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Island Mountain Mine (Dahrman group; Island Mountain Consolidated Mine; Annex; Leach; Merritt Gore; Shotgun), Moose Peak, Island Mountain, Coastal Range, Trinity Co., California, USA

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Latitude & Longitude (WGS84): 40° 1' 0'' North , 123° 28' 60'' West
Latitude & Longitude (decimal): 40.01667,-123.48333
Köppen climate type:Csb : Warm-summer Mediterranean climate

A former Cu-Au-Ag-Fe mine located in secs. 9, 10 & 15, T5S, R6E, HM, 0.1 km (200 feet) NNE of Moose Peak (coordinates of record) and 1.2 km (4,000 feet) SSE of Island Mountain (town), on the W bank of the South Fork Eel River, on private land (patented claims). Discovered 1942. Produced 1915-1930. Owned by John Mosier, California (1974). MRDS database stated accuracy for this location is 100 meters.

Mineralization is a Cu-Au deposit hosted in shale and siltstone. The ore body is lenticular at 36. 58 meters wide, 137.16 meters long at a thickness of 42.67 meters. Ore emplacement controls included a shear zone. Andesite is an associated rock. Alteration is local (silicification). The most abundant ore mineral is pyrite. The most important Cu mineral is chalcopyrite. Local rocks include Franciscan Complex, unit 1 (Coast Ranges).

Local geology: Shale and graywacke are members of the Franciscan Formation. Rocks strike N30W and dip steeply NE. Most widespread rock type in the area is the well-bedded graywacke. A zone of graywacke along Pine Creek, below the bridge, consists of alternate layers of fine-grained graywacke and graywacke containing rounded, roughly aligned pebbles of black shale. Most of the prominent ridges are composed of graywacke. The black shale bodies enclosed by the graywacke are narrow and lenticular with an average width of 100 feet. Shale has been silicifeid and re-cemented to form massive boulders in the landslide area near the mine. Other rock types in the vicinity include red and brown chert within graywacke, jagged pinnacles of greenstone, a small body of diabase 1/3 mile SE of the N end of the railroad tunnel, 3 small outcrops of andesite along with a larger mass of andesite farther W and several isolated masses of glaucophane schist.

Local geologic structures include a homocline with beds of shale and graywacke, paralleling the general strike of the graywacke and shale are several notable faults marked by depressions, shear zones and landslides.

Workings include surface and underground openings. The underground workings have a length of 2,721 meters, comprised of 6 adits and 5 shafts with several thousand feet of drifts, crosscuts and raises.

Production data are found in: O'Brien, J.C. (1965): 21.

production information: The principal Cu producer in Trinity County. Huge boulders of float ore contained 5 to 20% Cu, found below the main ore body. Average grade for first 5 years of production was 7% Cu.

Comments on the reserve resource information: About 158,000 tons of ore was estimated to remain in the mine in 1965.

Analyses results: to 1930 (131,600 tons of ore): Cu = 3.3%; Ag = 1.09 ounces/ton; Au = 0.065 ounces/ton.

Mineral List

29 valid minerals. 1 erroneous literature entry.

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 Macrostrat.org

Cretaceous - Jurassic
66 - 201.3 Ma

ID: 3190324
Mesozoic tectonic rocks

Age: Mesozoic (66 - 201.3 Ma)

Stratigraphic Name: Franciscan Complex

Comments: metamorphic protolith: mafic volcanics

Lithology: Shale

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]

Cretaceous - Jurassic
66 - 201.3 Ma

ID: 2942527
Franciscan Complex, unit 1 (Coast Ranges)

Age: Mesozoic (66 - 201.3 Ma)

Stratigraphic Name: Franciscan Complex; Dothan Formation; Honda Formation; San Luis Formation; Valentine Spring Formation

Description: Franciscan complex: Cretaceous and Jurassic sandstone with smaller amounts of shale, chert, limestone, and conglomerate. Includes Franciscan melange, except where separated--see KJfm.

Comments: Coast Ranges. Primarily Yolla Bolly, Central, and San Simeon terranes of Silberling et al. (1987). Consists primarily of variably deformed and metamorphosed graywacke, mudstone, and chert.

Lithology: Major:{sandstone,mudstone}, Minor:{chert}, Incidental:{limestone, conglomerate, greenstone, serpentinite, gabbro, amphibolite, schist, felsic volcanic}

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]

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

This page contains all mineral locality references listed on mindat.org. 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 mindat.org 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.


Sort by Year (asc) | by Year (desc) | by Author (A-Z) | by Author (Z-A)
Landon, Robert Emmanuel (1927), Roemerite from California: American Mineralogist: 12: 279-283.
Rand, L.H. (1931), The Mines Handbook, Mines Information Bureau: 18, part 1: 588.
Melhase, John (1934), A diversity of many fine minerals available in California for collectors: Oregon Mineralogist: 2(6): 1-2, 4.
Eric, J.C. (1948), Copper in California: California Division of Mines Bulletin 144: 350.
Trask, Parker Davies (1950) Geologic description of the manganese deposits of California. California Division of Mines Bulletin 152, 378 pp.: 314.
Palache, C., Berman, H., & Frondel, C. (1951), The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana, Yale University 1837-1892, Volume II. John Wiley and Sons, Inc., New York, 7th edition, revised and enlarged, 1124 pp.: 490, 507, 515 & 626.
Stinson, Melvin Clarence (1957a), Geology of the Island Mountain copper mine, Trinity County, California: California Journal of Mines and Geology (Report 53): 53(1&2): 9-33.
Van Loan, P. R., & Nuffield, E. W. (1959). An x-ray study of roemerite [California]. The Canadian Mineralogist: 6(3): 348-356.
O’Brien, J.C. (1965) Mines and mineral resources of Trinity County, California. Caifornia Division of Mines and Geology County Report 4: 125 pp.: 21-22, 62.
Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 85, 95, 104, 112, 128, 130, 134, 151, 156, 186, 205, 214, 258, 274, 283, 290, 312, 324, 357.
Fanfani, L., A. Nunzi, and P.F. Zanazzi (1970) The crystal structure of roemerite. American Mineralogist: 55: 79.
Moore, Lyman (1970), Costing Report for Island Mountain. (Could not find a backup file other than a 12 X 24 accounting sheet with product and reserve data written on it).
Official Railway Guide, The (1975) No. 4, National Railway Location Company, 424 West 33rd Street, New York, NY 1000.
Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 49, 70, 73, 77, 91, 93, 96, 99, 132, 228, 264, 270, 273, 274, 275, 296, 297.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10072353, 10190697 & 10287151.
U.S. Bureau of Mines, Minerals Availability System (MAS) file ID #0061050029 & 0061050782.

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