In February 2024, enCore Energy Corp. entered a joint venture with Boss Energy, Ltd. to develop and advance the Project. enCore retains ownership of 70% of the project and Boss Energy holds 30%.
The Project is located entirely within private land holdings of the Jones Ranch. The Jones Ranch is an approximately 380,000-acre ranch that was founded in 1897, and enCore controls over 200,000 of the 380,000 acres with mineral leases and options for uranium exploration and development.
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Summary:
The deposit type is being investigated and mined are sandstone hosted uranium roll-fronts, as defined in the “World Distribution of Uranium Deposits (UDEPO) with Uranium Deposit Classification”, (IAEA, 2009).
Surface Geology
In Jim Hogg County and across Mestena Grande Project area, the Eocene Jackson Group, the Miocene Catahoula and Frio Formations, the Pliocene Goliad Formation and Quaternary windblown deposits outcrop at the surface. In most of the county these units subcrop beneath a blanket of Holocene sediments brought inland by easterly and southeasterly winds. The Miocene age Oakville Formation and Lagarto Clays do not outcrop in this area.
Subsurface Geology
The deposits are roll-fronts, typical to 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 various host formations (Goliad, Oakville, and Catahoula). 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 vicinity of Alta Mesa include the Vicksburg Fault and the associated Vicksburg Flexure and Alta Mesa Dome. The Vicksburg Fault is a large-scale, deepseated 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 significant structural features in the vicinity of the Project include the Vicksburg and Midway Fault Zones, along with numerous, regional and local scale growth faults. Analyses of cross-sections indicate significant faulting has occurred during Catahoula and Oakville time, with the degree of faulting lessening upward into Goliad time. Lagarto sediments include thick fluvial sequences of bedload and mixed-load channel systems indicating increased fluvial processes were active during deposition in this region of south Texas.
Fluvial systems within the Catahoula, Oakville, Lagarto, and Goliad sequences all exhibit a significant reduction in energy toward the coast, with sediment size and process complexity decreasing in each to the east.
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 geoccesses; however, possibly from different reductant source.
The deposits at Mesteña Grande are characterized by vertically stacked roll-fronts controlled by stratigraphic heterogeneity, host lithology, permeability, reductant type and concentration, and groundwater geochemistry. Individual known roll-fronts are a few tens of feet wide, 2 to 10 feet thick, and often thousands of feet long. Collectively, roll-fronts are inferred to 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 known mineralization occurs at various depths, from 400 to over 1,200 feet.