141 samples. In about 10 percent of the area, the host rocks contain less than 3 ppb Au; this corresponds to normal gold contents in unmineralized greenstone belt igneous rocks (0.6-1.7 ppb Au). The 3-ppb Au contour line encloses a broad area believed to represent an ore-field halo; this threshold value is probably valid for the entire Val d'Or area of northwestern Quebec. The 10-ppb Au contour line envelops all known ore zones and defines ore-zone halos. The ore-zone halo enveloping the Main and East plug orebodies (ca. 4.0 X 10 6 troy oz Au) is extensive (ca. 4 km 2 ), while halos enveloping smaller ore zones (0.05-0.2 X 10 6 troy oz Au) are smaller (ca. 0.5 km 2 . Ore zone halos do not constitute exact drilling targets; other geophysical or geochemical methods are required to define the structure and attitude of gold-bearing veins within the halos.--Modified journal abstract.