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Direct dating of Fe oxide-(Cu-Au) mineralization by U/Pb zircon geochronology

¹Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X5, Canada
²Laboratory for Isotope Geology, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden

Correspondence: *E-mail: pvalley{at}mun.ca.

We present results for direct dating of Fe oxide-(Cu-Au) (iron oxide, copper, gold—IOCG) mineralization by U/Pb zircon geochronology. Constraining the timing of mineralization has important geodynamic implications for the processes involved in the genesis of these types of deposits and the tectonic evolution of rocks associated with these deposits. Hydrothermal zircon crystals were separated from four IOCG-type ore deposits associated with the Lyon Mountain Granite in the Adirondack Mountains, United States. Zircon grains from these low-Ti magnetite deposits reveal at least two periods of mineralization; one episode ca. 1039 Ma and a second between ca. 1015 and 1000 Ma. Previous age determinations of these deposits were constrained by the age of the altered host granitoids, and the assumption that Fe mineralization was directly related to pluton emplacement. Zircon crystals extracted from the ore yield ages that show that ore mineralization was episodic and younger than pluton emplacement.