Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province, Northeastern Brazil

Abstract

The Borborema Pegmatitic Province (BPP), northeastern Brazil, is historically important for tantalum mining and also famous for top-quality specimens of exotic Nb-Ta oxides and, more recently, for the production of gem quality, turquoise blue, 'Paraíba Elbaite.' With more than 750 registered mineralized rare-element granitic pegmatites, the BPP extends over an area of about 75 by 150 km in the eastern part of the Neoproterozoic Seridó Belt. The Late Cambrian pegmatites are mostly hosted by a sequence of Neoproterozoic cordierite-sillimanite biotite schists of the Seridó Formation and quartzites and metaconglomerates of the Equador Formation. The trace-element ratios in feldspar and micas allow to classify most pegmatites as belonging to the beryl-columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobiantantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the effectiveness of Nb-Ta oxide chemistry as a possible exploration tool, to trace the degree of pegmatite fractionation and to classify the pegmatites. The columbite group mineral composition allows to establish a compositional trend from manganoan ferrocolumbite to manganocolumbite and on to manganotantalite. This trend is typical of complex spodumene-and/or lepidolite-subtype pegmatites. It clearly contrasts with another trend, from ferrocolumbite through ferrotantalite to ferrowodginite and ferrotapiolite compositions, typical of pegmatites of the beryl-columbite phosphate subtype. Large scatter and anomalous trends in zoned crystals partially overlap and conceal the two main evolution patterns. This indicates that a large representative data set of heavy mineral concentrate samples, collected systematically along cross-sections, would be necessary to predict the metallogenetic potential of individual pegmatites. Other mineral species, e.g. garnets and/or tourmaline, with a more regular distribution than Nb-Ta oxides, would be more appropriate and less expensive for routine exploration purposes. The currently available Nb-Ta oxide chemistry data suggest the potential for highly fractionated Ta-Li-Cs pegmatites in the BPP, so far undiscovered, and encourages further, more detailed research.

The Borborema pegmatite Province, NE-Brazil revisited

“…Less fractionated, but still heterogeneous pegmatites were also recognized as being of the beryl subtype, free of phosphates, with magnetite, Ta-rich rutile and ixiolite as main ore minerals. Mineral chemistry data of Nb-Ta oxides from 29 pegmatites allowed Beurlen et al (2008) to identify pegmatites of both, the Beryl-ColumbitePhosphate and Complex Spodumene subtypes of rare element class of granitic pegmatites according to Černý & Ercit (2005). A diffi cult problem using mineral paragenesis, zonal structure, and mineral geochemistry (mainly feldspar and mica) for the pegmatite classifi cation is that in many of those pegmatites for which there is an available historical record, a strong vertical or longitudinal zoning is observed and would allow to classify different portions of a single body as belonging to different types or subtypes.…”

Post-collisional Pan-African granitoids and rare metal pegmatites in western Nigeria: Age, petrogenesis, and the ‘pegmatite conundrum’

“…The origin of these pegmatites is thus uncertain, 122 although the peraluminous plutons with which they are associated have not previously 123 been targeted for dating. Similar pegmatites occur in the Borborema Province, where 124 they are extensively mined for tantalum (Beurlen et al, 2008). As well as the tantalum 125 potential, gold deposits are known in the Nigerian schist belts, but their formation 126 may pre-date the Pan-African orogeny (Dada, 2008).…”

Phosphate sequence study of Boqueirão pegmatite (Rio Grande do Norte state, Brazil)

“…The western part may be divided into two main tectonic domains: Ceará and Médio Coreau domain. The area covered in this work is located in the southeastern part of Seridó fold belt (Beurlen et al, 2008 (Beurlen et al, 2008). …”

“…The BBP is known since the beginning of 20 th century for mica extraction and during the Second World War it became best known not only as one of the most important Ta-Be producers but also for its exotic Nb -Ta oxide production (Beurlen et al, 2008). Recently, it became an important supplier of raw material for the Brazilian ceramics industry (Beurlen et al, 2009).…”