GOLD-BEARING VEINS AND SHEAR ZONES
The gold deposits occur along an anastomosing system of veins and shear zones that extends from the Chesapeake and Ohio Canal near the east end of Widewater northward to the mouth of Cool Spring Branch, a distance of about 2.5 miles (pl. 2). Most of the veins are confined to a belt about 0.25 mile wide that trends approximately N. 10° W., but some prospecting and perhaps a little mining have been done on the Potomac vein and scattered smaller veins west of the main zone.
The quartz veins range in width from a few inches to as much as 20 feet. They are composed of sugary to vitreous quartz and commonly contain thin septa and irregular blocks of the wallrocks. In many places distinct sheeting in the quartz is defined by close-spaced sericite-coated shear planes parallel to the vein walls. The walls and shear planes generally display a strong, steeply plunging lineation defined by grooves and striae in the quartz and by streaks of fine mica on foliation planes in the wallrocks.
The quartz veins pinch and swell abruptly and commonly pass along strike into zones that consist of clayey fault gouge and altered and silicified schist which contains discontinuous pods and lenses of quartz and abundant disseminated sulfides. Lineation in the shear zones is similar to, and parallel with, lineation on the walls of the quartz veins. Because the shear zones are generally unrecognizable in float and are never exposed in natural outcrops, they can be mapped only where they have been prospected. They probably are much more extensive and continuous than indicated on plate 2.
The gold-bearing veins and shear zones have three principal orientations: (1) N. 5°15° W., with dips ranging between 80° W. and 80° E.; (2) N. 0°15° E., generally with dips ranging from vertical to 70° W.; (3) N. 20° W., with dips ranging from 60° W. to 85° W.
There are no obvious differences in mineralogy or internal structure between veins of these three sets. The first two sets form the mineralized zone that has been most intensively prospected and includes the sites of the only productive mines. The third set includes the Potomac vein and other veins west of the main zone; these veins have been explored in only a few places.
The two sets of northwest-trending veins clearly transect structures in the country rocks (fig. 6A). The north- or northeast-trending set is parallel or subparallel to foliation in the enclosing rocks of the Wissahickon Formation. Striae and grooves on the vein walls have diverse orientations, but most are transverse to regional lineation trends in the country rocks (Cloos and Cooke, 1953; Reed and Jolly, 1963). A statistical diagram (fig. 6B) indicates that lineations in and on the walls of the veins are generally parallel to intersections between the veins, but nowhere can this relation be demonstrated directly.
The amount of movement along the shear zones in the main area of mineralization cannot be readily determined because of lack of markers in the wallrocks. However, the apparent horizontal offset of the lamprophyre dikes along the fault that controls the alinement of the Potomac River southeast of Rocky Islands, and which is probably related to the vein system, is about 80 feet in a right-lateral sense.
Unfortunately, the intersection of the lamprophyre dikes with the fault surface is almost exactly parallel to the grooves and striae which presumably mark the movement direction in the veins and shear zones (fig. 6B). It is therefore impossible to make a reliable determination of the total movement along the fault. On the basis of offset in fold axes, Fisher (1963) estimated that the movement has been about 1,000 feet down on the southwest side of the fault. This estimate is compatible with the dike offset and with the movement direction inferred from grooves and slickensides. These relations suggest that the shear zones mark normal faults with displacements of a few hundred or a few thousand feet.
The principal sulfide in the veins and shear zones is pyrite; it occurs in cubes as much as 1 inch in diameter and as smaller disseminated grains and irregular masses, both in the quartz veins and in the altered rocks in the shear zones. Pyrite is also common in the immediately adjacent wallrocks. Chalcopyrite is locally associated with pyrite in the quartz veins. Galena is common in the richest parts of the vein in the Maryland mine, and some sphalerite and pyrrhotite have been reported (Zodac, 1947; Ingalls, 1960). In many places the veins and shear zones are devoid of sulfides. In float and in most surface exposures the sulfides have been largely removed by weathering, leaving only limonite-stained vugs or limonite pseudomorphs. Many of the vugs are lined with small crystals of secondary quartz. Various types of vuggy vein quartz and silicified schist were referred to by the miners as "honey comb," "gingerbread," and "hickory bark" ore.
Visible gold occurs in small irregular grains and wires in quartz without associated sulfides (fig. 7A) and in pyrite, both in the veins and in the altered schist in shear zones, even where quartz is absent (fig. 7B). Scattered grains are also found in the sulfide-bearing schist adjacent to the quartz veins. Large masses of coarse sheet and wire gold are commonly associated with galena in the Maryland mine (fig. 8). Ingalls (1960) reported that one 40-pound piece of ore from the Maryland mine yielded 125 ounces of gold, and another 30-pound piece yielded 87 ounces. Galena was reported also to have been associated with gold at the Watson mine, but it has not been found elsewhere in the vein system. An unknown, but probably appreciable, proportion of the gold apparently occurs in microscopic or submicroscopic grains, for spectrophotochemical analyses show that vein quartz and altered schist in which no gold is visible contain as much as 5 parts-per-million gold. Weed (1905) reported that a sample of solid pyrite from the 100-foot level of the Maryland mine, which contained no visible gold, assayed 4.25 ounces of gold per ton (149 ppm),and pyrite mill concentrates ran 0.74 ounce of gold per ton (26 ppm).
The distribution of gold in the veins is erratic, making it difficult to estimate the average gold content. Difficulty in blocking out ore shoots was one of the major obstacles to large-scale development of the deposits. Geochemical sampling of surface exposures indicates that in most places the veins and shear zones contain 0.1 to 5 parts-per-million gold; the highest value obtained was 18 parts per million. A. A. Hassan (unpub. report, 1915, kindly made available by E. T. Ingalls) estimated that the average tenor of the Ford vein was about 9.7 parts per million, but he quotes assays which indicate that ore shipped from the Ford mine in 1914 and 1915 had an average gold tenor of about 33 parts per million. An average of about 0.43 ounce of gold per ton (equivalent to 15 ppm) was recovered from ore from the Maryland mine milled between 1936 and 1940, but considerable flour gold and gold locked in sulfides probably was lost. The total production of the mine during those 5 years was 2,570 ounces; of that amount, 212 ounces (or more than 8 percent) came from only 70 pounds of ore (Ingalls, 1960, p. 13).
Very few details of the distribution of the ore shoots can be reconstructed from the presently available information. The only mines from which production has been recorded (the Maryland, Ford, and Watson mines) lie near the intersections of northwest- and (or) northeast-trending veins (pl. 2). Apparently, the richest ore shoots in the Maryland mine plunge steeply northwest (fig. 4) in about the same direction as the grooving and slickensiding in the veins and as the vein intersections. Ingalls (1960, p. 11) reported that most of the gold occurs along the hanging wall, in both the quartz and adjacent country rocks. No information is available on the orientation of the ore shoots in the Ford and Watson mines.
Last Updated: 28-Mar-2006