Geology Of South Texas Uranium Deposits

The South Texas Mineral Trend is now estimated to contain uranium reserves of 150 million pounds U308 . Within the past year, an estimated 10 million pounds U308 have been added to this gross reserve. It is probable that a similar amount has been identified in previously unknown orebodies that, as yet, have not been delimited or announced. Exploration that was limited in the past to a narrow band containing only the known trend has now expanded into older sediments updip and into younger units towards the coast. Uranium host formations are also now being explored at a considerable depth and distance eastward from known deposits. Only about 30 percent of the potential uranium host rocks in South Texas have been adequately explored. Geology The South Texas uranium deposits are confined mainly to sediments of the Tertiary system. Reserves are divided almost equally between the Whitsett Formation of the Jackson Group, the Catahoula Formation, and the Oakville Formation. A minor amount of the reserves occurs in sands of the Goliad Formation which may be either in the Tertiary or Quaternary Epoch. Figure 1 is a geologic column of the South Texas uranium host formations. These producing formations are marked with a mine symbol but there are also several prospect symbols that denote potentially favorable uranium host formations both younger and older from the producing formations. It is generally accepted by most workers in South Texas that the source for the uranium is the volcanic ash that is abundant within several of the formations. Likewise, the required reductant is considered to be hydrogen sulfide gas, derived from deeper seated hydrocarbon accumulations, that emanate upward along fault zones into favorable host-rock sand units. Within this basic framework of source, host, structure and hydrocarbons is where most of the reserves have been discovered and where most of the current exploration is either concentrating or expanding. Structure in South Texas is predominantly faults. Swarms of faults exist in zones paralleling the coast and running from the Rio Grande to the Sabine River. These faults are usually growth faults with the down-dropped block on the coastward side. Displacement may range from a few feet to a few hundred feet. Dips are near vertical in the younger rocks at the surface but become flatter as the fault cuts older beds in the subsurface. A map of the oil and gas fields in South Texas indicates a correlation between these fault swarms and accumulations of hydrocarbons. It is not coincidental that the known uranium trends closely follow the hydrocarbon accumulations and the faults swarms, all of which supports the theory of uranium concentration by groundwater movement through volcanic ash-rich beds into favorable host rocks impregnated with reducing hydrogen sulfide gases that migrate upward along fault planes from hydrocarbon accumulations. History Newcomers to South Texas are often amazed that active entry is possible in a district that has produced uranium for over 20 years. Understanding the conditions and occurrences of the past would explain why the opportunity still exists for companies not now active in South Texas to become active. Uranium was discovered in the middle 1950's in sandstone units of the Jackson Group at Tordillo Hill in Karnes County. This discovery was followed by a rush involving most of the major uranium exploration companies as well as several of the not-so-major. Those western prospectors who were used to numerous outcrops and neat land subdivisions were further discouraged by the small size and low grade of the deposits. Then after a brief blast, they left South Texas as they found it and returned to the richer diggings of New Mexico and Wyoming. Susquehanna Western was the only one to stay and develop mines in the area. Eventually they discovered enough ore along the Jackson outcrop to warrant constructing a small mill. They managed, with limited budget and diligent effort, to find enough ore to keep the mill going and eventually expanded into exploration in other formations. By the late 1960's, Susquehanna was mining from both the Jackson and Oakville deposits. About this time, the oil companies began to enter the uranium industry and found, that because of sound forward planning, they controlled the uranium on vast tracts of acreage. At this time, which was more than ten years after discovery, there was so little literature on South Texas uranium deposits that the oil companies began following the known trends and off-setting known orebodies. This, and a few kicks on some well gammaray logs, lead to the discovery of new areas in formations that previously had n o uranium discoveries. Still, the following of the trend as it crossed from one formation to another was the main geologic guide. Nowadays, we in South Texas feel that science has entered into the quest to discover new orebodies. The work of Galloway has indicated new pathways to explore. The understanding of multiple stages of oxidation and reduction has created some doubt about areas drilled in the past and abandoned. The expanded use of oil well logs and geochemical prospecting has lured the more progressive exploration companies off the mineral trend and into unexplored areas. Prognostication The fact that uranium exploration in South Texas has been active for only the past ten years is not an indication that South Texas is not a major uranium district. The geology of the South Texas uranium deposits as described here serves only to indicate that similar geology extends in all directions from the known mineral trend as can be seen on Figure 2.