The economic gold quartz veins and lenses at Yellowknife occur within shear zones in steeply dipping greenstones (amphibolites) of Precambrian age. The shear zones consist of chlorite schist that has been highly altered in the vicinity of the quartz veins and lenses. The zones of alteration enveloping the quartz bodies include an adjacent carbonate-sericite zone and an outer chlorite-carbonate zone that grades into the chlorite schist of the shear zone. Throughout the shear zones extensive chlorite-carbonate areas are present and these show no close relationship to the quartz bodies.Chemical analyses of samples taken across the strike and down the dip of the shear zones show that there is a marked loss of SiO 2 in the chlorite schist of the shear zones, when compared with the amphibolite of the wall rocks. This loss increases in the chlorite-carbonate zones and reaches a maximum in the carbonate sericite zones. In one shear zone where the quartz lenses occur above extensive carbonate-chlorite schist portions, a marked decrease in SiO 2 with depth is evident.Graphs of the chemical analyses across the width of the alteration zones indicate that the loss of SiO 2 is compensated for by the addition of CO 2 and H 2 O. The shape of the SiO 2 concentration line, exhibiting a low adjacent to quartz bodies, and increasing in value outward toward the wall rock, indicates that SiO 2 has migrated toward the present site of the quartz bodies.The chemical evidence strongly suggests that CO 2 and H 2 O from mineralizing solutions displaced SiO 2 from the alteration zones and chlorite-carbonate areas of the shear zones and that this SiO 2 migrated toward dilation zones within the shear zones where it was deposited as quartz veins and lenses.