Eoarchean to Neoproterozoic Detrital Zircons from the South of Meiganga Gold-Bearing Sediments (Adamawa, Cameroon): Their Closeness with Rocks of the Pan-African Cameroon Mobile Belt and Congo Craton

: The core of detrital zircons from the southern Meiganga gold-bearing placers were analyzed by Laser Ablation Split Stream analytical techniques to determine their trace element abundances and U-Pb ages. The obtained data were used to characterize each grain, determine its formation condition, and try to trace the provenance. The Hf (5980 to 12,010 ppm), Y (27–1650 ppm), U (25–954 ppm), Th (8–674 ppm), Ti (2–256 ppm), Ta, Nb, and Sr (mainly <5 ppm), Th/U (0.06–2.35), Ti zircon temperature (617–1180 ◦ C), ∑ REE (total rare earth element) (98–1030 ppm), and Eu/Eu* (0.03 to <1.35) are predominant values for igneous crustal-derived zircons, with very few from mantle sources and of metamorphic origin. Crustal igneous zircons are mainly inherited grains crystallized in granitic magmas (with some charnockitic and tonalitic afﬁnities) and a few from syenitic melts. Mantle zircons were crystallized in trace element depleted mantle source magmatic intrusion during crustal opening. Metamorphic zircons grown in sub-solidus solution in equilibrium with garnet “syn-metamorphic zircons” and in equilibrium with anatectic melts “anatectic zircons” during crustal tectono-metamorphic events. The U-Pb (3671 ± 23–612 ± 11 Ma) ages distinguish: Eoarchean to Neoproterozoic igneous zircons; Neoarchean to Mid Paleoproterozoic anatectic zircons; and Late Neoproterozoic syn-metamorphic grains. The Mesoarchean to Middle Paleoproterozoic igneous zircons are probably inherited from pyroxene-amphibole-bearing gneiss (TTGs composition) and amphibole-biotite gneiss, whose features are similar to those of the granites, granodiorites, TTG, and charnockites found in the Congo Craton, south Cameroon. The youngest igneous zircons could be grains eroded from Pan-African intrusion(s) found locally. Anatectic and syn-metamorphic zircons could have originated from amphibole-biotite gneiss underlying the zircon-gold bearing placers and from locally found migmatized rocks that are from the Cameroon mobile belt, which could be used as proxies for tracking gold. terrigenous sediments (alluvium, eluvium, colluvium, and terrace) of mostly unknown primary sources (Figure 1). The clastic gold particles (very ﬁne to coarse grained) are generally associated with some heavy minerals (e.g., zircon, magnetite, kyanite, ilmenite, and tourmaline) [1,2]. These weathering-resistant minerals are very useful in provenance studies, as they can register important information on their source rock petrogenesis, paleoenvironment, and tectonic reconstitution [3–9]. Some of these heavy minerals often ﬁngerprint information on the chemistry of their environment of crystallization, the nature of their source rocks, and on the pre-existing tectonic settings [4,5,8–11]. Zircon in particular is an important mineral in ﬁngerprinting source parameters [5,12–15]. One of the key tools to determine zircon source parameters is the combination of zircon geochemistry and U-Pb dating [4,8–10]. Each zircon has a characteristic age reﬂecting its genesis, and the population of detrital zircons in a sediment is a function of the age signature of source rocks in the proto-source terranes [12].


Introduction
Meiganga is one of the key areas for small scale gold mining activities in Cameroon. As in many areas in this country, gold is extracted from supergene assemblages and

Introduction
Meiganga is one of the key areas for small scale gold mining activities in Cameroon. As in many areas in this country, gold is extracted from supergene assemblages and terrigenous sediments (alluvium, eluvium, colluvium, and terrace) of mostly unknown primary sources ( Figure 1). The clastic gold particles (very fine to coarse grained) are generally associated with some heavy minerals (e.g., zircon, magnetite, kyanite, ilmenite, and tourmaline) [1,2]. These weathering-resistant minerals are very useful in provenance studies, as they can register important information on their source rock petrogenesis, paleoenvironment, and tectonic reconstitution [3][4][5][6][7][8][9]. Some of these heavy minerals often fingerprint information on the chemistry of their environment of crystallization, the nature of their source rocks, and on the pre-existing tectonic settings [4,5,[8][9][10][11]. Zircon in particular is an important mineral in fingerprinting source parameters [5,[12][13][14][15]. One of the key tools to determine zircon source parameters is the combination of zircon geochemistry and U-Pb dating [4,[8][9][10]. Each zircon has a characteristic age reflecting its genesis, and the population of detrital zircons in a sediment is a function of the age signature of source rocks in the proto-source terranes [12]. Meiganga is in the Central part of the Cameroon mobile belt [16], a mega-tectonic structure that formed during the Neoproterozoic, from a collision between the Saharan meta-craton and Congo Craton [17,18]. Recent research works carried out on rocks found in the west of Meiganga have revealed the existence of Archean and Paleoproterozoic inheritance [19][20][21]. Trace element geochemistry and U-Pb ages of detrital zircon from gold-bearing placers in the west of Meiganga show that they were mainly crystallized and sourced from Archean to Precambrian granitoids [9].
Gold-bearing sediments were found in some streams in the southern part of Meiganga. The gold particles are associated with zircon, tourmaline, magnetite, kyanite, and ilmenite [1]. The source rocks and crystallization processes of most of these heavy minerals are poorly constrained. Djou [1] suggested a gneissic origin for a part of the deposited clasts, based on the presence of kyanite within the heavy mineral suites. Detailed analyses have not yet been carried on those minerals to help understand their source history and constrain their provenance. In this paper, we present trace element abundance and U-Pb core age for zircons from this gold placer. These data are used to characterize each grain, understand its formation history, and try to locate its proto-source and source rock within the local and regional settings.

Brief Review of the Regional Geology
Basement formations in Cameroon ( Figure 2) comprise Archean, Paleoproterozoic (Eburnean), and Pan-African rocks (Table 1). Archean units (>2500 Ma) constitute the Congo Craton, while the Paleoproterozoic ones (2400-1800 Ma) include the West Central African Belt and Pan-African/Cambrian units that constitute the Oubanguide Belt in the Mobile Zone [22,23].

The Ntem Complex
The Archean Craton or Ntem Complex ( Figure 2) located in the northwestern end of the Congo Craton [24] is mainly composed of Archean rocks (Table 1) with some reworked Paleoproterozoic material that formed in early Proterozoic times [25]. It is structurally made up of two main units: the Ntem (at the center and south) and the Nyong (in the northwestern) [24]. The Ntem unit is essentially made up of tonalite, trondhjemite, and granodioritic suites (TTGs) and charnockites with TTGs cutting across charnockites and greenstone belts [26]. TTGs and charnockites enclose xenolithic remnants (3.1 Ga) of greenstone belts (banded iron formations and sillimanite-bearing paragneisses) [27,28]. Bounded Iron Formations found in the Ntem unit are locally intercalated with metasiltstones and meta-sandstones [29]. The TTGs and charnockites were intruded by K-rich granitoids (monzogranite and syenogranite) during the Archean [30,31] and cross-cut by metadoleritic dykes during the Eburnean [32,33] or Late Archean time [34]. The Nyong unit, ranging in age from Archean to Paleoproterozoic [32,[35][36][37], and part of the West Central African Fold Belt [23], is composed of migmatitic orthogneisses (TTGs), metagabbro, amphibolite, garnetite, eclogite, felsic gneiss of volcanic to volcano-sedimentary origin, quartzite, charnockite, meta-syenite, and BIF [22,23,37]. Migmatites, charnockites, and meta-sedimentary rocks are Archean in age [32,35]. Table 1. Summarized ages of plutonic and metamorphic rocks found in Congo Craton, Cameroon mobile belt, and Meiganga. Poli (520 ± 20 Ma) and Lom (498 ± 5 (Ma) (Rb-Sr age on whole -rock, [35]) Nkambe (530 ± 10 Ma and 510 ± 25 Ma; 569 ± 12 to 558 ± 24 Ma, and 533 ± 12 to 524 ± 28 Ma) [40,41] Ngondo (600 Ma) [42] Tonga (618 Ma) [43] Pyroxene-amphibolebearing gneiss (TTG composition) (  The Cameroon mobile zone or Central African Fold belt is a mega-tectonic structure underlying Cameroon, Chad, and the Central African Republic between the Congo Craton to the south and the Nigerian shield to the north [52]. It was formed during the Neoproterozoic, from the collision between the Saharan meta-craton and the Congo craton [17,18]. In the Cameroonian territory, the Central African Fold belt is made up of three main structural units: the Poli Group in its northern part, the Adamawa at the center, and the Yaoundé Group in the south [18]. Within the central African fold belt, several domains are recognized on the basis of field, petrographic, structural, and isotopic studies. These include the Paleoproterozoic gneissic basement, Mesoproterozic to Neoproterozoic schists, and gneisses of Poli, Yaoundé, and Lom, and Pan-African granitoids whose ages range from the early stage of the deformation (orthogneisses) to the late uplit stages of the belts [36]. Examples of geochronological data of some of these rocks are summarized in Table 1.
The western part of Meiganga is made up of pyroxene-amphibole orthogneiss, amphibole gneiss, biotite gneiss, amphibole-biotite gneiss, amphibolite, calc-alkaline and two mica-granites, and amphibole-biotite granite [16,[19][20][21]44,47,53]. Pyroxene-amphibole orthogneiss locally enclose mafic xenoliths [44]. The geochemical features of the orthogneiss and U-Pb zircon ages are similar to those of many TTGs and charnockites outcropping within the Archean Ntem complex in the south of Cameroon [20,44]. The ages of some rocks in the west of Meiganga presented in Table 1 range from Archean to Neoproterozoic. Zircons occurring in a gold-bearing placer in the west of Meiganga are inherited grains crystallized from Archean to Precambrian magmatic crustal evens with part of their source rocks being granitoids, TTG, and charnockites [9].
The southern part of Meiganga (Figure 1c) from where the studied zircon were sampled are composed of mainly undated graphite schists, amphibolites, mica-rich quartzites, amphibole-biotite gneisses, orthogneisses, migmatites, calc-alkaline granitic rocks, biotiteamphibole, and biotite-chlorite granites whose formation periods are assumed to be Precambrian as they also belong to the central part of the Cameroon mobile belt [53]. Hornfels are found at the contact between calc-alkaline biotite-chlorite granite and biotite-chlorite granite at the south eastern part of the locality (Figure 1c). Rocks found in valleys are locally covered by alluvial flats and terraces with part of the alluvium hosting gold.

Materials and Methods
In total, 111 zircons from gold-bearing alluviums in two areas (Gankoumbol and Yende: Figure 1) were analyzed to determine the trace elements composition and U-Pb core age at the University of California, Santa Barbara, CA, USA. The results from each zircon core were acquired by Laser Ablation Split Stream analytical techniques. The analyzed zircons were sampled upstream and in small size streams to be close to the source area.
They were separated from pre-concentrated heavy and light minerals mixtures obtained from 50 L of mainly very coarse-grained alluvium, at the bottom of the gold-bearing pits. Heavy mineral fractions were separated from light minerals using bromoform (Density: 2.7 g/cm −3 ) at the Department of Earth Sciences of the University of Yaoundé I, Cameroon. The separation procedure is similar to the one described in [54,55].
The gold-bearing heavy mineral fractions were sent for zircon separation, trace element analysis, and U-Pb dating at the Department of Earth Sciences of the University of California. The analytical procedures used to obtain the zircon trace element and U-Pb age data are the same as those presented in [9]. Each mounted grain is polished and analyzed following standard procedures 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 a plasma-mass spectrometer and an Agilent 7700S quadrupole inductively coupled plasma-mass spectrometer (for detailed methodology see [56][57][58]. Samples were abraded for 20 s using a fluence of 1.5 J/cm 2 , a frequency of 4 Hz, and a 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 the 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 [59]. "91500" 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: [60]) 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: [61]) 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 version 2.4 [62] and Density Plotter [63], respectively, using the 238 U and 235 U decay constants of [64]. 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.

Minor Elements
The relatively high Y and Hf contents in part of the studied zircons can reflect a crystallization in Hf-Y-rich melts with favorable conditions for Hf and Y to substitute Zr. The Hf/Y ratios (5.0−293.0) are mainly less than 30, with the highest values exclusively being those of zircons with very low Y contents.

U-Pb Dating
The U-Pb zircon core ages (

Igneous Affiliated Zircons
The trace and rare earth element abundances in the studied igneous affiliated zircons are generally less than those in some zircons found in Cameroon (e.g., [8,9,70]). For example, zircon inclusions in Mayo Kila gem corundum found in the NW region of Cameroon are composed of Hf (≤26,238 ppm), U (≤17,175 ppm), and Th (≤45,584 ppm) [70]. The ∑REE contents obtained for detrital zircons occurring with gem corundum in the Mamfe Basin, SW region of Cameroon are up to 1470 ppm [8]. These values are largely greater than those of the studied igneous zircons (Tables 2-4). They can, therefore, be classified as Hf-U-Th-REE-low zircons. Their Hf values are mainly close to those of magmatic zircons found in the western Meiganga gold-bearing placers (cf. [9]), and might show closeness in their crystallization history. The Hf contents in part of the studied zircons are compatible with the values (<11,000 ppm) in zircons crystallized in alkaline magmas [4,71], suggesting a crystallization in alkaline melts. Their plotted data in Figures 4 and 8 show some correlations, as some zircons are plotted together, suggesting a cogenesis and crystallization in the same/similar magma or in different magmas with similar features. This similarity is supported by the closeness of the values of other trace elements, Th/U ratios, and Ti zircon temperatures, and Eu/Eu*. magmatic zircons found in the western Meiganga gold-bearing placers (cf. [9]), and might show closeness in their crystallization history. The Hf contents in part of the studied zircons are compatible with the values (<11,000 ppm) in zircons crystallized in alkaline magmas [4,71], suggesting a crystallization in alkaline melts. Their plotted data in Figures 4 and 8 show some correlations, as some zircons are plotted together, suggesting a cogenesis and crystallization in the same/similar magma or in different magmas with similar features. This similarity is supported by the closeness of the values of other trace elements, Th/U ratios, and Ti zircon temperatures, and Eu/Eu*.  The elemental abundances, Th/U ratios, and Ti-zircon temperatures (617-1180 • C) in the igneous zircons distinguished those with relatively high and relatively low values. Relatively high elemental abundance zircons were probably crystallized in trace elements and REE-enriched melts, with favorable conditions for these elements to substitute Zr in each forming crystal. They are probably crustal-derived zircons, as zircon from crustal rocks generally have elevated contents of some trace elements (notably U, Th, and Y) and REE [4,5]. The relatively high Th (674 ppm), Y (1474 ppm), U (461 ppm), Ti (33 ppm), Nb (25 ppm), and Ta (20 ppm) in MSDZ085, for example, can relate its crystallization in a Y-Th-U-Ti-Nb-Ta-rich magma. The Zr substitution by Nb and Ta during this zircon crystallization was probably governed by Nb, Ta, and REE coupled mechanism (cf. [5]), as this grain also has significant total REE (990 ppm). The relatively high Ti (256 ppm in MSDZ055) may be due to crystallization in Ti-enriched environment with sufficient temperature for Ti to substitute Zr; alternatively, it can be due to Ti-rich mineral inclusion. Relating the high Ti content to a mineral inclusion is difficult, as no inclusion was visualized. The relatively high-elemental zircons are generally from granitoids, as their plots fall essentially in granitoid fields in Figures 9-11. The granitoid origin of those zircons is confirmed by the Y, U, Th, and Yb abundances, largely within the range limit in granitic zircons [4,11,71].
(461 ppm), Ti (33 ppm), Nb (25 ppm), and Ta (20 ppm) in MSDZ085, for example, can relate its crystallization in a Y-Th-U-Ti-Nb-Ta-rich magma. The Zr substitution by Nb and Ta during this zircon crystallization was probably governed by Nb, Ta, and REE coupled mechanism (cf. [5]), as this grain also has significant total REE (990 ppm). The relatively high Ti (256 ppm in MSDZ055) may be due to crystallization in Ti-enriched environment with sufficient temperature for Ti to substitute Zr; alternatively, it can be due to Ti-rich mineral inclusion. Relating the high Ti content to a mineral inclusion is difficult, as no inclusion was visualized. The relatively high-elemental zircons are generally from granitoids, as their plots fall essentially in granitoid fields in Figures 9-11. The granitoid origin of those zircons is confirmed by the Y, U, Th, and Yb abundances, largely within the range limit in granitic zircons [4,11,71]. Interpretations for very low to low elemental contents in part of the studied zircons can be approached in three ways: (1) elements in those zircon's forming melts are present, but good conditions to ensure that these elements go into their structure are lacking; (2) the depletion or absence of some elements in those zircons' environment of crystallization; or (3) the presence of other accessory minerals (e.g., apatite, xenotime, monazites, allanite, and titanite) [66,72,73] crystallizing in the same melt and competing for REE and other trace elements. The lowest Hf contents in relatively low elemental zircons are within the range limit (4576-6500 ppm) in magmatic zircons found in the western Mamfe corundum gem placers [6][7][8] and zircon mega-crysts found in alluvial gem corundum deposits associated with alkali basalts (e.g., [74]). These values are also within the range limit (Hf < 9000 ppm) in [10] magmatic zircons crystallized during tectonic rifting. Rifting cannot yet be suggested, as Hf isotopic data are lacking for a detailed interpretation. Zircons from basic and ultrabasic igneous rocks (mantle zircons) are generally depleted in U, Th, Y, and REE [10,75,76]; it is possible that part of the southern Meiganga zircons (e.g., MSDZ016, MSDZ031, MSDZ038, and MSDZ106) were crystallized in mantle source magma(s) as their features, namely U < 30 ppm, Th < 10 ppm, and some plots falling in the mafic rocks field (Figures 9-11), are within the range limit in mantle zircons. The Ti-zircon-temperature (<850 °C) for part of the very low U and Th zircons is less than the temperature (>1300 °C: [77]) for the primary mantle source magma. This temperature difference can complicate the affiliation of part of the very low U and Th zircons to mantle sources. They could be crystals that crystallized at the last stage of cooling mantle source magmas or crystals formed in cooling magmas that originated from the partial fusion of pre-existing mafic rocks. A mafic granulitic origin can be suggested, as part the temperatures are within the range limit (816 ± 12 °C to 798 ± 13 °C) proposed by [78]. Based on the plots of very low to low elemental contents zircons in Figures 9-11, three protosources are distinguished: granitoids, syenites, and mafic rocks.  Interpretations for very low to low elemental contents in part of the studied zircons can be approached in three ways: (1) elements in those zircon's forming melts are present, but good conditions to ensure that these elements go into their structure are lacking; (2) the depletion or absence of some elements in those zircons' environment of crystallization; or (3) the presence of other accessory minerals (e.g., apatite, xenotime, monazites, allanite, and titanite) [66,72,73] crystallizing in the same melt and competing for REE and other trace elements. The lowest Hf contents in relatively low elemental zircons are within the range limit (4576-6500 ppm) in magmatic zircons found in the western Mamfe corundum gem placers [6][7][8] and zircon mega-crysts found in alluvial gem corundum deposits associated with alkali basalts (e.g., [74]). These values are also within the range limit (Hf < 9000 ppm) in [10] magmatic zircons crystallized during tectonic rifting. Rifting cannot yet be suggested, as Hf isotopic data are lacking for a detailed interpretation. Zircons from basic and ultrabasic igneous rocks (mantle zircons) are generally depleted in U, Th, Y, and REE [10,75,76]; it is possible that part of the southern Meiganga zircons (e.g., MSDZ016, MSDZ031, MSDZ038, and MSDZ106) were crystallized in mantle source magma(s) as their features, namely U < 30 ppm, Th < 10 ppm, and some plots falling in the mafic rocks field (Figures 9-11), are within the range limit in mantle zircons. The Tizircon-temperature (<850 • C) for part of the very low U and Th zircons is less than the temperature (>1300 • C: [77]) for the primary mantle source magma. This temperature difference can complicate the affiliation of part of the very low U and Th zircons to mantle sources. They could be crystals that crystallized at the last stage of cooling mantle source magmas or crystals formed in cooling magmas that originated from the partial fusion of pre-existing mafic rocks. A mafic granulitic origin can be suggested, as part the temperatures are within the range limit (816 ± 12 • C to 798 ± 13 • C) proposed by [78]. Based on the plots of very low to low elemental contents zircons in Figures 9-11, three protosources are distinguished: granitoids, syenites, and mafic rocks. 798 ± 13 °C) proposed by [78]. Based on the plots of very low to low elemental contents zircons in Figures 9-11, three protosources are distinguished: granitoids, syenites, and mafic rocks.

Metamorphic Affiliated Zircons
The geochemical features in part of the southern Meiganga detrital zircons are compatible with those of metamorphic zircons grown in equilibrium with garnet (Th/U < 0.07, depletion in REE, Eu/Eu*: 0.24-0.63) (cf. [68]) and crystals grown in equilibrium with an anatectic melt (Th/U < 0.2; relatively trace element-enriched, depleted in MREE, steep REE patterns, positive Ce, and negative Eu anomalies) (cf. [5,9,11,67]). Only one zircon (MSDZ046) with Hf: 9790 ppm, Y: 155 ppm, U: 396 ppm, Th: 26 ppm, Th/U: 0.065, and ∑REE: 98.15 ppm, and Ti temperature: 690 • C, has features close to that of [68] metamorphic zircon grown in subsolidus solution in equilibrium with garnet. This zircon may have crystallized during syn-metamorphic crustal even in low-Th-REE melt. The other zircons (MSDZ015, MSDZ040, MSDZ067, and MSDZ093) have features of zircon grown in equilibrium with anatectic melts, as presented above. The Ti-in-zircon temperatures for these zircons range from 656 ± 60 • C to 778 ± 44 • C, with some values being close to experimental values for granulitic facies metamorphism presented in [78]. Relating their sources to granulitic facies metamorphism is difficult, as some analyses are still needed. MSDZ082, with its positive normalized Pr and Gd plots, is different from the other zircons, as it also has the highest Y (1650 ppm). This grain was plotted in granitoid fields in Figures 9-11, and its other features are close to those of granitoid zircons. It was probably crystallized in an anatectic melt of a granitic composition, as a geochemical feature of a metamorphic zircon grown in equilibrium with anatectic melt does not differ from that of igneous zircons (cf. [5]).

Detrital Zircon Geochronology and Fingerprinted Magmatic-Metamorphic Events
The recorded U-Pb ages ( Table 3, Figures 6 and 7) are mainly heterogeneous with some similarities. The heterogeneity of most of the ages show that they were crystallized at different periods and probably sourced from different protosources and/or source rocks. The crystallization periods of igneous crustal derived zircons, ranging from Eoarchean to Late Neoproterozoic (Figure 7), is composed of three main periods with the following peaks: (1) Figure 12) and fall in the same rock type field (in Figures 9-11).
Minerals 2020, 10, x FOR PEER REVIEW 25 of 31 that this syngenetic zircon was crystallized in a garnet-rich rock during Middle Neoproterozoic event, probably the Pan-African orogeny, which affected the Cameroon Mobile Belt. This age is close to those of some Pan-African rocks within the Cameroon Mobile Belt presented in Table 1.

Age Correlation, Potential Sources Rocks, and Deposition
The southern part of Meiganga from where the studied zircons were sampled is mostly made up of undated biotite-amphibole granites; biotite-amphibole gneisses; biotite granites; biotite-chlorite granitic rocks; and few amphibolites and hornfels (see [53]). With a lack of available data dating those rocks, it is difficult to do a local correlation to locate nearby proto-source(s) and source rocks for the southern detrital zircon Meiganga. However, at local and regional scales, the obtained ages are partly similar to those of zircons occurring in the western Meiganga gold-bearing placer presented in [9] Tables 2-4). The Hf contents in this group of zircons (mantle zircons) are all bellow 9000 ppm, and therefore, within the limit proposed by [10]  Paleoproterozoic, and Middle Neoproterozoic. The Neoarchean and Middle Paleoproterozoic zircons with anatectic melt zircon characteristics, could be grains whose proto-sources underwent metamorphism and partial melting (migmatization). They could be syngenetic zircon crystallized in migrating melts during the Neoarchean and Middle Paleoproterozoic periods. The 671 ± 12 Ma age of MSDZ015 and its geochemical features are similar to those of zircons grown in equilibrium with garnet, which shows that this syngenetic zircon was crystallized in a garnet-rich rock during Middle Neoproterozoic event, probably the Pan-African orogeny, which affected the Cameroon Mobile Belt. This age is close to those of some Pan-African rocks within the Cameroon Mobile Belt presented in Table 1.

Age Correlation, Potential Sources Rocks, and Deposition
The southern part of Meiganga from where the studied zircons were sampled is mostly made up of undated biotite-amphibole granites; biotite-amphibole gneisses; biotite granites; biotite-chlorite granitic rocks; and few amphibolites and hornfels (see [53]). With a lack of available data dating those rocks, it is difficult to do a local correlation to locate nearby protosource(s) and source rocks for the southern detrital zircon Meiganga. However, at local and regional scales, the obtained ages are partly similar to those of zircons occurring in the western Meiganga gold-bearing placer presented in [9] and to the ages of some rocks outcropping in the southwest, northeast, and west of Meiganga, and Congo Craton (see Table 1).
Crustal-derived igneous zircons with ages ranging from 3671 to 612 Ma have some age similarities with those of zircons from some igneous and meta-magmatic rocks found in other parts of Meiganga and in the Congo Craton in South Region of Cameroon (Table 5) [20,32]) found in the SW of Meiganga. The 206 Pb/ 238 U ages (612 ± 11 Ma: MSDZ085 and 619 ± 11 Ma: MSDZ074) are similar to zircon ages (614.1 ± 3.9 and 619.8 ± 9.8 Ma: [37]) for meta-diorite outcropping in the NE of Meiganga. The age of MSDZ043 (643 ± 11 Ma) is close to that of two micas granite (647 ± 46 Ma: [21]) outcropping in Doua, west of Meiganga. It is not easy in the current geologic setting to consider these rocks to be source rocks of the southern Meiganga crustal-derived magmatic zircons, as those rocks are often found very far from the sampling points of the studied zircons and their host gold bearing placer. They could be detritus from a nearby undated proto-source and source rock or could be polycyclic detritus from the above rocks.
Mesoarchean, as well as Neoarchean ages of crustal derived igneous zircons are often similar to those from rocks (e.g., charnockite, tonalite, granodiorite, syenite, and granite) found in the Ntem complex (Northern Congo Craton), with just a few links with those from rocks (e.g., garnet-bearing gneiss, meta-quartzite, clinopyroxene syenite, and orthopyroxene-garnet gneiss) of the Nyong Unit (Tables 1 and 5). Early and Middle Paleoproterozoic aged zircons are mainly similar to those from rocks (e.g., amphibolite, charnockite, meta-granodiorite, meta-syenite, and orthopyroxene-garnet gneiss) found within the Nyong Unit (Tables 1 and 5). Their presence in the studied area (within the Cameroon Mobile Belt) shows Archean to Paleoproterozoic inheritance, and post-Archean reworking. The Archean to Paleoproterozic igneous zircons inheritance in some metamorphic rocks found at the west of Meiganga was proven by [20] (see Table 1). Plotted in granitoid field ( Figures 9-11), they could be inherited grains from granite and granodiorite proto-sources with features similar to those of granitoids in the Congo Craton. Those zircons whose plots fall out the various discriminating fields could be inherited grains crystallized in charnockitic and tonalitic magmas, as their ages are close to that of charnockite and tonalite found in the Congo Craton. Those old Archean and Paleoproterozoic rocks were probably reworked with the conservation of some inherited zircons, during the two main tectonomagmatic and metamorphic events (the Eburnean and Pan-African) registered within the Cameroon Mobile Belt. The age (671 ± 12 Ma) of a metamorphic zircon grown in equilibrium with garnet (MSDZ015) is close to the youngest zircon age (675 Ma: [19]) for amphibole-biotite gneiss found in the west of Meiganga. This rock also hosts Early Paleoproterozic age zircons with some similarities to those of the studied zircons (Table 5). Undated amphibole-biotite gneiss cropping in the south of Meiganga (Figure 1) is the bed-rock of the studied zircon-gold bearing placers. If age extrapolation is possible, it can be suggested that amphibole-biotite gneiss found in the south of Meiganga may be the source rock of part of the detritus forming zircon-gold bearing placers. Indirect sources of the placers can also be pyroxene-amphibolebearing gneiss, meta-diorite, and two micas granites found within the local settings.
The primary sources of host gold grains are difficult to be directly constrained as gold crystals were not found in placer's rock fragments or in the underlying and surrounding rocks. The depositional periods of the studied zircons are also not easy to constrain. The unconsolidated nature of their host-sediments and their location in streams may suggest post-Neoproterozoic to recent deposition.

Conclusions
The southern Meiganga detrital zircon-gold bearing placers are composed of igneous (crustal derived and mantle origin) and metamorphic zircons (grown in equilibrium with garnet and those grown in equilibrium with an anatectic melt) with different histories of crystallization and from mainly different sources.
Crustal derived igneous zircons were crystallized in granitic magmas with some charnockitic and tonalitic affinities during Eoarchean to Late Neoproterozoic periods. Mantle igneous zircons were crystallized from mantle source magmas during Early Neoarchean to Middle Paleoproterozoic times.
The inherited igneous zircons of Mesoarchean to Middle Paleoproterozoic were probably sorted from pyroxene-amphibole-bearing and amphibole-biotite gneiss, with their features similar to those of rocks in the Congo Craton. Late Neoproterozoic zircons, with ages close to those of meta-diorite and two mica granite found in the NE and west of Meiganga, were probably eroded from unidentified nearby rocks formed in the same periods.
Metamorphic zircons grown in equilibrium with garnet were crystallized in low-Th-REE subsolidus solution during the Pan-African syn-metamorphic crustal event. Metamorphic zircons grown in equilibrium with an anatectic melt were probably crystallized during the Neoarchean and Middle Paleoproterozoic in migrated melts from partial fusion of metamorphic protoliths. These inherited zircons were probably sourced from amphibole-biotite gneiss underlying the zircon-gold-bearing placers.