Pilgangoora Lithium-Tantalum Project: deposit geology and new constraints on rare-metal pegmatite genesis

Summary The Pilgangoora lithium-tantalum project encompasses a globally significant Li-Ta pegmatite district which formed in the late-Mesoarchaean, along the western margin of the East Pilbara Terrane in Western Australia. Regional field mapping, coupled with local petrographic and micro-analytical investigation into lithium and tantalum mineralisation at Pilgangoora, have allowed a first step towards developing a mineral system-type model for rare-metal pegmatites, that looks beyond the internal features of the trap site, and considers factors such as the triggers, potential sources, and pathways of rare-metal pegmatite magmas. The Li-Ta pegmatite sheets and dykes at Pilgangoora are composite bodies of three paragenetically distinct magmatic phases, each showing differing degrees of ductile strain. The intrusions are structurally controlled and were emplaced within the strain halo of a regional shear zone that is kinematically distinct from, and overprinting of, deformational structures and fabrics belonging to earlier orogenic events in the East Pilbara Terrane. A crystallisation age of 2845 ± 4 Ma for tantalite, obtained during this study, is ~15 m.y. younger than a proximal pluton of the high-K Split Rock Supersuite. This seemingly rules out direct derivation of the rare-metal pegmatite magma from what has traditionally been considered the most likely parent granitic intrusive suite. Along with the evidence for multi-phase parageneses, such a timing relationship is at odds with the currently accepted paradigm for rare-metal pegmatite genesis. The new constraints on rare-metal pegmatite genesis identified as part of this study form valuable exploration criteria, which may be applied locally to locate additional resources and, longer term, more strategically to review and target lithium-bearing pegmatite fields throughout Western Australia.