| 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. |
