Dating of U-rich heterogenite: New insights into U deposit genesis and U cycling in the Katanga Copperbelt

摘要

The Katanga Copperbelt region of the Democratic Republic of Congo hosts world-class cobalt deposits accounting for ～50% of the world reserves. Heterogenite (CoOOH) is the most abundant Co-bearing secondary mineral in the region. Its occurrence is the result of oxidation of Cu–Co-sulfides and associated Co reprecipitation in the uppermost part of the deposits, during the Pliocene. In addition to sediment-hosted copper and cobalt ore deposits, the Katanga Copperbelt also hosts numerous uraniferous mineral occurrences and deposits, which can be associated with heterogenite. Within these deposits, heterogenite can have high concentrations of U (up to 3.5%) and Pb (up to ～4%). In situ SIMS U–Pb ages were obtained for heterogenite samples from the U deposits of Shinkolobwe, Kalongwe and Kambove. These analyses yield distinct Neoproterozoic ages, at ～876 Ma, ～823 Ma and in the ～720 to ～670 Ma age range. As the geological context prevailing at those times was not favorable for heterogenite formation, these ages most probably record geological events that are not the formation of the mineral itself. For instance, the heterogenites could have inherited the U–Pb signature of a U-rich mineral, most likely uraninite, formed and/or yet reworked at ～876 Ma, ～823 Ma and in the ～720 to ～670 Ma time interval and spatially associated with primary Co-sulfides. In this hypothesis, the ages obtained in this paper are significant for understanding the cycling and re-deposition of U at given moments in the regional geological history. In such context, the ～876 Ma and the ～823 Ma age are consistent with syn-early diagenetic concentration of uranium in sediments of the Katanga basin. The ～720 to ～670 Ma ages are interpreted as a phase of U remobilization related to hydrothermal fluid circulation induced by late Nguba proto-oceanic rifting or by early stages of Congo-Kalahari craton convergence, and associated hydrothermal circulation.