Richard L. Reynolds Robert O. Rye Martin B. Goldhaber 1978 <p><span>Petrologic and sulfur isotopic studies have been carried out on drill core samples from a roll-type uranium deposit in the mid-Tertiary Catahoula Tuff, Webb County, south Texas. Epigenetic iron disulfide minerals formed in two distinct stages. The first stage involved sulfidization of the host rock by sulfide (H&nbsp;</span>₂<span>&nbsp;S, HS-)-bearing solutions that emanated from a fault about 1.5 km downdip from and subparallel to the orebody. Pyrite was the dominant iron disulfide mineral formed from this fault sulfide. The isotopic composition (delta&nbsp;</span>³⁴<span>&nbsp;S) of first-stage iron disulfide is quite heavy (&gt;0 per mil), in part because the fault-derived H&nbsp;</span>₂<span>&nbsp;S was isotopically heavy. The development of the second-stage sulfides was related to processes that formed the uranium roll. Iron disulfide minerals produced during this second stage commonly occur as rims around the first-stage sulfides. The rims are exclusively marcasite in and adjacent to ore, but the pyrite content in these rims increases with increasing distance from ore. The sulfur of the second-stage sulfides in the vicinity of the roll front is isotopically light (--25 to --40 per mil). The virtual absence of organic carbon in the host sand precludes a bacterial origin for the ore-stage iron disulfide minerals and, therefore, eliminates bacterial metabolism as the mechanism for isotopic fractionation. Instead, the sulfur source for ore-stage sulfides was preore (first stage) sulfides, remobilized via partial oxidation to soluble metastable sulfur oxyanions.</span></p> application/pdf 10.2113/gsecongeo.73.8.1690 en Society of Economic Geologists Origin of a South Texas roll-type deposit; II, Sulfide petrology and sulfur isotope studies article