Placerville District

Site Name

Primary: Placerville District

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Commodity

Primary: Gold
Tertiary: Platinum

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Location

State: California
County: El Dorado
District: Placerville District

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: El Dorado County Planning Dept.

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer
Operation Type: Surface-Underground
Discovery Year: 1848
Years of Production:
Organization:
Significant: Y
Deposit Size: M

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Physiography

Not available

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Mineral Deposit Model

Model Name: Placer Au-PGE

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Orebody

Form: Irregular, lens

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Structure

Type: L
Description: Melones Fault Zone

Type: R
Description: Melones Fault Zone

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Alterations

Not available

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Rocks

Name: Sand and Gravel
Role: Host
Age Type: Host Rock
Age Young: Tertiary

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Analytical Data

Not available

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Materials

Ore: Gold
Gangue: Quartz
Gangue: Granite

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Comments

Comment (Geology): REGIONAL GEOLOGY The Placerville District is within the Sierra Nevada foothills of El Dorado County, where bedrock consists of north trending tectonostratigraphic belts of metamorphosed sedimentary, volcanic, and intrusive rocks that range in age from late Paleozoic to Mesozoic. Locally, the Mesozoic rocks are capped by erosional remnants of Eocene auriferous gravels and once extensive volcanic rocks of Tertiary age. The structural belts, which extend about 235 miles along the western side of the Sierra, are flanked to the east by the Sierra Nevada Batholith and to the west by sedimentary rocks of the Cretaceous and Jurassic Great Valley sequence. The structural belts are internally bounded by the Melones and Bear Mountains fault zones and are characterized by extensive faulting, shearing, and folding (Earhart, 1988). In the El Dorado County area, gold deposits occur in the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced in the county. There has also been substantial gold produced from the West Belt and East Belt (Clark and Carlson, 1956). The West Belt consists of widely scattered gold deposits located west of the Mother Lode vein system. Gold occurs in irregular quartz veins in schist and granitic rocks, altered mafic rocks, and as gray ore in greenstone. The West Belt has been further divided by some authors into an eastern component composed of an ophiolitic melange and a western component composed of Jurassic rocks of the Copper Hill volcanics (Duffield and Sharp, 1975; Saleeby, 1982; Clark, 1964). The Copper Hill volcanics consist of mafic to felsic flows and pyroclastic rocks that are metamorphosed to greenschist and amphibolite facies. The Bear Mountains fault zone separates the melange from the Copper Hill volcanics. The Mother Lode Belt consists of the upper Jurassic Logtown Ridge and upper Jurassic Mariposa formations. The Logtown Ridge Formation consists of volcanic and volcanic-sedimentary rocks of island arc affinity. These rocks are mostly basaltic and include flows, breccias, and a variety of layered pyroclastic rocks. The overlying Mariposa Formation contains a distal turbidite, hemipelagic sequence of black slate, fine grained tuffaceous rocks, and subvolcanic intrusive rocks. Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins. In El Dorado County, the belt trends north through Nashville, northeast through Placerville, and northwest to Garden Valley. At Garden Valley, the Mother Lode Belt splits. The west branch extends northwest through Greenwood, and the east branch extends north through Georgetown to the Georgia Slide area (Busch, 2001). The Mother Lode veins are generally enclosed in Mariposa Formation slate with associated greenstone. The vein system ranges from a few hundred feet to a mile or more in width. Within the zone are numerous discontinuous or linked veins, which may be parallel, convergent, or en echelon. The veins commonly pinch and swell. Few can be traced more than a few thousand feet. Mother Lode type veins fill voids created within faults and fracture zones and consist of quartz, gold and associated sulfides, ankerite, calcite, chlorite, and sericite (Clark and Carlson, 1956).

Comment (Workings): The main drifts were kept as straight as possible and in the center or lowest depression of the channel. To prospect the width of the channel, crosscuts at right angles to the drift were driven on each side to the rims of the channels or the limit of the paying lead. These were timbered and lagged in soft gravels, but not to the extent of the main drift. In wide pay leads, gangways paralleled the main tunnel to help block out the ore in rectangular blocks. In looser gravels, timbering was required and the main difficulty was preventing caving until timbering was in place. The looser gravels were excavated with pick and shovel. Up until the late 1800s, most workings were driven by hand, then later by machine and pneumatic drills. Working drifts in the gravel beds and pay leads themselves were larger than the bedrock tunnels and usually timbered due to their extended and long-term use. In wide gravel deposits, as a precaution against caving, gravel pillars from 20 - 40 feet wide were left on each side of the drift. When the main access tunnel was in bedrock following the line of the channel, pillars were not required, as the tunnel in the gravel was only for temporary use in mining the ground between its connections with the bedrock tunnel. Raises to access the gravel were made every 200 - 400 feet as necessary. The breaking out of gravel (breasting) was done from the working faces of drifts. Usually, 1-2 feet of soft bedrock and 3-4 feet of gravel were mined out to advance the face. When the gravels were well-cemented, blasting was required. Otherwise the material could be removed with picks. Boulder sized material was left underground and only the gravels and fines were removed from the mine. Bedrock swelling was a frequent problem. Tunnels on and within bedrock were sometimes affected by the upward swelling of the bedrock. In these cases, heavy timbering was required and the tunnel floor had to be periodically cut and lowered to keep the tunnel open. Soft or fractured slates were the most favorable bedrock. The surface was usually creviced and weathered enough that gold could be found to a depth of 1 foot in the top of the bedrock. Where sufficiently weathered and soft, this upper bedrock layer could be easily removed. If the surface of the bedrock was too hard to be worked, it was cleaned thoroughly, and the crevices and surface were worked with special tools to remove every particle of gold. According to the gravel's hardness, they were either washed through sluices or crushed in stamp mills. Much of the gravel was so highly cemented it had to be milled several times. Stamp mills with coarse screens were also found to be suitable for milling cemented gravel. For soft and uncemented gravels, a dump, sluices, and water supply under generally low pressure comprised the entire surface workings. Ventilation of mines was accomplished by direct surface connection through the use of boreholes and the mine shafts and tunnels. It relied on natural drafts, drafts by fire, falling water, or blowers. Within the mines, arrangements of doors were often used to direct the flow of air through the tunnels, drifts, and breasts. Ore was removed by ore cars of various capacity determined by available power and tunnel size. In smaller mines, small cars were often pushed by hand. In larger mines using horsepower or trains, larger two ton cars could be brought out in trains of 5-10 cars.

Comment (Geology): The district is situated on a ridge between the South Fork of the American River (American River) and Weber Creek, on which occur remnants of channel gravels deposited by southward flowing tributaries to the ancestral South Fork of the American River. Gravels deposited within the main channel of the American River, which approximately followed the course of Weber Creek, have been largely lost to erosion. The gravel deposits are largely overlain by overlain by thick beds of rhyolite tuff and andesite. In the Placerville area, the Valley Springs Formation coinsist of generally flat lying beds of rhyolite tuff. The tuff usually is fine grained and contains small crystals of black biotite. Smaller amounts of breccia, conglomerate, and siltstone are present (Clark and Carlson, 1956). The overlying Mehrten Formation occupies many of the interstream ridges and consists chiefly of andesitic volcanic debris composed of boulders, cobbles, and pebbles. Placerville District gravels are generally composed of pebbles and boulders of quartz, chert, granitic and volcanic rocks interbedded with clay and sand. Considerable amounts of placer gold is present at or near the bottom of these deposits (Clark and Carlson, 1956). Generally, the gold on bedrock was smooth and coarse. Associated with the gold in the placer deposits are magnetite and other heavy resistant minerals such as ilmenite, garnet, rutile, zircon, and platinum-group metals. The richest gravels in the Placerville District were those at Coon Hollow, just southwest of Placerville, and just west and downstream of the Mother Lode bedrock quartz veins (Lindgren, 1911). Lindgren (1911) describes a number of the individual deposits in greater detail. The ancestral Tertiary American River and its tributaries entered the district from the northeast. The most productive tributaries from east to west across the district were the Deep Blue Lead, Spanish Hill-Green Mountain Channel, and the Coon Hollow Channel. Deep Blue Lead Channel Of the numerous American River tributaries in this district, the best known was the Deep Blue Lead. It entered the northeast corner of the district and extended southward for about 2 miles from White Rock Canyon to Smith's Flat and then west-southwest through Texas Hill to Cedar Ravine. The Blue Lead channel consists of a deep, fairly straight channel flanked by broad sweeping and curving benches. This tributary followed a bedrock slate-granodiorite contact for several miles and is believed to have derived much of its gold from many small veins in the contact zone. The general course of the Deep Blue Lead was from north-northeast to south-southwest. At White Rock, the exposed gravels were hydraulically mined, producing about $5 million. The bedrock channel at White Rock was 30 feet wide with 12 feet of gravel. Two benches 40 feet high also yielded paying gravel (Lindgren, 1911). For about 2 miles from White Rock to Smith Flat, where the gravels were overlain by the Valley Springs and Mehrten formation rocks, the channel was almost continuously drift mined. From Smith Flat, the Deep Blue Lead trended west-southwest for another 2 miles to Texas Hill where it was again mined hydraulically. For most of this stretch, the channel was developed by the Bendfelt, Hook & Ladder, Lyon, Kumfa, Try Again, Texas Hill, Clark, Rivera, and Linden drift mines (Lindgren, 1911). The Lyon Mine produced $1.4 million. At the Benfield Mine, a gravel deposit some five feet thick and 50 to 120 feet wide yielded $2 to $8 per ton. Blue Lead gravels were generally not as rich as those to the west in channels crossing the Mother Lode quartz belt. Blue Lead gravels were deposited before the beginning of the later rhyolitic flows, for cobbles of rhyolite are contained in all of them (Lindgren, 1911).

Comment (Geology): 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 ancestral southern branch of the American River flowed southwestward towards Placerville and followed closely the modern Weber Creek drainage. Numerous tributaries of this ancestral south fork deposited the auriferous gravels that were so productive in the Placerville District. 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. 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. Detailed descriptions of the Tertiary channel deposits as well as a discussion of their geology are contained in U.S. Geological Survey Professional Paper 73 (Lindgren, 1973). Information can also be found in Auriferous Gravels of the Sierra Nevada (Whitney, 1880), California Mining Bureau Bulletin 92 (Haley, 1923), California's Gold-bearing Tertiary Channels (Jenkins and Wright, 1934), and California Division of Mines Bulletin 135 (Averill, 1946).

Comment (Workings): Hydraulic Mining Hydraulic mining methods were first applied in the Placerville District in 1854 at Spanish Hill. Hydraulic mining allowed the bulk processing of large volumes of low yield that would otherwise be unprofitable by other methods of mining. Hydraulic mining involved directing a powerful stream of high pressure water through large nozzles called monitors or "giants" at the base of a gravel bank, undercutting it and allowing it to collapse. Large gravel banks several hundred feet high were mined in this manner, but larger banks were often hydraulicked in two or more benches. In some cases, adits were driven into the exposed face and loaded with explosives to help break down the exposure. The resulting slurry of clay, sand, gravel, and gold was washed through sluice boxes to trap the gold. The sluice boxes were generally four feet wide and deep and often over a thousand feet long and lined with riffles or over devices to mechanically trap the gold. Mercury was added to amalgamate the finer gold. The remaining debris was indiscriminately dumped in the nearest available stream or river. One of hydraulic mining's highest costs was in the ditches, flumes, and reservoirs needed to supply sufficient volumes of water at high pressure. A mine usually needed its own system of ditches and flumes to deliver water from distant and higher reservoirs or rivers. A mine might have 10-20 or more miles of ditches as well as dams and reservoirs, flumes, and tunnels. At the mine site, the water fell through large iron pipes into the monitors. Hydraulic mining flourished for about 30 years until the mid-1880s when the Sawyer Decision curtailed debris disposal. Another expensive undertaking was often finding an outlet for the debris. As the gravels were washed lower and lower in the ancient channel beds, it was often necessary to drive a tunnel through the bedrock channel rim to drain the workings into a nearby valley.

Comment (Identification): The Placerville District, in west-central El Dorado County, is notable for its highly productive placer mines in gravels deposited by tributaries of the ancestral Tertiary South Fork of the American River. These deposits produced at least $25 million. Notable placer mines were the Coon Hollow ($10 million), Spanish Hill ($6 million), White Rock ($5 million), and Smith's Flat ($2 million). Smaller placer and drift mines in the district included the Diamond Springs, Green Mountain, Negro Hill, Sacramento Hill, Texas Hill, Benfield, Cedar Spring, Clark, Kumfa, Landecker, Lyon, Pascoe, Rivera, Try Again, and Union mines. The larger deposits were at first exploited by drift mining in the early 1850s, but this was replaced by large scale hydraulic mining in the mid 1850s to the 1870s. The district also included a number of lode gold mines in Mother Lode quartz veins. The most important lode mine was the Pacific Quartz Mine ($1,486,000), which is described separately in the Mineral Resources and Data System (MRDS). Several smaller lode mines produced between $100,000 - $200,000, including the Epley ($100,000+), Guilford ($200,000+), Harmon ($100,000+), Larkin ($125,000), Oregon ($100,000+), Sherman ($136,000), True Consolidated ($100,000), and Van Hooker ($100,000+) mines. Lesser lode mines included the Elliot, Griffith, Margurite, River Hill, and Superior mines. Total production of the lode mines is unknown, but is thought to exceed $2 million. Only the placer deposits in the district are the subject of this record. Historical references and discussions of the geology and operations of many of the placer and lode mines 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.

Comment (Location): Location selected for latitude and longitude is the Coon Hollow School symbol on the USGS 7.5 minute Placerville quadrangle. Coon Hollow was the most productive of the district?s mines.

Comment (Deposit): The Placerville District is one of the most important Tertiary placer-gold mining districts in the northern Sierra Nevada. The district is noted for several large hydraulic and drift mines within auriferous Eocene channel gravels deposited by numerous southwestward flowing tributaries of the ancestral South Fork of the American River. Ores consisted of channel lag and bench gravels deposited on the eroded bedrock surface and adjacent floodplains, and later elevated and exposed by uplift and downcutting of modern drainages. They were preserved under a sequence of volcanic deposits of the Tertiary Valley Springs and Mehrten formations, which blanket most of the gravels in the Placerville District. The gravels were laden with placer gold from the erosion of the bedrock gold-quartz veins through which the rivers flowed. Secondary deposits were encountered overlying the basal gravels in a section of younger interbedded channel gravels and volcanic flows. These "intervolcanic" gravels, while often barren, were sometimes charged with placer gold by erosion of the older auriferous channel gravels. The principle gravel deposit, the "Deep Blue Lead", is expressed as a bedrock erosional channel which transects the east half of the district from northeast to southwest. Other important gravel deposits include those of the Coon Hollow and Spanish Hill-Green Mountain channels on the west end of the district. These gravels are considerably richer than those of the Deep Blue Lead, having eroded the gold-bearing Mother Lode quartz veins in the central and western parts of the district. 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 serpentine.

Comment (Economic Factors): The value of the total output of the Placerville District is unknown, but the placer mines are estimated to have yielded at least $25 million, and lode mines at least $2 million. Clark (1970) reports the production of several of the placer mines: Coon Hollow $10 million, Spanish Hill $6 million, Smith's Flat $2 million+, and White Rock $5 million. The Pacific Mine was the most productive of the lode mines, having produced $1,486,000. Other significant lode mines included the Epley $100,000+, Guilford $200,000+, Harmon $100,000+, Larkin $125,000, Oregon $100,000+, Sherman $136,000, True Consolidated $100,000, and Van Hooker $100,000+.

Comment (Commodity): Commodity Info: Placer deposits: Placer gold dust to large nuggets. Lode deposits: Free-milling gold-bearing quartz veins and sulfides

Comment (Commodity): Ore Materials: Native gold and sulfides (lode)

Comment (Commodity): Gangue Materials: Quartz, volcanic, granitic, and metamorphic gravels

Comment (Development): Placer gold was discovered in Hangtown Creek in the Placerville area in July 1848. Initially, placer production from Hangtown Creek reached approximately 1,000 ounces of gold per week. As word spread, a rush to the area ensued and the town of Dry Diggings was established (nicknamed ?Old Hangtown? in reference to the hanging of three robbers here in October, 1849). The town was later renamed Placerville. By fall of 1849 the miners working Hangtown Creek had discovered Tertiary river gravel laying exposed on both sides of Spanish Hill. The Spanish Hill miners began "coyoting" under the hill where they found the auriferous gravel in channels generally running west by southwest. Names such as "Coon Hollow Channel" and the "Deep Blue Lead" were adopted by the miners to identify these gold producing channels. One 49er, J.M. Letts described the process of coyoting at Spanish Hill? "It was by digging holes or pits in the ground - generally into the base of the mountains - sometimes penetrating to a depth of 50 - 100 feet with an opening just sufficient to admit a man." (Noble, 2002). Mining at Spanish Hill paid extremely well but remained primitive until 1854 when the South Fork & Placerville Canal Company completed 16 miles of flume, which brought much needed water to Spanish Hill and Coon Hollow for hydraulic mining. By December of that year, the entire top of Spanish Hill had been washed out to a depth of 60 feet (Noble, 2002). At Coon Hollow, the gravel was removed by drifting from 1852 to 1861, and between 1861 and 1871 by hydraulic mining. From the middle 1850's through the 1870s, numerous hydraulic and drift mines were discovered and worked intermittently until as late as the 1930s. Quartz mining began in 1852 at the Pacific Mine, but the chief period of lode mining was from the 1880 to about 1915. Many of the mines (Lode and drift, and hydraulic) in the district came under the control of the Placerville Gold Mining Company, which was incorporated in 1911, and was a successor to the Placerville Gold Quartz Company, LTD., an English concern that was originally incorporated in 1878. (Clark and Carlson, 1956). The Placerville Gold Mining Company controlled 1,400 acres in all. Many of these claims were early day producers of which only fragmentary histories remain (Logan, 1934).

Comment (Workings): The Placerville District gravel deposits were generally worked by small- to medium-sized drift mines with access through shallow shafts or through adits generally less than 2,000 feet long, and/or large scale hydraulic mining operations. Consequently, there is meager information regarding specific mine workings. Limited information about some of the districts drift mines is contained in unpublished historical files of the California Geological Survey and other published sources (Noble, 2002). Specifics of the hydraulically mined deposits are even more scarce. Generic discussions of these mining techniques follow: Drift Mining Drift mining involved driving adits and tunnels along or close to the lowest point in the bedrock trough of an ancient channel and following it upstream along the bedrock surface. Some deeply buried drift mines were originally accessed through vertical shafts requiring timbering, headframes, hoisting, and pumping equipment. Larger shafts were seldom over 3 compartments Smaller mines often had single compartment shafts as small as 2 x 5 feet. Since considerable water was associated with the gravels, it was a serious problem in deeper shafts and costly pumping was required. By the 1890s, due to drainage problems and the expense of hoisting, most major drift mines were accessed through tramway and drain tunnels driven into bedrock below the channels. Channels were usually located by gravel exposures on hillsides and terraces. Exposures of upstream and downstream gravels were called "inlets" and "outlets," respectively. Where a ravine or canyon cut into, but not through an old channel, the exposure was called a "breakout." The preferred method of developing an inlet was to tunnel through bedrock under the channel at such a depth and angle as to break through into the bed of the channel providing natural drainage. The overlying gravels could then be accessed directly through the tunnel or by periodic raises and drifts. Development of an outlet involved following the bedrock channel directly into the hillside, the incline of the bedrock providing natural drainage. The tunnel entrances were usually in or near a ravine or gulch to aid in waste-rock disposal. Prospecting and developing a breakout was more difficult, since the exposed gravel could be in the basal channel or hundreds of feet up on the edge of the channel, making it impossible to locate a prospect tunnel with any certainty. The surest method of prospecting was to run an incline on the pitch of the bedrock. Another method was to sink a vertical shaft on the presumed channel axis. The former method proved superior since it involved less subjectivity and often uncovered paying bench gravels on edges of the old stream. Once the bed of the channel was located, it was prospected by drifts and cross cuts to ascertain width, direction, grade, and the location, extent, and quality of pay. Access tunnels were driven in bedrock to minimize timbering and ensure a stable roof, through which upraises were driven to work the placer gravels. Tunnels were generally run under the lowest point of the bed of the channel in order to assure natural drainage and to make it possible to take auriferous gravels out of the mine without having to hoist it.

Comment (Geology): Coon Hollow Channel The Coon Hollow channel was the richest of the Placerville district's placer deposits, having produced about $10 million. This channel is in the western part of the district and its deposits are thought to have been enriched by erosion of bedrock Mother Lode gold veins. The Coon Hollow deposits contained 3 gravel pay streaks, the first on bedrock, the second 25 feet above, and the third 60 feet above. The first and third were very rich (Lindgren, 1911). Coon Hollow gravels were much richer than the gravels of the Blue Lead or Spanish Hill - Green Mountain channels. Overall, the entire section of gravel was at least 100 feet thick and averaged about $1 per yard (Lindgren, 1911). A single claim of 20 acres (Excelsior claim) produced $5 million from gravels containing as much as $5 per cubic yard. Total width of the Coon Hollow bedrock channel was 2,000 feet. The third streak was reportedly 300 feet wide. Part of the gravel was cemented. Unlike the gravels farther east, there were no volcanic pebbles in the Coon Hollow gravels (Lindgren, 1911). Spanish Hill-Green Mountain Channel The Spanish Hill-Green Mountain Channel, in the south central part of the district, formed a well-defined tributary to the main river channel and was separated from the Deep Blue Lead to the east and the Coon Hollow channel to the west by pronounced intervening bedrock ridges (Lindgren, 1911). Several mines exploited these channel deposits including the important Spanish Hill hydraulic mine, which produced $6 million, Cedar Springs, and Green Mountain drift mines. In the Cedar Springs Mine, the bedrock channel was 300 to 600 feet wide with pay gravel 4 to feet thick.

Comment (Geology): 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. 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 usual 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. Continued uplift and increasing gradient during the Pliocene-early Pleistocene allowed the modern drainages 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. LOCAL GEOLOGY While the Placerville District includes some Mother Lode quartz lode mines, it is best known for its highly productive Tertiary channel deposits, the most important being at Coon Hollow, Diamond Springs, Smith's Flat, Texas Hill, and White Rock. The Placerville District straddles the Melones Fault zone and hence bedrock varies from east to west across the district. A belt of Mother Lode Mariposa Formation gray to black approximately one to two miles wide extends northward through the district. To the east, bedrock is schist and slate of the Calaveras Complex and granodiorite. To the west, bedrock is amphibolite and greenstone (Clark, 1970).

Comment (Geology): The Melones Fault zone separates the Mother Lode Belt from the East Belt. The East Belt lies in the south central part of El Dorado County, and traverses the county from the southern county line, north through Omo Ranch and Grizzly Flat, and apparently terminates near the Hazel Creek Mine east of Jenkinson Reservoir (Busch, 2001). The Eastern Belt is dominantly argillite, phyllite and phyllonite, and chert of Paleozoic age that have been assigned to the Shoo Fly Complex by most investigators. Lode deposits of the East Belt consist of many individual gold quartz veins within rocks of the Shoo Fly complex, or in granitic rocks. Most of the veins trend northward and dip steeply. East Belt veins are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. 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 intrusives. An important widespread unit of intercalated rhyolite tuffs and intervolcanic channel gravels is the Oligocene-Miocene Valley Springs Formation. The youngest volcanic unit, 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 sequence 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 younger 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 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).

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References

Reference (Deposit): DeGroot, H., 1890, Smiths Flat mines: California State Mining Bureau, 10th Annual Report of the Sate Mineralogist, pp. 179-180.

Reference (Deposit): Clark, W.B. and Carlson, D.W., 1956, Mines and mineral resources of El Dorado County: California Division of Mines, California Journal of Mines and Geology, v. 52, p. 422434.

Reference (Deposit): U.S. Geological Survey Professional Paper 410, 70 p.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 107-108.

Reference (Deposit): Clark, L.D., 1964, Stratigraphy and structure of part of the western Sierra Nevada metamophic belt, California:

Reference (Deposit): Busch, L.L., 2001, Mineral land classification of El Dorado County, California: California Geological Survey Open-File Report 2000-03.

Reference (Deposit): Additional information on various mines within the Placerville District is contained in File Nos. 339-8854 (Union Mine), 339-8844 (Try Again Mine), 331-5009 (Riveria Mine), 322-5969 & 331-7174 (Larkin Mine), 331-7169 (Landecker Mine), 332-0489 (Kumfa Mine), 322-5962 (Epley Mine), 332-0454 (Hook & Ladder Mine) (CGS Mineral Resources Files, Sacramento).

Reference (Deposit): Tucker, W.B., 1919, El Dorado County - Pacific Mine: California State Mining Bureau, 15th Annual Report of the State Mineralogist, pp. 293-295.

Reference (Deposit): Tucker, W.B., and Waring, C.A., 1916, Mines and mineral resources of El Dorado, Placer, Sacramento, and Yuba counties: California State Mining Bureau, 15th Report of the State Mineralogist, p. 283-299.

Reference (Deposit): Rowlands, R., 1894, Map of the principle gravel mines in the vicinity of Placerville, California: California State Mining Bureau, 12th Annual Report of the Sate Mineralogist, pp. 293-295.

Reference (Deposit): Noble, D., 2002, Mines of El Dorado County: El Dorado County Library website: http://www.eldoradolibrary.org/mines.htm
URL: http://www.eldoradolibrary.org/mines.htm

Reference (Deposit): Logan, C.A., 1938, Mineral resources of El Dorado County: California Division of Mines, 34th Report of the State Mineralogist, p. 251-253.

Reference (Deposit): Logan, C.A., 1935, El Dorado County - Harmon Group: California Division of Mines Bulletin 108, pp. 26-27.

Reference (Deposit): Logan, C.A., 1935, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 35, 52.

Reference (Deposit): Lindgren, W., 1911, Tertiary gravels of the Sierra Nevada: U. S. Geological Survey Professional Paper 73, pp. 171-180.

Reference (Deposit): Lindgren, W., and Turner, H.W., 1894, Placerville folio: U.S. Geological Survey Geological Atlas of the U.S., folio 3, 3 p.

Reference (Deposit): Irelan, W., Jr., 1888, El Dorado County, Van Hooker, Pacific, and Epley Consolidated mines: California State Mining Bureau, 8th Annual Report of the State Mineralogist, p. 181-187.

Reference (Deposit): Earhart, R.L., 1988, Geologic setting of gold occurrences in the Big Canyon area, El Dorado County, California: U.S. Geological Survey professional Paper 1576, 13 p.

Reference (Deposit): Saleeby, J., 1982, Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development: Journal of Geophysical research, v. 87, n0. 8, p. 1803-1824.

Reference (Deposit): Duffield, W.A., and Sharp, R.V., 1975, geology of the Sierra Foothills melange and adjacent areas, Amador County, California: U.S. Geological Survey Professional Paper 827, 30 p.

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California Gold

"Where to Find Gold in California" looks at the density of modern placer mining claims along with historical gold mining locations and mining district descriptions to determine areas of high gold discovery potential in California. Read more: Where to Find Gold in California.