Raul Minas Kuyumjian

THE GEOLOGY AND TECTONIC EVOLUTION OF THE TOCANTINS GRANITE-GREENSTONE TERRANE: ALMAS-DIANÓPOLIS REGION, TOCANTINS STATE, CENTRAL BRAZIL

The geology of Almas-Dianopolis region consists of a granite-greenstone terrane partially covered by Proterozoic metasedimentary units. The greenstone belt can be subdivided in two sequences. The first sequence, named Corrego do Paiol Formation, is composed of high-Fe basaltic flows, locally pillowed, and includes rare occurrences of komatiitic basalts. The second sequence, named Morro do Carneiro Formation, comprises a monotonous sequence of sericitic phyllites, as well as beds of chemical and clastic sediments. Both sequences were subjected to mediumto lowpressure metamorphism and variable temperatures in greenschist to amphibolite facies (Ml). Metamorphism was accompanied by a deformation event (Dn) that resulted in vertical sedimentary strata and contacts. Granitoid emplacement enhanced thermal conditions during Dn event, as suggested by amphibole exchange vectors edenite and tschermakite substitutions are dominant. These granitoid bodies form large granite gneiss complexes that intruded the greenstone belts during or at the close of the Dn event. The granitoids are grouped in two suites: Suite 1 comprises amphibole-bearing tonalite, granodiorite, trondhjemite, quartz-monzodiorite and quartz-diorite; and Suite 2 is composed of tonalite, trondhjemite, granodiorite and monzogranite with biotite. Both suites have calc-alkaline tonalitic-trondhjemitic chemical affinities. Contact relationships, deformation style of wall rocks, fabric development and different source rocks illustrate a complex history of granite-gneiss terrane evolution. Later Dn+i strike-slip shear zones partially obliterated Dn structural features in the greenstone belts, but their effects were restricted in the granite-gneiss domains.

KYANITE-STAUROLITE ORTHO-AMPHIBOLITE FROM THE CHAPADA REGION, GOIAS, CENTRAL BRAZIL

Abstract This study deals with the kyanite-staurolite assemblage from a metabasite (ortho-amphibolite) in the Chapada area of the Neoproterozoic Mara Rosa volcano-sedimentary sequence of Goiás state, Brazil. It is closely associated with hydrothermally altered rocks and contains the assemblage hornblende, epidote, quartz, kyanite, staurolite, plagioclase and rutile. The staurolite from the amphibolite shows lower values of SiO2 and higher values of MgO, MnO, ZnO and Xmg than those in staurolites from the metasediments of the same area. In the Al-Na-Ca-(Fe+Mg) diagram, the amphibolite plots in the aluminous part of the diagram and the phase volume encloses the area of mafic volcanic bulk compositions. Field relations and compositions in both kyanite-staurolite amphibolite and staurolite are consistent with formation of this amphibolite as a result of metamorphism of metasomatised basalts.

Geologia e controle estrutural dos depósitos de ouro do

The southwestern portion of the Amazonian Craton, in the Mato Grosso state, Central Brazil contains important gold concentrations that are associated with the tectonic and thermal evolution of the Aguapei folded belt. In these concentrations gold is associated with metasedimentary rocks of the Aguapei Group that cross the Brazilian border and has been called as Sunsas Group in Bolivia. The Lavrinha region at the central portion of the Aguapei folded belt, is located in the fold-hinge of the Cagado anticline and represents a domain of low angle tangencial tectonism that demonstrate an allochthonism of the Aguapei Group sediments from SW to NE. In this site, the deformation on the Aguapei group generates a boundary of this group with the basement rocks which is represented by granitic-gnaissic rocks. The boundary forms a 2 to 3 metres large shear zone, called Morro Solteiro Shear Zone. Nine gold deposits are mapped at the Lavrinha region, distributed in three levels (inferior, intermediate and superior). The inferior level is represented by the Morro Solterio Shear Zone, and has been considered to be the most important for gold exploration. The intermediate level is represented by a series of stick-slip movements along the bedding plane developed in the metaconglomerate and quartzites with metassiltites and metamudstone intercalation. The superior level is associated with metassiltites and metasandstone. Gold mineralization is generated by the percolation of hydrothermalmetamorphic fluids of aqueous to aqueous-carbonic composition presenting a elevated H 2 O/CO 2 ratio, low salinite and high sulphur activity. These fluids were generated by the end of the Sunsas-Aguapei orogeny (1.1 – 0.95Ga). The mineralogical assemblage is represented by pyrite, magnetite, hematite, ilmenite and chalcopyrite. The presence of pyrite boxworks and hematite associated with magnetite and leucoxene represents the weathering of the sulphide and oxide minerals. Thus, native gold disseminated in the hydrothermal altered mylonites indicate a secondary enrichment of gold at the Lavrinha region.

ALTERAÇÃO HIDROTERMAL PERALUMINOSA NA SEQUÊNCIA VULCANO-SEDIMENTAR PALEOPROTEROZÓICA DE SILVÂNIA, REGIÃO DE VIANÓPOLIS, GOIÁS

The small aluminossilicate deposit of Vianopolis, southeastern part of the state of Goias, is hosted by the paleoproterozoic island-arc-type Silvânia volcanic-sedimentary sequence and associated intrusions of granite and granodiorite. Pyrophyllite, kyanite, white mica, diaspore, tourmaline and chlorite are the products of the peraluminous hydrothermal alteration, inside a N50 o -60 o W trending shear zone. The presence of gold, pyrrotite and chalcopyrite in rocks from the alteration zone indicates potencial resources also for metallic mineral deposit in the Silvânia sequence.

The geology and tectonic evolution of the Tocantins granite-greenstone terrane, Almas-Dianópolis region

GREENSTONE BELT DE TOCANTINS: REGIAO DE ALMAS-DIAN6POLIS, ESTADO DE TOCANTINS, BRASIL CENTRAL O terrene granito-greenstone do sudeste do Estado do Tocantins e parcialmente recoberto por unidades metassedimentares proterozoicas. Na regiao de Almas-Dianopolis o greenstone belt pode ser subdividido em duas seqiiencias de rochas supracrustais. A primeira, denominada Formacao Corrego do Paiol, e composta por derrames basalticos de alto-Fe, localmente almofadados, e inclui raros derrames de basaltos komatiiticos. A segunda, denominada Formacao Morro do Carneiro compreende uma seqiiencia monotona de filitos sericfticos com camadas intercaladas de rochas sedimentares clasticas e quimicas. Ambas as seqiidncias foram afetadas por metamorfismo de baixa a media pressao e temperaturas variaveis do facies xisto-verde a anfibolito. O metamorfismo foi acompanhado por um evento deformacional (Dn) que resultou na verticalizacao dos estratos sedimentares e contatos geologicos. O posicionamento de corpos granitoides acentuou as condi?6es termais prevalecentes durante Dn, como sugerido pelos vetores de troca do anfibolio (substituicoes de edenita e tschermaquita). Estes granitoides constituem complexes granito-gnaissicos intrusivos nos greenstone belts sin a tardi Dn. Tonalitos, granodioritos, trondhjemitos, quartzo-monzodiorito e quartzo-diorito ricos em anfibolio foram agrupados na Suite 1. Tonalitos, trondhjemitos, granodioritos e monzogranitos com biotita foram agrupados na Suite 2. Ambas as suites sao calci-alcalinas tonalito-trondhjemiticas. Redoes de contato, estilo de deformafao das rochas encaixantes, desenvolvimento de fabrica e diferentes rochas fontes ilustram uma complexa estoria para a formagao dos complexes granito-gnaissicos. Zonas de cisalhamento direcionais (Dn+i) posteriores obliteraram parcialmente as estruturas Dn nos domfnios de greenstone belt, mas seus efeitos foram restritos nos domfnios granito-gnaissicos.