Portable X-ray fluorescence (pXRF) analysers are increasingly popular tools for geoscientific applications, including mineral exploration. One promising application, illustrated in the companion paper, is to obtain high-spatial resolution down-hole geochemical profiles using pXRF on unprepared exploration drill-cores. However, the precision and accuracy of pXRF analysers on such samples is not well studied. We have tested three Olympus Innov-X analysers, both on a sediment standard (NIST 2702, ‘Inorganics in Marine Sediment’) and in-situ on unmineralized rock cores from volcanic and intrusive, mafic to felsic lithologies. We conclude that pXRF is quite precise for a number of elements, but not very accurate using factory calibrations. For example, the 1σ precision of one Delta Premium analyser tested on a basaltic core, in mining plus mode, with a 60 s integration time, is better than 5 % for Al, Ca, Fe, K, Mn, S, Si, Ti, Zn and Zr. The same analyser, tested on a range of volcanic and intrusive core samples, yielded the following average systematic errors: Al -23 %, Ca -4 %, Fe +1 %, K -9 %, Mg -17 %, Mn -15 %, P +218 %, Si +4 %, Ti -23 %, Cu +220 %, Zn +151 %, and Zr +17 %. These systematic errors can largely be removed by the application of correction factors, which are unique to each analyser and each project. Without such corrections, the three analysers tested, including two ‘identical’ Delta Premium models, yield different results on the same sample. Another important finding is that within 20 cm long core samples, the effect of mineralogical heterogeneity on in-situ pXRF data is much larger than that of the instrument precision. Finally, with the Delta analysers, both the ‘mining plus’ and the ‘soil’ modes are needed to determine as many elements as possible with the best data quality possible.

A new geoscientific application of portable XRF (pXRF) analysers is the acquisition of high-spatial resolution down-hole geochemical profiles obtained in-situ on exploration drill-cores. One advantage of such profiles over traditional laboratory geochemistry, apart from the non-destructive aspect of pXRF, is that they are obtained quickly, in the field. So they can help exploration companies take important decisions such as “has a target stratigraphic horizon been reached, or should we drill deeper?” For example, in the Matagami mining camp, pXRF data permits the rapid distinction of two visually similar and variably altered rhyolites in the Persévérance area, based on a plot of Ti/Zr vs Al/Zr. The corrected pXRF data plot within the same fields as the traditional geochemical analyses for these rhyolites. Another advantage of pXRF profiles for exploration companies, geological surveys or academic researchers is the ability to locate lithological contacts better, and in general improve down-hole lithological discrimination, especially for fine-grained and/or hydrothermally altered lithologies. For example, in the Caber volcanogenic massive sulphide deposit area, there are abundant intrusions which makes it difficult to follow the volcanic stratigraphy between drill-holes and sections. In the drill-hole studied, the pXRF data, plotted as down-hole profiles of elements/oxides and ratios, allow several previously unidentified altered dykes to be distinguished from altered rhyolites.

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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

Multi-parameter data on drill cores has a range of useful applications for ore deposit modeling and mineral exploration. Density, magnetic susceptibility, eight major elements, seven trace elements (portable X-ray fluorescence, pXRF), five mineral groups (near-infrared spectrometry) and average visible light reflectance were determined on rock cores from several drill holes. These are located near the Archean Lemoine auriferous volcanogenic massive sulfide (VMS) deposit in the Chibougamau mining district, Abitibi Subprovince, Quebec, Canada. This case study focuses on one drill hole in particular which has been recently characterized in detail by conventional methods (laboratory geochemistry, petrography, stable isotopes); this provides validation for the multi-sensor dataset. The best variables and immobile element ratios to distinguish between different lithological units in the multi-parameter data are density, magnetic susceptibility, Ti/Zr, Al/Zr, and Zr/Y. Downhole profiles of these parameters allow the four different felsic units in LEM-37, many of which are visually similar, to be distinguished, and geological contacts to be positioned precisely. Such methods would be even more useful in environments where recognition of primary characteristics is strongly hampered by intense hydrothermal alteration and superimposed deformation or metamorphism. Volcanic and intrusive rocks can be placed on classic diagrams to assign them names and magmatic affinities based on the corrected pXRF data. Downhole profiles of major and trace elements, combined with infrared mineralogy, allow hydrothermal alteration to be characterized, yielding results comparable to those of conventional techniques for the same drill hole, but with a much tighter spacing and within potentially shorter timeframes.