Genesis of Emerald-Bearing Quartz Veins Associated With the Lened W-Skarn Mineralization, Northwest Territories, Canada

Abstract

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. 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. 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.