| Summary: | On the west of Chalkidiki peninsula and on the north of Evia Island, large magnesite deposits occur in dunite and its altered equivalent, brown serpentinite. In the Vavdos district on the west of Chalkidiki peninsula, magnesite veins up to 2m thick and with a lateral and almost uniform extent of more than 100m continue prominently at depths of about 100 m. Most of the substantial magnesite veins in brown serpentinite are surrounded by stockworks of smaller veins. In the Troupi area on Evia Island magnesite occurs as thin veins forming a stock- work-type of mineralisation in brown serpentinite and as the major component of a strong 2 to 3 metres thick, magnesite-serpentine breccia zone. The magnesite is cryptocrystalline, rarely microcrystalline, snow- white in colour, massive or nodular with porcelaneous lustre and conchoidal fracture. It is very pure and the small but variable amounts of Ca and Si and the traces of Fe and Mn shown in microprobe analyses are due to impuritie that occur between cryptocrystalline magnesite grains. The veins are filled with massive magnesite or with magnesite which exhibits smooth, irregular or rounded surfaces (nodular magnesite) with the internodular spaces being saturated with hydrous magnesium silicates. Tremolite, edenite, chlorite, antigorite, and talc are common in the selvages of veins filled with nodular magnesite but they are not genetically related to magnesite mineralisation. Talc is the only one of these minerals that suffered alteration during or shortly after magnesite mineralisation. Talc remnants and its alteration product, lizardite, together with chlorite and minor sepiolite fill the spaces between the magnesite nodules. Pre-existing talc-free, fractures or new structures created by the mineralising fluids were the sites of massive magnesite deposition. Field and laboratory evidence suggest that the deposits were formed from ascending, CO2-bearing fluids, that derived their magnesium content not from the immediately-surrounding country rocks, but from elsewhere and most probably from interaction with ultramafics at depth well below the observable zone of deposition. Experimental work on forsterite interaction with H2O-CO2-0.5 mNaCl solutions together with published data on the solubility of carbonates and silica suggest that ascending CO2-rich fluids that reacted with dunite at depth were able to fill open(ing) fractures with pure magnesite at a later stage in a high pressure-low temperature environment. Strontium isotope analyses indicate that the strontium in the magnesite came largely from carbonate sediments, and this in turn points to connate water or to any ground water isotopically equilibrated with carbonate sediments as the initial source of the mineralising fluids. The spatial relationship betweem the wall rocks of the magnesite and their plate tectonic environment suggest that it is likely that the deposits were formed during the final stages of the emplacement of the ultramafics in their present setting (Late Cretaceous-Eocene?).
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