A Review of Portable X-ray Fluorescence (pxrf) Applications for Exploration Geology

A pXRF spectrometer is a non-destructive testing equipment that can be used to detect and measure the concentration of elements in substances. pXRF works by striking a sample with x-rays instead of UV and it fluoresces by giving off lower energy x-rays. It is a well-established analytical technique for determining the chemical composition of a sample (Young et al. , 2016).

However, it should only be considered as a field tool for collecting element concentrations at the site while evaluating potential geological causes for the responses (Glanzman & Closs, 2007). Appropriate additional and more representative samples can then be collected for confirmatory laboratory analysis (Glanzman & Closs, 2007). According to Young et al. 2016 the handheld, battery-powered instruments are generally less than 4 W (versus the 50e100 W of power required to operate laboratory instruments), making the instruments safe to handle. The lower beam energies associated with the handheld configuration, makes it more challenging to detect and measure the lighter elements.

Silicon drift detectors, used in pXRF technology, have large surface areas and better resolutions than their alternatives (i. e. Si-PIN and CdTe detectors), meaning that they can differentiate between the broader X-ray peaks as well as detect and measure lower energy X-rays (Longoni et al. , 1998). pXRF has improved dramatically in recent years, due to advances in the semi-conductor industry, miniaturization of components such as the X-ray tube and the development of sensitive detectors such as the silicon drift detector (Hall et al. , 2014).

Nowadays, pXRF is used in many fields of studies including agriculture, archeology etc. It is now used to collect high spatial density, low cost data in numerous exploration settings and for grade recognition (Hughes and Barker, 2018). The advantages of pXRF include the smaller size (portability), lower cost, and faster data collection, making it suitable for on-site lithogeochemical analysis (Cao et al. , 2016). An advantage is that the user can also deploy the pXRF to triage samples, allowing them to select a sample suite tailored to their field and scientific objectives for return for future laboratory work(Young et al. , 2016). By deploying the pXRF in the field, the field geologist can streamline field operations, gain a real-time geochemical understanding of a field site, select which samples to collect, save time and money on laboratory analyses, and potentially minimize future return trips to the field site (Young et al. , 2016).

Early-stage exploration projects are often very budget-sensitive and being able to efficiently collect data and draw significant conclusions from pXRF data is important. The pXRF does not directly identify deposits, instead it detects geochemical pathfinder, trace or indicator elements that are associated with epithermal and base metal deposits. A pathfinder/indicator is a mobile element that is closely related to an ore mineral body. This report reviews the utility of the pXRF based on an examination of the accuracy and precision of this technology. It also highlights the limitations and best deployment practices in an effort to evaluate its utility in field geologic applications.

Calibration

Calibrations are necessary in order to improve precision and accuracy.

On most instruments, pXRF calibration is not the same for trace (soil mode) and major (mining mode) concentrations. The soil mode provides broad and easy coverage of low concentrations, and quantification is user calibration using the mining mode(Lemière, 2018). Some instruments provide a hybrid mode (Geochem) which provides semi-quantitative data over a larger range. This mode is useful for fast screening. Some elements may be available through only one mode, according to the manufacturer's calibration scheme. If both trace and major level concentrations are desired, a change of mode is needed without moving the analyser between two measurements.

This is longer and not very easy, but it improves accuracy (Lemière, 2018)Effect of sample preparation on accuracy and precisionFor accuracy and precision pXRF data where compared to laboratory data, as well as certified reference material. In most cases accuracy was limited by poor sample preparation, chemical matrix effects, unchecked element interference and sample matrix homogeneity. The poor sample preparation resulted from point-and-shoot measurements of the pXRF in contrast with laboratory homogeneity of laboratory instruments. Studies show that pXRF could provide accurate analyses if the sample preparation method was the same as for the laboratory (Hall et al. , 2012 as cited Lemière, 2018).

Decision making

Decision made using lab results interpretation are normally based on few samples because of high analyses costs. This increases sampling errors despite even with having low analytical uncertainty. Using pXRF with a large number of samples lowers analytical uncertainties and decreases sampling errors, as long as accuracy is monitored. The confidence level of the decision will be higher than with a small number of lab samples (Figure 3).

Samples with large concentration differences from the threshold values for given elements allow on-site decisions to be made without having to wait for laboratory results (Lemière, 2018). ConclusionAn increasing body of literature has shown that pXRF data is both reliable and robust (Durance, Jowitt, & K, 2014); Andrew and Barker, 2018; Young et al. , 2016). pXRF is used in mining for ore grade analyses. It is also used in exploration for fast site diagnoses, investigating anomalies. The portable instruments is sensitive to heavy elements, and in the case of light minerals the pathfinder, indicator minerals are explored instead. pXRF has high analytical uncertainties but with proper monitoring of the QA/QC schemes they are deemed highly reliable. It provides cost-effectiveness and it can be used to make rapid reliable decisions in the field.

Studies comparing pXRF data to laboratory data, yielded similar results. To further increase accuracy of pXRF it was concluded that if the pXRF samples should be homogenous rather than the shoot and point method. While laboratory analyses will remain the standard for providing the highest quality data possible, these measurements can be costly, and require intensive sample preparation and analysis.