Biogenic and nonbiogenic ore-forming processes in the south Texas uranium district; evidence from the Panna Maria deposit

Geochemical and petrographic studies of core samples from the Panna Maria uranium deposit, a roll-type orebody in the Eocene Jackson Group in Karnes County, Texas, yield important information on the origin of the deposit. Organic carbon content averages about 0.42 weight percent in reduced rock and correlates postively with sulfur content. Pyrite is the dominant iron disulfide (FeS (sub 2) ) mineral in most of the ore zone and throughout a surrounding zone of reduced barren ground, and it is commonly associated with organic debris. Marcasite is sparse except in ore adjacent to the altered tongue in one core and locally in mineralized lignite. Sulfur isotopic compositions (delta (super 34) S) of FeS (sub 2) minerals range broadly from -1 to -34 per mil; the lightest delta (super 34) S values (less than -20 per mil) were measured in samples from mineralized lignite and from the nose of the ore roll. Petrographic and geochemical characteristics of the Panna Maria deposit contrast greatly with those of three other south Texas roll-type uranium deposits (the Benavides, Felder, and Lamprecht deposits), which are devoid of organic carbon and which contain more sulfide than does the Panna Maria. These three deposits are characterized by abundant isotopically light ore-stage marcasite and by isotopically heavy preore (in the Benavides) or postore (in the Felder and Lamprecht) pyrite. We have concluded previously that sulfide-bearing fault-leaked solutions from underlying hydrocarbon accumulations were important in the formation of the Benavides, Felder, and Lamprecht deposits. Although the Panna Maria deposit shows an apparent alignment along a fault zone, and although underlying formations in the Karnes County area contain sour gas (delta (super 34) S [asymp] + 14 per mil) and produce oil, the deposit lacks characteristics indicating that its formation or preservation involved extrinsically derived reductants such as fault-leaked aqueous sulfide. Mineralization of the Panna Maria, rather, appears to have been controlled by intrinsically derived reductants related either directly or indirectly to the presence of organic matter.