Big Canyon Mine (Oro Fino), Mount Aigare, Shingle Springs Mining District, West Belt, El Dorado County, California, USAi

Regional Level Types
Big Canyon Mine (Oro Fino)	Mine
Mount Aigare	Mountain
Shingle Springs Mining District	Mining District
West Belt	- not defined -
El Dorado County	County
California	State
USA	Country

A former lode Au mine located in the NW¼NW¼ sec. 29, T9N, R10E, MDM, 1.4 km (0.8 mile) ENE of Mount Aigare (coordinates of record) (4½ miles S of Shingle Springs; 4½ miles NE of Latrobe), along the W side of Big Canyon Creek, on private land. Discovered before 1880. Operated prior to 1888, during the periods 1893 to 1901, 1915, and 1934 to 1940. MRDS database stated accuracy for this location is 100 meters.

The Big Canyon Mine is located in the Shingle Springs District of western El Dorado County in the foothills of the Sierra Nevada Mountains. The deposit was first mined from the early 1880's to 1901, then again between 1934-1940. The Big Canyon mine was one of the largest in El Dorado County, producing $720,000 in gold from 1893- 1901, and $2,380,000 from 1934-1940 (Clark and Carlson, 1956).

The Big Canyon Mine was discovered in the early 1880s. Little is known about the first few years of operation except that a twenty-stamp mill was on the property prior to 1888. Afterward, the mine became one of the most productive mines in El Dorado County having produced more than $3+ million in gold during two periods of operation, 1893-1901 and 1934-1940. Between 1893 and 1901, Hayward, Hobart, and Lane operated the mine, producing $720,000 from 180,000 tons of ore (Logan, 1938). During that period mining reached the 500 foot level.

In 1934, the Mountain Copper Company acquired the property. Mountain Copper sank a new shaft, erected a mill, and operated the mine on a large scale until 1940. From 1934-1940, Mountain Copper Company produced another $2,368,000 (period values). The workings were extended deeper and along strike; the average dip of the veins was about 35 degrees east and the strike was S25W. The ores from the upper levels were successfully treated by amalgamation, but that was not effective on the deeper pyritic ores, which were treated by flotation. On February 26, 1937, water from Big Canyon Creek flooded the mine. Within 52 hours, 30,000 gallons of water had poured into the mine and a pond several hundred feet long formed at the surface. Later in 1937 an open pit was developed to reach below the 100-foot level (Nash, 1988). The mine was forced to close in 1940 by the War Production Board Limitation Order. The mine has been idle since 1940 (Earhart, 1988).

In 1983, Gold Fields Mining Corp. acquired the mineral rights, with plans to develop the property as an open pit mine. Surface exploration and coring (about 12,000 meters of core) between 1983-1985 identified many zones with gold grades in the range of 3-7 grams/ton (Nash, 1988). Plans to open the open-pit mine were scuttled by environmental opposition. In 1986, an El Dorado County company acquired the mine and continued feasibility studies.

The deposit is structurally controlled by numerous closely spaced fractures in the Big Canyon Fault zone. Gold is associated with zones of pyrite-carbonate-albite alteration of mafic rocks and vein filling, both as free gold, auriferous pyrite, and pyrrhotite.

Earhart (1988) described the gold deposits of the Big Canyon area as follows: ?The area contains two distinct types of gold deposits. The older gold deposits are hosted by detached blocks of pyritic chert and associated banded iron-formation. These deposits , formed by sea-floor exhalative processes in an island-arc environment, were moved by gravity to their present chaotic setting during formation of the melange. Younger gold occurrences, including the Big Canyon deposit, are associated with hydrothermally altered fault zones that are younger than the formation of the melange. They are akin to the fault-controlled auriferous quartz veins in the Mother Lode belt east of the Big Canyon area.?

Mineralization is a vein-like 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 basaltic Paleozoic metatuff and metadiorite. The ore body is tabular, strikes N25E and dips 35E at a thickness of 18.29 meters. The deposit is a vein-like mass of sheared and brecciated meta-andesite in a shear zone with amphibolite to the W and a serpentine lens to the E. E of the serpentine are meta-sedimentary rocks of the Calaveras Group. The ore body dips up to 40E. Three periods of sulfide mineralization occurred. The first is of arsenopyrite, the second of barren pyrite and the third of auriferous pyrite. Ore materials include pyrite-carbonate-albite alteration of basaltic metatuff, metadiorite, and breccia with 5-15% auriferous pyrite carrying 80% of the gold. Free gold is the remaining 20%. The best ore Contained equal parts of albite, ankerite and quartz with albite favorable for gold deposition. Local alteration includes pyrite-carbonate-albite alteration of basaltic metatuff and metadiorite. Local rocks include pre-Cenozoic metasedimentary and metavolcanic rocks undivided.

Local geologic structures include the Big Canyon fault zone. Regional structures include the Melones Fault Zone.

The Big Canyon Mine lies within the eastern melange belt of the West Gold Belt. In the vicinity of the mine, the melange consists of detached blocks that were derived from oceanic crust and island arc terranes. The detached blocks are as much as 2 miles long, in a matrix of mostly serpentinite. Serpentinite and basalt that compose the matrix of the melange are vestiges of a dismembered ophiolite.

According to Earhart (1988), the Big Canyon deposit is at the intersection of three branching faults, which are parts of a major fault zone that parallels Big Canyon Creek. The steeply dipping ore zone is exposed for about 150 feet in a northerly direction along strike on the western side of a glory hole. The ore occurs along a fault that separates metasedimentary rocks on the east from basalt and basaltic tuff to the west. The host rock is a breccia that contains silicified and pyritized fragments primarily from the basaltic wall rocks. The breccia matrix consists of calcite, ankerite, quartz, chlorite, and tremolite.

According to Nash (1988), the deposit is structurally controlled by numerous closely spaced fractures withinin the east dipping Big Canyon fault zone. The Big Canyon structure is a series of sub-parallel high angle faults in a zone more than 100 meters wide with branching low angle faults that cut off blocks between the high angle faults (Nash, 1988). Gold occurs in veins and in faulted and brecciated Paleozoic basaltic metatuff and metadiorite displaying pyrite-carbonate-albite alteration zones up to 100 feet thick (Nash, 1988). The main ore body strikes N 25?E and dips 35? - 40? east and southeast. The ore shoot that was mined during the 1930s had a maximum length of 450 feet and was as much as 50 feet wide (Clark and Carlson, 1956).

Gold mineralization is consistently associated with fracture filling and pyrite-carbonate-albite alteration of basaltic metatuff and metadiorite wall rocks in are in fault contact with serpentinite, slate, and black argillite. PCA alteration was caused by metasomatic processes by which S, CO2, and Na were added to the rocks during both vein filling and replacement reactions.
The orebody formed by Cretaceous hydrothermal alteration of the fault zones by gold-bearing Mother Lode type mineralizing fluids which ascended along fractures within the east dipping Big Canyon fault zone (which may merge at depth with the Mother Lode Fault system four miles to the east). Fractures apparently acted as a structural control for ore deposition. The deposits are thought to have formed from alkaline, CO2 rich fluids of moderate temperature (300-400?C?) and pressure (1-2 kilobars?) of metamorphic origin (Nash, 1988). For information on geochemical controls, see Nash (1988), Earhart (1988), Loucks and Mavogenes (1999), and Bohlke and Kistler (1986).

Sulfide minerals occur as disseminations and fracture fillings in both the matrix and the fragments of the breccia, but are most abundant in the matrix. Surface exposures contain as much as 5% sulfides and intervals of 2-3 feet in drill cores contain as much as 15 percent sulfides. Gold values vary directly with sulfide content. Auriferous pyrite is the main sulfide, but some pyrrhotite and arsenopyrite is in the ore. Exploration coring by Gold Fields Corp in the 1980's identified many zones with gold grades in the range of 3-7 grams/ton (Nash, 1988).

Workings include underground openings comprised of a 200 foot vertical shaft and a 540 foot inclined 40 degree shaft. The ore was stoped to surface from the 500 level. Another 620 foot shaft was sunk on a 45 degree incline. The deposit was also mined by open pit. Stopes were 450 feet long and up to 60 feet wide.

The mine was originally developed by a vertical shaft sunk to a depth of 200 feet, and then sunk an additional 540 feet on a 40? incline to the east. The ore was stoped out to the surface from the 500-foot level of this shaft during the early operations of the mine. The mine had a 20-stamp mill before 1888. After the Mountain Copper Company acquired the mine in 1934, a new 620-foot three-compartment shaft on a 45? incline was sunk 400 feet north of the old shaft. Drifts were extended several thousand feet along the strike of the ore body. The ore was mined in open stopes. In 1937, after the underground workings were flooded, some ore in the upper portion of the mine was mined by open pit methods (Clark and Carlson, 1956).

From the mine, the ore was belt conveyed to the mill. The mill had a capacity of 300 tons/day. It was equipped with two 7 by 6 foot ball mills, two Dorr classifiers, and five Fagergren flotation cells and a cleaner cell. Concentrate was sent through a Dorr thickener and a Oliver filter. Concentrates were shipped to a smelter in Tacoma. A crew of 150 men worked at the mine and mill (Clark and Carlson, 1956).

Production data are found in: Clark, Wm. B. & D.W. Carlson (1956).

The Big Canyon Mine produced more than $3 million (period values) in gold from two main periods of operation (1893-1901 and 1934-1940). During 1934-40, ores yielding $5 (period values) or more per ton were considered economical.

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Link to USGS MRDS:	10006918

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