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 min erals of the hollandite-cryptomelane series from three Precambrian man ganese deposits (Ultevis/Sweden, Sitapar/India, Bachkoun/Morocco). Sam ples from the metamorphic occurrences Ultevis and Sitapar yielded Ar a ges of 1.8 Ga and 0.95 Ga, interpreted as the age of postmetamorphic c ooling (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 ''Sv ecokarelian'' and ''Satpura'' orogenic cycles. A date of 670 Ma obtain ed for a hollandite from a volcanogenic vein deposit (Bachkoun), howev er, contrasts with published extrusion ages of 580-560 Ma for the volc anic 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 establi sh a reasonable estimate of a mineralization age close to 580 Ma. Meas urement of fractional Ar losses during vacuum step heating (500-1600 d egrees C), although indicating good Ar retentivities, failed to define model diffusion parameters because of non-linear Arrhenius arrays. He lium diffusion results (200-1200 degrees C) indicated retention of rad iogenic He-4 by the samples, corroborated by U/He mineral dates betwee n 0.96 and 0.31 Ga. Potassium-bearing manganese oxides are therefore a ble to retain argon (possibly also He-4) through geological times and may thus provide ages of ore-forming processes (and perhaps later cool ing and alteration stages).