By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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Yuma County, in the southwest corner of Arizona, ranks fourth among the gold-producing counties of the State. The terrain includes many mountains of the fault-block type that trend north-northwest and are separated by broad desert plains. The bedrock of the mountains consists of schist, gneiss, and granite of Precambrian age, sedimentary and metamorphic rocks of Paleozoic, Mesozoic, and Tertiary age, granite of Tertiary age, and volcanic rocks of Cretaceous to Quaternary age (Wilson and others, 1934, p. 124).
Nine mining districts, mainly in the central and western parts of the county, have had a total output of more than 10,000 ounces of gold each. The mines of Yuma County produced a total of about 771,000 ounces of gold through 1959.
CASTLE, DOME DISTRICT
The Castle Dome district is in south-central Yuma County in the southern Castle Dome Mountains, about 20 to 25 miles north of Wellton.
Organized in 1863, the Castle Dome district has produced about equal amounts of placer and lode gold. The first discoveries were of silver-bearing lead ore; gold placers were found in 1884, and gold-quartz veins, although known for some time, received little attention until 1912 (Wilson, 1933, p. 85, 87; 1952, p. 23). Activity in the district has been sporadic, and from 1942 through 1959 the mines were dormant. Total gold production through 1959 was between 9,500 and 10,500 ounces.
In the Castle Dome district gneiss, schist, and granite, all probably Precambrian in age, are un-conformably overlain by thick-bedded shales and impure cherty limestones of Cretaceous (?) age. These rocks were intruded by numerous dikes of diorite porphyry. Broad areas of the older rocks are capped by volcanic rocks and cut by dikes of rhyolite porphyry (Wilson, 1933, p. 78-81).
The mineral deposits in the district are argentiferous galena-fluorite veins, gold-quartz veins, and some veins that carry copper, gold, and silver. The deposit of the Big Eye mine, one of the major gold producers, occurs in a sheared zone in volcanic rocks. The vein consists of brecciated yellow quartz interlaced with veinlets of calcite. The gold was probably free milling and did not continue in min-able quantities below depths of 30 feet (Wilson, 1933, p. 102-103).
CIENEGA DISTRICT
The Cienega district is in northwestern Yuma County, 5 to 8 miles northeast of Parker.
Some mining was done as early as 1870 (Wilson and others, 1934, p. 126). Gold-copper lodes developed during 1909-20 had small sporadic yields. Intermittent activity continued through 1957.
Nolan (in Hewett and others, 1936, p. 31) estimated that the district produced ore worth $80,000 (chiefly in gold) before 1908, but Elsing and Heineman (1936, p. 104) credited the district with $415,000 (about 20,000 ounces), from 1870 to 1933, most of which must have been mined before 1908 because recorded production from 1908 to 1933 was only 4,271 ounces. Total gold production through 1959 was at least 10,000 ounces.
A thick section of Paleozoic metamorphosed sedimentary rocks, consisting of limestone, shale, and quartzite, is the predominant bedrock in the district. These rocks are cut by intrusive bodies of granite and are overlain locally by basalt. Gold, chrysocolla, malachite, limonite, and specularite occur in brecciated pockets of sedimentary rock along shear zones.
The Dome (Gila City) district is at the north end of the Gila Mountains, about 15 miles east of Yuma.
Discovered in 1858, this placer district attracted a horde of prospectors who worked the rich gravels of Monitor Gulch and other gulches and benches near the newly founded settlement of Gila City, just west of the present town of Dome (Wilson, 1952, p. 18-19). By 1865 the high-grade placers were worked out, but spasmodic activity continued to 1950. Total gold production through 1959 was about 24,765 ounces, the bulk of which was mined before 1865.
ELLSWORTH DISTRICT
The Ellsworth (Harquahala) district is in the Little Harquahala Mountains, 5 to 10 miles south of Salome.
Small placer deposits in Harquahala Gulch were worked in 1886 and 1887, and the lodes of the Bonanza and Golden Eagle mines, from which most of the gold of the district has been mined, were found in 1888. The period of greatest activity was from 1891 to 1897, after which the ore bodies were considered to be worked out (Wilson and others, 1934, p. 128). Small production by lessees continued at intervals through 1957. Total gold production of the district through 1959 was about 134,000 ounces; nearly all production was from lodes.
Granite, of probable Precambrian age, is overlain by schist, quartzite, shale, and limestone, some of which may be as young as Carboniferous (Dar-ton, 1925, p. 221-223). Gold-bearing quartz veins are along shear zones in the sedimentary rocks and granite. The oxidized ores contain much free-milling gold; the sulfide ores, mined in more recent years, contain pyrite, galena, and local covellite (Bancroft, 1911, p. 106-114).
FORTUNA DISTRICT
The Fortuna district is on the west flank of the central part of the Gila Mountains, 21 miles southeast of Yuma. Discovered sometime between 1892 and 1895, the Fortuna mine has been the only profitable gold-mining venture in the district. The first period of operation was between 1896 and 1904, during which 123,050 ounces of gold was produced.
The mine was closed in 1904 after several fruitless attempts to locate the continuation of the vein beyond a fault (Wilson, 1933, p. 189-198). There was minor production in 1913, 1926, 1939, and 1940; but apparently no substantial segment of the vein was found. Total gold production of the district through 1959 was 125,332 ounces.
The Gila Mountains in the vicinity of the Fortuna mine are composed of schist and gneiss, of probable Precambrian age, and intrusive granite, amphibolite, and pegmatite and aplite dikes. These rocks are disrupted by a network of faults of several ages. Some of the older faults were mineralized, and the veins were then displaced by later faulting, as at the Fortuna mine.
The Fortuna ore body has been described as a vein that cropped out as two branches that joined at about 500 feet below the surface and as a southwestward-plunging chimney with two branches that joined at depth. The vein consists of coarse-grained quartz with disseminated native gold in little grains or particles. The quartz is locally stained with malachite and is transected by veinlets of hematite (Wilson, 1933, p. 190-194).
KOFA DISTRICT
The Kofa district is in the central part of the county, on the southwestern flank of the Kofa Mountains.
Nearly the entire gold output of this district came from the King of Arizona and the North Star lode mines, discovered in 1896 and 1906 respectively. The King of Arizona mine was operated until 1910 and the North Star until 1911 (Wilson, 1933, p. 109-113). A brief flurry of production occurred in the late 1930's, but during most of 1942-59 the district was idle. The total gold production of the district was about 237,000 ounces.
The principal bedrock exposed in the Kofa Mountains is relatively flat-lying rhyolite and andesite lavas, tuffs, and breccias of Tertiary (?) age, capped by olivine basalt flows. The major gold deposits are in brecciated zones and veins in the andesite. The deposit at the King of Arizona mine consists of anastomosing stringers of quartz and calcite in silicified andesite breccia. The gold occurs in finely divided particles.
Ore in the North Star mine is also in silicified andesite breccia, whose angular fragments are cemented by banded chalcedonic quartz containing fine-grained pyrite and adularia. Gold is present as very fine particles associated with the pyrite (Jones, 1916b, p. 154-159).
LAGUNA DISTRICT
The Laguna district is immediately north of the Gila River and east of the Colorado River, at the south end of the Laguna Mountains.
The important mineral deposits are gold-quartz veins and placers in the Las Flores area in the southeastern part of the Laguna Mountains, placers in the McPhaul area along the southern foot of the mountains, and placers in the Laguna Dam area on the west side of the mountains. Mexican and Indian placer miners were busy in the Las Flores area in the 1860's, and some activity was reported in gold-bearing veins before 1870. Efforts were made in 1884 or 1885 to dredge gravels in the Laguna Dam area, but the dredge was destroyed in a flood.
In the early 1900's small amounts of gold were recovered from potholes in gulches along the Colorado River. More recent operations were desultory, and the district was inactive from 1941 through 1959. Total gold production through 1959 was roughly 10,500 ounces, mostly from placers.
The gold-quartz veins are in zones of sheared and brecciated schist of Precambrian age. Locally the quartz is brecciated. The gold occurs in ragged grains in the quartz and is associated with iron oxides; no sulfides occur in the oxidized ore from the shallow workings (Wilson, 1933, p. 214).
Many arroyos have dissected the area, and placer gold has been found on benches as well as along the arroyo bottoms. In the Laguna Dam area rather coarse gold has been found in potholes as much as 100 feet above the river.
LA PAZ DISTRICT
The La Paz (Weaver) district, in west-central Yuma County, is 9 miles west of Quartzite and 6 miles east of the Colorado River, along the west side of the Dome Mountains.
Gold has come chiefly from placers, but a small amount has been mined from quartz veins. Indians gave a few nuggets to a trapper in 1862 and guided him and his party to the rich gold-bearing gravels. News of this spread quickly, and several hundred miners rushed to the new area. By 1864, however, the higher grade placers were exhausted.
The district was dormant until 1910, when plans were made to mine the gravels by hydraulic methods. These operations were thwarted when the land was included in an Indian reservation. Several later plans for large-scale mining were never carried out (Wilson, 1952, p. 25, 26). Lode deposits, probably discovered at about the same time as the placers, were worked intermittently and yielded about $100,000 worth of gold through about 1933 (Wilson and others, 1934, p. 136).
The placer gold production was estimated at about $2 million (96,800 ounces) in the first 5 years (Browne, 1868, p. 454-455). Total production from placers through 1959 was about 100,000 ounces, and total output from lodes was about 4,000 ounces.
The placers occur along gulches that drain the western slopes of the Dome Rock Mountains. These include the Goodman Arroyo and Arroyo La Paz and their tributaries, among which is Ferrar Gulch which contained the richest and most productive gravels of the district. The gold was recovered entirely by dry washing (Jones, 1916a, p. 49-52).
The rocks in the La Paz district are chiefly Precambrian schist and gneiss which were intruded by granitic rock of probable Mesozoic age (Wilson, 1952, p. 28). The gold occurs in quartz veins in the schist. Some of the veins are parallel to the foliation and others, referred to as gash veins, cut across. Those along the foliation are the larger; the gash veins are too small for exploitation (Jones, 1916a, p. 54-55).
PLOMOSA DISTRICT
The Plomosa district is near the town of Quartzite on La Posa Plain, between the Plomosa Mountains on the east and the Dome Rock Mountains on the west.
This is mainly a placer district; however, gold, copper, and lead have been produced from lode mines. In 1862, prospectors on their way west to the rich La Paz gravels found placers on the east side of the Dome Rock Mountains, at Oro Fino, La Cholla, and Middle Camp. These were worked intermittently until the 1950's, and several unsuccessful attempts were made to mine the gravels on a large scale (Jones, 1916a, p. 52).
Gold, copper, and lead veins were exploited after 1900 but their yield was small (Nolan, in Hewett and others, 1936, p. 33). Total gold production of the district through 1959 was about 24,570 ounces: about 19,400 ounces from placers and about 5,000 ounces from lodes.
The northern Plomosa Mountains, in which the auriferous veins occur, are composed of metamorphosed limestone, shale of probable Cretaceous age, and intrusive granite (Wilson and others, 1934, p. 134-135). The veins are along a fault zone in the shale. Gold occurs in fine flakes with hematite.
