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Mineralogical and geochemical characterization of drill hole samples of the lithiniferous pegmatite of Aldeia, Ribeira de Pena: Metallogenic implications and their importance for Li exploration
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Resumo(s)
Aldeia’s pegmatite swarm, part of the much larger Barroso-Alvão pegmatitic field, is composed
of several spodumene-bearing aplite-pegmatite bodies hosted in a sequence of micaschists and
quartzschists belonging to the parautochthonous unit of NE Iberia.
An appraisal of the Li-mineralization was carried out on drillhole samples from both host-rocks
and aplite-pegmatite bodies. 30 thin sections were produced. After a detailed petrographic analysis, 17
thin sections were selected for EMPA to assess mineral compositions.
The petrographic analysis revealed that spodumene-bearing aplite-pegmatites are poorly zoned,
and show a complex mineral assemblage developed along two main crystallization stages (Stage I and
Stage II), with a superimposed later metasomatic event.
Given the metallogenetic and economic implications of the reported variability for the Libearing phases within the Barroso-Alvão pegmatitic field, it is important to stress that: 1- although
usually deformed, analysed spodumene is extremely fresh, with no evidence of alteration or weathering
processes; 2- petalite is absent and there is no suggestion that spodumene results from the decomposition
of petalite, since SQI textures are inexistent in studied samples.
All the main mineral phases from both stages (including spodumene) exhibit micro-textures and
intra-crystalline high-T deformation features, suggesting deformation under crystallization and cooling
conditions, compatible with the regional Variscan deformation patterns observed in the
metasedimentary host-rocks.
Based on petrography and mineral chemistry, the studied Aldeia aplite-pegmatites can be
classified as Complex Spodumene Rare Element-Li pegmatites. They are texturally and compositionally
similar, showing a complex history with two main crystallization stages, with secondary phosphates
underlining late metasomatism. From a geochemical standpoint the system stands out due to elevated
enrichments in Na. Stage 1 should represent a Li-poor melt, with moderate P and F, and minor Fe, Mn,
and Sn. Stage 2, concomitant with spodumene crystallization, should represent a Li-rich and F-poor melt
with minor Nb and Ta.
Descrição
Tese de mestrado, Geologia (Geoquímica Mineralogia e Petrologia), 2023, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
Litio Espodumena Aplito-Pegmatito Portugal Elementos-Raros Teses de mestrado - 2023