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Metallographic and in situ compositional study on columbite–tantalite mining concentrates from placers at Maçainhas (Central-East Portugal): insights for tantalum exploration

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Abstract

Purpose

In the middle part of 20th century, the Portuguese Gaia-Maçainhas Sn–Ti–Ta placers were important tin and tantalum producers in Europe. The objective of this work was to recognize the mineralogy of ore concentrates containing colombotantalites, as well as to characterize the textural relations of the different minerals, in order to understand the metallogeny of the primary mineralization.

Methods

We used reflection microscopy and scanning electron microscopy with X-ray microanalysis (SEM–EDS). Analytical data were validated with electron microprobe analyses in selected grains.

Results

The following textures were identified: homogeneous, oscillatory zoning, patchy zoning, irregular mosaic-like intergrowths and biphasic oscillatory zoning. The main compositions identified are columbite-(Mn) and columbite-(Fe), but Fe- and Mn-tantalites are also important minerals as well as ixiolite–wodginite. Most of the minor mineral phases occur as small inclusions in columbite–tantalite and consist of cassiterite, zircon, struverite and microlite group minerals. Results indicate that magmatic-hydrothermal processes, followed by post-magmatic recrystallization, all contributed to the genesis of the Nb–Ta mineralogy in the source lithologies. These were most probably LCT (Li-Cs-Ta) pegmatites located in granitic country rocks which crop out a few kilometers from the placers.

Resumen

Objetivo

En la mitad del siglo XX, los placers portugueses de Sn-Ti-Ta de Gaia-Maçainhas fueron importantes productores de estaño y tantalio en Europa. El objetivo de este trabajo fue reconocer la mineralogía de los concentrados de mineral que contienen columbo-tantalitas, así como caracterizar las relaciones texturales de los diferentes minerales, para comprender la metalogénesis de la mineralización primaria.

Métodos

Se utilizó microscopía de reflexión y microscopía electrónica de barrido con microanálisis de rayos X (SEM-EDS). Los datos analíticos se validaron con análisis de microsonda electrónica en granos seleccionados.

Resultados

Se identificaron las siguientes texturas: homogénea, zonificación oscilatoria, zonificación irregular, zonificación remendada, intercruzamiento irregular tipo mosaico y zonificación oscilatoria bifásica. Las principales composiciones identificadas son columbita-(Mn) y columbita-(Fe), pero Fe- y Mn-tantalitas son también minerales importantes, así como ixiolita-wodginita. La mayoría de las fases minerales menores se presentan como pequeñas inclusiones en la columbo-tantalita y consisten en minerales del grupo de cassiterita, zircono, struverita y microlito. Los resultados indican que los procesos magmático-hidrotermales, seguidos por la recristalización post-magmática, contribuyeron para la génesis de la mineralogía con Nb-Ta en las litologías fuente. Estos fueron probablemente pegmatitas LCT (Li-Cs-Ta) localizadas en las rocas graníticas encajadoras que afloran a pocos kilómetros de los placers.

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Fig. 1

Adapted from the 1:500,000 Portuguese Geological Map (IGM 1995)

Fig. 2

Data collected from Parra and Filipe (2001) and Teixeira et al. (1965)

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Acknowledgements

We thank Daniela Silva and CEMUP (Materials Centre of the University of Porto) for SEM analysis and semi-quantification of the data. The Portuguese Science Foundation is thanked for financial support through the FCT UID/GEO/04683/2013 project. Ricardo Ribeiro’s permission to use the unpublished SEM data from his 2015 work is greatly appreciated. We are deeply grateful to Prof. Ana Neiva, Dr. Paulo Bravo and the two anonymous referees for careful reading the article, which improved the quality of the work. The valuable comments of the handling Editor Teresa Ubide also improved the quality of the manuscript.

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  1. Department of Geosciences, Environment and Land Planning, Faculty of Sciences, Porto University, Rua do Campo Alegre, 687, 4169-007, Porto, Portugal

    J. Fernandes

  2. Earth-Science Institute, Porto, Portugal

    A. Moura

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Fernandes, J., Moura, A. Metallographic and in situ compositional study on columbite–tantalite mining concentrates from placers at Maçainhas (Central-East Portugal): insights for tantalum exploration. J Iber Geol 43, 439–450 (2017). https://doi.org/10.1007/s41513-017-0029-z

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