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

Abstract

Purpose In the middle part of 20th century, the Portuguese Gaia-Ma double dagger ainhas 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.