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Lead Reservoirs and Metallization in Nigeria: Example of a Contrasting Geological Terrains

Silas Dada, Onema Adojoh*, Adisa Rahaman, Ibrahim Garba

University of Liverpool, School of Environmental Sciences, Department of Geography, Earth and Ecological Sciences, Roxby Building, Room 805 B, L69 7ZT, UK

Abstract: Whole rock and mineral extracts from Nigerian goldfields of late Proterozoic age are significantly more radiogenic than those from the much younger Mesozoic lead-zinc mineralization in the Benue Trough, their common formation by hydrothermal processes notwithstanding. Their marked contrasts in lead isotope compositions can be attributed to differences in source reservoirs, structural settings and geological history rather than their age of mineralization. The Mesozoic sulphides have a remarkable uniformly unradiogenic composition for their age, placing the deposits in the lower crustal geotectonic environment that indicate a source of much lower µ. In contrast, mineral extracts from the southwestern and northwestern Nigeria gold deposits show increasingly radiogenic composition and plot in the mixed and upper crustal geotectonic fields respectively, typical of sources enriched in U/Pb and Th/Pb ratios, with clear indication of larger upper crustal reservoir of much higher µ. Therefore, exploration efforts may need to emphasize near-source mineralization prospects of low radiogenic Pb composition for economic deposits.

Keywords: lead reservoirs, metallization, U/Pb and Th/Pb ratios, Benue trough, economic deposits

Received March 29, 2015; Revised April 07, 2015; Accepted April 13, 2015

Citation: Silas Dada, Onema Adojoh, Adisa Rahaman, Ibrahim Garba, Lead Reservoirs and Metallization in Nigeria: Example of a Contrasting Geological Terrains. Journal of Geosciences and Geomatics, 3 (2). 28-36. January 2015. doi:10.12691/jgg-3-2-1

1. Introduction

Auriferous fields within the Gondwana continental landmass in Nigeria and NE Brazil include Archaean to Early Proterozoic crystalline cores of the West African and Congo cratons, the Pan-African mobile belt between the cratons and their Phanerozoic cover rocks (Figure 1) (Dada, 1995).

Figure 1. Pre-drift Proterozoic belts and their Phanerozoic cover rocks between the cratons (Dada 2008)

The heterogeneous nature of the late Proterozoic continental collision thermotectonic event, particularly its all-pervasive structural and metamorphic obliteratory effect on the pre-existing older rocks, has necessitated the deployment of several approaches in the elucidation of its pre-drift evolution (Dada, 1989). Younger Phanerozoic cover rocks also host hydrothermally processed mineralization whose mineralizing fluids are related to deep seated fractures in older crystalline rocks (1995, Dada).

There are no reliable isotopic ages for Nigerian gold deposits but structural correlations suggest that gold mineralizing events are intimately related to the N-S trending semi-ductile to brittle strike-slip faults formed during late Proterozoic orogen-parallel wrenching (Garba, 2000). The kinematics indicate dextral and sinitral movements that are coeval with the formation of sulphide-bearing quartz veins and the late phase Pan-African Sn-Nb-Ta bearing pegmatites. In this study, Plumbotectonics modeling, which appears to be a function of the tectonothermal age of the crust (Rudnick & Goldstein, 1990), is used in constraining the Pb isotopic reservoirs of the diachronous deposits; particularly because the ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are indicative parameters for the source of the metals. The less radiogenic Pb isotopic composition of much younger Mesozoic Pb-Zn deposits of the Benue Trough (Maurin & Lancelot, 1987) contrasts the radiogenic Pb data presented herein from much older Upper Proterozoic metasedimentary terrains.

2. Geological Setting

Overlying the Archaean gneissic complex in Nigeria are widespread Proterozoic metasediments, which include variable volcanic components, well developed as elongated N-S trending belts in the western half of Nigeria. There are no affirmative ages of formation for metasediments in Nigeria and the Borborema province of NE Brazil. Inferred depositional ages for the Palaeoproterozoic series in these areas straddle from 2.2 to 2.0 Ga and predate the 1.9–1.8 Ga igneous activities (Grant, 1970; Rahaman & Emofurieta, 1983; Rahaman & Lancelot, 1984; Jardim de Sa et al., 1987; Annor, 1995; Neves et al., 2006). In addition to the well developed medium to high grade schists, the Neoproterozoic series of mainly low grade, weakly deformed metasediments occur in association with the thermal aureole of Pan-African granitoids and show upper greenschist to low pressure amphibolite facies metamorphism. More recent work on the same series in NE Brazil (Van Schmus et al., 2003; Neves et al., 2009), their structural relationships with Pan-African granitoids as well as their unequivocal reworking suggest a Neoproterozoic age of deposition. Gold and other metallic deposits occur in these rocks in Brazil and Nigeria and the Pan African is widely accepted as the minimum age of gold and associated mineralization in this belt.

Neoproterozoic granitoids in Nigeria consist of several contemporaneous petrologic groups. Their formation ages are well constrained between 640 and 580 Ma (Tubosun et al., 1984; Dada & Respaut, 1989; Dada et al., 1989; Rahaman et al., 1991). Pb isotope geochemistry on whole rocks and feldspars collected from plutons in north central Nigeria have been used to describe the evolution of the magmatic processes involved in their formation and the Pb reservoirs defined (Dada et al., 1995). The well defined Pan-African deformation pattern in the Nigerian schist belts has been related to gold and associated mineralization.

Figure 2a. Major areas of gold mineralization in the schist belts of western half of Nigeria. AFS Anka Fault System, KF Kalangai Fault, IF Ifewara Fault. 1 Maru, 2 Kwali, 3 Anka (Zuzurfa, Kuba), 4 Malele, 5 Bin Yauri, 6 Tsohon Birnin Gwari, 7 Kwaga, 8 Gurmana, 9 Okolom-Dogondaji, 10 Iperindo, after (Garba 2003)

Figure 2b. Geological map of Ife-Ilesha schist belt showing the location of Iperindo goldfield (Caby & Boesse 2001).

2.1. Mesozoic Sediments

Mesozoic sediments of Late Aptian to Maastritchian age are well developed in the Benue Trough subsequent to the opening of the south Atlantic. The coastal basin that developed in the South America and African landmasses developed a triple junction in the Guinean region with the formation of inland basin along the Benue with lead, sphalerite, brine and other associated mineralization. The origin and material sources of the widespread mineralization has been a subject of intense study and research. Available Pb-Pb isotope data is used in our comparative analysis with similar data from other geological terrains in Nigeria. We discuss our present understanding of metal sources in Nigeria; using new and pre-existing Pb-Pb data of ore pyrite, galena and chalcopyrite of the quartz-sulphide-gold veins from deposits in the Nigerian schist belts and the galena-sphalerite deposits of the Benue Trough (Figure 2 & Figure 3) in order to constrain the local source rocks of lead, identify the isotopic composition of the main Pb reservoirs, and by analogy, of the mineralization.

3. Methodology

3.1. Isotopic Data

Pb isotope data include recent analyses on sulphides from northwestern Nigerian goldfields carried out at the Canadian Activation Laboratories, existing data of Maurin & Lancelot (1987) on lead-zinc mineralization from the Benue Trough, with comparisons using granitic rocks from northern Nigeria (Dada et al., 1995), Iperindo goldfield Oyinloye, 2006) and aplites from Ibadan (Oversby, 1975).

Figure 3. Mesozoic Pb-Zn and related mineralization in the Nigerian Benue Trough, after (Burke et al. 1972)

3.2. Identification of Source End-members

Lead in sulphides of most ore deposits have been derived from underlying basement rocks and they give information on age and average Pb composition of such underlying basement and their constituents (1990, Rudnick). Nigeria consists of an amalgam of contrasted terrains with well developed metasedimentary rocks known as schist belts to the west and widespread crystalline, essentially granitic rocks under Mesozoic to Tertiary cover rocks to the east (1984, Rahaman). The entire Nigerian basement has been reworked during thermotectonic Pan-African orogeny (600 Ma), during which prominent strike-slip shears developed along-strike extensions southwards into the Borborema province of northeast Brazil (Dada, 2008). These two provinces obviously have several geological similarities including tectonic, mineralization, age, etc; parameters that are germane to our discussion in this study. Field evidence indicates that both the Proterozoic vein-gold mineralization in the metasediments and the Mesozoic galena-sphalerite and associated vein mineralizations in the Benue Trough are epigenetic (Ogunleye, 2003; Offodile, 1980; Maurin & Lancelot, 1987). Large mineralized veins are not common in the Nigerian Precambrian and are represented only by far-between quartz-sulphide gold and cassiterite-tantalite-bearing pegmatites and quartz veins that contain scheelite in association with talc schists in central Nigeria. On the other hand, the Nigerian Mesozoic Benue Trough is well endowed with solid minerals of great diversity, including metallic and industrial minerals. In general, there is direct correspondence between the Pb composition of the source rock and the ore deposits. We have therefore used the lead isotope compositions in Pb-Pb isotope diagrams (Figure 4, Figure 5, Figure 6 & Figure 7) of K-feldspars from basement rocks and sulphides from ore deposits to suggest possible geodynamic reservoirs.

Figure 4a. ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb of various rocks in Nigeria with the 600 Ma reference isochron

Figure 4b. ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb of various rocks in Nigeria with the average growth curve of Stacey & Kramers, SK (1975)

3.3. Lead Isotope Data

On ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb diagrams (Figure 4, Figure 5, Figure 6, Figure 7), the lead isotope data for whole rocks and mineralization plot in a series of echelon clusters along linear arrays. Geographical trends can be contoured (Isoplumbs) with increasing radiogenic whole rock lead to the southwest (Figure 4). Apart from those from northwest goldfields, all mineral extracts including K-feldspars plot along a linear array, regardless of the age of their host rocks. The only exception from northwest gold is the galena sample from Malele. This most probably implies that all other deposits were derived from source rocks of the same age that had initial Pb isotopic compositions similar to that preserved in Pan-African feldspars. When samples from NW goldfields are exempted, the slope of the linear array (0.1785) on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb gives an approximate age of 2.01Ga. This is likely to be a crustal residence average mixing age indicative of older source for both sulphide and K-fedspar lead. However the tectonic reservoirs involved have remained undefined. This has implications for exploration.

Table 1. Pb isotope composition of basement rocks from northern Nigeria

4. Discussion

4.1. Comparative Metallogenesis

Since we are interested in both the nature of the lead source and the identification of probable metal source rocks, the data presented in Table 1 & Table 2 give a comparison of ore-lead and rock-lead. The genetic and prospecting consequences are very obvious, particularly for the epigenetic hydrothermal deposits under study. However, we have decided to compare the ore-lead with the Pb isotopic composition in the feldspars, which unlike the host rock-lead, denotes a phase with low U/Pb (µ) and Th/Pb (W) ratios; whose isotopic composition will not continue to evolve. The galena samples from the Pb-Zn deposits of Abakaliki and Zurak areas plot in the same field with feldspars of the Pan African granitoids (with µ varying from 9.65 to 10.30). They have a remarkable uniformly unradiogenic composition for their age, although the Zurak group is slightly more radiogenic than the Abakaliki set. This places the deposits in the lower crustal geotectonic environment (Figure 7) of Zartman and Doe (1981), indicating a source of much lower µ. In contrast, mineral extracts from the northwestern Nigeria gold deposits plot distinctively in the upper crustal geotectonic field typical of sources enriched in U/Pb and Th/Pb. This is a clear indication of larger upper crustal reservoir with much higher µ. The pyrite samples from Malele and Tsohon Birnin Gwari are expectedly more radiogenic than the bulk galena/chalcopyrite from Gurmana and the Malele galena extracts. Even taking the galena sample, often taken as representative of their gold deposits (Ayuso et al., 2005), the northwestern Nigeria gold deposits must have derived from highly radiogenic sources as shown in both the uranogenic and thorigenic diagrams (Figure 4 to Figure 7). Examples of geologically related but dissimilar lead-isotope provinces have been identified in the galena-bearing ore deposits of south-western New Mexico (Stacey & Hedlund, 1983) and western and central Europe (Koeppel, 1984).

Table 2. Pb isotope composition of mineral extracts from various Nigeria geological terrains. ML are data from Maurin et Lancelot, 1987

Figure 5a. 207Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb of various mineral extracts in Nigeria with the 600 Ma reference isochron

Figure 5b. ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb of various mineral extracts in Nigeria with the average growth curve of Stacey & Kramers, SK (1975)

4.2. Constraints on Mineralizations

Sulphide ores containing galena, sphalerite, pyrite, and chalcopyrite that commonly preserve the initial, primitive isotopic composition of the ore fluid occur in both Proterozoic metasedimentary rocks of the Nigerian schist belts and in Cretaceous sediments of the Benue Trough. We infer the provenance of ore metals in precious (gold) and base (lead-zinc) metal deposits from lead isotope data from the Proterozoic schist belts and the Cretaceous of the Benue Trough. Our data on sulphides (Table 1 & Table 2) from Proterozoic goldfields of NW Nigeria show that their Pb compositions are much more radiogenic (²⁰⁶Pb/²⁰⁴Pb=19.30-19.94, ²⁰⁷Pb/²⁰⁴Pb=16.09- 16.98, ²⁰⁸Pb/²⁰⁴Pb=39.05-41.09) than those of K-feldspars from Pan African granitoids (Dada et al., 1995). This precludes the establishment of a direct genetic connection with the Pan African ganitoids. In comparison, pyrite from Iperindo goldfield in southwestern Nigeria have less radiogenic Pb composition (²⁰⁶Pb/²⁰⁴Pb=18.24-18.35, ²⁰⁷Pb/²⁰⁴Pb=15.72-15.74, ²⁰⁸Pb/²⁰⁴Pb=38.09-38.18). On the

other hand, Pb isotope compositions of sulphides from the younger Cretaceous Pb-Zn mineralization in the Benue Trough are even more significantly unradiogenic (²⁰⁶Pb/²⁰⁴Pb=17.98-18.13, ²⁰⁷Pb/²⁰⁴Pb=15.56-15.68, ²⁰⁸Pb/²⁰⁴Pb=38.29-38.67) and compare with K-feldspars from Pan African granitoids (Maurin & Lancelot, 1987). This led the authors to conclude for lead derived from a source strongly depleted in uranium such as detrital feldspars eroded from basement of Pan African age. The remarkable difference in isotopic compositions of the diachronous deposits cannot be explained by their age differences alone. In fact, crustal influences generally increase in younger ore associations (Bielichi & Tischendorf, 1991) in many geological settings and it would appear more logical for Cretaceous mineralizations to contain more radiogenic Pb than those of the Proterozoic. The nature of the sources for the deposits and probably the character of the ore-forming processes require further elucidation. While mineralizing fluids were mostly derived from devolatilization of the Pan African collisional upper crustal slabs, the higher radiogenic character of the lead in sulphides of the goldfields would suggest a source of upper crustal affinity. This reinforces the stable isotope (³⁴S, ¹³C, ¹⁸O) evidence for an upper crustal reservoir for the metals (Garba, 2000). Available U-Pb, Sm-Nd and Rb-Sr studies have revealed that NW Nigeria is underlain by Archaean basement (Bruguier et al, 1994, Dada, 1998) and their metasedimentary rocks contain a high proportion of Archaean-derived material. Such involvement of older rocks as source rocks for gold in Nigeria that was subsequently implicated in the Pan African thermotectonic event and mineralization is not in doubt.

4.3. Tectonic Environments and Implications for Exploration

Figure 6 & Figure 7 show the distribution of isotopic composition values (Table 1 & Table 2) in the curves for average Pb isotope evolution in the four Plumbotectonic domains of Zartman & Doe (1981). It is evident that the sulphides from northwestern goldfields and the K-feldspars from Ibadan aplites have a large component of ancient upper crustal material in the make-up of their rocks and mineralization; and rocks of Archaean age have been reported in both areas (Oversby, 1975; Bruguier et al, 1994, Dada, 1998). Further south in the southwestern Iperindo gold district, the data plot well below those of the NW goldfields and therefore indicates a much smaller involvement of older sialic material. The Iperindo sulphides plot below the Stacey and Kramers (1975) curve (SK) in Figure 5b and since the SK curve corresponds to the average orogene curve of Zartman & Doe, they can at best be regarded as haven derived from a mixture between upper mantle and crustal leads.

Figure 6. Plot of Pb isotopic composition of various rocks as in Figure 4. in the global geotectonic environments (Zartman & Doe, 1981). A=mantle, B=orogen, C=upper crust, D=lower crust

Data from the galena samples from the Abakaliki and Zurak Pb-Zn mineralization indicate a much smaller older basement component. The assertion of Maurin & Lancelot (1987) is corroborated by the isotopic evidence of Dada et al. (1995) and the contrasting evidence on Figure 7 that indicate upper mantle origin of subsequent evolution in the lower crust, without significant contribution from Archaean sources. Plutonic rocks responsible for mineralization in several geological terrains have been shown to have similar lead isotopic composition to that of the least sulphides in such regions (Stacey & Hedlund, 1983); with the consequence that the least radiogenic leads are often favourable indicators for large ore deposits (Doe & Stacey, 1974; Zartman, 1974). Exploration in Nigeria should therefore target and emphasize such near-source deposits of low radiogenic lead where erosion has not removed significant sedimentary and metasedimentary cover rocks.

Figure 7. Plot of Pb isotopic composition of mineralisation as in Figure 5 in the global geotectonic environments (Zartman & Doe, 1981). A=mantle, B=orogen, C=upper crust, D=lower crust

5. Conclusions

The development and evolution of the Nigerian crust from Archaean took place in several stages with the reworking of the older crust throughout the Proterozoic. Existing Sm-Nd, U-Pb, Rb-Sr and Pb-Pb data (Dada, 1998, 2008) indicate that a significant greater component of older material was incorporated into the metasedimetary pile in the western half of the country than into the eastern part during ancient subduction processes. Unradiogenic sources in the mantle but preferably in the lower crust have been inferred for the formation of Pan-African granitoids (Dada et al., 1995); and represent the most probable source reservoir for the less radiogenic Mesozoic Pb-Zn mineralization in the Benue Trough (Maurin & Lancelot, 1987). The reworked late Pan African pelites and phyllites of the upper crustal Younger Metasedimentary suite, that host the mineralized vein-quartz, are hereby interpreted as the ultimate source for the more radiogenic Pb of the western auriferous terrains. The isotopic characteristics of mineralization in Nigeria therefore have no direct relationship with age of formation but to structural settings and geological history. Pb-Pb isotope systematics suggests that metal sources are essentially from the crust implicated in well-defined structural discontinuities by hydrothermal processes during both the Pan African and Mesozoic. The correlation between radiogenic factor in Pb and size of deposit remains to be confirmed in the light of the low level of exploration activity.

Acknowledgments

Several people have contributed to this study, notably the Isotope Research groups at the University of Science and Technology (USTL), Montpellier and University of Paris 6 & 7 (IPGP) as well as Nigerian colleagues, including AOO (UNAD) with whom we had useful and stimulating discussions.

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