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Resolving monazite growth mechanisms in orogenic gold settings : a study from the Klondike Gold District, Western Yukon Stroh, Brodie A.

Abstract

Orogenic gold deposits form some of the world's largest gold sources; however, the timing of gold mineralization is difficult to constrain. The ~20 Moz of placer gold mined in the Klondike was derived from orogenic, gold-bearing quartz veins where the exact timing of gold mineralization is unknown despite previous age data from hydrothermal micas and rutile. U-Th-Pb dating of hydrothermal monazite can be a robust alternative to constrain the timing of orogenic gold deposits and associated geological events that has not been attempted in the Klondike until now. In orogenic gold settings or fluid-affected metamorphic terranes in general, monazite can grow by metamorphic and hydrothermal processes. The objective of this research is to distinguish between these monazite growth processes to provide constraints on orogenic gold mineralization in the Klondike. Vein material, altered wall-rock, and unaltered host-rock samples were investigated at the Virgin, Mitchell-Sheba, and Lone Star gold occurrences. Detailed petrographic analyses were integrated with LA-ICP-MS Th-Pb monazite dates and ThO₂ concentrations to identify metamorphic and hydrothermal monazite. The age of metamorphic monazite in the Klondike is between 189 and 151 Ma, with possible discrete pulses at approximately 175 and 160 Ma. These ages overlap with existing ⁴⁰Ar/³⁹Ar mica cooling ages and represent retrograde monazite growth during crustal exhumation. The age of hydrothermal monazite is between 178 to 117 Ma, with possible discrete pulses at approximately 169, 148, and 128 Ma. These ages represent episodic vein formation and provide approximate constraints on gold mineralization. The methods developed can be used to identify metamorphic and hydrothermal monazite globally to provide robust time constraints on metamorphism, vein formation, and gold mineralization. Metamorphic monazite occurs regardless of proximity to veins, is typically adjacent to or intergrown with other metamorphic minerals, and has variable ThO₂ concentrations that depend on the host-rock composition and metamorphic grade. In contrast, hydrothermal monazite occurs in or adjacent to veins, is typically adjacent to or intergrown with hydrothermal minerals, and has a distinctly low ThO₂ concentration

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