International Geologiical Congress - Oslo 2008


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


Rhenium and PGE in the marine and non-marine organic-rich sediments from Egypt


Hassan Baioumy, Central Metallurgical R & D Institute (Egypt)
Bernhard Peucker-Ehrenbrink, Woods Hole Oceanographic Institute (United States)


Jurassic non-marine coal and black shale, Cenomanian non-marine black shales and upper Cretaceous marine black shale from Egypt were analyzed for their Corg, N, Re and platinum group elements (PGE). The high Corg/ N ratios among the studied samples is attributed to the effect of both contribution of terrestrial organic matter and preferential release of N during diagenesis. Marine black shales have higher Re and Os contents compared to the non-marine coals and black shale. Both elements are related to the organic carbon fraction (hydrogeneous) in the marine black shale while they are more likely related to the detrital fraction in the non-marine coals and black shale. Except non-marine Jurassic black shale, samples are radiogenic with 187Os/188Os higher than the mean continental crust ratio. Marine black shales are mixtures of high Os and radiogenic end member that likely corresponds to the hydrogenous component and low Os and less radiogenic end member that likely corresponds to the detrital component. Radiogenic nature of the marine black shales is attributed to the high Re/Os contents in these shales at the time of their deposition. Non-marine coals and black shales show a mixing line between an end member of high Os and low 187Os/188Os ratios (ultramafic component) and an end-member of low Os and 187Os/188Os ratios (loess component). Rhenium-Os system showed that both marine and non-marine rocks experienced either loss or addition of Re and Os during weathering. Rhenium-Os system is more sensitive to weathering compared to organic carbon. Marine black shales in general have higher Rh, Pt and Pd contents compared to the non-marine sediments. Iridium has a negative correlation with Corg in both marine and non-marine samples suggesting its occurrence in the detrital fractions. Osmium/iridium ratios are generally higher in the marine black shales compared to non-marine coals and black shales. Uses of black shales weathering to calculate Re and Os budgets, isotopic composition and interpret geologic events should take into account the origin of such black shales and source of organic matter.


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