Overview

Post-subduction magmatism is a common phenomenon in former volcanic arcs and is increasingly recognised as an important control on the formation of exceptional ore deposits. Some of the world’s largest and/or highest grade copper and gold deposits are associated with arc magmatic systems that were active after subduction ceased. These post-subduction ore deposits are also notable for their enrichment in a wide range of trace elements and minerals, including “critical” elements such as Te, Pt, Pd, Bi and Sb.

Your project will investigate Lion One Metals’ Tuvatu project, Viti Levu, Fiji. Tuvatu is an epithermal gold deposit that represents one of a number of mines and prospects (e.g. Vatukoula, Mt Kasi) associated with post-subduction volcanism in Fiji. These deposits occur along a trend referred to as the Viti Levu lineament.

The shoshonites (potassic volcanic rocks) and monzonites of the Tuvatu caldera host a low sulfidation epithermal deposit with high-grade gold telluride-bearing veins. Lead supervisor Smith is leading an international consortium to study post-subduction Au-Te deposits, and this project represents a key part of that portfolio. The collaboration with Lion One Metals offers an exceptional opportunity to develop models of pre-ore igneous petrogenesis, hydrothermal fluid evolution, and the architecture of the resulting ore deposit. This project will build upon previous studies of mineralisation of Tuvatu.

The aim of this project is to unravel the magmatic evolution of the Tuvatu caldera, with comparison to centres elsewhere in Fiji, on- and off-trend of the Viti Levu Lineament. You will link the mineralisation to the magmatic evolution, and the subsequent interaction between hydrothermal fluids and the alkaline host rocks (following on from the models of Smith et al. 2017), to build a detailed, process-based genetic model for the Tuvatu deposit.

Key research questions include:

Why do alkaline, post-subduction magmas host exceptionally Au and Te rich ores?

Is the hydrothermal evolution of the Tuvatu deposit controlled by the potassic and alkaline nature of the host rocks?

Do the potassic post-subduction rocks of Fiji carry enhanced Au ad Te to the upper crust?

View from the Tuvatu deposit to the Sleeping Giant and interior of Viti Levu.

Methodology

You will work with Lion One Metals in Fiji to collect data and samples from Tuvatu, including surface samples and subsampling of drill core. These data will be supplemented by samples previously collected by the supervisors. The Tuvatu samples will be further supplemented by regional sampling on- and off- the trend of the Viti Levu Lineament in Fiji.

Analytical work will include detailed characterisation of unaltered igneous material, alteration zones from within the Tuvatu deposit, and samples with Au-Te mineralisation. Samples will be measured for bulk chemistry, mineralogy, texture and micro-analytical chemistry using world class instrumentation at the University of Leicester and the British Geological Survey, including optical microscopy, scanning electron microscopy with solid state elemental analysers, LA-ICP MS and 3D X-ray tomography. You will be trained in the use of these facilities, as well as supporting software packages for the collation and processing of data, including ArcGIS and Micromine.

Training and Skills

CENTA students benefit from 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes. 

You will become proficient in the use of analytical equipment including quantitative evaluation of minerals (UOL), SEM (UoL), and high-resolution mass spectrometry (UoL and BGS). This combination of state-of-the-art analytical methods to investigate novel and fundamental scientific questions will provide you with a unique set of skills that will be attractive to industrial and academic employers. You will join a thriving community of igneous and applied researchers, and work closely with members of two major NERC-funded projects (FAMOS - From Arc Magmas to Ore Systems, and TeaSe – Te and Se Cycling and Supply).

Timeline

Year 1: You will compile the existing datasets on the geology of Fiji and the post-subduction mineral deposits. You will compile a literature review of post-subduction magmatism and mineralisation to ascertain the “state of the art” in recognising and interpreting such systems. You will organise and carry out fieldwork with Lion One Metals. You will receive training in the use of key analytical facilities.

Year 2: You will continue to analyse and interpret data, and build a case for any further necessary sampling in Fiji. You will be supported in the development of manuscripts for publication as a key part of your career development. You will be embedded in the BGS for at least three months to experience a different research environment, and access unique facilities, expertise and training.

Year 3: Integration of data will provide a model for magma evolution and mineralisation in the Tuvatu system. Compare and contrast with other post-subduction sites (including data collected on previous Leicester projects in Colorado and Romania). Synthesise findings to define a holistic model for the genesis of post-subduction magmatism and mineralisation at Tuvatu.

Partners and collaboration (including CASE)

Dan Smith is a lead researcher on projects linking magmatic evolution and mineral deposit formation. His expertise includes geochemical modelling of hydrothermal water-rock interactions. Andrew Miles investigates magmatic processes in relation to crustal evolution using the chemistry of robust accessory minerals. Dave Holwell is an expert in economic geology, ore mineralogy and geochemistry. Jon Naden is an expert in hydrothermal ore deposits and leads the Science coordination team for SoS Minerals, a £15M, internationally-funded mineral deposit research programme. Stephen Mann is the managing director of Lion One Metals, and brings a wealth of expertise in mineral exploration and mining, the geology of Fiji, and economic gold mineralisation. Paul Spry is a world leader in gold telluride mineralisation, including deposits at Tuvatu and Vatukoula in Fiji.

Subject to confirmation, it is anticipated BGS will be a CASE partner in this PhD. BGS will provide additional funds to be used flexibly to support fieldwork, visits and placements at external laboratories, and attendance at international workshops. The student will be expected to spend at least 3 months at the BGS Keyworth site where they will receive training and use a variety of analytical facilities. They will also have access to a range of BGS in-house CPD-style training courses as described in the GeoSchool handbook (http://www.bgs.ac.uk/training/home.html).

Further Details

Contact Dr Dan Smith, University of Leicester, djs40@le.ac.uk