By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Mariposa County, the southernmost of the Mother Lode counties, has had a long and productive mining history. Most of the gold has come from lode mines on the Mother Lode and West Belt and lode and placer mines in the Hornitos district. Quaternary gravels have been productive along the Merced River and near Mormon Bar. Before 1900, unrecorded amounts of gold were mined from Tertiary placers in the Blanchard district in the Jawbone Ridge area and on the ridge between Moore and Jordan Creeks.
Gold production of Mariposa County for 1880-1959 was about 2,144,500 ounces: about 583,500 ounces came from placers and about 1,561,000 ounces came from lodes. Production before 1880 has not been determined.
Gold mining began at an early date in the county. Gravels along Agua Fria and Mariposa Creeks were worked before 1849 and were thoroughly mined out by the hordes of prospectors who overran the area during the gold rush of 1849. By July 1849 a stamp mill was processing ore from the first lode discovery in the county, the Mariposa mine on the Mother Lode (Bowen and Gray, 1957, p. 39, 43).
Lode mining in Mariposa County was inhibited by the controversial Las Mariposas land grant which gave title to 14 of the 24 miles of the Mother Lode in the county to Gen. John C. Fremont. This grant was unsurveyed and was made before gold was discovered. Long before Fremont attempted to establish his right, the grant was overrun with prospectors and miners, who understandably were reluctant to give up what they considered just claims. After years of conflict in and out of the courts, Fremont's claim to the grant was formally recognized. But by then the property was plagued by mismanagement and inefficiency and the mines never fulfilled the expectations of the authorities who evaluated them (Julihn and Horton, 1940, p. 95-96).
Another large estate, the Cook estate, which encompassed most of the mines along a 2-mile length of the Mother Lode in the Coulterville area, further complicated operations on the Mother JLode in Mariposa County (Julihn and Horton, 1940, p. 96-97).
Despite the early frustrations, lode mining in Mariposa County flourished and was especially successful in the late 1930's and early 1940's before most of the mines closed in compliance with War Production Board Order L-208 issued in October 1942. After World War II gold mining declined, and during 1950-59 the average annual gold output was less than 1,000 ounces.
The western two-thirds of the county is underlain by metasedimentary rocks and metavolcanics of Paleozoic and Late Jurassic age, and the eastern one-third is underlain chiefly by intrusives of Late Jurassic or Early Cretaceous age (Bowen and Gray, 1957, p. 45). The intrusive rocks consist of various types of granitic and peridotitic rocks, but biotite-hornblende granodiorite is predominant.
The Hornitos district, in western Mariposa County at lat 37Â°30' N. and long 120Â° 14' W., is noted for gold production from both placers and lodes.
In the early days the Quaternary gravels of Hornitos Creek yielded considerable gold, but these were nearly exhausted before 1900. The lode mines are all west of the Mother Lode, in the zone of veins referred to as the West Belt. Total production from the district is not known, but a minimum of 500,000 ounces seems to be a reasonable estimate.
The gold deposits of the West Belt are in veins that cut several rock types, chiefly metasedimentary rocks of the Mariposa Formation of Jurassic age. The Mariposa is intruded locally by serpentinized peridotite, pyroxenite, basic intrusives altered to hornblende schists, and acid intrusives such as granite and granodiorite (Julihn and Horton, 1940, p. 116-117). Most of the gold deposits are along contacts of igneous rocks and metasedimentary rocks.
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