Abstract Rare-element granitic pegmatites represent highly concentrated sources of rare metals, including Li, Rb, Cs, Be, Sn, Nb, Ta, Zr, Y, REE, and U. In today’s markets, pegmatites are the principal sources of Ta, and one pegmatite (Tanco, Canada) is the sole commercial producer of Cs for use as deep drilling fluid in the form of Cs formate solution. Growth in the demand for Li-based batteries has prompted exploration for spodumene-and petalite-bearing pegmatites, and several new Canadian prospects are slated for mining. Pegmatite bodies that contain minerals in which these elements are essential structural constituents constitute less than ~1 to 2% of all pegmatites in a given pegmatite-bearing terrane, and the economic production from many such bodies is limited by their small size (i.e., they may be economic in grade, but not for mechanized mining). Because pegmatites are found in cratons and orogenic belts, however, pegmatite-hosted resources are widespread and likely to be significant secondary, if not primary sources of rare metals for local economies or in times of disruption of global supplies from other types of deposits. Pegmatites are primarily igneous in origin, and the most likely processes that enrich them in rare elements include crystal-melt fractionation together with the creation of locally flux-and rare-element-enriched domains of melt in otherwise rather ordinary granitic melt. The mechanism of constitutional zone refining, in which fluxes and incompatible components are enriched in a boundary layer of melt adjacent to crystal growth fronts, represents the most effective means of concentrating rare elements. Whereas Rayleigh fractionation produces an exponential increase in the abundances of incompatible rare elements, constitutional zone refining leads to a sharp, “L”-shaped inflection in the concentration of incompatible elements with the progress of crystallization. The absolute concentrations of trace elements at the end of constitutional zone refining can be orders of magnitude greater that those that are attainable by Rayleigh fractionation (between mineral and bulk melt). In rare-element pegmatites, some trace-element enrichment patterns show the gradual increase in abundance that is expected of Rayleigh fractionation, whereas pegmatites in which the transition from ordinary mineral assemblages to those enriched in rare elements is sharply defined, more closely match the elemental fractionation that is derived from constitutional zone refining. Although pegmatites are igneous, pegmatite-forming melts crystallize well below their liquidus, and perhaps even below solidus temperatures. The textures and zonation that are hallmarks of pegmatites arise in response to the inception of crystallization from highly undercooled, viscous melt. Graphic granite, the one texture that is unique to pegmatites, constitutes prima facie evidence of such conditions. The crystallization of rare-element minerals, such as beryl, spodumene, tantalite, and pollucite can also be reconciled to the low-temperature crystallization of melts. Pegmatite-hosted ores are entirely endogenic, and many pegmatites exhibit little or no exogenic wall-rock alteration. Narrow and sporadic zones of alteration envelopes, unpredictable size in relationship to degree of fractionation, and sharply defined zonation of rare-element ores to inner units combine to make granitic pegmatites difficult targets for exploration.

Abstract Three major categories of ore deposits—granite-related sensu stricto—were selected for detailed analysis: disseminated magmatic mineralization in granites themselves (rare metal granites); hydrothermal deposits, imprinted on the granites and their immediate vicinity (vein and greisen type); and late-magmatic pegmatites largely spread around parental granitic plutons (rare element pegmatites). Rare metal granites with disseminated magmatic mineralization show broad diversity in the bulk composition and metal enrichment. Peralkaline (with REE, Zr, Nb, U, and Th mineralization), low phosphorus pera-luminous (with mainly Nb, Ta, and Sn), and high phosphorus peraluminous types (dominantly with Ta, Sn, and Li) are distinguished, approaching a continuum from mantle derived to crustal in origin. The shallow-seated, highly fractionated rare metal granites may be formed by protracted differentiation of huge magma chambers, but low-degree anatexis of source lithologies is a preferred model. It provides an initial enrichment in rare metals, facilitates fractionation and water saturation during ascent to shallow crustal levels, and permits saturation in ore minerals at the magmatic stage. Unmixing of a hydrothermal fluid may ultimately assist the most extreme enrichment in rare metals in the top of granitic cupolas, but the quantitative roles of enrichment versus a mere redistribution during this process are not well understood. Vein- and greisen-type hydrothermal mineralization is connected with shallow-seated granites with felsic, “minimum-melt” compositions. The granites are relatively K rich, metaluminous to peraluminous, magnetite (Mo-W) to ilmenite series types (Sn-W) and are highly fractionated, although less so than rare metal granites or the pegmatite-generating ones. The broad compositional variations in the Sn-, W -, and Mo-mineralized granites reflect the interplay of numerous variables, from the protoliths to the fractional crystallization and aqueous fluid-melt processes. Ore-forming aqueous fluids exsolved from the shallow intrusions generate the mineralization, but metal leaching from consolidated granites is locally documented. Regional zoning may be well expressed in the W-Sn-Mo succession, and also in the outer halo of Cu-Pb, Zn-Ag, Sb and similar sulfide assemblages. These low-temperature associations are locally generated by the high-heat–producing effects of the U- and Th-enriched granites on host rocks, but they are not related to the Sn-, W -, or Mo-bearing post-magmatic fluids. Economically significant granitic pegmatites belong to the LCT family of the rare element class, enriched in Li, Rb, Cs, Be, B, P, Sn, and Ta. Parent granites are dominantly peraluminous S and I types mobilized mainly from middle (to upper) crust. Differentiation produces highly evolved, Na-enriched leucogranitic melts that further fractionate to residual rare element pegmatite magmas. Multistage differentiation of huge batholithic granites, as well as a fractionation of small leucogranites pre-enriched by a relatively low percentage anatexis, are documented. The parent granite and pegmatite melts are volatile rich but water undersaturated until the very last stages of consolidation. The high proportion of volatile species in the final evolved melts facilitates ex-tensive dissolution of water and is suspected to promote exsolution of hydrosaline fluids. Low viscosity, thermal stability, and high mobility of the melts and of components within melt batches all promote fractionation; some rare element pegmatites represent the geochemically most evolved igneous rocks ever encountered. Mineralization is shown to be of primary magmatic descent, intrinsic to the magmatic to hydrothermal evolution of the pegmatites. Peraluminous rare metal granites with magmatic disseminated orebodies are locally associated with vein-and greisen-type mineralization, both requiring water saturation. However, significant hydrothermal deposits are generated dominantly from granites that lack magmatic disseminations. In contrast, substantial rare element pegmatites are not associated with either of these categories, as pegmatites form from deeper seated granitic magmas, water-rich but undersaturated until the very last stages of pegmatite consolidation. However, all three types of granite-related deposits treated here have a common link: current research supports internal derivation and enrichment of metals from the granitic melts themselves, as opposed to leaching from the host rocks, and the diversity of mineralization in each category is ascribed to the diversity of metamorphic protoliths from which the granites were mobilized.

The primary focus of this review is on P–T conditions, mineralogical indicators of melt or fluid composition and textural evolution; lesser treatment is given to pegmatite sources or to pegmatite–wallrock interactions. Investigations of stable and radiogenic isotopes have revealed that the source materials for pegmatites are likely to be more heterogeneous or varied than previously thought, especially for peraluminous pegmatites, but that overall pegmatites bear a clear intrusive relationship with their hosts, as opposed to an origin in situ. The P–T conditions of crystallisation of some lithium-rich pegmatites have been constrained by lithium aluminosilicate stability relations in combination with stable isotope or fluid inclusion methods. Experimental studies have elucidated the effects of components such as Li, B, P and F, which are common in some classes of pegmatites, to liquidus relations in the hydrous haplogranite system. Experimentation has also provided corroboration of an old concept of pegmatite crystallisation—that pegmatites owe their distinctive textures and mineral/chemical zonation to relatively rapid crystallisation of melt from the margins inwards at conditions far from the equilibrium (i.e. from supercooled liquids). The origin of aplites, whether alone, layered, or paired with pegmatites, remains an active area of research. Studies of fluid inclusions, crystal–vapour equilibria and wallrock alteration have helped to define the timing and compositions of vapour phases in pegmatites and to aid in the economic evaluation of deposits.

Abstract Paleomagnetic, petrographic, and geochemical results, as well as field relationships, are used to relate Late Paleozoic chemical remanent magnetizations (CRMs) to the migration of basinal fluids in Ordovician carbonates in the Arbuckle Mountains, southern Oklahoma. The Viola Limestone contains a pervasive Pennsylvanian synfolding CRM residing in magnetite and a localized Permian CRM which resides in hematite and occurs in alteration zones around veins mineralized with calcite and Mississippi-Valley-type oxides and sulfides. The vein mineralization precipitated from basinal fluids that were warm, saline, and radiogenic. Radiogenic 87 Sr/ 86 Sr ratios of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggest that the basinal fluids were also responsible for alteration in the limestones. The coincidence of the geochemical and remagnetization trends suggest that the Permian CRM dates the migration of the basinal fluids in the veins. Geochemical results from the Viola with pervasive CRM indicate that it is relatively unaltered, with no evidence for radiogenic basinal fluids. This suggests that a mechanism that does not necessarily require exotic externally-derived fluids is needed to explain the origin of the pervasive CRM. Liesegang-banded carbonate around calcite-filled fractures in the Kindblade Formation also contains a Permian CRM residing in hematite. The fluid that precipitated the hematite liesegang bands emanated from the fractures and, based on geochemical results, was basinal in origin. The results of this study suggest that basinal fluids migrated through the carbonates in the Arbuckle Mountains during the Permian, although perhaps in several episodes. The flow of basinal fluids was focused in veins and only locally altered the host carbonates.

The processes leading to the generation of pegmatite fabrics and rare-element ore deposits are evaluated in terms of pressure, temperature, and compositions of fluids through the combination of recent field, fluid inclusion, and experimental data. The results indicate that primary consolidation of miarolitic and massive rare-element (Li-Be-Ta) pegmatites occurs in the range of 700° to 450°C at 400 to 250 MPa. The model for pegmatite genesis proposed here involves disequilibrium crystallization of quartz and feldspars in a hydrous melt-crystal system that approaches chemical equilibrium but rarely attains it. Fluxing anionic components of boron, phosphorus, and fluorine are not essential to this model, but augment the zoning process by promoting expansion of the liquidus fields of quartz and K-silicates (feldspar and mica) and by driving residual melt toward alkaline, Na-rich compositions enriched in lithophile trace elements. Aqueous vapor, if present, may have a nominal affect on primary zonation and fabric development within pegmatites. Various lines of evidence suggest that rare-element pegmatites may not become vapor saturated until they approach solidus conditions, at which point internal retrograde recrystallization and wall-rock alteration occur. Recent experiments have succeeded in replicating most aspects of pegmatite geology, including fractionation trends, mineral fabrics, and spatial zonation, at vapor-undersaturated conditions.