Russell S. Harmon

Mineral exploration provides the commodities society requires, but continuing discovery and development of new critical mineral deposits is necessary to realize a sustainable, low-carbon future. Pegmatites in historic mining districts across North Carolina have been sites of mineral production for over a century, initially for mica and subsequently for spodumene. Most granitic pegmatites here are mineralogically simple feldspar, quartz, and muscovite pegmatites, with beryl, spodumene, and columbite-group minerals present in some chemically complex pegmatites. Analysis for Li, K, Zn, Ga, Rb, Sn and Cs content and K/Rb ratio by handheld LIBS has been undertaken for suites of muscovite samples from pegmatites in five historic mining districts across North Carolina—the Spruce Pine, Franklin-Sylva, and Cashiers districts in the Blue Ridge province and the Shelby district and Carolina Tin-Spodumene Belt (CTSB) in the Piedmont province, to assess their petrological character, degree of compositional evolution, and potential for rare element mineralization. Most pegmatites have only undergone moderate degrees of melt differentiation and compositional fractionation, and only rarely has melt evolution reached the extreme extent required for beryl or spodumene formation. The quartz-feldspar pegmatites of the Franklin-Sylva, Cashiers, and Shelby districts are poorly to moderately fractionated, whereas moderately to highly fractionated spodumene-bearing pegmatites are present in both the Spruce Pine district and Carolina Tin-Spodumene Belt. Muscovite from spodumene-bearing pegmatites is not unusually enriched in Li, but can be distinguished from its counterpart in common quartz-feldspar pegmatites by characteristically low K/Rb ratios of <40 and Li contents exceeding 0.05 wt. %.

Lithium (Li) is a critical metal for technology development which is driving demand and increasing exploration for new resources. Spodumene-bearing pegmatite deposits represent an important natural source of Li. Soil analysis during an exploration campaign can provide important insights into the presence of buried pegmatites. Here, we assert that a better understanding of soil and regolith formation and the behavior of Li and associated pathfinder elements can enhance the use of soils in mineral prospecting. This study applies a multifaceted pedological approach to investigate the physiochemical, mineralogical, and isotopic properties of soil-weathering profiles produced by spodumene-bearing pegmatites and surrounding country rock in the Carolina Tin-Spodumene Belt (CTSB) in south-central North Carolina, USA. Markedly higher Li concentrations are present in soils above Li-pegmatite dikes compared to off-dike soils, with over 80% of the original Li lost during weathering. Relic primary Li‑rich minerals, spodumene and muscovite, are the main soil minerals responsible for soil Li signatures overlying Li‑pegmatite dikes, whereas secondary mineral phases such as Fe, Al, and Mn oxides play a role in scavenging Li in off-dike soil profiles. Depth variability of Li in soil profiles reflects surface mixing, heterogeneity of bedrock lithology, and textural differences caused by eluviation and illuviation. The effectiveness of soil prospecting can be influenced by landscape position and soil-sampling depth. Integrating pedological and geochemical perspectives can, therefore, enhance the value of soil geochemical surveys in Li-pegmatite systems like CTSB. A pedogenetic context to mineral prospecting may well be a fruitful approach for critical mineral exploration.

This chapter (Laser‐Induced Breakdown Spectroscopy (LIBS)) is a contribution to the Geostandards and Geoanalytical Research Handbook of Rock and Mineral Analysis – an online textbook that is a fully revised and updated edition of A Handbook of Silicate Rock Analysis (P. J. Potts, 1987, Blackie, Glasgow).Chapter 13 (from Section 3 of the handbook dealing with microbeam techniques) provides first a history of the development of laser‐induced breakdown spectroscopy, and of the LIBS process, followed by an examination of the fundamental principles of LIBS and its instrumentation. Discussion is then provided on the preparation of sample material, LIBS matrix effects and signal processing. Different modes of compositional analysis that can be tackled by LIBS are described, including quantitative measurement (covering isotope measurements), compositional mapping, depth profiling and the determination of physical properties of geological materials. The recent tandem coupling of LIBS with laser ablation ICP‐MS instrumentation is explored. Finally, a suite of examples of LIBS analyses of silicate rocks and minerals is provided, demonstrating the utility of this measurement principle in rapid compositional assessment, detailed petrological studies and microgeochemical mapping.

Laser-induced breakdown spectroscopy (LIBS) is a type of optical emission spectroscopy capable of rapid, simultaneous multi-element analysis. LIBS is effective for the analysis of atmospheric gases, geological fluids, and a broad spectrum of minerals, rocks, sediments, and soils both in and outside the traditional laboratory setting. With the recent introduction of commercial laboratory systems and handheld analyzers for use outside the laboratory for real-time in situ analysis in the field, LIBS is finding increasing application across the geosciences. This article first overviews the LIBS technique and then reviews its application in the domain of mineral exploration and ore processing, where LIBS offers some unique capabilities.

A dataset of >1190 published compositional analyses of muscovite from granitic pegmatites of varying mineralogical types was compiled to reevaluate the usefulness of K-Rb-Li systematics of muscovite as a tool for distinguishing mineralogically simple pegmatites from pegmatites with potential Li mineralization. Muscovite from (i) common, (ii) (Be-Nb-Ta-P)-enriched, (iii) Li-enriched, and (iv) REE- to F-enriched pegmatites contain Li contents that vary between 10 and 20,000 ppm depending on the degree of pegmatite fractionation. Common pegmatites are characterized by low degrees of fractionation as exhibited by K/Rb ratios ranging from 618 and 25 and Li contents generally being 1700 ppm. Muscovite from moderately to highly fractionated Li-rich pegmatites exhibit a wide range of K/Rb ratios and Li values: (i) K/Rb = 84 to 1.4 and Li = 35 to >18,100 ppm for spodumene pegmatites, (ii) K/Rb = 139 to 2 and Li = 139 to >18,500 ppm for petalite pegmatites, and (iii) K/Rb = 55 to 1.5 and Li = 743 to >17,800 ppm for lepidolite pegmatites. Pegmatites that host substantial REE- and F-rich minerals may carry muscovite with K/Rb ratios between 691 to 4 that has Li contents between 19 to 15,690 ppm. The K/Rb-Li behavior of muscovite can be useful in assessing the potential for Li mineralization in certain granitic pegmatite types. The proposed limits of K/Rb values and Li concentrations for identifying spodumene- or petalite-bearing pegmatites as part of an exploration program is reliable for Group 1 (LCT) pegmatite populations derived from S-type parental granites or anatectic melting of peraluminous metasedimentary rocks. However, it is not recommended for application to Group 2 (NYF) pegmatites affiliated with anorogenic to post-orogenic granitoids with A-type geochemical signatures or that derived by the anatexis of mafic rocks that generated REE- and F-rich melts.

With the recent introduction of handheld instruments for field use, laser-induced breakdown spectroscopy (LIBS) is emerging as a practical technology for real-time in situ geochemical analysis in the field. LIBS is a form of optical emission spectroscopy that is simultaneously sensitive to all elements with a single laser shot so that a broadband LIBS spectrum can be considered a diagnostic geochemical fingerprint. Sets of LIBS spectra were collected for seven obsidian centers across north-central California, with data processed using multivariate statistical analysis and pattern recognition techniques. Although all obsidians exhibit similar bulk compositions, different regional obsidian sources were effectively discriminated via partial least squares discriminant analysis. Obsidian artifacts from seven archaeological sites were matched to their putative sources with a high degree of confidence.