Whole rock geochemistry and comparative leachability of variably bleached and red sandstones of the Permian Cutler Formation, Mesa County, Colorado

Forty samples of outcropping rock samples were collected from the Permian Cutler Formation exposed in the Sinbad Valley, Mesa County, Colorado by USGS volunteer Jon Thorson. This sample set includes 13 pairs of co-existing red unaltered and yellow-grey bleached sandstone. The decimal latitude and longitude locations of the sample collection sites are listed in the “All_Rock_Data” worksheet in the accompanying “All_Sinbad_Rocks” workbook. The samples were collected from rocks exposed on the eastern edge of a breached salt-cored anticline within about 1 to 100 meters from an active seep from which sulfurous brines supply water to Salt Creek. The co-existing red/bleached samples were collected within the same stratigraphic bedding plane, a few centimeters apart across the contacts between red and bleached tan to grey sandstones (Figure 1) as part of an effort to characterize the geochemical impact of bleaching on originally red sandstones. The study was designed to better understand the geochemical effects of bleaching of potential source rock for uranium and vanadium deposits on the Colorado Plateau as part of the development of a robust deposit model in support of the assessment of undiscovered uranium resources in this region.
Each sample was crushed, ground, and geochemically analyzed by AGAT laboratories using inductively coupled plasma-optical emission spectrometry (ICP-OES) and by inductively coupled plasma-mass spectrometry (ICP-MS) analysis. At AGAT labs, samples were fused at 750oC with sodium peroxide, and the fusion cake was dissolved in nitric acid resulting in total dissolution of samples. The results of the geochemical analysis are summarized in the “All Sinbad Rocks” workbook. Summary statistics, correlation, t-test, principal component, and factor analysis of the samples was completed using the XLSTAT Excel data add-on software program (version 2019.4.2.63677 www.xlstat.com). These analyses are presented in workbook “Sinbad_Rock_Statistics”. At the 95% confidence interval, t-tests show that the difference between the mean values of uranium for red and bleached rock are statistically significant, with more uranium in the bleached subsample set. Other elements show no statistically significant variation between the mean concentrations of the elements.
Sample splits of the 13 bleached/red paired rocks (26 total samples), were then leached using a 0.10 M NaHCO3 solution. This solution was chosen to: (1) attack loosely bound (ion-exchangeable or weakly sorbed) U and V; (2) provide an ionic strength buffer (pH 8.3); (3) provide a strong complexing agent to stabilize U in solution as uranyl carbonate or uranyl-Ca-carbonate complexes; (4) stabilize oxyanion species such as vanadate in solution; and (5) ensure that solutions approach equilibrium with calcite which is observed in the sandstones. Two grams of minus 200-mesh powder were combined with 20 ml of leach solution in plastic centrifuge tubes. The tubes were capped and shaken continuously for 24 hours on the benchtop (Figure 2). Additional two-gram aliquots of one bleached sandstone (sample 228558) were leached for periods of 1, 2, 4, 8, 24, 48 and 96-hours to compare to results from the 24-hour experiment. Mixtures of solid and leachate were centrifuged at 2500 revolutions per minute for 5 minutes, and liquids were withdrawn with syringe and filtered through 0.45-micron cellulose acetate syringe filters (18.5 ml of liquid was recovered). Solutions were acidified to pH &lt;2 using purified nitric acid and submitted for analysis by ICP-MS or ICP-OES, the method depending on the element (see Data Dictionary worksheet). 
The concentration of dissolved uranium in leachates of bleached rocks was consistently either similar to or greater than in leachates from red rocks. Vanadium results were not as consistent. Five bleached samples with visible sulfide yielded virtually no leachable V. For other bleached samples leachable V was similar (n = 1) greater (n = 4) or lesser (n = 3) than from red samples.
Figure 1. Example of bleaching of red sandstones in the Cutler Formation, Sinbad seep area, Mesa County, Colorado. Thirteen red/bleached sample pairs were collected from equivalent red/grey sandstone beds similar to those shown in this image. 
Figure 2. Centrifuge tubes being continuously shaken on a benchtop during the leaching portion of the study.

Tags: bleaching, colorado, geochemistry, geoscientificinformation, icp-ms-inductively-coupled-plasma-mass-spectrometry, icp-oes-inductively-coupled-plasma-optical-emission-spectrometry, mesa-county, red-bed, sandstone, sinbad-valley, uranium, usgs-627a57fcd34e8d45aa6e4e02, vanadium
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