The processes of stable isotope transfer between chemically contrasted boundaries are examined for two metacarbonate–granitoid contacts in the Quérigut complex, Pyrénées, France. External contacts, between the sedimentary basement and granitic intrusions, behaved like a fluid-dominated open system whereas septa contacts, between carbonate septa and the host granitoids, behaved like a closed system with respect to external fluids. Along external contacts, skarns of several decimetres to several metres were developed. δ18O values of calcite, buffered at 13.5–14‰, suggest an advection of metamorphic aqueous fluid with a minor contribution of low δ13C CO2 during skarn formation. Internal contacts are characterized by a thin centimetre-sized skarn layer. δ13C and δ18O values of calcite follow an evolution explained by decarbonation processes alone. δ18O exchange profiles across the contact show a typical diffusion profile with their inflection points slightly displaced towards the metacarbonate side, interpreted as a limited influx of magmatic fluids. Moreover, the shape of septa profiles varies according to the thermal energy budget induced by the intrusive rock: more isotopic alteration appears where the intrusion size is larger.