By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Santa Cruz County is in south-central Arizona along the Mexican boundary. Both lode and placer gold have been produced, but the placer output has been small. From 1900 through 1959, the county produced about 108,200 ounces of gold, mostly from the Oro Blanco district. From 1942 through 1959 gold production was very low.
ORO BLANCO DISTRICT
The Oro Blanco district is in western Santa Cruz County near Ruby, about 32 miles by road northwest of Nogales and about 70 miles by road south-southwest of Tucson. Deposits of gold and silver have attracted the most attention. Some of the gold deposits probably were worked by the Indians and early Spanish explorers. Placers and rich outcrops attracted early American prospectors who made their first locations in 1873 (Wilson and others, 1934, p. 188-189; Milton, 1913, p. 1005). The deposits were successfully exploited through the middle 1880's. Most of the mines were inactive from 1887 to 1893; thereafter mining was intermittent, and production in general was small. Production rose rapidly in 1934 but declined again in the early 1940's. From 1942 through 1959 the district was almost dormant. The gold mined in the district from 1873 through 1957 has been estimated as worth $2,626,000, which is equivalent to about 100,200 ounces (Wilson, 1962, p. 109). About $20,000 worth of placer gold was produced between 1896 and 1904. Production was not recorded for 1957-59.
Not much is known about the geology of the district. According to Wilson (Wilson and others, 1934, p. 189), metamorphosed sandstones, conglomerates, shales, and interlayered volcanic rocks of probable Cretaceous age overlie altered diorite, are intruded by dikes of basic to acid composition, and are complexly faulted.
The deposits are of three types: gold-bearing quartz veins, mineralized shear zones, and mineralized country rock. Pyrite, chalcopyrite, galena, and sphalerite are the common ore minerals, and the tellurides have been reported (Milton, 1913, p. 1006).
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