Detailed petrologic studies of oceanic gabbros have demonstrated that metamorphic recrystallization is related either to deformation and/or prolonged circulation of fluid during the progressive cooling of the gabbroic sequence (Honnorez et al 1984; Mevel 1987, 1988; Stakes et al 1991; Alt and Bach 2001). Magmatism in association with faulting plays a crucial role to understand deformation processes, magma migration paths and chemical differentiation processes, while hydrothermal circulation caused the post-crystallisation mineralogical changes within gabbro. Detailed studies of petrography and mineral chemistry of mylonitic gabbro and altered gabbro from Northern Central Indian Ridge (NCIR) have been undertaken to trace the hydrothermal alteration process of lower oceanic crust at slow spreading ridges. Hydrothermally altered plutonic rock samples also provide information about both the primary igneous intrusions that supplied heat to drive hydrothermal circulation and the geochemical evolution of hydrothermal fluids during rock-fluid interaction. Thus, the alteration in layer 3 crustal section always provides rare opportunity to our understanding on hydrothermal processes at spreading ridge system.