Sonora District

Site Name

Primary: Sonora District

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Commodity

Primary: Gold

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Location

State: California
County: Tuolumne
District: Sonora District

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: Tuolumne County Planning Department

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer
Operation Type: Surface
Discovery Year: 1848
Years of Production:
Organization:
Significant: Y
Deposit Size: M

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Physiography

Not available

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Mineral Deposit Model

Model Name: Placer Au-PGE

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Orebody

Form: Irregular

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Structure

Not available

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Alterations

Not available

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Rocks

Name: Sand and Gravel
Role: Host
Age Type: Host Rock
Age Young: Quaternary

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Analytical Data

Not available

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Materials

Ore: Gold
Gangue: Quartz

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Comments

Comment (Geology): The Sonora District is within the Sierra Nevada foothills, where bedrock consists of northerly trending tectonostratigraphic belts of metamorphosed sedimentary and volcanic rocks and associated intrusive rocks that range in age from Paleozoic to Mesozoic. The structural belts, which extend about 235 miles along the western side of the Sierra, are flanked to the east by the Sierra Nevada Batholith and to the west by sedimentary rocks of the Cretaceous and Jurassic Great Valley sequence. The structural belts are internally bounded by the Melones and Bear Mountains fault zones and are characterized by extensive faulting, shearing, and folding (Earhart, 1988). From El Dorado County southward into Mariposa County, lode gold deposits occur in three distinct belts - the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced. However, there has also been substantial gold production from the West Belt and East Belt. The West Belt in Tuolumne County consists of sparse, widely scattered gold deposits located west of the Mother Lode vein system, which represents the Mother Lode Belt. Gold occurs in irregular quartz veins and stringers in schist, slate, granitic rocks, altered mafic rocks, and as gray ore in greenstone. The West Belt is cut by the northwest-trending Bear Mountains fault zone, which separates an assemblage of metavolcanic and metasedimentary rocks of Jurassic age on the southwest from a more disrupted and diverse assemblage of metavolcanic, metasedimentary, plutonic, ultramafic, and melange rocks on the northeast. The metavolcanic rocks consist generally of volcanic and volcanic-sedimentary rocks of island arc affinity. These rocks are mostly mafic to intermediate in composition and include flows, breccias, and a variety of layered pyroclastic rocks. Some silicic rocks are present also. Various formation names assigned to the metavolcanic assemblages include Gopher Ridge, Copper Hill, Logtown Ridge, and Penon Blanco the metasedimentary rocks are dominantly distal turbidites and hemipelagic sequences of black slate. Assigned formation names include Mariposa, Salt Spring Slate, and Merced Falls Slate. The northwest-trending Mother Lode Belt traverses western Tuolumne County and is associated with the Melones Fault Zone. The rocks of this belt are typically metavolcanic, metasedimentary, and ultramafic, some of which have been hydrothermally altered to assemblages as described below. Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins and bodies of mineralized country rock adjacent to veins. Mother Lode veins are characteristically enclosed in Mariposa Formation slate with associated greenstone. The Mother Lode belt vein system ranges from a few hundred feet to a mile or more in width. Within the zone are numerous discontinuous or linked veins, which may be parallel, convergent, or en echelon. The veins commonly pinch and swell. Few can be traced more than a few thousand feet. Mother Lode type veins fill voids created within faults and fracture zones and consist of quartz, gold and associated sulfides, ankerite, calcite, chlorite, limonite, talc, chromium-bearing mica, and sericite. Stringer veins are commonly found in both adjacent footwall and hanging walls. Mother Lode ores are generally low- to moderate-grade (1/3 ounce of gold or less per ton), but ore bodies can be large. Ore shoots are generally short, 200-300 feet being the average stope length. However, they persist at depth, some having been mined to several thousand feet (Clark and Lydon, 1962). Ore shoots are commonly localized at bulges in veins, shear zones, vein intersections, or near abrupt changes in strike or dip.

Comment (Geology): Wall rocks have invariably been hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite with traces of rutile and leucoxene (Knopf, 1929). The mineralization is usually adjacent to the veins in ground that has been fractured and contains small stringers and lenses of quartz. Locally, greenstone bodies adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute what has been called "gray ore". Altered slate wall rock commonly contains pyrite, arsenopyrite, quartz, chlorite, and sericite with or without ankerite (Zimmerman, 1983). Large bodies of mineralized schist also form low-grade ore bodies throughout the Mother Lode. This ore consists of amphibolite schist that has been subjected to the same processes of alteration, replacement, and deposition that formed the greenstone gray ores. The altered schist consists mainly of ankerite, sericite, chlorite, quartz, and albite. Gold is associated with the pyrite and other sulfides that are present. Pyrite comprises about 8 percent of the rock. The average grade of mineralized schist is about 0.1 oz per ton. The Melones Fault zone separates the Mother Lode Belt from the East Belt. The East Belt is dominantly argillite, phyllite and phyllonite, chert, and metavolcanic rocks of Paleozoic-Mesozoic age. Carbonate rocks (marble) are also present locally. The phyllite and phyllonite are dark to silvery gray. The chert is mostly thin-bedded with phyllite partings. The Upper Paleozoic-Lower Mesozoic metasedimentary and metavolcanic rocks of the East Belt have been assigned to the Calaveras Complex by most investigators (Earhart, 1988). The Lower Paleozoic metamorphic rocks farther east have been assigned to the Shoo Fly Complex. More recently, some geologists have reinterpreted some assemblages along and immediately east of the Melones Fault Zone as separate Jurassic units (Schweickert and others, 1999). The metamorphic complexes are intruded in places by Mesozoic plutonic rocks. Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metamorphic rocks of possible Jurassic age, metamorphic rocks of the Calaveras Complex, metamorphic rocks of the Shoo Fly complex, or in granitic rocks. Most of the veins trend northward and dip steeply. An east-west set of intersecting faults may be a controlling factor in controlling deposition of ore. Ore deposits of the East Belt are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. Gold is usually associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite. LOCAL GEOLOGY The Sonora District is underlain by metasedimentary rock of the Calaveras Complex, which here consists of carbonate rock, argillite, phyllite, chert, and minor amphibolite, all of which are cut in places by a metavolcanic dike swarm. Remnants of Quaternary gravels are present on top of this complex and were the source of the placer gold in the district. The carbonate rocks harbored most of the gold because of their favorable physical character for trapping the gold. The numerous crevices and potholes produced by weathering and erosion of the highly soluble rock provided ideal sites for concentration of the gold carried across this karst landscape by streams.

Comment (Identification): This district was noted for its placer gold. It coincides approximately with the downtown section of the city of Sonora, which is underlain by a narrow north-trending belt of carbonate rock whose numerous crevices and potholes contained abundant placer gold.

Comment (Location): Location selected for latitude and longitude is the intersection of State Highways 49 and 108 in the city of Sonora on the USGS 7.5-minute Sonora quadrangle.

Comment (Workings): This district was initially developed as a placer-mining center. There is little published on the character of the placer mining in this district, although it is likely that standard small-scale hand methods were dominantly used. These processes were used to scour the crevices and potholes in the underlying carbonate rock, which served as traps for the placer gold. Later mining in the district and adjacent areas focused on underground mining of gold-quartz veins (described under other MRDS records).

Comment (Economic Factors): Julihn and Horton (1940) and Clark (1970) estimated that the total output from placer mining in the district was about $11 million.

Comment (Commodity): Gangue Materials: Metamorphic rock, igneous rock, quartz

Comment (Deposit): Julihn and Horton (1940) reported that over 95% of the placer gold yielded in Tuolumne County was obtained from Quaternary gravels, the content of which was derived in considerable part from erosion and reconcentration of gravels deposited by the earlier Tertiary streams. These streams deposited large amounts of coarse-grained gold in the Sonora area when the streams flowed over the karst terrain of the underlying Calaveras Complex. Here, the weathering and erosion of the highly soluble carbonate bedrock produced numerous crevices and potholes that served as ideal traps for the placer gold transported by the streams. In essence, this terrain functioned as a highly efficient natural sluice. The source of much of the gold was many nearby rich quartz veins as well as older Tertiary gravel deposits.

Comment (Commodity): Commodity Info: There is little published information on the character and conditions of the gold at this district probably because most of the placer mining was done during the gold rush when record-keeping was relatively poor. Nonetheless, the gold in the placer deposits of Tuolumne County was notably very coarse, with many large nuggets recovered from the various districts. The famous 28-lb Holden Chispa nugget was found here in the 1850?s.

Comment (Commodity): Ore Materials: Native gold

Comment (Development): The rich placer deposits at Sonora were discovered in 1848. They were intensively worked during the gold rush, but production likely was in steep decline by the 1870?s because of exhaustion of the placers.

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References

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 30.

Reference (Deposit): Clark. W. B., and Lydon, P.A., 1962, Mines and mineral resources of Calaveras County, California: California Division of Mines and Geology County Report No. 2, p. 72-73.

Reference (Deposit): Earhart, R.L., 1988, Geologic setting of gold occurrences in the Big Canyon area, El Dorado County, California: U.S. Geological Survey Professional Paper 1576, 13 p.

Reference (Deposit): Higgins, C.T., 1997, Mineral land classification of a portion of Tuolumne County, California, for precious metals, carbonate rock, and concrete-grade aggregate: California Division of Mines and Geology Open-File Report 97-09, 85 p.

Reference (Deposit): Julihn, C.E., and Horton, F.W., 1940, Mineral industries survey of the United States - Mines of the southern Mother Lode Region, Part II - Tuolumne and Mariposa counties: U.S. Bureau of Mines Bulletin 424, 179 p.

Reference (Deposit): Schweickert, R.A., Hanson, R.E., and Girty, G.H., 1999, Accretionary tectonics of the Western Sierra Nevada Metamorphic Belt in Wagner, D.L. and Graham, S.A., editors, Geologic field trips in northern California: California Division of Mines and Geology Special Publication 119, p. 33-79.

Reference (Deposit): Turner, H.W. and Ransome, F.L., 1897, Geologic atlas of the United States, Sonora Folio, California: U.S. Geological Survey Folio 41, scale 1:125,000.

Reference (Deposit): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, 88 p.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Principal gold-producing districts of the United States: U. S. Geological Survey Professional Paper 610, 283 p.

Reference (Deposit): Wagner, D.L., Bortugno, E.J., and McJunkin, R.D., 1990, Geologic map of the San Francisco-San Jose Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 5A, scale 1:250,000.

Reference (Deposit): Zimmerman, J.E., 1983, The geology and structural evolution of a portion of the Mother Lode Belt, Amador County, California: Unpublished M.S. thesis, University of Arizona, 138 p.

Reference (Deposit): Logan, C.A., 1949, Mines and mineral resources of Tuolumne County, California: California Journal of Mines and Geology, v. 45, no. 1, p. 47-83.

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California Gold

"Where to Find Gold in California" looks at the density of modern placer mining claims along with historical gold mining locations and mining district descriptions to determine areas of high gold discovery potential in California. Read more: Where to Find Gold in California.