Strontium-isotopic geochemistry of fluorite mineralization (Coahuila, Mexico)

The widespread fluorite deposits of Coahuila, Mexico, include limestone replacement deposits formed at the contact with Tertiary rhyolites and microsyenites, as well as mantos and veins that are not clearly associated with any known igneous body. In recent studies, it was concluded that most of the calcium in these deposits came from the limestone, whereas the fluorine was derived largely from the Tertiary igneous rocks. We have tested this model using strontium-isotopic data and simultaneously placed constraints on the mechanisms of fluorite deposition. Our results indicate that the ⁸⁷Sr/⁸⁶Sr ratio of the host-limestones ranges from 0.7075 to 0.7078, whereas that of the Tertiary rocks, at the time of mineralization, was 0.7074–0.7118. The ⁸⁷Sr/⁸⁶Sr ratios of the fluorite range from 0.7076 to 0.7083. The fluorites fall on two distinct mixing lines between Cretaceous limestone and Tertiary igneous rock. Between 70% and 98% of the Sr in the fluorite came from limestones. Relative abundances of Sr and F in the limestone and the igneous rocks suggest that between 70% and 96% of the fluorine came from the igneous rocks.