Copper content in sulfides on mid-ocean ridges (MOR) in controlled by the variation and abundance of Cu-rich minerals, that sulfides in ultramafic-hosted hydrothermal fields commonly contain higher Cu, compared with those in mafic-hosted hydrothermal fields. Recently, a new ultramafic-hosted hydrothermal field at 49.28°E was discovered on the ultraslow-spreading Southwest Indian Ridge (SWIR). Sulfides collected show remarkably high Cu contents (20.5–39.7 wt%, average 31.8 wt%), greater than those of typical ultramafic-hosted hydrothermal fields (0.07–33.98 wt%, average 17.94 wt%). In this study, we integrate sulfide mineralogy, bulk geochemistry and sulfur-isotope analysis of typical sulfide samples. Our findings indicate that the Cu-rich sulfides consist predominantly of primary chalcopyrite, bornite and secondary copper minerals, with bulk compositions enriched in Cu (∼26.88 ± 12.47 wt%, N = 19, 1SD), Au (∼5.1 ± 5.66 ppm), Co (∼730.96 ± 434.27 ppm), and Se (∼454.4 ± 247.21 ppm), while being depleted in Ni (∼7.46 ± 17.54 ppm). Additionally, these sulfides display relatively positive but uniform δ 34 S values, ranging from + 4.49 to + 6.9‰ (averaged + 5.49‰; N = 42), except for one value of + 2.3‰. These characteristics combined indicate deep ultramafic sourced metals, with restricted seawater entrainment (19–30%), and sulfide deposition occurring in a high-temperature (>300 °C), reducing, and relatively closed system. We propose that these conditions result from the immature Yuhuang detachment, which promotes early serpentinization of ultramafic rocks and low permeability, distinguishing from matured detachment controlled hydrothermal systems. Our results constrain hydrothermal formation at the end of immature detachment faults and provide new insights into ore-forming processes in ultramafic-hosted systems along ultraslow-spreading ridges.
Ao et al. (Mon,) studied this question.