ULTRA - Ultramafic-hosted mineral Resource Assessment

Lead Research Organisation: National Oceanography Centre
Department Name: Science and Technology

Abstract

Summary:
Hydrothermal seafloor massive sulphide (SMS) deposits on mid-ocean ridges (MOR) are paradoxical; their size seems to be inverse to the amount of volcanic activity. While hydrothermal SMS are more frequent at fast-spreading MOR, the largest deposits occur where volcanism appears to be a minimum. Here, at so-called amagmatic segments on slow- and ultra-slow spreading ridges, ultramafic rocks from the lower-crust and upper-mantle are exhumed by long-lived faulting; a process that is thought to affect 50% of the length of slow-spreading ridges. Ultramafic-hosted seafloor massive sulphides (muSMS) in these settings form some of the largest deposits known, hosting high metal concentrations of Au, Cu, Ni, E-tech elements (Co, Pt). Whereas the magmatic driving force for volcanic-hosted SMS deposits is well established, it remains contentious for the muSMS. Similarly, while there are models for the sub-surface structure and extent of volcanic-hosted SMS, little is known about muSMS. For some muSMS, vent fluid chemistry indicates the potential for extensive sub-seafloor metal precipitation, possibly by interaction with pH barriers due to serpentinisation of the host rock. Furthermore, the physical, chemical and microbial mechanisms affecting muSMS after their formation are poorly constrained.

Our study aims to test the hypothesis that muSMS deposits form extensive sub-surface mineralisation and undergo significant post-formational modification at and beneath the seafloor under the influence of highly variable pH conditions as a result of interaction with ultramafic rocks and during serpentinisation.

Our plan is to combine novel geophysical techniques (electromagnetic induction and inverted down-hole seismic tomography) with surface mapping and sub-seafloor drilling (recovering host rocks, sulphides, sediment and fluids) to image the 3D structure and composition of the deposit and its surroundings. The mineralogy, geochemistry and isotope signatures of the samples will reveal the paragenetic history of the deposits including formation, recrystallisation, metal mobilisation, alteration and penetration by seawater. Hydrothermal fluid samples will reveal the nature of the heat source driving deposit formation and host-rock interactions and, combined with studies of metalliferous sediment, constrain metal mobility during later alteration. Ages of these processes will be constrained by radiometric dating. Rates of processes will be constrained by in situ and lab-based, abiotic oxidation and microbial alteration experiments. We will draw these observations together using thermo-physio-chemical numerical modelling to construct a coherent understanding of the formation and preservation of these large polymetallic muSMS deposits in todays-oceans. Our approach requires two cruises to the largest known and best characterised muSMS field at 13degrees30minutesN, Mid-Atlantic Ridge (MAR). Despite being technically ambitious, our experience from the EU-funded Blue Mining project and the involvement of both academic and industrial partners, contributing in-kind data and costs, significantly de-risks the research.

Planned Impact

Impact Summary

'If it can't be grown, it has to be mined'; minerals underpin every aspect of our daily life. They are essential for supporting economic growth, and improving and maintaining quality of life. Demand for minerals is increasing as global population expands and minerals are used in a greater range of applications. The vast majority of minerals are currently derived from mining on land, which represents less than one third of the planet's surface. Set against the general trend of declining terrestrial ore grades and mineral deposit discovery rates, and a requirement to decouple metal production from carbon emissions it is essential to consider new resource types. The vast metal resources of the deep-ocean will be vital for resourcing future generations, and may represent a more sustainable source of supply. However, to meet this challenge new scientific knowledge is required on the magnitude of the metal resource and its global distribution, which relies on understanding mineral deposit evolution and preservation potential. Project ULTRA will have the following specific impacts:

Economic: Demand for metals is increasing and prices are predicted to increase in the medium- to long-term. Stable and secure metal supplies are vital to many industrial sectors (e.g. automotive, aerospace, energy), which are currently highly dependent on imports. There are specific concerns about the security of supply of metals of growing economic importance due to their use in high-technology and green energy applications. Diversification of supply through the exploitation of new and novel resource types (e.g. seafloor massive sulphide deposits) will help reduce supply risk and support economic growth. Huge growth is predicted in the deep-ocean mining sector and the European Commission expects global annual turnover of marine mining to reach about euros10 billion by 2030. The knowledge gained from ULTRA will directly contribute to UK R&D in this sector, improving competitiveness, and potentially attracting future global business.

Environmental: Decarbonisation of energy supply and meeting emission targets is dependent on the deployment of renewable energy technologies, and the development of new transport methods, many of which are highly metal intensive. Improved availability of metals, because of exploiting new sources will facilitate decarbonisation. Mining is currently one of the most energy intensive industrial sectors. Extraction of metals from deep-ocean mineral deposits may use less energy than traditional sources, directly contributing to a reduction in carbon emissions. This proposal will improve understanding of the size, mineralogy and metal tenors of these deposits, which is vital for making comparisons with land-based supplies. Our research will also provide new information on the composition and evolution of sea floor mineral deposits and hence their potential environmental impact on adjacent faunal habitats if they are mined.

Policy: Improved understanding of all of Earth's minerals resources is fundamental to sustainable development and balancing economic and environmental interests. ULTRA will yield new information on resource distribution that could help inform regulation of the sector, and help nations more effectively manage their marine resources. Results from our proposed research will enable evidence-based decisions by non-governmental organisations and policy makers scrutinising the sustainability of future extraction of sulphide mineralisation on the sea floor.

Social and educational: Transparent science and evidence collection, yielded by projects such as ULTRA, is key to gaining public acceptance of deep-ocean mining and social licence to operate. This multidisciplinary study of an extreme environment is visually exciting and will help entice the next generation into science and technology.

Publications

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