By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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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 gold - 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 —about 1,015,000 ounces from lodes and about 66,000 ounces from placers.AJO DISTRICT
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.
The oldest formation in the vicinity of the mineralized area is the Cardigan Gneiss, of Precambrian (?) age. It is bounded on the east along north-trending Gibson fault by the Concentrator Volcanics of Cretaceous (?) age. The Cornelia Quartz Monzonite of Tertiary age is a stock that occupies much of the northern part of the district and crosscuts the older rocks. The Locomotive Fanglomerate of Tertiary age overlies parts of the eroded surface of Cornelia Quartz Monzonite.
The rocks were faulted several times during Tertiary time (Gilluly, 1956, p. 57-58, 105-106). The ore body consists of chalcopyrite, bornite, and a little pyrite in veinlets and scattered grains in the quartz monzonite. Less abundant minerals are tennantite, sphalerite, molybdenite, magnetite, and specularite. The richest ore occurs where the rock is impregnated with orthoclase.
The oxidized zone ranges from 20 to 190 feet in thickness and its base terminates sharply at a horizontal plane. In this zone are abundant malachite, and small amounts of azurite, cuprite, tenorite, chrysocolla, hematite, and limonite (Gilluly, 1956, p. 2). Considerable supergene chalcocite has accumulated just below the oxidized zone.
Gold has not been seen in the sulfide ore. However, the close relationship of the ratios of recovered gold to total gold and recovered copper to total copper led Gilluly (1956, p. 87) to conclude that the gold in the ores is associated with copper sul-fides rather than pyrite.
The oldest formation in the vicinity of the mineralized area is the Cardigan Gneiss, of Precambrian (?) age. It is bounded on the east along north-trending Gibson fault by the Concentrator Volcanics of Cretaceous (?) age. The Cornelia Quartz Monzonite of Tertiary age is a stock that occupies much of the northern part of the district and crosscuts the older rocks. The Locomotive Fanglomerate of Tertiary age overlies parts of the eroded surface of Cornelia Quartz Monzonite.The rocks were faulted several times during Tertiary time (Gilluly, 1956, p. 57-58, 105-106). The ore body consists of chalcopyrite, bornite, and a little pyrite in veinlets and scattered grains in the quartz monzonite. Less abundant minerals are tennantite, sphalerite, molybdenite, magnetite, and specularite. The richest ore occurs where the rock is impregnated with orthoclase.
The oxidized zone ranges from 20 to 190 feet in thickness and its base terminates sharply at a horizontal plane. In this zone are abundant malachite, and small amounts of azurite, cuprite, tenorite, chrysocolla, hematite, and limonite (Gilluly, 1956, p. 2). Considerable supergene chalcocite has accumulated just below the oxidized zone.
Gold has not been seen in the sulfide ore. However, the close relationship of the ratios of recovered gold to total gold and recovered copper to total copper led Gilluly (1956, p. 87) to conclude that the gold in the ores is associated with copper sul-fides rather than pyrite.
GREATERVILLE DISTRICT
The Greaterville district is in southeastern Pima County, about 34 airline miles southeast of Tucson. It is chiefly a placer district, though for many years preceding the Civil War, silver and copper lodes were worked successfully in the Patagonia and Santa Rita Mountains south of the district.
In 1874 silver and lead lode deposits were discovered in Hughes Gulch in the Greaterville district, and later in the year placer gold was found which started a rush, during which most of the richer placers were mined out. By 1886 the district was practically dormant (Hill, 1910b, p. 11-12). From 1900 through 1959 there was only desultory activity and very small production.
According to Hill (1910b, p. 12), the placers yielded about $7 million in gold before 1900; however, Elsing and Heineman (1936, p. 98) estimated the total production was worth $650,000. From 1903 through 1959 only 4,146 ounces of gold was mined in the district.
The placer deposits occupy a triangular area of about 8 square miles on the lower east slope of the Santa Rita Mountains. The richest gravels are those along present stream courses, although placers are also in older gravels on benches and tops of ridges. The source of the gold was probably the auriferous pyritic-quartz veins of nearby Granite Mountain or the veins in Tertiary andesite that once covered the district (Schrader, 1915, p. 161-165).
