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

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.

U-Pb systematics of uranium-rich apatite from Adirondacks: Inferences about regional geological and geochemical evolution, and evaluation of apatite reference materials for in situ dating

“…Iron oxide-apatite deposits cross-cut all lithologies except late pegmatites, which cross-cut some ore bodies and are dated at about 1040 to 1020 Ma (LUPULESCU et al, 2011). Detailed geological descriptions of the deposits are given in VALLEY et al (2011), MCLELLAND et al (2013) CHIARENZELLI et al (2018, REGAN et al (2019) and references therein.…”

Syn-collisional exhumation of hot middle crust in the Adirondack Mountains (New York, USA): Implications for extensional orogenesis in the southern Grenville province

“…Rims showed a slightly tighter cluster of ages ranging from 1021 ± 8 Ma to 972 ± 32 Ma and yielded a weighted mean of 1011 ± 5.0 Ma (mean square weighted deviation [MSWD] = 2.8). Rims are interpreted as the result of fluid-mediated recrystallization processes based on texture (Harlov et al, 2011;Williams et al, 2011), an association with apatite (Harlov et al, 2011), and the correspondence of ages falling well within the realm of metasomatism within the region (Valley et al, 2011;Chiarenzelli et al, 2017;Regan et al, 2019). The large range in core dates is thus interpreted to represent igneous cores variably disturbed by fluid-mediated dissolution-reprecipitation reactions (Grand'Homme et al, 2016).…”

“…Fluid alteration has been interpreted to range in age from ca. 1050 to 980 Ma (Valley et al, 2011;Regan et al, 2019), which likely overlapped and outlasted leucogranite plutonism.…”