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.