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 some seminal work on understanding the process of gold precipitation and the impact of this to gold particle size and gold compositional variations
  • 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 to the causes of compositional variations, the controls on particle size of natural gold are not well understood. Most natural gold is formed by precipitation from hydrothermal solutions and this can occur in a wide range of geological environments. Generic ‘styles’ of gold mineralization are defined according to specific geological settings, characterized by variable pressure (P), temperature (T) and compositional (X) conditions. Gaining insights into the specific roles of these parameters in controlling the composition and size of gold particles is one aim of this study.

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 gold will also be examined using EBSD to establish the crystallography of newly precipitated material. Both these gold particles and some natural gold 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.

The outcomes of the project have two main areas of application. Firstly they will provide a new approach to interpreting the conditions of mineralization in hydrothermal systems through examination of the nature of metallic gold. Secondly, characterization of textural features according to their genesis in the hypogene and post hypogene environments will clarify which are appropriate to apply in indicator mineral methodologies using natural gold.

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).

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 relationships between lode gold and gold in local placers can be established to predict whether other hypogene sources remain undiscovered, (e.g. Chapman and Mortensen 2016). Identification of diagnostic mineral inclusion assemblages for alkalic and calc-alkalic porphryries (Chapman et al. 2017, 2018) provides the potential to develop an indicator mineral methodology based on gold for exploration for Cu-Au porphyries in areas where exposure is poor. In Yukon, Canada, this approach lead to the identification of a regional orogenic gold mineralizing event, whereas previous hypotheses favoured magmatic hydrothermal systems, (Chapman et al. in prep). Interpretation of the mineralogical characteristics of native gold can be used to illuminate the environment of precipitation, through constraining the conditions in which co-eval minerals (i.e. mineral inclusions within gold) precipitate.

Studies of gold crystallography (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-equilibriation 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).

In addition to the compositional and crystallographic characteristics of gold alloy, the mineralogical texture of host veins may vary considerably. The quartz – gold association is common, (e.g. Fig. 2A,B) but there is a wide range of vein textures within this category. Individual veins typically contain multiple generations of quartz, identified by cathodoluminescence (CL), but not all generations contain gold (e.g. Grimshaw, 2018). Where gold is found, the particle size of gold may vary from submicroscopic to nuggets weighing several ounces.

Examples of gold mineralisation

Figure 2: Examples of Different particle size of gold in auriferous quartz veins: A: Nuggety gold from Klondike District, Yukon, Canada, B: sub mm gold flecks in quartz vein: Co Mayo, Ireland

Approach

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.

Specific scientific questions to be addressed

  1. What parameters control the rate and resulting particle size 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 grain growth generate textures which correspond to those observed in natural gold, and what are the implications for our interpreting the paragenesis of gold bearing mineralization based on gold alloy textures in gold particles from the same environment?
  3. How do conditions of precipitation influence the detailed microstructure of gold alloy, and under what circumstances can this be altered post deposition?

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. Interpretation of results in terms of defining P-T-X conditions and timescales for hydrothermal gold deposits.
  5. Crytallographic studies of gold post precipitation under different P-T-X regimes. Reaction progression can be monitored live either in a flow through cell under an optical microscope or within a Scanning Electron Microscope (SEM).
  6. Differentiation of textures within natural gold vs. synthetic gold, relating to depositional and post-depositional processes
  7. Application of 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 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.

UoL is uniquely placed to interpret the results of the study on account of our existing library of natural gold samples. Various studies have characterized natural gold in terms of compositional and textural characteristics, but this project will establish which of these are of value in determining the nature of the environment of initial precipitation. Consequently there is an opportunity to develop a more sophisticated approach to using gold as an indicator mineral than has been hitherto possible.

Analysis of natural gold by EMP and LA-ICP-MS routinely involves measurements taken on a small amount of material exposed in polished section, on the assumption that the gold alloy is homogenous. This project will evaluate that premise and thereby inform fundamental approaches to gold analysis in general whilst also undertaking a pioneering study into the compositional microtextures in natural gold alloy. Consequently, we expect high ranking outputs in both academic and industry-facing journals.

Training

The student will work under the supervision of Dr Rob Chapman (Institute of Applied Geosciences) and Dr Thomas Müller (Institute of Geophysics and Tectonics), Prof Sandra Piazolo, and Dr David Banks (Institute of Geophysics and Tectonics), plus contributions from Dr Taija Torvela and Dr Dan Morgan who provide a raft of specialist expertise in structural geology and petrography respectively.

The student will be trained in planning and carrying out experimental series to work independently in the experimental petrology laboratory of UoL. The training will include sample preparation before and after the experiment as well as extensive use of cutting edge analytical tools (e.g. Cohen labs, electron optics, etc.) and experimental equipment available on UoL campus.

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/).

Professional development will be enhanced by membership of the Ores and Mineralisation Group (OMG: https://www.facebook.com/OMGLeeds; Twitter @OMGLeeds) which currently supports five 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.

Grimshaw, M.R., 2018, Gold mineralisation in the Lone Star area of the Klondike gold district, Yukon, Canada: Ph.D. thesis, Leeds, UK, The University of Leeds, 251 p.

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.

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

Mueller: http://www.see.leeds.ac.uk/people/t.mueller

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