Bonanza Mine

Site Name

Primary: Bonanza Mine

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Commodity

Primary: Gold
Secondary: Silver

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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: City of Sonora

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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: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Operation Type: Surface-Underground
Discovery Year: 1851
Years of Production:
Organization:
Significant: Y
Deposit Size: S

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular, pinch and swell

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Structure

Not available

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Alterations

Not available

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Rocks

Name: Granodiorite
Role: Host
Age Type: Host Rock
Age Young: Mesozoic

Name: Diorite
Role: Host
Age Type: Host Rock
Age Young: Mesozoic

Name: Metamorphic Rock
Role: Host
Age Type: Host Rock
Age Young: Mesozoic
Age Old: Paleozoic

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Analytical Data

Not available

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Materials

Ore: Calaverite
Ore: Sylvanite
Ore: Petzite
Ore: Telluride
Ore: Gold
Gangue: Pyrite
Gangue: Quartz

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Comments

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

Comment (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.

Comment (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.

Comment (Geology): 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.

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 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.

Comment (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.

Comment (Location): 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.

Comment (Workings): 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.

Comment (Commodity): 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.

Comment (Commodity): Ore Materials: Native gold, auriferous tellurides (petzite, sylvanite, calaverite)

Comment (Commodity): Gangue Materials: Quartz, pyrite

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References

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): 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): 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.

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

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

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): 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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