DIRECT DATING of LOW-Ti MAGNETITE DEPOSITS IN the LYON MOUNTAIN GNEISS by U/Pb ZIRCON GEOCHRONOLOGY

We present results of the first direct dating of Low-Ti, magnetite (IOCG-type) deposits by U/Pb zircon geochronology. Constraining the timing of Fe mineralization has important geodynamic implications for the processes involved in the orogenesis of IOCG deposits, other types of related mineral deposits, fluid-crust interaction and the tectonic evolution of rocks associated with these deposits. Zircon crystals were identified in thin section and subsequently separated from four Fe-ore deposit samples associated with the Lyon Mountain Gneiss (LMG) in the northeastern Adirondack Mountains. Zircon grains from these low-Ti magnetite deposits reveal at least two periods of mineralization, one episode at ~1039 Ma, and a second event between ~1020 and ~1000 Ma. An age difference of 20 m.y. between ore deposits that are geographically very close suggests fluids may be re-using preexisting structures. Previous age determinations of these deposits were constrained by the age of the host granitoids and the assumption that Fe mineralization was related to the waning stages of pluton emplacement. Zircon crystals extracted from the ore yield ages that show ore mineralization was as much as 50 m.y. younger than pluton emplacement. Host granites that intruded between 1059 Ma and 1049 Ma cannot be the source of heat or fluids required for Fe mineralization. The timing of the oldest mineralization event is coeval with regional extension and the intrusion of small leucogranite bodies and dikes throughout the Adirondacks. Fluid assisted partial melting and/or rapid uplift is evident from the presence of leucosomes in the host granite in and around many of the ore bodies. The range of U/Pb ages from the Fe ore deposits suggests extension was protracted and/or episodic in the northeastern Adirondack Mountains.