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1 Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
2 Yukon Geological Survey, Government of the Yukon, Box 2703 (K–10), Whitehorse, Yukon Territory Y1A 2C6, Canada
3 Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
4 Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
5 Institut de Recherche pour le Développement (UR154) et Centre de Recherches Pétrographiques et Géochimiques, Centre National de la Recherche Scientifique, BP 20, F–54501 Vandoeuvre-lès-Nancy, France
6 Archer, Cathro & Associates (1981) Limited, 1016–510 West Hastings Street, Vancouver, British Columbia V6B 1L8, Canada
E-mail address: lgroat{at}eos.ubc.ca
Aquamarine of distinctly dark blue color 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 syenite of Mississippian age. The True Blue showing is differentiated from other beryl occurrences in the northern Cordillera by the color of the beryl, the host rock, mineral associations, timing, and mineralizing fluid. The syenite was emplaced within an extensional setting into undeformed Paleozoic sediments of the Cassiar Platform and felsic volcanic rocks of the Pelly Mountain Volcanic Belt. Post-late-Triassic tectonics resulted in a number of northeasterly directed thrust panels that were subsequently cut by Cretaceous granitic magmatism. Accessory minerals in the veins include siderite, ankerite, allanite-(Ce), fluorite, and minor albite, sulfides, and Fe–Ti–Nb oxides. 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.%, CaO up to 0.11 wt.%, and H2O (calculated) up to 3.10 wt.%, whereas little to no Cr or V was detected. The darkest blue examples of beryl also have the highest concentrations of FeO. The allanite-(Ce) contains up to 26 wt.% REE2O3, and exhibits Fe2+ > Fe3+. The fluorite that coprecipitated with beryl from several veins has been dated using Sm–Nd geochronology at 171.4 ± 4.8 Ma. In situ and whole-mineral 
18O values of the beryl and whole-mineral 18O values of the quartz are variable; temperature estimates derived from these data suggest fluid temperatures between ~275 and ~400°C. Fluid-inclusion data from quartz, beryl, and fluorite suggest variable but high salinity (~6 to 24 wt.% NaCl equivalent) and CH4-absent mineralizing fluids. Conventional models to explain the formation of gem beryl, and consequently exploration parameters, applied in Yukon involve late-stage magmatic fluids. Evidence gathered in this study points to a metamorphic origin for the mineralizing fluid and a local derivation of vein constituents, which distinguish the fluids at True Blue from other intrusion- related beryl-forming fluids in the northern Cordillera.
Keywords: aquamarine, beryl, geology, mineralogy, geochemistry, geochronology, fluid inclusions, stable isotopes, Pelly Mountains, Yukon Territory, Canada.
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