Authigenic magnesite, hydromagnesite and huntite associated with intensely altered and serpentinized ophiolitic rocks in Attica (mainland Greece) occur predominantly as veinlets and nodules within a totally weathered former serpentinite groundmass. Carbonate δ18O values are consistent with post-geothermal fluid temperatures between 25 and 70°C, but mostly between 25 and 30°C, from a dominantly meteoric-sourced groundwater, indicating near-surface, low-temperature conditions. Despite the proximity of a volcanic centre with strong CO2 flux, 75% of the carbon isotope data imply little or no incorporation of this CO2 into the authigenic Mg-(hydro)carbonates. Indeed, many δ13C values are more negative than soil-zone calcrete values, and in this setting Mg-(hydro)carbonate δ13C values below −6‰ VPDB probably indicate disequilibrium effects in alkaline groundwaters. Geothermal fluids and groundwaters were mainly routed through structural conduits. Some of the low-temperature hydromagnesite subsequently dehydrated to magnesite under near-surface conditions, and huntite is probably a diagenetic transformation of hydromagnesite, forming close to the volcanic centre where fluid Mg/Ca ratios were low. The isotopic signatures are distinct from previously published Balkan–East Mediterranean magnesite data arrays but are consistent with many other ultramafic-associated magnesium carbonates worldwide; their association with likely fluid compositions provides important context for Mg-(hydro)carbonate formation as geothermal conditions cool to near-surface temperatures.