Controls of sulphide mineralisation at the kamoa copper deposit, with an emphasis on structural controls

Abstract

The Kamoa copper deposit, situated approximately 25 km west of the Kolwezi Cu-Co district of the Democratic Republic of Congo, is one of the largest high-grade undeveloped copper deposits in the world. Kamoa is a new discovery which occurs on the western edge of the Congolese portion of the Central African Copperbelt. Two main rock units are present at Kamoa, sandstone and conglomerate of the Mwashya Subgroup of the Roan Group, and overlying diamictite and interbedded siltstone-sandstone of the Grand Conglomérat unit of the Nguba Group. The deposit occurs along a stratigraphically controlled redox boundary at the base of the Grand Conglomérat. The Kamoa deposit occurs broadly within a NNE-trending structural high bound to the west by the West Scarp fault and to the east by the Kansoko trend. Within this area the host rocks dip gently away from two localized domes, which expose rocks of the Mwashya Subgroup. Drill core data indicate that the Kamoa deposit includes areas of higher and lower grade, some of which appear at the deposit scale to be related to known or interpreted faults. The aim of this study was to determine the role of structure(s) in the genesis and distribution of hypogene copper sulphides, and to identify the nature of those structures and their timing relative to mineralisation. Detailed structural logging in drill core has focused on analysis of foliations, folds, mesoscopic faults, strain shadows and strain fringes. True thickness isopach maps were used to investigate spatial relationships of mineralisation with facies/thickness changes and faults throughout the deposit. In the southern part of the Kamoa deposit, a linear NNW zone of abrupt change in stratal thickness named the “Mupaka Fault” is oblique to the localised domes and correlates well with higher copper grades. This fault may represent an important transport path for the hydrothermal fluids in the Kamoa deposit. Thickness variations, facies change, steep bedding and rotated mesoscopic faults demonstrate that the Mupaka Fault is a syn-sedimentary growth fault that occurs during the deposition of the Nguba Group rocks. The close relationship between copper mineralisation and structures demonstrates that structures played a significant role in the formation of ore at Kamoa. Zones of thickness variations of stratigraphic units may represent potential prospective areas for future exploration work. The development of S1 foliation, strain shadows and strain fringes is coeval and related to a NW shortening direction. The Kolwezian deformation event is believed to be responsible for the development of these structures. Strain fringes reveal a straight orientation of fibres during coaxial deformation; this supports the interpretation of a single phase of deformation within the Kamoa deposit. The non-deformed distal part of the fringe, the straight orientation of fibres, the lack of undulose extinction and recrystallisation features demonstrate the simple geometry of strain shadows and strain fringes. These characteristics suggest a new growth of minerals under low temperature and high pressure fluid. The analysis of minerals within fringes suggests little potential for remobilisation of minerals in strain shadows and strain fringes because none of the mechanisms of recrystallisation (bulging, subgrain rotation and grain boundary migration) have been observed. A syn- or post-orogenic origin of copper sulphides, probably related to the Lufilian orogeny is proposed.