Union-Church Mine

Site Name

Primary: Union-Church Mine
Secondary: Springfield Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: El Dorado
District: El Dorado District

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: El Dorado County Planning Dept.

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

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

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular

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Structure

Type: L
Description: Melones Fault Zone

Type: R
Description: Melones Fault Zone

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Alterations

Not available

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Rocks

Name: Slate
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Pyrite
Gangue: Calcite
Gangue: Quartz
Gangue: Slate

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Comments

Comment (Geology): REGIONAL GEOLOGY The Union Church Mine area is within the Sierra Nevada foothills, where bedrock consists of north-trending tectonostratigraphic belts of metamorphosed sedimentary, volcanic, and intrusive rocks that range in age from late Paleozoic to Mesozoic. Locally, the Mesozoic rocks are capped by erosional remnants of a once extensive conglomerate and tuff of Tertiary age. The structural belts, which extend about 235 miles along the western side of the Sierra, are flanked to the east by the Sierra Nevada Batholith and to the west by sedimentary rocks of the Cretaceous and Jurassic Great Valley sequence. The structural belts are internally bounded by the Melones and Bear Mountains fault zones. These steeply dipping major faults, along with an unnamed normal fault that parallels Big Canyon Creek, separate diverse geologic terranes of oceanic crust and magmatic arc affinities. All the belts are characterized by extensive faulting, shearing, and folding (Earhart, 1988). In the El Dorado County area, gold deposits occur in the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced in the county. There has also been substantial gold produced from the West Belt and East Belt (Clark and Carlson, 1956). The West Belt consists of widely scattered gold deposits located west of the Mother Lode vein system. Gold occurs in irregular quartz veins in schist and granitic rocks, altered mafic rocks, and as gray ore in greenstone. The West Belt has been further divided by some authors into an eastern component composed of an ophiolitic melange and a western component composed of Jurassic rocks of the Copper Hill volcanics (Duffield and Sharp, 1975; Saleeby, 1982; Clark, 1964). The Copper Hill volcanics consist of mafic to felsic flows and pyroclastic rocks that are metamorphosed to greenschist and amphibolite facies. The Bear Mountains fault zone separates the melange from the Copper Hill volcanics. The Mother Lode Belt consists of the upper Jurassic Logtown Ridge and upper Jurassic Mariposa formations. The Logtown Ridge Formation consists of about 6,500 feet of volcanic and volcanic-sedimentary rocks of island arc affinity. These rocks are mostly basaltic and include flows, breccias, and a variety of layered pyroclastic rocks. The overlying Mariposa Formation contains a distal turbidite, hemipelagic sequence of black slate, graywacke, conglomerate, fine grained tuffaceous rocks, and subvolcanic intrusive rocks. The thickness of the Mariposa Formation is difficult to ascertain due to structural complexities, but is estimated to be about 2,600 feet thick at the Cosumnes River (Earhart, 1988). The Union-Church Mine is located within black slates of the Mariposa Formation. Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins that traverse western El Dorado County. The belt trends north through Nashville, northeast through Placerville, and northwest to Garden Valley. At Garden Valley, the Mother Lode Belt splits. The west branch extends northwest through Greenwood, and the east branch extends north through Georgetown to the Georgia Slide area (Busch, 2001). The Mother Lode veins are generally enclosed in Mariposa Formation slate with associated greenstone. The vein system ranges from a few hundred feet to a mile or more in width. Within the zone are numerous discontinuous or linked veins, which may be parallel, convergent, or en echelon. The veins commonly pinch and swell. Few can be traced more than a few thousand feet. Mother Lode type veins fill voids created within faults and fracture zones and consist of quartz, gold and associated sulfides, ankerite, calcite, chlorite, and sericite (Clark and Carlson, 1956).

Comment (Development): Much information regarding the Church Mine was reportedly destroyed in the 1906 San Francisco fire. However, early annual reports of the California State Mineraloguist indicate that by 1888, the Church Mine was worked through a 315-foot vertical shaft and a tunnel two hundred and forty feet long reaching a vertical depth of 80 feet at 140 feet from the mouth and connected to the surface by a shaft 65 feet deep. At this depth, the mine was producing about 20,000 gallons of water per day. Ore was wet crushed in a stamp mill, amalgamated in a battery, and on outside plates. The mill was powered by a 6 foot Pelton wheel and contained a rock breaker and ten 850 pound stamps capable of crushing 16 ? tons per day. Four thousand pounds of concentrates were produced monthly (Irelan, 1888). By 1896, the mine had been developed to a depth of 1,200 feet on the main Kidney vein. In 1907, the Church Mine was shut down. At the time of shut down, the property consisted of a three compartment shaft to a vertical depth of 1,350 feet with the vein developed on the 100, 200, 300, 350, 500, 600 700, 850, 1,000, and 1,200-foot levels. On the 1,200 foot level, there was a crosscut from shaft to the vein 670 feet east, from which point a winze was sunk on the vein to a depth of 1,400 feet. The 600 and 1,200 foot levels were run clearv to the property line. The vein was stoped out on ore shoots from 1,200 foot level to the surface (Tucker and Waring, 1916). There was a poor zone between 350 and 500 foot levels, where the ore was replaced by gouge, but ore came in at 500 feet and continued to 1300 feet. Heavy gouge was encountered at the 1,300 foot level and little prospecting was done below that. Ore milled down to 1,300 feet level averaged $17 a ton before heavy gouge was encountered. The gouge was so soft and broken it could be shoveled. It carried a value of about about $4 a ton. On the 1,400 foot level, they drifted 80 feet but the ground ran and swelled overnite and could not be held open. At total depth, the mine was producing 75,000 gallons of water a day. In 1941, the mine was reopened by the Madre de Oro Mining Company. The shaft was rehabilitated and a new surface plant was built and a 20-stamp mill with flotation erected. Reported production for 1941 indicates that 496 tons of ore were milled from which 169 ounces of gold were recovered. The mine was shut down in 1942 and has been idle ever since. Between 1953-1954, the El Dorado Chrome Company used the mill as a custom Chrome mill, and the North American Tungsten Company used the mill to treat tungsten ore that it trucked in from the Garnet Hill Mine in eastern Calaveras County.

Comment (Economic Factors): Although the Union Mine has been credited as being the largest producer in the county, there is little definite information concerning its production; total output of the property will probably never be known. Early production from between the 1850s to the 1860s was never recorded. Later operators in the late 1800s and 1900s were particularly secretive and never released actual production data. While total production is unknown, the Union Mine is thought to have produced about $5 million (Clark, 1970). The Church Mine is thought to have produced another $1 million (Clark, 1970).

Comment (Location): Location selected for latitude and longitude is the Union Mine symbol on the USGS 7.5 minute Placerville quadrangle

Comment (Commodity): Ore Materials: Free-milling gold, auriferous pyrite

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

Comment (Commodity): Commodity Info: Union Mine ore averaged $5-$8 per ton with high grade to $25 per ton. Church Mine ore averaged $17 per ton with high grade to $30 per ton. Sulfides comprised 1 1/2 - 3% of ore with concentrates averaging $150 per ton Gold was 822 fine

Comment (Workings): Union Mine The Union Mine was developed by the 2,000-foot main Springfield shaft, which cut the Poundstone vein at 1,200 feet and the McCosmic vein at about 1,540 feet. About 750 feet north is the 900-foot Clement shaft and 200 feet beyond is the 500-foot Klondyke shaft. Also, there are several crosscut adits to the west, one 700 feet long near the Springfield shaft and another to the north near the Klondyke shaft about 600 feet long. More detail of the subsurface workings are described by Logan (1938). Church Mine The Church Mine was developed by a 1,350-foot vertical shaft with crosscuts to the vein. Levels were at 100, 200, 300, 350, 500, 600, 700, 850, 1000, and 1,200 feet. On the 1,200-foot level, a 670-foot crosscut extended east to the vein, from which a winze was sunk on the vein to the 1,400-foot level. On the 600- and 1,200-foot levels, drifts extend north and south to the property line (Clark and Carlson, 1956):

Comment (Development): Union (Springfield) Mine Placer gold deposits were discovered in the vicinity of the Union-Church Mine in the early 1850s. The area quickly became a rich and populous early placer mining district called Aurum City. Primarily a Mexican settlement, it hosted a population of about 2,000 people. After the placers were depleted, quartz float was worked in arrastras. Early production is unknown, but is thought to be considerable. In the early 1860s, a noted mining authority by the name of Professor Silliman was attracted to the rich float ore and induced friends to finance quartz mining. Soon after, the Union shaft was sunk. In the 1860's, Mr. Silliman, consolidated the Union mine with the Church Mine, whose original shaft lay about one half mile north.. By 1868, the two properties had produced more than $600,000 (Logan, 1934). Despite the good ore grade, the project failed and the mine lay idle for several years until it was purchased by Hayward and Hobart in 1871 and the Union Mine was reactivated independently. Hayward and Hobart renamed the mine the Springfield Mine and operated it for about 15 years until about 1887 (Logan, 1934). Under Hayward and Hobart's ownership, the shaft (renamed the Springfield shaft) was sunk to a depth of 1,640 feet and the Clement Shaft to a depth of 900 feet on an incline of 70?. Over 21,000 feet of drifts and crosscuts were reported to have been run. Ore shoots had been stoped out from the 1,300 foot level to the surface. During this time, there was a 15-stamp water-powered mill with a capacity of 26 tons a day. Ore was worked by amalgamation in batteries and collection on outside plates. The sulfides, worth about $150/ton, were collected on Frue vanners and processed by chlorination. After being idle for about 10 years, the Union Gold Mining Company took over the mine in 1896 and built a 30 stamp mill. In 1897, a production of $36,000 was reported. From then until 1909, only estimates of production were made public. These indicate that from 20,000 to 40,000 tons of ore was crushed annually, yielding from $5-$7 per ton. In 1909, the last year for which a figure is at hand, 12,000 tons yielded a little over $5 a ton (Logan, 1934). In 1909, the mine was shut down. The Union Mine was again reopened in 1914-1916 and some prospecting was done, but the results were disappointing (Logan, 1934). Between 1916 and 1934, the mine was idle. In late 1934, Gold Fields American Development Company began dewatering the Springfield workings for the purpose of further prospecting, and the Springfield shaft was deepened to 2,000 feet, and the lower workings were rehabilitated. About 30 men were employed, and electric power line was built, and pumps were started at the end of July (Logan, 1934). In 1936, Gold Fields suspended operations and the Union Mine was turned over to Apex Mining Company. During 1936-37, the mine was operated by the Montezuma-Apex Mining Company and the ore was trucked to the Montezuma Apex mill at Nashville. During their lease, the Apex Mining Company mined some ore, which was hauled to their mill at the Montezuma mine and gave "good results" (Logan, 1934). In 1937, the mine was turned over to its owners. Between 1937-1940, mining was sporadic and inconsequential. The mine has been idle since 1940. Church Mine The Church Mine lies about 0.5 mile north of the Union Mine. Like the Union Mine, the Church Mine was first worked on a small scale about 1850. During the 1860s, it was consolidated with the Union Mine, but was later worked separately. By 1868, the combined Union and Church properties had produced more than $600,000.

Comment (Geology): Church Mine The Church Mine developed three veins (West vein, Kidney vein, and East vein) within the Mariposa slates about ? mile north of the Union Mine. Similar to the Union Mine, the veins generally strike NNE-SSW, dip 74? E, and average 5-10 feet wide. The principal and middle Kidney vein was named for a number of kidney shaped ore shoots that were developed between the surface and the 500-foot level. The vein averaged 6 feet wide. While the dip of the Kidney vein was steep near the surface and at depth, it flattened to about 45? E between 350 feet and the 500 foot level where the vein had been largely cut out and replaced by fault gouge. Below the 500 foot level it resumed a dip of 74? SE and its strike trended more NE-SW (Tucker and Waring, 1916). The main ore shoot from the 500 level to the 1,200 level was continuous, but below the 1,200 level, the grade decreased greatly, and at 1,300 feet heavy gouge was encountered and little prospecting was done below that. The best Kidney vein ore graded as high as $30/ton and was mined during the earliest shallow operations. Ore down to the 1,300 foot level averaged about $17/ ton. Values decreased to about $4/ton or less in the underlying gouge. Little is recorded about the East and West veins with the exception that the East vein is known to have been worked in the neighboring Union Mine, and the West vein is known to be quite low grade. Ore from both mines is typical Mother Lode free-milling gold quartz ore with little auriferous pyrite comprising 1.5-3% of the ore. Concentrates carried about 35% of the gold saved. A typical lot carried about 3 ounces of gold and 1 oz of silver per ton. Bullion was 822 fineness.

Comment (Identification): The Union-Church Mine is located in the famous Mother Lode Gold Belt in the Sierra Nevada foothills of western El Dorado County. The mine consists of two adjacent but separate mines (the Union and Church mines) that were worked independently between their discovery in the 1850s and the 1860s, then consolidated until 1871, after which they were again worked independently. Total production from the mines is unknown, but the Union Mine is considered the largest gold producer in El Dorado County with estimated production of $5 million. The Church Mine is thought to have produced about $1 million. The deposits consist of steeply dipping typical Mother Lode quartz veins carrying free-milling gold and auriferous pyrite. The principle producing veins in the Union Mine are the Poundstone, McCosmic, and Klondyke veins. The Kidney vein is the main producer in the Church Mine.

Comment (Deposit): The Union-Church Mine produced from typical Mother Lode type low-sulfide gold-quartz veins. Ore consisted of free gold and auriferous pyrite within discontinuous ore shoots. The shoots were contained in fracture-filling mesothermal quartz veins enclosed in hard black slates of the Upper Jurassic Mariposa Formation. Quartz ore from the Union Mine generally averaged $5-8 per ton, but high grade ore from the McCosmic vein ran as high as $25 per ton. In the Church Mine, ore is said to have averaged about $17 per ton to a depth of 1,300 feet with shallow high grade values of up to $30 per ton. Sulfides formed 1.5 - 3% of ore, and carried about 35% of the gold recovered. Concentrates averaged $150 per ton.

Comment (Geology): The Melones Fault Zone separates the Mother Lode Belt from the East Belt. The East Belt lies in the south central part of El Dorado County approximately 15 miles east of the Mother Lode belt. The East Belt traverses the county from the southern county line, north through Omo Ranch and Grizzly Flat, and apparently terminates near the Hazel Creek Mine east of Jenkinson Reservoir (Busch, 2001). The Eastern Belt is dominantly argillite, phyllite and phyllonite, and chert of Paleozoic age. The phyllite and phyllonite are dark to silvery gray. The chert is mostly thin bedded with phyllite partings. Other rocks in the Eastern Belt include a Jurassic granodiorite pluton near the Cosumnes River and small bodies of Jurassic serpentinite, gabbro, diorite, and limestone. The Paleozoic metasedimentary rocks of the Eastern Belt have been assigned to the Shoo Fly Complex by most investigators (Earhart, 1988). Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metamorphic rocks of the Shoo Fly Complex, or in granitic rocks. Most of the veins trend northward and dip steeply. An east-west set of intersecting faults may be a controlling factor in controlling deposition of ore. Ore deposits of the East Belt are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. Gold is usually associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite (Clark and Carlson, 1956) LOCAL GEOLOGY Union Mine The Union Mine developed five significant gold quartz veins within black slate of the Mariposa Formation. The principal producing veins were the Poundstone (East Gouge), McCosmic (West Gouge), and Klondyke veins. The Poundstone vein was the greatest producer, followed by the McCosmic vein about 200 feet to the west, and the Klondyke vein which was developed north of the main shaft. The veins strike approximately N 16? E and dips vary between 60-79? E (Logan, 1934). The larger veins are linked by a series of lesser veins striking northeast. The veins generally varied in thickness from 5-10 feet in width, but the Klondyke vein narrowed to only 4-5 inches wide at the surface. The veins steepened in the lowest workings and are thought to join at depth. A series of lesser veins lie to the west of the main veins. The Poundstone vein included several ore shoots, which were worked from the 1,300 foot level to the surface. Ore shoots generally ranged from 100-250 feet long, but one was said to have been 800 feet long (Logan, 1934). To the north, the Poundstone vein split into thin stringers. Ore from the Poundstone vein yielded about $8 a ton. One large Poundstone ore shoot is credited with production of $5.1 million. The Klondike vein is credited with $350,000 (Logan, 1926). Ore from the McCosmic vein yielded up to $25 a ton. Ore recovered from the shallower workings in early days was considerably richer.

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References

Reference (Deposit): Logan, C.A., 1938, Mineral resources of El Dorado County: California Division of Mines, 34th Report of the State Mineralogist, p. 251-253.

Reference (Deposit): Saleeby, J., 1982, Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development: Journal of Geophysical research, v. 87, n0. 8, p. 1803-1824.

Reference (Deposit): Tucker, W.B., and Waring, C.A., 1916, Mines and mineral resources of El Dorado, Placer, Sacramento, and Yuba counties: California State Mining Bureau, 15th Report of the State Mineralogist, p. 283-299.

Reference (Deposit): Additional information on the Union-Church Mine is contained in File No. 339-8859 (CGS Mineral Resources Files, Sacramento)

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 21-22, 42-43.

Reference (Deposit): Busch, L.L., 2001, Mineral land classification of El Dorado County, California: California Geological Survey Open-File Report 2000-03.

Reference (Deposit): Clark, L.D., 1964, Stratigraphy and structure of part of the western Sierra Nevada metamophic belt, California:

Reference (Deposit): U.S. Geological Survey Professional Paper 410, 70 p.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 117.

Reference (Deposit): Clark, W.B. and Carlson, D.W., 1956, Mines and mineral resources of El Dorado County: California Division of Mines, California Journal of Mines and Geology, v. 52, p. 413-429.

Reference (Deposit): Duffield, W.A., and Sharp, R.V., 1975, geology of the Sierra Foothills melange and adjacent areas, Amador County, California: U.S. Geological Survey Professional Paper 827, 30 p.

Reference (Deposit): Earhart, R.L., 1988, Geologic setting of gold occurrences in the Big Canyon area, El Dorado County, California: U.S. Geological Survey professional Paper 1576, 13 p.

Reference (Deposit): Irelan, W., Jr., 1887, El Dorado County, Springfield Mine: California State Mining Bureau, 6th Annual Report of the State Mineralogist, p. 43.

Reference (Deposit): Irelan, W., Jr., 1888, El Dorado County, Church Mine: California State Mining Bureau, 8th Annual Report of the State Mineralogist, p. 191-193.

Reference (Deposit): Logan, C.A., 1926, El Dorado County: California State Mining Bureau, 22nd Report of the State Mineralogist, p.413

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