GOLD IN LOWER ORDOVICIAN IRONSTONES OF VALONGO ANTICLINE: CONTROL BY VOLCANISM, ALGAE AND STROMATOLITES

Lower Ordovician oolitic ironstones are well developed in Valongo Anticline, North Portugal and are significant gold bearing strata having been exploited since roman times. Ironstones are more developed along the normal limb of Valongo Anticline and in the periclinal zone and were reported from north to south in the areas of Ribeiro da Igreja and Vale do Inferno mines (Sb-Au), Fojo das Pombas, Moirama, Banjas and Poco Romano mines (Au-As) and in Ribeiro da Estivada mine (Pb-Zn(Ag)). Seven ironstones layers were distinguished in the Lower Ordovician in the surrounding areas of these mines. These layers control preferentially the Au-As type mineralization. The ironstones occur in a Lower Ordovician stratigraphic succession of Floian age (Santa Justa Formation) interbedded with quartzites, wackes and shales and overlapping the Lower Ordovician massive quartzite. Ironstones contain volcanic horizons and stromatolites. In the base of the succession rifting processes are reported and volcanism extends trough the Lower Ordovician, must having contributed to the metal concentration. Algae and stromatolites have also played an important role in the preconcentration of gold. Mineralogical, petrographic and geochemical data suggest the existence of a syndiagenetic generation of gold in ironstones prior to hydrothermal mineralizing episodes.