By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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 (WilÂ¬son 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).
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