Typology of Natural Hazards and Assessment of Associated Risks in the Mount Bambouto Caldera(Cameroon Line,West Cameroon)

Mount Bambouto is a polygenic stratovolcano of the Cameroon Volcanic Line, built between 21 Ma and 4.5 Ma. It is situated approximately 200 km NE of Mount Cameroon, between 09° 55＇ and 10°15＇ longitude east and, 05°25＇ and 05°50＇ latitude north. The volcano covers an area of 500 km^2 and culminates at 2740 m at Meletan dome and bears a collapsed caldera at the summit （13 × 8 km）. Mount Bambouto is characterized by several natural hazards of different origins： meteorological, such as landslides and rock falls; anthropogenic, such as bushfires, tribal wars and deforestation; and volcanological, such as volcanic eruption. The thematic map shows that 55-60% of the caldera has high probability of occurrence of mass movement. The caldera has a high population density （3000 inhabitants）, which increases the level of risk, evaluated at approximately $US3.8 million for patrimony, 3000 civilian deaths and destruction of biodiversity.

Petrogenesis of the Mayo-Darley tin formations,anorogenic complex of the Cameroon Line:implication for tin deposit

The Mayo-Darley massif is an anorogenic complex of the Cameroon Line that has a mantle origin.Tin-bearing rocks were analyzed by ICP-AES and INAA analytical methods.The purpose of this work was to provide new geochemical data for the Mayo-Darley tin formation and to understand its petrogenesis and the origin of tin mineralization.The Mayo-Darley tin deposit is made up of tin-granite,and tin-greisen,greisenification was developed on the borders of quartz dykes.These rocks belong to the alkaline series and were classified into acid(SiO2-=61.6%–73.8%;65.4%–98%respectively)and basic(42.9%–47%SiO_(2))rocks.They showed enrichment in HSFE,LILE,Ga/Al and chondrite normalized REE patterns indicating LREE enrichment relative to HREE with a negative Eu anomaly,only sample SB8 of tin-granite showed Eu/Eu*=1.11.Rocks display metaluminous,peralkaline,peraluminous,ferroan,high-K calc-alkaline to shoshonitic,alkalic to calcic affinity,and crystallized at800°C.The chemistry of this deposit reflects the primary composition of granite,quartz monzonite,gabbro,and foid gabbro.This complex experienced multi-stage sub-solidus hydrothermal fluid reactions and shows variable alteration of feldspars alkali mobility.The rocks are classified as A1-type granite,overlap with the OIB field,and were derived from a within-plate setting,similar to mantle non plumederived magmas.The origin of tin mineralization in MayoDarley has a complex evolution,tin mineralization was derived from hydrothermal and hydrogenous metal-rich deposits and shows Sn–Hf–Zr and Sn–Tl–Nb association.These new data confirm the complexity of the Mayo-Darley tin complex and elucidate the origin of tin mineralization.

A Typical Sublithospheric Mantle beneath the Tello Region, SE-Ngaoundéré (Cameroon Line)

Peridotite xenoliths,raised to the surface by alkaline basalts or kimberlites,provide us direct information on the processes and composition of the upper mantle.They are the major source of information on the state of stress,pressure and temperature in the deep mantle.They are thus a source of petrological and geochemical information that is generally not available on the Earth’s surface.Fresh spinel-lherzolite xenoliths exhibit a protogranular components of the Tello volcano.The Tello is the continental sector of the Cameroon Line,located in the South East of the town of Ngaoundéréat 75 km approximately between(N7°13’,N7°14’)and(E13°40’and E13°60’).Minerals’composition of the xenoliths is~64%olivine,~24%orthopyroxene,~11%clinopyroxene and~1%spinel.Significant variation in(Cr/Cr+Al)of the system shows the reciprocal nature of the spinel solution.The Tello spinel lherzolites show internal chemical homogeneity and represent a normal upper mantle.Their mineral chemistries suggest equilibrium condition of 830°-925°and 1.4 GPa-2.3 GPa.These data suggest that there is good correlation bracket between increasing activity of Al2O3 and decreasing of practionning of TiO2 into spinel.The AlIV and AlVI contents vary by 0.05-0.2 and 0.03-0.2 respectively.The majority of samples caracterise the lithospheric mantle.

Typology of Natural Hazards and Assessment of Associated Risks in the Mount Bambouto Caldera(Cameroon Line,West-Cameroun)

Mount Bambouto is a polygenic stratovolcano of the Cameroon Volcanic Line,built between 21 Ma and 4.5 Ma(Nkouathio et al.,2008).It is situated at about 200 km NE of mount Cameroon,at 09°55′and 10°15′East and,05°25′and 05°50′North.This volcano covers an area of 500 Km^2 and culminates at 2740 m at Meletan hill and bears a collapsed elliptical caldera at the summit(13×8 km).Mount

Petrology of Spinel-Lherzolite Xenoliths from Mazéléand Others Northen Xenoliths Localities of Cameroon Volcanic Line: Exchange Reactions and Equilibrium State

The alkaline volcanism of the Cameroon Volcanic Line in its northern domain has raised many fresh enclaves of peridotites. The samples selected come from five (05) different localities (Liri, in the plateau of Kapsiki, Mazélé in the NE of Ngaoundéré, Tello and Ganguiré in the SE of Ngaoundéré and Likok, locality located in the west of Ngaoundé). The peridotite enclaves of the above localities show restricted mineralogical variation. Most are four-phase spinel-lherzolites, indicating that this is the main lithology that forms the lithospheric mantle below the shallow zone. No traces of garnet or primary plagioclase were detected, which strongly limits the depth range from which the rock fragments were sampled. The textures and the wide equilibrium temperatures (884˚C - 1115˚C) indicate also entrainment of lherzolite xenoliths from shallow depths within the lithosphere and the presence of mantle diapirism. The exchange reactions and equilibrium state established in this work make it possible to characterize the chemical composition of the upper mantle of each region and test the equilibrium state of the phases between them. Variations of major oxides and incompatible elemental concentrations in clinopyroxene indicate a primary control by partial melting. The absence of typical “metasomatic” minerals, low equilibration temperatures and enriched LREE patterns indicate that the upper mantle below septentrional crust of Cameroun underwent an event of cryptic metasomatic enrichment prior to partial melting. The distinctive chemical features, LREE enrichment, strong U, Ce and Pr, depletion relative to Ba, Nb, La, Pb, and T, fractionation of Zr and Hf and therefore ligh high Zr/Hf ratio, low La/Yb, Nb/La and Ti/Eu are all results of interaction of refractory peridotite residues with carbonatite melts.

Geochemistry of the volcanic rocks from Bioko Island(“Cameroon Hot Line”)： Evidence for plume-lithosphere interaction

Bioko Island（3008 m a.s.l） is located in the presently more active volcanic zone of the Cameroon Line and composed essentially of alkaline basalts and hawaiites, and lesser mugearites. The rocks show microlitic porphyritic texture with phenocrysts of olivine（83% 〈 Fo 〈 87%） and clinopyroxene in a matrix of plagioclase, clinopyroxene and oxides. Hawaiites and mugearites also include phenocrysts of plagioclase（An62-67Ab35-32Or3-1）. Major element variation diagrams show an increase in Si O2, Al2O3, Na2 O and K2 O with increasing Mg O for the studied rock groups. The rocks are characterized by low（^86Sr/^87Sr）i ratios（0.70320e0.70406）, high 3Nd（t） values（2.56e4.33） and high（^206Pb/^204Pb）i ratios（20.032e20.035） values.Basalts are enriched in LILE and LREE, and have（Hf/Sm）N= 0.57e1.16. These geochemical signatures are similar to those of the Mount Cameroon rocks, and might be attributed to low degrees of partial melting from a garnet-amphibole-bearing mantle source. The trace elements and isotopic compositions suggest that the parental magma source might have involved HIMU- and EM1-components.

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line （CVL）, composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, het- erogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between Si02 58-64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability feld. Melting models suggest small degree （〈3%） partial melting of mantle bearing （6-10%） garnet for Mr. Etinde, the Ngaoundere Plateau and the Biu Plateau, and 〈5% of garnet for the oceanic sector of the CVL, Mr. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope sys- tematics suggest that mixing in various proportions of Depleted MORB Mantle （DMM） with enriched mantle 1 and 2 （EM1 and EM2） could account for the complex isotopic characteristics of the CVL lavas. Low Mg number （Mg# - 100 x MgO/（MgO ＋FeO）） and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in NbJTa and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle het- erogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

Mineralogy and Magmatic Processes of Cenozoic Intraplate Alkaline Volcanic Rocks of Bafang and Its Environs (Cameroon Volcanic Line, West Africa)

Alkaline basalts of Bafang and its environs are consisted of feldspars, olivines, pyroxenes and oxides which appear as phenocrysts, microphenocrysts and microcrysts. Feldspars are plagioclases (An67.97-15.84Ab69.19-30.43Or20.59-1.51) and anorthoclases (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Plagioclases are the most abundant amount these feldspars. Anorthoclases appear only in mugearite (BAF 3 and BAF 37) the most differentiated of the studied alkaline-basalts. In High Magnesian basalt, (HMg-B) plagioclases are labradorites (An67.97-59.30Ab38.74-30.43Or2.75-1.60) and sanidine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), whereas in Low Magnesian basalt (LMg-B) there are labrador (An67.4.75-51.96Ab44.98-33.72Or3.06-1.51), andesine (An45.44-31.82Ab62.66-51.79Or5.52-2.77), oligoclase (An26.65-15.84Ab69.19-63.57Or20.59-8.55) and anarthoclase (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Olivines are magnesian (Fo86.7-50.1) and ferriferous (Fo48.8-37.8). In HMg-B, olivine are only magnesian. These olivines are chrysolites and hyalositerite. In LMg-B, olivines are magnesian and ferriferous with the predominance of ferriferous. They are chrysolites, hyalositerite and hortonolite. Pyroxenes are Ca, Mg and Fe clinopyroxenes. There are diopsides (Wo51.94-45.02En44.41-33.16Fs16.42-10.70) and augites (Wo44.88-43.64En41.03-37.04Fs18.25-14.43). Oxides are magnetites represented by ulvospinel (Usp90-75Sp2-7Mt5-23). Fractionation of ferromagnesian minerals (opaque oxide, olivine and pyroxene) is the main differentiation process. Two stages of fractional crystallization can be distinguished: the first stage comes with basanites and the second with hawaiites to mugearites. Chemical compositions of phenocrystals in studied basaltics lavas record signatures of magma recharge by pulsatory intrusions of new magma into the existing magma reservoir before the eruptions.

Petrology of the Volcanic Rocks from Bioko Island(“Cameroon Hot Line”)

Bioko island (3008 m a.s.l.), is composed of the alkaline basaltic lavas (basalts and hawaiites) with xenoliths. These lavas have a microlitic textureand consisted of euhedral to subhedral phenocrysts?(>1 mm) of olivine (0.83 < Mg# < 0.87) and diopside ± Ti-augite. Plagioclase (An62-67Ab35-32Or3-1) phenocrysts are present only in hawaiites. In Harker diagrams, SiO2, Al2O3, Na2O and K2O contents of the lavas increase and Fe2O3, MgO and CaO decrease with increasing differentiation from basalts to hawaiites. The compatible elements Ni, Cr and V decrease strongly in basalts and remain at low levels in hawaiites. Basaltic lavas from Bioko lavas have low Hf contents (4.2 - 9.2 ppm) and consequently higher Hf/Zr ratios (50 - 90) than those (<50) of similar lavas from other volcanoes of the Cameroon Hot Line. Such high ratios are commonly observed in alkaline basaltic lavas associated with carbonatitic and/or nephelinitic magmatism. The Sr and Nd isotopic compositions point to a slightly depleted mantle source.

Variation in mantle lithology and composition beneath the Ngao Bilta volcano,Adamawa Massif,Cameroon volcanic line,West-central Africa

Mantle peridotites entrained as xenoliths in the lavas of Ngao Bilta in the eastern branch of the continental Cameroon Line were examined to constrain mantle processes and the origin and nature of melts that have modified the upper mantle beneath the Cameroon Line.The xenoliths consist mainly of lherzolite with subordinate harzburgite and dunite.They commonly contain olivine,orthopyroxene,clinopyroxene and spinel although the dunite is spinel-free.Amphibole is an essential constituent in the lherzolites.Mineral chemistry differs between the three types of peridotite:olivines have usual mantle-like Mg#of around 90 in lherzolites,but follow a trend of decreasing Mg#(to 82)and NiO(to 0.06 wt.%)that is continuous in the dunites.Lherzolites also contain orthopyroxenes and/or clinopyroxenes with low-Mg#,indicating a reaction that removes Opx and introduces Cpx,olivine,amphibole and spinel.This is attributed to reaction with a silica-undersaturated silicate melt such as nephelinite or basanite,which originated as a low-degree melt from a depleted source as indicated by low Al2O3 and Na2O in Cpx and high Na2O/K2O in amphibole.Thermobarometric estimates place the xenoliths at pressures of 11–15 kbar(35–50 km)and temperatures of 863–957
C,along a dynamic rift geotherm and shallower than the region where carbonate melts may occur.The melt/rock reactions exhibited by the Ngao Bilta xenoliths are consistent with their peripheral position in the eastern branch of the Cameroon Volcanic Line in an area of thinned crust and lithosphere beneath the Adamawa Uplift.

Mineralogy and geochemistry of pozzolans from the Tombel Plain,Bamileke Plateau,and Noun Plain monogenetic volcanoes in the central part of the Cameroon Volcanic Line

Pozzolans from the Tombel Plain,Bamileke Plateau,and Noun Plain,3 monogenetic volcanic fields in the central part of the Cameroon Volcanic Line(the Tombel Plain,Bamileke Plateau,and Noun Plain),were explored in order to constrain their petrology and make some predictions on their pozzolanicity.The rocks in this study include alkaline and subalkaline basalts,trachybasalts,and basanites.Most of these rocks present an overall composition that overlaps with primitive mantle,suggesting rapid ascent of magmas,limited crustal contamination and crystal fractionation of olivine,clinopyroxene,and feldspar.The pozzolans present enrichment of LREE relative to HREE and high chondrite normalized ratios of La/Yb and Tb/Yb.ranging between 7 and 20 and>1.9 respectively,similar to those of Ocean Island Basalts.Like other nearby volcanoes,partial melting in a dominantly garnet-bearing mantle zone can be assumed.Quantitative mineralogy by X-ray diffraction revealed various mineral phases with dominantly plagioclase,augite,olivine,and Fe-Ti oxides.The samples contains important amorphous phase up to 23,51,and 69 wt%in the Tombel Plain,Noun Plain,and Bamileke Plateau,respectively.This elevated amount of amorphous phases together with the sum of SiO2,Al2O3 and total Fe2O3(SAI=68.50-83.50>70 wt%)according to ASTM C618 standard and the sum of CaO,FeO,and MgO(CIM=14.5-30.52 wt%and 23.58-31.08 wt%)suggest interesting pozzolanicity character for the studied pozzolans.

The Bafoussam volcanic series:origin and evolution of the volcanism along the Cameroon volcanic line

The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.

Petrological and Geochemical Studies on the Si-Undersaturated Rocks of the Mount Cameroon: Genesis of the Camptonite and Nephelinite at the Cameroon Hot Line

The Cameroon hot line is dominated by magmatic rocks. The variations of magma and chemistry are generally due to the difference of physical conditions and chemistry in the magma source region during the ascent of magma. The Mt Etinde and the Mt Cameroon, both edifices belong to the Cameroon Hot line, have a particularity some rare rocks such as camptonite and nephelinite. The relationship between the silica undersaturated rocks in the both edifices is characterized by the lateral variation appear through the petrography of the different rocks. The concerned geochemical data allow to compare the Mount Etinde nephelinite and Mount Cameroon camptonite where the differentiation process reflects geochemical affinities from a basaltic magma source on the Cameroon hot line. The compatible elements between the Mount Etinde nephelinite and the Mount Cameroon camptonite and basalt correlate with the difference in modal compositions of mineral phases. The lateral variation of major and trace element contents in the Mount Cameroon camptonite and Mount Etinde nephelinite seem to be related to the difference in the fractional crystallization processes of mineral phases, the difference in the partial melting processes and the metasomatism source rich in volatile. The silica-undersaturated character of the camptonite and nephelinite could be attributed to assimilation of carbonate rocks within depth-level magma chambers. Trace element AFC modelling revealed that the parental magmas of both edifice volcanic rocks were mostly affected by fractional crystallisation coupled with metasomatism process in Ca rich source.

Exploration of Potential Ore Deposits along the Cameroon Volcanic Line from Gravity and Magnetic Studies

A gravity and magnetic study has been carried out along the continental part of the Cameroon Volcanic Line (CVL), with the aim to explore the possibility of ore’s presence into the basement of this region. Different processing techniques have been applied, including the isostatic residual, the analytic signal and the Euler deconvolution to compute the Bouguer and magnetic anomalies. Following Euler solutions (result of Euler deconvolution) and the analytic signal results, four profiles crossing main structures on the isostatic residual have been used to enhance the structure of intrusions in the studied area. Despite the lack of constraints in the studied region, the results show that the basement is intruded by bodies of different density (2.57 g/cm3 to 2.87 g/cm3), different size and shape, with depths between 1 and 10 kilometers. These bodies have been interpreted as old rocks then as potential reservoirs of rare ores. In addition, volcanic rocks modeled have constituted potential reserves of other ores like graphite, sulfur, copper, iron.

Magmatism of the Beka Volcanic Massifs (Cameroon Volcanic Line, West-Central Africa): New Petrographical and Mineralogical Data

The Beka volcanic massifs are located northeast of Ngaoundere region, within the Adamawa plateau. It consists mainly of basanites, trachytes and phonolites. The petrographic study shows that all the basanite lavas have porphyritic microlitic textures with a more pronounced magmatic fluidity than the felsic lavas displaying trachytic textures. The lavas are composed of phenocrysts, microlites and microphenocrysts of olivine, clinopyroxene, plagioclase and iron-titanium oxides for the basanites and of greenish clinopyroxene, alkali feldspar, and titanomagnetite for the felsic lavas. Chemical microprobe analysis indicates that the olivine crystals are magnesian (Fo73-78). Clinopyroxene crystals have a composition of diopside (Wo47-) in the basaltic lavas and diopside near the hedenbergite pole in the trachytes phonoliths and titanomagnetite (TiO2: 21.13% - 22.36% and FeO: 68% - 68%). Chemical analyses on whole rocks show that all the lavas belong to the same series and the felsic lavas come from the differentiation of basanite lavas by fractional crystallization of the minerals therein. The basanites originate from a low rate of partial melting of an OIB-type mantle. Contamination and mixing processes are suspected. Lavas of similar composition are found in other volcanic centres of the Adamawa plateau and the continental and oceanic sectors of the Cameroon Volcanic Line, in particular the Kapsiki plateau, Mounts Cameroon and Bamenda.

Baossi–Warack monogenetic volcanoes, Adamawa Plateau,Cameroon: petrography, mineralogy and geochemistry

Three monogenetic cones in the Baossi–Warack area, Ngaoundéré, Adamawa Plateau forming part of the Cameroon Volcanic Line(CVL) are documented in this study. Basaltic lavas(<1 km^3) scattered around these vents and restricted volcaniclastic deposits were emplaced by Hawaiian and mild strombolian style eruptions. The lavas are porphyritic, mainly composed of olivine(chrysolite) and clinopyroxene(diopside and augite) phenocrysts and plagioclase(andesine) microphenocrysts. Accessory minerals include titano-magnetite and titano-hematite, nepheline,apatite and amphibole xenocrysts. Sanidine occurs in some samples and sodi-potassic albite in others. Some olivines and clinopyroxenes exhibit resorbed margins and thin reaction rims while plagioclase displays oscillatory zoning, and sieved textures as a result of magma mixing. Whole-rock geochemistry data indicates that the lavas are silica-undersaturated, composed of basanites and basalts, showing little compositional variations(SiO_2: 39.20 wt.%–48.01 wt.%,MgO: 5.29 wt.%–9.70 wt.%). Trace elements patterns of these lavas suggest they are enriched in LILE including Pb,probably due to crustal contamination. REE patterns suggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes,and distinct sources below Warack volcano and nearby lavas.The lavas studied show affinity to high-μ(HIMU), enriched type Ⅰ(EM1) and Oceanic Island Basalt(OIB)-like mantle signatures and thus indicate a heterogeneous mantle source underneath the vents as noted at other monogenetic and polygenetic volcanoes along the CVL. Primary melts derived from low degrees of partial melting(0.5%–2%) and encountered low rates of fractionation, and crustal contamination coupled with magma mixing. These melts evolved independently through structural weaknesses in the basement.

Carbonate Enrichment in Volcanic Debris and Its Relationship with Carbonate Dissolution Signatures of Springs in the Sabga-Bamessing, North West, Cameroon

Sabga-Bamessing is a part of the Bamenda Mountains, an extinct volcanic center of the West Cameroon Highlands along the Cameroon Volcanic Line (CVL). The pristine volcanic rocks of the Sabga area are alkali mafic to felsic (basanites, phonolites, trachytes and rhyolites). Some weathered sections of a heterolithologic debris flow with a suppositious primary chemistry of the original volcanic rocks prior to weathering have shown significant calcium carbonate enrichment. CaO and LOI values of up 61.31% and 41.72% respectively show corresponding enrichment of 16.54 and 10.88, when compared with average fresh volcanic rocks. Na+ normalized molar ratios computed from the chemistry of springs and rivers show carbonate dissolution signature which is contrary to silicate dissolution expected in acid volcanic rocks. Saturation indices (SI) calculated with PHREEQC reveal that brackish to saline springs are supersaturated with Calcite (CaCO3), Aragonite (CaCO3), Dolomite (CaMg(CO3)2 and Hydroxyl apatite (Ca5(PO4)3OH). Recharging contributions to spring water chemistry deviate from those produced by rock weathering, precipitation and evaporation/crystallization. An enrichment process is therefore predicted as a recharging contributor to water chemistry.

Andosolization of Soils on a Strombolian Cone at Mount Bambouto, Cameroon

Morphology, mineral and geochemical investigations were carried out on two selected soil pedons (ZA and TO) developed on a late Quaternary-aged strombolian cone to better understand andosolization processes in Mount Bambouto, Cameroon. Both pedons have A-BC-C horizonation. They have thick surface (A) horizon with bulk density -3, Alo + 0.5Feo about 2% and P retention >90%. Mineral association consists predominantly of kaolinite, gibbsite, goe- thite, organometal complexes and trace amount of ferrihydrite and allophane. The Ki values between 1.1 and 1.6, the low TRB (45 to 67 mg kg-1), the important IMob (20% to 24%), the CIA between 60% and 70% and the EFs above 1.16 for Al and Fe, and below 0.6 for Si indicate sparingly hydrolysis process in subsoil during which released Al, Fe and Si form allophanic or ferrihydrite minerals undergo crystallization into kaolinite, gibbsite and goethite, respectively. At topsoil, part of released Al (and Fe) is organically bounded with organic acids to form organometal complexes.

Sedimentology and Geochemical Evaluation of Lignite-Argillite Sequences in a Named Basin in Bali Nyonga, Northwest, Cameroon

The sedimentology and geochemistry (major and trace element compositions) of lignite and argillite (carbonaceous shale and claystone) sequences in a Basin in Bali Nyonga, west of the Bamenda Mountain have been investigated to determine their sequences and the prevailing environmental?conditions which control their formation. Ten representative samples were obtained fromtrenches, pits, and river and stream valleys in the study area. These samples and their ashes were subsequently examined using X-ray fluorescence spectrometry (XRF), inductively coupled plasma spectrometry (ICP), and X-ray diffraction analysis (XRD). The geochemical results revealed that thelithophile, chalcophile and siderophile elements were dominantly epigenetic in origin, mainly?from detrital sources supported by high silica and alumina concentrations in all the samples. The mineral phases identified were quartz, kaolinite, illite, pyrite, hematite, and minor phases of feldspars,pseudorutile. The relatively high silica (54.10 wt%) and alumina contents (27.77 wt%) in these?samples can be explained by high detrital input during peat formation. The low contents of MgO and CaO in the analysed samples agree very much with the continental setting of the peat formating basin. A clayey microband derived from alkaline volcanic ashes was identified in the lignite and the dominant composition of these clayey microbands was mixed-layer clay minerals of illite and kaolinte, which were interlayered with organic bands. The modes of occurrence of ash bands indicated that the volcanic activities were characterized by multiple eruptions, short time interval?and small scale for each eruption during peat accumulation. The ratios of redox-sensitive traceelement (V/Cr versus Ni/Co and V/V + Ni versus Ni/Co), Sr/Ba, and major oxides ratio (CaO + MgO + Fe2O3)/(SiO2?+ Al2O3) from the analysed samples from Bali Nyonga indicate a terrestrial, reductive (oxic), littoral to brackish water environmental conditions which are characteristics of paludal-lacustrine basin that is filled by Tertiary volcanic materials.