Mafic dikes are abundant in the Quadrilátero Ferrífero and Southern Espinhaço regions, southeastern State of Minas Gerais. The dikes range from 1,7 Ga to 120 Ma and form at least three swarms that differ in trend, mineralogy, composition and age. The oldest swarnrhas a baddeleyite U/Pb age of 1,7 Ga (Ibirité Gabbro) and K-Ar age of about 1 ,7-1 ,5 Ga (schists), trends north-south, the dikes vary in width from centimeters to meters, and are deformed and metamorphosed to cnlorite-sericke-quartz-plagioclase schist or are some meters wide and contain plagioclase, augite,-ilmenite, quartz and traces of baddeleyite. These dikes are enriched in LREE [(La/Sm)N =3,68-6.39], and depleted to enriched in HREE [(Dy/Yb)N = 0,82-1,29]. These dikes (and sills also) are related to the opening of the Espinhaço Basin. Nprthwest-and northeast-trending mafic dikes have a U/Pb age of about 906 Ma. They are commonly ten meters wide, and are of metagabbro with relict igneous textures and containing altered plagioclase (Ans-52), amphibole, epidote, ilmenite, quartz, and apatite. Samples of this dike swarm are slightly enriched in LREE and HREE with a (La/Sm)N = 1 .32 to 2.2 and (Dy/Yb)N = 1 .36 to 1 .88, respectively, and also show small negative europium anomaly. Sills, stocks, plugs, laccoliths and phaccoliths with compositions similar to dikes are present in the study area. The Sm-Nd data indicate negative ENU (T) (+ 1 .34 and-3.20) and model ages (T DM) of ca. 2.59 and 1 .35 Ga. These dikes may be related the first stages of the Panafrican/Brasiliano event. North-south, east-west, northwest and northeast trending Phanerozoic dikes are of diabase composed of plagioclase (An«)), augite and magnetite. They show moderate LREE (La/Sm)N = 2,53, and slight HREE enrichments, (Dy/Yb)N = 1,47. These dikes have a K/Ar age of about 120 Ma and may be related to the fragmentation of Gondwanaland. The Quadrilátero Ferrífero and Southern Espinhaço dikes are tholeitic and exhibit prominent iron-enrichment. Some trace element distributions are similar to continental rift basalts. They represent significant amounts of mafic melts formed during the beginning of ample extensional tectonic regimes. There are significant variations in incompatible and partially compatible element abundances both within and between dike swarms, which is due not only to heterogeneous mantle sources, but also to crustal contamination. RESUMO ENXAMES DE DIQUES MÁFICOS DO QUADRILÁTERO FERRÍFERO E ESPINHAÇO MERIDIONAL, MINAS GERAIS, BRASIL Enxames de diques máficos são abundantes no Quadrilátero Ferrífero Quadrilátero Ferrífero e Espinhaço Meridional, sudeste de Minas Gerais. Três eventos de magmatismo básico ocorrem nestas regiões, cada qual com feições estruturais, petrográficas, geoquímicas e geocronológicas próprias. O enxame mais antigo tem idade de 1,7-1,5 Ga (U/P badeleíta-Ibirité Gabbro e K-Ar RT-xistos) e ocorre como sills e diques de direção NS, deformados e metamorfisados, ou como diques não deformados com a textura ignea preservada. Os deformados são clorita-sericita...

Resumo O Terreno Granito-Greenstone do Tocantins é composto por domos granito-gnáissicos e estreitas faixas de greenstone, formados em ambiente de subducção no Paleoproterozoico. As ocorrências de ouro nesse terreno são hospedadas em quartzo de veios associados a produtos de alteração hidrotermal de anfi bolito, formação ferrífera bandada e rochas graníticas em zonas de cisalhamento. Sugere-se que as ocorrências e depósitos de ouro do Terreno Granito-Greenstone do Tocantins formaram-se durante a orogênese Brasiliana, com a participação de fl uidos hidrotermais de origem metamórfi ca, com contribuição granítica. Os dados obtidos até o presente revelam critérios de exploração mineral para ouro no Terreno Granito-Greenstone do Tocantins.Palavras-chave: Tocantins, greenstone, ouro, exploração mineral. Abstract Geology of and gold occurrences in the Granite-Greenstone Terrane of Tocantins, Central Brazil: state of the art and guide lines for mineral exploration programs.The Paleoproterozoic Tocantins GraniteGreenstone Terrane is composed of granite-gneissic domes and narrow intervening greenstone belts. Tholeiitic metabasalts and calc-alkaline metatonalites have chemical composition characteristic of subduction tectonic environment. The main gold concentrations occur in iron-rich amphibolite, banded iron formation and granitic rocks. It is suggested that the Tocantins Granite-Greenstone Terrane gold occurrences and deposits were formed by dominantly metamorphic hydrothermal fl uids during the Brasiliano Orogeny, with granitic contribution. Guide lines for mineral exploration programs for gold in the Tocantins Granite-Greenstone Terrane are presented.

neoproterozdica. O minério ocorre intimamente associado a zonas de alteração hidrotennal, sendo que ambos são controlados por fraturamento, mais intense nas proximidades de contactos de corpos intrusivos e zonas de falhas regionais. O estagio inicial da mineralização consistiu de reações entre água do mar aquecida e basaltos da sequencia de Mara Rosa (fluido hidrotermal exalativo), com lixiviação de metais das rochas basalticas e deposição em zonas de alteração hidrotermal. Posteriormente, fluido hidrotermal, originado de intrusoes dioriticas pequenas lixiviou metais de rochas encaixantes, cuja deposição enriqueceu a mineralização. Finalmente, fluidos metamorficos, gerados durante a orogenese brasiliana, formaram veios de quartzo auriferos em aieas próximas a Chapada.

The Guarinos greenstone belt is one of the three Archaean volcano-sedimentary rock assemblages of Crixás, Goiás, and its northern half was maped by the authors in 1992. The belt was intruded by a tonalite of the Caiamar Complex in the west, has a transcurrent fault contact with gneisses of the Moquém Block in the east and north, and is tectonically covered by Late Proterozoic metassedimentary rocks of the Araxá Group in the south. Its northeastern portion was intruded by a trondhjemite. Rocks of the belt (Guarinos Group) are grouped, from base to top, into the Serra do Cotovelo Formation (metakomatiites), Serra Azul Formation (tholeiitic metabasalts), São Patricinho Formation (detrital metassedimentary rocks), Aimbé Formation (banded iron formations, metaconglomerates and metahydrothermalites), and the Cabaçal Formation, which is further subdivided into a lower sub-unit (carbonaceous phyllites, locally metachert and banded iron and manganese formations), and an upper sub-unit (rythmic metashales and metasiltites). The supracrustais are folded into a normal and horizontal antiform in the south, which gives place to westerly dipping inverse antiforms in the north and east. The trondhjemite intrusion in the northeastern portion of the belt deformed the supracrustal rocks into a dome. Primary structures in metavolcanic and metasedimentary rocks indicate that the stratigraphic sequence of the greenstone belt is overturned, due to early northward verging nappes, followed by Late Proterozoic, easterly verging folding. Regional metamorphism took place under greenschist facies conditions. Contact metamorphic parageneses formed under hornblende hornfels and albite-epidote hornfels facies, respectively in contact with the western tonalite and the northeastern trondhjemite. Mylonites are of greenschist to lower amphibolite facies. Mineral resources in the area comprise gold and banded iron and manganese formations, under several modes of occurrence.RESUMO O greenstone belt de Guarinos é uma das três faixas de rochas supracrustais arqueanas da região de Crixás, Goiás, cuja metade norte foi mapeada pelos autores em 1992. As supracrustais limitam-se com um tonalito intrusive do Complexo Caiamar a oeste, por falha transcorrente com gnaisses do Bloco do Moquém a leste e norte, com rochas metassedimentares neoproterozóicas a sul. Na sua porção nordeste, a seqüência foi intrudida por um corpo de trondhjemito. As supracrustais (Grupo Guarinos) foram divididas, da base para o topo, nas formações Serra do Cotovelo (metakomatiitos), Serra Azul (metabasaltos toleifticos), São Patricinho (rochas metassedimentares detríticas rítmicas), Aimbé (formações ferríferas, metaconglpmerados e metahidrotermalitos) e Cabaçal, a qual contem uma subunidade inferior (filhos carbonosos, subordinadamente metachert e formações ferríferas e manganesíferas) e uma superior (metafolhelhos, metassiltitos e metarenitos finos e rítmicos). A estrutura regional do greenstone belt de Guarinos é, a sul, um antiforme normal e horizontal que grada, para norte e leste,...

No abstract

No abstract

RESUMOOS ENXAMES DE DIQUES MÁFICOS DO MACIÇO DE GOIÁS, BRASIL CENTRAL. Relações de campo e litogeoquímica permitiram indivualizar três enxames de diques máficos no Maciço de Goiás. O enxame mais antigo compreende diques que secionam terrenos granhóide-gi-eenstone, são recobertos por rochas mctassedimentares proterozóicas e foram metamorfisados nas fácies xisto verde ou anfibolito. Esses diques mostram concentrações de TiO 2 inferiores a 2% e apresentam similaridade química com basaltos do tipo N-MORB. Diques não metamorfisados, mas localmente cisalhados, secionam rochas arqueanas e proterozóicas do Maciço de Goiás e foram posicionados, provavelmente, nos estágios finais de deformação do Ciclo Brasiliano (~600 Ma). Esses diques apresentam concentrações de TiO 2 entre 2 e 3% e são quimicamente semelhantes aos basaltos continentais da província do Deccan. O enxame de idade mais recente compreende diques sem deformação ou metamorfismo, mais ricos em TiO 2 (>3%) e com composição química também semelhante aos basaltos do Deccan. O posicionamento destes diques ocorreu, provavelmente, no Mesozóico, durante a fragmentação do Gondwana.Palavras-chave: Diques máficos, geoquímica, Maciço de Goiás, Brasil.ABSTRACT Three mafic dike swarms occur in lhe Goiás Massif. The earliest dikes cut Archaean granitoid-greenstone terranes partially overlain by Proterozoic metasedimentary units, underwent metamorphism of the greenschist to amphibolite fácies and consist of low-titanium (<2% TiO 2 ) basaltic rocks chemically similar to N-MORB. Unmetamorphosed but locally sheared dikes cut archaean and proterozoic rocks, have 2 to 3% TiO 2 and are more like Deccan continental flood basalts. They were probably emplaced during the late stage of the Brasiliano cycle. Younger mafic dikes of the Goiás Massif are unmetamorphosed and undeformed, enriched in TiO 2 (>3%0) and chemically similar to Deccan continental flood basalts. The emplacement of these dikes was probably related to the fragmentation of Gondwana during Mesozoic. The geochemical similarities between the Neoproterozoic and Mesozoic dikes of the Goiás Massif support the inference that the geochemical features of the Mesozoic mantle were inherited frorrï processes that occurred during the Proterozoic.

No abstract