Caracterização e origem das formações ferríferas e xistos grafitosos do Grupo Estrondo na Região de Xambioá/Araguanã-TO

Abstract

In the Xambioá/Araguanã region, located in the northern Neoproterozoic Araguaia Belt, iron-formations and graphite schists of the Estrondo Group are tightly associated with amphibolites and fine to medium-grained metagabbros, which are, respectively, interbedded in and intrusive into the metasedimentary essentially siliciclastic rocks of the cited unit. In this study, these different formations are described and compared with similar rocks and lithological associations from other regions in the world. Also, the origin and the depositional environment of the iron-formations and graphite schists, as well as their relationship with the basic rocks are discussed. The iron-formations are exposed in two principal zones, up to 5 km long, that are located along the northeastern border of the Lontra dome structure and the northern border of the Xambioá structure. These zones are discontinuous and consist of lenticular bodies, 200 to 1,500m long and 50 to 800m wide. Several more discrete iron-formations were recently identified in the study area. The iron-formations display a fine and fairly regular banding and are essentially composed of variably martitized magnetite and quartz, with highly subordinate pyrite and pyrrhotite. The studied samples are, as a rule, very rich in iron (mean Fe2O3tot=83.71%; maximum Fe2O3tot=96.77%) and revealed Pb, Zn, Cu and Co contents moderately higher than those detected in Carajás, Maru, Superior-type and Algoma-type banded ironformations. A prominent positive Eu anomaly (Eu* with mean=7.57 and maximum=15.54), the REE distribution pattern displaying LREE>HREE (mean La/Yb=18.54, maximum=82.83) and high ΣREE (mean=229 ppm, maximum= 371 ppm) suggest that the Xambioá/Araguanã iron formations are derived from chemical sediments of hydrothermalexhalative origin, in main part proximal. The graphite schists are interbedded in the micaschists of the Xambioá Formation and generally make up lenticular bodies tens to hundreds of meters long. However, to the west of the Lontra structure, graphite schists crop out for several kilometers north-south. These rocks are essentially composed of muscovite, quartz and graphite. Chemically, these schists are quite similar to other carbonaceous formations described in the litterature. Only vanadium and chromium contents are somewhat higher than those normally found in comparable rocks. Carbon, the content of which have mean=5,15% and maximum=9,41%, is likely of organic origin. The amphibolites and metagabbros are mainly hosted by micaschists and graphite schists. The former are often foliated, occasionally banded or microporphyritic, while the latter are normally isotropic or incipiently foliated and, frequently, display a porphyritic texture. These rocks are composed of hornblende-actinolite and plagioclase (labradorite and localling andesine in intrusive metamafics), with subordinate quartz, biotite, magnetite and hematite. In places, scapolite phenocrysts are fairly abundant. Sulphides (pyrite, chalcopyrite, chalcocite, and covellite, sphalerite and pentlandite) occur in traces. These metabasites are tholeiitic, sub-alkaline to slightly alkaline. Their REE distribution pattern suggest that, chemically, these rocks are similar to E-MORB. The association of iron-formations, graphite schists and basic rocks likely represents a sub-marine hydrothermal setting, developed during the distensive stage of the Araguaia basin evolution. In different sectors of the eastern part of the basin, basaltic flows and cogenetic gabbroic intrusions allowed the formation of convective systems. Percolating sea-water was progressively heated and evolved to hydrothermal fluids able to remove significant amounts of iron, silica and other metals from the basic volcanics and shallow intrusives. Chemical sediments rich in iron, resulting from this exhalative hydrothermalism were deposited in restricted sub-basins, close to venting zones. Diagenesis later transformed these metalliferous sediments into iron formations. The nutrient supply by the hydrothermal fluids and appropriate physico-chemical conditions close to these zones led to the proliferation of planctons. The organic matter accumulated in these sectors resulted in carbon enrichment of the siliciclastic sediments. During the metamorphic event that marked the late stage of the Araguaia Belt evolution, abundant aqueous solutions percolated the rocks of the Estrondo Group, removing most part of the silica from the iron-formations. Iron, almost insoluble in these probably oxidizing conditions, underwent a strong relative enrichment. At that same time, the carbonaceous matter was transformed into graphite.