Sulfide metallogenic model for the ultraslow-spreading Southwest Indian Ridge

Polymetallic sulfides present in mid-ocean ridges(MORs)have become important strategic resources for humans,and a scientific metallogenic model is necessary for the investigation and exploration of these resources.Compared to fast-and slow-spreading MORs,ultraslow-spreading MORs show substantial differences in magma supply,tectonic activity,and oceanic crust structures.However,information on hydrothermal circulation and a metallogenic model related to sulfides along the ultraslow-spreading ridges is still limited,which hinders further exploration of these resources.In this study,the distribution of hydrothermal activities,as well as the characteristics of the structures,heat sources,fluid pathways,host rock types,fluid properties,and sulfide assemblages in typical hydrothermal fields along the ultraslow-spreading Southwest Indian Ridge(SWIR),have been studied.It is concluded that the hydrothermal systems along the SWIR can be categorized into three types,including local enhanced magma-controlled,one-way detachment/high-angle large-offset fault-controlled,and flip-flop detachment-controlled types,which are further categorized into five subtypes based on their distinct geological backgrounds.Herein,we present a sulfide metallogenic model called Local Enhanced Heat Supply-Deep Faults(eHeat-dFault)for the SWIR.The overall spreading rate remains almost constant(14-18 mm/year),while the magma supply is heterogeneous in the segment scale along the SWIR.Over the past two decades,various hydrothermal systems and sulfide deposits have been identified along the SWIR.A deep magma chamber(4-9 km)is developed in the ridge segment with sufficient magma supply owing to the local enhanced magma supply,while long-lived active deep detachment faults(up to 13 km)with associated metallogenic belts are developed in ridge segments with poor magma supply.Hence,the ultraslow-spreading MORs fulfill the necessary conditions of a sustained heat source and stable hydrothermal pathway for the formation of large-scale polymetallic sulfide deposits.The number of hydrothermal fields detected in the investigation area is 2-3 times that predicted by the traditional Spreading Rate-Magma Flux model,demonstrating its significant endowment for sulfide resources.A balance between magma supply and faulting may influence the type and depth of hydrothermal circulation,the frequency of hydrothermal activity along the axis,and the scale of sulfide deposits.Spreading rate was previously believed to control heat sources,magma supply,and tectonic processes.However,for the SWIR,we suggest that local enhanced heat supply and deep detachment faults have a greater influence than the spreading rate on hydrothermal circulation and sulfide mineralization.The eHeat-dFault sulfide metallogenic model proposed herein could provide guidance for further exploration and research on polymetallic sulfides in ultraslow-spreading SWIR.