Geochemistry of Weathered Profiles over Syenite and Younger Granite in Pankshin Area , North Central Nigeria

Metallic ores of economic values often occur within profiles above basement rocks in tropical regions due to weathering and secondary enrichment. This study is carried out to determine the geochemistry and appraise the potential for metallic ore enrichment in the weathered profiles over Syenite and Younger Granite in Pankshin area. Top-soil, laterite and rock samples are collected from two lateritic profiles above Syenite and Younger Granite. The syenite rock is composed of alkali feldspars (perthite), nepheline, clinopyroxene, with minor amount of orthopyroxene, hornblende, plagioclase and few quartz grains, indicating oversaturation. The Younger Granite contains plagioclase feldspars, microcline, quartz, biotite, hornblende and reibeckite. Quartz (61.0%), kaolinite (32.0%) and microcline (7.0%) are the dominant minerals in the X-ray diffraction (XRD) of the decomposed rocks. The Chemical Index of Alteration (CIA) of both rocks is generally >90. There are enhancements of V, Sc, Zr and TiO2 in the Syenite profile and Fe2O3, Zr, V and TiO2 in the Younger Granite profile as reflected in the Accumulation Factor (AF), loss and gain of elements (K), though the values were too low for ore mineralization except for iron and titanium in the Younger Granite profile.

Weathered profiles over Syenite and Younger Granite are examined in this investigation.Metallic ores of economic values often concentrated within lateritic profiles above basement rocks in tropical regions due to supergene enrichment.This study is carried out to determine the geochemical characteristics of the lateritic profiles over syenite and Younger Granite in Pankshin area in order to appraise their economic potentials in terms of metallic ore enrichment.

Location and Description of the Weathered Profiles in the Study Areas
Profile 6 is overlying Syenite on the eastern side of Pankshin town (Figure 2).The Syenite occurs as intrusions within granite gneiss.A 3m thick profile is exposed in an abandoned laterite quarry at the back of the Local Government secretariat in Pankshin.Three horizons are identified based on their colour, textures and structures.They are the top soil, the laterite zone and the saprolite (Figure 3).The thin brownish top soil layer is 0.2 to 0.5m thick.It is rich in quartz and organic matter.The light brown laterite zone extends to a depth of about 2.5m, where it makes gradational contact with the saprolite zone as shown in Figure 3.The saprolite layer is pale yellow in colour with the textural characteristics of the parent rock well preserved.

Petrography
Syenite occurs as low-lying hills and ridges at the southeastern part of Pankshin town (Figure 2).It is grey in colour, medium to coarse grained without any evidence of deformation.The common variety at the eastern part of Pankshin is coarse and even grained.The rock is composed of alkali feldspars (perthite), nepheline, T o p so i l L at erite Sa p ro lit e W ea th ered p e g m at ite clinopyroxene, with minor amount of orthopyroxenes, bluish green hornblende, plagioclase and few quartz grains (Figure 5), indicating oversaturation.Epidote, biotite, apatite and opaque minerals occur in accessory quantity.The mineral grains are generally euhedral to sub-euhedral.
The Younger Granite is one of the most abundant rock types in the area covering about three quarters of the northeastern to the southeastern part.Most of the steeply inclined conical hills around Pankshin town are Younger Granite outcrops.The outcrops are on the average 2000m high above sea level.The rock is grey in colour and medium to coarse grained in texture.The Younger Granite contains feldspar, quartz, biotite, hornblende and reibeckite.The rock is porphyritic with plagioclase occurring as phenocryts within quartz matrix (Figure 6).

Mineralogy
The XRD of the soil and the laterites over Syenite (Figures 7a and 7b) show prominent peaks of kaolinite, microcline, and quartz.Bauxite minerals are often generated from the weathering of Syenite occurring in plateau regions, notably in India, Brazil, Australia and Arkansas in the United States of America.The dearth of bauxite minerals in the profiles around Pankshin could be attributed to the ruggedness of the terrain, which aided run-off and may not permit upward and downward alternate movement of the water table.The trend of weathering within the profile is towards iron enrichment (lateritization) rather than the expected aluminium accumulation (bauxitization) (Aleva 1994;Bolarinwa 2001 and2006).
The X-ray diffractograms of soil and laterite on the Younger Granite are illustrated in Figures 8a and 8b.They show prominent peaks of kaolinite, quartz, microcline, albite and muscovite.The presence of muscovite, albite and microcline in the weathered profile indicate incipient chemical weathering.With abundant sodic feldspars, biotite and amphiboles in the primary rock, the obvious end product of chemical weathering of such minerals in a well drained area, such as Pankshin is kaolinite.In weathering, topography and time is very important.The inability of the location to retain water for a period of time due to the nature of the slope could be partly responsible for the thin profile.

Geochemistry
The chemical compositions of the weathered profile over Syenite at Pankshin are presented in Table 1, while the average and range values are presented in Table 2 and Figure 9.The average contents of Fe 2 O 3(t) increased from 2.34% in the rock to 4.49%, and 4.94% in the laterite and soil; respectively (Table 2).The decrease in CaO, Na 2 O and K 2 O average contents from the bedrock to the soil is due to leaching of these oxides whereas, TiO 2 content slightly increases from 0.11% in the rock to 0.60% in the laterite.The total MgO + CaO average values are 2.13% in the rock and 0.27 and 0.12% in the laterite and soil layers.The Na 2 O + K 2 O content are 8.84 in the rock, 1.58 and 0.87 in the laterite and soil respectively (Figure 9).This conforms to the normal trend in chemically weathered rocks in tropical regions (Mattheis 1983, Hallberg 1984and Middelburg et al 1988).Average concentration of Ba (1179, 130, 159) ppm, Sr (206, 22, 22) ppm, Y (249, 58, 58) ppm and Be (5, 2, 2) ppm show depletions from bedrock through laterite to soil.On the other hand Sc (3, 6, 7) ppm and Zr (384, 594, 626) ppm increases from bedrock through laterite to soil.The absolute concentration of V in the rock (11 ppm) is lower compared to those of laterite (53 ppm) and soil (62 ppm).The silica-sequioxide ratio (SR) of the laterite (4.24) and soil (3.65) and the corresponding alumina-iron oxide ratio (AR) of 4.26 and 2.92 strongly suggest that the soil, which is produced from the weathering of Syenite, is non-lateritic according to the classifications of Nesbitt & Young (1984) and Aleva (1994).
Major and trace elements concentration in the profile over Younger Granite are presented in Tables 3 and 4. SiO 2 content decreases from 72.76% in the bedrock to 37.60% in the laterite and 48.42% in the soil (Table 4).Al 2 O 3 increased from 14.54% in the bedrock to 22.20% in the laterite, whereas Fe 2 O 3 content increases from 1.37% in the bedrock to 22.30% in the laterite and decrease to 20.55% in the soil due to leaching of the top soil.Iron bearing rock-forming minerals notably hornblende, biotite and ilmenite commonly release iron oxides/hydroxides during chemical weathering.Trace element data in Tables 3 and 4 show notable increment of Y (11 to 39 ppm), Zr (55 to 810 ppm) and V (26 to 242 ppm) from bedrock to the laterite.Other trace elements: Ba (537-260 ppm) and Sr (293-70 ppm) show depletion from bedrock to laterite.
Silica Ratio of the laterite (0.93) and soil (1.46) and the AR of 1.31 and 1.30 for the laterite and soil (Table 4) can be used to determine the types of soil formed from the chemical weathering of the parent rocks.According to Martin & Doyne (1927) and Nesbitt & Young (1984, 1989), true laterite is assigned a ratio of 1.33; lateritic soil 1.33 to 2.0 and non lateritic soil >2.00.Results from Tables 2 and 4 showed that the soil formed from Younger Granite with SR ratio of 1.46 is lateritic, while Syenite with 3.61 is non lateritic.The SR and AR values in this study strongly suggest that true laterite is produced only from the weathering of the Younger Granite.This is similar to results obtained by Nesbitt & Young (1984) and Aleva (1994).The total values of alkalis (MgO + CaO) is 1.77 in the rock and 0.40 and 0.21 in the corresponding laterite and soil, whereas the average values of (Na 2 O + K 2 O) is 8.07 in the rock, 0.16 and 1.03 in the laterite and soil respectively; thus indicating strong leaching of the regolith.The dominance of iron bearing minerals such as goethite and heamatite in the lateritic profiles and the high Fe 2 O 3 concentration in the chemical data suggests that the trend of weathering is towards iron enrichment (lateritization) and not aluminium enhancement (bauxitization).

Figure 1 .
Figure 1.Geological map of Nigeria showing the location of Pankshin area (After Nigeria Geological Survey Agency, NGSA, 2004)

Figure
Figure 2. G

Table 1 .
Chemical compositions of major (wt.%), trace (ppm) and oxide ratios of study profile above Syenite in Pankshin area

Table 2 .
Average chemical compositions of major (wt.%), trace (ppm) and oxide ratios of the study profile over Syenite in Pankshin area

Table 3 .
Chemical compositions of major (wt.%), trace (ppm) and oxide ratios of the study profile above Younger Granite in Pankshin area

Table 5 .
Accumulation Factor (AF), Loss and Gain (K) of elements of the study Syenite profile in Pankshin area

Table 6 .
Accumulation Factor (AF), Loss and Gain (K) of elements in the study Younger Granite profile in Pankshin area esr.ccsenet.