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Making Wealth: Precipitation of Gold from synthetic hydrothermal solutions

Dr Rob Chapman (SEE), Dr Thomas Mueler (SEE), Dr Dan Morgan (SEE), Dr Sandra Piazolo (SEE), Dr David Banks (SEE)

Contact email: r.j.chapman@leeds.ac.uk

This project provides you with the opportunity to produce some seminal work on an area that is surprisingly under researched. All previous studies on metallic gold have focussed on metal generated through a smelting process. We now know that the textures and compositional variations in gold precipitated form hydrothermal solutions are completely different. This is the first study which seeks to grow gold from synthetic solutions and to compare the resulting textures to those observed in samples from Leeds’ unique collection of natural gold. You will gain field expertise in specialist field skills for gold collection whilst developing new experimental procedures to generate synthetic gold for study in our state of the art electron optics suite. This new understanding of the controls on gold alloy heterogeneity will underpin increasingly sophisticated approaches to the use of detrital gold as an indicator mineral in exploration.

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, dental and electronic industries. These studies focus entirely on smelted metal, i.e., an alloy which has solidified from a molten state where the composition is determined 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 natural gold may be highly heterogeneous, but we have a very limited understanding of the causes and implications of this heterogeneity.

Most natural gold is formed by  precipitation from hydrothermal solutions in a wide range of geological environments. Generic ‘styles’ of gold mineralization are defined according to specific geological environments, that are characterized by variable pressure (P), temperature (T) and compositional (X) conditions. Both gold mineralogy (i.e. the incorporation of other metals such as Ag, Cu, Hg, Pd within the alloy) and particle size are controlled by the permissible range of P-T-X. Thus different styles of gold record their mineralization history through different ore mineralogy and the compositional range of the gold itself.  

At Leeds (UoL) we have been studying natural gold for over 20 years, building up an outstanding compositional data base describing over 30,000 gold grains. This work was initially undertaken in close collaboration with the British Geological Survey (BGS) (e.g Chapman et al. 2000) and grew more recently into an international network of collaborators in the context of Canadian Cordilleran and Alaskan metallogeny (e.g. Chapman et al. 2010, 2016, 2017). 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-abalation inductively coupled plasma-mass spectrometry (LA-ICP-MS) to reveal generic compositional signatures and to characterise compositional heterogeneity within individual gold particles (Fig. 1).  Several projects in different field areas have revealed a wide variety of gold precipitates ranging from economically important ores where the gold is ‘invisible’ but present as sub microscopic particles in quartz, or in intimate association with pyrite (Fig. 2A) to sub economic mineralization in which nuggets to several grams are commonplace, (Fig. 2B). The morphology of gold particles can be used to qualitatively evaluate the rate of precipitation. For example, dendritic forms are typically the result rapidly changing P-T-X conditions causing high rates of mineralization (Fig. 2C). This understanding of the nature of natural gold has been used in several ways, including to constrain the conditions under which mineralization occurs. Hitherto, our approach has been to interpret the physical and mineralogical characteristics in terms of the process of mineralization, given the geological constraints imposed by the style of mineralization. A quantitative assessment on what factors are controlling the mechanisms and rates of hydrothermal gold precipitation has not been attempted, however is urgently needed for effective gold prospecting and recovery.

Aim of the project

In this project we aim to illuminate the controlling factors for gold precipitation and nucleation. This will be achieved through a novel combination of well-constraint experiments, (Fig 3) high-end analytical chemical and structural analysis and comparison of experimental results and natural gold occurences. Specifically, we seek to generate gold alloy from synthetic hydrothermal solutions under various P-T-X regimes. By systematically varying the experimental P-T-X-t conditions it will be possible to identify the controlling parameters, quantify the rates of gold precipitation and to compare the resulting textures to inform our understanding of the library of features that we have already observed in natural gold.  The laboratory methods have already been established and used to investigate silicate and carbonate systems. 

Figure 1 examples of compositional heterogeneity in particles of natural gold revealed in SEM backscattered electron images. A: of Ag-rich gold (dark grey) mantling a pale grey gold core. (Violet Mine, Klondike, Yukon, Canada), B: Complex grain paragenesis: placer grain Similkameen River, British Columbia. C: Pd- Ag zones surrounded by Au-Ag alloy Placer grain, Brownstone, Devon, England. D: Au-Cu intermetallics surrounding a Au-Ag core. Detrital particle Kraskov, Czech Republic.

Systems to be studied

Two distinct mineralization regimes will form the basis of this study:

High temperature (>200 ºC) systems in a reduced Au-Ag-S-(low) Cl system

This system corresponds to hydrothermal regimes in an orogenic and intrusion related settings and to various stages in the evolution of porphyry- epithermal systems.

Low temperature (<200 ºC) systems in an oxidizing Au-Ag-Pd-Cl system. This system corresponds to various mineralizing settings where metal deposition is mainly controlled by redox conditions. (Chapman et al. 2009).

Figure 2: examples of gold morphology from different settings: A: very coarse gold intimately associated with pyrite, Klondike, Yukon, Canada,: B: 6.5g nugget found by the lead supervisor at Glengaber Burn, Southern Uplands Scotland C: nuggets from Leadhills, Scotland, D-F gold dendrites formed from rapid precipitation through lowering Eh in a low- temperature oxidizing chloride system, (Devon, England and Lammermuir Hills, Scotland).

Specific scientific questions to be addressed

  1. Are the changes in metal-contents of alloys (Ag, Pd, …) incoporporated in gold grains formed from synthetic hydrothermal solutions under different P-T-X conditions faithful to those predicted by theoretical consideration of equilibrium relations (e.g. Gammons and Williams-Jones 1995) ?
  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 on the basis of gold alloy textures?
  3. 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?

Project elements

  1. Characterization of selected sample of natural auriferous material (vein material, placer gold particles) using SEM, EMP (and LA-ICP-MS is appropriate)
  2. Field work to collect such materials as required by the project
  3. Laboratory- based synthesis of gold precipitation using rapid-quench cold seal apparatus and state-of-the-art flow through cells.
  4. Characterization of fluids and synthetic gold using SEM, EMP and ICP-MS
  5. Interpretation of results in terms of defining P-T-X conditions and timescales for hydrothermal gold deposits.

The project has the capacity to evolve in a variety of directions according to the interests and expertise of the candidate: these might include detailed microstructural studies of synthetic gold using EBSD and TDK analysis down to the nanoscale, investigations of the effect of trace elements within the system, or modelling of the theoretical conditions present in the system and their relationship to the characteristics of synthetic gold. 

Figure 3.  Cold seal apparatus with the option to perform rapid quenching to simulate abrupt changes in natural hydrothermal systems

Potential for high impact outcome

This project is the first to simulate gold precipitation mimicking natural settings under controlled, experimental conditions. Its value lies not only in this novelty but also the high potential of data exploration as UoL is uniquely placed to interpret the results of the study on account of the existing library of natural gold samples. Thus, the project 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 origin. This knowledge will inform design of exploration campaigns according to the constraints on geological setting provided by interpretation of gold particle textures.

The ability to extract the gold from an ore is of paramount importance to exploration and mining companies. Refractory ores, containing sub- micron sized gold require extensive pulverization in the processing operation which is both expensive and inefficient in terms of final gold recovery. Consequently even rich deposits where the gold is finely divided constitute unattractive targets. However such mineralization is clearly gold- rich, and changes in P-T-X could lead to changes in gold particle size and the presence of economically viable mineralization at other points in the hydrothermal system. This project will seek to establish vectors which may be applied in specific geological settings to aid the exploration process. Consequently we expect high ranking outputs in both academic facing and applied facing journals.

Training

The student will work under the supervision of Dr. Rob Chapman (Institute of Applied Geosciences) with a major contributions from Dr Thomas Müller (Institute of Geophysics and Tectonics) and plus contributions from other members of staff according to the evolution of the project (Dr Sandra Piazolo, Dr Dan Morgan and  Dr. David Banks) who provide a raft of specialist expertise.

The project will involve field work for sample collection, in auriferous areas where mineralization corresponds to the classification provided above. The localities of fieldwork will be determined as the project evolves.

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/).

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, is desirable but not essential.

References

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, R. J., Leake, R. C., Bond, D. P. G., Stedra, V., and Fairgrieve, B., 2009, Chemical and Mineralogical Signatures of Gold Formed in Oxidizing Chloride Hydrothermal Systems and their Significance within Populations of Placer Gold Grains Collected during Reconnaissance: Economic Geology, v. 104, no. 4, p. 563-585.

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 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.

Gammons CH, Williams-Jones AE (1995) Hydrothermal geochemistry of electrum; thermodynamic constraints. Economic Geology 90:420-432. doi: 10.2113/gsecongeo.90.2.420.

Jonas, L., 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

Le Guillou, C  Dohmen, R.,  Rogalla, D.,  Müller, T.,  Vollmer, C., Becker, H.W., (2015)  New experimental approach to study aqueous alteration of amorphous silicates at low reaction rates. Chemical Geology 412, 179-192

Related undergraduate subjects:

  • Chemistry
  • Earth science
  • Earth system science
  • Geology
  • Geoscience
  • Materials science