Unlike basalt-hosted hydrothermal sites, characterized by a lack of magnetization, the magnetic signature of ultramafic-hosted hydrothermal sites remains poorly known, despite their wide occurrence at slow-spreading ridges and their strong mineral potential. The first high-resolution magnetic surveys of such ultramafic-hosted sites, achieved by deep-sea submersible on four sites of the Mid-Atlantic Ridge, reveal positive magnetic anomalies, and therefore a strong magnetization at the largest sites. This observation reflects the presence of a wide mineralized zone beneath these sites, the stockwork, where several chemical processes concur to create and preserve strongly magnetized magnetite. Beyond pointing out the importance of subsurface chemical processes in hydrothermal activity, the aging of oceanic lithosphere, and the ocean chemical budget, our results have immediate application for detecting and characterizing economically valuable deep-sea mineral deposits.

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