Geological fate of seafloor massive sulphides at the TAG hydrothermal field (Mid-Atlantic Ridge)

Highlights • Generic geological model of hydrothermally extinct seafloor massive sulphide. • Sub-surface characterisation by combined drilling and geophysics. • New resource estimate for slow-spreading mid-ocean ridges. • Holistic approach to seafloor mineral deposits assessment. Abstract Deep-sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply. Increasing demand for these metals, many of which are required to enable a low-carbon and high-technology society and to relieve pressure on land-based resources, may result in deep sea mining within the next decade. Seafloor massive sulphide (SMS) deposits, containing abundant copper, zinc, gold and silver, have been the subject of recent and ongoing commercial interest. Although many seafloor hydrothermally systems have been studied, inactive SMS deposits are likely to more accessible to future mining and far more abundant, but are often obscured by pelagic sediment and hence difficult to locate. Furthermore, SMS deposits are three dimensional. Yet, to date, very few have been explored or sampled below the seafloor. Here, we describe the most comprehensive study to date of hydrothermally extinct seafloor massive sulphide deposits formed at a slow spreading ridge. Our approach involved two research cruises in the summer of 2016 to the TAG hydrothermal field at 26°N on the Mid-Atlantic Ridge. These expeditions mapped a number of hydrothermally extinct SMS deposits using an autonomous underwater vehicle and remotely operated vehicle, acquired a combination of geophysical data including sub-seafloor seismic reflection and refraction data from 25 ocean bottom instruments, and recovered core using a sub-seafloor drilling rig. Together, these results that have allowed us to construct a new generic model for extinct seafloor massive sulphide deposits that indicate the presence of up to five times more massive sulphide at and below the seafloor than was previously thought

Age and geochemistry of the Charlestown Group, Ireland:Implications for the Grampian orogeny, its mineral potential and the Ordovician timescale

Accurately reconstructing the growth of continental margins during episodes of ocean closure has important implications for understanding the formation, preservation and location of mineral deposits in ancient orogens. The Charlestown Group of county Mayo, Ireland, forms an important yet understudied link in the Caledonian-Appalachian orogenic belt located between the well documented sectors of western Ireland and Northern Ireland. We have reassessed its role in the Ordovician Grampian orogeny, based on new fieldwork, high-resolution airborne geophysics, graptolite biostratigraphy, U–Pb zircon dating, whole rock geochemistry, and an examination of historic drillcore from across the volcanic inlier. The Charlestown Group can be divided into three formations: Horan, Carracastle, and Tawnyinah. The Horan Formation comprises a mixed sequence of tholeiitic to calc-alkaline basalt, crystal tuff and sedimentary rocks (e.g. black shale, chert), forming within an evolving peri-Laurentian affinity island arc. The presence of graptolites Pseudisograptus of the manubriatus group and the discovery of Exigraptus uniformis and Skiagraptus gnomonicus favour a latest Dapingian (i.e. Yapeenian Ya 2/late Arenig) age for the Horan Formation (equivalent to c. 471.2–470.5 Ma according to the timescale of Sadler et al., 2009). Together with three new U–Pb zircon ages of 471.95–470.82 Ma from enclosing felsic tuffs and volcanic breccias, this fauna provides an important new constraint for calibrating the Middle Ordovician timescale. Overlying deposits of the Carracastle and Tawnyinah formations are dominated by LILE- and LREE-enriched calc-alkaline andesitic tuffs and flows, coarse volcanic breccias and quartz-feldspar porphyritic intrusive rocks, overlain by more silicic tuffs and volcanic breccias with rare occurrences of sedimentary rocks. The relatively young age for the Charlestown Group in the Grampian orogeny, coupled with high Th/Yb and zircon inheritance (c. 2.7 Ga) in intrusive rocks indicate that the arc was founded upon continental crust (either composite Laurentian margin or microcontinental block). Regional correlation is best fitted to an association with the post-subduction flip volcanic/intrusive rocks of the Irish Caledonides, specifically the late-stage development of the Tyrone Igneous Complex, intrusive rocks of Connemara (western Ireland) and the Slishwood Division (Co. Sligo). Examination of breccia textures and mineralization across the volcanic inlier questions the previous porphyry hypothesis for the genesis of the Charlestown Cu deposit, which are more consistent with a volcanogenic massive sulfide (VMS) deposit.</p

Origins of Si-Fe Cap Rocks at Extinct Seafloor Massive Sulphide (eSMS) Deposits from the TAG Hydrothermal Field (26 degrees N), Mid-Atlantic Ridge

Extinct seafloor massive sulphide (eSMS) deposits represent an understudied phenomena of modern seafloor hydrothermalism, and are thought to be a potential resource for base and precious metals if exploitation of seafloor mineral resources becomes economically viable in the future. The transition from active to inactive mounds poses important, but as of yet, unanswered questions about their preservation after hydrothermal venting ceases and oxygenated seawater circulates. This has the potential to destroy the metal tenor in SMS deposits, unless they are somehow protected. Here, we show the common occurrence of a silica-rich 'jasper' layer that forms the interface between unaltered sulphide below and oxidized metal-rich sediments above. The jasper layer is up to several m-thick and was encountered, in some form, at each of three extinct SMS deposits drilled in the TAG are of the Mid-Atlantic Ridge. The silicification events which have created these capping materials result in the decrease in permeability and is likely a common process during the waning stages of a hydrothermal cycle. As such, the Si-Fe cap could be a common product at eSMS deposits, and potentially provide an auto-preservation mechanism, restricting oxygenated seawater ingress and halmyrolysis of eSMS deposits

Origins of Si-Fe Cap Rocks at Extinct Seafloor Massive Sulphide (eSMS) Deposits from the TAG Hydrothermal Field (26 degrees N), Mid-Atlantic Ridge

Extinct seafloor massive sulphide (eSMS) deposits represent an understudied phenomena of modern seafloor hydrothermalism, and are thought to be a potential resource for base and precious metals if exploitation of seafloor mineral resources becomes economically viable in the future. The transition from active to inactive mounds poses important, but as of yet, unanswered questions about their preservation after hydrothermal venting ceases and oxygenated seawater circulates. This has the potential to destroy the metal tenor in SMS deposits, unless they are somehow protected. Here, we show the common occurrence of a silica-rich 'jasper' layer that forms the interface between unaltered sulphide below and oxidized metal-rich sediments above. The jasper layer is up to several m-thick and was encountered, in some form, at each of three extinct SMS deposits drilled in the TAG are of the Mid-Atlantic Ridge. The silicification events which have created these capping materials result in the decrease in permeability and is likely a common process during the waning stages of a hydrothermal cycle. As such, the Si-Fe cap could be a common product at eSMS deposits, and potentially provide an auto-preservation mechanism, restricting oxygenated seawater ingress and halmyrolysis of eSMS deposits