University of Nevada

Reno

Metamorphic Geology of a Portion

of the Bagdad Mining District

Yavapai County, Arizona

A thesis submitted in partial fulfillment of the

requirements for the degree of Master of Science

by

Daniel E. Collins III

May 1977 WiNEs U3RARY

m

© 1 9 7 8

DANIEL EDWARD COLLINS

ALL RIGHTS RESERVED The thesis of Daniel E. Collins is approved:

Thesis advisor

University of Nevada

Reno

May .1977 PLEASE NOTE:

This dissertation contains color photographs which will not reproduce well.

UNIVERSITY MICROFILMS INTERNATIONAL. 1

ACKNOWLEDGEMENT

The author is sincerely indebted to the Cyprus Mines

Corporation for its interest and generous financial support without which this thesis would not have been possible. I wish to thank in particular Bob Clayton and

Joe Sierakowsky for their advice and help while in the field. The guidance of Malcolm Hibbard and Don Noble at the University of Nevada was very much appreciated. I also wish to thank Arthur Baker III who first suggested the thesis area and provided many useful suggestions during the writing.

The Nevada Bureau of Mines and Jack Quade of NASA are thanked for access and instruction in the use of the x- ray analysis equipment.

I am deeply grateful to John and Constantine Zanarras and to my wife, Merilyn, for their constant companionship. I ABSTRACT

An estimated 2,150 meters of eugeosynclinal porphyritic andesites, basalts and volcanic sediments belonging to the Bridle formation were metamorphosed during the Mazatzal Revolution (?) to produce greenschist facies minerology, regional folding, and penetrative fabric elements. Prior to regional metamorphism, the Bridle formation was shallowly intruded by concordant masses of porphyritic trondjhemite and a differentiated Dick Rhyolite. The Dick Rhyolite may have vented onto the marine floor. Cooling of the volcanic-intrusive pile and deep water circulation brought mineralized solutions through the Bridle formation. These produced transecting chlorite alterative pipes with lenses of massive pyrite-sphalerite-chalcopyrite deposited along the stratigraphic bottom of the Dick Rhyolite.

High temperature hornfels and intrusive granites occurred toward the end of the metamorphic cycle. Bulk rock chemistry was considerably altered during regional metamorphism or by widespread iron-sodium-magnesium metasomatism prior to regional metamorphism. iv

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS i

ABSTRACT ii

TABLE OF CONTENTS iv

LIST OF FIGURES vi

INTRODUCTION 1 Location and Accessibility 1 Physiography 2 Purpose 2

METHOD OF INVESTIGATION 5

PREVIOUS WORK 8

GEOLOGIC SETTING 11

LITHOLOGY 16 Yavapai Series 16 Mafic Metavolcanic Rocks 16 Cordierite-Anthophyllite Hornfels 19 Intermediate Metavolcanic Rocks 20 Mica Schist 23 Felsic Metavolcanic Rocks 23 Tuffaceous Metasediments 25

Igneous Rocks 26 Metatrondjhemite 26 Contaminated Metatrondjhemite 27 Quartz-Biotite-CordieriteHornfels 28 Dick Rhyolite 29 Quartz-Biotite-AndalusiteHornfels 32 Intrusive Metarhyolite 33 Metagabbro 34 Metadiabase 35 Granodiorite Gneiss 36 Pegmatite 37

Alteration 38 Chlorite Schist 38 Quartz-Sericite Schist 40

Covered Areas 41 V

TABLE OF CONTENTS

Page

STRUCTURAL GEOLOGY 42 Metamorphic Structures 42 Mountain Springs Fault 47 Other Faults 49

METAMORPHIC PETROLOGY 51 Mineralogy 51 Regionally Metamorphosed Rocks 51 Plagioclase 52 Chlorite 53 Muscovite 56 Amphiboles 58

Thermally Metamorphosed Rocks 61 Cordierite 61 Andalusite-Sillimanite 62 Anthophyllite 63 Biotite 64 Magnetite 64 Staurolite 65 Chlorite and Sericite 65 Tourmaline and Apatite 66

Metamorphic Facies 68

CHEMISTRY 71 Basic to IntermediateMetavolcanic Rocks 72 Granitic Rocks 74 Contaminated Metatrondjhemite 83

SUMMARY OF METAMORPHIC HISTORY 86

THE MASSIVE SULPHIDE MINERALIZATION 91

BIBLIOGRAPHY 96

PHOTOGRAPHS AND PHOTOMICROGRAPHS 99 V i

TABLE OF CONTENTS

Page APPENDICES

A - Average Mineralogy of Mapped Rock Units 109 B - Chemical Analyses of Mafic to Intermediate 110 Metavolcanic Rocks C - Chemical Analyses of Metatrondjhemite 111 D - Chemical Analyses of Contaminated Metatrondjhemite 112 E - Chemical Analyses of the Dick Rhyolite 113

LIST OF FIGURES

1 - Location Map 4 2 - Geologic Map of a Portion of the Bagdad Mining Area, Yavapai County, Arizona (in pocket) 3 - Stratigraphic Column 15 4 - Contoured S-Pole Diagram of Metamorphic Foliation 46 5 - Plot of Barth's Sodium and Potassium Cations 78 6 - AFK Diagram of Metamorphosed Bridle Volcanics versus Unmetamorphosed Basalts and Andesites 79 1

INTRODUCTION

The Bagdad area is part of the Eureka mining district and is

located in west-central Arizona approximately seventy

kilometers west of Prescott. The mining community of Bagdad

is the nearest town. The Bagdad porphyry copper deposit and

Precambrian massive sulphide deposits are current producers

of copper, zinc, silver, and molybdenum in the area. Past

production dates back to the 1880's and also included the

recovery of gold, tungsten and lead metals. In the thesis

area are exposed Precambrian metamorphosed volcanic and

sedimentary rock that play host to massive sulphide ore

bodies.

Location and Accessibility

The thesis covers an area of approximately twenty-nine

square kilometers located southwest of the town of Bagdad.

This area includes the Old Dick and Bruce mines operated by

the Cyprus Mines Corporation (see Figure 1). The area is

accessible by dirt roads leading from U. S. Highway 93 or

State Highway 97. The dirt road connecting the Old Dick mine and the town of Bagdad is well maintained and traveled.

Within the thesis area are numerous jeep and dozer trails

that reflect the high level of mineral exploration activity

in the area. Bagdad is at the end of State Highway 97 which

connects Bagdad with the town of Hillside, forty-two

kilometers southeast. 2

Physiography

The Bagdad area is situated in the Mountain Region physiographic province of Ransome (1919). Rainfall averages fourteen inches annually with higher rainfalls occurring from December to March and from July to September. Flash flooding has been known to accompany heavy summer cloudbursts. Elevation ranges from a low of 950 meters in the south to a high of 1,200 meters in the northern part of the thesis area. The topography consists mainly of low relief, frequently steep, rounded hills carved by intermittent streams. Bridle Creek and Mountain Spring

Creek flow south from this area to form broad alluvial aprons.

Vegetation is sparse yet defines a zone of transition between arid desert and high mountainous regions.

Vegetation consists of a variety of cacti, grasses and mesquite at lower elevations, with juniper and ocotillo occurring at higher elevations.

Purpose

The purpose of this thesis was to determine the geologic relationships and metamorphic history of a limited area. It was intended also that a more clear model for ore genesis would be evolved and that the collected data would provide 3 useful information for future mineral exploration in the area.

The study area was chosen in part for the several occurrences of sulphide ore bodies and mineralized areas in an area where the geologic relationships are poorly understood. The study focused on the country rock rather than mineralization. No attempt was made to do a detailed study of the mineralization in the area except to note the occurrences and stratigraphic relationships. FIGURE 1

1

Las Vegas ARIZONA

Locati on M a p 5

METHOD OF INVESTIGATION

A geologic map was made of the area using aerial photograph

enlargements, then transferring the information onto a

compiled base map. Nearly twenty-nine square kilometers

were mapped at a scale of approximately 1:6,200 (1"=516').

The base map was compiled from available published and

unpublished topographic maps that were enlarged to the same

approximate scale as the aerial photographs used in the

mapping. The compiled map was reduced by over half to the

final scale of 1:13,340 (Figure 2, in pocket). Stream

drainage lines were added to the base map to provide

reference in those areas where no topographic information was available.

The area is bounded on the east and southwest by granitic

intrusive batholiths, and on the west by Late Tertiary colluvium deposits. The region north of the Old Dick and

Bruce mines was mapped previously by Anderson, Schultz and

Strobell (1955).

Field work was conducted from September to November, 1971 and in March 1972. Geologic units were differentiated on the basis of type of

parent material or by mineralogy. Bedding and foliation

attitudes were noted and measured wherever possible. The

prefix "meta", used during the geologic mapping, is used in

referring to a type of parent material which has been

metamorphosed. Since all the rocks are metamorphosed, the

"meta" is frequently omitted in the writing in order to

avoid its repetitious use. All the mapped geologic units

show some degree of regional dynamothermal metamorphism or

thermal metamorphism. Thus, a rhyolite is implied to be a

metarhyolite unless otherwise stated.

Stereo-pair aerial photographs were examined to help locate

possible faults that might be exposed in outcrop. These

inferred faults are distinguished from mapped faults on the

geologic map (Figure 2, in pocket).

Rock samples were collected for petrographic analysis and X-

ray spectrographic analysis. A total of 235 thin sections

were examined in order to determine mineral and fabric

relationships. Twenty-five samples were analyzed for bulk

chemical composition using a Siemens SR5-1 X-ray

spectrometer. X-ray samples were crushed first, using steel

gyratory crushers, then pulverized using rotary steel

plates. Volborth (1962) estimates that as much as 1% iron may be added to powder samples prepared in this manner. The 7 powdered samples were pressed into pellets about 2 millimeters thick and mounted in bakelite plastic. The contamination was considered to be relatively Low and constant for all the samples analyzed. No analytical corrections were made to eliminate the possible effects from

iron contamination. Unfortunately, X-ray spectrometry does not distinguish between elements in different valence states, therefore the total iron is reported as Fe202. The sum of the oxides is often less than 100% because no analyses were made for the minor amounts of C02, H20, MnO, and P20^ usually present. The limits and accuracy of the X- ray spectrometry and sample pelletizing methods are discussed by Volborth (1969). 8

PREVIOUS WORK

The early literature discussing the Bagdad area is concerned primarily with the extensive mining activity of the Eureka mining district which includes the Bagdad area. Production in the district dates back to the early 1880's. It was not until 1943 that production of zinc and lead began at the Old

Dick mine. Literature concerning the various mines and mining activities are too numerous to quote here but a good review of the earlier literature is found elsewhere in

Anderson (1950, 1955) and Baker and Clayton (1968).

Understanding of the geologic history of the area began in

1938 when Butler and Wilson correlated the metamorphic schists and amphibolites of the Bagdad area with the Yavapai schist of the Jerome-Prescott area. The name Yavapai schist was introduced by Jagger and Palache (1905) who mapped metamorphosed Precambrian rocks in the Bradshaw Mountains southwest of Prescott. Anderson, et al., (1955) adopted the term Yavapai series after they subdivided the Yavapai schist into three formations: the Bridle formation, Butte Falls tuff and Hillside mica schist. The report by Anderson, et al., (1955) is the single most significant and detailed account of the geology in the Bagdad area to date. In their work, they determined the structure, petrology and stratigraphic relationships of the rocks and related these 9 to mineralized occurrences within the area. The northernmost portion of the thesis mapping overlaps with a portion of Anderson's mapping. Baker and Clayton (1968) mapped in the immediate vicinity of the Old Dick mine and

Copper Queen mine working out the many useful stratigraphic and structural details of underground workings and petrographic descriptions of the country rock.

Isotopic lead dating of galena at the Old Dick mine produced an age of approximately 1,725+200 m. y. (Mauger, 1973). The lead date provides at least a minimum age for the Bridle formation host rock. The Lawler Peak granite, intruding the

Yavapai series just east of the thesis area, was dated at l, 410 +_ 90 m. y. by Damon (1959). Silver (1966) dated a oo­ genetic zircon suite from the Lawler Peak granite at 1,375 m. y.

An aeromagnetic map was made of the Bagdad area by Dempsey et. al. (1963) and covers the northern portion of the thesis mapping, extending south to one mile beyond the Old Dick mine. The structural discordance marked by the Mountain

Spring fault is clearly recognizable and coincides more or less with its mapped trace.

Small-scaled detailed maps around individual mines and prospect areas were made available by the Cyprus Mines 10

Corporation. With the exception of these maps, no other geologic mapping in the thesis area had been done prior to this investigation. Other mines, notably the Pinafore, Red

Cloud, and Zannaras mines, are located within this area about which no information could be found in this literature. The Pinafore mine will be discussed in a following section in relationship to the Old Dick mine and massive sulfide displacement. 11

GEOLOGIC SETTING

The Bagdad area occurs in metamorphosed terrain of volcanic

and epiclastic sedimentary rocks belonging to the

Precambrian Yavapai series. These were deposited in a

geosynclinal trough which extended northeast across the

North American continent (Wilson, 1962). The environment of

deposition was one of volcanism in a recurrently unstable

marine environment in which most of the sedimentary material

was derived from contemporaneous volcanic rocks (Gastil,

1953). This would be comparable to a modern day island-arc

environment.

The Yavapai series is divided into three formations - the

Bridle Formation, Butte Falls tuff and Hillside mica schist.

The Bridle formation, which underlies the thesis area,

consists of more than 900 meters of andesitic and basaltic

lava flows and intercalated water-deposited tuffaceous beds and terrigenous sediments (Anderson, et al., 1955).

Approximately 2,150 meters of Bridle calculated to be exposed the thesis area is considerably more than reported by Anderson. Overlying this is the Butte Falls tuff composed of about 760 meters of water-deposited and perhaps pyroclastic, volcanigenic sediments. The Butte Falls tuff grades upwards into the Hillside mica schist which consists of between 900 and 1,200 meters of sandstone and shale 12

(Anderson, et al., 1955). The Bridle formation is separated from the Butte Falls tuff by the Mountain Spring fault. The stratigraphic contact, be it unconformable or gradational between the two formations, has as yet to be reported or found. The Yavapai series was intruded by a wide variety of regionally concordant igneous rocks which include gabbro, diabase, trondjhemite and the Dick rhyolite. The entire trough assemblage was buried, deformed, and subsequently metamorphosed. Structures produced by deformation include northwest, north and northeast trending folds indicative of east-west compression (Anderson, 1952). The structure of the Bagdad area occupied by the Bridle formation and Dick rhyolite was interpreted by Anderson, et al., (1955) to comprise the overturned western limb of a postulated northeast trending major syncline. In the thesis area, the

Dick rhyolite separates the Bridle formation into two northeast-trending belts which bound the rhyolite.

Polyphase, dynamothermal metamorphism in the Precambrian is reported to have occurred in at least two major orogenic episodes. The older Arizona revolution occurred approximately 1,630 to 1,760 million years ago according to

Damon■(1968). The Arizona revolution was followed by the

Mazatzal revolution which affected Precambrian rocks approximately 1,370 to 1,450 million years ago.

Metamorphism culminated with the emplacement of large 13 granitic batholiths such as the Lawler Peak granite. The radiometric age of the Lawler Peak is approximately 1,400 m. y. before present which would attribute its origin to the

Mazatzal revolution (Damon, 1959). The Lawler Peak granite has likewise been dynamothermally metamorphosed (Anderson, et al., 1955). The grade of regional metamorphism ranges from low-grade greenschist metamorphic facies to locally high-grade sillimanite-bearing hornfels spatially related to the Lawler Peak granite (Anderson, et al., 1955).

Anderson (1955) discussed two possible periods of faulting which preceeded the intrusion of Precambrian batholithic granite, and at least two more periods of faulting after the emplacement.

Pre-Lawler Peak granite fault movement on the ancestral

Mountain Spring fault is said to be reverse. Post- metamorphic faulting on the younger fault trace exposed in the thesis area is stated to be normal, dipping to the west.

Paleozoic rocks are not represented in the Bagdad area. The geologic record resumes again in the Late Cretaceous or early Tertiary with the deposition of the Grayback Mountain rhyolite tuff. Late Cretaceous and early Tertiary granitic stocks, plugs and dikes also invaded the area which resulted also in renewed mineralization in the district (Mauger et. 14 al., 1965 and Anderson, et al., 1955). Tertiary and

Quaternary materials occurring in the Bagdad area include the Gila(?) conglomerate, Wilder formation composed of lavas and pyroclastic sediments, Sanders basalt, terrace deposits and recent alluvium. Quaternary terrace deposits and recent alluvium were mapped as "covered" areas in the thesis area. 15

FIGURE 3

METERS Butte Falls tuff; thickness unknown, outcrops 5,000 • east of fault

.Mountain Spring fault . Bridle formation below ■tuffaceous metasediments; up to 250 meters thick, may be equivalent to Butte Falls Tuff 4,000 •felsic metavolcanic rocks, 490 meters maximum

-micaschist; up to 520 meters thick; more or less interbedded with felsic metavolcanic rocks

intermediate metavolcanics, flows and epiclastic 3,000 1,035 meters maximum thickness

4* + + 4 Dick rhyolite, 470 to 1,575 meters thick; 4- + 4- + regionally concordant, schistose intrusive 2,000 + + + quartz porphyry grades into metatroudjhemite -V + + + +. + 4- + 4- + +

1,000 Metatroudjhemite, regionally concordant intrusive

basic metavolcanic rocks, 1,070 meters exposed thickness, dominantly flows, extensively intruded by metatroudjhemite porphyry 0

STRATIGRAPHIC COLUMN 16

LITHOLOGY

Yavapai Series

Two formations, the Bridle formation and Butte Falls tuff, were mapped in the thesis area. The Butte Falls tuff, crops out east of the Mountain Spring fault. The Dick rhyolite intrudes the Bridle formation, dividing it into two belts, one 800 meters thick on the west and the other about 1350 meters thick on the east. It is difficult to calculate the total thickness of Bridle formation mapped in the thesis area because of the intrusion of the Dick rhyolite and other igneous rock types which acted to remove or to push apart large areas of the Bridle. The stratigraphic column in

Figure 3 gives the maximum thickness of units mapped. The total pre-intrusive thickness of the Bridle Formation was probably no more than 2,150 meters if one adds together the two belts separated by the Dick rhyolite. The mineralogical composition of mapped units is presented in Appendix A.

Mafic Metavolcanic Rocks

Metamorphosed, mafic and intermediate volcanic flows outcrop west of the Dick rhyolite. In some instances it is possible to trace out individual flow units along strike for a limited distance. For the most part, metamorphism has altered the contact relationship between flow units. Flow units range from several meters to tens of meters in thickness. 17

Hand specimens are green to greenish-black, dense, very fine- to medium-fine grained rocks with textures ranging from granular to schistose depending upon the degree of recrystallization and deformation. The major rock forming minerals are andesine, chlorite, pargasite, tremolite, epidote, calcite and sometimes quartz and garnet.

Plagioclase and amphibole are common porphyritic minerals.

Other primary volcanic features include amygdules, vesicles, brecciation, flow layering, and compositional layering.

Compositional layering expressed by alternating light green and dark green layers is a widespread developed texture in the volcanic rocks. This texture is particularly well developed in the intermediate to felsic volcanic rocks east of the Dick rhyolite (see Photo 1). The layering is most likely primary in origin but regional metamorphism could also have produced some chemical and mineralogical variations within the layers.

Cataclastic deformation of the volcanics during regional metamorphism is regarded as non-uniform in intensity. The degree of deformation measured by the foliation and schistosity varied widely in intensity. Metamorphic fabrics ranged from granular, unfoliated rocks to those that are highly foliated and schistose. Variable schistosity may be in part a function of some primary feature such as a matrix 18 effect, degree of consolidation, amount of volcanic glass, relative competency between flows or sediments, flow orientation and chemical and mineral composition.

The metavolcanic rocks do not show any significant signs of contact, thermal metamorphism which might have accompanied intrusion of the Dick rhyolite unless, of course, the host rock alteration at the Old Dick and Copper Queen mines are attributed to the Dick rhyolite.

Mapping demonstrated that the metavolcanic rocks were intruded rather complexly by sills and pods of porphyritic trondjhemite (see Photo 19). Some contact thermal metamorphic effects such as silicification remain preserved.

The replacement of hornblende by tremolite-actinolite observed near some trondjhemite bodies suggests an amphibolite grade contact metamorphism may have been attained (see Photo 17).

The intrusion of porphyritic trondjhemite added quartz to the volcanic rocks. This is expressed mainly as an increase in groundmass quartz and quartz-filled vesicles near the trondjhemite. Vesicles usually contain just carbonate minerals and/or epidote. Quartz veining and felsic patches at one locality contained rounded quartz phenocrysts derived locally from the trondjhemite. This suggests the forceful 19

injection of the trondjhemite into the volcanic host rather

than a passive stoping.

Anderson, et al., (1955) also noted a quartz enrichment

phenomenon expressed by felsic spotted schists which he

attributed to the "soaking" of the volcanics by trondjhemite

(previously called "alaskite" by Anderson). Other evidence

for contact metamorphism which survived regional

metamorphism include increased biotite and growth of

andalusite porphyroblasts near to trondjhemite bodies.

Contaminations between trondjhemite and mafic volcanics is

covered more fully in a later section.

Cordierite-Anthophyllite Hornfels

The mafic volcanic rocks were subjected to a second period of thermal metamorphism which preceded regional metamorphism. This produced a hornfels with a dark-gray to almost black color and a medium to coarse grained granoblastic fabric. Cordierite, anthophyllite, quartz, pyrope and magnetite are the common rock forming minerals.

Pennite may entirely replace cordierite and clinochlore

(chlorite) replaces or mimics anthophyllite (see Photo 4).

The anthophyllite occurs as blades up to 20 millimeters in length often forming large radiating aggregates or bow-tie structures. 20

Cordierite-anthophyllite hornfels was mapped in two large

areas in the west-central portion of the geologic map as well as in numerous smaller elongate bodies in the general vicinity west of the Dick rhyolite. The contact of the hornfels is gradational more or less with the mafic volcanic

rocks. Hornfel rocks occurred in the same vicinity as bodies which were too small to be mapped. The cordierite- anthophyllite hornfels looks in places to be "interbedded" with the regionally metamorphosed rocks and chlorite schists, in layers between 0.5 and 2.0 meters wide. The apparent interbedding could have resulted from channeling of hydrothermal metasomatic fluids along certain beds or by primary chemical or mineralogic variations inherent to the bedding.

The weathered surface of cordierite-anthophyllite hornfels produces a spotted texture. This rock type bears the name of "dalmatianite" in the Abitibi volcanic belt. The dalmatianite was attributed in Royun-Noranda district in

Canada to thermal metamorphism of chlorite schist believed to represent former zones of hydrothermal alteration (de

Rosen-Spence,1969).

Intermediate Metavolcanic Rocks

Interbedded andesites and lesser amounts of basalt, rhyodacite, and volcanigenic sediments are mapped east of 21 the Dick rhyolite (see Photo 2). In microscope or hand specimen, there is little to distinguish the mafic and intermediate volcanic rocks east of the Dick rhyolite with those west of the Dick rhyolite. As a unit though, there are some notable differences. Compositional layering is more pronounced in these lavas than in those west of the

Dick rhyolite. Bedding is on the average thinner and interbedded volcanigenic sediments and felsic flows become more abundant near the top of the unit east of the Dick rhyolite. Hand samples are light to dark green, greenish- brown schists and phyllites.

Some 5% of this unit consists of sediments characterized by graded bedding and layering of fluvial origin. The sediments are poorly sorted and composed of rounded, coarse sand to gravel size lithic particles in a foliated chlorite- rich matrix (see Photo 5).

The compositional layering shown in Photo 6 is a common texture believed to be formed in part as a result of metamorphic recrystallization. The most common rock type is a mildly deformed schist with frequent amphibole and albite preserved in a relict igneous texture. These textures include flow structure, trachytic texture and porphyritic texture. Rocks having experienced more intense deformation produce a highly foliated chlorite-sericite schist. The 22 chlorite was derived primarily from the breakdown of

amphibole. Chlorite schists also contain major amounts of muscovite, epidote, and calcite. Most of the primary rock

texture has been destroyed by the development of schistosity. Recrystallized rocks are characterized by equigranular texture composed of epidote, amphibole, chlorite and quartz minerals. Epidote-rich rocks are light green in color and contrast rather sharply when interbedded with the darker amphibole schist or foliated chlorite schist.

A peculiar banded or layered variety of carbonate rocks were found in 0.5 to 3 meter beds within the intermediate metavolcanic rocks. The white to dark grey carbonate beds contained thin laminations of calcite or dolomite alternating with interlayers rich in epidote, chlorite or amphibole. The origin of this rock is somewhat of a puzzlement. Its sharp contact and laminated texture suggest a sedimentary origin, either clastic or chemical precipitation.

Interlayering between the four variants of intermediate metavolcanic rocks— amphibole, chlorite, epidote and calciterrich schists— may occur interlayered at a single locality or even on a microscopic scale to produce laminated textures. Much of the compositional layering is probably of primary origin. 23

Mica Schist

Two bands of mica schist up to 260 meters thick each were

mapped in the southeastern area interbedded with

intermediate metavolcanic rocks. The mica schist grades

laterally into the felsic metavolcanic rocks. Rock samples

light to dark brown and well—foliated containing

abundant oriented muscovite and chlorite. Garnet,

tourmaline, hornblende and quartz may also be distinguished

in thin section. The rock is massive and fine-grained; no

relict primary textures were observed. Under the

microscope, reconstituted iron-oxide is concentrated in

layers which probably reflects remnant bedding. Small

porphyroblasts of garnet, magnetite and andalusite (altered

to sericite) occur less frequently. Tourmaline less than

.01 millimeters in size occurs as an accessory mineral in

some specimens. The c-axis is usually observed to be

parallel to the plane of schistosity. Tourmaline was

reconstituted most likely from trace amounts of boron

included in the parent sediment. The parent material is

assumed to be an argillaceous or tuffaceous siltstone.

Felsic Metavolcanic Rocks

Rocks of this unit are light gray, brown or green schists with abundant relict porphyritic texture and thin layering

(see Photo 7). This unit was composed originally of about 24

half intermediate to felsic volcanic flows and half

interbedded volcanigenic sediments. Mapping shows the

felsic volcanic rocks to be interlayered with the mica

schist and with the upper part of the intermediate metavolcanic rock units.

Microscopic examination is useful in distinguishing between

a volcanic or sedimentary parent. Euhedral phenocrysts of

feldspar and quartz are responsible for the prophyritic texture of volcanic flows. Porphyritic minerals form euhedral and subhedral phenocrysts and frequently display flow oriented or rounded and embayed textures due to magmatic resorbtion. The porphyroblastic texture commonly developed in metasediments, on the other hand was formed secondarily by epiclastic porphyroblasts of phenocrysts which had been weathered out or mechanically separated from pre-existing volcanic rock. These epiclastic porphyroblasts are well-rounded from abrasion or angular due to mechanical breakage. Porphyroblasts are invariably oriented in the foliation plane with the groundmass having recrystallized and compacted to produce an augen texture as shown in

Photo 8. Epiclastic porphyroblasts occur over a wide range of sizes compared to the uniform size of phenocrysts in lava flows. They also tend to be concentrated into layers which further suggests some form of sedimentary control. A 3-meter thick bed of tourmaline schist composed of 96%

schorl is located just east of the Rudkins shaft. Unlike other sources of tourmaline, the tourmaline schist probably represents a hydrothermal replacement. Several quartz dikes and silicified zones located in the vicinity of the Rudkins might have provided the source of fluids.

Tuffaceous Metasediments

These rocks consist of white to grayish, bedded quartzites and mica schists composed mainly of muscovite, quartz and feldspar. The unit is almost identical to the felsic metavolcanic rocks and metasediments. It is distinguished, however, by more abundant massive quartzites and by S2 crenulations affecting the foliation.

The tuffaceous sediments east of the Mountain Spring fault belong to the Butte Falls tuff according to Anderson, et al., (1955). Tuffaceous mica schists are also located farther to the southwest across the Mountain Spring fault.

Mica schists and quartzites south of the fault are located stratigraphically at the top of the Bridle formation. The similarity of metamorphic rocks mapped as the Butte Falls tuff by Anderson, et al., (1955) and those near the top of exposed Bridle formation suggests the possibility that the

Bridle Formation grades upwards into the Butte Falls tuff in the area of mapping. 26

Igneous Rocks

Metatrondjhemite

Porphyritic trondjhemite was mapped extensively west of the

Dick rhyolite intruding into the mafic volcanic rocks of the

Bridle formation. The trondjhemite intruded as a complex of sills, dikes and large elongate bodies up to 1,000 meters across. It is a fine-grained, schistose, light greenish colored, porphyritic rock which weathers a yellowish-brown color. The trondjhemite contains rounded quartz and albite phenocrysts 2 to 3 millimeters in diameter. Quartz and albite occur in the groundmass as do muscovite, chlorite, iron oxides and biotite. The chlorite tends to be patchy or streaky giving the rock a more or less uniform speckled or foliated appearance in hand sample. Photos 13 and 14 are photomicrographs of trondjhemite samples.

The trondjhemite was originally called an alsakite porphyry by Anderson, et al., (1955). However, lack of alkali feldspars and low abundance of calcium or potassium in chemical analyses (Appendix C, D), indicate that the original rock was more like a soda-rich rhyolite. Thus, the name was changed for this report from alaskite to trondjhemite. 27

Contaminated Metatrondjhemite

A distinction was made in the field between fresh trondjhemite and trondjhemite contaminated through inter­ reaction with the mafic volcanic wall-rock. Hand specimens of fresh and contaminated trondjhemite appear almost identical to one another with the exception of a few minor differences. Mesoscopically, contaminated trondjhemite contains more chlorite and muscovite than does fresh trondjhemite. Moreover, the chlorite is characteristically streaky or patchy which produces gneissic banding in some localities. The patches, approximately 1 to 3 centimeters in size, are felt to represent inclusions of mafic volcanic rock which were partly resorbed and metamorphosed as was the rest of the rock. Anderson, et al., (1955) also discussed the significance of the patchy texture which he related to assimilation of mafic volcanic rock fragments. Assimilation caused slightly more noticeable amounts of chlorite, biotite and actinolite to form near the margins trondjhemite bodies.

Trondjhemite dikes display an ocellar texture in which needle-shaped crystals of actinolite radiate from a nuclear grain of a single quartz phenocryst. This secondary actinolite (see Photo 12) is believed to be an indication of contamination whereby iron, magnesia and silica were resorbed by the trondjhemite. This caused the growth of actinolite which nucleated at available sources of silica, 28

namely at the quartz phenocrysts. The contamination process

will be discussed again in a later section.

Quartz-Biotite-Cordier ite Hornfels

Like the Bridle formation, fresh and contaminated

metatrondjhemite were subjected to locally intense, post-

regional thermal metamorphism. The proximity of quartz- biotite-cordierite hornfels and cordierite-anthophyllite hornfels shows that they were formed in the same environment but of different parent materials.

Hand samples of quartz-biotite-cordierite hornfels are dark gray or brown in color with a coarse, hornfelsic fabric.

Muscovite and biotite are sometimes distinguishable with a hand lens. The occurrence of cordierite and quartz cannot be resolved without the aid of a microscope because the two are intergrown with each other. Other microscopically identifiable minerals include plagioclase, chlorite and sillimanite which occurs in very thin, needles or filaments.

Sillimanite is found associated with the light colored spots, a distinctive texture of the rock, which often attain a diameter of 2 to 5 millimeters. Differential weathering of the spots produces a distinct scoraceous-1ike texture on the weathered rock surface. 29

Dick Rhyolite

The Dick rhyolite was named by Anderson, et al., (1955) for

its occurrence at Dick Peak located just to the north of the

thesis area. It is the largest single mapped unit and extends from the Mountain Spring fault for over 8.5 kilometers to the southwest to some point beyond the limit of geologic mapping. The rhyolite locally cuts across the foliation and lithologic contacts but is largely concordant.

It occupies a stratigraphic position located between mafic volcanic rocks and intermediate volcanic rocks of the Bridle formation (see Figure 3). Total thickness of the rhyolite ranges from 470 meters to a maximum of about 1,580 meters.

The rhyolite contains rounded, bluish quartz phenocrysts up to 1 or 2 millimeters in diameter with a very fine-grained schistose matrix. Muscovite and chlorite are identifiable with a hand lens, but a microscope is needed to differentiate between abundant quartz and albite that comprise the groundmass. The rock is generally white or light green in color, locally stained reddish-brown due to trace amounts of iron oxides produced during sulfide weathering. Photo 9 shows the typical fine-grained, porphyritic texture and mineralogy of the rhyolite. This is contrasted against Photo 10 of the rhyolite in the southwest portion of the mapping where it appears to be more intensely deformed. The metamorphic textures include granulation of 30 quartz phenocrysts and well-defined lineation seen in the recrystallized muscovite.

The Dick rhyolite is mineralogically and texturally almost identical to the trondjhemite. The rhyolite on the average, however, contains slightly more muscovite and less chlorite and albite than the trondjhemite. Chemically, the rhyolite has more potassium than the trondjhemite, the excess potassium being tied up in the muscovite grains (see

Appendices C, D, E). The only feldspar observed in the rhyolite and trondjhemite is albite or andesine. Albite has been observed in phenocrysts of the trondjhemite whereas no albite forms in the rhyolite. The groundmass of the rhyolite often appears to be finer grained and less schistose than that of the trondjhemite further adding to the distinction between them (compare groundmass in Photo 9 with Photos 13 and 14).

The contact between the Dick rhyolite and the Bridle formation is highly visible owing to the color contrast brought out by the chemical and mineral composition. The contact between trondjhemite and rhyolite on the other hand is rather indistinct and mapped only with much hesitation.

The possibility of that contact being gradational is suspected rather strongly. The chemical and mineralogical similarity between the rhyolite and trondjhemite is reason 31

alone to postulate a close, co-magmatic relationship between

the two igneous rocks.

The possibility of the Dick rhyolite being extrusive has

been discussed by other investigators. The rhyolite does

not show having produced any widespread contact metamorphism

of the host rock or clear evidence of wall-rock alteration

other than at the Old Dick and Copper Queen mines. The case

is that the rhyolite was a shallow intrusion which cooled

rapidly as did the trondjhemite. The local mapping of

protrusions of rhyolite into the mafic and intermediate

volcanic rocks would present the strongest evidence for an

intrusive origin. The evidence nonetheless is ambiguous and

either origin, a surface extrusion or a near-surface

intrusion, is considered likely. Considering the thickness

of the rhyolite, there is little to distinguish the two

models of emplacement from each other petrologically.

The postulation of a co-magmatic relationship between the

Dick rhyolite and trondjhemite seems favorable at this time.

Such steps would help to explain some similarities between

the two igneous rocks. The almost identical texture and minerology is strongly in favor of a common genesis either

through fractional crystallization or differential wall-rock

reaction. The wall-rock assimilation prevalent in the

trondjhemite could have differentiated it from the rhyolite 32 by enrichment in iron, magnesium and sodium. The fine­ grained porphyritic nature of the igneous rocks suggests that both rock types were more or less rapidly intruded to a shallow depth near or at the earth's surface. The Dick rhyolite intruded nearer the surface than the trondjhemite if their relative stratigraphic position represents their level of intrusions. The underlying trondjhemite was effectively blanketed by the rhyolite allowing it to retain heat longer and by slower cooling processes, assimilate more of the wall-rock. Such an environment could result in a more complex intrusive pattern by the trondjhemite and quartz flooding noted in the mafic wall-rock aureoles. The grain size of the trondjhemite is often slightly coarser than that of the Dick rhyolite which would indicate a slower cooling rate. Initially, it was hoped that the chemical analyses would help demonstrate a co-magmatic relationship.

As it turned out, regional metamorphism did much to scramble the primary igneous chemical ratios and was not useful in testing the co-magmatic relationship.

Quartz-Biotite-Andalusite Hornfels

The Dick rhyolite, like the Bridle formation and the trondjhemite, was affected by a period of thermal metamorphism to produce a hornfels. Specimens of the hornfels are medium to dark grey, weathering a reddish-brown color that contrasts sharply against the white colored Dick 33

rhyolite. Quartz was recrystallized into a strong polygonal

mosaic texture which often masks out the original

porphyritic texture of the rhyolite. Some of the original

fine-grained matrix and quartz porphyritic texture are

preserved in inclusions of garnet and andalusite

porphyroblasts. Reddish-brown colored, Ti02 rich(?)

biotite, were identified occurring as large, randomly

oriented flakes often replaced by muscovite or chlorite.

Andalusite was almost totally replaced by very fine

sericite. Tourmaline is a common accessory often included

in biotite. Tourmaline forms as much as 30% of one rock

sample.

Intrusive Metarhyolite

Massive to weakly porphyritic, schistose, white to pinkish

colored rhyolite intrudes the Bridle formation in the

southeastern area of geologic mapping. The intrusive

relationship is demonstrated by cross-cutting contact

relationships and pods of rhyolite injected into the

intermediate volcanic rocks. The age relationship with the

Dick rhyolite or trondjhemite could not be determined by direct means. It occupies a stratigraphic position above that of the Dick rhyolite. This, of couirse, has nothing to say of its true age. The contact becomes somewhat indistinctive when in contact with the Dick rhyolite which closely resembles it. 34

The rock is composed of fine-grained microcline, albite,

quartz and biotite. The microcline and albite appears to

vary in abundance in inverse proportion to each other; when

albite is abundant, microcline is low and vice versa. The

reason for this relationship between microcline and albite

is not known. In one sample, microcline had been replaced

by quartz and muscovite leaving the albite untouched, which

partly explains the low abundance of microcline.

Pervasive weak thermal metamorphism is suggested by the

occurrence of relatively large muscovite and garnet grains

which contain inclusions of tourmaline and sphene. The

sphene also tends to be concentrated in albite grains.

Andalusite and garnet may occur as small porphyroblasts.

Metagabbro

Some five very small circular bodies of gabbro less than fifty meters in diameter were mapped in with the Dick rhyolite, trondjhemite and Bridle formation near the southern portion of the geologic map, (Figure 2, in pocket).

The gabbro is black to greenish-black in color and composed primarily of medium-grained, randomly oriented hornblende crystals, 5 to 8 millimeters in length. Other rock-forming minerals identified in thin section include minor relict diopside, plagioclase, biotite, zoisite, epidote, garnet, magnetite, chlorite and accessories. The hornblende occurs 35

as rectangular and equant shaped crystals suggesting a

pseudomorphic replacement of hornblende after pyroxene.

Regional metamorphism is represented by the hornblende,

epidote, chlorite mineralogy, and mild cataclastic

deformation. No well-developed planar schistosity or

foliation was observed in hand samples, or in thin section.

Deformation was expressed under the microscope by extensive

crystal breakage.

Metadiabase

Concordant sills of diabase up to 75 meters thick intrude

the Dick rhyolite and the Bridle formation. The unfoliated, dark green, medium-grained rock is composed of green hornblende, andesine plagioclase and minor amounts of magnetite, biotite, epidote, sericite and other accessories.

The contact with rhyolite and mica schists is sharp.

However, the similarity between diabase and mafic and intermediate volcanics rocks is enough that mapping of diabase within the volcanic rocks is difficult and many sills of metadiabase may have gone unnoticed.

Thin section reveals an ophitic texture created by the partial enclosure of subhedral plagioclase by later forming hornblende (see Photo 15). Hornblende crystals no more than

6 millimeters in length. All grains have experienced some degree of deformation by cataclastic destruction of primary grains and relict igneous fabric. Much of the fine-grained

groundmass experienced recrystallization. The diabase could

be related in origin with the gabbro but there is no direct

evidence for this.

Granodiorite Gneiss

Granodiorite intruded the upper part of the Bridle Formation

near the eastern edge of the geologic map. A dark fine­

grained, porphyritic phase was injected into the Bridle

formation along sills and dikes for some tens of meters into

the country rock. Pendants and slices of chlorite and mica

schists derived from the Bridle formation occur near the

border of the granodiorite intrusion.

The coarse phase of the granodiorite consists of white

elongated, ellipsoids or augen of quartz and feldspar minerals surrounded by a black, schistose matrix composed of microscopic biotite, chlorite, epidote, quartz, magnetite

and leucoxene. In thin section, the augen consist mainly of albite, microcline or microperthite phenocrysts ranging in size from 0.5 to 2.0 millimeters. The fabic development is that of an augen gneiss. The phenocrysts are well rounded due to the pervasive cataclastic deformation which affected the granodiorite. The rare occurrence of euhedral allanite, a metamict mineral, is partly replaced by epidote. mmm

The granodiorite was intruded by the Lawler Peak granite of

precambrian age (Anderson, et al., 1955). It is

significantly younger than the Yavapai series and the Dick

rhyolite which presumably had been folded by the time of

granodiorite intrusion postulated by Anderson to occupy the

fold axis of an overturned syncline.

Pegmatite

. Sills, dikes and irregular bodies of pegmatite intrude the

Bridle formation, Dick rhyolite, and trondjhemite in the

southern area of geologic mapping. The pegmatites are the

youngest igneous rocks in the area differing from other

igneous rock types by their discordant intrusive nature

controlled in part by jointing, and their lack of regional

metamorphic fabric features. ■

Geologic reconnaissance indicates that the pegmatites are

related to a medium grained, phaneritic, granite batholith

which truncates the Precambrian rocks a little southwest of ■ the thesis area. Pendants and xenoliths of the Precambrian

metamorphic rocks occur abundantly near the batholiths'

margin. The State Geologic Map indicates a Precambrian age

for the granite batholith (Wilson, 1962).

The pegmatite changes in character and mineralogy as a

function of distance from the batholith. From the 38 batholith, just south of the Pinafore mine, to roughly 1.5 kilometers northeast of the mine, the pegmatites are 1 to 2 meter-thick dikes and bodies composed of coarse grained quartz, muscovite, alkali feldspars and traces of tourmaline. From 1.5 to 3.0 kilometers northeast of the mine they become thin discontinuous veins or dikes of fine­ grained quartz and tourmaline which form prismatic aggregates. Beyond 3.0 kilometers the pegmatites no longer are mappable; nonetheless, they exist as very small, sporadic outcroppings of massive tourmaline. Massive tourmaline has been observed as distant as seven kilometers from the batholith.

The pegmatites experienced mild cataclasis shown by broken and stretched tourmaline crystals.

Alteration

Chlorite Schist

Chlorite-rich rocks mapped at the Dick mine do not show any obvious stratigrapic control, other than occurring chiefly within the mafic metavolcanic rocks. Some tend to cut across the bedding at low-angles in zones of what are felt to represent zones of hydrothermal alteration (Baker and

Clayton, 1968). With poor means for distinction, chlorite schists were mapped with difficulty since nearly identical chlorite schists are formed by regional metamorphism as 39 well. Abundant sphene observed in thin section helps to support a hydrothermal origin of some chlorite-rich schists.

Chlorite schists are greenish in color, well foliated which allow it to be easily broken with a hammer. They occurr mainly in the Bridle formation west of the Dick rhyolite.

Some were mapped occurring in the metatrondjhemite.

Chlorite schist layers in the metatrondjhemite are believed to represent altered xenoliths of Bridle mafic volcanic rock. Other chlorite schists within the intermediate volcanics and sediments east of the Dick rhyolite were produced by dynamothermal metamorphism of volcanic rocks which probably reflect in part on some primary chemical or mineralogical differences.

Chlorite schists occurred commonly in and around cordierite- anthophyllite hornfels. The hornfels are believed to represent the product of the thermal metamorphism of a pre­ existing chlorite schist. Retrograde metamorphism of the hornfels produced a late generation of chlorite which replaces anthophyllite, cordierite, garnet, and biotite.

The chemical composition of the chlorite schist in some instances suggests at least a chemical alteration from some earlier rock type. X-ray diffraction and thin section analysis of one sample of chlorite schist identified it to 40 be a felty matt of penninite chlorite (5 MgAl203 3Si02 4H20) with abundant inclusions of iron oxides and sphene. The iron oxides occur in the center of penninite grains and seem to have been exsolved from a prototype ferromagnesian chlorite prior to alteration (see Photo 16). The usefulness of chlorite schist as an indicator of alteration was rather limited because of the variety of ways in which a chlorite- rich schist may be formed other than by hydrothermal metamorphism.

Quartz-Sericite Schist

Quartz-sericite schist occurs at the Old Dick and Copper

Queen mines enveloping the massive sulfide ore bodies.

Their formation is attributed to hydrothermal alteration; however, much of the quartz-sericite schist at the Copper

Queen was also attributed to regional metamorphism of tuffaceous sediments of favorable composition (Baker and

Clayton, 1968). They are also indistinguishable from quartz-muscovite schists in the felsic metavolcanic and metasedimentary rocks of the Bridle formation and the Butte

Falls tuff. Both genera of quartz-sericite schist are whitish, foliated rocks often stained by iron oxides.

However, some bleached-white, nearly pure quartz-muscovite schists stood out as anomalous and were mapped separately on the basis of possible hydrothermal alteration. As with the chlorite schists, the usefulness of quartz-sericite schist 41

as a mapping unit is limited because of various ways in

which a quartz-sericite schist may be formed.

Covered Areas

The western limit of bedrock geologic mapping is determined

by late Tertiary colluvial deposits derived locally from

mass weathering of welded tuffs at Grayback Mountain. The

colluvium is between 0.5 to 3 meters thick, composed primarily of poorly sorted, unconsolidated, sand, gravel and

cobble sized particles of weathered tuff.

Covered areas located in other parts of the area consist of

recent colluvium and alluvium which effectively obscured

bedrock geologic relationships. 42

STRUCTURAL GEOLOGY

Metamorphic and Primary Structures

The orientation of metamorphic foliation was measured throughout the mapped area. Foliation is defined herein as consisting of all types of mesoscopically recognizable S- surfaces of metamorphic origin. Foliation covers a combination of elements including schistosity, mineralogical layering, slatey cleavage, and slip cleavage observed within the area in various degrees. In many instances, slatey cleavage and schistosity mimic the primary layering.

Cleavage sometimes cuts across bedding features particularly in areas of minor folding. Both axial plane and slip cleavage were observed.

Schistosity cuts across both the bedding and the cleavage.

Schistosity is well developed on a microscopic level even in seemingly aphanitic or granular rock types. The schistose fabric in rocks in the southern area of geologic mapping is more strongly developed with larger crystal grain sizes and

(compare Photo 9 with Photo 10). In this same area, a lineation is added to the metamorphic fabric, plunging steeply to the southwest in the plane of foliation.

Formation of hornfelsed rocks destroyed the earlier schistose fabric locally over the western portion of the area. Schistosity in the large igneous masses such as the

Dick rhyolite or grandoiorite gneiss, is rather uniformaly 43

developed. It is only in the interbedded felsic

metavolcanic and metasedimentary rocks that the intensity of

schistose fabric development tends to vary greatly.

Primary layering is commonly observed throughout the area.

Sedimentary bedding is more rare; however, enough is exposed

to determine direction of tops with the use of graded

bedding. Tops of beds consistently face east substantiating

in part Anderson's theory of the Bagdad area constituting an

overturned limb of part of a major east-lying syncline.

In the hornfelsed rocks, the metamorphic schistosity was obliterated by the strong development of a hornfelsic

fabric. Retrograde metamorphism which affected the hornfels

rocks resulted in the formation of new chlorite and muscovite. The fact that these platey minerals are unoriented, with the exception of certain mimic replacement textures, would suggest that the regional dynamothermal metamorphism had effectively died out prior to the retrograde alteration or even before the formation of the hornfels.

Lineation plunging moderately to steeply southwest was well developed over much of the southern portion of the area.

The lineation is often best defined by the elongation of chlorite mineral patches or the intersection of S-surfaces 44 which produced rod-like, mullion structures. Elongation of mineral patches is suggestive of differential shearing or plastic flowage during metamorphism. Differential plastic flowage is seen in some interbedded volcanics by occassional boudin structures. In the northern portion of the mapping, lineation was produced mainly by small crenulations of the schistosity. Crenulation is strongly developed in thinly laminated sedimentary rocks east of the Dick rhyolite.

Photo 18 is a photomicrograph of the sometimes strong crenulation.

Two hundred and thirty-nine S-poles measured normal to the regional foliation were plotted and contoured on a stereo net reproduced in Figure 4. The foliation measurements came from all types of lithologies, and throughout the mapped area. S-pole countours define a girdle of preferred orientation in the foliation. The girdle is characterized by two maxima which represent two generally preferred directions of orientation in the foliation. The maxima are concentrated in a N48°E, 86°W and N28°E, 84°W dip orientation. The range of foliation trend is from N40°W to

N80°E with steep westerly dips. The reason for the two maxima is not quite certain. They may be due to the intersection of two or more metamorphic S-surfaces such as relict bedding and slip cleavage, one S-surface perhaps trending N48°E and other N28°E. It could be that they may 45 represent two generations of metamorphic S-surfaces separated in time and under slightly different stresses. If one examines the foliation attitudes on the geologic map

(see Figure 2), foliation trends appear to bend or curve locally suggesting the possibility of secondary folding or crenulation of the regional foliation. The suggestion of rather broadly defined flexures seems to include all metamorphic S-surfaces and lithologic contacts, therefore post metamorphic in age.

Two flexure axes identified from the geologic map trend sub­ parallel to each other in a N40w direction. Flexuring caused the foliation on the limbs to bend some 20° from the parallel. This 20° difference in trend is the same amount which separates the two S-pole maxima in Figure 4. The flexuring is probably the last tectonic event of the area with the exception of possible ongoing faulting. The stress environment which produced the flexures corresponds to northeast-southwest compression.

Minor mesoscopic folding was observed locally, but was not that common a feature. Fold axes trend northeast and northwest plunging generally to the south. These folds were noted to be associated with steeply dipping axial plane cleavage which trended more or less parallel to the metamorphic foliation. These folds are the result of regional metamorphism. FIGURE 4

N

□ < 2/3% per 1 % Area 1 1 2/3% to 3%

3% to 6% 6% to 8%

CONTOURED S-POLE DIAGRAM OF METAMORPHIC FOLIATION 47

Mountain Spring Fault

The north trending Mountain Spring fault was mapped originally by Anderson, et al., (1955) and separates the

Bridle formation, Butte Falls tuff, Dick rhyolite and King

Peak rhyolite from the Lawler Peak granite and Hillside mica schist. The Mountain Spring fault is younger than the

Lawler Peak granite and coincides in part with the ancestral

Mountain Spring fault whose fault features predate regional metamorphism. The ancestral fault was postulated as having reverse displacement on a west dipping fault plane whereas normal offset was postulated for the younger Mountain Spring fault (Anderson, et al., 1955). The fault was implied to have movement on the order of hundreds of meters.

The fault is exposed in the northeast corner or the map trending south for a little over one mile to where it was last observed in outcrop midway between the Rudkins shaft and the Copper Queen mine. The fault zone consists of crushed rock and gouge with slickensides in a zone less than

10 feet wide, trending N10°E and dipping 65° to 75° to the west. The Dick rhyolite on the west side of the fault is considerably shattered, which decreases in intensity away from the fault zone. The location of the Mountain Spring fault is best expressed by the marked discordance of the foliation trends in the metamorphic rocks as one crosses the fault zone. Further to the south the fault trends parallel to the regional foliation and the pronounced discordance which marks the trace of the fault becomes lost. The structural discontinuity marked by the aeromagnetic anomally across the fault zone (Dempsey, et. al., 1963) also becomes regionally concordant along the southern projection of the fault.

A photogeologic interpretation was made in which two possible continuations of the fault zone were identified.

One possibility would project the fault along strike for another 1.75 miles, perhaps terminating within the grandiorite gneiss, but the field evidence for this is weak.

Another possibility suggests that the fault swings to the east possibly to rejoin the ancestral Mountain Spring fault which supposedly separates the Butte Falls tuff from the

Hillside Mica schist and Lawler Peak granite. Geologic mapping along the later projection indicates, however, that metatuffaceous sediments, believed to belong to the Butte

Falls tuff, are in stratigraphic contact with the Bridle formation. It should be noted that the petrology and texture of the upper part of the Bridle formation and the assumed Butte Falls tuff are so similar that they could easily be confused in the field. One or both of these projections may represent the continuation of the Mountain

Spring. In any case, the lack of any well defined features by which to identify the fault may be an indication that fault has diminished in magnitude or simply died out. 49

Other Faults

Minor shear zones and faults were observed in the area, but

could not be followed for any distance. These minor faults,

consist of narrow zones of gouge or brecciated rock often

containing epidote smeared onto slickenside surfaces. Other

faults were inferred where abrupt lithologic changes or

structural discordances were noted on air photos. These

features trend generally parallel to the northeast-trend of

metamorphic foliation but also trend north-south, and

northwest.

A maximum of 12 feet of right-lateral offset was inferred

from one traceable unit across a northwest trending fault.

The fault could not be traced or extended from the outcrop

location. The minor faults are post-metamorphic in age

having crushed and destroyed the earlier metamorphic fabric

and minerals.

The faulting described at the Old Dick mine by Baker and

Clayton (1968) which separates the Bridle formation from the

Dick rhyolite, may continue to the southeast for at least

one mile along the contact. This is based on two exposures

of minor gouge and brecciation on or near the contact.

Faulting, continuing southeast into the trondjhemite was

suggested by a fairly well defined lineament, but no

exposures were available to justify an extension of the

fault. 50

For the most part, fault recognition in a terrain characterized by multiphase regional metamorphism is difficult without additional subsurface information, good exposures or traceable geologic features. The air photo interpretation failed to produce any meaningful results. 51

METAMORPHIC PETROLOGY

Mineralogy

High temperature laboratory experiments over the last two decades have determined the stability fields of many but not all of important metamorphic mineral assemblages and paragenetic reactions. Laboratory experiments allow the investigator to place limiting factors on the physio- chemical conditions which affected the rocks during metamorphism by observing the mineral species and asemblages in the area of study. The following section discusses the mineralogy of the Bagdad area using primarily petrographic thin section analysis and relating it to published experimental results. Appendix A is a summary of mineral compositions of the different mapped rock units.

Regionally Metamorphosed Rocks

The minerals common to regionally metamorphosed basaltic and andesitic rocks include albite, chlorite, muscovite, hornblende, pargasite, epidote, and less commonly tremolite, quartz, garnet and calcite. The granitoid rocks such as the

Dick rhyolite and trondjhemite are more narrow in their chemical composition and are composed almost entirely of quartz albite and muscovite with minor biotite, chlorite and andalusite. These minerals are typical of greenschist facies regional metamorphism. 52

Plagioclase: Anorthite content in plagiocl-ase ranges from

An0 to An40 in nearly all rock types. The vast majority, however, is less than about An2^. Albite is most common in schistose rocks and is commonly associated with chlorite, muscovite and biotite. Primary grains and volcanic phenocrysts of albite are clouded with inclusions of epidote, sericite and less commonly chlorite. Recrystallized albite is clear, untwinned and equigranular.

Plagioclase in granitoid rocks in the Bagdad area are almost exclusively albite while the plagioclase in basic volcanic rocks may be either albite, oligoclase or andesine. The reaction of primary An-plagioclase to albite reaction is characteristic of the greenschist facies. Albite and epidote are associated together in lower temperature subfacies of greenschist metamorphism. The occurrence of oligoclase or andesine with epidote, associated with pargasite, actinolite, hornblende, chlorite, and garnet indicates a higher temperature greenschist subfacies

(Winkler, 1967). The higher temperature rock fabric is in general, more granoblastic and lineated than the lower temperature albite-epidote assemblage which tends to be more highly foliated.

The occurrence of reverse compositional zoning in oligoclase and andesine was observed in thin section of certain 53 contaminated trondjhemite dike rocks. Albite is believed to have been contaminated by calcium derived from the host basic volcanic rocks, to produce a slightly more calcic rim during recrystallization or cooling of the igneous rock.

Chlorite: Chlorite is the most common mafic mineral and occurs in nearly every rock type. Its non-occurrence is partly a function of unfavorable bulk composition or high- grade thermal metamorphism. Chlorite shows a weak to very strong preferred orientation which defines the plane of schistosity. Chlorite grains show clear to light grayish- green to green pleochroism. An unidentified species showed anomalous lavender-blue to purple interference color.

Common inclusions in chlorite grains are quartz, iron oxides magnetite or (limonite), sericite, hornblende, biotite and other accessory minerals. Chlorite in the Dick rhyolite and trondjhemite probably replaced primary biotite, but there is no evidence for this; essentially no biotite remains.

Chlorite in granitoid rocks may be randomly oriented, but is usually schistose. Chlorite clearly replaces biotite flakes

in the intermediate and felsic volcanic rocks, replacing biotite along cleavage planes and at grain boundaries.

Aggregates of chlorite suggest that some of the igneous biotite grew in syneusis aggregate. 54

Chlorite also replaces amphibole which formed as a result of

trondjhemite mafic wallrock assimilation. Actinolite nucleated at sources of quartz such as the quartz phenocrysts carried by the trondjhemite. Actinolite occurs

as thin radiating needles which form the ocellar texture noted in some of the contaminated trondjhemite rocks.

In the basic metavolcanic rocks chlorite replaces primary

amphiboles along cleavage planes and grain boundaries.

Relict hornblende grains have very serrated grain boundaries

and are fractured and broken from metamorphic deformation.

Chlorite replacing amphibole commonly contains excessive

inclusions of iron oxide and quartz compared to other paragenetic reactions and suggests that iron and silica were

released during the transformation. The paragenetic

reaction is given by the following equation:

Equation (1)

3 hornblende ----> 1 chlorite + 1 epidote + 9 FeO + 15 Si02 +CaO

Chlorite makes up over 50% of the rock mapped as chlorite

schist presumably formed in zones of hydrothermal

alteration. Hydrothermal chlorite species was identified as

penninite (5MgO*Al203•3 Si02*4H20) by X-ray diffraction. In

thin section, penninite contains inclusions of iron oxides suggesting that originally chlorite contained iron in its structure (see Photo 16). It is believed that the original iron-bearing chlorite became unstable with increasing temperature or oxygen fugacity during metamorphism which produced a magnesium-rich chlorite and separate Fe-oxide phase (Boyd, 1959).

Chlorites associated with muscovite in the Dick rhyolite and trondjhemite, but the mineral reaction between these two minerals could not be inferred from petrographic observations.

In interlayered mafic metavolcanic rocks, metadiabase and gabbro, the chlorite-amphibole ratio varied greatly between rock types. The degree to which chlorite replaces amphibole is a rate process controlled by such factors as temperature,

H2O partial pressure, availability and mobility of ions, and by the possibility of mutual interference of co-mineral reactions. Thus, when microchemical conditions are favorable, the reaction rate is stepped up and the chlorite- amphibole ratio (or the degree of which chlorite has replaced hornblende) is high. In rock specimens indicating even higher temperatures of formation hornblende reacts to form actinolite-tremolite, a factor that might result in a low chlorite-amphibole ratio. In no instance was amphibole observed replacing chlorite. This might be an indication that amphibole does not necessarily signify a prograde metamorphic facies progression formed at the expense of chlorite. Instead, the amphibole is felt to be relict primary or a seconday reaction involving pyroxene.

Muscovite: Muscovite in the Dick rhyolite and trondjhemite probably formed much the same way as the quartz-muscovite schists studied by Gresen and Stensrud (1974) in New Mexico.

They showed that muscovite schists formed as a result of hydrogen metasomatism during the regional metamorphism of rhyolitic rocks. Alkali feldspar normally comprise a major percentage of granitic rock types; however, none was observed in thin section in the thesis area. Widespread hydrogen metasomatism is believed to have accompanied regional metamorphism which converted nearly all the alkali feldspar into muscovite. The chemical relationship governing the alkali feldspar-muscovite reaction is identical to that described by Hemley and Jones (1964) excepting that the conditions are those of regional dynamothermal metamorphism rather than hydrothermal alteration. The equation for this is:

Equation (2) (Hemley and Jones, 1964)

3 Kspar + 2H+ ---> sericite + 2K+ + 6 quartz

The reaction equilibrium at the higher temperatures of metamorphism is almost entirely dependent upon the H+/K+ 57 ratio. This reaction also helps to explain the sometimes over abundance of quartz observed in many thin sections.

Muscovite and chlorite were formed during recrystallization of the tuffaceous matrix of metasedimentary and metavolcanic rock. Presumably the matrix was composed initially of some amount of volcanic glass. Microcline, orthoclase and perthite surrounded by a matrix of muscovite are abundant in many of the felsic metasediments east of the Dick rhyolite.

The alkali feldspar grains remained unaltered either because of their larger grain size and slower reaction rate or the

H+/K+ ration was not altered significantly enough to affect their stability. The later case would not necessarily inhibit the formation of muscovite in the matrix.

An indeterminant amount of muscovite probably occurred as primary minerals in some acid igneous rocks which remained stable during regional metamorphism. This is suggested by the rare occurrence of porphyritic muscovite in the intrusive metarhyolite unit.

Quartz-sericite schists were presumably formed during hydrothermal alteration prior to regional metamorphism.

Hydrothermal alteration at the Dick mine and Copper Queen mine was said to involve hydrogen-potassium metasomatism and the removal of iron and magnesium (Bake and Clayton, 1968). 58

The iron was not entirely removed, but partly fixed by sulfur presumably carried by the hydrothermal fluids to form pyrite. Late alteration of andalusite found in thermal metamorphic hornfels and regionally metamorphosed rocks produced late sericite replacement of andalusite.

Replacement sericite is fine-grained and unoriented having formed after regional metamorphism and the later thermal raetamorphism (see Photo 11).

Amphibole: The amphibole species observed in thin section include hornblende, pargasite, tremolite and actinolite.

Andesine, epidote, zoisite, magnetite, garnet and calcite are most commonly associated with amphibole bearing rocks.

Amphibole bearing rocks are strongly granoblastic in texture with euhedral to subhedral grain development. In some instances amphibole is elongated with the C-axis more or less parallel with the plane of foliation. The lineation might also reflect relict flow structuring.

The hornblende species is biaxial (-) with 2V alphas between

30° and 75°. Its pleochroism is a blue-green, green to light yellowish-green. Hornblende occurs in two distinct habits. One habit is as larger ragged-edged, irregular grains partly replaced by chlorite, magnetite and epidote.

This texture is felt to represent remnant hornblende which had not reacted completely to form chlorite during regional metamorphism. Hornblende also occurs, along with pargasite and tremolite, as small, euhedral prismatic crystals formed during thermal metamorphism. Rocks with recrystallized hornblende are strongly granoblastic in texture (see

Photo 17)

Pargasite is biaxial (+) with 2V alphas measured between

101° and 116°. The pleochroic scheme is variable because of variation in iron content. The pleochroism generally ranged from very light green (alpha), clear to bluish-green (beta) and light to dark green (gamma).

The pargasite-hornblende reaction observed in thin section is accompanied by the release of iron which forms abundant inclusions of euhedral magnetite grains:

Equation (3)

hornblende ---- > pargasite + Fe-oxides

Pargasite is unstable in the presence of excess quartz with which it reacts to form tremolite according to the following paragenetic reaction:

Equation (4) (Ernst, 1968)

pargasite + quartz ---* tremolite + plagioclase + minor enstatite

Tremolite replaces hornblende grains as seen in Photo 17. 60

Tremolite has 2V alphas between 75° and 85° and shows very weak green to clear pleochroism. The formation of tremolite from hornblende was probably a direct transition giving no indication of other mineral phases being involved in the transformation except possibly plagioclase. Tremolite was not observed in any of the amphibole bearing rocks east of the Dick rhyolite. This seems reasonable since the field mapping showed that only the Dick rhyolite and mapped units west of the rhyolite were affected by thermal metamorphism accompanying the trondjhemite intrusion and by a later, more intense episode occurring sometime after regional metamorphism.

Rare garnet is associated with amphibole bearing rocks believed to be formed by thermal metamorphism. Although textural evidence is lacking, garnet was probably produced from the break down of a primary iron-magnesium amphibole under conditions of elevated oxygen fugacity as suggested in the following reaction:

Equation (5) (Ernst, 1968)

ferrohastingsite ---- > magnetite + plagioclase + garnet + fluid 61

Thermally Metamorphosed Rocks

Thermal metamorphism affected numerous localities in the west central portion of the thesis area. Three hornfelsed rock types were mapped on the basis of mineralogical and other differences such as texture and color. The three types are (1) quartz-biotite-cordierite hornfels

(metatrondjhemite affinity), (2) quartz-biotite-andalusite hornfels (Dick rhyolite affinity), and (3) cordierite- anthophyllite hornfels (mafic metavolcanic affinity). The average mineral composition of these rock types is located in Appendix A.

Cordierite: Cordierite is optically positive and has no pleochroism. It occurs as clear euhedral grains and as porphyroblasts (poikiloblastic texture) with up to 50% inclusions of quartz, anthophyllite, garnet, magnetite, biotite and apatite. The ubiquitous accessory apatite suggests some element of hydrothermal alteration at one time. Conspicuously absent are plagioclase and muscovite when in the presence of cordierite suggesting some form of paragenetic reaction. One reaction which involves muscovite and chlorite in the presence of excess quartz is:

Equation (6), (Winkler, 1967)

chlorite + muscovitet quartz -----> cordierite + biotite + Al2Si05 + H20 62

This reaction would result in the formation of cordierite and andalusite together as has been observed in thin section of Dick rhyolite and metatrondjhemite affinity hornfels.

Biotite is produced mainly in the metatrondjhemite affinity hornfels which has higher content of iron and magnesium.

The reversibility of reaction (6) occurs at 515° +_ 10°C and

525 _+ 10°C at 1,000 bars and 2,000 bars respectively

(Winkler, 1967).

The co-production of anthophyllite and cordierite is demonstrated in the reaction:

Equation (7), (Akella, et al., 1966)

Al-rich chlorite + quartz ---- > anthophyllite + H20 + cordierite

The anthophyllite-cordierite assemblage is formed exclusively in the mafic volcanic affinity hornfels. Quartz would have had to be added to the reaction by some external source or by release from co-mineral reactions. The equilibrium data for equation (7) was determined to be 550°

+ 10°C and 560°C +_ 10°C with H20-pressures of 1,000 bars and

2,000 bars respectively by Akella, et al., (1966).

Andalusite and Sillimanite: A significant percentage (12% average) of Dick rhyolite affinity hornfels is composed of analusite. Andalusite is a co-product of the cordierite reaction in equation (6) at up to 525°C or more. At higher temperatures sillimanite is formed instead of andalusite.

Andalusite grains are clear when unaffected by retrograde alteration and occurrs interstitial with quartz, cordierite and sometimes plagioclase, although plagioclase is suggested to be a reactant mineral and therefore rare. Most andalusite has been replaced by retrograde metamorphic ser icite.

Sillimanite is found in the quartz-biotite-cordierite hornfels (metatrondjhemite affinity). Sillimanite forms as minute needles known as fibrolite and is observed to grow within muscovite and cordierite grains. Andalusite converts to sillimanite at 640*“ and at 655^ with 2,000 bars and 3,000 bars of pressure, respectively (Winkler, 1967). The quartz- biotite-cordierite hornfels appears to have formed at a higher temperature of thermal metamorphism than the quartz- biotite-andalusite hornfels where andalusite is the stable alumino-silicate polymorph.

Anthophyllite: Anthophyllite is quite distinct growing in

large, radiating blade-shaped grains up to 13 millimeters in

length. These grow commonly in clusters to form bow-tie

structures (ocellar texture). It occurs intergrown with

cordierite, magnetite, garnet, and quartz in the mafic

volcanic affinity hornfelsed rocks. Anthophyllite has a 2V-gamma optical angle between 104° and 109° and is non- pleochroic. The high 2V angle would indicate a very high magnesium content relative to iron and manganese which may

also substitute easily for magnesium. Anthophyllite formed

co-genetically with cordierite as presented in equation (7).

Biotite: Well-formed, clear flakes of biotite sometimes

form a significant amount of the rock for hornfels having a

metatrondjhemite or Dick rhyolite affinity. Biotite occurs

associated with staurolite, cordierite or andalusite.

Biotite is a product of equation (7) and (8). Biotite may

als; have formed directly from chlorite, however, this was

not observed in thin section. Instead chlorite replaces

biotite probably as a result of late stage retrograde

metamorphism.

Magnetite: Cordierite, anthophyllite, and biotite all

contain above average amounts of magnetite inclusions. It

shows an unusual arrangement within the cordierite crystal

which suggests some kind of mutual interference during

crystal growth. The non-pleochroism of cordierite and

abundant magnetite inclusions, suggest that the cordierite

is rich in magnesium. Iron, which normally forms a solid

solution with magnesium, in cordierite is believed to have

exsolved under a condition of high oxygen fugacity existing

during thermal metamorphism. This condition tended to 65 exsolve the iron from crystal lattice structures where iron might normally be incorporated. The iron became fixated by oxygen to form magnetite. The fact that anthophyllite, a magnesium rich amphibole, and biotite both contain abundant magnetite inclusions lends support to the theory. It is of further interest to note that magnetite inclusion in pargasite and chlorite suggests a similar condition existed during regional metamorphism. Magnetite is also reported in the gangue minerals of the Old Dick and Copper Queen mines

(Baker and Clayton, 1968).

Staurolite: Staurolite has clear to yellow pleochroism and occurs as minute interstitial grains in quartz-biotite- andalusite hornfels. The controlling mineralogical reaction is believed to involve chlorite and muscovite:

Equation (8), (Hoscheck, 1967)

chlorite + muscovite ----- > staurolite + biotite + quartz + I^O

The equilibrium data for this reaction is set at 540° 15°C at 7,000 bars H2O pressure and 560° +_ 15°C at 7,000 bars

(Hoscheck, 1967) .

Chlorite and Sericite: Chlorite occurs in various minor amounts usually replacing biotite, muscovite, anthophyllite, cordierite and accessory garnet. Chlorite and sericite 66 replacement is believed to represent a period of retrograde metamorphism or hydrothermal alteration which affected the hornfelsed rocks. Pseudomorphic chlorite may entirely replace anthophyllite preserving the bow-tie structure of the anthophyllite (see Photo 4). Cordierite is partly replaced at grain boundaries and along cleavage planes by a greenish colored, felty mixture of sericite and chlorite (or sometimes serpentine) known as pinite. Minor garnet noted in anthophyllite-cordierite hornfels and in quartz-biotite- andalusite hornfels is partly replaced by chlorite along fractures or veining which cut through the garnet.

Andalusite is almost entirely replaced by a felty matt of sericite (see Photo 11).

The retrograde metamorphic activity represented by the chlorite, pinite and sericite alteration required the addition of water to the hornfels. It is not certain whether retrograde metamorphism was the result of on-going regional metamorhpism or hydrothermal alteration. The replacement fabric suggests the latter case since no schistocity defined by chlorite and sericite was produced.

Those that are oriented are pseudomorphic replacements of anthophyllite by chlorite.

Tourmaline and Apatite: These accessory minerals are found particularly in the anthophyllite-cordierite hornfels and 67 quartz-biotite-andalusite hornfels showing evidence of retrograde metamorphic alteration. No thin section of the quartz-biotite-cordierite hornfelsed (metatrondjhemite affinity) appeared to show development of retrograde metamorphism except for the partial replacement of cordierite by pinite.

The formation of tourmaline and apatite is evidence that metasomatism accompanied a period of hydrothermal alteration which produced the retrograde metamorphism of the hornfels.

In the case of tourmaline the source of metasomatic fluids is linked to that of tourmaline bearing pegmatite sills and dikes which are in turn associated with a Precambrian batholith intruded into the Yavapai series southwest of the thesis map area. The pegmatites intrude both the hornfels rocks and regionally metamorphosed rocks and are relatively undeformed, presumably post- or late regional metamorphic in age. Tourmaline is best developed in anthophyllite- cordierite hornfels in areas where pseudomorphic replacement by chlorite is extensive (see Photo 4).

No indication of the paragenesis of apatite could be determined from thin section observations. It is conceivable that apatite was introduced during some event prior to thermal metamorphism such as a period of hydrothermal alteration which accompanied the formation of 68 massive sulfides.

Metamorphic Facies

Greenschist facies regional metamorphism is indicated by the dominant quartz-albite-epidote-chlorite-muscovite assemblages in nearly all rock types. The formation of biotite rather than stilpnomelane and the universal absence of garnet are indicative of low-pressure or Abukuma-type facies series. This facies is usually developed in metamorphic terrains which experienced a low pressure and a high geothermal temperature gradient. A depositional environment with a high degree of volcanic activity as reflected by the Bridle formation and Butte Falls tuff automatically assumes a high geothermal gradient which normally is associated with a volcanic province.

The temperature of greenschist facies metamorphism begins around 360° to 390°C at relatively low pressures to about

550°C whence rocks of the amphibolite facies begin to form

(Winkler, 1967). The occurrence of biotite in coexistence with muscovite and chlorite, and associated actinolite, epidote, and albite minerals corresponds to Winkler's quartz-albite-muscovite-biotite-chlorite subfacies. This subfacies represents a lower temperature greenschist subfacies than the quartz-albite-epitode-chlorite-muscovite 69

subfacies.

Higher greenschist facies temperature was suggested in some

rocks containing oligoclase or andesine associated with

epidote, actinolite, hornblende and chlorite of basic

metavolcanic rocks. No regional metamorphic facies changes

could be located in the course of the geologic mapping or

thin section analysis. The area is affected by locally

intense thermal metamorphism which may have altered or

interfered with any regional metamorphic facies pattern.

Hornblende hornfels facies and incipient higher temperature

orthoclase-cordierite hornfels facies were produced during

the late thermal metamorphism. In the hornblende hornfels

facies, cordierite is formed from chlorite, muscovite, and

quartz at 515° to 525°C with 1,000 and 2,000 bars of

pressure (Akella et. al., 1966). Calcic plagioclase is the

stable feldspar and epidote is absent. The higher

temperature, orthoclase-corierite hornfels is realized in

some thin sections by the first appearance of sillimanite

(fibrolite) and by a decrease in the amount of muscovite in

the rocks. Sillimanite is formed in place of andalusite

above approximately 640° and 655°C at pressures of 2,000 and

3,000 bars of pressure, respectively (Winkler, 1967). These

are taken to be minimum temperatures of formation of the

respective hornfels rock types. 70

A weak period of retrograde metamorphism affected the thermally metamorphosed rocks. Anthophyllite and garnet were replaced by chlorite, cordierite by pinnite and andalusite by sericite. 71

CHEMISTRY

During regional dynamothermal metamorphism, mineral phases recrystallize to form new mineral assemblages in order to achieve thermodynamic equilibrium in response to progressive changes in pressure and temperature. A fundamental principle in regional metamorphism is that the bulk chemistry of the rock remains relatively constant, thereby defining a closed system. In a closed system, material is neither gained nor lost with the exception of fluid phases which tend to migrate freely under influence of temperature and pressure gradients.

In the thesis area, there are essentially two chemical groupings of rock types which upon metamorphosm produce distinctly different mineral assemblages. One group consists of basaltic and andesitic metavolcanic rocks, while the other group consists of granitic rocks such as the Dick rhyolite and metatrondjhemite.

X-ray spectrochemical analysis was performed on the basic to intermediate metavolcanic rocks (see Appendix B), metatrondjhemite (see Appendix C and D) and the Dick rhyolite (see Appendix E) in order to determine their approximate chemical compositions. The X-ray analysis does not distinguish between ferric and ferrous ions, therefore, ACF and AKF diagrams often used to relate the bulk chemistry to metamorphic mineral assemblages could not be used for this study. A comparison of the chemistry with that of similar, unmetamorphosed rock types provided some valuable observations nonetheless.

Basic to Intermediate Metavolcanic Rocks

Chemical analysis was performed on six samples of metavolcanic rocks to provide some idea of the range of compositions within the Bridle formation. The results of the chemical analysis are presented in Appendix B. It is noted that sample #E-7 was collected from the series east of the Dick rhyolite and is considerably more silicic.

Sample #W-16 was collected west of the Dick rhyolite and appears to be a metabasalt. This reflects in part the field observation that the metavolcanic rocks east of the

Dick rhyolite consist of interbedded silicic units in comparison with those west of the Dick rhyolite, which contain more basic volcanic interbeds.

An approximate bulk chemical composition for metavolcanic rocks west of the Dick rhyolite was determined from a single analysis using a sample prepared from twenty-five individual samples. The individual samples were carefully selected at fifty-foot intervals across exposures west of the Dick rhyolite. Altered rock types, quartz veining and foreign rock materials were avoided. Each sample was reduced to a powder and equal portions weighed out and blended into one sample. This method provided a useful approximation of the bulk chemistry of the rock.

The metavolcanic rocks in the Bagdad area show some chemical relations to both andesitic and basaltic parent material (see Appendix B). Like andesitic rocks, the metavolcanic rocks in the Bagdad area tend to be relatively high in silica (50% to 66%) and low in calcium and titanium. Those having a basalt parent affinity are indicated by low aluminum and potassium and high iron and magnesium. Mineralogically, metabasalts contain abundant amphibole and chlorite minerals while meta-andesites are abundant in plagioclase and sericite minerals.

The use of cation ratios rather than weight percent of oxides was introduced by Barth (1948) which has the advantage of comparing cations in mineral reactions that are essentially iso-volumetric. Since oxygen accounts for almost all volume considerations, the total number of oxygen atoms too, remains more or less constant. Thus, using cation ratios, the total iron and magnesium is compared with the alkalis and aluminum as shown in

Figure 5. In comparison to the average unmetamorphosed basalt, the metavolcanic rocks of the Bagdad area are considerably lower in aluminum and have a higher proportion of combined iron and magnesium. The iron and magnesium content is interpreted as either initially very high or more likely, iron and magnesium was introduced to the rock during regional metamorphism. The lower amount of aluminum in the metamorphosed basalts and andesites could represent substitution of magnesium and ferrous iron for octahedral aluminum brought on by metasomatism.

However, the parent volcanic rock may have been low in aluminum initially, reflecting an AKF plot within a basaltic rather than the andesite field shown in

Figure 5. Anderson (1968) concluded that the same type of metasomatism affected the metamorphosed Precambrian silicic rocks in central Arizona. He attributed the enrichment to migration of iron and magnesium from nearby basic volcanic rocks during regional metamorphism. The source of iron and magnesium enrichment in the basic and

intermediate metavolcanic rocks in the Bagdad area is

speculative, however. The source could have been from

underlying basic rocks brought up to a higher

stratigraphic level during metasomatism.

Granitic Rocks

Chemical analyses of metatrondjhemite, contaminated

metatrondjhemite and Dick rhyolite are presented in

Appendices C, D, and E, respectively. Sample R 75

(Appendix E) is an approximate average bulk chemical

composition of the Dick rhyolite prepared in the

identical manner as described for the average metavolcanic rock of the Bridle formation from Appendix

B. A comparison of the x-ray chemical analyses was made with the average composition of alkali granite to which

the metatrondjhemite and Dick rhyolite are

petrographically related.

The chemical difference between the trondjhemite and the

rhyolite is subtle. This fact, and the similarity in

texture and mineralogy as discussed previously, suggests

a comagmatic relationship between the two. They both

contain identical rounded, bluish-colored quartz

phenocrysts in a fine-grained quartz-albite matrix. The

main mineralogical difference between the trondjhemite

and the rhyolite is in the relative amount or proportion

of muscovite (including sericite) and chlorite, and in

the occurrence of plagioclase as phenocrysts in the

trondjhemite. It was hoped initially that the chemical

analyses would bear out a comagmatic relationship, but

this was not the case. Instead, the analyses pointed to

a redistribution and/or loss of alkali cations and

enrichment in iron and magnesium to the Dick rhyolite and

metatrondjhemite not explainable by fractional

crystallization alone. 76

If we assume that the trondjhemite and rhyolite each represent end stages of magmatic crystallization, then initially the trondjhemite and rhyolite should have had a bulk chemical composition similar to that of the granite system. However, the metamorphosed granitic rocks of the

Bagdad area show enrichment in iron and magnesium relative to the average alkali granite. The granite contains 2.04% total iron (as Fe203) and 0.26% magnesium compared to 4.87% and 1.03% for the metatrondjhemite and

4.40% and 2.07% for the Dick rhyolite.

Rocks which crystallize in the granite system end up in a residual system in which quartz, albite and orthoclase crystallize out in a normative ratio of approximately 1/3 each (Tuttle and Bowen, 1958). Anderson (1968) made this same assumption when he compared the chemistry of metamorphosed Precambrian rhyolitic rocks in central

Arizona with that of young unmetamorphosed obsidians of low-water content. If the assumption is correct, then the trondjhemite and rhyolite should reflect a composition which would fall within the

NaAlSi308-KAlSi208-Si02 ternary system studied by Tuttle

and Bowen (1958). The ratio of sodium and potassium cations should therefore show a similarity between the metamorphosed granitic rocks and obsidian rocks, assuming

a closed chemical system exists. Figure 5 is a comparison of sodium and potassium cation ratio from the metamorphosed granitic rocks of the Bagdad area, and other Precambrian meta-igneous rocks from central

Arizona, with those of young unmetamorphosed obsidians.

The young unmetamorphosed obsidians plot in a well defined area which marks the composition of the granite eutectic minimum. The sodium and potassium cation ratios of metamorphosed rocks show a wide variation. This implies that some process later than fractional crystallization played an important part in affecting widespread chemical changes with the rock. Regional metamorphism is an obvious election; however, there are other factors such as the cooling, de-fluidizing and mixing process which commonly influence the chemistry of volcanic materials deposited in a volcanic-marine environment.

In both the trondjhemite and rhyolite, sodium and potassium were depleted relative to fresh obsidian. Six of the metatrondjhemite analyses contained higher amounts of sodium suggesting at least a partial amount of sodium metasomatism. Higher sodium content is reflected by

abundant albite in the groundmass of both rock types.

High iron and magnesium accompanied metasomatism and is

notably above average in mafic metavolcanics as well as Barth's K cations (in per cent) earnjsie f add area Bagdad of Metatrondjhsmite A eaopoe Dc rylt o h Bga area Bagdad the of rhyolite Dick Metamorphosed A O Precambrian metamorphosed rhyoiites, dacites and quartz quartz and dacites rhyoiites, metamorphosed Precambrian O on rylt osda (rm nesn 1968) Anderson, (from obsidian rhyolite Young • PLOT OF BARTH'S SODIUM AMD POTASSIUM CATIONS POTASSIUM AMD SODIUM BARTH'S OF PLOT opyis f h Yvpi uegop fo Adro, 1968) Anderson, (from Supergroup Yavapai the of porphyries IUE 5 FIGURE 7 8 79

FIGURE 6

A

o unmetamorphosed andesites from worldwide occurrences

A = 100% aluminum cations (after Barth,1948) F = 100% iron plus magnesium cations (") K = 100% sodium plus potassium cations (")

AFK DIAGRAM OF METAMORPHOSED BRIDLE VOLCANICS VERSUS UNMETAMORPHOSED BASALTS AND ANDESITES in the Dick rhyolite and raetatrondjhemite. The AKF diagram of Figure 6 shows the higher iron-magnesium to aluminum-sodium-potassium ratios of the metamorphosed

Bagdad rocks compared to the average basalt and andesite.

The alternative to Fe-Mg metasomatism is that the metavolcanics were intially more mafic than basalts being higher in Fe-Mg and lower in aluminum. The Fe-Mg metamosatism is more noticeable for the trondjhemite and the Dick rhyolite. That of the trondjhemite may be

attributed in part to partial assimilation of the mafic volcanic host rock.

The fact that muscovite is abundant and orthoclase is

almost totally absent within the trondjhemite and Dick

rhyolite points to yet another factor of metasomatism.

The formation of sericite or muscovite is controlled not

only be temperature and pressure, but also by the ionic

activity ratio between potassium and hydrogen (Hemley and

Jones, 1964). With pressure held constant, a decrease in

temperature causes the reaction of K-spar to form

sericite. The same reaction would occur if the ratio of

concentrations between H+ and potassium were increased.

The latter case can be considered as hydrogen metasomatism since hydrogen becomes fixated to the

sericite crystal lattice and potassium is released from

orthoclase. Regional metamorphism invariably involves the interaction of a fluid phase, dominantly as water, at several kilobars of pressure thus water becomes an important source of hydrogen necessary for the formation of index greenschist facies mineral namely muscovite, chlorite, and epidote. Hydrogen metasomatism in this respect is the same whether it be due to hydrothermal alteration or due to regional metamorphism. The widespread replacement of sericite or muscovite for K- spar leads to a fixation of hydrogen and the partial loss of potassium to regionally metamorphosed rocks (Gresens and Stensrud, 1974).

Major amounts of magnesium and iron in acqueous solution would tend to produce chlorite rather than muscovite.

The formation of chlorite instead of muscovite over a wide area may result in the direct loss of potassium from the system which would have normally been fixed in the muscovite. Where the potassium may have been disposed of is unknown. The source of iron, magnesium and sodium could have been from the surrounding intermediate to basic volcanic rocks. This source of iron and magnesium was favored by Anderson (1968) in the Precambrian metamorphosed granitic rocks of the Jerome area. It does not, however, fully explain the enrichment of iron and magnesium in the intermediate and basic metavolcanic rocks. There are other ways in which alkalis may be redistributed and iron and magnesium added to rocks without relying upon special factors during regional metamorphism. Other processes such as hydration of glass, cycling of connate water, hydrothermal alteration, reaction with sea water, and cooling rate variations are known to occur during the consolidation and cooling history of a volcanic pile which could have been

responsible for the chemical changes.

Anderson (1955) suggested also, that submarine alteration of the Bridle volcanic pile was a distinct possibility

and cited the formation of keratophyres in New Zealand

(Battey, 1955) as an example of this alkali enrichment.

One notable difference with the metamorphosed silicic

rocks of the Bagdad area is that they are considered to be intrusive rather than extrusive in origin. However,

if they had been intruded shallowly as suggested by their

fine-grained, porphyritic nature and shortly after the

accumulation of the Bridle formation, the trondjhemite

and Dick rhyolite might thus become part of the cooling

volcanic pile to be subjected to the same alteration,

redistribution processes, and massive sulfide mineralization that affected the volcanic pile. 83

Contaminated Metatrondjhemite

Contaminated trondjhemite is best distinguished from fresh trondjhemite by a higher proportion of muscovite and chlorite. The chlorite forms discrete folia and patches up to 5 or 10 millimeters in size believed to represent metamorphosed inclusions of Bridle formation metavolcanic rocks. Anderson, et. al. (1955) likewise recognized a "contaminated" phase which he describes as

"a light-colored foliated quartz-feldspar rock containing parallel flakes of biotite". He also described needles of actinolite partially replaced by chlorite, and granules of epidote, sphene, and blue tourmaline. The amphibole and biotite formed as clots and streaks which decrease in abundance towards the center of trondjhemite masses.

Photo 19 is an example of meta-andesite injected with trondjhemitic material occupying the light shaded areas.

The trondjhemite is identified by its rounded, bluish- colored quartz phenocrysts. Thin sections were made of this sample in order to study the mineralogical differences and petrographic differences between the

light-shaded, trondjhemitic areas and the dark shaded meta-andesite areas of the rock. The following relative proportions of minerals were noted: 84

dark patches light patches quartz 8% 21 plagioclase 15 42 hornblende 70 15 actinolite 5 20 magnetite 2 2

100 100

The plagioclase in the light shaded, trondjhemite areas is often reverse zoned. The core averages around An38 and the rim around An43. Normally zoned plagioclase grains are also present. Plagioclase grains within the dark shaded meta-andesite areas are unzoned. The intrusion of trondjhemite is believed to have assimilated a portion of the mafic wall rock which resulted in a more calcic rim of plagioclase being deposited around a sodic nuclear grain and by the secondary growth of actinolite which required the addition of iron and magnesium.

It was hoped initially that the chemical analyses would provide some indication as to the amount of contamination

from assimilation of intermediate to basic metavolcanic

rocks. Chemical analyses of trondjhemite and

contaminated trondjhemite are presented in Appendix C and

D, respectively. On the average, contaminated

trondjhemite contains less silica and potassium and

slightly more iron, magnesium and sodium than does m

uncontaminated trondjhemites. If the slight increase in iron and magnesium, as reflected by the higher percentage of chlorite (and sometimes actinolite) in the mode, is taken to represent assimilation of the Bridle formation, then up to 15% to 25% of metavolcanic material would have been needed in order to affect the chemical changes. The problem with this reasoning is that none of the other major oxides appear to be added or subtracted in that same proportion. In fact, the contaminated trondjhemite contains less calcium and more sodium than the uncontaminated trondjhemite which is the reverse of what would be expected from assimilation of rocks of basic composition. Assimilation probably occurred by partial resorbtion in which only certain minerals and oxides were resorbed by the trondjhemitic melt. Assimilation would appear to be less of a factor in affecting the chemistry than in the iron and magnesium metasomatism which apparently affected all the rock types in the Bagdad area. It could be that the streaky chlorite patches are metasomatic in origin as discussed in the preceeding section by hydrogen metasomatism.

There is sufficient petrographic evidence to conclude that material derived from the Bridle formation became mixed in with the trondjhemite, but the chemical resorbtion or assimilation which might have altered the chemistry of the trondjhemitie, played a negligible or minor role in the process.

SUMMARY OF METAMORPHIC HISTORY

Mafic and intermediate volcanic flows, volcaniclastic sediments and epiclastic sediments were deposited presumably in an intermittent subaerial-marine environment similar to a modern day island-arc situation.

Lesser amounts of felsic volcanic material are interbedded near the top of the Bridle formation which reaches a maximum thickness of some 2,150 meters in the thesis area. Sometime after accumulation the volcanic pile was intruded by fine-grained, porphyritic Dick rhyolite and trondjhemite emplaced relatively near the earth's surface. Little record remains of the chemical or mineralogical changes that affected the volcanic host rock as a result of the intrusions. Some effects of contact metamorphism produced by the trondjhemite remain preserved. The trondjhemite is somewhat different than the Dick rhyolite in being a little coarser grained and by having assimilated and "soaked" the Bridle mafic volcanic rocks. The trondjhemite presumably cooled more slowly than the rhyolite which was intruded stratigraphically higher than the trondjhemite. A period of hydrothermal alteration accompanied by ferromagnesian metasomatism and massive sulphide accumulation, is postulated to have occurred prior to regional metamorphism and perhaps prior to the intrusion of the prophyritic Dick rhyolite and trondjhemite. The timing of hydrothermal alteration is uncertain but seem likely to have occurred during de-gasing and hot water circulation within the cooling volcanic pile. The rocks were later buried and folded thereby initiating regional metamorphism.

Regional dynamothermal metamorphism produced greenschist facies mineral assemblages characterized by albite, chlorite, muscovite, epidote and sometimes amphibole and biotite minerals. A low-pressure, high temperature facies is suggested in part by the mineralogy and in part by the commonly higher geothermal gradient in areas of volcanic activity. The chemistry of the volcanic rocks became altered either during hydrothermal alteration or some other redistributive, metasomatic process prior to regional metamorphism or as a result of the regional metamorphism. A high oxygen fugacity situation is believed to have existed during regional metamorphism as evident by the formation of magnetite in paragenetic reactions involving ferromagnesiam minerals, namely pargasite and chlorite. The temperature-pressure range of the low-pressure greenschist facies series is from

200°C to 600°C and from 1 kilobar to 4 kilobar pressure. These pressures correspond to depth of less than 15

kilometers.

The grandiorite gneiss intruded the upper part of the

Bridle formation after the Dick rhyolite and trondjhemite

and before or during regional metamorphism. Nothing in

the way of contact thermal metamorphic effects from the

granodiorite were observed.

An episode of thermal metamorphism produced three

variations of hornfelsed rocks located in the central and

southwestern sections of the thesis area. The hornfels

overprints the schistose fabric thereby post-dating the

regional metamorphism. Hornfelsed bodies formed as

lensoidal masses cutting locally across the structural

grain. Minimum temperature of formation for the

cordierite-andalusite bearing assemblages is in the range

of 515°C and 550°C where cordierite begins to form.

Sillimanite forms in place of andalusite in the quartz-

biotite-cordierite hornfels at a minimum temperature of

640°C at 2,000 bars of pressure. The cordierite-

anthophyllite hornfels is also known as dalmationite

noted in massive sulfide terrains in Canada as having

formed from thermal metamorphism of chlorite schists

representing pre-metamorphic zones of hydrothermal

alteration. The source of heat for thermal metamorphism could not be determined from this investigation. Anderson attributed sillimanite bearing hornfels north of the thesis area, to proximity with the Lawler Peak granite which is younger than the granodiorite gneiss. No relationship between the hornfels bodies and the source of heat could be satisfactorily determined in the thesis area. It could be from an underlying igneous batholith not exposed at

the surface. It could also be that thermal metamorphism was telescoped from some distance along pre-existing hydrothermal alteration zones now represented by chlorite

schists.

Late, hydrothermal alteration promoted retrograde

metamorphism noted mainly in the hornfelsed rocks. The

hydrothermal alteration was characterized by boron

metasomatism which resulted in the formation of

tourmaline associated with retrogracke chlorite (see

Photo 4). The source of boron is believed to be from

tourmaline rich pegmatites occuring in the vicinity and

within the hornfels. The pegmatites were derived locally

from a Precambrian granite batholith located west of the

thesis area. Pegmatites and hydrothermal alteration

represent the last major noticeable event affecting the

petrology of Precambrian rocks in the thesis area. 90

Post-metamorphic movement on the Mountain Springs fault brought the Bridle formation and Dick rhyolite into a structural discordant position against the Hillside mica schist and the granodiorite gneiss. 91

THE MASSIVE

SULFIDE MINERALIZATION

Inferences presented here about the nature of massive sulfide mineralization stem mainly from the literature

(Baker and Clayton, 1968; Anderson, 1950; and Anderson, et. al., 1955) since no effort was made to study the ores themselves. An attempt is made, however, to understand ore genesis based in part on the understanding of its metamorphic history.

The massive sulfides at the Old Dick mine, the Bruce mine, and the Copper Queen mine are described as banded, strata bound, metal zoned deposits of massive iron, lead, zinc and copper sulfide minerals. A schistose fabric more or less parallel to the bedding and sulfide banding is assuredly of

metamorphic origin. Folding and crinkling of the banded

sulfides is also present, further suggesting metamorphic

deformation of the ores. If this be the case, then ore

deposition took place prior to regional metamorphism.

An epigenetic origin for the sulfide ore bodies associated

with the intrusion of the Dick rhyolite is a viable

arguement since most of the known copper, zinc and lead

occurrences (see Figure 2) are located at or near the

contacts of this rock. 92

The model for the intrusion of the Dick rhyolite and porphyritic trondjhemite presumes a shallow, near-surface

emplacement shortly after the deposition of the volcanic

pile represented by the Bridle formation. If this is true,

the age of any epigenetic origin for the ore related to the

Dick rhyolite approaches that of a syngenetic origin which

has often been stated to include some period of time after

deposition but before consolidation of the host rock.

The view taken herein is that the ores represent a form of

syngenetic-epigenetic origin such as has been proposed for

other volcanic, massive sulfide belts such as the California

Sierra Foothills (Kemp, 1977), the Abitibi volcanic belt in

Canada (de Rosen-Spence, 1969), and the Precambrian Ash

Creek Group in the Jerome-Prescott area (Anderson and Nash,

1972). The Ash Creek Group has an age of formation very

close to that of the Bridle formation (a minimum of 1,725 +.

200 m.y. Mauger, 1973).

The California Sierra Foothills represents a metamorphosed

Mesozoic pile of island-arc volcanic flows and pyroclastic

sediments deposited in a marine environment. Composition

ranges from basic to felsic volcanics with the massive

sulfide deposits having formed as a result of exhalations

and hot water circulation which migrated through the

volcanic rocks. Sulfide ores were precipitated where gross bulk chemical compositions are contrasted by interbedding of basic and felsic rock types (Kemp, 1977). A contrast in the bulk chemistry composition exists between the Dick rhyolite

and Bridle formation to form a similar condition in the

Bagdad area. If bulk chemical contrast in host rock is pre­

requisite to forming massive sulfide deposits then the ores were formed after the intrusion of the Dick rhyolite. This

does not necessarily mean an age of formation equivalent with that of the rhyolite but rather after its intrusion.

The redistribution of alkalies and iron and magnesium

metasomatism affect both the Bridle volcanics as well as the

porphyritic rhyolite and trondjhemite. There is the

possibility that the chemical alterations were accomplished

prior to regional metamorphism during the cooling cycle and

consolidation of the volcanic pile which includes the Dick

rhyolite and trondjhemite. Redistribution of alkalies and

magnesium metasomatism accompanied the metamorphism of

Archean felsic volcanic rocks related to sulfide

mineralization in Canada (Bennett and Rose, 1974). Bennett

and Rose suggested that alkali-magnesium alteration patterns

are more useful guides to ore deposit than would be the

primary composition of the host rock. The apparent host

rock chemical control exercised by the Dick rhyolite in the

Bagdad area, however, cannot be overlooked. 94

A strictly syngenetic origin for the massive sulfides in the

Bagdad seems unlikely because of the spatial relationship of the ores to the contact with the Dick rhyolite. The

Pinafore mine, located 7.4 km southwest of the Old Dick mine, is located on the same stratigraphic horizon as the

Old Dick and Bruce ore bodies. The Red Cloud, Copper Queen,

Queen and Rudkins mines are located on or near the contact with the Dick rhyolite in a position stratigraphically higher than the Pinafore and Old Dick mines. How the ore bodies east of the Dick rhyolite might relate to those west of the rhyolite is as yet uncertain.

A final analogy is made with massive sulfide deposits in the

Rouyn-Noranda district in Canada situated in a metamorphosed

terrain and associated with a local rock type known as

dalmatianite. Dalmatianite occurrences are hornfelsed

bodies of cordierite-anthophyllite mineralogy believed to

have formed in pre-metamorphic zones of hydrothermal

alteration. These zones were later affected by regional

metamorphism to produce chlorite schists. The occurrence of

dalmatianite (cordierite-anthophyllite hornfels) in the

Bagdad area suggests the same pattern: hydrothermal

alteration of the volcanics involving Fe-Mg metasomatism,

followed by regional metamorphsm to form chlorite schists,

and ending with the thermal metamorphism to produce

cordierite-anthophyllite hornfels. If this is the case, 95 then the Dick rhyolite and the trondjhemite are unlikely to represent sources of hydrothermal fluids for alteration and

Fe-Mg metasomatism because of their acid chemical composition which was initially very low in Fe-Mg.

The Precambrian metamorphosed volcanic rocks in the Bridle area were correlated with those of the Jerome-Prescott area by Buiter and Wilson (1938). The host rock age data of

1,820 m.y. for massive sulfide deposits at Jerome is similar to that of the Bagdad area (Anderson and Nash, 1972).

Anderson and Nash concluded that the Jerome massive sulfides, which correlate more or less in time and stratigraphic position with that of the Bagdad massive sulfide deposits, were formed essentially as syngenetic deposits related to hydrothermal brines discharged into a submarine basin. The sulfide ores at Bagdad may be later than a strictly syngenetic origin because of restrictions placed upon their age by the intrusion of the Dick rhyolite

and trondjhemite. 96

BIBLIOGRAPHY

Akella, J., and Winkler, H. G. F., (1966) Orthorhombic amphibole in some metamorphic reactions: Contr. Miner. Petrol., v. 12, p. 1-12.

Anderson, C. A., (1950) Lead-zinc deposits, Bagdad area, Yavapai County, Arizona: Arizona Bureau of Mines and Geology, Bulletin 156, p. 122-138.

Anderson, C. A., (1951) Older Precambrian structure in Arizona: Bulletin of the Geological Society of America, v. 62, p. 1331-1346.

Anderson, C. A., (1968) Metamorphic Precambrian silicic volcanic rocks in central Arizona: Geological Society of America, Memoir 116, p. 9-44.

Anderson, C. A., and Nash, J. T., (1972) Geology of the massive sulfide deposits at Jerome, Arizona— a reinterpretation: Economic Geology, v. 67, p. 845-863.

Anderson, C. A., Scholtz, E. A., and Strobell, J. D., Jr., (1955) Geology and ore deposits of the Bagdad area, Yavapai County, Arizona: U. S. Geological Survey, Professional Paper 278, p. 102.

Baker, A., Ill, and Clayton, R. L., (1968) Massive sulfide deposits of the Bagdad District, Yavapai County, Arizona: Ridge, S. D., (editor); Ore deposits of the United States, 1933-1967, p. 1312-1327.

Barth, F. W., (1948) Oxygen in rocks; a basis for petrographic calculations: Geology Journal, v. 56, p. 50-60.

Battey, M. H., (1955) Alkali metasomatism and the petrology of some keratophyrs: Geology Magazine [Great Britian], v. 92, p. 104-126.

Bennett, R. A. and Nash, I. R., Jr. (1974), Some compositional changes in Archean felsic volcanic rocks related to massive sulfide mineralization: Econ. Geol., v. 68, no. 6, p. 886-891.

Boyd, F. R., (1959) Hydrothermal investigation of amphiboles: _in Abe Ison, P. H. (editor), Researches on Geochemistry.

Bowen, N. L., and Tuttle, 0. F., (1958) Origin of granite in the light of experimental studies in the system NaAlSi30g-Si02-H20: Geological Society of America Memoir, v. 74, 153 p. 9

Butler, B. S., and Wilson, E. D., (1938) General features of some Arizona ore deposits: Arizona Bureau of Mines and Geology, Bulletin 145, p. 9-25.

Damon, P. E., (1959) Geochemical dating of igenous and metamorphic rocks in Arizona: Heindle, L. A. (editor), Southern Arizona Guidebook II, Arizona Geological Soc., p. 16-20.

Damon, P. E., (1968) Igneous and metamorphic rocks within the Basin and Ranges of the southwest: _in_ Titley, S. R. (editor), Southern Arizona Guidebook IIl7~p. 7-21.

Dempsey, w. J., Facklen, W. D., and others, (1963) Aeromganetic map of the Bagdad area, Yavapai County, Arizona: U. S. Geological Survey, Inv. Map GP-411.

Ernst, W. G., (1968) Ampiboles, Springer-Verlaq Inc., New York, 125 p.

Gastil, G., (1958) Older Precambrian of Diamond Butte quadrangle, Gila County, Arizona: Geological Society of America Bulletin, v. 69, p. 1495-1513.

Gilbert, M. C., (1966) Synthesis and stability relationships of ferropargasite, American Journal Science, v. 264, p. 698-742.

Greenwood, H. J., (1963) The synthesis and stability of anthophyllite: Journal Petrology, v. 4, 317-351.

Gresens, R., and Stensrud, H., (1974) Recognition of more metarhyolite occurrences in northern New Mexico and a possible Precambrian stratigraphy: The Mountain Geologist, v. 11, no. 3, p. 109-124.

Jaggar, T. A., Jr., and Palace C., (1905) Description of Bradshaw Mountains quadrangle, Arizona: U. S. Geological Survey, Geological Atlas, folio 126.

Kemp, Wayne (1977) The Foothills copper-zone belt Sierra Nevada, California-a massive sulfide province: Geologic Society of America, Abstracts with Program, v. 8, no. 3, p. 387-388.

Mauger, R. L., and others, (1965) Isotopic dating of Arizona ore deposits: American Inst. Min. Eng. Trans., v. 232, p. 81-87.

Mauger, R. L., (1973) A geologic interpretation and sulfer isotope study of the Old Dick and Copper Queen massive sulfides deposits, Bagdad area, Arizona (abs.): Econ. Geol, v. 68, no. 7, p. 1208. 98

Nockolds, S. R. , (1954) Average chemical composition of some igneous rocks: Geological Society of America Bulletin, v. 65, p. 1007-1032.

Ransome, F. L., (1919) The copper deposits of Ray and Miami, Arizona: U. S. Geological Survey, Professional Paper 115, 192 p. de Rosen-Spence, A., (1969)Genese des roches a cordierite- anthophyllite des gisements cupro-zinciferes de la region de Rouyn-Noranda, Quebec, Canada: Can. Jour. Earth Sci., v. 6, p. 1339-1345.

Silver, L. T., (1966) U-Pb isotope relations and their historical implications in Precambrian zircons from Bagdad, Arizona: Geological Society of Am. Program, Annual Meeting, p. 52.

Volboth, A., (1969) Elemental Analysis in Geochemistry, Elsevier Publishing Company, Amsterdam, 373 p.

Wilson, E. D., (1962) A resume of the geology of Arizona: Arizona Bureau of Mines Bulletin, v. 171, p. 140.

Winkler, H. G. F., (1967) Petrogenesis of Metamorphic Rocks, 2nd edition, Springer-Verlag New York Inc., 237 p.

Photo 3 - Remnant trachytic, volcanic.-texture in intermediate metavolcanic flow (25x, crossed nichols)

Photo 4 - MetasomatJ.c tourmaline associated with retrograde chlorite replaccment of cordiorite-anthophyili.te horntels 1 01

Photo 5 - Unsorted epiclastic metasediment with foliated chlorite matrix (25x} plain light)

Photo 6 -• Segregated mineral layering in laminated met a sedimentary rock (25x, crossed nichols) 102 .10 3

Photo 9 - Fine-grained, weakly foliated Dick rhyolite and embayed quartz phenocryst (in extinction) (25x, crossed nichols)

with quartz phenocryst (25x, crossed nichols) 104

Photo 11 - Retrograde sericite replacing andalusite in quartz- biotite-andalusite hornfels (lOOx, crossed nichols)

Photo 12 - Ocellar texture of actinolite .in contaminated metatrendjhemite (lOOx, crossed nichols) Photo 13 - Quartz and albite phenocryst in metatrondjhemite

Photo 14 - Strained quartz phenocrysts in metatrondjhemite. Compare the groundmass development with that of the Dick rhyolite in Photo 8 (2Sx» crossed nichcls) Photo 15 - Ophitic texture (hornblende enclosing plagioclase) in metadiabase (25x, crossed nichols)

Photo 16 - Penninite, iron oxides and sphere in chlorite schist 107

Photo 17 - Homfelsic fabric associated with tremolite-actinolite replacement of hornblende (lOOx, plain light)

Photo 18 - S7 crenulation of sericite in felsic metavolcanics sediment. Dark rounded grains are garnet. Angular porphyroblasts are replacement chlorite and quartz (25x, plain light) u

vucW tfW j k m , ft

iwe-ktia ■salt'

J

Photo 19 - Cut specimen of basic metavolcanic rock injected with trondjhemite (light areas) nb. e t i s a r a p ./ b rn o h e t i l o r u a t s l e t lli y h p o h t n a e t i c i r e s scov, u m Ci! tofrt s i rt o 170 N3 J3 o ro cr e s la c o i g a l p tn »-r; !S po If? c 1 , ry ji 9' P3 H* 'rt pr ! C i« h- rt H* {—i la irj rt o cn P> o rt| o i s o 1 P X a * a o rt X) r> v--.i ig H* H* -j .. n Cj rtj Q \ i Ih H» ’5 rt H* P3 rt ! it) ! rt is !rt :t rzl CD- rt rt rt rt N ^ j : p. Ih s CD 03 s rt rt CD j ^ 1 n> rt rt 1 3 H* rt n> O { j® ro‘ ..j, It CD . s [ H3 rt CD '0 j { ro i i O, 1 O | ! it j i CD A v e r a g e M i n e r o l o g y o f M a p p e d R e e k U n i t s 1 s j | . h - r _ b a s ic rt ... rt r^ to rt N. On f—1 k * ’ rtl NOrt rt 4>- tn metavolanics { L_ _ rt-rt rt CO rt to interm ediate hj H rt rt -fh to rt ti -Ph ON metavolcanics rt ON NO c h l o r i t e eo H CO COto rt O s ‘ s c h i s t I rt rt rt rt j H to NO NO f e l s i c rt rt- hi w* H !-< IO[5- M o ___ — metavolcanics rt rt rt rt rf . 1 N3 rt ON hi M rt 1-1 fO to ON to Dick rhyolite I 1 1 rt rt rt- 1 NO tnl to to to rt to metatrondj. j rt rt NO tn i hi rt S i—*| rt contam . i S rtieo cc ri ONH* m etatrondj. __ i__ I r*t rt rt ■ts rt . 1 j N3 cord. anth. ! ! hi ri H rt O -1 H N:j en . ! ”! I —j h o r n f e ls ! NO i r Jj j tn cjtz-biot-cord ! tCj tOj Nj>j toj MNJ O 1 i h o r n f e ls 1 i“T rt | r*- r t \ i | ON quartz-andalu- ! j r; f hi H :o rt ON j ON CO1 h" • t n i | | cite hornfels : i ! J _j _ pr i... ! ! j i i 1 I ! k H 1 :A \m ! | m etagabb ro i__L_. — __J I l _ __ n r1 H i J1 { i ! ! , 1 ! r" rt

APPENDIX B

Chemical Analyses of Mafic to Intermediate Metavolcanic Rocks

Avg„ Meta- Ande-- Ba­ Sample No. 319 317 E-ll E-7 W-16 99 volcanic*: site** salt*:

Si°2 50.65 53.75 50.85 66.10 44.60 53.50 53.90 54.20 50.83

Ti02 1.38 1.05 1.59 0.58 0.96 ND 0.97 1.31 2.03

a i 203 11.65 14.08 13.44 14.52 14.44 14.14 12.37 17.17 14.07

EFe203 18.44 9.60 12.88 7,30 16.75 14.25 14.74 9.60 12.88

MgO 5.62 4.91 6.33 1.98 12.75 8.98 7.84 4.36 6.34

CaO 8.66 8.93 4.47 2.12 5.51 4.34 6.28 7,92 10.42

Na20 1.84 3.21 5.10 4.09 0.83 3.42 2.60 3.67 2.23 k 2° 0.35 0.66 1.08 2.66 2.22 0.45 0.39 1.31 0.82

Total 38.59 96.19 95.74 99.35 98.05 99.08 99.09 99.54 99.62

♦sample prepared from 25 selected samples of metavolcanic rocks within the Bridle formation west of the Dick rhyolite

**data from Nockolds (1954) 1J1

APPENDIX C *

Chemical Analyses of Metatrondhjemite

Average Computed A. Ik a ii Sample No. 367 105 48A 77 5 205 Average G ranite1

Si02 75.20 76.20 74.40 78.30 75.50 76.60 76,10 73.86

11.33 12.82 11.60 13.75 A12°3 11.44 11.80 11.89 10.34 ZFe203 4.98 5.36 3.86 6.07 5.84 3.08 4.87 2.04

MgO 1.18 0.95 1.29 0.92 0.98 0-87 1.03 0.26

CaO 0.15 0.72 0.33 2.13 0.58 0.84 0.79 0.72

K a 2° 3.22 5.02 3.07 3.16 4.59 5.44 4.08 3.51

k 2° 0.77 0.27 0.33 0,41 1.52 1.41 0.79 5.13

\ Total 96.94 100.32 95.17 101.33 100.24 101.06 99.26 99.27

*data from Nockolds (1954) 3 1 2

Appendix D

Chemical Analyses of Contaminated Metatrondhjemite Average Uncontam~ Carputed. j noted Sample No. 107B 42B 35 70 112 Average Metatrond,*

Si02 78.90 73.40 71.60 71.10 69.80 73.00 76.10 11.09 11.38 10.82 11.07 A L 2 ° 3 12.48 11.37 11.60 l F e 2 0 3 2.49 5.80 5.09 9.53 3.42 5.27 4. 87 MgO 3.72 4.03 2.17 0.94 2.76 2.72 1.03 CaO 0.02 0.61 0.36 0.63 1.10 0.54 0,79 Na20 3.89 3.28 5.80 5.73 6.69 5.08 4.08 K2° 0.75 0.92 0.06 0.34 0.07 0.43 0.79

Total 100.86 99.42 95.90 99.34 96.32 98.41 99.26

*£rom Table 3 Appendix E

Chemical Analyses of the Die": Rhyolite

Average Alkali Sample No. 175 R7 R31 90 KL R* Granite** 75.90 Si02 79.60 76.55 71.20 75.90 76.75 73.86 10.73 fll2°3 15.50 11.77 13.02 14.50 11.91 13.75 5.84 0.89 EFe2°3 4.43 6.18 3.28 4.40 2.04 MgO 3.47 0.35 0.34 4.87 1.37 2.07 0.26 CaO 0.44 0.02 0.25 0.26 0.17 0.28 0.72 fe2° 1.94 0.25 3.83 0.98 1.66 2.01 3.51 k2° 0.86 3.63 1.32 3.34 3.23 2.78 5.13

Total 99.18 100.10 98.49 99.85 100.11 100.20 99.27

*sarnple prepared from 19 selected samples taken across the Dick rhyolite

**da,ta from Nockolds (1954)..