West side of the Plomosa Mountains, Tps. 3 and 4 N., R. 18 W.; T: 2 N., R. 17 W.
Quartzsite 15-minute quadrangle.
Miller, 1970, Geologic map of the Quartzsite quadrangle, Yuma County, Arizona, scale 1:62,500.
From Quartzsite, it is about 5 miles southeast on dirt roads to placer area.
The Plomosa placers are found at the western edge of the Plomosa Mountains in and near Plomosa Wash south of Scaddan Mountain (Tps. 3 and 4 N., R. 18 W.). The extent of the placers in the Plomosa district is not known, but some reports indicate that gold-bearing gravels are found for about 3-4 miles along the western edge of the mountains. Most large-scale placer activity was apparently concentrated in sees. 3 and 4, T. 3 N., R. 18 W., near the old town of Plomosa. Small placers were apparently worked in Cave Creek in the southern part of the range (T. 2 N., R. 17 W.).
The Plomosa placers were worked in the early 1860's about the same time as the La Paz placers. Early production is unknown, but the extensive diggings remaining from the early workers indicate that production was large. The Plomosa placers were actively sampled and mined during the 1910's by companies using large-scale drywashing machines; these operations attracted much attention from mining men at the time, and the resulting literature described the gravels and mining techniques in great detail.
Production records indicate that these operations were not a commercial success. Various reports of the average value of the gravels claim values as high as $20 per yard in placer gold, but more reasonable estimates range between 71 cents to $1.77 per yard. The placers worked by the Yuma Consolidated Co. and the Plomosa Placer Co. during the period 1915-16 were cemented gravels as thick as 140 feet; the gold was concentrated at various horizons in the gravels. The placers were steadily mined by individual drywashers until the 1950's.
Gold-bearing veins occur in the metamorphic rocks in the immediate vicinity of the placers and are considered to be the source of the gold in the gravels.
Allen, 1922: Quotes Heikes and Yale (1913).
Bancroft, 1911: Placer-mining techniques and operations; character of the placer gravels; source.
Heikes and Yale, 1913: Location; character and thickness of placer gravel; gold values per cubic yard; results of sampling gravels; placer-mining operations; information is abstracted from a professional report (unpublished) by John A. Church; production from 1904 to 1912.
Jones, 1915: Quotes Bancroft (1911); placer-mining activity in 1914.
Keiser, 1916: Describes placer-mining operations and techniques used by Yuma Consolidated and Plomosa Placer Cos.
Koschmann and Bergendahl, 1968: History; production.
Maltman, 1917: Placer-mining techniques and operations; character, thickness of average value of placer ground.
Mining and Scientific Press, 1916: Note on placer plants operated by Yuma Consolidated and Plomosa Placer Cos. Includes average value of different gravel horizons in 140-foot-deep hole on Yuma Consolidated land.
Plummer, 1916: Placer-mining techniques and operations by Yuma Consolidated Co. and Plomosa Placer properties; character, thickness, average value of placer gravels; distribution of gold.
Root, 1912: Gold values in gravels.
Wilson, 1961: Repeats information given by Bancroft (1911) and Heikes and Yale (1913); placer-mining activity during the period 1932-33.