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

Overview

Decarbonisation of global economies is leading to rapid increases in demand for a range of minerals (including those described as critical metals) that are used for modern technologies, industry, and green energy production. A wide range of metals are becoming increasingly important, from Cu and Zn for electrical infrastructure, to elements such as the REE, indium and tellurium for renewable energy sources. In view of the UK government’s recent net-zero strategy, it is timely to investigate domestic resources of these metals. Additionally, although Pb-Zn mineral veins are commonplace globally in a variety of tectonic settings, little research has been done to understand what association they may have with critical metals such as germanium and indium, which are known to be enriched in some deposits. Scotland, in particular the area around the Caledonian-age Strontian pluton in the northern Highlands, represents a natural testing ground for the inter-relationship of magmatism, structural geology, and mineralising episodes in the generation of Pb-Zn deposits.

The veins around Strontian have been mined for three centuries, historically for Pb and Zn and more recently (in the 1980s) for barite, whilst the Strontian granitoid pluton is quarried for aggregate at the Glensanda Superquarry. Despite quality exposure there has been virtually no modern research on its geology and mineral resources. Indeed, the absolute age of mineralised veins, their paragenesis, extent, their relationship to the pre-existing magmatism (e.g., high Ba-Sr signature of the pluton) and the structural controls on vein emplacement are essentially unknown. Whilst Pb and Zn are not critical metals, they are potentially important for green energy infrastructure; and critical commodities associated with Pb-Zn veins, such as germanium and indium, may be present. The potential for strontium mineralisation either in the form of strontianite (SrCO3), brewsterite ((Ba,Sr)Al2Si6O16.5(H2O)) or variably enriched within calcite, also needs to be explored, given it has recently been added to the European Commission list of Critical Raw Materials (September 2020).

The Northern Highlands terrane in which Strontian lies consists of Archaean-Proterozoic rocks affected by poly-deformation, metamorphism, and magmatism during several orogenic episodes, including the Ordovician-Silurian Caledonian Orogeny resulting from closure of the Iapetus Ocean. The latter stages of ocean closure and the onset of post orogenic collapse, and widespread reactivation of pre-existing structures during regional strike-slip faulting, resulted in emplacement of high Ba-Sr granitoid plutons from ~430-390 Ma, from hybridised mantle- and lower-crust-derived melts. Although the formation of Pb-Zn carbonate and baryte veins at Strontian may be associated with this magmatism, hydrothermal systems resulting from Carboniferous-Permian mafic to lamprophyric magmas may also be implicated. Such uncertainties over the age of mineralisation and the ultimate source(s) of Pb, Zn, Ba and Sr make the area particularly ripe for study. This project will therefore comprise a new multi-disciplinary assessment of the magmatic, geochronological, structural, microstructural and economic geology of the Strontian area, aiming to address the fundamental geological controls on mineralisation, and the prospects of critical metal enrichment. The primary objectives are to determine the:

1) Spatial-temporal distributions and structural relationships of magmatic and hydrothermal activity, particularly during and after emplacement of the Late Caledonian Strontian pluton.
2) Paragenesis, timing, structural setting and source of Pb-Zn carbonate-baryte mineralised veins around Strontian.
3) Potential of Strontian for metal extraction, including zinc, germanium, indium, and strontium.

Ultimately, the project begins from a local perspective, but can lead to the wider consideration of Scotland’s critical metal resources, particularly related to the Caledonian granites, and of processes applicable to mineralisation in poly-orogenic regions globally.

Methodology

The project will begin with a review of the geological context of the Northern Highlands, focusing on long-lived lineaments, the geodynamics of the Late Caledonian and later magmatic and structural events. The candidate will study existing mine maps, structural, lithological, geochemical, and geochronological data, as appropriate. They may 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 by request at the British Geological Survey in Keyworth, Nottingham. The key objectives of the project shall be fulfilled by:

Fieldwork and structural interpretation: a 25 km2 digital map of the northern end of the Strontian granite and surrounding lithologies will be completed, including a structural survey of minor intrusions, fault planes and mineral veins in order to determine cross-cutting relationships, kinematics, palaeostress configurations, and evidence for structural inheritance. An unrelated project at the Natural History Museum is currently working on whole rock geochemistry of the Strontian granite but there remains a notable lack of published geochronology and geochemistry from smaller-scale intrusions emplaced both before and after mineralisation. Therefore, minor intrusions and mineralised vein networks will be extensively sampled as a part of this field campaign. In-situ orientation and kinematic data will be analysed and a structural-tectonic model for magmatic and vein emplacement through relative time will be created using the stress inversion software package Wintensor™ and modelling packages Petrel™ and Move™.

Vein and magmatic geochronology: The candidate will take a multi-disciplinary approach to constrain the timing of magmatic and mineralising events around Strontian. Vein hosted calcite and epidote will be dated by U-Pb geochronology using laser ablation mass spectrometry (as per Holdsworth et al. 2020) while pyrite/chalcopyrite mineralization will be dated using Re-Os geochronology (as per Dempsey et al. 2021), both with collaborator Dr Dempsey at Hull. Having established the relationship of mineralisation to documented Caledonian or Permian-Carboniferous events, Zircon and titanite U-Pb laser ablation mass spectrometric analysis will then be used to explore magmatic evolution, emplacement, and cooling of the pluton and spatially-associated minor intrusions. Growth layers of zircon and titanite crystals analysed by U-Pb methods will also have their trace element abundances measured in order to constrain the timing of enrichment or depletion in various important elements such as strontium and the transition metals, building on the precedent of Gardiner et al. (2021). Lu-Hf radiogenic isotope studies of these layers will enable constraints to be placed on the extent of crustal involvement in melt sources and magmatic evolution over time.

Vein mineral genesis and metal distributions: vein minerals will be subjected to a petrographic, scanning electron microscope and electron microprobe survey in Glasgow and St Andrews to constrain ore paragenesis and the distribution of strontianite and other ore minerals. The carbonate veins will be analysed at microstructural scale using laser ablation mass spectrometry to determine how trace element concentrations vary across the mineralised region and if these can be matched to singular or multiple fluid types and sources. Stable isotope analysis (e.g., S, C, O) of vein materials can also be conducted to determine magmatic or meteoric sources of fluid. During geochronological analysis initial Pb and Os compositions will be determined, further revealing the origins of the metalliferous fluids.

Whole rock geochemistry: whole rock samples from minor intrusions will be sent externally for major and trace element analysis to determine their geochemical affinity in addition to any absolute U-Pb zircon dates. Of particular interest will be Ba-Sr-Ca concentrations in different phases of magmatism: is the Strontian pluton the ultimate source of alkaline earth metals or does younger magmatism also carry significant concentrations? Are such concentrations a product of mantle melting or crustal assimilation? In parallel, Pb-Zn vein mineralisation will be assayed to determine if there are high abundances of critical metals such as germanium and indium.

Project Timeline

Year 1

Literature review, fieldwork planning and execution, detailed structural analysis including stress inversion analysis, digitization of field data, rock sampling and dispatch for whole rock analysis. Sample preparation of calcite and extraction of zircons (Glasgow).

Year 2

The candidate will work with supervisors and laboratory staff to schedule appropriate continued sample preparation and laboratory analysis during this year, based on the techniques listed above. U-Pb zircon analysis will take place in Glasgow, whilst calcite, epidote pyrite/chalcopyrite and titanite dating will be undertaken at Hull, and zircon Hf isotope work will be done at St Andrews. Stable isotope analysis can be conducted at the Scottish Universities Environmental Research Centre in East Kilbride (SUERC). 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 analysis during the first half of the year. Interpretation and drawing together of results will address the key objectives of the project. Construction of a detailed 3D model of the structural framework of the orefield. There will be time to consider any additional sample collection, preparation, and analysis, considering the initial results. The student will attend a major international conference and shall begin preparing manuscripts for peer review alongside commencement of PhD thesis writing.

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.

The supervisory team are recognised experts across Scottish, regional, and global structural, magmatic, hydrothermal and resource geology, and will ensure the candidate gets access to a wide research network. Quantification of economic metals in magmas using zircons is at the cutting edge of Nick Gardiner’s exploration research (Gardiner et al. 2021). Dempsey is a field leader in the merging of detailed field based structural geology, microstructural geology, stress inversion analysis and geochronology in understanding orefield genesis (Dempsey et al. 2021) As part of their training, 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 different aspects of field and laboratory training from Iain Neill, Nick Gardiner, and Eddie Dempsey. Research technicians at Glasgow, St Andrews, Hull, and SUERC will assist with training in mineral extraction and preparation, scanning electron microscopy, electron probe micro-analysis, and laser ablation quadrupole and mass-collector mass spectrometry.

Applications for additional isotope support from the NERC Isotope Facilities Committee, 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. Finally, 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

Bruand et al. 2014 Journal of Petrology 55, 1619-1651 (Geochemistry of the Strontian pluton)

Dempsey et al. 2021 Journal of the Geological Society 178, jgs2020-226. (Example of orefield appraisal using structural and geochronological analysis)

European Commission 2020. Critical Raw Materials Resilience: Charting a Path towards greater Security and Sustainability. COM2020, #474

Gardiner et al. 2020 Chemical Geology 505, 20580 (Trace metals in zircon and application to ore genesis)

Holdsworth et al. 2019. Geology 47, 700-704 (Example of U-Pb calcite dating in understanding fracture fill in crystalline rocks)

Hutton 1988 GSA Bulletin 100, 1392-1399 (Discussion on emplacement of the Strontian pluton)

Kimbell 1986 British Geological Survey Report, nora.nerc.ac.uk/id/eprint/11813 (Geophysical Survey of Strontian)

Roberts and Walker 2016 Geology 44, 531-534 (U-Pb calcite dating techniques)

Upton et al. 2004 Geological Society Special Publications 223, 195-218 (Review of Permo-Carboniferous magmatism)

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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