Drill core structural analysis and extensional-contractional controls on the sulfide mineralization at the Ambrosia Sul zinc deposit, Vazante group, Western São Francisco craton, Brazil

Abstract

The Ambrósia Sul Zn-(Pb) sulfide deposit is located in the southern section of the Vazante Group, which is the border area between the Brasília belt and the southwestern edge of the São Francisco craton. This region is characterized by a typical karst environment in a tropical climate that is commonly represented by the intense development of thick and uneven soil cover, which hinders structural geologic mapping. A survey with oriented core samples allowed a full structural investigation and description of structures such as faults, fractures, veins, bedding, folds, foliations, and lineations. The structural analysis suggests an extensional phase with down block in the west, during which normal faults and high-angle veins were generated, followed by a contractional phase associated with the reactivation of bedding planes, the nucleation of reverse faults and low-angle veins, and east-vergent folding. The zinc sulfide ore bodies are preferentially hosted in high-angle veins and in cataclastic rocks generated during the extension. Low-angle veins, rarely mineralized with Zn, cut the high-angle veins, suggesting that the mineralization event lasted even during the tectonic inversion phase. The regional context indicates that the Ambrósia Sul deposit formed in the bulge region of the foreland basin and that it was affected first by an extensional phase during its nucleation and subsequently by a compressive phase with the advancement of thrusting in the Brasília belt to the east.