INSIGHTS INTO SPODUMENE CRYSTALLIZATION AND GROWTH FROM TRACE ELEMENT CONCENTRATIONS AND INTRACRYSTALLINE ZONING

Spodumene is the most important lithium ore mineral in granitic pegmatites with a Li₂O concentration of ~8 wt%. It incorporates trace to minor amounts of Fe, Mn, Sn, Ga, Ge, etc. into its pyroxene structure; however, the overall concentrations and intracrystalline distribution of these elements are poorly understood. Two factors that may influence spodumene geochemistry and zoning are the origins of the pegmatite melt and the spodumene crystallization processes. Li-rich pegmatite melts form either through direct products of anatexis (DPA) or as residual melts of granite magmatism (RMG)*. Spodumene crystallizes through (1) primary growth from magma, (2) primary growth from miarolitic fluid in pockets, or (3) secondary growth from petalite breakdown into spodumene-quartz intergrowth (SQI). It is possible that the trace element geochemistry of spodumene may preserve the signature of the original melt and crystallization process, but a systematic comparison of spodumene composition from different origins has not yet been conducted.

This study compares trace element concentrations and zoning of spodumene samples from >20 pegmatites, spanning the three crystallization types. When possible, the magmatic spodumene samples were further divided based on growth from RMG or DPA. Spodumene textures were first observed in hand samples and thin sections, then imaged using either benchtop µ-XRF at an ~20 µm spot size or cathodoluminescence (CL), depending on the crystal size, which ranges from >1 m to <1 cm. Trace element zones were quantified using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS).

Trace element zoning for both primary and secondary types follows a common pattern, with broad homogeneous cores surrounded by thin rims with oscillatory banding. On a preliminary basis, we found that the overall trace element contents are influenced not only by magma origin and crystallization processes, but also potentially by the local host rock, internal fractionation within the pegmatite, and the degree of alteration. Principle component analysis was used to simplify the elemental data and assess differences and similarities in spodumene from various origins.

*Wise, Müller, and Simmons (2022) Can. Min., 60, 229-248.