by Richard Belcher, Consulting Geologist at RWB Exploration Ltd and James Cleverley, Global Products Manager-Geosciences at REFLEX Instruments Europe Ltd/IMDEX Ltd
Portable X-Ray Fluorescent (pXRF) instruments have been utilized in mineral exploration for almost 20 years and are a common sight in field camps and mine sites the world over. Approaches to their use generally fall into two groups: as a qualitative tool to produce relative values for internal review and decision making, or as part of a workflow to generate quantitative data, for internal review and external reporting. Many explorers switch between these two end member cases depending on the requirements of the program and the nature of the decisions needed. Portable XRF technology offers users a rapid way to generate a chemical analysis that covers most of the critical elements of interest. The quick turnaround time for results means that there is potential to bring forward critical milestone decisions but also add dynamism to the project, allowing decisions to be made when they are the most valuable. The advances in pXRF technology mean that with modern instruments data quality is no longer related to the hardware but instead relates to the operation, data and workflow management and the quality of the sample.
Accuracy (how closely the analyzed value represents the true value) and the precision (repeatability of analysis) are the two key factors that are commonly discussed during reviews of the use of pXRF. The former is now significantly improved through technological and software advancements in the instruments. This has widened the spectrum of elements that can be analyzed, reduced Limit of Detection (LOD) and resolved element peak overlap and spurious values. In addition, instrument calibration and internal Quality Assurance-Quality Control (QA-QC) procedures have also helped address accuracy.
Precision, however, is primarily a function of the material presented to be analyzed, which for pXRF analyzers is a function of the samples grain size, moisture content, density, smoothness and/or airgap and homogeneity. The process of sample preparation and its presentation for analysis is often overlooked. It does not matter how comprehensive and rigorous the QA-QC procedure and the instrument calibrations are, this will not address precisions issues. The misunderstanding of these factors and how they contribute to the perception of poor-quality data by pXRF analysis lead to user frustration and the erosion of the value the technology can add to exploration programs. It is the inconsistency and noise in the data (related to precision) that is a far greater barrier to the routine adoption in mineral exploration now than lower ppm detection limits or speed of analysis.
For companies looking for qualitative or semi-quantitative results, then a workflow that includes some basic sample preparation or multiple sample analysis is common. Simple hand crushing to create a more representative sub-sample and/or multiple analyses of the sample and then using an average value, are examples of the attempt to mitigate these problems. These are common approaches in sieved soil and stream sediment samples, but variations of this approach can also be done on drill chips samples and the fines produced from sample cutting. However, issues around sample heterogeneity and presenting an ideal sample (fine-grained, dry, dense) still need to be addressed.
For companies looking for more representative and quantitative values, then the above approach needs refinement. The way to address this is to incorporate sample preparation and produce homogeneous, fine-grained, dense samples to present to the pXRF analyzer. In-house crushing and grinding of the samples using a hammer followed by a pestle-and-mortar partly addresses some issues, but not all. This approach comes down to a trade-off between the extra time and effort it takes to prepare the samples (and thus ultimately the cost) to the improvement in the results obtained. The use of a sample preparation facility is ideal, but logistically inconvenient if none is available nearby, and again the cost and time required to send samples, the facility to prepare and return needs to be taken into consideration. Again, this is a trade-off between the extra time, effort and cost and the improvement in the results.
Commonly, the method used is a function of the aim of the program and tailoring the procedure to fit the perceived deliverables needed. However, a data-driven exploration strategy that generates quality reliable data at all stages of the project lifecycle should be part of all exploration strategies. For example, collecting quality pXRF data supports lithogeochemistry (geological models, rocks types/sources, alternation and trends), element associations (both for use as pathfinders and geometallurgy) or could identify an association/feature not considered and thus change the perception of the project. This approach increases the understanding of the project and adds value by allowing more informed decisions to be made and the potential to bring key decision points earlier in the project lifecycle.
A new approach now available is the use of portable sample preparation equipment, which allows the quick preparation of samples on-site, and thus produces high quality, repeatable and auditable assay results with a pXRF to support the above data-driven strategy. REFLEX Instruments, one of the leading IMDEX brands, produce equipment that is specially designed for preparing the best possible samples to present for pXRF analysis. The equipment is portable, and is in three parts: a crusher for rock chips, <2 cm3 (<0.12 in3) producing <2 mm (<0.079 in) sample, a mill for pulverizing (produces 90% passing 100 microns), followed by a puck press for producing a dense sample which can then be analyzed with a pXRF (or another sensor). All the equipment comes in its own, ruggedized storage box. The equipment can be set up very quickly, anywhere with a power source, e.g., in a field camp running off a generator, back at an office or storage facility. QA-QC and instrument calibration is managed through REFLEX XRF CONNECT which connects directly to the Olympus VANTA pXRF. This data can then be uploaded to IMDEXHUB-IQ for on-line storage and remote data access/monitoring to allow, for example, the supervising geologist to review the data as it is produced when not on-site, and the data to be added into the company’s database as soon as it is validated and approved.
This process of hardware and software provides not only robust and reliable quantitative data, but it is also quick, cheap and available onsite wherever required. This data can be made available securely, anywhere, in a very short turn-around to aid decision-making and make programs more dynamic. The benefits of this in exploration are obvious, where programs are not rigid but adjust to new data as it becomes available, and real-time decision making is possible.
A commercial trial of the complete set-up was recently undertaken in the UK on rock chip and soil samples, which were taken as part of an early-stage exploration project. Rock chips were broken into smaller, manageable pieces with a lump hammer before going through the sample preparation procedure (crushed, milled and a press puck produced). Soil samples were initially dried and then sieved to remove larger stones and organic material and then milled and pressed. In this trial, a team of two undertook the sample preparation and analysis, completing approximately 50 samples per day, although optimization of the process is possible. This set-up produced a much larger robust dataset than what would have been produced following some of the approaches above, including geochemical data to support rock type classification and potential alteration styles, as well as trace element associations. Not only does this support the data-driven approach, but the data was available only a few days after the samples were taken allowing key decisions to be made during the program.