International Geologiical Congress - Oslo 2008


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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts


Re-Os ages of black shale in the glaciogenic Vazante Group: Evidence for Mesoproterozoic ice ages in Brazil?


Alan Kaufman, University of Maryland (United States)
Nick Geboy, University of Maryland (United States)
Richard Walker, University of Maryland (United States)
Kristen Miller, University of Maryland (United States)
Natalie Sievers, University of Maryland (United States)
Simon Poulton, Newcastle University (United Kingdom)
Tolentino de Oliviera, Votorantim Metais (Brazil)
Aroldo Misi, Federal University of Bahia (Brazil)


The Vazante Group in Brazil preserves lithologic evidence for at least two discrete ice ages. The basal Vazante diamictite contains faceted and striated basement cobbles indicative of glacial transport, while the upper glacial horizon includes carbonate breccia and black shale with ice rafted debris. The carbonate breccia hosts world-class Pb-Zn deposits and is recognized in cores from over 150 km of the basin margin. While these glacial deposits were previously associated with Neoproterozoic events, radiometric age constraints on the sedimentary succession were lacking. To determine the age of the Vazante Group and its associated glacial deposits, we used Re-Os techniques on three separate organic-rich shale horizons in the Serra do Garrote, Serra do Poço Verde, and Lapa formations. Insofar as the Serra do Poço Verde shale contain dropstones and is sandwiched between carbonate diamictite, the unit is interpreted as syn-glacial in origin. On the other hand, the basal Lapa shale lies stratigraphically above carbonate diamictite and sedimentary iron-formation, and overlying carbonates preserve the characteristic negative δ13C anomaly commonly associated with Neoproterozoic post-glacial transgression.
Time series elemental and isotopic analyses of the Vazante cores reveal enrichments in organic matter up to 4 wt. % in all three shale levels. Extractable biomarkers from the Serra do Poço Verde shale are interpreted to reflect a complex and productive microbial ecosystem, including both phototrophic bacteria and eukaryotes, living in a stratified ocean with thin or absent sea ice, oxic surface waters, and euxinic conditions within the photic zone. A remarkable 20‰ positive sulfur isotope anomaly is seen in the Serra do Poço Verde shale, which is associated with progressively more euxinic depositional and early diagenetic environments as interpreted from iron-speciation analyses. The sulfur isotope excursion is interpreted in terms of a progressive decrease in oceanic sulfate concentration in the depositional basin, consistent with the biomarker evidence for photic zone euxinia.
Rhenium and osmium concentrations in the shales range from ∼0.2 to 16 ppb and ∼0.04 to 1 ppb, respectively. The Re-Os data suggest this series of formations are temporally resolvable from each other and significantly older than previously considered. Successive isochron ages for the Serra do Garrote and Serra do Poço Verde formations are 1353 ± 69 Ma and 1126 ± 47 Ma, respectively.

These ages indicate a Mesoproterozoic age for the glacial deposits, a time previously considered to be ice free. Furthermore, paleomagnetic data suggest that the São Francisco craton was at tropical latitudes at this time.

These studies of Vazante Group black shale suggest that environmental conditions necessary for "Snowball Earth"-like events may have persisted intermittently throughout the Proterozoic Eon.


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