EXHALATIVE ORIGINS OF IRON FORMATIONS

Abstract

Iron formations are stratigraphic units which are largely composed of iron-rich chemical sedimentary rock, here called ironstone. Most aspects of iron formations continue to be controversial and so one must read voluminous literature to appreciate either the range of iron-formation characteristics or the conflicting interpretations of those characteristics. Most protagonists in the ongoing debate may be classified into two groups, i.e. those who support a shallow weathering source for the iron (weathering of land or surficial seafloor sediment) and those who invoke deep weathering (hydration of new crust or late diagenesis of sediments) followed by exhalation of ferriferous fluids through the seafloor. The present review concludes that deep weathering has been the source of all iron formations. Cherty iron formations are attributed to hydration of new crust. Noncherty iron formations are attributed to exhalation of late-diagenetic fluids which have been driven through a continental margin by seismic pumping.