Resumen
The specialized leuco-monzogranite of the La Chinchilla Stock is a small Carboniferous stock located
in the center of the Velasco Range, Pampean Province, La Rioja, Argentina. It is highly evolved and locally F- and
Be-bearing, and has the potential for hosting U mineralization. Three different facies can be identified in the granitoid:
border, porphyritic and equigranular facies. In all three facies the main minerals are quartz, microcline, plagioclase,
biotite, and muscovite. Accessory minerals present in all facies include fluorite, zircon, and apatite. In addition, monazite,
rutile, and uraninite occur as accessory minerals in the equigranular facies. Secondary minerals are muscovite, sericite,
kaolinite, and opaque minerals. Secondary uranophane occurs in the equigranular and border facies. In localized areas,
the equigranular facies contains small, green idiomorphic crystals of beryl (Be3
Al2
Si6
O18) as accessory mineral. One of
these beryl crystals was chemically analyzed for major and minor element contents using an electron microprobe and
this information, along with fractional crystallization models and comparison with compositions of non-pegmatitic beryl
from the literature, were used to understand the degree of evolution of the granitic melt. The chemical formula of beryl
from the La Chinchilla Stock can be written as: C(Na0.014-0.033, K0.001-0.002, Ca0.001-0.004) T(2)(Be2.978-2.987, Li0.016-0.022) O(Al1.889-1.967,
Fe0.045-0.103, Mg0.001-0.007, Mn0.001-0.007) T(1)(Si5.994-6.040O18). The alkali contents are low (Na2
O<0.18 wt%; K2
O<0.02 wt%), while
FeOt
is dominant among the divalent cations that substitute trivalent aluminum in the octahedral position of the mineral
(FeOt
/(MgO+MnO)>6; FeOt
<1.27 wt%). In a longitudinal geochemical profile, Al enrichment is observed at the border
while the highest Na content is found in an internal point. In a transversal geochemical profile, the highest concentration
of Al is seen in an internal point while Na remains almost invariable. Ferromagnesian elements vary randomly within
the crystal. This indicates compositional changes in the magma for Al, ferromagnesian elements, and Na. The FeOt
content of the analyzed beryl is within the compositional range of other disseminated beryl from granitoids but slightly
higher than that of beryl from hydrothermal veins and greisens. It contains similar to slightly lower amounts of FeOt
,
MgO, and Na2
O than beryl from medium to little evolved granitic pegmatites. Overall, the composition of beryl in the
La Chinchilla Stock is quite similar to that from medium to poorly evolved granitic pegmatites of the nearby Velasco
Pegmatite District. The formation of beryl in the La Chinchilla Stock is attributed to precipitation from a F-bearing,
highly fractionated, Al- and Si-rich melt saturated in BeO. A fractional crystallization model using Rb and Ba suggests
that the beryl-hosting rock crystallized from the parent melt after extreme fractionation and 75% crystallization. The
occurrence of beryl as a magmatic accessory mineral in the equigranular facies of the La Chinchilla Stock is indicative
of a very high degree of fractionation of the parental magma.