The Lom Series ( Figure 1(c) ) is a major pull-apart basin. It is characterized by transtensional movement [12] [25] , oblique to the main shear zones (CCSZ, SFZ) and normal faults defining its eastern and western limits. The Lom Series represents a sequence of metasedimentary and metavolcanic rocks of Pan-African age (~613 to 600 Ma, [24] ), including metatuffs, volcanoclastic and sedimentary-derived schists, staurolite-garnet mica schists and quartzites with sporadic conglomerate layers [10] [12] [24] [33] . Granitoids (~635 Ma, [17] ) intrude the schist and gneissic basements units, constituting the major outcrops in the Lom series [25] . The schists were metamorphosed under greenschist-facies conditions with the development of a low-pressure garnet-andalusite-staurolite regional metamorphic mineral assemblage with geochemical features attesting to derivation from argillaceous sediments [12] . Geochemical and geochronological studies carried out on the granitoids in the Lom Series reveal a crustal origin with S-type [33] and I-type geochemical signatures [17] [24] . The orientation of structures and rocks in this area follows the general N50 - N70 steeply dipping regional foliation (NE-SW trend of the CCSZ) related to D1 and D2 deformation phases during the Pan-African orogeny [12] [24] . The schist units (metasedimentary units) hosting the gold-bearing quartz veins [29] form a NE-trending corridor, dipping steeply towards the NW and subparallel to the regional foliation.

The samples used in this study were collected in the upper Lom Series within a shear zone ( Figure 1(c)-(d) ). The area was systematically mapped along transects ( Figure 2 ) to ensure all rock types were sampled and structural elements (foliation, lineation, shear planes, S-C fabric, boudins, folds and faults) measured. Samples spanning the spectrum from strongly altered metagranitoid- and metasediment-derived fault rocks to the least altered (fresh) fault rocks were collected for petrographic and geochemical analyses. Twenty-five (25) standard thin sections for petrographic study were prepared for all rock samples in order to decipher their mineralogy, texture, deformational fabric, as well as alteration signatures. The fault rocks in this study were classified using the classification scheme of [34] . With respect to the fault rock categorization using hydrothermal secondary minerals, the dominant mineral phase was used to name the rock purely from a petrographic perspective. All thin sections were cut in the XY and YZ structural plane (X-foliation and Z-lineation). The porphyroclast systems were analyzed using the classification scheme of [35] . The microstructural study of porphyroclast systems enabled the strain ellipsoid categories to be determined. The concept of strain ellipsoid analysis (finite strain technique) employed made use of the Rf/Ø method modified by [36] [37] , and the 2D space Flinn diagram. With the Rf/Ø data, Rf = a/b; where a and b are lengths of the maximum and intermediate principal axes of the ellipsoidal marker, and Ø is the angle between the maximum principal axis and any taken reference direction (direction of principal foliation). These ratios were then plotted against the orientation of the long axis. The geometries of strain ellipsoids were also represented in 2D space using a Flinn Plot, where the ratio a of the maximum to intermediate ellipsoid axes (a = X/Y) is plotted as ordinate and the ratio b of the minimum to intermediate axes (b = Y/Z) is plotted as abscissa. K represents the slope of a line from the data point to the origin at (1,1), so that K = a − 1/b − 1 with a = x/y and b = y/z. The modified Flinn Plot by [36] provides a more even distribution of points with increase in deformation, making data interpretation less difficult. Foliations, lineation and poles of the collected structural data were plotted in the lower hemisphere of Schmidt net diagrams following conventional techniques [38] .

A total of 15 samples were crushed, pulverized to below 75 µm mesh and analyzed for major and trace elements (Au inclusive) at Bureau Veritas Mineral (BVM) Laboratory Vancouver, Canada. These samples were decomposed by lithium tetraborate/lithium metaborate/sodium nitrate (Li₂B₄O₇/LiBO₂/NaNO₃) fusion discs using code XF 700, and whole rock major concentrations for the fault rocks, meta-granite and metasediment were determined by X-ray fluorescence spectrometry (XRF). The trace and Rare Earth Elements were analyzed by inductively coupled plasma-mass spectrometry (ICP-MS), after aqua regia extraction and total digestion (code, AQ250_EXT_REE), while Au concentration was determined by fire assay with a Ni-S finish. Loss on Ignition (LOI) was determined by heating the powdered samples at 850 °C for 3h. Accuracy and precision are better than 5% RSD (% relative standard deviation). Geostandards used in the analyses are STD OREAS184, STD SY-3(D), STD LOI471, STD LOI472, STD BVGEO01, STD OREAS262 and international rock standards, such as OREAS184 (4-acid), SdAR-M2 (USGS).

The rock types encountered in the Gankoumbol-Djouzami-Beka shear zone consist of granites, schists, metamorphosed unaltered rocks (metagranitoid and metasediments) and the faulted derivatives of the metagranitoid, showing varying intensity of deformation and hydrothermal alteration. The fault rocks grade outward from ultramylonite, at the center of the shear zone through mylonite, protomylonite, to cataclasite at the margins of the shear zone ( Figure 2 ). Strain compartmentalization with strongly ductile deformed samples at the center of the shear zone, grading out to brittle zone. Shearing is accompanied by progressive grain size reduction, strong mineral lineation and foliation as strain increases in the fault rocks towards the center of the shear zone ( Figures 3(a)-(c) ). The planes of schistosity are impregnated with quartz injections, healing the S1 foliations in mylonite ( Figure 3(b) ). These rocks are transected by quartz veins and fractures of different generations at the outcrop scale. The NW-SE second generation veins/fractures displace the dominant NE-SE structures in both dextral and sinistral manner ( Figures 3(d)-(e) ). Shear sense indicators and the deformation conditions also vary across the shear zone, with ultramylonites, mylonites and protomylonites presenting well-developed mylonitic foliation defined by preferred mineral orientation with a NE-SW trend ( Figure 3(a) and Figure 3(f) ). Faults, fractures, microfolds and S-C fabric constituted the dominant shear sense indicators in the study area both at the microscopic and microscopic scale ( Figure 3 and Figure 4 ). These shear sense indicators depicted both dextral and sinistral sense of shearing ( Figures 3(d)-(e) ). The S2 foliations are transformed to S3 foliations by microfolds in ultramylonite with increase in deformation strain ( Figure 3(f) ). These fault rocks outcrop define a N10E-N70E (NNE-SSW to NE-SW) trend which is aligned parallel to the dominant NE-SW trend of the CCSZ lineament.

1. Least Altered Metagranitoid and Metasediments

The least altered metagranitoid presents very limited features of deformation. They are coarse-grained and leucocratic ( Figure 4(a) ) with biotite, feldspars (microcline, orthoclase), quartz and minor muscovite as dominant minerals ( Figures 4(b)-(c) ). Microcline displays cross-hatch twinning with quartz, biotite and muscovite inclusions. The least altered metasediments, on the other hand, are fine to medium-grained, dark gray and mesocratic with foliations healed with quartz ( Figures 4(d)-(f) ). The mineralogical make-up consists of quartz, muscovite, biotite with minor feldspars. Silicification is the main alteration in the rock ( Figure 4(f) ).

2. Fault rocks (ultramylonite, mylonite, protomylonite and cataclasite)

These rocks range from very fine-grained ultramylonite to weakly foliated cataclasites at the margins of the shear zone ( Figure 2 and Figure 5 ). Under the microscope, the mylonitic fabric is defined by a strong foliation depicted by preferred orientation of minerals in the ultramylonite ( Figures 5(b)-(c) ), and alternating bands of microlithons with cleavage domains in the protomylonite ( Figures 5(e)-(f) ). Its mineralogy consists of quartz and feldspars porphyroclasts, sericite and muscovite. The sericite content of these fault rocks is remarkably high as it develops at the expense of feldspar ( Figures 5(e)-(f) ). Grain size reduction is prominent with the development of fine-grained quartz seams sub-parallel to sericite mosaics that ‘flow’ around the porhyroclasts ( Figures 5(e)-(f) ). Sericite occurs in association with disseminated sulphide minerals, defining NE-SW veins/veinlets ( Figure 5(b) , Figures 6(b)-(c) ). They also occur in patches and fractures in feldspars in the cataclasite zone ( Figures 5(e)-(f) ). K-feldspar alteration cuts across the shear zone, but dominant towards shear zone margins, and gives a pinkish color to the rock ( Figure 6(d) ). At the mylonitic zones where strain rate is high, K-feldspars occur as porphyroclast resulting from intense deformation and shearing towards the center of the shear zone ( Figure 5(f) and Figure 6(b) ). K-feldspars also depict evidence of syn-to-post magmatic deformation ( Figure 6(f) ).

Compositional layering is observed in protomylonite, with alternation of fine-grained chlorite/muscovite/sericite bands, and dynamically recrystallized quartz bands ( Figure 6(c) ). Sulphidation decreases with increasing K-feldspar alteration, associated with sericite mosaic (sericitization) and quartz seams (silicification) within the shear zone, but more pronounced within the protomylonitic and cataclastic zones, as euhedral to subhedral pyrite crystals ( Figures 6(b)-(c) ). Silicification occurs as dynamically recrystallized quartz sub-grains, with quartz defining porphyroclast towards the mylonite zone ( Figure 5(e) , Figure 6(c) and Figure 6(e) ). Silicification cuts across the shear zone, and constitutes the ground mass mantling the feldspars in the fault rocks. Silicification bands alternate with sulphidation bands towards center of shear zone ( Figure 6(c) ). Within the ultramylonite zone, quartz occurs as elongated ribbons defining lineation ( Figure 5(a) ). Microfolds of muscovite transform S2 foliations to S3 foliations in ultramylonite depicting ductile deformation ( Figure 5(c) ).

3. Microshear sense indicators and porphyroclast system

Microscopically, quartz and feldspar sigmoidal porphyroclast (“σ”), micafish, S-C fabric, microfolds and faults define the dominant shear sense indicators from the protomylonite to ultramylonite at the center of the shear zone ( Figures 7(a)-(f) ). Winged and non-winged porphyroclast systems are observed, with varying sizes and shape. The porphyroclast systems have elliptical grain shapes with long axes aligned subparallel to the main NE-SW foliation resulting from vertical stretching during deformation ( Figures 7(a)-(b) and Figure 7(f) ). The “σ” porphyroclast and snowball structures depict both sinistral and dextral sense of shearing ( Figure 4(a) and Figure 4(c) ). The finite strain ellipsoid analysis made used of quartz and K-feldspars porphyroclast with long axes inclined or parallel to the principal foliation. From the plots the samples show dominant prolate strain ellipsoids with constriction type of deformation ( Figures 8(a)-(b) ). Relics of oblate strain ellipsoids defining flattening type deformation were also observed in both Rf/Ø method and the 2D space Flinn diagram. The two methods employed yielded similar trends.

The major, trace, and rare earth elements (REE) geochemistry of the analyzed fault rocks, meta-granite and metasediment rocks are presented in Table 1 . The fault rocks are grouped into four (4) variably altered categories based on hydrothermal alteration (sericitized, silicified, sulphidized and K-feldspar altered-fault rock). The analyzed metagranitoid samples are subalkaline and plot in the granite and granodiorite fields on the TAS classification diagram ( Figure 9(a) ; [39] ). They show per-alkaline high K-calc-alkaline character ( Figures 9(b)-(c) [40] [41] ) and plot in the field of volcanic arc granitoid ( Figure 9(d) ) on the geotectonic discrimination diagram of [42] .

1. Least altered rocks

Least altered metagranitoid (CMRB-40, CMR-B45) display a silica content that vary between 71.46 and71.71 wt.%, high Al₂O₃ (14.54 wt.% - 15.2 wt.%), K₂O (3.63 wt.% - 3.94 wt.%) and K₂O + Na₂O contents that range from 7.6 to 7.84wt.%), moderate CaO (2.18 wt.% - 2.19 wt.%), and Na₂O content from 3.66 to3.67wt.%. They are characterized by low P₂O₅ (0.05 wt.% - 0.15 wt.%), FeOt (2.53 wt.% - 2.56wt.%), MgO (0.44 wt.% - 0.57 wt.%), TiO₂ (0.26 wt.% - 0.31wt.%) and MnO (0.03 wt.% - 0,05 wt.%), K₂O/Na₂O ratios (0.72 wt.% - 1.08 wt.%). The LOI value varies between 1.4 and 0.4 wt.% ( Table 1 ).

2. Fault rocks (altered samples)

The fault rocks in this study are grouped into four (4) categories based on hydrothermal alteration. The hydrothermal alteration includes: sericitization, silicification, sulphidation and K-feldspar alteration ( Figures 10(a)-(h) ). These altered samples represent the shear zone studied. The analyzed Fault rock-sericitized samples are characterized by a low to moderate silica content (63.11 wt.% - 71.64 wt.%), high Al₂O₃ (14.41 wt.% - 15.01 wt.%), LOI values from 1.1 wt.% - 4.4 wt.%, slight increase in FeOt (2.56 wt.% - 4.64 wt.%), low K₂O (2.73 wt.% - 3.65 wt.%), CaO (0.23 wt.% - 2.99 wt.%) and high K₂O + Na₂O (6.89 wt.% - 7.84 wt.%). The Na₂O values range from 3.25 to 4.86 wt.%. The concentration of TiO₂ (0.26 wt.% - 0.49 wt.%), MgO (0.44 wt.% - 1.97 wt.%), MnO (0.06 wt.% - 0.17 wt.%), P₂O₅ (0.09 wt.% - 0.15 wt.%), and K₂O/Na₂O ratios (0.60 wt.% - 1.35wt.%) are generally low.

The fault rock-silicified samples are characterized by high silica content (74.04 wt.% - 79.83 wt.%), low to moderate Al₂O₃ (11.22 wt.% - 14.64 wt.%), a decrease in K₂O (1.87 wt.% - 4.07 wt.%) FeOt (1.46 wt.% - 1.68 wt.%), CaO (0.15 wt.% - 0.78 wt.%), P₂O₅ (0.06 wt.% - 0.07 wt.%), low TiO₂ (0.07 wt.% - 0.11 wt.%), MgO (0.05 wt.% - 0.17 wt.%), and MnO (0.09 wt.% - 0.12 wt.%) and LOI (0.3 wt.% - 1.3 wt.%). They display moderate Na₂O (3.86 wt.% - 4.98 wt.%) content, K₂O + Na₂O (6.85 wt.% - 8.04 wt.%) and low K₂O/Na₂O ratios (0.38 wt.% - 1.05 wt.%). The fault rock-sulphidized sample is characterized by high SO₃ content (1.352 wt.%)m low silica content (64.88 wt.%), Al₂O₃ (14.75 wt.%), but displays an increase in FeOt (6.09 wt.%). Low K₂O (2.59 wt.%), CaO (1.16 wt.%), slight increase in TiO₂ (0.59 wt.%), MgO (2.16 wt.%), and MnO (0.33 wt.%). P₂O₅ (0.14 wt.%). They also display low K₂O/Na₂O ratios (0.62 wt.%), and K₂O + Na₂O (6.80 wt.%), and subtle increase in LOI values from 2.3 wt.%. Fault rock-K-feldspar samples are characterized by a moderate silica content (73.62 wt.% - 74.96 wt.%). Low Al₂O₃ (12.69 wt.% - 13.5 wt.%), FeOt (2.15 wt.% - 2.3 wt.%), CaO (0.5 wt.% - 0.77 wt.%) TiO₂ (0.28 wt.% - 0.29 wt.%), MgO (0.29 wt.% - 0.42 wt.%), MnO (0.04 wt.% - 0.06 wt.%), P₂O₅ (0.05 wt.%) and LOI value (0.4) are also observed. The Na₂O content is more or less constant (3.5 wt.%), with a slight increase in K₂O/Na₂O ratios (1.45 wt.% - 1.53 wt.%), and K₂O + Na₂O (8.59 wt.% - 8.96 wt.%).

The Least altered metasediment (CMRB-37) is characterized by a high silica content (78.88 wt.%) and moderate K₂O (4.2 wt.%). It displays a low Al₂O₃ (10.35 wt.%), Na₂O (3.12 wt.%) CaO (0.25 wt.%), P₂O₅ (0.01 wt.%), FeOt (1.31 wt.%), MgO (0.04 wt.%), TiO₂ (0.1), MnO (0.04 wt.%), K₂O/Na₂O ratios (0.99 wt.%), and LOI value of 0.2 wt.%.

Major element variations plot is presented in Figures 9(a)-(h) , Na₂O + K₂O, K₂O and Na₂O show a positive correlation with SiO₂, while LOI, FeOt, CaO, and Al₂O₃ decrease in concentration with increase in SiO₂. Na₂O also decreases with increase in K₂O. Al₂O₃ concentration is constant with sericitization, whereas sulphidation displays and increase in FeOt content.

Fault rock-silicified samples display Au content of 0.6 ppb, with higher Au concentrations of 1.8 ppb observed in fault rock sulphidized. These sulphidized samples with distinct ductile deformation features have the highest gold content and suggest that the domains with ductile deformation largely at the center of the shear zone are more prospective for gold: Invariably the brittle features are less endowed with gold. Sulphidation is mark by an increase in the transition trace elements (Cu, V, Mn, Co, Ni, Sc and Pb) alongside Au ( Table 1 ). Some selected elements concentrations of the fault rocks were normalized to the average least altered metagranites in the Gankoumbol-Djouzami-Beka area ( Figure 11 ). Enrichment peaks observed in Mn, Cu, S, Au, Te, Fe, Bi, Ba, Mo, Mg, and LREE (La, Sr). Ca, As, Sb, Ga, Cs, Sn and Nb are depleted. Generally, the concentration of Mn, and Ba is higher in the fault rock sulphidized, fault rock sericitized and fault rock silicified samples compared to the fault rock K-feldspars ( Table 1 ). The Mn and Ba content is least in the least altered metagranitoids and metasediments. Sr concentration is high in fault rock sulphidized and fault rock sericitized compared to fault rock silicified, fault rock K-feldspars, least altered metagranitoids and metasediments. Sulphidation, silicification and sericitization are associated with increased in base metals Mn, Sr, Ba and Au. On the other hand, Zn concentration is high in the least altered metagranitoids compared to the altered fault rocks.

On the variation plots of selected element concentration with Au for both least altered and altered fault rocks, S, Cu, Fe, Mn, As, Te, Ni and Ba show a positive correlation with gold in the sulphidation zone ( Figures 11(a)-(f) ). Fault rock-sericitized shows a constant concentration with gold mineralization.

On the C1 chondrite-normalized REE plot of [45] , the samples show LREEs enrichment (CeN/SmN: 1.98 - 7.45, LaN/YbN: 4.17 - 51.81) and HREEs depletion (GdN/YbN: 1.19-6.84), with a strong to weak negative Eu anomaly (0.79 - 0.04; Figure 10(a) ) a positive Ce anomaly and ƩREE = 30.7 - 103.2 ppm. On the multi-element variation plot ( Figure 10(b) ) normalized to values of [46] , the least altered metagranitoi display peaks at Cs, U, Ce, Nd, Sm and Dy, with depletions in Nb, Ba, P, Ti and Zr. The metasediment displays peaks at Rb, Th, Nd, Ce, Pr and Dy, with depletions in Ba, Sr, Zr, Ti, Ta and Cs. The fault rocks display peaks at Th, La, Nd, U and Pr, with negative Nb, Zr, Ti, P and Ba. Fault rock-K-feldspar and Fault rock-sericitized samples display peaks at Rb and Sm and depletion in Sr compared to the other fault rocks. Fault rock-sulphidized sample displays peak in Dy.

The small-scale Gankoumbol-Djouzami-Beka shear zone (GDBSZ) is about 400 m wide ( Figure 2 ). It is a typical brittle-ductile deformation zone with moderate to weak foliated low-strain fabrics towards the rims and strongly foliated higher-strain mylonitic/ultramylonitic zones at the center. It extends over several km along the NE-SW strike, straddling the boundary between the basement metasediments and the Pan African granitoids, and constitutes a segment of the CCSZ. The presence of both ductile and brittle structures ( Figures 3(a)-(f) ) within the investigated shear zone attests to the fact that the studied shear zone was developed at the brittle-ductile shear transition [11] [28] . Also, the presence of shear sense indicators depicting both sinistral and dextral sense of movement ( Figure 3 and Figure 7 ) is suggestive of the fact that the GDBSZ is characterized by polyphase deformation from D1-D4 [2] [12] [47] . The sinistral shear is dominant, with prolate type of strain ellipsoid, depicting deformation by constriction in a transtensional regime. The association of sinistral shear sense indicators with pyrite (sulphidation) at the microscopic scale ( Figures 7(a)-(c) ) may be suggestive of the fact that primary gold mineralization in the Upper Lom basin was initiated by sinistral shear movement. The NE-SW trending foliations together with sub-horizontal lineation characterized a strike-slip shear zone in a transtensional regime during the Pan-African tectonic evolution [2] . Grain size reduction from rim to core characterizes the shear zones [48] , with coarse-grained least deformed metagranite ( Figures 4(b)-(c) ), grading to extremely deformed fine-grained foliated ultramylonite at the core ( Figure 5 ). Furthermore, the development of a shear zone does not occur under-closed system conditions. Introducing or expelling fluids and mass during deformation has mineralogical consequences that control the rheology and the way the shear zone evolves. Fractures and lithological contacts (metagranitoid-metasediment interphase) represent pre-existing weaknesses that act as a site for shear zone nucleation and development. These structures also act as conduits for fast fluid flow, catalyzing metamorphic and metasomatic growth phases, allowing the propagation and development of shear zone via a physical-chemical feedback mechanism [49] . Shear zones began as discrete structures from nucleation on fractures, widening due to wallrock weakening given the abundant evidence of fluid flow and syntectonic metamorphism [49] . This interpretation is consistent with that of the present study.

Shear sense indicators are well developed in high strain zones ( Figure 3 and Figure 7 ). Shear sense reversal in brittle-ductile regime associated with the collisional strike-slip tectonic setting may result from multiple senses of shear movement within a single deformation phase, or reorientation of principal stress axes thereby imposing an orthogonal switch on the principal orientation [49] . However, shear sense reversal within the Lom Basin, is attributed to polyphase deformation from D1-D4 [2] [12] in a transtensional regime during the Pan African Orogenic Belt evolution. This interpretation is consistent with that of this study. The healed fractures within the feldspars depict sub-magmatic deformations ( Figure 6(f) ), and evidence that pluton emplacement was synchronous with the deformation event. The shearing and dynamic recrystallization of quartz and feldspars with etch-surfaces regarded as dissolution textures ( Figure 6(b) and Figure 6(f) ) probably occurred under hydrous conditions as confirmed by previous researchers [50] . The infiltration of fluids was a key contributor to the localization of strain along the fracture planes, and shear zones developed through a chemical and mechanical transformation of the anhydrous coarse-grained host.

Hydrothermal alteration is common in lode gold deposits and act as chemical traps for metal precipitation through water-rock interaction, and are generally favorable for mineralization [51] [52] . In hydrothermal ore deposits, alterations and metasomatism of parent rocks are crucial processes, because they record the details of ore formation through dissolution-reprecipitation mechanism. Alteration zones also mark the sharp interface between unaltered and altered mineralized zones. Gold mineralization associated with intense wallrock alteration is more or less structurally controlled [11] [53] . Sericitization, K-feldspar alteration, silicification and sulphidation constitute the alteration assemblages within the shear zone ( Figure 5 and Figure 6 ). These alteration assemblages trend NE-SW, defining the mineralogical-geochemical footprint towards Au mineralization. The hydrothermal alteration envelope shows some geochemical distinction from the least altered rocks to altered fault rocks. Diffusive sericitization is pervasive across the fault zone, with intensity increasing towards the ultramylonite/mylonite zone. Silicification is depicted by the high SiO₂ content (79.83 wt.%), low K₂O, CaO, P₂O₃ and LOI ( Table 1 ) resulting from the introduction of secondary silica into the system and the replacement of former igneous and secondary minerals by quartz [45] . However, the abundance of quartz in brittle-ductile zones indicates the presence of hydrothermal fluids saturated in silica [8] . Sulphidation is marked by an increased in S, Au and transition trace elements such as FeOt, Cu, Mn, Ni, Co, Cr, V, and Sr is attributed to sulphidization in the fault rocks [45] , and occurrence of disseminated sulphides associated with Au mineralization ( Figure 6(b) and Figure 6(f) ). The Au content in this zone reaches a high of 1.8 ppb ( Table 1 ). The low K₂O content in the Fault rock-sericitized is as a result of less K-feldspars, as sericite growth (sericitization) at the expense of K-feldspars. The slight Rb and Pb enrichments might be due to their preferential substitution for K⁺ and Na⁺ in the lattice of K-feldspar and sericite in the altered fault rocks during sericitization. The high concentration of Mn, Sr and Ba in the altered fault rocks depicts increasing hydrothermal alteration associated with Au mineralization. The high concentration of Zn in the least altered matagranitoids depicts a decrease in hydrothermal alteration.

Generally, the LOI content increases from the least altered samples (metagranite and metasediment) to altered fault rocks ( Table 1 , Figure 4(b) ) as alteration intensity increased, forming massive sericite and muscovite ( Figure 5(e) ). The concentration of Na₂O remained almost constant across the fault rocks resulting from partial consumption of feldspars such as albite [53] , indicating limited albitization ( Figure 10 ). Al₂O₃ content varied slightly (10.36 - 15.2) for the sampled fault rocks due to its immobility, suggesting maybe a slight change in ore-bearing fluid source, from under saturated to Al saturated fluids. The low Al₂O₃ content in the metasediments may be suggestive of the fact that the Al-rich fluids were derived from a granitic source [46] .

The predominant SiO₂ content in the major element concentration for the analyzed samples with concentrations ranging from (64.88 to 79.83 wt.%) is suggestive of a felsic source of the mineralizing fluids in the study [54] . The complex behavior of HFSE is seen in enrichment of Th, U and depletion of Nb and Ti. The negative Nb, Zr and Ti anomaly observed in the altered rocks is supportive of the breakdown/consumption of accessory zircon and titanite during alteration [45] ; Figure 12(b) , or maybe indicating a higher immobility of HREEs during fluid-rock interactions. The studied samples show LREE enrichment relative to the HREE and a negative Eu anomaly reflecting plagioclase fractionation [55] , and a reducing condition of mineralizing fluid ( Figure 12(a) ). The relatively low ƩREE content ( Table 1 ) in the studied fault rocks points to the fact that the REE bearing minerals such as zircon, monazite, and apatite were gradually consumed as alteration intensity increased with fluid fluxes, from K-feldspar alteration through silicification to sericitization [53] . Also, the lack of secondary minerals to accommodate the REE or limited complex ligands like SO₄²⁻, Cl⁻ and F⁻ to transport them may have contributed to their low concentrations. Sericitization, silicification provided suitable chemical traps, promoting Au mineralization through sulphidation, and is proposed to be the main mechanism for gold precipitation in orogenic gold deposit systems [11] [18] - [20] [28] . The Neoproterozoic granitoids described in the eastern goldfields in Cameroon are geochemically characterized as I-type, metaluminous to weakly peraluminous and calc-alkaline to shoshonitic, associated with gold mineralization along wallrock alteration zones and in auriferous hydrothermal quartz veins ( Figure 9 ; [11] [16] - [21] ). Gold mineralization in GDB area within the Upper Lom Series is shear zone and intrusion-related, with hydrothermal alteration controlling the mineralization. The significant enrichment of base metals Cu, Mn, Te, S, Bi; Sr, Mg and Au in fault rock sulphidized is suggestive of their significant role in gold mineralization in the GDB area. The enrichment of base metals Au-S-Cu-Mn-Te-Fe-Mo-Bi alongside gold in the fault rock goes to supports the influence of a postmagmatic fluid responsible for concentrating gold [18] .

Shear zones act as deep major channels that serve as conduits for hydrothermal ore-forming fluid circulation. Ore bodies are mostly associated with second and third-order fractures along shear zones [7] [11] . The role of a shear zone in gold mineralization has been emphasized in different structural settings, where mineralization is coeval with deformation, magmatism, shearing, fluid-rock interaction, and hydrothermal alteration [56] . The brecciated nature of the studied rocks in the GDB area provided a suitable pathway for hydrothermal fluid circulation and favorable conditions for intense hydrothermal alteration and precipitation of gold. Such features have been proven to exhibit great potentials for disseminated ores. Shear zones at the granitoid-metasediment contact are major structures with good prospects for hydrothermal gold mineralization. This interpretation is consistent with field and geochemical data from the GDB area in the Upper Lom Basin, with strongly altered fault rocks as a potential host of gold along the shear zone in the eastern goldfield in Cameroon. Future exploration in the Upper Lom Series should target altered fault rocks along contact granitoids. Therefore, the possibilities of locating economic low-grade lode gold concentrations in the area are high, considering that intensive eluvial/alluvial gold workings are ongoing in the area. Hydrothermal alteration processes, therefore, remain the main gold enrichment determining factor, remobilizing gold in early magmatic sulphide minerals into favourable fracture-controlled zones by post-hydrothermal fluids dominated by sericitization, silicification, and sulphidation alteration. The role of hydrothermal activity in gold precipitation in this area is in line with previous fluid inclusion and isotopic data [19] . These data show that the fluid is carbonic from metamorphic devolatization and the stable oxygen and carbon isotope data are tied to fluid derivation from magmatic and metamorphic sources with unmixing playing a role in Au precipitation. This could be a powerful enrichment process in shear-zone-related granitoids in SE Cameroon [11] [18] [19] . Geochemical indicators such as enrichment and depletion patterns and geochemical pathfinder assemblage from factor analysis can provide key exploration information enabling Au vectoring in target areas. Granitoids in orogenic systems acted as rheological competent rock units, undergoing brittle-ductile deformation and forming suitable structural controls (shear zones) which acted as fluid pathways propagating hydrothermal alteration/mineralization [8] [57] .