By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gold was first discovered in San Diego County in 1869 by discharged Confederate soldiers who worked small placers near Wynola. Some time later gold veins were discovered in the Julian and Banner areas which were later consolidated into the Julian district, the most important gold district in the county. Most of the data presented here are from Donnelly's (1934) report on the history, geology, and mines of the Julian district. After 1900 the Julian district had very little activity. A mild rejuvenation in the late 1930's and early 1940's produced about 1,500 ounces, but during 1950-59 the district was idle. The total gold production of San Diego County through 1959 was about 219,800 ounces, mostly from the Julian district. Only about 700 ounces of this is from placer deposits scattered throughout the county.
The Julian district is about 55 miles northeast of San Diego, near the center of the county. The oldest bedrock in the district is quartz-muscovite-biotite schist and quartzite composing the Julian Formation, of Triassic and Jurassic age. These rocks were invaded by three intrusives of Mesozoic age: the Stonewall Granodiorite, Rattlesnake Granite, and Cuyamaca Gabbro and norite of the Cuyamaca. The Stonewall Granodiorite is the oldest; the Cuyamaca basic intrusive is believed to be the youngest.
The most productive gold deposits are lenticular quartz veins in the Julian Schist. These are conformable in strike and dip with the foliation of the schist. Other deposits in the schist are in gold-bearing quartz lenses and V-shaped quartz rolls. The ore mineralogy of all three types is rather simple and uniform. Massive and banded quartz, accompanied by minor amounts of biotite, calcite, and sericite, comprises the gangue. The ore minerals are pyrrho-tite, arsenopyrite, pyrite, and native gold. The gold occurs most commonly in intimate association with pyrrhotite, but the coarser gold is embedded in quartz.
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