The outcrops (site S1) and the pit (site S2) from Pk27 show two main types of sedimentary facies. From the bottom to the top, one can distinguish: claystones and conglomerates with interstratified sandstones (Figures 2(a)-(d)). These sediments contain cobbles to fine-grained quartz, kyanite, grenat, rutile which suggest their terrestrial source. In general, conglomerates vary from yellow to yellowish brown color and claystones show light grey, yellowish-orange and red color. These sedimentary materials from the study area were grouped into five (05) main lithofacies including: four claystone facies and one conglomeratic facies. Facies have been described, interpreted and classified from coarsest (G) to finest (F) sediments and a code (Table 1) modified [12] [13] has been assigned to each facies.

The particle size distribution of clayey materials bearing pyrites is given according to the USDA classification system (gravel: 2 < d < 75 mm; sand: 50 µm < d < 2 mm; silt: 2 µm < d < 50 µm; clay: d < 2 µm). These materials have a small amount of sand (5.5% - 12.5%) and gravel (1% - 2%), and are essentially constituted of clays (51% - 56%) and silts (27% - 49%). Figure 4 shows similar patterns which indicate that these clayey materials have the same constituents with a rapid increase towards clays and silts fractions.

However, to analyze the sorting of these clayey materials and to determine the hydrodynamic conditions of their deposit, values of particle size parameters including: Quartiles (Q₁, Q₂, Q₃), Sorting index (So) and Coefficient of asymmetry (A) are showed on Table 2. The clayey materials bearing pyrite have the sorting index (So) higher than 4.5 and the coefficient of asymmetry (A) higher than 1 whose indicate respectively that these materials are poorly sorted and are the torrential type; whereas the sorting is orientated towards fine particles.

Figure 5(a) and Figure 5(b) represent the particle size distribution of pyrite in the sand fractions with diameter d > 80 µm. These histograms show that the grains, piles and aggregates pyrite are concentrated between 0.4 to 8 mm diameters with essentially a unimodal distribution for samples S1T1c, S1T1b, S2P2a and S2P2b or exceptionally a bimodal distribution centered in 4 mm and 0.1 mm diameters in S1T1a sample. In fact, the coarse pyrite of clayey materials is dominantly constituted with sand and gravel size.

X-ray diffraction patterns indicate that clayey materials of PK27 area (Figure 6) are essentially kaolinitic and il-

litic, followed by goethite, quartz, pyrite, halloysite and anatase. Rutile and hematite are accessory minerals. Furthermore, optical observations of heavy minerals of sand fractions show that in these sedimentary clayey materials, pyrite is accompanied by kyanite, grenat, rutile, diopside and tourmaline (Figure 7). These minerals are recognized as minerals of regional metamorphism.

Major elements of the studied samples (Table 3) show the following composition: silica (60.3% - 64.1%

SiO₂), alumina (12.6% - 16.7% Al₂O₃) and iron (2.28% - 4.1% Fe₂O₃). Iron contents are variable in the clayey materials in relation with the color (2.28% Fe₂O₃ for light grey clays, 4.1% Fe₂O₃ for light grey with red spots). Indeed, this variation characterizes the clayey materials of the eastern part of the Douala sub-basin [16] . The sum of bases (CaO, Na₂O, K₂O, MgO) is low, less than 1%. The SO₃ content varies between 2.15 and 4.03 %; whereas it’s abundant (50.86% - 57.64%) in pyrite minerals and is followed by Fe₂O₃ content (20.29% - 24.34%).

Trace element concentrations of pK27 analyzed samples are respectively reported in Table 3. This table shows the same behavior with trace elements in clayey materials and pyrite. These materials are systematically depleted in Large Ion Lithophile Element (LILE) like Sr (<51 ppm), High Field Strength Elements (HFSE) like Nb (<40 ppm) and Zr (<200 ppm in general with the exception of sample S1T1b). However, they are enriched in TTE or Transition Trace Elements like Cr, V, and Ni (in site S2), with exception of Co in S2P2b sample.

Two main types of sedimentary facies characterize the pyrite deposit of pK27: from the bottom to the top one can distinguish claystones and conglomerates with interstratified sandstones. Based on the facies analysis and sequences described above, the sedimentological character of the outcrops and the presence of abundant detrital material are indicative of alluvial fan-fan delta deposits. In fact, conglomerates (Gm facies) show no internal structure and result from high strength debris and gravity flows [13] ; whereas claystones (Fm facies) result from suspension in calm environment or from weak traction currents [12] .

The clayey materials bearing pyrite are essentially constituted. Moreover, to analyze the sorting of these clayey materials and to determine the hydrodynamic conditions of their deposit, some parameters such as sorting index (So) and coefficient of asymmetry were determined. In fact, Trask [14] shows that if So < 2.5, sediments are well sorted and correspond to marine sediments (beach) or fluvial sediments; if 2.5 < So < 3.5, sediments are normally sorted and the hydrodynamic conditions are difficult to determine; if 3.5 < So < 4.5, sediments are much sorted; and if So > 4.5, sediments are poorly sorted and are the torrential type. Moreover, the coefficient of asymmetry (A) also shows the sorting of sediments and the hydrodynamic conditions of their deposits; if A < 1, the sorting is orientated towards coarse particles and grains are more sorted. In this case, they are torrential deposits; whereas A = 1 indicates developed sediments and A > 1 a maximum sorting towards fine particles and corresponds to marine deposits. In general, all the analyzed clayey samples of Pk27 show that the sorting index (So) is higher than 4.5 and corresponds to poorly sorted sediments whereas their coefficient of asymmetry (A) higher than 1 indicates that the sorting is carried out towards fine particles (clays and silts).

The geological survey of pK27 area shows pyrite in clayey materials. Pyrite in sedimentary rocks has been also found in shale and marl in the world [1] [2] and in the others members of the N’Kapa Formation at Dizangue [17] and in the northern part of the Douala sub-basin at the Kompina pyrite deposit [6] . The mineralogical compositions of the host rocks are essentially constituted with kaolinite and illite, followed by goethite, quartz, pyrite, halloysite, anatase, hematite and rutile. All these minerals have been recognized as weathering products of the rocks schist around the coastal plain in the littoral part of Cameroon, and are the main characteristic of Paleocene-Eocene and Miocene-Pliocene clay sediments of Missole II and Bomkoul areas in the Douala sub-basin [16] . Also, the presence of kyanite, grenat, rutile, diopside, muscovite and tourmaline in these materials are indicative of regional metamorphism of the rocks source. However, high SO₃ content and trace elements contents like Ni, Co, Cu, Zn of clayey sediments of Pk27 area particularly characterize these sediments in the eastern part of the Douala sub-basin. These various trace elements may be concentrated in pyrites of different origin [18] . The concentration of metals in pyrite depends on the concentrations of the various metals in the precipitation environment. Metals that may circulate in a hydrologic column become fixed in sediments rich in organic material [19] .

In fact, many elements, such as Cu, Zn, As, Sb, Ni and Co, may be highly concentrated in sedimentary rocks, especially in anoxic sediments [20] . These elements may aid in elucidation of ore genesis [21] . For example, the Co and Ni contents of pyrites or their Co:Ni ratios, are primarily used as indicators of ore genesis [22] . The Co and Ni contents are extremely variable in hydrothermal pyrites and rather variable Co:Ni ratios. Pyrites in massive sulphide deposits have Co:Ni ratios of 5 - 50 (average 8.7), with high Co contents (average 480 ppm) and Ni contents less than 100 ppm (average 56 ppm) [22] . Sedimentary pyrites in shale have high Ni contents and low Co:Ni ratios (average 0.63) [22] . The Co contents of the studied clayey materials are in trace and are low in pyrites, but the Ni contents are high. The Co:Ni ratios of pyrites are approximately 0.25 to 0.8. These values are characteristic of sedimentary pyrites and also suggest an anoxic environment [19] -[22] . Both elements Co and Ni with another TTE such as Zn and Cu tend to be enriched in sulphide minerals [23] . In the sedimentary environment, Co and Ni are enriched in pyrite and marcasite [3] . The mineralogical data of pk27 show pyrite minerals. However, high Ni contents in some pyrite samples should also indicate the presence of the nickeliferous form of pyrite (bravoite). But the major oxides in pyrites, such as SiO₂, Al₂O₃, CaO, Na₂O, K₂O and MgO may have been derived from the clayey materials.