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Rare-metal pegmatite
Ta-Nb mineralization
Mufushan Granite complex
Fractional crystallization
Columbite-group minerals
Abstract:
The Mufushan area, which has abundant rare-metal pegmatites within and around the Mufushan Granite
Complex, has become a major target for Ta-Nb-(Li-Be) exploration in South China. The age and origin of the
pegmatites and associated rare-metal mineralization are still under debate. Here, we report the in situ U-Pb ages
and geochemical characteristics of granites and pegmatites in the Guanyuan and Duanfengshan districts, which
are located in the central and northern parts of the Mufushan Complex. Combined zircon, apatite and monazite
U-(Th)-Pb dating revealed that biotite, two-mica, and muscovite granites from the Guanyuan and Duanfengshan
districts were emplaced at 143–139 Ma, which overlaps with the U-Pb ages of columbite-group minerals (CGM)
from different internal zones of the Duanfengshan pegmatites (142–140 Ma). Whole-rock major and trace
element compositions and Sr-Nd-Hf isotope data reveal that the granites and pegmatites experienced continuous
evolution from biotite, two-mica, and muscovite granites to pegmatite and that the magma originated from the
partial melting of mica schists that are abundant in the Mufushan area. Temporal, chemical and mineralogical
evidence indicates a genetic link between muscovite granite and Ta-Nb pegmatites. The textures and chemical
compositions of CGM from different pegmatites exhibit features typical of magmatic CGM, indicating that
fractional crystallization was the driving force that promoted Ta-Nb enrichment. The increasing alumina satu-
ration index [ASI: molar Al/(Ca–1.67P + Na + K)] of pegmatitic melt due to albite crystallization may have been
the main factor controlling CGM deposition, explaining why major Ta-Nb mineralization is bound to albite
pegmatites. The Duanfengshan and other rare-metal pegmatites in the Mufushan area are derivatives of the most
evolved granitic facies (i.e., muscovite granite) of the Mufushan Complex. The Duanfengshan and Renli
pegmatite fields indicate that the Early Cretaceous (~140 Ma) may have been an important, underappreciated
epoch for the formation of pegmatite-related rare-metal resources in the Mufushan area and beyond in South
China.
