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

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.