The Lened emerald prospect, Northwest Territories, Canada: insights from fluid inclusions and stable isotopes, with implications for northern Cordilleran emerald

摘要

Vanadium-rich emerald at the Lened occurrence, Northwest Territories, is hosted within a fractured garnet-diopside skarn. The emerald is generally found in quartz-carbonate veins with mm-scale alteration haloes of muscovite and carbonate. The skarn is hosted within the Cambro-Ordovican Rabbitkettle Formation, which overlies black shales of the Devonian Earn Group. Skarn and subsequent quartz-carbonate veins are the result of contact metamorphisin related to the emplacement of the adjacent 93 Ma Lened pluton of the Selwyn Plutonic Suite. Fluid-inclusion studies reveal the presence of two distinct fluids, a CO_2-bearing fluid related to emerald precipitation and a brine limited to subsequent crystallization of quartz. The CO_2-bearing fluid is dominantly a dilute aqueous brine with approximately 4.5 mole percent C O_2 and minor amounts of CH_4, N_2, and H_2S. The pressure of formation is limited by estimates of the maximum lithostat to less than 320 MPa. The temperatures of formation, in the interval 200-610 deg C, are constrained by temperatures of fluid-inclusion homogenization and isochore intersections with the 320 MPa maximum lithostatic pressure. Preliminary stable isotope data from Lened are consistent with isotopic data for fluids originating from other nearby plutons of the Selwyn Plutonic suite, indicating that emerald at Lened may be derived from a magmatic source. Geological constraints are consistent with Lened being a Type-I (igneous-activity-related) occurrence of emerald. Oxygen isotope data, fluid compositions, pressures and temperatures of formation similar to those of the Regal Ridge occurrence in the Yukon suggest that emerald in the northern Cordillera is likely igneous-activity-related, although the Regal Ridge and Lened occurrences do display some characteristics of schist-type occurrences as well.