Effect of thermal metamorphism on the sedimentary iron formation of Orissa by basic igneous intrusives is described. Two mineral assemblages, quartz-cummingtonite-grunerite-magnetite with amosite and quartz-actinolite-zoisite-magnetite have been developed at the contact of the banded iron formation and the gabbro. The contact rocks show typical hornfelsic texture. The mineralogical assemblages indicate albite-epidote hornfels to hornblende hornfels facies of thermal metamorphism. The metamorphism took place under high water vapor pressure at a temperature of about 800° ± 50° C, which was confirmed by the decomposition temperature of amosite and also by the temperature of formation of cummingtonite.
The banded iron formation consists of alternate bands of fine-grained iron oxide and cherty or cryptocrystalline silica. The basic igneous complex has been differentiated into pyroxenites, olivine gabbro, normal gabbro without olivine, anorthositic gabbro, norite, granodiorite and leucogranite. From the chemical analyses of the contact rocks and their mineralogical and textural studies, it has been established that the metamorphism of the iron-formation was accompanied by the introduction of Ca and Mg from gabbro to the iron formation. Cummingtonite was formed by the reaction of quartz and magnetite. Chemical analysis and X-ray powder photograph data of the mineral amosite are also given.