Characterization of U(-V) deposits in the La Sal district, UT and CO and their relationship to Paradox Basin fluid flow

A new examination of tabular uranium (U)-vanadium (V) deposits within the La Sal district places mineralization in the context of the complex fluid flow history that has occurred in the Paradox Basin and Colorado Plateau. As the only currently producing district of this deposit type in the region, the La Sal district provides excellent opportunities to study deposit- and district-scale features that can serve as exploration guides where exposure is limited. The absence of major structures along a narrow and elongate mineral trend that runs E-W across the southern part of the Utah-Colorado border indicates a regional lithologic control. The trend was likely influenced by Jurassic drainage patterns, as indicated by paleocurrent directions shown in Fig. 1. Fluvial sandstone host rocks in the Jurassic Morrison Formation contain significant evidence of multiple superimposed fluid systems that have progressively changed local mineralogy. Apart from the presence of ore minerals, remnants of fluid systems are preserved as cements including quartz overgrowths, compositionally distinct carbonates, and sheet silicates. New geochronology methods provide constraints on the timing of initial mineralization and later remobilization. U-Pb ages of calcite associated with ore minerals provide evidence for Early Cretaceous mineralization, whereas textures and K-Ar ages show that early-formed V-minerals were replaced at the expense of V-bearing micas and clays during the Eocene. The paragenesis of minerals in U(-V) deposits of the La Sal district reflects the complexity of a sedimentary basin with more than 300 million years of fluid flow history. Introduction Attention directed toward low-carbon energy sources is renewing interest in U mining due to the commodity’s use as nuclear fuel. An important domestic supply of U is derived from tabular, sandstone-hosted deposits that are characteristic of the Paradox Basin and Colorado Plateau. The Uravan mineral belt of southwestern Colorado has been a historic principal source of U with significant V byproduct characteristic of this deposit type. Although the adjoining La Sal mining district contains many shared characteristics, it runs orthogonal to the Uravan belt and provides a useful contrast to study the area’s mineral endowment (Fig. 1). This combined with the availability of new field and analytical methods since the last true district-scale study makes the La Sal district highly suitable for reexamination. Fig. 1 Generalized geologic map of the Uravan mineral belt showing outcrops of the Morrison formation, paleoflow directions, and millions of pounds of U3O8 produced from various districts within the region. Background The Paradox Basin is a late Paleozoic flexural foreland basin that contains large volumes of evaporites due to arid environments in the Pennsylvanian. In addition to the abundant salt that is characteristic of the region, overlying terrestrial and marine sediments are host to a variety of natural resources including lithium-rich brines, hydrocarbons, copper-silver deposits and, of course, U(-V) deposits. All are a result of fluid-rock interactions that have been ongoing for the last several hundred million years. Apart from these unusual accumulations of commodities, the basin also contains extensive areas where red rocks have been bleached white by fluid passage. The spectacular color variations draws in millions of visitors each year for viewing on public lands. Bleaching, or the removal of red ferric iron phases, is just one instance of how fluids can significantly alter their surroundings with or without formation of ores. This study synthesizes observations related to bleaching and other alteration styles through detailed mapping, petrography and chemical analyses to relate them with prominent U(-V) deposits found in the La Sal district in the central part of the Paradox Basin. Character and distribution of Salt Wash sandstones and mineralization in the La Sal districtU(-V)-rich sandstones in the La Sal district are representative of those found throughout most of the Colorado Plateau, chiefly as tabular bodies hosted in fluvial sandstones at high stratigraphic levels of the Late Jurassic Salt Wash Member of the Morrison Formation. Recent drilling at the La Sal Mine by Energy Fuels Inc. provided for core-based observations of mineralogy and its distribution in space. The Salt Wash contains multicolored sandstones ranging from red/pink to white (see full paper for photo of the rocks representative of the different lithologies). Black or dark gray ore bodies are intimately associated with white sandstones. The distinguishing features between the different hues lies in their cement mineralogy, as described in Table 1. Notable paragenetic textures among ore and gangue minerals includes the intergrowth of U- and V-minerals with quartz and calcite, ferroan dolomite overgrowths on calcite, and the replacement of primary V-minerals by vanadian sheet silicates that are in turn replaced by supergene uranyl-vanadates. Gangue mineral distribution at the Morning Star Mine in La Sal Canyon varies in space depending on proximity to ore. For example, carbonates are less abundant or absent in the ore body in contrast with quartz cements that are volumetrically important within ore. Additional underground mapping is needed to examine these and other relationships in three-dimensional unweathered exposures. Though sandstones are the primary ore host, they are separated by thick beds of mudstone that can be red, green, gray or black. Red mudstones are commonly found adjacent to pink or red sandstones, and green mudstones with white sandstones. Like their coarser-grained equivalents, black mudstones contain ore. Novel geochronology methods were used to constrain the timing of different minerals’ formation. In situ U-Pb dating of calcite intergrown with coffinite yielded an age of 143.8±5.6 Ma. These Early Cretaceous ages are consistent with other geochronology of ores elsewhere on the Plateau. Multiple size fractions of V-illite-bearing ore produced two populations of ages: 41.5 to 36.6 Ma and approximately 4.6 Ma. The Eocene and Pliocene ages likely reflect formation of V-illites and uranyl vanadates, respectively. The Eocene ages parallel that of widespread bleaching and faulting within the Paradox Basin. Conclusions Mineral assemblages found in and around U(–V) ore bodies of the La Sal mining district contain several superimposed styles of alteration that vary in space and time. In tabular sandstone ores, paragenetically young pyrite indicates that redbeds were reduced to form sulfides from ferric oxides. Shortly thereafter, precipitation of U and V minerals occurred simultaneously with formation of abundant quartz and small volumes of calcite. Later ferroan dolomites are restricted in space, primarily in unmineralized sandstones where they are overgrown by illite, chlorite and kaolinite. V-rich sheet silicates with similar paragenetic textures are only present in ore bodies. Considering the similarity among different types of U(-V) deposits on the Colorado Plateau, the spatiotemporal observations made at La Sal can be used as first-order guidelines for exploring in areas with little to no exposure. Geochronology provides absolute ages for ore-stage calcite and V-bearing illite that demonstrate phase changes of primary to secondary ore minerals separated by about 100 million years. References A list of all references is available in the full paper.

Characterization of U(-V) deposits in the La Sal district, UT and CO and their relationship to Paradox Basin fluid flow

Authored by: Eytan Bos Orent, Mark D. Barton and Isabel F. Barton

To view this article you must be a member of SME/UCA.

Mining Engineering’s Weekly Rundown: Episode 73

Get your SME Foundation VIP Access Conference Pass to MINEXCHANGE 2027

“Check the box” and help support the SME Foundation

Mining Engineering’s Weekly Rundown: Episode 73

Get your SME Foundation VIP Access Conference Pass to MINEXCHANGE 2027

“Check the box” and help support the SME Foundation

To view this article you must be a member of SME/UCA.

Challenges in the battery raw materials supply chain: Achieving decarbonization from a mineral-extraction perspective

A novel job similarity index for career transition in the mining industry

Related Articles

Rare earth elements in the global economy: Usage, recovery and the quest for supply security — A review

A systematic evaluation of localization signals in the advancing openpit mine Aitik

From foundation to future: Revisiting AI integration in mining engineering education through current perspectives of students, educators and industry

Iron ore tailings reprocessing by electrostatic separation plus two-stage froth flotation

Mining Engineering’s Weekly Rundown: Episode 73

Get your SME Foundation VIP Access Conference Pass to MINEXCHANGE 2027

“Check the box” and help support the SME Foundation

Mining Engineering’s Weekly Rundown: Episode 73

Get your SME Foundation VIP Access Conference Pass to MINEXCHANGE 2027

“Check the box” and help support the SME Foundation

Use Single Sign On