The Paleoproterozoic porphyry copper–molybdenum deposit of Kourki (Liptako Province, western Niger)

Abstract

Abstract The Kourki porphyry Cu–Mo deposit is located in the southern part of the Gorouol greenstone belt in Western Niger. The potential Cu–Mo mineralization is hosted by granitoids (granodiorite and diorite) and veins, in Paleoproterozoic plutonic rocks, which are the main porphyry Cu–Mo system. The scientific objective of this study was to determine the characteristics of Kourki Cu–Mo mineralization and to compare them with other porphyry systems in the world and particularly with those of the West African Craton. The methods implemented consist of a field study and a polarizing microscope analysis in transmitted and reflected light on the rocks bearing Cu–Mo mineralization. The use of reference diagrams made it possible, on the one hand, to confirm the origin of the Cu–Mo mineralization and, on the other hand, to specify conditions of the formation of this mineralization. The geology of the study area shows a granodiorite–tonalite pluton in close contact with dioritic intrusions to the north, clastic sediments to the east, and volcano-sedimentary rocks to the SE. The pluton is crosscut by an intrusion of molybdenum porphyry dyke with irregular shape that can be observed in outcrop. The geochemical study shows that the granodiorite–tonalite pluton can be related to a calc-alkaline plutonic series which characterizes an arc type environment. Large areas of hydrothermal quartz breccia have been identified within the pluton. They were set up by hydraulic fracturing along the zones of weakness. Most of the Cu–Mo mineralization in the Kourki deposit occurs as hydrothermal breccia, filled with veins, and was disseminated in the porphyry diorite and quartz–diorite stockworks. The mineralogical assemblage is marked by a predominance of pyrite, molybdenite, chalcopyrite and quartz with some bornite and chalcocite. The alterations include potassic alteration, propylitic alteration, sericitization and epidotization. All of these data show that the Kourki porphyry represents a characteristic system of an environmental arc, quite comparable with that of the Andean Cordillera.