Publication Type:Journal Article
Source:Canadian MineralogistCanadian MineralogistCanadian Mineralogist, Volume 55, Number 4, p.561-593 (2017)
Keywords:cantung, cordillera, deposits, emerald, fluid inclusion, geochemistry, lened, mineral chemistry, northwest territories, oxygen-isotope fractionation, silicates, Stable isotopes, stable-isotope, system, tourmaline, tungsten, tungsten skarn
Emerald at the Lened occurrence in the western Northwest Territories is hosted by quartz veins cutting skarn near the Lened granite pluton and older Selwyn Basin strata. Euhedral beryl crystals (<0.5 x 5 cm) are present in approximately half of the 26 outcropping veins. Most of the crystals are opaque to translucent and colorless to yellowish and grassy green. Less than 5% of the beryl is transparent, bluish green, and can be considered pale emerald. Using field relationships, Ar-Ar dating, whole-rock geochemistry, stable isotopes (O, H, C, and B), and mineral chemistry, the sources of the emerald-forming fluids and chromophores have been assessed; the results clearly show that the ca. 100 Ma (Ar-Ar muscovite) Lened emerald occurrence is a Type I (igneous) skarn-hosted emerald deposit related to the proximal ca. 100 Ma (Ar-Ar biotite) Lened pluton. Beryllium and other incompatible elements (i.e., W, Sn, Li, B, and F) in the emerald, vein minerals, and surrounding skarn were derived during the terminal stages of crystallization of the proximal Lened pluton. Decarbonation during pyroxene-garnet skarn formation in the host carbonate rocks probably caused local overpressuring and fracturing that allowed ingress of magmatic-derived fluids and formation of quartz-calcite-beryl-scheelite-tourmaline-pyrite veins. Channel-water delta D values in emerald are depleted and range between -87.4 parts per thousand and -62 parts per thousand, similar to other granite-related emerald occurrences. The calculated delta O-18(H2O) for the vein fluid (similar to 10 parts per thousand, vein quartz, 350 degrees C) is compatible with a peraluminous granitic fluid source. The calculated delta O-18(H2O) of granite-derived fluids (similar to 12 parts per thousand; magmatic quartz, 600-700 degrees C) is slightly isotopically heavier than the vein fluid, which can be explained by fractionation during vein crystallization or mixing with O-18-depleted meteoric fluid. The delta B-11 values of accessory dravite in the emerald veins averages -4.9 + 0.3 parts per thousand (1 sigma, n = 10), which is compatible with a magmatic source, and the Al-Fe-Mg composition is that of tourmaline formed in sedimentary environments, with Mg likely derived from metasomatism of local marine carbonates. The vein fluid was largely igneous in origin, but the dominant emerald chromophore V (emerald vein = avg. 1560 ppm V versus 75 ppm Cr) was mobilized by metasomatism of V-rich sedimentary rocks (avg. 2000 ppm V) that underlie the emerald occurrence. Lened is a unique igneous skarn-hosted emerald occurrence that contributes to the understanding of emerald deposits and emerald exploration criteria in Canada and globally.
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