A review of the geology, mineralization, and geochronology of the Greenbushes Pegmatite, Western Australia

Abstract

The Greenbushes pegmatite is a giant pegmatite dike of Archcan age with substantial Li-Sn-Ta mineralization, including half the world's Ta resource. The pegmatite was intruded and crystallized synchronously with deformation and has a medium- to high-temperature and medium-pressure metamorphic setting. Four major compositional zones and four subsidiary zones have been recognized in the pegmatite. This zonation is unusual, and perhaps unique, in that those zones expected to crystallize last and in the center of the pegmatite (e.g., lithium zones) occur as footwall and hanging-wall marginal zones. Ore mineralogical studies have identified more than ten Ta-bearing phases; cassiterite is the main Sn-bearing phase and spodumene the main Li-bearing phase. The main ore shoots occur exclusively in the albite zones in the pegmatite and generally within tourmaline-rich subzones. Tin and tantalum mineralization appear to have crystallized synchronously with tourmaline in both these zones. The lithium ore zones comprise mainly spodumene, apatite, and quartz, with some ore zones returning upward of 5 percent Li20. Geochronological studies utilizing imprecise whole-rock Pb-Pb and precise U-Pb zircon techniques on granitoids within the same major shear zone as the pegmatite date the intrusion of the granitoids to about 90 m.y. before the intrusion of the pegmatite; the granitoids have been postulated as the source for the pegmatite. It is therefore unlikely that the pegmatite is genetically related to these granitoids, which show few mineralogically and geochemically characteristics of specialized granitoids. Three mineralizing events are recognized in the pegmatite. The first is related to initial crystallization of the pegmatite and metasomatism of the country rocks (at 2527 Ma). Later or continuing synkinematic and synmetamorphic hydrothermal alteration of the pegmatite produced a second mineralizing event (at ca. 2430 Ma), and finally remobilization of mineralization occurred during later deformation and metamorphism (at ca. 1100 Ma).