Sulfur and strontium isotope analyses have been used to test the possibility that barite veins in the Galeana district of northeastern Mexico formed where barium-bearing solutions intersected an overlying gypsum-bearing unit. Barite at Galeana occurs in essentially monomineralic veins in red beds of the Mesozoic-age Huizachal Group at and just below its contact with gypsum of the Olvido Formation. Sulfur isotope analyses confirm that the sulfur in the barite (delta 34 S = 12.3-17.4ppm) has the same range as Olvido gypsum (13.1-17.6ppm). The mean isotopic composition of the barite is about 1.2 per mil heavier than the mean gypsum composition, suggesting that fractionation took place during barite deposition. Strontium isotope compositions of Huizachal Group rocks fall on an isochron with an age of 227.5 + or - 3 m.y. and an initial 87 Sr/ 86 Sr ratio of 0.70816 + or - 0.00004, and the age-corrected, average Huizachal isotopic composition agreed with that of the Galeana barites (0.71345) about 30 m.y. ago, consistent with the geologic estimates of the age of mineralization. Compositions of whole rocks and dilute acid leachates from the Huizachal Group indicate that the strontium, and presumably also the barium, were derived from the bulk rock rather than the easily leachable carbonate cement and veinlets. The unusually homogeneous isotopic composition of strontium in the Galeana barite, when compared to data on the scale of strontium isotope heterogeneity in present-day formation waters, suggests that the Galeana mineralization was derived from a relatively small volume of sediment, similar to that of a single oil field. Solubility calculations and data on barium and strontium contents of formation waters indicate that a fluid volume of about 10 11 m 3 would be required if the mineralizing solution contained about 100 ppm barium. Depending on porosity, this volume of water could have come from a 100- to 200-m thickness of Huizachal Group red beds in the 40 X 20-km structural high underlying the Galeana district.