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The fluids found in sulfide-producing hydrothermal vents are rich in ore-forming elements. These elements usually precipitate as solid particles that are dispersed by the plumes and deposited as sediments around the hydrothermal field. To assess how the geochemical features of such sediments may be used in seafloor sulfide exploration, this study analyzed the surface sediment geochemistry in the Dragon Horn area on the Southwest Indian Ridge. The results indicate that the sediments are mainly composed of pelagic material, basalt and ultramafic debris, (Mn,Fe) oxyhydroxides and Mn oxides, hydrothermal components. The debris content in this area is lower than that in sediments found at fast- and medium-spreading mid-ocean ridges, with relatively high amounts of ultramafic components, indicating that ultramafic rock outcrops are present in the study area. Precipitated hydrothermal elements in the sediments exhibit zonation in the spatial distributions of Zn, Cu, Fe, and Mn. The sediments proximal to the hydrothermal field display high Cu, Zn, and Fe contents and low Mn contents. Hydrothermal Cu and Zn phases occurred within a limited range of the hydrothermal field, and Zn was found to precipitate earlier than Cu during dispersal from the plume. Hydrothermal Fe distributed similarly with Cu spatially but with a larger dispersion distance. Hydrothermal Mn was able to migrate beyond the ridge valley to precipitate in the ridge flank sediments. Two hydrothermal Cu and Zn anomalies were identified on the south ridge flanks, which suggest the presence of undiscovered hydrothermal field. The sediments in the study area are additionally characterized by relatively low Mn contents and high Fe/Mn molar ratios. The relatively reducing environment of the plume may facilitate the migration of hydrothermal Mn over large distances, which probably accounts for the lower Mn contents in the sediments. The Cu-Cu/Fe molar ratio model was proposed to identify mafic- and ultramafic-related hydrothermal field. The low Cu/Fe molar ratios and REE characteristics of the sediments suggest that ultramafic rocks are likely not the only source of the ore-forming element for hydrothermal mineralization. Low-temperature diffuse flow on the ridge flanks of the study area probably results from serpentinization of ultramafic rocks. [ABSTRACT FROM AUTHOR]

The distribution of hydrothermal vents and the biogeography of associated faunal communities in the Indian Ocean are still not well studied, particularly the ultraslow-spreading Southwest Indian Ridge (SWIR). Herein, we present the geological and morphological data for the first reported active hydrothermal field on the ultraslow-spreading center. In this context, we created a detailed seafloor map based on visual data obtained from human-occupied vehicle (HOV) dives. Longqi-1 vent field (LVF) is an off-axis, mafic-hosted but detachment fault-controlled, high-temperature hydrothermal area with at least 28 hydrothermal structures, and their contacting host rocks were identified and mapped. The morphologies of hydrothermal structures in the LVF exhibit unique characteristics, with Zone M characterized by chimneys grown on the relatively flat-lying sulfide mound and Zone S characterized by large sulfide mounds and steep-sided structures hosting large flanges, and isolated beehive structures occurred more often at the boundary between hydrothermal deposits and basalts outcrops in both M and S zone. Microscopic examination showed that anhydrite is the dominant sulfate phase and chalcopyrite, pyrite, pyrrhotite, and sphalerite are the dominant sulfide minerals. Subseafloor hydrothermal circulation controlled by the detachment fault and local fracture zone may have directly determined the morphology of the sulfide structure in the LVF. Our results provide a picture of the LVF as well as complement and expand on previous studies on metal resource evaluation along the SWIR and Indian Ocean ridges, a detailed scenario for studying the surface distribution characteristics of hydrothermal circulation and its relationship with the distribution of hydrothermal fauna, and further enhance our understanding of the formation of modern seafloor hydrothermal systems. • This study report geological and morphological data for the Longqi-1vent field along the ultraslow-spreading SWIR. • In this context, we created a detailed seafloor map based on visual data obtained from human-occupied vehicle dives. • Zones M and S are characterized by relatively flat-lying sulfide mound and large sulfide edifices with steep-sided flanges, respectively. • The Longqi-1 hydrothermal field is the first documented high-temperature venting field along the ultraslow-spreading SWIR. [ABSTRACT FROM AUTHOR]

Seafloor hydrothermal venting fields occur on all ocean spreading ridges (OSRs) regardless of spreading rates. However, the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow-spreading OSRs are poorly known. Chinese Dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system, with an array of turbidity sensors and a near-bottom camera, along the ultraslow-spreading Southwest Indian Ridge. Here we report the discovery of multiple hydrothermal plumes overlying segments 28, 29, and 30 between the Indomed and Gallieni fracture zones. From these data, and earlier explorations in segments 25–27, we identify nine active venting sites. The spatial density (F s , sites/100 km) of active sites along the 394 km of ridge axis in our study area is thus 2.8, nearly 3× higher than predicted by the global trend of F s for ultraslow OSRs in the InterRidge database. Previous studies concluded that an enhanced magma supply to the central Indomed–Gallieni supersegment 11–8 Ma is now limited to segment 27. Our results indicate that although hydrothermal activity may be most concentrated in segment 27, the discoveries of active venting in segments 25–30 implies the presence of additional magma bodies across a broad extent of the Indomed–Gallieni supersegment. • Site frequency is 3× higher than predicted by the global trend for ultraslow OSRs. • Increased hydrothermal activity is related to a melting anomaly. • Active site distribution implies magmatic heat sources beyond the known melt body. [ABSTRACT FROM AUTHOR]