Project highlights

  • Fieldwork examining ultramafic/mafic alkaline and carbonatites
  • First experimental determination of the partitioning of chalcophile metals into carbonatite melts, building on new empirical advances in the subject area
  • Implications for mineral exploration for Cu-Ni-PGE deposits in unconventional settings

Overview

Alkaline magmatic systems are the products of low-degree partial melting of their mantle source and produce intrusions that range in composition from alkaline ultramafic/mafic, through syenitic to granitic and carbonatitic. The low degree of partial melting means they are enriched in incompatible elements (e.g. REE) and volatiles (e.g. H2O, CO2 and halogens). While much work has been done to understand the REE behaviour and resource potential of both the alkaline silicate and carbonatite systems (e.g. Goodenough et al., 2021), the behaviour of their contained base and precious metals is much more poorly known. Mineralization can, however, be significant, as exemplified by the vast Cu (and PGE) resource in the Phalaborwa carbonatite, South Africa.

Recent work on the ultramafic/mafic parts of alkaline systems has identified a particular metallogenic ‘DNA’, with magmatic sulfides rich in Cu, Au and Te, but relatively low Ni and PGE compared with more conventional magmatic sulfides in tholeiitic-komatiitic systems (Holwell et al., 2019). In addition, these sulfides have an intimate association with volatile phases, including carbonates and it has been suggested CO2, can effectively increase the buoyancy of sulfide droplets, enhancing upward transfer of the metals into the crust (Blanks et al., 2020; Fig. 1). However, this association is likely lost in the mid-crust, where carbonatite melts tend to separate from the alkaline silicate portion of the magmas. Experimental studies show that the partitioning of REE into the carbonate melt phase is dependent on the timing of immiscibility (Nabyl et al., 2020).

This project will take a combined empirical and experimental approach to investigating the factors determining whether chalcophiles become concentrated in carbonatite (e.g. Phalaborwa) or remain in the alkaline ultramafic portions (e.g. Mordor Complex) during (ultra)mafic silicate-carbonate melt immiscibility. The UK’s only ultrapotassic igneous suite, the Lochs Borralan, Ailsh and Loyal complexes, together with a co-eval carbonatite at Loch Urigill, Scotland, will be used as an ideal natural laboratory for this study. This will expand to other locations globally and constrain the controls on chalcophile metal potential of alkaline ultramafic vs carbonatite systems that can be used in exploration for unconventional magmatic sulfide deposits.

Host

University of Leicester

Theme

  • Dynamic Earth

Supervisors

Project investigator

Co-investigators

How to apply

Methodology

The project will initially work on samples from the Loch Borralan, Ailsh, Loyal and Urigil intrusions in Scotland and involve a field visit. In addition, the BGS have cores from the Loch Borralan that can be worked on and sampled from Keyworth. Mineralogical and geochemical characterisation, using ZEISS quantitative SEM techniques and a combination of bulk rock XRF and in situ laser ablation analysis. Follow up fieldwork will involve more global case study areas. The origin of any carbonate and sulfur in the systems is important to constrain and so C-O and S isotopes will be analysed for all sample suite in the case study areas. The empirical observations and data will be then be tested with experimental work. This will be facilitated by Charlie Beard and collaborators.

Training and skills

The student will gain field sampling and mapping training from K Goodenough and others in the team. Training using ZEISS Mineralogic techniques, geochemical techniques (XRF and LA-ICP-MS) and stable isotope training will be given at UoL. Sulfur isotope analysis and training will be facilitated through an application to the NERC Isotope facilities. Experimental work will involve collaboration with external collaborators. The student will develop their transferrable skills with attendance and presentations at workshops and meeting that involve the minerals industry, presenting their work in academic and industrially relevant contexts.

Partners and collaboration

Partners for this project include the British Geological Survey (Kathryn Goodenough has extensive experience in researching alkaline magmatic systems, and fieldwork in Scotland and around the world). Partner universities around the world include Universite Grenoble Alpes (Charlie Beard is an expert in alkaline-carbonatite systems) and the University of Western Australia (Marco Fiorentini’s current work focusses on magmatic sulfides in alkaline ultramafic systems).

Further details

For further details, please contact Dr David Holwell [email protected]

To apply to this project please visit: https://le.ac.uk/study/research-degrees/funded-opportunities/centa-phd-studentships

Possible timeline

Year 1

Fieldwork in Scotland at Loch Borralan/Ailsh/Loyal/Urigil, and core facility at BGS. Mineralogical, geochemical at UoL. Identification of further case study areas. Preparation and planning of experimental work.

Year 2

Field visit to another case study location (TBC during Year 1). Isotopic analysis at UoL and application to NERC isotope facilities. Draft/submission of paper on Scottish example. Major part of work.

Year 3

Experimental work, further analysis of global case studies experimental. The student will be expected to present their work in progress in at least one UK and one international conference per year

Further reading

Blanks DE, Holwell DA, Fiorentini ML, Moroni M, Giuliani A, Tassara S, González-Jiménez J-M, Boyce AJ, Ferrari E. 2020. Fluxing of mantle carbon as a physical agent for metallogenic fertilisation of the crust. Nature Communications, 11, 4342, https://doi.org/10.1038/s41467-020-18157-6

Goodenough KM, Deady EA, Beard CD, Broom-Fendley S, Elliot HAL, van den Berg F, Ozturk H. Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings: Storehouses of Rare Earth Elements. Journal of Earth Science, https://doi.org/10.1007/s12583-021-1500-5

Holwell DA, Blanks DE. 2020. Emplacement of magmatic Cu-Au-Te(-Ni-PGE) sulfide blebs in alkaline mafic rocks of the Mordor Complex, Northern Territory, Australia. Mineralium Deposita,  https://doi.org/10.1007/s00126-020-01015-2

Holwell DA, Fiorentini M, McDonald I, Lu Y, Giuliani A, Smith DJ, Keith M, Locmelis M. 2019. A metasomatized lithospheric mantle control on the metallogenic signature of post-subduction magmatism. Nature Communications 10, 3511, https://doi.org/10.1038/s41467-019-11065-4

Nabyl Z, Massuyeau M, Gaillard F, Tuduri J, Iacono-MArziano G, Rogerie G, Le Trong E, Di Carlo I, Melleton J, Bailly L. 2020. A window in the course of alkaline magma differentiation conducive to immiscible REE-rich carbonatites. Geochimica et Cosmochimica Acta 282, 297-323.

Styles MT, Gunn AG, Rollin KE. 2004. A preliminary study of PGE in the Late Caledonian Loch Borralan and Loch Ailsh alkaline pyroxenite-syenite complexes, north-west Scotland. Mineral Deposita 39:240–255

 

COVID-19

COVID-19 may impact the ability to travel to field sites and external laboratories. The first field site is in the UK, and is accessible. In addition, cores are available to work on at the BGS in Keyworth. The supervisory team have a large sample suite that would be available if sampling is delayed. Almost all of the analytical work will be done at UoL. Visits to external collaborators and labs are planned, but if they are not possible, samples can be sent to be analysed in the respective labs.