Geologic description
The upper basin of Fourth of July Creek lies in Cretaceous to Tertiary sandstone, mudstone, and conglomerate derived from erosion of older metamorphic rocks (Dover and Miyaoka, 1988). The conglomerate is auriferous and probably is the source of all or most of the placer gold in Fourth of July Creek (Brooks, 1907). The placers immediately downstream from the upper basin also are underlain by conglomerate and other sedimentary rocks. The bench deposits on the northwest side of the valley are also auriferous but were not mined (Mertie, 1938). Platinum metals and silver are alloyed with the gold; a report of mercury is unverified. The mean of 22 assays of the gold indicates the average fineness is 892 parts Au per thousand, 99 parts Ag per thousand, and 9 parts dross per thousand. In 1942, one gold specimen assayed 0.23 percent platinum and iridium, with a trace of palladium (Mertie, 1938). The first claims along Fourth of July Creek were staked in 1898, and within 10 days most of the creek had been staked (National Park Service, 1990). There were about ten miners left working on the creek in 1904, and six in 1906. Production between 1898 and 1906 was between $25,000 and $30,000 (1906 dollars) (National Park Service, 1990). A hydraulic plant was installed along the creek in 1916 (Brooks, 1918). In 1938, the pay streak was reported to be 400 to 500 feet wide, and the bedrock was overlain by 6 to 10 feet of gravel and 2 to 7 feet of muck. The gold was mainly found on and in the top 2 feet of bedrock (Mertie, 1938). The last year that productive mining was done on the creek was 1951. Mining ended due to high production costs (National Park Service, 1990). |
