Hydrothermal Alteration in the 

Mount Hood Area, Oregon 

U.S. GEOLOGICAL SURVEY BULLETIN 2054 
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Hydrothermal Alteration in the 

Mount Hood Area, Oregon 

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

Melvin H. Beeson 

U.S. GEOLOGICAL SURVEY BULLETIN 2054 

A study of late Tertiary hydrothermal alteration corresponding 

to zeolite-facies to greenschist-facies metamorphism and 

Quaternary hydrothermal and fumarolic minerals from 

outcrops and geothermal drill holes near Mount Hood, Oregon 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

For sale by the Books and Open-File Report Sales 

U.S. Geological Survey 

Federal Center, Box 25286 

Denver, CO 80225 

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 

Text and illustrations edited by James W. Hendley II 

Library of Congress Cataloging in Publication Data 

Bargar, Keith E. 

Hydrothermal alteration in the Mount Hood area, Oregon I by Keith E. Bargar, Terry E.C. Keith, and Melvin H. Beeson. 

p. em.- (U.S. Geological Survey bulletin ; 2054) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2054 

1. Hydrothermal alteration-Oregon-Hood, Mount, Region. 2. Geology, Stratigraphic-Tertiary. I. Keith, Terry E.C. 

II. Beeson, Melvin H. III. Title. IV. Series. 

QE75.B9 no. 2054 

[QE390.5] 

557.3 s---dc20 

[549.9795'61] 93-12199 

CIP 
CONTENTS 

Abstract ............................................................................................................................. .. 

Introduction ........................................................................................................................ . I 

Analytical Methods .................................................................................................. . 2 

Acknowledgments ................................................................................................... .. 2 

Geologic Setting ................................................................................................................. . 3 

Stratigraphy .............................................................................................................. . 3 

Columbia River Basalt Group ........................................................................ . 3 

Rhododendron Formation ............................................................................... . 7 

Late Miocene Andesitic Lava Flows and Intrusive Plugs ............................ .. 7 

Still Creek and Laurel Hill Intrusions ............................................................ . 13 

Other Intrusive Rocks .................................................................................... .. 13 

Pliocene Andesitic Lava Flows ...................................................................... . 14 

Quaternary Volcanic Activity ......................................................................... . 14 

Hydrothermal Alteration Mineralogy ................................................................................ . 14 

Silica Minerals ......................................................................................................... .. 15 

Opal ................................................................................................................. . 15 

B-cristobalite ................................................................................................... . 16 

Chalcedony ..................................................................................................... . 16 

Quartz .............................................................................................................. . 16 

Carbonate Minerals .................................................................................................. . 17 

Dolomite ......................................................................................................... . 17 

Manganoan Calcite and Rhodochrosite .......................................................... . 17 

Siderite ............................................................................................................ . 18 

Cerussite .......................................................................................................... . 18 

Calcite ............................................................................................................. . 18 

Zeolite Minerals ....................................................................................................... . 18 

Wairakite ......................................................................................................... . 18 

Chabazite ......................................................................................................... . 19 

Epistilbite ........................................................................................................ . 20 

Ferrierite .......................................................................................................... . 20 

Harmotome ..................................................................................................... . 20 

Heulandite ....................................................................................................... . 23 

Laumontite ...................................................................................................... . 25 

Mordenite ........................................................................................................ . 26 

Scolecite .......................................................................................................... . 27 

Stilbite and Stellerite(?) .................................................................................. . 27 

Clay Minerals ........................................................................................................... . 29 

Kaolinite and Halloysite ................................................................. : ............... . 29 

Celadonite ....................................................................................................... . 31 

Smectite ........................................................................... : ..................... ~ ......... . 32 

Illite·······················································································.··························· 32 

Mixed-Layer Illite-Smectite ................................................................... : ....... . 32 

Chlorite ........................................................................................................... . 3_2 

Mixed-Layer Chlorite-Smectite .............................. .' ....................................... . 33 

Sepiolite(?) ..................................... · ............................................ : .................... . 33 

Sulfate Minerals ....................................................................................................... . 33 

Gypsum-Anhydrite ................................................................ : ................... .' ..... . 33 

Alunite-Natroalunite and Jarosite-Natrojarosite ............................. : ... .' ........... . 34 

Anglesite ......................................................................................................... . 34 

Sulfide Minerals ....................................................................................................... . 34 

III 
IV CONTENTS 

Galena and Sphalerite...................................................................................... 34 

Chalcopyrite..................................................................................................... 34 

Pyrite................................................................................................................ 34 

Iron Oxide-Hydroxide Minerals................................................................................ 35 

Amorphous Iron Oxide-Hydroxide................................................................. 35 

Goethite............................................................................................................ 35 

Hematite........................................................................................................... 35 

Magnetite . . . . .. . . . . . . . .. . .................. .. . . . . . ........... ... . . . .. . . . . . . . . . . ..... ... . . . . .. . . . . ... . ........ .. . . . 35 

Other Minerals........................................................................................................... 36 

Epidote............................................................................................................. 36 

Titanite(?)......................................................................................................... 36 

Garnet(?) . ................................ ........ .. ......... .. ..... .............. ............... .................. 38 

Truscottite(?) ... ..................... ........... ................. ............... ................. ................ 38 

Actinolite ............................................ ,............................................................. 38 

Prehnite . .................................... .. . . . . . ................................... ... . .. . . . . . . . ................ 39 

Adularia............................................................................................................ 39 

Tourmaline and Apatite ... . .. . ............ ............ .... . ............ ................... ................ 40 

Copper Minerals . .. . .. ... .. .. .. . .. . .. .. .. .. . . . . .. .. ... .. .. . .. .. . .................. ... . .. . .. . . . . ..... .... .. . . .. 40 

Fluid Inclusions................................................................................................................... 41 

Discussion........................................................................................................................... 42 

Tertiary Metamorphism............................................................................................. 45 

Greenschist- and Subgreenschist-Facies Metamorphism................................ 45 

Zeolite-Facies Metamorphism......................................................................... 46 

Quaternary Metamorphism . ..................... ...... . . . .. .. .. . . .. .. .. .. . . . . .. ... .... .. ... .. . . ........... ... . .. 48 

Conclusions......................................................................................................................... 49 

References Cited . . . . . .. .. . . . . . . . . . . . . . . .. .. . .. . .. . . . . .. .. . ... . . . . ........................ .. . . . . . . . . . . .. . . . . . . . . . . . ............... 49 

Appendixes: 

A. Lithologic Descriptions and Distribution of Hydrothermal Minerals in 

Selected Mount Hood Geothermal Drill Holes............................................... 56 

B. Hydrothermal Minerals Identified from Geothermal Drill Holes and 

Outcrops in the Mount Hood Area.................................................................. 70 

FIGURES 

I. Topographic map of the Mount Hood area, showing the location of geothermal drill holes included 

in this report and other geothermal gradient drill holes in the vicinity of Mount Hood ........................ ............ 4 

2. Photograph showing Mount Hood from Elk Meadows....................................................................................... 5 

3. Geologic map of the Mount Hood area................................................................................................................ 6 

4. Photograph showing lateral moraine deposits below Coe Glacier ..................................................................... 15 

5-22. Scanning electron micrographs showing: 

5. Botryoidal aggregates of rhombic dolomite crystals from the McGee Creek drill hole........................... 17 

6. An acicular cerussite crystal from a tailings pile in the Cheeney Creek Mining Area............................. 18 

7. Subhedral wairakite crystals and later chlorite vein filling in specimen 79CCQ-2001B.......................... 19 

8. Pseudocubic chabazite crystals coated by a thin layer of botryoidal blue-green chrysocolla 

that has been partially scraped off.............................................................................................................. 20 

9. Euhedral epistilbite crystals from the Zigzag River drill hole ................................................................. 23 

10. Lamellar crystals and prismatic needles of ferrierite from an outcrop near Barlow Pass ... .. .. .............. ... 24 

11. Harmotome (phillipsite group) crystals from the abandoned Cheeney Creek Mining Area..................... 24 

12. Blocky heulandite crystals from the McGee Creek drill hole . . .. ... .. .. .. .. ............. ... . .. .. . .. ..... .. .. .. . ............ .... 25 

13. Open-space filling of euhedrallaumontite prismatic crystals from the Pucci Chairlift drill hole............ 26 

14. Euhedral, acicular scolecite crystals from a fracture filling in a lava flow of the 

Rhododendron Formation . . . . .... . . . . . . . . ... . .. .. . . . . . . . . . . .. . . . . .. . . . . . . . . . . .. . . . . .. . . . . . .. . . . .. . . .. . . . . . . .. . . . . .. . .. . ... . . . . . . . . . . ............... 27 
CONTENTS v 

15. Euhedral, tabular stilbite crystals in drill cuttings from the Pucci Chairlift drill hole.............................. 29 

16. Euhedral, columnar stellerite(?) crystals from altered volcanic rocks of the Rhododendron Formation..... 29 

17. Euhedral columnar crystals of epidote from the Zigzag River drill hole.................................................. 36 

18. Imperfectly formed, trapezohedral garnet(?) crystal from the McGee Creek drill hole........................... 38 

19. Lamellar crystals of truscottite(?) deposited as radial concentrations on an erratic rock 

sample collected from the northeast side of Mount Hood......................................................................... 39 

20. Euhedral, equant adularia crystals from fracture fillings in Tertiary volcanic breccia near Iron Creek....... 40 

21. Radiating sprays of acicular tourmaline crystals in a lava flow of the Rhododendron Formation........... 41 

22. A grain of native copper from the Lost Creek drill hole .......................................................................... 41 

23-27. Photomicrographs showing: 

23. Liquid-rich, secondary fluid inclusions in hydrothermal quartz from a fracture in an 

outcrop of the Rhododendron Formation .... .............................. ........ .... .............................. ...................... 44 

24. Secondary, liquid-rich fluid inclusions in a hydrothermal quartz crystal that was deposited in 

open space of a Tertiary volcanic breccia .. .... .. .. .. .. .. .. .. .. .. .. . .... ...... .................... .... . .. .. .. .. .... .. .. .. .. .. .. .. .......... 44 

25. An isolated, monophase-vapor fluid inclusion having a negative crystal shape ...................................... 44 

26. A large, relatively saline, primary(?), liquid-rich fluid inclusion from a hydrothermal quartz 

crystal that was deposited near Iron Creek ....... .... .. .. .. . .... .. .. .. .. .. .. .. ... .. .. .. ...... .. .. . .. .. ...... ............ .... .. .... .. ...... 45 

27. A large, primary(?), vapor-rich fluid inclusion in hydrothermal quartz from a Tertiary 

volcanic breccia near Iron Creek................................................................................................................ 46 

28. Histograms of fluid-inclusion homogenization temperatures of six hydrothermal quartz samples 

from drill-hole cuttings and outcrops near Mount Hood..................................................................................... 47 

29. A plot of homogenization temperatures (Th) versus melting point temperature measurements (Tm) for 

fluid inclusions in hydrothermal quartz crystals from drill-hole cuttings and Tertiary outcrop 

samples collected near Mount Hood ...... . .. . ... .. ... . . . ... . . . . .. . . . . .. . . . . .. . . . . ..... ... . . .. ...................................... .... .. . . . . .......... 48 

TABLES 

1. Geothermal drill holes near Mount Hood, Oregon selected for hydrothermal mineralization studies .................. 5 

2. Chemical analyses of volcanic rocks from the Mount Hood area........................................................................... 8 

3. Electron-microprobe analyses of wairakite from outcrops associated with the Laurel Hill intrusion.................... 19 

4. Electron-microprobe analyses of chabazite from drill hole and outcrop samples associated with 

the Laurel Hill intrusion............................................................................................................................................ 21 

5. Water-content (in wt percent) and trace-element (in ppm) analyses of selected zeolite minerals 

from outcrops and drill holes near Mount Hood...................................................................................................... 22 

6. Electron-microprobe analyses of epistilbite from the Zigzag River drill hole (table 1, No. 8) near 

Mount Hood ...... . . . . . . . .. . .. . . ... . . . . . . . . .. ... . . .. . . . . . . . . . . . . ... . . . .. . . . . ................................... ... . . . . . . . . . . ... ..... . . . ... . . ..... . . . . . . ... . . ... ... . . . ... . . 23 

7. Electron-microprobe analyses of harmotome from the Cheeney Creek area, southwest of Mount Hood.............. 25 

8. Electron-microprobe analyses of heulandite from a fracture filling in a lava flow of the 

Rhododendron Formation near the Laurel Hill intrusion......................................................................................... 26 

9. Electron-microprobe analyses of laumontite from fractures in volcanic rocks of the 

Rhododendron Formation adjacent to the Laurel Hill intrusion.............................................................................. 27 

10. Electron-microprobe analyses of scolecite from outcrops and drill holes near Mount Hood................................. 28 

11. Electron-microprobe analyses of stilbite associated with the Laurel Hill intrusion................................................ 30 

12. Electron-microprobe analyses of stellerite(?) from the Zigzag River drill hole...................................................... 31 

13. Electron-microprobe analyses of pyrite from mineralized fault zones in Tertiary volcanic rocks near 

Mount Hood . . . . . . . . . . ... .. ... . . . . .. . . . . .. . . . . .. . . . . .. . . . . . . . . . . ... . . . . . . . . . . . .. .. .. . . . . . . . . . . ... ... .. . . . . . . . . . . .............................. ... ............... .......... 35 

14. Electron-microprobe analyses of epidote from drill holes and outcrops near Mount Hood................................... 37 

15. Electron-microprobe analyses of actinolite from volcanic rocks of the Rhododendron Formation 

associated with the Laurel Hill intrusion.................................................................................................................. 39 

16. Electron-microprobe analyses of adularia from near Mount Hood......................................................................... 40 

17. Fluid-inclusion heating/freezing data for hydrothermal quartz crystals from drill holes and outcrops 

near Mount Hood . .. . . . . . . . . . . .. . . . . .................. ............. ... . . .. . . . . .. . . . . .. . . . ... . . . . . . . . . . .. . . .. .. . . . . .. . . . . . . . . . . . ... . . .. . . . . .. . . . . ............ ..... ... . .. 43 

Hydrothermal Alteration in the 

Mount Hood Area, Oregon 

By Keith E. Bargar, Terry E.C. Keith, and Melvin H. Beeson 

ABSTRACT 

Thirty geothermal test drill holes were completed between 1976 and 1980 as part of an investigation of geothermal energy potential in the Mount Hood area. Active 

thermal features, consisting of near-summit fumaroles and 

warm springs at the southern base of Mount Hood, suggest 

the presence of a geothermal resource. The low flow volume of water in even the highest potential drill holes apparent! y has discouraged further geothermal studies. 

Nonetheless, the Mount Hood geothermal drilling program 

generated much geological, geophysical, geochemical, and 

mineralogical data. 

Hydrothermal alteration minerals identified from outcrop and drill-hole-cuttings samples in the Mount Hood 

area suggest that some late Tertiary volcanic rocks experienced geothermal temperatures corresponding to very low 

grade metamorphism over a temperature range comprising 

zeolite- to greenschist-facies metamorphism (-50°C to 

400°C). The higher greenschist-facies temperatures of this 

metamorphism appear to have been confined to narrow halos surrounding intrusions that have K-Ar ages of about 8 

to 11 Ma and -5 Ma. The late Miocene quartz diorite to 

quartz monzonite Laurel Hill and Still Creek intrusions on 

the southwest side of Mount Hood, and the late Tertiary 

volcanic rocks they intrude, exhibit typical greenschist 

metamorphic minerals including epidote, quartz, chlorite, 

magnetite, pyrite, actinolite, prehnite, adularia, calcite, and 

wairakite. Hornblende quartz microdiorite sills and 

dikes(?) that intrude late Tertiary volcanic rocks on the 

northwest side of Mount Hood were penetrated by two 

drill holes; many of the low-grade metamorphic minerals 

recovered in drill-core and cuttings samples were the same 

as those observed in the Laurel Hill and Still Creek outcrop and drill-hole samples. Near Iron Creek, to the southeast of Mount Hood, the presence of another intrusive 

body is suggested by geophysical studies; one dike from 

this area has a K-Ar age of -5 Ma. The late Tertiary intruded volcanic rocks in the Iron Creek area contain a 

greenschist-facies metamorphic mineral assemblage similar to the assemblage at Laurel Hill and Still Creek. Lower 

temperature, zeolite-facies minerals, consisting of various 

zeolite minerals, carbonate minerals, clay minerals, and 

hematite, are frequently superimposed upon the higher 

temperature, greenschist to subgreenschist mineral assemblages in the Mount Hood area. Late Tertiary zeolite-facies 

metamorphism occurs in several areas around Mount 

Hood and may be more widespread than observed in this 

investigation. 

Mount Hood Quaternary lava flows contain only sparse 

hydrothermal mineralization. Intense acid-sulfate alteration, producing silica minerals, sulfur, sulfate minerals, 

pyrite, and iron oxides and hydroxide, occurs in a fumarolic area (temperature -91 °C) near the summit of the 

mountain. Quaternary volcaniclastic debris units on the 

lower slopes of the mountain often contain redeposited fumarolically altered material. Geothermal test drill holes on 

the outer flanks of Mount Hood encountered mostly lowtemperature ( <23°C) waters. Thermal water between about 

60° and l13°C was found in four drill holes, which suggests that very low-temperature zeolite-facies metamorphism might presently be occurring at depth in the Mount 

Hood area. If present metamorphism in the Mount Hood 

area is similar to the late Tertiary metamorphic conditions, 

it can be predicted that any active higher temperature metamorphism must take place at considerable depth within 

narrow halos surrounding solidified intrusions. 

INTRODUCTION 

Mount Hood, an active volcano in the Cascade Mountains of northern Oregon, was designated a "Known Geothermal Resource Area" (KGRA) under the Geothermal 

Steam Act of 1970 (Godwin and others, 1971). Mount 

Hood was thought to have good potential for geothermal 

energy because of its late Pleistocene to Holocene volcanic activity (as recent as 200 to 300 years ago) (Crandell, 

1980), near-summit fumaroles, and the tepid ( <26°C) thermal springs near the base of the mountain on the south. 

Accordingly, a cooperative multidisciplinary geothermalresource-assessment program was begun in 1976 by the 
2 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Oregon Department of Geology and Mineral Industries 

(DOGAMI), U.S. Department of Energy (DOE), U.S. Forest Service (USFS), and U.S. Geological Survey (USGS). 

A major part of the assessment program included drilling 

several shallow geothermal gradient holes and a few deeper exploration holes. During the 1976 to 1980 program period, 30 geothermal drill holes were completed in the 

vicinity of Mount Hood (fig. 1); 15 drill holes were spudded by DOGAMI, 7 by Northwest Natural Gas Company 

(NWNG), 1 by DOE (which also provided funding for 

several other drill holes), and 7 by the USGS (Hook, 

1982; Robison and others, 1982). 

Temperature and geothermal gradient data have been reported for all Mount Hood geothermal drill holes (Hull 

and others, 1978; Blackwell and Steele, 1979; Blackwell 

and others, 1981a, b, c; Robison and others, 1981, 1982; 

Steele and others, 1982). Steele and others ( 1982) summarized the available heat-flow data for most drill holes and 

concluded that the data do not suggest the presence of a 

significant-sized shallow magma chamber beneath Mount 

Hood. An -5-km-diameter intrusive body that narrows upward to -1 km near the base of Mount Hood is suggested 

by aeromagnetic and gravity data (Williams and Keith, 

1982). Steele and others (1982) also indicated that, although the heat-flow data for the volcano reflect a very 

complex thermal system, heat-transfer data for much of 

the volcano cannot be adequately represented by the existing shallow drill-hole data. 

This report describes the hydrothermal alteration of numerous outcrop samples collected in the vicinity of Mount 

Hood, as well as drill cuttings from 13 of the geothermal 

drill holes (table 1) for which we were able to obtain sample splits. Several drill holes were not included in the 

study because the drilling reports suggested only minor or 

no hydrothermal mineralogy or because the drill cuttings 

were not located. The mineralogy of two other geothermal 

drill holes, also not included in this study, have previously 

been described (Timberline 3, White, 1979, and Old Maid 

Flat 7 A, Holdaway and Bussey, 1982). 

This study is also an outgrowth of a geologic and mineral survey of the Mount Hood Wilderness area in compliance with the Wilderness Act (Public Law 88-577, Sept. 3, 

1964), which requires that the U.S. Geological Survey and 

the U.S. Bureau of Mines evaluate the mineral resource 

potential of certain specified parcels of government-owned 

land. The Mount Hood Wilderness area comprises about 

47,100 acres, which include the summit of the volcano and 

its north, east and west flanks (Keith, Beeson, and Bargar, 

1982); in addition, much of Zigzag Mountain (fig. 1), to 

the west of Mount Hood, lies within the wilderness area. 

ANALYTICAL METHODS 

For most drill holes, drill cuttings were sampled at 

about 3-m intervals, with more closely spaced sample intervals in selected portions of a few drill holes; however, 

for the two earliest completed drill holes included in this 

report (table 1), samples were obtained at only about 12-m 

intervals. Cuttings splits were wet sieved through a 200-

mesh screen; generally, both the coarse and fine fractions 

were air dried and saved. Cuttings from each of the drill 

holes were systematically examined by binocular microscope; gravel- to silt-size grains were hand picked for sev-

.eral types of analyses. Thin sections were made of 

representative grains from the various lava flows and of 

volcaniclastic intervals penetrated by several drill holes. 

Selected whole-rock and hydrothermal mineral samples 

were hand picked for X-ray diffraction (XRD) analyses 

using a Norelco unit and Cu-Ka radiation. Rock and mineral samples were hand ground and prepared as slurries on 

glass slides. All samples were run at 1 o 28/min; selected 

samples were glycolated by placing the slides in an atmosphere of ethylene glycol at 60°C for 1 hour before Xraying. Euhedral hydrothermal mineral samples were carbon coated and placed in either a Cambridge Stereoscan 

180 or 250 scanning electron microscope (SEM) in order 

to obtain information on paragenesis, semiquantitative 

chemistry, and mineral morphology. Quantitative chemical 

analyses of several hydrothermal minerals from drill cuttings and outcrop samples were obtained by analysis of 

polished thin sections or mineral mounts using an ARLSEMQ electron microprobe. Double-polished thick sections of hydrothermal quartz from some drill cuttings and 

outcrop samples were prepared for fluid-inclusion studies 

using a Linkam THM 600 heating/freezing stage and TMS 

90 controller. 

Keith and others (1985) published K-Ar dates for 24 

rock samples from near Mount Hood in order to clarify 

the ages of several volcanic units mapped by Wise ( 1969); 

major- and trace-element chemical analyses for several of 

these dated rocks and other rock samples from the Mount 

Hood area are given in table 2. Major-element chemical 

analyses are by X-ray spectroscopy, and trace elements 

were analyzed by emission spectroscopy. Techniques for 

these two methods are as described in Baedecker ( 1987). 

ACKNOWLEDGMENTS 

We thank many of the past and present staff of the Oregon Department of Geology and Mineral Industries, particularly Donald A. Hull, Joseph F. Riccio, and George R. 

Priest, for allowing us to obtain splits of cuttings from several drill holes, for providing us with copies of drillers reports on all of the geothermal drill holes near Mount Hood, 

and for helpful discussions on the geology of the Mount 

Hood area. Scanning electron micrographs were provided 

by Robert L. Oscarson. James R. Boden, Michael L. 

Heard, Maura Hanning, and Joyce Miley assisted in some 

of the X-ray diffraction work. Reviews and constructive 
GEOLOGIC SETTING 3 

criticisms of the manuscript by Richard C. Erd and Robert 

I. Tilling are very much appreciated. We are especially 

grateful to John W. Hook for sharing his many geological 

insights on Mount Hood and the Oregon Cascades. 

GEOLOGIC SETTING 

Mount Hood, which lies in northern Oregon along the 

crest of the High Cascade Range, is a Quaternary stratovolcano that remains potentially active (Crandell, 1980). 

The summit (fig. 2), at an elevation of 3,425 m, is snow 

covered much of the year and is very conspicuous above 

the surrounding volcanic terrain (average altitude of about 

1,220 m). Intermittent volcanic activity has occurred in 

this area since the middle Miocene (Wise, 1969; Beeson 

and Moran, 1979; Covert and Meyer, 1979; Beeson and 

others, 1982, 1985). Rocks of the Columbia River Basalt 

Group are overlain by late Miocene andesitic lava flows 

and volcaniclastic rocks of the Rhododendron Formation 

that crop out in the Zigzag Mountain area immediately 

west of Mount Hood (fig. 3). The Rhododendron Formation is intruded by late Miocene andesite plugs that probably fed the andesite lava flows overlying the 

Rhododendron Formation. Andesitic lava flows of similar 

age occur all around Mount Hood and probably were extruded from several different vents (Wise, 1969). 

Roadcuts just south of Zigzag Mountain furnish excellent exposures of the Laurel Hill intrusion. This late Miocene quartz diorite intrusive body appears to be greater in 

extent than is indicated by surface outcrops (Wise, 1969); 

similar intrusive rocks have been found in drill holes as 

far as 2.2 km east and about 9.2 km north of the Laurel 

Hill outcrop (Gannett, 1982; Priest and others, 1982). 

Wise (1969) also suggested that the quartz diorite to 

quartz monzonite Laurel Hill and Still Creek intrusions 

(about 3 km to the south) might be related and are possibly coextensive beneath the surface. The Laurel Hill intrusion may have caused fractures and block faulting of older 

andesitic rocks on Zigzag Mountain. 

Pliocene andesite flows cap Zigzag Mountain and overlie the late Miocene andesite flows on all sides of Mount 

Hood. Like the older andesite lava flows, these late Pliocene andesite lavas were probably extruded from several 

different vents in the area. 

A glacially dissected Pliocene or Pleistocene (an age of 

3.2 Ma was reported by Wise (1969), but Keith and others 

(1985) gave an age range of 1.14 to 1.35 Ma) volcano, 

known as the Sandy Glacier volcano (fig. 3), is exposed on 

the west slope of Mount Hood about 4 km from the summit. 

Outcrops of basalt and andesite flows of this well-developed 

volcanic cone dip east and southeast beneath Mount Hood 

(Wise, 1969). Geophysical studies by Flanagan and Williams (1982) suggested the presence of two other volcanic 

cones buried beneath the present Mount Hood volcano. 

The present Mount Hood edifice is the product of volcanic activity over about the past 0.73 Ma, so inferred 

because no magnetically reversed rocks, older than the 

Brunhes-Matuyama polarity epoch boundary (Mankinen 

and Dalrymple, 1979), have yet been found (Priest, 

1982). The accumulation of layer upon layer of andesitic 

lava flows and pyroclastic rocks built the volcano to its 

present height or perhaps somewhat higher (Crandell, 

1980). According to Crandell's (1980) description of later stages of volcanic activity at Mount Hood, predominantly andesitic volcanism waned during the Fraser 

glaciation (-29 ka), and subsequent volcanic activity was 

dominated by hornblende dacite pyroclastic deposits, 

mudflows, and dome extrusion. The four most recent major eruptive periods are known as the Polallie (12 to 15 

ka), Timberline ( 1.4 to 1.8 ka), Zigzag (0.6 to 0.4 ka), and 

Old Maid (0.25 to 0.18 ka) eruptive periods (Crandell, 

1980; Cameron and Pringle, 1986, 1987). Crandell (1980) 

reported that during the Old Maid period a dome, a remnant of which is now known as Crater Rock, was formed 

just south of the summit of Mount Hood. Volcanic ejecta 

may have been deposited during mild activity reported in 

the mid-1800's (Crandell, 1980; Cameron and Pringle, 

1986, 1987). Historical activity and present-day fumaroles near Crater Rock (fig. 3) indicate that Mount Hood 

is still active. 

STRATIGRAPHY 

COLUMBIA RIVER BASALT GROUP 

The oldest rocks encountered in the drill cuttings we 

studied (Old Maid Flat 1 and Clear Fork drill holes; table 

1, Nos. 5 and 6) have been correlated with the -13.5- to 

16-Ma (McKee and others, 1977) tholeiitic Grande Ronde 

and Wanapum Basalts of the Yakima Basalt Subgroup of 

the Columbia River Basalt Group (Covert and Meyer, 

1979; Priest and others, 1982). Wise (1969) briefly described the petrology and major-element chemistry of the 

Columbia River Basalt Group near Mount Hood. Additional information on structure, stratigraphy, trace-element 

geochemistry, and geologic history of the Columbia River 

Basalt Group in the vicinity of Mount Hood can be found 

in Beeson and Moran (1979), Wollenberg and others 

(1979), and Beeson and others (1982, 1985). Priest and 

others' (1982) evaluation of the geothermal energy potential of the Mount Hood area indicated that the rocks of the 

Columbia River Basalt Group recovered from the Old 

Maid Flat (OMF) geothermal drill holes 1 (table 1) and 

7 A are largely unaltered and have open fractures. They 

concluded that the brittle, faulted (highly permeable) rocks 

of the Columbia River Basalt Group in the vicinity of 

Mount Hood might be a worthwhile reservoir for future 

geothermal exploration. 
4 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

45° 

30' / 

WASHINGTON 

.. / .. ) 

45o '{oo?' 

00' 

{) ..... · 

L.l 

.. ···· 

........ c~ 

i\ 

· ....... \_:;···.." 

·· .. ··· .. ) 

···········~ ..... \ 

Area of figure 1 

ttJ (::::..2ooo··········· 

1 0 Kilometers 

CONTOUR INTERVAL 2000 FT 

Figure 1. Topographic map of Mount Hood area showing the location of geothermal drill holes (dots) included in this 

report (numbered 1-13; see table 1) and other geothermal gradient drill holes (triangles) in the vicinity of Mount Hood. 

Abbreviations for names of topographic features referred to in the text of this report are BB, Barlow Butte; BP, Barlow 

Pass-Bennett Pass area; BR, Bluegrass Ridge; CSC, Cast Creek-Short Creek area; CH, Cheeney Creek; CC, Cloud Cap; 

CG, Coe Glacier; CR, Crater Rock; EH, Enola Hill; EM, Elk Meadows; GC, Government Camp; IC, Iron Creek; LCM, 

Last Chance Mountain; LH, Laurel Hill; LP, Lolo Pass; LM, Lookout Mountain, LC, Lost Creek; MCB, Mill Creek 

Buttes; P, The Pinnacle; PC, Polallie Creek; SCI, Still Creek intrusion; SG, Sandy Glacier; SGV, Sandy Glacier volcano; 

SWS, Swim Warm Springs; TL, Timberline Lodge; VR, Vista Ridge; and ZM, Zigzag Mountain. Base is modified from 

U.S. Geological Survey map of The Dalles, Oregon-Washington, 1971, 1:250,000. 
GEOLOGIC SETTING 

Table 1. Geothermal drill holes near Mount Hood selected for hydrothermal mineralization studies. 

[Data from Robison and others (1982). Abbreviation of drilling organization: DOGAMI, Oregon Depanment of Geology and Mineral Industries; NWNG, Nonhwest Natural 

Gas; USGS, U.S. Geological Survey] 

Map Year Drilling Drill-collar Total 

No . 1 Name of drill site2 Location3 Latitude Longitude drilled organi- altitude (m depth 

zation above m.s.l.) (m) 

I Eliot Branch I S/9E/26aba 45°27.6' 121°38.0' 1980 USGS 860 220 

2 Clear Branch I S/9E/31 aca 45°26.5' 121°43.0' 1980 USGS 1,250 311 

3 McGee Creek 2S/8Eilccd 45°25.1' 121°46.1' 1979, 1980 USGS 915 610 

4 Last Chance Mountain 2S/8E/9dba 45°24.5' 121°49.5' 1979 NWNG 890 152 

5 Old Maid Flat (OMF-1) 2S/8E/15cda 45°23.5' 121 °48.5' 1977, 1978 NWNG 838 1,220 

6 Clear Fork (OMF-3) 2S/8E/17cbd 45°23.3' 121°51.6' 1978 NWNG 658 402 

7 Lost Creek (OMF-1 0) 2S/8E/28aab 45°22.4' 121 °49.3 ' 1979 NWNG 811 131 

8 Zigzag River (Kiwanis Camp Road) 3S/8E/14bcb 45°18.9' 121°47.7' 1979 NWNG 951 287 

9 Laurel Hill 3S/8E/16cd 45°18.4' 121 °49.9' 1976 DOG AMI 762 125 

10 Still Creek 3S/8E/29ddc 4YI6.6' 121 °54.5' 1978 DOG AMI 722 152 

II Snow Bunny 3S/8.5E/25aac 45°17.2' 121°43.7' 1976 DOG AMI 1,168 82.5 

12 Mount Hood Meadows 3S/9E/3cca 45°20.0' 121 °39.6' 1980 USGS 1,665 355 

13 Pucci Chairlift 3S/9EI7dbb 45°19.3' 121 °42.8 ' 1979, 1980 USGS 1,628 1,220 

1Numbers refer to drill holes on figure I. 2Names in parentheses are alternate drill-hole designations. 

3Locations are given as township/range/section. Section letters a, b, c, and d refer toNE, NW, SW, and SE quarter sections, respectively in the 

sequence I /4, I /4 I /4, and I /4 I /4 I /4. For example: in 26aba, 26 is the section number, first a= NE I /4 section, b = NW I /4 of NE I /4 section, and 

second a = NE I /4 of NW I /4 of NE I /4 section . 

Figure 2. View of Mount Hood from Elk Meadows, about 7 km east-southeast of the summit. 

Max 

temp 

(C) 

10.3 

10.9 

60.0 

13.1 

82 

23.2 

10.8 

12 

10.2 

17.6 

7.2 

11.6 

65.5 

5 
EXPLANATION 

IN?q·ai)};j Alluvium (Holocene) 

~ Glaciers (Holocene) 

121°50' 

~pg~·~~ Glacial moraines (Holocene and Pleistocene) 

~~~Lavas of Mount Hood (Holocene and Pleistocene) 

~ l:;;;q~~;j Andesite of Cloud Cap (Pleistocene) 

8 H:O~P.H Andesite of The Pinnacle (Pleistocene) 

L·q~~v}j Andesite and basalt of Vista Ridge (Pleistocene) 

~a"gj Undifferentiated andesites (Pleistocene) 

w;q.:~R~:~m Sandy Glacier volcano (Pleistocene or Pliocene) 

f>"r.u~ ' :J Upper andesite flows (Pliocene) 

SJ::ia::::_,;;_3 Lower andesite flows (Miocene) 

45°25' I- I [:;::ti::;:j Intrusive rocks (Miocene) 

45°20' 

k:"li.~:;:J Laurel Hill intrusion (Miocene) 

[B Rhododendron 

Formation (Miocene) 

~ 

~ , Chenowith Formation of 

0 

Farooqui and others 

(1981) (Miocene) 

2 MILES 

0 1 2 3 KILOMETERS 

9. 

38. 

33 and • 39 

Qh 

121 °40' 

Figure 3. Geologic map of the Mount Hood area, Oregon (modified from Wise, 1969; Keith, Beeson, and Bargar, 1982). Numbers (1-41) are the locations of 

chemically analyzed rocks of table 2, except for rocks numbered 2, 7, 8, 18, and 19, which were collected outside the map area. 

0\ 

::r: 

>< I:' 

:-a 

~ ~ > t""' 

> 

~ 

~ 0 

z 

z 

~ s:: 

0 

c 

~ ::r: 

0 

0 

I:' 

~ ?> 

0 

~ 0 

~ 
GEOLOGIC SETTING 7 

Columbia River Basalt Group drill cuttings from the 

OMF-1 and Clear Fork drill holes (table 1, Nos. 5 and 6) 

contained moderate amounts of hydrothermal minerals including quartz, chalcedony, laumontite, stilbite, smectite, 

chlorite, siderite, calcite, hematite, and a 10-A mica group 

mineral. A highly fractured outcrop on the east side of 

Mount Hood (Tcrb, right-hand side, fig. 3), mapped as the 

Columbia River Basalt Group by Wise (1969), was considerably altered and contained abundant disseminated pyrite; fracture fillings of chlorite, magnetite, hematite, 

pyrite, quartz, chalcedony, and calcite; and chlorite 

groundmass alteration. This outcrop is very close to the 

Hood River Fault (Priest, 1982), which probably caused 

outcrop fracturing as well as provided a pathway for 

movement of mineral-depositing solutions. The permeability of outcrop and drill-cuttings samples of the Columbia 

River Basalt Group that we examined was significantly reduced by deposition of hydrothermal minerals. 

RHODODENDRON FORMATION 

Three drill holes (Last Chance Mountain, OMF-1, and 

Clear Fork; table 1, Nos. 4, 5, and 6) penetrated rocks that 

are correlated with the late Miocene Rhododendron Formation (Covert and Meyer, 1979; Priest and others, 1982; 

Trimble, 1963). In the Mount Hood area, this formation 

consists of as much as 600 m of predominantly andesitic, 

epiclastic, laharic deposits, pyroclastic tuffs, and lava 

flows (Gannett, 1982). According to Wise ( 1969) and Gannett ( 1982), the volcaniclastic deposits and lava flows of 

the Rhododendron Formation consist mostly of pyroxene 

(both hypersthene and augite) andesites and sporadic minor hornblende andesites. However, major-element chemical analyses of outcrop samples (Wise, 1969; Gannett, 

1982) and drill-hole material from OMF-1 and OMF-7 A 

(Priest and Vogt, 1982) also show the presence of several 

dacites. Trace-element data permitted Gannett (1982) to 

distinguish Rhododendron lava flows and breccia clasts 

from compositionally similar overlying late Miocene (designated as early Pliocene in age by Wise, 1969) volcanic 

rocks. Wise (1969) reported a K-Ar date of 7 Ma, which 

Gannett ( 1982) suggested may be too young, for a lava 

clast in the lower part of the Rhododendron Formation. 

Priest and others (1982) obtained K-Ar ages of 9.5, 10.6, 

and 12.1 Ma for altered whole-rock samples from the upper part of the Rhododendron Formation. Similar average 

K-Ar ages of 10.53 and 11.23 Ma were reported by Keith 

and others (1985) for andesitic lava flows (table 2, analyses 1 and 2) in the upper part of the formation. Thus, 

rocks of the Rhododendron Formation were probably extruded during about a 3- to 4-m.y. interval in the late Miocene (Gannett, 1982; Trimble, 1963). 

Virtually all outcrop and geothermal exploration drillhole samples that contain materials of the Rhododendron 

Formation exhibit extensive low-temperature hydrothermal 

alteration. In addition, Rhododendron tuffs and lava flows 

adjacent to intrusive bodies have been contact metamorphosed, propylitically altered, or have undergone minor 

epithermal mineralization. The most abundant hydrothermal minerals found in the Rhododendron Formation are 

laumontite and stilbite (less abundant zeolites include epistilbite, heulandite, harmotome, mordenite, scolecite, stellerite(?), and wairakite), smectite (less abundant clay 

minerals are illite, mixed-layer illite-smectite, chlorite, 

mixed-layer chlorite-smectite, celadonite, kaolinite-serpentine, and sepiolite), calcite (less abundant carbonates include siderite, ankerite, dolomite, cerussite, and 

manganoan calcite), sulfides (pyrite, chalcopyrite, galena, 

and sphalerite), silica minerals (quartz, chalcedony, and 

minor B-cristobalite), and iron oxides (hematite and magnetite); epidote, actinolite, adularia, chrysocolla, and anglesite also are present. 

On the east side of Mount Hood, the Dalles Formation 

(equivalent to the Chenoweth Formation of Farooqui and 

others, 1981) contains rocks lithologically similar to the 

Rhododendron Formation and is generally regarded to be, 

at least in part, contemporaneous with the Rhododendron 

Formation (Wise, 1969; Gannett, 1982). This correlation 

was based, in part, on a K-Ar age of 7.0 Ma for a lava 

flow that overlies the Dalles Formation (Wise, 1969). Keith 

and others (1985) obtained a K-Ar age of 8.18 Ma for a 

hornblende-pyroxene andesite flow (table 2, analysis 3), 

which is probably one of two interbedded andesite flows 

noted by Wise ( 1969) near the top of the formation. 

LATE MIOCENE ANDESITIC LAVA FLOWS AND 

INTRUSIVE PLUGS 

Pyroxene andesite lava flows that overlie the Rhododendron and Chenoweth Formations have K-Ar ages of 5.5 

Ma on Zigzag Mountain, 5.8 Ma near Lolo Pass on the 

west side of Mount Hood, and 7.0 Ma near Polallie Creek 

on the east side of Mount Hood (Wise, 1969). According 

to chemical and petrographic analyses by Wise (1969), 

these late Miocene volcanic rocks are mostly two-pyroxene andesites with some olivine andesites and basalts. 

Priest and others ( 1982) subdivided the late Miocene 

andesites at Last Chance Mountain near the OMF-1 and 

OMF-7 A drill holes and informally called the lower, 

coarse-grained two-pyroxene andesitic unit the Last 

Chance andesite. Major-oxide and trace-element chemical 

analyses for three of the andesitic rocks encountered in the 

OMF-7A drill hole were given in Priest and Vogt (1982); 

Priest and others (1982) reported K-Ar ages of 10.5 and 

10.7 Ma for the Last Chance andesite. Keith and others 

(1985) reported whole-rock K-Ar ages of about 5.4, 6.2, 

and 8.9 Ma for lava flows that overlie the Rhododendron 

Formation at Vista Ridge (northwest of Mount Hood), 
Table 2. Chemical analyses of volcanic rocks from the Mount Hood area. 00 

Chenoweth 

Rhododendron Formation Formation Late Miocene lava flows and intrusive plugs 

Anal;tsis number 1 2 3 4 5 6 7 8 9 

Sample number 81 MH-14 81 MH-21 79 EF-0016A 79 ZZL-1089 79 CCT-1094 79 LP-1108 79 KL-1117 79 KL-1120A 79 LL-1126C 

Major-element chemical analyses (wt percent oxides) I :I: 

--< 0 

Si02 ------------- 59.8 54.8 63.2 51.2 56.9 64.4 58.7 53.3 60.1 ~ 

A1203 ------------ 16.8 19.5 17.2 17.3 18.6 17.1 16.5 18.2 17.4 0 

1-j 

Ti02 ------------- 0.80 0.94 0.69 1.10 0.94 0.57 0.95 1.44 0.84 :I: 

Fe203------------ 6.15 7.02 5.49 9.42 6.30 4.66 6.75 8.63 6.43 trl 

~ 

MnO ------------- 0.09 0.10 0.08 0.16 0.09 0.08 0.14 0.13 0.10 ~ 

MgO ------------- 3.1 2.5 2.34 7.36 3.82 1.74 4.83 4.88 3.30 > t""' CaO -------------- 5.94 7.60 5.29 10.6 6.55 4.38 6.41 8.77 6.13 

~ Na20 ----------- 3.8 4.1 4.02 2.84 3.82 4.19 3.20 3.50 3.86 

K 20-------------- 1.00 0.98 1.41 0.28 0.18 1.32 0.61 0.80 1.29 

P20s ------------- 0.18 0.20 0.15 0.14 0.21 0.19 0.17 0.30 0.16 

Zr02 ------------- 0.014 0.014 0.02 0.01 0.03 0.03 0.02 0.02 0.02 ~ Cr02 ------------- 0.007 0.002 0.01 0.03 0.02 <0.005 0.02 0.01 0.01 0 - NiO -------------- 0.003 <0.002 0.003 0.009 0.008 0.002 0.01 0.004 0.005 z 

BaO -------------- 0.034 0.025 0.035 0.014 0.02 0.038 0.028 0.032 0.037 z 

SrO -------------- 0.040 0.052 0.055 0.047 0.07 0.064 0.048 0.097 0.05 1-j 

LOI (900°C) ---- __LLl_ __.2....QQ__ ___!1_62_ __Q,_l2._ __.2_.ll_ __l._6_L __.2_.M_ __!!,_8!_ ____L!l2_ ::r: 

trl 

Total---------- 99.51 99.84 100.64 100.63 100.37 100.39 100.63 100.95 100.75 ~ 

0 

c:: 

Trace-element chemical analyses (ppm)2 z 

1-j 

:I: 

Ba ---------------- 350 220 320 91 140 340 200 230 250 0 

Co---------------- 19 19 13 38 18 9.0 21 27 18 0 

Cr ---------------- 65 19 34 240 150 18 150 39 43 0 

> Cu---------------- 51 52 23 98 28 36 22 53 170 

Ga---------------- 22 23 17 13 18 20 14 18 16 ~ 

La ---------------- <10 <10 <20 <20 <20 <20 <20 <20 <20 ~ 

Mn --------------- 690 730 650 1000 630 630 980 920 770 0 

Ni ---------------- 22 14 22 81 60 13 68 34 31 ~ Pb ---------------- <10 <10 11 18 12 11 14 14 <10 0 

0 Sc ---------------- 15 15 11 42 21 <10 20 26 18 z 

S r ---------------- 410 520 390 250 370 430 290 620 320 

v ----------------- 120 120 69 220 170 38 150 190 120 

y ----------------- 23 24 14 28 21 10 22 24 21 

Zn ---------------- 74* 58* 57 96 80 72 63 96 75 

Zr ---------------- 144* 140* 190 140 250 200 260 200 240 

K-Ar date (Ma)3- 10.52±0.12 11.15±0.12 8.18:!::0.06 6.09±0.29 6.31±0.12 

10.54±0.11 11.32±0.12 6.25±0.39 6.11±0.22 

I Major-element chemical analyses are by X-ray spectroscopy. Analyses were made at various times and by various analysts: J.S. Wahlberg, J. Taggart, J. Baker, 

L. Espos, J. Kent, and M. Dyslin. 2Trace-element chemical ana~ses are by emission spectroscopy; values denoted by"*" are Kevex 0700 determinations. Analyst: T. Fries. 

3From Keith and others ( 198 ). 
Table 2. Continued. 

Still Creek 

Late Miocene lava flows and intrusive plugs Laurel Hill intrusion intrusion 

Anal~sis number 10 11 12 13 14 15 16 17 18 

Sample number 80 CF-0022A 80 ZM-1013 80ZM-1023A 80 ZM-1030 82 MH-1 79 LH-1058A 80 LH-1058 80 LH-1035 78 SC-5 

Major-element chemical anal~ses {wt 12ercent oxides}I 

Si02 ------------- 58.0 59.6 58.6 55.6 55.9 55.7 63.4 60.8 58.8 

Al203 ------------ 17.0 17.4 17.0 16.2 19.1 17.9 16.9 16.4 15.7 

Ti02 ------------- 0.93 0.76 0.95 0.93 1.12 1.09 0.66 0.89 0.86 

Fe203------------ 6.74 6.26 6.59 6.57 7.54 7.49 3.42 6.15 7.02 

MnO ------------- 0.08 0.10 0.13 0.10 0.11 0.15 0.04 0.09 0.04 

MgO ------------- 4.55 3.76 4.10 4.26 3.3 4.59 2.51 3.20 4.02 

CaO -------------- 6.76 6.53 6.70 5.64 7.06 7.74 4.45 5.69 6.17 

N a20 ------------ 3.64 3.77 3.31 3.22 4.2 3.83 5.34 4.35 3.42 

K20-------------- 1.14 1.13 1.30 1.97 0.50 0.65 0.93 1.60 1.64 a 

P205 ------------- 0.17 0.15 0.17 0.17 0.24 0.21 0.014 0.17 0.15 tT1 

0 

Zr02 ------------- 0.03 0.02 0.02 0.02 0.01 0.01 0.02 0.04 0.03 r 

Cr02 ------------- 0.02 0.01 0.01 0.02 0.00 0.01 0.01 0.01 0.04 0 

a NiO -------------- 0.007 0.005 0.008 0.007 0.002 0.006 0.005 0.005 0.016 (=) BaO -------------- 0.02 0.03 0.02 0.05 0.02 0.021 0.03 0.03 0.033 Cl".l 

SrO -------------- 0.05 0.05 0.05 0.05 0.07 0.057 0.06 0.04 0.049 

LOI (900°C) ---- __l.lL .....iLQ_ ___lAi_ ~ __QJ!5_ _ll22_ __b.H_ __Q:..6.l_ _ML ...., 

~ 

z 

Total---------- 100.85 100.33 100.41 100.14 99.22 100.44 99.93 100.08 98.00 a 

Trace-element chemical anal~ses {QQm}2 

Ba ---------------- 210 250 210 430 190 170 270 310 310 

Co---------------- 19 19 22 16 21 19 10 16 17 

Cr ---------------- 130 75 94 79 36 72 31 78 84 

Cu ---------------- 69 67 21 34 51 34 52 75 89 

Ga ---------------- 16 15 14 14 22 17 15 16 16 

La ---------------- <20 <20 <20 <20 <10 <20 <20 <20 <20 

Mn --------------- 650 780 920 670 820 970 370 690 420 

Ni ---------------- 60 37 52 43 18 49 20 37 44 

Pb ---------------- <10 17 15 15 <10 <10 17 16 <10 

Sc ---------------- 19 18 20 17 15 20 <10 16 19 

S r ---------------- 330 330 310 310 670 390 360 320 340 

v ----------------- 130 130 150 120 120 150 60 120 120 

y ----------------- 18 20 22 21 21 15 17 23 21 

Zn ---------------- 74 77 63 66 82* 56 60 <50 <50 

Zr ---------------- 220 190 220 220 126* 120 160 250 260 

K-Ar date (Ma)3- 7.65±0.10 5.44±019 8.60±0.14 

8.66±0.09 8.75±0.18 

9.63±0.07 

9.64±0.11 \0 
Table 2. Continued. -

0 

Still Creek Late Miocene to Pliocene lava flows Quaternary 

intrusion early Pliocene dike lava flow 

Anal~sis number 19 20 21 22 23 24 25 26 27 

Sample number 80 SC-1032 79 IC-2039A 79 MH-2039A 79 TF-0004 79-SWC-00 1 OA 79 ZM-0086 79 RH-1032 79 ZZL-1085 79 SGV-1085 

Major-element chemical analyses (wt percent oxides) 1 :I: 

-< 

Si02 ------------- 61.4 59.2 58.5 58.1 61.7 54.2 54.0 51.4 62.1 0 

~ 

Al203 ------------ 15.3 16.6 16.7 17.6 17.6 17.5 18.4 17.9 17.9 0 

>-1 Ti02 ------------- 1.04 1.15 1.15 1.17 0.92 1.28 1.43 1.68 0.83 :I: 

Fe203------------ 7.99 6.66 7.04 7.19 5.66 9.02 7.99 9.88 5.50 tr1 

~ MnO ------------- 0.06 0.10 0.11 0.11 0.08 0.13 0.12 0.15 0.08 s:: MgO ------------- 3.26 3.06 3.42 3.53 2.24 5.22 5.22 6.39 2.36 > 

CaO -------------- 5.42 5.98 5.81 6.47 5.57 7.63 8.01 8.81 5.48 t""" 

Na20 ------------ 3.40 3.30 3.37 3.89 4.20 3.76 4.03 3.46 4.22 > 

P205 

K 20--------------

------------- 0.20 

1.92 

0.22 

2.05 

0.24 

2.18 

0.25 

1.49 1.76 

0.20 0.30 

1.36 

0.29 

1.06 

0.26 

0.83 

0.17 

1.30 ~ Zr02 ------------- 0.04 0.03 0.03 0.03 0.02 0.02 0.02 0.02 0.02 ~ Cr02 ------------- 0.01 0.01 0.01 0.01 <0.005 0.02 0.02 0.02 <0.05 -0 NiO -------------- 0.005 0.004 0.005 0.004 0.002 0.01 0.007 0.008 0.003 z 

BaO 

SrO 

--------------

--------------

0.04 

0.04 0.07 

0.043 

0.07 

0.04 

0.078 

0.042 

0.081 

0.04 

0.10 

0.05 0.032 0.023 0.035 - 0.087 0.066 0.078 z 

>-1 LOI (900°C) ---- _L.QQ_ ___l_._ti_ ______L2.2_ __Q..1Q_ _illL __Q,1Q_ <0.01 <0.01 0.24 :I: 

tTl 

Total---------- 101.13 100.23 100.67 100.32 100.39 100.86 100.73 100.92 100.37 s:: 

0 

c::: 

Trace-element chemical analyses (ppm)2 z 

>-1 

:I: 

0 

Ba ---------------- 360 410 360 320 360 370 240 160 320 0 

Co---------------- 15 16 17 19 13 34 24 38 13 0 

Cr ---------------- 73 37 36 41 14 110 92 130 17 > 

Cu ---------------- 30 46 42 52 26 51 77 65 23 ~ Ga---------------- 15 16 16 15 16 16 20 16 20 ? 

La---------------- <20 <20 <20 <20 <20 25 <20 <20 <20 0 

Mn --------------- 530 760 750 750 670 940 850 960 650 ~ 

tTl 

Ni ---------------- 35 31 33 35 11 92 61 74 17 Cl 

Pb ---------------- <10 92 12 19 12 13 16 17 16 0 

Sc ---------------- 18 16 16 16 12 22 20 28 <10 z 

S r ---------------- 290 570 470 490 580 700 600 410 560 

v ----------------- 140 110 110 110 92 170 150 200 72 

y ----------------- 30 23 24 21 19 24 17 22 14 

Zn ---------------- <50 64 84 76 61 85 92 100 57 

Zr ---------------- 350 260 290 250 240 260 190 190 190 

K-Ar date (Ma)3- 5.26±0.44 1.68±0.06 2.7±0.03 2.24±0.10 1.09±0.06 2.40 ±0.05 0.40 ±0.05 

1.88±0.05 2.56±0.11 1.14±0.06 2.56 ±0.07 0.43 ±0.04 

1.48±0.05 3.28 ±0.21 0.52 ±0.04 
Table 2. Continued. 

Quaternary lava flows 

Anal~sis number 28 29 30 31 32 33 34 35 36 

Sample number 79 SGV-1111A 79 MH-1111A 80 BM-1002B 79 ZM-0078 79 LP-1109 79 ZZ-1048 80 LP-1033 80 LP-1034 MGCR-600 

Major-element chemical anal~ses (wt Qercent oxides}I 

Si02 ------------- 60.6 60.4 49.2 55.5 52.3 60.4 61.3 63.9 57.2 

AI203 ------------ 18.0 18.1 16.3 18.0 16.6 17.2 17.8 17.2 17.8 

Ti02 ------------- 0.80 0.80 1.78 1.19 1.35 0.94 0.80 0.63 1.05 

Fe203------------ 5.51 5.55 10.5 8.25 9.65 6.22 5.78 4.73 6.81 

MoO ------------- 0.10 0.12 0.15 0.12 0.15 0.09 0.09 0.07 0.11 

MgO ------------- 2.69 2.71 7.42 5.38 7.94 3.03 2.54 2.22 2.09 

CaO -------------- 6.13 6.12 8.24 7.33 8.94 6.17 5.45 5.17 6.28 

Na20 ------------ 3.86 3.81 3.03 3.83 3.28 4.09 4.22 4.02 3.58 

K 20-------------- 1.26 1.26 0.96 0.78 0.85 1.68 1.15 1.33 1.20 0 P20s ------------- 0.13 0.12 0.38 0.25 0.24 0.28 0.23 0.16 0.24 trJ 

Zr02 ------------- 0.01 0.01 0.02 0.02 0.02 0.01 0.02 0.02 0.02 0 

Cr02 ------------- <0.005 <0.005 0.03 0.02 0.04 0.01 0.01 <0.005 <0.005 t""' 

0 

NiO -------------- 0.003 0.003 0.012 0.01 0.016 0.004 0.004 0.003 0.002 0 

BaO -------------- 0.02 0.02 0.03 0.03 0.033 0.05 0.03 0.03 0.02 n 

SrO -------------- 0.10 0.10 0.06 0.074 0.08 0.12 0.07 0.08 0.12 en 

LOI (900°C) ---- ___LjJL_ ___l,_lL ____2A1_ ___Q_,_Ql_ ~ ___ll15_ _UL ___lll6_ ___1._80_ trJ 

~ 

:::3 

Total----------- 100.32 100.27 100.58 100.79 102.07 100.64 100.80 100.33 100.33 z 

0 

Trace-element chemical anal~ses (QQm)2 

Ba ---------------- 180 160 210 240 230 460 260 280 140 

Co---------------- 15 14 38 26 32 19 13 9.7 14 

Cr ---------------- 17 <10 190 130 290 41 27 23 <10 

Cu ---------------- 41 32 44 49 56 35 26 26 55 

Ga---------------- 18 19 14 18 15 18 20 18 20 

La---------------- <20 <20 <20 <20 <20 30 <20 <20 <20 

Mn --------------- 770 840 1000 910 950 730 680 550 870 

Ni ---------------- 20 19 120 79 150 33 19 18 16 

Pb ---------------- 20 20 15 14 19 15 10 14 <10 

Sc ---------------- 12 11 26 19 28 15 11 <10 11 

S r ---------------- 660 620 370 510 540 750 500 480 820 

v ----------------- 100 89 170 130 200 120 71 53 110 

y ----------------- 13 13 24 19 18 18 13 11 21 

Zn ---------------- 52 62 100 80 95 84 79 65 83 

Zr ---------------- 150 160 250 170 180 260 180 180 230 

K-Ardate (Ma)3- l.l4::t0.05 0.63::t0.06 l.30::t0.13 0.45::t0.06 

l.25::t0.05 0.79::t0.06 l.48::t0.15 0.52::t0.05 

l.30::t0.03 

l.35::t0.05 --
Table 2. Continued. -

N 

Quaternary lava flows 

Anal_ysis number 37 38 39 40 41 

Sample number 81 MH-10 81 MH-28 81 MH-29 82 MH-2 82 MH-12 

Maior-element chemical analyses (wt percent oxides) 1 ::c 

~ 

Si02 ------------- 57.6 59.5 60.8 59.4 56.5 t:' 

~ 

Al203 ------------ 18.4 18.3 17.3 18.8 18.5 0 

Ti02 ------------- 0.98 0.76 0.84 0.92 1.10 ~ 

Fe203------------ 6.86 5.76 5.54 5.72 7.51 ~ MnO 

MgO --------------------------

0.10 

3.4 

0.09 

2.9 

0.09 

2.5 

0.09 

2.5 

0.12 

3.5 ~ > CaO -------------- 6.72 6.68 5.92 6.48 6.84 t'"" 

N a20 ------------ 4.1 4.4 4.4 4.4 3.9 > 

P205 

K 20--------------

------------- 0.24 

0.96 

0.20 

0.96 

0.28 

1.62 

0.20 

1.02 

0.22 

0.90 ~ 

~ Zt{)2 ------------- 0.02 0.02 0.02 0.01 0.01 

Ct{)2 ------------- 0.01 Tr Tr Tr Tr 0 NiO -------------- Tr Tr Tr Tr Tr z 

BaO -------------- 0.04 0.03 0.05 0.03 0.03 z St{) -------------- 0.06 0.07 0.08 0,10 0.10 ~ LOI (900°C) ---- ~ __Qjill_ 0.00 ~ _!1ll_ ~ Total---------- 99.74 99.67 99.44 99.92 99.48 ~ 

0 

Trace-element chemical analyses (ppm)2 c:: 

z 

~ 

Ba ---------------- 430 350 530 270 200 ::c 

0 Co---------------- 20 17 17 15 23 0 

Cr ---------------- 47 51 30 23 37 0 

Cu---------------- 67 33 38 23 26 > 

Ga---------------- 22 23 22 22 22 ~ La---------------- <10 <10 16 <10 <10 ? 

Mn --------------- 880 780 730 780 1200 0 

Ni ---------------- 21 17 16 15 18 ~ Pb ---------------- 11 <10 <10 <10 <10 0 

Sc ---------------- 16 12 11 13 19 0 

S r ---------------- 520 700 880 1100 650 z 

v ----------------- 130 78 84 100 120 

y ----------------- 26 20 22 17 25 

Zn ---------------- 74* 64* 58* 56* 52* 

Zr ---------------- 172* 154* 192* 146* 146* 

K-Ar date (Ma)3- 0.49±0.06 0.57±0.10 0.42±0.42 1.22±0.04 0.15±0.02 

0.65±0.05 0.29±0.08 0.36±0.36 1.02±0.04 

0.63±0.07 0.82±0.13 

0.57±0.05 
GEOLOGIC SETTING 13 

Zigzag Mountain, and Last Chance Mountain, respectively. Chemical analyses of these rocks (table 2, analyses 4 

through 14) indicate that the late Miocene lava flows are 

mostly andesites, except for one olivine basalt from Zigzag Mountain. 

According to Wise ( 1969), the late Miocene volcanic 

rocks must have erupted from several vents surrounding 

the present-day site of Mount Hood. Available K-Ar age 

data suggest that the late Miocene volcanism on the north 

and west sides of the mountain was probably intermittent 

and ranged from about 5.4 to 10.7 Main age. Wise (1969) 

proposed several volcanic centers of this age, on the south 

and east flanks of Mount Hood, and suggested that there 

may be additional sites of volcanism of about the same 

age. Wise (1969) mapped six small pyroxene andesite intrusive plugs to the north and south of Zigzag Mountain, 

and similar small intrusions south and southeast of Mount 

Hood. These intrusions may have been feeders for extensive post-Rhododendron, late Tertiary volcanism. One 

such intrusive plug located at the base of Enola Hill (fig. 

1) has associated zeolite (chabazite, laumontite, mordenite, 

and stilbite ), smectite, hematite, and chalcedony hydrothermal mineralization. 

Wise ( 1969) mapped several hornblende andesite and 

dacite domes and flows at Mill Creek Buttes (northeast of 

Mount Hood) and Lolo Pass (northwest of Mount Hood) 

as being of Pleistocene age. K-Ar dating suggests that at 

least some of these units are older than the Pleistocene 

(1.6 Ma to 0.01 Ma). Hornblende dacite from one of the 

Mill Creek Buttes was dated at 7.5 Ma (White, 1979), and 

Keith and others ( 1985) obtained an age of about 6.21 Ma 

for one of the hornblende dacite domes from Lolo Pass 

(table 2, analysis 6). 

Two of the geothermal drill holes (Last Chance Mountain--data from this report-and Pucci Chairlift-data 

from Robison and others, 1981; table 1, Nos. 4 and 13, 

respectively) penetrated the late Miocene pyroxene andesite rocks. Hydrothermal minerals from the Tertiary sections of these two drill holes include pyrite, clay minerals 

(chlorite and smectite), calcite, epidote, gypsum, silica 

minerals (quartz, chalcedony, and B-cristobalite ), and zeolite minerals ( epistilbite, heulandite, laumontite, mordenite, 

and stilbite). 

STILL CREEK AND LAUREL HILL INTRUSIONS 

The Rhododendron Formation and some late Miocene 

andesites on Zigzag Mountain are intruded by the porphyritic hornblende-quartz diorite to quartz monzonite Laurel 

Hill and Still Creek intrusions (Wise, 1969). The Laurel 

Hill intrusion was dated at 8.0 and 8.4 Ma by Bikerman 

(1970) and 8.8 and 8.6 Ma by Keith and others (1985). 

Wise ( 1969) obtained an age of 11.6 Ma for the Still 

Creek intrusion; Wise (1969) and Bikerman (1970) regarded this age as too old. White (1979) reported an age of 9.3 

Ma for a lava flow cut by the Still Creek intrusion and 

suggested that the age may record the time of the Still 

Creek's emplacement. Wise (1969) indicated that the Laurel Hill and Still Creek intrusions might be the exposed 

parts of a much larger coextensive pluton. Plutonic rocks, 

lithologically similar to those of the Laurel Hill and Still 

Creek drill holes (table 1, Nos. 9 and 10), were found to 

the east in the Zigzag River drill hole (table 1, No. 8) (Covert and Meyer, 1979) and another drill hole (Thunderbird 

Lodge drill hole at Government Camp) (Steele and others, 

1982) not included in this study. Also, several quartz microdiorite sills (possibly dikes) were penetrated in the 

OMF-1 (table 1, No. 5) and OMF-7A drill holes, and a 

sample from 3, 750 m depth in the OMF-7 A drill hole has 

been dated at 9.3 Ma (Priest and others, 1982). Thus, radiometric age data suggest that the Laurel Hill and Still 

Creek intrusions were emplaced between about 8.0 and 

9.3 Ma. 

Rocks from the Laurel Hill and Still Creek intrusions 

sampled for this study consist of medium-dark-gray, medium- to fine-grained rocks that range in composition from 

quartz diorite to quartz monzonite (table 2, analyses 15 

through 19). These rocks contain phenocrysts of plagioclase 

and hornblende in a groundmass of plagioclase, potassium 

feldspar, quartz, hornblende, and magnetite. Most drill-hole 

cuttings and outcrops sampled were slightly to extensively 

altered; secondary minerals include actinolite, chlorite, 

epidote, hematite, clay minerals (smectite, illite, and mixedlayer chlorite-smectite), zeolite minerals (chabazite, 

epistilbite, heulandite, laumontite, scolecite, stilbite, and 

wairakite), sulfide minerals (chalcopyrite and pyrite), magnetite, quartz, adularia, prehnite, chrysocolla, and calcite. 

OTHER INTRUSIVE ROCKS 

Intrusive activity continued after the Laurel Hill-Still 

Creek plutonism because the Laurel Hill mass is itself intruded by andesite and basalt dikes, one of which has an 

age of 5.0 Ma (Bikerman, 1970). On the southeastern side 

of Mount Hood, Goldstein and others (1982), from a magnetotelluric study, suggested the presence of an elongate 

zone of resistive (presumably intrusive) rock extending 

from Lookout Mountain to Barlow Butte (fig. 1). They 

pointed out that the resistive zone is correlative with a 

Bouguer gravity high located by Couch and Gemperle 

(1979). 

The present study located several quartz diorite dikes 

(up to about 10 m in width) that intrude the late Miocene 

pyroxene andesites within the resistive zone. Keith and 

others ( 1985) obtained an age of 5.26 Ma on a plagioclase 

separate from one of the freshest dikes (table 2, analysis 

20). Extensive alteration was observed at several outcrops 

between Barlow Butte and Lookout Mountain. Secondary 
14 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

minerals in this area include pyrite, clay minerals (chlorite, illite, smectite, and kaolinite), silica minerals (quartz 

and chalcedony), prehnite, epidote, adularia, calcite, zeolite minerals (ferrierite, laumontite, and mordenite ), and 

iron oxides (hematite and goethite). 

PLIOCENE ANDESITIC LAVA FLOWS 

Basaltic to dacitic lava flows of late Pliocene age cap 

several of the ridges and buttes that partly encircle Mount 

Hood; they probably were erupted from several different 

vents (Wise, 1969). Andesites on the east side of Mount 

Hood (Lookout Mountain and an unnamed mountain a 

few kilometers to the southwest) have ages of 3.0 and 4.1 

Ma, respectively (Wise, 1969). Wise (1969) provided major-oxide chemical analyses and petrographic descriptions 

of several other Pliocene lava flows to the east of Mount 

Hood and from Zigzag Mountain on the west side of 

Mount Hood. Keith and others (1985) obtained a K-Ar age 

of about 1.8 Ma (average of two analyses) for pyroxene 

andesite lavas on top of Bluegrass Ridge and an age of 2. 7 

Ma for a hornblende-pyroxene andesite lava flow located 

about 3.2 km east of Bluegrass Ridge (fig. 1). At Zigzag 

Mountain, Keith and others (1985) obtained ages of about 

2.7 Ma for an olivine basalt flow and about 2.4 Ma for a 

pyroxene andesite flow (table 2, analyses 23 and 24). Major-oxide and trace-element chemical analyses of these 

rocks are given in table 2 (analyses 22 through 26). Keith, 

Beeson, and Bargar (1982) indicated that the late Pliocene 

lavas are less faulted and altered than the late Miocene 

and early Pliocene lavas. 

QUATERNARY VOLCANIC ACTIVITY 

Approximately 90 percent of the nearly 188 km3 of andesitic to dacitic lava flows, pyroclastic flows, and mudflows that compose Mount Hood formed prior to the 

Fraser glaciation, are normally magnetized, and must have 

been deposited during the period 700 ka to 13 ka (Wise, 

1969; Williams and others, 1982). Petrographic and geochemical studies of these rocks conducted by Wise (1968, 

1969), White (1979, 1980), Wollenberg and others (1979), 

and this study (table 2, analyses 27 through 41) suggest 

that the rocks are mostly unaltered. In a study of Timberline Lodge 3 drill-hole cuttings, White ( 1979) reported 

finding only a little pyrite mineralization. 

Basalt to andesite lavas that have erupted from satellite 

vents to the north and west of Mount Hood (Sandy Glacier 

volcano, Cloud Cap, The Pinnacle, Red Hill, Vista Ridge, 

and a vent near Lolo Pass) are believed to be of Pleistocene age because of field relations or K-Ar dating (Wise, 

1969; Keith, Beeson, and Bargar, 1982; Keith and others, 

1985). The latest periods of volcanic activity at Mount 

Hood occurred about 15 to 12 ka, 1.8 to 1.4 ka, 0.6 to 0.4 

ka, and 0.25 to 0.18 ka (Crandell, 1980; Cameron and 

Pringle, 1986, 1987). Harris (1976, 1988) compiled several accounts of minor volcanic activity at Mount Hood that 

were reported in the mid-1800's. 

Detailed heat-flow studies of the Mount Hood area 

show that the geothermal gradient increases toward the 

apex of the mountain; at a distance of about 5 km from 

the volcano the gradient is about double the mean regional 

heat-flow values (Steele and others, 1982). At present, the 

only indications of thermal activity at Mount Hood are the 

Crater Rock fumaroles near the summit of the mountain 

and Swim Warm Springs near its southern base (fig. 3). 

According to Wollenberg and others ( 1979), chemical 

analyses of these mildly warm (-20°-26°C) springs and 

several cold springs on Mount Hood suggest mixing of 

thermal water and surface runoff water. Calculations of 

mixing models and chemical geothermometers indicate 

that geothermal water at depth beneath Mount Hood may 

be as hot as 240°C (Wollenberg and others, 1979). Extensive fumarolic activity near the summit of Mount Hood 

has been mapped by infrared imagery, and the pattern of 

hot ground is believed to have resulted from structural 

controls (Friedman and others, 1982). Chemical analyses 

of the largely C02 and H2S fumarolic gases were reported 

in Wollenberg and others (1979) and Nehring and others 

( 1981 ). Hot rocks bathed by the fumarolic gases are subject to acid-sulfate leaching and deposition of silica minerals (opal and chalcedony), clay minerals (kaolinite and 

smectite), elemental sulfur, sulfate minerals (anhydrite, 

gypsum, alunite, and natrojarosite), iron oxide minerals 

(hematite, "specularite," and amorphous phases), and goethite (Beeson and others, 1980). 

Several erosional and mass-wasting processes, such as 

stream erosion, recent landslides and debris or mud slides 

(Gallino and Pierson, 1984), and glacial transport of volcaniclastic materials (fig. 4), have operated and continue 

to operate at Mount Hood. Eight of the 13 geothermal drill 

holes in this study penetrate Quaternary lava flows with 

intervening volcaniclastic debris units. The lava flows are 

generally unaltered; however, several reworked detritus 

layers contain siliceous, orange- to red-stained and whitebleached grains (frequently rounded) that consist of many 

minerals (opal, chalcedony, alunite, kaolinite, smectite, 

and hematite) attributed to fumarolic alteration at the summit of Mount Hood by Beeson and others (1980). 

HYDROTHERMAL ALTERATION 

MINERALOGY 

Drilling records indicate that hydrothermal minerals are 

sparse or absent in many of the 30 drill holes completed in 

the vicinity of Mount Hood. Lithologic descriptions and 

distribution of hydrothermal minerals are reported here for 

13 of the drill holes (appendix A; table 1). Brief lithologic 
HYDROTHERMAL ALTERATION MINERALOGY 15 

descriptions of the drill cuttings previously were given for 

the seven USGS drill holes (Robison and others, 1981); 

the hydrothermal mineralogy of one of these drill holes 

(Pucci Chairlift) was studied by Gannett and Bargar 

(1981). Bargar (1980a, b) described the hydrothermal mineral phases present in the Lost Creek and Snow Bunny 

drill holes. Geophysical data, lithologic descriptions, and 

some hydrothermal mineralogical data for drill cuttings 

from the OMF-1 drill hole have been published by the Oregon Department of Geology and Mineral Industries 

( 1978) and Priest and Vogt ( 1982); additional data on the 

hydrothermal mineralogy of drill hole OMF-1 are given in 

this report. White (1979) described the lithology and hydrothermal mineralization of the Timberline 3 drill hole. A 

comprehensive study of the OMF-7 A drill hole was made 

by . Priest and others (1982) and Holdaway and Bussey 

(1982). 

SILICA MINERALS 

Hydrothermal silica phases, identified in drill cuttings 

and rock samples collected near Mount Hood, include 

opal, B-cristobalite, chalcedony, and quartz. Using a binocular microscope, quartz can be readily distinguished from 

the other silica phases because of its distinctive habit of 

forming colorless crystals. The other three phases, in general, can only be distinguished with a petrographic microscope or by X-ray diffraction methods. In our study, opal 

was identified by a very broad X-ray reflection at about 

4.04 A, and B-cristobalite was identified by a broad reflection between 4.07 and 4.11 A and a single minor reflection 

near 2.50 A; the X-ray diffraction pattern for chalcedony 

(cryptocrystalline quartz) is the same as that of quartz. 

Two other silica phases that are present in the samples collected for this study are a-cristobalite and tridymite: acristobalite (sharp major X-ray reflection between 4.04 

and 4.07 A along with several minor peaks) is a product 

of devitrification of volcanic glass, and tridymite (major 

X-ray reflections at 3.82, 4.11, and 4.32 A, plus many minor reflections) forms by vapor-phase crystallization during initial cooling stages of the lava flows. 

OPAL 

Several exotic rock fragments, collected just south of 

Cloud Cap on the east side of Mount Hood and near Sandy 

Figure 4. Lateral moraine deposits below Coe Glacier on the north side of Mount Hood. 
16 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Glacier volcano on the west side of the mountain (fig. I), 

contain coatings or open-space fillings of opal that is colorless to white or is yellow to orange stained and sometimes 

has a colloform morphology. In addition to opal, the Sandy 

Glacier volcano samples contain associated specular hematite, kaolinite, alunite, gypsum, and anhydrite. X-ray diffraction analyses of a sample from the east side of Mount 

Hood show the presence of chalcedony plus reflections at 

3.15, 9.6, and 19.2 A, which might correspond to truscottite, a hydrated calcium silicate mineral. These rock samples are presumed to have formed and been altered close to 

the summit of Mount Hood by fumarolic processes (Beeson and others, I980) and subsequently tumbled down the 

side of the mountain to the collection sites. 

Drill cuttings from volcaniclastic debris intervals in six 

Mount Hood drill holes (table 1, Nos. 1, 2, 5, 7, 12, and 

13) contain bluish, white, cream, yellowish, or orangish 

monomineralic grains of opal, or grains composed of opal 

in association with alunite, natroalunite, hematite, kaolinite, smectite, gypsum, anhydrite, or pyrite. These minerals 

probably formed near the summit of Mount Hood because 

of near-vent fumarolic alteration and subsequently were 

transported downslope as colluvium, mudflow, landslide, 

fluvial, or pyroclastic deposits that now constitute the volcaniclastic debris intervals (Naboko, 1959; Crandell, 

1971). 

6-CRISTOBALITE 

B-cristobalite, probably similar in origin to opal, was 

identified in a few Quaternary volcaniclastic debris samples from drill cuttings in three (table 1, Nos. 2, 4, and 13) 

of the Mount Hood drill holes. Only a single grain of 

white B-cristobalite was found in drill cuttings from drill 

hole 13. In drill hole 2, orange-, red-, or green-colored 

drill-cuttings chips, consisting of B-cristobalite in association with minor hematite, smectite, calcite, or mordenite, 

apparently formed as vesicle fillings or cement coatings on 

volcaniclastic grains. White, colloform, B-cristobalite 

grains, sometimes associated with chalcedony and smectite, appear to fill vesicles in the volcaniclastic debris section of the Rhododendron Formation in drill hole 4; 

similar-appearing white B-cristobalite grains occur as hydrothermal fracture fillings in a late Miocene andesitic 

lava flow higher in this drill hole. 

CHALCEDONY 

Chalcedony was identified in 9 of the 13 Mount Hood 

drill holes studied (table I, Nos. 1, 3, 4, 5, 6, 7, 10, 11, and 

13). In the Quaternary volcaniclastic debris units, chalcedony mostly occurs as red- to orange-stained or whitebleached grains that frequently are found with one or more 

associated secondary minerals such as smectite, kaolinite, 

alunite, natroalunite, jarosite, hematite, B-cristobalite, opal, 

or pyrite. The mode of its occurrence and some of the minerals associated with it suggest that the chalcedony may 

have a similar origin as opal. Chalcedony commonly originates as the less stable amorphous silica (opal) phase and 

later is structurally converted to chalcedony. According to 

White and others (1988), the conversion process probably 

includes an intermediate B-cristobalite stage and takes 

place in a neutral to slightly alkaline environment. 

Cloudy, colorless, bluish, or white chalcedony, often 

with a colloform texture or banded structure, was identified in drill cuttings of Tertiary and Quaternary lava flows 

and Tertiary volcaniclastic units (Rhododendron Formation). Chalcedony from these drill-hole intervals appears 

to occur as open-space fracture or vesicle fillings and 

probably is of hydrothermal origin. Associated secondary 

minerals include smectite, hematite, calcite, pyrite, heulandite, stilbite, and laumontite. Milky white, microcrystalline quartz or chalcedony vein fillings occur in association 

with epidote, chlorite, mixed-layer illite-smectite, and an 

amphibole in two grains from widely separated intervals 

in the Quaternary volcaniclastic part of the Lost Creek 

drill hole (table 1, No. 7). The two grains appear to be 

from a Tertiary intrusive source, and the associated secondary minerals suggest a moderately high formation temperature (>200°C). Gray or white, colloform chalcedony 

deposits also fill fractures in surface outcrops of Tertiary 

lava flows and intrusive dikes near Mount Hood. 

QUARTZ 

Grains of colorless, massive quartz or euhedral quartz 

crystals were identified in drill cuttings from seven of the 

Mount Hood drill holes (table 1, Nos. 4, 5, 6, 8, 9, I 0, and 

13). A few quartz grains were found in Quaternary alluvial 

deposits, but they are much more plentiful, apparently as 

vesicle and fracture fillings, in drill cuttings of the older 

Tertiary intrusive rocks, andesite flows, Rhododendron 

Formation, and Columbia River Basalt Group. Hydrothermal minerals found at the same sample depths as quartz 

grains include pyrite, smectite, calcite, hematite, chalcedony, stilbite, epidote, and magnetite. Quartz grains in these 

drill holes usually are monomineralic, so the paragenetic 

relations between quartz and the above-mentioned hydrothermal alteration minerals were mostly not observed. 

Outcrops of Tertiary igneous rocks near Mount Hood also 

contain quartz crystals as vesicle fillings or fracture coatings and between breccia fragments. About half the secondary minerals listed in appendix B are found in 

association with quartz crystals in these outcrops. The 

quartz crystals frequently formed earlier than other secondary minerals, although some open-space fillings contain earlier minerals such as epidote and chlorite. 
HYDROTHERMAL ALTERATION MINERALOGY 17 

CARBONATE MINERALS 

DOLOMITE 

A dolomite-group mineral occurs as open-space fillings 

of colorless to white colloform clusters of rhombic crystals (fig. 5) or as white veins in six samples from the 67.0-

to 91.4-m-depth interval in the McGee Creek drill hole 

(table I, No. 3). Its classification as a dolomite-group mineral is suggested by the most intense d(l04) X-ray reflection, occurring at about 2.90 to 2.92 A. According to the 

Mineral Powder Diffraction File (MPDF) card 11-78, the 

d(104) reflection for dolomite is at 2.89 A, ankerite has a 

d(104) reflection at 2.90 A (MPDF card 33-282), and 

magnesium kutnohorite has a d(l04) reflection at 2.9I A 

(MPDF card 20-225). Semiquantitative chemical analysis 

of carbonate from 79.2 m depth in the McGee Creek drill 

hole, using an X-ray energy dispersive spectrometer (EDS) 

on a scanning electron microscope (SEM), indicated 

Ca>Mg>Mn; no Fe was observed (analysis of a nearby 

grain shows that Mn is the dominant cation with some Ca 

and a trace of Mg; the mineral probably is a manganoan 

calcite). Deer and others (1966) considered dplomite to 

have a Mg:Fe ratio of >4, whereas ankerite has a Mg:Fe 

ratio of <4. X-ray diffraction data suggests that the mineral may not be dolomite, and because no Fe was found in 

the EDS analysis, ankerite can be eliminated. From the 

above chemical and X-ray data, it appears that the mineral 

might be a magnesium kutnohorite (Tsusue, 1967); however, the index of refraction ( -1.68) is much too low. Hydrothermal minerals occurring in the same drill cuttings 

samples as this dolomite-group mineral include heulandite, 

smectite, hematite, and manganoan calcite. 

An obscure mining district known as the Cheeney Creek 

area, about 25 km southwest of Mount Hood, was described briefly by Callaghan and Buddington (1938, p. 81-

82). According to their report, the area was mined near the 

beginning of this century, but never yielded very much 

gold {<49 ounces); only a single adit, a minor excavation, 

and a nearby tailings pile were visited during this study. 

Two samples collected from the tailings pile have white 

carbonate vein material associated with quartz, sphalerite(?), and manganoan calcite. The white vein mineral has 

a major X-ray diffraction reflection near 2.90 A, and the 

index of refraction is -1.68. A semiquantitative chemical 

analysis by EDS on an SEM indicates that Ca>Mg>Fe, 

with a Mg:Fe ratio slightly greater than four; the mineral 

is tentatively identified as dolomite. Callaghan and Buddington (1938) reported finding dolomite in this area. 

Yellowish, massive to rhombic carbonate crystals, in association with calcite, cerussite, sphalerite, chalcopyrite, 

galena, pyrite, and illite, fill spaces between quartz crystals 

in a -0.3-m-thick quartz vein on the north side of Zigzag 

Mountain. The crystals have an index of refraction of 

-1.67 and a d(l04) X-ray peak near 2.89A. Semiquantitative chemical analysis by EDS on an SEM suggest that 

Ca>Mg>Fe>Mn, and the Mg:Fe ratio is less than four. 

Chemical data suggest that the mineral might be ankerite, 

but the index of refraction and X-ray data do not support 

this identification. 

Index of refraction measurements, X-ray diffraction 

analyses, and semiquantitative chemical data do not provide an unequivocal identification for any of these three 

dolomite-group minerals. Consequently, throughout this 

report the minerals are simply reported as dolomite. 

MANGANOAN CALCITE AND RHODOCHROSITE 

A single X-ray diffraction analysis of a bulk sample 

from I 03.6 m depth in the McGee Creek drill hole (table 

I, No. 3) contained a minor X-ray reflection at 2.84 A, 

suggesting the presence of rhodochrosite. Several other 

drill cuttings between depths of 67.I m and 118.9 min the 

same drill hole have X-ray reflections between 2.98 and 

3.01 A, corresponding to manganoan calcite (Krieger, 

Figure 5. Scanning electron micrograph showing botryoidal 

aggregates of rhombic dolomite crystals from 79.2 m depth in 

the McGee Creek drill hole. 
18 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

1930). One sample at 112.8 m depth has a double X-ray 

peak at 3.04 and 2.99 A, indicating that both calcite and 

manganoan calcite are present. Microscope observations of 

the drill cuttings from several samples near this depth suggest that the two carbonate phases may be present in other 

samples not examined in detail. 

Three samples of white vein material, collected from a 

tailings pile in the Cheeney Creek area (Callaghan and 

Buddington, 1938), have X-ray diffraction reflections at 

3.01 A, suggesting that the material consists of manganoan 

calcite (Krieger, 1930). Associated hydrothermal minerals 

include quartz, siderite, dolomite, and sphalerite(?). 

SIDERITE 

According to MPDF file card 29-696, siderite has a major X-ray reflection at 2.80 A. Four samples examined in 

this study have intense X-ray reflections between 2.79 and 

2.81 A and thus are probably siderite. A siliceous grain at 

45.7 m depth in the McGee Creek drill hole (table 1, No. 

3) consists of chalcedony, smectite, calcite, and siderite. 

Brown grains at depths of 338.3 m and 341.4 m in the 

Clear Fork drill hole (table 1, No. 6) are composed of siderite and quartz. One vein filling sample collected from 

the tailings pile in the Cheeney Creek area contains siderite, quartz, and manganoan calcite. 

CERUSSITE 

Cerussite was identified at two locations in the Mount 

Hood area. Several samples collected from a tailings dump 

in the Cheeney Creek area contain euhedral, colorless to 

white, acicular to columnar crystals (fig. 6) that were identified as cerussite by X-ray diffraction. Associated minerals include galena, anglesite, sphalerite, pyrite, limonite, 

calcite, and quartz. Similarly, white acicular cerussite crystals were identified in two samples from a quartz vein on 

the north side of Zigzag Mountain in association with calcite, dolomite, sphalerite, chalcopyrite, galena, pyrite, and 

illite. Cerussite commonly forms as an alteration product 

of galena, with anglesite as an intermediate stage, in oxide 

zones of lead-ore deposits (Park and MacDiarmid, 1964 ). 

CALCITE 

Calcite (major X-ray reflection near 3.03 A), a very 

common hydrothermal mineral in the Mount Hood area, 

was identified in drill cuttings of all except two of the 

geothermal drill holes; calcite is present in many of the 

outcrop samples collected near Mount Hood (appendix B). 

Many drill-cuttings samples contain subrounded to angular, colorless to white (rarely brown), massive to blocky, 

crystal fragments that appear to be vein or vesicle fillings 

in association with one or more other hydrothermal minerals (dolomite, manganoan calcite, pyrite, smectite, quartz, 

stilbite, chabazite, laumontite, chlorite, chalcedony, mixedlayer illite-smectite, siderite, mixed-layer chlorite-smectite, 

epidote, scolecite, and wairakite ). 

Outcrop samples of some rock units penetrated by the 

geothermal drill holes show details of crystal morphology 

and paragenesis not seen in the shallow geothermal-gradient drill-hole samples. Colorless or white (rarely brown) 

calcite crystal habits vary from massive to blocky or are 

bladed in open-space fillings of vesicles, microfractures, or 

between breccia fragments. Plagioclase phenocrysts in intensely altered areas have been almost completely converted to calcite. On the other hand, a few aggregates of 

quartz crystals appear to be pseudomorphs after bladed 

calcite. In addition to most of the hydrothermal minerals 

listed above as associated with calcite in samples of drill 

cuttings, calcite outcrop samples also contain illite, hematite, cerussite, adularia, sphalerite, and galena. 

ZEOLITE MINERALS 

WAIRAKITE 

Two vein fillings in an andesite flow of the Rhododendron Formation contain tiny subhedral wairakite crystals 

(fig. 7). Both samples were obtained just a few meters from 

the Laurel Hill intrusion (figs. 1 and 3), and extensive mineralization observed at the outcrop undoubtedly is related to 

Figure 6. Scanning electron micrograph showing an acicular 

cerussite crystal from a tailings pile in the Cheeney Creek Mining Area, southwest of Mount Hood. Striations may be due to 

polysynthetic twinning. 
HYDROTHERMAL ALTERATION MINERALOGY 19 

this quartz diorite intrusion. Other minerals identified in 

these two samples include epidote, smectite, chlorite, hematite, and chabazite. X-ray diffraction analysis of both vein 

deposits show a reflection near 13° 28 (Cu Ka) and double 

reflections at about 26.2° 28; both mineral samples appear 

to be wairakite, based on X-ray data reported by Coombs 

(1955), rather than analcime, which is the Na end member 

of the analcime-wairakite isomorphous series. Subsequent 

electron-microprobe analyses (table 3 ) confirmed the identification of the Ca end-member wairakite. Analyses for 

specimen 79CCQ-2001B in table 3 appear to be very near 

the stoichiometric formula for wairakite; however, analyses 

of specimen 79LH-0087F contain significant K20, which is 

present in much greater abundance than in other reported 

chemical analyses for wairakite (Gottardi and Galli, 1985). 

Experimental data of Zeng and Liou ( I982) suggest that 

wairakite forms at temperatures exceeding about 220°C. 

Drill cuttings in three of the Mount Hood drill holes 

(table I, Nos. 8, 9, and IO) contain small amounts of wairakite identified by X-ray diffraction. In all three drill holes, 

wairakite was found only in quartz diorite chips of the Laurel Hill or Still Creek intrusions, suggesting its probable 

origin as a hydrothermal alteration product of these intrusions. Other hydrothermal minerals in the wairakite-bearing 

drill cuttings include epidote, quartz, chabazite, laumontite, 

stilbite, calcite, chlorite, smectite, illite(?) and pyrite. 

CHABAZITE 

White to colorless, euhedral, pseudocubic, rhombohedral, chabazite crystals (fig. 8) were observed in drill cutFigure 7. Scanning electron micrograph showing subhedral 

wairakite crystals and later chlorite vein filling in specimen 

79CCQ-2001B in altered volcanic rocks of the Rhododendron 

Formation near the Laurel Hill intrusion. 

Table 3. Electron-microprobe analyses of wairakite from outcrops associated with the Laurel Hill intrusion. 

Sample number ----------- 79LH-0087F 79CCQ-200 I 8 

Analysis number---------- 4 7 

Major-element chemical analysis (wt percent oxides) 

Si02------------------------ 59.68 58.61 58.76 55.70 55.09 54.78 

AI203---------------------- 21.18 20.72 20.43 23.19 23.61 23.28 

Fe203---------------------- 0.00 0.03 0.00 0.04 0.00 0.04 

MgO------------------------ 0.51 0.80 0.88 0.05 0.04 0.03 

MnO------------------------ 0.00 0.01 0.00 0.01 0.00 0.00 

CaO------------------------- 9.51 8.77 8.54 12.66 12.99 13.13 

Na20----------------------- 0.16 0. 15 0. 14 0. 18 0.02 0.05 

K20------------------------- __1.,_11 _l..l3. __L.8Q .....QJll .....QJll .J1.QQ 

Total--------------------- 92 .38 90.62 90.55 91.84 91.76 91 .35 

Number of atoms on the basis of 96 oxygens 

Si--------------------------- 33.95 33.98 34.11 32. 16 31.68 31 .88 

AI--------------------------- 14.20 14.16 13.98 15 .78 16.00 15.97 

Fe--------------------------- 0.00 0.01 0.00 0.02 0.00 0.02 

Mg-------------------------- 0.43 0.69 0.76 0.04 0.03 0.03 

Mn-------------------------- 0.00 0.01 0.00 0.01 0.00 0.00 

Ca--------------------------- 5.80 5.45 5.31 7.83 8.00 8.21 

Na--------------------------- 0. 18 0.17 0. 16 0.20 0.03 0.06 

K---------------------------- 0.98 1.13 1.33 0.00 0.01 0.00 

Si +AI---------------------- 48.15 48 .13 48.08 47 .94 47 .68 47.85 Si I Al + Fe3+ ______________ 2.39 2.40 2.44 2.04 1.98 2.00 

Balance error 1------------- 4.42 4.25 2.45 -1.01 -0.61 -3.35 

1Deterrnined by the method of Passaglia ( 1970). 

tings from five of the Mount Hood drill holes (table I, 

Nos. 5, 7, 8, 9, and IO), as well as several outcrop samples 

obtained near Laurel Hill and Zigzag Mountain (fig. I). 

The chabazite occurs as fracture fillings in quartz diorites 

of the Laurel Hill intrusion and in altered volcaniclastic 

deposits and andesite flows of the Rhododendron Formation. Chabazite commonly was deposited later than chlorite, epidote, and quartz, and it probably formed at much 

lower temperatures than these minerals; in Icelandic geothermal areas, chabazite occurs at temperatures lower than 

1 00°C (Kristmannsd6ttir and Tomas son, I978). Laboratory 

studies have produced chabazite at atmospheric pressure 

and at temperatures below I 00°C (Tomita and others, 

1969). Other hydrothermal minerals identified in the chabazite-bearing samples include calcite, stilbite, stellerite(?), 

mixed-layer chlorite-smectite, laumontite, smectite, hematite, mixed-layer chlorite-vermiculite(?), epistilbite, prehnite(?), pyrite, wairakite, illite, heulandite, magnetite, 

scolecite, chalcopyrite, and chrysocolla (appendix B). 

The chemical composition of chabazite, especially chabazite of hydrothermal origin, is quite variable, and major 

cations can be Ca, K, Na, or Sr (Gottardi and Galli, 1985). 

Microprobe analyses of chabazite from near Mount Hood 

mostly are rich in Ca, have a significant K content, and 

generally are low in Na (table 4). However, chemical analyses for one specimen (KCR-760-I; table 4, analysis 1) 

from the Zigzag River drill hole (table I, No. 8) vary from 

near the schematic Ca-rich (very low Na and K) formula 
20 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

given by Gottardi and Galli ( 1985) to chabazite containing 

significant Na. Balance errors (Passaglia, 1970) of the 

chabazite analyses listed in table 4 are less than 7, and the 

analyses appear to be very good; however, if H20 content 

from a bulk analysis (table 5) is added to each electron 

microprobe analysis, the total weight percent of oxides 

sums to as much as 105 percent for individual samples. 

The same problem occurs in the chemical analyses of other zeolites in this report; we suspect that loosely bound 

H20 in the zeolite structure may be driven off by the electron beam, which results in chemical analyses that are 

slightly high for other elements. 

Laboratory studies (Barrer and Sammon, 1955; Mondale 

and others, I978; Semmens and Seyfarth, 1978) show that 

zeolite minerals can be effective in absorbing the same 

trace elements as are included in table 5. One quantitative 

trace-element analysis of a chabazite sample collected 

from an outcrop near the Laurel Hill intrusion (table 5) 

Figure 8. Scanning electron micrograph showing pseudocubic 

chabazite crystals coated by a thin layer of botryoidal blue-green 

chrysocolla that has been partially scraped off. Specimen 79LH0087H was collected from the contact zone between the 

Rhododendron Formation and the Laurel Hill intrusion. 

contains very large amounts of Sr and Pb, some Ba, Cu, 

Mn, Ti, V, and Zn, and minor Co and Ga. X-ray diffraction analysis of this sample also showed minor wairakite 

contamination; it is not known if the presence of any trace 

elements should be attributed to this contamination. 

EPISTILBITE 

Epistilbite was identified in drill cuttings from four of the 

Mount Hood drill holes (table 1, Nos. 7, 8, 9, and 13) by Xray diffraction; samples containing epistilbite also were collected in a quarry near the Laurel Hill intrusion. The small 

amount of epistilbite occurs as vesicle fillings and fracture 

coatings along with smectite, mixed-layer chlorite-vermiculite(?), wairakite, scolecite, laumontite, hematite, calcite, 

stilbite, quartz, mordenite, and epidote. Paragenetic relations generally were not observed; however, binocular microscope observations of one fracture surface showed that 

colorless to white epistilbite formed later than epidote. A 

sample from 64 m depth in the Zigzag River drill hole 

(table I, No. 8) contains stubby, prismatic epistilbite crystals that are partly coated by smectite (fig. 9). 

Several chemical analyses show epistilbite crystals from 

the Zigzag River drill hole (table 6) to be AI rich, Ca rich, 

and Si poor compared with published epistilbite analyses 

(Galli and Rinaldi, 1974; Gottardi and Galli, 1985; Onuki 

and others, I988). According to hydrothermal synthesis 

studies by Lo ( 1981 ), the silica content of epistilbite is 

somewhat variable and depends upon the silica content of 

the parent material. 

FERRIERITE 

One sample of fault gouge from a late Miocene andesite 

flow, near Barlow Pass at the southern base of Mount 

Hood, contained colorless, acicular to lamellar crystals of 

ferrierite in association with smectite (fig. I 0). No electron 

microprobe analysis of the mineral was obtained, but SEM 

EDS semiquantitative chemical analysis shows the presence of significant Mg along with K, Ca, AI, and Si. Ferrierite is one of the few zeolite minerals that may contain an 

appreciable amount of magnesium (Gottardi and Galli, 

1985). 

HARMOTOME 

Harmotome was identified in one sample of an altered 

andesite flow of the Rhododendron Formation collected 

from a tailings pile in the deserted Cheeney Creek mining 

area southwest of Mount Hood. The colorless, blocky harmotome crystals (fig. 11) were deposited later than stellerite(?) on a fracture surface. Other hydrothermal minerals 

in this sample include chlorite, pyrite, and calcite (altered 
Table 4. Electron-microprobe analyses of chabazite from drill hole and outcrop samples associated with the Laurel Hill intrusion. 

Sample number---------- LH240-2 KCR- 79LH- 79LH- 79LH- 79CCQ760-1 0087C 0087F 0087H 0088P-4 

--

Analysis number--------- 1 4 6 1 3 4 1 2 3 1 3 6 1 1 4 5 

::I: 

Major-element chemical analyses (wt percent oxides) ~ 

0 

~ 

0 

s i 02------------------------ 53.73 54.35 54.32 55.26 57.32 52.52 55.04 52.78 54.89 54.50 55.06 53.74 55.16 53.47 53.40 53.58 ~ 

Al203--------------------- 20.89 19.40 21.07 19.57 17.84 22.13 19.75 20.59 20.66 20.63 20.45 20.48 20.52 20.24 18.91 20.19 ~ Fe203 ---------------------- 0.12 0.05 0.08 0.00 0.00 0.00 0.05 0.04 0.06 0.00 0.00 0.00 0.05 0.02 0.00 0.02 ~ 

MgO------------------------ s:: 0.20 0.28 0.25 0.00 0.00 0.00 1.20 0.95 0.89 0.52 0.29 0.48 0.76 0.07 0.05 0.07 

M n > 0------------------------ 0.01 0.02 0.00 0.03 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.07 0.01 0.02 0.01 0.01 r 

CaO------------------------- 9.16 9.08 9.28 9.05 8.86 11.63 7.11 7.61 7.92 9.07 9.54 8.89 8.33 9.45 9.48 9.65 > 

N a20----------------------- 0.78 0.75 0.53 1.42 0.65 0.07 0.19 0.16 0.17 0.18 0.17 0.25 0.22 0.79 0.49 0.60 

K20------------------------ ____1.2_3. ~ __lA6 __Q,_U __!1ilii .J1.1Q _1._U ____.3....21 ____M2 __b_5A ___L_2.4 ____.3..1.3. ~ _1...8..3. ~ ~ ~ 

Total-------------------- 87.12 85.75 86.99 85.46 84.73 86.65 86.47 86.10 87.06 87.44 86.75 87.14 87.13 86.89 84.59 86.67 ~ 5 

z 

Number of atoms on the basis of 24 oxygens s:: 

z 

tTl 

Si--------------------------- 8.22 8.41 8.26 8.49 8.80 8.04 8.43 8.21 8.34 8.29 8.37 8.26 8.37 8.25 8.41 8.27 ~ A 1--------------------------- 3.77 3.54 3.78 3.54 3.23 3.99 3.57 3.78 3.70 3.70 3.66 3.71 3.67 3.68 3.51 3.67 r 

Fe--------------------------- 0.01 0.01 0.01 0.00 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0 

Cl M g -------------------------- 0.05 0.07 0.06 0.00 0.00 0.00 0.27 0.22 0.20 0.12 0.07 0.11 0.17 0.02 0.01 0.02 ~ 

M n-------------------------- 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 

Ca--------------------------- 1.50 1.51 1.51 1.49 1.46 1.91 1.17 1.27 1.29 1.48 1.55 1.47 1.35 1.56 1.60 1.60 

N a--------------------------- 0.23 0.11 0.16 0.42 0.19 0.02 0.06 0.05 0.05 0.05 0.05 0.07 0.06 0.24 0.15 0.18 

K ---------------------------- 0.44 0.18 0.28 0.03 0.01 0.06 0.61 0.79 0.47 0.49 0.24 0.63 0.40 0.56 0.23 0.50 

Si + AI---------------------- 11.99 11.95 12.04 12.03 12.03 12.03 12.00 11.99 12.04 11.99 12.03 11.98 12.03 11.93 11.92 11.94 

Si I AI + Fe3+ ______________ 2.18 2.38 2.19 2.40 2.73 2.01 2.37 2.18 2.25 2.24 2.28 2.23 2.28 2.24 2.40 2.25 

Balance errorl _____________ 0.43 2.94 5.78 3.29 3.43 2.52 0.54 -0.74 5.55 -1.12 3.83 -4.26 4.41 -6.78 -2.58 -5.97 

-

1 Determined by method of Passaglia ( 1970). 

N 

-
Table 5. Water-content (in wt percent) and trace-element (in ppm) analyses of selected zeolite minerals from outcrops and drill holes near Mount Hood. 

[Trace-element analyses for samples 79KLN-0003B, 79CTC-0025, 79CF-0103B, and CF-750-5 are semiquantitative by direct-current arc spectroscopy (analyst: R. Lerner, other analyses are quantitative 

by inductively coupled plasma-acid digestion (analysts: P.H. Briggs, and J. Cornell). Sample CF-750-5 is from 228.6 m depth in the Clear Fork drill hole (table I, No.6); other samples were collected 

from outcrops near Mount Hood. Dashed(--) where not determined] 

Mineral Sample number Ba Cd Co Cr Cu Ga Mn Ni Pb Sr Ti v Zn H201 

---

Chabazite2 79LH-0087F 170 <2 2 <1 42 7 230 <2 1100 7300 80 21 24 17.60 

Heulandite 79CCQ-0088C-1 273 <2 -- -- 9 -- 22 -- <4 6500 -- -- <4 15.93 

Laumontite 79CCQ-0088B-1 6 <2 2 <1 4 40 14 8 <4 250 100 3 <4 13.81 

Do. 79KLN-0003B <40 <4 <2 <20 <2 <20 <400 2 <20 240 <400 <20 <100 13.64 

Do. 79CTC-0025 <40 <4 <2 <20 280 32 <400 <2 <20 200 <400 <20 <100 13.57 

Scolecite 79CCQ-0088P-4 15 <2 -- -- 19 -- 43 -- <4 585 -- -- 8 14.80 

Stilbite 79LH-0087C 44 <2 1 I 110 5 130 <2 400 210 60 70 180 18.76 

Do. 79CCQ-0088D-2 9 9 -- -- <1 -- 14 -- 9 120 -- -- <4 18.76 

Do. 79CF-0103B <40 <4 <2 <20 <2 <20 <400 <2 <20 74 <400 <20 <100 18.26 

Do. 79CCQ-2001 H-2 9 <2 2 6 27 9 38 6 1400 I 10 100 3 6 16.08 

Do. CF-750-5 <40 <4 <2 <20 <2 <20 <400 <2 <20 <30 <400 <20 <100 17.94 

I H20 determined by the Karl Fischer titration method (analyst: S.T. Pribble). 

2Sample contains minor wairakite contamination. 

N 

N 

::z:: 

~ 

0 

:;:t) 

0 

~ 

::z:: 

trl 

~ ;1> 

r 

;1> 

:;:t) 

~ 

~ 

0 

z 

z 

~ 

~ 

3:: 

0 

c 

z 

~ 

::z:: 

0 

0 

0 

;1> 

~ 

.?> 

0 

~ Cl 

0 

z 
HYDROTHERMAL ALTERATION MINERALOGY 23 

plagioclase). Electron-microprobe analyses of these harmotome crystals (table 7) are very high in barium; these 

analyses plot within or very close to the field of other published harmotome analyses (Cerny' and others, 1977; Robert, 1988) on an exchangeable-cation diagram. 

HEULANDITE 

Heulandite occurs in five of the Mount Hood drill holes 

(table 1, Nos. 3, 5, 8, I 0, and 13). Colorless, platy or tabular heulandite crystal fragments were found in only a single sample of late Tertiary quartz diorite drill cuttings 

from each of drill holes 5, 8, and 1 0; associated hydrothermal minerals are laumontite, mixed-layer chlorite-smectite, 

calcite, chlorite, stilbite, epidote, scolecite, magnetite(?), 

and smectite. Similar colorless, lamellar heulandite crystal 

fragments from the Pucci Chairlift drill hole (table 1, No. 

13) were identified in two samples of late Tertiary volcaniFigure 9. Scanning electron micrograph showing euhedral epistilbite crystals from 64.0 m depth in the Zigzag River drill hole 

(table 1, No. 8). Short epistilbite crystals are partially to completely coated by later smectite. 

Table 6. Electron-microprobe analyses of epistilbite from the Zigzag River drill hole (tablel, 

No. 8) near Mount Hood. 

Sample number------ KCR-210-4 

Analysis number----- 3 4 5 

Major-element chemical analyses (wt percent oxides) 

s i 02-------------------- 50.30 52.24 50.00 

A1203------------------ 20. 18 21.30 20.51 

Fe203 ------------------ 0.01 0.00 0.00 

MgO-------------------- 0.01 0.00 0.01 

M n 0-------------------- 0.00 0 .02 0.01 

CaO--------------------- 11.10 10.94 10.80 

N a20------------------- 0.38 0.16 0.28 

K 20-------------------- _M.Q _Q,_Q_8_ __!l...8A 

Total--------------- 82.44 85 .34 82.45 

Number of atoms on the basis of 48 oxygens 

s i -----------------------

AI-----------------------

Fe-----------------------

M g ----------------------

Mn----------------------

Ca-----------------------

N a----------------------- K ------------------------

S i + Al------------------ Si I AI + Fe3+ ________ _ 

Balance error 1---------

16.22 

7 .67 

0 .00 

0 .01 

0 .00 

3.83 

0.24 

0 . 19 

23.89 

2.12 

-5 .34 

16.23 

7.80 

0.00 

0 .00 

0 .00 

3.64 

0.09 

0.27 

24.04 

2.08 

1.93 

16.15 

7 .81 

0.00 

0.00 

0.00 

3.74 

0.18 

0.35 

23 .95 

2.07 

-2.47 

I Determined by method of Passaglia ( 1970). 

clastic debris along with stilbite, chalcedony, and smectite. 

The McGee Creek drill hole (table 1, No. 3) contains evidence of extensive heulandite deposition in nearly every 

drill-cuttings sample from both late Tertiary and Quaternary lava flows and volcaniclastic debris intervals between 

depths of about 50 and 590 m. Colorless heulandite from 

this drill hole has a blocky habit (fig. 12) in contrast with 

the tabular habit observed for other Mount Hood samples. 

The McGee Creek heulandite formed earlier than smectite 

and calcite in vesicles; other secondary minerals in this 

drill hole include chalcedony, rhodochrosite, manganese 

calcite, dolomite, pyrite, spinel(?), alunite, and hematite. 

Heulandite was identified, along with celadonite, mordenite, smectite, and B-cristobalite, in one sample of highly 

altered volcanogenic rocks of the Rhododendron Formation that crops out about I 0 km west-northwest of Mount 

Hood. Colorless, tabular heulandite crystals (max. 0.5 em 

across) occur in several samples of an altered Rhododendron lava flow that was intruded by the Laurel Hill intrusion. The heulandite formed as late-stage fracture fillings 
24 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

along with several other hydrothermal minerals (chrysocolla, chlorite, magnetite, smectite, epidote, laumontite, actinolite, hematite, quartz, chabazite, and stellerite(?)). 

Samples were identified as heulandite by X-ray diffraction analysis, which included heating the sample overnight 

at 450°C (Mumpton, 1960). In X-ray diffraction analyses 

of these heated samples, the (020) X-ray reflection at -9 A was destroyed, indicating the presence of heulandite rather 

than another member of the heulandite group of zeolites 

(Mumpton, 1960). Heulandite is further distinguished from 

other heulandite-group minerals by having Ca>Na+K (Mason and Sand, 1960). Several electron-microprobe analyses (table 8) of one heulandite sample from volcaniclastic 

rocks of the Rhododendron Formation near the Laurel Hill 

intrusion clearly indicate that Ca is the predominant exchangeable cation and that the mineral is heulandite. Adding the H20 content (table 5) of this heulandite sample to 

Figure 10. Scanning electron micrograph showing lamellar 

crystals and prismatic needles of ferrierite (mordenite group) 

from an outcrop near Barlow Pass on the south side of Mount 

Hood. 

the electron-microprobe analyses' total weight percent oxides (table 8) indicates that some H20 is released owing to 

heat generated by the electron beam. A quantitative traceelement chemical analyses of this heulandite sample 

shows the presence of Ba, Cu, Mn, and significant Sr (table 5). Abundant SrO (7.9 weight percent) and BaO (5.6 

weight percent) have been previously reported for heulandite by Lucchetti and others (1982) and Miller and Ghent 

(1973), respectively. 

Most heulandite found in late Tertiary volcanic rocks 

near Mount Hood consists of colorless, lamellar or tabular 

crystals similar to the typical morphology reported in Gottardi and Galli (1985). Conversely, ubiquitous heulandite 

open-space fillings in the McGee Creek drill hole have a 

colorless, blocky habit and occur in drill cuttings from both 

late Tertiary and Quaternary volcanogenic rocks. The 

blocky heulandite crystals found in volcanic rocks of both 

time intervals probably were deposited late in the geologic 

history of Mount Hood and may have precipitated from 

still-ongoing processes. The maximum temperature recordFigure 11. Scanning electron micrograph showing harmotome 

(phillipsite group) crystals from the abandoned Cheeney Creek 

Mining Area, southwest of Mount Hood. 
HYDROTHERMAL ALTERATION MINERALOGY 25 

Table 7. Electron-microprobe analyses of harmotome from the Cheeney Creek area, southwest of Mount Hood. 

Sample number------- 79BM-20461 

Analysis number------ 2 3 9 

Major-element chemical analyses (wt percent oxides) 

Si02-------------------- 48.69 51.24 50.03 

Al203 ------------------ 19.22 18.97 18.82 

Fe203----------------- 0.00 0.00 0.00 

B aO--------------------- 20.76 21.84 22.81 

CaO--------------------- 1.44 0.54 0.44 

N a20------------------- 0.70 0.74 0.63 

K 20-------------------- ~ _____l..02 1.30 

Total----------------- 92.09 94.38 94.03 

Number of atoms on the basis of 32 oxygens 

s i ------------------------ 10.93 11.19 11.10 

A 1------------------------ 5.09 4.88 4.92 

Fe------------------------ 0.00 0.00 0.00 

Ba------------------------ 1.83 1.87 1.98 

Ca------------------------ 0.35 0.13 0.10 

N a------------------------ 0.30 0.32 0.27 

K ------------------------- 0.37 0.29 0.37 

Si + AI------------------- 16.02 16.07 16.03 

Si I AI + Fe3+ ---------- 2.15 2.29 2.26 

Balance error! __________ 1.52 6.18 2.22 

1 Determined by method of Passaglia ( 1970). 

ed near the bottom of the McGee Creek drill hole was 

60°C (Robison and others, 1982). Heulandite from Icelandic geothermal areas has been reported by Kristmannsd6ttir and T6masson (1978) to exist at temperatures somewhat 

lower than 70°C. 

LAUMONTITE 

Laumontite occurs in numerous samples of drill cuttings 

from six Mount Hood drill holes (table 1, Nos. 5, 6, 7, 8, 

10, and 13). Drill-hole samples of lavas from the Columbia 

River Basalt Group, lava flows and volcaniclastic units of 

the Rhododendron Formation, early Tertiary andesites, and 

the Laurel Hill and Still Creek intrusions all contain extensive laumontite deposits. Laumontite is notably absent from 

late Tertiary andesites and Quaternary volcanogenic rocks 

near Mount Hood. In drill-hole cuttings samples, laumontite commonly is found as open-space (fracture, vug, or cavities between breccia fragments) fillings and consists of 

intergrown euhedral crystals (fig. 13). Other hydrothermal 

minerals identified in the laumontite-bearing drill cuttings 

include hematite, goethite, pyrite, smectite, illite, chlorite, 

quartz, chalcedony, calcite, chabazite, stilbite, heulandite, 

prehnite, mordenite, native copper, epistilbite, stellerite(?), 

scolecite, chalcopyrite, epidote, wairakite, and magnetite. 

Laumontite was found in several outcrop samples of the 

Rhododendron Formation that appear to be altered because 

of later emplacement of the Laurel Hill and Still Creek 

intrusions; laumontite also occurs in association with a 

late Tertiary hornblende andesite intrusive plug on the 

south side of Zigzag Mountain. These laumontite deposits 

line fractures and vugs, fill open spaces between breccia 

fragments, and replace(?) plagioclase phenocrysts. The 

outcrop samples also contain many of the associated hydrothermal minerals identified in the drill-hole samples. 

Observed paragenetic relationships suggest that chlorite, 

epidote, and quartz formed earlier than laumontite and calcite is a later deposit. 

Electron-microprobe analyses oflaumontite from two outcrop samples collected from altered rocks of the Rhododendron Formation near the Laurel Hill intrusion show that Ca 

is the dominant exchangeable cation and that the samples 

contain very little Na and only minor K (table 9). Holdaway 

and Bussey (1982) reported similar chemical compositions 

Figure 12. Scanning electron micrograph showing blocky heulandite crystals from 64.0 m depth in the McGee Creek drill 

hole. Some crystals appear to be partly coated by later smectite. 
26 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Table 8. Electron-microprobe analyses of heulandite from a fracture filling in a lava flow of 

the Rhododendron Formation altered by the Laurel Hill intrusion. 

Sample number------ 79CCQ-0088C-1 

Analysis number----- 2 4 7 

Major-element chemical analyses (wt percent oxides) 

Si02-------------------- 59.49 58.42 58.74 

AI203------------------ 17.76 17.83 17.57 

Fe203------------------ 0.04 0.01 0.02 

MgO-------------------- 0.04 0.03 0.03 

M nO-------------------- 0.00 0.00 0.02 

CaO--------------------- 8.91 8.74 8.71 

N a20------------------- 0.57 0.83 0.52 

K 2 0-------------------- __!!._U _ll.TI __.Q.l.8. 

Total----------------- 87.04 86.13 85.79 

Number of atoms on the basis of 72 oxygens 

s i -----------------------

A 1-----------------------

Fe-----------------------

M g ----------------------

M n ----------------------

Ca-----------------------

N a-----------------------

K ------------------------

Si + Al------------------

Si I AI+ Fe3+ _________ _ 

Balance errotl ________ _ 

26.65 

9.38 

0.01 

0.03 

0.00 

4.28 

0.49 

0.13 

36.03 

2.84 

1.71 

26.49 

9.53 

0.00 

0.02 

0.00 

4.25 

0.73 

0.15 

36.03 

2.78 

1.32 

26.67 

9.40 

0.01 

0.02 

0.01 

4.24 

0.45 

0.10 

36.07 

2.83 

3.63 

1 Determined by method of Passaglia ( 1970). 

for laumontite from drill hole OMF-7 A, on the northwest 

side of Mount Hood; however, their laumontite chemical 

analyses contain more Na20 than K20 and they also reported minor BaO in one analysis. One quantitative trace-element chemical analysis (table 5, 79CCQ-0088B-1) of 

laumontite from near the Laurel Hill intrusion contained 

measurable amounts of Sr, Ti, and Ga and traces of Ba, Co, 

Cu, Mn, Ni, and V. Also, some Cu, Ga, and Sr, along with 

minor Ni, was found by semiquantitative trace-element 

chemical analyses of two laumontite samples collected from 

Zigzag Mountain and from propylitically altered rocks associated with the Still Creek intrusion (Wise, 1969) southwest of Mount Hood. Analysis of H20 in one laumontite 

sample (table 5, 79CCQ-0088B-1) that was also analyzed 

by electron microprobe is ambiguous in that the analysis 

totals, with H20 added, range from -97 to -103 percent. 

MORDENITE 

White, fibrous mordenite was identified in drill cuttings 

from three of the Mount Hood drill holes (table 1, Nos. 2, 

6, and 13). In the Clear Branch drill hole, mordenite occurs in early Tertiary lava flows and volcaniclastic debris 

units between 201 and 210 m depth and from depths of 

250 to 293 m along with alunite, smectite, pyrite, opal, 

calcite, and B-cristobalite. Volcaniclastic drill-cuttings 

samples of the Rhododendron Formation from depths of 

253 and 268 m in the Clear Fork drill hole contain mordenite as well as smectite, hematite, calcite, stilbite, and 

laumontite. Early Tertiary lava flows and volcaniclastic intervals in the Pucci Chairlift drill hole have numerous 

mordenite-bearing samples between depths of 896 and 

1,167 m; other hydrothermal minerals in this drill hole include chalcedony, smectite, calcite, laumontite, stilbite, 

and epistilbite. 

One sample from a volcaniclastic outcrop of the 

Rhododendron Formation northwest of Mount Hood contains white, fibrous mordenite along with heulandite, 

smectite, and celadonite. Two outcrops of early Tertiary 

rocks southeast of Mount Hood have vesicle fillings and 

open spaces between breccia fragments that are filled by 

Figure 13. Scanning electron micrograph showing open-space 

filling of euhedral laumontite prismatic crystals from about 1,128 

m depth in the Pucci Chairlift drill hole. Distinctive terminal 

ends of the laumontite crystals permit easy identification of the 

mineral. 
HYDROTHERMAL ALTERATION MINERALOGY 27 

Table 9. Electron-microprobe analyses of laumontite from fractures in volcanic rocks of the Rhododendron Formation altered 

by the Laurel Hill intrusion. 

Sample number-------- 79LH-0087D 79CCQ-0088B 

Analysis number------ 2 2 5 6 

Major-element chemical analyses (wt percent oxides) 

Si02--------------------- 53.11 53.42 52.51 50.84 53.65 52.74 

A I203 ------------------- 21.56 22.49 21.06 21.16 22.28 22.04 

Fe203------------------- 0.00 0.05 0.01 0.00 0.03 0.06 

MgO--------------------- 0.04 0.02 0.05 0.03 0.03 0.03 

M nO--------------------- 0.02 0.02 0.02 0.00 0.00 0.01 

CaO---------------------- 11.39 11.69 11.25 10.78 11.86 11.34 

N a20-------------------- 0.05 0.07 0.05 0.07 0.06 0.06 

K20--------------------- _iLM _lLQ1 ____!U1 ____Q,2J. ___.OM ____Q2Q 

Total---------------- 86.81 88.43 85.68 83.39 88.35 86.84 

Number of atoms on the basis of 48 oxygens 

s i ------------------------ 16.23 16.05 16.27 16.15 16.11 16.11 

A 1------------------------ 7.77 7.97 7.69 7.93 7.89 7.94 

Fe------------------------ 0.00 0.01 0.00 0.00 0.01 0.01 

Mg----------------------- 0.02 0.01 0.02 0.01 0.01 0.01 

M n----------------------- 0.01 0.01 0.01 0.00 0.00 0.00 

Ca------------------------ 3.73 3.77 3.73 3.67 3.82 3.71 

Na------------------------ 0.03 0.04 0.03 0.04 0.03 0.03 

K ------------------------- 0.25 0.26 0.29 0.21 0.17 0.22 

Si + AI------------------- 24.00 24.02 23.96 24.08 24.00 24.05 Si I AI + Fe3+ ______ ___ __ 2.09 2.02 2.12 2.04 2.04 2.03 

Balance errorL ________ -0.28 1.57 -1.85 4.09 0.46 3. 13 

1 Determined by method of Passaglia ( 1970). 

mordenite associated with quartz, calcite, and chlorite. 

Tiny mordenite fibers are difficult to analyze using an 

electron microprobe and no quantitative chemical analyses 

were obtained; however, Ca, Na(?), AI, and Si were detected in a sample from the Pucci Chairlift drill hole by 

qualitative analysis using the EDS capability of an SEM. 

SCOLECITE 

Although scolecite is a common hydrothermal mineral 

(Gottardi and Galli, 1985), it does not appear to be very 

abundant in hydrothermally altered rocks from the Mount 

Hood area. Bundles of white, radiating, fibrous scolecite 

crystals occur in small amounts in quartz diorite drill cuttings from two Mount Hood drill holes (table 1, Nos. 8 

and 1 0). Vein deposits of white, acicular, scolecite crystals 

(fig. 14) also were found in four volcanic rock samples of 

the Rhododendron Formation from near the Laurel Hill intrusion. Other hydrothermal minerals identified in these 

samples include quartz, laumontite, stellerite(?), chlorite, 

epidote, pyrite, calcite, wairakite, smectite, chabazite, 

chalcopyrite, actinolite(?), and magnetite(?). The only 

minerals observed in association with scolecite by SEM 

are stellerite(?) and smectite. Presumably, scolecite formed 

late in the sequence of alteration minerals; scolecite is reported to occur in low-temperature geothermal areas of 

Iceland at temperatures below 1 00°C (Kristmannsd6ttir 

and T6masson, 1978). 

Several electron-microprobe analyses of scolecite from 

four samples near Mount Hood (table 10) show that the 

mineral is nearly homogeneous. The chemical composition 

of Mount , Hood scolecite is very near the stoichiometric 

formula for scolecite, consisting predominantly of Ca, Al, 

and Si along with minor Na and little or no K, Mg, Fe, or 

Mn. A quantitative trace-element analysis of one sample 

(table 5) also showed the presence of minute amounts of 

Ba, Cu, and Zn, as well as significant Sr. Adding H20 

content (table 5) to each electron-microprobe analyses' 

weight percent oxides results in totals near 100 percent, 

which indicates that H20 and, presumably Na and K as 

well, are held fairly tightly within the scolecite crystal 

structure. 

STILBITE AND STELLERITE(?) 

Stilbite is a fairly common zeolite mineral in the Mount 

Hood area; several outcrop samples (near the Laurel Hill 

intrusion) and drill-hole-cuttings samples (table 1, Nos. 4, 

5, 6, 7, 9, 10, and 13) contain soft, white fragments; colorless, broken crystals; or euhedral, tabular crystals of stilbite (fig. 15). Colorless or white grains and crystals of 

stellerite(?) (our identification is tentative) were found 

only in drill cuttings from several depths in the Zigzag 

River drill hole (table 1, No. 8) and in one sample from a 

Figure 14. Scanning electron micrograph showing euhedrai, 

acicular scolecite crystals from a fracture filling in a lava flow of 

the Rhododendron Formation that was intruded by the Laurel 

Hill intrusion. 
28 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Table 10. Electron-microprobe analyses of scolecite from outcrops and drill holes near Mount Hood. 

Sample number-------- 79CCQ-0088P-41 79-CCQ-200 l H -4 KCR-360-4 791C-2032 

Analysis number------- 6 10 2 3 2 4 5 2 

Major-element chemical analyses (wt percent oxides) 

s i Oz-----~- -------------- 45.96 45 .37 45.24 44.68 44.59 44.63 45.64 45.07 44.52 45.21 45 .89 

A }z03 -------------------- 25.06 25.36 25.64 24.49 25.14 25.37 25.89 24.95 24.48 24.97 24.59 

Fe203 -------------------- 0.00 0.02 0.02 0.03 0.06 0.00 0.01 0.00 0.00 0.04 0.01 

MgO---------------------- 0.01 0.04 0.03 0.03 0.01 0.01 0.01 0.03 0.01 0.01 0.00 

M nO---------------------- 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

CaO----------------------- 14.25 13.81 13.44 13.99 13.76 13.84 13.87 13.21 13.53 13.95 13.97 

N azO--------------------- 0.23 0.25 0.29 0.10 0.13 0.16 0.15 0.47 0.26 0.14 0.23 

K 20---------------------- _Q.Ql 0.00 0.02 0.00 0 00 _QJlQ 0.01 0.00 0.01 0.00 0.00 

Total------------------ 85.52 84.85 84.70 83.32 83.69 84.01 85.58 83 .73 82.81 84.32 84.69 

Number of atoms on the basis of 80 oxygens 

s i -------------------------- 24.23 24.09 24.04 24.18 24.01 23.95 24.01 24.21 24.22 24.16 24.40 

A 1-------------------------- 15.57 15.87 16.06 15.62 15.96 16.04 16.05 15.80 15.70 15.73 15.41 

Fe-------------------------- 0.00 0.01 0.00 0.01 0.03 0.00 0.00 0.00 0.00 0.01 0.01 

Mg------------------------- 0.01 0.03 0.02 0.02 0.01 0.00 0.00 0.02 0.00 0.00 0.00 

M n------------------------- 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

Ca-------------------------- 8.05 7.85 7.65 8.11 7.94 7.95 7.82 7.61 7.88 7.99 7.96 

N a-------------------------- ·0.24 0.25 0.30 0.11 0.14 0.16 0.16 0.49 0.27 0.15 0.23 

K --------------------------- 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 

Si + AI--------------------- 39.80 39.96 40.09 39.80 39.96 39.99 40.06 40.01 39.91 39.89 39.81 Si I A! + Fe3+ _____________ 1.56 1.52 1.50 1.55 1.50 1.49 1.50 1.53 1.54 1.53 1.58 

Balance error2 ____________ -4.75 -0.89 2.46 -4.58 -0.33 -0.21 1.52 0.31 -2.23 -2.41 -4.55 

1Quantitative trace-element chemical analysis by optical spectroscopy (table 5) shows the presence of 43 ppm Mn, 15 ppm 

Ba, 19 ppm Cu, 585 ppm Sr, and 8 ppm Zn. 

2Determined by method of Passaglia ( 1970). 

quarry containing volcanic rocks of the Rhododendron 

Formation intruded by the Laurel Hill intrusion. Stilbite 

and stellerite(?) minerals occur in vesicles and as vein or 

fracture fillings in late Miocene volcanic rocks. Other 

minerals identified in the same samples include calcite, 

siderite, chabazite, epistilbite, heulandite, laumontite, 

mordenite, scolecite, wairakite, opal, quartz, chalcedony, 

smecite, chlorite, illite, mixed-layer chlorite-smectite, celadonite, hematite, goethite, magnetite, pyrite, native copper, chrysocolla, palagonite, prehnite, actinolite, epidote, 

and adularia. 

Stellerite is chemically and structurally similar to stilbite 

and the two minerals are difficult to distinguish by routine 

methods. Some differences occur in the common crystal 

morphology of these minerals (Gottardi and Galli, 1985): 

Most euhedral, tabular stilbite crystals are twinned and 

have pyramidal terminations (fig. 15), whereas orthorhombic stellerite crystals are rectangular and have very prominent {001} terminations (fig. 16). Galli and Passaglia 

(1973) indicated that stellerite and stilbite could be identified optically by differences in their extinction angles. Lack 

of double X-ray diffraction reflections between 23° and 24° 

28 is characteristic of stellerite. Chemical differences have 

also been used to differentiate the two minerals because 

stellerite usually is higher in Si and lower in Na than stilbite (Galli and Passaglia, 1973; Passaglia and others, 1978). 

Unfortunately, while differences in crystal morphology are 

generally indicative of these minerals, this criteria is not 

100 percent reliable because many morphological variations can occur. Identification of stellerite and stilbite from 

their optical properties is difficult; stellerite has parallel extinction and extinction of stilbite crystals may be extremely 

close to parallel (Passaglia and Pongiluppi, 1974); this is 

true for samples from the Mount Hood area. Also, chemical 

differences between stilbite and stellerite are not unequivocally distinct because there is a complete compositional series between the two minerals (Passaglia and others, 1978). 

The only method of unambiguous identification of stilbite 

and stellerite is by a single-crystal X-ray diffraction study 

(R.C. Erd, written commun., 1992). 

Because single-crystal X-ray analysis would be impractical for the multitude of mineral identifications required 
HYDROTHERMAL ALTERATION MINERALOGY 29 

in this study, crystal morphologies, routine X-ray diffraction analyses, and mineral chemistries were relied upon 

for stilbite-stellerite mineral identifications. Most stilbitestellerite crystals from the Mount Hood area appear to 

have crystal morphologies and X-ray diffraction reflections 

characteristic of stilbite; these characteristics are compatible with stellerite in only a few studied samples. However, 

some electron-microprobe analyses of stilbite crystals 

show about the same low Na and high Si contents as stellerite analyses (tables 11 and 12). Passaglia and Pongiluppi (1974) showed that the Si/Si+Al+Fe ratio should be 

about 0.77 for stellerite and about 0.72 for stilbite; although, in a later study, Passaglia and others (1978) 

showed that the two minerals form a continuous series. 

The Mount Hood stilbite and stellerite(?) crystals appear 

to lie in the middle of this compositional series, with ( 1) 

Si/Si+Al+Fe ratios ranging from 0.73 to 0.77 (tables 11 

and 12), (2) pseudo-orthorhombic(?) crystallinity as suggested by nearly parallel extinction angles for stilbite, and 

(3) similar Si and Na contents. 

Figure 15. Scanning electron micrograph showing euhedral, 

tabular stilbite crystals in drill cuttings from -884 m depth in the 

Pucci Chairlift drill hole. Most crystals have a pyramidal termination; however, a few crystals are terminated by a (I 00) crystal 

face (see arrow). 

Trace-element analyses were obtained only for stilbite 

samples and are mostly unremarkable except for Pb which 

is high in two of the analyses (table 5). One of these stilbite specimens (79LH-0087C) also contains measurable 

Ba, Co, Cr, Ga, Sr, and Ti, as well as significant Cu, Mn, 

V, and Zn. The other specimen (79CCQ-2001H-2) contains some Ba, Co, Cr, Cu, Ga, Mn, Ni, Sr, Ti, V, and Zn, 

along with the highest Pb value obtained in any traceelement analysis of Mount Hood zeolite minerals. Addition of H20 content (from table 5) to the stilbite electronmicroprobe analysis for sample 79CCQ-0088D-2 (table 

11) results in a very reasonable chemical analysis, which 

is close to the stoichiometric formula for stilbite (Gottardi 

and Galli (1985). However, adding H20 content (table 5) 

to the higher silica electron-microprobe analyses for stilbite from specimen 79LH-0087C (table 11) results in totals that are greater than 100 percent. 

CLAY MINERALS 

KAOLINITE AND HALLOYSITE 

Five of the Mount Hood geothermal drill holes (table 1, 

Nos. 1, 3, 7, 12, and 13) contain kaolinite. Orange (iron 

oxide stained), cream, and white (bleached) subangular to 

Figure 16. Scanning electron micrograph showing euhedral, 

columnar stellerite(?) crystals from altered volcanic rocks of the 

Rhododendron Formation south of Mount Hood. 
Table 11. Electron-microprobe analyses of stilbite associated with the Laurel Hill intrusion. 

Sample number-------- LH-240-2 1 79LH-0087A 79LH-0087C 79LH-0087D 

Analysis number------- I 2 4 3 4 5 1 3 5 1 3 

Major-element chemical analyses (wt percent oxides) 

s i 02---------------------- 62.26 61.41 62.08 59.81 60.52 60.15 60.11 57.46 59.88 57.82 58.91 Al203 ________ ; ___________ 17.81 17.13 17.90 18.26 17.90 17.10 17.18 17.27 17.54 15.20 17.57 

Fe203-------------------- 0.00 0.06 0.00 0.20 0.03 0.06 0.00 0.00 0.00 0.00 0.00 

M gO---------------------- 0.06 . 0.04 . 0.04 0.54 0. 13 0.15 0.16 0.15 0.12 0.07 0.12 

M nO---------------------- 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.01 0.01 0.00 

CaO----------------------- 8.78 8.52 8.68 8.81 8.66 8.45 8.56 8.14 8.35 7.87 8.49 

N a20--------------------- 0.51 0.55 0.87 1.00 1.03 0.87 0.77 1.62 1.10 0.52 0.58 

K 20----------------:------ _j)j)] 0.08 0.06 0.08 0.07 0 05 0 24 ~ 0.26 _Q.TI 0.38 

Total------------------- 89.49 87.79 89.63 88.70 88.34 86.84 87.02 84.96 87.26 81.72 86.05 

Number of atoms on the basis of 72 oxygens 

s i ------------------------- 27.00 27.14 26.92 26.36 26.69 26:94 26.90 26.49 26.76 27.14 26.68 

A 1------------------------- 9.10 8.92 9.15 9.48 9.31 9.03 9.06 9.38 9.24 8.51 9.38 

Fe------------------------- 0.00 0.02 0.00 0.07 0.01 0.02 0.00 0.00 0.00 0.00 0.00 

Mg------------------------ 0.04 0.02 0.02 0.35 0.08 0.10 0.11 0.10 0.08 0.05 0.08 

Mn------------------------ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.00 

Ca------------------------- 4.08 4.03 4.03 4.16 4.10 4.05 4.10 4.02 4.00 4.00 4.12 

N a------------------------- 0.43 0.47 0.73 0.85 0.88 0.76 0.67 1.45 0.96 0.48 0.51 

K -------------------------- 0.04 0.04 0.03 0.02 0.04 0.03 0.14 0.17 0.15 0.14 0.22 

Si + AI---~---------------- 36.10 36.06 36.07 35.84 36.00 35.97 35.96 35.87 36.00 35.95 36.06 

Si I AI+ Fe3+ ____________ 2.97 3.03 2.94 2.76 2.87 2.98 2.97 2.82 2.90 3.23 2.85 

"Si I Si +AI + Fe3+ ______ 0.75 0.75 0.75 0.73 0.74 0.75 . 0.75 0.74 0.74 0.76 0.74 

Balance error2 ___________ 4.57 3.66 3.12 -3.46 0.54 -0.62 -1.76 -5.24 -0.24 -2.43 2.69 

I Sample from 73.2 m depth in the Laurel Hill drill hole (table 1, No. 9). 

2Determined by method of Passaglia ( 1970). 

79LH-0087H 

7 2 3 - 5 

62.77 61.52 61.83 ·59.57 

16.84 18.48 17.81 17.90 

0.00 0.00 0.02 0.04 

0.24 0.02 0.14 0.11 

0.00 0.00 0.02 0.04 

8.62 8.77 8.45 8.02 

0.41 0.98 ·0.62 1.05 

_Q,_J_Q 0.10 _lll.A ___Q_J2 

89.24 89.87 89.03 86.85 

27.30 26.04 26.96 26.69 

8.63 9.65 9.16 9.45 

0.00 0.00 0.01 0.02 

0.15 0.01 0.09 0.08 

0.00 0.00 0.26 0.02 

4.02 4.16 3.95 3.85 

0.35 0.84 0.53 0.91 

0.20 0.06 0.08 0.07 

35.93 35.69 36.12 36.14 

3.16 2.70 2.94 2.82 

0.76 0.73 0.75 0.74 

-2.86 4.36 -0.49 6.75 

79CCQ-0088D-2 

1 2 

55.91 56.24 

15.97 15.62 

0.00 0.02 

0.10 0.04 

0.00 0.00 

7.69 7.78 

1.28 1.48 

.o 08 _lll.A 

81.03 81.32 

26.89 26.99 

9.05 8.84 

0.00 0.01 

0.07 0.03 

0.00 0.00 

3.96 4.00 

1.19 1.38 

0.05 0.04 

35.94 35.85 

2.97 3.05 

0.75 0.75 

-2.65 -6.68 

w 

0 

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0 

,...;j 

::r: 

m 

~ )-

r 

)-

~ 

~ '-1 

a 

z 

z 

,...;j 

::r: 

m 

a: 

0 

e 

z 

,...;j 

::r: 

0 

0 

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0 

~ a 

0 

z 
HYDROTHERMAL ALTERATION MINERALOGY 31 

Table 12. Electron-microprobe analyses of stellerite(?) from the Zigzag River drill hole (table 1, 

No.8). 

Sample number1-------- KCR-440-2 KCR-460-4 KCR-760-1 

Analysis number-------- 2 4 5 2 3 2 3 

Major-element chemical analyses (wt percent oxides) 

Si02---------------------- 62.16 62.74 61.61 57.25 57.51 58.09 61.90 61.45 62.20 

Al203--------------------- 16.70 15.93 16.71 15.74 15.48 15.94 16.34 15.68 16.17 

Fe203 --------------------- 0.00 0.00 0.00 0.00 0.00 0.00 0.03 0.00 0.00 

MgO---------------------- 0.06 0.05 0.04 0.18 0.05 0.11 0.03 0.05 0.02 

MnO---------------------- 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

CaO----------------------- 8.43 8.55 8.38 7.69 7.87 7.87 8.53 8.12 8.36 

N a20---------------------- 0.40 0.17 0.41 0.40 0.21 0.60 0.15 0.19 0.13 

K20----------------------- ___!lll ~ ____Q_._H __ll.!lB __!!.M __ll.!lB __!!.12 ___!l.Q_8, ___!l.Q_8, 

Total------------------- 87.87 87.59 87.29 81.34 81.18 82.69 87.14 85.57 86.96 

Number of atoms on the basis of 72 oxygens 

Si -------------------------- 27.39 27.69 27.33 27.25 27.40 27.23 27.49 27.72 27.62 

AI-------------------------- 8.67 8.29 8.74 8.83 8.69 8.81 8.55 8.34 8.46 

Fe-------------------------- 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 

Mg------------------------- 0.04 0.03 0.03 0.13 0.04 0.08 0.02 0.03 0.02 

Mn------------------------- 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 

Ca-------------------------- 3.98 4.04 3.99 3.92 4.02 3.96 4.06 3.92 3.98 

N a-------------------------- 0.34 0.14 0.35 0.37 0.20 0.54 0.13 0.26 0.11 

K --------------------------- 0.07 0.08 0.08 0.05 0.04 0.05 0.09 0.04 0.04 

Si + AI--------------------- 36.06 35.98 36.07 36.08 36.09 36.04 36.04 36.05 36.08 Si I AI+ Fe3+ ______________ 3.16 3.34 3.13 3.09 3.15 3.09 3.21 3.32 3.27 

Si I Si +AI+ Fe3+ ________ 0.76 0.77 0.76 0.76 0.76 0.76 0.76 0.77 0.77 

Balance error2 _____________ 2.80 -1.06 3.34 3.78 4.23 1.80 2.20 2.67 3.93 

1 Samples are from depths of 134.1, 140.2, and 231.6 m, respectively. 

2Determined by method of Passaglia ( 1970). 

well-rounded grains within drill cuttings from volcaniclastic debris intervals in these drill holes have kaolinite in 

association with one or more other alteration minerals: 

opal, chalcedony, jarosite, natrojarosite, alunite, natroalunite, pyrite, smectite, hematite, or calcite. Erratic cobbles 

and boulders in glacial moraine at an elevation of 1,878 m 

on the northwestern side of Mount Hood contain kaolinite, 

specular hematite, opal, alunite, gypsum, anhydrite, and 

sulfur. Such occurrences of kaolinite and associated minerals appear to be due to acid-sulfate alteration near the 

summit of Mount Hood and were transported downslope 

by various mechanisms including glacial, lahar, and fluvial 

processes. Similar silica minerals, iron oxides, iron hydroxides, sulfate minerals, and sulfur are presently being 

formed by acidic fumarolic alteration in vent areas near 

the top of Mount Hood (Beeson and others, 1980). 

Local acid conditions appear to hav~ existed previously 

in other parts of the volcano. White kaolinite coats quartz 

crystals that line open spaces in orange stained, altered, 

volcanic breccia from an outcrop on the north bank of Iron 

Creek (southeast of Mount Hood). Also, broad 7 A X-ray 

peaks, suggesting halloysite, occur in analyses of (1) an 

orange clay sample coating a joint surface in a quarry south 

of Mount Hood and (2) drill cuttings containing hematite 

and chalcedony from 49 m depth in drill hole 11 (table 1). 

One green clay sample, associated with quartz and 

mixed-layer illite-smectite, from the Cheeney Creek mining area has no (001) or (003) X-ray diffraction reflections 

and has strong (002) and (004) reflections at 7.12 and 3.56 

A, respectively. Probably, this mineral belongs to the serpentine-kaolinite mineral group. 

CELADONITE 

Drill cuttings of the Columbia River Basalt Group from 

the Clear Fork drill hole (table 1, No. 6) contain bluegreen celadonite-bearing grains between depths of 363 and 

369 m in association with chalcedony, smectite, and chlorite. Blue-green celadonite also was identified along with 
32 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

smectite, mordenite, and heulandite at one outcrop of altered Rhododendron Formation rocks from northwest of 

Mount Hood. Celadonite and associated secondary miner-

~s similar to those listed above commonly occur in basalt 

vesicles and altered volcaniclastic rocks owing to diage- , 

netic alteration or low-temperature zeolitic alteration 

(Wise and Eugster, 1964 ). 

SMECTITE 

Smectite is the _most widespread secondary mineral in 

the vicinity of Mount Hood. All but one drill hole (table 1, 

No. 11) and most sampled outcrops contain at least minor 

amounts of smectite as groundmass alteration, fracture linings, or vug coatings. The exchangeable cation in the 

Mount Hood smectites is Ca because X-ray diffraction 

analyses of more than 300 smectite samples show basal 

(001) reflections between about 14 and 16 A (average near 

15 A) (Grim, 1968) that expand to between 16.5 and 18 A 

after being heate~ at 60°C for 1 hour in an atmosphere of 

ethylene glycol. Basal spacings of a few samples heated to 

400°C for 0.5 hour collapsed to 10 A. Many of the clays 

are poorly crystalline and have low, broad, asymmetrical 

basal peaks, but some smectite occurrences, generally associated with late Tertiary rocks, have high, sharp peaks 

characteristic of well-crystallized smectite. Origin of the 

smectite probably is variable, ranging from weathering to 

fumarolic alteration, hydrothermal alteration, or diagenetic 

processes. 

ILLITE 

White-, gray-, black-, or orange-stained illite occurs in 

several Mount Hood drill holes [table 1, Nos. 4(?), 5, 7, 

8(?), 10, 12 and 13(?)] and numerous outcrops surrounding the mountain. X-ray diffraction analyses show a low, 

broad to fairly sharp, basal (001) peak rahging from about 

9.5 to 10.4 A (average of 31 measure~ents is 10.0 A). 

The basal peak contracts slightly to about 9.8 A after heating for 1 hour at 60°C in an atmosphere of ethylene glycol. Secondary minerals associated with illite near Mount 

Hood include calcite, pyrite, quartz, chlorite, smectite, 

hematite, epidote, and chalcedony. In outcrop samples, illite fills vesicles, coats fractures, or is deposited in open 

spaces between breccia fragments of late Tertiary volcanic 

rocks (Columbia River Basalt Group, Rhododendron Formation, and late Tertiary andesite lava flows). Drill-cuttings samples from two drill holes (table 1, Nos. 8 and 10) 

cq_ntain a small amount of illite in association with microq~artz diorite of the Still Creek and Laurel Hill intrusions. 

Illite in the remaining drill holes is found in Quaternary 

clastic . debris units that .consist of pyroclastic, avalanche, 

mudflow, or fluvial deposits (Wise, 1969). Illite and other 

minerals that cannot have formed at near-surface conditions on the lower slopes of Mount Hood probably originated, because of high-temperature hydrothermal alteration 

or fumarolic processes within or close to the vent, near the 

summit of the mountain and were transported downslope 

to the drill site. Frank (1983) reported that high-temperature hydrothermal minerals such as illite have been transported considerable distances from their source near the 

top of Mount Baker, Washington. 

MIXED-LAYER ILLITE-SMECTITE 

Drill-cuttings samples from drill holes 4, 7, and 13 (table 1) and a few rock outcrop samples from the north side 

of Zigzag Mountain and the Cheeney Creek mining area 

southwest of Mount Hood contain mixed-layer illite-smectite. Some of the white, light green, or black, mixed-layer 

clay samples have a fairly sharp -10 to 12 A X-ray diffraction peak that splits into two peaks at -11 to 14 A and 

-9 to 10 A after exposure to ethylene glycol at 60°C for 1 

hour. A few other samples have a broad basal X-ray peak 

near -10 to 11 A that becomes more asymmetrical and 

much broader following glycolation; however, the only 

measurable peak is near -9.7 A. These mixed-layer clays 

are allevardite ordered with the illite content ranging from 

about 70 to 90 percent (Hower, 1981; Horton, 1985). One 

X-rayed sample from drill hole 4 (table 1) has a 12.8 A 

d(001) peak that splits into -17 and 9.5 A peaks following 

glycolation. Another sample from the same drill hole has a 

15.3 A basal peak that splits into -16.9 and -9.5 A peaks 

after glycolation. Both clay samples have random interstratification with about 50 to 60 percent illite (Hower, 

1981). 

The majority of mixed-layer illite-smectite samples 

from the Mount Hood area were obtained from fractures 

and altered andesite lavas and tuffs in the late Tertiary 

Rhododendron Formation. Associated secondary minerals 

include limonite, hematite, quartz, chalcedony, smectite, 

chlorite, calcite, pyrite, and galena. Mixed-layer illitesmectite also occurs in two Quaternary clastic debris samples from the Pucci Chairlift drill hole (table 1, No. 13). 

Mixed-layer clay in these clastic deposits must have originated at a higher elevation on Mount Hood and was later 

transported downslope. Frank (1983) identified mixedlayer illite-smectite and numerous other secondary minerals from clay-size fumarolic ejecta and recent tephras 

erupted from Sherman Crater near the summit of Mount 

Baker, Washington, in 1975. 

CHLORITE 

Chlorite was identified in several drill holes (table 1, Nos. 

4, 5, 6, 7, 8, 9, 10, and 13) and numerous outcrops surround-
HYDROTHERMAL ALTERATION MINERALOGY 33 

ing Mount Hood. Most chlorite deposits are restricted to 

late Tertiary rocks of the Rhododendron Formation, Laurel 

Hill and Still Creek intrusions, lower andesite flows (unit 

Tla in fig. 3), and vein fillings associated with these rock 

units. Only two samples of reworked Quaternary detrital 

deposits (from drill holes 7 and 13 in table 1) contain 

chlorite-bearing vein deposits. A few thin sections show 

that chlorite formed from alteration of mafic minerals; 

however, most occurrences of chlorite near Mount Hood 

are found in vein fillings and were deposited from circulating fluids. The chlorite deposits are associated with 

minerals such as epidote and chabazite, which are stable at 

quite different temperature conditions (230°C and <100°C, 

respectively) (Seki, 1972; Kristmannsd6ttir and T6masson, 

1978). Most chlorite at Mount Hood probably formed at 

temperatures above 200°C during cooling of the many intrusive rocks in the area. It should be noted, however, that 

chlorite has been found at temperatures near 100° C 

(Hulen and Nielson, 1986) and the possibility of lower 

temperature chlorite cannot be discounted. 

X-ray diffraction analyses show that the (002) and (004) 

X-ray reflections of Mount Hood chlorite are much stronger than its (001) and (003) reflections, which suggests 

that the mineral is Fe-rich (Grim, 1968). Holdaway and 

Bussey (1982) presented electron-microprobe analyses of 

chlorite from the Old Maid Flat 7 A drill hole. They suggest that minor Ca and other chemical characteristics of 

the mineral indicate the presence of randomly interlayered 

saponite-chlorite. No chemical analyses of chlorite were 

attempted for this report, but one sample, examined by 

SEM with EDS, contains some Ca, K, and Mg in addition 

to Fe, AI, and Si. 

MIXED-LAYER CHLORITE-SMECTITE 

Occasional rock samples containing randomly interstratified mixed-layer chlorite-smectite were found in association with Tertiary quartz diorite outcrops on the south and 

east sides of Mount Hood. One sample of cuttings from 

drill hole 13 (table 1) also contains mixed-layer clay minerals in a Tertiary(?) andesite lava flow. The majority of 

randomly interstratified mixed-layer chlorite-smectite samples noted in this study were found in the Rhododendron 

Formation, as well as some drill-cuttings samples of the 

Columbia River Basalt Group from drill holes 4, 5, 6, and 

7 (table 1) on the northwest flank of the mountain. Randomly interstratified mixed-layer chlorite-smectite in the 

Mount Hood samples is characterized by a weak (002) Xray reflection and a strong (00 1) reflection at -14.2 A that 

expands to -15 A (expansion range for Mount Hood samples is 14.4 to 15.8 A) following exposure to ethylene glycol fumes for 1 hour at 60°C. The large range of expanded 

(001) values probably is due to variation in the ratio of 

chlorite to smectite in individual samples (Hower, 1981, 

table 3.6). Holdaway and Bussey (1982) identified randomly interstratified mixed-layer chlorite-smectite 

throughout much of the nearby OMP-7 A drill hole and 

suggested that it is converted to chlorite with increasihg 

temperature and pressure. They found a small 31-A peak 

in three samples, indicating a more or1.lered interstratification of the mixed-layer clay. 

The (00 1) reflection at -14 A did not expand with glycolation of a few Mount Hood samples. These mixed-layer 

clays may consist of chlorite-vermiculite, although no 28-

A peak was observed. According to Reynolds ( 1980) this 

higher order X-ray reflection is only visible when chlorite 

and vermiculite are present in nearly equal amounts. 

SEPIOLITE(?) 

The Last Chance Mountain (table 1, No.4) drill cuttings 

contain caramel-colored clay grains between depths of 

30.5 and 137.2 m that may consist of sepiolite. X-ray diffraction analyses show a very sharp peak at 12.1 A that 

changed very little with glycolation (max expansion -12.4 

A); the peak contracts to 11.8 A and is somewhat reduced 

in intensity when the sample is heated to 400°C for 0.5 

hour. After heating to 550°C for 0.5 hour, there is additional contraction to 11.3 A and considerable reduction in 

intensity although the peak is not completely destroyed. 

Such behavior with heating and glycolation appears to be 

similar to that of sepiolite (Starkey and others, 1984). 

The caramel-colored clay particles were not observed 

adhering to the volcanic fragments that compose the drill 

cuttings and thus may have been introduced during drilling; however, no samples of drill mud used in completing 

this drill hole were obtained for comparative analysis. Sepiolite usually occurs as an alteration product of magnesium carbonates or silicates (Phillips and Griffen, 1981 ); 

however, these minerals do not occur in the Last Chance 

Mountain drill hole. Other secondary minerals identified in 

the same samples as the caramel-colored clay grains include B-cristobalite, chalcedony, quartz, epidote, stilbite, 

calcite, smectite, chlorite, mixed-layer illite-smectite, pyrite, hematite, and amorphous iron oxide. 

SULFATE MINERALS 

GYPSUM-ANHYDRITE 

Five Mount Hood drill holes (table 1, Nos. 1, 2, 4, 11, 

and 13) contain small amounts of soft, colorless or yellow, 

needlelike crystals of gypsum. One gypsum-bearing Quaternary andesite sample from drill hole 11 is associated 

with common constituents (calcite, brownmillerite and 

thaumasite) of cement (Taylor, 1964 ), suggesting that the 

gypsum was introduced in the drill hole during drilling. 
34 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Most gypsum occurs in Quaternary debris units and one 

Tertiary andesite flow where the mineral was deposited on 

(or associated with) pyrite crystals. This gypsum probably 

formed by oxidation of the pyrite and could have been deposited after recovery of the drill-hole samples because 

gypsum precipitates at fairly low temperatures ( <70°C) 

(Holland and Malinin, 1979). Other minerals associated 

with the gypsum include alunite, natroalunite, jarosite, and 

natrojarosite; these sulfate minerals probably formed by 

acid-sulfate fumarolic alteration and were later transported 

by various mechanisms and deposited as Quaternary debris units. 

X-ray diffraction analyses of fumarolic alteration material collected in moraine deposits on the west side of 

Mount Hood show the presence of sulfur, kaolinite, opal, 

alunite, gypsum, hematite, and anhydrite. Sulfur is a common fumarolic mineral precipitate. Kaolinite, opal, alunite, 

anhydrite, and gypsum probably formed during surficial 

oxidation of sulfur; they have been collected from the active fumarole area of Mount Baker, Washington (Babcock 

and Wilcox, 19-77; Frank, 1983). Keith and others (1981) 

listed sulfur, gypsum, alunite, anhydrite, and hematite 

among the fumarole minerals deposited at temperatures 

below 250°C at Mount St. Helens Washington. 

ALUNITE-NATROALUNITE AND 

JAROSITE-NATROJAROSITE 

One or more of the sulfate minerals alunite (K-rich), natroalunite (Na-rich), jarosite (K+Fe), or natrojarosite 

(Na+Fe) were identified from Quaternary debris deposits 

in Mount Hood drill holes 1, 2, 3, 7, 12, and 13 (table I) 

and from surface samples of reworked debris on the west 

side of the mountain. All of these minerals probably form 

under similar conditions. In fact, extensive solid solution 

exists between alunite and natroalunite, and jarosite and 

natrojarosite; isomorphous substitution can also occur between alunite and jarosite (Brophy and others, 1962). 

Drill-cuttings samples containing these sulfate minerals 

usually have a bleached appearance or are orange stained. 

Associated secondary minerals include kaolinite, smectite, 

opal, gypsum, pyrite, hematite, and sulfur. Probably all of 

the minerals formed from fumarolic alteration near the 

summit of Mount Hood and were carried downslope by 

mudflows, debris flows, avalanche, glacial reworking, and 

other mechanisms. 

ANGLESITE 

A sample of fault breccia from the Rhododendron Formation in the Cheeney Creek Mining Area southwest of 

Mount Hood contains anglesite along with cerussite, galena, sphalerite, pyrite, quartz, calcite, amorphous iron oxide 

(limonite?), and mixed-layer illite-smectite. The nearly 

opaque, gray anglesite probably formed by oxidation of 

galena. 

SULFIDE MINERALS 

GALENAANDSPHALERITE 

A sample containing galena and sphalerite was collected 

on the north side of Zigzag Mountain from an epithermal 

vein deposit in volcanic rocks of the Rhododendron Formation (Keith and Causey, I982, I984). Associated secondary minerals include quartz, calcite, chlorite(?), illite, 

chalcopyrite, dolomite, and cerussite. Mineralization occurs within a 0.3-m-wide quartz vein along a steeply dipping north-northwest fracture zone. Semiquantitative 

chemical analysis of the mineralized vein sample showed 

(in ppm) Ag = I5, Cd = 200, Cu = 5,000, Pb = 7,000, and 

Zn > 10,000 (Keith and others, I980; Keith, Bargar, and 

Beeson, I982). Keith and Causey (1982) indicated that 

claims had been staked along this vein but no production 

was reported and that the area has little potential for economic deposits. 

CHALCOPYRITE 

Chalcopyrite was identified in drill holes 8 and 9 (table 

I) and at one surface location other than the sample discussed above with galena and sphalerite. The two drill 

holes penetrated quartz diorite of the Laurel Hill intrusion, 

and a grain of chalcopyrite was identified in one sample 

from each drill hole; other secondary minerals identified in 

these two samples include quartz, chlorite, epidote, magnetite, calcite and laumontite. Two very altered andesite 

samples of the Rhododendron Formation, containing vein 

fillings of tarnished chalcopyrite, magnetite, chlorite, 

quartz, epidote, oxidized bladed hematite, stilbite, and calcite, were collected at a rock quarry near the Laurel Hill 

intrusion south of Mount Hood. Secondary minerals in 

both drill cuttings and surface samples indicate that the 

rocks are predominantly affected by propylitic alteration 

due to pluton emplacement. 

PYRITE 

Pyrite, occurring in drill holes I through 8 and 13 (table 

I), as well as in several surface samples, is a common 

secondary mineral in the Mount Hood area. Some pyrite 

was deposited as thin crusts on bleached Quaternary debris grains in association with minerals such as gypsum, 

hematite, natroalunite and opal; these minerals probably 

formed in areas of furnarolic alteration (Babcock and Wil-
HYDROTHERMAL ALTERATION MINERALOGY 35 

Table 13. Electron-microprobe analyses of pyrite from fractures in Tertiary volcanic rocks near Mount Hood. 

[Analyses in wt percent of elements) 

Sample number 79BM-2046A 79BM-2046G 

Analysis number 2 3 2 3 

Fe----------------- 46.12 45.67 45.83 45.89 45.98 45.85 

Co----------------- 0.03 0.02 0.01 0.04 0.03 0.02 

N i ----------------- 0.00 0.00 0.00 0.00 0.00 0.00 

S------------------ ~ .ll.1l ~ l£..21 .51....5..4 ~ 

Total----------- 98.59 97.40 98.74 98.90 98.55 98.47 

S : (Fe + Co)---- 1.98 1.97 2.01 2.01 1.99 2.00 

Sample number 79BM-2046S 79IC-2033 

Analysis number 3 5 8 2 

Fe------------------ 46.09 46.59 45.74 45.46 45.49 

Co------------------ 0.02 0.01 0.06 0.07 0.05 

Ni ------------------ 0.00 0.00 0.00 0.00 0.00 

S-------------------- ..23....3.1 .5..1...11 52 87 .i2...U 22...18 

Total----------- 99.45 99.97 98.67 97.68 97.92 

S : (Fe + Co)----- 2.01 1.99 2.01 2.00 2.00 

cox, 1977) and were later transported downslope. Other 

pyrite-bearing grains from the Rhododendron Formation 

and. Tertiary andesite flows appear to have formed as epithermal vein fillings along with later smectite and calcite 

or quartz. Most pyrite crystals are cubic, but a few pyritohedrons were observed. The crystal size is usually small 

and was measured at 0.01 mm in one disseminated pyrite 

drill-cuttings sample. Although widespread in occurrence, 

few samples contain very abundant pyrite. Highly mineralized fault gouge in drill cuttings from the bottom of drill 

hole 5 (table 1), from outcrops in the Cheeney Creek area 

(southwest of Mount Hood), and along Iron Creek (southeast of Mount Hood) contain significant pyrite. Chemical 

analyses of pyrite from the last two areas (table 13) show 

minor Co in addition to Fe and S, but no Ni was detected. 

IRON OXIDE-HYDROXIDE MINERALS 

AMORPHOUS IRON OXIDE-HYDROXIDE 

Orange- and red-stained drill cuttings from drill holes 1 

and 4 (table 1) are amorphous to X-rays. The amorphous 

iron-oxide- or iron-hydroxide-stained grains occur as hydrothermal fracture coatings on Tertiary andesite in drill 

hole 4. In drill hole 1, orange-stained Quaternary debris 

grains probably were transported from fumarolic areas 

higher on Mount Hood; associated minerals include opal, 

kaolinite, smectite, pyrite, and sulfates (alunite, natroalunite, jarosite, and natrojarosite ). One sample from a mineralized fracture zone in the Cheeney Creek area also has 

considerable orange, earthy, amorphous-iron staining along 

with cerussite, sphalerite, and quartz. Because the orangered, iron-bearing deposits are amorphous, it is uncertain 

whether the minerals consist of iron oxide or iron hydroxide or both. 

GOETHITE 

One sample from the Iron Creek area has a vein deposit 

of tiny, euhedral quartz crystals that are coated by later, 

orange, earthy goethite (identified by X-ray). Other secondary minerals in the sample are pyrite and epidote; the 

goethite possibly formed by oxidation of pyrite. 

HEMATITE 

X-ray diffraction analyses show that iron oxide stains 

found in many Mount Hood drill holes (table 1, Nos. 2 

through 7, 11, and 12) and surface samples consist of 

hematite. The occurrence of hematite in all rock units 

sampled, ranging from lava flows of the Columbia River 

Basalt Group to the youngest Holocene fumarolic. deposits, shows that the origin of the hematite is quite varied. 

Many andesitic flows have reddish hematite alteration that 

appears to be deuteric and formed during cooling of the 

surface of lava flows by oxidation of mafic minerals. 

Some thin sections of these flows also show reddish oxidation rims that probably are hematite surrounding mafic 

minerals. Bladed hematite crystals were noted in pore 

spaces of some andesitic rocks along with vapor~phase 

tridymite. Many of the Quaternary debris units contain 

bleached and orange hematite-stained grains in association 

with secondary minerals, such as opal, kaolinite, and sulfate minerals, that suggest a fumarolic origin. Finally, 

hematite possibly formed from. oxidation of pyrite was 

identified in mineralized veins from several areas near 

Mount Hood. 

MAGNETITE 

Thin magnetite veins, in association with epidote, chlorite, and quartz, occur in quartz diorite of the Laurel Hill 

and Still Creek intrusions, both in drill cuttings from drill 

holes 8, 9, and 10 (table 1) and in outcrop samples. This 

propylitic mineralization undoubtedly occurred as a very 

late stage during cooling of the intrusions. 
36 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

OTHER MINERALS 

EPIDOTE 

Secondary epidote occurs in quartz diorite drill cuttings 

from the Laurel Hill and Still Creek intrusions penetrated 

by drill holes 8, 9, and IO (table I). A few samples of 

Quaternary debris from drill holes 7 and I 0 (table I) also 

contain traces of epidote. Moreover, epidote was identified 

in drill hole 4 (table I) in association with possible quartz 

diorite dike material; vertical dikes, less than I 00 m from 

this drill hole, are shown on a geologic map by Priest and 

others ( I982). Several surface samples of quartz diorite 

from the Laurel Hill intrusion contain epidote. Epidote 

was also found in propylitically altered rocks of the 

Rhododendron Formation collected close to this intrusion. 

Similar epidote-bearing propylitic alteration can be found 

in the late Tertiary andesites near Iron Creek and the Barlow Pass-Bennet Pass area southeast of Mount Hood. 

Bright-green, euhedral, acicular, epidote crystals (fig. 

I7) fill veins and vesicles in the late Tertiary intrusive and 

extrusive igneous rocks near Mount Hood; some also occurs in open spaces between breccia fragments and is disseminated in bleached rock adjacent to fractures. Most 

epidote from the drill holes and surface samples is associated with earlier(?) chlorite, and coeval(?) magnetite and 

quartz. Zeolites, clay minerals, and other secondary minerals in these samples are later deposits. 

Composition of Mount Hood epidote ranges from about 

I5 to 33 percent pistacite (Ps) (table 14) and averages 

about 23 percent Ps. Minor epidote reported from the Old 

Maid Flat 7 A drill hole (Holdaway and Bussey, 1982) is 

25 percent Ps. In this study, considerable differences in Ps 

values were observed both between individual crystals and 

within a single crystal. The analyses in table I4 show significant Fe3

+ I AI substitution in all analyzed samples. Occasional grains have patches that contain abundant Mn or 

Mg, but no systematic occurrences of these elements were 

observed. 

TITANITE(?) 

During electron-microprobe analyses of epidote, one of 

the samples (LH240-2) from drill hole 9 (table I) yielded 

a single analysis that contains significant titanium (34.07 

Si02, I1.29 AI20 3, 21.13 Ti02, 5.79 Fe203, 0.04 MgO, 

0.0 MnO, and 25.70 CaO; total weight percent oxides = 

98.02). Subsequent examination of the polished thin section in transmitted light showed that there are clayey(?) 

concentrations of a brown to nearly opaque mineral between many epidote crystals; in reflected light, the mineral 

appears to be yellowish to grayish in color and might be 

leucoxene. Leucoxene is a general term for an alteration 

product of ilmenite, and it may consist of one or more 

titanium-bearing minerals (Phillips and Griffen, I981 ). 

The only titanium-bearing mineral that might be even remotely suggested by our chemical analysis is titanite 

(sphene). A surface vein sample from the same location as 

drill hole 9 contains tourmaline, apatite, and another mineral observed only in SEM. EDS semiquantitative chemical analysis of this unidentified mineral showed significant 

Ca, Ti, and Si and smaller concentrations of AI, Fe, and 

Mn. On the basis of the Si, Ti, and Ca determinations, 

these two mineral occurrences are tentatively identified as 

titanite(?). 

Authigenic titanite has been reported in association with 

laumontite in New Zealand zeolite-facies metamorphosed 

rocks (Boles and Coombs, 1977). Hydrothermally altered 

sandstones of the Cerro Prieto geothermal field, Mexico, 

also contain titanite, which Schiffman and others ( I985) 

indicated probably formed at temperatures that did not exceed 150 °C. Boles and Coombs ( I977) indicated that their 

analyses confirmed that substitution of AI and Fe3

+ for Ti 

is one of the characteristics of sphene in very low grade 

metamorphic rocks. Schiffman and others ( 1985) described 

Figure 17. Scanning electron micrograph showing euhedral columnar crystals of epidote from -116 m depth in the Zigzag River drill hole (table 1, No. 8). 
Table 14. Electron-microprobe analyses of epidote from drill holes and outcrops near Mount Hood. 

Sample number-------- 79LH-0087A 79CCQ-2001 B 79CCQ-2001 F LH-40-2 

Analysis number------- I 2 3 I 3 5 1 2 4 I 3 

Major-element chemical analyses (wt percent oxides) 

SiOz--------------------- 37.81 38.87 39.48 38.20 37.54 37.04 38.09 38.44 37.65 38.54 38.48 

Alz03------------------- 23.44 28.27 23.92 27.49 26.14 22.51 25.36 24.59 22.31 25.69 24.65 

TiOz--------------------- 0.34 0.09 0.13 0.045 0.10 0.00 0.04 0.03 0.02 0.03 0.03 

Fez031 ___________________ 12.66 7.70 10.46 8.11 11.00 I4.25 10.92 11.33 13.70 9.74 11.44 

MgO--------------------- 0.10 0.04 1.46 0.04 0.12 0.06 0.03 0.04 0.05 0.07 0.05 

MnO--------------------- 0.01 0.07 0.06 0.17 1.11 0.12 0.20 0.91 0.02 0.01 0.00 

CaO---------------------- 2l.l2 ~ 2.Ul .2.1..8.1 22 12 2..1..51 23.14 22 48 2.3..11 ll@ 21.2.3. 

Total------------------ 97.55 98.61 98.22 97.92 98.13 97.50 97.78 97.82 97.47 97.68 97.88 

Number of atoms on the basis of 12.5 oxygens 

Si ------------------------ 3.01 3.00 3.09 2.98 2.96 2.98 3.01 3.04 3.02 3.03 3.04 

A )IV---------------------- 0.00 0.00 0.00 0.02 0.04 0.02 0.00 0.00 0.00 0.00 0.00 AJVI ______________________ 2.20 2.57 2.21 2.51 2.39 2.12 2.36 2.29 2. 11 2.38 2.29 

Ti------------------------ 0.02 0.02 0.01 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.00 

Fe3+ --------------------- 0.76 0.45 0.62 0.48 0.65 0.86 0.65 0.67 0.83 0.58 0.68 

Mg---------------------- 0.01 0.01 O.I7 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01 

M n---------------------- 0.00 0.00 0.00 0.01 0.07 0.01 0.01 0.06 0.00 0.00 0.00 

Ca----------------------- 1.98 1.95 1.90 2.00 1.87 2.03 1.96 1.91 2.04 1.99 1.96 Ps2 ______________________ 0.26 0.15 0.22 0.16 0.21 0.29 0.22 0.23 0.28 0.20 0.23 

-

ITotal iron is given as Fez03. 

2ps is pistacite component (in mole percent) determined by the calculation Fe I Fe + Al. 

LH-240-2 

5 1 2 5 

38.17 38.92 38.15 37.93 

23.28 25.88 25.69 22.24 

0.12 0.00 0.07 0.21 

13.04 9.21 10.48 13.49 

0.05 0.01 0.07 0.02 

0.00 0.18 0.16 0.00 

2.1..13. .ll.11 .u...w 2.1...6..2. 

97.89 97.73 97.72 97.58 

3.03 3.05 3.0I 3.04 

0.00 0.00 0.00 0.00 

2.18 2.39 2.39 2.10 

0.01 0.00 0.00 0.01 

0.78 0.54 0.62 0.81 

0.01 0.00 0.01 0.00 

0.00 0.01 0.01 0.00 

1.98 1.98 1.95 2.03 

0.26 0.18 0.21 0.28 

79IC-2032 

I 2 

37.78 38.07 

27.16 25.30 

0.04 0.12 

8.77 10.17 

0.04 0.06 

0.19 0.25 

2.1.1.Q ~ 

97.68 97.45 

2.97 3.01 

0.03 0.00 

2.48 2.36 

0.00 0.01 

0.52 0.61 

0.01 0.01 

0.01 0.02 

2.00 1.99 

0.17 0.21 

5 

37.49 

20.70 

0.05 

16.25 

0.06 

0.14 

n.l.Q 

97.85 

3.02 

0.00 

1.97 

0.00 

0.99 

0.01 

0.01 

2.00 

0.33 

::r: 

~ 

0 

:;c 

0 

~ 

~ 

~ > r 

> 

~ 

~ -0 

z 

s= 

-

~ 

~ r 

0 

0 

~ 

w 

-l 
38 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

their titanite analyses as "some of the most Al-rich authigenic sphenes that have been described." Although the 

Mount Hood titanium-bearing mineral has not been demonstrated to be titanite, its chemical composition and mode 

of occurrence are quite similar to the "very fine grained 

xenoblastic aggregates" associated with epidote in the Cerro Prieto geothermal field (Schiffman and others, 1985). 

The Mount Hood titanite(?) has slightly higher Al20 3 and 

significantly higher Fe20 3 values than the titanite reported 

by Schiffman and others (1985). This appears to suggest 

that the substitution of AI and Fe3

+ for Ti in the Mount 

Hood titanite(?) is more extreme than in the previous case. 

GARNET(?) 

A single subhedral dodecahedral(?) crystal from 79 m 

depth in drill hole 3 (table 1) was observed by SEM (fig. 

18). Semiquantitative chemical analysis by EDS shows 

that the mineral contains abundant Fe, Ca, and Si, as well 

as small amounts of V and Ti. The crystal morphology and 

chemistry suggest that the mineral may be andradite garFigure 18. Scanning electron micrograph showing an imperfectly formed, trapezohedral garnet(?) crystal from about 79 ·m 

depth in the McGee Creek drill hole (table 1, No. 3) . . 

net. Low-temperature andradite and grossular garnet that 

formed in close association with earlier analcime and 

thomsonite were reported in Scottish Paleozoic lavas intruded by a Tertiary dike (Livingstone, 1989). According 

to Livingstone (1989), chemical conditions must have 

been favorable for the formation of the garnets at temperatures considerably below 300°C. Also, heat generated by 

the intrusion was not sufficient to alter the preexisting zeolite minerals. 

Possible garnet in drill hole 3 (table 1) is difficult to 

explain. The mineral was found in a Quaternary pyroxene 

andesite lava flow that appears to have been subjected 

only to recent very low temperature (measured temperature at 605 m depth was 60.0°C), (Robison and others, 

1982) hydrothermal alteration; other hydrothermal minerals identified in this sample include hematite, smectite, 

heulandite, dolomite, and manganoan calcite. If the crystal 

shown in figure 18 is garnet, it must have formed at higher 

temperatures than occur at the present time. One possible 

explanation is that the garnet(?)-bearing grain may have 

been part of a xenolith. Xenoliths containing garnet were 

found in andesitic lavas at higher elevations on the north 

side of Mount Hood. 

TRUSCOTTITE(?) 

One rounded cobble in Quaternary reworked debris 

from about 2,200 m elevation on the northeast slope of 

Mount Hood contains a surface coating of white to colorless botryoidal opal, some of which appears to have altered to chalcedony. Several broken botryoids have an 

interior of yellowish-white, radiating, lamellar crystals 

(fig. 19). X-ray diffraction analysis suggests that the lamellar crystals may be truscottite; however, EDS semiquantitative chemical analysis using an SEM shows some 

AI and about equal proportions of K and Ca in addition to 

Si. Although truscottite is a calcium silicate hydrate mineral, significant K and AI have been reported in truscottite 

analyses from Yellowstone National Park (Bargar and Beeson, 1985). 

ACTINOLITE 

Drill holes 8, 9, and 10 (table 1) and several surface 

samples collected in the vicinity of the Laurel Hill intrusion 

contain pale-green acicular actinolite. Most of these samples consist of quartz diorite in which the hornblende has 

been recrystallized to actinolite or chlorite, as indicated by 

Wise ( 1969). A few samples from lavas of the Rhododendron Formation adjacent to the Laurel Hill intrusion contain 

actinolite that probably formed by replacement of pyroxene 

. phenocrysts during contact metamorphism. Actinolite also 

formed as an open-spac~ deposit with closely associated 

epidote and later zeolites. Electron-microprobe analyses of 
HYDROTHERMAL ALTERATION MINERALOGY 39 

two open-space actinolite crystals are given in table 15. The 

low total weight percent oxides may reflect nondetermination of alkalis and water. 

PREHNITE 

Prehnite is scarce in Mount Hood samples and was only 

detected by X-ray diffraction in drill cuttings from two 

drill holes (table 1, Nos. 5 and 7). In drill hole 5, white 

vein fillings or tan-green altered grains in drill cuttings of 

the Columbia River Basalt Group contain prehnite with 

laumontite, quartz, smectite, and chlorite. Prehnite was 

also identified in drill hole 7, with hematite, epistilbite, 

mixed-layer chlorite-smectite, and laumontite, in wholerock X-ray diffraction analyses of late Tertiary(?) hornblende andesite. A few surface samples of open-space 

fracture fillings and dikes, from the Laurel Hill intrusion 

and a zone of mineralization near Iron Creek southeast of 

Mount Hood, contain prehnite in association with quartz, 

epidote, smectite, chlorite and pyrite. In only one sample 

was prehnite observed (by microscope) as equant white to 

colorless crystals deposited later than quartz crystals in a 

large open fracture or vug(?); prehnite more commonly 

occurs as lamellar crystals. 

Fluid inclusions in quartz from the just mentioned sample were not analyzed; however, fluid inclusions from several nearby samples have mean homogenization 

temperatures between 250° and 270 °C. Prehnite has been 

Figure 19. Scanning electron micrograph showing lamellar 

crystals of truscottite(?) deposited as radial concentrations on an 

erratic rock sample collected from about 2,200 m elevation on 

the northeast side of Mount Hood. 

Table 15. Electron-microprobe analyses of actinolite from volcanic rocks of the Rhododendron Formation altered by the Laurel Hil1 

intrusion. 

Sample number-------- 79LH-0087A 

Analysis number------- 2 

Major-element chemical analyses (wt percent oxides) 

Si02----------------------

Al203---------------------

Ti02----------------------

FeO 1----------------------

MgO----------------------

MnO----------------------

CaO-----------------------

Total------------------

51.48 

7.33 

0.51 

7.90 

16.29 

0.08 

11..1.4 

97.33 

52.02 

4.31 

0.51 

9.56 

16.71 

0.02 

.ll..48 

95.79 

Number of atoms on the basis of 24 oxygens 

s i --------------------------

AI IV------------------------ AJVI_ ______________________ _ 

Ti--------------------------

Fe--------------------------

Mg-------------------------

Mn-------------------------

Ca--------------------------

1 00 Mg I Mg + Fe2+ -----

Fe I (Fe + Mg)------------

1Total iron as FeO. 

7.61 

0.39 

0.89 

0.06 

0.98 

3.59 

0.01 

2.18 

78.56 

0.21 

7.86 

0.14 

0.63 

0.06 

1.21 

3.76 

0.03 

2.02 

75.65 

0.24 

found in drill holes of several geothermal areas where 

measured subsurface temperatures were between 250° and 

350 °C (Bird and others, 1984). 

ADULARIA 

A few outcrops of the Laurel Hill intrusion and a zone 

of extensive mineralization adjacent to Iron Creek on the 

southeast side of Mount Hood contain fractures coated by 

colorless to white (one sample is pink), euhedral adularia 

crystals (fig. 20) in association with earlier quartz crystals. 

Other secondary minerals in the samples include smecti~e, 

illite, chlorite, stilbite, epidote(?), and calcite. Chemical 

analyses of three of the samples (table 16) show that the 

adularia contains little or no Ca and Na and that its composition is uniform between crystals from different areas 

as well as within a single crystal. 

Adularia can form by replacement of primary plagioclase phenocrysts, loss of C02 and increase in pH during 

boiling of geothermal water, or introduction of significant 

potassium to the thermal water (Browne and Ellis, 1970). 
40 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Table 16. Electron-microprobe analyses of adularia from near Mount Hood. 

[Dashed(--) where not analyzed] 

Sample number--------

Analysis number------

s i 02---------------------

Al203--------------------

FeO-----------------------

MgO----------------------

M nO----------------------

BaO-----------------------

CaO-----------------------

N a20---------------------

K 20----------------------

Total-----------------

Si--------------------------

A 1--------------------------

Fe--------------------------

Mg-------------------------

M n ------------------------- B a--------------------------

Ca--------------------------

N a--------------------------

K ---------------------------

79LH-1057D 79IC-2032 79IC-2033 

2 3 3 5 2 5 

Major-element chemical analyses (wt percent oxides) 

64.33 63.58 63.52 63.65 64.22 64.79 64.1 6 64.51 64.96 

18.41 18.36 18.58 18.33 18.54 18.27 17.95 18.50 17.47 

0.04 0.00 0.00 0.00 0.04 0.03 0.00 0.01 0.07 

0.04 0.01 0.03 0.02 0.03 0.05 

0.00 0.00 0.00 0.01 0.00 0.00 

0.36 0.63 0.24 

0.35 0.06 0.06 0.00 0.00 0.00 0.00 0.00 0.08 

0.19 0.17 0.13 0.24 0.11 0.16 0.18 0.21 0.22 

15_.__5_1 .l.6..illi 16....11 l6..ill. l.6...l5. lfi.11 .lQ.ll 16.21 15...12 

99.25 98.86 98.65 98.31 99.07 99.50 98.53 99.47 98.64 

Number of atoms on the basis of 32 oxygens 

11.78 11.65 11.58 

3.98 3.97 3.99 

0.01 0.00 0.00 

0.03 0.05 0.02 

0.07 0.01 0.01 

0.07 0.06 0.05 

3.64 3.75 3.75 

11 .53 11.73 11.89 11.66 11.84 11.78 

3.92 3.99 3.95 3.85 4.00 3.73 

0.00 0.01 0.00 0.00 0.00 0.01 

0.01 0.00 0.01 0.01 0.01 0.01 

0.00 0.00 0.00 0.00 0.00 0.00 

0.00 0.00 0.00 0.00 0.00 0.02 

0.09 0.04 0.06 0.06 0.08 0.08 

3.71 3.76 3.80 3.76 3.80 3.65 

The Mount Hood adularia does not replace plagioclase 

and fluid inclusions in the earlier formed quartz do not 

indicate that boiling occurred either during crystal formation or at a later time~ probably the adularia formed because of potassium-rich thermal water that flowed through 

fracture systems. 

TOURMALINE AND APATITE 

One surface sample of the Laurel Hill intrusion has a 

fracture coating consisting of radiating sprays of black, columnar tourmaline crystals (fig. 21). SEM semiquantitative 

chemical analyses by EDS show that the constituents are 

Si, AI, Fe, and minor Mg and Ca. Tiny, white needles of 

apatite, identified by X-ray diffraction, are intergrown with 

the tourmaline crystals. The fracture also has a late finegrained coating of smectite and stilbite~ hornblende crystals mostly are altered to chlorite. Coexisting titanite(?) 

(discussed above) was also tentatively identified on the basis of high Ti content (determined by SEM methods). 

COPPER MINERALS 

Figure 20. Scanning electron micrograph showing euhedral, 

equant adularia crystals from fracture fillings in Tertiary volcanic 

breccia near Iron Creek on the southeast side of Mount Hood. 

A single grain from drill cuttings in Quaternary reworked volcanic debris at 121.9 m depth in drill hole 7 
FLUID INCLUSIONS 41 

(table 1) contains four crystals of native copper (copper 

was confirmed by EDS). Generally, the copper crystals 

have a rounded morphology and appear to have an oxide(?) coating (fig. 22). The surface of one crystal displays 

parallel striations and another copper crystal is somewhat 

flattened. Panned concentrates of stream-sediment samples 

collected upstream from the drill hole 7 site have copper 

values that . are much higher than background values for 

copper at Mount Hood (Keith and others, 1980). 

A few surface samples from the Laurel Hill intrusion 

contain open-space fillings of bluish green chrysocolla. 

Figure 8 shows a thin botryoidal chrysocolla crust that 

formed later than chabazite on a fracture surface; other 

minerals identified from this fracture include iron oxide, 

smectite, chlorite, wairakite, epistilbite, and stilbite. 

FLUID INCLUSIONS 

Hydrothermal quartz crystals occur in several outcrop 

samples collected close to intrusions near Mount Hood, as 

well as in drill holes that penetrated these plutons. The 

Figure 21. Scanning electron micrograph showing radiating 

sprays of acicular tourmaline crystals in a lava flow of the 

Rhododendron Formation intruded by the Laurel Hill intrusion. 

plutons and the rocks they intrude are of late Tertiary age 

and are not related to the presently active geothermal system at Mount Hood. An examination of fluid inclusions in 

these secondary quartz crystals was included in this study 

to obtain an idea of temperatures and fluid salinities. involved with hydrothermal alteration observed in drill-hole 

and outcrop specimens. Fluid-inclusion homogenization 

temperature (T h) and final melting-point temperature (T m) 

data, determined by the freezing method (Roedder, 1962), 

were obtained for 16 sample chips from 6 double-polished, hydrothermal quartz samples (table 17). Fluid-inclusion heating/freezing measurements were made on fluid 

inclusions in quartz collected from a drill-cuttings sample 

(specimen SC-170-3) at 51.8 m depth in the Still Creek 

drill hole (table 1, No. 10) and from outcrop samples near 

the Laurel Hill intrusion (specimens 79CCQ-0088F&O). 

Fluid-inclusion data were also obtained for quartz associated with an intrusion southeast of Mount Hood near Iron 

Creek (specimens 79IC-20390 and 79IC-2040) and an 

outcrop (specimen 79WR-2010) -3 km northwest of this 

intrusion. (The Iron Creek intrusion was probably responsible for the mineralization observed in several kilometers 

of road cuts on the southeast flank of Mount Hood). A 

total of 279 T h measurements and 58 T m values were obtained initially using a Chaixmeca heating/freezing stage 

and later a Linkam THM 600 heating/freezing stage with a 

TMS 90 temperature control system. 

The majority of fluid inclusions located in samples near 

the Laurel Hill and Still Creek intrusions are liquid rich 

(fig. 23), with a vapor bubble that usually constitutes 

Figure 22. Scanning electron micrograph showing a grain of 

native copper from 121.9 m depth in drill hole 7 (table 1) that 

appears to be coated by a thin oxide(?) layer. 
42 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

about 10 to 30 percent of the volume of the inclusion. 

These inclusions, ranging in size from < 10 to -100 !Jill, 

formed along healed fractures and appear to be secondary, 

although a pseudosecondary origin (formed along healed 

fractures prior to a later quartz deposit that covers the exterior of the crystal) is possible for some of the fluid inclusions. A few monophase-liquid, monophase-vapor, or 

vapor-rich fluid inclusions appear to be closely related and 

may have formed by necking down (Bodnar and others, 

1985). Necking down refers to a process whereby quartz is 

dissolved and redeposited, changing the shape of the fluid 

inclusions with 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. An example of necking down is marked by an 

arrow in figure 24. A discontinuous, healed fracture in 

specimen 79CCQ-0088F consists entirely of small, pseudosecondary, monophase-vapor fluid inclusions that have a 

negative-crystal shape; these fluid inclusions along with 

occasional, large, isolated, negative-crystal inclusions 

filled by monophase vapor (fig. 25) may have formed at 

an early stage in the growth of the crystal and possibly are 

due to boiling conditions or effervescence as discussed in 

Roedder (1984). 

Fluid inclusions in quartz samples collected near the intrusion at Iron Creek consist mostly of secondary, liquidrich inclusions (fig. 24) that range in size from <10 to 

-1 00 !Jill and contain about 10 to 30 percent vapor. A few 

liquid- and vapor-rich fluid inclusions are as much as 294 

!Jill in size, appear to be isolated or unrelated to other fluid 

inclusions in the crystals, and may be of primary origin 

(fig. 26). A few closely associated vapor-rich and 

monophase-liquid fluid inclusions appear to be the result 

of necking down. Large, isolated, monophase-gas fluid inclusions and healed fractures containing monophase-gas 

inclusions, both with negative-crystal shapes, probably 

formed due to boiling or effervescence. Some healed fractures in specimen 79IC-2040 contain vapor-rich, secondary fluid inclusions (fig. 27) (vapor constitutes -50 to 80 

percent of inclusion volume) in association with the dominant liquid-rich inclusions. Three of these vapor-rich fluid 

inclusions homogenized (to the vapor state) near the critical temperature for pure H20 (374.1°C); these Th values 

are somewhat higher than the T h for liquid-rich fluid inclusions (fig. 28). 

Fluid-inclusion salinities in three quartz samples (table 

17) are quite low, -0.0 to 0.4 weight percent NaCl equivalent. Several fluid inclusions in another sample have a salinity of about 2.4 weight percent NaCI equivalent, and 

another sample contains fluid inclusions with quite variable salinities ranging from 0.0 to 5.8 weight percent NaCl 

equivalent. Plotting T h vs T m (fig. 29) shows two possible 

trends. A vertical trend at T m value 0°C shows simple 

cooling of very low salinity water present in the majority 

.of the fluid inclusions analyzed; another vertical trend occurs at Tm values of -1.4° and -1.9°C. A second, less pronounced, essentially horizontal trend (at- 270°C) that also 

appears to be present in figure 29 may suggest isothermal 

mixing of waters having different salinities. 

Figure 28 shows six histograms of T h data for fluid inclusions from the six quartz sample locations included in 

this study. The tabulated data (table 17) give the T h ranges 

and mean T h values for all 16 quartz chips analyzed from 

these six sample sites. The fluid inclusions in quartz crystals associated with the Laurel Hill, Still Creek, and Iron 

Creek intrusions all have mean T h measurements between 

251 o and 273°C. Specimen 79WR-2010, which was collected about 3 km closer to Mount Hood than the Iron 

Creek intrusion samples, has a mean T h of 229°C. 

One intriguing result of the fluid-inclusion study is that 

one of the specimens (79IC-20390) was found to have a 

few hydrothermal quartz crystals that contain from 1 to -4 

or 5 tiny ( -1 !Jill sized) moving particles that appear to 

have a threadlike or rodlike shape. These moving particles 

apparently were trapped inside at least eight of the fluid 

inclusions at the time the fluid inclusions formed. Two of 

these liquid-rich, secondary fluid inclusions homogenized 

at 250° and 258°C; melting-point temperatures were not 

determined for these two fluid inclusions, but six other fluid inclusions in the same sample all had T m values of 

0.0°C. Similar-sized and -shaped moving particles have 

been reported from fluid inclusions in hydrothermal quartz 

crystals that formed in fractures in a 150-ka rhyolite lava 

flow penetrated by a U.S. Geological Survey research drill 

hole in Lower Geyser Basin, Yellowstone National Park, 

Wyoming (Bargar and others, 1985). Homogenization temperatures for the Yellowstone fluid inclusions were between 190° and 280°C. The Yellowstone moving particles 

(movement of the particles appears to be due to Brownian 

motion) are hypothesized to be bacteria. If the particles are 

eventually proven to be bacteria, they would be very significant because they must have lived at far hotter temperatures than is believed possible for present-day bacteria. 

Baross and Deming (1983) reported thermophilic bacteria 

living in 350°C waters of a sea-floor hot spring at 21 o N. 

latitude on the East Pacific Rise; however, the results of 

this study have been disputed (Trent and others, 1984 ). 

Presently, the upper temperature limit for life to exist is 

not known but is thought to be between 110° and about 

200°C (Brock, 1985). 

DISCUSSION 

Our study of drill-hole and outcrop samples in the 

Mount Hood area found numerous metamorphic minerals 

that are believed to have formed by hydrothermal alteration at low to moderate temperatures. Such metamorphic 

mineral assemblages are described as "very low grade 
Table 17. Fluid-inclusion heating/freezing data for hydrothermal quartz crystals from drill holes and outcrops near Mount Hood. 

[Dashed(--) where not analyzed] 

Sample numberl Locality2 Associated Number of melting- Melting-point Salinity Number of homosecondary point temperature temperatures (wt percent genization temperminerals3 measurements Tm CC)4 NaCl equiv.) ature measurements 

79CCQ-0088F 1 Ep, Mt, Sm, St, He 7 -1.4 2.4 29 

79CCQ-00880 1 Ep, Mt, Ch, Sm, Sc, St 6 +0.1 0.0 28 

SC-170-3 2 La, Ep, Ca, Ch 2 -0.2 0.4 21 

79WR-2010 3 Cc, I, Ch, Py, Cl 0 -- -- 38 

79IC-20390 4 Ep, Ch 20 variable, 0.0 to -3.6 0.0 to 5.8 68 

79IC-2040 4 Ad 23 0.0 0.0 95 

I Sample SC-170-3 is from 51.8 m depth in the Still Creek drill hole (table 1, No. 10); remaining samples are from outcrops. 

2Localities (see fig. 1) are: 1 = Laurel Hill, 2 = Still Creek, 3 = Barlow Pass-Bennett Pass area, and 4 = Iron Creek 

Range of Mean 

homogenization homogenization 

temperatures temperature 

Th CC)4 Th CC)4 

192-320 270 

213-346 258 

212-324 273 

193-330 229 

151-315 251 

159-373 272 

3Mineral abbreviations are: Ad= adularia, Cc = calcite, Ca = chabazite, Cl =chalcedony, Ch =chlorite, Ep =epidote, He= heulandite, I = illite, La= laumontite, Mt = 

magnetite, Sc = scolecite, Sm =smectite, and St = stilbite. 

4Multiple calibration measurements, using synthetic fluid inclusions (Bodnar and Sterner, 1984) and chemical compounds with known melting-point temperatures recommended 

by Roedder (1984), suggest that the T m measurements should be accurate to within ±0.2·c, and that the Th values should be accurate to better than ±2.o·c. 

0 

En 

(") 

c 

C/.) 

C/.) 

8 

z 

.J::>. 

w 
44 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

metamorphism" by Frey and Kisch (1987), who used the 

term to refer to metamorphism that takes place in the temperature range of about 150° to 200° through 350° to 

400°C. Mineral assemblages occurring in this temperature 

range include "zeolite facies" minerals, which formed at 

the lower temperatures, and "greenschist facies" minerals, 

which were produced at the higher temperatures (Liou and 

others, 1987). Also present are several subgreenschist-fa- t\ 

cies mineral assemblages (Liou and others, 1987). The distinction between very low grade metamorphism and 

diagenesis is uncertain because definitions of diagenesis 

are ambiguous; Frey and Kisch (1987) indicated that such 

distinctions probably are not of great importance. 

Liou and others (1987) discussed five types of very low 

grade metamorphism: ocean floor, burial, subduction, thermal, and hydrothermal. Hydrothermal metamorphism best 

describes most alteration mineralogy observed in the 

Mount Hood area; however, thermal metamorphism does 

occur in rocks adjacent to intrusions. Wise (1969) described a hornblende hornfels zone (thermal halo) about 

60-m thick ~t the west end of the Still Creek intrusion. 

Figure 23. Photomicrograph showing liquid-rich, secondary 

fluid inclusions in hydrothermal quartz from a fracture in an outcrop of the Rhododendron Formation (specimen 79CCQ-0088F) 

altered by the Laurel Hill intrusion. 

Figure 24. Photomicrograph showing secondary, liquid-rich 

fluid inclusions in a hydrothermal quartz crystal that was deposited in open space of a Tertiary volcanic breccia (specimen 79IC2040) intruded by a pluton near Iron Creek to the southeast of 

Mount Hood. The arrow points to a small monophase-liquid 

necked-down fluid inclusion. 

Figure 25. Photomicrograph showing an isolated, monophasevapor fluid inclusion having a negative crystal shape (specimen 

79CCQ-0088F). 
DISCUSSION 45 

Similar thermal halos in country rocks surrounding intrusions can experience temperature ranges (higher than 

400°C) characteristic of low- to high-grade metamorphism 

and can result in the formation of zeolite minerals such as 

wairakite and laumontite (Liou and others, 1987). Beaufort 

and others ( 1990) described fractures containing several 

alteration minerals, including tourmaline and apatite, from 

a fossil hydrothermal system at Saint Martin, Lesser Antilles; studies of fluid inclusions in miarolitic quartz from 

these rocks indicate that quartz crystals formed at temperatures ranging from near 340°C to more than 51 0°C. In the 

Mount Hood area, closely associated tourmaline and apatite line a fracture in one very altered sample obtained 

from a quarry in which exposures of the Laurel Hill intrusion can be seen; these minerals were probably formed by 

thermal metamorphism, possibly at temperatures above 

400°C. 

TERTIARY METAMORPHISM 

GREENSCHIST- AND SUBGREENSCHIST-FACIES 

METAMORPHISM 

On the southwest side of Mount Hood, near the base of 

the mountain, are quartz diorite to quartz monzonite outcrops of the Laurel Hill and Still Creek intrusions. Wise 

(1969) indicated that the two intrusions may actually be 

part of a single larger stock. Similar hornblende-quartz microdiorite sills and dikes(?) were identified in the Old 

Maid Flat 1 and 7 A (Priest and Vogt, 1982) drill holes 

(northwest side of Mount Hood). According to K-Ar dating, summarized in Priest and others ( 1982), and supplemented by data from Keith and others (1985), these 

smaller intrusive units may be correlative with the larger 

Laurel Hill and Still Creek intrusions; K-Ar dates all range 

from about 8.0±0.6 Ma to 11.6±1.2 Ma. Bikerman (1970) 

Figure 26. Photomicrograph showing a large, relatively saline (4.5 wt percent NaCl equivalent), primary(?), liquid-rich fluid inclusion 

from a hydrothermal quartz crystal that was deposited on a fracture in a Tertiary lava flow (specimen 79IC-20390) intruded by the 

pluton near Iron Creek. 
46 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

reported a K-Ar date of 5.0±1.0 Ma for an andesite dike 

that intrudes the Laurel Hill intrusion. Another dike, dated 

at about 5.26±0.44 Ma (Keith and others, 1985), occurs to 

the southeast of Mount Hood near Iron Creek. Geophysical evidence suggests the presence of an intrusive body in 

the area of Iron Creek (Goldstein and others, 1982). From 

these age data, there appear to have been at least two periods of intrusion in the Mount Hood area during late Miocene and early Pliocene time. 

During late cooling stages, both the intrusive rocks and 

the margins of the intruded volcanic rocks would be expected to form alteration minerals over a wide temperature 

range. Alteration minerals such as tourmaline, epidote, 

quartz, chlorite, magnetite, and pyrite, encountered in association with small intrusive bodies throughout the Westem Cascade Range of Oregon, were thought by Peck and 

others (1964) to have formed by contact metamorphism 

(thermal metamorphism of Liou and others (1987)). Peck 

and others (1964) also included these same minerals (except for tourmaline), in addition to chalcedony, carbonate 

minerals (calcite?), celadonite, and alkali-feldspar (adularia?), in their list of propylitic alteration (hydrothermal alteration) minerals found in Oregon's Western Cascade 

Range. A few outcrop and drill-hole cuttings samples included in our study consist of late Miocene to early PlioFigure 27. Photomicrograph showing a large, primary(?), vapor-rich fluid inclusion in hydrothermal quartz from a Tertiary 

volcanic breccia (specimen 79IC-2040) intruded by the pluton 

near Iron Creek. The melting-point temperature of this fluid inclusion is O.o·c and the homogenization temperature measurement is 

372.2• C; the fluid inclusion homogenized to the vapor state. 

cene intrusive rocks (Laurel Hill intrusion, Still Creek 

intrusion, and Iron Creek intrusion) and intruded late Tertiary lava flows and volcaniclastic debris units; these samples contain the same minerals as listed in Peck and others 

(1964) plus several others (wairakite, actinolite, prehnite, 

and adularia) that formed by hydrothermal processes near 

cooling intrusions. Fluid-inclusion studies of hydrothermal 

quartz crystals, from fractures associated with small late 

Tertiary intrusions, yielded T h values that mostly fell in 

the range of about 250° to 300°C (fig. 28 and table 17). 

The fluid-inclusion studies indicate that these quartz crystals and associated hydrothermal minerals formed at temperatures consistent with subgreenschist- to greenschistfacies metamorphism. In the Mount Hood area, such 

metamorphism was only identified near intrusions and was 

not found to be of regional extent. If hydrothermal alteration produced by subgreenschist- to greenschist-facies 

temperatures currently occurs at Mount Hood, it would 

probably only be found at considerable depth in association 

with cooling dikes, sills, or a larger solidified magma body. 

Sparse native copper and chrysocolla were located near 

Mount Hood (appendix B). Ore-bearing minerals including 

chalcopyrite, sphalerite, galena, anglesite, and cerussite 

occur in quartz and carbonate veins that are found in association with late Tertiary intrusions in the Mount Hood 

area (Keith and Causey, 1982). Similar silver- and goldbearing epithermal vein deposits have been reported in association with other small late Miocene intrusions of the 

Western Cascade Range (Callaghan and Buddington, 

1938; Peck and others, 1964). At all these locations sulfide 

minerals probably formed at greenschist to subgreenschist 

temperatures, but ·the carbonate and sulfate deposits, as 

well as copper and chrysocolla, were produced later at the 

lower temperatures associated with oxidized zones. 

The Old Maid Flat 7 A geothermal drill hole on the 

northwestern side of Mount Hood encountered Eocene(?) 

greenstone between depths of about 1,725 m and the bottom of the drill hole at 1,837 m (Priest and others, 1982). 

According to Holdaway and Bussey (1982), these rocks 

underwent greenschist metamorphism at temperatures of 

about 300°±50°C, which produced chlorite, calcite, albite, 

and quartz mineralization. Most drill holes included in our 

study are much shallower than the Old Maid Flat 7 A drill 

hole, and similar metavolcanic greenstone deposits were 

not encountered. 

ZEOLITE-FACIES METAMORPHISM 

Most drill holes and outcrop locations in this study contain one or more zeolite minerals. Studies of active geothermal areas, such as the low- to high-temperature 

Icelandic geothermal areas, indicate that zeolite minerals 

can form over a wide temperature range extending from 

very low temperatures ( -50°C ) to more than 200°C 
DISCUSSION 47 

(Kristmannsd6ttir and T6masson, 1978). On the basis of 

these Icelandic hydrothermal alteration mineral studies, 

Kristmannsd6ttir (1982) showed the temperature range for 

zeolite-facies metamorphism to be between about 50° and 

200°C. Comparing the zeolite minerals identified in the 

Mount Hood area with the zeolite zones of Kristmannsd6ttir and T6masson (1978) suggests that (l) chabazite probably crystallized at the lower end of the zeolite-facies 

temperature range; (2) epistilbite, heulandite, mordenite, 

scolecite, and stilbite, and possibly ferrierite, harmotome, 

and stellerite(?) could have formed at intermediate temperatures in the range of -70° to 150°C; and (3) laumontite 

and wairakite probably were stable at high zeolite-facies 

10 

79CCQ-0088F 

0 

10 

79CCQ-00880 

0 

CIJ 

1-

z 

w 10 

~ SC-170-3 w 

a: 

::> 

CIJ 0 

<C 

w 

~ 

LL 10 0 79WR-2010 a: 

w 

m 

~ 0 ::> 

z 

10 

791C-20390 

0 

10 

791C-2040 

0 

100 200 

temperatures to low subgreenschist-facies temperatures, although laumontite has been reported precipitating from 

43°C waters (McCulloh and others, 1981). 

Hydrothermal carbonate minerals (dolomite, calcite, 

manganoan calcite, rhodochrosite, and siderite), clay minerals ( smectite, illite, chlorite, mixed-layer illite-smectite, 

and mixed-layer chlorite-smectite), hematite, and titanite(?) 

are found in late Tertiary and Quaternary rocks from several drill holes and outcrops in the Mount Hood area. Some 

of these minerals, such as calcite and chlorite, can form 

over a wide temperature range extending through both zeolite-facies and greenschist-facies temperatures, whereas 

other minerals, such as smectite, dolomite, and the mixed300 400 

Figure 28. Histograms of fluid-inclusion homogenization temperatures of six hydrothermal quartz samples from drill-hole cuttings 

and outcrops of small plutons near Mount Hood. Homogenization temperature data for the predominantly liquid-rich fluid inclusions 

are plotted for 2°C intervals. The only vapor-rich fluid inclusions for which homogenization temperatures were obtained are the three 

highest values for specimen 79IC-2040. 
48 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

layer clay minerals, generally have been found to occur at 

temperatures consistent with zeolite-facies metamorphism 

in active geothermal areas (Aumento and Liguori, 1986; 

Hulen and Nielson, 1986). Many minerals listed here occur 

in association with early greenschist to subgreenschist hydrothermal minerals and probably formed as intrusive 

rocks cooled into the temperature range of zeolite-facies 

metamorphism. Hydrothermal minerals associated with 

late Tertiary zeolite-facies metamorphism are fairly widespread in the Mount Hood area, rather than being confined 

to the greenschist to subgreenschist metamorphic halos that 

surround the late Tertiary intrusions. 

QUATERNARY METAMORPHISM 

Mount Hood, an active volcano, formed over about the 

past 700,000 years (Priest, 1982). The volcano experienced 

its last significant activity about 200 to 300 years ago during the Old Maid eruptive period (Crandell, 1980). Quaternary lava flows and volcaniclastic units from outcrops and 

drill-hole samples surrounding Mount Hood contain only 

minor evidence for very low temperature Quaternary metamorphism. Quaternary lava flows penetrated by one drill 

hole on the south side of Mount Hood (table 1, No. 11) 

have traces of low-temperature hydrothermal minerals 

(chalcedony, calcite, halloysite, gypsum, and hematite) and 

gibbsite, which probably formed from weathering. Active 

fumaroles with temperatures as high as 91 °C lie on the 

400 

300 

-(.) 

~ 200 

.s:::. 

..... 

100 

0 

0.0 

• 

-1.0 

• I I 

I 

-2.0 

Tm (• C) 

- I 

-3.0 -4.0 

Figure 29. A plot of homogenization temperatures (Th) versus 

melting-point temperatures (T m) for fluid inclusions in hydrothermal quartz crystals from Tertiary rock outcrop samples near 

Mount Hood. 

south side of the volcano within a few hundred meters of 

the summit (Nehring and others, 1981). An aerial infrared 

survey (Friedman and others, 1982) mapped active fumarole fields at Mount Hood and estimated total heat discharge to be about 5 to 12 megawatts. Friedman and others 

( 1982) also concluded that the heat source is deep seated. 

White (1980) indicated that a still-hot volcanic conduit beneath Crater Rock could be the heat source for present-day, 

near-summit geothermal activity at Mount Hood. Other 

thermal features at Mount Hood include Swim Warm 

Springs, at the southern base of the mountain, and elevated 

temperatures (as high as ll3°C at the bottom of the Old 

Maid Flat No. 7A drill hole) (Holdaway and Bussey, 1982) 

in some geothermal test drill holes. 

Intense acid-sulfate leaching by present-day fumarolic 

activity readily alters volcanic rocks to a "sticky, blue or 

gray clay" (Cameron, 1988), usually consisting of kaolinite. 

Other secondary minerals from the fumarole area identified 

in our study include silica minerals (opal, ~-cristobalite, 

and chalcedony), native sulfur, sulfate minerals (gypsum, 

alunite, natroalunite, jarosite, and natrojarosite ), pyrite, iron 

oxides (amorphous iron oxide and hematite), and goethite. 

Many of these fumarolic mineral assemblages have been 

observed in glacial erratics on the upper slopes of Mount 

Hood, as well as in drill cuttings from several geothermal 

test drill holes located on the lower slopes and near the base 

of the mountain. Drill-cuttings samples from most of the 

studied drill holes contain volcaniclastic chips that are subrounded to rounded, are composed of a mixture of chips 

from different volcanic rock units, and contain one or more 

of the fumarolic minerals listed above. Undoubtedly, such 

hydrothermal alteration minerals were transported from the 

present fumarole field, or similar past sites of fumarolic 

activity, near the summit of Mount Hood. The transport 

mechanisms probably were quite variable and may have 

included streamflow, mudflows, lahars, avalanches, debris 

flows, or pyroclastic flows. Some recent Mount Hood lahar 

and debris-flow deposits have been studied by Gallina and 

Pierson (1984) and Cameron and Pringle (1986, 1987). Detailed mineralogical studies of the deposits were not reported; however, Frank (1983) completed a study of the 

hydrothermal mineralogy of similar deposits at Mount Baker, Washington. He concluded that a hydrothermal mineral 

suite, similar to the fumarolic minerals reported here for 

Mount Hood, occurs in five large mudflow deposits that 

originated near the summit fumarolic area of Mount Baker. 

Higher temperature minerals such as illite or mixed-layer 

illite-smectite are also associated with the fumarolic minerals both at Mount Baker (Frank, 1983) and at Mount Hood 

(this study). 

Wollenberg and others (1979) completed a geochemical 

study of water from Swim Warm Springs, which are the 

only thermal springs in the vicinity of Mount Hood. They 

concluded that the warm springs (temperatures measured 

at several orifices are between 19° and 26.5°C ) have been 
REFERENCES CITED 49 

considerably diluted by cold, near-surface runoff. Several 

chemical geothermometers suggest that the temperature of 

unmixed water at depth may range from 104° to 170°C, 

although temperatures above 200°C may also be possible 

(Wollenberg and others, 1979). According to Wollenberg 

and others' ( 1979) model, water from melted snow and 

glacial ice is heated near the summit by contact with the 

volcanic conduit or "neck" of the volcano. The heated water percolates down slope through highly permeable lava 

flows and volcaniclastic debris units in which the thermal 

water is continuously mixed with cold meteoric water. 

When the water reaches Swim Warm Springs at the base 

of the mountain, the dilution factor is quite high and temperatures are low. 

Holdaway and Bussey (1982) indicated that the only hydrothermal alteration in the Old Maid Flat 7 A drill hole, 

attributable to present-day conditons, occurs at subzeolitealteration temperatures. They list stilbite, heulandite(?), 

smectite, chlorite, calcite, anhydrite, and hematite as alteration minerals consistent with measured temperatures up 

to 113°C (bottom of drill hole 7 A). The McGee Creek 

drill hole (table 1, No. 3), on the lower northwest flank of 

Mount Hood, has a bottom-hole temperature of 60°C. The 

drill hole apparently penetrated a mantle of Quaternary 

volcaniclastic debris (Wise, 1969) and late Tertiary lava 

flows and volcaniclastic debris units (J.H. Robison, L.S. 

Forcella, and M.W. Gannett, written commun., 1981). Hydrothermal alteration, consisting predominantly of heulandite, calcite, smectite, and hematite, occurs throughout 

most of this 610-m-deep drill hole below 47 m depth. 

Temperature measurements near the bottom of the Pucci 

Chairlift and Old Maid Flat 1 drill holes (table 1, Nos. 13 

and 5) are 65.5°C and 82°C, respectively. Most hydrothermal mineralization in these drill holes appears to be associated with lava flows and volcaniclastic deposits that are 

interpreted as being late Tertiary in age. 

CONCLUSIONS 

Hydrothermal minerals identified from outcrop and 

drill-hole samples in the Mount Hood area were probably 

formed in the late Tertiary and are mostly associated with 

alteration halos surrounding several cooling intrusions. 

However, hydrothermal alteration, consistent with zeolitefacies temperatures, appears to have been of somewhat 

more regional extent during this period. Some low-temperature zeolite minerals (chabazite, scolecite, heulandite, stilbite, and epistilbite) (Kristmannsd6ttir and T6masson, 

1978) and smectite are present in drill holes that penetrate 

Neogene volcanic units and in outcrops of Neogene age. 

Except for slumped fumarolic minerals, hydrothermal minerals in Quaternary lava flows and volcaniclastic debris 

units of Mount Hood are very sparse. Present temperatures 

in the majority of drill holes are below -23°C, a temperature which probably is too low even for formation of lowtemperature hydrothermal minerals listed above. Only in 

drill holes 3, 5, and 13 (table 1) are the bottom-hole temperatures (60° to 82°C) possibly warm enough for the formation of some of these low-temperature hydrothermal 

minerals; however, even in these drill holes hydrothermal 

minerals are likely to have formed in the late Tertiary. 

On the basis of available data from geothermal exploration in the Mount Hood area, evidence for an exploitable 

(by current technology) geothermal system does not appear to be very great. Late Tertiary geothermal systems 

near Mount Hood apparently were of very limited extent 

and probably cooled markedly within a short distance 

from the heat source; conversely, late Tertiary lower temperature hydrothermal mineralization appears to have been 

widespread. The present-day situation may be similar in 

that the heat source for thermal activity in the Mount 

Hood area is not believed to be very extensive. 

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Nehring, N.L., Wollenberg, H.A., and Johnston, D.A., 1981, Gas 

analyses of fumaroles from Mt. Hood, Oregon: U.S. Geological Survey Open-File Report 81-236, 9 p. 

Onuki, Hitoshi, Shiba, Masatoshi, and Tazaki, Akira, 1988, Notes 

on petrology and rock-forming mineralogy (19) vein-forming Ca-zeolites from the Miocene metabasites at Okiura, 

Kuroishi City, Aomori Prefecture: Journal of Mineralogy, 

Petrology, and Economic Geology, v. 83, p. 311-317. 

Oregon Department of Geology and Mineral Industries, 1978, 

Geophysical logs, Old Maid Flat No. 1, Clackamas County, 

Oregon: Oregon Department of Geology and Mineral Industries Open-File Report 0-78-6, 6 logs. 

Park, C.F., Jr., and MacDiarmid, R.A., 1964, Ore deposits: San 

Francisco, W.H. Freeman & Company, 475 p. 

Passaglia, Elio, 1970, The crystal chemistry of chabazites: American Mineralogist, v. 55, p. 1278-1301. 

Passaglia, Elio, Galli, Ermanno, Leoni, Leonardo, and Rossi, 

Giuseppe, 1978, The crystal chemistry of stilbites and stellerites: Bulletin de Mineralogie, v. 101, p. 368-375. 

Passaglia, Elio, and Pongiluppi, Daria, 1974, Sodian stellerite 

from Capo Pula, Sardegna: Lithos, v. 7, p. 69-73. 

Peck, D.L., Griggs, A.B., Schlicker, H.G., Wells, F.G., and Dole, 

H.M., 1964, Geology of the central and northern parts of the 

Western Cascade Range in Oregon: U.S. Geological Survey 

Professional Paper 449, 56 p. 

Phillips, W.R., and Griffen, D.T., 1981, Optical mineralogy; the 

nonopaque minerals: San Francisco, W.H., Freeman and 

Company, 677 p. 

Priest, G .R., 1982, Overview of the geology and geothermal resources of the Mount Hood area, Oregon, in Priest, G .R., 

and Vogt, B.F., eds., Geology and geothermal resources of 

the Mount Hood area, Oregon: Oregon Department of Geology and Minerai Industries, Special Paper 14, p. 6-15. 

Priest, G.R., Beeson, M.H., Gannett, M.W., and Berri, D.A., 

1982, Geology, geochemistry, and geothermal resources of 

the Old Maid Flat area, Oregon, in Priest, G.R., and Vogt, 

B.F., eds., Geology and geothermal resources of the Mount 

Hood area, Oregon: Oregon Department of Geology and 

Mineral Industries, Special Paper 14, p. 16-30. 

Priest, G.R., and Vogt, B.F., eds., 1982, Geology and geothermal 

resources of the Mount Hood area, Oregon: Oregon Department 

of Geology and Mineral Industries Special Paper 14, 100 p. 

Reynolds, R.C., 1980, lnterstratified clay minerals, in Brindley, 

G.W., and Brown, G., eds., Crystal structures of clay minerals and their X-ray identification: London, Mineralogical Society Monograph no. 5, Chapter 4, p. 249-300. 

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from the Plateau des Coirons, Ardeche, France: Bulletin de 

Mineralogie, v. 111, p. 671-677. 

Robison, J.H., Porcella, L.S., and Gannett, M.W., 1981, Data 

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Geological Survey Open-File Report 81-1002, 24 p. 

Robison, J.H., Keith, T.E.C., Beeson, M.H., and Bargar, K.E., 

1982, Map showing geothermal investigations in the vicinity 

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---1984, Fluid inclusions, in Ribbe, P.H., ed., Reviews in 

Mineralogy, v. 12: Washington, D.C., Mineralogical Society 

of America, 644 p. 

Schiffman, Peter, Bird, D.K., and Elders, W.A., 1985, Hydrothermal mineralogy of calcareous sandstones from the Colorado 

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light of natural occurrences: Geological Society of Japan 

Journal, v. 78, p. 405-413. 

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G.R., and Vogt, B.F., eds., Geology and geothermal resources 

of the Mount Hood area, Oregon: Oregon Department of Geology and Mineral Industries Special Paper 14, p. 31-42. 

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Academic Press, v. 1, 460 p. 

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Association of Mineralogists, Petrologists, and Economic 

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areas: U.S. Geological Survey Bulletin 1119, 119 p. 

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Department of Geology and Mineral Industries Open-File 

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Oregon Department of Geology and Mineral Industries Special Paper 8, 26 p. 

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Survey Professional Paper 1456, 84 p. 
REFERENCES CITED 53 

Williams, D.L., Hull, D.A., Ackermann, H.D., and Beeson, M.H., 

1982, The Mt. Hood region; volcanic history, structure, and 

geothermal energy potential: Journal of Geophysical Research, v. 87, p. 2767-2781. 

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and Hood River Counties, Oregon: U.S. Geological Survey 

Miscellaneous Field Studies Map MF 1379-D, 2 sheets, 

scale 1 :62,500. 

Wise, W.S., 1968, Geology of the Mount Hood volcano, in Dole, 

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98. 

---1969, Geology and petrology of the Mt. Hood area; a 

study of High Cascade volcanism: Geological Society of 

America Bulletin, v. 80, p. 969-1006. 

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nos. 7 and 8, p. 1031-1083. 

Wollenberg, H.A., Bowen, R.E., Bowman, H.R., and Strisower, 

Beverly, 1979, Geochemical studies of rocks, water, and 

gases at Mt. Hood, Oregon: Lawrence Berkeley Laboratory 

Report LB7-7092, 57 p. 

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of yugawaralite-wairakite equilibrium: American Mineralogist, v. 67, p. 937-943. 

APPENDIXES A AND B 
56 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

APPENDIX A. LITHOLOGIC DESCRIPTIONS AND DISTRIBUTION OF 

HYDROTHERMAL MINERALS IN SELECTED MOUNT HOOD 

GEOTHERMAL DRILL HOLES. 

[Mineralogical descriptions and mineral identifications are based on binocular microscope observations, X-ray diffraction analyses (XRD), and 

petrographic microscope studies. The distribution of hydrothermal minerals with depth in the drill holes is shown by short horizontal line segments in the 

appendix A figures. USGS, U.S. Geological Survey; NWNG, Northwest Natural Gas Company; OOGAMI, Oregon Department of Geology and Mineral 

Industries; CIPW norms, classification of igneous rocks based on the normative mineralogy; drill hole numbers refer to numbers used in table 1] 

Lithologic symbols used in appendix A figures 

Volcaniclastic debris-may consist of alluvial, landslide, colluvial, fluvial, glacial, mudflow, lahar, or pyroclastic deposits 

Andesite and basalt lava flows 

Quartz diorite to quartz monzonite intrusive rocks 

1. ELIOT BRANCH 

The USGS Eliot Branch drill hole was spudded in an unconsolidated Quaternary alluvial fan at the Eliot Branch of the Middle Fork 

of the Hood River. Drill cuttings from the 220-m-deep drill hole are composed predominantly of angular to subrounded clastic (mudflow) deposits mapped as unit Qhc (postglacial debris, in part) by Wise ( 1969); four porphyritic hypersthene andesite flows occur in the 

lower half of the drill hole (Robison and others, 1981 ). 

Depth (m) 

0-125 

125-127 

127-143 

143-146 

146-149 

149-159 

159-199 

Description of drill cuttings 

Volcaniclastic debris. Largely mudflow deposits 

consisting of angular to subrounded grains of 

light- to dark-gray pyroxene or hornblende andesite; other primary minerals are plagioclase and 

magnetite. a-cristobalite, formed by devitrification of volcanic glass, is present in most (XRD) 

analyses . Many samples contain white-bleached 

grains or orange- to red-stained grains (amorphous 

iron oxide) that consist mostly of hydrothermal 

minerals: opal, chalcedony, sulfate minerals (alunite, natroalunite, jarosite, natrojarosite, or gypsum), kaolinite, smectite, and pyrite. 

Medium-gray, porphyritic, hypersthene andesite 

flow. 

Volcaniclastic debris consisting mostly of fine hypersthene andesite grains; the interval contains a 

few bleached or orange-stained grains. 

Medium-gray, porphyritic, hypersthene andesite 

flow. 

Volcaniclastic debris. A mixture of varicolored 

fine andesite grains, a few bleached or orangestained grains, and plagioclase crystals. 

Light-gray, porphyritic, hypersthene andesite flow. 

Volcaniclastic debris. Subangular to subrounded, 

hypersthene andesite grains. A few grains are or199-201 

201-220 

0 

tw 

w 

LL. 

~ 

:i 

1-

ll. 500 w 

0 

ange or yellow stained, but only plagioclase, acristobalite, and orthopyroxene were identified by 

XRD. 

Porphyritic, hypersthene andesite flow. 

Clastic debris. Subangular to subrounded, varicolored, hypersthene andesite grains with some yellowish-stained grains; XRD analysis of one of the 

stained samples indicates that opal, tridymite, and 

a-cristobalite are present. 

w w t-

> t- Ci5 z z 0 w 0 w ::J a: 0 0 w t- ...J w < w t- z w < t- ..., ~ x (J t= i 0 ii) 0 ~ w 0 ...J ...J 0 :::J a: 0 a: ... < < ~ 0 t- a: t- D.. if z: 

~ :r; < _, < < < >- > 0 0 en ::.::: < z ..., z (!) Q. ~ 

0 en 

a: 

w = =:: =- tw 

~ 

100 ~ - :i 

tll. 

w 200 0 

Stratigraphic representation of the Eliot Branch drill hole showing 

the distribution of hydrothermal minerals. 
APPENDIX A. LITHOLOGIC DESCRIPTIONS AND DISTRIBUTION OF MINERALS 57 

2. CLEAR BRANCH 

The 311-m-deep Clear Branch USGS drill hole was sited near the Clear Branch of the Middle Fork of the Hood River in unit Plv 

(lower Pliocene basalts and andesites) as mapped by Wise (1969); Keith and others (1982) remapped the unit as Tla (lower andesite 

flows (Miocene)). The Clear Branch drill cuttings mostly consist of volcaniclastic debris similar to the postglacial clastic debris units 

penetrated by the Eliot Branch drill hole. Interspersed with the clastic intervals are four hypersthene andesite flows (Robison and 

others, 1981). 

Depth (m) 

0-79 

79-105 

105-173 

173-177 

177-232 

232-235 

Description of drill cuttings 

Volcaniclastic debris. A mixture of angular to 

rounded, varicolored, hypersthene andesite 

grains; several grains are stained orange red. 

Hematite plagioclase, tridymite, a-cristobalite, 

orthopyroxene, and magnetite were identified by 

XRD. 

Medium-gray, hypersthene andesite flow. 

Volcaniclastic debris consisting of angular to 

rounded grains of lithic-crystal tuff, orangish siliceous (6-cristobalite) cemented material, or hypersthene andesite; many andesite grains are 

bleached white or stained yellow orange. XRD 

analyses of the bleached or stained grains indicate that they consist of plagioclase, a-cristobalite or tridymite, hypersthene or hornblende, 

hematite, smectite, natroalunite, and pyrite. The 

interval also has some gypsum needles (associated with pyrite), which may have formed after retrieval of the drill core; minor calcite may be due 

to contamination by drilling cement . 

Black, hypersthene andesite flow. Plagioclase, orthopyroxene, magnetite, and a-cristobalite were 

identified by XRD. 

Volcaniclastic debris. A mixture of angular to 

rounded, hypersthene andesite grains; many 

grains are orange stained and consist of plagioclase, a-cristobalite or tridymite, magnetite, 

smectite, natroalunite, mordenite, and pyrite. One 

sample also contains opal and calcite. 

Brownish, hypersthene andesite flow. Plagioclase, a-cristobalite, orthopyroxene, magnetite, 

and smectite were identified by XRD. 

235-241 

241-246 

246-311 

1-

w 

w 

u. 

z 

- 500 

x 

1-

a.. 

w 

c 

Volcaniclastic debris consisting of angular to 

rounded, hypersthene andesite grains; a few 

grains are orange stained. 

Greenish-gray, hypersthene andesite flow. Minerals identified by XRD are plagioclase, orthopyroxene, magnetite, and a-cristobalite. 

Volcaniclastic debris. A mixture of varicolored, 

angular to rounded, hypersthene andesite grains; 

yellow-brown, lithic-crystal tuff with fibrous 

mordenite alteration; and many grains of buffred- or green-colored silica (6-cristobalite) or silica vesicle fillings in andesite. Some grains have 

orange hematite staining. One sample has minor 

calcite, and XRD reflections for smectite occur in 

two analyzed samples. 

w 

1- w :::i 1-

< z IXl w :::> 1- w w 0 w z 1- ..J 

1- < ~ !::: en 1- w i= 0 :::> w 1-

..J a: 0 0 0 a: en 1- ~ 

< 0 ..J a: w 1- 0.. a: :2 0.. J < 0 ~ ~ > >- w 0 IXl 0 ~ cn z 0 a.. :I: 

en 

a: 

w 

1-

w 

~ 

~ 

200 x 

1-

a.. 

w 

c 

Stratigraphic representation of the Clear Branch drill hole 

showing the distribution of hydrothermal minerals. 

3. McGEE CREEK 

A 610-m-deep USGS drill hole at McGee Creek was begun in unit Qhc (postglacial debris, in part) as mapped by Wise (1969). The 

upper few tens of meters may consist of this Quaternary volcaniclastic unit; however, pervasive hydrothermal alteration throughout the 

remainder of the drill hole suggests that the rocks penetrated during drilling may consist of older Tertiary andesite lava flows, clastic 

debris, and mudflows (Robison and others, written commun., 1981; 1982). 

Depth (m) Description of drill cuttings 

0-47 Volcaniclastic debris. A mixture of angular to 

rounded, greenish, brownish, or light- to darkgray, pyroxene andesite; black basalt or basaltic 

andesite; and orange, iron-oxide-stained grains. 

Most grains consist of plagioclase, a-cristobalite, orthopyroxene, and magnetite. Some grains 

have vapor-phase, tridymite crystals. Hydrothermal alteration minerals identified by XRD are 

47-64 

smectite, chalcedony, hematite, calcite, and siderite; one sample has a single pyrite crystal. 

Mostly light-gray, pyroxene andesite grains with 

some reddish, iron oxide staining. Plagioclase, 

a-cristobalite, clinopyroxene, hornblende, and 

magnetite were identified by XRD. Some grains 

are rounded, and a few are slightly vesicular 

with either buff smectite or tiny, colorless, heulandite crystals lining cavities. Hydrothermal 
58 

64-76 

76-128 

128-140 

140-155 

155-274 

274-311 

311-341 

341-357 

HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

minerals identified by XRD are hematite, heulandite, smectite, and calcite. 

Reddish, andesitic, lithic tuff is composed of 

plagioclase, a-cristobalite, tridymite, clinopyroxene, hornblende, and magnetite. Alteration 

minerals, mostly from a few bleached or orangestained grains, include hematite, smectite, 

calcite, heulandite, alunite, dolomite, and manganoan calcite . A few grains, containing a black 

mineral which was identified by XRD as spinel 

with associated smectite, hematite, and corundum(?), might be xenolith fragments. 

Reddish-gray, slightly vesicular, porphyritic, pyroxene andesite flow with plagioclase, a-cristohalite, hornblende, and pyroxene plus abundant 

disseminated magnetite crystals (partly altered to 

hematite). A few grains have yellow-orange or 

bleached alteration material; other grains contain 

veins or cavity fillings of colorless to white colloform masses of rhombic carbonate minerals 

(dolomite, manganoan calcite, rhodochrosite, or 

calcite). Colorless, euhedral, heulandite crystals 

line cavities and veins. Most samples contain 

some smectite identified by XRD. 

Light-gray to greenish-gray, pyroxene andesite 

flow with minor hematite, smectite, calcite, and 

heulandite hydrothermal mineralization. 

Volcaniclastic debris consisting of a mixture of 

angular to rounded, light- to medium-gray, andesite grains. A few reddish grains appear to be 

sedimentary mudstone or sandstone. Hydrothermal alteration minerals are calcite, heulandite, 

hematite, and smectite. 

Light- to medium-gray, hornblende and pyroxene andesite grains from several flows. Most 

grains are angular to subrounded, but several 

samples contain rounded or even well-rounded 

grains; the interval may also contain volcaniclastic debris units. Hydrothermal minerals include 

smectite, heulandite, calcite (in order of paragenesis), and chalcedony (in one sample). One grain 

has black spinel crystals and is probably from a 

xenolith. In tables 1 and 2, specimen MGCR600, analysis 36 is a whole-rock chemical analysis of hornblende andesite from a depth of about 

182.9 m. 

Reddish, andesitic tuff altered to red clay. Alteration minerals are smectite, calcite, hematite, and 

heulandite. 

Volcaniclastic debris. A mixture of hypersthene 

andesite grains composed of plagioclase, a-cristobalite, orthopyroxene and magnetite. Hydrothermal minerals are smectite, calcite, heulandite, 

and chalcedony. 

Brownish, pyroxene andesite flow. Hydrothermal 

alteration minerals are calcite, heulandite, smecite, hematite, and chalcedony. 

357-375 

375-378 

378-398 

398-427 

427-610 

w 

1-

(j) 

0 

a: 

J: 

(.) 

0 

0 

0 

J: 

a: 

0 p"15<i 

-~~~ 

- ~i 

- ~ 

- 500 -

- 1- - w 

w - u. - ""~'.!:;~ z 

- 1000 -'o'a• 

::C ~~:." -r.·ac 1-

Q. 

w -;@ 0 -~JV;-; 

- (J"G• 

1500 -w( f\".( 

-

-~!f 

-.- - :jf.i 

2000 etc 

Volcaniclastic debris consisting of a mixture of 

pyroxene andesite grains. Hydrothermal alteration minerals are hematite, smectite, calcite, 

heulandite, and chalcedony. At about 360 m 

depth, one grain contained spinel that is probably from a xenolith. 

Dark greenish-gray, pyroxene andesite flow. Hydrothermal minerals are heulandite, calcite, and 

smectite. 

Volcaniclastic debris(?). Fine-grained cuttings 

consist mostly of plagioclase crystals. Hydrothermal alteration minerals are calcite, heulandite, and smectite. 

Reddish-gray, hornblende andesite flow composed of plagioclase and hornblende phenocrysts 

(to 1.5 mm) in a matrix of tiny hornblende and 

magnetite crystals. Hydrothermal alteration minerals are calcite, heulandite, smectite, magnetite(?), hematite, and pyrite. 

Volcaniclastic debris(?) consisting mostly of a 

mixture of varicolored, andesite grains. A few of 

the fine-grained samples appear to be composed 

primarily of a single color of andesite grains 

(thin flows?) mixed with sloughed(?) andesite 

grains from higher in the drill hole. Hydrothermal minerals are calcite, pyrite, chalcedony, heulandite, smectite, and hematite. 

w 

1-

5 

_. 

c( 

(.) 

>- z z w 0 w c( 1-

0 0 i5 w w w 1- z w w w 1- 1- 1-

~ c( 1- 1- z z ~ w w ~ 1- (.) iE c( 1- w _. 0 (!' 5 _. :::i (.) z 1- c( z -< _. z w _. ::> 0 w ::> a: :::i: a: J: 0 -< 0 -< w -< :::i: _. >- w c( 

(.) 0 :::i: (j) (.) J: :.:: CI'J -< Q. J: (!' 

0 

- t-

-- ~ -· 1- iii • -

I - • -

I tt-- 200 

t- CI'J 

a: ~ w 

1-

t- w 

• • :::i: ~ ~ 

• II -- ~ I 

1-

t- Q. 

w 

=· t-- 400 0 

t-

• 1-

• ~ 

I ~ 

• 

- ;;;;;;; ~ 600 

Stratigraphic representation of the McGee Creek drill hole showing the distribution of hydrothermal minerals. 
APPENDIX A. LITHOLOGIC DESCRIPTIONS AND DISTRIBUTION OF MINERALS 59 

4. LAST CHANCE MOUNTAIN 

The 152-m-deep Last Chance Mountain drill hole, completed by NWNG, was sited in unit Plv (lower Pliocene basalts and andesites) 

of Wise (1969), which Keith and others (1982) remapped as unit Tla (lower andesite flows (Miocene)). The upper 58 m of drill cuttings 

appears to consist of this Neogene, pyroxene andesite flow. Below 58 m depth, the drill cuttings are composed of volcaniclastic debris 

layers and andesitic lava flows correlated with the late Miocene Rhododendron Formation (Priest and others, 1982; Trimble, 1963). 

Depth (m) 

0-58 

58-85 

85-88 

88-94 

94-98 

98-113 

113-125 

Description of drill cuttings 

Brownish-gray, hypersthene andesite flow composed of plagioclase, a-cristobalite, hypersthene, 

and magnetite. Hydrothermal vein or fracture fillings consist of chalcedony, 6-cristobalite, quartz, 

smectite, calcite, pyrite, amorphous iron oxide, and 

a zeolite mineral (stilbite?); one sample has a little 

gypsum associated with pyrite. Many samples contain caramel-colored grains of clay (sepiolite(?) + 

smectite) that may be contamination due to drilling. 

Between about 30 and 40 m depth, the drill cuttings 

contain a few grains of quartz diorite contsisting of 

plagioclase and quartz plus hydrothermal minerals: 

chlorite, actinolite(?), pyrite, and epidote. 

Volcaniclastic debris. A mixture of angular to subrounded grains of light- to dark-gray, pyroxene 

andesite and varicolored, lithic tuff. Hydrothermal 

minerals are 6-cristobalite, chalcedony, quartz, 

stilbite(?), calcite, smectite, chlorite, mixed-layer 

illite-smectite, hematite, pyrite, and gypsum. The 

cuttings contain some caramel-colored sepiolite 

(drilling contamination?). 

Light-reddish-gray-green, lithic tuff. Alteration 

minerals are hematite, smectite, chlorite, and 

mixed-layer illite-smectite. 

Dark-gray to black, amygdaloidal, andesite or basalt(?) flow with amygdule fillings of green clay 

(smectite and chlorite by XRD), and later calcite 

or chalcedony. 

White to light-gray, bedded(?), lithic tuff. Hydrothermal minerals are pyrite, calcite, and green 

clay (sloughed from above ?). 

Dark-gray to black, porphyritic, hypersthene andesite or basalt(?) flow; some smectite was identified by XRD in representative whole rock 

samples. Hydrothermal alteration minerals, calcite, pyrite, 6-cristobalite, smectite, and mixedlayer illite-smectite, are mostly associated with a 

few bleached (erratic?) grains. 

Volcaniclastic debris. A mixture of angular to 

rounded, dark gray to black, hypersthene andesite 

or basalt(?); black, vesicular basalt(?); and green125-128 

128-134 

134-137 

137-140 

140-149 

149-152 

w 

1-

:::i 

< 

m 

0 

1-

en 

~ 

0 

I 

m 

ish, bluish, or reddish tuffaceous grains. Alteration minerals are calcite, hematite, 6-cristobalite, 

smectite, quartz, and chlorite. 

Green, crystal to lithic tuff. Hydrothermal minerals 

are calcite, hematite, smectite, and euhedral to subhedral, quartz crystals ( -1 mm) and crystal clusters. 

Dark-brownish-gray to black, porphyritic, hypersthene andesite flow. Alteration minerals are 

smectite, calcite, and quartz. 

Reddish, lithic-tuff breccia Hydrothermal minerals include calcite, quartz, hematite, and smectite. 

Volcaniclastic debris. A mixture of angular to 

subrounded grains of reddish, tuff breccia; bluishgreen and white lithic tuff; and varicolored, mostly altered, andesite grains. Alteration minerals are 

calcite, quartz, white colloform 6-cristobalite, 

smectite, and caramel-colored sepiolite grains 

(drilling contamination?). 

Dark-brownish-gray, porphyritic, hypersthene andesite flow with fractures filled by hydrothermal 

minerals: calcite, smectite, chalcedony, quartz, 

pyrite, and illite. 

Volcaniclastic debris(?). A mixture of mostly finegrained drill cuttings (light-bluish-greenish tuff 

grains predominate in one sample). Hydrothermal 

minerals consist of abundant pyrite and some calcite, quartz, chlorite, smectite, and illite. 

w 

1-

>- i= z 0 w 0 w w :::!: w w c c 

w x N w w 1- en 1- 1- :::!: w 0 1- 1- 1- i= I ~ :::i :::::1 w 0 1-

_I a: 0 iii 0 w w 0 0 en 1- 0 

< < _I _I w 1- 1- _I a: Q. ~ z c :X: :::::1 < i= :::!: :::i :::i :X: w >- >- 0 a: 0 0 0 en en ::::! ::::! 0 en <!' Q. ff w 

Stratigraphic representation of the Last Chance Mountain drill 

hole showing the distribution of hydrothermal minerals. 

5. OLD MAID FLAT (OMF-1) 

A detailed stratigraphic log of the Old Maid Flat 1 drill hole is given in Priest and Vogt (1982). The drill hole penetrated a Holocene 

mudflow deposit, lava flows and volcaniclastic debris deposits of the Rhododendron Formation, hornblende quartz microdiorite, and 

basalt flows from several units of the Columbia River Basalt Group. Major-element analyses, CIPW norms, modal analyses, and traceelement analyses of drill cuttings and core from this drill hole are given in Priest and Vogt (1982). Priest and others (1982) compared 

this drill hole with the nearby Old Maid Flat 7 A drill hole. Geophysical logs for the Old Maid Flat 1 drill hole were published by the 

Oregon Department of Geology and Mineral Industries (1978). This report lists the secondary- alteration minerals identified in the Old 

Maid Flat 1 drill hole; however, a stratigraphic description of the drill cuttings is not reproduced. 
60 HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

>- w g w 

w 1- 1- ... 0 i= w ~ ~ w w 

w N w N z z w 1- ... 1-

(.) ... 1- < ~ 0 .... i= a: w i= % z 

....1 ....1 a: 0 m ....1 ~ m (.) w 0 1- < 1- :t 

< < < ....1 < :::> :::> ....1 w 1- ....1 a: ~ w w 

Q.. :t :::> < ::c w < i= ~ ~ :t > w 0 a: 0 (.) 0 (.) (.) :t ....1 (/) (/) :::! (.) Q. :t <!) Q. 

0 0 

200 

1000 

400 

(/) 

1- a: 

w w 

w 1-

u.. w 

~ 600 ~ 

2000 z :i 

1- :i a.. 

w 1-

a.. c w 

c 

BOO 

3000 

1000 

1200 

4000 

Stratigraphic representation of the Old Maid Flat drill hole showing the distribution of hydrothermal minerals. 
APPENDIX A. LITIIOLOGIC DESCRIPTIONS AND DISTRIBUTION OF MINERALS 61 

6. CLEAR FORK (OMF-3) 

A stratigraphic section of the 402-m-deep NWNG Clear Fork drill hole was given in Covert and Meyer (1979). The upper 37m of 

drill cuttings is described as mudftow debris. Between 37 and 317 m, the drill hole penetrated mostly tuffaceQUS materials of the 

Rhododendron Formation. Below 317m depth, the drill cuttings consist of fine grains that, according to Covert and Meyer (1979), are 

from basalt flows of the Columbia River Basalt Group and interlayered andesitic volcanic rocks. 

Depth (m) 

0-37 

37-317 

317-402 

Description of drill cuttings 

Volcaniclastic debris. A mixture of angular to 

well-rounded, varicolored andesite grains. A few 

grains of quartz, chalcedony, and soft white zeolite (laumontite plus stilbite by XRD) are included with the mudftow debris. 

Rhododendron Formation. Greenish-, grayish-, or 

brownish-colored crystal-lithic tuff. Crystals consist of plagioclase, quartz, magnetite, and occasionally pyroxene or hornblende. Some samples 

near the top of the interval contain a few andesite 

grains; below 232 m depth, most samples are a 

mixture of tuffaceous grains and angular to 

rounded andesite grains. Alteration minerals consist mostly of smectite and zeolite minerals (sillbite, laumontite, and minor mordenite ), along 

with some chalcedony, chlorite, magnetite, hematite, and pyrite, and minor quartz and chrysocolla. 

Columbia River Basalt Group. Samples are 

mostly fine-grained material and appear to consist of basalt grains, although Covert and Meyer 

(1979) reported andesite intervals. Hydrothermal 

quartz, chalcedony, stilbite and calcite grains are 

1-

w 

w 500 u. 

~ 

:i 

1-

Q. 

w 

c 

1000 

fairly abundant. Most XRD analyses contain 

smectite and minor laumontite, chlorite, siderite, 

hematite, and ceiadonite(?). 

>-

z w w w 0 1- 1-

i= 1- c z w w z w w N z 0 1- 1- w 1- 0 1- a: 0 c ....J a: u ::!: c iii < < < ....J w ::I a: ....J ....J 

:I: ::I < c < 0 i= w 0 0 0 en ....J ::!: en 0 

- -~ ~ -- 5ii 

-=iiiiiiiili 

~~:= 

w 

1-

i= 

0 

w 

::!: 

en 

I 

w w w 1- 1- 1-

i= a: a: 0 0 0 w ....J ....J 

::!: :I: :I: en 0 0 

w 

1- w i= 1- < a: ::!: >- w Q. :I: 

< ....J 

....J 

0 

0 

0 

en 

>-

a: 

:I: 

0 

en 

a: 

w 

1-

w 

::!: 

200 ~ 

:i 

1-

Q. 

w 

c 

,_~---=~L-----~=-~~----------;- 400 

Stratigraphic representation of the Clear Fork drill hole showing 

the distribution of hydrothermal minerals. 

7. LOST CREFK (OMF-10) 

The 131-m-deep NWNG Lost Creek drill hole penetrated unit Qhd (Quaternary reworked debris consisting of postglacial detrituspartly morainal and partly water transported) mapped by Wise (1968) and unit Qhc (postglacial debris, in part) mapped by Wise (1969). 

From 0 to 73 m, the drill cuttings are composed of this volcaniclastic debris. Below 73 m the drill cuttings consist of altered hornblende andesites(?) that may be correlative with the Rhododendron Formation. 

w 

1-

w i= 

0 1- w w 

> i= ::s 1-

z w w 0 rn z 

0 w 1- 1- w w I ::> 

0 1- m i= !:: w ~ w w ..J w w 

w w N z w 1- rn 1- 1- c( 1- w 1- a: 

0 1- c( ..J 0 1- z i= I a: a: 0 w i= 1- z w 

..J ..J 0 m i= ~ m :::::i 0 w 0 0 a: 1- c( 0 :t: 'l. 

c( c( ..J c( C/) ::> ..J 0 w 1- ..J ..J 1- a: ~ 0 w 'l. 

'l. :t: c( J: a: c( i= c( ~ :::::i :t: J: c( > w a: a: 0 0 0 0 0 w ..J rn ~ C/) = 0 0 z 'l. J: w 'l. 0 

1- 0 0 rn 

w ~tV. a: 

w -

l~~ - ~ w 

lL 

= - 1-

~ - ~ w 

= -- - !! • :I 

.z - ~ - ; - 1- == - 100 ~ ~ iii - 'l. 400 - !&! = iiiii - - - - w x 

0 .... 

a. 

w 

0 

Stratigraphic representation of the Lost Creek drill hole showing the distribution of hydrothermal minerals. 
62 

Depth (m) 

0-73 

HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Description of drill cuttings 

Volcaniclastic debris. A mixture of varicolored, 

angular to well-rounded, pyroxene andesite 

grains consisting of plagioclase, a-cristobalite, 

magnetite, clinopyroxene, orthopyroxene, and 

amphibole. Many samples have orange-to redstained or white-bleached grains that consist of 

hydrothermal chalcedony, smectite, natroalunite, 

kaolinite, opal, mixed-layer illite-smectite, and 

mixed-layer chlorite-smectite; minor epidote, 

chlorite, calcite, laumontite, pyrite, and hematite 

are also present. 

73-131 Light-gray, altered, hornblende andesite(?) containing plagioclase ( -1 mm) and hornblende 

(up to 2 mm) phenocrysts in a groundmass consisting of magnetite, plagioclase, hornblende, 

and chalcedony (discernable only in XRD). Minor clinopyroxene was detected in a few XRD 

analyses. Hydrothermal minerals include abundant mixed-layer chlorite-smectite, hematite, 

and zeolite minerals (laumontite, chabazite, 

stilbite, and epistilbite), as well as minor 

epidote, smectite, calcite, prehnite, and native 

copper. 

8. ZIGZAG RIVER 

(KIWANIS CAMP ROAD) 

Covert and Meyer (1979) showed a stratigraphic section of the 287-m-deep NWNG Zigzag River drill hole. The upper 12 m 

consists of a Quaternary Mount Hood andesite flow. Below 12 m depth, the drill hole penetrated quartz diorite of the Laurel Hill 

intrusion. 

Depth (m) 

0-12 

12-287 

1-

w 

w 

u.. 

z 

:I:-

1-

a.. 

w 

c 

Description of drill cuttings 

Medium- to dark-gray, hypersthene andesite flow 

consisting of plagioclase, orthopyroxene, and 

magnetite. 

Light- to medium-gray, porphyritic, hornblende, 

quartz diorite with plagioclase (up to 6 mm) and 

hornblende phenocrysts. Space between phenow w w 

w w 1- 1- 1-

1- 1- iii 0 i= 

N w :;;;:: N z 

1- 1- < ...J z 0 0 < i= < a: a: co ...J ::E 

< ...J < < en :::,) :::,) :::,) c( :I: a: w < 0 (.) ~ (.) w :I: ...J 

0 e;;;;;;: 

-

-

- - -

- 500 === 

-=== 

= - = - = - -

- -

w 

1-

0 

w 

...J 

0 

(.) 

en 

w 

1-

a: w 

1-

w i= ...J w ...J (.) 

w w 1-

1- ~ ::::i 

en en :::! 

-

- -

-

-

- -

crysts is filled by quartz and magnetite. Hydrothermal minerals are predominantly quartz, 

altered hornblende (chlorite and actinolite), epidote, magnetite, pyrite, and zeolite minerals (laumontite and stellerite(?) along with minor 

epistilbite, heulandite, scolecite, chabazite, and 

wairakite); some chalcopyrite, calcite, smectite, 

and illite(?) are also present. 

w 

t: 

a: w 

>- w 

w 1-

a.. 1-

t: 0 i= w ...J 

a: (.) w w 1- 0 

t: z 0 z 0 ...J 

...J < a: (!' c i= 

:I: :I: >- c( a: (.) 

(.) (.) a.. ::?! w c( 

0 = - = 

- - - en 

a: === - w = 1-

w == = ~ 

~ 

= ::t = 200 1- ;;;;;;;;;: a.. 

= w 

c = === === 

Stratigraphic representation of the Zigzag River drill hole showing the distribution of hydrothermal minerals. 
APPENDIX A. LITHOLOGIC DESCRIPTIONS AND DISTRIBUTION OF MINERALS 63 

9. LAUREL HILL 

The DOGAMI Laurel Hill drill hole encountered quartz diorite of the Laurel Hill intrusion throughout its 125m depth. 

Depth (m) 

0-125 

Description of drill cuttings 

Light- to dark-gray, hornblende, quartz diorite 

containing hornblende (up to 3 mm) and plagioclase (up to 2 mm) phenocrysts in a matrix of 

plagioclase crystals and very minute quartz 

grains. Hornblende alters to chlorite or is replaced by actinolite; epidote, magnetite, zeolite 

minerals (stilbite, chabazite, epistilbite, and wairakite ), and minor chalcopyrite occur as fracture 

fillings. 

Stratigraphic representation of the Laurel Hill drill hole showing 

the distribution of hydrothermal minerals. 

10. STILL CREEK 

The 152-m-deep DOGAMI Still Creek drill hole was spudded in a Quaternary stream deposit. This deposit comprises the upper 18 

m of drill cuttings; below 18 m depth, drill cuttings are a light- to dark-gray, hornblende, quartz microdiorite of the Still Creek 

intrusion. 

Depth (m) 

0-18 

Iw 

w 

u. 

z 

:I: 

Ia.. 

w 

0 

Description of drill cuttings 

Stream deposit consisting of a mixture of angular 

to rounded andesite and quartz microdiorite 

grains. Hydrothermal minerals include epidote, 

chlorite, magnetite, quartz, chalcedony, and 

smectite. 

>- w z w 

0 w w I- I0 t: I- 0 t= 

w N w N z 

I- ~ z 

(.) I- <C <C <C 0 

....1 a: 0 a: m ....I ::E 

<C <C ....I < <C ::::> ::::> :I: ::::> <C J: w <C 

(.) 0 (.) ~ (.) J: ....I 

0 ~:'2~ 

, '< == - ..:;..,.1\1-.L 

H ;;;;;;;;;;; 

- - -

- - t-1"" "'- -

- "v-:_"',.~~r.,.~o 

500 

..., 

,,, 

~..:. '-

-

18-152 

w 

I- w 

0 w II- w i= 

....1 iii (.) w 

0 ....1 w ..... (.) i= ~ :::i 

rn rn rn ::! 

-

-

-

-

- -

Light- to dark-gray, hornblende, quartz microdiorite. Hornblende is mostly altered to chlorite or is 

replaced by actinolite. Other alteration minerals 

are epidote and magnetite veins, quartz, zeolite 

minerals (chabazite, heulandite, laumontite, scolecite, stilbite and(or) stellerite(?), and wairakite), 

calcite, and clay minerals (smectite and illite). 

w w 

w I- II- :J i= w 

a: w I- 0 

0 z 0 z 

....I (!) 0 i= 

J: <C a: (.) 

(.) ~ w < 

0 rn = a: 

'" w = 1- ..... - = w - '" ::?! - -

= ~ 100 ~ 

- 1- :i - I- Ia.. 

w 

0 

Stratigraphic representation of the Still Creek drill hole showing the distribution of hydrothermal minerals. 

11. SNOW BUNNY 

DOGAMI's 82.5-m-deep Snow Bunny drill hole was one of the first geothermal drill holes completed in the Mount Hood area. This 

drill hole was sited in a Quaternary andesite flow (Wise, 1968, 1969). The seven samples of drill cuttings recovered from about 12-m 

intervals appear to be from three different andesite flows. 
64 

Depth (m) 

12.2 

18.9-61.0 

73.2 

HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Description of drill cuttings 

Reddish, slightly vesicular, porphyritic, pyroxene 

andesite flow consisting of colorless plagioclase, 

and brown hypersthene; a-cristobalite, tridymite, 

clinopyroxene, alkali-feldspar(?), and magnetite 

were also identified by XRD. The presence of 

alteration minerals hematite and gibbsite is probably due to weathering. 

Light-gray, porphyritic, pyroxene andesite flow. 

Phenocrysts are colorless plagioclase, brown hypersthene, and minor light-green olivine; a-cristobalite, tridymite, clinopyroxene, magnetite, and 

alkali-feldspar(?) were identified by XRD. Hydrothermal minerals are hematite(?), chalcedony, 

and halloysite. 

Black, vesicular, pyroxene andesite flow with 

colorless plagioclase and brown hypersthene 

phenocrysts; magnetite, a-cristobalite, and tridy1-

w 0 

w 

LL. 

~ 200 

::z::· 

1-

Q. 

w 

0 

mite were identified by XRD. The only alteration 

mineral is hematite. 

>-

z 

0 

0 w 

w 1-

~ C3 

< ....I 

::I: < 

(.) (.) 

w 

1-

c;; 

>-

0 

....I 

....I 

< ::I: 

0 ~ w 

1-

w 

~ 

z 

:i 

1-

Q. 

w 

0 

Stratigraphic representation of the Snow Bunny drill hole 

showing the distribution of hydrothermal minerals. 

12. MOUNT HOOD MEADOWS 

The 355-m-deep USGS Mount Hood Meadows drill hole was spudded in Quaternary andesite (Robison and others, 1982). The drill 

hole encountered numerous intervals of epiclastic debris units interlayered with thin andesitic or basaltic lava flows (Robison and 

others, 1981). The drill cuttings are nearly devoid of hydrothermal minerals. 

Depth (m) 

0-17 

17-24 

24-30 

30-45 

45-53 

53-64 

64-72 

72-82 

Description of drill cuttings 

Volcaniclastic debris. A mixture of varicolored, 

angular to rounded, pyroxene andesite grains 

consisting of plagioclase, orthopyroxene, magnetite, and a-cristobalite. A few orange-stained, 

clayey grains contain hematite and illite. 

Medium-gray, porphyritic, pyroxene andesite 

flow composed of plagioclase, clinopyroxene, 

magnetite, and a-cristobalite. 

Volcaniclastic debris. A mixture of angular to 

rounded, varicolored, andesite grains with some 

reddish iron oxide staining. 

Medium-gray, hypersthene andesite flow with 

reddish oxidation. Plagioclase, a-cristobalite, orthopyroxene, magnetite, and hematite were identified by XRD. 

Volcaniclastic debris. A mixture of angular to 

rounded, light- to medium-gray andesite grains 

with some reddish oxidation. 

Medium- to dark-gray, hypersthene andesite flow. 

Volcaniclastic debris. A mixture of angular to 

rounded, light- to dark-gray, pyroxene andesite 

grains. 

Light-gray, porphyritic, hypersthene, andesite 

flow. 

82-88 

88-195 

195-204 

204-234 

234-249 

249-256 

256-263 

263-269 

269-290 

290-296 

Black, basalt or basaltic andesite flow. 

Volcaniclastic debris. A mixture of angular to 

rounded, varicolored, andesite grains with a few 

bleached or orange-stained grains that consist of 

plagioclase, a-cristobalite, tridymite, illite, kaolinite, opal, and hematite by XRD. One grain of 

calcite. 

Dark-gray, basaltic andesite(?) flow. 

Volcaniclastic debris. A mixture of varicolored, 

andesite grains with a few bleached or orangestained grains composed of plagioclase, acristobalite, orthopyroxene, magnetite, smectite, 

kaolinite, hematite, and alunite by XRD. 

Medium-gray, hypersthene andesite flow. 

Volcaniclastic debris. A mixture of medium- and 

dark-gray, andesite grains with some reddish iron 

oxide staining. 

Dark-gray to black basalt flow. 

Volcaniclastic debris. Mixture of dark- to medium-gray, andesite grains; many grains have reddish-hematite staining. 

Dark-gray, hypersthene andesite flow. 

Volcaniclastic debris. A mixture of varicolored, 

andesite grains. Plagioclase, orthopyroxene(?), 

a-cristobalite, magnetite, and hematite by XRD. 
296-305 

305-335 

335-347 

347-355 

APPENDIX A. LITHOLOGIC DESCRIPTIONS AND DISTRffiUTION OF MINERALS 

Light- to medium-gray, hypersthene andesite 

flow. XRD analysis shows plagioclase, a-cristohalite, orthopyroxene, magnetite, and hematite. 

Volcaniclastic debris. A mixture of angular to 

rounded, medium-gray to black andesite grains 

plus a few bleached and orange'-stained grains. 

XRD analysis of orange-stained grains shows 

plagioclase, a-cristobalite, orthopyroxene, magnetite, hematite, illite, and smectite. 

Dark-gray, porphyritic, basaltic andesite(?) flow. 

XRD analysis shows plagioclase, a-cristobalite, 

orthopyroxene, and magnetite. 

Volcaniclastic debris. A mixture of varicolored, 

andesite grains plus yellow-orange smectite 

grains. Some reddish oxidation due to hematite. 

w w w w 1- 1- w 1- 1- z i= 1- i= ..J C3 ::::i 0 w z < < ..J 0 w 1- ::::::> ~ a.. < < ~ ::::i ..J w 0 0 ~ (/) :::! < J: 

0 0 

1- (/) 

w a: w w LL 500 ~ 

1-

w 

~ 

:i 

1-

a.. 

w 

c 

200 ~ 

:i 

1-

a.. 

w 

c 1000 

Stratigraphic representation of the Mount Hood Meadows drill hole showing the distribution of hydrothermal 

minerals. 

13. PUCCI CHAIRLIFT 

65 

The 1 ,220-m-deep USGS Pucci Chairlift drill hole was sited near the base of the Pucci ski chairlift in unit Qhd (Quaternary clastic 

debris consisting of reworked detritus) mapped by Wise (1968) and unit Qhc (Quaternary clastic debris, largely pyroclastic) mapped by 

Wise (1969). The upper 737 m of the drill hole contains several volcaniclastic intervals that are interbedded with 19 grayish hypersthene andesite flows. Below 737 m, Robison and others (written commun. 1981) indicated that the highly altered pyroxene andesite 

flows and interbedded volcaniclastic units are probably of Tertiary age. The description below is from Gannett and Bargar (1981). Gaps 

in the section (for example, 103.6-109.7 m) are due to lack of sample recovery. 

Depth (m) 

0-67.1 

67.1-79.2 

79.2-85.3 

97.5-103.6 

Description of drill cuttings 

Colluvium and other volcanic debris. A mixture of subangular to rounded, chiefly medium-gray and reddish-brown, pyroxene 

andesite grains. Phenocrysts are colorless 

plagioclase and brown orthopyroxene. Considerable magnetite and a-cristobalite, and 

traces of clinopyroxene and amphibole were 

identified by whole-rock XRD; vesicular oxidized clasts also contain hematite. Tridymite, 

alkali-feldspar(?), and opal were detected in 

XRD analyses of a few orange-stained and 

white-bleached pyroxene andesite grains. 

Gypsum was identified by SEM. 

Medium-gray, pyroxene andesite. Phenocrysts consist of 5 percent colorless plagioclase ( 1 to 2 mm) and 1 to 2percent clear 

brown orthopyroxene (up to 3 mm). a-cristohalite, tridymite(?), quartz, magnetite, and alkali-feldspar(?) were identified by XRD. 

Light-gray, slightly vesicular, pyroxene 

andesite. Contains up to 10 percent colorless 

plagioclase (3 mm) and 2 to 3 percent honeybrown orthopyroxene (1 mm). a-cristobalite, 

magnetite, alkali-feldspar(?), hematite, and 

tridymite were identified by XRD. 

Volcaniclastic debris. A mixture of subrounded to subangular, andesitic lavas; mostly 

109.7-125 

125-128.6 

134.7-140.8 

140.8-146.9 

light-gray, pyroxene andesite which has 5 to 

10 percent clear plagioclase and slightly altered pyroxene phenocrysts. Plagioclase, orthopyroxene, clinopyroxene, amphibole(?), 

tridymite, a-cristobalite(?), magnetite, and 

hematite were identified by XRD. 

Light-gray, microvesicular, pyroxene andesite. Has 8 to I 0 percent colorless plagioclase (3 mm) and 3 percent dark pyroxene (3 

to 4 mm) (clinopyroxene and orthopyroxene 

by XRD) phenocrysts. The groundmass contains disseminated magnetite; alkali-feldspar(?), a-cristobalite, and tridymite were 

identified by XRD. 

Medium-gray, highly porphyritic, pyroxene 

andesite. Phenocrysts are 15 to 20 percent colorless, often anhedral, plagioclase and 3 to 5 

percent pyroxene (both clinopyroxene and orthopyroxene by XRD) (I to 2 mm). Hematite, 

magnetite, tridymite, a-cristobalite, and alkali-feldspar(?) were also identified by XRD. 

Volcaniclastic debris. Mostly subrounded, 

pyroxene andesites. XRD analysis of one 

light-gray, andesite grain showed reflections 

for plagioclase, a-cristobalite, magnetite, amphibole, clinopyroxene, and orthopyroxene. 

Dark-reddish-gray to red-black, glassy, vesicular, pyroxene andesite. Phenocrysts are 5 
66 

146.9-165.2 

165.2-172.8 

184.4-202.7 

202.7-216.4 

216.4-374.9 

HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

percent colorless plagioclase (1 to 2 mm) 

and brown orthopyroxene. Magnetite, hematite, amphibole, clinopyroxene and a-cristohalite were also identified by XRD. 

Reddish-gray (oxidized), vesicular, hypersthene andesite. Phenocrysts consist of 3 to 5 

percent colorless plagioclase (4 mm) and 1 to 

2 percent brown hypersthene (1 mm). XRD 

reflections for a-cristobalite, magnetite, hematite, and clinopyroxene(?) were observed. 

Medium-gray, fine-grained, pyroxene andesite. Phenocrysts are 5 percent colorless plagioclase (2 to 3 mm) and <1 percent 

pyroxene (1 mm). Plagioclase, orthopyroxene, a-cristobalite, magnetite, and alkalifeldspar(?) were identified by XRD. 

Medium- to light-gray, pyroxene andesite. 

Has two sizes of colorless plagioclase phenocrysts (lxl mm and 1x0.1 mm) compose 3 

to 5 percent of sample; another 1 percent of 

phenocrysts are clear brown orthopyroxene 

(1 mm). XRD analyses contain reflections 

for a-cristobalite, clinopyroxene, magnetite, 

and alkali-feldspar(?). 

Medium-gray, pyroxene andesite. Phenocrysts are composed of 10 percent colorless 

plagioclase (1 to 2 mm) and 1 to 2 percent 

brown orthopyroxene (0.5 to 1 mm). Magnetite, tridymite, and a-cristobalite were identified by XRD. One sample from 215.5 m 

contained a trace of smectite. 

Volcaniclastic volcanic debris: colluvium and 

mudslide material typical of large stratovolcanoes. A mixture of angular to subangular, 

pyroxene, andesite grains that are various 

shades of gray, black, brown, and red with a 

few orange-stained and white-bleached 

grains sprinkled throughout. XRD and binocular microscope studies show that most of 

the grains consist of colorless plagioclase 

and brown orthopyroxene phenocrysts, magnetite, and a-cristobalite; minor constituents 

may include hematite, clinopyroxene, amphibole, tridymite, and alkali-feldspar(?). The 

interval was subdivided on the basis of the 

presence or absence of several hydrothermal 

alteration minerals. 

216.4-236.2 chalcedony, kaolinite, smectite, pyrite, I 0-A mica. 

236.2-272.8 unaltered except for some 

iron oxide staining. 

272.8-338.3 calcite, chalcedony, kaolinite, B-cristobalite, sme-ctite, 

natroalunite, opal, pyrite, 10-

A mica. 

338.3-356.6 unaltered. 

374.9-393.2 

393.2-513.6 

513.6-522.7 

522.7-554.7 

554.7-563.9 

563.9-573.0 

573.0-585.2 

356.6-374.9 chalcedony, pyrite. 

Light-gray, slightly vesicular, pyroxene andesite. Phenocrysts are colorless plagioclase, 

brownish orthopyroxene, and rare hornblende; the groundmass contains finely disseminated magnetite. Clinopyroxene, 

hematite, and a-cristobalite were identified 

by XRD. 

Volcaniclastic volcanic debris: colluviuim 

and landslide material; possibly minor fluvial 

material. Mixture of angular to subangular, 

mostly medium-gray, pyroxene andesite, and 

subordinant light-gray, dark-gray, black, reddish, and brown pyroxene andesite grains. 

Phenocrysts are colorless plagioclase, brown 

orthopyroxene, and green clinopyroxene. 

Magnetite, tridymite, amphibole(?), and alkali-feldspar(?) were detected by XRD. Most 

samples have a few orange-stained or whitebleached grains that contain one or more of 

the alteration minerals: calcite, chalcedony, 

or pyrite. 

Medium-dark-gray, pyroxene andesite. Phenocrysts consist of <5 percent colorless plagioclase (1 mm) and rare, small ( <0.5 mm), 

brown orthopyroxene crystals. XRD analyses have reflections for a-cristobalite and 

tridymite. 

Volcaniclastic debris. Mixture of subangular 

to subrounded grains of light- medium- and 

dark-gray, brownish, reddish vesicular, and 

black glassy andesite. Plagioclase, orthopyroxene, a-cristobalite, hornblende, tridymite, 

magnetite, and alkali-feldspar(?) were identified by XRD. A few frosted calcite and 

white- or pink-silica grains were observed. 

a-cristobalite, smectite, kaolinite, 10 A mica 

or illite, and hematite were identified by 

XRD of iron-stained and siliceous grains. 

Reddish-gray to medium-gray, porphyritic, 

pyroxene andesite. Phenocrysts consist of 15 

percent colorless plagioclase (1 mm), often 

occurring as glomerocrysts, and 3 to 5 percent brown hypersthene (0.5 mm). Also, acristobalite, tridymite, clinopyroxene and 

hematite were identified by XRD. 

Black- to brownish-black, diktytaxitic, olivine(?) basalt. Many grains are microvesicular and are frequently quite oxidized. 

Phenocrysts are 1 to 2 percent colorless 

plagioclase (0.5 mm) and very minor yellowgreen olivine(?). a-cristobalite and orthopyroxene identified by XRD. 

Volcaniclastic debris. A mixture of subangular to rounded, light- and medium-gray, 

andesite grains that contain plagioclase, acristobalite, orthopyroxene, clinopyroxene, 
585.2-597.4 

597.4-602 

602-611.1 

611.1-630.9 

630.9-640.1 

640.1-646.2 

646.2-653.8 

653.8-691.9 

691.9-698.0 

APPENDIX A. LITHOLOGIC DESCRIPTIONS AND DISTRIBUTION OF MINERALS 67 

tridymite(?), and hornblende identified by 

XRD analyses. Calcite grains also occur in 

most samples. 

Medium- to dark-gray, porphyritic, pyroxene 

andesite with 7 to 12 percent colorless plagioclase (0.5 mm) phenocrysts. a-cristobalite, 

clinopyroxene, hematite, tridymite, chalcedony, calcite, pyrite, and smectite were identified by XRD. 

Volcaniclastic material consisting of a mixture 

of varicolored, subangular to rounded andesite grains; some of which are altered. Plagioclase, a-cristobalite, tridymite, smectite, 

calcite and opal(?) were identified by XRD. 

Dark- to medium-gray, hypersthene andesite. 

Phenocrysts consist of 10 percent colorless 

plagioclase (1 to 1.5 mm) and 2 to 3 percent 

clear brown hypersthene ( <0.5 mm). The 

groundmass is crystalline and contains plagioclase, orthopyroxene, a-cristobalite, tridymite, and hematite identified by XRD. 

Volcaniclastic material. A mixture of 

varicolored, angular to rounded andesite 

grains. Minerals identified by XRD are acristobalite and several hydrothermal alteration minerals: calcite, chalcedony, smectite, 

hematite, natroalunite, and kaolinite(?). 

Medium- to dark-gray, pyroxene andesite. 

Phenocrysts consist of 10 percent colorless 

plagioclase ( 1.0 mm) and 2 percent hypersthene (0.5 mm). a-cristobalite, clinopyroxene and smectite also were detected by XRD. 

Has some oxidized clasts and calcite. 

Volcaniclastic debris consisting of varicolored, subangular to subrounded andesite 

grains. XRD analyses show the presence of 

calcite, hematite, chalcedony, and natroalunite 

hydrothermal minerals plus a-cristobalite. 

Brownish-gray, pyroxene andesite. Phenocrysts are -7 percent colorless plagioclase 

(0.5 mm) and <1 percent very small (0.1 

mm) pyroxene crystals. a-cristobalite and 

tridymite also were identified by XRD. 

Volcaniclastic debris. A mixture of varicolored, subangular to subrounded andesite 

grains (rarely rounded). a-cristobalite was 

identified by XRD analyses. Hematite is very . 

abundant; other hydrothermal minerals include calcite, chalcedony, natroalunite, kaolinite, smectite, quartz, and 10-A mica. 

Dark-gray, slightly vesicular, hypersthene andesite. Phenocrysts consist of colorless plagioclase (1 mm) (10 to 15 percent), brown 

hypersthene (0.2 to 0.5 mm), often as glomerocrysts, and rare larger augite crystals. 

Groundmass(?) a-cristobalite was detected 

698.0-713.2 

713.2-731.5 

731.5-737.6 

737.6-755.9 

755.9-780.3 

780.3-803.2 

by XRD. Hematite coats -20 percent of 

chips. Traces of calcite, green clay (smectite?), and chalcedony occur as hydrothermal 

alteration minerals. 

Volcaniclastic debris. A mixture of varicolored, subangular to subrounded andesite 

grains. Minerals identified by XRD include 

a-cristobalite plus hydrothermal alteration 

minerals:· smectite, and chalcedony. The interval also contains calcite, hematite, and 

green clay (smectite?). 

Dark-gray to orange-red (oxidized), vesicular, pyroxene andesite. Most chips are 75 

percent oxidized (hematite?). Phenocrysts 

are 25 to 30 percent colorless plagioclase 

( 1.0 mm) and 3 to 5 percent clear brown orthopyroxene (0.1-0.5 mm), often as glomerocrysts. The groundmass appears glassy, but 

contains some a-cristobalite identified by 

XRD. Hydrothermal alteration minerals 

identified by XRD include: chalcedony, hematite, calcite, and smectite. 

Volcaniclastic debris. A mixture of varicolored, subangular to rounded andesite grains. 

A few grains contain calcite, chalcedony(?), 

and white and green clay (smectite?). 

Orange, pyroxene andesite with some autoclastic aspects. Phenocrysts are white plagioclase. Interpretation of XRD analyses 

suggests that clinopyroxene has mostly been 

altered to green smectite, and if a-cristobalite were present, it has been altered to chalcedony. The groundmass is soft and altered 

to iron oxide and clay. Hydrothermal alteration minerals, deposited in open spaces of 

fractures and cavities, are: calcite, chalcedony (in part colliform), stilbite, laumontite, 

hematite, and smectite (identified by XRD). 

Dark-brownish-gray (rarely greenish gray), 

pyroxene andesite. Phenocrysts are 2 to 3 

percent colorless, red- or green-stained plagioclase (1 to 2 mm). Clinopyroxene, orthopyroxene(?), smectite, hematite, and quartz(?) 

or chalcedony(?) were identified from XRD 

analyses. Mafic minerals appear to be mostly 

altered to blue-green smectite. Several XRD 

analyses of fracture fillings (including green 

clay) have reflections for smectite, stilbite, 

laumontite, chalcedony, and calcite. One 

grain has a crust of euhedral magnetite crystals. 

Volcaniclastic debris. A mixture of hematitealtered, subangular to subrounded pyroxene 

andesite grains. Fracture fillings of white zeolite minerals (laumontite, heulandite, and 

stilbite), chalcedony, green clay (smectite), 

and calcite were identified by XRD. 
68 

803.2-851.9 

851.9-862.6 

862.6-885.4 

885.4-894.6 

894.6-912.9 

912.9-920.5 

920.5-932.7 

932.7-941.8 

HYDROTHERMAL ALTERATION IN THE MOUNT HOOD AREA, OREGON 

Dark-green- to dark-greenish-brown, hydrothermally altered, pyroxene andesites. Plagioclase phenocrysts are cloudy to white. 

Plagioclase, clinopyroxene, quartz(?) or 

chalcedony(?), smectite, and hematite were 

identified by whole-rock XRD analyses. 

Fracture-filling hydrothermal minerals consist of stilbite, laumontite, calcite, chalcedony, and smectite. One sample also has a 

cavity(?) filling of colorless, crystalline 

quartz. 

Volcaniclastic material consisting of a mixture of subangular to subrounded pyroxene 

andesite grains. Hydrothermal minerals in altered andesite grains, fractures, and vein fillings include calcite, chalcedony, hematite, 

heulandite, smectite, laumontite, and chlorite. 

Moderate brownish-red to grayish-green, 

fine-grained, pyroxene andesite. Plagioclase 

phenocrysts are colorless to cloudy; pyroxene is altered to bluish-green smectite. Alteration minerals are chalcedony, hematite, 

quartz, calcite, stilbite, laumontite, and smectite. 

Volcaniclastic material. A mixture of hydrothermally altered, subangular to subrounded 

andesitic-grains. Hydrothermal minerals 

identified by XRD include hematite, stilbite, 

chalcedony, smectite, calcite, and laumontite. 

Dark-greenish-gray to dark-brownish-gray, 

hydrothermally altered, pyroxene andesite. 

The groundmass is soft and has been altered 

to smectite. Rare 1 mm cloudy plagioclase 

phenocrysts are present; a-cristobalite was 

identified by XRD analysis. Drill cuttings 

contain green smectite fracture fillings. Other 

hydrothermal minerals, stilbite, chalcedony, 

quartz, hematite, calcite, and mordenite, 

were identified by XRD. 

Volcaniclastic debris consisting of hydrothermally altered, subangular to subrounded, andesite grains. Alteration minerals are 

chalcedony, stilbite, mordenite, smectite, 

hematite, and epistilbite. 

Moderate, reddish-brown, intensely altered 

andesite. The original texture is obscured, 

but it appears to have been porphyritic. Drillcutting samples contain much hydrothermal 

stilbite, calcite, chalcedony, mordenite, hematite, laumontite, and smectite. 

Epiclastic material. A mixture of subangular 

to subrounded, andesite grains. Hydrothermal 

minerals include stilbite, laumontite, mordenite, hematite, smectite, and chalcedony. 

941.8-1,039.4 A thick series of altered, dark -greenish-gray 

to grayish-olive-green, porphyritic, pyroxene 

andesites. Phenocrysts are 5 to 10 percent 

plagioclase (2 to 5 mm), rarely up to 8 mm; 

pyroxenes (orthopyroxene? by XRD) are seldom present and are mostly altered to green 

smectite. Groundmass glass probably was 

devitrified to a-cristobalite, but later was hydrothermally altered to chalcedony(?). Alteration varies from minor hematite and 

smectite altered groundmass to extremely intense and apparently fracture controlled alteration. Intensely altered clasts, which may 

account for -50 percent of some sample intervals, consist predominantly of varicolored 

mottled smectite with evidence of shearing 

and microbrecciation. Other hydrothermal 

fracture or vug minerals include mordenite, 

stilbite, chalcedony, calcite, and laumontite. 

1,039.4-1,091.2 A thick section of subangular to subrounded, 

epiclastic, andesitic material; probably a series of mudflows or colluvial debris. Most 

samples are a mixture of varicolored andesite 

grains, although some sample intervals are 

dominated by one grain color. Hydrothermal 

minerals are smectite, mordenite, chalcedony, stilbite, calcite, laumontite, hematite, and 

quartz. 

1,091-1,100.3 Dark-greenish-gray to grayish-black, hydrothermally altered, pyroxene andesite. 

Phenocrysts (10 percent) often consist of 

red-stained plagioclase (up to 1 em but generally smaller) and 5 to 7 percent pyroxene 

( -1 mm). Apparently a groundmass of devitrified a-cristobalite(?) was hydrothermally 

altered to chalcedony(?). Other hydrothermal 

minerals identified are smectite, calcite, stilbite, and mordenite. 

1,100.3-1,143 Volcaniclastic material similar to interval at 

1 ,039 .4 to 1 ,091.2 m. A mixture of subangular to subrounded, varicolored, andesitic 

grains. Contains some of the most intensely 

altered lavas encountered in the drill hole. 

Secondary minerals are calcite, stilbite, chalcedony, smectite, mordenite, laumontite, and 

pyrite. 

1,143-1,221.1 Thick, dark-grayish-green to grayish-brown, 

pyroxene andesite section. Contains 15 to 20 

percent cloudy or clear to slightly green or 

red-stained plagioclase phenocrysts ( -1 mm; 

some up to 2 mm) and 3 to 5 percent mostly 

altered pyroxene (hypersthene and augite) 

(0.5 mm). Groundmass is fairly coarse. Secondary minerals are smectite, stilbite, laumontite, calcite, chalcedony, pyrite, 

mordenite, and traces of chlorite. 
1-

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Stratigraphic representation of the Pucci Chairlift drill hole showing the distribution of hydrothermal minerals. 

69 

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70 APPENDIX B. HYDROTHERMAL MINERALS IDENTIFIED FROM DRILL HOLES AND OUTCROPS 

APPENDIX B. HYDROTHERMAL MINERALS IDENTIFIED FROM 

GEOTHERMAL DRILL HOLES AND OUTCROPS IN THE MOUNT HOOD AREA. 

[Identified by binocular microscope, petrographic microscope, and X-ray diffraction analyses. Leaders ( --), mineral not detected] 

Drill hole and 

outcrop designation 1 2 3 4 5 6 7 8 9 10 11 12 13 A B c D E F G 

Silica minerals 

Opal ----------------------------------------- X X X X X X 

8-cristobalite ------------------------------ X X X 

Chalcedony -------------------------------- X X X X X X X X X X X X X X 

Quartz -------------------------------------- X X X X X X X X X X X X X X 

Carbonate minerals 

Dolomite ----------------------------------- X X X 

Calcite ------------------------------------- X X X X X X X X X X X X X X X X 

Manganoan calcite ------------------------ X X X 

Cerussite ---------------------------------- X X 

Rhodochrosite ----------------------------- X 

Siderite ------------------------------------- X X X 

Zeolite minerals 

Chabazite ---------------------------------- X X X X X X X 

Epistilbite ---------------------------------- X X X X X 

Ferrierite ----------------------------------- X 

Harmotome -------------------------------- X 

Heulandite --------------------------------- X X X X X X X 

Laumontite --------------------------------- X X X X X X X X X -- Mordenite -------------------------------- X X X X -- X 

Scolecite ---------------------------------- X X X X -- Stellerite(?) -------------------------------- X 

Stilbite ------------------------------------- X X X X X X X X 

Wairakite ----------------------------------- X X X X 

Clay minerals 

Halloysite ---------------------------------- X 

Kaolinite ---------------------------------- X X X X X X X X -- Celadonite -------------------------------- X X 

Smectite ----------------------------------- X X X X X X X X X X X X X X X X X X 

Illite ---------------------------------------- X X X X X X X X X X X 

Illite-Smectite ----------------------------- X X X X X 

Chlorite ------------------------------------- X X X X X X X X X X X X 

Chlorite-Smectite -------------------------- X X X X 

Sepiolite ------------------------------------ X 

Sulfate minerals 

Gypsum ----------------------------------- X X X X X 

Alunite ------------------------------------ X X X 

Natroalunite ------------------------------ X X X X 

Jarosite ----------------------------------- X 

N atrojarosite ------------------------------- X 

Anglesite ----------------------------------- X 

Sulfide minerals 

Chalcopyrite ------------------------------- X X X X 

Galena ----------------------------------- X X 

Pyrite ------------------------------------- X X X X X X X X X X X X X X 

Sphalerite --------------------------------- X X 

Iron oxide and hydroxide minerals 

Amorphous FeO -------------------------- X X X X -- Goethite ----------------------------------- X X X X 

Hematite ----------------------------------- X X X X X X X X X X X X X 

Magnetite --------------------------------- X X X X X X 

Other minerals 

Epidote ------------------------------------ X X X X X X X X 

Titanite(?) --------------------------------- X X 

Garnet(?) ----------------------------------- X 

Actinolite -------------------------------- X X X X 

Prehnite ------------------------------------ X X X X -- Adularia ----------------------------------- X X -- Tourmaline --------------------------------- X 

Apatite -------------------------------:-------- X 

Chrysocolla -------------------------------- X 

Copper ----------------------------------- X 

I Drill holes numbered I to I3 are the same as those listed in table I; locations are shown in fig. I. Letter designations correspond to: A, Cheeney 

Creek mining area; B, Lost Creek area; C, Cast Creek-Short Creek area; D, Lolo Pass; E, Laurel Hill; F, Iron Creek; G, Barlow Pass-Bennett Pass area. 
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