North Bloomfield District

Site Name

Primary: North Bloomfield District

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Commodity

Primary: Gold
Secondary: Platinum
Secondary: Silver

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Location

State: California
County: Nevada
District: North Bloomfield 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

Owner Name: private owners

Owner Name: U.S. Forest Service

Owner Name: California Dept. of Parks and Recreation

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Production

Not available

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Deposit

Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer
Operation Type: Surface-Underground
Discovery Year: 1851
Years of Production:
Organization:
Significant: Y

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Physiography

Not available

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Mineral Deposit Model

Model Name: Placer Au-PGE

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Orebody

Form: Irregular

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Structure

Type: L
Description: Ramshorn Fault

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

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Alterations

Not available

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Rocks

Name: Sand and 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
Gangue: Chlorite
Gangue: Epidote
Gangue: Amphibole
Gangue: Pyrite
Gangue: Zircon
Gangue: Ilmenite
Gangue: Magnetite
Gangue: Quartz
Gangue: Siderite

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Comments

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 (Economic Factors): The total output of the Malakoff Diggings alone was about $3.5 million, and the Derbec pit yielded $1 - $2 million (Lindgren, 1911). Lindgren also estimated that 30 million cubic yards had been removed and 130 million remain at North Bloomfield. Jarman (1927) estimated that 40 million yards had been removed and that more than 50 million remained.

Comment (Deposit): Due to extensive erosion of the Valley Springs and Mehrten Formation formations, the North Bloomfield and neighboring districts are known for immense bodies of exposed auriferous gravels. The gravels were deposited by a main branch of the Tertiary Yuba River. This branch flowed southwestward from the vicinity of Snow Point in the adjacent Moore's Flat District and entered the North Bloomfield District near Derbec. It then continued southwestward, depositing the extensive placer deposits near North Bloomfield. A smaller tributary joined the river from Relief Hill to the southeast. The deposits can be divided lithologically and texturally into a lower and upper unit. Total thickness of both units within the district reached upwards of 600 feet. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. At the Malakoff Diggings, the lower unit consists of cobbles, pebbles, and boulders. At North Bloomfield, the best recoveries came from the lower 130 feet of blue gravels. The deepest gravels are generally well-cemented and quartz-rich. The upper gravels form the majority of the deposits. In the Malakoff pit, the upper gravel unit is at least 325 feet thick. 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. Upper gravels generally have significantly lower values than the deeper gravels and are commonly barren.

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 (Location): Location selected for latitude and longitude is the approximate center of the Malakoff Diggings hydraulic workings in section 1-T17N-R9E as shown on the USGS 7 1/2-minute North Bloomfield quadrangle

Comment (Development): Placer gold was first discovered in the North Bloomfield District in 1851 by three prospectors scouring the hills near San Juan Ridge. After discovering gold in the Tertiary gravels they set up camp and started mining. Running short of supplies, one miner went to town and attracted attention by paying for his drinks in the local saloon with gold nuggets. On his way back to camp he was secretly followed. The newcomers, however, after trying their luck ended up empty handed. They named the local creek Humbug Creek and moved on. Hydraulic mining of the gravel deposits began in 1853 and the town of Humbug was established. By 1855, hydraulic mining had become a major industry, and Humbug became a regional mining center. In search of a more respectable name, Humbug was renamed Bloomfield, but when it was discovered that a town named Bloomfield already existed in Sonoma County, it was again renamed North Bloomfield. North Bloomfield prospered and eventually had over 2000 residents. By 1860, many of the individual claims were purchased and consolidated into the North Bloomfield Gravel Mining Company, which pursued a coordinated effort in the Malakoff Diggings. Hydraulic mining techniques were fully developed in the Malakoff pit, which eventually became California's largest single hydraulic operation. To support the operation, as well as those in neighboring districts along San Juan Ridge, the three leading mining companies (North Bloomfield Gravel Mining Co., Milton Mining and Water Co., and Eureka Lake and Yuba Canal Co.) spent over $5 million constructing an extensive 320 mile system of ditches, flumes, and reservoirs to bring water to the mines from Bowman lake and other sources in the High Sierra. In order to manage the water deliveries, in 1878 the Edison Company built the world's first long distance telephone line at a cost of $6,000. The lines ran 60 miles from French Corral to the west to Bowman Lake and connecting the intervening mining areas of Birchville, North San Juan, Cherokee, North Columbia, Lake City, North Bloomfield, Moore's Flat, Graniteville, and Milton. To provide better drainage and disposal of tailings, the North Bloomfield Gravel Mining Company dug a 7,874- foot-long tunnel through bedrock under the Malakoff pit. Completed in 1874, the tunnel allowed the wholesale disposal of tailings directly into the South Yuba River rather than little Humbug Creek. With completion of the tunnel, the North Bloomfield Gravel Mining Company reached its zenith, continuously operating seven monitors, round the clock, and processing about 50 thousand tons of gravel a day. In 1880, electric lights were installed in the Malakoff pit and the world's first long distance telephone line was installed to service and monitor activities in the pit and the extensive water supply network. As more and more gravel was extracted, the pits became enormous. At the Malakoff Diggings, California's largest hydraulic mine, the pit measured more than 7,000 feet long, 3,000 feet wide, and originally up to 600 feet deep. Wholesale disposal of the tailings ultimately caused the downstream choking of the Sacramento and Yuba rivers making them unnavigable and flooding of important farmland in the Sacramento Valley. At Marysville, debris from the operations filled the Yuba River until the river bottom was higher than the adjacent town. Subsequent litigation resulted in the famous court case in 1884 (Woodruff vs. North Bloomfield Gravel Mining Co.). The final ruling, which came to be known as the Sawyer Decision, resulted in an injunction against dumping mine debris into the Sacramento and San Joaquin rivers and their tributaries. The Sawyer decision effectively spelled the end of large-scale hydraulic mining in the Sierra Nevada.

Comment (Geology): As is common in most of the auriferous Tertiary gravels of the northern Sierra Nevada, the deposits can be divided lithologically and texturally into a lower and upper unit. Total thickness of both units within the district reached upwards of 600 feet. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. Gold was .900 fine. At the Malakoff Diggings, the lower unit consists of cobbles, pebbles, and boulders. The deep gravels are generally well-cemented and quartz-rich. At North Bloomfield, the best recoveries came from the lower 130 feet of blue gravels, with average values in the Malakoff Diggings being approximately $0.56 per cubic yard ($35.00 oz. gold). At the Derbec working, the lowermost gravels yielded an average $6.30 per cubic yard ($35.00 oz. gold). Lower gravels are generally immature and composed of bluish-black slate and phyllite of the Calaveras Complex, weathered igneous rocks, and quartz. In the Malakoff Diggings, large boulders of siliceous phyllite 6-10 feet in diameter rest on bedrock within the lower gravels. The upper gravels form the majority of the deposits and, unlike the lower gravels, are often well-exposed in cliffs and bluffs along the old river channels. In the Malakoff pit, the upper gravel unit is at least 325 feet thick. 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 generally have significantly lower values than the deeper gravels and are often barren. Average values in the upper gravels at Malakoff ran $0.49 - $0.67 per yard ($35.00 oz. gold). Clay beds exposed in the old pit walls have frequently failed and mud and earth flows in the old pit are common.

Comment (Identification): The North Bloomfield District is in north-central Nevada County about 10 miles northeast of Nevada City. The district includes the workings adjacent to the town of North Bloomfield as well as the placer workings at Lake City to the west, Derbec to the north, and Relief Hill to the east. The district is renowned for the enormous size of some of its hydraulic pits, especially the famous Malakoff Diggings, the largest hydraulic mine in California. The pit ultimately measured 7,000 feet long and 600 feet deep. The district's hydraulic workings were so extensive that they were largely responsible for the silting up of the Sacramento and Yuba rivers leading to the Sawyer Decision in 1884 and the demise of hydraulic mining in the Sierra Nevada.

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): After the 1884 Sawyer Decision had brought an end to hydraulic mining at North Bloomfield, drift mining was implemented at several locations to recover the lower gravels. By 1888, 10,135 feet of tunnels had been driven under the former Malakoff hydraulic pit. The pay channel averaged 400 feet wide. The blue lead gravel was in places as much as 135 feet thick and extremely cemented. Gravel required blasting and breaking with sledges before washing. Washing was done with 3 monitors. A hydraulic gravel elevator raised the gravel to a flume in which it was washed and conducted to an impounding reservoir Hydraulic mining was also replaced by drift mining at Relief Hill. By 1919, the Union Tunnel had been driven 2,500 feet from which $30,000 to $90,000 was produced annually for a number of years. Gravels in the Derbec area were not hydraulicked. At the Derbec workings, drifts exploited a generally easterly trending section of the channel, which was buried deep under San Juan Ridge. The gravel deposits are 415 feet thick at the shaft. The basal gravels extracted ranged front 6-18 feet thick. The width of the channel varied greatly. The gravel was breasted to a width ranging from 150 feet to 600 feet. Bedrock in the tunnel consisted of slate, which at places swelled. The drift mine was accessed by a shaft 475 feet deep with a steam-powered hoist works to raise the ore. A main bedrock drift ran from the foot of the shaft up the bedrock channel. Careful timbering of the tunnel was required to shore up weak roof rocks. 4,500 feet up the drift from the shaft, an upraise to the surface allowed emergency escape. The ore was moved in 1,550 pound ore cars. A drain tunnel 5,000 feet long extended from the bottom of the shaft to the town of North Bloomfield. Cross drifts were run from the main pay channel to the west and the gravel breasted out. The pay gravel was not cemented and included fine gravel to boulders. Gold was extracted by washing. Gravel was first washed through a 600-foot flume then impounded to allow complete slaking. It was then flushed down a 4,000-foot flume with the tailings being impounded in a series of impoundments. Flumes were two feet wide by two feet deep. Monitors were used on the washing dump. The historic town of North Bloomfield and the Malakoff Diggings are now within the Malakoff Diggings State Historic Park administered by the California Department of Parks and Recreation. In 1966, the U.S. Geological Survey initiated an exploration program in the district to asses the extent and quantity of unmined gravel.

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. More information is contained in the Exploration and Development History section.

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): 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 NORTH BLOOMFIELD DISTRICT In the North Bloomfield District, basement rocks include metasedimentary rocks of the Lake Combie-Slate Creek Complexes on the west and metavolcanic rocks, slates, and argillites of the Calaveras Complex to the east. These differing basement rocks are juxtaposed along the Ramshorn Fault, which bisects the district in a north-northwesterly direction. Bedrock is overlain by thick Eocene auriferous gravels that are in turn overlain by Valley Springs and Mehrten Formation rocks on San Juan Ridge in the north part of the district. Elsewhere in the district, the Valley Springs and Mehrten rocks have been stripped by erosion exposing the underlying gravels. Basal Eocene Auriferous Gravels Due to extensive erosion of the Valley Springs and Mehrten Formation formations, the North Bloomfield and neighboring districts are known for immense bodies of exposed gravels. The North Bloomfield District contains several separate hydraulic workings including the extensive workings at Malakoff Diggings west of the town of North Bloomfield, and smaller operations at Lake City (1 mile west), Derbec (2 miles northeast), and Relief Hill (2 miles southeast). In part because of the volcanic capping of San Juan Ridge, which encroached upon the workings at Lake City, Derbec, and Relief, these workings were limited in scale relative to the Malakoff workings. The auriferous gravels were deposited by a main branch of the Tertiary Yuba River. This branch flowed southwestward from the vicinity of Snow Point in the adjacent Moore's Flat District and entered the North Bloomfield District near Derbec. It then continued southwestward, depositing the extensive placer deposits near North Bloomfield. A smaller tributary joined the river from Relief Hill to the southeast. The main channel curves west and north at North Bloomfield, then westward again before exiting the district.

Comment (Commodity): Commodity Info: Average value of lower gravel at Malakoff Diggings was $0.56 per cubic yard ($35/oz). Average value of upper gravel was $0.049-$0.067 per cubic yard ($35/oz). Average value of lower gravels at Derbec workings was $6.30 per cubic yard ($35/oz). Average value of the undifferentiated gravels at Relief Hill ranged from $0.15-$0.22 per cubic yard ($35/oz).

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References

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

Reference (Deposit): Hobson, J.B. and Wiltsee, E.A., 1893, North Bloomfield mining district; California State Mining Bureau Report 11, p. 311-312.

Reference (Deposit): Irelan, W., Jr., 1888, North Bloomfield mine: California State Mining Bureau Report 8, p. 454-459.

Reference (Deposit): Jarman, A, 1927, An investigation of the feasibility of any plan or plans whereby hydraulic mining operations can be resumed in this state: California State Mining Bureau Report 23, p. 103-111.

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. 139-141.

Reference (Deposit): MacBoyle, E., 1919, Nevada County: North Bloomfield mining district: California State Mining Bureau Report 16, p. 45-51.

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

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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.