Geology, mineralogy, and fluid inclusion geothermometry of the El Paso Gold Mine, Cripple Creek, Colorado

Author

Charles Albon Lane

Abstract

"The Cripple Creek district consists of a deeply- eroded, breccia-filled volcanic subsidence basin. Subsidence of the basin occurred along fissures formed by fracturing during the Laramide orogeny. The material filling the crater probably developed first as fissure flows. Erosion of the volcanics formed the material now filling the basin. Volcanic activity was accompanied by intrusion of numerous sills, dikes, and stocks of several different rock types into the breccia and surrounding granite, punctuated by occasional explosive activity. The majority of the mines in the district lie within the volcanic center; however, a few important ones, most notably the El Paso gold mine, lie outside in the surrounding granite.

The major structural control in that part of the district where the El Paso mine is situated is the north- northeast trending El Paso master shear zone. This shear zone influenced the outline of the main volcanic center and the emplacement of the Beacon Hill phonolite plug, a small volcanic crater. Many of the individual shears are mineralized and form the veins in the El Paso vein system.

The underground geology of the El Paso mine is characterized by a number of parallel veins along shear zones trending north-northeast and a tension fracture trending east-northeast to east which formed at an acute angle to the shears. Numerous phonolite sills and dikes occur, many of which are clearly related to the Beacon Hill phonolite plug. The principal contribution of this thesis research has been to map those igneous bodies and veins in the portions of the El Paso mine currently accessible. A number of phonolite bodies are irregular in shape and appear to have been emplaced partly by assimilation of the enclosing granite.

The major controls for ore deposition in the El Paso mine are junctions of veins. The intersections of veins with phonolite bodies also constitute very favored loci. Areas where veins show a curvature or change in dip may locally contain ore enrichments.

The primary ore minerals which have been mined are the gold tellurides, particularly calaverite and sylvanite. A secondary aspect of this thesis research involved ore microscopic examination of selected ore specimens. Calaverite and sylvanite occur as bladed crystals filling fractures or lining open fissures and vugs. Where they are oxidized they show replacement rims of a fine grained mixture of tellurite (tellurium oxide) and free gold. Pyrite and sphalerite are the most common sulfides accompanying the ores. Sphalerite and galena exhibit emulsion textures with chalcopyrite and bournonite blebs respectively. Pyrite locally replaced sphalerite, and bournonite commonly replaced galena. Other minerals observed in polished sections include tennantite, enargite, molybdenite, and two unidentified minerals.

The third purpose of this thesis research involved the determination of fluid inclusion homogenization temperatures for El Paso fluorite and barite. Fluorite yielded a temperature range of 178°C to 190°C. Temperatures determined for barite fall into two temperature ranges, a lower one of 162°C to 181°C and an upper range of 248°C to 261°C. Because vein textures indicate that deposition of fluorite followed that of barite, fluid inclusion geothermometry indicates a decrease in temperature during the deposition of those two minerals. The lower temperature range determined for barite occurs in secondary inclusions formed during or shortly after deposition of fluorite. The temperatures determined conform to those of epithermal deposits"--Abstract, pages ii-iv.

Advisor(s)

Hagni, Richard D.

Committee Member(s)

Grant, S. Kerry
Clifford, Roger K.

Department(s)

Geosciences and Geological and Petroleum Engineering

Degree Name

M.S. in Geology

Comments

Five plates, folded in the back pocket of the manuscript, are available for download as supplemental files below. Due to the large size, these files may take more time to download.

Publisher

University of Missouri--Rolla

Publication Date

1976

Pagination

xiii, 104 pages

Note about bibliography

Includes bibliographical references (pages 100-102).

Geographic Coverage

Cripple Creek, Colorado

Rights

© 1976 Charles Albon Lane, All rights reserved.

Document Type

Thesis - Open Access

File Type

text

Language

English

Thesis Number

T 4168

Print OCLC #

5988114

Recommended Citation

Lane, Charles Albon, "Geology, mineralogy, and fluid inclusion geothermometry of the El Paso Gold Mine, Cripple Creek, Colorado" (1976). Masters Theses. 3216.
https://scholarsmine.mst.edu/masters_theses/3216