Mineralogical and geochemical study of the Regal Ridge emerald showing, southeastern Yukon
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| Authors: | Groat, LA; Marshall, DD; Giuliani, G; Murphy, DC; Piercey, SJ; Jambor, JL; Mortensen, JK; Ercit, TS; Gault, RA; Mattey, DP; Schwarz, D; Maluski, H; Wise, MA; Wengzynowski, W; Eaton, DW |
| Year: | 2002 |
| Journal: | Can. Mineral. 40: 1313-1338 Article Link (DOI) |
| Title: | Mineralogical and geochemical study of the Regal Ridge emerald showing, southeastern Yukon |
| Abstract: | In September, 1998, one of the authors (WW) discovered a major occurrence of emerald in the Finlayson Lake district of southeastern Yukon. The Regal Ridge showing occurs in complexly deformed metavolcanic rocks in the Yukon-Tanana Terrane, near their contact with a mid-Cretaceous granitic pluton. The emerald crystals occur where quartz veins cut mica-rich layers in a shallowly dipping mica schist of the Upper Devonian Fire Lake mafic metavolcanic unit. At least eight such veins have been found. Most are surrounded by a much more extensive, overlapping mass of fine, dark tourmaline crystals. The tourmaline crystals are locally associated with minor amounts of scheelite, and small amounts of sulfides have been observed to occur within the quartz veins. A zone of sparse, disseminated sulfides apparently coincides with the tourmaline zone, which is surficially marked by ochreous products of oxidation. Green beryl crystals up to 4 cm in length occur in tourmaline zones and, rarely, in the quartz veins. Some of the smaller crystals, and sections of larger crystals, are of gem quality. The Cr content (average 3208 ppm) shows that it is the predominant chromophore. Fluid-inclusion data indicate that the emerald precipitated from a fluid whose maximum salinity was 3 wt.% NaCl equivalent. The oxygen isotopic composition of the emerald is highly variable (12.3 to 14.8parts per thousand), but there is little difference in corresponding deltaD values (-57.3 and-59.8parts per thousand, respectively), which suggests the presence of an isotopically homogeneous fluid that underwent isotopic exchange with the host rocks without achieving homogenization. The delta(18)O values for coexisting quartz and tourmaline from the quartz veins yield temperatures of formation of approximately 365 and 498degreesC. Based on fluid-inclusion isochoric data, these temperatures correspond to pressures of 1.0 to 2.5 kbar, and inferred depths of 3 to 7.7 km. The close proximity of the granite suggests that it is the source of the Be, although the Be content is low (12 and 13.2 ppm). The source of the Cr is the schist (520 ppm Cr). An Ar-40/Ar-39 age of 109 Ma for a mica sample from the schist could either reflect a thermal overprint age related to the event that produced the emerald crystals, or cooling following intrusion of the adjacent pluton, or both. |
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