Testing models of pre-GOE environmental oxidation: A Paleoproterozoic marine signal in platform dolomites of the Tongwane Formation (South Africa)

Abstract Carbonate facies of the upper Tongwane Formation preserve a largely overlooked record of pre-GOE Paleoproterozoic seawater. This inventory has survived despite a complex paragenetic history involving both diagenetic and contact metamorphic processes. BIF mineral assemblages are dominated by a medium-grade grunerite overprint and the formation of prograde and retrograde riebeckite. The massive dolomite member, a platform top carbonate which caps the succession, is characterized by marine REY SN patterns lacking significant negative Ce SN anomalies ( 13 C carb values of between +1.83 and +2.95 ‰ VPDB that are slightly elevated above Paleoproterozoic ‘normal marine’ values. Systematically decreasing δ 13 C carb values measured in monotonously recrystallized, talc bearing, slope carbonates reflect interaction with devolatilization fluids during contact metamorphism caused by the intrusion of the Bushveld Igneous Complex; slope carbonates offer no insight into marine paleoredox conditions. Thus, despite contact metamorphism, Paleoproterozoic marine signals are retained in platform-top dolomites, however they do not support claims for widespread oxygen accumulation in shallow depositional environments of the Transvaal Basin on the eve of the GOE.