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New LA-ICP-MS U-Pb Ages, Lu-Hf Systematics and REE Characterization of Zircons from a Granitic Pluton in the Betare Oya Gold District, SE Cameroon

Kevin I. Ateh*, Cheo E. Suh, Elisha M. Shemang, A. Vishiti, Enerst Tata, Nelson N. Chombong

Economic Geology Unit, Department of Geology, University of Buea, P.O. Box 63 Buea, South West Region, Cameroon

Abstract: A combination of whole rock geochemistry, Ti-in-zircon thermometry, geochronology and Lu-Hf isotope composition of zircon is employed in this study to depict the source of a granitoid from the Bétaré Oya Gold District, its formation temperature, age of emplacement and evaluate the role of petrogenesis and magmatic evolution of the granitic melt in hydrothermal fluid circulation and primary gold precipitation. In this contribution, zircon grains from a granitic pluton were analyzed for their internal structures using cathodoluminescence imagery and dated by U-Pb technique using LA-ICP-MS. Based on the zircon internal structure, magmatic and metamorphic zircons are distinguished. The granitoid reveal a mean age of 635 Ma similar to those obtained from granitic intrusions along the Central African Shear Zone (CASZ) in Cameroon. This age defines a narrow Pan African emplacement age for the pluton and a unique melting event synchronous with magmatism and deformation. It also depicts a new and older mineralization along the CCSZ at 620-635 Ma for the Bétaré Oya Gold District, SE Cameroon. Ti-in-zircon thermometry indicates their emplacement at a modal temperature range between 625°C and 775°C. Based on whole rock geochemistry alongside trace and REE composition of zircon, the pluton shows a granodioritic to tonalitic affinity. The granitoid is sub alkaline with a high K calc-alkaline affinity, peraluminous and of I-type. REE in zircon patterns display high Ce concentrations, negative Eu anomalies, HREE enrichment, ¹⁷⁶Lu/¹⁷⁷Hf ratio < 0.022 and negative εHf values that range from -5.29 to -0.12. Positive εHf values suggest a mafic crustal contribution. Across the Atlantic into NE Brazil, mineralization is often associated to late Pan African event. We disclose an early mineralization event at 635-620Ma in the region.

Keywords: Granitoid, geochemistry, Ti-in-zircon thermometry, zircon chemistry, geochronology

Citation: Kevin I. Ateh, Cheo E. Suh, Elisha M. Shemang, A. Vishiti, Enerst Tata, Nelson N. Chombong, New LA-ICP-MS U-Pb Ages, Lu-Hf Systematics and REE Characterization of Zircons from a Granitic Pluton in the Betare Oya Gold District, SE Cameroon. Journal of Geosciences and Geomatics, 5 (6). 267-283. January 2017. doi:10.12691/jgg-5-6-2

1. Introduction

Several hydrothermal ore deposits are formed within and at some distance from felsic to intermediate igneous intrusions although a direct genetic link with these intrusions is not always clear [1]. It is therefore necessary to constraint the ages and petrogenesis of plutons within metallogenic belts to allow for comparison of such felsic bodies with other intrusions associated with ore deposits worldwide. Skarn deposits in NE Brazil have often been related to granitic intrusions within a meta-sedimentary sequence (marble, quartzite and schist) characterized by late Braziliano-Pan African mineralization (W, Au, Mo, Pb-Zn, Sn). However across the Atlantic into the central African region the mineralization - granitoid relationship still remains obscure. Zircon is a stable mineral that can crystallize over a wide range of magmatic temperatures, essentially depending on the bulk composition of the host magma [2] and could be sourced from different rock types with different elemental compositions. The composition of zircon (trace/REE, HFSE, U, Th concentrations) and the shape and slope of chondrite-normalized patterns bear characteristic features of zircon grains of a particular magmatic system [3]. The chemical composition of zircon is used to determine various magmatic processes such as fractional crystallization, interaction with hydrothermal fluids and/or magma mixing [4]. The ability for zircon to incorporate trace and rare earth elements such as U, Pb and Hf in its crystal structure makes it ideal for geochemistry, geochronology and isotopic studies. According to [5], Lu-Hf systematics on zircon grains is of great significance when combined with the U-Pb method as it gives the possibility to characterize isotopically the host magma from which they crystallized and unravel the history of crustal growth. Cathodoluminesence Back Scattered Electron (CL-BSE) images of zircon grains reveal its chemical zonation and the internal growth structure essential in understanding the evolution of the magmatic system.

The eastern region of Cameroon is the most prospective area for gold in the country (Figure 1). This gold district extends beyond its borders into the Central African Republic. Research in this region has focused on understanding the style of mineralization [6]. Previous studies in the area also highlight the significance of granitic plutons in the genesis of gold mineralization [7,8]. According to [7] granitoids from the Batouri Gold District varies in age from 619 ± 2 Ma to 624 ± 2 Ma, are high K calc-alkaline and related to gold mineralization. The ages of granitoids related to mineralization in the Bétaré Oya Gold District are not known. This area is known for its alluvial gold mining. This paper therefore focuses on a granitic body from the Bétaré Oya area (Figure 1), one of several plutonic bodies along the Central Cameroon Shear Zone (CCSZ). The aureole of this pluton is gold-bearing and characterized by sheared lithologies. Recently obtained combined aeromagnetic and radiometric data (Figure 1a) from this area indicates the pluton as the most prominent feature. We therefore use zircon chemistry alongside Lu-Hf isotope data of zircon grains to depict the source, constraint the age of the granitoid emplacement and evaluate the role of petrogenesis and magmatic evolution of the granitic melt in hydrothermal fluid circulation and primary gold precipitation in the Bétaré Oya area. We further compare this to mineralized intrusive bodies along the CCSZ and around the world. Generally, aureoles around felsic igneous intrusions present prospective areas for mineral exploration. In this area the aureole around this plutonic body is currently being exploited for gold through artisanal and semi mechanized mining methods (Figure 1c).

2. Regional Geology

The Central Cameroon Shear Zone (CCSZ) is the SW extension of the Central African Shear Zone (CASZ) and constitutes a part of the Central African Fold Belt (CAFB). It is situated at the northern edge of the Congo Craton [9] and composed largely of Palaeoproterozoic basement, which can be traced through Cameroon across the Atlantic into northeastern Brazil and the Trans-Sahara Belt [10]. In Pre-drift reconstruction of West Gondwana, the Pan-African/Braziliano belts of Cameroon, Nigeria and the Borborema province are sandwiched between the Congo-Saõ Francisco and West African Cratons hence occupy a strategic position in unraveling the geodynamic evolution of these regions [11,12]. The CCSZ and its relay feature known as the Sanaga faults (SF) have a NE-SW orientation and is characterized by the abundance of granitic intrusions associated with major shear zones [7,13,14]. Although Pan-African granites constitute a common lithology in the country, their origin remains poorly understood. Intrusive bodies along the CCSZ are generally Pan-African (560 - 686 Ma) in age [7,15,16,17,18]. However, an earlier Tonien - Stenien overprint event at 911 to 1127 Ma [7] have been reported in the Batouri area indicating an extended unique melting event contemporaneous with the magmatism responsible for the emplacement of granitoid in the other domains of the CCSZ. The Lom basin is a syn-depositional Neoproterozoic pull apart basin [19]. It is sandwiched by strike slip faults known as the Sanaga Fault (SF) a relay of the CCSZ system [19,20]. The SF is a continental scale transcurrent fault. It is a potentially deep tapping crustal fault that has focused gold-bearing fluids into structures in the Lom Basin [21]. The Lom basin is composed mainly of meta-sedimentary rocks, grouped into two main structural and metamorphic units. These units include a monocyclic unit which is composed of volcano-clastic series, orthogneiss, quartzite and polygenic conglomerate [22] metamorphosed under green schist facies and associated with grabens. The polycyclic unit consists of staurolite micaschists, Lom bridge gneisses, and staurolite-chloritoid mylonites, closely related to the horst structures [19]. The mylonites are the main identifying features for the presence of the SF [22]. These units are intruded by quartz veins and granitoids (granite, monzonite and lamprophres) which show evidence of sinistral deformation. This region is known for its gold mineralization hosted within quartz veins [6]. The quartz veins are associated with schist and believed to be products of hydrothermal fluids which originated from nearby intrusive bodies. They generally have a N70°E foliation conformable to the CCSZ shear zone system.

3. Samples/Analytical Techniques

Three samples were collected from the study area; two (02) granitic rock chips and a pack (01) of granitic saprolite weighing about 5 kg (see Figure 1d). The granitoid is coarse-grained and displays a porphyritic texture with quartz and plagioclase mega phenocrysts set in a mica-quartz dominated groundmass. Mineralogically, the granitoid is composed of quartz (50-65%), K-feldspars (15-25%), biotite (15-20%) and hornblende (<5%). Accessory minerals present include: apatite, titanite, spene, zircons, secondary carbonates and opaque minerals. Plagioclase occurs as subhedral to anhedral crystals with variable inclusions of zircon, apatite, and biotite. Quartz occurs as closely packed grains and elongate ribbons that display undulatory extinction. K-feldspar occurs as fine to medium anhedral crystals interstitial to plagioclase and quartz and includes microperthitic microcline and orthoclase. Biotite occurs as pleochroic green to reddish brown flakes partly altered to chlorite. Hornblende occurs as large, subhedral prismatic crystals which are altered to chlorite in some places. Titanite occurs as fine to medium-grained idiomorphic crystals interstitial to plagioclase, hornblende, biotite, and quartz. These samples were shipped to the University of California, Santa Barbara, USA for zircon chemistry, geochronology and Lu-Hf isotope analyses.

Figure 1. a - Aeromagnetic and radiometric image of the Bétaré-Oya Gold District-BOGD, Lom basin, eastern Cameroon b - Elevation and drainage map of the study area c - Stream sediment sample location map with gold count per location (gold particles per 10 kg pan) in proximity to artisanal mining sites d - Rock sample location map with ages of dated samples displayed

3.1. Sample Preparation

The granitoid samples were crushed and zircons separated by standard techniques (Rogers’s water table, Frantz magnetic separation, and methylene iodide heavy-liquid separation) at the Department of Earth Science, University of California Santa Barbara, USA (UCSB). Zircons were mounted in separate epoxy plugs, ground down to expose grain interiors and polished with 6 µm and 1 µm diamond paste Prior to analytical work, zircons were examined for zoning using a cathodoluminescence (CL) system attached to a FEI Quanta400f Scanning Electron Microscope house at UCSB.

3.2. Zircon U-Pb and Trace Element Analyses

U-Th/Pb isotope and trace element concentration data were collected using a split stream inductively coupled plasma mass spectrometry (LASS-ICPMS) facility located at UCSB. Methods follow those outlined in [23] with modifications as presented in [24] and are briefly summarized here. Instrumentation consists of a Photon Machines 4-ns pulse duration 193-nm wavelength ArF excimer laser attached to a Nu Plasma HR multi-collector (MC) ICPMS set to measure U, Th, and Pb isotope ratios and an Agilent 7700S Quadruple ICPMS to measure trace element concentrations. Analytical conditions of the laser for zircon were a 25µm spot, 4Hz frequency, a laser energy of 100% of 3mJ (equating to a fluence of ~1.7 J/cm²) and 100 shots per analysis.

U-Th/Pb data reduction, including corrections for baseline, instrumental drift, mass bias, down-hole fractionation and uncorrected age calculations were carried out using Igor Pro and the plugin Iolite v. 2.5 (see [25] for details on data reduction methodology). Primary reference zircon "91500" [1065 Ma ²⁰⁶Pb/²³⁸U isotope dilution-thermal ionization mass spectrometry age, [26]] was used to monitor and correct for instrumental drift, mass bias, and down-hole inter-element fractionation. To monitor data accuracy, secondary reference zircon “GJ1” (601.7 ± 1.3 Ma, [27]) was analyzed concurrently (once every 8 unknowns) and mass bias- and fractionation-corrected based on measured isotopic ratios of the primary reference material. During the analytical period, repeat analyses of GJ-1 gave a weighted mean ²⁰⁶Pb/²³⁸U age of 599 ± 1 Ma, mean square weighted deviation (MSWD) = 0.8, ²⁰⁷Pb/²⁰⁶Pb age of 601 ± 3 Ma, MSWD = 0.8, n = 70. Zircon trace element concentrations were normalized to zircon GJ-1 (using the reference values of [28]), using ⁹⁰Zr as an internal standard and, based on the long-term reproducibility of multiple secondary reference zircons, are accurate to 3-5%. All uncertainties are quoted at 2σ and include contributions from the reproducibility of the reference materials for ²⁰⁷Pb/²⁰⁶Pb, ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U. U-Th/Pb data were plotted using the Redux v.3.0.4 [29]. The complete U-Th/Pb and trace element data are presented in Table 1 and Table 2.

3.3. Zircon Lu-Hf Isotope Analyses

The same zircon domains that had been analyzed for U-Pb were analyzed for their Lu-Hf isotopic composition at UCSB, following the method described in [23,30]. Data were collected on a Nu Plasma MC-ICPMS using a spot size of 40μm, laser energy of 3 mJ, at 10Hz, and 500 shots. Whenever possible the Lu-Hf laser spot was drilled directly on top of the LASS laser spot. Replicate measurements yield ¹⁷⁶Hf/¹⁷⁷Hf values of 0.282506 ± 0.00002 (2σ, n=25), 0.282473 ± 0.00007 (n=7), 0.282323 ± 0.00007 (n=5), and 0.282128 ± 0.00003 (n=7) for reference zircons Mud Tank, Plesovice, 91500, and synthetic zircon “MUN3” respectively. For calculation of ɛHft values the decay constant of 1.867 × 10−11 (average of [31] and [32]) and the values for the chondritic uniform reservoir (CHUR, ¹⁷⁶ Lu/ ¹⁷⁷Hf = 0.0336, ¹⁷⁶ Hf/ ¹⁷⁷ Hf = 0.282785; [33]) were used. The initial ¹⁷⁶Hf/ ¹⁷⁷Hf ratios and ɛHft for all analyzed zircon domains were calculated for the corresponding ²⁰⁶ Pb/ ²³⁸U age of the sample. A two stage depleted mantle Hf model age (TDM, in Ga) was calculated from the initial 176 Hf/ ¹⁷⁷Hf of each zircon at the time of crystallization (in terms of the apparent ²⁰⁶Pb/²³⁸U age) by using 176 Hf/ ¹⁷⁷Hf = 0.28325 and ¹⁷⁶ Lu/¹⁷⁷ Hf = 0.0384 for the bulk earth [33] and ¹⁷⁶ Lu/ 177 Hf = 0.0113 for the average crust. Ti-in zircon temperature calculated as outline in [34].

4. Results

4.1. Whole Rock Geochemistry

The whole rock geochemical data for the studied granitoid samples alongside the Batouri granitoid [7] and the two mica granite [16] from Ekomédion are represented in Table 1. Samples CMR06, CMR07 and CMR08 show a similar compositional range for SiO₂ (67.52, 66.42 and 65.38), Al₂O₃ (15.89, 16.25, and 16.6), Fe₂O₃ (3.5, 3.66 and 3.87) Na₂O (3.81, 3.98 and 4.21) K₂O (3.66, 3.33 and 2.73) and CaO (2.72, 0.23 and 3.74) respectively. According to the classification scheme of [35,36,37], the Bétaré Oya granitoid is sub-alkaline with a high-K calc-alkaline affinity (Figure 2a, Figure 2b). On the A/NK versus A/CNK classification plot [38], the granitoid is granodioritic to tonalitic in composition. They are peraluminous with all samples clustering within the ‘I-type’ field (Figure 2c). Nb - Y discrimination plot [39], reveals a volcanic arc setting for that the Bétaré Oya granitoid (Figure 2d).

Table 1. Major oxides and trace element composition for the Bétaré-Oya (this study), Batouri [7] and Ekomédion [16] granitoids

Figure 2. Major element geochemical classification plots for granitoid rocks from Bétaré Oya. (a) Na₂O + K₂O versus SiO₂ diagram of [35], showing the subalkaline compositions of the rocks. (b) K₂O versus SiO₂ diagram of [37], the rocks plot in the high-K field. (c) A/NK versus A/CNK diagram of [38] showing aluminosity and alkalinity. (d) Discrimination diagram of [39], showing that the rocks were formed in a volcanic arc setting. A/NK = molar Al₂O₃/Na₂O+K₂O; A/CNK= molar Al₂O₃/CaO+Na₂O+K₂O, Syn-COLG, Syn-collisional granitoid; WPG, within-plate granitoid; VAG, volcanic arc granitoid; ORG, oceanic ridge granitoid

4.2. Zircon Morphology and Composition

Representative CL images of zircon grains from the investigated granitic pluton are provided in Figure 3. Based on their internal structures five (05) different zircon populations have been identified: (i) Zircon grains with consistent oscillation zoning from core to rim, equant to elongated grains (Figure 3, 07Z05). The variation of CL intensity results in a light/dark zone alternation (06Z25). (ii) Zircon grains that are euhedral to subhedral with light and /or distorted core and a clear zoning towards the darker rim (Figure 3, 07Z20, 07Z24 and 08Z10). In some grains the core is zoned (Figure 3, 07Z19). (iii) Subhedral to euhedral grains characterized by relatively small, thin and elongated dark cores with thick light mantles and a thin dark rim (Figure 3, 07Z29). The dark core in some grains is clearly absent (Figure 3, 08Z06) whereas in others the dark core is thick and constitutes a thin light mantle (iv) Sub rounded crystals (Figure 3 (06Z24, 08Z26) with thick light core and an apparent zoning in some cases and a thin dark rim. (v) Distorted zircon grains some of which portray irregular to no zoning at all (Figure 3, 07Z07) and the others with a distorted core but zoned rims (06Z04). The boundary between the distorted core and zoned rim in some grains is marked by thin light band (07Z24, Figure 3). In this study Ti concentration in the zircon grains ranges from ~1 ppm to 330 ppm with approximately 83% of the analyzed grains with values ≤ 20 ppm (Table 2). Crystallization temperature for each grain varies from ~ 560°C to 1230°C with modal temperatures of 700±2°C Figure 4 (a to d). The concentrations of Rare Earth Elements (REE) vary greatly from La - Lu. Ce values in sample CMR06 range between 0.74 - 264 ppm, CMR07 between 1.33-1660 ppm and CMR08 between 4.14-1560 ppm. These values are generally higher than those of neighboring Pr, with all La values below the detection limits. Pr values range between 0.3-32.9 ppm in CMR06, 0.112-149 ppm in CMR07 and 0.52-367 ppm in CMR08. Eu values are generally depleted relative to Sm and Gd. Figure 5 (a, b and c) show REE patterns for CMR06, CMR07 and CMR08 respectively. These patterns generally portray high Ce values, negative Eu anomalies and HREE enrichment. However sample CMR08 shows a dual character of depletion and enrichment in Eu (Figure 5c). On Zircon/chondrite normalized patterns, REE of zircon grains from the three (03) samples investigated revealed three (03) patterns: (i) one with high concentrations in Ce accompanied by a negative Eu anomaly and HREE enrichment, (ii) high Ce concentrations, positive Eu anomaly and HREE enrichment and (iii) progressive enrichment from LREE to HREE.

Figure 3. CL images of zircon crystals analyzed from the Bétaré Oya granitic pluton

Figure 4. Zircon crystallization temperature of the various granitic samples obtained through Ti-in-zircon thermometry [34]. a - Temperature variation in zircon grains from sample CMR06. b - Temperature variation in zircon grains from sample CMR07 c - Temperature variation in zircon grains from sample CMR08. d - Temperature variation in all three samples combined. The mean temperature is 700 ± 2 °C

Figure 5. a - REE pattern of zircon grains from sample CMR06 showing negative Eu anomaly with HREE enrichment also observe are some grains with LREE enrichment, negative Eu anomaly with relatively stable HREE (5 and 15) whereas grains 11 and 29 show positive Eu anomaly with HREE enrichment. b - REE patterns of zircon grains for CMR07 showing high Ce concentrations, negative Eu anomaly and HREE enrichment. Discrepancy patterns for grains 5, 11, 29 with positive Eu anomalies followed by HREE enrichment and grains 20, 21, and 22 with negative Eu anomaly and HREE depletion can also be observed. c - REE patterns of zircon grains for CMR08, showing great variation in patterns with HREE enrichment, positive Eu anomaly, negative Eu anomaly and grains showing progressive enrichment from LREE to HREE

Table 2. Trace and Rare Earth Element composition of zircon grains form the investigated Bétaré Oya granitoid

4.3. U-Pb and Hf Isotopes Data

U-Pb data for the various samples are presented in Table 3. A total of 103 zircon grains were analyzed: CMR06 = 32 grains, CMR07 = 33 grains and CMR08 = 38 grains. This result show varying concentrations of U, Pb, Th and Th/U ratio. Sample CMR06 shows, U (508 - 4220 ppm), Pb (11 - 235 ppm), Th (47 - 399 ppm) and a Th/U ratio range of 0.04 - 1.1. Sample CMR07 reveals, U concentrations between 220 and 4740 ppm, Pb (19 - 212 ppm), Th (47 - 333 ppm) and a Th/U ratio between 0.03 - 0.26. CMR08 values vary for U (242 - 3590 ppm), Pb (2 - 192 ppm), Th (9 - 498 ppm) and a Th/U ratio range of 0.04 - 0.28. On the discriminant plots of [3], the Bétaré Oya samples reveal a granodioritic composition (Figure 6a, Figure 6b). Their Th/U ratios and Ce/Sm vs Yb/Gd plots indicate that the granitoids originated from the crust (Figure 6c, Figure 6d). U-Pb data were plotted with the aid of isoplot and concordia obtained as presented in Figure 7. Sample CMR06 has an upper intercept at 1838± 1000 Ma with a lower intercept at 587±100 Ma and a mean age of 635.1±5.7Ma (MSWD=6.7). CMR08 has an upper intercept at 879 ± 580 Ma, lower intercept at -10 ± 2100 Ma and a mean age of 633.1±4.5Ma (MSWD=15) whereas CMR07 intercept the isochron line at a single point with intercept at 651.7±7.3 (MSWD=13). It shows a mean age of 633.2±5.3 Ma. Hafnium concentrations vary from 6560 ppm to 17100 ppm. The lowest Hf concentration is observed in CMR08 which ranges from 6560 ppm to 15280 ppm with an average value of 12244 ppm. Sample CMR07 has Hf values ranging from 8570 to 17100 ppm with an average value of 12823 ppm whereas those of CMR06 range from 8680 to 16100 ppm with an average 12465 ppm. ¹⁷⁶Hf/¹⁷⁷Hf ratio is 0.28 for all analyzed samples while ¹⁷⁶Lu/¹⁷⁷Hf ratios are quiet low ranging from 0.0 to 0.002. εHft are significantly negative to positive ranging from -5.29 to 13.62. Hf model ages were also calculated and vary from 1010 - 1390 Ma for CMR06, 680 -1415 Ma for CMR07 and 964 -1425 Ma for CMR08 (Table 4).

Figure 6. REE discriminant diagrams for zircons. a - Y vs U in ppm and b -Y in ppm vs Yb/Gd jointly characterise the type of granitoid as a granodiorite and tonalite after [3] where 1 = Aplites leucogranite, 2 = granites and 3 = granodiorites and tonalites. c - U vs Th in ppm. d - Ce/Sm vs Yb/Gd chracterise the source of the granitoid as crustal after [42]

Figure 7. ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf isotope variations in three granitic samples from the Bétaré-Oya gold district and their corresponding concordia diagrams. ¹⁷⁶Lu/¹⁷⁷Hf ratios generally < 0.002

Figure 8. Histograms of 207Pb corrected 206Pb/238U ages for the granitoid samples revealing Pan African mean ages of 635.1±5.7Ma for CMR06 (a); 633.1±5.3Ma for CMR07 (b) and 633.2±4.5Ma (c)

Table 3. Zircon U-Pb analytical data by LA ICP-MS

Table 4. Lu/Hf isotopic composition of zircon grains from the Bétaré Oya granitoid

5. Discussion and Conclusion

5.1. Petrogenesis, Evolution and Significance of Granitoid Intrusions in the Bétaré Oya Gold District

Zircon external and internal structures have been used to distinguish pristine magmatic zircons and resorbed zircons derived from residual melts through mixing [40]. These categories of zircon populations exist in the granitoid investigated. The pristine magmatic zircons are generally euhedral displaying oscillatory and /or sector zoning whereas the resorbed zircons are characterized by corroded and resorption surfaces, recrystallized quartz and mineral inclusions. U vs Y and Y vs Yb/Sm discriminant plots (Figure 6a and Figure 6b) show that the Bétaré Oya granitic pluton varies from granodiorite to tonalite. Their REE normalized patterns dominantly show high Ce concentrations, negative Eu anomaly and enrichment in HREE which are characteristics of crustal derived magmas [41]. However, some of the zircons from sample CMR08 display REE patterns with positive Eu anomalies and we attribute this to zircon formation at different stages of magma evolution. Eu exists as a divalent (Eu²⁺) and a trivalent (Eu³⁺) cation. The Eu²⁺ readily substitutes for Ca²⁺ in plagioclase in a reducing environment fractionating Eu from the melt hence resulting in a negative Eu anomaly whereas Eu³⁺ is incompatible and will remain in the magma resulting to a positive Eu anomaly. The negative Eu anomaly exhibited by most grains Eu fractionation into plagioclase whereas the positive Eu anomaly depicts the influx of magma void of plagioclase. This therefore further suggests the phenomenon of magma mixing during the magma evolution. According to [42] the positive Eu anomaly suggests that the zircons crystallized from a highly evolved melt where most of the plagioclase was already fractionated. The Ce/Sm ratio is used as a representation of oxidation conditions while Yb/Gd ratio evaluates the evolution of magma by fractional crystallization. The Ce/Sm ratios range of 0 to 3 associated with variable Yb/Gd ratios upto 76 in zircons from the Bétaré Oya granitoid are characteristic features of crustal derived melt in a reducing environment. Th/U ratios between 0.1 and 1 with most values falling below 1.0 as well as a negative correlation observed between Th/U and Hf concentration and an almost constant Th concentration with increase in U/Ce ratio all confirm that the granitoids are derived from crustal recycling. The emplacement temperatures of this granitoid range from 625°C to 775°C and this is consistent with temperatures for S-type granitoids based on the classification of [43]. Such temperatures however have been reported in the northern parts of the CASZ by [18] for I-type granitoid of high K-calc alkaline composition using hornblende in plagioclase with temperature values of 698 and 720 °C for the Djourde granitoids, 698 - 728 °C for the Sinassi granitoids and 667- 670°C for the orthogneiss. Zircon morphology (the existence of dark rounded cores with magmatic overgrowths is clear evidence of inherited zircons) and chemistry coupled with the tectonic setting of the study area (a pull a part basin) provide evidence of metasedimentary contribution during magma generation. Hf as a minor element in zircon is a very sensitive tracer of crustal and mantle processes [44] hence its concentration is vital in unveiling the petrogenesis of the Bétaré-Oya granitic pluton. All three samples from this intrusion are characterized by high Hf concentration (>10000 ppm, Table 3). According to [45], residual and depleted mantle are characterized by Lu/Hf ratio > chondrite values and will be radiogenic with positive εHf value. In contrast to this, enriched ancient crust tends to have Lu/Hf ratios < chondrite values and is unradiogenic with negative εHf. ¹⁷⁶Lu/¹⁷⁷Hf ratio for the Bétaré Oya granitic samples are generally less than chondrite values (0.022) and along with negative εHf values are evidence of a crustal source whereas the few positive values just above CHUR and below the Depleted Mantle (DM1) line points to a mafic crust contribution (Figure 9).

Figure 9. Epsilon Hf versus ²⁰⁶Pb/²³⁸U ages diagram showing the studied samples together with crustal evolution trends of average crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.0113), mafic crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.022), and Depleted mantle evolution trends DM1 and DM2

Geochronological studies along the Central Cameroon Shear Zone (CCSZ) have reported various Pan African ages for granitic intrusions [13]. These ages vary from 626 to 654 Ma for leusosomes of the Yaoundé series [17], 603 - 618Ma for the Mayo Punko metadiorite and 560 - 611Ma for the Mayo IIou metagranodiorite [46] in the northern region of Cameroon. Recently, [18] dated the Sinassi batholith at 644 - 686 Ma while in the western domain, [15] dated the Fomopéa complex at 610Ma, the Ngondo plutonic complex at 602Ma [47]. The Ekomédion two mica granite reveal an age of 578 Ma [16,18]. Ages of the Batouri granitoid lie within the 640 - 620 Ma age bracket [7]. A mean age of 635 Ma was obtained for granitic pluton within the Bétaré-Oya Gold District. This age is attributed to the emplacement age of the pluton and falls within the age bracket obtained by [7]. The age range defines an extensive unique Pan African melting event contemporaneous with the magmatism and deformation (sinistral shear movement) responsible for the emplacement of granitic plutons along the CCSZ.

Mineralization along the CCSZ and the Patos/Pernamboco shear zones in NE Brazil are known to be associated with late granitoid intrusions. In NE Brazil mineralization is known to be associated with younger granitic plutons generally 570 - 580Ma. This age bracket is known for its tungsten, gold (skarn and quartz vein type) uranium and molybdonium deposits. This mineralization age is well represented in Cameroon by the U-Mo mineralization in Ekomédion. This study reveals a new and older mineralization along the CCSZ at 620 - 635Ma for the Bétaré Oya Gold District, SE Cameroon. This mineralization is seemingly absent across the Atlantic. We therefore suggest that exploration effort in the region should revisit older granitoids for Au, W, Mo, U mineralization amongst others.

5.2. Conclusion

1) Whole rock chemistry defines the Bétaré Oya granitoid as high K Calc-alkaline to sub-alkaline in nature. It is granodioritic to tonalitic in composition, peraluminous and of I-type emplaced within a volcanic arc setting at a temperature that varies between 625°C and 775°C

2) Internal structures such as zoning, resorption zone, corrosive surfaces and mineral inclusions were used in this study to distinguish between magmatic and metamorphic zircons. The co-existence of zircon grains with clear oscillatory zoning (characteristic of igneous zircon) associated with zircon grains showing corroded crystal outlines and resorption surfaces suggest recrystallization of zircon grains as a result of hot magma influx.

3) REE discriminant patterns clearly shows positive Ce concentrations, negative Eu anomaly and HREE enrichment coupled with Lu/Hf ratios <0.022 and negative εHf values points to a crustal source of magma with mafic crust contribution.

4) U-Pb age by LA ICP MS revealed that granitic intrusions within the Bétaré Oya Gold District are Pan African in age with a mean age of 635 Ma similar to previously obtained ages along the CCSZ and defines a unique melting event synchronous with magmatism and deformation for the emplacement of granitoids along this structure. The mean age of the granodiorite intrusion in the Oya Gold District at 635 Ma defines a new and older mineralization age along the Braziliano-Pan African region

Acknowledgements

This publication is part of the PhD thesis of the first author underway in the University of Buea. Prof. John Cottle of the Department of Earth Sciences & Earth Research Institute of the University of California assisted with the geochronological analyses. Prof. Bernd Lehmann is greatly thanked for helping with the isochron plots. Special thanks go to Ashu Jones Egbe for helping with the geologic maps used in this publication.

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