Neoproterozoic Roan Group in the Zambian Copperbelt: sequence stratigraphy, alteration and mineralization, The

Abstract

The Neoproterozoic Roan Group of central Africa is 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). Previous studies concerning the stratigraphy, sedimentology and mineral composition of the Roan Group have been largely confined to ore and the immediate ore-bearing interval. Rarely have these studies attempted to integrate the full extent of the succession. Similarly, while numerous petrographic studies have been carried out on individual ore deposits, only rarely have these attempted to define a consistent sequence of events from sedimentation, through burial diagenesis, to mineralization and metamorphism. One of the main objectives of this study was to define a sequence stratigraphic model that could provide the basis for reliable stratigraphic correlation and prediction at both the local and regional scale. A significant outcome was the recognition that the Roan Group can be divided into four sedimentary sequences (RG1 to RG4) which allows for continuity to be established between the Zambian and Congolese Copperbelts; traditionally an area of significant uncertainty and debate. In contrast to previous correlations, the sequence stratigraphic model suggests that the ore-bearing interval of the Congolese Copperbelt, namely the Mines Subgroup, is broadly equivalent to the (barren) Bancroft Dolomite Formation of the Zambian Copperbelt, herein defined as a succession of platformal carbonate rocks confined to the base of the Upper Roan Subgroup. The model also predicts that the underlying Copperbelt Orebody Member of Zambia has only limited basinward extent and is not significantly developed in the DRC. Other significant implications concern the regional distribution of salt in the Roan Group, and its effect on ore deposit distribution and subsequent stratal fragmentation. Significant in terms of the fundamental controls on sedimentary copper ore systems is the remarkable similarity between the Roan Group sequences and those of the younger, less deformed and non-metamorphosed, but equally productive, Zechstein Group of central Europe. This reaffirms the contention that stratigraphic architecture and composition is the main constraint on the development of these ore systems, rather than a pre-existing Cu source or superimposed tectonic event. Key criteria are defined as (1) a basal, rift-related, oxidized redbed sequence, (2) a transgressive, shallow-marine reduced sequence, and (3) a basinwide evaporite-carbonate sequence. Each is regarded as an essential component for optimal leaching, transport, focusing and deposition of metals. The evaporitic setting of the Roan basin is also confirmed as the fundamental control on the development of widespread alteration in the Roan Group. The petrographic observations described in this study suggest a consistent sequence of events that challenges the traditional view that metamorphism, recrystallization and sulfide remobilization destroyed primary mineral textures. Although complex and protracted, these events generally progressed from early diagenetic K-feldspar and quartz, along with early dolomite and sulfate and local albite-silica, to later magnesian alteration in the form of phengitic muscovite, Mg-chlorite and phlogopite. This characteristic suite of minerals is taken to reflect the evaporitic nature of the Roan basin and its initial enrichment in Ca, Mg, Na, K and SO4 from Neoproterozoic seawater. The ore sulfides consistently occupy a late stage in the paragenetic sequence and are locally intergrown with phlogopite, chlorite and talc in interstitial sites between authigenic cements or remobilized into cleavage planes and veins. The timing of initial sulfide growth prior to metamorphism remains enigmatic but is linked to secondary porosity created by the dissolution of anhydrite and dolomite, probably due to acid generation from the maturation of organic material. An antithetic relationship between the ore sulfides and anhydrite suggests that accumulations of thermochemically-derived H2S gas provided a key constraint on ore deposit formation.