The iron oxide – apatite (IOA) deposits from the eastern Adirondack Mountains are close to the classic Kiruna-type deposits. From immiscible Fe-rich fluids to extraction of Fe during the breakdown of the Fe-rich silicates during metamorphism to eruption of Fe oxide-rich magmas to the interaction of the surface-derived saline fluids with the rocks in the late stages of the pluton emplacement, several geologists discussed the origin of these iron deposits over time with no agreement.
The ores consist of intrusive sheets or dikes of low-Ti magnetite, fluorapatite, augite, and hedenbergite. Observations under transmitted light show polygonal and cumulate textures. The ore bodies, each with knife-edge contacts, are in some opinions closely associated in time with A-type leucogranites and granitic gneisses. Field, geochemical, and textural evidence support an igneous origin and subsequent long-lived, fluid-rock interaction for magnetite ores of the eastern Adirondack Highlands.