Controls on the high-grade gold-cobalt mineralisation at Rajapalot, Finland

 

Controls on the high-grade gold-cobalt mineralisation at Rajapalot, Finland

Supervisors: Dr Taija Torvela, Dr Rob Chapman

CASE partner: Mawson Gold Ltd.

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

  • Develop an understanding of the deposition and grade controls of a critical battery metal (cobalt) in hydrothermal deposits
  • Use a holistic approach integrating multiple methods and datasets
  • Field work in Finnish Lapland with an active exploration project
  • Contribute to the understanding of metallogenic systems in orogenic belts in general through an analysis of the tempo-spatial evolution of gold-cobalt mineralization
  • Opportunity to publish high-impact papers or focus on applied, industry-style approaches according to your career trajectory

 

Background

Cobalt is one of the crucial metals needed for the energy transition as Co is found in batteries in e.g. electric cars. Gold, in turn, remains a key economic commodity but it is also an important metal in non-corrosive electronic components. The Rajapalot area in Finnish Lapland, 35 km WSW of Rovaniemi, hosts disseminated, replacement and vein-type mineralisation with high Au-Co grades across several individual prospects (Fig. 1; Rantala et al., 2021). The most recent inferred resource estimate is 887 koz of gold and c. 4800 tonnes of cobalt for the entire Rajapalot area, based on extensive drilling (nearly 85 km total drill length by June 2021; Rantala et al., 2021).

The Precambrian basement of Northern Finland hosts a number of gold deposits of  “atypical metal association” characterized by redox-critical metals such as Co, Cu, Mo, U, V, and Cr. Late overprint by tectonically-driven orogenic or magmatic hydrothermal gold systems with the classic Au-W-As-Bi-Te (Hg) association is superimposed on the former, usually stratabound mineralisation events. The Rajapalot Au-Co mineralisation occurred in two phases, with an older, Co-only event (up to 1085 ppm Co in metavolcanic and calcsilicate-rich rocks) followed by a younger, high-grade Au-Co mineralisation event (Raič et al., in prep.). Broadly speaking, the earlier Co-only mineralisation is likely to be syngenetic with their host rocks; whilst the latter Au-Co event is structurally controlled, with interaction of reactive rock packages with hydrothermal fluids that probably sourced from 1.8 Ga granitoids and circulated through extensive fracture systems (Rantala et al., 2021).

It is likely that the Co in the Au-Co mineralisation is remobilised from the early-phase Co mineralisation, but overall the occurrence of Co in the second phase event remains enigmatic. The gold is usually found in fractures within pyrite (Raič et al., in prep.) either disseminated along specific horizons or in veins (Fig. 1) although the precise grade control is only partially understood; whilst the control(s) on the high cobalt grades in the Au-Co phase are unresolved. Raič et al. (in prep) show that the correlation between Au and Co is very weak, indicating different processes for the enrichment of these two elements. New unpublished internal research by Mawson indicates that addition of As to form cobaltite during the deposition of the gold is the most likely control for the distribution of cobalt minerals such as cobaltite, linnaeite and cobaltian iron sulphides at Rajapalot. However, the data also indicate that As is not co-precipitated with some of the gold.

The controls on grade of both Au and Co need to be constrained further with a detailed mineralogical, trace element, and textural study. There is also potential in this project to address some wider scientific questions on the partitioning of Co and various trace elements into sulphides during the evolution of the system. In particular, the study provides an opportunity to study how trace metal partitioning and Co deposition may be controlled by chemical and physical processes during fracturing and fluid flow (Fig. 2). Raič et al. (in prep.) conclude that the Rajapalot deposits do have unique trace element signatures, and their LA-ICP-MS spot analyses reveal that elements such as Co, Ni, As, Mo, Bi, Te, Au, Tl and W in pyrite have a capacity to discriminate between the Co-only and Au-Co zones, i.e. between the early and the late mineralisation. Whether these elements or sulphur isotopes are zoned within the pyrites (or other sulphides) cannot, however, be established with spot analyses. Establishing zoning is needed to understand possible fluctuation in the fluid chemistry or depositional conditions; a detailed paragenetic analysis is also needed from the veins to investigate the number and character of fluid pulses/generations that were involved in the formation of the mineralisation.

Fig. 1. Oblique view of Rajapalot project area (3D modelling in Leapfrog) looking northeast. The various prospects with their identified/interpreted ore shells (mineralised wireframes), with drill core intersections (in red), are illustrated. High-grade Au-Co mineralization at Rajapalot has been drilled to 540 metres deep at Raja and South Palokas prospects, but is not closed out at depth in any of the prospects. From Rantala et al., 2021.

 Main aims and objectives

The main aim is to investigate what controls the precipitation and the grade of Au and Co particularly in the second Au-Co mineralising event, and whether the event was likely to consist of a single event with fluctuating physical and/or chemical conditions, or separate events with unique signatures. In order to obtain a more detailed characterisation of the Rajapalot Au-Co system, your main tools will be the Scanning Electron Microscope (SEM) for textural and paragenetic investigation of the vein samples; and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) with a Time-of-Flight capability for detailed element and isotope mapping of the sulphides (Fig. 2). Specifically, you will:

  • Establish the textures and the paragenetic sequence within the veins using the Cathodoluminescence facility of the SEM; whilst also identifying any dateable minerals and whether they are inherited or not;and
  • Produce detailed LA-ICP-(ToF)MS trace element maps of the quartz vein generations mapped with SEM; and of the various sulphide phases in veins and in the disseminated ore in order to investigate any possible zonation and partitioning of Co and other elements between different minerals

Additional data and information can be gained to aid interpretation and application of the textural and element data. For example, dating of the Au-Co mineralisation more accurately (e.g., U-Pb dating on syn-depositional monazite) would help in understanding the geological context of the mineralisation. You will also be able to use Leapfrog 3D modelling to understand the spatial context of the mineralisation. Finally, you may also have an opportunity to pursue a more regional study, comparing gold alloy compositions and mineral inclusions in gold particles from the deposits with those from placer: such studies have been applied in other areas of poor outcrop exposure to illuminate regional metallogeny and prospectivity (Chapman et al. 2021; in press). For a study of gold geochemical compositions and inclusion suites you will use a combination of LA-ICP-MS, SEM/BSE/EBSD and Electron Microprobe Analyser EMPA (Fig. 3).

As a part of a DTP CASE studentship, you will have the opportunity to spend 3 months on site at Rajapalot, hosted by Mawson Gold Ltd. During this time, you will have the opportunity to familiarise yourself with the details of the geology, drill core, geophysics, and other material relevant to your study, as well as collect samples for laboratory analyses.

Fig. 2. An example of element mapping with LA-ICP-(ToF)MS. As and Co zonation is clearly imaged in the pyrite grains. The zonation can be linked to detailed sulphide growth history and is in this case interpreted to be related to rapid fluctuation in the fluid composition and/or physical deposition conditions, although the exact mechanism remains unknown. From Combes et al., 2020.

Fig. 3. Examples of analyses on gold grains; detailed data from gold can inform on the deposition process and any fluctuations in chemical or physical conditions. Top row, left to right: Heterogeneity in gold alloy composition viewed in BSE: numbered points indicate the paragenetic sequence of alloy (placer gold particle, Moosehorn Creek, Yukon); BSE image of a gold-rich rim with inclusion of pyrrhotite within both grain core and rim (Bonanza Creek, Yukon, Canada); SEM/EBSD map of placer particle from Bonanza Creek, Yukon, Canada, showing internal deformation of the grain depicted as colour changes as well as the bent twin boundaries. Bottom row: Examples of heterogeneity in LA-ICP-(ToF)MS maps in a gold particle from the Similkameen River, BC, Canada. From Chapman et al., 2021.

Training, support, and employability

You will work under the supervision of Dr Taija Torvela and Dr Rob Chapman at the Ores and Mineralisation Research Group OMG at Leeds (Facebook and Twitter: @OMGLeeds). The OMG draws upon a raft of expertise covering all necessary techniques. The project industry collaborator will provide logistical support, access to areas of interest, and geological and geophysical exploration data and samples. As a NERC DTP student, you will have access to NERC facilities such as the isotope laboratories at e.g. SUERC in Glasgow. You will also benefit from collaboration and support from existing PhD students at OMG working on various sulphide and gold deposits.

The project provides specialist training in: (i) state-of-the-art microanalytical and geochemical techniques; (ii) textural-structural and mineralogical analysis and field work; and (iii) industry-standard exploration and software skills. You will also have access to a spectrum of training both within the Leeds DTP and externally. The PhD study is equally suited to career pathways in academia or industry. The expected outputs of the project have global significance for illuminating a new economic source for an important battery metal, in a mineralization type where the deposit model and grade controls are still only partially understood. At the same time, exposure to industry-facing aspects through field work, relevant conferences and other interactions e.g. through the industry collaborator provides non-academic vocational experience. We anticipate that you will be able to publish up to three research papers, and attend both national conferences (e.g. MDSG) and international academic/ industry facing conferences (e.g. SGA, EGU, GSA, AGU, SEG, AMEBC Roundup) according to your career trajectory. The student would also be expected to contribute to the activities of the Leeds Chapter of SEG, with all the associated benefits of networking across industry and academia. You will also benefit from participation in the regular OMG research meetings and events that provide a forum to present and discuss your work in a supportive environment.

Student profile

The successful candidate will have at least a BSc with a high 2:1 or a 1st from a Geological Sciences or similar programme; an MSc/MGeol qualification is advantageous, as is experience of publication or other extra-curricular research activities. Excellent time management, critical thinking and analytical skills, and the ability to clearly communicate results are essential. Required existing subject-specific skills and experience can vary but suitable examples include e.g. ore deposit geology, preferably gold or cobalt deposit geochemistry, in either an academic or industrial context; textural and paragenetic analyses using SEM; and/or other microanalytical laboratory techniques relevant to the project. Training will be provided to develop and enhance all skills and knowledge.

 

References

Chapman, R.J. et al., 2021. Chemical and physical heterogeneity within native gold: implications for the design of gold particle studies. Miner. Deposita, doi.org/10.1007/s00126-020-01036-x

Chapman R.J. et al. In Press. A new approach to characterizing deposit type using mineral inclusion assemblages in gold particles. Economic Geology.

Combes, V., et al., 2021. Polyphase gold mineralization at the Yaou Deposit, French Guiana. Geological Society, London, Special Publications, 516, doi.org/10.1144/SP516-2020-29

Raič, S., Molnár, F., Cook, N., Vasilopoulos, M., O`Brien, H., Lahaye, Y., in prep. The powerful vectoring capacities of trace element signatures in orogenic Au-deposits in northern Finland.

Rantala et al., 2021. Mineral Resource Estimate NI 43-101 Technical Report, Rajapalot Property, Finland, for Mawson Gold Ltd. AFRY Report, Project Number: 101015822-001, 175 p.