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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): 40.0166666667, -123.483333333

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.

The above list 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.


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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