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Establishing the “play” concept in lode gold exploration

Dr Taija Torvela (SEE), Dr Rob Chapman (SEE)

Contact email: t.m.torvela@leeds.ac.uk

The lack of understanding of controls on the location of gold deposits within orogenic belts continues to discourage regional exploration strategies globally. This project addresses this fundamental problem in ore geology through a novel synthesis of regional structural geology with gold mineralogy. You will explore the relationship between the structural evolution of the Grampian terrane and the mineralogical markers of the gold it hosts, to develop the first framework focusing on detailed variations in metallogeny for regional gold mineralization. This project allows the development of an approach that could have applications in orogenic belts worldwide.

Project summary and motivation

Orogenic belts normally host gold mineralization of different styles as a consequence of both metamorphic and igneous processes. In some cases the presence of economically important mineralization greatly enhances our understanding of controls on both local and regional mineralization, but in other areas, whilst occurrences of placer gold indicate that gold mineralization appears widespread, there is no clear metallogenic framework to inform either mineralizing processes or exploration models. In short our ability to predict the location of economically important orogenic gold mineralization is exceedingly limited

Many studies have attempted to construct genetic and exploration models for gold-bearing belts ('provinces'). These metallogenic models are normally based on a concept of a single fluid source and/or geological control for the mineralization. Such a simplified approach has failed to conceive a unified model for gold deposits in orogenic systems that can be used efficiently in exploration targeting. Intense debate continues on both the origins and timing of fluid sources and ore trapping mechanisms. In hydrocarbons exploration, the concept of a 'play' is routinely used to refer to a group of prospects in a region that are controlled by the same set of geological circumstances. Crucially, these circumstances can vary in the same region, even if that region superficially possesses similar conditions for the prospect (such as would a 'traditional' ores province). Understanding these local variations of the geological controls have proven to be a significant asset for efficient targeting of hydrocarbons exploration efforts, and the work of Allan et al. (2013) show that this approach should be equally beneficial in the context of regional metallogeny.

Orogenic belts typically host different styles of ore deposits, ranging from metamorphic fluids-hosted ‘orogenic’ gold, to gold formed in magmatic hydrothermal systems. Within each metallogenic province there may be different deposits which share common genetic features, (i.e. ‘plays’) but exploration rarely focuses on distinguishing these despite the potential benefits both for the scientific understanding of the ores systems, and for the region-wide exploration models. Exploration efforts are commonly focused at a prospect level, without necessarily being preceded by a wider understanding of the ore province or a particular region within it – or, conversely, the models assume similar conditions of ore formation for the entire province (e.g. Marjoribanks, 2010). This project aims to address this problem, by aiming to identify different 'plays' within the auriferous Grampian Terrane of Scotland. The Grampian Terrane is analogous to other, globally important lode gold provinces in that it is geologically complex, with the potential to host gold mineralization of different styles, all of which exhibit some degree of structural control.

Gold formed in orogenic belts is typically classified as either ‘orogenic’, i.e. with no association with magmatism, or resulting from specific magmatic hydrothermal processes. Detailed analyses or the geochemical signatures of gold grains can be used to distinguish between gold from different sources. In each case, the location of the mineralization is commonly heavily influenced by structural considerations. Fundamental research into orogenic gold systems highlights the importance of crustal deformation processes in forming porosity and permeability in crystalline rocks that host important epigenetic gold deposits (e.g. Sibson et al. 1988; Cox et al., 2001; Micklethwaite et al. 2010). Critically, fluid pressure gradients and variations both in regional and local stress fields are key in driving both fluid flow and permeability evolution that is essential for mineralization systems. Although the principles of these relationships are reasonably well understood, a significant amount of important quantitative work remains to be done in order to elucidate this at all scales of ore-forming systems – and how this knowledge can be used to establish mineralization plays. Additionally, the interaction of fluid pathways with reactive lithologies can control the location of mineralization, and periods of rapid uplift can establish the deviation for normal thermal gradients necessary to drive rapid devolatilization and associated mineralizing systems (Yardley and Cleverley 2013). The Grampian Terrane is a well-studied crustal-scale strike-slip system hosting several known Upper Silurian to Lower Devonian gold occurrences (Fig. 1; e.g. Plant, 1989; Tanner, 2014a). Based on our understanding of known in situ deposits it seems likely that much of the regional mineralization occurs within second-order fault zones greatly resembling Riedel-geometries that would result from a regional N–S-trending maximum principal stress. Tanner et al. (2014a) suggested that the fluids that are responsible for orogenic gold mineralization and that have been suggested to originate from various different sources, were channelled into major orogen-parallel, strike-slip faults within this system, and then injected into these the secondary Riedel fractures/faults.

Fig 1. Overview map with significant fault zones and examples of known gold localities. Analysis results of many of the localities are included in the existing database. Figure modified from EDINA maps and Tanner (2014b).

Globally, orogenic belts host placer gold deposits, although frequently their relationship to source mineralization is not well understood. The development of microchemical approaches to characterization of detrital gold grains in terms of their alloy chemistry and mineral inclusions revealed in polished section has provided a methodology to predict the nature of the source (Fig. 2). This approach has been used to differentiate gold derived from orogenic and magmatic hydrothermal systems, (e.g. Chapman et al. 2017), and to characterise gold from different orogenic events in the same auriferous region (e.g. Chapman et al. 2010, 2016). There are relatively few known in situ occurrences of gold in the Grampian terrane, but detrital gold occurrences are widespread. Consideration of detrital gold abundance and mineralogy indicate that these occurrences indicate a corresponding number of in situ sources, which underlines the gold endowment of the region.

Fig. 2. Information from the abundance and morphology of gold grains may be combined with mineralogical information gained from study of polished sections of grains. Characterization of the inclusions (pale grey: galena, mid grey: bournonite, black; pyrite) permits reconstruction of the vein mineralogy of the (unknown) source.

The novel approach to exploring regional metallogeny through both structural and mineralogical studies can only be attempted when there is sufficient knowledge of gold occurrences. The project is uniquely placed in this regard as one of the supervisors (RC) has 25 years experience of gold collection in Scotland (Leake et al. 1997, Chapman et al. 2000). The student will have access to a unique, pre-characterized collection of gold grains from several Scottish localities, many of which are yet to be reported in the literature. Our knowledge of other potential localities has been informed both by collaboration with various exploration companies over the years, but also through informal relationships with amateur gold prospectors. 

Aims, objectives, and methodology

The play concept applied to ore exploration comprises a novel, systematic, holistic approach that takes into account detailed variations in gold genesis, i.e. the fluid sources, lithological variations, and the local geological/structural history. The specific research questions are:

  1. Which gold occurrences can be ascribed to magmatic vs. 'orogenic' hydrothermal systems based on the gold geochemistry? Are any particular episodes of magmatism more 'fertile' than others?
  2. What were the overall stress states and relative timings of the primary and secondary/tertiary faults at the various stages of the crustal deformation? How transient were the stress conditions along each fault/within each area?
  3. Where do the stress and deformation patterns modelled along the Grampian Terrane correspond to the locations of know gold occurrences, and where are lithological or magmatic controls more significant?
  4.  Will the vein and/or gold geochemistries show patterns in terms of their physical location, and do they map into the stress and deformation patterns?
  5. Can we use the models and the potential patterns as an exploration tool through defining lode gold plays in the Grampian Terrane? Can the play concept be extrapolated to other similar gold provinces globally?

These main objectives will aim at answering the above questions:

  1. To establish the structural and lithological framework (i.e. the fault and deformation model) for the Grampian Terrane, using existing geological and geophysical maps and literature; separating possibly intrusion-dominated areas from structurally controlled geology;
  2. To predict regional palaeostress maps and deformation history, through quantitative modelling;
  3. To characterise populations of gold grains form different placer (and where possible lode) localities in terms of their microchemistry, and to identify commonality in signatures suggestive of genetic controls.
  4. To correlate the microchemical gold data with new understanding of structural controls revealed by the stress and strain maps;
  5. To conduct a global comparison study of similar deposits described in the literature.

The ultimate aim is, in other words, to generate a better, more detailed understanding of the variations in the gold metallogeny of the Grampian Terrane, leading to a development of a novel synthesis of systematic approaches that will find use in similar orogenic belts worldwide.

The specific methodology involves modelling aspect and analytical aspects. The stress and deformation evolution will be modelled using Midland Valley's MOVE software, available at the School of Earth and Environment under academic licence. The structural models constructed for the Grampian Terrane will be "deformed" virtually in the software. The Stress Analysis Module of the software enables a rapid visualization and evaluation of the 3D stress states and potential fault and fracture activity in the models. The Fault Response Modelling Module can be used to predict highly fractured zones in the models, and to model stress perturbations around faults and other discontinuities. Combining the modelling results will allow prediction of the dilation sites, into which the fluids will tend to migrate, and the timing and longevity of these sites. Various realistic structural and kinematic models will be investigated in order to examine different scenarios for the Grampian Terrane.

Microchemical characterization of gold grains is achieved by use of state-of-the-art scanning electron microscope (SEM) facilities at Leeds to identify and characterize inclusions, and the electron microprobe (EMP) facilities for alloy analysis.

Potential for high impact outcome

Scientifically, the genesis of gold in orogenic is a hotly debated subject: the fluid sources, fluid pathways, gold source, timing of mineralisation, and trap mechanisms are all active research topics worldwide. Regardless of the fluid or gold source, all “orogenic” gold deposits are strongly structurally controlled, but the linkage between the structures, the tectonic evolution, and the fluid and gold sources (i.e. geochemistry) are poorly understood. Therefore, this project will be a significant contribution to the understanding of orogenic gold systems as it aims at a detailed analysis of the tempo-spatial evolution of this type of gold mineralization. We anticipate the project will generate several papers with wide interest, and at least one will be suitable for submission to a high impact journal.

Training and employability

The student will work under the supervision of Dr Taija Torvela and Dr Rob Chapman within the Ores and Mineralization Group (OMG) of the Institute of Applied Geosciences, with extended visits (gold sampling) to Scotland and Ireland. This project provides specialist scientific training in: (i) structural analysis, including 3D and 4D modelling of data at various scales; (ii) state-of-the-art microanalytical and/or geochemical techniques; and (iii) industry-standard software skills. In addition, the student will have access to a broad spectrum of training workshops both externally and internally, and participate in the OMG regular research meetings. The student will also learn the specialist field skills of the gold prospector, which are vital to the collection of gold grains for study.  The OMG involves a range of academic staff whose interests and expertise overlap ore deposits. The project will be supported by the existing post-graduate research projects at OMG addressing more detailed aspects of gold deposit genesis in the Scottish and Irish Caledonides. We anticipate that the student will attend both national conferences (e.g. MDSG) and international academic/ industry facing conferences (e.g. SEG, SGA, PDAC, Roundup) according to their career trajectory.

The PhD study is equally suited to career pathways in academia or industry. The expected outputs of the project have global significance for understanding of an economically important mineralization type where deposit models and deposit classification are still the subject of intense academic debate. At the same time, exposure to industry-facing aspects through relevant conferences and other interactions provides non-academic vocational experience. The student would also be expected to contribute substantially to the activities of the buoyant Leeds Chapter of SEG, with all the associated benefits of networking across industry and academia. Finally, the School has close links with the Minerals Deposit Research unit at UBC Vancouver, and Leeds postgraduate students regularly present their work at the annual Vancouver Exploration Roundup. Consequently the student will benefit from wide ranging and established institutional links with the Canadian exploration industry and academia. 

Student profile

The successful candidate will have a high 2:1 or 1st from a Geological Sciences or similar programme. A masters qualification is highly advantageous, as is experience of publication. Strong structural geology skills, and the ability to clearly communicate results are essential. Previous experience of ore deposit geology in either an academic or industrial context is desirable; additional specific experience in geological sampling, GIS, statistical analysis, gold deposits, microanalytical techniques, 3D modelling, and/or geochemical analyses is desirable but not essential.

References

Allan MM, Mortensen JK, Hart CJR, Bailey L, Sanchez MG, Ciolkiewicz W, Creaser RA, 2013. Magmatic and Metallogenic Framework of West-Central Yukon and Eastern Alaska. Society of Economic Geology Special Publication, 111-168.

Chapman RJ, Leake RC, Moles NR, Earls G, Cooper C, Harrington K, and 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, Crawford EC, Lebarge WP (2010) Microchemical Studies of Placer and Lode Gold in the Klondike District, Yukon, Canada: 2. Constraints on the Nature and Location of Regional Lode Sources. Economic Geology 105, 1393-1410. doi: 10.2113/econgeo.105.8.1393.

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.

Cox SF, Knackstedt MA, and Braun J, 2001. Principles of structural control on permeability and fluid flow in hydrothermal systems. In: Richards JP and Tosdal RM, Structural Controls on Ore Genesis. Reviews in Economic Geology 14, 1-24.

Leake RC, Chapman RJ, Bland DJ, Condliffe E, and Styles MT,1997b, Microchemical characterization of gold from Scotland: Institution of Mining and Metallurgy Transactions, v. 106, sec. B, p. B85–B98.

Marjoribanks, R, 2010. Geological methods in mineral exploration and mining. Springer, 237 p.

Micklethwaite S, Sheldon HA, and Baker T, 2010. Active fault and shear processes and their implications for mineral deposit formation and discovery. Journal of Structural Geology 32, 151-165.

Sibson RH, Robert F, and Poulsen KH, 1988. High-angle reverse faults, fluid pressure cycling, and mesothermal gold-quartz deposits. Geology 13, 551-555.

Plant J, 1989. Gold pathfinder elements As, Sb and Bi-their distribution and significance in the southwest Highlands of Scotland." Institution of Mining and Metallurgy Transactions. Section B. Applied Earth Science 98.

Tanner PWG, 2014a. Structural controls and origin of gold-silver mineralization in the Grampian Terrane of Scotland and Ireland. Geological Magazine 151, 1072-1094.

Tanner PWG, 2014b. A kinematic model for the Grampian Orogeny, Scotland. In: Corfu et al., (Eds), New Perspectives on the Caledonides of Scandinavia and Related Areas. Geological Society London, Special Publications 390, 467-511.

Yardley BW, and Cleverley JS, 2015. The role of metamorphic fluids in the formation of ore deposits. Geological Society London, Special Publications, 393, 117-134.

Related undergraduate subjects:

  • Earth science
  • Geochemistry
  • Geological science
  • Geology
  • Geoscience