Uranium-lead isotope systematics of roll-front ores in Miocene sandstone at the Felder and McLean uranium deposits (south Texas coastal plain) give a well-defined 207 Pb/ 204 Pb- 235 U/ 204 Pb isochron age of 5.07 + or - 0.15 m.y. The relatively slight degree of scatter of the points defining the isochron is probably due to initial Pb isotope inhomogeneity, and the resulting inferred persistence of closed system behavior for U and Pb is probably the result of the long-term presence of U- and Pb-immobilizing H 2 S. 206 Pb/ 238 U systematics are badly scattered owing to long-term migration of radioactive daughters of 238 U. Beta- and gamma-activity systematics of the ores consistently identify those with grossly anomalous Pb isotope systematics, however, and proved highly useful in identifying ores that have gained gross amounts of uranium daughters.FeS 2 minerals in the altered tongue of the host sandstone are characterized by abundant postore pyrite and heavy delta 34 S values, whereas FeS 2 minerals in mineralized and unaltered, barren rock are characterized by abundant ore-stage marcasite and by light delta 34 S values. The delta 34 S values of FeS 2 minerals in the altered tongue are similar to those defined for sour gas from the underlying Edwards Limestone of Cretaceous age, indicating the probable source for the sulfur of an inferred resulfidization event. The 5.07-m.y. isochron age probably reflects the end of roll-front development; we believe that the end was due to the resulfidization and does not preclude significant earlier periods of mineralization. Modern ground water in the area is also sulfide bearing (thus helping preserve the deposit), but with delta 34 S values distinct from those of sulfides in the resulfidized zone of the host rock.

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