USING HF ISOTOPES IN ZIRCON TO MONITOR FLUID PROCESSES AND IRON OXIDE-APATITE MINERALIZATION AND REMOBILIZATION DURING OROGENIC EXTENSION

In situ analyses of hafnium isotopes in zircon provide a "snapshot" of the fluid in which that zircon grew. The combination of Hf isotopes in zircon with U/Pb zircon ages, and trace element data, provide new information on the evolution of the fluids and fluid processes associated with the formation and remobilization of Kiruna-type iron oxide-apatite deposits and the tectonic processes in which they occur. The Lyon Mountain granite in the Adirondack Mountains of New York is the host to numerous zircon-bearing iron oxide-apatite deposits. Hafnium isotopic compositions and rare earth elements contents in individual zircon crystals were measured in situ in both host granites and ore bodies by laser ablation inductively coupled plasma – mass spectrometry. Ore bodies that contain apatite with low levels of Lu have zircon with initial εHf (t) (e.g., less than +7) indistinguishable from those of the their host granite and are typical of relatively juvenile Proterozoic crust. Ore bodies that contain high Lu apatite have zircon that are extremely radiogenic with respect to Hf and have initial εHf (t) values as high + 40. Previous work done to determine the U/Pb ages from the same zircon crystals, which were analyzed for the Hf isotopic composition in this study, revealed that the ore bodies containing zircon are 20 to 60 Ma younger than their respective granite hosts. Additionally ore zircon are enriched in heavy rare earth elements. These data suggest that early-formed ore bodies containing magnetite, low and high Lu apatite, and clinopyroxene were remobilized by secondary fluid alteration, which released Hf and Zr and rare earth elements.

Zircon ages are contemporaneous with the emplacement of dikes that crosscut the regional fabric of the Adirondack Highlands. These data suggest that fluids circulating during extension of the Adirondack Highlands are responsible for secondary mobilization of iron oxide-apatite deposits and the production of hydrothermal zircon.