Establishing the Conditions of Formation of Ishiagu Galena-Sphalerite Mineralisation, Southeastern Nigeria Using Stable Isotope

Abstract

The study area is located at southwestern extreme of the Benue Trough of Nigeria. The area is bounded by latitude N50 57’0’’ - N50 54’0’’ and longitude E70 32’ 0’’- N50 54’0’’. The Ishiagu galena-sphalerite deposit occurs in the Cretaceous (Albian) carbonaceous shale intruded by basic to ultrabasic igneous rocks. Tectonic events of the Santonian Era led to the folding, fracturing and faulting of the rocks of the Ishiagu area. The study tends to establish the conditions of formation of the hydrothermal deposits of Ishiagu.  Establishing the conditions of formation of a deposit is a key to understanding the genesis of the deposit for its proper characterisation. Apart from galena and sphalerite, the ore is composed of chalcopyrite with siderite, quartz and calcite as gangue minerals. Stable isotope of three ore bodies were studied in the area. 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 (PDB) and δ13C(PDB) values ranging from -10.01 0/00 to -9.50/00 and -1.930/00 to -1.380/00 respectively. The negative values of the δ34S and δ13C indicates the abundance of reduced sulfur (H2S) and carbon (CH4) respectively in the ore fluid and invariably the reduced nature of the hydrothermal system during the formation of the deposit. The range of negative δ34S and δ13C values of the deposit hosted by the intrusive which is -4.50/00 to -8.00/00 and -1.930/00 to -1.650/00 respectively and that of the shale host which is : -2.00/00 to -6.10/00 and -1.470/00 to -1.380/00 respectively reveal the more reduced condition of formation of the deposit hosted by the intrusive, the  close nature  of the geochemical environment  and also  the effect of the host rocks on the sulfur isotope compositional ratios.