The Mindouli (Republic of the Congo) mining district revisited (1). geological context and preliminary results on the formation of complex, multiphase, Cu-Pb-Zn deposits

The Niari Syncline (Republic of the Congo) hosts complex Cu-Pb-Zn deposits, associated with ENE-WSW-trending sets of regional faults (Maurin et al. 1990). Abandoned mines at Mindouli, Ntola, M’Passa Mines, Mfouati, Boko-Songho have been investigated, with a double objective: (1) get an improved understanding of the formation of these deposits; (2) get an improved understanding of the use of copper and lead exploited from these mines in pre-colonial metallurgy (Nikis and De Putter, this volume).The studied Cu-Pb-Zn deposits are hosted along or near the N60°E faulted contact between the Neoproterozoic stromatolitic limestone of the “Schisto-Calcaire” and the younger Neoproterozoic pink sandstone of the Mpioka Group, both in the West-Congolian Supergroup. At some post-depositional and pre-mineralization stage, the stromatolitic limestone has undergone extensive karstic dissolution, and later dolomitization/silicification.A hydrothermal origin has been proposed for these deposits (Buffet et al. 1987), which is substantiated by the presence of massive Fe-Cu-Pb-Zn sulphides (chalcopyrite, sphalerite, galena) and sulphide-cemented breccia at depth in most deposits. Further work is ongoing to characterize/date the post-Mpioka hydrothermal phase of these (MVT-like?) deposits, possibly allowing connections with major geodynamic events in the study area. In later phases, the deposits experienced supergene overprints that varied according to their specific settings. At Mindouli, the porous and permeable faulted contact between the karstic limestone and the Mpioka sandstone favoured the formation of a rich paragenesis of Cu-Pb-Zn secondary minerals, dominated by silicates (dioptase, plancheite, hemimorphite) and carbonates (malachite, azurite) in the upper part of the deposits. At Mfouati, the karstic cavities in the limestone are filled with a highly porous iron-rich alterite that hosts a paragenesis dominated by Pb-Zn minerals (hemimorphite, smithsonite, wulfenite), with accessory Cu minerals (dioptase, chrysocolla, malachite). At Boko-Songho, the deposits are capped by a thick iron-rich alterite and the paragenesis is dominated by void-filling malachite (crusts, botryoids: De Putter et al. 2010). Supergene overprints on the studied deposits need to be dated and further connected with major regional paleosurfaces (De Putter et al. 2015).Fluid circulation patterns through these deposits – both hydrothermal and meteoric – have resulted in a vertical elemental distribution: Pb deeper than Cu deeper than Zn. As in the Katanga Copperbelt (D.R. Congo), a “late” (Cenozoic?) meteoric phase has remobilized copper and allowed the formation of void-filling malachite in the subsurface. This mineral is present/abundant in most deposits and has been exploited to fuel pre-colonial metallurgy, at the vicinity of the mines (Nikis and De Putter 2016).