Gold mineralization in the hydrothermal field at the termination of a detachment fault: A case study of the Tianxiu Vent Field

Gold mineralization in hydrothermal systems at slow- to ultraslow-spreading ridges commonly occurs either in the hangingwall or the footwall of the detachment fault. However, the Tianxiu Vent Field (TVF) on Carlsberg Ridge is, to our knowledge, the only known example where the mineralization occurs directly at the termination zone of a detachment fault. Located approximately 5 km south of the rift axis near 3°48′N on the slow-spreading Carlsberg Ridge, TVF provides a unique opportunity for studying gold mineralization in this context. Detailed analyses of the mineralogy, mineral chemistry, and bulk geochemistry of massive sulfides from Tianxiu reveal several key findings: (1) both visible gold (native gold and electrum) and invisible gold are predominantly hosted in Cu-rich minerals such as isocubanite and covellite; (2) the content of Au (mean = 5.72 ± 4.38 ppm, n = 43) is positively correlated with Co, Cu, Bi, and Se; and (3) the gold mineralization occurs primarily at high-temperatures under strongly reducing conditions, with additional gold mineralization during late-stage silicification and seafloor weathering. When compared to other detachment-fault-associated deposits along slow- to ultraslow-spreading ridges, the ultramafic source rocks and the strongly reducing conditions at TVF appear to have facilitated Au mineralization. Additionally, the intensity of the fluid/rock interaction is possibly an important factor controlling the distribution of gold. The heterogeneous distribution of gold in Tianxiu is likely due to the spatial variability of fluid pathways within a highly permeable termination zone of the detachment fault. This study underscores a unique mineralization model of gold at the termination of a detachment fault on slow-spreading ridges, which has significant implications for the exploration of massive sulfide resource in off-axis regions.