Geochemical evolution of laterites from two areas of the semiarid region in Bahia State, Brazil

Abstract

Lateritic covers developed during the Pleistocene in two distinct areas (Gentio do Ouro District and Sento $6) of the State of Bahia, Brazil, have been investigated geochemically. Samples of the bedrock (gabbros, from the Gentio do Ouro District, and silicate facies iron formation and associated iron-rich banded carbonates, from the Sento $6 Area) and the lateritic products (latosols, cuirasses and pisolites ) were analyzed for major and selected trace elements (Co, Cu, Cr, Ni, V, Pb, Ba, Au and REE). The purpose of the research was to study the geochemical evolution of the laterites and the relationship of weathering processes to secondary gold mineralization. Four types of element mobilities are recognized, defined by progressive depletion (type A) or enrichment (type C) in the sequence latosol-cuirasse-pisolite or by extreme enrichment (type B) or depletion (type D) in the cuirasse. Cr and V have similar behavior (type C), whatever the type of parent rock; however, when the original rocks are gabbros, Co, Cu and Ni demonstrate behavior A, B and A, respectively. Also Ti has the same type of mobility (type A ), whatever the type of parent rock. The concentrations of some residual elements, e.g., Ti, V, Mn, Fe and Au, are inherited from the original rocks, that is, they exhibit higher values in the lateritic materials on parent rocks in which their contents are high. The enrichment of REE in the lateritic materials on iron-rich carbonates is attributed to relatively basic and reducing conditions at depth. This environment allowed the ions of these elements (Ce, Sm, Eu and Yb in the reduced ionic form) to stay in solution along with Fe 2+. At shallow depth, under oxidizing conditions, the iron oxyhydroxides adsorbed the REE ions. This adsorption was more effective for La, Ce and Yb ions. The REE contents in the lateritic products from gabbros indicate a depletion of HREE plus Eu. Nd and Sm are also depleted in cuirasses and Ce is depleted in pisolites. The loss of these REE and the slight concentration of LREE in the red latosol of the Gentio do Ouro District are attributed to partial leaching of REE by a relatively acidic solution. The adsorption of REE by iron oxyhydroxides was less intense here than in the weathering of the iron-rich carbonates at Sento $6 Area. It was also more effective for La and Ce (ions). This type of weathering solution may be generated from oxidation of the accessory pyrite present in the gabbros of the Gentio do Ouro District. It may dissolve primary gold which then precipitates on encountering basic conditions at depth.