Plomosa Mining District, Quartzsite Arizona

The Plomosa Placer Property is located within the Basin and Range Province, which comprises the southern third of the state of Arizona. The region is characterized by linear mountain ranges separated by downthrown, alluvium-filled basins. In southern Arizona, a "belt" of Precambrian "metamorphic core complex" ranges forms a sort of transition zone between the younger, predominantly volcanic desert mountains of the south and the folded and faulted highlands of central Arizona.

The Plomosa Mountains rise up just west of the Bouse Hills to the north and extend for tens of miles southward. The Plomosas form the eastern wall of the La Posa Plain for nearly its entire length. The northern half of the Plomosa Mountains is predominantly composed of Mesozoic sandstone, shale, conglomerate, and limestone. Slightly younger Cretaceous to early-Tertiary sediments crop out along the northernmost point of the range, and small outcrops of ancient Precambrian gneiss also occur in the area.

The alluvial placer gravels on the property consist of schist, granite, and volcanic rocks derived from the Plomosa Mountains to the east. The schist exposures in the Plomosa Mountains contain gold-bearing quartz veins and stringers, and probably were a significant source of the placer gold deposits. Photo 6 shows an outcrop of quartz vein similar to exposures which host gold mines in the region. These occurrences are geologically important because they are the host rocks of numerous prolific vein controlled gold mines in Arizona.

The property contains underlying gold placer gravels located within washes that drain the Plomosa
Mountains, and these drainages obtain alluvial material from the Plomosa Mining District. Photos 7 and 8 show drainages exposing thick abundant deposits of placer sand and gravel derived from the Plomosa Mountains. As discussed above, historic gold mines are located up-drainage from the property. The placer sand and gravel deposits extend for miles west of the Plomosa Mountains (Photos 9 and 10), and the Plomosa property overlies the drainages and washes that receive material derived from weathering of the Plomosas.

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.