Répertoire du personnel
Thierry De Putter
Sciences de la Terre
Géodynamique et ressources minérales
Géodynamique et ressources minérales
Détails
Cailteux, J., Muchez, P., De Cuyper, J., Dewaele, S. & De Putter, T. 2019. ‘Origin of the megabreccias in the Katanga Copperbelt (D.R. Congo)’. Journal of African Earth Sciences 140: 76-93. DOI: 10.1016/j.jafrearsci.2017.12.029. (PR).
Article dans une revue scientifique / Article dans un périodique
The megabreccias in the Katanga part of the Neoproterozoic Central African Copperbelt contain up to
several km-long blocks and fragments of the Mines Subgroup which host most of the stratiform Cu-Co
deposits. New observations, particularly on cores from boreholes drilled at Luiswishi indicate three types
of fracturing: 1) brittle post-folding in the Mines Subgroup; 2) hydraulic; and 3) ductile in soft incompetent
siltstones of the R.A.T. and Dipeta subgroups. These fracturing phases dislocated the Roan succession
into blocks and fragments and, in particular, clearly showed that there is an evolution from an in
situ hydraulic fracturing, to a heterometric brecciation implying some movement and abrasion of the
fragments. The process points to significant compression, and was accompanied by fluid expulsion and
precipitation of dolomite after decompression. Fluid inclusion microthermometry in dolomite grains
shows that the fluids were of high salinity and high temperature, suggesting dissolution of evaporites
most likely contained in the Roan sedimentary pile. These saline fluids allowed the fluidization of the
breccias, facilitating the displacement of the nappes, pinching out (extrusion-like) megabreccias along
thrust-faults, and resulting in intrusion of breccias between the blocks or into large fractures. Breccias
between the blocks are clearly identified as friction breccias. They contain a fine material, as part of the
matrix, resulting from abrasion of the fragments during transportation. Abrasion and attrition explain
the rounding of the fragments. A late cementation phase from less saline and lower temperature fluids
suggests the addition of meteoric water in the system, and the mixing with the ambient fluids. The
minimum burial depth of the meteoric water incursion is estimated at 2.8 km. Such under-saturated
fluids may have contributed to the dissolution of residual evaporites and of the evaporitic material
from the Kiubo rocks at the base of the nappes, and led to further brecciation, possibly explaining the
multi-phase features of the breccia. The megabreccias occur at the base of the thrusts sheets and are
marked by thrust-fault zones. Results of the study support a process of formation of the megabreccias
related to a fold-and-trust event, and invalidate a syn-orogenic sedimentary origin as an olistostrome
formed by subaqueous conglomeratic debris flows and clastic syn-orogenic sediments. They also
contradict a pure salt tectonic hypothesis that propose the extrusions and enlargements of allochthonous
evaporites-gigabreccia before the Lufilian deformation. However, the model is compatible with a “fluid
behaviour” of pressured saline fluids trapped in folds and/or thrust sheets, and resulting from evaporites
dissolution at variable depth.