Old Eureka Mine

Past Producer in Amador county in California, United States with commodities Gold, Silver

Geologic information

Identification information

Deposit ID 10310657
Record type Site
Current site name Old Eureka Mine
Alternate or previous names Hayward Mine, Eureka Mine, Hetty Green Mine, Badger Mine, Wolverine Mine, Amador Consolidated Mine

Comments on the site identification

  • The Old Eureka Mine is located within the town of Sutter Creek in the famous Mother Lode Gold Belt of western Amador County. The Central Eureka Mine adjoins the Old Eureka to the south and the Wildman-Mahoney and Lincoln Consolidated Mine lie to the north. While the mine is technically in the smaller Sutter Creek district, the uniform nature of gold mineralization with neighboring districts has caused some authors to consolidate the smaller neighboring districts into a single Jackson - Plymouth district (Clark, 1970). 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 Old Eureka Mine was first opened in 1852 and at various times during its history was known as the Hayward, Eureka, Hetty Green, Badger and Wolverine, and Amador Consolidated mines. The Old Eureka Mine was operated independently until 1924 when it merged with the Central Eureka Mine, the combined properties operating as the Central Eureka Mine. The combined Central Eureka Mine is credited with producing $36 million (Clark, 1970). The old Eureka properties are credited almost $28 million of that amount.

Geographic coordinates

Geographic coordinates: -120.8, 38.38788 (WGS84)
Elevation 405
Location accuracy 100(meters)
Relative position The Old Eureka Mine is located 1/4 mile south of Sutter Creek, CA
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Geographic areas

Country State County
United States California Amador

Public Land Survey System information

Meridian Township Range Section Fraction State
Mount Diablo 006N 011E 7 NE/4NE/4 California

Comments on the location information

  • Location selected for latitude and longitude is the Old Eureka Mine shaft symbol on the USGS 7.5 minute Amador City quadrangle.

Commodities

Commodity Importance
Gold Primary
Silver Secondary

Comments on the commodity information

  • Ore Materials: Free milling banded quartz-slate seams and hydrothermally altered greenstone with disseminated auriferous sulfides, primarily pyrite and arsenopyrite
  • Gangue Materials: Quartz, slate, ankerite, albite, sphalerite, greenstone

Materials information

Materials Type of material
Gold Ore
Pyrite Ore
Arsenopyrite Ore
Quartz Gangue
Slate Gangue
Ankerite Gangue
Albite Gangue
Sphalerite Gangue

Alteration

  • (Local) Wall rocks hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite. Locally, greenstone bodies adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute low-grade ore.

Mineral occurrence model information

Model code 273
USGS model code 36a
Deposit model name Low-sulfide Au-quartz vein
Mark3 model number 27

Host and associated rocks

  • Host or associated Host
    Rock type Metamorphic Rock > Metasedimentary Rock > Slate
    Rock unit name Mariposa Formation
    Stratigraphic age (youngest) Late Jurassic
  • Host or associated Host
    Rock type Metamorphic Rock > Metavolcanic Rock > Mafic Metamorphic Rock > Greenstone
    Rock unit name Mariposa Formation
    Stratigraphic age (youngest) Late Jurassic

Nearby scientific data

(1) -120.8, 38.38788

Geologic structures

Type Description Terms
Local Melones Fault zone
Regional Bear Mountains Fault zone, Melones Fault zone

Ore body information

  • General form Tabular, pinch and swell

Controls for ore emplacement

  • Ore shoots within mesothermal gold bearing quartz veins.

Comments on the geologic information

  • REGIONAL GEOLOGY The Old Eureka 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.
  • 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).
  • 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 Wolverine fault is the most prominent structural feature in the Old Eureka Mine. The fault strikes N 20? W and dips 65? east and displays measurable displacement of over 300 feet. The principal Old Eureka vein occurs within the fault fracture zone. In the upper portions of the mine, the faulting has occurred along the contact between Mariposa Formation slate and greenstone resulting in a slate foot wall and greenstone hanging wall. Below the 1700-foot level, the foot wall remains slate but the hanging wall becomes a series of dense slaty greenstones. Foot wall slates dip 70? - 80? east and strike northwesterly. Wall rock slates are intensely folded and buckled adjacent to the faults. The associated hanging wall greenstones vary from highly metamorphosed fine grained volcanic to coarse-grained tuffs. Slates are intimately interbedded with the greenstones and transition zones are gradational (Norman, 1939). The gouge accompanying the fault varies in width from a few inches to 5 feet. Three types of ore bodies occurred in the Old Eureka Mine: 1) fault ore bodies along the Wolverine fault consisting of irregular lenses of shattered quartz surrounded by gouge, 2) quartz veins in shear zones, and 3) bodies of mineralized greenstone or "gray ore" (Norman, 1939).
  • Rich ore bodies within the Wolverine Fault were the first ore bodies worked and the most productive. About $12 million was recovered from a 500 feet by 1700-foot ore shoot during early operations from 1852 to 1881. The richest part of the mine was about 1000 feet in depth, where the ore averaged $27 per ton. Elsewhere in the ore shoot values varied widely. In December, 1872, it averaged $35.19 per ton, and in December 1873 $6.65 per ton, averaging for 14 months $17.91 per ton (Logan, 1927). In the 1930s, small ore bodies along the same fissure were mined on the 2000, 2100, 2300, and 2500 foot north levels. The main Old Eureka vein consists mainly of milky white quartz with bands or ribbons of crushed wall rock slate. The bands are parallel to the walls and highly mineralized with pyrite and arsenopyrite. Ankerite, albite, sphalerite, chalcopyrite, and galena are present in lesser quantities. In places, intense crushing of the vein quartz is indicative of post mineralization movement along the fault. The bulk of the gold occurs as free milling gold, either in the quartz, in the ribbon structures, or around the borders of the pyrite (Norman, 1939). The vein varied in width from a few inches to 10 feet and the gouge a few inches to 3 feet. The quartz bodies were lenticular in shape, fraying out on the ends to a group of small quartz stringers where commercial values ended. The second type of ore body occurs in quartz veins within shear zones that intersects the Wolverine fault at 20?, the largest being on the 2500-foot level 900 feet north of the shaft . A series of shear zone quartz stringers occur within slaty greenstone for about 500 feet north beyond the intersection before the quartz vein abruptly develops. The ore body from this point northward is 500 feet long. The vein strikes north while the Wolverine fault diverges from it, being 120 feet west at the south end of the ore body and 250 feet west at the north end. At the north end, the vein is vertical and on the south end it dips 70? east. The convergence of the strike and dip of the vein and fault with depth resulted in the truncation of the ore along a line that rakes north and shortens the ore body with depth. From the 2500-foot level to the 2900-foot level the width of the ore increases from an average of 8 feet to a maximum of 24 feet. The fault and vein are together on the 3000 foot level. Between the 3000 and 3100 foot sublevel, the major part of the fault gouge passes through the ore and on the 3300 foot sublevel, the fault is in the hanging wall greenstone (Norman, 1939). Greenstone "gray ore" consisted of mineralized greenstone, most of which was developed in the hanging wall of the north drift on the 2100-foot level. Gray ore was usually adjacent to the veins in ground that has been fractured and contained small stringers and lenses of quartz. Alteration and replacement of the greenstone resulted in enough disseminated auriferous pyrite and arsenopyrite in large enough bodies to constitute low-grade ore. Gray ore Gold values are irregularly distributed in the greenstone, the pyrite and arsenopyrite content ranging from 2 to 5 percent (Norman, 1939).

Economic information

Economic information about the deposit and operations

Operation type Underground
Development status Past Producer
Commodity type Metallic
Significant Yes
Discovery year 1852

Mining district

District name Amador

Land status

Ownership category Private
Area name Amador County Planning Dept.

Comments on the workings information

  • Due to its long history of operation and various owners, much detail about the underground workings of the Old Eureka Mine is lost ore unavailable. Further, after the consolidation of the mine's workings with the adjoining Central Eureka Mine, accounts get confusing regarding the various levels in each mine. Few if any records are available regarding the very early workings of the mine by poorly capitalized parties. The first "comprehensive" description of the mine is included in records of the California Division of Mines and Geology which describe the then current workings of the mine in June, 1929. Prior to its acquisition by the Central Eureka Mining Company in 1924, the Old Eureka Mine consisted of a main 3500-foot shaft with levels at on the 800', 1200', 1700', 2100', 2950', and 3500-foot levels. Drifts: 1700' level: North 740 ft., South 980 ft. 2100' level: North 480 ft., South 865 ft. 2950' level: North 720 ft., South 965 ft. Crosscuts, raises, and winzes: 1700' level: hwcc 385 ft. and fwcc 425 ft. south of shaft, and 2 cc's north of shaft 2100' level: 2 cc's 25 or 30 ft. each north of shaft, and 2 cc's 30 or 40 ft south of shaft 2950' level: hwcc north of shaft 235 ft. and fwcc 120 ft. Also fwcc on south at face 40 ft. and hwcc 50 ft. back from face, 30 ft. long. 3500'level: cc 800 ft. west After acquisition by the Central Eureka Mining Company, drifts were run on the 2000', 2300', 2500' levels. The costs associated with mining small ore bodies from one level at a time necessitated the opening of the new ore on several levels simultaneously. The old shaft was cleaned out to the 3000-foot level and a drift was run northward 1800 feet and a raise driven to connect with a winze that had been started on the 2500-foot level. Sublevels were then run to several advantages levels. During its later years while operated by the Central Eureka Mining Company, mining practices were modified from those in use in most Mother Lode mines. Development drifts were run in the harder hangingwall rocks instead of along the heavy swelling ground in the fissures. Timbering was suited to the actual needs , avoiding the old practice of heavy timbering throughout. Ore bodies were developed by driving haulage levels from the shaft at 500 foot intervals and using sublevels to open sufficient stoping areas between for the economical mining of ore of a fairly constant value (Norman, 1939). Methods of exploration, stoping, general operations and milling details are provided by Norman (1939).

Comments on other economic factors

  • Clark (1970) credited the Consolidated Central Eureka Mine (Central Eureka and Old Eureka mines) with a total production of $36 million. Logan (1934) attributed $8.3 million of this amount to the original Central Eureka Mine before its consolidation with the Old Eureka mine and the cessation of work in the Central Eureka workings. Accordingly, the Old Eureka Mine property produced approximately $27.7 million prior to and after its merger with the Central Eureka.

Comments on development

  • The Old Eureka Mine was first opened in 1852 and comprised the Amador, Maxwell, Alpha, and Railroad claims. This mine has also been known at times as the Hayward, Eureka, Hetty Green, Badger and Wolverine, and Amador Consolidated mines. In 1852, a 10 stamp mill was and by 1855, an initial whim shaft had been sunk to about 95 feet with a lower level driven about 144 feet (Trask, 1855) and a 100 foot adit entered the mine from the west. The initial ore shoot commenced at the surface and extended about fifty feet in depth and was highly pyritiferous. Another ore shoot encountered at the bottom of the shaft was much more productive with almost no pyrite, but instead containing a veinlet of metallic gold (not disseminated) at time exceeding 3/8 inch wide. The same vein had been struck in an adjoining Badger Mine 1000 feet to the south. This is the only instance of a true vein of metallic gold having been found in the state (Trask, 1855). A new 20 stamp mill was built on the Old Eureka in 1856 and 20 more stamps were added in 1857. Alvinza Hayward acquired control of both the Old Eureka and Badger mines in 1859 and consolidated them under the Old Eureka Mine. The adjoining claim on the south was the Badger claim on which 10 stamps were built in 1854 (Logan, 1934). In 1857, the consolidated properties had a north shaft 1230 feet deep, a middle shaft 960 feet deep, and a southern shaft at 760 feet deep. By then there were 56 stamps operating in the two mills crushing 80 tons per day, and ore was also sent to custom mills (Logan, 1934). Hayward was very secretive and refused to divulge the details of his production, but local reports placed the output at $6 million up to 1867 (Logan, 1934). In 1866 it was said that 30,000 tons yielded $27 per ton at a working cost of $5 per ton. . In March 1869, the mine was purchased by the Amador Mining Company for $750,000. The Amador Mining Company operated the Old Eureka until 1881. For the year ending July 1, 1869, 30,361 tons of ore yielded $617,542 or $20.34 per ton. For 14 months ended April 1, 1875 the yield was $239,717 in bullion and $20,254 from concentrates, from 22,098 tons of ore indicating a decline in ore quality. In 1886, after the mine had attained a depth of about 2000 feet and had produced about $16,000,000, it was closed and remained idle for a period of 30 years during which time the mine was acquired by Hetty Green (at one time the country's richest woman). In 1916, a new company purchased the property for $500,000. The old shaft was in very bad shape having been sunk in Mariposa slate. About $200,000 was spent to reopen, unwater, and rehabilitate the shaft and on equipment and pumping. The shaft was deepened to 3500 feet (3212' vertical depth) and drifts were run north and south on the main fissure at 1700', 2100', 2950', and 3500 feet together with approximately 1500 feet of crosscuts (Norman, 1939). The 1200 foot level was also reopened On each of these, except the lowest, from 1450 feet to 1640 feet of drifts were run. A crosscut was run west 800 feet on the 3500 foot level. All of this exploration failed to reveal paying ore. Total drifts as of July 1, 1920 were 1720 feet on the 1700 foot level, 1345 feet on the 2125 foot level, and 1685 feet on the 2950 foot level. Crosscuts totaling about 1500 feet had been driven in the foot and hanging walls. Several new levels were driven without finding commercial ore in paying quantities. Work was discontinued in 1921.
  • In 1924, the Old Eureka was purchased by the Central Eureka Mining Company after threats of litigation over rights to the rich ore shoot being mined from the lower levels of the Central Eureka Mine on the south (Norman, 1939). Operations were merged with those of the Central Eureka Mine. From 1924 to 1930, when the lower levels of the Central Eureka were abandoned, the Central Eureka Mining Company confined its working to its principal ore body on the south line of the Old Eureka and to active exploration of the Old Eureka workings. After 1930, all of the ore produced from the Central Eureka Mine came from the Old Eureka properties. In 1930, mining on the old abandoned 2100-foot level was started. While the previous operators had directed their efforts to the south side of the Old Eureka shaft and adjacent to the Central Eureka Mine, the Central Eureka Mining Company started drifting northward. On the 2000', 2100', and 2300 foot levels, drifts were run in part along small contact ore bodies and in part along a shear zone in the hanging wall greenstones. In the later part of 1936, after stoping of the new ore above the 2500-foot level, a different approach to its development and mining was implemented. The unsuccessful efforts to profitably mine the small ore bodies on the long upper levels, mainly from one level at a time, necessitated the opening of the new ore on several levels simultaneously. The old shaft was unwatered and cleaned out to the 3000-foot level, from where a drift was run northward 1800 feet and a raise to connect with a winze that had been started on the 2500-foot level. Sublevels were then run to several advantageous levels. This program was successful and dividends of $108,000 and $240,000 were paid out in 1937 and 1938 respectively (Norman, 1939). As were all gold mines in the district, the Central Eureka Mine was shut down in 1942 for WW II, but in anticipation of its reopening, it was kept in working order during the war. Mining was resumed in 1946 but little ore was produced until 1948. In 1951, 39,440 tons of ore yielded a little more than $500,000, but because of greatly increased costs, the Central Eureka was permanently shut down in the early 1950s. Various reports place its final year of operation as 1951 or 1953. It was the last active major gold mine on the Mother Lode with a total production of approximately $36 million (Carlson and Clark, 1954).

Reference information

Bibliographic references

  • Deposit

    Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 69-77.

  • 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. 173-177.

  • 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.

  • Deposit

    Lambert, E.F., 1948, The geology of the Old Eureka Mine, Sutter Creek, Amador County, California: unpublished M.S. thesis, Univ. of California, Berkeley, 71 p.

  • Deposit

    Logan, C.A., 1921, Mines and mineral resources, Amador County - Old Eureka Mine: California State Mining Bureau, 17th Annual Report of the State Mineralogist, p. 411.

  • Deposit

    Logan, C.A., 1927, Amador County - Old Eureka Mine: California State Mining Bureau, 23nd Report of the State Mineralogist, p. 176-178.

  • Deposit

    Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 101-104.

  • Deposit

    Norman, L.A., 1939, Operations at the Old Eureka Mine: Mining Tech., v. 3, no. 6, p 1-15.

  • 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.

  • Deposit

    Spiers, J., 1931, Mining methods and costs at the Central Eureka Mine, Amador County, California: Bureau of Mines Information Circular 6512, 13 pp.

  • Deposit

    Storms, W.H., 1900, The Mother Lode region of California: California Mining Bureau Bulletin 18, p 64-65.

  • 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.

General comments

Subject category Comment text
Deposit The Old Eureka Mine produced primarily from a typical Mother Lode fracture filling mesothermal gold-quartz vein. The principal Old Eureka vein occupies the fissure created by the Wolverine reverse fault. The vein and strikes N 20? W and dips 65? east. In the upper portion of the mine the vein follows a contact between Mariposa Formation slate and greenstone resulting in a slate foot wall and greenstone hanging wall. Below the 1700-foot level, the foot wall remains slate but the hanging wall becomes a series of dense slaty greenstones. The largest and most productive ore shoot in the mine measured 500 by 1700 feet, its richest part occurring at a depth of about 1000 feet in and yielding ore averaging $27 per ton. Ore was also produced from quartz veins within shear zones intersecting the Wolverine fault. The main ore body of this type was found north of the shaft on the 2500-foot level where a series of shear zone quartz stringers suddenly developed into a thick quartz vein ore body 500 feet long. The convergence of the strike and dip of the vein and fault with depth resulted in the truncation of the ore along a line that rakes north and shortens the ore body with depth. The best ore occurred in milky white quartz with bands or ribbons of crushed wall rock slate highly mineralized with gold, pyrite and arsenopyrite. The bulk of the gold was free milling gold, either in the quartz, in the ribbon structures, or around the borders of the pyrite. Alteration and replacement of the greenstone wall rock adjacent to veins resulted in enough disseminated auriferous pyrite and arsenopyrite in large enough bodies to constitute low-grade ore (gray ore). Gray ore values were irregularly distributed, the pyrite and arsenopyrite content ranging from 2 to 5 percent (Norman, 1939).

Reporter information

Type Date Name Affiliation Comment
Reporter 15-MAR-2006 Downey, Cameron (Higgins, Chris, T.) California Geological Survey CGS (Formerly CDMG)
Editor 01-SEP-2007 Schruben, Paul G. U.S. Geological Survey Converted from S&A FileMaker format to Oracle. Edit checks on rocks, units, and ages with Geolex search, and other fields.