NERC SSCP DTP – Imperial College London / Natural History Museum
PhD Project 2015
Fingerprinting fertile porphyry magmas using apatite
Background and Motivation
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, and recognizing the
potential fertility of a particular belt of rocks before too much costly and invasive
exploration has taken place, 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. Four key parameters
are high water content, a high oxidation state, and an abundance of sulphur and
halogens which act as ligands for metal transport and, in the case of sulphur, is
required to form the common sulphide ore minerals.
Apatite (Ca5(PO4)3(F,Cl,OH)) is a common accessory mineral in igneous rocks. It is a
remarkable probe of magmatic evolution for a number of reasons. (1) Its chemistry
tracks primary magmatic parameters such as aluminosity. (2) It incorporates rare
earth elements in its structure, in particular Ce and Eu, which vary according to
magma oxidation state. (3) It incorporates SO3 and therefore tracks melt sulphur
content. (4) It incorporates halogens so can be used to calculate melt halogen
contents. (5) It is a faithful recorder of the Sr and Y content of melts which monitor
plagioclase fractionation; important because this is suppressed in water-rich melts.
Plagioclase also affects Eu behaviour so that extraction of a redox signal from the Eu
content of apatite requires an understanding of the plagioclase fractionation history.
The aim of this project is to establish apatite chemistry and crystallization history in
a suite of igneous rocks from selected magmatic arcs. These will include rocks from
arc segments that contain relatively abundant porphyry deposits (e.g. Miocene of
Central Chile) and those that are unprospective for deposits (e.g. NeogeneQuaternary of the Southern Chilean Volcanic Zone). In addition, Rio Tinto will
provide access to a remarkable collection of apatite grains from more than 25
variably mineralized domains worldwide. Analysis of these samples will allow us to
identify differences in magmatic evolution pathways in mineralized as opposed to
barren arcs and define an apatite fingerprint that is characteristic of fertile magmas.
This would provide an extremely powerful tool for assessing the potential
endowment of arc segments prior to extensive invasive exploration. Ultimately, our
goal is to develop a process model that explains how typical arc magmas become
transformed into ore generating systems.
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: 19th January, 2015
For further details contact: j.wilkinson@nhm.ac.uk