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PROVENANCE OF HEAVY-MINERALS IN THE PIRANHAS-AÇU RIVER, NORTHEASTERN BRAZIL

2000; Sociedade Brasileira de Geologia; Volume: 30; Issue: 3 Linguagem: Inglês

10.25249/0375-7536.2000303453456

2177-4382

MÁRCIA GOMES DA SILVA, HELENICE VITAL,

Geography and Environmental Studies

Clino-amphibole, epidote, garnet, sillimanite, tourmaline, staurolite, andalusite, zircon, rutile, augite, ilmenite, hematite and magnetite are the heavy-minerals recorded in the Piranhas-Açu River. Six assemblages were identified: (i) Garnet-hornblende-tourmaline with sillimanite, when cutting rocks of the Seridó Formation; (ii) Hornblende-garnet-zircon, when crossing rocks of the Caicó gneiss-migmatite Complex; (Hi) Hornblende-zircon-epidote-staurolite, when in rocks of the Jucurutu Formation; (iv) Hornblende-zircon-epidote, when rocks of the Açu Formation; (v) Hornblende-zircon-staurolite, when the lowermost Agu River channel cuts limestones of the Jandaira Formation, and (vi) Zircon-tourmaline-staurolite in the Açu River mouth (Cenozoic rocks) where coastal dynamics dominate. Mineral ratios that reflect differences in grain shape, density, and selective chemical decomposition were used to isolate the effects of "source" and "process" as controls of mineral variability. The reworking of the sediments was regionally effective in selective sorting; the more equant minerals (e.g. epidote) and heavier minerals (e.g. opaques) had a higher probability of being selected for permanent deposition during reworking. The process of selective decomposition dominates at the river mouth.