Hydrothermal silicification along rift border faults in the Rio do Peixe basin, Brazil

Abstract

This study investigates hydrothermal silicification along rift border faults in the Cretaceous Rio do Peixe Basin, Brazil, through field geology, petrography, mineralogy, and fluid inclusion microthermometry. We distinguish three main structural-diagenetic domains: (1) a non-silicified fault damage zone, (2) a silicified damage zone composed of deformation bands and centimeter-scale quartz veins in the hanging wall and footwall, respectively, and (3) a silicified fault core with meter-scale wide quartz veins and mosaic breccia textures. We recognize three main events of silicification in the fault zone. The first silicification event occurs after the widespread development of deformation bands in the hanging wall damage zone that may reach 270 m in width. During subsequent deformation events, multiple fracturing episodes were accompanied with precipitation of the second quartz generation in hydraulic mosaic breccias and the third quartz cement in vein stockworks. Shallow burial conditions of faulting (<2 km), combined with fluid inclusion homogenization temperatures of the second quartz generation, indicate that silicification occurred in a hydrothermal system characterized by fluid temperatures in the range of 71–391 °C (mean value of 141 °C). We conclude that hydrothermal silicification was imprinted during the late synrift stage, when the rift border faults were already established. This contribution has implications for understanding how structural processes may control the geometrical, petrographic and petrophysical attributes of hydrothermal products.