Handheld X-ray fluorescence (XRF) analyzers are now commonly used during exploration to rapidly evaluate a property’s rocks, soils, drill cuttings and cores in real time. These units can help confirm mineral deposit models, assist in recognizing new or unexpected types of mineralization and enable on-the-spot decisions to extend or infill drill holes. Occasionally, handheld XRF analyzers may provide significant and potentially material information about mineralization on a property. However, the qualified person (QP) preparing a disclosure must exercise professional judgement and be able to demonstrate the reliability of the information.
Because of sampling issues and other limitations of the handheld XRF method, there are concerns about the potential misuse of its results in a mineral resource database. Regulators generally deem handheld XRF results to be unsuitable for use in such a database.
It is important to remember that handheld XRF data do not replace traditional laboratory-based analysis. They do, however, provide an effective screening tool for selecting samples for traditional analysis. If a sufficient number and range of conventional analyses validate the accuracy of the handheld XRF analyzer, its results may be suitable for disclosure.
Under sections 3.2 and 3.3 of National Instrument 43-101, companies are required to include certain information pertaining to their exploration sampling and analytical results. This includes information about the nature and quality of sampling, factors that could affect the accuracy and reliability of data, the type of analytical or testing procedures, quality assurance programs, quality control measures applied and whether or not the data have been verified.
When exploration results based on handheld XRF analyzers are significant enough to require public reporting, it is essential to:
- Clearly indicate that results are based on data from a hand-held XRF analyzer.
- Make a balanced and non-misleading disclosure, providing results as a range and distribution of sample values as a measure of prudence.
- Include cautionary language regarding limitations of the hand-held XRF method, such as the sampling window, homogeneity of mineralization, penetration depth, possible surface effects, etc.
- Include a statement that results are subject to confirmation by chemical analysis from an independent laboratory and consider giving an estimate for the timing of confirmatory data.
- Provide information about the total number of samples, sample media and additional sample details, if applicable (float or bedrock, selected, random, representative of what dimensions, true width, etc.).
- Include the following in the disclosure:
- the analyzer name, model and precision, if available;
- the operator(s) of the unit and their relationship to the company; and
- a summary of the sampling procedures (site selection, whether samples are prepared or “in situ,” averaging procedure, treatment of background or very high readings, etc.).
- Discuss variances between laboratory and handheld XRF results and explain that any adjustment factors applied to the XRF results may be required in a followup news release, once sufficient confirmation laboratory results are available. Formal clarification or restatement of XRF results is required if assay data do not corroborate the previously disclosed results.
Before planning any handheld XRF sampling program, companies and QPs must understand the sampling limitations of the handheld XRF method and consider how to handle potential future disclosure of the results. If there is a breakdown of correlation between handheld XRF results and laboratory data, particularly in potentially economic grade ranges, the XRF results are unlikely to be suitable for public disclosure.
Unlike laboratory analysis, there is no hard backup, such as an independent “Certificate of Analysis,” to facilitate audit of the data. The suitability or otherwise of handheld XRF data for disclosure will depend upon verification of the reliability of the results based on a sufficient number and distribution of standard laboratory analyses.
Craig Waldie is a senior geologist with the Ontario Securities Commission Ian McCartney is a senior geologist with the British Columbia Securities Commission and reviews technical and scientific disclosure by mining and mineral exploration issuers.