Marine mineral exploration with controlled source electromagnetics at the TAG Hydrothermal Field, 26°N Mid‐Atlantic Ridge

Seafloor massive sulfide (SMS) deposits are of increasing economic interest in order to satisfy the relentless growth in worldwide metal demand. The Trans‐Atlantic Geotraverse (TAG) hydrothermal field at 26°N on the Mid‐Atlantic Ridge hosts several such deposits. This study presents new controlled source electromagnetic, bathymetric, and magnetic results from the TAG field. Potential SMS targets were selected based on their surface expressions in high‐resolution bathymetric data. High‐resolution reduced‐to‐the‐pole magnetic data show negative anomalies beneath and surrounding the SMS deposits, revealing large areas of hydrothermal alteration. Controlled source electromagnetic data, sensitive to the electrical conductivity of SMS mineralization, further reveal a maximum thickness of up to 80 m and conductivities of up to 5 S/m. SMS samples have conductivities of up to a few thousand Siemens per meter, suggesting that remotely inferred conductivities represent an average of metal sulfide ores combined with silicified and altered host basalt that likely dominates at greater depths

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

Axial morphology of the East-Pacific Rise crest at its intersection with the Mathematician hot-spot: results of the PARISUB'2010 cruise

International audienceThe PARISUB cruise was led in 2010 using the R/V L'Atalante, the Autonomous Underwater Vehicle (AUV) AsterX and the manned submersible Nautile (Ifremer). The goal was to investigate the processes that occur during the interaction between the Mathematicians hotspot and the East-Pacific Rise at 16°N. The present spreading axis has an elevation of at least 400m above the average depth of the North Pacific ridge, indicating a high magmatic production. Lava previously sampled in this area are enriched in incompatible elements and isotopically, revealing so the contribution of an enriched end member of the mantle, i.e. the plume Surface geophysical data (multibeam bathymetry, gravity, magnetism) and near-bottom data (high-resolution bathymetry, gravity, magnetism, plume mapping) acquired during the cruise are used to measure tectonic structures, to individualize volcanic flows in relation with axial density variations and magnetic micro-anomalies. Here we present the results of the first high-resolution mapping of the ridge crest at 16°N using AUV. The maps, combined with visual ground truthing, show that most of the flows originate at the axial summit graben. Most often lava has drained fully or partially such that the point sources of the flow can be localized. Despite an expected high effusion rate, lobate flows predominate over sheet flows. Pillow flows are also well-represented, they constitute the most prominent volcanic structures of the area. The structure of the axial summit graben strongly varies along-axis. It is segmented with segments that trend differently (up to 5° of difference in their orientation). In some areas, the axial graben consists in one unique, well-depicted narrow graben. In other places, it consists in two narrow and parallel grabens. At last, at 15°46'N, the axial summit graben is much wider and it is constituted of numerous normal faults. At that location, the tectonic deformation is much less localized and the number of normal faults that form the graben is higher than elsewhere in the studied area, and the tectonic structures are little obscured by volcanic flows, compared to other segments of the axial graben. This area of widely distributed deformation coincides with a global slight change in the spreading center orientation

Complex Dyke Emplacement at the Hyper-Inflated EPR 16°N Segment

International audienceThe EPR segment located between 15°22'N and 16°15'N (hereafter referred as EPR 16°N) presents a very wide (~13 km at the widest) and quite shallow (~2300 m) axial dome and is clearly over-inflated with respect to typical magmatically robust segments of the EPR. This segment is located at the intersection of the EPR with the Mathematician seamount chain, suggesting some kind of ridge-hotspot interaction which would account for the over-inflation. As a consequence of such an interaction, it has also been suggested that the ridge axis may episodically relocate further west to maintain the ridge-hotspot connection. Cruise Parisub of R/V L'Atalante took place in March-April 2010 and used Deep Sea Submersible Nautile and Autonomous Underwater Vehicle Aster-X to study this problem. The area was surveyed using L'Atalante new Simrad EM 122 echo-sounder, and a few sea-surface magnetic profiles were added to the existing data set. A higher resolution swath was achieved at a slower speed on the axial dome. AUV Aster-X completed a 30 x 4 km wide survey of the axial zone at an altitude of 70 m, collecting high resolution multibeam bathymetry and imagery, magnetics, and nephelometry. A total of 24 successful Nautile dives was carried out with videos and photos, rock sampling, and magnetic data collection, making an axial cross section up to 300 ka off-axis to investigate the inferred ridge jumps (10 Nautile dives), and a set of along axis dives to identify the active volcanic axis and search for active and fossil hydrothermal sites (13 Nautile dives). During these dives, several active and inactive hydrothermal vents have been discovered. In this presentation, we focus on the AUV survey. Unlike most fast spreading centres, which display one clear axial graben at the summit of the axial dome, the hyper-inflated dome at 16°N presents a complex set of several grabens, among which it is difficult to decipher which one are volcanically active and may be considered as the neovolcanic zone. To this end, we have used the magnetic data, adequately corrected for the effect of the AUV and reduced to total field anomaly. The resulting magnetic map shows a set on negative anomalies associated to some - but not all - grabens and help to discriminate between recently active and inactive grabens. Indeed, such negative anomalies have already been observed by Shah et al. (JGR, 2003) on the EPR at 17°28'S and 18°37'S and interpreted as marking the presence of hot dyke swarms, warmer than the Curie temperature, and the associated demagnetization of nearby basalt. Forward modelling by these authors suggests that such an anomaly can remain during about century. On the EPR 16°N, the presence of several anomalies suggests that, in some areas, several dyke swarms are "simultaneously" active (at the scale of a century). The intensity of these anomalies may reflect the size and/or the age of the dyke swarms, the stronger anomalies corresponding to the larger and/or the hottest (and therefore most recent) dykes

Multi Scales Exploration Strategy - Example of Research For Hydrothermal Mineralization in the French EEZ of Wallis and Futuna

As part of a Public Private consortium, explorations to locate sulfide mineralization in the French EEZ of Wallis and Futuna allowed to discover new hydrothermal fields. Three cruises organized between 2010 and 2012 have explored this area. The first phase focused on a regional mapping to find areas of recent volcanism using multibeam echosounders and acoustic imaging. Hydrothermal plumes produced by high temperature hydrothermal activity were located using chemical and physical anomalies in the water column. Using a CTD/rosette we collected water samples and measured physical parameters. Onboard methane and manganese concentrations and physical parameters studies helped to locate active fields with a precision of about 5 km. Physical anomalies caused by the hydrothermal plumes were also recorded by analyzing the multibeam backscattered signals of the water column. In a second phase, an AUV, operated at 70m above the seafloor, helped to establish high-resolution (1 m) bathymetric maps on targets identified during Phase 1. Near seafloor, magnetic anomalies were recorded using a deep magnetometer on the AUV. Analysis of AUV acoustic signals in the water column allowed to find the vertical part of the hydrothermal plumes and to pinpoint areas of fluids discharge with a precision of a few meters. During the third phase, manned submersible dives were conducted on the hydrothermal fields to study the hydrothermal fluids, the different types of mineralization and associated ecosystems. This multi-scale strategy allows gaining considerable time in exploration, and led to the discovery of several hundred of active and inactive fields and associated sulfides mineralization on an area not previously mapped. The AUV high-resolution mapping and detection of vertical plumes and inactive chimneys can thus be considered as a missing link of the exploration strategy to locate hydrothermal deposits