Improving trace metal characterisation of ore deposits through multi-modal, multi-scale, and multi-dimensional micro-analysis

The variety and amount of metals consumed by human society is ever increasing. Meeting the demand requires exploration for new ore deposits, efficient production of active mines, and improved efficiency in metal recycling. A key element in mining-related enterprises is the improvement of ore characterisation. The study of the geology and mineralogy of ore deposits allows us to infer the processes behind ore genesis. This knowledge guides important exploration and processing decisions. Over the last few decades, technological advancements have enabled ore characterisation at increasing levels of detail. This has brought the trace metal mineralogy of ore deposits into focus. In many cases, trace metals occur as extremely fine-grained minerals or as lattice-bound impurities in the more common minerals in ore deposits. Hence, their study requires the use of micro-analytical techniques. Trace metals and their minerals can carry crucial information on the conditions of ore formation. They can be of economic value, harmful to the environment, or of strategic economic and geopolitical interest (e.g. Critical Raw Materials). Trace metal characterisation is therefore highly relevant to research, industry, and society. In this project, micro-analysis was performed on the Liikavaara Östra Cu-(W-Au) deposit in northern Sweden to research the trace metal mineralogy of Au, Ag, Bi, Mo, Re, and W. The main goal of the project was the development, optimisation, and integration of various micro-analytical techniques for ore characterisation. The project was subdivided into four studies (scientific contributions): (1) Drill core logging, whole-rock geochemistry, and light microscopy were applied to identify lithology, alteration, and mineralisation of the deposit. An intrusion in the footwall, potentially related to ore genesis, was dated with LA-ICP-MS. Scanning electron microscopy with energy dispersive spectrometry was used to gain insight into the trace metal mineralogy of the deposit. This study provided an overview of the geology and mineralogy of the deposit and served as a basis for sample selection and data interpretation of subsequent studies. (2) A polished thin section of the ore containing trace metal minerals was scanned by automated mineralogy (QEMSCAN) at Boliden AB to assess the potential of trace metal mineral quantification in a production-focused environment. To delineate instrument limitations from operator input the same sample was also scanned at Camborne School of Mines, UK. Detection of trace metal minerals was generally difficult due to their fine-grained nature. Yet, quantification could be improved by optimisation of the mineral classification library. (3) Four polished epoxy-mounted drill core pieces of ore were analysed by automated mineralogy (Mineralogic) and x-ray computed tomography (XCT). In two samples, a smaller region of interest was drilled and re-analysed at higher resolution. Results from automated mineralogy were used to segment and interpret the XCT data. Vice versa, XCT data provided 3D spatial context for the 2D scans. (4) Three polished thin section pieces with grains of molybdenite, pyrite, and native Bi, all with Au-inclusions, were analysed by synchrotron radiation x-ray fluorescence mapping at the NanoMAX beamline of the MAX IV synchrotron facility in Lund, Sweden. Element fluorescence maps down to 50 nm pixel size revealed the distribution of micro- and nano-inclusions and lattice-bound impurities in the mineral grains. The studies demonstrated benefits and challenges of the various micro-analytical techniques, and how and what they may contribute to ore characterisation. Results allowed linking and integrating the techniques into a smart analytical flow to optimise the characterisation of trace metal minerals in ore deposits. This is useful for both ore geology research and the mining industry.