Comparison of lithium borate fusion and four acid digestions for the determination of whole rock chemistry – implications for lithogeochemistry and mineral exploration

Abstract

Mining and exploration companies routinely use four acid digestion, inductively coupled plasma, atomic emission spectra/mass spectrometry (4AD ICP-AES/MS) methods from commercial assay laboratories for analysing drill and rock samples for lithogeochemical assessment and resource reporting. This method is also known to exhibit lower recovery of elements hosted by resistate minerals. To assess the impact of lower recoveries on lithogeochemical interpretation, a suite of commonly used elements for lithogeochemical analysis (high field strength elements (HFSE) Zr, Hf, Nb, Ta, Ti and Eu and transition elements V and Sc) was analysed by a four-acid digestion (4AD) and an alkali fusion/acid digestion (AFAD). Lower recoveries in the 4AD relative to the AFAD were recorded for Zr, Hf, Nb, Ta, Ti and Eu. Scandium and vanadium reported similar concentrations for both decomposition methods. Despite the lower recoveries for Nb, Ta and Ti, element ratios were largely preserved with the 4AD method due to the recoveries covarying at a one-to-one ratio. A plot of Ti/Nb against V/Sc was found to be largely unaffected by decomposition method, producing similar compositional classifications between the two digestion methods. Use of the europium anomaly (Eu/Eu*) to determine plagioclase fractionation was also found to be unaffected by decomposition method. In contrast, a standard Zr/Ti vs Nb/Y discrimination plot produced incorrect classifications with the 4AD producing more mafic and alkaline classifications relative to the AFAD method. Magmatic fertility interpretations utilising Zr/Hf were also found to be affected in the 4AD results due to the lower recovery of Zr relative to Hf. This resulted in a bias in the 4AD results and produced false positive anomalism in fertility assessments. Multiple decomposition methods including combinations of acid and fusion methods are recommended for lithogeochemical analysis utilising large regions of the periodic table. However, if only 4AD data are available, plots such as Ti/Nb vs V/Sc and Nb/Ta which preserve their ratios can be used for lithogeochemical classification. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6444444