The Seal Lake area covers about 800 square miles in central Labrador. Rocks are Precambrian and consist of the younger Seal Lake group resting unconformably on the Letitia Lake group and granites and anorthosite.
The Seal Lake group consists of metasedimentary rocks, basalt flows, and diabase sills totaling at least 34,200 feet in thickness. Shallow-water conditions prevailed during deposition of most of the succession; the uppermost beds, however, are of terrestrial origin. During sedimentation basalt was extruded over great areas, and diabase intruded the lower and middle and, to a lesser extent, the upper formations.
Pressures directed from south to north folded the rocks into an east-west-striking syncline. A granite mass was thrust northward over the Seal Lake sedimentary rocks, causing overturning of the syncline to the north. Considerable shearing and overturning took place along the contact between the granitic and sedimentary rocks, and the basal beds of the southern limb now dip south under the granite. Thrust faulting has removed considerable thicknesses of some formations of the southern limb.
The formations show a higher degree of metamorphism along the south side of the basin than along the northern limb because of the thrusting. The greenschist facies reached in the rocks of the Seal Lake group on the south limb represents the highest degree of metamorphism.
Most of the 250 small occurrences of copper minerals recorded in the area occur in two of the upper formations, the Salmon Lake and Adeline Island formations, directly associated with basaltic lavas or diabase sills or sedimentary rocks adjacent to them. The mineralization occurs in tension fractures or local shears which appear to be unrelated to the faults of the area. The minerals, in increasing order of abundance, are chalcopyrite, chalcocite, bornite, and native copper, and minor pyrite. Bornite, chalcopyrite, and pyrite are commonly associated with diabase sills, chalcocite and native copper with the flows. No copper deposits of economic importance have been found. The association of copper with only the upper flows and sills in such a thick sequence of mafic and sedimentary rocks indicates that only the final outpourings of magma were copper-rich.
Low-grade niobium-thorium-rare-earth deposits occur in the Letitia Lake group rocks in the southwest corner of the area.