The clay mineralogy of selected fault gouges

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Authors Bladh, Kenneth Walter

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Link to Item http://hdl.handle.net/10150/554458 THE CLAY MINERALOGY OF SELECTED FAULT GOUGES

by

Kenneth Walter Bladh

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 3 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re­ quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg­ ment the proposed use of the material is in the interest of scholar­ ship. In all other instances, however, permission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

wa / 7 /f 7. fOHN W. ANTHONY?V J Datb Professor of Geosciences ACKNOWLEDGMENTS

I thank Dr. John W. Anthony for suggesting the top ic, for in­ valuable discussions, and for guidance during the study. Drs. Spencer

R. Titley and Joseph F. Schreiber, Jr. critically read the manuscript and offered useful suggestions. I thank Dr. Ralph W. G. Wyckoff for the use of X-ray diffraction and spectrographic analysis equipment and the general use of the resources of his laboratory „

M essrs. John T. Eastlick and David Johnson, geologists for the Inspiration Consolidated Copper Company, Inspiration, Arizona, are thanked for assistance in collecting samples. The Inspiration Consoli­ dated Copper Company is acknowledge for allowing the use of material collected on their property. Financial support from Training Grant 5 T01 DE 00126 of the

National Institute of Dental Research, National Institutes of Health, is also acknowledged. I thank my wife, Kathi, for suggestions, typing skills, and tolerance during the preparation of this manuscript.

iii TABLE OF CONTENTS

Page

LIST OF TABLES ...... vi

LIST OF ILLUSTRATIONS...... vii

ABSTRACT...... v iii

INTRODUCTION...... 1

GEOLOGY OF THE MIAMI-INSPIRATION PROPERTY...... 4 NOMENCLATURE...... 9

Faults and Cataclastic R ocks ...... 9 Clay Minerals ...... 10

METHOD OF S T U D Y ...... 13

Separation of Clay-size Fraction ...... 13 Preparation for X-ray Diffraction A nalysis ...... 18 X-ray Diffraction Identification of Clay M inerals ...... 19 Identification of the Non-clay Mineral F raction ...... 25

LITHOLOGY OF THE MIAMI FAULT CATAC LAS TICS...... 29

LITHOLOGY OF THE NO. 5 FAULT CATACLASTICS ...... 32 MINERALOGY OF MIAMI FAULT CATACLASTICS...... 34

Clay Minerals ...... 34 Non-clay Minerals ...... 40

MINERALOGY OF NO. 5 FAULT CATACLASTICS ...... 41

Clay Minerals ...... 41 Non-clay Minerals ...... 46

SPECTROGRAPHIC EXAMINATION OF CLAY-SIZE MATERIAL .... 48

Analytical Technique ...... 4 8 Results ...... 49 I r o n ...... 49 M agn esiu m ...... 52 M a n g a n e se ...... 52

iv V

TABLE OF CONTENTS—Continued

Page

Calcium „ „ . „ ...... 52 Sodium and Phosphorus 52 Copper ...... 53 Other Elements ...... 53 Applicability to Mineral Identification ...... 53

DISCUSSION ...... 56

No. 5 Fault ...... 56 Miami Fault ...... 59

CONCLUSIONS ...... 63 REFERENCES...... 67 LIST OF TABLES

Table Page

I. A Classification of the Clay Minerals 12 2 „ Summary of the X-ray Characteristics of Clay Minerals . . 26.

3„ D-spacings in Angstroms from Oriented Slides (Clay- size Fraction) from Miami Fault Cataclasties ...... 35

4. Mineral Distribution along the Miami Fault ...... 38 5. D-spacings in Angstroms from Oriented Slides (Clay- size Fraction) from No. 5 Fault Cataclastics ..... 42

6. Comparison of D-spacings of a Biotite Phenocryst from the Dacite Hanging Wall of the No. 5 Fault with Published Data for Biotite and Illite ...... 44

7. . Elemental Analyses of Gouge Samples and Standards (Clay-size Fraction unless Otherwise Noted) ..... 50

8. Comparison of Elemental Analyses of API Reference. Clay Minerals ...... 51

v i LIST OF ILLUSTRATIONS

Figure Page \ 1 „ Location of Drill Holes across the Miami Fault and Surface Exposure of the No. 5 Fault (Map View) . . . . 2

2 . Location of Sample Levels and Lithologies along the Miami Fault ...... 5 3. Geologic Section of the Inspiration Property, Arizona. . . 7

4 . Flow Diagram of the Analytical Procedure ...... 17

5 . Relative Concentrations of Clay Minerals along the Miami Fault ...... 39 6. Diagrammatic Representation of the Mineral Distribution across the No. 5 Fault ...... 43

v ii ABSTRACT

The clay mineralogy of gouge from two postmineralization nor- maTfaults associated with a porphyry copper deposit was determined by conventional X-ray diffraction techniques„ The No. 5 and Miami fault gouges from the Inspiration Consolidated Copper Company property at

Globe, Arizona, are almost identical mineralogically to the contiguous wall rocks. Most of the gouge contains angular to subrounded fragments of quartz and wall rock in sericitic matrix„ Kaolinite, muscovite, and smectite occur in varying relative abundances on the depths sampled across the Miami fault. The No. 5 fault gouge contains a larger suite of clay minerals than does the Miami fault, gypsum precipitated from ground waters, and genetically unexplained palygorskite. The faulting process created a nonequilibrated mixture’ of wall- derived clay minerals. No minerals were formed by the faulting process.

Generally, wall-rock type mineralogy decreases in abundance away from the source wall, stopping short of the opposite wall. The effects of sol­ utions traveling along the gouge zone are controlled by the local physico­ chemical environments. Chemically resistant minerals occur farther from the source wall than do easily leached minerals. Shearing is most in­ tense near walls or with depth, and the mechanically least resistant wall contributes the most material to the gouge . INTRODUCTION

The clay minerals literature, although extensive, does not in­ clude an adequate treatment of clay mineral occurrences in fault gouge„ The object of this study is to collect, examine, and describe the min­

eralogy of gouge material associated with faulting in rock. Because of

the lack of previous work in this area, the direction of this study de­ veloped and changed during the investigation. However, the goals re­

main to describe the minerals present in the fault gouge and, if possible, explain their origin.

Initially, a large number of faults were sought with the.inten­ tion of collecting gouge samples from each. Unfortunately, common as gouge seems to be, material suitable for study is rare. A relatively wide gouge zone (on the order of a few feet across) is required and should be nonlithified and unaffected by postfaulting solutions. Because the me­ chanical effects of faulting upon the mineralogy were of primary interest, gouge significantly altered by solutions and extensive surface weather­

ing was avoided. Faults recently exposed in open pit mining operations were examined for gouge of desired quality. After a number of unsuccessful examinations of faults, workable gouge material was collected across the Miami fault. The Miami fault is a pqstmineralization normal fault exposed at the Globe, Arizona, property of the Inspiration Consolidated Copper Company. Drill core

samples (Fig. 1) across the fault at various depths were furnished by the company. The extensive number of samples collected at this single No. 5/ Fault

Inspiration 37 Concentrator

Warrior Fault Miami / Fault

' Miami Concentrator C - l l l

Figure 1. Location of Drill Holes across the Miami Fault and Surface Exposure of the No. 5 Fault (Map View) locality immediately limited the scope of the study. It was thought that a detailed examination of gouge along the same fault at different depths would be instructive. A series of surface samples was also collected across the No. 5 fault on the same property. The recently bulldozed surface exposure of the No. 5 fault is in the floor of the Black Copper Pit (Fig. 1) and is relatively fresh. Gouge samples were collected across the fault perpendicular to the fault plane at 6-inch intervals. Representative wall-rock samples adjacent to the gouge were collected from both walls.

The previously split drill core material from the Miami fault was collected from core boxes. It is assumed that the core was in proper se­ quence and correctly labeled as to depth and drill hole. Gouge samples were collected across the fault zone defined in the core at approximately 1- to 2-foot intervals. Samples which contained a large proportion of loose breccia fragments were avoided. Preference was given to the fine­ grained gouge material. The crushed nature of the core material made wall rock difficult to distinguish from the cataclastics. Material with minor recognizable wall-rock fragments was assumed to be gouge. GEOLOGY OF THE MIAMI-INSPIRATION PROPERTY

The geology of the Miami-Inspiration ore body has been de­

scribed by Ransome (1903, 1919), Peterson (1962), Reed and Simmons

(1962), and Olmstead and Johnson (1966). The Miami fault, one of the major structures in the area, sepa­ rates the Globe Valley block (a graben) on the east from the Inspiration

block on the west (Fig. 1). This normal fault strikes N. 20° E. and dips

35° E. to vertical (Peterson, 1962). The exact amount of movement along

the fault is not known, but Reed and Simmons (1962) estimate the Globe

block to be downdropped 2,000 to 3,000 feet. Peterson (1962) estimates

1,500 feet of offset of dacite remnants along the fault. The thickness of

Gila Conglomerate indicates at least 5,000 feet of vertical movement be­

tween the Globe Valley and Inspiration blocks (Fig. 2). There niay have been some pre-porphyry movement (Reed and Simmons, 1962). The fault cuts dacite and Gila Conglomerate, which are younger than the secon­

dary enrichment, and rounded fragments of chalcocite and oxidized cap­

ping are found in the gouge (Rubly, 1938). Consequently, most of the

movement is postmineralization and occurred after secondary enrichment. had begun. The structural control for the faulting appears to be the in­ trusions of granite and granite porphyry into the Pinal Schist (Peterson,

1962). Rock types exposed along the Miami fault are shown diagram- matically in Figure 2, which was compiled from core log data of the

Inspiration Consolidated Copper Company. The main rock types are

4 Drill C - l l l 48 39 37 3500' Hole . Qtg Llevatioi 300' Level Ps \

\ \

\ \

\

\ \ * gp V v ,-V v 3900' Qtg Gila Conglomerate Level

V :9 tg v V V gp granite porphyry • • • v v— x ; 4500' Tsq Schultze Granite gp v Vv Vs- v» Level Qtg C-O/c v V Ps Pinal Schist Scale 1" = 1000' 5000' Tsqx ^ Level

Figure 2. Location of Sample Levels and Lithologies along the Miami Fault ■ 6 Pinal Schist,. Schultze Granite, granite porphyry, dacite, and Gila Con­ glomerate . The No. 5 fault is a northeast-trending normal fault with a dip of 30° to 40° E. (Fig. 1). It forms the surface contact between dacite and Pinal Schist (Fig. 3). There is at least 250 to 300 feet of dip-slip movement (Peterson, 1962). The PreCambrian Pinal Schist, derived from siliceou s sediments and granite intrusive rocks, has a nonuniform lithology that ranges from coarse-grained quartz muscovite schist to fine-grained quartz sericite schist (Ransome, 1919). Some biotite and chlorite are present; magne­ tite and ilmenite are the principal accessory minerals (Peterson, 1962).

The granite porphyry, a younger phase of the Schultze Granite, contains euhedral quartz, biotite, oligoclase, and orthoclase pheno- crysts. The Schultze Granite is actually a porphyritic quartz monzonite with orthoclase phenocrysts in a groundmass of quartz, oligoclase, or­ thoclase, and biotite. Hypogene mineralization is associated with these two intrusions (Reed and Simmons, 1962) . The age of the intrusions is approximately 60 m.y. (Creasey and Kistler, 1962).

The dacite is Tertiary (20 m.y. according to Creasey and

Kistler, 1962) and was erupted onto Pinal Schist near the ore body. It is light brown when fresh with an aphanitic groundmass containing small

(less than 2 mm) phenocrysts of feldspar, quartz, and black biotite.

Andesine is the most abundant feldspar, constituting twice the amount of orthoclase. Biotite is altered to a red brown. The dacite was erupted before deposition of the Gila Conglomerate, but remnants of dacite re­ main along the Miami fault below the conglomerate (Peterson, 1962). Td gp

No. 5 Ps Fault gp Tsq %\x • Qtg . Miami ' Qtg Gila Conglomerate Fault

Td dacite Fault

gp granite porphyry

Schultze GraniteTsq

Pinal SchistPs 1000 2000 3000 4000 feet Scale

Figure 3. Geologic Section of the Inspiration Property, Arizona.—After Olmstead and Johnson, 1966) 8 " ) The Quaternary Gila Conglomerate contains fragments of all

older rocks in the area, but schist and dacite fragments predominate

(Reed and Simmons, 1962). The conglomerate is the major aquifer in

the area (Peterson, 1962). The Inspiration deposits are the porphyry copper type in which relatively weak hypogene copper mineralization was concentrated by secondary enrichment by percolating ground waters (Reed and Simmons, 1962). Discussion of porphyry copper deposits and their characteristics can be found, for example, in Titley and Hicks (1966). The host rock was altered hydrothermally before mineralization was developed. The

Pinal Schist was intensely sericitized near ore with the loss of chlorite

(Peterson, 1962). Sericitization, the primary type of alteration in all rock types, was locally preceded by formation of secondary potassium

feldspar. In general, alteration was by high-potassium fluids (Olmstead and Johnson, 1966).

Throop and Buseck (1971) have studied the occurrence of black chrysocolla in the Black Copper Pit along the Warrior fault zone. (Fig. 1).

They explain the mineralization by ground water leaching silica from the overlying dacite, producing heulandite in the dacite, and subsequent combination with copper ions carried by ground waters from the primary mineralized areas to produce chrysocolla. NOMENCLATURE

Faults and Cataclastic Rocks

The concepts of faulting in rock are basic to geology but are

not considered in this study unless useful in explaining the observed

mineralogy of the gouge. These concepts are discussed in any good

structural geology text (Billings, 1954; Hills, 1963). However, clari­

fication of nomenclature is essential to avoid confusion. "Fault" refers to a naturally occurring break in rock along which there has been relative

movement of the walls. The "fault zone" includes the entire three- dimensional zone of fracturing and associated brecciation bound between

imaginary surfaces defined by relatively unaffected portions of the wall,

rock. As is the case with the Miami fault, many faults are wide zones

of subparallel breakage and movement.

The near-surface movement of rock against rock along a fault

zone usually causes brecciation and grinding of the wall rock immedi­

ately adjacent to the fault plane. This granulation produces cataclastic

rocks in the fault zone . Higgins (1971) divides cataclastic rocks into two groups, depending on the presence or absence of primary cohesion

within the rock. This study is concerned with the noncohesive group

which, in turn, is subdivided into fault breccia and fault gouge. "Fault

breccia," as defined by Higgins (1971, p. 4), is a cataclastic rock ' ■ • ■ . ■ " composed of angular to rounded fragments, formed by crushing or grind^-

ing along a fault. Most fragments are large enough to be visible to the

naked eyes, and they make up more than 30 percent of the rock." He 10 defines "fault gouge" as a "paste-like rock material formed by crushing or grinding along a fault. Most individual fragments are too small to be visible to the naked eye, and fragments larger than the average ground- mass make up less than 30 percent of the rock." Both rocks lack cohe­ sion but may be lithified by secondary cementation. These rock types form under low pressure-temperature conditions and are transitional with cohesive cataclastic rocks, such as mylonites, which are characteristic of higher temperature and pressure regimes (Higgins, 1971). These defi­ nitions and environments of formation are similar to those described by

Billings (1954). Gouge and breccia will be used with only the above connotations in this study.

Clay Minerals

The nomenclature associated with clay minerals is often con­ fusing and needs clarification in spite of exhaustive treatment in the literature. "Clay" is a general rock term for naturally occurring, very fine grained material that is plastic when wet and hard when dry (Carroll,

1970, p. 1). "Clay-size" is considered to be material of less than 2 p effective spherical diameter, as is customary with many clay mineralo­ gists . This break is a good practical split which yields a minimum of non-clay mineral components in the clay-size fraction (Grim, 1968, p. .

2). The two terms, "clay" and "clay-size," have no special composi­ tional or genetic connotation.

Clay minerals are hydrous aluminian phyllosilicates, except for the sepiolite-palygorskite group which has amphibolelike chain struc­ tures. The structures of clay minerals are well discussed in Grim (1968). 11

The significant point about the clay mineral concept is that there is crystalline material in clay that is usually identifiable and not found as larger size crystals (silt-size or larger). Clay mineral names used in this study are those of Grim (1968). Table 1 is a compiled classification of clay minerals. Table 1. A Classification of the Clay Minerals.—After Grim (1968) and Warshaw and Roy (1961)

Basal Structural Spacing (A) Type Group Subgroup Species Typical Formula

3.08 2:1 Pyrophylllte- Pyrophyllites Pyrophyllite Al2Si4O10(OH)2

3.12 talc Talcs Talc m 93S14o 102 3.18 2:1 Brittle micas Dioctahedral Margarlte CaAl2(Al2Si2O10)(OH)2 2.56 Trioctahedral Xanthophyllite CaMg^AlgSigOig) (OH) g

7 1:1 Kaolinite- Kaolinltes Kaolinite Al4Si4Ol0 (OH)8 serpentine Serpentines Chrysotile Mg6Si4O 10(OH)8

10 a 2:1 Mica Dioctahedral Muscovite KA12 (AlSi3O10) (OH)2 Trioctahedral Phlogopite KMg3 (AlSi3O10)(OH)2 >12 Mixed layer Regular minerals layered complex Irregular layered complex

12-15 2:1 Smectite Dioctahedral Montmorillonite ^ 13.3 4 i^9.66^S18O20^OH)4 N a#66 Trioctahedral Hectorite (Mg5.34Li .66)Si802o(OH)4 N a.66 14 2:1 Vermiculite Dioctahedral rare Trioctahedral Vermiculite (OH) 4 (MgCa)x (Sig_xAlx) (MgFe) 6O20 • yH20 x = 1 - 1.4 y = 8 14 2:1:1 Chlorite Dioctahedral rare Trioctahedral Clinochlore (MgFeAl)8(AlSi302o)(OH)g- general

10.5 — — Fibrous clay Palygorsklte (AlMgFe)5(AlSi7) 0 2o(OH)2 *8H20 12 minerals Sepiolite Mg9Sii20 3o(OH)6,10H2°

a . The position of illite is uncertain, but it probably should be included with the 10-A micas. METHOD OF STUDY

Separation of Clay-size Fraction Each raw sample was individually disaggregated by hand with an iron mortar and pestle „ Grinding was kept to a minimum to avoid any possible alteration of the clay minerals. Grim (1968, p. 469-471) re­ viewed the literature and summarized the possible consequences of grinding clay minerals of which the most important to this investigation is structural damage. Proper identification of the clay mineral compo­ nents of the gouge requires good quality X-ray diffraction patterns. The characteristic diffraction pattern for any crystalline substance is de­ pendent on the periodic structure of the material. If this structure is damaged, the X-ray diffraction pattern may be altered or entirely de­ stroyed. However, moderate, short-duration grinding is necessary to prepare a fine powder for separation of the clay fraction from the gouge , as will be described later. Investigators have found that long-duration mechanical grinding is required to alter significantly clay mineral struc­ tures (Grim, 1968). Nevertheless, a reasonable care should be exer­ cised in preliminary preparation of clay-bearing materials to protect the naturally weak structured clay minerals.

An additional problem of overgrinding is a tendency to produce finer particle size and increased surface area. Smaller particles floccu­ late faster than larger ones because of a reduced repulsive energy (van

Olphen, 1963). However, smaller particles tend to settle out of suspen­ sion more slowly than large particles (Stokes' law). The mild hand

13 14 grinding treatments used in these analyses should not have created un­ usually small clay mineral particles „ Each ground sample was individual­ ly hand sieved, using U.S. Standard brass sieves. Following several minutes of hand agitation, each sample was separated according to size.

Coarse material (larger than 500 > 1) was separated and not prepared be­ yond this step. Sample material finer than 500 ju was lightly re ground and sieved. All material finer than 63 ju (after the second sieving) was retained for sedimentation; material coarser than 63 ju was saved. When a sample size prohibited processing as one portion, the above procedure was applied to each of its parts until all of the sample was treated. A precaution observed during grinding was to keep approximately three- quarters of an inch of powder in the mortar at any one time to cushion the grinding effects of the pestle. Fragments that did not easily break down during grinding were not forceably disaggregated.

Prior to sedimentation, the minus 63 yU fraction was examined for the presence of carbonates, soluble salts, and organic material, all of which hinder dispersion of the clay-size material. As recommended by Jackson (1964, p. 247), organic material may be removed by hydrogen peroxide. Carbonates are dissolved in dilute hydrochloric acid, and soluble salts are removed by repeated washing in distilled water. Wash­ ing with distilled water may be ended when the flocculating effect of the soluble salts is no longer observed. Care must be taken to keep chemi­ cal treatment as gentle as possible to minimize replacement of inter­ layer cations or to avoid actual chemical alteration of the clay. A final rinse with distilled water is recommended to remove previously used re­ agents. Care should be exhibited during this procedure so that no clay- size material is discarded in the supernatant waste fluids. Waste water 15 should be clear. Most samples examined in this study contained no or­

ganic material or appreciable carbonates. Soluble salts were neither expected nor found.

Separation of the clay-size fraction from the minus 63 ju powder was accomplished by sedimentation in distilled water. Particulate mat­ ter settles out of suspension in water at a rate estimated by Stokes' law for settling of spherical particles. Larger particles settle out first. The time required to allow settling of all material larger than 2 ju (effective

spherical diameter) through a selected distance was calculated using

Stokes' law. In this study a 5-cm settling distance was used; that is, the time required to settle all particles larger than 2 ju to at least 5 cm below the top of the water level was calculated. When this amount of time had elapsed, the upper 5 cm of water and contained clay fraction were,removed from the container. An ordinary kitchen basting bulb was used in this study; others have used siphons to remove the liquid (Pip­ kin, 1964). Any method used to remove the water and clay suspension must not disturb the suspension. Small, screw-top jars were used to hold the clay-water suspension during settling. The two fractions, the clay-water suspension and the remainder of material in the settling jars, were individually dried in porcelain dishes in an oven to shorten the dry­ ing time. The oven temperature was kept below 50°C to avoid driving off interlayer water which begins to be lost about 100°C (Grim, 1968, p. 234). The dry, clay-size fraction was brushed or scraped from the dish and gently hand ground in an agate mortar to a powder and stored. The material larger than 2 ju (effective spherical diameter) that settled out during separation was dried, ground, and stored separately. 16 The difficult step in the separation procedure is the effective dispersion of the clay-size fraction in water because clay minerals,

readily flocculate in the presence of electrolytes. Other factors, such

as cementation, which affect flocculation of clay are discussed by Jack­ son (1964) and by van Olphen (1963). The clay examined in this investi­

gation was noncemented and did not contain appreciable amounts of

electrolytes (soluble salts). Consequently, dispersion was not a prob­

lem in most instances. An adequate clay-size fraction was obtained by

vigorous hand shaking of the minus 63 ju powder and water mixture for a

couple of minutes. The quality of the clay suspension was judged after

approximately one hour. If the water in the jar was clear, the dispersion was not good because the clay-size material had flocculated and settled from suspension before the calculated time. When such flocculation

occurred, the clear supernatant water was discarded, replaced with fresh

distilled water, and the new clay-water mixture was dispersed as above.

This procedure was repeated until an adequate suspension was obtained.

If hand shaking was inadequate , the use of a high shear dispensing ma­

chine, such as an electric blender, was used to suspend the clay (van

Olphen, 1963). As a last resort, a chemical dispersion agent was used.

Calgon (sodium hexametaphosphate and sodium carbonate) was used in

this study. In the few instances, where mechanical dispersive tech­

niques failed, chemical treatment with small amounts (a fraction of a

gram) of Calgon produced good dispersion and an adequate cla;^ yield.

Approximately 10 to 20 gm of minus 63 ju powder yielded, on the average,

2 to 3 gm of clay-size material by the above techniques. The procedure

is summarized in Figure 4. Other techniques for the separation of the Raw Sample

Sieve---- + 63 JLL i -63 ji Binocular Examination

X-ray Fragments

-2 JLL. Sedimentation

X-ray X-ray Examination Examination Spectrographic Oriented Examination Petrographic Slide Examination

Glycolated Slide Acid Treatment X-ray 350°C

550°C

Figure 4. Flow Diagram of the Analytical Procedure 18 clay-size fraction can be found in MacKenzie (1956), Pipkin (1964), Brown (1961), and Krumbein and Pettijohn (1938)

Preparation for X-ray Diffraction Analysis

The mineralogy of the two fractions separated by suspension in distilled water was determined by X-ray diffraction analysis (Fig. 4).

Examination of the clay-size fraction was emphasized in this study. Consequently, the mineralogy of the coarser fraction was studied only with reference to the genesis of the clay minerals.

The clay-size fraction was examined as a film on glass petro- graphic slides. Approximately 25 mg of the clay-size fraction was, used for each slide. This weight yielded sufficient clay film thickness to give good diffraction intensities (Brindley, 1961a). The powder for an indi­ vidual slide was dispersed in approximately 1 to 2 ml of distilled water and occasionally dispersion was aided by vibration in an ultrasonic cleaner for up to 15 minutes. The clay-water suspension was trans­ ferred to the glass slide with a medicine dropper. This technique causes problems when quantitative clay mineral concentrations are desired be­ cause minerals settle to the glass slide at different rates which results in layering due to settling velocity. The nonhomogeneous clay film created may give erroneous relative intensities for the clay minerals present (Gibbs, 1965). However, the effect would be uniform in all slides. The clay should all be applied to the slide at one time to obtain optimum orientation. Generally, the clay minerals have a platy habit and settle with this platiness parallel to the slide surface. As will be explained later, this orientation is the key to the routine identification 19 of clay minerals „ More than one application of clay-water suspension

to the same slide tends to disorient previously settled clay minerals

(Pipkin, 1964). Surface tension held the clay-water drop to the surface of the glass slide so it was essential that the slide be clean and sup­

ported by a flat substrate. A sheet of metal or plate glass serves this

latter function quite well, although the drop tends to flow from edge to

edge of the petrographic slide. The entire slide was, however, not analyzed by the X-ray beam. A waste of precious clay material was

avoided by drawing a line across the short dimension of the slide with a grease pencil to contain the clay-bearing drop to the area exposed to

the X-ray. The clay layer was allowed to dry overnight under natural at­

mospheric conditions and was usually quite stable once dry. Occasion­

ally one of the clay films curled up around its edges or severely cracked

during drying. Such samples were scrapped from the slide, re ground, resuspended, and reapplied to a clean slide. Once prepared, the slide was placed in the diffractometer with

the clay layer toward the beam. Slides that were to be heated as part of

the identification procedure were prepared on Vycor glass slides which

are resistant to high temperatures (Warshaw and Roy, 1961).

X-ray Diffraction Identification of Clay Minerals

The theory and practice of X-ray diffraction analysis is well

known and will not be described here. Good summaries can be found in Klug and Alexander (1954) and Cullity (1956). The techniques of X-ray

diffraction identification of clay minerals are specialized and well 20 discussed by Brown (1961), Grim (1968), and Carroll (19 70). The tech­ nique used in this study is derived mainly from the last three sources.

Other sources are Hathaway (1956) and Pipkin (1964).

Clay minerals were examined in this investigation., using a commercial Phillips X-ray diffractometer equipped with a monochrometer between the sample and detector using copper radiation. Intensities of diffraction were continuously recorded on chart paper in the conventional manner. As mentioned earlier, the clay films on glass slides were pre­ pared so that the natural tabularity of the clay minerals oriented itself parallel to the slide surface. This phenomenon resulted in a layer of clay on the glass slide which had the c crystallographic axis'of the clay minerals oriented approximately perpendicular to the plane of the slide. Consequently, (001) reflections from the clay minerals were pref­ erentially intensified relative to the other crystallographic directions in the structure when exposed to the X-ray beam. The random orientation usually desired in powder diffraction methods was avoided to make use of the characteristic periodicity of clay minerals along the c axis for identification.

Clay minerals are classified into groups according to the peri­ odicity along the stacking axis, c„ The major groups of clay minerals are 7 A, 10-11 A, 12-15 A, and clay minerals with greater than 15 A periodicity (Table 1). X-ray diffraction analysis was used to detect the presence of any clay minerals on the glass slide that had a periodicity falling into one of these groups. Once a mineral has been assigned to a group by X-ray diffraction analysis, other techniques, such as chemical analyses, differential thermal analysis, infrared absorption, or electron 22 on Vycor glass slides were used for the heating procedure „ The slides were supported on a flat, smooth substrate of fire brick to prevent warp­ ing at high temperatures„ The slides were cooled on the same substrate in a low-humidity dessicator and were stored there until analyzed. The slides were analyzed as soon as they could be handled and were scanned over the same interval as in previous steps, and changes in the diffrac- togram were noted. The slides were then heated at 550°C for 12 hours and reanalyzed. A useful tool in differentiating dioctahedral and tridctahedral clays is the interpretation of the (060) interplanar distance of the clay mineral. This spacing gives a direct measurement of Jb which is different for trioctahedral and dioctahedral minerals . For example, dioctahedral micas have (060) spacings of 1.50 to 1.52 A, whereas, trioctahedral micas have (060) spacings of 1.53 to 1.54 A. This reflection can be measured from an indexed random powder pattern, a procedure which can be time consuming when large numbers of samples are involved. An alternate method is one proposed by Rich (1957) that uses oriented films of clay mounted perpendicular to the normal sample position on a diffrac­ tometer. This technique utilizes the oriented clays to intensify the (060) reflection adequately for rapid identification. Unfortunately, this author could not get this technique to work adequately for dependable (060) re­ flection measurements.

Usually, at this point in the analysis, sufficient data had been collected to make identifications of the clay minerals present in the sample. Mixtures of clay minerals presented problems in identification caused by overlapping reflections and ambiguous combinations of 23

reflections. Distinctive characteristics of each clay mineral group and

procedures to identify components of mixtures are discussed below. .All

diffractograms taken in these procedures are from oriented films.

Kaolinite-type minerals are'easily distinguished by their 7 A basal reflections. Individual members of the group are differentiated by detailed examination of the entire diffractogram and comparison with known diffraction data. Halloysite is distinguished by its morphology as seen under the electron microscope. Confusion of kaolin-type minerals with chlorites can occur if the 14 A first-order chlorite reflection is not well developed. However, the third-order reflection at 4 .7 A distin­ guishes chlorite (Brindley, 1961b) . Also, chlorites are easily dissolved

in warm (80°C) dilute (2N) hydrochloric acid, while kaolinite is not. A weak 7 A reflection with no 14 A reflection after acid treatment is am­ biguous, because it could be from partially decomposed kaolinite, and other techniques may be needed to make positive identification (Hideomi and Kaoru, 1962). Heating to 550°C destroys the crystallinity of kaolin­

ite by removing hydroxols but itensifies the 14 A chlorite reflection

(Brindley, 1961b). The (060) spacing is 1.49 A for kaolinite and 1.54 A

for chlorites. Additional information on specific problems in identifica­ tion is found in Brown (1961), Carroll (1970), and Warshaw and Roy

(1961).

Minerals in the mica group are characterized by a sharp 10 A basal reflection that is not variable in position or intensity with mild heating or change in humidity. Individual micas are distinguished by detailed examination of the diffractogram and comparison to known dif­ fraction data. Dioctahedral micas have (060) close to 1.50 A, and the . 24 (002) reflection at 5 A is strong„ Trioctahedral micas have (060) close to 1.53 A, and the (002) reflection is weak or absent (Grim, 1968). Sub­ stitution of aluminum for silicon in the octahedral layer sites can cause large intensity variations of (001) reflections which can confuse identification (Grim, 1968, p. 145). Other 10 A minerals that may be confused with micas are mixed-layer minerals which have variable basal reflections depending on humidity. They can be identified by following the procedure of Warshaw and Roy (1961, Table 9). Minerals having basal reflections in the range 12 to 15 A are of four major types which are differentiated by treatment with ethylene glycol and observation of changes in position of basal reflections after treatment. Common chlorites and vermiculites will show no change in their characteristic 14 A basal reflection after treatment with ethylene glycol. Heating to 350°C causes the 14 A reflection of vermiculite to become a broad 9 A band, whereas common chlorite shows no change.

Some chlorites give an 18 A reflection after treatment with ethylene gly­ col which decreases to 14 A after heating to 500oC (MacEwan, 1961, p. 186). A basal reflection of 12 A before and after glycolation is char­ acteristic of sepiolite. If the natural basal reflection of the Clay mineral changes from 12 to 15 A to approximately 18 A after glycolation, a smec­ tite is indicated. The exact position of the post-glycol smectite reflec­ tion is dependent on the cations in the interlayer positions (MacEwan,

1961, p. 184). Heating a smectite to 250o-300° C will cause (001) to appear at 9 .7 to 10 A. Further identification of the specific smectite can be done according to the procedure of Warshaw and Roy (1961, Table 10). The fourth type of 12-15 A clay minerals is the mixed layer with 25 smectite components. These minerals show variations in c-axis peri­ odicity aifter treatment with ethylene glycol proportional to the amount of smectite in the structure. Additional identification criteria are found in Warshaw and Roy (1961) .

For mechanical mixtures of clay m inerals, WarshaW and Roy

(1961) give a sequential procedure for identification of all phases. Their

procedure is followed in this study, as outlined in Tables 11 and 12 of

their paper. Table 2 is a summary of the characteristic behavior of the

various clay minerals to the X-ray analytical procedure. A note of cau­

tion is advisable because the rate of heating reactions and temperatures

of breakdown for any mineral depend, on its crystal size and degree of crystal perfection. Temperatures given in the tables and discussions

are for the clay-size fraction of "perfectly" crystalline material. Smaller

particle size and greater imperfection can lower the temperature of break­

down or increase the rate of reaction.

Identification of the Non-clay Mineral Fraction

The non-clay-sized portion of the gouge sample was X-rayed as

a randomly oriented powder specimen on the diffractometer. The proce­

dure followed for the analysis is routine, as described in any good text on mineralogical X-ray diffraction methods (Cullity, 1956; Klug and

Alexander, 1954). Random powder orientation should be maintained to

maximize all possible reflection intensities. However, the tendency of

platy minerals like micas to.orient preferentially when packed into the

sample holder can cause deceptive relative diffraction intensities

(Cullity, 1956, p. 182). These incorrect relative intensities caused by Table 2. Summary of the X-ray Characteristics of Clay Minerals.—After Carroll (1970) and Warshaw and Roy (1961)

(Explanation of symbols: no = no change; *= broad peak.)

Basal Glycolation Minerals Spacing (A) Effect (A) Heating ..Effect (12 hours, °C)

Amorphous clay none nc dehydration and lo ss of weight Serpentine 7 nc nc to 400°, then loss of intensity Kaolinite 7.15 . . nc amorphous at 500° Kaolinite (disordered) 7.15* nc amorphous below 500°

Halloysite 2 H2 Q 7.2* nc dehydrates at 125°-150°, amorphous 575°

Halloysite 4 H2 O 10* nc dehydrates to 2 H2 O at 110° Illite 10* nc (001) increases in intensity to 700° Mica 10 nc (001) more intense to 700° Palygorskite 10.5 . nc stepwise dehydration Mixed-layer >10 if smectite variable, depends on layer types present Sepiolite 12.6 nc stepwise dehydration Smectite 12-15 17-18 (001) becomes 10 A at 300° Vermiculite 14 nc (001) becomes 9.3 A at 400° Chlorite 14 nc (001) increases in intensity to 800° 32 Chlorite 28 (001) becomes 14 A after 500° (swelling) 14 18 27 nonrandom orientation can cause identification problems if misinter­ preted. A scan of 3 to 90 degrees 2 © was used to identify the crystal­ line phases present in the gouge samples. The diffractogram recording the combined diffraction intensities of the minerals present in a sample was often complicated and difficult to interpret. The major problem was determining which reflections were from the same mineral. To facilitate rapid mineralogical identification of many samples, templates were made of the diffraction positions and relative intensities of common minerals. The templates Were constructed on the same scale as the diffractograms. Comparison of the gouge dif­ fractogram with the template readily revealed the presence or absence of the mineral represented on the template. When a positive identifica­ tion was made, the appropriate peaks were marked and the remaining peaks were then identified by a similar procedure. Peaks that were un­ identified after use of the templates were identified by conventional methods, utilizing Berry (1971), Borg and Smith (1969), and Berry and

Thompson (1962). Templates were constructed using data from the above sources. When necessary, the powders were examined in oils, using the polarizing microscope to identify mineral phases. This procedure was used when confusions or ambiguities occurred in the X-ray diffrac­ tion analysis. Another technique employed in the identification of the non-clay-sized minerals was separation of heavy minerals using tetra- bromoethane. This heavy fraction was examined by X-ray analysis and polarizing microscope as needed. In addition, individual mineral grains were separated from the coarse ( >63 ji) fractions and X-rayed in stan­ dard Debye Scherrer powder cameras (Straumanis mounting). LITHOLOGY OF THE MIAMI FAULT C ATACLAS TIC S

The cataclastic zone associated with the Miami fault is com­ prised of approximately parallel subzones of brecciation and grinding„

The cataclastics in these zones are a mixture of breccia and gouge, fol­ lowing the definitions of Higgins (1971) „ Wall-rock relationships are difficult to define from the core material; therefore, company core logs were used to supplement identifications „ The gross lithology of each depth interval collected along the fault is discussed separately. The depth below the surface of cataclastics from each drill hole was rounded off to the nearest hundred foot in the interval and the entire sample in­ terval called a level along the fault (Fig. 2). All sample numbers indi­ cate the depth below surface in feet.

The 300-foot level consists of samples that are virtually indis­ tinguishable from one another by binocular examination (SOX). Relative mineral distributions and abundances between samples vary, but gener­ ally, the number of quartz breccia fragments decreases below sample

256, and the number of schist fragments increases below sample 256.

However, both lithologies are present in all samples at this level.

Quartz fragments are subangular and schist fragments are subrounded.

Very fine grained gouge, composed largely of sericite and quartz, coats some of the larger breccia fragments and cements small quartz grains together. This cementation phenomenon, however, is more common in samples from other levels . Breccia fragments of wall rock are more abundant than gouge at this level.

. 29 30 Samples from the 3900-foot level are uniform in appearance except for sample 3898, which is a. white phaneritic igneous rock that is highly sericitized and contains fresh quartz and biotite phenocrysts „ This rock, which is not present in any other sample, is very distinctive and is probably Schultze Granite. The typical lithology of this level is

soft, gray fragments of schist that are striated and smoothed on all sur­ faces (slickensides). Quartz fragments are subrounded and some are

striated. None of these fragments show the same degree of alteration as sample 3989. Gray foliated gouge surrounds small quartz grains and lithic fragments. The proportion of schist fragments increases with depth from sample 3898, and the proportion of loose quartz decreases with depth from sample 3898. Some fragments contain fresh disseminated pyrite.

The 4500-foot level is as distinctive as the previous two but is lithologically uniform. Massive milky quartz containing inclusions of specular hematite is common. Hematite grains are altered around the edges to red iron oxides. Hematite also occurs as veinlets in quartz. Sericite coats many of the fragments, giving them a distinctive sheen.

There is much less gouge at this level; the rock is mostly unlithified breccia. No schist is present as identifiable fragments; however, the sericite is probably derived from schist.

The 5000-foot level is as uniform in lithology as are the other levels. Massive quartz fragments are subangular to subrounded.

Masses of foliated, fine-grained, red gouge have slickensides on some surfaces. There is welding of small quartz grains together and cementa­ tion of small grains by foliated red gouge. Sericite coats the surfaces of many fragments. However, the cataclastics are granular rather than lithified into true gouge with angular quartz, subrounded quartz, and lithic fragments mixed together. Quartz fragments are less abundant away from the walls (samples 5004 and 5022). There are loose biotite phenocrysts in the gouge (sample 5014). LITHOLOGY OF THE NO. 5 FAULT CATACLASTICS

The cataclastic rock produced by the No. 5 fault is a single zone approximately 5 feet wide. The wall-rock relationships are better defined than along the Miami fault, with the hanging wall altered dacite and the footwall Pinal Schist. Gouge material is more abundant than is breccia. The breccia fragments that are present are bound together by the abundant sericite matrix. Dacite fragments occur in the cataclasties immediately adjacent to the hanging wall and are present up to 1.5 feet away from the footwall (samples 9 to 7) Samples are numbered consecu­ tively at half-foot intervals away from the footwall toward the hanging w all. The abundance of sericite and muscovite in the schist favors development of foliated gouge near the footwall that is similar to that formed along the Miami fault at depth. Sample 2 shows the best develop­ ment of this foliation. Fragments of sheared granite (?) (containing fresh biotite) are incorporated into this gouge and are intermediate between unaffected rock and true foliated gouge. Also, sample 2 contains abun­ dant euhedral gypsum crystals (up to 5 mm) and some chalcocite coating gouge fragments. Sample 3, which marks the end of foliated gouge, is composed of subangular quartz fragments and sericite schist fragments loosely bound together by sericitic gouge. No foliation is evident, and there are numerous voids in the gouge. This type of cataclastic rock is developed in samples 3 to 6.

32 33 Dacite fragments become more abundant as the hanging wall is approached, and schist becomes less abundant away from the footwall„

The gouge immediately adjacent to the hanging wall is nonfoliated, ground dacite reduced to gouge size (samples 9 to 7). The color changes of the gouge across the fault directly cor­ respond to the lithologic changes „ Samples 9 to 7 are pale lavendar (dacite color), samples 6 to 3 are red brown to light tan, sample 2 is laminated red and white, and sample 1 is gray (schist color). The reds and tans of the gouge are derived from oxides and hydroxides of iron. I

MINERALOGY OF MIAMI FAULT CATACLASTICS

Clay Minerals

Table 3 lists basal spacings from oriented films prepared from

the clay-size fraction of each sample. The clay mineral species were

determined using the characteristic changes during treatment for each species as summarized in Table 2. The overall similarity among the d-

spacings from different samples in Table 3 reflects an overal similarity of clay mineralogy of the Miami fault cataclastics. The only significant difference among these samples is the absence of spacings larger than

10 A in material, from the 4500-foot level. The interpretation of these data is rather simple. The sharp, well-formed 10 A reflection that persists through all four of the treatments is characteristic of a mica. The mica species is revealed by random diffractograms to be 2M muscovite and in some samples the fine-grained variety sericite. These two species are con­

sidered to be identical and are classified as muscovite.

Another d-spacing that is constant through the first three of the treatments is 7.1 A. The disappearance of the 7.1 A d-spacing after heating at 550°C is indicative of kaolin-type minerals . Examination of randomly oriented diffractograms is usually inconclusive, because the reflections from this 7 .1 A mineral are obscured by reflections from the other minerals present. However, a few samples yielded three or four resolved reflections, other than the 7.1 A basal one, that are charac­ teristic of kaolinite. The (060) reflection is not sufficiently re solved or

34 35

Table 3. D-spacings in Angstroms from Oriented Slides (Clay-size Fraction) from Miami Fault Cataclastics (Explanation of symbols: ? = questionable diffraction position; * = poorly developed peak.)

Level Sample3 Untreated Glycolated 350°C 550°C

264 ? * 10 7.1 ? * 10 7.1 10 7.1 10 2 72 13* 10 7.1 17 10 7.2 10 7.1 10 2 80 ? * 10 7.1 17 10 7.2 10 7.1 10 300' 288 12 10 7.1 17 10 7.1 10 7.1 10 294 ? * 10 7.1 17 10 7.1 10 7.1 10 300 10 7.1 ? * 10 7.2 10 7.1 10

3904 ? * 10 7.1 17 10 7.2 10 7.1 10 3908 ? * 10 7.1 17 10 7.1 10 7.1 10 3910 ? * 10 7.2 17 10 7.1 10 7.1 10 3912 ? * 10 7.1 17 10 7.1 10 7.1 10 3900' 3914 ? * 10 7.1 17 10 7.1 10 7.1 10 3916 ? * 10 7.1 17 10 7.1 10 7.1 10 3923 ? * 10 7.1 17 10 7.1 10 7.1 10 3926 ?* 10 7.1 17 10 7.2 10 7.1 10

4471 10 7.1 ? * 10 7.2 10 7.2 10 4480 10 7.1 10 7.1 10 7.1 10 4501 10 7.1 10 7.1 10 7.1 10 4500' 4504 10 7.1 10 7.1 10 7.1 10 4507 10 7.1 10 7.1 10 7.1 10 4510 10 7.1 10 7.2 10 7.1 10

5010 14 10 7.1 17 10 8.7 7.1 10 7.1 9 .7 5014 14 10 7.1 17 10 8.5 7.1 10 7.1 9 .7 5014 b 15 10 7.1 17 8.5 7.1 10 7.2 9 .8 5000' 5016 12 10 7.1 17 10 7.1 10 7.1 10 5017 14 10 7.1 16 10 ? * 7.1 10 7.1 9 .8 5022 ? * 10 7.1 10 7.1 10 7.1 10

a. All sample numbers refer to sample depth below the surface. b. Hand-separated smectite fragment. 36 intense to be observed. To eliminate possible confusion of kaolinite with iron-rich chlorites, a few selected samples were treated with acid as described.previously and X-rayed. The persistence of the 7.1 A re­ flection after this treatment confirmed the presence of kaolinite rather than chlorite in these samples. The limited Sampling for the above test was justified by the great uniformity observed of the basal spacing from the Miami fault samples. The sharpness of the 7.1 A reflection after all treatments indicates a well-crystallized and well-ordered kaolinite.

The reflections remaining to be explained are larger than 10 A and the most characteristic of these is the 17 A spacing after treatment with ethylene glycol. It is indicative of variable-spaced clay minerals.

The decrease in this 17 A basal spacing to approximately 10 A after heat­ ing at 350°C and to 9.7 A after heating at 550°C is diagnostic of smec­ tites. The variability of. the basal reflection from untreated clay films is a function of the degree of hydration of the individual smectite. The

5000-foot level samples contain a clay mineral with a d-spacing at ap­ proximately 8.5 A after glyoolation. These reflections are interpreted as second-order peaks from the smectite basal spacing, which appear from samples of this level and not from those of the other levels because of differences in smectite concentration. Samples from the 5000-foot level, contain more smectite, consequently, the second-order peak is suffi­ ciently intense to be observed. The appearance of a 9 .7 A reflection after heating at 550°C is also the result of increased concentration of smectite relative to other minerals. This peak appears as a shoulder on the 10 A mica reflection in other samples. Samples from the 5000-foot level yield 9.8 A reflections with 10 A shoulders. 37 The identity of the smectite species cannot be definitely ascer­ tained from X-ray data because of peak interference from other phases in any given sample. However, some useful information was obtained from hand-picked fragments from sample 5014, which are mainly smectite and kaolinite in composition. A random diffractogram of these fragments con­ tained reflections at 15, 7.1, 4.46, 3.56* and 2.56.A. Acid treatment of this material showed that the 7.1 A phase persisted, and the 15 A phase responded to ethylene glycol like a smectite. Consequently, kaolinite is present which explains peaks of approximately equal intensity at 7.1 and 3.56 A. The remaining major reflections are hk reflections of the smectite material (4.46 and 2.56 A). Comparison of these reflections with hk reflections of known species (MacEwan, 1961) shows that the smectite could be beidellite (4.46, 2.56 A) or volkshonskoite (4.46,

2\58 A). Lithium-saturation followed by heating and X-ray analysis to determine composition within the. beidellite-montmorillonite group proved Z ambiguous (Greene-Kelly, 1953). Unfortunately, lack of good reflec­ tions renders these data inconclusive by themselves. The problem will be examined again under spectrographic analysis of the samples. There is no X-ray evidence from the oriented clay films for the presence of mixed-layer clay minerals in any of these samples.

The distribution of the clay minerals is shown in Table 4, and the relative abundances are shown in Figure 5. Erroneous clay mineral concentrations caused by the slide preparation technique are assumed to be constant in all slides; consequently, comparison of relative con­ centrations between levels is valid. Precision of the sample preparation technique has been confirmed by Gibbs (1965). Muscovite and kaolinite 38 Table 4. Mineral Distribution along the Miami Fault

(Explanation of symbols; xx = present in most samples; x = present in only a few samples; ? = feldspar of uncertain type.)

Level along the Fault (in feet)

Mineral 300 3900 4500 5000

Quartz XXXXXX XX

Orthoclase XX ? X

Plagioclase ? XX

Muscovite XXXXXXXX

Kaolinite XX XXXXXX

Smectite XX XX XX

Biotite X

Iron oxides XX XXXXXX

Specular hematite X XX X

Pyrite X Drill C-lll 48 39 37. 3500' Hole .-.Qtg Elevation m

3001 Level \ M \ \ m = muscovite \ k = kaolinite \ s = smectite \ \ \ \ major \ \ \ m k I minor \ N gp — not detected \ X/ ^ m 3900' •*. :. v Qtg Gila Conglomerate Level Ps ^ Qtg. v y- V V V 9P granite porphyry •N. • * 4500' m k prcr Level Tsq Schultze Granite v V gp ^ Ps Pinal Schist Scale 1" = 1000' . 5000' Level

Figure 5. Relative Concentrations of Clay Minerals along the Miami Fault 40

are.present in all samples „ Smectite is present in all samples but those of the 4500-foot level.

Non-clay Minerals

Table 4 is a summary of the minerals determined by X-ray anal­ ysis and optical observation. Quartz and muscovite, which are present

in all samples, yield reflections that overlie many of the characteristic feldspar positions. Consequently, identification of feldspars present in

small concentrations presents some problems because of peak ambiguity.

The feldspars are determined as to species whenever possible, but usually identification beyond "orthoclase" or "plagioclase" was not feasible. Most plagioclase reflections are indicative of a composition similar to oligoclase which is the major plagioclase observed in the wall rocks. Iron oxides are common in all samples as grain coatin gs. Sul­ fides and other accessory minerals are present in limited amounts and distributed, as shown in Table 4 . Minimum percent concentrations of common minerals detectable by X-ray analysis have been summarized by

Carroll (1970). These values are approximate limits of sensitivity of these analyses. MINERALOGY OF NO. 5 FAULT CATACLASTICS

Clay Minerals The mineralogy of the No. 5 fault cataclastics is more complex than that of the Miami fault. However, relationships between wall rock and mineral distribution in the gouge is more obvious in the No. 5 cata­ clastics. The d-spacings from oriented clay films are tabulated in

Table 5. A 10 A reflection is present in all samples and has the sharp­ ness and persistence characteristic of a mica. Examination of random powder diffractograms reveals the presence of two micas in the gouge. Biotite and muscovite (2M) are distributed across the fault zone, as shown in Figure 6. Table 6 records the d-spacings from biotite pheno- crysts removed from the dacite wall rock and the d-spacings of a typical biotite and trioctahedral illite. The sample biotite has spacings of both fresh biotite and illite, which suggests the incomplete conversion of biotite to illite. Illite reflections are observed in samples 8 and 9, but not in the biotite isolated from sample 2, indicating a different source for the latter. Sample 7 contains material that yields unique persistent spacings at 10.4, 6. 3, and 5.4 A. Random diffractograms confirm the mineral to be playgorskite. The peak positions change upon heating con­ sistent with the data of Hayashi, Otsuka, and Imai (1969) for palygor- skite.

A kaolin-type mineral is indicated by the 7.1 A reflections obtained from samples 1 through 6. Random powder diffractograms

41 ; ' Table 5. D-spacings In Angstroms from Oriented Slides (Clay-size Fraction) from No. 5 Fault Cataclastics

Sample Untreated Glycolated 350°C 550°C

1 24 20 14 10.4 10 26 23 17 10 8.6 7.1 ? 10 9.7 7.1 10

2 24 15 10 7.1 7 10 8.5 7.6 7.1 14.5 10.3 10 7.1 9.8

3 24 12 10.5 10 7.1 7 10 8.4 7.1 14.1 10 7.1 14 10 9.8

4 ? 12 10 7.1 4.7 22 17 10 14 10 7.1 14 10

5 14 12 10 7.6 7.1 7 14 10 7.6 7.1 14 10 7.1 14 10 9.7

6 12 10 7.1 7 10 7.1 10 7.1 10

7 13 10.4 6.3 5.4 7 10.4 8.3 6.5 10.3 9.1 6.3 10.5 6.2

8 13 10 7 10 8.4 14 10" 9.7 10 9.7

9 15 10 7 ? 8.5 ? 9.8 9.2

A to uin cos h N. Fault 5 No. the across bution

Footwall - Pinal Schist iue . igamtc ersnain f h Mnrl Distri­ Mineral the of Representation Diagrammatic 6. Figure | ixed-layer M | |_ | | —I I— h— ------Gypsum 4 6 8 9 8 7 6 5 4 3 2 1 | | ------

Chlorite Boie iotite-illite B Biotite 1 Chalcocite 1 uscovite M aolinite K ------Fut og zone ■Fault gouge ayosie | Palygorskite ape Number Sample - Quartz - Sanidine ------Plagioclase mectite Sm ------1 1 ------1 ------

1 1 ---- -■H h- H

Hanging wall - dacite Table 6. Comparison of D-spacings of a Biotite Phenocryst from the Dacite Hanging Wall of the No. 5 Fault with Published Data for Biotite and Illite

Biotite Phenocryst Biotite3 Trioctahedral Illite 3

d(A) I d(A) I d(A) I

9.9 10 10.1 vs 10.0 10

4 .5 band 4 .5 8 w 4 .4 7 9

3.6 5 3.68 2

3.3 8 3.36 vs 3.32 9

3.1 5 3.15 vw 3.16 1

2.90 5 2.91 vw 2.86 1

2.69 6 2.65 s

2.61 6 2.60 6 2.52 3 2.53 w 2.50 1

2.42 . 6 2.45 s 2.41 4

2.16 6 2.18 s 2.16 2

1.99 6 2.00 s 1.98 1 1.84 2

1.69 3 1.69 3

1.66 6 1.67 s

1.53 6 1.53 s 1.53 6

a . Data from Grim (1968). 46 relatively small intensity of these peaks indicates that the mixed-layer minerals are a minor phase in the gouge but also hinders adequate iden­ tification of the type of interstratifications present in the mineral. A

simple series of integral reflections from basal planes, characteristic of

regularly interstratified mixed-layer minerals, does not occur (MacEwan,

1961); therefore, the mixed-layer material present in the gouge is ran­ domly interstratified. The primary basal spacing of regularly alternating mixed-layer minerals is the sum of the thicknesses of each component of the repeat interval. For example, a chlorite (14 A)-smectite (12-15 A) mixed-layer should have a 26 to 29 A basal spacing. Various combina­

tions are possible to give basal spacings greater than 30 A. Unfortunate­ ly, the techniques used in this study could not resolve reflections in this low 2 © region from the scatter of the X-ray beam, so that informa­ tion corresponding to spacing greater than 30 A is lacking. The combi­ nation of sketchy diffraction information, poor peak resolution, and complexity of analytical technique needed for interpreting random inter-

stratifications makes the exact description of the nature of the mixed- layer mineral impossible. An important control on the development of the mixed-layer mineral must be the presence of chlorite because mixed- layer reflections occur only when chlorite is present. However, in

sample 5., chlorite occurs without mixed-layer minerals. The mixed- layer mineral appears to be better developed near the Pinal Schist con­ tact and is found in the wall rock.

Non-clay Minerals

Figure 6 shows the distribution of non-clay minerals across the fault, as determined by X-ray analysis and optical observation. Quartz 47

is present in all samples„ Muscovite is found only in samples 1 through

6 and is sericitic (fine-grained). Feldspars are identifiable in only

samples near the walls with sanidine present in samples 9 through 7, and plagioclase present in samples 1, 8, and 9. The absence of feld­ spars in the middle of the fault zone could be the result of lack of sen­ sitivity of the X-ray analysis to small concentrations (less than 5 percent according to Carroll, 1970, Table 12) or could have genetic significance „ Gypsum occurs in samples 1 through 4 in varying amounts but is mainly concentrated in sample 2 in which microscopic euhedral selenite crystals occur. Chalcocite coats some gouge material in sample 2. Iron oxides (hematite, goethite) coat fragments in all samples. SPEC TROGRAPHIC EXAMINATION OF CLAY-SIZE MATERIAL

Analytical Technique

To describe better the clay mineralogy of the fault gouge, the

clay-size fraction was examined with an Analytical Research Labora­

tories spectrographic analyzer (Model .26000-1). The theories of opera­

tion and analytical technique are well described in Harvey (1964, 1950)

and the manual supplied with the instrument. Ten mg of clay-size pow­ der were mixed with 10 mg of spectrographic carbon and then packed into

a carbon-cup electrode. The sample mixture was burned between carbon

electrodes in an electric arc, and the resultant emission spectra were recorded on photographic film. Instrumental settings were a logarithmic

intensity scale and 25 percent filter transmittance. The mixture was

burned until combusion was complete (2 to 3 minutes), and the film was

developed using conventional techniques.

The elements present in a sample were identified by comparison of the emission spectra recorded on film with a master film that recorded thd major spectral lines of all common elements. The comparison was

facilitated by the use of a film viewer that allowed proper alignment of

the sample film with the master film. Supplemental data were obtained

from tables of the relative intensities and spectral interference for each

spectral line of an element. Confirmation of the presence of an element

in a sample required the appearance of a number of the characteristic

spectral lines for that element on the film. The relative concentration of

48 49

an element in a given sample was estimated by comparing the degree of darkening on the film to the published line sensitivity „ The height of the line on the film was also measured and used as a guide to concentration. The log setting of the analyzer causes the spectral line heights recorded on film to be directly proportional to elemental concentration. However,

quantitative results are difficult to obtain using these spectrographic

techniques, and the results tabulated in Table 7 indicate the elements

present in a sample detectable by this technique and their relative con­

centrations (proportional to magnitude of log-scale value) in a gross

sense only. Two clay standards were analyzed to compare spectrographic readings from material of known chemical composition with values deter­ mined for the samples. These standards also provide a feeling for the

sensitivity of the analytical procedure for various elements. The pub­

lished composition (Kerr, 1950) and the composition of each standard

determined in this study are compared in Table 8.

Results

The following discussion is qualitative because of the approxi^-

mate nature of the elemental an alyses. Aluminum and silicon are major .

constituents of all samples.

Iron

The iron content of all samples is very similar. Most of this

iron is in mineral structures because of the relatively small amount of

iron oxides in the clay-size fraction. The determined iron content of the clay standard API #31 corresponds approximately to the values of the

samples. Published analyses of the standard show 2.5 percent iron 50 Table 7. Elemental Analyses of Gouge Samples and Standards (Clay-size Fraction unless Otherwise Noted) (Numerical value is the log-scale reading of the same spectral line for each respective element. M = major constituents.)

Element

Sample Si A1 Fe Mg Mn Ca Na Cu Ti Sr P Mo

Miami Fault

264 M M 80 75 45 40 — — 70 80 45 — — 10 294 MM 80 80 60 40 10 80 80 — — — ------3910 M M 75 60 10 40 — — 20 60 40 — — ------3923 MM 75 75 35 15 20 50 80 — — — — — 4504 M M 75 40 30 40 — — 65 45 70 — — — — 4507 M M 70 50 40 40 — — 70 60 75 ------5010 MM 80 80 35 40 30 80 65 50 ------— — 5014 MM 80 75 35 45 20 85 70 5014 (smectite) M M 90 75 30 55 15 85 70

No. 5 Fault

1 MM 70 65 50 60 50 90 50 5 35 *■ 1 (whole) MM 75 70 50 40 20 90 70 35 ------2 MM ? 60 40 95 20 30 20 75 ------— ” 3 MM 75 60 60 60 30 90 50 60 ------4 MM 75 65 45 50 70 80 45 50 — — — — 5 MM 80 60 45 65 30 80 50 55 15 5 6 MM 80 60 45 55 40 80 30 45 12 — — 7 MM 70 60 25 40 ------50 40 30 ------— — 8 MM 70 65 35 40 ------45 55 45 — — ------9 (whole) MM 75 80 65 45 40 50 85 60 ------10 9 MM 70 70 55 50 50 60 50 — 30 —— 9 (biotite) MM 75 75 60 25 ------30 95

Standards

API #9 MM 40 20 30 45 35 50 50 — ■ 5 — — API #31 MM 20 80 35 40 20 40 80 60 — — — — M uscovite MM 70 45 20 5 50

Analyses by K. W. Bladh 51 Table 8. Comparison of Elemental Analyses of API Reference Clay Minerals (Values are in weight percent.)

API #9 (kaolinite) API #31 (mixed-layer)

Element This Study Kerr (1950) This Study Kerr (1950)

A1 major major major major

Ba ? 0.001 ? 0.01

Ca present .004 present 1.0

Cr ? .001 ? .002

Co ? .001 ? .001

Cu trace .001 trace .01

Fe present .4 common 2.5 Pb Y .02 Y .04

Li Y .09 ? .09 Mg present .03 present .7

Mn trace .003 trace .03

Mo Y .003 ? .003

Ni ? .001 ? .002

K ? trace ? present

Rb ? .01 ? .01

Si major major major major Na trace trace present .3

Sr trace .03 present .1 Ti present .2 present .3

Zn Y .03 Y .03 52

(Table 8). This will be used as a rough estimate of the total iron in the sample clay-size fractions „

Magnesium Magnesium is present in all samples at approximately the same concentration. Comparison with the clay standards gives an estimated

concentration near 0.5 percent. Samples from the 4500-foot level are

slightly lower in magnesium content and are similar to the API #9, which

has approximately 0.03 percent magnesium.

Manganese The manganese concentrations are slightly variable but in the

trace element range for all samples . Comparison to the published stan­

dards gives a concentration range of 0.003 to 0.03 percent.

Calcium

The calcium content of the samples is less certain because of

an apparent lack of analytical sensitivity. The published values for stan­

dards range from 0.004 to 1.0 percent calcium, but the spectrograph used

in this study for analysis could not differentiate between these standards.

However, sample 2 from the Miami fault shows calcium enrichment as is

expected because of its high gypsum content. The value for pure musco­ vite is much lower than the values from the samples „

Sodium and Phosphorus

Sodium and phosphorus contamination can be expected in

samples separated by sedimentation aided by Calgon. All samples from the No. 5 fault, except 7, 8, and the biotite from 9, and the clay 53 standards were treated with Calgon in water. The other samples, al- , ■ t though not treated with Calgon, contain sodium but no phosphorus.

Sodium can be accepted by smectites as an exchange cation, and this is probably the cause of the slightly higher readings of samples treated with Calgon. Sodium is present in samples from all localities except from the 4500-foot level of the Miami fault. Incidentally, the 4500-foot level contains no detectable smectites. Sodium has limited diagnostic use because of the uncertain origin of the sodium in some of the samples.

Copper

Copper is a trace element in all samples with a maximum e s ti- . mated concentration of less than 0.1 percent. The copper concentration is smaller on the 3900-foot level of the Miami fault than other levels and increases slightly on any level toward the footwall. Across the No. 5 fault a definite decrease in copper content occurs in samples near the dacite hanging wall. Sample 2 is also low because of the high concen­ tration of. gypsum to the exclusion of clay minerals.

Other Elements

Other elements determined by the spectrographic analyses are . shown in Table 7, but their significance is minor and they are shown only for com pleteness.

Applicability to Mineral Identification Determination of the identity of the smectite species in the cla y -size fraction was a primary objective of the spectrographic anal­ yses. The three clay mineral phases present in the Miami fault gouge determined by X-ray analysis are kaolinite, muscovite, and smectite.

X-ray analysis revealed only kaolinite and muscovite in samples from

the 4500-foot level on the Miami fault; samples from the 5000-foot level were rich in smectite relative to the other two clay mineral components.

Comparison of the standard kaolinite and muscovite with the analyses of

samples from the 4500-foot level reveals a similar magnesium content.

Therefore, the magnesium content of the 4500-foot level samples can be explained completely by the presence of kaolinite and muscovite.

Samples from the 5000-foot level are richer in magnesium than those of the 4500-foot level. This magnesium enrichment on the 5000-foot level

is attributed entirely to the presence of smectite because the composition

of kaolinite and muscovite should be relatively constant with respect to magnesium. In addition, a hand-picked fragment from the 5000-foot level

(sample 5014) which gave only smectite and kaolinite X-ray reflections had an elemental distribution almost identical to the total gouge anal­ yses of that level. Also, slight iron enrichment is indicated in the smec­ tite fragment. The iron content is much lower, however, than that required for the iron-rich smectite , nontronite, which has an iron con­ tent of 20-30 weight percent (Grim, 1968, p. 529). The iron contents are more characteristic of low (less than 7 percent) iron, high-magnesium

smectites of the montmorillonite-beidellite series. The absence of cations, such as Cr, Ni, and Zn, excludes rarer smectites, such as volkshonskoite. Because of the similarity of mineralogy, elemental distributions, and lithology along the Miami fault, all the smectites along the section of fault sampled are assumed to be of the montmorillonite-beidellite type. 55 The No. 5 fault offers the same problems related to there being too many phases present in a sample to identify clearly the smectite species. Smectite is present in all samples with other clay minerals; however, in sample 9 only biotite phenocrysts occur with the smectite.

The elemental analysis of biotite phenocrysts from sample 9 and the analysis of the clay-size fraction of sample 9 are virtually identical.

Consequently, there are insufficient data to determine the smectite species. However, the similarity of elemental and X-ray analyses for material from the Miami and the No. 5 faults does indicate the beidellite- montmorillonite-type smectites may well be present along the No. 5 fault. In addition, the similarity of the iron, magnesium, manganese, and calcium contents in samples with and without chlorite suggests that perhaps chlorites formed at the expense of other minerals that carry these elements. Finally, the determinative accuracy of the spectro- graphic technique limits what can be confidently said about the miner­ alogy of the clay minerals of obscure identity . DISCUSSION

No. 5 Fault The investigation of the relationships between fault gouge mineralogy and the composition of the wall rocks was a primary concern of this study. The No. 5 fault cataclasties clearly show these relation­

ships. Examination of Figure 6 reveals a gradual decrease of wall-rock type mineralogy away from the source wall. The mineral distribution has definable limits away from the source wall that stop short of the opposite wall, apparently because there was no large-scale mechanical mixing of wall rock components to yield a homogeneous gouge. The mineralogical distribution and composition of the No. 5 fault cataclastics indicate that the gross mineralogy of the gouge is indeed controlled by the wall rock and is simply a fine-grained disaggregated extension of the walls.

The effect of faulting on the minerals in the gouge can be seen especially well by the behavior of the feldspars. The feldspars persist within the gouge for a distance away from the wall that is directly pro­ portional to their relative resistance to chemical weathering. Potassium feldspar, being more resistant than plagioclase to chemical weathering, is found farther from the source wall in the gouge. The maximum d is­ tance of the other minerals in the gouge away from the source can also be explained as being inversely proportional to their susceptibility to chemical weathering.

The occurrence of a sheared granite fragment in the gouge

(sample 2) opposite the schist wall indicates that some material in the

56 57 fault zone has been displaced along the fault zone away from its source.

The granite source in the wall is unknown, but the fragment must have moved along the fault because there is no other possible source. The sheared nature of the fragment indicates that it is being converted into gouge „ If more granite fragments had been incorporated into the gouge, a zone of anomalous mineralogic composition would have been created there. The effects of solutions upon the fault gouge are reflected in the gouge mineralogy . The evidences of the passage of solutions in­ clude the presence of gypsum crystals, chalcocite coatings, and the absence of feldspar in parts of the gouge that otherwise contain all other wall-rock minerals. Compared to the wall the gouge environment was oxidizing, as evidenced by the abundant iron oxides present. The waters were probably acidic because they were affected by the oxida­ tion of pyrite. The formation of iron precipitates and the taking into solution of sulfate ions during the leaching of upper portions of por­ phyry copper deposits are well known. The soluble sulfates produced by the oxidation of sulfide from pyrite probably combined with calcium in the ground waters to form the gypsum crystals in the gouge. The lack of either abundant secondary copper minerals or cementation indicates either a nonfavorable environment for deposition, insufficient water flow, or "too rapid a flow. Assuming that fragmental feldspar originally ex­ tended into the gouge as far as the recognizable wall-rock mineral frag­ ments, the absence of feldspars in the central portions of the gouge suggests leaching by acid ground waters. The greatly increased surface area of the gouge feldspar grains due to mechanical comminution would 58 significantly increase their susceptibility to chemical reaction and would require much less water flow to remove the feldspar by leaching. The waters that did flow along the fault traveled mainly along the central

and footwall portions of the gouge as indicated by the relative abun­

dance here of secondary minerals and the absence here of feldspar re­ moved by leaching. Along the dacite hanging wall, biotite phenocrysts are unaltered even though biotite is very susceptible to weathering in oxidizing environments. This biotite zone coincides with the non­

leached feldspar zone, which suggests that water did not flow through

this part of the gouge. The collection sites were above the present water table. The possibility exists that the gouge zone was unfavorable for the precipi­ tation of the minerals previously mentioned. If this were the case, the ground water would have carried the soluble leaching products down­ ward until a change of conditions was encountered and would have re­ sulted in leaching at the sample site more intense than is observed. Under these conditions/ the smectites and chlorites should not have been preserved in the gouge . Furthermore, there was no X-ray evidence of gibbsite or other aluminian hydroxides which might have accumulated

as insoluble residues during leaching of aluminian silicates.

The occurrence of palygorskite (sample 7) near the dacite foot­ wall without an obvious source from either immediate side can be ex­ plained by one of three possibilities. The palygorskite could be a remnant of an included wall fragment of dissimilar mineralogy which was carried along the fault near the wall, although there is no other evi­ dence of such a fragment in the gouge. Alternatively, it could be a 59 weathering product formed in place from a mineral in the dacite. The

precipitation of palygorskite from low-temperature solutions is the third

possibility. The isolated occurrence and absence of conventional pre­ cursors (inosilicates) in the gouge or wall rock support the first or third

possibilities. However, the similarity of minerals associated with the

palygorskite to minerals in the contiguous gouge favors either the second

or third alternative. Palygorskite occurrences have been described from

alkaline arid soils and playa lake sediments (Loughnan, 1969) and hydro- thermal associations (Carroll, 1970) . Palygorskite is stable only in an

alkaline environment (Carroll, 1970). The disappearance of the biotite-

illite association in the sample that contains palygorskite suggests for­

mation by the degradation of the mica structure. Brindley (1961b)

describes the structure of palygorskite as composed of micalike units,

but formation from a mica precursor is not mentioned in the literature.

The total evidence of palygorskite genesis is inconclusive.

Miami Fault The genetic relationships between wall rock and contiguous

fault gouge are less obvious in catac la sties from the Miami fault than

those from the No. 5 fault because of the similarity of mineral species

present on and across all'levels .of the Miami fault. Lateral variations

in the amount of wall-rock fragments in the gouge away from the source wall occur on all levels, even though the mineralogy of the gouge is

laterally consistent. Examination of Figure 2 reveals that along the fault there are three lithologies: granite porphyry, schist, and a conglomer­ ate (probably Gila) which is a combination of the previous two. The 60 mineralogy of gouge derived, without alteration, from the mineralogically similar walls should be approximately uniform„ However, differences between levels would occur in the relative concentrations of minerals

\ because of concentration differences in the wall-rock sources „

The Gila Conglomerate has accumulated on the downdropped block and now constitutes the hanging wall down to 5,000 feet (limit of information). Accordingly, it seems unlikely that much of the gouge ma­ terial was derived from grinding of the conglomerate „ However, because the conglomerate is composed of other rock types exposed along the fault, gouge derived from it would be indistinguishable from gouge de­ rived from other wall-rock type s „

The following comparison of mineral concentration variations between levels, rather than across any one level, was necessitated by the lateral mineralogic homogeneity of each level. Figure 5 shows that the major clay-size mineral on the 300-foot level is muscovite, with minor smectite and kaolinite present. This would be expected if the cataclasties were derived, without alteration, from the quartz-sericite schist wall. The 5000-foot level has a conglomerate and a granite wall, and on this level smectite is relatively more abundant than kaolinite or muscovite. The increase in smectite concentration is caused by a great­ er abundance in the granite (smectite being derived from feldspar). The

3900-foot and the 4500-foot levels have essentially identical wall types and have very similar mineral concentrations except for the absence of smectite on the 4500-foot level. This absence of smectite may be ex­ plained by the observed large proportion of smectite-poor quartz breccia on the 4500-foot level. 61 From the preceding discussion, it is apparent that gouge con­ tiguous with a schist wall is relatively enriched in muscovite. This is reasonable if adjacent walls supply the gouge because of the relative weakness to deformation of micas as .compared to the quartz and feldspars . of granite. The gouge is relatively enriched in the least mechanically resistant minerals „ Whether the presently adjacent wall is the source of the mica in the gouge cannot be determined because there are no unique markers in the source wall or gouge.

The presence of essentially one suite of minerals (kaolinite, smectite, muscovite, quartz, felspar), in varying relative abundances, in any rock type exposed along the Miami fault contributes to the gross mineralogic homogeneity of the gouge. The initial similarity of wall-rock types was accentuated by pre-faulting hydrothermal solutions. However, examination of the distribution of minerals along the fault (Table 4) re­ veals inhomogeneities in the gouge, especially with respect to the feld­ spars. These mineralogic discontinuities, combined with the lack of smectite on the 4500-foot level and with concentration variations, prove that even with large amounts (thousands of feet) of lateral movement definite unique mineralogic zones are preserved in the gouge. These mineralogic zones give additional support to the contention that the wall rock is the primary source of the fault gouge mineralogy.

The presence of feldspars at all depths in the gouge suggests that acid mine waters have not traveled along the fault in any appreci­ able quantities. An alternative explanation of feldspar stability is that they were preserved because of coarse grain size which provided kinetic barriers to reaction with the waters. The exact position of the water 62 table in this area is not known, but no evidence of variability of mineral degradation is noted along the fault segment sampled. Degradation above and below the water table in an acid-rich ground-water system should be significantly different. Oxidizing acid conditions should dominate above the water table and neutral to basic (possibly reducing) conditions should prevail below the water table. The minor degree of specular hematite oxidation (4500-foot level) and the presence of fresh pyrite (3900-foot level) indicate nonoxidizing conditions along the fault. No detectable concentrations of gibbsite, hydrous iron oxides or hydroxides, or other leaching residues were detected at any depth along the fault. In addi­ tion, there is no cementation of the gouge. The overlying conglomerate is a possible source of ground water because it is the major aquifer in the area, but there are no indications of solutions, such as enhanced chemical reactivity or cementation in the gouge near the conglomerate contact. The Miami fault gouge is a relatively nonoxidizing environment whose mineralogy is virtually unaffected by ground waters. CONCLUSIONS

General application of the following conclusions is restricted

by the limited sampling of this study. The faults that were sampled are younger than the hydrothermal alteration and primary mineralization that affected the wall rocks. Therefore, post-faulting alteration of gouge by hydrothermal solutions is not considered. The semi-arid climate of the

sampling sites limits the quantity of ground waters that is available for chemical reactions with gouge minerals. The climatic differences be­ tween areas must be considered when results of this investigation are compared to data of other gouges. The following conclusions about fault gouge, which are sum­ marized here, will be discussed in detail in the following paragraphs. 1 „ The wall rock is the source of all minerals in the gouge other

than precipitates from ground waters.

2. The faulting process creates no minerals in the gouge that were

not originally present in the wall rocks.

3 . The gouge is nonhomogeneous mineralogically and is character­

ized by a nonequilibrated mineral assemblage.

4. Ground waters percolating along the fault both remove and de­

posit material as regulated by the local physiochemical envir­ onments. ‘

5. Shearing, which is most intense near the walls and at depth

along a fault, is an indication of the mechanism of material

transport in the gouge .

63 64 Relationships between wall rock and gouge are genetically sig­ nificant, Gouge is a fine-grained disaggregated extension of the wall rock with the mechanically least resistant wall contributing most of the material in the gouge, Abundance of wall rock constituents in the gouge decreases away from each source wall and terminates short of the Oppo­ site wall, The gouge is not mixed by the faulting process because gouge mineralogy at any point is traceable to one of the source walls rather than to both. No heating effects related to faulting or depth of burial were noted in the gouge along either fault. No minerals charac­ teristic of low-pressure and low-temperature metamorphic zones were observed in the X-ray patterns, although small concentrations or diffrac­ tion interference from accompanying minerals might be the explanation for the absence. However, the stability of minerals other than feldspar

(clay minerals) in the gouges suggests that metamorphic temperatures were never attained. Faulting did not alter any of the wall-rock minerals, aside from reduction in grain size, and did not produce any recognizable new minerals in the gouge.

The heterogeneous miperalogy of the gouge is the result of dis­ equilibrium among the phases present. The fine granulation associated with faulting results in intimate local mixing of the gouge constituents and physical contact among practically all phases. In the presence of adequate quantities of water these phases would tend to equilibrate to a stable assemblage for the given physiochemical environment of the gouge zone. The gouge near the schist wall of the No. 5 fault contains chlorite, smectite, muscovite, kaolinite, quartz, and feldspar. Exami­ nation of equilibrium diagrams for these minerals (Helgeson, Brown, and 65 Leeper, 1969; Garrels and Christ, 1965) reveals that a stable associa­ tion of all of them is virtually impossible „ The author concludes that the

gouge here is a metastable assemblage preserved because of a lack of sufficient water flow. The minerals near the dacite wall and the minerals along the Miami fault are also metastable associations preserved by poor water circulation and nonoxidizing conditions . Ground waters are the primary degradational agent after the comminution associated with faulting and are the source of precipitates

in the gouge. Ground waters entering a fault gouge system carry ions in solution that are deposited as vug fillings and coatings if conditions are favorable. However, the effects of solutions are minimal with respect to weathering of clay minerals and deposition of authigenic minerals. Even though the gouge contains an abundant, fine-grained, highly reactive mixture of clay minerals, the mixture appears to be metastable over long periods of time (millions of years) . The persistence of minerals in the gouge away from the wall source appears to be directly proportional to that mineral's observed chemical weatherability (Goldich, 1938) . The limited amount of ground water and the rapid surface runoff characteris­ tic of the semi-arid climate probably contribute to the apparent lack of reactivity of the clay minerals in the gouge, This apparent lack of reac­ tivity and the metastable assemblages in the gouge system suggest that the system is closed to the intense oxidation associated with the leached capping of porphyry copper deposits. Foliation probably produced by shear is best developed in mica­ ceous gouge, but proximity to the footwall, as in the No. 5 fault, is also required because the equally micaceous central portion of the gouge is 66 nonfoliated. Foliation also increases with depth on a fault (Miami fault) given proper mica concentrations.

The nature of gouge movement along the fault zone is important to the understanding of genetic relationships between a wall and the contiguous gouge. Walls that are compositionally homogeneous along their entire length should yield a uniform gouge. If a wall is composed of different rock types, the gouge may be contiguous to a lithology dif­ ferent than its source „ This phenomenon occurs along the schist wall of the No. 5 fault where granite fragments are incorporated into the gouge.

The presence of these granite fragments in the zone of shearing near the schist wall shows that gouge is derived from the wall but not necessarily the wall adjacent to that gouge. The importance of this process no doubt varies locally and would depend on the amount of lateral movement and the rock types involved. The incorporation of lithologically different fragments derived from other depths along the fault into the gouge is of minor significance along the No. 5 fault. Similar effects are difficult to determine along the Miami fault, but the smectite-free 4500-foot level could be explained by a smectite-free slice carried along the fault zone . REFERENCES

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