Moore?s Flat District

Past Producer in Nevada county in California, United States with commodities Gold, Silver, Platinum

Sections on this page

Identification information
Geographic coordinates
Site location context
Geographic areas
Public Land Survey System information
Commodities
Materials information
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	10310651
Record type	District
Current site name	Moore?s Flat District
Related records	60001025

Comments on the site identification

The Moore's Flat District is in north-central Nevada County about 15 miles northeast of Nevada City. The district is both a lode and placer district, but the lode mines were of relatively little importance. Most production came from hydraulic mining of auriferous Tertiary river gravels at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point, and Snow Tent. The Alleghany District lies immediately northeast of the district. Moore's Flat was named after H.M. Moore who built a store there in 1851.

Geographic coordinates

Geographic coordinates:	-120.85073, 39.41909 (WGS84)
Elevation	1040
Relative position	15 miles northeast of Nevada City

Site location context

Political divisions (FIPS codes)

Nevada(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Alleghany(quadrangle 1:24,000 scale)

Truckee(quadrangle 1:100,000 scale)

Chico(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

Upper Yuba(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	Nevada

Public Land Survey System information

Meridian	Township	Range	Section	Fraction	State
Mount Diablo	018N	010E	15,16,21		California

Comments on the location information

Location selected for latitude and longitude is the Moores Flat site on the USGS 7/12-minute Alleghany quadrangle

Commodities

Commodity	Importance
Gold	Primary
Silver	Secondary
Platinum Critical	Secondary

Comments on the commodity information

Commodity Info: Average values of the undifferentiated upper and lower gravels at Snow Point and at Woolsey Flat and Orleans Flat are reported to have been $0.19/cubic yard and $0.25/cubic yard, respectively ($35/oz).

Hydraulic mining was reported to have produced $0.11-$0.15/cubic yard during the 1880s (Clark, 1970).
Ore Materials: Native gold: Fine - coarse gold and nuggets (.900 fine)
Gangue Materials: Quartz and metamorphic gravels; accessory minerals magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite

Materials information

Materials	Type of material
Gold	Ore
Magnetite	Ore
Ilmenite	Ore
Zircon	Ore
Pyrite	Ore
Amphibole	Ore
Epidote	Ore
Chlorite	Ore
Siderite	Ore
Quartz	Gangue

Mineral occurrence model information

Model code	119
USGS model code	39a
Deposit model name	Placer Au-PGE
Mark3 model number	54

Host and associated rocks

Host or associated Host

Rock type Unconsolidated Deposit > Sand and Gravel

Stratigraphic age (youngest) Tertiary

Nearby scientific data

(1)	-120.85073, 39.41909	ListMap

Economic information

Geologic structures

Type of structure	Local
Structure description	Goodyears Creek Fault Zone, Melones Fault Zone
Type of structure	Regional
Structure description	Ramshorn Fault, Goodyears Creek Fault, Melones Fault Zone

Ore body information

General form Irregular

General form	Irregular

Controls for ore emplacement

Mechanical accumulation on irregular bedrock riffles and within river- and stream-channel lag gravels, bars, and point bar deposits

Comments on the geologic information

REGIONAL SETTING

The northern Sierra Nevada is home to numerous important gold deposits. These include the famous lode districts of Johnsville, Alleghany, Sierra City, Grass Valley, and Nevada City as well as the famous placer districts of North Bloomfield, North Columbia, Cherokee, Foresthill, Michigan Bluff, Gold Run, and Dutch Flat. The geological and historical diversity of most of these deposits and specific mine operations are covered in numerous publications produced over the years by the U.S. Bureau of Mines, U.S. Geological Survey, California Division of Mines and Geology (now California Geological Survey), and others. The most recent geologic mapping covering the area is the 1:250,000-scale Chico Quadrangle compiled by Saucedo and Wagner (1992).

Stratigraphy

The northern Sierra Nevada basement complex has a history of both oceanic and continental margin tectonics recorded in sequences of oceanic, near continental, and continental volcanism. The complex has been divided into four lithotectonic belts: the Western Belt, Central Belt, Feather River Peridotite Belt, and Eastern Belt.

The Western Belt is composed of the Smartville Complex, an Upper Jurassic volcanic-arc complex, which consists of basaltic to intermediate pillow flows overlain by pyroclastic and volcanoclastic rock units with diabase, metagabbro, and gabbro-diorite intrusives. The Cretaceous Great Valley sequence overlies the belt to the west. To the east it is bounded by the Big Bend-Wolf Creek Fault Zone.

East of the Big Bend-Wolf Creek Fault Zone is the Central Belt, which is in turn bounded to the east by the Goodyears Creek Fault. This belt is structurally and stratigraphically complex and consists of Permian-Triassic argillite, slate, chert, ophiolite, and greenstone of marine origin.

The Feather River Peridotite Belt is also fault-bounded, separating the Central Belt from the rocks of the Eastern Belt for almost 95 miles along the northern Sierra Nevada. It consists largely of Devonian-to-Triassic serpentinized peridotite.

The Eastern Belt, or Northern Sierra Terrane, is separated from the Feather River Peridotite Belt by the Melones Fault Zone. The Northern Sierra Terrane is primarily composed of siliciclastic marine metasedimentary rocks of the Lower Paleozoic Shoo Fly Complex overlain by Devonian-to-Jurassic metavolcanic rocks. Farther east are Mesozoic granitic rocks of the Sierra Nevada Batholith.

The northern Sierra Nevada experienced a long period of Cretaceous to early Tertiary erosion followed by extensive late Oligocene to Pliocene volcanism. The oldest Tertiary deposits are Eocene auriferous gravels deposited by the predecessors of the modern Yuba and American rivers and preserved in paleochannels eroded into basement and on adjacent benches. In contrast to earlier volcanism, Tertiary volcanism was continental, with deposits placed on top of the eroded basement rocks, channel deposits, and Mesozoic intrusives. Two regionally important units are the Valley Springs and Mehrten Formations. The Oligocene-Miocene Valley Springs Formation is a widespread unit of intercalated rhyolite tuffs and intervolcanic channel gravels that blanketed and preserved the basal gravels in the valley bottoms. The younger Miocene-Pliocene Mehrten Formation consists largely of andesitic mudflows, which regionally blanketed all but the highest peaks and marked the end of Tertiary volcanism.

Pliocene-Pleistocene uplift of the Sierra Nevada caused the modern drainages to erode through the volcanic Valley Springs-Mehrten sequences and carve deep river gorges into the underlying basement rocks. During this process, the modern rivers became charged with placer-gold deposits from both newly eroded basement rocks and from the reconcentration of the eroded Tertiary placers. The discovery of these modern Quaternary placers in the American River at Sutter's Mill sparked the California Gold Rush.
Tertiary Channel Gravels

It has been estimated that 40 percent of California's gold production has come from placer deposits along the western Sierra Nevada (Clark, 1966). These placer deposits are divisible into Tertiary deposits preserved on the interstream ridges, and Quaternary deposits associated with present streams. Lindgren (1911) estimated that approximately $507 million (at $35.00/oz.) was produced from the Tertiary gravels. Almost all Tertiary gravel deposits can be divided into coarse basal Eocene gravels resting on basement, and overlying upper or "intervolcanic" gravels. While the gravels differ texturally, compositionally, and in gold values, no distinct contact exists between the two. The boundary is usually placed where pebble and cobble beds are succeeded by overlying pebble, sand, and clay beds.

Lower gravels contain most of the gold and rest on eroded bedrock that is usually smooth, grooved, and polished. Where bedrock is granitic, it is characterized by a smooth and polished surface. Where bedrock is slate, phyllite, or similar metamorphic rock, rock cleavage, joints, and fractures acted as natural riffles to trap fine to coarse gold. In many cases, miners would excavate several feet into bedrock to recover the trapped gold. The lower gravels, or "blue lead," of the early miners are well-cemented and characterized by cobbles to boulders of bluish gray - black slates and phyllites, weathered igneous rocks and quartz. Boulders may range upwards of 10 feet in diameter. In many deposits, disseminated pyrite and pyritic pebble coatings are common in the lower blue lead gravels. Adjacent to the bedrock channels, broad gently sloping benches received shallow but extensive accumulations of auriferous overbank gravels sometimes 1-2 miles wide.

The lower unit is also compositionally immature relative to the upper gravel unit as evidenced by their heavy mineral suites. Chlorite, amphibole, and epidote are common constituents in the basal gravels, but are conspicuously absent in upper gravels.

The upper gravels compose the bulk of most deposits, with a maximum measured thickness of 400 feet in the North Columbia District. These gravels carry much lower gold values (rarely more than a few cents per cubic yard) than the deeper sands and are often barren. Upper gravels are finer grained, with clasts seldom larger than cobble size, and contain abundant silt and clay interbeds. Cross-bedding and cut-and-fill sedimentary structures are abundant as well as pronounced bedding and relatively fair to good sorting. Compositionally they are much more mature, with quartz prevailing, and more stable heavy mineral components consisting almost exclusively of zircon, illmenite, and magnetite. Oxidation is common and often imparts a reddish hue to the gravels.

During the Cretaceous, the Sierra Nevada was eroded and its sediments transported westward by river systems to a Cretaceous marine basin. By the Eocene, low gradients and a high sediment load allowed the valleys to accumulate thick gravel deposits as the drainages meandered over flood plains up to several miles wide developed on the bedrock surface. The major rivers were similar in location, direction of flow, and drainage area to the modern Yuba, American, Mokelumne, Calaveras, Stanislaus, and Tuolumne Rivers. Their auriferous gravels deposits are scattered throughout a belt 40 - 50 miles wide and 150 miles long from Plumas County to Tuolumne County. In the northern counties, continuous lengths of the channels can be traced for as much as 10 miles with interpolated lengths of over 30 miles. The ancient Yuba River was the largest and trended southwest from headwaters in Plumas County. Its gravels are responsible for the placer deposits in the North Bloomfield, San Juan Ridge/North Columbia, Moore's Flat, and French Corral districts. Tributaries to the ancestral Yuba River were responsible for most of the other auriferous gravels in Nevada County.
Bedrock erosion degraded the rich gold-bearing veins and auriferous schists and slates as the rivers crossed the metamorphic belts of the Sierra Nevada. Upstream of the gold belts on the granitic Sierra Nevada batholith, channels are largely barren, but become progressively richer as they cross the metamorphic belt and the Mother Lode trend. They become especially enriched after crossing the gold-bearing "serpentine belt" (Feather River Peridotite Belt) upstream of many Tertiary placer districts. While the most gold is contained in the lower sand and gravel, the majority of rich material is within only a few feet of bedrock. Generally, in drift mines only these lower gravels were exploited; however, in hydraulic mines the whole gravel bed was washed. Lindgren (1911) estimated that on average, the hydraulic washing of thick gravel banks up to 300 feet, including both basal and upper gravels, yielded approximately $0.10 to $0.40/yard. Upper gravels alone might average $0.02 to $0.10/yard and lower gavels from $0.50 to $15/yard or more.

The bulk of the gold in the deposits was derived from gold-bearing quartz veins within the low-grade metamorphic rocks of the Sierra Nevada. Gravels that have the highest gold values contain abundant white quartz vein detritus and clasts of blue-gray siliceous phyllite and slate common to the gold-quartz vein-bearing bedrock of the region. Unusually high gold concentrations have also been documented immediately downstream of eroded qold quartz veins exposed in the scoured bedrock.

Most of the gold found in the gravels of the North Bloomfield and Moore's Flat districts is thought to have originated from the famous lode veins of the Alleghany Mining District. The veins in the Nevada City and Grass Valley districts have been proposed as possible sources for the gold in the gravels of the Sailor Flat and Blue Tent diggings.

Gold particles tend to be flat or rounded, shiny and rough, and range from fine and coarse gold to nuggets of 100 or more ounces. Large nuggets were especially prevalent in the Alleghany, North Columbia, Downieville, and Sierra City Districts. The gold particles are almost everywhere associated with black sands composed of magnetite, ilmenite, chromite, zircon, garnet, pyrite, and in some places platinum. Fine flour gold is not abundant in any of the Tertiary gravels. Lindgren (1911) and others have suggested that most of the flour gold was swept westward to be deposited in the thick sediments of the Great Valley.

Valley Springs Formation

After deposition of the Eocene channel gravels, Oligocene-Miocene volcanic activity in the upper Sierra Nevada radically changed drainage patterns and sedimentation. The first of many eruptive rhyolite flows filled the depressions of most river courses covering the Eocene gravels and diverting the rivers. Many tributaries were dammed, but they eventually breached the barriers and carved their own channels within the rhyolite fill. Ensuing intermittent volcanism caused recurrent rhyolite flows to fill and refill the younger channels resulting in a thick sequence of intercalated intervolcanic channel gravels and volcanic flows. In the Scotts Flat District, very little of the Valley Springs Formation remains, having been lost to erosion.

Mehrten Formation

Volcanism continued through the Oligocene to the Pliocene, with a change from rhyolitic to andesitic composition and a successively greater number of flows. During the Miocene and Pliocene, volcanism was so extensive that thick beds of andesitic tuffs and mudflows of the Mehrten Formation blanketed the Valley Springs. Thicknesses ranged from a few hundred to a few thousand feet. Pleistocene erosion removed much of these deposits, but remnants cap the axes of many existing ridges at mid-elevations.
Continued uplift during the Pliocene-early Pleistocene increased gradients allowing the modern drainages to cut through the volcanic mantle and auriferous gravel deposits and deeply into basement. The once-buried Tertiary river gravels were left exposed in outcrops high on the flanks of the modern drainage divides.

Structure

Most Upper Jurassic and younger basement rocks of the northern Sierra Nevada were metamorphosed and deformed during the Jurassic-Cretaceous Nevadan Orogeny. The dominant northwest-trending structural grain is a result of this period of compressive deformation, which produced thrust faults, major northwest-trending folds, and regional greenschist facies metamorphism. This episode also resulted in intrusions of granitic plutons that formed the Sierra Nevada.

Nevadan deformation structures within and between the northern Sierra Nevada lithotectonic blocks are steeply dipping northwesterly trending faults and northwesterly trending folds. These features are best developed in the Eastern, Central, and Feather River Peridotite Belts, where the faults have been collectively described as the "Foothills Fault System" (Clark, 1960). Where the attitude can be determined, most of the bounding faults dip steeply east and display reverse displacement.

The regional northwest-trending structural grain is also at approximately right angles to the prevailing direction of stream flow of both the ancient and modern channels. This grain, expressed in the form of foliation and cleavage in the metamorphic bedrock, served as a good trapping mechanism for the gold particles.

GEOLOGY OF THE MOORE'S FLAT DISTRICT

In the Moore's Flat District, basement rocks include slate and argillite of the Calaveras Complex and ultramafic rocks, amphibolite, and serpentinite of the Feather River Peridotite Belt. Bedrock is overlain by thick Eocene auriferous gravels that are in turn overlain by Oligocene to Pliocene Valley Springs and Mehrten Formation rocks on San Juan Ridge in the south part of the district. Elsewhere in the district, the Valley Springs and Mehrten rocks have been stripped by erosion exposing the underlying gravels.

The district straddles the junction of the Central Belt and the Feather River Peridotite Belt. Consequently, several faults of the Melones Fault Zone cut the basement rock in a north-south direction. The Goodyears Creek Fault, which trends north-south through the center of the district separates Calaveras Complex rocks from the ultramafic rock complex to the east.

Basal Eocene Auriferous Gravels

The Moore's Flat District contains several separate hydraulic workings including those at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point. and Snow Tent. The auriferous gravels were deposited by a southwesterly flowing branch of the ancestral Yuba River. This branch continued southwest from the vicinity of Snow Point and was responsible for depositing the famous placer deposits at Malakoff Diggings in the adjacent North Bloomfield District.

Similar to neighboring Tertiary gravel deposits, the deposits can be divided lithologically and texturally into a lower and upper unit. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. These deeper gravels are well-cemented and quartz-rich. The lower gravels are generally immature and composed of bluish-black slate and phyllite of the Calaveras Complex, weathered igneous rocks, and quartz. Lower gravels are more than 100 feet thick in places.

The upper gravels form the majority of the gravel deposits. Upper gravels are much finer, with clasts seldom larger than pebble size and characterized by an abundance of quartz sand and clay and silt beds. Large-scale cross-bedding and cut-and-fill features are common. Upper gravels are typically much leaner than the lower gravels. Upper unit gravels reach a thickness of about 200 feet at Woolsey Flat.
The average value of the undifferentiated upper and lower gravels is reported to have been $0.19 per yard and $0.25 at Snow Point and Woolsey Flat respectively.

At Moore's Flat, two bodies of gravel are exposed. The eastern deposit rests on amphibolite bedrock, while the western exposure rests on slate. The undifferentiated gravels vary from 100 - 130 feet thick. Of particular note are the large boulders ranging up to 10 feet in diameter in the hydraulic pit at Moore's Flat. These granitic boulders are thought to have been transported at least 8 miles, the distance to the nearest upstream exposure of similar lithology.

Smaller areas of exposed gravel occur at Orleans Flat and Snow Point. The gravel bank is 135 feet high at Snow Point, with the lower gravel unit being only 15 feet thick.

Economic information about the deposit and operations

Operation type	Surface-Underground
Development status	Past Producer
Commodity type	Metallic
Significant	Yes
Discovery year	1851

Mining district

District name	Moore?s Flat District

Land status

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

Ownership information

Type Owner

Owner Various private owners
Type Owner

Owner U.S. Forest Service

Type	Owner
Owner	Various private owners

Type	Owner
Owner	U.S. Forest Service

Comments on the workings information

Hydraulic Mining

Hydraulic mining methods were first applied in 1852 to the Yankee Jims gravels in the Forest Hill District of central Placer County. Its use and methods quickly evolved to where it was applied to most exposed Tertiary gravel deposits. Hydraulic mining involved directing a powerful stream of high pressure water through large nozzles (called "monitors") at the base of a gravel bank, undercutting it and allowing it to collapse. The loosened gravels were then washed through sluice boxes. The remaining tailings were indiscriminately dumped in the nearest available stream or river. Large banks of low-yield gravel could be economically mined this way. In some cases, adits were driven into the exposed face and loaded with explosives to help break down the exposure. One of hydraulic mining's highest costs was in the ditches, flumes, and reservoirs needed to supply sufficient volumes of water at high pressure. A mine might have many miles of ditches as well as dams and reservoirs, flumes, and tunnels. Hydraulic mining flourished for about 30 years until the mid-1880s when the Sawyer Decision essentially brought it to a halt.

Comments on other economic factors

In 1911, Lindgren estimated that 26 million cubic yards of gravel had been removed from the Moore's Flat District workings and that 15 million remained. Hydraulic mining was reported to have produced $0.11-$0.15/cubic yard during the 1880s (Clark, 1970).

Comments on development

Placer gold was discovered at Moore's Flat in 1851. In the same year, the town of Moore's Flat was established, being named after H.M. Moore who built a store there. Shortly thereafter, Moore's Flat became an important hydraulic mining town, with large-scale operations at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point, and Snow Tent. By 1880, the town had grown to a population of 500, but 1884 brought the Sawyer Decision, which effectively put an end to large-scale hydraulic mining in the northern Sierra Nevada. The district and town lived on while the emphasis changed from hydraulic mining to drift mining and the lode mines. The only significant drifting was done at Snow Point. The mines began to fail in 1895, and the town of Moore's Flat was abandoned.

In 1911, Lindgren estimated that 26 million cubic yards of gravel had been removed from the Moore's Flat District workings and that 15 million remained.

Reference information

Bibliographic references

Deposit

Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 93.
Deposit

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

Lindgren, W., 1911, Tertiary gravels of the Sierra Nevada: U.S. Geological Survey Professional Paper 73, p. 141.
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

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 Moore's Flat District contains several separate hydraulic workings including those at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point. and Snow Tent. The auriferous gravels were deposited by a southwesterly flowing branch of the ancestral Yuba River. This branch continued southwest from the vicinity of Snow Point and was responsible for depositing the famous placer deposits at Malakoff Diggings in the adjacent North Bloomfield District. Similar to neighboring Tertiary gravel deposits, the deposits can be divided lithologically and texturally into a lower and upper unit. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. These deeper gravels are well-cemented and quartz-rich. The lower gravels are generally immature and composed of bluish-black slate and phyllite of the Calaveras Complex, weathered igneous rocks, and quartz. Lower gravels are more than 100 feet thick in places. The upper gravels form the majority of the gravel deposits. Upper gravels are much finer, with clasts seldom larger than pebble size and characterized by an abundance of quartz sand and clay and silt beds. Upper gravels are typically much leaner than the lower gravels. They reach a thickness of about 200 feet at Woolsey Flat.

Subject category

Comment text

Deposit

The Moore's Flat District contains several separate hydraulic workings including those at Moore's Flat, Orleans Flat, Woolsey Flat, Snow Point. and Snow Tent. The auriferous gravels were deposited by a southwesterly flowing branch of the ancestral Yuba River. This branch continued southwest from the vicinity of Snow Point and was responsible for depositing the famous placer deposits at Malakoff Diggings in the adjacent North Bloomfield District.

Similar to neighboring Tertiary gravel deposits, the deposits can be divided lithologically and texturally into a lower and upper unit. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. These deeper gravels are well-cemented and quartz-rich. The lower gravels are generally immature and composed of bluish-black slate and phyllite of the Calaveras Complex, weathered igneous rocks, and quartz. Lower gravels are more than 100 feet thick in places.

The upper gravels form the majority of the gravel deposits. Upper gravels are much finer, with clasts seldom larger than pebble size and characterized by an abundance of quartz sand and clay and silt beds. Upper gravels are typically much leaner than the lower gravels. They reach a thickness of about 200 feet at Woolsey Flat.

Reporter information

Type	Date	Name	Affiliation	Comment
Reporter	10-SEP-2004	Downey, Cameron (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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