Detecting REE-rich areas in heterogeneous drill cores from Storkwitz using LIBS and a combination of k-means clustering and spatial raster analysis

Abstract

This paper presents a novel approach to calculate matrix-matched intensity limits for the spatially resolved detection of rare earth element (REE) enrichments in highly heterogeneous geological material from the Storkwitz carbonatite based on scanning Laser Induced Breakdown Spectroscopy (LIBS). A drill core from the Storkwitz carbonatite was mapped in detail with a LIBS drill core scanner. For reference purposes, μ-EDXRF was applied for qualitative REE detection, and a microprobe was used for quantitative REE analysis.Microprobe analysis of thin sections verified the existence of rare earth elements and revealed an accumulation of REEs around mineral rims. Microprobe measurements also revealed that the main carrier of rare earth elements in the analysed drill core are REE-carbonates with contents as high as 24.4 wt% for Ce, 15.4 wt% for La, and 9.2 wt% for Nd. Apatite and pyrochlore are carrier for rare earth elements as well, however, with significantly lower concentrations of Ce (1.0 wt%), La (0.3 wt%), and Nd (0.3 wt%).K-means clustering was applied on the LIBS mapping to separate classes that show similarities in chemical composition. The classes represent carbonates, silicates, and rock matrix, respectively. According to the microprobe results, most REE-carbonates were found to be related to elevated porosity or microfractures in the rock matrix, forming very thin rims of idiomorphic and hypidiomorphic minute crystals and rarely dense aggregates around exposed minerals and fractured rock fragments. Therefore, a buffer zone calculation was performed on the distinct mineral classes to extract pixel belonging to mineral rims only. Rims without any enrichment in rare earth elements were then used to calculate matrix-matched intensity limits for REE enrichments. Based on the observed similarity in chemical composition for rim pixel and rock matrix pixel, this intensity limit could be transferred to all pixels belonging to the rock matrix.Based on the calculated intensity limits for La, 1.4 area % of the sample was found to be enriched in REE-carbonates. This result was validated qualitatively by comparing it to a μ-EDXRF mapping of the same sample. Optical comparison showed good agreement and pixel counting confirmed similar zones of enrichment. The results reveal that La enrichments in REE-carbonates are reliably detected with LIBS. A mass balance calculation has shown that about 88% of the REE enrichments in the investigated drill core seem to occur as pure REE-carbonates. The fast detection of drill core areas that are enriched in REEs makes LIBS a viable addition to the currently used methods in REE exploration.