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Open AccessFeature PaperArticle

Age and Origin of Monazite Symplectite in an Iron Oxide-Apatite Deposit in the Adirondack Mountains, New York, USA: Implications for Tracking Fluid Conditions

Department of Geosciences, University of Alaska, Fairbanks, AK 99775, USA
New York State Museum, Albany, NY 12230, USA
Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA
Department of Geology, St. Lawrence University, Canton, NY 13617, USA
Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
Geosciences Department, Hamilton College, Clinton, NY 13323, USA
Author to whom correspondence should be addressed.
Minerals 2019, 9(1), 65;
Received: 7 December 2018 / Revised: 4 January 2019 / Accepted: 18 January 2019 / Published: 21 January 2019
Monazite crystals, intergrown with allanite, fluorapatite, and quartz from the Cheever Mine iron oxide-apatite (IOA-type) deposit in Essex County, New York, USA, display rare symplectite textures. Electron probe wavelength-dispersive spectrometry (WDS) mapping and major and trace element characterization of these features reveal a natural experiment in fluid-mediated monazite recrystallization. Two types of monazite with symplectite intergrowths have been recognized (Type I and II). Both types of symplectite development are associated with a decrease in HREE, Si, Ca, Th, and Y, but an increase in both La and Ce in monazite. Electron microprobe Th-U-total Pb analysis of Type I monazite with suitable ThO2 concentrations yielded a weighted mean age of 980 ± 5.8 Ma (MSWD: 3.3), which is interpreted as the age of monazite formation and the onset of symplectite development. Both types of monazite formed during a series of reactions from fluorapatite, and possibly britholite, to produce the final assemblage of monazite, allanite, and fluorapatite. Monazite formation was likely a response to evolving fluid conditions, which favored monazite stability over fluorapatite at ca. 980 Ma, possibly a NaCl brine. A subsequent transition to a Ca-dominated fluid may have then promoted the consumption of monazite to produce another generation of allanite and fluorapatite. Our results indicate that recrystallized monazite formed during fluid-mediated processes that, over time, trended towards an increasingly pure end-member composition. Regionally, these data are consistent with a magmatic-origin followed by fluid-mediated remobilization of select phases at subsolidus conditions for the Adirondack IOA deposits. View Full-Text
Keywords: monazite; metasomatism; IOA-deposit; Adirondack Mountains monazite; metasomatism; IOA-deposit; Adirondack Mountains
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Regan, S.; Lupulescu, M.; Jercinovic, M.; Chiarenzelli, J.; Williams, M.; Singer, J.; Bailey, D. Age and Origin of Monazite Symplectite in an Iron Oxide-Apatite Deposit in the Adirondack Mountains, New York, USA: Implications for Tracking Fluid Conditions. Minerals 2019, 9, 65.

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