Empire - North Star Mine

The Empire - North Star Mine is a gold mine located in Nevada county, California at an elevation of 2,707 feet.

About the MRDS Data:

All mine locations were obtained from the USGS Mineral Resources Data System. The locations and other information in this database have not been verified for accuracy. It should be assumed that all mines are on private property.

Mine Info

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Site Name

Primary: Empire - North Star Mine

Commodity

Primary: Gold
Secondary: Silver

Location

State: California
County: Nevada
District: Grass Valley

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: Nevada County Planning Dept.

Holdings

Not available

Workings

Not available

Ownership

Owner Name: Minerals: Newmont Mining Corp.

Owner Name: Surface: Calif. Dept. of Parks and Recreation

Production

Not available

Deposit

Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Operation Type: Underground
Discovery Year: 1850
Years of Production:
Organization:
Significant: Y

Physiography

Not available

Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

Orebody

Form: Tabular

Structure

Type: R
Description: Wolf Creek Fault Zone, Gillis Hill Fault, Melones Fault Zone

Alterations

Alteration Type: L
Alteration Text: Ankeritic, sericitic, and pyritic replacement of wall rocks adjacent to veins

Rocks

Name: Diabase
Role: Host
Age Type: Host Rock
Age Young: Mesozoic
Age Old: Paleozoic

Name: Granodiorite
Role: Host
Age Type: Host Rock
Age Young: Early Cretaceous

Analytical Data

Not available

Materials

Ore: Arsenopyrite
Ore: Gold
Ore: Pyrite
Ore: Galena
Gangue: Quartz
Gangue: Calcite
Gangue: Chalcopyrite
Gangue: Sphalerite

Comments

Comment (Geology): The Empire fissure is not a simple break that can be followed continuously on the strike and dip; rather, it is a zone of variable width and degree of shattering, within which the vein, as defined by the quartz filling, is confined. The complex vein structure often steps from one fracture zone to another. Width between main walls, the degree of shattering, the abundance of splits, and the amount and distribution of gouge are widely variable. The main walls of the vein are generally well defined and the average distance between them is between 3 and 6 feet. In some places the fracture narrows to less than 3 feet; elsewhere the distance between walls can be as much as 20 feet. The auriferous quartz vein averages 1.5 to 2 feet thick. The country rock between the main walls is moderately shattered. Gouge may be confined to one wall or to both walls or it may cross the vein material from wall to wall. Recurrent movement is recorded by gouge that cuts earlier quartz. The gouge itself is composed of ground country rock that has been replaced by sericite, carbonates, chlorite, and quartz. Quartz is the principal vein-filling mineral. In some places it fills the vein fracture from wall to wall; elsewhere it forms sheeted zones cementing fractured country rock between the main walls. Comb quartz textures predominate at the lower levels of the mine and both comb quartz and massive milky white quartz occur at the higher levels. Sheared and ribbon quartz are the most abundant ores, but coarse angular quartz breccias cemented by later quartz occur on many levels. Commonly three or four generations of vein filling can be identified by their intersection relations and associated gouge. Calcite and ankerite are widely distributed on both the upper and lower levels.. They are later than the quartz, with the calcite being younger than the ankerite. Pyrite is the principal vein sulfide with galena and sphalerite occurring in lesser amounts. Arsenopyrite is less common. Gold occurs in both the quartz and the sulfides.. It is more often found in sheared and ribbon quartz than in the comb and massive quartz. Pay shoots are irregularly distributed throughout the ore-bearing zone. The greatest barren zone in the mine was found from between the 1300 and 2100 levels in which practically no ore was encountered in 800 feet. In the sulfides, it fills minute cracks in the older fractured pyrite and appears to be contemporaneous with galena. The average gold content of the sulfides ranges from 2.5 to 10 ounces/ton. The average gold content of the ore from 1891-1928 was 0.56 ounces/ton. In the past, many pockets of high-grade ore were found, but in later years, most of the specimen gold of the Empire Mine came from the Pennsylvania vein. Newmont Vein Up until 1928, when Newmont Mining Co. acquired the Empire Mine, little exploration on the deep levels south of the Empire shaft had been done. Since then one of the most important new veins found was the Newmont vein. More development work has been done on the Newmont vein than any other south of the shaft. This vein strikes N 10? E and dips southeast. The vein is similar in structure and mineral content to others in the district. The vein fracture is complex with many changes in strike and dip. Though the quartz shows all the common textures, massive milky quartz and ribboned quartz are the most common. Ankerite is an abundant gangue mineral and in many places it cements brecciated quartz. Pyrite is the main sulfide and galena and sphalerite are less common. The vein ranges in width from a narrow seam to 3 feet.

Comment (Development): By 1930, the Empire mill and cyanide plant were again at full capacity as well as 50 of the 60 stamps at the North Star mill. An aerial tramway was constructed for transporting ore between the North Star and Empire allowing more flexibility in milling North Star ores. In 1937, the combined tonnage milled from the Empire, Pennsylvania, and North Star mines was 238,743 tons, with an average gold recovery of 0.449 ounces/ton. 1941 was a banner year for the Empire Star Mine. Despite a shortage of skilled miners, 248,300 tons of ore averaging 0.349 ounces per ton were produced and 25,641 feet of development work was completed. In the 1940's, the war forced the mine to shut down, but the Empire reopened briefly before mining operations in the Grass Valley District mines ended in 1956 due to a strike by the local labor union. Wage demands could not be met at the prevailing gold price, and ore reserves and quality were in decline. As a result, the plant equipment was dismantled and many buildings were torn down. An auction was held to dispose of usable equipment and the stamp mill was sold as scrap iron. At the time of the mine closure, the Empire-Star properties comprised 6,000 acres of mineral rights and about 4,000 acres of surface holdings. The workings went to an inclined depth of 11,007 feet with 367 miles of underground workings. During its one hundred years of operation the Empire-North Star Mine and its associated properties produced approximately 6 million ounces of gold worth over $135,000,000 to make it one of the richest gold-quartz mines in the world. After lying idle for years, in 1975, approximately 600 acres comprising the area of the original Empire Mine surface facilities were purchased by the California Department of Parks and Recreation from Newmont Mining Company for $1,250,000. Newmont retained the mineral rights below 100 feet in the event the reopening of the Empire Star Mine once again became feasible. The acquisition was designated the "Empire Mine State Historic Park." Today, visitors can see many of the mine's buildings, the owner's home and restored gardens, as well as the entrance to the underground workings. Visitors can also view a scale model of the entire mine's underground workings. This model is the actual working mine model maintained by the Empire Mine operators. HISTORY OF THE NORTH STAR MINE PRIOR TO ITS MERGER WITH THE EMPIRE MINE The North Star Mine was one of the three largest producers in the Grass Valley District. From the beginning of mining in 1851 through 1928, the last year of operations by the North Star Mines Co. (before it was merged with the Empire mine), it had produced over $29,000,000. Prior to its consolidation with the Empire Mine, the North Star comprised the original North Star Mine as well as the Massachusetts Hill, Gold Hill, New York Hill, New Rocky Bar, Granite Hill, Cincinnati Hill, and other mines. The discovery of what was to become known as the North Star vein was made by the Lavance brothers in 1851 and called the French lead on Lafayette Hill. Soon after, the Lavances and neighboring claims formed the Helvetia & Lafayette Gold Mining Company, which controlled 480 feet along the apex of the vein. In 1858, the Helvetia & Lafayette was sold at auction to satisfy a judgment for $8,000 to Edward McLaughlin. In February 1869, McLaughlin sold the property to J.C. Pascoe, Edward Coreman, and others for $15,000. These men also owned the North Star group, and in 1861 they formed a partnership called the North Star Quartz Mining Co. It is estimated the mine produced approximately $1,500,000 between the years 1851 and 1866 (Johnston, 1941). Between 1861 and 1866, this company bought many of the neighboring claims. By 1875, the North Star shaft had reached a depth of 1,200 feet on the incline. At this point, the ore decreased in value until it could no longer be profitably worked and the operation was idled.

Comment (Development): In 1884, the North Star Mining Co. was incorporated to take over the property of the North Star Gold Mining Co. and the mine was reopened. Between 1884 and 1894, the North Star shaft was sunk to the 2,300-foot level, and the North Star vein ore shoot that had been followed from the surface was stopped. Very rich rock was found on the 1800-foot level. Good ore was also found between the 1,900- and 2,100-foot levels, which made the yield in 1893 over $31/ton. This ore shoot alone produced approximately $5,250,000. At the 2,300-foot level, however, the quartz ceased and the vein below it became small and tight. It was then decided to sink a vertical shaft to intersect the vein at a depth of 4,000 feet on the dip. The new shaft, begun in 1897, cut the North Star vein at a vertical depth of 1,630 feet or at about 4,000 feet on the dip of the vein. Subsequent development showed the vein was wider and more extensive than in the upper levels, although the per-ton yield was lower. From 1900-1911, 182,000 tons of ore were stoped from the North Star vein, 18,000 tons of which averaged $37.50 per ton. From 1901 to the end of 1912, 43,192 feet of development work was done below the 2,700-foot level. These workings produced 635,028 tons of ore. In 1914, the central shaft was sunk to the 8,600-level or 3,700 feet vertically. A winze was sunk from the 8,600- level to a final depth of 11,007 feet on the incline. Development work on the 6,300-foot level disclosed a new vein, called the X vein or No. 1 vein, dipping southwest and conjugate to the North Star vein. Since the North Star vein was unproductive below the 5,300-foot level, development was begun on the new vein. Meanwhile a second new vein, the No. 2, which branched from the X vein, had been found.. An inclined winze was sunk from the 6,300-foot level on the No. 1 vein and had reached the 6,600-foot level in 1924, when a wholly new development plan was decided upon. The new plan involved 4 projects: 1) to sink the vertical central shaft, which bottomed at the 4,000-level, approximately 2,000 feet to intersect the No. 1 vein at the 8,600-foot level; 2) to continue sinking on the X vein until it was vertically under the central shaft; 3) to sink an inclined winze on the No. 2 vein, following a promising ore shoot, to the 8,600-foot level; 4) and to connect the vertical shaft with the bottoms of winzes 1 and 2 by drifts on the 8,600-foot level. This development plan was completed in 1927. During the two years of development 242,900 tons of ore worth $1,518,000 were produced. In 1928, a winze was sunk from the 8,600-foot level on a third new vein, called the No. 3, which lies in the footwall of the X vein and dips parallel to it. This winze bottomed at the 9,000-foot level, at a vertical depth of 3,700 feet. Other veins mined through the North Star shafts under the ownership of the North Star Mines Co. were the A or Alioth, New York Hill, Y, and Z veins. The spatial relationships if theses veins are shown in Johnson (1940, figure 48). In 1929, the Newmont Mining Co. purchased a controlling interest in the mine and commenced operating it in conjunction with their Empire Mine as the Empire-Star Mines Co., LTD.

Comment (Economic Factors): The Empire-North Star Mine and its associated properties have produced over $135,000,000.

Comment (Geology): REGIONAL GEOLOGY The Empire-North Star Mine is within the Grass Valley District, which is home to California's other largest underground gold mine, the Idaho-Maryland. The district is located in the northern portion of the Sierra Nevada Foothills Gold Belt. This belt averages 50 miles wide and extends for about 150 miles in a north-northwest orientation along the western slope of the Sierra Nevada range. The Foothills Gold Belt roughly coincides with the Foothills Metamorphic Belt, which can be subdivided into four major lithotectonic belts: Western Belt, Central Metamorphic Belt, Feather River Peridotite Belt, and Eastern Belt. The Grass Valley District lies within the Central Belt, where in the Grass Valley area it is marked by an 8-mile-wide north-trending assemblage of two accreted terranes that range from Late Triassic to Late Jurassic in age. The Central Belt is bounded on the east and west by regional-scale tectonic suture zones; the Wolf Creek Fault Zone on the west and the Gills Hill Fault/Melones Fault Zone on the east. The oldest rocks in the area are those of the Carboniferous-Triassic metasedimentary Calaveras Complex. Originally clastics, these rocks were converted to schistose or slaty rocks during the Late Paleozoic orogeny and locally into a contact-metamorphic biotite gneiss by intruded granodiorite during Late Mesozoic time. The slates of the Jurassic Mariposa Formation, which outcrop in a small part of the area, are relatively unaltered. Igneous rocks in the district include granodiorite, diabase, porphyrite, amphibolite schist, serpentinite, gabbro, diorite, quartz porphyry, and various dike rocks (Johnston, 1940). The veins of the Grass Valley and neighboring Nevada City districts are not connected with or continuations of the famous Mother Lode vein system to the south. The last veins of the Mother Lode end about 20 miles to the south. Also, the Grass Valley veins differ in general character from those of the Mother Lode. Generally, the Grass Valley veins are narrower and produce a higher-grade ore than those of the Mother Lode. The veins trend in two primary directions. One set trends N-S (dipping E or W), and the other trends E-W (dipping N or S). The major feature of the Grass Valley District is a body of Lower Cretaceous granodiorite and diabase five miles long from north to south and half a mile to two miles wide (probably the apex of a larger batholitic mass). It which is intruded into older sedimentary and igneous rocks, including diabase of the Mesozoic-Paleozoic Lake Combie Complex, and is itself cut by various dike rocks. Gold-quartz veins cut the granodiorite and diabase (and in some cases, serpentinite) throughout the district. Most of the veins strike generally north, parallel to the intrusive body, and display gentle dips averaging 35?. Others strike northwest, parallel to a diabase contact with the granodiorite. The veins fill minor thrust faults that occur within fracture zones of various width and degree of fracturing. The maximum measured reverse displacement is 20 feet (Johnston, 1940). In all veins, quartz is the principal vein material and occurs in four textural types: 1) Comb quartz that forms crustifications and lines vugs, 2) massive milky quartz with a granular texture that displays many sharp crystal faces and has not undergone deformation, 3) sheared quartz developed with little or no dilation of the vein fracture and commonly showing ribbon or shear-banding structures, and 4) brecciated quartz formed where vein movement dilated the interwall space (Johnston, 1940). Gold occurs in quartz and in sulfides, principally pyrite. Although specimen ore has been found, most ore from the district occurs as fine and coarse free-milling gold in ores averaging between 0.25 to 0.5 ounces per ton.

Comment (Development): By 1914, the Ophir shaft had reached a depth of 4600 feet with most of the development work and stoping being done on the 3000, 3400, 3700, 4200, and 4600 levels. Further sinking of the shaft was halted over litigation with the North Star Mine. The Empire shaft was extending under lands to which the North Star Mine had agricultural title and the North Star sued for invasion of territory and extraction of ore. The suit was settled out of court and the Empire continued development via a winze from the 4,600-foot level of the Ophir shaft to the 7,000 foot level. A new Wellman-Seaver-Morgan hoist operated by a 500-horsepower Westinghouse motor was installed. This hoist was designed for a 7500-foot depth with four-ton capacity skips. In 1915, an underground crosscut was driven from the 2,600-foot level of the Empire Mine to the Pennsylvania Mine, which combined the two mines and opened up new ore reserves. In the same year, the California Debris Commission issued a requirement for impoundment of mill tailings, which had previously been disposed of in adjacent Wolf Creek. As a result, the Empire constructed a 63-foot high by 835-foot long earthen dam to contain the tailings. In 1919, 20 more stamps were added to the Empire mill bringing the total to 80. The cyanide plant was also increased to a capacity of 400 tons per day. An electric railroad was completed in 1920 for ore haulage from the Pennsylvania Shaft to the Empire Mine. This allowed the Pennsylvania mill and cyanide plant to be shut down in favor of a central mill and cyanide plant at the Empire Mine. The Empire Mine and plant processed 142,000 tons of ore in 1920. By the early 1920s the effect of World War I on labor and material costs began to adversely affect the mine's operations and profitability. In 1924, the Empire entered into a purchase agreement for several important surrounding claims, including the Sultana Group, which included the Prescott Hill, Orleans, Osborn Hill, and other mines to the south of the Empire. The Empire also owned the Omaha, Lone Jack, and Homeward Bound, and Wisconsin mines, which were south of the North Star Mine and were inaccessible from the Empire workings. During 1927, development work continued with the notable achievement of the sinking of the main shaft to the 7,000-foot level (928 feet below sea level). On October 8, 1927, all mining at the Pennsylvania shaft was halted. The shaft was to remain in good repair and used to pump out the increasing inflow of water into the Empire Mine. At the same time, milling at the Empire was reduced by half, employing only 40 stamps. In 1929, Newmont Mining Company purchased the Empire Mine and other surrounding mines for $250,000. By then, the Empire and its associated claims had produced more than $35,000,000 worth of gold. On May 1, 1929, the Empire Mines and the North Star Mines Company were merged into a new company, the Empire Star Mines Company, LTD. All properties and facilities were conveyed to the new company. This merger consolidated over 3,700 acres of mineral rights under combined management. Newmont endeavored to join several mines and find all the gold veins that ran between them.

Comment (Development): Later in 1878, W.H. Bourn, Jr. (who had inherited the mine), who had questioned the earlier engineering report, formed the Original Empire Co. to work the Ophir vein, and by 1883, the mine was again profitable. In 1884, Bourn purchased the neighboring North Star Mine, which had been closed for several years. Under the management of John Hays Hammond, a bright young mining engineer, the North Star Mine once again became a major producer. Shortly thereafter, however, Bourn sold the North Star Mine to James D. Hague who represented wealthy eastern investors. Prior to this time, the Empire had used steam power, but in keeping with its policy of expansion, in 1886 the company installed water power, including Pelton wheels, flumes, and storage reservoirs, improved the surface plant, and increased the number of stamps from 20 to 40. In 1887, Mr. George Starr became the mine manager and under his supervision, the mine prospered. The 1890 introduction of compressed air drills greatly increased the rate at which exploratory and development work could be accomplished. In 1890, an extremely severe winter resulted in the flooding of the mine and operating delays, but by 1892, the main Ophir shaft had reached the 2,100 foot level on the incline. In 1893, George Starr resigned to go to South African gold mines, but returned in 1898 to find the mine again on the verge of exhaustion. He reassumed management upon the condition that $200,000 be made available for development work. A program of 600 feet of drifting a month was implemented, and a new plant and 40-stamp mill were constructed. This prospecting program was successful, and the mine again became profitable. Due to the discovery of rich ore, there was no need to spend any of the $200,000 allotted for improvements as the newly produced gold paid for all improvements and development work, as well as substantial dividends. Under Mr. Starr's management, profits increased and the Original Empire Co. became the Empire Mines & Investment Co. The mine became a model of efficiency, attracting geologists and engineers from the world over to view the latest technologies being used at the Empire. In 1910, the Empire Mine installed a new cyanide plant with a capacity of 150 tons per 24 hours. By then, the Empire Mine's plant and equipment were one of the best and most substantial installations in all of California (MacBoyle, 1919). The mine was equipped with a Union Iron Works hoist driven by two 10-foot Pelton wheels and capable of hoisting 750 tons per day from a depth of 5000 feet. In 1911, Bourn Jr., acquired an option to take over both the Pennsylvania Mine and the W.Y.O.D. mine, which proved to be a good investment. He equipped the Pennsylvania Mine with a new 20-stamp mill and a cyanide plant with a capacity of 28,000-30,0000 tons/year. By 1913, production from the Pennsylvania Mine was about equal to the Empire mine with double the profits. By 1915 the option was paid off and the Pennsylvania Mine became part of the Empire Mine. During 1913-1914, the Empire Mine upgraded its facilities, equipping the mill with an additional 20 stamps and remodeling the existing 40 stamps. This increased crushing capacity from 47,000 tons/year in 1912 to 85,000 tons/year in 1915. The mill was equipped with 1,575-pound stamps capable of crushing 5 tons of ore per day per stamp. The cyanide plant was also upgraded to treat the higher tonnage of sulfide concentrates.

Comment (Geology): An important structural feature in the district is a group of "crossing" vertical or steeply dipping fractures that strike northeast, about normal to the long axis of the granodiorite body. In places they are simple fractures; elsewhere they form sheeted fracture zones several feet wide. Some are tight, some are open and form watercourses, and few contain any quartz. Two main stages of primary or hypogene mineralization are recognized - 1) a hypothermal stage represented by one vein and one mineralized crossing, in which magnetite, pyrrhotite, pyrite, and specularite were deposited, and 2) a mesothermal stage, in which the gold quartz veins were formed. The mesothermal stage is further divided into two sub-stages - an older one, in which quartz is the principal gangue mineral, and a younger one, marked by the deposition of carbonates. Pyrite and arsenopyrite, deposited in the quartz stage, are the earliest sulfides of the gold-quartz veins. Sphalerite, chalcopyrite, and galena are somewhat later. No secondary or supergene minerals have been noted except limonite, calcite, and gypsum, which are being deposited in the oxidized zone. The distribution of gold in the ore shoots is extremely erratic and assays of adjacent vein samples commonly differ widely. Some ore shoots have a pitch length of several thousand feet, but most are much smaller. Adjacent to veins and crossing fractures, the wall rocks are generally highly altered. Ankerite, sericite, and pyrite have replaced the original rock-forming minerals. Lesser amounts of chlorite and epidote have been found. The wall rock has not been replaced by quartz. LOCAL GEOLOGY (EMPIRE MINE PORTION OF COMPLEX) In the upper part of most the mine, diabase and porphyrite form the wall rock. All wall rocks are altered near the veins. Sericite and carbonates have been introduced, and the extent to which they have replaced the original components of wall rocks is more or less dependent on the distance from the vein fracture, its size, and the amount of vein filling. At a depth of about 1,700 feet and throughout the lower part of the mine wallrock is granodiorite. In the upper levels of the northern part of the mine, slate of the Calaveras Complex is present. A crosscut on the 4,200 foot level enters serpentinite. All rocks are cut by aplite and basic dikes. Vertical or nearly vertical joints striking northeast are abundant throughout the mine. Generally, these crossings are closed and show no evidence of displacement. Less commonly they are open allowing the circulation of fluids. The principal veins are the Empire (Ophir), Newmont, and Rich Hill veins. Empire (Ophir) Vein The Empire (Ophir) vein is an extremely persistent vein that has been followed down dip for 7,000 feet and along strike for 5,000 feet. Although the average strike is about due north, segments strike northeast and northwest. The average dip on the vein is 35? to the west, but segments of the vein have both steeper and gentler dips.

Comment (Development): The principal vein in the Empire Mine is the Ophir vein, which was discovered in 1850, soon after the initial discovery of gold-earing quartz at Gold Hill. The original Ophir claim measured only 30 by 40 feet. In 1850, the Ophir claim and a few adjoining claims were sold to Woodbury & Parks, owners of a local custom mill, and in 1854, the Empire Mining Co. was incorporated to work the claims. Production between 1850 and 1854 was estimated at 15,000 ounces worth approximately $300,000. In 1864, the Empire Mine was sold to Captain S.W. Lee and A.H. Houton. The mine prospered, and the new owners added numerous improvements including a new $200,000 surface plant and a 30-stamp mill, which was considered to be the finest in the state. At this time, the quartz-mining industry in the Grass Valley District was booming, and by 1867 there were 30 quartz mills in operation and 1,600 men employed in the industry. In 1867, a half interest in the mine was sold to a group of San Francisco investors including Lake, Cronise, Horner, and others. This marked a turning point in the history of the Empire Mine with ownership going to individuals with sufficient resources to introduce the best mining equipment and best engineers to manage mine operations. In 1869, William Bowers Bourn, a San Francisco capitalist, who had been acquiring Empire stock for several years, took over control of the mine. The mill was destroyed by fire in 1870 and was replaced with a new 20-stamp mill. By 1878, with the mine bottomed at the 1,200 foot level, all the visible ore had been exhausted. An engineering report suggested that the mine was too deep for profitable working and recommended closure. Based on this report, the Ophir shaft was allowed to fill with water and work was begun on the Rich Hill vein.

Comment (Commodity): Ore Materials: Free milling coarse and fine gold in quartz. Auriferous pyrite, galena, and arsenopyrite.

Comment (Commodity): Gangue Materials: Quartz, calcite, chalcopyrite, sphalerite

Comment (Location): The location point selected for latitude and longitude represents the Empire Mine main shaft as shown on the USGS Grass Valley 7.5-minute quadrangle. The Empire-Star Mine is located 24 miles north of Auburn. Take State Highway 49 to the Empire Street exit in Grass Valley. It is located in Grass Valley at 10791 East Empire Street.

Comment (Identification): Empire Mine was one of the first -- and eventually the largest and most productive -- hard-rock mining operations in California. In existence for more than 100 years, some 5.8 million ounces were eventually produced by the mine before its closure in 1956. During its history, the Empire Mine acquired most of the significant neighboring claims and mines including the Pennsylvania, W.Y.O.D., Houston Hill, Massachusetts Hill, Osborn Hill, Prescott Hill, Sultana, Omaha, Lone Jack, Homeward Bound, and Wisconsin mines, before finally acquiring the North Star Mine in 1929 and becoming the Empire-North Star Mine. Today the site is maintained by the California Department of Parks and Recreation as the Empire Mine State Park. The park contains many of the mine's buildings, the owner's home and restored gardens, as well as the entrance to 367 miles of abandoned and flooded mine workings.

Comment (Workings): Underground workings of the Empire Mine as of 1940 are described by Johnston (1940, pls. 35, 36, 37). A three-dimensional model of the entire Empire-North Star Mine underground workings is on display at the Visitor Center of the Empire Mine State Park. This model is the actual working mine model maintained by the Empire Mine operators and was highly confidential. During the mine?s operation, the model was hidden in the "Secret Room" (named for its blacked-out windows). Few people knew the room existed while the mine was in operation. Many of the underground workings in the North Star Mine are shown in Johnston (1940, plates 26-34)

Comment (Geology): X Vein The X vein (also called the No. 1 vein) strikes N 45? W and dips an average 35? southwest, conjugate to the North Star vein. The X vein extends up dip from the 8,600-foot level of the North star Mine to the 700-foot level of the Pennsylvania Mine and has been worked from both mines. It has also been worked through a crosscut from the Empire shaft at the 4600-foot level of the Empire mine. The vein lies wholly in granodiorite and is characterized by the abundance and persistence of the gouge and breccia within the walls of the vein fracture, necessitating in places, much closer timbering than is required in other veins in the granodiorite.. Extensive post-quartz movement is indicated by gouge seams that cross the quartz from wall to wall and by much crushed and broken quartz that has not been cemented by later vein materials. Recurrent movement is also indicated by younger gouge cutting older gouge. The width of the main vein fracture is highly variable, ranging from 1 foot to at least 30 feet and averaging between 5 and 10 feet. In the North Star Mine, all workings below the 6,000-foot level, amounting to 16, 300 feet of drifts were driven on the X vein. The ore shoots are unusually patchy, the best ore having come from the stope on the 6,900-foot level where the vein split into two well defined branches. Below the 7,800-foot level, little ore was found. Y Vein The Y vein strikes east-west and dips south at an average of 30?. Although the general strike is east-west, it has the form of an arc convex toward the hanging wall. The eastern part of the arc, where the largest and principal ore shoot occurs, strikes N 75? W and the eastern part of the arc strikes N 70? to 75? W. The greater part of the Y vein in the mine workings lies within diabase and porphyrite, although tongues of granodiorite are also cut. Everywhere the contacts are sharp and without transitional zones. Three ore shoots in the vein were mined prior to Newmont's acquisition of the mine. The largest shoot was on the west end of the vein where it had a pitch length of 1,100 feet and a breadth of 600 feet. Two smaller ore shoots were stoped on the eastern part of the vein between the 3,700- and 3,400-foot levels. Z Vein The Z vein strikes N 30? E and dips about 20? NW. It lies in the footwall of the Y vein and strikes at right angles to it. Although quartz as much as 12 inches thick occurs in the vein it was too poor in gold to mine. No.2 Vein The No.2 vein, like the Z and No. 3 vein, strikes north to northwest and dips and average 35? west. The wall rock is granodiorite, except on the 7200-foot level about 1700 feet south of the winze where the vein passes through 200 feet of diabase. The thickness of the vein averages 1 to 10 feet. Some gouge is always present, but its amount and distribution vary. In some places it is confined to one wall, in others it is on both walls. A partial measure of the displacement on the vein is shown by the offset of quartz-filled crossing. Displacement is about 20 feet. All types of quartz are present in the vein. The most conspicuous combs found anywhere in the district were found on the 6,000-level where they graded into massive white quartz. Vugs a foot or more in diameter occurred on the 6300-level. Commonly, the quartz shows evidence of shearing, and banded textures predominate.

Comment (Geology): LOCAL GEOLOGY (NORTH STAR MINE PORTION OF COMPLEX) Wall rocks in the North Star Mine include both granodiorite and diabase. The contact between the diabase and the granodiorite occurs near the 4,000-foot level in both the vertical and inclined shafts. The contact between unaltered granodiorite and the diabase complex is unusually sharp and shows no evidence of assimilation by the granodiorite. Close to the vein however, where both rocks have been extensively replaced by sericite and carbonates, the contact is less obvious. Three large granodiorite dikes between 30 and 50 feet thick cut the diabase complex. Smaller quartz porphyry dikes also cut the country rocks. The principal veins in the North Star Mine are the North Star, A, X, Y, and Z veins North Star Vein The North Star vein strikes northwest and dips northeast, but displays many irregularities in strike and dip. The dip of the vein ranges from 11? to 38? and averages 26?. The vein is not a simple fracture, but a fracture zone whose main walls dip between 11? and 38? and whose width between walls ranges form a few inches to more than 20 feet. Between the main walls, the country rock is broken and replaced by sericite and carbonate. The main quartz-vein filling commonly follows either the hanging wall or the footwall and in some places both. In places, the walls come together and the vein is represented by a narrow fracture occupied by a thin seam of gouge. Although gouge is usually confined to narrow seams on either wall, in places it occupies the entire width of the vein fracture, and fragments of the wall rock contained in it. The quartz in the North Star vein does not differ from that of the other veins. Dense milky white quartz makes up the greater part of the vein filling, but euhedral crystals and combs are present. In places the quartz is sheared and broken, and at least two generations are discernable. Calcite and ankerite, where present in the vein, are later than the quartz. Pyrite, chalcopyrite, sphalerite and galena are the sulfide minerals. The sulfides are usually broken with gold occurring along fractures in them as well as in the sheared and granulated quartz.. Gold from the North Star vein averages between 0.850 and 0.870 fine. The average value of ore mined from the North Star Mine up to 1929 was $11.35 per ton, with the average of the North Star vein slightly higher. The shape and extent of the ore shoots in the North Star vein are shown in Johnston (1940, Plate 26) The veins are cut by hundreds of crossings all striking to the northeast. Crossings range from simple fractures to fracture zones as much as 20 feet wide. A Vein The A, or Alioth, vein strikes north-south, dips west at an average angle of 25? and lies mainly in granodiorite. Roof pendants of diabase are cut at the south end of the 3,400- and 4,000-foot levels. To the north, the A vein has been cut off by the New York Hill vein and has not been found beyond it. Near the junction, the A vein flattens and numerous splits turn into the footwall of the other vein. There is considerable variation in the character of the vein fracture. In places, a simple fracture with little gouge contains quartz as much as 3 feet thick, whereas elsewhere the vein is very broken and forms a stockwork of interlacing fractures as much as 12 feet from wall to wall. Although sheared vein material is abundant, in general, the quartz is less broken by post-quartz vein movement than the quartz of the North Star vein. Vugs and carbonate-filled quartz combs are found on every level. Separate production figures for this vein are not available.

References

Reference (Deposit): Clark, W.B., 1970, Gold districts of California: California Division of Mines and Geology Bulletin 191, p. 53.

Reference (Deposit): Johnston, W.G., Jr., 1940, The gold quartz veins of Grass Valley, California: U.S. Geological Survey Professional Paper 194, 101 p.

Reference (Deposit): Koschmann, A.H., and Bergendahl, M.H., 1968, Gold-producing districts of the United States: U.S. Geological Survey Professional Paper 610, 283 p.

Reference (Deposit): Lindgren, W., 1896a, Geologic atlas of the United States - Nevada City Special Folio: U.S. Geological Survey Folio 29.

Reference (Deposit): Lindgren, W., 1896b, Gold-quartz veins of Nevada City and Grass Valley: Seventeenth Annual Report of the U.S. Geological Survey, Part 2, p. 1-262

Reference (Deposit): MacBoyle, E.M., 1919, Mines and mineral resources of Nevada County: Sixteenth Annual Report of the State Mineralogist, California State Mining Bureau, p. 1-270.

Reference (Deposit): McQuiston, F.W., 1986, Gold: The saga of the Empire Mine: 1850-1956: Empire Mine Park Association, Grass Valley, California, Blue Dolphin Pub., 95 p.

Reference (Deposit): Starr, G.W., 1900, The Empire Mine, past and present: Mineral and Scientific Press, vol. 81, p. 120, 152, 184-185.

Reference (Deposit): Additional information on the Empire - North Star Mines is contained in File No. 331-9342 (CGS Mineral Resources Files, Sacramento)

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"Where to Find Gold in California" looks at the density of modern placer mining claims along with historical gold mining locations and mining district descriptions to determine areas of high gold discovery potential in California. Read more: Where to Find Gold in California.