Plumas-Eureka Mine (Mammoth Mine; Eureka Peak Mine; Rough and Ready Mine; Washington Mine; Seventy-Six Mine; Lawton vein; North vein; Eureka vein; Eureka claim; Director claim; Director 1 claim; Director 2 claim; Oceola claim; Oliver claim; Johns Extension Quartz claim; Eureka Peak Quartz claim; Mammoth claim), Johnsville, Johnsville Mining District, Plumas County, California, USAi

Regional Level Types
Plumas-Eureka Mine (Mammoth Mine; Eureka Peak Mine; Rough and Ready Mine; Washington Mine; Seventy-Six Mine; Lawton vein; North vein; Eureka vein; Eureka claim; Director claim; Director 1 claim; Director 2 claim; Oceola claim; Oliver claim; Johns Extension Quartz claim; Eureka Peak Quartz claim; Mammoth claim)	Mine
Johnsville	- not defined -
Johnsville Mining District	Mining District
Plumas County	County
California	State
USA	Country

A former lode Au-Pb-Ag-Cu-Zn-Fe-As mine located in secs. 23 & 24, T22N, R11E, MDM, 1.2 km (3,800 feet) WSW of Johnsville, on the divide between Eureka Lake and Jamison Creek, on California State Park land. Discovered 1851. MRDS database stated accuracy for this location is 100 meters.

The Plumas-Eureka Mine is one of the most productive gold mines in the northern Sierra Nevada and the preeminent gold mine in Plumas County. Extravagant claims of up to $20 million have been attributed to the Plumas-Eureka during its lifetime from 1851 to 1942. However, most high estimates are unsubstantiated and tend to be associated with assertions connected with the mine's promotion or sale. What is reasonably clear is that during operation by the Sierra Buttes Mining Company, between 1872 and 1902, the mine produced approximately $7 million. Records preceding 1872 and after 1902 are unavailable. Actual total production probably ranges between $10 -$15 million. Ore generally ranged from 0.5 to 1.45 ounces per ton during the mine?s heydays up until the 1890s, although ores were found to be much richer near the surface. Ore values declined to the 0.3-0.4 oz/ton range before the Sierra Buttes Mining Company sold the property. Thereafter, limited exploration allowed subsequent operators to exploit small ore bodies in the 0.4-0.5 oz/ton range. Sulfides, while generally making up a small fraction of the ore, could locally range as high as 30%. Yields from sulfide concentrates could average as much as $309/ton. Reports of the California State Mineralogist as late as 1928 and 1936 concluded that much potential prospective ground remained untested below the final mine workings. An independent mine report conducted in 1936 concluded that the mine did not run out of ore, but its underground workings were impaired by water, which existing pumps could not adequately handle. Thus, it is possible that significant reserves may remain within and below the lowermost workings of the mine. Given the current status of the site as a California State Park, this assumption will likely never be tested.

The Plumas-Eureka lode was discovered by a party of prospectors in search of a mythical Gold Lake in the Sierra Nevada. While two members of the party were climbing to the top of Gold Mountain (later renamed Eureka Peak) to reconnoiter, they found a rich outcropping quartz vein about 1,500 feet above the valley floor on May 23, 1851. The vein was 20 feet wide, 400 feet long, and stood 4 feet above the ground. The mountainside was also covered with rich quartz float. What they found was the exposed top of the Eureka Chimney, a gold-bearing vein that wouldn't be worked out until 1865. The prospectors recruited friends to open a gold mine on the mountain. They assembled a total of 36 members. They named the vein Eureka and formed the Eureka Company, the first company incorporated for mining purposes in California and which began operations in 1851. The Eureka Company staked 30-foot square claims for each of its members, took possession of the nearby Eureka Lake water rights, and erected some arrastras. In 1855 they put up a 12-stamp mill followed in 1857 by a 16-stamp mill to which 8 more were added in 1870. Both were below Eureka Lake Dam at the head of a steep ravine to take advantage of the water drop. As word spread, another group of 76 men located another quartz outcrop on the mountain. They organized their own company and named their mine the Seventy-Six Mine. They also located a mill site on Jamison Creek. Another group of 40 men located outcrops on the southeast side of the mountain and called themselves the Rough and Ready Company and their claims the Rough and Ready Mine. Yet another group of some 80 men formed the Mammoth Mine immediately north of the Eureka Claim. They were satisfied with what could be done with arrastra until the spring of 1856 when they erected a 12-stamp mill along Jamison Creek. As more and more newcomers arrived they became a majority and to accommodate them all, claims were reduced to 20-feet square. The various mines were not immediately successful. The Seventy-Six mine lasted one year. They had invested in a 16 stamp-mill on Jamison Creek and a 1,500-foot wooden chute to bring ore from the mine to the mill. They also laid out a townsite calling it the "City of 76". When they started the mill in 1852, it yielded only $200 from 42 tons of rock. The company failed, the mill was torn down, and the City of 76 never prospered. The Rough and Ready Mine also fared poorly. After building a 12-stamp mill on Jamison Creek, they worked the mine until 1854, suspended operations until 1857, and shut down again shortly thereafter. Both the Eureka and Mammoth continued on and eventually prospered, becoming the core of the Plumas-Eureka Mine. In 1870, a San Francisco banker purchased both the Seventy-Six and Rough and Ready Mines, having previously obtained the Eureka claims and the main Eureka Mill. In 1872 he sold the properties to the Sierra Buttes Mining Co Ltd (SBMC) of London England for $1,000,000. SBMC formed the town of Eureka Mills high on the mountain near the mine and mill to support these operations and by the fall of 1872, SBMC employed 326 men. In 1873, SBMC bought the Mammoth Mine for $50,000, and paid another $50,000 for a claimed northeast extension of a Mammoth lead. Thus all existing operations were consolidated into the Plumas-Eureka Mine, which at its peak comprised a total of 2500 acres including 788 acres of water rights and timber lands.

Shortly after their purchase in 1872, the Eureka Mill collapsed. This lead to the construction of the huge new Mammoth Mill with 40 stamps (later enlarged to 48), near the mouth of the upper Mammoth Tunnel. The Mammoth Mill relied on water power from Eureka Lake, but had auxiliary steam power. The company stockpiled 3,000 cords of wood, enough to run the works for a year if water became scarce. They also enlarged and retimbered the Mammoth Tunnel and prepared to drive a Lower Mammoth Tunnel, which would also provide ore handy to the mill. A flume to bring Eureka Lake water power to the mill was also installed. The new mill began crushing ore in 1873. Even the Rough and Ready came through when exploration of an old tunnel opened a new ore vein. Good leads were also developed in tunnels of the old Seventy-Six near the top of the peak. The discovery of new orebodies in the old tunnels was largely responsible for construction of the new Mohawk Mill and the erection of a high-line gravity cable tramway to bring the ore down. After the new Mammoth mill went on line in 1873, plans were made for a second huge 40-stamp mill. It began crushing ore with 20 stamps in December, 1878. The second 20 stamps went online in January 1879. The Mohawk mill operated in conjunction with the Mammoth mill. The Mohawk mill wheel was driven by water from a miners ditch, which tapped Jamison Creek two miles up the canyon. During 1876, William Johns (the mine manager) laid out a townsite along Jamison Creek to support the new mill operation at the base of the mountain. The town came to be known as Johnsville, which still survives to this day. Throughout the 1880s, ore values began to steadily decline. Through the last good year, the company cleared $7/ton on ore assaying $12/ton. But as the ore values dropped to $10, $8, to $6, work stopped at the Seventy-Six and the Rough and Ready; 20 of the Mammoth's stamps were moved to the Mohawk mill. Dividends, for long 15% a year, dropped to 10% in 1884, then 5%, and finally 2.5% in 1886. In 1888 average yield of ore was $7.92/ton with mill tailings assaying $2/ton. Employment had dropped to 200 men in 1888. By 1888, the main tunnel was 6,000 feet long and the vertical depth reached in the mine was 1,500 feet. Three gravity tramways (500 feet, 1,500 feet, and 1,700 feet long) were in operation to transport ore from the mines to the mills. Ore cars were brought out by horses and conveyed to the mill by the tramways. By 1890, the Plumas-Eureka Mine was in decline and employed 229 men. Through the years, the mine had been opened by five tunnels to tap the Plumas-Eureka vein to a vertical depth of 1,500 feet. Three of these had been worked out, and the remaining two were idle. Small quantities of ore were still being taken from the Seventy-Six vein, but the ore quality was poor. It averaged less than $5/ton, which scarcely paid for its extraction and processing, but was enough to keep the Mohawk mill operating. Recovery in the batteries averaged 60%, the remainder being found on the plates. The amalgam yielded 20% of its weight in gold when retorted. The pulp, after passing over the amalgamating plates and concentrators, was passed down the creek to be worked by almost 30 arrastras situated one below the other down the canyon. In March of 1892, the town of Johnsville was largely destroyed by fire. Due to the declining fortunes of the Plumas-Eureka, it was never completely rebuilt.
•In 1893, the Plumas-Eureka earned only $9,000. The ore had become low quality but was plentiful. The Chairman of the SBMC Board told shareholders that continued mining was exceedingly doubtful at these values. By 1894 workings were confined to near the summit of the peak in the area of the old Seventy-Six Mine. Only 30 of the 60 stamps at the Mohawk Mill were dropping, and the workforce had dropped to 130. By 1896 work was still confined to the Seventy-Six ground but as the ore played out, the number of working stamps declined to 15 and the workforce declined to 50 men. In 1896, SBMC was no longer prospecting for new ore bodies and by 1897 the Company was only crushing existing ore at ? capacity and employing 10-30 men. In December, SMNC ceased operations and leased the mine to its former employees. During SBMC's operation, most work was done by hand, air drills not generally in use at that date. Production was labor intensive and ore costs ran an average $4/ton. Thus the company ignored any ore that assayed less than $4/ton. So the company left hundreds of thousands of tons of $3 -$4/ton ore in the old workings. The SBMC operation was considered inefficient in that instead of working from the lowest level up, they developed the top of the hill down. Often ore had to be conveyed distances of 400-1,200 feet through surface tramways and around precipitous hillsides before it reached the mill. Supplies were also expensive, having to be imported from San Francisco or Reno. Between 1897 and 1901, the mine was operated by lessees during the summer months with a workforce of between 12-20 men. In October 1901 only ten stamps were still running. The lessees did not try to find any new ore. Instead they only worked the pillars and sweepings left by the Sierra Buttes Mining Co. By 1902, SBMC decided to sell the mine. The decision was not based on lack of ore, from which they were receiving royalties, but because the majority of corporation members had died and the heirs knew nothing of mining. Rather than investing the 30,000 pounds remaining in SBMC reserve funds to explore for more ore bodies, the remaining stockholders decided to withdraw the funds and sell the properties. In 1905, Rhode Island investors purchased the mine and formed the Johnston Graham Mining Company. The transaction included all 2,500 acres and all the timber and water rights. They operated the mine from 1905-1909, during which time they repaired the mine and installed 20 new stamps. They reopened the Eureka Tunnel and erected the aerial tramway from the upper Seventy-Six workings to the mill. Only small quantities of ore were mined each year. During their operation, cyanidization increased recovery to 96%. A second major fire in the town in 1906, effectively marked the end of Johnsville's growth. Owners were reluctant to rebuild, and rebuilding was piecemeal and slow. In 1909 the mine changed hands again and resumed operations as the Plumas-Eureka Mining Company. 1910 was spent repairing the mill by reconstructing the 20 stamps installed by the previous owner, and adding another 20 stamps for a total of 40. The Eureka tunnel was reopened. At this time, the main tunnel was 5,500 feet long and there were many miles of development work in adits, winzes, stopes, and inclines, 7 to 8 miles of which were considered to be in working shape. Only about $105,000 was produced during this period during which the mine was operated in an inefficient and "haphazard" manner. Nonetheless, in 1912 the Plumas-Eureka was second only to the neighboring Jamison mine in gold output in Plumas County.

Mining ceased in 1914. Plans were made to start a new tunnel to open up the property at considerable depth below the former workings, however, the company went bankrupt in August 1915. Reorganized as the Plumas-Eureka Corporation, work was resumed in 1916. A raise was started from the face of the 7,300-foot Eureka tunnel, the intention being to crosscut for a distance of 1100 ft and connect with the old Seventy Six workings above. The almost vertical raise was driven 534 feet in quartz porphyry. The raise intersected the Mohawk vein 358 feet south of any point previously worked on this vein so a drift was driven along the Mohawk Flat Floor vein to connect with the old workings. It never reached the intended Seventy-Six workings. During the next 8 years, the property was operated for an average of 2 months/year, and the mill was run for only a few week periods to test ore samples. No new ore bodies were developed. During the war years, the mine was hardly operated at all due to the labor shortage and increased supply prices. In 1925, the mine passed into the hands of trustees in Boston and remained idle for nearly a decade. A new company, Plumas-Eureka Mines, Inc., was formed and in 1934, power shovel mining was carried out on the gravel and vein float on the mountainside. The broken rock was hauled by truck to a screening and washing plant. It is reported that 50 tons of this material yielded 21 ounces of gold, 17 ounces of silver, and 2939 pounds lead. Other reports indicate that approximately 300 cubic yards per day were being exploited at a yield of $0.70/yard. A new 20-stamp mill was started in 1936. The operators also had plans to build a dam at Eureka Lake to raise the water level 30 feet, and by means of tunnels and pipelines to hydraulic the remaining ores and mill the tailings. In 1940 the mine was leased to the Portola Corporation, which opened up the old Eureka and Mammoth Tunnels, but explored no new ground. A new ball mill, jaw crusher, classifier, and flotation cells were installed. A recovery of 95% was claimed on an 8-day run on old tailings. No other information is available on this operation. A Hydraulic Mining Permit was issued to Dodge Construction in May 1940, which hydraulically mined limited slopes on Eureka Peak until 1941. All equipment was removed and sold in 1942, and the California Debris Commission approved the mine's closure. From May - November 1943, the Plumas-Eureka tailings were worked under lease. Sixty-seven tons of tailings were reportedly shipped to a smelter from which 57 ounces of gold, 55 ounces of silver, and 4,141 pounds of lead were recovered. When mining at Plumas-Eureka ceased in 1943, it left behind the most complete mining community in the state. Recognizing the historical value and state of preservation, the State of California established the Plumas-Eureka State Park in 1959. Mineral rights to the deposit were not determined during research for this record.

The Plumas-Eureka Mine consists of a consolidation of several early mines discovered in 1851 on the east flank of Eureka Peak (formerly Gold Peak) and operated independently until 1871 when the claims were purchased by the Sierra Buttes Mining Company, Ltd. (SBMC) of London, England. Previously known as the Eureka, Rough and Ready, Seventy-Six, and Mammoth Claims, SBMC consolidated and operated them under the banner of the Plumas-Eureka Mine. At the height of its operation in the 1890s, the Plumas-Eureka Mine property comprised approximately 2,500 acres, which included 788 acres of prime timber and water rights essential to the operation of the mine. In the 1890s, gold-bearing quartz deposits of the same lode were also found on a property approximately one mile south and adjoining the property of SBMC's Plumas-Eureka Mine. These outcrops were developed independently as the Jamison Mine by the Jamison Mining Company.

The location point selected for latitude and longitude is the main Eureka Tunnel adit symbol on the USGS Johnsville 7.5-minute quadrangle. The mine is located within the Plumas-Eureka State Park, which is owned and operated by the California Department of Parks and Recreation. Access is via County Highway A14 (Graeagle-Johnsville Road) approximately four miles southwest of Mohawk, California, to the Plumas-Eureka State Park Headquarters and Museum. The Mohawk Mill and Eureka Tunnel are located about 500 feet west of the headquarters.

Mineralization is a vein Au deposit (Model code 273 USGS model code 36a, Low-sulfide Au-quartz vein Mark3 model number 27), hosted in Late Devonian gabbro, porphyry, tuff and rhyolite of the Sierra Buttes formation. Mineralization occurs as ore shoots within mesothermal gold-bearing quartz veins in roughly parallel fracture zones. Gold concentrations increase at vein pinches and where veins flatten or roll over. Local rocks include Paleozoic metavolcanic rocks, unit 2 (Northeastern Sierra Nevada).

Comments on the geologic information:

The Plumas-Eureka Mine is located in the Johnsville Mining District in south central Plumas County. The district is both a lode and placer gold district at the north end of a belt of mineralization extending about 20 miles southward into Sierra County and including the Sierra Buttes Mine ($17-$20 million produced) at its south end. The district is noted for varied exposures of slate, schist, quartzite, metadacite, quartz porphyry, greenstone, and numerous mafic and felsic intrusives. Portions of the district are overlain by Tertiary andesite and/or glacial debris. The Plumas-Eureka Mine has the distinction of being one of the oldest lode gold mines in California as well as being the preeminent and most profitable gold mine in Plumas County. Operations were conducted from 1851 until the 1940s. REGIONAL SETTING The northern Sierra Nevada is home to numerous placer and lode gold deposits and includes many of the more famous gold mining districts such as Grass Valley, Nevada City, Allegheny, Downieville, Eureka, Rich Bar, La Porte, and Johnsville districts. The geological and historical diversity of most of these deposits is covered in numerous publications produced over the years by the U.S. Bureau of Mines, U. S. Geological Survey, California Division of Mines and Geology (now California Geological Survey), and others. Additional information on the geology and history of the Plumas-Eureka area is contained in publications and exhibits available at Plumas-Eureka State Park and in the archives of the Plumas County Museum. The most recent geologic mapping in the area is a 15-minute scale compilation of the Blairsden Quadrangle by Grose (2000). Stratigraphy The northern Sierra Nevada has a history of both oceanic and continental margin tectonics recorded in sequences of oceanic volcanism, near-continental volcanism, and continental volcanism that can be divided into four major lithotectonic belts. These belts are the Western Belt, Central Belt, Feather River Peridotite Belt, and Eastern Belt (Day, 1988). The Western Belt is composed of the Smartville Complex, a late Jurassic volcanic arc complex (Beard and Day, 1987). Rocks are basaltic to intermediate pillow lavas overlain by pyroclastic and volcaniclastic rock units with diabase, metagabbro, and gabbro-diorite intrusives. This belt is bounded to the north and east by the Big Bend-Wolf Creek Fault Zone. To the west, it is unconformably overlain by the Cretaceous Great Valley sequence. The Central Belt is bounded to the west by the Big Bend-Wolf Creek Fault Zone and to the east by the Rich Bar and Goodyears Creek faults. The Central Belt is structurally and stratigraphically complex and consists of metamorphosed Mississippian - Triassic chert, argillite, phyllitic argillite, ophiolite, and greenstone of marine origin. It is equivalent to the Calaveras Complex of central and southern Mother Lode of the Sierra Nevada. The Feather River Peridotite Belt is also fault bounded separating highly deformed Central Belt from the rocks of the Eastern Belt for almost 95 miles along the strike of the northern Sierra Nevada (Day, 1988). The Feather River peridotite belt consists largely of Devonian to Triassic serpentinized peridotite and dunite.

The Eastern Belt, more commonly called the Northern Sierra Terrane, underlies the Johnsville District and is separated from the Feather River Peridotite Belt to the west by the Melones Fault Zone. To the east are the Mesozoic plutonic granitic rocks of the Sierra Nevada Batholith. The Northern Sierra Terrane is primarily composed of Devonian to Jurassic metavolcanic rocks and the earlier Shoo Fly Complex. The Shoo Fly Complex is a sequence of siliclastic metasedimentary basement rocks of continental origin. The dominant provenance of Shoo Fly sedimentation was the North American continental crust. The Shoo Fly Complex also contains shale- matrix melanges containing serpentinite, limestone, phyllite, and chert that developed in a trench setting. Unconformably overlying the Shoo Fly Complex is a series of three superimposed upper Devonian to Jurassic island- arc sequences (Brooks, 2000). The earliest volcanism is recorded in the upper Devonian - Pennsylvanian (?) Taylorsville Sequence, which includes the Grizzly, Sierra Buttes, Taylor, and Peale Formations. Later episodes of volcanism are preserved in the Permian - Triassic Arlington, Goodhue, and Reeves Formations and the Jurassic volcanic rocks of the Sailor Canyon Formation. In the area of the Plumas-Eureka Mine, only the older rocks of the Shoo Fly Complex, Sierra Buttes Formation, and Taylor Formation are exposed where not obscured by extrusive Tertiary volcanic rocks or later Quaternary alluvium. Rocks of the Peale Formation and later island-arc sequences occur farther northeast. The Taylorsville Sequence consists of submarine volcanic and volcaniclastic rocks and radiolarian chert of the Grizzly, Sierra Buttes, and the lower member of the Peale Formation. The Grizzly Formation is present only locally along the unconformity between the Shoo Fly Complex and the Sierra Buttes Formation. Near its type location, the Sierra Buttes Formation consists of four major units. The lower three units are primarily rhyolitic to dacitic in composition. The upper unit is primarily andesitic. All four units contain thick, discontinuous beds of lapilli tuff, tuff breccia, tuffaceous turbidites, and argillite with interbeds of carbonaceous and phosphatic chert, and laminated tuff (Hanson, 1983) intruded by rocks ranging from gabbro to rhyolite. Hanson interpreted these deposits to be submarine mass-flow deposits produced by eruption or slumping of near-vent accumulations within a volcanic island-arc sequence. The Sierra Buttes Formation is conformably overlain by andesitic submarine volcanic and volcaniclastic rocks of the Taylor Formation. It consists primarily of andesite breccia, massive and pillowed andesite and basaltic andesite flows, and andesitic volcaniclastic turbidites. Overlying the Taylor Formation is the Peale Formation, which consists of a lower volcanic-volcaniclastic member and an upper chert member. The lower member contains massive trachytic to quartz latite flows, tuff breccia, tuff, tuffaceous siltstone, and sandstone (Harwood, 1988) and grades upward into siltstone, sandstone, and limestone. The lower member grades abruptly into a thin bedded black, red, green, and gray radiolarian chert member that serves as a widespread marker unit in the region. The Peale Formation records the waning stages of Late Devonian and Early Mississipian submarine arc volcanism (Grose, 2000). The chert member, which includes bedded radiolarian chert and other pelagic and siliceous sediments, reflects regional subsidence of the volcanic arc to abyssal ocean depths.

Regionally, the northern Sierra basement rocks are overlain by a series of Tertiary volcanic and sedimentary rocks of mostly Miocene and Pliocene age. The oldest of the Tertiary units in the area is a basal conglomeratic unit commonly referred to as Eocene auriferous gravels. The gravels are preserved in paleochannels eroded in the basement surface. In contrast to the earlier volcanism, Tertiary volcanism was continental and deposited on top of the eroded island arc rocks and Mesozoic intrusives. As many as eight different volcanic units blanket large portions of the area ranging from the lower Lovejoy basalt to widespread andesitic flows and breccias. The youngest deposits include Quaternary sediments. Extensive beds of Quaternary alluvium and Quaternary morainal and glaciofluvial debris blanket much of the region. Structure Most upper Jurassic and earlier rocks of the northern Sierra Nevada were metamorphosed and deformed during the Jurassic-Cretaceous Nevadan Orogeny (Clark, 1960), the primary expression of the Jurassic amalgamation of oceanic terranes with the Mesozoic continental margin (Moores, 1972; Saleeby, 1981; Schweickert and Cowen, 1975). The dominant northwest trending structural grain of the Northern Sierra Terrane is a result of this period of compressive deformation, which produced east-directed thrust faults, major northwest-trending folds, and regional greenschist facies metamorphism (Harwood, 1988). This episode also resulted in many of the intrusions of granitic plutons that formed the Sierra Nevada. However, all the main lithotectonic belts of the northern Sierra Nevada also contain rocks that have undergone significant pre-Nevadan deformation and metamorphism. While the nature and extent of pre-Nevadan deformation is poorly understood, the principal expression of this earlier deformation is a cleavage or metamorphic foliation that dips steeply and strikes north-northwest (Day, 1988). Nevadan deformation structures within the northern Sierra Nevada lithotectonic blocks are steeply dipping northwesterly trending faults and folds. These faults are best developed in the Eastern, Central, and Feather River Peridotite Belts, where they have been collectively described as the "Foothills Fault System" (Clark, 1960). They deform upper Jurassic rocks and are truncated by uppermost Jurassic and Cretaceous plutons. Where the attitude can be determined, most of the bounding faults dip steeply east and display reverse displacement. Significant structural deformation in the area was absent from the end of the Cretaceous to the Pliocene or Quaternary. Two fault zones of Pliocene(?) and Quaternary age occur in the area. The Mohawk Valley Fault Zone, which lies to the east of the Plumas-Eureka Mine, trends north-northwest, is normal and down to the west, and suggests over 1,000 feet of dip-slip displacement with a minor right slip component (Jennings, 1994). The Mohawk Valley Fault Zone has been ascribed as the eastern front of the Sierra Nevada, but more likely represents a zone within the Sierra Nevada that is transitional to the Basin and Range province (Grose, 2000). The Grizzly Valley Fault Zone is northeast of the Mohawk Valley Fault Zone and strikes northwesterly. About a mile wide, the zone is composed of down to the west, left-stepping, high-angle normal faults with estimated dip- slip displacement of a few hundred feet. Metallogeny The northern Sierra Nevada harbors many individual mining districts, each known for important deposits of lode and/or placer gold. Lode gold occurs primarily as native-gold ore shoots within hydrothermal quartz veins and, to a lesser degree, in low-grade altered wall rocks.

Most significant exposed deposits have probably been discovered given the intense scrutiny the area was subjected to during the latyer half of the 19th century. However, there are undoubtedly undiscovered deposits in the region. Deposits might exist where the veins do not crop out or where surface exposures do not reflect mineralized ore shoots at depth. Zones of barren quartz commonly separate known ore shoots within veins. More likely would be deposits concealed under the extensive covers of Tertiary volcanic rocks and Quaternary alluvium and glacial debris. In regards to the Plumas-Eureka Mine, additional low-grade ore and possibly high-grade shoots likely remain in the deeper reaches of the mine that were not exploited prior to the mine's closure. Since the mine and adjacent properties are now a State Park, further exploration or development is highly unlikely. GEOLOGY AT THE PLUMAS-EUREKA MINE A variety of Ordovician to Jurassic volcanic and sedimentary rocks comprising the lower sections of the Northern Sierra Terrane are exposed in the area of the Plumas-Eureka Mine. Rocks are metamorphosed and exhibit extreme schistosity and foliation. Rocks of the Shoo Fly Complex and portions of the lower Taylorsville Sequence including the Sierra Buttes Formation and Taylorsville Formation are exposed in a north-northwesterly trend. Rocks of the overlying Peale Formation are not exposed, and if present, are obscured by Quaternary glacial morainal material, alluvium, or Tertiary volcanic rocks. The west flank of Eureka Peak is composed of the Sierra City Melange Unit of the Shoo Fly Complex. This unit represents the structurally highest of four thrust blocks comprising the Shoo Fly subduction complex and is composed of blocks of serpentinite, gabbro, basalt, sandstone, limestone, mudstone, and chert within a sheared sandstone and shale matrix. Comprising Eureka Peak itself and the eastern flank of the mountain are rocks of the Sierra Buttes Formation, which consist locally of submarine rhyolitic to andesitic tuff interbedded with lenses of black phosphatic radiolarian chert and siliceous argillite. In the upper part, andesitic tuff and tuff breccia prevail. The Sierra Buttes Formation is highly disrupted by intrusions of gabbro, pyroxenite, quartz porphyry, and diorite. The principal intrusive is gabbro, which forms most of the exposed east slope of Eureka Peak and carries many of the veins. Pyroxenite forms most of the northeast slope. Elongate inclusions of quartz porphyry are often found within the gabbro and are commonly associated with the veins. East and north of Eureka Peak, are two extensive glacial moraines and related alluvium that cover the Sierra Buttes Formation to considerable depth. The Plumas-Eureka ore bodies consist of a complex system of at least five main fissure-filling hydrothermal quartz veins much like the typical gold-bearing quartz veins of the California Mother Lode of the Sierra Nevada foothills. Veins trend northeast-southwest, are roughly parallel with divergent dips, and cut the various lithologies in the Sierra Buttes Formation primarily, quartz porphyries, gabbro, and argillite. The principal Eureka Vein strikes northeast-southwest and dips 75? northwest. It varies from 2-40 feet wide, but averages six feet thick. It is more regular in its course than many of the other veins. For a good portion of its length the vein is at or near the contact between quartz porphyry and gabbro. This vein is the original outcropping vein discovered in 1851.

Other important veins include the Seventy-Six vein, which can be traced for 2400 feet towards the south and dips from 10 to 25 ? to the east, southeast and south; the Rough and Ready vein, which strikes a few degrees west of north and dips 71 degrees east; and the Jamison vein, which averages six feet thick, trends north 20? west, and dips 35-40? west. The Jamison vein also appears as a primary ore body in the neighboring Jamison Mine. All the veins vary in thickness throughout their lengths, from a few inches to more than 20 feet. Commonly from 2 - 6 feet of ore is found between the walls. One of the noteworthy characteristics of many veins is their sharp definition, with unbroken walls being traceable from one end of the vein to the other. In these cases, there is little waste found in the ore due to the regular and clean nature of the walls. An unusual phenomenon exhibited by some of the veins is their tendency to roll over, flatten, and reverse their dip taking on an anticlinal configuration. While this is more common in the neighboring Jamison Mine, this dip reversal also occurs in the Plumas-Eureka Mine within the Eureka Flat vein, Mohawk Flat Floor vein, and the Seventy-Six Flat vein, and Jamison vein. Higher gold values typically accompany this flattening. Ore is free-milling quartz vein material being about 0.814 fine (ore values consist mainly of gold with little silver). The gold is finely divided. A number of high-grade pockets were taken from the surface in the early days. The main sections of veins are commonly low-value, milky bull quartz. Gold values are generally lean in this material and increase along the vein walls. Occasionally, rich ore shoots are found within the veins. Gouge material sometimes occurs on the foot and hanging walls and also carries values. Sulfide content is variable, commonly consisting of abundant pyrite and varying amounts of galena, chalcopyrite, sphalerite, and arsenopyrite. Generally, sulfide content ranges from a trace to 1.5%, but where veins pinch, sulfides can run as high as 30%. Based on available information, sulfide mineralization was confined to the quartz veins and thin bordering gouge zones. No reference to or indication of sulfide mineralization or alteration of the wall rock is available. Sulfides were treated with chlorination until the early 1900s, after which they were treated by cyanidization.

Workings include surface and underground openings. •The workings of the Plumas-Eureka Mine are confined to the east side of Eureka Peak from near its summit to its base on the banks of Jamison Creek. The complexity of the many veins, offshoots, and flat veins make a description of the mine workings impossible without the aid of maps. Further, many of the details of early and later workings conflict, or are absent. Generally, however, five main tunnels were opened up over the years on the main Eureka vein, these being the Eureka, Mammoth, Mohawk, Railroad, and Cropping Hills tunnels. They vary in length from 7,300 to 450 feet in length, the respective lengths of the Eureka (lowermost) and Cropping Hill tunnels (uppermost). The upper tunnels are the Cropping Hill and Railroad Tunnels located between 1,200 and 1,500 feet above Jamison Creek in the valley below. Almost 900 and 600 feet above the valley floor are the Mammoth and Mohawk tunnels, respectively. The main Eureka Tunnel is at the base of the mountain near the Mohawk Mill. Including these tunnels, some 62 miles of underground workings are said to have been driven by the time the Sierra Buttes Mining Company ceased operations in 1902. Unlike conventional mining methods, the ore bodies of the Plumas-Eureka were exploited from the top down. This is largely in part due to the early 1851 discovery of the Eureka vein outcrop by the Eureka Company and the competing operations of the Rough and Ready, Mammoth, and Seventy-Six mines located adjacent to the original Eureka claims. These mines operated independently for over 20 years, each exploiting its own claims, until they were consolidated and operated as the Plumas-Eureka Mine by the Sierra Buttes Mining Company in 1873. While the Sierra Buttes Mining Company did open the Eureka Tunnel at the base of the mountain, they never made a concerted effort to connect all the overlying workings in a systematic manner and block out all the ore. SBMC did, however, sink two winzes 200 feet deep below the Eureka tunnel and drive a drift on the vein blocking out 115,000 tons of ore that was never removed. As late as 1910, it was recognized that considerable ore remained in the Eureka vein below the lowest working levels in the Eureka Tunnel. On more than one occasion, the Johnson Graham Mining Company and Plumas-Eureka Mines Inc. planned to open a new and deeper tunnel to access and stope the overlying ore, but the work was never completed. At present on Eureka Peak, physical access to all known workings that daylight is restricted by cyclone fencing, which was completed last year.

Free-milling gold-bearing quartz veins with auriferous sulfides. Gold averaged 0.814 fine with secondary silver. Ore averaged 0.5 to 1.45 ounces/ton, with richer pockets near the surface. Sulfides included abundant pyrite and varying amounts of galena, chalcopyrite, sphalerite, and arsenopyrite. Sulfide content ranged from a trace to 1.5%, but reached as much 30% where veins pinched.

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