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Reading Gold: Investigating the link between chemical composition, microstructure and geological history of gold

Dr Daniel Morgan (SEE), Dr Sandra Piazolo (SEE), Dr Robert Chapman (SEE), Dr David Banks (SEE), Dr Thomas Mueller (SEE)

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

This exciting project aims to shed light on the long standing problem of how we “read” gold. That is, how can we deduce from the chemistry, habit and microstructure of gold, its origin and geological history. This knowledge is urgently needed to improve our ability to understand how gold deposits from and therefore to find new gold deposits. You will benefit from the existing and unique suite of gold samples at SEE and supplement it with some targeted sampling. Much of the work will be lab-based microstructural and geochemical analyses. Depending on student’s interests investigations will be augmented by a choice of in-situ real-time experiments, and/or nanoscale chemical and/or  structural analysis.

Project background

Studies of natural gold grains have traditionally focused on the variation in alloy composition, and in particular the ratio of Au and Ag, the two main elements. Variations in this ratio are routinely reported, particularly in the context of placer mining where the miner’s revenue is dependent on the actual composition of the ‘gold’. The recognition of gold alloy variation underpinned studies of the relationships between lode gold and placer gold which have become progressively more sophisticated, i) as the range of minor alloying elements considered has increased (e.g. Antweiler and Campbell 1977), ii) as the role of mineral inclusions within the gold has been considered in tandem with the alloy composition (e.g. Chapman et al. , 2017) and iii) as trace element compositions are revealed by LA-ICP-MS analysis (Chapman et al. 2016).

During the past few years, parallel studies of natural gold have examined the internal structures of natural gold as revealed by electron back-scatter diffraction (EBSD) studies, (e.g. Hough et al. 2008, Pearce et al. 2016). This approach adds new information to the traditional chemical data, as the microstructure of a mineral such as gold can yield information on the post-depositional thermal history of the mineral through the annealed textures (e.g. Piazolo et al. 2005, 2006). To date, correlation between the detailed microstructure and composition has been explored only superficially in the context of plausible processes of formation of the near-ubiquitous gold-rich rims found on placer gold grains, and to a lesser extent regarding internal heterogeneity of both Au and Ag-rich tracks observed in many gold particles.

At Leeds (UoL) we have been studying natural gold for over 20 years, and have a compositional data base describing over 30,000 gold grains. This work was initially undertaken with the British Geological Survey, (BGS) (e.g. Chapman et al. 2000) and more recently with a range of collaborators in the context of Canadian Cordilleran and Alaskan metallogeny (e.g Chapman et al. 2010, 2016, 2017). We have unparalleled experience in the compositional variation in natural gold both within and between different styles of gold mineralisation.

This experience and sample base is augmented with top-of-the-range scanning electron microscopy (SEM) equipped with EDS and EBSD detectors and electron probe micro-analyser (EPMA) facilities along with laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) (Chapman et al. 2017b) to reveal generic compositional signatures and to characterise compositional heterogeneity within individual gold particles (Figure 1).  Most recently we have used time of flight (TOF) LA-ICP-MS in a close collaboration with Tofwerk, Switzerland, to generate trace element maps of gold particles which have revealed features of natural gold previously unrecorded, (Figure 2).  A upcoming  exciting addition to the laboratory capabilities will be a heating stage for the SEM to observe in real time the changes in the chemistry and microstructure of metals (e.g. gold) and salts (e.g. Piazolo et al. 2006, Borthwick and Piazolo, 2010) (Figure 3 and for example see movies ( http://dx.doi.org/doi:10.1016/j.tecto.2006.06.007 (supplementary materials)).

This project is the first to combine approaches of high end compositional study and gold microstructures. The project remit is to subject samples of natural gold to analysis by both approaches to establish which compositional textures are a function of the environment of precipitation and which are a consequence of post-depositional processes. Initial characterisation of gold by this route is expected to inform the choice of further specialised analytical approaches to further unravel the compositional complexities; these may include atom probe tomography (Piazolo et al. 2016a, Fougerouse et al.2016), synchrotron based experiments (Borthwick et al. 2012) and SIMS.

Project remit

The project will investigate features of natural gold both in the context of compositional and microstructural information. Figure 1 shows examples of commonly observed compositional heterogeneity within gold. It appears likely that some of these features may be a consequence of post depositional process rather than reflecting variation in conditions of mineralisation. Their true nature will be revealed through in-depth microstructural and –chemical studies.

Figure 1:  examples of compositional heterogeneity in particles of natural gold revealed in SEM backscattered electron images. A: A common association of a gold- rich (palest grey) track bordering an Ag-rich (darkest track) film. B complex array of zones and tracks where minor cross- cutting relationships can be observed.

Both small ‘exotic’ inclusions and the ‘clusters’ of trace elements observed by TOF-LA-ICP-MS may be formed either as a consequence of extreme partitioning at the point of crystallisation or as a consequence of post-depositional processes (Fig. 2). Correlation of the microstructure such as fractures, subgrains and grain boundaries of the gold particle with detailed compositional studies will illuminate the origins of these features. In addition, in-situ experiments within the SEM will help to develop an in-depth understanding how chemical changes are related to changes occurring during heating and/or deformation (Fig. 4).  

Figure 2: TOF-LA-ICP-MS image of a placer gold grain from The Cariboo gold district, British Columbia showing highly irregular distribution of Pd at trace levels.

Fig. 3 Changes in microstructure due to heating; here in-situ experiments of Ni thin film (Piazolo et al. 2004) shown as EBSD maps, different grain colours different crystallographic orientations, lines signify grain boundaries (black: high angle, yellow & blue: low angle;  left: before heating, right: after 3 hr of heating. Note the changes in grain boundary network which in gold are likely to result in redistribution of trace elements.

Main scientific questions to be tackled

  1. To what extent is natural gold alloy compositionally heterogeneous, and what processes are responsible for these features?
  2. Can we interpret gold grain heterogeneity in terms of depositional and post-depositional processes?
  3. What aspects of gold grain compositional variations have significant implications for industrial exploration methodologies?

Figure 4: Use of EBSD to delineate crystal domains within a gold grain. A: reflected light image of gold particle form vein, Klondike, Yukon, Canada. B: same grain showing straight twin boundaries with different crystallographic orientations colour coded. C:  Example of how compositional data and crystallographic data may be combined. Gold rich areas (green) on interior of gold particle lack new grain boundaries. Silver-depleted area results in slight distortion of straight twin (arrowed).


Project elements

Initially the backbone of the project will be based on the following:

  1. Characterisation of selected samples of auriferous material (vein material, placer gold particles) using SEM, EPMA (and LA-ICP-MS if appropriate)
  2. Correlation of compositional and crystallographic orientation information describing various features of natural gold to illuminate the nature of both depositional and post depositional processes
  3. In-situ experiments of selected samples, watching in the SEM the changes in chemical distribution along with changes in the microstructure.

The project has the capacity to evolve in a variety of directions according to the interests and expertise of the candidate: these might include application of the outcomes to scenarios relevant to the exploration industry (including field work and sampling), modelling of processes according to theoretical criteria for comparison with observational data sets involving different processes and modelling approaches including diffusion modelling (e.g. Morgan & Blake, 2006, Morgan et al., 2014) and/or microdynamic modelling (e.g. Piazolo et al. 2010), or detailed atomic scale chemical and structural analysis using atom probe tomography (Piazolo et al. 2016a). It is envisaged that the student will get the opportunity to visit collaborators with expertise in the field of choice, e.g. visit of CSIRO (Perth, Australia), the Australian Centre for Microanalysis and Microimaging (Sydney, Australia), University of Tuebingen (Germany).

Potential for high impact outcome

This project is the first to marry compositional studies at trace element level with detailed microstructural analysis at the same scale. In addition, it will concentrate on the dynamics of the processes influencing both chemical composition and microstructural development. This approach will inevitably lead to an improved understanding of the nature of a familiar and economically important element.

Additionally, UoL is uniquely placed to interpret the results of the study on account of the existing library of natural gold samples. A major incentive for the project is that it will make a powerful contribution to our ability to define mineralizing conditions at many localities worldwide where detrital gold particles provide the only source of information on their source. This knowledge will inform design of exploration campaigns according to the constraints on geological setting provided by interpretation of gold particle textures.

Training

The student will work under the supervision of the project team: Dr Daniel Morgan, Dr Sandra Piazolo, Dr David Banks (IGT) and Dr Rob Chapman (IAG) within the School of Earth and Environment, with possible contributions from other members of staff according to the evolution of the project (Dr Thomas Muller).

The project will involve gaining familiarity with a variety of state of the art analytical techniques. This will include SEM-based analysis (BSE imaging, CL imaging, EDS analysis, EBSD and TKD analysis (Piazolo et al. 2016b), electron microprobe, LA-ICP-MS along with the necessary sample preparation using equipment such as Focussed Ion Beam milling. Furthermore, the student will receive training in planning, conduction and interpretation of in-situ experiments. Depending on the chosen focus of the project this will be augmented with training in numerical modelling and atom probe tomography analysis.

Within the supervisory team there are experts for each of these fields; this along with the technical staff at the University of Leeds, will ensure excellent training.

In addition, the student will have access to a broad spectrum of training workshops offered in house e.g. image analysis,  presentation skills, through to ‘managing your degree’ and ‘preparing for your viva’ (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

The successful student will have a strong background in either applied or physical sciences demonstrated by high marks in relevant undergraduate and/or postgraduate modules or final dissertation project. The ability to clearly communicate results visually and in writing is also essential. Experience in gold mineralisation, laboratory techniques, microanalytical techniques, experiments and/or numerical modelling is desirable but not essential. Postgraduate study (MSc or undergraduate masters), industrial experience or any publication record are also highly regarded.

References

Antweiler J, Campbell W (1977) Application of gold compositional analyses to mineral exploration in the United States. Journal of Geochemical Exploration 8:17-29.

Borthwick VE; Schmidt S; Piazolo S; Gundlach C (2012) Quantification of mineral behavior in four dimensions: Grain boundary and substructure dynamics in salt, Geochemistry, Geophysics, Geosystems, 13, . doi: 10.1029/2012GC004057

Chapman R, Leake R, Moles N, Earls G, Cooper C, Harrington K, Berzins R (2000) The application of microchemical analysis of alluvial gold grains to the understanding of complex local and regional gold mineralization: a case study in the Irish and Scottish Caledonides. Economic Geology 95:1753-1773.

Chapman RJ, Mortensen JK (2016) Characterization of Gold Mineralization in the Northern Cariboo Gold District, British Columbia, Canada, Through Integration of Compositional Studies of Lode and Detrital Gold with Historical Placer Production: A Template for Evaluation of Orogenic Gold Districts. Economic Geology 111:1321-1345. doi: 10.2113/econgeo.111.6.1321.

Chapman, R.J., Mileham, T.J., Allan, M.A., Mortensen, J.K., (2017). A distinctive Pd-Hg signature in detrital gold derived from alkalic Cu-Au porphyry systems. Ore Geology Reviews. 83: 84-102.

Chapman, R.J., Banks, D.A. and Spence-Jones, C. (2017b): Detrital gold as a deposit-specific indicator mineral, British Columbia: analysis by laser-ablation inductively coupled plasma–mass spectrometry; in Geoscience BC Summary of Activities 2016, Geoscience BC, Report 2017-1, p. 201–212.

Fougerouse, Denis, Steven M. Reddy, David W. Saxey, William DA Rickard, Arie Van Riessen, and Steven Micklethwaite. "Nanoscale gold clusters in arsenopyrite controlled by growth rate not concentration: Evidence from atom probe microscopy." American Mineralogist 101, no. 8 (2016): 1916-1919.

Hough RM, Butt CRM, Fischer-Buhner J (2009) The crystallography,metallography and composition of gold. Elements 5:297–302

Morgan, D.J., and Blake, S. (2006) Magmatic residence times of zoned phenocrysts: introduction and application of the binary element diffusion modelling (BEDM) technique.   Contributions to Mineralogy and Petrology Volume: 151 Issue: 1 Pages: 58-70

Morgan, D. J.; Jollands, M. C.; Lloyd, G. E.; and Banks, D.A. (2014)  Using titanium-in-quartz geothermometry and geospeedometry to recover temperatures in the aureole of the Ballachulish Igneous Complex, NW Scotland.  Deformation Structures and Processes Within the Continental Crust, Special publication of the Geological Society no. 394, p.145-165

Piazolo S; La Fontaine A; Trimby P; Harley S; Yang L; Armstrong R; Cairney JM (2016a) Deformation-induced trace element redistribution in zircon revealed using atom probe tomography, Nature Communications, 7, . doi: 10.1038/ncomms10490

Piazolo S; Kaminsky FV; Trimby P; Evans L; Luzin V (2016b) Carbonado revisited: Insights from neutron diffraction, high resolution orientation mapping and numerical simulations, Lithos, 265, pp.244-256. doi: 10.1016/j.lithos.2016.09.011

Piazolo S; Jessell MW; Bons PD; Evans L; Becker JK (2010) Numerical simulations of microstructures using the Elle platform: A modern research and teaching tool, Journal of the Geological Society of India, 75, pp.110-127. doi: 10.1007/s12594-010-0028-6

Piazolo S; Bestmann M; Prior DJ; Spiers CJ (2006) Temperature dependent grain boundary migration in deformed-then-annealed material: Observations from experimentally deformed synthetic rocksalt, Tectonophysics, 427, pp.55-71. doi: 10.1016/j.tecto.2006.06.007

Piazolo S; Jessell MW; Prior DJ; Bons PD (2004) The integration of experimental in-situ EBSD observations and numerical simulations: A novel technique of microstructural process analysis, Journal of Microscopy, 213, pp.273-284. doi: 10.1111/j.0022-2720.2004.01304.x

Piazolo S; Prior DJ; Holness MD (2005) The use of combined cathodoluminescence and EBSD analysis: A case study investigating grain boundary migration mechanisms in quartz, Journal of Microscopy, 217, pp.152-161. doi: 10.1111/j.1365-2818.2005.01423.x

Pearce, M.A., Gazley, M.F., Grimshaw, M.R., Jane, M. 2016. Silver Depletion in Detrital Gold Particles During Weathering, Transport, and After Deposition: A Western Australian Example. Gold 2016. Extended Abstracts, AIG, Rotorua New Zealand.  

Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Geochemistry
  • Geological science
  • Geology
  • Geoscience
  • Materials science
  • Natural sciences
  • Physical science