The Eureka (Bagdad) district is in western Yavapai County, 42 miles west of Prescott. Most of the mines are near Bagdad in the southwestern part of the district.
Although the district is noted mainly for copper, its deposits were mined originally for silver, gold, and lead. The first claims were located in 1880, and mining began in 1887. Until 1917 most production was from ores rich in gold and silver, with subordinate lead and zinc, from the Hillside mine. Copper minerals were known in the district as early as 1882; however, sporadic exploration through the early 1900's failed to disclose any significant copper ore bodies until 1929 when the Bagdad mine began operations. Gold and silver production from the Hillside mine and several smaller properties continued until 1942, when the Hillside mine was closed. Meanwhile the Bagdad mine expanded due to the demand for copper during World War II. Large-scale activity continued after the war. The Hillside mine was reopened during 1948-51; open-pit mining increased the Bagdad mine production after 1947; and other properties were developed to mine tungsten and zinc. Copper output at the Bagdad mine continued to be significant through the 1950's, and in 1959 it was the largest copper producer in the county.
Total gold output of the district from 1887 through 1951 was 59,787 ounces, of which 58,748 ounces is attributed to the Hillside mine (Anderson and others, 1956, p. 46, 84). From 1952 through 1959 the district produced only 179 ounces of gold. The copper ores at the Bagdad mine yielded insignificant amounts of gold.
Most of the bedrock in the Eureka district is of Precambrian age and consists of metamorphosed volcanic and tuffaceous sedimentary rocks and intrusive masses of rhyolite, gabbro, anorthosite, quartz diorite, diabase, alaskite, granodiorite, and granite. Rhyolite tuff of Cretaceous (?) or Tertiary (?) age unconformably overlies the Precambrian rocks in the southwestern corner of the area, and small stocks, plugs, and dikes of quartz monzonite, quartz monzonite porphyry, and diorite porphyry, slightly younger than the rhyolite tuff, are scattered throughout the older terrain. A thick section of Tertiary and Quaternary clastic sedimentary rocks intercalated with basalt and rhyolite flows and tuffs caps the mesas and overlaps the older rocks (Anderson and others, 1956, p. 6-29). The Precambrian rocks show effects of folding, dynamic and thermal metamorphism, and several periods of faulting. Faulting continued during at least three periods in post-Precambrian time, when some of the major faults related to the mineral deposits were formed (Anderson and others, 1956, p. 29-39).
The gold-silver-zinc-lead deposit at the Hillside mine is a fissure vein in the Hillside fault, which trends N. 10 W. to N. 25 E. and dips steeply to the west in the mine vicinity. The mineralization was related to the quartz monzonite intrusions and occurred during Cretaceous or early Tertiary time. Postmineral faulting, resulting in gaps and overlaps of ore, is a factor to be considered in exploiting these deposits. The hypogene vein minerals are pyrite, arsenopyrite, galena, sphalerite, argentite, chalcopyrite, freibergite, and tetrahedrite in a gangue of quartz. Most of the gold is associated with pyrite and arsenopyrite. The upper part of the vein is oxidized to limonite-stained quartz and variable amounts of gold, silver, cerargyrite, cerus-site, malachite, chalcanthite, and goslarite (Anderson and others, 1956, p. 77-79).
HASSAYAMPA-GROOM CREEK DISTRICT
The Hassayampa-Groom Creek district is on the western slopes of the Bradshaw Mountains, 6 miles south of Prescott.
Gold placers were discovered in 1864 along the Hassayampa River, and shortly afterward many quartz veins were found. Considerable gold and silver was extracted from the shallow oxidized parts of these veins, and after 1895 the primary sulfide ore was mined for gold, silver, copper, lead, and zinc (Wilson and others, 1934, p. 41). The placers were worked most intensively between 1885 and 1890; thereafter, operations were carried out on a small scale (Wilson, 1952, p. 52). From 1953 through 1959 the district produced only a few ounces of gold per year from lodes and placers.
Total gold production through 1959 was about 127,000 ounces - 18,700 ounces from placers and 108,300 ounces from lodes. Schists of the Yavapai Series and the Bradshaw Granite, the oldest rocks of the district, were intruded by several small masses of diorite and granodiorite and dikes of rhyo-lite porphyry, all of Precambrian age (Lindgren, 1926, p. 114-115, pi. 2).
Most of the ore deposits are in fissure veins in the schist, a few are in the granite and diorite. The ore consists of pyrite, arsenopyrite, sphalerite, galena, chalcopyrite, and local tetrahedrite, in a gangue of quartz and a little carbonate. Gold is associated with quartz and sulfides, and in some deposits, with specularite. Some veins are of Precambrian age, others are of probable Tertiary age (Lindgren, 1926, p. 114-126).
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