The Geology and Hydrothermal Alteration of the Bear Creek Butte Area, Crook County, Central Oregon

Author

Richard Matthew Wilkening, Portland State University

Sponsor

Portland State University. Department of Geology

First Advisor

Michael L. Cummings

Term of Graduation

Spring 1986

Date of Publication

4-24-1986

Document Type

Thesis

Degree Name

Master of Science (M.S.) in Geology

Department

Geology

Language

English

Subjects

Geology -- Oregon -- Bear Creek Butte Region, Hydrothermal deposits -- Oregon -- Bear Creek Butte Region, Mineralogy -- Oregon -- Bear Creek Butte Region

DOI

10.15760/etd.5542

Physical Description

1 online resource (3, vi, 129 pages)

Abstract

The Eocene Clarno Formation, the Oligocene John Day Formation and basalts of the High Lava Plains are exposed in the Bear Creek Butte area in Central Oregon. In this area the Clarno Formation can be divided into a lower sequence composed of intermediate lava flows with intercalated mudflows and volcaniclastic sediments and an upper sequence of rhyolite and basalt flows and felsic ruffs. Separating the two units is a well developed saprolite. The change from intermediate to rhyolite-basalt volcanism reflects a change in the tectonic environment of the Cascade volcanic arc from compression to relaxation as subduction of the Farallon plate by the North American plate slowed, allowing extension of the continental plate margin to occur.

Hydrothermal alteration has affected rocks of the Clarno and John Day Formations in the Bear Creek Butte area. The Oronogo and Platner mines, and the Admunsen prospect, currently being worked, are hosted in the Clarno Formation. Alteration is most intense at the Platner mine where hydrothermal alteration forms an ellipse along N-NW-trending faults. Hydrothermal breccias are associated with silicified and kaolinitized felsic tuffs and a mafic intrusion at this mine. At the Admunsen claim, located south of the Platner mine, a mafic intrusion is argillized and cut by fine quartz veinlets and felsic tuffs have been silicified. At the Oronogo mine, the northernmost property, alteration zones are confined to fractured basaltic andesite along a NW-trending fault. Alteration consists of silicification with minor argillization of plagioclase phenocrysts in the basaltic andesite.

The Platner mine contains the highest concentrations of As and Sb. Trace element geochemistry indicates Sb concentrations (maximum 102 ppm, average 44 ppm) are highest in the area of the Platner mine and decrease northward along the controlling structure more gradually than to the south of the mine. Arsenic concentrations increase northward along the controlling structure from the Platner mine (maximum 45 ppm, average 12 ppm). Hg is irregularly distributed along the structure and has its highest concentration south of the Platner mine. Trace concentrations of U occur along the fault zones in the areas of alteration.

Altered rocks of the John Day Formation, which unconformably overlies the Clarno Formation, host uranium mineralization. The alteration zones are located along NW-WNW-trending faults. The alteration assemblages are similar to that at the mercury prospects; clays, predominately kaolinite, and fine grained quartz. Trace element geochemistry indicates anomalous U (maximum 59 ppm, average 24 ppm) and Mo concentrations.

The presence of cinnabar + hematite ± pyrite indicates that the hydrothermal system that operated at the mercury prospects was an oxidizing system. Deposition of U and Mo required a reducing environment. The differences in the environment of deposition for mercury and uranium indicate that two different systems were active in the study area.

Although alteration is extensive and hydrothermal breccias are present in the Bear Creek area, significant base and precious metal anomalies have not been detected along controlling structures.

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Comments

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Persistent Identifier

https://archives.pdx.edu/ds/psu/20642

Recommended Citation

Wilkening, Richard Matthew, "The Geology and Hydrothermal Alteration of the Bear Creek Butte Area, Crook County, Central Oregon" (1986). Dissertations and Theses. Paper 3656.
https://doi.org/10.15760/etd.5542