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
Résumé
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 siliciclastic Neoproterozoic metasedimentary rocks (Mines Subgroup) and are interpreted as syn- to latediagenetic 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 kyanitemicaschists metamorphosed in the upper amphibolite facies. Mineral inclusions of graphite, micas and sulfides in kyanite porphyroblasts of the Cubearing 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 (Ky1) define an inherited S0/1 foliation developed during the prograde part of the P-T path (D1 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 S2 schistosity and by chalcopyrite and bornite delineating a shallow-dipping S3 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 (Ky2) transposed in the S3 schistosity (Ky2-3), and is marked by progressive cooling from ca 620°C down to 580°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 C3 shear bands associated with rutile crystals yielding temperatures from ca. 610 to 540°C. This final remobilization is younger than ca. 500 Ma (youngest U-Th- Pb age on syn-S3 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 (D1), syn-orogenic exhumation (D2), and post-orogenic exhumation during gravitational collapse (D3).
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