By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Pima County, which lies in part along the southern border of Arizona, is a region of broad desert plains and mountain ranges that trend north-northwest. Only two districts have produced more than 10,000 ounces of goldâ€”the Ajo, where considerable amounts have been recovered as a byproduct from copper ores, and the Greaterville, where most of the gold was from placer deposits. Elsing and Heineman (1936, p. 98) credited the Papago district with a production of $250,000 in placer gold before 1933, but this probably is in error, for no other known account cites more than a very small amount. The total gold production of Pima County through 1959 was roughly 1,081,000 ounces â€”about 1,015,000 ounces from lodes and about 66,000 ounces from placers.
The Ajo district is in western Pima County, 125 miles west of Tucson.
Small-scale mining of copper deposits was done by Spaniards and Mexicans as early as 1750, and Indians used the red oxides and green carbonates from the Sierra Del Ajo to paint their bodies. Americans entered the area after the Gadsden Purchase of 1853 and located the Ajo mine. After a boundary dispute with Mexico was settled, numerous attempts at mining were made, but all ended in failure due to high freight costs and lack of water. In 1909 three companies conducted separate exploration programs, none of which was considered encouraging.
The Calumet and Arizona Copper Co. entered the district in 1911 and organized the New Cornelia Copper Co., which found a large tonnage of carbonate ore containing 1 to 2 percent of copper underlain by sulfide ore containing disseminated chalcopyrite and bornite. Drilling later revealed considerable ore on other properties. Experiments to leach and recover copper from the carbonate ore were started in 1912 and were concluded successfully in 1915. By 1917 a 5,000-ton leaching plant was built, permitting large-scale exploitation of the carbonate ores. After the exhaustion of the known reserves of carbonate ores, a 5,000-ton sulfide concentrator was put into operation in 1923, and production from the sulfide ores soon became predominant (Gilluly, 1956, p. 98-100). In 1931 the New Cornelia Copper Co. was merged with the Phelps Dodge Co. which continued to be the sole operator in the district through 1959.
Significant recovery of gold began with the production of copper from the sulfide ore of the New Cornelia mine. Prior to 1924 the district produced only 178 ounces. From 1924 through 1934 about 130,000 ounces was recovered (Gilluly, 1946, p. 101), and from 1935 through 1959 about 860,000 ounces was recovered. The total gold production of the district through 1959 was about 990,000 ounces.
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