Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Hewton, M.L., Marshall, D.D., Ootes, L., Loughrey, L.E., Creaser, R.A.
• Other Authors: Hanley, Jacob
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 2013
• Subjects: Dee; Diablo; Mackenzie mountains
• Online Access: http://dx.doi.org/10.1139/cjes-2012-0128; http://www.nrcresearchpress.com/doi/full-xml/10.1139/cjes-2012-0128; http://www.nrcresearchpress.com/doi/pdf/10.1139/cjes-2012-0128

Emerald in the Mackenzie Mountains is hosted in extensional quartz–carbonate veins cutting organic-poor Neoproterozoic sandstones and siltstones within the hanging wall of a thrust fault that emplaced these strata above Paleozoic rocks. Isotopic compositions of water extracted from emerald are typical of evolved sedimentary sulphate brines. Fluid inclusion studies indicate two saline fluid populations: a CO 2 –N 2 -bearing, high-salinity brine (20.4–25.8 wt.% NaCl equivalent), and a gas-free, saline brine (7.6–15.3 wt.% NaCl equivalent). Both populations display evidence of post-entrapment volume changes. δ 18 O VSMOW (VSMOW, Vienna standard mean ocean water) values for emerald, quartz, and dolomite yield averages of 17.3‰ (±0.9), 19.6‰ (±1.5), and 18.1‰ (±1.0), respectively. Dolomite δ 13 C VPDB (VPDB, Vienna Pee Dee belemnite) averages –6.8‰ (±1.0). Two pyrite samples returned δ 34 S CDT (CDT, Cañon Diablo troilite) values of 5.1‰ and 11.2‰. Triply concordant mineral equilibration temperatures determined from mineral pair δ 18 O VSMOW equilibration (quartz–emerald, quartz–dolomite, emerald–dolomite) range from 380 to 415 °C. Depth calculations based on mineral pair isotope equilibration and typical geothermal gradient indicate vein formation at 6–11 km depth. A Re–Os isochron age of 345 ± 20 Ma from pyrite indicates that mineralization was contemporaneous with estimated ages of some northern Cordilleran Zn–Pb occurrences. Emerald mineralization resulted from inorganic thermochemical sulphate reduction via the circulation of warm basinal brines through siliciclastic, carbonate, and evaporitic rocks. These brines were driven along deep basement structures and reactivated normal faults during the development of a trans-tensional back-arc basin during the late Devonian to middle Mississippian. The Mountain River emerald occurrence thus represents a variant of the Colombian-type emerald deposit model requiring thermochemical sulphate reduction.