Friday, May 31, 2013

The Giant King gold mine

Looking towards the mine portal within the Giant King mine.
In 2012, I visited the Giant King mine near the town of Washington and walked a small portion of the property (Figure 1).

Figure 1. Looking down a 40o slope above the
Giant King Mine.
The Giant King is in the Washington District of Nevada County a short distance from the town of Washington (~200 residents), and 14 to 16 miles east of Nevada City and Grass Valley. The claim maps provided by the mine owners indicated the mine has 480 acres of contiguous claims within 24 unpatented lode claims that enclose the Giant King and much of the Alpha hydraulic mine (Figure 2).

The Giant King mine lies along the Melones Fault Zone (MFZ) that includes the Mother Lode trend to the south and the 16-to-1 Mine to the north. Gold was recovered from both placer and lode deposits throughout this belt. Total gold production from California to date, has been approximately 118 million ounces – more than most countries in the world.

Figure 2. Lode claim map. The Alpha mine
lies on the eastern edge of the claims block.
A few hundred diamonds were also found nearby. Many were recovered from placers in the Oroville area 40 miles to the west-northwest. The Cherokee and French Corral gold placers in the Oroville region also yielded diamonds: Nevada County lies upslope from these. Other gems (benitoite and sapphire) were identified in a placer 25 miles to the north of the Giant King mine (Hausel, personal field notes). A possible source for such gems would be aluminous serpentinite and a source for diamonds has been postulated as tectonically emplaced, mantle-derived serpentinized peridotites (serpentinites) similar to those discovered at Beni Bousera in Morocco (Erlich and Hausel, 2000). Whether such high-pressure serpentinites exist in the Washington district was not investigated.

History & Geology
According to Western Mining History, the Washington district (which includes the Giant King mine) was caught up in the great California gold rush during the second half of the 19th century, and placers in the Middle Yuba River immediately downstream from the Giant King were mined for gold. The Omega and Alpha hydraulic mines were opened in the middle 1850s and worked on a major scale through the 1880s.

Lode mining began in the 1850s and continued until about 1915. There was renewed activity during the 1930s. The nearby Red Ledge mine was worked for gold and chromite. The Red Ledge produced some visible gold. According to a brief summary at, the gold-quartz veins at Red Ledge were in sedimentary rocks close to a contact with serpentinite (author’s note – this geology is similar the Giant King). At the Red Ledge, mariposite was reported to be associated with altered serpentinite as well as in close proximity to gold; and this appears to be a very important indicator of gold at the 16 to 1 mine north of the Giant King mine. Mariposite is distinctly colored, chrome-bearing mica.

Western Mining History’s website further describes the Washington district as being underlain by slate, schist and quartzite of the Blue Canyon Formation. A serpentine body one to two miles wide crops out in the central portion of the district. The Relief quartzite (Carboniferous) and amphibolite lie to the west and granodiorite to the east. The serpentine is a south extension of a belt that passes north-northwest through Alleghany and Goodyear's Bar in Sierra County to the north. Tertiary andesite overlies the main ridges to the north and south.

Figure 3. Geological map of the Giant King mine area (Saucedo and 
Wagner, 1992). The central purple band is serpentinized peridotite in 
the Melones fault zone. The light-blue band to the east is slate of 
the Shoo Fly complex (the primary host for the Giant King mine). 
To the west of the MFZ (light-blue band) are Calaveras Complex 
rocks (includes chert and argillites).
The auriferous Tertiary channel gravels at Alpha and Omega are part of the main channel that extends west and north to Relief and North Bloomfield. Jarmin (1927) estimated that, at Omega, 13 million yards were mined and yielded 13.5 cents in gold per yard. Lindgren (1911) estimated that 40 million yards remained. The quartz veins contain small but rich ore bodies, similar to those of the Alleghany district to the north, but are not as plentiful.

Except for arsenopyrite, sulfides are not abundant. 
Figure 4. Narrow fault with offset veins.
Following discovery of quartz veins in Washington Creek (a tributary of the Yuba River) in 1891, development of the Giant King mine and mill began. Some gold was mined on the property, which was closed during World War I and World War II. The history of the mine and events can be reviewed at the Giant King Mine’s website.

The mine was dug in metasedimentary rocks (slate) along the edge of the MFZ of the California gold belt (Figure 3). The Giant King encloses three main veins referred to as the Queen Vein, Central Vein, and King Vein in this report. Minor veins also occur in the mine and some may have potential as unrecognized mineralized zones. These need to be investigated for gold content. Recommendations are summarized in the last section of this report.

Based on the geological map compiled by Saucedo and Wagner (1992), the primary host for the Giant King is the northerly-trending Shoo Fly Slate. The slate is dark-gray, fissile, clayey metamorphosed sedimentary rock that weathers brown to tan and has strong foliation likely paralleling original sedimentary bedding. The rock units are tilted in the mine and sit on edge and may even be overturned locally. The slate is folded with several minor faults with only apparent minor offset (Figure 4). No major offsets were observed on any fault in the mine. The Slate lies in contact with serpentinite of the MFZ a very short distance west of the mine portals (the contact is likely marked by Washington Creek). 

Figure 5 Gold districts in the Sierra Foothills
California (from Sillitoe, 2008) (MFZ = Melones
fault zone) (BMFZ = Bear Mountains fault zone).
The MFZ is an important structure recognized in the Mother Lode district to the south, where it represents a major fracture of the foothills fault system of the western Sierra Nevada Mountains separating Mesozoic rocks to the west from Paleozoic rocks to the east. It is likely a conduit that tapped auriferous solutions at great depth during tectonic deformation in the geological past. Gold and silica (quartz) rose from these depths in the Mother Lode district as illustrated by some mines in the district that reached depths as great as 5,000 feet. It is likely that much of this gold was derived from the serpentinites of the MFZ.

Along the northern portion of the MFZ (and in the vicinity of the Giant King mine), this suture cuts Paleozoic stratigraphy exposing Paleozoic rocks on both sides of the fault. Movement along the MFZ is complex and may include both strike-slip and dip-slip components (Cebull and Russell, 1979).

The MFZ is likely important in controlling emplacement of gold-quartz veins in the Alleghany and the Mother Lode districts to the north and south of the Washington district, and should also be a gold target in the Washington district (Figure 5).

Figure 6. King vein (shear zone) exposed in mine drift.
Photo taken at manway connecting Lower Tunnel
(No. 1) to Middle Tunnel (No. 2).
The MFZ is mapped immediately west of the Giant King mine portals. Even so, the Lower Tunnel cuts a distinct shear zone characteristic of the MFZ that could represent a narrow off-shoot of the MFZ near the mine face. This shear encloses several; narrow, vertical quartz veins that were followed by a north-northeasterly drift for a short distance (Figure 6). The drift likely was the last development in the mine prior to closure during World War II and it does not appear that this suture was sampled or tested for gold (see Recommendations). This structure is identified as the King Vein in this report (see Figures 6 and 8).

Figure 7. Location of 1911 channel sample in the
Central Vein in east stope of Tunnel 2. Note
the abundant quartz and limonite (tawny-colored stains)
in the mine rib. The limonite likely originated from
oxidation of primary sulfides.

Figure 8. Cross-section and sketch map of mine workings at the Giant King with my notes and vein projections. There appears to be three parallel veins (targets) in the mine as well as a lack of samples in the Upper and Lower Tunnels on both the Central and King Veins. The King Vein may have been cut in the Central Tunnel just past the dam near the mine face. Otherwise, it may be just beyond the mine face.
References Cited
Cebull, S.E., and Russell, L.R., 1979, Role of the Melones fault zone in the structural chronology of the North Yuba River area, Western Sierra Nevada, California: Geological Society of America, Bulletin, v. 90, no. 3, p. 225-227.
Erlich, E.I., and Hausel, W.D., 2002, Diamond Deposits - Origin, Exploration and History of Discovery. Society of SME. 374 p.
Hausel, W.D., 1998, Diamonds & Mantle Source Rocks in the Wyoming Craton with Discussions of Other US Occurrences. WSGS Report of Investigations 53, 93 p.
Hausel, W.D., and Hausel, E.J., 2011, Gold: Field Guide for Prospectors and Geologists (Part 1 Wyoming). CreateSpace (Self-Published), 366 p.
Silitoe, R.H., 2008, Special Paper: Major gold deposits and belts of the North and South American Cordillera: Distribution, tectonomagmatic settings and metallogenic considerations: Economic Geology, v. 103, p. 663-687.
Saucedo, G.J., and Wagner, D.L., 1992, Geological map of the Chico Quadrangle, California (1:125,000)

Central vein complex exposed in the Giant King mine.
Narrow banded vein with adjacent stockworks in the Giant King
mine back.
At least two generations of quartz veins visible in the back of the mine tunnel, Giant King mine, California.
Limonite-stained mine rib, Giant King gold mine, California.
Ribbon vein in mine back at the Giant King.

Historical Alpha and Omega placer near the Giant King mine. Placers like
always contain gold nuggets missed by former miners. These are great places
to use metal detectors. And who knows how many diamonds or benitoite
gems may lie in these gravels.

Friday, February 12, 2010

California - Gold, Geology & Prospecting

Shorty pans for gold - sketch by the author.
California's rich gold mining heritage should not end with 'historical' mining: the State’s geology and mining history supports that rich deposits remain to be found, and many deposits that were mined left unexplored resources in place. But the naivety of the public and green movements, over-regulation by bureaucracies, costs and liabilities of mining, and exploration costs have left the state broke. California produced more than 118 million ounces of gold in the past worth about $130 billion at today’s gold prices (Hill and others, 2001)! In addition to gold, the state yielded by-product platinum, sapphire and benitoite from its serpentinite belt in the Sierra Nevada. And the accidental recovery of >600 diamonds from 15 different counties suggest this same terrain could provide an important source for diamonds (Hausel, 1998).

Gold was initially found by Spanish prospectors along the Colorado River between 1775 and 1780, but it was the later discovery of the precious metal at Sutter’s Mill in 1848 that produced one of the most important events in the history of the western US and led to a major gold rush that enveloped California with thousands of prospectors and merchants swarming to the Californian gold fields and settling large regions of the West. Gold was soon found in many locations that included Grass Valley, Jackson-Plymoth, Hammonton, Folsom, Columbia, La Porte, Oroville, Nevada City, Alleghany, French Gulch, Bodie, Sierra City, Angels Camp, Jamestown, Placerville, Carson Hill, Magalia, Big Oak Flat, Forest Hill, Mojave, Iowa Hill, Rand, Soulsbyville, Snelling and Polker Flat. It soon became evident that a major gold belt unparalleled in North America occurred in the north-central portion of the state east of Sacramento and west of the State’s elbow.

Alpha and Omega historical hydraulic gold mines, California
This belt, known as the Mother Lode, was outlined by gold discoveries in quartz veins in a region about 4 miles wide and 170 miles long that reached north from the Sixteen-to-One mine at Alleghany to Mormon Bar in the south. The precious metal was found in quartz veins in phyllite, schist, slate and greenstone. The more productive veins were discovered along at contacts between two different rock types. Where extensive erosion occurred, major placers formed downstream in Holocene and Tertiary gravels. Broad zones of mineralization found in weathered lodes that were hydraulically mined.

The Mother Lode was legendary for its incredibly rich pockets of gold. The richest contained crystalline gold within the Alleghany-Downieville region 75 miles northeast of Sacramento. In the 1920s, a pocket intersected at the Sixteen-to-One yielded nearly 95,000 ounces of gold! At today’s gold price, this one pocket would be worth $105 million! Another pocket found about the same time produced >45,000 ounces. Other mines in the Mother Lode belt noted for rich pockets included the Oriental, Alhambra, Four Hills, Keltz, Bonanza, Kate Hardy, Carson Hill, North Fork, Kenton, Angels, Green Emigrant, Finnegan, St. Patrick and Plumbago. But those of the Allegany district were especially rich (Clark, 1970).

Some pockets showed enrichment of iron, others did not. Many of the rich pockets were small but found at or adjacent to serpentinite-slate contacts. In some, quartz-mariposite (a green mica) was found. Near serpentinites, many of the quartz veins split or were bent. Such structures produced gold pockets in some cases. Other high grade pockets were found at vein intersections or in shears.

Another variety of pocket veins were termed ‘Seam Diggins’ that were essentially a form of stockworks. These were found at Placerville, nearly 50 miles to the south of the sixteen-to-one mine and close to Sutter’s Mill. Most were mined by hydraulic methods because of the broad zones of gold mineralization with numerous crisscrossing quartz veinlets. Not only did the veinlets contain precious metal, locally some fractures contained gold. Possibly the largest seam was mined at Georgia Slide north of Georgetown (8 to 10 miles north of Sutter’s mill). The seam in amphibolite, was nearly one mile long and as much as 400 feet wide (Ralph, 2010). Today, such deposits would be likely mined by open pit. Many of the veins produced rich pockets cut by iron-rich fault offsets.

Sample of milky quartz with visible gold containing considerable green mariposite.

By 2007, environmentalism and government had taken its toll on this rich gold state. California only produced 9,400 ounces of gold from two mines: the Mesquite and Briggs. Aerial examination of the giant gold placers suggest blocks of placer ground were missed along the American and Yuba rivers. In addition, because of technology, gold placers could only be mined to limited depths, thus unmined placers likely underlie several mined placers. And all lode gold deposits have not been found and additional exploration would result in more discoveries. Then there is the question of diamonds and sapphires found in the placers. The geology of California appears to be very favorable for Benioff and obducted related diamond-rich slabs and intrusive breccias. And while the author was exploring the Poker Flat region for Western Archon in 2003, several excellent sapphire and benitoite specimens were discovered and while searching for serpentinites in the Trinity River area, several chromian diopsides were discovered by the author in serpentinite belts in that region.

Grass Valley
Grass Valley was the most productive gold district in California. Total output from lode mines was estimated at $300 million (historical gold prices or 30 to 52 times lower than today’s gold price). Placer mines yielded a few million dollars. The two largest operations, the Empire-Star and Idaho-Maryland had outputs of 5.8 million ounces and 2.4 million ounces, respectively. The district lies within the Sierra Nevada 50 miles northeast of Sacramento. The Eureka and North Star Mines were consolidated into one of the largest gold mines in the world. The shafts were sunk on a 50 to 60 degree incline reaching a vertical depth of more than a mile with >11,007 feet of incline length. More than 367 miles of tunnels were dug in the development of the mine (Kelly, 1997). The mine operated from 1850 to 1957 and according to the historical park, only 20% of the gold was recovered! The Idaho-Maryland mine was the second largest underground mine in California and operated from 1861 until 1957. Both mines closed due to labor problems, increasing mining costs and due to a fixed price at Due to recent gold price increases, this property is in the process of being permitted for a 2,400 ton per day gold mill and 1,200 ton per day ceramics operation.

Right - A view down the incline of the Empire shaft

The area is underlain by an elongated 5-mile long (north-south), 0.5- to 2-mile-wide granodiorite that intrudes greenstones, metadiabase porphyry, amphibolite, serpentinite, gabbro, diorite and slate. To the north and southwest of the granodiorite , metasediments of the Calaveras Formation (Carboniferous to Permian) crop out. A number of intermediate to basic dikes are present along with aplite and granite porphyry dikes. Overlying part of the district to the east and northwest are Tertiary gravels, which are overlain by andesite.

This district includes many veins in a relatively small region. Two vein sets have been recognized: (1) those with gentle dip hosted by granodiorite-greenstone and (2) those with steep dips that cut serpentine-amphibolite. The veins range from one to 10 feet thick and consist of several generations of quartz with calcite and ankerite controlled by thrust faults. Numerous northeast-striking, vertical or steeply-dipping fractures are found that form boundaries of ore shoots. The ore contains free gold with varying amounts of pyrite and lesser galena, chalcopyrite, arsenopyrite, sphalerite and pyrrhotite.
The ore shoots vary in size and shape, and the distribution of gold within the shoots is erratic. Some have pitch lengths of several thousand feet with inclined depths as much as 1,000 feet. Much specimen ore was found, but milling ore averaged 0.25 to 0.5 opt Au (ounce per ton of gold). The erosion of auriferous veins from this district carried the precious metal in the Yuba River producing rich placers near Yuba City.

The ore shoots vary in size and shape, and the distribution of gold within the shoots is erratic. Some have pitch lengths up to several thousand feet and have been developed to inclined depths as much as 1,000 feet. Much specimen ore has been found, but milling ore usually averaged 0.25 to 0.5 opt Au (ounce per ton of gold). The erosion of the auriferous veins from this district carried the precious metal in the Yuba River producing rich placers near Yuba City.

Serpentinized breccia upstream from placer diamonds, sapphires and benitoite
at undisclosed locality in California.
Placerville lies 30 to 40 miles east of Sacramento in the Sierra Nevada. The district also includes placer deposits at Smith Flat, Diamond Springs, Texas Hill, Coon Hollow, White Rock and Placerville. Gold was discovered in Placerville in 1848. From the 1850s through the 1870s deposits were also mined by hydraulic and drift mining. One 20-acre claim at Coon Hollow yielded $5 million and the Spanish Hill area yielded $6 million (historical prices).

The district is underlain by a northerly-trending belt of gray to black slate of the Mariposa Formation (Upper Jurassic) that is one to two miles wide. The western portion of the district is underlain by greenstone and amphibolite and the eastern portion by schist and slate of the Calaveras Formation (Carboniferous to Permian) which is intruded by granodiorite. The South Fork of the American River and its tributaries enter Placerville from the east. In places, Tertiary gravels are overlain by thick rhyolite tuff and andesite flows.

Serpentinite with chromian diopside upstream from diamond deposits.
Of the numerous tributaries of the main Tertiary channel in this district, one of the richest was the Dee Blue Lead. This channel extends south from White Rock to Smith's Flat and then west-southwest through the Texas Hill area. Lode-gold deposits appear as massive quartz veins as much as 20 feet thick with numerous parallel stringers. The ore bodies are low to moderate grade and the veins were mined to depths of 2000 feet. The ore contains finely disseminated gold and small amounts of pyrite and occur chiefly in slate.

The Jackson-Plymoth district lies 35 miles east of Sacramento and 20 miles south of Placerville and forms a 20-mile-long belt enclosing the towns of Jackson, Sutter Creek, Amador City, Drytown and Plymouth. Most of the important lode deposits were discovered during the 1850s and includes the Original Amador, Keystone, Central Eureka and Kennedy mines. From the 1890s until 1942, this belt was one of the more important gold-mining districts in the nation with production that ranged from $2 to $4 million annually (historical gold prices).

The principal lode mines in the district progressed to great depths until costs became prohibited due to decreasing ore grades, increased requirements for timbering, dewatering, increased costs of ore haulage to the surface, and low gold prices. The veins were developed to depths of nearly a mile or more. The Argonaut mine reached a vertical depth of 5,570 feet and the Kennedy mine reached a vertical depth of 5,912 feet (the Homestake later surpassed these reaching depths >8000 feet). The Argonaut produced >$25 million in gold (historical gold prices) recovering 720,000 to 1.1 million ounces and the Kennedy mine produced $34.3 million in gold (historical gold prices) recovering 1.0 to 1.5 million ounces. The Central Eureka reached a depth of 4,965 feet and produced 1.8 million ounces. Other deep mines included the South Eureka and Plymouth Consolidated mines that reached >4000 feet. All of the mines were shut down in World War II. The Central Eureka reopened in 1945, but because of increased costs, closed in 1953. This was the last active major gold mine on the Mother Lode belt after a total output estimated at $180 million (historical gold prices). At today’s gold prices of 30 to 50 times that of the historical prices, it is likely that one of more of these could again be economic with government support and incentives.

The gold deposits in this district occur in a north- and northwest-trending belt of gray to black Mariposa Formation (Upper Jurassic) slate that is 1 mile wide with interbedded coarse and sheared conglomerate. To the west, massive greenstones of the Logtown Ridge Formation (Upper Jurassic) crop out. To the east are graphitic schist, metachert and amphibolite schist of the Calaveras Formation (Carboniferous to Permian). Several Tertiary auriferous channel gravels are exposed south of Jackson. The lodes occur in massive and sheared quartz veins with abundant fault gouge. The veins are primarily in Mariposa Formation slate and are sometimes tens of feet thick; in places the Keystone vein is as much as 200 feet thick. The ore bodies contain disseminated gold, pyrite, and minor amounts of other sulfides. Greenstones with disseminated auriferous pyrite occur adjacent to some quartz veins at depth. The ore averaged 0.14 to 0.3 opt Au.

Gold mine in California. Note the common limonite stains on the mine ribs
and back.

Hammonton (Yuba River)
The Hammonton district lies along lower Yuba River downstream from Grass Valley and from the famous 16 to 1 gold mine. The placers are 5 to 10 miles east of Marysville and Yuba City. This was a major placer district and was dredged for about 8 miles (portions of the placer were not mined). The river and streams were initially worked by small-scale placer methods and dredging began in 1903. The Yuba No. 20 dredge was one of the largest gold dredges in the world at the time of operation. The river was dredged almost continuously from 1903 to 1968 with total output estimated at 4.8 million ounces (about $5.2 billion at today’s price).

In 1968, the last dredge shut down after mining >1 billion yds3 of gravel. The extensive gravel pipes have become increasingly important sources of aggregate and likely contain considerable discarded gold, typical of dredge tailings. The gold-bearing gravels ranged from 60 to 80 feet deep on the upper end to 100 to 125 feet in the vicinity of Hammonton. The eastern part of the field is underlain by metamorphic rocks, while the central and western portions have gravels underlain by clay. Minor of platinum was recovered. Estimates suggest that 235 million yds3 of unmined gravel remains in the field.

Folsom (American River) district
Folsom lies 5 to 10 miles east of Sacramento and is a placer district that was dredged downstream from the town of Folsom for approximately 10 miles. The area was originally settled in 1849 and first known as Negro Bar. Bucket-line dredging began at Folsom in 1898. In 1916, 11 dredges yielded more than $2 million in gold. Dredging operations terminated at the outbreak of war, but resumed on a major scale after World War II. However, increased costs, the depletion of placer ground, and changing land values resulted in curtailed dredging operations. By 1960 there was only one active dredge, and this shut down in 1962. Folsom, one of the largest dredge fields in California had a total output estimated at $125 million (historical gold prices).

Recent stream gravels lie in and adjacent to the American River. To the south, sand and gravel deposits of the Victor Formation (Pleistocene) and silt, sand and gravel of the Laguna Formation (Plio-Pleistocene) are underlain by andesite of the Mehrten Formation (Pliocene). The paying gravels are in or along the American River and near the lower contacts of the Laguna and Mehrten Formations. Digging depths ranged from 30 to 110 feet. Along with gold, minor amounts of platinum were recovered. A few narrow gold-quartz veins are found in greenstone east of Folsom.

The Columbia district is located 60 miles southeast of Sacramento in the foothills of the Sierra Nevada. This was a placer district and includes Yankee Hill, Sawmill Flat, Squabbletown, Brown's Flat and Springfield areas. The Sonora district is just to the south and the American Camp district to the northeast.

Columbia was one of the richest and most famous placer-mining districts in California. Early in 1850 a group of Mexican miners struck it rich. During the 1850s and early 1860s, the diggings were enormously productive but the district declined in the late 1860s, but small-scale mining continued. Total production was estimated to be at least $87 million - some estimates suggest as high as $150 million (historical gold prices).

Columbia lies in a Tertiary valley, a flat valley underlain by crystalline limestone and dolomite of the Calaveras Formation (Carboniferous to Permian). The limestone has numerous potholes and cavities, which contained enormously rich gravel. Several large nuggets were found including one that weighed >50 pounds and several weighing >300 ounces. Slow degradation of the area. Gravels were hoisted from the potholes and washed in sluices and long toms on raised platforms.

The LaPorte district lies 25 miles south of Quincy and 50 miles northeast of Oroville. It was one of the great placer-mining districts where streams were mined early in the gold rush. Hydraulic mining began in the middle 1850s and continued through the 1880s. During this time the district was enormously productive; the output from 1855 to 1871 alone was $60 million.

Quartz vein in mine back.
The main Tertiary channel of the North Fork of the Yuba River, known as the La Porte channel, extended south-southwest from Gibsonville. The channel continued southwest and south and was joined by a branch from the east from the St. Louis-Table Rock area. The main channel continues south to the Poverty Hill and Brandy City districts. At La Porte, the channel is 500 to 1500 feet wide and as much as 500 feet thick. The lower gravels are quartz-rich and up to 80 feet thick. Most of the gold was recovered near bedrock. The gravels are capped by thick beds of sand and clay. During the heyday of mining, these lower gravels yielded 0.1 to 1.0 oz/yd3 gold. To the east, channel deposits are capped by 800 feet of andesite. Considerable faulting disturbed channel gravels. Bedrock is amphibolite, with a belt of slate and quartzite of the Calaveras Formation (Carboniferous to Permian). Some narrow gold-quartz veins were found in the district.

Oroville was a placer field extending from west of Oroville to the southwest along the Feather River to 5 miles east of Biggs. The field is one to two miles wide and nine miles long. The area was settled in 1849: and bucket-line dredging began in 1898. The field was highly productive from 1903 to 1916; in 1908 there were 35 dredges active in the district. Output declined, but dredging continued from 1936 to 1942 and 1945 to 1952. The total Output from dredging is estimated to be about 1,964,000 ounces of gold ($2.2 billion at 2010 gold price).

Gold occurs in river and adjacent terrace gravels on a flood plain. The gravels rest on andesite and rhyolite tuff. Coarse boulders, which become smaller downstream, are present with alternating sand layers. Digging depths ranged from 25 feet upstream to 55 feet downstream. The gold was fine. Minor amounts of platinum also were recovered along with diamonds.

In fact, on page 77 and 78 of his book on "Diamonds in the US", Hausel (1998) describes the situation for finding diamonds at Oroville:

Based on the geology and past discovery history, it is likely more diamonds will be found in California, and in particular, in the Oroville area. With any increased rains, erosion and flooding, the Oroville (as well as other regions of California) will likely release more diamonds from unknown source areas. But in addition to diamond, other gemstones will be washed out during flash flooding, thus gold prospectors need to learn to recognize some of these - such as jade, pyrope garnet, chromian diopside, sapphire and benitoite. Two books useful for assisting in the identification of diamond include Hausel (2011) and Hausel (2009). Although these two books focus on Wyoming, they equally apply to gemstones found anywhere in the world.

Nevada City
The Nevada City district is located 40 to 45 miles northeast of Sacramento and covers an extensive area from Indian Flat on the west to Canada and Banner Hills on the east. Both lode- and placer deposits were mined in the past. Gold was first found in Deer Creek in 1849. The placers were rich and some hydraulic mining occurred at American Hill. Drift mining began in the 1850s, and these mines were continuously active until 1900. Gold in quartz was discovered in 1850 when the Gold Tunnel vein was found. However, important production of lode gold did not commence until the early 1860s. By 1865, the output from lode mines averaged $500,000/year (historical gold prices). The Champion and Providence mines were the major producers during these years. Large-scale lode-gold mining resumed at the Lava Cap and Banner mines. Total output is estimated at more than $50 million and may have exceeded $70 million.

Nice looking potential host rock for gold - note all of the limonite and milky quartz. Site of a channel sample dug in the mine rib for assay.
The district is underlain by granodiorite with slate, mica schist and quartzite, most of which are part of the Calaveras Formation (Carboniferous to Permian). To the west and southwest are fairly extensive beds of massive greenstone, amphibolite and serpentinite. There are a number of fine- to medium-grained dioritic and aplitic dikes, some of which are associated with the gold-quartz, veins. In places these rocks are overlain by Tertiary channel gravels capped by rhyolite and andesite.

Several major gold-quartz vein systems traverse the district. In the western portion, one system extends northwest along a granodiorite-metasedimentary contact. In the southern and eastern portion of the district the veins strike nearly west and dip steeply. There are also a few northeast-striking and southeast-dipping veins. The veins usually are one to four feet thick, but in places a few are as much as 15 feet thick. The ore contains varying amounts of free gold, often abundant pyrite and smaller amounts of other sulfides. Some of the ore bodies are extensive; the ore body at the Providence mine persisted to an inclined depth of more than 2700 feet. Considerable high-grade ore was recovered.

Several important Tertiary channels were sources of ore-bearing gravels. One, the Harmony channel, which enters the district from the northeast, was extensively mined. The pay gravel in this channel was 150 to 200 feet wide, two to four feet deep, quartzitic, often containing sub-angular clasts that were well-cemented. These pay streaks yielded $1.55 to $2.50 in gold to the ton (historical gold prices). Northwest of town is the northwest-trending Cement Hill channel. In the southern part of the district is the Town Talk channel, which was narrow but rich in places. Much of the placer gold taken from the channels was coarse.

Cherokee lies 12 miles north of Oroville on the north side of Table Mountain in the vicinity of the town of Cherokee or Cherokee Flat. It was also known as the Spring Valley district. Most of the output was from a single hydraulic mine, that yielded about $15 million in gold.

The Tertiary placer deposits are associated with a west-trending channel about 700 feet wide. The sequence from bottom to top of the hydraulic pit is as follows: irregular greenstone gravel 5-10 feet thick that is lean in gold with local clay streaks and minor basalt blocks; a rich 20- to 30-foot layer of coarse fresh blue gravel with greenstone blocks, coarse and fine gold, small diamonds and minor platinum; several feet of decomposed gravel; 50 feet of sand and quartzitic gravel; 200 feet of clayey sand; and 50 to 75 feet of massive basalt. Between 400 and 500 diamonds were recovered from the gold-bearing gravels (Hausel, 1998). Several stones weighed >two carats and were of good quality, but most were small and had a pale-yellow tinge.

Poker Flat
Poker Flat was one of the least important gold districts at the northern edge of the Mother Lode belt. Even so, the area was mined by hydraulic methods in the 1800s. The district is underlain by amphibolite and serpentinite including slates of the Blue Canyon Formation. Portions of the area are overlain by andesite. Possibly the most interesting feature of this district was discovery of sapphire and benitoite in the streams at Poker Flat that are assumed to have eroded from the serpentinite belt. During exploration of this area in 2003, diamond indicator minerals were recovered from a serpentinite breccia. Several other gold districts are found in California.

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This blog is excepted from: Gold - Geology, Prospecting & Exploration by W. Dan Hausel, scheduled for publication in 2010. For more information, contact the GemHunter.