Petrella, and three co-authors, reported in 2014 that the Misery syenitic intrusion in northern Quebec is host to a potentially important, recently discovered rare earth element (REE)-Zr-Nb prospect, containing significant concentrations of both light and heavy REEs, and is conspicuous on aeromagnetic maps as a well-defined, ring-shaped anomaly. Felsic syenite composed of idiomorphic perthite with interstitial mafic minerals, comprising fayalite, hedenbergite, ferropargasite and annite dominates the exposed outer part of the intrusion (the core is covered by Misery Lake). This unit is accompanied by ferrosyenite containing up to 50 vol % mafic minerals, including cumulate fayalite. The ferrosyenite, which occurs as amoeboid-like inclusions in the felsic syenite, is interpreted to have formed by fractional crystallization of ferromagnesian minerals, leaving behind a residual magma which produced the felsic syenites.
This latter magma was progressively enriched in alkalis and silica, and only became saturated in ferromagnesian minerals at a very late stage of crystallization. The bulk of the REE mineralization was initially concentrated in pods and layers of fluorapatite that accumulated as a result of gravitational settling. The fluorapatite crystals were partly or completely replaced by britholite-(Ce)[often associated with allanite, see (more) for comparable mineralization], which further enriched the rocks in LREEs. Locally, thin quartz-fayalite dikes cut the felsic syenite. These dikes contain up to 10 vol % fergusonite-(Y) and 8 vol % zircon, and are interpreted to be products of an immiscible FeO-SiO2-rich magma into which Zr, Nb, and HREEs were preferentially fractionated from the conjugate felsic syenite magma.
The authors concluded that during magmatic crystallization, fluorapatite containing high concentrations of REEs accumulated by settling under gravity in ferrosyenite and also in felsic syenite. This mineral was partly or completely replaced by britholite-(Ce), which further concentrated the REEs during interation of the rocks with magmatic hydrothermal fluids. Locally, the coarse-grained syenites were altered by external fluids that had previously equilibrated with the host granitoids. Owing to the higher temperature of the syenites, these fluids albitized the perthite, forming microcrystalline albite, and remobilized the REEs in the rocks, crystallizing britholite-(Ce), monazite- (Ce) and REE fluorocarbonate minerals.
Managed & Built by I2M Consulting, LLC | © 2010-
