The crystallization temperature of granitic pegmatites: The important relationship between undercooling and critical metal prospectivity

Abstract

Granitic pegmatite deposits contain important critical metal resources, but the geologic processes that generate critical metal mineralization in these systems remain enigmatic. Previous research indicates that liquidus undercooling is one of multiple important controls on critical metal mineralization in granitic pegmatites, although other research has suggested that this process may be unnecessary. Here we investigate the influence of crystallization temperature and undercooling on pegmatite-hosted critical metal mineralization using a global compilation of naturally-measured crystallization temperatures from >200 granitic pegmatite occurrences that span various pegmatite classes, geothermometer types, and intrapegmatite zones.

Our analysis indicates that pegmatites generally crystallize between 400 and 700 °C, many of which crystallize between 400 and 600 °C. Pegmatite classes yield the following mean crystallization temperatures (±2SE): abyssal: ∼670 ± 50 °C, muscovite: ∼675 ± 50 °C, muscovite-rare element: ∼535 ± 25 °C, rare element: ∼525 ± 20 °C, and miarolitic ∼460 ± 25 °C. These variations indicate that critical metal-mineralized (i.e., rare element and miarolitic classes) pegmatites have a mean liquidus undercooling temperature of ∼175 °C and ∼240 °C, respectively, whereas barren pegmatites crystallize near the hydrous haplogranite solidus. Main-stage zone temperatures for different pegmatite families indicate that Nb-Y-F (NYF; ∼560 ± 20 °C) pegmatites crystallize at temperatures ∼50 °C higher than Li-Cs-Ta (LCT; ∼515 ± 20 °C) pegmatites, suggesting that the formation of different pegmatite families and associated commodities requires different petrogenetic processes. In addition, the subdivisions of the rare element class that are associated with different commodities also have different crystallization temperatures, where Li-mineralized pegmatites have a mean crystallization temperature of ∼510 ± 25 °C compared to Be- and rare earth element (REE)-mineralized pegmatites with mean temperatures of ∼550 ± 45 °C. In terms of intrapegmatite zoning, crystallization of the border to the core zone (main-stage zones) occurs at near-isothermal mean temperatures (∼530–500 °C), late-stage zones form at lower temperatures than the former (miarolitic cavities: ∼420 ± 45 °C, replacement: ∼465 ± 55 °C), and unzoned, typically unmineralized pegmatites crystallize at relatively high mean temperatures (∼680 ± 55 °C). However, albite-spodumene pegmatites, an unzoned, occasionally economic-grade rare element pegmatite subclass, have a mean temperature of ∼490 ± 70 °C. We also speculate that large intrapegmatite temperature variations may be important for forming pegmatite deposits such as the Tanco pegmatite. This study demonstrates that (1) large degrees of undercooling are necessary for pegmatite-forming melts to surpass the mineralogical barrier and host critical metal mineralization, and (2) pegmatite field zonation, or the apparent lack thereof, can be explained by variability in the degree of undercooling throughout a pegmatite field.