Earth and Planetary Sciences ETDs

Publication Date

4-10-1978

Abstract

The Hope Buttes volcanic province lies on the southwestern flank of the Black Mesa Basin in northeastern Arizona. Over 200 diatremes occur in this region. The diatremes fill funnel-shaped vents with maar-craters at the surface. They were formed by phreatomagmatic eruptions along existing northwest-southeast fractures during mid-Pliocene time. Anomalous concentrations of uranium occur in at least 20 of the diatremes. The diatremes and dikes which feed them are composed of monchiquite, a feldspar-free lamprophyre containing analcite, and limburgite, a lamprophyre with or without feldspar in a glassy matrix. The presence of zeolites, serpentine, calcite filling fractures, and the high Fe2O3/FeO ratios in the rocks strongly suggest that hydrothermal alteration has occurred. Tuff-breccias and agglomerates which rim many of the vents consist of sedimentary and igneous fragments in a limburgite matrix. These rocks also show the effects of hydrothermal fluids. The maars contain water-laid deposits of interlayered tuff, claystone, and marl. The tuffs are calcareous and much of the glass has been altered to clay. The limestones or marls which cap the maars are micritic, and contain as much as 25 percent clay. The lamprophyres are enriched in light REE, and may have been derived from a slight partial melting of the upper mantle. The tuff-breccias and tuffs have a similar REE pattern, suggesting their common derivation. A slight Ce anomaly in the lamprophyres and tuffs indicates that these rocks have been oxidized. No Eu anomaly is apparent. The average uranium concentration in the lamprophyres is 7.26 ppm; in the tuff-breccias, 10.54 ppm; in the tuffs, 23.95 ppm; and in the limestones, 58.23 ppm. The average Th/U ratio for the lamprophyres is about 2, and decreases in all rock types with increasing uranium content. There is a strong correlation between Th and Hf, but not between U and Hf. Uranium has apparently travelled in mobile phases in the system and was deposited along grain boundaries in the lamprophyres. Uranium in the tuffs and limestones may be the result of both syngenetic deposition and adsorption onto clay minerals. Localization of uranium in maar-type diatremes may have occurred as hydrothermal fluids percolated up through fractures in the vents and mixed with ground and surface waters. The concentration of uranium at the surface of maars and the downward vertical decrease in uranium content suggest that uranyl complexes in solution were precipitated, dominantly as carnotite, during evaporation of the basin-waters.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Douglas Gridley Brookins

Second Committee Member

Albert Masakiyo Kudo

Third Committee Member

J. Paul Fitzsimmons

Project Sponsors

The National Science Foundation and the Department of Energy (formerly ERDA; BFEC Subcontract #76-029 E)

Language

English

Document Type

Thesis

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