Caledonian magma genesis and lithium enrichment – a geochemical study

Overview

Global attention has been drawn to the source, extraction, and supply of lithium from igneous micas and lacustrine brines. Lithium is an essential component in batteries for low-carbon transport and energy storage solutions, however UK lithium potential has only recently been researched (British Geological Survey, 2020). Rising demand, the post-Brexit trading environment, and concerns over the environmental cost of global trade, mean it is timely to research the origins and potential for local extraction of critical raw materials, including lithium.

Lithium-bearing minerals, such as zinnwaldite, spodumene, lepidolite and tourmaline, are related to granitoid magmas derived from melting of, or interaction with, Al-rich sedimentary rocks. Occurrences are associated with a range of other minerals including valuable high field strength elements such as tungsten (British Geological Survey, 2020). In Cornwall, such “S-type” granites may soon be exploited via the extraction of lithium from geothermal brines as an offshoot from the production of geothermal energy (https://www.iom3.org/resource/cornish-lithium-grades—compelling-resource.html). The other principal location of interest is the NE Grampian Highlands of Scotland. Magmatism there is principally associated with the duration and aftermath of the Caledonian Orogeny from ~475-390 Ma. Unlike the bulk of mantle- and meta-igneous crust-derived Caledonian (I-type) granitoids elsewhere, many NE Grampian granitoids have S-type characteristics related to melting of Dalradian meta-sediments. Different Li-bearing minerals are reported from scattered localities over ~600 km2. These include Caledonian pegmatites found in float near Ballater at up to 1.7% Li (British Geological Survey, 2020), the historic Mn mine near the Lecht ski centre, and most extensively, a body of granite at Glen Gairn near Banchory (Hall and Walsh, 1972), where 5-20% of the Coilacreich facies of this granite comprises Li-bearing zinnwaldite (Smith et al. 2002) (Figure 1).

This study will address the currently incomplete picture of the timing and distribution of lithium resources and their genetic relationship to Caledonian magmatism in NE Scotland. It will take both a field and laboratory approach, building on evidence that a clear understanding of the timing and petrogenesis of granitoid magmatism is integral to tackling the broad relationship between granite petrogenesis and lithium metallogenesis (e.g., the Cornubian example of Simons et al. 2017). The two key components and five research objectives of this project are:

1) Granitoid petrogenesis:
i) geochemical characteristics of Li- rich and Li-poor granitoids, comparing S-and I-type examples.
ii) granitoid petrogenesis, including nature of source and assimilation processes.
iii) timescales of Caledonian magma residency and emplacement, relationship to regional geodynamics, deep crustal hot zone development, and ore mineralisation.

2) Metal distribution during the magmatic cycle:
i) timing of metal enrichment in the life history of each NE Grampian granitoid, addressing the relationship of mineralization event(s) to stages of magmatic history from melting, magmatic evolution, to later hydrothermal activity
ii) tracing mobility of Li and co-genetic transition metals though the magmatic cycle via mineral phase compositions, principally zircon.
iii) spatial and genetic relationship of Li mineralisation to Caledonian plutons and an assessment of future exploration possibilities.

Methodology

To address the objectives above, this project contains three work packages covering fieldwork and basic geochemical analysis, zircon U-Pb dating and geochemistry, and in-situ studies of Li mineralisation and other associated mineral species.

Work package A: Literature review, fieldwork, sample collection, and basic whole-rock geochemistry. The candidate will spend time visiting the known mineralised localities as well as any potentially-related Caledonian granitoids across the NE Grampian Highlands. Generic granitoid samples as well as mineralised rocks will be collected and sent for external major and trace element analysis to address points 1.i-ii, and GIS-based mapping of samples and mineralised locations will be used to address parts of 2.ii-iii.

Work package B: Zircon histories and chemistries addressing 2.i-iii. Magmatic evolution and emplacement timescales, and the chronological relationship between magmatism and mineralisation will be tackled using laser ablation mass spectrometry on zircons extracted from each targeted pluton or mineralised vein. U-Pb geochronology will be carried out largely at the University of Glasgow. The same zircon mounts will also be subject to trace element analysis at Glasgow or St Andrews, following the recent precedent of Gardiner et al. (2021), constraining the timing of metal enrichment during magmatism. Mounts will finally be analysed for Hf isotopes at St Andrews to categorise both melt sources and assimilation/interaction with local crust during magmatic evolution, helping to understand the source of metals. Additional whole rock Hf isotope work may be used to obtain the composition of the local Dalradian rocks. There may be opportunities to apply additional methods to accurately date vein mineralisation, e.g., U-Pb in calcite or Re-Os in sulphide.

Work package C: Quantifying mineralisation and exploration potential for 2.iii. A deeper study of mineralised localities will be conducted, by scanning electron microscope, electron microprobe, and/or laser ablation mass spectrometry, to constrain the extent of Li enrichment and establish the identity and paragenesis of lithium-bearing minerals and other economically significant ores.

Project Timeline

Year 1

Literature review, fieldwork planning and execution, sample preparation and sending for external whole rock analysis. Commencement of mineral separation and zircon mounting and textural studies by scanning electron microscope; commencement of preparation of polished stubs from mineralised samples.

Year 2

Completion of sample preparation, scheduling and conducting the majority of zircon U-Pb and trace element studies; scanning electron microscope and electron probe studies of mineralised samples. The candidate will attend a national conference this year or early next to highlight initial results, develop networks and gain additional feedback.

Year 3

Completion of remaining zircon Hf isotope and mineralised sample studies. The candidate will draw together results to address the key research questions/objectives and start thesis write-up as well as preparation of manuscripts for peer review. They will attend a major international conference and may also be involved in preparations for a focused local workshop on resource extraction in Scotland. There will be time to consider any additional sample collection, preparation and analysis considering the initial results.

Year 3.5

The candidate will complete interpretation and thesis write-up and continue with preparation and submission of manuscripts for peer review.

Training
& Skills

This project is suited to a candidate willing to work for periods in the field in Scotland, with a strong interest in laboratory preparation and analysis techniques to constrain magmatic and metallogenic processes. We would expect candidates to have external engagement with academics and public figures interested in rural environments, planning, economies, and populations: there is potential to help plan a workshop on resource extraction in Scotland during the later stages of the PhD.

The supervisory team are recognised experts in Scottish, regional and global structural, magmatic, hydrothermal and resource geology, particularly related to granitoid magmatism, and will ensure the candidate gets access to a wide research network. As part of this experience, the student will benefit from direct collaboration with the BGS, principally through supervisor Kathryn Goodenough at the Lyell Centre, including access to BGS-backed digital mapping tools. The student will receive training from Iain Neill and Nick Gardiner and research technicians at Glasgow and St Andrews, covering zircon extraction and preparation, scanning electron microscopy, electron probe micro-analysis, and laser ablation quadrupole and mass-collector mass spectrometry. Quantification of economic metals in magmas using zircons is at the cutting edge of Nick Gardiner’s exploration research (Gardiner et al. 2021).

Applications for isotope support from NERC, plus any additional opportunities, e.g., funding from the Society for Economic Geologists or Geological Society of London, will give valuable grant-writing experience. The candidate will be strongly encouraged to attend NERC-funded or other training workshops on analytical skills. As part of the IAPETUS DTP, the candidate will attend day or residential training courses with their cohort. The candidate will study a wide choice of credit-bearing courses in research methods at the University of Glasgow and will join a tight-knit and diverse cohort of MSc and PhD students in a friendly department.

References & further reading

British Geological Survey 2020: https://www2.bgs.ac.uk/mineralsuk/download/cmp/lithium.pdf
Gardiner et al. 2021: https://doi.org/10.1016/j.chemgeo.2021.120580
Hall and Walsh 1972: https://doi.org/10.1144/sjg08030265
Simons et al. 2017: https://doi.org/10.1016/j.lithos.2017.02.007
Smith et al. 2002: Geology of the Ballater District: memoir for 1:50 000 geological sheet 65E (Scotland). London: Stationery Office.

Further Information

Please feel free to contact any of the supervisory team by e-mail in the first instance. We will be happy to hear from you.

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