An enhanced understanding of the thermal and fluid history of a Variscan metallogenic province from critical metal investigations: The antimony and tungsten-bismuth deposits of South-west England (dataset)

Critical raw materials (CRM) are those that are at risk of supply interruption while also being economically vital; many are essential to the manufacturing of components for technologies needed to achieve net zero greenhouse emissions. Knowledge and understanding of the geological processes that lead to the formation of CRM deposits is essential in exploration and geometallurgy. In this study, three CRMs, tungsten (W), antimony (Sb) and bismuth (Bi), that occur in the South-west England metallogenic province were studied. Investigations were carried out on Bi occurrence and criticality, wolframite as a source of tungsten and two case studies of CRM ore deposits: Hemerdon W mineralisation and Herodsfoot Sb mineralisation. The mineralogy and textures of 92 vein samples from deposits across the South-west metallogenic province were determined using classic and more advanced techniques (SEM-EDS mapping and QEMSCANTM) to understand the petrogenesis of both W and Sb deposits. This was combined with mineral chemical analysis (EPMA, LA-ICP-MS), fluid inclusion microthermometry and stable (O, S) and radiogenic (Pb) isotope analyses to constrain ore-bearing fluid compositions and sources. Comparison of exo- and endogranitic occurrences of W mineralisation in Southwest England indicates that there are a wide range of processes that can impact mineral chemistry and geochemical evolution, even in a seemingly similar geological setting. This produces mixed indicator signatures for the different deposits and a distinct signal appears to be the exception rather than the norm. Combining microthermometry with careful petrographic analysis, the role of fluids as a controlling parameter in the formation of wolframite mineralisation has been further investigated. Each of the deposits appear to have formed as a result of different crystallisation histories and fluid emanation processes and pathways. In the case of the endogranitic Cligga Head and Bray Down systems, cooling, neutralising, fluids controlled wolframite precipitation. At Hemerdon, vapour-rich H2O-CO2-NaCl-metal-bearing fluids exsolving from granite magmas drove hydraulic fracturing and precipitated wolframite. At St Michael’s Mount, isothermal mixing of meteoric fluids was an important driver in mineralisation. In the exogranitic deposits, two different mechanisms of mineralisation are evident. At Redmoor, fluid mixing of S-rich fluids with primary magmatic fluids drove the formation of polymetallic base metal-rich deposits, while at Castle-an-Dinas there is no evidence for fluid mixing in this S-poor deposit. Wolframite crystallisation in both vein deposits was likely initiated as a result of fracturing and subsequent pressure release. A case study of W mineralisation at Hemerdon shows the complex interplay of multiple fluid generations, with nine different major vein types identified, and thirteen different vein types in total have been characterised. From radiogenic and stable isotope studies there were several generations of fluids identified, those associated with the initial mineralisation, where W and Sn were decoupled, and a later Sn-B-rich stage which overprints this and was formed from fluids derived from the Hemerdon Deep granite. The distribution of wolframite end members ferberite (Fe-dominant) and hübnerite (Mn-dominant) was controlled by the fractional crystallisation of primary magmatic fluids. Alteration of the mineralisation by subsequent, likely meteoric, fluid circulation driven by later magmatic activity resulted in the ferberitisation of the wolframite mineralisation in some vein sets (Type 2b veins). The latter has resulted in the formation of ore that is challenging to process. Bismuth occurs widely in a variety of geological deposits (Skarn; Granite-related vein and greisen; Reduced intrusion-related Au-Bi; Porphyry-related; Iron oxide copper gold; Pegmatite; Five-element vein; Orogenic gold; Volcanogenic Massive Sulphide; Sediment-hosted copper), sometimes in sufficient quantities to be extracted as a by-product. Production could be enhanced through a series of interventions at the exploration and mining stages in a ‘full value mining’ approach to reduce criticality. The origin of the Herodsfoot Sb deposit remains enigmatic, highlighting the challenges of understanding the sources of CRMs. Microthermometric and stable isotope analyses have classified this deposit as a post-orogenic Pb-Zn+Sb hydrothermal deposit that formed from cool (~120°C), saline (up to 27 wt% NaCl+CaCl2 equiv.), hydrothermal fluids. A proximal mafic lithology is not the likely source of Sb at Herodsfoot based on a comparison of S isotopic analysis but does not exclude the possibility of the reworking of proximal syn-rift stratabound base metal mineralisation as a potential source. Alternatively, the deposit could represent the uppermost parts of a post-orogenic hydrothermal deposit where mixing of basinal brine with another similar temperature and salinity fluid, possibly of meteoric origin, resulted in the selective precipitation of Sb in a shallow setting.