Critical metals and deep time: decoding the interplay of magmatism, structure and mineralization

Overview

Critical metals are commodities vital to industry, modern technology or green energy production. Our reliance on them is rising rapidly and supplies of some metals are already under severe stress. Although Pb-Zn mineral veins are commonplace globally, little research has been done to understand their association with critical metals such as germanium and indium, which are known to be enriched in some deposits. Scotland, and particularly the region around Strontian in the Highlands (Image 1), represents a natural testing ground for the inter-relationship of magmatism, structural geology and mineralising episodes in the generation of Pb-Zn deposits. This study will comprise a new multi-disciplinary assessment of the Strontian area, the type locality for the element strontium. The project aims to determine the:

1) Spatial-temporal distribution and structural relationships of magmatic and hydrothermal activity, particularly post-dating the Late Caledonian Strontian pluton.
2) Paragenesis, timing and source of Pb-Zn carbonate and baryte mineralised veins around Strontian.
3) Prospects of the Strontian area for critical metals such as germanium and indium needed for modern technology.

Mineral veins around Strontian have been mined for three centuries, historically for Pb and Zn and more recently (in the 1980s) for barite. The Strontian pluton is quarried for high-quality aggregate at the Glensanda Superquarry. Despite the quality exposure (Images 2 and 3) and economic importance of this area, there has been virtually no modern research on its geology and mineral resources. The absolute age of the veins, their paragenesis, extent, relationship to the pre-existing magmatism (e.g. the high Ba-Sr signature of the pluton), and the structural controls on vein emplacement are essentially unknown. Whilst Ba, Pb and Zn are not critical metals, the potential for critical commodities potentially associated with Pb-Zn veins needs to be explored. For Strontian, there are currently no data on the chemistry of the vein minerals. In summary these veins may contain a range of raw materials that are of importance for modern technology, and this project therefore represents a timely opportunity to make a step-change in our understanding of some of Scotland’s significant natural resources.

More widely, the Northern Highlands Terrane in which Strontian lies consists of the Proterozoic Moine Supergroup, originally deposited ~1000-870 Myr ago, cut by rift-related granitic and basaltic intrusions (~870 Ma) followed by poly-deformation and metamorphism during Late Proterozoic orogenesis. The Ordovician-Silurian Caledonian Orogeny, associated with closure of the Iapetus Ocean (~470-425 Ma), and the onset of major strike-slip faulting along the Great Glen Fault marked the last major mountain building episode in the region. The Caledonian Orogeny terminated with Iapetus slab break-off and emplacement of high Ba-Sr granitoid plutons, the largest of these being at Strontian. Mafic to lamprophyric magmas were subsequently emplaced during the Carboniferous-Permian, associated with crustal thinning. Hydrothermal systems related to this magmatism are thought to have resulted in the emplacement of Pb-Zn carbonate and barite veins at Strontian, but uncertainty over the age of mineralisation and the ultimate source(s) of Pb, Zn, Ba and Sr makes the area particularly ripe for study. Your aim will be to address the fundamental geological controls on mineralisation and the prospect of critical metal enrichment. This project can then begin to feed into a wider assessment of mineralising processes across Scotland or in poly-orogenic regions globally.

Click on an image to expand

Image Captions

Image 1: the mapping area between Corrantee and Worth Vein north of Strontian on the Morvern peninsula. The area is additionally cut by Caledonian appinites, Permian-Carboniferous mafic to lamprophyric and Palaeogene mafic dykes. Image modified from a map by the Northern Mine Research Society.

Image 2: example of old mine workings hosted in well-exposed Moine and granite gneiss, allowing good structural constraints and sampling opportunities. Photo: John MacDonald.

Image 3: excellent carbonate vein lithologies for geochemical and geochronological work. Photo: John MacDonald.

Methodology

You’ll review the geological context of the Northern Highlands, and study existing mine maps, structural, lithological, geochemical and geochronological data as appropriate. You can make short trips to Strontian (3 hrs from Glasgow) to strategically sample rock types, particularly vein minerals and geochronological targets, in advance of a major field season. Core from Strontian can be inspected at the British Geological Survey in Keyworth, Nottingham. The key aims of the project shall then be fulfilled by:

Fieldwork and structural interpretation: in collaboration with the BGS, you will make a digital map of the northern end of the Strontian granite and surrounding lithologies, including structural data from minor intrusions, fault planes and mineral veins. As there is almost no published geochronology and geochemistry from smaller-scale intrusions emplaced before and after mineralisation, these will be extensively sampled alongside mineral veins. Structural data will be plotted using standard software and a tectonic model for magmatic and vein emplacement through time will be created.

Geochronology: at the BGS in Keyworth (location subject to funding), you will map vein calcite for high U regions using SEM (scanning electron microscopy) and laser ablation techniques, then use targeted laser ablation U-Pb analysis to date mineralisation. The ‘age’ of many local minor intrusions and the youngest parts of the Strontian pluton have been determined qualitatively, often only petrographically, so U-Pb zircon analysis of these will be conducted in order to determine which magmatic event(s) match the mineralising episode(s). There is also good potential for Re-Os dating of sulphides.

Petrography: vein minerals from Strontian will be subjected to an extensive petrographic survey in Glasgow to constrain their paragenesis and identify if samples are suitable for fluid inclusion work, which may form an additional part of the project if desired.

Geochemistry: whole rock samples from minor intrusions will be prepared in Glasgow and sent for major and trace element analysis to determine their chemical affinity. Of particular interest will be Ba-Sr-Ca concentrations: is the Strontian pluton the ultimate source of alkaline earth metals or does younger magmatism also carry significant concentrations? Pb-Zn vein mineralisation will be assayed to determine if there are high abundances of critical metals such as germanium and indium. Carbonate veins will be analysed at the layer scale using laser ablation mass spectrometry in Glasgow. This study will determine how trace element concentrations vary across the mineralised region. Stable isotope analysis (e.g., S, C, O) of vein materials will also be conducted in Glasgow or at the Scottish Universities Environmental Research Centre to determine magmatic or meteoric sources of fluid.

All supervisors shall guide the student through planning, fieldwork and structural analysis. Drs Neill and MacDonald and professional staff at the University of Glasgow shall train the student in sample preparation and in-house analysis techniques such as zircon separation, SEM and laser ablation mass spectrometry. Dr Goodenough will accompany the student for parts of their fieldwork and will host the student at the Lyell Centre in Edinburgh for development of digital maps using the BGS-SIGMA system. The student shall additionally travel to Keyworth, Nottingham, to collaborate with Dr Nick Roberts and develop a proposal for U-Pb dating of calcite and zircon grains. If the proposal is successful with the Isotope Facilities Committee, the student will further work with Dr Roberts to produce U-Pb ages. All supervisors will be involved in interpretation of results.

Project Timeline

Year 1

Literature review, fieldwork, detailed structural analysis, digitization of field data, rock sampling and initial laboratory preparation. Preparation of proposal for U-Pb dating of calcite and zircon.

Year 2

Continued sample preparation and ongoing laboratory analysis. Submission of U-Pb dating proposal at the start of the year. Visits to Keyworth to conduct final stages of sample preparation and dating if successful. This work can alternatively be developed in-house at Glasgow or at other institutions such as the University of Hull. You will attend a national conference this year to highlight initial results, develop networks and gain additional feedback.

Year 3

Finishing off analysis and interpretation. Time to consider any additional sample analysis based on initial results. Attend at least one major international conference and begin preparing manuscripts for peer review alongside thesis writing.

Year 3.5

Complete write-up of thesis and preparation/submission of manuscripts for peer review.

Training
& Skills

You’ll be willing to plan and undertake fieldwork in Scotland, with broad interests in magmatism, structure and geochemistry and particularly mineral resources.

The supervisory team are recognised experts in Scottish, regional and global structural, magmatic, hydrothermal and resource geology and will ensure you access a wide research network. You’ll benefit from direct collaboration with the BGS, principally through supervisor Kathryn Goodenough at the Lyell Centre, and through Nick Roberts of the NERC Isotope Facility in Nottingham, as discussed above. All geochemical techniques will include appropriate training, giving you a wide range of analytical skills covering both silicate and carbonate minerals, elemental, isotopic, in-situ and whole-rock analysis.

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

References & further reading

Bruand et al. 2014 Journal of Petrology 55, 1619-1651; Hutton 1988 GSA Bulletin 100, 1392-1399; Roberts and Walker 2016 Geology 44, 531-534; Upton et al. 2004 Geological Society Special Publications 223, 195-218; Kimbell 1986 British Geological Survey Report, nora.nerc.ac.uk/id/eprint/11813.

Further Information

Applications: to apply for this PhD please use the url: https://www.gla.ac.uk/study/applyonline/?CAREER=PGR&PLAN_CODES=CF18-7316

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