Origin of the extreme polymetallic enrichment (Cd, Cr, Mo, Ni, U, V, Zn) of the Late Cretaceous–Early Tertiary Belqa Group, central Jordan

Abstract The sedimentary formations of the Late Cretaceous to Early Tertiary Belqa Group correspond to pure to slightly clayey limestones and minor black shales characterized by a remarkable enrichment in phosphorous and various redox sensitive elements (Cd, Cr, Mo, Ni, U, V, Zn). Phosphorous enrichment is related to the great phosphorite deposition event that has occurred at the southern margin of the Tethys Ocean during this period. The very low organic matter contents in the limestones, despite their strong enrichment in redox sensitive elements, is attributed to a deposition in anoxic water but under shallow water conditions permitting the bacterial degradation of a large part of the organic matter. Most of the elements constituting the Belqa Group sediments have been deposited from the sea water either directly by chemical precipitation (most Ca and redox sensitive elements) or indirectly through biogenic activity (P, part of Ca and U), except for the minor detritial contribution presents in some limestones and more significantly in the shales. Such an origin is also supported by the REE patterns of the limestones which are similar to that of the present sea water. The anomalously high concentrations of some redox sensitive trace metals (Cr, Ni and probably Cd) compared to worldwide black shales is explained by an exogenic metal flux, corresponding to the leaching of the huge amounts of ophiolites obducted during the collision between the African-Arabian and Eurasian plates at the same time as the deposition of the Belqa Group sediments. Uranium is substituted for calcium in biogenic apatite in the limestones, but was also directly precipitated from sea water in the black shales in relation with the suboxic environment with the periodic anoxia developed during their deposition in restricted deeper basins. The pyrometamorphism resulting from the burning of organic-rich levels in clayey limestones has created a further enrichment of redox sensitive elements in the resulting marbles by a volume loss resulting from decarbonatation reactions and the combustion of the organic matter.