Yuma County Arizona Gold Production
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.