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Multistage, magmatic and hydrothermal evolution of the Debert Lake Rare Earth Element (REE) prospect, Nova Scotia, Canada

University of British Columbia

The Debert Lake Rare Earth Element (REE) prospect shows high, potentially economic, rare metal concentrations but requires a multi-faceted approach to uncover the complex formative processes and resolve timing for the deposit. This study provides a genetic model for REE mineralization at Debert Lake based on whole-rock and mineral geochemistry, together with U-Pb radiometric dates from zircon and monazite. The Wentworth plutonic complex in the Cobequid Highlands is associated with the Debert Lake REE prospect and is composed of granites and gabbros and their volcanic equivalents. The anorogenic (A-type) Hart Lake-Byers Lake (HLBL) granites, dominated by feldspar, quartz, and interstitial arfvedsonite, host the prospect. They show appreciable rare metal mineralization with grades of 1.2 wt.% REE₂O₃, 3.4 wt.% ZrO₂, and 0.4 wt.% Nb₂O₅ in strongly altered samples. Episodic mineralization yielded distinct REE assemblages including Nb-oxide (pyrochlore and fergusonite) phases, chevkinite, zircon, thorite, monazite, and titanite, formed during multiple magmatic-to-hydrothermal stages in the rare-metal bearing granites. Zircon, as a persistent phase of the REE assemblages, occurs in all granites. In the REE-rich granite, REE mineralization is characterized by REE assemblage minerals displaying intergranular to interstitial textures. “Spongy” zircon grains occur in patchy domains reflecting late magmatic stage fluid interaction with primary minerals and concentration of Y, Th and Zr mobilized by the fluids. U-Pb dating of these zircons yields an age of 355 to 360 Ma, approximately coeval with igneous monazite crystallization, dated at 366 ± 5 Ma. These findings document an early, late-magmatic REE mineralization event. A subsequent pulse of post-magmatic hydrothermal mineralization associated with Zr-F-rich fluid complexes produced two zircon populations, dated at ~ 310 to 318 Ma, with unusual habits and atypical REE patterns that are interpreted to be of hydrothermal origin. Principal Component Analysis separates zircon populations based on element concentrations and element associations. Late magmatic stage zircon is characterized by depletion in LREE whereas LREE-rich post-magmatic stage zircon grains are enriched in either Na or Ca as a result of sodic and calcic (respectively) metasomatic fluids.

Text

2020-12-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76909

Earth and Environmental Sciences

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/