PhD Project
Mineral trace element vectors
to Zn-Pb mineralization in the Irish orefield
Dr Jamie Wilkinson
AIM To evaluate structural, mineralogical and geochemical criteria as indicators of proximity to mineralized
centres and of their likely metal endowment.
BACKGROUND
Studies over the past two decades have built up a detailed understanding of the
geological and geochemical processes that led to the formation of major Zn-Pb(-Ag) resources in the Irish
Midlands Basin in the Lower Carboniferous (e.g. Wilkinson et al., 2005). We know that two fluids were
involved in the ore-forming systems: partially
evaporated seawater that evolved into a
potential ore solution by circulating into rocks
beneath the basin succession, heating, and
extracting metals; and saline brines, produced by
high degrees of evaporation that became
enriched in reduced sulphur by the action of
sulphate-reducing bacteria. Mixing between
these two fluids in permeable and/or reactive
lithologies in structurally-controlled locations
resulted in the formation of economic
mineralization. Despite this knowledge, the
implications of this model, particularly in terms
of the geochemical fingerprint left by the oreThe Irish Midlands. Peat bog and farmland hide major Zn-Pb depos- forming process in rocks distal to the ore zone
its, formed in carbonate sediments beneath the seafloor around 350
itself, have yet to be fully explored for possible
million years ago. Exploration can be assisted by new tools that extract maximum information on the imprint of hydrothermal fluids, exploration tools. Much of the potential host
rock sequence in Ireland is covered by younger
preserved in drillcore.
deposits and peat bogs so expensive drilling of
blind targets is required. Thus, extracting the maximum amount of information from these drillcores that can
assist with targeting further drilling could result in improved exploration success.
OBJECTIVES The principal objective of the study is to identify trace element signatures in pyrite and
dolomite/calcite that define a zonation pattern centred on known mineralization. A secondary objective will
be to test the potential of Zn isotopes as a tool for fingerprinting the presence of hydrothermal Zn in whole
rock samples on the fringes of mineralized systems where Zn concentrations are indistinguishable from
background. For the trace element study we will focus on pyrite in the medial to distal positions lateral to
mineralization and in the hangingwall stratigraphy above mineralization. Pyrite is likely to concentrate certain
trace elements (e.g. Ni, As, Co, Pb, Mn) that may be sensitive discriminators of proximity to ore. The
distribution of trace elements within individual pyrite crystals will be mapped to characterise multi-stage
pyrite growth. Analyses of carbonates will focus on hydrothermal dolomite and veins/breccia cements above
known mineralization. Hydrothermal dolomite is known to be depleted in Mn, HREE and perhaps Sr and Na
proximal to mineralization so these are anticipated to be enriched in distal carbonates. Sampling at known
distances from mineralization may enable tools to be developed that not only predict direction to the
hydrothermal centre, but also the distance.
METHODS
An initial literature review of
sediment-hosted hydrothermal systems, both fossil
and active, will identify characteristic element
dispersion patterns on local (tens to hundreds of
metres) and wider (several kilometre) scales. A
compilation of available mineralogical and
geochemical data on the Irish orefield will be made
to help target parts of the system for further
focused study and identify potentially useful trace
elements. Fieldwork will primarily involve
Cartoon model for mineralization illustrating possible extent of
examination of geological information (cross- hydrothermally influenced pyrite and distribution of hydrothermal
sections, drillhole logs), coupled with drillcore carbonates in an Irish mineralizing system.
logging and sampling. Sampling will focus on black
and white hydrothermal dolomite, hydrothermal calcite and pyrite. Samples will be collected both from wellconstrained areas where the nature and extent of mineralization is well understood and from prospects for
comparative testing. Samples will be studied using conventional microscopy and cathodoluminescence (CL)
utilising the new CL-microscopy laboratory at Imperial College. Features such as extent and complexity of
bright luminescence zones in hydrothermal dolomite which may reflect quantitative precipitation of iron
sulphides in the ore deposit will be indentified and quantified. Fluid inclusion analyses will be carried out
where necessary to constrain the thermal and chemical properties of hydrothermal fluids. Mineral chemical
analyses will be carried out using the LODE laser ablation ICP-MS facility to
determine trace element signatures. Trace element mapping of selected samples
will determine zonation patterns. Zinc isotope analyses of trace Zn in wholerock
samples collected at various distances from mineralization will be carried out to
test whether this technique can be used to identify a hydrothermal signature and
vector towards mineralization.
WIDER IMPLICATIONS The research will help develop a better understanding
of element mobility in carbonate-hosted hydrothermal systems and will provide
new insights into mineralizing processes in the Irish orefield. The aim is to develop
new tools that provide reliable vectors toward hydrothermal centres and allow
discrimination between major and minor zones of mineralization.
STUDENT PROFILE 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/geochemistry and gain experience working within a major
international mining and mineral exploration company. 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 will be encouraged.
TRAINING
Banded sphalerite, galena
The successful student will join the LODE research group in and white calcite from the
geochemistry and ore formation in the attractive environment of South Galmoy Mine, Ireland.
Kensington, London, that includes researchers from Imperial College London and
the Natural History Museum (NHM). The student will have the opportunity to work in the state-of-the-art
analytical suite at the NHM. The student will receive training in core logging and sampling, laboratory best
practice, SEM techniques, laser ablation ICP-MS instrumentation and analysis, sample digestion and
multicollector ICP-MS analysis of Zn isotopes, data reduction and statistical analysis methods. Presentation of
results at regular knowledge transfer meetings with Teck geologists in Ireland will be required. Attendance
and presentation of results at major UK and international conferences will be supported in the research
programme. All postgraduates in the Department of Earth Science and Engineering have access to
workshops organised by the Graduate School of Engineering and Physical Science which include: personal
organisation and effectiveness; thesis writing and completing the PhD; technical writing; teamwork;
professional issues in science; research ethics; and presentation skills. There are also optional courses in
career planning, IT skills, media and entrepreneurship. Attendance at regular seminars on ore geology,
geochemistry and the wider Earth Sciences is compulsory.
FUNDING Funding for the project is currently being sought from industry. Applications for additional
support for conference and workshop attendance will be made to the Society of Economic Geologists
student grant program.
FURTHER INFORMATION If you are interested in the project and would like to have further details
please contact Jamie Wilkinson at j.wilkinson@imperial.ac.uk
http://www3.imperial.ac.uk/people/j.wilkinson
http://www3.imperial.ac.uk/earthscienceandengineering/research/lode