Strain fringes and strain shadows at Kamoa (DRC), implications for copper mineralisation

Abstract

The formation of strain shadows and strain fringes is described from diamictite and siltstone of the Grand Conglomérat unit (Nguba Group) at the Kamoa copper deposit located within the DRC at the western end of the Central African Copperbelt (CACB). Fringes reveal a straight orientation of fibres during a coaxial deformation; this supports the interpretation of a single phase of deformation during fringe growth within the deposit area. The non-deformed distal part of fringes and the straight orientation of fibres demonstrate the simple geometry of strain fringes. Quartz grains within strain fringes and strain shadows have generally straight or smoothly curved boundaries and lack undulose extinction. In strain fringes, muscovite and chlorite are generally observed on quartz grains where they define a foliation. These characteristics suggest a static recrystallisation that developed under relatively high temperature and wet conditions. A NW crustal shortening during the Kolwezian deformation event of the Lufilian orogeny is believed to be responsible for the coeval development of foliation, strain shadows and strain fringes. A syn-orogenic origin of the copper sulphides during the Lufilian orogeny is proposed.