Direct dating and characterization of the Pope's Hill REE occurrence

Abstract

The Pope’s Hill (PH) rare earth element (REE) occurrence, located in central Labrador, is a syenite-hosted, highly enriched unit with whole-rock geochemical analyses indicating REE contents ranging from <1 to 22 weight percent. Samples of ore, altered host rock, and unaltered country rock, were collected throughout the occurrence in order to: 1) determine which phases concentrate the REE and also quantify their abundances and distribution in the ore; 2) determine the host rock emplacement age; 3) determine the timing of the subsequent hydrothermal alteration and REE mineralization; and 4) constrain the source of the REE using in situ LA-ICPMS and ID-TIMS U-Pb geochronology and in situ Sm-Nd isotopes using LA-MC-ICPMS in monazite. The REE-enrichment occurs in millimeter- to decimeter-scale pods that are locally discontinuous and consist of primary and secondary REE minerals including a variety of silicate, phosphate, carbonate, and niobate phases; the most abundant being allanite-(Ce), monazite-(Ce), britholite-(Ce), and an unnamed LREE-enriched mineral similar in composition to titanite. ID-TIMS geochronology reveals monazite dates from ~1090 to 940 Ma that coincide with Grenvillian metamorphism and plutonism during continent-continent collision suggesting that locally derived REE in the PH syenite crystalized into primary REE-silicates either before or during Grenvillian events. Microtextures in thin section indicate that secondary non-silicate REE minerals were produced by the breakdown of primary REE-silicates, suggesting either hydrothermal fluid alteration, or metamorphism.