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Big Horn Mine

Past Producer in Los Angeles county in California, United States with commodities Gold, Silver, Copper, Lead, Zinc, Arsenic
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Sections on this page
  1. Identification information
  2. Geographic coordinates
  3. Site location context
  4. Geographic areas
  5. Public Land Survey System information
  6. Commodities
  7. Materials information
  8. Alteration
  9. Mineral occurrence model information
  10. Host and associated rocks
  11. Nearby scientific data
  12. Geologic structures
  13. Ore body information
  14. Controls for ore emplacement
  15. Economic information about the deposit and operations
  16. Mining district
  17. Land status
  18. Ownership information
  19. Bibliographic references
  20. General comments
  21. Reporter information

Geologic information

Identification information

Deposit ID 10310697
Record type Site
Current site name Big Horn Mine

Geographic coordinates

Geographic coordinates: -117.74361, 34.35639 (WGS84)
Elevation 2100
Location accuracy 100(meters)
Relative position Approx. 40 miles east of Los Angeles, CA; 6.6 miles due west of Wrightwood CA; 1.14 miles N 94.9? E of the summit of Mount Baden-Powell

Site location context

Political divisions (FIPS codes)

Los Angeles(county)

California(state)

United States(country)

North America(continent)

Land(continent)

USGS map quadrangles

Mount San Antonio(quadrangle 1:24,000 scale)

San Bernardino(quadrangle 1:100,000 scale)

San Bernardino(quadrangle 1:250,000 scale)

Hydrologic units (watersheds)

San Gabriel(hydrologic unit)

Ventura-San Gabriel Coastal(hydrologic accounting unit)

Southern California Coastal(hydrologic subregion)

California(hydrologic region)

Federal lands

Angeles National Forest(National Forest)

National Forest FS(Type of land area)

FS(Federal land areas administered by FS)

Sheep Mountain Wilderness(Wilderness)

Wilderness FS(Type of land area)

FS(Federal land areas administered by FS)

Geographic areas

Country State County
United States California Los Angeles

Public Land Survey System information

Meridian Township Range Section Fraction State
San Bernardino 003N 008W 7,8,17,18 California

Comments on the location information

  • The location point selected for longitude and latitude represents the adit symbol near the mine name on the Mount San Antonio 7.5 minute quadrangle.

    The Big Horn Mine is located in the San Gabriel Mountains, approx. 40 air miles east-northeast of Los Angeles and about 6 air miles west of the town of the town of Wrightwood. The mine is located 18 road miles west of Wrightwood via State Hwy 2 and an unpaved road to the mine.

    From the city of San Bernardino, CA, the mine can be reached by taking Interstate Hwy 15, State Hwy 138 and State Hwy 2 (Angeles Crest Highway) north to the town of Wrightwood, CA. From Wrightwood, take State Hwy 2 northwest for 9.4 miles to the junction of Big Pines Hwy. At this junction, continue west on State Hwy 2 for 6.8 miles to Vincent Gap where there is a locked gate at the entrance to the Big Horn Mine road. Continue south on the Big Horn Mine road for 1.5 miles to the Big Horn millsite ruins and No. 6 Adit.

    The mine is within the Sheep Mountain Wilderness, Angeles National Forest. Travel on the mine road is restricted by the U.S. Forest Service and the mine owners. The mine is located at an elevation of approx. 6900 feet (2103 meters). Access to the mine may also be restricted due to snow accumulation during winter months.

Commodities

Commodity Importance
Gold Primary
Silver Secondary
Copper Tertiary
Lead Tertiary
Zinc Critical Tertiary
Arsenic Critical Tertiary

Comments on the commodity information

  • Ore Materials: Native gold; sulfides (pyrite, chalcopyrite, galena, arsenopyrite)

  • Gangue Materials: Unmineralized host rocks: lower-plate Pelona Schist, upper-plate San Gabriel Gneiss, monzonite porphyry dikes and sills, andesite dikes.

Materials information

Materials Type of material
Gold Ore
Pyrite Ore
Chalcopyrite Ore
Galena Ore
Arsenopyrite Ore
Schist Gangue
Gneiss Gangue
Andesite Gangue

Alteration

  • (Local) Silicification
    Pyritization
    Carbonatization

Mineral occurrence model information

Model code 273
USGS model code 36a
Deposit model name Low-sulfide Au-quartz vein
Mark3 model number 27

Host and associated rocks

  • Host or associated Host
    Rock type Metamorphic Rock > Amphibolite
    Stratigraphic age (youngest) Precambrian
  • Host or associated Host
    Rock type
    Rock unit name San Gabriel Gneiss
    Stratigraphic age (youngest) Precambrian
  • Host or associated Host
    Rock type Tectonite > Cataclasite
    Stratigraphic age (youngest) Paleozoic
  • Host or associated Host
    Rock type Metamorphic Rock > Schist
    Rock unit name Pelona Schist
    Stratigraphic age (youngest) Mesozoic
    Stratigraphic age (oldest) Precambrian
  • Host or associated Host
    Rock type Metamorphic Rock > Gneiss
    Stratigraphic age (youngest) Precambrian
  • Host or associated Associated
    Rock type Metamorphic Rock > Gneiss
    Stratigraphic age (youngest) Precambrian
  • Host or associated Associated
    Rock type Metamorphic Rock > Schist
    Stratigraphic age (youngest) Mesozoic
    Stratigraphic age (oldest) Precambrian
  • Host or associated Associated
    Rock type Plutonic Rock > Granitoid > Quartz Monzonite
    Stratigraphic age (youngest) Miocene
  • Host or associated Associated
    Rock type Volcanic Rock (Aphanitic)
    Rock type qualifier Dikes

Nearby scientific data

(1) -117.74361, 34.35639
ListMap

Economic information

Geologic structures

Type of structure Regional
Structure description Vincent Thrust fault; San Andreas Fault; Punchbowl Fault, San Gabriel Fault; northwest striking normal faults in Big Horn Mine area; regional arching of the San Gabriel Mountains basement complex and Vincent Thrust along a northwest-trending axis in the region around the Big Horn Mine.

Ore body information

  • General form Tabular; gold mineralization occurs within a zone of brecciation and cataclasite formation along the gently northwest-dipping Vincent Thrust fault zone. Disseminated gold occurs in the Pelona Schist near the Vincent Thrust.

Controls for ore emplacement

  • Permeable, crushed rock between the upper and lower strands of the Vincent Thrust Fault.

Comments on the geologic information

  • Age of Mineralization:
    Middle Miocene, correlative with intrusion of aplite and quartz monzonite porphyry of Miocene age (Hamann, 1985; 14 to 16 m.y. by K-Ar method, Miller and Morton, 1977, in Ehlig, 1981).

    Host Rock Type:
    Gold mineralization occurs primarily in crushed amphibolite and cataclasite between the upper and lower traces of the Vincent Thrust, and, to a much lesser extent, disseminated locally in lower-plate Pelona Schist or in mylonitic upper-plate San Gabriel Gneiss, close to the Vincent Thrust.

  • Stratigraphy from oldest to youngest rocks (from Ehlig, 1981; Hamann, 1985)

    San Gabriel Gneiss (Precambrian):
    The San Gabriel Gneiss forms the upper plate above the Vincent Thrust. These rocks originated as part of a Precambrian craton. The oldest rocks consist of amphibolite facies quartzofeldspathic gneiss, and amphibolite at least 1715 ? 30 m.y. old (Ehlig, 1981).
    The most abundant mineral in the gneiss is quartz, constituting between 20 and 65% of the rock with an average abundance of 35%. Quartz grains, up to 1 cm in diameter, have sutured grain boundaries and undulose extinction. Plagioclase feldspar, ranging from albite to andesine, makes up about 25% of the rock. Potassium feldspar constitutes about 10% of the gneiss. All three form the leucorcratic bands of the gneiss. Both feldspars have been affected by calcite and sericite alteration.

    Muscovite grains, making up 5% of the gneiss, follows foliation predominantly in the leucocratic layers of the rock. Mafic minerals are albite and hornblende, making up 5 and 10% of the total rock composition, respectively. These minerals, ranging in size up to 1mm in length, have been extensively altered to chlorite. Chlorite, composing 15% of the rock, forms reaction rims around the large mafic grains and has entirely replaced some of the smaller ones. The mafic and chloritic zones form the melanocratic layers in the gneiss.

    Calcite, composing 10% of the total rock, occurs as secondary veins and as an alteration and replacement of feldspar. It increases in abundance to up to 20% approaching the Vincent Thrust.

    Pyrite, forming up to 1% of the rock near the thrust, is generally found associated with secondary calcite and quartz veins. It has been altered to limonite along the grain edges.

    Other minerals found in the gneiss in quantities less than 1% include apatite, sphene, and magnetite. Magnetite is very fine-grained and disseminated throughout the rock. It is probably derived from the remobilization of iron during metamorphism of the rock.

    Amphibolite (Precambrian):
    Crushed amphibolite is the most abundant rock type in the interval between the upper and lower strands of the Vincent Thrust in the Big Horn Mine area, and is the predominant host rock for gold mineralization. The amphibolite is mineralogically similar to the San Gabriel Gneiss and presumably was derived from it.

    In the amphibolite, quartz content varies from 35 to 55% of the rock, with an average volume of 45%. Up to half of the quartz is due to secondary silicification which varies markedly from 0 to 35% of the total rock composition. Pyrite and gold are associated with the quartz veinlets.

    Plagioclase, comprising up to 20% of the rock, ranges in composition from albite to andesine. These grains can have relict augen shapes but seem to have a gradational boundary with the surrounding material.

    Sericitic alteration occurs throughout the rock.

    Carbonates, calcite, ankerite, and dolomite, comprise between 3 and 15% of the amphibolite. The most abundant is calcite, making up approximately half of the total carbonate. It is found as veinlets and as a replacement of feldspar. Calcite veinlets vary in size from 0.1 to 5 mm and can have pyrite within them. The remaining carbonates, ankerite and dolomite, are secondary minerals. Pyrite and gold are found in association with ankerite veinlets. There appears to be a direct correlation between an increase in the volume of carbonate and an increase in the gold content.
  • Stratigraphy from oldest to youngest rocks (from Ehlig, 1981; Hamann, 1985) (continued)

    Hornblende comprises between 5 to 15% of the rock. The grains are partially to completely replaced by chlorite. Chlorite, as an alteration after amphibole and biotite, makes up 5 to 10% of the total composition.

    Pyrite is abundant in the amphibolite, comprising from 2 to 10% of the rock. It is much more abundant in the underground workings and drill cores than exposed at the surface. Pyrite grains vary in size from less than 0.01 to 4 mm in diameter. The small grains are confined to quartz-pyrite veinlets whereas the larger ones can be distributed at random throughout the samples. Gold is commonly associated with the quartz-pyrite veinlets. Epidote and fine-grained magnetite each make up 1% of the rock. The magnetite was probably deposited along with the ore fluids.

    Free gold observed in drill core samples is associated with secondary carbonate and quartz-pyrite veinlets; its grain size does not exceed 3 mm in diameter.

    Pelona Schist (Precambrian or Mesozoic):
    The Pelona Schist consists of a sedimentary assemblage of predominantly grayschists and lesser greenschists that underwent prograde regional greenschist facies metamorphism synchronous with deep burial beneath the Vincent Thrust. It consists mainly of thin-bedded white mica-quartz-albite schist derived from arkosic greywacke, siltstone, and shale. Greenschist derived from basaltic tuff is common in the upper part of the sequence. Thin beds of quartzite derived from chert are common, particularly in association with greenschist. The Pelona Schist protolith as being deposited on oceanic crust within a marine basin that received sediments mainly from a continental source of granitic composition, but with some sediment coming from an offshore volcanic source. He states the depositional age of the Pelona Schist has not been established but is probably Mesozoic, perhaps Late Cretaceous. The metamorphism of the Pelona Schist occurred about 60 m.y. ago in response to deep burial beneath the Vincent Thrust fault. This event probably relates to subduction, involving northeastward thrusting of a micro-continent or peninsula across a marginal basin or narrow gulf. Locally, metamorphism has progressed to the lower amphibolite facies in schist close to the Vincent Thrust. The greyschist protolith consists predominantly of well-bedded arkosic sandstone, siltstone, and claystone. In addition to the grayschists, Pelona Schist of the San Gabriel Mountains contains about 10% greenschist derived from basaltic tuff and 1% or 2% quartzite (with some interlaminated marble) derived from chert (Ehlig (1981, pg. 254).

    Pelona Schist near the mine is well-foliated, medium to dark grey to black. Black graphitic schist does not differ significantly in its mineralogy from the grey schist. Physically, the grey schist is harder and has a uniform schistosity, whereas the graphitic schist is tightly folded and friable.

    Feldspar is the most abundant mineral in the schist, primarily as inclusion-rich albite up to 1.5 mm in diameter, making up to 20-35% of the rock. Minerals included in the albite are: quartz, muscovite, amphibole and graphite, the latter giving the schist its dark color (Ehlig, 1958, in Hamann, 1985). Sericite and calcite extensively replace the feldspar.

    Quartz is the second most abundant mineral in the schist, making up about 25-30% of the rock. Quartz grains are up to 1 mm in diameter and if elongate, tend to parallel foliation. The grains have sutured boundaries and undulose extinction characteristic of metamorphic quartz but are inclusion-free and neither altered nor replaced by other minerals.

  • Stratigraphy from oldest to youngest rocks (from Ehlig, 1981; Hamann, 1985) (continued)

    Carbonates, in the forms of calcite, ankerite and dolomite, make up 10-15% of the schist. Calcite is common as an alteration of feldspar and in secondary veins, especially near the Vincent Thrust. It is not unusual to find pyrite associated with veinlets of calcite.

    Muscovite and chlorite form about 10-8% of the schist, respectively. The grains, up to 1 mm in length, are parallel to foliation. Muscovite is found in thin layers, often wrapped around quartz and albite. Where muscovite is included within albite, it commonly forms relict "S" shaped folds within the mineral.

    Chlorite is an alteration after amphibole and biotite, typically forming a reaction rim around these mafic minerals. Pyrite is commonly associated with chlorite.

    Epidote, forming up to 5% of the schist, follows foliation in the rock.
    Other minerals making up less than 1% include: actinolite, biotite, pyrite, limonite, graphite, sphene, and magnetite. Pyrite, common in secondary veinlets, is not seen away from the Vincent Thrust. It has been partially altered to limonite. Very fine-grained magnetite is probably derived from metamorphism and subsequent mobilization of iron.

    Mariposite, a chrome mica, was identified through X-ray diffraction and X-ray fluorescence. It was found in drillcore samples and in one underground location. The green mineral was found in the presence of Quartz, dolomite-ankerite and feldspar.

    Visible gold, ranging up to 1.5 mm in size, was observed in drill cores transecting the schist but not in hand samples of schist collected along the surface or underground. Gold is not restricted to secondary veining episodes, but appears to be randomly distributed (disseminated) throughout the graphitic schist near the thrust.

    Chlorite Schist (correlative with the Pelona Schist(?)):
    Up to 65% of the schist consists of fibrous chlorite. Second most in abundance is calcite, making up 25% of the rock. The calcite replaces feldspar in places and is also disseminated throughout the chloritic groundmass.

    Feldspar, making up 5% of the schist, has been moderately to entirely replaced by calcite. Some sericitic alteration is visible in thin section, especially along the feldspar cleavage planes. Because of the alteration, feldspar composition is difficult to determine. Some grains display Carlsbad twinning, suggesting oligoclase to andesine composition.

    Pyrite, composing 3% of the schist, is disseminated throughout the rock.

    Folds with amplitudes from 5 cm to 1 m are readily apparent in the chloritic schist.

    No gold mineralization was noted within the chloritic schist.

    Cataclasite (Paleocene; formed during movement along the Vincent Thrust):
    Cataclasite, from 2 to 5 meters thick has developed along the lower and upper strands of the Vincent Thrust, respectively. Cataclasite is the most abundant mineral constituent in the cataclasite, ranging in volume from 30 to 65%. The grains are small, seldom exceeding 2 mm in diameter. They display sutured grain boundaries and have undulose extinction in thin section.

    The second most abundant mineral within the cataclasite is feldspar, ranging in volume from 15 to 35% of the total rock composition. Through the use of a petrographic microscope Hamann (1985) identified the majority of the feldspar as plagioclase varying in composition from oligoclase to andesine. Less than 2% of the rock is potassium feldspar. Alteration of the feldspars to sericite and calcite was observed in all samples. The extent of alteration varies from slightly to moderately altered.


  • Stratigraphy from oldest to youngest rocks (from Ehlig, 1981; Hamann, 1985) (continued)

    Carbonates, making up to 20% of the total rock composition, consist of calcite, dolomite and ankerite. Calcite is found as secondary veinlets associated with pyrite and also as a replacement of feldspar. Pervasive alteration and degradation of the rock has made it impossible to distinguish between primary and secondary calcite mineralization. Ankerite mineralization wherever in volumes exceeding 10% of the rock, gives a characteristic orange-brown color to the cataclasite. The ankerite is found in small veinlets distributed throughout the rock.
    Two other minerals which exceed 1% of the total composition are pyrite and muscovite. Pyrite, making up 3% of the rock, is most commonly found associated with calcite and quartz veinlets but can also be disseminated randomly thorough the rock. The grain size of pyrite does not exceed 0.5 mm in diameter. Oxidation of the pyrite to limonite is common, especially along the edges of the grains. Muscovite compromises 3% of the cataclasite with the grains not exceeding 0.23 mm in length. Muscovite orientation tends to coincide with the rock foliation.

    Minerals which do not exceed 1% of the total composition of cataclasite are: amphibole, chlorite, limonite, apatite, sericite and biotite. Amphibole and biotite have been partially to completely altered to chlorite. Limonite is an oxidation after pyrite, and sericite is a replacement after feldspar.

    Free gold is distributed through the cataclasite. The gold was observed only in drillcores; no free gold was observed in outcrop. The gold is usually less than 2 mm in diameter and is associated with pyrite, quartz and carbonate veinlets.

    Quartz Monzonite Porphyry (Middle Miocene):
    The Pelona Schist, Vincent Thrust rocks, and all older rocks in the eastern San Gabriel Mountains have been intruded and locally hornfelsed by stocks of a light-colored, medium-grained granitic porphyry whose composition straddles the border between quartz monzonite and granodiorite. A swarm of more than thirty sills forms a prominent light-colored band, about 50 meters thick, beneath a greenschist member of the Pelona Schist between Mount Baden-Powell and the Narrows. Miller and Morton (1977, in Ehlig, 1981, pg. 277) have dated these rocks as Middle Miocene (14 to 16 m.y. by the K-Ar method).

    White to light grey dikes of quartz monzonite porphyry (classified according to Ehlers and Blatt, 1980, in Hamann, 1985), contain quartz and feldspar phenocrysts (up to 2 mm diameter) in an aphanitic matrix. The rock is very hard and breaks into irregularly shaped fragments.

    Forty (40) to 70% of the rock is made of an aphanitic groundmass, with the remaining constituent being the phenocrysts. Identification of the groundmass is complicated by secondary alteration and replacement of the original material by calcite.

    The most abundant mineral in the rock is plagioclase feldspar, constituting between 50-65% of the total composition. The composition of the feldspar ranges from oligoclase to andesine. The feldspar has been replaced by sericite and calcite.

    Calcite comprises between 10-35% of the rock, mostly as an alteration of feldspar but also as secondary veinlets crosscutting the rock.

    Quartz makes up 5% of the rock and is found mostly as euhedral crystals and subrounded phenocrysts. Some secondary quartz veinlets are developed through the rock.

  • Stratigraphy from oldest to youngest rocks (from Ehlig, 1981; Hamann, 1985) (continued)

    Other constituents within the porphyry, not everywhere present, include very fine-grained biotite (1%) and less than 1% of: pyrite, limonite, hornblende, muscovite and epidote.

    Aplite (Middle Miocene):
    Gold-bearing aplite dikes occur locally near the Vincent Thrust zone and cut the post-metamorphic porphyry sills (Ristorcelli, 1988).

    The aplite dikes near the mine vary in color from white to orange, depending on the extent of limonitic alteration of pyrite within the rock. The aplite is very hard and breaks into small, irregularly shaped chips after being subjected to numerous hammer blows.

    An aphanitic groundmass makes up about 85% of the rock. Identification of the groundmass is complicated by calcite replacing feldspar grains. Standard petrographic microscopy did not positively identify the remaining constituents.

    Feldspar accounts for up to 10% of the rock. It is so altered and replaced by sericite and calcite that its composition could not be determined with a petrographic microscope.

    Quartz grains, ranging in size up to 0.2 mm, compose up to 5% of the aplite. The grains are anhedral and show no alteration or replacement reaction. Quartz veinlets, up to 0.2 mm wide, crosscut the aplite.

    Pyrite, mostly oxidized to limonite, can make up 5% of the total composition of aplite. The pyrite grains do not exceed 0.5 mm in diameter. Limonite alteration is pervasive throughout the dikes, forming a halo around pyrite grains.

    Other minerals seen within the aplite are biotite and chlorite. The chlorite is formed as an alteration after biotite. These minerals make up less than 1% of the aplite.
    Andesite (Middle Miocene):
    The youngest intrusive geologic event is represented by andesite dikes, which clearly post-date mineralization (Van Nort, 1986, in Ristorcelli, 1988).

    Shallow emplaced dikes of andesite, diabase, and, less commonly, olivine basalt and quartz latite are locally common in the southern and east-central parts of the San Gabriel Mountains. These dikes intrude the Miocene porphyry in areas where the relationship can be observed. The dikes have not been dated but are assumed to be related to the Middle Miocene Glendora Volcanics along the southern edge of the range (Shelton, 1955, in Ehlig, 1981, pg 277).

    Andesite dikes near the mine vary in character from a porphyritic grey rock containing hornblende and feldspar phenocrysts up to 5 mm in length to a brown-colored rock, devoid of any phenocrysts. Petrographically, both rock types are classified as andesites (Ehlers and Blatt, 1980, in Hamann, W.E., 1985). The rock is moderately hard, fracturing into blocky pieces.

    The most abundant mineral is andesine, making up approximately 33% of the rock. It is extensively altered to calcite and sericite. Alteration has been so complete that many andesine grains cannot be distinguished from the fine-grained calcite matrix except by the pseudomorphs of calcite after feldspar.

    Calcite makes up 30% of the rock. It is entirely of secondary origin, replacing the larger andesine grains as well as the fine-grained groundmass. The extent of calcite replacement has made it difficult to determine the original mineralogy of the dikes.

    Hornblende needles, up to 5 mm in length, can compose up to 10% of the andesite. Hornblende has been altered to chlorite, especially along the outer edges of the grains.

    Other minerals seen in the andesite include muscovite (2%), chlorite (1%), pyrite (1%) and less than 1% of limonite as an oxidation of pyrite. The remainder of the rock is an aphanitic groundmass, presumably composed of andesine. The groundmass is predominantly calcite which has replaced the andesine.

  • Host Rock Age:
    Amphibolite (San Gabriel Gneiss):
    Precambrian; "at least 1715 ? 30 m.y. old" (Ehlig, 1981, pg. 254).
    Cataclasite:
    Tertiary (Hamann, W.E., 1985); Paleocene (The Vincent Thrust was active 60 m.y. ago based on a Rb-Sr mineral isochron age of 58.5 ? 4 m.y. (Conrad and Davis, 1977, in Ehlig, 1981, pg. 270.)

    Gold mineralization also occurs to a much lesser extent in the San Gabriel Gneiss (Precambrian) above the Vincent Thrust zone and in the Pelona Schist (Precambrian or Mesozoic) below the Vincent Thrust zone.

    Associated Rock Types (from Hamann, 1985, unless noted otherwise)
    Unmineralized San Gabriel Gneiss overlying the Vincent Thrust zone;
    Unmineralized Pelona Schist below the Vincent Thrust zone;
    Sills and dikes of quartz monzonite, aplite, andesite.

    Associated Rock Age:
    San Gabriel Gneiss
    Precambrian; "at least 1715 ? 30 m.y. old" (Ehlig, 1981, pg. 254).

    Quartz monzonite porphyry

    Middle Miocene (14 to 16 m.y. by the K-Ar method; Miller and Morton, 1977, in Ehlig, 1981, pg 277).
    Aplite dikes
    Middle Miocene; gold-bearing aplite dikes occur near the Vincent Thrust zone and cut the porphyry sills (Ristorcelli, 1988).
    Andesite dikes
    Middle Miocene. Shallow emplaced dikes of andesite, diabase, and, less commonly, olivine basalt and quartz latite are locally common in the southern and east-central parts of the San Gabriel Mountains. These dikes intrude the Miocene porphyry in areas where the relationship can be observed. The dikes have not been dated but are assumed to be related to the middle Miocene Glendora Volcanics along the southern edge of the San Gabriel range (Shelton, 1955, in Ehlig, 1981, pg 277).

    Host Rock Unit:
    San Gabriel Gneiss (crushed amphibolite within Vincent Thrust zone)
    San Gabriel Gneiss (cataclasite within Vincent Thrust zone)

    Host Rock Unit Age:
    San Gabriel Gneiss (Precambrian)
  • Tectonic Setting: .
    The Big Horn Mine is located near the crest of the San Gabriel Mountains, an east-west trending range along the southwest side of the San Andreas Fault in the central part of the Transverse Range Province of southern California. Uplift of the range began a few million years ago and is still going on as a product of convergence between the North American and Pacific Plates along the bend in the San Andreas Fault. It was accomplished by reverse faulting along the southern range margin and broad arching across the interior and northern margin, with uplift extending northeastward across the San Andreas Fault. The relatively simple pattern and style of uplift are superimposed upon the complex structure of rocks exposed within the range (Ehlig, 1981, pg.254).

    Process of gold concentration: Deposition of free gold and gold in sulfide minerals from a hydrothermal system in a structurally favorable location and favorable host rocks along the Vincent Thrust zone. Favorable structures possibly include structural arching of the Vincent Thrust zone to form a pathway for channeling and trapping of the hydrothermal system. The fluids appear to have been constrained within permeable host rocks between the upper and lower strands of the Vincent Thrust Fault, possibly by impermeable gouge developed along the fault surfaces, and by the less-permeable San Gabriel Gneiss in the upper plate of the fault and less-permeable Pelona Schist in the footwall of the fault. Favorable ground preparation included crushing and increased permeability of the host rock within the Vincent Thrust zone and formation of impermeable rock, including gouge, along the upper and lower Vincent Thrust sheets.

    Hamann (1985) used fluid inclusions, stable isotopes, and petrographic data to establish the chemistry of the hydrothermal system responsible for gold mineralization at the Big Horn Mine. Quartz-pyrite-gold and calcite-pyrite-gold veinlets were precipitated from a fluid averaging 5.5 percent equivalent weight NaCl at a temperature between 176? to 253? C. Oxygen fugacity was confined to -41.18 < log fO2 < -37.76 and sulfur fugacity to -15.4 < log fS2 < -12.85. The pH was between 5.5 < pH < 8.5.

    Stable isotope analyses of carbon, oxygen, and sulfur suggest that the hydrothermal system was derived from deep-seated metamorphic fluids. Hamann postulates that the hydrothermal system became enriched in gold during metamorphism and dewatering of the Pelona Schist protolith. Metamorphism of the Pelona Schist occurred concurrently with over-thrusting along the Vincent Thrust during the Pliocene, latest Cretaceous at the earliest (Ehlig, 1981). Movement along the Vincent Thrust caused brecciation of the gneiss and amphibolite in the fault zone prior to the intrusion of the aplite and quartz monzonite porphyry of Miocene age. These intrusives were dated at 14 to 16 m.y. by K-Ar method (Miller and Morton, 1977, in Ehlig, 1981).

    Hamann states that sulfur isotope data suggest that the Miocene intrusives supplied most of the sulfur to the hydrothermal fluid, forming a stable and mobile gold-sulfide complex ion, and generating heat, which allowed for convective flow of the system. He also suggests that gold deposition was caused by changes in the chemistry of the system. As ascending fluids reached near-surface conditions, temperature, Eh, and pH regimes presumably changed, causing the gold-sulfur complex ion to become unstable, resulting in deposition of native gold. He also suggests that late-stage(?) unmineralized quartz veinlets reflect a mixing of meteoric water with the deep seated metamorphic fluids.

Economic information about the deposit and operations

Operation type Underground
Development status Past Producer
Commodity type Metallic
Deposit size Small
Significant Yes
Discovery year 1891

Mining district

District name San Gabriel (Mount Baldy) District

Land status

Ownership category National Forest
Area name Sheep Mountain Wilderness (Angeles National Forest)

Ownership information

  • Type Owner-Operator
    Owner Siskon Gold Corporation
    Year 1997

Comments on the workings information

  • More than 7000 feet of drifts, raises, and winzes.

Comments on other economic factors

  • PRODUCTION

    Prior to 1901; a few tons of high-grade ore:
    According to Miller (1938), F. C. Fenner bought the Big Horn property from Vincent in July 1901. Fenner built a 10-stamp mill and made other improvements. Outside of a few tons of high grade ore shipped by Vincent from surface cuts, all of the ore extracted from Big Horn vein and treated was done by Fenner. An accurate account has been kept of the results obtained by plate amalgamation, and the mine receipts show the value of the bullion gold:

    " 1904-1906 production (ore and concentrates)
    a) Ore production: 15,564 tons ore; 1922 ounces Au; average ore grade, 0.198 oz Au/ton; 65% recovery

    Between July 1904 and October 1906, 15, 564 tons of ore from the Big Horn Mine were treated by Fenner using a 10-stamp mill and plate amalgamation, and $39,720.18 in gold was recovered (Miller, 1938, pg. 3). The price of gold during this period was $20.67 per ounce (Clark, 1998); Miller (1938) reports the gold price at $20.00 per ounce. Using $20.67 per ounce, the amount of gold recovered was 1922 ounces, giving an average recovery of 0.123 oz Au/ton; using $20.00 per ounce, the amount recovered was 1986 ounces, giving an average recovery of 0.128 oz Au/ton. Tailings taken during this time reportedly averaged about $1.40 per ton or 0.07 oz Au/ton lost to the tailings (at $20.00 per ounce). Using these figures, the average grade of ore was about 0.198 oz Au/ton with a recovery of 65%.

    b) Concentrates production: 16.2 tons concentrates; 29,178 oz Au; ore grade 1.80 oz Au/ton of concentrates, 0.60 oz Ag/ton of concentrates; Au/Ag ratio of concentrates, 3/1

    Miller (1938, pg. 3) states that the first mill had concentrating tables and 1.8 carloads of concentrates were shipped to Selby's in San Francisco. (The ore contained approximately 2% of concentrates.) This shipment amounted to 32,430 pounds (16.2 tons); assay value 1.80 oz Au/ton and 0.60 oz Ag/ton; total contents, 29,178 oz Au. These concentrates, therefore, ran 0.90 oz Au/ton. From these data, the Au/Ag ratio of 3/1 was calculated. Miller also reports the average value of an ounce of bullion as being: average fineness gold = 0.733; average fineness silver = 0.249. This yields an Au/Ag ratio of 2.94/1. Data from Miller (1938) is the only information pertaining to the Au/Ag ratio in ores at the Big Horn Mine found by this MRDS reporter.

    " 1934-1936; about 2000 tons of ore were milled using a pilot mill, and 12 shipments of concentrates were sent to the Selby Smelter in San Francisco (Ristorcelli, 1988). Miller (1938, pg. 4) reports 10,000 tons ore milled). The gold recovery was not reported.

    Miller (1938, pgs.3, 4) reports that the property had been worked steadily since September 1, 1914. At this time the firm of Mudd and Wiseman took an option on the property. Samples from the mine reportedly assayed $1.75 and $3.00 per ton gold ("Gold at $35.00"). [Note: that the price of gold during this time was $20.67 per ounce; which calculates to 0.08 oz Au/ton and 0.15 oz Au/ton, respectively.] Mudd and Wiseman relinquished their option in 1916, and Fenner continued work in the mine; assays yielded ore grades of 0.08 oz Au/ton and 0.15 oz Au/ton. Miller (1938, pg. 4) reports that in 1917 work at the mine ceased; shortly thereafter Fenner died; and, in 1934 a lease was given to "Eastern people" who milled about 10,000 tons of ore using a pilot mill installed in the old mill building (no Au production reported). The lessees failed to get into production and relinquished the property in 1936. In 1935 title to the property went to Fenner Mines, Inc. Beginning in 1936 and continuing to March 1937, American Metals Co. Ltd. conducted extensive sampling (1100 cannel samples) that yielded an average grade of 0.174 oz Au/ton and proven and probable ore reserves of 371,000 tons (64, 554 oz Au, 2.0 metric tons). There has been no production from the Big Horn Mine since 1936.

  • RESERVES

    Various ore-tonnage estimates made prior to the 1990s range from 303,981 tons of 0.193 oz Au/ton (1.8 metric tons) to 669,978 tons of 0.200 oz Au/ton (4.2 metric tons). Both estimates include measured, indicated, and inferred reserves. The deposit is open-ended, the northwest edge having a drill-hole intersection of 24.2 feet of 0.647 oz Au/ton. In 1935, American Metal Co. Ltd., and their extensive sampling (1100 channel samples) gave an average grade of 0.174 oz Au/ton and proven and probable ore reserves of 371,000 tons (Miller, 1938). This amounts to 64, 554 oz Au (2.0 metric tons).

    In the early 1990s, Siskon Gold Corp. contracted a feasibility study that resulted in a plan to mine Big Horn ore by modified room and pillar method. The Siskon study reported gold mineralization delineated by drilling, of 3,710,080 tons containing 434,079 ounces (13.5 metric tons) of gold (0.117 oz Au/ton average grade), within which is a proven/probable reserve of 1,211,440 tons containing 188,137 ounces (5.9 metric tons) of gold (0.115 oz Au/ton average grade).


    Small; based on reserve estimates ranging from 1.8 to 5.9 metric tons gold.

    Early 1990s estimate: resource = 13.5 metric tons gold; proven+probable reserves = 5.9 metric tons gold (Siskon Gold drilling program and feasibility study conducted from 1990-1992).

    1988: 38,000 oz gold reserves (estimate based on 200,000 tons ore at 0.19 oz gold per ton (1.8 metric tons gold) (from Centurion Properties report, October 21, 1988, in GCS Minefile Folder No. 330-3745).

    1986: Van Nort (1986, in Ristorcelli, 1988, pg. 14):
    Indicated reserves 582,800 tons 0.196 oz Au/ton = 114,229 oz Au
    Probable reserves 72,274 tons 0.214 oz Au/ton = 15,467 oz Au
    Inferred 14,904 tons 0.256 oz Au/ton = 3,815 oz Au
    Total 669,978 tons 0.199 oz Au/ton = 133,511 oz Au (4.2 metric tons)

    1935: American Metals Co. Ltd., and their extensive sampling (1100 channel samples) gave an average grade of 0.174 oz Au/ton and proven and probable ore reserves of 371,000 tons (Miller, 1938). This amounts to 64, 554 oz Au (2.0 metric tons).

  • GOLD/SILVER RATIO

    Miller (1938, pg. 3) states that the first mill had concentrating tables and 1.8 carloads of concentrates were shipped to Selby's in San Francisco. (The ore contained approximately 2% of concentrates.) This shipment amounted to 32,430 pounds (16.2 tons); assay value 1.80 oz Au/ton and 0.60 oz Ag/ton; total contents, 29,178 oz Au. These concentrates, therefore, ran 0.90 oz Au/ton. From these data, the Au/Ag ratio of 3/1 was calculated. Miller also reports the average value of an ounce of bullion as being: average fineness gold = 0.733; average fineness silver = 0.249. This yields an Au/Ag ratio of 2.94/1. Data from Miller (1938) is the only information pertaining to the Au/Ag ratio in ores at the Big Horn Mine found by this MRDS reporter.

    Economic Comments:
    Location of the Big Horn Mine at high elevations within the Sheep Mountain Wilderness Area, the mine's location on the south slope of the San Gabriel Mountains, the long distance to suitable ore-processing sites, the grade of the ore, and method of underground mining, have made development of the Big Horn Mine unfeasible during the period of relatively low gold price.

Comments on development

  • 1. Mid to late 1800s: the San Gabriel River was the scene of possibly the earliest recorded gold placer operation in California when the Franciscan Fathers began mining in 1836. Placer activity continued intermittently for many years thereafter with sufficient yield that Charles Vincent and N.D. Shippy, prospectors, resolved to find the upstream lode-source of the gold. In the fall of 1891 Vincent and Shippy located and staked the Big Horn lode. In 1892 Vincent started what became known as Tunnel No. 1 and dug a large, open cut from which ore was shipped my mule. During 1893, the property was under "bond" to a private Los Angeles group who drove Tunnels, No. 2, 3, and 4 before the property reverted to the owners owing to default in contractual payments by the lessees.
    2. 1900-1906: Except for assessment work by the owners, the property was idle until July 1901 when Col. F.C. Fenner, through his Lowell and California Mining Company, an Arizona corporation, bought the property and started driving Tunnel No. 5 and building a wagon road from the north side of the mountains to the mine. Tunnel No. 5 cut the lode in the spring of 1902. A 2-stamp mill operated for the following 18 months on ore from various parts of the property. Tunnel No. 6 was driven in 1903 with results that encouraged building a 10-stamp mill which began operation in June 1904, using amalgamation concentrating tables. The mill burned down the following June but was rebuilt by mid-September, 1905, and operated successfully until 1907. Tunnels No. 7, 8, and 10 were driven during this time as prospects, and Tunnel No. 9 was driven to connect with No. 6 for ventilation and alternate access. The 10-stamp mill treated about 40 tons ore per day.
    3. 1907-1917: the property was in litigation an idle, but resumed minor activity in September 1914, continuing to 1917. Seely Mudd, a noted mining engineer, optioned the property in 1914 or 1915, through his firm of Mudd and Wiseman, and did considerable exploration work which included driving raises from the Tunnel No. 6 level and starting a lower adit now known as the Fenner Tunnel. This option was relinquished in 1916. Thereafter Fenner continued the lower adit to a total distance of 1100 feet, but all work at the mine ceased in 1917, when Fenner died leaving the property to his wife.
    4. 1917-1936: Fenner's widow kept the idle property in good standing until 1934 when a lease was granted to an "eastern" group, Big Horn Mining Company, R.C. Huffman, president. Big Horn Mining rebuilt the road, cleaned out and retimbered workings, and, among other work, reconstructed a pilot mill in the old mill building. About 2000 tons of ore were milled and 12 shipments of concentrates were sent to the Selby Smelter in San Francisco; concurrently, some development work was done. Having failed to get into continuous production, the lessees gave up the property in 1936, leaving debts to local creditors. In 1935 title to the property was placed with a new company, Fenner Mines, Inc., an Arizona corporation, which was mainly owned by Mr. Fenner's widow and other heirs, and had less than twenty stockholders.
    5. 1936-1980: In 1936 to march 1937, American Metals made a comprehensive examination of the Big Horn including underground surveying, geological mapping, cutting and assaying of channel samples, and diamond drilling. Records of this work have survived, largely intact. In 1939 the Big Horn property was purchased by H.B Chessher of Reno, Nevada, and held by his firm, Security Industrial Corporation, until the deed was transferred to an associated Nevada company, Siskon Corporation, in November, 1966. Except for two raises driven from the 6900 level in 1940, very little new work was done during these years other than maintenance of road and related permits.
  • 6. 1981-1982: In April, 1981, Siskon Corporation and all its assets were sold to and became a subsidiary of the Hanna Mining Co., Cleveland, Ohio. Work during Hanna's tenure has principally been restoration of the road and access to underground workings, some re-sampling, metallurgical tests, preliminary mining and cost estimates, and examination of environmental concerns.
    7. 1983-1985: On April 25, 1983, Great Pacific Resources, Inc., Vancouver, British Columbia, Canada, entered into a contract with Siskon to earn up to 75% of the Big Horn property. Great Pacific established a grid system, mapping surface, doing limited surface sampling, and drilling 4535 feet of core from the surface and underground. A second surface diamond drilling program was initiated in June, 1984, and completed by Inspiration Mines, Inc. after drilling a total of 8542 feet. Inspiration entered into a joint venture agreement with Great Pacific in June 7, 1984, to earn 735 of Great Pacific's potential 75% interest. Inspiration Mines dropped their vested interest in 1985, after spending just over $2 million in exploration and development. Their work attempted to produce a favorable feasibility study on the Big Horn property, which was only in the early to mid-exploration stage.
    8. 1988: Centurion Minerals, Ltd. acquired Siskon Corporation from the Hanna Mining Co., and the Big Horn property was part of this package.
    9. 1990-1997: Siskon Gold Corp. conducted underground diamond drilling and contracted for a feasibility study to develop the Big Horn Mine (100% owned by Siskon). The study was completed in 1993. PAH reported proven and probable reserves of 1,211,440 tons containing 188,137 ounces (5.85 metric tons) of gold. The proposed mining plan called for underground mining with a modified room and pillar method. A processing plant was to be built off-site and the ore was to be trucked to the plant. A site, purchased in March 1995, in the City of Adelanto, has been quit claimed to the sellers following much opposition to the plan for processing of ore at the site. Siskon began environmental permitting for the Big Horn property, and the major permits were in place by June of 1995. During 1997, the price of gold declined to $290 per ounce, with no assurance that prices would recover to levels that would make mining at the Big Horn economical. Consequently, Siskon put Big Horn into a care and maintenance basis.
    10. 1997-2006: No records pertaining to the history of the Big Horn Mine during this period were found by this MRDS reporter.

    ADDITIONAL DETAIL

    Development Status: Idle as of 1997; status from 1997 to July 2006 unknown to this MRDS reporter.

    Year of Discovery: 1891

    Production Size:
    Production:
    Between July 1904 and October 1906, 15, 564 tons of ore from the Big Horn Mine were treated using a 10-stamp mill and plate amalgamation, and $39,720.18 in gold was recovered (Miller, 1938). The price of gold during this period was $20.00 per ounce; therefore, the amount of gold recovered was approximately 1986 ounces, giving an approximate average recovery of 0.128 oz Au/ton.

    Nature of Discovery:
    During the mid to late 1800s the San Gabriel River was the scene of possibly the earliest recorded gold placer operation in California when the Franciscan Fathers began mining in 1836. Placer activity continued intermittently for many years thereafter with sufficient yield that Charles Vincent and N.D. Shippy, prospectors, resolved to find the upstream lode-source of the gold. In the fall of 1891 Vincent and Shippy located and staked the Big Horn lode on the southeast flank of Mount Baden-Powell.

    Owner: As of 1997, Siskon Gold Corporation (100%). The Trust for Public Land offered $250,000 for the property for the purpose of removing it from private ownership and placing it in the public domain. The outcome of this offer is not known.

    Operator: As of December 1997, Siskon Gold Corp., Delaware

Comments on the environmental information

  • The mine is located at elevations of 6200 feet to 8200 feet near the crest of the San Gabriel Mountains; drainage is to the south into Mine Gulch and Vincent Gulch, which drain into the San Gabriel River, which drains into the San Gabriel Valley. The property is surrounded by the Sheep Mountain Wilderness, administered by the U.S. Forest Service; the region is home to an estimated 700 (in 1988) protected Nelson Big Horn Sheep. Offsite ore-processing, which is necessary for environmental mitigation, requires considerable truck traffic through the town of Wrightwood.

Reference information

Bibliographic references

  • Deposit

    Portions of various unpublished reports and reports from various Internet websites, contained in CGS Minefile Folder No. 330-3745.

  • Deposit

    Clark, W. B., 1988, Mount Baldy District in Gold Districts of California, Sesquicentennial Edition, California Gold Discovery to Statehood: CGS (formerly CDMG) Bulletin 193, Page 174.

  • Deposit

    Conrad, R. L., and Davis, T. E., 1977, Rb/Sr Geochronology of cataclastic rocks of the Vincent thrust, San Gabriel Mountains, southern California [abstract]: Geol. Soc. America Abstracts with Programs, v. 9, no. 4, p. 403-404.

  • Deposit

    Ehlers, E. G. and Blatt H., 1980, Petrology: Igneous, Sedimentary, and Metamorphic: W. H. Freeman and Co., San Francisco, 732 p.

  • Deposit

    Ehlig, P. L., 1981, Origin and tectonic history of the basement terrane of the San Gabriel Mountains, central Transverse Ranges; in Ernst, W. G., Editor, 1981, The tectonic development of California, Rubey Volume I: Prentice-Hall, Inc., Englewood Cliffs, New Jersey 07632, Pages 253-283.

  • Deposit

    Hamann, W. E., 1985, Geology and geochemistry of the Big Horn gold mine, San Gabriel Mountains, southern California: M.S. Thesis, University of California Los Angeles (UCLA).

  • Deposit

    Inspiration Mines, Inc., 1985, Big Horn Mine Feasibility Report.

  • Deposit

    Jones, A. G., 1983a, Report on the Gig Horn Mine for Harbour Management Ltd.

  • Deposit

    Miller, C. C., Jr., 1938, Report on Big Horn Mine, Los Angeles County, California, owned by Fenner Mines, Inc., 15 pages (The date of the report is written in pencil on the front of the report; the report is contained in CGS Minefile Folder No. 330-3745.)

  • Deposit

    Miller, F. K., and Morton, D. M., 1977, Comparison of granitic intrusions in the Pelona and Orocopia schists, southern California: U.S. Geol. Survey Jour. Res., v. 5, p. 643-649.

  • Deposit

    Ristorcelli, Steve, January 15, 1988, Summary report and recommendations on the big Horn property, Los Angeles County, California: report prepared for Centurion Minerals, Ltd. And contained in the CGS (formerly CDMG) Minefile archives.

  • Deposit

    Shelton, J. S., 1955, Glendora volcanic rocks, Los Angeles basin, California: Geol. Soc. America Bull., v. 66, p. 45-90.

  • Deposit

    Van Nort, S. D., 1986, Report on the big Horn Mine - An Evaluation of Recent Exploration - for Great Pacific Resources.

General comments

Subject category Comment text
Deposit Geology modified from Hamann, 1985; Ristorcelli, 1988 [Ristorcelli reports his general geology as being adapted from Jones (1983a), reports by Inspiration Mines, Inc. (1985), and Van Nort (1986)]; Ehlig, 1981; and miscellaneous information contained in CGS Minefile No. 330-3745:

The Big Horn Mine is an underground mine with over 7000 feet of workings. It is located along the trace of the Vincent Thrust in the San Gabriel Mountains of southern California. Mineralization occurs in the northeast-striking, 15? to 30? northwest-dipping Vincent Thrust zone. The thrust forms the contact between upper plate San Gabriel Gneiss (Precambrian) and lower plate Pelona Schist. (Precambrian or Mesozoic?). About 6000 feet of the Vincent Thrust zone transects the Big Horn property.

The Pelona Schist and San Gabriel Gneiss comprise a metamorphic assemblage several thousand feet thick in the Big Horn Mine area. Metamorphic foliation generally dips between 15? and 50? to the northwest; metamorphic structures include recumbent isoclinal folds, various lineation features, and augen texture. The gneiss becomes increasingly mylonitized toward the Vincent Thrust. Some schist contains graphite and small garnet, sillimanite or cordierite, all of which suggest a sedimentary origin for the protolith. The gneissic members are mineralogically similar to the schist but are more cohesive and less well-foliated.

Unmetamorphosed igneous dikes and sills in the area of the Big Horn Mine include pegmatite, quartz-feldspar porphyry, quartz-feldspar-hornblende granite, aplite, and andesite. The andesite dikes are the youngest of these rocks and post-date gold-mineralization at the Big Horn mine. Igneous dikes and sills are subordinate in volume to the metamorphic rocks in the vicinity of the mine. Elsewhere in the San Gabriel Mountains, large intrusions of granodiorite, quartz-monzonite, gabbro, and quartz-hornblende porphyry occur.

The Vincent Thrust fault transects the property and strikes generally N 40? E and dips generally 15?- 20? northwest sub-parallel to the fabric in the metamorphic rocks. It is an imbricate fault zone with a close spatial, if not genetic, relationship to the zone of gold mineralization. Other recognized major faults in the region lie outside the vicinity of the Big Horn property. The northwest-striking, right-lateral, strike-slip San Andreas Fault passes through the town of Wrightwood six air miles east of the mine. The Punchbowl Fault, a sub-parallel southern branch of the San Andreas Fault, lies about a mile north of the Big Horn property and passes through Vincent Gap where the mine road joins Highway 2 (Angeles Crest Highway). The Vincent Thrust is cut-off by the Punchbowl Fault.

In the northeast portion of the Bighorn Mine area, high-angle, northwest-striking faults displace the hanging wall and footwall rocks as well as the Vincent Thrust. Continuity of the layered complex of metamorphic strata and sills is also interrupted by at least two northwest-striking, steeply to moderately dipping smaller faults. Most, if not all, of these faults have normal and relatively small displacements. Numerous high-angle reverse faults observed in underground workings have unknown continuity and displacements. Reverse faults have been inferred in order to correlate projections of the Vincent Thrust from the surface to the underground workings. The youngest faults in the mine area include numerous vertical, northwest-striking, right-lateral faults with up to 3 meters of separation.
Deposit Geology (continued):

The hanging wall of the Vincent Thrust consists almost entirely of San Gabriel Gneiss and lesser lenses of schist. The footwall is composed principally of quartz-rich to quartz-graphite Pelona Schist. Post-metamorphic, hydrothermally altered, porphyry sills are also common. Locally, gold-bearing aplite dikes occur near the Vincent Thrust zone and reportedly intrude and are intruded by the porphyry sills. The youngest intrusive geologic event is represented by andesite dikes, which post-date mineralization (Van Nort, 1986, in Ristorcelli, 1988) and are thought to be correlative with the Middle Miocene Glendora Volcanics.

The Vincent Thrust zone crops out for over 6000 feet within the Big Horn property boundaries, and is up to 200 feet thick. Its locally brilliant orange color, due to oxidation and destruction of sulfides and carbonates, is obvious in most places. The absence of gold in samples from surface outcrops of the oxidized zone does not preclude the presence of gold at depth.

In the Big Horn Mine area, the Vincent Thrust has two strands separated by a zone composed primarily of crushed amphibolite derived from the San Gabriel Gneiss. The principal gold-bearing rocks occur within the zone of crushing, and, where amphibolite is more prevalent, both the zone and gold mineralization appear more continuous and thicker (Van Nort, 1986, in Ristorcelli, 1988).
Cataclasite forms a band up to 5 m thick beneath the upper strand of the Vincent Thrust. Cataclasite forms a band from 0 to 2 m thick above the lower strand of the Vincent Thrust. The cataclasite exhibits a relict San Gabriel Gneiss mineralogy. A gouge zone up to 10 cm thick separates the upper cataclasite from the overlying mylonitized San Gabriel gneiss and defines the upper strand of the Vincent Thrust. A gouge zone from 0 to 3 cm thick separates the lower cataclasite from the underlying Pelona Schist and defines the lower strand of the Vincent Thrust.

The mineralized zone occurs as a broad tabular band generally but not exclusively within the thrust zone. Gold mineralization occurs as free gold and in gold-bearing sulfides, and is associated with varying degrees of pyritization, silicification, and carbonatization. Silver is subordinate to gold. Miller (1938) reports that the 32,430 pounds (16.22 tons) of concentrates shipped to Selby's at San Francisco assayed 1.80 oz Au/ton and 0.60 oz Ag/ton. The gold/silver ratio as determined from this information is 3/1. This is the only information pertaining to the Au/Ag ratio at the Big Horn Mine found by this MRDS reporter. Analyses of ore concentrates indicate that gold is also associated with arsenic and lesser copper, lead and zinc. Up to 2% pyrite and lesser arsenopyrite and galena(?) have been recognized in mineralized samples.

The West Boundary Fault (aka West Fault, West Limiting Fault) is a major discontinuity within the mineralized band. It strikes N 45? - 60? W, dips 43? - 75? NE, and appears to have about 100 feet of normal displacement. It occurs west of the No. 6 Stope and apparently caused the termination of mining in that direction -- reportedly due to a decrease in gold-mineralization west of the fault. Another north-trending, though poorly defined fault, appears to cut off the mineralization on the east end of the No. 9 stopes. Gold-mineralization has been found beyond both of these faults, however, elsewhere along the crushed zone between the upper and lower strands of the Vincent Thrust.
Deposit Geology (continued):

Although the mineralized zone at the Big Horn Mine is up to 200 feet thick, mineable thicknesses are estimated to range between 10 feet and 50 feet due to the distribution of ore-grade mineralization in the zone. Gold mineralization, accompanied by the addition of quartz, pyrite, and ankerite, is confined primarily to the crushed amphibolite and cataclasite. Gold mineralization is associated with quartz-pyrite and calcite-pyrite veinlets, and appears to have preceded late-stage carbonate replacement of quartz and feldspar in the ore zone. Oxidation of pyrite occurred contemporaneously with late-stage carbonate replacement.

Hamann (1985) used fluid inclusions, stable isotopes, and petrographic data to establish the chemistry of the hydrothermal system responsible for gold mineralization at the Big Horn Mine. Quartz-pyrite-gold and calcite-pyrite-gold veinlets were precipitated from a fluid averaging 5.5 percent equivalent weight NaCl at a temperature between 176? to 253? C. Oxygen fugacity was confined to -41.18 < log fO2 < -37.76 and sulfur fugacity to -15.4 < log fS2 < -12.85. The pH was between 5.5 < pH < 8.5.

Stable isotope analyses of carbon, oxygen, and sulfur suggest that the hydrothermal system was derived from deep-seated metamorphic fluids. Hamann postulates that the hydrothermal system became enriched in gold during metamorphism and dewatering of the Pelona Schist protolith. Metamorphism of the Pelona Schist occurred concurrently with over-thrusting along the Vincent Thrust during the Pliocene, latest Cretaceous at the earliest (Ehlig, 1981). Movement along the Vincent Thrust caused brecciation of the gneiss and amphibolite in the fault zone prior to the intrusion of the aplite and quartz monzonite porphyry of Miocene age. These intrusives were dated at 14 to 16 m.y. by K-Ar method (Miller and Morton, 1977, in Ehlig, 1981).

Hamann states that sulfur isotope data suggest that the Miocene intrusives supplied most of the sulfur to the hydrothermal fluid, forming a stable and mobile gold-sulfide complex ion, and generating heat, which allowed for convective flow of the system. He also suggests that gold deposition was caused by changes in the chemistry of the system. As ascending fluids reached near-surface conditions, temperature, Eh, and pH regimes presumably changed, causing the gold-sulfur complex ion to become unstable, resulting in deposition of native gold. He also suggests that late-stage(?) unmineralized quartz veinlets reflect a mixing of meteoric water with the deep seated metamorphic fluids.

Gold mineralization at the Big Horn Mine predates the intrusion of the youngest igneous rocks in the area, which are unmetamorphosed, unmineralized andesite dikes, believed to be correlative with the Middle Miocene Glendora Volcanics.

Various ore-tonnage estimates made prior to the 1990s range from 303,981 tons of 0.193 oz Au/ton (1.8 metric tons) to 669,978 tons of 0.200 oz Au/ton (4.2 metric tons). Both estimates include measured, indicated, and inferred reserves. The deposit is open-ended, the northwest edge having a drill-hole intersection of 24.2 feet of 0.647 oz Au/ton. In 1935, American Metal Co. Ltd., and their extensive sampling (1100 channel samples) gave an average grade of 0.174 oz Au/ton and proven and probable ore reserves of 371,000 tons (Miller, 1938). This amounts to 64, 554 oz Au (2.0 metric tons).
Deposit Geology (continued):

In the early 1900s, Siskon Gold Corp. contracted a feasibility study that resulted in a plan to mine Big Horn ore by modified room and pillar method. The Siskon study reported gold mineralization delineated by drilling, of 3,710,080 tons containing 434,079 ounces (13.5 metric tons) of gold (0.117 oz Au/ton average grade), within which is a proven/probable reserve of 1,211,440 tons containing 188,137 ounces (5.9 metric tons) of gold (0.115 oz Au/ton average grade) (Siskon Gold drilling program and feasibility study conducted from 1990-1992).

The proposed plan to mine the Big Horn deposit includes an underground tunnel with an entrance on the north slope of the San Gabriel Mountains, to provide access to the ore body and avoid environmental and terrain problems. The plan included trucking ore through the town of Wrightwood to the City of Adelanto, approximately 50 road miles to the northeast, for offsite processing. Environmental permits were issued in 1995, but the then declining price of gold put Siskon's plan on hold indefinitely. The current (2006) status of the mine is unknown to this MRDS reporter.

The mine occupies parts of 15 contiguous patented lode claims, 2 patented millsite claims, 5 unpatented mining claims, and 2 unpatented association placer claims. The mine is surrounded by the Sheep Mountain Wilderness Area, Angeles National Forest.
Deposit Deposit Size:
Various ore-tonnage estimates made prior to the 1990s range from 303,981 tons of 0.193 oz Au/ton (1.8 metric tons) to 669,978 tons of 0.200 oz Au/ton (4.2 metric tons). Both estimates include measured, indicated, and inferred reserves. The deposit is open-ended, the northwest edge having a drill-hole intersection of 24.2 feet of 0.647 oz Au/ton. In 1935, American Metal Co. Ltd., and their extensive sampling (1100 channel samples) gave an average grade of 0.174 oz Au/ton and proven and probable ore reserves of 371,000 tons (Miller, 1938). This amounts to 64, 554 oz Au (2.0 metric tons).

In the early 1990s, Siskon Gold Corp. contracted a feasibility study that resulted in a plan to mine Big Horn ore by modified room and pillar method. The Siskon study reported gold mineralization delineated by drilling, of 3,710,080 tons containing 434,079 ounces (13.5 metric tons) of gold (0.117 oz Au/ton average grade), within which is a proven/probable reserve of 1,211,440 tons containing 188,137 ounces (5.9 metric tons) of gold (0.115 oz Au/ton average grade).


Small; based on reserve estimates ranging from 1.8 to 5.9 metric tons gold.

Early 1990s estimate: resource = 13.5 metric tons gold; proven+probable reserves = 5.9 metric tons gold (Siskon Gold drilling program and feasibility study conducted from 1990-1992).

1988: 38,000 oz gold reserves (estimate based on 200,000 tons ore at 0.19 oz gold per ton (1.8 metric tons gold) (from Centurion Properties report, October 21, 1988, in GCS Minefile Folder No. 330-3745).

1986: Van Nort (1986, in Ristorcelli, 1988, pg. 14):
Indicated reserves 582,800 tons 0.196 oz Au/ton = 114,229 oz Au
Probable reserves 72,274 tons 0.214 oz Au/ton = 15,467 oz Au
Inferred 14,904 tons 0.256 oz Au/ton = 3,815 oz Au
Total 669,978 tons 0.199 oz Au/ton = 133,511 oz Au (4.2 metric tons)

1935: American Metals Co. Ltd., and their extensive sampling (1100 channel samples) gave an average grade of 0.174 oz Au/ton and proven and probable ore reserves of 371,000 tons (Miller, 1938). This amounts to 64, 554 oz Au (2.0 metric tons).

Reporter information

Type Date Name Affiliation Comment
Reporter 21-JUL-2006 Hill, Robert L. California Geological Survey CGS (Formerly CDMG)
Editor 20-FEB-2008 Schruben, Paul G. U.S. Geological Survey Converted from S&A FileMaker format to Oracle. Edit checks on rocks, units, and ages with Geolex search, and other fields.

Beyond USGS

Supplemental information added by qvyshift.com. Not part of the original USGS MRDS record.

External references

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