Origin and depositional environment of Ordovician stratiform iron mineralization from Zamora (NW Iberian Peninsula)

Abstract

The Ordovician stratiform iron deposits at Zamora (NW Iberia) are arranged in several levels ranging between 0.2 and 1.5 m in thickness, which are interstratified in the upper member of the “Pielgo” Quartzites Formation (Arenig). The sandy nature, and trace-fossils corresponding to the ichnogenus Cruziana and Daedalus, together with major and trace element contents of this formation suggest an inter and subtidal, shallow marine depositional environment, which on a global scale formed part of a broad shelf situated in the northern margin of the Gondwana continent. The iron mineralization displays a foliated and banded structure due to the alternation of quartzitic, phosphatic (apatite), chamositic, chamositic-biotitic and ferriferous (magnetite and hematite) beds. They have high TiO₂, Ta, Sc, V, Nb, Co, Zn and Y contents. The magnetite contains unusually large amounts of TiO₂, V, Cr and Ni; there is also a clear depletion in Eu and the (Eu/Sm)_CN ratio is <1 and the (Sm/Yb)_CN ratio is >1. The chamosite contains high concentrations of Cr and V. These results suggest that iron was supplied from the weathering of a continental source, in combination with volcanic activity, such as within-basin basic volcanism or the presence of basic volcanic rocks in the exposed land. The physicochemical conditions of iron mineral crystallization calculated from chamosite compositions are the following: log f O₂: −38.8 to −30.7, log f S₂: −13.2 to −9.5 and T: 200 to 330 °C. These results together with the δ¹⁸O value (∼2‰) of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low-grade metamorphism under redox conditions below the magnetite-hematite buffer.