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.