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KAOLIN DEPOSITS FROM THE NORTHERN SECTOR OF THE CUNENE ANORTHOSITE COMPLEX (SOUTHERN ANGOLA)

¹ Dipartimento Ingegneria Chimica dei Materiali, delle Materie Prime e Metallurgia, Università degli Studi di Roma "La Sapienza", Via Eudossiana 18, 00184 Rome, Italy
² Dipartimento di Scienze della Terra, Università di Siena, Via delle Cerchia, 2, Siena, Italy
³ Department of Mineral Engineering, Faculty of Engineering, "A. Neto" University, C.P. 1756 Luanda, Angola

View larger version (56K): [in a new window]   Figure 1. Geology of SW Angola and the location of the studied kaolinite deposit (main excavation): (1) red sandstone and clay of the Kalahari Formation, Cenozoic; (2) red granitoids; (3) quartzite and conglomerate of the Humpata Formation, Upper Proterozoic; (4) Cunene anorthosite complex; (5) gneiss, migmatite, granitoids of the ‘Basal complex’, Upper Archean; (6) study area.

View larger version (43K): [in a new window]   Figure 2. Schematic geological map of the Mevaiela area. (1) Cunene anorthosite complex; (2) occurrences of kaolinite; (3) clay and sandy clay, sometimes with fibrous calcite-kaolin nodules; (4) faults; (5) main excavation; (6) outcrops of kaolinite.

View larger version (30K): [in a new window]   Figure 3. Geological cross-section (not to scale) showing the kaolin outcrops, the drill-hole locations, and sampling.

View larger version (25K): [in a new window]   Figure 4. XRD patterns: (a) represents a quarry sample (CAO15), (b and c) are from the kaolinized outcrops (CAO4 and CAO5), pattern (d) (anorthosite) is for the enclosing rocks. Note the good crystallinity of the sample CAO4 from the outcrop.

View larger version (19K): [in a new window]   Figure 5. DTA curves for: (a) the main-excavation sample (CAO15), (b) the kaolinized outcrop (CAO4), and (c) the radiating calcite nodules (CAO6).

View larger version (105K): [in a new window]   Figure 6. SEM images (secondary electrons) of kaolin samples. (a,b) Unprocessed kaolinite, with the former showing the natural fabric of the rock and coherent kaolin books, and the latter showing the weathering of feldspars into small elongate tubular kaolinite. (c–f) Samples processed by dispersion in water, filtering and subsequent coating with a thin graphite film. (c) >125 µm size fraction. Note the curved and short linear kaolin books. (d) 125 µm size fraction. Fewer and smaller kaolin crystals near books (bottom right). Note the single plates and the scalloped or slightly-crinkled edges of the clay flakes. (e) 75–125 µm size fraction. Rolled-up plates of what is probably halloysite, elongated, and small kaolinite plates. (f) 38–75 µm size fraction. Small, randomly oriented clay plates.

View larger version (6K): [in a new window]   Figure 7. Isotopic variation with depth; SM6, SM13 and SM15 are boreholes in the main excavation (see Figure 3).

View larger version (11K): [in a new window]   Figure 8. Current climatic data from IAEA ISOHIS Station of Malange and Menongue (Angola), plotted along with the Global Meteoric Water Line (GMWL) and the isotopic data from kaolinite of Mevaiela.

View larger version (9K): [in a new window]   Figure 9. D vs. 18O SMOW plot for kaolin samples from the study area (modified after Hassanipak and Eslinger, 1985). See text for details.