KEYNOTE SPEAKER: CHARACTERIZING THE METASOMATIC ALTERATION AND MINERALIZATION IN THE LYON MOUNTAIN GRANITE AND RELATED IRON OXIDE APATITE (IOA) ORES; CONSTRAINTS FROM U-PB GEOCHRONOLOGY, RADIOGENIC TRACER ISOTOPES AND TRACE-ELEMENT GEOCHEMISTRY

The Lyon Mountain granite (LMG) located in the northeastern Adirondack Mountains of New York State hosts numerous low-titanium iron oxide apatite (IOA) ore deposits. The majority of the deposits and their adjacent host rocks contain apatite that has unusually high rare earth element (REE) concentrations (total lanthanides > 20 wt. %, and up to 8 wt. % Y) and titanite from the host rocks that were metasomatically altered by hydrothermal fluids (total lanthanides up to 3.4 wt. %, and up to 1.4 wt. % Y). The ores are predominately hosted by perthitic granite, which has been extensively metasomatised to albite and microcline granite by Na- and K-bearing hydrothermal fluids.

One challenge obtaining a better understanding of the timing and evolution of the metasomatism and subsequent mineralization, and to develop a genetic model for the formation of these IOA deposits, is to define and characterize what is sensu stricto LMG and how that fits into the tectonic framework of the Adirondack Mountains and the Grenville Orogeny, and how the LMG and its related ores relate to other IOA deposits globally (e.g., Kiruna, El Laco) in space and time.

Several studies have been undertaken in the past ten years by our group that employed cathodoluminesence microscopy, whole rock major and trace element geochemistry, in situ EPMA and LA-ICPMS trace element microanalysis of rock-forming and accessory minerals, ID-TIMS U-Pb geochronology of apatite and titanite, SIMS U-Pb geochronology of zircon, LA-MC-ICPMS Sm-Nd analyses of apatite and titanite, and Lu-Hf isotope measurements of zircon in the host rocks and ore, to characterize the LMG IOA ores and their respective host rocks.

This integrated geochemical dataset indicates a multi-stage protracted history for the formation of these enigmatic deposits from the original emplacement of the host rocks, to the metasomatic alteration by hydrothermal fluids, to the later IOA mineralization. The U-Pb dating of apatite, titanite, and zircon suggests that many of these deposits formed during later fluid events, at least 40 Ma after the crystallization and emplacement of the LMG. The initial Nd isotopic composition of both ore and host-rock apatite, and host-rock titanite, suggests the apatite-hosted REE were scavenged from local country rocks.