Extending the amphibole sponge: The metasomatism of clinopyroxene in arcs

Lead Research Organisation: University of Leicester
Department Name: Geology

Abstract

Volcanic arcs, like those that form the Pacific Ring of Fire, are markers for the collision and subduction of tectonic plates. These volcanic arcs are typically characterised by water-rich magma, which gives them a distinctive mineralogy and chemistry when compared to volcanoes produced from water-free magma. This water-rich character is what leads to the explosive nature of arc volcanoes, and arguably makes arc volcanoes the most hazardous on Earth. It is also responsible for their ore forming potential, most notably in the formation of copper and gold deposits.

Researchers in arc environments have recognised common chemical signatures in the erupted rocks, and have suggested that the mineral amphibole crystallises from the original magmas and is left in the lower crust as residual crystal mush. Previous geochemical studies suggest it exerts strong chemical controls on the crystallising and evolving magmas. However, despite a suggested widespread role for amphibole in these magmas, it is not particularly common in the volcanic rocks erupted at surface. If it forms so readily from the magmas to produce a mush, why do we not see it in the crystal population of the magma once it has moved away from the mush?

Experimental work on candidate "parent" magmas suggest that it is not amphibole but another mineral, clinopyroxene, that will be expected to dominate the early crystallisation and thus be the main mineral in the crystal mush. Clinopyroxene is also commonly observed in the volcanic arc rocks. There is a mismatch between observations: chemistry suggests amphibole is the important mineral, but the mineralogy of the rocks at surface suggest a greater role for clinopyroxene.

What if amphibole does not form by direct crystallisation from the melt? What if it formed by reactions between the melt and already-formed clinopyroxene? In such a scenario amphibole forms at the melt-mush interface, by altering the clinopyroxene. As melts are periodically released from this melt-mush "reaction zone", the mush (now a mixture of clinopyroxene and amphibole) is left behind. The melt moves to shallower crustal levels, and outside of the pressure stability range for amphibole. Thus, a melt has formed amphibole by reaction, left it behind in the mush, and risen to levels where more amphibole cannot form. Amphibole is not abundant in the crystal content of the melts that reach the surface. The reaction process explains the observation that amphibole is not ubiquitous in volcanic arc rocks, but can it explain the observation that amphibole is a major driver of chemistry?

A suite of samples from Savo volcano, Solomon Islands arc, will allow us to test this process. The surface rocks contain nodules of preserved crystal mush material (which are usually left behind in the crust). Detailed chemical analysis of clinopyroxene and amphibole from Savo will determine the chemical effect the reaction has on the evolving melt. Two hypotheses will be tested: 1) chemical signatures of amphibole crystallisation can instead be developed by clinopyroxene mush-melt reactions, therefore reconciling the chemistry with the minerals observed in the rocks; 2) clinopyroxene mush in the crust acts as a sponge, drawing water and copper out of the evolving melts, and thus acting as a buffer or barrier for their transfer from the mantle to the upper crust and surface.

Rather unusually, the mush nodules at Savo contain two different amphiboles - as well as the amphibole replacing clinopyroxene in the mush nodules (as per the scenario above), high water and sodium contents of melts at Savo helped to stabilise amphibole, and so it forms by direct crystallisation. This direct crystallisation amphibole will be used as a frame of reference to critically assess the two hypotheses - are the chemical effects of the two processes and produced amphiboles identical, therefore allowing the reaction process to reconcile the conflicting observations made in arc rocks?

Planned Impact

This project will assist exploration geologists in the broad scale targeting of potential copper mineralisation targets, allowing for more focussed exploration and a better discovery rate, contributing in turn to better economic returns, lower carbon footprints and reduced environmental impacts of exploration programmes.

Grassroots exploration needs good genetic models of mineral deposit formation. This project is identifying processes that will contribute to our understanding of the precursor magmas and the first steps for the formation of a deposit (e.g. Richards 2003). Applying the clinopyroxene sponge model briefly to ore deposits: early arc magmas will be barren, due to tholeiitic chemistries, early sulphide fractionation, and water and copper loss to the juvenile clinopyroxene sponge. Later magmas will be more copper rich, due to the hydration of the sponge allowing more hydrous melts into the upper crust. Post-subduction magmas formed by the remelting of the sponge will be alkaline and copper and gold enriched (Richards 2009). Thus, exploration targets (Cu vs. Au-Cu porphyries) and host rocks (tholeitic vs. calc-alkaline vs. mildly alkaline) and the related alteration signatures are changed as a result of a genetic model. This project will therefore contribute to the wider industrial effort to understand and discover copper and gold mineralisation in arc terrains.

Discoveries of gold and copper deposits in particular have not kept pace with demand, despite high prices (even during the economic downturn). Hall (2010) calculated that the major gold discoveries of the period 1997-2007 contained 377 Moz total anticipated recoverable reserves - less than half that produced from active gold mines in the same period. Grassroots exploration (that is, exploration for mineralisation in frontier countries or areas without previously recognised mineralisation) in particular has declined (Sillitoe 2010). Improving discovery rate, cost per discovery, reducing the environmental impact and carbon footprint of exploration, and encouraging and supporting higher-risk greenfield exploration depends on a number of factors, but good underpinning science plays a crucial role. In summarising the exploration techniques that contributed to the major ore deposit discoveries of the last forty years, Sillitoe and Thompson (2006) stated that "perhaps the greatest geologic influence on exploration practice has resulted from radical reinterpretation or modification of the empirical and/or genetic models for particular ore deposits types".

Hall DJ (2010) Exploration and discovery: Paradigm shift required. SEG Newsletter (82):15-17
Richards JP (2003) Tectono-magmatic precursors for porphyry Cu-(Mo-Au) deposit formation. Econ Geol 98(8):1515-1533
Richards JP (2009) Postsubduction porphyry Cu-Au and epithermal Au deposits: Products of remelting of subduction-modified lithosphere. Geology 37(3):247-250
Sillitoe RH (2010) Grassroots exploration: Between a major rock and a junior hard place. SEG Newsletter (83):11-13
Sillitoe RH and Thompson JFH (2006) Changes in mineral exploration practice: consequences for discovery. SEG Special Publication 12: 193-219

Publications

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Smith DJ (2014) Clinopyroxene precursors to amphibole sponge in arc crust. in Nature communications

 
Description This research provided evidence for a potentially widespread step / process in the development of the lower crust, and its subsequent impacts on the evolution of volcanic suites.
Exploitation Route Potentially relevant to the mining and exploration industry: this research will onform models of magam evolution, and thus impacts on regional scale scoping studies, where terrains and licence areas are selected on the basis of interpreted magamtic history.
Sectors Other

URL http://www.nature.com/ncomms/2014/140708/ncomms5329/full/ncomms5329.html
 
Description This research was primarily academic in nature. Impacts realised in the mining industry through refined regional-scale scoping of exploration areas are likely to remain in confidence, and unpublicised.
First Year Of Impact 2014
 
Description NERC Highlight Topics
Amount £3,000,000 (GBP)
Funding ID NE/P017053/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 05/2017 
End 04/2021
 
Description Olympus 
Organisation Olympus
Country Global 
Sector Private 
PI Contribution Developing new proxies for mineral exploration
Collaborator Contribution Loans of equipment, networking.
Impact Ongoing collaborations and new research funding
Start Year 2014
 
Description Fermor Conference 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Conference hosted at the Geological Society of London. All content recorded and made freely available through YouTube. Conference brought together academics from fields of petrology, geochemistry, volcanology and ore deposits to deliver presentations on current state-of-the-art on copper porphyry mineralisation (and associated ore styles).
Year(s) Of Engagement Activity 2017
URL https://www.geolsoc.org.uk/fermor17
 
Description MDSG Special Session 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Special session in a UK based, internationally-attended conference on mineral deposit research. Attended by approximately 250 delegates, from academia (including research students) and industry.

The event triggered good discussions between academics and industrial contacts; future collaborations are in an ealry, informal stage but seem likely.
Year(s) Of Engagement Activity 2013
 
Description Mineralogical Society Distinguished Lecturer 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact The Mineralogical Society of Great Britain and Ireland has invited Dan Smith to tour the UK and Ireland as a 2017-18 Distinguished Lecturer. He has delivered multiple talks on his research and its impact to audiences from academia (all levels), professional societies, and amateur interest groups.
Year(s) Of Engagement Activity 2017,2018
URL https://www.minersoc.org/distinguished-lectures-17-18.html