Abstract
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: 0.09–2.42 × 10−8 cm3STP∙g−1; 3He: 0.06–3.28 × 10−13 cm3STP∙g−1; 3He/4He: 1.12–9.67 Ra) 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–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 δ34S values (3.0‰–9.8‰, ~5.9 on average, n = 49) than other basaltic-hosted active hydrothermal systems on the EPR. Overall, these He–Ar–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 δ34S 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.
Highlights
Because of their remote locations, ancient and active hydrothermal systems on the ultraslow-spreading Southwest Indian Ridge (SWIR) have been much less extensively explored until recently [19,20,21,22,23,24,25], when compared to those well-investigated vent sites located along the fast-spreading East Pacific Rise (EPR), and the slow-spreading Mid-Atlantic Ridge (MAR) [1,2,3,4]
These ratios represent typical mantle values and correspond to 3 He originating from the mid-ocean ridges (MORs) magma chambers that has degassed into the hydrothermal system [37,38,39]
Contain the highest 3 He and 4 He concentrations among the entire suite of tested samples, which are significantly higher than the Indian Ocean polymetallic sulphides
Summary
Over the past 40 years, submarine hydrothermal venting and associated polymetallic massive sulphides have been found in a variety of tectonic settings on the modern seafloor, including mid-ocean ridges (MORs) with different spreading rates [1,2,3,4,5,6], back-arc basins, and rift environments [7,8,9,10,11].These sulphide deposits represent an immense potential reservoir of various important metal resources, such as Fe, Cu, Zn, Pb, Au, and Ag, attracting worldwide attention and research interest [12,13,14].relatively little is known about the metallogenic characteristics of massive sulphide deposits from the Indian Ocean [15,16,17,18]. Over the past 40 years, submarine hydrothermal venting and associated polymetallic massive sulphides have been found in a variety of tectonic settings on the modern seafloor, including mid-ocean ridges (MORs) with different spreading rates [1,2,3,4,5,6], back-arc basins, and rift environments [7,8,9,10,11]. These sulphide deposits represent an immense potential reservoir of various important metal resources, such as Fe, Cu, Zn, Pb, Au, and Ag, attracting worldwide attention and research interest [12,13,14]. An integrated study of the He–Ar–S isotope system allows us to further develop our understanding of submarine hydrothermal systems at ultraslow-spreading ridges
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