Improving lithological discrimination in exploration drill-cores using portable X-ray fluorescence measurements: (1) testing three Olympus Innov-X analysers on unprepared cores

Abstract

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.

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

“…Choice of samples for precision and accuracy determinations; traditional geochemistry data For the seven Matagami rock core samples, ranging in composition from gabbro to rhyolite, used by Ross et al (2014a) to perform in situ measurements in mining plus mode, a representative fraction of the rejects -the unused coarsely crushed material from traditional geochemistry -were pulverized (see details below) and analysed in the mining plus mode only.…”

“…The internal capacity of the cups is a cylinder 1.8 cm high with a diameter of 2.6 cm; they were fully filled with powder and the Mylar ® film was in direct contact with the powder. Ross et al (2014a) describe instrumental precision and beam time testing on a sediment standard using both modes of analysis. They also report on a similar test on rock cores, in situ, using the mining plus mode only.…”

“…Such strategies can become time consuming and Fischer et al (2014) and Simandl et al (2014) noticed negligible drift, leading to doubts about the necessity of investing substantial time on measuring CRMs every hour or every 20 samples. This paper, building on the work of Ross et al (2014aRoss et al ( , 2014b, continues the performance study of two pXRF analyzers used on non-mineralized to weakly mineralized rock core samples. The main focus here is on comparing the performance of the devices on a series of rock core samples, both in situ and in powdered form.…”

“…In the latter field, the focus so far has been on analysing pulverized rock samples or performing the equivalent of assays on drill-cores (Glanzman & Closs 2007;Peter et al 2009;Fajber & Simandl 2011;Marsala et al 2011). Ross et al (2014aRoss et al ( , 2014b have also looked at performing, in situ, the equivalent of whole-rock analyses on drillcores to produce high spatial-resolution elemental profiles.…”

An innovative optical and chemical drill core scanner

“…Portable/handheld XRF instruments are gaining popularity and have been applied widely and successfully to analyse drill cores in a non-destructive way (e.g. Gazley et al, 2011Gazley et al, , 2012Fisher et al, 2014;Le Vaillant et al, 2014;Ross et al, 2014). Advantages of the new instrument over handheld XRF instruments are a better and more representative coverage (continuous scanning vs. point analyses), reduced labour, standardised analytical conditions, integrated routines, and advanced data handling.…”