UNLV Retrospective Theses & Dissertations

1-1-2000

Sodium metasomatism and magnesio- mineralization in the Wilson Ridge pluton

Deborah Ann Potts University of Nevada, Las Vegas

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Repository Citation Potts, Deborah Ann, " metasomatism and magnesio-riebeckite mineralization in the Wilson Ridge pluton" (2000). UNLV Retrospective Theses & Dissertations. 1155. http://dx.doi.org/10.25669/vxk6-khfc

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SODIUM METASOMATISM AND MAGNESIO-RIEBECKITE

MINER.ALIZATION IN THE WILSON RIDGE PLUTON

by

Deborah Ann Potts

Bachelor of Science University o f Nevada, Las Vegas 1993

A thesis submitted in partial fulfillment of the requirements for the

Master of Science Degree Department of Geoscience College of Sciences

Graduate College University of Nevada, Las Vegas May 2000

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Copyright 2000 by Potts, Deborah Ann

Alt rights reserved.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Copyright by Deborah A. Potts 2000 All Rights Reserved

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Thesis Approval UNIV The Graduate College University of Nevada, Las Vegas

March 29 2 0 ^ 0

The Thesis prepared by Deborah Ann P o tts

Entitled Sodium Metasomatism and Magneslo-R iebeckite M ineralization in the

Wilson Ridge Pluton

is approved in partial fulfillment of the requirements for the degree of Master of Science in Geology

Exantmation Committee Chair

Dean of the Graduate College

Examination Committee Member

Examination Committee Member

GraduéeradiÆe ColleiCollege Faculty Representative

PR/1017-53/l-«) u

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT

Sodium Metasomatism and Magnesio-Riebecidte Mineralization in the Wilson Ridge Pluton

by

Deborah A. Potts

Dr. Rodney V. Metcalf, Examination Committee Chair Associate Professor of Geoscience University of Nevada, Las Vegas

The Wilson Ridge pluton contains a rare occurrence of hydrothermal sodic

. Hydrothermal amphibole occurs as coatings, veinlets, dense

stockwork-type networks, and diffusive alteration fronts. Electron microprobe analyses

of the hydrothermal amphibole reveal a range of Na-rich compositions, most notably

magnesio-riebeckite. Magnesio-riebeckite in the Wilson Ridge pluton formed by Na-

metasomatism related to subsolidus hydrothermal alteration involving Na-rich fluids of

meteoric origin. Whole-rock geochemistry of samples from zones of hydrothermal

amphibole show enrichment ofNaiO (> 9 .6 wt. %) and depletion of K 2O (0 .1 5 wt. %)

and CaO (2.4 wt. %) relative to unaltered host rock. Secondary fluid inclusions

associated with zones of hydrothermal amphibole contain daughter of halite and

up to three other unknown crystals. Oxygen isotope studies suggest that granitoid rocks

containing hydrothermal have lower S ‘* 0 values (+3.68% o to + 6.97% o) than

unaltered granitoid rocks ( 5 ‘* 0 values +6.04% o to + 9.23% o).

Ill

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS

ABSTRACT...... iii

LIST OF FIGURES...... vi

LIST OF TABLES...... viii

ACKNOWLEDGEMENTS...... ix

CHAPTER! INTRODUCTION...... I Purpose ...... 2 Study Design and Instrumental Techniques ...... 3

CHAPTER 2 GEOLOGY AND GEOLOGIC HISTORY OF THE WILSON RIDGE PLUTON...... 7

CHAPTER 3 FIELD OCCURRENCES AND PETROGRAPHY OF AMPHIBOLE ALTERATION IN THE WILSON RIDGE PLUTON...... 11 Field Occurrences - Kingman W ash ...... 11 Petrography of Kingman Wash - Dike Rocks ...... 12 Petrography o f Kingman Wash - Granitoid Host Rocks ...... 13 Field Occurrences - Horsethief Canyon ...... 17 Petrography of Horsethief Canyon-Dike Rocks ...... 18 Petrography of Horsethief Canyon-Granitoid Host Rocks ...... 19 Additional Occurrences of Blue Amphibole in the Wilson Ridge Pluton ...... 19 Relationship of Blue Amphibole Mineralization to Structures ...... 20 Saline Fluid Inclusions ...... 25

CHAPTER 4 COMPOSITION OF BLUE AMPHIBOLE IN THE WILSON RIDGE PLUTON...... 29 X-ray diffractometry...... 29 Electron Microprobe Analyses of Blue Amphiboles ...... 29 Structural Formula Calculation ...... 32 Amphibole Classification and Compositional Variability ...... 33

CHAPTER 5 WHOLE-ROCK COMPOSITIONS...... 40 Whole-rock Geochemistry ...... 40

IV

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Whole-rock Oxygen Isotope Analyses ...... 40

CHAPTER 6 DISCUSSION: ORIGIN OF BLUE AMPHIBOLE IN THE WILSON RIDGE PLUTON...... 46 Evidence of Subsolidus Hydrothermal Origin ...... 46 Discussion of the Timing of Mineralization ...... 46 Evidence of Na-metasomatism ...... 47 Origin of Hydrothermal Water - Interpretation of Oxygen Isotope Data ...... 48 Source of Sodium ...... 51 Thermal Conditions of Blue Amphibole Growth ...... 52 Potential Cause of Na-metasomatism in the Wilson Ridge Pluton ...... 57

CHAPTER 7 CONCLUSIONS...... 61 Suggestions for Future W ork ...... 62

REFERENCES...... 64

APPENDIX A CHEMISTRY OF THE AMPHIBOLES...... 71

APPENDIX B ESTIMATION OF GIBBS FREE ENERGY OF FORMATION FOR MAGNESIO-RIEBECKITE...... 76

APPENDIX C ELECTRON MICROPROBE DATA AND STRUCTURAL FORMULA INTERPRETATION...... 81

APPENDIX D PETROGRAPHIC ANALYSES...... 89

APPENDIX E LIST OF SAMPLES AND ANALYSIS...... 99

APPENDIX F X-RAY DIFRACTOMETRY...... 101

VITA...... 104

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES

Figure 1 Location M ap ...... 6 Figure 2 Geologic Map of Wilson Ridge Pluton ...... 10 Figure 3 Photomicrograph of blue amphibole crystals ...... 14 Figure 4 Photomicrograph of chessboard feldspar ...... 15 Figure 5 Photomicrograph of minute, amphibole crystals ...... 15 Figure 6 Banded dike from Kingman Wash ...... 16 Figure 7 Blue amphibole veining in Horsethief Canyon ...... 21 Figure 8 Blue amphibole veining in Horsethief Canyon ...... 22 Figure 9 Horsethief Canyon rocks altered by metasomatism ...... 22 Figure 10 Photomicrograph of altered magmatic hornblende ...... 23 Figure 11 Photomicrograph of radiating cluster of amphibole grains ...... 24 Figure 12 Fluid inclusions in containing a daughter ...... 27 Figure 13 Fluid inclusions in quartz containing a daughter crystal ...... 28 Figure 14 a-b SEM images of microprobed amphiboles ...... 31 Figure 15 Graphical representation of calcic amphiboles ...... 35 Figure 16 Graphical representation of alkali amphiboles ...... 36 Figure 17 Graphical representation of sodic-calcic amphiboles ...... 37 Figure 18 a-b Graphically determined compositions for amphibole ...... 38 Figure 19 Graphical representation of sodium in amphibole structure ...... 39 Figure 20 Graphical representation of Na-vs-K ...... 44 Figure 21 Distribution of for various rocks ...... 49 Figure 22 Miocene-Pliocene evaporate deposits near southern Nevada...... 53 Figure 23 Thermal stabilit) limits for riebeckite and magnesio-riebeckite 56 Figure 24 Schematic representation of the amphibole structure ...... 65 Figure 25 Regression values for magnesio-riebeckite ...... 80

VI

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES

Table 1 Representative Electron Microprobe Analyses ...... 30 Table 2 Whole-rock XRF Analyses ...... 42 Table 3 Whole-rock Oxygen Isotope Analyses ...... 43 Table 4. Summary of site occupancies in amphiboles ...... 73 Table 5. Magnesio-riebeckite forming reactions ...... 78

vn

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGEMENTS

I thank my thesis advisor Rodney Metcalf for his instruction, guidance, and patience throughout this project. Without his support and knowledge of amphibole mineralogy , this study would not have been possible. 1 thank Gene Smith. Fred Bachhuber and Jean Cline for their patience and help, always at the last minute. I thank Clay Crow and Alex Sanchez for their help in preparation and X-ray Fluorescence. Finally. 1 wish to thank Anne Wyman for her guidance, friendship, enthusiasm, and love of geology, which she shared with everyone she met.

vni

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER I

INTRODUCTION

Magnesio-riebeckite, a sodic amphibole, has a large temperature and pressure

stability field, yet it's occurrence in nature is rare. Specialized conditions are necessary

for the formation of magnesio-riebeckite including an abundant source of sodium and

oxygen fugacity conditions necessary to produce coexisting Fe“" and Fe^T Sodium

metasomatism is generally characterized by the exchange reaction of Na^ for K"'” or Ca’’ in

mafic . Minerals such as hornblende may be partially to totally replaced by more

sodium-rich minerals. Although magnesio-riebeckite requires specialized conditions

leading to its formation, many occurrences have been documented. Magnesio-riebeckite

is most common in metamorphic terranes and Precambrian banded iron formations but

has also been identified in fenites (Kresten. 1988), carbonitites (Mian and LeBas, 1986),

porphyry copper systems (Battles, 1990; Carten, 1986; Dilles, 1987 and 1982). some

(Lyons, 1972; Collins et al, 1982; Kiimard. 1985) and in evaporate deposits in

the Green River Formation. Utah (Ernst. 1960). Rarely has magnesio-riebeckite been

documented in hydrothermal systems (Hawley, 1937; Frohberg, 1939; Strong and Taylor,

1984).

Blue amphibole, of apparent hydrothermal origin, is found associated with the

Wilson Ridge pluton. I use the term, blue amphibole, to represent all compositional

variations of sodic. sodic-calcic, and alkali amphiboles, including magnesio-riebeckite, as

1

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. defined by Leake (1978) and Hawthorne (1989). The Wilson Ridge pluton (Figure 1 )

formed during a period of mid-Miocene extension in the Lake Mead area of southern

Nevada and northwestern Arizona (Smith et a!.. 1990; Larsen and Smith, 1990; Metcalf

el al.. 1993). Wilson Ridge is the northernmost pluton in the Colorado River extensional

corridor (CREC; Howard and John, 1987). E. 1. Smith and E. M. Duebendorfer

(unpublished data, 1992) first documented the occurrence of blue amphibole associated

with the Wilson Ridge pluton. Field observations suggest that amphibole alteration is

spatially associated with north-trending faults and late-stage dikes which cross-cut the

pluton host rock. Regions o f sodium enrichment and potassium depletion have been

recognized in the Wilson Ridge pluton. suggesting removal or redistribution of potassium

and sodium by fluids which circulated through the pluton (Feuerbach. 1986; Larsen and

Smith. 1990; Smith et al.. 1990). This study is important because late-stage hydrothermal

systems in tectonically active (extensional) zones, such as the Wilson Ridge pluton.

generally are not studied unless associated with ore deposition. The processes which

produce barren systems are as important to our knowledge as those which produce, and

aid in exploration of economic ore deposits.

Purpose

This study was initiated to resolve the origin(s) of blue amphibole in the Wilson

Ridge pluton. The purpose of this thesis is to document and to test hypotheses regarding

the origin of the blue amphibole. The hypothesis I am testing is that blue amphibole is

the result of Na-metasomatism related to hydrothermal activity. The objectives of this

study are to document and describe the occurrence of blue amphibole in the Wilson

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ridge pluton by determining ( 1 ) the composition and possible compositional variation in

blue amphibole. (2) whether Na-enrichment was responsible for amphibole alteration and

whether Na-enrichment was coupled with K-depletion, (3) the relative timing of blue

amphibole formation with respect to pluton crystallization, dike emplacement, tilting, and

faulting of the pluton. (4) the geochemical environment that led to blue amphibole

formation in the Wilson Ridge pluton, and. (5) conducting oxygen isotope studies to

constrain the source of the hydrothermal fluids responsible for amphibole alteration.

Finally. I plan to use the collected data to suggest models for the formation of blue

amphibole in the Wilson Ridge pluton.

Study Design and Instrumental Techniques

The following methods were utilized to test the hypothesis that blue amphibole is

the result of Na-metasomatism related to hydrothermal activity. The initial phase of this

study documented the field occurrence of blue amphibole and its relationship to fault

zones and associated breccia zones. The association of blue amphibole to late-stage dikes

was also examined. Amphibole-bearing materials from veins, fault zones, and

surrounding host rock (altered and unaltered) were collected for analysis and imaltered

samples were collected for comparison. Existing data from previous studies of

published/unpublished work were also incorporated into this study. I used X-ray

diffraction (XRD) to confirm the presence of riebeckite in vein material. Mineral

associations and textural relationships in samples containing blue amphibole have been

determined by pétrographie analysis. The sampling strategy for the pétrographie study

involved collecting samples containing blue amphibole from the center of veins, faults

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4

and fractures, and then from varying distances outw ard from the center of most intense

alteration to areas with no alteration. Dikes containing diffusive blue banding (similar in

appearance to Liesegang banding) were also sampled. A pétrographie study was

completed on 30 thin sections of altered and unaltered granitoid rocks, dike rocks, and

breccias (Appendix D). Samples consist of 10 altered dikes. 1 unaltered dike. 8 altered

granitoids, and 3 visibly unaltered granitoids. Most samples are from the Kingman Wash

and Horsethief Canyon areas. Two severely altered samples (CL5-15/4 and CL5-15/5)

were collected from the eastern edge of the pluton. near the abandoned Cohenour Mine

shaft (36° 3' 55" latitude. 114° 34' 53" longitude), on Lake Mead National Recreational

Area road number 71. Samples containing rare crystals of blue amphibole were

examined using an Olympus SZ-PT Binocular microscope.

1 observed fluid inclusions containing daughter crystals, in quartz, that were

spatially associated with blue amphibole mineralization. This was done to estimate the

salinity of fluids that may be responsible for blue amphibole mineralization. Electron

microprobe analyses quantified the chemistry, zonation, and chemical variation in blue

amphibole and were used to compare these results with similar data for amphiboles in

unaltered rocks of the Wilson Ridge pluton. Whole-rock X-ray fluorescence

spectrometry (XRF) analyses identified NaiO values in altered and unaltered rocks and

consequently provided a test for Na-metasomatism. Altered rock samples were selected

on the basis of their close spatial relationship to blue amphibole veins. Whole-rock

oxygen isotope data from altered and unaltered samples were obtained to identify

potential sources of fluids responsible for blue amphibole mineralization.

X-ray diffraction and X-ray spectrometry were conducted at the University of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Nevada, Las Vegas. A Rigaku 3030 X-ray Spectrometer was used for X-ray fluorescence

analyses at the University of Nevada, Las Vegas, to analyse major and trace elements in

whole-rock samples. Fused glass disks were prepared for each sample for major element

analy sis, using the method outlined by Morikawa (1996). which consists of heating

1.7000 g ± 0.0005 g sample. 8.5000 g ± 0.0005 g lithium tetraborate, and 0.2740 g ±

0.0005 g ammonium nitrate at 1100 °C for thirty minutes in covered gold-platinum

crucibles and pouring the resultant melt into heated gold-platinum molds. Major element

standardization was performed using a fused disk of U.S.G.S. standard AGV-1 (dated

10/96). An unknown (NBS-688) was then run to check against published values. Trace

element standardization used a fused disk of U.S.G.S. standard BFIVO-1 (dated 10/96)

and MAG-1 (dated 10/96) as an unknown to check against published values.

Microprobe studies were conducted using a JEOL 733 Electron Microprobe at the

University of New Mexico. Operating conditions were 15 kV and 20 mA. The electron

beam was de focused to ~3 pm to prevent loss of Na^O and KiO. Natural silicate

minerals were used as standards.

Oxygen isotope analyses were conducted at the New Mexico Institute of Mining

and Technology. Soccorro. New Mexico, under the direction of Dr. Andrew Campbell.

Oxygen isotope values for igneous rocks from the Wilson Ridge pluton are compared to

standard mean ocean water (SMOW). Values of Ô^^O, which are positive, indicate that a

sample is enriched in ^^O relative to SMOW;

1 8 o /16q (SMOW) = 1.008 l^O/^^O (NBS-1) National Bureau of Standards

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 20 Miles

20 KiioiTMters

Wilson Ridge Pluton Paint Pots

McCulkMiQh

Mt. Perkins * 0 Pluton Nelson Pluton

NEVADA

ARIZONA

Figure 1. Location map of the Lake Mead area of southern Nevada and northwest Arizona Las Vegas Valley shear zone (LWSZ); Lake Mead fault system (LMFS); Boulder City pluton (BC); Hamblin Bay Fault (HBF); Hamblin-Cleopatra volcano (HCV); Hoover Dam volcanic rocks (HD); River Mountains stock (RMS). Map modified from Larsen and Smith (1990) and Metcalf et al. (1993).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2

GEOLOGY AND GEOLOGIC HISTORY OF

THE WILSON RIDGE PLUTON

The Wilson Ridge pluton, northwestern Arizona is an epizonal, calc-alkaline

pluton that formed during mid-Miocene extension in the Lake Mead area Nevada and

Arizona (Smith et al.. 1990; Larsen and Smith, 1990; Metcalf et al.. 1993; Anderson et

al.. 1994). The pluton lies approximately 20 km east of the River Mountains and appears

to have been separated from the River Mountains by movement along the Saddle Island

detachment fault (Weber and Smith, 1987; Duebendorfer et al.. 1990), and/or along the

Lake Mead fault system (Anderson et al., 1994). Geochemical correlation suggest a

cogenetic relationship between the Wilson Ridge pluton and volcanic rocks in the River

Mountains (Weber and Smith, 1987; Smith et al., 1990; Metcalf, unpublished isotopic

data; Duebendorfer et al., 1998).

The intermediate composition of the Wilson Ridge pluton has been shown by

petrography, and chemical and isotopic data to result primarily from the mixing o f mafic

and magmas with lesser amounts of fractional crystallization (Larsen and Smith,

1990; Smith et al., 1990; Metcalf et al., 1993). The northern and central parts of the

pluton (Figure 2) consist of quartz monzonite, quartz monzodiorite, and monzodiorite of

the Teakettle Pass suite (13.5 ± 0.4 Ma; K-Ar on biotite separates; Larsen and Smith,

1990). In the southern part of the pluton, the Teakettle Pass suite is in intrusive contact

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with Precambrian rocks. The Horsethief Canyon diorite (13.4 ± 0.4 Ma; dating method

K-Ar on biotite separates; Larsen and Smith. 1990). in the southern portion, intruded

Precambrian basement rocks (Larsen and Smith. 1990; Metcalf et al.. 1993; Anderson et

al.. 1994). The Teakettle Pass quartz monzonite consists of quartz, plagioclase,

orthoclase. biotite. and minor hornblende, sphene. and apatite, with trace zircon

(Feuerbach. 1986; Larsen and Smith. 1990). Quartz monzodiorite and monzodiorite

contain plagioclase, biotite. hornblende, and late forming quartz and orthoclase, with

accessory sphene. apatite, and trace zircon (Larsen and Smith. 1990). The Teakettle Pass

suite contains abundant mafic micro granitoid enclaves consisting of basalt and diorite.

The enclaves have crenulate. fine grained margins resulting from instantaneous chilling

upon contact with the cooler felsic melt (Larsen and Smith. 1990). Characteristics such

as compositional banding, schlieren, or pillow-like enclaves provide evidence of

incomplete mixing in the Teakettle Pass portion of the pluton. The intermediate phases

of the Teakettle Pass suite intruded the Horsethief Canyon diorite. producing net-veins

and incorporating angular xenoliths of the diorite (Larsen and Smith. 1990; Metcalf et al.,

1993).

Hornblende geobarometry (Metcalf and Smith, 1991) indicates a pressure range

of 1.9 to 3.1 ± 0.5 kbars from north to south across the pluton, corresponding to a depth

of crystallization of 6.9 to 11.3 ± 0.8 km for the deeper portion (south) of the pluton

(Metcalf. 1997. unpublished data), as well as the conclusion that 17° of post­

solidification northward tilt occurred in the pluton (Metcalf and Smith, 1991).

Emplacement of at least 400 north-trending dikes has resulted in approximately 20

percent east-west extension in the pluton (Feuerbach, 1986).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The northern portion of the pluton which is 12.5 Ma (^^ArP^Ar on biotite

separates from quartz monzonite. diorite. and a rhyodacite dike) is cut by northeast-

striking high-angle faults of the Lake Mead fault zone (Anderson et al.. 1994). North-

striking tabular dikes of , granite porphyry, . basalt, and biotite lamprophyTe

that have widths of 0.5-30 m. and north-striking high-angle normal faults cross-cut the

pluton (Feuerbach. 1986; Larsen and Smith. 1990). At Flamingo Cove (Figure 2).

Naumann (1987) observed low-angle faults cutting the Wilson Ridge pluton. The low-

angle faults also place the pluton in contact with lower Paleozoic sedimentary rocks

(Naumann. 1987).

The youngest igneous rocks in the area are 5.89 Ma to 4.61 - 4.3 Ma basalt flows

at Fortification Hill and Petroglyph Wash (Figure 2). respectively (4.3 Ma. Anderson et

al.. 1972; Feuerbach et al.. 1991; K-Ar on groundmass plagioclase separates). These

basalt flows are spatially associated with, and/or overlie major north-northwest striking

high-angle faults that cut the pluton (Smith et al.. 1990; Metcalf and others. 1993). In

Petroglyph Wash, basalt vents are not clearly associated with north-striking high-angle

normal faults, yet the basalt flows are unfaulted and overlie the faults. Movement along

the north-striking high-angle normal faults, therefore, must have occurred after pluton

crystallization and prior to the eruption of the basalt flows. North-northwest striking

high-angle faults cut the low-angle faults and the Wilson Ridge pluton, but not

Fortification Hill basalts. Movement along these faults occurred between 13.5 to 5.89

Ma.

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114-40* 114' 37'30 114* 35’ Miles 5 Kilometers Flamingo Cove Î

I 36"07'30" ' 36*07'30*'

Cohenour Mine and Cohenour Loop Rd Petroglyph -OS’ Canyon

2*30“. r 2’ 30"

36-00' - - 36*00'

Horsethief Canyon

114*40 * 114*37W 114*35’ Figure 2. Generalized geology of the Wilson Ridge pluton. Fortification Hill (FH). Modified from Feuerbach (1986), Naumann (1987), Larsen (1989), and Metcalf (1993). Explanation Quaternary, Pliocene & j Fine-grained Quartz I Horsethief Canyon Miocene sedimentary _ Monzonite I Diorite I Patsy Mine □ rocks s Quartz Monzonite I Volcanics Fortification Hill Precambrian Basalts Monzodiorite and Quartz Monzodiorite Basement Rocks High-angle Normal Faults

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3

FIELD OCCURRENCES AND PETROGRAPHY

OF AMPHIBOLE ALTERATION IN

THE WILSON RIDGE PLUTON

Two major occurrences of blue amphibole mineralization occur on the west side

of the pluton. one in Kingman Wash, and the other in Horsethief Canyon. At both

localities, blue amphibole is widespread within dikes and granitoid host rocks, as coatings

on joints and fault surfaces, and along dike edges. Blue amphibole also occurs in north-

south striking latite (QAPF diagram: Streckeisen. 1976) dikes that represent the last

phases of magmatism in the Wilson Ridge pluton. Complete pétrographie descriptions,

for all thin-sections studied, are included in Appendix D. and correspond to the following

descriptions.

Field Occurrences - Kingman Wash

Blue amphibole occurs in shear zones in the eastern Kingman Wash area and

is commonly fibrous to powdery. Another occurrence of blue amphibole mineralization

can be observed in a sub-horizontal, east-west trending dike of fine-grained latite (QAPF

diagram; Streckeisen. 1976). This latite dike contains minute needlelike to prismatic blue

crystals of blue amphibole throughout (sample KWlO-18/1). The dike intruded an

extensively fractured and/or sheared portion o f the pluton characterized by extensive

11

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amphibole alteration, but. the dike is not fractured and sheared as is the surrounding host

rock. Both the dike and the dike host contain blue amphibole.

Another pale-blue, fine-grained latite dike occurs near the western end of

Kingman Wash. This dike intruded a portion of the pluton characterized by extensive

development of blue amphibole. The dike strikes northwest and dips approximately 45°

east. Pétrographie examination of dike rock revealed minute needlelike to prismatic blue

crystals. Field relationships indicate that rocks of the Kingman Wash portion of Wilson

Ridge pluton have been extensively faulted and sheared and subjected to hydrothermal

alteration.

Samples containing rare crystals of blue amphibole (Figure 3) from a fault zone

within Kingman Wash (36° 2" 15' latitude. 114° 37" 12' longitude) exhibit a gradational

color change from dark green to black to a pale-blue on the opposite end of the crystal.

Petrography of Kingman Wash Dike Rocks

Dike rocks from the Kingman Wash area consist of latite (QAPF diagram;

Streckeisen. 1976). and generally contain anhedral quartz, subhedral to euhedral

plagioclase. anhedral K-spar. subhedral to anhedral hornblende, and minor amounts of

apatite, sphene. large sericite (> 10pm), and (Appendix D). Olive

green, euhedral magmatic hornblende grains generally have pale-blue overgrowths or

reaction rims, visible in plane polarized light, or appear as altered and ragged relict

grains. Plagioclase phenocrysts in most samples exhibit minor to moderate sericitic

alteration. Some feldspar phenocrysts have a chessboard texture (Figure 4). The

groundmass. generally consisting of plagioclase and potassium feldspar, contains minute

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(< 2pm width), acicular pale-blue needles (K.Wl-12/5. KW5-396/7. KW10-I8/1) of

amphibole throughout (Figure 5) and rare minute (< 1 pm) inclusions of red iron-oxide.

Blue amphibole also occurs in pale blue dikes (KW5-396/7) as diffusive alteration fronts,

similar to Liesegang banding, in Kingman Wash (Figure 6).

Petrography of Kingman Wash - Granitoid Host Rocks

Granitoid host rocks from Kingman Wash consist of monzonite. monzogranite.

quartz monzodiorite. diorite. and granite (QAPF diagram of Streckeisen. 1976).

Monzonite host rocks contain magmatic hornblende with pale-blue overgrowths or

reaction rims. Some hornblende is extensively altered to chlorite and/or pale-blue

amphibole. Biotite and opaque oxides (mostly magnetite), are found within remnant

crystals of altered hornblende. Some biotite is altered to chlorite. Veinlets of chlorite/

riebeckite occur throughout some samples. Most plagioclase crystals show only minor

sericitic alteration.

Monzo-granite contains anhedral quartz, subhedral to euhedral plagioclase.

anhedral alkali feldspar, extensively altered magmatic hornblende and biotite, and minor

amounts of apatite, sphene, and opaque minerals. Secondary chlorite veinlets are present

throughout. Plagioclase exhibits weak to moderate sericitic alteration. Magmatic

hornblende has been altered to chlorite with few a remnant hornblende grains. Less than

1 % of feldspar phenocrysts exhibit a chessboard texture. Opaque oxide minerals most

likely are magnetite because they lack the reddish cast on thin, outer edges characteristic

of hematite.

Quartz monzodiorite consists of anhedral quartz, euhedral to subhedral

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Figure 3. Rare crystals of minute, pale- to silvery- blue colored amphibole have been found as open-space filling in one fault zone in Kingman Wash. (Field of view 8.5 mm, reflected light).

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Figure 4. Photomicrograph (KW10-18/3b) of chessboard feldspar adjacent to vein of magnesio-riebeckite (not in photo). Microprobe analyses of eight points, including core and rim yield nearly pure (Abg»). (Field of view = 3.25mm, cross polarized light).

Figure 5. Minute, acicular crystals of disseminated blue amphibole. Sample KW4-27/5 (Field of view = 360 pm - plane polarized light).

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Figure 6. Blue diffusive banding in dike from the Kingman Wash area of the Wilson Ridge piuton. Bands are minute, needlelike to prismatic blue amphibole crystals. Six inch ruler for scale.

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plagioclase. subhedral to anhedral hornblende, subhedral biotite. and minor sphene and

opaque oxides. Minor sericitic alteration occurs in the core of some plagioclase

phenocr>'sts. Most magmatic hornblende phenociysts have overgrowths or reaction rims

of pale-blue amphibole.

Diorite consists of anhedral quartz (< 5%), subhedral to euhedral plagioclase,

subhedral hornblende, and minor euhedral to subhedral sphene. apatite, and opaque

minerals. Plagioclase is not affected by sericitic alteration. Hornblende does not appear

visibly altered and lacks the overgrowths or reaction rims of pale-blue amphibole

observed elsewhere.

Granite consists of anhedral quartz, subhedral plagioclase. anhedral K-feldspar.

subhedral biotite and hornblende, and minor sphene and opaque minerals. Plagioclase is

not altered by sericitic alteration. Hornblende appears visibly unaltered, without

overgrowths or reaction rims of pale-blue amphibole.

Field Occurrences - Horse thief Canyon

The occurrence of blue amphibole at Horsethief Canyon is similar to that of

Kingman Wash inasmuch as amphibole alteration is widespread as a coating on fractures,

joints, fault surfaces. Pale blue dikes containing minute (< 2 pm wide), prismatic pale-

blue amphibole also occur. In addition, blue amphibole in Horsethief Canyon is found in

veinlets and dense stockwork-type networks that permeate the siuroimding host rock

(Figures 7 and 8).

In Horsethief Canyon, diorite and pegmatitic diorite xenoliths are incorporated

into quartz monzonite, quartz monzodiorite, and monzodiorite of the Teakettle Pass suite

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(Larsen and Smith. 1990: Metcalf et al. 1993). Some diorite xenoliths. near stockwork-

t\'pe blue amphibole veining. whether fine crystalline or pegmatitic. exhibit a bluish color

throughout. In hand sample, euhedral hornblende crystals have a silvery-blue coating. In

some instances this blue alteration extends into more felsic rocks enclosing the diorite

xenoliths. creating a ver> noticeable blue color in the rock (Figure 9).

Petrography of Horsethief Canyon - Dike Rocks

Most dike rocks fi-om Horsethief Canyon are aphanitic or porphyritic latite

(QAPF diagram of Streckeisen. 1976). The Latite dikes generally consist of anhedral

quartz, subhedral to euhedral plagioclase. anhedral K-feldspar. subhedral to anhedral

hornblende, and minor amounts of apatite, sphene. and muscovite. Some opaque oxide

minerals exhibit cubic habit while others have red internal reflections characteristic of

hematite. Plagioclase phenocrysts are moderately to intensely altered to sericite.

Groundmass feldspars have weak to moderate sericitic alteration. Original magmatic

hornblende is partially altered and rimmed by blue amphibole (Figure 10) while other

hornblende phenocrysts are moderately to severely altered to chlorite. Biotite and opaque

oxide minerals are found within altered hornblende grains. Some biotite (< 5%) is altered

to chlorite. The groundmass of some dikes contains tiny acicular pale-blue amphibole

needles (HTl 0-27/1 ). Secondary veining commonly cross-cuts chlorite/riebeckite

veining in very altered samples of latite dike rocks from Horsethief Canyon (HTI-I3/9

and HTl-13/11).

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Petrography of Horsethief Canyon - Granitoid Host Rocks

Granitoid host rocks from Horsethief Canyon consist primarily of monzonite and

monzodiorite (QAPF diagram of Lemaitre et al.. 1989; Streckeisen. 1976) and contain

anhedral quartz, subhedral to euhedral plagioclase. anhedral K-feldspar. subhedral to

anhedral hornblende, opaque oxides, and minor amounts of apatite and sphene.

Plagioclase phenocrysts are moderately to intensely altered to sericite while

approximately 25% have sericitic alteration only within their cores. Less than 2% of

feldspar phenocrysts display a chessboard texture One sample contains feldspar with

tartan twinning, indicating that the feldspar is microcline. Hornblende and biotite are

extensively altered. Rims and interiors of hornblende phenocrysts are altered to pale-blue

amphibole. Some hornblende has been altered to chlorite. Blue amphibole also occurs as

radiating clusters (Figure 11) in severely altered granitoid host rocks. Opaque oxide

minerals are associated with severely altered hornblende phenocrysts and appear within

remnant amphibole phenocrysts. Chlorite veining occurs throughout dike host rocks.

Additional Occurrences of Blue Amphibole

in the Wilson Ridge Piuton

Other occurrences of blue amphibole are found in Gilbert Canyon and at James

Bay (Figure 2). Gilbert Canyon lies in the northeastern portion of the piuton, just south

of Lake Mead. On the eastern side of Lake Mead National Recreational Area (LMNRA)

road number 71 (northern portion) north of Petroglyph Canyon and south of the

abandoned Cohenour Mine shaft lie flows of Fortification Hill basalt. Blue amphibole

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mineralization occurs as veins or sheets at the base o f the basalt flow, adjacent to

LMNRA road 71.

The James Bay portion of the canyon. LMNRA road 73. is characterized by dike-

on-dike morphology as described by Feuerbach et al.. ( 1986). Blue amphibole is sparse

at this locality, but where observed, it appears as a very thin blue coating on fractures and

joints.

Relationship of Blue Amphibole

Mineralization to Structures

The occurrence of blue amphibole in Kingman Wash area is related to a number

of structures, including low-angle faults, high-angle faults, brittle fracture zones

associated with late-stage dikes, and as minute crystals in banded dikes. In the low-angle

structures, alteration is parallel to fault planes, extending outward into adjacent host rocks

fas much as 3 meters) and. in some cases, exhibiting alteration fronts as in the pale blue

dikes on the north and south side of Kingman Wash. In most localities, the alteration

forms a pale, silvery-blue coating on joints and fractures, although pods of pale-blue fault

gouge do occur in some sheared areas. Blue amphibole has been observed as a thin

coating on high-angle fault surfaces in Kingman Wash along the western flank of Wilson

Ridge. Blue amphibole also occurs as a rare hydrothermal breccia in one locality in

Kingman Wash (36° 2" 19' latitude, 114° 37" 12' longitude). This breccia occurs along

the edge of a nearly horizontal dike on the north side of Kingman Wash and consists of

clasts of angular feldspars and minor opaque oxides cemented in a pale-blue matrix.

Banded dikes, containing tiny acicular blue amphibole crystals in the groundmass, are

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m

Figure 7. Blue amphibole veining (grey veins in lighter host rock) is observed in north-trending brittle fracture zones of tectonic and/or hydrothermal origin. Rock hammer for scale.

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Figure 8 . Blue amphibole veining (shown as dark grey, adjacent to rock hammer) in brittle fracture zones at Horsethief Canyon, Wilson Ridge piuton, northwest Arizona.

% À

Figure 9. Felsic rocks and diorite enclaves in Horsethief Canyon exhibit a blue color (shown here as dark grey) as a result of metasomatic processes. Rock hammer for scale.

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4

Figure 10. Photomicrograph of magmatic hornblende (HTl 0-27/1 a) partially altered to and overgrown by "blue" amphibole. Microprobe analyses indicate magnesio-riebeckite overgrowth on tremolitic hornblende. (Field of view = 1.3mm, cross-polarized light).

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I

Figure 11. Radiating clusters of blue amphibole. Sample KW4-17/5 (Field o f view = 0 .8 mm - plane polarized light).

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found in Kingman Wash. Some dikes exhibit visible bands of blue, while others contain

red bands, which appear as microscopic red-orange iron-oxide minerals disseminated

throughout the groundmass. One dike. located at the eastern end of Kingman Wash road,

exhibited a pale grey-blue color, owing to the presence of microscopic blue acicular

amphibole crystals disseminated throughout the groundmass. In Horsethief Canyon,

blue-amphibole occurs along north-northwest striking high-angle faults, fractures, and

dike edges as coatings and alteration fronts extending away from the structures into the

host rocks. Na-metasomatism affects host rocks as far as 3 meters from the dike edge

(Samples HTl-13/9 and HTl-13/11).

Saline Fluid Inclusions

Pétrographie examination of thin-sections from the Kingman Wash and

Horsethief Canyon samples associated with magnesio-riebeckite mineralization reveal

that some fluid inclusions, in quartz, contain daughter crystals of halite, occasionally

sylvite. and up to three other imknown phases (Figures 12 and 13). The fluid inclusions

containing daughter crystals are liquid-rich and exhibit consistent liquid/vapor phase

ratios. As a minimum. 26 weight percent NaCl is required before a halite cube will

precipitate from a fluid as a separate phase within a fluid inclusion (Roedder. 1984);

inclusions with halite cubes, therefore, have salinities of > 26 weight percent NaCI

equivalent. The fluid inclusions containing daughter crystals have been characterized as

secondary inclusions, based on criteria by Roedder (1984). Fluid inclusions occur as

planar arrays and trains which terminate at grain edges. Many fluid inclusions containing

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daughter crystals are necked-down and exist in planes which terminate at the edge of the

quartz grain.

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Figure 12. Fluid inclusions containing daughter crystals in quartz, adjacent to fibrous vein of magnesio-riebeckite (dark field upper left) and minute acicular crystals of blue amphibole. Sample KWl-12/5 (Field of view = 0.73mm - plane polarized light).

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^ C tlto y. T m • ’9- . - r . ' V F. • ^

• \ i , « - i . '* ^ <Êh “ # .

Figure 13. Fluid inclusions in quartz containing multiple daughter crystals. Cube is halite, other solids unknown. Sample KWl-12/5 (Field of view = 255pm - plane polarized light).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4

COMPOSITION OF BLUE AMPHIBOLE

IN THE WILSON RIDGE PLUTON

X-ray diffractometry

X-ray diffractometry of two samples containing blue amphibole from Horsethief

Canyon confirmed the presence of riebeckite within vein material (Appendix F). Based

on these results, further study using electron microprobe was indicated to determine the

distribution of magnesio-riebeckite in the sample and the exact chemical composition of

the amphiboles.

Electron Microprobe Analyses of Blue Amphiboles

Electron microprobe studies were conducted on five polished thin-sections. two

from dikes and three from veined granitoid samples. Representative analyses are shown

in Table 1 and Figure 14, A-B. Amphibole crystals displaying microscopic patchy areas

with differing optical properties were chosen because they reflect differing mineral

compositions. Amphibole crystals, which were selected for microprobe analysis, have

green to blue-green cores grading to pale-blue rims. Blue, fibrous vein material was

selected for probe analyses due to the minute, acicular indigo-blue crystals, their

29

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■o I I

% I Table 1. Representative Flectron Microprobe Analyses of Amphibole and Structural I'ormula Interpretation. œC/) o'3 Areas alTccted by Na-alteration exhibit increased NaA) values. See Appendices A and C. O

CD Sample; KW 101813a 8 Point # Si02 AI203 Ti02 FeO* MgO CaO K20 Na20 MnO TOTAL 3 - core 48.40 5.47 1.14 12.59 15.53 11.37 0.54 1.28 0.50 96,82 â' 6 -rim 56.38 0.53 0 12.18 17.17 3.43 0,53 6.76 0 96.98 Structural Formula 3 Point# Na(A) K(A) A-total Na(M4) Ca(M4) Fe(M4) Ca(M1-3) Fe(M1-3) Fe3+ Mg Mn Ti Al(vi) Al(iv) Si T AI(T) CD 3 0.18 0.10 0.28 0.18 1.76 0.06 0 0,63 0.84 3.35 0.06 0.12 0 0.93 7.00 7.94 0.93 C 6 0.34 0.02 0.36 1.49 0.50 0.00 0 0.60 1.36 3.58 0 0 0 0.09 7.87 7.96 0.09 p.

CD Sample: HI 1-13/9a ■o O Point # Si02 AI203 Ti02 FeO* MgO CaO K20 Na20 MnO TOTAL CQ. 7 - rim 54.73 0.61 0.43 18.24 12.98 3.31 0.15 5.48 0 95.93 o 9 - core 51.46 1.92 0.54 14.70 13.88 8.51 0 2.79 0.39 94.20 16-rim 55.64 0.91 0.78 19.45 12.25 1.17 0.13 6.68 0 97.00 ■o o Structural Formula Point # Na(A) K(A) A-total Na(M4) Ca(M4) Fe(M4) Ca(M1-3) Fe(M1-3) Fe3+ Mg Mn Ti Al(vi) Al(iv) Si T AI(T)

CD 7 0.05 0.03 0.08 1.47 0.51 0.02 0 0.64 1.53 2.78 0 0.05 0 0.10 7.86 7.97 0.10 Q. 9 0.17 0 0.17 0.63 1.36 0.01 0 1.08 0.73 3.07 0.05 0.06 0 0.34 7,64 7.98 0.34 16 0.01 0.02 0.03 1.83 0.17 0 0 0.58 1.73 2.59 0 0.08 0.03 0.12 7.88 8.00 0.15

■D CD Sample: HT 10-27/1

C/) Point # Si02 AI203 Ti02 FeO* MgO CaO K20 Na20 MnO TOTAL C/) 4 - core 44.65 8.03 1.99 13.39 13.99 11.19 0.82 1.95 0.60 97.08 5-rim 53.90 0.67 0.80 14.26 14.65 3.61 0.24 5.70 0 93.83 11 - rim 55.14 0.65 0.42 17.21 13.85 2.07 0.12 6.55 0 96.00 Structural Formula Point # Na(A) K(A) A-total Na(M4) Ca(M4) Fe(M4) Ca(M1-3) Fe(M1-3) Fe3+ Mg Mn Ti Al(vi) Al(iv) Si T AI(T) 4 0.35 0.15 0.50 0.21 1.76 0.02 0 0.87 0.76 3.07 0.08 0.22 0 1.40 6.58 7.98 1.40 5 0.19 0.04 0.23 1.42 0.56 0.01 0 0.51 1.22 3.19 0 0.09 001 0.12 7,86 7.98 0.12 11 0.16 0.02 0.18 1.65 0.32 0.03 0 0.38 1.64 2.94 0 0.04 0 0.11 7.85 7.96 0.11 o 31

3 2 4

Figure 14 A-B. SEM image o f amphiboles analyzed by electron microprobe. Numbers in photos refer to individual analyses found in Appendix (Table 1 and Appendix C). A. Replacement of magmatic amphibole (KW10-18/3a) by late-stage sodic amphibole. B. Fibrous, vein-filling amphibole (HT l-l3/9a).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. anomalous extinction, , and pleochrosim in shades of blue which is

characteristic of riebeckite and magnesio-riebeckite and not magmatic hornblende.

Microprobe results indicate that amphibole crystals with dark green to blue-green

cores more closely correspond to magmatic amphiboles than do outer rims of pale-blue

material. Cores contain approximately 11.0 weight percent CaO and 5.5 to 8.0 weight

percent AI 2O3. NaiO and K 2O of values for core compositions are generally 2.5 weight

percent and 0.5 weight percent, respectively. Rim compositions are very different with

average CaO values substantially less than cores at 2.5 weight percent, average AI 2O3

values were reduced to 0.5 to 0.9 weight percent. Na 2 0 values for rim compositions were

as high as 9.7 weight percent, and K 2O values were reduced as compared to cores,

generally 0.2 weight percent. One chessboard feldspar was analyzed throughout the

crystal and determined to be nearly pure albite (composition AI 599 ).

Structural Formula Calculation

Total iron content (ferric + ferrous) can be determined by electron microscopy,

however, valence state cannot be resolved. Several methods have been devised to

estimate the Fe'^/Fe^^ ratio from microprobe data. Each method requires assumptions

which may affect their validity, including the assumption of 23 oxygen atoms in the

structtu-al formula of the amphibole (Hawthorne. 1981). The following method, used by

Spear (1980. 1981) uses the cation normalization structural formula calculation to

estimate the abimdance of Fe^\ Maximum and minimum estimates of Fe^^ may be

calculated by normalizing the total number of cations to 13 and 15, respectively, and then

totaling the anions (oxygen). When normalizing to 13 cations, K, Na, and Ca are

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omitted, while only K and Na are omitted when normalizing to 15 cations. Frequently

the result is less than the required niunber of anions (23). When this occurs, the deficient

amount of oxygen is totaled and twice that amount is assigned to Fe^*. A spread sheet for

manipulating raw microprobe data (Metcalf, unpublished data) was used to resolve the

problem of the valence state of iron contained within the sodic-amphibole (See

Appendices .A and C). Thirteen cation normalizations were used to estimate ferric iron

contents. Thirteen cation formulas calculated with all iron as Fe‘* yielded total oxygen

less than 23. Fe^* was estimated by adjusting Fe^"/(Fe^^+Fe"^ to yield 23 total oxygen

atoms.

Amphibole Classification and Compositional Variability

The exact chemical composition of an amphibole crystal determines the

nomenclature for naming the amphibole. This is important to this study because the

composition of altered amphibole grains aids in determining whether amphibole

alteration was sodic. calcic, or alkali. Compositional variation depends on the amount of

Na. Ca. K. Mg, and Fe'/Fe^^ ratio within a given sample. Whether an amphibole is

classified as calcic, alkali, or sodic-calcic is determined by specific weight percentages of

Ca. Na and Ca + Na in the M4 group cation occupancy (Leake, 1978; Hawthorne, 1989)

in the structural formula (Appendix A). Microprobe examination of amphibole crystals

from Horsethief Canyon and Kingman Wash samples identify a compositional trend from

core (magnesio-homblende) to rim (), with increasing silica values, and plot as

calcic-amphibole, while others plot as alkali and sodic-calcic amphibole. The calcic

amphibole group is characterized by (Ca + Na) > 1.34 weight percent and Na < 0.67

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weight percent (Figure 15). Amphiboles belonging to the alkali group have Na > 1.34

weight percent in the M4 group cation (Figure 16). Amphiboles belonging to the sodic-

calcic amphibole group contain (Ca + Na) > 1.34 weight percent and 0.67 < Na < 1.34

weight percent in the M4 group cation (Figure 17). Figures 15. 16. and 17 indicate the

composition of altered amphibole only, for the purpose of identifying the type of

alteration.

.Analyses indicate that hydrothermal amphibole compositions, in the Wilson

Ridge piuton. vary significantly within some samples and range from a sodic tremolite to

magnesio-riebeckite (Figure 18a). The composition of altered amphibole (Figure 18a)

compared with imaltered magmatic amphibole (Figure 18b). clearly shows increasing

amounts of Na in the M4 site. Sodium is introduced primarily on the M4 (Figures 18 and

19) site by a magnesio-riebeckite exchange mechanism, i.e.

(Ca-~.|Na*~.;-i)fVi 4 ]-where FM = Fe^^ + Mg-^.

The linear trend in Figure 19 reflects a 1:1 addition of Na^ in the M4 site with Fe^"*" in

the M l-3 cation site. The apparent deviation from the linear trend in Figure 19 may

result at low Na content when Fe^^ enters the M l-3 site by some mechanism other than

the above magnesio-riebeckite exchange mechanism. Sodiiun and appear to

substitute primarily in the A site (Appendix A) by an edenite exchange mechanism, i.e.

(—_i(Na*'^or K'^)+|)[/\], (Si'*\|Al^\,)[y] [T] represents the tetrahedral site _ represents a vacancy in the A-site in the structural formula

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(Na + K)^< 0.50; Ti< 0.50 1.0 Tremolitic “ ■VI Tremolite 0.9 Hornblende 0 t , 0.8 V 0.7 + 0.6 Magnesio- Hornblende 0.5 " I t %4 Ferro- 0.4 £ Homblende + 0.3 Ferro- O) Actinolite s 0.2 O) a Horsethief Can> . 1 1 S 0.1 □ O Kingman Wash 0.0 6.0 6.3 6.5 6.8 7.0 7.3 7.5 7.8 8.0 Si (number of cations in the structural formula)

Figure 15. Determination of calcic amphibole compositions from Horsethief Canyon and Kingman Wash by evaluation of microprobe data. Calcic amphibole group is based on the M4 group cation occupancy; (Ca +Na) > 1.34 and Na < 0.67 (Leake. 1978; Hawthorne. 1989).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 36

(Na + K)a < 0.50 .0

Glaucophane

+ ▲ Magnesio- Riebeckite S . 0.5 u. + O) Ferro- Giaucophane Riebeckite o> A 4 ■ Horsethief Canyon □ 0 ® Kingman Wash 0.0 0.0 0 . 0.7 1.0 Fe3/ Fe3+ Al(vi)

Figure 16. Determination of alkali amphibole compositions from Horsethief Canyon and Kingman Wash by evaluation of microprobe data. Alkali amphibole group is based on the M4 group cation occupancy; Na> 1.34 (Leake, 1978; Hawthorne, 1989).

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(Na ♦ K)^ < 0.50 1.0

m « Barrosite W inchite m 0.5

u. + Ferro-Barrosite Ferro- O) Winchite a t A 6 ■ Horsethief Canyon □ 0 * Kingman Wash 0.0 6.0 6.5 7.0 7.5 8.0 Si (number of cations in the structural formula) Figure 17. Determination of sodic-calcic amphibole compositions from Horsethief Canyon and Kingman Wash by evaluation of microprobe data. Sodic-calcic amphibole group is based on the M4 group cation occupancy; (Ca +Na) > 1.34 and 0.67 < Na < 1.34 (Leake. 1978; Hawthorne. 1989).

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Kingman Wash (north) Horsethief Canyon (south) o veins and patches ■ veins and patches in granitoid host ♦ in granitoid host 0 altered magmatic A altered magmatic amphibole in dike amphibole in dike

riebeckite magnesio-riebeckite OT oC om 0.8

0.6

0.4

0.2

0.0 0.0 0.2 0.4 0.6 0.8 1.0 ferro-actinoiite tremolite Mg/(Mg + F©^ ) on M 1.3 cation sites

c o

O + (Q Unaltered sam ples z

0.0 0.2 ferro-actinolite Mg/(Mg + Fe^*) on M 1-3 cation

Figure 18. (a) Microprobe analyses of altered samples from Kingman Wash and Horsethief Canyon areas confirms the presence o f magnesio- riebeckite. Data indicate a compositional trend between tremolite and magnesio-riebeckite. (b) Microprobe analysis of unaltered magmatic amphibole from Wilson Ridge piuton (Metcalf, unpublished data). Symbols indicate individual samples.

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Kingman Wash (north) Horsethief Canyon (south) O veins and patches ■ veins and patches in granitoid host ^ in granitoid host

0 altered magmatic ▲ altered magmatic amphibole in dike amphibole in dike

Na (M4)

Figure 19. Microprobe analyses of amphiboles graphically showing the linear relationship of Na in the M4 site versus Fe3+ in the M l-3 site. Symbols indicate individual samples.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5

WHOLE-ROCK COMPOSITIONS

Who le-Rock Geochemistry

Fifteen samples of the Wilson Ridge piuton. including both altered and unaltered,

were analyzed for major and trace elements. Whole-rock X-ray fluorescence analyses

show that altered rocks from Wilson Ridge piuton have higher Na^O concentrations

compared to unaltered rocks. Results (Table 2) were plotted with published analyses of

unaltered Wilson Ridge granitoid rocks (Figure 20; Larsen and Smith. 1990) lacking blue

amphibole to determine the degree of sodium and/or potassium enrichment. Data

indicate that rocks containing blue amphibole are enriched in Na 2 0 with a corresponding

depletion of K 2 O. Some unaltered samples lacking magnesio-riebeckite also show Na-

enrichment while others show K-enrichment.

Whole-Rock Oxygen Isotope Analyses

Whole-rock oxygen isotope studies were conducted to determine the source of the

fluids responsible for sodium metasomatism. Eighteen samples were chosen for analysis,

twelve representative of altered rocks from Kingman Wash and Horsethief Canyon, four

relatively unaltered samples from Horsethief Canyon and the southern margin of the

piuton. and one basaltic dike. Oxygen isotope analyses of altered and unaltered samples

40

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have ô O values that range from +3.68 to +7.0%o and unaltered samples have ô'*0

values that ranee from +5.7 to +9.396o.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ■DCD Q.O C g Q.

■D CD

C/) Table 2. Whole-Rock XRF Analyses for the Wilson Ridge Piuton C/)

KW-BD Sample HT1-13/9 HT1-13/11 HT10-27/1 HT10-27/2 KW10-18/1 KW5-396/1 KWiO-18/3 banded Number dike host dike host biue dike dike host pink dike granitoid blue dike ■D8 dike (O' 8102 60.28 68.67 70.49 62.13 72.93 63.68 69.66 73.40 Ti02 1.45 0.47 0.40 0.92 0.35 1.17 0.36 0.41 AI203 14.03 15.30 15.27 16.60 15.02 17.18 13.95 15.83 MgO 3.93 1.21 0.66 2.56 0.57 3.01 0.71 0.68 3. 3" FeO 7.27 2.74 1.97 5.14 1.92 CD 2.25 2.03 2.26

CD CaO 3.79 1.28 0.65 2.42 0.25 2.56 0.33 0.58 ■D Na20 7.37 6.54 8.16 8.36 8.16 Q.O 8.70 8.53 9.07 C K20 0.41 1.55 a 0.21 0.46 0.08 0.45 0.23 0.18 3O MnO 0.06 0.01 0.00 0.06 0.01 0.02 0.02 "O o P205 0.36 0.19 0.13 0.33 0.11 0.55 0.10 0.10 Total wt.% Q.CD (LOI free 98.90 98.00 98.00 99.00 99.40 99.60 96.00 102.00 totals) ■D CD Trace Elements in ppm Rb 11.53 43.14 5.76 13.03 4.53 (/) 24.12 11.69 16.50 Sr 178.46 236.15 163.31 222.14 107.93 203.97 131.09 103.04 Y 36.27 16.09 22.70 29.42 25.05 49.18 23.24 23.77 Zr 174.30 217.57 258.94 250.22 202.14 309.99 198.27 216.22 Nb 18.90 16.05 31.25 21.12 16.62 25.80 14.57 20.62 Cr 170.00 20.96 <10 48.02 18.23 35.36 16.73 22.31 Ni 56.38 <48 <48 <48 <48 <48 <48 <48 .t; ■o û.o c 3 Q.

■O CD Table 2. (Continued)

KWi-12/3 *LL-42 Sample KW10-18/2 altered *9010-3 *9010-3e *912-68 Teakettle CD Number dike host 8 granitoid Pass ■o ê ' 8102 71.44 62.65 53.52 48.25 59.30 69.70 TI02 0.42 1.56 1.20 1.69 0.97 0.44 AI203 15.45 15.27 19.53 19.61 19.05 15.07 MgO 1.16 3.80 2.99 4.91 2.42 1.11 p- FeO 2.52 3.12 6.53 8.97 5.33 3.72 3" CD CaO 2.08 2.38 7.42 7.30 5.22 2.64 CD ■D Na20 3.90 9.56 4.87 3.68 3.98 O 3.91 Q. C K20 4.05 0.15 1.84 3.11 2.28 3.84 Oa MnO 0.01 0.01 0.09 0.14 0.08 0.04 3 ■D P205 0.11 0.58 0.52 0.54 0.50 0.15 O Total wt.%

CD (LOI free 101.00 99.10 98.51 98.20 99.13 Q. 100.60 totals) Trace Elements In ppm ■O CD Rb 168.66 5.61 39.02 92.61 82.00 80.90 en Sr 425.85 127.60 en 1327.60 1398.20 852.00 762.00 Y 16.35 30.25 39.99 23.58 20.73 n/a Zr 200.15 204.80 393.25 339.00 237.00 n/a Nb 18.10 25.68 37.65 23.34 15.20 n/a Cr 17.39 48.99 0 0.00 0.00 n/a NI <48 <48 28.31 38.87 40.20 n/a * Previous study by R V Metcalf (1991), Previous study by Larsen (1989) 4^ 44

Whole-rock analyses of Wilson Ridge Piuton

* "unaltered" samples lacking magnesio-riebeckite (Larsen and Smith, 1990; Smith et al., 1990) ^ Horsethief Canyon samples ^Kingman Wash samples with vein magnesio-riebeckite with vein magnesio-riebeckite

10 Na-enrichment 8 & K-deple-depletion

6 K-enrichment Na depletion 4

21- field of unaltered granitoid compositions 10 ..i ■ j ■ » » * 10 8 10 K g O

Figure 20. Sodium vs potassium in altered and unaltered rocks of the Wilson Ridge piuton. Diagram from Larsen and Smith (1990). and Smith et al. (1990).

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Table 3. WTiole-rock oxygen isotope data for the Wilson Ridge piuton including altered and unaltered samples. Horsethief Canyon (HT). Kingman Wash (KW).

ô i* o Sample NO. (per mil)*

KWlO-18/2 (granitoid host) 6.97 KWlO-18/3 (blue dike) 3.81 KW5-396/7 (banded dike) 3.68 KWl-12/3 (altered granitoid) 5.7 HTl-13/9 (granitoid host) 5.6 HT 10-27/1 (blue dike) 5.16 HT 10-27/2 (granitoid host) 5.6 HT4-17/2 (brittle fracture zone) 6.9 HT4-17/3 (brittle fiacture zone) 7.0 HT4-17/4A (brittle zone-blue) 6.4 HT4-17/4B (brittle zone-bleached) 6 . 6 HT-Vein (amphibole separates) 4.3 9010-lb 9.23 9010-3 b 5.72 9010-3eb 7.82 LL8-42b 6.63 912-68® 6.04 78-218<* 6.73

•The values are relative to standard mean ocean water (SMOW). **Relatively unaltered samples of Wilson Ridge piuton from Horsethief Canyon (Samples from L. Larsen and R. V. Metcalf). Relatively unaltered sample from the eastern edge of the southern margin of the Wilson Ridge piuton (Sample supplied by R. V. Metcalf). **Basaltic dike rock, Fortification Hill Basalt (Sample supplied by R. V. Metcalf).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6

DISCUSSION: ORIGIN OF BLUE AMPHIBOLE IN

THE WILSON RIDGE PLUTON

Evidence of Subsolidus Hydrothermal Origin

Evidence of a subsolidus hydrothermal origin for blue amphibole in the Wilson

Ridge piuton consists of the following ( 1 ) blue amphibole fills fractures in granitoid

rocks and is spatially associated with post-magmatic faulting. ( 2 ) textures suggest blue

amphibole replaced magmatic magnesio-homblende crystals. (3) presence of haloes and

alteration fronts associated with dikes cutting granitoid rocks. Further evidence of

hydrothermal alteration is observed in Horsethief Canyon and the Kingman Wash areas

where grainitoid host rocks are bleached and vuggy.

Discussion of the Timing of Mineralization

Extension began in the vicinity of Lake Mead approximately 16 to 9 Ma

(Duebendorfer and Wallin, 1991; Beard, 1993; Faulds. et al., 1995b). During this

period, the Wilson Ridge piuton (13.4 - 12.5 Ma) was emplaced (Larsen and Smith,

1990; Anderson et al.. 1994). The area was extending during the last stays of Wilson

Ridge emplacement, as evidenced by at least 400 north-trending dikes resulting in

approximately 2 0 percent east-west extension in the northern portion of the piuton

46

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(Feuerbach. 1986) High-angle normal faulting became active about 10 Ma at Calville

Mesa, north of Lake Mead, where 10.46 Ma basalt flows are cut by high angle faults, but

not 8.49Ma basalt flows are not (Feuerbach. 1986). The Fortification Hill basalt was

dated at 4.3 - 5.88 Ma (Anderson et al.. 1972; Feuerbach et al.. 1991 ) and was erupted

after major extension and faulting had ceased. Basalt used the high-angle faults as

conduits to the surface (Feuerbach. 1986). Previous work by Weber and Smith (1987),

Smith et al. (1990). Larsen (1989). and Duebendorfer et al. (1989). demonstrated that

low-angle faulting in the Wilson Ridge piuton predated the occurrence of high-angle

normal faulting. Naumann (1987) stated that Na-metasomatism. in the northernmost

portion of the piuton. is spatially related to low-angle faults which were cut by the high-

angle faults. In the central and southern portions of the piuton, Na-metasomatism is

spatially related to both high-angle and low-angle faults. Na-metasomatism also occurred

in pre-existing structurally weak areas, such as dike margins in Horsethief Canyon and

Kingman Wash. The Fortification Hill basalt is not faulted, indicating that high-angle

faulting ceased by 5.89 Ma. Field observations indicate that the intensity of the blue

color associated with magnesio-riebeckite (blue-amphibole) mineralization is most

intense in areas near Fortification Hill Basalts: however. Fortification Hill basalt is not

altered, therefore, Na-metasomatism must have occurred after activation of high-angle

faults (10 Ma) and prior to emplacement of the Fortification Hill basalt (5.89-4.61 Ma),

constraining the age of Na-alteration to between 10 Ma and 5.89 Ma.

Evidence of Na-metasomatism

Whole-rock geochemistry of altered rocks fiom the Wilson Ridge piuton indicate

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substantially increased NaiO values with comparable decreases in K 2 O in altered rocks

relative to unaltered rocks. Further evidence is the conversion of K-feldspar to

chessboard patterned feldspar (Abgg) in the Wilson Ridge piuton which commonly occurs

in rocks affected by sodic and sodic-calcic metasomatism (Gilluly. 1933; Battles and

Barton. 1995). Blue amphibole occurrences are associated with increases in whole-rock

NaoO values and therefore appear to be the product of Na-metasomatism.

Origin of Hydrothermal Water - Interpretation

of Oxygen Isotope Data

In general, igneous rocks exhibit 5**0 values of +5 to +15%o. with higher values

related to increased levels of SiO^ (Taylor. 1974). Ultramafic rocks have 6**0 values of

-5.4 to -^6.6 96o. gabbros. basalts, and anorthosites have 5**0 values of +5.5 to +7.4%o

(Faure. 1986). Granitic rocks and contain high concentrations of SiO? and

therefore have higher 5**0 values (+7 to +1396o) (Taylor, 1974; Faure, 1986). Taylor

(1974) has found that not all igneous rocks fall within the predicted range of 5**0 values

and that some show 5**0 depletions and range from less than *5 to +10%o (Figure 21).

The depletion of 5**0 values from the norm in some igneous rocks has been attributed to

the interaction of magmatic rocks with meteoric or formation waters. Interaction of hot

igneous fluids with cool meteoric waters produces changes in the 5**0 values of both

fluids (Taylor, 1979) and thus, corresponding changes in 5**0 values of host rocks.

When interaction occurs between igneous rocks and meteoric waters, 5**0 values are

depleted in the rocks whereas the meteoric waters become enriched in 5**0; this is

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■ D CD

C/) C/)

Figure 21. Distribution for varions rocks, including the Wilson Ridge piuton. Solid dots represent 8 altered samples whereas b, c, and d refer to unaltered samples from the Wilson Ridge piuton in fable 3. 5 c q ' Wilson Ridge Piuton ••• Granitic Rocks Ultramafic Rocks p- 3" Gabbro, basalt CD ■OCD Sedimentary Rocks ■^35?'oo O chert, carbonates, shales Q. I C (0 aO Most igneous and 3 Metamorphic Rocks O 3" Pegmatites

& Meteoric Waters 2 0 ^ ^ Locally ~ -11 to -12 O C ■o CD -6 -4 -2 0 2 4 6 8 10 12 14 16 (gC/) Data compiled by Taylor (1968a, 1974, 1979); Epstein and Taylor, 1967; lloefs, 1980; Larsen, 1989; O 3 Smith, unpublished data; and Metcalf, unpublished data. 50

known as the **0-shift (Taylor. 1979). The 8**0 values for meteoric waters in Nevada

have remained unchanged since the early Tertiary (Sheppard et al.. 1969; Taylor. 1973),

therefore, the present-day 8**0 values of meteoric waters may be used as an

approximation of the meteoric fluids which may have contributed to amphibole alteration

in the Wilson Ridge piuton. A study by L. Metcalf (1997) of groimdwater near Lake

Mead. Nevada yielded 8**0 values of " 1 196o to "14%o.

Soloman and Taylor ( 1 9 9 0 ) suggested that 8**0 values as low as +6 to +9% o for

igneous rocks may be due in part to the source rock from which magmas are derived.

Low 8 * * 0 values such as those reported above are common for imaltered granites derived

from Precambrian cratonic gneisses from western Utah and Colorado, or from other low

5 * * 0 crustal sources or fluids (Soloman and Taylor, 1 9 9 0 ). In a study of the Ruby-East

Humboldt Range metamorphic core complex, Wickham et al. (1 9 9 3 ) has suggested that

meteoric waters were drawn downward in response to rapid uplift. Rocks located just

below a major detachment fault that were subjected to water-rock interaction have the

lowest 8 * * 0 values ( + 2 to +6% o) in the Ruby-East Humboldt Range (Wickham et

al.. 1 9 9 3 ).

Unaltered rocks of the Wilson Ridge piuton have 8**0 values + 5 .7 2 to + 9.23% o.

consistent with most igneous rocks (Taylor, 1 9 7 4 ), although the values are at the low end

of the range. One of the unaltered samples (Table 3; 78-218) is a basaltic dike, included

in this study to illustrate that more mafic samples have lower 8**0 values than typical

granitic rocks. Altered rocks from the Wilson Ridge piuton have 8**0 values of + 3 .6 8 to

+ 6.97% o. which lie between typical unaltered values for plutonic rocks and meteoric

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waters. Low ô ' *0 values from whole-rock samples of altered igneous rocks from the

Wilson Ridge piuton may indicate that the crystallized piuton interacted with low 5**0

fluids of probable meteoric origin. Meteoric waters have negative 5**0 values, between

< -20%o and zero (SMOW). and reduce magmatic values in granitoid rocks when water-

rock interaction occurs (Field and Fifarek. 1993).

Source of Sodiiun

The formation of magnesio-riebeckite requires a specific geochemical

environment. Oxidizing conditions sufficient to form Fe^^ and hypersodic conditions are

necessary for magnesio-riebeckite formation (Emst. 1968). Evidence for a possible

source of sodium from within the Wilson Ridge piuton can be observed in whole-rock

geochemistry (Larsen and Smith. 1990; Smith et al., 1990). Some rocks are enriched

with respect to sodium and depleted in potassium and vice versa (Chapter 5, Figure 20),

leading to the possibility that at least some sodium was stripped from within the piuton.

Late Tertiary. (Miocene-Pliocene) nonmarine, evaporate deposits of primarily

halite stretch from eastern Arizona to the Lake Mead region (Faulds et al., 1997). Faulds

et al. (1997) studied non-marine Miocene salt deposits (13-9 Ma) in northwest Arizona,

near Lake Mead. They stated that non-marine salt deposits are common in extensional

terranes and that halite deposits greater than 500 m thick exist in the Detrital and Virgin

basins (Mannion, 1974; Faulds et al., 1997). Detrital wash lies to the east o f the Wilson

Ridge piuton and Virgin basin to the north (Figure 22). These evaporate deposits are 13-

9 Ma and hydrothermal activity post-dates 13.5 Ma, old enough to have been encoimtered

by post Wilson Ridge piuton hydrothermal fluids, but not old enough to have been

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intruded by the Wilson Ridge piuton itself, thus providing a possible source of additional

sodium to the hydrothermal system. Roddy et al. (1988) states that meteoric water (5**0

< *2096o to 0) which accumulates in an evaporate basin in an arid climate will become

progressively more enriched in 5**0. due in part to evaporation of 5*^0 enriched water.

Pétrographie studies of altered rocks containing blue amphibole indicate the

presence of secondary fluid inclusions in quartz phenocrysts. The quartz appears to be of

secondary origin due to the sparse nature of quartz in unaltered rocks. The presence of

fluid inclusions containing halite daughter crystals were not observed in quartz from

unaltered rocks in samples from Horsethief Canyon and Kingman Wash (Metcalf,

unpublished data), therefore, the fluids responsible for the high salinity fluid inclusions

do not appear be the same fluids that precipitated quartz in the unaltered samples. Most

liquid-rich fluid inclusions in altered rocks contain one or more daughter crystals, most

commonly halite cubes. The presence of halite cubes indicates that the fluid which was

trapped within the fluid inclusion contained a minimum amount of NaCl, generally taken

to be approximately 26 weight percent (Roedder. 1984). Fluid inclusion evidence,

therefore, indicates the presence of a highly saline fluid.

Thermal Conditions of Blue Amphibole Growth

Magnesio-riebeckite can form by conversion of magmatic amphibole by ionic

exchange and replacement. In the Wilson Ridge piuton, pétrographie evidence suggests

that magmatic hornblende was converted to magnesio-riebeckite on grain boundaries

(Figure 10 and 14A). Microprobe analyses indicate that rim compositions vary

significantly within some samples and range from sodic tremolite to magnesio-riebeckite.

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ID WY

Nevada Jurassic M ariue Deposits

Lake California Mead

Arizona

Figure 22. Cenozoic evaporate deposits in the vicinity of Lake Mead. Detrital and Virgin Basins (DV). Grand Wash Trough (GT), Hulapai Basin (HE). (Battles and Barton, 1995; Faulds et al., 1997).

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By finding the equilibrium temperature of the reaction where magmatic hornblende is

converted to magnesio-riebeckite. an approximate temperature at which magmatic

hornblende is converted to magnesio-riebeckite may be found. The composition of end-

member tremolite has been substituted for magmatic hornblende (Wilson Ridge piuton

samples) to simplify calculations of AG (Gibbs free energy of formation). This

substitution is legitimate as a first approximation of near end-member compositions. By

beginning with the general formula for the amphiboles. fVo./ X2 Y5 Zg O:: ((OH)', F', Cl',

O ^ 'h - magnesio-riebeckite can be formed from magmatic tremolite (as shown below) by

exchange and substitution in the IT. X. T. and Z sites. Tremolite (CaiMgsSisOT^fOH)!)

can be converted to magnesio-riebeckite (NaiMg^^aFe^^^SigOzzfOH): by the coupled

substitution o f C a 'b y Na^ in the X site and Mg "^5 by (Mg '^ 3 Fe^%) in the ¥ 5 site to

maintain charge balance. Using exchange vectors, these substitutions can be written as:

Tremolite + (Ca'^.z Mg"". 2)(Na% 2 (Mg^% Fe^^z)) = Magnesio-riebeckite

The structural formula of riebeckite and magnesio-riebeckite contains Fe'^ and

Fe'’*. The co-existence Fe“* and Fe^^ is possible over a wide range of oxygen fugacities

(magnetite-wustite up to magnetite-hematite buffers). Trivalent iron requires oxidizing

conditions which may occur if the piuton encounters meteoric water during emplacement.

Sodium-metasomatism most likely occurred during uplift and associated faulting, when the

piuton encountered meteoric waters, post-dating emplacement of the piuton. The main

controlling factors for conversion appear to be (1) a source of Na and, (2) oxidizing

conditions controlling the ionic state of iron and other elements. Fe^^ is present even at

oxygen fugacities (JO 2 ) below the magnetite-hematite buffer, such associations must be

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common given common occurrence of magnetite in magmatic rocks including the Wilson

Ridge piuton.

The stability field of pure, end member magnesio-riebeckite and pure riebeckite

(Figure 23) was experimentally investigated by Emst (1960. 1962) over a range of fluid

pressures (0-2000 bars) and temperatures (0-1000 °C). Under appropriate bulk

compositions, magnesio-riebeckite has an upper thermal stability limit of 928 ± 5° C (1000

bars). Riebeckite has an upper thermal stability limit of approximately 496± 5° C ( 1000

bars). The existence of magnesio-riebeckite and riebeckite as authigenic minerals,

however, suggests that these minerals are also stable at very low temperatures, implying a

large stability field (Emst. 1960).

Petrography and microprobe data indicates that the following reaction occurred

whereby magmatic homblende (using tremolite as a proxy) was converted to magnesio-

riebeckite:

Tremolite + 2Na'*"-r 2Fe^‘^ -* Magnesio-riebeckite 4- 2Ca-"^ + 2Mg"^.

The assumption of pressure (P) = 1 bar w as used for all calculations because magnesio-

riebeckite mineralization post-dates faulting which produced uplift, as such, probably

occurred at low pressures. Given that all thermodynamic base equations are done at

standard state, including the assumption of P = 1 bar. the temperature calculated below

may not be realistic in geologic terms.

All required thermodynamic data for the calculations are available in the literature

(Robie et al.. 1978) except the Gibbs free energy of formation for magnesio-riebeckite.

The Gibbs free energy of formation for magnesio-riebeckite was estimated using the

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Comparison of Upper Thermal Stability Limits for Riebeckite and Magnesio-riebeckite

Mag nesio-riebeckite

H+Mg+Qtz+Ac+Fluid 2 3 (A ± I Q. •o 3 u. H+Mf+OI+Ma+L+FluidJ ^ in

H+Mf+OI+Ma+Ac+Fluid

600 700500 800 900 1000 Temperature (°C)

Figure 23. Thermal stability limits for riebeckite and magnesio-riebeckite. Liquid (L) in the daigram refers to a small amount of pure water which was added to the experiment by the researcher. Hematite (H). magnetite (M), quartz (Qtz). acmite (Ac), magnesioferrite (MO, olivine (Ol), Na20-5(Mg, Fe)0-12Si02(Ma). pyroxene (Px). (From Emst, 1960. 1962, 1968).

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method described by Chen (1975) (Appendix B) and was then used to calculate the 1 bar

equilibrium temperature (Tg) of the reaction (Appendix B);

AH = AG + TAS: -9978321 = -10150384 +(298.15)(577.39) (J/mol) P = 1 bar

The values were then used to calculate the temperature at equilibrium, using the initial

equation for the formation of magnesio-riebeckite. where

Tg = AH / AS Te = 223.47/0.386 Te = 578.9 K = 306° C

The temperature at equilibrium for magnesio-riebeckite was estimated to be

306 °C at 1 bar. This is the minimum temperature at which tremolite is converted to

magnesio-riebeckite.

Potential Cause of Na-metasomatism in

the Wilson Ridge Piuton

Miocene plutons of the Colorado River Extensional Corridor (Howard and John.

1987) share similar histories of magma mixing, faulting and uplift, and influx of meteoric

waters. Among these Miocene plutons Na-metasomatism appears to be unique to the

Wilson Ridge piuton. What factors lead to Na-metasomatism at Wilson Ridge piuton and

not at the other plutons?

One explanation for Na-metasomatism at Wilson Ridge piuton is related to its

possible position in the lower plate of a low-angle detachment fault system. In

detachment fault systems, sodium metasomatism is generally associated with lower plate

rocks whereas potassium metasomatism affects upper plate rocks (Reynolds and

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Lister. 1987; Roddy et al.. 1988). Lower plate rocks from detachment zones are subject to

infiltration of meteoric fluids and. as such, whole-rock oxygen isotope studies would

yield depleted Ô ' *0 values. The Wilson Ridge piuton is geochemically and

geochronologically correlated with the River Moimtains volcano (Weber and Smith.

1987: Smith et al.. 1990: Metcalf, unpublished isotopic data; Duebendorfer et al., 1998).

The Wilson Ridge piuton is thought to have been separated from upper plate rocks

containing the cogenetic River Mountains volcanic center by movement along the low

angle Saddle Island detachment fault (Weber and Smith, 1987; Duebendorfer et al.. 1990;

Duebendorfer and Black. 1992: Duebendorfer et al.. 1998). If the model proposed by

Weber and Smith (1987). Duebendorfer et al. (1990). Duebendorfer and Black (1992).

and Duebendorfer et al. (1998). is correct and the Saddle Island detachment fault projects

to the Wilson Ridge piuton. then the model that lower plate rocks, including the Wilson

Ridge piuton. have undergone Na-metasomatism is a valid explanation for amphibole

alteration and sodium enrichment in the piuton.

Anderson et al. (1994) state that they found no evidence of a detachment fault in

the northern Black Mountains containing the Wilson Ridge piuton and they do not

believe that the Saddle Island fault is a major structure. They believe that the major

feature that separated the River Mountains from the Wilson Ridge piuton was an offset

along the left-lateral Lake Mead fault system (Anderson et al.. 1994)

An alternative explanation for Na-metasomatism at Wilson Ridge piuton is that it

may be a failed porphyry copper system. Sodic, sodic-calcic, sericitic. and potassic

alteration are typically associated with porphyry systems in the western United States

(Titley and Beane, 1981; Carten, 1986; Dilles, et al., 1992; Battles and Barton, 1995).

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Large scale iron-oxide alteration accompanied by hematite, barite and manganese

mineralization are characteristic of intense K-metasomatism (Spencer and Welty. 1986;

Roddy et al.. 1988) and are found in the Paint Pots piuton and along the eastern margin of

Wilson Ridge piuton. The Paint Pots piuton could represent the potassically altered cap

of a porph>Ty copper system, associated with the Wilson Ridge piuton or a distinct

system, separated from the main body of the piuton along a fault, or detachment. Two of

the most important features of a porph>T>' copper system are the presence o f a porphyry

and significant fracturing (Titley and Beane; 1981). At Wilson Ridge piuton. porphyritic

dikes are a common feature (Feuerbach. 1986) and significant fracturing occurs at all

visible structural levels, furthermore, the presence of as many as 400 dikes (Feuerbach,

1986). allow for chaimelized flow along the dike edges throughout the piuton.

The Wilson Ridge piuton appears to follow a line of porphyry copper deposits

which trend northwest, beginning in southeast Arizona, and ending just south of the

Wilson Ridge piuton at Mineral Park. Arizona. Titley and Beane (1981) have suggested

a correlation between the composition of the ferromagnesian minerals, or lack thereof,

and the degree of economic copper mineralization. Plutons containing more homblende

and biotite and rare to absent pyroxene, as in the Wilson Ridge piuton. seem to be good

sources for porphyry copper type mineralization (Titley and Beane. 1981). Although the

Wilson Ridge piuton does not contain economic quantities of copper, field observations

do indicate minimal copper mineralization along the periphery of the piuton.

The presence of a failed porphyry copper system may be a reasonable explanation

for Na-metasomatism in the Wilson Ridge piuton and not in other plutons in the area

which have the same tectonic historv and available meteoric fluids. Therefore, whether a

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detachment fault is present (Weber and Smith. 1987; Duebendorfer et al.. 1990;

Duebendorfer and Black. 1992; Duebendorfer et al.. 1998) or if it is not. as Anderson et

al ( 1994) suggest, the Wilson Ridge is unique because it is the only piuton in the area that

may be related to a possible porphyr) system.

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CONCLUSIONS

This study was designed to test the hypothesis that blue amphibole alteration is

the result of Na-metasomatism related to hydrothermal activity. The major conclusions

of this thesis are:

1. Blue amphibole in the Wilson Ridge piuton is of post-magmatic hydrothermal origin.

2. Blue amphibole in the Wilson Ridge piuton exhibits a range of sodium-rich

compositions including magnesio-riebeckite. winchite, and crossite (Figures 15. 16,

17. 18).

3. Blue amphibole occurrences are associated with increases in whole-rock Na^O values

and therefore appear to be the product of Na-metasomatism.

4. Na-metasomatism occurred after activation of high-angle faults (10 Ma) but prior to

the emplacement of the Fortification Basalt (4.61-5.89 Ma), constraining the age of

Na-metasomatism to between 5.89 Ma and 10 Ma.

5. The presence of halite daughter crystals in fluid inclusions indicates that the fluid

which was trapped within the inclusions contained a minimum amount of NaCl

(approximately 24 weight percent), therefore indicating the presence of a highly

saline fluid.

6. Whole-rock 5'*0 values for granitoid rocks (+7%o to +13%o) likely reflect a negative

shift from magmatic values (approximately +10%o) by water-rock interaction with a

61

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fluid of probable meteoric origin (approximately -11%© to -14%o). The data are

consistent with the hypothesis that Na-metasomatism is the result of hydrothermal

activitv related to water-rock interaction involving meteoric fluids.

Suggestions for Future Work

1. A detailed study o f Ô1 and 5D values of mineral separates from altered and

unaltered portions of the Wilson Ridge piuton would provide a more through means

to determine the source and temperatures of the fluids responsible for sodic alteration.

-A map of the piuton showing 6^*0 contours would visually reveal the limits of

magmatic/meteoric fluid interaction. This type of map could also show the extent of

the Oo-shift in adjacent country rocks, which can extend as far as three piuton widths

from the piuton (Taylor. 1974. 1979).

2. A detailed fluid inclusion study to better determine the temperature at which

amphibole alteration occurred.

3. Microprobe examination of biotite and feldspars is necessar) to determine to what

extent they have been affected by potassium, sodic. or sodic-calcic metasomatism.

4. Geochemical evaluation of altered and unaltered rocks to evaluate the amount of

mobilization and redistribution of rare earth elements (REE) in the altered Wilson

Ridge piuton. REE distribution is directly related to alteration type. K-metasomatism

causes light REE and heavy REE depletions whereas Na-metasomatism causes

enrichment in heavy REE relative to unaltered host rocks (Kinnard, 1985).

5. A detailed study of the Paint Pots piuton to examine if it is potassically enriched, if

so. then to determine the extent of potassiiun-metasomatism and it's relationship to

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the Wilson Ridge piuton. It has been suggested that the Paint Pots piuton may be the

potassically enriched cap of a porphyry system, further study is necessary to

determine if this is true and if it related to the Wilson Ridge piuton.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64

References Cited

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Anderson. R. E.. Bamhard. T. P.. and Snee. L. W., 1994. Roles of plutonism, midcrustal flow, tectonic rafting, and horizontal collapse in shaping the Miocene strain field of the Lake Mead area. Nevada and Arizona: Tectonics, v. 13. p. 1381-1410.

Barton. M. D.. Ilchik. R. P.. and Marikos. M. A., 1991. Contact Metasomatism: Kerrick, D. M.. ed.. Reviews in Mineralogy. Mineralogical Soc. Am., v. 36. p. 321-349.

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Carten. R. B.. 1986, Sodium-calcium metasomatism: Chemical, temporal, and spatial relationships at the Yerington, Nevada, porphyry copper deposit: Economic Geology, v. 81, p. 1495-1519.

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Charoy, B., Pollard, P. J., 1989, Albite-rich, silica=depleted metasomatic rocks at Emuford, northeast Queensland: Mineralogical, geochemical, and fluid inclusion constraints on hydrothermal evolution and tin mineralization: Economic Geology, V . 84. p. 1850-1874.

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Chen. Chao-Hsia. 1975. A method of estimation of standard free energies of formation of silicate minerals at 298.15°K; American Journal of Science, v. 275. p. 801-817.

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Dilles. J. H.. Soloman. G. C.. Taylor. H. P.. and Einaudi. M. T.. 1992. Oxygen and hydrogen isotope characteristics of hydrothermal alteration at the Ann-Mason porphyry copper deposit. Yerington. Nevada: Economic Geology, v. 87. p. 44-63.

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Duebendorfer. E. M.. Se wall. A. J.. and Smith. E. I.. 1990. The Saddle Island Detachment fault, an evolving zone in the Lake Mead area of southern Nevada, in Wernicke. B.. ed.. Basin and Range extensional tectonics near the latitude of Las Vegas. Nevada: Geological Society of America Memoir 176.

Duenbendorfer. E. M.. Beard. S.. and Smith. E. I.. 1998. Restoration o f Tertiary extension in the Lake Mead region, southern Nevada: The role of strike-slip transfer zones, in Faulds, J. E. and Stewart, J. H.. eds.. Accommodation zones and transfer zones: The regional segmentation of the Basin and Range Province: Geological Societ>' of America Special Paper 323, p. 127-148.

Epstein. S.. and Taylor. H. P.. Jr., 1967. Variation of 018/016 in minerals and rocks: in .A.belson. P. H.. ed.. Researches in Geochemistry, v. 2, p. 29-62.

Ernst. W. G.. 1960, The stability relations of magnesioriebeckite: Geochimica et Cosmochimica Acta. v. 19, p. 10-40.

Ernst. W. G.. 1962. Synthesis, stability relations, and occurrence of riebeckite and riebeckite- solid solutions: Journal of Geology, v. 70, p. 689-736.

Ernst. W. G..1968, Amphiboles: Springer-Verlag, New York. 125 p.

Faulds. J. E., Feuerbach, D. L.. Regan, M. K., Metcalf, R. V., Gans, P., and Walker, J. D., 1995b. The Mount Perkins block, northwestern Arizona: An exposed cross section of an evolving preextensional to synextensional magmatic system: Journal of Geophysics Research, v. 100, p. 15,249-15,266.

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Faulds. J. E.. Schreiber, B. C., Reynolds. S. J.. Gonzalez, L. A., and Okaya, D.. 1997. Origin and paleogeography of an immense, nonmarine Miocene salt deposit in the Basin and Range (Western USA): Journal of Geology, v. 105. p. 19-36.

Faure, G.. 1986. Principles of Isotope Geochemistry. Second Edition: John Wiley and Sons. New York. 589 p.

Field. C. W., and Fifarek. R. H.. 1985, Light stable-isotope systematics in the epithermal environment: in Berger. B. R. and Bethke. P. M., eds.. Geology and Geochemistry of Epithermal Systems: Society of Economic Geology, Reviews in Economic Geology, v. 66, p. 48-62.

Feuerbach. D. L.. 1986, Geology of the Wilson Ridge pluton; a mid-Miocene quartz monzonite intrusion in the northern Black Mountains. Mohave County. Arizona: M.S. Thesis. University of Nevada. Las Vegas. 79 p.

Feuerbach. D. L.. Smith. E. 1.. Shafiqullah. M.. and Damon. P. E., 1991. New K-Ar dates for late Miocene to early Pliocene mafic volcanic rocks in the Lake Mead area. Nevada and Arizona: Isochron/West. v. 57, p. 17-20.

Frohberg. M. H.. 1939. Occurrence of riebeckite in the Michipicoten District. Ontario: .American Mineralogist, v. 6. p. 382-387.

Gilluly. J.. 1933. Replacement origin of the albite granite near Sparta, Oregon: U. S. Geological Survey Professional Paper 175-C. p. 63-81.

Hawley. J. E.. 1937. Riebeckite in quartz veins from the Michipicoten District. Ontario: .American Mineralogist, v. 22. number 11, p. 1099-1103.

Hauthome. F. C.. 1989, Crystal chemistry of the amphiboles: in D. R. Veblen. ed.. Amphiboles and other hydrous pyriboles: Reviews in Mineralogy, v. 9A. p. 1-102.

Helgeson. H. C.. Delany. J. M., NesbitL H. W., and Bird. D. K., 1978. Summary and critique of the thermodynamic properties of rock-forming minerals: American Journal of Science, v. 178-A, p. 1 -229.

Hoefs. J.. 1980. Stable Isotope Geochemistry, Second Edition: Springer-Verlag, New York. 208 p.

Howard. K. A.. John, B. E.. 1987, Crustal extension along a rooted system of imbricate low-angle faults: Colorado River extensional corridor, California and Arizona: in Coward, M. P., Dewey, J. F.. and Hancock, P. L. editors. Continental Extensional Tectonics, Geological Society of London Special Publication 28, p. 299-311.

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Karpov . I. K., and Kashik. S. A.. 1968. Computer calculation o f standard isobaric- isothermal properties of silicates by multiple regression from a crystallochemical classification: Geochem. Internat., v. 5. p. 706-713.

Kinnaird. J. A.. 1985. Hydrothermal alteration and mineralization of the alkaline anorogenic ring complexes of Nigeria: Journal of African Earth Science, v. 3, p. 229-252.

Klein. C.. and Hurlbut. C. S.. Jr., 1977, Manual of Mineralogy, Twentieth Edition: John Wiley and Sons. New York. 596 p.

Kresten. P., 1988. The chemistry of fenitization: Chemical Geology, v. 68, p. 329-349.

Larsen. L. L.. 1989. The origin of the Wilson Ridge pluton and its enclaves, northwestern Arizona: Implications for the generation of a calc-alkaline intermediate pluton in an extensional environment: M.S. Thesis. University of Nevada. Las Vegas, 81 p.

Larsen. L. L.. and Smith. E. 1.. 1990, Mafic enclaves in the Wilson Ridge Pluton, northwestern Arizona: Implications for the generation of a calc-alkaline intermediate pluton on an extensional environment: Journal of Geophysical Research, v. 95, p.17693-17716.

Leake. B. E.. and Winchell, H.. 1978. Nomenclature of amphiboles: American Mineralogist, v. 63, p. 1023-1052.

Lyons. P. C.. 1972. Significance of riebeckite and ferrohastingsite in Microperthite granites: American Mineralogist, v. 57. p. 1404-1412.

Mannion. L. E., 1974. Virgin River salt deposits. Clark County, Nevada, in Coogan, A. H.. ed.. Fourth symposium on salt: Cleveland. Northern Ohio Geological Society, v. 1, p. 166-175.

Metcalf. L.. 1995, Ground water-surface water interactions in the lower Virgin River area. Arizona and Nevada. M.S. Thesis, University of Nevada. Las Vegas, 79 p.

Metcalf. R. V., and Smith. E. L, 1991, Hornblende geobarometry from two Miocene plutons: Implications regarding uplift and block rotation during Basin and Range extension: Geological Society of America, abstracts w/ programs, v. 23, no. 5, p. A245.

Metcalf, R. V., Smith, E. 1.. Martin, M. W., Gonzales, D. A , and Walker, J. D., 1993. Isotopic evidence of source variations in commingled magma systems: Colorado River extensional corridor: Geological Society of America, abstracts w/ programs, v. 25. no. 5, p. 210.

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Metcalf. R. V.. Smith. E. I., and Mills. J. G.. 1993, Magma mixing and commingling in the northern Colorado River extensional corridor; Constraints on the production of intermediate magmas—part 1: in Lahren. M. M.. Trexler. J. H., Jr. and Spinosa. C.. eds.. Crustal evolution of the Great Basin and Sierra Nevada: Cordilleran / Rocky Mountain section. Geological Society of America Guidebook. Department of Geological Sciences. University of Nevada. Reno. p. 35-55.

Mian. 1., and LeBas, M. J., 1986. Sodic amphiboles in fenites from the Loe Shilman carbonatite complex, NW Pakistan: Mineralogical Magazine, v. 50, p. 187-197.

Miyano, T.. and Klein. C.. 1983. Conditions of riebeckite formation in the iron-formation of the Dales Gorge Member. Hamersley Group, Western Australia: American Mineralogist, v. 68, p. 517-529.

Morikawa. S.. 1996. UNLV XRF Procedures Manual. April. 1994 (revised January 1996): Unpublished Manual. University of Nevada. Las Vegas. 26 p.

Naumann. T. R.. 1987. Geology of the central Boulder Canyon quadrangle. Clark County, Nevada. M.S. Thesis. University of Nevada. Las Vegas, 68 p.

Pirajno. P.. 1992. Hydrothermal Mineral Deposits: Principles and Eimdamental Concepts for the Exploration Geologist: Springer-Verlag. Berlin, 709 p.

Reynolds. S. J ., Lister. G. S.. 1987, Structural aspects of fluid-rock interactions in detachment zones: Geology, v. 15. p. 362-366.

Robie. R. A.. Hemingway. B. S., and Fisher. J. R., 1978, Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (10^ Pascals) pressure anc at higher temperatures: U.S. Geological Survey Bulletin 1452, 456 p.

Roedder. E., 1984, Fluid Inclusions, Ribbe, P. H., ed.. Reviews in Mineralogy. Mineralogical Society of America, v. 12, 646 p.

Roddy. M. S., Reynolds, S. J.. Smith. B. M., and Ruiz. J. 1988. K-metasomatism and detachment-related mineralization, Harcuvar Mountains. Arizona: Geological Society of America Bulletin, v. 100, p. 1627-1639.

Sheppard, S. M. F., Nielsen, R. L.. Taylor, H. P., 1969, Oxygen and hydrogen isotope ratios of clay minerals from porphyry copper deposits: Economic Geology, v. 64:7. p. 755-777.

Smith, E. 1.. 1982, Geology and geochemistry of the volcanic rocks in the River Moimtains, Clark County, Nevada, and comparisons with volcanic rocks in nearby areas, in Frost, E. G., and Martin, D. L., (eds.), Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada: San Diego, California, Cordilleran Publishers, p. 41-54.

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Smith. E. !.. Feuerbach, D. L.. and Naumann, T. R.. and Mills. J. G.. 1990. Mid-Miocene volcanic and plutonic rocks in the Lake Mead area of Nevada and Arizona; Production of intermediate igneous rocks in an extensional environment: in Anderson. J.L.. (ed.). The nature and origin of the Cordilleran magmatism: Boulder. Colorado. Geological Society of America Memoir 174. p. 169-194.

Soloman. G. C.. and Taylor. H. P.. Jr.. 1990. Isotopic evidence for the origin of Mesozoic and Cenozoic granite plutons in the northern Great Basin: Geology, v. 17, p. 591-594.

Spencer. J. E., and Welty. J. W., 1986. Possible controls of base- and precious- metal mineralization associated with Tertiary detachment faults in the lower Colorado: Geology. V. 14. p. 195-198.

Streckeisen. A.. 1976. To each plutonic rock its proper name: Earth Science Reviews. International Magazine for Geo-Scientists, Amsterdam, v. 12. p. 1-33.

Strong. D. F.. and Taylor. R. P., 1984. Magmatic-subsolidus and oxidation trends in composition of amphiboles from silica-saturated peralkaline igneous rocks: Tschermaks Mineralogische und Petrographische Mitteilungen, v. 32, p. 211-222.

Taylor. H. P.. Jr.. 1968. The oxygen isotope geochemistry o f igneous rocks: Contributions to Mineralogy and Petrology v. 19, p. 1-71.

Taylor, H. P.. Jr.. 1973. 0'*/0** evidence for meteoric-hydrothermal alteration and ore deposition in the Tonopah, Comstock Lode, and Goldfîeld mining districts, Nevada: Economic Geology, v. 68. p. 747-764.

Taylor. H. P.. Jr.. 1974a. The application of oxygen and hydrogen isotope studies to problems of hydrothermal alteration and ore deposition: Economic Geology, v. 69. p. 843-883.

Taylor. H. P.. Jr.. 1974b. Oxygen and hydrogen isotope evidence for large-scale circulation and interaction between ground waters and igneous intrusions, with particular reference to the San Juan volcanic field. Colorado; in Hofhnan, A. W., Giletti. B. J., Yoder. H. S., Jr., and Yund, R. A., (eds.), Geochemical Transport and Kinetics: Carnegie Institution of Washington Publication 634, p. 299-323.

Taylor. H. P.. Jr.. 1979, Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits; in Barnes, H. L., (eds.) Geochemistry of Hydrothermal Ore Deposits, Second Edition: John Wiley and Sons, New York. p. 236-277.

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Weber. M. E.. and Smith. E. !.. 1987. Structural and geochemical constraints on the reassembly of mid-Tertiary volcanoes in the Lake Mead area of southern Nevada: Geology, v. 15. p. 553-556.

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CRYSTAL CHEMISTRY OF THE AMPHIBOLES

71

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Amphiboles are widespread in igneous and metamorphic rocks, occurring as

principal and accessory minerals. The basic structural unit of the amphiboles is a double chain of comer-linked, oxygen sharing, tetrahedra (SLO,,) that run parallel to the c crv'stallographic axis (Fig. 24). The general chemical formula for the amphibole group.

W fl-/ X : Ys Zs O 2 2 ((OH)~, F~. Cl~, 0 ^ ) 2. permits a variet) of cation coordinations.

IT represents the A site and accepts Na" and K \ or is vacant. X represents the M4 sites and accepts Ca“~. Na*. Mn" . Fe~^. Mg"~. and Li*, and Trepresents the M l. M2, and M3 sites and accepts Mg’*. Fe^*, Fe^*. Al^*, Cr**. Zn“*. and Ti"** (Klein and Hurlbut. 1977).

The tetrahedral (T) sites, represented by Z, accept Si‘‘* and Al^* (Klein and Hurlbut. 1977).

Figure 24. Schematic representation of the amphibole structure perpendicular to the c crystallographic axis (modified from Ernst, 1968).

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The amphibole structure contains six cation sites; A. Ml. M2. M3. M4. and T

sites. IT-t\"pe cations have 10 to 12 coordination with oxygen and (OH). T-type cations

share edges to form octahedral chains parallel to the c crystallographic axis, where Ml

and M3 have four-fold coordination and M2 has six-fold coordination. T-type cations

have six- to eight-fold coordination with oxygen and Z-type cations have 4-fold

coordination with oxygen (Klein and Hurlbut. 1977).

The ability of the X site (M4) to incorporate a wide range of elements allows us a

basis in which to group the amphiboles; (Ca) calcic amphiboles , (Na) sodic amphiboles ,

and (Fe-Mg) iron-magnesium amphiboles (Ernst. 1968). The calcic amphiboles dominate

in abundance and number of species due to their broad pressure and temperature stability

limits. The occurrence of Na-amphiboles is more restricted than Ca-amphiboles due to

required Na-rich conditions and/or relatively high pressures and low temperatures that are

required for their formation (Ernst. 1968). The occurrence of iron-magnesium amphiboles

is restricted to metamorphic terranes (Ernst. 1968).

Structural Cation Cation Representative Formula Site Coordination Elements

ITq-I A 10- 12 Na+, K+ M4 6 - 8 Ca2+. Mg2+, Na+, Mn^^, . Fe2+. Li+ ^5 Ml. M2. M3 6 Mg2+ Fe2+. Fe2+, a P+. Ti4+, Cr3+. Zn2+ Z8 tetrahedral site 4 Si4+, A13+ (OH) hydroxyl site (OH)-, q 2-. F-. Cl-

Table 4. Summary of site occupancies in amphiboles.

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The chemical complexity of the amphiboles is a result of ionic substitution (Table 4). The extent to which substitution occurs, is the result of several factors including ionic radii and charge on the ion species (Klein and Hurlbut. 1977). Substitution commonly occurs in the M4 site between Na* and Ca’* and among Fe’\ Mg’*, and Mn’*. The M1. M2, and M3 sites permit limited substitution between Fe^* and .AT*, and between Ti"** and other Ttype ions (Mg’*. Al^*. Fe’*, Fe^^ C r’*. Zn’*) (Klein and Hurlbut. 1977). The hydroxyl (OH)" site commonly exhibits partial substitution of F-- C1-, and O’ (Klein and Hurlbut. 1977). Beginning with the composition o f tremolite (CazMgsSigOzzfOH):). the Ca-series

and Na-series of amphiboles can be produced by ionic exchange and substitution on fV. -T. T. and Z sites. Ferro-actinolite (CaiFesSigOjjfOH)!) is produced by simple Fe-Mg exchange from tremolite (CaiMgsSigOzzfOH)?). may also be produced from a starting composition of tremolite although not as simply as ferro-actinolite. Glaucophane (Na2Mg3Al2Si80z2(0H)2) requires the coupled substitution of Ca’* by Na"*"

and Mg’* by AP *2 to maintain charge balance. Using exchange vectors, these substitutions can be written as:

Tremolite + (□0.jSi^'*'.i)(Na^‘'‘+i Al^’*‘+j) = Edenite

Tremolite + (Ca2+_2Mg2+_2)(Na"'"+2Al2+.,_2) = Glaucophane Glaucophane + (Al2+_2Fe2++2) = Magnesio riebeckite Magnesio-riebeckite + (Mg2+_g F e2++g ) = Riebeckite

Q denotes a vacancy

Cation sites in the amphibole structure can accept a variety of elements which may have variable valence states. The presence of Fe^* and/or Fe^* in the M1-M4 cation sites is of interest when dealing with sodic amphiboles. The end member composition of

riebeckite (Na 2Fe^* 3Fe^* 2Sig0 2 2 (0 H)2) contains both Fe^^ and Fe^^ in the T site whereas

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Mg“ *3 substitutes for Fe' 3 in magnesio-riebeckite (Na2Mg"*3Fe^*2Si8022(0H)2). The coupled substitution (Ca‘*.iFm“*i) (Na'.iFe^*.:). where Fm represents Mg"* or Fe“~. is required to make the transition from Ca-amphiboles to the Na-amphibole magnesio- riebeckite.

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ESTIMATION OF GIBBS FREE ENERGY OF

FORMATION FOR MAGNESIO-RIEBECKITE

7 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77 In order to determine the temperature of conversion of magmatic hornblende to

magnesio-riebeckite for the system below, it became necessary to create thermodynamic

data.

Tremolite + 2Na* + Fe^*

— Magnesio-riebeckite + 2Ca“* + 2Mg“*

I began to search the literature for thermodynamic data relevant to the above

system. I was unable to locate the standard molar Gibbs free energy (AG°f) and enthalpy

(AH°f) of formation for magnesio-riebeckite. A paper by Helgeson et al., (1978) did

include standard molar volume (V) and entropy (S°) data for magnesio-riebeckite. The

Gibbs free energy (AG°f) for magnesio-riebeckite combined with the data from Helgeson

et al.. (1978) permitted me to algebraically determine values for enthalpy (AH°f). 1 could

not find the required AG°f. therefore 1 was at a standstill. A study by Miyano and Klein,

(1983). on the conditions of riebeckite formation in the iron-formation of the Dales Gorge

Member. Hamersley Group. Western Australia, applied a method used by Chen (1975) to

estimate the standard free energies of formation to determine the standard Gibbs free

energy (AG°f) of riebeckite (-9323.2 kJ/mol ±63 kJ/mol). Chen. (1975) applied the

methods of Karpov and Kashik, (1968) to estimate the AG°f of silicate minerals. Chen

( 1975) states that the percent error falls within the range of 0.6 or less for this method of

estimating AG°f.

By following the steps that Chen (1975) applied, I was able to estimate AG®f for

magnesio-riebeckite. The method requires writing a series of magnesio-riebeckite

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forming equations (Table 5). The simplest equation should be made from the oxides,

follovs'ed by simple silicates, and so forth, to more complex equations.

Table 5. Magnesio-riebeckite forming reactions. (All reactions: 1 bar pressure and 298 K)

Rank Reaction £AG°f (kcal/mol K) -AG °r 0 Na2Si03 + SMgSiO] + 2FeSi04 + 2(OH) -2123.04 306.71 1 Na20 + 3MgO + 2FeO + 8Si02 + 2(OH) -2330.53 99.22 2 N a20 + 2MgSi03 + Mg(OH) 2 + Fe203 -r 6Si02 -2392.87 36.88 2 Na20 + 3MgSi03 + Fe2 0 3 + 5Si02 + H2 O -2394.84 34.91 3 Na2Û + Mg3Si40io(OH)2 + Fe2 0 3 + 4Si02 -2409.17 20.58 4 2NaOH + MgFe2 0 4 + 2 MgSi0 3 + 6Si02 -2422.50 7.25 5 2NaOH + Fe2 0 3 + 3 MgSi0 3 + 5Si02 -2429.75 6 Magnesio-riebeckite N/A 0.00

The SAG°f is determined for each equation. The next step is to rank the equations in terms of decreasing ZAG°f values, from zero (0) through nine (if you have ten

reactions) and assign each reaction a value according to its rank. The reaction with the largest SAG°f value is assigned the number zero (0), the next reaction is assigned the

number one ( 1 ), and so forth, in descending order. The difference in kcals between the reaction with the most negative ZAG°f and the reaction with the next higher SAG°f is the “AG°r. and this value is entered into the corresponding column. The —AG°r shows how the value of SAG°f behaves exponentially. The values of LAG°f are then plotted against

their rankings, grouping similar values together as one rank (as in above reactions). Graphing (Figure 25) at this point gives an approximation of the exponential, or linear, curve. If the plotted values of £AG°f form an exponential trend then the data may be fit

into the following equation:

S A G ° f 4 = (a ^*** ) + c

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The constants a. b. and c are determined by regression analysis. The value "c" represents

the ZAG°f for the mineral you are interested in. "X" represents the value assigned when

ranking the reactions according to ZAG°f values. 1 used a software package that provides

for regression analysis to obtain the values of the variables. 1 plotted the values of ZAG°f

for the five reactions 1 had written along with the estimated values gained in the

regression which were fit into the above equation. The result is shown in Figure 25,

where -2425 kcals corresponds to the ZAG°f for magnesio-riebeckite. To test the

accuracy of the method. 1 wrote several tremolite forming reactions. The values of ZAG°

f approached the recorded value for tremolite (2873 kcal).

The ZAG°f value for magnesio-riebeckite was then used to determine the enthalpy

of formation, where

AH = AG + TAS: -9978321 = -10150384 +(298.15)(577.39) (J/mol) P = 1 bar

The values were then used to calculate the temperature at equilibrium, using the initial

equation for the formation of magnesio-riebeckite. where

Tg = AH / AS

Tg = 223.47/0.386

Tg = 578.9 K = 306° C

Thermodynamic data from Robie et al., 1978.

The equilibrium temperature (Tg) above gives an estimate of the temperature at

which tremolite was converted to magnesio-riebeckite.

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CJ

C/Î 55

Ocr.

C5 > 0-' c w S.

t/i =i £?

fSIf) % ec3

I o o e lO «y

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ELECTRON MICROPROBE ANALYSES OF AMPHIBOLES

AND STRUCTURAL FORMULA INTERPRETATION

81

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■D CD

C/) C/) APPENDIX C: Electron Microprobe Analyses of Amphiboles and Structural Formula Interpretation

SAMPLE point# Na20 K20 CaO FeO* MgO MnO TI02 AI203 SI02 TOTAL 10-27-1 a 1 1.85 0.74 11.28 13.89 14.26 0.55 1.58 7.93 45.52 97.58 8 10-27-la 2 1.89 0.60 11.59 12.91 14.89 0.49 1.37 7.07 46.48 97.29 ■D 10-27-1a 3 1.60 0.75 11.37 13.21 14.64 0.64 1.15 6.84 46.88 97.08 10-27-la 4 1.95 0.82 11.19 13.39 13.99 0.60 1.99 8.03 44.65 96.61 10-27-la 5 5,70 0.24 3.61 14.26 14.65 0.00 0.80 0.67 53.90 93.83 10-27-la 6 5.92 0.00 1.92 17.63 13.56 0.00 0.32 0.65 53.34 93.34 10-27-la 6 6.69 0.16 2.02 17.79 13.94 0.00 0.47 0.56 55.66 97.29 10-27-la 10 6.54 0.22 2.07 15.13 14.47 0.26 0.36 0.54 54.66 94.25 3. 3" 10-27-la 11 6.55 0.12 2.07 17.21 13.85 0.00 0.42 0.65 55.14 96.00 CD 10-27-la 12 6.74 0.21 1.66 15.16 11.15 0.00 0.24 4.43 56.86 96.45 ■DCD 10-27-la 13 6.02 0.00 1.97 16.82 12.97 0.00 0.35 1.84 55.70 95.68 O Q. 1027-2-1 1 1.41 0.56 11.38 14.80 13.89 0.47 0.97 5.98 47.20 50.47 C a 1027-2-2 2 1.15 0.67 11.59 14.25 13.86 0.59 1.10 6.03 46.97 51.21 3O 1027-2-3 7 0.99 0.48 11.64 15.10 13.67 0.40 0.23 5.96 49.24 55.46 "O 1027-2-4 11 1.05 0.27 11.22 O 14.14 14.41 0.56 0.50 4.09 50.65 57.24 1027-2-5 14 6.75 0.00 1.20 18.49 12.35 0.00 0.49 0.36 53.78 53.64 HT113-9A 1 6.62 0.00 1.29 20.61 10.99 CD 0.00 0.80 0.67 53.45 94.43 Q. HT113-9A 2 5.55 0.13 2.18 19.67 11.06 0.00 0.85 0.76 52.83 93.02 HT113-9A 3 6.04 0.00 3.72 16.76 14.13 0.00 0.53 0.82 56.10 98.09 HT113-9A 4 6.84 0.00 0.92 20.13 11.27 0.00 1.28 0.65 54.64 95.73 ■D HT113-9A 5 1.77 0.18 11.24 13.34 16.75 0.00 0.32 3.36 54.17 101.13 CD HT113-9A 6 6.27 0.00 1.51 20.30 11.37 0.00 0.77 0.78 53.63 94.62 C/) HT113-9A C/) 7 5.48 0.15 3.31 18.24 12.98 0.00 0.43 0.61 54.73 95.93 HT113-9A 8 4.14 0.00 6.19 17.24 15.04 0.00 0.27 0.68 56.16 99.72 HT113-9A 9 2.79 0.00 8.51 14.70 13.88 0.39 0.54 1.92 51.46 94.20 HT113-9A 10 6.40 0.17 0.86 20.23 10.84 0.00 0.94 0.83 53.72 93.98 HT113-9A 11 6.37 0.00 0.95 20.72 10.59 0.00 1.06 0.93 53.72 94.33 HT113-9A 12 6.20 0.00 1.65 19.73 11.93 0.00 0.79 0.53 54.01 94.83 HT113-9A 15 7.03 0.00 0.95 19.47 11.20 0.36 1.29 0.68 53.76 94.74 HT113-9A 16 6.68 0.13 1.17 19.45 12.25 0.00 0.78 0.91 55.64 97.00 HT113-9C 1 0.57 0.00 11.59 15.02 14.47 0.00 0.00 1.96 51.67 95.27 WOC 83

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C/) (Continued) C/) 23 OXYGEN FORMULA Fe2-3

SAMPLE p oint# Na(A) K A NaM4 CaM4 FeM4 M4 CaM13 FeM13 Fe3 Mg Mn Ti Al(vl) M13 10-27-1 a 1 0.29 0.14 0.43 0.23 1.76 0.02 2.00 0.00 0.74 0.93 3.09 0.07 0.17 0.00 5.00 10-27-la 2 0.35 0.11 0.47 0.18 1.81 0.01 2.00 0.00 0.85 0.71 3.23 0.06 0.15 0.00 5.00 ■D8 10-27-la 3 0.22 0.14 0.35 0.23 1.77 0.00 2.00 0.01 0.80 0.80 3.18 0.08 0.13 0.00 5.00 10-27-la 4 0.35 0.15 0.50 0.21 1.77 0.02 2.00 0.00 0 8 7 0.76 3.07 0.08 0.22 0.00 5.00 10-27-la 5 0.19 0.04 0.23 1.42 0.56 0.01 2.00 0.00 0.51 1.22 3.19 0.00 0.09 0.00 5.00 10-27-la 6 0.09 0 0 0 0.09 1.58 0.30 0.12 2.00 0.00 0.00 2.04 2.94 0.00 0 0 4 0.00 5.01 10-27-la 8 0.20 0.03 0.23 1.63 0.30 0.07 2.00 0.00 0.35 1.68 2.93 0.00 0.05 0.00 5.00 10-27-la 10 0.16 0.04 0.20 1.67 0.32 0.01 2.00 0.00 0.26 1.55 3.11 0.03 0.04 0.00 5.00 3. 10-27-la 11 0.16 0.02 0.18 1.65 0.32 0.03 2.00 3" 0.00 0.38 1.64 2.94 0.00 0.04 0.00 5.00 CD 10-27-la 12 0.12 0.04 0.16 1.74 0.25 0.01 2.00 0.00 1.16 0.63 2.36 0.00 0.03 0.82 5.00 ■DCD 10-27-la 13 0.07 0.00 0.07 1.60 0.30 0.10 2.00 0.00 0.71 1.21 2.77 0.00 0.04 0.28 5.00 O 1027-2-1 1 0.22 0.10 0.32 0.18 1.79 0.03 Q. 2.00 0.00 0.97 0.82 3.04 0.06 0.11 0.00 5.00 C 1027-2-2 2 0.17 0.13 0.30 a 0.16 1.83 0.01 2.00 0.00 1.05 0.70 3.05 0.07 0.12 0.00 5.00 3O 1027-2-3 7 0.07 0.09 0.16 0.21 1.79 0.00 2.00 0.01 1.10 0.73 2.95 0.05 0.02 0.14 5.00 "O 1027-2-4 11 0.03 0.05 0.08 0.26 1.74 0.00 2.00 0.01 1.03 0.69 3.11 O 0.07 0.05 0.04 5.00 1027-2-5 14 0.15 0.00 0.15 1.78 0.19 0.03 2.00 0.00 0.53 1.70 2.71 0.00 0.05 0.00 5.00 HT113-9A 1 0.11 0.00 0.11 1.78 0.20 0.02 2.00 0.00 0.74 1.77 2.41 0.00 0.09 0.00 5.00 CD Q. HT113-9A 2 0.00 0.02 0.02 1.60 0.35 0.01 1.96 0.00 0.85 1.59 2 4 6 0.00 0.10 0.01 5.00 HT113-9A 3 0.20 0.00 0.20 1.44 0.56 0.00 2 0 0 0.00 0.73 1.24 2.96 0.00 0.06 0.01 5.00 HT113-9A 4 0.04 0 0 0 0.04 1.87 0.13 0.00 2 0 0 0.01 0.73 1.70 2.42 0.00 0.14 0.00 5.00 HT113-9A ■D 5 0.13 0.03 0.16 0.34 1.66 0.00 2.00 0.00 0.84 0.69 3.43 0.00 0.03 0.00 5.00 CD HT113-9A 6 0.04 0.00 0.04 1.74 0.24 0.03 2.00 0.00 0 5 9 1.86 2.47 0.00 0.08 0.00 5.00 HT113-9A 7 C/) 0.05 0.03 0.08 1.47 0.51 0.02 2.00 0.00 0.64 1.53 2.78 0.00 0.05 0.00 5.00 C/) HT113-9A 8 0.14 0.00 0.14 0.97 0.92 0.12 2.00 0.00 0.48 1.39 3.10 0.00 0.03 0.00 5.00 HT113-9A 9 0.17 0.00 0.17 0.63 1.36 0.01 2.00 0.00 1.08 0.73 3.08 0.05 0.06 0.00 5.00 HT113-9A 10 0.00 0 0 3 0.03 1.82 0.13 0.00 1.96 0.00 0.75 1.74 2.37 0.00 0.10 0.04 5.00 HT113-9A 11 0.00 0 0 0 0.00 1.81 0.14 0.00 1.95 0.01 0.76 1.78 2.31 0.00 0.12 0.03 5.00 HT113-9A 12 0.07 0.00 0.07 1.68 0.26 0.06 2.00 0.00 0.53 1.80 2 5 8 0.00 0.09 0.00 5.00 HT113-9A 15 0.16 0.00 0.16 1.84 0.15 0.02 2.00 0.00 0.82 1.55 2 4 4 0.05 0.14 0.00 5.00 HT113-9A 16 0.01 0.02 0.03 1.83 0.17 0.00 2.00 0.00 0.58 1.73 2.59 0.00 0.08 0.03 5.00 HT113-9C 1 0.02 0.00 0.02 0.14 1.83 0.03 2.00 0.00 1.17 0.65 3.18 0.00 0.00 0.00 5.00 00 Os 87

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PETROGRAPHIC ANALYSES

89

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Percentages are based on visual estimations of mineral abundance's in

thin-section. What are refered to as alkali feldspar are most likely secondary and

represent partial of complete replacement of primary magmatic orthoclase during Na-

metasomatism.

Sample Number - KW1-I2/3 Location: 114°40'30". 36=37'00" Rock Name - monzoniie Degree of Crystallinity - holocry stalline Grain Size - phaneritic Absolute Grain Size - medium Relative Grain Size - granular Color - leucocratic Shape - subhedral to anhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Plagioclase (49 %) shows minor sericitic alteration, hornblende (15%). chlorite (30%). apatite (<1%). opaques (5%). Hornblende is very altered, to chlorite and/or magnesio-riebeckite. and occurs as veins throughout the sample.

Sample Number - KWl-12/3b Location: 114°40'30". 36=38’00" Rock Name - diorite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - fine to medium Relative Grain Size - granular Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (<5%), plagioclase (79%), hornblende (6%). secondary calcite veining (5%). euhedral and aiihedral sphene (2%), apatite (<1%). opaques (2%). Hornblende is not visibly altered. Plagioclase is not altered to sericite.

Sample Number - KWl-12/5 Location: 114°40'40". 36=02 30 ' Rock Name - latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - crystalline Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral

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Description - Quartz (<5%), plagioclase (25%). hornblende (4%). apatite (<1%). opaques (5%). muscovite (3%). groimdmass (57%) contains some plagioclase laths, evidenced by parallel extinction and albite twinning. Groimdmass contains tiny acicular blue needles and tiny inclusions of red iron-oxides throughout. Olive-green hornblende grains have pale-blue overgrowths or reaction rims. Fluid inclusions in quartz, adjacent to the contact between the dike and the dike host rock, contain at least one daughter crystal, occasionally more.

Sample Number - KW2-7/9 Location: 114=40 20 ". 36°02'00" Rock Name - quartz latite dike Degree of Crystallinity - holocrystallme Grain Size - aphanitic Absolute Grain Size - crystalline Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral phenocrysts with anhedral groundmass Description - Quartz (7%), plagioclase (7%). biotite (1%). apatite (<1%), opaques (3%), groimdmass (81%; 13% alkali feldspar. 68% plagioclase). Minor sericitic alteration of plagioclase. Odd phenocrysts of chessboard twinned feldspar phenocrysts.

Sample Number - FCW4-27/3 Location: 114=38'53", 36°02'08" Rock Name - granodiorite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - medium to fine Relative Grain Size - granular Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (25%). plagioclase (20%), alkali feldspar (42%), biotite (5%), hornblende (5%), sphene (<1%), opaques (5%). Weak to moderate sericitic alteration of plagioclase. Fluid inclusions (liquid-rich and vapor-rich) in quartz contain at least one daughter crystal, but in some cases liquid-rich fluid inclusions contain up to five daughter crystals. Some fluid inclusions contain a biréfringent daughter crystal. Opaque minerals most likely magnetite because they do not have a reddish cast on thin, outer edges as does hematite.

Sample Number - KW5-396/1 Location: 114=00'21", 36=55 00" Rock Name - latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain S ize - fine Relative Grain S ize - granular

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Color - mesocratic Shape - euhedral to anhedral Description - Quartz (<5%), plagioclase (40%). alkali feldspar (26%), biotite (5%), hornblende (5%). opaques (5%). sphene (2%). chlorite (1%). Hornblende rimmed and being replaced by pale-blue amphibole. Some biotite is altered to chlorite. Secondary quartz in vugs. Minor chlorite veining.

Sample Number - KW5-396/7 Location: 114°39'30". 36=0218 " Rock Name - latite dike Degree of Cry stallinity - holocrystalline Grain Size - phenocrysts in an aphanitic groimdmass Absolute Grain Size - medium to fine Relative Grain Size - granular Color - mesocratic Shape - subhedral Description - Quartz (<5%), plagioclase (25%), alkali feldspar (14%), biotite (2%). hornblende (3%). sphene (<1%). opaques (3%), groimdmass (50%). Hornblende phenocrysts have pale-blue rims. Groimdmass contains tiny acicular pale-blue needles throughout. Plagioclase laths show minor sericitic alteration. Banded blue dike.

Sample Number - KW5-396/11 Location: 1I4=39'30". 36=02'20" Rock Name - andésite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - fine to medium Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - anhedral to subhedral Description - Quartz (7%). plagioclase (25%), biotite (5%), hornblende (3%). sphene (<1%). opaques (3%), sericite (<1%). groundmass (55%). Sericitic alteration in the cores of some plagioclase phenocrysts. Most hornblende phenocrysts are rimmed by a pale- blue mineral.

Sample Number - KWlO-18/1 Location: 114=39'54", 36=02'09 " Rock Name - latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic phenocrysts within a cryptocrystalline groundmass Absolute Grain Size - fine to aplitic Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral to euhedral

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Description - Quartz (<3%) in groundmass. plagioclase (36%). biotite (<1%). hornblende (2%). muscovite (7%). groundmass (50%). Hornblende is extensively altered, mostly relict grains. Groimdmass contains tiny acicular pale-blue needles throu^out.

Sample Number - KW10-18/2 Location: 114°39'51". 36=02'10" Rock Name - granite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - fine to medium Relative Grain Size - equigranular Color - mesocratic Shape - anhedral to euhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (20%). plagioclase (30%). alkali feldspar (37%). biotite ( 10%). hornblende (2%), sphene (<1%). Minor sericitic alteration of plagioclase grains. Severely altered hornblende, mostly relict grains.

Sample Number - KWlO-18/3 Location: 114=39'H", 36=02’30" Rock Name - latite dike Degree of Crystallinity - holocrystalline Grain Size - phenocrysts in a microcry stalline groundmass Absolute Grain Size - fine to medium Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral Description - Quartz (5%) in groundmass. plagioclase (35%). biotite (5%), hornblende ( 10%), opaques (3%), groundmass (42%). Hornblende is very altered and ragged in appearance, mostly relict grains. Groundmass contains tiny acicular pale-blue needles throughout. Some feldspar phenocrysts exhibit a chessboard texture. Plagioclase phenocrysts exhibit moderate sericitic alteration.

Sample Number - KW3-18/2 Location: 114°39’44". 36°02'16" Rock Name - granodiorite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - medium to fine Relative Grain Size - granular Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (25%), plagioclase (50%), alkali feldspar (10%), hornblende (3%), chlorite (7%), sphene (3%), apatite (<1%), opaques (1%). Hornblende is extensively

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altered, mostly altered to chlorite. Minor sericitic alteration of plagioclase laths. Some feldspar phenocrysts exhibit a chessboard texture. Chlorite veinlets present throughout.

Sample Number - HTl-13/8 Location: 114°38'14". 35°59’56" Rock Name - quartz monzodiorite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - medium to fine Relative Grain Size - granular Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (<5%), plagioclase (59%). alkali feldspar (3%). biotite (3%). hornblende (15%). chlorite (10%), sphene (1%). opaques (5%). Moderate sericitic alteration of plagioclase crystals. Chessboard texture observed in some feldspar crystals. .Altered hornblende is associated with opaques and secondary sphene and contains biotite inclusions. Some hornblende has been altered to chlorite and some is rimmed and replaced by blue amphibole. Chlorite veining prevalent.

Sample Number - HTl-13/9 Location: 114=3759". 35°59'57" Rock Name - latite dike Degree of Cry stallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - fine Relative Grain Size - equigranular Color - mesocratic Shape - subhedral to anhedral Description - Quartz (<5%), plagioclase (35%), biotite (<5%), hornblende (30%) very altered, chlorite (15%). opaques (10%). Hornblende very altered, mostly to chlorite. Moderate sericitic alteration of most plagioclase phenocrysts. Secondary microcrystalline calcite veining accompanies magnesio-riebeckite/chlorite veining. Most opaques have a cubic habit.

Sample Number - HTl-13/10 Location: 114=3759". 35=59'57" Rock Name - latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic with cryptocrystalline groundmass Absolute Grain Size - fine Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral to anhedral

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Description - Quartz (<5%). plagioclase (35%). alkali feldspar (17%). biotite (10%). hornblende (5%), sphene (<1%). opaques (3%). sericite (

Sample Number - HTl-13/11 Location: 114=3757". 35=59'59" Rock Name - quartz latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - fine to medium Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral Description - Quartz (20%). plagioclase (30%), biotite (7%), hornblende (7%), calcite (<1%). apatite (<1%). opaques (3%), groundmass (32%). The larger plagioclase phenocrysts are moderately to severely altered to sericite. Groundmass feldspars do not contain much sericitic alteration. Hornblende phenocrysts contain biotite inclusions, opaques and sphene. Minor secondary calcite veinlets. Opaque minerals show red internal reflections on thin edges, suggestive of hematite. Generally, fluid inclusions (liquid-rich) in quartz contain at least one daughter crystal, occasionally more.

Sample Number - HTl-13/12 Location: 114=3758". 35=59’58" Rock Name - quartz latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - medium to fine Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral Description - Quartz (20%). plagioclase (20%), biotite (7%), hornblende (5%). apatite (<1%). opaques (3%). groundmass (44%). Intense sericitic alteration of plagioclase phenocrysts, in most cases masking twinning and zoning, some cores remaining. Weak to moderate sericitic alteration of groundmass plagioclase. Hornblende altered to biotite, opaques, and sphene. Euhedral biotite not associated with hornblende alteration.

Sample Number - HTl-13/13 Location: 114=3757". 35°59'58" Rock Name - quartz latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - medium to fine Relative Grain Size - inequigranular porphyritic Color - mesocratic Shape - subhedral

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Description - Quartz (20%). plagioclase (25%). biotite (7%). hornblende (5%). calcite (<1%). apatite (<1%). opaques (2%). groundmass (39%). Moderate to intense sericitic alteration of plagioclase phenocrysts. Groundmass plagioclase has minor sericitic alteration. Hornblende phenocrysts are moderately to severely altered. Anhedral quartz in the groundmass contain fluid inclusions (liquid-rich) with one daughter crystal, occasionally more.

Sample Number - HTl 0-27/1 Location: 114=3735". 36=00'07" Rock Name - latite dike Degree of Crystallinity - holocrystalline Grain Size - aphanitic to microcrystalline Absolute Grain Size - fine to aplitic Relative Grain Size - inequigranular seriate Color - mesocratic Sbape - subhedral phenocrysts with anhedral groundmass Description - Quartz (<5%). plagioclase (25%), biotite (3%). hornblende (5%), sphene (<1%). muscovite (<1%). opaques (1%). groundmass (60%). Minor to moderate sericitic alteration of plagioclase. Groundmass contains liny acicular pale-blue needles throughout.

Sample Number - HTl 0-27/2 Location: 114=3755". 36=00'07" Rock Name - monzodiorite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - fine Relative Grain Size - granular Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (<5%), plagioclase (60%). alkali feldspar (16%), biotite (10%). hornblende (10%). sphene (<1%). apatite (<1%). opaques (3%). Hornblende and biotite are intensely altered. Moderated to intense sericitic alteration of plagioclase. Some feldspar phenocrysts exhibit chessboard twinning.

Sample Number - HT4-17/2 Location: 114=3755". 35°59'58" Rock Name - quartz monzonite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - medium to fine Relative Grain Size - equigranular Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular

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Description - Quartz (15%). plagioclase (40%). alkali feldspar (33%). hornblende (5%). sphene (3%). apatite (<1%). opaques (3%). Minor sericitic alteration of plagioclase. Some feldspar grains (5%) have chessboard twinning. Relict hornblende associated with opaque oxides and sphene.

Sample Number - HT4-17/3 Location: 114°37'55". 35°59'58" Rock Name - granite Degree of Crystallinity' - holocrystalline Grain Size - phaneritic Absolute Grain Size - fine to medium Relative Grain Size - equigranular Color - mesocratic Shape - subhedral to anhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (20%). plagioclase (35%). alkali feldspar (25%), hornblende (5%), biotite ( 10%). sphene (2%). opaques (3%). Minor to moderate sericitic alteration of plagioclase. Some feldspars have cores that have not been altered by sericitic alteration. Most feldspar is untwinned. Very little chessboard twinning observed. Relict hornblende associated with opaques and sphene.

Sample Number - HT4-17/4b Location: 114=3755". 35°59’58" Rock Name - monzonite Degree of Cry stallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - fine to medium Relative Grain Size - granular Color - mesocratic Shape - anhedral to subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Quartz (<5%), plagioclase (60%). alkali feldspar (27%). biotite (<1%). sphene (3%). opaques (3%), apatite (<1%). chlorite (<1%). Minor sericitic alteration of plagioclase. Some feldspars have albite twinning but most is untwinned. Some feldspar phenocrysts (3%) exhibit chessboard twinning. Relict hornblende associated with minor biotite inclusions, opaques and sphene. Some hornblende has been altered to chlorite. Chlorite and/or amphilxjle veining throughout.

Sample Number - CL5-15/4 Location: 114=34'30", 36°04'00" Rock Name - monzodiorite Degree of Crystallinity - holocrystalline Grain Size - phaneritic Absolute Grain Size - medium to fine Relative Grain Size - equigranular Color - mesocratic

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Shape - euhedral to subhedral Rock Textures Depending on Mineral Shapes - idiomorphic granular Description - Plagioclase (75%). hornblende (25%). Intense sericitic alteration of plagioclase. Some hornblende altering to chlorite. Opaque oxides associated with severely altered hornblende phenocrysts.

Sample Number - CL5-15/5 Location: 114=34’53". 36°03’55" Rock Name - monzodiorite Degree of Crystallinity - holocrystalline Grain Size - aphanitic Absolute Grain Size - fine Relative Grain Size - inequigranular seriate Color - mesocratic Shape - subhedral Rock Textures Depending on Mineral Shapes - hypidiomorphic granular Description - Plagioclase (67%). hornblende (25%). calcite (1%). opaques (5%). The cores of some plagioclase show sericitic alteration. Hornblende is extensively altered, some rims and interiors of crystals are altered to a pale-blue color. Other homblende crystals have been altered to chlorite. Some feldspar crystals exhibit microcline twinning.

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LIST OF SAMPLES AND ANALYSES

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List of Samples and Analyses

Sample Number Type XRF Microprobe Oxygen Isotope

KWl 0-18/2 (granitoid host) ✓ ✓ KWlO-18/3 (blue dike) ✓ ✓ ✓ KW5-396/7 (banded dike) ✓ ✓ KWl-12/3 (altered granitoid) ✓ ✓ ✓ H Tl-13/9 (granitoid host) ✓ ✓ ✓ HTlO-27/1 (blue dike) ✓ ✓ ✓ HT 10-27/2 (granitoid host) ✓ ✓ ✓ HT4-17/2 (brittle fracture zone) ✓ HT4-17/3 (brittle fracture zone) ✓ HT4-17/4A (brittle zone-blue) ✓ HT4-17/48 (brittle zone- bleached) ✓ HT-Vein (mineral separates) ✓ 9010-]b (unaltered) ✓ 9010-3 (unaltered) ✓ ✓ 9010-3eb (unaltered) ✓ ✓ LL8-42b (unaltered) ✓ ✓ 912-6BC (unaltered) ✓ ✓ 78-218^ (unaltered) ✓

^Relatively unaltered samples of Wilson Ridge pluton from Horsethief Canyon ( Larsen, 1989; Metcalf, unpublished). *^Relatively unaltered sample from the eastern edge of the southern margin of the Wilson Ridge pluton (Metcalf, unpublished). ^Basaltic dike rock (Smith et al., 1990).

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X-RAY DIFFRACTOMETRY - SPECTRUM SCANS

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X-Ray Diffractometr>’ - Spectrum Scans Reflect Riebeckite Peaks Only Compare with Published Values on Following Page

Spectrum Scan of Sample HT-794/1 two theta angle d-spacing’* net intensity relative (60 - 7 degrees) height 10.45 8.457 395.89 10Ô 28.45 3.134 340.46 86 32.94 2.716 336.5 85 24.21 3.672 186.06 47 30.25 2.951 142.52 36 35.31 2.539 126.68 32 26.24 3.392 118.76 30 27.14 3.281 79.17 20 34.49 2.597 75.21 19 55.5 1.654 67.3 17 19.65 4.513 63.34 16 58.28 1.581 51.46 13 31.75 2.814 43.54 11 39.44 2.282 43.54 11 18.11 4.891 43.54 11 44.81 2.020 35.63 9 48.0 1.893 35.63 9 56.68 1.622 23.75 6

Spectrum Scan of Sample HT-794/3 two theta angle d-spacing* net intensity relative (60 - 7 degrees) height 24.14 3.682 375.41 100 30.38 2.939 195.21 52 28.36 3.143 120.13 32 10.42 8.476 120.13 32 32.94 2.716 116.37 31 35.05 2.557 93.85 25 48.08 1.890 52.55 14 45.72 1.982 41.29 11 41.57 2.169 33.78 9 43.39 2.083 30.03 8 19.48 4.551 30.03 8

*d-spacing (1.54 - 12.6129 degrees) in 2120 steps of -.025 degrees.

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Published Values for Riebeckite Spectrum Scan

d-spacing relative d-spacing relative height height

9.02 4 2.031 8 8.40 100 2.079 6 4.89 10 2.000 4 4.51 16 1.888 4 3.88 10 1.866 6 3.66 10 1.805 6 3.42 12 1.659 10 3.27 14 1.635 6 3.12 55 1.617 8 2.976 10 1.593 10 2.801 18 1.583 8 2.726 40 1.576 6 2.602 14 1.520 4 2.541 12 1.509 4 2.324 12 1.504 4 2.301 4 1.429 6 2.268 10 1.352 4 2.191 4 1.333 6 2.176 16 1.301 6

Values from Selected Powder Diffraction Data for Minerals, in Barry, L. G. et al. eds.. 1974. Joint Committee on Powder Diffaction Standards. Swarthmore, 833 p.

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Graduate College University of Nevada, Las Vegas

Deborah A. Potts

Local Address: Deborah A. Potts 1572 Elizabeth Avenue. Apt. 1 Las Vegas. Nevada 89119

Home Address: Deborah A. Potts 114 Ventoura Vista Toronto. Ohio 43964

Degrees: Bachelor of Science, Geology, 1993 University of Nevada, Las Vegas

Publications: Potts. D. A., and Metcalf, R. V., 1997. Sodium metasomatism and riebeckite mineralization in an extensional terrane: Wilson Ridge pluton, northwest Arizona [Abs.]: Geological Society of America 93rd Annual Cordilleran Section, Abstracts with Program, v. 29. no. 5, p. 57.

'Hs. D. A.. and Cline. J.S.. 1992, Preliminary pétrographie and fluid inclusion data irom the Castle Mountain gold deposit. Hart Mining DistricL San Bernardino County, California [Abs.]: Fourth Biennial Pan American Conference of Research on Fluid Inclusions, Program and Abstracts, Lake Arrowhead, California, p. 66.

Duebendorfer. E.M., Black, R.A., Simpson, D.A., Tuma, T.L., Potts, D.A.. Osmond, E.W.. 1992. Structural chronology of Miocene deformation near the east end of the Las Vegas Valley shear zone, Nevada [Abs.]: Geological Society of America, Abstracts with Program, v. 24, p. 8-9.

Thesis Title: Sodium Metasomatism and Magnesio-riebeckite Mineralization in the Wilson Ridge Pluton

Thesis Examination Committee: Chairperson, Rodney V. Metcalf, Ph D. Committee Member, Eugene I. Smith, Ph.D. Committee Member, Frederick W. Bachhuber, Ph D. Graduate Faculty Representative, Sidkazem Taghva, Ph D.

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