| dc.description | Strata-bound, gold-bearing quartz-breccia zones at Gordon Lake, near Yellowknife, N.W.T., occur within carbon-rich siltstone beds of the Archean Burwash Formation, and are located within the hinge domain of a subvertical regional refold. Structural analysis indicates that although some quartz veining developed before peak metamorphism and formation of a regional cleavage (S$\sb3$), more significant quartz veining, brecciation, and gold mineralization occurred later (late-D$\sb3$ to syn-D$\sb4$), concurrent with formation of a crenulation cleavage (S$\sb4$) and counter-clockwise rotation of the east limb of the refold. Microfractures within the quartz-breccias have abundant fluid inclusions of high salinity ($\sim$25 wt% NaCl-CaCl$\sb2$) and hydrocarbon concentration (e.g., CH$\sb4$ and C$\sb2$H$\sb2$). Gold in the quartz breccias occurs with arsenopyrite, pyrite, and pyrrhotite, and was probably emplaced at ca. 450 $\pm$ 40$\sp\circ$C and a depth of 7-12 km. Hydrothermal alteration enriched the siltstone host in Au, As, Sb, (Br), S, K, Ca, and Na. The high C and low SiO$\sb2$ content of the host siltstone is probably a primary feature of the metasediments rather than a product of alteration. $\sp{40}$Ar/$\sp{39}$Ar dating of a metamorphic hornblende and a muscovite from a post-metamorphic pluton constrain the time of (D$\sb3$) peak metamorphism to ca. 2600 Ma. A plateau age of 2573 $\pm$ 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 compatible with a genetic model that derives the gold and the fluids from the arc-related volcaniclastic metaturbidite pile, transports the gold as chloride complexes in a moderately saline (and oxidized?) fluid, and deposits the gold, quartz, and sulphides in hydraulically-induced brittle fractures within the reducing carbon-rich silstones. | en_US |
