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Pocket Belt Region, Mother Lode Belt, Tuolumne Co., California, USA

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Latitude & Longitude (WGS84): 37° 59' 28'' North , 120° 24' 38'' West
Latitude & Longitude (decimal): 37.99111,-120.41056
Köppen climate type:Csa : Hot-summer Mediterranean climate


This is a file covering a named region within Tuolumne County. There are numerous individual localities within the region, spread over several mining districts. This file serves to document the existence and description of the "region." The coordinates provided by the MRDS database file were 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.

The Pocket Belt region encompasses a loosely defined area bounded on the west by Tuttletown, Sonora-Bald Mountain on the east, Columbia-Stanislaus River on the north, and Jamestown on the south (T1 & 2N, R14E, MDM).

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.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.

The region is 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. Locally, where the quartz veins swell, there can be concentrations of coarse gold. Also, concentrations may be present where the small, late-stage seams of quartz and calcite intersect other rock types or structures.

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).

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.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.

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.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.

Julihn and Horton (1940) estimated that about $5.5 million of gold (period values) 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 (period values).

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.


No minerals currently recorded for this locality.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Mesozoic - Paleozoic
66 - 541 Ma



ID: 2945485
Undivided pre-Cenozoic metavolcanic rocks, unit 2 (undivided)

Age: Phanerozoic (66 - 541 Ma)

Stratigraphic Name: Hodge Volcanic Series; Franklin Canyon Formation; Slate Creek Complex

Description: Undivided pre-Cenozoic metavolcanic rocks. Includes latite, dacite, tuff, and greenstone; commonly schistose.

Comments: Klamath Mountains, Sierra Nevada, Mojave Desert, and other areas. Miscellaneous volcanic and metavolcanic rocks, probably mostly Jurassic in age

Lithology: Major:{mafic volcanic,greenstone}, Minor:{felsic volcanic}, Incidental:{amphibolite, gabbro}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Devonian - Silurian
358.9 - 443.8 Ma



ID: 3185269
Paleozoic sedimentary and volcanic rocks

Age: Paleozoic (358.9 - 443.8 Ma)

Comments: Mccloud Arc

Lithology: Mudstone-carbonate-sandstone-conglomerate

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License



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References

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Tucker, W.B. (1916), Tuolumne County: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 132-172.
Knopf, A. (1929), The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, 88 p.
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.
Logan, C.A. (1949), Mines and mineral resources of Tuolumne County, California: California Journal of Mines and Geology (Report 45): 45(1): 47-83.
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.
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.
Clark, W. B. (1970a), Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 121.
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.
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.
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.
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.
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.
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.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10310669.

 
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