Mineralogical Characteristics of Greywackes at Lake Bosomtwe Area in the Ashanti Region of Ghana

The mineralogical content and texture of greywackes in the Lake Bosomtwe area of Ghana’s Ashanti Region are examined in this petrographic study. The Graywackes are fine- to medium-grained, poorly sorted, firmly indurated sedimentary rocks. Quartz is the dominant framework component of the studied specimens, and it is both monocrystalline and polycrystalline. The average mineral compositions of the samples are quartz (53%), feldspar (16%), biotite (4%), opaque minerals (2%), and lithic fragments (1%) and classified as feldspathic wacke. The other predominant minerals include orthoclase and plagioclase feldspars. Other constituents include biotite, opaque minerals, and iron oxides. The characteristics of an advanced sedimentary environment with moderate mobility are evident in the rocks. The discoveries broaden our understanding of the Ashanti Region’s geology and have ramifications for resource development, environmental management, and geological study.

Introduction

Greywacke is a sedimentary rock consisting of poorly sorted angular to sub-angular grains of quartz, feldspar, and lithic fragments (wide variety of dark rock and mineral fragment) set in a compact clay-rich matrix. It is characterized by its hardness. Greywacke is usually formed from sediments deposited in submarine avalanches or from strong turbidity currents, creating mixed-sediment slurries [1]. The formation cycle is a result of the rock cycle of erosion, transportation of eroded materials by water current deposited on sea floors, and the pushing up by plate tectonics. The formation of greywacke usually reflects an environment in which erosion, deposition, transportation, and burial were so rapid that chemical weathering did not occur [2]. Greywacke is sometimes referred to immature sandstone due to the angular to sub-angular nature of the edges of the lithic fragments that make up the rock. Currently, a number of scientific disciplines are quite interested in studying urban environments. Urban environments are changing as a result of human activities such urbanization, carbon emissions, and biodiversity changes [3]. In the context of safety and environmental protection in all aspect research and development underscores the importance on operational sustainability and highlighting the environmental and financial implications of different protection methods [4]–[8]. This emphasizes the significance of continuous environmental assessment to protect the health and livability of urban and rural residents [9]–[17], [26]–[33].

The success of this work will help establish the mineralogy of the exact rock types that occur at the Lake Bosomtwe area. It will also help improve the geology of Ghana.

The main aim of this research is to determine the mineral composition of the greywacke in the Bosomtwe area. The specific objectives are:

1. to determine the mineralogy of the rock and
2. to determine the texture.

Description of the Study Area

Ghana is made up of sixteen (16) administrative regions. One of these regions is the Ashanti region, whose capital is Kumasi. There are thirty administrative districts in the Ashanti Region. The study area falls in the Bosomtwe District, the district capital of Kuntanase. The Bosomtwe District is in the central part of the Ashanti Region and is found within latitudes 6° 24′ N and 6° 43′ N, and 1° 15′ W and 1° 46′ W. To the north of the Bosomtwe District are the Kumasi Metropolitan Assembly, Ejisu-Juaben to the east, Amasie to the south, and Atwima-Kwanawoma District to the west. The total land surface area of Bosomtwe District is approximately 422 km², with a population density of 275.2 per km² [18].

The lake Bosomtwe, found in the Bosomtwe District, is a lake formed by an ancient meteorite strike in the Ashanti Region. The impact crater measures 10.5 km in diameter, which is partially larger compared to the current lake’s diameter of 8 km. It is believed to be approximately 1.7 million years old. The depth of the crater is approximately 380 m but about 750 m with the depth of sediments. The lake rises as a result of heavy rains filling the crater with water.

Local Geology

The Paleoproterozoic supercrustal and intrusive rocks and the Neoproterozoic to early Cambrian lithologically diverse system sediments are the two main lithostratigraphic complexes that are present in the research area [19]. The Paleoproterozoic rocks consist of the Birimian Supergroup (made up of Birimian sediments and volcanic). In the Bosomtwe District, the Birimian consists of phyllite, schist, greywacke, and tuff.

Greywacke

The primary constituents of greywacke, a sedimentary rock, are feldspar, quartz, and lithic pieces of dark rocks and minerals [20]. Its hardness, black hue, and tiny rock of lithic pieces mixed in a compact, fine-grained clay and muddy matrix are its defining characteristics. It also contains poorly-sorted angular quartz and feldspar grains.

Greywacke can be categorized into three groups according to the proportion of main components of quartz, feldspar, and rock fragments:

1. Lithic greywacke containing less than 95% quartz and more rock fragments than feldspar.
2. Feldspathic greywacke containing less than 95% quartz and more feldspar than rock fragments.
3. Quartz greywacke containing greater than 95% quartz [1].

Mineralogy

The main elements of Greywacke are mostly quartz, feldspar, and lithic fragments. Depending on the sandstone’s mineralogical maturity, other minerals could potentially be present [21], [22]. Based on these minerals, research on the provenance of sandstones is possible. The following are the common minerals and fragments in sandstone:

Quartz

Quartz is one of the major minerals found in greywacke. The reason for this includes:

1. Quartz is mechanically resistant due to its high hardness and cleavages,
2. Chemically, quartz is persistent under circumstances discovered at the earth’s surface [23], [24].

Quartz, as mentioned earlier, is a major found in greywacke and has the chemical formula SiO₂ with a hexagonal crystal system. Quartz can be present as either monocrystalline grains or polycrystalline grains. Sand-sized polycrystalline quartz is a stronger indicator of a metamorphic origin, particularly when there are more than five distinct crystals. Quartz occurs in virtually in every color with a colorless streak [25], [26]. It does not cleave but undergoes conchoidal fracture with a hardness of seven on the Mohs scale [27].

Monocrystalline quartz is composed of a single-unit boundary. Polycrystalline quartz are single quartz grains composed of two or more optically different quartz crystalline units but looks like a single grain under polarized light [28], [29]. It is an important tool for identification of source rock [30]. Most polycrystalline quartz is larger than monocrystalline quartz. In the present study of polycrystalline quartz, they commonly exhibit two crystals per grain with straight to unclosed extinction. Undulose extinction in optical mineralogy is the irregular or wavy extinction seen in thin sections of mineral grains on rotation. As the microscope stage is rotated, individual grain mineral appears black when polarization due to the minerals prevent any light from passing through.

Feldspar

This is another major constituent of greywacke found in most igneous and metamorphic rocks, although less stable than quartz at conditions near the earth’s surface. Feldspar has a hardness of 6 to 6.5. It is a silicate of sodium [31], [32], potassium, calcium, or a combination of these elements. Feldspar in greywacke consists of:

• Plagioclase: The name of a group of feldspar minerals that form a solid solution series ranging from Albite to Anorthite with an increasing Calcium (Ca) and a decreasing sodium (Na) content as you move from Albite to Anorthite. The plagioclase is often used instead of one of the more specific names of the individual types of plagioclases. This is because the minerals of the plagioclase series are very similar and difficult to tell apart without laboratory testing. Plagioclase has two directions of perfect cleavage [33].
• Orthoclase: This is a feldspar mineral, which is a member of the alkali feldspar series and one of the most abundant rock-forming minerals of the continental crust. It can also be called potassium feldspar or K-feldspar with the chemical formula KAlSi₃O₈, making it belong to the silicates. This feldspar has a hardness of 6 in the Mohs hardness scale. It has perfect cleavage in two directions intersecting at 90°, which is a diagnostic property of it.

Lithic Fragments

Any type of rock particle might be observed in greywacke. These components present in the greywacke give the best clues to provenance. But some lithic fragments may be more than others due to the various reasons:

• Areal extent of the source’s outcrop region where the lithic bits are generated. It is more likely to appear in sediment made from that source the larger the outcrop region that is the source that produces the lithic particles.
• The drainage basin’s location and relief. Lithic fragments from the source are more likely to be identified in the sediment if they are situated near the depositional basin. Thus, erosion rates will be higher in a source location with significant topographic relief because lithic particles from the source are more likely to be found in the sediment.
• Rock fragment stability in a sedimentary environment. Mineral found in the source rock with low stability under conditions present on the earth’s surface will be rare in the sandstone formed. However, granitic fragments are more common in sandstones due to being stable under conditions present near the earth’s surface.
• Crystal size in the fragments. A lithic fragment in sandstones requires the mineral grains in it to be smaller than the sediment grains in order for it to be present.

Accessory Minerals

Given that the degree of mineralogical maturity in greywacke may determine the provenance of any given mineral, sandstone can also contain an extensive number of other minerals. These minerals can be generally called heavy minerals (which include zircon, homblend, garnet, epidote, topaz, etc.).

Methodology

The research flow chat has been adopted, as seen in Fig. 1.

Fig. 1. A research methodology flow chat (Modified after [35]).

Sampling

Geological field mapping was carried out in the Abono geographic area and along the lake Bosomtwe. Fifteen (15) representative samples were collected in polythene bags and with the help of a geological hammer. The collected samples were then taken to the laboratory for thin section preparation and for petrographic analysis as well.

Thin Section Preparation

The process of making a thin section of a rock involves several steps and equipment. The rock sample is initially cut into a portable size using a cut-off saw, creating a flat surface. The slab is then ground with various sandpaper grit sizes to eliminate projections and achieve a smooth surface. Glass slides are frosted and ground to enhance epoxy adhesion. Epoxy is mixed and applied to the polished side of the rock slab, and a frosted glass slide is carefully positioned to avoid air bubbles. After 24 hours of curing, a clip of rock firmly attached to the slide is obtained. The excess clip is cut off using a thin section machine and trim saw, and the slide is labeled. The final step involves grinding the slide to a thickness of 30 microns to obtain the desired thin section.

Petrographic Analysis

After the thin sections had been prepared, they were studied under a petrographic microscope to determine their texture and modal composition. Lecia (DM750P) was the microscope used for the petrographic analysis. The samples were also critically analyzed with a hand lens before cutting. Modal percentages of the selected samples were reckoned using the comparison chart of [34].

Results and Discussion

Petrography

The greywackes sampled from Abono are fine to medium grained and poorly sorted. They are grey in color, with the majority of their grains being sub-angular to sub-rounded and firmly indurated. Quartz and feldspar (plagioclase and orthoclase) are the major minerals present, with very little or no rock fragment. Quartz is the predominant mineral in the greywacke. The two types of quartz (monocrystalline and polycrystalline) in the greywacke exhibit straight extinction and undulatory extinction.

Quartz

Quartz (Fig. 2) is the most abundant mineral in the greywacke, accounting for an average of 53% of the total framework grains. Generally, quartz is sub-angular and occurs as monocrystalline and polycrystalline quartz. Monocrystalline quartz is the most occurring quartz, with an average percent of about 73%, while polycrystalline quartz has the remaining 27% of the total quantity of quartz. The two broad types of detrital quartz in the greywackes exhibit undulatory extinction and straight extinction.

Fig. 2. Monocrystalline quartz in (A) PPL and (B) XPL, a polycrystalline grain of quartz with more than two crystal units in (C) PPL and (D) XPL. Tcf6t.

Feldspar

Feldspars are the second leading minerals found in the greywacke. Two major kinds of feldspars were identified namely orthoclase and plagioclase feldspars of which some were identified being fresh and others deformed.

Orthoclase Feldspar

Orthoclase (Fig. 3) appears to be similar to quartz but exhibits a diagnostic property of confetti of dirt or sparkles of dirt under crossed polarized light (XPL) giving evidence of been deformed to resulting in sericitization. Its grains are sub-angular to sub-rounded indicating it has travelled an appreciable distance away from source.

Fig. 3. Grain of sub-rounded orthoclase viewed in (E) PPL and (F) XPL.

Plagioclase

Plagioclase (Fig. 4) is the dominant feldspar present in the greywacke samples. The grain observed were sub-angular to sub-rounded indicating it has not been transported very far from its source. Both fresh plagioclase and moderately deformed ones are present. In the fresh state well preserved polysynthetic twinning, that is the composition surface are parallel to one another was much conspicuous. Whereas in the deformed state, plagioclase has been moderately altered making the polysynthetic twinning less conspicuous showing evidence of sericitization. In most cases, plagioclase exist as phenoclast.

Fig. 4. Moderately deformed plagioclase showing less conspicuous polysynthetic twinning in (G) PPL and a grain of plagioclase in its fresh states with well-preserved polysynthetic twinning in (H) XPL.

Biotite

Biotite (Fig. 5) present is sporadic with cleavage planes altered with pleochloric haloes within them. Alteration of biotite usually begins from the edges and move gradually inside. Chloritization is rapid. Evidence of this were the light green colors seen in most of the grains under plane polarized light.

Fig. 5. Grains of biotite viewed under in (I) PPL and (J) XPL.

Opaque Minerals

Grains of opaque minerals (Fig. 6) in the rock appear to be sporadic ranging from sub-angular to sub-rounded. Viewing the opaque minerals both under plane polarized light and cross-polarized light, they always appear dark and does not change in color upon rotating the stage. The reason for this is that the opaque minerals block light passing through them.

Fig. 6. Opaque mineral viewed in (K) PPL and (L) XPL.

Iron Oxide

Iron oxides (Fig. 7) occur sometimes as cementing material in the greywacke. It appears as a reddish brown in color found mostly on the boundaries of the minerals. The reddish brown color may be as a result of water or moisture reacting with the edges of the loosed packed grains, leading to the formation of oxides. The iron oxide usually anastomoses the quartz grains.

Fig. 7. Photomicrographs of iron oxide in (M) PPL and (N) XPL, anastomosing the quartz grains.

Discussion

The greywackes found in the Lake Bosomtwe area of the Ashanti Region, Ghana, are subject to a detailed petrographic study, revealing a nuanced mineralogical composition and texture. The predominant minerals are quartz and feldspar, specifically orthoclase and plagioclase, with additional components such as biotite, opaque minerals, and iron oxides. Quartz, both monocrystalline and polycrystalline, constitutes a significant portion of the rock, demonstrating features like straight and undulatory extinction, indicative of its mechanical durability and potential metamorphic origin. Feldspar, particularly plagioclase, displays fresh and moderately deformed grains, with well-preserved polysynthetic twinning in the former and evidence of sericitization in the latter (See Table I). Biotite occurs sporadically, exhibiting altered cleavage planes with pleochloric haloes. Opaque minerals, appearing dark and unresponsive to light, are interspersed sporadically, while iron oxide acts as a cementing material, anastomosing quartz grains and contributing to the rock’s reddish-brown color. The overall texture of the greywackes is characterized as fine to medium-grained, poorly sorted, and firmly indurated, indicating maturity and resistance to weathering. The sub-angular to sub-rounded shapes of grains suggest moderate transportation, and lithic fragments offer valuable insights into the provenance and characteristics of the source rocks.

Table I. Calculated Modal Percentages of the Greywacke

Sample	Qmt (%)	Qpt (%)	Qt (%)	Ft (%)	Lt (%)	M (%)	Bt (%)	Opq (%)	Fe2O3 (%)	QFL (%)			QmFL (%)
										Q	F	L	Qm	F*	L*
ROJ 01	37.5	12.5	50	10	1	30	4	2	3	82	16	2	77.4	20.6	2
ROJ 02	35	15	50	15	1	20	5	4	5	76	22	2	68.6	29.4	2
ROJ 03	35	15	50	15	1	25	3	2	4	76	22	2	68.6	29.4	2
ROJ 04	44	11	55	10	1	20	8	2	4	83	15	2	80	18.2	1.8
ROJ 05	35.8	29.2	65	10	0.5	17	3	2.5	2	86	13	1	77.3	21.6	1.1
ROJ 06	36	24	60	15	0.5	20	1	2	1.5	79	20	1	69.9	29.1	1
ROJ 07	39	21	60	18	1	15	2	2	2	76	23	1	67.2	31.1	1.7
ROJ 08	32.5	17.5	50	14	0.5	30	2	1	2	78	21	1	69.1	29.8	1.1
ROJ 09	36	4	40	8	1	45	3	1	2	82	16	2	80	17.8	2.2
ROJ 10	29.8	5.2	35	12	1	45	3	1	2	73	24	3	69.7	28	2.3
ROJ 11	22.5	7.5	30	10	1	50	4	3	2	72	26	2	67.2	29.9	2.9
ROJ 12	36	4	40	12	1	40	3	2	2	75	23	2	73.5	24.5	2

Conclusion and Recommendation

Conclusion

A thorough understanding of the mineralogical composition of the greywackes in the Ashanti Region’s Lake Bosomtwe area is provided by petrographic investigation. The predominance of orthoclase and plagioclase feldspars together with monocrystalline and polycrystalline quartz highlights the sedimentary origin of these rocks. The mineralogical diversity is further enhanced by the presence of iron oxides, opaque minerals, and biotite. A mature sedimentary environment is indicated by the texture, which is notable for having fine to medium-grained, poorly sorted, and firmly indurated features. The varied mineral composition and textural features provide information on the origins, geological history, and modes of transit of the greywackes. This information is essential for deciphering the Ashanti Region’s intricate geological evolution, advancing more general geological research, and possibly guiding resource development and environmental management in the region.

Recommendation

1. To ascertain precise ages and obtain knowledge about the time of sedimentation and geological events, take into account isotope research, such as radiometric dating.
2. A sedimentological research should be done to provide a comprehensive understanding of depositional environments by analyzing grain size, sedimentary structures and bedding features.