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UBC Theses and Dissertations

Source and timing of alkali metasomatism and associated critical element mineralization in the Pool Creek map area, southeastern Yukon Naber, Tiera Venesse


Rare-earth element (REE) and high field strength element (HFSE) bearing Eocene and Neoproterozoic intrusions occur throughout southeastern Yukon, Canada. Poorly exposed outcrops of a REE-HFSE bearing fenite of unknown origin are found adjacent to the Neoproterozoic Pool Creek nepheline syenite, which is superimposed by Eocene volcanism. Fenites hold critical information about the original composition of their source intrusions and have implications as a vector towards associated REE-HFSE mineralization; however, fenites associated with syenitic intrusions are understudied in the literature. Rock and mineral textures and compositions of the Pool Creek nepheline syenite, altered syenite suites, fenites, and host units (quartzite and argillite) were compared to determine the source and timing of the fenitization event and to contribute to the collective knowledge of fenites associated with syenitic intrusions. The altered syenite and fenite suites show geochemical and mineralogical compositions that are transitional between their respective host units and the Pool Creek nepheline syenite. Interestingly, the fenitized units hosted in argillite have REE-HFSE contents up to 15x greater than those hosted in quartzite. The style of mineralization observed in this study is consistent with that of a silicate roof zone type deposit, indicating the orientation of the intrusive system. Relationships between unaltered and altered zircon textures and dates show that the ages of crystallization of multiple intrusive units and associated fenitization occurred between 620 to 660 Ma. Overlapping ages of different syenitic units suggest that the Pool Creek nepheline syenite forms a composite alkaline-silicate complex, rather than a single intrusive event. These lines of evidence suggest that the source of the fenitization that produced the alteration observed in the altered syenite and fenite suites is the Neoproterozoic Pool Creek nepheline syenite, and therefore that fenitization was driven by contact metasomatism between the nepheline syenite and host units. These findings show that the characteristics of the host rocks strongly influence the composition and distribution of fenite aureoles and REE-HFSE mineralization associated with syenitic systems. These conclusions demonstrate that understanding the compositional relationship between the source intrusion, associated fenite, and host yields a plethora of information about the magmatic system and controls on associated mineralization.

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