Geological Settings of Archean Rare-Metal Pegmatites

The geological evolution of Archean Cratons is of importance for understanding the origin of LCT pegmatites, especially those with massive pollucite mineralisation as at Bikita. The pegmatites of Bikita are hosted by metabasalts and metadiorites of the Upper Bulawayan Group in the Masvingo greenstone belt in the SE part of the Zimbabwe Craton. At about 2670 Ma the southern part of this craton was affected by deformation and up to granulite-facies metamorphism along the Northern Limpopo Thrust Zone. Partial melting during this event led to the intrusion of the 2601 Ma Chilimanzi Suite granitoids with their temporal and spatial association to the pegmatites. The Yilgarn Craton in Western Australia is subdivided into six terranes which host many LCT pegmatites. The Londonderry and Mount Deans pegmatite fields form part of the Kalgoorlie Terrane, whereas the Cattlin Creek pegmatites belong to the Southern Cross Terrane in the southeastern part of the craton. Two main contrasting models for the seven Archean geodynamic events (D0–D6) are discussed: modern-style plate tectonics with a westward directed subduction type scenario, or vertical crustal movements induced by constantly upwelling of hot mantle material. The emplacement of the LCT pegmatites in various greenstone belt lithologies is related to the D5 event (2650–2630 Ma) with widespread intrusions of a low-Ca granitoid suite and formation of Au-mineralisation. The Pilbara Craton in the northern part of Western Australia represents Paleo- to Meso-Archean ages (3530–2830 Ma) and one of the world’s major tantalum pegmatite provinces. Well preserved low-grade Paleo-Archean greenstone belt successions with classic dome-and-keel architecture rest between dome-like granitic complexes that were emplaced during tectonic events (D1–D10) with numerous distinct episodes of magmatism. In the Eastern Pilbara Craton, the Wodgina pegmatite district is located within the Wodgina greenstone belt, surrounded by the large granitoids of the Yule Granitoid complex. The emplacement of the Wodgina pegmatites is associated to an event D10 (2930–2830 Ma) with the Sn-Ta-Li bearing post-tectonic granitoid intrusions of the Split Rock Supersuite at 2890–2830 Ma.