Plymouth Consolidated Mine

Site Name

Primary: Plymouth Consolidated Mine
Secondary: Plymouth Mine
Secondary: Phoenix Mine
Secondary: Empire Mine
Secondary: Pacific Mine

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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
Operation Type: Underground
Discovery Year: 1852
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: L
Description: Melones Fault zone

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

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Alterations

Not available

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Rocks

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: Pyrite
Ore: Arsenopyrite
Ore: Chalcopyrite
Ore: Arsenopyrite
Ore: Chalcopyrite
Gangue: Quartz
Gangue: Slate
Gangue: Calcite

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Comments

Comment (Development): On January 24, 1888 a fire was reported on the 1200-foot level and the mine was flooded to extinguish the blaze. In 1889, the Pacific workings were reopened and produced $215,515 by 1892 when operations ceased. The mine lay idle until 1911 when, with British capital, the California Exploration Company Ltd. was formed to explore the mine. In 1914, after the California Exploration Company had identified $100,000 tons of ore assaying $6.35 a ton, the Plymouth Consolidated Mine Company Ltd was incorporated to reopen and operate the mine. The operation was capitalized with 240,000 shares of one pound sterling, of which 204,482 pounds sterling was paid for the property (Logan, 1934). The Pacific shaft was reopened and a winze was sunk from below the 1600-foot level to 2000 feet. A 30-stamp mill was built and began operating in July, 1914. The adjacent Plymouth Gold Quartz and new London claims were purchased to obviate any danger of apex litigation (Knopf, 1929). Operations continued profitably until 1920, during which time the company milled 1,037,373 tons of ore yielding $5,718,000 and paid $758,000 in dividends. During this period, about 125 men were employed underground and 60 on the surface. After 1920, however, costs escalated to the point where the mine was no longer profitable and the mine was forced into receivership. By that time, the workings had reached an inclined depth of 4300 feet (the shaft below 1600 inclined of 60?). In 1925, the Plymouth Consolidated Mine was sold to the Argonaut Mining Company for less than $75,000 (Knopf, 1929). The Argonaut Mining Company deepened the main shaft an additional 150 feet making a total depth of 4450 feet. Some ore was milled between 1925 and 1928 which yielded $525,292. Based on these results they decided not to rehabilitate the mine ore sink any more workings. Very little work was done between 1928 and 1939. Some exploration was conducted on the 400 and 500-foot levels between 1928 and 1931 and a combined flotation and cyanide plant was put into operation by the Argonaut Mining Company to treat tailings. The plant was shut down in 1943 as a result of World War II. In 1946, the Empire workings were reopened by the Argonaut Mining Company. This was a resumption of the prospecting interrupted by WWII. On the 400-foot level, an old drift was extended north 700 feet. Only a small amount of ore was found and the mine was permanently closed in 1947. The head frame, tailings, and stamp mill foundation remain, and the most notable structure in the town of Plymouth, the Empire building, was the mining company's office. Total production of the properties comprising the Plymouth Consolidated Mine was about $13.5 million. The average value of all ore milled between 1860 and 1924 was $6.75 per ton (Logan, 1927).

Comment (Economic Factors): Clark (1970) reported that the Plymouth Consolidated Mine produced $13.5 million. The average value of all ore milled between 1860 and 1924 was $6.75 per ton (Logan, 1927).

Comment (Commodity): Commodity Info: 70% of the produced gold having been recovered by milling and amalgamation. Sulfides comprised 1.25% to 1.5% of the ore, contribute the remaining 30%. Sulfides concentrates assayed $100 - $200 per ton. Gold is 800 fine.

Comment (Commodity): Ore Materials: BFree milling banded white gold quarts seams composed of ribbons or bands containing free gold, slate, auriferous pyrite, arsenopyrite, and minor chalcopyrite.

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

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 (Geology): The Pacific vein strikes N 20? W and dips between 70? to nearly vertical, conforming more with the cleavage of the slate wall rocks. The Pacific vein merges with the Empire vein near the surface in the vicinity of the Empire North and Empire South shafts Three ore shoots were worked in the Pacific vein between the 835-foot and 1600-foot levels above which they pinched out. The ore bodies rake towards the Empire shoot, so that the Empire shoot on the north and Pacific shoots on the south merged at about the 2900-foot level. Their occurrence was attributed to the junction of the two main veins (Logan, 1927). Due to the nature of their junctures, the Pacific vein is considered a spur of the Empire vein, leaving it a little north of the Empire North shaft and striking a little east of south. The Empire vein extended southward to the New London claim. The Reese and Woolford vein, while located only 500-700 feet east of the Pacific vein, did not yield any paying ore Ore shoots appear to have occurred at and near vein intersections or the junctions of spur veins. Also, contact zones between slate and interbedded andesites seem to have been a controlling factor in determining the location of veins, as well as ore shoots. The occurrences of open spaces in the main fault fissures or adjacent spurs was another factor (Logan, 1934). In general, ore shoots were found at or near the intersection of main veins and various spur and stringer veins. Ore occurs as seams or series of seams of banded white quartz, the ribbons or bands containing free gold, slate, auriferous pyrite and arsenopyrite, and minor chalcopyrite. Ore is free milling with 1.25% to 1.5% sulfides, the concentrates assaying $100 - $200 per ton (Irelan, 1888). Gold is 800 fine. Most of the gold is free gold with 70% of the gold recovered by amalgamation. The remaining 30% came from concentrates. Gold content of concentrates fluctuated between 24%-32% (Knopf, 1929). .

Comment (Identification): The Plymouth Consolidated Mine is located within the town of Plymouth, California in the famous Mother Lode Gold Belt of the Sierra Nevada foothills. The mine is the most northern of the major Mother Lode Mines in Amador County and one of the preeminent mines of the Jackson-Plymouth district. The Jackson-Plymouth district was the most productive district of the Mother Lode belt with an estimated total production of about $180 million (Clark, 1970). The Plymouth Consolidated Mine came into being in 1883 with the consolidation of the Plymouth, Empire, and Amador Pacific mining companies. The Plymouth Consolidated Mine ultimately produced about $13.5 million (Clark, 1970). The Plymouth Consolidated Mine holdings comprised 126.3 acres including the Plymouth, Southerland, Oaks, Pacific, Simpson, Aden, Reese, Phoenix East claims, the Phoenix mill site, and interests in the Reese and Woolford, Indiana, Rising Star, Conville, and Beta claims.

Comment (Geology): REGIONAL GEOLOGY The Plymouth Consolidated 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 (Deposit): The Plymouth Consolidated Mine produced gold from two principal Mother Lode gold-quartz vein systems in fault and fracture systems within Mariposa Formation black slate. The more productive of the two, the Empire vein, strikes northerly and dips about 62? east. To the east, the Pacific vein strikes N 20? W and dips from between 70? to 90? east. The veins merge within the northern end of the property. The main Empire ore shoot was claimed to be 800 feet long by an average 30 feet wide with ore assaying between $6 to $8 a ton. A number of smaller ore shoot s were worked in the Pacific vein. Ore shoots primarily occurred at or near the intersections of the main veins or spur veins off the major veins. Ore occurs as seams of banded white quartz, the ribbons or bands containing free gold, slate, auriferous pyrite, arsenopyrite, and minor chalcopyrite. Gold is 800 fine. Most of the gold is free milling gold with 70% of the produced gold having been recovered by amalgamation. Sulfides, which comprise 1.25% to 1.5% of the ore, contribute the remaining 30%. Sulfides assayed $100 - $200 per ton.

Comment (Location): Location selected for latitude and longitude is the Pacific Mine shaft symbol on the USGS 7.5 minute Amador City quadrangle (the Pacific Mine was one of the claims consolidated into the Plymouth Mine).

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 Mother Lode Mariposa slate belt at the Plymouth Consolidated Mine is one mile wide, within which there are three parallel vein systems called the Empire, Pacific, and Reese and Woolford veins. The Empire and Pacific veins were productive, while no paying ore was developed in the easternmost Reese and Woolford vein. All three veins have a general north-south strike and dip from 62? to 90? east. The most important vein is the Empire which occupies a reverse fault fissure within the Mariposa slates. The vein strikes north and dips 62? east, cutting across the schistosity of the slate wall rocks at a small angle and strike. In the deeper levels it is a typically composed of ribboned structures of crushed slate-quartz stringers (Logan, 1934). The large Empire ore shoot was located in this vein. The Empire shoot raked to the south into adjacent claims resulting in litigation and the resultant formation of the Plymouth Consolidated Mining Company. From the surface to 400 feet it was only average grade, and from there to 1000 feet even lower, so that much of it between 600 and 1000 feet was left unstoped. From 1000 to 1500 feet it became richer, and in 1886 this ore resulted in a profit of $254,402.13 from a gross production of $ 447,547.13. Dividends of $225,000 were paid out that year. The ore shoot at the time was claimed to be 800 feet long. The vein averaged 30 feet wide and carried 1.5% sulfide concentrates worth $135 a ton. The average value of the ore was $6 to $8 per ton (Logan, 1934). The Empire ore shoot ended to the north at a greenstone dike entering from the footwall side. The dike crops out 2850 feet north of the Pacific shaft and persisted to the lowest level of the mine at 4450 feet. The dike was about 100 feet where encountered in crosscuts. In the lower mine levels, the dike turns away from the shaft on a curved course suggesting a lens shape. On the 4450 level, the northern end of the ore body is 800 feet north of the main shaft (Logan, 1934). A longitudinal cross section of the Plymouth Mine in the plane of the Empire vein is given by Logan (1934).

Comment (Workings): Prior to the Consolidation of the Plymouth, Empire, and Pacific Mines, several independent shafts opened each mine. Little information is available regarding the underground workings of these early mines. After 1883, the Plymouth Consolidated Mine was opened by at least 4 shafts including the Pacific shaft, the Empire North shaft, and Empire South shaft. The North Empire shaft was a 1065 foot inclined shaft located about 350 feet north of the 1280 foot inclined Empire South shaft. The Pacific shaft ultimately became the main working shaft for the mine and was ultimately sunk to a depth of 4450 feet in 1925. It consisted of a 3-compartment vertical shaft to 1600 feet, below which the balance was inclined eastward at 60? (Carlson & Clark, 1954). No comprehensive descriptions of all the workings of the Plymouth Consolidated or its earlier component mines is available. Logan (1934) presented a longitudinal section in the plane of the Empire vein showing the underground workings of the Pacific and Empire shafts in that plane. Records contained in the California Geological Survey Mineral Resources Files and historic California State Mineralogist reports provide limited snapshots of some of the workings at various points in time. During the mine's heyday between 1883 and 1889, the Plymouth Consolidated mills were considered the second largest quartz mills in the world. The mills had an aggregate of 160 stamps crushing 400 tons a day (Knopf, 1929).

Comment (Development): The Plymouth Consolidated Mine came into existence in 1883 by way of the consolidation of the Plymouth, Empire, and Amador Pacific mines. Prior to 1883, these mines operated independently, some having started as early 1852. Among the Plymouth Consolidated mine holdings were the Plymouth, Southerland, Oaks, Pacific, Simpson, Aden, Reese, Phoenix East claims, the Phoenix mill site, and interests in the Reese and Woolford, Indiana, Rising Star, Conville, and Beta claims. The mine controlled a total of 126.3 acres (Logan, 1934). The Plymouth Mine was discovered and first developed in 1852, but very little work was done until 19 years later. In 1871, Alvinza Hayward purchased an interest in the Plymouth Mine. Hayward was a veteran mining man, having made his fortune in the Sutter Creek and Comstock mines. He constructed a new stamp mill and began extensive exploration and deeper mining. By 1878, the Plymouth Mine was producing $30,000 to $50,000 a month in gold. The onslaught of new miners with wages to spend resulted in a boom for the nearby town of Plymouth. The Phoenix Mine was opened in 1859 on the Southerland, Simpson, and Aden claims, and worked continuously until 1875. The Phoenix Mine had 2 shafts 200 feet apart and 900 feet deep. Ore occurred in a body of low-grade quartz. Soon after, the Phoenix Mine was sold to the Empire Company and renemed the Empire Mine. By 1879 the 2 shafts (renamed the Empire North and Empire South shafts) deepened to 1200 feet and 1280 feet deep (1020 and 100 vertical feet respectively). During this period, ore averaging $9 a ton was being mined between the 1000-foot and 1100-foot levels on the main Empire ore shoot. About the same time, the Amador Pacific Mining Company was developing the Pacific Mine on the Pacific vein south of the Empire Mine. The Pacific vertical shaft (which became the main working entry of the Plymouth Consolidated Mine) was started in May 1880, and was 1000 feet deep by October 1880. The southward rake of the Empire ore shoot carried it under the holdings of the Pacific Mine where it was mined by the Empire Company. The ensuing lawsuit, while won by the Amador Pacific Company, lead to the consolidation of the Empire, Amador Pacific, and Plymouth mining companies on June 1, 1883. The new venture was operated by the Plymouth Consolidated Mining Company (Irelan, 1888; Logan, 1934). Production of these mines prior to consolidation was reported to be about $2.5 million (Logan, 1934). Soon after they were consolidated, the property became the principal producer in the County. As many as 147 stamps were operated by water power. From June 1, 1883 to January 1, 1888, the company produced $2,804,499 from which 55 dividends totaling $2,200,000 were paid (Logan, 1934). Under the Plymouth Consolidated Mining Company, the main shaft was sunk 900 feet from 3400 feet to 4300 feet (Loga, 1927). For some time prior to this, ore had been mined through a winze, hoisted to the 3400 foot level then trammed to the shaft to be hoisted to the surface. Both shaft and winze were in poor shape, necessitating this change.

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References

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

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. 190-192.

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.

Reference (Deposit): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, p. 49-52.

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., 1921, Amador County, Central Eureka Mine: California State Mining Bureau, 17th Report of the State Mineralogist, p. 411.

Reference (Deposit): Logan, C.A., 1927, Amador County, Central Eureka Mine: California State Mining Bureau, 23rd Report of the State Mineralogist, p. 179-180.

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 106-111.

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): Storms, W.H., 1900, The Mother Lode region of California: California Mining Bureau Bulletin 18, p 82-83.

Reference (Deposit): Tucker, W.B., 1914, Amador County, Original Amador Mine: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 41-43.

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): Additional information on the Plymouth Mine is available in fiile nos. 339-1329, 331-9829, and 339-1334 (CGS Mineral Resource files, Sacramento).

Reference (Deposit): Irelan, W., Jr., 1888, Amador County Gold: California State Mining Bureau, 8th Annual Report of the State Mineralogist, p. 42-49.

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California Gold

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