Lumwana polyphased Cu-mineralization in the high-grade internal zone of the Lufilian belt (Zambia) : petrographic, microstructural and geochronological constraints

Abstract : The Pan-African Lufilian orogenic belt hosts world-class syn- to epigenetic Cu-Co deposits and epigenetic U mineralizations within the evaporitic and siliciclastic Neoproterozoic metasedimentary rocks (Lower Roan group). The first Cu mineralizing event is syn- to late-diagenetic of these sequences, controlled by BSR processes and preserved in the External fold-and-thrust belt (DRC). The second one is related do TSR processes and occurs in or close to the Domes region (Zambia). It is synchronous to the Pan-African metamorphism affecting the Lower Roan sequences. Cu and U occurrences in the Domes region are hosted by kyanite-micaschist. The timing of the Cu mineralization and the nature of the protolith of the micaschist are debated. Petrography, deformation relative chronology and geochronology evidence : - a sedimentary kyanite-micaschist protolith for Lumwana as shown by inclusions of graphite and sulfides in first kyanite generation (Ky1) (peak of pressure). Sulfides define an inherited S0-n schistosity. The next step will be to associate this metasedimentary sequence with the Neoproteozoic Katanga metasedimentary cover which may evidence the link with the Cu-U mineralization in RDC, or with a metasedimentary part of the Paleoproterozoic basement. - a polyphased Cu mineralization through the Lufilian orogenic evolution marked by a second Cu-mineralization linked to the synorogenic exhumation. Evidences are the sulfides associated to the Sn+1 schistosity and second kyanite generation (Ky2) (peak of temperature). The two generations of sulfides display different geochemical signatures. This second mineralization occurs between ca. 580 and 630°C (geothermometry on rutile and garnet/biotite), and 550-510 Ma (Th-Pb ages on syn-metamorphic monazite). Moreover, the circulation of metamorphic Cu- and U-rich brines in Lolwa and Mitukuluku has been dated around ca. 530 Ma. It is consistent with the age on uraninite grain from the Lumwana mine dated at ca. 540 to 530 Ma by the U-Pb method.