Identifying Framboidal Pyrrhotite: A Proxy for hydrothermal alteration of Organic-Rich Sediments

Framboidal pyrrhotite, in sharp contrast to framboidal pyrite, has been rarely reported, and its formation remains poorly understood. Here we report its clear identification in a shale‐gas well core and explore its potential as a proxy for diagenesis or low‐grade metamorphism of organic-rich sediments. A range of complementary results including petrography, geochemistry, rock magnetism and paleomagnetism, collectively support the identification of framboidal pyrrhotite, whose coexistence with other framboidal minerals indicates pseudomorphic replacement of framboidal pyrite. A strong correlation between total organic carbon and natural remanent magnetization, together with its restriction to organic-rich layers, highlights organic matter's role in its genesis. Paleomagnetic and vitrinite reflectance data further link its formation to magmatic heating (∼274°C). We therefore propose hydrothermal replacement of framboidal pyrite by framboidal pyrrhotite, involving heating and organic matter. This study highlights its diagnostic features, key conditions, and proxy potential for hydrothermal alteration and low‐grade metamorphism in organic-rich sediments.