Globe Hills, Globe Hills District, Globe-Miami District, Gila Co., Arizona, USA

In all of the veins the principal hypogene minerals are quartz, pyrite, chalcopyrite, bornite, and specularite. Cuprite that is intergrown with specular hematite may also be hypogene. Sphalerite and galena occur locally in very small amounts, and a little tetrahedrite and enargite have been reported in the ores of the old Dominion Mine.

The veins were formed by replacement of breccia and wall rock along faults and fissures that cut upper Precambrian and Paleozoic sedimentary rocks and bodies of diabase intruded into them. Undoubtedly the veins continue downward into the underlying Pinal schist, but the few that have been followed down to the schist were foubnd to be poorly mineralized. Some of the veins, the Old Dominion for example, are along faults that have displacements of several hundred feet, whereas others follow fissures with little or no displacement. The largest and most continuous orebodies are along faults or segments of faults having relatively large displacements.

In general, the ore does not appear to be limited to definate shoots of great vertical extent but rather seems to be localized in areas of the fault zones that show a definate relationship to the formations traversed by the faults; that is, the character and volume of the vein matter differ from place to place, according to the type of wall rock. Except for limestones of the Paleozoic age, the differences are due mainly to the physical characteristics of the rocks that affect the permeability of the fault zones rather than to the chemical character of the rocks.

Very abrupt changes occur where veins pass from one type of rock to another. The Paleozoic limestones were the most readily replaced and commonly contain thick lenses of very rich ore. In some places, flat tabular orebodies extend outward along certain especially favorable beds as far as a hundred feet from the vein fault. Extensive orebodies have been mined between walls of mescal limestone and between walls of the various quartzite and sandstone formations or any combination of these formations. Good orebodies may be found also where diabase forms one wall and any of the sedimentary formations the other; but where both walls are of diabase, the vein fault is likely to be tight and poorly mineralized or entirely barren.

Apparently the veins pinch and swell and range froma single quartz stringer to broad zones of wholly or partly replaced breccia, or lodes comprised of several to many irregular, discontinuous stringers. The character of the vein changes abruptly with changes of wall rocks. Where the vein faults traverse quartzite, the veins are zones of loose, angular breccia in which the vein minerals occur as replacement films or crusts coating the rock fragments and locally, fill the spaces between the fragments. In other places, the vein faults contain only sporadic deposits of quartz, coarse granular pyrite intergrown with some chalcopyrite, and specularite. Specularite is commonly present in otherwise barren parts of the vein faults, and it may continue far beyond the limits of areas characterized by quartz and sulfides.

Most of the copper veins have been affected by supergene enrichment, much that probably occurred while the veins were exposed during the interval of erosion that preceded the eruption of dacite, because rich chalcocite ore and some leached gossan are present under thick cover of dacite and Gila conglomerate.

The copper-bearing veins of the Globe Hills area have yielded less than 20% of the total metal produced from the district, but nevertheless, their contribution amounts to nearly 1 billion pounds of copper and more than $9,000,000 in gold and silver (period values).

References:
Peterson, N.P. (1962), Geology and ore deposits of the Globe-Miami District, Arizona, USGS PP 342: 97-120.