Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime
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| Authors: | Hewton, ML; Marshall, DD; Ootes, L; Loughrey, LE; Creaser, RA |
| Year: | 2013 |
| Journal: | Can. J. Earth Sci. 50: 857-871 Article Link (DOI) |
| Title: | Colombian-style emerald mineralization in the northern Canadian Cordillera: integration into a regional Paleozoic fluid flow regime |
| Abstract: | 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 CO2-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. delta O-18(VSMOW) (VSMOW, Vienna standard mean ocean water) values for emerald, quartz, and dolomite yield averages of 17.3 parts per thousand (+/- 0.9), 19.6 parts per thousand (+/- 1.5), and 18.1 parts per thousand (+/- 1.0), respectively. Dolomite delta C-13(VPDB) (VPDB, Vienna Pee Dee belemnite) averages -6.8 parts per thousand (+/- 1.0). Two pyrite samples returned delta S-34(CDT) (CDT, Cafion Diablo troilite) values of 5.1 parts per thousand and 11.2 parts per thousand. Triply concordant mineral equilibration temperatures determined from mineral pair delta O-18(VSMOW) equilibration (quartz-emerald, quartz-dolomite, emerald-dolomite) range from 380 to 415 degrees 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. |
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