By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Pinal County, in south-central Arizona, is characterized by broad alluvial plains and scattered mountain ranges, which are composed of Precambrian schist and granite unconformably overlain by younger Precambrian and Paleozoic sedimentary rocks and by Tertiary volcanic rocks. Dikes, irregular bodies, and stocks of granitoid rocks and rhyolite of Cretaceous and Tertiary ages have intruded the Paleozoic and older rocks. Large areas are covered by sedimentary rocks of Cenozoic age.
The principal mining districts from which gold is produced are the Mammoth, Ray, and Superior. Most of the gold is a byproduct of copper ores, although a small amount has come from placers. Total gold production from 1858 through 1959 was about 893,350 ounces.
The Mammoth (or Old Hat) district is in southeastern Pinal County on the east flank of the Black Hills, about 50 miles northeast of Tucson.
The history of mining in the district is focused on the development of two mines - the Mammoth which produced mainly gold and, for a short time, molybdenum and the San Manuel which is in a disseminated copper deposit. The first claims were located in the district in 1879. The Mammoth mine was operating on a large scale by 1888, and continued to be active until 1901, when the workings caved. Demand for molybdenum during World War I created new interest in Mammoth because of the wulfenite content of the ores that previously had been mined for gold alone. For a few years almost the entire molybdenum output of the United States came from this area. Between the end of World War I and 1934 the district was practically dormant (Peterson, 1938, p. 25-30). The increase in the price of gold rejuvenated the district from 1934 through 1943. Production of the Mammoth mine declined after 1944, but the important developÂ¬ment of the great San Manuel copper deposit in 1943 assured the district a prosperous future.
Total gold production of the district through 1959 was roughly 403,000 ounces, of which about 40,000 ounces was a byproduct of the San Manuel copper ores.The Mammoth district is underlain by the Oracle Granite (quartz monzonite) of Precambrian age, which is cut by dikes and irregular bodies of monzonite porphyry, diabase, and rhyolite of late Mesozoic to Tertiary age. Much of the area is covered by the Gila Conglomerate of Tertiary and Quaternary age, which unconformably overlaps the older rocks. All the rocks are cut by strong northwest-trending faults, the most prominent of which is the San Manuel fault (Schwartz, 1953, p. 7-16). The vein deposits are along faults and brecciated zones in rhyolite and Precambrian quartz monzonite. The veinfillings consist of quartz and calcite with sphalerite, galena, and a little chalcopyrite, chalcocite, and pyrite. Wulfenite, vanadinite, chrysocolla, cerus-site, malachite, smithsonite, and hematite are fairly common in the oxidized ore bodies. Native gold is associated with quartz and coats breccia fragments in the hypogene deposits (Peterson, 1938, p. 30-38). The San Manuel ore body consists of chalcopyrite and pyrite disseminated in quartz monzonite, monzonite porphyry, and diabase. The ores have been oxidized to variable depths, and in places zones of supergene enrichment are at the base of the oxide zone (Schwartz, 1953, p. 46-55).
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