Colosseum Mine

Past Producer in San Bernardino county in California, United States with commodities Gold, Silver, Copper, Lead, Arsenic, Antimony, Zinc

Geologic information

Identification information

Deposit ID 10310702
Record type District
Current site name Colosseum Mine
Alternate or previous names Ivanpah Consolidated Mine
Related records 10159351

Geographic coordinates

Geographic coordinates: -115.56556, 35.57 (WGS84)
Elevation 1800
Location accuracy 100(meters)
Relative position 45 air miles S 20.13? W of Las Vegas, NV; 7 air miles N 11.29 ? W of Mountain Pass, CA (Mountain Pass is located at the junction of Interstate Hwy 15 and the Mountain Pass Rare Earths Mine road).
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Geographic areas

Country State County
United States California San Bernardino

Public Land Survey System information

Meridian Township Range Section Fraction State
San Bernardino 017N 013E 10, 15 California

Comments on the location information

  • Longitude and latitude represent the shaft symbol approx. 1480 feet SW of the peak at 1858 meters (6096 feet) elevation on the Clark Mountain 71/2 minute quadrangle.

Commodities

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

Comments on the commodity information

  • Commodity Info: Minor Commodities: Copper (0.1%); 0.25% Cu was recovery from concentrates sent to the smelter in the early days of underground mining. Trace Commodities: Lead (3500 ppm), Arsenic (1500 ppm), Antimony (8300 ppm), Zinc (7500 ppm), (2300 ppm)
  • Ore Materials: Native gold, auriferous pyrite, electrum, chalcopyrite, galena, silver ores.
  • Gangue Materials: Quartz, barren pyrite, siderite, iron and manganese oxides (goethite), sericite, barite, felsite; felsite breccia (fragments of felsite, Tapeats Quartzite, Precambrian granite and gneiss, andesite); rubble breccia (fragments of dolomite, Precambrian granite and gneiss; comminuted rock flour with composition essentially the same as the felsite and igneous breccia with higher carbonate content; cobbles of Paleozoic Tapeats Quartzite, chips of Bright Angel Shale, boulders and slabs of Goodsprings Dolomite, and fragments of Mesozoic felsite and igneous breccia).

Materials information

Materials Type of material
Gold Ore
Electrum Ore
Chalcopyrite Ore
Galena Ore
Silver Ore
Quartz Gangue
Pyrite Gangue
Siderite Gangue
Iron Gangue
Goethite Gangue
Sericite Gangue
Barite Gangue
Quartzite Gangue
Granite Gangue
Gneiss Gangue
Andesite Gangue
Granite Gangue
Gneiss Gangue
Quartzite Gangue
Shale Gangue
Dolomite Gangue

Alteration

  • (Local) Quartz-sericite-pyrite

Host and associated rocks

  • Host or associated Host
    Rock type Volcanic Rock (Aphanitic) > Felsic Volcanic Rock
    Rock type qualifier Felsite breccia
    Stratigraphic age (youngest) Early Cretaceous
  • 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) Precambrian
  • Host or associated Associated
    Rock type Sedimentary Rock
    Stratigraphic age (youngest) Paleozoic
  • Host or associated Associated
    Rock type Volcanic Rock (Aphanitic) > Intermediate Volcanic Rock > Andesite
    Rock type qualifier dikes

Nearby scientific data

(1) -115.56556, 35.57

Ore body information

  • General form Pipe

Controls for ore emplacement

  • Economic gold mineralization occurs within two felsite (rhyolite) breccia pipes. Gold occurs as fine, rounded inclusions inside of coarse, euhedral pyrite or in contact with pyrite as fracture fillings or along grain boundaries (Corbett, 1980, cited in Sharp, 1984, pg. 138). The more numerous occurrences are as fillings and boundary coatings. The gold was further determined to be alloyed with silver and of a size range of 1 to 30 microns with the majority from 5 to 20 microns. As reported by Davis and others (1989), gold at the Colosseum Mine is generally submicroscopic and associated with sulfide mineralization, chiefly pyrite. It occurs as free gold, with minor alloyed silver. It is primarily in contact with pyrite in fractures in the pyrite or along pyrite grain edges. Secondarily, it occurs as isolated particles in quartz and other gangue minerals but spatially always close to pyrite, and rarely as particles encased in euhedral pyrite. The pyrite mineralization and minor base metal sulfides occur in three distinct styles: (1) as disseminations, (2) as open space filling or vein/fracture filling, and (3) in breccia clasts replacing dolomite. The pyrite megascopically ranges from absent to 30% of rock volume within the breccia pipes (Davis and others, 1989). Although the gold is apparently spatially and geochemically associated with pyrite, there is only a general proportional relationship between pyrite content and gold content. Sometimes, a high volume percentage of pyrite has only geochemically anomalous gold. This makes visual pyrite only a general grade control tool and not a specific tool This further makes the ore control process all the more difficult.

Comments on the geologic information

  • INTRODUCTION The Colosseum Mine is located in the Clark Mountain mining district 45 air miles southwest of Las Vegas, Nevada, in San Bernardino County, California. The district includes the Mountain Pass rare earth mine seven miles south of the Colosseum Mine, numerous abandoned copper mines, and scattered fluorite, antimony, and tungsten prospects. Most gold and silver deposits in the district are within the northeast quadrant of the district north of Clark Mountain, and are associated with emplacement of a breccia complex into Precambrian basement rocks. The complex is comprised of two felsite (also called "rhyolite" and "rhyolite felsite" by other authors) breccia pipes that form a northeast-southwest elongate complex, which contains mineralized zones of disseminated auriferous pyrite. The historic Colosseum Mine was developed as underground workings within the southwest pipe, also referred to as the west pipe; the northeast pipe, also referred to as the east pipe was not mined until the advent of modern mining. The modern Colosseum Mine is developed as two open pits: the South Pit, excavated in the west pipe, and the North Pit, developed in the east pipe. Exploration in the Clark Mountain district began in the late 1860s. The district was organized on July 18, 1865, by John Moss, owner of several mines, including the historic Colosseum Mine (Crossman, 1980, cited in Sharp, 1984). More than $3.5 million worth of silver ore was produced from the Beatrice and Monitor mines between 1870 and 1880. Less significant values of gold, copper, lead, tungsten, and fluorite ores have been produced from the district.
  • STRUCTURAL GEOLOGY AND STRATIGRAPHY Regional Tectonic Setting The Colosseum Mine is located at the southern end of the Sevier thrust belt (foreland thrust belt) in the southern Basin and Range Province. Gold mineralization occurred about 100 Ma in earliest Late Cretaceous time (post-Sevier thrusting/pre-Basin and Range extensional tectonics), probably in association with one of several shallow-level, early Late Cretaceous stocks that appear to represent northeastern outliers of the magmatic belt that comprises the broadly calc-alkaline series of six plutons of the Teutonia Batholith (Haxel and Miller, 2006). Mesozoic deformational features of regional extent reflect shortening shown by brittle-style thrust plates developed in the foreland of the Cordilleran thrust belt and ductile-style nappes in southeastern California and Arizona (Burchfiel and Davis, 1971, 1977, 1981; Howard and others, 1980; Snoke and Miller, 1988; Miller and Barton, 1990; all cited in Haxel and Miller, 2006). Generally east-directed thrust faults, present in the Clark Mountain Range area, may be Middle Triassic(?) through Early Jurassic (Burchfiel and Davis, 1980; cited in Haxel and Miller, 2006(?)). Some east-directed thrust faults in the Clark Mountain Range were cut by small dioritic plutons originally dated at 190 and 200 Ma by K-Ar (Burchfiel and Davis, 1981) but now known as Late Jurassic. Local Structure and Stratigraphy The Colosseum Mine is located within the Clark Mountain thrust complex at the southern end of the Sevier foreland thrust belt, in the southern part of the Basin and Range Province. The age of thrusting ranges from late Permian to late Cretaceous (Armstrong, 1968, cited in Sharp, 1984, p. 120). Major thrust faults near the mine include, from west to east, the northwest-trending Mesquite Pass and Keystone thrusts; the Keystone thrust reportedly is a decollement structure. Crustal development of the region is further complicated by Tertiary Basin and Range crustal extension and regional uplift accompanied by low-angle gravity sliding and by high-angle normal faulting that modified earlier-formed structures. Gravity sliding along the Keystone thrust, for example, is interpreted by Sharp (1984) to have occurred during Tertiary crustal extension prior to the Keystone thrust being offset by the Clark Mountain high-angle normal fault during the late Tertiary. Normal faulting has also resulted in the thrust faults being exposed repeatedly in separate fault blocks. The Mesquite Pass and Keystone thrusts have been mapped in the fault block that lies west of the Clark Mountain normal fault located west of the Colosseum Mine. The Mesquite Pass thrust has also been mapped in the fault block that lies east of the Ivanpah normal fault located east of the mine.
  • STRUCTURAL GEOLOGY AND STRATIGRAPHY (continued) Local Structure and Stratigraphy (continued) Basin and Range faulting has also strongly influenced the present-day physiography of the region. The Colosseum Mine is situated between 5000 and 6000 feet elevation at the northern edge of a saddle located in hilly and plateau-like terrain along the north-northeast flank of the Clark Mountains. It is located in an uplifted horst of Precambrian (1,700 m.y.) crystalline basement that is bounded on the west by the northwest-trending, 65? to 75? west-dipping, Clark Mountain normal fault and on the east by the northwest-trending, steeply dipping, Ivanpah normal fault. The mine is developed in two late Cretaceous (100 m.y.) mineralized felsite (rhyolite) breccia pipes within the Precambrian crystalline basement. The basement consists of biotite-gneiss interlayered with granite gneiss, both of which have been intruded by alaskite and pegmatite dikes that generally parallel the regional foliation but locally are discordant. Localized zones of quartz-mica schist occur parallel to the gneissic layering. The felsite breccia pipe complex intrudes and cuts across the northwesterly trending fabric of the gneissic layers. Early Cretaceous andesite dikes, one to four feet in width, also cut across the foliation in the layered Precambrian rocks. The andesite dikes are cut by younger felsite dikes of the pipe complex. The trend of the andesite dikes is congruent with that of the felsite breccia pipe complex, suggesting that the breccia pipe complex may have intruded along the same fracture system (zone of weakness) as that along which the andesite dikes intruded. Paleozoic carbonate rocks, mantled by Mesozoic sedimentary and volcanic rocks, crop out in the area west of the Colosseum Mine. Thrust faulting during the Sevier orogeny thickened a pre-existing pile of miogeosynclinal carbonate rocks of Paleozoic and Mesozoic age overlying the crystalline basement rocks within the vicinity of Clark Mountain. The stratigraphic thickness of the carbonate section prior to the Sevier orogeny was 13,000 feet east of the Ivanpah fault, thickening westward towards the Kingston Range to 20,000 feet, and farther west beyond the Kingston Range to 30,000 feet (Hewett, 1965, cited in Sharp, 1984, pg.121). The thickness of the post-thrust faulted sedimentary rocks at Clark Mountain and vicinity, prior to uplift and denudation, is estimated by Hewett to have been 20,000 to 30,000 feet. The present erosion surface exposes 1500 feet of Paleozoic carbonate rocks. A reconstruction of the carbonate section reveals their present total thickness below the Mesquite Pass thrust to be about 4500 feet.
  • FELSITE BRECCIA PIPE COMPLEX (Sharp, 1984) The two felsite (rhyolite) breccia pipes within the northern portion of the Clark Mountains mining district crop out as resistant knobs that form a northeast-trending ridge across less resistant inter-layered gneiss and granite gneiss of the Precambrian basement. The outcrops of both pipes are similar in shape and size. In plan, the west pipe has a teardrop shape, 500 feet wide and 700 feet long, with its tapered eastern end thinning to a 150-foot wide, arcuate breccia dike that connects with the east pipe. The east pipe is also teardrop shaped, 500 feet wide and 700 feet long, and is tapered on its eastern end, from which emanates a 1- to 3-foot-wide felsite dike. Two additional felsite dikes radiate outward from the east pipe, one trending northwest parallel to the fabric in the Precambrian rocks, and the other southwest, across the fabric. Another felsite dike radiates southwestward from the west pipe. Similar felsite dikes occur regionally within the Precambrian block as far south as Mountain Pass (Olson and others, 1954, cited in Sharp, 1984, pg. 127). Within the vicinity of the breccia pipe complex, felsite dikes have been traced for two miles east, a mile south, and a half mile west of the pipes. Felsite also occurs as sills in the Tapeats Quartzite. The breccia pipes are multiphase breccia events. The east pipe comprises an early felsite phase and a slightly later igneous breccia phase. The west pipe also has an early felsite phase and a slightly younger igneous breccia phase, same as the east pipe, but was made composite by an even younger rubble breccia event. The lithologies of fragments within the breccia pipes were used to reconstruct the pre-existing stratigraphy above the pipes, and to determine the height to which each breccia pipe had stoped. Breccia fragments of quartzite in the felsite and igneous breccia are evidence that during early events in their evolution the east and west pipes intruded at least through the Precambrian basement and into the then overlying Tapeats Quartzite. The rubble breccia of the west pipe characteristically contains cobbles of quartzite, chips of shale, and pebbles to slabs of dolomite. The rubble breccia of the west pipe, therefore, clearly stoped at least to the base of the Goodsprings Dolomite. The breccia events of the pipes are proposed to have developed through fluidization and brecciation of older rock units. The Goodsprings Dolomite fragments within the rubble breccia suggest the reconstructed pipe complex to have once been 1300 to 1400 feet higher than its present outcrops. This reconstruction also requires the Tapeats Quartzite to have once been from 500 to 600 feet above the present land surface, and the base of the Goodsprings to have been from 1300 to 1400 feet above the present land surface. Deep diamond drilling confirms the continuation of the breccia pipes to a depth of at least 2300 feet, making the total restored vertical dimension of the pipe complex to have been at least 3600 feet. Drilling of the pipe complex confirmed the presence of commercial values of disseminated gold mineralization to depths of at least 500 feet.
  • Felsite and Igneous Breccia Felsite (rhyolite) is the oldest rock in both pipes of the breccia complex. Felsite occurs as both a well-foliated rock and as a massive structureless rock. The foliated variety, where it occurs near contact zones, has a foliation oriented parallel to the contact. In the interior regions of the pipes, distorted swirl foliations are more typical. The massive variety occurs only within the interior regions of the pipes. The felsite is gray to white and, where foliated, has a taffy-like appearance. The felsite is composed of nearly equal proportions of quartz, potash-feldspar, and sericite with accessory siderite. The carbonate content of the felsite averages about 6% of total rock volume. Disseminated, minus 1/16-inch pyrite always occurs in the felsite. Manganese and iron oxides occur as stains in varying amounts throughout the complex with the manganese oxide occurring in greater abundance. Occasional muscovite phenocrysts occur within the felsite. Some of these mica phenocrysts were separated from the felsite and used for age determination. Samples from the east and west pipes gave age dates of 99.8 ? 3.6 m.y. and 102 ? 4 m.y., respectively (Krueger, 1979, cited in Sharp, 1984, pg. 128); hence the age of the pipe complex is approximately 100 m.y. In the area of the west breccia pipe, the felsite dikes have been severely contorted by northeast-southwest compressional forces. These contortions occur megascopically as irregular folds and areas of differential thinning of the dikes. Microscopic distortions within the dikes are exhibited as hairline, sine-wave-shaped fractures (Delaney, 1972, cited in Sharp, 1984, pg.128). The folded and foreshortened felsite dikes are thought to indicate as much as 150 feet of compressional thinning of the Precambrian rocks west of the breccia pipe complex. Dikes trending easterly from the complex also exhibit differential thinning but little or no folding. The igneous breccia portion of the pipes formed as a result of brecciation of the preexisting felsite. The matrix of this breccia is felsitic, and the major breccia fragments are felsite, while minor quartzite, granite, gneiss, and andesite also occur as fragments. The composition of the matrix of the igneous breccia is essentially the same as the felsite, differing only in its carbonate content, which ranges from 5 to 20%. Rubble Breccia The rubble breccia portion of the pipe complex is the most spectacularly mineralized rock of the complex. The highest grade of mineralization and the most diverse accumulation of breccia fragments are associated with this unit. Mineralization ranges from disseminated pyrite in the matrix of the breccia to massive replacement of dolomite breccia fragments by coarse, cubic, auriferous pyrite, and sphalerite, siderite, and chalcopyrite. Eight fragments selectively sampled from the rubble breccia averaged 0.125 ounces gold, 0.172 ounces silver, 969 ppm copper and 4800 ppm zinc. The matrix of the rubble breccia is comminuted rock flour with an overall composition essentially the same as the felsite and igneous breccia, differing only in its higher carbonate content which ranges to a high of 35%. Breccia fragments within the rubble breccia range from Precambrian granite and gneiss to cobbles of Paleozoic Tapeats Quartzite, chips of Bright Angel Shale, boulders and slabs of Goodsprings Dolomite, and fragments of Mesozoic felsite and igneous breccia. Diamond drill cores reveal the presence of dolomite fragments within the breccias of the west pipe at a hole depth of 1900 feet, thereby indicating mixing of fragments within this pipe over a vertical distance of 3200 feet.
  • Emplacement of the Felsite Breccia Pipes The process of fluidization is commonly called upon as the mechanism by which breccia pipes are formed. The composition of the fluidizer varies from pipe to pipe, with the only chemical evidence of the fluidizer remaining in a frozen mineral state. Carbon dioxide gas combined with steam and fluorine gas was postulated to be the fluidizer for the Redwell Basin breccia pipe complex in Colorado The breccia pipes of the Clark Mountain mining district appear texturally and compositionally the same as the breccia pipes of the Redwell Basin except for the absence of fluorite and hydrous minerals in the Clark Mountain pipes (Sharp, 1978, cited in Sharp, 1984, pg. 128; Delaney, 1971, cited in Sharp, 1984, pg. 128). The carbonate content of the breccias at Clark Mountain is many times greater than in the breccias at Redwell, suggesting that the fluidization mechanism at Clark Mountain was through the release of only carbon dioxide gas, as opposed to the multi-compositional fluidizer at Redwell Basin. The carbonate content of the Clark Mountain breccia pipes increases with each progressively younger breccia event (6% for felsite, 20% for igneous breccia, and 30% for rubble breccia). The increase in carbonate content is believed to reflect the progressively higher level of stoping of each breccia event into the roof rocks. The igneous breccia stoped mostly into the low- to no-carbonate content Tapeats Quartzite and Bright Angel Shale, while the younger rubble breccia stoped higher into the Goodsprings Dolomite.
  • Structural Evolution of the Breccia Pipe Complex The pre-felsite structural setting near Clark Mountain and vicinity during early Cretaceous time is interpreted by Sharp (1984) to have consisted of a folded, and by then repetitively thrust-faulted, stack of Paleozoic sedimentary rocks unconformably overlying a Precambrian metasedimentary complex. The regional dip of this Paleozoic sedimentary pile was approximately ten degrees towards the west at the time of the breccia pipe development, and the Precambrian foliation was high-angle to vertical. Intrusion of felsitic magma into the Precambrian basement rocks about 100 m.y. ago marked the beginning of the development of the breccia pipe complex. Exposed outcrops of felsite dikes from Clark Mountain to Mountain Pass, a distance of 7 miles, attest to the regional extent of this intrusive event. As a result of invasion of this magma (as reported in Clark County, Nevada, by Kane 1963, cited in Sharp, 1984, pg. 129), the Precambrian basement rocks were uplifted and tilted slightly westward. Following the complete crystallization of the early felsite and igneous breccia phases of the breccia pipe complex, additional invasion of magma resulted in further uplift and tilting of the region and emplacement of the late phase rubble breccia within the pipe complex. Sharp proposed that, by the end of the late Cretaceous, the felsite breccia pipe complex had invaded to its maximum structural level, the intruded Precambrian basement had been elevated approximately 1000 feet, and the entire Paleozoic section and crystalline basement had been rotated downward to the west an additional ten degrees. Basin and Range tectonics of late Tertiary age superimposed additional structural complications on the region through renewed uplift associated with the development of the "Ivanpah Upland" of Hewett (1956, cited in Sharp, 1984, pg. 130) located east of the trace of the present-day Ivanpah fault. This renewed uplift of the region is interpreted by Sharp to have resulted in westward directed gravitational gliding along the Keystone decollement surface, reversing the motion along this earlier thrust fault and decapitation of the rubble breccia pipe and its associated high-level, vein-silver mineralization. A gravitational glide of 2800 feet would be sufficient to have displaced the vein-silver mineralization from a position directly above the pipe complex to its present position west of the pipe complex. Continued uplift and tilting of the region produced localized structural deformation of the felsite dikes of the breccia complex and crystalline Precambrian basement rocks. Deep fracturing across foliation west of the pipe complex is postulated to have occurred in the basement rocks in response to vertical uplift. An uplift of 800 feet and an additional down-to-the-west rotation of ten degrees are suggested for this period of structural adjustment. The waning stages of structural development of this portion of the Clark Mountain thrust complex produced the features observable today. The Clark Mountain normal fault formed drag folds within, and additional tilting of the Tapeats Quartzite. This accounts for the slice of quartz-pyrite mineralized Bright Angel Shale being positioned side by side with silver-bearing calcite-dolomite veins in the Goodsprings Dolomite. An additional upward movement of 400 feet on the Precambrian horst lying between the Clark Mountain fault to the west and the Ivanpah fault to the east would complete the postulated 2200 feet of vertical uplift and twenty degrees of rotational westward tilting required to produce the present-day structural configuration for this portion of the Clark Mountain thrust complex.
  • METAL/MINERAL ZONATION IN THE NORTHEAST QUADRANT OF THE CLARK MOUNTAIN MINING DISTRICT Sharp (1984) identified four distinct metal/mineral zones: 1) a vein-silver zone, 2) a gold zone, 3) a tungsten zone, and 4) a fluorite zone, in the area surrounding the Colosseum Mine. The vein-silver and gold zones are interpreted by Sharp as being genetically related to emplacement of the felsite breccia pipe complex. Sharp is not clear on whether or not the tungsten zone is related to the breccia pipe complex, but geochemical evidence suggests that anomalous tungsten in some veins near the complex might be related to emplacement of the complex. According to Sharp, the fluorite zone is not related to emplacement of the breccia pipes. In map view, the geometric pattern of zoning has been complicated by post-mineral gravity sliding along the Keystone thrust and by post-mineral Basin and Range faulting. Mineral Zonation Associated with the Felsite Breccia Pipes Sharp's (1984) interpretation is that the original vertically zoned mineralization associated with the felsite breccia pipes has been faulted into horizontally juxtaposed zones by post-Sevier, low-angle gravitational gliding on the Keystone thrust fault, and by high-angle Basin and Range normal faulting on the Clark Mountain fault. The present zoning, from west to east, represents displaced slices of these once vertically stacked zones. Vein-silver zone This zone, the westernmost zone of mineralization, occurs within the Goodsprings Dolomite west of the Colosseum Mine area, and represents, prior to faulting and gravity sliding, the original highest level of mineralization associated with the felsite breccia pipes. The westward-displaced vein-silver zone lies west of the trace of the Keystone thrust fault. The silver veins are restricted to the Goodsprings Dolomite. The veins are composed of a gangue of calcite-dolomite and quartz, while the ore mineralization is reported to have occurred as pods and blebs of stromeyerite with minor azurite and malachite (Hewett, 1956, cited in Sharp, 1984, pg. 125). The veins trend northwest, and dip steeply to the northeast. Sharp interprets Hewett's description of silver mineralization and post-mineral fractures at the Allie Mine to be compatible with Sharp's interpretation that the silver mineralization represents a zone of silver mineralization that was situated above the felsite breccia pipes prior to the zone being displaced to the west by gravity sliding along the Keystone thrust, followed by displacement down to the west along the Clark Mountain normal fault (Hewett, 1956, cited in Sharp, 1984, pg. 125). The vein-silver zone and its Goodsprings Dolomite host rock have been removed by erosion from the area east of the Keystone thrust, the area that includes the Colosseum Mine and breccia pipe complex.
  • METAL/MINERAL ZONATION IN THE NORTHEAST QUADRANT OF THE CLARK MOUNTAIN MINING DISTRICT (continued) Gold zone The uppermost portion of the zone of gold mineralization (upper gold zone) is interpreted by Sharp as occurring within the Bright Angel Shale-Tapeats Quartzite quartz-pyrite zone. It represents an intermediate level of mineralization associated with the felsite breccia pipes. A portion of this zone is preserved in a semicircular area within the Bright Angel Shale immediately west of the trace of the Clark Mountain fault west of the Colosseum Mine. Mineralization within the Bright Angel Shale is characterized by quartz-pyrite veins that both cross-cut and parallel the shale lamination. Gold mineralization in the Bright Angel Shale was situated above the felsite breccia pipes prior to the zone being displaced to the west by gravity sliding along the Keystone thrust, followed by displacement down to the west along the Clark Mountain normal fault. In the area east of the Clark Mountain fault, which includes the area around the Colosseum Mine and breccia pipe complex, the upper portion of the gold zone and its host rocks, the Bright Angel Shale and Tapeats Quartzite, have been removed by erosion. The displaced remnant of the upper portion of the gold zone west of the Clark Mountain Fault is not economic. The lower portion of Sharp's gold zone (lower or main gold zone) occurs within the Precambrian basement in an uplifted block (horst) between the Clark Mountain normal fault on the west and the Ivanpah normal fault on the east. In map view, the zone forms a nearly circular area that surrounds and includes both of the felsite breccia pipes and their enclosing Precambrian wall rocks. Prior to displacement of the upper portion of the gold mineralization along faults, the lower portion of the gold zone was situated directly beneath the Bright Angel Shale-Tapeats Quartzite quartz-pyrite zone. The main gold zone is characterized by two distinct types of gold occurrences: 1) within the southeast half of the circular zone, the gold occurs primarily in association with sufide minerals, chiefly pyrite, of the breccia pipes; and 2) within the northwest half of the zone, the gold occurs as a constituent of quartz-barite and quartz-pyrite veins and veinlets, which form a complex network surrounding the northernmost felsite dikes of the breccia complex. The historic Colosseum Mine was developed as underground workings in the west breccia pipe within the lower (main) gold zone, and constitutes the largest historic gold mine within the district. No recorded production occurred until the 1930s, with production of about 615 ounces gold (Beatty, 1989b). Recorded production for the mine also indicates that $45,000 in gold and copper was produced prior to 1940 (Hewett, 1956, cited in Sharp, 1984, pgs. 125-126). The main historic developments at the mine consist of a vertical stope in the central interior of the rubble breccia portion of the west breccia pipe and semicircular workings on the westernmost edge of the west pipe.
  • METAL/MINERAL ZONATION IN THE NORTHEAST QUADRANT OF THE CLARK MOUNTAIN MINING DISTRICT (continued) Mineral Zonation Not Associated with the Felsite Breccia Pipes Tungsten Zone: the major portion of the tungsten zone occurs in a northwest trending belt within the Precambrian basement rocks east of the Clark Mountain fault, with truncated slices in Paleozoic rocks west of the Keystone and Clark Mountain faults. Dobbs (1961, cited in Sharp, 1985, pg. 126) describes the tungsten mineralization occurring at the Mojave Mine as "(consisting) of wolframite, scheelite, gold, silver, pyrite, azurite and malachite in a gangue of quartz and subsidiary calcite." Rock chip geochemistry of veins in the Precambrian basement around the breccia pipe complex reveals that quartz-pyrite veins contain tungsten ranging from non-detectable to 500 ppm. Sharp is not clear on whether or not this tungsten mineralization might be associated with the breccia pipes. Fluorite Zone: Fluorite mineralization occurs within low-angle shears and fractures developed parallel to the Keystone and Mesquite Pass thrust faults in the area west of the traces of the Keystone and Clark Mountain faults. Massive, friable, and schistose fluorite with sericite are characteristic of this mineralization (Crosby and Hoffman, 1951, cited in Sharp, 1984, pg. 126). Varying amounts of pyrite, copper carbonates, silver, and tungsten occur sporadically with fluorite. Fluorite occurs throughout the Clark Mountain mining district and beyond, and is not considered by Sharp to be part of the mineral zonation associated with the felsite breccia pipes.
  • GEOCHEMISTRY OF THE BRECCIA PIPE COMPLEX The distribution of metals around the breccia pipe complex was determined by chip sampling of rocks of the complex and by selective sampling of the outlying quartz-pyrite, quartz-barite, calcite-barite, and calcite-dolomite veins. The veins are located northwest of the main pipe complex and are grouped around an east-west-trending felsite dike. The results are presented separately for the veins and for the breccia pipes. Rock Chip Geochemistry of the Breccia Pipes Horizontal Zoning of gold, zinc, copper, and lead Contouring of the metal content of rocks within the pipes and within the wallrock and veins surrounding the pipes shows various patterns of anomalous values as summarized below: 1. The pattern of highest values for gold greater than 1.5 ppm crudely overlap the contacts of the breccia pipes with their wall rocks, an association probably due to the more highly fractured nature of the contacts having provided greater permeability through which late-phase mineralizing solutions percolated and deposited their gold. The greater than 1.5 ppm areas over the west pipe average 6.56 ppm (0.193 oz Au/ton), while over the east pipe they average 8.43 ppm (0.248 oz Au/ton). The sites of deposition for gold were mainly on or within micro-fractures in pyrite, the most abundant sulfide in the system. 2. The patterns obtained for zinc are crudely elliptical shaped areas overlying the east and west pipes, and average greater than 1300 ppm and 2000 ppm, respectively. 3. Copper values of greater than 100 ppm are distributed similar in size, shape, and distribution to the zinc patterns; the west pipe averages greater than 150 ppm copper and the east pipe about 400 ppm copper. 4. Anomalous lead values of greater than 200 ppm are associated with the rocks of the east pipe; only a few erratically high values for lead occur within the west pipe. High gold values accompany iron and copper. The strong association of iron with the gold mineralization reflects the high auriferous pyrite content of the deposit, while most of the copper present in the deposit occurs in chalcopyrite in solid solution with pyrite.
  • GEOCHEMISTRY OF THE BRECCIA PIPE COMPLEX (continued) Vertical Zoning of copper, lead, zinc, and gold Samples from a line of vertical holes across the west pipe were selected for analysis to assess vertical zoning within the pipe. The four metals that showed the best continuity for zoning were copper, lead, zinc, and gold; results are summarized below. 1. Copper distribution exhibits a similar pattern as lead. The higher copper values occur below the water table and below the zone of oxidation. A poorly defined barren area is interpreted to correlate with the barren core of the gold zonation. 2. The highest grade copper zone averages 600 ppm and occurs as an overprint of the rubble breccia; a lower grade zone overlaps the igneous breccia and felsite and averages 100 ppm. The barren zone that correlates with the barren core of the gold zonation averages 50 ppm copper. 3. The lead content of the west pipe is low, less than 100 ppm for all rocks, except within that part of the rubble breccia pipe presently below the water table, where the lead content averages 100 ppm. 4. Zinc within the west pipe exceeds in amount and aerial distribution all of the other four metals. The zinc content of the rubble breccia averages 7500 ppm in the sulfide zone, which is three times as great as the average zinc content of other rock units of the pipe complex. Zinc is distributed in zones that form a geometric overprint of the rock units within which it occurs. The higher grade zone generally overprints the rubble breccia, and the lower grade zone the igneous breccia and felsite units. The only exception to this zoning is the +500 ppm zinc zone that occurs along the north footwall and central interior of the rubble breccia pipe. This interior zone of the lower zinc content lies within a "barren core" as defined by the total absence of gold mineralization within this zone. 5. Gold distribution within the west pipe was defined by three assay values: 0.01-0.029 oz/ton, 0.030-0.099 oz/ton, and greater than 0.1 oz/ton gold. Gold is distributed in the form of a stalk-like central mass within the rubble breccia pipe from which emanates arcuate branches that in three dimensions form an annulus within the contact zone of both breccia pipes. These annular zones are merged on the west end of the complex where mineral events associated with the rubble breccia pipe overlap mineral events associated with the igneous breccia pipe. From west to east, the annuli spread away from the rubble breccia pipe, forming in plan view a horseshoe-shaped zone open to the east. The central mass of gold mineralization is elliptical in plan, elongate east and west; in cross-section it forms an irregular vertical cylinder. 6. Gold mineralization is divided into an oxidized zone and a sulfide zone. The depth of oxidation is about 300 feet, with the visually estimated degree of oxidation averaging about 80%. Supergene enrichment is not mineralogically or economically important. 7. A barren core within the gold zone lies along the northern footwall and interior region of the rubble breccia pipe. This barren core is essentially void of gold mineralization, yet contains minor to major concentrations of pyrite, zinc (2350 ppm) and copper (480 ppm). The barren core is a well silicified and impervious rock, and it is speculated that the late gold mineralization was unable to percolate through this rock to reach and deposit gold within favorable sulfide mineral sites.
  • GEOCHEMISTRY OF THE BRECCIA PIPE COMPLEX (continued) Rock Chip Geochemistry of Veins in Precambrian Basement around the Breccia Pipe Complex (Sharp, 1984) Four vein types were recognized as an integral, yet separate, part of the gold zone which surrounds and includes the pipe complex. The veins occur in the Precambrian crystalline rocks as a weak stockwork from 600 to 2300 feet northwest of the pipes. The veins range in thickness from less than an inch to several feet. The four vein types include: quartz-pyrite, quartz-barite, calcite-barite, and calcite-dolomite; their geochemistry is summarized below: 1. Quartz-pyrite and quartz-barite veins have the highest precious metal content, both groups averaging about 0.2 oz Au/ton and 0.5 oz Ag/ton. 2. All of the quartz-pyrite veins contain varying amounts of tungsten, ranging from non-detectable to 500 ppm. Quartz-pyrite veins also occur west of the Clark Mountain fault in the Bright Angel Shale. 3. Calcite-barite veins within the gold zone are low in precious metals. 4. The calcite-dolomite veins contain no gold but run more than 0.2 oz Ag/ton. Calcite-dolomite veins occur in the Bright Angel Shale and the Goodsprings Dolomite west of the Clark Mountain fault. The silver ores contain trace amounts of gold (0.005 oz/ton) and appear to belong to the tetrahedrite-tennantite group. Tungsten occurs sporadically in minor amounts in all of the observed calcite-dolomite veins. Scarce exotic veins occur, which contain unusual metal combinations; their genetic significance is unknown. One such vein contained 2000 ppm lead, 10,000 ppm antimony, 10,000 ppm tungsten, and 200 ppm zinc with minor silver. Metals that occur in detectable amounts in all zones of the district are gold, silver, tungsten, antimony, lead, zinc, and copper. The environment of deposition for the various combinations of these metals ranges from structurally-high, epithermal deposits in the Ivanpah vein-silver zone to structurally lower mesothermal deposits within the pipe complex. Summary of Rock Chip Geochemistry Trace Commodities, av. ppm: Pb As Sb Cu Zn Ba W Vein-silver (epithermal zone) 3500 1500 8300 7300 700 displaced to the west of the breccia pipes. Veins assoc. with upper gold 325 1250 750 2250 3500 350 zone displaced to the west of the breccia pipes. Veins assoc. with lower gold 1700 1000 2300 zone surrounding breccia pipes. Felsite dikes & sills 560 60 300 950 Rocks within breccia pipe complex. Igneous breccia 80 850 2100 720 Rubble breccia 100 75 7500 300 Alteration: All lithologies in and around the breccia pipes exhibit moderate to intense quartz-sericite-pyrite alteration.
  • Reserves: 1984 Reported Reserves Estimates reported by Sharp, Draco Mines (1984); reserves are based on 10,000 feet of drilling within the west breccia pipe and 2500 feet of drilling within the east pipe: 1. Proven reserves, west pipe: 6,572,251 million tons of ore at 0.061 oz Au/ton based on a 0.01 oz Au/ton cut-off grade = 400,907 oz Au (12.47 metric tons); 2. Proven reserves, east pipe: 6,540,939 million tons of ore at 0.033 oz Au/ton based on a 0.01 oz Au/ton cut-off grade = 215,851 oz Au (6.71 metric tons); 3. Total proven reserves (west + east pipes) = 13,113,190 oz Au at 0.047 oz Au/ton based on a 0.01 cut-off grade = 616,320 oz Au (19.17 metric tons Au). 1986 Reported Reserves Estimates reported by Dingwall (1986), reporter for the Northern Miner; reserves of both breccia pipes (west and east pipes) are combined and based on a cut-off grade of 0.03 oz Au/ton; most reserves occur in the west pipe: 1. Geologic reserves: 17.2 million tons of ore averaging 0.057 oz Au/ton = 980,400 oz Au (30.49 metric tons); 2. Mineable reserves: 7.6 million tons of ore averaging 0.081 oz Au/ton = 615,600 oz Au (19.15 metric tons). 1989 Reported Reserves Estimate reported by Beatty (1989a; preprint), chief metallurgist, Bond Gold Colosseum Inc.: 1. Mineable reserves = 10,539,000 short tons with an average grade of 0.062 oz Au/ton = 653,418 oz Au (20.32 metric tons). Estimate reported by Beatty (1989b; published paper), chief metallurgist, Bond Gold Colosseum Inc.: 2. Mineable reserves estimate = 10,500,000 short tons with an average grade of 0.061 oz Au/ton = 640,500 oz Au (19.92 metric tons). July 12, 1990, Reported Production and Remaining Mineable Reserves Reserves of both pipes (west and east pipes) are combined (U.S. Environmental Protection Agency, 1992; reportedly based on Bond Gold Colosseum Inc., Colosseum Mine Data Sheet): 1. As of July 1990 the mine had produced over 170,000 oz Au (5.29 metric tons); 2. Estimated total reserves remaining in 1990: 3.9 million tons ore averaging 0.040 oz Au/ton = 156,000 oz Au (4.85 metric tons Au); 3. Total production + mineable reserves = 326,000 oz Au (10.14 metric tons). Summary of Total Reported Reserves (table needs formatting) Date reported Reserves Ounces Au Metric Tons 1984 1986 1989 (Preprint) 1989 (Published) 1990 Mineable 616,320 Geologic 980,400 Mineable 615,600 Mineable 653,418 Mineable 640,500 Mineable 326,000 19.17 30.49 19.15 20.32 19.92 10.14
  • Total Production No recorded production occurred until the 1930s. 1930s: Recorded production = about 615 ounces gold (Beatty, 1989b). Recorded production for the mine also indicates that $45,000 in gold and copper was produced prior to 1940 (Hewett, 1956, cited in Sharp, 1984, pgs. 125-126). 1987-1993: 344,000 oz Au (10.70 metric tons); about ? of the reported mineable reserves. Gold to Silver Ratio Silver assays indicate a gold to silver ratio of 1.5 to 1. If all the assayed silver is alloyed with gold, the gold-silver mineral would be classified as argentiferous electrum. Assays performed in conjunction with metallurgical testing suggest that a portion of the silver might be in solid solution with manganese oxides (Odekirk, 1974, cited in Sharp, 1984, pg. 136). Mineralogy of the Gold (Sharp, 1984; Davis and others, 1989) Gold occurs as fine, rounded inclusions inside of coarse, euhedral pyrite or in contact with pyrite as fracture fillings or along grain boundaries (Corbett, 1980, cited in Sharp, 1984, pg. 138). The more numerous occurrences are as fillings and boundary coatings. The gold was further determined to be alloyed with silver and of a size range of 1 to 30 microns with the majority from 5 to 20 microns. As reported by Davis and others (1989), gold at the Colosseum Mine is generally submicroscopic and associated with sulfide mineralization, chiefly pyrite. It occurs as free gold, with minor alloyed silver. It is primarily in contact with pyrite in fractures in the pyrite or along pyrite grain edges. Secondarily, it occurs as isolated particles in quartz and other gangue minerals but spatially always close to pyrite, and rarely as particles encased in euhedral pyrite. The pyrite mineralization and minor base metal sulfides occur in three distinct styles: (1) as disseminations, (2) as open space filling or vein/fracture filling, and (3) in breccia clasts replacing dolomite. The pyrite megascopically ranges from absent to 30% of rock volume within the breccia pipes (Davis and others, 1989). Although the gold is apparently spatially and geochemically associated with pyrite, there is only a general proportional relationship between pyrite content and gold content. Sometimes, a high volume percentage of pyrite has only geochemically anomalous gold. This makes visual pyrite only a general grade control tool and not a specific tool This further makes the ore control process all the more difficult.
  • Paragenesis (Sharp, 1984) The mineralogy of the dolomite breccia fragments within the rubble breccia pipe revealed the following gangue mineralization and mineral paragenesis. The gangue minerals in order of decreasing abundance are: siderite, goethite, quartz, and sericite. The overall paragenesis is as follows: 1. Initial coarse barren pyrite and minor quartz; 2. Major coarse second-stage pyrite with co-precipitated(?) included gold, chalcopyrite, sphalerite, bornite(?), and/or pyrrhotite; 3. Shattering and fracturing of coarse and brittle earlier pyrite; 4. Second gold event: intruding fractures and interstices in earlier-formed pyrite, thought to account for major proportions of total gold mineralization, accompanied by sphalerite, chalcopyrite, and galena; 5. Third stage: fine-grained, barren pyrite; 6. Late stage: siderite replacement and flooding of the breccia matrix and; 7. Final stage: localized veining by quartz and fine-grained pyrite (Corbett, 1980, cited in Sharp, 1984, pg. 138). The mineralogic observations and metallurgical testing support the observations that the gold is disseminated throughout the deposit and is closely associated and co-mingled with pyrite, the most abundant sulfide in the pipe complex; that only an insignificantly small portion of the gold is locked up in pyrite or silica; and that electrum mineralization is a later event than the main stage sulfide mineralization. ROCK UNITS 1. Precambrian (1,700 m.y.) crystalline basement of biotite-gneiss inter-layered with granite gneiss, both intruded by alaskite and pegmatite dikes; localized zones of quartz-mica schist. 2. Felsite as reported by Sharp (1984; also reported as "rhyolite," "rhyolite felsite," and "granite felsite" by various authors). 3. Igneous Breccia: matrix is felsitic; major breccia fragments are felsite, with minor fragments of quartzite, granite, gneiss, and andesite; carbonate content ranges from 5 to 20%. 4. Rubble breccia: matrix is comminuted rock flour with an overall composition essentially the same as the felsite and igneous breccia, differing only in its higher carbonate content, which ranges to a high of 35%; breccia fragments are Precambrian granite and gneiss, cobbles of Paleozoic Tapeats Quartzite, chips of Bright Angel Shale, boulders and slabs of Goodsprings Dolomite, and fragments of Mesozoic felsite and igneous breccia. STRUCTURAL GEOLOGY (See above: Structural Geology and Stratigraphy; Regional Tectonic Setting; Local Structure and Stratigraphy)
  • MINERALIZATION Precious Metals Economic gold mineralization at the Colosseum Mine occurs within two felsite (rhyolite) breccia pipes. Sub-economic gold mineralization occurs within mineralized veins in the Precambrian wallrock surrounding the breccia pipes, and in the Tapeats Quartzite and Bright Angel Shale that were situated above the present breccia pipes prior to westward displacement of the upper portions of the pipes as a result of tectonic movement along normal faults and gravity sliding along older thrust faults. Gangue minerals Barren pyrite, quartz, barite, siderite, sericite, manganese and iron oxides (goethite), felsite; felsite breccia (fragments of felsite, Tapeats Quartzite, Precambrian granite and gneiss, andesite); rubble breccia (fragments of dolomite, Precambrian granite and gneiss; comminuted rock flour with composition essentially the same as the felsite and igneous breccia with higher carbonate content; cobbles of Paleozoic Tapeats Quartzite, chips of Bright Angel Shale, boulders and slabs of Goodsprings Dolomite, and fragments of Mesozoic felsite and igneous breccia). Alteration Moderate to intense quartz-sericite-pyrite alteration. Mineralization Controls Economic gold mineralization at the Colosseum Mine occurs within two felsite (rhyolite) breccia pipes. As reported by Davis and others (1989), the gold is generally submicroscopic and associated with sulfide mineralization, chiefly pyrite. It occurs as free gold, with minor alloyed silver. It is primarily in contact with pyrite in fractures in the pyrite or along pyrite grain edges. Secondarily, it occurs as isolated particles in quartz and other gangue minerals but spatially always close to pyrite, and rarely as particles encased in euhedral pyrite. The pyrite mineralization and minor base metal sulfides occur in three distinct styles: (1) as disseminations, (2) as open space filling or vein/fracture filling, and (3) in breccia clasts replacing dolomite. The pyrite megascopically ranges from absent to 30% of rock volume within the breccia pipes (Davis and others, 1989). Although the gold is apparently spatially and geochemically associated with pyrite, there is only a general proportional relationship between pyrite content and gold content. Sometimes, a high volume percentage of pyrite has only geochemically anomalous gold. This makes visual pyrite only a general grade control tool and not a specific tool This further makes the ore control process all the more difficult. Gold mineralization is divided into an oxidized zone and a sulfide zone. The depth of oxidation is about 300 feet, with the visually estimated degree of oxidation averaging about 80%. Supergene enrichment is not mineralogically or economically important. A barren core within the gold zone lies along the northern footwall and interior region of the rubble breccia pipe. This barren core is essentially void of gold mineralization, yet contains minor to major concentrations of pyrite, zinc (2350 ppm) and copper (480 ppm). The barren core is a well silicified and impervious rock, and it is speculated that the late gold mineralization was unable to percolate through this rock to reach and deposit gold within favorable sulfide mineral sites. GEOCHEMISTRY (See above: Geochemistry of the Breccia Pipe Complex; Rock Chip Geochemistry of the Breccia Pipes; Rock Chip Geochemistry of Veins in Precambrian Basement around the Breccia Pipe Complex)
  • OVERALL SUMMARY Age of Mineralization: Approx. 100 m.y. BP (Early Cretaceous). Host Rock Age: Approx. 100 m.y. BP (Early Cretaceous). Associated Rock Types: 1. Precambrian crystalline basement: biotite-gneiss and granite gneiss, alaskite and pegmatite dikes; localized zones of quartz-mica schist; 2. Paleozoic sedimentary rocks: Goodsprings Dolomite, Tapeats Quartzite; 3. Andesite dikes (age unknown but older than the felsite dikes of the felsite breccia complex). Host Rock Unit: Felsite (rhyolite) breccia complex. Tectonic Setting: The Colosseum Mine is located at the southern end of the Sevier foreland thrust belt in the southern Basin and Range Province. Gold mineralization occurred about 100 Ma in earliest Late Cretaceous time (post-Sevier thrusting/pre-Basin and Range extensional tectonics), probably in association with one of several shallow-level, early Late Cretaceous stocks that appear to represent northeastern outliers of the magmatic belt that comprises the broadly calc-alkaline series of six plutons that comprise the Teutonia Batholith (Sharp, 1984; Haxel and Miller, 2006). Alteration: Quartz-sericite-pyrite. Ore control: Economic gold mineralization occurs within two felsite (rhyolite) breccia pipes. Gold occurs as fine, rounded inclusions inside of coarse, euhedral pyrite or in contact with pyrite as fracture fillings or along grain boundaries (Corbett, 1980, cited in Sharp, 1984, pg. 138). The more numerous occurrences are as fillings and boundary coatings. The gold was further determined to be alloyed with silver and of a size range of 1 to 30 microns with the majority from 5 to 20 microns. As reported by Davis and others (1989), gold at the Colosseum Mine is generally submicroscopic and associated with sulfide mineralization, chiefly pyrite. It occurs as free gold, with minor alloyed silver. It is primarily in contact with pyrite in fractures in the pyrite or along pyrite grain edges. Secondarily, it occurs as isolated particles in quartz and other gangue minerals but spatially always close to pyrite, and rarely as particles encased in euhedral pyrite. The pyrite mineralization and minor base metal sulfides occur in three distinct styles: (1) as disseminations, (2) as open space filling or vein/fracture filling, and (3) in breccia clasts replacing dolomite. The pyrite megascopically ranges from absent to 30% of rock volume within the breccia pipes (Davis and others, 1989). Although the gold is apparently spatially and geochemically associated with pyrite, there is only a general proportional relationship between pyrite content and gold content. Sometimes, a high volume percentage of pyrite has only geochemically anomalous gold. This makes visual pyrite only a general grade control tool and not a specific tool This further makes the ore control process all the more difficult. Depth of Mineralization: Economic gold mineralization is confined to two felsite (rhyolite) breccia pipes and to specific zones within the pipes; drilling of the pipe complex confirmed the presence of commercial values of disseminated gold mineralization to depths of at least 500 feet.

Economic information

Economic information about the deposit and operations

Operation type Surface-Underground
Development status Past Producer
Commodity type Metallic
Deposit size Medium
Significant Yes
Discovery year 1865

Mining district

District name Clark District

Land status

Ownership category BLM Administrative Area
Area name BLM Needles Field Office

Ownership information

  • Type Owner
    Owner Lac Minerals, Ltd. (1993)
    Year 1993
  • Type Operator
    Owner Colosseum, Inc. (1993)
    Year 1993

Comments on the workings information

  • Workings Type: 1. Historic underground workings: open stope and pillars. 2. Modern open-pit workings: two open pits. 1. Historic Mine Workings (Tucker, W.B. and Gowman, C.H., 1942): Historic workings consisted of two adits: a) lower adit, elevation 5700 feet, 470 feet of drifts; b) upper adit, elevation 8577 feet, 725 feet of drifts, 250 feet of raises. a) Number 1 vein/ore body: 100 feet long, average 60 feet wide; stopes, 100 feet long, 40 feet wide, 40 feet high; "positive" (proven?) reserves, 92,000 tons, $7.82 (0.22 oz Au/ton; 20,240 oz; 0.63 metric tons). b) Number 2 vein/ore body: 100 feet long, average 12 feet wide; stopes, 100 feet long, 12 feet wide, 40 feet high; "positive" (proven?) reserves, 18,500 tons, $8.00 (0.23 oz Au/ton; 4255 oz; 0.13 metric tons). "Probable" ore reserves in No. 1 and No. 2 ore bodies: 58,100 tons (0.22 oz Au/ton; 12,782 oz; 0.40 metric tons). Mine equipment: One 180-h.p. Fairbanks-Morse diesel; one 60 KVA generator; one 3-h.p. gas engine; 1 20-h.p. Westinghouse generator; One Ingersoll-Rand 2 stage air compressor (12" x 11") (71/2" x 12"); Sullivan drill sharpener; three mine cars; air drills and steel; oil forge; blacksmith shop; assay office; 100-ton ore bin. Reduction equipment: 3D Gates gyratory crusher; "Challendge" ore feeder; 24-foot bucket elevator; 36" x 96" vibrating screen; 5' x 4' Marcy ball mill; 50-h.p. Westinghouse motor; 18" x 48" trammel cone classifier; 2 Wilfley tables; 25-h.p. motor; 2 Groch flotation cells; 12' Rake classifier; 16' conical settler; 20' Dorr thickener; 20-ton concentrate bin; three 8' tanks. Water supply equipment: International diesel unit with generator; 15-h.p. motor; one Triplex pump; 5-h.p. motor; concrete storage tank, 20' x 8' x 60' capacity, 35,000 gallons. Two men at the mine. Daily production: 40 tons; shipped, 1000 tons, value $18 to $40 per ton. Ore shipped to U.S. Smelting Co.: 0.73 oz Au/ton; 1.10 oz Ag/ton (Au:Ag of concentrates = 1:1.5); 0.25% Cu; Au recovery, 60%. Proven reserves within the west pipe total in excess of 6.5 million tons of 0.060 oz Au/ton based on a 0.01 oz Au/ton cut-off grade. Tonnage reserves are based on 10,000 feet of drilling within the west breccia pipe and 2500 feet of drilling within the east pipe. Silver assays, although not available for all drillhole sampling, indicate a gold to silver ratio of 1.5 to 1. If all the assayed silver is alloyed with gold, the gold-silver mineral would be classified as argentiferous electrum. Assays performed in conjunction with metallurgical testing suggest that a portion of the silver might be in solid solution with manganese oxides (Odekirk, 1974, cited in Sharp, 1984, pg. 136). 2. Modern mine workings consisting of two open pits, a South Pit developed in the west felsite breccia pipe (west pipe), and a North Pit developed in the east felsite breccia pipe (east pipe) (U.S. Environmental Protection Agency, 1992): The two breccia pipes (the ore bodies) were mined through two open pit excavations, the North and South pits. Colosseum used a contract mining company, Industrial Contractors Corporation, to drill, blast, excavate and transport the waste rock and ore from the pits. Blastholes with a 6.5-inch-diameter were drilled on a 15-foot by 15-foot square pattern using a down-hole hammer. ANFO (ammonium nitrate mixed with fuel oil) was used as the blasting agent. Four blasts were conducted each week, with no more than one per day. A 13-cubic yard front-end loader excavated the broken rock an placed it into 85-ton haulage trucks for transport to either the waste rock piles, the low-grade stockpile, or the primary crusher. Fifty-ton haulage trucks were used in the South Pit due to the restricted access of the narrow pit configuration at the bottom of the pit.
  • The stripping ratio for both pits was 3.97:1 (waste to ore). In the South Pit, Colosseum maintained a 2:1 (waste to ore) stripping ratio. Colosseum reported a 6:1 stripping ratio on some 20 benches in the South Pit. The stripping ratio in the North Pit was 1:1 (waste to ore). The interslope angel of the South Pit was 53 degrees. The interslope angle of the North Pit was 45 degrees. Twenty-foot benches were maintained in both pits. Safety benches were left every 60 feet in the South Pit and every 40 feet in the North Pit. At the time of the EPA visit to the mine (1992), the South Pit was at an elevation of 5280 feet and was approximately 760 feet deep; the finished elevation was projected to be 5240 an equivalent pit depth of 800 feet. The greatest distance across the South Pit was estimated to be 1600 feet. Ore removed from the South Pit yielded approximately 80 ounce of gold per day (up to 200 ounces per day when higher grade ore was mined). Mining at the South Pit was scheduled to end in June 1992. The North Pit bottom was at an elevation of 5740 feet with a depth of 300 feet at the time of the EPA visit. Prior to 1991, the North Pit was used mostly as a "relief area," mined when the South Pit was being drilled and blasted. In 1991, heavy mining began in the North Pit. Ore removed form the North Pit reportedly yielded 40 to 60 ounces gold per day. Colosseum personnel estimated that mining would cease in the North Pit at the end of August 1992. On August 4, 1992, Colosseum notified EPA that mining had terminated in both pits on July 10, 1992.

Comments on other economic factors

  • PRODUCTION Historic: No recorded production occurred until the 1930s, with production of about 615 ounces gold (Beatty, 1989b). Recorded production for the mine also indicates that $45,000 in gold and copper was produced prior to 1940 (Hewett, 1956, cited in Sharp, 1984, pgs. 125-126). The mine produced about 17,000 ounces Au in 1987, about 54,000 to 70,000 ounces Au annually from 1988-1990, approximately 16,000 ounces Au annually during 1991 and 1992, and about 9500 ounces Au in 1993; mining ceased in 1993. At its peak, the mine produced 70,000 ounces gold and 30,000 ounces silver annually. Modern Open-pit: 1987-1993, 344,000 oz Au (10.70 metric tons), about ? of the reported mineable reserves. RESERVES 19-20 metric tons mineable Au reserves; 30.49 metric tons geologic Au reserves. Approximately 9 metric tons Au originally considered mineable reserves were not mined due to geologic/economic reasons. Gold:silver ratio: 1.5:1 (1.5 Au to 1 Ag) Deposit Size Medium, based on: 1. Total production of 344,000 ounces (10.7 metric tons) gold. 2. Mineable reserve estimate of 10,539,000 short tons (st) with an average grade of 0.062 oz Au/st (Beatty, 1989a), which amounts to 653,418 oz Au (20.32 metric tons). 3. Geologic reserves of 17.2 million tons of ore averaging 0.057 oz Au/ton, which amounts to 980,400 oz Au (30.49 metric tons), based on a cut-off grade of 0.03 oz Au/ton (Dingwall, 1986).

Comments on development

  • OVERVIEW (from Sharp, 1984; Britton, Vickie, 1986; Mining Engineering, September 1987, US and International Mineral News Briefs, pg. 844; Beatty 1989b) The Colosseum mine site has been active intermittently since the 1870s. Initially mined for other metals, gold was discovered in the 1860s. In following years, there was only limited underground development. The peak of early historic production occurred in the late 1920s and 1930s, when high-grade ore was produced from the west pipe by underground mining. No recorded production occurred until the 1930s, with production of about 615 ounces gold (Beatty, 1989b). Recorded production for the mine also indicates that $45,000 in gold and copper was produced prior to 1940 (Hewett, 1956, cited in Sharp, 1984, pgs. 125-126). The main historic developments at the mine consisted of a vertical stope in the central interior of the rubble breccia portion of the west breccia pipe and semicircular workings on the westernmost edge of the west pipe. Further exploration was conducted in the west pipe in 1940 and 1941; however, the mine was shut down in 1942 when all non-strategic metal operations were closed by the War Production Board (War Production Board Limitation Order L-208). Activity at the mine resumed in 1972 when sampling and geological mapping were done by Draco and Placer Amex. Drilling and mapping continued until 1980. The mine was taken over in the early 1980s by Amselco, who attempted to open the mine, but instead opted to sell to Colosseum California. In 1986, the property was acquired by Dallhold Resources Inc., which became part of Bond International Gold, Inc. Construction and modern open-pit mining began in 1986 with mineable reserves estimated to be 10,539,000 short tons with an average grade of 0.062 oz Au/ton (653,418 oz Au; 20.32 metric tons) (Beatty, 1989a). A 3400 tons/day Carbon-In-Pulp mill was completed in September 1987, and the mill began operating in January 1988. With an average stripping ratio of 4:1, active mine life was estimated to be nine years. Mill capacity was 3,400 tons per day or 1.2 million tons per year. Approximately 110 workers were employed, 50 of whom were contract miners. The mill initially worked two shifts per day, seven days per week, and the mill operated 24 hours per day, seven days per week. In May 1992, mining removed 15,000 tons of rock (waste plus ore) per day, five days a week. This rate was down from the 29,000 tons of rock removed per day, six days a week in 1990. The State permitted the mine to excavate up to 31,000 tons per day. The mine produced about 17,000 ounces Au in 1987, about 54,000 to 70,000 ounces Au annually from 1988-1990, approximately 16,000 ounces Au annually during 1991 and 1992, and about 9500 ounces Au in 1993; mining ceased in 1993. At its peak, the mine produced 70,000 ounces gold and 30,000 ounces silver annually. From 1987-1993, the mine produced a total of 344,000 oz Au (10.70 metric tons), about ? of the reported mineable reserves. Capital costs to bring project into production were estimated at $30.8 million; the expected operating cost per ounce/Au was $175.00/oz Au.
  • Mine History (from Tucker and Gowman, 1942; Ely, 1982; Sharp, 1984; Beatty, 1989b; U.S. Environmental Protection Agency, 1992; CGS Minefile Folder No. 322-5560 and Sunshine Mine Files, Colosseum Mine, Box #3) 1860 - 1865: Exploration in the Clark Mountain district began in the late 1860s. The district was organized on July 18, 1865 by John Moss, owner of several mines, including the historic Colosseum Mine, which was discovered in 1865 (Crossman, 1980, cited in Sharp, 1984). 1900 - 1906: Developed by Devereaux Brothers, Ivanpah Consolidated Mining Co. 1923 - 1938: Purchased by C.H. Gowman, September 1923; operated by Colosseum Mines, Inc. from 1923 to February 1938. The historic Colosseum Mine was developed as underground workings in the West breccia pipe within the lower portion of the gold zone, and constitutes the largest historic gold mine within the district. No recorded production occurred until the 1930s, with production of about 615 ounces gold (Beatty, 1989b). Recorded production for the mine also indicates that $45,000 in gold and copper was produced prior to 1940 (Hewett, 1956, cited in Sharp, 1984, pgs. 125-126). The main historic developments at the mine consist of a vertical stope in the central interior of the rubble breccia portion of the West breccia pipe and semicircular workings on the westernmost edge of the West pipe. 1938 - 1942: Under lease to Harold Chase and Walter Lineberger, of Santa Barbara from February 1938 to February 1942; 30 unpatented claims, 2 patented claims, Colosseum No. 1 and Colosseum No. 2; 640 acres. The mine was closed in 1942 as a nonessential industry during World War II. 1970s - 1985: During this 15-year period the Colosseum property was held by California Gold Properties, which leased some of the older claims and located an additional 176 claims. A series of exploration ventures on the Colosseum property was conducted by Draco Mines, Placer AMEX, and Amselco Exploration using modern methods. Amselco leased the property from Draco Mines in 1982 and conducted extensive drilling and feasibility studies between 1982 and 1984. Amselco began the required permit applications in 1983, and a Final EIR/EIS was approved in July 1985. This work resulted in delineation of ore reserves associated with the southwestern most of two rhyolite ("felsite" of Sharp, 1984) breccia pipes to a depth of 750 feet. 1986 - 1989: the property was acquired by Dallhold Resources, Inc. in mid 1986. Sometime after this purchase, Dallhold Resources became part of Bond International Gold, Inc., controlled by Australian investor, Alan Bond. In November, 1986, Royal Resources acquired a 25% interest in the property. Modern open-pit mining began in 1986 with mineable reserves estimated to be 10,539,000 tons with an average grade of 0.062 oz Au/ton, which totals 653,418 oz Au (20.32 metric tons) (Beatty, 1989a). Construction of a 1.1 million carbon-in-pulp, cyanide mill started in November 1986, and was completed in September 1987; mineable reserves were estimated to be 10,539,000 short tons (st) with an average grade of 0.062 oz Au/st). This amounts to 656,766 oz (20.5 metric tons Au). The mining facilities occupied 284 acres with another 3316 acres held as private land and unpatented mining claims. Mining was conducted in two open pits, the South Pit and the North Pit. Most of the mining facility lies on unpatented Federal land under the jurisdiction of the Bureau of Land Management (BLM); but two patented claims are located in the South Pit area. 1989 - 1993: Colosseum Inc., a subsidiary of Lac Minerals Ltd., operated the Colosseum Mine until July 10, 1993, after Lac Minerals acquired the properties of Bond International Gold, Inc. On July 10, 1992, Colosseum ceased mining after 4 ? years of operation. Milling of stockpiled ore continued until May of 1993.
  • Ore processing and gold-silver recovery (from Beatty, 1989b) 1. Ore was hauled from the open-pit mine to the crushing facility, crushed to minus 6-inches with a single-stage Fuller-Traylor jaw crusher, then to a 35,000 short ton reclaim ore pile; 2. Lime was added to the crushed product as it was conveyed to a single 21-foot by 13-foot Hardinge Semi-Autogenous grinding (SAG) mill followed by a single 14-foot by 24-foot Hardinge O.F. ball mill; 3. After grinding, the slurry was thickened and pre-aerated for three hours; 4. The ore was then leached with NaCN at a pH of 11 for three hours; 5. The slurry then went through a 7-stage CIP (Carbon in Pulp) circuit for 21 hours; 6. The tailings slurry was pre-aerated for three hours prior to cyanide destruction using the Inco process; 7. Loaded carbon was eluded using an Anglo-American strip, followed by electrowinning on stainless steel wire; 8. The wire was transferred to refinery cells where the gold and silver were plated onto stainless steel plates; 9. Following electro-refining, the gold-silver foil was removed, melted, and poured into dore bars for shipment.
  • MINING AND PROCESSING METHODS The historic Colosseum Mine was developed as underground workings within the southwestern most of two felsite (rhyolite) breccia pipes. The southwest pipe is also referred to in the literature as the west pipe. The northeast pipe, referred to in the literature as the east pipe was not mined until the advent of modern open-pit mining. The modern Colosseum Mine was developed as two open pits: the South Pit, excavated in the west breccia pipe, and the North Pit, excavated in the east breccia pipe. Modern open-pit mining at the Colosseum Mine began in 1986 with mineable reserves estimated to be 10,539,000 short tons (st) with an average grade of 0.062 oz Au/st (653,418 oz Au; 20.32 metric tons) (Beatty, 1989a). From U.S. Environmental Protection Agency (1992): Colosseum used a contract mining company, Industrial Contractors Corporation, to drill, blast, excavate and transport the waste rock and ore from the pits. Blastholes with 6.5-inch-diameter were drilled on a 15-foot by 15-foot square pattern using a down-hole hammer. ANFO (ammonium nitrate mixed with fuel oil) was used as the blasting agent. Four blasts were conducted each week, with no more than one per day. A 13-cubic yard front-end loader excavated the broken rock an placed it into 85-ton haulage trucks for transport to either the waste rock piles, the low-grade stockpile, or the primary crusher. Fifty-ton haulage trucks were used in the South Pit due to the restricted access of the narrow pit configuration at the bottom of the pit. The stripping ratio for both pits was 3.97:1 (waste to ore). In the South Pit, Colosseum maintained a 2:1 (waste to ore) stripping ratio. Colosseum reported a 6:1 stripping ratio on some 20 benches in the South Pit. The stripping ratio in the North Pit was 1:1 (waste to ore). The interslope angel of the South Pit was 53 degrees. The interslope angle of the North Pit was 45 degrees. Twenty-foot benches were maintained in both pits. Safety benches were left every 60 feet in the South Pit and every 40 feet in the North Pit. At the time of the EPA visit to the mine, 1992, the South Pit was at an elevation of 5280 feet and was approximately 760 feet deep; the finished elevation was projected to be 5240 feet, an equivalent pit depth of 800 feet. The greatest distance across the South Pit was estimated to be 1600 feet. Ore removed from the South Pit yielded approximately 80 ounces of gold per day (up to 200 ounces per day when higher grade ore was mined). Mining at the South Pit was scheduled to end in June 1992. The North Pit bottom was at an elevation of 5740 feet with a depth of 300 feet at the time of the EPA visit. Prior to 1991, the North Pit was used mostly as a "relief area," mined when the South Pit was being drilled and blasted. In 1991, heavy mining began in the North Pit. Ore removed form the North Pit reportedly yielded 40 to 60 ounces gold per day. Colosseum personnel estimated that mining would cease in the North Pit at the end of August 1992. On August 4, 1992, Colosseum notified EPA that mining had terminated in both pits on July 10, 1992. Ore processing consisted of the following circuit: Primary crusher (-6 inch) SAG / Ball Mill Conventional CIP (Carbon-In-Pulp) 8 Agitation Leach tanks AARL (Anglo American Research Laboratory) elution system (Elution - the process of extracting one material from another by washing with a solvent to remove adsorbed material from an adsorbent.) Electrowinning/Electrorefining (gold foils)
  • MINING AND PROCESSING METHODS (continued) Recovery: Recovery was reported to be 92% From Beatty (1989b): Ore was hauled 1 mile from the open-pit to the primary crusher and reduced to -6 inches with a single-stage 60-inch by 48-inch 250-hp, Fuller Traylor jaw crusher. The ore was then stockpiled in a 35,000-short ton (st) reclaim ore pile. Lime was added to the crushed product as it was conveyed to a single 21-foot by 13-foot Hardinge semiautogenous grinding (SAG) mill, followed by a single 14-foot by 24-foot Hardinge overflow ball mill. After grinding, the slurry was thickened and then preaereated for three hours. Following preaereation, the pulp was leached with NaCN at a pH of 11, also for three hours. The slurry then went through a seven-stage Carbon-In-Pulp (CIP) circuit for 21 hours. The tailings slurry was preaereated for three hours before cyanide destruction using the Inco process. Loaded carbon was eluted using an Anglo American strip, followed by electrowinning on stainless steel wire. The stainless steel wire was transferred to refinery cells where the gold and silver was plated onto stainless steel plates. Following electrorefining, the gold-silver foil was removed, melted, and poured into dore bars for shipment. The crushing, milling, refining, and maintenance process required 37 people. RECLAMATION The Colosseum Mine was one of several mines chosen by the U. S. Environmental Protection Agency (EPA) to be part of its program to assist states in improving their mining reclamation programs. EPA visited the Colosseum Mine on May 7, 1992, and prepared a comprehensive report on the operation of and reclamation plans for the mine (U. S. Environmental Protection Agency, Site Visit Report; Colosseum Mine; 2006). Mining ceased on July 10, 1992, and milling of stockpiled ore continued until May 1993. Post mining/mineral processing reclamation activities at the mine have been implemented. The Lead Agency for reclamation compliance at the Colosseum Mine is the County of San Bernardino.

Reference information

Bibliographic references

  • Deposit

    Beatty, R.L., 1989a, Bond Gold Colosseum Inc., crushing and grinding: an update: Society of Mining Engineers Preprint No. 89-85 for presentation at SME Annual Meeting, Las Vegas, NV Feb. 27 - Mar. 2, 1989, 6 pgs.

  • Deposit

    Beatty, R.L., 1989b, Crushing and grinding at Bond Gold's Colosseum mine: an update: Society for Mining, Metallurgy, and Exploration, Mining Engineering, August 1989, pgs. 823-826.

  • Deposit

    Britton, Vickie, 1986, Colosseum Mine plans to open: California Mining Journal, September 1986, pgs. 50-51.

  • Deposit

    Burchfiel, B.C. and Davis, G.A., 1988, Mesozoic thrust faults and Cenozoic lowangle normal faults, eastern Spring Mountains Nevada, and Clark Mountains thrust complex, California in This Extended Land: Geological journeys in the southern Basin and Range, Field Trip[ guidebook, Geological Society of America, Western Cordilleran Section, Las Vegas, Nevada, p. 8710.

  • Deposit

    California Polytechnic University - Pomona (CSU), site visited August 2006, Colosseum Mine in Ivanpah Mining District: CSU Geological Sciences Department, Pomona, http://geology.csupomona.edu/drjessey/fieldtrips/mtp/coloss.htm.

  • Deposit

    Davis, B.M., Trinble, J., and McClure, D., 1989, Grade control and ore selection practices at the Colosseum gold mine: Society for Mining, Metallurgy, and Exploration, Mining Engineering, August 1989, pgs. 827-830.

  • Deposit

    Dingwall, Laima, 1986, Aussie-made for N. America new Colosseum Gold Mines: Northern Miner, July 21, 1986.

  • Deposit

    Dobbs, P.H., 1961, Geology of the central part of the Clark Mountain range, San Bernardino County, California, Unpublished M.S. Thesis, University of Southern California, 116p.

  • Deposit

    Ely, Marion, 1982, Notice of reclamation plan completion, new Colosseum mine: Notice dated January 5, 1982, and submitted to the California Division of Mines and Geology; copy contained in CGS (formerly CDMG) Minefile Folder No. 322-5560.

  • Deposit

    Garside, L.J., and Bonham, H.F., Jr., 1987, Precious metal deposits in southeastern California and southern and west-central Nevada in Johnson, J.L., Ed., 1997, A Symposium on Bulk Mineable Precious Metal Deposits of the Western United States, Guidebook for Field Trips, Field Trip 2: Geological Society of Nevada, April 6-8, 1987, Symposium, Field Trip 2, pgs. 70-71.

  • Deposit

    Haxel, G. B., and Miller, D. M., 2006, Mesozoic rocks: Internet site, http://www2.nature.nps.gov/geology/USGSNPS/mojave/MESOmoja.pdf.

  • Deposit

    Hewett, D.F., 1956, Geology and mineral resources of the Ivanpah Quadrangle, California and Nevada: U. S. Geological Survey Professional Paper 275, 172 p.

  • Deposit

    Hill, R.L., 1998, Gold in the California desert, past, present, and future: California Division of Mines and Geology (aka California Geological Survey), California Geology, vol. 51, no. 1, pgs. 14-25.

  • Deposit

    http://www.epa.gov/epaoswer/other/mining/techdocs/gold.htm; Chapter 3: Site Visit Report: Colosseum Mine [PDF, 48 pages, 578 KB]; http://www.epa.gov/epaoswer/other/mining/techdocs/gold/goldch3.pdf.

  • Deposit

    McClure, D.L., and Schull, H.W., 1988, Colosseum Gold Mine, Clark Mountain range, San Bernardino County, California: Society of Mining Engineers Preprint Number 88119, SME Annual Meeting, Phoenix, AZ, 4p.

  • Deposit

    Mining Engineering, September 1987, US and International Mineral News Briefs, pg. 844.

  • Deposit

    Moyle, P.R., and Cather, E.E., 1992, Colosseum mine another new discovery in U.S. Bureau of Mines Open File Report 62-92, pgs. 38-43.

  • Deposit

    Portions of various unpublished reports, and information from various Internet websites, contained in CGS (formerly CDMG) Minefile Folder No. 322-5560.

  • Deposit

    Sharp, J.E., 1984, A gold mineralized breccia pipe complex in the Clark Mountains, San Bernardino County, California: Arizona Geological Society Digest, Vol. 15, pgs. 119-139.

  • Deposit

    Skillings Mining Review, Feb. 21, 1987, pg. 7.

  • Deposit

    Sunshine Mine Files, Colosseum Mine, Box #3, housed with CGS (formerly CDMG) Minefile Data.

  • Deposit

    Tucker, W.B. and Gowman, C.H., December 29, 1942, Field Report No. 371: State Mining Bureau in CGS Minefile Folder #322-5560.

  • Deposit

    U. S. Environmental Protection Agency, 2006, Technical Resource Document: Extraction and Beneficiation of Ores and Minerals: Gold

  • Deposit

    U.S. Environmental Protection Agency (EPA), 1992, Site Visit Report: Colosseum Mine: U.S. EPA, Office of Solid Waste, 401 M Street SW, Washington, DC 20460, 47 pgs.

General comments

Subject category Comment text
Deposit The Colosseum deposit is located at the southern end of the Sevier foreland thrust belt in the southern Basin and Range Province. Gold mineralization occurred about 100 Ma in earliest Late Cretaceous time (post-Sevier thrusting/pre-Basin and Range extensional tectonics), probably in association with one of several shallow-level, early Late Cretaceous stocks that appear to represent northeastern outliers of the magmatic belt that comprises the broadly calc-alkaline series of six plutons that comprise the Teutonia Batholith (Sharp, 1984; Haxel and Miller, 2006). The deposit is associated with emplacement of a breccia complex into Precambrian basement rocks. The complex is comprised of two felsite (also called "rhyolite" and "rhyolite felsite" by other authors) breccia pipes that form a northeast-southwest elongate complex, which contains mineralized zones of disseminated auriferous pyrite. Sub-economic gold mineralization occurs within mineralized veins in the Precambrian wallrock surrounding the breccia pipes, and in the Tapeats Quartzite and Bright Angel Shale that were situated above the present breccia pipes prior to westward displacement of the upper portions of the pipes as a result of tectonic movement along normal faults and gravity sliding along older thrust faults. As reported by Davis and others (1989), gold at the Colosseum deposit is generally submicroscopic and associated with sulfide mineralization, chiefly pyrite. It occurs as free gold, with minor alloyed silver. It is primarily in contact with pyrite in fractures in the pyrite or along pyrite grain edges. Secondarily, it occurs as isolated particles in quartz and other gangue minerals but spatially always close to pyrite, and rarely as particles encased in euhedral pyrite.
Environment The mine is surrounded by the Mojave National Preserve (MNP), administered by the National Park Service (NPS); the MNP is surrounded by BLM lands. The area around and including Clark Mountain to the south of the mine, is designated Wilderness managed by the NPS.

Reporter information

Type Date Name Affiliation Comment
Reporter 31-AUG-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.