Geochemistry of Precambrian black shales from Ossa-Morena Zone (Portugal): depositional environment and possible source of metals

Abstract

The paleodepositional environments related to Ediacaran black shales from the Série Negra succession in Ossa-Morena tectonostratigraphic zone were investigated using their geochemical composition to identify the sedimentary sources and depositional conditions. Their potential for assisting the genesis of metal deposits was also discussed. The sedimentary sources of the black shales are related to the break up of a Cadomian magmatic-arc developed in the North Gondwana realm, where the Ossa-Morena Zone was located during late Ediacaran times, showing acidic to mixed signatures. Some ratios were calculated based on redox sensitive elements, indicating both anoxic and oxic paleoenvironmental conditions. The anoxic conditions were predominant and they were identified on samples with higher organic carbon content, emphasizing that the redox conditions favored organic matter preservation and the accumulation of selected metals. Considering the mode of occurrence, the elements Ag, Cd, Cu, Pb and Se are assumed to be preferentially associated with sulfides, whereas Mo and U are preferentially organically bound. The results reveal that the black shales from the Série Negra succession in Ossa-Morena Zone might represent a source of metals, which were probably remobilized during Variscan events.