Trace Element and U – Pb Core Age for Zircons from Western Meiganga Gold Placer , Cameroon : Their Genesis and Archean-Proterozoic Sources

Trace element concentrations and U–Pb ages were obtained using Laser Ablation Split Stream Method from the core of 115 zircon grains from the western Meiganga gold placer deposit. The data was used to characterize zircon, to understand the history of crystallization and to locate source rocks within the local and regional geological settings. Zircon trace element geochemistry was used to distinguish between magmatic and metamorphic affinity. The magmatic zircons have characteristics compatible with their probable origin from granitoid, syenite, tonalite, charnockite and mafic to ultramafic rocks. The metamorphic zircons composition is compatible with growth from anatectic melts and by sub-solidus crystallization in equilibrium with garnet. The zircon ages reveal Archean, Paleoproterozoic, Mesoproterozic, and Neoproterozoic events with the principal source could mainly belong to Paleoproterozoic magmatic lineage. Some of the Paleoproterozoic magmatic zircons were probably sourced from two mica granite found within the local geology, whereas the remaining zircons have features indicating source rocks within the Congo Craton. We suggest that the geologic history of these zircons is related to crustal-scale magmatic and/or tectono-metamorphic events, possibly linked to Eburnean and Pan-African orogeny.


Introduction
Placers are mechanical concentrations of economically viable dense/heavy minerals (e.g., corundum, ilmenite, cassiterite, diamond, rutile, monazite, xenotime, and zircon) and important elements (e.g., gold and platinum, as well as Nb, Ta, and U [1]) sorted and sourced from various areas of erosion within the local and the regional geological settings [2,3].They are mainly allochthonous in nature, transported and deposited proximally to distally from their source rock [1].The presence of specific dense and heavy minerals (e.g., zircon, rutile, tourmaline, kyanite, sillimanite, andalusite, and monazite) in placers can be used to track the geological history of the bedrock in the source region [4][5][6][7][8][9][10][11][12][13][14][15].Zircon in particular plays a prominent role in provenance studies of displaced and accumulated clastic materials [16][17][18].Detrital zircon can be used to make inferences about the source history, for paleogeographic and tectonic reconstitution [19][20][21][22][23][24].The use of trace-element geochemistry of trace-element geochemistry and U-Pb dating to characterize and interpret the provenance of detrital zircons is well established for constraining source parameters, a commonly used technique in sedimentological and tectonic studies [17,18,25,26].Hence, geochemical study of detrital or xenocrystic zircon in conjunction with U-Pb geochronology on the same grain is an extremely powerful tracer tool for provenance studies [27].
Gold, zircon and other heavy minerals are found in many areas in Cameroon (e.g., Betaré-Oya in the east; Minton in the south; and Meiganga in the Adamawa).In the Meiganga Sub-division (Figure 1), gold occurs in the south, east, and west.These gold occurrences are mainly found in alluvium and soils, but rarely in primary rocks.In the west of Meiganga Sub-division, gold grains are scattered in sediments from some streams and rivers.There are no known primary gold occurrences in this area, so their source areas have still not been found.In this paper, we used single grain zircon trace element geochemistry and U-Pb core dating to assess the most likely source of the detrital materials associated with gold that is found in substantial quantities in small streams in the west of Meiganga sub-division.

Materials and Methods
In total, one hundred and fifteen zircon grains were analyzed for their trace element contents and U-Pb ages at the University of California, Santa Barbara, CA, USA.The analyzed zircons were sampled from small, nameless, first order tributary streams found in the study area (Figure 1c), similar to the sampling strategy employed in [42].According to [42], during gold exploration, it is better to sample small streams (first-order tributaries) where the streams are <5 km in length rather than larger rivers, because the former yield more valuable information on the provenance of placer gold than samples from large rivers.Collected samples (5 kg × 10) in each point (from gravel layer) and from three localities (Békasiki, Doua, and Kaladi: see Figure 1c) were washed (by panning) immediately in the field and on the spot to obtain substantial quantity of heavy mineral concentrates.Each concentrate was packaged separately, numbered and carried to the laboratory for gold identification and heavy mineral separation.Heavy mineral concentrates with gold grains were separated using dense liquid (bromoform: D ≥ 2.7 g/cm −3 ) at the Department of Earth Sciences at the University of Yaoundé I, Cameroon.The mineral separation protocols are similar to those described in [43] and [44].A pre-concentrated gold-host heavy mineral assembly (from Békasiki) was sent to the University of California (Santa Barbara, CA, USA) for the zircon grains separation and analyses by Laser Ablation Split Stream.
Zircon U/Pb geochronology and trace element data were acquired simultaneously using a laser ablation "split stream" setup consisting of a Photon Machines Excimer 193 nm laser ablation unit coupled to a Nu Instruments, "Nu Plasma" multi-collector inductively coupled plasma-mass spectrometer and an Agilent 7700S quadrupole inductively coupled plasma-mass spectrometer, housed at the University of Santa Barbara, CA, USA (for detailed methodology see [45][46][47]).Samples were analyzed for 20 s using a fluence of 1.5 J/cm 2 , a frequency of 4 Hz, and spot size of 20 µm diameter resulting in crater depths of ~9 µm.Utilizing a standard-sample bracketing technique, analyses of reference materials with known isotopic compositions were measured before and after each set of seven unknown analyses.Data reduction, including corrections for baseline, instrumental drift, mass bias, down-hole fractionation, and age and trace element concentration calculations were carried out using Iolite v. 2.1.2[48]."91,500" zircon (1065.4± 0.3 Ma 207 Pb/ 206 Pb ID-TIMS age and 1062.4 ± 0.4 Ma 206 Pb/ 238 U ID-TIMS age: [49]) served as the primary reference material to monitor and correct for mass bias, as well as Pb/U down-hole fractionation and to calibrate concentration data, while "GJ-1" zircon (608.5 ± 0.4 Ma 207 Pb/ 206 Pb and 601.7 ± 1.3 Ma 206 Pb/ 238 U ID-TIMS ages: [50]) was treated as an unknown in order to assess accuracy and precision.Twenty-three analyses of GJ-1 zircon throughout the analytical session yield a weighted mean 207 Pb/ 206 Pb date of 593 ± 5 Ma, MSWD = 0.8 and a weighted mean 206 Pb/ 238 U date of 603 ± 2 Ma, MSWD = 1.0.Concordia and Kernal Density Estimate (KDE) plots were calculated in Isoplot v 2.4 [51] and Density Plotter [52], respectively, using the 238 U, and 235 U decay constants of [53].All uncertainties are quoted at 95% confidence levels or 2 s level and include contributions from the external reproducibility of the primary reference material for the 207 Pb/ 206 Pb and 206 Pb/ 238 U ratios.For plotting and age interpretation purposes, the 207 Pb/ 206 Pb dates are used for analyses older than 1000 Ma, whereas the 206 Pb/ 238 U dates are used for analyses younger than 1000 Ma.

Zircon Geochemistry
Twenty three elements (Table 1) quantified from the zircon cores are heterogeneous, as is expected with their elemental concentration ranges similar to those reported in the literature.Within these elementary suites, Hf and Y are minor, whereas other 21 elements (U, Th, Pb, Ti, Nb, Ta, Sr, and REE) are mainly at trace levels.The minor and trace elements concentrations are presented separately.Hafnium varies between 6390 and 13,020 ppm, which is in general higher than values reported for detrital zircons from western Mamfe area [10][11][12].Most of the values are above 9000 ppm with very few being below 7500 ppm.Some zircons have similar abundances (e.g., Hf = 9160 ppm in MW092 and MW093), suggesting same degree of Hf substitution (degree of fractionation) in these zircons.Due to the high heterogeneity of the Hf contents, the zircon grains can be classified into three main groups: (1) relatively low-Hf zircons (Hf ≤ 8910 ppm); (2) moderate-Hf zircons (9010 ppm ≤ Hf ≤ 10,120 ppm); and (3) high-Hf grains (Hf > 10,120 ppm).The Hf values in group 1 are within the range in magmatic zircons crystallized in a continental rift setting (Hf < 9000 ppm: cf.[27]) whereas those in the other group could be that of zircons grew out of rifting.
Yttrium content ranges from 82 to 1867 ppm with the lowest value found in NW081, and the highest in MW093.They are generally greater than 250 ppm, with a few values exceeding ppm.Four groups can be distinguished; (1) zircons with relatively low Y (<160 ppm); (2) zircons with moderately low Y (204-491 ppm); (3) zircons with relatively high Y (505-988 ppm); and (4) zircons with very high Y (≥1014 ppm).The Y values are mostly within the range noted in [54] and [55] for crustal derived zircons, with some being close to values found in [19] high-Si granitoids.The calculated Hf/Y ratios for the zircon grains vary from 4 to 117 with the lowest values found in MW093 and the highest in MW081.

Trace Elements
The U values, listed in Table 1, ranges from 11 to 1042 ppm with the lowest values mainly found in MW100 and MW113 and the highest in MW023 and MW096.Some zircons have similar contents (e.g., U = 210 ppm in MW017 and M030, or U = 222 ppm: MW11 and U = 223 ppm: M016), suggesting same degree of U substitution in those grains [11,12].Those with very low U (<30 ppm) values are within the range limit in [54] kimberlitic and [55] syenitic zircons.Zircons with U > 300 ppm are close to those in detrital zircons classified as from high-Si granitoids [19], and within the range noted by [56] for granitic zircons.
Thorium content ranges from 2 to 859 ppm, with some values being much higher than others.They can form three groups: (1) made up of zircons with Th ≤ 70 ppm; (2) zircons with Th (70-101 ppm), and (3) composed of grains with Th ≥ 106 ppm.Correlation exists between the Th and U values in Figure 2a, for zircons whose Th and U contents are <520 ppm and <400 ppm, respectively.The calculated Th/U ratios (0.01 to 2.80) are highly heterogeneous.Due to the heterogeneity of the Th/U ratios, the zircons can be grouped into two: (1) those with Th/U ratio (<0.07) compatible with metamorphic zircons [57] and (2) those with magmatic zircons (0.2 ≤ Th/U ≤ 1: [58]).The Th/U versus Th binary diagram (Figure 2b) shows positive correlation, whereas no correlations are evident in the Th/U versus U diagram (Figure 2c).1), with most of the lowest temperatures calculated for zircon with very low Th/U ratio (≤0.09), and the highest temperature (≈1065 • C) calculated for zircon with low Th/U ratio (≈0.25).Some grains have similar temperature or very close values; this may reflect crystallization at the same conditions.Niobium (0.2-7.0 ppm) and Ta (0.09-8.0 ppm) contents are very low.Most Nb contents are close to that of Ta and show a positive correlation in Figure 2d.The Nb/Ta ratios vary from 0.40 to 2.23 with most of values being slightly greater than 1.

Rare Earth Elements
The quantified rare earth element abundances are highly heterogeneous (Table 2).Within the LREE suite, La is predominantly below detection, except in MW001 and MW115.The Ce content ranges from below detection to 330 ppm, with the lowest quantified values being below 5 ppm (e.g., in MW013 and MW025), and the higher values being greater than 149 ppm (e.g., in MW002 and MW007).The calculated Ce/Ce* for MW001 is 268.2.The Pr content varies from bdl to 7.8 ppm, with most values being ≤0.1 ppm.Within the MREE suite contents (in ppm): Nd ranges from bdl to 1020 ppm with most values being ≤2; Sm (0.02 to 38.0) is generally ≥1.6; Eu (0.06 to 14.0) is dominantly <1.0; whereas Gd (1.0 to 50.0) is mostly ≥9.0.The highest Nd value was obtained in MW006, which exceptionally has the highest Pr (7.8 ppm), Sm (37.1 ppm), and Eu (13.9 ppm) contents.The MW006 normalized chondrite plot in Figure 3a, shows positive Nd and Eu anomaly, not visible for other zircons.The calculated Eu/Eu* anomalies (0.05 to 2.1) and normalized chondrite plots (Figures 3 and 4) predominantly exhibit negative Eu anomalies with a few showing positive to the lack of an anomaly.Heavy rare earth elements are mainly the highest in zircons from the western Meiganga gold placer.High HREE contents are common in zircon, as these REEs mainly substitute Zr in zircon structure [27].Within this group of REE, Yb (22 to 555 ppm) and Er (12 to 222 ppm) contents are generally the highest, whereas those of Ho (2 to 53 ppm), Tm (3 to 48 ppm), and Tb (0.7 to 13.1 ppm) are the lowest.The Dy contents (6 to 148 ppm, with most values exceeding 30 ppm) and that of Lu (5 to 74 ppm; with most values exceeding 20 ppm) are moderately high.The calculated Gd/Yb and Lu/Hf ratios range from 0.01 to 0.25 and 0.0005 to 0.010, respectively.The highest Gd/Yb (≈0.24) and Lu/Hf (≈0.010) ratio was respectively obtained in MW002 and MW093; with MW093 having the highest Lu (92 ppm), Yb (555 ppm), Tm (59 ppm), Er (291 ppm), and Ho (177 ppm) contents.The calculated Lu n/ Sm n ratios range from 7 to 7177 and that of Yb/Sm, from 20 to 4827.Except for MW013 (with Th/U ≈ 0.4, Lu n/ Sm n ≈ 7176.5, Yb/Sm ≈ 4827), the highest values were mainly obtained for zircon with Th/U < 0.07.

U-Pb Dating
The 206 Pb/ 238 U (575 to 2927 Ma, dominantly ≥1900 Ma, Figures 5 and 6) and 207 Pb/ 206 Pb (565 to 3088 Ma; dominantly >2000 Ma) dates are highly heterogeneous, with some zircons having the same age (Table 3).Due to this high heterogeneity, the 206 Pb/ 238 U ages are used to group the zircons into six sets: Heavy rare earth elements are mainly the highest in zircons from the western Meiganga gold placer.High HREE contents are common in zircon, as these REEs mainly substitute Zr in zircon structure [27].Within this group of REE, Yb (22 to 555 ppm) and Er (12 to 222 ppm) contents are generally the highest, whereas those of Ho (2 to 53 ppm), Tm (3 to 48 ppm), and Tb (0.7 to 13.1 ppm) are the lowest.The Dy contents (6 to 148 ppm, with most values exceeding 30 ppm) and that of Lu (5 to 74 ppm; with most values exceeding 20 ppm) are moderately high.The calculated Gd/Yb and Lu/Hf ratios range from 0.01 to 0.25 and 0.0005 to 0.010, respectively.The highest Gd/Yb (≈0.24) and Lu/Hf (≈0.010) ratio was respectively obtained in MW002 and MW093; with MW093 having the highest Lu (92 ppm), Yb (555 ppm), Tm (59 ppm), Er (291 ppm), and Ho (177 ppm) contents.The calculated Lun/Smn ratios range from 7 to 7177 and that of Yb/Sm, from 20 to 4827.Except for MW013 (with Th/U ≈ 0.4, Lun/Smn ≈ 7176.5, Yb/Sm ≈ 4827), the highest values were mainly obtained for zircon with Th/U < 0.07.

Discussion
Geochemical and U-Pb geochronological results are used to characterize each zircon grain and to understand its genetic history.They are correlated with rocks within the local and regional setting in order to locate their primary source (s).

Geochemistry of Zircon and Possible Magmatic Segregation
Uranium, Th, Hf, Y, and REE contents are commonly used to characterize zircons and understand the history of their genesis [27,57,58,60,61].Heaman et al. [27] demonstrated that zircon chemistry can reflect the composition of the magma from which it is crystallized and its tectonic setting.Out of 115 zircon grains, a total of nine trace elements and REEs are highly heterogeneous with equal values found in some.The heterogeneity in elemental contents suggests that the zircons were crystallized in different rock types with some of the sources being more chemically enriched than others.The equal elemental contents found in some of the zircons may relate their crystallization to an environment with the same abundance of these elements.
The Th/U of Meiganga zircons fall within the reported envelope for zircons of both igneous [54,58,61] and metamorphic [57] origins.Three main groups distinguished are: (1) magmatic zircons (Th/U ≥ 0.2); (2) metamorphic affiliated zircons (Th/U < 0.07); and (3) grains with Th/U ratios ranging from 0.09 to < 0.2.The highest Th/U ratios (>1.0) obtained in some of the group 1 zircons could suggest preferential incorporation of Th into those zircon lattices over U, or relate the crystallization of their source under amphibolite or eclogite melting conditions [62].The nature of zircons in group 3 is not easy to determine.They may be metamorphic zircons, with high Th/U [60], originating from high-grade metamorphic rocks (e.g., [63]).In this study, a grain with Th/U (≈0.09) is considered metamorphic as its other features are similar to those of metamorphic zircons in group 3.Those with Th/U ratios (>0.09) and (<0.2), quite close to magmatic grains, were affiliated to magmatic zircons in group 1.They may be zircons from partial melt segregation, as their Th/U ratios are within the range (0.13-0.37) in some partial melt segregation zircons studied by [64].The partial-melt segregation origin of those grains (MW067, MW081, MW082, and MW108) is supported by Ti thermometric data (618 ± 87 to 744 ± 66 • C), which are similar to those obtained by [64] for zircon that grew by partial melt segregation process.The other features that characterize those grains is their position within various plots, situated out of various fields in Y versus U, Y versus Nb/Ta, and Y versus Yb/Sm binary diagrams (Figures 7-9).The significant variation in Y (81-626 ppm), U (115-442 ppm), Th (23-169 ppm), Pb (30-158 ppm), and ∑REE (59-474 ppm) contents in those zircons suggests crystallization in different environments with these environments being Y, U, Th, and REE are enriched than others.The Hf abundance (9430-11,460 ppm) is greater than values (<9000 ppm: [27]) in zircons formed during continental rifting.The Eu/Eu* (0.08-0.63), their large positive and negative Ce anomalies (Figure 3e,f,h) and chondrite-normalized REE patterns are close to that of magmatic zircons, which helps to classify them as magmatic.These may also represent zircon that crystallized in anatectic melts, as their geochemical features are similar to those of anatectic zircons.[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Magmatic Affiliated Zircons
The variable Hf contents (6390-13,020 ppm) in the western Meiganga magmatic zircons, suggest a different degree of Hf substitution in the zircon lattice.The same interpretation is consistent for the U, Th, and Y contents, as they also show consistent variations.The Pb (42-629 ppm), Ti (2-133 ppm), Nb (0.2-7.0 ppm), and Ta (0.2-8.0 ppm) contents in the zircons, are significantly low to very low.The presence of relatively-high Pb concentrations (e.g., 618 ppm: MW036; 629 ppm: MW093) in some of these zircons may be due to high concentration of this element in source magma coupled with Pb from Th and U radiogenic decay [65]; In contrast, the very low Pb content may be due loss of radiogenic Pb by diffusion during the geological history of low-Pb zircon source rock (cf.[66][67][68][69].Titanium does not frequently substitute for Zr in the zircon structure and the concentration is temperature dependent [70,71].The low Ti in most of the zircons may be due of low degree of Ti substitution, and the absence of favorable temperature for its maximum concentration in those zircons.The relative high Ti content found in MW048 (Ti: 133 ppm) differentiates this zircon from others and relates its crystallization to a Ti-enriched environment, with significantly high temperature to facilitate Ti substitution in its structure.
The very low to low Nb and Ta contents in the western Meiganga zircons are probably due to the fact that these two elements do not easily substitute into the zircon structure.The highest Nb (6.9 ppm) and Ta (7.8 ppm) in some of those zircons can be explained by Nb, Ta, and REE coupled substitutions {(Y, REE) 3+ + (Nb, Ta) 5+ ↔ 2 Zr 4+ : [65]}, as those grains also have the highest ∑REE values.As shown in Figure 2d, there is a positive correlation between Nb and Ta showing that the degree of Nb and Ta substitution is the same, as most obtained Nb values are close to those of Ta.
The studied zircons are mostly REE enriched, as the ∑REE (generally below 800 ppm) are below those obtained ∑REE (up to 2161 ppm) in the Western Mamfe clastic zircons as studied by [12].Within the REE suites, particular features were noted: the absence of La, extreme variation of Ce contents, high Nd in MW006 and Eu anomaly.The Zr 4+ radius (0.84 Å) is close and matched the smaller HREE radii than larger LREE (e.g., La 3+ : 1.60 Å) such that the LREE are generally  [19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Magmatic Affiliated Zircons
The variable Hf contents (6390-13,020 ppm) in the western Meiganga magmatic zircons, suggest a different degree of Hf substitution in the zircon lattice.The same interpretation is consistent for the U, Th, and Y contents, as they also show consistent variations.The Pb (42-629 ppm), Ti (2-133 ppm), Nb (0.2-7.0 ppm), and Ta (0.2-8.0 ppm) contents in the zircons, are significantly low to very low.The presence of relatively-high Pb concentrations (e.g., 618 ppm: MW036; 629 ppm: MW093) in some of these zircons may be due to high concentration of this element in source magma coupled with Pb from Th and U radiogenic decay [65]; In contrast, the very low Pb content may be due loss of radiogenic Pb by diffusion during the geological history of low-Pb zircon source rock (cf.[66][67][68][69].Titanium does not frequently substitute for Zr in the zircon structure and the concentration is temperature dependent [70,71].The low Ti in most of the zircons may be due of low degree of Ti substitution, and the absence of favorable temperature for its maximum concentration in those zircons.The relative high Ti content found in MW048 (Ti: 133 ppm) differentiates this zircon from others and relates its crystallization to a Ti-enriched environment, with significantly high temperature to facilitate Ti substitution in its structure.
The very low to low Nb and Ta contents in the western Meiganga zircons are probably due to the fact that these two elements do not easily substitute into the zircon structure.The highest Nb (6.9 ppm) and Ta (7.8 ppm) in some of those zircons can be explained by Nb, Ta, and REE coupled substitutions {(Y, REE) 3+ + (Nb, Ta) 5+ ↔ 2 Zr 4+ : [65]}, as those grains also have the highest ∑REE values.As shown in Figure 2d, there is a positive correlation between Nb and Ta showing that the degree of Nb and Ta substitution is the same, as most obtained Nb values are close to those of Ta.
The studied zircons are mostly REE enriched, as the ∑REE (generally below 800 ppm) are below those obtained ∑REE (up to 2161 ppm) in the Western Mamfe clastic zircons as studied by [12].Within the REE suites, particular features were noted: the absence of La, extreme variation of Ce contents, high Nd in MW006 and Eu anomaly.The Zr 4+ radius (0.84 Å) is close and matched the smaller HREE radii than larger LREE (e.g., La 3+ : 1.60 Å) such that the LREE are generally incompatible in the zircon structure [65].The absence of La in the western Meiganga magmatic zircons may be due to the non-substitution of this element in their lattice, whereas the extreme variation of Ce abundance can be interpreted differently.For example, the below detection of Ce contents in part of the zircons may typify the lack of substitution of Ce and the presence of Ce 3+ (incompatible in zircon structure) and very low oxidization state in the source magma [65].The low Ce (1-18 ppm) and relatively high (150-330 ppm) contents found in some zircons are due to different oxidation state of Ce substitution in those grains.The low recorded Ce value can be due to limited Ce 4+ concentrations (low oxidization condition) in their source magma, whereas the relatively high values may typify high Ce 4+ concentration (high oxidization state) [72].
The calculated Eu anomalies range from 0.08 to 1.8 with most of those values being below 1 (negative anomalies), indicating reducing condition.The Nd content (1020 ppm in MW006) is the highest obtained value for REE in the studied zircons.This feature differentiates the grain from others, as it also encloses the highest Pr, Sm, and Eu (positive anomaly: Figure 3a).It is difficult to discern a particular interpretation to this feature as data on fluid/melt and mineral inclusions are absent.The very pronounced Eu negative anomalies (Eu/Eu* ≤ 0.1) for the magmatic zircons, characterize reducing conditions and may reflect a crystallization of a mantle source magma as the Eu anomalies in this magma are very low [65].The low negative Eu anomaly (0.1 < Eu/Eu* < 1; reducing condition) and relatively high positive Eu anomalies: Eu/Eu* = 1(no anomaly), and Eu/Eu* > 1 (positive anomaly: oxidizing condition) are the dominant features found in magmatic zircons that grew within the crust.

Source and Tectonic Frame Work
Hafnium, U, Th, Y, Ti, and REE abundances in zircons have been used to study the tectonic setting and identified a specific source rock type [19,27,73,74].Heaman et al. [27] described the Hf content in zircons crystallized within a continental rift setting as below 9000 ppm.Hafnium contents in the western Meiganga detrital zircons are predominantly >9000 ppm; with part of the zircons grew during continental rifting.For [74] the Hf content in some zircons from oceanic crust is greater than 1.1 wt %.The Hf contents in part of the studied Hf-enriched zircons are within the range limit noted by [74] in oceanic crust zircons.However, it is not easy to associate the crystallization of these zircons (Hf > 10,000 ppm) to an oceanic crust setting, as some analyses/works are still needed to unequivocally support this hypothesis.Hafnium contents in zircons from alkaline magmatic rocks are below 11 wt % (e.g., [19,54]).
Element contents in part of the studied zircons are within the range limit in alkaline magmatic zircons; relating their crystallization in alkaline rocks.The group 1 (Hf ≤ 8910 ppm) are within the range (6000-8000 ppm) in nepheline-syenite zircons of [27], higher than the value (Hf = 5900 ppm) in syenites studied by [63].Relating their crystallization to nepheline syenite is not possible, as they were not plotted in nepheline syenite field in any of the three diagrams in Figures 7-9, but in kimberlite, syenite, granitoid, or mafic rocks.Those greater than 9000 ppm, are within the range in zircons from mafic rocks [27], dolerites [54], and some gabbro [74].Those in group 3 (Hf ≥ 10,000 ppm) are within the range (dominantly within 10,065 and 12,981 ppm) in zircons from felsic rocks studied by [27], with some of those values close to those in zircons from [54].The granitoid affiliation of some of those zircons is supported by their U abundance mostly within the range (154-4116 ppm) in [56] granitic zircons.
The U content of kimberlite lies between 3 and 69 ppm, Y between 4 and 194 ppm, and Yb between 0.16 and 36 ppm [54].The geochemical features in grains (MW100 and MW113) are within the range limit of kimberlitic zircons, as they are plotted within the kimberlite field in Figures 7-9.The dominant crustal origin of the western Meiganga zircons is confirmed with their U (>65 ppm) and Th (>45 ppm), Y and ∑REE contents, which generally fall within the range limit in [27], [54], and [65] crustal zircons.The Y (67-1867 ppm) values for the studied zircons are within the range limit in crystals from igneous rocks (e.g., 911-17,258 ppm: syenite pegmatite; 3243-20,612 ppm: nepheline-syenite pegmatite; 120-171: syenite; 83-2342 ppm: basalts; 821-4415 ppm: dolerite; 376-67,922 ppm: granitoids) [54].Zircon saturation temperatures are a function of crystallization temperature and melt composition [75], and their relationship to Th/U ratios help to understand magmatic melt condition [60].The calculated Ti-zircon-temperatures in Table 1, are highly heterogeneous with equal values obtained for some zircons (700 • C for MW033 and MW045), suggesting their crystallization at the same temperature.Those values compared to those for primary zircons published in [73,74], and show some correlation.The lowest values (≤697 • C) are more close to temperatures obtained for zircons from tonalites and granites, whereas higher values (700 ≥ T ( • C) ≤ 787) are close to data for zircons from tonalites, rhyolitic, and dioritic rocks.Relatively high to very high values: 815 to 892 and 915 to 1065 • C, respectively, are mostly characteristics of zircons from gabbroic rocks, although the obtained temperature (815 • C for MW024) is equal to the maximum value for zircons from anorthosite found Kikkertavak Island, Nain Anorthosite Complex, Labrador studied by [73].The Ti-zircon temperature correlations for the Western Meiganga magmatic affiliated zircons show that they are mainly composed of zircons from granitoids, anorthosite, tonalites, gabbroic and unidentified felsic intermediate rocks.

Metamorphic Affiliated Zircons
The Hf contents in the metamorphic affiliated zircons from the western Meiganga gold placer range from 9050 to 12,580 ppm, with some of the values being close to those of magmatic zircons found in this same placer.This similarity in Hf content shows that the degree of Hf substitution was almost the same in metamorphic and part of igneous zircons.The relative high Hf content (up to 12,580 ppm) shows that as for igneous zircons, they were crystallized in Hf-enriched melts.The Hf content in these zircons is greater than values that are found in zircons grown within continental rift settings [27].They may be zircons crystallized within crustal settings during syn-tectonic to syn-metamorphic events, as their U contents are generally greater than values found in mantle-derived zircons.
The Y contents are dominantly greater than 250 ppm and are close to those found in some of the magmatic zircons from this placer, the same is valid for Nb, Ta, Sr, Ti and U contents, and Ti-in-zircon temperatures.This similarity in elemental content and temperature may be due to their similar crystallization conditions.The main difference is that the Th (2-17 ppm) and Pb (1-8 ppm) contents which are, generally, lower than quantified values in magmatic zircons, but are close or similar to some values published by [57] for zircons from Ultra-HP eclogites and metasediments.Relating the crystallization of those metamorphic zircons to Ultra-HP eclogites and metasediments is not easy as more analyses are needed.The depletion in Th and Pb contents may relate their crystallization within Th and Pb depleted melts with low degree of Th and Pb substitution.They may be re-crystallized grains, as re-crystallized zircons are often Hf-enriched and Th-Pb-depleted [65].Other features distinguishing the metamorphic zircons from the magmatic grains is the enrichment in HREE with respect to LREE is much more variable than that observed in magmatic zircon, and the low MREE.The low MREE in metamorphic zircons is interpreted to be due to the relative depletion of these elements in their crystallization milieu [49].Some of the zircons that possess relatively depleted HREE abundances and very small negative Eu anomalies (Eu/Eu* = 0.24-0.63),have features found in zircons grown by net transfer reactions in high-grade metamorphic rocks [57].A negative Eu anomaly is a common feature of HT metamorphic rocks [75].The calculated Eu anomalies and plotted data in Figure 4 for these zircons are negative and positive, with positive values found in zircon with highest Ti-zircon temperatures; contradicting the above information.Those with positive Eu anomalies were probably grown in oxidizing conditions at high temperature.
The composition of metamorphic zircon in equilibrium with an anatectic melt does not differ greatly from igneous zircons.The zircons are enriched in trace elements and have steep REE patterns increasing from La to Lu with positive Ce and negative Eu anomalies [65].The features presented above are characteristics of MW001, and relate its crystallization in an anatectic melt.The normalized-chondrite REE data presented in Figure 4 and based on [65] studies, distinguished two main generations of zircons: those produced by sub-solidus growth in equilibrium with garnet (e.g., MW015, MW022, and MW052) and grew in equilibrium with anatectic melt (e.g., MW001).Two groups are distinguished within the first generation: (1) zircons with positive Eu anomaly and (2) grains with negative Eu anomaly.The features in group 2 zircons are common in metamorphic zircons [57], but those presented in group 1 are difficult to interpret as data on source rocks are lacking.For [76] in petrogenesis of metamorphic rocks, during high temperature metamorphism within crustal environments and at the pressure of 500 MPa, and in the presence of an aqueous fluid, granitic rocks begin to melt at a temperature of about 660 • C, whereas basaltic rocks need a much higher temperature of about 800 • C.
The measured Ti-zircon temperature in the metamorphic zircon grains ranges from 571 to 665 • C with the highest value being close to temperatures recorded at the beginning of fusion of granitic rocks.Those with relative high temperature (621 to 665 • C: close to some magmatic zircons), may be the grains crystallized at the beginning of melting of granitic parent rocks during high temperature regional metamorphism.The type of the metamorphism (subduction type or collisional type) is difficult to be determined by simple examination of geochemical features and temperature in clastic zircons, although the recorded temperature for those zircons are much closer to the highest temperatures (up to 700 • C) obtained during subduction-type orogenic metamorphism [76].

Detrital Zircon Geochronology and the Potential Source of Gold Placer
The recorded 206 Pb/ 238 U ages ranging from 576 ± 10 to 2927 ± 52 Ma (Table 3 and Figures 5  and 6), are highly heterogeneous, with the youngest ages (≤611 Ma) recorded mostly in metamorphic zircons and oldest ages in mainly those of a magmatic origin.Major age peaks are: Neoproterozoic, late Mesoproterozoic, late Paleoproterozoic, early Paleoproterozoic, and Archean.The metamorphic zircons were mainly crystallized during the Neoproterozoic, whereas the magmatic grains grew during Archean to early Neoproterozoic.Those suggested to crystallize by partial melt segregation were formed during Archean to Paleoproterozoic (2585 ± 45 to 1958 ± 39 Ma) with most of them being of Paleoproterozoic.The mostly magmatic nature of the zircons shows that their source areas are dominantly made of magmatic rocks and/or metamorphic rocks with inherited igneous zircons.The ages of some of the magmatic zircons are similar, suggesting that they have been crystallized at the same time and derived likely from the same rock type.
The age of the magmatic zircon grains is extremely variable, with the peak of crystallization of early Paleoproterozoic, followed by late Paleoproterozoic, late Mesoproterozoic and Archean.This shows that rocks of these ages dominantly provided part of the detritus forming the western Meiganga gold placer.A few were eroded from late Mesoproterozoic and Neoproterozoic age rocks.Within these 5 groups (Archean to early Neoproterozoic), 7 sub-groups can be distinguished with the peak being that of the Rhyacian to Orosirian (2050-1807 Ma).If they are from different source rocks, it should reflect the existence of different magmatic history in their source areas.It started during Rhyacian and continues up to Orosirian.This hypothesis is confirmed by the geochemical characteristics presented above.A single grain (MW105: Hf = 8970 ppm; age: 1922 Ma) is suggested to have crystallized within a continent rift setting, whereas the other grains (Hf > 9000 ppm) are not related to continental rifting.The same interpretation can be applied to other Paleoproterozoic zircons, which are also composed of a population crystallized within rift settings and non-rifting settings.From a general point of view, with the exception of the Mesoproterozoic and early Neoproterozoic age zircons that were exclusively not crystallized during rifting, the other zircon sub-groups are composed of those crystallized during rifting and out of rifting.What is remarkable is that the oldest zircons found in the Archean sub-group were crystallized within a continental rift setting.This shows that the magmatic history in their source of area of the Archean zircons started with rifting and mainly continued by crustal crystallization for the youngest grains, although late and post-Archean rifting were also registered.
The metamorphic zircons ranging in age from 1915 to 576 Ma ( 206 Pb/ 238 U ages) are also variable; they are dominantly composed of grains crystallized during the Neoproterozoic (Ediacarian) with The source rocks of those detrital zircons are the same nature as those described above (syenitic, doleritic, charnockitic, or granitic) that are distinguished from their geochemical variations.The Neoproterozoic ages (611-576 Ma) of the youngest zircons found within the western Meiganga placer overlap the range of ages for Pan-African zircons found along the Cameroon mobile belt, indicating their crystallization period and has implications on tracking gold placer source(s).

Figure 1 .
Figure 1.Sketch geologic map locating West of Meiganga within the local and regional settings.Figure 1. Sketch geologic map locating West of Meiganga within the local and regional settings.

Figure 1 .
Figure 1.Sketch geologic map locating West of Meiganga within the local and regional settings.Figure 1. Sketch geologic map locating West of Meiganga within the local and regional settings.
Minerals 2018, 8, x FOR PEER REVIEW 7 of 28versus Th binary diagram (Figure2b) shows positive correlation, whereas no correlations are evident in the Th/U versus U diagram (Figure2c).

Figure 2 .
Figure 2. Geochemical correlation of various elements/ratios within the western Meiganga detrital zircons (a): Th versus U; (b): Th/U versus Th; (c): Th/U versus U; (d) Nb versus Ta; blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 2 .
Figure 2. Geochemical correlation of various elements/ratios within the western Meiganga detrital zircons (a): Th versus U; (b): Th/U versus Th; (c): Th/U versus U; (d) Nb versus Ta; blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).
Minerals 2018, 8, x FOR PEER REVIEW 8 of 28 zircons.The calculated Eu/Eu* anomalies (0.05 to 2.1) and normalized chondrite plots (Figures 3 and 4) predominantly exhibit negative Eu anomalies with a few showing positive to the lack of an anomaly.

Figure 3 .
Figure 3.The western Meiganga magmatic zircon grains are normalized to [59] chondrite values and are plotted on Log10 versus element (La-Lu) diagrams.

Figure 3 .
Figure 3.The western Meiganga magmatic zircon grains are normalized to [59] chondrite values and are plotted on Log 10 versus element (La-Lu) diagrams.

Figure 3 .
The western Meiganga magmatic zircon grains are normalized to[59] chondrite values and are plotted on Log10 versus element (La-Lu) diagrams.

Figure 4 .
Figure 4.The western Meiganga metamorphic zircon grains are normalized to [59] chondrite values and are plotted on Log10 versus element (La-Lu) diagrams.

Figure 4 .
Figure 4.The western Meiganga metamorphic zircon grains are normalized to [59] chondrite values and are plotted on Log 10 versus element (La-Lu) diagrams.

Figure 6 .
Figure 6.Plot showing spatial distribution of U-Pb ages for the western Meiganga detrital zircons.Figure 6. Plot showing spatial distribution of U-Pb ages for the western Meiganga detrital zircons.

Figure 6 .
Figure 6.Plot showing spatial distribution of U-Pb ages for the western Meiganga detrital zircons.Figure 6. Plot showing spatial distribution of U-Pb ages for the western Meiganga detrital zircons.

Figure 7 .
Figure 7. Y versus U plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 8 .
Figure 8. Y versus Nb/Ta plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 7 .
Figure 7. Y versus U plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 7 .
Figure 7. Y versus U plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 8 .
Figure 8. Y versus Nb/Ta plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 8 .
Figure 8. Y versus Nb/Ta plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 9 .
Figure 9. Y versus Y/Sm plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Figure 9 .
Figure 9. Y versus Y/Sm plot of western Meiganga magmatic zircons (adapted from[19,54], blue and white diamond-shape plots are for magmatic zircons; red and white square plots are for metamorphic zircon).

Table 1 .
Minor and trace elementary abundance (in ppm) in the western Meiganga detrital zircons.

Table 2 .
Rare earth element abundance (in ppm) in the western Meiganga detrital zircons.

Table 3 .
Isotopic geochemical data and U-Pb core age (in Ma) for the west Meiganga detrital zircons.