Ophir Mining District (Auburn Mining District; Duncan Hill Mining District), Placer Co., California, USAi

Regional Level Types
Ophir Mining District (Auburn Mining District; Duncan Hill Mining District)	Mining District
Placer Co.	County
California	State
USA	Country

Location: A Au-Ag-Pb-Cu mining area located in sec. 18, T12N, R8E, MDM. The center of this district is about two miles west of the old town area of Auburn. Discovered in 1851. 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.

This lode-gold district is located in southwestern Placer County, in the vicinity of the old town of Ophir and 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. It extends east to Auburn and west to Gold Hill and also includes what was known as the Duncan Hill Mining District, which was centered about half way between the towns of Ophir and Auburn. It adjoins the Penryn District on the NE.

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.

History: 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 1860's, 1870's, and 1880's, but mining activity in the district declined after that. The mines were active again from the early 1900's through the 1930's with substantial production, but little has been done since. The value of the total output of the district is estimated at more than $5 million (period values).

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 1860's, 1870's, and 1880's, but mining activity in the district declined after that. The mines were active again from the early 1900's through the 1930's with substantial production, but little has been done since. The value of total output of the district is estimated at more than $5 million (period values).

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 [period values]). 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.

Geology: The mineralized zone is on the NE flank of a granodiorite and quartz-diorite stock that is intrusive into amphibolite schist. A series of west-northwest-striking and south-dipping quartz veins occur in the granitic rocks or along the granitic rock-amphibolite schist contact. A few veins are in the amphibolite. The ore contains free gold in places with
often abundant pyrite, galena, and chalcopyrite; much of the ore is base. Milling ore ranged from ½ to 1 ounce of gold per ton. Considerable high-grade ore was taken close to the surface during the early day's. The veins range from one to five feet in thickness, and several were mined to depths of more than 1,000 feet. The ore shoots had stoping lengths of up to 250 feet.

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.

Geologic structures: Regional structure include the Bear Mountains Fault Zone.

Mineralogy: Mineralization is Cretaceous lode Au veins in amphibolite and granitic rocks. Ore bodies are generally tabular. Mineral occurrence model information: Model code: 273; USGS model code: 36a; Deposit model name: Low-sulfide Au-quartz vein; Mark3 model number: 27. Host rocks include the Early Cretaceous (Chronological age: 139) quartz diorite of the Penryn pluton; the Late Jurassic amphibolite of the Copper Hill Volcanics; and Cretaceous diorite dikes. Controls for ore emplacement included quartz veins filling fractures developed in quartz diorite and metamorphosed mafic to intermediate volcanic rock. Lindgren (1892) observed that where the quartz veins intersected zones of metavolcanic rock enriched in pyrite and copper sulfides (termed "iron belts"), the veins were more enriched in gold. Also, some localities where veins intersected were the sites of pockets or rich ore shoots. Local alteration includes carbonatization, calcite, sericite, pyrite, minor chlorite formed as selvages a few inches to a few feet wide in wallrock immediately adjacent to quartz veins. Local rocks include Mesozoic granitic rocks, unit 3 (Sierra Nevada, Death Valley area, Northern Mojave Desert and Transverse Ranges)

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).

Commodity Information: 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 to .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. Ore Materials: Native gold (mainly as electrum), auriferous and argentiferous sulfides (pyrite, chalcopyrite, galena, sphalerite, arsenopyrite, tetrahedite, molybdenite), native silver, stibnite, tellurides, selenides. Gangue Materials: Quartz, calcite, chalcedony.

Workings Information: 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.

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

Mines: Belmont, Black Ledge, Centenial ($150,000+), Conrad ($50,000), Crater ($750,000), Doig, Eclipse ($100,000+), Gold Blossom ($216,000+), Grass Ravine, Green ($150,000+), Green Emigrant ($150,000+), Hathaway ($336,000), Julian, Mina Rica ($55,000), Moore ($180,000), Oro Fino ($500,000+), Pine Tree, Rock Creek ($200,000 ?), St. Lawrence, St. Patrick ($148,000+), Three Stars ($415,000).

References

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Link to USGS MRDS:	10310713

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