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Investigative Methods for Controlling Groundwater Flow to Underground Mine Workings
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| Figure 10—The Charter Oak Mine in the lower reach of the Little Blackfoot River south of Elliston, MT, is one of several mines and prospects in the area. |
The Charter Oak Mine (figure 10) and nearby mill, T. 9 N., R. 7 W., sec. 36 (tracts CCB), are on land administered by the Helena National Forest along the Little Blackfoot River south of Elliston, MT. Although the Charter Oak Mine’s mill tailings and some waste have been removed and placed in a repository, two small adits arsenic, cadmium, copper, zinc, and other metals to the environment. Methods by which the flow could be reduced, if not eliminated, were considered.
From 1916 to 1966, the mine and mill intermittently produced 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, at least, is vertical; the orientation of the other is unknown (Pardee and Schrader 1933). These veins are referred to as the front and back veins. Mineralization consists principally of argentiferous galena and boulangerite (a lead-antimony sulfide) (Pardee and Schrader 1933) along with arsenopyrite, sphalerite, and plumbojarosite in quartz. McClernan (1975) reports 300 feet of underground workings were accessible in 1968. A northwest-trending drift split as it encountered the northeast-striking, 88 degrees southeast-dipping, fault-controlled vein. Elliot and others (1992) say that five adits were driven along shear zones in Cretaceous andesite. A major northeast-trending fault known as the Dog Creek Fault parallels the Little Blackfoot River and Hat Creek to the southwest (Schmidt and others 1994). The southeast block has only 427 feet (130 meters) of downward displacement. Movement postdates the Elkhorn Mountains volcanics but predates the younger volcanics to the southwest.
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