Pocket Belt Region

Site Name

Primary: Pocket Belt Region

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Tuolumne
District: Various

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Land Status

Land ownership: BLM Administrative Area
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: Folsom Field Office

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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: Region
Operation Category: Producer
Deposit Type: Hydrothermal vein
Operation Type: Surface-Underground
Discovery Year: 1850
Years of Production:
Organization:
Significant: Y
Deposit Size: L

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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

Alteration Type: L
Alteration Text: None reported

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Rocks

Name: Porphyry
Role: Host
Description: Porphyritic Igneous Rocks
Age Type: Host Rock
Age Young: Mesozoic

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

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

Name: Metamorphic Rock
Role: Host
Age Type: Host Rock
Age Young: Jurassic

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: Gold
Ore: Pyrite
Ore: Calaverite
Ore: Sylvanite
Ore: Petzite
Ore: Telluride
Gangue: Quartz
Gangue: Calcite
Gangue: Pyrite
Gangue: Chalcopyrite
Gangue: Galena

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Comments

Comment (Workings): These deposits were exploited initially by shallow surface-extraction methods from exposed, weathered portions of the quartz veins. Later, mining was accomplished by standard underground workings (shafts, drifting, crosscutting, etc.). Collectively, these processes were referred to as ?pocket? mining because of the relatively small but rich concentrations of the ore.

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 Pocket Belt consists of individual gold deposits situated between the Mother Lode Belt and the East Belt of gold mineralization. They are concentrated mainly at Bald Mountain near Sonora and at Jackass Hill near Tuttletown. Those at Bald Mountain are largely within metamorphic rocks of the Calaveras Complex, while those at Jackass Hill are within metavolcanic and metasedimentary rocks of what has been mapped as the Sullivan Creek Terrane.

Comment (Commodity): Gangue Materials: Quartz, calcite, pyrite, chalcopyrite, galena

Comment (Deposit): The Pocket Belt is characterized by widely separated, very narrow seams of quartz and calcite that swell in places to contain rich pockets of coarse gold. The deposits also contain telluride minerals including petzite, sylvanite, and calaverite. These deposits are noted for their small but spectacularly rich concentrations of gold, some of which is beautifully crystalline. The seams do not persist individually, but appear to be related to very minor, almost insignificant fissuring that was later invaded by rich gold-bearing solutions. Rich seams may persist for only a few feet. In some cases, the seams are associated with igneous dikes, such as at the Bonanza Mine. Some of these seams have been superimposed on older, less-rich gold-bearing mineralized zones typical of the Mother Lode Belt and East Belt. The Pocket Belt is distinct from ?typical? Sierran gold mineralization because the gold is present as a mix of native form and telluride minerals. The deposits do not show consistent enrichment in the ore shoots. Sulfides, although present typically as pyrite, are not as prominent as in the Mother Lode Belt and East Belt. Because the veins of quartz and calcite do not appear to be disturbed by later fissures and seams, these deposits probably formed very late in the regional sequence of gold mineralization. Also, the veins in the belt may represent a very localized source of enrichment compared to the areas of less-rich quartz-veining to the north and south. The above characteristics suggest that the origin of the Pocket Belt was relatively unique in the overall process of gold mineralization of the Sierra Nevada. These deposits have not been sufficiently studied, however, to conclusively establish their origin. Although tellurides most commonly form in lower-temperature environments, they can form also in higher-temperature environments. Because of the characteristics displayed by these deposits, they are genetically classified here as low-sulfide gold-quartz deposits, but the possibility that they may have originated by epithermal processes is not ruled out at this time.

Comment (Development): The first deposits in this belt were reportedly discovered in 1850 (Sugarman Mine at Bald Mountain) and 1851 (Bonanza Mine at Sonora). Julihn and Horton (1940) identified the Bonanza as probably the most famous pocket mine in the United States. Mining of these individual deposits has continued sporadically since that time. Because of the small size of individual deposits in the Pocket Belt, they have not been amenable to large-scale mining techniques. Rather, their exploitation has been the domain of individual miners or very small operators. Small-scale mining probably still continues in some of the underground mines.

Comment (Economic Factors): Julihn and Horton (1940) estimated that about $5.5 million of gold was produced by direct mining of the Pocket Belt veins. They also emphasized that erosion of this gold-bearing belt was the major source of the placer gold found in nearby districts such as Columbia, Shaws Flat, Sonora, and Jamestown. They estimated this process provided about two-thirds of the placer gold produced in the county, or more than $100 million.

Comment (Geology): The Pocket Belt gold mines are 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): The Pocket Belt is characterized by widely separated, very narrow seams of quartz and calcite that swell in places to contain rich pockets of coarse gold. The deposits also contain telluride minerals including petzite, sylvanite, and calaverite. These deposits are noted for their small but spectacularly rich concentrations of gold, some of which is beautifully crystalline. The seams do not persist individually, but appear to be related to very minor, almost insignificant fissuring that was later invaded by rich gold-bearing solutions. Rich seams may persist for only a few feet along seams that are inches in width. In some cases, the seams are associated with igneous dikes, such as at the Bonanza Mine. Some of these seams have been superimposed on older, less-rich gold-bearing mineralized zones typical of the Mother Lode Belt and East Belt. Based on reported observations, it appears that structural attitudes of the seams do not show a regional consistency, although additional study is needed to confirm this observation. Because the veins of quartz and calcite do not appear to be disturbed by later fissures and seams, these deposits probably formed very late in the regional sequence of gold mineralization. Also, the veins in the belt may represent a very localized source of enrichment compared to the areas of less-rich quartz-veining to the north and south of this area.

Comment (Commodity): Commodity Info: Famous for numerous small but rich pockets of specimen (crystallized) gold. Notable also for the presence of auriferous telluride minerals including petzite, calaverite, and sylvanite. Sulfides are minor compared to other Sierra Nevada gold deposits.

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

Comment (Identification): This region consists of individual deposits situated in the vicinity of the towns of Columbia, Sonora, and Tuttletown all between the Mother Lode Belt and the East Belt of gold mineralization. The deposits were noteworthy for their small, but very rich, gold-quartz veins. The most productive mines were situated at geographic features known as Bald Mountain (Sonora) and Jackass Hill (Tuttletown). The many individual pocket deposits on and near Bald Mountain were thought to be major contributors of gold to the very large placer deposits at Columbia, Shaws Flat, and Sonora.

Comment (Location): Location selected for latitude and longitude is the intersection of Jamestown Road and Racetrack Road on the USGS 7.5-minute Sonora quadrangle. This location does not represent a deposit, but rather it represents very approximately the geographic center of the belt.

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References

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): 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): 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.

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): 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): 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): 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.

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California Gold

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