Geology and geochemistry of the Las Cuevas fluorite deposit, San Luis Potosi, Mexico

The Las Cuevas fluorite deposit, which is one of the largest high-grade fluorite deposits known, is located in the Zaragoza-Rio Verde fluorite district of central Mexico. K-Ar and Rb-Sr data indicate that the mineralization at Las Cuevas took place about 30 m.y. ago, which coincides with major Cenozoic volcanism in northern Mexico. The deposit is at the contact between Lower Cretaceous limestone and Tertiary rhyolite breccia and was formed by replacement of the limestone followed by open-space filling. It consists almost entirely of fluorite with minor amounts of calcite and silica. There is a distinct compositional zonation within the deposit, with silica increasing toward the rhyolite breccia and calcite increasing toward the limestone. Fluid inclusions in open-space-filling fluorite, which constitutes about 40 percent of the total fluorite in the deposit, homogenize at temperatures of 60 to 130C and exhibit salinities close to 0 weight percent (equivalent NaCl). Isotopic ratios of strontium in fluorite, igneous rocks, and limestone indicate that all the calcium in the fluorite came from the limestone. The fluid inclusion data show that the ore-forming solutions were essentially heated meteoric waters. The close spatial and temporal association between the mineralization and the volcanic rocks argues for these as the heat and fluorine source. The lack of calcite in the initial stages of fluorite deposition and the presence of kaolinite as the alteration of the wall rock indicate initially acid solutions. A change in pH, temperature decrease, and addition of calcium to the ore-forming solutions were the causes of deposition.