ORE-FORMING PROCESSES WITHIN GRANITIC PEGMATITES (Invited Presentation)

Rare-element granitic pegmatites represent the end stages of crystallization of much larger volumes of granitic magma. Simple Rayleigh fractionation models predict that 95-99+% of an anatectic melt must crystallize to achieve the lower limits of saturation in rare-element ores. Even so, extreme liquidus undercooling prior to the onset of crystallization is necessary to precipitate exotic minerals. Based on its bulk composition, the Tanco pegmatite, Manitoba, became saturated in beryl, columbite, montebrasite, pollucite, tantalite, and tourmaline from ~ 525°C. 

The Li-rich pegmatites are derived from a sedimentary component added to I-type granites at subduction zones, or from the anatexis of marine sediments (S-type granites) during the waning stages of continental collisions. The REE-rich pegmatites are associated with weakly alkaline A-type granites that are derived from a lower crustal or mantle source in regions of continental extension. Igneous processes concentrate ores in both families of pegmatites, but hydrothermal redistribution of rare elements in the alkaline pegmatites is pervasive.

The processes that give rise to sharply bounded mineralogical and textural zones in pegmatites stem from liquidus undercooling of ~ 150°-250°C (i.e., at ~ 425°-525°C) prior to the onset of crystallization. The formation and localization of rare-element ores further depend on the width of the magma body, the level of emplacement as it relates to heat loss from the melt, and on the nature of boundary-layer pile up of excluded elements along the crystallization front, in which fluxing components (H, B, P, and F) play major roles. 

Boundary layer pile-up has two consequences for the formation or rare-element ores: (1) the crystallization front is alternately saturated in an ore mineral, or (2) constitutional zone refining leads to an accumulation of rare-element and fluxing components in the boundary layer liquid. In the first case, minerals such as beryl and columbite may appear repeatedly in a succession of assemblages. In the second case, constitutional zone refining leads to a final melt that is exceedingly enriched in rare-elements and fluxing components. Crystallization of that last liquid yields the latest primary assemblage of albite ± lepidolite with the major proportion of rare-metal ore minerals.