Geometria fractal dos veios de quartzo da serra de Ouro Preto, flanco sudeste do anticlinal de mariana, Quadrilátero Ferrífero/MG

Abstract

Fractal analysis statistics combined with textural and structural data of quartz vein sets have been used to address their evolution in the tectonic context of the Ouro Preto Range, which is the southern limb of the Mariana Anticline. Fractal statistics have become a convenient way to deal with the problem of the scale analysis of vein sets considering that vein dimensions (length, thickness and spacing) have analogous relationships and length and thickness vary according to a power law distribution. The vein sets have been mapped in three sections across the Ouro Preto Range, 2 km from each other. 613 veins have been mapped on the surface and in abandoned mine galleries. Measured data comprise thickness, length and spacing. 161 veins are hosted in quartzite, 60 in phyllite and 392 in the iron formation (BIF). Data have been plotted in cumulative log-plots of number of veins versus thickness, number of veins versus . length, thickness versus length and number of veins versus spacing. The older vein sets are parallel or subparallel to the regional cleavage. These veins underwent dynamic recrystalization and have been formed as a consequence of an extensional event. The younger vein set displays a crystalline fabric not truncated by the regional cleavage. The largest veins cross the quartzite layers and have larger axial ratios than the smaller ones that cross the phyllite and iron formation. Analysis of shape factors (thickness and length) using the fractal dimension D revealed that both vein sets display a flat shape (prolate in section) in the western portion of the studied area changing to a nodule shape (oblate in section) towards the east. In addition, vein density is greater forlarger values of D and D increases eastward for both sets of veins. These data may suggest that the source of the fluids is closer to the axis of the Mariana Anticline and/or that contractional strain increases towards the east as reported by regional geology data.