Assessing the origin of old apparent ages derived by Pb stepwise leaching of vein-hosted epidote from Mount Isa, northwest Queensland, Australia

Epidote metasomatism affected large areas of tholeiitic metabasalts of the ~1,780 Ma Eastern Creek Volcanics in the Western Fold Belt of the Proterozoic Mount Isa inlier. Hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant [S.sub.2] fabrics which formed during the regional metamorphic peak at ~1,570 Ma associated with the Isan orogeny. Previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin (with an evaporitic component). Application of the Pb stepwise leaching technique to the epidote does not separate radiogenic [Pb.sup.4+] and common [Pb.sup.2+], generating little spread in [sup.206]Pb/[sup.204]Pb (between 16.0 and 30.5). The causes for this relatively low range are twofold: There is little radiogenic Pb in the epidotes (the most radiogenic steps account for Keywords Epidote * Pb stepwise leaching * Mount Isa * Uranium * Copper * Mineralization * Age inheritance
Introduction Regionally extensive epidote-rich hydrothermal alteration assemblages in the Proterozoic Mount Isa Inlier formed during regional metamorphism ([D.sub.2] event) at ~1,575 Ma (Heinrich et al. 1995; Hand and Rubatto 2002; [...]