Provenance Studies of Alluvial Tin Deposits in Parts of Ropp Younger Granite Complex, Northcentral Nigeria

. The study area in this research is within the Ropp Complex, part of the Nigerian Mesozoic Younger Granite province. The study aims to interpret the depositional environment and establish the provenance of the alluvial cassiterite deposits in the study area. Boreholes/mining pits were logged for this study, and stanniferous sandstone samples were collected, which were used for textural and mineralogical studies. The mineral assemblages documented in the samples include ilmenite (3 to 27%). Cassiterite (2 to 14 %), Zircon (2 to 16%), magnetite (0 to 17%), tourmaline (5 to 11%), rutile (2 to 8%) and monazite (2 to 7%). The ZTR Index calculated from the result of heavy minerals analysis for the selected pieces is 59%. Mineralogical studies revealed that quartz is the most dominant detrital mineral averaging about 93-99%, indicating that the stanniferous sandstones are compositionally matured and have experienced a high degree of chemical weathering. The quartz grains have grain sizes ranging from coarse to very coarse. They are poorly sorted, sub-angular to sub-rounded, with low sphericity. This also indicates a closeness to the source and textural immaturity. The occurrence of relatively very few feldspar grains suggests a slow sedimentation rate, very high rate of chemical weathering and composition maturity. The bivariate plots, univariate grain size parameters and probability plots, and the absence of fossils and trace fossils suggest deposition in a fluvial environment. The results of the granulometric analysis indicate that the study area's stanniferous sandstone was deposited in a fluvial environment by a low-energy fluvial (river) depositional system and the deposition in proximal (close to the source). This study suggests that the Basement complex and Younger Granite are the sources of the stanniferous placer deposits.


INTRODUCTION
Provenance studies involve the interpretation of the lithologic source of sediments and/or sedimentary rocks [21]. It is one of the key elements of basin analysis, as it provides basic information regarding the tectonic origin of a sedimentary basin using compositional and textural properties of sediments. Information about the source of sediments may be obtained from examining the various clasts present [23,1]. The most crucial aspect of provenance studies is identifying source rock, relief, the climate in the source area, tectonic settings, transport history and digenetic modifications. Provenance studies of sedimentary rocks, especially sandstones, have been carried out by many workers to extract information from compositional and textural features of sandstones, and a thorough review of the subject is given by [24]. Standard petrographical approaches to identifying source rocks of sandstone investigations of undulocity and polycrystallinity of quartz grains, types of feldspar present [23] and rock fragments [24]. The relief and climate of the source area can be inferred from grain roundness and the average degree of feldspar alteration [10]. Tectonic settings can be determined from the relative proportion of quartz, feldspar and rock fragments [6]. Clues of sandstone's transport history can come from examining the roundness and sphericity of grains and textural and mineralogical maturity [24].
Alluvial tin deposits have been reported in the Younger Granites of mostly North Central Nigeria. [27,28,22]. Some of these Younger granite occurrences have also been reported in Northwestern Nigeria. [14]. The major sources of cassiterite in Nigeria are the placer (alluvial and eluvial) deposits from the biotite granites within the Jurassic alkaline ring complex (the Younger Granites) of the Jos Plateau. Placers are probably one of the earliest types of mineral deposits to have been utilized by man. Author [2] pointed out that the tin placer deposits were rarely treated as serious topics within classical tin geology, even though [4] estimated that 80 % of the world's tin production was derived from placer deposits. The lack of understanding of tin placers may be illustrated by examining the inferred distance of transport of cassiterite from the source rock. According to [8], the median transport distance from bedrock source for economic tin placer deposits is only 8 km. Other undiscovered bedrock mineralization sources were possibly contributing en route, and the dispersion could be even shorter.
Most of the published and unpublished literature on the occurrence of tin in the Younger granite focuses on the primary mineralization of tin; this shows that only a little knowledge is highlighted about the alluvial tin mineralization in the complex even though over 95% of the tin (cassiterite) produced in Nigeria was mined in the Younger Granite Province and were won from alluvial deposits derived from the tin-bearing granites and lodes [15,16]. The rich and extensive placers of the Jos Plateau were mainly formed from the erosion of the basement rocks. During a period of elevated base level, streams aggraded their valleys and buried the placers beneath a blanket of clay and alluvium.

Study area
The study area is in parts of Ropp Complex, located southeast of the Jos Bukuru Complex in North Central Nigeria. The complex is known for its whitish riebeckite granites, extensive kaolin and tin mineralization, and long mining history. It was the second largest tin producer in the Nigerian Younger Granite Province [3]. The study ar-ea is situated in Barkin Ladi LGA within latitude 9030'00'' to 9015'00"N and longitude 8045'00" to 8052'30"E of the Federal Survey Map sheet 189 Kurra NE ( Figure 1).
The study covers an area of approximately 375 km 2 . The area is accessible by tarred roads, untarred roads, footpaths and cattle tracks. The area is marked by high topography averaging about 1350 m above sea level. The complex is bounded by extensive actuate and polygonal ring dykes, which enclose basement rocks and the prominent central massif of rugged younger granite hills. The drainage pattern of the area is radial and essentially controlled by the distribution of younger granite outcrops. The central river system in the study area is the River Kurra and its tributaries, draining into Monguna and Tente provinces, where dams are constructed.

MATERIALS AND METHODS
The methods employed in this research work were fieldwork and laboratory analyses. The fieldwork involved description, logging, measurement and sample collection of various lithologic units. Samples were collected for laboratory analyses. The laboratory work involves textural, petrographic and heavy minerals analyses of cassiterite-bearing sandstone samples.
Granulometric analysis. This analysis was used to determine and evaluate the grain size distribution and texture of selected samples of the stanniferous sandstones of the study area. Friable and unconsolidated arenaceous models were the only ones subjected to sieve analysis. The samples were first disintegrated into individual grains using the ceramic mortar and pestle. Each sample was soaked with hydrogen peroxide for about 2-3 hours in a beaker. This is done to aid the disintegration of the samples further to eliminate the effect of cementation.
Each sample was placed in a sieve shaker and sieved for about 10 minutes using standard sieve openings of 2, 1, 0.6, 0.5, 0.3, 0.25, 0.15, and 0.075 mm and pan. The percentage mass retained and cumulative percentage of the group retained for each sample were determined. The methods then computed the statistical parameters (mean, standard deviation, skewness, and kurtosis) of the grain size frequency distribution [12,18]. A probability scale was used in plotting the standard cumulative curves of grain size distribution following the procedures of [7,26].
Petrography. Thin-section petrography of the stanniferous sandstones was carried out at the geological laboratories of Ahmadu Bello University Zaria on the ten representative samples for microscopic examination of their mineralogical composition. The samples were impregnated in epoxy before cutting and mounted on glass slides using Canada balsam. The rock slice was glued to a glass slide and was ground to 30 microns thick.
The prepared slides were then labelled and examined with the aid of transmitted light under the flat stage of the petrographic microscope. The studied thin sections were described in terms of grain contacts, roundness, grain size and sorting.
Point counting. Modal analyses of the samples were carried out using Jmicrovision v1.27 Image Analyzer. In each thin section, 300 points were counted to calculate the composition. The objective of point-counting is to identify grain types (framework), cement, matrix and their proportion for each thin section. Classification of the sandstone was determined using [10,23] methods. Major detrital framework components of the sandstone quartz, feldspar and lithic fragments were used to construct a QFL ternary diagram.

Heavy mineral analysis
The heavy mineral analysis was carried out in the Sedimentology Laboratory of the Department of Geology, Ahmadu Bello University Zaria. The sandstones were sieved to obtain the very fine to fine sand fractions (4 phi), which are commonly analyzed because it is the size fraction likely to contain the highest percentage of heavy minerals. Two grams from each sample were weighed and poured into the centrifuge tubes containing 10 ml of the heavy liquid (bromoform). The centrifuge tubes were inserted in their respective holes in the centrifuge, and the cover was closed before running the machine for 10 minutes. Acetone was used to clean and deodorize the bromoform in the heavy minerals.
Furthermore, Magnetic separations were carried out to identify opaque heavy minerals. Although the Frantz electromagnetic separator may be used to separate minerals, it is slow. The sample can be contaminated by impurities such as rust and leftover minerals from the previous separation. So, a hand-magnet was used, which was invaluable in telling the difference between magnetite, ilmenite, and other opaque minerals.
Furthermore, the heavy minerals were identified and examined with a binocular microscope. The percentage of heavy minerals in each sample was determined using the Fleet method by counting all the grains in the mount (manually) [9]. Whenever possible, at least 200 heavy minerals were identified and counted from each slide, which is a sufficient number for characterizing the abundances of common species. The "ZTR" index was calculated using the percentage of the combined Zircon, Tourmaline and Rutile grains for each sample according to the formula below.

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The calculated index is expressed in percentage to ascertain the mineralogical maturity of the sediment. ZTR <75% implies immature to submature sediments, and ZTR >75% indicates mineralogical matured sediments. Apart from the ZTR index, various frequency percentage plot of both pie charts were made for each sample.

Granulometric analysis
Eleven samples were used for the grain size analysis using standard sieve analysis techniques as outlined in [11].
Grain size statistical parameters. Table 1 shows the calculated values of various grain size parameters: the mean, median, skewness, kurtosis and standard deviation of the samples. The calculation of the multiple parameters follows [11].  Kurtosis. The kurtosis expresses the peakedness of the grain size distribution. The kurtosis values of the samples range from 0.80 to 0.96 Φ, i.e. from platykurtic to mesokurtic, respectively (Table 1). The samples analyzed show a dominance of mesokurtic sediments (7 pieces). Although, little geologic information is derived from kurtosis values [24].
Probability plots. The probability plots are of environmental significance. They are indicative of either fluvial, beach, or wave zone, according to [26] characterization. Two sand populations indicate a fluvial setting, three sand populations indicate wave zone bars, and four sand populations show a beach setting. The probability curves were plotted from the data obtained from the sieve analysis. This is actually established from the plots of the cumulative weight percent against grain size in (phi, Φ).
From the Cumulative probability distribution curves of the analyzed samples, it is observed that all curves have only two sand population distributions (straight line segments).i.e. saltation and suspension. Thus, indicating a fluvial environment. [26,7,25].
Bivariate grain size parameters. Graphic Mean, Standard Deviation, Skewness and graphic First Percentile are the parameters used to separate sands based on origin according to standard plots devised by various workers. These bivariate plots include the field of mean versus the first percentile, standard deviation versus first percentiles, and standard deviation versus mean [13,19]. Plots for each bivariate plot show that 100 % of the samples fall in the river sand environment.

Petrography
Textural characteristics. Description of chosen thin sections of the stanniferous sandstone with focus on grain size, sorting, roundness, sphericity and grain contacts shows in Table 3.    Table 4.
Sandstone classification. The results from the point counting (Table 4) were used to determine the relative proportions of quartz, feldspar and lithic fragments in Table 5.   These were plotted on QFL and QmFLt diagrams to investigate provenance and highlight possible differences in sandstone composition (Figure 7). The design of the sandstones was classified in ternary QFL and QmFLt charts following the descriptive terms of [24] (Figure 7 A-B). Additionally, the same values were plotted on ternary QFL and QmFLt diagrams similar to [5] to highlight the samples' provenance (Figure 7 C-D). For the QFL diagram, all quartzose grains are plotted together, monocrystalline and polycrystalline quartz. Contrarily, the QmFLt diagram only re-gards monocrystalline quartz as quartzose grains. Thus, in the QFL diagram used in this study, lithic clasts and opaque minerals are considered lithic fragments following [5]. For the QmFLt diagram, lithic fragments include polycrystalline quartz and opaque minerals, which agrees with the QmFLt chart [5] and is similar to the classification [20].
The texture of sediments provides information about the medium, mode of transport, and energy conditions at the deposition time. The coarse to very coarse-grained nature of the sandstones encountered, bivariate analyses, univariate grain size parameters and probability plots, and the absence of fossils and trace fossils suggest deposition in a low-energy fluvial environment.
The petrological studies revealed that the stanniferous sandstones have similar mineralogical compositions and lithology types, although with few variations. The studies showed that quartz is the most dominant detrital mineral averaging about 93-99%. This is an indication that the sandstones are compositionally matured and texturally immature. It also implies a high degree of chemical weathering and closeness to the source. The quartz grains have grain sizes ranging from coarse to very coarse. They are poorly sorted, sub-angular to sub-rounded, with low sphericity. This also indicates a closeness to the source and textural immaturity. The grain contact is dominated by floating grains, with few point contacts and no sutured contact. Most grains show the presence of iron oxide rimming their edges, inclusions, and vacuoles, which could be attributed to a low-temperature source such as a hydrothermal vein. Quartz is the common silicate mineral acting as cement in the analytes, although there are a lot of patches of iron-oxide glue. These cementing materials are chemically attached to the crystal lattice of existing quartz grains forming rims of cement (overgrowths). The mineral content of cement is silica and iron-oxide. Feldspars were not observed in most thin sections, and the few presents have been significantly weathered, giving them a brown colouration. Very little feldspar suggests a slow sedimentation rate and a very high chemical weathering rate and indicates composition maturity.
The matrix is about 4 to 11% of the detrital fraction and consists of fine silty particles that appear brownish in some samples. This brownish colouration, coupled with the presence of iron oxide, is a strong indication of weathering and ferruginisation or probably due to percolation from the lateritic cover. Also, the reduction or oxidation process of the iron oxide may be responsible for this.
Judging from the ternary diagram, Both the QFL and QmFLt diagrams for sandstone classification indicates the dominance of quartz arenites, and following [5], the stanniferous sandstone samples are indicative of dominantly craton interior and few recycled orogen source area. Terrigenous sediments derived from the craton interior are more felsic, which explains why quartz minerals are more dominant.
From the study of the heavy mineral assemblages of the stanniferous sandstones of the studied area, it is found that zircon, tourmaline, rutile, cassiterite, ilmenite, magnetite, monazite and garnet occur persistently in all the studied samples. The heavy minerals were primarily derived from igneous and metamorphic rocks.
Opaque minerals (magnetite and ilmenite) indicate the chemical leaching of the sediments. Opaque minerals are mainly derived from crystalline igneous rocks, both acidic and basic. The higher proportion of opaque minerals indicates an igneous source. Therefore, the above study shows that the bulk of the heavies of the stannif-erous sandstone are mineralogical immature and have been derived from igneous rocks and a little part from low to medium grade metamorphic rocks.

CONCLUSIONS
Stanniferous sandstones of the study area are dominantly angular to subrounded, coarse to very coarse-grained, poorly sorted, positively skewed, and mesokurtic sandstone. They were deposited in a fluvial environment by a low-energy fluvial (river) depositional system, and the deposition in proximal (close to the source). The heavy mineral study shows that the stanniferous sandstone is mineralogical immature and has been derived mainly from igneous rocks and a little part from low to medium-grade metamorphic rocks.
Furthermore, the stanniferous sandstones are compositionally matured, textural immature, close to the source and have experienced a high degree of chemical weathering.