The origin of propylitic alteration halos in porphyry systems

A close up image of Gersdorffite

Gersdorffite is associated with hydrothermal veins and magma derived from sulphite deposits © The Trustees of the Natural History Museum, London

Understanding the origin of district-wide and localised propylitic alteration in porphyry systems and constrain the nature of the fluids that flux metals through these zones.

Project background 

Porphyry systems represent the world's principal source of copper and molybdenum and are major repositories of gold and silver (Cooke et al., 2014a).

These deposits originate from huge volumes of metal-bearing hydrothermal fluid that exsolved from crystallising crustal magma reservoirs. Recent studies have shown that the propylitic alteration halo – the most extensive zone of alteration associated with porphyry centres – can extend for more than 5 km from the ore deposit itself, and that magmatic fluids are likely to contribute to its development even over such large distances (Pacey et al., 2020).

We also now know that some of the alteration minerals that develop within these halos, such as epidote and chlorite, can crystallise with characteristic compositions that are typical of the porphyry environment and which can vary systematically with distance from the centre of the system (e.g. Cooke et al., 2014b; Wilkinson et al., 2015, 2017, 2020).

Despite this new understanding (see Hollings and Orovan, 2020), we still do not know how such huge volumes of alteration develop, in terms of the origin and nature of the fluids involved and their flowpaths. For example, there are no published data on the chemistry of propylitic fluids, derived by LA-ICP-MS analysis, and very few geochronological studies that constrain the timing and possible multi-stage origin of these alteration systems. There is only one extensive modern study of the isotopic composition of propylitic minerals.

Project objectives

In order to understand the origin of propylitic halos on a district scale it is necessary to integrate mapping with large scale sampling, petrography, mineral chemistry, geochronology and fluid inclusion studies.

This will allow a model to be developed that constrains the relative timing of fluid flow events, the structural and lithostratigraphic controls of fluid flow, and the pressure-temperature-compositional evolution of the fluids involved. Numerical modelling may be utilised to test alternative scenarios that can account for the observations.

Project methods

An initial literature review of propylitic alteration in porphyry systems will compile data on the scale, mineralogical zonation and mineral chemistry patterns observed.

A suitable field area will then be identified, likely to be in collaboration with an industry partner, which will be the focus of the study.

Fieldwork will involve extensive mapping of the propylitic alteration assemblages in relation to their host volcanic and intrusive igneous sequences and associated structures. Where necessary, intrusions will be dated using the zircon U-Pb LA-ICP-MS method in order to pin key geological events. Samples collected from across the district will form the basis of subsequent mineralogical, geochemical, geochronological and fluid inclusion analysis.

Samples will be studied using conventional microscopy, hot cathode cathodoluminescence (CL) and electron beam instruments housed at the Natural History Museum in order to establish mineral assemblages and paragenesis.

Analysis of minerals by analytical SEM and LA-ICP-MS methods will determine the residence of major and trace metals to link metal fixing and release to specific fluids. Dating of alteration assemblages will be done using novel titanite, apatite, epidote and potentially hydrothermal zircon U-Pb geochronology by LA-ICP-MS. Fluid inclusions from propylitic veins will be studied using detailed microscopy and CL methods to constrain their relative timing of trapping, and their compositions, density and P-T trapping conditions determined by conventional microthermometry and LA-ICP-MS at the Natural History Museum.

Wider implications

The research will provide new insights into the origin of district scale alteration associated with porphyry centres and the connections to the long-lived magmatism that typically precedes porphyry ore-forming events.

There will be significant implications for porphyry exploration in terms of better models for interpreting mineral chemistry zonation patterns that are now widely applied by industry, improved geochronology of alteration events and better prediction of porphyry fertility signals.

Who we are looking for

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/geochemistry and gain experience in a range of mineralogical and geochemical analytical methods.

Excellence in geochemistry and mineralogy are essential; experience of microanalytical techniques and statistical data evaluation are desirable. A desire for involvement with the Imperial Student Chapter of the Society of Economic Geologists and outreach activities will be beneficial.

Training

The successful student will join the London Centre for Ore Deposits and Exploration (LODE) research group in the attractive environment of South Kensington, London, that includes researchers from 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 Natural History Museum.

The student will receive training in field mapping, core logging and sampling, laboratory best practice, 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 in the Department of Earth Science and Engineering have access to workshops organised by the Graduate School of Engineering and Physical Science which include: personal organisation and effectiveness; thesis writing and completing the PhD; technical writing; teamwork; professional issues in science; research ethics; and presentation skills. There are also optional courses in career planning, IT skills, media and entrepreneurship.

Attendance at regular seminars on ore geology, geochemistry and the wider Earth Sciences is compulsory.

How to apply

Applications for this PhD are processed through the Natural History Museum.

To apply please send the following documents to the Postgraduate Office at postgradoffice@nhm.ac.uk:

  • Curriculum vitae.
  • Covering letter outlining your interest in the PhD position, relevant skills training, experience and qualifications for the research, and a statement of how this PhD project fits your career development plans.
  • Names of two academic referees.

The deadline for applications is 4 January 2021.

Apply for this project

Find out more about the Science and Solutions for a Changing Planet Doctorate Training Program on their website

To apply, send your application to postgradoffice@nhm.ac.uk.

Application deadline: 4 January 2021

Any questions?

Natural History Museum

Main supervisor: Prof Jamie Wilkinson

Supervisors

Imperial College London

Prof Jamie Wilkinson

 

Science and Solutions for a Changing Planet (SSCP) doctoral training partnership

This is a joint project between The Science and Solutions for a Changing Planet (SSCP) Doctoral Training Partnership at Imperial College London and The Natural History Museum.

Funded by 

References

Cooke, D.R., Hollings, P., Wilkinson, J.J., and Tosdal, R.M., 2014a, Geochemistry of porphyry deposits in Holland, H.D., and Turekian, K.K., eds., Treatise on Geochemistry, 2nd Edition, v. 13, Oxford, Elsevier, p. 357-381.

Cooke, D.R., Baker, M., Hollings, P., Sweet, G., Chang, Z., Danyushevsky, L., Gilbert, S., Zhou, T., White, N., Gemmell, J.B., and Inglis, S.,  2014b, New advances in detecting the distal geochemical footprints of porphyry systems - Epidote mineral chemistry as a tool for vectoring and fertility assessments, in Kelley, K.D., and Golden, H.C., eds., Society of Economic Geologists, Special Publications, v. 18, p. 127-152.

Orovan, E. and Hollings, P, 2020, Exploring the Green Rock Environment: An Introduction. Economic Geology, 115, 695-700. 

Pacey, A., Wilkinson, J.J., Boyce, A.J. and Millar, I.L., 2020, Oxygen, hydrogen and strontium isotopic constraints on the origin of propylitic alteration in porphyry deposits: insights from the Northparkes district, NSW, Australia. Economic Geology, 115, 729-748. 

Wilkinson, J.J., Chang, Z., Cooke, D.R., Baker, M.J., Wilkinson, C.C., Inglis, S., Chen, H., and Gemmell, J.B., 2015, The chlorite proximitor: A new tool for detecting porphyry ore deposits: Journal of Geochemical Exploration, v. 152, p. 10-26.

Wilkinson, J.J., Cooke, D.R., Baker, M.J., Chang, Z., Wilkinson, C.C., Chen, H., Fox, N., Hollings, P., White, N.C., Gemmell, J.B., Loader, M.A., Pacey, A., Sievwright, R.H., Hart, L.A., and Brugge, E.R.,  2017, Porphyry indicator minerals and their mineral chemistry as vectoring and fertility tools, in McClenaghan, M.B. and Layton-Matthews, D., eds., Application of indicator mineral methods to bedrock and sediments: Geological Survey of Canada, Open File 8345, 2017, 67-77. .

Wilkinson, J.J., Baker, M.J., Cooke, D.R. and Wilkinson, C.C., 2020, Exploration targeting in porphyry Cu systems using propylitic mineral chemistry: a case study of the El Teniente deposit, Chile. Economic Geology, 115, 771-791.