Cheo Emmanuel Suh

Geology and geochemical aspects of lode gold mineralization at Dimako–Mboscorro, SE Cameroon

The Dimako–Mboscorro lode gold deposit has two styles of mineralization: quartz veins (4.5–54 ppm Au) and altered wallrock (up to 2.6 ppm Au). The veins occur in steeply dipping brittle–ductile shear zones in a foliated biotite metagranite. The alteration envelop around the gold-bearing quartz veins shows lateral zoning comprising a silicified/ ferruginized and mylonitic zone (SFZ) at the centre, through a transitional sericite zone (SZ), to an outer, slightly sheared, but mineralogically less altered, though bleached, metagranite zone (BZ) at the shear zone boundaries. Hydrothermal alteration in these zones is characterized by silicification (SFZ, SZ), hematite formation (SFZ), alkali metasomatism with muscovite/sericite formation (SFZ, SZ, to a much lesser extent BZ), and pyritization (SFZ, SZ). The auriferous quartz veins are concordant to the host structure but discordant to the foliation in the metagranite. The main ore minerals in the veins and in the SFZ and SZ zones are gold, hydrothermal hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, covellite and colloidal supergene hematite. Gold occurs as inclusions in sphalerite (15.7±0.4 wt% Ag; 83.6±0.2 wt% Au), as coarse grains associated with euhedral hematite pseudomorphs (18.5±1.0 wt% Ag; 82.0±1.1 wt% Au), as stringers within fractures in quartz (13.3±0.1 wt% Ag; 87.1±0.5 wt% Au), as well as fine to coarse grains related to limonite, goethite and colloidal hematite (8.1±1.8 wt% Ag; 92.1±2.2 wt% Au). A hydrothermal metal association of Ag, Au, As, Ba, Pb, V, W, and Zn is identified in the wallrock and quartz veins. These metals, together with K-enrichment, are potential pathfinder elements for wider lithogeochemical exploration in this region, especially when combined with the lateral wallrock zonation pattern. Preliminary fluid inclusion data from the auriferous quartz vein samples indicate that gold was probably deposited from low salinity (<1 to 8 wt% eq. NaCl, with mean at 5.52 wt% eq. NaCl) H2O–CO2 fluids that have total homogenization temperatures of 263–335°C.

Morphology and structure of the 1999 lava flows at Mount Cameroon Volcano (West Africa) and their bearing on the emplacement dynamics of volume-limited flows

The morphology and structure of the 1999 lava flows at Mount Cameroon volcano are documented and discussed in relation to local and source dynamics. Structures are analysed qualitatively and more detailed arguments are developed on the processes of levee formation and systematic links between flow dynamics and levee–channel interface geometry. The flows have clear channels bordered by four main types of levees: initial, accretionary, rubble and overflow levees. Thermally immature pahoehoe lava units with overflow drapes define the proximal zone, whereas rubble and accretionary levees are common in the distal region bordering thermally mature aa clinker or blocky aa flow channels. Pressure ridges, squeeze-ups and pahoehoe ropes are the prevalent compressive structures. Standlines displayed on clinkery breccias are interpreted to represent levee–channel interactions in response to changing flow levels. These data complement previous knowledge on lava flow morphology, thus far dominated by Etnean and Hawaiian examples.

Geochemistry and geochronology of the ~620 Ma gold-associated Batouri granitoids, Cameroon

The Batouri gold mining area in southeastern Cameroon is part of the Adamawa–Yadé Domain of the Central African Fold Belt (Pan-African). It is underlain by a variety of granitic rocks, including alkali-feldspar granite, syeno-monzogranite, granodiorite, and tonalite. Geochemical data suggest that these rocks formed by differentiation of I-type tonalitic magma under oxidizing conditions in a continental volcanic arc setting. U–Pb dating of zircons from gold-associated monzogranite-granodiorite at Kambélé gave concordant ages of 619 ± 2 and 624 ± 2 Ma, while Ar–Ar dating of alkali-feldspar granite yielded a non-plateau maximum age of 640–620 Ma. These ages imply that the Batouri granitoids were emplaced during the collision of the West African Craton and the Congo Craton. The geochemical characteristics of the Batouri granitoids as well as their oxidized state (magnetite series) are typical of gold-associated felsic rocks in subduction settings elsewhere. The similarities in age, composition, and geochemical affinities of these granitoids with those reported from other localities in the Adamawa–Yadé Domain reinforce the earlier assumption that the granitic rocks of this domain represent parts of a regional-scale batholith, with commonly small-scale, high-grade auriferous quartz veins in structurally favourable sites. The spatial and temporal association of gold mineralization and the Batouri granitoids may suggest potential for regional-scale, high-tonnage, granite-related gold ore.

Host rock geology and geochemistry of the Zona uranium occurrence, Peta Gulf Syncline (Upper Benue Trough), northeast Nigeria

Abstract The Peta Gulf Syncline (Upper Benue Trough, northeast Nigeria) is a fault-bounded pull-apart sub-basin. The boundary faults are mainly northeast-southwest-trending en echelon strike-slip faults, truncated along their lengths by normal and tear faults with stepovers. The eastern marginal faults underwent rotation during sedimentation, whereas the steeply dipping western marginal faults were inactive. The Peta Gulf Sub-basin is filled by the Bima Sandstone Formation (Lower Cretaceous) which has three siliciclastic members: (i) B 1 : medial fan coarse-grained to microconglomeratic sandstones; (ii) B 2 : full fluvial medium-grained sandstones with minimal fines; and (iii) B 3 : lacustrine and flood basin deposits comprising alternating fine-grained sandstones and siltstones/claystones. Sediment supply was from east to west and facies changes show a general fining in this direction. B 3 offers the most favourable environment/lithology for U concentration. The only significant U occurrence in the Peta Gulf Syncline is the Zona U anomaly, which occurs within transitional B 2 -B 3 brecciated sandstones with wall rock alterations zones. The mineralised zone has elevated SiO 2 , Fe, As, Ba and W levels but is depleted in the alkalis, Zr, Rb and Sr. This chemical zonation supports the epigenetic orgin of this anomaly.

Trace Element Soil Quality Status of Mt. Cameroon Soils

The concentrations of Cu, Co, Zn, Ni, V, and Cr in topsoils at six sites located along the lower slopes of Mt. Cameroon were assessed for their potential toxicity to humans and the ecosystem. Soils were collected from horizons down to a depth of 70 cm and analysed for trace element concentration by ICP-MS technique. The Dutch soil quality standards which use %clay/silt and organic matter content to derive target values were used to assess the contamination levels of the soils. The content of these soils was also compared to the United Kingdom ICRCL “soil trigger” values. Zinc and Cu values were persistently below the normal value (A) and occurred in the lower elevation, the region of extreme weathering, while Cr and V values were above the intervention (C) values. The high content of Cr in common fertilisers poses a potential risk in toxicity in the higher elevations experiencing lower weathering rates, where soil Cr levels are elevated.

Geochemistry of Ferriferous, High-K Calc-Alkaline Granitoids from the Banefo-Mvoutsaha Massif (NE Bafoussam), Central Domain of the Pan- African Fold Belt, Cameroon~!2009-11-12~!2010-01-05~!2010-03-11~!

The ferriferous high-K calc-alkaline rocks of the Banefo-Mvoutsaha Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a sinistral strike-slip shear zone of Pan-African age. The rock sequences consist of orthogneises made of coarse-grained granites, quartz-monzonites, and medium- to fine-grained granodiorites, ranging from ca. 55 to 75 wt.-% SiO2. They display characteristics of shoshonitic and high-K calc-alkaline series. Orthogneises are metaluminous to peraluminous and show characteristics of I-type granitoids from a ferriferous series. Trace element distribution patterns reveal rocks enriched in LILE when compared to HSFE and distinctively depleted in Th, Nb, Ba, Sr, Ti and Ta. The data indicate that this granitic rock assemblage did not result from the simple differentiation of a common parental magma, but show that Banefo-Mvoutsaha plutonic rocks were derived from different crustal protoliths. Major and trace element composition are consistent with the magmatism which may have involved remelting of (1) a composite metagreywackes protolith in the upper crust and (2) amphibolitised high-K calc-alkaline basaltic andesites in the central domain of the PANEFB (Pan-African North Equatorial Fold Belt). The plutonic rocks of Banefo-Mvoutsaha area resemble other Neoproterozoic high-K calc-alkaline syntectonic plutons in Western Cameroon. They also display strong similarities with high-K calc-alkaline plutons of the Pernambuco shear zone in NE Brazil and Eastern Nigeria.

Petrogenesis of peraluminous magmas from the Akum-Bamenda Massif, Pan-African Fold Belt, Cameroon

Peraluminous intrusives of the Akum-Bamenda Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a Pan-African sinistral strike-slip shear zone. The rock sequences consist of medium-grained leucogranites, fine-grained leucogranites, and orthogneisses of biotite granite composition; in aggregate, they cover a range from about 65 to 74 wt.% SiO2, defining a continuous chemical evolutionary trend and displaying characteristics of the high-K and medium calc-alkaline series. Leucogranites are strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, whereas orthogneisses are metaluminous and plot in the field of I-type granitoids. Major and trace element compositions and the Rb/Sr isotopes of the leucogranites indicate crustal derivation by remelting of a composite metapelite + metagreywacke protolith similar to the metasedimentary rocks of the central domain of the Cameroon Pan-African North-Equatorial fold belt.

Host rock geochemistry, texture and chemical composition of magnetite in iron ore in the Neoarchaean Nyong unit in southern Cameroon

ABSTRACT A combination of petrography, whole-rock geochemistry, geochronology and compositional variation in magnetite is employed in this study to elucidate the nature and origin of enigmatic magnetite mineralisation hosted within gneissic rocks in the Nyong Unit in southern Cameroon. The mineralisation occurs as magnetite-bearing calc–silicate gneisses. The host rock mineral assemblage comprises quartz–plagioclase–biotite–amphibole–chlorite–clinopyroxene–garnet that provides evidence of medium-grade metamorphism and retrograde alteration. Textural and chemical analyses of the magnetite grains show variable textural and trace element chemical characteristics attributed to metamorphic-hydrothermal overprint and weathering. Magnetite occurs as disseminations and stringers commonly intergrown with amphiboles. It is also observed to show vermiforms wrapped around quartz and clinopyroxenes within a biotite–chlorite- plagioclase groundmass. Massive-granoblastic magnetite is rare and mainly observed within vein-like domains. On backscattered secondary electron images the magnetite grains are anhedral, with minor spinel exsolution lamellae. Electron microprobe analysis on magnetite suggests both a hydrothermal skarn and banded iron formation (BIF) affinity. The lack of negative Ce anomalies excludes a Proterozoic BIF setting, but it is in agreement with Archaean BIF. Sensitive high resolution ion microprobe U-Pb isotope data on zircon in the magnetite gneiss define an identical Wetherill concordia and Tera–Wasserburg Neoarchaean age of 2699 ± 7 Ma (1σ; MSWD (mean square weighted deviation) = 1.3; n = 13), and Pan-African disturbance at about 500 ± 200 Ma. The Neoarchaean age is in accordance with the known onset of BIF deposition at the northern edge of the Congo Craton and therefore constrains the maximum age of formation of the Nyong magnetite gneisses.

Hydrothermal Alteration of Basaltic Rocks at Eruptive Vents on Mount Cameroon Volcano, West Africa

The study of changes in rocks due to interaction with hydrothermal fluids at active volcanoes provides insights into wall rock alteration associated with ore deposits formed in the geological past. Therefore, the nature of mineral alteration and chemical changes experienced by wall rocks can be investigated at eruptive sites on active volcanoes and the results used to better constrain ore-forming processes. In this study, we investigated the alteration at eruptive sites at Mount Cameroon volcano. These eruptive vents lie along NE-SW-trending fissures that define the Mount Cameroon rift. The vents are surrounded by cones composed largely of pyroclastic materials and to a lesser extent lava. Fumaroles (volcanic gases) rising through the vents during and after the 1999 eruption have resulted in the alteration of the pyroclastic robble along the fissures and the inner walls of the cones. Consequently, altered basaltic materials are covered with thin films of reddish, yellowish to whitish secondary minerals. These coatings result from an interaction between the surfaces of the basaltic glass with volcanically-derived acidic fluids. Petrographic investigations and XRD analysis of the basalts have identified primary mineral phases, such as olivine, pyroxene (mainly augite) and feldspars. Alteration products revealed include ubiquitous silica as well as gypsum, magnetite, feldspar, alunite and jarosite. Jarosite occurrence indicates that SO2 is the primary volcanically-derived acid source involved in coating formation. High contents of sulfur identified in the basalts indicate that melts at Mount Cameroon can be sulfur saturated as backed by previous melt inclusion data. Whole rock geochemical analysis shows a gain in silica in the altered samples and this ties with the mass balance calculations although minor gains of Al2O3, , MgO, MnO, CaO and K2O are shown by some samples.

Wallrock alteration categories and their geochemical signatures in gold-bearing Neoproterozoic granitoids, Batouri gold district, southeastern Cameroon

Hydrothermally altered granitoids in the Batouri district host gold mineralization. Gold and associated metals occur as disseminated, stockwork and veins. The granitoids range from quartz-alkali granitoids sensu stricto to diorite with various types of wallrock alterations including K-feldspar alteration, sericitization, silicification, and sulphidation/ferruginization. Most gold-bearing samples are extensively brecciated. Gold mineralization is accompanied with sericitization, silicification, and sulphidation/ferruginization alterations. Gold concentrations reach a high of 103.7 ppm. The granitic rocks are sub-alkaline. They show enrichment in the LREE, a negative Eu anomaly and a depletion in the HREE reflecting the breakdown and mobility of the initial plagioclase feldspar bearing HREE during fluid-rock interaction. On multielement variation diagrams, spikes at K, Ba, Pb, and Th are depicted resulting from selective enrichment during alteration. Mass gains/losses during alteration calculated using the immobile element method indicate, amongst others, gains in SiO2 (silicification), K2O (K-feldspar alteration), SO3 and Fe2O3 (sulphidation/ferruginization) with losses in Na2O linked to sericitization. The samples show Pd and Pt as high as 2 ppm. Gold mineralization is associated with wallrock alteration zones with elevated contents of As-Ba-Cu-Pb-Rb-Sr-Zn and Zr due to the neominerals developed during hydrothermal alteration. Au-Ag-Zn defines a potential pathfinder element cluster in the Batouri district.