Kaolin Deposits from the Northern Sector of the Cunene Anorthosite Complex (Southern Angola)

Abstract

The Mevaiela kaolin deposits are located in the northern part of the anorthositic-gabbro massif within the Cunene complex (southern Angola) and were formed by the alteration of basic anorthosites and gabbros. The Mevaiela area is part of an elevated region which is located between two extensive NNW-SSE fracture systems. Several kaolinite samples were collected from a quarry (main excavation) and from drill-holes as well as from surficial occurrences in the Cunene complex. Chemical analyses, X-ray diffraction, differential thermal analysis, scanning electron microscopy and isotope analyses were performed in order to model the kaolinite occurrences. The alteration of the anorthosite to kaolin approaching the main excavation is characterized by significant decrease in alkaline-earth and transition metals (Ca, Mg, Fe, Co, Ni and Mn) between the average anorthosite and the kaolin. The crystallinity indices suggest that the kaolin contains kaolinite with a reasonably well ordered structure and near the transition between T (triclinic) and pM (pseudo monoclinic).Mineral exploration tools have been evaluated during this study to assist in future kaolin exploration in the Cunene anorthosite complex.Isotopic analysis of O and D indicates that Ca-feldspar alteration is essentially due to meteoric fluids, over a different range of temperatures. Furthermore, the presence of quartz-feldspar veinlets in the kaolinite bodies could be the result of hydrothermal activity linked to post-anorthosite granite intrusions of the so-called ‘red granite’. Kaolinite from Cunene plots on or close to the kaolinite line into the ‘warm temperature in tropical region’ area (surficial samples). Samples from drill-holes plot on the left and show the largest displacement from the KS line; these samples also have a relatively reduced δD range of values (−65 to −98%). However, if supergene processes take place in the presence of waters of meteoric origin at temperatures similar to typical surface temperatures, the clays thus formed should plot either in the vicinity of the KS line or be displaced towards lower δO18 and higher δD, depending on both the temperature and relative proportion of clay to water.

Fourier transform infrared spectroscopy and thermal analyses of kaolinitic clays from South Africa and Cameroon

“…Compared to Mayouom kaolins, utilised for white firing ceramics and refractories Nkoumbou et al, 2009), exothermic temperatures for MK 1 and 2 were much lower whereas the rest of the samples recorded comparatively higher values (Table 2). With the exception of MK 1 and MK 2, exothermic temperatures of the studied samples were close to that of Mavaiela kaolins, used for bricks, tile and paper (Saviano et al, 2005;Ekosse, 2010). Apart from MK content and weak H-O-H bands.…”

“…Apart from MK content and weak H-O-H bands. The lack of symmetry displayed by the DSC curves, kaolinite dehydroxylation temperatures (500 -650 °C) together with goethite dehydroxylation reactions observed in the studied kaolins, suggests moderate degrees of crystallinity (Saviano et al, 2005). According to Ekosse (2008), oxides and other associated clay minerals (e.g mica and smectite) may equally have influenced the reaction dynamics.…”

“…The dehydroxylation temperature of most kaolinites ranges from 498 °C to 605 °C, whereas the dehydroxylation of dickite and nacrite occurs at 680 °C. According to Ece et al (2003) and Saviano et al (2005) well-ordered kaolinites have broad endothermic peaks at approximately 590 °C however, reported endothermic peaks for well-ordered kaolinites from Mayouom at 520 -The positions and assignments of the transmittance bands of representative samples from South Africa and Cameroon compared against that of theoretical kaolinite is summarized in Table 1. Three characteristic peaks at 3688/3687 cm -1 , 3651/3652°C°C cm°C -1 and 3690/3692 cm -1 were observed in ZEB 1, ZEB 2, DUT 1 and DUT 2 ( Fig.…”

“…4, 5 and 7 respectively) was attributed to dehydroxylation reactions of goethite (α-FeO.OH) resulting in the formation of hematite as represented by the equation below: 522 °C. Conversely, major weight losses during TG experiments are typically observed between 400 and 600 °C (Saviano et al, 2005).…”

Assessment of kaolin resources using electrical resisitivity method in part of Ekiti state southwest Nigeria

“…The geological setting of the kaolin deposits in lower Cretaceous fluvial Chaswood formation, Nova Scotia, was interpreted from high resolution seismic-reflection profiles and boreholes, and mineralogical studies on one reference borehole [4]. It has been documented by [5] the use chemical analyses, X-ray diffraction, differential thermal analysis, scanning electron microscopy and isotope analyses to model kaolinite occurrences in Southern Angola. Electrical geophysical evaluation of Kaolin in part of Ekiti state, Nigeria has been recorded by [6].…”