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. 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.
Published Version
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