Summary:
The Alta Mesa Project is in the South Texas Uranium Province, which is known to contain more than 100 uranium deposits (Nicot, et al., 2010). Within the South Texas Uranium Province, uranium mineralization is primarily hosted in the Miocene/Pliocene Goliad Formation, Miocene Oakville Formation, Oligocene/Miocene Catahoula Formation, and the Eocene Jackson Group.
The deposits are roll-fronts, typical of others found in the South Texas Uranium Province. The ore bodies are isolated within several sand units, which occur within the middle portion of the Goliad Formation.
Genesis of the ore deposits are related to the presence of chemical reductants trapped in the Goliad host formation. Reductants are believed to be associated with natural gas and/or hydrogen sulfide seepage from deeper formations through localized faulting.
The significant structural features in the area are the Vicksburg Fault and the associated Vicksburg Flexure and Alta Mesa Dome. The Vicksburg Fault is a large-scale, deep-seated growth fault, mainly affecting deeper stratigraphic units. Little, if any, displacement has occurred in Goliad and younger units. Activity on the Vicksburg Fault and related structural features has, however, influenced sedimentation patterns in the Goliad.
The Alta Mesa Dome is a deep-seated, non-piercement shale diapir structure associated with the Vicksburg Flexure. Deformation of the subsurface strata is considerable at depth but at the Goliad level, maximum uplift is on the order of only 100 to 125 feet. The location of the ore deposit closely coincides with the top of the dome at the Goliad stratigraphic level. Domal uplift is believed to have been active but subdued during deposition of the Goliad Formation. The rate of uplift was insufficient to divert fluvial deposition but did limit its extent.
As a result, strata thin over the dome and thicken off the dome. Clay interbeds are more abundant and more continuous over the dome. At the Goliad stratigraphic level, symmetry of the dome is broken on the western and northwestern flanks by a pair of subparallel, normal faults. These appear to be zones of structural failure associated with sporadic reactivation of domal uplift. The throw of these faults is opposite to each other, creating an intervening graben structure. Surface expression of faulting did not occur until after the ore mineralization phase.
The eastern fault of the two faults referenced in the paragraph above is the Jones Fault. The downthrown block lies to the west of the fault plane (an up-to-the-coast fault). Vertical displacement is up to 50 feet at the Goliad level and increases with depth. At the Goliad stratigraphic level, the vertical displacement along the fault disappears as the fault trends across the dome where structural integrity of the dome is preserved. The extension of the same fault plane continues in the far northern limits of the project area.
The Figueroa Fault formerly referred to as the Garcia-Ramos Fault by Chevron, a previous leaseholder, occurs just west of the Jones Fault. Its orientation is roughly northeast-southwest and trends parallel to the Jones Fault. Displacement at the Goliad structural horizon is roughly 20 feet, downthrown to the east. Subsurface interpretation indicates the Figueroa Fault is antithetic to the Jones Fault, intersecting and terminating on the Jones Fault at depth.
Mineralization
Uranium mineralization occurs primarily as uraninite with some coffinite and like other deposits within the South Texas Uranium Province, is stratabound in clay-bounded sandstone packages. Mineralization occurs as roll front type deposits with “C” shaped configurations in cross section and elongated sinuous ribbons in plan-view. Deposits are diagenetic and/or epigenetic forming because of a geochemical process whereby oxidized surface water leaches uranium from source rocks (Finch, 1996). Source rocks of the south Texas deposits are generally agreed to be Miocene and Oligocene age volcanic ash from west Texas and/or Mexico (Galloway et al, 1977 and Aguirre-Diaz and Renne, 2008).
This ash was deposited by wind and fluvial systems and uranium was leached from the ash by oxygenated surface waters. Uranium bearing waters were transported to outcrop areas where sandstone formations were exposed and began to move downdip as groundwater. The movement of uranium continued in groundwater until a reductant source was encountered, such as hydrogen sulfide gas, pyrite or carbonaceous material resulting in uranium precipitating out of solution.
At Alta Mesa, uranium bearing groundwater moved from northwest to southeast and encountered a reduction zone associated with the Alta Mesa oil and gas field, caused primarily by hydrogen sulfide gas introduction through faults and fractures. Mineralization away from the oil and gas field occurs by the same geochemical processes; however, possibly from different reductant source.
The deposits are characterized by numerous vertically stacked roll-fronts controlled by stratigraphic heterogeneity, host lithology, permeability, reductant type and concentration, and groundwater geochemistry. Individual roll-fronts are a few tens of feet wide, 4 to 10 feet thick, and often thousands of feet long. Collectively, roll-fronts result in an overall deposit that is up to a few hundred feet wide, 50 to 75 feet thick and continuous for miles in length.
Depth of mineralization ranges from 500 to 600 feet.