Granitic pegmatites are coarse-grained igneous rocks that contain abundant crystals with skeletal, graphic or other strongly directional growth-habits, or anisotropic layered mineral fabrics (London, 1992, 2018). Giant or megacrystic crystals may also be present.

Several classification schemes exist for granitic pegmatites (Cerný and Ercit, 2005; London, 2008; Simmons and Webber, 2008) — the simplest scheme divides them into common pegmatites and rare-element pegmatites. More complex schemes are based upon the presence of different rare-metal mineral assemblages. The rare-element pegmatites have anomalous contents of Be, Li, Ta, Sn and Cs. Beryllium is most commonly present as beryl, Li occurs as spodumene or lepidolite, Ta as columbite–tantalite, Sn as cassiterite and Cs as pollucite (Bradley et al., 2017). Pegmatites are also often mined for high-purity quartz, potassium feldspar, albite, kaolinite, white mica, gem beryl, gem tourmaline and museum-quality specimens of many rare minerals.

Rare-element pegmatites are divided into two end-member petrogenetic/compositional families (Cerný, 1991; Cerný and Ercit, 2005) as a simple chemical division to emphasise key differences in the geological processes responsible for rare-element mineralization:

• Lithium–caesium–tantalum (LCT) pegmatites are enriched in Li, Cs, Ta, Be, B, F, P, Mn, Ga, Rb, Nb, Sn and Hf. Examples of major LCT pegmatite deposits include the Tin Mountain pegmatite in the United States; Tanco pegmatite in Canada, Altai Number 3 pegmatite in China; the Greenbushes, Wodgina and Pilgangoora pegmatites in Western Australia; Bikita pegmatite in Zimbabwe; and the Kenticha pegmatite district in Ethiopia (e.g. see summaries of Cerný et al., 2005; Bradley et al., 2017).
• Niobium–yttrium–fluorine (NYF) pegmatites are enriched in Be, Sn, B, Nb > Ta, Ti, Y, rare earth elements (REE), Zr, Th, U, Sc and F, but are depleted in Li, Cs and Rb. Biotite is more common in NYF pegmatites, whereas muscovite is dominant in LCT pegmatites. Notable NYF pegmatite deposits, as summarized by Ercit (2005), include the South Platte granite and pegmatite system in Colorado (Simmons et al. 1987), the Grötingen granite and Abborselet and other associated pegmatites in Sweden (Kjellman et al., 1999), the Lac du Bonnet biotite granite and Shatford Lake pegmatite group in Canada (Buck et al., 1999), and the Stockholm granite and Ytterby pegmatite group, Sweden (Kjellman et al., 1999).
• Mixed or ‘hybrid’ rare-element pegmatites have blended rare-element signatures and are considered to be products of contamination of NYF pegmatites at the magmatic or postmagmatic stage. For example, they have been suggested to result from remelting of newly formed NYF pegmatites by metasomatic fluids rich in Li, B, Ca and Mg (Cerný and Ercit, 2005; Martin and De Vito, 2005). Some examples of mixed pegmatites include those at Kimito in Finland (Pehrman, 1945 ), the Tørdal district of Norway (Bergstøl and Juve, 1988; Cerný, 1991) and the O'Grady batholith in Canada (Ercit et al., 2003).

Lithium–caesium–tantalum pegmatites are present in all continents and span three billion years of Earth history. Their global age distribution mirrors those of orogenic granites and detrital zircons, corresponding to times of supercontinent assembly and major collisional orogenic events (Tkachev, 2016; Bradley et al., 2017). In Proterozoic–Phanerozoic settings, where collisional tectonic processes are well documented, LCT pegmatites most likely formed in orogenic hinterlands related to plate convergence (Bradley et al., 2017). Arc-related processes that control pegmatite generation include: i) overthickening of continental crust, ii) slab breakoff, iii) slab delamination, iv) extensional collapse occurring late in the collisional event and involving decompression melting. Lithium–caesium–tantalum pegmatites are consequently hosted in metamorphosed supracrustal rocks (e.g. greenstone belts). Intrusions are emplaced at midcrustal levels late during orogeny and are controlled by existing faults, fractures, foliation and bedding in country rocks. Pegmatites exposed to these conditions are tabular, whereas at lower levels of the crust, ductile hydrostatic conditions promote lensoid to irregular pegmatites (Brisbin, 1986). In the Proterozoic and Phanerozoic, LCT pegmatites are products of extreme fractional crystallization of S-type granites, derived from melting of metasedimentary rocks in continental collision zones (Cerný and Ercit, 2005). Specific examples include pegmatite fields in South Norway (Müller et al. 2015), Namibia (Fuchsloch et al., 2018), Maine (Webber et al., 2019) and in the Italian Alps (Konzett et al., 2018). An alternate process proposed for pegmatite generation is by direct melting of rocks with the appropriate composition (e.g. metasedimentary rocks with evaporite sequences: Simmons and Webber, 2008; London, 2008, 2018).

In Archean settings such as the Pilbara and Yilgarn Cratons, S-type granites are scarce and the largest known deposits hosted by LCT pegmatites (e.g. the Wodgina, Pilgangoora and Greenbushes deposits) are associated with highly fractionated I-type granites (Sweetapple and Collins, 2002; Sweetapple, 2017). Although contentious, some form of plate tectonics is generally agreed upon for the Archean (e.g. Cawood et al., 2013). In this context, progressive partial melting of trondhjemite–tonalite–granodiorite precursors is one possible method for the enrichment of rare-elements in melts that act as the parental sources of mineralized pegmatites (Sweetapple, 2017). These pegmatites are most commonly emplaced into mafic or ultramafic host rocks within greenstone belts (e.g. the Pilbara Craton, Sweetapple and Collins, 2002; Yilgarn Craton, Witt, 1992). In these Archean settings, regional-scale structures control the distribution of pegmatites, being responsible for focusing and transporting fluids and magmas (e.g. Sweetapple and Collins, 2002; Demartis et al., 2011; Deveaud et al., 2013).

Most LCT pegmatite melts are enriched in fluxing components (H₂O, F, P and B) that depress the solidus temperature, lower the magma density and increase rates of ionic diffusion. Hence, LCT pegmatites form relatively thin dykes with large crystals at lower temperatures (350–550°C) compared to common granitic melts (London, 2008, 2018). Rates of crystallization modelled experimentally are remarkably short (days to years; Webber et al., 1997; London, 2008, 2018).

Pegmatites are located within 10 km of cogenetic peraluminous granites and leucogranites (as modelled experimentally by Baker, 1998). The roof zones of large plutons are the most favourable positions (London, 2018). Proximal pegmatites are the least evolved and are poorly mineralized, containing only the general rock-forming minerals (Fig. 1). More distal and evolved pegmatites may include beryl, beryl and columbite, tantalite and Li aluminosilicates, and pollucite in the most evolved pegmatites. The spatial zonation of pegmatites around a common granitic source is a fundamental starting point for exploration models (London, 2018).

Niobium–yttrium–fluorine pegmatites are identified in most continents and their crystallization ages correspond to major intervals of global continent assembly from the Archean to the Neogene, with a peak at ~1000 Ma corresponding to the Grenville orogeny in Laurentia (McCauley and Bradley, 2014).

Niobium–yttrium–fluorine pegmatites are products of pronounced differentiation of anorogenic, A-type granites, which are a common product of bimodal gabbro-granite magmatism in rift zones. Geological processes controlling the genesis of A-type granites include: i) fractionation of direct partial melts from the upper mantle, ii) remelting of basalts that accumulate beneath the thinned lithosphere, iii) partial melting of lower crustal gneisses (Eby, 1990; Christiansen and McCurry, 2008). In the advanced rift setting where A-type granites are commonly generated, the mafic and felsic melts are mostly metaluminous. The melts are near or above silica saturation, with the granites notably depleted in Ca and P, and possessing heavy rare earth element (HREE) enrichment (London, 2018).

Like the LCT pegmatites, NYF pegmatites are often controlled by structures, fabrics and bedding in country rocks. However, regional zonation patterns around parental granites do not appear to occur in NYF pegmatite fields (Simmons and Webber, 2008). Rather, the NYF pegmatites are commonly hosted within granites (e.g. in the Pilbara Craton; Sweetapple and Collins, 2002).

Derived layers are grouped based on their critical features:

SOURCE – Formation of fertile magmas

PATHWAY – Structures, foliation and bedding in country rocks

TRAP – Cooling and chemical diffusion in fractionating melts

PRESERVATION – Erosion and uplift of rare-element pegmatites

The Mineral System Tree is the graphical display of a mineral systems analysis showing the link between critical/constituent processes and their recommended targeting features and GIS layers.

 Mineral System Tree: Rare-element pegmatite

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See the included reference list.

Subject matter experts:

• Marcus Sweetapple (consultant)