The selective chemical extraction of CO2 is essential for correct measurements of isotopic compositions in rocks containig multiple carbonates. Recent experimental work with pure and artificial samples by Al-Aasm on acid attack for isotopic analysis suggests that traditional laboratory procedures are not efficient and that there is a real danger of contamination of the gases extracted from different coexisting carbonates. The present paper discusses data obtained by traditional methods on the δ13C and δ18O of a Proterozoic Veitsch-type magnesite deposit near Brumado, southern Bahia, Brazil. The new data are compatible with those reported in the literature for similar rocks. One of the studied samples exhibited a veiy high Mg/Ca ratio (223) and was virtually ali magnesite, as confirmed by X-ray spectrometry. This sample was considered a "natural standardMof magnesite for several laboratory experiments to test the speed of reaction of dolomite and magnesite with H3PO4 at 25 and 50ºC in order to verify the best analytical procedures for obtaining carbon and oxygen isotopic ratios in magnesite ores. The problem of contamination is investigated in this same sample, with complementary studies with a dolostone sample. The results with the magnesite sample point to the application of traditional procedures but the dolostone data confirms the danger of contamination, principally in samples rich in dolomite and poor in magnesite.
