Navegando por Assunto "Rochas sedimentares - Tocantins"

Agora exibindo 1 - 1 de 1

Acesso aberto (Open Access)
Petrografia, caracterização química e significado geológico dos metassilexitos e formações ferríferas do Grupo Tocantins, centro oeste do Cinturão Araguaia
(Universidade Federal do Pará, 2013-11-19) COSTA, Nivia Oliveira da; VILLAS, Raimundo Netuno Nobre; http://lattes.cnpq.br/1406458719432983
Metacherts and iron-formations from de Agostinho, Grande, Jabuti, Pau Ferrado and Salto hills that occur in the central-western portion of the Araguaia fold belt in the Tocantins state have been investigated. These rocks are interlayered with slates, phyllites and metagreywackes (Tocantins Group), derived from siliciclastic sediments that were deposited in a Neoproterozoic proto-oceanic basin. They are locally associated with chloritites and more commonly with lenses of serpentinized or talcified ultramafic rocks. Metabasalts occur near Araguacema town, in the western part of the studied area, where they exhibit well preserved pillowed structures. The metacherts are massive to laminated and dominantly grey-colored. They show microcrystalline texture and normally are cross cut by quartz veinlets. Quartz is the chief mineral, although subordinate amounts of magnetite, hematite, talc, chlorite, chromite, muscovite, kaolinite and rutile are present. In the banded iron-formations, mm- to cm- scale bands rich in microcrystalline quartz alternate regularly with bands rich in hematite and magnetite, associated with minnesotaite, stilpnomelane, rutile, chromite and tourmaline. Goethite replaces hematite to a lesser or greater extent. Chemically, the metacherts present SiO2 contents higher than 90% and Fe2O3 contents that range from 2 to 8.4%. Also variable are the MgO concentrations that reach 9.55%, although most values are less than 0.1%. Al2O3 contents are normally below 0.6%, but in three samples they record >2%. These variations reflect basically the modal composition of these rocks. Concerning the trace elements, only Ni, Co and Cu have some significance with averages of 222 ppm, 122 ppm e 40 ppm, respectively. Gold reveals low values, except in a sample (27.4 ppb). REE total (in general, ≤ 67 ppm) is lower than the standard North American Shale Composite (NASC). Only two samples contain higher ETR values (154 and 237 ppm). The REE distribution pattern, normalized to the NASC, varies significantly from a metachert sample to another. The LREE tend to be enriched with respect to the HREE, although the contrary is also observed. Most samples present important Ce negative anomalies, whereas others show slightly positive or no anomalies at all. Similar finding is valid for Eu, although most values are positive. In the ironformations, the average Fe2O3(total) content is 76%. Silica concentrations are higher than 14% (average of 21,3%), but in just one sample it is as low as 2,95%. Al2O3 contents vary from 0.21% to 2.35%. Other major components reveal very low contents. Regarding the trace elements, the iron-formations differ from the metacherts in that the contents of Ni and Co are highly variable (averages of 20 and 30 ppm, respectively) and Co averages 23 ppm. Copper concentrations are low, but exceed 120 ppm in a few samples. REE total in iron- formations is comparable to that of the metacherts (≤ 89 ppm). However, the distribution of these elements, normalized to the Post-Archean Average Australian Sedimentary Rocks (PAAS) standard, displays a more systematic pattern than in the metacherts. The geological context related to a proto-oceanic basin, the anomalous contents of trace elements such as Ni, Zn, Co and Cr, the negative Ce and positive Eu anomalies frequent in both metacherts and iron-formations, suggest that the protoliths of these rocks formed by exhalative activity in a submarine environment. Nevertheless, the chemical sediments did not precipitate from high temperature hydrothermal fluids, but underwent some contamination from terrigenous sediments. The positive Y anomalies, also detected in the iron-formations samples, are common in modern seawater and suggest that the precipitation of the ferruginous material was relatively fast and favored by the migration of reducing, slightly acid marine waters to shallow environments where the water was more alkaline and oxidizing. The association of some metacherts and iron-formations with chloritites, serpentinites and hydrothermally altered mafic/ultramafic rocks supports the interpretation of a ocean-floor environment and allows considering the investigated sequences as slices of ophiolitic complexes that have been tectonically dismembered and emplaced into the Tocantins Group rocks. As such characterized, this portion of the Araguaia fold belt is potentially favorable to the exploration of both exhalative and podiform chromitite deposits.