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

Abstract

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.

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