Geochemistry of Manto Fluorite Deposits,

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-kin 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 supergene(?) gypsum. Sulfides are extremely rare in these deposits. Wall-rock alteration around the mantos involved fiuoritization and locally intense silicification and formed halos about one foot wide. Primary inclusions in fluorite from these mantos contain liquid + vapor _ petroleum material _+ an unidentified daughter crystal. Homogenization temperatures on nearly 300 such large, primary inclusions range from about 110 ø to 330 ø with a strong maximran at 150 ø + _ 20øC. Salinities of these inclusion fluids range from 8 to 18 equiv wt % NaC1, 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