From the 4170 heavy mineral grains analyzed for the Les Sarras gold placers, cassiterite is the most abundant at all sites, followed by garnet, ilmenite, rutile, columbite-tantalite, monazite and gold ( Figure 3 ).

The description and identification of mineral species were based on their morphological characteristics (transparency, color, shape/section, luster, and other features). Based on these criteria, seven mineral species were identified: four species from the oxide group (ilmenite, rutile, cassiterite, and columbo-tantalite), one species from the silicate group (garnet), one species from the rare earth elements group (monazite), and one species from the native elements group (gold). Ilmenites are opaque, dark grey in color, elongated rhombohedral tablets with longitudinal striations; the edges are either sharp or sub-angular; the luster is metallic, and the grains are very weakly magnetic ( Figure 4(A) ). Rutile grains are opaque, dark-gray prisms with an elongated, flattened shape and a metallic luster ( Figure 4(B) ). Cassiterite grains are opaque, dark brown, angular in shape, with a metallic luster, displaying both longitudinal and transverse striations ( Figure 4(C) ). Garnets are translucent, pink in color, sub-angular and elongated in shape, with a vitreous luster ( Figure 4(D) ). Monazites are translucent, yellow brown in color, sub-angular and elongated, with a greasy to vitreous luster ( Figure 4(E) ). Gold grains are opaque, metallic yellow in color, flattened, sub-crystalline, with very irregular outlines, a metallic luster, and show phenomena of peptization, quartz grain inclusions, and impact marks ( Figure 4(F) ).

Geochemical analyses show that the placers are dominated by oxides, notably ilmenite, cassiterite, columbo-tantalite, and rutile.

The oxides are dominated, in order of abundance, by cassiterite, ilmenite, rutile, and columbotantalite. The cassiterite grains, with an average composition of 97.3% by weight of SnO₂, exhibit trace amounts of significant concentrations of Nb and Ta. The niobium pentoxide (Nb₂O₅) shows average concentrations of 0.2% by weight, tantalum pentoxide (Ta₂O₅) 0.33% by weight, MnO 0.03% by weight, and FeO 0.18% by weight ( Table 2 ). Normalization to Apfu reveals a bimodal distribution of trace elements, suggesting a dual origin (magmatic and metamorphic). Ilmenites show average concentrations of 56.4% by weight of TiO₂, and 41.4% by weight of FeO. The trace elements indicate an abundance of tungsten oxide (WO₃) at 0.11% by weight, uranium oxide (UO₂) at 0.04%, and chromium oxide (Cr₂O₃) at 0.02% by weight ( Table 2 ). Furthermore, the presence of zirconium oxide (ZrO₂) at an average concentration of 0.06% by weight suggests the presence of zinciferous ilmenites in the area [44] . Zinciferous ilmenite develops during contact metamorphism associated with granite intrusions [44] . The TiO₂ content of rutile reaches an average of 96.5% by weight. The trace elements identified in rutile grains show average concentrations of 0.97% by weight of iron, 0.69% by weight of niobium, and 0.04% by weight of tantalum ( Table 2 ). The presence of these elements helps determine the origin and formation conditions of the rutile.

Ilmenites exhibit very low proportions of MnO and MgO (ranging from 0.4 to 3 wt% and 0.02 to 0.2 wt%, respectively), suggesting a possible partial substitution of Fe²⁺ by Mg²⁺ and Mn²⁺ within the ilmenite structure ( Figure 5(A) ). However, this substitution is more strongly influenced by increasing temperature than by pressure during metamorphism [44] .

FeO is present in low concentrations (0.2 to 3 wt%) in rutile, along with trace amounts of Nb₂O₅ and ZrO₂, suggesting high-pressure formation conditions ( Figure 5(B) ).

High concentrations of Mn and Fe are observed in some cassiterite grains, while others show elevated Nb and Ta contents ( Figure 5(C) ). This may reflect distinct hydrothermal and magmatic origins.

The analyzed columbo-tantalite grains reveal two distinct groups. The first group is characterized by very high concentrations of niobium pentoxide (Nb₂O₅), ranging from 67.8 to 71.18 wt%, and low concentrations of tantalum pentoxide (Ta₂O₅), between 4.5 and 6.4 wt% ( Figure 6 ). The second group shows high Ta₂O₅ contents, ranging from 62.21 to 75.28 wt%, and lower Nb₂O₅ contents, between 7.42 and 10.10 wt% ( Table 3 ).

The niobium and tantalum contents suggest the presence of two distinct columbo-tantalite populations. One is niobium-rich (columbite), with FeO contents ranging from 16.33 to 16.63 wt%, corresponding to the ferrocolumbite group. The other is tantalum-rich (tantalite), with FeO contents between 14.03 and 18.46 wt%, representing the ferrotantalite group [45] . The TiO₂ concentrations (0.75 - 0.85 wt%) in the first three samples and those (0.22 - 9.99 wt%) in the last two, along with Mn# values (7.80 - 8.62) for the first group and Ta# values (84.43 - 89.93) for the second, suggest a diversity of origins for the columbo-tantalite [46] [47] . This diversity may be linked to granite intrusions (Les Saras granite and Mfoubou granite) previously reported in the area by [17] - [22] . The second group could serve as a guide for tantalum exploration in the sector.

Apfu: Analyses of Atoms per Formula Unit.

Among the silicate group, only garnet was analyzed. The results show that the garnets exhibit high FeO contents (29 - 36 wt%) and MgO contents (3 - 6 wt%) ( Table 4 ), along with low MnO (1.0 - 1.3 wt%) and CaO (1.0 - 1.09 wt%) contents, indicating that the garnets are depleted in Mn²⁺ and Ca²⁺.

Figure 7. Position of the garnets analysed in the ternary diagrams of [49] [50] : (A) almandine-pyrope-grossulaire and (B) almandine-spasartine-grossulaire (normalised apfu content).

These compositions suggest that the garnets are primarily of the almandine (Alm) and pyrope (Pyr) types ( Figure 7 ). Trace elements include low concentrations of MnO and CaO (1.0 - 1.27 wt% and 1.0 - 1.07 wt%, respectively), confirming their metamorphic origin [48] .

The rare earth element group is represented by monazite. Monazite grains are enriched in P₂O₅ (29 - 37 wt%) and exhibit high concentrations of lanthanum (21 - 28 wt%) and cerium (8 - 11 wt%). The weak correlation between CaO and P₂O₅ concentrations suggests chemical zoning, reflecting variations in the formation environment ( Figure 8 ).

Gold is the principal native element observed in the Les Saras placers. The gold grains exhibit well-defined angular features ( Figure 9 ).

The chemical composition of gold grains from the Les Saras area ( Table 5 ) reveals that the gold is primarily alloyed with silver, bismuth, copper, and traces of arsenic, cobalt, and antimony. The gold (Au) content in the analyzed grains ranges from 67.74% to 98.47%, with a high average of 87.94%. Conversely, silver (Ag) content varies inversely from 1.13% to 33.42%, with an average of 12.22%. This negative correlation between gold and silver contents reflects a wide compositional range, from nearly pure gold to silver-rich electrum. A similar inverse relationship is observed between fineness and silver content: the higher the Ag content, the lower the fineness. This relationship is expected, as fineness is directly defined by the relative proportion of gold in the alloy, making it a key indicator for estimating the formation temperatures of gold grains [51] . Figure 10 illustrates this relationship between gold and silver contents and inferred formation temperatures. The data points from the Les Saras samples are distributed primarily between the medium- and high-temperature fields, indicating variability in metallogenic conditions. This trend supports previously suggested differences in formation contexts [20] - [22] . Bismuth (Bi) is detected in low amounts, ranging from 0.42% to 1.01%, suggesting a minor association with hydrothermal processes [43] [51] [52] . Additionally, trace elements such as arsenic (As), copper (Cu), iron (Fe), cobalt (Co), and antimony (Sb) are present in concentrations below 0.1%. Although their levels are low—which is typical for native gold—their presence remains significant, as they provide insight into the formation conditions of the auriferous mineralizations ( Figure 10 ). In this study, the fineness of the gold grains ranges from 670 to 988, with an average of 878 ( Table 5 ). Fineness values below 800 correspond to silver-rich electrum, while values above 900 indicate nearly pure gold. This bimodal or scattered distribution suggests multiple primary sources for the gold grains, implying at least two distinct types of deposits [51] [52] . Regarding the Au/Ag ratio, values range widely from 2.0 to 85.9, with an average of 7.2. This ratio is a useful indicator of the nature and origin of the gold [51] - [53] : high values (>30) are indicative of very pure gold, typically associated with hypogene deposits, whereas low values (~2 - 4) are characteristic of silver-rich electrum, which may originate from polymetallic sulfide deposits or near-surface altered zones [52] .

Table 6 shows that at the grain boundaries, the gold content varies from 50.6% to 100% (average 93%), silver content ranges from 0.16% to 49.1% (average 7%), and bismuth content varies from 0.46% to 0.9% (average 0.72%). The composition suggests leaching of the chemical elements accompanying the gold [51] - [53] .

Geochemical analyses show that the minerals accompanying the gold in the Les Saras placers provide information about the phenomena that led to the genesis of the source rocks.

The ilmenites rich in TiO₂ and FeO, along with the nearly pure rutile, indicate high-temperature and high-pressure metamorphism, as previously demonstrated in the work of [18] on the Mayombe. The cassiterites, on the other hand, exhibit a dual origin: grains enriched in Nb and Ta suggest a magmatic origin, while those rich in Mn and Fe point to a hydrothermal contribution [54] . The columbotantalites display reasonable concentrations of niobium and tantalum in the grains, but the small number of grains analyzed does not allow for hypotheses to be made regarding their origins.

Almandine-pyrope garnets and monazites enriched in lanthanum and cerium confirm high-temperature metamorphism [48] - [50] and [54] . The zonations observed in monazite grains reflect variations in chemical composition linked to distinct crystallization cycles.

The strongly angular morphology of the gold grains indicates limited transport within the basin, as suggested by the works of [55] - [62] . The geochemical signature, predominantly composed of Au, Ag, Bi, Cu, and Sb, reveals chemical homogeneity at the core of the grains and an enrichment in gold at the boundaries compared to the center. This phenomenon is attributed to the selective dissolution of the gold’s accompanying elements in the grains [63] [64] . The geochemical signature of the gold grains in the Les Saras area, along with their fineness, suggests a dual origin:

These two types of signatures suggest the existence of at least two primary sources, corresponding to two distinct types of deposits. This hypothesis is further supported by the simultaneous observation of two types of grains within the same basin: (i) electrum grains with silver content > 15% and fineness < 850; and (ii) native gold grains with silver content < 10% - 15% and fineness > 850. According to [51] [52] [68] [69] , this coexistence in the same sedimentary environment constitutes a strong indication of a dual origin, reflecting the overlap of metamorphic and hydrothermal processes.

Geochemical analyses highlight two types of gold grains in the Les Saras samples: native gold grains, characterized by high fineness (up to 988) and silver content below 5% ( Table 5 ). This type of gold is typical of high-temperature hydrothermal deposits. On the other hand, electrum-type gold grains display lower fineness (down to ~670) and elevated silver content, between 20% and 33% ( Table 5 ). These grains generally form in shallower, more oxidizing conditions or in silver-enriched environments ( Figure 12 ) [51] [52] . Additionally, the presence of relatively high bismuth concentrations (~0.5% to 1%) also suggests a possible magmatic origin. This chemical signature may indicate an association with bismuth-bearing mineralization linked to magmatic intrusions of the gold-bismuth type. The lack of previous data on gold fineness in the Les Saras area makes these results particularly valuable. Indeed, they provide the first insights into the formation temperatures of gold based on observed fineness values [51] . The data indicate that the presumed gold deposits in the study area formed at high temperatures and considerable depth. The gold present in the ores appears to have high fineness, characteristic of Paleoproterozoic metamorphic rock series belonging to the Loeme, Loukoula, and Bikossi groups. It is also likely that multiple and prolonged metallogenic events contributed to the transformation, purification, and progressive enrichment of the gold. This secondary enrichment possibly results from leaching, particularly observable at the grain boundaries, which removes impurities from the primary gold-bearing minerals. This process increases the gold fineness and alters its geochemical signature. Finally, it is observed that ores poor in sulfides generally display higher fineness compared to those rich in sulfides, confirming the influence of formation conditions [42] [43] [51] [52] . Figure 12 clearly illustrates the relationship between fineness and temperature in auriferous mineralizations.

Impurities observed in gold grains provide valuable insights into the depositional environment and the nature of the source fluids. The results obtained suggest that the Les Saras area was influenced by several successive geodynamic events, including regional metamorphism, hydrothermal activity, and various depositional processes. The mineralogical characteristics of ilmenites and garnets confirm the presence of high temperature regional metamorphism, consistent with the regional tectonic framework described by [22] and [32] . Moreover, the enrichment in niobium (Nb), tantalum (Ta), and rare earth elements (REE) detected in the studied grains indicates the involvement of hydrothermal fluids associated with magmatic intrusions. The analysis of gold grain fineness supports the hypothesis of a polymetallic system with a dual origin, involving two distinct types of gold mineralization:

1) Magmatic-hydrothermal origin (high temperature): This type is characterized by very pure native gold (fineness > 950), frequently associated with quartz veins located near felsic intrusions (granitoids, tonalites), formed under hypothermal conditions, at great depth and high temperature ( Figure 12 ). This interpretation aligns with the observations reported by [20] - [22] . The presence of bismuth as a trace element in some gold grains suggests a gold-bismuth mineralization type linked to intrusive settings, as discussed by [51] and [52] .

2) Volcanogenic or sedimentary origin: This second type is represented by electrum with low fineness (670 - 780), enriched in silver, formed under low-temperature conditions. It is typically associated with sulfide mineralization or secondary alteration processes affecting originally silver-rich rocks. This kind of gold is typical of epithermal transition zones, often located in volcanic settings or reworked sedimentary basins [52] . However, the mineralogy observed shows no sulfides, and there is no evidence of a link with a sedimentary basin. An exhaustive study of the mineralogical record and fluid inclusions is needed to elucidate this problem.

The coexistence of these distinct geochemical signatures within the same area in Les Saras strengthens the hypothesis of a complex metallogenic system involving multiple mineralizing phases, combining deep-sourced inputs with shallow remobilization processes.

The geochemical and morphological characteristics of the Les Saras placers provide valuable clues for mineral exploration. The angular morphology of the gold grains indicates proximity to primary sources. The geochemical signatures of gold, oxides, and silicates suggest a magmatic-hydrothermal and/or volcano-sedimentary origin. These findings can inform exploration efforts within the Loukenene watershed and surrounding areas, supporting the identification of potential targets for exploitation through the application of these signatures in geochemical prospecting.

These results contribute to a better understanding of the mineralogical distribution and geological processes that have influenced the Les Saras placers. Additional analyses will further enhance the interpretation of the origin and mining potential of these formations.