Spodumene petalite eucryptite: Mutual relationships and pattern of alteration in Li-rich aplite-pegmatite dykes from northern Portugal

The co-occurrence, at the thin section scale, of the three anhydrous Li-aluminosilicates spodumene, petalite and eucryptite, is not a common feature. Such an association occurs in some aplite-pegmatite dykes of the Covas de Barroso district, northern Portugal. We describe their mutual relationships: where spodumene is an early magmatic phase, petalite precipitates directly from a late orthomagmatic fluid, the spodumene remaining metastably, whereas eucryptite is obviously hydrothermal and secondary. Replacement of the primary Li-minerals is the rule, but is diversified and selective: spodumene is mainly replaced by albite and muscovite, petalite by K-feldspar and eucryptite. Muscovite is widespread, and the late hydrothermal paragenesis is dominated by quartz associated with various mixtures of phosphates. The assemblage eucryptite, K-feldspar and albite is obviously in chemical disequilibrium with the early phases. The compositional evolution of the hydrothermal fluids (Na, K, Li) is tentatively bracketed. There is disequilibrium among thermodynamically conflicting mineral phases in terms of pressure, temperature and fluid:rock ratio, in a mobile and partly open system. A genetic affiliation of the Li-enriched pegmatites with the nearby two-mica granites is difficult to prove. The tectonically controled intrusion of biotite granites to the east of the pegmatite belt would seem to be responsible of the drop in pressure that accounts for the spodumene-petalite transition and the development of the widespread replacement by feldspars, not present in the western part of the belt.