Forest Hill 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	10310614
Record type	District
Current site name	Forest Hill District
Alternate or previous names	Dardanelles, Mayflower, Paragon, Baker Divide, Smith Point, Maus, New Jersey, Jenny Lind, Independent, Baltimore, Excelsior, San Francisco, Pond?s Channel, Todd?s Valley, Peckham Hill, Gray Eagle, Georgia Hill, Yankee Jims, Small Hope, Orono, Green Spring, Hope, Black Hawk, Live Oak, Spring Garden, Centennial

Comments on the site identification

The Forest Hill Mining District is centered around the town of Foresthill (note different spelling) in south central Placer County, California. The district includes all hydraulic, drift, and lode gold mines between Baker Ranch on the northeast and Drivers Flat to the southwest along the crest and flanks of the Forest Hill Divide (to the northeast is the adjacent Michigan Bluff Mining District). The Forest Hill Divide is a northeast-southwest trending drainage divide separating the modern 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 channel gravel deposits. Significant hydraulic mines include those at Yankee Jims, Peckham Hill, and Todd's Valley, while important drift mines are the Dardanelles, Mayflower, and Paragon Mines. Only a limited amount of production came from quartz lode mines on the flanks of the divide including the Eureka, Consolidated International, and Three Queens Mines.

Geographic coordinates

Geographic coordinates:	-120.81873, 39.02069 (WGS84)
Elevation	730
Location accuracy	100(meters)
Relative position	15 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

Foresthill(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)

Geographic areas

Country	State	County
United States	California	Placer

Public Land Survey System information

Meridian	Township	Range	Section	State
Mount Diablo	013N	010E	03-08, 18	California
Mount Diablo	014N	010E	13,14, 23-28, 32-35	California
Mount Diablo	014N	011E	18, 19, 30	California

Comments on the location information

The Forest Hill District includes many individual mines widely distributed throughout an area encompassing nearly 25 square miles of a portion of the Forest Hill Divide. Since the town of Foresthill geographically lies near the center of the district and is itself underlain by several important mines, it was chosen to represent the location of the district. The location latitude and longitude identify the approximate center of the town of Foresthill on the USGS Foresthill 7.5 minute quadrangle (coincident with the C N/2 NE/4, Sec. 35, T14N, R10E, MDBM). The town of Foresthill is reached by taking the paved Foresthill Road from Auburn, California. The road traverses the crest of the divide to the northeast for approximately 20 miles before reaching the town.

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: Gravels of mixed metamorphic and igneous composition; quartz

Materials information

Materials	Type of material
Gold	Ore
Gravel	Gangue
Quartz	Gangue

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) > Felsic Volcanic Rock > Rhyolite

Rock type qualifier

tuff

Rock unit name

Valley Springs Formation

Stratigraphic age (youngest)	Miocene
Stratigraphic age (oldest)	Oligocene

Host or associated

Host

Rock type

Volcanic Rock (Aphanitic) > Pyroclastic Rock > Tuff

Rock type qualifier

Rhyolite

Rock unit name

Valley Springs Formation

Stratigraphic age (youngest)	Miocene
Stratigraphic age (oldest)	Oligocene

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)	-120.81873, 39.02069	ListMap

Economic information

Geologic structures

Type of structure	Regional
Structure description	Melones Fault Zone, Gillis Hill Fault Zone
Type of structure	Local
Structure description	Foresthill Fault, Volcano Canyon Fault, 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 Forest Hill deposit is synonymous with the Forest Hill District. The district is primarily a placer gold district containing many important hydraulic and drift mines, with minor production from a few small 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 as well as the early famous placer districts of La Porte, North Columbia, Cherokee, Michigan Bluff, and Forest Hill. 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 are the 1:250,000-scale Chico Quadrangle, compiled by Saucedo and Wagner (1992), and the 1:250,000-scale Sacramento Quadrangle, compiled by Wagner and others (1981). 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, which 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), consisting 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 and 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 Foresthill 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. In the Forest Hill District, it includes argillite and chert of the Calaveras Complex and slate of the Jurassic Mariposa Formation.

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, a portion of which underlies the eastern end of the Forest Hill District.

The Eastern Belt, also called the Northern Sierra Terrane, is separated from the Feather River Peridotite Belt to the west by the Melones Fault Zone of Clark (1960). To the east are Mesozoic granitic rocks of the Sierra Nevada Batholith. The Northern Sierra Terrane is primarily composed of Devonian-to-Jurassic metavolcanic rocks and siliciclastic metasedimentary rocks of the Lower Paleozoic Shoo Fly Complex. The dominant source of Shoo Fly sedimentation was the North American continental crust; however, it also includes shale-matrix melanges, serpentinite, limestone, phyllite, and chert that developed in a trench setting. The Upper Devonian-to-Jurassic rocks unconformably overly the Shoo Fly Complex and are of island-arc origin (Brooks, 2000). They consist of the Devonian-Permian Taylorsville Sequence, Permian-Triassic Arlington, Goodhue, and Reeves Formations, and the Jurassic Sailor Canyon Formation.
Regionally, the northern Sierra Nevada experienced a long period of Cretaceous to early Tertiary erosion, after which it underwent extensive Oligocene to Pliocene volcanism. The oldest of the Tertiary units are basal Eocene auriferous gravels, which were preserved in paleochannels eroded into basement and adjacent bench gravels deposited by the predecessors of the modern Yuba and American Rivers. In contrast to the earlier volcanism, Tertiary volcanism was continental and deposited on top of the eroded basement rocks, channel deposits, and Mesozoic intrusive rocks. As many as eight volcanic units blanketed portions of the northern Sierra Nevada, ranging in composition from older rhyolite to widespread younger andesite. An important widespread sequence of intercalated rhyolite tuffs and intervolcanic channel gravels is the Oligocene-Miocene Valley Springs Formation. The youngest volcanic sequence, the Miocene-Pliocene Mehrten Formation, consists largely of andesitic flows overlying the Valley Springs Formation.

Pliocene-Pleistocene uplift of the Sierra Nevada caused existing drainages to cut down 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 Quaternary and Holocene deposits associated with present streams. Tertiary gravels can be further divided into basal Eocene, or ?auriferous? gravels, which almost invariably rest on basement, and younger ?intervolcanic? gravels, which are within the overlying continental volcanic units. Tertiary gravels have been mined chiefly by hydraulicking or drift mining, and the Quaternary-Holocene deposits by dredging and small scale placer methods (pan, rockers, long toms).

After the Jurassic Nevadan Orogeny, the Sierra Nevada was eroded and its sediments transported westward by river systems to a Cretaceous marine basin occupying 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 streams 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 (Merwin, 1968). In the northern counties, the channels can be traced for many 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 channel 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 north branch of the ancient American River can be traced from east of the Forest Hill District through Auburn to the southwest. An ancient southern branch trended upstream towards Placerville and followed closely the modern South Fork of the American River. Continuous lengths of channels have been traced for as much as 10 miles, with interpolated lengths of over 30 miles. Farther south, the ancient Mokelumne River emptied into northern Amador County from headwaters near Mokelumne Peak, and the ancient Calaveras River drained an area between its headwaters in the Dardanelle area near Sonora Pass and its mouth near Valley Springs. Aggregate length of the ancient channels has been estimated at 400 miles (Hammond, 1889).

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 and on the granitic Sierra Nevada batholith, channels are barren, but become progressively richer as they cross the metamorphic belt and the Mother Lode area. At Forest Hill, they are further enriched from crossing the gold-bearing "serpentine belt" (Feather River Peridotite Belt) at Forest Hill (Lindgren, 1911). Large volumes of liberated gold accumulated in the basal gravels. These gravels were composed largely of resistant quartz and metamorphic bedrock fragments. Considerable siliceous blue-black slate and schist fragments imparted a bluish hue to the gravels, hence the name ?blue leads,? or blue gravels. These basal channels have been the most productive as a whole and grade upward into quartz 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.

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. The gold particles are everywhere associated with black sands composed of magnetite, ilmenite, chromite, zircon, garnet, pyrite, and in some places platinum.
Valley Springs Formation

After deposition of the basal Eocene channel gravels, 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 auriferous gravels and diverting the rivers. Many tributaries were dammed but 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 as they repeatedly tried to reestablish themselves. This resulted in a thick sequence of intercalated intervolcanic channel gravels and volcanic flows hundreds of feet thick known as the Valley Springs Formation, which covered an area much more extensive than the original river channels. 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. Occasionally, they cut entirely through the basal gravel deposits to depths of 50 to 100 into bedrock. Where the channels eroded the bedrock gravels they often became especially rich as they reconcentated the placer gold. Shallower channels that didn't 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 grayish blue gravel (blue leads) of bedrock material 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. 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 light colored tuff and clay beds.

The Valley Springs sequence of intercalated channel gravels and rhyolitic flows grades upward into flows of increasing andesitic composition.

Mehrten Formation

Volcanism continued through the Oligocene to the Pliocene with a change from rhyolitic to andesitic composition and a successively greater number of thin flows filling the river valleys. During the Miocene and Pliocene, volcanic activity intensified with the result that thick generally horizontal beds of andesitic tuffs and mudflows of the Mehrten Formation filled most remaining depressions and almost completely blanketed the Valley Springs deposits throughout Nevada, Sierra, Placer, El Dorado, Calaveras, and Amador Counties. The flows were so extensive that only a few peaks remained exposed. Thicknesses ranged from a few hundred to a few thousand feet. Pleistocene erosion removed the larger part of these deposits, but remains cap the axes of most existing ridges to a depth of several hundred feet. As a rule, the greatest thicknesses overlie the old channels, while the adjacent bedrock hills may have been only superficially covered. The most common form of deposit is a tuff breccia.

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 allowed the modern drainages to establish themselves. The increasing gradient caused the modern rivers to cut through the volcanic mantle and auriferous channels and deeply into basement. The erosion of the ancient channels helped enrich the modern placers. 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 zone composed 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 under the far eastern end of the district and continues northward through Alleghany in Sierra County.

Structure

Most Jurassic rocks of the northern Sierra Nevada were metamorphosed and deformed during the Jurassic-Cretaceous Nevadan Orogeny (Clark, 1960). The dominant northwest-trending structural grain is a result of this period of compressive deformation, which produced east directed thrust faults, major northwest-trending folds and regional greenschist facies metamorphism (Harwood, 1988). This episode also resulted in the 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 they 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 latter 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 gravels were ignored in preference of the richer gravels, which could be profitably mined by drift mining. Merwin?s (1968) investigation of the Tertiary gravels concluded that the demise of hydraulic mining resulted in more than half of the then known gravels remaining 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 FOREST HILL DISTRICT

Rocks of the Forest Hill District can be divided into four units: 1) basement rocks of the Calaveras Complex-Mariposa Formation and Feather River Peridotite Belt, 2) Eocene auriferous gravels, 3) interbedded volcanic rocks and gravels of the Valley Springs Formation, and 4) an uppermost Mehrten Formation volcanic cap. These units are prominently exposed on the flanks of the Forest Hill Divide where the North and Middle Forks of the American River have cut deep canyons hundreds of feet lower than the pre-Tertiary basement surface.

Basement

Forming the main body of the divide are north-northwest-trending foliated rocks of the Calaveras Complex and Mariposa Formation. The Foresthill Fault, a steep easterly dipping thrust fault, bisects the district and juxtaposes a terrane of slate, argillite, phyllite, metavolcanic rock and chert on the west against a terrane of metavolcanic rocks on the east. Chandra (1961) mapped these terranes as the Capehorn Formation and Tightner Formation, respectively, before they were recognized later as lithotectonic assemblages of the Calaveras Complex. Metasedimentary rocks of the western terrane strike north-northwest and dip 75?- 85? east and are interspersed with local belts of serpentinite and diorite dikes. On the west end of the district, the steeply east-dipping Gillis Hill Thrust Fault separates the Calaveras Complex from sandstone and slate of the Jurassic Mariposa Formation. The easternmost end of the district coincides with the Melones Fault Zone, which cuts the divide in the vicinity of Baker Ranch and Michigan Bluff. The fault zone is represented by a one- mile wide belt of partially to completely serpentinized peridotite of the Feather River Peridotite Belt. The bounding fault on the east side of the belt is the Volcano Canyon thrust (Chandra, 1961). Farther east, basement is composed of sandstone, siltstone, and slate of the Lower Paleozoic Shoo Fly Complex.

Basal Eocene Auriferous Gravels and the Forest Hill Channel

The primary Eocene channel system within the district is the "Forest Hill Channel", a part of an ancient American River system that meandered from east to west through the district, incising a distinct channel in the exposed bedrock. The channel gravels and associated bench gravels yielded the bulk of the district's gold production. The gravel-filled bedrock channel is preserved in a sinuous trend almost six miles long in which three of the district?s most prolific drift mines are located (Paragon, Mayflower, and Dardanelles Mines). The river entered the district from the Michigan Bluff area to the northeast. Its channel was exposed in cross-section on the south side of the divide at the Paragon Mine where it was hydraulically mined before drifts were driven to follow the channel underground. From the Paragon Mine, the buried channel curved north under the divide to the Mayflower Mine; then southwest through the Independence, Baltimore, and Dardanelles Mines before being abruptly truncated in the Dardanelles Mine by the younger Orono Channel. Based on extrapolation, Lindgren (1911) suggested that the Forest Hill Channel continued under the center of the divide and reappeared in the hydraulic workings at Yankee Jims on the north side of the divide. Chandra (1961) contended that the Forest Hill Channel ran southwest from the Dardanelles Mine and included gravels worked at the San Francisco and Pond's Channel Mines. This issue remains unresolved, and much of the Forest Hill Channel is probably still unexplored. Chandra also mapped a second, smaller bedrock Eocene channel, which skirted the north side of the district and entered the district at the Keystone Mine. It then trended southwest through the Keystone and Small Hope Mines and the Yankee Jims placer deposits before exiting the district to the southwest near the Grey Eagle Mine.
The bedrock expression of the Forest Hill Channel is a flat trough shaped depression with gently inclined flanking benches. Near Bath, the bedrock trough is 500 feet wide and 100 feet deep. The bedrock surface is irregular with ridges, swales, and potholes conducive to trapping placer gold. The gravels are composed of well-rounded boulders, cobbles, pebbles, and sand composed of the more resistant bedrock materials such as quartz and siliceous metamorphic rock, metavolcanic rock, and slate. They are well-cemented with precipitated opal or chalcedony and require blasting and crushing. A blue-gray hue imparted by the slate and other metamorphic rock resulted in the terms ?blue gravels? or ?blue leads.? Blue gravels are commonly overlain with white or red gravel composed almost entirely of quartz. These quartz gravels are less cemented and generally don't require crushing. The bedrock channel gravels are often capped by a widespread layer of weathered rhyolite or "pipe clay."

Gravels within 3 feet of bedrock are usually the richest, with rounded gold nuggets, flakes, and dust. Pay gravels may run the full width of the channel, but more often occur as a narrow ribbon-like belts meandering from side to side due to the currents or meandering in the former channels. Barren stretches are common and yields vary considerably. Important placer deposits are also found on bench gravels flanking the channels. Bench gravels consist of sand, pebbles, cobbles, and some boulders and are commonly interbedded with sandy shales and sands. Bench gravels are leaner than bedrock gravels, but truly barren gravels are rare.

Valley Springs Intervolcanic Channels

Overlying the Forest Hill Channel are varying thicknesses of intercalated rhyolite tuffs and intervolcanic channel gravels of the Valley Springs Formation. Towards the top of the formation the tuffs become progressively more andesitic before being blanketed by the overlying Mehrten Formation andesites. Including the Forest Hill Channel gravels, thicknesses of up to several hundred feet of gravel, sand, and pipe clay, can extend well beyond the limits of the lowest channel depression. At Bath and Mayflower, over 100 feet of rhyolitic tuff and intervolcanic gravels occur. Along the flank of the divide south of Foresthill, the exposed thickness of rhyolite tuff and intervolcanic gravels ranges from 40-130 feet.

On the Forest Hill Divide, 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 a volcanic event bringing each channel event to an end. The frequent diversion and reestablishment of the intervolcanic channels (often called cement channels), and subsequent erosion of earlier channels makes it very difficult to establish correlative channels with any certainty. Channels of this period are notably narrower and deeper, the flanks steeper, and the accumulations of bedrock gravel significantly less than those of the older basement channels. Gravel thickness in these channels is generally several inches to fifteen feet and is usually dominated by volcanic gravel unless that stream cut deeply enough to erode older deposits or basement.

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 include gray, blue, green, reddish brown, and white depending on the source material and oxidation of the gravel and/or cementing material.
Between the first rhyolite flow and the later andesitic flows, the Sierra was elevated, which increased the gradient. In response, many intervolcanic channels eroded deeply into older auriferous gravels either partially or wholly removing them. The Forest Hill Channel is frequently cut by intervolcanic channels. Where this has occurred, they are generally rich in gold. Locally, the uppermost channels in the andesite tuffs may contain small amounts of gold, but not enough to warrant mining.

The most important intervolcanic channel in the district, the Orono, has cut small canyons as deep as 150 feet into bedrock. This channel enters the district near Michigan Bluff where it is exposed in the Hazard Shaft near the northeast corner of the district. The sinuous channel has been identified under much of the district and can be traced in surface and subsurface for almost 14 miles from the northeast end of the district to its outlet at the southwest end of the district at Peckham Hill. It has been encountered in the Hazard Shaft, Paragon Mine, Mayflower Mine, Live Oak Tunnel, Dardanelles Mine, Georgia Hill Tunnel, Gray Eagle Mine, and Peckham Hill. The Orono Channel has been encountered cutting the Forest Hill Channel deposits in the Paragon, Mayflower, and Dardanelles Mines, in some cases cutting deeply into the underlying basement. The Orono Channel is particularly rich in these areas and was often worked in lateral drifts from the main Forest Hill Channel drifts. In the Mayflower Mine, it eroded to bedrock level where it was worked for almost 3,500 feet along its length. The Orono Channel was also worked in the Live Oak Tunnel (just below the Mayflower Tunnel portal) along the north flank of the divide for 2,000 feet. In the Paragon Mine, the Orono Channel was also mined for 1,800 feet where it cut into, but not completely through, the Forest Hill Channel desposits. In the Dardanelles Mine, the Orono Channel cut 100 feet into bedrock, completely truncating the Forest Hill Channel a half-mile into the mine. At Peckham Hill, the Orono gravel did not pay, while in the Grey Eagle Mine (2 miles north), Orono pay was spotty, the gravel thin, but very rich in places.

Several smaller volcanic channels, often with local names, also occur but are too small or erratic for accurate correlations. On the flank of the divide below the town of Foresthill, a series of such channels and bench gravels were worked in several drift mines including the Jenny Lind and New Jersey Mines. Some of these channels were extremely rich. In the New Jersey Mine alone, a single claim of only 800' x 300' is reported to have yielded $1,500,000. Collectively, this localized area is reported to have produced about $5,000,000 (Lindgren, 1911).

Mehrten Formation

Throughout the Forest Hill District, the Valley Springs Formation is overlain by the Mehrten Formation. Once covering the whole region, it is now confined to the flat-topped divides between drainages such as the Forest Hill Divide. Throughout the Colfax and Foresthill quadrangles, it ranges from a few feet to 700 feet thick. It consists of hard and dense massive layers of light gray, reddish brown, and dark colored andesitic mud flows, tuffs, breccias, and volcanic conglomerates. Intervolcanic channels are locally present in the Mehrten Formation, but they are almost universally barren.

Lode Gold Deposits

The Forest Hill District also contains lode gold mines, including the Dry Hill Quartz, Eureka Consolidated Quartz, International Quartz, and Three Queens Quartz Mines. While some of the gold-quartz veins were rich (especially those occurring near serpentinite), they were of limited extent and thickness. Veins are usually 3-4 feet thick and strike northwesterly. Consequently, these mines were not significant producers, having collectively produced something more than $100,000.
In the Three Queens Mine (Sec. 31, T14N, R11E), located in Volcano Canyon and the district's principal lode mine, the gold occurs in a white, mesothermal quartz vein 3-4 feet wide along the contact of a serpentinite body and black schist within the Feather River Peridotite Belt. The vein strikes northwest and dips 30-40? NE. Gold occurs as free-milling gold within the quartz and as "pocket" ore bodies. In the Eureka Consolidated Quartz Mine (Sec. 1, T13N, R9E), quartz stringers strike N 11? W and dip 45? E within amphibolite of the Calaveras Complex. Ore assayed $5/ton. In the Dry Hill Quartz Mine (Sec. 31, T14N, R11E), quartz stringers bearing free crystallized gold occur in slate of the Calaveras Complex. In the Consolidated International Quartz Mine (Sec. 12, T13N, R9E), free gold occurs in a vein along the footwall of a 50-foot-wide porphyry dike. Assays ran $3.75 - $15.00/ton. A quartz vein 4 feet thick was encountered in the main tunnel of the Paragon Mine at the contact of serpentinite and amphibolite. The vein strikes N 65? E and dips 66? SE. The Forest Hill channel gravel was greatly enriched downstream of the vein.

Most of the quartz mines were short lived. The last of them was closed down during World War II and never reopened.

Structural Disturbance

With the exception of the westward regional tilting of the Sierra Nevada, there is very little evidence of significant post-Cretaceous structural disturbance in the Forest Hill District. A small, almost vertical, fault was encountered across the bed of the Yankee Jim Channel. The strike was N 35? W with a throw to the northeast of 15 feet.

Economic information about the deposit and operations

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

Mining district

District name	Forest Hill District

Land status

Ownership category	Private
Area name	Placer County Planning Department

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 methods were first applied to the Yankee Jims gravels in 1852. Crudely applied at first, hydraulic mining 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. The loosened gravels were then washed through sluice boxes placed below the gravel banks. The remaining debris was 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.

The primary hydraulic mining operations in the Forest Hill District were at Yankee Jims, Bath, Youngs Pond, Georgia Hill, Peckham Hill and Foresthill. Smaller scale operations were conducted at numerous areas of lesser surface gravel exposures on the flanks of the Forest Hill Divide.

Drift Mining

The Forest Hill Divide is the most famous of the drift mining areas in California. While limited mining of the channel gravels by means of shafts and adits commenced soon after their discovery, underground mining flourished after the Sawyer Decision of 1884. 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 1890, due to drainage problems and the expense of hoisting, all the major drift mines were accessed through tramway and drain tunnels driven into bedrock below the channels. At the Mayflower Mine, a tunnel nearly 6,000 feet long was driven to replace the original shaft through which the discovery was made.

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 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 Valley Springs 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. In the Forest Hill Channel, the gravel was sufficiently cemented to stand with little timbering and required blasting for removal. Up until the late 1800s, all workings were driven by hand, then later by machine and pneumatic drills.

Working drifts in the gravel beds and pay leads themselves were larger than the bedrock tunnels and usually timbered due to their extended and long-term use. In wide gravel deposits, as a precaution against caving, gravel pillars from 20 - 40 feet wide were left on each side of the drift. When the main access tunnel was in bedrock following the line of the channel, pillars were not required, as the tunnel in the gravel was only for temporary use in mining the ground between its connections with the bedrock tunnel. Raises to access the gravel were made every 200 - 400 feet as necessary.

The breaking out of gravel (breasting) was done from the working faces of drifts. Usually, 1-2 feet of soft bedrock and 3-4 feet of gravel were mined out to advance the face. When the gravels were well-cemented, blasting was required. Otherwise the material could be removed with picks. Boulder sized material was left underground and only the gravels and fines were removed from the mine.

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.

Some channels were followed and mined for several miles. 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.

In most drift mines, ore was removed by ore cars, which were available in many capacities. 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 Forest Hill District

Paragon Mine

Early hydraulic mining of surface deposits at the Paragon Mine in 1852 exposed the Forest Hill Channel on the south flank of the divide about 2 miles northeast of the town of Foresthill. Hydraulic mining was replaced by drift mining, which followed the channel underground. The channel was mined over 8,000 feet into the divide along the strike of the channel through a bedrock tunnel in amphibolite and serpentinite. In 1859 the tunnel was connected with the Mayflower mine making almost five miles of continuous workings in the channel. The channel averaged 300 feet wide and the width of the gravel breasted in the channel thalweg was 50 feet. Pay gravel averaged 2 - 7 feet thick and yielded an average $10/ton, but in some stretches yielded as much as $125 per running foot. The gravel was particularly rich downstream of a four-foot-wide gold-quartz vein encountered at an amphibolite/serpentinite contact in bedrock. The hard cemented gravel required crushing in the mine's steam- powered 10-stamp mill. An auriferous-gravel channel lead (Paragon Channel) was also discovered 150 feet above bedrock. This channel was 45 feet wide with gravels averaging about $10/ton. It was followed for 2,000 feet until it was cut off by a younger intervolcanic channel filled with barren andesitic tuff. The Orono Channel was encountered about 4,000 feet into the mine where it cut into, but not through the Forest Hill Channel. The Orono Channel was reportedly worked at least 1,800 feet perpendicular to the main drift. Pay gravel was 250 feet wide, 5 feet thick, and yielded $4.50-$5.00/ton. By 1911, the Paragon Mine was almost worked out, and by that time had produced approximately $1,425,000 from the Forest Hill Channel ($500,000 from hydraulic mining and $950,000 from drifting), $900,000 from the upper lead, and $75,000 from the Orono Channel. Total cumulative production was estimated at $2,650,000.

Mayflower Mine

The Mayflower Mine was discovered in 1884 when a vertical exploratory shaft was sunk a mile north of Foresthill near the center of the divide. The shaft intersected the Forest Hill Channel downstream of the Paragon Mine. In working the mine through the shaft, six pumps were employed underground to drain the mine. The high cost of pumping and material hoisting led to the abandonment of the shaft. Instead, a 6,000-foot bedrock tunnel was driven from the north flank of the divide southward to intersect the channel at which point a perpendicular bedrock tunnel almost 15,000 feet long was driven to follow the channel both upstream and downstream. Periodic raises were driven into the channel to work the overlying gravels. In the course of operations, the Orono Channel was encountered and exploited. Ultimately, the mine controlled 3,000 acres of claims.
The Forest Hill Channel was 1,500 feet wide in much of the mine, and the average width of breasted gravel was 75 feet. A thickness of 2-14 feet of gravel was extracted and yielded an average $7.00/ton. Actual yields ranged from $3-$50/ton, but in one extraordinary 3,900-foot stretch it yielded $150.00 per running foot. As in the Paragon Mine, the basal gravel was well-cemented, and breasting was done by drilling and blasting. Little timbering was required due to the hardness of the gravel, which required crushing at the Mayflower's 20-stamp mill at the mouth of the tunnel. The Orono Channel averaged 40 feet wide, and a separate 3,500-foot bedrock tunnel was driven to mine it. The Mayflower was worked chiefly from 1888 to 1894 for a distance of 3 miles connecting it with the Paragon workings to the south. By 1895, the Forest Hill Channel was largely worked out, and pay gravel was coming largely form the Orono Channel. The mine reportedly produced between $1,000,000 and $1,500,000 before closure.

Dardanelles Mine

The Dardanelles Mine, which is about a mile south of Foresthill, commenced operations in 1853 and was originally worked as a drift mine and later by hydraulicking. The Forest Hill Channel was drifted for a little more than a half mile north from exposures on the south flank of the divide before the channel was truncated by the southwest trending Orono channel. The Forest Hill Channel was up to 2,500 feet wide in bedrock slates but the breasted pay gravel was only 75 feet wide and 5 feet thick. The cemented gravel averaged about 40% cobbles and boulders, the largest material being left underground. The mine was worked through a 3,000-foot bedrock tunnel with periodic raises and a terminal incline into the overlying gravel beds. Unlike other mines in the district with hard cemented gravel, the entire width of the pay was worked in one curved breast without running a main tunnel ahead or cross drifts from it. This method was considered inferior since it provided no opportunity to prospect the gravel ahead of the working face, and no ore was blocked out to process while exploring barren stretches. It also left the mine prone to caving. Cemented gravel was crushed in the mine's steam-powered 5-stamp mill and yielded an average $1.50/ton. The Dardanelles Mine is credited with producing over $2,000,000.

Grey Eagle Mine

The Grey Eagle Mine developed the downstream equivalent of the Orono Channel near the southwest end of the district. The channel was accessed via a 3,000-foot tunnel in slate bedrock driven from the north side of the divide. After intersecting the east-west trending channel, drifting was carried out at least 4,000 feet along the channel. The auriferous gravel averaged 5 feet thick and 50 feet wide. Ore carts were drawn by horses to the mine's steam- powered 10-stamp mill. The gold was generally fine, the larger pieces being worth about $1.00.

Yankee Jims, Georgia Hill, and Smith?s Point

Approximately two miles northwest of Foresthill, on the northwest flank of the divide, are three gravel exposures that were worked hydraulically during the early years of the Forest Hill District's development. At Georgia Hill, a 100-foot thick layer of gravel is exposed below the overlying lava cap. A larger area of gravel occurs at Yankee Jims where it ranges from 40-100 feet thick before disappearing under the lava to the east. A mile to the east are similar exposures at Smith's Point. These auriferous gravels are fairly loose and composed of metamorphic rocks and quartz fragments. Due to low average yields, it was not economic to pursue these by drift mining. It is estimated that 8,630,000 yards of gravel have been removed from these three exposures, yielding $5,000,000.
Todd's Valley and Pond's Channel

Surface exposures of bench gravels up to 75 feet thick at Todd's Valley and Pond's Channel on the south side of the divide, southwest of Foresthill, experienced extensive hydraulic mining. At Pond's Channel, a sequence of partly cemented, poorly sorted auriferous gravels forty feet thick were washed until the overlying Mehrten Formation lavas became too heavy to handle. By 1888, about 11,000,000 cubic yards of gravel yielded approximately $5,000,000. For a short time thereafter, small-scale mining yielded approximately $2,500 per year. As was the case at Yankee Jims, the gravels were not rich enough to warrant drift mining.

Comments on other economic factors

Since production figures were not compiled during the most prolific production years (1852-1884), no accurate figures are available for the mines of 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. Root (1927) reported at least $19,550,000 had been produced from the larger drift mines in the Forest Hill District (includes lode production from veins encountered in drifts). Clark (1970) estimated that an additional $100,000+ was produced from the Dry Hill, Eureka, Consolidated International, Mitchell, and Three Queens quartz lode mines. A summation of other accounts suggests that the district produced over $25,000,000 by the time all operations ceased.

Merwin (1968) concluded that the 1884 Sawyer Decision's adverse impact of hydraulic mining resulted in more than half of the then known gravels 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/yd 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, which still remained concealed under the volcanic cover.

In an effort to spur development of these reserves by encouraging hydraulic mining, federal funds were appropriated in 1930 to build the Englebright and North Fork Dams to serve as debris dams for hydraulic tailings. Unfortunately, water rights and distribution canals had long ago been sold to water and power companies, which precluded any significant mining.

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. Hydraulic mining of the Yankee Jims and Georgia Hill deposits commenced in 1852 using a primitive canvas hose and a small quantity of water under a head of only 50 feet. The Yankee Jims discovery was soon followed by others at Todd Valley and Peckham Hill and to the east in the neighboring Michigan Bluff District. Ultimately, most of the major and many of the smaller surface placer deposits in the district were hydraulically mined.

Hydraulic mining in the Sierra Nevada contributed enormous quantities of sediment to the Sacramento River causing flooding and silting of downstream farmlands. The Sawyer Decision of 1884 severely curtailed the dumping of sediment in the rivers resulting in a precipitous decline in hydraulic mining. Declining gold content and difficulties in dealing with the overlying volcanic cap also contributed to its demise. 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 auriferous basal Eocene gravels (especially those of the Forest Hill Channel) into the hillsides via drift mining. Unable to remove the thick cover, they resorted to tunneling, the gravels being brought to the surface and washed for their gold. It was found that some of the overlying intervolcanic gravels could be rich enough to drift mine. Eventually, skilled miners to began to infer the location of channels from existing drainage patterns, and tunnels and crosscuts were driven to locate and mine them.

The Forest Hill Channel was first identified in outcrop at the Paragon Mine and worked primarily through tunnels from the south side of the divide. Drift mining involved driving adits along or below the bedrock trough of the channel. Drift mining reached its zenith in the mines of the Forest Hill Divide and adjacent districts. Although hydraulic mining continued into the 1880s, drift mining became the dominant method up into the early 1900s. The Forest Hill District was the most productive drift mining camp in the state. The district was most active during the late 1850s and 1860s. By 1859, it was shipping $100,000 of gold a month, and by 1868 was credited with production of over $10,000,000.

Pay gravels were originally accessed and worked in tunnels along the bedrock surface or by shafts connected to tunnels on the bedrock surface. Eventually, shafts were abandoned and access adits were driven in the more competent bedrock below the channel with raises and inclines to access the working tunnels within the channels. This method didn't require much timbering and correct placement of the bedrock adits had the added advantage of naturally draining the workings. By 1890, all major drift mines were worked through bedrock tunnels with raises into the overlying gravels.
By 1917, little gold was being produced from the district although active mines included the Dry Hill Quartz, Eureka Consolidated Quartz, International Quartz, Mitchell Quartz, Dardanelles Drift, Excelsior Drift, Florida Drift, Grey Eagle Drift, Maus Drift, Mayflower Drift, and Paragon Drift. By 1920 little gold was being produced throughout Placer County and most of that produced came from dredges on the American River and quartz mines in the Ophir District. The district saw renewed activity during the Depression, and many mines were active during the 1930s and 1940s. Operation of all mines ceased in 1942 with the War Production Order. Only a few mines, including the Paragon Mine and Three Queens Mine have been worked since. No significant mines are in production today.

Today, the remains of many of the hydraulic mines in the district are clearly visible. The extensive pit walls and debris fields are still evident having not fully revegetated themselves. What were once forested areas, they appear as cliffs, slopes, and hummocky flats of white to light tan sands and rhyolite beds devoid of mature vegetation. Large boulders are common on the flats. Sparse and immature stands of pine and manzanita as well as native grasses help identify these areas. The remains of the drift mines are far from evident. While virtually all drift mines are on posted private property and were not visited, only a few were even remotely visible from any nearby accessible viewpoint.

Comments on the environmental information

The Forest Hill District is the area encompassing a portion of the Forest Hill Divide in south central Placer County approximately 35-40 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. It is one of many northeast-southwest trending ridges present along the western flank of the northern Sierra Nevada.

The area is generally rural, with the town of Foresthill (pop. 2,000) located near the center of the district. The larger city of Auburn (pop.13,300) is located approximately 16 miles southwest. Most of the historic mine workings are on private property.

Topography is dominated by heavily forested and mountainous terrain punctuated by riverine canyons, which support a cover of mixed oak, conifers, and manzanita. The Forest Hill Divide generally trends northeast, and its flanks are strongly dissected by small gullies and ravines, which support mostly ephemeral streams. Relief from the Middle Fork of the American River to the top of the divide near the town of Foresthill exceeds 2,100 feet. The former hydraulic and drift mine adits are located on the upper flanks surrounding the divide where the Tertiary gravels were exposed by regional uplift and downcutting by the modern American River. A few gold-bearing quartz veins in the bedrock flanks were mined, but none were important producers.

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 Divisions 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

Wagner, D. L. and others, 1987, Geologic map of the Sacramento Quadrangle: California Division of Mines and Geology Regional Map Series Map No. 1A, scale 1:250,000.
Deposit

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

General comments

Subject category	Comment text
Deposit	The Forest Hill District is one of the most important Tertiary placer-gold mining districts in the northern Sierra Nevada. The district is famous for hydraulic and drift mines within auriferous Eocene channel gravels of the ancient American River. Discovery of these gravel deposits high above the North Fork of the American River on the flanks of the Forest Hill Divide led to some of the first hydraulic mining in the state at Yankee Jims on the north flank of the divide. Hydraulic mining prevailed until the Sawyer Decision of 1884, which severely curtailed hydraulic mining, after which drift mining of the buried channels became the primary means of exploration and development. While a few small quartz lode deposits were developed in the basement rocks on the flanks of the divide, the predominant ores were Eocene channel lag and bench gravels deposited on the eroded bedrock surface and later elevated and exposed by uplift and downcutting of 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 from the erosion of 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 primary Eocene bedrock channel, the "Forest Hill Channel," is expressed as a flat trough-shaped depression that meanders in wide bends from northeast to southwest through the district. Generally between 300 and 1,500 feet wide, it widened to 2,500 feet in the Dardanelles Mine. The depth of the bedrock channel was commonly over 100 feet from thalwag to rim. Auriferous bench gravels up to several miles wide often spilled over the channel rims covering floodplains up to several miles wide. The best bedrock gravels were recognized by the abundance of quartz and metamorphic bedrock fragments, the latter imparting a bluish cast to the gravels. Deposits included gravels, cobbles, and quartz boulders of up to many tons, which were well-cemented and required blasting and crushing. Pay zones 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 large 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, ilmenite, chromite, and pyrite derived from basic bedrock such as diabase, gabbro, and serpentinite.

Subject category

Comment text

Deposit

The Forest Hill District is one of the most important Tertiary placer-gold mining districts in the northern Sierra Nevada. The district is famous for hydraulic and drift mines within auriferous Eocene channel gravels of the ancient American River. Discovery of these gravel deposits high above the North Fork of the American River on the flanks of the Forest Hill Divide led to some of the first hydraulic mining in the state at Yankee Jims on the north flank of the divide. Hydraulic mining prevailed until the Sawyer Decision of 1884, which severely curtailed hydraulic mining, after which drift mining of the buried channels became the primary means of exploration and development.

While a few small quartz lode deposits were developed in the basement rocks on the flanks of the divide, the predominant ores were Eocene channel lag and bench gravels deposited on the eroded bedrock surface and later elevated and exposed by uplift and downcutting of 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 from the erosion of 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 primary Eocene bedrock channel, the "Forest Hill Channel," is expressed as a flat trough-shaped depression that meanders in wide bends from northeast to southwest through the district. Generally between 300 and 1,500 feet wide, it widened to 2,500 feet in the Dardanelles Mine. The depth of the bedrock channel was commonly over 100 feet from thalwag to rim. Auriferous bench gravels up to several miles wide often spilled over the channel rims covering floodplains up to several miles wide.

The best bedrock gravels were recognized by the abundance of quartz and metamorphic bedrock fragments, the latter imparting a bluish cast to the gravels. Deposits included gravels, cobbles, and quartz boulders of up to many tons, which were well-cemented and required blasting and crushing. Pay zones 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 large 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, ilmenite, chromite, and pyrite derived from basic bedrock such as diabase, gabbro, and serpentinite.

Reporter information

Type	Date	Name	Affiliation	Comment
Reporter	15-APR-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.

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