Greenlee County Arizona Gold Production
The Clifton-Morenci district is in west-central Greenlee County near the towns of Clifton and Morenci.
The first ore discovery was made in 1872, but early development was hampered by lack of transportation and the activities of hostile Indians. The completion of the Southern Pacific Railroad in 1881 lowered transportation costs sufficiently to permit large-scale mining of the copper ores. The discovery in 1893 of large low-grade copper ores at Copper Mountain at Morenci assured a certain degree of stability and permanence to the future of the district (Lindgren, 1905, p. 33-34).
At first several companies were involved in development and mining, but after several mergers and consolidations, the Phelps Dodge Corp. was the major operator from 1921 through 1959. Total gold production from 1873 through 1959 was about 203,000 ounces; nearly all production was recovered as a byproduct of the copper ores.
The rocks of the district consist of Precambrian granite and schist unconformably overlain by an aggregate thickness of 1,000 feet of limestone, shale, and quartzite which ranges in age from Cambrian to Carboniferous. The Paleozoic rocks are unconformably overlain by shale and sandstone of Cretaceous age. The Cretaceous and older rocks are faulted and intruded by stocks, irregular masses, dikes, and sills of granitic, monzonitic, and dioritic porphyry. The mineral deposits are probably genetically related to these intrusions. The sedimentary rocks and intrusive porphyries are capped by a series of lavas, tuffs, and breccias of Tertiary age. Overlapping all the preceding rocks are extensive deposits of Gila Conglomerate, of Tertiary and Quaternary age.
The ore deposits are of three general types: tabular bodies in limestone or shale near contacts of stocks or dikes, fissure veins, and irregular disseminations in porphyry, quartzite, or other rocks (Lindgren, 1905, p. 97-99). The most important of these are disseminated deposits which have been oxidized and enriched by the supergene copper sulfides. These ore bodies are capped by an oxidized gossan of limonite, secondary quartz, and minor amounts of copper oxides and carbonates. The zone of secondary enrichment, below the oxidized zone, contains abundant chalcocite. The protore consists of pyrite, and small amounts of chalcopyrite and sphalerite (J. B. Tenney, in International Geological Congress, 1935, p. 218-221). Second in importance are the tabular replacement deposits in limestone and shale. Ore bodies are in the oxidized parts of the deposits and consist of masses of malachite, azurite, and cuprite and small amounts of native copper and chalcocite in a gangue of decomposed lime silicate minerals. The fissure vein deposits, which are nearly exhausted, are in fissures in Precambrian granite and Cambrian quartzite closely associated with dikes. The ore minerals are pyrite, chalcopyrite, sphalerite, molybdenite, chalcocite, cuprite, chrysocolla, brochantite, and malachite.
The ores of the district contain relatively insignificant quantities of gold, and the mode of occurrence and mineral associations of the gold are not clearly understood.