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Noble gases have become a powerful tool to constrain the origin and evolution of ore-forming fluids in seafloor hydrothermal systems. The aim of this study was to apply these tracers to understand the genesis of newly discovered polymetallic sulphide deposits along the ultraslow-spreading Southwest Indian Ridge (SWIR). The helium, argon, and sulphur isotope compositions of metal sulphide minerals were measured for a number of active/inactive vent fields in the Indian Ocean. The helium concentrations and isotopic ratios in these ore samples are variable (⁴He: 0.09&#8315;2.42 &#215; 10^&#8722;8 cm³STP∙g^&#8722;1; ³He: 0.06&#8315;3.28 &#215; 10^&#8722;13 cm³STP∙g^&#8722;1; ³He/⁴He: 1.12&#8315;9.67 R_a) and generally greater than the modern atmosphere, but significantly lower than those in massive sulphides from the fast-spreading East Pacific Rise (EPR), especially for three Cu&#8315;Fe-rich samples from the ultramafic-hosted Tianzuo and Kairei vent fields. On the contrary, most of the SWIR sulphide deposits have somewhat higher ⁴⁰Ar/³⁶Ar ratios of trapped fluids (ranging from 290.6 to 303.4) when compared to the EPR ore samples. Moreover, the majority of sulphide minerals from the Indian Ocean have much higher &#948;³⁴S values (3.0&#8240;&#8315;9.8&#8240;, ~5.9 on average, <i>n</i> = 49) than other basaltic-hosted active hydrothermal systems on the EPR. Overall, these He&#8315;Ar&#8315;S results are well within the range of seafloor massive sulphide deposits at global sediment-starved mid-ocean ridges (MORs), lying between those of air-saturated water (ASW) and mid-ocean ridge basalt (MORB) end members. Therefore, our study suggests that the helium was derived mainly from the MORB mantle by degassing during the high-temperature stage of hydrothermal activity, as well as from a mixture of vent fluids with variable amounts of ambient seawater during either earlier or late-stage low-temperature hydrothermal episodes, whereas the argon in ore-forming fluids trapped within sulphide minerals was predominantly derived from deep-sea water. Additionally, relatively high &#948;³⁴S values exhibit a great estimated proportion (up to nearly 40%) of seawater-derived components. In summary, sub-seafloor extensive fluid circulation, pervasive low-temperature alteration, shallow seawater entrainment, and mixing processes, may make a larger contribution to the SWIR hydrothermal ore-forming systems, compared to fast-spreading centres.