Department of Earth Science and Engineering, Imperial College London
and Department of Earth Sciences, Natural History Museum
PhD Project 2016
Titanite occurrence and chemistry as a monitor of melt redox state, magmatic
fertility and hydrothermal alteration in porphyry ore systems
Ore deposits are exceedingly scarce. Only one in every one thousand prospects that
are explored by companies is eventually developed into a mine, and the location of
such prospects has already consumed significant time and resources. Consequently,
understanding what controls the location of ore deposits, recognizing the potential
fertility of a particular belt of rocks before too much costly and invasive exploration
has taken place, and exploring efficiently within such domains are important for
reducing risk, energy use and environmental impact.
Porphyry-type ore deposits source much of the copper, molybdenum, gold and
silver utilized by humankind and are the largest and most complex geochemical
anomalies known in the Earth’s crust. They are related to magma generation in
subduction zones, the place where most of Earth’s continental crust is created. Most
of the time, the magmas produced are erupted or are emplaced at depth as barren
plutons. Rarely, they are transformed in such a way that they become capable of
generating abundant, metalliferous hydrous fluids. At present, there is a limited
understanding of what triggers this change in magma fertility although one key
parameter is a high oxidation state. Once fluids are released into the surrounding
rocks, they produce characteristic alteration patterns with the most widely
developed but most cryptic being the propylitic alteration zone. Recent work has
shown that trace element abundance in propylitic minerals can be used to identify
proximity to a hydrothermal centre and can therefore be a powerful tool for
exploration (Wilkinson et al., 2014; J. Geochem. Explor., v. 152, p. 10-26). However, a
better understanding of the controls of such trace element patterns is essential if
this is to be applied routinely by the minerals industry.
Titanite (CaTiSiO4)(O,OH,F) is a common accessory mineral in igneous rocks and is
particularly abundant in more oxidized and alkaline, metaluminous compositions in
the intermediate to felsic range (diorites to granodiorites) and under hydrous
conditions. It is common in propylitic environments, particularly as a byproduct of
the reaction Ti-biotite + Ca2+ → chlorite ± titanite. It is potentially a remarkable
probe of magmatic and hydrothermal evolution, not only because of its redox
sensitivity but because it can substitute other major ions into the Ti site (e.g. Al3+,
Fe3+, Sn4+, V4+, Zr4+, Nb5+, Ta5+), the Ca site (e.g. Mg2+, U, Th, Mn and Pb), undergo
coupled substitutions of the REE into the Ca site (e.g. 2Ca2+ = Na+Y3+ or K+Y3+), or
undergo substitution of one of the oxygens in the unit cell by OH- or F- in
conjunction with substitution of Ti4+ by Al3+ or Fe3+. In addition, the incorporation of
U and Th allows suitable samples to be dated by radiometric methods. Low grade
metabasites (such as may be difficult to distinguish from hydrothermally altered
rocks in the propylitic domain of porphyry systems) typically contain low U
concentrations so there is potential to use this as a discriminator of propylitic
titanite. Because it is fairly hard, it can be weathered out and become concentrated
in stream sediments, thus has potential in exploration screening programmes.
The aim of this project is to establish titanite chemistry and crystallization history in
a suite of igneous rocks and associated, propylitically altered, country rocks from
selected magmatic-hydrothermal systems. Timing and conditions of formation of
titanite will be constrained using a range of geothermobarometers and
oxybarometers, and titanite chemistry will be modelled utilizing crystal-chemical
considerations. By comparing results from barren granite-related environments
(e.g. Augusta, Maine) with mineralized porphyry systems (e.g. Oyu Tolgoi, Mongolia)
we will be able to assess whether a diagnostic fingerprint exists for titanite from
mineralized systems. The chemistry of titanite in the propylitic domain will be
considered in terms of precursor biotite or hornblende composition and the
conditions of formation. Where possible, we aim to date titanite grains in order to
provide a first-order constraint on models for the development of propylitic
alteration halos.
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.
Excellence in geochemistry and mineralogy are essential; experience of
microanalytical techniques and statistical data evaluation are desirable.
Involvement with the Imperial Student Chapter of the Society of Economic
Geologists and outreach activities at the Natural History Museum will be
encouraged.
Closing date for applications: 15th January, 2016
For further details contact: j.wilkinson@nhm.ac.uk