Using Magnetic Susceptibility to examine lithium deposits in granitic intrusions

Granite intrusions are key targets for deep geothermal energy and for key strategic resources needed for mobile and renewable technologies (e.g., Lithium, Tin, Tungsten, Tantalum, Ceasium and Rubidium). The way these resources are concentrated geologically is related to both emplacement and crystallisation of the granite body and subsequent exposure to hydrothermal systems. Hydrothermal fluids can originate from within the granite magma from volatiles concentrated as it crystallises and from meteoric fluids that circulate through fault zones that intersect the intrusion. The key exploration targets are hydrothermally altered zones within and around the granite intrusion where economically important minerals are found in exploitable concentrations.

Understanding of the structures and focussing mechanisms of economic domains from exposed rocks is needed enhance the geological models and predictive targets for new deposits within granites.

Marginal pegmatite rich, aplitic and leucocratic zones in granite plutons often exhibit markedly lower magnetic susceptibility (e.g., Stevenson et al., 2007; Stevenson 2009). Unpublished pilot data from the Tregonning Granite, Cornwall, show negative magnetic susceptibilities in those areas. In these cases the accumulation of volatiles has altered the granite and reduced the amount of iron bearing minerals such as magnetite, leading to a lower magnetic susceptibility. In this project we will study the magnetic properties of the regions within an immediately around granite intrusions affected by hydrothermal alterations, which typically carry economically relevant minerals. The overarching aim of this project is to assess the viability of magnetic susceptibility as an exploration tool. To this aim, our objectives are to carry out detailed rock magnetic and structural geological analyses that will establish the connection between magnetic mineralogy, faults (which controls hydrothermal alteration paths) and mineralisation in granite bodies The events will be linked to radio-isotopic geochronological data to interpret how and when these features evolved from magma to late alteration (a 4-dimensional view).

Work will include field work involving structural and mineralogical mapping of key granite sections. This mapping will use digital mapping and drone survey techniques. 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.