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Sonora District

Past Producer in Tuolumne county in California, United States with commodity Gold
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Public Land Survey System information
  6. Commodities
  7. Materials information
  8. Mineral occurrence model information
  9. Host and associated rocks
  10. Nearby scientific data
  11. Ore body information
  12. Controls for ore emplacement
  13. Economic information about the deposit and operations
  14. Mining district
  15. Land status
  16. Bibliographic references
  17. General comments
  18. Reporter information

Geologic information

Identification information

Deposit ID 10310682
Record type District
Current site name Sonora District

Comments on the site 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.

Geographic coordinates

Geographic coordinates: -120.382, 37.98286 (WGS84)
Location accuracy 100(meters)
Relative position Coincides with the downtown section of the city of Sonora.

Site location context

Political divisions (FIPS codes)

Tuolumne(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Sonora(quadrangle 1:24,000 scale)

Oakdale(quadrangle 1:100,000 scale)

San Jose(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Upper Tuolumne(hydrologic unit)

San Joaquin(hydrologic accounting unit)

San Joaquin(hydrologic subregion)

California(hydrologic region)

Geographic areas

Country State County
United States California Tuolumne

Public Land Survey System information

Meridian Township Range Section Fraction State
Mount Diablo 002N 014E 36 California

Comments on the location information

  • 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.

Commodities

Commodity Importance
Gold Primary

Comments on the commodity information

  • 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.
  • Ore Materials: Native gold
  • Gangue Materials: Metamorphic rock, igneous rock, quartz

Materials information

Materials Type of material
Gold Ore
Quartz Gangue

Mineral occurrence model information

Model code 119
USGS model code 39a
Deposit model name Placer Au-PGE
Mark3 model number 54

Host and associated rocks

  • Host or associated Host
    Rock type Unconsolidated Deposit > Sand and Gravel
    Stratigraphic age (youngest) Quaternary

Nearby scientific data

(1) -120.382, 37.98286
ListMap

Economic information

Ore body information

  • General form Irregular

Controls for ore emplacement

  • Mechanical accumulation within stream-channel lag gravels, bars, and point-bar deposits as well as erosional lag on bedrock surfaces in basins.

Comments on the geologic information

  • 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.
  • 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.

Economic information about the deposit and operations

Operation type Surface
Development status Past Producer
Commodity type Metallic
Deposit size Medium
Significant Yes
Discovery year 1848

Mining district

District name Sonora District

Land status

Ownership category Private
Area name City of Sonora
Ownership category Private
Area name Tuolumne County Planning Department

Comments on the workings information

  • 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).

Comments on other economic factors

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

Comments on 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.

Reference information

Bibliographic references

  • Deposit

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

  • 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.

  • 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.

  • 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.

  • 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.

  • Deposit

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

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

General comments

Subject category Comment text
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.

Reporter information

Type Date Name Affiliation Comment
Reporter 16-AUG-2006 Higgins, Chris T. California Geological Survey CGS (Formerly CDMG)
Editor 01-SEP-2007 Schruben, Paul G. U.S. Geological Survey Converted from S&A FileMaker format to Oracle. Edit checks on rocks, units, and ages with Geolex search, and other fields.

Beyond USGS

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