Discovery and definition of sedimentary exhalative mineralization enhances lead-zinc-silver resources in the Benue Trough, Nigeria
Introduction
Occurrences of lead, zinc, silver and barites have been recorded in parts of the Benue Trough [Abakaliki, Ishiagu, Arufu/Akwana areas]. Useful contributions include McConnel 1957, Pargeter 1959, Olade 1976. Mining of lead, zinc and associated silver had been carried out in the Abakaliki area by Europeans from early 1940s to 1965: Indigenous small-scale mining companies have extracted the same commodities in the Abakaliki and Ishiagu areas from 1950’s to date.
Mineralization is in fracture/veins (i.e. fracture and vein fillings by hydrothermal fluids). No records of reserves are available, but estimates indicate substantial quantities of the metallic sulphides. Sedimentary exhalative lead- zinc- silver and barites mineralization was noted for the first time by Nwabufo-Ene (1980). Further definition of this type of mineralization were presented in Nwabufo-Ene [1995, 2006].
The discovery and definition of the mineralization of this ore genetic type has greatly enhanced the resources of these minerals.
Lead, zinc, silver minerals industry in retrospect
a) Geological features
– Mineralization: fracture/vein filling
– Host rock
i) Abakaliki Ishiagu areas: shales
ii) Arufu/Akwana area: silcified limestone
– Gangue minerals
i) Abakaliki Ishiagu areas: siderite and barites
ii) Arufu/Akwana area: fluorite
An important feature in the Arufu area is that the limestone had a neutralizing effect on the mineralizing fluid. The galena occurs in a continuous lode approximately 939m long, striking EW. Faulting is proved by the linear dyke like nature of outcrops of limestone as well as the slacken siding and brecciation.
b) Mining
Open pit operations; mechanization has been minimal.
c) Marketing strategies
Official records are not available. There are no indications of production, export or revenue derived.
Sedimentary exhalative mineralization
Sedimentary exhalative lead zinc-silver-barites occurrences were first noted by Nwabufo-Ene (1980). Further characterization and definition are given in Nwabufo-Ene (1993, 2005, 2006 and in Press]
Important features
a) The environment of deposition of the host sediments was the shallow inland sea during the Albian while the tectonic setting is a rift structure.
b) The stratabound Pb-Zn-Ag deposits are syngenetic but post date pyrite formation
c) Isotope studies indicate that separate sulphur sources contributed to the formation of pyrite (biogenic) and reduction of sea-water sulphates (sphalerite and galena)
d) Mineral zoning is a characteristic feature of the deposits. The proportions of lead and zinc may change laterally and vertically within a deposit. The zoning may be related to faults from which the ore fluids have issued and the composition of the fluid.
e) A number of silver minerals occur and these are mostly associated with the galena. The most common is freigerbite.
There are also:
Pyragyrite
Stephanite
Some metallurgical implications are noted.
i) In some localities the galena encloses freigerbite. For very fine varieties, extraction of the silver may not be complicated.
ii) Elsewhere, the lead-zinc ores are associated with a number of silver bearing ores. This may present complex metallurgical problems.
Ore genetic episodes
Nwabufo-Ene (1980, 1993) classified the ore genetic episodes as follows
Episode 1
Sedimentary exhalative lead-zinc-silver deposits.
Episode 2
Fracture/vein fillings. Some of the sedimentary exhalative deposits may have been remobilized and deposited in the fracture and veins.
Exploration guide
The nature of the host sediments and ore deposits indicate substantial paleogeographic, structural and stratigraphic studies. Geochemical studies (stream water/sediment and soil surveys) will be useful.
Geophysical techniques may provide useful data. Induced polarization (IP) and gravimetric surveys may be the most appropriate.
Economics of the sedimentary exhalative deposits
The disseminated nature of the deposits make them easier to extract than the fracture/vein fillings. They may also be easier to refine. The mineralogy of these deposits seem more diverse than the fracture/vein fillings:
Galena
Sphalerite
More variety of silver minerals
Barites
Fluorite [only in the Arufu/Akwana area]
Minor chalcopyrite and bornite
Conclusions
The sedimentary exhalative lead-zinc-silver mineralization appears substantial and enhances considerably the resources of the metallic sulphides in the Benue Trough, Nigeria[i.e. combination of deposits of fracture/vein filling ore type and deposits related to the sedimentary exhalative mineralization]
References
Farrington J. L. 1952 Preliminary description of the Nigerian lead zinc field-Econ Geol 47 pp583-608
McConnell R. B. 1957 Report on the geology of the silver, lead deposits of Arufu and Akwana, Benue Province.Geol.Surv. of Nigeria. Report No 853 11p
Nwabufo-Ene 1980 Albian Foraminifera from south-eastern Nigeria: Unpub PhD Thesis University of Wales,U.K802p
Nwabufo-Ene, K. E 1993 Non oil mineral exploration in Nigeria Journ. Sci. and Tech Vol.1 No. 1 pp 15-30
Nwabufo-Ene K. E. 2006 Industrial Development based on Minerals from South-eastern Nigeria Special Paper at Nigerian Mining and Geosciences Society Economic Workshop Abakaliki, Nigeria 29p
Olade M.A 1976 The genesis of lead-zinc deposits in Nigeria’s Benue Rift (Aulacogin) Are interpretation Journ. Min and Geol. Vol.13 No 2 pp20-27
Pargeter R. C. 1959 The geology of the lead deposits at Arufu and Akwana, Benue Province Geological Survey of Nigeria Rept. No
