PETROLOGIC CHARACTERIZATION OF METASEDIMENTS AT THE KANSANSHI MINE, ZAMBIA

The Kansanshi Cu-Au mine in the metamorphosed Domes region of the North-Western Province of Zambia is the largest copper mine in Africa. Recent deep drilling penetrated a nearly complete sequence of Katangan metasedimentary rocks providing an opportunity to characterize the sequence from the basal Mindola Clastics through the Nguba Group, including talc-kyanite “whiteschists,” and alteration related to Cu-Au mineralization. Rocks were investigated using traditional thin section petrography, FE-SEM, and TESCAN Integrated Mineral Analyzer (TIMA) automated mineralogy; where available, these results were compared to Corescan Hyperspectral imagery. Results show a sequence of lower-amphibolite facies metasediments with an abundance of Mg-, Na-, and Ca-rich phases suggestive of metasomatism by saline, carbonate-rich metamorphic fluids. The deepest Roan Group metasediments commonly contain albite, phlogopite, Mg-chlorite, and scapolite. Local talc-kyanite “whiteschist” assemblages occur in distinct zones. Kyanite is also present in narrow 1-2 cm thick quartz-kyanite alteration zones rimmed by biotite within garnet-biotite-quartz schists. At the top of the sequence, Nguba Group interlayered schists and phyllites display a quartz-carbonate-biotite-muscovite-(garnet) mineral assemblage with local development of scapolite and large randomly oriented hornblende porphyroblasts. Rutile porphyroblasts are common throughout the entire sequence. Alteration minerals associated with chalcopyrite bearing veins in siliciclastic metasedimentary rocks include Fe-carbonate, albite, white mica, rutile, apatite, green mica-V, and occasionally scapolite. In carbonate/evaporitic rocks, metasomatic alteration consists of recrystallized carbonate minerals, albite, and rutile. Overall, rocks at Kansanshi underwent widespread Mg-metasomatism and localized Na-Ca alteration, with the most pervasive alteration occurring in the deepest rocks.