Fremont - Gover Mine

Site Name

Primary: Fremont - Gover Mine
Secondary: Fremont
Secondary: Gover
Secondary: North Gover
Secondary: Loyal Lode

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Amador
District: Jackson - Plymouth

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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: Amador County Planning dept.

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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: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein, replacement
Operation Type: Underground
Discovery Year: 1867
Years of Production:
Organization:
Significant: Y

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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, pinch and swell

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Structure

Type: R
Description: Bear Mountains Fault zone, Melones Fault zone

Type: L
Description: Melones Fault zone

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Alterations

Alteration Type: L
Alteration Text: Hydrothermally alteration of greenstone and schist wallrocks yileding enough disseminated auriferous pyrite iand arsenopyrite to constitute low-grade ore.

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Rocks

Name: Schist
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

Name: Greenstone
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

Name: Slate
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Arsenopyrite
Ore: Pyrite
Ore: Arsenopyrite
Gangue: Quartz
Gangue: Slate

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Comments

Comment (Geology): Ore Genesis Several mechanisms have been suggested as the source of the Mother Lode gold deposits. The most widespread belief is that plutonic activity magmatically differentiated vein constituents or provided the heat to circulate meteoric fluids or to metamorphose the country rocks to liberate the vein constituents. Knopf (1929) proposed that carbon dioxide, sulfur, arsenic, gold, and other constituents were emitted from a crystallizing magma but the components were carried by meteoric water in a circulation system driven by plutonic heat. Most theories suggest that gold deposits formed at temperatures of 300 to 350 degrees centigrade with a possible magmatic or metamorphic origin. Zimmerman (1983) proposed that the Mother Lode veins were generated by and localized near a major late Nevadan shear zone, the mechanism of ore genesis being the shearing and redistribution of mass within a major fault zone. He suggested that the early reverse faults had strike slip component, which is evident in the correlation of expected strike-slip dilatant zones with the geometries and steeply raking attitudes of the ore shoots. Fault movement and shearing would cause recrystallization of the rocks within the fault zone, releasing the more mobile elements including gold and most of the other vein constituents. Moreover, the heat generated by shearing would contribute to the metamorphism of the rocks in the fault zone and cause fluid circulation in the fault zone. Mineral laden auriferous fluids generated by this shearing channeled into the fault fracture system into dilatant zones, which represented avenues of increased flow and lower strain. LOCAL GEOLOGY The Fremont-Gover mine developed two veins, the Mother Lode vein (or contact vein) and the East vein (or gray ore fissure), as well as large bodies of mineralized greenstone and schist (gray ore). The Mother Lode and the East veins are separated by a zone of slate that narrowed to only a gouge in places, but thickened to as much as 90 feet in places. The Mother Lode vein occupies a fault fissure striking N 20?- 25? W and dipping 45?- 50? east. The fault displays a measurable offset of 375 feet. From the surface to the 170 feet, the vein is in slate while from that depth to the 600-foot level, the vein has a greenstone hanging wall and a slate footwall. Below the 600-foot level, the vein is in greenstone. The vein is generally narrow in the slates, but widens with depth, 50 to 60 feet being commonly reached (Knopf, 1929). The contact vein next to the greenstone produced the best ore. Gold was produced from pockets in quartz stringers extending into greenstone. Auriferous pyrite and arsenopyrite were common. The principal Mother Lode vein ore shoot was about 300 feet long by 20 feet wide and pinched out at a depth of 800 feet. While the Mother Lode vein continued at depth, it was of too low a grade to extract

Comment (Geology): The East vein or gray ore fissure vein occupies a facture that strikes approximately N 10? W and dips 55? E. The vein is composed of slate and quartz exhibiting much auxiliary fissuring and shattering. Above the 900-foot level, it is a well-defined quartz vein, generally narrow and containing a foot or so of gouge. In some places, however, it expands to large bodies of coarse white quartz up to 6 feet thick that were stoped. The largest ore shoot was 280 feet long. Below the 900-foot level and extending to the 2300-foot level, the East vein entered hanging wall greenstone. As the vein entered greenstone, the nature of the ore changed to a body of mineralized schist and greenstone gray ore in the enclosing wall rocks. This ore was composed to hydrothermally altered schist and greenstone carrying 3% to 5% disseminated auriferous sulfides, mostly pyrite and arsenopyrite, which carried three fourths to five sixths of the gold. The largest gray ore shoot was nearly 300 feet long, extending to the 2300-foot level. It averaged 30 feet wide, but thickened to as much as 70 feet (Logan, 1927). The ore shoot consisted of a large mass of auriferous greenstone and schist occupying the wedge between the Mother Lode vein and the East vein at their junction. Contained sulfides were chiefly pyrite and arsenopyrite with trace amounts of galena and sphalerite. Several other lenses of gray ore were mined between the 800 and 2500-foot levels. These varied in width, their wider sections being 10-25 feet thick, with stope lengths up to 100 feet (Logan, 1934). The gray ore bodies yielded the bulk of the output from the Fremont-Gover Mine. Gray ore values varied from a trace to $20 a ton, but in general were quite low. (Knopf, 1929).

Comment (Identification): The Freemont-Gover Mine is located one and one-half miles north of Amador City in the famous Mother Lode Gold Belt of the Sierra Nevada foothills. The Fremont and Gover mines operated independently until they were merged in 1872. The mine is part of the Jackson - Plymouth district, which was the most productive district of the Mother Lode belt with an estimated total production of about $180 million (Clark, 1970). The Fremont-Gover Mine is credited with producing about $5.0 million. (Clark, 1970)

Comment (Location): Location selected for latitude and longitude is the Fremont Mine shaft symbol on the USGS 7.5 minute Amador City quadrangle.

Comment (Geology): Mineralization is characterized by steeply dipping massive gold-bearing tabular quartz veins striking north to northwest and dipping between 50 to 80? east. Veins are discontinuous along both strike and dip, with maximum observed unbroken dimensions of 6,500 feet in either direction (Zimmerman, 1983), but individual veins more commonly range from structures 3,000 feet long and 10 to 50 feet wide to tiny veinlets. In rare instances, veins are known to reach as much as 200 feet thick (Keystone Vein). Veins may be parallel, linked, convergent, or en echelon, and commonly pinch and swell. Few can be traced more than a few thousand feet. At their terminations, veins pass into stringer zones composed of numerous thin quartz veinlets or into gouge filled fissures (Knopf, 1929). Ores consist of hydrothermally deposited minerals and altered wall-rock inclusions. Gold occurs as free gold in quartz and as auriferous pyrite and arsenopyrite. Quartz is the dominant mineral component in the veins, comprising 80-90% or more with ankerite, arsenopyrite, pyrite, albite, calcite, dolomite, sericite, apatite, chlorite, sphalerite, galena, and chalcopyrite in lesser amounts of a few percent or less. Cumulative sulfides generally range 1% - 3% of the rock (Carlson and Clark, 1954; Zimmerman, 1983). Ore grade material is not evenly distributed throughout the veins, but was localized in ore shoots, which tend to occur at vein intersections, at intersections of veins and shear zones, or at points where the veins abruptly change strike or dip (Moore, 1968). Ore shoots generally display pipe-like geometries raking steeply in the veins at 60-90%. Horizontal dimensions of the ore shoots are commonly 200-500 feet, but pitch lengths were often much greater, and often nearly vertical. Pockets of high grade ore are relatively abundant. Single masses of gold containing over 2,000 ounces and single pockets containing more than 20,000 ounces have been found. Silver is subordinate. Gold fineness averages 800. While most of the Mother Lode ore shoots mined have been less than 300 feet in strike length, many have extended down dip for many thousands of feet. In the deeper mines, mining continued to almost 6,000 feet on the dip of the vein with no evidence of bottoming. Cessation of operations in the deep Kennedy (5912') and Argonaut (5570') mines was caused by increasing costs at the greater depths rather than an absence of ore. Milling ore was generally low to moderate in grade (1/7 to 1/3 ounce per ton). Alteration 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 usually adjacent to the veins in ground that has been fractured and contains small stringers and lenses of quartz.. Locally, greenstone bodies (altered volcanic rocks) adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute what has been called "gray ore". Altered slate wallrock 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 which 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 (Moore, 1968).

Comment (Deposit): The Freemont-Gover Mine produced from Mother Lode type mesothermal gold-quartz veins and gray ores consisting of hydrothermally altered and mineralized schist and greenstone wall rocks. Two principal veins, the Mother Lode and East veins consisted of fault and fracture filling quartz which contained largely free milling quartz ore bodies in the shallower workings to a depth of about 900 feet. While the Mother Lode vein became very low grade or barren below this depth, the East vein passed into greenstone where hydrothermal alteration and mineralization of the enclosing greenstone and schist wall rocks formed large bodies of low grade gray ore which provided the bilk of the mine's production.

Comment (Geology): REGIONAL GEOLOGY The Fremont-Gover Mine is located within the Sierra Nevada foothills, where bedrock consists of north trending tectonostratigraphic belts of metamorphosed sedimentary, volcanic, and intrusive rocks that range in age from late Paleozoic to Mesozoic. Locally, the Mesozoic rocks are capped by erosional remnants of Eocene auriferous gravels and once extensive volcanic rocks of Tertiary age. 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. In Amador County, the structural belts are internally bounded by the Melones and Bear Mountains fault zones. Schweickert and others (1999) provide one interpretive overview of the regional geology of this part of the Sierra Nevada. Gold deposits in the Plymouth - Jackson district occur within the north and northwest trending mile-wide Mother Lode Belt, which is dominated by gray to black slate of the Upper Jurassic Mariposa Formation and associated greenstone and amphibolite schist bodies assigned to its Brower Creek Volcanics member. In Amador County, the Mother Lode Belt approximately parallels Highway 49 southeastward from Plymouth through the town of Jackson. The geology of this segment has been mapped by Zimmerman (1983) and Duffield and Sharp (1975). The lode gold deposits along this stretch are responsible for most of the gold production in the county, which has been reported to be 7.68 million ounces (Koschman and Bergendahl, 1968). Clark (1970) placed the value of this production at $180 million. The Amador County portion of the belt was one of the most productive gold mining areas in the United States, and the Plymouth - Jackson district in Amador County was the most productive part of the belt. The Mariposa Formation contains a distal turbidite, hemipelagic sequence of black slate, amphibolite, schist, and fine-grained tuffaceous rocks, and volcanic intrusive rocks. The thickness of the Mariposa Formation is difficult to ascertain due to structural complexities, but is estimated to be about 2,600 feet thick at the Cosumnes River. Massive greenstone of the Upper Jurassic Logtown Ridge Formation lies west of the Mother Lode Belt. The contact between the Logtown Ridge and Mariposa Formation is generally gradational (Zimmerman, 1983). The Logtown Ridge Formation consists of over 9,000 feet of volcanic and volcanic-sedimentary rocks of island arc affinity. These rocks are mostly basaltic and include flows, breccias, and a variety of layered pyroclastic rocks. Metasedimentary rocks, chiefly graphitic schist, metachert, and amphibolite schist of the Calaveras Complex (Carboniferous to Triassic) are to the east. Mother Lode Gold Quartz Veins Mother Lode-type veins fill voids created within faults and fracture zones. The Mother Lode Belt consists of a vein system ranging from a few hundred feet to a mile or more in width. The vein system consists of a fault zone containing several parallel veins separated by hundreds of feet of highly altered country rock containing small quartz veins and occasional bodies of low-grade ore. Veins are generally enclosed within numerous discontinuous fault fissures within Mariposa Formation slate, associated greenstone, amphibolite schist, or along lithologic contacts. Mineralized fault gouge is abundant.

Comment (Workings): The Fremont-Gover Mine was developed through the Fremont shaft extending 2950 feet on a 51? incline, and the Gover shaft, an inclined shaft 1500 feet in length located 1430 feet north of the Fremont (Knopf, 1929). The 1500-foot level of the Gover connected with the 1350-foot level of the Fremont. Levels were 200 feet apart. Drifting was extensive from both shafts aggregating several miles, the vein having been followed 2500 feet north from the Fremont shaft. Mining was done partly by square setting and partly with stulls, all stopes being filled (Carlson & Clark, 1954). The mill contained 40 stamps. Concentration was on Frue vanners. The mill handled about 200 tons daily and produced about 200 tons a month of concentrate worth $65 to $100 a ton. Recovery was about 85% of assayed values. From 1914 to 1920, the tailings were worked on a royalty basis by the California Slimes Concentrating Company. Tailings were treated by regrinding, concentration, and cyanidation (Carlson & Clark, 1954).

Comment (Commodity): Ore Materials: Free milling quarts with associated auruiferous pyrite and arsenopyrite Hydrothermally altered greenstone and schist wallrock containing disseminated auriferous pyrite and arsenopyrite.

Comment (Commodity): Gangue Materials: Quartz, slate, greenstone

Comment (Development): At its peak, the Fremont-Gover Mine covered 4200 feet along the strike of the Mother Lode, and included the Fremont, Gover, and North Gover claims, Loyal Lode Mine and mill site, Bona Esperanza Quartz Mine and mill site, one-half interest in the Bona Fortuna Quartz Mine and mill site, and several parcels of non-mineral land (Logan, 1934). There is little information about the earliest history of the properties and later histories are sketchy, but they are known to have been operated independently until their later consolidation. The Loyal Lode mine is known to have been in operation prior to 1867 (Carlson & Clark, 1954). Work on the Loyal Lode claim had been confined to mining rich stringers and small high-grade bodies of or at or near the surface. The surface quartz ore was originally worked in arrastras which were replaced by a 20-stamp mill sometime before 1867. By 1876, the mine was idled at a depth of 100 feet. The Fremont and Gover mines were also early producers with Gover Mine having been partially mined to a depth of 1500 feet (1050 feet vertical) when it was shut down after its 40-stamp mill was destroyed by fire. The Fremont and North Gover claims also were largely undeveloped. In 1872, the Fremont and Gover Company was formed and consolidated these two properties. During the 1880s and 1890s, work focused primarily on the Gover claim (Logan, 1934). The only figures available indicate an annual output of from $50,000 to $70,000 in the 1880's and early 1890's with the last reported production in 1894. In 1888, ore yielded $9-$13 a ton (Logan, 1934). The Loyal Lode Mine was reopened in the 1880s and in 1886 it was producing ore that yielded $7-$8 a ton in free gold. In 1888, two ore shoots were reported, one 200 feet long and another 80 feet long. Fourteen men were working and the stamp mill was crushing 15 tons a day (Logan, 1934). The 200-foot ore shoot had been opened its full length and was reportedly 20 feet wide. The work was done through 3 adits, the deepest 300 feet long and 120 feet deep at the face. There are no later records, and no details of the output (Logan, 1934). The Fremont shaft was started in 1900 and in 1903, a new 40-stamp mill was erected. The upper levels from 400 to 700 feet deep, developed the Mother Lode contact vein. From the 800-foot level downward, the gray ore associated with the East vein was mined. Fremont Consolidated Mining Company worked to a depth of 2950 feet on a 51? incline. The mine was closed in December, 1918 due to excessive costs and a labor scarcity brought on by World War I. Late in 1920, the Metals Exploration Company began unwatering the mine and prospected it from 1920 - 1923, producing some ore. In the latter year, 20 stamps were put in operation but work was stopped in August. A new Company, Fremont-Gover Mines Company, was incoprporated at once by former employees, but they were unable to finance work for long, and quit in 1925 after only a small amount of work. Shortly after, Black Hills Fremont Mines Company did a little work in a shallow adit and a winze 100 feet deep on the Gover claim 600 feet north of Gover shaft. The venture was short lived (Logan, 1934). Between 1900 and 1925, there was a reported output of about 930,000 tons which is said to have averaged about $4 a ton recovered. Shortly after, Black Hills, Fremont Mines Company did a little work in an adit and in a 100-foot winze (Carlson & Clark, 1954). Early in 1937, the property was reopened by Amador Mother Lode Mining Company (Carlson & Clark, 1954). In September 1937, the property was purchased by the Fremont Gover Company who operated it until April 1940. The Gover shaft was reopened to the 865 level and the Fremont shaft to the 770 level. Several thousand feet of new drifts, crosscuts and raises were driven (Carlson & Clark, 1954).

Comment (Development): Total production from the combined properties was about $5 million from which $316,000.00 in dividends are known top have been paid.

Comment (Economic Factors): Clark (1970) reported that the Fremont-Gover Mine produced $5.0 million.

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References

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): Tucker, W.B., 1914, Amador County, Original Amador Mine: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 28-29.

Reference (Deposit): Additional information on the Fremont Mine is available in file no. 322-5926 (CGS Mineral Resources Files, Sacramento).

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): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, p. 52-54.

Reference (Deposit): Logan, C.A., 1921, Amador County, Central Eureka Mine: California State Mining Bureau, 17th Report of the State Mineralogist, p. 411.

Reference (Deposit): Koschman, 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): Logan, C.A., 1927, Amador County, Central Eureka Mine: California State Mining Bureau, 23rd Report of the State Mineralogist, p. 165-166.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 69-76.

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 82-84 .

Reference (Deposit): Carlson, D.W., and Clark, W.H., 1954, Mines and mineral resources of Amador County, California: California Division of Mines and Geology, 50th Report of the State Mineralogist, p. 179-181.

Reference (Deposit): Irelan, W., Jr., 1888, Amador County Gold: California State Mining Bureau, 8th Annual Report of the State Mineralogist, p. 51-57.

Reference (Deposit): Duffield, W.A. and Sharp, R.V., 1975, Geology of the Sierra foothills melange and adjacent areas, Amador County, California: U.S. Geological Survey Professional Paper 827, 30 p.

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