Timothy R. Stokes

Gold-bearing quartz-breccia zones at Gordon Lake, near Yellowknife, N.W.T., constitute an important and new type of metaturbidite-hosted gold deposit in the region. The mineralized quartz breccias are stratabound, restricted to carbon-rich siltstones of the Archean Burwash Formation, and occur within the hinge domain of a subvertical regional refold. Structural analysis indicates that some quartz veining developed before peak metamorphism and formation of a regional cleavage (S3), but that more significant quartz veining, brecciation, and gold mineralization occurred later (late-D3 to syn-D4), concurrent with formation of a crenulation cleavage (S4) and counter-clockwise rotation of the east limb of the refold. Microfractures within the quartz-breccias have abundant fluid inclusions of high salinity (~25 wt% NaCl-CaCl₂) and hydrocarbon concentration (e.g. CH₄ and C₂H₂). Gold in the quartz breccias occurs with arsenopyrite, pyrite, and pyrrhotite, and was probably deposited at 450 + 40oC and a depth of 7-10 km. Associated hydrothermal alteration enriched the siltstone host in Au, As, Sb, (Br), S, K, Ca, and Na. The high C and low SiO₂ content of the host siltstone is probably a primary feature of the metasediments rather than a product of alteration. 40Ar/39Ar dating of a metamorphic hornblende and a muscovite from a post-metamorphic pluton constrain the time of (D3) peak metamorphism to approximately 2,600 Ma. A plateau age of 2,573 + 9 Ma for a biotite from the largest quartz-breccia zone gives a minimum age for mineralization, consistent with the late metamorphic timing of the brecciation event. The data are consistent with a genetic model that derives the gold and the fluids from the arc-related volcaniclastic metaturbidite pile, transports the gold in (oxidized?) chloride-rich fluids, and deposits gold (together with quartz and sulphides) in hydraulically induced brittle fractures within the carbon-rich siltstones.

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Supervisor: Marcos Zentilli