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Lead-zinc fluorite veins occur at Arufu and Akwana within the Lower Cretaceous (Albian) carbonate sequence of the middle Benue Trough. The veins occur as fracture controlled bodies within a sequence of carbonate rock types ranging from recrystallized micrite, peloidal wackestone, oncoidal peakstone, oolitic grainstone and carbonate breccia. The principal ore minerals in the veins are galena, sphalerite, tetrahedrite, native silver and fluorite. Minor minerals of the veins include marcasite and chalcopyrite. These minerals are commonly covered by malachite, limonite, smithsonite and pyromorphite in the supergene zone. Apart from the mineralized veins, ore minerals also occur as stratiform stringers and open-space infillings of the interstitial, fracture and channel porosities of the highly silicified carbonate host. Filled vugs and porespaces are also in close proximity to the mineralized fractures within the carbonate host. This relationship suggests that the fracture controlled veins acted as feeders to the open space sulphide infillings. The carbonate wall rocks are extremely silicified in areas adjacent to veins to varying degrees. Microthermometry of fluid inclusions in fluorite yield homogenization temperatures from 106 to 148°C with a mean of 132°C in the two deposits. Pressure corrections should be minimal for these temperatures. Salinity determinations from the final ice melting temperatures (Tm-ice) of the inclusions yield values between 15 to ca 23 equivalent weight percetn NaCl for the ore fluid. Eutectic temperature (Te-ice) data suggest very high values of CaCl2 in the ore fluid. The low temperatures of the ore forming fluid compares with the low temperatures reported from the sphalerite and quartz inclusions in the Ishiagu and Abakaliki Pb + Zn (−Ag, −F) in the southern Benue Trough. The presence of silver and fluorite in the Arufu and Akwana deposits contrast mineralogically with those deposits in the southern Benue Trough where mineralization is dominantly sphalerite and galena. Our results suggest that the lead-zinc-fluorite mineralization resulted from heated NaClCaCl2H2O basinal brines. The mineralization may be ascribed to the “fluoritic subtype” of the Mississippi Valley Type (MVT) deposits. |
