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Mineralogical and geochemical study of the True Blue aquamarine showing, Shark Property, southern Yukon Territory Turner, David James


Aquamarine of distinctly dark blue colour was discovered during the summer of 2003 in the Pelly Mountains, southern Yukon Territory, Canada. The beryl is found within quartz veins that fill sigmoidal tension gashes, which cut a Mississippian syenite. The True Blue showing is differentiated from other beryl occurrences in the northern Cordillera by the colour of the beryl, host rock, mineral associations, timing, and mineralizing fluid. The syenite was emplaced within an extensional setting into undeformed Paleozoic platform sediments of the Cassiar Platform and felsic volcanics of the Pelly Mountain Volcanic Belt. Post late-Triassic tectonics resulted in a number of northeast-directed thrust panels that were subsequently cut by Cretaceous granitic magmatism. Accessory minerals in the veins include siderite, ankerite, allanite, fluorite, and minor albite, various sulfide minerals, and Fe-Ti-Nb oxide minerals. Electron microprobe analyses of beryl (n = 192) revealed that FeO values range up to 5.92 wt.%, Na₂O up to 2.66 wt.%, MgO up to 3.42 wt.%, and CaO up to 0.11 wt.%, while little to no Cr, Sc or V were detected. The darkest blue beryl has the highest concentrations of FeO. Allanite is of the Ce-dominant variety, contains up to 26 wt.% REE₂O₃, and exhibits Fe²⁺ > Fe³⁺. Fluorite from several veins that co-precipitated with beryl has been dated using Sm-Nd geochronology at 171.4 ±4.8 Ma. In situ and whole-mineral δ¹⁸O[sub SMOW] values from beryl and whole-mineral δ¹⁸O[sub SMOW] values from quartz are variable and temperature estimates derived from this data suggests fluid temperatures between ~275 and ~425 °C. Conventional gem beryl formation models, and consequently exploration parameters, applied in Yukon invoke late-stage magmatic fluids. Evidence gathered in this study points to a metamorphic origin for the mineralizing fluid and local derivation of vein constituents, thus differentiating the fluids at True Blue from other intrusion-related beryl-forming fluids in the northern Cordillera.

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