Formation and resulfidization of a South Texas roll-type uranium deposit

Abstract

Core samples from a roll type uranium deposit in Live Oak County, south Texas have been studied and results are reported for Se, Mo, FeS/sub 2/ and organic-carbon distribution, sulfide mineral petrology, and sulfur isotopic composition of iron-disulfide phases. In addition, sulfur isotopic compositions of dissolved sulfate and sulfide from the modern ground water within the ore bearing sand have been studied. The suite of elements in the ore sand and their geometric relationships throughout the deposit are those expected for typical roll-type deposits with well-developed oxidation-reduction interfaces. However, iron-disulfide minerals are abundant in the altered tongue, demonstrating that this interval has been sulfidized after mineralization. Iron disulfide minerals in the rereduced interval differ mineralogically and isotopically from those throughout the remainder of the deposit. The resulfidized sand contains dominantly pyrite that is enriched in /sup 34/S whereas the sand beyond the altred tongue contains abundant marcasite that is enriched in the light isotope, /sup 32/S. Textural relationships between pyrite and marcasite help to establish relative timing of iron disulfide formation. Based on this evidence, the deposit formed in at least two stages: (1) a pre-ore process of host rock sulfidization which produced disseminated pyrite as the dominant iron disulfidemore » phase; and (2) an ore-stage process which led to the development of the uranium roll with emplacement of the characteristic suite of minor and accessory elements and which produced abundant isotpically light marcasite. The host rock was modified by a post-ore stage of resulfidization which precipitated isotopically heavy pyrite. Sulfur isotopic compositions of sulfide and sulfate present in modern ground water within the host sand differ greatly form sulfur isotopic composition of iron disulfides formed during the resulfidization episode.« less