Why are barren porphyry systems unmineralized?

Project Highlights

• First detailed case study characterization of a well-drilled but barren porphyry centre.
• Identification of key process differences between barren and mineralized systems.
• Development of mineral proxies to discriminate barren from fertile porphyry systems.

Overview

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 igneous accessory minerals such as zircon and apatite can retain critical chemical information that allows us to distinguish magmas that are predisposed to form porphyry ore deposits (e.g. Loader et al., 2017; Nathwani et al., 2020, 2021). However, we do not really understand processes operating at the deposit level that result in the failure of a mineralizing system to develop, even when the right type of magmas are involved.

Key research questions

This project aims to answer the following key research questions:

1. What igneous petrogenetic features, physico-chemical properties of fluids, fluid-rock interactions or timescales differ between barren and well-mineralized porphyry centres?
2. Are the fingerprints of these processes preserved in igneous accessory or alteration minerals that can be used as proxies for the discrimination of barren and fertile systems?
3. How can the established porphyry ore genetic model be modified in the light of the research findings?

Methodology

Fieldwork will be carried out on a case study site (potentially in Peru) where 3D sampling can be done on a barren porphyry system. Drillcores will be logged in detail to identify intrusive units, describe contact relationships and define a vein and breccia – and associated alteration – chronology.

Analysis will involve conventional whole rock multielement lithogeochemistry, mapping of sections in PPL, XPL and mineral mapping using automated SEM to define paragenetic relationships and identify grains for analysis. SEM-EDS, microprobe and LA-ICP-MS will be used for multielement mineral chemistry. Representative samples of key stages will be used for fluid inclusion microthermometry to constrain fluid properties and P-T conditions. Geochronology of targeted phases will be carried out to constrain timing and duration of hydrothermal activity.

Results will be compared with relationships and parameters derived from well-studied porphyry deposits as published in the literature. Phases identified as potential proxies for key processes (or lack thereof) will undergo further analysis to build a sufficient dataset for the development of machine learning algorithms for system discrimination.

Possible timeline

Year 1: Literature review, development of porphyry geology understanding, possible field training course (SEG or other). Main field campaign, start to put samples through the preparation workflow and initial petrographic work. Report on petrographic study at conference 1.

Year 2: Major analytical phase, including the majority of imaging and mineral chemistry analysis. Identify samples for fluid inclusion study and get sample preparation underway, plus initial cathodoluminescence and fluid inclusion analyses. Identify samples for geochronology and initiate study. Report on mineral chemistry results at conference 2.

Year 3: Complete fluid inclusion microthermometric study and analyse selected inclusion samples by LA-ICP-MS. Complete geochronology. Extend database on target minerals and develop discrimination tools. Report on main findings of research at conference 3. Plan thesis structure and begin write-up.

Training and skills

TARGET researchers will participate in a minimum of 40 days training over the 3.5 years of study composed of:

• an annual one-week workshop dedicated to their year group, and tailored to that cohort’s needs in terms of skills development – for the first three years of their study;
• an annual all-TARGET workshop with cross-year interactions, advanced training and opportunities to specialise in particular areas – all years of study;
• a number of one-day workshops;
• additional online events and in-person workshops attached to relevant conferences.

At NHM, the student will receive training in lab safety and occupational health, sample preparation and instrumental methods. Optional courses in transferable and various technical skills including use of software and coding, and public engagement opportunities, will be available. At Imperial College, technical training in machine learning methods will be provided. Training in fieldwork and porphyry geology will be done by Wilkinson, attendance on field training courses (e.g. SEG) and by BHP technical staff. Training in rock petrography and mineral analysis will be mainly completed at the

NHM using instruments in the Imaging and Analysis Centre supervised by Wilkinson and the lab support team.

Partners and collaboration (including CASE)

The student will be based primarily at NHM but will work with the co-supervisor and other collaborators at Imperial College and the CASE partner as appropriate and will be part of the student cohorts both at NHM and Imperial. Demonstrating opportunities for undergraduate classes at Imperial will also be available. It is expected that the student will present the results of the research at a workshop for technical teams in BHP in person at their head office in Toronto, as well as to the local technical teams in-country.

Requirements

Applicants should have a First Class or good Upper Second Class MSci/MGeol degree or BSc + relevant Masters in geology/geoscience, with particular interests in hydrothermal mineralogy, geochemistry and igneous petrology and a passion for mineral exploration and ore genesis.

Further details

For further details please contact Jamie Wilkinson