Studies of the mineralogy, sulfur isotope geochemistry, and uranium-lead isotope geochronology of a roll-type orebody in the Felder deposit, south Texas, have indicated important constraints on the genesis of this deposit. Post-mineralization sulfidization of the host Oakville Sandstone (Miocene) resulted in precipitation of pyrite and marcasite in the altered tongue that differ in relative abundance and in isotopic composition from pyrite and marcasite elsewhere in the host rock. The altered tongue is characterized by a predominance of pyrite over marcasite and by heavy ^dgr³⁴S values (-5.2 to +20.6 per mil), whereas reduced-barren and mineralized rock is characterized by abundant ore-stage marcasite and by light ^dgr³⁴S values (-29.9 to -47.7 per m l). In reduced-barren and mineralized rock, two generations of pyrite are present: (1) pre-ore pyrite grains that are commonly enclosed by ore-stage marcasite; and (2) post-ore pyrite that is genetically equivalent to pyrite in the altered tongue. Resulfidization of the deposit by hydrogen sulfide-bearing solutions introduced along one or more nearby faults is indicated by the similarity in isotopic compositions of pyrite in the altered tongue to that of sour gas in the underlying Edwards Limestone (Cretaceous). The time of this resulfidization is probably indicated by the unusually precise ²⁰⁷Pb/²⁰⁴Pb-²³⁵U/²⁰⁴Pb isochron age of 5.09 ± 0.11 m.y. The lack of significant scatter of the points on the isochron diagram must reflect the immobili y of uranium and lead over the last 5 m.y. due to the continued presence of hydrogen sulfide. The 5-m.y. isochron age indicates that no significant roll-forming processes have occurred in the Felder deposit since this time.

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