Massive seafloor mounds depict potential for seafloor mineral deposits in the Great South Basin (GSB) offshore New Zealand.

Seafloor mounds are enigmatic features along many continental margins and are often interpreted as gas hydrate pingoes, seep deposits, mud volcanoes, or hydrothermal mounds. When such mounds occur in basins with past volcanic activities, they have the potential to host seafloor metalliferous deposits, which is generally overlooked. Using geophysical datasets, we document the fluid plumbing systems that promoted the formation of seafloor mounds in the Great South Basin (GSB), offshore New Zealand. We also investigate these mounds as potential seafloor metalliferous deposits. Our results reveal 9 seafloor mounds (~ 137 m high) above gigantic (~ 5.4 km high) fluid escape pipes that are associated with deeper crystalline rocks. The structural make-up of the mounds, their geospatial relationships with the pipes and intrusive rocks, and geophysical properties suggest a primary volcanic or hydrothermal origin for the culpable fluids and mounds respectively. Fluids derived from deeper coal beds and shallow foraminiferal oozes in the basin constitute secondary fluid sources focused along polygonal faults and lateral flow cells. A main sub-vertical and minor lateral fluid plumbing patterns are proposed. The relationship between the mounds, pipes, underlying intrusive rocks, and upward routing of mineral-rich fluids could have implications for the formation of ore-grade mineral deposits on the seafloor in the GSB.