Detrital mineral records of fertile porphyry systems

Aim

The study would focus on the combined use of key detrital minerals (zircon, apatite, titanite, magnetite) for the assessment of porphyry exploration districts utilising both igneous and hydrothermal fertility signals and geochronology.

In previous studies, we have developed chemical/textural criteria for distinguishing between igneous and hydrothermal apatite, titanite and magnetite and these would be developed and expanded to include hydrothermal zircon. Experiments would aim to assess the number of grains/analyses required for a robust assessment and develop multivariate and machine learning tools that combine coupled zircon-apatite and other mineral datasets.

Preliminary work (M. Loader, R. Sievwright and E. Brugge PhD projects) has shown that hydrothermal apatite and magnetite chemistry can reflect both alteration assemblage (potassic, propylitic, sericitic) and local grade. This will be explored further to develop a system centre targeting tool of use in both in situ and detrital samples. Quantification of composite particle types and distribution could also be explored.

Deliverables

• Compilation and expansion of databases on zircon, apatite, titanite, magnetite chemistry from a variety of porphyry ore and non-ore systems
• Develop individual mineral tools for porphyry discrimination, igneous/hydrothermal origin, alteration assemblage, fertility and grade signatures
• Develop combined multivariate/ML tools for fertility assessment
• Quantify grain numbers/analytical measurements required for robust assessments
• Refine workflows for rapid and cost-effective analysis of detrital samples

Methods

An initial literature review of igneous and hydrothermal accessory mineral chemistry in porphyry systems will compile data on the scale, mineralogical zonation, geochronology and spatiotemporal mineral chemistry patterns. Fieldwork will involve mapping and sampling of igneous units in the selected case study district, and sampling of key drainage catchments.

Where necessary, intrusions will be dated using zircon U-Pb LA-ICP-MS in order to pin key geological events. Both in situ and drainage samples collected from across the district will form the basis of subsequent mineralogical, geochemical and geochronological analysis.

Samples will be studied using conventional microscopy, hot cathode cathodoluminescence (CL) and electron beam instruments housed at the Natural History Museum (NHM), in order to establish mineral relationships, textures and chemistry. Analysis of minerals by automated (TESCAN TIMA) SEM, analytical SEM, microprobe and LA-ICP-MS methods (including LA-ICP-MS mapping) will determine the residence of major and trace elements. Where necessary, dating of rock-hosted and detrital zircon, apatite, titanite and epidote will be done using U-Pb geochronology by LA-ICP-MS.

Wider Implications

The research will provide new insights into the multi-million-year and district-scale evolution of igneous complexes that produce economic porphyry-copper deposits. The project will be of direct and immediate benefit to BHP’s exploration programmes globally.

There are currently few case studies that look at the incorporation of mineral chemistry into the assessment of detrital mineral assemblages, and none that look at samples from a holistic perspective (they typically focus on one or a couple of minerals). The study will also provide a template for equivalent work that must be completed for other mineralisation styles where drainage sampling could be used to accelerate the path to discovery.

Student Profile

We are looking for a well-qualified and highly motivated Earth Sciences/Geology graduate who wishes to carry out a cutting-edge PhD in economic geology/mineralogy/geochemistry and gain experience in a range of mineralogical and geochemical analytical methods. Being a team player and having excellence in geochemistry and mineralogy are essential. Experience in microanalytical techniques and statistical data evaluation, including data analytics such as machine learning methods, is desirable. A desire for involvement with MSci student project supervision and outreach activities will be beneficial.

Training

The successful student will join the London Centre for Ore Deposits and Exploration (LODE) research group, based at the Natural History Museum, in the attractive environment of South Kensington, London, and will be registered for their PhD degree at Imperial College London.

The LODE group includes researchers from University College London, Imperial College London and the Natural History Museum. The student will have the opportunity to work in the state-of-the-art analytical suite at the NHM and will receive training in field mapping, core logging and sampling, detrital mineral sampling, laboratory best practice, mineral separation, SEM techniques, laser ablation ICP-MS instrumentation and analysis, geochronological methods, data reduction and statistical analysis.

Attendance and presentation of results at major UK and international conferences will be supported in the research programme. All postgraduates have access to transferable skills workshops at Imperial College London and additional professional development and public engagement opportunities at NHM. Attendance at regular seminars on ore geology, geochemistry and the wider Earth Sciences is required.

Funding

The project will be fully funded by BHP Minerals. Applications for further support for conference and workshop attendance will be made to the Society of Economic Geologists student grant program and other sources.

Further information

If you are interested in the project and would like to have further details, please contact j.wilkinson@nhm.ac.uk. Applications will be via the NHM applications portal.