Role of fluid mixing and fault-related sulfide in the origin of the Ray Point Uranium District, south Texas

The Lamprecht and Felder deposits are two of many roll-type uranium deposits hosted by the Miocene Oakville Sandstone in the Ray Point (south Texas) uranium district. The deposits in this district are aligned parallel to or are intersected by the Oakville fault which suggests control by the faults on their origin and/or preservation. The Lamprecht and Felder deposits show fundamentally similar patterns of geochemical zonation and of distribution of authigenic iron disulfide (FeS2) minerals. Distribution of U, Mo, and Se are typical of roll-type deposits that result from invasion of geochemically reduced rock by oxygenated uranium-bearing solutions. Sulfur content of these deposits is high (as much as 9%), with almost all sulfur occurring as FeS2 minerals. Organic carbon is virtually absent. Four distinct stages of FeS2 mineral formation are recognized: (1) a generation of isotopically light (delta34S < -20%o) preor pyrite, (2) a generation of isotopically light (delta34S < -20%o) marcasite that formed mostly before, but also during, ore deposition, (3) a postore generation of isotopically heavy pyrite (delta34S > 0%o), and (4) a yet later generation of marcasite with isotopically light sulfur close to the values of stages 1 and 2. The altered tongue in these deposits is anomalous in that it contains FeS2 minerals in contrast to other Texas and Wyoming roll-type deposits which typically have an oxidized (ferric iron-bearing) altered tongue. The dominant FeS2 mineral in the altered tongue is isotopically heavy pyrite (stage 3) with subordinate amounts of late marcasite (stage 4)