LAUSR

Abstract The iron oxide-apatite (IOA) deposits of the eastern Adirondack Mountains consist of intrusive sheets or dikes of magnetite, fluorapatite, augitic pyroxene, quartz, and microcline. Other trace mineral phases include ilmenite with hematite exsolution, V-rich titanite rimming magnetite, zircon, monazite-(Ce), stillwellite-(Ce), lanthanite-(Ce), allanite-(Ce), and thorite. Observations under transmitted light show polygonal and cumulate textures. The ore bodies, each with knife-edge contacts with the host gneisses, are closely associated in time with A-type leucogranites and granitic gneisses ( ca . 1070–1050 Ma). Backscattered electron (BSE) images highlight the following types of fluorapatite-monazite-(Ce) relations that formed as a result of metamorphism and fluid-rock interaction: (1) areas of relatively low BSE intensities containing tiny secondary monazite-(Ce) and thorite crystals developed within brighter apatite grains and along crystal margins and fractures; (2) areas of low BSE intensity within larger fluorapatite grains; (3) oriented rods of quartz in fluorapatite; (4) monazite-(Ce) rimming fluorapatite; and (5) multi-domain clusters of fluorapatite in unzoned fluorapatite. Fluorapatite samples from these IOA deposits were analyzed by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) for major and trace elements. All grains contain high concentrations of light and heavy rare earth elements and yttrium, and samples from some deposits are exceptionally enriched in heavy rare earth elements and yttrium. The crystal structures of the fluorapatites from Cheever, Mineville, Palmer Hill, Arnold Hill, and Rutgers mine were analyzed. Rare earth elements (REE) are dominant at the Ca2 site, and in the most REE-rich sample, from Cheever, 5.7% of the Ca2 sites and 3.5% of the Ca1 sites are occupied by REEs. We consider that the most likely geological scenario for the incorporation of the REEs in fluorapatite includes high concentrations of incompatible elements in a dense iron- and phosphorus-rich melt that formed by immiscibility with the silicate melt of the Lyon Mountain granite. The high REE concentration appears to have been accommodated in the fluorapatite structure through a coupled substitution with Si 4+ . A later, low-temperature stage of fluid infiltration, probably at greenschist facies conditions, re-mobilized the REE and produced secondary minerals within the ore, including low-actinide bearing monazite-(Ce), tremolite, ferro-actinolite, chlorite, rutile, and hematite.