By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gila County, in mountainous east-central Arizona, ranks eighth among the gold-producing counties of the State with a total of about 240,500 ounces produced through 1959. Most of the gold has been a byproduct of copper ores mined from the Globe-Miami district; a lesser amount has come from copper ores of the Banner district. Placers have yielded an insignificant amount.
The Banner (Christmas) district lies in the extreme southern tip of Gila County at the southeast end of the Dripping Springs Mountains.
Many of the deposits have been known and worked intermittently since the 1870's, but little ore was shipped before 1900 (Ross, 1925, p. 29). The district is noted for its copper mines from which lead, silver, and gold were produced as byproducts. The Christmas mine, discovered in 1880 and operated intermittently through 1954, is the major mine in the district. Total gold production from 1905 through 1959 was about 26,000 ounces.
Small patches of Precambrian granite are exposed beneath a thick section of the Apache Group of late Precambrian age, Martin Limestone of Devonian age, and Tornado Limestone of Carboniferous age. The area of the Christmas mine is blanketed by sandstone, breccia, andesite, and basalt of Cretaceous age, which are overlain by patches of Tertiary bedded rocks consisting of tuff, conglomerate, basalt, and rhyolite.
The Paleozoic and Cretaceous rocks throughout the district are cut by dikes and small masses of quartz-hornblende diorite and quartz-mica diorite of Cretaceous age. The rocks were slightly folded in post-Pennsylvanian time; more pronounced folding occurred in Late Cretaceous time. This was followed by faulting which continued through much of Tertiary time (Ross, 1925, p. 6-29).
The important deposits of the district are pyritic gold deposits in shear zones and contact metamorphic deposits such as those at the Christmas and Landon-Arizona mines. The pyritic gold deposits are principally in Cretaceous volcanic rocks, whereas the contact metamorphic deposits are mostly in Paleozoic carbonate rocks. Both types are near or adjacent to bodies of quartz-mica diorite.
Pyrite and local chalcopyrite, magnetite, and specularite are the principal minerals of the pyritic deposits. The contact metamorphic deposits contain a variety of minerals, including magnetite, specularite, chalcopyrite, pyrite, sphalerite, galena, fluorite, chalcedony, and lime silicate minerals. In both types the richest ore has been in the oxidized parts (Ross, 1925, p. 32-39).
The Globe-Miami district, in the foothills of the Pinal and Apache Mountains in the southwestern part of Gila County, is noted primarily for its copper deposits which have yielded considerable amounts of gold, silver, and lead.
The discovery of the Globe claim in 1874 marked the first activity in the area, and for a time thereafter interest centered on small silver and gold prospects. In 1882 copper deposits on the Old Dominion and Buffalo veins were mined. Development was considerably stimulated in 1898, when the first railroad reached Globe.
In 1904 development was begun on the large low-grade disseminated copper deposits, which by 1911 were mined on a large scale (Ransome, 1919, p. 19-21). These operations continued with undiminished vigor through 1959 and resulted in an output of copper, lead, silver, gold, and zinc worth more than a billion dollars (Peterson, 1962, p. 81, 82). Total gold production through 1959 was 191,801 ounces.
Lower Precambrian rocks, consisting of the Pinal Schist, Madera Diorite, Ruin Granite, and an unnamed granite, are the oldest rocks exposed in the district. These are overlain by the Apache Group and Troy Quartzite, of late Precambrian age, and are intruded by dikes and sills of diabase of later Precambrian age (A. F. Shride, oral commun., 1962). The Paleozoic System is represented by the Devonian Martin Limestone, the Mississippian Escabrosa Limestone, and the Pennsylvanian Naco Limestone.
Late Cretaceous and early Tertiary time was marked by igneous intrusives including the Solitude Granite, Willow Spring Granodiorite, biotite granodiorite of Gold Gulch, and Lost Gulch Quartz Monzonite. These events were followed in later Tertiary time by faulting, intrusion of porphyry dikes, and then emplacement of the Schultze Granite and of a granite porphyry. Extensive mineralization followed this granitic intrusion.
The Whitetail Conglomerate of Tertiary (?) age and younger volcanic tuffs and dacite flows or welded tuffs (Peterson, 1962, p. 40-41) unconformably overlie all the older rocks. Faulting again occurred, after which the alluvial Gila Conglomerate of Tertiary and Quaternary age was deposited and later basalt flows were extruded over part of the area.
The most important ore deposits of the Globe-Miami district are disseminated copper deposits in the granite porphyry of the Schultze Granite and in the adjacent country rocks. More than 80 percent of the value of metals mined in the district has come from such deposits, of which the major examples are the Miami-Inspiration, Castle Dome, Copper Cities and Cactus deposits. In mineralized areas the rocks are shattered, and the closely spaced fractures are filled with quartz, pyrite, chalcopyrite, and molybdenite.
In areas of more intense mineralization the rocks are argillized and sericitized, and much pyrite has been replaced by chalcocite. Most ore bodies are the result of supergene enrichment in which copper has been leached by ground water from an oxidized zone and redeposited as chalcocite and covellite (Peterson, 1962, p. 82-83). Very small amounts of gold are contained in these ores.
Before 1904 the important deposits of the district were copper-bearing veins of the Old Dominion vein system, in the Globe Hill area. These veins are along faults and fissures that cut Precambrian and Paleozoic sedimentary rocks, and the ore shoots are localized in intervals of favorable host rock, mainly Paleozoic limestone.
The principal hypogene minerals of these deposits are quartz, pyrite, chalcopyrite, bornite, and specular hematite; sphalerite, galena, tetrahedrite, and enargite are locally present in small amounts. These ores were also enriched in copper by supergene processes. Considerable native gold was recovered from the gossan of these ores (Peterson, 1962, p. 69, 97, 98).
Deposits of copper silicates and carbonates formed by meteoric waters are important sources of copper in the district, but no gold has been reported from them.
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