Pressure, temperature and fluid conditions during emerald precipitation, southeastern Yukon, Canada: fluid inclusion and stable isotope evidence

The Crown emerald veins are somewhat enigmatic, displaying characteristics that are common to emerald deposits of tectonic–hydrothermal origin and of igneous origin. The veins cut the Fire Lake mafic meta-volcanic rocks, occurring within 600 m of an outcrop of Cretaceous S-type granite. Field work and vein petrography are consistent with a polythermal origin for the veins. The primary vein mineralogy is quartz and tourmaline with variable sized alteration haloes consisting of tourmaline, quartz, muscovite, chlorite and emerald. The veins weather a buff brown colour due to jarosite, scheelite and minor lepidocrocite, which were precipitated during the waning stages of vein formation. Microthermometic studies of primary fluid inclusions within emerald growth zones are consistent with emerald precipitation from H2O–CO2–CH4 (±N2±H2S) bearing saline brines. The estimated fluid composition is approximately 0.9391 mol% H2O, 0.0473 mol% CO2, 0.0077 mol% CH4 and 0.0059 mol% NaCl (∼2 wt.% NaCl eq.). Fluid inclusion and stable isotope studies are consistent with vein formation in the temperature range 365–498 °C, with corresponding pressures along fluid inclusion isochore paths ranging from 700 to 2250 bars. These data correlate with a very slow uplift rate for the region of 0.02–0.07 mm/year. d deposits are generally formed when geological conditions bring together Cr (±V) and Be. Cr and V are presumed to have been derived locally from the mafic and ultramafic rocks during hydrothermal alteration. The Be is most likely derived from the nearby Cretaceous granite intrusion.