Copper transport in porphyry ore deposits
Dr. Andrew Berry (Australian National University) & Dr. Jamie Wilkinson
AIM To determine the species responsible for complexing with and transporting copper in porphyry ore
deposits and constrain mechanisms for copper sulphide precipitation.
BACKGROUND
Porphyry systems represent the world's principal source of copper. These deposits
originate from huge volumes of metal-bearing hydrothermal fluid that exsolved from crystallising crustal
magma chambers. Critical to the understanding of the transport of metals in this system are the mechanism
and timing of fluid exsolution, the partitioning of Cu between the melt and fluid, and subsequent Cu
partitioning between vapour and brine should the fluid "boil". The behaviour of Cu in all these processes is
governed by how it is complexed in the fluid.
The importance of metal complexing, or
speciation, in the porphyry context can
be
illustrated
by
our
limited
understanding of the possible link
between porphyry and high sulphidation
epithermal deposits. It is traditionally
believed that porphyry ores form from
high-density brines although recent fluid
inclusion data suggest that low salinity,
View of the giant open pit at the Bingham porphyry-copper deposit, Utah, USA
intermediate density liquids and vapours
may also be important (e.g. Audetat et al., 2008). The subsequent evolution of such vapours and their
migration to shallower crustal depths has been proposed to explain the formation of spatially associated
epithermal deposits (Heinrich et al., 2004). This model requires that copper can be transported in the vapour
phase, possibly as a volatile sulphide (HS-) species. Depositional mechanisms are also linked to the type of
metal complex in solution: boiling (loss of H2S) or the crystallisation of pyrite (loss of dissolved S) may be
important precipitation mechanisms for metals transported as sulphide complexes, while cooling or dilution
may result in the breakdown of chloride complexes. However, our understanding of these key transport and
deposition questions is hindered by a lack of experimental data.
OBJECTIVES The principal objective of the study is to determine the speciation of Cu in natural and
synthetic fluid inclusions at hydrothermal conditions using X-ray absorption spectroscopy (XAS).
METHODS Samples of the fluid that formed porphyry ore deposits are preserved as micron-sized fluid
inclusions in minerals such as quartz. These can be analysed using techniques such as laser ablation ICP-MS
to determine composition and XAS to determine speciation (Berry et al., 2009). Inclusions from selected
porphyry systems will be reheated to the entrapment temperature allowing the speciation of the metals to
be determined at hydrothermal conditions. XAS experiments will be undertaken at the newly completed
Diamond Light Source synchrotron (www.diamond.ac.uk). Synthetic inclusions will be prepared in the
experimental petrology laboratory in the Royal School of Mines Building at Imperial College. These will also
be analysed to constrain speciation in simple, controlled chemical systems and will allow the effects of
parameters such as temperature, pressure and salinity to be differentiated. Subject to access approval, more
detailed study involving field sampling will be carried out to evaluate transport and depositional processes in
a particular porphyry ore deposit.
WIDER IMPLICATIONS The results of this work will improve our understanding of how porphyry deposits
form, impacting on exploration models, and will provide broader insights into Cu transport in crustal fluids.
STUDENT PROFILE We are looking for a well-qualified and highly motivated Earth Sciences/Geology
graduate with a strong interest in geochemistry and hydrothermal mineralization. Experience of geochemical
analytical methods would be an advantage.
TRAINING The successful student will join a vibrant research group in geochemistry at Imperial College.
The student will receive training in laboratory best practice, sample preparation methods, fluid inclusion
petrography and analysis, X-ray absorption spectroscopy and data reduction methods. The project will
involve collaborations with the Natural History Museum and the University of Tasmania.
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
REFERENCES
Audétat, A., Pettke, T., Heinrich, C.A., and Bodnar, R.J., 2008, The composition of magmatic-hydrothermal
fluids in barren and mineralized intrusions: Economic Geology, v. 103, p. 877-908.
Berry, A.J., Harris, A.C., Kamenetsky, V.S., Newville, M., and Sutton, S.R., 2009, The speciation of copper in
natural fluid inclusions at temperatures up to 700 ˚C: Chemical Geology, v. 259, p. 2-7.
Heinrich, C.A., Driesner, T., Stefansson, A., and Seward, T.M., 2004, Magmatic vapor contraction and the
transport of gold from the porphyry environment to epithermal ore deposits: Geology, v. 32, p. 761-764.