Origen y significado de las “pizarras moradas” de la Faja Pirítica Ibérica

Laburpena

The Iberian Pyritic Belt (IPB) is a world-class metallogenic province with huge accumulations of massive sulphide depo- sits. Despite this importance, some geological features, even related to the stratigraphic record, are still poorly known. This applies specifically to rocks non related to the sulphides. The “purple shales” constitute the only cartographic guide level at regional scale, because it outcrops almost continuously throughout the entire region. Three sections of “purple shales” have been studied on the western, central and eastern parts of the Spanish IPB. Stratigraphic, geochemical, mineralogical and petrographic data indicate that they do not conform a homogeneous level neither at regional nor at local scale. The “purple shales” are composed of lithofacies of volcanic and sedimentary rocks showing frequent lateral and vertical facies changes. The distinctive purple/red colour was caused by a post-sedimentary oxidation process that unevenly affected these rocks. The oxidation was not accompanied by Fe enrichment nor involved significant changes in the geochemistry of the deposits. This occurred because the Fe3+ responsible for the purple colouring came directly from the rock, with no external source. This oxidative process forms part of the global Carbo- niferous-Permian oxidation event that, in the IPB, was controlled by the basin palaeogeography in turn responsible for the distribution and nature of the deposits.