A hydrogeological conceptual model for the groundwater dynamics in the ferricretes of Capão Xavier, Iron Quadrangle, Southeastern Brazil.

Abstract

In the Iron Quadrangle (IQ), southeastern Brazil, ferricretes usually overlie the banded iron formations (BIFs) of the Cauˆe Formation, rocks that not only host the most important iron ore deposits of this region but are also its major groundwater source. Little is known about the water dynamics in ferricretes and therefore how they in- fluence the recharge of the subjacent aquifer (Cauˆe aquifer). The objective of this research was to identify fer- ricrete typologies and investigate their water dynamics in the Cap ̃ao Xavier region, northwest of IQ. Field work consisted of macroscopic description of ferricretes, infiltration tests and multitemporal electrical resistivity surveys. Two ferricrete typologies were distinguished: structured ferricretes, which are older and occur on top of plateaus, and detrital ferricretes of colluvial-alluvial origin, which were deposited downslope and in topographic lows. The structured ferricretes preserve the medium- to high-angle foliation inherited from the underlying BIFs, whereas the detrital ferricretes constitute thick packs subparallel to the topographic surface and are intercalated with argillaceous layers in areas of karstic depressions. The structured ferricretes are more porous (20.8% to 24.4%) than the detrital ferricretes (14.4% to 17.6%), hence they have higher mean basic infiltration rates (8.3x10-6 m/s) than detrital ferricretes (2.8x10-6 m/s). Despite these low mean basic infiltration rates, infiltration advances thanks to the karstic-fissural porosity (fractures and joints widened by karstification), and superficial cavities that are connected to caves in the interface between the ferricrete and the subjacent, less resistant, transition horizon. Karstification is controlled by the subvertical foliation in the structured ferricretes and sub- parallel layering in the detrital ferricretes. Some factors that favor infiltration and deep percolation of water are superficial rugosity in structured ferricretes, and karstic depressions in both typologies. The recharge of the Cauˆe aquifer is diachronic, firstly taking place via the karstic-fissural system and later via the ferricrete matrix.