Mineral-Textural Characteristics of Lithium Pegmatite Ores of Western Australia

Mineralogical and geochemical characterization of some of the main lithium-cesium-tantalum (LCT) pegmatite intrusions of the Archaean Yilgarn and Pilbara cratons, Western Australia, was undertaken to establish the key parameters that distinguish these important Li-ion battery resources. The majority of Western Australia pegmatites investigated belong to one of three main complex subtypes: (1) spodumene—Greenbushes, Kathleen Valley, Dome North, Mount Marion; (2) petalite—Londonderry, Dome North; and (3) lepidolite—Sinclair cesium. Examples of less common pegmatite types included Mount Cattlin, Bald Hill, and Pilgangoora (albite-spodumene type) and the Dalgaranga pegmatite (albite type). Spodumene shows a near-stoichiometric LiAlSi2O6 composition with a Li2O content of ~8.0 wt %. Impurities of commercial importance, Fe (+ Mn) varied up to 1 to 1.2 wt % with Na (500–1,200 ppm), as the only other trace element of significance detected in spodumene. Structural deficiencies of Li on the M2 site in the pyroxene structure contribute to the susceptibility of spodumene to alteration and to the preferential removal of Li, relative to Al and Si, during postcrystallization, and hydrous alteration resulting in reduced Li contents of 5.50 to 5.84 wt % Li2O. Spodumene is universally affected by two key types of alteration: a less common, postcrystallization, pseudomorphic replacement of spodumene by a massive, dark-green-to-black, fine-grained, Li-bearing mica-chlorite (cookeite) assemblage (Mount Cattlin and Bald Hill pegmatites); and a more widespread alteration characterized by symplectitic assemblages of graphic-textured, spodumene-quartz intergrowth (SQUI) along the crystal margins of spodumene in contact with Na/K-feldspar. Related to the former alteration style is a pervasive, secondary sericite-like vein alteration, developed along internal fractures and cleavage planes of spodumene. In all cases, alteration leads to the loss of Li from spodumene, and, in relation to the pseudomorphic replacement and vein alteration, introduces significant K and lesser trace element impurities such as F, Mn, Fe, Mg, and Rb. Mineral-textural associations revealed a more coarsely textured but unrelated SQUI developed in the upper petalite zone at the Dome North deposit and in the Li zone in the Greenbushes pegmatite formed by the decomposition of precursive petalite (confirmed) and virgilite (inferred), respectively. Changes in mica (muscovite and lepidolite) composition followed well-correlated trends with Li wt % positively correlated with F wt % and Al/Si negatively correlated with the Li content. The K, Rb, and Cs composition systematics of mica in Western Australia and worldwide pegmatites indicate a complex fractionation mechanism than cannot be explained alone by simple Rayleigh fractionation, which may operate during pegmatite crystallization. A new zircon U-Pb age of 2631 ± 4 Ma for the Greenbushes pegmatite is older than the previously determined age 2527 Ma and suggests that emplacement of the Greenbushes pegmatite was contemporaneous with other pegmatites in the Yilgarn craton with a maximum age range, ca. 2650 to 2620 Ma. Reported Pb-Pb dating of Ta-Nb-Sn oxides in Pilbara craton pegmatites (e.g., Wodgina and Pilgangoora) defines an emplacement window of 2850 to 2830 Ma, establishing the pegmatites as significantly older (ca. 200 m.y.) than the Yilgarn craton pegmatites. The younger 2629 ± 13 Ma U-Pb zircon age for the Pilgangoora pegmatite of the current study conflicts with the Meso-Archaean age reported for Pilbara craton pegmatites and is attributed to Pb loss associated with regional deformation and metamorphism, resetting zircon to an isotopically younger age. Further geochronology research is merited to establish a regional, temporal framework of pegmatite crystallization in the Pilbara craton.