Damascus District

Site Name

Primary: Damascus District
Secondary: Sunny South

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Commodity

Primary: Gold
Secondary: Platinum
Secondary: Silver
Tertiary: Zinc
Tertiary: Copper
Tertiary: Indium
Tertiary: Lead
Tertiary: Iron

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Location

State: California
County: Placer
District: Damascus District

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Land Status

Land ownership: National Forest
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: Tahoe National Forest (U.S. Forest Service)

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Holdings

Not available

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Workings

Not available

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Ownership

Owner Name: U.S. Forest Service

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Production

Not available

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Deposit

Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer?Hydrothermal vein
Operation Type: Underground
Discovery Year: 1855
Years of Production:
Organization:
Significant: Y

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein
Model Name: Placer Au-PGE

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Orebody

Form: Irregular?tabular, lens

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Structure

Type: L
Description: Melones Fault Zone

Type: R
Description: Gills Hill Fault, Foresthill Fault, Melones Fault Zone

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Alterations

Not available

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Rocks

Name: Schist
Role: Host
Age Type: Host Rock
Age Young: Devonian
Age Old: Ordovician

Name: Slate
Role: Host
Age Type: Host Rock
Age Young: Devonian
Age Old: Ordovician

Name: Gravel
Role: Host
Age Type: Host Rock
Age Young: Tertiary

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Chlorite
Ore: Epidote
Ore: Amphibole
Ore: Pyrite
Ore: Zircon
Ore: Ilmenite
Ore: Magnetite
Ore: Quartz
Ore: Siderite

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Comments

Comment (Deposit): The primary Damascus placer deposits were deposited by a north-south-trending early Tertiary tributary channel, which flowed southward into the adjacent Michigan Bluff District where it joined the ancestral Middle Fork of the American River. This channel can be traced from Damascus to Gas Hill, a distance of 4-1/2miles, and it has been drifted almost continuously for over 4 miles from Damacus to the Hidden Treasure Mine. The channel is a wide (> 300 feet), flat depression in soft swelling clayey slate bedrock, which required substantial timbering to keep the tunnels open. The channel was filled with almost 200 feet of uncemented quartz gravel, sand, and clay with some quartz boulders. The Damascus District also contains lode gold mines, the most important being the Pioneer Mine, which was developed to a depth of 1400 feet and produced $1 million. The gold quartz veins occur in slate and are of limited extent and thickness. Veins trend northwest and northeast dipping steeply to the east. They range from 2 to 8 feet in thickness and contain free gold and often abundant sulfides. The ore is usually low to moderate in grade, but the ore shoots had stoping lengths of up to several hundred feet.

Comment (Development): The streams in the Damascus District were originally placer-mined during the Gold Rush in 1849 and drift mining began in the late 1850s. The Hidden Treasure Drift Mine was discovered in 1875 and was worked on a major scale through the early 1900s. There was some mining activity in the district in the 1930s, and there has been intermittent prospecting ever since.

Comment (Economic Factors): The most productive mine in the district was the Hidden Treasure placer mine, which produced more than $4 million. Clark (1970) reported other significant producers including the Pioneer lode mine ($1 million) and the Mountain Chief placer mine ($700,000+).

Comment (Workings): The main drifts were kept as straight as possible and in the center or lowest depression of the channel. To prospect the width of the channel, crosscuts at right angles to the drift were driven on each side to the rims of the channels or the limit of the paying lead. These were timbered and lagged in soft gravels, but not to the extent of the main drift. In wide pay leads, gangways paralleled the main tunnel to help block out the ore in rectangular blocks. In looser gravels, timbering was required and the main difficulty was preventing caving until timbering was in place. The looser gravels were excavated with pick and shovel. Up until the late 1800s, most workings were driven by hand, then later by machine and pneumatic drills. Working drifts in the gravel beds and pay leads themselves were larger than the bedrock tunnels and usually timbered due to their extended and long-term use. In wide gravel deposits, as a precaution against caving, gravel pillars from 20 - 40 feet wide were left on each side of the drift. When the main access tunnel was in bedrock following the line of the channel, pillars were not required, as the tunnel in the gravel was only for temporary use in mining the ground between its connections with the bedrock tunnel. Raises to access the gravel were made every 200 - 400 feet as necessary. The breaking out of gravel (breasting) was done from the working faces of drifts. Usually, 1-2 feet of soft bedrock and 3-4 feet of gravel were mined out to advance the face. When the gravels were well-cemented, blasting was required. Otherwise the material could be removed with picks. Boulder sized material was left underground and only the gravels and fines were removed from the mine. Bedrock swelling was a frequent problem. Tunnels on and within bedrock were sometimes affected by the upward swelling of the bedrock. In these cases, heavy timbering was required and the tunnel floor had to be periodically cut and lowered to keep the tunnel open. Soft or fractured slates were the most favorable bedrock. The surface was usually creviced and weathered enough that gold could be found to a depth of 1 foot in the top of the bedrock. Where sufficiently weathered and soft, this upper bedrock layer could be easily removed. If the surface of the bedrock was too hard to be worked, it was cleaned thoroughly, and the crevices and surface were worked with special tools to remove every particle of gold. According to the gravel's hardness, they were either washed through sluices or crushed in stamp mills. Much of the gravel was so highly cemented it had to be milled several times. Stamp mills with coarse screens were also found to be suitable for milling cemented gravel. For soft and uncemented gravels, a dump, sluices, and water supply under generally low pressure comprised the entire surface workings. Ventilation of mines was accomplished by direct surface connection through the use of boreholes and the mine shafts and tunnels. It relied on natural drafts, drafts by fire, falling water, or blowers. Within the mines, arrangements of doors were often used to direct the flow of air through the tunnels, drifts, and breasts. Ore was removed by ore cars of various capacity determined by available power and tunnel size. In smaller mines, small cars were often pushed by hand. In larger mines using horsepower or trains, larger two ton cars could be brought out in trains of 5-10 cars.

Comment (Geology): Bedrock erosion degraded the rich gold-bearing veins and auriferous schists and slates as the rivers crossed the metamorphic belts of the Sierra Nevada. Upstream of the gold belts on the granitic Sierra Nevada batholith, channels are largely barren, but become progressively richer as they cross the metamorphic belt and the Mother Lode trend. They become especially enriched after crossing the gold-bearing "serpentine belt" (Feather River Peridotite Belt) upstream of many Tertiary placer districts. While the most gold is contained in the lower sand and gravel, the majority of rich material is within only a few feet of bedrock. Generally, in drift mines only these lower gravels were exploited; however, in hydraulic mines the whole gravel bed was washed. Lindgren (1911) estimated that on average, the hydraulic washing of thick gravel banks up to 300 feet, including both basal and upper gravels, yielded approximately $0.10 to $0.40/yard. Upper gravels alone might average $0.02 to $0.10/yard and lower gavels from $0.50 to $15/yard or more. The bulk of the gold in the deposits was derived from gold-bearing quartz veins within the low-grade metamorphic rocks of the Sierra Nevada. Gravels that have the highest gold values contain abundant white quartz vein detritus and clasts of blue-gray siliceous phyllite and slate common to the gold-quartz vein-bearing bedrock of the region. Unusually high gold concentrations have also been documented immediately downstream of eroded qold quartz veins exposed in the scoured bedrock. Most of the gold found in the gravels of the North Bloomfield and Moore's Flat districts is thought to have originated from the famous lode veins of the Alleghany Mining District. The veins in the Nevada City and Grass Valley districts have been proposed as possible sources for the gold in the gravels of the Sailor Flat and Blue Tent diggings. Gold particles tend to be flat or rounded, shiny and rough, and range from fine and coarse gold to nuggets of 100 or more ounces. Large nuggets were especially prevalent in the Alleghany, North Columbia, Downieville, and Sierra City Districts. The gold particles are almost everywhere associated with black sands composed of magnetite, ilmenite, chromite, zircon, garnet, pyrite, and in some places platinum. Fine flour gold is not abundant in any of the Tertiary gravels. Lindgren (1911) and others have suggested that most of the flour gold was swept westward to be deposited in the thick sediments of the Great Valley. Valley Springs Formation After deposition of the Eocene channel gravels, Oligocene-Miocene volcanic activity in the upper Sierra Nevada radically changed drainage patterns and sedimentation. The first of many eruptive rhyolite flows filled the depressions of most river courses covering the Eocene gravels and diverting the rivers. Many tributaries were dammed, but they eventually breached the barriers and carved their own channels within the rhyolite fill. Ensuing intermittent volcanism caused recurrent rhyolite flows to fill and refill the younger channels resulting in a thick sequence of intercalated intervolcanic channel gravels and volcanic flows. In the Scotts Flat District, very little of the Valley Springs Formation remains, having been lost to erosion. Mehrten Formation Volcanism continued through the Oligocene to the Pliocene, with a change from rhyolitic to andesitic composition and a successively greater number of flows. During the Miocene and Pliocene, volcanism was so extensive that thick beds of andesitic tuffs and mudflows of the Mehrten Formation blanketed the Valley Springs. Thicknesses ranged from a few hundred to a few thousand feet. Pleistocene erosion removed much of these deposits, but remnants cap the axes of many existing ridges at mid-elevations.

Comment (Geology): Continued uplift during the Pliocene-early Pleistocene increased gradients allowing the modern drainages to cut through the volcanic mantle and auriferous gravel deposits and deeply into basement. The once-buried Tertiary river gravels were left exposed in outcrops high on the flanks of the modern drainage divides. Structure Most Upper Jurassic and younger basement rocks of the northern Sierra Nevada were metamorphosed and deformed during the Jurassic-Cretaceous Nevadan Orogeny. The dominant northwest-trending structural grain is a result of this period of compressive deformation, which produced thrust faults, major northwest-trending folds, and regional greenschist facies metamorphism. This episode also resulted in intrusions of granitic plutons that formed the Sierra Nevada. Nevadan deformation structures within and between the northern Sierra Nevada lithotectonic blocks are steeply dipping northwesterly trending faults and northwesterly trending folds. These features are best developed in the Eastern, Central, and Feather River Peridotite Belts, where the faults have been collectively described as the "Foothills Fault System" (Clark, 1960). Where the attitude can be determined, most of the bounding faults dip steeply east and display reverse displacement. The regional northwest-trending structural grain is also at approximately right angles to the prevailing direction of stream flow of both the ancient and modern channels. This grain, expressed in the form of foliation and cleavage in the metamorphic bedrock, served as a good trapping mechanism for the gold particles. GEOLOGY OF THE DAMASCUS DISTRICT Rocks of the Damascus district can be divided into four units consisting of basement complex, Eocene auriferous gravels, interbedded volcanics and gravels of the Valley Springs Formation, and an uppermost Mehrten Formation volcanic cap. The main body of basement rocks within the district consists of sandstones, siltstones, slates, and schist of the Shoo Fly Complex within the Sierra Nevada Eastern Belt. In the extreme southwest corner of the district, the Volcano Canyon Fault, within the Melones Fault zone, juxtaposes partially to completely serpentinized peridotites against the Shoo Fly Complex rocks. Basal Eocene Auriferous Gravels The primary Damascus placer deposits were deposited by a north-south-trending early-Tertiary tributary channel, which flowed southward into the adjacent Michigan Bluff District where it joined the ancestral Middle Fork of the American River. This channel can be traced from Damascus to Gas Hill, a distance of 4-1/2miles, and it has been drifted almost continuously for over 4 miles from Damacus to the Hidden Treasure Mine. The ancestral Middle Fork flowed northwest-southeast following the same general course as the modern Middle Fork of the American. Gravels from this large channel are also important producers in the famous Forest Hill District 10 miles to the southwest. Both the tributary channel and the main channel incised deeply into the underlying bedrock. The bedrock surface is irregular with ridges, swales, and potholes conducive to trapping placer gold. In contrast to the gravels in the Forest Hill District, which are generally a blue gray due to a concentration of slate and metamorphic rock, the Damascus and Michigan Bluff district gravels are almost exclusively white quartz gravels with occasional rounded white quartz boulders of up to 20 tons. This difference is attributed to their location upstream of Calaveras Complex bedrock. The tributary quartz gravels are generally less cemented than many Eocene gravels in the Northern Sierra Nevada and generally don't require crushing.

Comment (Commodity): Commodity Info: Placer deposits: fine to coarse gold to large nuggets Lode deposits: free-milling gold-bearing quartz veins

Comment (Commodity): Ore Materials: Native gold - Fine to coarse gold and nuggets (.900 fine)

Comment (Commodity): Gangue Materials: Quartz and metamorphic gravels; accessory minerals magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite

Comment (Geology): Lode Gold Deposits The Damascus District also contains lode gold mines, the most important being the Pioneer Mine, which was developed to a depth of 1400 feet and produced $1 million. The gold quartz veins occur in slate and are of limited extent and thickness. Veins trend northwest and northeast dipping steeply to the east. They range from 2 to 8 feet in thickness and contain free gold and often abundant sulfides. The ore is usually low to moderate in grade, but the ore shoots had stoping lengths of up to several hundred feet.

Comment (Geology): Tertiary Channel Gravels It has been estimated that 40 percent of California's gold production has come from placer deposits along the western Sierra Nevada (Clark, 1966). These placer deposits are divisible into Tertiary deposits preserved on the interstream ridges, and Quaternary deposits associated with present streams. Lindgren (1911) estimated that approximately $507 million (at $35.00/oz.) was produced from the Tertiary gravels. Almost all Tertiary gravel deposits can be divided into coarse basal Eocene gravels resting on basement, and overlying upper or "intervolcanic" gravels. While the gravels differ texturally, compositionally, and in gold values, no distinct contact exists between the two. The boundary is usually placed where pebble and cobble beds are succeeded by overlying pebble, sand, and clay beds. Lower gravels contain most of the gold and rest on eroded bedrock that is usually smooth, grooved, and polished. Where bedrock is granitic, it is characterized by a smooth and polished surface. Where bedrock is slate, phyllite, or similar metamorphic rock, rock cleavage, joints, and fractures acted as natural riffles to trap fine to coarse gold. In many cases, miners would excavate several feet into bedrock to recover the trapped gold. The lower gravels, or "blue lead," of the early miners are well-cemented and characterized by cobbles to boulders of bluish gray - black slates and phyllites, weathered igneous rocks and quartz. Boulders may range upwards of 10 feet in diameter. In many deposits, disseminated pyrite and pyritic pebble coatings are common in the lower blue lead gravels. Adjacent to the bedrock channels, broad gently sloping benches received shallow but extensive accumulations of auriferous overbank gravels sometimes 1-2 miles wide. The lower unit is also compositionally immature relative to the upper gravel unit as evidenced by their heavy mineral suites. Chlorite, amphibole, and epidote are common constituents in the basal gravels, but are conspicuously absent in upper gravels. The upper gravels compose the bulk of most deposits, with a maximum measured thickness of 400 feet in the North Columbia District. These gravels carry much lower gold values (rarely more than a few cents per cubic yard) than the deeper sands and are often barren. Upper gravels are finer grained, with clasts seldom larger than cobble size, and contain abundant silt and clay interbeds. Cross-bedding and cut-and-fill sedimentary structures are abundant as well as pronounced bedding and relatively fair to good sorting. Compositionally they are much more mature, with quartz prevailing, and more stable heavy mineral components consisting almost exclusively of zircon, illmenite, and magnetite. Oxidation is common and often imparts a reddish hue to the gravels. During the Cretaceous, the Sierra Nevada was eroded and its sediments transported westward by river systems to a Cretaceous marine basin. By the Eocene, low gradients and a high sediment load allowed the valleys to accumulate thick gravel deposits as the drainages meandered over flood plains up to several miles wide developed on the bedrock surface. The major rivers were similar in location, direction of flow, and drainage area to the modern Yuba, American, Mokelumne, Calaveras, Stanislaus, and Tuolumne Rivers. Their auriferous gravels deposits are scattered throughout a belt 40 - 50 miles wide and 150 miles long from Plumas County to Tuolumne County. In the northern counties, continuous lengths of the channels can be traced for as much as 10 miles with interpolated lengths of over 30 miles. The ancient Yuba River was the largest and trended southwest from headwaters in Plumas County. Its gravels are responsible for the placer deposits in the North Bloomfield, San Juan Ridge/North Columbia, Moore's Flat, and French Corral districts. Tributaries to the ancestral Yuba River were responsible for most of the other auriferous gravels in Nevada County.

Comment (Workings): Brief descriptions of the workings of several noteworthy mines are contained in the reports of the California Mining Bureau. Irelan (1888) described the geology and early workings of California's most productive drift mine, the Hidden Treasure Mine as well as the Mountain Gate, Pioneer, Golden River, and other mines. Logan (1936) also provides an interesting description of the Hidden Treasure Mine. A generic discussion of drift mining techniques follows: Drift Mining Drift mining involved driving adits and tunnels along or close to the lowest point in the bedrock trough of an ancient channel and following it upstream along the bedrock surface. Some deeply buried drift mines were originally accessed through vertical shafts requiring timbering, headframes, hoisting, and pumping equipment. Larger shafts were seldom over 3 compartments Smaller mines often had single compartment shafts as small as 2 x 5 feet. Since considerable water was associated with the gravels, it was a serious problem in deeper shafts and costly pumping was required. By the 1890s, due to drainage problems and the expense of hoisting, most major drift mines were accessed through tramway and drain tunnels driven into bedrock below the channels. Channels were usually located by gravel exposures on hillsides and terraces. Exposures of upstream and downstream gravels were called "inlets" and "outlets," respectively. Where a ravine or canyon cut into, but not through an old channel, the exposure was called a "breakout." The preferred method of developing an inlet was to tunnel through bedrock under the channel at such a depth and angle as to break through into the bed of the channel providing natural drainage. The overlying gravels could then be accessed directly through the tunnel or by periodic raises and drifts. Development of an outlet involved following the bedrock channel directly into the hillside, the incline of the bedrock providing natural drainage. The tunnel entrances were usually in or near a ravine or gulch to aid in waste-rock disposal. Prospecting and developing a breakout was more difficult, since the exposed gravel could be in the basal channel or hundreds of feet up on the edge of the channel, making it impossible to locate a prospect tunnel with any certainty. The surest method of prospecting was to run an incline on the pitch of the bedrock. Another method was to sink a vertical shaft on the presumed channel axis. The former method proved superior since it involved less subjectivity and often uncovered paying bench gravels on edges of the old stream. Once the bed of the channel was located, it was prospected by drifts and cross cuts to ascertain width, direction, grade, and the location, extent, and quality of pay. Prospecting also included projecting the grade and direction of existing channel segments for distances up to several miles. Thus having determined a potential location, a prospect adit or shaft was driven to evaluate it. This was a common method of finding old channels where there were no surface exposures. Access tunnels were driven in bedrock to minimize timbering and ensure a stable roof, through which upraises were driven to work the placer gravels. Tunnels were generally run under the lowest point of the bed of the channel in order to assure natural drainage and to make it possible to take auriferous gravels out of the mine without having to hoist it.

Comment (Identification): The Damascus District is in east-central Placer County at the site of the old town of Damascus, about seven miles southeast of Dutch Flat. It includes the lode mines of the Pioneer-Humbug Bar area on the north and the extensive placer deposits that extend from Damascus through Forks House to the Sunny South-Gas Hill area (Clark, 1970). The Gas Hill Mine is generally taken to mark the southern limit of the district. The Michigan Bluff District lies immediately to the south. Total output of the drift mines alone was greater then $12 million, while the lode mines produced several million more. Important mines in the district include the American Eagle, Black Hawk, Central, Diver, Floyd, Lynn, Mars, North Star, Pioneer, Rawhide, Southern Cross, Bullion, Cameron, Comet, Gas Hill, Golden River, Hermit, Hidden Teasure, Mountain Chief, Mountain Gate, Rainbow, and Tickell.

Comment (Location): Location selected for latitude and longitude represents the old Damascus site on the USGS 71/2-minute Westville quadrangle

Comment (Geology): Deposits within the tributary channel can be traced for several miles within the district to a southern limit at the Gas Hill Mine. North of this mine, the channel is truncated by a deeper intervolcanic channel, but reappears 1 mile farther north in the Hidden Treasure Mine (Sec. 35, T15N-R11E), where it was called the "White Channel" and produced over $4 million. From the Hidden Treasure Mine, the tributary can be traced almost continuously to Damascus (Lindgren, 1911), a distance of almost 4 miles. Upon entering the Michigan Bluff District, its deposits have been largely removed by erosion in El Dorado Canyon. The channel is a wide (> 300 feet), flat depression in soft swelling clayey slate bedrock, which required substantial timbering to keep the tunnels open. The channel was filled with almost 200 feet of uncemented quartz gravel, sand, and clay with some quartz boulders. The gravel is markedly finer and more quartzose than that of the main American River Channel into which it drained. Breasting could be done with pick and caving and blasting was limited to the removal of some large boulders. In places the channel widened to 800 feet with rims rising gradually to 16 feet above the thalweg. The width of the gravel breasted was 250 feet, and 4-7 feet of the lowermost gravel, including 1 foot of bedrock, was extracted. The gold was generally coarse, with gravels yielding only $0.50 to $1.75/ton. Only the unusually low cost of production allowed this mine to profitably produce this grade of material via drift mining. Valley Springs Intervolcanic Channels Overlying the Eocene channels are varying thicknesses of intercalated rhyolite tuffs and intervolcanic channel gravels (often called "cement" or "blue" channels) of the Valley Springs Formation. Towards the top of the formation the tuffs become progressively more andesitic. The thickness of the sequence is highly variable. Thicknesses of up to several hundred feet of gravel, sand, and pipe clay, can extend well beyond the limits of the lowermost bedrock channel depression. Little information is available regarding specific thicknesses within the Damascus or neighboring Michigan Bluff districts, but in the Forest Hill District, exposed thicknesses of rhyolite tuff and intervolcanic gravels range from 40-130 feet. Many intervolcanic channels eroded deeply into older auriferous gravels either partially or wholly destroying them. In these cases the intervolcanic channels can be locally very rich, having cannibalized the gold content of the older channel. Clay beds are common in the upper portions of intervolcanic gravels. Petrified and/or lignitized cedar and oak tree trunks are not uncommon. Some gravel layers have become highly cemented by percolation of siliceous and calcareous waters and colors range from gray, blue, reddish brown, to white depending on the source material and oxidation of the gravel and/or cementing material. The Valley Springs intervolcanic channel sequence is capped by andesites of the Mehrten Formation along the crest of the Foresthill Divide, which traverses the district in a northeasterly direction. Gold particles tended to be flat or rounded and ranged from fine to coarse gold and nuggets, although large nuggets were unusual. A little fine or flour gold was found in the upper sands and clays that covered the gravels. The gold particles are in some places associated with platinum and almost invariably associated with black sands composed of magnetite, illmenite, chromite, and pyrite derived from basic bedrock such as diabase, gabbro, and serpentine.

Comment (Geology): REGIONAL SETTING The northern Sierra Nevada is home to numerous important gold deposits. These include the famous lode districts of Johnsville, Alleghany, Sierra City, Grass Valley, and Nevada City as well as the famous placer districts of North Bloomfield, North Columbia, Cherokee, Foresthill, Michigan Bluff, Gold Run, and Dutch Flat. The geological and historical diversity of most of these deposits and specific mine operations are covered in numerous publications produced over the years by the U.S. Bureau of Mines, U.S. Geological Survey, California Division of Mines and Geology (now California Geological Survey), and others. The most recent geologic mapping covering the area is the 1:250,000-scale Chico Quadrangle compiled by Saucedo and Wagner (1992). Stratigraphy The northern Sierra Nevada basement complex has a history of both oceanic and continental margin tectonics recorded in sequences of oceanic, near continental, and continental volcanism. The complex has been divided into four lithotectonic belts: the Western Belt, Central Belt, Feather River Peridotite Belt, and Eastern Belt. The Western Belt is composed of the Smartville Complex, an Upper Jurassic volcanic-arc complex, which consists of basaltic to intermediate pillow flows overlain by pyroclastic and volcanoclastic rock units with diabase, metagabbro, and gabbro-diorite intrusives. The Cretaceous Great Valley sequence overlies the belt to the west. To the east it is bounded by the Big Bend-Wolf Creek Fault Zone. East of the Big Bend-Wolf Creek Fault Zone is the Central Belt, which is in turn bounded to the east by the Goodyears Creek Fault. This belt is structurally and stratigraphically complex and consists of Permian-Triassic argillite, slate, chert, ophiolite, and greenstone of marine origin. The Feather River Peridotite Belt is also fault-bounded, separating the Central Belt from the rocks of the Eastern Belt for almost 95 miles along the northern Sierra Nevada. It consists largely of Devonian-to-Triassic serpentinized peridotite. The Eastern Belt, or Northern Sierra Terrane, is separated from the Feather River Peridotite Belt by the Melones Fault Zone. The Northern Sierra Terrane is primarily composed of siliciclastic marine metasedimentary rocks of the Lower Paleozoic Shoo Fly Complex overlain by Devonian-to-Jurassic metavolcanic rocks. Farther east are Mesozoic granitic rocks of the Sierra Nevada Batholith. The northern Sierra Nevada experienced a long period of Cretaceous to early Tertiary erosion followed by extensive late Oligocene to Pliocene volcanism. The oldest Tertiary deposits are Eocene auriferous gravels deposited by the predecessors of the modern Yuba and American rivers and preserved in paleochannels eroded into basement and on adjacent benches. In contrast to earlier volcanism, Tertiary volcanism was continental, with deposits placed on top of the eroded basement rocks, channel deposits, and Mesozoic intrusives. Two regionally important units are the Valley Springs and Mehrten Formations. The Oligocene-Miocene Valley Springs Formation is a widespread unit of intercalated rhyolite tuffs and intervolcanic channel gravels that blanketed and preserved the basal gravels in the valley bottoms. The younger Miocene-Pliocene Mehrten Formation consists largely of andesitic mudflows, which regionally blanketed all but the highest peaks and marked the end of Tertiary volcanism. Pliocene-Pleistocene uplift of the Sierra Nevada caused the modern drainages to erode through the volcanic Valley Springs-Mehrten sequences and carve deep river gorges into the underlying basement rocks. During this process, the modern rivers became charged with placer-gold deposits from both newly eroded basement rocks and from the reconcentration of the eroded Tertiary placers. The discovery of these modern Quaternary placers in the American River at Sutter's Mill sparked the California Gold Rush.

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References

Reference (Deposit): Browne, R.E., 1890, The ancient rivers of the Foresthill Divide: California State Mining Bureau Report 15, p. 435-465.

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 42-43.

Reference (Deposit): Crawford, J.J., 1894, Golden River and Hidden Treasure mines: California State Mining Bureau Report 12, p. 208-210.

Reference (Deposit): Irelan, W., Jr., 1888, Placer County: California State Mining Bureau Report 8, p. 468-472.

Reference (Deposit): Lindgren, W., 1900, Colfax folio: U.S. Geological Survey Atlas of the U.S., Folio 66, 10 p.

Reference (Deposit): Lindgren, W., 1911, Tertiary gravels of the Sierra Nevada: U.S. Geological Survey Professional Paper 73, p. 150-159.

Reference (Deposit): Logan, C.A., 1936, Gold mines of Placer County, Hidden Treasure Mine: California State Mining Bureau Report 32, p. 65-66.

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

Reference (Deposit): Waring, C. A., 1919, Placer County, Damascus District and Placer County drift mines: California State Mining Bureau Report 15, p. 317 and 352-375.

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California Gold

"Where to Find Gold in California" looks at the density of modern placer mining claims along with historical gold mining locations and mining district descriptions to determine areas of high gold discovery potential in California. Read more: Where to Find Gold in California.