ABSTRACT

The Lower Ordovician ironstones of the Iberian Massif (Zamora) and of the Armorican Massif (central Brittany) occur in units of varying thickness (0.2-2.5 m). They are interstratified in mainly sandy formations, with numerous sedimentary structures and trace fossils showing that they were deposited in a nearshore environment (intertidal or subtidal). Mineralogically, they consist of quartz, magnetite, hematite, apatite, and chamosite, associated with biotite in Zamora and with stilpnomelane in central Brittany.

These mineralizations were probably formed together with the other ironstones of the ferriferous belt of southwest Europe, within a broad shelf that extended along the northern margin of the Gondwana continent during the Early Ordovician. In the sedimentary basin, the iron, leached out of emergent continental rocks, gave rise to direct precipitation of iron oxyhydroxides (goethite and hematite) under oxidizing conditions from sea water. Below the sediment-water interface small crystals of magnetite and/or iron phyllosilicates (berthierine?) were formed under reducing conditions. During later diagenesis and metamorphism, the goethite and hematite and the berthierine were probably transformed into magnetite and chamosite, respectively, the second generation of chamosite and/or magnetites were precipitated, and those minerals that already existed were recrystallized.