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 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 (eSMS) deposits formed at a slow spreading ridge. Our approach involved two research cruises in the summer of 2016 to the Trans-Atlantic Geotraverse (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 robotic lander-type seafloor drilling rig. Together, these results that have allowed us to construct a new generic model for extinct seafloor massive sulphide deposits indicate the presence of up to five times more massive sulphide at and below the seafloor than was previously thought.
Highlights
Demand for mineral raw materials is increasing as a result of population growth, rising living standards, urbanisation and, more recently, the transition to a low-carbon economy (Zepf et al, 2014)
The central part of the Trans-Atlantic Geotraverse (TAG) hydrothermal field is characterized by hummocky volcanic lavas that coalesce into mounds, 100–500 m in diameter, forming a neovolcanic zone ∼1 km to the west of the extinct seafloor massive sulphide (eSMS) mounds (Fig. 1B)
In the centre of the TAG hydrothermal field, a zone of extensional faulting is dominated by NNE-SSW strikes (Fig. 2A)
Summary
Demand for mineral raw materials is increasing as a result of population growth, rising living standards, urbanisation and, more recently, the transition to a low-carbon economy (Zepf et al, 2014). Published bulk geochemical data from 95 of these modern SMS deposits suggest a median grade of 3 wt.-% Cu, 9 wt.-% Zn, 2 g/t Au and 100 g/t Ag (Hannington et al, 2011; Monecke et al, 2016) These analyses are mainly derived from recoverable surface grab-samples, such as high-temperature sulphide chimney and related talus material, and may not be representative of the average composition of the deposits at depth. It is likely that the vast majority of SMS deposits are no longer hydrothermally active and are increasingly covered by pelagic sediment as they age It remains unknown what low temperature geological and environmental processes affect hydrothermally extinct SMS (eSMS) deposits once the flow of chemically reduced hydrothermal fluid ceases, whether the metal tenor becomes enriched, depleted or disappears with time, and what the structure and composition of these deposits is beneath the seafloor
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