U.S. Department of the Interior

U.S. Geological Survey

Hydrothermal Mineralogy of Core from

Geothermal Drill Holes at Newberry

Volcano, Oregon

Professional Paper 1578
Hydrothermal Mineralogy of Core from

Geothermal Drill Holes at Newberry

Volcano, Oregon

KEITH E. BARGAR AND TERRY E. C. KEITH

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1578

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1999
DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

 is for descriptive purposes only and does not imply endorsement

by the U.S. Government.

Manuscript approved for publication, April 20, 1997

Text and illustrations edited by Jeff Troll

For sale by

U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

 Library of Congress Cataloging-in-Publication Data

Bargar, Keith E.

Hydrothermal minerology of core from geothermal drill holes at

Newberry Volcano, Oregon / Keith E. Bargar and Terry E.C. Keith.

p. cm -- (U.S. Geological Survey professional paper ; 1578)

Includes bibliographical references.

Supt. of Docs. no.: I 19. 16: 1578

ISBN 0-607-91905-1

1. Hydrothermal alteration--Oregon--Newberry Volcano Region.

2. Minerals--Oregon--Newberry Volcano Region. 3. Geothermal

resources--Oregon--Newberry Volcano region. I. Keith, Terry E.C.

II. Title. III. Series.

QE390.5.B375 1999

551.2'3'09795--dc21 99-32015

 CIP
V

CONTENTS

Abstract---------------------------------------------------------------------------------- 1

Introduction------------------------------------------------------------------------------ 2

Analytical methods---------------------------------------------------------------- 4

Acknowledgments----------------------------------------------------------------- 6

Geologic setting------------------------------------------------------------------------- 6

Geothermal exploration---------------------------------------------------------------- 7

Drill holes within the caldera---------------------------------------------------- 7

USGS-N2-------------------------------------------------------------------- 7

Lithology------------------------------------------------------------ 8

Hydrothermal alteration------------------------------------------------- 8

Surface to 320-m depth--------------------------------------------- 8

320- to 697-m depth------------------------------------------------- 9

697-m to 932-m depth---------------------------------------------- 12

Fluid inclusions----------------------------------------------------------- 16

RDO-1----------------------------------------------------------------------------- 22

Lithology----------------------------------------------------------------- 22

Hydrothermal alteration------------------------------------------------- 23

Fluid inclusions----------------------------------------------------------- 23

Drill holes outside the caldera---------------------------------------------------------- 24

GEO-N1---------------------------------------------------------------------------- 24

Lithology------------------------------------------------------------------ 24

Hydrothermal alteration------------------------------------------------- 26

GEO-N2---------------------------------------------------------------------------- 27

Lithology------------------------------------------------------------------- 28

Hydrothermal alteration------------------------------------------------- 28

Fluid inclusions----------------------------------------------------------- 30

 GEO-N3---------------------------------------------------------------------------- 30

Lithology------------------------------------------------------------------- 30

Hydrothermal alteration-------------------------------------------------- 32

GEO-N4----------------------------------------------------------------------------- 32

Lithology------------------------------------------------------------------- 32

Hydrothermal alteration-------------------------------------------------- 32

GEO-N5----------------------------------------------------------------------------- 32

Lithology------------------------------------------------------------------- 33

Hydrothermal alteration-------------------------------------------------- 33

SF NC-01--------------------------------------------------------------------------- 35

Lithology------------------------------------------------------------------- 35

Hydrothermal alteration-------------------------------------------------- 35

Fluid inclusions----------------------------------------------------------- 36

SF NC72-03------------------------------------------------------------------------ 38

Lithology------------------------------------------------------------------- 38

Hydrothermal alteration-------------------------------------------------- 38

Fluid inclusions----------------------------------------------------------- 39

Other drill holes-------------------------------------------------------------------- 40

Hydrothermal mineralogy of drill core specimens----------------------------------------- 41

Drill holes within the caldera------------------------------------------------------------ 41

Drill holes outside the caldera----------------------------------------------------------- 41

Hydrothermal minerals------------------------------------------------------------------------- 42
CONTENTS

Zeolite minerals-------------------------------------------------------------------- 42

Analcime--------------------------------------------------------------------- 42

Chabazite--------------------------------------------------------------------- 43

Clinoptilolite----------------------------------------------------------------- 44

Dachiardite ------------------------------------------------------------------ 44

Erionite----------------------------------------------------------------------- 45

Faujasite---------------------------------------------------------------------- 45

Heulandite-------------------------------------------------------------------- 45

Laumontite------------------------------------------------------------------- 45

Mordenite--------------------------------------------------------------------------- 46

Phillipsite--------------------------------------------------------------------------- 46

Calcium silicate hydrate minerals------------------------------------------------------ 47

Gyrolite------------------------------------------------------------------------------ 47

Okenite------------------------------------------------------------------------------ 47

Carbonate minerals---------------------------------------------------------------------- 48

Ankerite-dolomite----------------------------------------------------------------- 49

Aragonite---------------------------------------------------------------------------- 50

Calcite------------------------------------------------------------------------------- 50

Kutnohorite------------------------------------------------------------------------- 51

Magnesite--------------------------------------------------------------------------- 51

Rhodochrosite---------------------------------------------------------------------- 51

Siderite------------------------------------------------------------------------------ 52

Clay minerals------------------------------------------------------------------------------ 53

Kaolinite and halloysite----------------------------------------------------------- 53

Smectite----------------------------------------------------------------------------- 54

Mixed-layer illite-smectite-------------------------------------------------------- 56

Illite---------------------------------------------------------------------------------- 56

Mixed-layer chlorite-smectite---------------------------------------------------- 57

Chlorite------------------------------------------------------------------------------ 58

Silica minerals----------------------------------------------------------------------------- 59

Opal---------------------------------------------------------------------------------- 59

Cristobalite-------------------------------------------------------------------------- 59

Quartz and chalcedony------------------------------------------------------------ 60

Sulfide minerals--------------------------------------------------------------------------- 62

Pyrrhotite---------------------------------------------------------------------------- 62

Pyrite-------------------------------------------------------------------------------- 63

Marcasite---------------------------------------------------------------------------- 64

Sphalerite--------------------------------------------------------------------------- 65

Sulfate minerals--------------------------------------------------------------------------- 65

Anhydrite--------------------------------------------------------------------------- 65

Barite-------------------------------------------------------------------------------- 66

Jarosite, natroalunite, and natrojarosite----------------------------------------- 66

Gypsum----------------------------------------------------------------------------- 67

Iron oxide and iron hydroxide minerals------------------------------------------------ 67

Hematite and lepidocrocite------------------------------------------------------- 67

Goethite----------------------------------------------------------------------------- 67

Magnetite--------------------------------------------------------------------------- 68

Amorphous iron oxide------------------------------------------------------------ 68

Other minerals----------------------------------------------------------------------------- 69

Apatite------------------------------------------------------------------------------- 69

Apophyllite------------------------------------------------------------------------- 69

Copper------------------------------------------------------------------------------- 69

Epidote------------------------------------------------------------------------------ 70

Hydrogrossular--------------------------------------------------------------------- 71

VI
Discussion--------------------------------------------------------------------------------- 71

Factors controlling hydrothermal alteration-------------------------------------- 71

Permeability------------------------------------------------------------------- 72

Temperature------------------------------------------------------------------- 73

Fluid composition ----------------------------------------------------------- 74

Rock composition------------------------------------------------------------ 75

Summary and conclusions--------------------------------------------------------------- 79

References cited-------------------------------------------------------------------------- 83-86

Appendixes:

1. Lithologic description of drill core USGS-N2 from within the

caldera of Newberry volcano------------------------------------------------------ 87-90

2. Lithologic description of drill core GEO-N1 from the

south flank of Newberry volcano------------------------------------------------- 91-92

FIGURES

1. Map showing location of Newberry volcano and major Quaternary volcanic centers in Cascade

Range of western United States and British Columbia, Canada------------------------------------------------------- 3

 2. Generalized geologic map of Newberry caldera------------------------------------------------------------------------- 4

3. Map showing location of geothermal exploration drill holes at Newberry volcano-------------------------------- 5

4. Comparison of lithologic units in upper part of USGS-N2 drill hole with those of RDO-1 drill hole----------- 9

 5. Distribution of hydrothermal alteration minerals in core from lower two-thirds of USGS-N2

drill hole----------------------------------------------------------------------------------------------------------------------- 10

 6. Photograph of fine-grained thin-bedded volcaniclastic sandstone drill-core specimen from

315.1-m depth in USGS-N2 drill hole------------------------------------------------------------------------------------- 11

7. Generalized paragenetic sequence of selected hydrothermal minerals in core from USGS-N2 drill

hole----------------------------------------------------------------------------------------------------------------------------- 11

8. Photographs of drill-core samples from upper and middle parts of USGS-N2 drill hole-------------------------- 15,16

9. Photographs of drill-core samples from lower part of USGS-N2 drill hole----------------------------------------- 17,18

10. Photomicrographs showing fluid inclusions in hydrothermal quartz crystals from lower part of

USGS-N2 drill hole---------------------------------------------------------------------------------------------------------- 21

11. Diagram showing fluid-inclusion Th values for quartz and calcite specimens from lower part of

USGS-N2 drill hole---------------------------------------------------------------------------------------------------------- 22

12. Distribution of hydrothermal alteration minerals in cuttings samples from drill hole RDO-1--------------------- 23

13. Diagram showing fluid-inclusion Th values for quartz specimens from RDO-1 drill hole------------------------- 24

14. Distribution of hydrothermal alteration minerals in core from drill hole GEO-N1--------------------------------- 25

15. Generalized paragenetic sequence of hydrothermal alteration minerals deposited in flank drill holes

of Newberry volcano--------------------------------------------------------------------------------------------------------- 28

16. Distribution of hydrothermal minerals in core from drill hole GEO-N2--------------------------------------------- 29

17. Diagram showing fluid-inclusion Th values for quartz and calcite specimens from GEO-N2 drill hole-------- 30

18-20. Distribution of hydrothermal alteration minerals in core

18. From GEO-N3 drill hole---------------------------------------------------------------------------------------------------- 31

19. From GEO-N4 drill hole---------------------------------------------------------------------------------------------------- 33

20. From GEO-N5 drill hole---------------------------------------------------------------------------------------------------- 34

21. Scanning electron micrograph of fracture filling that contains crystals of sphalerite and later massive

siderite------------------------------------------------------------------------------------------------------------------------- 35

22. Scanning electron micrograph of botryoidal chalcedony open-space deposit with clusters of late

quartz crystals----------------------------------------------------------------------------------------------------------------- 35

23. Distribution of hydrothermal alteration minerals in core from SF NC-01 drill hole-------------------------------- 37

24. Photomicrograph of opal with disseminated pyrite crystals------------------------------------------------------------ 38

25. Diagram showing fluid-inclusion Th values for quartz and calcite from SF NC-01 drill hole--------------------- 38

26. Distribution of hydrothermal alteration minerals in SF NC72-03 drill hole----------------------------------------- 39

27. Diagram showing fluid-inclusion Th values for quartz and calcite from SF NC72-03 drill hole------------------ 40

CONTENTS VII
28-36. Scanning electron micrographs of Newberry drill-core samples showing:

28. Large trapezohedral analcime crystal deposited later than smectite in altered volcaniclastic

sandstone--------------------------------------------------------------------------------------------------------------- 42

29. Phacolitic chabazite crystals and fractured and flaking faujasite crystals------------------------------------- 44

30. Lath-shaped clinoptilolite crystals partly coated by smectite and fibrous mordenite(?)-------------------- 45

31. Fibrous to acicular dachiardite crystals in altered lithic tuff---------------------------------------------------- 45

32. Bundle of stubby acicular erionite crystals and smectite deposited in pore spaces of basaltic

sediment---------------------------------------------------------------------------------------------------------------- 46

33. Fractured and flaking faujasite crystals---------------------------------------------------------------------------- 46

34. Tabular to blocky heulandite crystals------------------------------------------------------------------------------- 47

35. Fibrous mordenite crystals------------------------------------------------------------------------------------------- 47,48

36. Spherical aggregate of platy gyrolite crystals in volcaniclastic sandstone------------------------------------ 48

37. CaCO3-MgCO3-FeCO3+MnCO3 ternary diagram for electron microprobe analyses of carbonate

minerals from USGS-N2 drill core----------------------------------------------------------------------------------------- 49

38. Scanning electron micrograph of blocky and discoidal aggregates of rhombic ankerite-dolomite

crystals------------------------------------------------------------------------------------------------------------------------- 50

39. Scanning electron micrograph of columnar aggregates of rhombic ankerite-dolomite crystals------------------- 50

40. Scanning electron micrographs and photograph of calcite crystals---------------------------------------------------- 52,53

41. Photomicrograph showing cross section of discoidal aggregate of siderite crystals-------------------------------- 53

42. CaCO3-MgCO3+FeCO3-MnCO3 ternary diagram for electron microprobe analyses of discoidal

siderite crystal aggregate---------------------------------------------------------------------------------------------------- 53

43. Scanning electron micrographs and photograph showing different morphologies of siderite crystal

clusters------------------------------------------------------------------------------------------------------------------------- 54-56

44. Scanning electron micrographs showing clusters of smectite crystals------------------------------------------------ 57

45. Scanning electron micrographs showing mixed-layer chlorite-smectite crystal aggregates------------------------ 58

46. Scanning electron micrographs and photograph of aggregates of chlorite crystals---------------------------------- 61

47. Scanning electron micrograph of botryoidal opal------------------------------------------------------------------------ 62

48. Scanning electron micrograph of hemispherical aggregates of bladed cristobalite crystals deposited on

calcite--------------------------------------------------------------------------------------------------------------------------- 62

49. Photomicrographs of vesicle fillings in basaltic breccia fragments---------------------------------------------------- 63

50. Scanning electron micrographs of pyrrhotite and pseudomorphous replacement of pyrrhotite by pyrite

and marcasite------------------------------------------------------------------------------------------------------------------ 64

51. Part of phase diagram for Fe-S system below 350 ̊C showing electron microprobe compositions of

monoclinic pyrrhotite-------------------------------------------------------------------------------------------------------- 65

52-55. Scanning electron micrographs showing:

52. Octahedral pyrite crystal and marcasite crystals with a cockscomb habit-------------------------------------------- 66

53. Blocky anhydrite crystals partly coated by mixed-layer chlorite-smectite rosettes---------------------------------- 67

54. Backscatter image of intergrown tabular jarosite crystals--------------------------------------------------------------- 67

55. Hexagonal tabular natrojarosite crystals----------------------------------------------------------------------------------- 68

56. Photograph of part of large cavity filling of red botryoidal hematite and later colorless calcite------------------- 68

57-60. Scanning electron micrographs showing:

57. Hexagonal, tabular to columnar apatite crystals with later chlorite---------------------------------------------------- 69

58. Striated grain of native copper partly filling vesicle--------------------------------------------------------------------- 69

59. Clusters of euhedral twinned epidote crystals---------------------------------------------------------------------------- 70

60. Subhedral hydrogrossular crystals in volcaniclastic sandstone--------------------------------------------------------- 72

61. AFM diagram showing chemical variation of basaltic to rhyolitic core samples and surface

rocks from Newberry volcano---------------------------------------------------------------------------------------------- 75

62. Chemical variation diagrams showing differences between precaldera and postcaldera rocks from

Newberry volcano------------------------------------------------------------------------------------------------------------ 76

63. SiO2 vs. CaO diagram for chemical analyses of rocks from geothermal drill holes and outcrops at

Newberry volcano------------------------------------------------------------------------------------------------------------ 77

64. MgO variation diagrams for chemically analyzed rocks from geothermal drill holes and outcrops

at Newberry volcano--------------------------------------------------------------------------------------------------------- 77,78

VIII CONTENTS
65. H2O vs. SiO2 diagram for chemically analyzed rocks from Newberry volcano------------------------------------- 78

66. Zr variation diagrams for major- and selected trace- element contents of rocks from

Newberry volcano------------------------------------------------------------------------------------------------------------ 79-81

TABLES

1. Geothermal drill holes at Newberry volcano selected for hydrothermal mineralogy studies---------------------- 8

2. Chemical analyses of USGS-N2 drill-core samples--------------------------------------------------------------------- 12,13

3. Spectroscopic trace-element analyses of USGS-N2 drill-core samples----------------------------------------------- 14

4. INAA trace-element analyses of USGS-N2 drill-core samples-------------------------------------------------------- 14

5. Electron microprobe analyses of phenocryst and groundmass plagioclase crystals from

USGS-N2 drill core---------------------------------------------------------------------------------------------------------- 19

6. Fluid-inclusion heating/freezing data for Newberry volcano drill-core samples------------------------------------ 20

7. Chemical analyses for drill-core samples outside Newberry caldera------------------------------------------------- 26,27

 8. Electron microprobe analyses of zeolite minerals from USGS-N2 drill core---------------------------------------- 43

9. Electron microprobe analyses of carbonate minerals from USGS-N2 drill core------------------------------------ 49

10. Trace-element composition of carbonate and sulfide minerals from USGS-N2 drill core------------------------- 51

11. Electron microprobe analyses of phyllosilicate minerals from USGS-N2 drill core-------------------------------- 60

12. Electron microprobe analyses of sulfide minerals from USGS-N2 drill core---------------------------------------- 65

13. Trace-element analyses for drill-core samples outside Newberry caldera-------------------------------------------- 70

14. Electron microprobe analyses of epidote from 916.7-m depth in USGS-N2 drill core----------------------------- 71

15. Optical and crystallographic data for epidote from 916.7-m depth in USGS-N2 drill core------------------------ 72

IX
Library of Congress Cataloging-in-Publication Data

Bargar, Keith E.

Hydrothermal minerology of core from geothermal drill holes at

Newberry Volcano, Oregon / Keith E. Bargar and Terry E.C. Keith.

p. cm -- (U.S. Geological Survey professional paper ; 1578)

Includes bibliographical references.

Supt. of Docs. no.: I 19. 16: 1578

ISBN 0-607-91905-1

1. Hydrothermal alteration--Oregon--Newberry Volcano Region.

2. Minerals--Oregon--Newberry Volcano Region. 3. Geothermal

resources--Oregon--Newberry Volcano region. I. Keith, Terry E.C.

II. Title. III. Series.

QE390.5.B375 1999

551.2'3'09795--dc21 99-32015

 CIP
1

ABSTRACT

Twenty geothermal exploration drill holes, funded by both

industry and various government agencies, were completed

between 1976 and 1986 on the flanks and within the caldera

of Newberry volcano, central Oregon. These holes were drilled

in order to evaluate the geothermal energy potential of this

young Cascade volcano. Part of the evaluation process involved studies of the hydrothermal alteration mineralogy in

nine of the drill holes. Geothermal drill holes GEO-N1, GEON4, and GEO-N3 were located on the south, east, and north

flanks of the volcano, respectively. Four holes (GEO-N5,

GEO-N2, SF NC-01, and SF NC72-03) were drilled on the

west flank of Newberry volcano, and two holes (USGS-N2

and RDO-1) were completed within the volcano’s caldera.

Maximum temperatures measured at the bottoms of the

flank drill holes GEO-N1, GEO-N3, GEO-N4, and GEO-N5

were below 100 ̊C. Fractures, vesicles, spaces between breccia fragments, and glassy drill-core samples usually exhibit

little or no hydrothermal alteration in the upper parts of these

drill holes. Similar open spaces in the lower parts of the drill

holes contain low-temperature hydrothermal minerals (carbonates, silica minerals, zeolites, clay minerals, sulfates, iron

oxides, and sulfides) that could have formed at the temperatures measured during drilling. Temperatures in the three remaining flank drill holes (GEO-N2, SF NC-01, and SF NC

72-03) exceeded 150 ̊C at the hole bottoms. The extent of

hydrothermal alteration in these drill holes is low to moderate, and the hydrothermal minerals include laumontite, quartz,

and mixed-layer chlorite-smectite that are consistent with temperatures above 150 ̊C.

The extent of hydrothermal alteration is greater in some

intracaldera drill-hole specimens from depths where the measured temperatures were significantly higher than temperatures encountered at equivalent depths within the flank drill

holes. Measured temperatures above about 300-m depth in

the USGS-N2 drill hole ranged between about 18 and 39 ̊C,

and hydrothermal alteration minerals are nearly absent in the

drill core. The temperatures increased from 31 ̊C at 300-m

depth to 265 ̊C at 930-m depth, and hydrothermal alteration

is slight to extensive. The more pervasive alteration occurs

along fractures and fracture margins and in the more permeable brecciated and volcaniclastic zones. Hydrothermal zeolites, apophyllite, gyrolite, carbonates, apatite, hydrogrossular,

clay minerals, silica minerals, sulfides, sulfur, anhydrite, epidote, and iron oxides and hydroxides were identified. Temperatures in the RDO-1 drill hole were greater than 158 ̊C at

about 350.5-m depth. Many of the hydrothermal alteration

minerals (analcime, aragonite, siderite, rhodochrosite, calcite,

smectite, chlorite, quartz, mordenite, pyrite, pyrrhotite, and

hematite) occur at shallower depths than in the nearby USGSN2 drill core.

The most important controls of hydrothermal alteration in

the Newberry volcano drill holes are permeability, temperature, and fluid composition. Hydrothermal fluids flow more

readily through the interflow breccias and have altered these

breccias and other volcaniclastic layers where initial permeability was highest; massive lava flows having low

permeabilities are generally unaltered or only slightly altered

except where fractured or vesicular.

Fluid-inclusion studies of quartz and calcite crystals from

the lower parts of the three west-flank drill holes indicate

that past temperatures were hotter. Temperatures were significantly hotter-commonly by 20 to 40 ̊C and by as much as

160 ̊C-in the SF NC72-03 drill hole, which is near the ring

fracture on the west side of the volcano. The ring fractures

may provide conduits for movement of deep geothermal water to shallower levels. Hydrothermal alteration minerals identified from the two intracaldera drill holes suggest that past

temperatures could have been similar to the measured temperatures. However, fluid-inclusion heating studies indicate

that temperatures within both drill holes have been significantly hotter (by as much as 80 ̊C in RDO-1 and by ~100 ̊C

at the bottom of USGS-N2) than the measured temperatures.

Lacustrine deposits in the upper parts of these drill holes suggest that transient higher past temperatures may have resulted

from higher pressure caused by the additional weight of an

intracaldera lake about 100 to 150 m deep.

The few published chemical analyses of waters from the

Newberry volcano drill holes and the low measured temperatures in the upper parts of all the drill holes indicate that descending cold meteoric waters exert a substantial control over

any alteration in the shallower levels of the drill holes. Limited fluid-inclusion melting-point temperature measurements

suggest that the geothermal waters become more saline with

depth, increasing from 0 at about 801-m depth to 1.9 weight

percent NaCl equivalent near the bottom of the USGS-N2

HYDROTHERMAL MINERALOGY OF CORE

FROM GEOTHERMAL DRILL HOLES AT

NEWBERRY VOLCANO, OREGON

By Keith E. Bargar and Terry E. C. Keith

1
2 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

drill hole. Salinity also appears to be higher near the caldera

ring fractures where it ranges from 0.2 weight percent NaCl

equivalent in GEO-N2 to as much as 0.7 weight percent NaCl

equivalent in SF NC72-03.

Comparison of major- and trace-element contents of chemically analyzed rocks from the GEO-N1, GEO-N5, and SF

NC72-03 drill holes with those of published analyses of unaltered surface rocks shows little or no evidence of element

mobility resulting from hydrothermal alteration of core from

these flank drill holes. Conversely, comparison of the published analyses with chemical data from many USGS-N2 drillcore specimens suggests that hydrothermal alteration caused

insignificant to moderate decreases in SiO2, Na2O, and K2O

as well as possible slight increases in total Fe and CaO. Considerable scatter of the data for Al2O3 may indicate both loss

and gain of this element. Gains or losses involving MgO, TiO2,

and total Fe are somewhat difficult to discern because analyses of these elements show strong trends that result from differences in the composition of precaldera and postcaldera

lavas and volcaniclastic rocks. The concentration of several

trace elements, as well as P2O5 and MnO, appears to vary

according to whether the rock is basaltic or rhyolitic; traceelement abundance in several USGS-N2 drill-core samples

having intermediate compositions appear to complete the differentiation trend.

Possibly the Newberry hydrothermal system is very young

and has not evolved to a stage where trace elements and some

major elements have been mobilized and reconcentrated in

significant amounts. The Newberry hydrothermal system

appears to be very youthful because unaltered glassy rocks

are present at elevated temperatures. The lack of indications

of self-sealing, together with the absence of multiple or crosscutting fracture systems, also provides evidence that the hydrothermal system is young.

INTRODUCTION

Newberry volcano is in central Oregon about 60 km east

of the north-south-trending crest of the Cascade Range (fig.

1). This broad shield-like volcano rises only about 1,100 m

above the surrounding terrain, but its extensive deposits have

a volume of approximately 450 km3 that is spread over an

area of about 1,600 km2 (MacLeod and Sherrod, 1988). Enclosed within the 5- by 7-km summit caldera of Newberry

volcano are the scenic Paulina and East Lakes (fig. 2). Hot

springs on the northeast shore of Paulina Lake have temperatures of 50 and 52 ̊C (Sammel, 1983). Temperatures as high

as 62 ̊C are reported for hot springs along the south shoreline

of East Lake, and a temperature of 35.5 ̊C was measured in a

warm-water well in a campground on the east side of Paulina

Lake (Mariner and others, 1980). Weak fumaroles are also

present on the northeastern edge of the Big Obsidian Flow

(fig. 2), which is the youngest of several Holocene silicic lava

flows and eruptive centers within the caldera (Peterson and

Groh, 1969; Friedman, 1977). The thermal features and abundant evidence of at least 25 eruptions of silicic volcanic rocks

in the Holocene epoch (as young as 1,350 years B.P.)

(MacLeod and others, 1995) make this large complex volcano an attractive target for geothermal energy exploration.

Since 1976, numerous geologic and geophysical studies

(summarized by Fitterman, 1988), including several deep drill

holes, have been completed at Newberry volcano in an attempt to understand the volcano’s structure and geothermal

system. The location of all of the geothermal drill holes in

the vicinity of Newberry caldera are shown on a map published by Olmstead and Wermiel (1988) and in figure 3 of

this report. Access to information obtained from these drill

holes varies (table 4 in MacLeod and others, 1995), depending partly on whether private or public funds were used to

complete the holes.

This report describes the hydrothermal alteration of core

and drill cuttings from the two drill holes within Newberry

caldera (USGS-N2, and RDO-1) and seven core holes (GEON1, GEO-N2, GEO-N3, GEO-N4, GEO-N5, SF NC-01, and

SF NC72-03) located outside the caldera rim. Core samples

were not recovered from the upper part of USGS-N2, so drill

hole USGS-N3 was completed nearby to obtain samples and

information on the shallow part of the stratigraphic section.

Brief observations on core from USGS-N1, CE NB-3, and

CE NB-4 drill holes on the northeast, southeast, and northwest flanks of the volcano (fig. 3) also are included.

The 932-m-deep USGS-N2 drill hole is by far the hottest

of all of the Newberry drill holes; in fact, the 265 ̊C nearbottom temperature is hotter than temperatures reported for

any other geothermal drill hole in the Cascade Range within

the United States. Only the Meager Mountain area in British

Columbia, Canada (fig. 1), where 270 ̊C is reported from a

3,000-m-deep drill hole (Souther, 1985), has a higher reported

temperature. Interpretations of the Newberry volcano hydrothermal system rely heavily on data from the USGS-N2 drill

hole. Some preliminary data on the hydrothermal alteration

of this drill core were reported by Bargar and Keith (1984).

Light stable-isotope measurements on selected mineral and

whole-rock samples from USGS-N2 drill core indicate that

the hydrothermal minerals probably precipitated from the

present geothermal waters (Carothers, Mariner, and Keith,

1987). On the basis of hydrothermal mineralogical data, Keith

and others (1984a, b) and Keith and Bargar (1988) concluded

that Newberry volcano has a young evolving geothermal system. A comparison of hydrothermal mineralogy in the RDO1 drill hole with that in the USGS-N2 drill core suggests that

rocks within the caldera become less altered at equivalent

depths as distance increases from the southern ring fracture

(Keith and others, 1986).

Preliminary hydrothermal alteration studies (Bargar and

Keith, 1986) show only minor low-temperature alteration,

consistent with temperatures reported by Swanberg and

Combs (1986) for the GEO-N1 drill hole on the southern flank

of Newberry volcano. Wright and Nielson (1986) compared

electrical resistivity anomalies with alteration in drill core
INTRODUCTION 3

Figure 1.—Map showing location of Newberry volcano and major Quaternary volcanic centers in

Cascade Range of western United States and British Columbia, Canada (Sherrod and Smith, 1990).

GEO N-1 and advised caution in interpretation of electrical

geophysical surveys because of an inability to distinguish

between rocks that have been altered by high-temperature

fluids (USGS-N2) and rocks subjected to low-temperature

alteration (GEO-N1). Similar low temperatures are reported

for drill holes on the north (GEO-N3), east (GEO-N4), and

southwest (GEO-N5) flanks of the volcano (Swanberg,

Walkey, and Combs, 1988; Walkey and Swanberg, 1990).

Volcanic deposits

Volcanic center

Hot springs 

0 100 200 KILOMETERS

EXPLANATION

Lassen Peak

Medicine 

 Lake 

 volcano

Mount Shasta

Caribou volcanic

 field

CALIFORNIA

NEVADA

OREGON

IDAHO

WASHINGTON

BRITISH COLUMBIA

Meager Mountain

Mount Cayley

Mount Garibaldi

Mount Baker

Mount Rainier

Mount Adams

Mount Jefferson

Mount McLoughlin

Three Sisters

Crater Lake

Mount Saint

Helens

Simcoe volcanic field

Glacier Peak

Mount Hood

Newberry volcano

CANADA

UNITED STATES

Pacific Ocean

50°

45°

40°

125° 120°
4 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Higher temperatures and more intensive hydrothermal alteration are reported for Newberry’s west-flank drill holes GEON2 (Walkey and Swanberg, 1990), SF NC-01, and SF NC72-

03 (Arestad, Potter, and Stewart, 1988). In 1990, Newberry

volcano was designated as the Newberry National Volcanic

Monument (Public Law 101-522) (Collins, 1991). Because

future geothermal exploration and possible development of

the Newberry geothermal system will be restricted to areas

outside the national monument boundaries, the west flank of

the volcano may be of considerable interest to the geothermal industry.

ANALYTICAL METHODS

Drill cuttings were sampled at either 3- or 6.1-m intervals

during drilling of the RDO-1 drill hole and were logged by

M.W. Gannett and A.F. Waibel, who used a binocular microscope at the drill site (Keith and others, 1986). The authors

examined the entire USGS-N2 drill core (plus the adjacent

offset drill core USGS-N3) by hand lens and collected a subset of the USGS-N2 core, consisting of about 640 specimens

that are representative of rock units, vug fillings, fracture

coatings, and whole-rock alteration, for laboratory studies of

the hydrothermal alteration. GEO-Newberry Crater, Inc.,

logged and photographed the GEO-N1 and GEO-N3 drill

cores and sent a split of both cores to Energy and Geoscience

Institute of the University of Utah (EGI). Subsets of the splits

of these two cores (262 specimens from GEO-N1 and 118

specimens from GEO-N3), including vein or fracture and vug

fillings, and both altered and unaltered representative samples

of stratigraphic intervals, were obtained from the EGI core

library for this study. GEO-Newberry Crater, Inc., also logged

and photographed the GEO-N2, GEO-N4, and GEO-N5 drill

cores and made the drill-core and drill-hole data from all of

their Newberry drill holes available for this study. The company had acquired the geothermal assets of Santa Fe Geothermal, Inc., at Newberry volcano, which included core from

drill holes SF NC72-03, and SF NC-01, and data from them

was also available. Photographs provided by GEO-Newberry

Crater, Inc., for each core box (drill holes GEO-N2, GEON4, GEO-N5, SF NC-01, and SF NC72-03) were used to locate possible alteration zones. These zones were inspected,

and representative core samples (88 specimens from GEON2, 72 from GEO-N4, 59 from GEO-N5, 95 from SF NC-01,

and 118 from SF NC72-03) were collected for this investigation (Bargar and others, 1990); they included fresh rock and

hydrothermally altered drill core (vug fillings, fracture coatings, and whole-rock samples). Unfortunately, we were unable to spend the time necessary to thoroughly examine all

the core from these five drill holes.

Subsets of specimens from the nine Newberry geothermal

drill holes were obtained for this study at several different

times between 1982 and 1990. Prior to the collection of

samples, a lithologic log for each drill hole was made available, usually by the drilling organization. From these logs,

we selected subsets of appropriate samples for various types

of laboratory analyses. Some core specimens from the USGSN2 and GEO-N1 drill holes were prepared as thin sections

for petrography studies. Core specimens were generally selected for chemical analysis on the basis of being representative of either the freshest available rock of a unit or flow of

particular interest (not all flow units were analyzed) or a highly

altered part of the same flow unit. Major- and trace-element

analyses were obtained for drill-core specimens of two lava

flows from each of the GEO-N5 and SF NC72-03 drill holes

and from several depths in the GEO-N1 and USGS-N2 drill

holes. Major elements were analyzed by X-ray spectroscopy

and conventional rock analysis methods, and trace elements

were analyzed by energy dispersive X-ray fluorescence spectroscopy, optical spectroscopy, and instrumental neutron activation analysis. Techniques for these methods are described

in Baedecker (1987). In this report, the chemically analyzed

rocks are classified, according to weight percent SiO2, as

basalts (<52 percent), basaltic andesite (52-57 percent), andesite (57-62 percent), dacite (62-67 percent), rhyodacite (67-72

percent), and rhyolite (>72 percent).

Figure 2.—Generalized geologic map of Newberry caldera, Oregon

(after MacLeod and others, 1995) showing locations of drill holes

USGS-N2, USGS-N3, and RDO-1, hot springs, fumarole, caldera

ring fractures, caldera rim, and some of the younger volcanic deposits. Northeast Obsidian Flow and obsidian flow southwest of Big

Obsidian Flow are Pleistocene. Unit in which fumarole is heated

may be latest Pleistocene or Holocene; remainder of units in figure

are Holocene (post-Mazama ash bed, about 6,845±50 C14 yr BP),

relative ages of which are provided by MacLeod and others (1995).
5

The sample subset for each drill hole was examined by

binocular microscope, and detailed descriptions of the occurrence of the hydrothermal alteration minerals and their paragenetic relations were noted. At this time, we selected appropriate hydrothermal alteration mineralogy samples for X-ray

diffraction (XRD), scanning electron microscope (SEM),

fluid-inclusion, and electron microprobe analyses.

The mineralogy of many altered and unaltered drill-core

specimens was determined by XRD using a Norelco X-ray

diffractometer equipped with a graphite monochrometer and

Figure 3.—Map showing location of geothermal exploration drill holes at Newberry volcano (after

Olmstead and Wermiel, 1988).

ANALYTICAL METHODS

GEO–N3

P-3

CE NB-4 P-5

P-6

USGS–N2, N3

RDO–1

U83-9

U83-8

GEO–N2

GEO–N5

CE NB-3

GEO–N1

6000

6000

5000

5000

7000

Paulina Peak

Paulina

Lake

East

Lake

121°20'

43°

50'

43°

45'

43° 40'

121°15' 121°10'

0 2 KILOMETERS

CONTOUR INTERVAL

1000 FEET

EXPLANATION

Ring fractures

Drill hole discussed

 in this report

Other drill hole

USGS–NI

SF NC–01

SF NC72–03

U82-9 GEO–N4

U82-10

NEWBERRY

NATIONAL

VOLCANIC

MONUMENT

5000
6 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Ni-filtered Cu-Kα radiation. Rock and mineral specimens

were prepared as slurries on glass slides and run at 1 ̊2θ/min;

selected zeolite and clay samples were heated in a furnace at

450 or 550 ̊C and X-rayed a second time or were glycolated

by placing the slides in an atmosphere of ethylene glycol at

60 ̊C for 1 hour before X-raying again. In later analyses, the

Norelco XRD was controlled by a Dapple computer automation system.

Several hydrothermal mineral specimens were carbon or

gold-palladium coated and mounted in a Cambridge Stereoscan 180 or 250 SEM (equipped with an X-ray

energy-dispersive spectrometer or EDS) in order to obtain

detailed information on the morphology, paragenesis, and

semiquantitative chemistry of very small hydrothermal mineral specimens.

Doubly polished thick sections of hydrothermal quartz and

calcite were prepared from as many drill core specimens as

possible in order to obtain data on past subsurface temperatures and fluid salinities at Newberry volcano. Measurements

of fluid-inclusion homogenization temperatures (Th) and final melting-point temperature (Tm), determined by the freezing method (Roedder, 1962), for specimens from drill holes

GEO-N2, SF NC72-03, SF NC-01, and RDO-1 were made

by using a Linkam THM 600 heating/freezing stage and TMS

90 temperature-control system. A Chaixmeca heating/freezing stage was used to obtain fluid-inclusion data for the USGSN2 drill hole. Successive calibration runs, using synthetic fluid

inclusions (Bodnar and Sterner, 1984) and chemical compounds with known melting points recommended in Roedder

(1984), suggest that the accuracy of the homogenization temperature (Th) measurements should be within ±2.0 ̊C and that

the final melting-point temperature (Tm) values should be

accurate to at least ±0.2 ̊C for the Linkam heating/freezing

stage. The Chaixmeca heating/freezing stage has similar accuracy. Salinity of the fluids trapped inside the fluid inclusions was calculated, in weight percent NaCl equivalent, by

an equation given in Potter, Clynne, and Brown (1978). Selected specimens were crushed with a device manufactured

by M.K. Latham Enterprises in order to observe the behavior

of the gas bubble when the fluid inclusions were breached.

By this means, some information can be obtained about the

nature of the vapor phase within the fluid inclusions (Roedder,

1984).

Quantitative chemical analyses for several hydrothermal

minerals from the USGS-N2 drill core were obtained with an

ARL-EMX electron microprobe by using both natural and

synthetic mineral standards. A defocused 15-25 μm beam was

utilized at an accelerating voltage of 15 kV, a sample current

of 15 nA, and a counting time of 10 seconds.

ACKNOWLEDGMENTS

Our knowledge of Newberry volcano geology, hydrology, and geochemistry was greatly enhanced through discussions with W.W. Carothers, S.E. Ingebritsen, N.S.

MacLeod, R.H. Mariner, E.A. Sammel, and D.R. Sherrod.

We thank M.J. Johnson, C.A. Swanberg, and W.C. Walkey

of GEO-Newberry Crater, Inc. for providing us with preliminary data from the GEO-N1 and GEO-N3 drill holes

and allowing us to sample the three other GEO-Newberry

Crater, Inc., and the two Santa Fe Geothermal, Inc., drill

cores. P.M. Wright of the Energy and Geoscience Institute

of the University of Utah permitted us to sample GEONewberry, Inc. drill cores GEO-N1 and GEO-N3 and Cal

Energy CE NB-3 and CE NB-4 for this study. The assistance of M.H. Price, L. Stevens, and S. McKnight with X-ray

diffraction, computer manipulations, and diagrams, S.

Wessells with some of the drafting, and R.L. Oscarson with

scanning electron microscope studies is gratefully acknowledged. We especially acknowledge the manuscript reviews

by J.M. Donnelly-Nolan, R.C. Erd, and D.R. Sherrod, which

greatly improved this report.

GEOLOGIC SETTING

Newberry volcano, one of the largest of the Cascade volcanoes, is also one of the largest Quaternary volcanoes in the

conterminous United States (MacLeod and Sherrod, 1988).

Many physical, chemical, and age similarities between

Newberry volcano and the much larger Medicine Lake volcano in northern California suggest that the two volcanoes

have similar origins and are related to the Cascade Range of

volcanoes (Donnelly-Nolan, 1988). Newberry volcano is at

the western end a group of Miocene to Holocene volcanic

vents that extend northwestward across much of the southeast quarter of Oregon. Radiometric ages indicate that the

volcanic activity is progressively younger toward the west,

culminating in the Holocene volcanism at Newberry (Walker,

1974; MacLeod, Walker, and McKee, 1976). The flanks of

Newberry volcano are mantled by more than 400 cinder cones

and fissure vents, several rhyolitic domes, flows, and pyroclastic deposits, as well as basaltic to basaltic-andesite flows

and pyroclastic deposits, all of Quaternary age (MacLeod and

others, 1981). Paleomagnetic studies of the surficial basaltic

to rhyolitic lava flows extruded from Newberry volcano and

of flows recovered from the USGS-N2 drill hole show only

normal polarities; thus the volcano probably is younger than

0.7 Ma (MacLeod and Sammel, 1982). Drill holes GEO-N1

and GEO-N3 did penetrate rocks having reversed polarities;

however, these older rocks and other nearby, reversely polarized domes and flows may predate Newberry volcano

(MacLeod and others, 1995). MacLeod and Sammel (1982)

and MacLeod and Sherrod (1988) describe multiple sets of

ring fractures within the Newberry caldera, which indicate

that caldera collapse may have occurred more than once following major eruptions between about 0.3 and 0.5 Ma. Postdating the last caldera collapse are several intracaldera volcanic deposits, including the Big Obsidian Flow, which overlie
7

the ~6.8-ka Mazama ash bed (Bacon, 1983; MacLeod and

Sherrod, 1988).

Geophysical studies (summarized in Fitterman, 1988) suggest that the interior of Newberry volcano is composed of

three distinct zones. The upper zone consists predominantly

of younger unaltered silicic to mafic lava flows and tuffs along

with their feeder pipes and dikes. In this zone, electrical

resistivities are high (Fitterman, Stanley, and Bisdorf, 1988),

and seismic velocity (Catchings and Mooney, 1988) and densities (Gettings and Griscom, 1988) are low. An intermediate

zone contains older, hydrothermally altered, silicic to mafic

lava flows and tuffs, abundant dikes, and some small intrusive pod-like bodies. The geophysical studies show that the

intermediate zone has moderate density, intermediate seismic velocity, and low electrical resistivity. Feeder dikes beneath the ring fractures are interpreted from high gravity values. The lowermost zone, characterized by high electrical resistivity, high seismic velocity, and high density, may contain

numerous sills, dikes, and small pods that are mostly solidified. Seismic tomography studies by Achauer, Evans, and

Stauber (1988) show one low-velocity zone that they interpreted as a small molten magma body about 3 to 5 km beneath the surface of the caldera. A later analysis of compressional-wave-attenuation tomography using the data of

Achauer, Evans, and Stauber (1988) led Zucca and Evans

(1992) to reinterpret this feature as a recently solidified hot

intrusion. Stauber, Green, and Iyer (1988) constructed a threedimensional velocity picture of the crust beneath Newberry

volcano using a teleseismic P residual method; they indicate

that a high P velocity zone, possibly attributable to molten

or partially molten intrusions, extends from about 10 to 25

km beneath the caldera and western flank of Newberry volcano. Fitterman (1988) and MacLeod and Sherrod (1988) conclude that Holocene volcanism at Newberry volcano has resulted from the intrusion of many small pods and magma

chambers rising from deep within the crust; this model is

similar to the one proposed by Donnelly-Nolan (1988) for

Medicine Lake volcano.

GEOTHERMAL EXPLORATION

Since 1976, twenty drill holes have been completed at

Newberry volcano (Olmstead and Wermiel, 1988) in order to evaluate the geothermal energy potential of this

young active volcano. Data for seven of the drill holes

has not been released; location, depth, temperature, and

other data for ten drill holes are given in table 1. Specimens from three drill holes within the caldera and seven

drill holes outside the caldera were obtained in order to

better understand the physical and chemical processes

occurring at several locations within the volcano. The

numerous hydrothermal minerals that precipitated within

various parts of the volcano, along with their compositions, provide information about the thermal and chemical conditions present at the time the minerals formed.

Fluid-inclusion analyses of hydrothermal calcite and

quartz crystals give information about past and present

temperatures of hydrothermal alteration.

DRILL HOLES WITHIN THE CALDERA

USGS-N2

In the summer of 1978, the U.S. Geological Survey began

drilling the USGS-N2 research drill hole in the south-central

part of Newberry caldera (Sammel, 1981) (fig. 3) to obtain

stratigraphic, heat-flow, and hydrologic information (Muffler, Bacon, and Duffield, 1982). The drill hole was located

at an altitude of 1,935 m about 400 m northeast of the Big

Obsidian Flow (fig. 2) and was completed in several stages.

During 1978, the hole was drilled to a depth of 312 m by the

mud-rotary method; it was progressively deepened by the

wireline-coring method during the summers of 1979 and 1981

and was completed to 932-m depth in September, 1981

(MacLeod and Sammel, 1982). In 1980, a second offset drill

hole (USGS-N3) was spudded just south of USGS-N2 to provide drill core from the upper part of the stratigraphic section. The USGS-N2 and USGS-N3 drill holes are treated in

this report as a single drill hole called USGS-N2. Core recovery from 98-m to about 300-m depth ranged from 40 to

90 percent; below 300-m depth in USGS-N2, core recovery

was about 90 percent (MacLeod and Sammel, 1982).

The hydrologic system at Newberry volcano, including

data from drill hole USGS-N2, was described by Sammel

(1983) and Sammel and Craig (1983). Low temperatures

measured in the upper part of USGS-N2 probably are due to

dilution by downward-percolating cool meteoric water

(Sammel, 1983). Some changes in the temperature-depth profile constructed from the drill-hole data can be correlated with

differences in permeability of the various rock units penetrated

in the drill hole (Sammel, 1981; MacLeod and Sammel, 1982).

The maximum near-bottom temperature measured following

drilling was 265 ̊C (Sammel, 1981; MacLeod and Sammel,

1982). A hypothetical conductive thermal gradient of 285 ̊C/

km is given for the drill hole, and a thermal gradient of 706 ̊C/

km was calculated for the lower 250 m of it (Sammel, 1981;

MacLeod and Sammel, 1982). The conductive heat flux for

the drill hole is more than 10 times greater than the regional

average value (MacLeod and Sammel, 1982).

Drill hole USGS-N2 was flow tested in late September 1981. An initial wellhead pressure of 5,700 kPa decreased to 900 kPa after 20 hours of testing (Sammel,

1981). Sammel (1981) and Sammel and Craig (1983) reported a gas phase that consisted predominantly of CO2

with minor H2S, CH4, and other gas phases. Initially, the

dilute fluids collected from the drill hole during flow testing were thought to be contaminated by drilling fluids

(Keith and others, 1984b); however, later isotopic studies

demonstrated that the drill-hole fluid sample was differGEOLOGIC SETTING
8 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

ent from the drilling fluid (Ingebritsen and others, 1986).

Fluid at the bottom of the drill hole is thought to consist

predominantly of steam and CO2 (Ingebritsen and others,

1986).

LITHOLOGY

A lithologic description of the USGS-N2 drill core is given

in appendix 1. A preliminary lithologic log for the drill core

(MacLeod and Sammel, 1982) has been slightly modified on

the basis of later geochemical and petrographic data (Keith

and Bargar, 1988). A stratigraphic column for the upper part

of the drill core is shown in figure 4, and a stratigraphic representation of the lower two-thirds of the drill core is shown

in the left column of figure 5. Keith and Bargar (1988) divided the rock units of USGS-N2 into three lithologic zones.

Zone I comprises the upper 320 m of drill core, which contains lacustrine sediment and unconsolidated pumiceous

caldera fill interrupted by a 56-m thick obsidian flow. Zone II

extends from 320- to 697-m depth. The section above 501 m

consists mainly of lacustrine or fluviatile pumiceous ashy sand

and gravel, pumiceous tuff and lithic tuff, tuff breccia, a 10-

m-thick rhyodacite sill, and two thin glassy dikes; the section

from 501 to 697 m contains coarse dacite breccia and dacite

and andesite lava flows. Zone III, which extends from 697- to

932-m depth, is mostly made up of subhorizontal andesite,

basaltic andesite, and basalt lava flows and interflow breccias.

HYDROTHERMAL ALTERATION

SURFACE TO 320-M DEPTH

Local palagonitization and some hydration of glass are

about the only alteration products seen in the upper 300 m of

the drill core, in which the measured temperatures ranged

from 18 to 39 ̊C (Sammel, 1981). XRD analyses also show

the presence of minor siderite at 135-m depth, amorphous

silica at 181-m depth, and traces of smectite at depths of 174

and 185 m. Beginning at about 300-m depth (fig. 5), scarce

zeolites and other hydrous silicates locally replace basaltic

Table 1.—Geothermal drill holes at Newberry volcano selected for hydrothermal mineralogy studies.

[m.s.l., mean sea level. —, not available]

Drill Latitude Longitude Year Drilling organization Drill-collar Total Max. Referhole drilled altitude (m depth temp. ences1

above m.s.l.) (m) ( ̊C)

Drill holes within the caldera

USGS-N2 43 ̊ 42.5' 121 ̊ 13.5' 1978–1981 U.S. Geological Survey 1,935 932 265 1,2,7

USGS-N3 43 ̊ 42.4' 121 ̊ 13.5' 1980 do. 1,960 186 —- 2,7

RDO-1 43 ̊ 42.3' 121 ̊ 13.3' 1983 Sandia National Laboratories 1,969 424 158+ 3,7

Drill holes outside the caldera

GEO-N1 43 ̊ 38.3' 121 ̊ 14.5' 1985 GEO-Newberry, Inc. 1,780 1,387 71 5,6,7

GEO-N2 43 ̊ 43.6' 121 ̊ 18.7' 1986 do. 1,779 1,337 164 6,7

GEO-N3 43 ̊ 4 9.9' 121 ̊ 14.7' 1986 do. 1,753 1,220 57 5,6,7

GEO-N4 43 ̊ 42.7' 121 ̊ 08.2' 1987 do. 1,905 703 18 6,7

GEO-N5 43 ̊ 40.8' 121 ̊ 18.6' 1987 do. 1,731 988 69 6,7

SF NC-01 43 ̊ 43.6' 121 ̊ 18.0' 1984 Santa Fe Geothermal, Inc. 1,859 1,219 170 4,7

SF NC72-03 43 ̊ 42.1' 121 ̊ 19.8' 1983, 1984 do. 1,993 1,372 155 4,7

1Sources of data: 1, Sammel (1981), 2, MacLeod and Sammel (1982), 3, Black, Priest, and Woller (1984), 4, Arestad, Potter, and

Stewart (1988), 5, Swanberg, Walkey, and Combs (1988), 6, Walkey and Swanberg (1990), 7, MacLeod and others (1995).
9

glass in subhorizontal layers of sandstone and siltstone (fig.

6) and fill small fractures and intergranular pore spaces. Calcite and aragonite crystals were deposited later (fig. 7) in

steeply dipping fractures as much as 2 mm wide. Although

evidence for hydrothermal alteration is lacking for most of

the 300- to 320-m interval, a chemical analysis of basaltic

glass siltstone at 311.4-m depth shows more than 22 weight

percent volatiles. This is the most highly altered core specimen analyzed (table 2). The sample also contains substantially more Cr, Cu, and Ni than any other rock analyzed from

Newberry volcano (tables 3 and 4).

320- TO 697-M DEPTH

The interval from 320- to 697-m depth is mostly silicic,

ranging from rhyolite having a volatile-free SiO2 value of

73.8 weight percent, in a light-colored pumiceous lithic tuff

at 474.6-m depth, to massive and brecciated rhyodacite to

andesitic lavas from 501- to 697-m depth that have volatilefree SiO2 values as low as 60.8 weight percent (table 2 and

appendix 1). Rhyolitic to rhyodacitic pumice-rich tuff and

lithic tuff from 320- to 501-m depth include abundant amorphous, hydrated glass, detected by XRD, much of which is

incipiently to totally replaced by smectite (fig. 8A). Siderite,

along with local magnesite and ankerite-dolomite, is abundant as a late-deposited coating on fractures, breccia fragments, and pore-space infilling throughout this interval (figs.

5, 8B, C, and D).

Pumiceous tuff immediately above and below the

rhyodacite sill at 460- to 470-m depth has been subjected to

more intense, higher temperature alteration due to injection

of the sill into wet tuff. The rhyodacite sill is devitrified to a

cryptocrystalline mixture of cristobalite and alkali feldspar;

however, the perlitic texture remains. Although the rhyodacite

sill itself is very dense, secondary permeability is provided

by cross-cutting fractures (fig. 8E). Fine-grained quartz, chalcedony, pyrrhotite, pyrite, marcasite, and siderite were deposited along fractures in the sill, and white smectite replaced

groundmass of the brecciated parts. The tuff at the upper contact of the sill is baked. A decrease in temperature within the

adjacent rock outward from the top and bottom of the sill is

reflected by a corresponding decrease in the intensity of alteration, as small amounts of quartz, pyrrhotite, pyrite, calcite, and siderite are deposited in pore spaces, and chlorite

and mordenite replace glass in the tuff.

Below the rhyodacite sill, two thin (less than 1-m thick),

glassy dikes intersect the pumiceous lithic tuff at depths of

471 m and 479 m. These dike rocks are hydrated and partly

replaced by mordenite, and they were injected at sufficiently

high temperature to alter the immediately adjacent pumiceous

tuff. Thin veinlets lined with calcite and filled with later siderite emanate into the tuff perpendicular to the dike. The contacts of these thin dikes with the tuff are not baked, but the

irregular incursion of dike material into the tuff indicates that

the tuff was wet and slightly fluidized when intruded.

Rhyodacite, dacite, and andesite lava flows between 501-

and 697-m depth are little altered; however, the rock has been

bleached in the five breccia zones and adjacent to fractures

throughout this interval. Micro-bleaching effects adjacent to

the thin veinlets or hairline cracks are seen in thin section.

However, thin sections also show that much of the rock is

Figure 4.—Comparison of lithologic units in upper part of USGSN2 drill hole with those of RDO-1 drill hole (after Keith and others,

1986). Drill-hole elevations in meters above mean sea level; TD,Total

depth of drill hole.

GEOTHERMAL EXPLORATION

470

0

0

100

200

300

400

424m

TD

932m

TD

100

200

300

400

USGS–N2

1935m

RDO–1

1969m

Pumiceous ash 

 and lapilli, rhyo-

 litic with minor 

 basalt

Pumiceous ash

 and lapilli 

Palagonite tuff

Obsidian flow

Obsidian flow

Basaltic tuff

Lithic and

pumiceous 

tuff

Basaltic sand-

 stone, siltstone,

 and mudstone

Basaltic tuff and

 tuff breccia

Basaltic sand

 and grit, silt-

 stone, and

 mudstone

Pumiceous

 sand and

 gravel

Pumiceous

 lapilli tuff

 and lithic 

 breccia

Pumiceous

 lapilli tuff

 and lithic

 breccia

Rhyodacite sill
10 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Figure 5.—Distribution of hydrothermal alteration minerals with depth in lower two-thirds of USGS-N2 drill core (after Keith and

Bargar, 1988). Left column shows generalized stratigraphic section of rock units in drill hole (MacLeod and Sammel, 1982).

Horizontal lines in right column indicate distribution of samples studied. Width of mineral columns shows approximate mineral

abundance based on X-ray diffraction and microscope observations. Vertical continuity in mineral presence between samples is

assumed except where mineral abundance becomes zero, in which case zero point is arbitrarily placed 0.3 m from last occurrence

of the mineral. Solid curve shows measured temperature profile for drill hole (Sammel, 1981; MacLeod and Sammel, 1982).
11

very tight and unaltered, that there was virtually no replacement of primary minerals in the lava flows, and that many

vapor-phase cavities lack hydrothermal minerals (fig. 8F).

Pyrrhotite was deposited on some fracture surfaces but has

been mostly altered to siderite and pseudomorphous marcasite and pyrite (fig. 8G). Scarce opal, cristobalite, and chalcedony were deposited locally on fracture surfaces and in

vesicles. Abundant siderite commonly coats fracture surfaces

and cements breccia fragments, although much open space

between fragments remains in some of the breccias.

The measured temperatures through this zone increased

from 40 ̊C at 300-m depth to 100 ̊C at about 425-m depth,

slowly decreased to 90 ̊C at 500-m depth, continued to decrease to 75 ̊C at about 550-m depth, and then increased again

to 85 ̊C at 679-m depth (fig. 5). The temperature increase

between depths of 350 m and 500 m in USGS-N2 occurs at a

horizon comparable to the position of a 158+ ̊C hot-water

aquifer that was penetrated by drill hole RDO-1 (fig. 12) at a

depth of 379.5 m to 397 m. Previous lateral flow of thermal

waters between the two holes seems likely (Keith and others,

1986). Mineralogical evidence indicates that the interval of

the temperature bulge has been heated to present temperatures or greater by a lateral influx of warm waters. The heating was accompanied by oxidation that resulted in the alteration of pyrrhotite to marcasite, pyrite, and siderite. A residue of the reaction, consisting of sulfur, iron hydroxide, and

iron oxide, is present on a fracture surface in the rhyodacite

sill. Isotope studies (Carothers, Mariner, and Keith, 1987)

show that the siderite in this interval is associated with present

temperatures and fluids. Similarly, the presence of iron oxide

and natrojarosite and a small temperature inflection measured

following drilling suggest that there may have been a major

depositional break or an aquifer at 605-m depth in the tuff

breccia.

Figure 6.—Fine-grained thin-bedded volcaniclastic sandstone; drillcore specimen from 315.1-m depth in USGS-N2 drill hole. Rock

consists of basaltic glass with incipient zeolitization and is of lacustrine origin (MacLeod and Sammel, 1982).

Figure 7.—Generalized paragenetic sequence of selected hydrothermal minerals in USGS N-2 drill core from within caldera of

Newberry volcano (after Keith and Bargar, 1988). Most common

mineral occurrence in parentheses: O, open-space precipitate; R,

replacement of glass or primary minerals. Length of horizontal line

has no relation to length of depositional period.

GEOTHERMAL EXPLORATION
12 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

The paragenetic sequence of secondary minerals (fig. 7)

began with the incipient and partial replacement of glass in

the tuff by smectite and zeolites between 320- and 505-m

depth. Subsequent intrusion of the sill and dikes into wet tuffs

caused local baking effects and deposition of the higher temperature alteration minerals pyrrhotite, chlorite, quartz, pyrite, and calcite in small fractures and pore spaces in and adjacent to the sill. Fractures and open-space fillings between

about 505- and 697-m depth contain early deposits of pyrrhotite followed by increased oxidation by the present thermal regime to pyrite, marcasite, and siderite. Silica minerals

were deposited earlier than carbonate minerals, possibly at

the time of pyrrhotite oxidation.

697- TO 932-M DEPTH

Subhorizontal lava flows and breccia units below 697-m

depth usually are more mafic (table 2) and include andesite,

basaltic andesite, and basalt. The character of hydrothermal

alteration in the interval between 697-m depth and the bottom of the drill hole at 932-m depth is different from that of

the shallower alteration. Flow breccias, interflow breccias,

and pumice layers are altered by replacement of groundmass,

especially interstitial glass, to smectite, mixed-layer chloritesmectite, and chlorite. Locally, plagioclase and mafic phenocrysts have been partly replaced by calcite and (or) clay

minerals. Generally the dense parts of the andesite to basalt

lava flows are little altered. In contrast, the flow tops and

basal flow breccias, vesicular zones, and interflow breccias,

as well as a thin interval of pumiceous tuff and tuff breccia,

are substantially altered (fig. 5).

The dense andesite flow between depths of 697 m and

758 m is highly fractured and altered to light-green clay minerals from 737- to 745-m depth. Several stages of vein minerals consisting of pyrrhotite, pyrite, clay minerals, quartz,

and calcite have been deposited (figs. 9A, B, and C), and

there has been partial replacement of calcite by quartz. In

spite of extensive alteration and mineral deposition, the fractures are not completely filled, and this zone may have been

a young aquifer for lateral migration of thermal waters. The

fractured andesite flow overlies interflow breccia and pumice layers that are also extensively altered.

Below 758-m depth, dense massive lava flows are sporadically intersected by steeply dipping hairline fractures that

are typically filled with blackish-green clay (mixed-layer chlorite-smectite) and, locally, later calcite (fig. 9D). Wide fractures are scarce, but where present they contain black-green

(mixed-layer chlorite-smectite) clay and euhedral quartz and

calcite crystals. Vesicles are locally filled with similar blackish-green clay, quartz, and calcite (fig. 9E), and the groundmass in the vesicular zone is more altered due to relatively

high permeability. Leaching has occurred in interflow breccias (fig. 9F). In the interval from 697- to 930-m depth, the

Table 2.—Chemical analyses of USGS-N2 drill-core samples.
13

temperature increased in a quasi-linear fashion from 100 ̊C

to 264 ̊C (fig. 5).

Near the bottom of the interval, epidote and anhydrite occur in pore spaces and cavities as late-stage deposits (fig. 5).

Plagioclase phenocrysts and groundmass have been partly replaced by calcite and probably provided the chemical components for the chlorite and epidote. Microprobe analyses of

cross sections of plagioclase phenocrysts (table 5) in the basal

lava flows show slight patchy differences in the Na:Ca:K ratios, but the phenocrysts are consistently higher in Ca and

lower in Na and K relative to groundmass plagioclase. Significant albitization of plagioclase has not occurred in these

rocks. The clay minerals gradually change structurally and

chemically with increasing depth and temperature from predominantly smectite to mostly mixed-layer chlorite-smectite

with local chlorite.

The bottom 2 m of USGS-N2 drill core, 930- to 932-m

depth (fig. 9G), is highly bleached flow breccia and vesicular

basalt. Groundmass is partly replaced by pyrrhotite, pyrite,

chlorite, quartz, and calcite, and vesicles are partly to totally

filled with well-crystallized chlorite, illite, pyrrhotite, pyrite,

quartz, calcite, and a small amount of epidote. Plagioclase

phenocrysts are partly replaced by calcite and illite; chlorite

and epidote fill open spaces left by leaching of the phenocrysts.

Bleaching is conspicuous in the basal parts (approximately

1 m thick) of the lowest two lava flows and decreases rapidly

upward into unbleached dense lava. The basal bleaching is due

to subhorizontal fluid flow; low vertical permeability restricted

fluid access to dense parts of the flows. Bleaching appears to

cause little change in mineralogy in the rock, probably only

the leaching of iron. However, in some bleached zones

pyrrhotite is present, again showing that it forms in lowpermeability rocks where fS2 of fluids is low for a given fO2

(and fH2) relative to pyrite (Browne, 1970). Sammel (1981) noted

gas, probably CO2, in the vesicular parts of these lowest two

flows during drilling, and the flow test of USGS-N2 produced

a fluid that was about 20 weight percent CO2 (Ingebritsen and

others, 1986; Sammel, Ingebritsen, and Mariner, 1988).

The general paragenetic sequence of secondary minerals

below 697-m depth in USGS-N2 (fig. 7) began with clay alteration of interstitial glass and replacement of mafic phenocrysts by mixed-layer chlorite-smectite. Locally, small amounts

of iron oxide were also deposited early. Small fractures were

Table 2.—Chemical analyses of USGS-N2 drill-core samples—Continued.

GEOTHERMAL EXPLORATION
14 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Table 3.—Spectroscopic trace-element analyses of USGS-N2 drill-core samples.

Table 4.—INAA trace-element analyses of USGS-N2 drill-core samples.
15

Figure 8.—Drill-core samples from upper and middle parts of USGS-N2 drill hole. A, Pumiceous tuff of unaltered glass with some incipient

smectite crystallization, from 351.9-m depth. B, Siderite-cemented lithic-tuff breccia with several unfilled spaces between breccia fragments

from 415.1-m depth. C, Pumiceous tuff with incipient smectite replacement and late siderite deposition, from 448.7-m depth. Arrow on

specimen points to altered pumiceous glass fragment of siderite and smectite analyzed by XRD. D, Breccia cemented by siderite, from

470.6-m depth. Spaces between breccia fragments are only partly filled. Arrow points to area from which specimen was obtained for XRD

analysis. E, Rhyodacite sill from 468.8-m depth; fractures partially filled by secondary minerals. F, Vapor-phase cavities oriented parallel to

flow banding in dacite flow from 588.9-m depth. Elongate cavities are coated with vapor-phase tridymite. G, Siderite and pyrite plus

marcasite pseudomorphs after pyrrhotite (dark specks) that coat irregular fracture surface in dacite flow at 520.6-m depth.

GEOTHERMAL EXPLORATION
16 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

filled with mixed-layer chlorite-smectite and pyrite, which

were deposited shortly before and in part were overlapped by

deposition of quartz, illite, and the other silica minerals. Most

of the calcite precipitated later than quartz and illite, although

some codeposition and fluctuating deposition of quartz and

calcite appears to have occurred. Anhydrite, epidote, and

scarce iron oxide were late minerals deposited in the lower

part of the core.

FLUID INCLUSIONS

Quartz and calcite crystals from seven depths (table 6) in

the lower 200 m of the USGS-N2 drill core contain mostly

secondary, liquid-rich fluid inclusions; the vapor phase ranges

from nearly 0 to more than 50 percent of the inclusions. For

this study, liquid-rich fluid inclusions having nearly equal

vapor-to-liquid ratios (fig. 10A) were selected for heating/

freezing measurements. Vapor-rich monophase-vapor and

monophase-liquid fluid inclusions occur in some samples.

Homogenization temperature (Th) measurements were obtained for 17 fluid inclusions in one calcite sample and 139

fluid inclusions in nine quartz crystals (table 6). The Th values, ranging from 173 to 367 ̊C, are plotted in figure 11 along

with the measured temperature profile for the drill hole

(Sammel, 1981). Melting-point temperature measurements for

19 fluid inclusions range from 0.0 to -0.2 ̊C (0 to 0.4 weight

percent NaCl equivalent) in samples from 929-m depth and

Figure 8.—Continued.

Figure 8.—Continued.

F

Figure 8.—Continued.
GEOTHERMAL EXPLORATION 17

Figure 9.—Drill-core samples from lower part of USGS-N2 drill hole. A, Fractures in altered interflow basaltic-andesite breccia from 741.3-m

depth. Filling is chlorite, smectite, pyrrhotite, pyrite, and calcite. B, Partly filled fracture system in altered interflow basaltic-andesite breccia

from 742.3-m depth. C, Fracture partly filled with bladed calcite and colorless euhedral quartz crystals in breccia from 742.5-m depth. Note

feeder fracture in lower right part of sample. D, Basalt lava flow with hair-line fractures filled by mixed-layer smectite-chlorite, from 922.0-m

depth. Rock is self sealed and impermeable. E, Vesicular basaltic andesite lava flow from 835.2-m depth. Vesicles lined or filled with mixedlayer chlorite-smectite, quartz crystals, and rhombic calcite (sample number N2-2740 represents depth in feet). F, Basaltic andesite interflow

breccia from 844.3-m depth; mostly leached (light gray areas) but retains dark patches of rock not leached by circulating geothermal fluids. G,

Partly leached vesicular basaltic andesite from 930.9-m depth. Vesicles lined with chlorite, quartz, calcite, pyrite, and illite.

C D
18 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

higher. At 931-m depth, the Tm of -0.8 to -1.1 ̊C (1.4 to 1.9 weight

percent NaCl equivalent) in 10 fluid inclusions suggests that

this lower sample was deposited from a separate more saline

fluid than the higher samples. Crushing of a few quartz and

calcite fluid inclusions in refractive index oils shows a gradual

expansion of the vapor bubble to fill the fluid-inclusion cavity,

which indicates the presence of a noncondensible gas such as

CO2 (Roedder, 1984). According to Ingebritsen and others

(1986) and Sammel, Ingebritsen, and Mariner (1988), the

present bottom fluids from the USGS-N2 drill hole contain

high concentrations of CO2. Hedenquist and Henley (1985)

found that the presence of CO2 in fluid inclusions may result

in erroneous Tm values and significantly high salinity estimates,

but CO2 could not account for the difference in Tm measurements between the lowermost and the upper USGS-N2 fluidinclusion samples.

Taguchi and Hayashi (1982) and Taguchi and others (1984)

studied fluid inclusions from drill holes in geothermal areas

of Japan and found that at a given depth minimum Th values

are generally the same or slightly warmer than the present

measured temperatures; they suggest that minimum Th values can be used to estimate present temperatures. They also

found that maximum Th values are nearly as high as a theoretical reference boiling-point curve drawn to the present

water-table depth. Thus, the fluid-inclusion Th values often

provide a record of decreasing thermal conditions since the

Figure 9.—Continued.

Figure 9.—Continued.

Figure 9.—Continued.
GEOTHERMAL EXPLORATION 19

Table 5.—Electron microprobe analyses of phenocryst (ph) and groundmass (gr) plagioclase crystals from USGS-N2 drill

core.

host crystals formed. In a few drill holes, the maximum Th

values exceeded the reference boiling-point curve, and

Taguchi and Hayashi (1982) and Taguchi and others (1984)

indicate that, at the time the fluid inclusions formed, this curve

must have been much higher than the current reference boiling-point curve and water table. An extreme example is given

by Muramatsu (1984) where the observed Th values greatly

exceed the reference boiling-point curve, and calculations

indicate the erosion of about 900 m of overburden since the

fluid inclusions formed.

Minimum fluid-inclusion Th values for the lower part of

drill hole USGS-N2 generally are close to the present measured temperature curve (fig. 11). However, the maximum Th

values (as much as 367 ̊C at 931-m depth) greatly exceed the

theoretical reference boiling-point curve drawn to the present

ground surface as shown in figure 11. Except for one quartz

sample at 929-m depth, these fluid inclusions uniformly are

liquid rich, without any vapor-rich coeval fluid inclusions.

The single quartz crystal from 929-m depth has several

monophase-vapor fluid inclusions along healed fractures and

a few vapor-rich inclusions that appear to be associated with

monophase-liquid fluid inclusions (fig. 10B), in addition to

abundant liquid-rich inclusions. These coeval(?) monophaseliquid and vapor-rich fluid inclusions may have formed due

to necking down rather than boiling conditions (Bodnar,

Reynolds, and Kuehn, 1985). Necking down is a process

whereby quartz is dissolved and redeposited, changing the

shape of the fluid inclusions over a period of time. During

this process, large fluid inclusions can be divided into several smaller inclusions; some of these smaller fluid inclusions

may contain only a single phase (vapor or liquid) whereas

others contain both vapor and liquid with variable vapor-toliquid ratios as discussed in Roedder (1984). Fluid inclusions

that have necked down can often be recognized by thin tubelike connections between fluid inclusions (fig. 10C) or fluid

inclusions with short tails (fig. 10D). If the initial Th mea-
20 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Table 6.—Fluid-inclusion heating/freezing data for Newberry volcano drill-core samples.
GEOTHERMAL EXPLORATION 21

Figure 10.—Fluid inclusions in hydrothermal quartz crystals from lower part of USGS-N2 drill core. A, Liquid-rich fluid inclusions

from 928.7-m depth. B, Coeval(?) liquid-rich (lr), monophase-vapor (v), vapor-rich (vr), and monophase-liquid (l) fluid inclusions

from 928.7-m depth. C, Liquid-rich fluid inclusion that is almost completely necked down, from 800.9-m depth. D, Liquid-rich fluid

inclusions plus vapor-rich fluid inclusion from 928.7-m depth; tails are characteristic of necking down.
22 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Figure 11.—Fluid-inclusion homogenization temperature (Th) values for quartz (shaded histograms) and calcite (unshaded histogram) specimens from lower part of USGS-N2 drill core within

Newberry caldera. Data are from table 6. Baseline for histograms

is sample depth. Dashed line shows a theoretical, reference boiling-point curve for pure water assuming hydrostatic conditions

controlled by an overlying column of cold water. Measured temperature data (solid line) from Sammel (1981).

surements that plot above the boiling-point curve were due

to boiling conditions, the data would be easy to explain. However, in order to unequivocally demonstrate boiling conditions it is necessary for coeval vapor-rich and liquid-rich fluid

inclusions to homogenize at the same temperature (Roedder,

1984); no such evidence was observed in this study.

Adjusting the boiling-point curve in order to account for

the highest Th in drill core from USGS-N2 would require about

1,140 m of erosion. If a mean Th value (318 ̊C for one set of

fluid inclusions in table 6), at 931-m depth, is utilized, the

calculations become much more reasonable and removal of

only about 160 m of overburden would be required. Williams

(1935) indicated, that while fluvial and glacial erosion undoubtedly occurred at Newberry, the effects of these two

agents are fairly minimal; erosion by glaciers and water probably would not account for removal of even 160 m of overburden. Lack of glacial moraines at Newberry (Williams,

1935; MacLeod and others, 1995) also suggests that glacial

ice cover probably was not extensive and would not allow

raising the theoretical boiling-point curve enough to account

for the high Th (Bargar and Fournier, 1988).

Alternatively, Th exceeding the present reference boilingpoint curve might result from localized high pressure due to

bounding impermeable lava layers. Many fractures in the

lower part of the USGS-N2 drill core are completely filled,

and local pressures might have been closer to lithostatic rather

than hydrostatic at the time these fluid inclusions formed.

However, a preferred explanation is that at the time the fluid

inclusions formed, the caldera of Newberry volcano was filled

by a deep intracaldera lake. The presence of a lake is indicated by lacustrine sediments from 290- to 320-m depth in

the USGS-N2 drill core (MacLeod and others, 1995). A depth

of 160 m for this previous intracaldera lake is reasonable considering that the present-day Paulina Lake is 80 m deep, and

it may have been at least 60 m deeper in past times (MacLeod

and others, 1995). Another Cascade intracaldera lake at Mount

Mazama (Crater Lake), Oregon, presently is 622 m deep

(Nelson, Carlson, and Bacon, 1988).

Mineralogical and textural data for USGS-N2 drill core

indicate that temperatures higher than about 265 ̊C could not

have persisted for a long time. If temperatures had been as

hot as 318 ̊C (mean Th at 931-m depth in USGS-N2) for very

long, chlorite would be deposited instead of mixed-layer chlorite-smectite, epidote would probably be more abundant, and

actinolite and (or) prehnite would probably be present (Bird

and others, 1984). In addition, oxygen isotope data for quartz

and calcite from the USGS-N2 drill core do not support such

high temperatures (Carothers, Mariner, and Keith, 1987). If a

deep intracaldera lake was present within Newberry caldera,

subsequent volcanic activity probably caused the lake to be

at least partially drained so that higher temperature owing to

the high pressure of the lake water was relatively short-lived.

RDO-1

During the fall of 1983, a second intracaldera drill hole

(RDO-1), located about 0.5 km southeast of USGS N-2 at

1,969-m elevation (figs. 2 and 3), was rotary drilled to a depth

of 424 m by Sandia National Laboratories (Keith and others,

1986). This hole penetrated a shallow hot-water aquifer with

a temperature exceeding 158 ̊C between depths of 379 and

397 m (Black, Priest, and Woller, 1984). On the basis of drill

cuttings, Keith and others (1986) determined that the same

hydrothermal minerals found in USGS N-2 were deposited at

shallower depths in drill hole RDO-1. This suggests that past

temperatures also were greater at higher levels in drill hole

RDO-1, which is closer to the ring fractures in the southern

part of Newberry caldera (fig. 3). Modeling studies by Sammel

(1983) suggest that the thermal water reached near-surface

strata or a few localized surface hot springs by movement

along ring fractures or vents for Holocene extrusive rocks.

LITHOLOGY

Drill cuttings from RDO-1 were studied by M.W. Gannett

and A.F. Waibel (unpub. data, 1983) and Keith and others
23

(1986). The upper 20.7 m of drill cuttings consist of white to

tan pumice with some clasts of lava and welded tuff. From

20.7- to 39.6-m depth, the drill cuttings are mostly basaltic

vesicular-glass lapilli in a palagonite matrix. A thin layer of

reworked epiclastic volcanic material occurs at 39.6- to 41.2-

m depth. A thick obsidian flow lies between 41.5- and 111.3-

m depth. From 111.3- to 307.9-m depth, the RDO-1 cuttings

consist of basaltic vesicular-glass lapilli with clinopyroxene,

plagioclase, and minor orthopyroxene crystals, as well as lithic

inclusions in the tuff. Clayey to sandy volcanic sediments

occur between depths of 307.9 m and 328.6 m; primary and

devitrification minerals identified by XRD include plagioclase, K-feldspar, and cristobalite. From 328.6- to 424.0-m

depth are lithic and pumiceous tuffs that M.W. Gannett and

A.F. Waibel (unpub. data, 1983) subdivided into six units on

the basis of differences in the amount of lithic and pumiceous

material in the tuffs. Most X-rayed samples contain primary

plagioclase and (devitrification?) K-feldspar and cristobalite.

A good correlation can be made between rocks of Sandia

National Laboratories drill hole RDO-1 and the upper part of

the USGS N-2 drill core (Keith and others, 1986) (fig. 4).

HYDROTHERMAL ALTERATION

Above 285-m depth in the RDO-1 drill cuttings, the only

secondary minerals Keith and others (1986) found appear

to be deuteric and consist of minor colorless to white amorphous silica and thin greenish clay vesicle coatings. They

also identified traces of smectite and pyrite between depths

of 285 m and 307.9 m. In the lower part of the drill hole,

hydrothermal alteration minerals (smectite, chlorite, quartz,

mordenite, and pyrite) occur both as replacement of groundmass and phenocrysts and as open-space deposits in fractures, vesicles, and intergranular pore spaces; calcite,

pyrrhotite, and small amounts of analcime, aragonite, siderite, rhodochrosite, and hematite are open-space fillings

(Keith and others, 1986) (fig. 12).

Keith and others (1986) indicated that hydrothermal

alteration is most intense between depths of 379.5 m and

397 m. Iron sulfide minerals appear to be early deposits

along with smectite; mordenite, calcite, quartz, and chlorite were deposited later, although some of these minerals

may be codepositional (Keith and others, 1986).

FLUID INCLUSIONS

Drill cuttings from depths of 335 m and 381 m in RDO1 contain small (<1 mm) broken quartz crystals containing

fluid inclusions. The fluid inclusions formed along healed

fractures, but it is uncertain if they are secondary or

pseudosecondary because no complete crystals were located. The inclusions primarily are liquid rich with vapor

bubbles forming about 5 to 10 percent of the volume. The

quartz crystal from 335-m depth also contains some

monophase-vapor inclusions and closely associated

monophase-liquid inclusions; these single-phase fluid inclusions may have formed due to necking down as described

by Roedder (1984). The monophase inclusions are in close

proximity to the liquid-rich fluid inclusions used in this

study. However, the monophase inclusions are more equant,

GEOTHERMAL EXPLORATION

Figure 12.—Distribution of hydrothermal alteration minerals with

depth in drill hole RDO-1 (after Keith and others, 1986). Left column shows lithologic log for drill chips. Temperature data (Black

and others, 1984) shown as points connected by a solid line. Most

common mineral occurrence in parentheses: O, open-space precipitate; R, replacement of glass or primary minerals.
24 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

are faceted, and approach a negative crystal appearance

unlike the apparently unrelated set of elongate, flattened,

liquid-rich inclusions for which Th and Tm measurements

were obtained.

Melting-point temperatures were not measured for the

quartz crystal at 335-m depth because the small fragment was

polished only on one side and the clarity within the crystal is

not sufficient for Tm measurements. In addition, the fluid inclusions were mostly less than 10 μm in size, and the exact

temperature at which the last ice melted could not be seen.

Tm values for fluid inclusions from the two quartz samples

from 381-m depth are -0.4 and -0.7 ̊C, which corresponds to

fluid salinities of about 0.7 and 1.2 weight percent NaCl

equivalent, respectively.

Homogenization temperature measurements for 39 liquidrich fluid inclusions in the three hydrothermal quartz crystals

ranged from 210 to 252 ̊C and averaged about 238 ̊C (table

6). These temperatures greatly exceed the highest measured

temperature of more than 158 ̊C for the RDO-1 drill hole

(Black, Priest, and Woller, 1984) and indicate that past temperatures were considerably warmer than these measured temperatures. In fact, about half of the Th values of fluid inclusions from 335-m depth plot above a reference boiling-point

curve drawn to the present ground surface (fig. 13). The presence of monophase-vapor fluid inclusions and liquid-rich fluid

inclusions are not thought to suggest boiling conditions because the two types of fluid inclusions do not appear to be

coeval. Th values reported in table 6 are for liquid-rich fluid

inclusions with nearly constant vapor-to-liquid ratios. If boiling conditions did not exist when these fluid inclusions

formed, the theoretical reference boiling-point curve at the

time the fluid inclusions formed must have originated at a

somewhat higher elevation (water table) than at present because of a deep intracaldera lake (as discussed above for

USGS-N2 fluid inclusions). Two quartz crystal fragments

from 381-m depth contain only liquid-rich fluid inclusions

with nearly constant vapor-to-liquid ratios, and Th values all

plot at slightly lower temperatures than the present reference

boiling-point curve (fig. 13).

DRILL HOLES OUTSIDE THE CALDERA

GEO-N1

In 1985, GEO-Newberry Crater, Inc. completed the GEON1 drill hole on a cost-sharing basis with the U.S. Department of Energy (Earth Science Laboratory, 1986, 1987). GEON1 was sited at an altitude of 1,780 m (Walkey and Swanberg,

1990) about 4.9 km south of the southern rim of Newberry

caldera (fig. 3) near the vent of a Holocene basalt flow

(Swanberg, Walkey and Combs, 1988) and within a soil mercury anomaly shown by Priest and others (1983). The hole

was rotary drilled to 148-m depth and cored to a depth of

1,387 m (Swanberg and Combs, 1986; core recovery was

about 95 percent (Earth Science Laboratory, 1986).

LITHOLOGY

A generalized stratigraphic description of the GEO-N1 drill

core, partly based on data provided by GEO-Newberry Crater, Inc. (Earth Science Laboratory, 1986), is given in appendix 2. The lithologic log of GEO-Newberry, Inc. (see Earth

Science Laboratory, 1986) contains detailed lithology notes

and tentative rock nomenclature that generally will be followed in this report. Modifications of the descriptions are

based on chemical analyses, our observations of the entire

split of drill core housed in the EGI core library, and additional drill-core samples collected at the GEO-Newberry Crater, Inc., core-storage facility in Bend, Oregon. Several variations of a stratigraphic column for the GEO-N1 drill core

were reported previously in Bargar and Keith (1986),

Swanberg and Combs (1986), Wright and Nielson (1986),

and Swanberg, Walkey, and Combs (1988).

The GEO-N1 drill core consists mostly of lava flows; associated flow breccia, ash-fall, and ash-flow material occur

between lava flows (appendix 2). Most lava flows in the upper ~1,125 m of drill core are basalts and basaltic andesites

(fig. 14). Below that depth the analyzed flows (table 7) are

basaltic andesites, andesites, or rhyodacites. Primary minerals in the lava flows vary with the chemical composition of

the lava but predominantly consist of plagioclase with varying amounts of olivine, clinopyroxene, orthopyroxene, and

magnetite. Devitrification cristobalite is identified in many

XRD analyses of the drill-core samples. Several shallow lava

flows contain vapor-phase tridymite, alkali-feldspar(?), and

magnetite that has altered to hematite. Groundmass K-feldspar along with trace amounts of primary quartz were identiFigure 13.—Fluid-inclusion Th values for quartz specimens from

RDO-1 drill hole within Newberry caldera. Explanation is the

same as for figure 11. Measured temperature data from Black,

Priest and Woller (1984).
GEOTHERMAL EXPLORATION 25

Figure 14. —Distribution of hydrothermal alteration minerals with depth in geothermal

drill core GEO-N1 on south flank of Newberry volcano. Left column shows generalized

stratigraphic section of rock units in drill hole: Basaltic-to-andesitic (<62 weight percent

SiO2) intervals are unpatterned, and dacitic to rhyolitic intervals (>62 weight percent

SiO2) have wavy pattern. Right column shows distribution of samples studied. Solid curve

shows measured temperature with depth in drill hole (after Swanberg and Combs, 1986;

Swanberg, Walkey, and Combs, 1988; Walkey and Swanberg, 1990; Wright and Nielson,

1986).
26 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

fied in some XRD analyses of the silicic lava flows. GEON1 lava flows commonly are vesicular at the top and bottom,

dense in the interior, and have intervening fractured intervals

consisting of steeply dipping tight fractures. Ash and cinder

layers and lithic tuffs from the upper part of the drill hole

appear to have good permeability where unaltered. At deeper

intervals, below 830-m depth, ash-flow tuffs are pervasively

altered to smectite and the present permeability is presumably much lower. K-Ar ages for ten selected rocks from the

GEO-N1 drill hole range from 0.027 Ma (late Pleistocene)

near the top of the drill hole to 1.63 Ma (approximately the

late Pliocene-early Pleistocene boundary) in the lower part

of the drill hole (Swanberg, Walkey, and Combs, 1988). In

addition, a 14C date of 5,835±195 years B.P. was obtained for

charcoal collected from a shallow surface excavation near

the top of the GEO-N1 drill hole (Swanberg and Combs, 1986;

Swanberg, Walkey, and Combs, 1988).

HYDROTHERMAL ALTERATION

A maximum temperature of about 90 ̊C at the bottom of

the GEO-N1 drill hole was reported by Walkey and Swanberg

(1990). Hydrothermal alteration minerals identified from the

drill core are consistent with these low temperatures. Although

the temperature gradient begins to increase at about 1,000-m

depth, there are no changes in hydrothermal mineralogy at

that depth. Twelve moderate to steeply dipping basaltic dikes

(as much as an apparent core thickness of about 12 m) between the depths of 622 m and 719 m indicate the presence

of a transient heat source during the time the dikes intruded

the volcanic pile. The dikes occur near the upper stratigraphic

limit of the formation of most hydrothermal alteration minerals in the GEO-N1 drill core. Chilled margins are preserved

for some of the dikes, but there is no evidence of significant

alteration directly adjacent to the contacts.

The earliest deposited secondary minerals in drill hole

GEO-N1 are high-temperature vapor-phase tridymite and

vapor-phase or deuteric(?) magnetite and hematite, which

formed during cooling of the lava flows and pyroclastic intervals. A few scattered occurrences of ~10Å illite in the upper half of the drill core probably formed during cooling of

the volcanic deposits and also may be deuteric. During latestage cooling and prehydrothermal circulation of meteoric

waters, amorphous iron hydroxides or iron oxides and amorphous clay-like deposits were precipitated.

Table 7.—Chemical analyses for drill-core samples outside Newberry caldera.

[Major-element chemical analyses by X-ray spectroscopy; analyst for most samples: T. Frost, U.S. Geological Survey, Denver,

Colo. Samples denoted by * analyzed by D.F. Siems and J.E. Taggart, U.S. Geological Survey, Denver, Colo. FeO, H2O+,

H2O-, and CO2 determined by conventional rock analysis methods; analyst: S.T. Pribble, U.S. Geological Survey, Menlo Park,

Calif. Volatile free SiO2 values calculated by normalizing to 100 percent after subtracting LOI or H2O+, H2O-, and CO2.

LOI, loss on ignition. —, not determined]
27

Hydrothermal smectite and carbonate minerals were

deposited later than vapor-phase minerals. Smectite alteration

of the pyroclastic layers occurred prior to carbonate

deposition. Most open-space smectite also formed earlier than

the carbonate minerals; however, locally, smectite was

deposited later than carbonate minerals (fig. 15). The

numerous carbonate phases are not confined to discrete zones.

Instead, the minerals vary from fracture to fracture. Such

abrupt changes are especially true for calcite and kutnohorite.

The fluids from which these minerals were deposited appear

to have varied somewhat in chemical composition in adjacent

fractures. It is likely that fluctuations in cation content (Mg,

Ca, Mn, and Fe) occur over a period of time because as many

as four different carbonate minerals (aragonite, siderite,

calcite, and dolomite) were deposited in a single open-space

filling. Aragonite is always the last carbonate mineral to be

deposited, but, locally, it appears to have been converted to

the more stable calcite phase. Some replacement of

plagioclase by calcite occurs near 1,200-m depth.

Silica minerals, zeolite minerals, and okenite all formed

later than some smectite; only one sample was found that

contains early cristobalite in association with later smectite,

siderite, and calcite. Pyrite formed later than hydrothermal

hematite in the single vein occurrence, and it formed later

than smectite but earlier than carbonate minerals in the

disseminated occurrences.

The hydrothermal minerals in drill core GEO-N1 compose

a mineral suite that is similar to the minerals occurring at

temperatures of less than 100 ̊C in the upper 650 m of the

USGS-N2 drill core. In both drill holes, hydrothermal silica,

zeolite, carbonate, and clay minerals were deposited from

migrating fluids, mostly in open spaces of vugs, fractures, and

voids in flow breccias. Permeable ash-flow tuff and lithic tuff

locally display more extensive alteration of glass to smectite.

GEO-N2

Drill hole GEO-N2 was spudded at an elevation of 1,779

m on the west flank of Newberry volcano about 2.8 km outside the western rim of the caldera (fig. 3). The drill site apparently was chosen on the basis of a geophysical anomaly;

the sites for most of the west flank drill holes at Newberry

volcano were selected because of electromagnetic or surface

resistivity anomalies (Walkey and Swanberg, 1990). A temperature profile for the geothermal well (drilled by GEONewberry Crater, Inc.) shows a bottom-hole temperature of

167 ̊C (Walkey and Swanberg, 1990). The SiO2 content of

GEOTHERMAL EXPLORATION

Table 7.—Chemical analyses for drill-core samples outside Newberry caldera—Continued.
28 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

sampled fluids increases substantially near the bottom of the

GEO-N2 drill hole, which Walkey and Swanberg (1990) conclude is due to the influx of thermal fluids.

LITHOLOGY

Lithologic descriptions and chemical data for the GEON2 drill core have not been published by GEO-Newberry

Crater, Inc. Drill-core samples obtained for this study emphasize secondary alteration and are not sufficient to provide more than a very generalized description of the

stratigraphy of the GEO-N2 drill core; therefore, a detailed

stratigraphic column for the drill hole is not included in

this report. The core samples consist of basaltic to rhyolitic

lava flows with intervening flow breccia, lithic tuff, and

volcanic sandstone. Breccias produced by hydrofracturing

and tectonic processes are also present. At least one thin

basaltic dike intrudes a vesicular mafic lava flow at 1,186-

m depth. Some of the collected samples display flow banding and vapor-phase partings; many samples are fractured

or contain spherical to elliptical vesicles. Plagioclase is

the dominant primary mineral; some magnetite and

clinopyroxene(?) are present, and groundmass quartz and

K-feldspar occur in the silicic lava flows. In most samples,

glass is devitrified to cristobalite. Some lava flows also

contain concentrations of tridymite, K-feldspar, and magnetite as vapor-phase minerals. Spaces between breccia fragments, vesicles, and fractures are usually partly to completely filled by hydrothermal alteration minerals.

HYDROTHERMAL ALTERATION

One of the earliest secondary minerals in the GEO-N2 drill

core appears to be native copper (fig. 16) which occurs in

vesicles of one sample from 1,186-m depth; presumably, the

copper was deposited as a vapor-phase mineral during cooling of the basalt dike host. Octahedral, vapor-phase magnetite

crystals that formed in some vesicles at 1,180.2-m depth are

partly covered by later mixed-layer chlorite-smectite deposits.

Various colors of smectite (white, bluish, green, orange,

brown, black) are deposited in vesicles, on fractures, and between breccia fragments of core from this drill hole; smectite

also occurs as altered glass in some lithic tuffs from the GEON2 drill core. Smectite was deposited in several generations

and can be seen to be both earlier and later than cristobalite

in a sample at 679.1-m depth. Texture of the smectite deposits varies from waxy to fibrous. Other open-space clay minerals in this drill core include light- to dark-green mixed-layer

chlorite-smectite which also can be fibrous or waxy in texture. A few green clay vesicle-filling samples show no expansion in XRD analyses following glycolation and are chlorite.

Siderite is the earliest carbonate mineral in the GEO-N2

drill core. Some siderite forms tiny caramel-colored rhombic

crystals or columnar stacks of rhombic crystals, but most commonly it occurs as hemispheres that may be either earlier or

later than smectite. In some places, similar-appearing altered(?) hemispheres consist of minerals such as smectite,

mixed-layer chlorite-smectite, hematite, calcite, magnesite,

or ankerite-dolomite. Rhodochrosite was detected along with

ankerite-dolomite on two XRD analyses of a carbonate patch

at 224.9-m depth and with bladed calcite(?) that is replaced

by quartz at 1,031.7-m depth. Open-space bladed, blocky, and

needle-like calcite crystals were also observed, and some of

them formed later than smectite and chalcedony.

Colorless, white or bluish botryoidal vesicle fillings of

some GEO-N2 samples were identified as cristobalite in XRD

analyses, and many of these deposits formed later than siderite. XRD analyses show that other similar siliceous botryoidal

open-space deposits consist of chalcedony. Colorless euhedral

quartz crystals formed later than the other silica deposits and

are locally associated with earlier smectite or ankerite-dolomite.

Bladed pyrrhotite and green mixed-layer chlorite-smectite

in the groundmass of the drill core from 1,104.6-m depth appear to have formed from altered mafic minerals. The pyrrhotite itself is partly oxidized. Fractures and vesicles of a

few samples near the bottom of the drill hole are coated by

crystals of pyrite, some of which are slightly oxidized at

1,278.3-m depth.

Figure 15.—Generalized paragenetic sequence of hydrothermal alteration minerals deposited in flank drill cores of Newberry volcano. Queried where very uncertain.
GEOTHERMAL EXPLORATION 29

Figure 16.—Distribution of hydrothermal alteration minerals with depth in geothermal drill hole

GEO-N2 on west flank of Newberry volcano. Explanation is same as for figure 14. Measured temperature data from Walkey and Swanberg (1990).
30 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Colorless, tabular, anhydrite crystals were deposited

in vesicles and fractures at depths of 1,128.4 to 1,131.4 m

and at 1,323.1-m depth. Hematite or amorphous iron oxide deposits in the groundmass of some samples stain vapor-phase crystals and sulfides owing to alteration of mafic crystals. At 1,122.3-m depth, a brick-red iron oxide

fracture deposit formed later than smectite and an unidentified carbonate mineral.

FLUID INCLUSIONS

Double-polished thick sections for four open-space deposits of hydrothermal quartz and calcite from the GEO-N2 drill

core contain monophase-liquid and liquid-rich fluid inclusions having a vapor phase consisting of only about 1 to 2

percent. The fluid inclusions usually occur along healed fractures and appear to be secondary inclusions.

Melting-point temperatures for most of the specimens

either could not be determined or were very erratic; Tm values range between -0.3 and +5.8 ̊C. In many of the fluid

inclusions, the vapor bubble disappeared during freezing and

only reappeared following melting of the ice, which suggests the presence of metastable fluids (Roedder, 1984). One

quartz sample from 1,223-m depth yielded consistent Tm

values of -0.1 ̊C, which indicates that the salinity of this fluid

is about 0.2 weight percent NaCl equivalent (Potter, Clynne,

and Brown, 1978).

Only 61 Th measurements were obtained from the 6 mineral chips that were heated. Minimum Th values for three of

the sample depths plot very close to the measured temperature curve given in Walkey and Swanberg (1990) (fig. 17).

Two Th measurements from the lowermost sample plot as

much as 11 ̊C lower than the measured temperature curve,

which suggests that past temperatures may have been somewhat lower at this depth. Maximum Th measurements indicate that the fluid in this drill hole previously has been in the

range of about 20 to 40 ̊C warmer than the present measured

temperatures; a single Th value of 195 ̊C at 925-m depth may

be significant, although additional data would be necessary

before reaching any conclusions regarding this value.

GEO-N3

Drill hole GEO-N3, located at an elevation of 1,753 m

about 9.7 km north of the northern rim of Newberry caldera,

was completed in 1986 (Swanberg, Walkey, and Combs, 1988;

Earth Science Laboratory, 1987). The hole was drilled by the

rotary method to 138-m depth and cored by wireline method

to a total depth of 1,220 m; core recovery ranged from 50 to

95 percent (Earth Science Laboratory, 1987). A maximum

temperature of 57 ̊C was recorded at the bottom of the drill

hole (Earth Science Laboratory, 1987).

LITHOLOGY

Drill core from the upper half of the GEO-N3 hole was

not sampled for this study because the core specimens appeared to be devoid of secondary mineralization (fig. 18).

According to W.C. Walkey (written commun., 1987), the upper part of the drill core consists largely of basaltic to basaltic

andesite lava flows with some glassy obsidian flows, interflow

breccia, air-fall tuff, ash-flow tuff, scoria, and a few thin dikes.

Most of the lower half of the drill core consists of basaltic to

basaltic andesite lava flows, scoria, and interbedded ash-flow

tuff, although more silicic flows and glassy interflow breccias occur from about 974- to 1,137-m depth. The bottom of

the GEO-N3 drill core is composed of basaltic to basaltic

andesite lava flows, scoria, and interbedded ash-flow tuff.

Figure 17.—Fluid-inclusion Th values for quartz and calcite specimens from GEO-N2 drill core. Explanation is same as for figure 11.

Measured temperature data from Walkey and Swanberg (1990).
GEOTHERMAL EXPLORATION 31

Figure 18.—Distribution of hydrothermal alteration minerals with depth in GEO-N3 drill core on

north side of Newberry volcano. Explanation is same as for figure 14. Measured temperature data from

Swanberg, Walkey, and Combs (1988), and Walkey and Swanberg (1990).
32 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Swanberg, Walkey, and Combs (1988) reported K/Ar ages

for seven GEO-N3 drill core samples that range between about

0.109 Ma and 1.54 Ma.

Primary mineralogy of the mafic lava flows is dominantly

plagioclase, with some clinopyroxene and magnetite. Devitrification cristobalite was identified by XRD analyses of several whole-rock samples. The more silicic lava flows contain

K-feldspar and vapor-phase tridymite in addition to plagioclase and magnetite. Some magnetite appears to be of secondary origin (vapor-phase?), although much of these deposits has been altered to hematite. Textures of the more silicic

lava flows tend to be glassy, dense, or flow banded, while the

mafic flows are more open with diktytaxitic, vesicular, or scoriaceous textures. The denser lava flows typically are unaltered; however, where the flows are vesicular, are scoriaceous,

or contain vertical to horizontally oriented fractures, the open

spaces are partly to completely filled by secondary minerals.

HYDROTHERMAL ALTERATION

Secondary mineralization in the GEO-N3 drill core consists mostly of deposition in vesicles, fractures, and open

spaces between breccia fragments. Black sooty, clayey, or

botryoidal magnetite in fractures and vesicles was deposited

earlier than other secondary minerals and probably is of

deuteric origin. Some magnetite open-space fillings are partly

altered to hydrothermal(?) hematite. In addition, hematite

stains several scoria units, and tuff beds and was deposited as

hexagonal vapor-phase crystals. The only other iron-oxide

mineral in this drill core is minor goethite, which probably is

a low-temperature hydrothermal deposit.

Three samples contain illite or mixed-layer illite-smectite

in association with the iron-oxide minerals. The clay minerals appear to be authigenic and do not result from drill mud

contamination. One mixed-layer clay sample at 1,219.0-m

depth appears to be associated with cooling of a 5-mm-wide

glassy dike(?) that intruded the lava flow. The two other clay

deposits may have a similar higher temperature origin.

Colorless, botryoidal cristobalite occurs in two vesicle

fillings. Sulfate minerals (jarosite, natrojarosite, and

natroalunite), deposited in a few vesicles, fractures, and open

spaces of scoria, suggest acid-sulfate hydrothermal conditions.

Kaolinite in a vesicle filling at 1,148.0-m depth is compatible with formation in an acid environment.

Pyrite crystals appear to be closely associated with siderite and smectite, although the order of deposition for these

three minerals may not always be the same. Caramel-colored hemispherical crystal aggregates of siderite sometimes

have a black magnetite core. Some other caramel-colored

siderite hemispheres, observed in cross sections cut parallel to the planar view, consist of alternating concentric rings

of caramel and white- or cream-colored calcite or

manganoan calcite. XRD analysis of one siderite sample

at 1,211.6-m depth shows the presence of magnesite and

ankerite-dolomite (or possibly kutnohorite). Colorless

acicular aragonite crystals were deposited later than other

secondary minerals in open spaces of the GEO-N3 drill

core.

GEO-N4

GEO-Newberry Crater, Inc. completed a single core hole

(GEO-N4) at a surface elevation of 1,905 m 3.2 km outside

the caldera rim on the east flank of Newberry volcano

(Walkey and Swanberg, 1990). The maximum bottom-hole

temperature of this drill hole is about 30 ̊C at a depth of

703 m (Walkey and Swanberg, 1990).

LITHOLOGY

Except for two dacitic lava flows, the drill core consists of basalt, basaltic andesite, and andesite lava flows,

lithic- to vitric-pumiceous tuff, scoria, and volcaniclastic

debris flows (C.A. Swanberg and W.C. Walkey, written

commun., 1990). Plagioclase and magnetite are the dominant primary minerals; minute quartz peaks are present

in a few XRD analyses. Pyroxene probably is present, but

no peaks were observed in XRD. Vapor-phase tridymite

and devitrification cristobalite occur in several XRD

analyses.

HYDROTHERMAL ALTERATION

The GEO-N4 drill core samples contain only minor

secondary alteration (fig. 19). Glassy tuffs are mostly fresh,

and fractures and vesicles are only partly filled. Initial

secondary mineralization of the GEO-N4 drill core consists of open-space deposits of vapor-phase minerals, such

as tridymite and hexagonal hematite crystals. Amorphous

iron oxide (deuteric?) stains several tuffs, cinders, scoria,

and other volcanic rocks in the drill core. The earliest hydrothermal minerals appear to be light-yellow to dark-caramel-colored spherical and hemispherical clusters of siderite. Some of the spheres are broken and show a sequence

of concentric shells consisting of alternating siderite and

calcite (reacts with HCl). Smectite was mostly deposited

later than siderite, but occasional smectite deposits are

earlier, which suggests that one or possibly both of the minerals formed during two generations. Colorless acicular

aragonite crystals were deposited later than other associated hydrothermal minerals.

GEO-N5

Drill hole GEO-N5 is located on the southwest flank of

Newberry Volcano 3.9 km outside the caldera rim at an elevation of 1,731 m (Walkey and Swanberg, 1990). The drill

hole was completed to 988-m depth and had a bottom-hole

temperature near 80 ̊C (Walkey and Swanberg, 1990).
GEOTHERMAL EXPLORATION 33

Figure 19.—Distribution of hydrothermal alteration minerals with

depth in GEO-N4 drill core on northeast side of Newberry volcano.

Explanation is same as for figure 14. Measured temperature data

from Walkey and Swanberg (1990).

LITHOLOGY

The GEO-N5 drill core consists of nearly equal proportions of silicic (dacitic to rhyolitic) (table 7) and mafic (basaltic to andesitic) (fig. 20) lava flows, obsidian flows, flow

breccias, and obsidian breccias, intercalated lithic-vitric tuff,

scoria, debris flows, palagonitic soils, air-fall tuff, and a few

dikes (C.A. Swanberg and W.C. Walkey, written commun.,

1990). Many of the samples collected for this study are flow

banded or vesicular. Glass frequently is devitrified to

cristobalite or is hydrothermally altered, but several samples

contain unaltered glass. XRD analyses of mafic whole-rock

samples show plagioclase and cristobalite, whereas in the silicic rocks K-feldspar and cristobalite were identified in most

analyzed samples. Silicic rocks also contain vapor-phase mineralization consisting of tridymite and alkali-feldspar and

quartz (granophyric?). Open spaces between breccia fragments, vesicles, and fractures contain small amounts of hydrothermal alteration minerals.

HYDROTHERMAL ALTERATION

A few intervals in the GEO-N5 drill core contain

subhorizontal cavities in which vapor-phase crystals of

tridymite, alkali feldspar, cristobalite, and magnetite (mostly

altered to hematite or amorphous iron oxide) were formed

during cooling of the lava flows. Between depths of about

622 m and 646 m, a silicic lava flow also has euhedral quartz

crystals lining the vapor-phase cavities; the quartz probably

is granophyric (Keith and Muffler, 1978) because it formed

before vapor-phase K-feldspar and tridymite.

Various colors (yellow, orange, brown, pink, red, or black)

of iron oxide staining are scattered throughout the GEO-N5

drill core. Red, iron-oxide stained cindery intervals or breccias marking the upper or lower parts of some of the lava

flows undoubtedly are deuteric and formed during cooling of

the lava flows. Most of these iron oxide deposits are amorphous in XRD analyses, although sporadic poorly crystalline

hematite was identified. Magnetite usually occurs as tiny primary groundmass crystals or vapor-phase deposits; however,

at 972.3-m depth vesicles are lined with black, spherical, or

hemispherical deposits of magnetite (partly altered to red iron

oxide) that also may be deuteric. XRD analyses show that

soft red clayey material lining a few vesicles and fractures

consists of both smectite and hematite and probably is of hydrothermal origin.

A thin carbonate vein at 789.4-m depth contains a few

black sphalerite crystals (fig. 21) that formed earlier than the

siderite and calcite vein filling. In the upper half of the drill

core, virtually all of the obsidian and ash glass is unaltered.

Much glass in the lower half of the drill core remains unaltered, but some tuffaceous beds are altered to smectite

(halloysite in lithic tuff at 583.1-m depth). Blue-green mixedlayer illite-smectite with disseminated pyrite crystals was

deposited on vapor-phase minerals in a cavity at 785.2-m

depth. Pyrite is frequently deposited in fractures and vesicles

that also contain siderite. Brown, red, green, gray, white, and

orange smectite coats many fractures and vesicles and appears to have been deposited in more than one generation;

some open-space smectite is earlier than calcite, whereas

smectite coats calcite in other deposits. Both siderite and calcite may also have been deposited in more than one generation. In some samples calcite appears to form from siderite

with the release of iron oxide; however, in other samples siderite crystals are deposited on top of calcite. The two carbonate minerals commonly are deposited as hemispherical crystal clusters or concentric rings in tight fractures. Both minerals occur as rhombic crystals or massive deposits; siderite

may form disc-shaped crystal clusters, whereas in some occurrences calcite needles were deposited. Circular buff car-
34 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Figure 20.—Distribution of hydrothermal alteration minerals with depth in GEO-N5 drill core on southwest side of

Newberry volcano. Explanation is same as for figure 14. Measured temperature data from Walkey and Swanberg

(1990).
GEOTHERMAL EXPLORATION 35

Figure 21.—Scanning electron micrograph of fracture filling at

789.4-m depth in GEO-N5 drill core; contains a few sphalerite

crystals and later massive siderite.

Figure 22.—Scanning electron micrograph of botryoidal chalcedony open-space deposit with clusters of late quartz crystals (center), from 756.5-m depth in GEO-N5 drill core.

bonate deposits on a fracture at 576.7-m depth were identified as rhodochrosite in XRD analysis. Colorless, rhombic

crystals from two open-space deposits near the bottom of the

drill hole were identified as ankerite or dolomite by XRD

analysis. Vesicles in one silicic core sample from 756.5-m

depth are lined by light-gray botryoidal chalcedony and a few

later quartz crystals (fig. 22). Similar frosted botryoidal silicavesicle deposits, some of which are later than calcite, were

identified as cristobalite by XRD. Three zeolite minerals

(dachiardite, heulandite, and mordenite) appear to be late

deposits in fractures, vesicles, and open spaces between breccia fragments in the lower part of the GEO-N5 drill core;

mordenite, along with smectite, also replaces glass in tuffs

and breccias.

SF NC-01

Drill hole SF NC-01 was spudded in 1984 at an elevation

of 1,859 m about 2 km outside the western rim of Newberry

caldera by Santa Fe Geothermal, Inc. (previously known as

Occidental Geothermal, Inc.) (Arestad, Potter, and Stewart,

1988; Walkey and Swanberg, 1990). The drill hole was completed to 1,219-m depth and had a near-bottom-hole temperature of 170 ̊C (Arestad, Potter, and Stewart, 1988).

LITHOLOGY

The drill hole penetrated mafic (basalt and basaltic andesite) lava flows and cinders in the upper 183 m (Arestad, Potter, and Stewart, 1988). From 183- to 762-m depth, some

basaltic andesite and andesite lava flows and cinders are

present, but the volcanic ash and lithic tuff intervals, volcanic breccia, and dense and vesicular lava flows are mostly of

dacite and rhyodacite composition (Arestad, Potter, and Stewart, 1988). Below 762-m depth the drill core consists of a

sequence of andesitic, vesicular to dense lava flows, cinders,

and interflow breccias that are increasingly mafic in composition toward the bottom of the drill hole (Arestad, Potter,

and Stewart, 1988). XRD analyses show that plagioclase and

magnetite are the primary minerals in the mafic rocks; other

minerals undoubtedly are present, but were not detected by

XRD. Devitrification cristobalite was identified in XRD patterns for both mafic and silicic drill core samples. The silicic

rocks contain K-feldspar and quartz, in addition to plagioclase and magnetite. Concentrations of vapor-phase minerals

consist of euhedral tridymite and K-feldspar crystals that were

deposited within subhorizontal to subvertical linear cavities,

usually with some remaining open space.

HYDROTHERMAL ALTERATION

Unaltered glass matrix and obsidian clasts do occur in

poorly indurated tuffs from the upper part of the SF NC-01
36 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

drill hole; however, hydrothermal alteration of these deposits

is more extensive with depth. Open spaces of vapor-phase

partings, fractures, vesicles, and between breccia fragments

usually contain hydrothermal mineral deposits below about

200-m depth in the drill hole (fig. 23). One of the earliest

deposited minerals frequently is a clay that is dominantly

smectite throughout the upper 1,000 m of drill-core samples.

While smectite may be orange, brown, gray, white, or green,

the majority of smectite samples are green. Some open-space

fillings consist of two or more layers or generations of different-colored smectite deposits. Obsidian and pumice intervals

are mostly unaltered in the upper half of the drill core, but

glass in sampled ash layers from the bottom half of the drill

hole are altered to smectite. A few clay deposits between about

277-m and 290-m depth are amorphous in XRD analyses.

One of these clay deposits from a fracture at 277-m depth

contains tiny disseminated pyrite crystals and later disc-shaped

aggregates of siderite. Beneath this cream-colored amorphous

clay is colorless to dark-brown isotropic opal (refractive index <1.47), which also contains disseminated pyrite crystals

(fig. 24). Below 1,000-m depth, few smectite deposits were

seen in the drill core; however, the dominant clay mineral

there is green mixed-layer chlorite-smectite. Open spaces

between breccia fragments at 1,124.4-m depth are partly filled

by green botryoidal chlorite-smectite. One white clay vesicle

filling and one green clay fracture deposit consist of mixedlayer illite-smectite as seen in XRD analyses. Three green

clay deposits were identified as chlorite by XRD. At 1,120.4-

m depth, a wormy-looking cluster of green chlorite crystals

(similar to those in figure 45B) was deposited between breccia fragments.

Beneath clay layers in several fractured and vesicular drillcore samples are coatings of yellow-brown-orange-red amorphous iron oxide or red hematite. Massive vapor-phase magnetite in a vesicle at 1,191.2-m depth has red hematite staining, and the groundmass mafic crystals are altered to iron

oxide.

Scattered open-space fillings in the upper 1,000 m of the

drill core contain crystals or crystal aggregates (some hemispherical) of siderite that appears to have formed during two

generations. The earlier siderite deposit is sometimes caramel

colored and was followed by light-yellow siderite in a sample

from 758.0-m depth. A few samples such as the hemispherical

deposits at 617.5-m depth have an intervening smectite layer

and later light-yellow siderite. Some samples contain more than

one generation of calcite. Colorless rhombic calcite crystals

formed earlier than siderite at 758.0-m depth but were deposited later than siderite in several other samples. Vesicles in a

lava flow at 1,204.7-m depth are filled, in order of paragenesis, by (1) a botryoidal calcite layer, (2) green clay, (3) colorless calcite, (4) chalcedony, and (5) quartz crystals. Other drillcore samples contain blocky calcite crystals that were deposited later than quartz crystals. Calcite morphology includes

massive, bladed, blocky, and scalenohedral forms. Calcite (reacts with HCl) at least partly replaces plagioclase phenocrysts

near the bottom of the drill hole. Ankerite-dolomite, the only

other carbonate mineral in the SF NC-01 drill core, was identified by two XRD analyses of calcite-bearing samples.

A few botryoidal silica open-space deposits above 1,000-

m depth consist of cristobalite. Spherical clusters of

cristobalite crystals also coat calcite at 900.0-m depth. The

cristobalite open-space fillings appear either to be gradually

converted to chalcedony or to consist of alternating chalcedony and cristobalite layers; XRD analyses show that several

of the bluish-frosted, banded silica deposits consist of both

minerals. Colorless, euhedral, quartz crystals formed later than

many of the botryoidal silica deposits.

Two vesicular samples from just above 1,000-m depth

contain late deposits of white fibrous mordenite. The only

other zeolite mineral identified in this drill core is colorless,

fibrous to acicular laumontite, which occurs at 1,120.4-m

depth along with earlier wormy-looking green chlorite and

blocky calcite.

FLUID INCLUSIONS

Fluid inclusion data were obtained for nine sample chips

from five sample depths in the SF NC-01 drill hole. Two

bladed calcite samples from 900-m depth have abundant

monophase-liquid and liquid-rich fluid inclusions (about 1 to

2 percent vapor) that appear to have formed along crystal

growth planes parallel to the crystal faces and may be primary. Most other fluid inclusions in this drill hole appear to

have formed along healed fractures and are secondary. At

1,216-m depth, a quartz deposit may be later than fluid inclusions along a healed fracture, in which case the fluid inclusions would be pseudosecondary.

Only ten usable Tm measurements, ranging between -0.2

and 0.0 ̊C, were obtained for hydrothermal calcite and quartz

from depths of 900 m and 1,149 m, and the salinity of these

fluid inclusions is about 0 to 0.4 weight percent NaCl equivalent. The vapor phase of fluid inclusions from one hydrothermal quartz sample from 900-m depth all disappeared at about

-42 ̊C during freezing. In seven of the continuously monitored fluid inclusions, the vapor bubble suddenly reappeared

at temperatures between +1.7 and + 4.9 ̊C, but melting of the

last piece of ice was not observed; these fluids probably are

metastable and do not provide a reliable estimate of the fluid

salinity (Roedder, 1984). Melting-point temperatures were not

attempted for several samples because the fluid inclusions

were too small, because leakage occurred during heating, or

because the samples were too cloudy for accurate measurements.

Homogenization temperature measurements for 117 fluid

inclusions in quartz and calcite from five sample depths

straddle the measured temperature curve of Walkey and

Swanberg (1990) (fig. 25). From these measurements, it appears that the rocks sampled in this drill core have been subjected to past temperatures that ranged from as much as about

20 ̊C lower than the present measured temperatures to about

45 ̊C higher than the down-hole temperature measurements.
37

Figure 23.—Distribution of hydrothermal alteration minerals with depth in SF NC-01 geothermal drill

hole on west flank of Newberry volcano. Explanation is same as for figure 14. Measured temperature

data from Arestad, Potter, and Stewart (1988) and Walkey and Swanberg (1990).

GEOTHERMAL EXPLORATION
38 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

SF NC72-03

Santa Fe Geothermal, Inc., began drilling the SF NC72-03

hole at an altitude of 1,993 m, very near the caldera rim on the

west side of Newberry volcano (fig. 3) in 1983 and completed

it in 1984 (Arestad, Potter, and Stewart, 1988; Walkey and

Swanberg, 1990). A near-bottom temperature of 155 ̊C was

measured in the 1,372-m-deep drill hole and an average geothermal gradient of 137 ̊C/km is given (Arestad, Potter, and

Stewart, 1988).

LITHOLOGY

A generalized stratigraphic column for this drill hole shows

basaltic to rhyolitic rocks consisting of massive to very vesicular lava flows, pumice, lahars, vitrophyre, cinders, lithic

tuff, and interflow breccia (Arested, Potter, and Stewart, 1988).

The more siliceous volcanic rocks occur between about 160-

to 240-m depth and 550- to 700-m depth (fig. 26, table 7).

Several thin microdiorite sills(?) were encountered at depths

of 924 to 929 m, 1,141 m, 1,279 m, and 1,356 m. Plagioclase

is the dominant primary mineral; mafic minerals appear to

have been altered to clay and hematite, although magnetite is

seen in some whole-rock XRD analyses. Silica minerals (primary quartz, devitrification cristobalite, and vapor-phase

tridymite) are found in the silicic volcanic rocks.

HYDROTHERMAL ALTERATION

The shallowest sample collected for this study was from

about 480-m depth. Hydrothermal minerals were found lining vesicles and fractures and between breccia fragments of

virtually every sample examined below that depth. A few fractures and vesicles in the SF NC72-03 drill core contain small

amounts of massive to spherical secondary magnetite that

usually is at least partly altered to red hematite or amorphous

iron oxide. Red iron oxide is an early alteration mineral in

many open-space hydrothermal deposits in this drill core and

may consist of completely altered vapor-phase or deuteric

magnetite. Some red cindery interflow deposits are oxidized

to amorphous iron oxide, and groundmass mafic minerals in

some lava flows have red iron oxide staining. At 762-m depth,

a ~2-cm-thick reddish clay interval consists of smectite and

hematite as shown by XRD analysis. There is some evidence

for a later stage of red iron oxide alteration in a few botryoidal

samples, such as at 1,164.9-m depth where red iron oxide

appears to be later than both calcite and dark-green smectite.

Many fractures, vesicles, and open spaces between breccia fragments contain an early deposit of light- to dark-green,

Figure 24.—Opal with disseminated cubic, pyritohedral, and

rounded to irregular-shaped pyrite crystals (0.02 mm or less in size)

from 227.1-m depth in SF NC-01 drill core.

Figure 25.—Fluid-inclusion Th values for quartz and calcite from

SF NC-01 drill hole. Explanation is same as for figure 11. Measured temperature data from Arestad, Potter, and Stewart (1988)

and Walkey and Swanberg (1990).
39

m depth also is identified as a mixed-layer chlorite-smectite

by XRD analysis; a semiquantitative chemical analysis of the

white clay by EDS on the SEM shows that there is much

more Mg than Fe in the deposit.

Another early deposit in this drill core is siderite, which

occurs in open spaces of several samples between depths of

485.2 m and 1,187.2 m. Siderite occurs as individual tiny rhombic crystals, stacks of rhombic crystals, and disc-shaped or

hemispherical crystal aggregates and appears to have formed

in several generations. Often siderite is closely associated with

calcite; at 1,181.6-m depth, botryoidal carbonate deposits consist of alternating layers of white calcite and brown siderite.

Calcite, which occurs in most of the collected drill core samples,

is much more abundant than siderite. Calcite also formed during more than one episode; early calcite deposits tend to be

massive or botryoidal, whereas later generations may consist

of bladed, blocky, or scalenohedral crystals. The latest carbonate mineral to be deposited in open spaces of this drill core is

ankerite-dolomite, which usually occurs as tiny colorless rhombic crystals between depths of 762.2 m and 1,187.2 m.

White, bluish, frosted, or colorless, massive to botryoidal

silica deposits in vesicles and fractures below about 840-m

depth usually consist of chalcedony, although several X-ray

analyses also show the presence of cristobalite. Tiny colorless

euhedral quartz crystals generally are a late deposit; however, quartz is earlier than calcite in vesicles at 1,125.6-m

depth, and quartz crystals formed earlier than green clay and

calcite in a fracture at 1,356.1-m depth.

One vesicle at 1,131.0-m depth contains an early-deposited grain of native copper. At 1,351.5-m depth, a fracture

surface contains a few hexagonal, bladed, iron sulfide crystals that may be pyrrhotite(?). Small amounts of tiny pyrite

crystals are deposited in open spaces of a few drill-core

samples; sometimes, the minute crystals are disseminated in

clay or silica deposits. At the very bottom of the drill core,

open spaces between breccia fragments are filled by quartz,

calcite and late white prismatic laumontite crystals. A single

vesicle in this core sample also contains colorless acicular

crystals that might be mordenite(?).

FLUID INCLUSIONS

Fluid inclusions were located in five prepared double-polished thick sections of hydrothermal quartz and calcite crystals from open-space deposits of the SF NC72-03 drill core.

All of the fluid inclusions appear to be secondary; most are

liquid-rich inclusions, but the shallower samples contain

monophase-liquid fluid inclusions. Vapor-bubble volume varies from about 1 to 2 percent in the shallowest sample, to

about 5 to 10 percent in the intermediate-depth samples, and

to about 15 percent in the deeper samples.

Many of the fluid inclusions were too small for Tm

determinations. At 945-m depth, the vapor bubbles

disappeared during freezing and reappeared only after the

temperature was raised to +0.1 to +2.5 ̊C, indicating that the

Figure 26.—Distribution of hydrothermal alteration minerals with

depth in SF NC72-03 drill hole just outside western ring fracture of

Newberry volcano. Explanation is same as for figure 14. Measured

temperature data from Arestad, Potter, and Stewart (1988) and

Walkey and Swanberg (1990).

brown, red, orange, or white smectite. Several samples contain evidence for more than one generation of smectite deposition; at 766.0-m depth, vesicles are coated by three different colors of clay deposits. Similarly, a ~1.5-cm vesicle at

1,015.6-m depth is filled by green clay, later pyrite, a second

generation of green clay, and finally bluish chalcedony. A few

green clay deposits were identified in X-ray analyses as chlorite and mixed-layer chlorite-smectite. One vesicle-filling

white clay in a dark-red iron oxide-altered lava flow at 1,342.0-

GEOTHERMAL EXPLORATION
40 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

fluid is metastable (Roedder, 1984). A Tm value of -0.4 ̊C

(0.7 weight percent NaCl equivalent) was only observed for

a single fluid inclusion from 1,287-m depth. Melting-point

temperatures for the flank drill holes are too sparse to warrant

drawing any sweeping conclusions concerning the salinity

of the fluid from which the calcite and quartz crystals formed.

However, the available fluid-inclusion salinity data appear

to suggest a slight fluid salinity increase near the ring fracture

system on the west side of Newberry volcano. Fluid moving

up the fracture probably is slightly saline but becomes diluted

by mixing with meteoric water during lateral movement

outward from the caldera.

Ninety-six Th measurements were obtained from fluid

inclusions in eight quartz and calcite samples from five

depths in the SF NC72-03 drill core (fig. 27). The shallowest

sample, at 945-m depth, has minimum Th values that plot

very close to the measured temperature curve and range up

to about 15 ̊C higher. Nearly all of the remaining Th

measurements plot much higher than the temperatures

measured during drilling (Walkey and Swanberg, 1990).

Homogenization temperatures from near the bottom of the

drill hole are about 160 ̊C higher than the measured

temperature curve, indicating that rocks recovered from this

drill hole have cooled considerably since the hydrothermal

quartz and calcite crystals were formed.

OTHER DRILL HOLES

The locations of several other geothermal drill holes on

the flanks of Newberry volcano that were not studied for

this report are shown on figure 3. Some of these holes were

very shallow and would not be expected to contain

substantial hydrothermal alteration. In 1976, three

temperature-gradient drill holes (maximum depth about 61

m) were spudded by Phillips Petroleum Company on the

north and northeast flanks of Newberry volcano (P-3, P-5,

and P-6) (Olmstead and Wermiel, 1988). Results from these

wells were never released.

In September 1977, the U.S. Geological Survey completed

drill hole USGS-N1 to a depth of 386 m at an altitude of

about 1,900 m on the upper northeast flank of Newberry

volcano (Sammel, 1981; MacLeod and Sammel, 1982). The

maximum temperature recorded in this drill hole was only

17 ̊C at 384-m depth (Blackwell, Black, and Priest, 1981;

MacLeod and Sammel, 1982). According to MacLeod and

Sammel (1982), geothermal gradients for the drill hole

(about 50 ̊C/km) are lower than the mean regional gradient

(65°C/km) estimated by Blackwell and others (1978). The

drill core consists of rhyolitic to basaltic lava flows and

interbedded ash-flow tuffs, ash-fall deposits, breccia,

volcaniclastic sediments, and cinders (Sammel, 1981;

MacLeod and Sammel, 1982). Magnetic studies indicate that

these rocks have not been heated by geothermal fluids

(Griscom and Roberts, 1983). No alteration was observed

in thin sections, nor was any alteration of this drill core

reported by MacLeod and Sammel (1982).

In 1982 and 1983, Union Oil Company of California

drilled four 610-m-deep drill holes on the east and west

flanks of Newberry volcano (U 82-9, U 82-10, U 83-8, and

U 83-9). Results from these drill holes remain unpublished.

California Energy Company, Inc., drilled two holes

(1,325 m and 1,225 m deep) on the south and northwest

slopes of Newberry volcano in 1986 (Olmstead and Wermiel,

1988) (CE NB-3 and CE NB-4). Drill core from these holes

are available for study at the Energy and Geoscience Institute of the University of Utah core storage facility in Salt

Lake City, Utah. We made a cursory hand-lens examination

of core from the deeper parts of the two holes. Core samples

from CE NB-3 contain iron-oxide coatings on fractures,

calcite breccia infillings, open-space deposits of green

smectite(?), and undifferentiated (Mg, Fe, Ca, or Mn?)

carbonate minerals in vesicles and other open spaces between 1,216-m depth and the hole bottom at 1,325 m. Drillcore specimens from CE NB-4 contain similar hydrothermal mineral deposits from 1,162-m depth to the hole bottom at 1,225 m. The hydrothermal mineralogy and extent

of alteration in these drill cores are similar to those of the

Figure 27.—Fluid-inclusion Th values for quartz and calcite from

SF NC72-03 drill hole. Explanation is same as for figure 11. Measured temperature data from Arestad, Potter, and Stewart (1988) and

Walkey and Swanberg (1990).
41

other flank drill holes located at similar distances from the

caldera ring fractures. No samples were obtained for laboratory mineralogy studies.

HYDROTHERMAL MINERALOGY OF

DRILL-CORE SPECIMENS

Secondary minerals deposited along fractures and in

vesicles in the shallow levels of the Newberry drill cores,

especially in flank drill holes, probably are not of hydrothermal origin. Yellow, green, or pale-blue-gray, soft amorphous

clay-like open-space deposits in the upper, low-temperature

part of a few drill holes are similar in appearance and texture

to hydrothermal smectite coatings and may be a precursor

smectite-clay phase. Low-temperature zones may also contain minor white, frosted, or colorless amorphous silica deposits that are similar to hydrothermal opal. Some orangereddish amorphous iron oxide staining may result from lowtemperature oxidation of primary magnetite. These types of

deposits are also found in fresh lavas as a result of degassing

during cooling (the cooler stages).

Some drill-core samples contain tridymite vapor-phase

mineralization and sometimes other minerals such as magnetite or ilmenite that formed during initial degassing of lava

flows. Reddish iron oxide (amorphous iron oxide, iron hydroxide, or hematite) open-space deposits or pervasive staining of some tuffs, as well as botryoidal, massive, or powdery

black magnetite found in some fractures and vesicles (drill

hole GEO-N3) probably also formed during later cooling

stages of the lava flows and ejecta.

DRILL HOLES WITHIN THE CALDERA

In the upper 300 m of USGS-N2 drill core, alteration minerals are notably sparse; X-ray diffraction analyses show the

presence of minor siderite at 135-m depth, amorphous silica

at 181-m depth, and traces of smectite at depths of 174 m and

185 m. Measured temperatures in the rhyolitic ash, obsidian,

and basaltic tuff, breccia, and sedimentary intervals penetrated

in this part of the drill hole ranged from 18 to 39 ̊C (Sammel,

1981). Below 300-m depth, the measured temperatures ranged

from 31 ̊C at 300-m depth to 265 ̊C at 930-m depth (Sammel,

1981) (fig. 5), and hydrothermal alteration is slight to extensive, the more pervasive alteration occurs along fractures and

fracture margins and in the more permeable brecciated and

volcaniclastic zones (Keith and Bargar, 1988). The following minerals were identified in the rhyolitic to basaltic breccia, tuff, sediments, and lava flows recovered from the lower

two-thirds of the hole: hydrothermal zeolite minerals (analcime, chabazite, clinoptilolite, dachiardite, erionite, faujasite,

and mordenite), apophyllite, gyrolite, carbonate minerals

(ankerite, aragonite, calcite, dolomite, magnesite, and siderite), apatite, hydrogrossular, clay minerals (smectite, mixedlayer illite-smectite, illite, mixed-layer chlorite-smectite, and

chlorite), silica minerals (opal, cristobalite, chalcedony, and

quartz), sulfide minerals (pyrrhotite, pyrite, and marcasite),

sulfur, anhydrite, epidote, and iron oxides and hydroxides

(hematite, goethite, and lepidocrocite) (fig. 5).

The RDO-1 drill hole, located about 0.5 km southeast of

drill hole USGS-N2, encountered temperatures greater than

158 ̊C at about 350.5-m depth (fig. 12) (Black, Priest, and

Woller, 1984; Keith and others, 1986). Many of the hydrothermal alteration minerals (analcime, aragonite, siderite, rhodochrosite, calcite, smectite, chlorite, quartz, mordenite, pyrite,

pyrrhotite, and hematite) occur at shallower depths in the RDO1 hole than in the USGS-N2 drill hole (Keith and others, 1986).

DRILL HOLES OUTSIDE THE CALDERA

Measured temperatures for drill holes on the north, south,

east, and southwest flanks of Newberry volcano (GEO-N1,

GEO-N3, GEO-N4, and GEO-N5) were less than 100 ̊C. Thin

fracture coatings, vesicle fillings, spaces between breccia fragments, and glassy drill-core samples from these holes usually exhibit little or no secondary alteration in the upper parts

of the drill holes. The lower parts of these drill holes contain

low-temperature hydrothermal minerals that are compatible

with the measured temperatures. Hydrothermal minerals identified from the four drill cores are dominated by carbonate

minerals (ankerite-dolomite, aragonite, calcite, kutnohorite,

magnesite, rhodochrosite, and siderite), smectite, and hematite, with local minor amounts of silica minerals (opal,

cristobalite, and chalcedony), zeolites (chabazite, dachiardite,

heulandite, mordenite, and phillipsite), okenite (one occurrence of the calcium silicate hydrate mineral), other clay minerals (mixed-layer illite-smectite and halloysite), sulfates (barite, gypsum, jarosite, natrojarosite, and natroalunite), and sulfides (pyrite and local sphalerite) (figs. 14, 18, 19, and 20).

The three remaining flank drill holes (GEO-N2, SF NC-01,

and SF NC72-03), located on the west side of Newberry volcano, encountered near-bottom-hole temperatures in excess

of 150 ̊C. Hydrothermal alteration in the three drill holes is

slight to extensive (Arestad, Potter, and Stewart, 1988) and

reflects the higher temperatures. Hydrothermal minerals identified in one or more of the drill cores include, in addition to

several of the above listed minerals, laumontite, mixed-layer

chlorite-smectite, chlorite, copper, cristobalite, quartz, anhydrite, and pyrrhotite (figs. 16, 23, and 26).

Figure 15 shows a very generalized paragenetic diagram

for hydrothermal mineralogy from all of the Newberry geothermal drill holes located outside the caldera that were

sampled for this study. Iron oxides and clay minerals are usually early deposits; carbonate minerals, silica minerals, sulfates, and zeolites were deposited later but do not necessarily

coexist. Several minerals, particularly smectite, siderite and

calcite, were deposited in more than one generation.

HYDROTHERMAL MINERALOGY OF DRILL CORE SPECIMENS
42 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

HYDROTHERMAL MINERALS

ZEOLITE MINERALS

Zeolite minerals are sparsely distributed in drill holes on

the outer flanks of Newberry volcano; only minor amounts

of chabazite, dachiardite, heulandite, laumontite, mordenite,

or phillipsite were identified in four of the drill holes (figs.

14, 20, 23, and 26). Except for mordenite, these mostly calcium-rich zeolite minerals were associated with mafic rocks.

Somewhat more abundant zeolite minerals in the USGS-N2

drill core occur in two zones at depths of 308.9 to 320 m, and

436.5 to 496.2 m (fig. 5). In the upper zone, analcime,

chabazite, erionite, and faujasite were identified as vein fillings and altered glass in basaltic sediments. The lower zone

contains analcime, clinoptilolite, dachiardite, and mordenite

as open-space deposits and alteration products of glass in rhyolitic tuff and tuff breccia. A few scattered occurrences of

mordenite persist to about 748.2-m depth. Traces of analcime

and some mordenite were found in drill cuttings from the

RDO-1 drill hole (fig. 12).

Electron microprobe and scanning electron microscope

(SEM) semiquantitative energy-dispersive X-ray spectroscopy

(EDS) analyses of some of the USGS-N2 zeolites show variations in the chemistry of the minerals that partly reflect the

chemistry of the rocks in which they reside. Alkali-rich

clinoptilolite and dachiardite occur in the more silicic rocks,

whereas calcium-rich chabazite, analcime, and erionite are

found in mafic rocks. The cation composition of other analcime deposits, faujasite, and mordenite are either variable or

inconsistent with the type of rock in which they are found.

Measured temperatures for the depths at which the above

zeolite minerals occur appear to be compatible with temperatures at which these zeolites are found in other geothermal

areas (Kristmannsdóttir and Tómasson, 1978). Minerals such

as analcime, laumontite, and mordenite can form over a wide

temperature range that can exceed 200 ̊C; in Newberry drill

cores these zeolite minerals were found at temperatures presently ranging between about 50 and 160 ̊C. Chabazite,

clinoptilolite, heulandite, and phillipsite were identified at

present temperatures of about 100 ̊C or less in the Newberry

drill cores, which is consistent with temperatures reported for these minerals in Icelandic geothermal areas

(Kristmannsdóttir and Tómasson, 1978). Erionite was found

at about 50 ̊C in one of the Newberry drill cores and is reported at temperatures below 110 ̊C from Yellowstone National Park (Honda and Muffler, 1970). Dachiardite occurs in

two Newberry drill cores at measured temperatures less than

about 100 ̊C, but the mineral has been found at temperatures

as high as 200 ̊C in drill cores from Yellowstone’s geothermal areas (Bargar and others, 1987). Faujasite, identified in

one of the Newberry drill cores at a temperature of about

50 ̊C, is thought to form under hydrothermal conditions

(Gottardi and Galli, 1985), but it apparently has not been reported from other modern-day geothermal areas.

ANALCIME

Two fine-grained volcaniclastic drill-chip samples at depths

of 329 and 332 m in drill hole RDO-1 (fig. 12) contain analcime, as shown by whole-rock XRD analyses. Analcime was

found at two locations in the USGS-N2 drill core (fig. 5). In

the upper zone (about 313- to 320-m depth; temperature 40-

50 ̊C), euhedral trapezohedral analcime crystals (fig. 28) occur as open-space deposits in coarse basaltic sediments. A

trace of analcime also was detected by XRD at 443.8-m depth

(temperature ~l00 ̊C). These low temperatures are reasonable

for the formation of analcime, which appears to be somewhat

independent of temperature constraints; analcime has been

found over a wide temperature range (70-300 ̊C) in several

geothermal drill holes ( Honda and Muffler, 1970; Keith,

White, and Beeson, 1978; Kristmannsdóttir and Tómasson,

1978). Analcime formation appears to be favored by increasing the pH and Na+ concentration of the associated fluid

(Kusakabe and others, 1981).

Electron microprobe analyses of analcime from a depth of

318.5 m (table 8, analyses 4-8) show that the mineral is a

sodium-rich pure analcime end member.

Figure 28.—Scanning electron micrograph of large trapezohedral

analcime crystal deposited later than smectite in altered

volcaniclastic sandstone at 315.1-m depth in drill core USGSN2. A second, unidentified zeolite(?) or clay(?) mineral precipitated later than euhedral analcime crystal.
43

Additional chemical analyses of analcime from the interval 314.6-315.1-m depth (table 8, analyses 1-3) show substantial amounts of CaO in addition to Na2O. A continuous

solid-solution series exists between Na-bearing analcime and

Ca-bearing wairakite (Gottardi and Galli, 1985). Lack of a

doublet (400) XRD reflection at 3.41 Å (run at 1/4 ̊ 2θ/min.)

indicates that this occurrence of the mineral probably should

be classified as calcium-rich analcime rather than sodiumrich wairakite (Coombs, 1955). Formation of the calciumrich analcime specimen probably was due to calcium release

during hydrothermal alteration of basaltic sediments at this

depth.

CHABAZITE

Chabazite was identified by XRD analysis of fracture and

pore-space fillings in samples of basaltic sandstone between

depths of 308.9 and 318.5 m in drill hole USGS-N2 (fig. 5).

The colorless, intergrown, twinned hexagonal crystals have

a phacolitic lenticular habit (fig. 29A and B) and were observed in SEM to be a late deposit on top of faujasite and

calcite in a sample from 308.9-m depth. Phacolite (colorless

lenticular twinned chabazite) crystals apparently are uncommon in altered rocks of the Cascade Range; instead, the usual

crystal habit for chabazite from these rocks is a pseudocubic

rhombohedron (fig. 9 in Bargar, 1990). Semiquantitative EDS

analysis on the SEM shows that chabazite from 308.9-m depth

consists of Si, Al, Ca, K, and Na. Trace amounts of colorless

to white twinned pseudocubic rhombohedral chabazite crystals coat fractures along with calcite, smectite, and phillipsite

in a mafic lava flow between 801- and 802-m depth in drill

core GEO-N1 (fig. 14). In both drill holes, chabazite occurs

at depths where measured temperatures were less than 60 ̊C.

Chabazite is a characteristic hydrothermal mineral in lowTable 8.—Electron microprobe analyses of zeolite minerals from USGS N-2 drill-core.

HYDROTHERMAL MINERALS
44 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

temperature alteration zones (<75 ̊C) of Icelandic geothermal areas (Kristmannsdóttir and Tómasson, 1978).

CLINOPTILOLITE

Lath- to acicular-shaped clinoptilolite crystals (fig. 30)

were identified by XRD in six samples between depths of

436.5 and 443.8 m (temperature ~99 ̊C) in drill hole USGSN2 (fig. 5). All six samples were heated overnight at 450 ̊C

in order to distinguish between clinoptilolite and heulandite

by the X-ray method of Mumpton (1960). No change in intensity or position of the (020) X-ray reflection at ~9.0 Å was

observed in any of the samples; thus, the mineral was identified as clinoptilolite. The clinoptilolite probably forms as an

alteration product of pumiceous fragments in the rhyolitic

tuff breccia. Electron microprobe analyses (table 8, analyses

9-11) from 442.3 m show great variation in Na2O and K2O

contents for different crystals in the same sample. One

clinoptilolite crystal (analysis No. 11 in table 8) is high in

potassium—8.34 percent. We are unaware of K2O that high

for clinoptilolite from any other hydrothermal deposit.

Clinoptilolites with such high K2O contents usually are produced by sedimentary (Stonecipher, 1978) or diagenetic

(Ogihara and Iijima, 1990) processes rather than by hydrothermal alteration. Keith, White, and Beeson (1978) and

Bargar and Beeson (1985) reported clinoptilolite K2O contents of 5.73 and 4.99 weight percent, respectively, in rhyolitic drill-core specimens from thermal areas of Yellowstone

National Park.

DACHIARDITE

Clusters of fibrous-, acicular-, or lath-shaped dachiardite

crystals (fig. 31) were found in a single sample of rhyolitic

tuff from 443.2-m depth in drill hole USGS-N2 (temperature

is ~98 ̊C) (fig. 5). Dachiardite is a fairly rare zeolite mineral

that appears to have formed, along with smectite, from alteration of pumice fragments. An electron microprobe analysis

(table 8, analysis 12) shows that the Newberry dachiardite is

a sodium-rich mineral and probably should be referred to as

sodium dachiardite rather than dachiardite (Bargar and others, 1987). One volcanic breccia sample in drill core GEON5 contains tiny colorless dachiardite crystals along with

mordenite and smectite in open spaces between breccia fragments at 886.7-m depth (temperature ~70 ̊C) (fig. 20).

Figure 29.—Scanning electron micrographs of phacolitic chabazite crystals from 308.9-m depth in USGS-N2 drill hole. A, Twinned

crystals deposited later than fractured and flaking faujasite (F) that coat veinlet. B, Phacolitic habit of chabazite.
45

ERIONITE

Bundles of tiny prismatic erionite crystals (fig. 32) appear

to have formed by alteration of glass in three samples of basaltic sediments between depths of 315 m and 318.5 m in drill

hole USGS-N2 (fig. 5). The presence of Mg, Ca, Fe, K, Na,

Al, and Si is indicated by EDS analysis using the SEM.

Erionite occurs at a depth where the temperature measured

following drilling was about 50 ̊C. In the drill core from

Yellowstone National Park, erionite is found at temperatures

<110 ̊C (Honda and Muffler, 1970).

FAUJASITE

Faujasite, a rare zeolite mineral, was identified by XRD

analyses of colorless to white vein fillings and intergranular

open-space deposits of 11 porous samples of basaltic sediments between 308.9- to 320-m depth in drill hole USGS-N2

(temperature about 40-50 ̊C) (fig. 5). Scanning electron micrographs of faujasite from this drill core (fig. 33) suggest

crystal deterioration with extensive cracking and flaking of

the crystal surfaces. Electron microprobe chemical analyses

of two faujasite crystals from 314.4-m depth (table 8, analyses 13 and 14) show considerable variability in Na2O and

CaO contents which, according to Gottardi and Galli (1985),

is typical for faujasite.

HEULANDITE

One vesicular lava flow in drill hole GEO-N5 contains

tiny, colorless, tabular to blocky, euhedral heulandite crystals

(fig. 34) in fractures and vesicles in association with smectite,

mordenite, and calcite at 951.3- to 954.6-m depth (temperature ~80 ̊C) (fig. 20). XRD analysis of one multicomponent

sample shows a peak at ~9.0 Å; semiquantitative chemical

analysis by EDS on the SEM shows Si, Al, and Ca, along

with minor K, Mg, and Na, and the mineral is tentatively identified as heulandite rather than clinoptilolite.

LAUMONTITE

Single volcanic breccia samples near the bottoms of drill

holes SF NC-01 and SF NC72-03 (1,120.4- and 1,370.1-m

depth, respectively) contain tiny, white, prismatic, laumontite crystals deposited in open spaces between breccia fragments in association with earlier quartz, chlorite, calcite, or

mixed-layer chlorite-smectite. Measured temperatures at these

depths are about 150 to 160 ̊C (figs. 23 and 26). Laumontite

commonly occurs between temperatures of 100 and 200 ̊C

(Kristmannsdóttir and Tómasson, 1978), but it can also form

at lower temperatures (McCulloh and others, 1981).

Figure 30.—Scanning electron micrograph of lath-shaped

clinoptilolite crystals partly coated by smectite and fibrous

mordenite(?), from 442.3-m depth in drill core USGS-N2.

Figure 31.—Scanning electron micrograph of fibrous to acicular

dachiardite crystals in altered lithic tuff from 443.2-m depth in USGSN2 drill core.

HYDROTHERMAL MINERALS
46 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Figure 32.—Scanning electron micrograph showing bundle of

stubby acicular erionite crystals and smectite deposited in pore

spaces of basaltic sediments from about 315-m depth in USGSN2 drill core.Figure 33.—Scanning electron micrograph of fractured and flaking faujasite crystals from 308.9-m depth in drill core USGS-N2.

MORDENITE

White fibrous radiating hemispherical crystal clusters

(fig. 35A) or mattes of acicular mordenite crystals were deposited in vesicles of lava flows, open spaces between breccia fragments, or perlitic cracks of partly altered glass in the

lower part of the GEO-N5 drill core (fig. 20). An EDS

semiquantitative chemical analysis of this mordenite shows

Si, Al, and Ca>Na. The only other mordenite identified in

drill cores from the flanks of the volcano occurs near the bottoms of the SF NC-01 and SF NC72-03 drill holes where

white fibrous mordenite crystals were deposited in a few cavities. Temperatures measured during drilling of these two holes

were about 140 to 150 ̊C; however, temperatures in the lower

part of the GEO-N5 drill hole ranged from about 50 to 80 ̊C.

In Icelandic geothermal areas, mordenite forms over a wide

temperature range (~75-230 ̊C) (Kristmannsdóttir and

Tómasson, 1978).

Mordenite, the most abundant zeolite mineral in the

USGS-N2 drill core, occurs as open-space deposits from

depths of 443.7 to 460.2 m, and 469.1 to 496.2 m, and in

three widely scattered samples from between 711.2 and 748.2

m (fig. 5). In the upper zone, mordenite is confined to the

uppermost and lowermost part of a rhyodacite sill and rhyolitic tuffs where the fibrous zeolite was produced by alteration of glass. Figure 35B shows tiny fibrous mordenite crystals that formed perpendicular to the walls of a vesicle. A

semiquantitative chemical analysis by EDS shows the presence of Si, Al, Ca, and K.

Mordenite was identified in drill cuttings from the RDO-1

drill hole as an open-space filling and in altered tuffs at 332-

to 338-m depth along with smectite. Between depths of 393

and 423 m, mordenite is more abundant, and the fibrous to

acicular crystals are associated with later-deposited bladed

calcite (fig 35C), subhedral quartz crystals, and chlorite in

open-space fillings; mordenite also replaces glass in altered

pumice and lithic tuffs. Temperatures were not measured at

the bottom of the RDO-1 drill hole (Keith and others, 1986)

(fig. 12); the maximum temperature at which this mordenite

formed must have exceeded 160 ̊C.

PHILLIPSITE

In drill hole GEO-N1, tiny, colorless, prismatic crystals

from a lava flow at 756-m depth and three samples of open-
47

Figure 34.—Scanning electron micrograph of tabular to blocky

heulandite crystals from 951.3-m depth in drill core GEO-N5.

Associated smectite appears to have been deposited both earlier and later than heulandite.

Figure 35.—Scanning electron micrographs of mordenite

crystals from Newberry volcano. A, Clusters of radiating fibrous mordenite crystals that partly fill open space in core

from 748.4-m depth in GEO-N5 drill hole. B, Vesicle completely lined by short mordenite fibers at 496.2-m depth in

USGS-N2 drill core. C, Fibrous and acicular mordenite crystals and later bladed calcite from about 393-m depth in drill

cuttings from RDO-1 drill hole.

space fillings in a lava flow between depths of 801 and 804

m were identified as phillipsite by XRD (fig. 14). Phillipsite

occurs with chabazite in two of the samples; calcite and

smectite are the only other associated hydrothermal minerals. Measured temperatures at these depths are below 10 ̊C.

Phillipsite formed between temperatures of 60 and 85 ̊C in

Icelandic geothermal areas (Kristmannsdóttir and Tómasson,

1978), but it has been reported at temperatures as low as 37 ̊C

in a drill hole at Surtsey volcano (Jakobsson and Moore, 1986).

CALCIUM SILICATE HYDRATE MINERALS

GYROLITE

Clusters of lamellar gyrolite crystals line a fracture at 314.5-

m depth in the drill core from USGS-N2 (fig. 5). Gyrolite

also fills pore spaces in basaltic sandstone at 318.5-m depth

(fig. 36) along with calcite, smectite, erionite, hydrogrossular,

analcime, chabazite, apophyllite, and faujasite. An electron

microprobe analysis of gyrolite from 314.5-m depth gave the

following percentages of major-element oxides: 54.88 SiO2,

1.60 Al2O3, 0.02 Fe2O3, 0.03 MgO, 33.37 CaO, 0.45 Na2O,

and 0.03 K2O (total weight percent oxides = 90.38). Gyrolite

typically is associated with zeolite minerals in basalts

(Merlino, 1988). Basaltic drill-hole specimens from a few

Icelandic geothermal areas contain gyrolite (Kristmannsdóttir

and Tómasson, 1978), and gyrolite occurs in the rhyolitic drill

core from one research geothermal drill hole in Yellowstone

National Park (Bargar, Beeson, and Keith, 1981). In both of

these occurrences, the temperatures were considerably higher

than the <50 ̊C measured temperature at which gyrolite was

found in the USGS-N2 drill hole.

OKENITE

Okenite occurs as a soft white vug filling in a basalt lava

flow core specimen from 857-m depth in drill core GEO-N1

(temperature <10 ̊C) (fig. 14). The mineral appears to be rare

in modern geothermal systems but typically is found in baHYDROTHERMAL MINERALS
48 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Figure 35.—Continued. Figure 35.—Continued.

Figure 36.—Scanning electron micrograph of spherical

aggregate of platy gyrolite crystals in smectite-altered

volcaniclastic sandstone from 318.5-m depth in drill core

USGS-N2.

salt cavities in association with zeolite minerals (Heller and

Taylor, 1956). Platy okenite crystals have also been reported

from a single cavity (temperature ~130 ̊C) in a rhyolitic drillcore specimen from Yellowstone National Park (Bargar,

Beeson, and Keith, 1981).

CARBONATE MINERALS

Every geothermal core hole, both on the flanks of

Newberry volcano and within the caldera, contains at least

three carbonate minerals that were deposited in vesicles, on

fractures, or between breccia fragments. Calcite and siderite

are the most abundant of the carbonates; other minerals include aragonite, ankerite-dolomite, kutnohorite, magnesite,

and rhodochrosite. Most mineral identifications were made

by XRD analyses. XRD identification of carbonate minerals

is based on the mineral powder diffraction file of the Joint

Committee on Powder Diffraction Standards (JCPDS). No

internal standard was used in any of the XRD measurements;

however, accuracy of the measurements should be within

about ±0.02 Å. The most intense X-ray reflection was found

to be significantly different in nearly all the carbonate minerals probably owing to variation in the composition of the

minerals. Electron microprobe analyses of several carbonate

minerals from the USGS-N2 drill core show considerable vari-
HYDROTHERMAL MINERALS 49

Table 9.—Electron microprobe analyses of carbonate minerals from USGS-N2 drill-core.

Figure 37.—CaCO3-MgCO3-FeCO3+MnCO3 ternary diagram for

electron microprobe analyses of carbonate minerals from USGSN2 drill core.

ability in the Mg, Ca, Mn, and Fe cation content (table 9 and

fig. 37). In fact, two of the carbonate minerals (ankerite and

dolomite) were found to be very close both structurally and

morphologically and are not readily distinguishable except

by their Mg:Fe ratio (Deer, Howie, and Zussman, 1966).

ANKERITE-DOLOMITE

Colorless to white rhombic ankerite-dolomite crystals or

massive fillings were deposited in open spaces between breccia fragments, vesicles, or fractures in all seven of the flank

drill cores and in the USGS-N2 core hole within the Newberry

caldera (figs. 5, 14, 16, 18, 19, 20, 23, and 26). Morphology

of the crystalline deposits varies from individual rhombs to

disk-shaped (fig. 38), columnar (fig. 39), or hemispherical

clusters of rhombic crystals. XRD analyses of these carbonate deposits have a major (104) reflection ranging between

about 2.89 Å (dolomite) and 2.90 Å (ankerite) making it difficult to determine which of the two minerals is present from

the X-ray analysis. Electron-microprobe analyses of crystals

from three samples from the USGS-N2 drill core (table 9,

analyses 6-10) indicate that the crystals are inhomogeneous

and range from ferroan dolomite to ankerite. The crystals are
50 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

slightly Ca-rich in most analyses; however, in three analyses

Mg is greater than Ca, and the chemical data plot outside the

ankerite-dolomite field (fig. 37). Minor Mn, Sr, and Ba were

detected in the electron-microprobe analyses. Trace-element

analysis of one sample from USGS-N2 (table 10) also shows

the presence of minor Zn, as well as measurable Ce, Co, Ga,

and Yb. Measured temperatures at the depths where ankerite-dolomite was found in the drill holes range from <10 to

~150 ̊C. Dolomite was found in one drill-core sample from

Yellowstone National Park at a depth where the measured

temperature was 190 ̊C (T.E.C. Keith, unpub. data, 1991). In

some Salton Sea geothermal drill holes, dolomite and ankerite are reported at depths where measured temperatures range

from less than 100 ̊C to more than 200 ̊C (Muffler and White,

1969) or even as high as about 250 ̊C (McDowell and Paces,

1985).

ARAGONITE

Aragonite occurs in core samples of three drill holes from

the north, east, and south flanks of Newberry volcano (figs.

14, 18, and 19) and is found in both of the intracaldera drill

holes (figs. 5 and 12). Colorless acicular (as long as ~2.0 cm)

aragonite crystals or fan-shaped aggregates of aragonite crystals were deposited in scattered vesicles and fractures where

the measured temperatures are <50 ̊C. In a few samples,

vesicles are completely filled by colorless, massive aragonite; however, white powdery or cauliflower-like aragonite

deposits were also verified by XRD analyses. Electron-microprobe analysis of aragonite from one sample in the USGSN2 drill core (fig. 37 and table 9 analysis 1) suggests minor

substitution of Sr, Ba, Mn, Mg, and Fe for Ca in the aragonite

structure.

CALCITE

Calcite was found in all of the Newberry drill cores; it

usually is the predominant carbonate mineral, but siderite is

more abundant in the USGS-N2 and GEO-N4 drill holes.

Vesicles, fractures, and open spaces between breccia fragments contain white massive or colorless crystalline calcite

deposits. Calcite replaces plagioclase phenocrysts near the

bottoms of the GEO-N1 and USGS-N2 drill holes. Crystal

morphology of the calcite is variable and ranges from individual thin-bladed crystals (fig. 35C) to equant (fig. 40A) or

needle-like crystals (fig. 40B). Some calcite is associated with

Figure 38.—Scanning electron micrograph of blocky and discoidal aggregates of rhombic ankerite-dolomite crystals and a

small spherical cluster of smectite (Sm) crystals from 693.7-m

depth in USGS-N2 drill core.

Figure 39.—Scanning electron micrograph of columnar aggregates of rhombic ankerite-dolomite crystals from 841.6-m

depth in SF NC72-03 drill core.
51

KUTNOHORITE

Colorless, white- to light-yellow, botryoidal, acicular, or

massive vesicle and fracture fillings in the lower half of the

GEO-N1 drill core were identified as kutnohorite (fig. 14).

These deposits have a major (104) XRD reflection at about

2.91 to 2.98 Å; the wide range in peak position is a function

of Ca or Mg as the predominant cation (Gabrielson and

Sundius, 1965; Tsusue, 1967). Kutnohorite has the dolomite

structure (Frondel and Bauer, 1955); however, the X-ray peaks

that are characteristic of dolomitic structure may be too weak

to detect, at least in Ca-kutnohorite (Gabrielson and Sundius,

1965). Consequently, some difficulty occurred in distinguishing between calcite and kutnohorite because abundant manganese in the calcite structure expands the range of positions

of the most intense X-ray reflection from ~3.03 Å (typical

calcite) to about 2.95 Å (Krieger, 1930). In this study, carbonate minerals were identified as kutnohorite if the most

intense X-ray peak ranged between 2.91 and 2.98 Å and as

calcite if the reflection occurred from 2.99 to 3.05 Å.

Kutnohorite and calcite in the GEO-N1 drill core were not

deposited in distinctive zones; instead, the two minerals overlap in their distribution throughout the lower half of the drill

hole (fig. 14). In some fractures or vugs, calcite and

kutnohorite are found together or are closely associated with

one or more additional carbonate minerals (ankerite-dolomite,

aragonite, rhodochrosite, or siderite). This distribution suggests that the cation content of the fluids that deposited these

carbonate minerals probably varied somewhat with time.

MAGNESITE

Magnesite was found by X-ray diffraction analyses of a

few scattered rhyolitic volcaniclastic and tuffaceous samples

from 349.3 to 365.2-m depth and between depths of 409.0

and 421.8 m in the USGS-N2 drill core (fig. 5). Magnesite

also occurs in vesicles of one sample from near the bottom

of the GEO-N3 drill core (fig. 18) and was deposited in

vesicles, fractures, and between breccia fragments of a few

samples between 876.3- and 961-m depth in the GEO-N2

drill hole (fig. 16). Many of the samples also contain closely

associated siderite, ankerite-dolomite, and calcite. According to electron-microprobe analyses (fig. 37 and table 9

analyses 11 and 12) of magnesite from the USGS-N2 drill

core, the mineral is not a pure magnesium carbonate; substantial Ca and Fe and minor Mn, Sr, and Ba are present in

an analysis of the mineral.

RHODOCHROSITE

Rhodochrosite is a sparse hydrothermal mineral in the drill

cores from Newberry volcano. Samples collected from three

of the flank drill holes (GEO-N1, GEO-N2, and GEO-N5)

(figs. 14, 16, and 20) contain vesicle or fracture fillings of

HYDROTHERMAL MINERALS

Table 10.—Trace element composition (in ppm) of carbonate and

sulfide minerals from USGS-N2 drill core.

[Analyses by emission spectroscopy, Analysts: P.H. Brigg and J. Cornell, U.S. Geological Survey, Denver, Colo. –, not determined]

earlier and later red iron oxide and green (iron-rich?) smectite

(fig. 40C); it is surprising that siderite did not form rather

than calcite in the presence of abundant iron.

The most intense XRD (104) peak for calcite typically

ranges from about 3.02 to 3.05 Å. However, some of the

Newberry calcite deposits have their most intense X-ray reflection between 2.99 and 3.01 Å, which indicates that the

mineral contains significant manganese (Krieger, 1930; Bargar

and Beeson, 1984). In the drill core from USGS-N2, the Mn

content appears to increase with increasing depth (table 9

analyses 2-5, and fig. 37); significant Fe and Mg also are

present in some of the analyzed samples. The trace elements

Ba and Sr occur in minor amounts along with detectable Co

and Zn (tables 9 and 10).
52 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

ite deposits in vesicles, fractures, and open spaces between

breccia fragments. The siderite varies greatly in color from

pale yellow to caramel to dark reddish orange, and the color

appears to reflect the chemical composition of the mineral.

Lighter caramel or pale-yellow siderite crystals have their

most intense (104) XRD reflection at about 2.82 Å, corresponding to siderite with high manganese content (Rosenberg,

1963). In darker caramel-colored siderite crystals (Fe-rich),

the most intense X-ray reflection occurs between about 2.78

and 2.80 Å.

Morphology of siderite in the Newberry drill cores is varied. Occasionally, massive vesicle fillings or individual rhombic siderite crystals are found; however, siderite more commonly forms randomly oriented disk-shaped aggregates of

rhombic crystals (fig. 43A). Just as common are spherical or

hemispherical crystal clusters that differ in the size or arrangement of the rhombic crystals (fig. 43B, C, D, and E). Rarely,

siderite crystals form unusual shapes such as columns of

stacked crystals, fan-shaped crystal aggregates, boomerang-shaped crystal aggregates, crystal aggregates arranged

in an open-textured configuration resembling lace, or a

rather staghorn-shaped arrangement of clustered siderite

Figure 40.—Scanning electron micrographs and photomicrograph of calcite crystals from Newberry volcano. A, Blocky aggregates of

calcite crystals and associated chlorite that line fracture at 847.6-m depth in USGS-N2 drill core. B, Needle-like rhombic calcite crystals

from fracture filling in basaltic sandstone at 308.9-m depth in USGS-N2 drill core. C, Photomicrograph of ~1-cm-long vesicle filling of

multiple alternating layers of calcite (Cc), green (iron-rich?) smectite (Sm), and red iron oxide (Fe). From 906.8-m depth in drill core SF

NC-01.

colorless to light-yellow, bladed, columnar, or botryoidal carbonate deposits that have an intense XRD reflection between

2.82 and 2.86 Å. Closely associated minerals are ankeritedolomite, kutnohorite, and siderite. Both drill holes within

the caldera contain rhodochrosite; caramel-colored botryoidal

coatings of clastic fragments at 308- to 311-m depth in samples

from the RDO-1 drill hole (fig. 12) were identified by XRD

as rhodochrosite. Some disk-shaped and spherical clusters of

siderite crystals from the USGS-N2 drill core have a dark,

nearly opaque core that is surrounded by concentric rings

when viewed in cross section (fig. 41). Two beam scans, ~90°

apart, from core to rim across a slabbed surface of a spherical

crystal cluster at 590.6-m depth were made with the electron

microprobe. The results show that the core is Mn- and Carich (Ca-Fe rhodochrosite) and Fe- and Mg-poor (fig. 42 and

table 9 analysis 17). The Mn and Ca content decreases toward the rim, and Mg and Fe increase outward and are most

concentrated in the rim.

SIDERITE

All of the Newberry geothermal drill cores (except the

RDO-1 drill cuttings) examined for this study contain sider-
53

crystals (fig. 43F). XRD analyses of some of the exotically

shaped deposits indicate that the predominant mineral is

siderite; however, other carbonate minerals such as ankerite-dolomite, kutnohorite, and magnesite also are locally

present. A group of concentric rings on a fracture surface

(fig. 43G) consists of alternating dark layers of siderite and

lighter layers of calcite or aragonite that react with HCl.

Similar concentric rings or hemispherical shells of carbonate deposits in other drill cores sometimes have a core of

clay or even black magnetite or reddish iron oxide.

There is considerable inhomogeneity of Mg, Ca, Mn,

and Fe contents in individual siderite crystals and crystal

clusters (fig. 37 and table 9 analyses 13-16). Some siderite

crystals contain substantial Sr and Ba. Trace-element analysis of a siderite sample from 570.0-m depth in drill core

USGS-N2 (table 10) also showed small amounts of Cd, Ce,

Co, Ga, La, Nd, Pb, Th, Yb, and Zn.

CLAY MINERALS

KAOLINITE AND HALLOYSITE

Halloysite and smectite were identified by XRD from an

ash-flow tuff core sample at 950-m depth in the GEO-N1

drill hole (Wright and Nielson, 1986). A peach-colored tuffaceous deposit that is completely altered to halloysite was

HYDROTHERMAL MINERALS

Figure 40.—Continued.

Figure 41.—Cross section of discoidal aggregate of siderite crystals from 659.9-m depth in USGS-N2 drill core.

Core of crystal aggregate is rich in Mn and Ca (Ca-Fe

rhodochrosite); Mg and Fe content increases progressively

away from core toward rim.

Figure 42.—CaCO3-MgCO3+FeCO3-MnCO3 ternary diagram for

electron microprobe analyses of discoidal siderite crystal aggregate

from 590.6-m depth in USGS N-2 drill core. Two traverses, represented by solid squares and open circles, were made 90 ̊ apart from

core to rim of crystal aggregate.

Rim

Core

 MgCO3 + FeCO3 

CaCO3

MnCO3
54 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

found at 583.1- to 586.0-m depth in core hole GEO-N5 (fig.

20). The only other kaolinite-group mineral (~7Å XRD peak

that is destroyed by heating at 550 ̊C for 1/2 hour, Starkey,

Blackmon, and Hauff, 1984) found in the Newberry drill cores

occurs in basalt vesicle fillings at 1,148.0-m depth in drill

core GEO-N3 (fig. 18) where magnetite, smectite, kaolinite,

siderite, and calcite (in order of formation) were identified in

XRD analyses. Measured temperatures at the depths where

kaolinite-group minerals were identified in all three drill holes

are less than 50 ̊C. The origin of these scarce kaolin deposits

from the flanks of Newberry volcano is uncertain; however,

several sulfate minerals were deposited at a little shallower

depth than kaolinite in the GEO-N3 drill core, and nearby

drill-core samples contain pyrite. Possibly local acidic conditions were created by the oxidation of pyrite (Steiner, 1977) in

rock adjacent to the drill-core sample. If so, the acidic conditions must have been very brief because siderite was deposited

somewhat later than the kaolinite. Wright and Nielson (1986)

suggest that the GEO-N1 ash-flow tuff alteration may result

when extrusive volcanic material interacts with local ground

water or geothermal water; if so, the formation temperature

undoubtedly was much higher than the measured temperatures.

SMECTITE

Smectite is the most abundant clay mineral in all of the

Newberry drill holes; white-yellow, blue-green, brown-black,

massive to crystalline clay deposits coat open spaces of

vesicles and fractures, fill voids between breccia fragments,

and replace glass in tuffaceous samples and the matrix of lava

flows. Paragenetic relations between smectite and other hydrothermal minerals frequently show more than one generation of smectite deposition. Detailed XRD studies of smectite

show (001) basal reflections that range from poorly crystalline (low broad X-ray peaks) to well-crystallized with sharp

(001) X-ray peaks that generally fall within the range from

~13 to 16 Å; the basal spacing expands to between ~16 and

18 Å after exposure to ethylene glycol vapors at 60 ̊C for 1

hour and contracts to ~10 Å after heating at 550°C for 30

Figure 43.—Scanning electron micrographs and photograph of siderite morphologies at Newberry volcano. A, Cluster of discoidal aggregates of siderite crystals coating fracture at 565.7-m depth in drill core USGS-N2. B, Cluster of spherical aggregates of siderite crystals

deposited on vapor-phase parting at 603.2-m depth in drill core GEO-N4. Very tiny individual crystals are not discernible at this scale. C,

Spherical aggregates of rhombic siderite crystals from 1,291.7-m depth in GEO-N3 drill core. Small parts of overlapping rhombic siderite

crystals can be distinguished. Areas between some spherical siderite crystal clusters contain scattered individual rhombic siderite crystals.

D, Roughly spherical aggregates of rhombic siderite crystals from depth of 690.2 m in GEO-N4 drill core. E, Rosette of rhombic siderite

crystals filling part of vesicle at 1,296.6-m depth in drill core GEO-N1. F, Branching aggregate of siderite crystals that resembles antler

or staghorn from 987.9-m depth in drill core GEO-N5. G, Photograph showing group of concentric rings deposited on fracture surface at

648.9-m depth in drill core GEO-N4. Rings consist of alternating dark siderite and light calcite or aragonite (reacts with HCl).
HYDROTHERMAL MINERALS 55

Figure 43.—Continued. Figure 43.—Continued.

Figure 43.—Continued. Figure 43.—Continued.
56 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

minutes. The range of unglycolated (001) basal reflections

suggests that there is some variation in the exchangeable cations (Grim, 1968). Semiquantitative chemical analyses of

several clay samples by EDS shows the presence of Si, Al,

and Fe in all the samples analyzed; Ti, Mg, Ca, Na, and K are

also present in one or more of the analyzed clays. The mean

index of refraction for one of the green Fe-rich clay samples,

~1.58 to 1.59, suggests that the smectite species probably is

nontronite. Crystal habit generally takes the form of randomly

oriented, open-textured, sheet-like or platy crystals (fig. 44A)

or densely packed platy crystals (fig. 44B). However, green

smectite (identified by XRD) vesicle fillings in one sample

from the GEO-N2 drill core have a fuzzy or fibrous surface

coating (fig. 44C). According to Phillips and Griffen (1981),

nontronite may sometimes have a fibrous crystal habit.

MIXED-LAYER ILLITE-SMECTITE

Light-green and white mixed-layer illite-smectite was identified as fillings between breccia fragments and as an alteration product of phenocryst plagioclase in a narrow, very altered breccia zone at depths of 793.5 and 795.5 m in drill

core USGS-N2 (fig. 5). XRD data for two samples show a

(001)I/(001)S spacing of ~10.7 Å that contracts to ~9.7 Å

(smectite peak not seen) after exposure to ethylene glycol for

1 hour at 60 ̊C. These two samples appear to be randomly

interstratified illite-smectite (Hower, 1981). A third sample

has a peak at ~11.4 Å that splits into ~16.4 and ~9.0 Å peaks

after glycolation and is probably an allevardite-ordered

interstratified illite-smectite (Hower, 1981). All three samples

appear to have about 80 to 90 percent illite layers (Hower,

1981; Horton, 1985). According to data given in Horton

(1985), these mixed-layer clays could have formed very near

the present measured temperature of about 180 ̊C.

Three of the flank drill cores contain mixed-layer illitesmectite deposits. A low, broad, asymmetrical, XRD reflection at about 10.8 Å, which separates into two reflections at

about 9.8 and 12.0 Å following glycolation, was seen for a

single sample at 1,219.0-m depth in the GEO-N3 drill core

(fig. 18). The mineral is tentatively identified as an allevarditeordered mixed-layer illite-smectite (Hower, 1981). It is associated with a 0.5-cm-wide vertical glassy dike(?) that is mostly

altered to magnetite, hematite, pyrite(?), siderite, and graygreen mixed-layer illite-smectite. A blue-green clay deposit

on a vapor-phase cavity containing tridymite crystals at 785.2-

m depth in drill core GEO-N5 (fig. 20) has a ~11.5 Å peak

that splits into ~17 and ~9.7 Å peaks following glycolation

and is a randomly interstratified mixed-layer illite-smectite

(Hower, 1981). Both the GEO-N3 and GEO-N5 clay deposits occur at depths where the measured temperatures are

<60 ̊C. One sample is associated with vapor-phase mineralization and the second with a narrow glassy dike; both samples

probably are deuteric and formed during cooling of the lava

flow or intrusion at temperatures that were considerably higher

than the measured temperatures. In drill core SF NC-01, a

light-green fracture filling from 1,090.3-m depth and white

vesicle fillings at 1,164.0-m depth (fig. 23) have XRD peaks

at about 12 to 13 Å that split into two peaks at ~13 to 14 Å

and at 9.5 to 9.7 Å, respectively, after glycolation. The measured temperature at the depths where the two SF NC-01

allevardite-ordered mixed-layer illite-smectite (about 65-80

percent illite, Hower, 1981) clay samples were deposited is

about 150 ̊C. According to Horton (1985, fig. 9), mixed-layer

illite-smectite with about the same illite component occurs in

other geothermal areas at about the same temperature.

ILLITE

Light-green illite (mean index of refraction ~1.55-1.56),

a mica-clay mineral showing no change in the 10 Å XRD

peak following glycolation (Grim, 1968), is mostly confined

to breccia zones at depths of 752.1 to 761.4 m, 772.4 m, 907.8

to 908.8 m, and 930.1 to 931.8 m in the intracaldera USGSN2 drill core (fig. 5). Measured temperatures at these

depths ranged between about 150 and 265 ̊C. An EDS

semiquantitative chemical analysis of illite from 907.8 m

shows the presence of Si, Al, Fe, and K. XRD analyses of

lithic tuffs, lava-flow vesicle fillings, and fracture fillings from

depths of 310.0 to 362.0 m, 568.1 to 568.5 m, and 789.0 m in

drill core GEO-N1 (fig. 14) show that white, yellow, orange,

Figure 43.—Continued. and greenish-brown clay alteration has a very low, broad ~10
57

Å peak. The basal (001) peak does not shift when the sample

is glycolated, indicating that the mineral is illite. Measured

temperatures at these depths in the GEO-N1 drill hole are

<10 ̊C; illite undoubtedly did not form at such low temperatures. Wright and Nielson (1986) attributed the formation of

halloysite at depths of 948.8 to 951.0 m in this drill hole to

the interaction of cooling volcanic deposits and meteoric

water. Probably, the illite formed similarly before the extrusive

volcanic rocks cooled to the measured drill-hole temperatures.

MIXED-LAYER CHLORITE-SMECTITE

Scattered samples, below 742.5-m depth in drill core

USGS-N2 contain green mixed-layer chlorite-smectite or

corrensite (fig. 5). Ordered interstratified chlorite-smectite is

found lining fractures, filling vesicles, and replacing interstitial glass in andesite and basalt. Mixed-layer chlorite-smectite

has a sheet-like morphology and occurs as rosettes of platy

crystals at 915.2-m depth (fig. 45A). XRD analyses show that

ordered interstratified chlorite-smectites from this intracaldera

drill core contain higher order 28 to 29 Å X-ray reflections in

addition to 14 and 7 Å reflections that exhibit a slight expansion with glycolation (Hower, 1981). From the X-ray data,

this clay mineral appears to contain random proportions of

HYDROTHERMAL MINERALS

Figure 44.—Scanning electron micrographs of smectite morphologies at Newberry volcano. A, Open-textured spherical aggregate of

randomly oriented sheet-like smectite crystals deposited on dolomite-ankerite crystals in open space between breccia fragments at

690.4-m depth in USGS-N2 drill core. B, Vesicle filling of closely spaced dark-green platy smectite crystals from 884.5-m depth in SF

NC-01 drill core. C, Olive-green fibrous smectite crystals filling vesicle at 679.0-m depth in GEO-N2 drill core.
58 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

chlorite and smectite with no discernible pattern. Electron

microprobe analyses of mixed-layer chlorite-smectite from

916.7-m depth (table 11 analyses 10-17) show that the mineral is very rich in Mg and would plot in the talc-chlorite

field of the Si vs. Fe/Fe+Mg diagram of Hey (1954, p. 280).

Mixed-layer chlorite-smectite was identified in the lower

200 to 400 m of the three higher temperature (measured temperatures between ~110 and 160 ̊C) drill holes (GEO-N2, SF

NC-01, and SF NC72-03) on the west flank of Newberry volcano (figs. 16, 23, and 26). Vesicles, fractures, and open spaces

between breccia fragments are coated with mostly light- to

dark-green clay that sometimes has a waxy, botryoidal, or

wormy (fig. 45B) texture. EDS semiquantitative chemical

analysis of several of the mixed-layer clay deposits generally

show Mg, Al, Si, Fe, and variable amounts of Ca. Most of the

analyses suggest that Mg content is somewhat greater than

Fe; however, one white clay deposit at 1,342.0-m depth in

drill core SF NC72-03 (fig. 26) contains Mg, Al, Si, K, Ca,

and Fe, with Mg being much more abundant than Fe. All of

the mixed-layer chlorite-smectite deposits appear to be well

crystallized and have very sharp (001) and (002) XRD peaks

at an average of about 14.7 and 7.3 Å that show slight expansion to about 15.3 and 7.5 Å following exposure to ethylene

glycol at 60 ̊C for 1 hour. One sample also has a first-order

peak at 29.9 Å, which expands to about 32.4 Å after

glycolation and is characteristic of corrensite (Bailey, 1982).

Heating to 400 ̊C for 1/2 hour causes slight contraction of the

(001) peak to about 14.0 Å. Hydrothermally altered basaltic

rocks in drill holes from the Reykjanes, Iceland, geothermal

area contain mixed-layer chlorite-smectite and swelling chlorite that formed between temperatures of 200 ̊ and 270 ̊C

(Kristmannsdóttir, 1976). Kristmannsdóttir subdivided these

interstratified clay minerals into 5 different swelling chlorites and mixed-layer chlorite-smectite. In the present study,

several differences were noted for the behavior of the (001)

X-ray peak with various treatments, but no systematic study

or attempts at subdivision were made.

CHLORITE

Dark-green chlorite precipitated in open spaces and in altered pumiceous to lithic tuff in drill-hole samples between

depths of 357 and 424 m in the RDO-1 drill core (fig. 12). At

390-m depth, open-space botryoidal clusters of randomly oriented sheet-like chlorite formed later than calcite. Chlorite

appears to be the latest hydrothermal mineral deposited in

Figure 45.—Scanning electron micrographs of two mixed-layer chlorite-smectite forms at Newberry volcano. A, Rosettes of platy mixedlayer chlorite-smectite crystalline aggregates that coat fracture at 915.2-m depth in USGS-N2 drill core. B, Wormy-textured clusters of

mixed-layer chlorite-smectite crystals deposited on calcite in cavity at 1,140.9-m depth in SF NC72-03 drill core.
59

the RDO-1 drill-hole specimens. The measured temperature

for the upper part of the chlorite zone is a little more than

160 ̊C (fig. 12). Chlorite identified by XRD at depths of 308.9

m, 448.7 to 477.2 m, and below 707.4 m in the USGS-N2

drill core (fig. 5) is primarily an open-space deposit on fractures and in vesicles; it occurs mostly as randomly oriented

subhedral platy crystals, rosettes of radiating sheet-like crystals (fig. 46A), or books of euhedral hexagonal platelets (fig.

46B). Electron-microprobe analyses of chlorite from 930.0-

m depth (table 11, analyses 1-9) show that it would plot in

the pycnochlorite field of a Si vs. Fe/Fe+Mg diagram (Hey,

1954, p. 280). Chlorite, swelling chlorite, and randomly

mixed-layer chlorite-smectite have been described in Iceland’s

Reykjanes geothermal area at temperatures of 200 to 260 ̊C

(Kristmannsdóttir, 1976). In the USGS-N2 drill core, there

appears to be a progression of smectite to randomly

interstratified chlorite-smectite to ordered interstratified chlorite-smectite to chlorite over the measured temperature range

of about 150 to 265 ̊C (fig. 5).

Chlorite was only identified as fracture coatings, vesicle

fillings, and fillings between breccia fragments in the three

west-flank drill cores (GEO-N2, SF NC-01, and SF NC72-

03) that also contain mixed-layer chlorite-smectite; it also

was found as altered mafic minerals in one sample from

the SF NC72-03 drill core (fig. 26) . Most of the chlorite

deposits consist of randomly oriented platy crystals, but

spaces between breccia fragments at 1,120.4-m depth in

the SF NC-01 drill core (fig. 23) are partly filled by a matte

of intergrown wormy clusters (about 2 mm in length) of

sheetlike crystals (fig. 46C; see also fig. 45B). The green

clay mineral has (001) and (002) XRD reflections at about

14 and 7 Å that do not expand with glycolation and are not

appreciably affected by heating to 400 ̊C for 1/2 hour. Measured temperatures at the depths where chlorite was found

in the west-flank drill holes range between about 140 and

160 ̊C (figs. 16, 23, and 26); these temperatures correspond

well with the minimum temperature for chlorite in the lower

part of the USGS-N2 drill core (fig. 5).

SILICA MINERALS

Drill cores from within Newberry caldera and on the flanks

of the volcano contain silica minerals that span the range of

crystallinity from noncrystalline opal to poorly crystallized

cristobalite (XRD peak at 4.11 Å) to better ordered cristobalite

(XRD peak at 4.04 Å) to chalcedony—a cryptocrystalline variety of quartz—to well-crystallized quartz. Geothermal drill

holes in Yellowstone National Park show the same depositional sequence of silica minerals (Keith, White, and Beeson,

1978). The solubilities of these silica minerals at various temperatures, and indications that the less well-crystallized silica

phases can be converted to better-crystallized silica minerals

through solid-state recrystallization as in the conversion from

opal to cristobalite or from poorly ordered cristobalite to wellordered cristobalite, are discussed by (Keith, White, and

Beeson, 1978). However, solution and redeposition are

thought to be required in converting chalcedony to quartz and

in the formation of chalcedony from cristobalite (Murata and

Larson, 1975). Several silica samples in the Newberry drill

cores consist of both chalcedony and cristobalite in XRD

analyses. These deposits appear to consist of chalcedony that

is enveloped by cristobalite and may result from changing

physical or chemical conditions during silica deposition.

OPAL

A few scattered fractures between 554.1- and 641.9-m

depth in drill core USGS-N2 (fig. 5) are lined by white powdery to granular, massive orange, or white to colorless

botryoidal (fig. 47) noncrystalline opal (isotropic, index of

refraction = ~1.46). EDS chemical analysis on the SEM shows

only Si. A fracture at 354.0-m depth in the GEO-N4 drill hole

(fig. 19) on the east flank of Newberry volcano contains a

colorless to frosted botryoidal silica coating that is isotropic

and has an index of refraction <1.46; however, a few of the

silica fragments viewed in oils have an index of refraction

>1.46 and a spotty low-order birefringence that might indicate partial conversion to cristobalite. At 277-m depth in core

hole SF NC-01 (fig. 23), a fracture is coated by as much as

~2.5 mm of colorless to dark-brown, massive to botryoidal

isotropic silica with an index of refraction <1.47. Gray and

cream-colored amorphous clay is a later deposit in the fracture; both the opal and clay fracture coatings contain disseminated, yellowish, cubic and pyritohedral metallic pyrite crystals (see fig. 24). XRD analysis of one white fracture coating

at 322.2-m depth in the GEO-N1 drill core (fig. 14) indicates

that the material is amorphous and probably consists of opal.

Similar, white to light-yellow-green, thin fracture and cavity

coatings of three other core samples in the upper part of the

GEO-N1 drill core also appear to be opal.

CRISTOBALITE

Colorless to white botryoidal cristobalite (index of refraction = ~1.47, weak birefringence) coats a few fractures from

depths of 556.0 to 563.1 m and 700.4 to 725.1 m in the USGSN2 drill core (fig. 5). XRD analyses of several samples also

show the presence of peaks for quartz or cryptocrystalline

chalcedony, but none was observed in refractive-index oils.

Cristobalite was identified in six of the flank drill cores

(all except GEO-N4). In core hole GEO-N1 (fig. 14), several

cavities in samples from depths of 1,124 to 1,172 m, open

spaces of two breccia samples at 1,301.3- and 1,339.6-m

depth, and a fracture filling at 1,363.4-m depth are partly

coated by bluish botryoidal cristobalite. A massive, 2- to 3-

mm-thick green fracture filling at 1,178-m depth consists of

cristobalite in XRD analysis. Cristobalite also was identified

in an X-ray trace of a soft, spongy-clayey, fracture filling at

1,185-m depth.

HYDROTHERMAL MINERALS
60 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

In the GEO-N2 drill core (fig. 16), vesicles, fractures, and

spaces between breccia fragments contain colorless, white,

or bluish botryoidal or spherical silica deposits at 514.5-m

depth and scattered samples between depths of 656.7 and

1,040.9 m. XRD analyses of these siliceous open-space fillings show the presence of both poorly crystallized cristobalite

and well-crystallized cristobalite; the former has a major low

broad X-ray peak close to 4.11 Å, and the latter has high sharp

peaks near 4.04 Å (Keith, White, and Beeson, 1978).

Cristobalite is the dominant mineral above about 810-m depth

but also occurs in scattered samples below that depth. Peaks

for cryptocrystalline quartz or chalcedony are also present in

several X-ray analyses of the botryoidal silica deposits below 810-m depth. Well-crystallized cristobalite was not identified in any of the other flank or intracaldera drill cores.

Frosted to colorless botryoidal cristobalite lines open

spaces in scoriaceous drill core material at 1,141.5-m depth

and coats vesicles at 1,146.2-m depth in the GEO N-3 drill

core (fig. 18). It is the only silica mineral identified in core

samples from this drill hole. A few scattered samples below

~750-m depth in the GEO-N5 drill core (fig. 20) show

cristobalite in XRD analyses of altered glass in lithic tuff and

obsidian breccia, as botryoidal silica deposits in fractures and

vesicles, and as fillings between breccia fragments.

The drill core from the SF NC-01 drill hole has a few bluish-gray to frosted botryoidal cristobalite deposits in fractures

and vesicles from 693.4- to 930.1-m depth (fig. 23). SEM

studies usually show that the botryoidal silica has a smooth

surface with no evidence of crystallinity, similar to the opal

of figure 47. However, one sample of cristobalite deposited

on calcite at 900.1-m depth shows that the botryoidal silica

consists of spherical clusters of bladed crystals (fig. 48). A

similar habit has been observed for hydrothermal cristobalite

in drill cores from Yellowstone National Park (Bargar and

Beeson, 1981) and for diagenetic cristobalite in the Miocene

Monterey Shale in California (Murata and Larson, 1975).

The SF NC72-03 drill core contains cristobalite between

depths of 662.9 and 1,187.2 m (fig. 26). Fractures, vesicles,

and open spaces between breccia fragments are partly filled

by soft white silica, by colorless, frosted, or bluish-white

botryoidal silica, or by similarly colored massive silica deposits. SEM observations of a cristobalite sample from 867.2-

m depth shows botryoidal clusters of bladed(?) crystals similar to those in figure 48. Many of these cristobalite deposits

contain associated chalcedony.

QUARTZ AND CHALCEDONY

Colorless euhedral quartz crystals (as long as 7 mm) were

deposited in vesicles and fractures between depths of 461.5

and 468.9 m and below 713.5-m depth in the USGS-N2 drill

Table 11.—Electron microprobe analyses of phyllosilicate minerals from USGS-N2 drill core.
61

core (fig. 5). A few samples show more than one generation

of quartz crystals. The general paragenetic sequence consists

of pyrite, green clay (chlorite or mixed-layer chloritesmectite), ± bluish botryoidal chalcedony (cryptocrystalline

quartz), crystalline quartz, bladed or blocky calcite (fig. 49),

± very minute quartz crystals coating calcite. Hydrothermal

quartz was identified both in the groundmass of the pumiceous to lithic tuffs and as open-space fillings throughout most

of the RDO-1 drill cuttings below 332-m depth. Euhedral

open-space quartz crystals (as long as 2 mm) were deposited

later than pyrrhotite, mordenite, and calcite but formed earlier than some of the chlorite.

The four west-flank drill holes (GEO-N2, GEO-N5, SF

NC-01, and SF NC72-03) all contain quartz crystals in the

lower parts of the drill cores (figs. 16, 20, 23, and 26). Most

of these quartz deposits consist of tiny euhedral to subhedral

crystals that formed on top of botryoidal chalcedony (see fig.

22) in vesicles and fractures or between breccia fragments.

However, several core samples contain thin quartz veins or

tiny colorless crystals that formed in open spaces without

earlier chalcedony deposits.

Colorless, white, or bluish-gray cryptocrystalline chalcedony (mean index of refraction = ~1.53) lines vesicles, coats

fractures, and fills space between breccia fragments at depths

of 556.0 to 563.1 m, 701.0 to 744.0 m, and at scattered locations between 811.2- and 899.8-m depth in the USGS-N2 drill

core (fig. 5). The open-space cryptocrystalline silica deposits

generally have a massive, banded, or botryoidal texture and

have XRD peaks for chalcedony (quartz) and cristobalite in

the upper zone; below ~701-m depth, XRD analyses of the

HYDROTHERMAL MINERALS

Figure 46.—Scanning electron micrographs and photograph of crystalline chlorite at Newberry volcano. A, Rosettes of sheet-like chlorite

crystalline aggregates that coat fracture at 740.7-m depth in drill core USGS-N2. B, Randomly oriented aggregates of stacked, hexagonal,

platy chlorite crystals from vesicle filling at 931.0-m depth in drill core USGS-N2. C, Wormy-textured aggregates of chlorite crystals

deposited between breccia fragments along with acicular laumontite crystals at 1,120.4-m depth in SF NC-01 drill core.
62 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

microcrystalline to microfibrous silica deposits only show the

presence of chalcedony. Many of the chalcedony open-space

fillings are covered by later-formed tiny quartz crystals.

Five of the flank geothermal drill cores at Newberry volcano contain chalcedony deposits. A single sample of yellowish botryoidal chalcedony coats flow-breccia fragments

at 555.2-m depth in drill core GEO N-1 (fig. 14). Scattered

vesicle, fracture, or cavity fillings in the lower part of the

GEO-N2 drill core (fig. 16) contain colorless, white, or bluish, massive, banded, or botryoidal chalcedony. A few samples

in the upper part of the chalcedony zone also have XRD peaks

for cristobalite. Vesicle fillings of one core sample from 756.5-

m depth in drill hole GEO-N5 (fig. 20) are composed of lightgray botryoidal chalcedony and later, poorly developed quartz

crystals (see fig. 22). Open-space fillings in the lower part of

core hole SF NC-01 (fig. 23) contain similar colorless, frosted,

white, or bluish powdery, massive, banded, and botryoidal

chalcedony. Cristobalite also is indicated by XRD analyses

of a few samples from the upper part of this chalcedony zone.

Vesicles, fractures, and spaces between breccia fragments of

samples from the lower part of the SF NC72-03 drill hole

(fig. 26) have chalcedony with the same morphology and colors as the above flank drill holes; cristobalite is also identified in XRD analyses of these silica deposits.

SULFIDE MINERALS

PYRRHOTITE

Tabular pseudohexagonal bronze pyrrhotite crystals (as

long as 3 mm) (fig. 50A) line fractures and cavities and are

disseminated in the USGS-N2 drill core from depths of 460.9

to 466.8 m, 696.2 to 792.5 m, and 907.8 to 931.8 m (fig. 5).

Pyrrhotite generally is one of the earliest hydrothermal minerals deposited in these sections of drill core. A few core

samples in the upper zone contain replacement pyrite and

marcasite that appear to be pseudomorphous after pyrrhotite

(fig. 50B). Electron microprobe analyses of four pyrrhotite

samples from the two upper pyrrhotite zones are listed in table

12 (analyses 1-4); additional analyses are given by Bargar

and Keith (1984; appendix 7). Trace-element analyses show

that pyrrhotite from 713.2-m depth contains some As along

with minor Co, Cr, Cu, Mn, Pb, V, Y, and Zn (table 10). A plot

of atomic percent iron versus temperature (phase diagram for

the Fe-S system below 350 ̊C, fig. 51) indicates that the

USGS-N2 pyrrhotite should be monoclinic (type 4c) (Kissin

and Scott, 1982). Nine additional samples from the three pyrrhotite zones were X-rayed at 1/4 ̊/min.; in all nine X-ray

Figure 47.—Scanning electron micrograph of botryoidal opal

deposit at 613.3-m depth in USGS-N2 drill core.

Figure 48.—Scanning electron micrograph of hemispherical

aggregates of bladed cristobalite crystals deposited on calcite

at 900.1-m depth in SF NC-01 drill core.
63

diffractograms, doublet X-ray reflections (102) and (202) of

nearly equal intensity suggest that a hexagonal component, if

present, could only be of minor abundance (Arnold, 1966).

Euhedral bladed pyrrhotite crystals line open spaces of several drill cuttings chips between depths of 332 and 408 m in

the RDO-1 drill hole (fig. 12). The pyrrhotite crystals frequently are associated with quartz prisms and appear to be

the earliest hydrothermal mineral deposited in the RDO-1

drill-hole material.

The groundmass of a pale-gray-green, bleached, moderately porous dacitic core sample from 1,104.6-m depth in the

GEO-N2 drill hole (fig. 16) contains oxidized bladed sulfide

that was identified as pyrrhotite by an X-ray-powder camera

film. This sample is the only pyrrhotite positively identified

in any of the flank drill cores. However, a fracture at 1,351.5-

m depth in the SF NC72-03 drill core (fig. 26) contains very

tiny hexagonal bladed (pyrrhotite?) crystals like those in figure 50A; EDS analysis using the SEM shows that they are

composed of Fe and S.

Browne and Ellis (1970) and Steiner (1977) report finding pyrrhotite in the drill cores from New Zealand geothermal areas at temperatures ranging from 152 to 268 ̊C. Pyrrhotite in drill cores from Yellowstone National Park occurs

at drill hole temperatures of 130 to 152 ̊C (Bargar and Beeson,

1981). In the USGS-N2 drill hole, the measured temperatures

the three pyrrhotite zones were about 97 ̊C, 110 to 180 ̊C,

and 250 to 265 ̊C. Measured temperatures in the pyrrhotite

zone of the RDO-1 drill hole were above 130 ̊C. The measured temperatures at the depths where the pyrrhotite was

located in drill holes GEO-N2 and SF NC72-03 were about

140, and 150 ̊C, respectively.

PYRITE

Cubic pyrite crystals are present in the USGS-N2 drill core

at depths of 383.7 m, 388.6 m, 446.2 to 828.9 m, and in scattered concentrations below 828.9 m (fig. 5). Pyrite is closely

associated with siderite, smectite, and marcasite in the main

sulfide zone. Microprobe analyses of six pyrite samples, listed

in table 12 (analyses 5-10), show minor Co in addition to Fe

and S. Significant amounts of the trace elements As, Co, Cu,

and Zn, as well as some Cd, Ce, Cr, La, Mn, Nd, Ni, Pb, Sn,

V, Y, and Yb, are present in an analysis of pyrite from 930.1-

m depth in the drill hole (table 10). Several samples from

between depths of 338 and 399 m in the RDO-1 drill cuttings

(fig. 12) contain disseminated pyrite or pyrite in thin veins,

but it is not very abundant in any of the samples.

Pyrite is present in all of the flank drill holes except for

drill hole GEO-N4. Small patches of disseminated, very

HYDROTHERMAL MINERALS

Figure 49.—Vesicle fillings (A and B) in basaltic breccia fragments at 793.1-m depth in USGS-N2 drill core. Bottoms of vesicles have

horizontal layer of pyrite and green chlorite; pyrite and traces of chlorite also are deposited along walls of vesicles. Disseminated pyrite

and very minor chlorite occurs throughout groundmass of basaltic breccia fragments. Vesicles were later filled with euhedral quartz

crystals and finally calcite.
64 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

minute (~ 0.02 mm) yellow metallic cubic pyrite crystals were

identified only at 943-, 945-, and 1,068-m depths in the GEO

N-1 drill core (fig. 14). In the lowest sample, a pyrite veinlet

crosscuts iron oxide deposits, and pyrite crystals formed later

than smectite in vesicles. Pyrite occurs in three zones (1,030.8

m, 1,128.4 to 1,131.4 m, and 1,278.3 to 1,332.3 m) in the

GEO-N2 drill core (fig. 16) as tiny crystals on fractures and

in vesicles. Samples from the middle zone also contain colorless anhydrite crystals. Some of the open-space pyrite deposits in the other two zones appear to be slightly oxidized

with a surrounding halo of orange-brown staining. Pyrite lines

fractures and vesicles and coats open spaces between rock

fragments in scattered core samples below 1,009.0-m depth

in the GEO-N3 drill core (fig. 18). Most of the pyrite is very

fine grained; a few samples have larger cubic crystals that

are about 1 mm diameter. At depths of 1,020.2, 1,020.3, and

1,040.0 m, tiny white fibrous gypsum crystals probably

formed due to post-drilling oxidation of pyrite.

Tiny cubic pyrite crystals were deposited on fractures,

in vesicles, and between breccia fragments in the lower part

of the GEO-N5 drill core (fig. 20). At 958.6-m depth, the

apparent bladed habit of the pyrite suggests replacement of

a precursor sulfide mineral. Pyrite crystals were found in

open spaces of three zones in the SF NC-01 drill core (fig.

23). One sample in the upper zone contains a fracture filling with opal and smectite that both contain disseminated

cubic and pyritohedral pyrite crystals (see fig. 24). Euhedral

to subhedral pyrite crystals (fig. 52A) are commonly associated with siderite and smectite in this drill core. Several

samples from the lower half of the SF NC72-03 drill core

(fig. 26) contain pyrite on fractures and in vesicles. Some

pyrite is disseminated in smectite and cryptocrystalline silica

(cristobalite and quartz or chalcedony in X-ray analyses)

open-space deposits (see fig. 24).

MARCASITE

Marcasite occurs as cavity and fracture deposits, in close

association with pyrite and pyrrhotite, at scattered locations

between depths of 452.8 and 627.0 m in the USGS-N2 drill

core (fig. 5). Marcasite typically has a tabular-prismatic habit

(fig. 50B) and locally shows the characteristic cockscomb

morphology of twinned crystals (fig. 52B). Marcasite is

Figure 50.—Scanning electron micrographs of pyrrhotite, marcasite, and pyrite at Newberry volcano. A, Laminar aggregate of hexagonal pyrrhotite crystals and later chlorite partly filling vug at 907.8-m depth in USGS-N2 drill core. B, Pseudomorphous replacement of

large bladed pyrrhotite(?) crystals by small tabular marcasite and massive to blocky pyrite (identified by XRD) crystals on fracture at

534.8-m depth in USGS-N2 drill core.
65

metastable with respect to pyrite at low temperature (Craig

and Scott, 1974) and is converted to pyrite at temperatures

greater than 160 ̊C in the Salton Sea geothermal system of

California (McKibben, 1979). Marcasite occurs at temperatures of about 80 to 170 ̊C in drill cores from Yellowstone

National Park (Bargar and Beeson, 1984) and at temperatures slightly below ~140 ̊C at Steamboat Springs, Nevada

(Sigvaldson and White, 1962). The present temperature range

at which marcasite occurs in the USGS-N2 drill core is ~74

to 98 ̊C. Several electron microprobe analyses of marcasite

from 565.7-m depth (appendix 9 in Bargar and Keith, 1984;

and table 12, analysis 11 of this report) show that the mineral

is fairly homogeneous with respect to both Fe and S and contains no detectable Co or Ni.

SPHALERITE

Thin fracture fillings at 789.4-m depth (temperature <60 ̊C)

in the GEO-N5 drill core (fig. 20) contain early black euhedral

crystals of sphalerite (see fig. 21) in association with later

soft brown siderite and colorless calcite. EDS chemical analysis of the sphalerite using the SEM shows Zn and S with significant Fe and is the basis for the sphalerite identification.

Sphalerite occurs at temperatures above 300 ̊C in geothermal drill holes of the Salton Sea area, California (McKibben

and Elders, 1985). Minor sphalerite also has been found in

some geothermal areas of New Zealand (Browne, 1969;

Browne and Lovering, 1973; Steiner, 1977) at temperatures

of 120 to 298 ̊C. The waters from which the New Zealand

sphalerite precipitated have low salinity and only trace

amounts of Zn (Browne and Ellis, 1970). The Newberry

sphalerite probably formed during some stage of cooling of

the volcanic rocks at somewhat higher temperatures than occur at present.

SULFATE MINERALS

ANHYDRITE

Colorless blocky anhydrite crystals formed on fractures

at 914.7 m to 915.9 m in drill hole USGS-N2 (fig. 5), along

with later mixed-layer chlorite-smectite (fig. 53), where

the temperature measured during drilling was about 258 ̊C.

The presence of anhydrite at depth in the Newberry drill

cores probably is due to oxidation of H2S in the fluids.

Subsequent fluid chemistry must have changed because

the anhydrite appears to have been partly dissolved either

before or during deposition of the mixed-layer clay. Anhydrite also was identified in fractures and vesicles of the

GEO-N2 drill core at depths of 1,128.4 to 1,131.4 m and at

1,323.1 m (fig. 16). The anhydrite crystals are colorless and

tabular, have a mean index of refraction of about 1.57, and

HYDROTHERMAL MINERALS

Table 12.—Electron microprobe analyses of sulfide minerals from USGS-N2 drill core.

Figure 51.—Monoclinic pyrrhotite and monoclinic pyrrhotite-pyrite fields of phase diagram for Fe-S system below 350 ̊C (after Kissin

and Scott, 1982) showing electron microprobe compositions of pyrrhotite from four depths in USGS-N2 drill core. USGS-N2 temperatures at depths where pyrrhotite was identified are from Sammel (1981).

300

250

200

150

100

48 47 46

Monoclinic pyrrhotite – 

pyrite

Field boundary

Mono-

 clinic

pyrrhotite

Field boundary

modified by 

Kisson and

Scott (1982)

ATOMIC PERCENTAGE Fe IN Fe-S SYSTEM 

350

TEMPERATURE, IN DEGREES CELSIUS

Pyrite
66 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

are usually associated with pyrite crystals that appear to be

partly oxidized.

BARITE

One cavity at 1,293.4-m depth in the GEO-N1 drill core

(fig. 14) contains very tiny colorless crystals that were identified as barite by XRD analysis. Barite characteristically

forms in hydrothermal environments associated with metalliferous deposits (Phillips and Griffen, 1981; Krupp and

Seward, 1987); it is rarely reported from nonmetalliferous

modern geothermal areas.

JAROSITE, NATROALUNITE, AND NATROJAROSITE

Several sulfate minerals were identified in the GEO-N3

drill core (fig. 18). Six samples between depths of 809.2 and

918.7 m have vesicles, fractures, or open spaces in scoria that

are coated by yellow, orange, or greenish-yellow, intergrown,

finely crystalline hexagonal tabular jarosite crystals (fig. 54).

Similar white or yellow, hexagonal, tabular, natrojarosite

crystals (fig. 55) line vesicles, fractures, and open spaces of

scoria at depths of 846.8 to 851.3 m, 893.1 to 896.5 m,

and 995.9 m in the GEO-N3 drill core. Another white

or yellow sulfate mineral, natroalunite, also is found

in vesicles, fractures, and other open spaces of seven

scattered samples between 837.6 m-depth and the bottom

of this drill core.

Jarosite [KFe3

+3(SO4

)

2

(OH)6], natrojarosite [NaFe3

+3(SO4

)

2

(OH)6] and natroalunite [NaAl3(SO4)2(OH)6] are all members of the alunite group of minerals. These minerals generally form in near-surface hydrothermal acid-sulfate or fumarolic conditions (Hemley and others, 1969). The drill core

in this zone and above has the appearance of fumarolic

conditions with yellowish-orangish-reddish iron oxide

deposits and staining in open spaces. However,

pyrite occurs in many drill-core samples beneath the sulfate

zone. The three alunite-group sulfate minerals, jarosite,

Figure 52.—Scanning electron micrographs of pyrite and marcasite crystals at Newberry volcano. A, Octahedral pyrite crystal with

earlier(?) siderite and later smectite coating fracture surface at 326.1-m depth in SF NC-01 drill core. B, Marcasite crystals with

cockscomb habit accompanied by later smectite (pyrite and siderite also identified by XRD), from fracture at 565.7-m depth in USGSN2 drill core.
67

natrojarosite, and natroalunite, in the GEO-N3 drill hole may

have formed from either acid-sulfate conditions due to oxidation of pyrite or precipitation from fumarolic gases.

GYPSUM

Gypsum was identified at depths of 1,020.2 and 1,040.0 m

in the GEO-N3 drill core (fig. 18). Sprays of tiny white fibrous

gypsum crystals occur in open spaces in association with pyrite. Gypsum probably formed as a result of minor oxidation

of pyrite crystals, but we are not certain if the gypsum precipitated before or after the drill core was brought to the surface.

Minor gypsum was occasionally observed deposited on top of

pyrite crystals in a few of the other drill cores where the mineral does not appear to be related to the present hydrothermal

conditions within the drill holes.

IRON OXIDE AND IRON HYDROXIDE MINERALS

Several iron oxide or iron hydroxide minerals occur in

the drill cores from the flanks of Newberry volcano or from

inside the caldera. In the mineral distribution diagrams, these

minerals are lumped together under the general heading of

iron oxide.

HEMATITE AND LEPIDOCROCITE

Hematite was identified by XRD analyses of material from

a few vesicles, fractures, altered mafic minerals, and oxidized

zones at depths of 599.8 to 600.5 m, 804.5 m, 833.9 to 860.1

m, and 914.7 to 917.1 m in the USGS-N2 drill core (fig. 5). A

narrow zone from 464.5- to 466.0-m depth contains oxidized

crusts that coat pyrrhotite crystals. XRD analyses show that

the crusts are composed of sulfur, goethite, and lepidocrocite

in addition to pyrrhotite, pyrite, and marcasite. Presumably,

the two iron oxide minerals and sulfur are by-products of the

alteration of pyrrhotite to pyrite and marcasite. Red iron oxide stains were observed in three samples from the lower part

of the RDO-1 drill hole (fig. 9); the stains are identified as

hematite by Keith and others (1986).

GOETHITE

XRD analyses of three brown to orange clayey fracture

coatings and breccia open-space fillings from depths of 906.0,

HYDROTHERMAL MINERALS

Figure 53.—Scanning electron micrograph of blocky anhydrite

crystals partly coated by later mixed-layer chlorite-smecite rosettes on fracture surface at 915.2-m depth in USGS-N2 drill

core.

Figure 54.—Backscatter image (on scanning electron microscope) of intergrown tabular jarosite crystals from vesicles at

918.7-m depth in GEO-N3 drill core.
68 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

917.7, and 921.6 m in the GEO-N3 drill core show the presence of smectite, natroalunite, and goethite (fig. 18). Goethite is an iron hydroxide mineral that commonly forms due

to weathering or low-temperature hydrothermal alteration of

iron-rich sulfides and oxides (Deer, Howie, and Zussman,

1966). No sulfide or iron oxide minerals were observed in

these drill core samples; either these minerals were completely

altered or the goethite and natroalunite formed due to the

oxidation of iron sulfide in rock adjacent to the drill-core

samples.

MAGNETITE

A few cavities containing euhedral, high-temperature, vapor-phase minerals that formed during cooling of the lava at

1,180.2-m depth in the GEO-N2 drill core (fig. 16) have associated black to red blocky crystals. These crystals may have

been high-temperature, vapor-phase, magnetite crystals that

are partly to completely altered to red hematite or amorphous

iron oxide. Vapor-phase titanomagnetite crystals (significant

Ti in EDS analysis using the SEM) also rim a vesicle at

1,299.1-m depth in this drill core.

Several drill-core samples from below 1,132.2-m depth in

the GEO N-3 drill hole (fig. 18), at 972.3-m depth in drill

hole GEO-N5 (fig. 20), and at depths of 1,136.6, 1,164.9,

and 1,288.7 m in drill hole SF NC72-03 (fig. 26) have fractures and vesicles that are lined by black sooty or clayey deposits and black botryoidal precipitates that were identified

as magnetite in XRD analyses. Botryoidal magnetite appears

to have formed earlier than other secondary minerals, and

some of the black magnetite spheres and hemispheres appear

to form the core for later botryoidal siderite crusts. The secondary magnetite frequently is partly altered to reddish amorphous iron oxide or hematite (fig. 56). These magnetite openspace deposits probably formed during cooling of the lava

flows and are of vapor phase or deuteric origin.

AMORPHOUS IRON OXIDE

Yellow, orange, brown, red, and black iron oxide stains

the interflow breccias, scoria deposits, ash layers, and tuffaceous beds in most of the Newberry flank drill cores. XRD

analyses of these deposits usually indicate that the iron oxide

is amorphous. Most of the staining probably forms by oxidation of primary magnetite during cooling of the volcanic deposits and is deuteric; however, some staining may be due to

Figure 55.—Scanning electron micrograph of hexagonal tabular natrojarosite crystals from fracture at 995.9-m depth in

GEO-N3 drill core; euhedral natrojarosite crystals are partly

coated by smectite.

Figure 56.—Part of large cavity filling of red botryoidal hematite (individual spheres are ~0.5 mm in diameter) and later

colorless calcite from 1,343.6-m depth in SF NC72-03 drill

core.
69

weathering processes. XRD studies of iron oxide-stained vapor-phase crystals (including magnetite), groundmass magnetite grains, and botryoidal deuteric(?) magnetite deposits

generally show the presence of hematite, although sometimes

the deposits appear to be amorphous iron oxide. These hematite and amorphous iron oxide deposits could be deuteric or

they could form due to weathering or low-temperature hydrothermal alteration. Other clayey hematite fracture and

vesicle fillings and altered siderite crystals undoubtedly were

produced by hydrothermal alteration.

OTHER MINERALS

APATITE

Crystals of a hexagonal, tabular to prismatic apatite group

mineral (fig. 57) line a cavity at 909.8-m depth in the USGS

N-2 drill core where the temperature measured following drilling was ~250 ̊C (fig. 5). Similar euhedral hydrothermal apatite crystals occur in Yellowstone National Park drill core Y6, where the temperature measured during drilling was about

100 ̊C (Bargar and Beeson, 1984). Steiner (1977) found hexagonal prismatic hydrothermal apatite crystals (no temperature given) that are pseudomorphous after mafic phenocrysts

in the Wairakei geothermal area of New Zealand. Apatite also

is present in the Larderello geothermal area, Italy, along with

epidote and datolite, which suggests that the minerals precipitated at more than 200 ̊C (Cavarretta, Gianelli, and Puxeddu,

1980).

APOPHYLLITE

The identification of an apophyllite group mineral at 318.5-

m depth in USGS-N2 drill core (fig. 5; temperature ~50 ̊C) is

based on a single multicomponent XRD pattern. To the best of

our knowledge, apophyllite has not been reported previously

from a geothermal system; however, the mineral is not rare and

is frequently associated with zeolite minerals in basalts (Dunn,

Rouse, and Norberg, 1978).

COPPER

Native copper was identified in vesicles at 1,186.0-m depth

in drill core GEO-N2 (fig. 16) and at 1,131.0-m depth in the SF

NC72-03 (fig. 26) drill core (fig. 58). Both drill holes are on the

west flank of Newberry volcano. Measured temperatures at the

copper-bearing sample depths of the two drill holes are about

120 to 150 ̊C. Fluid-inclusion homogenization temperatures in

quartz and calcite from the SF NC72-03 drill hole range from

near the present conditions up to about 311 ̊C. Native copper

has been observed in a few other geothermal drill holes in the

Cascade Range of Oregon (Bargar, 1980, 1990; Bargar, Keith

and Beeson, 1993; Keith and Boden, 1981). Native copper commonly occurs in basaltic lavas owing to chemical reactions beHYDROTHERMAL MINERALS

Figure 57.—Scanning electron micrograph of hexagonal, tabular to columnar apatite crystals with later chlorite that line vesicle

at 909.8-m depth in USGS-N2 drill core.

Figure 58.—Scanning electron micrograph of striated grain of native copper partly filling vesicle at 1,131.0-m depth in SF NC72-03

drill core.
70 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

tween copper-bearing low-temperature hydrothermal waters and

hematite or other iron-bearing minerals (Hurlbut, 1971). In the

USGS-N2 drill-hole samples, the more silicic lavas have lower

Cu values (sample nos. 1065 to 2467.5 in table 3). The Cu concentration for one basalt specimen from the SF NC72-03 drill

hole is greater than Cu values for more silicic drill core samples

from the flank drill holes (table 13).

EPIDOTE

Euhedral yellow-green epidote crystals (fig. 59) line vugs

and fractures from depths of 911.5 to 912.6 m and 916.4 to

916.7 m in the USGS-N2 drill core (fig. 5). The twinned epidote crystal clusters occur in association with earlier quartz and

calcite and later(?) chlorite or mixed-layer chlorite-smectite;

some core samples also have patches of reddish hematite. Multiple electron microprobe chemical analyses across three epidote crystals (table 14) suggest some Fe-Al substitution with

very slight Fe enrichment and Al depletion near the centers of

two of the crystals. Selected optical and crystallographic data

for epidote from 916.7-m depth in the USGS-N2 drill hole are

given in table 15 (R.C. Erd, written commun., 1991).

The measured temperature at which epidote was found in

the USGS-N2 drill hole was about 258 ̊C, which is well above

the 230 ̊C minimum stability limit given for epidote that forms

at low-pressure conditions characteristic of geothermal areas

(Seki, 1972). Bird and others (1984) subsequently documented

calc-silicate temperature data from several well-studied natural

systems and indicated that epidote most commonly is deposited at temperatures between 200 and 250 ̊C.

Table 13.—Trace element analyses (in ppm) of drill-core samples

outside Newberry caldera.

Figure 59.—Scanning electron micrographs showing clusters of euhedral twinned epidote crystals (A and B) from cavity at 916.4-m

depth in USGS-N2 drill core.
71

HYDROGROSSULAR

A whole-rock XRD analysis of reddish basaltic sediments

from about 315-m depth in drill hole USGS-N2 (fig. 5) gives a

pattern nearly identical to that of tricalcium aluminate hexahydrate [Ca3Al2(OH)12] or nonsilicic hydrogrossular (Zabinski,

1966). The X-ray pattern is very similar to that of synthetic

Ca3Al2(OH)12 (JCPDS 24-217). SEM studies of the USGS-N2

sample show that the mineral forms tiny dodecahedral(?)

crystals (fig. 60) in pore spaces of the caldera-fill materials.

According to Zabinski (1966), a true silica-free hydrogrossular has not been found to occur naturally. However, Passaglia

and Rinaldi (1984) described a hydrogrossular group mineral

(katoite) which has much lower silica content than other members of this garnet group. A single electron microprobe analysis

of the USGS N-2 drill core materials gives (in weight percent

oxides) 4.30 SiO2, 38.24 Al2O3, 1.97 TiO2, 13.99 FeO, 0.38 MgO,

0.05 MnO, and 38.69 CaO (total oxides = 97.62). This analysis

is substantially lower in silica than the katoite analysis and has

extraordinarily high alumina and iron contents compared with

reported analyses of hydrogrossular (Passaglia and Rinaldi,

1984).

Garnet has been reported from several modern geothermal

systems where it apparently formed at temperatures above 300 ̊C

(Bird and others, 1984). The measured temperature at which

the USGS-N2 garnet occurs is only about 50 ̊C, and there is no

evidence that temperatures in this part of the drill core ever approached 300 ̊C. Zabinski (1966) indicates that hydrogrossular

of composition Ca3Al2[OH]12 may form as a crystalline phase

in the hydration of Portland cement. Cement and other foreign

materials are routinely used in completing geothermal drill holes

in the Cascade Mountains. The hydrogrossular crystals found

in the USGS-N2 drill core sample appear to be a late phase and

may have been introduced during drilling.

DISCUSSION

FACTORS CONTROLLING HYDROTHERMAL

ALTERATION

Factors that influence the distribution and kind of mineral

assemblages present in hydrothermal systems include permeability, rock and water composition, temperature, pressure, and

duration of hydrothermal alteration (Browne and Ellis, 1970).

These factors are largely interdependent, but the effects of one

or more of the factors can exert a dominant influence in the

location and extent of hydrothermal alteration. Permeability of

the rocks controls the access of thermal fluids, which cause hydrothermal alteration of the rocks and precipitation of secondary minerals in open spaces. Obviously rocks that have very

restricted permeability or are completely impervious to fluids

will be only slightly altered. Crystallinity of the host rock is of

HYDROTHERMAL MINERALS

Table 14.—Electron microprobe analyses of epidote from 916.7-m depth in USGS-N2 drill core.
72 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Table 15.—Optical and crystallographic data for epidote from

916.7-m depth in USGS-N2 drill core.

[Determined by R.C. Erd, U.S. Geological Survey, Menlo Park,

Calif. (written commun., 1991)]

Specimen No.: N2-3007.5

Crystal Size:

Length (Y) ----------- 0.21 mm

Width (X) ------------ 0.055 mm

Width (Z) ------------ 0.063 mm

Indices of refraction:

α ----------------------- 1.745 ±0.003

β ----------------------- 1.774 ±0.003

γ ----------------------- 1.787 ±0.003

Birefringence (γ - α) ----------- 0.042

2Vx ------------------------------- 68 ±1 ̊ measured

Dispersion ----------------------- r > v, strong

Pleochroism --------------------- X = light greenish brown

Y = greenish brown

Z = yellow-green

Y ≥ Z > X

Optical orientation ------------- X : c = + 8 ̊

Y : b

Z : a = + 33 ̊

importance because glass is altered more easily than crystalline

rock. Chemical composition of the host rock determines the

availability of cations to form alteration minerals. Similarly, the

cation and anion content of the fluids interacting with the rocks

influences the composition of alteration minerals. Temperature

may be the most significant factor in hydrothermal alteration

because many of the chemical reactions require elevated temperatures. Pressures at the depths penetrated by most of the

Newberry flank drill holes probably are not sufficient to greatly

affect the hydrothermal alteration (Browne and Ellis, 1970).

However, the effects of past higher pressure within the caldera,

possibly owing to an overlying deep intracaldera lake, apparently resulted in higher fluid-inclusion homogenization temperatures than can be accounted for by the present geothermal

system. Duration of the hydrothermal alteration at Newberry

volcano is not precisely known. Hydrothermal alteration is

undoubtedly occurring at the present time and likely extended

back through the Holocene and probably into the late Pleistocene following the first volcanic eruptions from Newberry

volcano.

Data on the factors controlling hydrothermal alteration are

most complete for the USGS-N2 drill hole, which was studied in much greater detail than the others (Keith and Bargar,

1988). Conclusions based on the study of the USGS-N2 drill

core may also be applicable to the other drill cores.

PERMEABILITY

On the flanks of Newberry volcano, pronounced lateral

and vertical permeability were expected prior to drilling on

the basis of the available information concerning the types of

rocks that constitute this volcano (Black, 1983). Lava flows

in the flank drill holes generally are vesicular at the top and

bottom and dense in the interior; intervening intervals contain steeply dipping hairline fractures. Volcanic deposits between most lava flows include flow breccia, vitric tuff, lithic

tuff, volcanic sandstone, and scoria or cinders. Fractures,

vesicles, and void spaces between breccia fragments are partly

to completely filled in the lower parts of the seven sampled

drill holes located outside the caldera. Partial filling of the

open spaces reduces permeability somewhat but does not

completely eliminate the passage of water through the rocks.

The upper parts of most of these flank drill holes contain unfilled open spaces. Ash and cinder layers and tuffaceous strata

appear to have high permeability where unaltered; at deeper

Table 15.—Optical and crystallographic data for epidote from 916.7-

m depth in USGS-N2 drill core.

Figure 60.—Scanning electron micrograph of subhedral

hydrogrossular crystals in volcaniclastic sandstone from 314.6-

m depth in USGS-N2 drill core.
73

intervals some of these deposits are pervasively altered to

clay, and the present permeability is presumably quite low.

MacLeod and Sammel (1982) indicated that vertical permeabilities within Newberry caldera are low except for fluid

movement along faults, brecciated intrusion conduits, and ring

fractures. They further suggest that lateral fluid flow is restricted to permeable zones having access to vertical fracture

zones that contain water. In the USGS-N2 drill hole, several

intervals appear to have maintained substantial subhorizontal

permeability, allowing passage of both cold meteoric and thermal fluids (MacLeod and Sammel, 1982). High to moderate

primary permeability was present in tuff, tuff breccia, and

flow breccia, and, more locally, in highly vesicular parts of

lava flows. Primary permeability within lava flows was very

low because of the dense, fine-grained crystalline nature of

the rocks. Consequently, the lava-flow interiors are much less

altered than the more permeable rock units. Secondary permeability in dense lava flows was provided by hairline fractures that were rapidly sealed by deposition of hydrothermal

minerals (mostly clay minerals), which resulted in a decrease

or cessation of fluid flow through many rock units penetrated

by the drill hole (especially below 697-m depth). These dense

lava flows may form an impermeable cap for a deeper thermal convection system for the USGS-N2 area, as proposed

by Sammel, Ingebritsen, and Mariner (1988).

Unfilled open spaces in the drill-chip samples suggest that

permeability of most of the volcanic units penetrated by the

RDO-1 drill hole is quite high. Below 307.9-m depth, hydrothermal minerals are deposited in open spaces and replace

glass, which locally reduces the permeability. Vesicles may

be completely filled by mordenite and chlorite, but bladed

calcite and euhedral quartz crystals only partly fill open spaces

and the rock remains pervious. The presence of unaltered

pumice and the high permeability led Keith and others (1986)

to suggest that the volcanic units penetrated by the RDO-1

drill hole are in an early stage of hydrothermal alteration.

T E M PER AT U R E

Temperature profiles for the seven drill holes on the flanks

of Newberry caldera are shown in figs. 14, 16, 18, 19, 20, 23,

and 26. Hydrothermal alteration minerals from drill holes

GEO-N2, SF NC-01, and SF NC72-03 (figs. 16, 23, and 26),

located on the west side of the volcano, include carbonate

minerals, iron oxide, and smectite, in addition to higher temperature minerals such as quartz, mixed-layer chloritesmectite, and chlorite. Measured bottom-hole temperatures

for these drill holes are higher than 150 ̊C, and fluid-inclusion studies of calcite and quartz (figs. 17, 25, and 27) show

that the past temperatures were significantly hotter than the

present-day temperatures. This is especially true near the ring

fracture, where fluid-inclusion homogenization temperatures

(Th) are mostly greater than 200 ̊C and some are above 300 ̊C.

Measured temperatures throughout the GEO-N1, GEON3, GEO-N4, and GEO-N5 drill holes, located on the south,

north, east, and southwest flanks of the volcano, are less than

100 ̊C. Hydrothermal alteration mineralogy of the four drill

holes is dominated by carbonate minerals, smectite, and iron

oxide (figs. 14, 18, 19, and 20). The presence of these minerals and lack of higher temperature minerals, such as quartz,

chlorite, or mixed-layer illite-smectite (except for a single

sample at the bottom of the GEO-N3 drill hole), indicate that

temperatures resulting from circulating thermal fluids probably never were very much hotter than the highest temperatures measured following drilling.

Temperatures in three other geothermal drill holes (USGSN1, CE NB-3, and CE NB-4) on the flanks of Newberry volcano were below 100 ̊C. Hydrothermal alteration in these

drill cores either is absent or is similar to that of the other

low-temperature flank drill holes.

Measured temperatures in the USGS-N2 drill hole correlate closely with the temperature ranges indicated by hydrothermal minerals deposited throughout the drill core (fig. 5).

Clay minerals are the best indications of temperature in the

drill core. Smectite forms at lower temperatures; mixed-layer

chlorite-smectite is deposited at intermediate temperatures;

and chlorite precipitates at higher temperatures. However,

these clay minerals are not strictly dependent on temperature. The structure of clay minerals may vary depending on

whether the clay replaces glass or fills open spaces. Clay minerals with different structures may even coexist; mixed-layer

chlorite-smectite in the groundmass interstices coexists with

chlorite vesicle fillings at the bottom of the USGS-N2 drill

core. Above 697-m depth, the hydrothermal alteration mineralogy suggests that temperatures probably have not been

higher than the present measured temperatures except in the

contact zones of the rhyodacite sill and thin dikes. Alteration

is more intense in these zones, and the hydrothermal minerals were formed at higher temperatures than the secondary

minerals in the adjacent tuffs. Alternatively, the present temperature maximum of about 100 ̊C between 400- and 425-m

depth (fig. 5) suggests a lateral influx of thermal water. This

water probably was as hot as the waters that deposited the

chlorite-quartz-calcite-mordenite assemblage below 355-m

depth in drill hole RDO-1 (Keith and others, 1986). Minimum fluid-inclusion Th values for the lower part of drill hole

USGS-N2 generally are warmer or nearly coincident with

the measured temperature curve (fig. 11). However, the maximum Th values of the liquid-rich fluid inclusions (as high as

367 ̊C at 931-m depth) greatly exceed the theoretical reference boiling-point curve. It appears likely that these high-Th

measurements resulted from increased pressure in the vicinity of the USGS-N2 drill hole at the time the fluid inclusions

formed because of an overlying deep intracaldera lake. Lacustrine sediments between 290- and 320-m depth in the USGSN2 drill core tend to support this hypothesis.

In the RDO-1 drill hole, smectite occurs between depths

of about 308 m and 350 m where the temperature measured

during drilling was about 80 to 150 ̊C (fig. 12). Chlorite was

identified below the smectite zone where temperatures probably exceeded 158 ̊C (Keith and others, 1986). The present

HYDROTHERMAL MINERALS
74 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

aquifer temperature for the lower part of the RDO-1 drill hole

is presumed to be greater than 158 ̊C (Black, Priest, and

Woller, 1984). However, Th measurements of liquid-rich fluid

inclusions in hydrothermal quartz crystals at 335- and 381-m

depth suggest that past temperatures were as high as about

252 ̊C (fig. 13). These high-Th values plot above a reference

theoretical boiling-point curve drawn to the present ground

surface, and they appear to support the conclusion that the

fluid inclusions formed at a time when the pressure was greater

because of a deep intracaldera lake. Present temperatures in

the RDO-1 drill hole are hotter than at the same depths in the

USGS-N2 drill hole. The RDO-1 drill hole is closer to the

southern ring fracture, which suggests that hot fluids probably move upward along the ring fractures and then spread

out laterally within the caldera-fill deposits.

FLUID COMPOSITION

Complete chemical analyses of fluids from the seven flank

Newberry geothermal drill holes are lacking. The only published water-chemistry data for these drill holes are given in

Swanberg, Walkey, and Combs (1988) and Walkey and

Swanberg (1990) who reported silica values for fluids collected from drill holes GEO-N1, GEO-N2, and GEO-N3. For

drill hole GEO-N1, Swanberg, Walkey and Combs (1988)

indicated that the use of fluid chemistry is inconclusive in

locating depths of geothermal fluids. On the other hand, they

indicated that SiO2 data for the lower part of the GEO-N3

drill hole do show the presence of a geothermal fluid. Also,

according to Swanberg, Walkey, and Combs (1988), low values for SiO2 and other fluid constituents throughout much of

the GEO-N3 drill hole suggest a dilution effect by a rain curtain of cold descending groundwater. High concentrations of

SiO2 and other fluid constituents for two parts of the GEON2 drill hole are interpreted by Walkey and Swanberg (1990)

as showing the presence of pore fluids rather than circulating

geothermal waters.

Melting-point temperature (Tm) measurements of fluid

inclusions in quartz and calcite from the GEO-N2, SF NC01, and SF NC72-03 drill cores are sparse (table 6). The limited data show values of 0.2, 0 to 0.4 , and 0.7 weight percent

NaCl equivalent for the salinities of these respective drill

holes. The data suggest a slight increase in the fluid salinity

with decreasing distance toward the ring fracture system on

the west side of Newberry volcano. Slightly saline fluid moving up the ring fractures would be quickly diluted by mixing

with meteoric water during lateral movement outward from

the caldera.

Tm values for fluid inclusions in two quartz samples from

381-m depth in the RDO-1 drill core indicate that the fluid

salinities are in the range of about 0.7 to 1.2 weight percent

NaCl equivalent. Near the bottom of the USGS-N2 drill hole,

salinities of the fluid inclusions are less than 0.4 weight percent NaCl equivalent except for the lowermost sample at 931-

m depth where the salinities are about 1.4 to 1.9 weight percent NaCl equivalent. The highest salinity water in USGSN2 drill hole fluid-inclusion specimens appears to represent

fluid that has been least diluted by meteoric water. The RDO1 drill hole is closer than the USGS-N2 drill hole to the southern ring-fracture system, which might account for higher salinity, hotter fluids at shallower levels in the RDO-1 well.

Chemical analyses of water from the RDO-1 drill hole were

not obtained before the well was abandoned. Dilute liquid

(pH ranging from 5.3 to 6.1) sampled at the separator during

a 20-hour flow test of the USGS-N2 drill hole (Sammel, 1981;

Keith and others, 1984b) was steam condensate that probably originated as a mixture of steam and CO2 in the bottom

2 m of the drill hole (Carothers, Mariner, and Keith, 1987;

Ingebritsen and others, 1986). However, it is uncertain whether

the steam originated from a true two-phase fluid system in

the rock formation or resulted from flow testing. The high

CO2 content of the sampled bottom fluid may indicate high

partial pressure of CO2 (pCO2) in porous zones throughout

the USGS-N2 drill hole. This high pCO2 could account for

the scarcity of epidote (Browne, 1978) at higher temperatures near the bottom of the drill core, as well as the paucity

of zeolites (Muffler and White, 1969) at lower temperatures

in the glassy tuffs of the upper part of the drill core where

zeolites would be expected to be abundant.

Fluids with elevated oxygen fugacity (fO2) are suggested

by the formation of iron oxide at permeable horizons (fig. 5)

and by deposition of sulfate minerals—natrojarosite (not

shown in fig. 5) and anhydrite—at depths where the influx of

more oxidized thermal meteoric water has occurred. However, such oxidation can result from boiling fluids in an open

system. At 250 ̊C, only 2 percent boiling is necessary to cause

extensive separation of gases (H2S and CO2) from the liquid

to the extent that the volume of vapor is equal to the volume

of liquid, leaving the liquid in an oxidizing state (Ellis, 1970;

Drummond and Ohmoto, 1985). On the basis of evidence from

New Zealand (Tulloch, 1982), the bladed morphology of calcite crystals might indicate that boiling occurred in more permeable intervals of the USGS N-2 drill core. However, colorless bladed calcite crystals also occur near the bottom of

the SF NC72-03 drill hole where the temperature does not

appear to have ever exceeded the boiling point.

The occurrence of pyrrhotite has been attributed

to high H2 levels relative to H2S and O2 in the Broadlands

geothermal field, New Zealand (Browne, 1970; Browne and

Ellis, 1970), and at Krafla, Iceland (Steinthórsson and

Sveinsbjörnsdóttir, 1981). At the Broadlands field, pyrrhotite

crystallizes in low-permeability rocks in a chemical environment caused by confined boiling (Browne, 1970). At Krafla,

the early deposition of pyrrhotite is interpreted as being indicative of the underlying magmatic processes (Steinthórsson

and Sveinsbjörnsdóttir, 1981). Pyrrhotite occurs in low-permeability rocks of the lower part of the USGS-N2 drill core,

as well as in the rhyodacite sill between 460- and 470-m depth.

Fluid samples were not collected near the depth of the sill;

however, magmatic sulfurous gases may have accompanied

the intrusion of the rhyodacite sill and were confined to the
75

sill following its emplacement. Pyrrhotite in the rhyodacite

sill is depleted in Fe and S relative to pyrrhotite in the deeper

part of the drill core. This depletion, as well as alteration to

pyrite and marcasite (and siderite), is caused by the introduction of thermal meteoric waters that are more oxidizing than

the initial fluids. Pyrrhotite from the lower part of USGS-N2

does not appear to be altered. Sulfur isotope (∂34S) values of

+1.6 parts per thousand for pyrite from 930-m depth and of

+1.7 parts per thousand for pyrrhotite from 713-m depth (Keith

and others, 1984b) are within the range of magmatic sulfur.

Bleaching due to hydrothermal alteration of the lowest 2

m of USGS-N2 drill core, as well as at the basal parts of the

lower two basalt lava flows, occurs where two-phase fluids

are present in a confined-boiling or vapor-dominated zone.

The dense basalt lava flows form an impermeable barrier

above the permeable basal flow breccias. Local boiling separates steam from the fluid, which is confined to the interflow

breccia, and the low-pH steam condensate bleaches the rock

at the base of the overlying lava flow. In contrast, the liquid

part of the boiling fluid becomes increasingly oxidizing as

indicated by a small amount of late iron oxide (hematite) deposition. Increased oxidation moves the fluid into the stability

field for hematite, and pyrrhotite and pyrite are no longer

deposited. Additional evidence for the increasingly oxidizing hydrothermal fluids is provided by the change from deposition of sulfide minerals to precipitation of late-forming sulfate (anhydrite).

ROCK COMPOSITION

Many published chemical analyses of surface rock samples

that were collected on the flanks of Newberry volcano and

within its caldera show that the volcano is composed of rocks

ranging in composition from basalt to rhyolite (Williams,

1935; Higgins, 1973; MacLeod and Sherrod, 1988; and

MacLeod and others, 1995). Higgins (1973) subdivided the

rocks of Newberry volcano into four groups (precaldera,

syncaldera, postcaldera, and flank) on the basis of the geologic history of the volcano and the geographic position of

the rock specimens. However, he observed that chemical

analyses of the rocks show two trends: (1) Iron-enriched

precaldera rocks and (2) alkali-enriched postcaldera rocks.

The differences are attributed to the presence or absence of

lake water within the caldera of Newberry volcano. During

precaldera conditions, the caldera region was filled by relatively dry lavas and volcaniclastic rocks, whereas the large

depression at the volcano’s summit has been occupied by lake

water following formation of the caldera. A large volume of

water overlying the magma chambers greatly affected the

lavas’ oxygen fugacities, which are influenced by pressure

and water content, during differentiation of the magmas

(Higgins, 1973). The volume of water trapped within the

caldera may have been even greater in the past than that contained within the two present-day 50- and 80-m-deep lakes

(MacLeod and others, 1995).

Chemical variation diagrams (Higgins, 1973, figs. 13, 14,

and 15) show different trends for several elements in

precaldera and postcaldera rocks. An AFM diagram of

Higgins’ data, as well as data from MacLeod and others

(1995), shows a trend toward iron enrichment of the precaldera

basaltic rocks and a second trend toward greater magnesium

content of the postcaldera rocks (fig. 61). Superimposed on

figure 61 are plots of the chemical analyses of rocks from the

USGS-N2 drill hole (from table 2) and three flank drill holes

(from table 7). This diagram shows that some of the basaltic

rocks from the flank drill holes have compositions similar to

those of the postcaldera rocks of Higgins (1973), and many

of the basaltic USGS-N2 drill-core samples are even more

iron enriched than those of Higgins’ precaldera basalts. This

double trend can also be seen in several other plots (figs. 62A,

B, and C) involving different elements (including total Fe vs.

MgO, MgO vs. SiO2, and TiO2 vs. SiO2) as indicated by

Higgins (1973). These three diagrams are among those that

Higgins says are useful to show differences in major-element

chemistries of precaldera and post caldera rocks at Newberry

volcano.

Trends in the chemical diagrams of figures 61 and 62 appear to be influenced somewhat by hydrothermal alteration;

for example, plots of oxides such as MgO and TiO2 versus

SiO2 suggest some depletion of SiO2 in the USGS-N2 drill

core specimens when compared to the analyses of unaltered

rocks given in Higgins (1973) and MacLeod and others

(1995). Similarly, a plot of CaO versus SiO2 (fig. 63) shows

SiO2 loss in most analyzed specimens from the USGS-N2

drill hole. SiO2 loss coupled with some depletion of CaO in

HYDROTHERMAL MINERALS

USGS–N2

Flank holes

MgO

Fe (total)

Precaldera

trend

Postcaldera

trend

Higgins (1973)

MacLeod and

others (1995)

 Na2O + K2O 

Figure 61.—AFM diagram showing chemical variation of basaltic

to rhyolitic rocks from geothermal drill holes on flanks of Newberry

volcano and within the caldera (USGS-N2), compared with analyses of surface rock samples from Higgins (1973) and MacLeod and

others (1995). Lines showing precaldera and postcaldera trends from

Higgins (1973, fig. 13B).
76 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

the USGS-N2 drill core samples could have resulted in the

observed pronounced trend; however, loss of CaO does not

appear to be very great in any of the analyzed USGS-N2

samples when compared with the MgO content of unaltered

specimens (fig. 64A). MgO variation diagrams for other oxides of major elements (Al2O3, P2O5, K2O, and MnO, figs.

64B, C, D, and E, respectively show minor to considerable

scatter for both USGS-N2 and unaltered Newberry rocks that

may reflect some gains or losses of these elements during

hydrothermal alteration; however, the dispersion of data points

more likely results from variations in MgO content of

precaldera and postcaldera rocks. An MgO versus Na2O diagram (fig. 64F) suggests some loss of Na2O in many of the

USGS-N2 drill core specimens. A plot of MgO versus TiO2

(fig. 64G) suggests that the TiO2 content of USGS-N2 core

samples has not been affected by hydrothermal alteration.

Instead, the plot appears to show separate precaldera and

postcaldera trends that illustrate Higgins’ (1973) concept of a

two-trend chemical relation at Newberry volcano.

Evidence for hydrothermal alteration is suggested by the

high water content of many of the USGS-N2 analyzed rocks,

although several analyses of unaltered Newberry rocks from

Higgins (1973) and MacLeod and others (1995) also contain

substantial H2O (fig. 65). One analyzed USGS-N2 specimen

from 311.4-m depth contains nearly 22 weight percent total

H2O and is extensively hydrothermally altered. With only 39.5

Figure 62.—Chemical variation diagrams for samples from USGS-N2 (table 2) and flank geothermal drill holes GEO-N1, GEO-N5, and SF

NC72-03 (table 7) compared with analyses of surface rocks from Higgins (1973) and MacLeod and others (1995). A, MgO vs. total Fe. B,

SiO2 vs. MgO. C, SiO2 vs. TiO2.
77

weight percent SiO2, this core sample is extremely depleted

in SiO2 and plots alone on any SiO2 variation diagram (see

figs. 62B, C and 63).

Depiction of the chemical effects of hydrothermal alteration for both major and trace elements typically are shown

on variation diagrams of the elements versus immobile eleHYDROTHERMAL MINERALS

Figure 63.—SiO2 vs. CaO diagram for chemical analyses of rocks

from drill holes USGS-N2, GEO-N1, GEO-N5, and SF NC72-03

and outcrops at Newberry volcano. Diagram shows a loss of SiO2

resulting from hydrothermal alteration in nearly all analyzed rocks

from USGS-N2 core.

Figure 64.—MgO variation diagrams for chemically analyzed rocks

from geothermal drill holes USGS-N2, GEO-N1, GEO-N5, and SF

NC72-03 and outcrops at Newberry volcano. A, Comparison of

samples from unaltered surface rocks with rocks from geothermal

drill holes suggests that little if any CaO has been gained or lost

owing to hydrothermal alteration. B-E, Al2O3, P2O5, K2O, and MnO

contents appear to be little affected by hydrothermal alteration, although data are considerably scattered in some diagrams. Hydrothermal alteration has caused definite loss of Na2O (F) and possibly

some loss of K2O (D) in rocks from USGS-N2 drill hole. G, MgO

vs. TiO2 diagram clearly shows the double trend of unaltered

precaldera and postcaldera rocks.
78 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

ments, such as Zr or TiO2 (Finlow-Bates and Stumpfl, 1981).

Several plots of Zr versus major and trace elements for the

analyzed Newberry drill-core specimens (from tables 2, 3, 4,

7, and 13) were compared with chemical analyses of unaltered Newberry rocks from Higgins (1973) and MacLeod and

others (1995). These diagrams show considerable scatter for

Al2O3 (fig. 66A), apparent loss of SiO2, Na2O, and K2O (figs.

66B, C, and D), and an increase in CaO (fig. 66E) and possibly total Fe(?) (fig. 66F) for several of the USGS-N2 analyzed drill-core specimens. The higher total Fe values in the

Zr versus total Fe diagram (fig. 66F), the suggestion of a

double trend in a Zr versus TiO2 diagram (fig. 66G), and a

concentration of data points at ~4 weight percent MgO on a

Zr versus MgO diagram (fig. 66H) may reflect differences in

precaldera versus postcaldera rock chemistry rather than hydrothermal alteration. Higgins (1973) indicated that the abundance of these three elements may have been strongly influenced by the presence (postcaldera) or absence (precaldera)

of lake water within the caldera of Newberry volcano.

Higgins (1973) observed that on chemical variation diagrams of some trace elements (Ba, Rb, and Sr) the data plot

into two groups according to whether the rocks are basalts or

rhyolites rather than of precaldera or postcaldera origin. The

data of Higgins (1973) and MacLeod and others (1995) are

plotted along with data from tables 3, 4, and 13 of this report

as Zr versus Ba, Cu, Rb, Sc, and Sr diagrams (figs. 66I, J, K,

L, and M, respectively). Two groupings of the data from

Higgins (1973) and MacLeod and others (1995) occur in these

diagrams because of differences in trace elements in the basaltic and rhyolitic rocks. Many of the analyzed rocks from

Figure 64.—Continued.

Figure 65.—H2O vs. SiO2 diagram for chemically analyzed rocks

from Newberry volcano drill holes USGS-N2, GEO-N1, GEO-N5,

and SF NC72-03. Diagram shows high H2O content resulting from

hydrothermal alteration in many USGS-N2 drill-core samples.
HYDROTHERMAL MINERALS 79

Figure 66.—Zr variation diagrams for major- and selected traceelement contents of rocks from Newberry volcano drill holes USGSN2, GEO-N1, GEO-N5, and SF NC72-03. Diagrams A to H appear

to show more depletion of SiO2, Na2O, and K2O in many samples

from USGS-N2 drill core than in samples of unaltered surface rocks.

There may be both depletion and enrichment of Al2O3, CaO, and total

Fe in USGS-N2 specimens; however, there is considerable scatter of

the USGS-N2 drill hole are of intermediate composition (see

table 2), and the separation between basaltic and rhyolitic

rocks on these diagrams is at least partly filled to form a nearly

continuous differentiation series. Minor to considerable scatter in the data points of the Zr variation diagrams may indicate gains or losses of trace elements owing to hydrothermal

alteration (Kristmannsdóttir, 1983); however, substantial scatter of data points also occurs for some trace elements in the

unaltered rocks.

For drill holes GEO-N2, GEO-N3, GEO-N4, SF NC-01,

and RDO-1, no major or trace element chemical analyses are

presently available. Eighteen drill-core samples from drill hole

GEO-N1 were analyzed for major elements along with two

drill-core samples from each of the GEO-N5 and SF NC72-

03 drill holes (table 7). Analyses of selected trace elements

were obtained for three of the GEO-N1 samples and the four

GEO-N5 and SF NC72-03 samples (table 13). A few of the

analyzed drill-core samples from the GEO-N1 hole are slightly

altered, but most of them were selected for analysis because

they are relatively unaltered. Although the H2O (H2O+ + H2Oor LOI) content for four of the flank drill core specimens is

greater than 3.0 (table 7; fig. 65), most chemical analyses of

specimens from the flank drill holes plot with the unaltered

surface-rock samples on SiO2, MgO, and Zr variation diagrams (figs. 62, 63, 64, and 66). Figures 61 and 62 suggest

that both precaldera and postcaldera rocks are represented in

core specimens from the flank drill holes.

SUMMARY AND CONCLUSIONS

Numerous geothermal drill holes have been completed at

Newberry volcano. Hydrothermal alteration of specimens

from several of these drill holes has been studied, and the

results are reported here and in previous publications (Bargar

and Keith, 1984; Keith and others, 1984a, b; Keith and others, 1986; Bargar and Keith, 1986; Carothers, Mariner, and

Keith, 1987; Keith and Bargar, 1988; Arestad, Potter, and

Stewart, 1988; Bargar and others, 1990). Holes GEO-N1,

GEO-N4, and GEO-N3 were located on the south, east, and

north flanks of the volcano, respectively. Four wells (GEON5, GEO-N2, SF NC-01, and SF NC72-03) were drilled on

the west flank of Newberry. Two drill holes (USGS-N2 and

RDO-1) were completed within the volcano’s caldera. Of the

nine drill cores used for this study, only the USGS-N2 drill

core was studied in great detail. The drill holes penetrated

basaltic to rhyolitic rocks that have been affected by only

mild post-emplacement temperatures (<100 ̊C ) in the upper

parts of USGS-N2 and RDO-1 and in all the flank drill holes

except for the lower parts of GEO-N2, SF NC-01, and SF

NC72-03. Alteration minerals deposited in open spaces

(mostly carbonates, smectite, zeolites, iron oxide, and sulfates) reflect these low temperatures. Conversely, the lower

parts of the three higher temperature west-flank drill holes

(150 to 170 ̊C) and the two intracaldera drill holes (158+ ̊C

in RDO-1 and 265 ̊C in USGS-N2) contain alteration minerdata points in some of the diagrams. Scattering of data for total Fe,

TiO2, and MgO may be caused by differences in amounts of these

elements in precaldera and postcaldera rocks; this also appears to be

especially true for TiO2. Trace-element contents of basaltic and

rhyolitic analyses of Higgins (1973) plot in two groups according to

rock type. Plots of trace elements in diagrams I to M reflect many

rock compositions intermediate between basalt and rhyolite.
80 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

Figure 66.—Continued. Figure 66.—Continued.
HYDROTHERMAL MINERALS 81

Figure 66.—Continued.
82 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

als (quartz, mixed-layer chlorite-smectite, and chlorite) that

formed at significantly higher temperatures. The most notable difference in hydrothermal mineralogy between the two

intracaldera drill holes is that in RDO-1 alteration minerals

(chlorite, quartz, mordenite, pyrite, and pyrrhotite) occur at

shallower depths than in USGS-N2 because of significantly

higher temperatures at equivalent depths.

Nearly all of the hydrothermal alteration minerals identified

in the Newberry drill cores are found in the USGS-N2 drill

core. They first appear at a depth of 300 m, where scarce

zeolites and smectite replace basaltic glass and the fractures

contain late-formed calcite and aragonite. Measured

temperatures above 300-m depth did not exceed 35 ̊C.

Hydrothermal alteration between 300- and 697-m depth

consists mainly of incipient alteration of pumice to smectite,

but much glass remains hydrated and otherwise unaltered. A

rhyodacite sill between 460- and 470-m depth intruded wet

tuffs and caused local, more intense alteration of the enclosing

tuffs to chlorite and mordenite, with open-space deposition

of quartz, calcite, pyrrhotite, pyrite, and marcasite. A late

hydrothermal heating event throughout the interval resulted

in deposition of siderite along fractures, in pore spaces, and

as cement in breccias. Measured temperatures between 320-

and 697-m depth increase from 35 to 100 ̊C at 410-m depth

and then show a reverse thermal gradient to 74°C at 550-m

depth. Temperatures range from 74 to 85 ̊C at depths from

550 to 675 m.

Hydrothermal alteration from 697-m depth to the bottom

of the USGS-N2 drill hole at a depth of 932 m increases

with depth and increasing temperature along a steep

conductive thermal gradient from 110°C at 697-m depth to

265 ̊C at 930-m depth. Hydrothermal minerals in this zone

are mainly smectite, mixed-layer chlorite-smectite,

pyrrhotite, pyrite, calcite, and quartz. As temperatures

increase toward the bottom of the drill hole, mixed-layer

chlorite-smectite and chlorite become the dominant clay

minerals and smectite abundance decreases. Where different

clay structures coexist, the mixed-layer clay replaces

groundmass and lines thin irregular fractures, whereas the

chlorite coats vesicles and appears to be a later deposit.

Scarce amounts of epidote, anhydrite, and hematite are latestage minerals near the base of the section and are compatible

with the measured temperatures of 250 to 265 ̊C. In the

highly altered rocks at the bottom of the drill hole, primary

plagioclase is partly replaced by calcite and illite.

The most important controls of alteration in USGS-N2

are permeability, temperature, and fluid composition. The

massive subhorizontal lava flows that make up the lower part

of the section are little altered except where locally fractured

or vesiculated. Hydrothermal fluids have clearly been

confined mainly to the subhorizontal interflow breccias and

volcaniclastic layers.

Th of fluid inclusions in quartz and calcite from the lower

part of the USGS-N2 drill hole range between the present

measured temperatures and about 367 ̊C with no evidence of

boiling. However, mineral assemblages, textures, and isotopic data indicate that temperatures throughout the drill hole

could not have been significantly hotter than at present for a

long period of time. A probable explanation for the hotter Th

is that the system attained higher temperatures because of

greater pressure of an overlying deep intracaldera lake at the

time the fluid inclusions formed. Lacustrine sediments in the

upper part of the drill hole attest to the presence of a past

overlying lake. Differences in the chemistry of precaldera and

postcaldera rocks at Newberry volcano also are attributed to

the presence of an intracaldera lake overlying the magma

chambers (Higgins, 1973).

According to models of the caldera hydrology, meteoric

waters in the shallow porous caldera fill in the vicinity of the

USGS-N2 drill hole have maintained cool temperatures except for the incursion of thermal fluids between 350- and 500-

m depth (Sammel, Ingebritsen, and Mariner, 1988). These

thermal fluids apparently spread laterally from an upflow zone

such as the caldera ring fracture or a feeder dike. The fluids

produced from the bottom 2 m of the USGS-N2 drill hole

were probably a mixture of steam and CO2, along with a significant amount of H2S (Sammel, 1981). Isotope studies of

the fluids produced from the bottom of the drill hole indicate

that the liquid was a steam condensate (Carothers, Mariner,

and Keith, 1987). The isotope studies, together with modeling studies of the fluids (Sammel, Ingebritsen, and Mariner,

1988), support the hypothesis that the fluid encountered at

the bottom of the hole during drilling was largely steam and

CO2. If two-phase fluids existed in the flow breccia and vesicular basalt of the basal 2 m of the drill hole, such fluids

probably were confined by the overlying impermeable lava

flows. The flow breccia at 910-m depth appears to have contained similar two-phase fluids. Hydrothermal mineralogy

studies indicate that hydrothermal fluids at the bottom of the

USGS-N2 drill hole are evolving to a more oxidizing state.

Comparison of chemically analyzed rocks from the USGSN2 drill hole with published analyses of unaltered surface

rocks (Higgins, 1973; MacLeod and others, 1995) show an

insignificant to moderate decrease in SiO2, Na2O, and K2O,

as well as possible slightly increased total Fe, and CaO; MnO,

and MgO appear to be unchanged, while considerable scatter

of the data points on chemical variation diagrams of Al2O3

and P2O5 may indicate both loss and gain of these elements.

H2O, CO2, and S contents of some of the USGS-N2 drill-hole

analyses range to significantly greater values than the unaltered surface rocks. Some of the trends observed on chemical

variation diagrams for major elements apparently result from

differences between precaldera and postcaldera lavas as explained by Higgins (1973). Conversely, Higgins (1973) found

that trace-element analyses for basaltic and rhyolitic rocks

plotted in distinctly separate fields. Trace-element analyses

for rocks of intermediate composition from the geothermal

drill holes appear to complete the differentiation trend.

The age of the hydrothermal system penetrated by the

USGS-N2 drill hole is unknown. The latest caldera collapse

occurred more than 10,000 years ago (MacLeod and Sammel,

1982). Local intracaldera volcanic activity since that time
HYDROTHERMAL MINERALS 83

indicates ongoing although probably intermittent thermal injections into the caldera fill; the hydrothermal system below

697-m depth could have developed prior to the latest caldera

collapse. Petrologic and mineralogic studies of the USGSN2 geothermal system shows that it is a young evolving system associated with the evolution of Newberry volcano. The

1.35-ka age of the youngest dated silicic lava and the numerous small volcanic features of post-Mazama age within the

caldera (MacLeod, Sherrod, and Chitwood, 1982) indicate

an active volcanic system. The youth, large volume of both

silicic and basaltic volcanic rocks, and the thermal springs of

Newberry volcano provide a favorable setting for a significant geothermal system (MacLeod, Walker, and McKee,

1975). Except for much deeper (3,000 m), slightly hotter

(270 ̊C) temperatures at Meager Mountain in British Columbia, Canada (Souther, 1985), the 265 ̊C temperature at 930-

m depth at the USGS-N2 drill hole is as yet the highest temperature reported in a Cascades geothermal system. The volume of geothermal fluids may be restricted, however, because

of localization within narrow interflow zones.

REFERENCES CITED

Achauer, U., Evans, J.R., and Stauber, D.A., 1988, High-resolution

seismic tomography of compressional wave velocity structure at Newberry volcano, Oregon Cascade Range: Journal of

Geophysical Research, v. 93, no. B9, p. 10,135-10,147.

Arestad, J.F., Potter, R.W., II, and Stewart, G.E., 1988, Stratigraphic

test drilling in the Newberry crater KGRA, Oregon: Geothermal Resources Council Bulletin, v. 17, no. 10, p. 3-8.

Arnold, R.G., 1966, Mixtures of hexagonal and monoclinic pyrrhotite and the measurement of the metal content of pyrrhotite by

X-ray diffraction: American Mineralogist, v. 51, p. 1,221-1,227.

Bacon, C.R., 1983, Eruptive history of Mount Mazama and Crater

Lake caldera, Cascade Range, U.S.A.: Journal of Volcanology

and Geothermal Research, v. 18, p. 57-115.

Baedecker, P.A., ed., 1987, Methods for geochemical analysis: U.S.

Geological Survey Bulletin 1770, 156 p.

Bailey, S.W., 1982, Nomenclature for regular interstratifications:

American Mineralogist, v. 67, p. 394-398.

Bargar, K.E., 1980, Lithologic log of drill cuttings for Northwest

Geothermal Corporation drill hole at Lost Creek near Mount

Hood, Oregon: U.S. Geological Survey Open-File Report

80-1166, 19 p.

——1990, Hydrothermal alteration in geothermal drill hole CTGH1, High Cascade Range, Oregon: Oregon Geology, v. 52, no.

4, p. 75-81.

Bargar, K.E., and Beeson, M.H., 1981, Hydrothermal alteration in

research drillhole Y-2, Lower Geyser Basin, Yellowstone National Park, Wyoming: American Mineralogist, v. 66, p. 473-

490.

——1984, Hydrothermal alteration in research drill hole Y-6, Upper Firehole River, Yellowstone National Park, Wyoming: U.S.

Geological Survey Professional Paper 1054- B, 24 p.

——1985, Hydrothermal alteration in research drill hole Y-3, Lower

Geyser Basin, Yellowstone National Park, Wyoming: U.S.

Geological Survey Professional Paper 1054-C, 23 p.

Bargar, K.E., Beeson, M.H., and Keith, T.E.C., 1981, Zeolites in

Yellowstone National Park: The Mineralological Record, v. 12,

p. 29-38.

Bargar, K.E., Erd, R.C., Keith, T.E.C., and Beeson, M.H., 1987,

Dachiardite from Yellowstone National Park, Wyoming: Canadian Mineralogist, v. 25, p. 475-483.

Bargar, K.E., and Fournier, R.O., 1988, Effects of glacial ice

on subsurface temperatures of hydrothermal systems in

Yellowstone National Park, Wyoming: Fluid-inclusion evidence: Geology, v. 16, p. 1077-1080.

Bargar, K.E., and Keith, T.E.C., 1984, Hydrothermal alteration mineralogy in Newberry 2 drill core, Newberry volcano, Oregon:

U.S. Geological Survey Open-File Report 84-92, 62 p.

——1986, Hydrothermal mineralization in GEO N-1 drill hole,

Newberry volcano, Oregon: U.S. Geological Survey Open-File

Report 86-440, 12 p.

Bargar, K.E., Keith, T.E.C., and Beeson, M.H., 1993, Hydrothermal

alteration in the Mount Hood area, Oregon: U.S. Geological

Survey Bulletin 2054, 70 p.

Bargar, K.E., Keith, T.E.C., Swanberg, C.A., and Walkey, W.C., 1990,

Hydrothermal alteration in cores from geothermal drill

holes on the flanks of Newberry volcano, Oregon [abs.]: Eos,

Transactions, American Geophysical Union, v. 71, no. 43, p.

1691.

Bird, D.K., Schiffman, P., Elders, W.A., Williams, A.E., and

McDowell, S.D., 1984, Calc-silicate mineralization in active

geothermal systems: Economic Geology, v. 79, p. 671-695.

Black, G.L., 1983, Newberry hydrology, in Priest, G.R., Vogt, B.F.,

and Black, G.L., eds., Survey of potential geothermal exploration sites at Newberry volcano, Deschutes County, Oregon:

Oregon Department of Geology and Mineral Industries OpenFile Report O-83-3, p. 28-36.

Black, G.L., Priest, G.R., and Woller, N.M., 1984, Temperature data

and drilling history of the Sandia National Laboratories well at

Newberry caldera: Oregon Geology, v. 46, no. 1, p. 7-9.

Blackwell, D.D., Black, G.L., and Priest, G.R., 1981, Geothermal

gradient data (1978): Oregon Department of Geology and Mineral Industries Open-File Report O-81-3A, 63 p.

Blackwell, D.D., Hull, D.A., Bowen, R.G., and Steele, J.L., 1978,

Heat flow of Oregon: Oregon Department of Geology and Mineral Industries Special Paper 4, 42 p.

Bodnar, R.J., Reynolds, T.J., and Kuehn, C.A., 1985, Fluid-inclusion systematics in epithermal systems, in Berger, B.R., and

Bethke, P.M., eds., Geology and geochemistry of epithermal

systems: Society of Economic Geologists Reviews in Economic

Geology, v. 2, p. 73-97.

Bodnar, R.J., and Sterner, S.M., 1984, Synthetic fluid inclusions in

natural quartz: I. Compositional types synthesized and applications to experimental geochemistry: Geochimica et

Cosmochimica Acta, v. 48, p. 2,659-2,668.

Browne, P.R.L., 1969, Sulfide mineralization in a Broadlands geothermal drill hole, TaupoVolcanic Zone, New Zealand: Economic Geology, v. 64, p. 156-159.

——1970, Hydrothermal alteration as an aid in investigating geothermal fields, in United Nations Symposium on the development and utilization of geothermal resources, Pisa, Italy, September 22-October 1, 1970, Proceedings: Pisa, Italy, Istituto

Internazionale per le Ricerche Geotermiche, Geothermics, Special Issue 2, p. 564-570.

——1978, Hydrothermal alteration in active geothermal fields: Annual Reviews in Earth and Planetary Sciences, v.6, p. 229-250.

Browne, P.R.L., and Ellis, A.J., 1970, The Ohaki-Broadlands hy-
84 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

drothermal area, New Zealand, mineralogy and related

geochemistry, American Journal of Science, v. 269, p. 97-131.

Browne, P.R.L., and Lovering, J.F., 1973, Composition of sphalerites from the BroadlandsGeothermal Field and their significance to sphalerite geothermometry and geobarometry: Economic Geology, v. 68, p. 381-387.

Carothers, W.W., Mariner, R.H., and Keith, T.E.C., 1987, Isotope

geochemistry of minerals and fluids from Newberry volcano,

Oregon: Journal of Volcanology and Geothermal Research, v.

31, p. 47-63.

Catchings, R.D., and Mooney, W.D., 1988, Crustal structure of east

central Oregon: relation between Newberry volcano and regional crustal structure: Journal of Geophysical Research, v.

93, no. B9, 10,081-10,094.

Cavarretta, G., Gianelli, G., and Puxeddu, M., 1980, Hydrothermal

metamorphism in theLarderello Geothermal Field:Geothermics,

v. 9, p. 297-314.

Collins, S.D., 1991, Newberry National Volcanic Monument making a consensus process work: Geothermal Resources Council

Transactions, v. 15, p. 417-424.

Coombs, D.S., 1955, X-ray observations on wairakite and non-cubic analcime: Mineralogical Magazine, v. 30, p. 699-708.

Craig, J.R., and Scott, S.D., 1974, Sulfide phase equilibria, in Ribbe,

P.H., ed., Sulfide mineralogy: Mineralogical Society of America

Short Course Notes, v.1, p. CS1-CS110.

Deer, W.A., Howie, R.A., and Zussman, J., 1966, An introduction to

the rock-forming minerals: London, Longman, 528 p.

Donnelly-Nolan, J.M., 1988, A magmatic model of Medicine Lake

volcano, California: Journal of Geophysical Research, v. 93,

no. B5, p. 4,412-4,420.

Drummond, S.E., and Ohmoto, H., 1985, Chemical evolution and

mineral deposition in boiling hydrothermal systems: Economic

Geology, v. 80, p. 126-147.

Dunn,P.J.,Rouse, R.C., and Norberg, J.A., 1978, Hydroxyapophyllite, a new mineral, and a redefinition of the apophyllite group:

I. Description, occurrences, and nomenclature: American Mineralogist, v. 63, p. 196-202.

Earth Science Laboratory, 1986, Announcement of open file data

release: University of Utah Research Institute, Salt Lake City,

Utah, Cascades Newsletter, no. 2, 2 p.

——1987, Announcement of open file data release: University of

Utah Research Institute, Salt Lake City, Utah, Cascades Newsletter, no. 3, 3 p.

Ellis, A.J., 1970, Quantitative interpretation of chemical characteristics of hydrothermal systems, in United Nations Symposium

on the Development Utilization of Geothermal Resources,

Pisa, Italy, September 22-October 1, 1970, Proceedings: Pisa,

Italy, Istituto Internazionale per le Ricerche Geotermiche,

Geothermics,Special Issue2,p.516-528.

Finlow-Bates, T., and Stumpfl, E.F., 1981, The behavior of so-called

immobile elements in hydrothermally altered rocks associated

with volcanogenic submarine-exhalative ore deposits:

Mineralium Deposita, v. 16, p. 319-328.

Fitterman, D.V., Stanley, W.D., and Bisdorf, R.J., 1988, Electrical

structure of Newberry volcano, Oregon: Journal of Geophysical Research, v. 93, no. B9, p. 10,119-10,134.

Friedman, Irving, 1977, Hydration dating of volcanism at Newberry

crater, Oregon: U.S. Geological Survey Journal of Research,

v. 5, no. 3, p. 337-342.

Frondel, C., and Bauer, L.H., 1955, Kutnahorite: a manganese dolomite, CaMn(CO3

)2: American Mineralogist, v. 40, p. 748-760.

Gabrielson, O., and Sundius, N., 1965, Ca-rich kutnahorite from

Langban, Sweden: Arkiv för Mineralogi och Geologi, v. 4, p.

287-289.

Gettings, M.E., and Griscom, Andrew, 1988, Gravity model studies

of Newberry volcano, Oregon: Journal of Geophysical Research, v. 93, no. B9, p. 10,109-10,118.

Gottardi, G., and Galli, E., 1985, Natural zeolites: Berlin, SpringerVerlag, 409 p.

Grim, R.E., 1968, Clay mineralogy: San Francisco, McGraw-Hill

Book Co., 596 p.

Griscom, Andrew, and Roberts, C.W., 1983, Gravity and magnetic

interpretation of Newberry volcano, Oregon, in Priest,

G.R., Vogt, B.F., and Black, G.L., eds., Survey of potential geothermal exploration sites at Newberry volcano, Deschutes

County, Oregon, Oregon Department of Geology and Mineral

Industries Open-File Report O-83-3, p. 68-81.

Hedenquist, J.W., and Henley, R.W., 1985, The importance of CO2

on freezing point measurements of fluid inclusions: evidence

from active geothermal systems and implications for epithermal

ore deposition: Economic Geology, v. 80, p. 1379-1406.

Heller, L., and Taylor, H.F.W., 1956, Crystallographic data for the

calcium silicates: London, Her Majesty’s Stationery Office,

79 p.

Hemley, J.J., Hostetler, P.B., Gude, A.J., and Mountjoy, W.T., 1969,

Some stability relations of alunite: Economic Geology, v. 64,

p. 599-612.

Hey, M.H., 1954, A new review of the chlorites: Mineralogical Magazine, v. 30, p. 277-292.

Higgins, M.W., 1973, Petrology of Newberry volcano, central Oregon: Geological Society of America Bulletin, v. 84, no. 2, p.

455-488.

Honda, S., and Muffler, L.J.P., 1970, Hydrothermal alteration in core

from research drill hole Y-1, Upper Geyser Basin, Yellowstone

National Park, Wyoming: American Mineralogist, v. 55, p.

1714-1737.

Horton, D.G., 1985, Mixed-layer illite/smectite as a paleotemperature

indicator in the Amethyst vein system, Creede district, Colorado, USA: Contributions to Mineralogy and Petrology, v. 91,

p. 171-179.

Hower, J., 1981, X-ray diffraction identification of mixed-layer clay

minerals, in Longstaffe, F.J., ed., Short course in clays and the

research geologist: Toronto, Mineralogical Association of

Canada, Short course handbook 7, p. 39-59.

Hurlbut, C.S., Jr., 1971, Dana’s manual of mineralogy (18th ed.):

New York, John Wiley & Sons, Inc., 579 p.

Ingebritsen, S.E., Carothers, W.W., Mariner, R.H., Gudmundsson,

J.S., and Sammel, E.A., 1986, Flow testing of the Newberry 2

research drill hole, Newberry volcano, Oregon: U.S. Geological Survey Water Resources Investigations Report 86-

4133, 23 p.

Jakobsson S.P., and Moore, J.G., 1986, Hydrothermal minerals and

alteration rates at Surtsey volcano, Iceland: Geological Society of America Bulletin, v. 97, p. 648-659.

Keith, T.E.C., and Bargar, K.E., 1988, Petrology and hydrothermal

mineralogy of U.S. Geological Survey Newberry 2 drill core

from Newberry Caldera, Oregon: Journal of Geophysical Research, v. 93, no. B9, p. 10,174-10,190.

Keith, T.E.C., Bargar, K.E., Howe, S.S., Carothers, W.W., and Barnes, Ivan, 1984a, Mineralogical studies of the hydrothermal

system in Newberry Volcano drill hole 2, Oregon: Geothermal

Resources Council, Transactions, v. 8, p. 125-128.
85

Keith, T.E.C., and Boden, J.R., 1981, Volcanic stratigraphy and alteration mineralogy of drill cuttings from EWEB 1 drill hole,

Linn County, Oregon: U.S. Geological Survey Open-File Report 81-250, 11 p.

Keith, T.E.C., Gannett, M.W., Eichelberger, J.C., and Waibel, A.F.,

1986, Lithology and hydrothermal alteration of drill hole RDO1, Newberry caldera, Oregon: Oregon Geology, v. 48, no. 9, p.

103-107, 110.

Keith, T.E.C., Mariner, R.H., Bargar, K.E., Evans, W.C., and Presser,

T.S., 1984b, Hydrothermal alteration in Oregon’s Newberry

volcano No. 2, fluid chemistry andsecondary mineral distribution: Geothermal Resources Council Bulletin, v. 13, no. 4, p.

9-17.

Keith, T.E.C., and Muffler, L.J.P., 1978, Minerals produced during

cooling and hydrothermal alteration of ash-flow tuff from

Yellowstone drill hole Y-5: Journal of Volcanology and Geothermal Research, v. 3, p. 373-402.

Keith, T.E.C., White, D.E., and Beeson, M.H., 1978, Hydrothermal alteration and self-sealing in Y-7 and Y-8 drill holes in

northern part of Upper Geyser Basin, Yellowstone National

Park, Wyoming: U.S. Geological Survey Professional Paper

1054-A, 26 p.

Kissin, S.A., and Scott, S.D., 1982, Phase relations involving pyrrhotite below 350°C: Economic Geology, v. 77, p. 1739-1754.

Kreiger, P., 1930, Notes on an X-ray diffraction study of the series

calcite-rhodochrosite: American Mineralogist, v. 15, p. 23-29.

Kristmannsdóttir, H., 1976, Types of clay minerals in hydrothermally altered basaltic rocks, Reykjanes, Iceland: Jökull, v. 26,

p. 30-39.

——1983, Chemical evidence from Icelandic geothermal systems

as compared to submarine geothermal systems, in Rona, P.A.,

Bostrom, K., Laubier, L., and Smith, K.L., Jr., eds., Hydrothermal processes at seafloor spreading centers: New York, NATO

Conference Series IV, Marine Sciences, 12, Plenum Press, p.

291-320.

Kristmannsdóttir, H., and Tómasson, J., 1978, Zeolite zones in geothermal areas of Iceland, in Sand, L.B., and Mumpton, F.A.,

eds., Natural zeolites-occurrence, properties, use: New York,

Pergamon Press, p. 277-284.

Krupp, R.E., and Seward, T.M., 1987, The Rotokawa geothermal

system, New Zealand: An active epithermal gold-depositing

environment: Economic Geology, v. 82, p. 1109-1129.

Kusakabe, H., Minato, H., Utada, M., and Yamanaka, T., 1981, Phase

relations of clinoptilolite, mordenite, analcime and albite with

increasing pH, sodium ion concentration and temperature: Tokyo, University of Tokyo, Scientific Papers of the College of

General Education, v. 31, p. 39-59.

MacLeod, N.S., and Sammel, E.A., 1982, Newberry volcano, Oregon, A Cascade Range geothermal prospect: Oregon Geology, v. 44, no. 11, p. 123-131; published simultaneously in California Geology, v. 35, no. 11, p. 235-244.

MacLeod, N.S., and Sherrod, D.R., 1988, Geologic evidence for a

magma chamber beneath Newberry volcano, Oregon: Journal

of Geophysical Research, v. 93, no. B9, p. 10,067-10,079.

MacLeod, N.S., Sherrod, D.R., and Chitwood, L.A., 1982, Geologic

map of Newberry volcano, Deschutes, Klamath, and

Lake Counties, Oregon: U.S. Geological Survey Open-File

Report 82-847, scale 1:62,500.

MacLeod, N.S., Sherrod, D.R., Chitwood, L.A., and Jensen, R.A.,

1995, Geologic map of Newberry volcano, Deschutes, Klamath,

and Lake Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-2455, scales 1:62,500

and 1:24,000.

MacLeod, N.S., Sherrod, D.R., Chitwood, L.A., and McKee, E.H.,

1981, Newberry volcano, Oregon, in Johnston, D.A.,

and Donnelly-Nolan, Julie, eds., Guides to some volcanic terranes in Washington, Idaho, Oregon, and northern California:

U.S. Geological Survey Circular 838, p. 85-103.

MacLeod, N.S., Walker, G.W., and McKee, E.H., 1976, Geothermal significance of eastward increase in age of upper Cenozoic rhyolitic domes in southeast Oregon, in Second United

Nations Symposium on the Development and Use of Geother -

mal Resources, San Francisco, Calif., 1975, Proceedings:

Washington, D.C., U.S. Government Printing Office, v. 1, p.

465-474.

Mariner, R.H., Swanson, J.R., Orris, G.J., Presser, T.S., and Evans,

W.C., 1980, Chemical and isotopic data for water from thermal springs and wells of Oregon: U.S. Geological Survey OpenFile Report 80-737, 50 p.

McCulloh, T.H., Frizzell, V.A. Jr., Stewart, R.J., and Barnes, Ivan,

1981, Precipitation of laumontite with quartz, thenardite, and

gypsum at Sespe Hot Springs, Western Transverse Ranges,

California: Clays and Clay Minerals, v. 29, no. 5, p. 353-364.

McDowell, S.D., and Paces, J.B., 1985, Carbonate alteration minerals in the Salton Sea geothermal system, California, USA: Mineralogical Magazine, v. 49, p. 469-479.

McKibben, M.A., 1979, Ore minerals in the Salton Sea geothermal

system, Imperial Valley, California, U.S.A.: Riverside, University of California, M.S. thesis, 90 p.

McKibben, M.A., and Elders, W.A., 1985, Fe-Zn-Cu-Pb mineralization in the Salton Sea geothermal system, Imperial Valley,

California: Economic Geology, v. 80, no. 3, p. 539-559.

Merlino, Stefano, 1988, Gyrolite: its crystal structure and crystal

chemistry: Mineralogical Magazine, v. 52, p. 377-387.

Muffler, L.J.P., Bacon, C.R., and Duffield, W.A., 1982, Geothermal

systems of the Cascades, in Pacific Geothermal Conference,

Auckland, New Zealand, November 8-12, 1982, Proceedings:

Auckland, New Zealand, University of Auckland Geothermal

Institute, Part 2, p. 337-343.

Muffler, L.J.P., and White, D.E., 1969, Active metamorphism of

Upper Cenozoic sediments in the Salton Sea geothermal field

and the Salton Trough, southeastern California: Geological

Society of America Bulletin, v. 80, p. 157-182.

Mumpton, F.A., 1960, Clinoptilolite redefined: American Mineralogist, v. 45, p. 351-369.

Muramatsu, Y., 1984, Fluid inclusion study in the Takinoue geothermal field, Iwate Prefecture, Japan: an application to the

estimate of the present underground temperature, in New

Zealand Geothermal Workshop, 6th, Auckland, New Zealand,

1984, Proceedings: Auckland, New Zealand, University of

Auckland Geothermal Institute, p. 21-25.

Murata, K.J., and Larson, R.R., 1975, Diagenesis of Miocene siliceous shales, Temblor Range, California: U.S. Geological Survey Journal of Research, v. 3, p. 553-566.

Nelson, C.H., Carlson, P.R., and Bacon, C.R., 1988, The Mount

Mazama climactic eruption (~6900 yr B.P.) and resulting convulsive sedimentation on the Crater Lake caldera floor, continent, and ocean basin, in Clifton, H.E., ed., Sedimentologic

consequences of convulsive geologic events: Geological Society of America Special Paper 229, p. 37-57.

Ogihara, S., and Iijima, A., 1990, Exceptionally K-rich clinoptiloliteheulandite group zeolites from three offshore boreholes of northREFERENCES CITED
86 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

ern Japan: European Journal of Mineralogy, v. 2, p. 819-826.

Olmstead, D.L., and Wermiel, D.E., 1988, Newberry Crater geothermal resource area, Deschutes County, Oregon: Oregon Department of Geology and Mineral Industries Open-File Report

O-88-3, scale 1: 24,000.

Passaglia, E., 1970, The crystal chemistry of chabazites: American

Mineralogist, v. 55, p. 1,278-1,301.

Passaglia, E., and Rinaldi, R., 1984, Katoite, a new member of the

Ca3Al2(SiO4)3-Ca3Al2(OH)12 series and a new nomenclature for

the hydrogrossular group of minerals: Bulletin de Mineralogie,

v. 107, p. 605-618.

Peterson, N.V., and Groh, E.A., 1969, The ages of some Holocene

volcanic eruptions in theNewberry volcano area, Oregon: Ore

Bin, v. 31, no. 4, p. 73-87.

Phillips, W.R., and Griffen, D.T., 1981, Optical mineralogy: the

nonopaque minerals: San Francisco, W.H. Freeman and Company, 677 p.

Potter, R.W., II, Clynne, M.A., and Brown, D.L., 1978, Freezing

point depression of aqueous sodium chloride solutions: Economic Geology, v. 73, p. 284-285.

Priest, G.R., Hadden, R.M., Woller, N.M., and Brand, C.B., 1983,

Preliminary soil-mercury survey of Newberry volcano, Deschutes County, Oregon, in Priest, G.R., Vogt, B.F.,

and Black, G.L., eds., Survey of potential geothermal exploration sites at Newberry volcano, Deschutes County, Oregon: Oregon Department of Geology and Mineral Industries Open-File

Report O-83-3, p. 45-67.

Roedder, E., 1962, Studies of fluid inclusions: I. Low temperature

application of a dual-purpose freezing and heating stage: Economic Geology, v. 57, p. 1045-1061.

——1984, Fluid inclusions: Mineralogical Society of America Reviews in Mineralogy, v. 12, 644 p.

Rosenberg, P.E., 1963, Subsolidus relations in the system CaCO3-

FeCO3: American Journal of Science, v. 261, p. 683-690.

Sammel, E.A., 1981, Results of test drilling at Newberry volcano,

Oregon—and some implications for geothermal prospects in

the Cascades: Geothermal Resources Council Bulletin, v. 10,

no. 11, p. 3-8.

——1983, The shallow hydrothermal system at Newberry volcano,

Oregon: a conceptual model: Geothermal Resources Council

Transactions, v. 7, p. 325-330.

Sammel, E.A., and Craig, R.W., 1983, Hydrology of the Newberry

volcano caldera, Oregon: U.S. Geological Survey Water Resources Investigations Report 83-4091, 52 p.

Sammel, E.A., Ingebritsen, S.E., and Mariner, R.H., 1988, The hydrothermal system at Newberry volcano, Oregon: Journal of

Geophysical Research, v. 93, no., B9, p. 10,149-10,162.

Seki, Y., 1972, Lower grade stability limit of epidote in the light of

natural occurrences: Geological Society of Japan Journal, v.

78, p. 405-413.

Sherrod, D.R., and Smith, J.G., 1990, Quaternary extrusion rates of

the Cascade Range, northwestern United States and southern

British Columbia: Journal of Geophysical Research, v. 95, no.

B12, p. 19,465-19,474.

Sigvaldson, G.E., and White, D.E., 1962, Hydrothermal alteration

in drill holes GS-5 and GS-7, Steamboat Springs, Nevada: U.S.

Geological Survey Professional Paper 450-D, p. D113-D117.

Souther, J.G., 1985, Geothermal potential of the Garibaldi Belt,

Mount Meager and Mount Cayley, British Columbia, in

Guffanti, M., and Muffler, L.J.P., eds., Proceedings of the Workshop on Geothermal Resources of the Cascade Range, May

22-23, 1985, Menlo Park, Calif.: U.S. Geological Survey OpenFile Report 85-521, p. 51-52.

Starkey, H.C., Blackmon, P.D., and Hauff, P.L., 1984, The routine

mineralogical analysis of clay-bearing samples: U.S. Geological Survey Bulletin 1563, 32 p.

Stauber, D.A., Green, S.M., and Iyer, H.M., 1988, Three-dimensional P velocity structure of the crust below Newberry volcano, Oregon: Journal of Geophysical Research, v. 93, no. B9,

p. 10,095-10,107.

Steiner, A., 1977, The Wairakei geothermal area, North Island, New

Zealand, its subsurface geology and hydrothermal rock alteration: New Zealand Geological Survey Bulletin 90, 136 p.

Steinthórsson, S., and Sveinsbjörnsdóttir, A.E., 1981, Opaque minerals in geothermal well no. 7, Krafla, northern Iceland:

Journal of Volcanology and Geothermal Research, v. 10, p. 245-

261.

Stonecipher, S.A., 1978, Chemistry of deep-sea phillipsite,

clinoptilolite, and host sediments, in Sand, L.B., and Mumpton, F.A., eds., Natural zeolites–occurrence, properties, use:

New York, Pergammon Press, p. 221-234.

Swanberg, C.A., and Combs, Jim, 1986, Geothermal drilling in the

Cascade Range, preliminary results from a 1387-m core hole,

Newberry volcano, Oregon: Eos, Transactions, American Geophysical Union, v. 67, no. 29, p. 578-580.

Swanberg, C.A., Walkey, W.C., and Combs, Jim, 1988, Core hole

drilling and the “rain curtain” phenomenon at Newberry volcano, Oregon: Journal of Geophysical Research, v. 93, no. B9,

p. 10,163-10,173.

Taguchi, S., and Hayashi, M., 1982, Application of the fluid inclusion thermometer to some geothermal fields in Japan: Geothermal Resources Council Transactions, v. 6, p. 59-62.

Taguchi, S., Hayashi, M., Mimura, T., Kinoshita, Y., Gokou, K., and

Abe, I., 1984, Fluid inclusion temperature of hydrothermal minerals from the Kirishima geothermal area, Kyushu, Japan: Japan Geothermal Energy Association Journal, v. 21, no. 2, p.

119-129.

Tsusue, A., 1967, Magnesian kutnahorite from Ryujima mine:

American Mineralogist, v. 52, p. 1,751-1,761.

Tulloch, A.J., 1982, Mineralogical observations on carbonate scaling in geothermal wells at Kawerau and Broadlands, in Pacific

Geothermal Conference, Auckland, New Zealand, November

8-12, 1982, Proceedings: Auckland, New Zealand, University

of Auckland Geothermal Institute, Part 1, p. 131-134.

Walker, G.W., 1974, Some implications of late Cenozoic volcanism

to geothermal potential in the High Lava Plains of south-central Oregon: Ore Bin, v. 36, no. 7, p. 109-119.

Walkey, W.C., and Swanberg, C.A., 1990, Newberry volcano, Oregon:

new data supports conceptual hydrologic model: Geothermal

Resources Council Transactions, v. 14, pt. 1, p. 743-748.

Williams, Howell, 1935, Newberry volcano of central Oregon: Geological Society of America Bulletin, v. 46, p. 253-304.

Wright, P.M., and Nielson, D.L., 1986, Electrical resistivity anomalies at Newberry volcano, Oregon; comparison with alteration

mineralogy in GEO corehole N-1: Geothermal Resources

Council Transactions, v. 10, p. 247-252.

Zabinski, W., 1966, Hydrogarnets: Polska Akademia Nauk, Komisja

Nauk Mineralogicznych,Prace Mineralogiczne, v. 3, p. 1-61.

Zucca, J.J., and Evans, J.R., 1992, Active high-resolution compressional wave attenuation tomography at Newberry volcano,

central Cascade Range: Journal of Geophysical Research, v.

97, no. B7, p. 11,047-11,055.
87

APPENDIX 1.

LITHOLOGIC DESCRIPTION OF DRILL CORE USGS-N2 FROM

WITHIN THE CALDERA OF NEWBERRY VOLCANO

Depth (m) Description of drill core

0-42 Unconsolidated rhyolitic pumiceous ash and lapili.

42-98 Obsidian flow.

Predominantly light- to medium-gray lapilli tuff, tuff

breccia, and subaqueous volcaniclastic beds in shades

of gray, green, brown, and black. Volcanic fragments

are mostly hydrated basaltic glass, but locally the

fragments are finely crystal line with some phenocrysts of plagioclase and olivine, as well as sporadic

clinopyroxene, magnetite, quartz, and cristobalite.

Lower part of unit grades downward within a few

millimeters into glassy basaltic sandstone and grit

from 290 to 301 m and into siltstone and mudstone

from 301 to 320 m. Hydrothermal minerals identified near base of interval include aragonite, calcite,

siderite, analcime, chabazite, faujasite, erionite,

gyrolite, apophyllite, garnet?, apatite, chlorite, and

smectite.

 302.8 Light-greenish-gray, fine-grained, crudely bedded

lacustrine siltstone with parallel and irregularly oriented fractures filled by calcite and siderite. Composed predominantly of unaltered glass shards with

a few small plagioclase laths.

 313.3 Orange-brown, well-indurated, lacustrine siltstone

with about 1 cm thick layer of pumice-rich coarse

sand grains. XRD analysis of a whole-rock sample

shows plagioclase, faujasite, analcime, and chabazite. In thin section, rock consists primarily of glass

pervasively altered to zeolite; however, grain size is

small and individual minerals were indiscernable. Orange-brown color appears to result from glass altered

to amorphous clay.

320-360 Nearly all of dominantly rhyolitic pumiceous sand

and gravel composing this unit appears to have been

water sorted or reworked and was probably deposited in an intracaldera lake. Lithic material consists

of multicolored, angular to subangular, aphanitic or

porphyritic volcanic fragments. Local layers contain

mixed lithic and pumice clasts in a finer grained

glassy or ashy groundmass. Minor calcite and magnesite were identified by XRD analysis in a few

samples; siderite and smectite are somewhat more

abundant.

 357.8 Light-gray lithic-rich pumiceous tuff with a mostly

unaltered glassy groundmass containing crystals of

plagioclase, clinopyroxene, and magnetite. Very

slight alteration of pumiceous groundmass to brownish smectite; small rounded to irregular-shaped blebs

of siderite also formed in groundmass. Some lithic

fragments contain slight to moderate smectite alteration; a few magnetite crystals are partly altered to

hematite. Quartz, olivine(?), and cristobalite identified in XRD analysis of this and other samples in

this interval probably are associated with lithic fragments.

360-460 Predominantly rhyolitic pumiceous lithic tuff and

local pumice-lapilli tuff with abundant lithic fragments. Pumice and lithic fragments are mixed in an

ashy or sandy pumiceous groundmass. Although proportion of pumice to lithic clasts is variable throughout unit, more pumice is found in upper and lower

parts, where clast size is small, and more lithic clasts

are found in middle part, where clast size is large.

Most XRD analyses of samples from upper part of

interval show that amorphous glass is prevalent. Other

minerals identified are K-feldspar, plagioclase,

clinopyroxene, magnetite, quartz, devitrification

cristobalite, and vapor-phase tridymite. Hydrothermal minerals are predominantly siderite and smectite;

however, near base of interval the alteration assemblage (analcime, clinoptilolite, dachiardite, mordenite, ankerite-dolomite, calcite, magnesite, chlorite,

quartz, pyrrhotite, marcasite, and pyrite) becomes

more abundant and varied due to injection of an underlying rhyolite sill.

 408.7 Light-gray coarse-grained sandstone with abundant

fragments of pumice and lava along with crystals of

plagioclase, clinopyroxene, and magnetite. Glassy

matrix and pumiceous lithic fragments are mostly unaltered. Small cubic pyrite crystals and minor brown

patches of smectite occur throughout; diverse shapes

of siderite crystal aggregates line vesicles and are

superimposed on pumice fragments and glassy matrix.

 442.3 Olive-green crystal-lithic tuff with pumice and lavaflow lithic fragments,plagioclase, K-feld spar, minor

magnetite (some hematite alteration), and altered pyroxene(?) crystals. Matrix is extensively altered to

greenish smectite with patches of calcite. XRD analyses also show clinoptilolite and devitrification

cristobalite.

 454.2 Light-green crystal-lithic tuff with abundant plagioclase and a few magnetite crystals. Matrix glass and

pumice fragments are altered to green chlorite and

later fibrous mordenite; XRD analysis also shows devitrification cristobalite. Some red-orange patches

consist of calcite partly replaced by later siderite.

460-470 Moderately fractured, light- to medium-gray

rhyodacite sill that is massive, dense, and locally flow

banded in its central part. Sill may have been a hot

intrusion into water-saturated pumiceous-lithic tuff.

Glassy top and brecciated base were chilled and subsequently devitrified with concentric hydration cracks

in cryptocrystalline matrix. Sill contains a few small

plagioclase phenocrysts and small plagioclase-rich

mafic xenoliths. Hydrothermal alteration minerals

consist of smectite, siderite, quartz, mordenite, pyrite, marcasite, and pyrrhotite. Some core samples

contain pyrrhotite that is partly oxidized, forming

lepidocrosite, goethite, and sulfur.

1

Lithologic dcscriptions of Newberry drill cores are based on binocular

microscope observations of core samples and petrographic studies of thin

sections. For some intervals, a summary of the thin-section data is provided

without regard to specific sample depths.

98-320

REFERENCES CITED
88 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

 468.5 White, leached, fine-grained rhyodacite with lightgray banding. Contains a few plagioclase phenocrysts in groundmass of tiny plagioclase laths. Devitrification cristobalite and K-feldspar are present in

XRD analyses. Siderite occurs as vein-fillings and

irregular patches; some murky patches are probably

smectite.

 469.1 Dark-gray, glassy to devitrified rhyodacite. A few plagioclase phenocrysts and pyrite grains occur in glassy

groundmass that is partly devitrified to cristobalite.

Perlitic cracks are filled with siderite, mordenite, and

smectite.

470-501.4 White, gray, tan, and light-green, pumiceous-lithic

tuff with local pumice-lapilli tuff similar to that overlying the rhyodacite sill. Two thin (less than 1-m

thick) glassy dikes intersect tuff at 471 and 479 m.

Multicolored lithic fragments are mostly dark aphanitic volcanic rocks, although some fragments contain plagioclase phenocrysts and magnetite crystals.

K-feldspar, devitrification cristobalite, and unaltered

glass are identified in XRD analyses. Predominant

hydrothermal minerals in this interval are siderite,

smectite, and pyrite; calcite, chlorite, and mordenite

are locally abundant.

 496.2 Light- to medium-gray crystal-lithic tuff. Crystals are

mostly plagioclase with a few magnetite grains; Kfeldspar and devitrification cristobalite were noted

in XRD analysis but were not observed in thin section. Some lithic fragments have quartz veining

thought to be unrelated to the present hydrothermal

system. Obsidian and pumice fragments are completely altered to grayish smectite and fibrous

mordenite which also are prevalent in the altered

groundmass. Cubic and pyritohedral pyrite crystals,

associated with rims of lithic fragments, vein fillings,

and open-space deposits in altered glass fragments,

are partly oxidized to iron-oxide stains and are closely

associated with brownish siderite.

501.4-552 Massive to flow-banded, dense rhyodacite lava flow,

brecciated at its top and in its basal 5 m. Fractures

throughout, some coated by hydrothermal minerals,

are oriented both parallel to and across flow banding; local unfilled vapor-phase partings are parallel

to flow banding in massive part of flow. Groundmass

of lava flow is cryptocrystalline to aphanitic and contains approximately 15 percent plagioclase and

clinopyroxene phenocrysts ranging up to about 1.5

cm in length; clinopyroxene content increases near

base of flow. XRD analyses show minor quartz and

K-feldspar, as well as vapor-phase tridymite and devitrification cristobalite. Hydrothermal minerals deposited on numerous fractures and vesicles consist

of orange, reddish-brown, or pale-yellow siderite

mostly deposited in disk-shaped crystal aggregates,

smectite, and hexagonal bladed marcasite that is

partly altered to pyrite.

 515.7 Light-gray rhyodacite flow with plagioclase phenocrysts and a few associated subhedral magnetite

crystals. Fractures in a few plagioclase crystals are

filled with siderite and pyrite. Groundmass consists

of small plagioclase laths and minute magnetite

grains. Scattered pyrite crystals and crystal clusters are partly oxidized and have orange halos. Irregular orange-stained patches and siderite deposits are scattered throughout groundmass.

 548.2 Dark-gray rhyodacite flow containing glomeroporphyritic clots of plagioclase, clinopyroxene,

and magnetite phenocrysts in fine-grained groundmass of tiny plagioclase laths and magnetite crystals. Siderite with earlier tridymite fills vapor-phase

cavities and veins that cut through both plagioclase

and clinopyroxene crystals. Most clinopyroxene phenocrysts have brown-black rims and fracture fillings

of iron oxide. K-feldspar and devitrification cristobalite were detected in XRD analysis.

552-552.5 Thin interlayer of pervasively altered bedded ash and

pumice lapilli. XRD analyses show plagioclase and

hydrothermal alteration minerals: siderite, smectite,

and pyrite.

552.5-600 Light-gray to black dacite lava flows with brecciated tops and local intra- or interflow breccias less

than 0.5 m thick. Flows have local zones of vaporphase colorless tridymite crystals associated with

needle-like crystals of a brown amphibole that formed

in open spaces along partings parallel to flow banding. Dacite groundmass is cryptocrystalline with as

much as 10 percent plagioclase and less than 3 percent clinopyroxene and magnetite phenocrysts. Hydrothermal minerals deposited on fractures and in

cavities consist predominantly of orange to pale-yellow siderite, buff, gray, or green smectite, and tiny

cubic pyrite crystals. Some amorphous opal, chalcedony, cristobalite, calcite, marcasite, and hematite

were identified by XRD.

600-603 Orangish, nonsorted, lithic-rich tuff with ashy

groundmass. Partly oxidized, angular lithic fragments

are composed of plagioclase and devitrification cristobalite. Oxidation to hematite and presence of natrojarosite indicate this zone may have been exposed

to surface weathering for a substantial period of time

or that it might have been an aquifer for late-meteoric thermal water. Other secondary minerals are siderite, smectite, and pyrite.

603-607 Large brecciated fragments of black dacite in a buff

to white ashy matrix. Plagioclase and devitrification

cristobalite were identified in XRD analyses of both

dacite fragments and the matrix material. Hydrothermal minerals are siderite,pyrite, and smectite.

607-656.5 Light-gray to black, dense, massive, locally fractured

dacite lava flows. Groundmass of dacite is cryptocrystalline to aphanitic with thin plagioclase laths and

specks of magnetite; locally hydration cracks are apparent. Dacitic lavas contain about 15 percent plagioclase phenocrysts (many of which are zoned or

partly resorbed), 5 percent clinopyroxene and magnetite phenocrysts, and 1 percent glomeroporphyritic

clots of plagioclase and clinopyroxene. Most wholerock XRD analyses show devitrification cristobalite.

Drill core from this interval usually contains siderite, pyrite, and smectite; marcasite occurs in several

samples, but amorphous opal was found in only two

fracture fillings.
89

 631.9 Dark-gray dacite flow with fine-grained groundmass

consisting of plagioclase laths, tiny magnetite crystals, and rare clinopyroxene grains. Mostly euhedral

phenocrysts consist of plagioclase, magnetite, and

clinopyroxene that locally form glomeroporphyritic

clots. Thin fracture filling consists of colorless, rhombic(?) siderite crystals. Fine-grained siderite and

brownish smectite form alternating bands or concentric layers of a sphere.

 656.2 Dark-gray, vesicular, dacite flow. Fine-grained

groundmass consists of plagioclase laths and tiny

magnetite crystals. Mostly euhedral phenocrysts of

plagioclase, clinopyroxene, and magnetite frequently

form glomeroporphyritic clots. Some vesicles are

partly filled by colorless, disk-shaped clusters of

rhombic siderite crystals. Tiny cubic pyrite crystals

were deposited in a few vesicles and are associated

with reddish-brown to opaque crystals (in transmitted light) that are grayish in reflected light and may

be leucoxene resulting from alteration of magnetite.

656.5-697 Light-gray to black dacite breccia with large fragments. Breccia fragments contain plagioclase, clinopyroxene, and magnetite phenocrysts in finegrained groundmass of plagioclase laths and magnetite crystals. Most, but not all, magnetite phenocrysts

appear to be altered to leucoxene(?) with veins of

pyrite. Devitrification cristobalite was identified in

several samples by XRD.Green, buff, or gray smectite and yellow to orange siderite crystals formed in

cavities between breccia fragments. Some diskshaped crystals of siderite in cross sections appear to

have formed as concentric layers around a neucleus

of brown clay(?). Fractures at base of interval contain bronze-colored hexagonal pyrrhotite platelets;

white rhombic crystals of ankerite-dolomite were

identified in matrix of three samples.

 659.9 Light-gray dacite breccia fragments have fine-grained

groundmass of plagioclase laths and magnetite crystals. Phenocrysts of plagioclase and clinopyroxene

form some glomeroporphyritic clots with magnetite

that has altered to leucoxene(?) and pyrite. Green

smectite and colorless siderite crystal clusters were

deposited in open spaces between breccia fragments.

697-743.5 Two dark-gray, massive dense andesitic lava flows

and associated breccias. A flow-top breccia from 697

to 702.6 m and a massive flow with few fractures

from 702.6 to 707.4 m are relatively unaltered. Conversely, highly fractured, leached, interflow breccia

between 707.4 and 715 m is intensely altered. A second massive, dense but fractured andesitic flow occurs from 715 to 735 m; brecciated, fractured, altered, leached basal part of this flow continues from

735 to 743.5 m.Conspicuous flow banding occurs in

upper flow unit, but otherwise flows are petrographically very similar. Both flows have dark-brown finegrained groundmass (74 percent) with tiny plagioclase laths (An59). XRD analyses of most whole-rock

samples contain cristobalite. Phenocrysts consist

of about 20 percent subhedral to euhedral, locally

zoned plagioclase (An61), 5 percent subhedral to

euhedral clinopyroxene, and 1 percent magnetite.

Glomeroporphyritic clots of plagioclase and

clinopyroxene make up about 1 percent of rock. Hydrothermal alteration becomes more abundant and

varied in lower part of unit. Radiating fibrous

mordenite crystals line cavities in a few samples.

Calcite is late-fracture and vesicle filling; siderite

gradually diminishes in abundance and finally ceases

to form below about 715-m depth. Smectite is sporadically abundant throughout interval, whereas chlorite becomes more plentiful in lower part of andesitic lavas and breccias. Mixed-layer chlorite-smectite

was detected in XRD analysis of one sample. Fracture fillings and open spaces between breccia fragments are lined by colorless colloform cristobalite,

locally in association with colorless to white chalcedony, and colorless quartz crystals and massive deposits. Pyrrhotite is disseminated in groundmass of

some andesite flows and was deposited on many fractures. Pyrite occurs in most samples and may have

formed, at least in part, from alteration of pyrrhotite.

743.5-757.5 Light-green andesitic crystal- and crystal-lithic tuffs.

Plagioclase crystals are unaltered in upper part of

interval but are partly altered to calcite in samples

near base. Calcite also fills vesicles and fractures

along with earlier tiny quartz grains. Small quartz

grains also fill open space in altered pumice fragments. Groundmass of most samples has altered to

green chlorite; however, some white smectite occurs

in upper part of unit and illite was identified on XRD

analyses from lower part. A few bronze-colored hexagonal pyrrhotite crystals and cubic pyrite crystals

were observed.

757.5-932 Massive, dense, locally vesicular but sparsely fractured, subhorizontal basaltic-andesite to basalt lava

flows and interflow breccias make up lower part of

USGS-N2 drill core. Petrographically, flows have

different groundmass characteristics (grain size and

textures), proportions of phenocrysts, and SiO2 contents. Rounded clinopyroxenite xenoliths occur in

lower basalt flows. Flow tops and basalt breccias at

792.8 to 795.5,809.2 to 810.8 m, 850.4 to 852.8 m,

910.4 to 911.1 m, and 930 to 930.5 m are altered and

bleached but are not extensively fractured. Interval

between 930-932 m, at bottom of drill hole, is extensively altered and bleached distinctively different

from overlying lava flows.

757.5-761.7 Light-greenish-gray basaltic breccia. A few breccia

fragments have unaltered(?) tiny groundmass plagioclase laths and abundant tiny opaque grains

(leucoxene?) that are light gray in reflected light.

Phenocrysts are notably absent, but sporadic rectangular calcite deposits suggest that plagioclase has

been completely replaced. Vesicles, matrix material

between breccia fragments, and most lithic fragments

are completely altered primarily to quartz crystals

and calcite along with green chlorite. Tiny cubic pyrite crystals are disseminated throughout one sample;

smectite and illite were identified in XRD analyses

of some samples.

761.7-792.8 Medium-gray to greenish-black basalt or basaltic

andesite lava flow with trachytic to felty texture conAPPENDIX
90 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

sisting of approximately 2 percent plagioclase and

less than 1 percent clinopyroxene phenocrysts.

Groundmass contains plagioclase laths and tiny

clinopyroxene and magnetite crystals. Hydrothermal

alteration is more intense in upper and lower parts

of flows. Fractures and vugs are coated by colorless

quartz crystals as long as 7 mm, white to colorless

rhombic and bladed calcite crystals as much as 1 cm

across, bronze-colored pyrrhotite platelets, and some

cubic pyrite crystals. Green clay deposits on fractures and in groundmass of a few samples consist of

smectite, mixed-layer chlorite-smectite, chlorite, and

illite in decreasing order of abundance. A few samples

have calcite or patches of brownish iron-oxide(?)

groundmass stains.

795.5-809.2 Light-greenish-gray to black basalt lava flow has

holocrystalline groundmass composed of plagioclase

laths and anhedral clinopyroxene and magnetite;

it is porphyritic with about 5 percent plagioclase

phenocrysts, some of which are zoned, about 5 percent clinopyroxene phenocrysts, and 1 percent

glomeroporphyritic clots of clinopyroxene and plagioclase. Hydrothermal alteration is more intense in

upper part of flow where plagioclase crystals are altered to calcite and white illite-smectite mixed-layer

clay. Clinopyroxene crystals appear to be completely

altered to green chlorite. Patches of calcite and gray

clay and crystals of tiny cubic pyrite are disseminated in groundmass of several samples. Fractures

and vesicles generally are filled by quartz, calcite,

and either chlorite, smectite, or mixed-layer chlorite-smectite.

810.8-850.4 Thick dark-greenish-gray to black basalt flow is highly

vesicular (locally as much as 40 percent vesicles

between 822.4 and 844.3 m). Groundmass is

holocrystalline with plagioclase laths, clinopyroxene,

and magnetite. Locally zoned and partly resorbed

plagioclase phenocrysts make up 10 percent of rock

near top of flow and about 50 percent at base.

Clinopyroxene phenocrysts make up about 3 percent

of rock in upper part of flow and increase slightly

in abundance toward base; olivine phenocrysts

altered to iddingsite are present at base of flow.

Glomeroporphyritic clots of clinopyroxene and

plagioclase make up less than 1 percent of rock.

Colorless and white calcite is a late fracture and vug

filling and partly replaces plagioclase crystals.

Smectite was identified by XRD analyses in a few

green clay samples from fractures and vugs, but most

of these clays consist of chlorite or mixed-layer

chlorite-smectite. Bluish colloform chalcedony was

deposited on a few fractures. Colorless euhedral

quartz crystals are early open-space fillings in most

samples but formed later than chlorite in some

samples. In a few samples more than one generation

of quartz crystals occur and late quartz replaces

calcite. Cubic pyrite crystals are plentiful in a few

samples. Reddish hematite partly replaces magnetite

in lower half of interval.

852.8-910.4 Dark greenish-black basalt flow similar to flow

above but is coarser grained with more abundant

phenocrysts. Intergranular groundmass is composed

of anhedral clinopyroxene and magnetite and

subhedral plagioclase laths. Flow consists of about

30 to 60 percent zoned plagioclase, about 3 percent

clinopyroxene phenocrysts, and 5 percent olivine

phenocrysts that are altered to iddingsite (mostly

brown-green chlorite and hematite, but one sample

also contains calcite). Minor vesicles occur at top

and base of flow. White, pink, or orange calcite fills

vesicles and fractures and partly replaces plagioclase

phenocrysts in a few samples. One sample contains

white hexagonal prisms of apatite in cavities. Minor

illite was detected in XRD analyses of two wholerock samples. Chlorite and mixed-layer chloritesmectite occur as open-space fillings and groundmass

alteration throughout the basalt flow. Bluish

colloform chalcedony and colorless massive to

crystalline quartz were deposited in many fractures

and vesicles. Cubic pyrite crystals occur in several

samples, but bronze, hexagonal, tabular, pyrrhotite

crystals coat cavities and are disseminated in one

sample from lava flow. Hematite replaces spherical

magnetite deposits in one fracture filling.

 911.1-930 Medium-gray to greenish-black basalt flow is

texturally similar to flow above but has about 40

percent vesicles at its top with vesicularity decreasing

rapidly to massive basalt within about 3.5 m. Flow

includes about 25 percent plagioclase phenocrysts

that are partly zoned and slightly to extensively

resorbed, 5 percent olivine phenocrysts altered to

iddingsite, 3 percent clinopyroxene phenocrysts, and

<1 percent rounded xenocrystic clots of anhedral

clinopyroxene. Calcite partly replaces plagioclase

phenocrysts, and colorless euhedral rhombic calcite

crystals are deposited in vesicles and fractures. XRD

analysis of one whole-rock sample shows smectite.

XRD analyses also show chlorite or mixed-layer

chlorite-smectite deposited in vesicles, fractures, and

groundmass of most samples. Colorless euhedral

quartz crystals or massive quartz are formed in

vesicles and fractures. Bronze-colored platy

pyrrhotite crystals are in one sample. Pyrite fracture

fillings and disseminated crystals occur in a few

samples. Euhedral yellow-green epidote crystals are

deposited in vesicles and, with calcite, replace

plagioclase phenocrysts. Colorless euhedral anhydrite

crystals were deposited in open spaces of one sample.

Red rim is present around a clinopyroxene phenocryst

in one sample.

 930.5-932 Light- to medium-gray leached vesicular lava flow

at bottom of drill hole is pervasively altered and

distinctively different from overlying lava flows.

Plagioclase phenocrysts and holocrystalline

groundmass are partly altered to calcite. Bladed or

blocky calcite crystals also were deposited in vesicles.

Olivine is altered to green clay; both chlorite and

illite were identified in whole-rock XRD analysis.

Green fibrous chlorite lines vesicles and was earliest

deposit. Some small euhedral quartz crystals were

deposited later. Pyrite is disseminated throughout

bottom sample, and pyrrhotite was deposited on a

fracture surface.
91

APPENDIX 2.

LITHOLOGIC DESCRIPTION OF DRILL CORE GEO-N1 FROM

THE SOUTH FLANK OF NEWBERRY VOLCANO

Depth (m) Description of drill core

0-148.4 No core recovery.

148.4-357.2 Several, light-, medium-, and dark-gray, basalt to

andesite lava flows; flow breccias, cinder layers, and

thin (<1m) tuff beds occur between lava flows.

 180.7 Fine-grained holocrystalline basaltic andesite lava

flow; groundmass consists of plagioclase laths and

anhedral clinopyroxene and magnetite grains.

Clinopyroxene phenocrysts are scarce; some plagioclase phenocrysts are zoned. Rare olivine phenocrysts were observed in several other samples from

this interval (Earth Science Laboratory, 1986).

Deuteric iron oxide stains much of the breccias. Except for one occurrence of calcite and a few white

(amorphous silica?) fracture and vesicle coatings,

core samples are unaltered.

357.2-362.1 Horizontally bedded, light-brown to bright-orange,

soft friable dacitic-rhyodacitic tuff interval with

fiamme. XRD analysis shows mostly plagioclase with

minor devitrification cristobalite and hematite.

362.1-658.9 Medium- to dark-gray andesitic to basaltic lava flows

that are usually dense, although highly fractured with

some vesicular or flow-banded intervals. Between

lava flows are flow breccias or green, brown, and

orange lithic-tuff beds. Interval is cut by seven thin

(<1 m to 2.5 m thick) dark-gray to black aphanitic

dikes that commonly have glassy chill margins. Some

dikes display minor vesiculation, and at least one

contains inclusions of light-gray volcanic fragments.

Whole-rock XRD analyses show plagioclase,

clinopyroxene, and minor devitrification cristobalite.

Sparse olivine was noted in published description

(Earth Science Laboratory, 1986) for some core

samples. Flow breccias and many fractures have darkred to orange deuteric to hydrothermal(?) iron-oxide

staining; XRD analyses show that iron oxides range

from amorphous to well-crystallized hematite. Some

fractures and open spaces of vesicles or between breccia fragments contain white, yellow, or orange

smectite. Lithic-tuff interval just above dike at about

572 m contains mixed-layer illite-smectite. White

blocky calcite and yellowish botryoidal chalcedony

were identified by XRD analysis in open spaces of

two flow breccias.

658.9-664.2 Pink rhyodacitic ash-flow tuff. XRD analysis shows

minor hematite alteration, primary plagioclase and

magnetite, devitrification cristobalite, and vaporphase tridymite.

664.2-1,132.0 Series of thin basaltic lava flows (includes a few basaltic andesite and andesitic flows) with intervening

flow breccias, ash and cinder beds, lithic tuff, tuff

breccia, lapilli tuff, and ash-flow tuff. Core from this

interval is cut by several basaltic dikes (<1 m to ~12

m thick). Brief hand-specimen and thin-section descriptions of some chemically analyzed samples are

included below.

 703.3 Dark-gray holocrystalline basalt dike with glassy chill

margins and vertical flow banding marked by trains

of tiny unfilled vesicles. Groundmass consists of

euhedral plagioclase laths and anhedral clinopyroxene

and magnetite crystals. A few plagioclase phenocrysts are slightly larger than groundmass crystals.

Minor devitrification cristobalite and fracture coatings of hydrothermal smectite were identified by XRD

analysis.

 744.3 Dark-gray to black medium-grained vesicular basalt

lava flow with ophitic texture. Groundmass plagioclase laths and subhedral magnetite crystals are enclosed by large, optically continuous clinopyroxene

phenocrysts. Larger plagioclase and olivine phenocrysts are scarce. Devitrification cristobalite was identified by XRD analysis. Colorless calcite and green

smectite line one sampled fracture from this flow; a

vesicle-filling sample contains tiny colorless

phillipsite crystals.

 789.0 Dark-gray vesicular basalt flow with plagioclase laths

ranging from groundmass to phenocryst size. Elongate clinopyroxenes enclose plagioclase laths (ophitic

texture) and are partly altered to green (iron-rich?)

smectite and exsolved subhedral magnetite; magnetite is partly altered to red iron oxide, possibly hematite. Partly altered olivine phenocrysts form several

percent of rock. Vesicles, many having horizontal

elongation, are lined with orange or green stubby fibrous smectite, sometimes forming boytryoids, and

later botryoidal, massive, or acicular carbonates

(ankerite-dolomite, calcite, and kutnohorite in XRD

analyses).

 843.4 Light-gray, medium- to fine-grained basalt flow.

Groundmass consists of plagioclase laths and abundant anhedral clinopyroxene and anhedral to

subhedral magnetite crystals partly altered to hematite. Centers of some large euhedral zoned plagioclase

phenocrysts have a sieve texture. A few plagioclase

crystals are extensively fragmented. Large subhedral

olivine phenocrysts with reddish rims (magnetite altered to hematite) form glomeroporphyritic clots with

plagioclase. XRD analysis of this sample also shows

devitrification cristobalite. Fractures and vesicles contain green clay and colorless needles or massive deposits of aragonite and calcite.

 859.4 Medium-gray basalt flow near base of above lava

flow. Groundmass consists of small anhedral magnetite grains and plagioclase laths within large, distinctively separated anhedral clinopyroxene crystals

(ophitic texture). Sieve-textured plagioclase phenocrysts occur as at 843.4 m. Occasional large olivine

crystals, partly altered to green smectite, are

glomeroporphyritic with plagioclase. Colorless calcite and radiating needles of aragonite coat a steep

fracture. Calcite and okenite were identified in XRD

analyses of other samples from this basalt flow.

 932.7 Dark-gray medium-grained porphyritic basalt flow

with ophitic texture. Groundmass contains plagioclase laths enclosed by anhedral clinopyroxene,

subhedral blocky magnetite partly altered to hematite, and dark-brown interstitial glass altered to alHYDROTHERMAL MINERALS
92 GEOTHERMAL DRILL HOLES AT NEWBERRY VOLCANO, OREGON

tered to iddingsite. Various sizes of plagioclase phenocrysts display zoning, twinning, or albitized edges,

or are extensively shattered. Sample is cut by a nearvertical fracture lined by tiny white crystal aggregates of kutnohorite and later dark-green clay. Fracture fillings in other samples from this lava flow also

contain calcite and ankerite-dolomite in addition to

kutnohorite.

 939.7 Medium-gray vesicular basalt flow with palagonitized and devitrified glass (cristobalite identified byXRD analyses). Groundmass has small scattered plagioclase laths and very dark devitrified

glass with vesicles partly to completely filled with

yellow-brown palagonite(?) or smectite. A few

phenocrysts of fresh plagioclase and clinopyroxene

and one glomeroporphyritic clot of clinopyroxene,

plagioclase, and magnetite occur. Other samples

from this

lava flow contain fracture and vesicle fillings of

kutnohorite, siderite, smectite, and aragonite. A few

tiny pyrite crystals coat flow partings of one sample

along with smectite and later siderite.

 953.0 Dark-gray to black, medium- to fine-grained

microvesicular basalt dike (~2 m thick). Groundmass

consists of large needle-like plagioclase laths in dark

glassy matrix that appears to be partly altered to clay,

although smectite was not identified in whole-rock

XRD analysis. Sample has a few small plagioclase

phenocrysts. Numerous small vesicles occur in

steeply dipping trains; many vesicles are partly to

completely filled by botryoidal siderite. Siderite may

have brownish iron oxide(?) or clayey(?) core with

concentric layering or radial structure that has a very

thin brownish boundary between growth layers. Some

vesicles are coated by colorless botryoidal isometric

opal. A few vesicles contain both siderite and opal;

relations between these two secondary minerals are

unclear, but siderite may be later.

 1,015.0 Dark-gray to black aphanitic basalt or basaltic andesite flow. Groundmass of tiny anhedral magnetite

grains, some with reddish hematite alteration, and

plagioclase laths in dark glassy matrix. Anhedral to

subhedral clinopyroxene phenocrysts are partly in

glomeroporphyritic association with rounded, elongate to stubby and twinned plagioclase crystals.

Vesicles are filled or coated with fibrous brown to

green smectite; a few vesicles contain carbonate

(kutnohorite?) fillings.

 1,033.6 Medium-gray microporphyritic andesite flow.

Groundmass of small plagioclase laths, anhedral

clinopyroxene crystals, and anhedral magnetite grains

(minor hematite alteration). Clinopyroxene phenocrysts are small. Many anhedral to subhedral plagioclase phenocrysts are sieve textured; others are

twinned or zoned, and a few have overgrowths of

later plagioclase. A few vesicles are filled by brown

fibrous smectite or siderite. Light-caramel-colored

siderite and pale-green smectite coat a vertical fracture. Colorless aragonite also coats fractures in other

samples from this lava flow.

 1,068.3 Light-gray, medium- to fine-grained vesicular basalt

or basaltic andesite. Groundmass contains plagioclase

laths and abundant small anhedral clinopyroxene

crystals and euhedral to anhedral magnetite grains

(some hematite alteration). Plagioclase phenocrysts

are elongate. Vesicles are mostly unfilled, but siderite (radial structure) associated with opaque iron oxide (magnetite?) coats a few vesicle walls and occurs between successive siderite layers. Fracture fillings in other samples from this lava flow contain

deuteric(?) botryoidal magnetite (some altered to hematite) with later pyrite, siderite, and calcite.

1,114.8 Medium-gray basaltic andesite flow with pilotaxitic

texture. Groundmass consists largely of equigranular

plagioclase laths oriented parallel to flow direction;

interstices between plagioclase crystals are filled by

small anhedral clinopyroxene crystals and magnetite grains. A few plagioclase phenocrysts are present.

Scarce vesicles are filled by reddish-brown,

botryoidal iron oxide (hematite?), later radial carbonate minerals (aragonite, ankerite-dolomite, and calcite in XRD analysis) in concentric growth rings, and

later green smectite.

1,132.0- Lowest part of drill hole contains a few, dark-gray,

dense, basaltic lava flows, but most samples consist

of medium-gray dacitic to rhyolitic lava flows

lithologically variable, including porphyritic,

vesicular, vitrophyric, and flow-banded zones.

Between some lava flows are medium-gray to

brick-red lithic tuffs, black glassy tuffs with

perlitic, pumiceous, or scoriaceous textures, or

brecciated zones containing a few black glassy

clasts or consisting entirely of obsidian fragments.

Thin-section notes are given below for two

chemically analyzed samples from this drill-core

interval.

 1,150.8 Medium-gray, porphyritic rhyodacite with very fine

grained groundmass containing tiny plagioclase laths

and short linear trains of opaque magnetite grains.

Phenocrysts of large plagioclase crystals, partly altered olivine, and clinopyroxene that usually forms

glomeroporphyritic clots with plagioclase and magnetite phenocrysts. Magnetite only very slightly altered to reddish iron oxide. Light-caramel-colored

hemispherical crystal clusters of siderite and later

deposits of calcite or kutnohorite(?) and gray smectite

were deposited on vapor-phase partings.

1191.5 Medium-gray, porphyritic rhyodacite with buff to

green mottling around irregular vapor-phase cavities.

Groundmass is cryptocrystalline with small plagioclase laths and stringers of magnetite grains; XRD

analysis also shows cristobalite, K-feldspar, tridymite,

and quartz(?). Plagioclase phenocrysts are abundant,

and a few zoned or twinned euhedral to subhedral

crystals form glomeroporphyritic clots with subhedral

magnetite and a mafic mineral (olivine?) that is

mostly altered to brownish and greenish smectite and

a carbonate mineral (siderite or calcite?). Vapor-phase

cavities contain brownish or greenish smectite and

siderite filling spaces between tridymite crystals.

1,386.8