Portable X-ray fluorescence in the assessment of rare earth element-enriched sedimentary phosphate deposits

Abstract

Phosphate deposits are the main source of raw materials used by the phosphate fertilizer industry and, in recent years, they have also been considered as potential sources of rare earth elements (REEs) and fluorine (F). Based on 160 portable XRF (pXRF) measurements taken on 32 pulps of naturally occurring phosphate rocks from the Fernie Formation (British Columbia, Canada) with a representative range of P and REE concentrations, the pXRF analyser was able to provide data with acceptable precision, accuracy, and excellent coefficients of determination (r 2 ≥ 0.85) relative to the laboratory data for Ba, Mo, Y, Sr, U, Rb, Zn, Fe, Ca, P, Si and S. Results with coefficients of determination of 0.5 ≤ r 2 < 0.85 between the pXRF and laboratory-derived datasets were achieved for Nd, Ce, La, Zr, W, and Al. From an exploration geologist’s point of view, P and Fe were measured accurately enough by pXRF using the factory-set calibration. Portable XRF analysis for Nd, Pr, Ce, La, Ba, Mo, Zr, Y, Sr, U, Rb, Zn, Ca, K, Al, Si and S had to be recalibrated relative to laboratory results. Using factory pXRF settings, light REEs (La, Ce, Pr, Nd) were subject to systematic overestimation relative to laboratory results whereas Y was subject to systematic underestimation. This study presents the first stage of an orientation survey (pulp analyses) for sedimentary phosphate occurrences containing REEs in southeastern British Columbia. If sample preparation (to pulps) and field recalibration are carried out, pXRF can identify zones of phosphate rocks enriched in REEs, and delimit zones with unacceptable levels of deleterious elements, such as uranium.

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

“…When measuring field samples, however, precautions have to be taken regarding sample homogeneity. Larger errors are to be expected in "real" geological samples [15,25]. Test runs are required (i) to determine whether a desired critical element can be measured in the matrix of interest; (ii) to establish limits of detection; and (iii) to identify potential spectral interferences.…”

“…Hence, in order to correct inaccurate data, users have to establish their own calibrations based on matrix-matched standards and well-characterised samples containing the elements of interest at different concentration levels. Commonly, the data is corrected and re-calibrated by post-processing using correction factors based on simple linear regression (e.g., [7,10,15,25,26]). This may work for elements that show only minor to moderate inaccuracy, i.e., bias, however, this method may not be appropriate for elements yielding highly erroneous results, like false positives (e.g., Bi, W, Co in our dataset; Zr, Ce in carbonatites).…”

“…The potential of quick in situ analysis coupled with the instant display of results makes this technology particularly appealing for the mining industry. Most of the mineral resources-related pXRF projects have focussed on base and precious metals and have shown that this technology, when appropriately used, may yield data of sufficient quality for exploration and geochemical comparison [7][8][9][10][11][12][13][14][15].…”

“…However, portable XRF spectrometers have proven suitable to detect REEs, Ta, and Nb in phosphates and carbonatites [13][14][15], W, Nb, Ta, La, Ce, and Zr in Sn-W ore-bearing granites [20], Ta and Zr in soil overlying granites [12], and P, Ba, Nb, V, Zr, and Y in various mafic to felsic igneous rocks [7,[21][22][23][24]. Consequently, critical elements can be detected and reliably quantified in geological materials devoid of interfering elements or when enriched.…”

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

“…To counter this potential problem, the most common strategy is to analyse CRMs (Certified Reference Materials) periodically to monitor any drift and correct it , Hall et al 2014, Le Vaillant et al 2014. 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.…”