By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Cochise County, third among the gold-producing counties of Arizona, produced approximately 2,-723,000 ounces of gold from the beginning of mining in the county in about 1879 to the end of 1959. Of this amount, about $24,275,000 (1,174,408 ounces) was a byproduct of copper ores, mainly from the Bisbee district (Wilson and others, 1934, p. 117), and about 950 ounces was from placers. Other districts that have produced more than 10,000 ounces of gold are the Turquoise (Courtland, Gleeson), Dos Cabezas, and Tombstone.
The Bisbee (or Warren) district is in the southeastern Mule Mountains, in the southern part of the county, immediately north of the Mexican border. Although the Bisbee district was the largest gold producer in Arizona in 1959, most of its gold was a byproduct of copper ore.
Though lead carbonate ore was discovered in the district about 1876, there was little activity in the area until after 1880 when rail connections, generally favorable business conditions, and copper prices encouraged prospecting.
The Copper Queen ore body, found in 1877, was developed in 1880. In subsequent years the Copper Queen Mining Co., under the control of Phelps, Dodge, & Co., acquired other properties in the district and became the leading producer (Ransome, 1904, p. 13-15). In 1900 the Calumet and Arizona Co., another major producer, was organized. In 1902 a custom smelter was erected at Douglas, and some of the smaller mining companies, among them the Shattuck and Denn, were started.
At the end of 1931 the two largest companies, the Copper Queen and the Calumet and Arizona, were consolidated as the Phelps Dodge Corp., Copper Queen Branch (J. B. Tenney, in International Geological Congress, 1935, p. 222). In 1947 the Denn mine was sold to the Phelps Dodge Corp., and in 1949 the custom mill of the Shattuck Denn Mining Corp. was closed, leaving the Phelps Dodge Corp. as the only large producer in the district. Phelps Dodge maintained large-scale operations through 1959.
The gold production of the Bisbee district before 1895 was not ascertained. From 1895 through 1929 the district produced 1,110,058 ounces of gold (J. B. Tenney, in International Geological Congress, 1935, p. 222) and from 1930 to 1959, a total of 1,082,765 ounces was produced. Total gold production through 1959 was about 2,193,000 ounces.
The oldest rocks in the Bisbee district are the Pinal Schist and a granite of Precambrian age. These are unconformably overlain by about 5,250 feet of Paleozoic rocks which in turn are unconformably overlain by about 4,750 feet of Cretaceous sedimentary rocks. The Paleozoic rocks consist of about 430 feet of Cambrian quartzite, succeeded by about 4,800 feet of limestones of Cambrian, Devonian, and Carboniferous age.
In pre-Cretaceous time, folding and faulting occurred, and in post-Cretaceous time the rocks were warped and dislocated by thrusts and normal faults of moderate throw (Ransome, 1904, p. 24-73, 106-108). Dikes, sills, and stocks of granite porphyry intrude the Paleozoic rocks, but their relation to the Cretaceous rocks is not clearly revealed (Tenney, in Ransome and others, 1932, p. 46-47; O. N. Rove, in New-house, 1942, p. 211-212).
The main stock of granite porphyry is exposed on Sacramento Hill, the principal and most productive center of mineralization. The stock was intruded in the plane of the east-trending Dividend fault, a dominant structural feature of the district.
Surrounding the intrusive mass is a zone, ranging from a few feet to 1,000 feet in thickness, of contact breccia composed of rounded and angular fragments of the intruded rocks. The border of the porphyry, the contact breccia, and the adjacent limestone are all silicified. Surrounding this silicified zone is a chloritized zone that grades outward into a marbleized zone in the limestone.
The most productive ore zone lies south of the Dividend fault and the ore bodies are arranged in a semicircle around the stock on Sacramento Hill. The copper ore occurs in irregular replacement bodies in the Paleozoic limestones, in the contact breccia, and as disseminated sulfides in the granite porphyry (Tenney, in Ransome and others, 1932, p. 56-57; C. Trischka, in Arizona Bureau of Mines, 1938, p. 38-40). A few deposits are at some distance from Sacramento Hill and are associated with small porphyry bodies (J. B. Tenney, in International Geological Congress, 1935, p. 228).
The oxidized ores consist of a blanket of copper carbonates, cuprite, copper, limonite, and local chalcocite that extends from the surface to depths of several hundred feet, and in one place to more than 2,000 feet (Tenney, in Ransome and others, 1932, p. 61). The zone of secondary sulfide enrichment contains bornite, chalcopyrite, chalcocite, and a little sphalerite and galena.
The disseminated primary ore contains quartz, pyrite, chalcopyrite, and bornite. Most of the gold recovered in current mining operations is very fine grained and is probably allied with the sulfides. Ransome (1904, p. 121) reported concentrations of native gold in the Cretaceous Glance Conglomerate and Morita Formation as well as in Recent placers derived from weathering of these formations.
The main ore deposit at Sacramento Hill forms an inclined blanket which is enriched toward the bottom, where the contact with the sparsely mineralized, sericitized porphyry is sharp (J. B. Tenney, in International Geological Congress, 1935, p. 228).
DOS CABEZAS DISTRICT
The Dos Cabezas district is 18 miles southeast of Wilcox in the Dos Cabezas Mountains.
Gold deposits discovered before the Civil War were worked intermittently after the 1870's and yielded about $182,000 (8,835 ounces) through 1932 (Wilson and others, 1934, p. 117). The first workings were probably in the Teviston placers on the north side of the mountains, though most of the gold came from lodes rich in copper, silver, and lead near the village of Dos Cabezas.
The district was most active during 1914-20 and 1931-36. No production was reported for 1956-59. Total gold production was at least 15,000 ounces.
In the Dos Cabezas Mountains a complexly folded and faulted section of Precambrian granite and gneiss and Paleozoic and Cretaceous sedimentary rocks is cut by granitic intrusives of Late Cretaceous or Tertiary age (Cooper and others, 1959). The ore deposits occur in veins in the Precambrian granite and Cretaceous sedimentary rocks and are apparently related to rhyolite porphyry and diabase dikes of Tertiary age.
Vein minerals are galena, pyrite, sphalerite, and chalcopyrite in a gangue of coarse-textured, white to grayish-white quartz. Most of the gold is in the galena.
The Tombstone district, about 20 miles northwest of Bisbee in the Tombstone Hills, includes a group of low scattered mountains that extend northwestward from the Mule Mountains. Ores rich in silver were discovered in the Tombstone district in 1877, and the mines and camp developed rapidly.
Tombstone produced more than $5 million worth of ore per year in 1881 and 1882, but by 1886 many of the larger ore bodies were either mined out or mined to water level and production decreased sharply, although the district was a steady producer through 1953. During 1879-86 the yield of silver, gold, and lead ore was valued at about $19 million (Butler and others, 1938, p. 38).
By 1900 many of the properties had been combined under one ownership, and an attempt was made to develop the deposits below water level, but this did not prove profitable and was abandoned in 1911 (Butler and others, 1938, p. 38-48). Production was stimulated during World War I and by the increased price of gold in 1934, but from 1948 through 1959 the district was unimportant.
Total gold production through 1959 was about 271,200 ounces, most of which was mined from 1879 through 1932 (Wilson and others, 1934, p. 122).
The oldest rocks in the district are scattered patches of Precambrian Pinal Schist and of albite granite, which are overlain by Paleozoic sedimentary rocks that include the Bolsa Quartzite and Abrigo Limestone of Cambrian age, the Martin Limestone of Devonian age, the Escabrosa and Horquilla Limestones of Mississippian and Pennsyl-vanian ages respectively, the Earp Formation of Pennsylvanian and Permian age, and the Colina Limestone and Epitaph Dolomite of Permian age. These rocks were folded and faulted in post-Paleozoic pre-Cretaceous time, and then the Bisbee Formation of Cretaceous age was deposited.
At the end of Cretaceous time the rocks were cut by thrust faults that trend east and northwest (Gilluly, 1956, p. 128-132). The Uncle Sam Porphyry of early Tertiary age was injected along a thrust, and slightly later the Schieffelin Granodiorite of probable early or middle Tertiary age (Gilluly, 1956, p. 104) intruded the area. Patches of volcanics of Miocene age are exposed to the east of Tombstone. In Pliocene time the rocks were again faulted, this time by great normal faults that are responsible for the present major topographical features (Gilluly, 1956, p. 158-160).
The ore deposits are associated with dikes that are believed to be related to the Schieffelin Granodiorite (Butler and others, 1938, p. 26-28). Ore occurs as replacement bodies in limestones and porphyry, and as fissure fillings. The oxidized ores contain hematite, limonite, cerussite, horn silver, gold and locally abundant argentiferous galena, sphalerite, pyrite, alabandite, malachite, chrysocolla, psilomelane, and wulfenite. Most of the gold occurs as native gold in very fine particles (Wilson and others, 1934, p. 123-124).
The Turquoise (Courtland, Gleeson) mining district lies on the east side of the Dragoon Mountains, about 14 miles due east of Tombstone and about 18 miles north-northeast of Bisbee. During the 1880's mines near Gleeson produced oxidized ore rich in gold, silver, lead, and copper, and in 1901 mining of copper deposits near Courtland was started.
Mixed oxide-sulfide ore was mined on a large scale from 1912 through 1918, but thereafter activity declined and remained at a low level through 1955. The district was idle from 1956 through 1959. Early gold production figures were not ascertained, but from 1908 through 1955 the district produced about 70,000 ounces.
The northwest-trending Dragoon Mountains are composed primarily of contorted and faulted Paleozoic sedimentary rocks and intrusive masses of monzonitic and granitic rocks of Triassic or Jurassic and Cretaceous or Tertiary age. The Paleozoic formations are the Bolsa Quartzite and Abrigo Limestone of Cambrian age, the Escabrosa Limestone of Mississippian age, the Horquilla Limestones of Pennsylvanian age, the Earp Formation of Late Pennsylvanian and Permian age, and the Colina Limestone and Epitaph Dolomite of Permian age (Gilluly, 1956, p. 14-49).
In the interval between the end of the Paleozoic and the beginning of the Cretaceous the rocks were deformed and intruded by masses of Gleeson Quartz Monzonite, Copper Belle Monzonite Porphyry, and Turquoise Granite, all of Triassic or Jurassic age. The Sugarloaf Quartz Latite was probably intruded at the end of Cretaceous time. In early Tertiary time the rocks were displaced by strong northwest-trending thrust faults, and in Pliocene time normal faulting occurred which formed the major topographic features of the present (Gilluly, 1956, p. 159, 160).
The ore bodies are pyritic replacement deposits in limestone, shale, and porphyry along thrust faults. Some of the deposits are oxidized and consist of masses of iron and copper oxides containing cavities lined with chrysocolla, malachite, and azurite. The unoxidized deposits are mainly pyrite and chalcopyrite with local accumulations of bornite, sphalerite, and galena (Ransome, 1913). The gold occurs as very finely divided particles in all the ores; in the oxidized deposits some gold is contained in cerargyrite (Wilson, 1927, p. 39, 50).