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

TitleAge and Origin of Monazite Symplectite in an Iron Oxide-Apatite Deposit in the Adirondack Mountains, New York, USA: Implications for Tracking Fluid Conditions
Publication TypeJournal Article
Year of Publication2019
AuthorsRegan, S, Lupulescu, MV, Jercinovic, M, Chiarenzelli, J, Williams, M, Singer, J, Bailey, D
JournalMinerals
Volume9
Issue1
Pagination65
Date PublishedJan-21-2019
KeywordsAdirondack Mountains, IOA-deposit, metasomatism, monazite
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.

URLhttp://www.mdpi.com/2075-163X/9/1/65
DOI10.3390/min9010065
Short TitleMinerals