Iowa Hill District

Site Name

Primary: Iowa Hill District

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Commodity

Primary: Gold
Secondary: Platinum
Secondary: Silver

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Location

State: California
County: Placer
District: Iowa Hill District

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Land Status

Land ownership: BLM Administrative Area
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: Folsom Field Office

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Holdings

Not available

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Workings

Not available

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Ownership

Owner Name: Various private owners

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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: 1853
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: Forest Hill Fault, Melones Fault Zone

Type: R
Description: Gills Hill Fault, Forest Hill 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 (Deposit): The district produced from extensive auriferous channel gravels, most of which were deposited by a main channel of the ancestral Tertiary American River, which traversed the district from northeast to southwest. The deposits consist of a richer lower unit and a leaner upper unit. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. The deep gravels are generally well-cemented and quartz rich. Lower gravels are generally immature and composed of bluish-black slate and phyllite. There were also numerous other branches and contemporaneous and younger tributary channels. Where preserved, these channels contain varying thicknesses of intercalated rhyolite tuff and intervolcanic, frequently auriferous, channel gravels of the Valley Springs Formation. The beds of gravel, sand, and pipe clay, can extend well beyond the limits of the lowermost bedrock channel.

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): One of the most important mines in the district was the Big Dipper mine (also called the Waterhouse & Dorn Mine). This mine worked the upstream portion of the Morning Star Channel (the Morning Star Mine worked the downstream part of the same channel). This gravel deposit was hydraulically mined from 1858 to 1882, during which time it produced $100,000 - $200,000. Thereafter it became an important drift mine and was operated continuously between 1890 and 1902. During these twelve years the mine produced over $1 million. The channel was drifted 4700 feet downstream until it intersected the Morning Star Mine workings. The drifts followed the highly cemented blue-lead gravel in the lowest part of the bedrock channel. In the Big Dipper Mine, the gravel was extracted through a bedrock tunnel 37 feet below the channel with raises into the gravel. Only the lower 6 feet of gravel was drifted. Breasts as much as 370 feet wide were opened with posts eight feet apart. In 1890, the lower gravel yielded as much $9 per ton with the average said to be $6/ton. The last work done on the Big Dipper Mine was done between 1917 - 1920 by the Stanislaus Development Company. Prospect shafts were sunk, and an open-pit cut 600 feet long by 4 feet wide and up to 50 feet deep was made in search of unworked gravel. Only a few thousand tons of low-grade gravel from prior operations was milled. Brief descriptions of the workings of several noteworthy mines are contained in the reports of the California State Mining Bureau. Irelan (1888) described the geology and early workings of the Morning Star Mine. Hobson (1890) gives a brief description of the workings in the important Big Dipper Mine. Logan (1936) discusses the geology and operations in several mines including the Jupiter and Long Point mines. A generic discussion of hydraulic and drift mining techniques follows: 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.

Comment (Identification): The Iowa Hill District is in central Placer County in the vicinity of the old mining town by the same name. The town of Colfax is about 5 miles to the west. It is a large placer and drift mining district covering an area north of the American River, along Shirttail Canyon and Indian Creek in the vicinity of the Iowa Hill Divide. It includes workings at Roach Hill, Monona Flat, Strawberry Flat, Succor Flat, Grizzly Flat, Shirttail Canyon, and the Kings Hill area. A few small gold-quartz veins occur in the district, but are of only minor importance. Many mines operated in the district including the Big Dipper, Blue Wing Quartz, Brunn, Buckeye, Campbell, Canyon, Carey, Copper Bottom, Dewey Consolidated, Drummond, Elizabeth Hill, Excelsior, Fitzpatrick, Gleason, Golden Star, Golden Streak, Goodwin, Haymes, H and H, Iowa Hill, Irish and Byrne, Jupiter, Keystone, King's Hill Point, King's Hill Quartz, Lebanon, Mohawk, Morning Star, Occidental, Jupiter, Penn Valley, Randal, Roach Hill, Star United, Strawberry, Twenty One, Welcome, Winchester, Wisconsin Hill, and Union.

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 (Workings): 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. 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.

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

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 IOWA HILL DISTRICT Throughout most of the district, only the basement rocks of the Calaveras Complex are exposed. However, thick sections of Oligocene to Pliocene Valley Springs and Mehrten Formation rocks overlying Eocene auriferous gravels are present in the east and northeast parts of the district, particularly along Strawberry Hill Divide and extending southwestward to Grizzly Flat and Prospect Hill. The main body of basement rocks consists of a belt of north-northwest-trending steeply dipping, slate, argillite, amphibolite, phyllite, chert, and metavolcanic rocks. Gabbroic and serpentinite intrusions are common. The Foresthill Fault, a steep easterly dipping thrust fault trends north-south through the district and cuts the Calaveras Complex. Immediately east of the district, the Melones Fault Zone (Clark, 1960) separates the Calaveras Complex from partially to completely serpentinized peridotite of the Feather River Peridotite Belt. Basal Eocene Auriferous Gravels The district produced from extensive auriferous channel gravels, most of which were deposited by a main channel of the ancestral Tertiary American River, which traversed the district from northeast to southwest. There were also numerous other branches and contemporaneous and younger tributary channels. These channels included the Succor Flat intervolcanic channel, which entered the district from the northeast, the westerly trending Morning Star and Grizzly Flat deep channels, and lesser channels known as the Golden Gate, Wolverine, Glencoe, Long Point Volcanic, and Vigilante volcanic channels. The extensive deposits consist of a richer lower unit and a leaner upper unit. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. The deep gravels are generally well-cemented and quartz rich. Lower gravels are generally immature and composed of bluish-black slate and phyllite. Chlorite, amphibole, and epidote are common accessory mineral components.

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

Comment (Location): Location selected for latitude and longitude is the community of Iowa Hill (adjacent to the old hydraulic workings) on the USGS 7 1/2-minute Foresthill quadrangle

Comment (Geology): The deep and lowermost gravels at Morning Star and Grizzly Flat rest directly on bedrock in well-defined, incised bedrock channels. These gravels are well-cemented and in places yielded 0.5 ounce of gold or more per yard. Generally, the lowest 7 feet were the richest, but some equally rich bench gravels were encountered in bench gravels peripheral to the deep channels. In places, deep and rich bedrock potholes occur in the phyllite and slate. Bedrock is uneven and in part polished and hummocky with slates sometimes being in part soft and decomposed. Harder slates alternating with beds of softer slate make effective riffles for trapping gold nuggets. The fill in the Morning Star deep channel is composed of rounded cobbles, boulders, and gravel of highly cemented metamorphic clasts (blue-lead gravels) overlain by a thick sequence of uncemented finer metamorphic gravels. These basal gravels were worked for several thousand feet in the Big Dipper Mine. Both units are highly auriferous and reach an aggregate thickness of about 300 feet. The Succor Flat deep channel shows a high percentage of serpentinite boulders and gravel indicating its traverse of the nearby Feather River Peridotite Belt to the east. Intervolcanic Channels Where preserved, overlying the basal Eocene channels are varying thicknesses of intercalated rhyolite tuff and intervolcanic, frequently auriferous, channel gravels of the Valley Springs Formation. The thickness of the sequence is highly variable and these beds of gravel, sand, and pipe clay, can extend well beyond the limits of the lowermost bedrock channel. The geometry of the intervolcanic paleochannels can be complex, with each channel representing a periodic displacement of the stream, a distinct cut with a deposit of gravel, and finally a volcanic event that filled the cut and buried the gravel. The frequent diversion and reestablishment of the intervolcanic channels, and subsequent erosion of earlier channels makes it very difficult to correlate these channels with any certainty. Intervolcanic channels were deposited during a period of increasing gradient and are characteristically narrower and deeper, the flanks steeper, and the accumulations of bedrock gravel significantly less than those of the older basement channels. Gravel thicknesses in the smaller of these channels are generally several inches to fifteen feet and are generally dominated by rounded volcanic gravel unless that stream cut deeply enough to erode older deposits or basement. There are a few gold-quartz veins in the district.

Comment (Development): Placer mining began along the American River and its tributaries soon after the discovery of gold in 1848 at Sutter's Mill and the onset of the Gold Rush. In 1853, miners prospecting the North Fork of the American River discovered gold along the north flank of a ridge between the North Fork of the American River and Indian Canyon. The miners made camp atop the ridge and christened it Iowa Hill and from then on the ridge became known as the Iowa Hill Divide. Further prospecting revealed that the rich Tertiary gravel deposits were perched above the American River on the flanks and underlying the divide. The camp grew around the Jamison claim, which was first drift mined and later hydraulically mined. As new exposures were located along the divide more mining camps sprung up in the district including Independence Hill, Rich Hill, Grizzly Flat and Succor Flat. During the first few years drift mining prevailed, with many miles of drifts being driven under the divide to exploit the rich blue lead gravels. The North Star Mine was the first in the state to use a stamp mill to crush the hard cemented basal blue-lead gravels. When these gravels began to play out, attention turned to hydraulic mining of the finer gravels along the flanks of the divide. By 1856, the district was producing more than $100,000 a week. In 1857, the town of Iowa Hill was leveled by fire. Never rebuilt to its prior state, the town burned down twice more in later years. By 1880, more than $20 million had been produced, shortly after which hydraulic mining was curtailed by the Sawyer Decision in 1884. After cessation of hydraulic mining, drift mining became popular again and continued through the early 1900s. The district saw a significant burst of activity again in the 1930s. In 1934, the Iowa Hill Mining Company reopened an old adit and advanced it to 1600 feet with a raise 70 feet into gravel. Work was suspended in 1935 after producing only 1,000 yards. Equipment included an 8-foot Price mill, Dorr classifier, trommel, two Deister concentrators, compressor, ore car, tractor, trucks, drill sharpener, electric generator and several buildings. The Big Dipper, Occidental and other mines were worked on a small scale as recently as the 1960s.

Comment (Economic Factors): Two of the most noteworthy of the Iowa Hill district mines were the Big Dipper and Gleason, which produced $1.2 million and $1 million+, respectively.

Comment (Workings): 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.

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References

Reference (Deposit): Lindgren, W., 1911, Tertiary gravels of the Sierra Nevada: U.S. Geological Survey Professional Paper 73, p. 148-149.

Reference (Deposit): Logan, C.A., 1936, Gold mines of Placer County: Big Dipper, Morning Star and Succor Flat Channel mines: California Division of Mines Report 32, p. 52-54, 65-66, and 79.

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: Iowa Hill District: California State Mining Bureau Report 16, p. 38.

Reference (Deposit): Hobson, J.B., 1890, Iowa Hill mining district: California State Mining Bureau report 10, p. 419-425.

Reference (Deposit): Jarman, A., 1927, Iowa Hill: California State Mining Bureau Report 23, p. 86-87.

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

Reference (Deposit): Chandra, D.K., 1961, Geology and mineral deposits of the Colfax and Foresthill quadrangles: California Division of Mines Special Report 67, 50 p.

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 67-69.

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