The San Juan Ridge is a gold mine located in Nevada county, California at an elevation of 2,493 feet.
About the MRDS Data:
All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.
Elevation: 2,493 Feet (760 Meters)
Primary Mineral: Gold
Lat, Long: 39.3625, -120.99250
Map: View on Google Maps
San Juan Ridge MRDS details
Primary: San Juan Ridge
Secondary: Columbia Hill Mine
Secondary: Badger Hill Diggings
District: North San Juan District
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 indicate a claim status and does not necessarily indicate an area is open to prospecting.
Administrative Organization: Nevada County Planning Dept.
Record Type: District
Operation Category: Past Producer
Deposit Type: Stream placer
Operation Type: Surface
Discovery Year: 1851
Years of Production:
Mineral Deposit Model
Model Name: Placer Au-PGE
Description: Big Bend - Wolf Creek Fault Zone, Melones Fault Zone
Name: Sand and Gravel
Age Type: Host Rock
Age Young: Tertiary
Comment (Identification): San Juan Ridge comprises the North Columbia, Badger Hill, and North San Juan districts in north-central Nevada County about 7 miles northeast of Nevada City. In this application, "San Juan Ridge" refers to the extensive auriferous placer deposits laid down by the ancestral Yuba River just south of the town of North Columbia and along the axis of San Juan Ridge. The extensive gravel deposits cover many square miles and represent the largest deposit of Tertiary gravels in the region. Total production is unknown, but the placer deposits are thought to have produced between $2 and $3 million in the North Columbia District alone, primarily from hydraulic mining operations conducted between 1852 and 1884.
Comment (Geology): 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 is 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. The lower unit is compositionally immature as evidenced by their heavy mineral suites. Chlorite, amphibole, and epidote are common. Disseminated pyrite and pyritic pebble coatings are common. Adjacent to the bedrock channels, broad gently sloping benches received shallow but extensive accumulations of auriferous overbank gravels sometimes 1-2 miles wide. 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) 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. 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 are often found downstream of the gold-rich Feather River Peridotite Belt, and in some cases immediately down stream of eroded qold quartz veins exposed in the scoured bedrock. Most of the gold found in the gravels of the North Columbia, North Bloomfield and Moore's Flat districts is thought to have originated from the famous lode veins of the Alleghany mining district to the northeast. Most of the gold occurs in the lower sand and gravel, particularly within only a few feet of bedrock. Generally, in drift mines only these lower gravels were exploited; however, in hydraulic mines the entire gravel deposit 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. 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, 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.
Comment (Workings): 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.
Comment (Geology): GEOLOGY OF THE SAN JUAN RIDGE DEPOSITS At San Juan Ridge, basement rocks are composed primarily of metasediments of the Delhi Formation within the larger Lake Combie-Slate Creek Complex. Bedrock is overlain by thick auriferous gravels. The Valley Springs and Mehrten Formations overly the gravels farther east along San Juan Ridge divide, but erosion in the San Juan Ridge area and the neighboring North Bloomfield and Badger Hill districts has exposed the gravel deposits. The deposits are the most extensive in the region due to their having been deposited at the confluence of the ancestral Yuba River and one of its largest tributaries. The main river channel flowed westward from neighboring North Bloomfield District though the North Columbia District continuing onward through the Badger Hill District to the west. In the middle of the main channel, gravel deposits can reach 400 - 500 feet thick. The tributary flowed northward and is responsible for the important auriferous gravels in the Dutch Flat, You Bet-Red Dog, Blue Tent, and Scotts Flat districts to the south. The gravel deposits can be divided lithologically and texturally into an upper and lower unit; the contact between the two is often ill-defined. The lower unit, or blue lead of the early miners, rests directly on bedrock, and contains the richest ores. While the unit ranges from 70 -140 feet thick, the richest gravel occurrs within 40 feet of bedrock. It consists of coarse gravels, cobbles, pebbles, and boulders. Boulders up to 10 feet in diameter have been encountered in lower unit gravels in nearby hydraulic pits. The deep gravels are generally well cemented and quartz-rich. Generally, the lower gravels have a red tint above the water table where they are oxidized, while below the water table these gravels take on a darker blue-gray color and contain 1-2% secondary pyrite. Lower gravels are generally immature and composed of bluish-black slate and phyllite, weathered volcanic rocks, and quartz representing the upstream and underlying basement complexes and the local Delhi Formation basement. A USGS study of the San Juan Ridge gravels conducted in the 1960s, while identifying values as high as $6.35 per cubic yard ($35.00 gold), concluded that rarely do the lower gavels exceed $1.00 per yard ($35.00 gold). Most of the gold particles are 1 -2 mm in diameter and 0.1 - 0.2 mm thick. Gold coarser than 1 mm was not detected more than 80 feet above bedrock during the USGS study. The upper gravels are well-exposed in cliffs and bluffs around San Juan Ridge. In contrast to the lower gravels, upper gravels are much finer, with clasts seldom larger than pebble size. They are compositionally mature with milky white quartz and quartzite clasts predominating and a heavy mineral assemblage almost exclusively of zircon, illmenite, and magnetite. Clay and silt beds are common. Oxidation has given these deposits a reddish tint. Large-scale cross-bedding and cut-and-fill features are common. Carbonized and silicified wood fragments are characteristic of this unit. The upper gravels generally have significantly lower values than the deeper gravels, but were still considered profitable. The USGS concluded these gravels seldom contain more than $0.02 per yard ($35.00 gold).
Comment (Geology): The northern Sierra Nevada experienced a long period of Cretaceous to early Tertiary erosion during which its sediments transported westward by river systems to Cretaceous Great Valley marine basin. By the Eocene, low gradients and a high sediment load allowed the river valleys to accumulate thick gravel deposits as the drainages meandered over flood plains up to several miles wide developed on the bedrock surface. The oldest of these deposits are Eocene auriferous gravels deposited by the predecessors of the modern Yuba, American, Calaveras, Stanislaus, and Tuolumne rivers and preserved in basement paleochannels. The major rivers were similar in location, direction of flow, and drainage area to their modern counterparts. 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 Columbia, North Bloomfield, San Juan Ridge, Moore's Flat, and French Corral districts. Subsequent volcanism blanketed the eroded basement rocks and channel gravels. 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 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 and younger 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 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 replaced by overlying pebble, sand, and clay beds.
Comment (Location): Locattion selected for latitude and longitude is the approximate center of the large former San Juan Ridge hydraulic workings and corresponds to the C S/2 Sec. 5-T17N-R9E
Comment (Geology): REGIONAL SETTING The northern Sierra Nevada is home to numerous important placer gold deposits. These include the famous placer districts of North Columbia, North Bloomfield, 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 and Structure 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 consisting of basaltic to intermediate pillow flows overlain by pyroclastic and volcanoclastic rocks with diabase, metagabbro, and gabbro-diorite intrusives. 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. 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 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 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.
Comment (Commodity): Ore Materials: Native gold - Fine to coarse gold and nuggets (.900 fine)
Comment (Commodity): Gangue Materials: Quartz and metamorphic gravels; accessory minerals magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite
Comment (Deposit): The San Juan Ridge placer gold deposits consist of thick Eocene fluvial sands and gravels deposited in incised bedrock channels by the ancestral Yuba River and its tributaries. The placer gold was eroded from the gold-quartz veins that lace the bedrock through the Sierra Nevada gold belts and are the primary lode ores in the important Alleghany Mining District to the northeast. The sand and gravel sequence is up to 500 feet thick and consists of lower and upper units. The lower unit is up to 140 feet thick and consists of gravel to boulder size material within incised channels in the metasedimentary and igneous bedrock. Most of the gold occurs within 80 feet of bedrock and consists of particles 1 -2 mm in diameter and 0.1 - 0.2 mm thick or smaller. A USGS study of the San Juan Ridge deposits conducted in the 1960s identified values as high as $6.35 per cubic yard ($35.00 gold), but concluded that rarely do the lower gavels exceed $1.00 per yard ($35.00 gold). The lower unit is overlain by a finer pebble to sand and silt upper unit, which comprises the bulk of the deposits and is generally lean (seldom more than $0.02 per yard) or barren.
Comment (Development): Placer gold was first discovered and placer mined in the San Juan Ridge area in 1851. Extensive hydraulic mining of the deposits began in the mid-1850s and continued to 1884 when the Sawyer decision injunction prohibited tailings disposal in tributaries of the Sacramento River. During the 1800s, the main property was operated as the Columbia Hill Mine. While the hydraulic operations were extensive, in general they only removed the upper 100-150 feet of the deposits. Three hundred to three hundred fifty feet of rich lower gravels remain and are considered one of the largest unexploited Tertiary gravel deposits in the Sierra Nevada. From the 1890s to early 1900s, Chinese miners conducted some small scale placer mining operations. The 1900s saw several periods of activity. In 1917, the Haman Company conducted an exploratory program on the deposits involving 7,100 feet of borings, but no mining took place. Renewed exploration efforts were undertaken in 1939, with an additional 23,000 feet of 10-inch churn-drill borings. During 1968 -1969, the US Bureau of Mines and the US Geological Survey also drilled several churn holes in the San Juan Ridge and neighboring deposits. In 1979, the Placer Service Company (PSC) leased the property from its owner (San Juan Gold Co.) with the intent of exploring and mining the gravels. PSC drilled more than 20 three foot shafts and over 5,781 feet of additional boreholes. The initial exploratory phase identified over 18 million cubic yards of pay gravels. While PSC planned an 8-year operation of exploration and open-pit mining, all operations were ended in 1984 due to local opposition and low gold prices.
Comment (Economic Factors): Total production is unknown, but the placer deposits at North Columbia are thought to have produced between $2 and $3 million. Lindgren (1911) estimated that 25 million cubic yards had been removed and 165 million remain at San Juan Ridge. In the 1960s, they USGS estimated that about $140,000,000 ($35 gold) distributed in 800,000,000 cubic yards of gravel remained in the San Juan Ridge area between the Malakoff Diggings to the east and Badger Hill to the west. An exploratory program conducted by the Placer Service Company in the 1980s concluded that approximately 187,000 ounces of proven and probable reserves, 139,000 ounces of possible reserves, and 633,000 ounces of potential reserves remained within a 2,200-acre parcel under lease.
Reference (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.
Reference (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.
Reference (Deposit): Additional information on San Juan Ridge is contained in File No. 331-9312 (CGS Mineral Resources Files, Sacramento)
Reference (Deposit): Clark, L. D., 1960, Foothills fault system, western Sierra Nevada, California: Geological Society of America Bulletin, v. 71, p. 483-496.
Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 193, p. 101.
Reference (Deposit): Hobson, J.B. and Wiltsee, E.A., 1893, Columbia Hill district: California State Mining Bureau Report 11, p. 305-308.
Reference (Deposit): Irelan, W., Jr., 1888, Columbia Hill district: California State Mining Bureau Report 8, p. 444-447.
Reference (Deposit): Lindgren, W., 1900, Colfax folio: U.S. Geological Survey Atlas of the U.S., Folio 66, 10 p.
Reference (Deposit): Lindgren, W., 1911, Tertiary gravels of the Sierra Nevada: U.S. Geological Survey Professional Paper 73, p. 139.
Reference (Deposit): MacBoyle, E., 1919, Nevada County, North Columbia mining district: California State Mining Bureau Report 16, p. 48-51.