Trace-element, carbon, oxygen and lead-isotope analyses were carried out to determine the formation mode and crystallization age of magnesite from the Budd ultramafic complex of the Archaean Barberton Greenstone Belt, South Africa. Its significantly high Ti contents probably relate to a soluble Ti-rich accessory mineral, probably dissolved during magnesite precipitation. Primitive mantle-normalized REE patterns of the magnesite show negative Ce and Eu anomalies induced by two events: (1) the Eu anomaly indicates reducing conditions probably induced by the emplacement of the ultramafic source rock; (2) the Ce anomaly implies oxidizing conditions, probably during a hydrothermal event that favoured the precipitation of magnesite in veins of the rock. This Ce anomaly is among the oldest in Earth history. Negative δ13C (−2.9 to −3.3‰ PDB) and high δ18O (30.5–31.2‰ SMOW) suggest low-temperature precipitation in a near-surface Kraubath-type epithermal system. The magnesite contains considerable amounts of radiogenic lead (206Pb/204Pb is 125.5–153.7 and 207Pb/204Pb is 41.3–47.7). The resulting Pb–Pb age of 3043 ± 59 Ma is interpreted as dating a hydrothermal event related to the extensive plutonism episode that has been reported in the Barberton area at about 3.1 Ga. Previous models that proposed formation of the magnesite under recent climatic conditions can be discarded.