Ophir District

Site Name

Primary: Ophir District
Secondary: numerous individual mines in this district

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Commodity

Primary: Gold
Primary: Silver
Secondary: Lead
Secondary: Copper

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Location

State: California
County: Placer
District: Ophir 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: Placer County Planning Department

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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: Hydrothermal vein
Operation Type: Underground
Discovery Year: 1851
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: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular

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Structure

Type: R
Description: Bear Mountains Fault Zone

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Alterations

Alteration Type: L
Alteration Text: Carbonate; calcite, sericite, pyrite, minor chlorite; formed as selvages a few inches to a few feet wide in wallrock immediately adjacent to quartz veins.

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Rocks

Name: Diorite
Role: Host
Description: dikes
Age Type: Host Rock
Age Young: Cretaceous

Name: Amphibolite
Role: Host
Age Type: Host Rock
Dating Method: K-Ar
Age Young: Late Jurassic

Name: Diorite
Role: Host
Description: Quartz
Age Type: Host Rock
Age in Years: 139.000000+-
Age Young: Early Cretaceous

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Analytical Data

Not available

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Materials

Ore: Silver
Ore: Sphalerite
Ore: Galena
Ore: Chalcopyrite
Ore: Pyrite
Ore: Tetrahedrite
Ore: Arsenopyrite
Ore: Gold
Ore: Electrum
Ore: Gold
Ore: Telluride
Ore: Stibnite
Ore: Molybdenite
Gangue: Chalcedony
Gangue: Calcite
Gangue: Quartz

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Comments

Comment (Geology): REGIONAL GEOLOGY (continued) Mother Lode ores are generally low- to moderate-grade (1/3 ounce of gold or less per ton), but ore bodies can be large. Ore shoots are generally short, 200-300 feet being the average stope length. However, they persist at depth, some having been mined to several thousand feet (Clark and Lydon, 1962). Ore shoots are commonly localized at bulges in veins, shear zones, vein intersections, or near abrupt changes in strike or dip. Wall rocks have invariably been hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite with traces of rutile and leucoxene (Knopf, 1929). The mineralization is generally adjacent to the veins in ground that has been fractured and contains small stringers and lenses of quartz. Locally, greenstone bodies adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute what has been called "gray ore.? Altered slate wall rock commonly contains pyrite, arsenopyrite, quartz, chlorite, and sericite with or without ankerite (Zimmerman, 1983). Large bodies of mineralized schist also form low-grade ore bodies throughout the Mother Lode. This ore consists of amphibolite schist that has been subjected to the same processes of alteration, replacement, and deposition that formed the greenstone gray ores. The altered schist consists mainly of ankerite, sericite, chlorite, quartz, and albite. Gold is associated with the pyrite and other sulfides that are present. Pyrite comprises about 8 percent of the rock. The average grade of mineralized schist is about 0.1 oz per ton. The Melones Fault Zone separates the Mother Lode Belt from the East Belt. The East Belt is dominantly argillite, phyllite and phyllonite, chert, and metavolcanic rocks of Paleozoic-Mesozoic age. Carbonate rocks (marble) are also present locally. The phyllite and phyllonite are dark to silvery gray. The chert is mostly thin-bedded with phyllite partings. The Upper Paleozoic-Lower Mesozoic metasedimentary and metavolcanic rocks of the East Belt have been assigned to the Calaveras Complex by most investigators (Earhart, 1988). The Lower Paleozoic metamorphic rocks farther east have been assigned to the Shoo Fly Complex. More recently, some geologists have reinterpreted certain assemblages along and immediately east of the Melones Fault Zone south of El Dorado County as separate Jurassic units (Schweickert and others, 1999). The metamorphic complexes are intruded in places by Mesozoic plutonic rocks. Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metamorphic rocks of possible Jurassic age, metamorphic rocks of the Calaveras Complex, metamorphic rocks of the Shoo Fly complex, or in granitic rocks. Most of the veins trend northward and dip steeply. An east-west set of intersecting faults may be a controlling factor in controlling deposition of ore. Ore deposits of the East Belt are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. Gold is generally associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite (Clark and Carlson, 1956).

Comment (Geology): LOCAL GEOLOGY The Ophir District is situated at the western edge of the Sierra Nevada metamorphic belt. Here, metamorphosed Jurassic island-arc volcanic rocks have been intruded by quartz diorite of the Lower Cretaceous Penryn Pluton (Olmsted, 1971; Wagner and others, 1981). The mines of the district are developed in a system of quartz veins along and adjacent to the contact between the plutonic rock and metavolcanic rock. Both types of rock serve as hosts for the quartz veins. Younger igneous dikes of dioritic composition intrude the plutonic and metavolcanic rocks and are also associated with the quartz veins in places. In general, the vein system of the district strikes west-northwest, and dips of individual veins are about 40-80SW. There is a lesser set of veins within the central part of the system, which strike northeast and dip 20-60SE. Lindgren (1892) reported that the westerly striking set of veins was more important economically. Thicknesses of veins in the district range from a few inches up to 10 or 20 feet; the average is about one foot. Several veins are traceable for 1.5 miles or more. The veins are present in both the quartz diorite and metavolcanic rocks, and commonly cross the contact. In the quartz diorite, the hanging walls of the quartz veins typically are composed of the diorite dikes mentioned above, although some ore bodies are not associated with dikes. The dikes are seldom over 1-2 feet thick and in some places are altered to amphibolite (Lindgren, 1892). The veins are typically characterized by ribbon structure and contain vugs and cavities locally, which are strong evidence for origination of the veins by filling of open fissures rather than by replacement of wallrock. Undulatory extinction and comminution of quartz in the veins suggest movements along some of the veins. Gangue consists almost entirely of quartz with very minor calcite and chalcedony. Lindgren (1892) reported that the wallrocks of quartz diorite and amphibolite are intensely altered; the zones of alteration are a few inches to a few feet wide and are composed of grayish to white fine-grained rock where formerly metavolcanic rock and grayish green rock where formerly quartz diorite. Alteration minerals in the quartz diorite and metavolcanic wall rock consist of aggregates of carbonate (mostly calcite), sericite, and euhedral pyrite. Pyrite is more abundant in the altered rock than in the veins and unaltered rock. Lindgren (1892) interpreted the alteration to be similar in both types of wallrock and was the result of carbonatization. In some places, the altered rock contains small amounts of gold and silver even though no quartz veins are present. In other places, small veinlets of quartz traverse the rock. Lindgren (1892) observed that the character of the ores in the two main types of host rock are somewhat different. Those in the quartz diorite are low grade, rich in silver, and very extensive. Those in the metavolcanic rock are richer, contain less silver, and are of small extent (commonly present as ?pockets?). Concentration of sulfides was generally considered an indicator of richness of the gold-quartz ore; galena is an especially good indicator. Milling ore contained 1.5 to 3% sulfides. Pyrite is the most common sulfide. Of note near the northwest edge of the district are the orientations of a third set of veins, which were developed at the Three Star Mine and a few other mines. This vein system trends north-northeast rather than west-northwest.

Comment (Location): Location selected for latitude and longitude is the former site of the town of Ophir on the USGS 7.5-minute Auburn quadrangle. The district is oblong shaped and covers several square miles. It trends west-northwest and is about 5 miles long by 2 miles wide. Access is via paved rural roads.

Comment (Commodity): Commodity Info: the district is notable for the large proportion of silver in the ore - most native gold mined was in the form of electrum. Gold seldom exceeded 700 in fineness and was as low as 500-600; the average fineness in California was about 850 (Lindgren, 1892). Regarding sulfides, Lindgren (1892) reported that gold was typically found in the pyrite, while silver was found in the tetrahedrite, galena, and sphalerite. Some specimen gold was produced in this district.

Comment (Development): As reported by Clark (1970): ?Gold-bearing surface gravels were discovered at Ophir and Auburn in 1848 and for several years yielded substantial amounts of the mineral. The quartz veins were then developed, and the Ophir and Duncan Hill districts were organized. Appreciable amounts of high-grade gold ore were recovered during the 1860s, 1870s, and 1880s, but mining activity in the district declined after that. The mines were active again from the early 1900s through the 1930s with substantial production, but little has been done since. The value of total output of the district is estimated at more than $5 million.? Processing of ores from this district in the early days was hindered by the presence of abundant sulfides. A chief period of production was in the 1870?s. From the mid-1930?s to 1942, the district was also very productive, but between 1942 to 1959, less than 100 ounces per year were produced (Koschmann and Bergendahl, 1968). Mining activity subsequent to that time is not known, but is assumed to have been very minor. There are at least 50 known lode mines in the Ophir District. The most productive mines were the Bellevue, Crater Hill, Gold Blossom, Hathaway, Oro Fino (Oro Fina), and Three Star. The Crater Hill may have been the largest producer in the district (estimated by Clark to be $750,000). The Oro Fina was the last-worked important producer in the district as of the 1930?s. Amalgamation and flotation processes were used at mines in this district. Concentrates (sulfide minerals) were typically shipped elsewhere for processing.

Comment (Workings): This district contains numerous small underground mines, which were developed along the extensive system of quartz veins that cut the Penryn Pluton and Jurassic metavolcanic rocks. Workings typically consisted of shafts, adits, drifts, and crosscuts with associated stoping of the ore shoots. The Crater Hill Mine may have the deepest workings in the district; the shaft was at least 800 feet deep in the early 1890?s. Logan (1936) reported, however, that the shaft of the Oro Fina Mine reached a depth of 860 feet on a 72-degree incline during the 1930?s. Workings of most mines were probably limited to less than 500 feet deep. Waring (1915) provided a figure that shows the claims of the district as of the early 20th century.

Comment (Commodity): Ore Materials: Native gold (mainly as electrum), auriferous and argentiferous sulfides (pyrite, chalcopyrite, galena, sphalerite, arsenopyrite, tetrahedite, molybdenite), native silver, stibnite, tellurides, selenides

Comment (Commodity): Gangue Materials: Quartz, calcite, chalcedony

Comment (Geology): REGIONAL GEOLOGY The Ophir District is within the Sierra Nevada Mountains, where bedrock consists of northerly trending tectonostratigraphic belts of metamorphosed sedimentary and volcanic rocks and associated intrusive rocks that range in age from Paleozoic to Mesozoic. 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 western Placer County-El Dorado County region, gold deposits are present in the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced. There has also been substantial gold produced from the West Belt and East Belt (Clark and Carlson, 1956). The West Belt of the Sierra Nevada foothills consists of widely scattered gold deposits located west of the Mother Lode vein system, which represents the Mother Lode Belt. The Ophir District is within this belt. Gold occurs in irregular quartz veins and stringers in schist, slate, granitic rocks, altered mafic rocks, and as gray ore in greenstone. The West Belt is cut by the northwest-trending Bear Mountains Fault Zone, which separates an assemblage of metavolcanic and metasedimentary rocks of Jurassic age on the southwest from a more disrupted and diverse assemblage of metavolcanic, metasedimentary, plutonic, ultramafic, and melange rocks on the northeast. The metavolcanic rocks consist generally of volcanic and volcanic-sedimentary rocks of island-arc affinity. These rocks are mostly mafic to intermediate in composition and include flows, breccias, and a variety of layered pyroclastic rocks. Some silicic rocks are present also. Various formation names assigned to the metavolcanic assemblages include Gopher Ridge, Copper Hill, Logtown Ridge, and Penon Blanco. The metasedimentary rocks are dominantly distal turbidites and hemipelagic sequences of black slate. Assigned formation names include Mariposa, Salt Spring Slate, and Merced Falls Slate. Mesozoic plutonic rocks in this belt are generally of intermediate to mafic composition. The Ophir District is associated with both the Mesozoic plutonic rocks and the Jurassic metavolcanic rocks. The northwest-trending Mother Lode Belt traverses western El Dorado County and is associated with the Melones Fault Zone. 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 as a continuation of the Melones Fault Zone through Georgetown to the Georgia Slide area. The rocks of this belt are typically metavolcanic, metasedimentary, and ultramafic, some of which have been hydrothermally altered to assemblages as described below. Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins and bodies of mineralized country rock adjacent to veins. Mother Lode veins are characteristically enclosed in Mariposa Formation slate with associated greenstone. The Mother Lode belt 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, limonite, talc, chromium-bearing mica, and sericite. Stringer veins are commonly found in both adjacent footwall and hanging walls.

Comment (Identification): This lode-gold district includes many individual mines developed along a west-northwest-trending belt of quartz veins, which formed at the contact between granitic and metavolcanic rocks a few miles west of the town of Auburn. It is the most productive lode-gold district in Placer County. This description also includes what was known as the Duncan Hill Mining District, which was centered about half way between the towns of Ophir and Auburn.

Comment (Deposit): The ore bodies of the Ophir District are developed in gold-bearing quartz veins emplaced in quartz diorite of the Lower Cretaceous Penryn Pluton and in older Jurassic metavolcanic rocks, which are mafic to intermediate composition. In places, dioritic dikes are associated with the quartz veins. The veins are present in three sets. The dominant set trends west-northwest, has dips mainly to the southwest, and was the most important source of gold in the district. A second set, which intersects the central part of the dominant set, trends northeast, with dips to the southeast. The third set is minor and trends northerly along the west edge of the district. The quartz veins are typically characterized by ribbon structure and contain vugs and cavities locally, which are strong evidence for origination of the veins by filling of open fissures rather than by replacement of wallrock. Undulatory extinction and comminution of quartz in the veins suggest movements along some of the veins. Gangue consists almost entirely of quartz with very minor calcite and chalcedony. Ore was typically found in shoots that extended down-dip within the quartz veins. Where the veins intersect zones of metavolcanic rock enriched in pyrite and copper sulfides (termed ?iron belts?), miners observed that the veins were more enriched in gold. Ore consists mainly of native gold (electrum) and sulfides that are auriferous and argentiferous. The ore bodies are noteworthy for their high silver content. Concentration of sulfides was generally considered an indicator of richness of the gold-quartz ore; galena is an especially good indicator. Milling ore typically contained 1.5 to 3% sulfides, with pyrite the most common sulfide. Narrow zones of altered wallrock adjacent to the quartz veins also contain sulfides, some of which carry minor amounts of gold and silver. An extensive summary of the lode-gold deposits of the Ophir District was presented by Lindgren (1892).

Comment (Economic Factors): Clark (1970) estimated production in this district to be in excess of $5 million, which may include both lode and placer sources. Koschmann and Bergendahl (1968) estimated lode production to be about 255,500 ounces.

Comment (Environment): The district is situated among hills that are subdivided by semi-rural residential development. Vegetation consists of a mixture of grass and oak woodland. The topography of the area underlain by the metavolcanic rock is rugged in places, while that underlain by the quartz diorite is more subdued.

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References

Reference (Deposit): Wagner, D.L. and others, 1981, Geologic map of the Sacramento Quadrangle, California: California Department of Conservation, Division of Mines and Geology Regional Geologic Map Series, Map No. 1A, scale 1:250,000.

Reference (Deposit): Waring, C.A., 1915, Placer County: 15th Annual Report of the State Mineralogist, California State Mining Bureau, p. 318-319.

Reference (Deposit): Zimmerman, J.E., 1983, The Geology and structural evolution of a portion of the Mother Lode Belt, Amador County, California: unpublished M.S. thesis, University of Arizona, 138 p.

Reference (Deposit): Lindgren, W., 1892, The gold-silver veins of Ophir, California: U.S. Geological Survey 14th Annual Report, Part 2, p. 243-284.

Reference (Deposit): Lindgren, W., 1894, Sacramento Folio: U.S. Geological Survey Atlas of the U.S., Folio 5, 3 p.

Reference (Deposit): Logan, C.A., 1927, Placer County: 23rd Annual Report of the State Mineralogist, California State Mining Bureau, p. 247-251.

Reference (Deposit): Logan, C.A., 1935, Placer County: 31st Annual Report of the State Mineralogist, California Journal of Mines and Geology, p. 17-18.

Reference (Deposit): Logan, C.A., 1936, Gold mines of Placer County: 32nd Annual Report of the State Mineralogist, California Journal of Mines and Geology, p. 28-31.

Reference (Deposit): Olmsted, F.H., 1971, Pre-Cenozoic geology of the south half of the Auburn 15-minute quadrangle, California: U.S. Geological Survey Bulletin 1341, 30 p.

Reference (Deposit): Schweickert, R.A., Hanson, R.E., and Girty, G.H., 1999, Accretionary tectonics of the Western Sierra Nevada Metamorphic Belt in Wagner, D.L. and Graham, S.A., editors, Geologic field trips in northern California: California Division of Mines and Geology Special Publication 119, p. 33-79.

Reference (Deposit): Irelan, W., Jr.,, 1888, Auburn District: 8th Annual Report of the State Mineralogist, California State Mining Bureau, p. 460-462.

Reference (Deposit): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, 88 p.

Reference (Deposit): Hobson, J.B., 1890, Ophir Mining District: 10th Annual Report of the State Mineralogist, California State Mining Bureau, p. 426-434.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Principal gold-producing districts of the United States: U.S. Geological Survey Professional Paper 610, 283 p.

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. 408.

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 102-103.

Reference (Deposit): Clark. W.B., and Lydon, P.A., 1962, Mines and mineral resources of Calaveras County, California: California Division of Mines and Geology County Report No. 2, p. 40.

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.

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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.