ORIGIN AND DEPOSITIONAL ENVIRONMENT OF ORDOVICIAN STRATIFORM IRON MINERALIZATION FROM ZAMORA (NW IBERIAN-PENINSULA)

The Ordovician stratiform iron deposits at Zamora (NW Iberia) are arra nged in several levels ranging between 0.2 and 1.5 m in thickness, whi ch are interstratified in the upper member of the ''Pielgo'' Quartzite s Formation (Arenig). The sandy nature, and trace-fossils correspondin g to the ichnogenus Ci 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 for med part of a broad shelf situated in the northern margin of the Gondw ana 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 hematit e) beds. They have high TiO2, Ta, Sc, V, Nb, Co, Zn and Y contents. Th e magnetite contains unusually large amounts of TiO2, V, Cr and Ni; th ere is also a clear depletion in Eu and the (Eu/Sm)(CN) ratio is <1 an d the (Sm/Yb)(CN) ratio is >1. The chamosite contains high concentrati ons of Cr and V. These results suggest that iron was supplied from the weathering of a continental source? in combination with volcanic acti vity, such as within-basin basic volcanism or the presence of basic vo lcanic rocks in the exposed land. The physicochemical conditions of ir on mineral crystallization calculated from chamosite compositions are the following: log fO(2): -38.8 to -30.7, log fS(2): -13.2 to -9.5 and T: 200 to 330 degrees C. These results together with the delta(18)O v alue (similar to 2 parts per thousand,) Of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low-grade metamorphism under redox conditions below the magnetit e-hernatite buffer.