Detecting hydrothermal alteration of granite from magnetic susceptibility

Project highlights

The magnetic susceptibility of granite is affected by the action hydrothermal fluids that serves to leach out or deposit new magnetic minerals.
Hydrothermal alteration can occur during emplacement and crystallisation, associated with the alteration of granitic mineral (greissenisation), deposition of key resources (Li, U) and be an indicator of geothermal potential.
Clear sustainability and resource potential applications and links.

Overview

Granite intrusions are key targets for deep geothermal energy and for key resources needed for mobile and renewable technologies (e.g., Li and U). The way these resources are concentrated geologically is related to both emplacement and crystallisation of the granite magma and subsequent exposure to hydrothermal systems. Hydrothermal fluids can originate from within the granite body from volatiles concentrated as the magma crystallises and from meteoric fluids that circulate through fault zones that intersect granite. The key exploration targets are hydrothermally altered zones where important minerals become concentrated by the activity of hydrothermal fluids.

Marginal pegmatite rich, aplitic and leucocratc zones in granite plutons often exhibit markedly lower magnetic susceptibility (e.g., Stevenson et al., 2007; Stevenson 2009). Unpublished data from the Tregonning Granite, Cornwall (Fig .1), show negative magnetic susceptibilities. In these cases the accumulation of volatiles has altered the granite and reduced the amount of iron bearing minerals such as magnetite. In this project you will study the effect of hydrothermal alteration on the magnetic properties of granite. The aim is to assess the viability of magnetic susceptibility as an exploration tool. As part of this aim, your objectives are to carry out detailed rock magnetic analyses that will establish the magnetic mineralogy and along side this to study the mineralisation history and hydrothermal alteration.

Work will include field work involving structural and mineralogical mapping of key granite sections. This mapping will use digital mapping and drone survey techniques (e.g., Fig. 1). Field work will primarily aim to collect a suite of oriented block samples form a range of hydrothermal granite outcrops. Potential field areas include Cornwall, Scotland, Ireland and can make use of existing samples in house and in collaboration with other ongoing projects.

3D photogrammetric model of Rinsey Cove, Cornwall, part of the Tregonning Granite: https://skfb.ly/o7KJI

Figure 1: 3D photogrammetric model of Rinsey Cove, Cornwall, part of the Tregonning Granite.

Host

University of Birmingham

Theme

Dynamic Earth

Supervisors

Project investigator

Dr. Carl Stevenson, University of Birmingham ([email protected])

Co-investigators

Dr. Marco Maffione, University of Birmingham ([email protected])

How to apply

Each host has a slightly different application process.
Find out how to apply for this studentship.
All applications must include the CENTA application form.
Choose your application route

Methodology

Rock magnetic analyses: detailed magnetic characterisation of the magnetic minerals. These require oriented sampling of granite outcrops. In the lab AMS is carried out on numerous sub samples. This technique is relatively rapid but the focus in this project is to establish as much detail as possible on the magnetic properties of the granite samples to enable useful exploration tools using basic susceptibility measurements. Magnetic characterisation involves measuring different magnetic properties including the remanence, partial acquisition remanence spectra, anisotropy of anhysteretic remanence (AARM) and susceptibility measured at a range of temperatures.

Structural and mineralogical mapping of granite sections: Digital capture techniques including digital mapping using FieldMOVE app, Gigapan and drone based photogrammetry 3D modelling. Structural models based on detailed field mapping will be carried out using MOVE suite.

Training and skills

Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.

Rock magnetic lab analyses: range of rock magnetic analyses and laboratory methods including oriented field sampling and lab preparation. Other laboratory skills involve demagnetisation steps, operation of spinner magnetometer and kappabridge apparatus. Low temperature thermomagnetic analyses require the handling of liquid nitrogen.

Structural mapping using FieldMOVE app, structural modelling using MOVE structural software.

Drone surveys will use DJI Mavic Pro 2 for field digital capture and Agisoft Metashape for photogrammetric 3D modelling.

Further details

If you wish to apply to the project, applications should include:

A CENTA application form, downloadable from: CENTA application

A CV with the names of at least two referees (preferably three and who can comment on your academic abilities)

Submit your application and complete the host institution application process via: https://sits.bham.ac.uk/lpages/LES068.htm. and go to Apply Now in the PhD Geography and Environmental Science (CENTA) section. Please quote CENTA23_B14 when completing the application form.

Applications to be received by the end of the day on Wednesday 11^th January 2023.

Additional information for international applicants

All international applicants must ensure they can fulfil the University of Birmingham’s international student entry requirements, which includes English language requirements. For further information please visit https://www.birmingham.ac.uk/postgraduate/pgt/requirements-pgt/international/index.aspx.
Please be aware that CENTA funding will only cover University fees at the level of support for Home-fee eligible students. The University is only able to waive the difference on the international fee level for a maximum of two successful international applicants.

Possible timeline

Year 1

Background theory, granite structural geology, magnetic theory, fieldwork planning and location selection, field sampling training, drone survey training, initial field campaign, initial magnetic analyses.

Year 2

Magnetic analyses and characterisation experiment design, further fieldwork, structural modelling and analyses.

Year 3

Completion of magnetic analyses and consolidation of granite hydrothermal and magnetic properties (publication on this), assessment of exploration methodology based on magnetic properties and development of calibration or key learnings and outputs (publication)

This project may also work with industry partners. Part of the work plan will involve initiating partnerships and there is therefore the opportunity to set up and carryout placements with relevant partners.

COVID-19

If there is a complete lockdown then field work can lab work cannot proceed and we will have to consider how much theoretical background reading is worthwhile as well and how much we can use existing samples or acquire samples from collaborators. Depending on the nature of any covid related restrictions, the project field programme can adapt to work locally and accommodate social distancing, hygiene, avoid public transport and wearing of face covering. Field work otherwise does not require close working and so can proceed under most restrictions provided training can be accomplished. Most training can be done remotely and if training is curtailed we can make use of remote field supervision using remote mobile wifi.