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Mineralogy and Texture of the Weathered Products from Silicic Lavas in the Sabga Area, North West Cameroon

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Mineralogy and Texture of the Weathered Products from Silicic Lavas in the Sabga Area, North West Cameroon International Journal of Geosciences, 2021, 12, 605-624 https://www.scirp.org/journal/ijg ISSN Online: 2156-8367 ISSN Print: 2156-8359 Mineralogy and Texture of the Weathered Products from Silicic Lavas in the Sabga Area, North West Cameroon Edith Etakah Bate Tibang1*, Gideon Anoma Waji2, Elisha Mutum Shemang3, Cheo Emmanuel Suh1,2 1Department of Geology, Mining & Environmental Science, Faculty of Science, University of Bamenda, Bambili, North West Region, Cameroon 2Economic Geology Unit, Department of Geology, University of Buea, Buea, South West Region, Cameroon 3Department of Earth and Environmental Sciences, Botswana International University of Science and Technology, Palapye, Botswana How to cite this paper: Tibang, E.E.B., Abstract Waji, G.A., Shemang, E.M. and Suh, C.E. (2021) Mineralogy and Texture of the Wea- Products of weathering usually clay minerals, are commonly characterized thered Products from Silicic Lavas in the through mineralogical, chemical and geochemical analysis, emphasizing their Sabga Area, North West Cameroon. Inter- implications in industrial applications, initiation of landslides and their im- national Journal of Geosciences, 12, 605-624. pact on surface water geochemistry. In this study the vertical variations in https://doi.org/10.4236/ijg.2021.126034 textural characteristics and clay mineral type within weathered profiles in the Received: May 13, 2021 Sabga area were investigated. Two exposed weathered profiles were logged Accepted: June 27, 2021 from bed rock to topsoil and each horizon was sampled separately. Granulo- Published: June 30, 2021 metric analysis on the samples indicated mixtures of clay, silt and sand size Copyright © 2021 by author(s) and particles in all horizons. Smectite and kaolinite were identified by X-ray dif- Scientific Research Publishing Inc. fractometry (XRD) and scanning electron microscopy (SEM). The two logs This work is licensed under the Creative show that the lower horizons display whitish colors with fine laminations, Commons Attribution International middle horizons are brownish and the upper horizons display dark brown License (CC BY 4.0). colors. Graphic mean values for both sections gave values that range between http://creativecommons.org/licenses/by/4.0/ φ φ Open Access 2.02 - 3.7 with an average standard deviation value of 1.9 , indicating that the grains are poorly sorted sand, silt and clay. SEM micrographs show laths of smectite in the lower horizons, flakes of kaolinite and laths of smectites in the middle horizons while the upper horizons show flakes of kaolinite with microlites of quartz + feldspars. XRD patterns show broad basal reflections for kaolinite at 2θ 36.8˚ (3.15 Å), smectite at 2θ 33.5˚ (3.34 Å), both conta- minated with quartz at 2θ 36.5˚ (3.32 Å) typical for these minerals. Keywords Mineralogy, Texture, SEM, Clay DOI: 10.4236/ijg.2021.126034 Jun. 30, 2021 605 International Journal of Geosciences E. E. B. Tibang et al. 1. Introduction The weathering of rocks is the most fundamental geomorphic process which modifies the Earth’s surface, and it is one of the vital processes in soil mineral cycling. The intensity and products of chemical weathering in tropical regions are controlled generally by climatic factors (e.g., variations in temperature and rainfall), chemistry, mineralogy, texture and structure of parent rocks, drainage, topography and time [1] [2]. Despite being controlled greatly by climatic factors (variations in temperature and rainfall) [1], is afunction of the chemistry, mineralogy, texture of the parent rock, drainage system and topography, structures, vegetation andtime [2]. For instance studies carried out on weathered profiles of basaltic rocks from the French Massif Central indicated the presence of iddingsite from forsterite, iron oxyhydroxide from augite and halloysite from plagioclase feldspars [3]. [4] stu- died tropical weathering of recent basalts from Cameroon and described the evolution of the weathered product from parent rock through allophone, halloy- site, metahalloysite to gibbsite, as a function of climate, drainage and time. On well drained, upper and middle sections of slopes on suitable rocks, kaolinite develops. On the contrary, downslope or in hindered drainage conditions, kaoli- nite is gradually replaced by illite/smectite [5]. With the dependence of chemical weathering on the parent rock, the products of weathering on silicic and mafic lavas will differ to a greater extend, since their chemical compositions also differ [1]. For instance study carried out on wea- thered basalts in Eastern Australia revealed that the minerals altered to a mixture of allophanes, iron oxyhydroxides and clay minerals [6]. [7] showed that Quartz, feldspars, micas and opaque minerals that make up the mineral constituents of silicic lavas (rhyolite to dacite) alter to smectite, illite, kaolinite, chlorite and ep- idote. The variability of the geological basement of Cameroon and its location has favored the development of thick weathering profiles [8] [9]. These profiles are generally characterized by the presence of clay deposits which are some of the most important industrial materials as millions of tons are utilized in a vast va- riety of applications including ceramics, oil drilling and the paper industry. Nu- merous studies have been carried out on these weathered profiles particularly along the Cameroon Volcanic Line (CVL: e.g., [9]-[16]). These studies have ex- plored the products of weathering, usually clay deposits, with focus on characte- rizing the type of clay minerals, emphasizing their implications in medicinal and industrial applications including ceramics, oil drilling, pharmaceutical and the paper industry. Most of these works focused more on geochemical and minera- logical quantification of homogenized weathered products without laying em- phasis on the bed rock type, vertical variations in texture characteristics and clay mineral type within weathered profiles. Other weathering-related studies have examined the weathering on igneous rocks under specific environmental and climatic conditions with the aim of evaluating their implications in landslides DOI: 10.4236/ijg.2021.126034 606 International Journal of Geosciences E. E. B. Tibang et al. development [17] [18]. This study aims to show vertical variations in texture, clay mineral type and their morphology within weathered profiles on rhyolitic rocks in the Sabga area located in the North West Region Cameroon. This allows us to have a better understanding of the systematic evolution of clay minerals within weathered profiles developed from silicic volcanic bed rock. As previous studies in the area also focused on clay mineral characterization on homogenous clay deposits de- veloped on trachyte without paying attention to vertical variation of the clay minerals. X-Ray Diffractometry (XRD) analyses and the scanning electron mi- croscopy (SEM) are used to identify the clay minerals and their morphology in the various horizons. 2. Geologic Setting The Sabga region is part of the Bamenda Mountains an extinct volcano of West- ern Cameroon highland [19] which in terms of volumetric importance is the fourth largest volcanic massif along the Cameroon Volcanic Line (CVL) (Figure 1). The CVL is a near 1700 km, SW-NE trending, African intraplate “fan shaped”, alkaline volcano-plutonic rift zone of variable width (<200 km), with a ca. 66 Ma history of magmatic activity without any systematic internal pattern of age vari- ation [20]-[25]. The CVL structure is a consequence of a series of parallel fis- sures oriented N30˚E and some transversal events [26]. The study area lies midway between Mt. Bambouto to the South-west and the Oku Massifs in the North-east, all being volcanoes along the central part of the continental sector of the CVL. It is inferred to be underlain by older Paleoproterozoic and Pan-African metagranitic basements [25] [27] [28] which is covered by sequences of alternating felsic and mafic units [25] [29]. The felsic lavas are more abundant than their mafic counterparts and are comprised dominantly of rhyolitic lava flows, trachytic, dacitic and rhyodacitic flows as well as welded ignimbrites. The mafic units are mainly alkalis basalts, hawaiites and basanites [25]-[32]. The rhyolitic ignimbrites consist of welded and non-welded massive lapilli tuff and lithic breccias [33]. The welded ignim- brites vary in colour ranging from welded grey, welded greyish brown and dark grey units [25]. The felsic lavas are massive, blocky, moderately to deeply wea- thered, occur as tongue-like lava flows (coulee), and as domes. The lavas vary in colour from light-grey, grey, dark-grey, dark green to black. They are highly lo- calised and comprised of both coherent and autobrecciated facies which occur at the base and top of coherent facies. The coherent facies are characterised by rhyo- litic flow banded and non-banded varieties. Hand samples indicate the presence of dark inclusions, millimetric to centimetric flow bands which are rhythmic and either planar or highly contorted forming flow folds. Some bands display an alignment of vesicles whereas others show alignment of alternating dark and light colour variations (Figures 2(a)-(c)). Microscopically the coherent lava facies are flow banded, porphyritic with DOI: 10.4236/ijg.2021.126034 607 International Journal of Geosciences E. E. B. Tibang et al. Figure 1. Location of the Sabga area along the Cameroon the Cameroon Volcanic Line (CVL). (a) Location of the study area along the Cameroon Volcanic Line with
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