The Virginius Vein Ore Deposit, Northwestern San Juan Mountains, Colorado: A Study of the Mineralogy, Structure, and Fluid Inclusions of an Epithermal Base-Metal and Silver Vein in a Volcanic Environment

Author

Berton Woodward Coxe

Publication Date

Winter 12-5-1985

Abstract

The Virginius ore deposit, Ouray and San Miguel Counties, Colorado, yielded 14.5 million oz of silver between 1880 and 1912. The deposit consists of several quartz, base-metal, and silver mineralized fissures which is hosted by Tertiary volcanic rocks of the San Juan volcanic field and was deposited in fractures radial to the northwest margin of the Silverton caldera.

Four stages of mineralization have been recognized: (I) quartz, galena, sphalerite, chalcopyrite, and argentiferous tetrahedrite; this stage comprises a bulk of the total vein material and all of the silver; (II) rhodonite, rhodochrosite, quartz, magnetit, galena, chalcopyrite, and pyrite; this stage is observed only in the lower levels of the mine; (III) quartz, calcite and pyrite which replace stage I minerals; (III-A) barren quartz deposited locally crosscutting state II mineralization.

Vein-related wall rock alteration is confined to a 2.0 to 3.0 m envelope surrounding the vein and consists of sericitization, slicification, and pyritization. Tetrahedrite is the primary silver-bearing phase and averages 15.5% silver. Silver is negatively correlated to arsenic in tetrahedrite. Galena is conspicuously non-argentiferous. Homegenization temperatures of fluid inclusions average 218 degrees Celsius for stage I mineralization and 101 degrees Celsius for state III mineralization. No evidence for boiling was recognized in the samples studied. Salinities of the ore-forming fluids were mostly between 2.0 and 3.0 % NaCl equivalent. Oxygen isotope data indicate that the Virginius ore-forming fluids underwent extensive isotopic exchange with the country rock. The most notable aspects of the Virginius ore deposit include the overall high grade of silver (40+oz/ton), the general lack of mineralogical or metallogenic zoning, the extreme vertical range of the ore shoot (700 m), the lack of evidence for boiling of the ore-forming fluids, and the total amount of silver produced from such a narrow structure.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Lee A. Woodward

Second Committee Member

Douglas G. Brookins

Third Committee Member

Klaus Keil

Project Sponsors

Ranchers Exploration and Development Company, NASA, UNM Graduate Student Association

Language

English

Document Type

Thesis

Recommended Citation

Coxe, Berton Woodward. "The Virginius Vein Ore Deposit, Northwestern San Juan Mountains, Colorado: A Study of the Mineralogy, Structure, and Fluid Inclusions of an Epithermal Base-Metal and Silver Vein in a Volcanic Environment." (1985). https://digitalrepository.unm.edu/eps_etds/180