GEOLOGIA E GEQQUÍMICA DOS ELEMENTOS MAIORES DOS DEPÓSITOS DE MAGNESITA PRÉ-CAMBRIANA (~1.8Ga) DA FAIXA MÓVEL ORÓS ( CEARÁ)

Abstract

The five most important magnesite mines and some small occurrences of the Ceara State form an array of discontinuous lenses extending for about 140 km. The magnesite-rich rocks are hosted in metadolomites with lutecite, fibroradiated quartz pseudomorph after sulfate nodules, scapolite and dissolution breccias. The marbles are hosted by a metavolcanosedimentary sequence of the greenschist to amphibolite facies, and were intruded by basic sills and granites of size, form, composition and age that vary from the Meso to Neoproterozoic. Two types of magnesite marbles may be distinguished: (1) magnesite marble of medium grain-size (1 to 9mm) and (2) sparry magnesite marble (1 to 15cm). The sparry magnesite marble has porphyric, rosette, layered, and palisade textures, the latter having remains of the original sedimentary features. In spite of deformation, the sparry crystals are hipidiomorphic and pinolitic. The rocks vary from white to light gray or dark gray or even red. The dark banded terms have traces of microfossils and stromatolitic structures. The texture and color of the medium-grained magnesite marbles are more homogeneous and the crystals are more anhedral. The sparry magnesite marble is in general poorer in SiO 2 , Fe 2 O 3 , Al 2 O 3 and CaO and richer in MgO as compared to the medium-grained types. The paleogeographic interpretation suggests that these rocks formed near a paralic system (lagoon), with strong evaporitic conditions. The depressions were of variable depth and length and probably isolated by stromatolite barriers, dried and flooded by seawater with continental inflows. The largest and deepest deposits correspond to the sparry magnesite. Regionally, the magnesite deposits give place to dolomitic marbles, which grade into almost pure calcitic marbles, indicating a chemical differentiation within the paralic system, by which the lagoon waters turned progressively poorer in Ca by the precipitation of calcium carbonate, thus increasing the Mg/Ca ratio and leading to the precipitation of magnesite. The magnesite marble is than interpreted as of sedimentary origin, and underwent strong diagenesis before metamorphism and Neoproterozoic deformation.