Forum Comment
doi: 10.1130/G38086C.1 Nb-Ta fractionation in peraluminous granites: A marker of the magmatichydrothermal transition
Aleksandr S. Stepanov1, Sebastien Meffre1, John Mavro2
1
genes , and Jeff Steadman
1
ARC Centre of Excellence in Ore Deposits (CODES), School of
Physical Sciences, University of Tasmania, Private Bag 79, Tas 7001,
Australia
2
Research School of Earth Sciences, Australian National University,
Canberra, ACT 0200, Australia
On the basis of geochemical and mineralogical features, Ballouard
et al. (2016) argue that magmatic fractionation alone cannot explain
the formation of leucogranites with low Nb/Ta ratios. Instead, they
propose that low ratios are better explained by post-magmatic
interaction with F-bearing fluids. Although fluids may certainly play
an important role in the formation of rare metal leucogranites, the
model proposed by Ballouard et al. lacks key details. We contend that
the principal features they attribute to magmatic-hydrothermal
processes are better explained by the magmatic fractionation model.
Ballouard et al. propose greater mobilization of Nb than Ta during
post-magmatic alteration. Although fluid removal of Nb may decrease
Nb/Ta ratios, it does not explain the high Ta concentrations in
leucogranites, which vary from crustal values (≈1 ppm) to >100 ppm
in the most evolved leucogranites (Stepanov et al., 2014; Ballouard et
al., 2016, their figure 1B). In fluid-driven models, Ta enrichment in
leucogranites may either be attributed to removal of silicate component (the removal scenario) or to addition of Ta by the fluid (the
addition scenario). The removal scenario requires the extraction of 90
wt% of the silicates from the rock mass for a tenfold enrichment in Ta,
which is not feasible. The Ta addition scenario is equivalent to the
formation of Ta mineralization by hydrothermal processes. However,
Ta-rich granites have igneous contacts with their host rocks, and do
not display textures typical for hydrothermal ores (e.g., London,
2008).
Stepanov et al. (2014) demonstrated that the fractionation of micas
and ilmenite have an opposite effect on Nb/Ta ratios. Ballouard et al.
take this argument too far by proposing that effect of mica fractionation can be counterbalanced by the fractionation of 0.5 wt% of
ilmenite. However, peraluminous granites evolving to low Nb/Ta
values commonly have Ti contents much lower than 0.26 wt% TiO2,
which is equivalent to 0.5 wt% of ilmenite (Stepanov et al., 2014;
Ballouard et al., 2016). Moreover, biotite and muscovite are significant
hosts of Ti, and their presence further reduces the amount of produced
ilmenite. Therefore, peraluminous granites evolving to low Nb/Ta
contain insufficient ilmenite to counteract the decrease in Nb/Ta due to
fractionation of mica.
Leucogranites with low Nb/Ta also contain low concentrations of
Fe, Mg, Zr, light rare earth elements (LREEs) and Ti (Stepanov et al.,
2014). If this were due to post-magmatic alteration, as proposed by
Ballouard et al., fluid removal of all these “immobile” elements and
simultaneous enrichment in “mobile” elements (e.g., Sn, Li, Cs, and F)
would be required. By contrast, the magmatic differentiation model
explains decreasing concentrations of Fe, Mg, Zr, LREEs, and Ti
through the fractionation of minor and accessory minerals present in
granites (London, 2008; Stepanov et al., 2014), while enrichments of
Sn Li, Cs, and F are attributed to incompatible behavior during
crystallization.
The metallogenic arguments put forward by Ballouard et al. are also
problematic. Whereas Li, Cs, Rb, and Be can be transported by fluids,
the highest concentrations of these elements are found in magmatic
intrusions of pegmatites and leucogranites, and are commonly
associated with elevated Ta (London, 2008). On the other hand,
granite-related hydrothermal Sn and W deposits are not known to be
significant sources of Li, Cs, Rb, Be, and Ta. Ballouard et al. argue
that negative correlations of Sn contents with Nb/Ta ratios in granites
are “markers of magmatic–hydrothermal alteration.” However,
Lehmann (1990) demonstrated that magmatic fractionation of tin
granites increases Sn content, while fluid loss and alteration decreases
Sn concentrations in granites, contrary to the proposal by Ballouard et
al.
Ballouard et al. further argue that the correlation of decreasing
Nb/Ta with increasing alteration of micas supports Nb-Ta fractionation by hydrothermal fluids. However, magmatic fractionation
increases the concentrations of water and F in the melt. Upon
exsolution, these components impose increasing post-magmatic
alteration of fractionated leucogranites. Therefore, both decreased
Nb/Ta and post-magmatic alteration can be the result of magmatic
evolution.
Tantalite overgrowths on columbite in ongonite grains (Dostal et al.,
2015) are cited by Ballouard et al. as an example of Ta hydrothermal
transport. This contradicts their own statement that Nb is more mobile
than Ta in hydrothermal fluids. Zonation with an increasing Ta from
core to the rim of tantalite-columbite grains is common in leucogranites and pegmatites, and this zonation is best explained by the lower
solubility of columbite relative to tantalite in melts (Linnen, 1998).
Extreme granite fractionation is required to explain the genesis of
rare metal pegmatites (London, 2008) and Sn granites (Lehmann,
1982). Occam’s razor demands that the simplest explanation should be
preferred. That magmatic processes explain most features of rare metal
leucogranites and pegmatites suggests that hydrothermal processes
may not be required to fractionate Nb from Ta, although there are still
many unknowns regarding the origin of rare metal granites.
REFERENCES CITED
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