Application of portable XRF to the direct analysis of till samples from various deposit types in Canada

Abstract

In this study, results by direct portable XRF (‘pXRF’) on unsieved till samples were compared with those by established laboratory methods (aqua regia or fusion ICP-MS and ICP-ES) on the <0.063-mm fraction to determine if the application of direct pXRF in the field would serve as an acceptable guide for immediate follow-up work. Four test sites in Canada were chosen: the Halfmile Lake Cu-Pb-Zn VMS deposit; the intrusion-hosted W-Mo Sisson deposit; a Pb-Zn Mississippi Valley–type (MVT) deposit in the Pine Point district; and the Triple B kimberlite. Unsieved till samples from the GSC archive collection were used for this study and included samples from background areas, immediately overlying, and at various distances down-ice of each deposit. Ziploc® and Whirl-Pak® bags that were used to contain the samples in the field were tested for their properties of X-ray attenuation and contamination. In general, the performance of pXRF in the four test areas was very good where concentrations of elements of interest (indicator or pathfinder elements) were substantially above detection limits by this technique (in the low ppm range for many elements). The following elements, shown to be useful indicator elements (important constituents of the ore/commodity) or pathfinder elements (those associated with the commodity elements) by the established methodology, showed similar patterns by pXRF on the unsieved material: Zn, Cu, Pb, and As at Halfmile Lake; W, Mo, Cu, Zn, Pb, and As at the Sisson deposit; Zn, Pb, and Fe at Pine Point; and Ca, Sr, Cr, and Ni at Triple B. Pathfinder elements whose concentrations were too low for determination by pXRF include: Ag and Sb at Halfmile Lake; Ag and Cd at Sisson; Cd, S, and Se at Pine Point; and Co, Mg, P, U, and Th at Triple B. The high background for Bi by pXRF, equivalent to c . 50 ppm, and its noisy signal precluded its use at Halfmile Lake and Sisson. Elements which tended to show poor precision (three analyses each sample) by pXRF in some samples due to sample heterogeneity include Sn, V, and W. Mercury was erroneously reported for the majority of samples in the low ppm range by pXRF whereas its concentration in fact was in the low ppb range. Several Pb-, Zn- ( c . 1% Pb, Zn) and Fe-rich (up to 16% Fe) samples demonstrated spectral interferences by: Pb on As, Th and Se; Zn on Cu; and Fe on Co. Results for six till samples analysed in Ziploc® and Whirl-Pak® bags showed that Ziploc® absorbs fewer low-energy photons and hence is preferable for determining light elements such as Si, K and Ca. Supplementary material: Four data-sets are available at http://www.geolsoc.org.uk/SUP18897

Supplementary material to "Vectors to ore in replacive VMS deposits of the northern Iberian Pyrite Belt: mineral zoning, whole rock geochemistry, and use of portable XRF"

“…60 s per filter or 200-240 s in total) to measure prepared samples (e.g. Conrey et al, 2014;Le Vaillant et al, 2014;McNulty et al, 2018, Hall et al, 2016. However, such long counting times are unpractical if the method is to be applied in real exploration situations on rock surfaces or hand specimens.…”

“…Many exploration studies use two analytical modes (e.g. Le Vaillant et al, 2014;Hall et al, 2016;Ross et al, 2016) to optimize analytical precision and detected limits for all elements of interest. However, in this study, for analytical convenience during active exploration it was decided to use only one test mode at each spot.…”

Possible Pitfalls in the Analysis of Minerals and Loose Materials by Portable XRF, and How to Overcome Them

“…Reference [27] investigated several types of plastic bags commonly used, but not cloth (calico) bags, with signal attenuation from −20 to −90% depending on bag type. Reference [28] compared the attenuation by LDPE bags, a proprietary type of bag and Prolene TM film on cups. They concluded a minor effect on Zn (representing heavier elements), an observable but acceptable effect on K and Ca, and a major effect on Si.…”

Vectors to ore in replacive volcanogenic massive sulfide (VMS) deposits of the northern Iberian Pyrite Belt: mineral zoning, whole rock geochemistry, and application of portable X-ray fluorescence

“…2 Geological setting 2.1 Vectors to ore in VMS systems Upwelling high-temperature (>350 • C) hydrothermal fluids in VMS systems react with the surrounding rocks and sediments, producing chemical and mineralogical changes in the form of alteration halos. These are most developed in the footwall around the feeder zone that underlies the massive sulfides, although they can also form, to a lesser extent, around the deposit and in its hanging wall, especially in subseafloor replacive deposits (Large et al, 2001a;Franklin et al, 2005;Hannington, 2014) (Fig. 1).…”

“…1). The alteration assemblage is controlled by factors such as rock composition, water / rock ratio, temperature, and fluid composition, pH, and redox state (Hannington, 2014). Changes in these controlling factors with depth and distance from the centre of the hydrothermal system typically result in distinct mineralogically and geochemically zoned alteration halos, which are the base for most vectoring tools used in the exploration and characterization of VMS systems (Large et al, 2001a;Gibson et al, 2007) (Fig.…”