Looking for lithium-bearing pegmatites
Boulder, Colo., USA: The commercial importance of lithium is ever growing,
and its production is globally dominated by lithium-cesium-tantalum (LCT)
pegmatites. These are spectacular rocks, featuring impressive ultra-coarse
textures, but they are very elusive due to a combination of factors: they
are small, and until a decade ago they were often regarded as nothing more
than a geological curiosity. As a result, our knowledge of LCT pegmatites
from an exploration standpoint is limited, which makes them hard to find.
The starting point of mineral exploration is typically a targeted hunt for
that mineral’s formation environment. But while in the case of many other
ores, such as copper and gold, several genetic models inform our search,
when it comes to LCT pegmatites the available models are limited. Lot
Koopmans et al. aims at lifting the veil that has thus far been enshrouding
lithium exploration. “Knowing that certain environments have the right
geological history to enable LCT pegmatite formation is paramount to finding
them,” he explains.
Two classic hypotheses that explain the genesis of pegmatites as the product
of either extreme fractionation of a parental granitic body or low-degree
partial melting of a metamorphic rock. Koopmans et al. used petrological
modeling to evaluate whether significant lithium enrichment on the scale of
an economic deposit could be achieved during these processes, and found that
it could not—at least not starting with common rock compositions.
While studying pegmatites in Zimbabwe and the USA, Koopmans et al. kept
finding field evidence that just couldn’t be explained through the classic
models. How else can LCT pegmatites form? The authors did not come across
any satisfactory alternative explanation in the literature. From stimulating
discussions at discipline-specific conferences, they got to work
synthesizing their observations and thermodynamic calculations into a novel,
multi-stage petrogenetic model: first, metasedimentary rocks undergo partial
melting during prograde metamorphism, producing a granitic melt modestly
enriched in lithium that crystallizes as a granitic intrusion; at a later
time, this granite is then remelted, forming a highly enriched melt that
ultimately crystallizes as a Li-rich pegmatite. This proposed mechanism
satisfies some geochemical and geochronological constraints that previous
models could not, providing an efficient mechanism to increase lithium
enrichment and generate economic lithium deposits. Koopmans et al.’s work
was just published in Geology.
“Finding the proof of concept is the next step, and that comes down to
finding the right field area and method to study these pegmatite-forming
processes—perhaps incorporating novel isotopic systems,” says Koopmans. “Hopefully fellow geologists can now go to the field, find pegmatites, and
gather the evidence to test whether they have formed according to the model
we proposed.”
FEATURED ARTICLE
The formation of lithium-rich pegmatites through multi-stage melting
L. Koopmans, T. Martins, R. Linnen, N.J. Gardiner, C.M. Breasley, R.M. Palin, L.A. Groat, D. Silva, and L.J. Robb
Contact: Lot Koopmans, University of Oxford, lot.koopmans@univ.ox.ac.uk
https://doi.org/10.1130/G51633.1
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