By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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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.
MAMMOTH DISTRICT
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 development 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).
RAY DISTRICT
The Ray (or Mineral Creek) district is in northeastern Pinal County about 17 miles south of Miami. It lies between the Dripping Springs Range to the east and the Tortilla Range to the west. Copper is the major commodity of this district; gold is a byproduct.
The district was organized by silver prospectors, probably before 1873, and the first locations were made about 1880 (Arizona Bureau of Mines, 1938, p. 80-81). The first copper company was organized in 1883, but attempts at exploitation over the next 23 years failed, owing to the generally low grade of the ore.
In 1906 some high-grade copper ore was mined. In 1907 the Ray Consolidated Copper Co. was organized, and extensive surface drilling and underground exploration revealed enormous copper ore bodies which were mined on a large scale in the spring of 1911 (Ransome, 1919, p. 17-19). Ray Consolidated soon became the largest producer in the district. The property continued to be an important source of copper, though ownership was changed to Ray Division of Kennecott Copper Corp.
The Ray district has produced a surprisingly small amount of gold, considering the large production of copper. Total gold production through 1959 was about 35,250 ounces.
The rocks exposed in the Ray district are similar to those of the Globe-Miami district. The oldest rocks are granitic intrusives and Pinal Schist of Precambrian age. Unconformably overlying them are altered sedimentary rocks of the Apache Group and the Troy Quartzite of late Precambrian age. Great sills of diabase were intruded into the Apache Group and the older rocks (A. F. Shride, oral com-mun., 1962).
In the eastern part of the district lower Paleozoic sedimentary rocks are exposed in a few fault blocks. Dikes, sills, and irregular bodies of quartz diorite, quartz monzonite, and granite, of probable early or middle Tertiary age intrude the Precambrian and Paleozoic rocks. Conglomerate and a dacite flow of late Tertiary age and the Gila Conglomerate of Tertiary and Quaternary age discordantly overlap the older rocks (Ransome, 1919, p. 123-126).
The rocks in the eastern part of the district are displaced by a mosaic of normal faults. West of Mineral Creek, which is in general parallel to the Ray fault (the major structural element in the district), Precambrian and Tertiary rocks are exposed and are considerably less faulted than the rocks east of Mineral Creek (Ransome, 1919, p. 127, 128).
The ore deposits consist of disseminated chalcocite of secondary origin associated with primary pyrite and are chiefly in the Pinal Schist and in diabase adjacent to quartz monzonite intrusives and in the intrusives themselves. The primary deposits, which underlie the chalcocite ore, contain pyrite and chalcopyrite. The chalcocite ore is generally overlain by a leached capping of variable thickness which locally is rich in chrysocolla and malachite. The ore bodies are undulate, flat-lying masses of irregular outline and thickness (Ransome, 1919, p. 12).
SUPERIOR DISTRICT
The Superior (Pioneer) district is about 15 miles southwest of Miami and 12 miles northwest of Ray. Most of its gold has been a byproduct from copper ores of the Magma property; however, some gold ore has been mined south of the main copper mines.
The first significant mineral discovery in the Superior district was of nugget silver in 1873 or 1874 at the Silver Queen mine, now known as the Magma mine, and the initial locations were made in 1875. Rich silver ore was mined in the early years and the camp was active until 1893 when a drop in the price of silver halted operations. Several unsuccessful attempts at silver mining were made in later years (Short and others, 1943, p. 59-75, 139-141).
Exploration in the old Silver Queen mine by the newly organized Magma Copper Co. in 1912 revealed large bornite-chalcopyrite ore bodies which effected a rejuvenation of the district that was sustained through 1959. Gold is produced from the copper ores and also from auriferous quartz veins in the old Lake Superior and Arizona workings (Gardner, 1934, p. 1-2).
Prior to 1912 the output of gold from the district was small, probably less than 500 ounces. From 1914 through 1959 the recorded production was 397,700 ounces.
Rocks of the area range in age from Precambrian through Tertiary. The oldest is the Pinal Schist, unconformably overlain by the Apache Group and Troy Quartzite of late Precambrian age. Thick diabase sills, considered to be of Precambrian age, intrude the foregoing rocks (A. F. Shride, oral commun., 1962). An aggregate thickness of about 2,000 feet of Paleozoic strata, predominantly limestone, overlies the Precambrian rocks.The Paleozoic rocks were faulted and invaded by dikes and stocks of quartz monzonite porphyry and quartz diorite of late Mesozoic or Tertiary age. Parts of the district are covered by conglomerate and thick dacitic flows and tuffs of Tertiary age and by conglomerate of Tertiary and Quaternary age. Additional crustal movement involving tilting and faulting occurred during middle and late Tertiary time (Short and others, 1943, p. 12-15). Small plugs, flows, and dikes of basalt were intruded locally during Pliocene or Pleistocene time.
The Magma deposits are a series of disconnected ore shoots in replaced shattered country rock between two east-trending shear zones. The richest ore bodies are found along the Magma fault, where it intersects diabase. The principal ore minerals are pyrite, bornite, chalcopyrite, and enargite, with subordinate tennantite and hypogene chalcocite. In places sphalerite is the predominant sulfide; small amounts of galena accompany the sphalerite. Most of the ore bodies were enriched by supergene copper sulfides (Short and others, 1943, p. 74-78).
A considerable amount of gold ore has been mined from the Lake Superior and Arizona property and lesser amounts from similar gold lodes in the Belmont-Queen Creek area. The gold occurs in small lenticular ore bodies 10 to 20 feet above the base of the Martin Limestone (Devonian) and adjacent to faults. Gold, malachite, and chrysocolla occur in a gangue of iron and manganese oxides and quartz. Silver is associated with the copper minerals and gold (Short and others, 1943, p. 138).
