Petrogenesis and Experimental Petrology of Granitic Rocks

Highly fractionated Himalayan leucogranites and associated rare-metal mineralization

In their last stages of evolution (ie at the
magmatic-hydrothermal transition), peraluminous
granitic melts exsolve a large amounts of fluids
which can modify the chemical composition of
whole-rock granitic samples and lead to the
deposition of economicaly significant mineralization
(Sn, W)
Nb and Ta are lithophile elements considered to
be “geochemical twins” because they have the same
charge and a similar ionic radius However, Nb/Ta
ratios are highly variable in granites Some authors
have demonstrated that the Nb/Ta ratios decrease in
granites during fractional crystallization [1] Other
studies have suggested that Nb and Ta could be
fractionated in evolved peraluminous granites during
the interaction with late magmatic fluids [2]
In this study [3], we demonstrate, using a
compilation of whole-rock geochemical data
available in the litterature that fractional
crystallization alone is not sufficient to explain the
distribution of Nb-Ta in most peraluminous granites
However, we notice that most of the granitic samples
displaying evidence of interactions with fluids have
Nb/Ta < ∼5 We propose that the decrease of the
Nb/Ta ratio in evolved melts is the consequence of
both fractional crystallization and sub-solidus
hydrothermal alteration We suggest that the Nb/Ta
value of ~5 fingerprints the magmatic-hydrothermal
transition in peraluminous granites Furthermore, a
Nb/Ta ratio of ~5 appears to be a good marker to
discriminate mineralized from barren peraluminous
granites

Nb-Ta fractionation in peraluminous granites: a marker of the magmatic-hydrothermal transition

Genesis of the Cuonadong tin polymetallic deposit in the Tethyan Himalaya: Evidence from geology, geochronology, fluid inclusions and multiple isotopes

Mineralogical evidence for fractionation processes in the Himalayan leucogranites of the Ramba Dome, southern Tibet

The Himalayan leucogranite occurs as two extensive (>1000 km) E-W trending belts on the Tibetan Plateau with the unique features. The leucogranite comprised biotite granite, two-mica/muscovite granite, tourmaline granite and garnet granite, which have been identified in previous studies, as well as albite granite and granitic pegmatite that were identified in this investigation. Fifteen leucogranite plutons were studied and 12 were found to contain rare-metal bearing minerals such as beryl (the representative of Be mineralization), columbite-group minerals, tapiolite, pyrochlore-microlite, fergusonite, Nb-Ta rutile (the representative of Nb-Ta mineralization), and cassiterite (the representative of Sn mineralization) mainly based on the field trip, microscope observation and microprobe analysis. The preliminary result shows that the Himalayan leucogranite is commonly related to the rare-metal mineralization and warrants future investigation. Further exploration and intensive research work is important in determining the rare-metal resource potential of this area.

A preliminary study of rare-metal mineralization in the Himalayan leucogranite belts, South Tibet

Spodumene pegmatites from the Pusila pluton in the higher Himalaya, South Tibet: Lithium mineralization in a highly fractionated leucogranite batholith

Highly fractionated leucogranites in the eastern Himalayan Cuonadong dome and related magmatic Be–Nb–Ta and hydrothermal Be–W–Sn mineralization

Abstract Beryllium is a critical metal typically concentrated in highly fractionated granitic rocks such as the leucogranites in the Himalaya. Here, we report beryl mineralization that was continuous from the earlier and less-evolved two-mica granite to the highly evolved albite granite and pegmatite in a typical leucogranite pluton at Pusila in the central of Himalaya. Textural and mineral chemical evidence support a magmatic origin for beryl, and the trends of beryl crystal chemistry indicate magma differentiation. Despite low to moderate fractionation of the biotite granite and two-mica granite in the Pusila leucogranite pluton, the Be contents (~7 μg/g, beryl-free and ~22 μg/g, beryl-bearing, respectively) of these granites are much higher than the average for biotite- and two-mica granites worldwide (~3–4 and 5–10 μg/g, respectively), indicating that the initial magma had a relatively high-Be concentration. The gneisses of Greater Himalayan System, considered the protolith, also show a higher Be abundance (~4–6 μg/g) than the mean value of pelitic rocks worldwide (~2–3 μg/g), which could be the source reservoir of Be. The source contributed the initial Be to the melt, and fractionation resulted in the onset of beryl crystallization from the interstitial residual melt in the two-mica granite. The ubiquity of beryl in two-mica granite to pegmatite stages of the Pusila pluton is explained by a continuous crystallization model, although there was a delay in the onset of beryl crystallization in the two-mica granite. Modeling based on Rayleigh fractionation indicates that Be becomes compatible once saturation is attained because of the beryl crystallization. Our findings indicate that the enrichment of critical elements (e.g., Be) is controlled not only by fractional crystallization but also by the buffering action of a saturating phase (e.g., beryl) on the concentration of the critical element in the melt.

Chen Liu

Papers

Highly fractionated Himalayan leucogranites and associated rare-metal mineralization

A preliminary study of rare-metal mineralization in the Himalayan leucogranite belts, South Tibet

Spodumene pegmatites from the Pusila pluton in the higher Himalaya, South Tibet: Lithium mineralization in a highly fractionated leucogranite batholith

Highly fractionated leucogranites in the eastern Himalayan Cuonadong dome and related magmatic Be–Nb–Ta and hydrothermal Be–W–Sn mineralization

Continuous Be mineralization from two-mica granite to pegmatite: Critical element enrichment processes in a Himalayan leucogranite pluton