Structural controls on syntectonic metasomatic tremolite and tremolite-plagioclase pods in the Molanite Valley, Mt. Isa, Australia

In the Molanite Valley west of Mt. Isa, tremolite-bearing metasomatic pods, showing sharp contacts with enclosing rocks, occur over a strike length of 7.4 km in the biotite-zone Bortala Formation. In relatively low-strain rocks of the western part of the valley, plagioclase—tremolite pods have replaced psammite or mica schist surrounding the terminations of buck quartz veins, whereas in intensively sheared rocks to the east, pods of medium to coarse-grained massive tremolite (commonly containing greater than 95% tremolite) have replaced marble, quartzite and psammite. Talc-chlorite schists, formed by dissolution of carbonates during shearing of marble, are associated with some of the massive tremolite pods. rect relationship between structural domains and metasomatism suggests that migration of externally-derived fluids was controlled by fractures and the shear zone. The fluids, having migrated along fractures, interacted with wall rocks to produce plagioclase-tremolite pods. In the shear zone, buck quartz veins, some tremolite-bearing, show no obvious geometric relationship to massive tremolite pods. It is postulated that the metasomatic fluids fluxed through shear domains and penetrated progressive shortening domains to replace the host rocks and form the massive tremolite pods. Both styles of metasomatic rocks developed from late S2 to early S3 during a progressive E-W shortening event, which corresponds to the regional metamorphism. The metasomatic rocks represent only a small part of widespread metasomatism across the Mt. Isa Inlier.