Textural and mineralogical analisys of morro do nÍquel rocks (Mg) and correlations with serpentinization

Abstract

Morro do Níquel, a serpentinitic massif located in the southern extension of the Southern Brasilia Belt, in the Passos Nappe context, was the subject of a detailed textural and mineralogical study, developed on a drillhole, aiming to characterize the rocks present there. This massif has an ellipsoidal shape, with dimensions of 400 x 800 meters and thickness greater than 300 meters, composed only by serpentinite. This serpentinite has a predominance of mesh, mesh-ribbon and ribbon textures, consisting of lizardite / chrysotile and pseudomorphs that refer to preterit olivine crystals. The homogeneity of the studied samples indicates that the protolith was a rock with granular texture, medium to coarse granulation (> 0.5 cm) and constant mineralogy, possibly a dunite. The conditions described for that massif indicate two metamorphic events, an older one that developed in amphibolite facies, and a younger one, developed in prehnite-pumpellyite facies, but that partially preserved features of the past event. The presence of awaruite and heazlewoodite suggests a serpentinization developed in low O2 and S2 fugacity. Petrographic, lithological and metamorphic analysis shows that this body has distinct characteristics from serpentinites of the Greenstone Belt Morro do Ferro, an Archean metavolcanossedimentary unit to which it is currently associated.