Central Eureka Mine (Summit Mine), Sutter Creek, Jackson-Plymouth Mining District, Amador County, California, USAi

Regional Level Types
Central Eureka Mine (Summit Mine)	Mine
Sutter Creek	Creek
Jackson-Plymouth Mining District	Mining District
Amador County	County
California	State
USA	Country

A former Au-Ag-Fe-Zn-Pb-Cu mine located in the SW¼ sec. 8, T6N, R11E, MDM, 1.2 km (0.7 mile) SSE of Sutter Creek, on private land. Discovered in 1855. Operator Central Eureka Mining Co. MRDS database stated accuracy for this location is 100 meters.

The Central Eureka, originally the Summit, was worked from 1855 to 1875 and reopened in 1895 as the Central Eureka, which then operated intermittently until 1924, when consolidated with the Old Eureka, which had been worked intermittently since 1852. It was then worked intermittently from 130 to 1942. Mining resumed in 1946 but little was done until 1948.

The Central Eureka Mine is situated in the most productive part of the Sierra Nevada Mother Lode belt, the 10-mile portion that lies between the towns of Plymouth and Jackson in Amador County. The Central Eureka Mine was one of the three largest mines on the Mother Lode (Central Eureka, Argonaut, and Kennedy mines). 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 Jackson - Plymouth district (Clark, 1970), which, in sum, was the most productive district of the Mother Lode belt with an estimated total production of about $180 million (Clark, 1970).

Prior to 1924, the Central Eureka Mine (formerly the Summit Mine) was operated independently, having produced about $8.3 million by that time. Following a decline in its ore values and increasing costs, the Central Eureka Mining Company purchased the adjoining Old Eureka Mine Company in 1924 and consolidated it with the Central Eureka Mine. Between 1924 and 1930, the Central Eureka Mining continued to mine both properties. From 1930 until its closure in the 1950's, all produced ore came from the former Old Eureka Mine workings. In total, the consolidated Central Eureka Mine is credited with producing $36 million (Clark, 1970).

Most of the important lode gold deposits in Amador County Mother lode were discovered in the 1850s while rich Tertiary placer deposits were being worked. While many of the neighboring mines were very profitable by 1875, the Central Eureka Mine (and nearby Argonaut, Kennedy, Bunker Hill, Fremont-Gover, and Lincoln Consolidated mines) did not become a major producers until the 1880s and 1890s.

The Central Eureka Mine was located in 1855 as the Summit Mine. Eleven years later, a shaft was sunk to a depth of 550 feet and about 1000 tons of ore worth $30,000 was produced from a depth of 165 feet. However, the bottom workings failed to find additional ore. A second shaft, the Central Eureka shaft, was sunk to a depth of 700 feet but also failed to find ore. By 1875 mining ceased and the mine machinery was dismantled and sold.

In 1893, the Summit property was sold for $6,000 and incorporated under the name of the Central Eureka Mining Company. Since 1893, with the exception of several minor shutdowns, operations remained relatively continuous until its final closure in either 1951 or 1953. A 10-stamp mill was built in 1900, but production started before this, the ore being crushed at that nearby Zeila mill. The Central Eureka shaft was deepened from 700 to 2500 feet by 1907.

The mine operated profitably until 1907, paying large dividends. The operations during this period were on the hanging wall vein between 1000 to 2500 feet inclined depth. The mine closed down for one year in 1907, but it was reopened in 1908. For the next ten years, the mine operated at a loss. Ore values declined to between $3 - $4 per ton, and numerous assessments were levied on the shareholders. By 1914, the shaft had reached 2825 feet. By 1916, only a few short, low-grade ore shoots had been found, the plant was getting old, and the shaft was falling into disrepair (Logan 1927). Also, due to the converging of the end lines in the direction of the dip, the length of holdings along the hanging wall vein was shortening on each successive level. The retiring directors concluded the mine was exhausted and recommended the mine be closed (Logan, 1934). However, the incoming board of directors decided to go forward. An evaluation of the property recommended that the east and west parts of the property be explored since the former work had been confined to the central part of the lode. By early 1919, the 3500-foot level had been reached and a crosscut on this level revealed a good ore shoot. In a winze below the 3500-foot level, a sublevel was turned and run both ways in good ore (Logan, 1934). By this time, while the stockholders were facing their 48th assessment, the mine entered a bonanza period. During the following 11 years, the mine was very productive with each year resulting in dividends to shareholders.

In 1924, the adjoining Old Eureka Mine (comprising the Amador, Maxwell, Alpha, and Railroad claims and 136 acres of adjoining land) was purchased, a new steel head frame was built, and many other upgrades and improvements were made (Logan, 1934). The Old Eureka Mine was first opened in 1852. In 1886, after reaching a depth of about 2000 feet, and having produced about $16,000,000, it was idled for a period of 30 years. In 1916, the property was purchased for $500,000. The Old Eureka shaft was reopened and extended it to a depth of 3500 feet inclined distance (3212 feet vertically), and several new levels were driven without finding commercial ore in paying quantities. Work on the Old Eureka was again discontinued in 1921 (Spiers, 1931) until reactivated by the Central Eureka Mining Company.
Below the 4500-foot level, the ground turned out to be so badly broken that shaft and drift maintenance and other underground repairs were costing more than breaking the ore. Nonetheless, the veins continued to be developed deeper and deeper until the Central Eureka bottomed at 4855 feet. Together with a declining gold content in the ore, these conditions led to a substantial operating loss in 1930. The lower workings of the Central Eureka were abandoned and all pipes, rails, pumps, and other equipment from the deeper levels and the tracks in the shaft below 3900 feet were taken up. Having produced $8,321,925 in gold and paid out $1,263,973 in dividends, this ended all mining operations on the original Central Eureka Mine claims. After 1930, all ore produced by the Central Eureka Mining Company came from the Old Eureka Mine workings (Carlson and Clark, 1954). Development of the Old Eureka workings proved to be quite profitable, and by 1939, the Central Eureka Mine was the best paying mine on the Mother Lode.

In 1930, a partnership took an option on the Central Eureka mine tailings between 1932 and 1938, and a total of 880,000 tons of tailings were treated. Assays varied from $1.50 - $3.00 per ton, with recovery of about 70%. The tailings were hydraulically sluiced to a classifier. Both sands and slimes were then treated with 3 pounds of coal tar per ton to prevent precipitation from the carbonaceous content. The sands were treated in leaching vats. The slimes were directed to an agitator, thickener, and filters. Sand and slime residues were discarded. Cyanide precipitates were acid treated, roasted, and melted in to gold bars.

As were all gold mines in the district, the Central Eureka Mine was shut down in 1942, but in anticipation of its reopening, it was kept in working order during WWII. 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 1950's. Various reports place its final year of operation as 1951 or 1953. It was the last active major gold mine on the Mother Lode.

Mineralization is a vein deposit (Mineral occurrence model information: Model code: 273; USGS model code: 36a; Deposit model name: Low-sulfide Au-quartz vein; Mark3 model number: 27) hosted in Late Jurassic slate and andesite of the Mariposa Formation. The ore body is tabular/pinch & swell in form, strikes N40W and dips 71NE at a thickness of 6.1 meters. Ore controls included a shear zone. The ore bodies consist of quartz-ankerite-albite and massive quartz stringers, lenses and veins. The ore bodies are in black slate at the contact with meta-andesite in the upper portion of the mine. The hanging wall is meta-andesite but both it and slate are found on both walls due to the veins crossing interbedded lenses of both rock types. The ore vein at depth is a black gouge with quartz stringers. The average thickness of the mineralized black gouge zone is 8 feet. One ore body parallel to the ore vein is 420 feet long and 280 feet wide. A large wedge 850 feet long, 35 feet wide and as great as 125 feet contains several ore lenses. Au alloyed with Ag is in the free-milling state or the Au is disseminated with pyrite. Ore included free-milling ribboned quartz and slate seams and mineralized greenstone "gray ore". Gray ore contained a few percent auriferous sulfides, principally pyrite and arsenopyrite. Pyrite and arsenopyrite are common while sphalerite, chalcopyrite and galena are minor. Local alteration is hydrothermal alteration of meta-andesite causing blocky meta-andesite to mix with quartz in the ore veins. Meta-andesite is altered to schist in places. Local rocks include Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains).

The principal vein worked in the early workings was the so-called hanging wall, or contact, vein, which occupied the main fault plane fracture zone. The vein is not straight in strike but rolls in a series of long swells, and gouge is always found on the footwall side. Long, lenticular masses of quartz within the vein form the ore shoots. Some ore shoots in this vein reportedly averaged about $70 per ton. In the upper workings, this vein consisted of a simple single fissure cutting in strike and dip the black slates and greenstone wall rocks (Storms, 1900). Above the 1000-foot level, this vein was enclosed in slate walls. Below 1000 feet, the hanging wall slate turned to greenstone. The ore shoot on this vein extended from near the 1000-foot level to the 2540-foot level and ranged from 1-12 feet thick, with its greatest stope length about the 1500-foot level. Several short, narrow ore shoots were found at a depth of about 1000 feet, and these increased in length and width until they merged to form one large ore shoot at 1850 feet (Logan, 1934). The exact tonnage from this shoot is unknown but was probably about 300,000 tons, which was worked between 1896 and 1907, averaging about $7.16 per ton (Logan, 1934). At a depth of 3300 feet, ore shoots within this vein became too low grade to mine profitably and mining of the Contact vein ceased at a depth of 3350 feet. The average yield between 1908-1919 was only $3-$4 a ton, and the size of the ore bodies, even at this grade, kept decreasing. Between the 2540 and 3425-foot levels, most of the rock milled did not pay expenses and what ore was encountered soon gave out (Logan, 1934).

Low-grade mineralization of the greenstone wall rock was common upon nearing veins and ore shoots. Mineralization was generally in the form of disseminated sulfides, chiefly pyrite and arsenopyrite. In some cases mineralization was sufficient to form low-grade but profitable ore bodies.

Footwall vein:

On the 3,500-foot level, a new ore-bearing vein was discovered in the footwall. From 30 to 70 feet of intervening Mariposa slate separate the two veins (Spiers, 1931). The footwall vein ore shoot on the 3760-3900 foot levels was 340 feet long and averaged 9-10 feet wide, filled with quartz ribbon rock and gouge. On the 3500-foot level, ore was in two shoots, a north and a south shoot, separated by 160 feet of low-grade filling. During the year ending April 22, 1920, a total of 34,705 tons of ore were milled from the 3500 foot, 3600 foot, and 3700 foot levels with an average assay value $13.15 with a recovery rate of 94.31% (Logan, 1921). About one quarter of the ore came from the 3500-foot level south shoot, but this shoot was spotty and apparently faulted between the 3500 and 3700 foot levels. Ore from the north shoot averaged over $20 per ton (Logan, 1921).

The most important and productive level of the mine was the 4550-foot inclined level in the footwall vein. At this level a spur vein striking N25W joins the footwall vein. This spur vein provided good ore and was stoped for 165 feet from its junction with the main vein, which it joined about the center of the main footwall ore shoot which measured 425 feet long. Ores from these levels assayed $10 -$14 per ton.

Regional geologic structures include the Bear Mountains Fault zone and the Melones Fault zone. Local structures include the Melones Fault zone.

Workings include underground openings comprised of 2 inclined shafts averaging 65 degrees at 4,855 feet and 3,500 feet, respectively. The 3,500 foot shaft has a winze at its base to 4,150 feet. Water was con fined to this shaft. Levels were at 2900, 3100, 3400, 3500, 3600, 3700, 3800, 3900 & 4150 feet with a total of several thousand feet of drifts. Stopes were timbered close to the faces due to considerable gouge on the walls of most levels. Cut & fill stoping methods were employed.

The Central Eureka Mine was accessed through the main Central Eureka shaft, a 3-compartment shaft (2 skipways and 1 manway). This shaft struck the hanging wall, or contact, vein at 500 feet and followed it to about 2000 feet where the shaft passed into footwall slate and then continued to a total depth of 4855 feet. After acquisition of the Old Eureka Mine in 1924, the mine also utilized the Old Eureka shaft which was 3500 feet deep with an 11 x 5 foot inclined winze sunk from the 3500 to 4150 feet for working the lower levels at 3700', 3800', 3900', and 4150 feet. Ore was hoisted up the Central Eureka shaft while the haulage of men, supplies, timber, and waste was confined to the Old Eureka shaft (Carlson and Clark, 1954). The Central Eureka shaft was 1800 feet south of the Old Eureka shaft. Each had an average incline of 65?-75? east. Additionally, the 2700-foot inclined South Eureka shaft, on an adjoining property, was maintained for ventilation and pumping.

Levels In the Central Eureka shaft were turned at intervals of about 100 to 200 feet depending on conditions. The deepest level was 4855 feet. From 1919 to 1930, the mine was deepened from 3500 feet to the bottom with levels at 2900, 3100, 3400 3500'. 3600', 3760', 3900', 4100', 4250',4400', 4550', 4700', and 4855 feet (Logan 1934). During this time, the South Eureka Mine was under option and was prospected on the 3350', 3900', and 4100' levels. The 3900 foot level was run south 1180 feet from the shaft, extending some distance into the South Eureka, and a crosscut was run 740 feet east in the slate from this level through several vein formations and gouge seams, without finding ore (Logan, 1934). The 4100 foot level was also extended into South Eureka ground (Logan, 1934).

Between 1935 and 1938, the Central Eureka Mining Company also ran exploration drifts north from the 2100 and 2500-foot levels of the Old Eureka Workings into the adjoining Lincoln Consolidated Mine property. A total of 2467 feet of drifting and crosscutting was completed on both levels without finding any commercial ore (Carlson and Clark, 1954).

Illustrations of some of the Central Eureka Mine workings are given by Spiers (1931) and Zimmerman (1983).

Due to the heavy ground conditions, the cut and fill method of stoping was used. Ore was extracted by square set stoping for support in conjunction with waste filling. Ore was removed in horizontal cuts across stope faces up to 30 feet wide and 100 feet long. Timbers were placed in position as soon as room was available and no opening over 2 sets (16 feet) in height was made without filling with waste (Spiers, 1931). Originally waste rock was used for backfilling, but in 1946 sand from spent tailings was used to fill the stoped out areas.

Ore from the lower stopes was loaded into mine cars, trammed to the transfer chutes, and hoisted up the 3500 winze to the main haulage way on the 3500-foot level. Electric locomotives transported ore cars 3000 feet south to the Central Eureka shaft where a double drum steam hoist brought the ore to the surface in 3-ton skips.
In its final years of operation, ore was run through a jaw crusher, 30-stamp mill of 9 tons per hour capacity, jig classifier, conditioner, and flotation cells. The gold and concentrate from the jig was delivered to two Knudsen bowls, an amalgam barrel, retorted, and sent to the U.S. Mint in San Francisco. The gold and concentrate slimes, as well as the slimes from the flotation cells were tabled and sent to a Dorr thickener, after which the gold concentrates were delivered to a 12-foot hydro-separator. The products from the separator went to a cyanide plant or were reground in a pebble mill and sent to another Knudsen bowl. The fines from the bowl were sent to the amalgam barrel while the slimes were run over a corduroy table and recirculated through the hydro-separator. Mill recovery averaged 95% with approximately 70% of the gold recovered by amalgamation and 30% recovered through concentrates. The mill sand discharge was delivered to settling tanks where one pound of aluminum sulfate was added to each ton of discharge to aid in settling. Thirty percent of the discharge was discarded on the tailings pond, while the remaining 70% of the sand was mixed with water and pumped down the Old Eureka shaft for backfilling. This process could deliver up to 30 tons of sand per hour for use as backfill (Carlson and Clark, 1954).

Production data are found in: Carlson, D.W. & W.B. Clark (1954).

Total production (both mines) was $36,000,000 (period values), approximately to the end of 1951. 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. The mine produced 39,440 tons of ore yielding more than $500,000 in Au & Ag in 1951.

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