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Guest Blog – Jo Miles

JO MILES

BRITISH GEOLOGICAL SURVEY/UNIVERSITY OF BRISTOL
Jon Naden, Stephen Grebby, Brian Tattich, Frances Cooper, Dan Smith

Undertaking an alteration sampling transect along a paleo-fumarole, Milos island, Greece, May 2017

 

Jo is undertaking a BGS-hosted PhD titled ‘Epithermal paleosurface evolution in emergent volcanoes: implications for shallow submarine mineral deposit exploration and preservation’. She is the UK SEG Student Chapter Rep with interests in exploration models and approaches, and using high resolution hyperspectral remote sensing datasets focusing on alteration mapping using the short-wave infrared spectrum (SWIR).

@geologyjomiles

 

 

Exploration is often guided by ore-deposit models – but what happens when concepts evolve or a deposit doesn’t quite fit?

An ore deposit model is a conceptual idea, to allow consistent assessment among geologists and improve our predictability of deposit occurrence. Models encompass essential characteristics and with sufficient exposure, via drillcore, submersibles or outcrops, they have allowed us to develop our understanding of ore forming processes. We use models to display how deposits can form and are essential for exploration as we move into more challenging environments in search for future resources.

Overemphasis on deposit classification leads some (often the lesser experienced, which was all of us at some point), to think a deposit must fit into the scheme we have learnt during undergraduate lectures. With any natural system, variation occurs, leading us to the question – what happens when a deposit does not quite fit one model? The earliest classifications were dominantly based on metal content, as opposed to more modern concepts, which focus upon metal source, physical and chemical conditions and processes.

With time, concepts and associated geological jargon can evolve. Terrestrial epithermal systems are a great example where we witness this confusion in literature (Fig. 1) and misclassification.

The end-members of epithermal systems can be generally split between geothermal systems such as Rotorua (Migon and Pijet-Migon, 2016), New Zealand (Fig. 2) in comparison to volcanic-hydrothermal systems such as White Island (Hedenquist et al., 1993), New Zealand, which we typically assign to intrusion-centred settings (Fig. 3).

Figure 2 Pohutu Geyser eruption, Rotorua (Migon and Pijet-Migon, 2016)

Figure 3 Native sulfur from the Whakaari/White Island volcanic hydrothermal system (Heap et al., 2017)

 

Whilst the early classifications were probably fit for purpose during the time of conception, (e.g. Bonham, 1986), they fail to help us understand the larger systems and how we may go exploring such terrains for future deposits. We do not know the controls on mineralisation and why similar systems are barren. Using a classification by sulfide assemblage is limiting in exploration, because often but not, the mineralisation is the last thing you find. Often you are interested in an area because of the larger alteration footprint and from there you begin to narrow down your targets. From an exploration approach, Heald’s (1987) alteration end-member system remains the most helpful when in the field – it reflects the types of fluids present, allowing you to guestimate conditions e.g. pH. It is also worth noting that even though high- and low-sulfidation assemblages are end-members, they are not mutually exclusive – an intermediate-sulfidation assemblage can contain mineralogy from both. It is possible to have minor high-sulfidation minerals e.g. enargite associated with adularia-sericite alteration, likely reflecting local fluctuations in available sulfur or that the system never gained equilibrium.

Volcanic arcs are a prime setting for exploration of epithermal-style mineralisation. Yet, volcanic islands began life in the submarine environment prior to emergence, potentially hosting massive sulfide mineralisation (VMS). The depth of mineralisation is a local feature, controlled by the basin from which it emerges. The shallow submarine realm varies from 1650 – 200 m depth at the Brothers hydrothermal system (de Ronde et al., 2005), whereas for emergent islands in the Aegean volcanic arc, maximum depth of mineralisation is 500 m. It is likely that at shallow depths, we do not have the hydrostatic pressures required (like that observed at deep mid-ocean ridges), to allow successful sulfide accumulation. It results in a very unstable environment and mass wasting (Naden et al., 2016). Nevertheless, emergent volcanic islands can preserve both submarine and subaerial mineralisation – is this a hybrid system, or are the submarine features overprinted by subaerial paleosurfaces? Should they be considered as a new deposit model?

To begin to answer this question, we use analogues in the geological record to further our understanding of the deposits that do not necessarily fit a model.

The Eskay Creek stratiform deposit, located in British Columbia, formed during Phanerozoic volcanism and arc rifting. It exhibits similar characteristics to Kuroko-type VMS deposits, with Au deposition indicative of shallow sub-seafloor boiling (1500 m water depth), which we typically associate with epithermal systems (Sherlock et al., 1999). Clastic textures (Fig. 4) indicate it likely began life as a seafloor mound but underwent mass wasting and reworking. The deposit likely formed in shallow water, as the volcanic host contains shallow-water fauna and fossilised wood (Nadaraju, 1993). In the opposite end-member environment, the Kita-Bayonnaise submarine calderas of the Izu-Ogazawara arc, are host to numerous barite and pyrite-chalcopyrite-sphalerite +/- galena deposits associated with several ore forming events and are too associated with elevated Au grades < 2.7 ppm (Iizasa, 1993).

Figure 4 Rip up clasts and sulfide-rich rubble due to destabilisation within a shallow environment (Sherlock et al., 1999)

The Manus back-arc basin hosts enargite-luzonite hydrothermal chimneys at the summits of submarine volcanoes (Dekov et al., 2016) – mineralogy characteristic of high-sulfidation epithermal mineralisation. Associated alteration is acid-sulfate in nature where dacitic host rocks have undergone pervasive vapour-derived advanced argillic alteration, similar to the lithocaps that host Au-Ag veining in high-sulfidation epithermal systems (Hedenquist and Taran, 2013) – which leads us to the question could a porphyry copper deposit be at depth?

Figure 5 Enargite-luzonite active flange (Dekov et al., 2016)

Although these are only a few examples, it is apparent the mineralisation styles reflect a VMS-epithermal continuum (Sillitoe et al., 1996), indicating a likely hybrid model (Keith et al., 2018) that currently is not satisfied by current epithermal or VMS models. More research is needed to understand this variation in mineralisation styles within this hybrid system. We are dealing with dynamic environments and it is important to understand whether mineralisation is likely to be preserved.

Exploration is often guided by ore-deposit models – but what happens when concepts evolve or a deposit doesn’t quite fit?

To summarise:

(1) Go back to basics – consider the mineralogy, alteration and whether your observations suit the setting. Don’t get hung up on using geological jargon, just describe what you see.

(2) We are still learning and researching ore-forming processes, how they are displayed at the surface and we continue to improve upon ore-deposit models – they are not set in stone. We still don’t understand everything and that is the beauty of research.

 

References:

Berger and Henley, 1989. Economic Geology, 6, 405-423. Bonham, 1986. Proceedings of the 1986 International Volcanology Congress, Symposium 5, 13-17. Dekov et al., 2016. Chemical Geology, 438, 36-57. de Ronde et al., 2005. Economic Geology, 100, 1097-1133. Einaudi et al., 2003. Society of Economic Geology Special Publication, 10, 285-313. Hedenquist et al., 1993. Geology, 21, 731-734. Hedenquist and Taran, 2013. Economic Geology, 108, 1523-1540. Heald et al., 1987. Economic Geology, 89, 1-30. Heap et al., 2017. Journal of Volcanology and Geothermal Research, 332, 88-108. Hedenquist, 1987. Transactions of the 4th Circum-Pacific Energy and Mineral Resources Conference, Singapore, 1986. Hedenquist et al., 2000. Reviews in Economic Geology, 13, 245-277. Iizasa, 1993. Marine Geology, 112, 271-290. Keith et al., 2018. A new submarine hybrid-type between VHMS and epithermal-porphyry deposits, Brothers volcano, Kermadec arc. Presentation at Mineral Deposits Studies Group (MDSG) AGM 2017-18 – University of Brighton Migon and Pijet-Migon, 2016. Proceedings of the Geologists’ Association, 127, 413-421. Naden et al., 2017. Mass wasting events and their impact on the formation and preservation of submarine magmatic-hydrothermal ore deposits. Presentation at Fermor Meeting 2017: Factory Earth – The Geological Society, London. Nadaraju, 1993. Unpublished MSc thesis, University of British Columbia, 268 p. Sherlock et al., 1999. Economic Geology, 94, 803-824. Sillitoe et al., 1996. Economic Geology, 91, 204-212.