Black Mountain Mine (Black Mountain group), Black Mountain, Poway, San Diego Co., California, USA
|Latitude & Longitude (WGS84):||32° 58' 59'' North , 117° 6' 50'' West|
|Latitude & Longitude (decimal):||32.98306,-117.11389|
A former As-Au mine located in sec. 5, T14S, R2W, SBM, 1.0 km (3,400 feet) NNE of Black Mountain (coordinates of record), on its N slope, and 7.6 km (4.7 miles) WNW of Poway, on state park land. Note the property is located within the Black Mountain Open Space Park and is closed to collecting. Alternate coordinates provided: 32°59'24"N, 117°06'40"W
According to 2009 article in the San Diego Union Tribune, "in the 1920s, there was a short-lived boom in white arsenic because it was an ingredient in a pesticide that attacked boll weevils, which were infesting Southern cotton crops. Frank Hopkins, an Escondido rancher, rodeo cowboy and actor, used his Hollywood contacts to fund an arsenic mine on the north slope of Black Mountain. Several years of cold winters and dry summers in the South effectively eliminated the boll weevils, so demand for white arsenic fell. The mine was abandoned in 1927, but its concrete dust chamber, shafts and oven remain."
However according to a contemporaneous article, Mr. Hopkins and his partners may also have had interest in another mineral at this site - gold. According to a 1939 article, "known to be conservative" J.N. McLeod "a nationally known mining engineer", "obtained 150 samples from test holes and reports they assayed from $13.60 a ton at the extreme south end of the workings to $113 at the north end." In 1939, the US Official gold price was set at $35 per troy ounce.
In 1926, the 180-foot-long concrete stack was constructed for processing the arsenic. The arsenic was mined, crushed, and heated to a vaporizing temperature. The condensed arsenic vapor was collected from the baffles inside the stack. Downhill of the stack, is the mine's oven, a few collection bins (perhaps for cyanide leaching?) and collapsed outbuildings.
To the west of the oven and in front of what seems to be a collapsed mine adit, is a small pile of arsenopyrite ore, some with small, unremarkable pyrite crystals. Getting to the location also requires off trail bush-wacking, another activity frowned on by the park rangers, as dangers at this site include bees, ticks, thorns, mountain lions (2011 sightings), snakes, heat exhaustion and unstable structures. And as readers of this website likely already know, elemental arsenic and arsenic compounds are classified as “toxic” and “dangerous for the environment” in the European Union under directive 67/548/EEC.
The Park is located in the geological area known as the “Poway Quadrant” which consists of rock units called “Santiago Peak Volcanics.” The Santiago Peak Volcanics comprise an elongate belt of mildly metamorphosed volcanic, volcaniclastic, and sedimentary rocks that crop out from the southern edge of Los Angeles Basin southward towards Mexico (California Division of Mines, 1975).
Local rocks include Mesozoic volcanic rocks, unit 4 (Peninsular Ranges) (per MRDS database).
1 valid mineral.
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
66 - 252.17 Ma
|Metamorphosed and unmetamorphosed volcanic and sedimentary rocks, undivided|
Age: Mesozoic (66 - 252.17 Ma)
Description: Low-grade (green-schist facies) metasedimentary rocks (conglomerate, sandstone, and siltstone) interlayered and mixed with metavolcanic rocks consisting of flows, tuffs, and volcaniclastic breccia.
Reference: Kennedy, M.P. and S.S. Tan. Geologic Map of the San Diego 30' x 60' Quadrangle, California. Department of Conservation, California Geological Survey. 
|Early Cretaceous - Late Jurassic|
100.5 - 163.5 Ma
|Mesozoic volcanic rocks, unit 4 (Peninsular Ranges)|
Age: Mesozoic (100.5 - 163.5 Ma)
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.
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. 
Weber, F.H., Jr. (1963a) Mines and mineral resources of San Diego County, California. California Division of Mines and Geology County Report 3, 309 pp.: 49.
Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 87.
Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 77.
USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10262361.
U.S. Bureau of Mines, Minerals Availability System (MAS) file ID #0060730632.