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
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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.
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