REGIONAL AND LOCAL GEOLOGY
The French Corral Mine project is located in a complex zone of Paleozoic and Mesozoic rocks, within the western slope region of the Sierra Nevada Range. Bedrock consists of plutonic rocks that range in composition from diorite to granodiorite as well as metamorphic rocks ranging from slates to amphibolite schist. (MacBoyle 1919)(Wagner 1982)(Saucedo 1992).
A series of gold-bearing paleochannels developed during the early Tertiary period and have been designated as the Ancestral Yuba River system, encompassing portions of Lassen, Plumas, Sierra, Nevada, Placer, and El Dorado Counties within the Sierra Nevada region (Yeend 1974)(Lawler 1995).
The Tertiary age, extrusive volcanic mudflow deposits (designated as the Mehrten Formation) buried the Ancestral Yuba paleochannel system under several hundred feet of andesitic mudflow elsewhere along the western slope of the Sierra Nevada Mountains are not present at this mine site.
Ancestral Yuba Paleochannel
The Ancestral Yuba River paleochannel sediments are divisible into three generalized stratigraphic units: an upper clay unit, a middle sandy unit, and a basal, unsorted cobble, sand and clay unit (Figure 5). The basal unit often contains economic concentrations of placer gold (Lawler 1988). Often, a complex 3-dimensional stratigraphic relationship exists between these units, due to local downcutting and redepositional processes, which significantly influence the creation of placer gold deposits or “pay streaks” within the paleochannel.
The French Corral Mine project is located on the main Ancestral Yuba Channel, downstream from the major southern and northern paleochannel tributaries.
The best sites where the Ancestral Yuba sediments are exposed are located on the north and west hydraulic pit walls of the main pit. The depth of the channel in the unmined portion of the deposit is approximately 60-120 ft (Yost Report - no date). Large diameter granite boulders are exposed at the base of the hydraulic pit and provide a good indication as to the maximum boulder size that will be encountered during subsequent mining of the basal cobble-boulder unit.
ECONOMIC GEOLOGY
Placer Gold Resources
The French Corral Mine project is located on the main stem of the Tertiary age Ancestral Yuba River paleochannel. This gold-bearing paleochannel system has been well documented historically (Averill 1946, Clark 1970, Lindgren 1911, Garside 2005, Haley, 1923, Lawler, 1995, and Whitney 1880. It has produced an estimated 12 million ounces of placer gold over the last +150 year period. San Juan Ridge contains the largest reserves of unmined auriferous Tertiary age paleoplacer deposits in California (Monroe 1929).
The French Corral-Birchville portion of the paleochannel is believed to have produced in excess of 300,000 ounces of placer gold from an estimated 30 million cubic yards of Tertiary gravel. (Haley 1923).
Monroe (1929) reports that hydraulic mining of 4.2 million cubic yards of channel gravels from top to bottom in the Birchville-French Corral District during the period 1865 to 1883 and yielded values of 41.5 cents (@$20/ oz Au). This equates to the recovery of 78,000 ounces with an average gold grade of .012 ounces/ton.