Rare Metal Granites and Pegmatites

International audienceRare-metal granites and pegmatites are highly evolved rocks that form in a wide range of different tectonic settings. Ore element associations may vary widely, depending on source rocks, conditions of source rock melting, extent of fractionation, and conditions of melt-hydrothermal transitions. The range of textural variation and of associated mineral deposits largely depend on local constraints at the level of emplacement of these highly fractionated granites. For Sn, W, Ta(-Nb) and Li specific rare metal granites, the protoliths are dominantly sediments, characterized by low Ca and Na contents, and for this reason they are commonly referred to as “S-type” granites. The predominance of Sn over W or Li-Cs-Ta in mineral deposits related to these granites reflects different melting conditions in the source region and partitioning of these elements at the melt-hydrothermal transition. After the separation of fluid from the granite magma, Sn and W are transported in separate high temperature (250–450 °C) aqueous solutions whose physicochemical parameters control deposition mechanisms and determine the nature of the hydrothermal deposits that ultimately form (i.e., veins, greisen or skarn)