Mineralogy and geochemistry of bauxite and bentonite deposits from Mozambique

Abstract

Results of mineralogical and geochemical studies of bauxites, kaolinitic clays and bentoniteS from Mozambique are presented in this thesis. The bauxite and kaolinitic clay deposits in Penhalonga area (in the central western part of Mozambique) are associated with Precambrian magmatic rocks and the bentonites resulted from weathering of volcanic rocks, belonging to the Lebombos Mountain range in the Boane district (in the south of Mozambique). The bauxites and kaolinitic clays are the main products resulting from weathering of rock types with anorthositic, gabbroic composition and metabasalts. These parent rocks have been subjected to different degrees of weathering witnessed by the presence in the profiles of the minerals gibbsite, kaolinite, hydroxides and oxides of iron, as well as illite in variable amounts. The distribution of major and trace elements, including the Rare Earths, in the weathering profiles provides gives indications for the main development trends of the bauxitization-Iateritization in Penhalonga area. Bauxite formation took place over the main rocks of the area mineralogically with feldspars and pyroxenes and/or amphiboles as the dominant primary components under influences of processes ofweathering in tropical conditions. Although most of the weathering sequences are truncated, due to superficial erosion, the general behaviour of the chemical elements reflects the composition of the source rock and, to some extent, the dominant conditions for the geochemical mobility and fractionation of the elements. This guides to the separation of the easily mobile elements like the alkalis and alkaline earths and the less mobile or immobile elements. The later group is represented by elements as AI, Fe, Ti, Zr, which are relatively retained in the weathered profiles, in almost all stages. Remarkable in the weathering sequences is the behaviour of the Rare Earth elements (REE), specially the elements Ce and Eu, which display anomalies, due to their chemistry. On the other hand, taking into consideration the activities ofNa+, K+, ci+ and I4Si04, the composition of the waters draining the area also provides evidence of the principal mineral assemblages present in the weathering sequences. In the Mozambican territory, associated with the volcanics of the Lebombos mountains, smectite-rich bentonites constitute deposits of economic interest. The principal mineral is montmorillonite in concentrations ranging up to an average grade of 75-79% and invariably associated with silica phases, chiefly cristobalite. According to referenced discrimination diagrams the bentonites have close affinity with parent rocks of rhyolitic and rhyodactic compositions. However, the mechanisms of the bentonite formation are still poorly understood, although, from field relationships and the geological setting of the bentonitic masses, it seems that they have originated from miscellaneous processes of alteration of the rhyolitic parent rocks. The bentonites are chemically of the Ca-type with high silica SIOz and generally low contents in Alz03 and MgO, features, which are strictly related to the parent rocks of dominantly rhyolitic composition. In some places occurrences of indurated calcium carbonate horizons are observed in a vertical profile. Geochemically the bentonites show a uniform composition, which is also reflected in the trace element content. The behaviour of the chemical elements is consistent with the general effects of an alkaline environment on the mobilization and fixation of elements propitiating formation of bentonites instead of zeolites.