Back-arc extension in the Aegean Sea has been accommodated by several large-scale detachments such as the West Cycladic Detachment System (WCDS) in the Oligocene and Miocene. The WCDS especially crops out on Serifos Island (Cyclades) with a synkinematic granodioritic intrusion. As skarns represent metasomatic reactions near the contact between intrusions and the host rocks, they sign the position of the main drains used by fluids. While the mineralogy of the Serifos skarns is well known, geometrical and kinematic relations between the detachments and ore bodies remains poorly studied. This study allows us to distinguish different types of skarn. High Temperature skarns correspond to (1) massive garnet endoskarns, (2) ribbon and “bubble” garnet-pyroxene endoskarns, (3) garnet-pyroxene cracks exoskarns, whereas medium-temperature pyroxene ± ilvaite bearing skarn breccias within the Meghàlo Livadhi and Kàvos Kiklopas detachments belonging to the WCDS. Our observations show that skarn formation is associated with the activity of detachments and the syntectonic pluton emplacement. Endo and exoskarn deposits formed coeval with the ductile and brittle structures resulting from the activity of the WCDS, such as echelon veins, veins with antithetic shear and boudinaged veins wrapped within sheath folds, with a top-to-the SSW or SW shear senses compatible with the regional kinematics. Some skarn breccias formed within detachment planes far from the contact of the main granodioritic body, attesting for the intense flow of magmatic fluids along these discontinuities. These over-pressurized fluids play a major role in the mechanical behaviour of the detachments and strain localization. The two detachments acting as preferential crustal-scale drains, the pattern of magmatic fluid flows is not centred on the intrusion. In this context, iron-rich skarns and associated primary magnetite deposits have been remobilized during late brittle increments of motion along the detachments, together with surface-derived fluids responsible for deposition of the economic Fe/Ba mineralization.