Geologic Relations Among Uranium Deposits, South Texas, Coastal Plain Region, U.S.A.

Abstract

Uranium has become an important energy resource in the South Texas Coastal Plain, where it occurs in tuffaceous sandy sedimentary rocks of late Eocene to Pliocene age. The uranium ore contains small amounts of molybdenum and selenium. Deposits range from small, irregular bodies of highly oxidized “yellow ores” near the outcrop to un- oxidized “black ores” at depths of from 60 feet (18. 3 meters) to at least several hundred feet. The oxidized ore is highly susceptible to leaching accompanied by migration down the dip of permeable sands. The deep unoxidized deposits exhibit well-developed roll-type ore bodies along oxidation-reduction boundaries.

Genetic factors presumed to be common to these deposits are (1) a source of uranium from the diagenetically altered tuffaceous sedimentary rocks, (2) dissolution and transport of uranium by the alkaline carbonate pore-waters of the ash, (3) precipitation of uranium in reducing environments caused principally by hydrogen sulfide and in part by organic matter where waters were retarded by lenticular, clayey or structural barriers, and (4) preservation from leaching by such features as a caliche cap developed during an arid climate.

Four principal geologic environments of uranium ore in South Texas are (1) in near- shore sandstones sandwiched between less permeable beds and overlain unconformably by tuffaceous deposits, (2) in sandstones interfingering with claystones along the sides of major paleo-stream-channel deposits, (3) in sandstone near faults along which natural gas containing hydrogen sulfide has migrated, and (4) in sandstone above the sulfurous caprock of a salt dome.

Publication authorized by the Director, U. S. Geological Survey.