Deciphering the Central African Copperbelt sedimentary succession: Chemostratigraphy of the Neoproterozoic metasedimentary sequences at Lubambe, Zambia

A +1.6 km borehole, southeast of the Lubambe Copper Mine, Zambia has intersected a seemingly continuous sedimentary sequence spanning the Neoproterozoic Kantanga Supergroup sedimentary succession within the Central African Copperbelt. This extensive sequence encompasses the Mindola Clastics Formation of the Lower Roan Subgroup, through the Upper Roan Subgroup, and the lower Nguba Group (including the Grand Conglomérat of the Mwale Formation and overlying Kakontwe Limestone). Lithofacies and mineralogical studies of this drillcore have revealed numerous sedimentary cycles, with clear evidence for evaporite formation during sedimentation and diagenetic processes throughout (e.g., bedded/vein anhydrite, chicken-wire textures, relic cements & pseudomorphs). Siliciclastic red-bed lithologies at the base transition into repeated cycles of shales/siltstones-dolostone-evaporite, succeeded by deeper water carbonate and siltstone intervals. Diamictites of the Mwale Formation (Grand Conglomérat unit) cap these cycles at the base of Nguba Group, interbedded with several thick carbonate beds (peloidal grainstones, dolostones +/- bioherm textures).

To explore the Upper Roan to Nguba Group carbonate successions, detailed mineral chemistry mapping (Tescan TIGER MIRA3 FEG-SEM, µXRF, pXRF) and C-O isotopic analysis were undertaken, with a focus on understanding global trends associated with the onset of diamictite formation. Mineral chemistry analysis revealed the influence of post-depositional processes on the sequence, including dolomite and silica alteration. Silicification selectively preserved peloids above the first diamictite occurrence while the original carbonate matrix dissolved, with later cement formation (hydrothermal?) associated with elevated Mn-Fe relative to earlier carbonates. However, most of the carbonate units from the upper successions are composed of dolomite grains/cement, with original textures often preserved (e.g., stromatolites, disrupted reefs).

Isotope sampling targeted the ‘least-altered’ carbonates from the upper Roan to lower Nguba Group, to minimize post-depositional alternation effects. The initial analysis revealed variable δ¹³C isotopic values, with ~negative trending excursions below the main diamictite body. The lack of notable covariance with corresponding O isotope values suggests representation of the original seawater composition, not impacted by local diagenetic/hydrothermal alterations. Further, the largest negative excursion is associated with a lithofacies change from non-stromatolite- to stromatolite-bearing dolomitic siltstones, suggesting local lithofacies changes may influence the isotope profile. Specifically, δ¹³C isotopic values drop from ~+4.2 ‰ to +0.37 ‰, before returning to ~+4.7 ‰ below the diamictite contact. While these units do not record the more extreme negative excursion (<-5 ‰) typically associated with the Sturtian glaciation recorded elsewhere, the overall curve may represent the Islay anomaly recorded globally in Cryogenic sections beneath the Sturtian diamictite. This negative excursion, in line with global C isotope trends (<-5 ‰) has also been identified within Copperbelt on both the Zambian and Democratic Republic of Congo sides (ongoing work), but with a more significant decrease in C isotope values, in line with global trends. The lack of exact duplication may be the result of local sedimentological factors, as shown by the association in lithofacies variation, an incomplete sample record, or perhaps the influence of the Sturtian is not fully recorded in the stratigraphy underlying these diamictites.