Portable X-ray fluorescence measurements on exploration drill-cores: comparing performance on unprepared cores and powders for ‘whole-rock’ analysis

Abstract

One geoscience application of pXRF technology is acquiring 'whole-rock' analyses of unmineralized or weakly mineralized rock cores for major oxides and trace elements, to fill the gaps between traditional laboratory analyses and/or obtain geochemical data more quickly. But the question of whether the samples actually need to be crushed and pulverized before analysis to produce useful results has not been extensively studied. In this paper pXRF data quality is compared on unprepared rock cores and on powders in three ways: instrumental precision (relative standard deviation, RSD, of a series of measurements on the same spot), sample precision (for unprepared samples, RSD of a series of measurements on different spots on the core), and accuracy (average pXRF value versus laboratory geochemistry). Two Olympus Innov-X Delta Premium pXRF devices were tested on 27 core samples of dense, non-mineralized, fine-to medium-grained, Precambrian volcanic and intrusive rocks from Canada. In general, sample preparation does not improve instrumental precision or accuracy. The significant advantage of powders is to avoid mineralogical heterogeneity. However sample precision for in situ data is improved by averaging multiple measurements of different points on the sample: a significant gain is obtained between three and seven measurements. The sample precisions at 25 points -which is about the most measurements one can make during the same amount of time used for powdering a rock core sample -are better than the instrumental precision on powders for most elements. For high spatial resolution downhole element profiles on entire drill holes, in situ pXRF measurements with smoothing (e.g. three to five point moving averages) provide fit-forpurpose data; the alternative of turning the entire drill-core into powder is not realistic.. INTRODUCTIONDiamond drilling is a major component of advanced mining exploration programs. Two types of traditional laboratory geochemical analyses are often performed on exploration drill-cores by the mining industry, geological surveys and university researchers: (1) assays of mineralized or potentially mineralized samples, for elements such as base metals, precious metals, rare earth elements, etc. (e.g. Moon et al. 2006); and (2) 'whole-rock' analyses of unmineralized or weakly mineralized samples for major oxides and trace elements (e.g. Ross 2010; Mercier-Langevin et al. 2014;Rogers et al. 2014). The first type of analysis is carried out within selected intervals, often on c. 1 mlong sections of split or cut cores, to quantify the grades of orebodies. The second type of analysis can be

Geochemical sourcing of volcanic materials imported into Teti'aroa Atoll shows multiple long‐distance interactions in the Windward Society Islands, French Polynesia

“…Overall, the analyses of the samples’ surfaces with pXRF provided comparable results with those using ICP‐AES, especially when analysing the fine‐ to medium‐grained volcanic rocks and averaging multiple measurements on different spots of a clean surface of the artefact. These results are in agreement with other assessments of the analytical accuracy of this technique (Bourke & Ross ). However, the analyses of coarse‐grained material and/or weathered surfaces are more divergent (especially in the measurement of MnO, Sr and Y), which is probably related to textural variation and chemical weathering as shown in the online Supporting Information Figure .…”

A review of pXRF (field portable X-ray fluorescence) applications for applied geochemistry

“…For till samples, and Sarala (2016) used three repeated measurements to increase accuracy and to diminish matrix and heterogeneity effects. Bourke and Ross (2016) reported that repeating measurements (3 to 7 times) at the surface of a core produced the same accuracy as milling, for fine to medium-grained metavolcanics.…”

The Influence of Spectral Interferences on Critical Element Determination with Portable X-Ray Fluorescence (pXRF)

“…Consequently, critical elements can be detected and reliably quantified in geological materials devoid of interfering elements or when enriched. Generally, pXRF data need to be corrected or re-calibrated to yield accurate results, which is mostly achieved through linear correlation (e.g., [3,5,10,[25][26][27][28]. This is frequently done without checking the energy spectra corresponding to pXRF measurements for spectral interferences and, in particular, for false positives.…”