Resumo

EPHRAIM, Bassey Edem. Investigation of the geochemical signatures and conditions of formation of metacarbonate rocks occurring within the Mamfe embayment of south-eastern Nigeria. Earth Sci. Res. J. [online]. 2012, vol.16, n.2, pp.121-138. ISSN 1794-6190.

Hitherto unknown metacarbonate deposits constitute parts of the Cretaceous Mamfe embayment which straddles the border between south-eastern Nigeria and western Cameroon. The rock is characterised by a high concentration of LOI, CaO and MgO and depleted content of various insoluble components. Assuming all the CaO and MgO content of the rock were related to calcite and dolomite phases, these two minerals would account for around 22.5 wt% and 76.3 wt% on average, respectively. Among the trace elements investigated, only Ba, Cs, Rb, Sr, Nb, Pb, Zr, Cd, Cu, Ni, U, Y and Zn display concentrations beyond their detection limits. Chondrite normalised rare earth element patterns show that the rock under investigation show moderate to strong fractionation of light rare earth elements (LREE_N) over heavy rare earth elements (HREE_N) and distinct negative Eu anomaly. Multivariate statistical treatment and variation plots revealed several geochemical interrelationships, among which are the SiO₂ -Al₂O₃ -K₂O -TiO₂ Fe₂O₃ -Ba -Nb -Rb -Zr links which is associated with the rock’s silicate fraction. The carbonate fraction comprises CaO, MgO, Sr, Pb and Cu. The overall geochemical signatures support development of the metacarbonate deposit from sedimentary carbonate materials that was deposited in a saline, shallow-marine, low-energy seawater environment. The consistency of the rock’s chemical properties can be attributed to the relative stability experienced during the parent sedimentary materials’ deposition.

Palavras-chave : metacarbonate; geochemistry; depositional environment; Mamfe embayment.

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