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The Ishiagu galena - sphalerite vein mineralization was investigated for its stable isotope composition to determine the characteristics and source of the mineralizing fluid. The δ34S of galena, sphalerite and chalcopyrite range from -5.00/00 to -8.00/00, -2.00/00 to -4.90/00 and -5.50/00 respectively. Siderite samples have δ18O(SMOW) and δ13C(PDB) values ranging from 20.590/00 to 23.190/00 and -1.930/00 to -1.380/00 respectively. The range of values for the stable isotope compositional ratios suggest that connate brine from the deeply buried , slightly metamorphosed sedimentary Albian Shales of the study area are the sources of sulfur, oxygen and carbon in the ore fluid. Comparing the stable isotope ratios of the study area to other deposits of the Benue Trough revealed that although the source of sulfur and carbon is same for all the deposits of the Benue Trough, the sulfur isotope in the deposits of the Southern Benue contain more of the lighter sulfur isotope (32S) than that of the Central and Northern Benue Trough deposits which is the main reason for absence of barite in the deposit of the study area unlike the Central and Northern Benue deposits. The oxygen isotope compositional ratio of the Southern Benue deposits depicts connate brine while that of the Central Benue deposit is meteoric.