Abstract
Hydrothermal activity results in the formation of hydrothermal mineral deposits, including seafloor massive sulfide deposits, at oceanic spreading ridges, arcs, and back-arcs. As hydrothermal systems age, the mineral deposits eventually become severed from the heat source and fluid-flow pathways responsible for their formation and become extinct. The timescales and processes by which this cessation of activity occurs, and the resultant distinction between hydrothermally active and inactive deposits has recently taken on policy implications related to the potential issuance of exploitation leases for seafloor massive sulfide deposits by the International Seabed Authority in Areas Beyond National Jurisdiction. Here, we discuss the scientific rationale behind designating hydrothermal systems as active, inactive, or extinct, with the aim of applying a scientific underpinning to ongoing policy discussions, which often lack a common set of criteria and use the same descriptions for opposing phenomena. We apply the simple definition that active vent fields currently exhibit fluid flow above ambient seawater temperatures, inactive vent fields are not currently exhibiting fluid flow but may potentially become active again, and extinct vent fields are not expected to become active again. We suggest these terms can only be correctly applied at the vent field scale and define a vent field as a geologically continuous entity that may include both actively and formerly venting hydrothermal deposits. Finally, we propose criteria and techniques for determining activity and reasonably bounding the extent of a vent field for classification purposes.
Highlights
Hydrothermal vents are sites of fluid discharge on the seafloor that occur along or associated with submarine tectonic boundaries such as mid-ocean ridges and subduction zones
If the hydrothermal vents are discharging high-temperature (>~250 C), metal- and sulfur-rich fluids, a seafloor massive sulfide (SMS) deposit may develop over time on and/or immediately below the seafloor from the precipitation and accumula tion of metal-rich minerals that precipitate from the fluid when it mixes with cold seawater [1,2]
This has been followed by assertions that mining of active hydrothermal vents would likely take place in a limited manner, if at all, and that inactive deposits will be the main target of this emergent industry [13]
Summary
Hydrothermal vents are sites of fluid discharge on the seafloor that occur along or associated with submarine tectonic boundaries such as mid-ocean ridges and subduction zones. A ban on mining of active SMS deposits has been suggested by parts of the scientific community, due to the minimal areas of the seafloor these vents occupy, their scientific value, and low estimations of their resource potential [10,11,12] This has been followed by assertions that mining of active hydrothermal vents would likely take place in a limited manner, if at all, and that inactive deposits will be the main target of this emergent industry [13]. Known active hydro thermal systems are diverse, and many do not result in SMS formation We discuss those associated with high-temperature sub-seafloor fluid circulation that has the potential to mobilize and deposit base and precious metals as sulfide minerals (e.g., temperatures above 250 C) [14]. We do not include systems that would not result in SMS formation, and so exclude low-temperature, low-flux systems associated with off-axis areas and seamounts
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