Environment and structural controls on the intrusion of the giant rare metal Greenbushes Pegmatite, Western Australia

Abstract

The Greenbushes pegmatite district has been the main center of production of alluvial tin in Western Australia since the beginning of this century, and more recently, for the production of tin, tantalum, and lithium from one of the largest rare metal pegmatite deposits in the world. The intrusive rocks in the Greenbushes pegmatite district are concentrated along the ancient crustal-scale, Donnybrook-Bridgetown shear zone, analogous to the present San Andreas fault system, and are characterized by steeply dipping planar mylonitic fabrics with horizontal stretching lineations, asymmetric folds, asymmetric pressure shadows, and shear bands suggesting sinistral strike-slip movement. The Greenbushes pegmatite occurs in a higher temperature and higher pressure metamorphic terraine than would be expected for pegmatites containing rare element mineralization. The pegmatite contains the same shear fabrics as its host rocks and has evidence for syntectonic crystallization of minerals such as tourmaline, fantalite, garnet, and cassiterite.It is proposed that intrusion of the Greenbushes pegmatite magma was controlled by the Donnybrook-Bridgetown shear zone. Any melts or fluids present during movement along the shear zone would have been channeled into it, leading to intrusion of the pegmatite. The fluid pressure in the pegmatite magma may then have increased causing further failure, zones of structural weakness, and further intrusion of pegmatite generally parallel to the mylonitic fabric in the shear zone.According to present models and classifications, the Greenbushes pegmatite group should be unmineralized. However, it is apparent from this study that giant rare metal pegmatites can occur in higher grade metamorphic terranes and that these types of pegmatites need not have obvious parental granitoids. They may contain a variety of mineralization and are likely, at least in the Archean, to be associated with tectonism along crustal-scale fault systems.