The principal country rocks in the Great Falls area are interlayered mica schist and metamorphosed graywacke cut by dikes, sills, and small irregular plugs of light-colored granite and, locally, by dikes of lamprophyre. The schist and graywacke are part of the Wissahickon Formation, a thick sequence of metamorphosed sedimentary rocks that crops out over wide areas in the Piedmont plateau in Virginia, Maryland, and Pennsylvania, including much of western Montgomery County, Md. (Cloos and Cooke, 1953). Recent studies indicate that the Wissahickon probably is of late Precambrian age and that it was metamorphosed and invaded by granitic rocks during the early Paleozoic (Hopson, 1964).
Outcrops of the country rocks are generally sparse in the immediate vicinity of the gold deposits, but enough exposures are present to show that the rocks are identical with those spectacularly exposed a few thousand feet to the south along the Potomac River gorge and described in detail by Cloos and Anderson (1950), Fisher (1963), Reed and Jolly (1963), and Hopson (1964).
The schist and graywacke are intimately interlayered in all proportions, but schist is the predominant rock type in most of the area. Where fresh, it is a lustrous medium- to fine-grained bluish- or greenish-gray rock composed predominantly of quartz, plagioclase, muscovite, biotite, and chlorite. Locally, it contains relict grains of sillimanite, kyanite, garnet, and potassic feldspar that crystallized during the climax of early Paleozoic metamorphism; in many places, however, these minerals have been largely converted to chlorite and fine-grained white mica that formed during later shearing and lower temperature metamorphism. Much of the schist is crisscrossed by light-colored seams and veinlets of quartz and feldspar (albite) that locally form as much as 50 percent of the rock and give it a "shredded" appearance. Similar veined schist along the Potomac River gorge was interpreted by Reed and Jolly (1963) to be the result of partial melting of the schist, but Hopson (1964) has shown that the veinlets were more likely formed by segregation of the quartz and feldspar during metamorphism, without fusion.
The graywacke, a medium- to fine-grained light-bluish-gray massive rock, is composed principally of quartz and plagioclase feldspar and contains lesser amounts of muscovite and biotite. Beds range in thickness from a few inches to as much as 3 feet; many are finely laminated, and a few have graded bedding.
The schist and graywacke have been complexly folded, but because of the scarcity of outcrops, the pattern of the folds cannot be worked out in detail in the area of the gold-bearing veins. Mapping along the Potomac River gorge indicates that the folds of the principal set are nearly isoclinal and have axial planes that strike N. 10°-20° E. and are vertical or dip steeply east (pl. 2). The fold axes plunge gently to moderately northeast or southwest. Axes of the major folds are spaced 2,000-4,000 feet apart, and the amplitude of the folds probably is about 3,000 feet. Attitudes of structures in the scattered outcrops of country rocks near the gold veins (pl. 2; fig. 6A) are consistent with that fold pattern. Bedding in graywacke and foliation in schist generally strike a few degrees east of north and are vertical or dip steeply east or west. Commonly, the schist has two subparallel foliations. The older, which probably is parallel to bedding, is folded into minute isoclines, many of which have sheared apart and remain only as rootless fold hinges. The later foliation parallels the limbs of these microfolds but transects their noses; it probably is parallel to the axial planes of the major isoclinal folds. Axes of the microfolds and intersection of the two foliations in the schist produce a lineation in the schist which plunges gently to moderately south and probably is parallel to the axes of the major folds. Small crenulations in micaceous partings in the graywacke have a similar orientation. A second lineation (fig. 6A) in the schist is marked by steeply plunging mineral streaks, grooves, and crenulations on the foliation planes. This lineation is apparently younger than that parallel to fold axes and was probably produced during late shearing and retrogressive metamorphism (Reed and Jolly, 1963).
The granitic rock that cuts the schist and graywacke is identical with the Bear Island Granite of Hopson (1964). It is a fine-grained white rock that ranges in composition from quartz monzonite to quartz diorite and contains small amounts of muscovite and, locally, of biotite. The granite is particularly susceptible to weathering and is generally exposed only in the underground workings. Granite dikes and sills are widespread throughout the area, and some large bodies are several hundred feet in diameter, but lack of exposures makes it virtually impossible to map the distribution of the granite in detail.
The granite was emplaced after the enclosing schist and graywacke were folded and metamorphosed, but before the latest episode of shearing and low-temperature metamorphism. Radiometric age determinations summarized by Hopson (1964) suggest that regional metamorphism in that part of the Piedmont reached a climax in Ordovician time (about 440 million years ago) and that the youngest granites, including the granite in the Great Falls area, were emplaced no later than 370 million years ago (Devonian). No firm date can be placed on the late shearing and low-temperature alteration, but a few scattered age determinations suggest that they must have occurred before 300 million years ago (Pennsylvanian). This shearing and alteration predated emplacement of the gold-bearing veins.
A group of narrow, closely spaced lamprophyre dikes is exposed along the Potomac River near the northern end of Bear Island (pl. 2). The dikes range in thickness from a few inches to 2 feet, and two to five dikes occur in a zone about 25 feet wide. The dikes are composed of a fine-grained dark-green nonfoliated rock that contains phenocrysts of fresh biotite as much as 5 mm in diameter. The matrix consists principally of biotite, actinolite, albite, epidote, and calcite. These dikes follow cross joints normal to axes of the major folds. They are completely undeformed, but are offset by a fault which is parallel with, and probably related to, the gold-bearing veins and shear zones. The lamprophyre dikes are exposed only in the cliffs at the edge of the river and for a short distance on the western shore, but a distinct magnetic low associated with them has been traced for several hundred feet to the east (pl. 2). The age of the dikes is uncertain. They are clearly post-tectonic and may be as old as Ordovician or as young as Triassic.1
Last Updated: 28-Mar-2006