United States Geological Survey (USGS) Bulletin 135 “Placer Gold Deposits of Arizona” indicates gold-bearing placer gravels extend for up to four miles along the western edge of the Plomosa Mountains and production was large in the 1860’s. When gold was valued at around $21 per ounce, the value per yard ranged from 71 cents to around two dollars with a high of $20 per yard, which is equivalent to about one ounce per yard. At least 18,000 ounces of gold was produced from the early small-scale placer operations at Plomosa.
Arizona Geological Survey Open-File Report 85-12 “Ore Grades for Metallic Mineral Districts of Arizona” provides list of the average grades of mines in the Southern Plomosa District of La Paz County. The average gold grade was .295 ounces per ton, and the average silver grade was 7.487 ounces per ton.
Arizona Geological Survey Bulletin 168 “Gold Placers and Placering in Arizona” states the coarse gold content per cubic yard in the Plomosa District averaged from ten cents to several dollars when gold was valued at $21 per ounce. This gold content is equivalent to approximately .120 ounces per yard at the upper end. During 1934 to 1949, production from the district was valued at $176,042 from small dry-placer mining operations that typically only excavated to a depth of about 15 feet. Additionally, some pits were sunk into the placer gravels 5 to 50 feet or more, often encountering cemented gravel (often called caliche) with a much richer gold content.
Arizona Bureau of Geology and Mineral Technology Bulletin 192 “Index of Mining Properties in Yuma County, Arizona” states production from the La Paz placers west of the Plomosa property produced over 50,000 ounces gold from small-scale operations in the two year period from 1862 to 1864. Production from the Plomosa district was at least 18,000 ounces gold.
The Delos Toole’s book “Where to Find Arizona Placer Gold” indicates that since 1862 over two million ounces of gold has been produced from the La Posa Plain just west of the Plomosa property. Some of this gold originated from the Plomosa Mountains, which host the property.