Fluid infiltration in the southern segment of the Red River Fault, China: Insights from rock magnetic, mineralogical, and geochemical analyses of fault rocks

Fluid infiltration within fault zones is intimately linked to the physical and chemical attributes of fault rocks, thereby playing a critical role in deformation and evolution of faults. Magnetic properties of fault rocks have proven to be an emerging source of information on faulting processes. To document evidence of fluid infiltration within the Red River Fault (RRF), detailed rock magnetic measurements in combination with mineralogical, and geochemical analyses are conducted on fault rocks collected from the Matouzhai outcrop along the range-front fault of the southern segment of the RRF. The results reveal that the ferrimagnetic fraction in the fault rocks is dominated by magnetite, with a small amount of hematite present in (proto-)cataclasites and fault gouges. Magnetic grain size and concentration decrease significantly from host rocks (mylonitized gneiss), via (proto-)cataclasites to fault gouges. Fault gouges are enriched in volatiles (CO₂, LOI, H₂O⁺), rare earth elements (REEs), and calcite, but are depleted in high-field strength (HFS) elements and exhibit negative δEu and δCe anomalies. These results indicate pervasive infiltration of the fault zone by CO₂-rich oxidizing hydrothermal fluids, leading to the depletion of magnetite and oxidation of magnetite to hematite, with dissolution of silicates and precipitation of abundant calcite (high pH). Magnetic parameters, particularly the high-field magnetic susceptibility, show significant correlations with HFS elements and REEs. These observations suggest that the rock magnetic properties of fault rocks are highly sensitive to fluid infiltration, and could serve as indicators of fluid conditions and fluid-rock interactions within fault zones.