Heavy Mineral Distribution in the Lokoja and Patti Formations, Southern Bida Basin, Nigeria: Implications for Provenance, Maturity and Transport History

In total, 13 sediment samples (six from Lokoja Formation and seven from Patti Formation) were subjected to heavy mineral analysis. The analysis was carried out at the Sedimentological Laboratory, Department of Geosciences, University of Lagos, Nigeria.

Heavy mineral analysis in this study followed the procedure of Suzuki [20] and Mange and Maurer [21] for heavy mineral separation. The heavy liquid, Bromoform (CHBr3) with a density of 2.9 g/cc and viscosity of 0.068 poises was used. The samples were air-dried for a week to dispel any moisture as they were collected during the rainy season. Thereafter, the sediments were gently disaggregated by squeezing between fingers and filter paper or mortar and pestle in the case of hard samples to liberate individual grains. Each sample of 70 g was weighed and sieved to obtain 62.5–500 microns size grains. The individual sample was then poured into the bromoform and stirred thoroughly to free the samples of air bubbles. The particles were allowed to settle for about 20 minutes, stirring periodically to prevent the particles from adhering to funnel wall. The heavy crop was repeatedly washed in excess acetone and distilled water and air-dried and labelled. In all, 13 permanent mounts were made on slides using coupled resin. Mineral identification on the basis of their optical properties as proposed by Mange and Maurer [21] and Lindholm [22] was conducted on the samples.

More than 100 grains were counted from each slide for statistical analysis. Rock fragments, unidentifiable grains as well as authigenic and opaque minerals were excluded from the total sum to obtain uniform, comparable data on transparent assemblages for the characterization of mineralogical provinces. The sum of transparent minerals was recalculated to a value of 100% and the abundance of each heavy mineral species was scaled accordingly. The “ZTR” index which is the combined percentage of zircon, tourmaline and rutile among the non-opaque heavy minerals, omitting micas and authigenic species was calculated using [23] formula below:

ZTR index was calculated for the samples to ascertain their mineralogical maturity. According to Hubert [23], ZTR index <75% implies immature to sub-mature sediments and ZTR >75% indicates mineralogically matured sediments.

Heavy mineral concentration in the Lokoja and Patti Formations is classified into opaque and non-opaque constituents. The opaque components generally referred to as iron stained heavy minerals require further chemical analysis to allow investigation under petrographic microscope. This chemical analysis, however, is beyond the scope of the present investigation and no further serious attention apart from statistical consideration is accorded to this class of heavy minerals herein. The recovered non-opaque components common to the two formations include the following: zircon, tourmaline, rutile, garnet, staurolite, epidote, kyanite, titanite, lawsonite, cassiterite, sillimanite, hornblende, hypersthene and andalusite (Plates 1–14). Kyanite, cassiterite, hypersthene, staurolite, hornblende and lawsonite show rare occurrences in the Lokoja Formation, except in a sample (Lok2 S6) that has significant amount of hornblende. Also, Kyanite, titanite, hornblende and garnet show rare occurrences in the Patti Formation, kyanite occurs only in PAT S7 sample. The proportion of heavy mineral recovered to the volume of sediment analysed is generally low and may not support any significant economic prospect. Information on the proportion of the heavy minerals recovered from the two formations is provided in Tables 1 and 2. Also, Tables 3 and 4 show the calculated ZRT% index results.

The summary of the identification/classification criteria and the significant characteristics of each of the recovered heavy mineral are as follow:

Zircon: Zircon grains (Plates 2, 3 and 6) are prismatic, rounded to sub-rounded and sometimes contains fluid and mineral inclusions. Prismatic grains frequently showed zonation identified by fine bands parallel to the crystal boundary (Plate 2). The colourless varieties are more in abundance than the coloured varieties. The grains are easily identifiable owing to their very high relief and they are surrounded by black halo (Plates 2, 3 and 6). They show weak pleochroism and strong birefringence.

Tourmaline: Tourmaline grains (Plates 1, 2, 4 and 5) are prismatic, elongated, oval-shaped, spherical, euhedral, sub-hedral to irregular in shapes, with terminations at one or both ends of prismatic variety. Moderate relief, mineral inclusions (Plate 9) and overgrowths were frequently observed (Plate 1). Colours vary from pale to pink, dark green (Plate 4), pale yellowish, dark brown (Plate 4), very dark (almost opaque, may be iron bearing; Plates 1 and 5) and sometimes colourless. Colour zoning is frequent, pleochroism sharp and distinctive. Varicoloured varieties were also present (one portion of the grain displays striking different shades to the other).

Rutile: Rutiles (Plate 7) appeared as small and sub-rounded slender prisms with well-developed terminations or breakage patterns. A thick halo surrounds the grains because of their extremely high refractive indices. Grains are mostly brownish red to brownish black in colours and show distinct pleochroism (Plate 7).

Kyanite: Kyanite grains (Plates 8 and 12) are angular, or prismatic, dominantly colourless, weakly pleochroic (uneven coloured pleochroism) and exhibit characteristic cross-fractures and step-like features (Plates 8 and 12). The step-like uneven thickness resulted in a spectacular arrangement of the interference colours appearing in blue and grey on thicker and thinner parts, respectively.

Sillimanites: Sillimanites (Plates 9, 11 and 13) are long, slender, prismatic or irregular in shapes. Sillimanites show distinct cleavage, pleochroic (pale brown to pale yellow), parallel extinction, shows brilliant second- and third-order interference colours with yellow, green and pink as a dominant shades; slight twinkling is noticeable on rotation.

Andalusite: Andalusite (Plates 2–4, 6 and 7), mostly colourless and frequently enclose carbonaceous impurities. The rare manganoan variety, viridine which is green in colour was observed (Plate 4). They occur as prismatic, sub-rounded to angular or of irregular morphology; cleavage traces are poorly displayed. Some grains are non-pleochroic, whereas others display inhomogeneous pattern of pleochroism. Due to their irregular shape and uneven thickness, they display disorderly patterned interference colours. Weak interference colours are second-order orange and greenish blue (Plates 2–4, 6, and 7).

Garnet: Garnets (Plates 1, 3–6), easily identifiable because of their high relief and isotropic nature are euhedral, rounded, sub-rounded or irregular grains with uneven or conchoidal fractures. Colours vary from pinkish brown, pale red, pale pink to colourless.

Titanite: Titanite (Plate 12) grain has a high resinous lustre. Relief shows a slight twinkling on rotation of the stage. The grain has a weak pleochroism and good cleavage. High-order interference colours in golden yellow and yellowish white.

Staurolite: Staurolite (Plates 4, 11 and 14) occurs as irregular or angular grains. Colour ranges from pale yellow, golden yellow to dark yellowish brown. They exhibit distinct pleochroism (colourless to different shades of yellow).

Epidote: Epidote (Plates 2, 6 and 8) occurs as irregularly shaped grains, yellowish green to green coloured with fairly high relief. They exhibit distinct pleochroism (colourless, pale yellow, yellowish green).

The percentage proportions of the non-opaque heavy mineral constituents (Figures 4 and 5) in Lokoja and Patti Formations show the dominance of zircon. Tourmaline is the second most abundant of the recovered minerals. Epidote comes next in the Lokoja Formation with staurolite in the Patti Formation. Epidote is present in all the samples of the Lokoja Formation but only in three (3) samples of the Patti Formation. Staurolite was recovered in three (3) samples each of the Lokoja and Patti Formations. The minerals with the lowest percentage proportion are cassiterite, staurolite, kyanite and hypersthene in the Lokoja Formation, whereas the lowest in the Patti Formation is kyanite. Cassiterite, lawsonite and hypersthene are totally absent in the Patti Formation. In the heavy mineral stability index of [24,25], hornblende, hypersthene and andalusite are classified as unstable, epidote, kyanite, garnet, sillimanite, titanite, lawsonite, cassiterite as moderately stable, garnet, staurolite as stable and zircon, tourmaline, rutile as ultra-stable minerals. Consequent to the proportion of the constituent heavy mineral in the studied sections, the sediments of the Lokoja and Patti Formations are considered generally stable as they are dominated by ultra-stable and stable minerals.

Figure 4

Figure 5

The calculated ZTR% index for the two formations ranges from 30.2% to 69.4% in the Lokoja Formation (Figure 6), whereas in the Patti Formation, it ranges from 28.4% to 76.0% (Figure 7). All, except one sample from the Lokoja Formation, have ZTR% index of <50%. This low ZTR index indicates that the samples of the Lokoja Formation are mineralogically immature. The ZTR index of the Patti Formation shows only slight improvement with two samples having ZTR >50% and three having ZTR% index of <50%. The mean percentage of ZTR index for Lokoja Formation is 42.8%, whereas that of Patti Formation is 54.0%. These show the comparative mineralogical maturity advantage of the sediments of the Patti Formation over their Lokoja counterparts.

Figure 6

Figure 7

Generally, very few rounded grains were encountered in all the samples studied (Plates 1–14). Most of the grains are either angular or sub-angular, suggesting that the sediments are from nearby sources and may not have been transported far away from weathering location. Hence, indicating textural immaturity of the sediments of both formations.

The abundance of ultra-stable and stable heavy minerals in an assemblage of texturally immature sediments grains (angular to sub-angular) is suggestive of the prevalence of chemical weathering of the source rocks contributing the heavy minerals to the studied formations.

Based on the constituent minerals in the studied sediments, mixed source ranging from high-to low-grade metamorphic and non-metamorphic sources is suggested. The possibility of contribution from low-grade metamorphic and non-metamorphic sources is indicated with dominance of zircon and tourmaline [26] in the two formations. Mange and Maurer [21] have linked the presence of lawsonite, hypersthene, hornblende, cassiterite, kyanite, andalusite, sillimanite, titanite, epidote, staurolite, garnet to high-grade metamorphic terrain. Also, Diekmann and Kuhn [27] attributed the dominance of low maturity heavy mineral such as green hornblende and garnet to high-grade metamorphic source. Rutile is generally linked to high-grade metamorphic source.