San Juan Mine (San Juan Ridge Mine), North Columbia, North Columbia Mining District (Columbia Hill Mining District), Nevada County, California, USAi

Regional Level Types
San Juan Mine (San Juan Ridge Mine)	Mine
North Columbia	- not defined -
North Columbia Mining District (Columbia Hill Mining District)	Mining District
Nevada County	County
California	State
USA	Country

A former placer Au-Ag-Pt occurrence/mine located in sec. 16, T17N, R11E, MDM, 1.3 km (0.8 mile) SSW of North Columbia (7 miles NE of Nevada City), on Federal Wildlife Service land (Sacramento River National Wildlife Refuge). Discovered in 1851. Owned & operated by the Battle Mountain Gold Company (70%), Texas (1985-1992). Owned & operated by the Siskon Gold Corporation (100%), California (1992-1997>). MRDS database stated accuracy for this location is 1,000 meters. NOTE: USGS MRDS databse file #10072366 presents erroneous coordinates for this locality.

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.

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.

Mineralization is a placer Au deposit (Mineral occurrence model information: Model code: 119; USGS model code: 39a; BC deposit profile: C01. C02; Deposit model name: Placer Au-PGE; Mark3 model number: 54), hosted in unconsolidated Tertiary sand & gravel. The ore body is irregular in form. Controls for ore emplacement included the mechanical accumulation on irregular bedrock riffles and within river and stream channel lag gravels, bars, and point bar deposits. Local rocks include Quaternary alluvium and marine deposits and/or Eocene nonmarine rocks, unit 1 (Northern and Central California).

Regional geologic structures include the Big Bend-Wolf Creek Fault Zone and the Melones Fault Zone.

Geology of the San Juan Ridge Deposits: At San Juan Ridge, basement rocks are composed primarily of meta-sediments 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 occurs 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 1960's, 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 cubic 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, ilmenite, 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 cubic yard ($35.00 gold).

Commodity information: Ore materials: Native gold - fine to coarse gold and nuggets (.900 fine). Gangue materials: Quartz and metamorphic gravels; accessory minerals include magnetite, ilmenite, zircon, pyrite, amphibole, epidote, chlorite, and siderite.

Production data are found in: Comapnies, p. 166; MINING ENGINEERING. SAN JUAN RIDGE GOLD MINE BEGINS PRODUCTION. 12/96. PP.26-31.

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 1960's, 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 1980's 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.

Production statistics: Year: 1994 (estimated): 40,000 Troy ounces/year Au (1,244,139 grams); Year: 1995: 25,000 Troy ounces Au (777,587 grams);
Year: 1996: 9,651 Troy ounces/year (300,180 grams; First half of 1996: (estimated) 7,000 Troy ounces Au (217,724 grams). Estimated production is 30,000 to 40,000 ounces per year 1995). Over 7,000 ounces Au was produced during the first half of 1996.

Reserve-Resource data are found in: Randol Mining Directory (1994-1995), U.S. Mines and Mining Companies: 238; Siskon Gold Corporation (1996), Annual & Form 10-SKB Report: 6-7.

Reserves: Type: in-situ, estimate year: 1997: Demonstrated reserves: 2,880,000 metric tons ore @ 3.15 grams/metric ton Au. Equivalent to 16.4 million cubic yards Au-bearing material, 0.034 ounce Au/ton. The 257,707 ounces Au (1996/97 Randol Mining Directory), was converted from ounces/cubic yard to grams/metric ton using a specific gravity of 2.2.

Other Databases

Link to USGS MRDS:	10310673

Other Regions, Features and Areas containing this locality

References