Gamma Probe eU Data QA-QC Procedures

The most common method used to determine in-situ uranium grades from drillholes is down-hole gamma logging, and the conversion of probe-measured counts-per-second (CPS) to “equivalent” in-situ uranium grades (eU% or eU3O8%). Although it is an indirect measurement of uranium grade that requires diligent attention to calibration and correction factors, the eU-determined grade is essentially equivalent to any other analytical method, and should have quality assurance and quality control (QA/QC) protocols in place to guarantee the accuracy and precision of results. Understanding gamma-log QA/QC procedures and providing recommendations for implementation provides SRK’s clients with better quality information for their uranium database.

Standards: Where the analytical process for chemicals would incorporate standard reference materials (SRMs), the gamma probe’s equivalent would be the probe calibration report from a known drillhole in a specially-constructed test pit. These calibration holes are typically located at government-run nuclear testing facilities. The U.S. Department of Energy maintains a calibration facility for radiologic instruments in the western U.S. at Grand Junction, Colorado. A calibration report from the facility will provide the K-factor (conversion factor) for determining eU% grade, and other correction factors for the specific gamma probe in use. Primary calibration should be done on a regular basis; yearly as a minimum. Secondary forms of calibration can be carried out in the field at the project site, and should be a regular part of the gamma-logging process. Portable calibration sleeves containing a low-level radioactive source, such as thorium, can and should be used before logging each drillhole. In addition, where possible, a control hole with mineralization should be designated for each project as a hole to remain open for re-logging at the beginning of each day the probe is in operation. These secondary forms of calibration allow the detection of instrument error or instrument drift, and provide verification of depth readings.

Blanks: A drillhole in known barren rocks can provide a measure of background radiation and a check of the gamma probe instrumentation. A control hole with mineralization can often act as a hole that measures background radiation as well.

Duplicates: Duplicate assay equivalents are essential, and should include:

• Re-logging a percentage of drillholes with the same gamma-probe (duplicates), and
• Re-logging a percentage of drillholes with a different gamma-probe, by the same contract logging company (replicates).

Outside (secondary lab) checks include:
• Re-logging a percentage of drillholes with a third-party gamma-probe, using a different contract logging company (outside check on duplicates), and
• Re-logging a percentage of drillholes with a spectral probe (K, U, Th), providing verification of grade from uranium.

Other checks include:
• Prompt fission neutron (PFN) logging a percentage of drillholes for a) verification of uranium grades, and b) state of equilibrium in comparison with gamma logs, and
• Globally comparing eU data distribution from gamma logging with the global chemical assay-data distribution of drillhole samples. Both sets of measurement represent the deposit, but by using different sampling methods and different sample volumes, it is possible to achieve a comparative check on methodologies and an indication of equilibrium.

Equilibrium: A state of equilibrium, or the ratio of chemical U to radiometric U (U/eU) for the same sample volume, is best done on core or reverse circulation (RC) samples. A common method is called “closed can” radiometric analysis, where a sample is allowed to equilibrate in a canister for approximately 3 to 4 times the half-life of radon gas, and the radiometric eU is, therefore, back-calculated and compared to an ICP or XRF analysis for the sample.

Allan Moran: amoran@srk.com



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