Geochemistry of Callovian Ironstone in Kutch and Its Stratigraphic Implications

A detailed study of Callovian golden oolites of Jhumara Formation from Keera dome, Kutch basin reveals a similarity in the chemical signature of Jurassic ironstones across the globe. An integrated sedimentological and stratigraphical approach involving petrographical, mineralogical, textural and geochemical analysis was carried out to understand the origin of iron ooids. X-ray diffraction analysis indicates a predominance of goethite, calcite and chamosite. Petrographic investigation reflects oolitic-grainstone microfacies originating in high-energy littoral conditions. The ooids are composed of alternate layers of chamosite and goethite over the carbonate grains, bioclasts and peloids. The geochemical analyses suggest oxidation of chamosite to goethite indicating a fluctuating redox environment from sub-oxic to oxic. The negative correlation of Fe2O3 (total) with Al2O3, SiO2 and MgO suggest removal of Al, Si and Mg with increasing Fe. The enrichment of V, Cr, Zn, As, Pb and Th in golden oolites suggests a possible hydrothermal origin of iron. The golden oolites originate by the direct precipitation of hydrothermally-sourced Fe from seawater and accretion of Si, Al and other oxides on suspended nuclei of ooids. The origin and geochemistry of iron oolites in Kutch basin relate to the Jurassic greenhouse condition.