California State University, Northridge

Home , Aplite, Arica Mountains, Autochthon (geology), Chemehuevi Mountains, Dead Mountains, Kilbeck Hills

CALIFORNIA STATE UNIVERSITY, NORTHRIDGE

GEOLOGY AND DIKE SWARMS OF

THE HOMER MOUNTAIN AREA,

SAN BEP~ARDINO COUNTY, CALIFORNIA

A thesis submitted in partial satisfaction of the requirements for the degree of Master of Science in Geology

Kit H. Custis

January, 1984

i The Thesis of Kit H. Custis is approved:

D. Carlisle, University of California, Los Angeles

P.L. hllg, State Un1vers1ty, Los Angeles

Un1vers1ty,

G. { .3() California State University, Northridge

California State University, Northridge

ii ACKNOWLEDGEMENTS

The author gratefully acknowledges the advice and review by the thesis committee. Dr. G.C. Dunne is especially thanked for his guidance and suggestions.

Dr. P.W. Weigand is thanked for his tireless effort to keep the XRF computer running. Dr. D. Carlisle is thanked for suggesting the project. Dr. P.L. Ehlig is thanked for his reviews and field instruction. Special thanks to Dr. M.J. Walawender, San Diego State Universit~ for allowing me access to the SDSU XRF. The California

State University Foundation, Northridge Students Project

Committee, is gratefully acknowledged for providing funding for the K-Ar age dating and plate graphics. Peter

Ertman and Albert Endo, Bureau of Land Management, River side, California, are thanked for arranging the loan of the air photos. Torn Anderson and Eugene Hsu, California

Division of Mines and Geology, Los Angeles, California, are thanked for their effors to include my data in the

Kingman 2° Geologic Sheet. Yaw Agyakawa, University of

California, Los Angeles, provided helpful discussions on the dikes of the New York Mountains. Bob Griffis,

California State University, Northridge is especially thanked for developing the computer programs for the geochemical analysis and plotting.

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TABLE OF CONTENTS

ABSTRACT~ ..... 1:1 "' ••••• So • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • X •

INTRODUCTION. 1.

Location . ... ·· ...... , . 1.

Purpose and Scope. • • • • • • • • • • • • • • • • • • • • • • • • • • • e • e 1.

Methodology ... • • • 0 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • 4 .

Previous Work ...... " . . . . . 5.

GEOGRAPHY ...... 7.

Surface and Cultural Features...... 7.

Climate and Vegetation. 9.

Pioneer Settlements .... 10.

GEOLOGY • ...... •.•...••••••••..•••••••••••••• 11.

Regional Geologic and Tectonic Setting. 11.

Geochemical Analysis ..... 20.

Analytical Procedure. 20.

Rock Classification. 24.

Results of Geochemical Analyses ...... 24.

Plutonic Rocks ..... 26.

Volcanic Flows and Breccias. 29.

Dikes of Mafic and Intermediate Composition ...... 29.

Felsic Dikes and Sills. 40.

K-Ar Age Dating ...... 43.

Litho logy ...... 43.

Precambrian and Hesozoic-Early Tertiary Plutonic Rocks ...•.... 43.

General Statement. 43.

Precambriam Granitoid Gneiss. 47.

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Teutonia Quartz Monzonite Pluton...... 49.

Homer Mountain Quartz Monzonite Pluton ...... ~~~~.... 51.

Leucocratic and Mesocratic Aplite Dikes ...... ~~ ...... 52.

Quartz Veins and Pegmatite Dikes ...... •. 55.

Altered Crystalline Rock of the Detachment Terrane ...... •...•.• 59.

Tertiary Volcanic Flows and Breccias ...... 61.

Mafic and Intermediate Composition Flows ...... • 61.

Eastern Rhyolitic Dikes and Breccias ...... ~~~ ...... 65.

Tertiary Hypabyssal Dikes ...... ••...... •....•. 66.

General Statement ...... •...... 66.

Altered Quartz Gabbro Dikes •...... •... 68.

Altered Quartz Andesite Dikes ...... 69.

Granophyric Rhyodacite Porphyry Dikes and Sills ...... •..•.. 72.

Quartz Andesite to Quartz Andesite Porphyry Dikes...... 73.

Hornblende Quartz Andesite Porphyry Dikes...... 74.

Biotite Rhyodacite Porphyry Dikes...... 76.

Quartz-Orthoclase Porphyritic Rhyodacite Dikes ...... •...... •... 78.

Quartz Rhyodacite Porphyry Dikes ...... ••... 81.

Reddish Rhyodacite and Rhyodacite Porphyry Dikes...... • . . . . . • . . . . 8 2.

v Tertiary Sedimentary Units...... 83.

Homer Mountain Conglomerate...... 83.

Quaternary Sedimentary Units...... 85.

General Statement...... 85.

Consolidated Alluvium (Qc) ...•...... •. 86.

Older Alluvium (Qoal)...... 89.

Recent Alluvium {Qal) ...... • . 90.

Structures...... 90.

Foliations and Mylonitic Zones .•.... ~...... 90.

Joints ...... 94.

East-West-Trending Joints...... 94.

North-Trending Secondary Joints ...... •. 101.

Faults...... 101.

Northwest- and Northeast- Trending Faults ...... 101.

Low-Angle Normal (Detachment) Faults...... 107.

High-Angle Normal (Basin and Range) Faults ...... •.....•. 116.

Quaternary Faults ...... • 121.

Dike Structures ...... •.•••.... 123.

Economic Geology . . , ...... 124.

Mineralization in the Surrounding Region ...... •.....• 124.

Mineralization in the Homer Mountain Area ...... •...... • 125.

Transverse Structures of the Southern Cordillera...... 12-a.

SUMMARY. . • . . • • • • • . • • • . • . • • • . • • • • • • • • . • . • • . • • • • • • • • • • 13 8 •

vi References ...... ~. & • • • • • • • • • • • • • • • • • 144-154.

APPENDICES

APPENDIX A

Table A-1: Major Oxides and Normative Minerals ...•..•...... •.... 155-165.

APPENDIX B

Loss On Ignition Values ...... •.. 166-168.

ILLUSTRATIONS Figure Number

1. Index Map ...... 2-3.

2. Normative Mineral QAP Plots of Plutonic Rocks .....•...... 27-28.

3. Normative Mineral QAP Plots of Andesitic Flows and Rhyolitic Breccias .. 30-31. 4. Normative Mineral QAP and AFM Plots of Mafic and Intermediate Composition Dikes ...... 33-34. 5. Classification Diagrams of Tertiary Mafic, Intermediate, and Felsic Dikes ... 35-36. 6 . Harker Diagrams of Tertiary Mafic, Intermediate, and Felsic Dikes ... 37-38.

7. Normative Mineral QAP and AFM Plots of Felsic Dikes ...... 41-42. 8. Plots of Leucocratic Aplite dikes, Rose and Schmidt Net ...... •...... 53-54.

9. Plots of Quartz Veins, Rose and Schm.id t Net ...... •....•...... 57-58. 10. Plots of Andesitic and Quartz Andesite Flow Layering, Schmidt Net ...... •• 63-64.

vii 11. Plots of Mafic and Intermediate Dikes, Rose and Schmidt Net .•.•••....•.. 70-71. 12. Plots of Felsic Dikes, Rose and Schmidt Net ...... •..•.....•... 79-80.

13. Plots of Foliations and Mylonite Lineations, Schmidt Net ...... •.••.. 92-93.

14. Plots of East-West-Trending Joints, Rose and Schmidt Net ...... ••.....•.• 96-97.

15. Joints Commonly Found in Folded Rocks and Pinnate Fractures...... 98-99.

16. Plots of North-Trending Secondary Joints, Rose and Schmidt Net...... 102-103.

17. Plots of Northwest- and Northeast Trending Paul ts, Rose and Schmidt Net... 105-106.

18. Plots of Detachment Faults, Schmidt Net...... 109-110.

19. Graphic Calculations of Amount of Granophyric Rhyodacite Dike Displacement by Detachment Fault ...... 113-114.

20. Plots of High-Angle Basin-Range Style Faults, Rose and Schmidt Net ...... 117-118.

21. Transverse Structures of the Southern Cordillera, Western United States ...... 134-135.

TABLES

Table Number 1. Tertiary Dike Swarms-Lower Colorado River Region ...... 21-23. 2. Summary of Rock Classifications Based on Geochemical Analyses •.....•.••. 25. 3. Potassium-Argon Dates on Igneous Rocks From the Horner Mountain Area ...•.. 44.

4. Transverse Structures...... 129-133.

viii 5. Summary of Regional and Local Tectonic Event~ •..•...... •.•.•••.••.. 139.

PLATES

Plate Number 1. Geologic Map and Sections of the Horner Mountain Area, San Bernardino County, California ...... •.•..... in pocket

2. Dike Swarms of Horner Mountain Area, San Bernardino County, California..... in pocket

ix Q •

A B S T R A C T

GEOLOGY AND DIKE SWARMS OF THE HOMER MOUNTAIN AREA,

SAN BERNARDINO COUNTY, CALIFORNIA

Kit H. Custis

Master of Science in Geology

Homer Mountain is located in the northern portion of the eastern Mojave Desert Province. Basement rock exposed in the 250 km 2 study area consists of Precambrian large K-spar granitoid gneiss which is intruded by two

Late Cretaceous peraluminous quartz monzonite plutons

(Teutonia and Homer Mountain plutons). These plutons are similiar to the Mid Hills adamellite of the Teutonia batholithic complex of the New York Mountains. Developed within the eastern portion of the Precambrian basement, prior to the late Mesozoic plutonism, are a small number of thin, north-trending , east-dipping mylonitic zones.

An early Tertiary or older system of generally north trending altered tholeiitic quartz gabbro dikes are exposed in the central portion of the study area and intrude these basement rocks. Northwest- to northeast trending conjugate leucocratic aplite dikes are inter~ preted to have been intruded along with scattered mineralized quartz veins during the final stage of Late

X @ •

Cretaceous plutonism.

Late Cretaceous to early Tertiary (?) submarginally

economic molybdenium-copper-tungsten mineralization in

the western portion of the study area accompanied em

placement of the quartz veins along a northwest-trending

shear zone and east-west-trending joints. Younger middle

Tertiary age hydrothermal mineralization appears to have

accompanied the intrusion of numerous east-west-trending

felsic dikes. A similar pattern of mineralization

associated with east-west-trending diking is developed

in the New York Mountains and further west in the Cima

Dome and Granite Spring area.

During latest Oligocene time calc-alkaline basalt to

quartz andesite flows were extruded onto the western

portion of the study area and today cap the highest hills.

These flows are associated with the volcanics of the

southern Piute Range. A sample of the andesitic flows

has yielded a K-Ar age of 27.7 + 1.4 m.y.B.P.

As these andesitic flows were being extruded , the

Homer Mountain conglomerate, a cobble- to boulder-sized monolithic conglomerate with minor sandstone and volcanic

interbeds, was being deposited in the eastern portion of

the study area within a probable steep-walled graben.

This conglomerate is intruded by alkalic rhyolitic breccia

plugs which have yielded a latest Oligocene K-Ar age of

26.0 + 1.7 m.y.B.P. The graben into which the Homer

xi Mountain conglomerate was deposited was probably formed during the earliest phase of low-angle detachment faulting which was prevalent during the middle Tertiary in the

present day lower Colorado River region.

Also in the eastern portion of the study area, north

trending granophyric rhyodacite porphyry dikes and/or

sills were intruded into the Precambrian and Mesozoic basement subparallel to the pre-Teutonia mylonitic zones.

A sample of this granophyric dike has yielded a latest

Oligocene K-Ar age of 27.2 ~ 2.7 rn.y.B.P.

The most important geologic structure of the Horner

Mountain area is a series of east-west-trending dikes of

intermediate to felsic composition which extend the basement approximately 20% in a N-S direction. These dikes appear to have intruded subparallel to a regionally developed east-west-trending joint system. The dikes are exposed across most of the central and southern portions of the study area in a zone that is about 8. krn wide and

15 krn long. Based on a geochemical composition and

field relationships these dikes appear to belong to two groups which may reflect separate parent magmas. The oldest group of dikes consists of aninterrnediate compo sition sequence of three types of quartz andesite dikes

(altered quartz andesite, quartz andesite to qua.rtz andesite porphyry, and hornblende quartz andesite) and a youngest rhyodacite dike (biotite rhyodacite porphyry). A

xii sample of the biotite rhyodacite dike has yielded an

Early Miocene K-Ar age of 19.4 ~ 1.4 m.y.B.P.

The second and younger group of dikes are of felsic

composition and are the most abundant dikes in the study

area. This group consists of three rhyodacite dikes

(quartz-orthoclase porphyritic rhyodacite, quartz rhyo-

dacite porphyry, and reddish rhyodacite and rhyodacite

porphyry) which are all peralu~inous. These two groups

of dik~s appear to have been intruded while regional ·

north-south extension was dominant in the study area.

~. This extension direction is transverse to the proposed

northeast-3outhwest direction of extension proposed for

the middle Tertiary detachment terranes of southeastern

California and southwestern Arizona.

The southern and eastern flanks of Horner Mountain

are distinguished by a group of at least three sinuous

south-~ east-dipping, low-angle, normal displacement

detachment faults which may be part of the headwall or

breakaway zone for the detachment faults of the meta

morphic core complex terranes to the east.

The two lowermost allochthonous plates of the detach

ment terranes consist of hydrothermally altered chloritic

Teutonia Quartz Monzonite which has been intruded by

numerous andesitic dikes prior to alteration. The upper

most detachment plate contains the Horner Mountain

conglomerate. This upper plate is not as altered as the

xiii lower plates. Bedding in the upper plate strikes north

·to northeast with northwest dips which are interpreted to result from tilting during movement on listric low-angle normal faults. An offset granophyric d~ke suggests about

1.6 km of net-slip along the basal detachment fault with about500 mof down-to-the-east vertical separation.

Uplift along a north- to northwest-trending axis is postulated for the Homer Mountain area during Late

Miocene-Pliocene time. This uplift was accompanied by high-angle oblique-slip reactivation of the northwest trending shear zone in the western portion of the study area. Numerous east-west-trending dikes have been right laterally separated. and the andesitic flows have been faulted and tilted to the northwest. Associated with this uplift is a northeast-trending zone of down-to the-east normal faulting along the southern Piute Range.

High-angle normal faulting has displaced the eastern detachment terranes and the autochthonous basement. The most recent movement appears to have been during the

Pleistocene because alluvial fans of that age are displaced while Recent fans are not.

xiv I N T R 0 D U C T I 0 N

LOCATION

Homer Mountain is located about 35 km northwest of

Needles, California, west of U.S. Highway 95 and north of u.s Interstate Highway 40 (Fig. 1). The study are is a 2 250 km region which lies mostly within the Kingman 2°

sheet between longitude 114°55' W. and 115°10' W., and . 0 0 latitude 34 58' N. and 35 05' N. The study area includes

T. 11 N., Rs. 18 and 19 E., and about one section across

adjacent townships to the north and south. These

townships lie within the Lanfair Valley and Homer

Mountain 15' quadrangles.

PURPOSE AND SCOPE

This study focuses on general geologic mapping of

the Homer Mountain area, the description and classi-

fication of dike rocks and other intrusive basement rocks

by geochemical analysis, and their relative ages or

absolute ages, where available. The structural and

temporal relationships between a dike swarm emplaced along

an east-west-trending joint system and similarly oriented

subeconomical mineralization are discussed. The Homer

Mountain dike swarm is briefly compared with other

Tertiary dikes swarms in the surrounding region and with numerous transverse structures of the southern

1 2

Figure 1. Index map of the location of the Homer Mountain dike swarm, and their relationship to other dikes and faults in the lower Colorado River region. List of abbreviations:

AM Arica Mountains NH Newberry Mountains BLM Buckskin Mountains NYM New York Mountains BM Black Mountains OWM Old Woman Mountains BMM Big Maria Mountains PLM Plomosa Mountains CD Cima Dome PM Piute Mountains CDM Castle Dome Mountains PR Piute Range CKM Clark Mountains PRM Providence 11::mntains CM Chemehuevi Mountains RM Rawhide Mountains DDM Dead Mountains RV Rivers Mountains DM Dome Mountains RVM Riverside Mountains EDM ElDorado Mountains SHF Slaughter House Fault FH Fenner Hills SLM Steplatter fuuntains GM Granite Mountains SM S acrarrento Mountains GS Granite Springs SPM Spring Mountains HB Hackberry Mountains TM Turtle Mountains HCM Harcuvar Mountains TR Trigo Mountain HQM Harquahala Mountains VH Vontrigger Hills IM Iron Mountains WM Whipple Mountains KH Kilbeck Hills KM Kofa Mountains KTF Keystone Thrust Fault LM Lake Mead LMM Little Maria Mountains MH Mid Hills MM Mojave Mountains MR Mopah Range MS Marl Springs MeR McCullough Range 3

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NEVADA UTAH

CALIFORNIA ARIZONA'

eLOS ANGELES

P~OENIX

~ THRUST FAULT compraealonal ,-/ T ACHMENT FAULT OE a~ttanelonal ~ STRIKE-St.! p FAI.A. T

DIKE- SWARM 20 4

Cordillera of the western United States.

METHODOLOGY

Sixty-three days of fieldwork were conducted during

the winter of 1981-82. Geologic data were plotted on overlays of color air photos, BLM-C-MOOO series, at a

scale of approximately 1:25,000. Data were then trans

ferred to an enlargement of the USGS Lanfair Valley and

Horner Mountain 15' quadrangles, at a scale of 1:24,000.

Two geologic maps, Plates 1 and 2, were made from the data because of the complexities in plotting the dike swarms.

Plate 1 is a geologic map of the area with none of the east-west dikes plotted. Plate 2 is a dike swarm map which differentiates seven east-west-trending dikes, quartz veins, and two north-trending dikes previously plotted on Plate 1.

Fifty-nine rock samples were analyzed for major oxides by x-ray flourescence spectrometry. Rock names were determined from QAP plots of normative minerals.

Intrusive rocks were given plutonic names if their ground mass was phanerocrystalline and volcanic names if their groundmass was aphanitic. A more detailed discussion of the geochemical procedures is given in the geochemical analysis section. 5

PREVIOUS WORK

The western portion of the study area surrounding the Leiser Ray mine and Tungsten Flat areas lies within the southeasternrnost corner of the area covered by

Hewett (1956) in USGS Professional Paper 275 on the

Ivanpah quadrangle. Hewett described the granitic basement rock at the Leiser Ray mine as belonging to the

Teutonia QUartz Monzonite with a megascopic appearance sirniliar to the Teutonia rocks exposed in the Mid Hills and New York Mountains located 32 krn to the northeast.

The geology of the Horner Mountain area as shown on the . 20 Klngrnan sheet of the Geologic Map of California by

Jennings (1961) is from an unpublished regional geologic map by Bonham and others (1960) which was made for the

Land Department of the Southern Pacific Company, with reconnaissance mapping of incomplete portions by Jennings and Strand of the California Division of Mines and Geology.

Beckerman and others (1982) have described the Teutonia batholith of the Ivanpah Mountains. Mid Hills, New York

Mountains, and Cirna Dome areas as being comprised of at least six non-cornagrnatic suites of Jurassic to Cre- taceous age, rnetalurninous to weakly peraluminous granitic plutons. Although their study did not extend south- eastward into the Horner Mountain area, it is the most detailed study of the Teutonia batholithic complex to date. 6 .

Hewett (1956) noted a similarity in the megascopic

appearance of monzonite dike swarms in the following areas:

Mid Hills east of Elora: the New York Mountains southeast of Brant: the eastern New York Mountains near Lecyr Wells:

and the Leiser Ray mine in the western portion of the study

area. Burchfiel and Davis (1977) regard the northwest

trending porphyritic dikes of the eastern New York Moun

tains as similar to the quartz monzonite to monzonite

basement, and obtained from one dike sample a K-Ar age of

71.7 ± 0.8 m.y.B.P. Spencer and Turner (1982) compare the east-west-trending dikes of the eastern portion of the

study area to the dikes of the Sacramento Mountains,

located 19 km to the south. Based on modal analyses, they

distingish four types of dikes: biotite rhyolite; biotite quartz latite; biotite (± hornblende) andesite: and basalt. Spencer (1983) has determined K-Ar ages of 16 to 19 m.y.B.P.

(Middle Miocene) for some of the east-west-trending dikes of the Sacramento Mountains, Homer Mountains, and Piute

Range. Ash-flow tuffs which are interbedded with tilted conglomerates yield K-Ar ages of 15 to 18 m.y.B.P., and overlying basalt has been dated at 14.6 m.y.B.P. (Spencer, 1983) . Spencer believes that the N80°W ± 10° trend of the dike swarms indicates a N10°E + 10° orientation for the

least compressive stress in the autochthon and believes

the dikes were emplaced at the sametime the allochthonous

terrane of the metamorphic core complexes to the east were undergoing east-west extension. Spencer and Turner (1982) also describe a sequence of three low-angle normal faults on the southern and eastern flanks of Homer Mountain.

Work is in progress by Y.N. Agyakwa (personal conununication, 1983) on the age of emplacement, and the structural relationship of mineralization along a prominent east-west-trending joint and dike system in the New York Mountain. Carlisle and others (1982) have postulated that hydrothermal porphyry mineralization in the New York Mountains, Homer Mountain-Signal Hill, and

Granite Spring (near Holloran Springs} areas are temporally and spacially associated with porphyritic dike swarms that have intruded along a system of generally east-west-trending joints. These joints form a zone with a width of 5 to 8 km, and an overall length of about 80 km.

Carlisle and others speculate that the hydrothermal systems of the New York Mountains and the Homer Mountain

Signal Hill areas might have been right-laterally offset along the Slaughterhouse fault of Burchfiel and Davis(l977).

G E 0 G R A P H Y

SURFACE AND CULTURAL FEATURES

Horner Mountain is located in San Bernardino County, within the northeasternmost corner of the Mojave Desert

7 8

Province (Fig.l). The nearest town is Goffs, California

(pop ~ 28) which is located about 6 km southwest of the

study area. Goffs is located on the Old Trail road

which runs adjacent to the Santa Fe railroad; food, gas,

water and repairs are available. Running northward from

Goffs is the Ivanpah road which runs across Lanfair

Valley, intersecting the Old Government road about mid

valley. The Old Government road runs west to the Mid Hills,

and east through Piute Spring, then on to the Colorado

River. These roads provide access to the trails which

lead into the study area.

The western edge of the study area is bound by a

major unnamed wash which drains Lanfair Valley to the

south. Runoff from the Homer Mountain area drains either

to the Piute Wash on the east or to the unnamed wash

which follows the Atachison, Topeka, and Santa Fe

railroad to the south of the study area, and then joins

with the Piute Wash at the southern end of Dead Mountains

where they drain to the Colorado River.

Homer Mountain rises to an elevation of 1140m

and is about 540m·3bove the alluvial plain that

surrounds the mountain on three sides (Plate 1). The

west side of Homer Mountain decends about 275 m to a

broad saddle, then rises to an elevation ofl070 mat

Signal Hill. North of Signal Hill there are a number of basalt capped knobs whose elevations are between 9

Q •

1070 to 2000 m. The northwesternmost portion of the

area contains a line of basalt-capped ridges that are

extensions of the southern Piute Range, and these range

in elevation from 1240 to lOBO m . The most unique

topographic features of the Homer Mountain area are the

numerous generally east-west trending spine-like ridges

which rise from 3 m to as much as 60 m above surrounding

weathered basement rock. These ridges are supported

by siliceous dike rocks that are resistant to weathering.

Past mining activities were concentrated in the west

ern portion of the study area around the Leiser Ray mine and Tungsten Flats. Mining was most active

between 1905 and 1916 (Hewett, 1956). At present,

there is a heapleach gold mine, the Rattlesnake mine, being operated by the Wright-Bryant Mining Company on the northeastern edge of the Vontrigger Hills, about

5 km northwest of the study area.

CLIMATE AND VEGETATION

The Homer Mountain area has an elevation generally greater than 610 m and average elevation of about

915 m. Hewett (1956) reports an average rainfall in Las Vegas, elevation 620 m, of 12 em per year, and an average of 22 em for Searchlight, Nevada, elevation

1960 m. The average rainfall for the study area probably lies between these values. Rainfall is greatest during the months of December through March, with local 10 ,, . thunderstorms during July to September. Temperatures reach highs of 46° C during the summer and lows of near freezing during the winter.

Vegetation is typical of the California-Arizona high desert. Mesquite, creosote, seasonal grasses, cholla, and smallbarrel cactus are common throughout most of the area. Desert animals observed include rabbit, coyote, bobcat, quail, owl, rattlesnake, and an unusual abundance of desert tortoise. Special caution should be taken when traveling on the south and east flanks of Homer Mountain because the area was apparently used as an artillery range during WWII. Numerous unexploded artillery shells were encountered.

PIONEER SETTLEMENTS

The first white man known to pass near the Homer

Mountain area was Father Garces, a Spanish Priest, in

March of 1776 (Hewett, 1956). Father Garces traveled westward from the Colorado ~ver, across the Dead Moun tains to Vontrigger Spring and Lanfair Valley on his way to the Mojave River and the San Gabriel Mission.

The next recorded settler to pass near the study area was Lt. A.W. Whipple who followed Father Garces' route from spring to spring, through Piute Spring and Lanfair

Valley.

In 1858 Fort Mojave was constructed along the Colorado River. In the winter of 1860-61, Dr. Copper, attached to the Commission of the United States and

California Boundary Survey, followed the now well established Piute Spring-Lanfair Valley route. Sometime after 1868 the government established an army post at

Piute Spring and Government Hole.

Mining activity increased in southern Nevada in the

1860's with the discovery of silver in the Ivanpah district (Hewett, 1956). Development of mining in the region was restricted because of the 300 krn distance to

San Bernardino, the nearest railroad. In 1883 the

Atachison, Topeka, and Santa Fe Railway was completed to Mojave, and in 1892 a branch was completed from Goffs to the Vanderbilt gold district in the New York Mountains.

Mining in the study area was concentrated around the

Leiser Ray mine, and a mill was constructed between 1905 and 1915. The foundation of the mill still~ stands.

G E 0 L 0 G Y

REGIONAL GEOLOGIC AND TECTONIC SETTING

The Horner Mountain area is located in the northern portion of the geologically complex eastern Mojave Desert

Province. Here the boundaries between the Mojave Desert

Province and the northernand southern Basin and Range Provinces are not well defined because of overlapping and

11 12

interrelated Cenozoic histories. Burchfiel and Davis

(1981) have arbitararily drawn the boundary between the Mojave Desert and southern Basin and Range along the

Colorado River. Dokka (1983) has defined an eastern limit of the Mojave Desert structural block based on geophysical criteria as a line extending south from the

Death Valley fault, 120 km west of the study area, south east through the Granite Mountain fault. The northern boundary between the Mojave Desert and the Basin and

Range is also vague, and is defined by Nelson (1981) as the uncertain extension of the Garlock fault east from the intersection with the Death Valley fault. The structural history of Homer Mountain is most closely related to the northeastern Mojave Desert Province to the northwest, but a series of low-angle normal faults in the eastern part of the study area suggest that the boundary between the Mojave Desert and the southern

Basin and Range may be through the Homer Mountain region.

During the late Precambrian and Paleozoic Era, the

Homer Mountain area was near the rifted northeast trending edge of a Precambrian craton (Burchfiel and

Davis, 1972). Miogeoclinal sediments of the late

Precambrian and Paleozoic Era extend southwest of the study area into the Mojave Desert (Stone and others,

1981; Stewart and Poole, 1974). The late Precambrian to Paleozoic history of the region surrounding Homer 13

Mountain is complicated by the overprinting of multiple periods of Mesozoic thrusting and plutonism.

The southwestern portion of the North American mio geocline was truncated several times during Middle

Jurassic to Late Cretaceous time along northwest trending sinistral-slip megashears (Burchfiel and Davis,

1975; Silver and Anderson, 1974,1983; Dickinson, 1983).

Early to late Mesozoic southeast-trending magmatic arcs overlapped in the region surrounding Homer Mountain

(Armstrong and Suppe, 1973; Burchfiel and Davis, 1975).

Both Mesozoic arcs had in their foreland$ southeast trending east-directed thrust faults.

The New York Mountains,located about 32 krn to the northwest, are underlain mostly by intrusives of the

Jurassic to Late Cretaceous Teutonia batholithic complex

(Hewett, 1956; Beckerman and others, 1982). The eastern flank of the New York Mountains in the Sagamore

Canyon-Slaughterhouse Spring area contains a 2000-m thick sequence of thrusted and folded Paleozoic and

Mesozoic metasedimentary and metavolcanic rock that has been intruded by the Teutonia batholith (Burchfiel and Davis, 1977). These deformed rocks are thought to be correlative with those in the upper plate of the

Keystone thrust which occurs in the late Mesozoic

Cordilleran foreland thrust beltof the Spring Mountains. The nearest exposure of pre-Tertiary sedimentary 14

rocks is 19 km northwest of the study area within

Lanfair Valley. Here Precambrian crystalline rocks

are in fault contact with the Bird Spring Formation

of Pennsylvania age, and either Jurassic Aztec Sand

stone (Burchfiel and Davis, 1977), or Lower Cambrian

Tapeats Sandstone (Hewett, 1956). The fault that

separates the basement and sedimentary rock is thought

to be the southern extension of the Slaughterhouse fault

(Burchfiel and Davis, 1977). This fault strikes north

south, dips at moderate angles to the west, and is

thought to have had strike-slip movement sometime after

the intrusion of the Teutonia batholith but before the

extrusion of Late Miocene-Pliocene(?) volcanics in the

New York Mountains which cover it. No displaced rocks

units have been found to indicate the amount or sense

of movement on the fault (Burchfiel and Davis, 1977).

A possible southward reappearance of the Mesozoic

foreland thrust belt may be represented by a small

exposure of deformed Paleozoic rocks present 38 krn

south of the study area in the Piute-Old Woman

Mountains (Miller and others, 1982). Ductile thrusting of the Precambrian basement during the deformation at

the Piute-Old Woman Mountains suggest that the thrust belt cuts southeast out of the miogeocline and is controlled by the ductility of the basement, not the miogeoclinal stratigraphy as is the case to the 15

north (Miller and others, 1982; Burchfiel and Davis,

1972).

Soon after the Teutonia batholith was intruded magmatic activity ended across southeastern California and swept inland to the Rocky Mountain region (Coney and Reynolds, 1977) . In southwestern Arizona a broad northeast-draining regional upwarp developed in the early Tertiary and produced a regional erosional unconforrnicy of Eocene age (Shafiquallah and others,

1980; Young and McKee, 1978).

A major change in the direction of convergence between the Farallon plate and North America occurred at the end of the Laramide Orogeny, about 53 rn.y.B.P.

(Engebretson and others, 1982). The motion of the

Farallon plate jumped from an eastward direction to a northeast direction and became progressively more northerly until 10 m.y.B.P. This change in plate motion was followed by a "flare up" of calc-alkaline magmatism which swept westward back across Arizona and by 25 m.y.B.P. had reached the southwestern portion of the state (Lipman and others, 1972; Coney and

Reynolds, 1977). To the north calc-alkaline volcanism swept southward across the Great Basin in broad east west-trending belts (Stewart and Poole, 1977). These two waves of Cenozoic magmatism terminated in south eastern Utah and along the Colorado River below Hoover

Darn about 21-20 m.y.B.P. and were separated by a 100-km- 16

wide amagamtic corridor (Anderson, 1981).

The structural transition from Mesozoic-early

Tertiary compressional deformation to middle- to late

Tertiary extensional deformation is poorly understood in the region surrounding the study area because development of the metamorphic core complexes and low-angle normal faulting which spans this transition time have created a complex geologic record.

Thirty-two kilometers southeast of the study area is the Sacramento-Chemehuevi-Whipple Mountain chain. This area is part of the Cordilleran Tertiary metamorphic core complex terrane (Coney, 1980; G.A. Davis and others, 1980,

1982 ; John, 1982; McClellan, 1982). Metamorphic core complexes can generally be described as uplifts of metamorphic and plutonic rock which are separated from overlying unrnetamorphosed cover by a zone of low-angle, listric normal faults, termed detachment faults. The lowest detachment fault marks a ~one of thermal and tectonic transition from the brittle deformation of the cover rock to the ductile deformation of the metamorphic core (Coney, 1980). A detachment fault is often in contact with a mylonitic zone which appears to be older, but both may be end members of a complex deformational history ("Rehrig, 1982). Commonly superimposed along both sides of the detachment fault is a "chlorite breccia" zone the development of which may be coeval 17 <1 ' with or precede detachment faulting (G.A. Davis, 1982).

In the lower Colorado River region the detachment

terranes have a strikingly consistant northeasterly direction of movement.

In addition to their complex strain histories, metamorphic core complexes typically have a complex history of plutonic intrusions (Coney, 1980). In the

Piute-Old Woman and Whipple Mountains, Jurassic to Late

~etaceous metaluminous and peraluminous granitic plutons and sills have intruded during early stages of mylonitization (Miller and others, 1982 ; G.A. Davis and others , 19 8 0 , 19 8 2) .

The timing of the formation of the mylonitic zones and the detachment faulting is complex, and as yet not fully understood. In the Whipple Mountains mylonitic rocks found in the Oligocene-Miocene debris flows yield

K-Ar and fission track dates of 78.5 and 82.9 m.y.B.P., respectively, and they are assumed to be unaffected by younger age-resetting (G.A. Davis and others, 1980). K-Ar dates from crystalline rocks directly below the Whipple

Mountain detachment fault yield ages as young as 15 m.y .B.P ., and increase in age with structural depth (G.A. Davis and others, 1980, 1982}. In the allochthonous upper plate at

Whipple Mountain, a high-angle unconformity between the

Gene Canyon Formation (33 to 26 m.y.B.P.) and the Copper

Basin Formation (20 to 16 m.y.B.P.} suggests that detachment faulting was very active prior 18

Q '

to 20 m.y.B.P. (Frost, 198l).Mylonites of the core com

plexes formed at depths of 10 to 12 krn and were raised from

a regime of ductile deformation to one of brittle block

fault deformation as Tertiary Basin and Range extension

progressed (Hamilton, 1983). The transition to high-

angle basin-and-range style faulting is thought to have

occurred before 15-16 m.y.B.P. in southwestern Arizona (Rehrig, 1982).

In the mountain ranges west of the Sacramento-Che

meuvehi-Whipple Mountains are a series of high-angle nor

mal faults which trend along the eastern portions of the

Turtle-Steplatter and Piute-Old Woman Mountains (Fig.l).

These faults are thought to be a headwall or breakaway

zone for the detached terrane to the east because they are

the westernmost known detachment-related normal fault (G.

A. Davis and others, 1980; Howard and others, 1982; Miller

and others, 1982). The normal faults of the easternmost

portions of the study area are on trend with this headwall

fault zone and may be its continuation northward to the

El Dorado Mountains of Nevada (Spencer snd Turner, 1982;

G.A. Davis and others, 1980).

A transition from low-angle detachment faulting to high-angle basin-and-range style faulting followed the

deposition of the Osborne Wash Formation (13.5 m.y.B.P.)

in Late Miocene time. High-angle faulting was most active

in southwestern Arizona between 14 and 8 rn.y.B.P. 19

(Shafiqullah and others, 1980). Significant high-angle

faulting occurred after 13 m.y.B.P. southeast of the study

area (Eberly and Stanley, 1978) and between 12.7 and 11.1

rn.y.B.P. to the northeast (Anderson and others, 1972).

The north-south to northwest trends of southwestern

Arizona mountain ranges and high-angle faults suggests

continued east-west to northeast-southwest directed exten

sion. A regional erosional unconformity developed across

southwestern Arizona between Middle and Late Miocene time

(17 to 12 rn.y.B.P.) and is thought to be genetically re

lated to the beginning of Basin and Range high-angel

structures (Eberly and Stanley, 1978; Shafiqullah and

others, 1980).

Basaltic volcanism which was syntectonic with basin

and-range style faulting ceased in southern Arizona

between 8 and 4 rn.y.B.P. (Shafiqullah and others, 1980).

About 70 krn north of Horner Mountain, the Fortification

Hill Basalt Member of the Muddy Creek Formation was

extruded into a closed basin 5.8 rn.y.B.P. which pre-dates

the Colorado River (Longwell, 1963; Shafiqullah and others,

1980). The Colorado River began its present southward drainage coarse sometime after the earliest Pliocene.

Pleistocene faulting in the region surrounding the

study area is rare. Southeast of the study area, about 20 krn north of Parker Darn, is a continous zone of small

faults which cut Plio-Pleistocene alluvial fans (Carr, 1981). These may be related to the major northwest- 20

trending Havasu Springs fault which is known to cut the

Bouse Formation of Middle to Late Pliocene age (G.A. Davis

and others, 1980).

The dike swarms of Homer Mountain are not unique to

the lower Colorado River region (Table 1). Numerous

northwest-trending and less common east-west- to north

east-trending dike swarms are known (Rehrig, 1981). The

emplacement of the northwest-trending dikes is thought

to be related to the northeast direction of extension, but

the origins of the east- tonortheast-trending dikes is not clearly understood. Table 1 is listing of major dike swarms of the lower Colorado River region. These dike swarms are plotted on the index map (Fig. 1).

GEOCHEMICAL ANALYSIS

&~ALYTICAL PROCEEDURES

Fifty-nine rock samples were analyzed for major oxides by utilizing fused-glass discs with x-ray flour escence spectrometry (XRF), and are listed in Table A-1

(Appendix 1). A computer-controlled four-position Diano instrument was used for the analyses. Fused-glass discs were prepared by heating powdered and dried rock samples with a flux of lithium metaborate in a ratio of lsample:

6LiB02 . The drying of the rock sample prior to weighing and mixing with the lithium metaborate eliminated the need to add the loss-on-ignition value to the total weight percent oxides. OXide weight percents of the starrlarjs TABLE 1: TERTIARY DIKE SWARMS-LOWER COLORADO RIVER------REGION EAST-WEST TO EAST-NORTHEAST-TRENDING Location Description References

New York Mountains E-W diabase to rhyolite dikes and quartz veins Carlisle and Marl Spring in New York Mts. to Marl Spring, swings NW others, 1982 Granite Spring, at Granite Spring, zone typically 5 to 8 Ca km wide, total length at least 80 km intrudes Jurassic-Late Cretaceous Teutonia batholith; hydrothermal porphyry mineraliz ation associated with dikes and veins.

Sacramento Mount ains, Ca W-NW bas~lt to rhyolite dieks in NW portion, Spencer and N75 W + 10° trending zone 10 km wide; K-Ar Turner, 1982; age of-16-19 m.y.B.P. Spencer, 1983

Newberry Mountains, E-W andesite and rhyolite dikes; zone 8 km Volbroth, 1973 NV wide by 7 km long; intrudes Pre e gneiss; part of mineralized block dropped between two E-W trending faults.

River Mountains, E-W alkalic andesite and dacite dikes in cent- Smith, 1982 NV ral and southern portions of range; predate NW high-angle faulting; foliations in Pre b schist suggest E-W axis of folding.

Harcuvar and E-NE muscovite granite dikes; cut Late Cretac- Reynolds, 1982 Harquahala eous deformation fabrics; older than mylon- Mountains, AZ itic foliations with E-NE lineations; ranges have NE trending Tertiary folds; Rb-Sr age of Eocene.

N ...... TABLE 1 continued : TERTIARY DIKE_ SWARMS-LOWER COLORADO RIVER PEGION NORTHWEST-TRENDING Location Description References

Mojave Mountains, NW to W-NW gabbro to rhyolite dikes, Nakata, 1982; Pike AZ 45°-70°NW dips; intrudes Pre€ rocks and Hansen, 1982; around Crossman Peak; pre-date Howard and others, rotation of detached plate; cut by NW 1982 trending faults; detached surface warped by E-NE trending folds.

Kofa Mountains, NW intermediate to felsic dikes, 60°SW dips; Dahm and Hawkins, AZ some dikes associated with-mineralization, 1982 W-NW, 60°S at King of Arizona mine and E-W, 60°N at North Star mine; dikes older than 19.5 m.y.B.P. rhyolite-dacite flows.

Harcuvar and NW to N-NW microdiorite dikes; cut middle Reynolds, 1982 Harquahala Tertiary mylonite foliations and E-W Rehrig and Mountains, AZ dikes (see above); average K-Ar age of Reynolds, 1980; 25 m.y.B.P.; locally associated with Shafiqullah and copper-gold mineralization. others, 1980

Castle Dome N to NW dacite porphyry and rhyolite porphyry Gutman, 1982; Mountains, AZ dikes; near vertical dips; rhyolite (20.4 Logan and Hirsch, m.y.B.P.} but not dacite (19.0 m.y.B.P.) 1982 cuts overlying Tertiary strata; mineral ized fissures associated with dikes.

N N

e TABLE 1 continued TERTIARY DIKE S~'VARHS~LOvlER COLORADO RIVER REGION NORTHWEST TRENDING Location Description References

Whipple Mountains, N to Nl5°w dikes, zone 10 km wide; forms in G. A. Davis and Champers Wells, ward dipping fan; older high-K, calc others, 1980; CA alkaline andesite-to-dacite series pre 1982 dominates over younger olivine tholeiitic diabase series; emplaced during and after formation of mylonite.

Chemehuevi NW to NE gabbroic to dacite porphyry dikes in John, 1982 Mountains, CA SW portion and E-W dikes in central and northern portion; intrude Cretaceous(?) Chemehuevi pluton and Pre€ wallrock; NE to E-W dikes older

Turtle Mountains, N to NE quartz porphyry rhyolite dikes; cut Howard and CA Late Cretaceous Castle Rock and Turtle others, 1982 plutons but not Oligocene-Miocene volca nics; dikes converge at N. Turtle Mts. suggest source area; gabbro-diabase dikes Pre~ (?) age trends Eat Turtle Mts., N at Steplatter Mts.

El Dorado, N to NW rhyolitic, andesitic, pegmatite, and Volbroth, 1973 Newberry and aplite dikes; intrude Pre€ gneiss to Northern Dead early Tertiary granite; zone up to 16 km Mountains, NV wide, 64 km long; mineralization younger than NW dikes; NW dikes Miocene (?) age; some E-W dikes (see above).

N w 24

" . were recalculated to remove the loss-on-ignition percentages. A counting standard was utilized during analyses to correct for machine drift.

ROCK CLASSIFICATION

Igneous rock names are determined from quartz alkali feldspar-plagioclase plots {QAP) of normative minerals (Table A-1). Plutonic and volcanic names follow the Strekeisen {1967) nomenclature. Igneous rocks are assigned plutonic or volcanic names based on the hand specimen texture of the ground mass. Those rocks with a phanerocrystalline groundmass are given plutonic names, while those with an aphanitic groundmass are given volcanic names, modified for distinctive mineral assemblages or colors. Igneous rocks have been further classified by plotting oxide values on alkalis-silica

{K 0 + Na o vs. Sio ), potash-silica (K o vs. Sio ), 2 2 2 2 2 AFM, and alumina to lime plus alkaline vs . silica

(Al ; [CaO + Na o + K oJ , abbreviated Al/CNK) diagrams. 2 o3 2 2 Table 2 summarizes the rock classifications based on these diagrams.

RESULTS OF GEOCHEMICAL ANALYSES

Based on field relationships and geochemical analyses the igneous rocks of the study area can be divided into four groups: (a} plutonic: (b) volcanic flows and breccias: (c) dikes of mafic to intermediate composition; 11\1\U ~,: C:,lJMM/IIlY Ill Hill K l'l fl','_, If II Ill IIIW, IJII'•I II lit~ 1;1 Ill Ill 1411 Ill /INI\1_ Y',l ',

____Rock Unit ''I ]' (' k I~ i ,;o' I I 1\1 M 1\ II·

1111<1 r 1 1111 f.· Teulonia pluton t mnn1 it 1

Homer Mountain pluton qtJartz motlLnnite f'<1lc-alkal ic ·;11lJ i!lkal i' meti-K rhynlitf? IJP.Piluminolls leucocratic aplite dikes quarl7 monzonite calr:-alkal ic still a l ka 1 i c hi -K rhyolite peraluminous mesocralic aplite dikes qtJarl; monzonite t~alc-alkalic su h ;1) k

N U1

"" 26

(d) dikes and/or sills of felsic composition. The

following sections will briefly discuss the chemistry of

each of these groups which is summarized on Table 2. A more detailed discussion of the field relationships,

ages, and petrography of each rock type is given in

the discussion of rock lithologies. For brevity structural

analyses of the trends of the dike rocks are included in

the discussions of the dike rock lithologies.

Plutonic Rocks

Two plutons (Teutonia and Homer Mountain) and two

types of aplite dikes (leucocratic and mesocratic) make

up the early Tertiary-Late Cretaceous intrusive basement

in the study area. Based on normative rnineralology all

of these rocks fall in the quartz monzonite field of the

QAP diagram (Fig.2a), except one granodiorite sample

(Horner Mountain pluton). Alkalis-silica relationships

indicate that all of these rocks are sub-alkaline and are cal~ alkaline on the AFM diagram (Table 2) . All samples lie in the high-K field on the potash-silica diagram. All of these plutonic rocks are peraluminous based on their Al/CNK ratios (Table A-1). The mesocratic aplite dikes have the lowest peraluminous ratio (1.01), and the Teutonia pluton has the highest (1.09 to 1.18).

The chemical compositions of the Teutonia and Homer

Mountain plutons are similar and may be genetically related (Fig. 2b). 27

Figure 2a. Normative mineral QAP plot of Late

Cretaceous to early Tertiary plutonic rocks; (~)

Teutonia pluton, ( •) Homer Mountain pluton,

( 0) leucocra tic aplite dikes, ( fil) mesocra tic aplite

dikes.

Figure 2b. Harker diagrams of plutonic rocks. Symbols: TQ-Teutonia Quartz Monzonite pluton

TM-Homer Mountain Quartz Monzonite pluton

AL-leucocratic aplite dikes

~1-mesocratic aplite dikes 28

a (a)

(b)

cao vs. Si02 ~~ FeO total vs. Si02

{rro \ \ \ "-.~HM '_) ...... ! \ ' ~c;AL 6" 68 72 76 ao 68 72 78 80 5102 wl'!l. SI02-

K20 vs. Si02 ~~ MgO vs. Si02 .:::::...~ '""~-- AM ~t:._~HM .. .., "-.J i"' ~Al '_.) f ,"-ro

68 72 76 12 TIS 80 5102 wl'!lo 8102- 29

p '

Volcanic Flows and Breccias

Basalt and quartz andesite flows (Fig. 3a) extend

from the southern Piute Range across the western portion of the study area capping the highest hills (Plate 1). These rocks are subalkaline on the alkali-silica diagram and calc-alkaline on the AFM diagram (Table 2). They fall within the medium-K basalt and high-K andesite field of the potash-silica diagram and are metaluminous. These flows are similiar in age (27.7 m.y.B.P.) to a group of fine-orained rhyolitic dikes and breccia plugs (27.2. m.y.

B.P.) found in the easternmost portions of the study area.

These eastern rhyolitic rocks are alkaline on the alkalis silica diagram (Table 2). They lie in the high-K rhyolite field of the potash-silica diagram and are slightly pera luminous. These eastern rhyolitic dikes and breccia plugs are chemically different from the younger felsic dikes (see discussion of felsic dikes and Fig. 5).

Mafic to Intermediate Composition Dikes

This group consists of four structurally and temporally related dikes of intermediate composition

(altered quartz andesite, quartz andesite porphyry, horn blende quartz andesite porphyry , and biotite rhyodacite porphyry), and one group of structurally unrelated mafic dikes (altered quartz gabbro) which are included here for brevity. The intermediate-composition dikes 30 ,, '

Figure 3. Normative mineral QAP of andesitic

flows and rhyolitic dikes and breccia plugs;

(•) andesitic flows, (y) rhyolitic dikes and breccia

plugs. 31 ,, .

I I I I I I I I I I I I +I I 1 .... I I 32

are part of the earliest phase of east-west-trending diking, while the mafic dikes are cross-trending and are the oldest dikes in the study area. The biotite rhyodacite dikes are included in the intermediate composition group because they are structurally associated with the hornblende andesite porphyry dikes, they appear to be older than any of the felsic dikes (except for the granophyric sills), and their chemical composition is more closely related to that of the intermediate composition dikes than that of the felsic dikes.

The oldest dikes in the study area are classified as quartz gabbros based on a QAP plot of the normative minerals (Fig 4a and Table A-1) They have been slightly metamorphosed (see discussion in lithology section) and may have originally been gabbros. Chemical analyses of these dikes indicates that they are sub-alkaline on the alkalis-silica diagram (Fig. Sa) and tholeiitic on the

AFM diagram (Fig. 4b). They fall in the medium-K field of the potash-silica diagram (Fig. 5b). These dikes are metaluminous (Fig. 5c). Harker diagrams of the major oxides are presented in Figure 6.

The oldest dikes of the intermediate-composition group are the altered quartz andesites. Although these dikes are structurally related to the other east-west trending intermediate-composition dikes, they are closer in chemical composition to the altered quartz gabbro dikes. 33

Figure 4a. Normative mineral QAP plot

of mafic- and intermediate-composition dikes;

( 0) altered quartz gabbro, ( (j)) altered quartz

andesite, (•) quartz andesite and quartz andesite

porphyry, (~) hornblende quartz andesite porphyry,

(@) biotite rhyodacite porphyry.

Figure 4b. AFM plot of mafic- and intermediate

composition dikes; symbols same as above. 34 Q '

F 35

Figure 5. Classification diagrams of Tertiary

age mafic, intermediate, and felsic dikes; (a)

alkalis-silica diagram, (b) potash-silica diagram,

QA-altered quartz andesite

AN-quartz andesite and quartz andesite

porphyry

HP-hornblende quartz andesite porphyry

BP-biotite rhyodacite porphyry

PQ-quartz-orthoclase porphyritic rhyodacite

GP-granophyric rhyodacite porphyry

QP-quartz rhyodacite porphyry

RW-reddish rhyodacite and rhyodacite

porphyry

RE- eastern rhyolitic dikes and

breccia plugs 36

N - K20+Na20 vs. Si02 ~E alkaline # w -!II ?51.~-':'-;;-e:z---~p o"' • /1').-:_•.L... I C\1 ·~a~ a zIll + 0 :.::C\1

48 84 72 80 8102 wtllb

0 - K20 vs. Si02 basalt I andesite dacite

#CD i 0 :.::C\1

N medium-K

low-K ee u 72 80 8102 wtllb

48 88 u 72 80 8102 wtllb 37

Figure 6. Harker diagrams of major oxides of

Tertiary age mafic, intermediate, and felsic dikes.

Symbols same as Figure 5. 38

MgO vs. 5102 FeO total vs. 5102

48 •• e4 Sl02 w"

Al203 vs. 5102 CaO vs. 5102

.,.., \ .AN HP .,.., ~ OA ;- ;- OA ~ (.'>( (") 0 0 .. ~.,.:_OA N 0 ;( ---0 AN~ ~~?·_PO ~ ~ OP"" • c~~ . HP...... - w •• e• n eo 4e •• .. T2 eo •• 9102 wl'l. 8102 wt~

.; ., NA20 vs. 5102 .. (\ ;\ Tl02 vs. 5102 ~\ '#o .,.., )tz; c1\\o• i• ~ ~'.. ___,C., 0 .. • i.: 0 N N.. OAY/ 0 ~ z YV \, •• >::f/~~~ ;::: . (__:,} OA ., f.?~~p .. PO ~r RW 0 _.;.o_P..Siio...,._eeo 72 eo •e 12 Sl02•• wi'W, Sl02 wl'!lo 0

Mn02 vs.5102 f'205 YS. 5102

N 0 0:: ~ I~ OA .0

OP 4e ee •• Sl02•• wt'l(, eo 8102 WI'!(, 39

A QAP plot (Fig. 4a) of the normative minerals shows

these dikes lie close to the quartz gabbro field, and

two of the three samples fall in the tholeiitic field

of the AFM diagram (Fig. 4b). They are subalkaline

on the alkalis-silica diagram (Fig. Sa), and lie in the medium-K basalt to high-K andesite field of the potash

silica diagram (Fig. Sb). They are metaluminous (Fig.Sc).

Harker diagrams of the major oxides also show a similarity

with the altered quartz gabbro dikes (Fig. 6).

The other three dikes of intermediate-composition

appear to be a differentiated sequence. A QAP plot

(Fig. 4a) of the normative minerals shows these dikes

range from quartz andesite to rhyodacite, with two of

the hornblende porphyry samples falling in the dacite

field. These dikes are mostly sub-alkaline on the alkalis-silica diagram (Fig. Sa), and calc-alkaline on the AFM diagram (Fia. 4b). They lie across the high-K andesite and high-K dacite fields of the potash

silica diaaram (Fig. Sb). The quartz andesite porphyry dikes are clearly metaluminous, but the others are on the border with the peraluminous field (Fig. Sc). The

Harker diagrams (Fig. 6) of major oxides show a grouping of these three dikes with a general decrease in oxide percentage with increased silica. The grouping of these three dikes on the classification diagrams, and the gradation of oxides on the Harker diagrams suggest 40

that they may have come from the same parent magma.

Felsic Dikes and Sills

The most abundant dikes in the study area are felsic

in composition. Three petrographically distinct groups of east-west-trending dikes (quartz-orthoclase porphyritic rhyodacite, quartz rhyodacite porphyry, and reddish rhyodacite to rhyodacite porphyry), and a group of north to northeast-trending dikes and/or sills (granophyric· rhyodacite porphyry) crop out in the study area. The east-west-trending dikes are the youngest of the dikes

(less than 19 m.y.B.P.), while the north-trending dikes and/or sills may be older than the intermediate composition dikes (27.2 m.y.B.P.). A QAP plot of the normative minerals shows all rock samples to be rhyo dacites, except one reddish rhyodacite sample of rhyolite composition (Fig. 7a). All felsic dikes are subalkaline on the alkalis-silica diagram (Fig. Sa) and calc-alkaline on the AFJ-.1 diagram (Fig. 7b). These rocks lie in the high-K rhyolite field of the potash-silica diagram

(Fig. Sb). All rock samples are peraluminous (Fig. Sc).

Harker diagrams of major oxides (Fig. 6) indicate that these felsic dikes are generally similiar in chemical composition and may have come from the same parent magma (except the granophyric rhyodacite porphyry which are older). The chemical composition 41 ~ .

Figure 7a. Normative mineral QAP plot of

felsic dikes; (@) quartz-orthoclase porphyritic

rhyodacite, (•) quartz rhyodacite porphyry,

(~) reddish rhyodacite and rhyodacite porphyry,

(~) granophyric rhyodacite porphyry.

Figure 7b. AFM plot of felsic dikes; symbols

same as above. 42

F 43

of the quartz-orthoclase porphyritic rhyodacite dikes is close~t tothe chemical composition of the intermediate composition dikes and partially overlaps the biotite rhyodacite porphyry fields. This suggest a possible genetic link between the two dikes.

K-Ar AGE DATING

Four rock samples from the study area have been dated at the San Diego State University Geochronology

Lab utilizing K-Ar methods, and the results are presented in Table 3. These ages indicate a major pulse of igneous activity at the Oligocene-Miocene boundary.

Andesitic flows (27.7 m.y.B.P.) were closely followed by granophyric rhyodacite dikes and/or sills (27.2 m.y~ and the extrusion of rhyolitic dikes and breccias

(26.0 m.y.B.P.). The east-west-trending diking event followed during the Early to Middle Miocene. The Middle

Miocene age (19.4 m.y.B.P.) of the youngest(?) inter mediate-composition dike suggest that the earliest phase of east-west-trending diking occurred in the late-

Early Miocene.

LITHOLOGY

PRECAMBRIAN AND MESOZOIC-EARLY TERTIARY PLUTONIC ROCKS

General Statement

Basement rocks into which the Tertiary dike swarms TABLE 3: POTASSIUM-ARGON DATES ------ON IGNEOUS ROCKS FROM THE HOMER MOUNTAIN AREA Sample Rock Type Material Dated K 40Ar* 40Ar** Age*** number (wt %) (mole/g) (%) (m.y.)

HM-519 granophyric rhyodacite hornblende 0.89 4.22919 85 27.2 + 2.7 porphyry dike

HM-538 biotite rhyodacite biotite 7.25 24.53915 78 19.4 + 1.4 porphyry dike

HM-544 rhyodacite breccia whole rock 4.46 2.30988 86.6 26.0 + 1.7

HM-563 andesite flow whole rock 1.68 8.13009 32 27.7 + 1.4

-11 * moles/gram x 10 ** atmospheric argon *** lambda = 5.543 x 10 10 1 year

"'" "'" 45

intruded consist of a porphyritic granitoid gneiss of

probable Precambrian age, and two quartz monzonite plutons of probable Mesozoic age which may be related to the Late

Cretaceous Teutonia batholith of the New York Mountains (Hewett, 1956). The Precambrian granitoid gneiss generally occurs in relatively thin sections of country rock lying

just above the intrusive contacts. Only along the western flanks of Homer Mountain does .the granitoid gneiss have

substantial vertical thickness. The general impression is

that most of the study area is within the roof zone of the

Hesozoic plutons.

The Teutonia pluton is mostly a distinctive medium- to coarse-grained quartz monzonite with variable amounts of large white orthoclase phenocrysts. In the easternmost

study area there is a small pluton which differs from the

Teutonia pluton because of its uniform medium-grained, equigranular texture. This pluton is given the informal name Homer Mountain pluton. Occurring within 5 km to the west of the gradational contact between these two plutons are a number of randomly oriented mesocratic quartz monzonite aplite dikes. A second group of leucocratic quartz monzonite aplite dikes commonly occurs in northeast and northwest-trending conjugate sets, while some are sub parallel to the east-west-trending dike swarm of Cenozoic age.

Quartz veins are commonly found in the western portion of the study area. The larger veins are commonly cut by 46

the east-west-trending middle Tertiary dikes, but locally

small quartz veins intrude these same dikes. This cross

cutting relationship suggest that the veining and diking

are interrelated. Although the quartz veins and pegmatite

dikes, found in the eastern portion of the study area, are

not genetically related, they are discussed together in the

plutonic rock section because of their similar relative

ages. Both rocks intrude the Teutonia pluton and are cut

by the middle Tertiary dike swarm. This places them

temporally in the same relative time period, but no direct

evidence was found to relate the two.

A series of intermediate to felsic east-west-trending

dikes cuts across the Precambrian granitoid gneiss, Teutonia

pluton, and aplite dikes with equal intensity, but they do

not penetrate the Homer Mountain pluton except for a few

altered quartz andesites which are the oldest east-west

trending dikes. A small number of north-trending altered

quartz gabbro dikes crop out in the central portion of the

study area and appear to be the oldest dikes in the area

because they are cut by dikes of all other dike groups. Trending north and dipping east near the contact between

the Teutonia and Homer Mountain plutons are a series of

granophyric rhyodacite dikes and/or sills that are of

Oligocene age (27.2 m.y.B.P.) and are probably older than

the east-west-trending dikes. Several pre-Teutonia mylon

itic zones subparallel these granophyric dikes/sills and may have structurally controlled their emplacement. 47

Precambrian Granitoid Gneiss

The Teutonia pluton has intruded into the granitoid gneiss country rock of the study area. This country rock varies from dark-brown to brown in color, and is generally biotite rich. A porphyritic variety is the most common and has numerous purplish, large (2 to 8 em), euhedral orthoclase phenocrysts with 5-10% quartz and 25-40% biotite in the matrix. A less common augen gneiss has similiar mineralogy but the orthoclase phenocrysts are generally smaller and subrounded. Foliation within this unit is generally defined by a rough alignment of the large flat-lying orthoclase phenocrysts; nowhere is typical gneissic guartz-feldspathic layering well developed. Although previous authors have called this rock unit granitic (Bonham and others, 1960; Spencer and Turner, 1982), I consider this unit to be a coarse grained porphyritic granite to granitoid gneiss which is genetically more metamorphic than intrusive. The protolith of this unit may have been a granite, but the widespread occurrence of the tabular orthoclase in rough alignm~nt that locally subparallels rare mylonite zones suggest a metamorphic orgin for the foliations.

The mylonite crops out on the southern and western flanks of Homer Mountain (Plate 2} as rare 8- to 1~~ wide, north-trending, northeast-dipping zones. These 48

Q • zones occur only within the Precambrian gneiss, and they are truncated by the Teutonia pluton and Tertiary dikes.

Although these mylonite zones are narrow, and are exposed for lengths of only a few tens of meters between trunca tions by younger structures, they seem to have structural importance because the granophyric rhyodacite porphyry dikes/sills were intruded subparallel to the mylonite zones and the foliations in the granitoid gneiss are locally subparallel.

Similar Precambrian rocks have been described as far north of the study area as the Virgin Mountains (150 km; theGold Butte rapakivi pluton, Volborth, 1962), and as far south as the Big Maria Mountains (120 km; Hamilton,

1982). Mathis (1982) and Volborth (1962, 1973) describe the Precambrian rocks in the Newberry Mountains, located

13 km northeast of the study area, as rapakivi granites.

Mathis notes that the "perthitic microcline phenocrysts" are mantled by "fine-grained allotriomorphic quartz, microcline, and albitic plagioclase". The othoclase phenocrysts of the Homer Mountain porphyritic granitoid gneiss also have a thin mantle, but the cryptocrystalline texture hides the composition.

The absolute age of the Precambrian granitoid gneiss is somewhat hard to define, and age dating has not been published for nearby areas. Mathis (1982) suggests 49

that the rapakivi granites of the Newberry Mountains are part of the 1.4-1.5 b.y.B.P. rapakivi granite suite of

Silver and others (1977). Volbroth (1962) has dated the

Gold Butte rapakivi pluton by Rb-Sr methods at 1.06-

1.09 b.y.B.P. Finally, Burchfiel and Davis (1981) indicate that rocks of the Mojave Desert were deformed and metamorphosed about 1.7 b.y.B.P. The age of the

Precambrian granitoid gneiss at Horner Mountain probably ranges within the Precambrian Y era sometime between 1.06 and 1.7 b.y.B.P.

Teutonia Quartz Monzonite Pluton

The majority of the intrusive basement rock in the study area is a leucocratic, medium- to coarse-grained, hypidiornorphic-granular biotite quartz monzonite which commonly has up to 10% euhedral phenocrysts of white orthoclase. These large phenocrysts vary from 1 to 5 em in length. An equigranular variety is sirniliar to the groundmass of the porphyritic variety, and occurs locally near the contact with the Precambrian country rock. Biotite is the most abundant accessory mineral and varies in abundance from 3% to 5%. Muscovite

(sericite) is common along joints and is due to hydro thermal alteration. In thin section, minute laths of muscovite are a common alteration product of the feldspars, but some interstitial muscovite does occur, 50 ,, '

and may be primary.

Hewett (1956) considers the plutonic rocks of the

New York Mountains, Mid Hills, and Leiser Ray mine area

to belong to the Teutonia Quartz Monzonite batholith whose type area is the Cima Dome, located 50 km north

west of the study area. Hewett indicates that pink

orthoclase phenocrysts are characteristic of the Cima

Dome quartz monzonite, but that in the New York-Mid Hills

and Leiser Ray areas the orthoclase is white. More

recent studies by Beckerman and other (1982) have divided

the 3000 km 2 Teutonia batholithic complex of Hewett

into six plutons of Jurassic to Cretaceous age. The

eastern New York Mountains and Mid Hills areas are

considered to be underlain by a Cretaceous age Mid Hill

adamellite. This pluton has K-Ar ages ranging from

104.5 to 83.5 m.y.B.P., and a crosscutting north trending mylonitic zone yields a K-Ar age of 73.4 m.y.B.P.

(Beckerman and others, 1982).

The uniformity of the petrography and texture of the intrusive basement rock over the entire study area

(except the small Homer Mountain pluton discussed below) with the basement surrounding the Leiser Ray mine, and the petrographic and geochemical similarities with the basement in the New York-Mid Hills area indicate that the majority of the intrusive basement rocks in the

Homer Mountain area are part of the Teutonia Quartz 51

Monzonite batholithic complex or the Mid Hills Adamellite

of Beckerman and others (1982), and is probably of Late

Cretaceous age.

Homer Hountain Quartz Monzonite Pluton

On the eastern flank of Homer Mountain is a small

(4 km 2 ) pluton of white to tan, medium-grained, equi

granular, hypidiomorphic-granular, muscovite-biotite

bearing quartz monzonite which is given the informal

name of the Homer Hountain pluton. This pluton is

texturally different from the Teutonia pluton. There

are no large orthoclase phenocrysts, and the overall

color of the pluton is slightly browner due to an increase

in biotite which makes up from 3% to 10% of the rock. In

thin section there is about 2% muscovite. Host muscovite

grains appear to be primary because they form interstitial wavy laths, but some occur on altered feldspars and are

clearly secondary.

A distinction between the two plutons can also be

seen in the weathering patterns. The Teutonia typically weathers uniformly without developing any distinctive

jointing pattern. The Homer Mountain pluton, on the other hand, weathers to a bouldery surface developed on a distinctive, nearly perpendicular joint system.

The similarity between the geochemical character

istics of the Homer Mountain pluton and those of the 52

Teutonia pluton (Table 2, Table A-1, and Fig. 2) suggests

that these two plutons are genetically related. The

generally lower percentage of MgO, TiO, and FeO-total,

the slightly higher silica content, and the contact

relationships (see discussion of mesocratric aplite dikes)

all support the interpretation that the Homer Mountain

pluton is younger than, but comagmatic with, the Teutonia

pluton.

Leucocratic and Mesocratic Aplite Dikes

Two types of aplite dikes are common in the study area. The most common is a white to pinkish, fine- to medium-grained, allotriomorphic-granular quartz monzonite.

Accessory minerals include biotite (2%), sericite (<1 %), and opaques {<2%). The pinkish color appears to be due to the presence of weathered pyrite and is common in these dikes near the western mining area. The leucocratic aplite dikes generally occur in steeply dipping, northeast- northwest sets that are interpreted to be conjugate, with a minor number following the east-west-trending fractures. The strikes of the dikes concentrate at

The leucocratic aplite dikes are truncated by all other dikes, except the altered quartz gabbro dikes which they intrude. The relative age and the geochemical similarity with the Teutonia pluton (Table A-1 and Fig.2) suggest that the leucocratic aplite dikes were probably 53

Figure 8. Plots of leucrocratic aplite dikes:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal

area plot of 64 dike poles;

contour interval 2, 4, 6, and 8%.

Legend of patterns:

R\\:m:m:i 2% m 4% § 6% ~8% w 10% ~ 12% ffi]] 15% D 20% 54

(a)

N (b)

liM 55

intruded duri~g the late cooling phase of the Teutonia pluton.

A second, less common, type of aplite dike is a mesocratic, gray to dark-grey, fine-grained, allotrio morphic-granular, biotite-rich (25 to 40%) quartz monzonite. These dikes occur only on the eastern and southeastern flanks of Homer Mountian within about

5 km of the Homer Mountain pluton, but not within this pluton. They occur only in the Precambrian granitoid gneiss and Teutonia pluton. These mesocratic aplite dikes are generally randomly oriented with irregular thickness ranging from a few inches to several feet. The lack of these aplite dikes in the Homer Mountain pluton suggest that the Teutonia pluton is the older of the two plutons.

Quartz Veins and Pegmatite Dikes

In the western portion of the study area near the

Leiser Ray mine are an abundance of east-west- and northwest-trending quartz veins. These veins vary in thickness from a few inches up to 3m and have lengths of up to 0.2 km and 0.8 km, respectively. The quartz is generally milky white with only minor amounts of coarse grained minerals, mostly muscovite, and scattered finely disseminated copper and vanadate mineralization. Sericite is common along the edges of the dikes where alteration of the country rock has occurred. The northwest-trending 56 p •

quartz veins commonly occur in the vicinity of northwest trending faults. Northwest-trending quartz veins strike

from Nl0°w to N50°w, whereas east-west-trending veins vary in strike from N80°w to NB0°E (Fig. 9). All of the larger veins dip from 45° to 70° NE. The northwest trending faults have strikes that are similiar to the veins, but their dips are closer to vertical (see discus sion of faults). Weak steeply plunging slickensides on the east-west quartz vein at the Leiser Ray mine seem to indicate that some dip-slip movement has occurred along the vein. The copper mineralization is greatest where east-west-trending veins and northwest-trending veins and/or faults intersect.

A minor number of quartz veins and pegmatite dikes occur elsewhere in the study area. Northwest of Homer

Mountain there are several shallow shafts excavated at the intersection of east-trending and northwest-trending shears and quartz veins. About 2.5 km to the east a

"silver" mine has been excavated along a similiar shear/ vein system.

Other pegmatite dikes are common south of Homer

Mountain near the west-trending contact between the autochthon and the allochthonous detachment terrane.

These dikes are randomly oriented with very irregular contacts, unlike the quartz veins. They typically contain abundant coarse muscovite clusters and small 57

Figure 9. Plots of quartz veins:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt

equal-area plot of 77 vein

poles; contour interval 2,

4, 6, and 8%. Legend of

symbols on Figure 8. 58

N (b) 59

garnets. No geochemical analyses of the quartz veins or pegmatite dikes were made for this study. Geochemical analyses for economic mineralization have been made on rocks from around the Leiser Ray min~ and the results are discussed under the heading of economic geology.

Altered Crystalline Rocks of the Detached Terrane

On the eastern and southern flanks of Horner Mountain are a series of south- and east-dipping low-angle normal faults which may be genetically related to the detachment terranes of the lower Colorado River (Spencer and Turner,

1982). Lying structurally above a basal fault is a sequence of allochthonous rocks consisting of altered crystalline basement rock, a tilted angular conglomerate, and local volcanic breccia plugs. The allochthonous conglomerate and volcanic breccias will be discussed below under separate headings. The allochthonous crystalline rocks are highly fractured and have been affected by low-grade hydrothermal alteration. Chlorite and epidote are abundant, as are minor localized areas of iron and copper sulfide enrichment. The alloch thonous c~ystalline rocks appear to be part of the

Teutonia pluton because of the large orthoclase pheno crysts and a sirniliar chemical composition (Sample

HM-250). Precambrian granitoid gneiss is also a minor component of the allochthonous terrane. It is often 60

difficult to determine if an exposure of gneiss is part of the basement or of the overlying angular conglomerate, which is composed almost exclusively of the same granitoid gneiss. Only those exposures of Precambrian basement which are clearly not part of the overlying conglomerate unit which have been mapped as altered granitoid gneiss of the lower allochthonous plate (Plate 1).

The allochthonous crystalline rock was intruded prior to alteration, and probably prior to faulting, by a number of east-west-trending dikes of intermediate composition that appear to be related to the autochthonous altered quartz andesite dikes of Tertiary age. No other intrusions associated with the east-west-trending dikes are recognized in the allochthonous terrane. A similar singular occurrence of altered quartz andesite is found intruding the Homer Mountain pluton. The importance of this unique dike occurrence is presently unclear.

In the Whipple Mountains to the south of the study area, a "chlorite breccia" zone is commonly associated with the Whipple Mountain detachment fault (G.A. Davis and others, 1980). The altered crystalline rock at Homer

Mountain may be a similar feature. The timing of the formation of the chlorite breccia is thought to be before or syntectonicwith tie major period of detachment faulting (G.A. Davis and others, 1980; 1982). The 61

restriction of the alteration at Homer Mountain to only

the allochthonous rock likewise suggests that it is

genetically related to the detachment faulting. However, most or all of the alteration may have occurred prior to

the latest faulting because no alteration is found in the autochthonous rock, as is the case in the Whipple Mountains

(G.A. Davis and others, 1980).

TERTIARY VOLCANIC FLOWS AND BRECCIAS

Mafic- and Intermediate-Composition Flows

Mafic to intermediate volcanic flows are most exten sive in the western portion of the study area, but a small number of exposures occur in the eastern detachment terrane (Plate 2). The western flows are generally of intermediate-composition varying from andesite to quartz andesite (Table A-1), whereas the eastern flows are more mafic varying from basalt to andesite. These flows are bluish-grey to dark-grey in color and aphanitic to scoria~ ceous in texture. Some of the andesitic lavas contain up to 10% medium-grained olivine phenocrysts, but most do not contain any visible phenocrysts. In thin section the groundmass is commonly felty but pilotaxitic textures are not uncommon. The vesicles in some scoriaceous varieties have been partially filled with zeolite.

In the western portion of the study area these ande sitic flows cap the highest hills, such as Signal Hill 62 " '

and Billie Mountain, and are southwest extensions of the southern Piute Range volcancis (Plate 1) . Lying generally at the base of and sometimes interbedded with the andesite flows is a red-brown cinder and mud conglomerate or lahar.

This lahar unit is commonly less than 5 m thick, but locally it thickens to as much as 15 m.

Andesitic flow layering in the western study area generally dips at angles less than 35°NW (Fig. 10). Flows appear to be gently folded along broad northwest-trending fold axes. It is unclear whether these gentle folds are primary flow structures, or if they result from later faulting or arching. On the eastern side of the southwest

Piute Range, these andesitic flows cap several closely spaced hills and dip 25° to 45° to the east and southeast.

These dips are opposite to the westerly dips of the south ern Piute Range and may reflect north- to northwest-trend ing antiformal warping {see discussion of foliations) .

Hewett (1956) may have used these southward dips as the basis for his interpretation that an anticline oriented

N60°E trends across the northern study area. No evidence for this fold was found in the dips of the flow layers.

Along the major unnamed southward draining wash in the western portion of the study area there are locally several northwest-striking, near vertical flow layers which rapidly fold over into shallow dipping layers.

These zones may be feeder dikes or flow channels. They 63

Figure 10. Plot of andesite and quartz andesite

flow layering: lower hemisphere

Schmidt equal-area plot of 50 layering

poles; contour interval 2, 4, 6, 10, and

12%. Legend of symbols on Figure 8. 64 65

trend subparallel to the major northwest striking en echelon

fault zone, yet no major mafic dikes were observed to have

northwest strikes. Several northwest-trending high-angle

faults cut the western flows.

A sample of the western andesite {HM-563) has yielded

a Late Oligocene K-Ar age of 27.7 m.y.B.P. {Table 3). No dates on the eastern flows of mafic to intermediate

composition were made. However, Spencer {1983) has reported a K-Ar age of 14.6 rn.y.B.P. for basalts of the eastern Horner and Sacramento Mountains. This would suggest

that the western and eastern flows are not genetically

related. Nevertheless these intrusives have been mapped as one unit because of a K-Ar age date take from a

seemingly structurally related eastern rhyolitic breccia which partially surrounds a small mafic extrusion. This breccia has likewise yielded a Late Oligocene age {see discussion of eastern rhyolitic breccia).

Eastern Rhyolitic Dikes and Breccias

In the southeastern corner of the study area and extending south beyond the map boundary are a number of volcanic breccias and radiating fine-grained dikes. The breccias are generally red-brown in color, vary in compo sition from rhyodacite to rhyolite {Table A-1), and 66

consists mostly of angular lapilli- to block-sized pumiceous volcanic fragments. This breccia occurs in irregular shaped plugs or domes which range in diameter from 60 to

600 rn. The radiating dikes are white, fine-grained rhyolties. Both of these volcanic rocks are exposed only within the allochthonous detached terranes and mostly within the upper plate conglomerates (Plate 1). These upper plate conglomerates are poorly bedded and therefore it is not clear whether the whitish rhyolite dikes paral lel or crosscut bedding. It is clear that the breccias have intruded the detached terrane and some of the white rhyolite dikes do radiate outward from them suggesting that they are intrusive.

A sample of the rhyolitic breccia (HM-544) has yielded a Late Oligocene K-Ar age of 26.0 rn.y.B.P. (Table 3).

This date conflicts with the 15 to 18 rn.y.B.P. K-Ar ages reported by Spencer (1983) for ash-flow tuff and basalt interbeds in the allochthonous conglomerates of the

Sacramento and Horner Mountain areas. My data suggest that the allochthounous conglomerate at Horner Mountain is no younger than Oligocene age because these Late Oligocene rhyolitic breccias intrude the upper portions of the unit.

TERTIARY HYPABYSSAL DIKES

General Statement

Trending generally east-west across the central 67

portion of the study area is a series of seven sets of

dikes which are temporally and compositionally related.

Trending northward in the central and easternmost portions

of the study area, respectively, are two unrelated groups

of mafic and felsic composition dikes which are older than

the east-west-trending dikes. The east-west-trending

dikes can be divided into two general groups of interme

diate and felsic composition based on their geochemical

similarities and their relative ages of emplacement.

All of the east-west-trending dikes .appear to have

intruded along a pervasive high-angle joint system.

These dikes have extended the basement in a north-south

direction from 5% to as much as 40% in the central portion~

with an average of about 20%. The dips of these dikes

form a complex crosscutting pattern when viewed in cross

section (Plate 2), but a majority of the younger felsic

dikes generally fan inward with depth, particularly at

Homer Mountain. The dike pattern is further complicated

in the western portion of the study area by multiple

periods of slip on a northwest-trending en echelon system

of faults (see discussion of faulting). Economically

submarginal copper-molybdenum-tungsten mineralization associated with quartz veins appears to be related to the emplacement of the dikes (see discussion of economic geologyl

The following discussion is organized according to

the relative ages of the dikes based mostly on field 68

mapping and two K-Ar age dates (Table 3) • Therefore the granophyric rhyodacite dikes are discussed after the oldest dikes of intermediate composition because their K-Ar age suggest that they are older than the remaining east west-trending dikes.

Altered Quartz Gabbro Dikes

In the central portion of the study area are a small number of widely scattered, steeply dipping, north- to ~- northwest-trending, black to purplish-black, coarse-graineq subophitic altered quartz gabbro dikes. The largest of these dikes is about 60 m wide and 1 km long. These dikes occur mostly within the Precambrian granitoid gneiss. A small number of gabbro dikes occur in the northwestern portion of the study area within the Teutonia pluton and are adjacent to a small exposure of Precambrian gneiss.

These dikes may be xenoliths rather than intrusions.

Individual dikes are generally coarser grained in the central portions and become fine grained near their con tacts with the country rock. Contacts with the basement are generally sharp, but they have been displaced by numerous younger intrusives and shears. In thin section primary pyroxene grains have been totally replaced by minute laths of green hornblende. Plagioclase grains are severly altered, but a few relict grains reveal composi tions ranging from An 44 to An 56 . Apatite is very common. 69 v .

The age of these dikes is unclear. The small number of gabbroic dikes which appear to have intruded the

Teutonia pluton in the northwest portion of the area suggest that these dikes are of early Tertiary age.

However, if these northwestern dikes are xenoliths, then they may be as old as Precambrian. Except for the granophyric rhyodacite dikes which strike subparallel to them, the gabbroic dikes are intruded by all other dike groups which indicates that they are no younger than Early

Miocene.

Altered Quartz Andesite Dikes

The intrusion of the plutonic rocks and the altered quartz gabbro dikes was followed by a major episode of intrusion of dark-grey to greenish-grey, fine- to medium grainec quartz andesite dikes along a steeply dipping, generally east-west-trending joint system. These dikes are commonly aphanitic, but coarse varieties have medium-grain ed laths of plagioclase and globular hornblende and biotite, that together make up as much as 10% of the rock. These dikes strike between N70°W and N80°E, and they dip more steeply than 70° both to the north and south (Fig. 11).

These dikes vary in thickness from 0.3 rn to about 6 m.

Their exposures are commonly only a few hundred feet long, but some extend up to 1 km before being truncated by younger dikes. These dikes are commonly found adjacent to 70

Figure 11. Plots of mafic and intermediate dikes:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal-area

plot of 84 dike poles; contour

interval 2, 4, 8, 12, and 15%.

Legend of symbols on Figure 8. 71

N (b)

+ 72

and between younger dikes which appear to have intruded along the same joint system. These intermediate dikes weather easily. They show extensive alteration. Notably, they are the only dikes which unequivocally intrude the

Homer Mountain pluton and the allochthonous crystalline basement.

Their truncation by other intermediate- and felsic composition dikes indicates that they are the oldest of the east-west-trending dikes. As discussed in the geochem ical analyses section, their dissimilarity with other intermediate-composition dikes suggest that they may repre sent a separate older intrusive phase. Their age is probably Early Miocene, but their exposure east of and possibly structurally above the Late Oligocene age grano phyric rhyodacite sills suggest that they may be early

Tertiary in age.

Granophyric Rhyodacite Porphyry Dikes and/or Sills

On the eastern flank of Homer Mountain are a number of north- to northeast-trending, east-dipping, light-grey to light-brown, coarse-grained granophyric rhyodacite porphyry dikes and/or sills. In hand specimen these dikes appear similiar to the younger quartz rhyodacite porphyry dikes, but in thin section the groundmass consists of distinctive microgranophyric grains. Phenocrysts comprise up to 40% of the rock, and consist of subhedral quartz, 73

orthoclase and plagioclase (An to An ). These dikes 22 32 have highly variable widths ranging from a few meters to tens of meters. Their contacts with wall rocks are both linear and sinuous, indicating that they may have intruded more as sills than as dikes following linear fractures.

They strike mostly to the north, dip from 10° to 50° to the east, and commonly have been eroded to form dip slopes. A sample (HM-519) of the granophyric dikes has yielded a

Late Oligocene K-Ar age of 27.2 m.y.B.P. (Table 3).

The westernmost dike/sill appears to have controlled the structure of the intrusion of the other intermediate and felsic, dikes, except for the altered quartz andesites because none of the younger dikes cross the granophyric dikes. This structural control may reflect in part the fact that the granophyric dikes are subparallel to the east dipping mylonite zones (see discussion of foliations).

These granophyric dikes may have been intruded during or soon after detachment faulting because: (1) their age is within the Late Oligocene time of the deposition of the detached eastern comglomerates which are thought to have been deposited in faulted basins; and (2) they have been displaced by the detachment faults (see discussion of low-angle normal faulting).

Quartz Andesite and Quartz Andesite Porphyry Dikes

In the western portion of the study area are a number 74

of light-grey to brown quartz andesite dikes. These dikes vary in mineralogy and texture, ranging from aphanitic to porphyritic with up to 20% phenocrysts. The porphyritic varieties have from 5 to 15% medium- to coarse-grained plagioclase phenocrysts (An28 to An 48 ), minor amounts of globular orthoclase and biotite, or hornblende and biotite

(2%). Quartz generally occurs as interstitial blebs but there are rare subhedral embayed grains up to 12 mm long.

These dikes occur mostly south of the Leiser Ray mine in the western portion of the study area. They strike from N70°W to N80°E and dip from 60° to vertical, mostly to the south (Fig. 11). These dikes are more continous than the altered quartz andesites, and they commonly extend up to 1.5 km.

These quartz andesite dikes are the oldest of a prob able sequence of three dikes sets of intermediate composi tion ti~tare interpreted to be geochemically, structurally, and temporally related (see discussion of geochemical anal yses). The age of these quartz andesite porphyry dikes is probably similar, but slightly older, than the Early

Miocene age, 19.4 m.y.B.P., of the biotite rhyodacite dikes. These later rhyodacite dikes are the youngest of the three intermediate composition dikes.

Hornblende Quartz Andesite Porphyry Dikes

In the north-central portion of the study area are a 76

small number of light- to dark-greenish grey, hornblende

rich quartz andesite porphyry dikes. They are distinctive

in that they contain 5 to 15% medium- to coarse-grained

laths of hornblende in an aphanitic groundmass. The hornblende laths range in length from 1 to 12mm. In thin

section the groundmass exhibits microphenocrysts of plagio clase (An ), a minor amount of microcline and globular 35 hornblende and biotite. These dikes have strikes similar

to the other intermediate composition dikes (Fig. 11).

They vary from 6 to 15 m in width and have discontinuously exposed lengths of up to 1.5 km.

The age of these dikes is not clear because no dis tinct crosscutting relationships were found with the bio tite rhyodacite porphyry dikes with which they have a similar Early Miocene age. Their relative age is based on a weak argument that the intermediate composition dikes represent an ideally differentiated parent magma, and the hornblende andesite porphyry dikes are the middle number.

Biotite Rhyodacite Porphyry Dikes

In the north-central and northeast portions of the study area, beneath the crest of Homer Mountain, are several east-west-trending, light- to dark-grey, biotite rich rhyodacite porphyry dikes. These dikes weather to a distinctive "pox-marked" desert varnished surface which is 77

Q •

caused by the weathering on the biotite phenocrysts that make up to 20% of the rock. Medium- to coarse-grained, subhedral to euhedral plagioclase phenocrysts occur in some varieties of these dikes and comprise up to 20% of the phenocrysts. In thin section the groundrnass is typic ally 70 to 80% of the rock and consists mostly of micro granophyric grains with minor interstitial quartz. Plagio clase phenocrysts vary in composition from An to An . 22 34 Biotite phenocrysts are commonly partially rimmed by hornblende. Alteration of the biotite to chlorite is common.

These dikes have strikes similar to the other inter mediate-composition dikes, ranging from N70°W to N80°E, and they dip at angles generally greater than 65° both to the north and south forming an upward convergent fan beneath Horner Mountain (Plate 2). Differential weathering of these dikes creates prominent high, narrow spine-like ridges. This differs from the other intermediate-composi tion dikes which weather easily.

A sample of the biotite rhyodacite porphyry dikes

(HM-538) taken from Horner Mountain yields an Early Miocene

K-Ar age of 19.4 rn.y.B.P. (Table 3). These dikes are assumed to be the last group of the intermediate-composi tion dikes based on their geochemical similarities (see discussion of geochemical analyses). 78

Quartz-Orthoclase Porphyritic Rhyodacite Dikes

In the western portion of the study area are a rela-

tively few light-brown, coarse-grained, quartz- and ortho-

clase-rich, porphyritic rhyodacite dikes. These dikes

consist of 40 to 60% phenocrysts which gives them a

coarse-grained almost granitic texture, but because of

their aphanitic groundmass they have been given a volcanic

rock name. Quartz phenocrysts are medium- to coarse-

grained, subhedral to euhedral, commonly embayed. These

quartz phenocrysts make up to 15% of the rock. Feldspar

phenocrysts consist of up to 15% coarse-grained subhedral

plagioclase (An to An ) and from 20 to 30% orthoclase. 25 39 These dikes generally strike from N60°W to E-W and

dip at angles greater than 75° both to the north and south

forming an upward convergent fan (Fig. 12 and Plate 2).

The dikes are typically 15 to 30 m wide and are exposed

for lengths of up to 900 m. They weather fairly easily and form rounded, low-lying ridges.

The quartz-orthoclase porphyritic rhyodacite dikes

are probably the oldest of the group of felsic dikes (see discussion of geochemical analyses) . They are truncated by the reddish rhyodacite porphyry dikes and appear to be cut by the quartz rhyodacite porphyry dikes. Their age is probably Early to Middle Miocene and they are probably younger than the biotite rhyodacite dikes (19.4 m.y.B.P) which are the youngest of the intermediate composition 79

Figure 12. Plots of felsic dikes;

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal

area plot of 85 dike poles;

contour interval 2, 6, 10, 15,

and 20%;

Legend of symbols on Figure 8. 80

N (b) 81

dikes.

Quartz Rhyodacite Porphyry Dikes

The most abundant dikes in the Homer Mountain area are a texturally varied group of gray to light-brown, medium to coarse-grained, quartz-rich rhyodacite porphyries. The percentage of phenocrysts in these dikes is higly vari able. In general, the phenocrysts comprise less than 30% of the rock, with up to 10% being coarse-grained, doubly terminated, embayed quartz phenocrysts which are distinc tive of this group of dikes. Other phenocrysts include up to 10% subhedral to euhedral, coarse-grained orthoclase, up to 7% plagioclase (An to An }, and up to 3% biotite. 10 34 The dikes vary in width from 3 to 60 m, and are exposed for lengths ranging from 30m to up to 2.5 km. These dikes form prominent, narrow ridges which in places are as much as 60 m above the surrounding terrane.

These dikes generally strike form N60°w to N90°W and dip at angles steeper than 65° {Fig 12}. Their dips are mostly to the south in the western portion of the study area, and they form a downward convergent fan at Homer

Mountain (Plate 2).

Hewett (1956) has described the quartz monzonite porphyry dike which surround the Leiser Ray mjne as being similiar to dikes in the New York Mountains. Y.N. Agyakwa

(personnal communication, 1983) indicates that a number of 82

porphyritic quartz monzonite dikes occur in the central

New York Mountains, and that they are associated with the hydrothermal mineralization there. A radiometric age has not been determined for the central New York Mountain dikes. Quartz rhyodacite porphyry dikes at Homer Mountain do not appear to be as old as the Late Cretaceous (71.7 m.y.B.P.) quartz monzonite dikes of the eastern New York

Mountains (Burchfiel and Davis 1977) because the Homer

Mountain dikes cross cut Early Miocene (19.4 m.y.B.P.) biotite rhyodacite dikes. Spencer has obtained K-Ar age dates which range from 19 to 16 m.y.B.P. for the east-west- trending dikes at Homer and Sacramento mountains. The quartz monzonite porphyry dikes probably have a similar

Early to Middle Miocene age.

Reddish Rhyodacite and Rhyodacite Porphyry Dikes

A minor number of reddish-brown, fine-grained rhyodacite and medium- to coarse-grained rhyodacite porphyry dikes occur mostly in the western portion of the study area near the Leiser Ray mine. These porphyry dikes typically have 5 to 15% subhedral orthoclase and quartz phenocrysts.

They strike within the range of the other felsic dikes,

N60°W to N90°W, but in general their strikes are more towards the northwest (Fig. 12). They dip at steep angles, greater than 70,0 mostly to the south (Plate 2).

These dikes vary in width from 6 m to about 31 m 83

and are exposed for distances up to 915 rn. Around the

Leiser Ray mine the reddish-brown color of these dikes can be mistaken for zones of intense pyrite alteration which is common in the Teutonia pluton basement.

The reddish rhyodacite dikes appear to be the young~ est of the generally west-trending group of felsic dikes.

Their more northwesterly trend causes them to crosscut many of the other felsic dikes. Northwest of the Leiser

Ray mine several of these dikes change trend for lshort distances to become subparallel to the northwesterly faults.

TERTIARY SEDIMENTARY UNITS

Horner Mountain Conglomerate

In the southeastern portion of the study area, the uppermost detachment plate consists of brown to reddish brown, angular, cobble- to boulder-sized conglomerate with minor coarse-grained sandstone and rare water-laid tuff interbeds. This unit is given the informal name of the

Horner Mountain conglomerate for this report. Most clasts in the conglomerate are weathered Precambrian porphyritic granitoid gneiss similar to that found in the autochthon.

The western portions of this unit which are in contact with the lower detachment plate have very angular clasts and appear to be broken and shattered Precambrian basement against which the sedimentary rocks to the east were 84

deposited. The Homer Mountain conglomerate is poorly bedded, but that bedding which can be identified is north to northeast striking and dips 10° to 55°NW, or back into the detachment surface (Plate 1). To the east, or down section, from the basal detachment surface, the clasts of this unit become finer grained and less angular with sand and clay becoming more abundant in the matrix. Several locally extensive reddish-brown, coarse-grained sandstone and white water-laid tuff units are present. These units are not laterally continuous. The bedding is upright to the west based on ripups and grading within these sandy units.

The Homer Mountain conglomerate probably is correla tive with similar red-brown sedimentary deposits on the east side of Piute Valley at the base of the Dead Moun- tains. In the northern Sacramento Mountains are two similar allochthonous sedimentary units (Spencer and

Turner, 1983) which consist of fault-bounded sequences of interbedded sandstone in the uppermost unit. Spencer

(1983) has dated overlying basalts as Middle Miocene

(14.6 m.y.B.P.), and he suggests that this is the youngest possible age for the conglomerate units at Homer and

Sacramento mountains. The age of the Homer Mountain conglomerate is probably Oligocene. This unit is older than the Late Oligocene (26.0 m.y.B.P.) rhyolitic breccias which have intruded it. The rhyolitic breccias have 85

intruded the structurally highest, and therefore youngest, portion of the Homer Mountain conglomerate. This conglo merate is temporally correlative with the Gene Canyon

Formation in the Whipple-Rawhide-Buckskin mountains

(G.A. Davis and others, 1980) . The Gene Canyon Formation was deposited in local basins and consists of coarse grained volcanic and clastic rocks which are thought to be derived in part from syntectonic fault scarps. The coarseness, angularity, and uniformity of clast composi tion suggest that the Homer Mountain conglomerate may have been deposited in a similar tectonic environment. The abundance of clasts of Precambrian granitoid gneiss sug sests a structurally high source rock which the Cretaceous granitic plutons did not intrude.

QUATERNARY SEDIMENTARY UNITS

General Statement

In the Homer Mountain area there are at least three sedimentary units of Quaternary age (Plate 1). The ages of these units can only be estimated based on geomorphic features. An attempt is made below to determine relative ages for the alluvial units in the Homer Mountain area by correlating then with ages of similar units determined in studies of southeastern Mojave Desert geomorphology by

Shlemon and Purcell {1976). These authors have developed a four part alluvial classification scheme of 86

which the three youngest classes (Q , 0 , and Q ) appear 1 2 3 to be applicable to the alluvial units in the Horner Moun- tain area.

The youngest class (Q ) is that of modern washes. 1 The next oldest class is that of the "active fans" (Q ) 2 which is a unit of active sediment transport. These fans have not developed a desert pavement, patina is lacking, and the caliche horizons are poorly developed.

The third alluvial class (Q ) has three subdivisions, 3 all of which are considered to be non-active. All three subdivisions are characterized by moderate to well developed desert pavement and an indurated carbonate layer corn- rnonly at a depth of 120 ern. The youngest subgroup is found within a few feet above the active fans and has a relative age of 15,000 to 35,000 y.B.P. The oldest sub group has a well developed paved and strongly patinated surface which is preserved only along narrow remnant divides and has a relative age of 200,000 to 500,000y.B.P.

Consolidated Alluvium (Qc)

The oldest Quaternary unit in the Horner Mountain area is a poorly to moderately well-bedded, light- to dark-brown, moderately well-consolidated, pebble-to-cobble conglomerate with sandstone interbeds. This unit occurs throughout the study area as an erosional remnant, standing 2 to 8 rn above the modern alluvium. This unit may 87

consist of fans ofseveralages, but it is here discussed as one unit.

In the western part of the study area this unit is distinguished by its deeply eroded surface. There is little remaining of the original fan surface which is now separated by wide and deep gullies that lie between elongated well-rounded ridges. Caliche is commonly well developed within the upper 1 m and forms a hard pan in some areas. Along the western edges of the volcanic capped hills north of Signal Hill is a flat-lying, well paved and patinated alluvial surface that covers a wide bench. This surface is about 6 m above the surrounding consolidated alluvium, and it may be a remnant of the original fan surface. A 0.3- to 1-m-thick water-laid tuff is interbedded with this unit, and crops out in the slope of the modern wash just north of Signal Hill. This tuff dips about 5°SW away from the volcanic-capped hills adjacent to the north.

In the detachment terrance of the eastern part of the study area, there is another consolidated alluvial unit which has slightly different characteristics than those in the west. This eastern unit also has a broad, well paved, and moderately well-patinated fan surface, but it has only been slightly eroded, except in modern washes.

Caliche development is similar to the western area, except hard pan in not common. This difference in the degree of 88

erosion of the fan surface and the development of caliche indicates that the eastern fans are younger than those in the west.

In the north-central portions of the detachment ter rane the consolidated conglomerate fans have developed at the base of Homer Mountain near the basal detachment fault which the fans overlap. These fans extend eastward from the basal detachment fault for up to 1.6 km within a down dropped block, or "head graben", before terminating against a linear north-trending uplifted block of lower plate altered crystalline rock. East of this up~ifted block the fan surface begins again in a narrow "trough" and extends eastward for about 300 to 450 m before termi nating against another 1.6-km-wide uplifted block of altered crystalline rock.

The narrow "trough" and "head graben" extend for about 2.5 km in a north-south direction, widening to the south. The "head graben" fans are underlain1 by alter ed crystalline rock. Cropping out in the "trough" are patches of Homer Mountain conglomerate which apparently under~ies the alluvium and which is elsewhere known to be in fault contact with the underlying altered crystalline rock.

The fans within the "trough" coalesce into modern wash channels which have cut deeply through the fan surface. This suggest that the trough has not been 89 Q •

actively subsiding since the deposition of the upper fan surface. The age of the soils within the trough and head graben should be as old as the movement that formed these features, unless uplift has caused erosion or prevented deposition of older soils. There are no older alluvial units underlying these fans. Therefore it can be assumed that the grabens formed sometime near the beginning of fan development.

If the rate of fan development at Homer Mountain is considered similar to that found in the southeastern

Mojave Desert by Shlemon and Purcell (1976), then the age of this oldest consolidated alluvial unit is similar to their 0 3 class and ranges from 15,000 Y·B.P. to possibly as old as 500,000 y.m.B.P.

Older Alluvium (Qoal)

The most areally extensive Quaternary unit in the

Homer Mountain region is a poorly consolidated, light- to yellow-brown, sandy to gravelly older alluvium. This unit does not have a well developed desert pavement, is poorly patinated, and has only minor caliche development. This unit occurs as slightly incised coalescing fans which are dissected by modern washes and shallow gullies. If the

Homer Mountain area can be correlated with the fans of the southeastern Mojave Desert, then these older fans are probably similar in age to the fans of Shlemon and o2 90

Purcell (1976) and are younger than 15,000 y.B.P.

Recent Alluvium (Qal)

Recent alluvium is the least extensive Quaternary unit and occurs mostly in narrow sand- and silt-filled washes or deep channels. The largest modern wash bounds the western edge of the region and drains Lanfair Valley.

In the northeast portion of the study area, the modern washes donot maintain their narrow channels and seemingly die out in the valley. In the detachment terrane the modern washes cut channels up to 6 m deep into the con solidated alluvium which indicates a recent lowering of the stream's base level. This unit is correlative with the o1 class of Shlemon and Purcell {1976).

STRUCTURES

FOLIATIONS AND MYLONITIC ZONES

Foliation is restricted to the Precambrian granitoid gneiss of Homer Mountain. The foliation is poorly to moderately well developed and is defined by the subparallel alignment of large tabular orthoclase phenocrysts within a matrix of coarse-grained biotite and interstitial quartz.

Locally the feldspars coalesce to form short continuous feldspathic layers. Foliation is bestdevelopedat Homer

Mountain and becomes less defined toward the contact with the Teutonia pluton. Foliations vary in strike from Nl0°W 91

" '

to N30°E with dips that concentrate around 25°NE (Fig. 13).

These foliations define on the stereonet a girdle that is suggestive of north-trending open folds which are not apparent in outcrops.

The girdle may in fact result from tilting of folia tion in fault blocks. However, data from surrounding area suggest that the girdle results from a regional arching of the Homer Mountain area. Hewett (1956) has described the foliations to the west in Precambrian gneiss of the

Vontrigger Hills as oriented about N20°W, 60°sw. The andesitic flows which form thin caps on the western hills dip at shallow to moderate angles to the west, except the northeasternmost hills which dip to the east. These andesitic flows are cut by several northwest-trending faults which locally appear to have had normal-slip with down-to-the-west displacement. The detachment terrane in the eastern portion on the study area has moved eastward off of Horner Mountain. The origin of orientation of these structures may have resulted from a regional upwarp about 15 to 20 krn wide along a north- to northwest-trend ing axis. This may be an extension of the middle Tertiary upwarping proposed for the Whipple-Chemehuevi-Sacramento mountains to the southwest (G.A. Davis and others, 1980;

John, 1982; McClelland, 1982).

Several narrow, laterallydiscontinuous mylonitic zones occur within the Precambrian gneiss and are subparal- 92

Figure 13. Plot of foliation and mylonite lineation:

lower hemisphere Schmidt equal-area plot

of 83 gneiss foliation poles; 7 mylonite

foliation poles, and 12 lineations within

mylonite; contour interval 2, 4, 6, and

8%. Legend of symbols on Figure 8. 93

~ :lln•etlona e pol• of lllylonlte t_!)llation 94

lel to the foliation in the southern and western flanks of

Homer Mountain. These mylonitic zones are terminated by either the Teutonia pluton or Tertiary dikes which suggest that they are pre-Late Cretaceous in age. Lineation with in these mylonitic zones is defined by poorly developed small rods of quartz. The lineations generally plunge less than 30° to the northeast with a minor number plung ing to the southeast (Fig. 13). The mylonite lacks well developed folds, therefore it can not be determined if the lineations formed parallel to or perpendicular to the direction of shearing and/or extension within the mylonite.

However, these lineations are parallel to those of Whipple

Mountain which are thought to be the result of early to middle Tertiary northeast-southwest-directed extension

(G.A. Davis and others, 1980). The quartz rods are typically several inches long and only a fraction of an inch thick. It is interesting that the foliations of mylonitic zones are subparallel to the foliations developed by large orthoclase phenocrysts in the Precambrian gneiss.

Their similar orientation suggests that they may be gene tically related. No mylonite zones were found in the detachment terrane.

JOINTS

East-West Jointing

The predominant joint system in the Horner Mountain 95

area trends generally east-west and dips at high-angles, mostly to the south. The joints strike N85°W ± 25° with dips concentrating around 80°SW (Fig. 14). The joints are subparallel to the numerous middle Tertiary dikes and appear to have controlled the orientations of the dikes and quartz veins. These joints are probably tensional in orgin. One hypothesis for the formation of the east west-trending fractures is illustrated in Figure 15a

(joint set no. 3). These fractures may have formed paral lel to major east-west-oriented fold axes (Carlisle and others, 1982).

An east-west-trending fracture system probably existed when the quartz veins were emplaced. Short east west-trending veins are commonly terminated on their western ends by northwest-trending veins and/or faults. I interpret the east-west-trending veins as having been em placed into a number of large pinnate fractures or feather joints created by movement along a northwest-trending left slip fault system (Fig. 15b). I favor this model because northwest-trending veins are subparallel to a major fault system and the short east-west-trending veins are commonly terminated on their western ends by those northwest-trend ing faults. However, no conclusive evidence for left-slip on the fault system was found. Although I favor the later model for the emplacement of the quartz veins, I support the model of the development of east-west-trending frac tures parallel to regional fold axes or axes of uplift. 96 0 '

Figure 14. Plots of east-west-trending joints:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal-area

plot of 127 joint poles; contour

interval 2, 4, 6, and 8%. Legend

of symbols on FigureS. 97 0 '

(b)

+ 98

Figure 15. {a) Joint sets commonly found in

folded rocks: (1) and {4) are

conjugate joint sets with intersecting

poles either horizontal or inclined,

similar to fractures containing

leucocratic aplite dikes at Homer

Mountain; (3) are radial joints parallel

to fold axis, similar to east-west

trending joints at Homer Mountain (after

Hobbs and others, 1976, Figure 7.4).

(b) Pinnate fractures or feather

joints; arrow shows sense of movement;

acute angle between fault and joints

points towards direction of movement;

similar origin is proposed for east

west-trending quartz veins at Leiser

Ray mine (after Hobbs and others, 1976,

Figure 7.6). 99

(a)

(b) 100

Development of the regional east-west-trending joints and the conjugate northwest-northeast-trending joints can be explained by a model of an east-west elongated

Teutonia pluton progressively uplifting as it intruded.

A conjugate set of fractures oriented about a vertical plane could be formed and be intruded by aplite dikes during the final phase of pluton crystallization (Fig. lSa, joint set no. 2). Continued uplift like that common during the early Tertiary would cause a tensional radial jointing pattern to form parallel to the elongation of the pluton (Fig. lSa, joint set no. 3). This model is not inconsistant with a regional least principle stress orientation of north-south which probably existed during the east- to northeast-directed compression proposed for the Laramide orogeny.

Spencer (1982) has proposed an alternate uplift model for the east-west jointing and subparallel intrusion of dikes at Homer Mountain. He suggests that northeast trending "decompressional folds" were created when denuda tion during the early Tertiary decreased the vertical compressive stresses. Northeast-trending folds were created when regional elongation in a northwest direction was constrained between two "fixed" end points. The folding caused fracturing to occur parallel to the north east-oriented fold axes. 101

North-Trending Secondary Jointing

Exposed throughout the Homer Mountain area is a second set of crosscutting joints and fractures. These joints strike N5°E + 20° with dips that concentrate around 35°SE (Fig. 16). These crosscutting joints are rarely intruded by dike material which suggests that they are younger than the diking event. They are seldom so closely spaced enough to dominate over the east-west trending jointing. These north-trending joints may have formed by any or all of the following methods (a) as extensional joints formed perpendicular to an east-west axis of the Teutonia pluton (Fig. 15a, joint set no. 1);

(b) in response to northwest and northeast faulting; (c) as radial jointing formed parallel to a north oriented regional arch (see discussion of foliations); (d) as shear joints whose movement was syntectonic with the eastern detachment and high-angle faulting.

FAULTING

Northwest- and Northeast-Trending Faults

A major northwest-trending system of en echelon faults cuts the plutonic and gneissic basement in the western portion of the study area. This fault system appears to have had multiple periods of movement during the Tertiary.

In the northern and central portions of the study area are a small number of short, northeast-trending faults whose 102

Figure 16. Plots of north-trending secondary

joints:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal-area

plot of 124 joint poles; contour

interval 2, 4, 6, and 8%. Legend

on Figure 8. 103

Q '

N 104

age is similar to the northwest-trending faults, but evi dence of multiple movements on these faults can not be discerned.

The northwest-trending faults strike N40°W + 10° and dip generally greater than 70°NE (Fig. 17). This system of faults is associated with the zones of greatest mineral ization. East-west-trending quartz veins are truncated on the .west by northwest-trending faults and/or veins indicating a possible younger phase of faulting. North west-trending veins are cut by near vertical northwest trending shears which adds to the evidence for a younger phase of faulting. This second phase of faulting began after the extrusion of the Late Oligocene andesitic flows

(27.7 m.y.B.P.) which are offset by these younger faults and probably after the intrusion of the east-west-trending felsic dikes which are likewise offset. Reddish rhyoda cite porphyry dikes appear to be right-laterally offset for up to 150 m, while the andesitic flows commonly are offset with apparent down-to-the-west slip.

The crosscutting northeast-trending faults which are exposed in the central portions of the study area appear to have had less movement than the northwest-trending faults because they generally do not offset the northwest trending faults, are less numerous, and are shorter in length, commonly less than 2 km long. The age of these northeast-trending faults is not certain. There are some 105

Figure 17. Plots of northwest- and northeast

trending faults:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal-area

plot of 38 fault poles; contour

interval 2, 4, 8, and 15%. Legend

of symbols on Figure g. 106

+ 107

aplite dikes which subparallel these faults, but their

emplacement was probably more controlled by the jointing

(see discussion of jointing) .

In the north-central portion of the study area

(T. 12 N., R. 18 E., Sees. 34 to 36) there is a 4-km-long,

northeast-trending system of faults which is exposed on the

slopes below the southern Piute Range andesitic flows.

These faults dip between 50° to 70°SE and are marked by a

3- to 6-m-thick zone of clayey gouge along their north

easternmost exposures. These faults mark a boundary be

~n pyritic, altered Teutonia pluton on the south and

unaltered Teutonia pluton on the north. Several small

hills south of this fault zone are capped by andesitic

flows which dip eastward opposite to the northwest dip direction of the Piute Range flows. The basal contact of

the flows on the small hills is approximately 180 m lower

than the same contact in the Piute Range which indicates a down-to-the-east normal sense of slip on the fault.

Low-Angle Normal (Detachment) Faulting

Exposed on the southern and eastern flanks of Homer

Mountain are allochthonous rocks that probably are tectonically related to the middle Tertiary detachment terranes of the lower Colorado River region (G.A. Davis and other, 1980, 1982; Mathis, 1982; Rehrig, 1982; Spencer and Turner, 1982). A sinuous low-angle normal 108

fault marks the contact between relatively unaltered

autochthonous gneissic and plutonic rocks and allochthonous,

highly fractured and faulted hydrothermally altered,

chloritized crystalline rocks of the lowermost allochtho-

nous plate. The trend of the major detachment fault is

sinuous, but the overall strike trends between N20°E and

N40°E with dips from 20° to 45°SE (Fig. 18). Fault zones

or shear surfaces within the altered crystalline rock are

typically wavy, narrow zones containing from 2 to 15 em

of faulted rock ranging from siliceous-cemented cataclastic

microbreccias to uncemented clayey gouge. The allochtho-

nous crystalline rocks are cut by numerous slickensided

shear surfaces whose orientations are highly varied by

have a general east- to southeast-direction of plunge.

The westernmost exposure of the basal detachment fault

trends nearly east-west and dips at a slightly higher

angle, up to 70 0 S. This portion of the basal fault may

have a large component of strike-slip movement, especially

if the allochthonous terrane moved predominantly to the east.

The detachment terrane appears to be broken into two or three plates. The lowermost plate is in the northwestern portion of the detachment terrane. It is separa

ted on the east from the middle plate by a fault about

200 m long. Both the lowest and middle plates consist of similar chloritized crystalline basement. It is 109

Figure 18. Plot of detachment faults: lower

hemisphere Schmidt equal-area plot of 42

poles; contour interval 2, 4, 8, and 12%.

Legend of symbols on Figure 8. 110

N 111 Q •

unclear whether the intervening fault separates two dis

tinct plates or just subdivides one larger plate.

The Horner Mountain conglomerate rests in fault

contact above the crystalline lower and middle plates

and forms the highest detachment plate. The bedding in

this conglomeratic unit generally trends northeast and

dips to the northwest, or back into the southeast-dipping

fault surface. This backwards rotation of the bedding

is taken as evidence for listric normal fault movement

to the southeast or east.

The amount of normal slip across the basal detachment

fault can be estimated from the separation of a distinctive

north-trending granophyric rhyodacite dike {located in

T. llN.,R. 20 E., Sec. 23). The left strike separation

is a minimum of 800 rn and possibly as much as 1,500 krn.

If the slip direction of the faulting is assumed, then the

amount of throw, heave, and net slip can be approximated.

The fault strikes about N70°W and dips 30°SW between the

two closest offset granophyric dikes which may be assumed

to have been continuous prior to faulting. The dikes are

idealized as striking north and dipping 50°E. The pitch of the net slip is not precisely known, but it should be

less than the pitch of the line of intersection between

the fault and the dikes, that is,less than 55° to the east. If the pitch of the net slip were greater, then

the two dikes would not have been left-laterally offset 112

by normal-slip movement. A graphic solution of the amounts of net-slip, throw, and heave as a function of the pitch of the net-slip is presented in Figure 19. This figure shows that even with mostly strike-slip movement, yielding a pitch of 5 0 , about 850 m of net-slip, 45 m of throw, and

60 m of heave are needed for the dikes to be joined.

The lack of east-west-trending felsic dikes in the detachment terrane suggests that the detached rock has had substantial vertical movement or throw. With this in mind, a more likely pitch of 30° indicates about 1555 m of net- slip, 460 m of throw, and 660 m of heave.

Spencer and Turner (1982) have discussed the detachment terrane southeast of Homer Mountain and have divided it into three plates. Their lowest allochthonous plate is represented by the crystalline basement exposed in two north-trending fault blocks. I agree that these blocks expose either the lowest or possibly the middle plate rocks, but I interpret these blocks as being bounded by high-angle faults, not by low-angle detachment faults. The middle and upper plates of Spencer and Turner consist of undifferentiated porphyritic granite and poorly sorted conglomerates. I have choosen to separate the crystalline rock from the conglomerates because a fault separates them, and therefore I have mapped them as separate allochthonous plates. In the western detachment terrane their middle plate rests directly on the autochthon. I have mapped a 113

Figure 19. Graphic calculation of amount of

granophyric rhyodacite porphyry dike

displacement:

(a) orthographic projection of line of

intersection between the fault

surface and the dike;

(b) graphic calculations of the net-slip,

throw, and heave based on various

pitch angles of the net-slip.

pitch of net-slip. + 0 Dip slip = X X sin 55° D - ll\ Throw = dip slip x sin 35° Heave = dip slip x cos 35°

(b) pilch of

.(). .Jt('0 ( c ) n 0~ G_ (/) •rl H .l)&t -:--1 Qj (/) -4-' (/) (/) Q) H H .s I. ~ E Q) Q) Q) (/) ~ ~ -l.o I Q) Q) c H I Q) E E (j_ ~ 0. 0 Q) •rl I I I E rl L (/) :5: Q) I u I 0 > -4-' ~ H ro ·rl Q) L Q) 0. lxl c ~ L 5 90 850 43 60 10 200 915 95 1351 15 340 1020 160 230j 20 500 1145 235 340' 25 700 1310 330 47d 50 980 1555 460 660 9 30~m 35 1350 1920 635 9051 40 2000 2515 940 13401 strike slip =

...... ~ 115 Q •

structurally lower fault in this area which is about

600 to 900 m to the north. I consider this fault to be

the basal detachment fault which underlies my lowest

plate. I have mapped the basal fault contact of the

lowest plate based on the termination of chloritic

alteration along a widely diffuse zone of moderately

steep east-west-trending shears.

South of Homer Mountain in the Piute-Old Woman-Turtle

Steplatter Mountains is a series of north-trending, east

dipping normal faults which have been interpreted as the

headwall or breakaway zone for the detachment terranes to

the east (G.A. Davis and others, 1980; Howard and others,

1982; Miller and others, 1982). This headwall fault may

extend northward to connect with the Homer Mountain detach ment fault (Fig. 1). No major low-angle normal fault is

known west of Homer Mountain (Spencer and Turner, 1982).

To the south, the headwall is typified by an abrupt westward lessening of the degree of backward tilt of the

strata in the allochthon (G.A. Davis and others, 1980;

Howard and others, 1982). The poorly defined stratifi cation of the Homer Mountain conglomerate make testing of this model difficult.

Movement on the detachment faults at Homer Mountain may have begun during the Oligocene and is probably responsible for the basin into which the Homer Mountain conglomerate was deposited. Rhyolitic breccias of Late 116

Oligocene (26.0 m.y.B.P.) age intrude the structurally highest, and therefore youngest, section of the conglom

erate, suggesting that the basin was well developed by

Early Miocene time. Movement on these low-angle faults apparently ceased by Middle Miocene time when a 14.6 m.y.B.P. siliceous dike cut an assumed related low-angle

fault on the western flank of the Newberry Mountain,

just east of the study area (Spencer, 1983).

High-Angle Normal (Basin and Range) Faulting

In the north-central portion of the detachment terrane there are a series of north-trending high-angle faults. These faults displace the detachment terrane and appear to extend into the autochthonous terrane.

The faults bound two north-trending uplifted blocks of lower and middle plate allochthonous crystalline rock and they have created a narrow alluvial filled graben between and to the west of the blocks. These faults trend between N20°W and N20°E and have dips which average

70°E (Fig. 20). The westernmost uplifted block is bounded by east-dipping faults which cut the basal detachment fault. The east side of the easternmost uplifted block is bounded by a major high-angle east-side-down fault that also extends northward cutting the basal detachment fault. The northern portion of this eastern fault appears to be following the trend of the older detachment fault 117

,, '

Figure 20. Plots of high-angle basin-range

style faulting:

(a) rose diagram of strikes;

(b) lower hemisphere Schmidt equal-area

plot of 62 fault ples; contour

interval 2, 4, 6, 10, and 12%.

Legend of symbols on Figure 8. 118

N (b)

+ 119

because it separates unaltered fron altered crystalline rock (section C-C', Plate 1). Two possible tectonic interpretations of these steeply dipping fau1ts are discussed here.

As stated above, the detachment terrane of Homer

Mountain may be part of the headwall or breakaway zone for the detachment terrane to the east (see discussion of detachment faults). If the headwall fault zone can be generally compared to the head of a block-glide land slide, then certain characteristics of lateral movement, well studied in landslides, can be applied. In the head region of a block-glide landslide, grabens commonly form which trend perpendicular to the direction of movement.

These grabens form only where the basal rupture surface has a marked downward decrease in curvature or slope which causes greater horizontal movement per unit vertical movement in the center of the slide mass than at the head

(Rib and Liang; 1978). The dips of the headwall faults south of Homer Mountain are thought to shallow with depth and eventually join with a basal detachment or decollement fault (G.A. Davis and others, 1980; Howard and others, 1982). Such mechanics of sliding may have occur red at Homer Mountain and would explain the development of uplifted blocks and grabens in the allochthon. However, the northerly faults extend into the autochthon and cut the basal detachment fault which suggest that a landslide 120

model does not represent the latest movement on the high

angle normal faults. A more likely model for these high

angle faults is that they are part of a phase of basin

range style faulting which postdates the low-angle

detachment faulting.

These north-trending high-angle faults appear to

have pivotal movement with greater displacement in the

northern portions, based upon the observation that the

number of narrow alluviated channels crossing the uplifted

blocks increases to the south and eventually covers the

crystalline rock. The grabens between the uplifted

blocks containlower-platecrystalline rock, upper-plate

conglomerate, and andesitic volcanics which are overlain

by a seemingly normally faulted downward consolidated

alluvium that may be as old as 500,000 y.B.P. The

location of these high-angle faults may, in part, be

controlled by the pre-existing weaknesses of the detach ment surfaces, the granophyric dikes/sills, or the mylonite zones, but their time of latest movement is too

recent to be directly related to the middle Tertiary detachment tectonics. For these reasons, I conclude

that these high-angle faults appear to be more closely

related to the range bounding basin-range style faulting described by Stewart (1971).

Homer Mountain juts abruptly eastward into Piute

Valley, narrowing the width of the alluvial valley floor 121

from 18 km to 10 km. Piute valley north of Homer Mountain is slightly asymmetrical with topography on the east side being more subdued than the volcanic terrane of the Piute

Range on the west side. The Piute Range rises rapidly between 180 to 400 m off the valley floor at a slope of

20° to 30°. Flow layering of the southern Piute range generally dips to the northwest. Published studies further north are lacking, but nearly flatlying dips are suggested by the topography. The abrupt rise of the Piute

Range suggests that a range-bounding normal fault trends north along the east side of the range. A poorly expres sed north-trending fault in the central part of the study area may be an extension of such a fault. Spencer and

Turner (1982) have given this fault a greater importance than this study. They indicate that it has a continuous length of about 3 km and down-to-the-east displacement.

I have not found such strong evidence for the continuity and sense ofdisplacementon this fault. Piute Valley is probably a westward tilted graben formed by normal fault movement along high-angle faults on the western side of the valley. The abrupt extension of the basement into the valley at Homer Mountain makes the eastern "nose" of

Homer Mountain a likely site for high-angle faulting.

Quaternary Faulting

The most recent faulting in the study area occurred 122

on the eastern flanks of Horner Mountain and has been

briefly discussed above (see discussion of consolidated

alluvium and high-angle faulting). The two north-trending

grabens in the detachment terrane are covered by a

consolidated alluvium that may range in age from 15,000

to 500,000 y.B.P. In order to determine the age of the

most recent faulting which formed these grabens it would

be necessary to determine the age of the sediments that

are deposited within them. Deposition of sediments will

commence immediately following graben initiation. The

lack of upper-plate conglomerate in the westernmost graben

suggests that during the late Tertiary and/or Quaternary

the detachment terrane was highly eroded. If these

grabens were formed as the result of detachment faulting,

then the high degree of erosion would probably have pene

plained the surface. Alternatively, if these grabens were high standing so that sediments were being removed until recently ( 500,000 y.B.P.), then deeply incised major stream channels might be expected to be backfilled with alluvium as the stream base-level changed to allow deposition of the consolidated alluvium. The present major stream channels do not appear to be filled with deep alluvium; in fact, in situ crystalline basement commonly crops out along the channel bottoms suggesting recent down cutting. The latest movement on thehigh~angle faults probably occurred during the Late Pleistocene based 123

on the age of the fans that fill the grabens. The level of the fans surfaces within the grabens are nearly equal, or at least not significantly displaced, suggesting that no movement has occurred along these high-angle faults since the upper fan surface developed, at least 15,000 y.B.P.

DIKE STRUCTURES

The east-west-trending dikes of Homer Mountain typically have linear contacts with host rock. Their margins are commonly chilled indicating that the dikes were intruded into relatively cool country rock. Some dikes appear to have had multiple intrusions because they contain several chilled margins. Most of the dikes do not exibit any distinct flow structures or lineations, perhaps because of their coarse-grained textures. The hornblende quartz andesite porphyry dikes do have local lineations of hornblende laths which suggest sub-vertical flow.

Most of the dikes are jointed subparallel to their contacts with the country rock. The development of this jointing is probably from cooling or continued tensional stresses. Other sets of conjugate joints having traces trending perpendicular to the dike walls and whose inter section is horizontal are common. The acute bisectrix of these joints is often subhorizontal and indicates that 124

,, '

they may have formed during unroofing when the least principle stresses were sub-vertical, but jointing from cooling can not be ruled out.

ECONOMIC GEOLOGY

MINERALIZATION IN THE SURROUNDING REGIONS

The California mine, primarily a copper producing mine, is located about 8 km west of the Leiser Ray mine in the Vontrigger Hills. Hewett (1956) describes the mineralization zones of this mine as forming short lenses

3 to 6 m thick.which lie almost parallel to the general

N20°W, 60°SW trend of the gneiss layering in the country rock. No Teutonia Quartz Monzonite was found. The copper-bearing minerals include chalcopyrite, malachite, azurite, with chalcanthite and brochanitite near the water level of the mine.

The New York Mountains have a structural pattern of east-west-trending veins (Carlisle and others, 1982).

Like the Leiser Ray mine, the molybdenum-copper minerali zation occurs in an elongated central quartz-seritite pyrite alteration zone that is bounded by regions contain ing abundant huebnerite-bearing quartz veins. The only dikes in the New York Mountains similar to the Leiser

Ray mine area that are associated with the mineralization are the quartz monzonite porphyry, aplite, and quartz veins. 125

Wilkins and Heidrick (1982) have described the

Cu-Au-Mn mineralization in the Whipple-Buckskin Mountains located 100 km southeast of the study area. Here the major prospects are all spacially related to a regional detachment fault surface. The mineralization is concen trated in thick sections of allochthonous rocks which lie within northeast- to east-northeast-trending troughs or megagrooves in the detachment surface. The magagrooves trend subparallel to regionally developed northeast trending antiformal arches found commonly to the south

(Rehrig and Reynolds, 1980).

MINERALIZATION IN THE HOMER MOUNTAIN AREA

Mineralization in the Homer Mountain area is great est in the western portion, near Signal Hill and Billie

Mountain, where several old mines and numerous adits and pits are present. The largest is the Leiser Ray mine which is described by Hewett (1956) as reaching an inclined depth of 60 m. Numerous shallow workings are present as much as 4 km to the north in the Tungsten

Flat area, and 3 km to the south at the Old Glory mine.

Hewett (1956) states that exploration at the Leiser Ray mine occurred prior to 1891, but that the mill sites were established between 1905 and 1915. The newest mill, the foundation of which still remains was constructed for the recovery of vanadates. Production at the Leiser Ray 126

during 1916 and 1917 only reached in total output 40 tons of ore with an average grade of 17 percent copper, 1.2 percent lead, 0.28 ounces/ton silver, and 0.15 ounces/ton gold.

Mining excavations in the area surrounding the tungs ten mineralization consist of shallow shafts and pits which are concentrated along northwest-trending veins, most com monly where these intersect east-west-trending veins. The northwest-striking veins dip to the northeast at moderate angles but are sheared by younger, near vertical en echelon faults. At the Leiser Ray mine the main--vein strikes west and dips about 45° to 50°N. Vein minerals include very minor anounts of chalcopyrite, galena, hematite, and minor yellow, brown, green, and black vana date oxides of desclozite, cuprodescloisite, and wulfenite

{Hewett, 1956; Carlisle and others, 19B2). Recent studies by Carlisle and others (1982) show that mineralization and hypogene alteration at the Leiser Ray-Signal Hill district is zoned. Coarse sericitic alteration and sparse hydro thermal pyrite occur in an oval area extending west from the Leiser Ray about 1 km. The quartz veins and adjacent jointing commonly have minor narrow sericitic envelopes.

In addition to the minerals found at the Leiser Ray mine, huebnerite and scheelite are found with the quartz veins to the north in Tungsten Flat and to the south in the Old

Glory mine. Gechemical analysis of the quartz veins shows peaks of 1310 ppm Mo, 343 ppm Cu, 8300 ppm Pb, 127

1600 ppm F, 0.15 oz Au and 1.6 oz Ag per ton (Carlisle and others, 1982).

Hewett (1956) states that at the Leiser Ray mine three types of pre-mineralization dikes, a monzonite porphyry, aplite, and andesite, are present. He based his intrepertation that they are pre-mineralization on their lack of quartz phenocrysts. A fourth type of dike that is lamprophyric is said to be post-mineralization because it cuts the main quartz vein five times along the main shaft. I agree with Hewett that these dikes are associated with the mineralization, but I disagree with his order of their occurrence. The aplite dikes are associated with the mineralized quartz veins. The altered quartz andesite dikes of my study are probably equivalent to the lamprophyre dikes of Hewett and clearly cut the quartz veins. The principal disagreement between my study and that of Hewett is whether or not the andesitic and monzonite porphyry dikes (quartz-monzonite rhyodacite and/or quartz rhyodacite porphyry dikes of my study) are younger or older than the mineralization. I found these dikes to be younger than the lamprophyre dikes (altered quartz andesite of my study), whereas Hewett interpreted them to be older. The scattered occurrence of sericite and pyrite within dikes intruded after the aplite and quartz veins suggest a second and younger phase of hydrothermal alteration. 128

The only other mining activity in the study area is

located just north of Horner Mountain. A series of shafts

and pits have been dug along an east-west-trending, south

dipping quartz veins where it is intersected by northwest

trending quartz vein and shears. The topographic map

indicates that a shaft and well are located at this site

(Plate 1).

TERTIARY TRANSVERSE STRUCTURES OF THE SOUTHERN CORDILLERA

The Cenozoic structure of the Cordilleran orogen of

the western United States is dominated by north-south- to

northwest-trending structures of middle Tertiary andyounger

age. However, there are a number of transverse structures

which predate or are synorogenic with these Cenozoic struc

tures. Transverse structures are defined as those having

orientations of east-northeast to east-west. These two

orientations are combined because there are no data to

determine whether or not the structures have been rotated

clockwise into their present orientations. These trans

verse structures include faults lineaments, horsts and

grabens, elongated plutons, mineralized belts thatcontain

linear veins and dikes, regional gravity and magnetic

anomaly patterns, and approximately linear belts of igneous

centers. Table 4 is a tabulation of most major e~rly to middle Tertiary transverse structures of the southern

Cordillera of the western United States, and Figure 21 ======Tl\BLE 4: 'I'RANSVERSE STHUCTURES------Name and Location Description_ References

1. Oquirrh-Uinta ENE-trending mid-Tertiary belt of mineralization that Hilpert and mineral belt, follows the Uinta arch of northern Utah; about 160 Roberts, 1964 Utah km long; contains the Bingham and Park City mining Stewart and districts. others, 1977

2. Deep Creek-Tintic E-W-trending mid-Tertiary belt of mineralization in Hilbert and mineral belt, central Utah-Nevada; oldest intrusive is mid-to Roberts, 1964 Utah-Nevada late Eocene, youngest Miocene; 20-30 km wide and Haybey and 200 km long; contains Tintic mining district. Morris, 1967

3. Wah Wah-Tushar E-W-trending Late Cretaceous to mid-Tertiary belt of Hilpert and mineral belt, mineralization in SW Utah-SE Nevada; 50 km wide and Roberts, 1964 Utah, Nevada 200 km long; contains Pioche and San Francisco Schmoker, 1970 mining districts; gravity and aeromagnetic anomaly Tschanz and northern boundary San Francisco Mts. is an E-W Pampeyan, 1970 trending fault predates intrusives. James and Knight, 1979

4. Colorado mineral NE-trending Late Cretaceous to Eocene belt of miner Tweto and belt, Colorado alization in central Colorado; 15 to 55 km wide and Sims, 1963 400 km long; intrusives thought to have followed Precambrian shear zone.

5. (a) Big Bug, (b) ENE-elongated Laramide age plutons, 75 to 58 m.y.B.P. Rehrig and Granite Wash, with ENE-trend~ng mineralized joints, veins, and Heidrick, (c) Schultz, (d) dikes; N50°-70 E trends predominate with younger 1972 Cornelia, (e) Nl0°-40°W trends; mineralization not economical. Slate, (f) Amole, (g) Copper Creek, (h) Whetstone stocks Arizona

...... N 1.0 TABLE 4 continued: TRANSVERSE STRUCTURES

Name and Location Description------References 6. (a) Ithaca Peak, ENE-trending Laramide age plutons and mineralization Rehrig and (b) Bagdad, (c) 58-75 m.y.B.P. 6 with ENE-trending joints ve~ns, Heidrick, 1972 Sierrita mining and dikes; N50 -60°E at Ithaca Peak, N60 6 -80 E at Lacy, 1959 districts, Arizona Bagdad, ENE to E-W at sierrita.

7. Vulture Mountains NE-trending Wickensburg batholith, 68.4 m.y.B.P., Rehrig, and Maricopa County, with NE-trending comagmatic dikes, veins, and others, 1980 Arizona joints; 5-8 km wide and 20 km long; N to NW-trend ing mid-to-late Miocene dikes crosscut; Precam brian basement has NE-trending foliation

8. Tucson Mountains, E-W-trending Silver Lily latite porphyry dike swarm; Brown, 1959 Arizona Tertiary age; 2 km wide and 10 km long; Old Yuma mine mineralized along NE-trending veins.

9. New York, Homer, ENE- to E-W-trending dikes and veins have intruded Burchfiel and and Sacramento Late Cretaceous plutons; 71.7 m.y.B.P. dikes in Davis, 1977 Mountains, New York mts, 16-19 m.y.B.P. dikes in Homer and Spencer and California Sacramento Mts. Turner, 1982 Spencer, 1983

10. Igneous belts, 3 E-W-trending arcuate belts of igneous rocks Stewart and Nevada-Utah migrating and younging southward; began near others, 1977 lat. 40 0 N 43-34 m.y.B.P., ends lat. 37 0 N between 17-6 m.y.B.P.

1-' w 0 TABLE 4 continued: TRANSVERSE STRUCTURES

Name and Location Description References

11. Soda Mountains, N70°W-trending diabase and younger felsic dikes Grose, 1959 California intrude Late Cretaceous Teutonia pluton; .3-30m wide dikes are 30m-3km long; 1/5 exposed pre Pliocene rock west of Avawatz fault ~s dike; Precambrian foliations trend N60°-70 W.

12. Indian Peak N45°-65°E-trending mid-to-late Tertiary rhyolite Grant, 1979 porphyry to andesite dikes; dikes correlate with extrusive rocks in surrounding area.

13. Jemez Zone, NE-trending Cenozoic volcanic field extends from Mayo. 1958 Arizona, New Jamez Caldera-Mt. Taylor areas to NW side of Eastwood, 1974 Mexico, Datil volcanic field; may extend to Pinacate Colorado volcanic field, Sonora, Mexico

14. Magnetic 3 E-W- to ENE-trending aeromagnatic anomly patterns; Stewart and anomalies, related to mineral belts and calc-alkaline rocks; others, 1977 Nevada-Utah western boundary at N-S-trending "quiet zone" of low magnetic intensity in central Nevada.

15. Bouguer anomaly, E-W-trending Bouguer gravity anomaly near lat. 37°N; ·Stewart and Nevada-Utah occurs along southern boundary of volcanic belt others, 1977 and may reflect low density volcanic material Eaton and others, 1980 Diment and others, 1961

16. Garlock fault, NE- to E-W-trending left-lateral strike-slip fault; Davis and California 260 km long; 48~64 km offset of pre-Cenozoic rock; Burchfiel, thought to be intracontinental transform fault. 19.73

1--' w 1--'

"' TABLE 4 continued: TRJ\NSVERSF. STRUCTURES

Name and Location Description References

17. Chocolate and E-W-trending generally left-lateral strike-slip faults Crowell and Orocopia Mts., Pleistocene to early Holocene offsets; includes Crowe, 1976 California Salton Creek, Pinto Mt., Blue Cut, Porcupine Wash, Chiriaco, Corn Spring, Mammoth Wash, Black Eagle mine faults.

18. Cedar Canyon and NE-trending faults; Cedar Canyon fault has at least Hewett, 1956 an unnamed fault 900 ft down-on-south displacement and extends from Swanson and New York Mts., New York Mts. to Kelso on SW; unnamed fault N-side others, 1980 California New York Mts. left-lateral offset Clark Mt. thrust 8-9 mi.

19. Lake Mead shear NE-trending Tertiary left-lateral strike-slip fault; Anderson, zone, Nevada pre-Late Miocene movement; 48-64 km offset proposed. 1973

20. Pahranagat shear 3 E-W-trending strike-slip faults; early right-lateral Tschanz and system, Nevada offset of 48 km with younger post-Miocene left- Pampeyan, lateral offset of 16 km. 1970

21. Nevada Rift, ENE-trending vertical faults offset 14-17m.y.B.P. NNW Zoback and Nevada trending Nevada Rift; 3-4 km dip-slip indicates a Thompson, clockwise change in axis of extension between 1978 14-16 m.y.B.P. Zoback and others, 1981

22. Candelaria fault ENE-trending graben of Oligocene age, 25-22 m.y.B.P. Speed and and trough, basalts and tuffs fill trough; Candelaria fault Cogbill, Nevada strikes E-W with left-lateral offset; 15 km long. 1979a, 1979b

1-' w N TABLE 4 continued: TRANSVERSE STRUCTURES======Name and IJocation Description References

23. Lemington fault, N60°E-trending right-lateral strike-slip tear Morris and Utah fault on S edge of Nebo-Charleston thrust plate; Shepard, 38 km long. 1964

24. East Tintic N55°-60°E-trending vertical right-lateral strike Morris and Mts., Utah slip tear fault suggested by drill hole data and Shepard, offset strata. 1964.

25. Gila trough, NE-trending faulted graben; pre-Miocene age sedi Eberly and Arizona ments at base; 25 km wide and 150 km long; NW Stanley, ·trending late Cenozoic structures overprint. 1978.

26. Seismic zone, E-W- to NE-trending zone of seismicity cuts Basin Smith and SW-Utah and and Range normal faulting; 200 km long; strike Sbar, 1974 S-Nevada slip movement.

27. Colorado line NE-trending belts of faults are thought to be Pre Warner, 1978a, ament, Colorado cambrian wrench faults; zone 160 km wide and 1100 1978b Utah, Arizona km long; Colorado Mineral Belt on NE end. Dutch, 1978 Hickman, 1979

28. Cortez-Uinta E-W-trending arch, disrupts Paleozoic rocks; contains Stewart and Oquirrh-Uinta Mineral Belt. Poole, 1974

29. (a) Cache Valley, 4 E-W-trending lineaments, possible pre- to syn- Kelly and (b) Basaltic, Laramide structures, may be Precambrian shear zones; Clinton, (c) Rico, (d) lengths from 150-300 km; left- and right-lateral 1960 Rattlesnake strike-slip movement on (a), (c), and (d); {b) Utah and Colorado alignment of San Francisco, Hopi, and Taylor volcanic fields.

1-' w w 134

Figure 21. Transverse structures of the southern

Cordillera, western United States.

Numbers correspond to numbers in

Table 4. uo!/0

~I 0 ~I t OJ

~"'

f •• ••\ ..II. ··" •• • . "'-27'" ... •• •• ••• •• •• •• ..... t;,~ ...... 2Ri1. •...... ••I •G

} -a. J.J"13 LEGEND .... • •• Mineral belt 5b PH (H)Q lgneoue belt --'" ..... -o- Geophyelcar anomaly ;rt~~i -.r"-"'· •••• Lineament - §(I •• •;:&e 60 ...... _ Thruet laufl .• •• 61 .. Jl ....

..&.t.U Normar fault 0 ' . ~OJrm , ~~

,_, w ln 136 @ .

is a graphic presentation.

The east-west-trending dike swarm of Homer Mountain

is part of these transverse structures. Dike swarms in

the Sacramento and New York Mountains are similar in age,

composition, and trend with those in Homer Mountain.

Together these dikes are part of a region that is about

80 km long and 65 km wide where early(?) to middle

Tertiary rocks were commonly intruded by dikes that strike

perpendicular to the general northerly orientation of

southern Cordilleran structures. In addition, hydrother mal copper-molybdenum mineralization is associated with

the dike swarms of Granite Spring, Homer Mountain, and

New York Mountains areas (Carlisle and others, 1982).

The occurrence of Laramide-age mineralizatio~ in particular copper porphyries, along transverse structures in the

southern Cordillera is well documented (Table 4).

Although the economic value of the mineralization is presently marginal in the study area, I interpret the area to be another example of ore enrichment along a transverse structural trend.

If it can be assumed that subduction and the genera tion of magma are interrelated, then the association of intrusions and/or mineralization in transverse alignment become important because a plate tectonic model can be created to explain the occurrence of features like the dike swarm at Homer Mountain. Of importance to the 137

il '

tectonic history of Horner Mountain is the timing of the end of Farallon Plate subduction and the location major transverse structures in the subducting slab, such as the Mendocino and Murry fracture zones. There is growing evidence that the boundary between slabs which are sub ducting at different angles can cause transverse struc tures within the overlying craton or arc. In the Andes boundaries between lateral tectonic segments in the forearc and magmatic belt coincide closely with breaks in the subducting slab (Jordan and others, 1983). In north ern Central American and Japan transverse faults and lineaments of active volcanoes are aligned with breaks in subducting slabs (Carr and others, 1972; Carr, 1976)~

The degree to which such transverse structures develop seems to be dependant on the rate of subduction, the amount of the differences in the slab dips, and the ductility of the craton or arc. The eastward sweep of the Laramide orogeny appears to have caused regional heating and an increase in the ductility of the hinderland in southeastern California, Arizona, and eastern Nevada allowing the development of overthrusts and possibly the metamorphic core complexes.

The Mendocino fracture zone with its present day offset of over 800 km of magnetic anomalies of Oligiocene age is clearly a major break in the oceanic slab that must have extended back to the middle Tertiary. Recon- @ .

struction of the positions of major breaks in the Farallon

Plate (Mendocino and Murry fracture zones) and their influence on cratonic structures is complicated by the northward migration of these breaks following the cessa tion of subduction at about 29 m.y.B.P. {Atwater, 1970).

Recent studies of the Cenozoic volcanic terranes of the western Transverse Ranges indicate that calc-alkaline volcanism and assumed corresponding Farallon Plate subduction continued off southern California as recently as 14 m.y.B.P. (Weigand, 1982). This timing indicates that transverse structural influence of any breaks in the subducting Farallon Plate may beasyoung as Middle Miocene, and does not preclude the possibility that development of transverse diking structures, such as those of the Homer

Mountain, New York Mountain, and Granite Spring areas which are 200 km inland from the subduction zone, might be directly related to the subduction of a major trans verse structure such as the Mendocino or Murry fracture zone.

S U M M A R Y

A brief summary of the geologic history of the study area is given belo~ in Table 5 which compares local with regional events.

Basement rocks exposed in the Homer Mountain area consist of granitoid gneiss of Precambrian Y age and two

138 .------>------·------·------; ---.------___:_1.:..:11~1111 '>: ',lJMMIIHY Ill Hli,IIINIII IINil 1111'/11 Ill 1111111 I VI t{\:,_ ------1 <'I y------1 HI I; lllN/11 ~~~ ~N ~~--~-V~N-!~::::=:::...:=-=:r------i~~Mf~::~;~-~ ~~-iN· II llmW~--l ~f N I •• n: ltl PEHIOD MPI.amorphic & I~Jrl!.•ou;. I v•·11h ',t l'lll'llllill .\ lllill'r l Vtellt •, t~danH,rpliil !'. l~~--~~~ ':,'.nwlllr

Recent 15,000 y. 1 r,,ft iiii.L·rllcd!~d i11 lhn~e m.y.- .... Pliocene ~Bimodel rhynl i te-uasa It .. 1--t- 7 m.y. assemblayes 1\d'>ill nding sills H-Regional erosion? • 21.7 m.y. Andesilic flow , P~leocen~ tt:Teutonia batholithic ! , 65 m.y. complex intruded in S. Piute Ranqe ':.._ . . E-d i red ed compression (Jtz veins/mineral izat.ion Poss1hle left.- Cretaceous : New York Mts., CA deform~ New York Mt,CA ~ I NW-lr~nrhng 156 m.y. ~ ~ IPillnnin "- Homer Mt. lateral faultJnq .Jurassic llverlap of NW-trending [ : plutons&spliledikes :______., . . f d t--Mojave-Sunora Megashear :-'~Y lYO m.y. , magmnlic arcs 1on1 11c zones orme ' Triassic ' J-"NW-lrenrlinq l.runcat.inn . ns m.y.- ol' North America ', Permian H Ll tl lo (') Minqeoclinnl sediment Ltl uliun western 15 Cambrian . 570 m.y. I . El-Q) t--~ NF -t rendinq crstonic y Porphyritic graniloirl Porphyritic granitoid n'• marq in form~~~~ t ''~iss to Rapdkivi !Jilf:isS X ~ranlle formed

1-' w \0 140

peraluminous quartz monzonite plutons (Teutonia and Homer

Mountain plutons) of Late Cretaceous (?) age. These plutons have chemical and petrographic similarities with the Teutonia batholithic complex of the New York Mountain and were probably intruded at a similar time. No Paleo zoic or Mesozoic sedimentary rocks are exposed in the study area. A minor number of north-trending, east dipping mylonite zones within the Precambrian gneiss are the only remnants of any pre-Teutonia structural events.

These mylonitic zones have northeast-southwest-oriented lineations which may indicate a direction of tectonic transport.

During the early Tertiary the study area was probably undergoing deep erosion similar to most of the southwest

United States. Andesite flows of latest Oligocene age which cap the highest hills in the western portion of the study area are underlain locally by a few local thin lahar deposits, but commonly lie directly on the plutonic basement. Also during the Late Oligocene, the Homer

Mountain conglomerate was being deposited in the eastern portion of the study area into a faulted graben. This graben is assumed to have been formed during the early phase of detachment faulting. The easternmost portion of the study area is allochthonous and is bounded on the west by a basal detachment fault which appears to be part of a headwall or breakaway zone for the detachment ter- 141

ranes to the east. This breakaway zone is probably

continuous with a similar structural feature in the

Piute-Old Woman Mountain to the south. Southeast of the

study area detachment faulting stopped during the late

Middle Miocene (14 m.y.B.P.) and a similar termination

age is assumed for detachment faulting at Homer Mountain.

Displacement of a Late Oligocene granophyric rhyodacite

dike/sill by the Homer Mountain basal detachment fault

suggests that 660 m of vertical displacement and about

1.5 km of down-to-the-east net-slip.

In the early Tertiary there was molybdenum-copper

tungsten mineralization in the western portion of the

study area which accompanied the emplacement of quartz

veins along northwest and east-west trends. The quartz

veins were emplaced subparallel to a northwest-trending

left-lateral shear zone and associated east-west-trending

pinnate fractures. These quartz veins are thought to

have been emplaced during the final phase of the Teutonia

pluton solidification. A second phase of hydrothermal mineralization probably accompanied the intrusion of the

east-west-trending dikes.

These northwest-trending faults were reactivated

probably during Late Miocene-Pliocene time as arching of the study area occurred along a north- to northwest

trending axis. This arching may be related to the late

phase of detachment faultin~ but offset of the felsic 142

dike (19-16 m.y.B.P.) suggest a possible younger age.

The most dominant geologic structures of the Homer

Mountain area are east-west-trending dike swarms of Early to Middle Miocene age. These numerous linear dikes vary from intermediate to felsic in composition. Geochemical and field relationships suggest that these dikes origina ted from two parent magmas. The intrusion of these dikes in a generally uniform orientation suggests that they are indicators of a regional extensional stress. A north south extensional orientation is seemingly transverse to the northeast-southwest extension direction postulated for the lower Colorado River region during the Miocene. The structural relationship between these dike swarms and the detachment terranes is presently unresolved. There is evidence that this transverse structure continues westward in the New York Mountains, with a right-step offset of about 32 km to the northwest. The Homer Mountain dike swarm is part of the easternmost portion of a 65 km wide and 80 km long transverse mineralized zone. These transverse dikes may have formed within structures that developed as response to transverse structures within the subducting oceanic slab.

The late Tertiary was dominated by north-trending high-angle faulting of basin-range style. This faulting has displaced both the allochthonous detachment terrane and adjacent autochthonous rock. High-angle faulting 143

probably occurred as recently as the Late Pleistocene and has displaced the oldest of three Quaternary alluvial units. 144

0 '

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Young, R.A., and McKee, E., 1978, Early and middle Cenozoic drainage and erosion in west-central Arizonia; Geol. Soc. America Bull., v. 89, p. 1745- 1750.

Zoback, M.L., Anderson, R.E., and Thompson, G.A., 1981, Cainozoic evolution of the Basin and Range province of the western United States: Philisophical Transcripts of the Royal Society of London, v. A-300, p. 407-434.

Zoback, M.L., and Thompson, G.A., 1978, Basin and Range rifting in northern Nevada: Clues from a mid Miocene rift and its subsequent offsets: Geology, v. 6, p. 111-116. 155

A P P E N D I X A

TABLE OF MAJOR OXIDES AND NORMATIVE MINERALS TABLE A-1: Major Oxides and Normalive.Minerals Mesozoic and earl Tertiar lulonic rocks Homer Mountain quartz monzonite mesocratic aRlite

HM-349 HM-522 HM-526 HM-5 9 I:IM-2:2:2 1:1~-2908 Si02 72.98 7Lt. 99 73.03 72. l 7 71. uo 67.17 Al203 14.40 lit. 33 14. 6lt 13.84 13.06 14. 13 Fe203* • 91 .43 .84 .98 l. 64 2.08 FeO* 1. 24 .58 l. 14 1. 33 2.21 2.81 MgO .67 .63 .73 .. 69 .64 .80 CaO 1.46 1. 26 1.69 1. 53 1. 30 1. 89 Na20 3.59 4.45 lL07 3.24 2.37 2.64 1<20 3.71 4.02 3.62 4.09 6. 19 5.75 Ti02 .16 .05 .23 .20 .55 .90 P205 .06 .03 .06 .12 • 1•3 .26 MnO .05 .03 .04 .05 .05 .05 Sum** 99.23 100.80 100.29 98.23 99.15 98.47 QZ 33. 6" 29.6 30.4 33.7 29.5 24.7 co 2.0 . 4 1.1 1.6 .4 .8 OR 22.1 23.6 21.4 24.6 36.9 34.5 AB 30.6 37.4 34. 4· 27.9 20.2 22.7 AN 6.9 6.0 8.0 6.9 5.6 7.8 HY 3.0 2.2 2.9 3.2 3.5 4. 1 MT 1.3 .6 1.2 1.4 2.4 3. 1 IL • 3 . 1 .4 .4 1.1 1.7 AP . 1 . 1 . 1 .3 . 3 .6 Norm AN 18.40 13.85 18.84 19.9 21.83 25.58 Norm 4.7 2.9 4.6 5.0 II 7.0 8.9 c. I. Mol. Al*** C+N+I< 1 ' 14 1.03 1.07 1.10 II 1. 01 1. 01 *calculated from Fe-total; **loss on ignition not added to total; C.I.= color index); *** molecular ratio of Al to ( CaO + Na + 1< 0 ) 2o3 2o 2

1-' l11 0"1 TABLE A-1 continued: Majur OxidP.~ld Nn_rmalive __ t1ciDerr:Jls Mesozoic and earl Terliart__[!lutnnir: rocks leucocraff~ a~lile --- letJonra-q~lartz monzonn:e

HM-136 HM-1199 HM-5cY5B HM-250 fit~- 3 rJ h HM-549 Si02 75.32 75.89 16.21 71.91 71.85 70.7I Al203 13. 51 13.30 J 3. I 5 14.B7 15.09 15. I 2 Fe203* .40 .32 ..32 .83 .75 l. 08 FeO* .53 . 4ll .ld l. 13 l. 01 l. 46 MgO .47 .44 .45 . 7 1 .68 .73 CaO .62 .49 .54 l. 57 1 . 116 l. 67 Na20 3.73 2.73 3.57 3.29 3.59 ILOO K20 4.98 6.29 5.72 4.03 3.94 3.84 Ti02 .06 .05 .04 . 2 1 .22 .29 P205 .01 .00 .02 .08 .09 .10 MnO .01 . 0 l . 01 .04 .04 .04 Sum** 99.64 99.95 99.96 98.73 98.71 99.05 QZ 32.4 34.0 33.5 33.1 32.0 28.0 co .9 1.1 . 7 2.5 2.5 1.6 OR 29.5 37.2 30.9 24.1 23.6 22.9 AB 31.7 2 3. 1 30.2 28.2 30.8 34.2 AN 3.0 2.4 2.5 7.4 6.7 7.7 HY 1.8 1.6 1.6 2.9 2.7 3.2 MT .6 .5 . 5 1.2 1.1 1.6 IL • 1 . 1 . 1 . 4 . 4 .6 AP .0 -- .0 . 2 • 2 • 2 Norm AN 8.78 9.45 7.78 II 20.70 17.97 18.40 Norm c. I. 2.4 2. 1 2. 1 4.5 4.2 5.4 Mol. Al 1.05 1.18 1.18 1.10 C+N+K 1.07 1.09 --·--··-- ~

I-' l11 -.....1

"' TABLE 1\-l continued: Ma 'or Oxides and Normalive Minerals - - lutonic rocks Tertiar~ volcanic flows Teuton1a uartz monzonite easler~_19__! i U c breccia HM-555A HM-562 HM-2:36 HM-7""'!2 HM-2368 Si02 72.57 71.70 72.AO 77.40 71.4 3 Al203 15. 18 13.52 13.84 13.46 13. 31 Fe203* .73 l. 41 .82 . 8 It .80 FeO* .99 l. 90 1.11 l. 13 1.09 MgO . 7 1 .69 .48 .50 .47 CaO 1. 62 l. 49 .59 .72 .59 Na20 3.A1 2.53 3. 19 2. 10 3. 11 K20 3.81 5.06 6.93 7.63 6.83 Ti02 .20 .40 .33 . 1 6 .33 P205 .07 . 1 3 .15 .03 . 1 3 MnO .03 . 05 .06 .04 .06 Sum** 99.73 98.88 100.30 99.01 98.15 QZ 31.1 33.3 25.6 28.5 25.7 co 2.0 1.5 . 4 . 5 .0 OR 22.6 30.2 40.8 45.5 41.1 AB 32.3 21.6 26.9 17.9 26.8 AN 7. 6· 6.6 2.0 3.4 2. 1 HY 2.7 3. 5 2. 1 2.5 2. 1 MT 1.1 2. 1 1.2 1.2 1.2 IL .4 .8 .6 . 3 .6 AP .2 . 3 .3 . 1 • 3 Norm 15.97 7.32 AN 19.04 23.41 3.9 Norm 4.2 6.4 6.8 4.0 3.9 C. I. II Mol. Al 1.14 1.09 1.00 1.03 .98 C+N+K II

I-' lJ1 (X) TAf1LE A-1 continued: Major Oxides and Normative Minerals

Tertiary vnJ~anic flows quartz andesite a od andes i L..a_Ll.ruls HM-2058 HM-346 HM-47R HM-556 HM-563 HM-564 Si02 48.95 51.6 7 52.54 54. 19 54.92 54.88 Al203 15.77 16.38 1 7. 14 16.05 16.37 15.98 Fe203* 4.29 3.92 3.36 3.33 3.21 3. 14 FeO* 5.80 5.30 4.54 4.50 4.34 4.24 MgO 6.91 4.97 5. 15 4.34 3.42 4.42 CaO 11.20 8.89 9. 13 8. 16 7.03 8.26 Na20 2.60 2.97 3. 81~ 3.61 3.52 3. 60· K20 .91 1.09- 1. 72 1. 87 1. 93 1. 99 Ti02 1. 21 1. 35 1. 48 1. 26 1. 27 1. 29 P205 .28 .32 .56 .48 .37 .40 MnO . 1 5 . 1 6 . 1 4 . 1 4 • 1 4 .12 Sum** 98.07 97.02 99.60 97.93 96.52 98.32 QZ --- 5.8 . 1 5.2 8.7 5.4 OR 5.5 6.6 10.2 11.3 11.8 12.0 AB 22.4 25.9 32.6 31.2 30.9 31.0 AN 29.2 29.0 24.5 22.5 24.0 21.9 01 20.5 11.5 13.8 12.5 7.8 13.7 HY 12.5 11.9 9.7 8.8 8.7 8.0 OL . 5 MT 6.3 5.9 4.9 4.9 4.8 4.6 IL 2.3 2.6 2.8 2.4 2.5 2.5 AP • 7 .8 1.3 1.2 .9 1.0 Norm 56.58 52.83 42.90 41.94 42.82 41.45 AN Norm 28.8 C ·.I • 42.2 31.9 31.2 28.7 24.3 Mol. Al .74 .69 .70 .78 .69 C+N+K .62

1-' l11 ~ TARLE A-1 continued: Major Oxides nnrl Normative Minerals

intermerli~te rnmposilion rlikes altered quarl7 andesite allc[~f!il rjtJarlz gabbro

HM-158A HM-160 HM-525 HM-0311\ IIM-51~7 HM-551 - Si02 54.47 49.63 511.28 48.63 50. l l 50.27 Al203 15.35 15.81 16.22 16.06 ]').66 16. 17 Fe203* 3. 10 5.67 5. 19 s.ns 5. 51 5. 19 FeO* 4.19 7.66 7.02 6.82 7.45 7.01 MgO 3.03 4.35 4.67 5.87 5. 1 5 4.63 CaO 6.56 8.05 8.55 9.94 9.05 8.48 Na20 3.29 3.11 2.90 2.63 2.67 3.00 K20 2.35 1. 02 1 . () 3 .84 .59 1.04 Ti02 1.07 2.44 l. 97 l. 56 1. 71 l. 96 P205 .43 .44 .56 .29 .29 .56 MnO .12 . 1 8 . 1 9 .18 .18 .18 Sum** 94.96 98.36 98.58 97.90 98.37 98.48 QZ 10.0 5.0 5. 5 2.0 6.2 5.2 OR 14.8 6. 1 6.2 5. 1 3.5 6.2 AB 29.6 26.8 24.9 22.7 23.0 25.8 AN 21.5 26.6 28.6 30.2 29.5 28.0 DI 8.2 9.0 8.7 14.7 11.5 8.9 HY 7.9 12.4 13.4 14. 1 14.2 13.2 MT 4.8 8.4 7.6 7.5 8. 1 7.6 IL 2.2 4.7 3.8 3.0 3.3 3.8 AP 1.1 1.1 1.3 . 7 . 7 1.3

Norm 49.86 53.47 57.65 56.21 52.08 AN 42.02 II Norm 23.0 34.5 33.5 II 39.1 37.2 33.5 c. I. Mol. Al .77 .76 .76 .69 .73 .75 C+N!-K II

1-' m 0 TABLE A-1 continued: Ma j o r 0 x i d ~ s and No r ma t _i v e_ Mj n P r a l s in t e rrne d i ate c O~.fJO s i tl._9_!1_~J<~2 quartz andesite and quartz andesitf:__Q_orphyry HM-088A IIM-093A HM-370 HM- 3 71 HM-3/5 HM-40') HM-489 Si02 60.98 57.23 61.17 58.86 62.22 58.72 66.51 Al203 1 5. 10 17.49 15.79 15.97 1~.36 16.29 15.1~5 Fe203 4 2.80 2.59 2.21 2. 81 2.35 2.67 1 . 116 FeO* 3.78 3.50 2.99 3.80 3. l 7 3.61 1. 97 MgO 1. 65 1. 49 1. 2 3 2. l 5 l . 1 3 1. 55 .97 CaO 3.63 4.69 3.66 5.56 3. 12 4.85 2.64 Na20 5.12 3.97 3.93 3.25 4.25 3.51 4.04 K20 2. 11 3.45 4.19 3.30 3.76 3.56 4.44 Ti02 .87 1.04 .96 1. 12 .88 1. 12 .60 P205 . 18 .36 .39 .45 . 18 .45 • 1 7 MnO .10 • 1 4 .08 . 10 .11 .09 .06 96.53 96.41 98.31 Sum* 4 96.32 95.95 96.60 97.37

QZ 13.4 9.0 13.7 13.6 15. 1 13. 1 19.2 OR 12.9 21.3 25.6 20.0 23.0 21.8 26.7 AB 45.0 35.0 34.4 28.2 37.3 30.8 34.8 AN 12.4 20.5 13.5 19.8 17. 1 18.9 11. 1 01 4. 1 1.0 2.2 4.5 2.2 2.5 .9 HY 5.7 6.3 4.4 6.4 4.6 5.6 3.6 MT 4.2 3.9 3.3 4.2 3.5 4.0 2.2 IL 1.7 2. 1 1.9 2.2 1.7 2.2 1.2 AP .4 .9 1.0 1.1 . 4 1.1 .4 Norm 28.21 41. 16 24.59 37.97 24.19 AN 21.67 36.98 Norm 15.8 13.3 11.8 17. 3 12.0 14.3 7.8 c. I. Mol. Al .84 .92 .88 .95 C+N+K .87 .93 .89

1-' 0"1 1-' TABLE /\-1 continued: Major Oxides and Normative Minerals

inlerme~iate composit!on dikes ho rnb 1 enc~ua r t z

Ht,-082A HM-51~6 IIM-S~'IJ HM-372 HM-057A - . Si02 63.09 67.56 64. 1 7 63.Rl 61.47 Al203 16.59 15. 12 16.62 1().911 15 .ld Fe203* l. 87 1 . 0 5 l.Bn l. 7 5 2.09 FeD* 2.53 1. 42 2.43 2.37 2.83 MgO l. 05 .70 l. 10 1. 06 l. 41 CaD 2.76 1. 51 3. 14 2.98 4.39 Na20 4.55 4.37 3.71 ll. 4 4 . 3. 37 K20 3.68 4.94 4.01 3.82 2.86 Ti02 .72 .50 . 7 l .69 .72 P205 .30 .09 . 3 1 .30 .26 MnO . 1 1 .07 .12 .10 .07 Sum** 97.25 97.33 98. 11 98.22 94.90 QZ 15.4 19.2 19.2 15.6 20.4 co . 8 . 1 1.2 .8 OR 22.4 30.0 24.2 23.0 17.8 AB 39.6 38.0 32.0 38.3 30.0 AN 12. 1 7. 1 13.8 13. 1 19.5 Dl ------1.3 HY 4.9 2.9 4.8 4.7 5.6 MT 2.8 1.6 2.7 2.6 3.2 IL 1.4 1.0 1.4 1.3 1.4 AP . 7 • 2 . 7 . 7 . 6 Norm 15.73 30: 16 25.45 39.38 AN 23.36 Norm 9.1 5.4 8.9 8.6 11.6 c. I. Mol. Al .99 1.03 1. 00 .93 C+N+K 1. 01

1--' ~ N TABLE A-1 continued: Ma~ r 0 x t des n ~ d Norm at. i v e Mj n e r Cl] s

in_t:ermedlale r.:..n~w,l Uon dikes b i ol 1 L~~o 1~~-c 1 t e p U_!:J~_!"2.Y_!:_Y HM- ::Sit"> HM-538 HM-054 HM-142 HM-309 - Si02 67.34 64.57 65.95 69.48 66.78 A1203 14. '9 5 15.78 15.27 15.rJ3 15.67 Fe203* 1. 35 l. 6 5 1. 69 1 . ()9 1. 51 FeO* 1.82 2.23 2.79 I . 4 7 2.04 MgO .93 1. 13 l. 06 .7R 1. 05 CaO 2. lt4 3.28 3. OLt 1. 90 2.94 Na20 3.44 3. lt8 3. 31 3.47 3.38 K20 4.60 4. 10 4.72 4.84 4. 18 Ti02 .55 .66 .67 .39 .60 P205 . l 3 . 19 .22 .09 • 1 7 MnO .06 .06 --.07 .04 .06 Sum** 97.60 97. 14 97.79 98.58 98.38 QZ 23.4 20.2 22.7 25.6 23.4 co • 2 . 1 . 3 .9 • 7 OR 27.9 24.9 25.5 29.0 25.1 AB 29.8 30.3 28.6 29.8 29.1 AN 11.5 15.5 14.0 9.0 13.7 HY 3.8 4. 7 4.6 3.2 4.3 MT 2.0 2.5 2.5 1.6 2.2 Il 1.1 1.3 1.3 . 8 1.2 AP . 3 . 5 . 5 • 2 .4

Norm 33.80 32.7() 23. 14 32.03 AN 27.89 Norm 6.9 8.5 8.4 5.6 7.7 c. I. Mol. Al .98 .98 1.05 1.02 C+N+K .99

I-' 0"1 w

"" TABLE A-1 continued: . Major Oxides and Normative Minerals felsic dikes uartz rh odacite uarlz-orthoclase ~or~htritic rhtodacife HM-069A HM-3458 HM-396 HM-390 HM-416 HM-561 HM-3688 Si02 74.36 77.50 77.26 68.97 72.33 72.35 66.85 Al203 12.41 11.72 12.26 14.79 13.36 13.88 14.41 Fe203* .59 .64 .52 1. 14 1. 13 1. 18 1. 65 FeD* .79 .87 . 7 1 1. 54 1. 52 1. 60 2.23 MgO .48 .49 .48 .65 .56 .62 .94 CaO .66 .69 .67 1. 35 1.00 1. 22 2.50 Na20 3.25 3.63 3.06 3.80 3.59 3.66 3.10 K20 5.02 3.13 4.84 4.59 3.88 4.44 4.25 Ti02 .05 . 1 3 . 1 1 .41 .28 .39 .64 P205 0.00 .02 0.00 . 1 1 .05 .09 . 1 5 MnO .04 .02 .03 .04 .07 .05 .07 Sum** 97.65 98.84 99.94 97.39 97.77 99.48 96.80 QZ 34.5 42.3 38.5 25.8 33.8 30.3 26.2 co . 4 1.2 . 8 1.4 1.6 1.1 . 5 OR 30.4 18.7 28.6 27.9 23.5 26.4 26.0 AB 28.2 31.1 25.9 33.0 31.1 31.1 27.1 AN 3.3 3.3 3.3 6. 1 4.7 5.5 11.8 HY 2.2 2.1 1.9 3.0 3.0 3.0 4.3 MT .9 .9 .8 1.7 1.7 1.7 2.5 IL . 1 .2 . 2 .8 . 5 . 7 1.3 AP --- .0 --- . 3 . 1 .2 .4 Norm 9.7 15.68 13.24 15.00 30.34 AN 10.60 11.36 II Norm 3.2 3.3 2.9 II 5.5 5.2 5.4 8.0 c. I. Mol. Al 1.04 1.10 1.07 1.08 1.12 1.06 1. 01 C+N+K

1-' 0'1 ol::o TABLE A-1 continued: Major Oxides and Normative Minerals felsic dikes reddish rh odacile

HM-357 HM-.3~8 HM-539 HM-.388 HM-509 HM-557 Si02 72.Lil 73.72 74.49 77.21 75.09 76.26 Al203 13.69 13. 12 L3. 75 12.56 12.66 12.42 Fe203* .86 .80 .87 .66 .94 .66 FeD 1. 16 1.09 1. 17 .89 1. 27 .89 MgO .53 .60 .48 .39 .43 .47 CaO .87 1.13 .62 .28 .44 .60 Na20 3.42 3.47 3.03 1. 96 3.88 3.42 K20 5. 12 4.68 5.45 6.55 4.30 4.82 Ti02 .30 .26 .37 .10 . 1 3 • 1 1 P205 .09 .48 .07 .01 .02 .01 MnO .07 .05 .06 .03 .03 .02 Sum** 98.52 99.40 10D. 35 100.64 99.18 99.68 QZ 30.5 33.4 33.6 38.9 34.2 35.6 co 1.2 1.4 1.9 1.7 .9 . 5 OR 30.7 27.8 32.1 38.5 25.6 28.6 AB 29.4 29.5 25.5 16.5 33.1 29.0 AN 3.8 2.5 2.6 1.3 2.1 2.9 HY 2.4 2.5 2.1 1.9 2.5 2. 1 MT 1.3 1.2 1.3 .9 1.4 1.0 IL .6 . 5 . 7 .2 . 2 . 2 AP .2 1.1 . 2 .0 .o .o Norm 7.77 9.27 7.43 5.88 9.18 AN 11.41 II Norm 4.3 4.2 4. 1 II 3.08 4.1 3.3 c. I. Mol. Al 1.02 1.15 1.16 1. 07 1.04 C+N+K 1.07 II

1-' ~ l11 166

A P P E N D I X B

LOSS_ON IGNITION VALUES TABLE B-1: Loss On Ignition Values Sample Numbers Rock Unit Loss On Ignition Percentage HM-250 HM-306 HM-549 HM-555A HM-562 Teutonia pluton 0.63% 0.64% 0.76% 0.61% 0.50% HM-349 HM-522 HM-526 HM-529 Homer Mountain pluton 0.68% 0.34% 0.51% 0.65% HM-136 HM-499 HM-5558 leucocratic aplite dikes 0.41% 0.35% 0.26% HM-533 HM-5408 mesocratic aplite dikes 0.64% 0.60% HM-2058 HM-346 HM-478 HM-556 HM-563 HM-564 andesitic flows 1.90% 1.56% 2.12% 0.72% 0.46% 0.83% HM-232 HM-236 HM-2368 eastern rhyolite breccias 1.57% 1.70% 11.12% altered quartz HM-031A HM-547 HM-551 gabbro dikes 1.84% 1.46% 2.00% altered quartz HM-158A HM-160 HM-525 andesite dikes 1.50% 2.19% 1.80% quartz andesite HM-088A HM-093A HM-370 HM-371 HM-375 HM-405 HM-489 porphyry dikes 5.40% 6.07% 2.36% 4.06% 4.15% 2.60% 2.89% hornblende quartz HM-057A HM-082A HM-372 HM-546 HM-550 andesite porphyry dikes 2.06% 1.74% 2.38% 3.39% 3.00%

r-> 0"1 -...) TABLE B-1 continued: Loss On Ignition Values Sample Numbers Rock Unit Loss On Ignition Percentage biotite rhyodacite HM-054 HM-142 HM-309 HM-345 HM-538 porphyry dikes 1.31% 0.59% 0.66% 0.74% 0.60% quartz-orthoclase porphy- HM-3688 HM-390 HM-416 HM-561 ritic rhyodacite dikes 2.47% 1.78% 1.68% 1.67% quartz rhyodacite HM-069A HM-3458 HM-396 porphyry dikes 1.06% 0.74% 1.02% reddish rhyodacite HM-388 HM-509 HM-557 porphyry dikes 0.75% 0.80% 1.16% granophyric rhyodacite HM-357 HM-358 HM-539 porphyry dikes/sills 0.69% 1.23% 0.91%

I-' 0'1 00