Rare-Element Granitic Pegmatites

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.

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