DISTRIBUTION AND ORIGIN OF NOBLE-GASES IN MAGNETITES FROM THE PRECAMBRIAN BANDED IRON-ORE FORMATIONS

Under study were contents and isotopic composition of noble gases in magnetites from jaspilites of various age from 550 to 3.250 m. y. of mostly sedimentary (Kursk magnetic anomaly, Minor Khingan) and volcanic-sedimentary (Intermediate Near - Dnepr, Kola peninsula, Karelia) origin. Magnetic separation with subsequent manual extraction of pure magnetite with binocular microscope was used. A value of Ar-40/Ar-36 of all studied samples evidences on presence of radiogenic Ar. Its percentage varies from 36 to about 100%. Isotopic composition of Xe does not differ from the atmospheric one. He-3/He-4 ratio determined in one of the samples from Kursk magnetic anomaly corresponds practically to pure radiogenic helium (1.4 x 10(-8)). Radiogenic gases are mostly captured but some part of them was generated in situ. Besides radiogenic gases in magnetites are present atmospheric ones supplied by the meteoric waters. Distribution of nonradiogenic noble gases in magnetites of the skarn and carbonatite types is close to that observed in magnetites from jaspilites.