Metallogenesis of the Bakoshi-Kundila gold deposit in northern West Nigerian Subshield: Insights from pyrite chemical and sulfur isotopic compositions and zircon U-Pb geochronology

Abstract

The Bakoshi-Kundila gold deposit, located in northern West Nigerian Subshield, in the southern trans-Saharan Orogenic Belt of West Africa, is a recent discovery and artisanally mined deposit. Like other Nigerian gold deposits, the mechanism and timing of gold precipitation remains unknown. In this study, we report the results of an integrated study of pyrite chemical and isotopic compositions and U-Pb on zircon geochronology to determine the gold precipitation mechanisms, fluid sources, and formation age of the gold mineralization. Two mineralization stages are recognized: (i) the Au-pyrite-quartz vein-type mineralization, including two generations of pyrite referred to as Py1a and Py1b, and (ii) wall rock disseminated-type Au-pyrite mineralization, comprising three pyrite types, Py2a, Py2b, and Py3. Py1 is Au-rich and As-poor pyrite and contain lattice-bound Au (Au+), nanoparticle Au (Au0), and native Au grains adsorbed on Py1b surface. Associated with Py1 are Ag-Te-Au bearing minerals. In contrast, Py2 and Py3 are As-rich and are less in Au; they contain only lattice-bound Au. Py1 grains have higher contents of Ag, Pb, Bi, Sb, Tl, and Cu which correlate with Au compared to those from Py2 and Py3. Py1 generations has lower δ34SV-CDT from –5.43 to –2.11 ‰ compared to the higher values of +3.63 to +5.47 ‰ obtained for the Py2 and Py3. These sulfur isotopes data are within the δ34S values of 0 ± 5 ‰ of the magmatic reservoir. U-Pb on zircon from Au-pyrite-quartz vein stage yielded older ages of 644.5 ± 2.2 Ma and 681.5 ± 1.8 Ma, compared to younger age of 606. 4 ± 1.8 Ma yielded by zircon grains from wall rock-type mineralization. Additionally, zircon grains from Au-bearing Tourmalinite yielded ages of 679 ± 1.3 Ma and 609.6 ± 7.1 Ma. We propose that Au mineralization at the Bakoshi-Kundila occurred during two periods in the Neoproterozoic: (i) an early ∼ 682 Ma (Cryogenian), possibly related to the emplacement of 683 Ma Yettiti granite and (ii) a second-stage Au disseminated in wall rock of granites and might have experienced 606–610 Ma Ediacaran metamorphism during the late tectonic events in West Nigerian Subshield.