David R. Webb

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The existing stratigraphic and structural subdivisions are refined, and temporal constraints indicate two prograde metamorphic events at Yellowknife. Four generations of structures are identified: (1) brittle syn-volcanic, (2) brittle-ductile, late first metamorphism, (3) brittle, post-first metamorphism, pre second metamorphism, and (4) brittle, post second metamorphism, Proterozoic faults. Gold mineralization occurs in P-shears which became dilatant during seismic-aseismic events after a passive ductile state had been achieved, and is reflected at all scales, from hand sample, stope, orebody, ore zone, to mining camp-scales.;Structural studies at Yellowknife, and geochemical analyses of the igneous rocks are consistent with greenstone belt development in a back-arc basin that accreted obliquely onto an older crustal terrain, with emplacement of shoshonitic lamprophyres during late convergence, post-granite. Geochronological work places this framework into; (1) synvolcanic, at 2,683 Ma, (2) syn-I-type batholith, syn first metamorphism at 2,620 Ma, (3) mineralization between 2,620-2,585 Ma, (4) second metamorphism at 2,585 Ma, and (5) Proterozoic dykes.;Ore-related metasomatic alteration is imparted upon structurally and chemically modified volcanic rocks of the Yellowknife Bay Formation. The chemical composition of these rocks, now chlorite schists reveals a fluid history consistent with metasomatic alteration by large volumes of weakly saline, near neutral, CO{dollar}\sb2{dollar}/CH{dollar}\sb4{dollar}-rich fluids at temperatures near 390{dollar}\sp\circ{dollar} {dollar}\pm{dollar} 15{dollar}\sp\circ{dollar}C, and log f{dollar}\sb{lcub}02{rcub}{dollar} near the pyrite-pyrrhotite transition. Gold mineralization was enhanced by fluid-gas unmixing induced by pressure fluctuations, causing periodic excursions in the Eh of the remaining aqueous fluids, resulting in destabilization of soluble gold complexes, changes in mineral phase relationships and chemistries, and shifts in the {dollar}\delta\sp{lcub}13{rcub}{dollar}C of related carbonates. Pressure fluctuations were promoted by structural discontinuities generated by the intersection of type 1 and 2 structures; the former was reactivated during type 3, to generate the present distribution of mineralization. Related metasomatic effects can be detected up to 100 m from ore zones, within macroscopically unaltered chlorite schists. Later, surface-related fluids locally modified the {dollar}\delta\sp{lcub}18{rcub}{dollar}O of less robust minerals. (Abstract shortened by UMI.)



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