Hydrothermal nickel sulfide hosted in Neoproterozoic carbonate and evaporitic rocks of the Menda Central prospect, Democratic Republic of Congo

Abstract

The Menda diapir, located in the southern Katanga Province of the Democratic Republic of Congo contains a number of megaclasts of sedimentary rocks from the Neoproterozoic Roan Group. Several of the megaclasts have undergone mineralization with copper, uranium, and/or nickel. Nickel mineralized megaclasts at the Menda Central prospect occur within hydrothermally altered Mines and Dipeta Subgroup carbonate and evaporitic rocks of the Roan Group. The host rocks underwent early magnesian alteration with the conversion of most of the carbonate to magnesite and growth of Mg-chlorite and/or talc in argillaceous rocks with synchronous or slightly later silicification. Magnesian alteration appears to have occurred prior to and following halokinetic formation of the Menda diapir. Nickel was deposited within the Menda diapir in megaclasts of Roan Group hematitic, evaporitic siltstone units, termed the R.A.T. Subgroup and within the matrix and clasts of the halokinetic breccia. Nickel in these rocks is bound within Mg-chlorite that is thought to have formed during the early magnesian alteration event. No mafic or ultramafic igneous rocks are recognized in the vicinity and the source of nickel within the Mg-chlorites is unknown. Nickel sulfide mineralization in the Mines and Dipeta Subgroup rocks was associated with a post-halokinetic calcic alteration event that resulted in local conversion of magnesite to dolomite and formation of dolomite veins. The sulfide assemblage is dominated by vaesite but includes less abundant siegenite, millerite, and chalcopyrite. Sulfides occur in veins, irregular altered zones, and as bedding parallel disseminations. Nickel sulfide mineralization differs from the more typical copper-(cobalt) mineralization of the Congolese Copperbelt in having occurred after formation of the diapiric breccia. The source of nickel in the Mines Subgroup sulfides is thought to be from local remobilization of nickel within the R.A.T. Subgroup and breccia. Similar nickel occurrences in the R.A.T. and Mines Subgroups are found along a 125 km trend from the Shinkolobwe uranium deposit in the east to the Kambunji prospect in the west, including the Menda diapir. The total amount of nickel within these rocks is immense, indicating significant amounts of the metal were transported by basinal fluids. The Menda Central prospect shares many similarities with the newly discovered Enterprise nickel deposit in northwest Zambia, the historic Shinkolobwe uranium deposit to the East and is an further example of a newly emerging sediment hosted hydrothermal nickel deposit type.