Big Four Mine, Leadville, Lake County, Colorado, USAi

Regional Level Types
Big Four Mine	Mine Dump (Inactive)
Leadville	Municipality
Lake County	County
Colorado	State
USA	Country

The Big Four mine is opened by a shaft whose collar is at an altitude of 11,348 feet (old datum). The shaft has been sunk to a depth of 600 feet, and from this point a drill hole was put down for an additional 600 feet.

The records of the shaft are unsatisfactory, as no clear distinction has been drawn between the "transition shales" beneath the White limestone and the various porphyries which have been intruded into them. The geologic section must therefore be inferred largely from the information gained from the surrounding workings.

The workings of the Big Four mine have not at any time been accessible to the writers, and the accompanying description is therefore compiled from the statements furnished by the management.

The collar of the shaft is 20 feet south of the outcrop of the Colorado Prince fault. The fault here dips northward at an angle of 51°. The geologic structure is shown by section B—B', Plate 28. On the south side of the Colorado Prince fault the upper portion of the shaft is in the White limestone or the underlying "transition shales," which are turned upward at a steep angle against the Colorado Prince fault. It is probable that the rocks cut in the upper first 320 feet of the shaft belong mainly in the "transition shales" beneath the White limestone, although they are recorded by the company as porphyry. These shales have been frequently mistaken for porphyry.

The workings of the third level of the Ibex mine, which have been visited, extend northward to the workings of the Big Four mine and clearly indicate the sedimentary nature of the geologic formations cut in the upper levels of this mine. The record of rocks cut by the drill' hole below the shaft is also open to question. The material supposed by the company to be the lower sheet of White porphyry and the quartzite beneath it may prove to be granite, and if so the succession conforms to that on the south side of the Rawlings fault.

The country rocks north of the fault are not cut in any of the workings of the mine, all of which lie within the upturned strata on the footwall side of the fault. The Rawlings fault crosses the shaft at a depth of 190 feet. The dip slip along this fault has not been determined in the Big Four mine but is about 175 feet from the Fannie Rawlings mine. This fault meets the Colorado Prince fault at the bedrock surface immediately west of the Big Four shaft. Levels were run eastward and westward from the Big Four shaft at depths of 190, 246, 306, 385, 455, 524, and 599 feet.

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