COMPARATIVE-STUDY OF THE LOWER ORDOVICIAN IRONSTONES OF THE IBERIAN MASSIF (ZAMORA, SPAIN) AND OF THE ARMORICAN MASSIF (CENTRAL BRITTANY, FRANCE)

The Lower Ordovician ironstones of the Iberian Massif (Zamora) and of the Armorican Massif (central Brittany) occur in units of varying thic kness (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, apati te, and chamosite, associated with biotite in Zamora and with stilpnom elane in central Brittany, These mineralizations were probably formed together with the other ironstones of the ferriferous belt of southwes t Europe, within a broad shelf that extended along the northern margin of the Gondwana continent during the Early Ordovician, In the sedimen tary basin, the iron, leached out of emergent continental rocks, gave rise to direct precipitation of iron oxyhydroxides (goethite and hemat ite) under oxidizing conditions from sea water, Below the sediment-wat er interface small crystals of magnetite and/or iron phyllosilicates ( berthierine?) were formed under reducing conditions, During later diag enesis 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 precipitate d, and those minerals that already existed were recrystallized.