GEOCHEMICAL FRACTIONATION OF Nb-Ta OXIDES IN Li-BEARING PEGMATITES FROM THE BARROSO-ALVAO PEGMATITE FIELD, NORTHERN PORTUGAL

Abstract

The Barroso-Alvão granitic pegmatite field, located in northern Portugal, is known for its numerous occurrences of Li-bearing pegmatites, especially spodumene-and petalite-bearing bodies. Recent work on this pegmatite field characterized other types of pegmatites, and complex mineralization and mineral associations. Minerals of the columbite-tantalite group are found in all the LCT (Li-Cs-Ta) pegmatites from Barroso-Alvão: spodumene pegmatites, petalite pegmatites, and lepidolite pegmatites. In the petalite and lepidolite pegmatites, the columbite-tantalite group of minerals are found spatially associated with cassiterite. Cassiterite is not present in the spodumene pegmatites. The columbite-group minerals have Ta/(Ta + Nb) and Mn/(Mn + Fe) values ranging from 0.20 to 0.60, except in the lepidolite pegmatites, where Mn/(Mn + Fe) is close to 1. The fractionation trend starts in the Mn-poor columbite-(Fe) field (spodumene pegmatites) toward Mn-and Ta-enrichment (lepidolite pegmatites). Our results suggest that Mn enrichment is not controlled by silicate or phosphate phases, but instead by the increasing activity of fluorine. The Ta enrichment, on the other hand, may well be controlled by fractional crystallization. We also observe a decrease in the Ti of the columbite-group minerals from the spodumene to the lepidolite pegmatites, which we consider an evolutionary trend.

Micas, feldspars and columbite-tantalite minerals from the zoned granitic lepidolite-subtype pegmatite at Namivo, Alto Ligonha, Mozambique

“…Garnet, zinnwaldite and gahnite containing Fe and Mn also only occur in the OIZ and cannot also have caused the increase of the Mn/Fe in the melt. The dominant Mn enrichment, before Ta enrichment started in the columbite-tantalite composition from some granitic pegmatites, is attributed to an increased alkali-fluoride activity at moderate mHF (Č erný et al, 2004) and an increase in the activity of fluorine (Martins et al, 2011). Each Al-richer mica (muscovite, lithian muscovite and ''mixed form'') from the Namivo granitic pegmatite shows an increase in F content from the OIZ to the IIZa, IIZb and Cc (Table 1).…”

Uncommon Nb-tantalate from the Cachoeira mine, Araçuaí pegmatite district (Minas Gerais)

“…Minerals from the columbite-tantalite series are the most common Nbtantalates found in granitic pegmatites, mainly occurring as solid solutions. Their generic formula is AB 2 O 6 , where the A site is normally occupied by Fe or Mn, and the B site is generally filled by Nb, Ta, Sn and Ti cations Martins et al, 2011). They are orthorhombic, crystallizing with Pbcn structure, and their true cell has: a ~14.4 Å, b ~5.8 Å and c ~5.1 Å (Grice et al, 1976).…”

“…In such series, it is reported that the Mn:Fe ratio increases according to the fractionation within a body or peg-matite group. Likewise, it is expected that the Ta:Nb ratio growth follows the chemical fractionation degree (Martins et al, 2011). According to Burke (2008), end-members of this series are currently designated as tantalite-(Fe), tantalite-(Mn), columbite-(Fe) and columbite-(Mn).…”

“…Ixiolite represents a columbite substructure, with the cations in disordered arrangement; its generic formula can be written as (A,B,C) 4 O 8 , where all metals occupy the same site, A=Fe 2+ ,Mn 2+ ; B=Nb,Ta; C=Sn,Ti (Palache et al, 1944;Nickel et al, 1963;Grice et al, 1976). According to Wise et al (1998), orthorhombic ixiolite structure reverts, after heating, to an ordered (monoclinic) wodginite cell.…”

Infrared and Raman spectroscopic characterization of the phosphate mineral fairfieldite – Ca2(Mn2+,Fe2+)2(PO4)2·2(H2O)

“…The [21]. Other occurrences were later described in the Cigana mine, Brazil [22]; Tanco mine, Canada [23] and Barroso-Alvão pegmatite, Portugal [24]. The crystal structure of fairfieldite was refined by Fanfani et al [3].…”