Thematic Articles

Pegmatites accentuate the trace element signatures of their granitic sources. Through that signature, the origin of pegmatites can commonly be ascribed to granites whose own source characteristics are known and distinctive. Interactions with host rocks that might modify the composition of pegmatites are limited by the rapid cooling and low heat content of pegmatite-forming magmas. The trace element signatures of most pegmatites clearly align with those of S-type (sedimentary source, mostly postcollisional tectonic environment) and A-type (anorogenic environment, lower continental crust ± mantle source) granites. Pegmatites are not commonly associated with I-type (igneous source) granites. The distinction between granites that spawn pegmatites and those that do not appears to depend on the presence or absence, respectively, of fluxing components, such as B, P, and F, in addition to H2O, at the source.

Pegmatites are sources of gem-quality crystals of beryl, tourmaline, topaz, spodumene, and spessartine. Historic localities are found in Brazil, Madagascar, Russia, and the United States, but important deposits have recently been discovered in Africa and Asia. Most high-quality gem minerals occur in miarolitic cavities found near the centers of pegmatite bodies or in reaction zones between pegmatites and ultramafi c host rocks. The most important gem-bearing granitic pegmatites formed at shallow levels in the continental crust during the latest stages of collisional plate tectonic events. Single, spectacular miarolitic cavities in some pegmatites have produced tons of gem crystals valued in excess of $50 million.

Rare-element granitic pegmatites are well recognized for the diversity and concentrations of metal ores that they host. The supply of some of these elements is of concern, and the European Commission recently designated metals such as tantalum and niobium as “critical materials” or “strategic resources.” Field relationships, mineral chemistry, and experimental constraints indicate that these elements are concentrated dominantly by magmatic processes. The granitic melts involved in these processes are very unusual because they contain high concentrations of fluxing compounds, which play a key role at both the primary magmatic and metasomatic stages. In particular, the latter may involve highly fluxed melts rather than aqueous fluids.

Granitic pegmatites are mined for feldspar, quartz, mica, lithium aluminosilicate minerals, and kaolin. These industrial minerals have a myriad of uses, some as mundane as glasses, porcelains, and bulk fillers, and others that are critical to the most advanced electronic devices. The chemical fractionation that produces pegmatites refines these industrial minerals to a purity that is not achieved in other geologic environments. The high chemical purity of their constituents and the fact that they contain nearly 100% of minable rock make large granitic pegmatites some of the most valuable sources of industrial minerals.

Virtually every conceivable model to explain the internal evolution of granitic pegmatites had been proposed by the 1920s. Two of these hypotheses have prevailed: (1) the fractional crystallization of fluxbearing granitic melt inward from the margins of the pegmatite body to the center, and (2) the buoyant separation of an aqueous fluid from the silicate melt and its effects on the redistribution of components. A recent model combining aspects of both concepts invokes the formation of a flux-enriched boundary layer of silicate liquid in advance of a crystallization front. Though most of the internal chemical and textural features of pegmatites can now be reconciled, the puzzle of pegmatites is far from solved.