Análise estrutural de ocorrências de gossans associadas à grafita-xistos do complexo metamórfico porongos

Abstract

Gossans are supergenic deposits resulting from oxidation of sulfide rocks. Gossans having been used in the exploration of base metals as Zn, Cu, Ag, and others. The study of Gossans associated with Graphite-Schist within Porongos Metamorphic Complex aimed to understand the structural relations of these rocks to the host rocks. The study area is in the Santana da Boa Vista Dome, a structural unit of Porongos Metamorphic Complex. In this study, structural analysis and field magnetometry were employed. The structural analysis was performed based on the definition of the geometric aspects of structures, involving descriptive analysis and acquisition of attitudes of planar and linear structures. Data were tabulated and stereographic diagrams were made as well as structural and geophysical maps for spatial data analysis. Samples collected in the field were described and all information was organized into geo-referenced database. Measured values of Total Magnetic Field (nT) were corrected for diurnal variation. The magnetmometric maps allow the identification of linear magnetic anomalies associated with the Gossan aligned according to the N55E direction. Through geological-structural study conducted at 1: 5.000 scale, five litotypes were identified with their respective planar and linear structures formed in ductile and brittle conditions. Structural analysis showed that the metamorphic foliation and axial plane of folds occur in the NE-SW direction and the fold axes plunge to SW. Already, fractures have NW-SE preferred orientation, with moderate to high angle of dip. The macroscopic study of Gossan showed that botryoidal goethite occur in Boxworks left by the dissolution of pyrite. Hematite, sometimes oxidized, was identified in the massive portion of Gossan. The brecciated facies is dominated by graphite-schist intensely affected by hydrothermal alteration with silica or carbonate as cement. Fractures display strong oxidation. Sulphides were identified only in fractures of the graphite-schist which crop out in the drainage system, partially spared from oxidation. The study shows that the mineralization of Gossans is associated with fold axial planes and also in the NW direction of fracture planes.