Abstract

Economically important manto fluorite deposits in northern Coahuila, Mexico, are found at the contact between uppermost Lower Cretaceous Georgetown limestone and disconformably overlying Del Rio shale or Buda limestone in nine districts within a 5,000-km 2 area. The deposits formed by replacement followed by open-space filling and consist of fluorite with minor calcite and quartz and local celestite and supervene (?) gypsum. Sulfides are extremely rare in these deposits. Wall-rock alteration around the mantos involved fluoritization and locally intense silicification and formed halos about one foot wide. Primary inclusions in fluorite from these mantos contain liquid + vapor + or - petroleum material + or - an unidentified daughter crystal. Homogenization temperatures on nearly 300 such large, primary inclusions range from about 110 degrees to 330 degrees with a strong maximum at 150 degrees + or - 20 degrees C. Salinities of these inclusion fluids range from 8 to 18 equiv wt % NaCl, and Na/K atomic ratios of inclusion leachates range from 2.5 to 3.14 with all leachates containing large amounts of calcium. Evidence for boiling is seen in many deposits.The deposits are spatially associated with mid-Tertiary (?) rhyolite (anomalously high in fluorine), and some chimneys, veins, and contact deposits are immediately adjacent to these rhyolites. One such contact deposit contains fluorite with polyphase inclusions (salinity of over 40%) that homogenize in the 370 degrees to 430 degrees C range and have a Na/K atomic ratio of 1.14 (with abundant calcium). Sulfur from arsenopyrite in this deposit has an isotopic composition of +0.8 per mil, whereas sulfate from manto celestite has sulfur as heavy as + 13.2 per mil. Sulfur in gypsum and pyrite in the Del Rio shale have compositions of -18.8 and -31.3 per mil, respectively.Consideration of these data in their geologic context suggests that the manto deposits formed when a zone of formation water trapped below the Del Rio shale was intersected by rhyolite magmas, which contributed fluorine, heat, and possibly maginatic water and sulfur to the formation water zone. On a regional scale, the northern Coahuila fluorite district appears to be related to a zone of mid-Tertiary alkaline igneous rocks that extends from the Big Bend area to Tampico and includes another important Mexican fluorite district. These results lend credence to unexposed magmatic affiliations postulated for other fluorite districts in North America.

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