The rare element uranium (U) has an average concentration of 2.7 ppm on the earth’s crust.
U primarily occurs as U-238 in over 99 % of cases, while its fissionable counterpart, U-235, makes up only around 0.7 %, and U-234 represents less than 0.1 % of natural U.
U’s large atom size typically prevents it from entering the early-crystallizing minerals found in magma. It tends to be concentrated in final magma crystallization products found in silica-rich rocks like granites, most notably in the minerals present within granites such as sphene, zircon, and apatite.
U in these rocks can potentially reach tens or even hundreds of ppm, resulting in the formation of a mineral called uraninite or pitchblende (UO2).
Several methods of U ore mining exist, including open pit, underground, in-situ leaching, and borehole mining via high-pressure water jets.
Low-grade U ore is generally comprised of 0.01 to 0.25 % uranium oxides (typically reported as U3O8), but deposits can exhibit U concentrations as high as several percent.
U occurs in a handful of deposit types, including vein deposits, Olympic Dam-type deposits, unconformity-related deposits, quartz-pebble conglomerate deposits, sandstone deposits, intrusive deposits, surficial deposits (calcrete deposits), volcanic and caldera-related deposits, and metasomatite deposits.
The naturally occurring radioactive chemical element Thorium (Th) is also fissionable. This element is around three to four times more abundant than U in the earth’s crust (9.6 ppm) and is typically refined from monazite sands as a by-product of rare earth metal extraction.
Handheld XRF Analyzers in Mining
Handheld XRF (HHXRF) analysis delivers accurate, rapid results with little or no sample preparation. This useful technology applies to various stages of mining activity, from grassroots exploration, exploitation, and ore grade control to environmental investigations.
Thermo ScientificTM NitonTM Handheld XRF analyzers can be used to investigate a wide range of elements from magnesium to U.
The instruments’ high-power X-ray tube is coupled with dynamic current adjustment functionality and silicon drift detectors, allowing users to benefit from rapid data acquisition times while maintaining low detection limits and high accuracy.
When combined with a test stand, these analyzers can be used on prepared samples in the lab, and results can be verified using certified reference materials (Figure 1).
Figure 1. Verification of U and Th measurement accuracy on powdered ores and soil-certified reference materials. Image Credit: Thermo Fisher Scientific – Handheld Elemental & Radiation Detection
Method
This study was performed in the Nopal area of northeast Mexico by geologists from Servicio Geologico Mexicano. Systematic analysis was completed on a total of 1,334 samples in 31 stations on 40 linear sections with 100 m spacing.
Each sample was prepared, and its U content was measured using the radiometric method, laboratory analysis, and handheld XRF analysis.
Scintillometers were used in the radiometric method. These instruments are widely employed in U-Th exploration, and some models can provide semi-quantitative data. Figure 2 shows the geochemical maps of U based on these three methods.
Figure 2. Geochemical map of U and Th measured by radiometry, lab, and field portable XRF. Image Credit: Thermo Fisher Scientific – Handheld Elemental & Radiation Detection
Results
There was a significant overlap between the geochemical anomalies of U identified by lab analysis and HHXRF analysis (Figure 2). While HHXRF analysis features the same anomalies for Th as for U, sufficient laboratory data was not available for comparison.
Scintillometers are highly effective tools for the rapid identification of radioactive samples, but their data may not be as dependable as HHXRF or lab analyses due to their U and Th anomalies not overlapping with those from HHXRF or lab analyses. Scintillometers have the potential to be more effective when employed in the field in conjunction with HHXRF.
Acknowledgments
Produced from materials originally authored by Thermo Scientific.
This information has been sourced, reviewed, and adapted from materials provided by Thermo Fisher Scientific – Handheld Elemental & Radiation Detection.
For more information on this source, please visit Thermo Fisher Scientific – Handheld Elemental & Radiation Detection.