Insights into extinct seafloor massive sulfide mounds at the TAG, Mid-Atlantic Ridge

Over the last decade there has been an increasing interest in deep-sea mineral resources that may contribute to future raw metal supply. However, before seafloor massive sulfides (SMS) can be considered as a resource, alteration and weathering processes that may affect their metal tenor have to be fully understood. This knowledge cannot be obtained by assessing the surface exposures alone. Seafloor drilling is required to gain information about the third dimension. In 2016, three extinct seafloor massive sulfide mounds, located in the Trans-Atlantic Geotraverse (TAG) hydrothermal area of the Mid-Atlantic Ridge were drilled. A mineralogical and textural comparison of drill core and surface-grab samples revealed that in recent ceased mounds high-temperature copper assemblages typical for black smoker chimneys are still present whereas in longer extinct mounds the mineralogy is pre-dominated by an iron mineral assemblage. Zinc becomes remobilized early in the mound evolution and forms either a layer in the upper part of the mound or has been totally leached from its interior. Precipitation temperatures of sphalerite calculated using the Fe/Zn ratio can help to identify these remobilization processes. While the Fe/Zn ratios of primary sphalerites yield temperatures that are in very good agreement with fluid temperatures measured in white smokers, calculated temperatures for sphalerites affected by remobilization are too high for SMS. Overall drilling of SMS provides valuable information on the internal structure and mineralogy of the shallow sub-surface, however, additional drilling of SMS, at a greater depth, is required to fully understand the processes affecting SMS and their economic potential

Structural control, evolution, and accumulation rates of massive sulfides in the TAG hydrothermal field

The Trans‐Atlantic Geotraverse (TAG) hydrothermal field on the Mid‐Atlantic Ridge is one of the best‐studied hydrothermal systems to date. However, high‐resolution bathymetric data obtained in 2016 by an autonomous underwater vehicle (AUV) reveal new information about the distribution of active and inactive hydrothermal deposits, and their relation to structural features. The discovery of previously undocumented inactive vent sites contributes to a better understanding of the accumulation rates and the resource potential of seafloor massive sulfide deposits at slow‐spreading ridges. The interpretation of ship‐based and high‐resolution AUV‐based data sets allowed for the determination of the main tectonic stress regimes that have a first‐order control on the location and distribution of past and present hydrothermal activity. The data reveal the importance of cross‐cutting lineament populations and temporal variations in the prevalent stress regime. A dozen sulfide mounds contribute to a substantial accumulation of hydrothermal material (~29 Mt). The accumulation rate of ~1,500 t/yr is comparable to those of other modern seafloor vent fields. However, our observations suggest that the TAG segment is different from many other slow‐spreading ridge segments in its tectonic complexity, which confines sulfide formation into a relatively small area and is responsible for the longevity of the hydrothermal system and substantial mineral accumulation. The inactive and weakly active mounds contain almost 10 times the amount of material as the active high‐temperature mound, providing an important indication of the global resource potential for inactive seafloor massive sulfide deposits

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

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

Origins and implications of Si-Fe cap rocks from extinct seafloor massive sulphide deposits from the TAG Hydrothermal Field, 26˚N, Mid-Atlantic Ridge

The deep ocean is considered the largest unexplored environment left on the earth. Since the discovery of active hydrothermal venting on the seafloor over 40 years ago, deep ocean research has furthered our understanding on the role that hydrothermal systems have in transferring heat and elements from the earth’s interior, and the unique chemosynthetic biota that inhabit vents provide a potential analogue for the origins of life on earth. With a global shift towards the development of green technologies, and the general decreasing grades of on-land mineral deposits, mineral reserves in the deep ocean could prove to be of economic interest in the future. Seafloor massive sulphide (SMS) deposits form at hydrothermal vent sites at a wide range of ocean spreading centres, however due to their high venting temperatures and unique life, are not considered viable for mineral exploitation. However, extinct seafloor massive sulphide (eSMS) deposits are an understudied aspect of modern seafloor hydrothermal activity, and are thought to be a potential resource for base and precious metals. The transition from active to inactive sulphide deposits poses important, but as yet, unanswered questions about their preservation as hydrothermal venting ceases. Upon cessation of hydrothermal activity, oxygenated ocean water has the potential to destroy the metal tenor and grade in SMS deposits, unless they are somehow protected. Surface study of inactive mounds from the TAG Hydrothermal Field has been undertaken, and reveals that with one hydrothermal field, multiple eSMS deposits present a range of surface weathering features, indicating that if deposits remain on the seafloor they begin to oxidise. However, upon drilling three eSMS deposits, Si-Fe caps, in the order of 3-5 m thick, were discovered at each deposit, directly overlying the massive sulphide ore bodies. This project brings together historical data based on comparable Si-Fe material from volcanogenic massive sulphide deposits (the geological equivalent of SMS deposits), and uses a range of petrological and geochemical techniques to assess their origins, to identify the range of processes which have combined to form these cap rocks, and to determine what effect these caps may have on the preservation of eSMS deposits. Petrological assessment of the Si-Fe cap rocks and the overlying sediments has identified a range of comparable textures and mineralogy to determine that the cap is a product of silicification of hydrothermal sediments. Preserved textures imply that the sediments have formed by a combination of seafloor weathering processes including mass-wasting, and likely involve microbial mediation and a biological precipitation of iron oxides. Late-stage, low temperature, reduced, diffuse hydrothermal fluids are interpreted to have silicified these sediments, thus forming the SiFe cap. Three dimensional connected porosity and permeability simulations undertaken on Si-Fe cap rock samples show that they are impermeable to vertical fluid movement (i.e. seawater ingressor hydrothermal upflow) in their current form. Two of the eSMS deposits show evidence of hydrothermal resurgence which has resulted in deep bleaching and sulphide precipitation within the base of the Si-Fe cap, and provides evidence that the formation of the Si-Fe cap has imparted a significant control on the hydrological regime of the eSMS deposit. Ultimately, the processes that are interpreted to have formed these Si-Fe cap rocks are generic and are likely to occur at other seafloor hydrothermal deposits. Combined with the fact that the Si-Fe cap is impermeable to fluid flow, their presence is identified as a potential auto-preservation mechanism to protect eSMS deposits from losing their metal tenor and grade as they move off-axis. Therefore, occurrence of these Si-Fe features in eSMS deposits could have significant implications for the potential for exploitation of eSMS deposits, and the future of deep-sea mining

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