Synmetamorphic Cu remobilization during the Pan-African orogeny: Microstructural, petrological and geochronological data on the kyanite-micaschists hosting the Cu(-U) Lumwana deposit in the Western Zambian Copperbelt of the Lufilian belt

The Pan-African Lufilian orogenic belt hosts world-class Cu deposits. In the Congolese Copperbelt (DRC), Cu(-Co) deposits, are mostly hosted within evaporitic and silicidastic Neoproterozoic metasedimentary rocks (Mines Subgroup) and are interpreted as syn- to late-diagenetic deposits. In this paper, we present new data on Cu(-U) deposit hosted in metamorphic rocks of the internal zone of the Lufilian belt known as the Western Zambian Copperbelt in which a primary Cu mineralization is overprinted by a second syn-metamorphic Cu mineralizing event. This mineralizing event is synchronous with the Pan-African metamorphism affecting both the pre-Katanga basement and the Katanga metasedimentary sequence. Cu(-U) occurrences in the Western Zambian Copperbelt are hosted by kyanite-micaschists metamorphosed in the upper amphibolite fades. Mineral inclusions of graphite, Micas and sulfides in kyanite porphyroblasts of the Cu-bearing kyanite-micaschists in the Lumwana Cu deposit point to a sedimentary protolith with relics of an inherited Cu stock. Based on petrologic, microstructural and geochronological evidence, we propose that this initial Cu-stock was remobilized during the Pan-African orogeny. Graphite, micas and sulfides preserved in a first generation of kyanite poikiloblasts (Ky(1)) define an inherited S-0/1 foliation developed during the prograde part of the P-T path (D-1 deformation-metamorphic stage) reaching HP-MT metamorphic conditions. Remobilization during the retrograde part of the P-T path is evidenced by chalcopyrite-pyrrhotite and chalcopyrite-bornite delineating a steep-dipping S-2 schistosity and by chalcopyrite and bornite delineating a shallow dipping S-3 schistosity associated with top to the south kinematic criteria. This retrograde path is coeval with ductile deformation in the kyanite field as evidenced by a second generation of synkinematic kyanite porphyroblasts (Ky(2)) transposed in the 53 schistosity (Ky(2-3)), and is marked by progressive cooling from ca. 620 degrees C down to 580 degrees C (rutile geothermometry). Syn-S2-3 metamorphic monazite grains yield U-Th-Pb ages ranging from ca. 540 to 500 Ma. Final retrogression and remobilization of Cu is marked by recrystallization of the sulfides in top to the north C-3 shear bands associated with rutile crystals yielding temperatures from ca. 610 to 540 degrees C. This final remobilization is younger than ca. 500 Ma (youngest U-Th-Pb age on syn-S-3 recrystallized monazite). These data are consistent with successive Cu remobilization for more than 40 Ma during Pan-African reworking of sediment-hosted deposits either from the basement of the Katanga sedimentary sequence or from the Katanga sequence itself marked by burial (D-1), syn-orogenic exhumation (D-2), and post-orogenic exhumation during gravitational collapse (D-3). (C) 2015 Elsevier B.V. All rights reserved.