Institute of Disposal Research > Departments > Mineral resources > Research > Metal enrichment in black shales and the oxidation state of the oceans

 


  
We study a spectrum of black shales in South China, where the environmental conditions in the Early Cambrian (about 540 million years ago) were such, that upwelling ocean currents on the passive continental margin of the Yangtze Platform produced biologically mediated chemical sediments with very little clastic input from continental weathering. High biological activity in the near-surface oxic ocean led to progressively less oxic to euxinic (oxygen-deficient und sulfide-rich) conditions in deeper basins, similar to the present-day Black Sea. Redox-sensitive trace metals were precipitated in the water column and on submarine hardground (seafloor crusts), and formed a broad spectrum of synsedimentary-early diagenetic metal enrichments, from phosphorite, baryte and  V-U shale in suboxic to Mo-Ni-Au-PGE mineralization in euxinic environments.

The isotope composition of molybdenum, which was quantitatively scavenged from the seawater, has preserved the signature of the Early Cambrian oceans and reflects a much less oxidized global marine environment than today. Mo isotope data may allow to trace the paleo-redox state of the oceans, and indicate rapidly changing oxidation states at the Precambrian/Cambrian boundary.
 

  • Wille M, Nägler TF, Lehmann B, Schröder S, Kramers JD (2008) Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary. Nature 453: 767-769
  • Lehmann B, Nägler TF, Holland HD, Wille M, Mao J, Pan J, Ma D, Dulski P (2007) Highly metalliferous carbonaceous shale and Early Cambrian seawater. Geology 35: 403-406
  • Lehmann B, Mao J, Li S, Zhang G, Zeng M (2003) Re-Os dating of polymetallic Ni-Mo-PGE-Au mineralization in Lower Cambrian black shales of South China and its geological significance - a reply. Economic Geology 98: 663-665
  • Mao J, Lehmann B, Du A, Zhang G, Ma D, Wang Y, Zend M, Kerrich R (2002) Re-Os dating of polymetallic Ni-Mo-PGE-Au mineralization in Lower Cambrian black shales of South China and its geological significance. Economic Geology 97: 1051-1061



 

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