Origin of a South Texas roll-type uranium deposit; I, Alteration of iron-titanium oxide minerals

The detrital Fe-Ti oxide minerals and their authigenic alteration products in samples from cores spanning 1.7 km across a roll-type uranium deposit in the mid-Tertiary Catahoula Tuff, south Texas, record important information on host-rock preparation and on development of the altered tongue of the deposit. In reduced rock, in front of and enveloping the altered tongue, iron disulfide minerals (pyrite and marcasite) have partially to completely replaced titanomagnetite and, to a lesser extent, titanohematite. Subsequent oxidation of these sulfidized Fe-Ti oxides, by processes that formed the altered tongue and uranium roll, produced ferric oxide and hydroxide phases (limonite) pseudomorphous after the sulfides. Fe-Ti oxides 1.0 km updip from the roll front were never sulfidized; titanomagnetite (partly replaced by hematite) and martite constitute nearly half of the heavy mineral fraction and limonite is absent. Oxidized rocks 210 m and closer to the nose of the roll front, however, are nearly devoid of titanomagnetite and contain abundant limonite. The redox interface that bounds the altered tongue, therefore, moved more than 210 m but less than 1.0 km. Detrital Fe-Ti oxide minerals are increasingly sulfidized in the downdip direction indicating that initial sulfidizaton was more intense in that direction. Organic carbon content is uniformly very low throughout the host sandstone, which implies that heterotrophic sulfate-reducing bacteria were not involved in the generation of sulfide. The H (sub 2) S for the initial sulfidization may have been derived from oil and gas in formations that underlie the Catahoula Tuff. The H (sub 2) S probably invaded the Catahoula along a fault that is 1.5 km downdip from the present roll front.