Cu-Zn isotope compositions of hydrothermal sediments from the southern Mid-Atlantic Ridge: implications to the sediment formation and mineral exploration

Six hydrothermal sediment samples were collected from the Xunmei and Tongguan hydrothermal fields along the southern Mid-Atlantic Ridge during the China Ocean Cruise DY46 in 2017. Sulfides and oxides in the samples were separated, and Cu and Zn isotope compositions were analyzed. Results show that the ranges of delta 65Cu values of the bulk sediments, sulfides, and oxides were 0.36 parts per thousand-2.46 parts per thousand, -0.21 parts per thousand-1.10 parts per thousand, and 0.68 parts per thousand-1.52 parts per thousand, respectively. The delta 65Cu values of sulfides in four samples (46II-14, 46II-30, 46III-06, and 46II-09) were relatively low (-0.21 parts per thousand-0.50 parts per thousand), corresponding to the delta 65Cu values of sulfides from inactive old hydrothermal chimneys in northern Mid-Atlantic Ridge (nMAR), suggesting that the sulfides in the sediments were originated from collapsed dead chimney mainly. While the delta 65Cu values of the other two samples (46III-02 and 46III-08) were relatively high (1.10 parts per thousand-0.96 parts per thousand), corresponding to the delta 65Cu values for active hydrothermal chimneys sulfides in nMAR, which indicated that the sulfides in these two samples might mainly come from sulfide particles settled from active hydrothermal plume. Because of the high density of sulfide particles, they tended to settle near the hydrothermal vents first. Therefore, high delta 65Cu values of sulfides in 46III-02 and 46III-08 implied that undiscovered active hydrothermal vents near the sampling positions of 46III-02 in the Xunmei hydrothermal field and 46III-08 in the Tongguan hydrothermal field. The delta 66Zn values of hydrothermal sediments and sulfides ranged 0.11 parts per thousand-0.43 parts per thousand and 0.29 parts per thousand-0.67 parts per thousand, respectively. In the four samples from the Xunmei hydrothermal field, a positive correlation was found between the distance of the sampling position from sulfide mineralized spot and the Zn isotopic ratio, showing that the greater the distance from the mineralized spot, the heavier the Zn isotope composition as seen in two samples (46II-30 and 46II-14) of the Xunmei-3 spot. This result aligned with the findings of Wilkinson et al. (2005) and Baumgartner et al. (2023), suggesting that the lower the Zn isotope composition, the closer it is to the hydrothermal vent. However, in the Xunmei hydrothermal field, the Zn isotope composition in the other two samples (46III-02 and 46III-06) showed the opposite trend. This indicated that there might be an active hydrothermal vent near the sampling location of sample 46III-02. This observation aligned with the Cu isotope analysis results. This study showed that Cu-Zn isotopes are good indicators for understanding the formation mechanisms of hydrothermal sediments and for locating active hydrothermal vents.