Making wealth: precipitating gold from synthetic hydrothermal solutions to understand gold deposition processes

Making wealth: precipitating gold from synthetic hydrothermal solutions to understand gold deposition processes

    • Produce seminal work on understanding the process of gold precipitation through application of high- end analytical techniques to both natural and synthetic gold.
    • Design and undertake new experimental procedures to generate synthetic gold veins in the first systematic study of its kind
    • Develop a novel approach to gold particle characterization combining mineralogical and crystallographic techniques.
    • Apply new understanding to refine methodologies for exploration geology leading to close contact with industry

    Project background

    There is a considerable body of work which addresses the properties of gold alloys as they find wide ranging use in the jewellery, medical and electronic industries. These studies focus entirely on smelted metal, i.e. an alloy solidified from a molten state where the composition is engineered by additions to the smelted charge. There has been a tendency to assume that the compositional characteristics of natural gold are similar to these synthetic metals. However, work undertaken at Leeds over the last 20 years has shown that alloy within natural gold particles may be chemically highly heterogeneous, exhibiting complex textures of gold alloys with varying amounts of other elements (Ag, Cu, Hg, Pd). In addition, various studies have proposed modifications to gold alloy, and even gold growth in surficial environments (e.g., Shuster and Reith 2018). Therefore it is important to be able to link specific features with their genetic origins to ensure that interpretations are valid.

    In this project you will undertake experiments in which gold is precipitated from synthetic ‘ore fluids’ under well constrained laboratory conditions. Experiment design will permit co-deposition of common accessory minerals such as quartz, carbonates and pyrite.  Examination of the resulting gold alloy and synthetic vein textures will illuminate the detailed controls on gold particle composition, size and vein texture which will act as a template to interpret those observed in natural gold (such as those depicted in Figure 1).

    Synthetic and natural gold will also be examined using a range of high-end analytical techniques (ToF-LA-ICP-MS, atom probe tomography, EBSD) to establish the micro- relationships between element distribution and crystallography.  Gold particle can be further treated by heat and pressure to induce grain boundary migration and associated compositional changes. This process mimics environmental changes caused by post-deposition metamorphism of pre – existing gold particles and will establish which textures relate to the primary material and which are a consequence of residence in the lithological host. At Leeds you will be able to do experiments of grain boundary migration using gold grain clusters. In addition, grain boundary migration can be modelled numerically using the numerical platform Elle (e.g. Piazolo et al. 2019) extending process understanding to a predictive tool. The tandem of modelling and experiments of grain boundary migration will be permit interpretation of features observed by EBSD and ToF- LA-ICP-MS in naturally occurring gold from a variety of ore deposit styles.

    The initial scope of the project is large, and the focus may subsequently focus on some of the aspects described here.

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    Figure 1. A-D, BSE images of gold particles with grayscale indicative of Ag content. A: Ag and Au rich tracks in homogenous grain, B: High Ag/Hg alloy mantling Au-rich core, C: complex polygonal zones of heterogeneity sympathetic to grain boundaries, D: examples of mineral inclusions in alloy, E: heterogeneity at trace level revealed by Pd analysis in cross section of gold particle by Time of Flight LA-ICP-MS, F: Colour coded crystallographic map of detrital gold grain showing small grains associated with recrystallization near margins, G: Simulation of compositional changes induced by grain boundary migration generates the same texture as seen in ‘C’

    Gold studies at Leeds

    At Leeds (UoL) we have been studying natural gold for over 20 years, building up an outstanding compositional database describing over 40,000 gold grains. This work was initially undertaken in close collaboration with the British Geological Survey (BGS; e.g. Chapman et al. 2000) but grew into an international network of collaborators studying Cordilleran metallogeny (e.g. Chapman et al. 2010, 2016, 2017, 2018). We have unparalleled experience in characterizing compositional variations in natural gold both within and between different styles of gold mineralization. We have used state-of-the-art techniques such as scanning electron microscopy (SEM) based analyses, electron microprobe (EMP) and laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) to reveal generic compositional signatures related to specific styles of mineralization. Compositional heterogeneity within individual gold particles may present in alloy as different textures (Fig.1 A, B, C) or as inclusions of other minerals (Fig 1D). In addition, recent exploratory work has shown that the gold alloy may be highly heterogeneous with respect to trace element content (Fig. 1E), and this new understanding provides a platform to undertake some seminal studies on both the compositional and crystallographic characteristics of natural gold using some high – end analytical approaches.

    Characterization of populations of gold grains according to their alloy composition and suite of mineral inclusions has been developed at Leeds, and the approach has found several applications. The features of hypogene gold particles are inherited by detrital particles following erosional process, and therefore studies of placer gold can shed light on the source even where this remains undiscovered.  The potential to use gold as an indicator mineral in mineral exploration has underpinned several publications (e.g. Chapman et al. 2017, 2018) and is the focus of an ongoing project funded by Geoscience BC (Canada) which seeks to create the first regional compositional Atlas of gold particles for use by the exploration industry (http://www.geosciencebc.com/i/pdf/SummaryofActivities2019/Minerals/Project%202018-013_Minerals_SOA2019.pdf).

    Studies of crystallographic relationships in gold (e.g. Hough et al. 2009) have revealed complex arrays of crystal domains within individual gold particles. Crystal orientation identified by EBSD provides a history of recrystallization both during residence in the original lithological host post precipitation and in the surface environment (Stewart et al. 2017), (e.g. Fig 1F). Grain boundary migration induced by a reduction in surface energy may be simulated for environments where fluid out of equilibrium with gold alloy penetrates grain boundaries (e.g. Piazolo et al. 2016a, 2016b). Variations in alloy composition reflect the grain boundary migration and the re-equilibration of the alloy with the fluid (Fig 1G). The patterns generated show a striking similarity to textures observed in some natural gold particles (Fig 1C).

    Approach

    This project will extend our understanding of the nature of heterogeneity of natural gold and its causes through development of three novel and interconnecting research strands.

    1. You will engage in the first experimental study globally to precipitate synthetic gold particles from hydrothermal fluids, in order to explore the conditions of gold precipitation. General pressure/temperature conditions of gold precipitation and associated fluid compositions have been reported in literature for many individual deposits, based on fluid inclusion studies and interpretation of vein petrography. This information will establish the starting parameters of the experimental conditions. The laboratory methods have already been established and used to investigate other silicate and carbonate systems (Mueller et al. 2017) and we expect the experimental program to evolve with increasing experience of these auriferous systems. We aim to illuminate factors controlling gold particle size, and the destiny of other metals present in the fluid. Synthetic samples so produced will be characterized by the same array of techniques previously applied to natural gold.
    2. Characterization of synthetic and natural gold from the UoL collections using ToF-LA-ICP-MS, atom probe tomography, and EBSD.
    • Simulation and experiments of post – depositional changes in gold particles through T&P changes and observation of resulting associated crystallographic and associated compositional modifications in real time. Correlations of crystallographic changes with predictions from modelling exercises.

    Specific scientific questions to be addressed

    1. What parameters control the rate and compositional features of gold co-precipitated with quartz, calcite and pyrite? How important is the rate of T, P and chemical changes?
    2. Do changes in P-T-X applied during gold particle growth generate textures and compositional heterogeneity which correspond to those observed in natural gold? Under what circumstances can these be altered post deposition?
    3. Is it possible to develop generic templates of trace element compositional features linked to the various conditions of primary ore formation, post depositional modification in hypogene settings, and modification in the surficial environment?
    4. Can a more complete understanding of spatial, compositional and crystallographic heterogeneity find application in targeting ore deposits?

    Project objectives

    1. Literature survey to establish ranges of parameters underpinning experimental work
    2. Laboratory-based synthesis of gold – (other mineral) precipitation using rapid-quench cold seal apparatus and state-of-the-art flow through cells.
    3. Characterization of fluids and of the synthetic gold and co-precipitated minerals using SEM, EMP, LA-ICP-MS, EBSD.
    4. Characterization of natural gold particles formed in different geological settings by ToF-LA-ICP-MS, atom probe tomography and EBSD.
    5. Crystallographic studies of gold post – precipitation under different P-T-X regimes. Live monitoring of reaction progression either in a flow through cell under an optical microscope or within a Scanning Electron Microscope (SEM). Corellation of experimental results with ‘Elle’ simulation exercises.
    6. Synthesis of information from all project streams to i. enhance our understanding of gold depositional processes, ii. identify crystallographic and compositional characteristics associated post-depositional processes
    7. Application of all project results to refining indicator mineral methodologies based on gold.

     Potential for high impact outcome

    This project is the first to simulate gold–quartz precipitation mimicking natural fracturing processes and/or fluid – rock interactions under controlled experimental conditions. Consequently, there is excellent potential to make a major contribution to fundamental understanding of how specific conditions influence gold precipitation and the resulting microchemistry and microtexture of the alloy.

    The study will be the first to characterize heterogeneity in gold from different environments at a micron scale and according to trace element signatures. The novelty lies not only in characterization of gold precipitated in the natural environment but in linking compositional modification driven by subsequent grain boundary migration.  UoL is uniquely placed to undertake this study on account of our large and unique library of natural gold samples. Various studies have characterized natural gold in terms of some compositional and textural characteristics, but this project will systematically study the nature and origins of compositional variation at trace element level. This new knowledge will be fundamental to gaining a better understanding of ore-forming processes. Additionally, establishing the controls on formation of various features of gold particles permits interpretation of these  features when observed in detrital particles whose origins are unclear. Hence the study also  provides a platform for development of a powerful exploration tool.  Consequently, we expect high ranking outputs in both academic and industry-facing journals and the possibility for  future impact studies.

    Training

    The student will work under the supervision of Dr Rob Chapman (Institute of Applied Geosciences), Prof Sandra Piazolo, and Dr David Banks (Institute of Geophysics and Tectonics). Dr Thomas Müller (University of Goettingen) will act as an external supervisor and advise on both experimental procedures and modelling. It is planned that the student will visit the University of Goettingen for two extended periods allowing utilization of additional analytical and experimental equipment and interaction with Dr. Mueller. Other colleagues from the Ores Group with expertise in structural geology and petrography may also contribute as the project progresses

    The student will be trained in planning and carrying out experimental series to work independently in the experimental petrology laboratories of Goettingen and UoL. The training will include sample preparation before and after the experiment as well as extensive use of cutting edge analytical tools (e.g. electron optics) and experimental and numerical equipment available on UoL campus (dynamic microstructural modelling suite, Experimental Petrology Laboratory). Experiments to synthesize ore will be undertaken in Leeds and complemented by experiments in Goettingen. 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/). There will be potential to undertake field work and to acquire specialised skills used in gold collection if appropriate to the evolving project.

    Professional development will be enhanced by membership of the Ores and Mineralisation Group (OMG: https://www.facebook.com/OMGLeeds; Twitter @OMGLeeds) which currently supports six post-graduate students. OMG students benefit from the networking opportunities provided by the activities of the Leeds Chapter of the Society for Economic Geology, (e.g. Leeds staff and students attend the Vancouver Exploration Roundup), and the extensive industry, academic and alumni networks of supervisors. The applied nature of some project outcomes are compatible with high levels of contact with industry during the project lifetime and the associated networking opportunities.

     Student profile

    The successful student will have 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 mineralization, laboratory techniques, microanalytical techniques, and a publication record is desirable but not essential.

    References

    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 RJ; Allan MM; Mortensen JK; Wrighton TM; Grimshaw MR (2018) A new indicator mineral methodology based on a generic Bi-Pb-Te-S mineral inclusion signature in detrital gold from porphyry and low/intermediate sulfidation epithermal environments in Yukon Territory, Canada, Mineralium Deposita, 53, pp.815-834.

    Chapman RJ, Mileham TJ, Allan MM, Mortensen JK (2017) A distinctive Pd-Hg signature in detrital gold derived from alkalic Cu-Au porphyry systems. Ore Geol Rev 83: 84-102

    Chapman RJ, Mortensen JK, Crawford EC, Lebarge W (2010) Microchemical Studies of Placer and Lode Gold in the Klondike District, Yukon, Canada: 1. Evidence for a Small, Gold-Rich, Orogenic Hydrothermal System in the Bonanza and Eldorado Creek Area. Economic Geology 105:1369-1392. doi: 10.2113/econgeo.105.8.1369.

    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.

    Hough, R.M., Butt, C.R. and Fischer-Bühner, J., 2009. The crystallography, metallography and composition of gold. Elements, 5(5), pp.297-302.

    Müller, T., Dohmen, R., Immenhauser, A., Putlit, B., (2017) Hydrothermal replacement of biogenic and abiogenic aragonite by Mg-carbonates – Relation between textural control on effective element fluxes and resulting carbonate phase. Geochimica et Cosmochimica Acta 196, 289-306

    Piazolo, S., Kaminsky, F. V., Trimby, P., Evans, L., & Luzin, V. (2016b). Carbonado revisited: Insights from neutron diffraction, high resolution orientation mapping and numerical simulations. Lithos, 265, 244-256

    Piazolo, S., La Fontaine, A., Trimby, P., Harley, S., Yang, L., Armstrong, R., & Cairney, J. M. (2016a). Deformation-induced trace element redistribution in zircon revealed using atom probe tomography. Nature communications, 7, 10490.

    Shuster, J. & Reith, F. 2018. Reflecting on gold geomicrobiology research: thoughts and considerations for future endeavors. Minerals8, 401.

    Stewart J, Kerr G, Prior D, Halfpenny A, Pearce M, Hough R,Craw, D (2017). Low temperature recrystallisation of alluvial gold in paleoplacer deposits. Ore Geol Rev 88:43-56

    Links to supervisor websites:

    Chapman: http://www.see.leeds.ac.uk/people/r.chapman

    Piazolo: http://www.see.leeds.ac.uk/people/s.piazolo

    Banks: http://www.see.leeds.ac.uk/people/d.banks