Surface sediment geochemistry and hydrothermal activity indicators in the Dragon Horn area on the Southwest Indian Ridge

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.