AR-40/AR-39 AGES OF PRECAMBRIAN MANGANESE ORE MINERALS FROM SWEDEN, INDIA AND MOROCCO

In order to test their chronometric potential, Ar-40/Ar-39 stepheating- (and He-4-) analyses have been carried out on five manganese ore minerals of the hollandite-cryptomelane series from three Precambrian manganese deposits (Ultevis/Sweden, Sitapar/India, Bachkoun/Morocco). Samples from the metamorphic occurrences Ultevis and Sitapar yielded Ar ages of 1.8 Ga and 0.95 Ga, interpreted as the age of postmetamorphic cooling (Hollandites/Ultevis) and of an early, K-introducing alteration process subsequent to amphibolite facies metamorphism (cryptomelanes/Sitapar). Both data are consistent with known chronologies of the ''Svecokarelian'' and ''Satpura'' orogenic cycles. A date of 670 Ma obtained for a hollandite from a volcanogenic vein deposit (Bachkoun), however, contrasts with published extrusion ages of 580-560 Ma for the volcanic host rocks (''Ouarzazate Series''), probably due to incorporation of excess argon. The use of the Ar-40/Ar-39 technique, together with multiple isotope systematics, made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 Ma. Measurement of fractional Ar losses during vacuum step heating (500-1600 degrees C), although indicating good Ar retentivities, failed to define model diffusion parameters because of non-linear Arrhenius arrays. Helium diffusion results (200-1200 degrees C) indicated retention of radiogenic He-4 by the samples, corroborated by U/He mineral dates between 0.96 and 0.31 Ga. Potassium-bearing manganese oxides are therefore able to retain argon (possibly also He-4) through geological times and may thus provide ages of ore-forming processes (and perhaps later cooling and alteration stages).