A REVIEW OF THE GEOLOGY, MINERALIZATION, AND GEOCHRONOLOGY OF THE GREENBUSHES PEGMATITE, WESTERN-AUSTRALIA

The Greenbushes pegmatite is a giant pegmatite dike of Archean age wit h substantial Li-Sn-Ta mineralization, including half the world's Ta r esource. The pegmatite was intruded and crystallized synchronously wit h deformation and has a medium- to high-temperature and medium-pressur e metamorphic setting. Four major compositional zones and four subsidi ary zones have been recognized in the pegmatite. This zonation is unus ual and perhaps unique, in that those zones expected to crystallize la st and in the center of the pegmatite (e.g., lithium zones) occur as f ootwall and hanging-wall marginal zones. Ore mineralogical studies hav e 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 gen erally within tourmaline-rich subzones. Tin and tantalum mineralizatio n appear to have crystallized synchronously with tourmaline in both th ese zones. The lithium ore zones comprise mainly spodumene, apatite, a nd quartz, with some ore zones returning upward of 5 percent Li2O. Geo chronological 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 post ulated as the source for the pegmatite. It is therefore unlikely that the pegmatite is genetically related to these granitoids, which show f ew mineralogically and geochemically characteristics of specialized gr anitoids. Three mineralizing events are recognized in the pegmatite. T he first is related to initial crystallization of the pegmatite and me tasomatism of the country rocks (at 2527 Ma). Later or continuing synk inematic and synmetamorphic hydrothermal alteration of the pegmatite p roduced a second mineralizing event (at ca. 2430 Ma), and finally remo bilization of mineralization occurred during later deformation and met amorphism (at ca. 1100 Ma).