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The evolution of porphyry-epithermal gold systems using trace element mobility and concentration

Dr Dan Morgan (SEE), Dr David Banks (SEE), Dr Rob Chapman (SEE), Dr Steven Stackhouse (SEE)

Contact email: d.j.morgan@leeds.ac.uk

There is a significant amount of effort associated with understanding the genesis and evolution of copper- gold (Cu-Au) porphyry systems and their associated epithermal expressions,  due to their commercial importance based on dual commodities, (e.g. Halter et al. 2002, Kessler et al. 2002, Williams-Jones and Heinrich, 2005)) .  Consequently, this mineralisation is frequently well characterised at a regional and deposit scale. However, the strong association of critical elements such as Palladium (Pd), (Tarkian and Strybny, (1998), Thompson et al., (2002)), Selenium, (Se) or Tellurium (Te)) in parts of the mineralised system, (Fig 1) is poorly understood. Variability of trace elements and ligands may point to variability in the physico-chemical state of the mineralising system which affects transport and deposition of the economic metals.  The trace element composition of Cu-Au porphyry-epithermal systems may also be related to the tectonic environment and initial state of the intrusions. This project provides an opportunity to address questions of trace element mobility and concentration within evolving magmatic hydrothermal systems. A key principle underpinning this study is that much as minor and trace elemental analysis has revolutionised the interpretation of igneous geological systems since the 1960's, such analysis has not yet been applied to mineralised systems such as Cu-Au porphyries.

Figure 1, Examples of minor element assovciations in Cu-Au porphyries. left: temagamite (Pd3HgTe3) (white) in chalcopyrite, Afton alkalic Cu-Au porphyry, BC, Canada. right: Gold particle coated with Bi telluride, Nucleus porphyry, Yukon, Canada.

Objectives

This project focuses on three questions:

  1. What are the trace elemental signatures associated with magmatic hydrothermal gold mineralisation?
  2. What can they tell us about the evolving chemical environment during ore genesis?
  3. Which other (often critical) metals can be co-genetic with gold, and what are the controlling factors in their enrichment?

Approach

This study will investigate a suite of samples of in-situ gold mineralisation across the porphyry-epithermal transition.  Specific case studies could include Canadian alkalic and calc-alkalic porphyry systems such as Copper Mountain, BC Canada, (e.g. Fig 2), the Klaza porphyry-epithermal transition (Yukon, Canada) or porphyry-epithermal systems in Europe.  Studied systems will be characterised in terms of fluid evolution and associated mineralisation using a raft of techniques including petrography, electron microscopy, electron beam microanalysis, fluid inclusion analysis and trace elemental analysis. In addition, the project will apply laser ablation ICP-MS to Au grains to inform trace element partitioning in the first systematic study of its kind. Questions of elemental mobility, speciation, partitioning and the sequence of enrichment, mobilisation and deposition events within the porphyry systems will be addressed, to establish the physico-chemical controls of both ore and trace element distribution.

Fig 2 Copper Mountain  Cu-Au porphyry, British Columbia

This project in particular considers a broad-spectrum analysis of trace element abundances in natural gold particles forming from hydrothermal fluids.  In examining changes in trace element abundances from field and fluid perspectives, a P-T-X view of the mineralising system will be attained. The study of trace element behaviour provides opportunity to develop new insights into the fluid evolution and associated mineralisation of complex but economically important hydrothermal systems, and to create powerful tools for understanding their origins. There exists a further possibility to use some ab-initio modelling to gain a different perspective on the behaviour and speciation of ions in solution.

Potential for high impact

The project addresses the fundamental behaviour of elements within porphyry hydrothermal systems and as such will generate new understanding which may find application in many areas;

  1. Improved targeting of mineralised environments which have the potential to yield critical metals as by products,

  2. Development of analytical expertise applying LA-ICP-MS to Au alloy within implications for enhanced ‘fingerprinting’ of gold from different environments, This expertise could find application in diverse fields including gold provenancing (for ethical or archaeological purposes), and exploration, where the detailed trace geochemistry of gold specific to particular environments could aid exploration. This aspect builds on an existing large body of work undertaken at Leeds.

  3. New understanding of trace element partitioning will aid interpretation of  surficial samples routinely collected during reconnaissance exploration

Training

Candidates will benefit from a range of academic expertise in analytical techniques, ore deposit geology, geochemistry and igneous petrology.  Specifically the student will gain:

  1. Analytical skills, (LA-ICP-MS, SEM, EMP, fluid inclusion studies)

  2. Core geological skills: (RL and TL microscopy,)

  3. Field skills, (sample collection, logging)

  4. IT skills:  (ARC-GIS, Iogas, Leapfrog).

The candidate will undertake field studies in Canada and possibly Turkey, with all the associated implications for future employability. A field season of two summer months is standard for such work.

In addition, a wide range of training will be provided by the Faculty of Environment (http://www.emeskillstraining.leeds.ac.uk/). The broad spectrum of geological sciences represented in the project will provide the student with various possibilities to develop teaching skills through contribution to teaching as a demonstrator in both lab-based classes and on field courses. Research findings will be disseminated through presentations at both national conferences (e.g..MDSG) and international conferences, some of which are industry facing, such as the Vancouver Exploration Roundup. In this way the student will have access to first class networking opportunities in both academia and industry during their postgraduate career. The publishable outputs will include contributions to the flagship ore deposits literature (Economic Geology, Mineralium Deposita) , but the multidisciplinary nature and novelty of the research will also generate outputs for high impact journals such as Nature Geoscience and Geology.

The student will join a small team of ore deposits facing postgraduate students whose interests span critical metals, and Au formation in mineralisation associated with orogens. He/she will play a full role within the Ores and Minerals Group in IAG which holds regular in house academic seminars and benefits form a strong association with the SEG Chapter.

References

Halter, W. E., Pettke, T., Heinrich, C.A  2002 The Origin of Cu/Au Ratios in Porphyry-Type Ore Deposit   Science,  v. 296  no. 5574, pp. 1844 1846

Kesler, S. E., Chryssoulis,  S. L. , Simon, G.,  2002. Gold in porphyry copper deposits: its abundance and fate. Ore Geol Rev 21: p103–124

Tarkian, M. and Stribrny, B., 1999. Platinum-group elements in porphyry copper deposits: a reconnaissance study. Mineralogy and Petrology, v.65 p. 161-183

Thompson, J.F.H., Lang, J.R., and Stanley, C.R., 2002, Platinum group elements in alkaline porphyry deposits, British Columbia: British Columbia Ministry of Energy, Mines and Petroleum Resources, Exploration and Mining in British Columbia-2001, p. 57–64.

Williams –Jones, and A.E., Heinrich, C.A., 2005. Vapor Transport of Metals and the Formation of Magmatic-Hydrothermal Ore Deposits. Economic Geology 100, 7, p. 1287-1312.

Related undergraduate subjects:

  • Geochemistry
  • Geology
  • Natural sciences