AFC OF LCT, NYF & REE: UNTANGLING THE GEOLOGIC PROCESSES BEHIND THE ALPHABET SOUP OF PEGMATITE CLASSIFICATIONS

Pegmatites can be geochemically classified as LCT (Lithium-Caesium-Tantalum), NYF (Niobium-Yttrium-Fluorine), REE (Rare-Earth Elements), among others. Many pegmatites however do not fit into a single classification, which complicates exploration for the critical minerals resources they contain. Examples of these are the Delmoe Lake pegmatites, Montana, associated with the Boulder Batholith and the Ray pegmatite in the Spruce Pine region, North Carolina. This study presents combined field, mineralogical and mineral chemical analyses to untangle the processes related to assimilation, fractionation and contamination (AFC) that generated these complex pegmatites.

The Delmoe Lake pegmatites are associated with a broad range of accessory minerals (allanite, topaz, beryl, phenakite and tourmaline) within miarolitic cavities in the aplite-pegmatites. Previous work indicates large amounts of allanite occur, indicating an REE classification, although tourmaline could correspond to LCT while the topaz may correspond to NYF. The Ray pegmatite is unusual in the Spruce Pine region, which typically corresponds to NYF pegmatites. Ray also has a broad range of accessory minerals (columbite-group, lepidolite, beryl, fluorite, elbaitic tourmaline, amazonite, pollucite, and apatite). Analysis of a sample by micro-XRF highlights the complexity at Ray with zoned columbite-group minerals with a Nb-rich core and Ta-rich rims. Thus, even within a single crystal Ray displays both NYF and LCT characteristics.

This complex mineralisation is interpreted to be related to the igneous processes that developed the residual melt from which the pegmatite crystallised. The model we propose involves AFC during solidification of the Spruce Pine granodiorite to produce a residual melt of NYF affinity, followed by localised contamination at a late stage to crystallise minerals of an LCT affinity. Similarly, for Delmoe pegmatites we propose mixing of residual melts may be key to forming the range of minerals present. Understanding these complex processes is essential for evaluating the mineral resource potential of pegmatites.