Summary:
Velvet-Wood
Uranium mineralization in the Velvet and Wood areas is found in sandstone units within the Cutler Formation.
The dominant feature in the Velvet-Wood area is the Lisbon Valley Anticline. The Lisbon Valley Anticline is a northwest/southeast feature about 20 miles long that was formed when salt in the Paradox Formation was mobilized. The up-warping and subsequent erosion of the anticline has exposed Pennsylvanian to Cretaceous age rocks along the length of the anticline.
Three Step Hill is composed of three mesas, each progressively higher than the last. The VelvetWood Deposit is under the lowest mesa and on the margin of the second. The dips on the lower plateau are about 6 to 8 degrees and dips on the upper plateau are about 3 to 5 degrees.
Locally, uranium mineralization is found in the Permian Cutler Formation. The Cutler formation in Lisbon Valley is composed predominantly of fluvial arkosic sandstones, siltstones, shales, and mudstones that were deposited by meandering streams that flowed across a flood plain and tidal flat. This flood plain was occasionally transgressed by a shallow sea from the west, resulting in the deposition of several thin limestones and marine sandstones. Wind transported sand along the shoreline of the shallow sea, forming dunes (Campbell and Mallory, 1979). The marine and eolian sandstones are usually finer grained, better sorted, and cleaner than the fluvial arkosic sandstones. The fluvial sandstones are medium to very coarse grained and have abundant feldspar and biotite.
The unconformity at the top of the Cutler has truncated the southward dipping Cutler beds, the mineralized sandstone bed at the Velvet-Wood Deposit is stratigraphically a few hundred feet above that at the Big Buck Mine in the northern end of Lisbon Valley.
The upper portion of the Cutler Formation, which is the primary host of known uranium mineralization in the Velvet-Wood Area, is composed of intervals of siltstone interbedded with thin-bedded, fine-grained sandstone. In places there are thicker, more resistant sandstone beds up to 47 feet thick. The thickness and frequency of sandstone beds increases downward, and siltstone is less common. Thick mudstone intervals separate the sandstone beds. A few limestone and conglomerate beds occur in the bottom third of the formation. The rocks are mostly greenish-gray, reddish-brown, or reddish-orange. The limestone beds are usually olive-gray (Campbell and Mallory, 1979).
Slick Rock Project
The Slick Rock district lies in the Paradox Basin at the southern edge of the salt anticline region also called the Paradox Fold and Fault Belt (Kelley, 1958). The district, which covers approximately 570 square miles of the Colorado Plateau, is underlain by about 13,000 feet of sedimentary strata which lies on metamorphic and igneous rocks of a Precambrian basement. The sedimentary formations range in age from Cambrian to Late Cretaceous (Shawe, 1970).
The Slick Rock project is located in the proximal Disappointment Valley syncline. The syncline plunges gently to the southeast and lies between the collapsed Gypsum Valley anticline to the northeast and the Dolores anticline to the southwest. Sedimentary rocks that outcrop in the Slick Rock district range from the Permian Cutler Formation up to the late Cretaceous Mancos Formation with a maximum thickness of approximately 4,700 feet (Shawe, 2011). The Jurassic Morrison Formation is the host of uranium/vanadium deposits in the Slick Rock district. It is widely recognized as an aggrading, terrigeneous clastic, fan-shaped fluvial sequence of sediments. While the precise location of the sediment source is unknown due to erosion, most authors agree that the sediment source area for the fan is the modern-day south-central Utah and north-central Arizona area (Page et al., 1956). The proximal fan is dominated by a high percentage of coarse clastics in braided stream sediments. The energy of the depositional environment decreases distally, as does the grain size of the sediments. The Slick Rock district occupies the medial fan facies. From the apex of the fan, the stream flow was in a northern, northeastern, and eastern direction. In the Slick Rock district, the direction of stream flow was generally to the northeast while local paleo topography controlled the flow direction.
Major folds in the Slick Rock district are broad, open, and trend about north 55 degrees west, and are parallel to the collapsed Gypsum Valley salt anticline which bounds the northeast edge of the district. The Dolores anticline lies about ten miles southwest of the Gypsum Valley anticline. The Disappointment syncline lies between the two anticlines (Williams, 1964).
Within the Slick Rock project area, the Morrison is divided into two Members: the upper Brushy Basin Member and the lower Salt Wash Member. The Salt Wash Member is composed of fluvial sandstone and mudstone averaging about 350 feet thick, and is further divided into three parts: the top and bottom units that are composed of fairly continuous layers of sandstone interbedded with thin layers of mudstone, and a middle unit that is primarily mudstone but contains scattered discontinuous lenses of sandstone (Rogers and Shawe, 1962 MF-241).
The following four geologic characteristics were indicative of favorable grounds for a uranium deposit:
• Most mineralized deposits are in or near thicker, central parts of sandstone lenses and, in general, the thickness of the sandstone decreases moving away from the mineralized deposits. Sandstone less than 40 feet thick is generally not favorable for large ore bodies.
• Sandstone in the vicinity of the mineralized deposit is colored light brown, but moving away from the mineralized deposit an increasing proportion of sandstone has a reddish color, which is indicative of unfavorable ground.
• The mudstone in the mineralized sandstone near and immediately below the deposit changes from a red to gray color. The amount of altered mudstone decreases further outward from the deposit.
• Sandstone in the immediate vicinity of the deposit contains more carbonized plant fossils than similar beds further away from the mineralized zone. This suggests that mineralization is localized in the vicinity of abundant carbonaceous material (Weir, 1952).
Mineralization in either case, tabular or roll deposits, averages about 0.25% U3O8 and 1.5% V205 within the impregnated sandstone. The mineralized bodies have an average thickness of 2 ft to 4 ft and range in size from a few feet wide to several hundred feet wide (Fischer and Hilbert, 1952). These deposits can contain a few tons of ore to several thousand tons in the larger ore bodies.
The uranium- and vanadium-bearing minerals occur as fine-grained coatings in detrital grains; they fill pore spaces between the sand grains and replace carbonaceous material and some detrital grains (Weeks et al., 1956).
The primary uranium minerals are uraninite (UO2) with minor amounts of coffinite (USiO4OH). Montroseite (VOOH) is the primary vanadium mineral, along with vanadium clays and hydromica. Metal sulfides occur in trace amounts. Mineralization occurs within tabular to lenticular bodies that are peneconcordant within sedimentary bedding. Mineralization may also cut across sedimentary bedding to form highly irregular shapes. The mineralized bodies have an average thickness range of 2-4 ft and range in size from a few feet wide to several hundred feet wide. The length can also vary from a few feet to several hundred feet. Secondary minerals: calcium uranyl vanadate (Tyuyamunite) (Ca(UO2)2(VO4)2 5-8(H2O)) and potassium uranyl vanadate (Carnotite (K(UO2)2(VO4)2 1-3(H2O)) occur in shallow oxidized areas and on outcrop.