Geological Survey Bulletin 1286 Gold Veins Near Great Falls, Maryland

The gold-bearing veins and shear zones transect wallrock structures formed during early Paleozoic regional metamorphism and cut granitic rocks emplaced at least 370 million years ago. The mineralization must, therefore, be of post-Devonian age. The only other structures in that part of the Appalachian Piedmont with similar trends are postmetamorphic dikes and faults that cut Triassic rocks in the nearby Leesburg Basin (Toewe, 1966). Thus, the gold deposits near Great Falls may be of Triassic or post-Triassic age.

FIGURE 8.—Gold associated with galena. A (top), Sheet and wire gold (Au) in quartz associated with galena (g). From 135-foot level, Maryland mine. Specimen from the collection of E. T. Ingalls displayed at the Gold Mine Museum, Potomac, Md. B (bottom), Gold (Au) disseminated in galena (g) and quartz. USNM 104638, labeled "Montgomery Co., Md., donor John W. Langdale, Georgetown University." Probably from Maryland mine. Photograph by David Massey, U.S. Geological Survey.

Although some spectacularly rich pockets of ore have been found in the Maryland mine, the average gold content of the veins is marginal under present economic conditions. The generally low gold content of the veins, the erratic distribution of gold within the veins, the fact that some of the richest parts of the vein system lie on Federal land, and the high value of the remaining lands for commercial development will probably preclude further development of the gold deposits near Great Falls. Nevertheless, examination of the Great Falls vein system has yielded some clues to the age and origin of the deposits, which may be useful in locating and interpreting similar deposits in other parts of the Appalachian Piedmont.