Using JORC and CIM Guidelines for Uranium Resource Estimation
Undertaking a resource estimation on a uranium deposit is no different from any other estimate in terms of the methodology used, however one aspect unique to uranium deposits is the ability to use equivalent grade data obtained through down hole radiometric probing of drill holes to supplement chemical assay databases (equivalent grades are denoted by prefix “e”). Often, radiometric data provides the majority of the data in the database, with chemical assay only performed on a small percentage. The radioactive decay of uranium into various uranium isotopes and associated daughter products produces radioactivity that can be measured with an assortment of down hole logging equipment and probes. Down hole logging, such as Prompt Fission Neutron (PFN) and gamma techniques, play a vital role in terms of fast, low-cost drilling methods.
The radiometric data can be converted into equivalent grade data by determining an appropriate conversion factor, referred to as the “K factor”, for the type of logging equipment used and the characteristics of the borehole (i.e. open hole vs. cased hole, diameter of borehole, fluid filled vs. air filled borehole, etc.) in which the probing was conducted. The reliability of equivalent grade data must be demonstrated through rigorous protocols that address: continuous calibration of equipment, equipment testing prior to each use, and direct validation of radiometric data against chemical assay data. Contamination of the borehole can occur by the smearing of mineralized material along the borehole wall or drill rods through which the probing takes place, or by the diffusion of radon within the borehole. Twinning of drill holes should be undertaken where possible and chemical sampling should be carried out to confirm the radiometric logs. When the precision of the equivalent assay has been demonstrated, it may be merged with chemical assay data for the purpose of estimating the mineral resources.
Where radiometric logging has been used, the presence of uranium-bearing minerals should be established and relationships with uranium mineralogy identified along with associated gangue mineralogy from drill hole samples. Where equivalent grade data comprises the majority of the grade information for a deposit, it is essential to have good spatial distribution of chemical assay data across the deposit for validation purposes. By comparing the radiometric data with the chemical data, over the full grade spectrum, small-scale variability’s and overall sampling error can be determined. Validation of radiometric data against chemical assays is essential to ensure that contamination of the borehole was not introduced during the drilling process, and to determine the degree of disequilibrium that may exist within a deposit. Disequilibrium occurs between uranium and its daughter isotopes when there is an imbalance between the uranium content and the radioactivity emitted by a mineralized rock. A common cause of this phenomenon is the removal of more soluble uranium from a deposit through groundwater interaction, resulting in an overestimation of the uranium content based on the radiometric data.
Quality Assurance and Control (QA/QC) procedures applied to other commodities should be applied to uranium deposits. Quality control of radiometric data can only be achieved through a rigorous program of calibrating individual assaying and logging tools. Representative holes must be cored, radiometrically logged for calibration purposes, and rock samples collected to provide information on density. Bulk densities are important and have high significance in logging correction factors between the test models and the natural rock environments; however, it can be quite difficult to determine and the results may be quite variable in soft sediments.
If it has been determined that the uranium deposit should be mined using in-situ recovery (ISR) techniques, it is important to note that the reporting codes, such as JORC and CIM, have amended their reporting guidelines to take this method of extraction into consideration. Minimum mining width, cut off bulk density and dilution are less applicable to ISR; however, the weight of the assay is critical. Recovery is of particular importance in these environments, and factors which may affect this include permeability, porosity, hydrologic confinement, amenability of minerals to dissolution, and the ability to return groundwater to its original baseline quality. Metallurgical, stratigraphical, petrophysical, hydrological and geochemical studies are important, if not critical. For ISR operations the quantity, quality and recovery should be reported based on facts from field tests and trials. These factors play a large part in determining the classification using the JORC and CIM codes.
As with all other deposits, the spacing of holes for ISR deposits is determined by the formation, structure and continuity of the deposit; however, the porosity and permeability will also influence spacing, while ground water level, quality and transmissivity are critical to ISR projects.
With uranium once again becoming a favoured commodity, many countries continue to change their opinions and develop legislation concerning the mining of uranium. Renewed interest in uranium has grown with the drive towards sustainable energy and initiatives towards reducing emissions linked to climatic change. A gap in uranium exploration experienced during the 1980s and 1990s has led to many historical uranium discoveries being revisited. A great deal of historical data exists for many of these discoveries, all of which must be understood and tested before it can be accepted and used as the basis of an up-to-date resource estimation.
Having experience working with differing types of uranium hosts, SRK understands how these factors can affect a project and take them into account when classifying resource estimations using JORC and CIM guidelines.
Tracey Laight: firstname.lastname@example.org
Cliff Revering: email@example.com
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