Abstract
Laser-induced breakdown spectroscopy (LIBS), which has recently emerged as tool for geochemical analysis outside the traditional laboratory setting, is an ideal tool for Li exploration because it is the only technique that can measure Li in minerals, rocks, soils, and brines in-situ in the field. In addition to being used in many products essential to modern life, Li is a necessary element for a reduced carbon future and Li–Cs–Ta (LCT) granitic pegmatites are an important source of Li. Such pegmatites can have varying degrees of enrichment in Li, Rb, Cs, Be, Sn, Ga, Ta>Nb, B, P, and F. We focus here on the LCT pegmatites of the Carolina Tin-Spodumene Belt (CTSB) situated in the Kings Mountain Shear Zone, which extends from South Carolina into North Carolina. The CTSB hosts both barren and fertile pegmatites, with Li-enriched pegmatites containing spodumene, K-feldspar, albite, quartz, muscovite, and beryl. We illustrate how handheld LIBS analysis can be used for real-time Li analysis in the field at a historically important CTSB pegmatite locality in Gaston County, N.C. in four contexts: (i) elemental detection and identification; (ii) microchemical mapping; (iii) depth profiling; and (iv) elemental quantitative analysis. Finally, as an example of a practical exploration application, we describe how handheld LIBS can be used to measure K/Rb ratios and Li contents of muscovite and rapidly determine the degree of pegmatite fractionation. This study demonstrates the potential of handheld LIBS to drastically reduce the time necessary to acquire geochemical data relevant to acquiring compositional information for pegmatites during a Li pegmatite exploration program.
Highlights
Introduction published maps and institutional affilLithium is a metal widely used in the production of pharmaceuticals, glass, lubricants, and alloys; is present in most modern electronic devices; and used in many chemical processing and manufacturing processes [1]
This study demonstrated that valuable information about the distribution of elements in minerals can be obtained from Laser-induced breakdown spectroscopy (LIBS) elemental mapping, especially when combined with emission intensity data derived from matrix-matched calibrations
Riberio et al [62] used a portable X-ray fluorescence (XRF) analyzer and a bespoke laboratory LIBS system to examine the same locations on quartz, montebrasite, and turquoise in a slab cut from a hydrothermal vein at the Argemela Tin Mine in Central Portugal
Summary
LCT pegmatites are largely considered to be late magmatic products of extreme fractionation of peraluminous S-type granites [8], the anatectic melting of supracrustal and mantle-related source lithologies has been proposed as an alternative process for the generation of some LCT pegmatite populations [10,11,12]. The population of LCT pegmatites cogenetic with fertile S-type granites ranges from barren bodies that lack significant rare-element mineralization to pegmatites that display varying degrees of enrichment in Li, Rb, Cs, Be, Sn, Ga, Ta > Nb, B, P, and F This moderate to extreme fractionation of pegmatite melts typically results in rare-element mineralization that includes beryl, columbite-group minerals, the compositionally complex borosilicate tourmaline, and Li-minerals, such as triphylite-lithiophilite, amblygonite-montebrasite, spodumene, petalite, and lepidolite. Low values of K/Rb (~2–400) and K/Cs (~20–2000) in K-feldspar, K/Rb (~1–50) and K/Cs (~10–1000) in muscovite, Na/Li (~2–20) in beryl, Fe/Mn in garnet (~0.007–2.0), Nb/Ta (~0.001–24) in columbite-group minerals, and Zr/Hf (~0.01–13) in zircon are encountered in pegmatites that have undergone moderate to extreme levels of fractionation and accumulation of rare lithophile elements [16,17] These geochemical indicators of fractionation have been successfully used to identify parental granitoids, establish regional trends of rare-element enrichment in large pegmatite populations, and distinguish barren from mineralized pegmatites suitable for the extraction of potential rare-element ore minerals
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