Hydrothermal Alteration and Self-Sealing 

in Y-7 and Y-8 Drill Holes 

in Northern Part of Upper Geyser Basin, 

Yellowstone National Park, Wyoming

GEOLOGICAL SURVEY PROFESSIONAL PAPpR 1054-A
Hydrothermal Alteration and Self-Sealing 

in Y-7 and Y-8 Drill Holes 

in Northern Part of Upper Geyser Basin, 

Yellowstone National Park, Wyoming

By TERRY E. C. KEITH, DONALD E. WHITE, and MELVIN H. BEESON 

HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1054-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1978
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Keith, Terry E.

Hydrothermal alteration and self-sealing in Y-7 and Y-8 drill holes in

northern part of Upper Geyser Basin, Yellowstone National Park, Wyoming.

(Hydrothermal studies in Yellowstone National Park, Wyoming) (Geological

Survey Professional Paper 1054-A)

Bibliography: p. 25-26

1. Hydrothermal deposits-Wyoming-Upper Geyser Basin. 2. BoringsWyoming-Upper Geyser Basin. I. White, Donald Edward, 1914- 

joint author. II. Beeson, Melvin H., joint author. III. Title: Hydrothermal 

alteration and self-sealing... IV. Series. V. Series: United States.

Geological Survey. Professional Paper 1054-A.

QE475.A2K43 551.3'5 77-608183

FOT sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-03049-7
CONTENTS

Page

Abstract __________________________________A1

Introduction ______________________________ 1

Stratigraphy -_______________________ 2

Sinter________________________________________ 2

Obsidian-rich sediments _________________ 3

Biscuit Basin flow ____________________________ 5

Flow breccia_________________________ 5

Pumiceous tuff ___________________________ 6

Hydrothermal alteration ____________________ 6

Y-7 drill hole __________________________________________ 6

Sinter_______________________________ 6

Obsidian-rich sediments ________________ 6

Flow breccia of the Biscuit Basin flow___________ 12

Y-8 drill hole _________________________________ 13

Sinter____________________________ 13

Page

Hydrothermal alteration Continued 

Y-8 drill hole Continued

Obsidian-rich sediments __________-__ _. _ -___A13

Flow breccia of the Biscuit Basin flow _____________ 15

Pumiceous tuff of the Biscuit Basin flow___ _________ 17

Significance of minerals and mineral assemblages . ________ 18

Obsidian __________________________________ 18

Clay minerals ____________________.. ________ 19

Silica minerals _____________________________ 20

Zeolites ________________________________ 22

Potassium feldspar______________________________ 23

Calcite _____________________________ 24

Relative importance of temperature and fluid compositions _ - 24 

Summary of evidence for self-sealing _________________ 24

References cited __________________________________ __ 25

ILLUSTRATIONS

	Page

FIGURE 1. Index map of Upper Geyser Basin, Yellowstone National Park, showing locations of research drill holes _____ ______ A2

2. Photomicrograph of perlitic vitrophyre of the Biscuit Basin flow _______________________________________ 5

3. Distribution of fresh obsidian and hydrothermal minerals with depth in Y-7 drill hole _________ ___ _ ___ 7

4. Distribution of fresh obsidian and hydrothermal minerals with depth in Y-8 drill hole ________________________ 8

5. Section through Y-7 and Y-8 drill holes showing pressures, temperature contours, and inferred "self-sealing" _____ 9 

6-13. Photomicrographs showing:

6. Rims of clinoptilolite surrounding clastic grains _______________________________________ 10

7. Initial stages of alteration of obsidian to celadonite and clinoptilolite adjacent to perlitic cracks ___________ 12

8. Progressive increase in alteration toward veinlet __________________________________________ 12

9. Pore spaces between clasts in conglomerate filled by opal, /3-cristobalite, and chalcedony _____________ 13

10. Overgrowth of hydrothermal potassium feldspar on clastic plagioclase grain _____________ __________ 14

11. Late pore filling of calcite deposited on analcime in sandstone ___________________________________ 14

12. Veinlet consisting of celadonite, /3-cristobalite, and clinoptilolite _______________________ ______ 17

13. Frothy pumice texture preserved and outlined by celadonite _________________________________________ 17

14. Triangular diagram showing ranges in cation compositions of clays ______________________________ 20

15. Triangular diagram showing cation compositions of clinoptilolite ___________________________________ 22

TABLES

Page

TABLE 1. Stratigraphic units penetrated by Biscuit Basin drill holes compared with other research holes in Upper Geyser Basin _ A3

2. Chemical analyses of cores from Y-7 and Y-8 drill holes compared with their unaltered equivalents __________ 4

3. Chemical analyses of altered sedimentary rocks from Y-7, Y-8, and unaltered rhyolite of the Summit Lake flow,

recalculated to anhydrous compositions _____________________________________________ 5

4. X-ray and glycolation characteristics of clay minerals from Y-7 drill hole ___________________________ _ 10

5. Distribution and sequence of deposition of hydrothermal minerals in veinlets of Y-7 drill hole _____________ _. _ 11

6. X-ray and glycolation characteristics of clay minerals from Y-8 drill hole _________ ____ _____ ___ ___ 15

7. Distribution and sequence of deposition of hydrothermal minerals in veinlets of Y-8 drill hole ____ __________. 16

8. Electron microprobe analyses of clay minerals from Y-7 and Y-8__________________________________ 19

9. Electron microprobe analyses of zeolites from Y-7 and Y-8__-_---_____-__---________________________ 22

HYDROTHERMAL ALTERATION AND SELF-SEALING IN Y-7 

AND Y-8 DRILL HOLES IN NORTHERN PART OF UPPER 

GEYSER BASIN, YELLOWSTONE NATIONAL PARK,

WYOMING

By TERRY E. C. KEITH, DONALD E. WHITE, and MELVIN H. BEESON

ABSTRACT

Of the 13 research holes drilled in Yellowstone's thermal areas in 

1967 and 1968, 2 holes, Y-7 and Y-8, are the most closely spaced 

pair, thus providing opportunities for detailed correlations and interpretations of mineralogy and chemistry as affected by differences 

in original rocks (generally minor), temperatures, pressures, and 

compositions of the fluid phases. All of these factors, except the detailed fluid chemistry, are considered here.

Y-7 drill hole, with moderately high temperatures and no excess 

fluid pressures, is outside the present local upflow system. Y-8 drill 

hole, 130 m away, has temperatures that exceed the simple hydrostatic boiling curve and water overpressures of more than 2.1 kg/cm2 , 

indicating direct involvement of upflowing fluids. Permeability in 

obsidian-bearing sands and gravels between the two holes has decreased greatly in response to devitrification, solution, and deposition of hydrothermal minerals, largely in initial pore spaces.

The principal hydrothermal minerals of Y-7 are clinoptilolite, 

opal, cristobalite, mordenite, and montmorillonitic clays. The more 

intense alteration of Y-8 includes zones of the same minerals, but in 

other prominent zones, quartz and analcime have replaced the less 

stable (more soluble) clinoptilolite and cristobalite; celadonite is the 

dominant clay mineral. Below -24.8 m in Y-8, hydrothermal potassium feldspar is associated with the analcime-quartz zones, probably 

having formed from excess potassium as potassium-rich clinoptilolite 

altered to analcime, quartz, and potassium feldspar.

Local concentrations of calcite in Y-8 are associated with 

analcime-quartz-potassium feldspar zones in the altered sediments. 

Rapid upward decrease in fluid overpressure caused boiling, which 

resulted in partitioning of CO2 into the vapor phase, and an increase 

in pH and conversion of bicarbonate to carbonate in the water. Thus, 

calcite formed by reaction of calcium with carbonate from the water.

The sediments from both drill holes are more altered than the 

underlying flow breccia of the Biscuit Basin flow, because the initial 

permeability of the sediments was much greater than that of the flow 

breccia. The pumiceous tuff of the Biscuit Basin flow is also completely altered to potassium feldspar, quartz, mordenite, and celadonite. The tuff probably had high initial permeability owing to its 

pumiceous texture, but its permeability is now generally very low.

A detailed study of veinlet mineralogy and paragenesis indicates 

that first-formed minerals are generally the most soluble and least 

stable and, in time, are converted into more stable minerals. This 

succession is most strongly indicated for the five common species of 

silica minerals.

Major factors influencing formation of the hydrothermal minerals 

are abundance and reactivity of obsidian as the dominant unstable

starting material, temperature, permeability, silica activity, and 

fluid composition.

The initial lithologies of Y-7 and Y-8 are very similar and indicate 

high horizontal permeabilities. Thus, present contrasts in wellhead 

pressures between the two drill holes at equivalent depths provide 

strong evidence for horizontal self-sealing.

INTRODUCTION

Thirteen diamond-coring research holes were drilled 

in major thermal areas of Yellowstone National Park 

in 1967-1968. The physical data obtained from these 

holes have been described by White, Fournier, Muffler, 

and Truesdell (1975), but detailed results of petrographic and mineralogic studies have been published 

only for Y-l drill hole in Black Sand Basin, in the 

southwestern part of Upper Geyser Basin (Honda and 

Muffler, 1970). The local geology has been mapped by 

Christiansen and Blank (1974), by Waldrop (1975), and 

by L. J. P. Muffler, D. E. White, and A. H. Truesdell 

(unpub. data).

This report concerns drill holes Y-7 and Y-8 of Biscuit Basin in the northern part of Upper Geyser Basin, 

about 2.5 km north of Y-l (fig. 1) and 3.4 km northnorthwest of Old Faithful geyser. Y-7 and Y-8 are the 

most closely spaced of all the holes drilled by the U.S. 

Geological Survey, because Y-7 failed to encounter 

temperatures as high as expected (White and others, 

1975, p. 16-17). Y-8 was sited 130 m south of Y-7 near 

a cluster of hot springs and small geysers, thus ensuring at least localized vigorous upflow and high temperatures. A comparison of the physical data from the two 

holes (White and others, 1975) provides strong support 

for the existence of processes that have been called 

self-sealing, a concept first developed by Bodvarsson 

(1964) but identified by this term by Facca and Tonani 

(1967). The present report describes the original rocks 

and sediments and their conversion by hydrothermal 

alteration into less permeable rocks by deposition of 

minerals in fractures and pore spaces.

Al
A2 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

110 50

44 28 -

1 KILOMETER

MONTANA

IDAHO

J

YELLOWSTONE ( 

NATIONAL PARKj

WYOMING

FIGURE 1. Index map of Upper Geyser Basin, Yellowstone 

National Park, showing locations of Y-7 and Y-8 research drill holes relative to other holes drilled in the 

basin. Y-1, Y-7, and Y-8 were drilled by the U.S. 

Geological Survey in 1967 and C-l by the Carnegie Institution of Washington in 1929 (Fenner, 1936).

Core recovery from both holes was excellent, except 

from poorly cemented sediments in the upper parts of 

Y-7 and Y-8 and an apparently poorly cemented zone 

from -45.7 to -52.4 m at the base of the sedimentary 

unit in Y-7. Approximately 20 percent of the total core 

was selected for laboratory study, with emphasis on 

obtaining one or more representative samples from 

each cored interval, generally about 1.5 m, as well as 

any unusual samples that included veinlets, vugs, or 

textural extremes. All of the data reported in this 

paper were obtained from this "skeleton" core. All drill 

core samples were marked and cataloged by the depth, 

in feet, at which they were taken in the drill hole;

therefore, the sample numbers used in this report refer 

to downhole depth in feet.

Detailed mineralogical study of the core was made 

by T. E. C. Keith, utilizing 20 thin sections and more 

than 70 X-ray diffractograms for Y-7 and 34 thin sections and more than 100 X-ray diffractograms for Y-8. 

Chemical analyses of minerals were made by M. H. 

Beeson with an ARL EMX-SM electron microprobe, 

using natural and synthetic silicate mineral standards. 

All data were collected on X-ray dispersive spectrometers and corrected for background, matrix absorption, characteristic fluorescence, and atomic 

number using a modified version of the FRAME IV 

data reduction program developed at the U.S. National 

Bureau of Standards (Yakowitz and others, 1973). An 

excitation potential of 15 kV, a beam current of 0.01 ^ia 

(on brass), and a beam diameter of 5 to 10 ^im were 

used in an attempt to produce acceptably high counting 

rates without destroying the sample. The defocused 

beam tends to even out inhomogeneities in the samples, but still the average homogeneity index for most 

elements is relatively large, indicating an inhomogeneous distribution of elements. Chere Bargar provided valuable laboratory assistance in the X-ray 

studies.

STRATIGRAPHY

The sequence of rocks penetrated by Y-7 and Y-8 

drill holes is the same as that of Y-1 (Honda and Muffler, 1970; White and others, 1975) and the Carnegie 

hole drilled west of Old Faithful in 1929 (Fenner, 

1936). The thickness of each stratigraphic unit is similar but not identical in the four holes (table 1).

In this section, we emphasize the original characteristics of the rocks and sediments prior to changes 

induced by circulating hot water.

SINTER

The shallowest unit is white, gray, and tan sinter, 

initially consisting of layers of amorphous opaline 

silica deposited on the ground surface from hot-spring 

water (as in present-day springs and discharge channels near the drill holes). A few layers, especially from 

Y-8, are primary sinter consisting of continuous thin 

beds of amorphous silica that have undergone no conspicuous change since deposition. However, the most 

abundant type, as in most sinter areas (White and 

others, 1964, p. B32-B33), consists of angular fragments of primary sinter cemented by amorphous silica. 

The sinter of both drill holes also contain abundant 

silicified plant stems, roots, and casts as well as 

diatoms that are generally rare but locally abundant.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A3

TABLE 1. Stratigraphic units penetrated by drill holes in Biscuit Basin compared with other research holes in Upper Geyser Basin

Altitude, surface 

Sinter, thickness

Altitude at base 

Sediments, thickness

Altitude at base 

Biscuit Basin flow

Flow breccia, thickness

Pumiceous tuff, thickness 

Total depth of hole

Y-7 

Biscuit Basin

2215.2 m (7,266 ft)

1.8 m (6 ft)

2213.4m (7,260ft)

50.9m (167ft)

2162.5 m (7,093 ft)

>21.0m (>69ft)

Not found 

73.8 m (242 ft)

Y-8 

Biscuit Basin

2217 m (7,272ft) 

-1.5 m (~5ft)

2215.5m (7,267ft) 

53.7m (176ft)

2161.9m (7,091 ft)

7.6m 

>90.6 m ( 

153.4m

(25 ft) 

>297 ft) 

(503 ft)

Y-l 1 

Black Sand Basin

2223.7 m (7,294 ft)

3.7m (12ft)

2220 m (7,282 ft)

61.0m (200ft)

2159.0m (7,082ft)

>1.2m 04 ft)

Not found 

65.5 m (215 ft)

Cnrnegie I,2 

Myriad Gioup

2242.7m (7,356ft)

-2.2 m (~7ft) 

2240.5 m (7,349 ft)

64.9 m (213 ft) 

2175.6m (7,136ft)

-13.4m (-44ft) 

>43.3m 0142ft.) 

123.8m (406ft)

'Honda and Muffler (1970)

2Fenner 119361 with additional data from White, Fnurnier, Muffler, and Truesdell (1975>

A buff-colored silica mud that contains fragments of 

sinter and fine-grained clastic sediments extends from 

 1.1 to -1.7 m in Y-7. The variety of sinter known as 

geyserite is completely absent, thus indicating an absence of geysers and perpetual spouters in the immediate area.

OBSIDIAN-RICH SEDIMENTS

Much of Upper Geyser Basin is underlain by deposits 

of silt, sand, and gravel. Particle sizes used in this 

paper are in accordance with the Wentworth scale 

(Dunbar and Rodgers, 1951, p. 161), except in a few 

places where our so-called silt actually includes some 

fine sand. Detrital black obsidian fragments, some 

with phenocrysts of quartz, sanidine, and plagioclase, 

comprise the bulk of the sediments. Rounded to subangular rhyolitic fragments, usually pink, purple, or 

green and consisting of phenocrysts in a groundmass of 

a-cristobalite 1 and sanidine, are moderately abundant. Detrital mineral grains, originally phenocrysts, 

are dominantly quartz and sanidine, with subordinate 

plagioclase and scarce clinopyroxene, magnetite, and 

zircon. Except for slight cementation, the upper few 

meters of sediments in both drill holes are similar to 

the unaltered "black sands" of Upper Geyser Basin except that in Y-7, silicified plant roots are abundant. 

Deeper in both drill holes, hydrothermal activity has 

completely reconstituted the obsidian and cemented 

the detrital fragments into greenish-gray porous 

sandstone, siltstone, and conglomerate. Coarse- to 

fine-grained sandstone is the dominant lithologic type;

'In this report the teim n-cnttobalite is used tor the ordered form of cristobalite with 

major X-ray peak at 4 04 A and with many other distinct subsidiary peaks The term 

0-cnstobahte refers to the disordered form with a major broad X-ray peak, generally horn 

4.06 to 4 12 A, a second broad peak near 2 50 A, and commonly with no other distinct peaks 

According to Murata and Larson (1975), a complete gradation exists between n- and 0-cristobalite, and we find e\ idence of this in the hydrothermal cristobalites of both Y 7 and Y- S, 

where the major peak position varies between 411 and 4.04 A

siltstone is relatively abundant as thin interbeds just 

above the middle, and conglomerate is sporadic in the 

upper and middle parts and common near the base of 

the sedimentary units.

Graded bedding and crossbedding are conspicuous in 

the sands and silts. The bedding is usually subhorizontal or gently dipping, but some crossbeds dip as much 

as 30° from horizontal (the drill holes are assumed to 

have been approximately vertical). In Y-7 local smallscale slumping and small faults with as much as 10 

mm displacement occur in fine-grained sandstone in 

core from -21.6 m, and highly contorted beds occur at 

-23.5 m. Similar small offsets on fractures occur in 

core from Y-8 at -19.7 m and from -24.8 to -25.0 m.

The sediments were probably outwash stream deposits from and adjacent to receding glaciers of late 

Pinedale age (Waldrop, 1975; their probable age is 

from 10,000 to 15,000 years B.P. according to Pierce 

and others, 1976). The original sediments were mainly 

silt, sand, and gravel that were sufficiently well sorted 

to have high porosities and relatively high permeabilities. Even now, the average porosity of 

sedimentary rocks from the two drill holes, as calculated from bulk and powder densities (table 2), is about 

30 percent.

The most likely sources for the obsidian-rich detritus 

were the Summit Lake flow, which is part of the Central Plateau Member of the Plateau Rhyolite, and the 

Biscuit Basin and Scaup Lake flows, both of which are 

part of the Upper Basin Member of the Plateau Rhyolite (Christiansen and Blank, 1972), The obsidian-rich 

detritus was probably derived largely from the glassy 

rhyolitic Summit Lake flow, which covers a large area 

west and southwest of Upper Geyser Basin and has an 

eruption age of about 127,000 years B.P. (this and 

other K/Ar ages from J. D. Obradovich, written commun., 1975). Support for this conclusion, as opposed to
A4 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING 

TABLE 2. Chemical analyses of cores from Y-7 and Y-8 drill holes with their unaltered equivalents

Scaup Summit 

Lake Lake

flow1 flow 1

Y-7 

Altered 

Rhyolite Rhyolite sediments 

of 7)

3

Sediments of Upper Basin and dominant source rocks

Y-7-21 

Black 

obsidian 

gravel 

and SiOa

3

Y-7- 142 

Volcanic 

cobble

3

Y-8 

Altered 

sediments 

( average 

of 9)

Y-8- 14.7 

Least 

altered 

gravel 

and SiO2

3

Y-8^32.5 

Analcimequartz 

sandstone

Y-8-160.5 

CalciteanalcimequartzK-feldspar 

sandstone

Fresh 

flow 

breccia

Biscuit Basin flow

Y-7 

Altered 

flow 

breccia 

(average 

of 5)

2, 3

Y-8 

Altered 

flow 

breccia 

(average 

of3)

2, 3

Y-8 

Altered 

pumiceous 

tuff 

(average 

of 4)

2, 3

Major oxides (percent)

SlO2 -___

AUOs --- 

Fe2 Os - 

FeO_ _ -- 

MgO 

CaO_- ___

H2O+- ... 

HaOTiO2 __ _ 

P2 O.. _____ 

MnO _ 

COa. -__ _ 

Total S as S

72.95

12.50

1.53

.22

19

.84

3.20

5.10

2.37

.10

.26

.12

.04

.01

.01

Total ____ 9944

Bulk density _

Powder density ________ __

Calculated porosity, percent

76.51

12.05

.47

1.11

.01

.42

3.65

5.10

.23

.02

.13

01

.04

.01

9976

71.5

11.5

.74

.44

.03

.56

3.5

3.9

5.3

2.7

16

.05

.03

.03

.02

100

1.58

2.21

29

78.9

9.8

.50

.64

.00

.35

2.8

4.0

2.7

.01

.12

.04

.03

.03

.03

100

2.06

2.25

8

74.6

12.2

1.0

.36

.00

.26

3.8

5.3

.58

.20

.12

.05

.02

.03

.03

99

2.31

2.44

5

70.8

12.1

.80

.41

.05

.55

5.3

3.3

5.1

1.7

.15

.07

.03

.04

.01

100

1.61

2.34

31

78.6

8.6

.53

.52

.00

.32

2.9

3.3

2.8

1.0

.10

.06

.02

.02

.01

99

2.11

2.38

11

64.7

16.7

.91

.36

.07

.26

8.6

2.9

5.4

.30

.16

.07

.04

.02

.01

100

1.51

2.35

36

72.2

11.1

1.1

.44

.07

2.5

3.9

5.2

1.5

.13

.19

.06

.04

1,6

.00

100

1.60

2.54

37

69.86

13.11

1.17

2.14

.30

1.51

3.27

4.69

2.67

.13

.45

.09

.07

.00

.00

99.46

66.3

13.0

1.6

1.6

.30

1.4

3.5

4.2

5.3

2.1

.46

.14

.07

.03

.01

100

1.95

2.34

17

64.9

13.1

1.9

1.2

.24

2.0

5.3

1.9

7.1

2.6

.47

.15

.05

.02

.00

101

1.99

2.28

13

64.2

14.3

2.5

1.4

.44

1.8

4.5

4.2

4.8

2.0

.53

.16

.06

.03

.02

101

1.41

2.32

39

Semiquantitative spectrographic analyses (weight percent)

-0005 N

.12 .010

N .0003

.0003 N

.0003 .0001

.0026 .0018

010 .013

N """0667

.006 .0079

.004 .0032

.005 .0004

.0015 N

.0071 .0065

0008 .0009

034 .038

<0.0015 

.028 

.0003 

.014

N

.0001 

.0002 

.002 

.010

N

<.0003 

.003 

.003 

.003

N

.006 

.0005 

.019

0.0015 

.015 

.0005 

.010 

N 

N

.00015 

.003 

.007 

N

.0003 

.002 

.003 

.001 

N

.005 

.0005 

.015

N

.010 

.0003 

.010 

N 

N

.0001 

.002 

007 

N

.0003 

.005 

.003 

N 

N

.005 

.0005 

.02

<0.0015

.021

.0017

.013

N

.0001

.0003

.0018

.008

-.015

N

.003

.003

.003

N

.005

.0005

.018

0.003

.010

.0002

.015

N

N

.00015

.0015

.005

N

N

.002

.0015

.0010

N

.003

.0003

.015

N

.02

.005

.015

N

.0002

.0003

.002

.010

N

N

.003

.003

.003

N

.007

.0005

.02

N

.02

.002

.010

N

.00015

.0001

.0015

.010

.02

N

.003

.005

.003

N

.007

.0007

.02

N

.12

N

<.0007

.0007

.0005

.0026

<.005

N

.003

.003

.012

.002

.0057

.0007

.038

N

.082

.0003

.015

0001

.0006

.0008

.002

.008

N

.0002

.002

.002

.015

.0006

.007

.0005

.03

N

.073

.0002

.012

.0001

.0006

.0006

.002

.009

N

N

.002

.003

.02

.0011

.007

.0005

.03

N

.065 

.0004 

.010 

.0002 

.0005 

.0008 

.0025 

.007

N

N

.003 

.003 

.015 

.0010 

.008 

.0007 

.04

'Chemical analyses of fresh rocks, from Christiansen and Blank (unpub. data) but excluding F and Cl; analyses of major constituents by standard methods of rock analysis with 

minor constituents by quantitative spectrographic methods.

2Unweighted averages of each major stratigraphic unit; rapid-rock chemical analysis and semiquantitative spectrographic analyses.

"Individual analyses by rapid-rock method of Shapiro (19671, Lowell Artis, analyst, and semiquantitative spectrographic analyses, Chris Heropoulos, analyst.

possible derivation from the local Biscuit Basin flow 

(about 527,000 yrs B.P.) or the Scaup Lake flow (about 

265,000 yrs B.P.) which crops out east of Upper Geyser 

Basin, is found in the petrographic and mineralogic 

characteristics of the volcanic rocks of the area (R. L. 

Christiansen, oral commun., 1975). The Summit Lake 

flow has abundant quartz and sanidine phenocrysts 

and scarce plagioclase phenocrysts, the Scaup Lake 

flow has a moderate amount of quartz, sanidine, and 

plagioclase phenocrysts, and the Biscuit Basin flow has 

abundant plagioclase and minor quartz and sanidine 

phenocrysts. The relative abundance of clastic quartz 

and sanidine grains and paucity of plagioclase grains 

in the sediments thus support their derivation mainly 

from the Summit Lake flow. A few of the clastic plagioclase grains in the sedimentary rocks have the same

internal sieve texture that characterizes the underlying Biscuit Basin flow, suggesting that this flow 

supplied a minor amount of the clastic debris.

Additional evidence on the source of the obsidianrich sediments is provided by chemical and spectrographic analyses of fresh volcanic rocks and hydrothermally altered sediments from the drill holes, 

summarized in tables 2 and 3. The individual analyses 

of core from each drill hole are relatively uniform in 

many constituents in both the sediments and the Biscuit Basin flow. The altered rocks are most nearly similar to the Summit Lake flow in many constituents 

when corrected for differences in EbO, suggesting that 

this rhyolite flow and other chemically similar young 

flows (Christiansen, unpub. data) supplied most of the 

sediments. The relatively immobile constituents such
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A5

TABLE 3. Chemical analyses of altered sedimentary rocks from Y-7, 

Y-8, and unaltered rhyolite of the Summit Lake flow, recalculated 

to anhydrous compositions (from table 2)

Rhyolite of

Summit Lake

flow

Y-7 altered

sediments with

clinoptilolite

(average of 7)

Y-8 altered

sediments with

clinoptilolite

(average of 3)

Y-8 altered 

sediments with

analcime 

(average of 3)

SiO2 __ __

AbOs ___ _ 

FtsOs ____ 

FeO __

MgO

CaO ______

Na2O ____

K2O __ ____

TiO2 _ ____

MnO_

Total 

Bulk density

Powder density 

Calculated poros 

oercent

76.89 

12.11 

.47 

1.12 

.01 

.42 

3.67 

5.13 

.13 

.04

99.99

ity,

77.4 

12.5 

.8 

.5 

.03 

.6 

3.8 

4.2 

.17 

.03

100.0 

1.58 

2.21

29

75.8 

12.7 

1.0 

.4 

.07 

.6 

5.6 

3.5 

.19 

.03

99.9 

1.42 

2.24

37

73.0 

14.6 

.9 

.4 

.06 

.4 

7.1 

3.4 

.17 

.04

100.0 

1.63 

2.45

34

as TiO2, P2Os, Ba, Cr, and V are especially useful, 

suggesting that the Scaup Lake flow or the older Biscuit Basin flow may have supplied a small part of the 

clastic debris. The only immobile constituent that is 

not easily explained by derivation from the analyzed 

lava flows is zirconium, which is deficient in the sediments by nearly 50 percent. R. L. Christiansen (oral 

commun., 1975) has suggested that most zirconium in 

the rhyolites occurs as zircon phenocrysts; these 

phenocrysts may have been freed and enriched in 

near-source sediments.

BISCUIT BASIN FLOW

FLOW BRECCIA

The contact between sedimentary rocks and the underlying flow breccia is abrupt in both holes. This flow 

breccia and associated pumiceous tuff (in Y-8) are correlated with the Biscuit Basin flow (Christiansen and 

Blank, 1974), which crops out immediately west of Biscuit Basin and elsewhere in and around Upper Geyser 

Basin. The pumiceous tuff has not been found in outcrop.

The flow breccia extends from 52.7 m to the bottom 

of the hole at -73.8 m in Y-7 (table 1), and from -55.2 

m to 62.8 m in Y-8. The flow breccia now consists of 

subrounded fragments of green to black vitrophyre 

with abundant concentric hydration cracks. The subrounded fragments are enclosed in a similar but lighter 

green, more altered groundmass. Much obsidian persists in the flow breccia of Y-7, but the flow breccia of 

Y-8 is more thoroughly altered. The initial flow breccia 

evidently consisted of angular obsidian blocks (on the 

order of 5 cm diameter) with interstitial granulated

obsidian; the rounded obsidian fragments resulted 

from hydration that progressed inward from the borders of the angular blocks.

The local flow breccia is pervasively fractured and 

granulated, and its initial permeability was evidently 

high enough to permit access of enough water to hydrate the obsidian (table 2) but not high enough for 

much continued circulation. Initial porosity is not 

known, but it may have been close to the present porosity of about 14 percent, as calculated from the bulk and 

powder densities of the drill core.

The obsidian from Y-7 is' amorphous to X-rays but 

has a refractive index ranging from about 1.48 to about 

1.50 consistent with the extensive hydration indicated 

by the chemical analyses. A small amount of acristobalite is evident in some X-ray diffraction traces, 

probably as a product of partial devitrification during 

initial cooling and prior to the hydrothermal regime. 

Scarce spherical patches of incipient crystallization of 

obsidian to a-cristobalite are seen in thin sections of 

unaltered flow breccia, and an X-ray diffractogram of 

the same rock shows a small amount of a-cristobalite, 

comparable to that in the black obsidian of Y-7.

The vitrophyre from the drill holes has a glomeroporphyritic texture with clusters of plagioclase, 

clinopyroxene, and magnetite; abundant plagioclase 

and scarce quartz and sanidine occur as solitary 

phenocrysts. The composition of the plagioclase phenocrysts is about Arm-35, as determined by the method of 

Smith and Yoder (1956). An electron microprobe 

analysis of a phenocryst in core from Y-8 indicates a 

composition of AmeAbeeOrs. Most of the solitary plagioclase phenocrysts are corroded and have an internal 

sieve texture (fig. 2). This texture with fresh obsidian

0.5mm

FIGURE 2. Photomicrograph of perlitic vitrophyre of the Biscuit 

Basin flow, showing internal sieve texture characteristic of plagioclase phenocrysts. Y-7-238.3 (-72.7 m); partly crossed polars.
A6 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

filling the holes is characteristic of plagioclase-rich 

rhyolites of the Plateau Rhyolite, one of which is the 

Biscuit Basin flow, one of several rhyolite flows within 

the Upper Basin Member (Christiansen, unpub. data). 

This texture has clearly resulted from reactions prior 

to eruption of the lava flow and is unrelated to hydrothermal alteration.

Clinopyroxene phenocrysts of the flow breccia are associated with small euhedral to subhedral magnetite 

grains and zircon crystals. These mafic clots of crystals 

occur in unaltered rhyolite of the Biscuit Basin flow 

and are also primary.

PUMICEOUS TUFF

The pumiceous tuff of the Biscuit Basin flow extends 

from -63.0 m to the bottom of the hole at - 153.4 m in 

Y-8. The contact with the overlying flow breccia is 

fairly sharp, occurring within 0.3 m. The pumiceous 

tuff is light and porous, consisting entirely of pumice 

fragments generally less than 5 cm in diameter. The 

porosity of the tuff as calculated from bulk and powder 

densities is now about 39 percent, which may not differ 

greatly from initial porosities. The tuff is pale green 

mottled with dark green; its original color is not known 

because it does not crop out. Glomeroporphyritic clots 

of plagioclase, clinopyroxene (now largely replaced by 

hydrothermal minerals), and solitary plagioclase 

phenocrysts with the sieve texture shown in figure 2 

and an electron microprobe analysis of An26 Abes Ore 

provide strong evidence for a close relation between the 

flow breccia and the pumiceous tuff. The chemical data 

of table 2 also provide supporting evidence. Most constituents of low mobility, such as titanium, phosphorous, chromium, and zirconium, have nearly identical 

abundances in the altered pumiceous tuff and in the 

fresh and altered flow breccia.

The pumiceous tuff has very uniform characteristics 

at depth in Y-8, with only two exceptions. A thin zone 

near -113.1 m is fine grained with contorted and 

slumped bedding, and a crystal-rich layer near 119.5 

m (perhaps as thick as 1 m) contains angular to 

rounded crystals of quartz, plagioclase, scarce sanidine 

and zircon, and a few lithic rhyolite fragments, but 

pumice fragments are absent. More thoroughly altered 

rocks occur in the lower part of Carnegie I drill hole 

(table 1) and are considered equivalent to the pumiceous tuff of Y-8.

HYDROTHERMAL ALTERATION

The drill core samples from Y-7 and Y-8 are extensively affected by the hydrothermal activity. The duration of the hydrothermal activity in this area is an

unsolved problem at present but is likely to have continued for the past 12,000 years or more. The 

mineralogical changes that are evident, not only in the 

bulk rocks of Y-7 (fig. 3) but also in associated veinlets, 

are considerably simpler than in Y-8 (fig. 4) and other 

previously described core samples from Upper Geyser 

Basin (Fenner, 1936; Honda and Muffler, 1970). The 

summary of temperatures, pressures, and implied 

flow-channels given in figure 5 shows why the rocks of 

Y-7 should be less affected than Y-8 and thus is described first.

In both drill holes a major focus of interest is the 

response of rhyolitic obsidian of the "black sand" and 

the vitrophyric lavas to the thermal environment. Obsidian is a metastable undercooled liquid; in different 

environments it may remain as obsidian that hydrates 

in time, it may devitrify to more stable phases, or it 

may dissolve and allow more stable phases to precipitate in available pore spaces and open fractures. All of 

these alternatives occur in the Biscuit Basin core.

In this report, the mineralogy of veinlets is discussed 

separately from that of each hydrothermally altered 

rock unit. During the earliest hydrothermal activity, 

the initial sediments were highly permeable. At least 

partial cementation was first required before crosscutting fractures could form. Thus, the mineralogy of the 

veinlets may represent somewhat later stages of hydrothermal development than that of the bulk 

sedimentary rocks.

Y-7 DRILL HOLE

SINTER

Little change has occurred in the sinter since it was 

first formed. Some of the opal linings of open spaces 

were deposited from subsurface thermal waters after 

initial burial.

OBSIDIAN-RICH SEDIMENTS

Obsidian granules, generally hydrated to perlite 

with concentric hydration cracks, are abundant in 

sand-sized detritus in the upper part of the hole, and in 

a conglomeratic zone near 6 m. In general, clastic 

fragments increase in size downward from 40 m to 

 48 m with diameters generally ranging from approximately 1 mm to 25 mm. Obsidian remains in the centers of hydrothermally altered clasts in a conglomerate 

zone at -48.0 m, and as unaltered, probably hydrated, 

granules in coarse sandstone at 52.7 m. The larger 

initial volume of glass in individual fragments was 

probably influential in preserving their centers from 

attack other than hydration. Elsewhere, most grains 

are cut by curved fractures that are lined by a variety 

of minerals, with little or no remaining obsidian.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A7

TEMPERATURE, IN DEGREES CELSIUS 

100 150

100 

EXPLANATION

150 — Flow breccia, 

Biscuit Basin flow

200 —

= -60

--70

SILICA ZEOLITES CLAYS

FIGURE 3. Distribution of fresh obsidian and hydrothermal minerals in groundmass of core from Y-7. Column on left shows 

generalized stratigraphic section. Horizontal lines at right indicate samples studied in detail, and the starred samples 

are chemically analyzed. Width of mineral columns shows relative abundance based mainly on X-ray diffraction data 

and petrographic study. Interpolation between data points assumes linear variation. The dashed line connects data 

points of bottom-hole temperatures, and the solid line is the reference boiling curve for pure water.
A8 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

TEMPERATURE, IN DEGREES CELSIUS 

1 00 150

EXPLANATION

250 —

SILICA ZEOLITES CLAYS

FIGURE 4.—Distribution of fresh obsidian and hydrothermal minerals in groundmass of core from Y-8; data shown in same manner as for 

Y-7, figure 3. Depths below —82.3 m not shown because of general uniform alteration mineralogy from —63.0 to —153.4 m.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A9

Spherical cavities of similar size and cavities coated 

with tiny crystals and traversed by curving septa of 

birefringent minerals are also common. These textures 

and relict textures characteristic of perlite provide evidence for abundant obsidian granules that were first 

hydrated to perlite. Hydrothermal minerals then filled 

the hydration cracks and replaced adjacent hydrated 

obsidian. Still later, any remaining obsidian either was 

hydrothermally devitrified or was dissolved, leaving 

the curving septa as relicts of former hydration cracks 

and also providing evidence for the sequence of events. 

Clear botryoidal amorphous opal occurs as intergranular cement and as thin coatings on most clastic 

grains in the uppermost conglomeratic zone; many of 

the grains are obsidian, presumably hydrated. The refractive index of the opal ranges from 1.45 to 1.46. Very 

small spheres of a mineral of low birefringence are included with opal in figure 3; these spheres may be cristobalite that is too scarce to be identified on X-ray diffraction traces. No amorphous opal was found in the 

sediments at depths below about —7.5 m. However,

/Water table

veinlet fillings and linings of white to green, nearly 

isotropic material with the X-ray characteristics of 

/3-cristobalite are especially common from -24.3 to

—32.8 m. Because amorphous opal in sinter (White and 

others, 1964) and diatomite (Murata and Larson, 1975) 

is converted to /3-cristobalite after burial at increased 

temperatures, we suspect that all /3-cristobalite of Y-7 

was initially amorphous opal.

The only other silica mineral normally in these sediments is a-cristobalite, which occurs in lithic fragments throughout the unit as a high-temperature devitrification product of the source rhyolite, and also as 

a hydrothermal mineral. Hydrothermal a-cristobalite 

is present from —6 to —7 m as small clear spheres of 

radiating bladelike crystals deposited on an irregular 

veinlet surface. Chalcedony is a scarce hydrothermal 

silica mineral, occurring as small white cryptocrystalline patches in intergranular spaces from -5.8 to

-6.4 m and as thin coatings of small cavities in a volcanic cobble at — 43.3 m (discussed below, and not 

shown on fig. 3).

N

ALTITUDE,

FEET 

-7300 Firehole River. 

,Pond \

Sand and gravel # $ 

(originally highly permeable) ?

Flow breccia 

(originally permeable)

2100-

2075-

2050-I

25

, . (originally permeable?)

50 METERS

EXPLANATION

2.37

-7200

—7100

7000

-6900

L-6800

Water overpressure in kg/cm2 , Direction and qualitative

as measured at surface rate of thermai water flow

-0.11 ... ....

Water levels below ground ex- .' •!; .'•. •': ."

pressed as negative pressure Decreased permeability due

relative to surface, in kg/cm2 f() deposition of hydro -

—— 120°—— thermal minerals 

Temperature contour, °C

FIGURE 5.—Section through Y-7 and Y-8 drill holes, showing pressures (in kg/cm2 ), temperature contours, and inferred "self-sealing" by 

deposition of hydrothermal minerals in originally permeable ground (from White and others, 1975, p. 21).
A10 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

Hydrothermal alteration and cementation of the 

sedimentary rocks from -13 to —48 m are relatively 

uniform (fig. 3). Obsidian either is leached or, more 

commonly, is replaced by clinoptilolite and montmorillonite. Unfortunately, no core was recovered from the 

intervals at the top and bottom of this section to provide evidence on mineralogical transitions.

Clinoptilolite is not only a common replacement of 

obsidian but is also the dominant pore filling mineral 

that has formed cemented rocks from the clastic sediments. This zeolite also surrounds clastic grains as 

thin rims of tabular crystals, generally oriented perpendicular to the grain surfaces (fig. 6). Mordenite occurs as hairlike radiating fibers in some pore spaces; it 

is a subordinate mineral that increases somewhat in 

amount with depth (fig. 3) and is distinguished optically and by X-ray from the earlier clinoptilolite.

Green montmorillonite occurs throughout the 

sedimentary section, commonly outlining hydration 

fractures in altered obsidian and mantled by later 

clinoptilolite. Concentrations of green to brown 

montmorillonite also occur in some lithic fragments 

and altered mafic grains. The X-ray and glycolation 

characteristics of the clay minerals of Y-7 are summarized in table 4.

Veinlets, dominantly of clay minerals (table 5), are 

more prominent in the sedimentary rocks of Y-7 than 

in those of Y-l (Honda and Muffler, 1970). A little

FIGURE 6.—Photomicrograph showing rims of clinoptilolite (cpt) surrounding clastic grains, generally as tiny tabular crystals with 

long dimension oriented perpendicular to the grain surfaces. 

Y-7-156.4 (-47.7 m); plane polarized light.

olive-green montmorillonite of normal characteristics 

(table 4) was deposited in a veinlet at —6.4 m on earlier 

a-cristobalite. In veinlets at greater depth, the earliest 

and most prominent mineral is probably montmorillonite, but its X-ray and glycolation characteristics are 

anomalous for montmorillonite. In untreated samples, 

this olive-green (or rarely brown) mineral has a 10 A 

basal spacing, characteristic of mica and mixed-layer

TABLE 4.—X-ray and glycolation characteristics of clay minerals from Y-7 drill hole, with identifications used in this report

Sample No. 1

Sedimentary

unit

Y7-21.0-4

Y7-47.2-1

Y7-64.0-1

Y7-64.0-2

Y7-73.8-1

Y7-94-2

Y7-94-3

Y7-94-4

Y7-115-1

Y7-119.5-1

Y7-124-1

Y7-149-1

Y7-157.5-1

Y7-173

Y7-173

Flow breccia,

Biscuit Basin

flow

Y7-174-3

Y7-180.5-1

Y7-190

Y7-198-1

Y7-205-1

Y7-209-1

Y7-209-2 

Y7-211

Y7-217-1

Y7-229.2-1 

Y7-233.6-1 

Y7-237-4 

Y7-241.6-1

Description

Olive-green clay in veinlet 

do.

do.

do.

do.

Brown clay in veinlet

Olive-green clay in veinlet 

Red Fe-stained clay in veinlet

Olive-green clay in veinlet

do.

do.

Altered perlite fragment

Green clay in veinlet 

Silt, groundmass

do.

Altered groundmass 

Thin coating gray-green clay 

Groundmass

Orange-yellow altered glass 

Yellow waxy coating

Green clay in veinlet

Groundmass

do.

Veinlet coating

Groundmass

do.

Dark-green clay from veinlet 

Dark-green veinlet

Untreated

12.6 A (broad) 

10.0 A

10.05 A

10.0 A

10.2 A

10.0 A

10.05 A 

10.07 A (broad)

10.6 A

10.05 A

10.5 A

11.0 A

10.9 A 

16.99 A

7.13 A

13.3 A (broad) 

12.2-14.4 A (broad) 

12.2 A (broad)

11.6 A 

11.9 A (weak)

No distinct maximum

11.6 A

11.0 A (broad)

11.9 A

12.9 A (broad)

12.9 A

10.6 A 

10.28 A

Glycolated Vi 

hour at 60°C

17.6 A 

17.6 A

17 A

19.6 A

19.2 A

19.6 A (broad)

17 A 

17 A

16.9 A

17 A

17.3 A

10 A and broad at

higher d-spacing 

16.9 A 

16.99 A

17.3 A 

17.3 A 

17.3 A

11.3 A (broad) 

17.6 A (broad, weak)

Shifts to higher

d-spacing and 10.0 A

18.4 A (broad)

17.3 A, 9.8 A (weak)

12.2 A and broad

at higher d-spacing

17.6 A

18.4 A (broad)

9.8 A 

10.04 A

Heated Vi 

hour at 400°C

_.

9.9 A

9.8 A

9.9 A 

10.2 A

9.8 A

9.8 A

9.8 A 

9.8 A

7.13 A

10 A""" 

9.8 A

11.3 A (broad) 

10 A (small)

9.8 A

11.0 A (broad)

9.9 A

12.1 A

10 A"""

Identification

Montmorillonite 

10 A expanding clay

Do.

Do.

Do.

Do.

Do. 

Do.

Do.

Do.

Do.

Interlayered montmorilloniteceladonite 

10 A expanding clay 

Montmorillonite

Kaolinite

Montmorillonite 

Do. 

Do.

Montmorillonite (?) 

Montmorillonite

Interlayered montmorilloniteceladonite

Montmorillon ite

Interlayered montmorilloniteceladonite

Do.

Montmorillonite

Do.

Celadonite 

Do.

'Sample number corresponds to depth in feet.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING All

illitic clays (table 4). However, upon glycolation, the 

mineral expands to or near the 17 A spacing expected 

of montmorillonite, with little or no indication of a 10 

A mica component. Because of uncertain identity, this 

mineral is called "10 A expanding clay". In core from 

—6.4 m, —52.7 m, and from the underlying flow breccia, most of the expanding clays show the normal 

characteristics of montmorillonites with untreated 

d-spacings from about 12 to 15 A, similar to those of 

Y-l (Honda and Muffler, 1970).

Several veinlets in the middle and lower part of the 

sedimentary section contain /3-cristobalite, either as 

the only mineral or later than the 10 A expanding clay. 

Tiny crystals of pyrite, where present, are the latest 

hydrothermal mineral. Pyrite has not been identified 

in the groundmass of the sedimentary rocks. Zeolites 

are notably absent in veinlets from this drill hole, in 

contrast to the abundant clinoptilolite and common 

mordenite of the adjacent sedimentary rocks.

The core from -43.3 m, consisting entirely of dense 

dark-green fine-grained rhyolite, is interpreted to be a 

lithic volcanic cobble rather than a thin flow within the 

sediments. It is described here rather than in the 

foregoing systematic discussion of the sedimentary 

rocks because its mineralogy and chemical composition

(table 2) indicate high-temperature devitrification and 

vapor-phase reactions associated with extrusion in a 

rhyolite flow, with only minor hydrothermal effects 

after its incorporation as a cobble in the sediments. A 

few phenocrysts of sanidine and quartz occur in a devitrified groundmass of a-cristobalite and sanidine 

that probably formed during initial cooling. Euhedral 

tridymite and sanidine occur in small irregular 

cavities and are interpreted as high-temperature 

vapor-phase precipitates in vesicles. Overlying these 

minerals in some small cavities are thin layers that 

were deposited in the sequence of celadonite, clinoptilolite, mordenite, and chalcedony, all of which were 

probably deposited in the active geothermal system. 

The key minerals supporting this interpretation are 

the euhedral tridymite and sanidine in cavities; the 

high bulk density and low water content (table 2) also 

point to high-temperature devitrification, and the 

chemical aspects of the cobble are similar to rhyolite of 

the Summit Lake flow.

A critical gap in recovered core extends from —48.0 

to —52.7 m. A single piece of core from —52.7 m contains an upper layer of coarse- to medium-grained 

sandstone consisting dominantly of black obsidian 

clasts. The underlying layer is light-buff siltstone that

TABLE 5.—Distribution and sequence of deposition of hydrothermal minerals in veinlets of Y—7 drill hole

[Numerical order indicates sequence of deposition in each veinlet. Query indicates uncertain order of deposition. _, not found]

Sample 1 

No.

Sedimentary

unit

Y7-21.0

Y7-47.2

Y7-64.0

Y7-64.4

Y7-73.8

Y7-79.6

Y7-94.0

Y7-107.5

Y7-115.0

Y7-119.5

Y7-124.0

Y7-124.5

Y7-157.5

Flow breccia,

Biscuit Basin

flow 

Y7- 198.0

Y7-199

Y7-200

Y7-209

Y7-211

Y7-213

Y7-217

Y7-219

Y7-219a

Y7-219b

Y7-220.3

Y7-223

Y7-224.8

Y7-227.7

Y7-229.2

Y7-233.6

Y7-237

Y7-238.3

Y7-239

Y7-239a

Y7-239.3

Y7-240.6

Y7-241

Interlayered montmorillonite 

celadonite

_.,

__

__

T

i

Y

i

i

i

i

i

i

i

i

-

Montmo- 10 A ex- Celadorillonite Pandmg ; 

clay

2

1

1

1

1

II T II

'.'. Y _I i

i

i

..i

-I II 2

~2~ II \\

~2

1

1

1

I

1

1

I

1

Chalce- a-cristodony halite

1

3? 2?

3

2 3~

2

-

/3-cristobalite

Y

2

1

"2

1

1

1

4

4

1

3

2

2

~2~

3

2

2

2

6

~2~

2

2

Opal

~5

4

3

3

~3

3,5

7

~3

3

3

Clinoptilolite

2

6~

5

2

5

4

2~

2

2

2

3

4

3

4

4

4

3

~4~

4

Cal- Morde- Acicular Pvrite 

cite nite unknown y

II II II ~2~

I II II ~3

."! II II 3

y i" ~~- ii

6

3

--

.- 3~ 4~ "

3 4

-3 4

4

-.5

4

5? 6?

6

5

4

4

5

5? 6?

"Sample number corresponds to depth in feet
A12 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

now consists mainly of montmorillonite (fig. 3 and 

table 4) with subordinate a- and /3-cristobalite, quartz, 

sanidine, and kaolinite. No zeolites or mixed-layer 

clays were detected. A chemical analysis (not included 

in table 2) shows major and minor constituents that are 

intermediate in content between the overlying sediments and the underlying Biscuit Basin flow, suggesting local derivation of some of the clastic debris. Kaolinite was identified from this depth (table 4 and fig. 3) 

but not elsewhere in this drill hole.

FLOW BRECCIA OF THE BISCUIT BASIN FLOW

The vitrophyric flow breccia of Y-7 is extensively 

altered in its upper part, least altered from —62 to —67 

m, and increasingly altered downward to the bottom of 

the hole at —73.7 m. Much of the groundmass consists 

of thoroughly hydrated obsidian, which is amorphous 

to X-rays and has an average total water content of 

about 7 percent (table 2). The altered groundmass from

—53.0 to —59.8 m contains light-green montmorillonite that has totally replaced obsidian. Light-blue or 

gray-green montmorillonite fills hydration cracks in 

the remaining obsidian. The montmorillonites of various colors are poorly crystalline and have similar 

X-ray characteristics (table 4). The groundmass from

—60.4 to —62.5 m is altered to an orange-yellow material that is nearly amorphous to X-rays, is optically 

isotropic, and is possibly equivalent to the light-green, 

poorly crystalline groundmass montmorillonite at 

deeper levels.

From —63.7 m to the bottom of the hole at —73.7 m, 

clinoptilolite is the dominant groundmass mineral

along with subordinate green montmorillonite. Below 

—67 m, mordenite becomes conspicuous, and celadonite 

appears to increase below —71.2 m. In order of deposition, the paragenetic sequence consists of (1) green 

montmorillonite (and celadonite below —71.2 m) replacing obsidian along hydration cracks; and (2) 

clinoptilolite progressively replacing part of the remaining obsidian and crystallizing in the hydration 

cracks as subhedral crystals on the montmorillonite 

and oriented perpendicular to the cracks. Residual obsidian was locally leached, with clinoptilolite and (or) 

mordenite then being deposited in the leached cavities. 

The initial stages of alteration of obsidian are illustrated by figure 7, and the progressive filling of an 

adjacent pore space, part of a veinlet serving as a 

permeable channel, is illustrated by figure 8.

Obsidian that initially filled the holes of sieved plagioclase phenocrysts (see fig. 2) is now replaced by 

a-cristobalite and finely crystalline green interlayered 

celadonite-montmorillonite in the upper part of the 

unit, and by celadonite in the lower part. Fibers of 

mordenite may fill the remaining parts of the nonfeldspar framework. The plagioclase shows no alteration or change in composition and is evidently more 

stable than the obsidian. Clinopyroxene phenocrysts 

are not stable, generally being partly replaced by green 

clay minerals. Irregular and subhedral grains of 

magnetite and euhedral crystals of zircon are primary 

phenocrysts, although the magnetite may have minor 

hydrothermally deposited overgrowths of an unidentified opaque oxide.

The mineralogy and sequence of deposition in the 

veinlets of the flow breccia are summarized in table 5.

FIGURE 7.—Photomicrograph showing initial stages of alteration of 

obsidian (g) to celadonite (ce, dark) and clinoptilolite (cpt, birefringent) adjacent to perlitic cracks. Y-7-238.3 (-72.6 m); partly 

crossed polars.

FIGURE 8.—Photomicrograph of the area adjacent to figure 7, showing progressive increase in alteration toward veinlet filled with 

celadonite (ce), clinoptilolite (cpt), /3-cristobalite (/3), and mordenite 

(m). Y-7-238.3 (-72.6 m); partly crossed polars.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A13

The veinlets range in width from less than 0.5 mm to 

about 8 mm; the wider veinlets are not everywhere 

completely filled with hydrothermal minerals. The 

mineral assemblages in these veinlets are generally 

more complex than those in the groundmass of the flow 

breccia and in the veinlets of the overlying sedimentary unit. The veinlet mineralogy of the flow breccia 

does not everywhere correlate with the groundmass 

mineralogy (for example, a thin coating of clinoptilolite 

forms a veinlet at —60.4 m, but the adjacent groundmass contains no zeolites). Silica minerals, along with 

green clay minerals, are the most abundant constituents of the veinlets, but clinoptilolite (also with 

green clay minerals) is the dominant replacement of 

the obsidian groundmass.

The earliest mineral of the veinlets is generally 

green mixed-layer celadonite-montmorillonite near 

the top of the flow breccia (tables 4 and 5) and, below 

— 70 m, celadonite with little or no mixed layering. 

Pale-green and blue-white varieties of /3-cristobalite 

are generally the next minerals in sequence, commonly 

followed by a thin layer of clear isotropic opal that 

seems still to be amorphous. Locally, chalcedony and 

a-cristobalite precede the j8-cristobalite. Clinoptilolite 

is almost universally present, generally occurring as 

small tabular crystals on earlier /3-cristobalite. Radiating fibers of mordenite are later than clinoptilolite and, 

in the middle and lower parts of the unit, are locally 

followed by tiny clear radiating acicular crystals of an 

unidentified mineral. In its five known occurrences in 

the drill hole, the unidentified mineral is closely associated with mordenite, and, from its optical proper-

*vyVrt - -.. 7

FIGURE 9.—Photomicrograph showing pore spaces between clasts in 

conglomerate filled by spheres of early-formed opal (o), thin bands 

of nearly isotropic j6-cristobalite ()6), and later chalcedony (ch) from 

nearby veinlet. Y-8-18.0 (-5.5 m); partly crossed polars.

ties and X-ray powder camera data, is probably also a 

zeolite.

Calcite occurs locally as a late veinlet mineral at

-63.7 and -64.9 m. Pyrite, as in the bulk rock, is absent.

Y-8 DRILL HOLE

SINTER

The uppermost 0.2 m consists of layers of coherent 

mottled white and gray fragmental sinter with a few 

thin bands of primary sinter. Deeper core to --0.6 m 

consists entirely of gray fragmental sinter. As at Y-7, 

geysers and perpetual spouters were probably absent 

in the immediate area.

No core was recovered from —0.6 to -4.3 m, but a 

sample of loose grains that include both obsidian sand 

and sinter was obtained from — 2.1 m, so that contact is 

presumably at a slightly shallower depth, arbitrarily 

placed at —1.5 m. The fragments of sinter are cemented 

with clear opal, thus indicating some effects of circulating thermal waters after burial.

OBSIDIAN-RICH SEDIMENTS

As in Y-7, obsidian is preserved mainly in the upper 

part of the sedimentary section, preferentially in the 

larger clasts. A little obsidian persists in sand-size 

grains near -12.2 m but was not detected in any of the 

deeper sedimentary layers (fig. 4). Elsewhere in Y-8, 

relict concentric hydration cracks indicate a former 

abundance of obsidian that hydrated to perlite.

a-cristobalite is probably present in part as a hightemperature devitrification product within the obsidian grains, but its qualitative abundance in the zones 

in which it occurs (fig. 4) suggests that it is also in part 

a hydrothermal mineral. Amorphous opal occurs locally in the upper 9 m of Y-8 (fig. 9). /3-cristobalite was 

identified in the groundmass only in the upper 6 m of 

Y-8 and is absent at greater depths, in striking contrast to its distribution in Y-7. Chalcedony and quartz 

are abundant in some zones of Y-8.

Of the four zeolite minerals identified in the 

sedimentary section, erionite is the rarest, occurring 

only in a single sample from —4.9 m. This rarity is 

similar to its sparse occurrence only in the upper 15 m 

of Y-l (Honda and Muffler, 1970).

Clinoptilolite is the dominant zeolite in most of the 

middle and upper parts of the sedimentary section, 

where it replaces obsidian and also occurs in pore 

spaces as tabular crystals 20 to 70 /um long, generally 

with long dimensions oriented perpendicular to clastic 

grain surfaces. Clinoptilolite is strikingly absent below

-41 m, where analcime is abundant. Mordenite is a 

common but generally not an abundant associate of
A14 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

clinoptilolite; mordenite also accompanies analcime in 

the upper part of the hole, but these two zeolites are 

rarely associated at depths below —24 m.

As noted by Honda and Muffler (1970) in Y-l, analcime is typically associated with hydrothermal quartz 

or chalcedony but rarely accompanies the less stable 

(more soluble) forms of silica. The association pairs of 

analcime-quartz (or chalcedony) and clinoptilolitecristobalite (either variety, but mainly a-cristobalite) 

are nearly mutually exclusive and alternate irregularly in subhorizontal zones with depth. These zones 

are roughly parallel to the sedimentary bedding, and 

boundaries are rather sharp. However, a few analcime 

grains extend very slightly into clinoptilolite zones, 

but clinoptilolite crystals have not been observed extending into analcime-quartz zones. The thickness of 

individual zones ranges from about 2 mm (in the intervals from -5.5 to -7.9 m, and from -22 to -23.2 m) to 

12.5 m. The zones show no clear relation to initial particle size or composition in the clastic sediments.

Conspicuous overgrowths of hydrothermal quartz 

and potassium feldspar occur on clastic mineral grains, 

beginning at about —24.8 m and continuing downward 

throughout the sedimentary unit in the analcimequartz zones. The most common overgrowth is quartz 

on clastic quartz in optical continuity, but hydrothermal potassium feldspar occurs as overgrowths on clastic sanidine and also on clastic plagioclase (fig. 10). The 

overgrowths generally differ slightly in optical relief 

from the clastic grains and also generally include other 

tiny hydrothermal mineral grains, especially near the 

original grain boundaries. In some thin sections from 

quartz-analcime zones, tiny euhedral hydrothermal

potassium feldspar crystals about 20 to 40 /xm wide and 

with rhombic cross sections typical of adularia are visible along with the euhedral quartz as cavity linings.

Green clay minerals occur throughout the sedimentary section as replacements of perlitic glass along concentric hydration cracks and also as cryptocrystalline 

massive clots that apparently replace volcanic lithic 

fragments and clastic ferromagnesian minerals. In 

thin section several types of clay are apparent: (1) 

light-green color and moderately low birefringence, 

probably a montmorillonite; (2) brown to orange-brown 

color and moderate birefringence, probably a montmorillonite; and (3) strong deep-green color and moderately high birefringence, probably celadonite. These 

identifications are supported by X-ray diffraction and 

glycolation characteristics, summarized in table 6. The 

clay types coexist as separate minerals and probably 

also as mixed-layer celadonite-montmorillonite. However, the distribution of montmorillonite and celadonite is shown in figure 4 as separate clays, mainly from 

thin-section observations.

Calcite, absent in the sedimentary section of Y-7, is 

abundant in Y-8, especially in the lower part of the 

thick basal analcime-quartz zone from —50 to —55 m. 

The calcite typically occurs as white blades with pearly 

luster, generally as a late filling of pore spaces (fig. 11).

Veinlet minerals in the sedimentary section of Y-8 

consist dominantly of chalcedony and, rarely, quartz, 

sometimes followed by analcime, mordenite, or rhombic calcite (table 7). All of the veinlets are in analcime-quartz zones except at —22.1 m, which is in 

a clinoptilolite zone but immediately below an 

analcime-quartz zone. This veinlet is also unusual in

FIGURE 10.—Photomicrograph showing overgrowth of hydrothermal FIGURE 11.—Photomicrograph showing late pore filling of calcite (cc) 

potassium feldspar (kf) on clastic plagioclase grain (p), with inter- deposited on analcime (a) in sandstone. Y-8-169.5 (-51.7 m); 

granular quartz (q) and analcime (a). Y-8-169.5 (-51.7 m); partly partly crossed polars. 

crossed polars.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A15

containing chalcedony with clinoptilolite. Analcime in 

Y-8 veinlets occurs where the adjacent groundmass 

contains analcime. The dominant silica mineral of the 

veinlets is usually chalcedony, whereas quartz is 

usually dominant in the adjacent groundmass. 

/3-cristobalite was identified in a veinlet only at —7.0 

m, in contrast to its distribution to -32.8 m in Y-7. 

Clay minerals are notably absent in the veinlets of Y-8 

in contrast to Y-7, where clays are the dominant veinlet fillings and zeolites are absent.

FLOW BRECCIA OF THE BISCUIT BASIN FLOW

Vitrophyric flow breccia extends from -55.2 m to 

—63.0 m, except for a thin interbed of pumiceous tuff or 

granulated pumice at -59.9 m. From -63.0 m to the 

bottom of the hole at —153.4 m, the rocks consist of 

relatively homogeneous pumiceous tuff, all of which 

has been extensively altered hydrothermally.

The flow breccia was initially similar to the same 

unit previously described from Y-7. Obsidian that is 

only partially devitrified and hydrothermally altered is 

characteristic of the middle third of the unit, where as 

much as 50 percent of the obsidian persists. The upper 

and lower parts of the flow breccia have generally decreasing proportions of residual obsidian toward the

contacts of the unit (fig. 4) as the hydrothermal minerals increase in abundance.

Quartz is prevalent in the groundmass from —55 to 

— 56 m and is generally sparse or absent elsewhere. 

Below -56.4 m, a-cristobalite and clinoptilolite are the 

characteristic groundmass minerals of hydrothermal 

origin, having replaced obsidian. Calcite, abundant in 

the immediately overlying sedimentary rocks, decreases strikingly downward in the flow breccia and 

was not identified below —55.8 m. Deep-green celadonite is the characteristic clay mineral of the unit (table 

5); chlorite was identified only from —55.6 m, where 

chlorite and celadonite occur together. Thin layers of 

bright-green celadonite fill hydration cracks of the altered groundmass. Clinoptilolite, directly replacing the 

glass, forms a layer of fibrous and sometimes tiny tabular crystals on celadonite oriented perpendicular to the 

cracks. Later tabular clinoptilolite crystals, and in a 

few places fine fibrous mordenite, were deposited on 

earlier clinoptilolite where the obsidian granules are 

completely replaced or dissolved.

Obsidian that initially filled the holes of the sieved 

plagioclase phenocrysts has been replaced by 

a-cristobalite and celadonite (fig. 2). Mordenite is a 

late-stage deposit in some holes. Pure potassium 

feldspar occurs as patches forming internal rims or

TABLE 6.—X-ray and glycolation characteristics of clay minerals from Y-8 drill hole, with identifications used in this report

Sample1 

No.

Sedimentary

unit

Y8-15.7-1

Y8-27.8-1 

Y8-32.2-2 

¥8-64.6^1 

Y8-64.6-2 

Y8-69.7-1

Y8-81.4

Y8-82-1

Y8-85.4-1

Y8-140-2

Y8-177.6-1

Flow breccia,

Biscuit Basin

flow

Y8-182.2-2

Y8-182.2-3

Y8-185.3-1

Y8-185.3-2

Pumiceous tuff.

Biscuit Basin

flow

Y8-304-1

Y8-327-1

Y8-327-2

Y8-327-3

Y8-331.2-1

Y8-335

Y8-371-1

Crystal tuff 

Y8-391.8-1

Y8-391.8-2

Description

Light yellow-green clay

in porous areas

Very minor clay in groundmass 

Hard green veinlet 

Green clays in groundmass

do.

do.

do.

Groundmass clays 

do. 

do. 

do.

Groundmass clays

do.

Dark-green veinlet 

Green clay in groundmass

Dark-green veinlet 

Red center of veinlet 

Green borders of same veinlet 

Groundmass clay

do.

do.

Black, fine-grained zone 

in groundmass

Buff groundmass clay 

Green groundmass clay

Untreated Glycolated V2 

hour at 60°C

11.6 A 17.8 A

Heated Va 

hour at 400°C

10.0 A

Identification

10.2 A

10.0 A, 11.0 A. 11.9 A

10.2 A, 12.45 A

9.9 A, 11.1-13.5 A 

9.9 A, with broad

shoulder to greater

d-spacing

10.0 A, with broad

shoulder to greater

d-spacing

10.0 A

10.1 A, 12.6 A

10.1 A, 13.1 A

10.1 A, 13.5 A

10.0 A, 17.6 A

9.9 A, 11.0 A (broad)

10.2 A, 16.3 A

10.0, 13.3, 16.3 A 

9.0 A, 12.6-13.19 A

10.0 A, 16.0-17.6 A

(broad)

10.0 A, 17.6 A

10.0 A, 13.8 A

10.1 A, 14.1 A

10.2 A, 16.9 A

10.0 A, 11.0 A

10.2 A

10.0 A, 11.0 A 

10.0 A

10.0 A~~~

_____

10.0 A, 14.0 A, 7.0 A

10.0 A, 7.0 A (small) 

10.2 A

10.1 A

10.1 A

10.2 A

10.2 A

10.1 A

10.1 A

10.2 A

10.2 A

12.1-14.1 A (broadl

7.02 A 

10.5 A, 7.08 A

9.9 A, 14.0 A, 7.0 A 

9.9 A, 7.1 A (small) 

10.0 A 

9.9 A

10.0 A

9.8 A

9.9 A

10.0 A 

10.0 A

9.9 A, 11.9-14 A 

(broad)

10.0 A

16.6 A, 14.0 A, 7.05 A 

9.9 A, 7.08 A

9.9 A

10.0 A

10.0 A 10.0 A 10.0 A

10.0 A"

9.9 A

10.0 A, 7.05 A

Montmorillonite

Celadonite and montmorillonite 

Celadonite and montmorillonite (?) 

Celadonite and montmorillonite

Do.

Do.

Do.

Do. 

Do. 

Do. 

Do.

Celadonite and chlorite

Do. 

Celadonite

Do.

Celadonite

Do.

Do.

Do.

Do.

Celadonite with minor interlayered montmorillonite 

Celadonite

Montmorillonite and chlorite 

Celadonite and chlorite

'Sample number corresponds to depth in feet.
A16 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

overgrowths in the holes within the sieved plagioclase 

and also fills microfractures extending outward from 

the holes. Beam scans with the electron microprobe 

confirm the composition and distribution of the potassium feldspar and thus its probable hydrothermal 

origin.

Clinopyroxene phenocrysts are partially to completely replaced by celadonite. Associated magnetite 

and zircon, as in Y-7, are probably primary phenocrysts. However, an unidentified opaque phase (hematite?) is seen in reflected light filling microfractures in 

magnetite grains and as irregular partial overgrowths 

or replacement at grain boundaries. This opaque phase 

is very scarce but indicates some hydrothermal alteration of the magnetite.

Veinlets of hydrothermally deposited minerals are 

relatively abundant in the flow breccia, generally 

ranging from 0.5 to 4 mm in width; the wider veinlets

are commonly not completely filled. The most abundant minerals are celadonite, /8-cristobalite, opal, and 

mordenite. The sequences of deposition are generally 

consistent in trend in the different veinlets, with only a 

few reversals (table 7). No single veinlet contains the 

full sequence; a few veinlets are monomineralic (generally celadonite), and three veinlets contain as many 

as seven of the ten species recognized in the unit.

The flow breccia adjacent to the wider veinlets is 

generally completely altered, with the hydrothermal 

effects generally decreasing away from the veinlets, as 

in Y-7.

The earliest mineral of nearly all veinlets is darkgreen botryoidal celadonite (fig. 12). Near the top of the 

flow breccia, celadonite occurs with fine-grained quartz 

(table 7), but elsewhere celadonite is either alone or is 

followed by cristobalite and one or more zeolites, most 

commonly mordenite.

TABLE 7.—Distribution and sequence of deposition of hydrothermal minerals in veinlets ofY-8 drill hole

[Numerical order indicates sequence of deposition in each veinlet. Query indicates uncertain order of deposition.—, not found]

Sample1 CeladoNo. nite

Sedimentary

unit

Y8-18.0

Y8-21.5

Y8-23

Y8-32.2a

Y8-32.2b 1?

Y8-32.5

Y8-33

Y8-69.7

Y8-70

Y8-72.5

Y8-76

Y8-81.4

Y8-166

Y8-170.5

Y8-171

Y8-180

Flow breccia.

Biscuit Basin

flow

Y8-181.9

Y8-185.3 1

Y8-185.3a 1

Y8-187 1

Y8-187a

Y8-188 1

Y8-188a 1

Y8-190 1

Y8-191.1 1

Y8-191.1a 1

Y8-192.3 1

Y8-192.3a 1

Y8-194.5 1

Y8-195.5 1

Y8-197.3

Y8-197.7 1

Y8-199 1

Y8-199a 1

Y8-202 1

Y8-204.3 1

Y8-205 1

Pumiceous tuff, 

Biscuit Basin

flow

Y8-226.3

Y8-236

Y8-247

Y8-294 1

Y8-304 1

Y8-327 1

Y8-331.2

Y8-350 1

Y8-354

Goethite Quartz

II ~2

1

1

II 2~,5

" "2

II "2

II ~3

2? I"

1

Chalce- a-cristodony balite

1

1

1

2? \\

1

1

1

2

1

1

1

1

3

II ¥

2

II ¥

2*

3?

2

-

8-cristo- n , Clinop- . ]„•„_> Laumon- Morde- Calcite balite °Pal tilolite Anal«me tite nite (rhombic)

2 II II II II II II

1 2

3

__ 2 __3

_____ 2 3

1 3

2

13" _-4

II II II II ''- 2 II

2

4

__ 3 2

3

__ 1 2

3 4

3 ~4 2,5 __ 6 7 __

2 3 4

¥ ¥ II II -'^ 4~ —

3~ 4 2 II II

2

12—- 3

4 6? 5?

__ 3 4 __ 5? 6?

II II II II ~-^- 3 II

II II II II II ¥ II

__ 1 2

1 . 2

1

2

1

Calcite p..-;*- 

(bladed) Pynte

__

__

1

2

2

__

II 3

3~ II

¥ II

'Sample number corresponds to depth in feet.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A17

The five recognized species of silica minerals are 

never present in a single veinlet, but three species 

occur together at -57.3 and -60.3 m; most other veinlets have two species. The sequence of deposition is 

generally from the most stable phase, quartz, to the 

least stable (most soluble) phase, opal. j8-cristobalite is 

the most abundant silica phase and forms a bluishwhite botryoidal layer (fig. 12).

Mordenite is moderately abundant and is the last 

mineral to have been deposited in most veinlets; it is 

not associated with any particular assemblage. 

Neither clinoptilolite nor analcime is common in veinlets, in contrast to the abundant clinoptilolite that has 

replaced obsidian of the flow breccia. Laumontite, not 

previously described from the rhyolitic rocks of Yellowstone National Park but common elsewhere in 

many calcium-rich geothermal systems, was identified 

in three veinlets from -58 to -61 m by X-ray diffraction and optical properties. The laumontite occurs as 

very small prismatic crystals and is mixed with 

mordenite.

A minor amount of subhedral pyrite occurs in a veinlet at -59.6 m. The groundmass of the crystal tuff at 

— 119.5 m contains the only other pyrite identified in 

Y-8.

PUMICEOUS TUFF OF THE BISCUIT BASIN FLOW

The contact of flow breccia and underlying pumiceous tuff occurs at about —63.0 m, but the actual contact was not recovered in core. The whole unit of 

pumiceous tuff is relatively uniform and consists of

soft, punky, fine-grained tuff with glomeroporphyritic 

clusters of plagioclase and clinopyroxene, and with 

only a few crosscutting veinlets. The tuff is pale green 

mottled with dark-green patches that are enriched in 

celadonite and abundant white patches that are 

largely mordenite.

The hydrothermal alteration of the pumiceous tuff is 

extensive and relatively uniform. The groundmass 

consists dominantly of hydrothermal celadonite and 

mordenite that have replaced initial glass of the 

pumice (fig. 13). In most of the other Yellowstone drill 

core, mordenite is a common minor constituent of zeolitic assemblages, but in the pumiceous tuff, it is the 

only identified zeolite. The frothy pumice texture is 

preserved although alteration is complete. Anhedral to 

euhedral hydrothermal quartz crystals, averaging 

about 30 /am in length, are present throughout the 

unit, a-cristobalite is present from about -92.7 to 

—99.7 m as hemispheres deposited between celadonite 

and some of the later mordenite and is again characteristic below —120 m.

Tabular euhedral to subhedral crystals of potassium 

feldspar, about 20 to 40 /am long, are associated with 

mordenite fibers throughout the tuff, and all are presumed to be hydrothermal. Analysis by electron microprobe of the crystals from -92.9 m indicates a composition of OrvrAbsAno, which provides very strong support 

for a hydrothermal rather than devitrification or 

vapor-phase reaction origin.

Tiny clusters of chlorite have crystallized on celadonite that has entirely replaced augite phenocrysts from

FIGURE 12.—Photomicrograph of veinlet consisting of early dark 

green celadonite (ce) and later botryoidal /3-cristobalite (/3) and 

clinoptilolite (cpt). Perlitic glass (g) of groundmass is almost entirely replaced by celadonite and clinoptilolite. Y-8-190 (-57.9 m); 

partly crossed polars.

FIGURE 13.—Photomicrograph showing frothy pumice texture preserved and outlined by dark-green celadonite (ce) which has replaced initial glass. Tiny euhedral crystals of quartz (q) and potassium feldspar (kf) have crystallized on celadonite, and mordenite 

(m) has filled much of the pore space (v). Y-8-304B (-92.7 m); 

plane polarized light.
A18 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

—68.9 to -69.5 m. No other chlorite has been identified 

in the pumiceous tuff. A few dark-green aphanitic volcanic rock fragments enriched in celadonite are scattered throughout the tuff.

Many solitary plagioclase phenocrysts have the 

same sieve texture as described for the vitrophyric flow 

breccia of Biscuit Basin, but the obsidian within the 

feldspar framework has been dissolved and mordenite, 

celadonite, and hydrothermal potassium feldspar deposited. Clinopyroxene phenocrysts are completely 

replaced by celadonite, which crudely retains the 

clinopyroxene crystal outlines. Associated magnetite, 

as in other parts of this Biscuit Basin flow, is probably 

primary.

A thin zone near —119.5 m is enriched in subrounded 

crystals of quartz, sanidine, plagioclase, and minor 

magnetite and zircon. Most of the crystals are close to 

0.75 mm in diameter. Also included are a very few 

rhyolite fragments, but no obsidian or pumice was recognized. Intergranular hydrothermal minerals are 

montmorillonite, celadonite, and chlorite (table 5). 

Scarce intergranular pyrite is dispersed throughout 

the zone. Internal bedding appears to be nearly horizontal, although neither the upper nor the lower contact was recovered. This local zone is probably a crystal 

tuff.

From —120 m to the bottom of the drill hole at

— 153.4 m, the pumiceous tuff is very uniform in appearance. The characteristic hydrothermal minerals 

are mordenite and celadonite, and minor potassium 

feldspar, a-cristobalite, and quartz.

Veinlets are not abundant in the pumiceous tuff, but, 

where present, they affect the adjacent groundmass. As 

in the overlying flow breccia, celadonite and mordenite 

are the most abundant veinlet minerals (table 7), but 

celadonite is not nearly so abundant. The mineral assemblages tend to be less complex than in the open 

veinlets of the overlying flow breccia. No veinlet contains more than three of the six mineral species recognized in veinlets of the unit. The veinlets range in 

width from less than 0.5 mm to about 4 mm, and most 

are completely filled with hydrothermal minerals. 

Veinlets are completely absent in the basal 34 m of 

pumiceous tuff below the crystal tuff at —119.5 m.

Celadonite, where present, is the earliest veinlet 

mineral. Mordenite, the only common zeolite, was deposited on clear euhedral analcime in two veinlets at

—68.9 and —72.0 m. Analcime also occurs in thin subparallel veinlets as much as 20 mm below the thickest 

veinlet at —68.9 m. Goethite, not recognized elsewhere, 

is a relatively early mineral in two veinlets near

—100.6 m (table 7). Later chalcedony is the only recognized silica species. Calcite is known to occur only in 

one veinlet at —101.0 m, and the adjacent tuff is

silicified 6 cm from the veinlet to a hard gray rock 

mottled with green celadonite. A silicified zone about 8 

cm thick at —113.1 m consists of steeply dipping, hard, 

dark-gray layers of chalcedony.

SIGNIFICANCE OF MINERALS 

AND MINERAL ASSEMBLAGES

OBSIDIAN

Obsidian was initially abundant in the sediments 

and in the Biscuit Basin flow. Because glass is metastable (an undercooled liquid) it tends to be highly 

reactive and provides constituents for minerals that 

are stable (or less unstable) in the existing environment. The first reaction is hydration of nonperlitic obsidian, initially with less than 0.3 percent water (table 

1, Summit Lake flow; Friedman and Smith, 1958). Perlitization occurs when water is absorbed from the environment, and the consequent progressive increase in 

volume causes outer shells to expand and spall, forming the onion-skin layers. Most perlites in nonthermal 

environments have water contents ranging from about 

2 to 5 percent. The least altered of the obsidian-bearing 

sediments from Y-7 and Y-8 (table 2) contains 2.7 percent total HaO, and the least hydrated vitrophyric flow 

breccia contains 5.8 percent total HaO; both of these 

are in large part perlite.

Only a little undevitrified obsidian or perlite remains in the sedimentary section, persisting down to 

-7.0 m in Y-7 and -4.6 m in Y-8 (figs. 3 and 4). However, minor obsidian occurs sporadically at greater 

depths in the sediments of both drill holes (figs. 3 

and 4).

Obsidian is still abundant in the flow breccia, especially in the central parts of the unit in both drill holes, 

and thus has persisted at temperatures that are considerably higher than in most of the overlying sediments where the obsidian granules are completely reconstituted. Temperature is obviously not the only 

factor that controls the conversion of obsidian to more 

stable minerals.

Hydrothermal alteration of obsidian granules progresses inward from grain boundaries and along and 

adjacent to hydration cracks. An increase in volume 

occurs when anhydrous obsidian is hydrated, and 

further increases occur during reconstitution to zeolites and clay minerals. Clinoptilolite is one of the few 

hydrothermal minerals that require little chemical 

change other than addition of water as obsidian is devitrified. Most hydrothermal minerals require a major 

local chemical change, with some constitutents being 

added from the pore fluid as the other constituents are 

removed, either by means of a through-going fluid or 

by precipitation nearby as new minerals. The grain
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A19

density of the rock decreases progressively through 

hydration and reconstitution wherever hydrothermal 

alteration changes do not add quartz, calcite, and 

feldspar in sufficient proportions to affect the decreasing density.

Permeability of the sediments and the borders of the 

flow breccia must have been initially high, judging 

from fresh viscous zones, flows, and flow breccia 

elsewhere. The preservation of abundant undevitrified 

but hydrated obsidian in the vitrophyric flow breccia at 

temperatures as high as 170°C is probably a consequence of the decreased porosity and permeability that 

resulted from initial hydration and incipient alteration 

along hydration cracks. The absence of wholesale replacement of obsidian by clinoptilolite in the sediments, as well as the preservation of hydrated obsidian 

in the vitrophyric flow breccia, may also be due to an 

inadequate supply of additional water; an increase in 

water content from about 5 to 9 percent is necessary to 

produce clinoptilolite from hydrated obsidian. Diffusion rates are too slow to supply much of the needed 

water, except over the short distances adjacent to the 

rare permeable channels such as veinlets. We have no 

good explanation for the complete absence of zeolites in 

the upper 7.6 m of the flow breccia of Y-7 (fig. 3); zeolites do occur in this part of the unit in Y-8 (fig. 4).

CLAY MINERALS

Clay minerals were the first direct replacements or 

reconstitutions of obsidian in and adjacent to fractures 

and hydration cracks. Clay patches were also deposited 

in pore spaces prior to most zeolites.

X-ray data on the groundmass clays were difficult to 

obtain owing to their very small grain size and poor 

crystallinity, but data on clays in veinlets are good.

Optical data on clays in any sample are quite variable 

and not diagnostic except for the well-crystallized 

celadonites in the Biscuit Basin flow. The clays are 

generally distinguished as montmorillonite or celadonite, as determined by X-ray diffraction (tables 4 and 

6). Irregularly mixed layer clays are probably also 

commonly present, but these can seldom be distinguished.

The poorly defined mineral identified in tables 4 and 

5 as "10 A expanding clay" is the most abundant clay of 

the sedimentary unit in Y-7 and is included with normal montmorillonite in figure 3. Scanty chemical and 

electron microprobe data indicate that much potassium 

has been absorbed without producing a nonexpanding 

illite structure. We cannot be certain that any of the 

clays analyzed with the electron microprobe are the 

"10 A expanding clay". However, we have two 

semiquantitative spectrographic analyses on samples 

studied by X-ray diffraction, and these both show 

potassium as the dominant large cation. We suspect, 

but cannot prove, that this mineral is an intermediate 

step between montmorillonite and celadonite; we do 

not understand its restriction to this unit in Y-7 or 

why similar clays have not been recognized elsewhere.

Electron microprobe analysis shows the clays in the 

sedimentary unit to have a large compositional variation between and within grains (table 8, fig. 14). The 

average homogeneity index. *L. , (Boyd, 1969) indicates large variations within grains. Only CaO has a 

homogeneity index low enough to be due to statistical 

variation in the counting system of the electron microprobe. All of the green clays contain appreciable but 

variable amounts of K2O, and none of the analyzed 

green clays contains less than 3.26 percent K2O. 

Whether the low-potassium clays represent montmorillonite "stuffed" with KzO, or whether they are

TABLE 8.—Electron microprobe analyses of clay minerals from Y-7 and Y-8

[Analyst: M. H. Beeson]

SiCb

AkOs _________

MgO ________

CaO__________

Naao

Average1 

brown clay 

from sedimentary 

unit

._ ----- - _ 61.3

Q 4

____________ .05

_____________ 3.19

____________ .77

___ ______ 2.65

- __ ______ 2.41

Range

60.6 61.9

6.0 10.9

.04- .06

1.4 5.0

.43- 1.1

2.6 2.7

2.2 2.7

Average2 

green clay 

from sedimentary 

unit

64.8

8.7

.25

4.02

.53

1.50

5.1

Range

53.3 80.3

4.0 11.6

0 Q9

8 94

97 Q4.

.05- 4.0

3.26- 8.33

Average3

<T

vl

for green 

clay

14.3

6.7

6.3

8.6

5.8

12.0

11.2

Y-8-194.5 

Dark-green 

celadonite 

in veinlet

55.9

10.7

1.64

12.3

94.

.18

7.8

Y-8-194.5 

Clay "blebs" 

in replacement clinoptilolite

40.3

10.2

1.12

15.3

.57

.16

7.4

Y-8-194.5 

Celadonite 

after clinopyroxene

59.2

10.1

1 QO

11.6

.54

.23

6.2

Sum_ 79.8 84.87 88.8 75.1

Average of 2 analyses (Y-8-24 and Y-7-148.7).

2Average of 8 analyses (2 each from Y-8-24, Y-8-117.1, Y-7-148.8 and Y-7-129).

3Homogeneity index (Boyd, 1969), averaged over 8 analyses of green clay. A value larger than 3 indicates inhomogeneous distribution of the constituent.

•"Total iron as FeO.
A20 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

mixed-layer montmorillonite-celadonite cannot be established with the microprobe; if they are mixed 

layers, the scale of mixing is smaller than the resolution of the instrument.

Montmorillonite and occasionally interlayered 

montmorillonite-celadonite occur in the groundmass, 

partly replacing clinopyroxene phenocrysts, and in 

veinlets in the Biscuit Basin flow breccia of Y-7 (table 

4), where temperatures are higher than in the 

sedimentary rocks; celadonite occurs with montmorillonite in this hole at depths below — 70 m. Celadonite is 

commonly the only clay mineral in the Biscuit Basin 

flow of Y-8, where it occurs along hydration cracks in 

the altered obsidian and replacing clinopyroxene 

phenocrysts. Well-crystallized celadonite is the earliest 

deposited veinlet mineral in the flow breccia of Y-8 

(tables 7 and 8).

Celadonite is not abundant in the sedimentary rocks, 

perhaps because these rocks contain only about 1 percent of total reported iron oxides (table 2), but celadonite is much more abundant in the Biscuit Basin flow, 

which contains more than 3 percent total iron oxides. 

The distribution of celadonite in both drill holes is related at least in part to local availability of both ferrous 

and ferric iron from the rocks. In addition, the greater 

abundance of celadonite in all units of Y-8 relative to 

Y-7 suggests that the hotter upflowing waters of Y-8 

may be supplying some potassium.

From our few analyses of clay minerals that show 

wide and apparently unsystematic compositional variations, we do not know whether the clays of one drill

EXPLANATION

Ca + Mg + Fe

FIGURE 14.—Triangular diagram showing ranges in cation compositions of clay minerals in Y-7 and Y-8.

hole or the other are consistently higher in K^O. These 

clays are poorly crystallized, irregularly interlayered 

structures that eventually develop into celadonite, but 

we have no evidence supporting evolution into the illite structure.

SILICA MINERALS

The varieties of hydrothermal silica minerals that 

we recognize include quartz, chalcedony, a-cristobalite 

(ordered), /3-cristobalite (disordered), and opal (X-ray 

amorphous). Murata and Norman (1976) have recently 

recognized a complete gradation between wellcrystallized quartz and imperfectly crystallized chalcedony and have devised an X-ray method for rating 

the degree of crystallinity on a scale of 10. Complete 

gradations are also recognized by Murata and Larson 

(1975) within the cristobalite series from disordered 

/3-cristobalite to well-ordered a-cristobalite. Murata 

and Larson (1975) have also noted tridymite as a minor 

associate of the disordered cristobalite. They recognize 

no gradation between the amorphous silica of diatomite and the earliest stage of disordered cristobalite, 

which occurs as a dense rock with few if any relict 

diatoms. Marked changes in 18O occur during this conversion (Murata and others, 1977), and other marked 

changes in 18O occur where ordered cristobalite is converted to chalcedony or quartz. These two zones of 

change indicate extensive reaction between water and 

solid phases and thus strongly support solution and 

redeposition. In contrast, no systematic change in 18O 

occurs during the ordering of cristobalite, or in the 

stages of conversion of chalcedony to quartz; for these 

phases, the authors favor solid-state ordering without 

solution and redeposition.

In the relatively high-temperature, low-pressure, 

hot-spring environment, a similar series of silica minerals is recognized, but with a few important differences. The freshly deposited sinter, amorphous opal, 

seems to grade completely into /3-cristobalite by means 

of solid-state ordering with no evidence of solution and 

redeposition (White and others, 1964; D. E. White, unpub. data). Tridymite has not yet been identified as an 

associate of /3-cristobalite in hydrothermal systems, 

but instead it occurs only as an early vapor-phase mineral related to initial eruption of lava flows and ashflow tuffs and not related to the active hydrothermal 

systems, a-cristobalite is not common in the shallow 

parts of hot-spring systems, but we have evidence of 

continuous solid-state ordering between the two recognized cristobalite species in that the major X-ray peak 

position may range anywhere between 4.11 A ((3- 

cristobalite) and 4.04 A (a-cristobalite). Our silicamineral series agrees with that of Murata and Larson 

(1975) in requiring solution and redeposition in the
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A21

conversion of cristobalite to chalcedony or quartz.

The distribution of the silica minerals in Y-7 and 

Y-8 is first considered in order of increasing stability. 

Opal is the dominant silica phase in the hot-spring 

sinter of both drill holes and is also an abundant cementing mineral of the near-surface sediments; opal is 

sparse or absent elsewhere throughout the drill holes. 

At depth, opal occurs only locally in veinlets in the flow 

breccia of Y-8. Both recognized species of cristobalite 

are rather erratic in distribution (figs. 3 and 4; tables 5 

and 7). /3-cristobalite is most common in veinlets in the 

sediments of Y-7 and in veinlets in the flow breccia of 

both drill holes, a-cristobalite is present in the 

groundmass of most core from Y-7 but is absent from 

significant intervals of Y-8; a few veinlets include 

a-cristobalite as a relatively early mineral. Chalcedony is very rare in groundmass and veinlets of Y-7 

and also in the groundmass of Y-8 but is characteristically the earliest veinlet mineral in the sediments of 

Y-8. Quartz has not been reported in Y-7 but is sparse 

to abundant in the groundmass and veinlets of Y-8. 

Thus, the hotter environment of Y-8 clearly favors 

quartz and chalcedony, and with rare exceptions these 

minerals are abundant where opal and /3-cristobalite 

are absent.

This complex picture of the distribution of the silica 

minerals can best be understood by focusing on their 

occurrence and relative ages in veinlets. Of the 21 

veinlets listed in tables 5 and 7 that have two or more 

silica species, the relative ages from oldest to youngest 

in 18 of these are quartz, chalcedony, a-cristobalite, 

/S-cristobalite, and opal; note that this sequence is also 

in the order of highest to lowest thermodynamic stability. Y-7-209 (-63.7 m) is a probable exception, as are 

the veinlets from Y-8-185.3 (-56.5 m) and Y-8-197.7 

(-60.3 m), where quartz is deposited on chalcedony. 

The rarity of these exceptions is surprising in view of 

the common deposition of quartz on chalcedony at a 

very late stage in cavities in chalcedonic sinter and 

elsewhere (White and others, 1956).

The usual sequence from most stable to least stable 

is also in the order of increasing solubility in water at 

constant temperature (Fournier, 1973). At 100°C, for 

example, the solubility of quartz is about 50 ppm, chalcedony about 87 ppm, a-cristobalite about 115 ppm, 

/S-cristobalite about 250 ppm, and amorphous silica 

about 370 ppm. Chemical geothermometers indicate 

that the thermal waters of Upper Geyser Basin rise to 

the surface from a reservoir that is probably rather 

homogeneous at a temperature close to 200°C (Fournier, 1973, p. 134; Fournier and Truesdell, 1973). At 

200°C, the solubility of quartz is 260 ppm. With 

adiabatic cooling (all excess heat converting water to 

steam) to an eventual surface termperature of 93°C,

the maximum content of silica in the residual water is 

about 330 ppm; if boiled to an intermediate stage of 

adiabatic cooling at 150°C, the residual water has 290 

ppm of silica. From these relations, we conclude that 

water from the reservoir is strongly supersaturated 

with respect to quartz, chalcedony, and a-cristobalite 

at all temperatures in the drill holes, but is only 

slightly supersaturated with respect to /3-cristobalite 

and is unsaturated with respect to opal at all temperatures higher than about 80°C (at depths below -15 m 

in Y-7 and -6 m in Y-8).

The indicated occurrence of opal at temperatures as 

high as 140°C in Y-7 and 170°C in Y-8 is inconsistent 

with these conclusions. Three possible explanations 

are: (1) the deep opal was deposited when reservoir 

temperatures and the silica contents of upflowing 

water were much higher; conversion to cristobalite has 

not yet occurred; (2) the deep opal is everywhere associted with undevitrified obsidian, which could have a 

solubility as high as that of amorphous silica (R.O. 

Fournier, oral commun., 1976); the hot water has locally equilibrated with obsidian and has then risen and 

cooled enough to deposit opal; or (3) the deep opal identified only by its optical properties (isotropic; index of 

refraction about 1.46), is nowhere abundant enough to 

isolate and confirm as being amorphous by X-ray; it 

may actually be isotropic /3-cristobalite of low crystalUnity. We suspect, but cannot prove, that the third 

alternative is correct.

How are the two strong tendencies for paragenesis 

from most stable to least stable, and for mutual exclusion of quartz and chalcedony from cristobalite and 

opal, best reconciled? Were all silica minerals initially 

deposited in their present form or does the sequence 

indicate conversion through time from less stable 

forms to more stable forms? White, Brannock and 

Murata (1956) and Murata and Larson (1975) have 

concluded that each silica mineral forms from water 

only slightly supersaturated with respect to that mineral; a water supersaturated with respect to opal tends 

to precipitate opal and not other forms, and a 

chalcedony-supersaturated water does not precipitate 

quartz. If this concept is valid, the present mineral assemblages are best explained by consistently high 

silica contents in the waters of the main upflow channels, but with eventual self-sealing and isolation of individual small channels, veinlets, and rock masses. 

With such isolation, or semi-isolation, each unit of rock 

volume can then behave much like a closed system, 

with pore water evolving toward lower silica contents 

as more stable silica minerals are in succession reconstituted from less stable minerals. With respect to each 

successive mineral, the pore water is successively 

supersaturated, just saturated, and then undersatu-
A22 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

rated, starting with the least stable silica mineral that 

can precipitate and evolving toward the more stable, 

with each requiring a lower silica content in the water. 

This model of evolution implies, for example, that the 

sequence from —41 to —55 m in Y-8 (generally with 

quartz as the only silica mineral) has either been an 

isolated closed system throughout recent activity or, as 

a favored alternate possibility, this interval was only 

semiclosed, permitting limited inflow of high-silica 

water that mixed convectively with lower-silica water 

already adjusted to the mineral assemblage of the unit. 

The limited inflow of new silica permits continuing 

precipitation of quartz, but saturation with respect to 

chalcedony is not ordinarily reached (in the profile of 

the drill hole).

ZEOLITES

In order to understand interrelations of the zeolites, 

their chemical composition is critical but also difficult 

to obtain. Electron microprobe analysis, which is the 

best available method for fine-grained intermixtures, 

is often unsatisfactory and frustrating to the analyst. A 

low beam current and a slightly defocused beam (about 

10/u.m) are usually used to keep the sample from volatilizing. In addition, each grain has a large y^ (table 

9), indicating that all elements other than calcium are 

inhomogeneously distributed. Also, most grains are so 

small that only a few separate spots can be counted in 

each grain, making a statement of precision meaningless.

EXPLANATION

o 

Replacement

a 

Tabular

A 

Veinlet

FIGURE 15.—Triangular diagram showing cation compositions of 

clinoptilolite in Y-7 and Y-8. Tie lines represent analyses of 

clinoptilolite replacing glass and of recrystallized (tabular) clinoptilolite from the same sample; sample numbers indicate depth (in 

feet).

Individual electron microprobe analyses are averaged in table 9 and are plotted in figure 15. The calcium content of clinoptilolite from Y-7 is greater than 

that from Y-8 in both replacement and tabular types. 

The clinoptilolite deposited as tabular euhedral crysTABLE 9.—Electron microprobe analyses of zeolites from Y-7 and Y-8

[Analyst: M. H. Beeson. Looked for but not found: Ba]

SiO2 _____ _______

AkOs _____ __. _,_

CaO ________________

NaaO

Y-8-117.1

Analcime

— _ _ 59.79

_____ 18.36

_____ .00

_____ 10.83

_____ .00

Y-7- 148. 7'

Replacement

clinoptilolite

71 9

10.3

1.21

3.22

3.09

Y-7-148.71

Tabular

clinoptilolite

67.1

11.4

.82

2.03

4.28

Y-8-241

Replacement

clinoptilolite

66.3

12.1

.54

4.42

3.87

Y-8-241

Tabular

clinoptilolite

70.3

11.9

.45

2.64

3.11

Y-8-194.5

Veinlet

clinoptilolite

75.3

8.9

.13

1.23

5.73

Av -— -

\ ^

4.6

3.4

2.1

11.6

8.7

Sum ________ 88.98 

Ions/formula based on 72 oxygens

89.7 85.6 87.0

Si

Al

Ca

Na

j£

2Ca+Na+K

Al

Si/Al ___ ________________

An ____________________

Ab ______________________

Or

26.49

9.59

.00

9.31

.00

1.029

2.76

100.0

30.81

5.20

.56

2.67

1 49

1.004

5.93

12.0

57.6

30.6

30.20

6.07

.40

1.76

2.45

.824

4.98

8.6

38.2

53.2

29.58

6.38

.26

3.95

2.20

1.026

4.64

4.2

61.0

34.8

88.4

30.38

6.05

.21

2.22

1.73

.811

5.02

5.1

53.5

41.4

91.3

31.64

4.42

.06

1.00

3.07

.9357

7.16

1.4

24.3

74.3

"Average of 2 analyses.

''Homogeneity index (Boyd, 1969) averaged over all analyses of each sample. A value larger than 3 indicates that a particular element has an inhomogeneous distribution.
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A23

tals in open spaces and pores is enriched in KaO over 

that replacing obsidian, especially that from Y-7. The 

one analyzed clinoptilolite from a veinlet in Y-8 is 

even higher in KzO with 5.73 percent. To our knowledge, clinoptilolite with such high KzO content has not 

been reported previously, but Sheppard, Gude, and 

Munson (1965) found diagenetic clinoptilolites that 

ranged from 3.86 to 5.43 percent KsO in tuffacous rocks 

of the Mojave Desert, and Kashkai and Babaev (1976) 

report a clinoptilolite containing 5.30 percent K2O 

from a zeolitized tuff from the Aydag deposit in the 

Azerbaidzhan SSR.

Analcime is absent from Y-7 but is abundant in 

parts of Y-8, where it occurs in contact with clinoptilolite on the borders of alternating thin bands in the 

upper 24.4 m (fig. 4). Elsewhere in Y-8 the two are 

mutually exclusive, with analcime everywhere occurring with quartz, and clinoptilolite nearly everywhere 

being associated with one or more of the less stable 

forms of silica, usually a-cristobalite. The strong tendencies for analcime to be associated with quartz and 

for these two minerals to occur without clinoptilolite 

and the less stable silica minerals were emphasized by 

Honda and Muffler (1970) as being characteristic of 

Y-l, with most exceptions also occurring in the upper 

part of that drill hole.

Considering analyses of all zeolite species, clinoptilolite, although variable, tends to be relatively high in 

silica, and analcime, which is also variable, is lower in 

silica. All of the clinoptilolites of Y-7 and Y-8 are the 

high-silica varieties. A representative analcime sample from Y-8 is a pure sodium variety with a silica-toalumina ratio of 2.76:1, which is very high for analcime (table 9; the ratio in common analcime is close to 

2:1). The high silica content is consistent with the relations between the silica minerals and silica contents of 

the pore water, as previously discussed, in that the 

lower silica phase (analcime) is not deposited if the 

fluid is supersaturated with respect to the higher silica 

phase (clinoptilolite).

Mordenite coexists with clinoptilolite and to a lesser 

extent with analcime and is also intermediate between 

the two in relative silica content. If silica content were 

the only controlling factor, we might also expect 

mordenite to be intermediate in time of deposition. 

Where age relations can be deciphered with confidence 

(tables 5 and 7), mordenite does follow clinoptilolite 

but, for reasons that are not evident, mordenite also 

occurs after analcime as a partial filling of open spaces 

in veinlets and pores. Several attempts were made to 

analyze mordenite with the electron microprobe, but 

even with a very low sample current and a defocused 

beam, the mineral volatilized too rapidly for satisfactory analysis.

POTASSIUM FELDSPAR

Hydrothermal potassium feldspar, as in other hotspring systems, is monoclinic; from its optical and electron microprobe compositions, it is chemically near the 

pure potassium end member. Potassium feldspar is a 

minor but persistent mineral in the alteration of Y-8, 

where it is associated with quartz and analcime zones 

in the altered sediments at depths below —24.8 m, and 

is also present in minor amounts throughout the Biscuit Basin flow. It has not been found in core from Y-7 

or in veinlets from Y-8. For comparison, a little hydrothermal potassium feldspar was found in Y-l 

(Honda and Muffler, 1970), and it is relatively abundant in veinlets and groundmass from Carnegie-I drill 

hole of the Myriad group of Upper Geyser Basin (Fenner, 1936; confirmed in Fenner's core by present authors).

Hydrothermal potassium feldspar is given special 

prominence by Browne and Ellis (1970) in their study 

of core and cuttings from the Broadlands system of 

New Zealand. They conclude that hydrothermal potassium feldspar indicates the permeable upflow channels 

of the hottest water, which is usually highest in potassium. With decreasing pressure the water boils, losing 

COz to the vapor phase, thereby increasing the pH and 

the K/H ratio and shifting the water into the potassium 

feldspar stability field (Hemley and Jones, 1964). In 

general we agree with these conclusions, which provide 

an excellent explanation for the relatively abundant 

potassium feldspar in Carnegie-I drill hole and in Yellowstone's Y-13 drill hole (T. E. C. Keith, unpub. data; 

White and others, 1975, p. 41-45). However, the uniform distribution of minor potassium feldspar in Y-8 

(fig. 4) is better explained as a product of the conversion of clinoptilolite to analcime. Clinoptilolite is one of 

the few zeolites that readily accepts potassium and 

may contain as much as 5.73 percent KzO (table 9). In 

contrast, analcime is consistently low in potassium 

(except for phenocryst analcime in high-potassium volcanic rocks). This contrast suggests that as clinoptilolite high in potassium is converted to analcime, the 

excess potassium precipitates in part as potassium 

feldspar. The close relation of potassium feldspar to 

analcime (fig. 4) and to the absence of clinoptilolite is 

thereby explained. Much of the potassium in the original rocks is also fixed in the clay minerals, to some 

extent as montmorillonite and mixed-layer clays but to 

a greater extent as celadonite (table 8). However, none 

of these potassium-bearing clays seems to increase in 

abundance as potassium is released in reconstituting 

clinoptilolite to analcime.

The whole-rock chemical compositions of the two 

zeolitic assemblages differ considerably, with enrichment of Na2O and decrease of KsO in the analcime-rich
A24 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

sediments relative to the clinoptilolite-rich sediments 

(table 3). However, the analcime-rich sediments below 

—35.7 m, in which potassium feldspar crystals are 

common, have a considerably higher proportion of KzQ 

than in analcime-rich rocks without hydrothermal 

potassium feldspar (compare analyses Y-8-32.5 

(-9.9 m) and Y-8-160.5 (-48.9 m) of table 2). Table 3 

also shows higher AbOs and slightly lower SiCte and 

CaO in the analcime-rich sediments with no hydrothermal potassium feldspar.

CALCITE

Calcite is absent or very rare in Y-7 but is common 

in parts of the sedimentary section of Y-8. Two habits 

are recognized in Y-8; rhombic calcite is the latest 

mineral in some veinlets from -9.8 to -21.3 m, and 

bladed calcite, also as the latest mineral, occurs in 

veinlets and groundmass from about —50 to —55 m in 

the lower part of the quartz-analcime-potassium 

feldspar zone previously discussed.

If other parameters are constant, the solubility of 

calcite increases with decreasing temperature. In general, we view the scarcity of calcite as due to decreasing 

temperature upward in the principal flow channels. 

Precipitation of calcite can be due to several different 

circumstances; an especially important one in Yellowstone National Park is related to the high fluid 

overpressures at depth, with boiling occurring as pressures decrease irregularly upward (White and others, 

1975). The depth of the most rapid pressure change in 

Y-8 cannot be determined precisely from the physical 

data but is close to —55 m, where fluid overpressures 

decreased abruptly upward from about +2.2 kg/cm2 to 

+0.6 kg/cm2 (White and others, 1975, p. 18-19). Water 

flowing upward through a restricted channel at a 

temperature near 168°C and a pressure near 2.2 

kg/cm2 is about 11°C above boiling when its pressure 

suddenly decreases to 0.6 kg/cm2 (White and others, 

1975, p. 18-19). About 2 percent of the water is suddenly boiled to steam, and COz is selectively lost to the 

vapor phase, consequently increasing the pH, converting bicarbonate to carbonate, and precipitating calcite.

RELATIVE IMPORTANCE OF TEMPERATURE 

AND FLUID COMPOSITIONS

We conclude, in agreement with Honda and Muffler 

(1970), that the mineral associations of Y-7 and Y-8 

cannot be explained by temperature differences alone, 

even though temperature is obviously important. We 

have already cited the outstanding demands for more 

than temperature alone: (1) the distribution of undevitrified obsidian, still prevalent at or near the 

maximum temperatures of each hole; (2) coexistence or

close association of minerals that are obviously not in 

equilibrium with each other, including various combinations of silica minerals; (3) temperature-independent alternation of clinoptilolite-cristobalite and 

analcime-quartz with depth.

Erionite occurs in only one sample from about — 4.9 m 

in Y-8 (fig. 4), where its temperature of formation was 

probably between 75 and 80°C. Erionite with similar 

limited distribution was also suspected by Honda and 

Muffler (1970) to be restricted to low temperatures; the 

maximum temperature of formation at which erionite 

was observed in Y-l was about 110°C.

Laumontite is a relatively rare zeolite in siliceous 

low-calcium geothermal systems. Its scarcity in the 

Upper Geyser Basin drill holes may be more closely 

related to the low availability of calcium than to temperature restrictions; all laumontite of Y-8 occurs in 

veinlets at temperatures near 170°C.

Honda and Muffler (1970) found that obsidian as 

well as opal and /3-cristobalite occurred in Y-l at 

temperatures of less than about 110°C, but these restrictions are not effective in Y-7 and Y-8, where environments of these minerals are as hot as 170°C. Y-l 

was clearly much more permeable than Y-7 and Y-8, 

and hence opal and /3-cristobalite would be less likely 

to persist. Chlorite may be favored by high temperature, but relative scarcity of magnesium and ferrous 

iron in the altering rocks is likely to be more important. Celadonite is absent from the sedimentary rocks 

of Y-7, where temperatures range up to 117°C, but 

occurs at higher temperatures in the underlying Biscuit Basin flow. In Y-8, celadonite occurs in all rocks 

deeper that —7.6 m and at temperatures above 80°C 

and is especially abundant in the Biscuit Basin flow. 

These data indicate that availability of magnesium 

and iron is clearly important, judging from the abundance of celadonite in the Biscuit Basin flow of both 

drill holes as compared to the overlying sediments, 

which are lower in these elements (table 2).

Thus, many chemical reactions occurring in these 

rocks are favored by high temperature, but permeability and an adequate supply of through-flowing fluids 

seem to be equally important for most reactions in supplying some constituents and redistributing or removing others. Our conclusions are nearly identical to 

those of Honda and Muffler (1970).

SUMMARY OF EVIDENCE FOR SELF-SEALING

The physical evidence for self-sealing of the original 

sediments of Y-7 and Y-8 was reviewed by White and 

others (1975, p. 22). In summary, the original sediments (as interpreted from drill core) contain no strata 

likely to have been initially so fine grained and low 

enough in permeability to account for the observed ver-
HYDROTHERMAL ALTERATION AND SELF-SEALING, YELLOWSTONE NATIONAL PARK, WYOMING A25

tical pressure differences (core recovery through the 

sedimentary section was 76 percent from Y-7 and 88 

percent from Y-8). However, the lithologies as represented by patterns in figures 3 and 4 have correlative 

zones near -15 m and from -21.3 to —27.4 m that 

contain silt-sized clastic sediments of moderately low 

initial permeability, thus decreasing bulk vertical 

permeability.

Strong evidence for horizontal self-sealing by hydrothermal alteration and mineral deposition exists in the 

contrasts in wellhead water pressure between Y-8 and 

Y-7 at equivalent depths. All of the sediments were 

rather well sorted by streams, and the coarser clastic 

zones are correlative between the two holes (fig. 3 and 

4). The depth ranges of individual zones in the two 

holes are (in meters, with Y-8 listed first): (1) 1.5-8.5 

and 4.0-7.0; (2) 25.3-28.4 and 23.5-24.4; (3) 35.1-36.3 

and 32.0-32.6; (4) 41.2-42.7 and 40.2-42.7; 

(5) 47.6-54.9 and 44.8-48.2 (and probably to 52.7 but 

no core was recovered). If the depth intervals are corrected for differences in wellhead altitude, with Y-7 

being 1.8 m lower than Y-8, the gravel zones are at 

almost the same altitude. If these zones are assumed to 

be both correlative and originally permeable, the large 

pressure differences observed today between the two 

holes provide convincing physical evidence for selfsealing. The pressure difference in the basal sediments 

from Y-8 to Y-7 is now about 1.49 kg/cm2 per 100 m, or 

a gradient of 0.17, with pressure converted to equivalent meters of water (White and others, 1975, p. 22). In 

contrast, the pressure gradient from Y-7 to the 

Firehole River, both of which are outside of the principal area of self-sealing, is only about 0.01.

The photographic evidence in this report demonstrates extensive dissolution of obsidian granules. 

Some of the reconstituted new minerals were deposited 

in the relict granule spaces, but probably an equal fraction was deposited in the pore spaces between 

granules. For any specific sample, we cannot determine 

with certainty whether an interstitial space was originally a void or whether the sediments were initially 

poorly sorted with fine clastic grains interstitial to the 

larger grains, and now completely destroyed. The present sediments of unaltered kame terraces of the Upper 

Geyser Basin (Waldrop, 1975) are considered to be 

equivalent to the original sediments of the drill holes. 

These obsidian-bearing sands and gravels are so porous and open textured that, except locally, precipitation does not drain off in surface channels and gullies 

but sinks underground. Thus, the total physical, chemical, and petrographic evidence points conclusively to 

hydrothermal self-sealing as an important nearsurface factor in the development and evolution of this 

geothermal system.

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A26 HYDROTHERMAL STUDIES IN YELLOWSTONE NATIONAL PARK, WYOMING

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. S. GOVERNMENT PRINTING OFFICE: 1978 - 789-107/63 9-II