Chemical denudation and geochemical degradation of the relief in the Quadrilátero Ferrífero region, Brazil: An approach from an important hydrographic microbasin up to the geological unity

Abstract

The Quadrilátero Ferrífero is one of the most important metallogenic provinces in the world, and most of these mineral resources are from geological units belonging to the Minas Supergroup. The present study investigates the chemical denudation and the lowering of the relief of the Bule Stream microbasin, which covers an area of 38 km2 and is composed almost entirely of units of the Minas Supergroup. As the study area has no anthropic influence, it can represent a natural background. Surface river water samples were collected at 15 points in the watershed in 4 periods: Dec/2015 and Apr/2018, representing the rainy period (summer), and in Aug/2016 and Aug/2017, representing the dry period (winter). Analyzes of their dissolved chemical compositions (Ca, Mg, Na, K, silica, and the main constituent metals) were performed, determining the total dissolved solids (TDS), in addition to flow and pH calculations. The main influencing source of water chemistry in the microbasin is rainfall, associated with the materials geological “flushing” process, in both periods. The main control factor for denudation is the lithological control and its erodibility, where an average of 246.4 tons of material removed and transported in the watershed per year was estimated. Ca, Mg and Si, the main constituents of rocks present in the microbasin, such as dolomite, phyllite, and quartzite, presented the highest denudational values. The ‘Colonia sector’, which drains a quartzite area, and the ‘west sector,’ which drains an area of carbonate rocks, presented the highest denudation rate values, verifying that the size of the drainage area of these sectors is not a considerable parameter determining the denudational behavior of the hydrographic basin. The extrapolation of denudation and degradation for the entire Minas Supergroup, considering no anthropic influence, provided values of 12,475.62 ton/yr of material removed through weathering with a relief degradation of 2.14 m/Ma.