Geology and uranium deposits of the Caribou area, Boulder County, Colorado

Abstract

Pitchblende was discovered in the Caribou mine, Boulder County. Colo., in 1948 by Consolidated Caribou Silver Mines, incorporated during the reopening of the 1040-level. Since 1948, a small quantity of pitchblende has been produced from the Radium vein during exploratory work that was done under contract with the Atomic Energy Commission. The Caribou mine, primarily a silver mine, develops five veins (Caribou. No Name, Poorman, Sherman and Silver Dollar) that were formerly worked as separate mines. Three other veins, the Radium, Elmer, and Nelson, are exposed in the Caribou mine only at and below the 920-level. The general geology of the Caribou area, known since 1869 as a silver district, has been discussed by Bastin and Hill (1917) and more recently by Lovering and Goddard (1950). In 1937 a detailed study of the petrography of the Caribou stock was made by Smith (1938). In 1948 and 1949, R.U. King (1950) of the U. S., Geological Survey examined the Caribou mine to evaluate the uranium deposit. In 1950, D.M. Sheridan and E.N. Hinrichs of the U. S. Geological Survey made a radioactivity reconnaissance of the mine dumps at Caribou and vicinity. This reconnaissance included examination of most of the accessible workings of the Caribou mine. Other reports on the area are listed in the bibliography. As a result of King’s preliminary examination and the later exploration for uranium on the property, the U.S. Geological Survey in 1951 began a detailed investigation of the Caribou area on behalf of the Division of Raw Materials of the Atomic Energy Commission, the principal purpose of which was to evaluate the uranium occurrences in the area. The study was begun by E. P. Kaiser and W. S., Cavender, who established a triangulation net, using a base line in Caribou Park for horizontal control and a U.S. Geological Survey bench mark on the top of Caribou Hill for vertical control. Upon the assignment of Kaiser to another project in August 1951 the investigation was continued until November 1951, by F.B. Moore and W. S. Cavender. During the study of the district the writers prepared a geologic map at a scale of 1 inch to 100 feet of an area of about one square mile. The mapped area includes most of the mines in the Caribou district and embraces the eastern half of Caribou Hill and adjacent areas to the east and north. In addition, all accessible underground workings were mapped at a scale of 1 inch to 40 feet. The writers wish to acknowledge the cooperation of the staff of the Consolidated Caribou Silver Mines incorporated, who made the Caribou mine available for examination at all times and who furnished maps and suggestions that were of great assistance. Mr. A.E. Blakesley, owner of the Comstock mine, was also most cooperative in making possible the examination of his mine. Thanks are due Dr., E.E. Wahlstrom of the University of Colorado and to the Boulder Daily Camera for the use of their files containing information on the Caribou mine. To S. Lovering of the U.S. Geological Survey made many valuable suggestions on the identification of alteration products in thin section.