Regional Report: Globe-Miami District, Gila Co., Arizona, USA

A major Cu-Mn-Au-Ag-Pb mining area located in the northern foothills of the Pinal Mountains and the Globe Hills, about 90 road miles east of Phoenix. It is almost entirely within the Inspiration and Globe quadrangles and comprises the Miami-Inspiration sub-district in its western side and the Globe Hills sub-district on its eastern side. The topography is fairly rugged and the altitude ranges from 3,400 feet at Miami to 5,060 feet on Needle Mountain near the southern edge of the Inspiration quadrangle. The oldest rock of the region is the Pinal Schist, a complex of metamorhosed sedimentary rocks of early Precambrian age. Also of Precambrian age, but much younger than the Pinal Schist, and separated from it by a major unconformity, are rocks of the Apache group. Widespread igneous intrusions occurred during the interval between the Pennsylvanian epoch and the deposition of the Tertiary (?) Whitetail conglomerate. A thick flow of dacite younger than the Whitetail conglomerate covered the entire region. In later Tertiary and Quaternary time the Gila conglomerate was deposited by great coalescing alluvial fans and stream deposits filling valleys and spreading more thinly over much of the higher parts of the region. All of the rocks of the district are cut by a complex pattern of faults, described by Ransome (1903) as "regional brecciation." The deformation of the rocks by faulting appears to have been continuous from Precambrian time until after deposition of the Gila conglomerate, when many of the largest displacements of the rocks occurred. Great volumes of diabase magma was intruded into the earlier rocks probably during Mesozoic time. The diabase forced its way between beds of sedimentary rock as sills and occupied many of the faults. Great blocks of strata, particularly those of Precambrian and Cambrian age, were pushed apart and in places completely enveloped in diabase. Several other intrusions of igneous rocks, ranging from granodorite to quartz monzonite, took place probably during late Mesozoic and early Tertiary time. The latest of these is the mass of Schultze granite, which underlies the southern part of the district, and numerous smaller bodies of granite porphyry which may be offshoots of the main Schultze granite mass. The mineralization of the district is most nearly contemporaneous with the Schultze granite and the granite porphyry and is probably genetically related to them. This district is known mainly for its large disseminated copper deposits and the copper-bearing veins of the Old Dominion system. The lead and zinc deposits of the district are small and are mainly near the outer limits of the mineralized area related to the Old Dominion vein system. The value of past production of Pb-Zn (circa 1950) was probably not in excess of $250,000 (period values). As of 1950 the total past and future production of metals from these deposits was estimated to exceed $1,000,000,000.

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