Timing and evolution of metasomatic alteration and mineralization in the Lyon Mountain Granite and related iron oxide apatite (IOA) ores; constraints from apatite and titanite U-Pb geochronology, Sm-Nd isotopes and trace-elements

Abstract

The Lyon Mountain granite (LMG) in the northeastern Adirondack Mountains of New York State hosts numerous low-titanium iron oxide apatite (IOA) ore deposits, with most deposits containing apatite that has unusually high rare earth element (REE) concentrations (total lanthanides > 20 wt. %, and up to 8 wt. % Y) as does titanite from the metasomatically altered host rocks (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. To better understand the timing and evolution of the metasomatism and subsequent mineralization, and to develop a genetic model for the formation of REE-IOA deposits, U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) dating of apatite and titanite, laser ablation multi collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Sm-Nd analyses of apatite and titanite from the ore and host rocks and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon geochronology were combined with LA-MC-ICPMS Lu-Hf isotope measurements. In addition to field and petrologic observations and newly obtained major- and trace-element data for rock forming and accessory minerals present in the ores and hosts, this integrated dataset indicates a multi-stage protracted history for the origin of these enigmatic deposits. U-Pb ID-TIMS dates of apatite and titanite show that these minerals formed during a later fluid event, likely the same event which formed the LMG ores and the initial Nd isotopic composition of both ore and host-rock apatite, and host-rock titanite, suggests a local source for the REE.