Hardy Jost

Stratigraphy of the greenstone belts, Crixás region, Goiás, central Brazil

Abstract Archean rocks in the Crixas region, central Brazil, comprise polydeformed low-grade metamorphic greenstone belt volcano-sedimentary sequences of 2.8 Ga arranged in three subparallel belts separated by domal granitoid gneiss blocks. From west to east the supracrustal belts are named Crixas, Guarinos and Pilar de Goias. A revised stratigraphy is proposed for each belt. The Crixas Group is composed of: (a) the Corrego Alagadinho Formation, consisting of an approximately 500 meter thick sequence of komatiites interlayered with banded iron formations; (b) the Rio Vermelho Formation (350 m), locally pillowed metabasalts with intercalations of oxide and/or silicate facies banded iron formations; and (c) the Riberrao das Antas Formation (700 m), with a lower sub-unit of carbonaceous phyllite-manganiferous metachert assemblage, a middle carbonaceous phyllite-marble assemblage, and an upper metapelite sub-unit. The Guarinos Group is subdivided into: (a) the Serra do Cotovelo Formation (500 m), made up of komatiites with intercalations of carbonaceous phyllite; (b) the Serra Azul Formation (300 m), consisting of metabasalts with intercalations of carbonaceous phyllite; (c) the Sao Patricinho Formation (300 m), metagraywacke interlayered with chlorite phyllites and locally banded iron formations and orthoquartzite; (d) the Aimbe Formation (50 m), a stratigraphic marker made up to diverse banded iron formations; and (e) the Cabacal Formation (400 m), consisting of carbonaceous phyllite. The Pilar de Goias Group contains four sub-units: (a) the Corrego Fundo Formation (900 m), komatiites interlayered with thin banded iron formations; (b) the Cedrolina Formation (500 m), consisting of metabasalts with thin iron and/or manganese formation layers; (c) the Boqueirao Formation (200 m), made up of 50 meters of lower metachert grading up into calc-silicate rocks with dolomitic marble lenses close to the top; and (d) the Serra do Moinho Formation (400 m), composed of phyllite with variable proportions of carbonaceous matter. Each of the greenstone belts has a gross stratigraphic framework made up of a lower volcanic and an upper sedimentary sequence. Major stratigraphic differences occur among the upper sedimentary sections of the three belts; these are interpreted as resulting from different paleographic settings. Vertical facies change from deep to shallow water facies in each belt. Lateral facies change from an outer shelf euxinic environment in the Crixas greenstone belt, eastward into slope graywackes in the Guarinos greenstone belt, and deep water cherts and calc-silicate rocks in the Pilar de Goias greenstone belt. A carbonaceous sequence on top of both the Guarinos and Pilar de Goias Groups may represent either a blanket of euxinic shelf prograding onto deep water facies or allochtonous slices of outer shelf type metasediments transported during late Archean thrusting.

Neoproterozoic anatexis of 2.9 Ga old granitoids in the Goiás-Crixás archean block, Central Brazil: evidence from new SHRIMP U-Pb data and Sm-Nd isotopes

No presente estudo sao reportadas as primeiras idades U-Pb SHRIMP para rochas granito-gnaissicas dos complexos Uva e Caicara, na parte sul dos terrenos arqueanos de Goias. Combinados com analises isotopicas Sm-Nd, os dados U-Pb permitem aprofundar o conhecimento a respeito da evolucao geologica daquela parte da Faixa Brasilia. Cristais de zircao do gnaisse tonalitico de Uva (GOV-4) indicam a idade de 2934 ± 5 Ma para a cristalizacao do protolito igneo. Um grao de zircao metamorfico sugere idade de 2793 ± 3 Ma para o metamorfismo que afetou a rocha, enquanto que um grao herdado de zircao com idade de 3092 ± 9, aliado a valores de µNd(T) variando entre +0,4 e -4,6, sugere contaminacao com crosta continental mais antiga. Granito leucocratico exposto a norte do greenstone belt de Crixas no Complexo Caicara apresenta idade de cristalizacao de 626 ± 7 Ma, dada por sobrecrescimentos igneos cristalizados em torno de nucleo com idade de 2893 ± 12 Ma. O resultado, combinado com valores fortemente negativos de µNd(T) (entre -28,0 e -29,0), indica que o granito e produto de refusao de rochas arqueanas com ca. 2,89 Ga de idade. A idade do granito e identica as idades U-Pb SHRIMP e convencional do alcali-granito Itapuranga e quartzo sienito de Uruana, expostos a norte da area estudada. Trata-se, portanto, do primeiro registro confiavel de magmatismo Neoproterozoico no interior dos terrenos arqueanos de Goias e pode sinalizar que varios dos pequenos corpos e diques de leucogranitos encontrados em meio aos terrenos TTG podem representar magmas gerados durante a orogenese brasiliana.

Características estruturais, petrográficas e geoquímicas de enxame de diques máficos intrusivo em rochas metassedimentares do greenstone belt de Crixás, Goiás

Mafic dikes and stocks are a common feature in the Archean terrain of Goias, Central Brazil, where they occur as swarms of 2.3 - 2.5 Ga within the granite-gneiss complexes (2.8 e 2.7 Ga), as well as intrusions related to the komatiite and basalt flows of the greenstone belts lower stratigraphic units, but were unknown within the upper metasedimentary units. Detailed study of core sections from several drill-sites in the area of the Crixas greenstone belt gold deposits showed that dike intrusion occurred after the main Paleoproterozoic deformation and metamorphism of the metasedimentary units, and literature data indicate that the magmatic zircons from the dikes yielded an age of 2,170 ± 17 Ma. Petrographic and geochemical data show that they have the composition of epicratonic high-Ti diabases, which are similar to the large continental flood basalts of the Parana Basin. The age of the intrusions may be correlated with the short time-interval of the Rhyacian (2.20 to 2.17 Ga), during which successive localized episodes of mantle plume volcanism occurred on the Earth.

Multistage gold deposition in the Archaean Maria Lázara gold deposit (Goiás, Brazil)

The Maria Lázara gold deposit in the Archaean Guarinos greenstone belt, central Brazil, consists of quartz-carbonate veins formed in a ductile shear zone. Study of the sulphides and gold-bearing quartz-carbonate veins and veinlets of the potassic hydrothermal alteration zone reveals two stages of ore paragenesis. The first stage corresponds to S-rich arsenopyrite deposition and the second one to the precipitation of As-rich arsenopyrite and Bi-Te-Au-S phases. Modes of occurrence, textures and Ag contents of native gold define two types of gold. The first type is native gold with Ag contents > 5 wt % postdating the S-rich arsenopyrites. The second type has Ag contents < 3 wt % and is contemporaneous with Bi-Te-S phases.