Quartz is increasingly considered a strategic mineral in the world market because it represents the raw material for special applications in high-tech industries [1]. High purity quartz ( HPQ ) is the prime source of silica and it has a vast range of applications: in the production of metallurgical silicon, used in the manufacturing of solar panels, specialty chemicals, special filaments, breast and tissue implants, and other optical applications including fiber optics [1] [2] [3].

HPQ is often thought of as a relatively easily available commodity but it is very rare in economic and near-economic quantities. Most deposits around the world occur in quartz-rich granitic pegmatite and hydrothermal quartz veins [4] [5]. However, very few deposits are suitable in volume, quality, and amenability to tailored refining methods for specialty high purity applications (the Purs Pine Deposit North Carolina USA) [2]. The demand for HPQ is increasing strongly due to the rapid development and expansion of the HPQ consuming industries [6] [7].

There is a great price margin between low-quality quartz, which costs 15 to 20 US$ per ton, and HPQ which costs 2000 US$ per ton. Quartz is considered to be high purity when it contains < 50 µg per gram of structurally incorporated trace elements [2] [8] [9]. Transformation of raw quartz into refined high-purity products involves several refinement steps which need to be adapted to effectively minimize the specific impurities of the individual raw quartz feed to comply with stringent end-use specifications [6]. Indeed, refining seeks to achieve HPQ with total impurity levels of less than 20 ppm, thereby creating a highly valuable raw material that commands up to 5 EUR/kg [2].

Impurities within quartz crystals (intercrystalline impurities) are the principal control of the quality of quartz, although they can be removed to some extent by processing. Such impurities include lattice-bound trace elements, submicron (<1 µm) inclusions, mineral and fluid micro-inclusions (>1 µm). In addition, intercrystalline impurities may occur along grain boundaries in the form of mineral coatings or micro-crystals.

The commercial concentration requirements for critical elements which are detrimental to the HPQ products vary considerably. Thus, the definition of HPQ from a commercial point of view is based on the concentration limits of elements that interfere with the quality of melted HPQ products, which are predominantly silica and quartz glass products [1]. Classified HPQ is based on the sum of the concentrations of nine elements (Na, K, Li, Al, Ca, Fe, Ti, B, and P) analyzed on quartz crystals (single grain analysis) or processed quartz sand (bulk product analyses) to be <50 µg per gram.

Today, the US-based Unimin Corp./Sibelco still dominates the global high purity quartz market from operations in North Carolina [8]. One of the few alternative suppliers, Norsk Mineral’s Norwegian Crystallites, has been producing high purity quartz from its Drag plant in western Norway and several underground and open-pit mines since mid-1996 when the company changed ownership. Potential new entrants into the high purity quartz world market are still under development such as the quartz resource in the Serra de Santa Helena Formation, Brazil [8], and a few more. The size of the worldwide market is estimated at 30,000 tons per year for the 99.99% purity and above material [2] and the present producing mines have an estimated shortfall of about 15,000 tons. There is consequently a need to find new sources of HPQ, especially in sub-Saharan African countries where the use of solar panels is expected to grow exponentially in the not-too-distant future. This, therefore, is the pivotal motivation for this study.

Recently, large quartz veins have been encountered in basement rocks of the Supe area of SW Cameroon [10]. However, the nature of the veins and the purity of the quartz are not well known as this area is still relatively unexplored although earlier attempts to map this region have been reported from the Tombel Graben by [11] and [12] [13]. The present study sets out to characterize rutile/tourmaline-bearing quartz veins in the Pan-African belt of SW Cameroon in the Supe area. This requires napping of quartz veins and their host rocks with emphasis on the structural context, petrography, and their mineral inclusions therein, and investigation of the potentials of extracting HPQ from these veins by EMPA analysis. Understanding the nature of these veins will enhance the regional interpretation of principal stress orientation during polyphase deformation and allow the evaluation of the purity of the quartz and the origin of their mineral inclusions.

The Supe area is part of the Pan-African mobile belt in SW Cameroon (Figure 1). This region is part of the Neoproterozoic fold belt (NFB) of Cameroon, a major part of the Neoproterozoic Braziliano belt [14] [15] and it is affected by the Central African Shear Zone (CASZ) of Braziliano/Pan-African age (600 ± 50 Ma), that is a pre-drift extension of the Pernambuco shear zone in NE Brazil [14] [16]. This transcontinental shear zone is a major crustal discontinuity extending from NEBrazil into Cameroon (Figure 1(a)).

Movements along the Pan-African structures have created basins such as the Lom Basin [16] and a series of shallow grabens such as the Tombel Graben SE of the Supe area. The Tombel Graben is one of a series of grabens that defines the alternation of horsts and grabens along the Cameroon Volcanic Line (CVL) [17], which is a linear array of numerous volcanic centers and anorogenic complexes [18] [19] [20]. It has been postulated that the linear trend of volcanoes along the CVL is structurally controlled [21] [22] by Braziliano/Pan-African structures

that predate the onset of volcanism. Indeed, the CVL is affected by a trans-lithosphere fault zone which can be traced to pre-date the opening of the South Atlantic Ocean, in Brazil as the Pernambuco dextral transcurrent fault system [23]. The basement of the continental sector of the CVL is composed of Precambrian and Pan-African age rocks. Syn-tectonic alkaline, granites, crystalline schists, and gneisses with rare syenites, gabbros, and diorites belonging to Pan African Orogeny have been dated as 620 - 480 Ma [23]. These rocks outcrop mostly in the central and NW part of the CVL.

This mobile belt is known to host several mineral deposits of economic importance such as the U-Mo mineralization in the Pan-African granites of the Ekomédion prospect [27] [28], the U mineralization at Kitongo, N Cameroon [29], the detrital rutile in the Yaounde region, the lode gold mineralization at Dimako-Mboscorro, SE Cameroon [30], and the Belikombone hill gold prospect (E) Cameroon [31] and references therein.

Lithologic units of the Supe area are made up of three major rock types: metamorphic rocks that cover ~65% of the area and that are dominated by gneiss, igneous rocks covering ~30% and consisting mainly of basalt, and sedimentary rocks that cover ~5% of the area (Figure 1(c)).

The major metamorphic rocks in this area are gneisses, migmatites, and schists. The gneisses are generally light-colored, coarse-grained, and have granoblastic textures. Their main minerals are biotite, muscovite, quartz, and feldspar (alkali feldspars and plagioclase) in various proportions. These minerals are commonly aligned in a parallel manner to each other forming alternating light and dark bands of 0.5 - 1.5 cm thick but occasionally without a clear banding. The migmatites mostly occur in association with gneisses. They are commonly light-colored, and coarse-grained with minerals that include quartz, feldspars, and micas. They are moderately foliated with very thick bands of ~7 cm in the outcrop. The schists encountered here are light-colored with continuous schistosity and they are made up of muscovite, biotite, quartz, and feldspar, which are strongly aligned.

Foliation surfaces (Figure 2(a) and Figure 2(b)) were mapped in gneisses, schists, and some migmatites. Data from these surface planes were plotted on stereonets (Π diagrams and β diagrams) and on rose diagrams (Figure 1(d)) showing that the gneissose, schistose, and migmatitic foliations have a NE-SW general trend between N231E-N240E. Fractures tend to crosscut the foliations at various angles although some are sub-parallel. It appears from the Π, β, and rose diagrams (Figure 1(e)) that the fractures have a dominant NW-SE trend between N131E-N140E and a somewhat less pronounced NE-SW trend between N180E-N190E. Microfaults were also encountered (Figure 2(c) and Figure 2(d)). Some of the fractures are open while others are filled by quartz veins that range from a few cm wide to about 1.5 m and about 35 - 40 m long. Stereographic projections of the conjugate fractures (Figure 1(f)) indicate a NE compression in the principal stress directions, δ₁, and a corresponding NW-SE extension direction. Folds in the area have axial planes that generally trend in a NW-SE direction (Figure 3).

Basalts are the dominant igneous rocks of the area (amygdaloidal and porphyritic basalt). They occur as flows (aa and blocky lava) and dyke-like masses with vertical and horizontal columns and joints. However, some dolerites were encountered in the contact with gneisses and migmatites. Granites in the Supe area occur as stock-like masses. They are massive, light-colored with coarse grains and a graphic texture.

Sandstones occur alternating with shales dipping in a NE direction and outcrop along river beds. The shales are dark grey, fine-grained, and fissile. The sandstones are light-colored (whitish) with poorly sorted with variable grain sizes. They are friable with sub-angular grains and minerals such as quartz, orthoclase feldspar, biotite, and muscovite. They have very little cementing material, which is probably calcite or iron oxide.

Quartz-rich veins of about 35 - 40 m long and 1 - 1.5 m wide were mapped. They have a NE-SW trend and are lensoid in shape (Figure 4). Some of the veins have yellowish quartz grains which are sugary to saccharoidal in texture and thus crumble easily. These crumbly veins have a diverging, ENE-WSW, trend and are dominated by coarser (~4 cm), more compact, and homogeneous white quartz. The quartz crystals range from clear to white with glassy to greasy lusters. Large mineral inclusions such as muscovite flakes, black slender rutile crystals, and dark-blue to black tourmaline needles are common (Figure 5). The transition into the gneiss shows a concentration of rutile and tourmaline crystals bound by a quartz matrix-forming porphyritic rutile-rich black tourmaline.

Three deformation events (D₁ - D₃) are recorded by the structures in this. D₁ is marked by the development of the strong S1 foliation defined by the gneissose layering and schistosity. D₂ is a NE-SW ductile to brittle sinistral shear that controlled the formation of tension gashes that served as subsequent pathways for quartz-rich hydrothermal fluids to circulate and eventually precipitate quartz. A D₂ sinistral shear with a distinct Pan-African signature has been reported in the CASZ portion of the Tombelgraben [12] [13] [17] which resulted in a NE-SW mylonitic foliation that is subparallel to the regional trend of the North Equatorial fold belt (NFB) and Braziliano belt. These structural similarities show that the Supe area is also part of the NFB of Cameroon, a significant contribution to the tectonics of this area.

Representative quartz samples obtained from the veins are white, coarse, compact, and homogeneous. The rock samples were crushed, and selected grains were examined under a binocular microscope, subsequently polished and coated with carbon before SEM imaging and analysis. The polished sections were analyzed using a Quanta 600F scanning electron microscope (SEM). X-ray tomography was applied at the University of Ghent, Belgium, for analysis of the internal features and the nature of the inclusions, following n [32] [33]. Hundreds

of radiographs were acquired for one scan from different rotation angles between 0˚ and 360˚. Larger samples were scanned using a Hamamatsu L9181 tube set at 130 kV, 35 W source, with down to 5 μm spot size, because of its high power; smaller samples (resolution < 3 μm) were scanned with a Hamamatsu L1711 tube set at 100/160 kV (LaB6/W filament), 3 W, down to 400 nm spot size, because of its small spot size. The combination of both tubes covers all resolutions optimally, thus producing 2-D and 3-D images of the samples.

The chemical nature and other textural features of these samples were subsequently analyzed by scanning electron microscopy (SEM) and electron microprobe analysis (EMPA) [34] [35] at Activation Laboratory in Canada. Backscattered secondary electron images (BSE) were taken from inclusion-free quartz grains as well as from those with tourmaline/rutile inclusions. By a combination of image analysis employing atomic number contrast imaging (from back-scattered electron– or BSE-signal intensity) and Energy Dispersive Spectrometry (EDS) using Bruker 5010 SDD detectors, the tourmaline/rutile and quartz were directly measured through SEM. BSE signal intensity was proportional to the mean atomic number of the minerals. A Field Emission Gun was used at an accelerating voltage of 25 kV and a spot size of 5.5. ETD imaging was also used. Electron probe microanalysis measurements were performed in one analytical session where six quartz samples were analyzed as well as their tourmaline inclusions. These measurements were performed with a JEOL microprobe set at 15 kV, with a beam diameter of 5 μm.

The following conclusions can be drawn from this study. The tourmaline inclusions have a hydrothermal origin with a metamorphic overprint while the rutile and micas might have a primary or secondary origin. A primary origin could have formed during vein crystallization within the quartz crystals (exsolution) as a result of decompression during tectonic uplift and cooling, or secondary due to hydrothermal extraction into the veins. The Supe quartz veins are metamorphic/hydrothermal in origin. The quality and quantity of the HPQ veins in this area suggest a high potential for HPQ exploration in the region.

The data presented are from parts of Arnold Chi Kedia’s master thesis supervised by Cheo Emmanuel Suh and supplemented by Afahnwie Ngambu Aloysius and Cyrille Sigue. Financial supports from coordinated institutional that grants to Elisha Mutum Shemang and Cheo Emmanuel Suh are acknowledged. The authors acknowledge support from colleagues at the University of Buea and the Remote Sensing Unit.

The authors declare no conflict of interest.

Afahnwie, N.A., Kedia, A.C., Suh, C.E., Sigue, C. and Shemang, E.M. (2022) The Potential of Quartzitic Veins in SW Cameroon for High-Purity Quartz. International Journal of Geosciences, 13, 281-302. https://doi.org/10.4236/ijg.2022.134015