The Colosseum Mine

The now inactive Colosseum Mine is located approximately 70 km southwest of Las Vegas, Nevada in the northeast corner of the Ivanpah Mining District. The mine is situated 15 km due north of the Mountain Pass rare earth mine. Prior to open pit mining two knob-like rhyolite domes marked the locations of the ore bodies.

Gold mineralization was first noted on the property in 1865, but no production was recorded until the 1930's. Hewett (1956) reports production of 615 ounces of gold, minor silver, copper and lead prior to World War II. Subsequent to the war little activity was reported until 1970 when three companies, Draco Mines, Placer AMEX and Amselco Exploration initiated a series of exploration ventures over the next fifteen years. In 1986 Dailhold Resources, Australia acquired the property and began construction of a mine and mill complex. The first gold bullion was poured in the fall of 1987. The mine was in continuous operation until 1993 producing approximately 7000 ounces of gold per month. The mine was acquired in 1990 by Lac Minerals of Canada. Site reclamation has cost Lac Minerals in excess of $30,000,000 to date.

Stratigraphy

The basement consists of Precambrian (1.7 BP) gneiss intruded by younger alaskite and pegmatite dikes. The basement complex is overlain by Cambrian Tapeats quartzite, Bright Angel shale and Bonanza King carbonates. These units strike northwest and dip gently to moderately to the southwest. The basement complex has been intruded by two felsite (rhyolite) breccia pipes of Cretaceous age which crosscut the regional structural trend. These pipes host the gold mineralization at the Colosseum Mine.


Structure

The local structure remains enigmatic. Several studies of the area around the mine have been undertaken and each has chosen to interpret the geology differently. Hewett (1956) concluded that the exposed Precambrian basement represents a horst­like block bounded to the west by the Clark Mountain fault and the east by the Ivanpah fault. Dobbs (1961) disagreed with the work of Hewett and mapped the Clark Mountain Fault as a thrust. Sharp (1984) mapped the faults as shown on Figure 5. He believed that the Keaney­Mollusk Mine thrust lies at the contact between the Cambrian Bonanza King and Bright Angel Formations. The Bright Angel­Tapeats contact was mapped as the Clark Mountain fault. The latter interpretation appears to have been based solely on attenuation of the Tapeats quartzite. Burchfiel and Davis (1988) reluctantly acknowledged the interpretation of Sharp, but felt that the Keaney­Mollusk Mine thrust itself has acted both as a Mesozoic thrust and Cenozoic normal fault and the Tapeats­Bright Angel contact is a normal depositional contact.

Mine Site Geology

The ore hosts at the Colosseum Mine are two breccia pipes age dated at 100 MY. The pipes, and associated dikes of similar composition, intruded the crystalline basement along a northeast structural trend. Both pipes are small with diameters of less than 225 meters. Both dip to the southeast and can be traced down dip approximately 300 meters. The two pipes, termed the East and West pipes, are connected by a narrow arcuate dike of similar composition to the pipes.


The Precambrian host rocks have been sheared and altered within 30 meters of the intrusive. Alteration takes the form of sericite, pyrite and less commonly clays. The intrusive is mapped as felsite, but geochemistry indicates the rock is a rhyolite. The rhyolite represents the first phase of intrusion. Subsequently, the rhyolite, gneissic host and overlying clastic sedimentary units were brecciated and the fragments incorporated in a second pulse of rhyolitic magma. For the purposes of mapping the mine staff termed this second unit a felsite­granite breccia. The West pipe underwent yet a third phase of brecciation which resulted in a rubble breccia of carbonate and clastic sediment clasts in a rock flour matrix. This unit is termed a sediment clast breccia and is easily recognized by the extensive pyrite replacement of the carbonate clasts. Restoration of the Paleozoic section indicates a mixing of fragments over a vertical distance of 1000 meters (McClure, 1988).

Ore Mineralization and Alteration

Mineralization consists predominantly of free gold particles generally less than 20 microns in diameter. The gold shows a close association with the principle gangue sulfide, pyrite, but locally samples containing more than 10% pyrite contain almost no gold (McClure, 1988). The gold occurs as coatings and fracture fillings and as encapsulated grains within pyrite suggesting at least two periods of precious metal mineralization. Galena, chalcopyrite, and sphalerite have been reported in minor amounts. Silver is present, but the 1:1 ratio with gold make it of little economic interest.

Pyrite occurs in three distinct styles. Disseminated pyrite occurs as fine to coarse euhedral crystals locally comprising up to 10% of the rock. Vein or fracture controlled pyrite occurs as massive, euhedral aggregates of crystals filling open spaces between breccia clasts and as finer grained films on fracture surfaces. Replacement pyrite is most common in the sediment clast breccias where carbonate clasts, in particular, have undergone partial to total replacement by pyrite.

Alteration consists of near surface oxidation, significant sericitization and moderate argillization. Quartz is an abundant gangue mineral commonly occurring as veinlets and with pyrite as replacements of sediment clasts. Adularia is present and may be locally abundant. Barite, dolomite, siderite and alunite are also present.

REFERENCES CITED

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

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, 115 p.

Hewitt, D.F., 1956, Geology and mineral resources of the Ivanpah quadrangle, California and Nevada: U.S. Geol. Survey Professional Paper 275, 172 p.

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, 4 p.

Sharp, J.E., 1984, A gold mineralized breccia-pipe complex in the Clark Mountains, San Bernardino County, CA - Gold-silver deposits of the Basin and Range Province, western U.S.A.: Arizona Geological Survey Digest, v. 15, p. 119-139.