Lava Cap-Banner-Central Group

Site Name

Primary: Lava Cap-Banner-Central Group
Secondary: Central Consolidated
Secondary: South Banner
Secondary: North Banner

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Commodity

Primary: Gold
Primary: Silver

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Location

State: California
County: Nevada
District: Nevada City 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: Nevada County Planning Department

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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; hydrothermal stringer zone
Operation Type: Underground
Discovery Year: 1860
Years of Production:
Organization:
Significant: Y
Deposit Size: M

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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

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Structure

Type: R
Description: Weimar Fault Zone, Gillis Hill Fault

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Alterations

Alteration Type: L
Alteration Text: None specifically reported in documents researched

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Rocks

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

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

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

Name: Granodiorite
Role: Associated
Age Type: Associated Rock
Age Young: Jurassic

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Analytical Data

Not available

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Materials

Ore: Arsenopyrite
Ore: Pyrite
Ore: Silver
Ore: Gold
Ore: Galena
Ore: Pyrargyrite
Ore: Freibergite
Ore: Sphalerite
Gangue: Quartz
Gangue: Pyrrhotite
Gangue: Pyrite

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Comments

Comment (Deposit): The Lava Cap-Banner-Central Group of mines exploits a set of north-trending gold- and silver-bearing quartz veins that is situated largely in metasedimentary rocks near the southern edge of a younger body of granodiorite. The Banner Vein and Central Vein contain the main ore bodies. The Banner Vein strikes approximately north-south and dips about 50E, while the Central Vein strikes about N25W and dips 51NE. The two veins intersect about 2,500 feet north of the Central Mine (Lava Cap) main shaft. The Banner Vein has widths of 25 feet or more in places, with an average width of about 7 feet. The Central Vein has widths up to 15 feet or more, but averages about 3 feet. Four small northerly trending quartz veins contain the ore bodies in the North Banner Mine. The vein complex is at least 1.5 miles long. It appears that both main veins formed along faults that were later filled with quartz and sulfides. There is evidence of extensive fracturing and shearing along the Banner Vein, both before and after deposition of the quartz. Wallrock consists of slate and argillite. Older igneous dikes are present along both the Banner and Central veins, but their relationship to ore shoots and ore formation is not clear. There were two main ore zones mined in the Banner-Central complex. Ore is present in lenses of quartz and in quartz stringers. Ore minerals consist of native gold and various gold- and silver-bearing sulfides including pyrite, galena, arsenopyrite, sphalerite, freibergite,and ruby silver (probably pyargyrite). Grade of ore is indicated by the amount of sulfides present in the ore. At the North Banner Mine, silver was reportedly present in large proportions near the surface compared to gold, but decreased with depth, while the percentage of gold increased. The ore shoots in the Banner ore zone generally rake flatly to the north. The extensive fracturing along both veins has resulted in large horses of wallrock that have produced quartz -filled splits in the veins. In some locations where these splits converge at the edges of the horses, ore of the highest grade has been found. Ore shoots tend to be large and have lengths up to 300-400 feet in the Banner, but are not as large in the Central. The Central has longer dimensions down-dip, however.

Comment (Workings): Workings during the early history of the group were developed at the Banner, North Banner, and Central mines. These consisted of standard shafts, adits, drifts, crosscuts, and stopes. Much of the waste rock was used to fill stopes. From most to least important, the following types of stoping were used: Cut and fill, square sets with fill, and open stopes with pillar support. Mining operations of the Lava Cap Gold Mining Corporation during the 1930?s and 1940?s were centered on the workings associated with the Banner shaft and Central shaft (known by some as the Lava Cap shaft). Both of these were connected by a tunnel, which served as a main haulage way for ore to be transferred from the Banner workings to the Central Mine shaft. The ore was lifted through the Central shaft to a mill adjacent to the Central Mine shaft collar. The Central shaft reportedly was 2,300 feet deep on the incline and serviced 14 levels, while workings at the Banner were reportedly about at least 800 feet deep with approximately 8 levels. Chandler (1941) provided some information on workings. As of the early 1940?s, the Banner-Central vein complex had been drifted for a length of 8,000 feet. Extensive timbering was required in places on the Banner Vein because of its highly fractured and sheared character.

Comment (Geology): REGIONAL GEOLOGY The Lava Cap-Banner-Central Group of mines is within the Nevada City District, which is adjacent to the Grass Valley District, home to California's two largest underground gold mines, the Empire and the Idaho-Maryland. The district is situated in the northern portion of the Sierra Nevada Foothills Gold Belt. This belt averages 50 miles wide and extends for about 150 miles in a north-northwest orientation along the western slope of the Sierra Nevada Mountains. The Foothills Gold Belt approximately coincides with the Foothills Metamorphic Belt, which in this area can be subdivided into four major lithotectonic belts: Western Belt, Central Belt, Feather River Peridotite Belt, and Eastern Belt. The Nevada City District lies within the Central Belt, which in the Nevada City-Grass Valley area is marked by a 15-mile-wide north-trending assemblage of two accreted terranes that range from Paleozoic-Late Triassic to Late Jurassic in age. The Central Belt is bounded on the east and west by regional-scale tectonic zones: the Wolf Creek Fault Zone on the west and the Melones Fault Zone on the east. Although some early geologists considered the mines at Nevada City and those on Banner Hill as forming two separate districts, all of these mines are considered here to be part of the "Nevada City District." The oldest rocks in the Central Belt in this area are those of the Paleozoic-Triassic metasedimentary Calaveras Complex, which is exposed in the east part of the belt. Derived from marine sedimentary rocks, these were converted to schistose or slaty rocks during the Late Paleozoic orogeny and locally into a contact-metamorphic biotite gneiss by granodiorite intruded during the Late Mesozoic. Exposed in the west part of the belt, and underlying the two mining districts, are metamorphic rocks of the Jurassic Lake Combie Complex. Slate of the Jurassic Mariposa Formation is exposed in a small area southeast of the Nevada City-Grass Valley area. Igneous and metaigneous rocks in this area include granodiorite, diabase, porphyrite, amphibolite schist, serpentinite, gabbro, diorite, quartz porphyry, and various dike rocks (Johnston, 1940). The main host rocks for the ore deposits of the two districts are rocks of the Lake Combie Complex and younger granitic rocks that intrude them. The veins of the Nevada City and neighboring Grass Valley districts are not connected with, nor are they continuations of, the famous Mother Lode vein system to the south. The northernmost veins of the Mother Lode end about 20 miles to the south. The Nevada City District is at the southern end of a large body of Jurassic granodiorite called the Yuba Rivers Pluton (Saucedo and Wagner, 1992). Here, the granodiorite is in intrusive contact with a north-trending belt of older metamorphic rocks (Lake Combie Complex of Tuminas, 1983) that consist of slate, schist, diabase, gabbro, and serpentinite. The east part of the district is covered with younger Tertiary volcanic rocks in places. The veins in the vicinity of Nevada City are concentrated along or near the contact of the granodiorite body with the metamorphic complex and are arranged in two main systems: one trends west-northwest with steep dips to the north or south, while the other trends northerly with medium dips to the east and contains the most productive veins. In general, according to Koschmann and Bergendahl (1968), the veins of the Nevada City District are mineralogically similar to those of the Grass Valley District. There are several differences in the character of the veins between the two districts, however. One is that the Grass Valley veins contain larger amounts of coarse gold. The following characteristics of the Nevada City District are summarized from Lindgren (1896b), Hobson and Wiltsee (1893), and Johnston (1938):

Comment (Commodity): Gangue Materials: Quartz, pyrite, pyrrhotite

Comment (Commodity): Commodity Info: Lindgren (1896b) reported that there is considerable silver in the sulfides of the Banner Hill area; in some mines here, the quantity of silver actually exceeded that of gold. At one point, the Banner-Lava Cap Mine was one of the most productive silver mines in California. The Central Mine had a gold:silver ratio by weight of 1:15 for several years of production (Johnston, 1938).

Comment (Commodity): Ore Materials: Native gold, auriferous and argentiferous sulfides (pyrite, galena, arsenopyrite, sphalerite, freibergite,and ruby silver-pyargyrite)

Comment (Geology): LOCAL GEOLOGY The Lava Cap-Banner-Central Group of mines is situated largely in metasedimentary rocks of the Lake Combie Complex near the southern edge of the Yuba Rivers Pluton; at the North Banner MIne, wall rock is granodiorite. Unconformably overlaying these crystalline rocks in places are ridge-capping Cenozoic volcanic and sedimentary rocks. The metasedimentary rocks are cut by north-trending gold- and silver-bearing quartz veins, which have been exploited by this group of mines. This vein complex is at least 1.5 miles long. The Banner Vein and Central Vein contain the main ore bodies. The Banner Vein strikes approximately north-south and dips about 50E, while the Central Vein strikes about N25W and dips 51NE. The two veins intersect about 2,500 feet north of the Central Mine (Lava Cap) main shaft. The Banner Vein has widths of 25 feet or more in places, with an average width of about 7 feet. The Central Vein has widths up to 15 feet or more, but averages about 3 feet. Four small northerly trending quartz veins contain the ore bodies in the North Banner Mine. The Banner Vein eventually intersects the footwall of the Central Vein to the south. It appears that both veins formed along faults that were later filled with quartz and metals. There is evidence of extensive fracturing and shearing along the Banner Vein, both before and after deposition of the quartz. Wallrock consists of slate and argillite of the Lake Combie Complex. Chandler (1941) reported the presence of older igneous dikes along both the Banner and Central veins, but concluded that they had no relationship to ore shoots along the veins. There were two main ore zones mined in the Banner-Central complex. Ore is present in lenses of quartz and in quartz stringers. Ore minerals consist of native gold and various gold- and silver-bearing sulfides including pyrite, galena, arsenopyrite, sphalerite, freibergite,and ruby silver (probably pyargyrite). Grade of ore is indicated by the amount of sulfides present in the ore. At the North Banner Mine, silver was reportedly present in large proportions near the surface compared to gold, but decreased with depth, while the percentage of gold increased. The ore shoots in the Banner ore zone generally rake flatly to the north. The extensive fracturing along both veins has resulted in large horses of wallrock that have produced quartz -filled splits in the veins. In some locations where these splits converge at the edges of the horses, ore of the highest grade has been found. Ore shoots tend to be large and have lengths up to 300-400 feet in the Banner, but are not as large in the Central. The Central has longer dimensions down-dip, however.

Comment (Identification): This summary discusses a group of originally separate mines that were later consolidated and operated by the Lava Cap Gold Mining Corporation during the 1930?s and early 1940?s. This group is one of the two most productive in the Nevada City Mining District. The Banner Mine is about 1.3 miles north of the Lava Cap Mine and is connected to the latter by a tunnel. The ?Lava Cap Mine? shown on the USGS Chicago Park 7.5-minute quadrangle is the same site as the historic Central Mine (Central Consolidated). A short distance to the north of the Banner, across Deer Creek, is the North Banner group of mines, which had workings on the Woodville, Dunnington, Tinny, and Reindeer veins.

Comment (Geology): REGIONAL GEOLOGY (continued) The most productive veins in the vicinity of the town of Nevada City are near or along the contact between the metamorphic rocks and granodiorite. They do not tend to directly coincide with the contact itself. The veins of the district are generally larger than those at Grass Valley; they range from a few to as much as 40 feet in width with an average of about 2 to 6 feet. Overall, the veins of the district contain more silver than those at Grass Valley, and generally the contained sulfides are richer in gold. Sulfides are generally auriferous and can constitute as much as 20% of the vein material. Pyrite is the dominant sulfide, with lesser amounts of galena, sphalerite, chalcopyrite, arsenopyrite, and tetrahedrite. Molybdenite and tellurides are locally present. Compared to Grass Valley, however, the veins are lower in grade and rarely contain coarse or specimen gold. The gangue is mainly quartz, although calcite and ankerite are abundant and later than the quartz; minor gangue minerals locally include sericite, chlorite, epidote, and chalcedony. Study of the paragenesis of the district by Johnston (1938) indicated that vein-filling developed in two main stages: quartz first, then carbonate, with a transistion of deposition between the two types. The sulfides and gold were deposited during the quartz stage. Hydrothermal alteration of the wallrock consists of carbonate, sericitic, and pyritic. Replacement of wallrock by silica was not noted, although small fissures filled with carbonates and quartz can be common. In places, the veins are associated with dikes. All of the veins have some gouge, which indicates that they were emplaced along faults. Detailed studies of several veins indicate that they were formed by successive movement along the faults with subsequent deposition of quartz; at least four separate cycles of movement and deposition have been recognized. Ore shoots have been followed down-dip as much as 2,700 feet. The nearby Banner Hill part of the Nevada City District has some distinct differences from those closer to Nevada City proper. Among these differences are the narrowness of the veins and their prevailing east-west strike with low to moderate dips to the north or south; locally, north-south-striking veins are present such as at the Lava Cap-Banner-Central group of mines. A few of the east-west-striking veins have nearly vertical dips. The east-west set appears to generally follow a system of joints or sheeting in the granodiorite country rock. Ores are commonly of high grade, with much silver and sulfides.

Comment (Location): Location selected for latitude and longitude is the Lava Cap Mine shaft symbol on the USGS 7.5-minute Chicago Park quadrangle. This shaft is that of the original Central Mine.

Comment (Development): Although individual mines of the Lava Cap-Banner-Central Group began operation in the 1800?s, the most productive period of mining was in the 1930?s-1940?s. The early period of production appears to have ended around 1918. Recovery of gold during this period was hindered by the presence of abundant sulfides. When the flotation process was instituted in later years, recovery increased to over 95 percent. No significant work was done at this group of mines between 1918 and 1932, the year when the Lava Cap Gold Mining Corporation commenced operations. Large-scale mining took place from 1934 to 1943, when the mine closed because of World War II. At closure, the Lava Cap Mine was the largest gold producer in California. All historic production was from above 2,100 feet. Just before the start of World War II, over 300 men were employed at the Lava Cap operations. Chandler (1941) provided an extensive discussion of mining and milling by the Lava Cap Gold Mining Corporation at the site during this highly productive period. In the 1970?s, concerns were raised about possible discharge of sediment and elevated levels of arsenic from the Lava Cap Mine. In the 1980?s, the Franco Nevada Mining Corporation and Keystone Copper proposed reopening the Banner Mine and Lava Cap Mine as a joint operation. The proposal generated significant controversy, mainly because of its location in a semi-rural residential area. During a referendum in the 1980?s, a measure to re-zone the mine site from residential to one that would allow mining was rejected. In the 1990?s, the Lava Cap Mine was placed on the EPA Superfund list for remediation related to arsenic contamination. Major cleanup at the site by EPA was initiated in May 2006. (See Website listed in ?References? section.) Amalgamation, chlorination, and cyanide processes were used at this group of mines. At one point, sulfide concentrates were shipped to the Selby smelter near San Francisco. Disruption of service at Selby stimulated construction of a cyanide plant at the Lava Cap Mine in 1940.

Comment (Environment): The Lava Cap-Banner -Central Group of mines is situated on the flanks of a westerly trending ridge crowned by Banner Mountain. The Lava Cap Mine (Central Mine) is on the south flank, while the Banner Mine is on the north flank, just west of Banner Mountain. The ridge is heavily forested and contains semi-rural residential development. Issues related to arsenic contamination from the Lava Cap Mine and its status as an EPA Superfund site are discussed below.

Comment (Economic Factors): Total production from the Lava Cap Mine during 1933-1942 was estimated by Clark (1970) at about $12,000,000. Chandler (1941) reported annual production of as much as 300,000 ounces of silver during the pre-World War II activity. Estimated reserves in the early 1980?s were 136,000 metric tons with a grade of 0.343 ounces gold per ton and 3.45 ounces silver per ton (Metals Economics Group, 1983).

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References

Reference (Deposit): http://yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/2e3c0ceec080b048882573290078b56a/7f29f3b3740158ca88257007005e944f!OpenDocument
URL: http://www.wise-uranium.org/uousawy.html

Reference (Deposit): Hobson, J.B., 1890, Nevada City Mining District: Tenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 387-389.

Reference (Deposit): Hobson, J.B. and Wiltsee, E.M., 1893, Nevada City Mining District: Eleventh Annual Report of the State Mineralogist, California State Mining Bureau, p. 296.

Reference (Deposit): Crawford, J.J., 1896, Nevada County: Thirteenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 257.

Reference (Deposit): Crawford, J.J., 1894, Nevada County: Twelfth Annual Report of the State Mineralogist, California State Mining Bureau, p. 194-195.

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 97-101.

Reference (Deposit): Chandler, J.W., 1941, Mining methods and costs of the Lava Cap Gold Mining Corporation, Nevada City, California: California Division of Mines, 37th Annual Report of the State Mineralogist, p. 409-425.

Reference (Deposit): Tuminas, A., 1983, Structural and stratigraphic relations in the Grass Valley-Colfax area of the northern Sierra Nevada, California: Ph.D. dissertation, University of California, Davis, 415 p.

Reference (Deposit): Lindgren, W., 1896b, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Annual Report of the U.S. Geological Survey, Part 2, p. 1-262

Reference (Deposit): Lindgren, W., 1896a, Geologic atlas of the United States - Nevada City Special Folio: U.S. Geological Survey Folio 29, 7 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): Johnston, W.G., Jr., 1940, The gold-quartz veins of Grass Valley, California: U.S. Geological Survey Professional Paper 194, 101 p.

Reference (Deposit): Johnston, W.G., Jr., 1938, Vein-filling at Nevada City, California: Geological Society of America Bulletin, v. 49, no. 1, p. 23-34.

Reference (Deposit): Irelan, W., Jr., 1888, Nevada County: Eighth Annual Report of the State Mineralogist, California State Mining Bureau, p. 420-422.

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): Metals Economics Group, 1983, The MineSearch annual: California, Oregon, and Washington, vol. VIII, 636 p.

Reference (Deposit): Saucedo, G.J. and Wagner, D.L., 1992, Geologic map of the Chico Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 7A, scale 1:250,000.

Reference (Deposit): MacBoyle, E.M., 1919, Mines and mineral resources of Nevada County: Sixteenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 1-270.

Reference (Deposit): Logan, C.A., 1935, Nevada County: Thirty-first Annual Report of the State Mineralogist, California Journal of Mines and Geology, p. 13, 16-17.

Reference (Deposit): Logan, C.A., 1924, Nevada County: Twentieth Annual Report of the State Mineralogist, California State Mining Bureau, p. 9.

Reference (Deposit): Logan, C.A., 1930, Nevada County: Twenty-sixth Annual Report of the State Mineralogist, California Division of Mines, p. 101-102.

Reference (Deposit): Logan, C.A., 1923, Auburn Field Division: Eighteenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 7.

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