Using muscovite and tourmaline to trace the origin and evolution of rare metal pegmatites in the Mufushan batholith, South China

Granitic pegmatites are renowned as significant sources of rare metals (e.g., Li, Ta, Nb, Be, Cs). However, the origins and mechanisms underlying the enrichment of rare metals in granitic pegmatites remain debatable. This study provides comprehensive petrography, major and trace element analyses of muscovite and tourmaline, and isotopic data from pegmatite dikes associated with the Early Cretaceous Mufushan composite batholith, South China to elucidate geochemical fractionation processes and the mechanisms responsible for rare metals mineralization.

Rare-metal pegmatites and barren pegmatites are found within the metasedimentary strata and the granitoid batholith, respectively. Both types of pegmatite dikes exhibit internal zoning, featuring three distinct structural zones with varied mineral assemblages, where muscovite and tourmaline are ubiquitously present. Both rare-metal pegmatites and barren pegmatites show a limited monazite ε_Nd(t) range between -9.0 and -7.6, which is aligned with the apatite ε_Nd(t) values of -9.8 to -7.8 from distinct monzogranites, indicating that they were derived from a similar magmatic source. The most primitive units of both pegmatites show differentiation degrees of muscovite and tourmaline both lower than the muscovite monzogranite, implying that the pegmatites and monzogranites might represent independent evolutionary products.

For rare metal pegmatites, buffered Be and Nb contents, and continuously increasing Li and Ta contents in early-stage muscovites and tourmalines indicate that early-stage fractional crystallization promoted the precipitation of beryl and columbite group minerals, and the enrichment of rare metals (e.g. Li and Ta) in the residual pegmatitic liquids. Meanwhile, highly evolved melts become saturated in magmatic volatile phases (e.g., H₂O, F, and B) corresponding to incremental differentiation. Whereas the late-stage evolved melt and coexisting aqueous fluid phases drive further enrichment of both fluid-mobile elements (e.g., Li, Na, Sn, Pb, Bi) and fluid-immobile elements (e.g., Be, Al, Mn, Nb, and Ta), and subsequent saturation and crystallization of lepidolite, Cs-rich lepidolite, manganotantalite, and microlite.

Melt compositions (Li, Rb, and Cs) in equilibrium with muscovites for pegmatites are quantitatively calculated, evaluated, and aligned with a Rayleigh fractional crystallization model. Our modeling indicates that the formation of barren pegmatite requires merely 50% Rayleigh fractional crystallization degree, whereas the intermediate zone, marking the initial conspicuous occurrence of rare metal minerals in rare metal pegmatites, necessitates ∼90% Rayleigh fractional crystallization degree.

The δ¹¹B values of tourmalines show narrow ranges in biotite monzogranites (-14.6‰ ~ -14.1‰), muscovite monzogranites (-16.7‰ ~ -14.0‰), and barren pegmatites (-14.7‰ ~ -14.5‰), but relatively larger variations in rare-metal pegmatites (-17.1‰ ~ -11.9‰). The calculated δ¹¹B values (-17.5‰ ~ -16.0‰) of primary magmatic melts for four host-rock units are indistinguishable due to co-crystallization of tourmaline and mica, while the relatively heavier and more variable boron isotopic compositions in rare-metal pegmatites reflect Rayleigh degassing and intense fluid activities. Compared with the slight change in boron isotopes of tourmalines, trace elements in both muscovites and tourmalines are much more sensitive to magmatic-hydrothermal evolution and the associated mineralization processes, which can distinguish rare-metal mineralization in pegmatite systems.