Bonanza Mine

Past Producer in Tuolumne county in California, United States with commodities Gold, Silver

Sections on this page

Identification information
Geographic coordinates
Site location context
Geographic areas
Public Land Survey System information
Commodities
Materials information
Mineral occurrence model information
Host and associated rocks
Nearby scientific data
Ore body information
Controls for ore emplacement
Economic information about the deposit and operations
Mining district
Land status
Bibliographic references
General comments
Reporter information

Geologic information

Identification information

Deposit ID	10310593
Record type	Site
Current site name	Bonanza Mine

Comments on the site identification

The Bonanza Mine is probably the most famous pocket mine in the United States. It is in the city limits of Sonora within the Pocket Belt of gold mineralization in Tuolumne County.

Geographic coordinates

Geographic coordinates:	-120.3869, 37.98841 (WGS84)
Location accuracy	500(meters)
Relative position	Location reportedly at northern end of downtown part of 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	NW	California

Comments on the location information

Location selected for latitude and longitude is an approximate site of the Bonanza Mine shaft estimated from research. Julihn and Horton (1940) reported the location to be on the ?north slope of Piety Hill within the city limits of Sonora.? A CGS field report described the location as 3/4 mile north of the Sonora railroad station (which was at the south end of town) on Sonora Flat.

Commodities

Commodity	Importance
Gold	Primary
Silver	Secondary

Comments on the commodity information

Commodity Info: Famous for many high-grade pockets. Notable for presence of auriferous tellurides, which is relatively uncommon in Sierra Nevada gold mineralization. Sulfides minor compared to other Sierra Nevada gold deposits.
Ore Materials: Native gold, auriferous tellurides (petzite, sylvanite, calaverite)
Gangue Materials: Quartz, pyrite

Materials information

Materials	Type of material
Gold	Ore
Telluride	Ore
Petzite	Ore
Sylvanite	Ore
Calaverite	Ore
Quartz	Gangue
Pyrite	Gangue

Mineral occurrence model information

Model code	273
USGS model code	36a
Deposit model name	Low-sulfide Au-quartz vein
Mark3 model number	27

Host and associated rocks

Host or associated Host

Rock type Metamorphic Rock

Rock unit name Calaveras Complex

Stratigraphic age (youngest) Mesozoic

Stratigraphic age (oldest) Paleozoic
Host or associated Host

Rock type Plutonic Rock > Mafic Intrusive Rock > Diorite

Stratigraphic age (youngest) Mesozoic
Host or associated Host

Rock type Plutonic Rock > Granitoid > Granodiorite

Stratigraphic age (youngest) Mesozoic

Nearby scientific data

(1)	-120.3869, 37.98841	ListMap

Economic information

Ore body information

General form Tabular, pinch and swell

General form	Tabular, pinch and swell

Controls for ore emplacement

Native gold and auriferous tellurides are found in pockets where both iron-stained horizontal seams and small quartz stringers intersect either in the interior of a diorite dike or in quartz sheathings on the walls of the dike.

Comments on the geologic information

The Bonanza Mine 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 Bonanza Mine is developed in metamorphic rocks of the Calaveras Complex, which have been intruded here by a diorite dike and, at greater depth, by plutonic rock.

The rich pockets of the Bonanza Mine were found along a narrow quartz seam in the diorite dike, which is 10-16 feet thick, and in quartz sheathings on each wall of the dike. The dike, which strikes N30E and dips either 20-35NW or 20-35NE (there are discrepancies in dip reported in the records researched), was intruded along a contact between mica schist and carbonate rock in the Calaveras Complex. It is commonly cut by narrow quartz stringers and small, early horizontal seams heavily stained with iron oxide. Generally, the pockets are found where both the iron-stained seams and quartz stringers meet, either in the interior of the dike or in the quartz sheathings on the walls of the dike. In the pockets, native gold is found with telluride minerals (petzite, sylvanite, calaverite) and with pyrite.

Economic information about the deposit and operations

Operation type	Surface-Underground
Development status	Past Producer
Deposit size	Small
Significant	Yes
Discovery year	1851

Mining district

District name	Sonora District

Land status

Ownership category	Private
Area name	City of Sonora

Comments on the workings information

Initially, the deposit was developed by surface workings. In the 1870?s, underground mining began with development of an inclined shaft and drifts along the diorite dike. The mine workings reportedly bottomed in granodiorite at an inclined depth of about 2,700 feet.

Comments on other economic factors

Julihn and Horton (1940) reported a production of $2 million in gold, whereas Clark (1970) reported a production of $1.5 million.

Comments on development

The deposit was discovered in 1851. The most famous production in this mine came in 1879 when it is said that 990 pounds of gold, valued at $300,000 were taken out in one week. Mining continued sporadically until at least 1914. Since then, the history of the mine is unclear.

Reference information

Bibliographic references

Deposit

Clark, W. B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 121.
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

Tucker, W.B., 1916, Tuolumne County: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 132-172.
Deposit

Wagner, D.L. and others, 1981, Geologic map of the Sacramento Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 1A, scale 1:250,000.
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	The rich pockets of the Bonanza Mine were found along a narrow quartz seam in a diorite dike, which is 10-16 feet thick, and in quartz sheathings on each wall of the dike. The dike, which was intruded along a contact between mica schist and carbonate rock, is commonly cut by narrow quartz stringers and small, early horizontal seams heavily stained with iron oxide. Generally, the pockets are found where both the iron-stained seams and quartz stringers meet, either in the interior of the dike or in the quartz sheathings on the walls of the dike. In the pockets, native gold is found with telluride minerals (petzite, sylvanite, calaverite) and with pyrite. The above characteristics suggest that the origin of this deposit was relatively unique in the overall process of gold mineralization of the Sierra Nevada. The deposit has not been sufficiently studied, however, to conclusively establish its origin. Although tellurides most commonly form in lower-temperature environments, they can form also in higher-temperature environments. Because of the characteristics displayed by this deposit, it is genetically classified here as a low-sulfide gold-quartz deposit, but the possibility that it may have originated by epithermal processes is not ruled out at this time.

Subject category

Comment text

Deposit

The rich pockets of the Bonanza Mine were found along a narrow quartz seam in a diorite dike, which is 10-16 feet thick, and in quartz sheathings on each wall of the dike. The dike, which was intruded along a contact between mica schist and carbonate rock, is commonly cut by narrow quartz stringers and small, early horizontal seams heavily stained with iron oxide. Generally, the pockets are found where both the iron-stained seams and quartz stringers meet, either in the interior of the dike or in the quartz sheathings on the walls of the dike. In the pockets, native gold is found with telluride minerals (petzite, sylvanite, calaverite) and with pyrite.

The above characteristics suggest that the origin of this deposit was relatively unique in the overall process of gold mineralization of the Sierra Nevada. The deposit has not been sufficiently studied, however, to conclusively establish its origin. Although tellurides most commonly form in lower-temperature environments, they can form also in higher-temperature environments. Because of the characteristics displayed by this deposit, it is genetically classified here as a low-sulfide gold-quartz deposit, but the possibility that it may have originated by epithermal processes is not ruled out at this time.

Reporter information

Type	Date	Name	Affiliation	Comment
Reporter	22-AUG-06	Higgins, Chris T.	California Geological Survey CGS (Formerly CDMG)
Editor	01-SEP-07	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

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

External references

Wikidata: Q49401402 (Bonanza Mine)

Maps centered on this site

Authoritative California resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.