Paper No. 247-3
Presentation Time: 1:40 PM
A SEQUENCE STRATIGRAPHIC VIEW OF THE ROAN GROUP – HOST TO THE WORLD’S LARGEST SEDIMENTARY ROCK-HOSTED STRATIFORM COPPER DISTRICT, CENTRAL AFRICA
The Neoproterozoic Roan and Nguba groups of central Africa are host to the world’s largest and highest-grade sedimentary-rock hosted Cu (-Co) deposits in the Copperbelts of Zambia and the Democratic Republic of Congo (DRC). Deposits are hosted primarily in siliciclastic and carbonate rocks of the Roan Group and the basal portion of the overlying Nguba Group; the deposits are fundamentally controlled by oxidation-reduction boundaries. Available temporal constraints on Roan Group sedimentation include a U–Pb zircon date of 883 ±10 Ma for underlying granitic basement, and ~760 Ma volcanic rocks within the upper portion of the Roan Group. Thus, the Roan Group, with a maximum total preserved thickness of approximately 2-3 km appears to have been deposited in a span of ~100 m.y. Recent work has demonstrated that the succession contained significant amounts of evaporites that underwent halokinesis and are currently missing from the preserved section. Even accounting for potentially several kilometers of missing evaporites, a combined sediment package of ~ 5 km deposited over 100 m.y. suggests geologically unreasonably slow depositional rates or previously unrecognized significant disconformities or unconformities within the section. Traditionally the Roan Group has been subdivided utilizing a lithostratigraphic rather than a sequence stratigraphic approach. In sequence stratigraphic terms, the Roan Group can be subdivided into four unconformity-bounded sedimentary sequences (RG1 to RG4) with major unconformities most likely at the top of the RG1 sequence and at the top of the RG3 sequence. The sequence stratigraphic model allows for improved correlations between the autochthonous and halokinetically disrupted parts of the Roan Group, and helps to explain the stratigraphic distribution of ore deposits within the basin. The “Ore Shale,” the dominant host to deposits of the Zambian Copperbelt, occurs near the base of the RG2 sequence, while the Mines Subgroup, the host to most deposits in the Congolese Copperbelt, occurs near the top of the RG3 sequence. Redox boundaries associated with the regional unconformity above the RG4 sequence, near the base of the glaciogenic Grand Conglomerate Formation of the Nguba Group, also control major ore deposits, especially in basin margin positions.