Report on Distribution, Characterization & Genesis of ...

[Pages:10]Distribution, Characterization, and Genesis of Mordenite in Miocene Silicic Tuffs at Yucca Mountain, Nye County, Nevada

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Prepared in cooperation with U.S. Department of Energy

Distribution, Characterization, and Genesis of Mordenite in Miocene Silicic Tuffs at Yucca Mountain, Nye County, Nevada

By RICHARD A. SHEPPARD, ARTHUR J. GUDE, 3d, and JOAN J. FITZPATRICK

Prepared in cooperation with U.S. Department of Energy

U.S. GEOLOGICAL SURVEY BULLETIN 1777

DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director

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UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1988

For sale by the Books and Open-File Reports Section U.S. Geological Survey Federal Center Box 25425 Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Sheppard, Richard A. Distribution, characterization, and genesis of mordenite in Miocene silicic tuffs

at Yucca Mountain, Nye County, Nevada.

(U.S. Geological Survey bulletin; 1777)

Bibliography: p.

Supt. of Docs. no.: 119.3:1777

1. Mordenite-Nevada-Yucca Mountain Region. 2. Volcanic ash, tuff, etc.-

Nevada-Yucca Mountain Region. 3. Geology, Stratigraphic-Miocene.

4. Geology-Nevada-Yucca Mountain Region. 5. Radioactive waste disposal in

the ground-Nevada-Yucca Mountain Region. I. Gude, Arthur J. (Arthur James),

1917- . 11.Fitzpatrick, Joan J. III. United States. Dept. of Energy. IV. Title.

V. Series.

QE75.B9 no. 1777

557.3 s

87-600028

[QE391.M81

[621.48' 381

CONTENTS

Abstract 1 Introduction 1

Acknowledgments 2 Geologic setting 3

Stratigraphy 3 Analytical methods 3 Diagenetic mineralogy of the tuffs 6

Distribution of mordenite 7 Description of mordenite 13

Chemical composition of mordenite 13 X-ray powder diffraction data for mordenite 15 Paragenesis 16 Chemical changes during zeolitization 17 Genesis of mordenite 18 References cited 21

FIGURES

1. Location map showing Yucca Mountain, Nevada Test Site boundary, and major calderas 2

2. Generalized geologic map of Yucca Mountain in vicinity of drill holes discussed in text 4

3. Diagram showing diagenetic zonation in drill holes USW-G2, USW-GI, USW-G4, and UE25b-lH 7

4-8. Photomicrographs of tuff showing: 4. Well-preserved vitroclastic texture 13 5. Clinoptilolite that grew on a thin layer of compact fibers of mordenite 13 6. Large pseudomorph of a vitric particle that consists of two layers of fibrous mordenite 14 7. Hollow pseudomorph of a vitric particle that consists, from margin to center, of smectite, mordenite, and clinoptilolite 14 8. Spherulitic mordenite 14

9-15. Scanning electron micrographs of tuff showing: 9. Filiform mordenite that projects into hollow interior of a pseudomorph of a shard 14 10. Filiform mordenite draped across euhedral clinoptilolite 15 11. Tangle of filiform mordenite draped across lepispheres of opal 15 12. Filiform mordenite draped across authigenic alkali feldspar and quartz 15 13. Tangle of filiform mordenite in a pseudomorph of a pumice fragment 15 14. Filiform mordenite draped on etched clinoptilolite 18 15. Crystals of mordenite and partially dissolved clinoptilolite 18

16. Schematic diagram showing the relationships of clinoptilolite, mordenite, analcime, and albite as functions of temperature, pH, and sodium ion concentration 21

Contents III

TABLES 1. Stratigraphy of Miocene volcanic rocks penetrated by drill holes 5 2. Mineralogic composition of tuffs as estimated from X-ray diffractometer

patterns of bulk samples of core 8 3. Chemical analyses and composition of unit cell of mordenite 16 4. X-ray powder diffraction data for mordenite 16 5. Cell parameters for mordenite 17 6. Chemical analyses and chemical comparison of unaltered glass and

mordenite-rich tuffs 19

IV Contents

Distribution, Characterization, and Genesis of Mordenite in Miocene Silicic Tuffs at Yucca Mountain, Nye County, Nevada

By Richard A. Sheppard, Arthur J. Gude, 3d, and Joan J. Fitzpatrick

Abstract

Tuffs at Yucca Mountain in the southwestern Nevada volcanic field are being investigated as a possible deep repository for high-level radioactive wastes. A sequence, as much as about 3,000 meters thick, of Miocene silicic ash-flow tufts, bedded tufts, lavas, and flow breccias was derived chiefly from the Timber Mountain-Oasis Valley caldera complex. Previous studies by others of core from several drill holes have shown that much of the original vitric material of the volcanic and volcaniclastic rocks was altered during diagenesis to clay minerals, silica minerals, zeolites, and feldspars. Unaltered glass still persists in the upper part of the sequence, but zones characterized by clinoptilolite and mordenite, analcime, and albite follow in succession with depth.

Information on the distribution, characterization, and genesis of mordenite in selected samples of core from four drill holes was obtained by transmitted light microscopy, scanning electron microscopy, X-ray powder diffraction, and chemical analysis. In samples of tuft from the clinoptilolite-mordenite zone, mordenite makes up 0-90 percent ofthe tuft and iscommonly associated with clinoptilolite. Pseudomorphs of mordenite after shards and pumice fragments are common. Mordenite occurs chiefly as a tangle of filiform crystals, as spherulites, and as layers of compact fibers. Individual mordenite crystals are ................
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