Depósitos minerales

Miner Deposita (2008) 43:207–228 DOI 10.1007/s00126-007-0152-4

ARTICLE

Nb–Ta–(Ti–Sn) oxide mineral chemistry as tracer of rareelement granitic pegmatite fractionation in the Borborema Province, Northeastern Brazil
Hartmut Beurlen & Marcelo R. R. Da Silva & Rainer Thomas & Dwight R. Soares & Patrick Olivier

Received: 30 November 2005 / Accepted: 10 June 2007 / Published online: 26 July2007 # Springer-Verlag 2007

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 overan 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 theberyl–columbite phosphate subtype. Electron microprobe analyses (EMPA) of columbite, tapiolite, niobian– tantalian rutile, ixiolite and wodginite group minerals from 28 pegmatites in the BPP are used to evaluate the
Editorial handling: S. Hagemann H. Beurlen (*) : D. R. Soares Programa de Pós-Graduação em Geociências, Universidade Federal de Pernambuco (UFPE), Rua Acadêmico Hélio Ramos s.n., CidadeUniversitária, 50740-530 Recife, PE, Brazil e-mail: beurlen@terra.com.br M. R. R. Da Silva : P. Olivier Departamento de Geologia Universidade Federal de Pernambuco (UFPE), Rua Acadêmico Hélio Ramos s.n., Cidade Universitária, 50740-530 Recife, PE, Brazil M. R. R. Da Silva e-mail: marcelor@ufpe.br R. Thomas GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany

effectiveness of Nb–Ta oxidechemistry 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 anothertrend, 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 alongcross-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–Cspegmatites in the BPP, so far undiscovered, and encourages further, more detailed research. Keywords Nb–Ta–Ti–Sn oxides . Rare-element granitic pegmatites . Borborema Pegmatitic Province . Brazil

Introduction Niobium–Ta–(Ti–Sn) oxide mineral chemistry data, based mainly on minerals of the columbite, tapiolite and ixiolite groups, are commonly used in pegmatite provinces worldwide as tracers of pegmatitemagma fractionation, for the prediction of the metallogenetic potential or petrological

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classification purposes (Černý and Ercit 1985, 1989; Tindle and Breaks 2000; Černý 1989b, 1992, 1998). This kind of study may gain in importance because the potential for high grade Ta, Li and Cs ore increases with the degree of pegmatite magma fractionation....