Sediment geochemistry reveals abundant off-axis hydrothermal fields on the ultraslow-spreading Southwest Indian Ridge

•We identified up to 14 potential off-axis fields by anomaly threshold average + SD.•Two fields were validated after this systematic sediment geochemistry survey.•The occurrence frequency of inactive fields is at least 2 times higher than reported active fields.•Sulfides tonnage on SWIR were likely comparable to previous estimate on neovolcanic zone. Off-axis inactive hydrothermal fields may harbor significant sulfide deposits. However, their distribution and abundance remain poorly constrained due to the challenges in their detection, as they lack associated hydrothermal plumes. Here, we present the first assessment on the abundance of off-axis inactive fields along four segments of the ultraslow-spreading Southwest Indian Ridge, based on a systematic sediment geochemistry survey. We found distinct dispersion patterns of hydrothermal Cu and Zn between magmatic- and detachment fault- controlled hydrothermal fields. These patterns are likely attributed to multiple factors, including variable physicochemical conditions of the hydrothermal plume,seafloor topography and bottom current. Up to 15 potential fields and their approximate locations were identified by the dispersion distance of hydrothermal Zn, Cu, and Fe. Among these fields, 8–14 are located off-axis, and 3–7 are likely newly identified inactive fields. The latter may have been active within the last 10 ka, with more than 60% spatially associated with detachment faults. The number of inactive fields is at least 2 times greater than previously reported for active fields within 40 km of the axial zone of the studied segments. As the studied sediments represent only the last 10 ka, significantly more buried inactive hydrothermal fields would be expected to be identified over longer time intervals. These findings significantly revise previous estimates of sulfide resources on ultraslow-spreading ridges.