Gila County Arizona Gold Production
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.