No abstract

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 SiO 2 (silicification), K 2 O (K-feldspar alteration), SO 3 and Fe 2 O 3 (sulphidation/ferruginization) with losses in Na 2 O 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.

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-chloriteclinopyroxene-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.

The deposition of BIF is linked to a contemporaneous process in which clastic sediments from the continent were being deposited in the ocean along with the upwelling of hydrothermal fluids at the Mid-Ocean-Ridge (MOR) (Bau and Möller, 1993; Clout and Simonson, 2005 and Beukes and Gutzmer, 2008). Lavas intercalated with BIF provide a direct tool for constraining the timing of BIF formation in a paleobasin. The determination of BIF depositional age is vital to date key events in the evolution of the biosphere. Precise age constraints on the iron formation across the world both advance our understanding of BIF genesis and enable current stratigraphic and tectonic models to be evaluated. This paper dates for the first time BIF deposition within the Ntem Complex at the Northern edge of the Congo Craton by SHRIMP U-Pb method using zircons separated from associated volcanic material (Fig. 1). In addition, zircons separated from a surface BIF sample in the region are also dated to determine if other BIF depositional events occurred after the Archean phase of BIF formation. These results provide new insights for the correlation of Gondwana fragments in Africa (Congo craton) with those of South America (São Francisco craton) and also allow a better comprehension of the relation between volcanism and BIF deposition in pre-Gondwana break-up setting. Overview of geology The Ntem Complex represents the extension of the northern edge of the Congo Craton in Cameroon. Mesoarchean to Neoarchean charnockitic rocks usually of TTG composition (Takam et al., 2009 and Shang et al., 2010) and greenstones associated with metasedimentary formations make up the basement rock suit of the Ntem Complex (Shang, 2001; Pouclet et al., 2007 and Shang et al., 2007). Rocks of the TTG suite are strongly mylonitized and retrogressed along the thrust boundary with the Pan-African Yaounde Group (Takam et al., 2009). Late high-K granites and dolerite dykes generally younger than the TTG and greenstone belts occur as intrusions clearly distinct from the older TTG (Shang et al., 2007, 2010) (See Fig. 2). These have been interpreted as a heat source for remelting TTG and charnockites to generate high-K granites thus suggesting the coeval nature of doleritic magmatism and high-K New SHRIMP data for zircons separated from a metadacite interbedded with banded iron formation (BIF) in the Mbalam region of southern Cameroon provide a 2883±20Ma age for the commencement of BIF deposition in the Congo craton. Separated zircons from the associated metadacite are brown and euhedral to subheral in form with distinct oscillatory zoning typical of magmatic signatures. These zircons lack internal resorption features and serrated zone boundaries as should be expected of zircons inherited from the surrounding basement rocks or multiple magmatic plumbing events. However, zircons from a surface BIF sample collected ~ 60km west of the sequence from which the metavolcanics were sampled show a different age spectrum (Palaeoproterozoic, 1.0-2.0 Ga) suggesting a latter stage of B...

Banded Iron Formations (BIFs) were formed by contemporaneous events of active sediments supply and the venting of a hydrothermal fluid source at the Mid-Ocean-Ridge. BIFs within the Ntem Complex at the northern edge of the Congo Craton are intercalated with metasandstones and siltstones. SHRIMP U-Pb analysis on detrital zircons obtained from these metasediments gave variable ages from over 3000 Ma to 1000 Ma with the maximum age of deposition clustered around 1200 Ma and the peak of deposition at 1800 Ma. This age range suggested that the sub-basin was opened sometime in the Archean and remained active up till the Neoproterozoic. Zircons with Archean ages have a provenance linked to the charnockitic suite and the high-K granites within the Ntem Complex. The Paleoproterozoic ages are attributed to clastic inputs from the neigbouring Nyong Series west of the Ntem Complex. Also the peak of deposition in the Proterozoic could probably be explained by the globally recognized intense crust-forming processes in the Early Proterozoic time. The provenance of the younger Neoproterozoic ages is tied to various lithologies within the northern mobile belts of the Adamawa-Yade massifs and correlates with Neoproterozoic sedimentation ages in the Yaoundé, Lom and Poli series. The Neoproterozoic ages obtained are comparable to those obtained from metasediments of the Amazonian Craton and provide evidence of Pre-Gondwana assemblage of the Congo and the S?o Francisco Cratons.