OVERVIEW OF LEAD-ZINC IN NIGERIA

Lead-Zinc occurrences in Nigeria are associated with saline water intrusion in the sedimentary basins or fractured / shear zones in crystalline rocks. The mineralisation is often associated with minor to significant amounts of copper and silver, occurring in lodes filing the fractures within the sedimentary rocks in the Benue Trough as well as in crystalline basement rocks as shown in recent studies by Nigerian Geological Survey Agency. The Trough, which is believed to have originated as a failed arm of an aulacogen at the time of the opening of the South Atlantic oceans during the separation of the African plate and the South American plate, is partitioned into the Lower, Middle and Upper region with Lead-Zinc mineralization occurring in almost the entire 800km Length. The Lead-Zinc are localized along the Northeast-Southwest trending belt of slightly deformed volcanic and sedimentary Cretaceous sequences (Albian Asu River Group) which is about 500m thick, and they occur in the form of veins and veinlets associated with the host rock. The Lead-Zinc is often associated with other gangue minerals such as marcasite, pyrite, quartz, barites e.t.c.

LOWER BENUE TROUGH LEAD-ZINC MINERALIZATION

The general geology of lower Benue Trough in Abakaliki area is made up of thick sequences (500m) of slightly deformed cretaceous sedimentary rocks made up of essentially of albian shales, subordinate siltstones of the Asu River Group. There is also the presence of Volcanic and Pyroclastic materials forming elongated conical hills in the cores of the anticlinal Structures. The Abakaliki Lead-Zinc is believed to be of hydrothermal origin emplaced at a low temperature of about 140°C and it is made up of primarily four lodes namely Ishiagu, Enyigba, Ameri and Ameka in the lower Benue Trough located in Ebonyi State.

MIDDLE BENUE TROUGH LEAD-ZINC MINERALIZATION

The middle Benue Trough veins are located mainly in Akwana and Arufu. This mineralization is hosted in silicified limestone sequence and also belongs to the Asu River Group. Dips of these veins are generally steep to vertical with a width of between 0.5m-10m and length of approximately 100m along the strike length of the bodies. Contacts with wall rocks are sharp and ore grades vary from 3% to 5% combined Lead and Zinc. Vein constituents include galena, sphalerite, tetrahedrite and occasionally native silver. At Azara, the veins are rich in barites with occasional cubic galena. Limestone wall rock at Arufu and Akwana is highly silicified, which appears to be related to the mineralization processes as the intensity of the silicification decreases away from the vein.

UPPER BENUE TROUGH LEAD-ZINC MINERALIZATION

The Upper Benue Trough is made up of sedimentary sequences represented by medium to fine grained sandstone which is divided into Bima sandstone (Upper Albian age) and Yolde sandstone (Cenomanian age), shales of Yolde formation which underlies the alluvium which is associated with the sandstones as intercalations .These mineralizations are located in and around Isamiya, Diji and Gidan Dari in Bauchi State.

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 NaCl---CaCl2---H2O basinal brines. The mineralization may be ascribed to the "fluoritic subtype" of the Mississippi Valley Type (MVT) deposits.