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Adit Discharge Summary for the Elkhorn and Charter Oak Mines, MT

The Charter Oak Mine

The Charter Oak Mine and nearby mill (T9N-R7W, 36CCB) are on lands administered by the Helena National Forest along the Little Blackfoot River near Elliston, MT. This site was identified in 1998 by the Montana Bureau of Mines and Geology and the Forest Service as having several potential environmental impacts (Hargrave and others 1998, figure 1). In particular, the concentration of several dissolved constituents in the lower adit discharge exceeded secondary and primary drinking water standards (that protect people) as well as acute and chronic aquatic life criteria (that protect wildlife).

Figure 1—This is a typical adit that has not collapsed. The opening
is partially submerged and is discharging about 10 gallons per minute
of acid mine drainage. Dissolved arsenic and metals
make the water very toxic.

The Charter Oak Mine operated intermittently from 1916 to 1966, producing 9,127 tons of ore that yielded 382 ounces of gold, 39,146 ounces of silver, 10,041 pounds of copper, 672,046 pounds of lead and 168,270 pounds of zinc (McClernan 1976). The dominant rock type at the Charter Oak Mine is an andesite that hosts two main veins. One vein, at least, is vertical and the other’s orientation is unknown (Pardee and Schrader 1933). Mineralization consists principally of argentiferous galena and boulangerite (a lead-antimony sulfide) (Pardee and Schrader 1933) along with arsenopyrite, sphalerite, and plumbojarosite in quartz. Robertson (1956) describes the Charter Oak Mine as the Hopkins Mine. McClernan (1976) provides a 1-inch to 50-foot-scale underground mine map. The map shows 300 feet of a northwest-trending drift splitting as it encounters the northeast-striking, 88 degrees east southeast-dipping fault-controlled vein. Elliot and others (1992) say that five adits were driven along shear zones in Cretaceous andesite. Pardee and Schrader (1933) report that some of the ore here was treated at a small mill onsite, while some was shipped directly to the smelter. McClernan (1969) refers to a “Hopkins Mill” in the vicinity of the Charter Oak Mine.

The discharge from the adit at the Charter Oak was monitored with a 90-degree V-notch weir and a chart recorder. The chart recorder was placed on a temporary foundation consisting of plywood and concrete blocks (figures 2a, 2b, and 2c). Because the ground near the portal was unstable, the weir was installed about 50 feet downstream (figure 3). The discharge ranged from about 0.4 gallons per minute to about 10 gallons per minute during the period of record. The greatest discharge occurred in early May during snowmelt and storm events; some of this additional discharge was due to runoff from outside the portal, but most appeared to be from within the portal. The discharge generally declined throughout the rest of the year (figure 4).

Figure 2a—The V-notch weir at the Charter Oak Mine with the
chart recorder and field-parameter recorder.

Figure 2b—The adit is about 50 feet upstream (to the right). The thin
cylinder is the recording device that measures the physical
characteristics of the water. The float is the pan-shaped object
connected by a wire to the chart recorder shown above.

Figure 2c—The float measures the height of water flowing over the
V-notch weir. This height is converted to flow.

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