The magnesite deposit of Euboea, Greece: a mineralizing process "per descensum" in ultramafic rocks

Magnesite from Euboea (Greece) occurs in mafìc and ultramafic rocks, which have generally been subjected to intense hydrothermal alteration. The hydrothermal activity led to the remobilization of both major and minor elements, and to the formation of minerals such as serpentine, quartz, smectite, calcite, dolomite andmagnesite. The type of mineral present and its abundance depend on the nature and intensity of the alteration. Over 150 samples representing thedifferent types and intensities of alteration (mineralization), have been analyzed for their Sr isotope composition,SiO2, and CaO contents, to obtain information on the nature of the fluids responsible for the alteration and mineralizing processes. he results of these investigations may be summarized as follows: (i) 87Sr/86Sr ratios range from 0.70470±3 to 0.70879±3 depending on the intensity of alteration and the type of associated minerals. (ii)SiO2and CaO are notcorrelated; Ca and Mg descended towards the lower parts of the deposit where they occur in pods and veins, whereas SiO2remained in the upper part of the orebodies to form stockworks. Comparison of Sr isotope data from alterated Euboea ophiolites with published data suggests that the fluids that caused the alteration had a marine origin. The Sr isotope values obtained at Euboea are comparable to published values for Miocene seawater, andhigher than seawater at the time of formation of the Mediterranean Ophiolite Belt, during the Jurassic. It can thus be postulated that the water-rock interaction processes, andhence hydrothermal alteration, probably started during theJurassic and continued until tectonic emplacement occurred at the end of the Cenozoic.