Rare Element Pegmatites

Pegmatites enriched in rare elements are holocrystalline rocks composed of igneous rock-forming minerals, typically of granitic composition and by their highly variable grain size, including extremely coarse crystals. They are considered to form by fractional crystallization from the residual melt highly enriched in incompatible elements and volatiles. Granite pegmatites occur as dykes, lenses, and smaller segregations, often as swarms within and around some granitic plutons. Only a small proportion of pegmatites (< 1 %) possess assemblages that contain uncommon minerals known as rare element pegmatites. Rare element pegmatites are mineralogically complex and enriched in lithophile and incompatible elements, such as beryllium, lithium (spodumene and petalite), rubidium (lepidolite), cesium (pollucite), tin (cassiterite), tantalum and niobium (Ta-oxide minerals), rare-earth elements, and uranium. The concentration of these elements may be highly enriched in pegmatites in contrast to the bulk continental crust and some enrichment factors are greater than 10,000 to 100,000. Many of these elements are also "strategic metals" or "critical materials". Critical materials are those that are of greatest risk to supply disruptions or are important to a country's economy or defence. They are primarily used for electronic, aerospace, and energy applications. The pegmatite bodies also contain industrial minerals such as kaolin, feldspar, mica, and quartz. 

Classification of pegmatites (modified from Galeschuk & Vanstone 2007; Cerny & Ercit 2005;                      Cerny 1990, 1991; and Ginsburg 1984)

There are two families of rare element pegmatites: Li-Cs-Ta enriched (LCT) pegmatites and Nb-Y-F enriched (NYF) pegmatites. LCT pegmatites are associated with S-type, peraluminous (Al-rich), quartz-rich granites with low oxidation state. The rocks are characterized by biotite and muscovite, and the absence of hornblende. In contrast, NYF pegmatites show enrichment in rare earth elements (REE), U, and Th. They are associated with A-type, subaluminous to metaluminous (Al-poor), quartz-poor granites or syenites. The granitic rocks are characterized by Fe-rich micas, amphiboles, and pyroxenes. The pegmatites are compositionally granites, with quartz, K-feldspar, and albite as major minerals but without either biotite or hornblende. Other minerals may include muscovite, rare metal minerals, and volatile element bearing minerals such as phosphates and tourmaline. 

 

Pegmatites are structurally controlled and occur as dyke-like bodies or sheet intrusions (sills). They vary clearly in both shape in size. In high grade metamorphic rocks they form tabular, ellipsoidal, or irregular bodies concordant to the foliation of the wall rocks. In lower grade metamorphic rocks they may form concordant and also discordant bodies in crosscutting structures such as tension faults. Pegmatites vary considerable is size from a few metres up to a few km in length and from less than 1 cm up to hundreds of metres wide.  According to the varying size, most pegmatites that have been commercially exploited range from thousands to millions of tonnes.

Two sheet intrusions (sills) of Li-Cs-Ta-Sn-bearing pegmatites, Spain: one in foreground beneath the prominent quartzitic layer (hammer for scale) and the other in background on the lower half of the slope composed of phyllites/schists. 

Dyke swarms of granitic aplite-pegmatites in metasedimentary rocks with drift, Portugal. The aplite-pegmatites are almost vertically and  vary from less than 10 cm up to 40 cm in thickness. The outcropping aplite-pegmatites are almost all moos-covered (vertical moss-covered traces).

The geological setting of pegmatites and the parental and cogenetic fertile granites worldwide range from Precambrian to Cenozoic. The rare element pegmatites occur in orogenic belts in association with late- to post-tectonic granites and large regional faults in upper-greenschist- to lower-amphibolite-facies rocks postdating the peak of regional metamorphism. They are commonly peripheral to larger plutons. These granites represent in many cases the parental fertile granite enriched in the ore and volatile elements, from which the pegmatite was derived. In a parental fertile granite the siliceous melt first crystallizes several different units, due to an evolving melt composition and forms a large stratified pluton. This process is known as fractional crystallization. The residual melt rich in silica, alumina, alkali elements, water and other volatile elements, and incompatible elements, such as rare metals can then migrate into the wall rocks to form pegmatite dykes, sills, or pipe-like bodies. 

 

A siliceous melt is a composition out of different minerals, each which their own melting temperatures. Hence, the entire rock does not instantly melt during heating and this process is known is partial melting. Partial melting of preexisting sedimentary rocks produces an S-type granite melt and when the melt crystallizes the magma chamber contain a mixture of solid crystals and liquid melt in equilibrium. Elements that prefer to partition into crystals of common rock-forming minerals over the coexisting melt phase are called compatible. Elements that partition preferentially into the melt phase over coexisting crystals are called incompatible. This group contains high field strength elements (HFSE, e.g. Ta, Nb, P, and REE), large-ion lithophile elements (LILE, e.g. Cs, Rb, Ba, and Sr), and "pegmatophile elements" (e.g. Li, Be, and B) that are enriched in the residual melt phase.  Other elements also behave incompatibly and are enriched in granitic melts and pegmatites as volatile phases, e.g. F, P, B, carbonate and bicarbonte ions, and water, act as fluxes and hence reduce the solidus temperature of the siliceous melt. A high concentration of fluxes will also lower melt viscosity and enable segregation of the residual melt.

 

Fractional crystallization and the evolution from barren to fertile of a siliceous melt is an important process in concentrating incompatible lithophile elements in the highly differentiated residual melt. The major control on the ultimate composition of fertile granite and its subsequently evolution to form rare element pegmatites are the source rocks that underwent partial melting. Undepleted upper crustal rocks generate fertile peraluminous S-type granite melts that give rise to LCT pegmatites. Depleted lower crustal rocks will result in metaluminous to peralkaline A-type granite melts associated with NYF pegmatites. As a consequence, on the one hand fertile granites require a high degree of partial melting of the source rocks to generate the magma. On the other hand is the degree of fractionation critical to concentrate incompatible rare elements and volatiles in a granitic melt in order to crystallize a rare element-rich pegmatite. 

Schematic section of a regional zoned fertile granite-LCT pegmatite system. Pegmatites increase in degree of evolution with increasing distance from the parent granite, i.e. increasing fractionation, volatile enrichment, complexity of zoning, and extent of replacements. The maximum distance of pegmatites from

the source granite is on the order of kilometers. Modified from Cerny (1989) and Linnen et al. (2012).

The composition of the fertile cogenetic granite changes as a function of increasing fractionation and evolution, i.e. from biotite granite to two-mica leucogranite, coarse-grained muscovite leucogranite, and finally to pegmatitic leucogranite with layers of sodic aplite and potassic pegmatite. There is also a noticeable increase in grain size through the evolution. 

 

The process of rare-element enrichment in pegmatites appears to proceed, in an essentially closed system,

from a small fraction of residual silicate liquid derived from a much larger magma body. The granitic magmas crystallize at first anhydrous minerals such as K-feldspar, plagioclase, mica, and quartz leaving an increasing water rich magma containing rare elements through its evolutionary path. Lithophile incompatible elements and volatiles are also enriched in this residual melt fraction due to their large ionic radii and their high field strength that prevent them in entering crystals structures of rock-forming minerals. As minerals crystallizes and separate, the resulting crystallization products evolve from barren to fertile. Within a pegmatite field, the enrichment of the different rare metals in the pegmatites is a function of their distance from the cogenetic intrusion. With increasing distance the mineral assemblage changes and contains the following minerals: (1) barren to beryl, (2) beryl, columbite, phosphates, and (3) Li-bearing minerals (spodumene, petalite), tantalite, cassiterite, and pollucite. The maximum distance of pegmatites from the source granite is on the order of kilometers and the environment of formation of rare elements pegmatites is about 2 - 4 kbars and 650°-500° C. 

Above: Outcrop of Li-Nb-Ta-Cs-Sn-bearing aplite pegmatite in metamorphic rocks striking parallel to the batholith contact, Spain.

Below: Detail of a pegmatite sill with albite (light brown), quartz (greyish-whitish), spodumene mainly altered to pink kunzite, and muscovite, Spain.

View into a stope, Portugal. The Li-Nb-Ta-Sn-bearing aplitic pegmatite was exploited along strike and upwards to the surface in the so-called "glory hole technique". The aplitic pegmatite has sharp transition to the wall rocks and a thickness of about 2.5 to 3.5 metres.