also indicate that aqueous sulfate formed from isotopically light H₂S, as it did at Hansonburg and Carlsbad. However, the occurrence of the cave below the level of jarosite formation, suggests that the fluids from depth were already acidic before H₂S was oxidized at higher levels that implies cave fonnation by hydrofluoric acid speleogenesis.