RE-APPRAISAL OF THE GENESIS OF ISHIAGU LEAD-ZINC ORE DEPOSITS, SOUTHERN BENUE TROUGH, NIGERIA
Faculty: Physical Sciences
Okeke, A. I.
Ezeh, H. N.
The aim of this research work is to use stable isotopes to unravel the genesis of the Ishiagu Lead-Zinc ore deposits. The objectives were to map the study area showing the Lead-Zinc veins, collect samples of ores and rocks associated with the deposit and carry out stable isotope and geochemical studies in order to create a genetic model for the Ishiagu Lead-Zinc deposits. The methods utilized include: geological field mapping, mass spectroscopy for stable isotope geochemistry and X-Ray Fluorescence (XRF) for igneous and carbonate rock geochemistry. Results of geological field mapping identified NW-SE fractured carbonaceous shale, intruded by some basic- ultrabasic igneous rocks. One of the lodes studied cut across the basic intrusive. The SiO2, Fe2O3, and MgO contents of the igneous rock range (in wt%) from 37.7 to 52.1, 8.24 to 16.34 and 4.0 to 14. 93 respectively. The CaO and LOI contents of the carbonate rock range (in wt%) from 48.06 to 49.08 and 37.65 to 38.42 to respectively. δ34S of the sulphide ore ranges from -8.20/00 to -2.00/00. δ18O and δ13C of siderite range from 20.590/00 to 23.190/00 and -1.930/00 to -1.380/00 respectively. Igneous rock geochemistry established that the intrusive rocks of the area are mainly alkaline gabbros with few ultrabasic varieties. Comparing the δ34S, δ18O and δ13C values from the sulphides to the Natural Sulphur, Oxygen and Carbon Isotope Reservoirs respectively, revealed the similarities between the stable isotope compositions to that from the anchimetamorphosed to unmetamorphosed sedimentary rocks. The range of the sulphur, oxygen, and carbon isotope compositional ratios showed that the ore was deposited under anoxic, slightly acidic and isotopic partial equilibrium conditions. The temperature calculated from isotope geothermometry of the contemporaneous mineral pairs range from 152oC to 240oC with the fractionation factor between the mineral pairs inversely proportional to the temperature of formation. Temperature calculated using carbon isotope is 134 oC. Isotope ratios revealed that one of the lodes was formed in a more reduced condition than the other two. It is therefore proposed from these genetic models that the Ishiagu Lead-Zinc lodes were generated from the anchimetamorphosed to unmetamorphosed shales of the Asu River group of the area and the ores deposited under a reduced, isotopic equilibrium condition, with the temperature of formation ranging between 152oC to 240oC .
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