Michigan Bluff District

Past Producer in Placer county in California, United States with commodities Gold, Silver, Platinum, Copper, Zinc, Iron, Lead, Indium

Sections on this page

Identification information
Geographic coordinates
Site location context
Geographic areas
Public Land Survey System information
Commodities
Materials information
Alteration
Mineral occurrence model information
Host and associated rocks
Nearby scientific data
Geologic structures
Ore body information
Controls for ore emplacement
Economic information about the deposit and operations
Mining district
Land status
Ownership information
Bibliographic references
General comments
Reporter information

Geologic information

Identification information

Deposit ID	10310649
Record type	District
Current site name	Michigan Bluff District
Alternate or previous names	Adams, Anna Sue, Argonaut & Sunset, Baker Ranch, Baker Divide, Beehive, Big Gun, Bogus Thunder, Boston, Bowen, Bower, Britt, Buckeye, Burnham, Burns, Burroughs, De Maria, Drummond, Eastman, Eldorado Hill, Franklin, Georgia Consolidated, Golden Chief, Golden Gate, Golden Gem Placer, Golden Sheaf, Gorman, Hazard, Hermit, Hoffman, Horseshoe Bar Placer, Imperial, Lightfoot, Manhattan, Marian, Mary Anna, Mountain Chief, Muir Tunnel Consolidated, North American, Oro, Pleasant Bar, Rainbow Land, Red Hill, Russell, Sage Hill, South Dakota, Swift Shore, Thompson & Powell, Turkey Hill Consolidated, Van Emon, Washburn, Washington, Weeks, Weske, Wills, Volcano, American Bar Quartz, Bunker & Nihill Quartz, Champion Quartz, Daniel Webster Quartz, Golden Sheaf Quartz

Comments on the site identification

The Michigan Bluff Mining District is located 5 miles east of the town of Foresthill in south central Placer County, California. The district includes all placer and quartz gold mines in the Byrds Valley and Chicken Hawk Ridge areas from Baker Ranch on the west to approximately 2 miles east of Michigan Bluff, and between the Middle Fork of the American River on the south and the Gas Hill Mine on the north. The Gas Hill Mine (Sec. 2, T14N, R11E) marks the southern limit of the adjacent Damascus Mining District to the north. The Forest Hill and Last Chance mining districts lie to the west and northeast, respectively. The district trends northeast-southwest along the crest and southeast flank of the Forest Hill Divide, a northeast-southwest trending drainage divide separating the North and Middle Forks of the American River. The district is primarily a placer-gold district, the majority of production having come from drift and hydraulic mining of Tertiary gravel deposits. Very little was produced from gold-quartz lode mines within the bedrock complex.

Geographic coordinates

Geographic coordinates:	-120.73681, 39.04183 (WGS84)
Elevation	365
Location accuracy	100(meters)
Relative position	20 miles northeast of Auburn.

Site location context

Political divisions (FIPS codes)

Placer(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Michigan Bluff(quadrangle 1:24,000 scale)

Truckee(quadrangle 1:100,000 scale)

Chico(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

North Fork American(hydrologic unit)

Lower Sacramento(hydrologic accounting unit)

Sacramento(hydrologic subregion)

California(hydrologic region)

Federal lands

Tahoe National Forest(National Forest)

National Forest FS(Type of land area)

FS(Federal land areas administered by FS)

Geographic areas

Country	State	County
United States	California	Placer

Public Land Survey System information

Meridian	Township	Range	Section	Fraction	State
Mount Diablo	13N	11E	3-5	N/2	California
Mount Diablo	14N	11E	2, 3, 9-11, 14-17, 20-23, 26-29, 32-35	All	California

Comments on the location information

The Michigan Bluff District includes a number of individual mines distributed throughout an area encompassing approximately 18-20 square miles. Since the majority of mines and claims were located around the community of Michigan Bluff, the community itself was chosen to represent the district?s location. The location latitude and longitude identify the intersection of Michigan Bluff Road and Gorman Ranch Road near the center of town on the USGS Michigan Bluff 7.5-minute quadrangle (within the W/2 W/2, Sec. 22, T14N, R11E, MDBM). Michigan Bluff is reached by taking the paved Foresthill Road from Auburn, California, for a distance of about 25 miles, then turning south on the paved Michigan Bluff Road and continuing on for an additional 3 miles.

Commodities

Commodity	Importance
Gold	Primary
Silver	Secondary
Platinum Critical	Secondary
Copper	Tertiary
Zinc Critical	Tertiary
Iron	Tertiary
Lead	Tertiary
Indium Critical	Tertiary

Comments on the commodity information

Commodity Info: Placer deposits: Placer gold dust to large nuggets.
Lode deposits: Free-milling gold-bearing quartz veins.
Ore Materials: Native gold
Gangue Materials: Quartz gravels; quartz

Materials information

Materials	Type of material
Gold	Ore
Gravel	Ore
Quartz	Ore

Alteration

(Local) None?Negligible - none described

Mineral occurrence model information

Model code	119
USGS model code	39a
Deposit model name	Placer Au-PGE
Mark3 model number	54
Model code	273
USGS model code	36a
Deposit model name	Low-sulfide Au-quartz vein
Mark3 model number	27

Host and associated rocks

Host or associated Host

Rock type Unconsolidated Deposit > Sand and Gravel

Stratigraphic age (youngest) Tertiary

Host or associated

Host

Rock type

Volcanic Rock (Aphanitic) > Pyroclastic Rock > Tuff

Rock type qualifier

Rhyolite

Stratigraphic age (youngest)	Tertiary

Host or associated

Host

Rock type

Volcanic Rock (Aphanitic) > Felsic Volcanic Rock > Rhyolite

Rock type qualifier

tuff

Stratigraphic age (youngest)	Tertiary

Host or associated

Host

Rock type

Metamorphic Rock > Metasedimentary Rock > Slate

Rock unit name

Calaveras Complex

Stratigraphic age (youngest)	Triassic
Stratigraphic age (oldest)	Permian

Host or associated Host

Rock type Metamorphic Rock > Serpentinite

Stratigraphic age (youngest) Paleozoic

Nearby scientific data

(1)	-120.73681, 39.04183	ListMap

Economic information

Geologic structures

Type of structure	Local
Structure description	Volcano Canyon Fault, Melones Fault Zone.
Type of structure	Regional
Structure description	Melones Fault Zone

Ore body information

General form Irregular; Tabular, lens

General form	Irregular; Tabular, lens

Controls for ore emplacement

Mechanical accumulation on irregular natural bedrock riffles and within river and stream channel lag gravels, bars, and point bar deposits. Deposits are often enriched downstream of eroded gold quartz veins in the bedrock channels. ? Ore shoots within mesothermal gold-bearing quartz veins.

Comments on the geologic information

INTRODUCTION

The Michigan Bluff deposit is synonymous with the Michigan Bluff District. The district is primarily a placer gold district, the bulk of its production coming from just a few important hydraulic and drift mines. Many lesser hydraulic and drift mining operations are scattered throughout the district, but did not significantly contribute to the district's output. Of lesser importance yet were a few small, scattered lode mines. The majority of the district's production has come from hydraulic and drift mining operations within a series of buried Tertiary auriferous gravel-filled channels.

REGIONAL SETTING

The northern Sierra Nevada is home to numerous placer and lode gold deposits. It includes the famous lode districts of Johnsville, Alleghany, Sierra City, Grass Valley, and Nevada City and the famous placer districts of La Porte, North Columbia, Cherokee, Michigan Bluff, and Forest Hill. The geological and historical diversity of these districts as well as specific mining operations are covered in numerous publications 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). Detailed mapping of the Foresthill and Colfax 7.5-minute quadrangles was conducted by Chandra (1961).

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 (Day and others, 1988).

The Western Belt is composed of the Smartville Complex, a late Jurassic volcanic-arc complex (Beard and Day, 1987), 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 a fault known farther north of Michigan Bluff as 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. Western portions of the Michigan Bluff District in the vicinity of Baker Ranch and Volcano Canyon are part of this belt, which is underlain here by metavolcanic rock of the Calaveras Complex.

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 (Day and others, 1988). It consists largely of Devonian-to-Triassic serpentinized peridotite and dunite and underlies much of the Michigan Bluff District.

The Eastern Belt, also known as the Northern Sierra Terrane, is separated from the Feather River Peridotite Belt to the west by the Melones Fault Zone of Clark (1960). 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.
Regionally, the northern Sierra Nevada experienced a long period of Cretaceous to early Tertiary erosion, after which it underwent extensive late Oligocene to Pliocene volcanism. The oldest Tertiary deposits are basal Eocene auriferous gravels, which are preserved in paleochannels eroded into basement and on adjacent benches; both were deposited by the predecessors of the modern Yuba and American Rivers. 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 is what sparked the California Gold Rush.

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 mined chiefly by hydraulic and drift mining, and Quaternary deposits, present along modern streams and mined by dredging and small-scale placer methods (pan, rocker, long tom). Tertiary gravels can be further divided into basal Eocene "auriferous" gravels, which almost invariably rest on basement, and younger "intervolcanic" gravels, which are within the overlying continental volcanic units.

During the Cretaceous, the Sierra Nevada was eroded and its sediments transported westward by river systems to a Cretaceous marine basin in the area of today's Great Valley and Coast Ranges. By the Eocene, the Sierra Nevada was more hilly than mountainous and of lower relief than present. Low stream gradients and a high sediment load allowed shallow 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 gravel 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. Farther south, they are more fragmentary due to erosion or smaller original drainages.

The ancient Yuba River was the largest of the rivers and trended southwest from headwaters in Plumas County. Its gravels are responsible for the large placer deposits at San Juan Ridge and North Columbia in Nevada County. In Placer County, the remnants of a significant north branch of the ancient American River can be traced from the Ralston Divide area westward through the Michigan Bluff and Forest Hill districts to Auburn to the southwest.
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, channels are largely barren on the granitic Sierra Nevada Batholith, but become progressively richer as they cross the metamorphic belt and the Mother Lode area. They become especially enriched in the Michigan Bluff and Forest Hill districts from crossing the gold-bearing ?serpentine belt? (Feather River Peridotite Belt) near Michigan Bluff (Lindgren, 1911). Large volumes of placer gold accumulated in the basal gravels composed largely of resistant quartz and metamorphic bedrock fragments. Blue-black slate and schist fragments often imparted a bluish hue to the gravels, hence the term "blue leads" or "blue gravels.? These basal channels have been the most productive as a whole and grade upward into white quartz gravel, sand, and laminated clays. Adjacent to the bedrock channels, broad gently sloping benches received shallow but extensive accumulations of auriferous overbank gravels sometimes 1-2 miles wide.

While the richest placers have been in the basal gravels within 3 feet of bedrock, the gold is not equally distributed. The richest concentrations occur as lenses or irregular streaks ("pay leads") caused by current and velocity conditions during deposition. Outside the pay leads, gravels yielding $0.10 per ton would be considered rich by a hydraulic miner, however, drift miners would have to leave behind gravels yielding $0.75 - $2.00 per ton as unprofitable (Lindgren, 1911). Gold was so often encountered in bedrock riffles and crevices that almost all drift mines worked bedrock to a depth of at least one foot. Gold particles tend to be flat or rounded and range from fine flour gold to nuggets of 100 or more ounces. Large nuggets were especially prevalent in the Alleghany, Columbia, Downieville, and Sierra City districts. Generally, however, gold in the larger channels was generally fine to medium, with coarse gold typically the size of a wheat kernel. Regionally, the placer gold of the main Tertiary gravels averaged .920 fine with the remaining 8% composed almost entirely of silver. Locally, trace amounts of lead, copper, and platinum metals are present. The gold particles are everywhere associated with black sands composed of magnetite, ilmenite, chromite, zircon, garnet, and pyrite.
Valley Springs Formation

After deposition of the basal Eocene channel gravels, volcanism in the upper Sierra Nevada radically changed drainage patterns and sedimentation. The first of many eruptive rhyolite flows filled the depressions of the river valleys, covering the auriferous gravels and diverting the rivers. Many tributaries were dammed, but eventually breached the barriers and carved their own channels within the rhyolitic fill. Ensuing intermittent volcanism caused recurrent rhyolite flows to fill and refill the younger channels as the drainages repeatedly tried to reestablish themselves. This resulted in a thick sequence of intercalated intervolcanic channel gravels referred to as "cement channels" and volcanic flows often hundreds of feet thick known as the Valley Springs Formation. Some of the intervolcanic channels cut narrow and deep channels, sometimes following and partly obliterating the older channel deposits, while other times crossing and leaving the deeper portions of the older channel deposits intact. In places, they cut entirely through the basal gravel deposits to depths of 50-100 feet into bedrock. Where younger intervolcanic channels eroded bedrock gravels they were often enriched by the reconcentration of the placer gold. Shallower channels that did not reach the basal gravels were generally lean or barren, their gravels being predominantly rhyolitic. In parts of Placer County, the intervolcanic gravels can exceed 300 feet in thickness. Intervolcanic deposits are frequently characterized by sporadic grayish blue gravel (blue leads) of bedrock material reflecting localized erosion of earlier gravels mixed with gravels of volcanic origin. In many places, the gravels are covered by extensive light-colored, fine-grained sandy or clayey beds, called pipe clay. Overall, the rhyolitic rocks are light gray-pink and fine-grained, ranging from massive near their source in the High Sierra and grading successively downslope to a tuffaceous form then to tuff and clay beds.

Mehrten Formation

Volcanism continued from the late Oligocene to the Pliocene, with a change from rhyolitic to andesitic composition and a successively greater number of flows. During the Miocene and Pliocene, volcanic activity was so intense that thick horizontal beds of andesitic tuffs and mudflows of the Mehrten Formation blanketed the Valley Springs throughout the northern Sierra Nevada. Thicknesses ranged from a few hundred to a few thousand feet. Pleistocene erosion removed most of these deposits, but remains cap the axes of many existing ridges. As a rule, the greatest thicknesses overlie the old channels, while the adjacent bedrock hills may have been only superficially covered. This widespread Miocene-Pliocene volcanism forced the rivers and streams to seek entirely new channels and brought to a close the system of Tertiary drainage and volcanism. In some of the Tertiary river valleys, thick sequences of up to 1,500 feet of Valley Springs and Mehrten beds remain.

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.
Lode Quartz

Gold and silver also occur in fracture-filling mesothermal quartz veins, which generally occur in three northwesterly trending bedrock zones. The Melones Fault Zone of the Mother Lode Gold Belt bifurcates south of the Forest Hill Divide. A western branch comprised of Mariposa Formation slates follows the Gillis Hill Fault Zone and passes west of the Forest Hill District and extends to Colfax, where it continues northward within diabase, granodiorite, and amphibolite through Nevada County and into Yuba County. East of the Gillis Hill Fault there are two zones of mineralization within rocks of the Calaveras Complex and the Feather River Peridotite Belt. The eastern branch of the Melones Fault Zone includes serpentinite, gabbrodiorite, and amphibolite of the Feather River Peridotite Belt where it passes through the Michigan Bluff District northward through Alleghany in Sierra County.

Structure

Most Upper Jurassic and younger basement rocks of the northern Sierra Nevada were metamorphosed and deformed during the Jurassic-Cretaceous Nevadan Orogeny (Clark, 1960). The dominant structural grain is a result of this period of compressive deformation, which produced northwest-trending thrust faults, major northwest-trending folds, and regional greenschist facies metamorphism (Harwood, 1988). 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). They deform Upper Jurassic rocks and are truncated by uppermost Jurassic and Cretaceous plutons. 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, locally expressed in the form of foliation and cleavage in the metamorphic bedrock, served as a good trapping mechanism for the eroded particles of gold as they were transported downstream from primary sources in the lode deposits.

Metallogeny

The northern Sierra Nevada contains many mining districts, each known for important deposits of lode and/or placer gold. Most significant exposed lode deposits have probably been discovered given the intense scrutiny the area was subjected to during the later half of the 19th century. Still of significance are the placer deposits in the Tertiary gravels. Lengths of buried Tertiary channels remain undeveloped, underdeveloped, or undiscovered. The complex interbedded nature of some of the channels and the extensive cover of Valley Springs and Mehrten Formations undoubtedly obscure many rich placer gravels. Further, after the 1884 Sawyer Decision curtailed hydraulic mining, lower-grade gravel deposits were ignored in preference of the richer gravels, which could be profitably mined by drift mining. Merwins (1968) concluded that the demise of hydraulic mining resulted in more than half of the then-known gravels to remain unexploited. He valued these reserves at $750 million - $1 billion dollars (1966 dollars at $35.00/oz.). At today's price of approximately $300/oz, this equates to $6.4 to $8.6 billion.

GEOLOGY OF THE MICHIGAN BLUFF DISTRICT

Rocks of the Michigan Bluff District can be divided into four units, which consist of basement complex, Eocene auriferous gravels, interbedded volcanic rocks and gravels of the Valley Springs Formation, and an uppermost Mehrten Formation volcanic cap.
The main body of basement rocks within the district is the north-northwest-trending Feather River Peridotite belt comprised of a 1-3 mile wide belt of partially to completely serpentinized peridotites. The Feather River Peridotite Belt coincides with the northern extension of the Melones Fault Zone of the Sierra Nevada Mother Lode (Clark, 1960). The Volcano Canyon Thrust Fault forms the eastern boundary of the belt where sandstone, siltstone, and slate of the Shoo Fly Complex are juxtaposed against ultramafic rock and serpentinite on the east side of the district. On the western side of the district, rocks of the Feather River Peridotite belt are faulted against metavolcanic rocks and slate of the Calaveras Complex.

Basal Eocene Auriferous Gravels

The Michigan Bluff District is at the intersection of two major Eocene channels. The primary channel entered from the southeast from the Ralston Divide District about 15 miles away. A smaller tributary channel entered the district from the Damascus District to the north (Lindgren, 1911) where it was highly productive. Within the district, however, the tributary deposits have been largely lost to erosion.

The Eocene drainage system included an ancient counterpart to the current Middle Fork of the American River, which followed the same general course as the modern drainage. It flowed westward through the Ralston Divide District and skirted the southern part of the Michigan Bluff District where its gravel deposits were discovered on Michigan Bluff. Between the two districts the channel has been lost to erosion. West of Michigan Bluff, the channel has again been eroded, but reappears in the Paragon Mine in the Forest Hill District where it is known as the Forest Hill Channel.

The main gravel deposits in the district are those at Sage Hill and Michigan Bluff adjacent to and below the Michigan Bluff townsite. The gravel exposure at Michigan Bluff covered approximately 40 acres and proved to be one of the most valuable deposits in the district. By 1880, almost all the smaller claims on this deposit were consolidated by the Big Gun Mining Company and operated as the Big Gun Mine, which ultimately produced $1 million by 1882. This deposit has been correlated with the Eocene Forest Hill Channel in the Forest Hill District to the west and the Long Canyon Channel to the east in the Ralston Divide District. In the Forest Hill District, it can be traced for almost 6 miles and produced more than $6.1 million from the district's three main mines, the Paragon, Mayflower, and Dardanelles. Channel morphology is characterized by a flat, trough-shaped channel depression incised in bedrock. Where best exposed at the Paragon Mine near Bath, the bedrock channel was 500 feet wide and 100 feet deep. The bedrock surface is irregular with ridges, swales, and potholes conducive to trapping placer gold. In contrast to the equivalent gravels in the Forest Hill District, which are generally a blue-gray due to a concentration of slate and other metamorphic rock, the Michigan Bluff and Sage Hill gravels are almost exclusively white quartz gravels with some white quartz boulders of up to 20 tons. This difference is likely due to their location upstream of Calaveras Complex bedrock and/or to the loss of the deeper thalweg deposits to erosion south of Michigan Bluff. The Michigan Bluff gravels appear to be channel rim and bench gravels deposited along the north side of the old Forest Hill Channel. The quartz gravels are less-cemented and generally do not require crushing. The gravels were said to be as much as 80 feet thick, but averaged 40 feet. Reportedly, some 6 million cubic yards of gravel were hydraulically mined from the Sage Hill and Michigan Bluff exposures yielding approximately $5,000,000.
A tributary of the Forest Hill Channel flowed southward from the Damascus District and appears to have merged with the main channel near Michigan Bluff. Unfortunately, throughout most of the district its deposits have been removed by erosion in Eldorado Canyon. They are, however, present at the Gas Hill Mine (Sec. 21, T14N, R11E) on the southernmost edge of the Damascus District. Here, the gravels are almost identical to those at Michigan Bluff. North of this mine, the channel is truncated by a deeper intervolcanic channel, but reappears one mile farther north in the Hidden Treasure Mine (Sec. 35, T15N, R11E), where it was called the "White Channel" and produced over $4 million. From the Hidden Treasure Mine, the tributary can be traced almost continuously to Damascus (Lindgren, 1911), a distance of almost 4 miles. The channel is a wide, flat depression in soft, swelling clayey slate bedrock, which required substantial timbering to keep the tunnels open. The channel was filled with almost 200 feet of uncemented quartz gravel, sand, and clay with some quartz boulders. The gravel is markedly finer and more quartzose than that in the Forest Hill Channel to the west. Breasting could be done with pick, and caving and blasting were limited to the removal of some large boulders. In places the channel widened to 800 feet, with rims rising gradually to 16 feet above the thalweg. The width of the gravel breasted was 250 feet, and 4 to 7 feet of the lowermost gravel, including 1 foot of bedrock was extracted. The gold was generally coarse, with gravels yielding only $0.50 to $1.75/ton. Only the unusually low cost of production allowed this mine to profitably produce this grade of material via drift mining.

The exact location of the confluence of the Forest Hill Channel and the tributary remains unclear having apparently been lost to erosion south of Michigan Bluff. While the Michigan Bluff gravels resemble more closely those of the tributary in terms of texture, lithology, and yield, they are also consistent with flanking bench gravels of the Forest Hill Channel.

Valley Springs Intervolcanic Channels

Overlying the Eocene channels are varying thicknesses of intercalated rhyolite tuff and intervolcanic channel gravels (often called ?cement? channels) of the Valley Springs Formation. Toward the top of the formation the tuffs become progressively more andesitic. The thickness of the sequence is highly variable. Thicknesses of up to several hundred feet of gravel, sand, and pipe clay, can extend well beyond the limits of the lowermost bedrock channel depression. Little information is available regarding specific thicknesses within the Michigan Bluff District, but in neighboring Forest Hill District, exposed thicknesses of rhyolite tuff and intervolcanic gravels range from 40-130 feet.

The network of intervolcanic paleochannels is complex, 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 period 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 volcanic gravel unless that stream cut deeply enough to erode older deposits or basement.
Many intervolcanic channels eroded deeply into older auriferous gravels either partially or wholly destroying them. The Forest Hill Channel is commonly cut by intervolcanic channels, in which case the channels are locally rich in gold. Clay beds are common in the upper portions of intervolcanic gravels. Petrified and/or lignitized cedar and oak tree trunks are not uncommon. Some gravel layers have become highly cemented by percolation of siliceous and calcareous waters, and colors range from gray, blue, reddish brown, to white depending on the source material and oxidation of the gravel and/or cementing material.

The most important intervolcanic channel in the district is the equivalent to the Orono Channel of the Forest Hill District. In that district, it could be traced for almost 14 miles; locally it cut the Forest Hill Channel deposits in the Paragon, Mayflower, and Dardanelles mines. The Orono Channel was found to be particularly rich downstream of those areas and was often worked in lateral drifts from the main Forest Hill Channel drifts.

The Orono Channel can be traced upstream from the Forest Hill District, across Volcano Canyon to the Michigan Bluff District where almost 3 miles of the channel are preserved. From its inlet in Eldorado Canyon, which was drifted in the Weske Tunnel, it trends almost a mile west before tuning sharply south for one mile. It then turns west again and can be traced to Volcano Canyon where it was drifted 3,000 feet upstream in the Hazard Mine. Called the "Weske" channel in the Weske Tunnel, it was drifted downstream for over 5,000 feet, requiring pumps and stopes. The channel was about 100 feet wide and cut into bedrock for most of its mined length. The channel gradient was steep, with many steep drops and potholes (Lindgren, 1911). The thin gravel was overlain with volcanic tuff in which were found several tree trunks in an upright position.

A smaller intervolcanic channel crosses the Orono Channel near the mouth of the Weske Tunnel and trends about one mile northward. Locally dubbed the Manhattan Channel, it was filled with heavy volcanic gravel and was little worked with the exception of a short drift in the Manhattan Tunnel.

Several other fragments of intervolcanic channels are also present, but are too small or erratic for accurate correlation. North of the Weske Tunnel, one short segment of a channel was worked in the Oro and Bowen tunnels. Along the face of Eldorado Canyon are several isolated gravel hills, the relicts of a complicated intervolcanic channel system. These deposits were hydraulically mined at Drummond Point, Eldorado Hill, and Bachelor Hill.

The Valley Springs intervolcanic channel sequence is capped by andesites of the Mehrten Formation in the northwest corner of the district and locally at Sage Hill and Michigan Bluff. It consists of hard and dense massive layers of light gray, reddish brown, and dark colored andesitic mud flows, tuffs, breccias, and volcanic conglomerates.

Lode Gold Deposits

The Michigan Bluff District also contains a few small lode gold mines, such as the American Bar Quartz (Sec. 33, T14N, R11E), Champion Quartz (Sec. 15, T14N, R11E), Bunker & Nihill Quartz (Sec. 22-T14N, R11E), and Daniel Webster Quartz (Sec. 33, T14N, R11E). In the Champion and Daniel Webster mines, workings were limited to a few hundred feet of tunnels. The American Bar Quartz Mine is reported to have had total workings of 1,950 feet of tunnels. No information is available regarding the extent of the Bunker & Nihill Mine. The gold-quartz veins were of limited extent and thickness. Veins trended northwest and northeast dipping steeply to the east. The pay occurred as free-milling gold in white mesothermal quartz veins 2 - 5 feet thick within slate with varying amounts of associated sulfides, especially galena. No production information is available for any of the mines
Structural Disturbance

With the exception of the westward regional tilting of the Sierra Nevada, there is very little evidence of any significant post-Cretaceous structural disturbance in the vicinity of the Michigan Bluff District.

Economic information about the deposit and operations

Operation type	Surface-Underground
Development status	Past Producer
Commodity type	Metallic
Deposit size	Small
Significant	Yes
Discovery year	1850

Mining district

District name	Michigan Bluff District

Land status

Ownership category	Private
Area name	Placer County Planning Department
Ownership category	National Forest
Area name	Tahoe National Forest

Ownership information

Type Owner-Operator

Owner Various private owners

Type	Owner-Operator
Owner	Various private owners

Comments on the workings information

HYDRAULIC MINING

Hydraulic mining allowed the bulk processing of large volumes of low-yield gravels that would otherwise prove unprofitable by other methods of mining. Hydraulic mining methods were first applied to the gravels in the Michigan Bluff District in 1853. Crudely applied at first, it evolved to a point where a powerful stream of high- pressure water was directed through large monitors at the base of a gravel bank, undercutting it and allowing it to collapse. Large gravel banks several hundred feet high were mined in this manner, but larger banks were often mined in two or more benches. Often, adits were driven into the exposed face and crosscuts parallel to the face were loaded with dynamite to help break down the exposure. The loosened gravels were then washed through long sluice boxes lined with riffles or over devices to mechanically trap the gold. Mercury was added to amalgamate the finer gold. The remaining debris was indiscriminately dumped in the nearest available stream or river. 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 usually needed its own system of ditches and flumes to deliver water from distant and higher reservoirs or rivers as well as dams, pipes, and tunnels. Another costly undertaking was finding an outlet for the debris. As the gravels were washed lower and lower in the ancient channel beds, it was often necessary to drive a tunnel through the bedrock channel rim to drain the workings into a nearby valley. Hydraulic mining flourished for about 30 years until the mid-1880s when the Sawyer Decision curtailed debris disposal.

The primary hydraulic mining operations in the Michigan Bluff District were on the bluff just below the town of Michigan Bluff. Several smaller operations originally worked these deposits, but they were ultimately consolidated as the Big Gun Hydraulic Mine (Secs. 22, 27 T14N, R11E). Smaller-scale operations were conducted on isolated deposits along Eldorado Canyon at the Batchelder Pit (Sec. 11, T14N, R11E), Drummond Pit (Sec. 15, T14N, R11E), and Eldorado Hill (Secs. 22, 23, T14N, R11E).

DRIFT MINING

Drift mining in the Michigan Bluff District was not as extensive as in the neighboring Forest Hill or Damascus districts. However, a few significant drift mines including the Hazard and Weske were developed in the intervolcanic Orono Channel. 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 up or down stream along the channel thalweg. While some deeply buried channels were originally accessed through vertical shafts, drainage problems and the expense of hoisting led to all the major drift mines being 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. 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. The tunnel entrances were usually in or near a ravine or gulch for easy waste- rock disposal.

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 raises 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. Working upstream in a channel with a uniform grade, the main tunnel could be run on the surface of the bedrock.

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 intervolcanic 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.

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.

Some mines were plagued by bedrock swelling. Both 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 one 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, before the boulder waste was thrown back on it.

According to the hardness of the gravels, they were either washed through sluices or crushed in stamp mills. Most of the gravels in the Michigan Bluff District were not highly cemented and did not require milling. Instead, most surface works consisted of no more than a dumping and slaking area, riffled sluices, and a water supply under low pressure. Mercury was added for amalgamation.

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.

In most drift mines, ore was removed by ore cars of 1- to 2-ton capacity. Car capacity was largely determined by the 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.

NOTEWORTHY MINES IN THE MICHIGAN BLUFF DISTRICT

Unlike the neighboring Forest Hill District in which there were a number of significant mines, the Michigan Bluff District was largely composed of many small mines and claims, some of which were consolidated into only a few significant mines. Consequently, there is very little specific information available about the district's mines.

Big Gun Mine (Sec. 22 & 27, T14N, R11E)

Drift and hydraulic mining on Michigan Bluff began in 1853 as a series of small operations on independent claims. In the early years, over 20 claims were worked profitably including the Big Gun, Thompson & Powell, Red Hill, North American, and Van Emon Placer Mines. By 1858, the hydraulic workings on the bluff below the town were so extensive that they undermined the town causing it to settle and slide downhill. The community had to be rebuilt farther uphill in 1861. In 1867, with the introduction of nitroglycerin, blasting was introduced to the hydraulic operation and is said to have resulted in as much work being done by 15 men as had previously been done by a crew of 28. By 1880, most of the smaller claims were bought by the Big Gun Mining Company and consolidated as the Big Gun Mine. The company used 400 inches of water under a 300-foot head. While reliable estimates of early production are lacking, it is estimated that the consolidated Big Gun Mine had produced $1 million by 1882. Hydraulic mining ceased in 1883. The mine was again operated intermittently between 1896 and 1900 with permits from the California Debris Commission; however, no production records are available for this period. As late as 1926, estimates of as much as 2,000,000 cubic yards of unworked gravel are said to have remained, with negotiations in progress at the time to reactivate the mine (Logan, 1927). No records of any subsequent activity could be found.
Hazard Mine (Sec. 20, T14N, R11E)

The intervolcanic Orono Channel was drifted in the Hazard Mine for about 3,000 feet from a 180-foot bedrock shaft on the east side of Volcano Canyon. Other than the channel being narrow with some rich gravel deposits, no additional mine-specific information is available. However, based on information from the Baker Divide Mine on the opposite side of Volcano Canyon, the Orono Channel in this area usually measured 30-40 feet wide, but narrowed to 8-10 feet in places. The gravel was 6 inches to 3 feet thick. The yield of the gravel was in part dictated by the hardness of the bedrock. The best pay was found where the channel crossed the softest bedrock.

Turkey Hill Consolidated Mine (Secs. 9, 10, 15, T14N, R11E)

The Turkey Consolidated Mine includes the earlier Boston, South Dakota, Weske, and Manhattan claims. The primary drifting was done in the Orono intervolcanic channel, which was locally called the Weske Channel for its workings in the Weske Tunnel. The Weske Tunnel was driven more than one mile west from a portal on the east side of Chicken Hawk Ridge. The tunnel was driven in bedrock for about 1,500 before breaking into channel gravel. The bedrock gradient was steep, with many steep drops and potholes. The channel itself was drifted downstream, which necessitated the use of pumps and stopes. The gravel channel was 100 to 300 feet wide, and the gravel was generally thin and overlain by volcanic tuff in which were found several tree trunks in an upright position. Approximately 4,000 feet from the portal, an incline was driven downstream into the neighboring Muir claim. The Weske Tunnel itself is reported to have produced $750,000 by 1911 (Lindgren, 1911). The mine was fully equipped with a surface plant and a locomotive for hauling ore trains.

A smaller intervolcanic channel crossed the Orono Channel near the mouth of the Weske Tunnel and was drifted about one mile northward in the Manhattan Tunnel. Locally called the Manhattan Channel, it was filled with heavy volcanic gravel. No detailed records of the workings or production are available.

Comments on other economic factors

In the Michigan Bluff District, it has been estimated that hydraulic mining of Sage Hill and Michigan Bluff worked approximately 6 million cubic yards of gravel and yielded $5 million; the Big Gun Mine alone reportedly produced $1 million by 1882 (Logan, 1936). At the Big Gun, it has been estimated that as much as 2,000,000 cubic yards of unworked gravel remain (Logan, 1927). By some accounts, Michigan Bluff is considered to have also included some of the most profitable drift mining in terms of yield. In one case, two men were reported to have recovered 1,200 ounces of gold during one week of drift mining. On the Franklin claim, 2,000 square feet of drifting produced $37,000 (Logan, 1936).

Since production figures were not compiled during the most prolific production years (1852-1884), no accurate figures are available for most of the mines on the Forest Hill Divide, or collectively for the Tertiary channels of the Sierra Nevada. Lindgren (1911) conservatively estimated that approximately $507 million had been produced from Tertiary channels statewide by 1911.

Merwin (1968) concluded that the 1884 Sawyer Decision's adverse impact on hydraulic mining resulted in more than half of the then known gravels statewide being left unworked. He characterized these gravels as one of the largest known reserves of gold in the United States. Based on Gilbert's (1917) volumetric calculations of produced Tertiary gravels, Lindgren's (1911) production information, and average yield information, Merwin estimated that a total of 3-4 billion cubic yards of gravel with an average yield of $0.25/yard and worth $750 million - $1 billion dollars (at $35.00/oz.) remained; the majority of this volume of gravel was contained in the deposits of the ancient Yuba and American Rivers. At today's price of approximately $300/oz this estimate equates to $6.4 to $8.6 billion. His estimate did not include allowance for unknown channel segments of possible value of which we know nothing and which still remained concealed under the volcanic cover.

Comments on development

Shortly after the discovery of gold in the American River at Sutter?s Mill in 1848, placer deposits were discovered in the North and Middle Forks of the American River flanking the Forest Hill Divide. Miners soon realized that much of the gold was eroded from rich Tertiary channel gravels perched high on the flanks of the divide. In 1850, the first ancient channels and gravel benches were identified hundreds of feet above the current rivers. In the same year, the community of Michigan City was settled just above an exposure of auriferous quartz gravel on a bluff overlooking Eldorado Canyon on the south flank of the divide. One of the early residents of Michigan City was Leland Stanford, future Governor of California and a founder of the Central Pacific Railroad. Having arrived in 1852 as a miner, he quickly opened a general store then served as justice of the peace before moving to Sacramento in 1855.

The Michigan Bluff gravels were first worked on a small scale by drift mining. Hydraulic mining of the deposits commenced in 1853 and continued until 1883. During the 1850s and 1860s Michigan Bluff was one of the most prosperous camps on the Forest Hill Divide and one of the foremost placer mining districts in the county. Between 1853 and 1858, gold shipments reportedly ran as high as $100,000 a month. In 1857, Michigan City was destroyed by fire, but the town was quickly rebuilt only to be moved again when, in 1858, the extensive workings on the bluff below caused the townsite to settle and slide downhill. A new townsite was selected farther uphill, and by 1861 most of the town was relocated and renamed Michigan Bluff (Clark, 1970). Activity began to taper off after 1870, but some work continued intermittently through the early 1900s and again in the 1930s. By 1880, most of the smaller hydraulic claims covering the bluff had been bought by the Big Gun Mining Company and consolidated with the Big Gun Mine, which was reported to have produced $1 million by 1882 and continued to operate sporadically until 1900.

The extensive hydraulic workings at Michigan Bluff and elsewhere in the Sierra Nevada dumped enormous quantities of sediment that choked local streams and eventually affected the Sacramento River. This waste resulted in extensive flooding and silting of downstream farmlands and litigation between the farming and mining interests. In 1884, the resulting Sawyer Decision injunctions, which curtailed sediment dumping in rivers, precipitated a plunge in hydraulic mining activity. Hydraulic mining so declined that by 1908, the total production of hydraulically mined gold in California had declined to $170,000 (Lindgren, 1911).

Miners persevered by following the richest auriferous channel gravels underground by drift mining. Generally, only the basal Eocene gravels were rich enough to warrant drift mining, but several intervolcanic gravels including those of the Orono Channel were found to be profitable by drift mining.

The most significant drift mines in the Michigan Bluff District were the Turkey Consolidated and Hazard Mines. While no date of discovery is known, the Turkey Consolidated Mine started out as the independent Boston, South Dakota, Weske, and Manhattan claims, which were later consolidated. The main drift was originally driven on the Weske claim in the Orono (Weske) Channel. The channel was mined for more than a mile west from a portal on the east side of Chicken Hawk Ridge. A second intervolcanic channel was drifted on the neighboring Manhattan claim where the Manhattan Tunnel was driven approximately one mile north with limited results.
By 1917 very little production was coming from the mines in the Michigan Bluff District, and dredging on the Middle and North Forks of the American River had become the primary means of gold recovery in Placer County. Only a few small placer mines were producing in the Michigan Bluff District. These included the Bogus Thunder and De Maria mines. Several small drift mines were also reportedly still in operation doing assessment work, but none was producing. These mines included the Franklin Drift, Golden Sheaf Drift, Gorman Drift, Marian Drift, Swift Shore Drift, and the Turkey Hill Consolidated Drift. Quartz mining, was also dormant with only a few mines conducting assessment work. These included the Bunker & Nihill Quartz, Champion Quartz, and Daniel Webster Quartz mines.

The district saw a little activity during the Depression, but interest waned by the1940s. Operation of all mines ceased in 1942 with the War Production Order. No mines are active today.

Today, little visible evidence remains of Michigan Bluff?s heydays. While the remains of some of the hydraulic mining operations are visible, the remains of the drift mines are far from evident to the casual observer. Since virtually all former drift mines are on posted private property, permission must be obtained to access these sites.

Comments on the environmental information

The Michigan Bluff District encompasses a portion of the crest and southeast flank of the Forest Hill Divide in south central Placer County approximately 50-55 miles northeast of Sacramento, California. The divide separates the North and Middle Forks of the American River and can be traced for almost 50 miles from an elevation of more than 6,500 feet west of Lake Tahoe to about 1,000 feet near Auburn, California. Marking the east edge of the district are El Dorado Canyon and the North Fork of the Middle Fork of the American River. The mouth of Volcano Canyon marks the southwest corner of the district.

The area is generally rural. The small community of Michigan Bluff, once the home of Leland Stanford, Governor of California and a founder of the Central Pacific Railroad, lies near the center of the district. The town of Foresthill (pop. 2,000) and the City of Auburn (pop.13,300) are located about 5 and 20 miles southwest, respectively. Most of the historic mine workings are on private property and inaccessible. Poorly vegetated scars from the former hydraulic pits are visible along the lower flank of the divide below Michigan Bluff.

Topography is dominated by heavily forested and mountainous terrain punctuated by riverine canyons, which support a cover of mixed oak, conifers, and chaparral. The Forest Hill Divide generally trends northeast, and its flanks are strongly dissected by small gullies and ravines, which support mostly ephemeral streams. In the vicinity of Michigan Bluff the divide is dissected by Volcano Canyon, which separates the crest of the main divide from smaller Chicken Hawk Ridge. Relief from the top of the divide near Michigan Bluff to the Middle Fork of the American River exceeds 3,000 feet. The former hydraulic mines and drift mine adits are located along the southeast flank of the divide where the auriferous gravels were exposed. A few gold-quartz veins were also exposed in bedrock outcrops.

The climate is intermediate between the Mediterranean climate of California's Central Valley and the alpine climate of the higher mountains to the east. Temperatures range from freezing in the winter to 100 degrees F in the summer, with average winter low temperatures of between 38 - 40oF and average summer highs in the mid 90s. Mean annual precipitation is approximately 14 inches, most of which falls during the rainy winter months between November and May.

Reference information

Bibliographic references

Deposit

Averill, C. V., 1946, Placer mining for gold in California: California Division of Mines Bulletin 135, p. 377.
Deposit

Beard, J. S. and Day, H. W., 1987, The Smartville intrusive complex, Sierra Nevada, California: The core of a rifted volcanic arc: Geological Society of America Bulletin, v. 99, no. 6, p. 779-791.
Deposit

Brooks, E. R., 2000, Geology of a late Paleozoic island arc in the northern Sierra terrane, in Brooks, E. R. and Dida, L.T., editors, Field guide to the geology and tectonics of the northern Sierra Nevada, California Division of Mines and Geology Special Publication 122, p. 53-110.
Deposit

Browne, R. E., 1890, Ancient river beds of the Forest Hill Divide: California State Mining Bureau 10th Annual Report of the State Mineralogist, p. 435-465.
Deposit

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

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

Clark, W. B., 1966, Gold, in Mineral resources of California: California Division of Mines and Geology Bulletin 191, p. 179-185.
Deposit

Clark, W. B., 1970 Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 49-50.
Deposit

Day, H.W. and others, 1988, Metamorphism and tectonics of the northern Sierra Nevada, in Ernst, W. G., editor, Metamorphism and crustal evolution of the western United States (Rubey Volume VII): Prentice-Hall, Englewood Cliffs, New Jersey , p. 738-759.
Deposit

Dunn, R. L., 1888, Drift mining in California: California State Mining Bureau 8th Annual Report of the State Mineralogist, p. 736-770.
Deposit

Gilbert, G. K., 1917, Hydraulic mining debris in the Sierra Nevada: U.S. Geological Survey Professional Paper 105, 155 p.
Deposit

Hamilton, F., 1920, Placer County: California State Mining Bureau 17th Report of the State Mineralogist, p. 442-451.
Deposit

Hammond, J. H., 1889, The auriferous gravels of California: California State Mining Bureau 9th Report of the State Mineralogist, p. 105-138.
Deposit

Harwood, D.S., 1988, Tectonism and metamorphism in the northern Sierra terrane, northern California, in Ernst, W. G., editor, Metamorphism and crustal evolution of the western United States (Rubey Volume VII): Prentice-Hall, Englewood Cliffs, New Jersey, p. 764-788.
Deposit

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

Lindgren, W., 1911, The Tertiary gravels of the Sierra Nevada of California, U. S. Geological Survey Professional Paper 73, 226 pp.
Deposit

Logan, C. A., 1927, Placer County: California Division of Mines 23rd Report of the State Mineralogist, p. 235-279.
Deposit

Logan. C. A., 1936, Gold mines of Placer County: California Division of Mines 32nd Report of the State Mineralogist, p. 7-96.
Deposit

Logan, C. A., 1948, History of mining and milling methods in California, in Jenkins, O.P. and others, editors, Geologic guidebook along highway 49 - Sierran gold belt - The Mother Lode Country: California Division of Mines Bulletin 141, p. 31-34.
Deposit

Merwin, R. W., 1968, Gold resources in the Tertiary gravels of California: U.S. Bureau of Mines Technical Progress Report 3, 14 p.
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.
Deposit

Waring, C. A., 1917, Placer County: California State Mining Bureau 15th Report of the State Mineralogist, p. 309-399.
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.

General comments

Subject category	Comment text
Deposit	The Forest Hill Divide is one of the most important drift and hydraulic mining areas in the Sierra Nevada. Perched on top of the divide, the Forest Hill, Michigan Bluff, and Damascus districts were all famous for their hydraulic and drift mines within the auriferous Tertiary gravels of the ancient American River. Hydraulic mining prevailed until the Sawyer Decision of 1884 curtailed hydraulic mining, after which drift mining became the primary means of exploration and development. While a few small lode deposits were developed in the basement rocks below the divide, the primary ores were Eocene channel lag and bench gravels deposited on the eroded bedrock surface and later elevated by uplift of the Sierra Nevada and exposed through downcutting by modern drainages. They were preserved under a thick overlying sequence of volcanic deposits of the Mehrten Formation, which makes up the backbone of the Forest Hill Divide. The gravels were heavily laden with placer gold eroded from the auriferous bedrock and gold-quartz veins through which the rivers flowed. Secondary placer deposits were encountered overlying the basal sand and gravel in a section of younger interbedded channel gravels and volcanic flows of the Valley Springs Formation. These "intervolcanic" gravels, while often barren, were sometimes charged with placer gold by erosion of the older auriferous channel gravels. The auriferous gravels in the Michigan Bluff District were deposited in bedrock channels near a confluence of the Eocene American River and a tributary coming from the north. In contrast with the Eocene quartz and metamorphic gravels of neighboring Forest Hill District, which were deposited in the main channel thalweg, the gravels at Michigan Bluff are almost exclusively white quartz gravels with quartz boulders of up to 20 tons; Lindgren (1911) interpreted them to be bench gravel. Gravel beds are up to 80 feet thick and are loosely cemented. Bedrock consists of slate, schist, and ultramafic rock/serpentinite. Up to 200 feet of tributary gravel was encountered due north of the district where the channel widened to a maximum 800 feet. Pay zones within the tributary gravels were commonly erratic, but often meandered from one side of the channel to the other reflecting relict current velocities. Gold particles tended to be flat or rounded and ranged from fine flour gold to coarse gold and rare nuggets. A little fine or flour gold was found in the sands and clays that covered the gravels. Gravel beds within the intervolcanic series contained economic quantities of gold when, during their deposition, the channels eroded adjacent beds or older channels that were gold-laden. The gold particles are in some places associated with platinum and almost invariably associated with black sands composed of magnetite, illmenite, chromite, and pyrite derived from basic bedrock such as diabase, gabbro, and serpentinite.

Subject category

Comment text

Deposit

The Forest Hill Divide is one of the most important drift and hydraulic mining areas in the Sierra Nevada. Perched on top of the divide, the Forest Hill, Michigan Bluff, and Damascus districts were all famous for their hydraulic and drift mines within the auriferous Tertiary gravels of the ancient American River. Hydraulic mining prevailed until the Sawyer Decision of 1884 curtailed hydraulic mining, after which drift mining became the primary means of exploration and development.

While a few small lode deposits were developed in the basement rocks below the divide, the primary ores were Eocene channel lag and bench gravels deposited on the eroded bedrock surface and later elevated by uplift of the Sierra Nevada and exposed through downcutting by modern drainages. They were preserved under a thick overlying sequence of volcanic deposits of the Mehrten Formation, which makes up the backbone of the Forest Hill Divide. The gravels were heavily laden with placer gold eroded from the auriferous bedrock and gold-quartz veins through which the rivers flowed. Secondary placer deposits were encountered overlying the basal sand and gravel in a section of younger interbedded channel gravels and volcanic flows of the Valley Springs Formation. These "intervolcanic" gravels, while often barren, were sometimes charged with placer gold by erosion of the older auriferous channel gravels.

The auriferous gravels in the Michigan Bluff District were deposited in bedrock channels near a confluence of the Eocene American River and a tributary coming from the north. In contrast with the Eocene quartz and metamorphic gravels of neighboring Forest Hill District, which were deposited in the main channel thalweg, the gravels at Michigan Bluff are almost exclusively white quartz gravels with quartz boulders of up to 20 tons; Lindgren (1911) interpreted them to be bench gravel. Gravel beds are up to 80 feet thick and are loosely cemented. Bedrock consists of slate, schist, and ultramafic rock/serpentinite. Up to 200 feet of tributary gravel was encountered due north of the district where the channel widened to a maximum 800 feet. Pay zones within the tributary gravels were commonly erratic, but often meandered from one side of the channel to the other reflecting relict current velocities.

Gold particles tended to be flat or rounded and ranged from fine flour gold to coarse gold and rare nuggets. A little fine or flour gold was found in the sands and clays that covered the gravels. Gravel beds within the intervolcanic series contained economic quantities of gold when, during their deposition, the channels eroded adjacent beds or older channels that were gold-laden. The gold particles are in some places associated with platinum and almost invariably associated with black sands composed of magnetite, illmenite, chromite, and pyrite derived from basic bedrock such as diabase, gabbro, and serpentinite.

Reporter information

Type	Date	Name	Affiliation	Comment
Reporter	10-JUL-2002	Downey, Cameron I. (Higgins, Chris, T.)	California Geological Survey CGS (Formerly CDMG)
Editor	01-SEP-2007	Schruben, Paul G.	U.S. Geological Survey	Converted from S&A FileMaker format to Oracle. Edit checks on rocks, units, and ages with Geolex search, and other fields.

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

Maps centered on this site

Authoritative California resources

These are landing pages for further research — the state agencies don't currently expose per-mine deep links.