Metal enrichment in pegmatites: insights from Mataketake Region, New Zealand

The use of lithium in high-capacity energy storage systems and rechargeable lithium-ion batteries has made it a necessary component to facilitate the energy transition. Hard-rock sources such as pegmatites and lithium-enriched granites comprise a significant portion of economic lithium resources and are commonly derived from anatexis of metamorphic rocks. Crustal melting, via mica dehydration reactions, is assumed to be a driving factor in the enrichment of lithium. However, the specific mechanisms and reactions that control lithium mobilisation and enrichment are not well constrained. This research intends to enhance knowledge on the mechanisms for lithium enrichment using samples of Alpine Schist and Mataketake Pegmatite (New Zealand) as a case study. Through in-situ petrological analysis of mineralogy, textures, and micro-textures, followed by geochemical analyses of mica and feldspar compositions we aim to establish melting reactions, mechanisms, and processes, including which phase(s) are melting and availability of fluids. Deformation textures are abundant in the pegmatite, in the form of fractured porphyroblasts, mica-fish, kink-banding, sub-grain deformation, deformation twinning, and highly disequilibrium inclusion textures, often occurring as ‘strings of beads’, interpreted as annealed myrmekite. This results in a protomylonitic texture within the pegmatite. Therefore, the intersection of strain and low-degree melting exert dominant controls on the petrography, and consequently will have important implications for the geochemistry. Using Laser Ablation ICP-MS, we supplement these observations using texturally constrained analysis of major and trace elements in key phases, such as micas and feldspars, to understand elemental concentrations in the Alpine Schist and the produced melt (Mataketake Pegmatite). We apply an empirical Ti-in-biotite geothermometer to constrain biotite closure temperatures within analysed samples. This thesis suggests a temperature range of 539-635ºC and is consistent with existing thermometry for crustal anatexis associated with the Mataketake Pegmatites. This is important as it removes the possibility for biotite melting, which initiates ~740°C (Kunz et al., 2022). We calculate whole-rock Sr, Rb and Ba and normative Ab-An-Or composition for the Alpine Schist and Mataketake Pegmatite. Our data align with recent literature and suggest H2O-present muscovite dehydration melting. Modelling of major elements K, Fe and Mg, followed by trace elements Sr, Ba, Pb, La and Li, intends to further constrain melting scenarios. Within the analysed system, K is solely hosted in significant quantities in micas, which therefore make them vital in determining melt concentrations of K. With the possibility for biotite melting in notable proportions inhibited, we look to accessory phases to aid in sufficient enrichment of Fe and Mg. This modelling therefore facilitates the refining of possible melting reactions to include accessory phases containing Fe and Mg. Trace element modelling further confines the proposed reaction. With Ba contents elevated in muscovite (averaging 2607ppm in the schist), this limits the extent of muscovite breakdown. Like our major element modelling, our trace element modelling points to the inclusion of accessory phases in the melting fraction, notably monazite and epidote for the liberation of La. Using all evidence, we propose that a low-temperature, fluid-present muscovite dehydration partial melt has formed the Mataketake Pegmatite. We finalise our proposed melting reaction as: Muscovite + Quartz + Plagioclase (Ab) + H2O ± (garnet) ± (epidote) ± (monazite) ± (chlorite) ± (ilmenite) = melt. Our proposed reaction contains muscovite, quartz and plagioclase as the melting phases, and aligns with the observed depletion in Li in the pegmatite relative to the schist, with calculated whole rock Li concentrations at 34.7ppm (Alpine Schist) and 21.5ppm (Mataketake Pegmatite). This is due to the melting phases hosting insufficient Li. Achieving Li enrichment would require biotite melting, which is inhibited by temperature constraints. Additionally, fluid-presence limits the extent of muscovite melting, which contains Li in higher quantities than quartz and plagioclase. We highlight the necessity for sufficient melting of micas, particularly biotite, for enrichment in unfractionated melts. We consider other analysed metals, (e.g., V, Cs and W), which are enriched in the Alpine Schist relative to the produced pegmatite. Integrating this work with previous literature, this can be explained by the absence of higher temperature melting reactions, particularly biotite breakdown, which facilitate enrichment of critical metals. This aids understanding of the processes dictating how lithium, and other trace elements, are mobilised during partial melting. Identifying the key controls on lithium enrichment and improving understanding of the mobilisation of lithium has significant implications for understanding the links between hard-rock lithium resources and geothermal fluids, which is vital for the development of mineral-deposit models and facilitating the transition to renewable energy sources.