By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Gold deposits are scattered throughout San Bernardino County. They occur in the Slate Range in the northwest, the Whipple Mountains in the southeast, the San Gabriel Mountains in the southwest, and the Clark Mountain area in the northeast. The report of Wright and others (1953, p. 53-86) has been the source of information on the geology and gold deposits of this county.
As early as the 1850's, placer gold was mined at several localities, but lode mines have been the most important sources of gold. Of the many mining districts in the county, the most important are the Dale, Holcomb, and Stedman. Production from 1880 through 1959 was 517,000 ounces; nearly all production was from lode mines. Production has declined rapidly in recent years. Only 102 ounces of gold was produced in 1958, mostly as a byproduct from silver ore in the Rand district. Production for 1959 was combined with other counties.
San Bernardino County is characterized by numerous steep mountain ranges separated by arid basins, many of which are undrained. Exposed in the higher ranges are metasedimentary rocks of Archean and Algonkian age. These rocks are overlain by thick sections of Paleozoic sedimentary rocks that range in thickness from 9,000 feet in the northeastern part of the county to more than 22,000 feet in the north-central part. Mesozoic sedimentary-rocks as much as 4,000 feet thick are found in the northeastern part of the county. Metavolcanic rocks of Triassic or Jurassic age are found in a few localities in the western part of the county. Numerous quartz monzonite stocks and small batholiths of late Mesozoic age intruded the sedimentary rocks, and most of the ore deposits are genetically related to mineralizing solutions that emanated from these plutons. Tertiary volcanic rocks form several mountain ranges in the northeastern, west-central, and south-central parts of the county. Numerous gold and silver deposits are associated with these rocks. Large basins were formed in Tertiary time, and in these, thick deposits of interbedded saline and alluvial sediments accumulated.
The Dale district, in T. 1 S., Rs. 11 and 12 E., about 15 miles southeast of the town of Twenty-nine Palms, began producing gold as early as 1893 from the Brooklyn mine. In the early 1900's production from the Supply and Nightingale mines made the district one of the more prosperous in the county. Production declined after World War II, and the district has been almost dormant in recent years. Total gold production through 1959 was about 63,500 ounces.
Bedrock in the district, as noted in mine descriptions by Wright, Stewart, Gay, and Hazenbush (1953, p. 70-86), consists of quartz diorite, ande-site porphyry, and granodiorite. The gold deposits are in quartz veins cutting these rocks. Gold occurs in the free state with iron and manganese oxides in some mines; in others, the gold is associated with sulfides.
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