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Minerals 2019, 9(1), 65; https://doi.org/10.3390/min9010065

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

1
Department of Geosciences, University of Alaska, Fairbanks, AK 99775, USA
2
New York State Museum, Albany, NY 12230, USA
3
Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA
4
Department of Geology, St. Lawrence University, Canton, NY 13617, USA
5
Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
6
Geosciences Department, Hamilton College, Clinton, NY 13323, USA
*
Author to whom correspondence should be addressed.
Received: 7 December 2018 / Revised: 4 January 2019 / Accepted: 18 January 2019 / Published: 21 January 2019
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Abstract

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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