WHETHER LI-RICH OR LI-POOR, GRANITIC PEGMATITES ARE ENDLESS SOURCES OF WONDER! (Invited Presentation)

Fractionated granitic pegmatites are main sources of lithium, cesium, rubidium, tantalum, etc., but their origin remains controversial. Giant crystals in pegmatites, even before Jahns (AmMin, 1953) coined, have continued to puzzle geoscientists. After all, what special magmatic conditions does it take to create >10 meter long single crystals of spodumene or feldspar? Here we present 1) an overview of recent advances of our understanding of pegmatite formation from undercooled hydrous silicate liquids and of associated mineral deposits at the magmatic-hydrothermal transition and 2) novel analytical and exploration approaches.

Pegmatites have unique textural features that vary widely between extremes. Grain sizes range from submillimeter to meter-long. Mineral habits vary from skeletal, such as intergrown quartz-feldspar in ‘graphic granite’, to perfectly formed, gem-quality tourmaline, beryl, or topaz in miarolitic cavities. Unidirectional solidification textures (UST), stellate, or banded aggregates standout within hipidiomorphic matrix. Compositional zoning occurs at individual crystal scale, but also as zones of distinct minerals within highly fractionated bodies, that crystallized sequentially from their borders to quartz-rich cores. At district scale, mineralized bodies can be scattered among far more numerous barren pegmatites.

To explain the distinct properties of pegmatite liquids and physicochemical conditions that can lead to such a wealth of textures and mineral resources, we explore the crystal nucleation and growth through dynamic-crystallization experiments. A positive correlation is proposed between the length of the largest giants and the width of the magma body. This function is extrapolated from crystallization experiments conducted in containers ranging from 0.2 to 30 mm. Can this correlation be used to estimate the size of incompletely exposed mineralized pegmatites? Geochemistry (K/Rb, K/Cs, Nb/Ta, etc.) has been traditionally used in exploration of LCT pegmatites. If properly calibrated, rapid testing using portable X-Ray Fluorescence and/or Laser Induced Breakdown Spectroscopy can speed up exploration, by screening common minerals and soils to discriminate among mineralized and barren pegmatites.