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Idaho-Maryland Mine (Idaho Maryland prospect; Maryland Mine; Howard Hill/Morning Dew; Idaho Maryland-Brunswick), Idaho-Maryland group, Grass Valley, Grass Valley District, Nevada Co., California, USA

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Latitude & Longitude (WGS84): 39° 13' 25'' North , 121° 2' 15'' West
Latitude & Longitude (decimal): 39.22361,-121.03750
Köppen climate type:Csa : Hot-summer Mediterranean climate

A former lode Au-Ag-Pb-Zn-Cu occurrence/mine located in secs. 25 & 26, T16N, R8E, MDM, about 2.3 km ENE of Grass Valley, on private (patented) land. Discovered in 1865. Owned by the Emgold Mining Corp., 570 Granville St., Vancouver, British Columbia Canada VGC 3P1 (1983-2002>). MRDS database stated accuracy for this location is 100 meters. The location point selected by the USGS for latitude and longitude represents the Idaho-Maryland Mine main shaft as shown on the USGS Grass Valley 7.5-minute quadrangle. Adjoins the Eureka claim to the E.

Additional names which apply to this locality: Eureka Construction/Haphine/Tracy; South Idaho Consolidated; Schofield/Baby Consolidated/Black Hawk; Morning Dew/Rip Van Winkle; Gambler Gold and Silver/Morehouse.

The Idaho-Maryland is the second largest underground gold mine in California behind the Empire Mine. The mine was located in 1865. The first period of mining extended from 1867, when the Idaho Quartz Mining Company was organized, until 1893. In 1867, a shaft was sunk to 300 feet and encountered a rich pay shoot (the eastern extension of the Eureka -Idaho Ore Shoot). The Idaho Quartz Mining Company worked this same pay shoot stoping it for a distance of 3500 feet along strike until 1893 when the eastern limit of the Idaho claims were reached. Total output up to this time was $11,638,000, with ore ranging $12.76 to $35.00/ton and averaging $20.00/ton. Most of the gold came from the Eureka-Idaho shoot, which averaged 2.5 feet wide yielded 1 ounce/ton.

The Idaho Mine was developed by a shaft inclined at 70 degrees to the 1000-foot level and an inclined winze raking to the east, called the Canyon shaft, that bottomed at the 1600-foot level at a vertical depth of 2,180 feet.

In 1893, in settlement of a dispute over the eastward extension of the Eureka-Idaho shoot, the Maryland Company acquired the Idaho Quartz Mining Company for $85,000. From 1893 until 1901, the Maryland Company operated the mine and produced $1,250,000 in gold. A winze was sunk from the 1,600- to the 1,900-foot level. However, in 1894, a fire destroyed the hoist and the mine was flooded and workings below the 1,600-foot level were not reclaimed. In 1901, partly because of the poor condition of the workings, the mine was closed.

The mine remained idle until 1903 when it was bonded to the Idaho-Maryland Development Company, which worked it until 1914. The company succeeded only in reopening it to the 1600-foot level, and the $300,000 in gold they produced came mainly from old stopes and pillars left behind by the earlier operators.

A fourth period of activity extended from 1918 to 1925, when the mine was operated by the Metals Exploration Company. At a cost far exceeding the $500,000 in gold it produced, the main shaft was extended downward 1,000 feet to the 2,100-foot level at a vertical depth of 2,000 feet. Drifts on the 2,000- and 2,100-foot levels were driven from the main shaft, the Dorsey winze, and a new winze was sunk 850 feet below the 2,000-foot level. Failing to find a new ore shoot, the company suspended operations in 1925.

Control of the property changed hands in 1926 when Errol MacBoyle and Edwin Oliver created holdings that included the Idaho-Maryland, Brunswick, and Morehouse mines. Work resumed the same year under the Idaho- Maryland Mines Company. New ore was found and the mine entered a period of prosperity. During the three years 1930, 1931, and 1933, the mine produced $1,681,887. Total production for the period 1868 - 1933 was almost $16,000,000. From 1926 to 1942, the mine produced 650,000 ounces of gold from 1.1 million tons of ore. The mines were closed in 1942 due to enactment of the Federal War Production Board's Limitation Order L-208, but were reopened again in 1945. Production was hampered by depleted operating funds, rising costs, labor shortages, and negligible exploration. All mining ceased in 1957. At the time of closure, the mine was owned by Idaho-Maryland Industries.

In 1983, Emgold Mining Corp., through its subsidiary Emperor Gold (U.S.) Corp., obtained a lease and option to purchase all mineral rights formerly held by Idaho-Maryland Industries. From 1993 to 2000, Emgold spent $7,000,000 evaluating the Idaho-Maryland properties. The project was put on hold for 1.5 years while the lease and option were renegotiated. The revised Agreement includes a mining lease and option to purchase the property consisting of 2,750 acres of minerals and minerals rights and approximately 37 acres around the old New Brunswick shaft. The term of the lease is 5 years commencing on June 1, 2002. As of March, 2004, Emgold had conducted a boring and assessment program, but has not yet commenced mining operations.

Mineralization is a vein deposit (Mineral occurrence model information: Model code: 273; USGS model code: 36a; Deposit model name: Low-sulfide Au-quartz vein; Mark3 model number: 27) hosted in Early Cretaceous granodiorite and Mesozoic-Paleozoic diabase and serpentinite. The ore body is tabular and forms shoots, seams, stringers, ribbon-like structures and fissures. It strikes N77W and dips 70E at a thickness of 9.14 meters. The veins average 2½ to 3 feet in thickness with an average of 2% sulfides. The quartz indicates ribboned layers 1 inch (2.5 cm) thick. Intruded by granite at depth. Controls for ore emplacement included mineralization that occurs as erratic shoots within mesothermal gold-bearing quartz deposited within fracture zones. Local alteration included ankeritic, sericitic, and pyritic replacement of wall rocks adjacent to veins. Local rocks include Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains).

By far the most important vein in the Idaho-Maryland Mine was the Eureka-Idaho-Maryland vein. This vein strikes N 77? W and has an average dip of 70? SW, ranging between 50? and 80?. The hanging wall is composed of diabase and gabbro, and the footwall is serpentinite. All of the rocks are highly altered and contain abundant ankerite. Mariposite commonly occurs in the serpentinite.

The famous Eureka-Idaho ore shoot had a pitch length of almost 1 mile and a breadth of 500 to 1,000 feet. The width of the shoot averaged 2.5 feet but reached 8 feet in places. The average gold content in the Eureka-Idaho shoot was 1 ounce/ton. Most of the gold was free gold, and much specimen ore came from this famous shoot. Between 1-2% of the ore was sulfides, pyrite being the most abundant. Lesser amounts of galena, chalcopyrite, and sphalerite are present. The sulfides yielded between 5 and 20 ounces of Au per ton. A diagram of the Eureka-Idaho ore shoot is given in Johnston (1940, fig. 64).

Regional geologic structures include the Wolf Creek Fault Zone, Gillis Hill Fault and the Melones Fault Zone. Local structures include 3 faults that cut the veins. The diabase dike is 20 to 30 feet thick.

Workings include surface and underground openings with a length of 72,418.5 meters and an overall depth of 3,450 meters. They are comprised of vertical and inclined shafts, drifts, stopes, crossscuts, raises and 11,650 feet of diamond drill holes plus winzes. A detailed account of the underground workings of the Idaho-Maryland Mine is not available at this time.

Ore materials include free-milling coarse and fine gold in quartz (850 fine) plus auriferous pyrite and galena. Gangue materials include quartz, calcite, chalcopyrite and sphalerite.

Yearly production values between 1868 and 1893 are presented in MacBoyle (1919), page 187. Other yearly values are presented in Johnston (1940), page 95. Also in Logan, C.A. (1935); Wolfin, H. M. (1941) and Goodwin, Joseph Grant (1957).

Total gold production from the Idaho-Maryland group was estimated by Clark (1970) at about $70,000,000 (period values).

Mineral List

Mineral list contains entries from the region specified including sub-localities

16 valid minerals.

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on

145 - 201.3 Ma

ID: 2757888
Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains)

Age: Jurassic (145 - 201.3 Ma)

Stratigraphic Name: Copper Hill Volcanics; Gopher Ridge Volcanics; Logtown Ridge Formation; Mariposa Formation; Monte de Oro Formation; Oregon City Formation; Peaslee Creek Volcanics; Penon Blanco Formation; Brower Creek Volcanic Member; Smartville Complex

Description: Undivided Mesozoic volcanic and metavolcanic rocks. Andesite and rhyolite flow rocks, greenstone, volcanic breccia and other pyroclastic rocks; in part strongly metamorphosed. Includes volcanic rocks of Franciscan Complex: basaltic pillow lava, diabase, greenstone, and minor pyroclastic rocks.

Comments: Western Sierra Nevada and western Klamath Mountains. Mostly basaltic to andesitic breccias, flows, and tuffs, metamorphosed but with primary volcanic features generally recognizable. Minor associated sandstone and conglomerate. Largely or entirely of marine origin. Includes some rocks interpreted as ophiolites (Smartville complex)

Lithology: Major:{mafic volcanic}, Minor:{felsic volcanic}, Incidental:{chert, sandstone, conglomerate}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

Jurassic - Triassic
145 - 252.17 Ma

ID: 3189515
Mesozoic sedimentary and volcanic rocks

Age: Mesozoic (145 - 252.17 Ma)

Lithology: Mudstone-carbonate-sandstone-conglomerate

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Data and map coding provided by, used under Creative Commons Attribution 4.0 License

Localities in this Region


This page contains all mineral locality references listed on This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


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Lindgren, Waldemar (1896a), The gold-quartz veins of Nevada City and Grass Valley districts, California: USGS 17th. Annual Report, part 2: 1-262; […(abstract): A.I.M.E. Transactions: 14: 667-668 (1897-98); …(abstract): Zeitschr. Prakt. Geologie, Jahrg. 7: 210-213 (1899)]: `115.
Lindgren, Waldemar (1896b), Description of the Nevada City, California, special sheet: USGS Geol. Atlas, Nevada City special folio (Folio No. 29), 7pp.; […(abstract): Journal of Geology: 5: 409-411 (1897)].
MacBoyle, Errol (1919), Mines and mineral resources of Nevada County: California Journal of Mines and Geology, California Mining Bureau (Report 16): 185-191.
Logan, Clarence August (1930), Nevada County: California Mining Bureau. (Report 26): 26(2): 114-118.
Logan, C.A. (1935), Review of Gold Mining in East-Central California: California Journal of Mines and Geology (Report 31): 31(1): 15-16.
Johnston, William Drumm (1940), The gold-quartz veins of Grass Valley, California: USGS PP 194, 101 pp.: 94-96, figure 64.
Farmin, Rollin (1941), Host-rock inflation by veins and dikes at Grass Valley, California: Economic Geology: 36: 173.
Wolfin, H. M. (1941), Mineral Resources of Nevada County, California Journal of Mines and geology (Report 37): 37(3): 403-406.
Eric, J.C. (1948), Tabulation of Copper Deposits in California in: Copper in California: California Division of Mines Bulletin 144, part 3: 278.
Goodwin, Joseph Grant (1957), Lead and zinc in California. California Journal of Mines and Geology, Division of Mines (Report 53): 53(3&4): 580.
Koschman, A.H. and Bergendahl, M.H. (1968) Principal gold-producing districts of the United States. USGS Professional Paper 610, 283 pp.
Clark, Wm. B. (1970a) Gold districts of California: California Division Mines & Geology Bulletin 193: 53.
Emperor Gold Corporation (1996), George Cross News Letter. Final Permits Issued for De-watering and Renewed Exploration of California's 2nd Largest Au Producer. - Underground Work to Start Mid-March. 2/3/96, 4 pp.
Guenther, Ross - Project Manager, Geological Summary of the Idaho-Maryland Mine, 4 pp.
Randol Mining Directory (1996/97), U.S. Mines and Mining Companies: 158.
Emperor Gold Corporation (1995), Annual Report: 8.
Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 218, 248, 256, 278, 286.
Mineralogical Record (1982): 13: 386.
Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 109, 10, 336, 426.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10007405 & 10310630.
USGS quadrangle Grass Valley, California, 7.5-minute topo map
California Geological Survey Mineral Resources files, Sacramento, California, file No. 331-9392.

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