Scotts Flat District

The Scotts Flat District is a gold mine located in Nevada county, California at an elevation of 2,592 feet.

About the MRDS Data:

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

Mine Info

Name: Scotts Flat District  

State:  California

County:  Nevada

Elevation: 2,592 Feet (790 Meters)

Primary Mineral: Gold

Lat, Long: 39.2809, -120.91340

Map: View on Google Maps


Satelite image of the Scotts Flat District

Scotts Flat District MRDS details

Site Name

Primary: Scotts Flat District


Commodity

Primary: Gold
Secondary: Platinum
Secondary: Silver


Location

State: California
County: Nevada
District: Scotts Flat District


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 indicate a claim status and does not necessarily indicate an area is open to prospecting.
Administrative Organization: Nevada County Planning Department


Holdings

Not available


Workings

Not available


Ownership

Owner Name: Private


Production

Not available


Deposit

Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer
Operation Type: Surface-Underground
Discovery Year: 1850
Years of Production:
Organization:
Significant: Y


Physiography

Not available


Mineral Deposit Model

Model Name: Placer Au-PGE


Orebody

Form: Irregular


Structure

Type: R
Description: Wolf Creek Fault Zone, Ramshorn-Foresthill Fault, Melones Fault Zone

Type: L
Description: Ramshorn-Foresthill Fault


Alterations

Not available


Rocks

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


Analytical Data

Not available


Materials

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


Comments

Comment (Workings): Hydraulic Mining Hydraulic mining methods were first applied in 1852 to the Yankee Jims gravels in the Forest Hill District of central Placer County. Its use and methods quickly evolved to where it was applied to most exposed Tertiary gravel deposits. Hydraulic mining involved directing a powerful stream of high pressure water through large nozzles (called "monitors") at the base of a gravel bank, undercutting it and allowing it to collapse. The loosened gravels were then washed through sluice boxes. The remaining tailings were indiscriminately dumped in the nearest available stream or river. Large banks of low-yield gravel could be economically mined this way. In some cases, adits were driven into the exposed face and loaded with explosives to help break down the exposure. One of hydraulic mining's highest costs was in the ditches, flumes, and reservoirs needed to supply sufficient volumes of water at high pressure. A mine might have many miles of ditches as well as dams and reservoirs, flumes, and tunnels. Hydraulic mining flourished for about 30 years until the mid-1880s when the Sawyer Decision essentially brought it to a halt. Drift Mining While limited mining of the Tertiary channel gravels by means of shafts and adits commenced soon after their discovery, underground mining flourished after the Sawyer Decision. 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.

Comment (Development): The various mines of the Scotts Flat District were extensively hydraulicked from the 1850s through the mid-1880s when the Sawyer Decision curtailed hydraulic mining debris disposal. Some drifting of the harder lower blue-lead gravels was also conducted in some parts of the district, but little information is recorded about these operations. The thick deposits at Quaker Hill were hydraulically mined until a declining gradient prevented the use of sluices. This prevented much of the deeper and richer gravels from being worked, but a shaft was sunk and some drifting was done. As a result, much of the lower deposit at Quaker Hill remains unworked. Lindgren (1911) estimated that 140 million yards remained unworked in this deposit alone. An insufficient gradient at Scotts Flat also hampered the complete development of these deposits. At Hunts Hill, bedrock was high enough to allow the entire deposit to be hydraulically mined without elevators. However, the lowermost 50 feet of gravel was so well cemented that it required crushing in a stamp mill before sluicing. Water for the Hunts Hill and Quaker Hill workings came from two ditches, the Union Ditch and the Jacobs and Sergeant Ditch. The relatively small deposit of upper gravel at Buckeye Hill was completely removed by hydraulic mining After hydraulic mining ended in the 1880s, many of the tailings were worked by Chinese miners.

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 (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.

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 (Commodity): Commodity Info: Upper gravels at Scotts Flat and Buckeye Hill averaged $0.10 - $0.11 per cubic yard ($35/oz). Undifferentiated upper and lower gravels averaged $0.255 per cubic yard ($35/oz).

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 (Identification): The Scotts Flat District is in west-central Nevada County about eight miles east of Nevada City. It includes the Tertiary placer deposits at Scotts Flat, Quaker Hill, Hunts Hill, Buckeye Hill, and Burrington Hill. The You Bet-Red Dog District lies immediately to the south, the Nevada City District to the west, and the Blue Tent District to the north. The original site of Scotts Flat is currently beneath the present day Scotts Flat Reservoir.

Comment (Location): Location selected for latitude and longitude is the village of Scotts Flat shown on the Colfax Quadrangle of Lindgren?s (1900) Colfax Folio. The site is now underwater within Scotts Flat Reservoir.

Comment (Deposit): The Scotts Flat District contains several separate hydraulic workings including the "diggings" at Scotts Flat, Quaker Hill, Hunts Hill, Buckeye Hill, and Burrington Hill. The auriferous gravels mined here were deposited by a tributary to the Tertiary Yuba River. The tributary flowed northward from the neighboring Dutch Flat and You Bet-Red Dog districts, through the Scotts Flat District and continued northward through the Blue Tent District to its confluence with the Yuba River within the North Columbia District. In the primary workings at Hunts Hill and Quaker Hill, the main channel is nearly 600 feet deep with overlying and widespread bench gravels up to 300 feet deep. Similar to neighboring Tertiary gravel deposits, the deposits can be divided lithologically and texturally into a lower and upper unit. The lower and richest unit, or blue lead of the early miners, rests directly on bedrock and is confined to the incised bedrock channel. These deeper gravels are quartzose, well-cemented, and locally very rich in gold. The overlying upper gravels form the majority of the gravel deposits. At Buckeye Hill, they formed the entire deposit. They overlie the coarser gravels in the bedrock channel and extend as bench gravels well beyond the edges of the bedrock channel.

Comment (Geology): Similar to neighboring Tertiary gravel deposits, the deposits can be divided lithologically and texturally into a lower and upper unit. The lower and richest unit, or blue lead of the early miners, rests directly on bedrock and is confined to the incised bedrock channel. These deeper gravels are quartzose, well cemented, and locally very rich in gold. The lower gravels are generally immature and composed of bluish-black slate and phyllite of the Calaveras Complex, weathered igneous rocks, and quartz. Large boulders are common. Chlorite, amphibole, and epidote mineral grains are also common components. Underlying bedrock is slate and argillite with some chert. Bedrock is in part polished and hummocky with slates being in part soft and decomposed. The lower gravels were extensively drifted mined in neighboring districts and at Quaker Hill the lowermost 4-16 feet were rich enough to allow some profitable drifting. The overlying upper gravels form the majority of the gravel deposits. At Buckeye Hill, they formed the entire deposit. They overlie the coarser gravels in the bedrock channel and extend as bench gravels well beyond the edges of the bedrock channel. These gravels are well-exposed in cliffs and bluffs along the old river channels. This unit's thickness varies significantly within the district. These gravels are much finer, with clasts seldom larger than pebble size and characterized by an abundance of quartz sand and clay and silt beds. Large-scale cross-bedding and cut-and-fill features are common. Upper gravels are mature; quartz predominates, and the heavy-mineral content consists almost exclusively of zircon, ilmenite, and magnetite. Average values of the upper gravels at the Scotts Flat and Buckeye Hill workings averaged $0.10 - $0.11 per cubic yard ($35.00 gold). Average values of the lower gravels are not available, but an averages for undifferentiated upper and lower units has been reported to be about $0.25 per cubic yard.

Comment (Economic Factors): In 1911, Lindgren estimated that 12 million cubic yards of gravel had been removed from Scotts Flat and another 35 million from Quaker Hill. He also estimated that as much as 140 million cubic yards remained at Quaker Hill. He also estimated that 6.75 million yards had been removed, and 4-5 million remained, at Hunts Hill.

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 SCOTTS FLAT DISTRICT In the Scotts Flat District, basement rocks of the Calaveras Complex and the overlying Eocene auriferous gravels are overlain by Valley Springs and Mehrten Formation rocks in the northern part of the district and on Quaker Hill. The main body of basement rocks within the district consists of a belt of north-northwest-trending steeply dipping, slate, chert and argillite of the Calaveras Complex. Gabbroic and serpentinite intrusions are common. Much of the district lies between two northerly branches of the Foresthill Fault, a steep northerly trending and easterly dipping thrust fault that cuts the Calaveras Complex. To the east of the district, the Melones Fault Zone (Clark, 1960) separates the Calaveras Complex from the ultramafic rocks of the Feather River Peridotite Belt. Basal Eocene Auriferous Gravels The Scotts Flat District contains several separate hydraulic workings including the "diggings" at Scotts Flat, Quaker Hill, Hunts Hill, Buckeye Hill, and Burrington Hill. Throughout much of the district, erosion of the Valley Springs and Mehrten Formations has exposed large areas of Tertiary gravels. At Quaker Hill and Burrington Hill, the auriferous gravels are overlain by Valley Springs and Mehrten deposits. The auriferous gravels were deposited by a tributary to the Tertiary Yuba River. The tributary flowed northward from the neighboring Dutch Flat and You Bet-Red Dog districts, through the Scotts Flat District and continued northward through the Blue Tent District to its confluence with the Yuba River within the North Columbia District. A smaller southwesterly draining tributary also enters the Scotts Flat district from the Burrington Hill area and joins the lager tributary at Hunts Hill. In the primary workings at Hunts Hill and Quaker Hill, the main channel is nearly 600 feet deep with overlying and widespread bench gravels up to 300 feet deep. The Hunts Hill gravels are continuous with those at Quaker Hill, but the overlying ridge of Mehrten volcanics separates the pits. The deep main channel is believed to be continuous all the way from Hunts Hill to the Blue Tent District, a distance of seven miles (Clark, 1970).

Comment (Workings): 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. 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.


References

Reference (Deposit): Yeend, W.E., 1974, Gold-bearing gravel of the ancestral Yuba River, Sierra Nevada, California: U.S. Geological Survey Professional Paper 772, 44 p.

Reference (Deposit): Clark, L. D., 1960, Foothills fault system, western Sierra Nevada, California: Geological Society of America Bulletin, v. 71, p. 483-496.

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 114-115.

Reference (Deposit): Jarman, A, 1927, Hunts Hill, Quaker Hill, and Buckeye Hill: California State Mining Bureau report 23, p. 100-101.

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. 143-144.

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.