What controls critical metal (Sn-W-Ta) deposit formation in granite batholiths? Unravelling the magmatic-hydrothermal evolution of the Bushveld complex and Cornubian ore-field

Overview

Large granite provinces that extend over 100s km are the refineries for many lithophile elements such as tin, tungsten, tantalum, and even lithium, that make up critical resources essential for new green technologies. Huge volumes of silicic magmas are formed during crustal injection of mafic magmatism and the melting of different crustal source regions over millions to tens of millions of years.

These granitic melts are transported to the mid and upper continental crust and incrementally emplaced forming reservoirs of melt where they can further concentrate metals of interest during fractional crystallisation. Ultimately these melts exsolve volatiles that may go on to form magmatic-hydrothermal ore deposits that contain key metals such as Sn, W, Li and Ta which can be extracted to provide the essential raw materials we need for transitioning to the low-carbon green economy.

It is however uncommon for the emplaced magmas to form economic ore deposits. Despite the protracted million-year timescales of magmatic system, the individual ore-forming events are short-lived and discrete. We know little about why and when ore deposits are formed during incremental batholith emplacement, or what role the emplacement rate and amount of melt present within the magmatic system have on the likelihood of forming an ore deposit. We don’t fully understand the mechanisms that preferentially concentrate metals in the magmatic system, and sometimes we don’t even know which magmatic event the observed hydrothermal ores are related to. Answering these questions is fundamental for new geological models of ore formation.

By using novel high-precision analytical techniques yielding new mineral chronometers (e.g. cassiterite U-Pb) we now have the potential to define precise and accurate timings (<0.1%) for hydrothermal events in granite systems by directly dating the ore. This exciting new opportunity in chronology will allow us to link the record of economic deposit formation of Sn (and associated metals) to high-precision temporal records of hosting granite system construction via zircon U-Pb dating at unprecedented resolution.

The zircon crystals we use to date the magmatic record also contain an important record of melt evolution. In-situ trace-elements including economic metals of interest and tracer isotopic analyses (e.g. Lu-Hf) can be used to track the concentration of metals and changes in source inputs in the magmas. Combined with geochronology these techniques allow us to evaluate how the rates of pluton construction and melt volumes can influence ore formation potential and what chemical signatures this leaves behind in the mineral record.

This project will apply these analytical techniques to two study areas where large-scale granite production is associated with abundant economic tin mineralisation. Fieldwork will be focused on the granites and Sn-deposits of the rapidly emplaced (~2 Myrs?) Bushveld complex (2055 Ma, South Africa) and the protracted magmatism (~20 Myrs) of the Early Permian Cornubian batholith and associated Sn-province (SW England), where mineral exploration in both terranes is currently undergoing a resurgence.

These areas represent anorogenic and post-orogenic granitic systems respectively and have excellent exposure allowing sample collection across the hosting rocks and through a number of economic deposits. Comparisons and contrasts will be drawn between the magmatic regimes and their world-class critical metal mineral systems.

The project will be hosted by the British Geological Survey and registered at the University of St Andrews. To accommodate the laboratory based ID-TIMS U-Pb geochronology, the student will split their time approximately 2:1 (BGS: St A.) over the course of the project. Regular travel between institutes will be strongly encouraged.

Click on an image to expand

Image Captions

IMG_20140521_105244010 cligga.jpg – “The Cligga Head W-Sn stock cross-cut by planar hydrothermal veins, Cornubian batholith, SW England. High-precision zircon and cassiterite U-Pb geochronology constrains their close genetic relationship (within ~200 kyrs), does this relationship occur elsewhere within the batholith?”

DSC_1038.JPG- “Granite-hosted cassiterite pipes in the Zaaiplaats tin deposit, Bushveld, South Africa. How much granitic melt was present at the time of their formation?”

Methodology

Field observations and sampling in the 2 study areas (Bushveld, South Africa and Cornwall, UK) will focus on the emplacement history of granite units using cross-cutting relationships and deep transects to build 3-D emplacement and ore histories.

Analytical work will be carried out at both at the British Geological Survey (BGS), and at the University of St Andrews.

At the Geochronology and Tracers Facility, BGS, the student will have access to instrumentation including: a newly installed SELFRAG (high-voltage fragmentation system) instrument for mineral separation; 2 SEMs with CL imaging capabilities; and laser ablation (LA) MC and SF -ICP-MS instruments (U-Pb characterisation and Lu-Hf isotopes). The major component of the analytical programme will be based at the BGS in the low-Pb blank clean laboratory suite, and use state-of-the-art TIMS with novel low noise amplifier technology to define high-precision (CA)-ID-TIMS U-Pb timescales for granite emplacement and ore events.

Mineral geochemistry on zircon (and other accessory phases) by laser ablation will be used to track crystallisation histories recorded by mineral growth within individual samples, and will be undertaken at the University of St Andrews, where the St Andrews Isotope Geochemistry Laboratory (StAIG) is equipped with a variety of solution-based and laser ablation MC-ICP-MS facilities to undertake in-situ trace element and isotopic analyses of mineral phases, including zircon U-Pb geochronology and Hf isotope analysis, as well as whole-rock geochemical and isotopic analysis. A range of other facilities including a new electron microprobe and SEM are also available to students.

Project Timeline

Year 1

Complete literature review, Field season 1 (Cornwall/South Africa) to collect samples.
Sample characterisation (petrography and chemistry) and mineral separation for geochronology.
LA-ICPMS U-Pb cassiterite geochronology training.
Attend national conference, poster presentation

Year 2

Field season 2 (Cornwall/South Africa) and sample characterisation (petrography and chemistry).
Zircon trace element analyses by LA-ICPMS.
Training in (CA)-ID-TIMS U-Pb geochronology for cassiterite and zircon.
Present initial results at national conference.

Year 3

Complete ID-TIMS U-Pb geochronology and tracer isotope analyses (Lu-Hf, O) where appropriate.
Prepare manuscript for publication.
Present results at international conference

Year 3.5

Timeline – Year 3.5 (6 months only)
Complete thesis writing, prepare further manuscripts for publication

Training
& Skills

The student will be provided with extensive training in radio-isotopic geochronology, mass spectrometry, data reduction/interpretation and H&S practices. Upon completion they will have developed field-leading analytical expertise.

The student will spend time both at the Geochronology and Tracers Facility BGS, Keyworth and at the University St Andrews, where they will join the Planetary Geodynamics Research Group. They will receive training in proposal writing, contributing to proposals for NEIF and EMIF isotope support grants.

It is anticipated a number of publications will arise from this study, and training in scientific manuscript preparation will be given.

The student will be provided with extensive training in radio-isotopic geochronology, mass spectrometry, data reduction/interpretation and H&S practices. Upon completion they will have developed field-leading analytical expertise.

The student will spend time both at the Geochronology and Tracers Facility BGS, Keyworth and at the University St Andrews, where they will join the Planetary Geodynamics Research Group. They will receive training in proposal writing, contributing to proposals for NEIF and EMIF isotope support grants.

It is anticipated a number of publications will arise from this study, and training in scientific manuscript preparation will be given

In addition to training offered through IAPETUS2 the student will be encouraged to undertake relevant courses available through the BGS such as: 4×4 driver training, R, MATLAB and Geostatistics.

References & further reading

Tapster, S.R. and Bright, J.W.G., 2020. High-precision ID-TIMS Cassiterite U-Pb systematics using a low-contamination hydrothermal decomposition: implications for LA-ICP-MS and ore deposit geochronology. Geochronology Discussions, pp.1-39.

Vonopartis, L., et al., 2020. Evaluating the Changes from Endogranitic Magmatic to Magmatic-Hydrothermal Mineralization: The Zaaiplaats Tin Granites, Bushveld Igneous Complex, South Africa. Minerals, 10(4), p.379.

Smith, W., et al., 2019. Zircon perspectives on the age and origin of evolved S-type granites from the Cornubian Batholith, Southwest England. Lithos, 336, pp.14-26.

Simons, B., et al., 2017. Fractionation of Li, Be, Ga, Nb, Ta, In, Sn, Sb, W and Bi in the peraluminous early permian Variscan granites of the Cornubian Batholith: Precursor processes to magmatic-hydrothermal mineralisation. Lithos, 278, pp.491-512.

Gardiner, N.J., et al., 2018. The crustal architecture of Myanmar imaged through zircon U-Pb, Lu-Hf and O isotopes: Tectonic and metallogenic implications. Gondwana Research, 62, pp.27-60.

Gardiner N.J., et al. 2017. Contrasting Granite Metallogeny though the Zircon Record: A Case Study from Myanmar. Scientific Reports 7, 748

Further Information

If you have any questions or would like to express interest in the project, please contact Simon Tapster: Simont@bgs.ac.uk

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