Mineralogy, geochemistry, and origin of Late Cretaceous ironstone deposits, Oman Mountains

Abstract

Late Cretaceous lateritic ironstones (ferricrete) cap the ophiolitic mafic-ultramafic rocks in different locations of the Oman Mountains. Seven localities were studied to generate the first detailed report on the ooidal ironstones in Oman. The studied ironstone horizons vary in thickness from a few meters to about 13m. Field, petrographic, and mineralogical investigations of the ironstones confirm their oolitic and laminated to massive textures, which are dominated by hematite, goethite, and magnetite. Subordinate minerals include clays, calcite, dolomite, gypsum, birnessite, and minor ilmenite and apatite. Inherited chromite and Ni-Co-Fe sulfides are also observed. The sum of Fe 2 O 3 , SiO 2 , and Al 2 O 3 oxides represent more than 70 wt% of the total constituents. The high abundance of compatible trace elements, including Cr, Ni, Co, V, and Sc, are mainly derived from the mafic minerals such as olivine and pyroxenes present in the mafic-ultramafic protolith. The analyzed detrital chromite grains from the oolitic ironstones show intermediate Cr-number (Cr# = Cr/(Cr+Al) atomic ratios ∼0.5), which is considered a general characteristic of the upper mantle peridotites of the Oman ophiolite. The major oxides relationships coupled with the paleosol mineralogy and their oolitic texture suggest that the studied ironstones precipitated mainly in a shallow marine environment and are of dominantly hydrogenetic origin with minor contributions from seawater. The persistence of negative and positive Ce anomalies (Ce* = 0.4 to 2.9), a slight positive Eu anomaly (1.12 to 1.29), and low Nd contents of the studied ironstones support the model of formation by fluctuations in water depth during transgressive and regressive episodes in oxic shallow marine to slightly anoxic deeper marine environments with a relatively high sedimentation rates where oxygen levels were low. Scandium abundance in the oolitic ironstones may be of economic interest alongside other critical metals (Ni, Co, and V) in the underlying lateritic soil under appropriate mining conditions.