Shorter Contributions

to C*L y •

GEOLOGICAL
Shorter Contributions

to Mineralogy and Petrology,

1979

The Vermilion Granitic Complex—A New Name for Old Rocks in Northern 

Minnesota

By D. L. SOUTHWICK and P. K. SIMS

Potassium-Argon Ages from the Mount Taylor Volcanic Field, New Mexico 

By PETER W. LIPMAN and HARALD H. MEHNERT

Crystals of Coexisting Alunite and Jarosite, Goldfield, Nevada 

By WILLIAM J. KEITH, LEWIS CALK, and R. P. ASHLEY

Zeolitization of Tertiary Tuffs in Lacustrine and Alluvial Deposits in the RaySan Manuel Area, Pinal and Gila Counties, Arizona

By MEDORA H. KRIEGER

Drake Peak-A Structurally Complex Rhyolite Center in Southeastern Oregon 

By RAY E. WELLS

Palladium, Platinum, and Rhodium Concentrations in Mafic and Ultramafic 

Rocks from the Zhob Valley and Dargai Complexes, Pakistan

By NORMAN J. PAGE, JOSEPH HAFFTY, and ZAKI AHMAD

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1124-A-F

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Meiiard, Director

Library of Congress catalog-card No. 79-600186

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

Washington, D.C. 20402

Stock Number 024-001-03247-3
CONTENTS

[Letters designate the chapters]

(A) The Vermilion Granitic Complex—A New Name for Old Rocks in Northern 

Minnesota, by D. L. Southwick and P. K. Sims

(B) Potassium-Argon Ages from the Mount Taylor Volcanic Field, New Mexico, by 

Peter W. Lipman and Harald H. Mehnert

(C) Crystals of Coexisting Alunite and Jarosite, Goldfield, Nevada, by William J. 

Keith, Lewis Calk, and R. P. Ashley

(D) Zeolitization of Tertiary Tuffs in Lacustrine and Alluvial Deposits in the Ray-San

Manuel Area, Final and Gila Counties, Arizona, by Medora H. Krieger 

(E) Drake Peak-A Structurally Complex Rhyolite Center in Southeastern Oregon, by

Ray E. Wells 

(F) Palladium, Platinum, and Rhodium Concentrations in Mafic and Ultramafic Rocks

from the Zhob Valley and Dargai Complexes, Pakistan, by Norman J. Page,

Joseph Haffty, and Zaki Ahmad
CONVERSION FACTORS

Metric unit Inch-Pound equivalent

Length

millimeter (mm) 

meter (m) 

kilometer (km)

= 0.03937 inch (in) 

= 3.28 feet (ft) 

= .62 mile (mi)

Area

square meter (m2 ) 

square kilometer (km2 ) 

hectare (ha)

= 10.76 square feet (ft2 ) 

= .386 square mile (mi2 ) 

= 2.47 acres

Volume

cubic centimeter (cm3 ) 

liter (L) 

cubic meter (m3 ) 

cubic meter 

cubic hectometer (hm3 ) 

liter 

liter 

liter 

cubic meter

cubic meter

= 0.061 cubic inch (in3 ) 

= 611.03 cubic inches 

= 35.31 cubic feet (ft») 

= .00081 acre-foot (acre-ft) 

= 810.7 acre-feet 

= 2.113 pints (pt) 

= 1.06 quarts (qt) 

= .26 gallon (gal) 

= .00026 million gallons (Mgal or 

109 gal) 

= 6.290 barrels (bbl) (1 bbl = 42 gal)

Weight

gram (g) 

gram 

metric tons (t) 

metric tons

= 0.035 ounce, avoirdupois (oz avdp) 

;= .0022 pound, avoirdupois (Ib avdp) 

= 1.102 tons, short (2,000 Ib) 

= 0.9842 ton, long (2,240 Ib)

Specific combinations

kilogram per square 

centimeter (kg/ cm2 ) 

kilogram per square 

centimeter 

cubic meter per second 

(m3/s)

_ 0.96 atmosphere (atm)

— .98 bar (0.9869 atm) 

= 35.3 cubic feet per second (ft3/s)

Metric unit

Specific

liter per second (L/s) 

cubic meter per second 

per square kilometer 

[(mVs)/km2 ] 

meter per day (m/d)

meter per kilometer 

(m/km) 

kilometer per hour 

(km/h) 

meter per second (m/s) 

meter squared per day 

(m2/d) 

cubic meter per second 

(m.3/s) 

cubic meter per minute 

(m3/min) 

liter per second (L/s) 

liter per second per 

meter [(L/s)/m] 

kilometer per hour 

(km/h) 

meter per second (m/s) 

gram per cubic 

centimeter (g/crn3 ) 

gram per square 

centimeter (g/cm2 ) 

gram per square 

centimeter

Inch-Pound equivalent

combinations — Continued

= .0353 cubic foot per second 

= 91.47 cubic feet per second per 

square mile [(ft3/s)/mi2 ]

= 3.28 feet per day (hydraulic 

conductivity) (ft/d) 

= 5.28 feet per mile (ft/mi)

— .9113 foot per second (ft/s)

— 3.28 feet per second 

= 10.764 feet squared per day (ft2/d) 

(transmissivity) 

= 22.826 million gallons per day 

(Mgal/d) 

= 264.2 gallons per minute (gal/min)

= 15.85 gallons per minute 

= 4.83 gallons per minute per foot 

[(gal/min) /ft]

— .62 mile per hour (mi/h)

— 2.237 miles per hour

= 62.43 pounds per cubic foot (Ib/ft3 ) 

= 2.048 pounds per square foot (lb/ft2 ) 

= .0142 pound per square inch (lb/in2 )

Temperature

degree Celsius (°C) 

degrees Celsius 

(temperature)

= 1.8 degrees Fahrenheit (°F) 

= [(1.8X°C)+32] degrees Fahrenheit
The Vermilion Granitic Complex 

A New Name for Old Rocks 

in Northern Minnesota

By D. L. SOUTHWICK and P. K. SIMS

SHORTER CONTRIBUTIONS TO MINERALOGY AND 

PETROLOGY, 1979

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1 1 2 4 - A

Prepared with the cooperation of 

the Minnesota Geological Survey

CONTENTS

	Page

Abstract _______________________________________ — ____-- A!

Introduction ______________________________________________________________ 1

Subdivisions of the Vermilion Granitic Complex ______________-___-_----_— 4

Note on the terminology of migmatites _______________ — _______________ 5

Lac La Croix Granite ___________________________ — ______ — _______ 5

Granite-rich migmatite related to the Lac La Croix Granite _____ — _______ 6

Schist-rich migmatite related to the Lac La Croix Granite _____ — __ — _____ 7

Amphibolite and amphibolite migmatite _______________ — ______ — — _____ 7

Intrusive rocks that are geologically older than the Lac La Croix Granite __ 8

Migmatite geologically older than the gray granodiorite __________________ 8

Burntside Gneiss _________________________________________________ 8

Biotite schist _____________________________________________________ 9

Pegmatite _________________________________________________ — _____— 9

Age of the complex _________________________________ ——— _——— 10

References cited __________________________________________________ 10

ILLUSTRATIONS

Page 

FIGURE 1. Generalized geologic map, showing the Vermilion Granitic Complex ________________________________ A2

2. Schematic diagram of the interrelationships among the lithologic subdivisions of the Vermilion Granitic

Complex __________________________________________________________________________________ 4

TABLES

Page

TABLE 1. Average modal composition of the Lac La Croix Granite and neosome of granite-rich migmatite _________ A6

2. Chemical analyses of the Lac La Croix Granite (includes neosome of granite-rich migmatite) _______ 6

3. Average modal composition of gray granodiorite, tonalite, and trondhjemite _______________ 8

4. Chemical analyses of gray granodiorite and trondhjemite, including the Burntside Gneiss ____________ 9

in

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

THE VERMILION GRANITIC COMPLEX— 

A NEW NAME FOR OLD ROCKS IN NORTHERN MINNESOTA

By D. L. SouTHWiCK1 and P. K. SIMS

ABSTRACT

The name Vermilion Granitic Complex is introduced for 

the heterogeneous granitic and migmatitic rocks of Archean 

(formerly called Precambrian W) age that occur north of the 

Vermilion district and south of the Kabetogama peninsula in 

northern Minnesota. The complex consists of the following 

subdivisions: Lac La Croix Granite, granite-rich migmatite, 

schist-rich migmatite, quartz-feldspar gneiss, hornblende 

quartz diorite and diorite, granodiorite and trondhjemite, 

amphibolite and amphibolite migmatite, older migmatite, biotite schist, Burntside Gneiss, and pegmatite. Because the 

name Vermilion Granitic Complex is proposed as a more 

inclusive group term, the more restricted name Vermilion 

Granite (Grout, 1923) is hereby abandoned. The new name 

Lac La Croix Granite is proposed for the uniform, light-pink 

biotite granite that occurs widely in the eastern and central 

parts of the complex. It is denned as having less than 5 percent of schistose or gneissic inclusions and is therefore more 

restricted than the Vermilion Granite of Grout (1926), which 

included sustantial amounts of migmatitic rocks. The name 

Burntside Gneiss is adopted as a replacement for the older 

term Burntside Granite Gneiss, originally named by Grout 

(1926). This change is required by the conclusion that the 

rock is a metamorphosed dacite and not a metamorphosed 

granite, as formerly interpreted.

INTRODUCTION

The name Vermilion Granitic Complex is proposed for the heterogeneous granitic and migmatitic 

rocks of Archean (formerly called Precambrian W) 

age that occur niorth of the Vermilion district and 

south of the Kabetogama peninsula in northern Minnesota (fig. 1). It replaces the informal name Vermilion granite-migmatite massif assigned to these 

rocks by Southwick (1972). The Vermilion Granitic 

Complex is understood to have group status in the 

formal stratigraphic sense, in accordance with the 

proposed Article 9 (g), amended, of the Code of 

Stratigraphic Nomenclature (Sohl, 1977). The complex includes, but is not restricted to, the Vermilion

1 Minnesota Geological Survey, St. Paul, Minn.

Granite as defined by Grout 1923) ; it includes some 

rocks previously included in the abandoned terms 

"Vermilion series, Vermilion schist, and Vermilion 

group," as used by Winchell and Winchell (1887, 

p. 4, 192, 355, 357) and Van Hise and Clements 

(1901).

The greater part of the Vermilion Granitic Complex consists of pink to grayish-pink biotite granite 

(as classified by Streckeisen (1973) and as used by 

Minnesota geologists for decades), and genetically 

associated granite-rich migmatite. These rocks constitute the Vermilion Granite of Grout (1923). 

Within the area dominated by granite, lesser volumes of other igneous and metamorphic rocks are 

intimately involved with the granite; moreover, on 

the north and west the granitic terrane passes gradationally through migmatites rich in schist into 

nonmigmatized biotite schist. We view the minor 

lithologies within and the migmatite around the 

granitic terrane to be logically parts of a larger rock 

body. Accordingly, we recommend the Vermilion 

Granite as used by Grout (1923) be elevated in rank 

and broadened to the Vermilion Granitic Complex, 

as described below. The complex is named for the 

Vermilion River, along which most of its lithologic 

subdivisions are exposed.

The northern boundary of the Vermilion Granitic 

Complex is gradational and is arbitrarily placed 

where granitic rocks of the complex become sparse 

or absent from the flanking unnamed biotite schist 

of the Kabetogama peninsula and its eastward extension into Canada (fig. 1). The southern boundary 

is defined mainly by high-angle faults, which provide 

a sharp break between the granitic rocks and associated amphibolite-facies rocks of the complex and 

the greenschist-facies volcanic and sedimentary 

rocks of the Vermilion district (Sims, 1976). We

Al
A2 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

93°00' 92°30'

^w^';^ 

^^^^

48° 00

Base modified from U. S. Geological Survey 

1- x 2-degree topographic sheets, 1:250,000 

International Falls, 1954, Nibbing and 

Two Harbors, 1954-64

FIGURE 1.—Generalized geologic map showing Vermilion Granitic Complex. Compiled by D. L. South wick, in 1977, from maps
VERMILION GRANITIC COMPLEX—A NEW NAME FOR OLD ROCKS IN NORTHERN MINNESOTA A3

92°00' 91°30'

CORRELATION OF MAP UNITS

Archean

DESCRIPTION OF MAP UNITS

VERMILION GRANITIC COMPLEX

Lac La Croix Granite 

Quartz-feldspar gneiss 

Granite-rich migmatite

Schist-rich migmatite

Hornblende quartz diorite 

and diorite

Burntside Gneiss

Amphibolite and amphibolite 

migmatite

Biotlte schist

UNNAMED 8IOTITE SCHIST

METASEDIMENTARY AND METAVOLCANIC ROCKS 

OF VERMILION DISTRICT-lncludes Giants Range 

Granite south of North Kawlshlwi fault

EXPLANATION

^

Contact-Dashed where concealed by water

High-angle fault-Dotted where concealed by water. 

U, upthrown side; D. downthrown side. Arrows 

indicate relative horizontal movement

Locality referred to in text

Locality referred to on tables 2 and 4

20 KILOMETERS 

I

Duluth Complex 

1,100 m.y. old 

(Proterozoic Y)

by Green (1973), Southwick and Ojakangas (1973), Sims and others (1970), Sims (1973), and Sims and Mudrey (1978).
A4 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

include in the Vermilion Granitic Complex the rocks 

north of the Vermilion, Haley, and Shagawa Lake 

faults, as shown on figure 1. The limits of the complex to the west and east are impossible to define 

precisely with present map data. West of International Falls, Minn., the Precambrian bedrock is 

covered by thick glacial drift, and existing bedrock 

mapping is sketchy. We tentatively conclude that the 

complex terminates against the westward extension 

of the Vermilion fault (Southwick and Ojakangas, 

1973). In Canada the complex merges into and appears to be virtually equivalent to the Quetico belt 

(Mackasey and others, 1974) and presumably continues eastward along strike for many kilometers.

SUBDIVISIONS OF THE VERMILION GRANITIC 

COMPLEX

On figure 1, eight subdivisions of the Vermilion 

Granitic Complex are delineated. In addition we recognize three other units that cannot be shown at the 

map scale but are probably significant to the petrogenetic history of the complex. These are (1) an 

early intrusive suite consisting of light-gray granodiorite, quartz diorite, and trondhjemite; (2) a migmatite that is geologically older than the early intrusive suite; and (3) late dikes of pegmatite. Relations among the subunits are shown diagrammatically in figure 2.

The east-central part of the complex (fig. 1) is a 

uniform, light-pink biotite granite. This remarkably 

homogeneous granite, together with rocks we map as 

granite-rich migmatite (discussed in a following 

section), was called the Vermilion Granite by Grout 

(1923). Because we propose to use Vermilion 

Granitic Complex as a more inclusive group term, 

the more restricted name Vermilion Granite is here^- 

by abandoned. Accordingly, we propose the name 

Lac La Croix Granite for the distinctive, wideCUTS

Light-gray granodiorite, 

tonalite, and trondhjemite

CUTS 

I CUTS

Dark-gray HORNBLENDE QUARTZ

DIORITE AND DIORITE

vqd

Dikes of basalt, diabase, and 

gabbro younger than, 

and unrelated to, the 

Vermilion Granitic Complex

CUT

Late-stage straight sharp-walled 

pegmatite dikes

CUT 

I

LAC LA CROIX GRANITE 

vl on figure 1

Quartz-feldspar 

gneiss (border 

phase)vqg

GRANITE-RICH MIGMATITE

related to Lac La Croix Granite

vgm CUTS

Early migmatite

SCHIST-RICH MIGMATITE 

related to Lac La Croix 

Granite

vsm

AMPHIBOLITE MIGMATITE 

related to Lac La Croix 

Granite

vam

RELATED BIOTITE SCHIST 

vbs

AMPHIBOLITE 

vam

Paleosome; equivalent to lower grade 

supracrustal rocks of the flanking 

greenstone belts

BURNTSIDE GNEISS 

vb

FIGURE 2.—Schematic diagram of the interrelationships among the lithologic subdivisions of the Vermilion Granitic Complex. Intrusive units are arranged with the youngest at the top of the diagram. Names spelled out in capital letters 

are the more abundant components of the complex.
VERMILION GRANITIC COMPLEX—A NEW NAME FOR OLD ROCKS IN NORTHERN MINNESOTA A5

spread, uniform, light-pink biotite granite that crops 

out south of La.c La Croix; these outcrops are the 

type area. Those granitic rocks that contain more 

than about 5 percent of gneissic or schistose inclusions are shown as granite-rich migmatite or schistrich migmatite on figure 1 and are not included 

formally in the Lac La Croix Granite. Contacts between Lac La Croix Granite, granite-rich migmatite, 

and schist-rich migmatite are broadly gradational 

and are therefore somewhat subjectively determined 

in practice, especially in areas of sparse outcrop.

NOTE ON THE TERMINOLOGY OF MIGMATITES

The term "migmatite" has been used for decades 

by European geologists for variably gneissose rocks 

that consist of granite intimately mixed with nongranitic, generally dark colored metamorphic rocks. 

The tortuous history of the term is discussed by 

Mehnert (1968, p. 1-8), and his definition (p. 355) 

is adopted here: "Migmatite [is a] megascopically 

composite rock consisting of two or more petrographically different parts. One is the country rock 

in a more or less metamorphic stage, the other is of 

pegmatitic, aplitic, granitic, or generally plutonic 

appearance." In discussing migmatites it is convenient to have terms for the two parts; the terminology of Dietrich and Mehnert (1961) has been 

widely followed and is adopted here. They refer to 

the "older part of a composite rock (i.e., the remaining or pre-existing part)" as the paleosome, and to 

the "younger part of a composite rock (for example, 

the injected, exuded, or metasomatically introduced 

material)" as the neosome.

The structures and textures of migmatites are 

varied and complex, and inasmuch as the rocks are 

neither purely igneous no>r purely metamorphic in 

origin, the descriptive structural and textural terms 

for igneous and metamorphic rocks are not strictly 

appropriate. Consequently the literature has been 

burdened by an excessive number of specialized 

terms for the structures and textures of migmatites, 

many of which have strong genetic connotations. 

Although we deplore this proliferation of jargon, we 

have found that some special terms illustrated and 

defined by Mehnert (1968, p. 8-42, especially p. 10- 

11) are useful in the field and are free of genetic 

overtones.

Many migmatites, for example, are layered, but 

not all layering in migmatites is the same. Migmatites with stromatic structure are layered, are 

chiefly paleosome, and have concordant pegmatoid 

neosomes that tend to pinch and swell; they are on 

the order of 0.1-1 m thick, and they commonly have

selvages rich in biotite or hornblende. Migmatites 

with phlebitic structure are layered on a fine scale, 

highly contorted, and have neosomes that are both 

concordant and discordant to the foliation in the 

paleosome. Migmatites with schlieren structure are 

layered, but the proportion of granite is so high that 

the layering is variable to indistinct and commonly 

frayed or wispy in appearance. All these varieties 

are commonly interspersed with one another in the 

field, and also grade into layered migmatites having 

simple plane-parallel layering, for which no special term is required.

A second large class of migmatites has a blocky 

structure consisting of more or less equant to slabshaped inclusions surrounded by more or less homogeneous granite. Considerable variation in the shape 

and degree of transformation of the included blocks 

occurs within this group. Specifically, migmatites 

with nebulitic structure have only vague, indistinct 

inclusions that approach the surrounding granite in 

modal composition. Commonly the ghostlike inclusions make up a quarter or more of the bulk rock 

(and rarely as much as half) ; the whole rock has a 

blotchy appearance that is aptly described as nebulous. Nebulitic migmatites are especially widespread 

in the granite-rich migmatite of the Vermilion 

Granitic Complex.

LAC LA CROIX GRANITE

The Lac La Croix Granite contains quartz, oligoclase, and perthitic microcline in subequal amounts, 

together with small amounts of biotite and (or) 

chlorite. Pegmatite dikes are sparse or absent, and, 

by definition, the granite contains less than about 

5 percent of gneissic to schistose inclusions. Most of 

the granite is equant and hypidiomorphic granular, 

with crystals in the size range of 5-10 mm. Also 

common are weakly porphyritic rocks containing 

microcline megacrysts that are only slightly coarser 

than the enclosing groundmass. The average modal 

analysis listed in table 1 is of samples from 19 localities distributed widely over the interior of the 

complex. As the extremes and standard deviations 

show, the variation about the mean composition is 

remarkably slight. A chemical composition computed from the mean mode compares very well with 

the few available chemical analyses of the granite, 

as shown in table 2.

Weak deuteric and (or) hydrothermal alteration, 

together with slight shearing and granulation, have 

affected much of the granite. Typically, the plagioclase shows some sericitic alteration, biotite shows 

variable degrees of alteration to chlorite and Fe-Ti
A6 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

TABLE 1. — Average modal composition, in volume percent, of 

the Lac La Croix Granite and neosome of granite-rich migmatite, based on 19 modal analyses, 2,100 points each

[Leaders ( — ) indicate not determined. Tr, trace]

Mean Standard Extremes 

deviation

Quartz ——————————————— 28.3 6.7 20.7-36.4

Perthitic K-feldspar 1 -- 32.4 5.7 22.9-52.1

Plagioclase (An ) ———— 33.3 4.9 21.5-39.0 

Biotite —————————————— 2.36 1.6 0.1- 5.4

Muscovite 2 ——————————— 1.48 1.2 0.4-4.5

Chlorite ————————————— 1.06 .8 0.1-2.3

Epidote and allanite ——— .21 .23 0.0-2.3

Opaques —————————————— .30 .22 0.1-0.7

*Perthite contains 17 percent exsolved lamellae of

albite. 

2Chiefly secondary.

oxides, and quartz shows undulose extinction. Near

fault zones the alteration and granulation are more 

severe; within 6-8 km of the Vermilion fault (Sims 

and others, 1968; fig. 1), the granitic rocks are extensively sheared and recrystallized to a variety of 

cataclastic gneisses. 

Numerous exposures of the Lac La Croix Granite 

occur east of Echo Lake and south of Lac La Croix. 

The road cuts are readily accessible along the Echo 

Trail just south of the Little Indian Sioux River 

(Shell Lake 7 ^-minute Quadrangle) and are designated as a reference locality, shown as point A on 

figure 1. 

A light-gray to grayish-pink granite gneiss exposed along the south shore of Namakan Lake was 

described by Grout (1925, p. 472-473) and has been 

interpreted by Southwick (1972, p. 114-116) as 

metasomatically replaced wall rock near the contact 

of the Vermilion Granite (Lac La Croix Granite as 

here defined). This rock is mapped on figure 1 as 

quartz-feldspar gneiss. It is well exposed near the 

mouth of Long Slough on Namakan Lake (Hale Bay 

7 1/->-minute Quadrangle), which is designated as a 

reference locality for quartz-feldspar gneiss border 

facies of the Lac La Croix.

GRANITE-RICH MIGMATITE RELATED TO THE LAC LA 

CROIX GRANITE

The Lac La Croix Granite grades into a rock that 

is mainly granite but contains 5-25 percent of paleoTABLE 2. — Chemical analyses, in percent, of the Lac La Croix 

Granite (including neosome of granite-rich migmatite). See 

figure 1 for sample localities

[Leaders ( — ) indicate not determined]

Sample 

Number —— 2-1 2-2 2-3 2-4 2-5

Si02 ———— 74.0 71.73 72.06 73.8 73.2 

A1 203 ——— 13.8 14.76 16.00 14.6 14.8

Fe 2 O 3 ——— .36 .58 .46 .88

Fe0 ————— 1.30 1.35 .72 — .9

MgO ————— .30 .62 .97 .20 .5

CaO ————— .87 1.18 .86 1.40 1.6

Na 2O ———— 3.85 3.58 4.56 4.40 3.8

l<r n _ _ _ e: in A co -} C.A A Ar\ A c.

H 20+ ——— .25 .64 .39 — .2

H 2O- ——— .03 .20 .05 .03

Ti02 ———— .13 .53 .12 .09

P 2O5 ———— .03 .14 .09 .03

MnO ————— .02 .03 .06 .01

CO2 ————— .11 

Other ——— ~ .28 .36

Total— 100.2 100.25 100.24 99.8 99.5

SAMPLE DESCRIPTIONS

2-1. Echo Trail mass, Echo Trail near crossing 

over Little Indian Sioux River; locality 

A, f ig. 1 (Arth and Hanson, 1975, p. 332, 

col. 26). 

2-2. Grassy Bay mass, Sand Point Lake (Grout, 

1926, p. 48, col. 1) . 

2-3. Neosome of granite-rich migmatite, southeast of Pelican Lake; precise location 

not known (Grout, 1926, p. 48, col. 2). 

2-4. Neosome of granite-rich migmatite, U.S. 

highway 53, 2.6 km west of St. Louis 

County road 122 (Arth and Hanson, 1975, 

p. 332, col. 25) . 

2-5. Analysis calculated from average mode of 

Lac La Croix Granite, table 1.

somatic inclusions. These inclusions range from 

clearly identifiable blocks of quartz diorite, biotite 

schist, and amphibolite to highly digested, barely 

discernable "ghosts." This granite-rich migmatite 

is the dominant rock in the west-central part of the 

complex. Although the granite neosome is similar to 

the related Lac La Croix Granite, its texture and 

modal composition are slightly more variable. The 

neosome of the granite-rich migmatite is streaky, 

locally pegmatoid, or weakly gneissic over substantial areas. Where gneissic, the rock is slightly richer 

in biotite, microcline, or both; where pegmatoid
VERMILION GRANITIC COMPLEX—A NEW NAME FOR OLD ROCKS IN NORTHERN MINNESOTA A7

zones are abundant, the nonpegmatoid granite matrix tends to be weakly to strongly porphyritic. 

Structurally the larger part of the granite-rich migmatite is nebulitic. The integrity of inclusions increases in the direction of the gradational contacts 

with schist-rich migmatite. The inclusions become 

more angular and better foliated; they gradually 

adopt a preferred orientation as they increase in 

abundance.

Numerous exposures of granite-rich migmaitite 

occur south and southwest of Crane Lake. The road 

cuts are easily accessible near the junction of the 

Echo Trail and St. Louis County highway 23 (Kabustasa Lake 7Vl>-minute Quadrangle) and are designated as a reference locality, shown as point B on 

figure 1.

SCHIST-RICH MIGMATITE RELATED TO THE LAC LA 

CROIX GRANITE

The schist-rich migmatite contains 25 percent or 

more of schistose to gneissic paleosome. Depending 

on the relative quantity of granite, the rock ranges 

from an assemblage of discrete inclusions engulfed 

in granite to a more or less uniform schist shot 

through with thin sheets and dikes of granite. The 

commonest paleosome is dark-gray biotite schist 

composed of biotite, quartz, and sodic plagioclase. 

Garnet, chlorite, and muscovite occur in uncommonly 

aluminous layers. Amphibolitic rocks make up no 

more than 5 percent of the paleosome in the northwest part of the complex, but they increase in 

abundance toward the south and east. Typically the 

amphibolite contains blue-green hornblende, andesine, epidote, and variable but small amounts of 

quartz. The schist-rich migmatite masses southwest 

of Crane Lake and west of Buyck contain abundant 

schist with subequal amounts of biotite and hornblende, as well as the more common hornblende-free 

variety of schist. Megacrysts of microcline have 

grown in some schist that has been intimately 

permeated by thin stringers of granite, but these 

megacrysts are not ubiquitous. Some of the schistose 

rocks in schist-rich migmatite are identical to metagraywacke outside the Vermilion Granitic Complex 

and almost certainly were derived from it. Others 

are largely replaced or veined by granitic material 

and cannot be linked directly to known wall-rock 

types.

The granitic fraction of schist-rich migmatite is 

white to light-pink coarse-grained leucogranite. The 

ratio of perthitic microcline to quartz and plagioclase is variable in stringers thinner than 20 or 

30 mm, but the composition of thicker layers is

quite constant and averages near the granite minimum. The texture is generally coarse but highly 

variable. Vague zones and clots of very coarse quartz 

and feldspar occur within somewhat finer grained 

rock, and subhedral to anhedral megacrysts of perthitic microcline are locally abundant. Some very 

thin neosomatic stringers contain microcline 

megacrysts that are larger than the width of the 

stringer; these crystals invade the schist on both 

sides without disturbing its foliation, indicating that 

they grew by replacement. Commonly the contacts 

between paleosome and neosome in thinly layered 

migmatite are wavy and marked by a zone a few 

millimeters thick that is anomalously rich in 

biotite. Such rocks typically have a braided appearance because of intertwined mafic and feldspathic 

wisps.

The schist-rich migmatite typically is layered. 

Rocks with plane-parallel layering are less common 

than rocks with stromatic, phlebitic, or schlieren 

structure; however, all the layering variants commonly occur close together. Layering is more or less 

continuous and coherent in migmatites that are less 

than about half granite; the granite in these rocks 

has a strong tendency to occur as conformable sheets. 

Migmatites that are more than half granite commonly exhibit some degree of discordance between 

granite contacts and foliation in the paleosome, and 

the overall structure grades into an assemblage of 

blocky inclusions surrounded by more or less uniform granite. Little rotation of blocks occurs until 

the granite fraction reaches about 75 percent, however, and it is possible to map regional structures 

from planar elements in even the most granitic of 

the schist-rich migmatites.

Good exposures of schist-rich migmatite occur 

along the shores of Kabetogama and Namakan 

Lakes. Particularly good exposures occur on the 

north side of Kabetogama Lake near the mouth of 

Lost Bay (Daley Bay TV^-minute Quadrangle), 

which is designated as a reference locality and is 

shown as point C on figure 1.

AMPHIBOLITE AND AMPHIBOLITE MIGMATITE

Amphibolitic rocks are moderately abundant at 

and near the southeast margin of the complex. 

Where amphibolite is intruded by many dikelets of 

granite, the resulting mixed rock is termed amphibolite migmatite. Most amphibolite migmatite is 

blocky in structure, and these amphibolite blocks are 

no different from unmigmatized amphibolite in texture and mineralogy. Locally, however, as in the 

area east of Pelican Lake, the amphibolite migmatite
A8 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

displays stromatic layering or schlieren structure. 

The amphibolite in the southeast part of the complex 

is composed of both layered and massive varieties. 

The layered varieties are layered on a scale of a few 

millimeters or centimeters; the layering is given by 

color contrasts that reflect different proportions of 

hornblende, oligoclase-andesine, quartz, and biotite. 

The massive variety contains variable amounts of 

hornblende and plagioclase and lesser amounts of 

quartz and biotite. Associated with the massive varieties are numerous small bodies of metagabbro. The 

massive amphibolite is interpreted to be a metamorphosed basalt; the associated: metagabbro is probably 

a subvolcanic equivalent of the basalt (Sims and 

Mudrey, 1978).

With increasing granite the amphibolite grades 

along and across strike into blocky amphibolite migmatite, which grades into granite. These relationships can be seen at several localities in the Shagawa 

Lake T^-minute Quadrangle (Sims and Mudrey, 

1978), about 1 km north of Burnside Lake. The 

granitic fraction of the amphibolite migmatite is 

dominantly pink leucogranite, virtually identical 

with the Lac La Croix Granite at its reference 

localities.

INTRUSIVE ROCKS THAT ARE GEOLOGICALLY OLDER 

THAN THE LAC LA CROIX GRANITE

Several kinds of plutonic igneous rocks that have 

little or no potassium feldspar are sharply crosscut 

by dikes or irregular masses of pink biotite granite 

related to the Lac La Croix Granite. These older 

plutonic rocks make up only a small fraction of the 

total exposed area of the complex. They tend to be 

small, irregular bodies in complicated migmatites, 

and except for the bodies of hornblende diorite and 

quartz diorite shown on figure 1, can be delineated 

only on detailed geologic maps.

The oldest intrusive rocks are hornblende diorite 

and dark-gray biotite-hornblende quartz diorite 

that in turn are intruded by light-gray granodiorite, 

tonalite, and trondhjemite. The hornblende diorite 

and quartz diorite are exposed near the Ash River 

and at a few localities near the south edge of the 

complex; only the larger masses appear on figure 1. 

Younger biotite granodiorite and trondhjemite are 

more widespread but occur in smaller bodies. Irregular dikes, sills, and plugs of granodiorite and 

trondhjemite are especially common in the schistrich migmatite belt between Cusson and Buyck (fig. 

1) and in the migmatite bodies at the west end of 

Namakan Lake. These light-gray, medium- to coarsegrained rocks typically vary considerably in texture

TABLE 3.—Average modal composition, in volume percent, of 

gray granodiorite, tonalite, and trondhjemite, based on 14 

modal analyses, 2,100 points each

[Leaders (—) indicate not determined. Tr, trace]

Epidote and allanite — -

Mean

24.7

9.4

56.0

7.06

1.44

.22

.51 

Tr

.10

.10

.10

.20

Standard 

leviation

5.47

8

"> 'S

8

r r\

2.92

1.23

.25

.80

Ex

9

0

40

2

0

0

0

0

0

0

0

treme

8 _"?9

.6-26

.3-65

.0-11

.1- 4

.1- 0

.1- 2

.1- 0

.1- 0

.1- 0

.1- 1

s

.3

.9

.9

.9

.5

.1

.5

.6

6

within distances of a few meters. Average mode, 

modal range, and chemical compositions are listed 

in tables 3 and 4.

MIGMATITE GEOLOGICALLY OLDER THAN THE GRAY 

GRANODIORITE

Complex, highly contorted schist-rich migmatite 

that is intruded by gray granodiorite is exposed near 

Buyck and Cusson. This migmatite is composed of 

compact garnet-biotite-quartz-plagioclase schist and 

granofels, biotite-poor quartz-plagioclase granofels, 

and minor amphibolite that are complexly interlayered with pegmatoid leucogranite. Structurally the rock has all variants of migmatite layering 

within short distances, and the neosome has been 

intensely boudined. Numerous thin stringers of gray 

granodiorite are concordant with the layering of the 

migmatite, but cross-cutting relations are common 

also. Both the migmatite and gray granodiorite are 

invaded on all scales by pink granite. This older 

magmatite is spatially associated with younger 

schist-rich migmatite and appears to represent a 

phase of migmatite generation that affected the 

paleosome of that rock. In general it cannot be delineated as a separate unit except on very detailed 

geologic maps.

BURNTSIDE GNEISS

Grout (1926, p. 29) named the Burntside Granite 

Gneiss for exposures around Burntside Lake, northwest of Ely. He mapped the lakeshore exposures in 

some detail and interpreted the gneiss to be an early 

phase of the Vermilion Granite (Grout, 1926, p. 30).
VERMILION GRANITIC COMPLEX—A NEW NAME FOR OLD ROCKS IN NORTHERN MINNESOTA A9

TABLE 4.—Chemical analyses, in percent, of gray granodiorite 

and trondhjemite, including the Burntside Gneiss

[Leaders (—) indicate not determined]

Sample

Number—— 4-1 4-2 4-3 4-4 4-5 4-6

Si02 ———— 67.70 68.25

A1 20 3 ——— 15.86 16.94

Fe 2 0 3 ——— .71 .70

FeO———— 2.45 1.92

MgO———— 1.02 .75

65.70 67.2 68.54

17.27 18.4 17.89

.77 — 1.77

2.56 1.57 .52

1.42 .67 1.22

CaO —— — - 

Na 2O ———— 

K2 0 —— — - 

H 20+ — — 

H 20- — —

Ti02

MnO —— —— 

CO2 —— —— 

Other -——

3.79

4.91

1.16

.91

.33

.17

.03

1.08

2.92

5.52

1.17

.81

.28 

.10 

.04 

.44

3.39

5.05

1.62

.90

.47 

.17 

.06 

.31

3.89 4.02

6.32 5.14

.92 1.05

.46

.03 .18

69.3

16.5

1.6

2.7

5.4

1.8

.3

.23 

.12 

.03

.20 

Tr 1

.06

Total— 100.12 99.84 99.69 99.4 101.05 98.5

lrTr, trace

SAMPLE DESCRIPTIONS

4-1.

4-2.

4-3.

4-4.

4-5.

4-6.

Biotite trondhjemite (1.5 percent K-feldspar), 

Duluth, Winnipeg, and Pacific Railroad, 5 km 

north of Ash Lake, Ash River SW 7 !j-minute 

quadrangle.

Biotite trondhjemite (0.6 percent K-feldspar), 

U.S. Forest Service Vermilion River road at 

Camp 40 Creek, Kabustasa Lake 7 !j-minute quadrangle. 

Light-gray biotite granodiorite (6.6 percent 

K-feldspar), U.S. highway 53 at Moose Lake Road, 

1.6 km north of Orr, Orr 7 !j-minute quadrangle. 

Burntside Gneiss, Echo Trail just north of beach 

on east side of Burntside Lake;, locality D, fig. 

1 (Arth and Hanson, 1975, p. 332, col. 17). 

Burntside Gneiss, portage between Burntside and 

Little Long Lakes (Grout, 1926, p. 29). 

Analysis calculated from average mode of gray 

granodiorite and related rocks, table 3.

Later, Goldich and others (1961, p. 58-59) suggested that the gneiss might be older than the Ely 

Greenstone and therefore substantially older than 

the Vermilion Granite of Grout (1926), which intrudes the Ely and interstratified metasedimentary 

rocks in the Vermilion district. Mapping1 by Sims 

and Mudrey (1972) confirmed Grout's original conclusion as to relative age. They interpreted the 

gneiss to be a cataclastically foliated leucocratic 

tonalite (trondhjemite) that intrudes amphibolite 

and schist correlative with Ely Greenstone and related rocks; the gneiss is in turn cut by apophyses 

of pink biotite granite related to the Lac La Croix 

Granite, as herein defined. From more detailed subsequent studies, Sims has concluded that the Burntside Granite Gneiss of Grout (1926) is metamorphosed dacite. During the metamorphism, which was 

associated with the emplacement of the major 

granitic components of the Vermilion Granitic Complex, the dacite was partially remelted and intruded 

into adjacent amphibolite and biotite schist. Because 

of its trondhjemitic composition and its apparent 

metavolcanic origin, we propose that Burntside 

Granite Gneiss be renamed Burntside Gneiss. The 

best exposures of the gneiss are on islands in and 

along the shores of Burntside Lake; the reference 

locality at the east end of the lake is designated D 

on figure 1.

The Burntside Gneiss is a light-gray gneiss having 

a moderate or strong cataclastic foliation. It contains conspicuous quartz eyes of light-blue color in a 

medium-grained matrix of oligoclase, quartz, lesser 

K-feldspar, and biotite. The biotite is strongly 

chloritized. Two chemical analyses of the gneiss are 

listed in table 4; representative modes were published previously (Sims and Mudrey, 1972, p. 100).

BIOTITE SCHIST

Several small areas of biotite schist are mapped 

within the boundaries of the Vermilion Granitic 

Complex northeast of Burntside Lake (fig. 1). The 

schist is typically dark gray on fresh surfaces but 

brown where weathered, fine to medium grained, and 

well foliated. Bedding is prominent locally. The commonest rock is composed of biotite, quartz, and sodic 

plagioclase; aluminous layers contain sparse garnet 

and muscovite as well. The schist is equivalent to 

somewhat less metamorphosed metasedimentary 

rocks (chiefly volcanogenic metagraywackes) that 

are abundant south of the Burntside Lake fault. It 

also is identical with the paleosome of adjacent 

schist-rich migmatite, into which it grades as granitic wisps and stringers oriented parallel to the 

foliation become more abundant.

PEGMATITE

Two distinct types of pegmatite occur in the Vermilion Granitic Complex. Irregular patches and indistinct, gradationally bounded dikes of simple 

quartz-alkali feldspar pegmatite are abundant in the 

granite-rich migmatite. These masses range in size 

from a few tens of millimeters to several tens of 

meters; typically the texture at the contact grades 

from granitoid to pegmatitic over distances of a few 

tens of millimeters to perhaps half a meter. The second type of pegmatite occurs in sharp-walled, 

straight dikes that postdate all other rocks of the 

complex. These pegmatite dikes commonly contain
A10 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

traces of muscovite and garnet where they traverse 

aluminous schistose wall rocks. Northwest of Echo 

Lake (fig. 1) some of the pegmatite bodies contain 

as much as 5-10 percent of primary magnetite 

(Grout, 1923, 1926) ; these pegmatites were explored in the early 1920's as possible iron-ore deposits.

AGE OF THE COMPLEX

The first systematic radiometric age study of the 

Vermilion Granitic Complex was by Goldich and 

others (1961), who reported K-Ar mineral ages on 

biotite from seven samples and a Rb-Sr mineral age 

on K-feldspar from one. All reported ages were between 2,390 and 2,620 m.y. (million years), with the 

mean about 2,550 m.y. (Goldich and others, 1961, 

table 14, p. 53; samples 47B, 46B, 239B, 249B, 250B, 

94B, and 4B). The samples used were from older 

migmatite, younger migmatite related to the Lac 

La Croix Granite, and the Lac La Croix Granite as 

herein defined; the several subunits of the complex 

were indistinguishable on the basis of radiometric 

age. Later, Peterman and others (1972) reported a 

Rb-Sr whole rock-isochron age of 2,680 ±95 m.y. for 

the Vermilion Granite, with an initial Sr87 /Sr86 of 

0.7005 ±0.0012. The isochron was based on three 

samples studied earlier by Goldich and others (1961) 

and on eight new samples collected from the Lac La 

Croix Granite and its associated migmatites as here 

defined. A refined Rb-Sr whole rock-isochron age of 

2,700 ± 50 m.y. has been reported for the complex by 

Jahn and Murthy (1975). They used eight data 

points from Peterman and others (1972) and three 

new analyses to define the isochron. Their initial 

Sr87 /Sr86 ratio of 0.7004 ±0.0003 is less uncertain 

than that reported by Peterman and others (1972), 

because two of the three new samples are relatively 

nonradiogenic mafic migmatite paleosomes. The RbSr ages were calculated usng a decay constant for 

Rb87 of 1.39X10- 11 yr- 1 .

The Rb-Sr whole-rock age of 2,700 m.y. agrees 

well with a U-Pb zircon age of 2,710 m.y. for geologically similar granite complexes in the Rainy Lake 

area (Peterman and others, 1972, p. 200-201), and 

it is accepted as the age of emplacement for the 

major Lac La Croix Granite phase of the Vermilion 

Granitic Complex. However, the ages of the rocks 

within the complex that are geologically older than 

the Lac La Croix Granite, as indicated by crosscutting relationships, have not been determined. By 

analogy with determined radiometric ages of the 

greenstone-granite complexes in the broad region 

along and adjacent to the Minnesotar-Ontario border

(Goldich, 1972, p. 34), however, it is probable that 

most of the older rocks in the complex are in the 

range of 2,700-2,750 m.y. old.

REFERENCES CITED

Arth, J. G., Jr., and Hanson, G. N., 1975, Geochemistry 

and origin of the early Precambrian crust in northeastern Minnesota: Geochimica et Cosmochimica Acta, v. 39, 

no. 3, p. 325-362.

Dietrich, R. V., and Mehnert, K. R., 1961, Proposal for the 

nomenclature of migmatites and associated rocks: International Geological Congress, 21st, Copenhagen, Report, 

pt. 26 (Supplementary vol.), p. 56-57, 73-76.

Goldich, S. S., 1972, Geochronology in Minnesota, in Sdms, P. 

K., and Morey, G. B., eds., Geology of Minnesota—A 

centennial volume in honor of George M. Schwartz: 

Minnesota Geological Survey, p. 27-37.

Goldich, S. S., Nier, A. 0., Baadsgaard, Halfdan, Hoffman, 

J. H., and Krueger, H. W., 1961, The Precambrian 

geology and geochronology of Minnesota: Minnesota Geological Survey Bulletin 41, 193 p.

Green, J. C., 1973, Geologic map of Minnesota, Two Harbors 

sheet: Minnesota Geological Survey Open-File Map, scale 

1:250,000.

Grout, F. F., 1923, The magnetite pegmatites of northern 

Minnesota: Economic Geology, v. 18, no. 3, p. 253-269.

————1925, The Vermilion batholith of Minnesota: Journal 

of Geology, v. 33, no. 5, p. 467-487.

————1926, The geology and magnetite deposits of northern 

St. Louis County, Minnesota: Minnesota Geological Survey Bulletin 21, 330 p.

Jahn, Bor-Ming, and Murthy, V. R., 1975, Rb-Sr ages of the 

Archean rocks from the Vermilion district, northeastern 

Minnesota: Geochimica et Cosmochimica Acta, v. 39, p. 

1679-1689.

Mackasey, W. 0., Blackburn, C. E., and Trowell, N. F., 1974, 

A regional approach to the Wabigoon-Quetico belts and 

its bearing on exploration in northwestern Ontario: Ontario Division of Mines Miscellaneous Paper 58, 29 p.

Mehnert, K. R., 1968, Migmatites and the origin of granitic 

rocks: Amsterdam, Elsevier, 393 p.

Peterman, Z. E., Goldich, S. S., Hedge, C. E., and Yardley, 

D. H., 1972, Geochronology of the Rainy Lake region, 

Minnesota—Ontario, in Doe, B. R., and Smith, D. K., 

eds., Studies in mineralogy and Precambrian geology, a 

volume in honor of John W. Gruner: Geological Society 

of America Memoir 135, p. 193-215.

Sims, P. K., 1973, Geologic map of western part of Vermilion 

district, northeastern Minnesota: Minnesota Geological 

Survey Miscellaneous Map Series No. 13, scale 1:48,000.

————1976, Early Precambrian tectonic-igneous evolution in 

the Vermilion district, northeastern Minnesota: Geological Society of America Bulletin, v. 87, no. 3, p. 379- 

389.

Sims, P. K., Morey, G. B., Ojakangas, R. W., and Griffin, 

W. L., 1968, Preliminary geologic map of the Vermilion 

district and adjacent areas, northern Minnesota: Minnesota Geological Survey Miscellaneous Map Series, M-5, 

scale 1:125,000.

Sims, P. K., Morey, G. B., Ojakangas, R. W., and Viswanathan, S., 1970, Geologic map of Minnesota, Hibbing sheet: 

Minnesota Geological Survey, scale 1:250,000.
VERMILION GRANITIC COMPLEX—A NEW NAME FOR OLD ROCKS IN NORTHERN MINNESOTA All

Sims, P. K., and Mudrey, M. G., Jr., 1972, Burntside Granite 

Gneiss, Vermilion district, in Sims, P. K., and Morey, G. 

B., eds., Geology of Minnesota—A centennial volume in 

honor of George M. Schwartz: Minnesota Geological 

Survey, p. 98-101.

Sims, P. K., and Mudrey, M. G., Jr., 1978, Geologic map of 

the Shagawa Lake quadrangle, St. Louis County, Minnesota: U.S. Geological Survey Geologic Quadrangle Map 

GQ-1423, scale 1:24,000.

Sohl, N. F., 1977, Stratigraphic Commission, Note 45-Application for amendment concerning terminology for igneous 

and high-grade metamorphic rocks: American Association of Petroleum Geologists Bulletin, v. 61, no. 2, p. 

248-252.

Southwick, D. L., 1972, Vermilion granite-migmatite massif, 

in Sims, P. K., and Morey, G. B., eds., Geology of Minnesota—A centennial volume in honor of George M. 

Schwartz: Minnesota Geological Survey, p. 108-119.

Southwick, D. L., and Ojakangas, R. W., 1973, Geologic map 

of Minnesota, International Falls sheet: Minnesota 

Geological Survey Open-File Map, scale 1:250,000.

Streckeisen, A. L., chairman, 1973, Plutonic rocks—classification and nomenclature recommended by the IUGS Subcommission on the Systematics of Igneous Rocks: Geotimes, v. 18, no. 10, p. 26-30.

Van Hise, C. R., and Clements, J. M., 1901, The Vermilion 

iron-bearing district: U.S. Geological Survey 21st Annual Report, pt. 3, p. 401-409.

Winchell, N. H., and Winchell, Alexander, 1887, The Geological and Natural History Survey of Minnesota: Minnesota Geological and Natural History Survey 15th Annual Report, 496 p.

Potassium-Argon Ages from the

Mount Taylor Volcanic Field, New Mexico

By PETER W. LIPMAN and HARALD H. MEHNERT

SHORTER CONTRIBUTIONS TO MINERALOGY AND 

PETROLOGY, 1979

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1124-B

Most volcanic activity occurred between 

43 and 1.5 million years ago

CONTENTS

Page

Abstract _______________---________ ———— —————————— ———— B!

Introduction _____________________________________--_---_----------------- 1

Acknowledgments _____________________---___-_------------------------ 1

Volcanic history ____________________--_______-__--_----_------------------ 1

Discussion _______________________________________________—____________— 5

References cited _____________________________-___-_-__----_--------------- 7

ILLUSTRATIONS

Page

FIGURE 1. Generalized geologic map of volcanic rocks in the Mount Taylor area, New Mexico, showing localities of 

dated samples and generalized section showing stratigraphic correlations in the Mount Taylor volcanic 

field _______________________________-__________________________—————— B2

2. Diagrammatic cross section, showing stratigraphic relations between dated volcanic units on the south side

of Mount Taylor and adjacent mesas _________________ — ___________________-_---_--_-_---------- 4

3. Map and graph showing the space-time relations of Cenozoic volcanism along the Springerville-Raton

zone ____________________________________________—______________--_---_----------------_-- 6

TABLE

Page

TABLE 1. K-Ar ages from volcanic rocks from the Mount Taylor area, New Mexico __________ — __ — — _______ B4

in

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

POTASSIUM-ARGON AGES FROM THE 

MOUNT TAYLOR VOLCANIC FIELD, NEW MEXICO

By PETER W. LIPMAN and HARALD H. MEHNERT

ABSTRACT

Fourteen new K-Ar dates for volcanic rocks of the Mount 

Taylor field, New Mexico, indicate that most activity occurred 

between 4.3 and 1.5 m.y. (million years) ago. Peak activity 

was at about 3.0-2.5 m.y., both on the central andesite-rhyolite shield volcano and on the surrounding alkali basalttrachyte volcanic plateau, and occurred concurrently with an 

episode of NNE-trending basin-range faulting. The K-Ar 

dates also indicate that the regional Ortiz pediment surface, 

graded to the ancestral Rio Grande, existed in the Mount 

Taylor area as recently as 3 m.y. ago and that 250-400 m of 

erosional downcutting has occurred in subsequent time. 

Growth of the Mount Taylor field was also concurrent with 

peak volcanic activity along the northeast-trending Springerville-Raton zone, a major late Cenozoic volcanic belt that is 

considered to reflect the presence of a regional structural discontinuity of Precambrian age in the North American craton.

INTRODUCTION

The general sequence of volcanic events in the 

Mount Taylor area of north-central New Mexico, 

well established from the reconnaissance work of 

Hunt (1938), has recently been the subject of detailed mapping and petrologic studies (Baker and 

Ridley, 1970; Lipman and Moench, 1972; Grumpier, 

1977). None of these studies has provided close 

chronologic control on the volcanic events, however, 

and accordingly we here report and interpret 14 new 

K-Ar dates on rocks of late Cenozoic age of the 

Mount Taylor field. These dates were determined in 

conjunction with mapping of the Mount Taylor 7 1/X>- 

minute quadrangle (Lipman and others, 1979) ; the 

interested reader may wish to consult this map. The 

new dates demonstrate that virtually all volcanic 

activity in the field occurred during a 2 m.y. (million 

year) span between about 4.3 and 1.5 m.y. ago, and 

peak activity was about 3.0-2.5 m.y. Flanking basaltic rocks were erupted before, during, and after 

growth of the central Mount Taylor cone.

ACKNOWLEDGMENTS

We thank John Pallister for assistance in collecting samples and making mineral separations. Discussions with Robert Moench and George Bachman 

of the U.S. Geological Survey have contributed to 

our understanding the volcanic history of the Mount 

Taylor field and its geologic setting. Larry Grumpier, 

while a graduate student at the University of New 

Mexico, made available some preliminary results of 

his field and petrologic study on central Mesa 

Chivato.

VOLCANIC HISTORY

Upper Cenozoic volcanic rocks of the Mount 

Taylor field presently cover an area of about 1,000 

km2 in north-central New Mexico, near the southeast edge of the Colorado Plateau (fig. 1A). Mount 

Taylor is a central stratovolcano consisting of rocks 

ranging in composition from alkali andesite to rhyolite (Baker and Ridley, 1970) ; the central cone is 

surrounded by an eroded basalt-capped plateau consisting of flows erupted before, during, and after the 

growth of the central cone (fig. IB). Many of these 

flows are silicic alkalic basalt (hawaiite), but flows 

are present in a nearly continuous range from basar 

nite to trachyte (Lipman and Moench, 1972; 

Grumpier, 1977).

The Mount Taylor cone has been deeply eroded, 

and prevolcanic sedimentary rocks of Cretaceous age 

are exposed in its core (Hunt, 1938; figs. 1, 2). The 

earliest volcanic unit exposed in this area is a 

trachyte lava dome (Lipman and others, 1979), 

which has yielded a K-Ar age of 4.37 ±0.27 m.y. 

(table 1, loc. no. 1). This is the oldest seemingly reliable radiometric age thus far determined for any 

rock in the volcanic field, and it thus provides a

Bi
B2

108°

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

45> 30> Samples 2, 7, 8

107 rj 1 5'

EXPLANATION

PJo Q bl ° 01

Qa

HOLOCENE AND PLEISTOCENE BASALTS

Tholeiitic basalt

Alkalic Zuni Canyon flow

PLEISTOCENE BASALTS

Of low mesas

Upper basalt

PLEISTOCENE ANDESITIC FLOWS, PLAGIOCLASE BASALT, 

AND PYROCLASTIC FLOWS OF MOUNT TAYLOR

PLEISTOCENE AND PLIOCENE RHYOLITE DOMES AND TUFFS

PLIOCENE LOWER BASALT

Flows

Plugs

CRETACEOUS AND OLDER SEDIMENTARY ROCKS

Contact

•j——— Fault--Bar and ball on downthrown side

* Basalt vent

o14 Sample locality

FIGURE 1.—A, Generalized geologic map of volcanic rocks in the Mount Taylor area, New Mexico, showing localities of

dated samples.

lower limit for the beginning of the growth of the 

Mount Taylor cone.

Another trachyte occurs as a large dome-flow complex on Mesa Chivato, the volcanic plateau north of 

Mount Taylor (fig. 1). This trachyte has yielded a 

somewhat younger age of 3.30 ±0.20 m.y. (table 1, 

no. 2). Subsequent to determination of this age,

several basaltic units were mapped underlying this 

trachyte by Grumpier (1977), so rocks may exist on 

the northern plateau as old as or older than the 

trachyte in the core of the Mount Taylor volcano. 

The dated trachyte on the northern plateau is also 

significant because it predates a 12-km-wide zone of 

normal faulting of basin-range type that trends
POTASSIUM-ARGON AGES FROM THE MOUNT TAYLOR VOLCANIC FIELD, NEW MEXICO B3

SOUTHERN MESAS MOUNT TAYLOR CONE MESA CHIVATO

•/;Thoeiitic basat (038) l-V

s\Upper basalt of high mesas--

^XX\-X.\\.\\\.N.X. XX.X.X.X.X.X.X.

ofjow 

flows 1 2-1 3'

Upper andesitic flow

Late pyroclastic flow (2.51-2.73)

XXXXXXXXXXXXXXXXXXXX

Upper basalt--flow 6 1 (265) Basalt flows and cinder cones

Plagioclase basalt--flow 4

Andesitic flows of the central 

cone (2.4-2.9) 2

Upper rhyolite domes and tuffs 

including rhyolite of Grants 

Ridg«"(3.34)

Lower basalt 

flow 1 1 (2

Trachyte dome (3.22)

Terminology of Moench and Schlee (1967) 

' Dates from Bassett and others (1963) 

'Most reasonable of two determinations.

FIGURE 1.—B, Generalized stratigraphic correlations in the Mount Taylor volcanic field (modified from Lipman and others, 

1979). K-Ar ages, in millions of years, given in parentheses. Patterns correspond to those used in A.

north-northeast across the mesa; the age of this 

trachyte accordingly provides a lower limit on the 

age of this faulting.

In the erosional basin within Mount Taylor and 

on the south flank of the volcano (figs. 1, 2), the 

trachyte dated at 4.37 m.y. is overlain by rhyolitic 

ash-fall tuffs related to upper rhyolite lava domes 

and by distinctive basanite basalt flows (lower 

basalt, upper flows) that, when traded to the southeast, are underlain by one or two silicic basalt flows 

(lower basalt, lower flows). The upper flows are 

everywhere conformable with the lower flows and 

lack intervening sedimentary deposits; They appear 

closely related in age; accordingly, both are probably younger than the 4.37-m.y. trachyte. Two 

radiometric ages determined from lower flows of 

the lower basalt are 9.87 ±1.52 and 2.97 ±0.86 m.y. 

(table 1, nos. 3-4). The older age, although analytically reproducible, is considered to be of doubtful 

geologic significance for the reasons cited above, 

although such an age for one of the lower flows cannot rigorously be excluded on stratigraphic evidence. 

These flows tend to be weakly deuterically altered 

(Lipman and Moench, 1972) ; the alteration may in 

some way be related to the apparently anomalous 

date. The relatively low radiogenic argon contents 

of both samples (9.2 and 13.5 percent) also suggest 

the possibility of unreliable ages.

The lower basalts are overlain both within the 

Mount Taylor cone and around its south flank by 

rhyolite lava domes and associated pyroclastic ashflow and ash-fall tuffs of the upper rhyolite (fig. IB). 

Two rhyolitic vent areas have been mapped: one 

within the core of the Mount Taylor cone and the 

other on Grants Ridge to the west (fig. 1). Pyroclastic rocks from both vent areas merge at a common
B4 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

NW SE

Mount Taylor

, Upper andesite flow

Mud flows and pyroclastic flows 2.51-2.73 m.y. 

Plagioclase basalt (6.03 m.y.)

Upper basalt, upper flow 1.56 m.y.

Upper basalts 2.87, 2.75, 2.65 m.y.

Trachyte dome Lower basalt, 

4.37 m.y. upper flows

Pre-Tertiary rocks

Basalt of low mesas 

2.42 m.y.

Late tholeiitic basalt 

0.38 m.y.

FIGURE 2.—Diagrammatic cross section, showing stratigraphic relations between dated volcanic units on south side of Mount 

Taylor and adjacent mesas. Dates are K-Ar ages; seemingly unreliable ages in parentheses.

TABLE 1.—K-Ar ages from volcanic rocks from the Mount Taylor area, New Mexico

[K4 °X = 0.581xlO~ 10/yr; A = 4.962xlO~ 10/yr; atomic abundance K40/K = 1.167xlO~4 ]

Sample 

locality Unit 

Number

11

6

Laguna .

flow 11.

flow 13.

Late pyroclastic flow, —

Mount Taylor.

flow 6.

Cerros de Guadaloupe .

end Mesa Chivato.

Mount Taylor .

Ridge.

lower flows.

Cerros de Guadaloupe .

Mount Taylor.

Field 

Number

74L-134B

74L-118B

Location 

(lat N. , long W.)

35°13' ,

35°11',

107°39'

107°31'

Rock type or 

mineral

biotite ————

plagioclase-

(percent)

.80,

5.51,

.83

5.48

(percent) moles/gram)

1.73,

7.84,

1.50,

.63,

1.72

7.95

1.49

.62

.311

.054

*» 40 *Ar

(percent)

36.1

31.1

Age m.y. 

12a

2.7310.16

1 6. 0310. 60

Geologically inconsistent date.

stratigraphic horizon between the lower and the 

upper basalts. A whole-rock obsidian date for the 

rhyolite on Grants Ridge is 3.34 ±0.16 m.y. (table 

1, no. 5), which is only slightly older and not analytically discordant with the preferred date of 2.92 ± 

0.86 for the underlying lower basalt flow that occurs 

below this rhyolite horizon to the east.

The thick cone-forming sequence of porphyritic 

andesite flows on the Mount Taylor volcano was not 

dated by us, but an early published date of relatively 

low analytical precision for a sample of this unit is 

2.4-2.9 m.y. (Basset and others, 1963, table 1, no. 

8). This date is in reasonable agreement with limiting ages determined by us for underlying and overlying units (fig.

A thick sequence of basaltic flows that were 

erupted following growth of the main Mount Taylor 

cone includes (1) distinctive, coarsely porphyritic 

plagioclase basalt flows erupted from high on the 

Taylor cone late in its growth, (2) sequences of as 

many as eight silicic alkalic basalt flows preserved 

on mesas around the east, south, and west sides of 

Mount Taylor (upper basalt, table 1), and (3) 

complexly interfingering sequences of basalts and 

more silicic alkalic flows on Mesa Chivato to the 

north (Grumpier, 1977).

An attempt to date the plagioclase basalt was unsuccessful: a date of 6.03 ±0.60 m.y. (table 1, no. 6) 

would seem too old by about 3 m.y., as indicated by 

the inconsistency with dated underlying units, in-
POTASSIUM-ARGON AGES FROM THE MOUNT TAYLOR VOLCANIC FIELD, NEW MEXICO B5

eluding the cone andesites, the upper rhyolite, the 

lower basalt, and the trachyte (fig. IB; table 1). We 

have no direct explanation for this inconsistent date, 

but some dates from volcanic plagioclase phenocrysts have been interpreted as being anomalously 

old owing to excess argon (Damon and others, 

1967).

A flow of upper basalt that directly overlies the 

plagioclase basalt southeast of Mount Taylor yielded 

an age of 2.65±0.15 m.y. (table 1, no. 9). This 

sample helps bracket the age of the main Mount 

Taylor cone to between about 3.0 and 2.5 m.y., in 

reasonable agreement with the older published age 

of 2.3-2.8 m.y. (Basset and others, 1963).

Two basalt samples from Mesa Chivato to the 

north yielded roughly similar ages of 2.87 ±0.20 and 

2.75 ±0.30 m.y. (table 1, nos. 7-8). The younger 

date is from a spatter cone in Cerros de Guadaloupe 

that rests on the trachyte from the same locality 

dated at 3.30±0.20 m.y. (table 1, no. 2). This basalt 

cone is alined along a major north-northeast-trending fault that displaces the trachyte at least 20-30 m, 

but the cone and its related flow are displaced by the 

fault at most about 2 m. These relations indicate 

significant fault movement during a relatively restricted time interval about 3 m.y. ago, concurrent 

with peak activity in the Mount Taylor volcanic 

field. The older date, from the lowest flow at the 

northern end of Mesa Chivato (Bachman and Mehnert, 1978), indicates that all basaltic activity this 

far north was as young as .or younger than the main 

cone^building stage of Mount Taylor to the south.

An intermediate-composition pyroclastic flow on 

the southwest flank of Mount Taylor, which postdates the main cone-building phase and overlies the 

capping basalts of Horace Mesa (fig. 1), yielded 

K-Ar ages of 2.73 ±0.16 and 2.51 ±0.25 m.y. for 

biotite and plagioclase phenocrysts (table 1, nos. 

10-11). These dates provide further confirmation 

that growth of the main cone was essentially complete by about 2.5 m.y. ago. The capping basalt flows 

on Horace Mesa, which underlie the dated pyroclastic flow and wedge out against the Mount Taylor 

cone, are also accordingly bracketed as having been 

erupted within a geologically brief time about 2.5 

m.y. ago, preceding emplacement of the pyroclastic 

flow. Widespread pyroclastic deposits—mainly mudflow breccias and conglomerates—and a single late 

flow of porphyritic andesite from the central cone 

underlie and interfinger with the upper basalts on 

the mesas east of Mount Taylor (fig. 2; Lipman and 

others, 1979) ; these also would appear to have been 

emplaced mainly about 2.5 m.y. ago, as indicated by

ages on underlying and overlying basalt flows (fig.

On the southeast side of Mount Taylor, two basalt 

flows occur as mesa-capping remnants of a pediment 

surface about 50 m lower than the high mesas (the 

Wheat Mountain surface of Moench and Schlee, 

1967). The upper of these two flows yielded a K-Ar 

date of 2.42 + 0.18 m.y. (table 1, no. 12), indicating 

that significant erosion had occurred around the 

margins of the volcanic plateau by this time.

The highest mapped flow of upper basalt, on the 

mesa southeast of Mount Taylor (flow 11 of Moench 

and Schlee, 1967), yielded a relatively young age of 

1.56 ±0.17 m.y. (table 1, no. 13). This flow overlies 

all the mudflow conglomerates derived from the 

Mount Taylor cone and does not extend to the mesa 

rim. If this K-Ar date is taken at face value (and we 

see no obvious reason to reject it), it would indicate 

that basaltic activity continued in central parts of 

the volcanic plateau while margins of the mesas 

were being eroded. Some younger basalt units 

mapped farther north on Mesa Chivato by Grumpier 

(1977) overlie the dated 2.75 m.y. flow there (table 

1, no. 8) and may also be relatively young. Such late 

high-mesa basalts would seem relatively uncommon, 

however, as is indicated by the general absence of 

observable channel-and-fill relations or plasters 

across erosional scarps at mesa rims.

The youngest dated basalt flows in the region occur around the south and southwest side of the 

Mount Taylor field, to within a few meters above 

present drainage levels. Of this group, a distinctive 

tholeiitic basalt near Laguna (fig. 1) yielded a 

K-Ar age of 0.38 ±0.25 m.y. This flow, derived from 

an unidentified vent further west, is probably genetically related to a complex basalt field centered in the 

Zuni Mountains that includes the McCartys, Zuni 

Canyon, and Bluewater flows (fig. 1 ; Lipman and 

Moench, 1972; Laughlin and others, 1972).

DISCUSSION

The new K-Ar dates, while not completely resolving all details of the volcanic history of the Mount 

Taylor field, demonstrate that the bulk of volcanic 

activity in this area occurred within a geologically 

brief interval between about 4.3 and 1.5 m.y. ago. 

A voluminous alkali basalt-trachyte sequence was 

erupted over a large region to form a volcanic 

plateau, with concurrent eruption of rocks ranging 

from alkali andesite to rhyolite, to form the central 

Mount Taylor cone. Peak volcanic activity both on 

the cone and on the surrounding plateau was be-
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TIME BEFORE PRESENT. IN MILLIONS OF YEARS

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Gila Bend

White Mountains

Zuni

_ Mount Taylor

Jemez Mountains

Ortiz 

Mountains

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Cimarron 

Mountains

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POTASSIUM-ARGON AGES FROM THE MOUNT TAYLOR VOLCANIC FIELD, NEW MEXICO B7

tween about 3.0 and 2.5 m.y. ago, concurrent with a 

significant episode of north-northeast-trending 

basin-range faulting in the same area.

The oldest volcanic rocks of the Mount Taylor 

field were deposited on a broad surface of low relief 

that probably constitutes part of the regional Ortiz 

surface (Bryan, 1938; Bachman and Mehnert, 1978), 

a regional, widespread pediment in central New 

Mexico graded to the ancestral Rio Grande. The new 

dates indicate that this surface existed as recently 

as about 3 m.y. ago, and that drainage levels adjacent to the Mount Taylor field have been lowered 

approximately 250-300 m in areas adjacent to the 

Mount Taylor field since that time.

Major activity in the Mount Taylor area also coincides with the time of a widespread volcanism in 

New Mexico and Arizona along a northeast-trending 

belt (fig. 3)—the Springerville-Raton zone, or the 

Jemez zone of Mayo (1958). Volcanic activity of 

roughly similar age occurred to the northeast in the 

Jemez Mountains, on the Taos Plateau, and on several parts of the High Plains of New Mexico and to 

the southwest in the Zuni Mountains, in the White 

Mountains, in the Springerville area, near Gila Bend, 

and as far as the Pinacate volcanic field in northernmost Mexico. These alined volcanic fields have recently been interpreted by Suppe, Powell, and Berry 

(1975, p. 512-513) as marking the trace of a mantle 

plume, but the lack of any consistent age progression 

•of volcanism along the belt (fig. 35) makes this 

hypothesis improbable. More likely, the volcanic belt 

follows an old zone of weakness in the lithosphere 

(Mayo, 1958; Laughlin, 1976) that parallels the 

northeast-trending Precambrian structures, which 

are dominant in the southwest (Shoemaker and 

others, 1974), and coincides closely with a boundary 

between major Precambrian age provinces (L. T. 

Silver, unpub. data, cited in Cor dell, 1978).

REFERENCES CITED

Bachman, G. 0., and Mehnert, H. H., 1978, New K-Ar dates 

and the late Pliocene to Holocene geomorphic history of 

the central Rio Grande region, New Mexico: Geological 

Society of America Bulletin, v. 89, no. 2, p. 283-292.

Baker, I., and Ridley, W. I., 1970, Field evidence and K, Rb, 

Sr data bearing on the origin of the Mt. Taylor volcanic 

field, New Mexico, U.S.A.: Earth and Planetary Science 

Letters, v. 10, no. 1, p. 106-114.

Bassett, W. A., Kerr, P. F., Schaeffer, 0. A., and Stoenner, 

R. W., 1963, Potassium-argon ages of volcanic rocks 

near Grants, New Mexico: Geological Society of America 

Bulletin, v. 74, no. 2, p. 221-226.

Bryan, Kirk, 1938, Geology and ground-water conditions of 

the Rio Grande depression in Colorado and New Mexico, 

in U.S. National Resource Planning Board, The Rio 

Grande Joint Investigation in the upper Rio Grande 

basin: Washington, U.S. Government Printing Office, v. 

1, no. 2, p. 197-225.

Cordell, L., 1978, Regional geophysical setting of the Rio 

Grande rift: Geological Society of America Bulletin, v. 

89, no. 7, p. 1073-1090.

Grumpier, L. S., 1977, Alkali basalt-trachyte suite and volcanism, northern part of the Mount Taylor volcanic 

field, New Mexico: Albuquerque, University of New 

Mexico M.S. thesis, 131 p.

Damon, P. E., Laughlin, A. W., and Percious, J. K., 1967, 

Problems of excess argon-40 in volcanic rocks, in Radioactive dating and methods of low-level counting, IAEAICSU Symposium, Monaco, 1967, Proceedings: Vienna, 

International Atomic Energy Agency, p. 463-482.

Doell, R. R., Dalrymple, G. B., Smith, R. L., and Bailey, R. 

A., 1968, Paleomagnetism, potassium argon ages, and 

geology of rhyolites and associated rocks of the Valles 

Caldera, New Mexico: Geological Society of America 

Memoir 116, p. 211-248.

Elston, W. E., Damon, P. E., Coney, P. J., Rhodes, R. C., 

Smith, E. I., and Bikerman, M., 1973, Tertiary volcanic 

rocks, Mogollon-Datil province, New Mexico, and surrounding region—K-Ar dates, Patterns of Eruption, and 

Periods of Mineralization: Geological Society of America 

Bulletin, v. 84, no. 7, p. 2259-2273.

Hunt, C. B., 1938, Igneous geology and structure of the Mount 

Taylor volcanic field, New Mexico: U.S. Geological Survey Professional Paper 189-B, p. 51-80.

Laughlin, A. W., 1976, Late-Cenozoic basaltic volcanism 

along the Jemez zone of New Mexico and Arizona: Geological Society of America Abstracts with Programs, v. 

8, no. 5, p. 598.

Laughlin, A. W., Brookins, D. G., and Causey, J. D., 1972, 

Late Cenozoic Basalts from the Bandera Lava Field, 

Valencia County, New Mexico: Geological Society of 

America Bulletin, v. 83, no. 5, p. 1543-1551.

Lipman, P. W., Pallister, J. S., and Sargent, K .A., 1979, 

Geologic map of the Mount Taylor quadrangle, Valencia 

County, New Mexico: U.S. Geological Survey Geological 

Quadrangle Map GQ-1523, scale 1:24,000.

Lipman, P. W., and Moench, R. H., 1972, Basalts of the Mount 

Taylor Volcanic Field, New Mexico: Geological Society 

of America Bulletin, v. 83, no'. 5, p. 1335-1343.

Mayo, E. B., 1958, Lineament tectonics and some ore districts 

of the Southwest: Mining Engineering, v. 10, no. 11, p. 

1169-1175.

Merrill, R. K., 1974, The late Cenozoic geology of the White 

Mountains, Apache County, Arizona: Tempe, Arizona 

State University Ph. D. thesis, 202 p.

Moench, R. H., and Schlee, J. S., 1967, Geology and uranium 

deposits of the Laguna district, New Mexico: U.S. Geological Survey Professional Paper 519, 117 p.

Shoemaker, E. M., Squires, R. L., and Abrams, M. J., 1974, 

The Bright Angel and Mesa Butte fault systems of 

northern Arizona, in Karlstrom, T. N. V., Swann, G. A., 

and Eastwood, R. L., eds., Geology of Northern Arizona, 

Part 1, Regional studies: Published for Geological Society 

of America Rocky Mountain Section Meeting, Flagstaff,
B8 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

Arizona p. 355-392. [Available from Northern Arizona 

University Bookstore, Flagstaff, Arizona.] 

Suppe, J., Powell, C., and Berry, R., 1975, Regional topography, seismicity, Quaternary volcanism and the presentday tectonics of the western United States: American 

Journal of Science, v. 275-A, p. 397-436.

U.S. Geological Survey, 1965, Geologic map of North America: U.S. Geological Survey, 2 sheets, scale 1:5,000,000.
Crystals of Coexisting 

Alunite and Jarosite, 

Goldfield, Nevada

By WILLIAM J. KEITH, LEWIS CALK, and R. P. ASHLEY

SHORTER CONTRIBUTIONS TO MINERALOGY AND 

PETROLOGY, 1979

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1124-C

CONTENTS

Page

Abstract ____________________________________________ Cl

Introduction ________________________________________________________ 1

Geologic setting _____________________________________________________ 1

Occurrence and properties ________„_____________________________________ 1

Electron microprobe data _______________________________________________ 3

Discussion ___________________________________________________________ 4

References cited _______________________________________________________ 5

ILLUSTRATIONS

Page

FIGURE 1. Photomicrograph of an optically continuous single crystal composed of alunite and jarosite that projects

into a vug __________________________________________________________________________ Cl

2. Map of Goldfield, Nev., and vicinity showing areas of hydrothermal alteration and ore deposits ________ 2

3. Photomicrograph of a comblike growth of alunite forming around silicified breccia fragment _________ 3

4. Photomicrograph of two crystals of alunite capped by jarosite showing step scan paths across contact __ 3

5. Graphs showing the distribution of aluminum, iron, and potassium across alunite-j arosite boundary in two

crystals _____________________________________________________________________________ 4

6. Generalized Eh-pH diagram showing relative stability fields for alunite and jarosite _______________ — _ 5

TABLE

Page

TABLE 1. Free energy used in calculations of generalized Eh-pH diagram for alunite and jarosite ___________ — __ C5

ill

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

CRYSTALS OF COEXISTING ALUNITE AND JAROSITE, 

GOLDFIELD, NEVADA

By WILLIAM J. KEITH, LEWIS CALK, and R. P. ASHLEY

ABSTRACT

Alunite (KAla (SO4 ),(OH)o) and jarosite (KFe:! (SO4 ) £ 

(OH)o) coexist in optically continuous crystals in a sample 

from the Goldfield mining district, Esmeralda County, Nev. 

Beam scans with an electron microprobe indicate that no 

intermediate minerals of the solid-solution series alunitejarosite occur in these crystals. An Eh-pH diagram demonstrating the equilibrium relations between alunite, jarosite, 

and geothite suggests the crystals most likely developed by a 

rise in Eh, which oxidized ferrous to ferric iron, thus allowing jarosite to precipitate.

FIGURE 1.—Optically continuous single crystal composed of 

alunite (A) and jarosite (J) that projects into a vug. 

Plane-polarized light.

INTRODUCTION

Alunite and jarosite coexist in the same optically 

continuous crystal (fig. 1) in a hand specimen from 

the Goldfield mining district (fig. 2), Esmeralda 

County, Nev. Brophy, Scott, and Snellgrove (1962) 

have shown that, although the intermediate minerals 

are rare, a solid solution exists between these two 

minerals in nature. The purpose of this study is to 

determine whether any intermediate varieties are 

present in the Goldfield sample and to propose a 

model explaining how these two-phase crystals might 

have formed.

GEOLOGIC SETTING

The epithermal precious-metal deposits of the 

Goldfield mining district produced over 130,000 kg of 

gold between 1903 and 1961. The district is part of a 

Tertiary volcanic center in which the rocks range 

from andesite to rhyolite. The ore bodies occur within a hydrothermally altered area (fig. 2) that covers 

approximately 39 km2 (Ashley and Albers, 1975, p. 

A3). The sample described in this report is from 

Preble Mountain, in the southern part of the area 

(fig. 2). Preble Mountain is composed largely of 

trachyandesite and rhyodacite flows and tuffs (Ashley, 1975) that have been argillized and silicified 

(Ashley and Keith, 1976).

OCCURRENCE AND PROPERTIES

Alunite and jarosite are relatively common sulfate 

minerals of the alunite group and are frequently 

assiociated with hydrothermal mineral deposits, although they are by no means limited to them. The

Cl
C2 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

R. 43 E.

Surface projection of 

ore-beoring area

FIGURE 2.—Map of Goldfield, Nev., and vicinity showing areas of hydrothermal alteration and ore deposits.

general formula for the group is AB3 (SO4) 2 (OH) 6 , 

where "A" represents potassium and "B" represents 

aluminum in alunite and ferric iron in jarosite. 

Sodium ions, and occasionally others, may substitute 

for potassium in varying amounts. Where the sodium-potassium ratio exceeds 1, the mineral is called 

natroalunite or natrojarosite. Alunite and jarosite 

both belong to the space group R3m and, therefore, 

have the same crystal symmetry. The alunite and 

jarosite in this study have developed perpendicular 

to the c crystallographic axis.

Alunite and jarosite in the Preble Mountain area 

are thought to be of hypogene origin because a 

sample of coarse-grained alunite intergrown with 

jarosite gave a K-Ar age of 20±0.4 million years, 

which is concordant with the age of mineralization 

determined by dating post- and premineralization

rocks (Ashley and Silberman, 1976). The normal sequence of crystallization is alunite followed by jarosite (Ashley and Keith, 1976, p. BIO; Ashley and 

Silberman, 1976). Initially, alunite formed comblike 

growths surrounding silicified rock breccia fragments (fig. 3) and extending into fractures and vugs. 

Then both alunite and jarosite formed crystal aggregate linings in vugs. Small vugs were completely 

filled in places by a cream to yellow-brown aggregate 

of the two minerals. The alunite crystals found in 

these aggregates are typically larger than the jarosite and commonly appear to be surrounded by finegrained jarosite. In the final stage, jarosite alone 

encrusted remaining vein walls, covering alunitejarosite aggregates where they were present. Where 

the aggregates were absent, jarosite filled open 

spaces among alunite crystals and formed termina-
CRYSTALS OF COEXISTING ALUNITE AND JAROSITE, GOLDFIELD, NEVADA C3

FIGURE 3.—Comblike growth of alunite (A) forming around 

silicified breccia fragment (B). Plane-polarized light.

tions on some (fig. 1). Fracture fillings of coexisting 

alunite and jarosite have been seen at more than 60 

localities in a 3-km2 area at Preble Mountain, but 

notably coarse-grained alunite or jarosite or both 

occur at only a few of these localities.

Where jarosite forms optically continuous terminations on alunite crystals, the planar boundary 

between the two phases is quite sharp and appears 

to parallel the terminating faces (fig. 1). The boundaries do not show embayments or other irregularities typical of replacement of one mineral by another.

ELECTRON MICROPROBE DATA

An Applied Research Laboratories EMX-SM1 

electron microprobe was used to examine compositional changes across two of the two-phase crystals. 

Step scans were performed across the phase boundaries of the crystals (fig. 4) using 3-/*m intervals and 

a counting period of 10 sec. The crystals were analyzed for potassium, aluminum, and iron.

The data from the microprobe scans (fig. 5) show 

that iron and aluminum are relatively homogeneous 

in both phases although they change in quantity from 

one phase to the other. Iron does not increase in alunite as the phase boundary is approached, and, conversely, aluminum does not increase in jarosite. The

1 The use of trade names and trademarks in this publication is for descriptive purposes only and does not constitute endorsement by the U.S. 

Geological Survey.

FIGURE 4.—Two crystals of alunite (A) capped by jarosite 

(J) showing step scan paths across contact. Plane-polarized 

light. For data on A and B refer to figure 5A and B, respectively.

potassium is relatively constant throughout, changing little from one phase to the other. Alunite is 

much less stable than jarosite under the electron 

beam (fig. 4). The beam craters are much more pronounced in alunite, suggesting that potassium may be 

more abundant in alunite than we determined. Nonetheless, the stable quantities of iron or aluminum 

indicate that intermediate species between alunite 

and jarosite do not exist in these crystals.
C4 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

B

10 20 30 40 

STEP DISTANCE, IN MICROMETERS

FIGURE 5.—Distribution of aluminum (squares), iron (triangles), and potassium (circles) across alunite-jarosite 

boundary in two crystals: A (shown in fig. 44) and B 

(shown in fig. 4B).

DISCUSSION

The two-phase crystals were probably formed in 

one of two ways:

1. All of the Preble Mountain localities were deficient in iron and then were flooded with ironrich solutions.

2. The late-stage hydrothermal fluids presumed to 

have formed these minerals underwent a 

change in Eh and pH, thus oxidizing existing 

iron to the Fe3 + state and causing the precipitation of jarosite.

The possibility of flooding with iron-rich solutions 

cannot be ruled out although it is unlikely that iron 

was totally absent during the formation of alunite. 

The sequence of deposition from one locality to another in the Preble Mountain area is generally consistent, although one or two stages may be absent. 

This consistency suggests that alunite and jarosite 

formed sequentially from solutions that had similar 

compositions at all of the localities. There is no independent evidence in the Preble Mountain area that 

would support the hypothesis of sudden flooding with 

iron-rich solutions. Thus, the hypothesis of changing 

Eh and pH seems more plausible.

A generalized Eh-pH diagram (fig. 6) showing the 

approximate stability fields for alunite and jarosite 

was constructed for 25° C at 101.3 kPa using the procedure outlined by Garrels and Christ (1965). Freeenergy data used in the calculations for figure 6 are 

listed in table 1. The activities were derived from 

data modified from Brown (1971) and were chosen 

because they have been shown to occur in nature and 

also because the concentrations exceed the solubilities of alunite and jarosite. An increase in iron 

would expand the jarosite stability field and also 

lower the minimum Eh.

Three hypothetical paths are shown in figure 6 

that would produce the two-phase crystals of this 

study. In an unbuffered solution, a rise in Eh should 

also lead to a decrease in pH and produce crystallization sequences similar to ones indicated by the 

diagram. On all three paths (A-A', B-B', C-C'), a 

rapid increase in Eh or decrease in pH would produce the two-phase crystals because the solutions 

would not remain outside the alunite or jarosite field 

long enough to precipitate anything else. Path A-A' 

is self-explanatory, and the crystals observed could 

have been produced in this manner. The same crystals could also have been formed following path 

B-B' because there is nothing to precipitate between 

alunite and jarosite. Path C-C' should produce crystals including a third phase, which has been observed. A few of the crystals show a thin red film of
CRYSTALS OF COEXISTING ALUNITE AND JAROSITE, GOLDFIELD, NEVADA C5

FIGURE 6.—Generalized Eh-pH diagram showing relative stability fields for alunite and jarosite. Activity of species used 

in calculations are as follows: (ass ) =10~-m; (avFe -+' F<.S+) 

rrlO^m; (aAi a+)=:10~;;m; (aK+)=10~3m. Dashed lines represents (arFe2+' Fe3+ )=10'^. Dotted lines represent hypothetical 

crystallization paths. (See text for explanation.)

TABLE 1.—Free energy data used in calculations of generalized 

Eh-pH diagram for alunite and jarosite

Species

KA1 (SC\) 2 (OH) 6 

(alunite) . 

FeOOH (goethite) ——

KFe 3(30^)2 (OH) 6 

(jarosite) .

A1 3 +

T, = 2 +

P=3+

v+

S 2 ~

qn 2 ~ SO

AGf° kcal/mol 

at 25°C, 101.3 kPa

-1113.600

-116.878 

-56.690

-794.00 

-116.000

-20.300

-2.520 

-67.700 

+20.500 

-177.970

-179.94

Reference

Hemley, Hostetler, Gude, 

and Mountjoy (1960) . 

Berner (1969). 

Robie and Waldbaum (1968) . 

Brown (1971).

Robie and Waldbaum (1968). 

Do 

Do 

Do 

Do 

Do 

Weast (1969).

hematite or goethite between the alunite and jarosite; the film does not appear to be an exsolution 

phenomenon.

A rise in Eh or a decrease in pH could be accomplished in the Preble Mountain area by the entry of 

downward moving, deeply circulating meteoric 

waters as the rocks cooled during the later part of 

hypogene alteration. Alunite and jarosite typically 

formed in voids produced by local shattering of the 

silicified rocks after silicification was complete. 

These voids would greatly facilitate permeation by 

descending water.

REFERENCES CITED

Ashley, R. P., 1975, Preliminary geologic map of the Goldfield 

mining district, Nevada: U.S. Geol. Survey Misc. Field 

Studies Map MF-682, scale 1:24,000.

Ashley, R. P., and Albers, J. P., 1975, Distribution of gold 

and other ore-related elements near ore bodies in the 

oxidized zone at Goldfield, Nevada: U.S. Geol. Survey 

Prof. Paper 843-A, p. A1-A48.

Ashley, R. P., and Keith, W. J., 1976, Distribution of gold and 

other metals in silicified rocks of the Goldfield mining 

district, Nevada: U.S. Geol. Survey Prof. Paper 843-B, 

p. B1-B17.

Ashley, R. P., and Silberman, M. L., 1976, Direct dating on 

mineralization at Goldfield, Nevada, by potassium-argon 

and fission-track methods: Econ. Geol., v. 71, p. 904-924.

Berner, R. A., 1969, Goethite stability and the origin of red 

beds: Geochim. et Cosmochim. Acta 33, p. 267-273.

Brophy, G. P., Scott, E. S., and Snellgrove, R. A., 1962, Sulfate studies II—Solid solution between alunite and jarosite: Am. Mineralogist, v. 47, p. 112-126.

Brown, J. B., 1971, Jarosite-goethite stabilities at 25°C, 1 

atm: Mineralium Deposita, v. 6, p. 245-252.

Garrels, R. M., and Christ, C. L., 1965, Solutions minerals, 

and equilibria: New York, Harper and Row, 450 p.

Hemley, J. J., Hostetler, P. B., Gude, A. J., and Mountjoy, W. 

T., 1960, Some stability relations of alunite: Econ. Geology, v. 64, no. 6, p. 599-612.

Robie, R. A., and Waldbaum, D. R., 1968, Thermodynamic 

properties of minerals and related substances at 

298.15°K and 1 atm pressure and at higher temperatures: U.S. Geol. Survey Bull. 1259, 256 p.

Weast, R. C., ed., 1969, General Chemical, section D, of Handbook of chemistry and physics—A ready-reference of 

chemical and physical data: Cleveland, The Chemical 

Rubber Co., p. D1-D218.

Zeolitization of Tertiary Tuffs 

in Lacustrine and Alluvial Deposits 

in the Ray-San Manuel Area, 

Final and Gila Counties, Arizona

By MEDORA H. KRIEGER

SHORTER CONTRIBUTIONS TO MINERALOGY AND 

PETROLOGY, 1979

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1124-D

Tuff beds were studied to determine the 

approximate mineralogic composition 

and type of alteration. Thicker beds 

containing at least 80 percent zeolite 

are potentially minable

CONTENTS

	Page

Abstract ____________________________________________ Dl

Introduction ___________________________________—___ 1

Results of studies of tuffs in the Ray-San Manuel area _————————————— 4

Tuffs in the Quiburis Formation ______________________——____ 4

Tuffs in deposits in Dripping Spring Valley _________——_———————— 6

Tuffs in the Big Dome Formation _________________—________———— 7

Tuffs in the San Manuel Formation _________________________————_ 7

Aravaipa Member of the Galiuro Volcanics _____________—_————— 8

Tuff in the Whitetail Conglomerate ___________________—_—- 8

Original composition of the tuffs and changes due to zeolitization ____ — — — — 8

Economic importance of the zeolites __________________________--_ 10

References cited _________________________________________________ 10

ILLUSTRATIONS

Page

FIGURE 1. Map showing the distribution of formations and sample locations of zeolitized and nonzeolitized tuffs —__ D2 

2. X-ray diffractograms of selected specimens of tuff, showing type of peak heights involved in the estimation of composition _______________________________________________________ — _________ 6

TABLES

Page

TABLE 1. Present mineralogical composition of specimens of tuff from the Quiburis Formation, deposits in Dripping

Spring Valley, and the Big Dome Formation ___________———————————————— ——— — D4

2. Present minerological composition of specimens of tuff from the San Manuel Formation, the Aravaipa

Member of the Galiuro Volcanics, and the Whitetail Conglomerate ____________________________— 5

3. Chemical composition of selected tuffs, Ray-San Manuel area, Arizona ________—__—_________—_—_— 8

4. Chemical analyses and comparisons of unaltered and altered specimens of the Aravaipa Member, Galiuro

Volcanics _______________.._________________________________________ —— __ — — — — — _ 8

5. Semiquantitative spectrographic analyses of minor elements in specimens of tuffs, Ray-San Manuel area,

Arizona ___________________________________________________________________——__—___ 9

in

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

ZEOLITIZATION OF TERTIARY TUFFS IN LACUSTRINE AND 

ALLUVIAL DEPOSITS IN THE RAY-SAN MANUEL AREA, 

FINAL AND GILA COUNTIES, ARIZONA

By MEDORA H. KRIEGER

ABSTRACT

Specimens of tuff in the Ray-San Manuel area, southeastern 

Arizona, have been X-rayed to determine the present approxic 

mate mineralogic composition and the type and degree of 

alteration of the tuffs. Some of the tuffs consist largely of 

glass or have been partly or completely altered to calcium 

montmorillonite; many of them have been almost completely 

zeolitized or contain a zeolite and some clay. Only three 

zeolitized specimens contain a little glass. With four exceptions, the zeolitie is clinoptilolite; the exceptions are one 

altered to chabazite, one to mordenite, one to erionite, and one 

to erionite, clinoptilolite, and chabazite. The tuffs are mostly 

of airfall origin and of rhyolitic or probable rhyolitic composition and generally contain negligible percentages of 

crystal and lithic fragments. Two specimens are tuffaceous 

sedimentary rocks, and two are ash-flow tuffs. The tuffs range 

in age from early Pliocene or late Miocene to Oligocene. 

Some were deposited in lacustrine or playa environments; 

others were deposited in freshwater alluvial environments. All 

the beds examined in the lower Miocene and older formations 

have been zeolitized. Zeolitization of beds in younger formations depends on the environment of deposition. Zeolitized 

tuffs that contain at least 80 percent zeolite and are in beds 

more than one-third meter thick are considered potentially 

minable.

INTRODUCTION

Tuffs of Teritiary age in the Ray-San Manuel 

area, Final and Gila Counties, southeastern Arizona are interbedded in the Quiburis Formation, unnamed deposits in Dripping Spring Valley, Big 

Dome Formation, San Manuel Formation, the Araviapa Member of the Galiuro Volcanics, and the 

Whitetail Conglomerate. The formations range in 

age from early Pliocene or late Miocene to early 

Oligocene and were deposited in lacustrine, playa, 

and alluvial environments. The distribution and age 

of the formations and locations of specimens studied 

are shown in figure 1.

More than 60 specimens of tuff were X-rayed 

(tables 1, 2). The study was undertaken originally 

as an aid in separating the San Manuel Formation 

from the overlying Big Dome Formation in the 

Crozier Peak (Krieger, 1974d) and the Kearny and 

Grayback (Cornwall and Krieger, 1975a, b) Quadrangles. Preliminary X-ray studies showed that the 

tuffs in the Big Dome Formation in the Kearny 

Quadrangle had remained vitric or had partly altered 

to calcium montmorillonite, whereas those in the 

San Manuel Formation in the Crozier Peak Quadrangle had been largely replaced by zeolite. It was 

hoped that X-ray analyses of tuffs would be an 

effective way of separating the two formations in 

areas where they are conformable and nearly identical in composition and appearance.

Additional studies, however, did not bear out the 

early indications that zeolitization occurred only in 

tuffs in the older formation. Tuffs in the Big Dome 

Formation in the Crozier Peak Quadrangle, some in 

the Hayden (Banks and Krieger, 1977) and Kearny 

Quadrangles, and some in the younger deposits within Dripping Spring Valley and in the Quiburis 

Formation have also been zeolitized. Studies by Hay 

(1966), Sheppard and Gude (1964, 1965, 1968, 1969, 

1973), and Sheppard, Gude, and Munson (1965) of 

zeolitization of tuffs in sedimentary deposits have 

shown that silicic glass generally is not altered to a 

zeolite if it is deposited in fresh water, has not been 

affected by moderately alkaline or moderately saline 

ground water, and has not been deeply buried. Age 

is also a factor, as few deposits older than the 

Cenozoic contain relic glass. Age and depth of burial 

have had little effect on the amount of zeolitization

Dl
D2 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979 

m°00' 110°45'

33°15'

33°00

32°45'

10°30'

WinVelman • ' . j

FIGURE 1.—The distribution of formations and sample
ZEOLITIZATION OF TERTIARY TUFFS, RAY-SAN MANUEL AREA, ARIZONA

EXPLANATION

D3

YOUNGER ROCKS Contact

QUIBURIS FORMATION (Early Pliocene or 

late Miocene)-Hemphillian age vertebrate fossils 

(Krieger, 1974c)

Alluvial facies

Lakebed fades-Tuff beds, Q-1 to Q-3, table 1. Gypsum 

deposit shown by dashed boundary

DEPOSITS IN DRIPPING SPRING VALLEYMay correlate in age with Big Dome Formation or 

possibly with Quiburis Formation (Cornwall and 

Krieger, 1977)

Alluvial facies

Lakebed facies-Tuff beds, D-1 to D 6, table 1

BIG DOME FORAAATION (Middle or early 

Miocene)—Tuff beds, B-1 to B-11, table 1. Mostly 

alluvial deposits. B-6A gave K-Ar age of 17 m.y. 

(hornblende), 14 m.y. (biotite) (Cornwall and 

others, 1971; Banks and others,1972)

SAN MANUEL FORAAATION (Early Miocene 

or late Oligocene)—Tuff beds, S-1 to S-30, table 2. 

Alluvial and playa deposits. S-2 gave K—Ar age 

of 17.111 m.y. (biotite) (Cornwall and Krieger, 

1975a). S-10 gave 18.0±0.5 m.y. (biotite), 

24.1±0.7 m.y. (sanidine), (Krieger, 1974a)

GALIURO VOLCANICS, ARAVAIPA MEMBER (Late Oligocene to early Miocene)-Ash-flow 

tuff, G-1 to G-3, table 2. Unaltered part gave K-Ar 

age of 25.7±0.8 m.y. (sanidine) 22.9+0.8 m.y. 

(biotite)(Creasy and Krieger, 1977)

Or'/ OLDER ROCKS

WHITETAIL CONGLOMERATE (Early Oligocene)-Tuff bed, W-1, table 2. W-1 gave K—Ar age 

of 32 m.y. (biotite) (Cornwall and others, 1971; 

Banks and others, 1972)

Mine or quarry

Nonzeolitized tuff-Glass and (or) clay

Zeolitized tuff-Zeolite is clinoptilolite, 

except for chabazite in D-6, erionite in S-21 

erionite with chabazite and clinoptilolite in 

S-30, and mordenite in G-2A

Quadrangle names and sources of data:

B Brandenburg Mountain (Krieger, 1968a)

BM Black Mountain (Krieger, 1974b)

C Crozier Peak (Krieger, 1974d)

Qh Christmas 15-minute (Willden,1964)

CR Clark Ranch (Creasy, 1967)

E El Capitan (Cornwall and Krieger, 1977)

G Grayback (Cornwall and Krieger, 1975b)

H Hayden (Banks and Krieger, 1977)

HJ Holy Joe Peak (Krieger, 1968b)

K Kearny (Cornwall and Krieger, 1975a)

L Lookout Mountain (Krieger, 1968c)

M Mammoth (Creasy, 1965, 1967)

P Putnam Wash (Krieger, 1974e)

S Sonora (Cornwall and others, 1971)

SM Saddle Mountain (Krieger, 1968d)

W Winkelman (Krieger, 1974c)

locations of zeolitized and nonzeolitized tuffs.
D4 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

TABLE 1.—Present mineralogical composition of specimens of tuff from the Quiburis Formation, deposits in Dripping Spring

Valley, and the Big Dome Formation

[Estimated from X-ray diffractometer patterns of bulk samples: •, abundant, more than 50 percent; o, 10-50 percent; x, trace to 10 percent; -, looked 

for but not found. Arranged in approximate order of increasing age. See figure 1 for distribution of formations and location and name of quadrangles]

Quad. Long.

Spec . 

No.

Field 

No. Glass Clay Zeolite 1 Quartz Feldspar Biotite2

Quiburis Formation

SM 32°55' 110°40' Q-l H-24

HJ 45' 36' Q-2 H-366

CR 38' 32' Q-3 M-2

Rhyolite tuff, 0.3- to 0.6-m-thick beds, unmapped, vitric shards,

altered matrix 

Tuffaceous sandstone, unmapped. Contains hornblende and

calcite 

Rhyolite tuff bed, unmapped, associated with diatomite. Shard

structure preserved

Deposits in Dripping Spring Valley

H

E

E

E

E

H

K

K

K

K

K

K

33°8

9

8

7

8

7

5'

7'

6'

6'

6'

6'

47

47

45

45

47

46

54'

55'

55'

55'

55'

52'

D-l

D-2

D-3

D-4

D-5

D-6

B-l

B-2

B-3A

B-3B

B-4

B-5

EC-1 • o

254-55 • o

275-16 o o

276-10 • o

253-2 • x

253-1 - x

43-17A • o

43-17W o •

45-21A •

45-21B o •

45-3 • o

198-1 o o

-

-

x

X

X

• X

Big Dome

x

X

X

X

X

X

_

-

X

-

X

X

-

X

X

X

X

-

Formation, lapilli

x

-

-

X

X

X

Big Dome Formation,

K

K

H

H

H

5'

5'

2'

2'

5'

55'

55'

52'

52'

52'

B-6A

B-6B

B-7A

B-7B

B-8

43-13A • o

43-13B • o

Ha-4 •

Ha-4a o •

DH-1 - •

X

X

X

X

X

X

X

X

-

X

Big Dome Formation

H

H

s

C

C

2'

2'

11'

32°58'

58 '

58'

48'

48'

59'

56'

56'

56'

B-9

B-10

B-ll

B-12

B-13

B-14

240-5 - x

240-6 - x

79/78/1 • o

3 - x

4 x

8 - -

X

X

x

o

X

X

X

X

X

o

X

X

-

-

X

-

-

X

Rhyolite tuff composed largely of shards

Rhyolite tuff composed of shards. Same bed as D-l

Rhyolite tuff bed, unmapped

Rhyolite tuff, unmapped. Identical to D-2. Finer grained

beds contain more clay

Rhyolite tuff, 0.3- to 0.6-in bed, unmapped. Shard structure

Rhyolite tuff , 0.3 m thick, unmapped. Zeolite is chabazite.

Shard structure preserved

tuff

Rhyolite tuff interbedded in lapilli tuff

White structureless pumice lapilli. Gray lapilli with tube

structure has same composition

Unaltered pumice lapilli; n. 1.496

White matrix is largely Ca-montmorillonite. Unaltered pumice

lapilli has n. 1.498

Lapilli tuff

Do.

ash -flow tuff unit

X

X

X

X

X

, relations

x

-

X

-

X

X

Upper pink part. Chemical analysis 3 of specimen 75/80/1 from

same area

Lower white part. Contains more clay than overlying pink part

Upper pink part . Contains hornblende

White area surrounding lithophysae

From drill hole in mineralized area. Glass and much of

feldspar altered to Ca-montmorillonite. Contains hornblende

and calcite. Sample submitted by Neil Cambell, Ray Mines

Division, Kennecott Copper Corp.

unknown

Rhyolite tuff, 15 cm thick, unmapped

Massive rhyolite tuff bed, 0.3 m thick, unmapped

Air-fall tuff 3

Rhyolite tuff, unmapped. Contains hornblende

Do.

Do.

Zeolite is clinoptilolite, except for chabazite in D-6. 

Some X-rays show hornblende, see description. 

3 See table 3 for chemical analysis.

that has occurred in tuff beds in the Ray-San 

Manuel area.

Diffractograms of whole-rock tuffs (fig. 2) illustrate the resolution of individual minerals and mineral aggregates. Phenocrystic quartz, biotite, and 

unaltered feldspar can be distinguished in glassy, 

zeolitized, and argillized tuffs. Quantitative determination of the amounts of zeolites in altered tuffs is 

difficult, because of preferred orientations of minerals in the X-ray mounts, different degrees of 

crystallinity, and masking by amorphous matrix 

material. The X-ray diffraction of the whole-rock 

tuffs, however, clearly shows the relative abundance 

of the alteration products, and many alteration mineral identifications were relatively easy owing to the 

nearly monomineralogic composition of some samples. Sheppard and Gude (1964) by using X-ray 

diffraction were able to recognize zeolite concentrations as low as 10 percent in some altered rocks.

RESULTS OF STUDIES OF TUFFS IN RAYSAN MANUEL AREA

TUFFS IN THE QUIBURIS FORMATION

The Quiburis Formation, of which about 200 m is 

exposed (Heindl, 1963 Krieger, 1968a-d, 1974c; 

Creasey, 1965, 1967), was deposited in a long, narrow, closed basin that extended for many kilometers 

southeast of the area mapped on figure 1. The formation consists of an alluvial and a lakebed facies 

separated by a narrow transition zone. The alluvial 

facies occurs around the margin of the basin and 

consists of sandy pebble conglomerate and some 

interbedded sandy and silty beds. The lakebed facies 

occupies the center of the basin and consists of thin, 

evenly bedded clay, silt, marl, and very fine grained 

sandstone. Thick deposits of gypsum occur in the 

lakebed facies north and south of present-day Aravaipa Canyon (fig. 1). Their occurrence indicates 

that Aravaipa Creek and Putnam Wash, west of the
ZEOLITIZATION OF TERTIARY TUFFS, RAY-SAN MANUEL AREA, ARIZONA D5

TABLE 2.—Present mineralogical composition of specimens of tuff from the San Manuel Formation, Aravaipa Member of

the Galiuro Volcanics, and the Whitetail Conglomerate

[Estimated from X-ray diffractometer patterns of bulk samples: •, abundant, more than 50 percent; o, 10-50 percent; x, trace to 10 percent; -, looked 

for but not found. Arranged in approximate order of increasing age. See figure 1 for distribution of formations and location and name of quadrangles]

Quad.

K

K

K

G

K

K

G

G

G

C

C

C

C

C

C

C

C

C

C

C

C

K

K

K

H

W

P

P

P

P

P

P

Lat.

33°2'

2'

2'

5'

1'

1'

3'

4'

2'

32°58'

59'

57'

57'

58'

57'

58'

58'

58'

58'

58'

58'

33°0'

1'

2'

0'

0'

32°49'

49'

49'

49'

48'

48'

Long.

110°59'

59'

59'

111"!'

110°59'

59'

111 0 !'

1'

0'

110°57'

57'

55'

55'

55'

55'

56'

56'

56'

56'

56'

56'

57'

54'

54'

51'

59'

46'

46'

48'

48'

48'

48'

Spec. 

No.

S-l

S-2

S-3

S-4

S-5A

S-5B

S-^6

S-7

S-8

S-9

S-10

S-11A

S-11B

S-12

S-13

S-14

S-15

S-16

S-17

S-18

S-19

S-20

S-21

S-22

S-2 3

S-24

S-25

S-26

S-27

S-28

S-29

S-30

Field 

No. Glass Clay Zeolite 1

12-105 - x •

12-100 - x •

13-40B x •

14-67 - o •

12-79C oo o

12-79B xx x

12-106 x •

176-45 - o •

174-45 o o

W-143B - o •

W-158 o •

W-136 o •

10 - x •

11 •

W-138 - •

1 - x •

2 o •

5 - x o

6 - x •

7 - o o

9 - x •

37-59 - o •

67-24 - x •

41-37 - o •

Ha-3 x •

W-71 - x •

W-12B - o •

W-14 _o o

W-16A - x •

W-394 - x •

W-52 - x •

W-395A - - •

Quartz

San

x

x

X

0

X

-

X

X

X

X

X

0

X

X

X

0

X

0

X

X

0

X

-

X

X

X

X

o

X

X

o

X

Feldspar Biotite2 Description

Manuel Formation

x

X

X

0

X

-

X

X

0

X

X

X

x

X

0

X

x

X

X

x

0

X

-

X

X

X

X

o

X

X

X

X

X

X

X

X

X

-

X

X

X

-

X

X

X

-

X

X

X

X

X

X

X

-

-

-

X

-

X

X

-

-

X

-

Rhyolite tuff in lower part of upper granitic conglomerate.

Contains hornblende

Rhyolite tuff 3 from top of tuffaceous sandstone and

conglomerate unit. Contains hornblende

Rhyolite tuff near base of tuffaceous sandstone and

conglomerate unit in this area

Rhyolite tuff at base of tuffaceous sandstone and conglomerate

unit in this area. S-4 to S-7 are from different parts of

the same lapilli tuff unit, not the same bed as S-3

Rhyolite tuff near middle of 1.2-m-thick bed of lapilli tuff.

Glass probably is in pumice

Large (3.5 cm) pumice lapillus near base of lapilli tuff.

Largely replaced by calcite. Shard structure preserved

Lapilli tuff in tuffaceous sandstone and conglomerate unit

Lapilli tuff at base of tuffaceous sandstone and conglomerate

unit. Tube structure preserved

Rhyolite tuff near base of lower granitic conglomerate,

unmapped. Contains hornblende

Thin bed of rhyolite tuff, unmapped

Thin bed of rhyolite tuff, unmapped 3

Tuffaceous sandstone unmapped, with pumice lapilli as much as

2 cm long

Tuffaceous sandstone with pumice lapilli, unmapped

Rhyolite tuff, unmapped. Contains hornblende

Thin bed of lapilli tuff, unmapped

Thin rhyolite tuff bed , unmapped

Do.

Rhyolite tuff with lapilli, unmapped

Tuffaceous sandstone, unmapped. Contains hornblende

Do.

Tuffaceous sandstone, unmapped. Contains calcite and

hornblende

Concretionary tuff, unmapped

Rhyolite tuff, 5-6 cm thick, unmapped, in dark playa deposits. 

Zeolite is erionite

Rhyolite tuff, 0.3 m thick, unmapped, in dark conglomerate

Rhyolite tuff in dark conglomerate. Contains hornblende

Rhyolite tuff 0.6 m thick, unmapped. Good shard structure.

Contains hornblende

Rhyolite tuff, unmapped, in alluvial conglomerate. Pumice

lapilli. Shard structure preserved

Rhyolite tuff, unmapped, in alluvial conglomerate. Contains

hornblende

Pumice lapilli in tuffaceous sandstone, unmapped, in granitic

playa deposits. Contains three parts calcite

Rhyolite tuff, unmapped, in granitic playa deposits. Composed

almost entirely of shards

Concretionary tuff in alluvial conglomerate

Large pumice lapillus 3 in tuffaceous sandstone, unmapped.

Contains erionite and some chabazite and clinoptilolite

Aravaipa Member, Galiuro Volcanics

G-l H-202L 

G-2 A-4

Devitrified zone 1*

33' G-3A H-343M 

33' G-3B H-343A

Welded tuff, Aravaipa Member

Distal end of Aravaipa Member5 (ash-flow tuff). Zeolite is

clinoptilolite 

Distal end of Aravaipa Member5 (north of Aravaipa Canyon).

Below columnar-jointed unit zeolite is entirely mordenite 

Distal end of Aravaipa Member, above columnar-jointed unit.

Zeolite is entirely of clinoptilolite

Whitetail Conglomerate

W-l 79/78/2 Rhyolite tuff 3 , unmapped, near top of Whitetail Conglomerate

Zeolite is clinoptilolite except of erionite in S-21, mordenite in G-2 and erionite, chabazite, and clinoptilolite in S-30. 

2 Some X-rays show hornblende, see description. 

3 See table 3 for chemical analysis.

"*Composed of cristobalite, feldspar, quartz and a little clay and biotite. See table 4 for chemical analysis. 

5 See table 4 for chemical analysis.

San Pedro River, probably did not exist when the 

gypsum was forming or did not supply much fresh 

water to the lake. The sulfur to form the gypsum 

may have been supplied through erosion of the San 

Manuel and Copper Creek ore deposits.

Only three specimens of tuff from the Quiburis 

Formation were X-rayed. Two of them had remained 

vitric; the other contains some clinoptilolite. The

vitric character of one of the specimens (Q-l, table 

1) from the alluvial margin of the basin and the 

zeolitized character of the altered specimen (Q-2) 

from the central lakebed are as to be expected 

(Shepard and Gude, 1968). The third specimen 

(Q-3), from a thick section of rhyolite tuff and 

diatomite was collected about equidistant from the 

eastern and western margins of the lakebed facies.
D6 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

20, IN DEGREES

10 20 30

8.8 4.4 3.0 

INTERPLANAR SPACINGS, IN ANGSTROMS

The presence of the diatomite and the vitric character of the tuff suggest that the lake water here 

must have been fresh at the time the tuff was deposited and remained so since. This area was probably near the intake end of the lake.

TUFFS IN DEPOSITS IN DRIPPING SPRING VALLEY

Sedimentary deposits, which are at least 450 m 

thick, in Dripping Spring Valley (Banks and 

Krieger, 1977; Cornwall and Krieger, 1977; Cornwall 

and others, 1971; and Willden, 1964) accumulated 

in a closed basin as did the Quiburis Formation; the 

deposits are a coarse alluvial conglomerate around 

the margins and a fine-grained material, mostly clay 

containing some silt and fine sand, in the center 

(fig. 1). The two facies are separated by a narrow 

transition zone on the southwest side of the basin 

and by a wide transition zone on the northeast side 

of the basin. The wide transition zone is included in 

the lakebed facies mapped on figure 1 because of the 

abundance of freshwater limestone, much of which 

is coarsely conglomeratic. The limestone was formed 

probably when calcium-bearing fresh water was 

brought by streams from surrounding Paleozoic 

limestone; the calcium was precipitated as calcite 

when it came in contact with the alkaline and possibly saline waters of the lake (Sheppard and Gude, 

1968). All specimens of tuff X-rayed from this 

transition zone (D-l through D-5, table 1) are vitric 

(fig. 2A) ; two have undergone some clay alteration. 

Freshwater in the pores and the freshness of the 

ground water to which the tuff was subsequently 

exposed, prevented zeolitization of the tuff beds. 

One specimen of tuff (D-6) from a 0.3-m-thick white 

tuff bed exposed in a roadcut in the lakebed facies 

is composed almost entirely of chabazite (fig. 2B). 

The presence in this area of thick deposits of montmorillonitic clay, in which gypsum fills little fractures, suggests that the tuff bed was deposited in 

an alkaline or saline environment.

FIGURE 2.—X-ray diffractograms of selected specimens of tuff, 

showing type of peak heights involved in the estimation of 

composition. Cu/Ni radiation, XCuKa=1.5418A. Scan from 

4° 26 at 1° 26/minute. A, Dripping Spring Valley, D-l; 

glass and clay. B, Dripping Spring Valley, D-6; altered to 

chabazite. C, San Manuel Formation, S-5A; glass, clay, and 

clinoptilolite. D, San Manuel Formation, S-21; altered to 

erionite. E, San Manuel Formation, S-29; altered to clinoptilolite. F, San Manuel Formation, S-30; altered to 

erionite, chabazite, and clinoptilolite. G, Aravaipa Member, 

Galiuro Volcanics, G-3A; altered to mordenite. (B, biotite; 

Cl, clay; C, clinoptilolite; Cb, chabazite; E, erionite; F, 

feldspar; M, mordenite; Q, quartz)
ZEOLITIZATION OF TERTIARY TUFFS, RAY-SAN MANUEL AREA, ARIZONA D7

TUFFS IN THE BIG DOME FORMATION

The Big Dome Formation, which is about 600 m 

thick (Krieger and others, 1974; Cornwall and 

Krieger, 1975a; Banks and Krieger, 1977; Krieger, 

1974a, c-e), is an alluvial deposit comprising several 

lithologic units, including two mappable tuff units. 

The upper one, about 12 m thick is a lapilli tuff and 

interbedded rhyolite tuff, tuffaceous sandstone, and 

conglomerate. It is extensively exposed in the northeastern part of the Kearny Quadrangle; its easternmost exposure is northeast of Kearny near the west 

edge of the Hayden Quadrangle. The lower unit, 

about 6 m thick, is a nonwelded quartz-latite ashflow tuff that crops out intermittently from about 

3.2 km northwest of Kearny to the west edge of the 

Hayden Quadrangle just north of the Gila River. The 

tuff was also found in a drill hole in a mineralized 

area of the western part of the Hayden Quadrangle 

a short distance southeast of the easternmost exposure of the lapilli tuff. Other tuffs studied in the 

Big Dome Formation are in the south-central part 

of the Hayden Quadrangle and north-central part of 

the Crozier Peak Quadrangle; all are thin, unmapped 

beds. A more extensive tuff bed is northeast of Ray. 

The relations of these tuff beds to the lapilli tuff 

and ash-flow tuff are unknown.

None of the X-rayed specimens of the lapilli tuff 

(B-l through B-5, table 1) contains any zeolite. 

They are largely vitric or contain generally minor 

amounts of calcium montmorillonite; locally, matrix 

or lapilli have been largely altered to clay. Ash-flow 

tuff specimens (B-6 through B-8) also contain no 

zeolite. B—6 and B-7 are vitric but have been somewhat altered to calcium montmorillonite; the clay is 

locally more abundant in the lower part of the ash 

flow or surrounding the lithophysae. X-ray and thinsection studies show that glass and much of the 

feldspar in the specimen obtained from the drill hole 

(B-8) has been altered to calcium montmorillonite; 

biotite and hornblende, however, are unaltered. The 

tuff bed near Ray (B-ll) is largely vitric but contains some clay and traces of clinoptilolite. It is one 

of the few tuffs studied that contains both glass and 

a zeolite in a single sample; all the other tuff beds 

from the Big Dome Formation have been almost 

completely altered to clinoptilolite. These beds in 

the Hayden Quadrangle (B-9 through B-10) may 

have been zeolitized because ground water became 

more alkaline as it circulated through the enclosing 

conglomerate, which is composed largely of andesitic 

volcanic clasts. Zeolitization of the tuff beds in the 

Crozier Peak Quadrangle (B-12 through B-14) may

be related to whatever conditions zeolitized the tuff 

beds in the underlying San Manuel Formation.

TUFFS IN THE SAN MANUEL FORMATION

The San Manuel Formation, which is at least 

3,000 m thick (Heindl, 1963; Krieger, 1974a, c-e; 

Krieger and others, 1974; Cornwall and Krieger, 

1975a, b; Banks and Krieger, 1977), consists of 

alluvial and playa deposits that have been separated 

into more than nine members. Numerous curled mud 

chips in playa sandstone and mud cracks in conglomerate beds attest to the alternate wet and dry 

condtions. Many of the tuff beds studied are enclosed in granitic alluvial deposits. The associated 

fine-grained granitic sandstone and conglomerate 

may have been deposited in a playa environment. A 

few of the tuffs are associated with dark nongranitic 

alluvial or playa deposits.

The most abundant tuffaceous material, and probably the youngest part of the formation, is in the 

downf aulted graben along Ripsey Wash in the Grayback and Kearny Quadrangles (fig. 1; S-l to S-8, 

table 2). Here the granitic tuffaceous sandstone and 

conglomerate and the overlying and underlying 

granitic conglomerate contain many relatively pure 

tuff beds (0.3-3 m), several of which have been 

traced from 0.2 to more than 3 km. One lapilli tuff 

unit was traced for about 8 km. Except for two thin 

tuff beds (S-22 and S-24) that were traced for short 

distances, all the other specimens of tuff studied are 

from thin unmapped beds.

All the tuffs studied from the San Manuel have 

been zeolitized (table 2) ; most contain a little clay, 

and four (S-5, S-8, S-18, S-26) contain nearly as 

much clay as zeolite. Only two (S-5A, S-5B) contain 

some vitric material, which is believed to be concentrated in large pumice lapilli. A large pumice lapillus 

(S-5B) has been largely replaced by calcite. The 

zeolite is clinoptilolite in all except two tuffs Xrayed; in one the zeolite was erionite (S-21), and 

in the other it was an assemblage of erionite (the 

most abundant), chabazite, and clinoptilolite 

(S-30).

The reason for zeolitization of the tuffs in the 

San Manuel Formation is uncertain. Some of the 

tuffs may have been deposited in a playa environment in which the water was sufficiently alkaline or 

saline to cause zeolitization. Those that were deposited apparently in an alluvial freshwater environment must have been zeolitized by later changes in 

ground water.
D8 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

ARAVAIPA MEMBER OF THE GALIURO VOLCANICS

The Aravaipa Member, whose thickness is GO90 m except for 15-30 m at the distal margin 

(Krieger, 1968a, b; 1979; Creasey and Krieger, 

1977; Simons, 1964), is an ash-flow tuff of rhyolite 

composition; it has a well-formed zonal pattern that 

indicates that it was a simple cooling unit and probably a single ash flow. Horizontal and vertical zonations are completely exposed in canyon walls. Dense 

welding, characteristic of the lower part of the 

interior of the ash flow, decreases vertically as well 

as horizontally away from the interior of the sheet. 

Near the member's distal margin, south of Aravaipa 

Canyon, slightly welded columnar-joined tuff pinches 

out and is replaced and surrounded by nonwelded 

tuff (about 20 m thick). In the only section of nonwelded tuff studied north of the canyon, about 25 m 

of welded tuff separates about 25 m (of nonwelded 

tuff below from 5 m of nonwelded tuff above.

No zeolite is present in any of the welded part of 

the ash flow (Gl, table 2). Except for the basal 

vitrophyre all of the welded part has been devitrified. The nonwelded part of the ash flow has been 

completely zeolitized except for tiny unaltered crystal 

fragments. South of Aravaipa Canyon the zeolite is 

clinoptilolite (G-2). North of the canyon it is clinoptilolite above the columnar-jointed tuff (G-3B) 

but is mordenite below it (G-3A). X-ray studies of 

many specimens (Krieger, 1979) show that all the 

nonwelded tuff south of the canyon has been altered 

to clinoptilolite.

Zeolitization of the nonwelded parts of the Aravaipa Member must have been caused by later 

changes in ground water, as the formation appears 

to have been deposited in an alluvial environment.

TUFF IN THE WHITETAIL CONGLOMERATE

One specimen of an unmapped tuff was collected 

from the Whitetail Conglomerate (Cornwall and 

others, 1971). It has been altered to clinoptilolite 

and traces of clay and unaltered crystal fragments 

(W-l,table2).

ORIGINAL COMPOSITION OF THE TUFFS 

AND CHANGES DUE TO ZEOLITIZATION

The tuffs in the general area mapped on figure 1 

are chiefly rhyolites or tuffs of probable rhyolite 

composition. On the basis that rhyolitic glass has a 

refractive index of less than 1.500 (George, 1924), 

tuffs from the Quiburis Formation (Ql, Q3, table 

1), from basin deposits in Dripping Spring Valley 

(D-l through D-5), and from the Big Dome Formation (B-3A, B-3B) are considered to be rhyolite. 

The ash-flow tuff in the Big Dome Formation is a 

quartz latite based on its chemical analysis (B-6, 

table 3).

Table 3 contains chemical analyses of seven tuffs; 

five are strongly zeolitized, and two are largely vitric 

but three have undergone some clay alteration. 

Table 4 shows chemical analyses of three specimens

TABLE 3.—Chemical composition of selected tuffs, Ray-San 

Manuel area, Arizona

Major oxide analyses, weight percent, under direction of L. Shapiro; by methods 

described by Shapiro and Brannock (1962), supplemented by atomic absorpiton, 

except no. D-6, by methods described by Shapiro (1967)]

Location 

Field No. 

Laborator

Fe2 0 3 ——

P2 0 5 ——

co2 —

Total -

Sample de 

D-6 - 

B-6 - 

B-ll - 

S-2 - 

S-10 - 

S-30 -

W-l -

No. --D-6 

———— 253-1 

y No.-M122781W

——— - 100

2

16 

8 

1

4 

2 

H 

6 

06 

08 

03 

07

B-6 

75/80/1 

M105898W

67

1 

1 

3 

3 

1 

3

100

7

26 

2 

7 

0 

4 

6 

6 

<*4 

12 

08 

05

B-ll S-2 S-10 S-30 

79/78/1 12-100 W-158 W-395A 

M105895W H113934W M104768W M107983W

68.7

.18 

.91 

1.8 

2.6 

3.8 

4.5 

2.1 

.25 

.05 

.07 

.05 

100

script ion : 

Air-fall tuff composed largely of

Air-fall tuff, 

Air-fall tuff, 

Air-fall tuff, 

Pumice lapilli, 

and clinoptilol 

Air-fall tuff.

some clay a 

largely alt 

largely alt 

composed o 

ite 

altered to

59

1

5 

2 

1 

7 

3

1 

100

8 59

88 1 

4 2 

9 3 

5 2 

4 1 

4 5 

0 4 

31 

10 

04 

6 < 

100

i* 56

7 

0 1 

7 4 

6 2 

3 1 

6 11 

6 7 

64 

17 

12 

05 

100

2 

4 

81 

) 

H 

2 

5 

4 

3 

4 

10 

00 

15 

20

W-l 

79/78/2 

M105896W

64.5 

12.4 

1.5 

.06 

2.1 

3.3 

.55 

1.5 

4.8 

8.8 

.24 

.06 

.05 

.08 

100

chabazite

Iteration 

ered to clinoptilolite 

ered to clinoptilolite 

f about equal parts of e

clinoDtilolite

.rionite , chabazite,

TABLE 4.—Chemical analyses and comparisons of unaltered 

and altered specimens of the Aravaipa Member, GaHuro 

Volcanics

[See table 5 for minor-element analyses. Major-oxide analyses 

(weight percent), no. 1 by P. Elmore, S. Botts, and G. Chloe; 

by rapid methods described by Shapiro and Brannock (1962); 

nos. 2, 3 by H. Smith by methods described by Shapiro (1967)]

Original

Si02 ——

A120 3 ——

Fe2 0 3 ——

FeO

MgO ——

CaO

Na2 0 ——

K2 0 ——

H2 0

H20

Ti02 ——

P2 0 5 ——

MnO

C02

Total —

1

73.4

13.6

1.2

.08

.47

.44

3.6

5.2

.93

.93

.24

.06

.08

<.05

100

analyses Recalculated

2

67.0

12.5

1.2

.12

1.0

2.6

.78

3.5

7.6

4.0

.16

.15

.07

.22

100

3

63.6

12.1

1.0

.08

.40

3.0

1.9

3.0

7.9

5.6

.12

.04

.06

.06

99

1

74.6

13.8

1.2

.08

.48

.45

3.7

5.3

—

—

.24

.06

.08

—

99.99

without

2

75.2

14.0

1.4

.13

1.1

2.9

.88

3.9

—

—

.18

.06

.08

~

99.83

H2 0 and C02

3

74.4

14.1

1.2

.09

.47

3.5

2.2

3.5

—

—

.14

.05

.07

—

99.72

Sample description

1. G-l. Devitrified tuff, H202L, Lab 159531

2. G-2. Tuff altered to clinoptilolite, A-4, Lab M126691W

3. G-3A. Tuff altered to mordenite, H343M, Lab M126692W
ZEOLITIZATION OF TERTIARY TUFFS, RAY-SAN MANUEL AREA, ARIZONA D9

TABLE 5.—Semiquantitative spectrographic analyses of minor elements in specimens of tuffs, Ray-San Manuel area-, Arizona

[Minor-element analyses in parts per million by Semiquantitative spectrographic methods by R. E. Mays (D-6, 

B-6, B-ll, S-2, W-l); by C. R. Heropoulos (S-10, S-30, G-2, G-3A): and I. H. Barlow (G-l). Results are 

to be identified with geometric brackets whose boundaries are 1.2, 0.83, 0.56, 0.38, 0.26, 0.18, 0.12, 

and so forth, but are reported arbitrarily as midpoints of these brackets, 1, 0.7, 0.5, 0.3, 0.2, 0.15, 

0.1, and so forth. The precision of a reported value is approximately plus or minus one bracket at 

68-percent confidence, or two brackets at 95-percent confidence. N, not detected, —, not looked for; 

<, less than. Looked for but not found: As, Au, Bi, Cd, Eu, Ge, He, In, Li, Pd, Re, Sb, Sm, Ta, Te, Th, 

Tl, U, W]

Location No. —— D-6 B-6 B-ll S-2 S-10 

Field No. 253 1 75/80/1 79/78/1 12 100 W 158

Laboratory No.- M122718W M105898W M105895W M113934W M10476

Ag —————————— 1 N N N N 

B —————————— 30 20 N N N

Ba ————————— 200 700 300 700 1000

Be ————————— N 3 2 N 3

Ce —————————— N N N N 150

Co —————————— N N N N 70

Cr —————————— 7 N 15 50 7

Cu —— ——————— 20 10 15 10 20

Ga ————————— 10 20 15 15 15

Gd

La ————————— N N N N 50

Mo —————————— N N N N N

Nb __ ———————— N N N N 30

Nd —————————— -- — — -_ N

Ni —————————— 7 N 3 5 3

Pb —————————— N 20 20 10 30

Sc —————————— N 5 N 3 15 

Sn ————————— N N N N

Sr - ————————— 500 300 200 700 5000

V —————————— 20 20 15 30 50

Y —————————— N 30 20 15 30 

Yb —————————— N 3 2 1.5 3

Zn ————————— N N N N N

Zr ————————— 70 150 70 100 300

Sample description: 

D-6 - Air-fall tuff composed largely of chabazite 

B-6 - Quartz-latite ash-flow tuff, some clay alteration 

B-ll - Air-fai:_ tuff, some clay alteration 

S-2 - Air-fall tuff, largely altered to clinoptilolite 

S-10 - Air-fall tuff, largely altered to clinoptilolite 

S-30 - Pumice lapilli, composed of erionite, chabazite, and 

G-l - Unaltered Aravaipa Member, devitrified zone, probably 

G-2 - Aravaipa Member, altered to clinoptilolite 

G-3A - Aravaipa Member, altered to mordenite 

W-l - Air-fai;. tuff, altered to clinoptilolite

S-30 

W-395A 

• 8W M107983W

N 

N 

150 

5 

N 

N 

3 

20 

15

N 

N 

50

7 

30 

3 

N 

500 

N 

N 

1.5 

N 

70

G-l 

H-202L 

159531

<.7 

<30 

500 

5 

100 

N 

<3 

3 

20 

N 

50 

<3 

20 

N 

10 

30 

3 

N 

70 

10 

30 

3 

N 

200

G-2 

A-4 

M126691W

N 

7 

200 

7

N 

1.5 

5

50 

N 

30

.7 

30 

5 

N 

100 

7 

30

30 

300

clinopt ilolite 

close to original composition

G-3A 

H-343M 

M126692W

N 

10 

200 

7

N 

3 

7

100 

N 

50

.7 

30 

7 

N 

200 

20 

30

30 

300

of magma

W-l 

79/78/2 

M105896W

N 

N 

1000 

2 

N 

N 

10 

30 

10

N 

N 

N

N 

30 

N 

N 

1000 

20 

20 

2 

N 

100

(two altered and one unaltered) from the Aravaipa 

Member. Table 5 gives spectrographic analyses of 

all ten specimens. The chemical compositions of the 

fresh equivalents of the five zeolitized tuffs (D-6, 

S-2, S-10, S-30, and W-l, table 3) are unknown, 

although they were probably rhyolites. 1 In comparison with tuff samples slightly altered to clay (B-6A, 

B-ll), the zeolitized samples have lost Si02 and K2 0 

and gained H2 0, CaO, and MgO. Na2 0 is unchanged, 

except in D-6 (altered to chabazite), in which it 

shows a gain. The change in Si02 and Na2 0 in the 

zeolitized tuffs, however, may have been greater 

than that shown in table 3, as some Si02 and Na2 0 

may have been lost in B-6 A and B-ll by ground1 Specimen S-2 is probably also a rhyolite, although it was called 

rhyodacite (Cornwall and Krieger, 1975a) because of its chemical composition, which is due to z<»litization.

water leaching, which is most advanced in porous 

glassy tuffs including nonwelded ash-flow tuffs 

(Lipman, 1965).

More specific information on the composition of 

fresh and altered tuffs from the Aravaipa Member 

of the Galiuro Volcanics is available from analyses 

of G-l (unaltered, but devitrified), G-2 (altered 

to clinoptilolite), and G-3A (altered to mordenite) 

(table 4). The major change has been a large increase in H2 0. To more readily compare the nonvolatile constitutents, the analyses have been recalculated on a water-free basis. In addition to H2 0, 

the major changes during alteration to both clinoptilolite and mordenite have been an increase in CaO 

(greater when altered to mordenite) and losses in 

Na2 0 (greater when altered to clinoptilolite) and
D10 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

K2 0 (greater when altered to mordenite). Clinoptilolite shows a small increase in MgO and Si02 , whereas mordenite show little change.

ECONOMIC IMPORTANCE OF THE ZEOLITES

Nearly monomineralic zeolitized tuffs are common 

in Cenozoic rocks. Those that contain at least 80 percent zeolite and are more than 0.3 m thick are 

considered potentially minable (Sheppard, 1971; see 

also Hawkins and Mumpton, 1976). Many of the tuff 

beds examined are very thin, or their thickness and 

extent were not determined because of poor exposures.

Only one tuff (D-6) in the Dripping Spring Valley 

is zeolitized (to chabazite), and it is exposed only 

in a roadcut; it might be of considerable extent. 

Numerous thin unmapped zeolitized tuff beds are in 

the San Manuel Formation (S-14 to S-19) and the 

overlying Big Dome Formation (B-12 to B-14) in 

the Crozier Peak Quadrangle. Some of the beds are 

composed largely of clinoptilolite. The zeolite here 

may be present in economic amounts. The greatest 

concentration of zeolite in the San Manuel Formation is along Ripsey Wash (S-l to S-8), where thin 

beds of nearly pure clinoptilolite have been mapped 

and are associated with numerous unmapped zeolitized tuffaceous sedimentary rocks. The Aravaipa 

Member of the Galiuro Volcanics contains the greatest known amount of zeolite in the Ray-San Manuel 

area. South of Aravaipa Canyon the nonwelded 

zeolitized tuff covers an area of nearly 2 km2 . A 

nearly horizontal section, about 0.8 km long and 

nearly 20 m thick, appears to be composed almost 

entirely of clinoptilolite. The known amount of mordenite is confined to a vertical section north of the 

canyon and is nearly 20 m thick. The rest of the 

nonwelded tuff north of Aravaipa Canyon has been 

altered to clinoptilolite (Krieger and others, 1979).

REFERENCES CITED

Banks, N. G., Cornwall, H. R., Silberman, M. L., Creasey, 

S. C., and Marvin, R. F., 1972, Chronology of intrusion 

and ore deposition at Ray, Arizona—Part 1, K-Ar ages: 

Economic Geology, v. 67, no. 7, p. 864-878.

Banks, N. G., and Krieger, M. H., 1977 [1978], Geologic map 

of the Hayden Quadrangle, Final and Gila Counties, 

Arizona: U.S. Geological Survey Geologic Quadrangle 

Map GQ-1391, scale 1:24,000.

Cornwall, H. R., Banks, N. G., and Phillips, C. H., 1971, 

Geologic map of the Sonora Quadrangle, Final and Gila 

Counties, Arizona: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1021, scale 1:24,000.

Cornwall, H. R., and Krieger, M. H., 1975a, Geologic map of 

the Kearny Quadrangle, Final County, Arizona: U.S.

Geological Survey Geologic Quadrangle Map GQ-1188, 

scale 1:24,000.

— 1975b, Geologic map of the Grayback Mountain Quadrangle, Final County, Arizona: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1206, scale 1:24,000.

1977 [1978], Geologic map of the El Capitan Mountain Quadrangle, Gila and Final Counties, Arizona: U.S. 

Geological Survey Geologic Quadrangle Map GQ-1442, 

scale 1:24,000.

Creasey, S. C., 1965, Geology of the San Manuel area, Final 

County, Arizona: U.S. Geological Survey Professional 

Paper 471, 64 p.

———— 1967, General geology of the Mammoth Quadrangle, 

Final County, Arizona: U.S. Geological Survey Bulletin 

1218, 94 p.

Creasey, S. C., and Krieger, M. H., 1977 [1978], The Galiuro 

Volcanics in southeastern Arizona: U.S. Geological Survey Journal of Research, v. 6, no. 1, p. 115-131.

George, W. 0., 1924, The relation of the physical properties 

of natural glasses to their chemical composition: Journal 

of Geology, v. 32, no. 5, p. 353-372.

Hawkins, D. B., and Mumpton, F. A., 1976, The world of 

zeolites, report on international conference on natural 

zeolites: Geotimes, v. 21, no. 12, p. 25-27.

Hay, R. L., 1966, Zeolites and zeolitic reactions in sedimentary 

rocks: Geological Society of America Special Paper 85, 

130 p.

Heindl, L. A., 1963, Cenozoic geology in the Mammoth area, 

Final County, Arizona: U.S. Geological Survey Bulletin 

1141-E, 41 p.

Krieger, M. H., 1968a, Geologic map of the Brandenburg 

Mountain Quadrangle, Final County, Arizona: U.S. Geological Survey Geologic Quadrangle Map GQ-668, scale 

1:24,000.

———— 1968b, Geologic map of the Holy Joe Peak Quadrangle, 

Final County, Arizona: U.S. Geological Survey Geologic 

Quadrangle Map GQ-669, scale 1:24,000.

———— 1968c, Geologic map of the Lookout Mountain Quadrangle, Final County, Arizona: U.S. Geological Survey 

Geologic Quadrangle Map GQ-670, scale 1:24,000.

——— 1968d, Geologic map of the Saddle Mountain Quadrangle, Final County, Arizona: U.S. Geological Survey 

Geologic Quadrangle Map GQ-671, scale 1:24,000.

———— 1974a, Generalized geology and structure of the 

Winkelman 15-minute quadrangle and vicinity, Final and 

Gila Counties, Arizona: U.S. Geological Survey Journal 

of Research, v. 2, no. 3, p. 311-321.

———— 1974b, Geologic map of the Black Mountain Quadrangle, Final County, Arizona: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1108, scale 1:24,000.

———— 1974c [1975], Geologic map of the Winkelman Quadrangle, Final and Gila Counties, Arizona: U.S. Geological Survey Geologic Quadrangle Map GQ-1106, scale 

1:24,000.

———— 1974d [1975], Geologic map of the Crozier Peak Quadrangle, Final County, Arizona: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1107, scale 1:24,000.

———— 1974e [1975], Geologic map of the Putnam Wash 

Quadrangle, Final County, Arizona: U.S. Geological Survey Geologic Quadrangle Map GQ-1109, scale 1:24,000.

————1979, Ash-flow tuffs of the Galiuro Volcanic in northern Galiuro Mountains, Final County, Arizona: U.S. 

Geological Survey Professional Paper 1104.
ZEOLITIZATION OF TERTIARY TUFFS, RAY-SAN MANUEL AREA, ARIZONA Dll

Krieger, M. H., Cornwall, H. R., and Banks, N. G., 1974, The 

Big Dome Formation and revised Tertiary stratigraphy 

in the Ray-San Manuel area, Arizona: U.S. Geological 

Survey Bulletin 1394-A, p. A54-A62.

Krieger, M. H., Johnson, M. G., and Bigsby, P. R., 1979, 

Mineral resources of the Aravaipa Canyon Instant Study 

Area, Final and Graham Counties, Arizona: U.S. Geological Survey Open-File Report 79-291, 183 p.

Lipman, P. W., 1965, Chemical comparison of glassy and 

crystallized volcanic rocks: U.S. Geological Survey Bulletin 1201-D, p. D1-D24.

Shapiro, Leonard, 1967, Rapid analysis of rocks and minerals 

by a single-solution method, in Geological Survey Research 1967: U.S. Geological Survey Professional Paper 

575-B, p. B187-B191.

Shapiro, Leonard, and Brannock, W. W., 1962, Rapid analysis of silicate, carbonate, and phosphate rocks: U.S. 

Geological Survey Bulletin 1144-A, p. A1-A56.

Sheppard, R. A., 1971, Clinoptilolite of possible economic value 

in sedimentary deposits of the conterminous United 

States: U.S. Geological Survey Bulletin 1332-B, p. 

B1-B15.

Sheppard, R. A., and Gude, A. J., 3d, 1964, Reconnaissance 

of zeolite deposits in tuffaceous rocks of the western 

Mohave Desert and vicinit, Caligornia, in Geological 

Survey Research 1964: U.S. Geological Survey Professional Paper 501-C, p. C114-C116.

—— 1965, Zeolitic authigenesis of tuffs in the Ricardo 

Formation, Kern County, California, in Geological Survey Research 1965: U.S. Geological Survey Professional 

Paper 525-D, p. D44-D47.

—— 1968, Distribution and genesis of authigenic silicate 

minerals in tuffs of Pleistocene Lake Tecopa, Inyo 

County, California: U.S. Geological Survey Professional 

Paper 597, p. 1-36.

—— 1969, Diagenesis of tuffs in the Barstow Formation, 

Mud Hills, San Bernardino County, California: U.S. 

Geological Survey Professional Paper 597, p. 1-34.

1973, Zeolites and associated authigenic silicate minerals in tuffaceous rocks of the Big Sandy Formation, 

Mohave County, Arizona: U.S. Geological Survey Professional Paper 830, 36 p.

Sheppard, R. A., Gude, A. J., 3d, and Munson, E. L., 1965, 

Chemical composition of diagenetic zeolites from tuffaceous rocks of the Mojave Desert and vicinity, California: American Mineralogist, v. 50, nos. 1-2, p. 244- 

249.

Simons, F. S., 1964, Geology of the Klondyke Quadrangle, 

Graham and Pinal Counties, Arizona: U.S. Geological 

Survey Professional Paper 461, 173 p.

Willden, Ronald, 1964, Geology of the Christmas Quadrangle, 

Gila and Pinal Counties, Arizona: U.S. Geological Survey Bulletin 1161-E, p. E1-E64.

Drake Peak—

A Structurally Complex Rhyolite Center in

Southeastern Oregon

By RAY E. WELLS

SHORTER CONTRIBUTIONS TO MINERALOGY AND 

PETROLOGY, 1979

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1124-E

CONTENTS

Abstract __________________________________

Introduction _______________________

Acknowledgments ________________

Regional setting __________________________

Older Tertiary volcanic rocks _______________

Lower andesitic sequence ________________

Basaltic andesite of Twelvemile Peak ____

Basalt and andesite of Crook Peak ______

Lower tuff ________________________

Middle (?) Miocene and younger volcanic rocks 

Miocene flood basalt _____________.

Rhyolite of Drake Peak _______________.

Flows _______________________

Intrusions ______________________

Tuff of Honey Creek ______________

Page

El 

1 

1

1

2

5

6

6

7

7

7

7

Page

Middle (?) Miocene and younger volcanic rock—Continued

Upper basalt __________ — — __ — — __ — __ — — __ E9

Tertiary basalt intrusive rocks ____————————— 10

Surficial deposits ________________________—___ 10

Petrography and petrochemistry of the rhyolite of

Drake Peak _______————————————— 10 

Petrography __________—_____—____________— 10

Petrochemistry ______ — ___________ — — _____— 10

Structure of the Drake Peak volcanic center _____— 13

Drake Peak dome ______ —— —— —— ———— —— 13

Other domal structures in the Drake Peak area __• 14 

Origin of the central ring intrusion _——————— 14 

Basin-and-range structure in the Drake Peak area

Summary __________________-----___- — __ — __ — ___ 14

References cited _______ — ———— — _ —— _______—— 15

ILLUSTRATIONS

FIGURE 1. Location map of southeastern Oregon showing area of investigation _____________—__—____—_________

2. Sketch of view looking north across Drake Peak volcanic center from summit of Drake Peak _ —— ____

3. Geologic map of Drake Peak area _______________________________________________________ — ____

4. Diagram of normative compositions of rhyolite of Drake Peak plotted in Q-Ab-An-Or system __________

5. Graph showing major and trace element compositions of rhyolite of Drake Peak plotted against Differ­ 

	entiation Index ____________________________________________________________________

6. North-south cross sections through Drake Peak volcanic center at various stages in its evolution ___ — __

Page

E2

3

4

12

13

15

TABLES

TABLE 1. Potassium-argon age determinations from the Drake Peak area, Oregon _______. 

2. Major-element and normative compositions, in percent, of the rhyolite of Drake Peak

Page

E6 

11

III

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE 

CENTER IN SOUTHEASTERN OREGON

By RAY E. WELLS

ABSTRACT

The Drake Peak volcanic center of middle Miocene age, 

located about 25 km northeast of Lakeview, Oreg., is a 

structurally complex eruptive center that resulted from several episodes of intrusion and extrusion of rhyolite. Two 

thousand meters of andesite and basalt flows, lahars, and 

volcaniclastic rocks of late Eocene age, and of basaltic andesite, tuff, and flood basalts of Eocene to middle Miocene age 

were structurally domed by the piston-like intrusion of a 

large body of rhyolite. The eruption of rhyolite flows at 

14.3±2 million years followed the structural doming, and 

upper Miocene tuffs and basalt flows lapped against the 

dome and the rhyolite. A felsitic rhyolite ring intrusion 3 

km in diameter, emplaced during the doming, is now exposed in the deeply eroded core of the dome. The rhyolites 

range in Si02 content from 69 to 76 percent and are peraluminous. The nonporphyritic ring intrusion is more silicic 

than the flows erupted later, which contain up to 23 percent 

phenocrysts of plagioclase, hypersthene, biotite, clinopyroxene, and quartz. The progressive depletion of Mg, Ca, 

Sr, Fe, Ti, and Al and the enrichmnet in silicon and rubidium are compatible with the generation of the observed 

rhyolite types by tapping a compositionally zoned magma 

chamber.

INTRODUCTION

More than 150 centers of Cenozoic silicic volcanism are indicated on reconnaissance maps of 

southeastern Oregon (Walker, 1963, 1973; Walker 

and Repenning, 1965, 1966; Walker and others, 

1967; Greene and others, 1972), and the ages of 

many are reported by MacLeod, Walker, and McKee 

(1975). The geology and petrology of individual 

centers, however, are not well known. Many appear 

to be relatively simple groups of cumulodomes and 

flows, but a few have more complicated histories involving several episodes of volcanism and largescale deformation of the underlying volcanic rocks. 

One of the more complex silicic vents is centered 

near Drake Peak about 30 km northeast of Lakeview, Oreg. (fig. 1). There, rhyolite of variable 

composition has been erupted, and several structural 

domes and a variety of silicic intrusions, including a

ring dike 3 km in diameter, are exposed beneath a 

carapace of rhyolite flows. This report briefly describes the geologic setting, petrography, petrochemistry, structure, and eruptive history of this 

vent area, which is informally referred to as the 

Drake Peak volcanic center.

ACKNOWLEDGMENTS

This work was funded in part by Geological Society of America Penrose Grant 1858-74 and by University of Oregon student research grants. The potassium-argon age determinations were made by 

E. H. McKee as part of an investigation of the geothermal potential of Oregon. N. S. MacLeod and 

B. H. Baker made helpful comments on earlier versions of the manuscript.

REGIONAL SETTING

The Oregon Basin and Range covers 57 000 km2 

of southeastern Oregon and consists almost entirely 

of volcanic rocks of Eocene * to Holocene age. Postmiddle Miocene basalt and rhyolite in a bimodal association cover much of the area. Older Tertiary volcanic rocks consisting of andesite, dacite, and 

rhyolite are locally exposed beneath the voluminous 

basalt-rhyolite sequence. Middle Miocene to Holocene normal faulting has created several northtrending grabens where facing escarpments expose 

up to 2200 m of Tertiary volcanic rocks. Pre-Tertiary rock is exposed in two small areas in the Pueblo 

and the Trout Creek Mountains near the Nevada 

border.

Miocene and younger rhyolite domes form two 

west-northwest-trending belts across southeastern 

Oregon (Walker, 1973). Domes younger than 10

1 Ages of epochs used in this report are from Berggren (1972). They 

are younger than those traditionally used in reports on eastern Oregon, 

which are based on radiometric ages of mammalian stages (Evernden and 

others, 1964).

El
E2 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

121° 120o 119° 118°

Area of Drake Peak 

rhyolite center

CALIFORNIA

Cenozoic rhyolite domes

NEVADA

EXPLANATION

50 KILOMETERS

Contact Normal fault Major range-bounding normal fault 

Hachures on downfhrown side

FIGURE 1.—Location map of southeastern Oregon showing area of investigation. Cenozoic silicic domes and faults from 

Walker and King (1969), Walker (1973), and MacLeod, Walker, and McKee (1975).

m.y. (million years) show a well-defined southeastto-northwest age progression from approximately 

10 m.y. at Duck Butte and Beatys Butte to less than 

1 m.y. at Newberry caldera (fig. 1) ; older domes, 

mostly about 15 m.y. old, are widespread but are 

fewer in number than the younger domes and show 

no age progression. The Drake Peak volcanic center 

is in the southern belt of domes, and radiometric 

ages presented here indicate that it belongs to the 

older group.

The volcanic center is situated near the western 

escarpment of the north Warner Mountains block 

and lies southeast of Abert Rim, one of the largest 

fault scarps in southeastern Oregon. It forms a 

striking circular group of peaks clustered around 

the headwaters of Twelvemile and McDowell Creeks. 

The peaks form two concentric rings 2100-2400 m 

in elevation separated by a 300- to 600-m-deep circular valley. The outer ring, about 8 km in diameter, 

consists of fault blocks that dip radially outward.

The inner ring, about 3.2 km in diameter, is composed of intrusive rhyolite (fig. 2).

OLDER TERTIARY VOLCANIC ROCKS

Middle Miocene and older volcanic rocks form the 

structural foundation for the later silicic flows and 

intrusions of the Drake Peak volcanic center. Miocene structural doming and later erosion have exposed a 2100-m sequence of basaltic and andesitic 

lava, volcanic breccia, ash-flow tuff, sedimentary tuff, 

and flood basalt in its core (fig. 3). The three oldest 

units, informally termed the lower andesitic sequence, the basaltic andesite of Twelvemile Peak, 

and basalt and andesite of Crook Peak, are exposed 

only in the strongly uplifted blocks of the structural 

domes. Oligocene or Miocene tuff and Miocene flood 

basalt that form the upper part of the foundation 

volcanic sequence are widely exposed throughout the 

Abert Rim area.
DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON E3

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E4 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

42°27'30"

I20°02'l5"

I20°I5' 

42°22'30

42° 15'

FIGURE 3.—Geologic map of Drake Peak area.
DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON E5

EXPLANATION

SEDIMENTARY DEPOSITS

Pediment gravel 

EXTRUSIVE ROCKS INTRUSIVE ROCKS

Flood basalt

Basalt and andesite 

of Crook Peak

Divided into Tab 3,

basalt and andes/fe;

Tab 2, basalt; and

Tab 1, andes/fe

Lower andesitic 

sequence

Dacite

Includes minor 

flows

Volcanic and

sedimentary rocks,

undivided

0°

Contact 

Dashed where inferred; queried where uncertain

Fault

Dashed where inferred; dotted where concealed; 

queried where uncertain. Bar and ball on downthrown side

Strike and dip of beds

Rhyolite flow foliation 

From aerial photographs

Inclined Vertical 

Strike and dip of flow foliation

Summit

Location of satellite

intrusion of rhyolite

see text

FIGURE 3.—Continued.

LOWER ANDESITIC SEQUENCE

The lower andesitic sequence consists of andesite 

flows, flow breccia, mudflows, ash-flow tuff, fluvial 

tuff, conglomeratic sandstone, and minor basalt 

flows. It crops out over a wide area in the middle of 

the Drake Peak volcanic center, where 650 m is exposed in the lowest parts of the eroded updomed

blocks surrounding the central ring intrusion. Its 

base is nowhere exposed, so its total thickness is not 

known. The oldest exposed strata are probably the 

conglomeratic volcanic arkose and lithic sandstone 

that crop out along the east flank of the amphitheater inside the central ring intrusion. They are 

generally very coarse grained and consist of variable
E6 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

amounts of labradorite, andesitic rock fragments, 

amphibole, and clinopyroxene and lesser amounts of 

microcline, muscovite, and polycrystalline quartz. 

The conglomerate lenses contain well-rounded cobbles of andesite, basalt, and quartzite.

The sedimentary rocks interfmger upsection with 

hornblende and pyroxene andesite flows and lahars. 

Andesitic flow breccias and thin basalt flows are 

sporadically interbedded with the lahars. Fine- to 

coarse-grained epiclastic tuffs and ash-flow tuffs that 

crop out near the base of Crook, McDowell, and 

Twelvemile Peaks form the upper part of the 

sequence.

A K-Ar age of 40.2 ±4 m.y. was determined for a 

hornblende andesite near the base of the exposed 

sequence (table 1). This age is compatible with those 

obtained on other older andesitic sequences in the 

region. A sequence similar to that of the Drake Peak 

area crops out in the Warner Mountains 90 km to 

the south in California (Russell, 1928). Axelrod 

(1966) reports an age of 40 m.y. on an andesite flow 

low in the central Warner Mountains section, and 

Duffield and McKee (1974) reported an age of 32 

m.y. on andesitic ash-flow tuffs about 450 m above 

the base of the exposed section in the south Warner 

Mountains. An older andesitic sequence of similar 

character exposed in the Paisley Hills 34 km northwest of Drake Peak is intruded by a 33-m.y.-old 

granodiorite plug (Peterson and Mclntyre, 1970). 

The lower andesitic sequence correlates in part with 

the Clarno Formation of central Oregon (see K-Ar 

ages in Enlows and Parker, 1972).

BASALTIC ANDESITE OF TWELVEMILE PEAK

A distinctive basaltic andesite overlies the lower 

andesitic sequence and forms a thick lenticular accumulation that is well exposed in the upturned

TABLE 1.—Potassium-argon age determinations from the 

Drake Peak area, Oregon

[Potassium-argon dates by E.H. McKee]

Unit and Mineral 

dated

Hornblende andesite,

lower andesite sequence

(hornblende)

Rhyolite of

Drake Peak; flow

(plagioclase)

Field no.

Latitude

Longitude

K2 0 (wt. percent)

Radiogenic ^ 

(moles/g)

Percent ^ 

Age (m.y.)

DP-154

42°20'54"N

120°07'12"W

0.363

2.1804xl(T n

17.3 

40.2±4

DP-108 

42°18'30"N 

120°08'50"W

0.871

0.910

15.96

14.312

blocks of the Drake Peak volcanic center. The unit 

is about 1000 m thick along the east face of Twelvemile Peak and thins northeastward to about 310 m 

on the Fort Creek-McDowell Creek divide. Flow 

units consist of flow breccia and range from 3 to 13 

m in thickness. Discontinuous zones of massive lava 

are common within the flows, and baked soil horizons are found between some flows. Tabular units of 

black unaltered nonvesicular holocrystalline basalt 

occur sporadically throughout the unit and probably 

represent sills intruded into the volcanic pile. Also 

found are lesser amounts of hornblende andesite and 

locally thick accumulations of zeolitized red volcanic 

breccia. The andesite is highly vesicular and porphyritic, averaging 46-percent phenocrysts of milky 

white plagioclase (~An50 ), black clinopyroxene, and 

red altered olivine. The rocks are pervasively altered, and vesicles and fractures are filled with green 

clay, calcite, and zeolites.

No similar rocks are exposed on any of the major 

fault scarps in the vicinity. The absence of exposures 

outside the complex, even in the deeply incised 

Crooked Creek canyon only 4 km to the west, suggests that the basaltic andesite is a localized accumulation. The lenticular outcrop pattern and the onlapping of overlying units to the south indicate a 

possible vent center near Twelvemile Peak.

The age of the basaltic andesite is uncertain. It 

overlies strata of late Eocene age and is in turn 

overlain by a varied volcanic sequence over 1000 m 

thick that separates it from overlying tuffs of middle to late Miocene age.

BASALT AND ANDESITE OF CROOK PEAK

The basalt and andesite of Crook Peak is exposed 

above the basaltic andesite on Crook and McDowell 

Peaks. About 530 m is exposed on Crook Peak, and, 

toward the northeast, the sequence thickens to 720 

m. Three subdivisions of nearly equal thickness 

have been made in the sequence. The lowermost unit 

consists of red-brown hornblende andesite, gray 

pyroxene andesite, and reddish-gray olivine basalt 

flows. Black to greenish-black aphyric (nonporphyritic) basalt making up the middle unit weathers 

to a distinctive orange, and it is generally without 

vesicles or flow structures. Gray to purplish-gray 

aphanitic basalt and olivine basalt form the upper 

part of the group. Most flows in the upper unit are 

quite vesicular and display well-developed platy 

jointing along the flow tops.

A few similar flows are interbedded in the upper 

Oligocene or lower Miocene lower tuff along the 

western margin of the area, and the upper part of
DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON E7

the lower tuff overlies the upper andesitic sequence 

in the upturned blocks of the structural dome. The 

basalt and andesite of Crook Peak is therefore considered to be of late Oligocene or early Miocene 

age.

LOWER TUFF

The lower tuff is a widespread composite unit 

consisting of ash-flow tuff, tuffaceous sediment, and 

interbedded basalt that is widely exposed west of 

the Drake Peak volcanic center. The upper 300 m of 

the lower tuff crops out in the canyons of Crooked 

Creek and its tributaries. The thickness of the unit 

decreases eastward, where in the upturned blocks 

of the dome it is represented by less than 30 m of 

strata.

The lower tuff is composed primarily of intermediate to silicic ash-flow tuff units. The lower part 

of the section in Crooked Creek Canyon consists of 

light-brown lithic tuff, pink crystal-rich welded tuff, 

and pale-gray vitric welded tuff. They are overlain 

by several unwelded massive buff to light-brown 

pumice lapilli ash-flow tuffs at least 65 m thick.

The base of the lower tuff is not exposed in the 

west half of the study area, but in the east half, 

where the unit is much thinner, it rests on the basalt 

and andesite of Crook Peak and possibly also on the 

basaltic andesite of Twelvemile Peak. The variation 

in thickness of the unit from west to east suggests 

that it laps onto the preexisting high of basaltic 

andesite. Basalt similar to that in the basalt and 

andesite of Crook Peak is interbedded in the lower 

tuff in the west part of the study area, which, along 

with the rapid east-to-west fades change, suggests 

that the lower tuff may be in part equivalent in age 

to the basalt and andesite of Crook Peak and somewhat younger than the basaltic andesite of Twelvemile Peak. Plant remains found 2 km west of the 

study area indicate a late Oligocene or early Miocene age for the lower tuff (Walker, 1960). This tuff 

unit probably correlates in part with the John Day 

Formation of central Oregon (Swanson, 1969).

MIDDLE(?) MIOCENE AND YOUNGER 

VOLCANIC ROCKS

MIOCENE FLOOD BASALT

The most widely distributed unit in the Drake 

Peak area is a flood basalt sequence similar in appearance and stratigraphic position to the Steens 

Basalt (Fuller, 1931; Piper and others, 1939) exposed on Steens Mountain, 110 km to the east. This 

well-exposed flow sequence, up to 600 m thick, caps 

the tilted blocks of the Drake Peak dome and forms 

the inner canyon walls along Honey and Little Honey 

Creeks, north of the volcanic center. The flows are 

typically less than 6 m thick, and many are 2 to 3 

m. The absence of tuffaceous interbeds, soil horizons, 

and top and bottom flow breccias suggests that the 

basalt was very fluid and that the flows were erupted 

in rapid succession. Light-gray ophitic diktytaxitic 

(crystal-bounded vesicles) olivine basalt is the most 

common type in the Drake Peak area. Some flows are 

coarsely porphyritic and contain plagioclase phenocrysts up to 20 mm long. The plagioclase content 

varies regularly within a flow sequence; coarsely 

porphyritic flows are overlain by successive flows in 

which plagioclase becomes less abundant.

The thickness of the flood basalt sequence varies 

widely within the Drake Peak area. A 610-m section 

on the east flank of the complex thins and disappears to the southwest, suggesting that the basalt 

flowed over a surface of considerable relief. The 

basalt unconformably overlies the basaltic andesite 

of Twelvemile Peak, lower tuff, and basalt and 

andesite of Crook Peak.

The geomagnetic polarities of the flood basalt 

were determined with a portable fluxgate magnetometer. Sections of five to seven flows on McDowell 

Peak, in Honey and Little Honey Creek canyons, 

and on the basalt ridge on the east flank of the complex are all reversely polarized. Reversely polarized 

flow sequences can be traced west to the Abert Rim 

escarpment, where they presumably correlate in part 

with a reversely polarized section of 16 flows, described by Watkins (1965). The Steens Basalt, however, exhibits a polarity transition from reversed 

to normal upsection that has been dated at 15.1 ±0.3 

m.y. (Baksi and others, 1967). The flood basalt of 

the Drake Peak area is roughly correlative with the 

Steens Basalt, as it is onlapped by tuffaceous sediments containing a Barstovian (middle Miocene) 

fauna in Honey Creek canyon (Walker, 1960) and is 

underlain by the upper Oligocene or lower Miocene 

lower tuff. It may or may not be of the same reversed-polarity epoch as the lower Steens Mountain section.

RHYOLITE OF DRAKE PEAK

The rhyolite of Drake Peak consists of a differentiated suite of rhyolite intrusions and flows that were 

emplaced into or erupted from the Drake Peak volcanic center.
E8 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

FLOWS

The flows consist mostly of massive or flow-foliated light-gray to black porphyritic vitrophyre and 

stony rhyolite containing plagioclase, biotite, and 

hypersthene phenocrysts. Potassium-argon dating of 

plagioclase from a flow sample gives an age of 14.3 ± 

2 m.y. (table 1). The flows form a massive apron or 

wedge of coalescing lobes that cover about 29 km 2 

on the southern flank of the structural dome. Erosion 

has extensively modified the shape of the flow mass, 

but several flow lobes still persist. The main rhyolite 

body rests on the upthrown blocks of the foundation volcanic sequence and has 750 m of relief from 

base to top. The average thickness of the main body 

is difficult to determine because of the irregularities 

of the underlying topography, but rough estimates 

on the large southeastern lobe suggest a thickness 

there of approximately 300 m. The absence of visible 

stratigraphic breaks in the main rhyolite mass suggest that it is the product of a single eruption or a 

rapid series of eruptions.

Outliers of rhyolite on the west flank of the dome 

are erosional remnants of a formerly larger body 

that covered the west slope of the structural dome. 

The largest detached mass, about 100 m thick, caps 

Twelvemile Peak. Two other lobate masses, about 

30 m thick, lie about 2 km northwest of Twelvemile 

Peak. A separate flow covering about 2.6 km 2 lies 

slightly east of the main rhyolite edifice and apparently originated from a separate vent. On the northeast side of the dome are two small flow remnants 

associated with small silicic intrusions.

The rhyolites unconformably overlie the structurally domed foundation and were erupted after 

doming occurred. The base of the rhyolite is at 

widely different elevations around the rim of the 

outer ring, indicating that the rhyolite flowed over 

a terrain of differentially uplifted (or eroded) blocks 

having considerable relief.

Viscous flow has produced a variety of internal 

structures. Flow layering is conspicuous and generally occurs throughout the flow mass except where 

obscured by later devitrification. A thick weathered 

zone obscures much of the flow geometry on the 

west slope of the dome, but on the east slope it is 

well exposed and suggests a flow direction south and 

east away from Drake Peak. Moderate- to steepdipping ramp structures are well exposed on the 

distal parts of the flow lobes. On Light Peak, large 

east-west-trending synformal flow folds, over 100 m 

wide, suggest channelized flow, possibly through 

gaps between tilted blocks of the structural dome. 

A source east of Light Peak and north of Drake

Peak is suggested by the progressive steepening of 

flow layering to vertical in that area.

The cooling zones in a rhyolite flow described by 

Christiansen and Lipman (1966) are also developed 

in the Drake Peak flows. Ideally, the zones include 

a surface layer of pumiceous breccia and flow-layered 

pumice, an envelope of massive flow-layered vitrophyre, a core of massive flow-layered devitrified 

rhyolite, and a basal flow breccia of vitrophyric and 

pumiceous blocks. Erosion has stripped the pumiceous surface layer from the flows, exposing the thick 

envelope of massive vitrophyre. Extensive erosion 

between Light and Drake Peaks has exposed the 

devitrified rhyolite core. A basal flow breccia consisting of about 35 m of flow-banded cobbles and 

boulders imbedded in a fine-grained white matrix 

is exposed on the ridge north of Light Peak where 

the flow inclines steeply upward. The contact of the 

breccia and the underlying flood basalt is also exposed there. Other breccia zones have been observed 

in the flows, but they differ from the basal breccia 

and were not formed by successive eruptions of 

rhyolite onto previously cooled flows. The most common intraflow breccia contains distorted augenlike 

rhyolite clasts in a red glassy matrix and is probably the result of reincorporation of solid fragments 

into the hot flow interior. Vertical breccia dikes 

about 15 m thick on the north face of Drake Peak 

just below the summit consist of flow-banded fragments up to 150 mm across embedded in a finely 

divided white glassy matrix. The dikes are near a 

probable vent and may have been created by violent 

degassing through the rhyolite pile.

INTRUSIONS

At least 13 separate rhyolite intrusions are exposed within the Drake Peak volcanic center; the 

largest is the felsitic ring intrusion exposed in its 

eroded core. The semicircular ring has an east-west 

diameter of 3 km and a north-south diameter of 3.3 

km. Remnants of the intruded lower andesitic sequence still cling to the flanks of the intrusion resulting in an irregular contact. The rhyolite is not 

well exposed; extensive jointing produces widespread talus that effectively obscures outcrops. The 

few good exposures consist of sugary light-gray to 

white aphyric felsite. In some areas, flow banding is 

well developed, and, where exposed in outcrop, it is 

generally vertical. Several smaller intrusions of 

identical aphyric felsite in the interior of the complex are probably attached to the main felsite mass 

of the ring intrusion. The felsite consists of a 

spherulitic interlocking mat of quartz and alkali
DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON E9

feldspiar lightly dusted with clay alteration products; 

it is texturally and mineralogically quite different 

from the flows that cover the flanks of the structural 

dome.

Intrusions west and south of the ring intrusion 

are similar to the flows. They contain phenocrysts 

of plagioclase, hypersthene, and biotite in a devitrified microlitic matrix. The intrusion at the base of 

Twelvemile Peak has a breccia zone along its contact with the ring intrusion that incorporates rhyolite fragments similar to those of the ring.

Small intrusions on the northeast and southeast 

flanks of the main structural dome fill the source 

vents for small flows isolated from the main rhyolite 

mass. The northeast intrusions are plagioclase-oxyhornblende(?) dacites that have deformed the surrounding country rock. Onlapping of the Miocene 

flood basalts onto the deformed rocks indicates that 

the dacites are older than the rhyolite.

TUFF OF HONEY CREEK

The tuff of Honey Creek is named for outcrops 

along Honey Creek from Sherman Valley east to its 

confluence with Snyder Creek, about 7 km northeast 

of the Drake Peak volcanic center. It consists of 

silicic tuffaceous sediments, welded tuff, and a few 

intercalated basalt flows that surround and apparently lap against the Drake Peak structural 

dome. The tuff of Honey Creek overlies the Miocene 

flood basalts with a slight angular unconformity and 

is in turn overlain by several widespread basalt 

flows. Near the volcanic center the tuffaceous rocks 

are very coarse grained crudely stratified sandstone 

containing fragments of flow-banded vitrophyre, 

felsite, and diktytaxitic ophitic basalt in a matrix 

of silicic glass shards. In places, the tuff contains 

large boulders of basalt apparently derived from the 

uplifted Miocene flood basalts of the structural 

dome.

Welded tuff occurs at several localities, the most 

prominent being an extensive rimrock exposure 5 

km northeast of the complex on the south side of 

Honey Creek. Isolated outcrops also occur at the 

2200-m level on the north shoulder of Twelvemile 

Peak and on either side of Honey Creek about 2 km 

east of Sherman Valley. All are eutaxitic vitrophyre 

containing partially collapsed pumice fragments and 

phenocrysts of sanidine, plagioclase, or anorthoclase, 

and amphibole. The welded tuff units are thin (less 

than 6 m) and mineralogically unlike the rhyolite 

of Drake Peak, indicating they originated outside 

the Drake Peak area.

At greater distances from the volcanic center, 

thin-bedded pumicite and friable fine-grained tuff 

rich in plant fragments suggest lacustrine or floodplain deposition. The coarse debris forming an apron 

around the Drake Peak center was derived from erosion of the structural dome and is younger than the 

intrusive activity in the dome. However, the welded 

tuff remnant high on Twelvemile Peak must have 

been uplifted by the doming, indicating that part of 

the tuff of Honey Creek predates the Drake Peak 

volcanic center. Vertebrate remains collected near 

the confluence of Honey and Snyder Creeks indicate 

a Barstovian (middle Miocene) age for the tuff of 

Honey Creek (Walker, 1960).

UPPER BASALT

Remnants of an extensive sheet of rim-capping 

basalt, termed the upper basalt in this report, are 

widely exposed on the upland surfaces surrounding 

the Drake Peak volcanic center. The unit is from 3 

to 60 m thick, with individual flows averaging about 

3 m. Most flows are highly vesicular and range from 

light-gray diktytaxitic olivine basalt to platy black 

aphanitic basalt. The upper basalt overlies the tuff 

of Honey Creek and the Miocene flood basalts with 

a slight angular unconformity, and in sections where 

the intervening tuff is not exposed, the two basalts 

are virtually indistinguishable. Paleomagnetic studies by Larson (1965) suggest that in the areas north 

and east, of the Drake Peak dome, the upper basalt 

may be distinguished from the reversed Miocene 

flood basalts by its normal polarity. Field polarity 

checks of the upper basalt at several localities (Mud 

Creek Campground, the Drake Peak Lookout Road, 

isolated exposures south of Sherman Valley, exposures north and south of Honey Creek, and rimrock north of Vee Lake) all indicate the basalt is 

normally polarized.

The upper basalt is in part equivalent to the 

"vesicular basalt flows" and "basalt," as mapped by 

Walker (1963) in the Klamath Falls quadrangle, 

for widespread rim-forming basalt generally having 

dips of less than 5°. It is also equivalent to Larson's 

(1965) upper basalt and young upper basalt, which 

he correlated eastward as far as the Plush escarpment.

The upper basalt in the Drake Peak area overlies 

the middle or upper Miocene tuff of Honey Creek and 

forms the uppermost volcanic unit in the section. It 

apparently flowed around the high ground of the 

Drake Peak volcanic center along its northern and 

southern margins and consequently is younger than 

the rhyolite of Drake Peak. About 5 km northeast
E10 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

of the Drake Peak area, a welded ash-flow tuff correlated by Larson (1965) with the Danforth Formation of Piper, Robinson, and Park (1939) overlies 

the upper basalt. Several ash-flow tuff units recognized within the Danforth Formation range in age 

from 9.2 to 6.8 m.y., and the tuff of Double 0 Ranch 

(6.8 m.y.) is the most likely candidate for correlation with Larson's (1965) ash-flow tuff (Greene 

and others, 1972; Greene, 1973). The upper basalt 

must then be of middle or late Miocene age.

TERTIARY BASALT INTRUSIVE ROCKS

Basaltic dikes are locally abundant along- the western margin of the map area. Medium- to fine-grained 

olivme-plagioclase-phyric (porphyritic) dikes, up to 

50 m thick, intrude the tuff of Honey Creek and 

probably represent feeders for the rim-capping middle or late Miocene upper basalt. A complex of dikes 

and sills of probable middle Miocene age crops out 

along Crooked Creek and along Abert Rim, 2 km 

west of the map area. A differentiated sill, at least 

70 m thick, occupies the bottom of the canyon, and 

a large olivine-phyric ophitic dike connected to the 

sill cuts through the lower tuff but does not cut the 

tuff of Honey Creek. These large intrusions are 

petrographically similar to the middle Miocene flood 

basalts and probably represent feeders for the flows.

SURFICIAL DEPOSITS

A variety of surficial deposits occur in the Drake 

Peak area, but they are not discussed in any detail 

in this report. They include the following: Pliocene 

or Pleistocene pediment gravel veneering a high-level 

erosion surface east of the dome; Pleistocene g-lacial 

moraines on the north side of Drake Peak; lacustrine 

deposits in unusual rimmed basins formed on flood 

basalt; large Pleistocene (?) landslide deposits, 

especially along- Crooked Creek; and Holocene sediments forming alluvial flats where local base levels 

have been maintained by resistant basaltic units.

PETROGRAPHY AND PETROCHEMISTRY OF 

THE RHYOLITE OF DRAKE PEAK

PETROGRAPHY

The rhyolite flows contain 10 to 23 percent phenocrysts of plagioclase, hypersthene, and biotite and 

minor amounts of quartz, amphibole, clinopyroxene, 

and opaque minerals in a microlitic or vitrophyric 

matrix. The modal composition of 27 samples is 83 

percent groundmass, 13 percent plagioclase, 1 percent each of biotite and hypersthene, 0.7 percent 

opaque minerals, and less than 0.2 percent each of

quartz, amphibole, and clinopyroxene. Zoned plagioclase, An38_50 , is ubiquitous and forms isolated 

euhedral laths and glomeroporphyritic clots up to 

10 mm across, with common honeycomb resorption 

in the more calcic zones. Both normal and oscillatory 

zoning are common. Hypersthene forms strongly 

pleochroic subhedral to euhedral prisms up to 1 mm 

long and also occurs as microlites in the groundmass 

of some samples. Biotite occurs in nearly all samples as isolated plates up to 1 mm across. Commonly 

it is strongly resorbed and rimmed or completely 

replaced by granular iron oxides. Quartz occurs as 

rare, strongly resorbed, spherical grains in a few 

samples. Sparse phenocrysts of lig-ht-green clinor 

pyroxene, green euhedral amphibole, and opaque 

minerals occur in a few samples. Glomeroporphyritic 

clots of plagioclase, hypersthene, clinopyroxene, and 

opaque minerals are common. Groundmass textures 

are hyalopilitic to trachytic, with olig-oclase microlites typically forming 50 percent of the groundmass. 

The mafic mineral assemblag-es vary among the 

rhyolite flows and intrusions. The major part of the 

main flow mass is hypersthene + biotite rhyolite, as 

are several of the intrusions south and west of the 

ring intrusion. The two detached flow lobes on the 

northwest side of the volcanic center are hypersthene rhyolite containing little or no biotite. In contrast, the isolated flow on the southeast flank consists of biotite rhyolite containing no hypersthene. A 

small intrusion just east of the glacial moraine is 

also composed of biotite rhyolite with minor amphibole. The central ring and associated intrusions contain no mafic phases. The differences in the mafic 

phenocryst assemblage may be controlled by variations in partial pressure of H2 0 or by the temperature of the mag-ma. The sequence of eruption, determined from field relations, suggests that the 

aphyric rhyolite was erupted first and was followed 

by the biotite ± amphibole rhyolite and finally by the 

hypersthene+biotite rhyolite. This seems consistent 

with the successive tapping of hotter magmas, perhaps representing deeper levels of a vertically zoned 

magma chamber.

PETROCHEMISTRY

Thirteen samples representing a variety of rhyolite types were analyzed for major and selected trace 

elements (table 2). The rhyolite flows and intrusions 

are peraluminous and have K2 0 in excess of Na 2 0. 

The major-element chemistry is comparable to some 

of the older domes (5-10 m.y.) described by MacLeod, Walker, and McKee (1975) in southeastern 

Oregon, although Ti, Ca, and Mg are somewhat more
DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON Ell 

TABLE 2.—Major-element and normative compositions, in percent, of the rhyolite of Drake Peak

[Chemical analyses by x-ray fluorescence methods of Norrish and Hutton (1969); Na 2 0 by INAA methods of Gordon and others

(1968). Analyst: R.E. Wells]

Sample no.

Si0 2

A1 2 0 3

1 Fe 2 0 3

MgO

CaO

Na2 0

K2 0

Ti0 2

MnO

P 2 0 5

2 Loss

Total

83

71.6

15.2

2.3

1.2

1.8

3.7

4.6

.41

.05

.20

.54

101.60

103

72.2

14.7

2.6

.90

1.9

3.8

4.4

.40

.05

.14

.65

101.74

103A

72.3

14.6

2.2

.74

1.8

4.0

4.6

.40

.06

.18

.85

101.73

109 112

Major

69.3 69.6

14.3 14.4

2.5 2.4

.88 .79

1.9 1.9

3.8 3.7

4.6 4.7

.40 .40

.07 .06

.13 .13

2.2 1.9

100.05 99.90

122 125 131 134

oxides (weight percent)

70.6 70.2 72.1 74.6

14.6 14.8 14.4 13.4

2.1 2.5 2.3 1.6

.57 .60 .40 .52

1.8 1.9 1.6 1.2

4.1 4.0 3.8 3.5

4.5 4.6 4.7 4.9

.39 .42 .38 .28

.06 .06 .05 .05

.11 ..14 .11 .08

.53 .56 .57 .52

99.36 99.78 100.41 100.65

139

73.6

13.7

1.7

.53

1.3

3.7

4.7

.28

.03

.07

.61

100.22

144

75

13

3

4

99

.5

.4

.51

.29

.85

.9

.6

.08

.02

.03

.56

.74

141

72.3

14.1

2.2

.8

1.8

3.6

4.5

.36

.06

.13

.99

100.84

149

76.5

13.6

.38

.21

.81

3.9

4.7

.10

.04

.00

.56

100.80

205

-

-

-

-

-

-

-

-

-

-

-

-

Catanorms 3

q

or

ab

an

cor

en

fs

hy

mt

il

ap

''Diff. Index

24.4

27.0

32.9

7.8

1.6

3.3

1.4

4.7

.7

.6

.4

84.3

24.8

26.1

34.1

8.8

.5

2.4

1.6

4.0

.8

.6

.3

85.0

24.4

27.2

35.3

8.0

.3

2.0

1.3

3.3

.7

.5

.4

86.9

23.9 23.3

28.0 28.7

34 . 0 34 . 0

8.7 8.6

.3 .2

2.0 2.2

1.5 1.5

3.5 3.7

.7 .7

.6 .6

.3 .3

85.5 86.0

23.3 22.9 26.0 29.2

27.1 27.4 27.8 29.3

37.0 36.0 34.5 32.0

8.3 8.5 7.3 5.7

.7 .3 .5 .3

1.6 1.7 1.1 1.5

1.2 1.5 1.4 1.0

2.8 3.2 2.5 2.5

.6 .8 .7 .5

.6 .6 .5 .4

.21 .3 .2 .1

87.4 86.4 88.3 90.5

28.3

28.1

33.6

6.2

.3

1.5

1.0

2.5

.5

.4

.1

90.0

30

27

35

4

1

93

.8

.7

.4

.1

.6

.9

.3

.1

.2

.1

.04

.8

27.0

27.0

33.0

8.0

.5

2.1

1.4

3.5

.7

.5

.3

86.6

31.2

27.8

35.2

4.0

.6

.6

.2

.8

.1

.1

-

94.3

-

-

-

-

-

-

-

-

-

Trace elements (parts per million)

Rb

Sr

Zr

Y

Nb

J A11 iron as

53

216

133

10

7

Fe 2 0 3 .

_

-

-

-

-

68

176

105

15

8

55 60

273 278

154 160

14 18

7 9

2 Loss on Ignition.

Sample localities:

83 42°18'

103 42°20'

103A 42°20'

109 42°18'

112 42°18'

122 42°18'

125 42°18'

00"N;120

10"N;120

10"N;120

30"N;120

50"N;120

10"N;120

18"N;120

°07'10"W

°08'23"W

°08'25"W

°08'50"W

°09'00"W

°07'23"W

°07'22"W

Quadrangle :

Drake Peak 7%'

Crook Peak 7%'

do.

do.

do.

Drake Peak 7%'

do.

61 - 62 70

268 - 252 165

162 - 160 85

16 - 17

8 - 9 -

3 Fe 3+ /Fe 2+ set at 0.4 for

72

167

90

-

-

99

25

36

normative

-

-

61

270

160

19

11

100

56

60

18

7

100

50

60

18

10

calculations.

''D.I. of Thornton and Tuttle (1960).

Sample localities:

131 42°18'46"N;120 0 05'

134 42°19'02"N;120°06'

139 42°19'50"N;120°05'

144 42°19'16"N;120°07'

141 42°18 I 46"N;120°07 I

149 42°19'4X)"N;120°06 I

205 42°21 I 36"N;120°06'

Quadrangle :

54"W

45"W

00"W

30"W

54"W

56"W

45"W

Drake Peak

do.

do.

7V

Crook Peak 7%'

do.

Drake Peak

do.

7V
E12 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

abundant for a given Si02 content in the rhyolite of 

Drake Peak.

The absence of potassium feldspar and the general paucity of quartz result from the relatively 

high calcium content of the mag-ma. The normative 

bulk compositions of the rhyolite are plotted in the 

Q-Ab-An-Or system in figure 4. The samples contain approximately 5 to 10 percent normative An and 

plot in the plagioclase field well above the two-feldspar surface. The rare, strongly resorbed quartz 

phenocrysts indicate that initial crystallization began at moderate crustal depths and higher water 

pressures, as inferred from Tuttle and Bowen's 

(1958) studies in the Q-Ab-Or-H2 0 system. Resorption of quartz occurred when pressure was released, 

probably during the rise of the magma to shallower 

crustal levels.

When weight-percent oxides of the rhyolite of

Ab7n Q

/ 

Ab

\ 

Or

FIGURE 4.—Normative compositions of rhyolite of Drake 

Peak plotted in Q-Ab-An-Or system. A, Compositions of 

rhyolite of Drake Peak plotted on Q:!0 plane in Q-Ab-AnOr system at PH2 o=Ptotai=l kbar. Position of plagioclasesanidine cotactic is from James and Hamilton (1969). 

Schematic Q-Ab-An-Or system (after Carmichael and 

others, 1974) shown in inset, with dotted line indicating 

plane of projection. B, Rhyolite compositions plotted on 

base of Q-Ab-An-Or system; conditions as in A. Cotectics 

for systems with 3 and 5 percent An component from 

James and Hamilton (1969). Compositional trend follows 

isobaric fractionation path as inferred from Tuttle and 

Bowen (1958).

Drake Peak are plotted against the Thornton-Tuttle 

Differentiation Index (D.I.) (Thornton and Tuttle, 

1960) (fig. 5), the compositions fall into three 

groups that are consistent with groupings defined 

in the field. The most differentiated rhyolites 

(D.I. —94) are those of the central ring intrusion, 

which on the basis of field evidence were the first 

to be emplaced. The two samples with intermediate 

indices (D.I. —90) are from a peripheral intrusion 

southeast of the main structural dome and a small 

isolated intrusion south of the central ring. The direction of the flow from the southeast intrusion suggests that it was emplaced after the formation of the 

main structural dome. Those samples with indices 

of 84 to 88 are from the main flow mass on the south 

side of the volcanic center, which was probably the 

last rhyolite body to be erupted.

The regular variation in major-element chemistry 

is complemented by a smooth variation in abundances of selected trace elements with D.I. (fig. 51?). 

Strontium is relatively abundant, and Rb/Sr increases with D.I. from about 0.2 to 4. This ratio is 

an order of magnitude less than in some other highsilica rhyolites from the Basin and Range Province 

(Noble and others, 1972) and indicates that the 

rhyolite of Drake Peak has not undergone extreme 

fractionation. The major- and trace-element compositions correlate directly with an observed phenocryst assemblage of plagioclase, pyroxene, biotite, 

opaque minerals, and zircon that constitutes up to 

23 percent of the later rhyolite flows. The eruption 

of progressively less differentiated rhyolite with increasing crystal content is comparable to the inverted compositional zonation commonly observed in 

ash-flow sheets (Lipman, 1966).

Although the mineralogical and chemical variations at Drake Peak are qualitatively consistent with 

differentiation by crystal fractionation, other processes may be responsible for the compositional zonation of the magma chamber. Hildreth (1976) demonstrated compositional zonation in the Bishop Tuff 

(from the Long Valley caldera in eastern California) 

that cannot be explained by crystal fractionation 

models. He postulated a process of thermogravitational diffusion to account for observed compositional gradients in large rhyolite magma bodies.

Structural relations in the Drake Peak volcanic 

center require the existence of a large magma body 

beneath it. The chemical variation in the rhyolite 

probably resulted from differentiation within a simple magma chamber rather than from the production of several physically and chemically discrete 

magmas from separate sources.
DEAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON E13

SAMPLE NUMBER

AI 2 03

Na 9 0

Fe 2 0 3

85 86 87 88 89 90 9) 92 93 94 95

DIFFERENTIATION INDEX

SAMPLE NUMBER

Rb

Sr

Nb

90

DIFFERENTIATION INDEX

92 93 94 95

STRUCTURE OF THE DRAKE PEAK VOLCANIC 

CENTER

DRAKE PEAK DOME

The largest of the three structural domes in the 

Drake Peak volcanic center is about 9 km across at 

its base, and the felsitic ring intrusion is exposed in 

its deeply eroded core. About 1600 m of strata has 

been stripped from the domal core, and remnant 

blocks of the foundation volcanic rocks form a ring 

of hills with quaquaversal dip slopes. Radial faults 

have broken the dome into several structural blocks. 

McDowell Peak, Crook Peak, and smaller blocks 

east of Crook Peak are bounded by radial faults, and 

McDowell Creek, Fort Creek, and Twelvemile Creek 

follow radial fault zones for long distances. Displacements observed across the faults indicate dip-slip 

and rotational movement. Radial faults such as those 

bounding McDowell Peak are normal faults having 

a small component of rotational movement away 

from the center of the dome. Others like the McDowell Creek fault and the east bounding fault of 

the Crook Peak block have a large component of rotational movement, and adjacent blocks commonly 

dip between 60° and 80° away from the center of 

the dome. In the core of the dome, a compound 

graben formed by closely spaced block faults has 

been complicated by the intrusion of the rhyolite 

ring.

The fault pattern of the Drake Peak dome is the 

result of extensional failure of the foundation rocks 

during their doming and stretching by the vertically 

rising rhyolite magma. The fault pattern is analogous to the pattern over salt domes, where extensive 

radial and normal faulting occurs in response to the 

diapiric rise of salt. Although tensional stresses 

dominated the early stages of dome formation, the 

eventual rise of magma into the dome displaced the 

overlying blocks upward and outward from the intrusive body. The rotational displacement of the 

foundation volcanic sequence to nearly vertical attitudes and the incremental outward and upward displacement of units adjacent to the intrusion are the 

result of forceful intrusion of magma rather than 

the tensional collapse across the domal crest. The

FIGURE 5.—Major and trace element compositions of rhyolite 

of Drake Peak plotted against Differentiation Index 

(Thornton and Tuttle, 1960). A, Weight percent majorelement oxides of rhyolite of Drake Peak plotted against 

Differentiation Index. B, Trace-element concentrations of 

rhyolite of Drake Peak plotted against Differentiation 

Index.
E14 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

limited, nearly circular region of doming is consistent with rise of a roughly cylindrical magma body 

rather than a body of broadly conical shape.

OTHER DOMAL STRUCTURES IN THE 

DRAKE PEAK AREA

An intrusion of dacitic magma 4 km northeast of 

the Drake Peak dome (location A on fig. 3) has deformed the older Tertiary volcanic rocks but apparently has not deformed the Miocene flood basalts. 

The flood basalt unconformably overlies strata that 

strike parallel to the intrusive contacts and dip more 

than 30°. Also unconformably overlying pre-middle 

Miocene strata are remnants of dacite flows presumably erupted from one of the small plugs in the 

area.

On the southeast flank of the Drake Peak dome, a 

satellite intrusion of rhyolite (location B on fig. 3) 

has updomed the foundation volcanic rocks in an 

unusual manner. A block of the overlying strata has 

been moderately tilted to the east where it meets 

steeply east-dipping strata along a hinge line concavely curved towards the intrusion. The structure 

is interpreted as a monoclinal flexure, with the uplifted block analogous to a trapdoor hinged along 

the flexure and open to the west. The rhyolite that 

erupted onto the surface preferentially flowed southeast, suggesting that the southeast dome may have 

formed on the flank of the preexisting Drake Peak 

dome.

ORIGIN OF THE CENTRAL RING INTRUSION

The ring intrusion is 3 km in diameter and consists of a uniform felsitic rhyolite having vertical or 

nearly vertical flow foliation in all observed outcrops. The intrusive contact is not well exposed, but 

the inner contact apparently dips toward the center 

and the outer contact away from the center of the 

ring. The ring intrusion itself does not seem, large 

enough to account for the observed scale of doming. 

The large-scale deformation accompanying the intrusion suggests that the rhyolite ring and associated intrusions of similar composition are part of 

a larger buried mass. The block-faulted country rock 

in the center of the dome is thus interpreted 

as a relatively thin roof covering the core of the 

intrusion.

The structural setting of the ring intrusion is 

somewhat different from those of ring intrusions in 

large central igneous complexes. Most ring dikes are 

associated with cauldron subsidence of several thousand meters (Billings, 1945; Smith and Bailey, 

1968; Roberts, 1970), but in the Drake Peak center

there is no evidence for extensive cauldron subsidence in connection with the ring intrusion. The 

structural doming argues for the forceful emplacement of the intrusion under excess magma pressure, 

with tensional failure along arcuate, inward-dipping 

fractures occurring above the intrusive mass. The 

rising magma invaded the tensional fracture zones 

and probably enlarged them by explosively venting 

to the surface. Piecemeal stoping apparently was not 

an important intrusive mechanism, judging from the 

absence of xenolithic material in the rhyolite. Minor 

subsidence in the center of the dome may also have 

contributed to the formation of the ring dike. Venting to the surface and the resultant reduction of 

magma volume may have allowed the center of the 

dome to sag into the magma, leaving an annular 

ridge of magma between the subsiding central blocks 

and the periphery of the magma chamber (see Smith 

and Bailey, 1968, p. 656).

BASIN-AND-RANGE STRUCTURE IN THE 

DRAKE PEAK AREA

Two sets of normal faults of moderate displacement occur in the area; the dominant set trends N. 

15°_45° w., and a subordinate set trends N. 10- 

45° E. Most faults offset the upper Miocene upper 

basalt, but some earlier faults in Honey Creek Valley displace rocks no younger than the middle Miocene flood basalts. North of the volcanic center the 

middle Miocene flood basalts dip 3° E., and the upper 

basalt dips 1.5°. The intervening tuff of Honey Creek 

thins to the west against the tilted Miocene flood 

basalts, indicating some of the displacement on the 

faults is at least as old as middle Miocene. Throughgoing regional faults are absent in the Drake Peak 

volcanic center, although it is somewhat older than 

most of the regional faulting. It is possible that 

most of the regional stress was taken up in readjustments along the many preexisting faults in the 

volcanic center. The apparent faulting of the upper 

basalt northwest of McDowell Peak may be the result of reactivation of the radial faults bounding 

McDowell Peak. Reactivation of faults along Fort 

and McDowell Creeks may also explain the radial 

faults that extend as much as 8 km from the volcanic center.

SUMMARY

The Drake Peak volcanic center is a structurally 

unusual vent complex that has undergone several 

periods of shallow intrusive activity, structural 

doming, and eruption of rhyolite flows. Exposed in 

its eroded domal core is a 2000-m section of upper
DRAKE PEAK—A STRUCTURALLY COMPLEX RHYOLITE CENTER IN SOUTHEASTERN OREGON E15

N

The

Trf

Tit

Jfb The

Eocene to middle Miocene volcanic rocks that records the early volcanic history of the area. An interpretation of the volcanic and structural history 

of the Drake Peak center is shown diagrammatically 

in figure 6.

A variety of plagioclase-phyric and aphyric 

rhyolites ranging from 69 to 76 percent Si02 were 

intruded into or erupted from the vent complex. 

They form a differentiated sequence in which the 

last flows to be erupted have the most crystals and 

are the least differentiated. The rhyolites were probably erupted from a compositionally zoned magma 

chamber.

REFERENCES CITED

Axelrod, D. I., 1966, Potassium-argon ages of some western 

Tertiary floras: Am. Jour. Sci., v. 264, p. 487-506.

Bailey, R. A., Dalrymple, G. B., and Lanphere, M. A., 1976, 

Volcanism, structure and geochronology of Long Valley 

caldera, Mono County, California: Jour. Geophys. Research, v. 81, no. 5, p. 725-744.

Baksi, A. K., York, Derek, and Watkins, N. D., 1967, The 

age of the Steens Mountain geomagnetic polarity transition: Jour. Geophys. Research, v. 72, p. 6299-6308.

Berggren, W. A., 1972, A Cenozoic time-scale—Some implications for regional geology and paleobiogeography: 

Lethaia, v. 5, no. 2, p. 195-215.

Billings, M. P., 1945, Mechanics of igneous intrusion in New 

Hampshire: Am. Jour. Sci., v. 243A, p. 41.

Carmichael, I. S. E., Turner, F. J., and Verhoogen, J., Jr., 

1974, Igneous petrology: New York, McGraw-Hill, 739 p.

Christiansen, R. L., and Lipman, P. W., 1966, Emplacement 

and thermal history of a rhyolite lava flow near FortyFIGURE 6.—North-south cross sections through Drake Peak 

volcanic center at various stages in its evolution. A, Latest 

Eocene (~40 m.y.) and Oligocene: deposition of andesitic 

sandstone and quartzitic conglomerate, andesitic lahars, 

andesite and basalt flows, and ash-flow and stratified silicic 

tuff of lower andesitic sequence (Taf); eruption of flow 

breccia and highly vesicular basaltic andesite of Twelvemile Peak (Tba). B, Late(?) Oligocene through early 

middle Miocene: successive onlapping of basalt and andesite of Crook Peak (Tab), silicic ash-flow and stratified 

tuff of the lower tuff (Tit), middle Miocene flood basalt 

(Tmb), and ash-flow of lower part of tuff of Honey Creek 

(The) onto basaltic andesite of Twelvemile Peak. Beginning of basin-and-range faulting. C, Middle Miocene I: 

structural doming by intrusion of aphyric felsite phase 

of rhyolite of Drake Peak (Tiri); probable eruptions of 

aphyric rhyolite followed by collapse of domal core; deposition of parts of tuff of Honey Creek (Thc). D, Middle 

Miocene II: eruption of plagioclase-phyric rhyolite (Trf) 

onto flank of structural dome; some additional doming; 

deposition of upper part of tuff of Honey Creek (Thc). E, 

Late Miocene: erosion of the dome and flows; eruption of 

the upper basalt (Tub). F, Latest Miocene to Holocene: 

continued erosion of dome; exposure of ring intrusion; 

glaciation above 2000 m during Pleistocene.
E16 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

mile Canyon, southern Nevada: Geol. Soc. America Bull., 

v. 77, p. 671-684.

Duffield, W. A., and McKee, E. H., 1974, Tertiary stratigraphy and timing of Basin and Range faulting of the 

Warner Mountains, northeast California [abs.]: Geol. 

Soc. America Abs. with Programs, v. 6, no. 3, p. 168.

Enlows, H. E., and Parker, D. J., 1972, Geochronology of the 

Clarno igneous activity in the Mitchell quadrangle, 

Wheeler County, Oregon: Ore Bin, v. 34, p. 104-110.

Evernden, J. F., Savage, D. E., Curtis, G. H., and James, C. 

T., 1964, Potassium-argon dates and the Cenozoic mammalian chronology of North America: Am Jour. Sci., 

v. 262, p. 145-198.

Fuller, R. E., 1931, The geomorphology and volcanic sequence of Steens Mountain in southeastern Oregon: 

Seattle, Washington Univ. Pub. Geology, v. 3, p. 1-130.

Gordon, G. E., Randle, K., Goles, G. G., Corliss, J., Beeson, 

M. H., and Oxley, S. S., 1968, Instrumental neutron 

activation analysis of standard rocks with high resolution 7-ray detectors: Geochim. et Cosmochim. Acta, v.

32. p. 369-396.

Greene, R. C., 1973, Petrology of the welded tuff of Devine 

Canyon, southeastern Oregon: U.S. Geol. Survey Prof. 

Paper 797, 26 p.

Greene, R. C., Walker, G. W., and Corcoran, R. E., 1972, 

Geologic map of the Burns (AMS) quadrangle, Oregon: 

U.S. Geol. Survey Misc. Geol. Inv. Map 1-680, scale 

1:250,000.

Hildreth, W., 1976, The Bishop Tuff: Compositional zonation 

in a silicic magma chamber without crystal setting 

[abs.]: Geol. Soc. American Abs. with Programs, v. 8, 

no. 6, p. 918.

James, R. S., and Hamilton, D. L., 1969, Phase relations in 

the system NaAlSi:!Os-KAlSi3 08-CaAl2Su.0 8-Si02 at 1 

kilobar water vapor pressure: Contr. Mineralogy and 

Petrology, v. 21, p. 111-141.

Larson, E. E., 1965, The structure, stratigraphy, and paleomagnetics of the Plush area, southeastern Lake County, 

Oregon: Boulder, Colorado Univ., Ph. D. thesis, 166 p.

Lipman, P. W., 1966, Water pressures during differentiation 

and crystallization of some ash-flow magmas from 

southern Nevada: Am. Jour. Sci., v. 264, p. 810^826.

MacLeod, N. S., Walker, G. W., and McKee, E. H., 1975, 

Geothermal significance of eastward increase in age of 

upper Cenozoic rhyolitic domes in southeastern Oregon: 

U.S. Survey Open-File Rept. 75-348, 21 p.

Noble, D. C., Korringa, M. K., Hedge, C. E., and Riddle, G. 

0., 1972, Highly differentiated subalkaline rhyolite from 

Glass Mountain, Mono County, California: Geol. Soc. 

America Bull., v. 83, p. 1179-1184.

Norrish, K., and Hutton, J. T., 1969, An accurate X-ray 

spectrographic method for the analysis of a wide range 

of geological samples: Geochim. et Cosmochim. Acta, v.

33. p. 431-453.

Peterson, N. V., and Mclntyre, J. R., 1970, Reconnaissance 

geology and mineral resources of eastern Klamath

County and western Lake County, Oregon: Oregon 

Dept. Geology and Mineral Industries Bull. 66, 70 p.

Piper, A. M., Robinson, T. W., and Park, C. F., 1939, Geology 

and-ground-water resources of the Harney Basin, Oregon: U.S. Geol. Survey Water-Supply Paper 841, 189 p.

Roberts, J. L., 1970, The intrusion of magma into brittle 

rocks, in Newell, G., and Rast, N., eds., Mechanisms of 

igneous intrusion: Liverpool, Gallery Press, p. 287-338.

Russell, R. J., 1928, Basin and Range structure and stratigraphy of the Warner Range, northeastern California: 

California Univ. Dept. Geol. Sci. Bull., v. 17, no. 11, p. 

387-496.

Smith, R. L., and Bailey, R. A., 1968, Resurgent cauldrons, 

in Studies in volcanology—A memoir in honor of Howel 

Williams: Geol. Soc. America Mem. 116, p. 613-662.

Swanson, D. A., 1969, Reconnaissance geologic map of the 

the east half of the Bend quadrangle, Crook, Wheeler, 

Jefferson, Wasco, and Deschutes Counties, Oregon: U.S. 

Geol. Survey Misc. Geol. Inv. Map 1-568, scale 1:250,000.

Thornton, C. P., and Tuttle, 0. F., 1960, Chemistry of 

igneous rocks, pt. 1, Differentiation index: Am. Jour. 

Sci., v. 258, p. 664-684.

Tuttle, 0. F., and Bowen, N. L., 1958, The origin of granite 

in light of experimental studies in the system NaAlSigOsKAlSi;;Of -SiO~-H20: Geol. Soc. America Mem. 74, 153 p.

Walker, G. W., 1960, Age and correlation of some unnamed 

volcanic rocks in south-central Oregon: U.S. Geol. Survey Prof. Paper 400-B, p. B293-B300.

————1963, Reconnaissance geologic map of the eastern half 

of the Klamath Falls (AMS) quadrangle, Lake and 

Klamath Counties, Oregon: U.S. Geol. Survey Mineral 

Inv. Field Studies Map MF-260, scale 1:250,000. 

-1973, Preliminary geologic and tectonic maps of Oregon east of the 121st meridian: U.S. Geol. Survey Misc. 

Field Studies Map MF-495, scale 1:500,000.

Walker, G. W., and King, P. B., 1969, Geologic map of Oregon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-595, 

scale 1:2,000,000.

Walker, G. W., Peterson, N. V., and Greene, R. C., 1967, 

Reconnaissance geologic map of the east half of the 

Crescent quadrangle, Lake, Deschutes, and Crook Counties, Oregon: U.S. Geol. Survey Misc. Geol. Inv. Map 

1-493, scale 1:250,000.

Walker, G. W., and Repenning, C. A., 1965, Reconnaissance 

geologic map of the Adel (AMS) quadrangle, Lake 

Harney, and Malheur Counties, Oregon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-446, scale 1:250,000.

————1966, Reconnaissance geologic map of the west half of 

the Jordan Valley quadrangle, Malheur County, Oregon: 

U.S. Geol. Survey Misc. Geol. Inv. Map 1-457, scale 

1:250,000.

Watkins, N. D., 1965, A paleomagnetic observation of Miocene geomagnetic secular variation in Oregon: Nature, 

v. 206, no. 4987, p. 879-882.
Palladium, Platinum, and Rhodium 

Concentrations in Mafic and Ultramafic Rocks 

from the Zhob Valley and Dargai Complexes, 

Pakistan

By NORMAN J. PAGE, JOSEPH HAFFTY, and ZAKI AHMAD

SHORTER CONTRIBUTIONS TO MINERALOGY AND 

PETROLOGY, 1979

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1124-F

CONTENTS

Page

Abstract ______________________________________________ Fl

Introduction _________________________________________________________ 1

Geologic setting of areas ____________________________________________ 1

Palladium, platinum, and rhodium analyses __________________________ 3

Interpretation and conclusions _________________________________________ 4

References cited ____________________________________________________ 5

ILLUSTRATIONS

Page

FIGURE 1. Map showing location of the Zhob. Valley and Dargai complexes in Pakistan _________________________ Fl

2. Geologic sketch map of Zhob Valley complex, Pakistan, showing location of analyzed samples ____ — ___— 2

TABLES

Page

TABLE 1. Palladium, platinum, and rhodium analyses, sample descriptions, and locations of rock samples from the

Zhob Valley ultramafic complex and the Dargai ultramafic complex, Pakistan _____________________ F4

2. Comparison of average palladium, platinum, and rhodium contents of Zhob Valley and Dargai rocks with

average contents from other alpine ultramafic localities __________________________________________ 5

SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

PALLADIUM, PLATINUM, AND RHODIUM CONCENTRATIONS

IN MAFIC AND ULTRAMAFIC ROCKS FROM 

THE ZHOB VALLEY AND DARGAI COMPLEXES, PAKISTAN

By NORMAN J PAGE,, JOSEPH HAFFTY, and ZAKI AHMAD

ABSTRACT

The Zhob Valley and Dargai complexes, Pakistan, consist 

of harzburgite and dunite tectonites containing chromite deposits, pyroxenite, wehrlite, and gabbro. Both are ophiolite 

complexes. Palladium, platinum, and rhodium were found in 

concentrations of up to 170, 200, and 22 parts per billion, 

respectively. Average concentrations for both complexs and 

all rock types collected are 28 ppb palladium, 33 ppb platinum, and 9 ppb rhodium. The Pt:Pt+Pd ranges from 0.41 

to 0.67 and averages 0.57. The concentration levels and ratios 

of these metals are similar to other alpine ultramafic bodies 

that have been analyzed by modern techniques.

INTRODUCTION

Inasmuch as alpine ultramafic bodies, interpreted 

to be parts of ophiolitic complexes, are thought to 

represent parts of the oceanic crust and upper 

mantle, examination of the concentrations of platinum-group elements in these rocks could provide 

data to determine the sources, concentration processes, and models for the derivation of these elements in other magmatic rocks or within alpine 

complexes themselves. As part of a survey of the 

platinum-group elements in alpine rocks, two areas 

in Pakistan were sampled in 1974 during the field 

excursions of the CENTO (Central Treaty Organization) Working Group on Volcanic and Intrusive 

Rocks and Their Associated Ore Deposits. The two 

areas are the Zhob Valley complex northeast of 

Quetta and the Dargai complex northeast of Peshawar (fig .1).

The geologic framework of both areas is briefly 

described here as a background for interpretation 

of the platinum, palladium, and rhodium concentrations in ultramafic, mafic, and chromite-bearing 

rocks. Data from these rocks are compared with 

rocks from other similar environments on which

60°

25°^

FIGURE 1.—Location of Zhob Valley and Dargai complexes 

in Pakistan.

modern analytical techniques for the determination 

of these platinum-group elements have been applied.

GEOLOGIC SETTING OF AREAS

The Zhob Valley igneous complex (Bilgrami and 

Ingamells, 1960; Bilgrami 1961, 1963), consisting 

of ultramafic and mafic rocks, was emplaced in Late 

Cretaceous to Paleocene time by major thrust faulting along the central axial tectonic belt of Pakistan 

(Rossman and others, 1971; D. L. Rossman, Z. 

Ahmad, and H. Rahman, unpub. data, 1970). Figure 

2 shows the major subdivisions of the complex and

1 Geological Survey of Pakistan, Islamabad, Pakistan.

Fl
F2 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

68°00'

30°45'h

UNCONSOLIDATED AND CEMENTED GRAVELS 

(Pleistocene and Holocene)

SHAIGALU LIMESTONE (Oligocene and Miocene)

NASAI LIMESTONE (Eocene)

ZHOB VALLEY IGNEOUS COMPLEX-Emplaced 

Late Cretaceous and Paleocene

HARZBURGITE AND DUNITE TECTONITE 

PYROXENITE AND WEHRLITE 

GABBRO

PARH GROUP (Cretaceous)

ALOZAI GROUP, LORALAI LIMESTONE, AND 

PARH GROUP (Triassic and Cretaceous)

~~~r~: AMPHIBOLITE

Contact, approximately located 

Fault, approximately located

Thrust fault, approximately located-Sawteeth 

on upper plate

Sample location—See table 1 for analysis

FIGURE 2.—Geologic sketch map of Zhob Valley complex, Pakistan, showing location of analyzed samples.

its tectonic setting as compiled by the Photographic 

Survey of Corporation, Ltd. (1958a, b), detailed 

mapping by Rossman, Ahmad, and Rahman (1971) 

and by D. L. Rossman, Z. Ahmad and G. Abbas (unpub. data, 1970). Some of the structural interpretations and rock relations shown in the figure are

based on field examinations made during the CENTO 

field excursion.

The Zhob Valley igneous complex overlies sedimentary rocks of Triassic to Cretaceous age and 

was extensively eroded and deeply weathered, and 

a laterite developed by mid-Eocene time. Locally, the
PALLADIUM, PLATINUM, AND RHODIUM CONCENTRATIONS, PAKISTAN F3

Nasai-Saplai Tor Ghar mass is overlain disconformably by remnants of shale and limestone of Eocene 

age.

The Zhob Valley igneous complex occurs in two 

detached masses. One, the Jung Tor Ghar mass, consists predominantly of harzburgite and dunite tectonites; the other, the Nasai-Saplai Tor Ghar mass, 

consists of harzburgite and dunite tectonites, pyroxenite, wehrlite, and gabbro. In both masses, podiform chromite deposits occur within the harzburgite 

and dunite tectonites.

The pyroxenite and wehrlite unit in the NasaiSaplai Tor Ghar mass contains repetitive sequences 

of rocks consisting of olivine, clinopyroxene, and 

olivine plus clinopyroxene. Textures of the rocks 

suggest that these sequences were formed by cumulate processes. All the petrologic features of the 

Zhob Valley igneous complex are those of an ophiolite complex, which probably represents part of the 

oceanic crust and upper mantle.

Descriptions of the analyzed samples from the 

Zhob Valley complex and their locations are given 

in table 1, and their locations are shown on figure 2. 

All the samples are grab samples weighing 1-2 kg.

The Dargai complex, which extends for 26 km 

from east to west and about 6 km from north to 

south, consists of ultramafic and mafic rocks that 

are separated from country rocks by faults. The 

complex probably was emplaced along a major eastwest-striking thrust fault whose trace parallels the 

front of the Himalaya Mountains.

The country rocks are chlorite, mica, and graphite 

schist, and marbles of an undetermined age; the 

south side of the complex is overlapped by sedimentary rocks and gravel of Pleistocene and Holocene 

age.

The complex contains predominately harzburgite 

and dunite with minor wehrlite and has a small 

amount of leuco- and melanogabbroic rocks exposed 

on its northern margin. The harzburgite and dunite 

are interlayered on the scale of centimeters to hundreds of meters. Crests of folds can be observed locally in the layered ultramafic rocks, the associated 

chromitite occurrences, and the gabbroic rocks. Most 

of the ultramafic part of the complex is tectonite 

similar to the harzburgite and dunite tectonite of 

the Zhob Valley complex. Although there are apparently more clinopyroxene-bearing rocks (Iherzolite and wehrlite) mterlayered with the dunite and 

harzburgite in the Dargai complex than in the Zhob 

Valley complex, the Dargai complex also probably 

represents part of an ophiolite complex and could 

be interpreted as a piece of the upper oceanic

mantle. Samples from the Dargai complex are described and their locations given in table 1.

PALLADIUM, PLATINUM, AND 

RHODIUM ANALYSES

The analyses of rocks from the Zhob Valley and 

Dargai complexes given in table 1 were performed 

by the method of Haffty and Riley (1968). Of the 

samples analyzed, 73 percent contain detectable rhodium, 35 percent contain detectable palladium, and 

33 percent contain detectable platium. Palladium 

concentrations range from <4 to 170 ppb and average 27 ppb for samples in which it was detected; 

platinum concentrations range from <10 to 200 ppb 

and average 46 ppb; and rhodium concentrations 

range from < 5 to 22 ppb and average 10 ppb. The 

average Pt:Pt+Pd is 0.57 and ranges from 0.41 to 

0.67. Significant characteristics of the distribution 

of the three metals in both complexes (table 1) are 

as follows:

1. Of 25 samples of the chromitites, 16 contain more 

than 7 ppb rhodium; no mafic or ultramafic 

rocks contain rhodium above the limit of determination (5 ppb or greater).

2. Of 11 samples of the mafic and ultramafic rocks, 

9 contain 4 ppb or more palladium; palladium 

was detected and determined in only 4 of 25 

samples of the chromitites.

3. Of 11 samples of the mafic and ultramafic rocks, 

7 contain more than 10 ppb platinum; no 

chromitites contain platinum at or above this 

level of concentration.

The data on platinum, palladium, and rhodium 

from the Zhob Valley and Dargai complexes can be 

compared with results from analyses of ultramafic 

and mafic rocks from (1) southwestern Oregon, (2) 

Red Mountain, Calif., (3) Burro Mountain, Calif., 

(4) the Eagle quadrangle, Alaska, (5) Eklutna 

complex, Alaska, (6) the Urals and other parts of 

the U.S.S.R., and (7) Mount Albert complex, 

Quebec. All seven areas contain alpine ultramafic 

rocks similar to those from Pakistan. Table 2 lists 

average concentrations for samples with palladium, 

platinum, and rhodium above detection limits from 

the above seven areas for comparison with data 

from the Zhob Valley and Dargai complexes. The 

values listed contain a strong bias toward high 

values because palladium, platinum, and rhodium 

were not detected in about half of the rocks analyzed 

from California, Oregon, the Eagle quadrangle, and 

Pakistan. Comparison of the average values in table 

2 for the Zhob Valley and Dargai complexes with
F4 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

TABLE 1.—Palladium, platinum, and rhodium analyses, sample descriptions, and locations of rock samples from the Zhob 

Valley ultramafic complex and the Dargai ultramafic complex, Pakistan

[Rock classification after Streckeisen (1976)• <, not detected at that level; Tr., trace. In calculations, "traces" of Pd, Pt, and Rh were 

assigned values of 2, 7, and 3 parts per billion, respectively. Analysts: Joseph Haffty and A. W. Haubert]

Pd Pt Rh ZPt+Pd+Rh 3oinp i e nuiiiuer (parts per biill ion) — acniipie ucsi»i ipLiuu cuiu iui»dLiuu

ZHOB VALLEY COMPLEX

1JG74

9JG74

10JG74

11JG74

12AJG74

12JG74

13JG74

14JG74

15JG74

16JG74

17JG74

18JG74

19JG74

20JG74

21JG74

22JG74

23JG74

24JG74

25JG74

26JG74

27JG74

28JG74

29JG74

30JG74

3UG74

32JG74

34JG74

36JG74

37JG74

40JG74

14

<4

<4

<4

7

<4

<4

<4

<4

<4

<4

<4

<4

<4

<4

<4

Tr.

<4

6

<4

27

20

62

<4

<4

<4

20

170

9

4

24 <5

<1 0 14

<10 5

<10 12

<10 <5

<10 13

<10 14

<10 20

<10 9

<10 11

<10 22

<10 13

<10 9

<10 12

<10 5

<10 11

<10 5

<10 <5

12 <5

Tr. 9

54 <5

36 <5

<10 5

<10 16

<10 Tr.

<10 Tr.

19 <5

200 5

11 <5

Tr. 7

38

14

5

12

7

13

14

20

9

11

22

13

9

12

5

11

7

18

16

81

56

67

16

3

3

39

375

20

18

Foliated dunite, mylonitized olivine, northwest end of Jung Tor Gar.

Layered chromitite in dunite, north of Mine 213-

Layered chromitite.

Chromitite, Mine 166.

Serpentinized harzburgite, near Mine 166.

Massive chromitite, Mine 166.

Massive chromitite.

Massive chromitite from mine dump.

Disseminated chromitite, from dump of 7ML mine.

Massive chromitite, from ore pile of 4ML mine.

Massive chromitite, from ore pile of 9ML mine.

Massive chromitite, from ore pile of 8ML mine.

Massive chromitite, from ore pile of 5CPL mine.

Massive chromitite, from ore pile of Mine 81, Shinakhowra mines.

Layered chromitite, from ore pile of Mine 604, Shinakhowra mines.

Massive chromitite, from ore pile of Mine 41, Shinakhowra mines.

Massive chromitite, from dump of Mine 153.

Wehrlite, relict cumulate textures.

Cl inopyroxenite.

Massive chromitite from mine dump.

Foliated melanogabbro.

Do.

Massive chromitite, from pillar in Mine PCM41.

Friable chromitite, from ore pile Mine 8ML/2.

Massive chromitite from Mine 8ML/1.

Massive chromitite from Mine 5CPL.

Foliated leucograbbro.

Interlayered melanograbbro and anorthosite.

Foliated leucogabbro.

Layered chromitite.

DARGAI COMPLEX, PESHAWAR DIVISION

1DG74

2DG74

3DG74

5DG74

6DG74

7DG74

9

<4

<4

<4

10

<4

Tr. Tr.

<10 5

<10 <5

<10 8

Tr. 6

Tr. 8

19

5

8

25

15

Harzburgite, adjacent to Heru Shah Chrome mine.

Massive chromitite, Heru Shah Chrome mine.

Dunite, near Heru Shah Chrome mine.

Massive chromitite, Landiraudi Chrome prospect.

Concentrate of chromite nodules, Landiraudi Chrome prospect.

Massive chromitite, from ore pile, Qila mine.

those of the other complexes suggests that, except 

for a few anomalously high individual concentrations in rocks not included in the averages, there is 

not much difference in concentration of palladium, 

platinum, and rhodium in the rock types of alpine 

complexes studied to date. Pt:Pt + Pd ranges from 

0.23 to 0.81 when calculated with average concentrations for the alpine ultramafic rocks. Most of the 

Pt:Pt+Pd for the various areas studied are greater 

than 0.58.

INTERPRETATION AND CONCLUSIONS

If the Zhob Valley and Dargai complexes are interpreted as parts of the oceanic crust and upper
PALLADIUM, PLATINUM, AND RHODIUM CONCENTRATIONS, PAKISTAN F5

TABLE 2.—Comparison of average palladium, platinum, and rhodium contents of Zhob Valley and Dargai rocks with average contents from other alpine ultramafic localities

[<, less than; —, not detected; number in parentheses is number of samples with detectable concentrations]

Description, location, and reference

Average 

(parts per bi 1 1 ion)

Pd Pt Rh

Sum of 

average 

Pd+Pt+Rh

Pt/Pd+Pt 

(percent)

CHROMITITE

DUNITE, PERI DOT ITE

Harzburgite, southwestern Oregon (Page and others, 1975) —— - 

Dunite and harzburgite, Burro Mountain, California

20 (4) 7 (4) 

7.5(10) X 26.3 (8) 

10 (2) 36 (2) 

11 (16) 22.2(14)

10.3(25) 

25.4(23) 

9.5 (2) 

17 (50)

37 

59-2 

55.5 

50

25.9 

77.8 

78.3 

66.7

, AND SERPENT! NI ZED EQUIVALENT

10 (3) 16 (2) 

8.4 (5) 8.3 (6) 

3.7(18) 12.8(18)

<3 (1) 

<5 

<5

29 

16.7 

16.5

61.5 

49-7 

77-6

(Loney and others, 1971).

Serpentinized ultramafic rocks, Eagle quadrangle, 

Alaska (Keith and Foster, 1973).

Ultramafic rocks, Eklutna, Alaska (Clark and Greenwood, 

1972).

Dunite and peridotite, Urals, Russia (Fominykh 

and Khvostova, 1970).

Dunite and peridotite, Mount Albert, Quebec 

(Crocket and Chyi, 1972).

Weighted average

2 7.5(16) 2 11.3 (8) 2 (I) 20.8 

HO (12) 42 (12) -- 182.0 

18 (8) 73 (8) 1.99(8) 92.99 

9.4(13) 

29 (75) 28 (54) 2 (10) 59

60.1

23.1

80.2

1*9-1

PYROXENITE AND GABBROIC ROCKS

Zhob Valley and Dargai, Pakistan (this report) ————————— 42 (6)

Pyroxenite, southwestern Oregon (Page and others, 1975) —— 2.0 (1)

Weighted average ———————————————————————————— 36 (7)

48 (7) 5 (1)

7.0 (1) <5

43 (8) <5 (0

90.7

9.0

84

53.7

77.8

54.4

J Average does not include two samples containing 450 and 850 ppb Pt. 

2Average does not include one sample containing 300 ppb Pt and 200 ppb Pd.

mantle, as is implied by the ophiolite model, what do 

the concentrations of palladium, platinum, and 

rhodium indicate about the composition of the upper 

mantle? A decision must be made as to whether 

these rocks represent mantle that has been depleted 

or partly depleted in platinum-group elements or 

mantle that could be a source rock for magmas that 

could have generated platinum-enriched deposits. 

Naldrett and Cabri (1976) assumed that they represented the source rocks, but the fact that there 

are scattered anomalously high values (4-20 times 

greater than average values) in some samples from 

each complex suggests to us that these rocks may 

instead represent a mantle depleted or partially depleted in platinum-group metals. In addition, the 

large number of rocks that contain concentrations 

below the limits of determination suggests that the

mantle or depleted mantle has a much lower average 

platinum, palladium, and rhodium content than reflected by values in table 2.

Although the highest platinum plus palladium 

plus rhodium content found is 375 ppb (0.012 oz/ 

ton), which is not of economic grade, the possibility 

of discovering concentrations of platinum-group 

metals in the gabbroic part of the Nasai Tor Ghar 

area should not be overlooked. In order to evaluate 

this possibility more extensive sampling should be 

done there.

REFERENCES CITED

Bilgrami, S. A., 1961, Distribution of Cu, Ni, Co, V, and Cr 

in rocks of the Hindubagh igneous complex, Zhob Valley, West Pakistan: Geol. Soc. America Bull., v. 72, p. 

1729-1738.
F6 SHORTER CONTRIBUTIONS TO MINERALOGY AND PETROLOGY, 1979

———— 1963, Further data on the chemical composition of 

Zhob Valley chromites: Am. Mineralogist, v. 48, p. 573- 

577.

Bilgrami, S. A., and Ingamells, C. 0., 1960, Chemical composition of the Zhob Valley chromites, West Pakistan: 

Am. Mineralogist, v. 45, p. 576-585.

Clark, A. L., and Greenwood, W. R., 1972, Geochemistry and 

distribution of platinum-group metals in mafic to ultramafic complexes of southern and southeastern Alaska, 

in Geological Survey research, 1972: U.S. Geol. Survey 

Prof. Paper 800-C, p. C157-C160.

Crocket, J. H., and Chyi, L. L., 1972, Abundances of Pd, Ir, 

Os, and Au in an alpine ultramafic pluton: Internat. 

Geol. Cong., 24th, Montreal, Canada, 1972, Sec. 10, p. 

202-209.

Fominykh, V. G., and Khvostova, V. P., 1970, Distribution of 

platinum metals in rock-forming minerals of the Gusevogorskoye deposit: Akad. Nauk SSSR Doklady, v. 200, 

p. 182-183.

Haffty, Joseph, and Riley, L. B., 1968, Determination of 

palladium, platinum, and rhodium in geologic materials 

by fire assay and emmission spectrography: Talanta, v. 

15, p. 111-117.

Keith, T. E. C., and Foster, H. L., 1973, Basic data on the 

ultramafic rocks of the Eagle quadrangle, east-central 

Alaska: U.S. Geol. Survey open-file rept. 4 sheets.

Loney, R. A., Himmelberg, G. R., and Coleman, R. G., 1971, 

Structure and petrology of the alpine-type peridotite at 

Burro Mountain, California, U.S.A.: Jour. Petrology, 

v. 12, p. 245-309.

Naldrett, A. J., and Cabri, L. J., 1976, Ultramafic and related mafic rocks—Their classification and genesis with 

special reference to the concentration of nickel sulfides 

and platinum-group elements: Econ. Geology, v. 71, p. 

1131-1158.

Page, N. J, 1969, Platinum content of ultramafic rocks, in 

U.S. Geological Survey heavy metals program progress 

report 1968—Topical studies: U.S. Geol. Survey Circ. 

622, p. 5.

Page, N. J, Johnson, M. G., Haffty, Joseph, and Ramp, Len, 

1975, Occurrence of platinum-group metals in ultramafic 

rocks of the Medford-Coos Bay 2° quadrangle, southwestern Oregon: U.S. Geol. Survey Misc. Field Studies 

Map MF-694, scale 1:250000.

Photographic Survey of Corporation, Ltd., 1958a, Reconnaissance geology of part of west Pakistan: Toronto, 

Canada, Photographic Survey Corp. Ltd., in cooperation 

with Geol. Survey Pakistan, Geol. Map 26, Quetta 34 

J-F, scale 1:253440.

———— 1958b, Reconnaissance geology part of west Pakistan: 

Toronto, Canada, Photographic Survey Corp. Ltd., in 

cooperation with Geol. Survey Pakistan, Geol. Map 27, 

Loralai 39 B-F, scale 1:243 000.

Rossman, D. L., Ahmad, Zaki, and Rahman, Hamidur, 1971, 

Geology and economic potential for chromite in Zhob 

Valley ultramafic rock complex, Hindubagh, Quetta Division, West Pakistan: (Project rep. (IR) PK-50) U.S. 

Geological Survey open-file rept, 63 p.

Streckeisen, Albert, 1976, To each plutonic rock its proper 

name: Earth Sci. Rev., v. 12, p. 1-33.

•frU.S. GOVERNMENT PRINTING OFFICE: 1979 O— 311-344/138