COMPOSITION AND ORIGIN OF HYDROTHERMAL IRONSTONES FROM CENTRAL PACIFIC SEAMOUNTS

Ironstones recovered from five Late Cretaceous seamounts in the centra l Pacific region probably formed during late-stage edifice-building vo lcanism. Ironstones are dense and compact with the appearance of brown chert. The ironstones are characterized by a goethite mineralogy with FeOOH contents up to 88%, extreme fractionation of Fe and Mn, low tra ce-element and rare earth element abundances, low Co/Zn ratios, and is otopic equilibration temperatures of about 20-45-degrees-C. These char acteristics indicate that the ironstones formed from hydrothermal flui ds. Ironstones probably formed below the seawater-seafloor interface, as indicated by their occurrence as a proximal hydrothermal deposit, p resence of primary goethite cement, pervasive replacement of rocks by goethite, and absence of interbedded pyroclastic beds.