Abstract
The Benue Trough of Nigeria is an intracratonic rift basin hosting several vein-type base metal deposits. The Akiri Cu ± (Ag) deposit represents a distinct sub-class of sediment-hosted Pb-Zn-Cu-Ba mineralization found throughout the Benue Trough. The deposit is hosted in bleached red beds of the Keana Formation and in shale-siltstones and carbonates of the Ezeaku Formation in the Middle Benue Trough, North-Central Nigeria. Mineralization at the Akiri deposit occurs as vein in-fillings in a series of NE-SW and E-W trending faults and fractures in the Early- to Late-Turonian Keana and Eze-Aku sedimentary rocks. To better constrain the sources of ore minerals and structural controls on the formation of this sediment-hosted Cu ± (Ag) mineralization, we report combined geologic, geochemical, mineralogical, and stable isotopic data for the Akiri Cu ± (Ag) deposit. Major ore-stage sulfides at Akiri are chalcopyrite and pyrite, which were accompanied by several types of alteration, including silicification, hematization, limited pyritization, and bleaching of mineralized sandstone bodies. In-situ trace element and sulfur isotopic data distinguishes early-stage pyrite (Py1) from late-stage pyrite (Py2). The late-stage Py2 co-exists with chalcopyrite suggesting coeval precipitation. Early-stage pyrite (Py1) contains lower Ag (avg. 0.04 ppm) but higher Au (avg. 3.03 ppm) than the late-stage pyrite (Py2) (avg. Ag = 2.78 ppm; Au = 0.424 ppm). The δ34S values of the early-stage sulfide (Py1) vary from 19.07‰ to 25.99‰ (avg. 22.20‰), suggesting that sulfur was largely derived from thermochemical reduction (TSR) of seawater sulfate. The δ34S values for co-existing Py2 and chalcopyrite range from 9.83‰ to 11.24‰ (avg. 10.32‰) and from 7.37‰ to 10.69‰ (avg. 8.96‰), respectively, suggesting a derivation of sulfur from TSR of seawater sulfate with contributions from magmatic sulfur. Based on structural features and ore textures, we propose that sulfide precipitation at Akiri was facilitated by sulfur-rich fluids circulating through pre-existing structures (fractures and faults) under fairly high (>200 °C) to moderate (<170 °C) temperature conditions. Geological, mineralogical, geochemical and isotopic data from this study support the classification of the Akiri Cu (+Ag) deposit as an epigenetic sandstone-hosted copper deposit.
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