This talk brings together four years of data collection including survey design, instrument development, two cruises and deep-seafloor drilling, during the EU-funded Blue Mining project focused on the TAG hydrothermal field at 26°N, Mid-Atlantic Ridge. The geological and geophysical data and their interpretation presented in this talk represent the most detailed, three-dimensional characterisation of hydrothermally extinct, seafloor massive sulphide (eSMS) deposits, hosted by volcanic systems, at slow spreading mid-ocean ridges. Our results indicate a substantial increase in the resource estimates for these deposits, which are targets for deep-sea mining.
Our research clearly demonstrates a distinct difference between the structure and composition of hydrothermally active SMS deposits on slow spreading ridges, where that model is mainly informed by drilling data from the active TAG mound, and eSMS deposits. The main differences are found in the change from active SMS deposits, where anhydrite (calcium sulphate) dominates an interior comprising massive sulphide overlying a silicified altered basalt-hosted stock-work of sulphide, to the extinct eSMS deposits that have a 1-5m thick carapace of iron-rich oxide sediments overlying a 3-5m thick silica cap that in turn overlies a highly conductive and dense massive sulphide ore body of about 100m thickness, in which the anhydrite has been dissolved and lost. This overlays a 100m-thick lower conductive and moderate density stock-work zone of altered basalt and sulphide surrounded by a low density, resistive host of altered basaltic lava.
The presence of highly conducting and high-seismic velocity material down to 50-100mbsf indicates that the main ore body forms a lens that is massive in character, probably with little gangue material. None of the CSEM (controlled source electromagnetic data) data penetrate below the main ore body, but the seismic evidence for relatively high velocity material below the main ore lens extends to 200mbsf where it forms a downwards-narrowing cone. We interpret this as a stock-work of sulphide in an altered basalt matrix, possibly with some silica overprint, that reduces the conductivity without making a significant impact on the velocity.
In general, our data indicate a resource that is 3 to 6 times larger than that predicted from the surface expression of the eSMS deposits alone. This is even larger if we include the metal rich sediment-apron that surrounds the eSMS mounds. Estimates by members of the Blue Mining project indicate that there are about ten times more extinct SMS deposits exposed on the seafloor or under a few meters of sediment, within 20km each side of the mid-ocean ridge axis, compared with the number of known hydrothermally active systems. Of these, 350 are known, with another 1000 estimated yet to be discovered. If we consider the eSMS deposits alone, then the accessible inventory may be of the order of 15,000 deposits worldwide.