Iowa Hill Mining District, Placer County, California, USAi

Regional Level Types
Iowa Hill Mining District	Mining District
Placer County	County
California	State
USA	Country

A former Au-Ag-Pt mining district located in secs. 1-12, 17, 18 & 19, T14N, R10E, and in secs. 22-28 & 32-36, T15N, R10E, MDM, in central Placer County, 5 miles E of Colfax. Discovered in 1853.

Location: The Iowa Hill district is in central Placer County in the vicinity of the old mining town of that name. It is an extensive placer-mining district that includes the Roach Hill, Monona Flat, Strawberry Flat, Succor Flat, Grizzly Flat, Shirttail Canyon, and Kings
Hill areas.

History: Placer mining began along the American River and its tributaries soon after the beginning of the gold rush. Hydraulic and drift mining apparently began here in 1853, and by 1856 the output was as high as $100,000 (period values) per week. By 1880 more than $20 million (period values) had been produced from the district. Drift mining continued through the early 1900's, and there was appreciable activity again in the 1930's. Most of the town was destroyed by fire in 1922. The Big Dipper, Occidental and a few other mines have been intermittently worked in recent years. Also snipers and skin divers have been active in the district.

Comments on 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 1900's. The district saw a significant burst of activity again in the 1930's. 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.

Geology: A main Tertiary channel of the American River crosses the area. There are numerous branches and intervolcanic channels, including the Succor Flat intervolcanic channel, which comes in from the northeast, and the west-trending Morning Star and Grizzly Flat deep channels. The deep channel gravels are well-cemented and in places yielded ½ ounce of Au or more per cubic yard. The lowest seven feet were the richest but there also were some rich benches. The bedrock is uneven, and consists of hard slate and phyllite of the Cape Horn Formation (Carboniferous) and amphibolite, which contains a number of deep and rich potholes. To the east the gravels are overlain by thick beds of andesite. There are a few gold-quartz veins in the 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.

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.

Ore bodies are irregular. Mineral occurrence model information: Model code: 119; USGS model code: 39a; BC deposit profile: C01. C02; deposit model name: Placer Au-PGE; Mark3 model number: 54. Host rocks include unconsolidated Tertiary sand and gravels. Controls for ore emplacement included mechanical accumulation on irregular bedrock riffles and within river- and stream-channel lag gravels, bars, and point bar deposits. Local rocks include Jurassic marine rocks, unit 1 (Western Sierra Nevada and Western Klamath Mountains).

Regional geologic structures include the Gills Hill Fault, Forest Hill Fault, and the Melones Fault Zone. Local structures include the Forest Hill Fault and the Melones Fault Zone.

Commodity Information: Ore Materials: native gold - fine to coarse gold and nuggets (.900 fine); gangue materials: Quartz and metamorphic gravels; accessory minerals magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite.

Major Mines and General Comments: 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 to $200,000 (period values). 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 (period values).

The channel was drifted 4,700 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 (period values) per ton with the average said to be $6/ton (period values).

The last work done on the Big Dipper Mine was done between 1917 to 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-1880's 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 1890's, 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.

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 to 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 to 2 feet of soft bedrock and 3 to 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.

Mines: Big Dipper ($1.2 million), Blue Wing Quartz, Brunn, Buckeye, Campbell, Canyon, Carey, Copper Bottom, Dewey Consolidated, Drummond, Elizabeth Hill, Excelsior, Fitzpatrick, Gleason ($1 million+), Golden Star, Golden Streak, Goodwin, Haymes, H and H, Iowa Hill, Irish and Bryne, Jupiter, Keystone, King's Hill Point, King's Hill Quartz, Lebanon, Mohawk, Morning Star, Occidental, Old Jupiter, Penn Valley, Randall, Roach Hill, Star United, Strawberry, Twenty One, Welcome, Winchester, Wisconsin Hill, Union.

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Link to USGS MRDS:	10310631

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References