Francisco Egidio Cavalcante Pinho

Early Proterozoic Evolution of the Alto Jauru Greenstone Belt, Southern Amazonian Craton, Brazil

The Alto Jauru Greenstone Belt in west-central Brazil comprises three belts of Early Proterozoic volcano-sedimentary sucessions that were invaded by Early to Middle Proterozoic intrusions, including tonalites, gabbros, and granites. Volcanic rocks represent a bimodal suite with ultrabasic-basic rocks of komatiitic-tholeiitic affinities at the base and intermediate-felsic calc-alkaline lavas and pyroclastic units on the top. Chemical differences exist between basic volcanic rocks from the Jauru Belt and those from the Cabacal Belt, but the volcanic rocks and the Cabacal Tonalite appear to be related to an island-arc environment and possibly were generated from the same mantle source. The volcanic-volcanoclastic sequence in the Jauru Belt hosts important deformed, gold-rich volcanogenic massive sulfide deposits.

Nova unidade litoestratigráfica registra glaciação ediacarana em Mato Grosso: formação Serra Azul

A new succession of diamictites and siltstones was found above post-Marinoan carbonates from the Araras Group in the northern Paraguay Belt (Brazil), in discontinuous outcrops. This new stratigraphic unit, named Serra Azul Formation in this work, has a variable thickness reaching up to 300 meters. The diamictites were deposited in the first 70 m and present glacial evidences, while the siltstones represent the upper part of the succession and show some sandstone layers towards the top of the succession. This glacial record is the youngest Neoproterozoic glacial event that has been found on South America and probably is related to Gaskiers Glaciation (582 Ma).

Petrology of potassic alkaline ultramafic and carbonatitic rocks from Planalto da Serra (Mato Grosso State), Brazil

The Planalto da Serra igneous rocks form plugs, necks and dykes of carbonate-rich ultramafic lamprophyres (aillikites and glimmerites with kamafugitic affinity) and carbonatites (alvikites and beforsites). Phlogopite and/or tetraphlogopite, diopside and melanitic garnet are restricted to aillikitic rock-types, whereas pyroclore occurs only in carbonatites. Aillikites and carbonatites are altered to hydrotermalites, having chlorite and serpentine as dominant minerals. Planalto da Serra igneous rock association has kamafugitic affinity (i.e. effusive, ultrapotassic. High LREE/HREE fractionation, incompatible elements data and Sr-Nd isotopes, suggest that the K-ultramafic alkaline and carbonatite rocks originated from a variably metasomatized mantle source enriched in radiogenic Sr. Crustal contamination is negligible or absent. Age values of 600 Ma rule out the geochronological relationship between the investigated intrusions and the Mesozoic alkaline bodies from the Azimuth 125° lineament. The TDM model ages allow to conclude that Planalto da Serra magma is derived from the partial melting of a mantle source metasomatised by K-rich carbonatated melt during the Early to Late Neoproterozoic. On the basis of alkaline magmatism repetitions at 600 Ma and 90–80 Ma we question the subsistence of a stationary mantle plume for so long time.

Granito Terra Nova - petrologia e geocronologia: um granito tipo-A da Província Aurífera Alta Floresta - Cráton Amazônico

O Granito Terra Nova faz parte da Suite Intrusiva Teles Pires e se localiza em torno do municipio homonimo, na Provincia Aurifera Alta Floresta. O granito em questao ocorre em forma de um batolito alonga- do de direcao NE-SW, cujas rochas sao de coloracao roseo-avermelhado a vermelho-tijolo com granulacao media a grossa, homogeneas, mostran- do localizadamente enclaves maficos microgranulares e cavidades miaroli- ticas preenchidas com fluorita, quartzo e sulfetos. Microscopicamente, as rochas estudadas mostram texturas xenomorfica a hipidiomorfica inequi- granular, sendo comum o intercrescimento pertitico e grafico. Os minerais essenciais que compoem a rocha sao feldspato alcalino pertitico e quartzo com proporcoes inferiores de plagioclasio, biotita e minerais acessorios. Es- timativas da composicao modal permitem plotar as amostras no campo dos alcali-granitos com variacao para o campo de sienogranito. O resultado das analises quimicas para elementos maiores e tracos feitas em 17 amostras, indicaram natureza sub-alcalina, carater metaluminoso a levemente peralu- minoso e padrao de elementos terras raras com suave enriquecimento de leves sobre pesados e anomalia de Eu negativa de moderada a forte. Dia- gramas multielementares mostram enriquecimento de Rb e Th e anomalias negativas de Ba, Nb, Ta, Sr, P and Ti. As temperaturas do magma parental foram estimadas entre 748°C a 855°C. Os dados de campo, a composicao mineralogica e o comportamento geoquimico das rochas sao coerentes com um ambiente tectonico do tipo anorogenico ou pos-colisional para o grani- to em questao. O quimismo do Granito Terra Nova permite ainda classifi- ca-lo como um granito rapakivi do tipo A, oxidado. Dados U-Pb em zircao resultaram em uma idade de cristalizacao 1727 ± 42 Ma e e Nd com valores entre +1,07 e -0,98 para a idade modelo Nd de 2,0 a 2,17 Ga.

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.