Geology and mineralization in the Saginaw hill area, Pima County, Arizona
Item Type text; Thesis-Reproduction (electronic); maps
Authors Frank, Thomas Russell, 1943-
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/554066 GEOLOGY AND MINERALIZATION IN
THE SAGINAW HILL AREA,
PIMA COUNTY, ARIZONA
by
Thomas Russell Frank
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE WITH A MAJOR IN GEOLOGY
In the Graduate College
THE UNIVERSITY OF ARIZONA
1 9 7 0 STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of re quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg ment the proposed use of the material is in the interests of scholar ship. In all other instances, however, permission must be obtained from the author.
SIGNED
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below: LCLfT JOHN M/GUILBERT D aje Associate P rote as or of Geology ACKNOWLEDGMENTS
The author would like to thank Dr. John M. Guilbert, under whose direction this thesis was written, for his help and suggestions.
Sincere appreciation and thanks go to Mr. Arthur Jacobs for providing the author with drill core and geochemical information and for arranging permission with The Anaconda Company to examine diamond drill core and to select samples for study. Thanks are also due to Dr.
Richard F. Wilson for arranging financial assistance from The Univer sity of Arizona Geology Department used to defray a portion of the laboratory work.
Dr. John W. Anthony and Dr. Donald E. Livingston gave valuable advice and suggestions.
Thanks are also due to the graduate students of the Geology
Department, especially Mr. John R. Wilson and Mr. Dieter A. Krewedl, for their critical review and discussions during this thesis study.
Not the least valuable of all was the constant encouragement of my wife Charlene, without whose help this thesis would never have been completed.
iii TABLE OF CONTENTS
P age
LIST OF ILLUSTRATIONS...... vi
LIST OF TA BLES...... ix
ABSTRACT...... x
INTRODUCTION ...... 1
Previous Work ...... Purpose and Procedures . Climate and Vegetation .
Mining History .... cn cr>
GENERAL G E O L O G Y ...... 13
Sedimentary and Volcanic R ocks ...... 14 Limy Unit (K al) ...... 14 Graywacke Unit (Kag)...... 15 Shale Unit (Kas) ...... 16 Arkose Unit (K aa) ...... 16 Tucson Mountain Chaos (Ktmc)...... 17 Cat Mountain Rhyolite (Kcr)...... 18 Intrusive Igneous Rocks...... 20 Saginaw H ill Q uartz L atite Porphyry ( T s h p ) ...... 20 Saginaw Mine Quartz Monzonite Porphyry (Tsmp) . . . . 26 Q uartz Latite Porphyry D ikes ( T q l d ) ...... 34 The Calcite Problem ...... 40 Age and C orrelation ...... 41 Paleozoic Rocks ...... 41 Mesozoic Rocks ...... 43 Cretaceous Amole Formation...... 44 C retaceous-T ertiary B o u n d a ry ...... 49 Structural G e o lo g y ...... 51 Regional S tr u c tu r e ...... 51 Local Structure ...... 52 F o ld in g ...... 52 F a u l t i n g ...... 53 Brecciation ...... 58 J o i n t s ...... 61 Thermal Metamorphic Effects ...... 64
iv V
TABLE OF CONTENTS—Continued
P ag e
Vein Systems ...... 66 Q uartz V e i n s ...... 69 C alcite V e i n s ...... 77 Quartz-calcite V eins ...... 77
ALTERATION AND MINERALIZATION...... 78
A ltera tio n ...... 80 A lteration in Igneous R o c k s ...... 80 Propylitic Alteration ...... 80 Quartz-sericite Alteration ...... 86 Summary of Igneous Rock Alteration...... 88 Alteration in Sedimentary and Metasedimentary Rocks . . 88 M i n e r a l iz a t i o n ...... 90 D issem inated M in e r a liz a tio n ...... 90 Meta sedimentary Rocks...... 91 Saginaw Hill Quartz Latite Porphyry ...... 91 Saginaw Mine Quartz Monzonite Porphyry ...... 92 Quartz Latite Dikes ...... 92 Vein M in e r a liz a tio n ...... 92 Replacement M ineralization ...... 94 Skarn Z o n e s...... 94 P a ra g e n e s is ...... 95 Z o n in g ...... 97 G eochem ical A n a l y s e s ...... 104
C O N C L U S IO N S ...... 117
Pre-Saginaw Stock Interpretations ...... 117 Discussion of Volcanism and Emplacement of the Saginaw Stock ...... 119 Saginaw Stock Im p lic a tio n s ...... 121 D iscu ssio n of the Saginaw Stock C o m p o sitio n ...... 124 D iscu ssio n of M ineralization and A lte ra tio n ...... 129 P otential of the Saginaw A r e a ...... 133
REFEREN CES...... 134 LIST OF ILLUSTRATIONS
Figure Page
1. Aerial Photograph of the Saginaw Hill A rea ...... 2
2. Index M a p ...... 3
3. Geology of the Saginaw Hill Area, Amole District, Pima County, Arizona ...... in pocket
4. The Saginaw Mine Viewed from the Southeast ...... 7
5. The Papago Queen M ine ...... 10
6 . The Palo Verde Mine ...... 10
7. Saginaw Hill Quartz Latite Porphyry ...... 24
8. Least Altered Saginaw Hill Quartz Latite Porphyry in Thin S e c ti o n ...... 25
9. Saginaw Mine Q uartz M onzonite Porphyry...... 27
10. Least Altered Saginaw Mine Quartz Monzonite Porphyry in Thin S e c ti o n ...... 30
11. Geologic Cross Sections for Figure 3 ...... in pocket
12. Saginaw Mine Quartz Monzonite Porphyry D ik e ...... 33
13. Saginaw Mine Quartz Monzonite Porphyry Fragm ental Z one...... 33
14. Q uartz Latite D ik e...... 35
15. Least Altered Quartz Latite Dike in Thin Section ...... 36
16. D ike-m etasedim ent C o n ta c t...... 39
17. Correlation Chart of the Amole Formation ...... 42
18. N. 60° E . -trending Joints and S h e a r s ...... 55
19. Pebble Dike in O utcrop...... 59
20. Pebble Dike from Diamond Drill Core...... 59
v i v ii LIST OF ILLUSTRATIONS—Continued
Figure Page
21. Pebble Dike in Thin S ectio n ...... 60 *
22. Rose Diagram of 275 Joints in the Composite Saginaw H ill S t o c k ...... 62
23. M aster J o in ts ...... 63
24. Approximate Limits of Thermal Metamorphic Effects . . . 65
25. M ultiple Q uartz V e i n s ...... 68
26. Single Q uartz V e i n s ...... 70
27. Massive Quartz ...... 71
28. M assive Q uartz Showing M aster J o in ts ...... 71
29. Ribbon R o c k ...... 73
30. Expanded Sketch of Infilling Z o n e ...... 74
31. Massive Quartz Dike in Thin Section ...... 76
32. Argillic A lte ra tio n ...... 81
33. Propylitically Altered Quartz Latite Porphyry Dike in Thin S e c tio n ...... 83
34. Sericitized Saginaw Mine Quartz Monzonite Porphyry in Thin Section ...... 87
35. Sericitized Saginaw Hill Quartz Latite Porphyry in Thin S e c ti o n ...... 87
36. Paragenetic Sequence of Hypogene Minerals in the Saginaw Hill A rea ...... 96
37. D issem inated Pyrite and C h a lc o p y r ite ...... 98
38. Disseminated Chalcopyrite ...... 98
39. S p h a lerite -C h a lc o p y rite -G ale n a ...... 99
40. Pyrite-Chalcopyrite-Sphalerite-Galena ...... 99
41. T e n n a n tite -C h a lc o p y rite ...... 100 v iii
LIST OF ILLUSTRATIONS—Continued
Figure Page
42. G a le n a -C h a lc o p y rite -T e n n a n tite ...... 100
43. Pyrite-Sphalerite-Chalcopyrite ...... 101
44. Galena-Sphalerite-Chalcopyrite ...... 101
45. P yrite-G alen a-T en n an tite-C h alco p y rite...... 102
46. T e n n a n tite -S p h a le rite ...... 102
47. Sphalerite-Chalcopyrite-Pyrite-Tennantite ...... 103
48. Copper Geochemical M a p ...... 106
49. Compatibility Diagram for Propylitic Alteration ..... 128 ABSTRACT
The Saginaw area is located on the western pediment in the
southern portion of the Tucson Mountains, Pima County, Arizona.
Northeast-southwest compression during Late Cretaceous time folded a sequence of Permian limestones and Cretaceous sub
aerial sediments into a large synclinorium with a northwest-trending
axis. Associated N. 60° E. faulting of unknown nature or displace
ment also occurred. After this deformation, the Tucson Mountain
chaos and the Cat Mountain Rhyolite, dated at about 70 m .y., were
unconformably deposited on the folded Cretaceous rocks. A composite
quartz latite-quartz monzonite stock passively intruded the weakened
central portion of the synclinorium and was emplaced near the contact
between the Limy and Graywacke units. Reactivation of the N. 60° E.-
trending faults during intrusion produced strike-slip movement along the faults allowing the emplacement of porphyry along them. Subse
quent quartz latite dikes were also emplaced along these zones of
w e a k n e ss.
The area adjacent to the stock was subjected to metasomatism
and thermal metamorphism which is reflected in propylitic alteration,
silicification, and metallization plus recrystallization of the sedimen
tary rocks. Hypogene propylitic alteration is pervasive in the intrusive
igneous rocks with quartz-sericite alteration developed along strong
fracture zones.
x x i
Hypogene sulfide mineralization occurs in veins and dissemi nations in all rock types and as replacements in calcareous shales and limestones. Copper mineralization is most intense in the stock and along sheared and brecciated zones associated with quartz-sericite alteration. Lead and zinc veins and replacements are generally con fined to calcareous metasedimentary rocks. Both mineralization and alteration were caused by hydrothermal solutions derived from the cooling magma which formed the composite stock. The area immedi ately adjacent to the Saginaw stock has been thoroughly tested and no large-tonnage economic mineralization was noted. However, the pos
sibility of large replacement deposits in the Limy unit of the Am ole For mation and in the Permian limestones or disseminated mineralization
north of the Saginaw mine associated with the northward-plunging stock
still exists. INTRODUCTION
The Saginaw mine area is located on the west flank of the southernmost extension of the Tucson Mountains in secs. 1, 2, 11, and 12, T. 14 S ., R. 12 E. on the San Xavier Mission quadrangle. The study includes a roughly symmetrical area around a subcircular, 2,000- foot wide, mineralized. Tertiary (?), porphyritic composite intrusion, locally known as Saginaw Hill. The study was deisgned to better under stand the nature, distribution, and origin of mineralization, alteration, and hydrothermal activity in the area.
The Tucson Mountains are included in the Basin and Range province of the southwestern United States. The range is about 25 miles long and about 7 to 10 miles wide in the central part, narrowing at both ends. The maximum relief, from Wasson Peak to the Santa Cruz
River, is about 2,500 feet. The main part of the range trends approxi mately N. 30° W ., but the smaller portion to the south of the Saginaw
area trends about S. 20° W. (Mayo, 1968). This change in trend may have been an important factor in the complex geology of the Saginaw
area. The topography of the thesis area is very gentle, with the ex
ception of Saginaw Hill, which rises about 250 feet above the surround
ing pediment (Fig. 1).
As shown on Figure 2, the thesis area can be reached by travel
ing west on Ajo Road to its junction with Camino de Oesta, south on
Camino de Oesta to Irvington Road, and west on Irvington to Buttes
1 2
Figure 1. Aerial Photograph of the Saginaw Hill Area
Oblique aerial photograph taken at 3,000 feet of elevation from the south. The low hill in the foreground on the eastern border of the photograph is composed of Cat Mountain Rhyolite. Saginaw Hill is located in the central western portion of the photograph. ARIZONA
TUCSON
REPORT AREA
PASS
SAGINAW HILL
VALENCIA ROAD
2 M L E S ? ____ !_ SCALE
Figure 2 .• Index Map 4
Road. Following Buttes Road south for approximately 1.25 miles one
reaches Saginaw Hill.
Previous Work
The Tucson Mountains were first studied by Guild (1905), who
briefly described some of the rock types. Tolman (1909), Jenkins and
Wilson (1920), and Allen (1920) reported on the general geology of the
range and mining activities in the southern portion of the Amole mining
district. Brown (1939) published the first comprehensive report covering
all aspect of Tucson Mountains geology. His report and accompanying
maps provided a strong base for this thesis study. Bryant (1952) pub
lished a resume of the stratigraphy of the Tucson Mountains. Feth (1947)
and Bryant (1955) reviewed the Permian rocks at Snyder Hill. Britt (1955)
reported on the stratigraphy of the Twin Peaks area, and Bennett (1957)
reviewed the geology of the Sedimentary Hills. Kinnison (1958, 1959a,
1959b) has published the most complete work to date on the geology of
the southern portion of the Amole mining district. His work, together
with Brown's (1939), provided invaluable information in the preparation
of this thesis. The reader is referred to Mayo (1968) for a more complete
summary of investigations in the Tucson Mountains.
Purpose and Procedures
The study was approached with the expectation that an under
standing of the distribution of hydrothermal activity and its relationships
to the igneous intrusive rocks, alteration, mineralization, and ore con
trols would provide a more complete understanding of the porphyry copper
environment generally and of the Saginaw Hill area in particular. 5
A reconnaissance of the field area showed that there was strong mineralization at the summit of Saginaw Hill and little mineralization
elsewhere in the area. Satellitic stocks and dikes were found intruding the sedimentary rocks and in some cases the main Saginaw Hill stock.
Spotty quartz-sericite alteration was noted adjacent to quartz veins and
fracture zones. The bulk of the igneous and metamorphic rocks appeared to be altered to a propylitic-type mineral assemblage.
A random sample of rocks was collected from the area and
studied under the binocular microscope along with selected thin sec tions in order to determine field criteria to facilitate mapping.
Field mapping (Fig. 3, in pocket) was done on aerial photo
graphs at a scale of 1 inch = 300 feet and 1:62,500 topographic maps
blown up to a scale of 1 inch = 300 feet. Approximately 15,500 feet of
drill core were logged and studied in detail. One hundred twenty-five
thin sections of selected surface and drill core samples were analyzed
petrographically. Thirty-eight samples of sulfide mineralization were
processed as polished surfaces and studied under the mineragraphic microscope.
Climate and Vegetation
The climate of the Saginaw Hill area is typical of the Basin
and Range province of southern Arizona and is similar to that of the
Tucson area. Temperatures range from winter lows of 20°F to over 100°F
in the summer months. Rainfall varies from 10 to 13 inches, with an
annual average of 11.85 inches, most of which occurs in July, August,
and December. Vegetation is typical of southern Arizona and consists of cacti, small bushes, trees, and sparse grasses. Sahuaro, barrel, prickly pear, and chollas are the most common cacti found in the area. Mesquite, palo verde, cats cl aw, and ocotillo are found growing on the slopes and along the banks of dry arroyos, while creosote bush grows between the arroyos. Grass is scarce and is usually found in small clumps several feet apart.
Mining History
The following information on mining history was obtained from
Jenkins and Wilson (1920), Allen (1920), and Kinnison (1958) as well as from personal communications with Mr. Arthur Jacobs, present locator of the Saginaw claims.
The area that is presently known as Saginaw Hill was originally referred to as "Cerro de el Aquila" (Hill of the Eagle) by early Mexican miners and later renamed Gold Mountain. The area has also been called the Sam Hughes Gold mine, Papago Queen area, Palo Verde area, and the Am ole area. The earliest recorded reference (Jacobs, personal communication) to mining operations in the area reports a small adobe smelter operated by Thomas Hughes on the road between Tucson and the
San Xavier Mission in 1873. The ore is reported to have come from mines three miles west of the smelter in the southern end of the "Sierra de
Tucsons," probably Saginaw Hill.
On August 20, 1887, the Saginaw mine (Fig. 4) was located by
Mr. D. Snyder. In his location notice he states that the Saginaw mine is a relocation of the Sam Hughes Gold mine. Until 1898, the area was Figure 4. The Saginaw Mine Viewed from the Southeast 8 reported to have little activity. Late in 1898, the Saginaw Mining Com pany was organized by Capt. J. S. Burgess of Buffalo Bill fame. He installed a small mill consisting of three huntingtons, six concentrating tables, a smelter, and four miles of pipeline to bring water from the
Santa Cruz River. Portions of the pipeline can still be seen to the east of the Saginaw mine.
The Arizona Daily Star (November 8, 1899, p. 4) states: "There are some very rich developments being made not far south of Tucson. A very rich body of gray copper has been struck at a depth of 100 feet in a claim not far from Saginaw. It is said to be an exceeding rich and promising strike." Subsequent reports indicate that high zinc content hampered the smelting of ore causing a suspension of mining operations in December of 1899. However, Kinnison (1958) reports that copper ore was brought from Mineral Hill, 25 miles to the south, in order to pro mote the property to local stockholders. Discovery of the fraud caused the mine to suspend operations. No matter which account is correct, the Saginaw Mining Company was essentially inactive by 1900.
The Arizona Daily Star of January 13, 1906 reports on page 6:
The Montgomery-Arizona Company has purchased the Saginaw Mines and for some days has been putting the property in a workable condition. Several tons of lumber and supplies were taken to Saginaw this week. Pumping machinery for removing water from the deep workings of the mine will be installed as soon as the same arrives. C. Christenson was for several years the owner of the properties. He sold the same to the Montgomery-Arizona Company. Ten men are employed at present. The force will be increased as fast as room is made for them. These mines were developed by Capt. Burgess un der the old Saginaw Company. The ores are good for concen tration. There is considerable more in the properties and under the proper management and proper treatment of the ore there is no good reason why the Montgomery-Arizona Company should not make a more satisfying success. 9
Further reports from the property were not made until 1913, indicating the Montgomery-Arizona Company did not fare very well.
During 1913, the Calumet and Arizona Mining Company sunk
shafts ranging from 180 feet to 300 feet in depth in order to develop gold-bearing veins in shale and limestone. Reports indicate that sloping was done on three-foot to six-foot veins that assayed $10.00 to $20.00 per ton in gold.
In 1914, the Calumet and Arizona Mining Company conducted a drilling program on Saginaw Hill. Five holes, totaling about 1,500 feet, were drilled. It was reported that the mineralization was similar to that
of Ajo, but less intense (Allen, 1920). All the holes indicated some
copper mineralization which was not of economic grade.
Mr. A1 Donan of Tucson opened the Papago Queen Mining
Company in 1917. He worked the quartz veins on Saginaw Hill for sili
ceous copper flux ore (Fig. 5). The ore, which was shipped to Douglas,
assayed three percent copper. Kinnison (1958) reports that minor ton
nages of lead-silver ore was also shipped from the Palo Verde mine
during 1918 (Fig. 6).
In 1922, the Arizona-Tucson Copper Company drilled two 1,000-
foot diamond drill holes in the area north and west of the Palo Verde in
cline shaft. A "good sized" (Jacobs, personal communication) ore body
of lead, silver, and zinc was reportedly found. During 1923, the
Arizona-Tucson Copper Company extended the Palo Verde incline shaft and developed the ore body discovered by the drill holes. A mine acci
dent killed two men, and the company could not pay the damages
awarded the widows and suspended operations late in 1923. 10
Figure 5. The Papago Queen Mine
The mine is located along the footwall of the N. 60° E.- striking Papago Queen fault. The exposed rocks are mostly Saginaw Hill quartz latite porphyry, the massive quartz (Q) , and one quartz latite dike (D).
Figure 6. The Palo Verde Mine
Views from the southeast. The mine is located along the Palo Verde fault in the Limy unit metasediments. The outcrops in the fore ground are Saginaw Hill quartz latite porphyry. 11
Kinnison (1958) reports that in 1924 new interests were develop ing the Palo Verde mine, but installation of a new power line halted operations. The company planned to sink a 1,000-foot shaft of which
200 feet were completed to remove ore that assayed $10.00 gold and
silver, five percent lead and fifteen percent zinc per ton. The opera tions were never completed.
M essrs. C. M. d'Autrement and Ira Joralemon reopened the
Palo Verde mine to the 215-foot level in 1945, and 172.tons of ore were
shipped to the Eagle-Picher Mill at Sahuarita, Arizona. This ore con
tained 5 ounces gold, 399 ounces silver, and 200 pounds zinc for a total of $3,172. During 1946, the Palo Verde mine is reported to have
shipped 1,574 tons of lead-zinc ore.
In 1954, M essrs. Coombs and Martin removed ten tons of ore
from the Palo Verde mine that assayed 15 percent zinc. Mr. Arthur
Jacobs relocated the old Papago Queen mine and leased the property to
Harris and Strong in 1955. About 2,000 tons of siliceous copper flux
ore were shipped to the Phelps Dodge smelter in Douglas. Mr. Jacobs
has retained control of the property to the present time.
Ventures Ltd. conducted an exploration program in 1956 and
1957, during which three holes totaling about 1,000 feet were drilled
in the Saginaw area.
During 1961, Bear Creek Mining Company conducted an exten
sive exploration program in the Saginaw area. Ten drill holes totaling
7,385 feet were sunk around and on Saginaw Hill. Detailed geochemi
cal and geophysical studies were conducted. 12
In 1964, The Anaconda Company deepened Bear Creek's number five hole to 3,296 feet from 1,130 feet. They also drilled three addi tional holes for a total of 5,982 feet. Since 1964, no active interest has been reported in the area. GENERAL GEOLOGY
The Tucson Mountains are composed of sedimentary, meta sedimentary, and igneous rocks of Precambrian through Recent age.
The older Precambrian rock, represented by the Pinal Schist, is uncon- formably overlain by Paleozoic rocks. These formations are exposed only in the northern part of the range. The southern part of the Tucson
Mountains consists of small outcrops of Permian rocks overlain uncon- formabl.y by the Cretaceous Amole Arkose and Cretaceous-Tertiary vol canic rocks. Local intrusive rocks cut the older units and, in some places, the volcanic rocks also.
During the Late Cretaceous period, the Amole Arkose and older rocks in the Saginaw area were folded into a broad synclinorium, asym metrically inclined toward the northeast and trending approximately
N. 45° W. Local overturning of beds and faulting have since compli cated this major structure. The Tucson chaos, a mega breccia, and a thick Cretaceous-Tertiary volcanic sequence overlie the folded rocks.
The breccia and volcanic rocks gently dip eastward and are cut by numerous high-angle faults. Flat-lying basalts and tuffs of Quaternary age border the east and south margins of the range. The range is bor dered by Basin and Range block faults, but due to the development of a broad pediment these faults are not exposed. Saginaw Hill itself is located on the steeply dipping west limb of the synclinorium adjacent to the broad alluvial plain of Avra Valley.
13 14
In the following discussion, the map symbols used on Figure 3
(in pocket) are given in parentheses.
Sedimentary and Volcanic Rocks
Limy Unit (Kal)
The Limy unit of the Amole Arkose is the oldest rock unit ex posed within the map area (Fig . 3). This portion of the Amole Forma tion is defined by the great abundance of limestone, calcareous arkoses and shales, and siltstone beds. The lower portions of this unit are covered by alluvium to the west, and the upper contact is masked by metamorphic effects against the intrusive rocks to the east. Conse quently, the true thickness is not known, but conservative estimates indicate at least 600 feet to be present in the Saginaw area.
The Limy unit is composed of thin- to medium-bedded, brown calcareous siltstones, brown to black calcareous shales, thin beds of blue-black, massive to laminated limestones, and thin- to medium- bedded, yellowish-brown and olive-green sandstone and arkose. The individual units are usually only 2 to 10 feet thick and are characterized by rapid and repeated changes of lithology.
The limestone beds are rarely over two feet thick, but they may be grouped rather closely over a short distance. A 10-inch thick, yellow-brown, platy limestone was found that contained numerous small unidentified pelecypod fossils about 130 feet below the contact between the Limy and Graywacke units. The pelecypod unit has been reported by
Brown (1939), Bennett (1957), Kinnison (1958), and Assadi (1964) in other areas in the Tucson Mountains. 15
The sandstones and arkoses are light gray, yellowish buff, and greenish brown. They are usually dense and often show cross-bedding and ripple marks. Bedding varies from massive to medium, and the individual beds seldom exceed five feet in thickness.
The shales are rarely exposed except in drainages or immedi ately adjacent to resistant arkosic units. Where observed, shales are dark gray to black to gray green and usually interbedded with argilla ceous platy limestone.
Graywacke Unit (Kag)
Overlying the Limy unit with apparent conformity is a thick sequence of interbedded graywackes and arkoses, with subordinate interbedded olive-green to gray-black to buff shales and siltstones.
The Graywacke unit is distinctive because it contains a greater per centage of graywacke and arkosic beds than do other units of the Am ole
Formation. Thickness of the Graywacke unit is approximately 2,200 feet in the Saginaw Hill area (Fig. 3, in pocket).
The graywacke beds are composed of angular to subangular quartz and rock fragments set in a fine-grained matrix of quartz, feld
spar, and clay minerals. A few limestone pebbles up to a quarter of an inch in diameter were noted. One discontinuous, 2-foot thick, gray- black limestone bed crops out within 100 feet of the upper contact.
The olive-green shales and siltstones appear to derive their color from detrital chlorite. They are fine grained, massive to thin bedded, and commonly break with incipient conchoidal fracture. These beds are easily weathered and are usually only exposed in drainages 16 and adjacent to the more resistant graywackes and arkoses. The indi vidual beds vary in thickness from about eight inches up to about six feet.
Shale Unit (Kas)
The Shale unit is discernible as a mappable portion of the
Amole Formation because of the abundant fine-grained shale and silt-
stone beds. Due to the lack of resistant beds, approximately 85 per
cent of the area underlain by this unit lacks outcrop. The lack of
outcrops leaves the relationships with the overlying and underlying
units in question, but it appears that they are conformable (Fig. 3) .
In areas where outcrop is available, shale and siltstone crops
out in drainages and adjacent to more resistant beds. The shales and
siltstones vary from olive green to a yellow buff, massive to thin
bedded, and may exhibit faint cross-bedding. They can only be dis
tinguished from the shales of the Graywacke unit by their stratigraphic
position. The arkose and sandstone beds are usually yellow buff to
gray white and are composed of subrounded quartz grains set in a
fine-grained matrix of quartz, feldspar, and clay minerals. The total
thickness of the Shale unit is about 800 feet.
Kinnison (1958) and Assadi (1964) describe very similar units
which contain fossil wood fragments. No wood remains were found in
the Saginaw area.
Arkose Unit (Kaa)
The Arkose unit is defined as consisting of those sediments
that overlie the Shale unit and underlie the Tucson Mountain chaos or 17
the Cat Mountain Rhyolite. The outcrops are poor, and the area is
usually heavily covered except near the upper and lower contacts.
Where outcrops were found, the proportion of shale to arkose appeared
to be about equal. The rocks consist of interbedded green and gray
shales and siltstones and yellow-buff to brown arkose. Many of the
sediments exhibit weak ripple marks and cross-bedding. The total
thickness of the unit is unknown, but about 700 feet has been estimated
to be present in the Saginaw area (Fig. 3).
Tucson Mountain Chaos (Ktmc)
Kinnison (1958) established the Tucson Mountain chaos forma
tion which was described by Brown (1939) as an imbricate thrust sheet.
Kinnison reported that the Tucson Mountain chaos is a tabular unit
lying above the Am ole Arkose and below the Cat Mountain Rhyolite and
composed of large fragments of all pre-Laramide rocks in chaotic arrange
ment. Kinnison (1958, 1959a, 1959b) suggests that the chaos originated
by erosion and slumping of a thrust mass, an origin which indicates a
sedimentary-tectonic origin for the chaos. Bikerman (1962) proposed
that the chaos represents an initial.phase of the emplacement of the
Cat Mountain Rhyolite and in reality is a pyroclastic flow breccia of
nuee ardente type. Mayo (1963) concludes that the chaos was emplaced
by uplift of an intrusive magma and fluidization of the chaotic fragments.
For a more complete review of the Tucson Mountain chaos the reader is
referred to the cited literature and to Mayo (1968).
In the Saginaw area, the Tucon Mountain chaos crops out at
three places along the Amole Formation-Cat Mountain Rhyolite contact 18
(Fig. 3). The chaos is composed of fragments of Amole-type rocks, gray to black limestone, and red shales set in a fine-grained matrix. The fragments vary in size from a few inches up to about 30 feet in diameter.
Cat Mountain Rhyolite (Kcr)
The Cat Mountain Rhyolite is exposed in the eastern portion of the thesis area and forms a series of resistant hills and ridges that mark the Amole Formation-Tucson Mountain chaos or Cat Mountain
Rhyolite contacts (Fig. 3). Bikerman (1962) presents an extensive geo logical and geochemical study of the Cat Mountain Rhyolite. Watson
(1964) and Mayo (1968) also have reported on the Cat Mountain Rhyolite.
The reader is referred to this literature for a comprehensive review of the Cat Mountain Rhyolite.
Bikerman (1962) concludes that the Cat Mountain Rhyolite is composed essentially of two main phases: the tuffaceous, poorly welded to non-welded phase and the harder, more competent, ridge-forming welded phase. He reports that the lower non-welded phase grades down ward into the Tucson Mountain chaos formation of Kinnison (1958) and upward into the welded tuff phase. He believe that the Cat Mountain
Rhyolite is an ignimbrite or an ash flow tuff. Rising magmas stoping through Paleozoic sediments and younger rocks slightly dome a pene- planed surface on the Amole Arkose and gave rise to a basal conglomer ate reported by Kinnison (1958). When the magma reached the surface, it erupted in a violent Pelean-type explosion, giving rise to the Tucson
Mountain chaos. Following this eruption, collapse of the surface was accompanied by extensive ash flows. The final phase of the development 19 of the Cat Mountain Rhyolite consisted of intrusion of the spherulitic rhyolite reported by Brown (1939). 8afford Tuff and Shorts Ranch Andesite of post-Cat Mountain Rhyolite age were deposited and, in turn, were followed by Basin and Range block faulting. Samples of the Cat Moun tain Rhyolite have been dated by the K/Ar method by Bikerman and Damon
(1966). The dates given are:
Sample Rock and Constituent Apparent Age MB-1-62 Cat Mountain Rhyolite, feldspar 70.3+ 2.3 m.y. MB-3-62 Cat Mountain Rhyolite, feldspar 65.2 + 2.0 m.y.
These dates indicate that the Cat Mountain Rhyolite in the Tucson Moun tains represents the volcanic activity that marks the Laramide time over much of southern Arizona.
In the Saginaw area, the Cat Mountain Rhyolite is found in what appears to be gradational contact with the Tucson Mountain chaos
and in unconformable contact with the underlying, steeply dipping Amole
Formation (Fig. 3). The rhyolite appears to dip gently towards the east.
Both phases of the rhyolite, as described by Bikerman (1962), are pres
ent in the Saginaw area, but they were not differentiated during field
mapping. Reconnaissance of the Cat Mountain Rhyolite to the east of
the map area indicates that nearly north-trending normal faults predomi
nate, in direct contrast to the strong N. 60° E. faults found near Sagi
naw Hill. Kinnison (1958) also reported these north trends east of the
Saginaw area. A brief analysis of jointing indicates that the major trend
is about N. 30o-50° E ., which approximately coincides with what
Bikerman (1962) reports. 20
Intrusive Igneous Rocks
The two major intrusive igneous rocks found in the thesis area are the Saginaw Hill quartz latite porphyry and the Saginaw Mine quartz monzonite porphyry, which together form the composite intrusion that is locally known as Saginaw Hill. This composite igneous intrusion is cut by a series of minor quartz latite porphyry dikes. All the intrusive rocks crop out on Saginaw Hill and near the Saginaw mine except for the por phyry intruded along major N. 60° E.-trending faults (Fig. 3). A sum mary of the petrographic descriptions of the igneous rocks discussed is given in Table 1.
Saginaw Hill Quartz Latite Porphyry (Tshp) The term Saginaw Hill quartz latite porphyry is applied to the
exposures of the pofphyritic igneous rocks that predominate in Saginaw
Hill (Fig. 3). The weathered porphyritic rock is tannish brown to grayish
brown to grayish white with phenocrysts of fresh, gray to flesh-colored
orthoclase and gray glassy subhedral quartz. Megascopically the rock
contains about 30 to 40 percent quartz, 25 percent orthoclase, 15 per
cent plagioclase, and about 20-30 percent very fine grained unidentified
minerals. Mafic minerals are noticeably absent.
Samples were collected every 20 feet along three north-south
traverses, 500 feet apart, over Saginaw Hill. In addition to the surface
samples, 11,823 feet of diamond drill core from holes collared on Sagi
naw Hill were logged, sampled, and studied in detail. Nine thin sec
tions of the least altered quartz latite porphyry specimens which retained
recognizable outlines of original minerals were used for analysis. Table 21
TABLE 1. Summary of Petrographic Analysis of Igneous Intrusive Rocks in the Saginaw Hill Area
Saginaw Hill Saginaw Mine Quartz Latite Quartz Monzonite Quartz Latite Porphyry Porphyry D ikes
M inerals (Vo. %) Phenocrysts Q uartz 15.1 — trace Plagioclase An30-35 12.3 15.3 8.5 O rthoclase 7.8 8 .5 6 .5 Biotite trace 1.1 2 .8 Hornblende — 0 .7 1.5 Total 35.2 25.6 19.3
Groundmass Quartz 29.1 36.3 35.6 Plagioclase An30-35 14.8 15.2 13.8 Orthoclase 15.5 18.1 29.1 Biotite 1.1 0 .4 0.4 Hornblende — 0.3 0.3 C alcite 4.1 3 .4 1.3 M uscovite —— trace Accessories trace trace trace O paques 0 .8 1.3 0.3 65.4 75.0 79.3 Groundmass to Phenocryst Ratio 2:1 2:1 4:1 Distinctive Characteristics Large subhedral Medium-sized Medium-sized to euhedral quartz phenocrysts of phenocrysts of orthoclase and plagioclase. plagioclase, plagioclase orthoclase, orthoclase, phenocrysts; no biotite, and biotite, and visible mafic hornblende; hornblende; m inerals; no quartz rare quartz usually gray- usually gray- usually gray to white or gray- w hite pinkish-maroon pink 22
2 is based on the nine least altered samples and represents the original modal composition of the Saginaw Hill quartz latite porphyry.
TABLE 2. Modal Analysis of the Saginaw Hill Quartz Latite Porphyry
Volume Volume Volumetric Total Percent in Percent as Mineralogic M inerals Groundm ass Phenocrysts Composition
Quartz 29.1 15.1 44.2
O rthoclase 14.8 7.8 22.6
Plagioclase (Angg-ss) 15.5 12.3 27.8
Biotite 1.1 Trace 1.1
C alcite 4.1 — 4.1
Opaque Minerals o (mostly pyrite) 0 .8 Trace 00
Zircon Trace — Trace
Apatite Trace — Trace
Sphene Trace — Trace
65.4 3 5 .2 100.6
The Saginaw Hill quartz latite porphyry is a holocrystalline
aphanite with a porphyritic texture composed of three distinct grain
sizes: a fine-grained (less than 0.5 mm) intergrown groundmass of
quartz, orthoclase, and plagioclase; phenocrysts of plagioclase,
quartz, and orthoclase that range from greater than 0.5 mm to about
5 mm; and orthoclase phenocrysts that reach 3 cm in length and average
about 1 cm. The texture and composition of the rock is that of a quartz 23 latite porphyry. Figure 7 is a photograph of an example of typical
Saginaw Hill quartz latite porphyry.
Quartz occurs as fine anhedral crystals intergrown with ortho- clase, plagioclase, and biotite in the groundmass or as subhedral to euhedral phenocrysts. The larger grains are usually embayed and cor roded by finer grained interstitial quartz. Minute inclusions of quartz, feldspar, and, very rarely, biotite are found in many of the larger quartz grains. Essentially all quartz grains exhibit moderate to strong undula- tory extinction.
The plagioclase is strongly altered to kaolinite and montmoril- lonite except in zones of intense fracturing and veining where sericite has developed. The characteristics and nature of the alteration will be discussed later in the thesis. The groundmass plagioclase has been entirely altered, but in fresher rock samples albite twinning in plagio clase phenocrysts of about Angg composition is retained. All pheno crysts are moderately embayed and corroded by interstitial quartz.
Figure 8 is a photomicrograph of a sample of propylitized Saginaw Hill quartz latite porphyry.
The Saginaw Hill quartz latite porphyry is in both intrusive and fault contact with the Am ole metasediments. The stock is finer grained where it is in intrusive contact with the metasediments (Fig. 3) than it is in the central portion where phenocrysts may reach 3 cm in length. Along the contacts, partially assimilated inclusions of Amole
Formation sediments are found. Drill core information indicates that these inclusions are not found more than a 125 feet into the Saginaw
Hill quartz latite porphyry. Some fragments indicate digestion along 24
Figure 7. Saginaw Hill Quartz Latite Porphyry
Shows characteristic porphyritic texture with quartz and plagioclase phenocrysts. Quartz grains are round and generally darker blue in the photograph (gray in hand sample): plagioclase is blue white in photograph and gray white in hand sample. XI. 5; core is two inches in diameter. 25
Figure 8. Least Altered Saginaw Hill Quartz Latite Porphyry in Thin Section
Montmorillonite, kaolinite, and rare sericite occurs as flecks replacing plagioclase (?) phenocrysts. Biotite (B) is replaced by chlorite (C) and leucoxene (L) . Fresh orthoclase (O) contains inclusions of al tered plagioclase. The intergrown groundmass (G) composed of quartz and K-feldspar corrodes the plagioclase and quartz (Q) phenocrysts. Crossed nicols; X 17. 26 their edges, but most fragments exhibit sharp boundaries with the por phyry being chilled at the contact.
The extensions of the Saginaw Hill quartz latite porphyry that occur in the Papago Queen fault and the southernmost east-trending apophysis of Saginaw Hill (Fig. 3) are of essentially the same composi tion as the composite intrusive rock. Both extensions are located along
Major N. 60° E.-trending strike-slip faults. Kinnison (1958) reports that in the Palo Verde mine, located adjacent to the N. 60° E.-trending
Palo Verde strike-slip fault (Fig. 3), a porphyritic intrusive rock similar to the Saginaw Hill quartz latite porphyry is found. The Saginaw Hill quartz latite porphyry has subsequently been intruded by the Saginaw
Mine quartz monzonite porphyry and quartz latite dikes.
Saginaw Mine Q uartz M onzonite Porphyry (Tsmp)
The term Saginaw Mine quartz monzonite porphyry is applied to
the exposures of porphyritic igneous rocks that crop out primarily in the
vicinity of the Saginaw mine (Fig. 3). The weathered rock is tannish
brown to gray, porphyritic, with medium-sized phenocrysts (0.5 to 5 mm
in diameter) of altered plagioclase and mafic minerals. Megascopically,
the rock contains about 5 percent quartz, 10-15 percent orthoclase, 30
percent plagioclase, 5 percent chlorite after biotite, and 40 percent
unidentified fine-grained minerals. The lack of quartz and large, fresh
orthoclase phenocrysts and the presence of visible mafic minerals make
the Saginaw Mine quartz monzonite porphyry readily identifiable in the
field (Fig. 9). The plagioclase has been altered to kaolinite and
montmorillonite except in intensely fractured zones or areas of strong 27
Figure 9. Saginaw Mine Quartz Monzonite Porphyry
Shows characteristic porphyritic texture and the presence of mafic minerals and the lack of quartz phenocrysts. Mafic minerals are yellow brown and the plagioclase is blue white (gray white in hand sample). Pyrite is blue black. X I.2; sample is 2 inches long. 28 vein development where argillic to quartz-sericite assemblages occur.
The mafic minerals, essentially biotite, have been altered to chlorite.
Numerous surface samples were collected from the outcrops near the Saginaw mine and about 2,287 feet of diamond drill core were
studied in detail. Four thin sections of the least altered quartz mon-
zonite porphyry which retained recognizable outlines of original min
erals were used for analysis. Table 3 is based on the four least altered
samples and presents the original modal composition of the Saginaw
Mine quartz monzonite porphyry.
TABLE 3. Modal Analysis of the Saginaw Mine Quartz Monzonite Porphyry
Volume Volume Volumetric Total Percent in Percent as Mineralogic M inerals Groundm ass Phenocrysts Composition
Q uartz 36.3 — 36.3
O rthoclase 18.1 8.5 26.6
Plagioclase (Angg-ss) 15.2 15.3 30.5
Biotite 0 .4 1.1 1.5
Hornblende 0.3 0 .7 1.0
C alcite 3 .4 — 3 .4 Opaque Minerals (mostly pyrite) 1.3 — 1.3 Zircon Trace — Trace
Apatite Trace — Trace
Sphene Trace — — Trace
75.0 25.6 100.6 29
The Saginaw mine intrusive body is a holocrystalline-phaneritic igneous rock with a porphyritic texture. The matrix is composed of inter- grown quartz, plagioclase and orthoclase (in an approximate 2:1:1 ratio), which are generally greater than 0.1 mm in size. Phenocrysts of plagio clase, subordinate orthoclase, muscovite, biotite, and rare quartz, which average about 2 mm to 3 mm in size, are set in the finer grained matrix. The texture and composition are those of a quartz latite to a quartz monzonite porphyry. For purposes of clarity in the thesis, the rock has been classified as a quartz monzonite porphyry. Figure 10 is a sample of typical Saginaw Mine quartz monzonite porphyry.
Quartz occurs as fine anhedral grains intergrown with ortho clase and plagioclase in the groundmass or as extremely rare subhedral to euhedral phenocrysts. The larger groundmass quartz grains and the phenocrysts are corroded along their crystal margins by the finer ground- mass quartz. Inclusions of plagioclase, muscovite, and orthoclase are found in the larger quartz grains. Most groundmass quartz and quartz phenocrysts exhibit moderate to strong undulatory extinction.
The plagioclase is altered to kaolinite and montmorillonite ex cept in strongly fractured zones where quartz-sericite alteration occurs.
In the fresher samples the plagioclase exhibits albite twinning of ap proximately AngQ-gs composition. The plagioclase crystals have been corroded and scalloped along their borders by groundmass quartz. The orthoclase feldspars in the groundmass are usually very fresh while the larger phenocrysts are slightly altered to clay minerals. Inclusions of plagioclase, quartz, and muscovite are fairly common in the orthoclase 30
Figure 10. Least Altered Saginaw Mine Quartz Monzonite Porphyry in Thin Section
Plagioclase (!) is replaced by kaolinite and montmorillonite. Biot it e (B) is replaced by chlorite and leucoxene. The intergrown groundmass (G) is composed of quartz, orthoclase, and plagioclase. Crossed nicols; X 17. 31 crystals. The crystal margins of the orthoclase is strongly embayed by the groundmass quartz.
The Saginaw Mine quartz monzonite porphyry is in both intru sive and fault contact with both the Amole Formation metasediments and the Saginaw Hill quartz latite porphyry. Exposures of the Saginaw Mine quartz monzonite porphyry are essentially confined to the area around the Saginaw mine (Fig. 3)., but drill core data show that the porphyry is found as irregular dikes and pods through the Saginaw composite intrusive body (Fig. 11, in pocket). Drill holes BC-2, BC-3, BC-4,
BC-5, BC-7, TAC-11, TAC-12, TAC-13, and Vent-3 (Fig. 3) all inter
sect dikes and pods of the Saginaw Mine quartz monzonite porphyry. At
Saginaw Hill about 15 percent of the intrusive rock intersected by the drill holes was Saginaw Mine quartz monzonite porphyry. It occurred
as thin stringers (less than 2 feet thick), lenses (2 feet to 20 feet thick),
and pods (greater than 20 feet thick), which are listed in order of de
creasing frequency. Drill core data show that maximum intersection of
Saginaw Mine quartz monzonite porphyry occurred in hole TAC-12 in which 718 feet was noted. However, the average intersection was about
12 feet. Surface exposures on Saginaw Hill were difficult to find due to
the similar weathered appearance of the Saginaw Hill quartz latite por
phyry and Saginaw Mine quartz monzonite porphyry. The only exposures
of the Saginaw Mine quartz monzonite porphyry that are readily observ
able on Saginaw Hill occur in the composite dike filling the Papago
Queen fault (Fig. 3). Near the Saginaw mine (Fig. 3), 100 percent of
the exposed igneous rocks consist of the Saginaw Mine quartz monzonite
porphyry. Drill hole BC-4 intersected 1,104 feet of Saginaw Mine quartz 32 monzonite porphyry. From drill hole data and the limited surface expo
sures it appears that Saginaw Hill is essentially composed of quartz latite porphyry while north of the Palo Verde fault the quartz monzonite porphyry predominates.
In drill core, the contact between the two porphyries is repre
sented by thin stringers of the Saginaw Mine quartz monzonite porphyry
cutting the Saginaw Hill quartz latite porphyry rock or a fragmental zone with the fragments invaded by the Saginaw Mine quartz monzonite por
phyry (Fig. 11). The inner portions of the thicker lenses and pods of the
Saginaw Mine quartz monzonite porphyry are essentially free of inclu
sions of the host rock. Rarely is this relationship exposed at the surface.
Exceptions are the irregular pods and lenses of the Saginaw Mine quartz
monzonite porphyry that intrude the Saginaw Hill quartz latite porphyry
along the Papago Queen fault. About 95 feet east of the Papago Queen
mine, along the strike of the fault (Fig. 3), an irregular, discontinuous
lens of the Saginaw Mine quartz monzonite porphyry crops out (Fig. 12).
It is about 6 feet wide, and the exposures of Saginaw Mine quartz mon
zonite porphyry are recurrent along the entire length of the dike. Frag
ments of partially assimilated Amole Formation sediments and rare
fragments of Saginaw Hill quartz latite porphyry mark .the outcrops of
the Saginaw Mine quartz monzonite porphyry (Fig. 13). Because of the
irregular nature of the outcrops, their limited size, and poor exposures
at Saginaw Hill, no attempt was made to map the individual stringers,v
lenses, and pods of the Saginaw Mine quartz monzonite porphyry. 33
Figure 12. Saginaw Mine Quartz Monzonite Porphyry Dike Saginaw Mine quartz monzonite porphyry with inclusions of Amole Formation metasedimentary rocks emplaced in the Saginaw Mine quartz monzonite porphyry along the Papago Queen fault.
Figure 13. Saginaw Mine Quartz Monzonite Porphyry fragmental Zone Shows inclusion of Amole Formation meta sedimentary rock from fragmental zone between the Saginaw Hill quartz latite porphyry and Saginaw Mine quartz monzonite porphyry contact. X I.5; core is 1.5 inches in diameter. 34
Quartz Latite Porphyry D ikes (Tqld)
The quartz latite porphyry dike system is exposed in limited outcrops on Saginaw Hill and in the east-trending southernmost exten sion of the Saginaw Hill quartz latite porphyry (Fig. 3), although it is commonly encountered in the diamond drill core. The thickness of the dikes averages about 13 feet with an upper limit of 30 feet and a lower limit of about 6 inches. In hand sample (Fig. 14) the rock is grayish pink to gray white and porphyritic with phenocrysts of feldspar, biotite, and hornblende set in an aphanitic, intergrown groundmass with about a 1:1 ratio of quartz to orthoclase. The rock is dense and usually appears to be very fresh. Twelve thin sections of the dike rock were cut and studied in detail. Table 4 is a modal analysis of the least
altered thin sections and represents the original modal composition of the ro ck .
The rock is aphanitic, holocrystalline, and porphyritic with a
completely intergorwn groundmass of anhedral quartz and orthoclase in
nearly equal proportions. The plagioclase and orthoclase phenocrysts
are subhedral to euhedral while the biotite and hornblende phenocrysts
are subhedral to anhedral. The phenocrysts are usually about 2 to 3 mm
in size. The composition and texture of the rock is that of a quartz
latite porphyry. Figure 15 is a photomicrograph of a sample of the dike
rock.
Quartz occurs in the groundmass intergrown with orthoclase and
as phenocrysts. The anhedral groundmass quartz exhibits strong undula-
tory extinction. The larger euhedral phenocrysts, although rare, contain
inclusions of plagioclase, orthoclase, and biotite and are somewhat 35
Figure 14. Quartz Latite Dike
Shows characteristic texture with plagioclase and mafic mineral phenocrysts. Plagioclase is blue gray in photograph (gray white in hand sample) and mafics are blue black in photograph. Characteristic pink to pink-gray color of the rock appears bluish pink in photograph. X I.5; core 2 inches in diameter. 36
Figure 15. Least Altered Quartz Latite Dike in Thin Section
Twinned and zoned plagioclase (P) phenocrysts set in a fine grained groundmass (G) of quartz, orthoclase, and plagioclase. Quartz (Q) occurs rarely as phenocrysts. Crossed nicols; X 17. 37 corroded and scalloped along their borders by the groundmass quartz.
The orthoclase crystals are very fresh and exhibit only slight alteration to kaolinite giving them a cloudy appearance. The larger phenocrysts of orthoclase are slightly corrded along their borders by the matrix quartz and usually exhibit strong undulatory zoning.
TABLE 4. Modal Analysis of the Quartz Latite Porphyry Dikes
Volume Volume Volumetric Total Percent in Percent as Mineralogic M inerals Groundm ass Phenocrysts Composition
Quartz 35.6 Trace 35.6
Plagioclase (Angg-ss) 13.8 8.5 22.3
O rthoclase 29.1 6 .5 35.6
Biotite 0 .4 2 .8 3 .2
Hornblende 0.3 1.5 1.8
C alcite 1.3 — 1.3
M uscovite Trace — Trace
Opaque Minerals (mostly pyrite) 0 .3 — — 0 .3
Sphene Trace — Trace
Apatite Trace —— Trace
Zircon Trace — Trace
8 0 .8 19.3 100.1
Plagioclase occurs rarely in the groundmass and more commonly
as subhedral to euhedral phenocrysts up to 2 mm in size. It is frequently
twinned by albite, carlsbad, and pericline laws, and normal zoning is 38
developed to a slight degree (Fig. 15). The plagioclase has been some what propylitically altered except in zones of intense fracturing and
vein mineralization where argillic and quartz-sericite alteration occurs.
Subhedral biotite is pleochroic from light to dark greenish brown and
exhibits partial alteration to chlorite and epidote. Subhedral to anhedral
hornblende averages about 0.5 mm to 1.5 mm in size and appears to be relatively fresh with the exception of slight chlorite alteration along
some of the crystal margins. Muscovite is rare and usually occurs as
subhedral phenocrysts about 1.0 mm in size.
The dikes are intrusive in character and usually form sharp
contacts with the invaded country rock. The dikes are found intruding
the Saginaw Hill quartz latite porphyry, the Saginaw Mine quartz mon-
zonite porphyry, and one 10-inch dike was found intruding the Amole
Formation metasediments (Fig. 16).
The quartz latite porphyry dike that crops out adjacent to the
Papago Queen fault (Fig. 3) represents the thickest and most continuous
surface exposure of this rock type in the Saginaw area. The dike is about
35 feet thick at the Papago Queen mine and rapidly thins eastward from
the mine and westward from the crest of Saginaw Hill (Fig. 3). The dikes
are in intrusive and fault contact with both the Saginaw Hill quartz latite
porphyry and the Saginaw Mine quartz monzonite porphyry. The intrusive
contact zones are sharp, and the dikes exhibit a chilled border that con
tains rare inclusions of the host porphyries and Amole Formation meta
sediments. Alteration of the host rocks adjacent to the dikes is not
apparent. 39
Figure 16. Dike-metasediment Contact
Quartz latite dike in intrusive contact with metasedimentary rock of the Amole Formation. X 1.5; core is 1.5 inches in diameter. 40
The Calcite" Problem
As previously noted, the intrusive rocks in the Saginaw area appear to contain anomalously high percentages of calcite (Table 1).
During the petrographic analysis of the igneous rocks it was attempted to differentiate between calcite associated with the calcite and quartz- calcite veins, calcite that was intimately associated with the altera tion of plagioclase, and interstitial calcite that was intergrown with the groundmass and exhibited no secondary relationship to other min
erals. Only the interstitial calcite occurrence was included in Table 1.
The least altered samples of the various intrusive rocks contained from
about one percent to a maximum of 11.5 percent calcite while strongly propylitized and sericitized samples contained about 4.5 percent to
18.5 percent calcite. Table 1 also shows that the amount of interstitial
calcite appears to decrease somewhat in the younger intrusive rocks.
The following hypothese are proposed to explain this anomalous
calcite and will be discussed later in the thesis: (1) the calcite was
added to the system during emplacement of the calcite and quartz-calcite
veins; (2) the calcite added to the system was controlled by the altera tion of plagioclase; (3) the original magma was oversaturated with CaO
resulting in the crystallization of magmatic calcite; (4) during intrusion
blocks of the Limy unit sediments and possibly underlying Paleozoic
limestones were assimilated by magmatic sloping which resulted in an
increase of CaO and COg in the melt and subsequent crystallization of
calcite; or (5) the calcite resulted from an early phase of propylitic al
teration in which the altering fluids were somewhat rich in CaO. 41
Age and Correlation A summary of previous and present information will be presented on the Cretaceous stratigraphy of the Saginaw area and its relationships to subsequent deformation and igneous activity. It is extremely unfor tunate that the detailed stratigraphy of this portion of the Cretaceous has never been studied. Consequently a multitude of local stratigraphic terms and confusion in the correct stratigraphic sequence for the Creta ceous System in the Tucson Mountains exist. Without a detailed strati graphic effort, the Cretaceous Amole Formation will plague future workers in the area as they attempt to determine the complex structural history and attempt to correlate it with other poorly described Cretaceous rocks in southern Arizona. Figure 17 attempts to correlate the revised strati graphic sequence proposed for the thesis area with Assadi's (1964) find in g s.
Paleozoic Rocks
The only Paleozoic rocks exposed in the vicinity of Saginaw
Hill crop out at Snyder Hill, approximately two and one-half miles north west of Saginaw Hill, and near the Braun mine, about two and one-half miles northeast of Saginaw Hill. At Snyder Hill, dark-gray to black limestone and dolomite form a topographic high. Bryant (1955) assigns the lower limestone unit to the upper Concha Formation and the overlying dolomite unit to the upper Rainvalley Formation. The outcrops near the
Braun mine are similar in appearance to the rocks at Snyder Hill which led Brown (1939) to assign them a Late Permian age. rpi odr rpsd y insn (1958). Kinnison by proposed order graphic
F OR M AT I ON rtLECYFODS — USN MOUNTAIN TUCSON CATMOUNTAIN SAGINAW HILL GRAYWACKE THI S) IS S E H T IS H (T IY UNIT LIMY CHAOS • RHYOLITE ARKOSE SHALE UNIT UNIT UNIT AREA h nmes n aetee rpeet h oiia stati tra s original the represent parentheses in numbers The iue 7 Corlto Cat fte ml Formation Amole the of Chart orrelation C 17. Figure EETOS — PELECTPOOS— SANDSTONE UNIT ODN GATE GOLDEN A MOUNTAINCAT GRAYWACKE OE ARKOSE LOWER SILTSTONE RHYOLITE UNIT UPPER UNIT UNIT SHALE AMOLE MINING CO VALLEY ECHO CAT MOUNTAIN USN MOUNTAIN TUCSON OS HOUSEMOUSE KINHISON (1361) PELtCYPOOS - FORMATION FORMATION FORMATION FORMATION ED COW DEAD RHYOLITE DISTRICT BRAUN HO \ CHAOS ( ( ( (I) 2 4 3 ) ) )
FCLCCVFOOS— — IY UNIT LIMY SEDIMENTARY SOUTHERN SOUTHERN BENNETT(1857) NORTHERN ARGILLIC UNIT HILLS c crDS — S tcrroD rcL — FORMATION RECREATION UNNAMED) E BEDS RED B R O W N( 1 8 % # ) STATION RANGER AMOLE (UNITS i
43
The marine Paleozoic rocks of the Snyder Hill formation are separated from the subaerial Mesozoic rocks by an unconformity (Bryant,
1955); this unconformity is regional in extent and has been reported from most areas in southeastern Arizona.
Mesozoic Rocks
The oldest known rocks of Mesozoic age in southeastern
Arizona are red beds in the Canelo H ills, which Hayes and Drewes (1968) report to be of Triassic age. They also suggest that the Recreation Red- beds, which underlie the Amole Arkose, are similar, if not equivalent, to the Canelo Hills red beds. Recent work by Mayo (1963) and Damon and others (1967) strongly support a Triassic age for the Recreation Red- beds. Mayo (1963) reports on an andesite porphyry that intrudes the
Recreation sequence near the Arizona-Sonora Desert Museum. Damon and others (1967) assign a 150 m.y. K-Ar age to the porphyry. This date would indicate that the Recreation Redbeds are Jurassic to Triassic or even Late Permian in age and not Cretaceous as previously believed.
In the Saginaw area the contact between the Permian Snyder
Hill Formation and the overlying Mesozoic Amole Formation is covered by Quaternary alluvium to the west of the map area (Fig. 3). Near the
Braun mine, Kinnison (1958) reports that a 2 to 10 foot thick conglom erate unit, composed of angular to subangular pebbles of Permian(?) limestone and other unknown formations, unconformably overlies the
Permian(?) Snyder Hill formation. He also noted that the conglomerate was in gradational contact with the overlying arkosic units of the Amole
Form ation. 44 During the Jurassic period, widespread plutonic activity fol lowed by uplift and erosion formed a widespread unconformity in south eastern Arizona (Hayes and Drewes, 1968). Hayes and Dr ewes further conclude that the contact between the Recreation Red Beds and the
Amole Formation may represent this unconformity in the Tucson Moun tains. The work of Brown (1939), Colby (1958), and others in the range disputes this conclusion. They feel that the Recreation-Amole contact is gradational. At the Braun mine the Amole Formation lies directly on the Permian limestone with no evidence of the presence of the Recreation
Red Beds. Reconnaissance field checking of the Recreation-Amole con tact indicated mostly fault contacts, and only rare and questionable gradational contacts were observed.
From the available literature and the relationships observed in the field it appears that the Recreation Red Beds represent isolated areas of deposition and were contemporaneously deposited with the lower
Amole Formation. It is also possible that the Recreation Red Beds repre
sent pre-Jurassic deposition with subsequent erosion occurring before the deposition of the Amole Formation.
Cretaceous Amole Formation. Brown (1939) mapped and named the sediments which overlie the Recreation Red Beds and underlie the Cat
Mountain Rhyolite as the Amole Formation. He measured about 2,300
feet of the lower Amole Arkose near the Ranger Station and assigned this
unit a Late Cretaceous age based on fossil evidence (Fig. 17). Reeside
(McKee, 1951) also assigned a Cretaceous age to the Amole Formation
based on fossil evidence. 45 Kifmison (1958) believed that the Amole Formation should be raised to group status and divided into four formations in the Amole mining district; these formations are, from oldest to youngest, Braun formation.
Dead Cow formation, Mouse House formation, and Echo Valley forma tion. Assadi (1964) subdivided the Amole Formation into seven units in the Golden Gate area of the Tucson Mountains; from oldest to youngest they are: Lower Arkose unit, Siltstone unit, Sandstone unit. Limey unit.
Graywaeke unit, Fossilwood unit, and Upper Shale unit. Assadi (1964) attempted to correlate his work with that of Kinnison, and Figure 17 is a
summary of Assadi's (1964) correlation chart with an interpretation of the stratigraphy in the Saginaw area.
The Saginaw area represents the thickest and most continuous
exposure of the Amole Formation found anywhere within the southern
section of the Amole mining district. However, the uniformity and repet
itive nature of the lithologic units, coupled with complex folding and
faulting, make the formulation of an accurate stratigraphic sequence
impossible, short of a comprehensive detailed stratigraphic study. The
stratigraphic interpretations of Assadi (1964) appear to correlate with
the relationships found in the Saginaw area, and his terminology will
be used instead of Kinnison1 s in an attempt to standardize descriptions within the Amole Formation.
The lowermost units of Assadi1 s (1964) stratigraphic section
include a lower arkose unit, a siltstone unit, and a sandstone unit (Fig.
17). Assadi correlated these units with Kinnison's (1958) Braun forma
tion. In the Saginaw area the only rocks in the correct stratigraphic
position that appear to be similar to the descriptions of Assadi and 46
Kinnison crop out in the westernmost portion of the map area. Due to poor exposures and complex structure of the area, however, they have been grouped with the overlying Limy unit.
Kinnison (1958) reports that the Dead Cow formation overlies the Braun formation and concludes that the contact is gradational. Fos
sils collected from an ostracod-bearing limestone bed were identified
by Peck (Kinnison, 1958), who concluded that they were "chiefly Morri
son but they do go as high as the Draney limestone of the Gannet group
(Aptian) of southeast Idaho." Kinnison concluded that the Dead Cow
must represent Lower Cretaceous sedimentation. He also reports that
about 300 feet below the top of the Dead Cow formation an apparent
angular unconformity was located. This discordance, according to
Kinnison, represents the break between Lower Cretaceous and Upper
Cretaceous sedimentation.
Kinnison also concluded that the Mouse House formation over-
lies the Dead Cow formation and describes the upper and lower contacts
as being not well established. Anderson (Kinnison, 1958) reports that
several grains of Betula (birch) pollen were isolated from a shale sample
which represents deposition of Late Cretaceous age or younger. In
addition to the birch pollen, a single "squashed" grass pollen grain
of very questionable Tertiary age was isolated.
Kinnison (1958) defines the Echo Valley formation as that unit
which overlies the Mouse House formation and underlies the Tucson
Mountain chaos. Compositionally, it is similar to the Braun formation
and is only recognizable because of its stratigraphic position. Kinnison
concludes that the Braun and the lower portion of the Dead Cow 47 formations represent Lower Cretaceous sedimentation which was followed by minor tectonic deformation with subsequent deposition of upper Dead
Cow, Mouse House, and Echo Valley sediments.
In 1964, Assadi attempted to correlate Kinnison's stratigraphic formations in the southern Amole mining district to his units in the
Golden Gate area. His conclusions may be summarized as follows:
1. The Braun formation may be equivalent to the lower Arkose
unit, the Silt stone unit, and the Sandstone unit.
2. The Mouse House formation correlates with the Limey unit.
3. The Dead Cow formation appears to be similar to the Fossil
Wood un it.
4. The Echo Valley formation does not correlate with the upper
shale unit but may be explained by faulting.
5. The Mouse House formation is stratigraphically older than the
Dead Cow formation.
Figure 17 shows the interpretations proposed by Assadi.
Four major stratigraphic units at Saginaw Hill are defined in this thesis. From oldest to youngest they are: Limy unit, Graywacke
unit. Shale unit, and Arkose unit. Figure 17 correlates this stratigraphy
proposed at Saginaw Hill with Assadi's (1964) work.
At Saginaw Hill, the Limy unit (Kinnison's Mouse House forma
tion) is exposed in the western part of the map area. The lower portions
are composed primarily of sandstone and may represent Assadi's Sand
stone unit, but because of poor exposures, these beds were included within the Limy unit. The rocks are well bedded and composed of cal
careous shales, siltstones, arkose, and limestone units. One limestone 48 bed contains small pelecypod fossils. Lithologically, the Limy unit at
Saginaw is the same as Assadi's Limey unit at Golden Gate Mountain.
Overlying the Limy unit with apparent conformity is the Gray- wacke unit. Kinnison (1958) has previously mapped these beds as un differentiated Amole Arkose. Compositionally, the Graywacke unit appears to be identical to those described by Assadi at Golden Gate
M ountain.
Overlying the Graywacke unit is the Shale unit. Composition- ally, it is extremely similar to that which Assadi defined as the Fossil
Wood unit except for the lack of the wood remains. The lack of wood remains may be explained by a facies change within the unit, an over sight in field mapping, or by the fact that the Fossil Wood unit is not present and the group of rocks belongs to the Graywacke unit or the overlying Arkose unit. Only a detailed stratigraphic study of this area will solve the problem. These rocks, hov/ever, are distinctly mappable and have been assigned to the Shale unit for lack of better evidence.
Kinnison (1958) mapped the lower contact of his Dead Cow for mation (Graywacke-Shale contact) as being the upper plate of the Jig
Thrust fault. Careful examination of the contact revealed no evidence of this thrust. It is proposed that this zone is only the contact between the resistant Graywacke unit and the less resistant Shale unit.
The contact between the Shale unit and the overlying Arkose unit is covered in most of the area. Where it can be observed, it ap pears to be gradational in character, as opposed to Kinnison's thrust fault contact. Usually the Arkose unit is only exposed near the contact with the overlying Tucson Mountain chaos or Cat Mountain Rhyolite and 49 in small drainages. Where this unit is exposed, it is composed essen tially of interbedded green to gray shales and yellow to brown arkose; very similar to Assaid's Upper Shale unit at Golden Gate Mountain and not the Braun formation as Kinnison (1958) indicates.
Cretaceous-Tertiary Boundary
A mid- to Late Cretaceous unconformity has been described in southeastern Arizona by Darton (1925), Epis (1952), and Gilluly (1956).
These workers describe this surface as being represented by conglom erate units, breccia units, and nearly flat-lying volcanic rocks over- lying isoclinally folded Early Cretaceous sediments. Hayes and Drewes
(1968) and Richard and Courtright (1960) correlated the conglomerate unit and the Tucson Mountain chaos of Kinnison (1958) with similar units that occur beneath Late Cretaceous volcanic units in the Empire Mountains and elsewhere in southern Arizona. They conclude the the chaos repre sents Late Cretaceous sediments that discordantly overlie the Amole
A rkose.
Overlying the Late Cretaceous sediments in most of south eastern Arizona are andesite and dacite breccias, rhyodacite tuff and welded tuff, and some sedimentary rocks (Richard and Courtright, 1960).
Samples of the Cat Mountain Rhyolite, which are reported to conformably overlie the Tucson Mountain chaos, have been dated as 70.3 + 2.3 m.y. by the K-Ar method (Bikerman and Damon, 1966). These dates verify the
Late Cretaceous age for the Tucson Mountain chaos proposed by Richard and Courtright (1960). 50 During Laramide time, southeastern Arizona was intruded by plutons of quartz diorite or granodiorite, coarse-grained quartz monzo-
nite, and quartz latite porphyry. Radiometric dating of the Saginaw in trusive bodies has not been done, and only field evidence can be used to determine the time of emplacement. Kinnison (1958) concluded that
essentially all the ore deposits and related intrusions are "post-Shorts
Ranch andesite age" and may even be " contemporaneous with tilting and
block faulting." Kinnison notes that the Shorts Ranch is the uppermost
Tertiary extrusive rock in the Amole mining district and if "tilting and
block faulting" refers to the Basin and Range orogeny, emplacement of
the intrusions occurred in late Oligocene and early Miocene time.
Kinnison (1958) based his conclusions on the observations that
most of the deposits in the Amole mining district are strikingly similar
and that they all contain a high percentage of silica indicating they
were comagmatic. In his mapping of the Saginaw area he noted that
"the main mass tapers to a root-like shape, and then narrows to a dike
about five feet wide" (Kinnison, 1958, p. 79), which cuts the Cat
Mountain Rhyolite. This cross-cutting relationships would indicate a
post-Laramide age of emplacement according to Kinnison. He also
reports quartz veins at the Old Pueblo mine cutting the Shorts Ranch
Andesite, which further indicates that emplacement of the intrusive
rocks and subsequent mineralization occurred during post-Laramide time.
More recently radiometric dating (Bikerman and Damon, 1966)
assign ages of 56.8 + 1.9 m.y. and 70.3 + 2.3 m.y. to the Shorts Ranch
Andesite and the Cat Mountain Rhyolite, respectively. As previously
discussed, the "root-like" dike of Kinnison (1958) is a composite 51 intrusive dike that was intimately associated with the emplacement of the Saginaw Hill intrusive body. This substantiates Kinnison's conclu sion that emplacement of the intrusive body occurred during post-Cat
Mountain time, which now indicates a Laramide age for emplacement of the Saginaw intrusive rocks.
Structural Geology
Regional Structure
The Tucson Mountains can be thought of as an eroded horst be tween the grabens of Avra Valley to the west and the Santa Cruz Valley to the east. West (1970) reports that a gravity study in the Avra Valley area located a major scarp which may represent Basin and Range faulting.
The scarp trends approximately N. 30° W. and is located about two and one-half miles southwest of Saginaw Hill. This would indicate that
Saginaw and Snyder Hills lie on the thinly covered pediment of the
Tucson Mountains. West also indicated that another major scarp was located to the north of the Sierrita Mountains. This scarp trends about
N. 60° E. and probably intersects the N. 30° W.-trending scarp near the Del Bac Hills, which are located about three miles south of Saginaw
Hill near the shift in trend of the Tucson mountain range noted by Mayo
(1968).
Brown (1939) considered that the Tucson Mountains consist of three main structural domains: (1) a basement block of Paleozoic and
Cretaceous rocks that had been folded into a broad open northwest trending synclinorium; (2) a series of tilted Tertiary volcanic rocks; and
(3) a series of nearly flat-lying Tertiary or Quaternary basaltic rocks. 52
The Saginaw Hill area lies on the west limb of the synclinorium first described by Brown (1939). About one mile east of Saginaw Hill is the gently tilted Late Cretaceous Cat Mountain Rhyolite, while westward is the thinly covered pediment described by West (1970).
The central and northern portions of the Tucson Mountains trend approximately N. 30° W ., while the smaller southern portion trends about S. 20° W. (Mayo, 1968). The Saginaw area lies about two and one-half miles north of this shift in trend. It has been previously noted that near this same area, the Del Bac Hills, West (1970) reported a major N. 60° E.-trending scarp, which he feels may reflect Basin and
Range faulting. The Pima County geologic map (Wilson, Moore, and
O'Haire, 1960) shows a strong northwest trend for structures in the
Tucson Mountains. This trend appears to be very prominent throughout southern Arizona. button (1958) and Wilson (1962) proposed Laramide time was one of strong northeast-southwest compression, which may account for this northwest trend.
Local Structure
Folding. The Saginaw area lies on the steeply dipping western limb of an asymmetrically inclined synclinorium, which had been reported by Brown (1939), Kinnison (1958), and Assadi (1964). The metasediments have a regional strike of approximately N. 35o-40° W. and a regional dip of about 30o-40° NE. Kinnison (1958) recognized four orders of folds in the Am ole mining district, which are also present in the Saginaw area. He noted that second-order folds with wavelengths of 200 feet to over 1,000 feet are superimposed on the limbs of the major synclinorium. 53
On the limbs of the second-order folds are drag folds which, in turn, have smaller folds on their limbs. In the vicinity of the Palo Verde mine and along the western side of Saginaw Hill, the second-, third-, and fourth-order folds are readily observed in the sediments of the Limy unit
(Fig. 3). All the folds observed appear to be asymmetrically inclined toward the west and plunge southto southeast. The second-, third-, and fourth-order folds may exist in the other stratigraphic units; that none were recognized may be due to the poor exposures, the lack of distinc tive stratigraphic units, or the isoclinal nature of the folds themselves.
Minor faults are generally found near the crests or troughs of the second-order folds, although these faults die out within ten feet of outcrop and rarely indicate more than a few inches to a foot of displace ment. The crests of these folds appear to have been tightened along their axial planes resulting in the faulting and associated brecciation.
Faulting. The dominant faults in the Saginaw area are N. 50°-
70° E.-trending, nearly vertical strike-slip faults of pre-Cat Mountain age (Fig. 3). Most of these faults are terminated by the contact between the Amole Formation and the Cat Mountain Rhyolite or die out rather rapidly within the rhyolite. The faults that crosscut this contact also crosscut the Saginaw Hill quartz latite porphyry and the Saginaw Mine quartz monzonite porphyry. This probably indicates reactivation of an older fault system during emplacement of the intrusive rocks. These re activated faults are either terminated by north to N. 30° W .-trending normal faults or extend under the Recent sands and gravels. The N. 60°
E.-trending faults may have experienced dip-slip movement prior to emplacement of the intrusive body, but because of poor exposures of 54 the fault scarps and subsequent strike-slip movement, this dip-slip movement could not be determined. The total displacement on the faults is also not clearly understood. The sedimentary rocks show about 700 feet of displacement which may have occurred during pre-intrusive time or may be a total of the displacement that occurred before and during intrusion. The intrusive rocks indicate little or no displacement (Fig.
3). The three major faults in the Saginaw area that are associated with mineralization and alteration are the Papago Queen fault, the Palo
Verde fault, and the Saginaw Mine fault (Fig. 3). The Papago Queen fault is well exposed in the Papago Queen mine, where it strikes about
N. 60° E. and dips about 80° N. (Fig. 5). Slickensides indicating
strike-slip movement were noted along portion of the hanging wall of the fault in the mine. The fault is filled with Saginaw Hill quartz latite porphyry and appears to be a locus of subsequent intrusion of Saginaw
Mine quartz monzonite porphyry (Fig. 12) and the quartz latite dike.
The fault cuts through the Saginaw Hill composite intrusive body, the
Amole sediments, and the Cat Mountain Rhyolite. The porphyry that fills
the fault is strongly sheared and brecciated, which indicates reactiva
tion of the fault during and after intrusion (Fig. 18). The sediments
show displacements of about 500 feet while the intrusive rocks and Cat
Mountain Rhyolite show very little movement (Fig. 3).
The Palo Verde fault was mapped and named by Kinnison (1958),
but he made no reference to the nature or extent of displacement of the
fault. He did note that the Palo Verde mine (Fig. 3) is located along a
nearly vertical fault contact between Amole Formation sediments and the 55
Figure 18. N. 60° E.-trending Joints and Shears
Strong N. 60° E. jointing and shearing of Saginaw Hill quartz latite porphyry and Saginaw Mine quartz monzonite porphyry filling the Papago Queen fault, viewed from the east and looking along N. 60° E. joints parallel to Papago Queen fault. 56
Saginaw Hill quartz latite porphyry. At the present time the Palo Verde mine is inaccessible, and the fault relationships described by Kinnison
(1958) could not be observed. It seems that his underground fault is the
same fault inferred at the surface. Surface exposures of the fault itself are nonexistent, but field observations and interpretations of aerial photographs of the Amole Formation sediments indicate that the fault
strikes about N. 60° E. and has about 450 feet of lateral displacement.
The attitude of this fault appears very similar to that of the Papago Queen
fault, and the two faults were probably formed at the same time in re sponse to similar forces. The eastern extension of the fault is covered
by Recent gravels, but it probably also cuts the Cat Mountain Rhyolite.
The third major fault zone was also mapped by Kinnison (1958)
and named by him the Saginaw Fissure (Fig. 3). He reports this fault
zone to be a fractured and beared zone a few feet wide which contains
slight traces of alteration and some green copper stain. Detailed mapping
of the fracture zone indicates that minor strike-slip and possibly some
dip-slip movement occurred in this shear zone. The shear zone strikes
about N. 65° E. and dips nearly vertically at the Saginaw mine where it
crops out. The exposed metasedimentary and intrusive rocks that are
cut by this zone are thoroughly brecciated and moderately altered to
calc-silicate minerals and quartz-sericite, respectively. Fracturing and
brecciation decrease in intensity east and west along the strike of the
from the Saginaw mine. Little or no displacement is noted where the
fracture crosses the metasedimentary rocks.
Many other unmineralized and unaltered strike-slip faults paral
lel the trend of the major fault zones (Fig. 3). Only those faults that 57
showed prominent displacement were mapped. These faults are readily observable on aerial photographs and, with some difficulty, can be
located on the ground. Generally, these faults appear to exhibit total
displacements which vary from about 10 feet to 700 feet and appear to
be terminated near the contact between the Amole Formation and the Cat
Mountain Rhyolite.
Many radial faults border the Saginaw Hill intrusive body and
the outcrops of the Saginaw Mine quartz monzonite porphyry (Fig. 3),
although most of these faults generally parallel the trend of the major
fault system. These faults are extremely difficult to map and are usually
not recognized except within 100 to 200 feet of the intrusion-sediment
contact. The faults seem to be significantly related to mineralization
and alteration and will be discussed in greater detail later in this thesis.
Extensive bedding plane slippage was noted in the drill cores and in the
prospect pits throughout the area. Poor exposures did not permit these
slippage planes to be mapped.
Kinnison (1958) mapped two thrust faults which cross the Sagi
naw area: the Burger thrust fault and the Jig thrust fault. His geologic
map indicates that these faults are thrusts and in the text he states
(Kinnison, 1958, p. 85):
The Burger Fault is inferred to explain the structural relation ships seen at the surface. Its association with overturned beds, and the formations in contact, indicate a thrust fault. The ex tension of the Burger Fault and the overturned section of the Dead Cow formation southeast beyond the middle of section 1 is based on speculation rather than on direct evidence.
Referring to the Jig fault, Kinnison (1958, p. 86) states: "The
Jig Fault cuts slightly across the bedding of the Amole group, and
pinches out the Mouse House formation. Its importance or magnitude 58 is not evident even in the vicinity of Wyoming Street, where it can be m apped."
Mapping where these faults were proposed produced no direct or indirect evidence of their existence. These fault zones were found to be the contact between resistant and nonresistant isoclinally folded rock units within the Amole Formation. The reversal in stratigraphy pro posed in this thesis would explain the relationships described by
Kinnison without the assumption of thrust faulting.
Brecciation. A two-foot thick, north-trending pebble dike crops out in a small adit on the south side of Saginaw Hill (Fig. 19).
The dike cannot be traced southward, and it is cut by the Papago Queen fault to the north. It is composed of angular to subrounded pebbles of
Saginaw Hill quartz latite porphyry, Saginaw Mine quartz monzonite porphyry, and Amole Formation-type metasediments. Minor copper oxides and traces of pyrite are found within the matrix and the pebbles.
Similar pebble and breccia dikes (Fig. 20) were encountered in diamond drill holes BC-1, BC-4, BC-5, BC-10, TAC-11, TAC-12, and TAC-13.
The breccia dikes contain angular to subrounded fragments of
Saginaw Hill quartz latite porphyry, Saginaw Mine quartz monzonite porphyry, and Amole Formation-type metasediments. The matrix is com posed of fine-grained interstitial rock fragments cemented by an inter- grown mosaic of quartz and orthoclase (Fig. 21). Some of the fragments are mineralized, especially those composed of porphyry and calcareous metasediments, while other fragments are barren of any sulfides. The groundmass also contains some mineralization. Bulk assays of the
breccia dikes indicate averages of 0.03 percent copper, 0.6 percent 59
Figure 19. Pebble Dike in Outcrop
The pebble dike is located on the south side of Saginaw Hill in a small adit. Host rock is Saginaw Hill quartz latite porphyry.
The sample was intersected in drill hole TAC-11 at 843 feet. The fragments are Saginaw Hill quartz latite porphyry (S) and Am ole Formation metasedimentary rocks. X I.5; core is 2 inches in diameter. 60
Fragments of altered Saginaw Hill quartz latite porphyry (S) and Amole Formation metasedimentary rocks (A) in a matrix of fine grained rock fragments cemented by an intergrown mosaic of quartz (Q) # orthoclase (O), and sulfide minerals (black). 61 zinc, 0.45 percent lead, and 0.08 ounces of silver per ton; no gold assays were available. One analysis of a random sample of the larger pebbles of porphyry was obtained. The analysis indicated 0.10 percent copper, trace lead and zinc, and no detectable gold.
Joints. A rose diagram based on 275 strike measurements
(Fig. 22) summarizes the joint directions and their relative importance in the igneous intrusions. There are two sets of master cross joints which seem to reflect the major N. 60° E. faulting and the N. 30° W. trend of the sediments in the Saginaw area. The subordinate second- order cross joints trend N. 20° E. and N. 75° W. and appear to cut the master joints.
The predominant N. 60° E.- and N. 35° W .-trending master joints arc of particular interest (Fig. 23). They parallel the trend of the strike-slip faults and bedding, respectively. The intensity of the
N. 60° E. joint system increases as the major faults are approached, indicating that the faults influenced the direction and character of jointing (Fig. 3). The N. 30° W.-trending joints may represent a com plimentary set of joints that are perpendicular to the N. 60° E. joints are are parallel to bedding within the Am ole metasediments. The second-
order joints trend about N. 20° E. and N. 75° W. and appear to crosscut and occasionally displace the master joints up to about 12 inches.
Field evidence suggests that both the master joints and the second-
order joints are tensional in nature because the quartz veins filling the joints exhibit classical open-space filling textures. 62
N3o.W.
M/5W..
Scale 1/10 inch = 1 reading 275 readings
Figure 22, Rose Diagram of 275 Joints in the Composite Saginaw Hill Stock 63
Figure 23. Master Joints
Viewed from east looking parallel along N. 60° E.-trending joints in the Saginaw Hill quartz latite porphyry (N. 60° E.). Subor dinate N. 30° W. master joints are also present. 64
Thermal Metamorphic Effects
Thermal metamorphic effects in the Saginaw area are confined to those sedimentary rocks that border the igneous intrusive rocks.
Figure 24 is a map showing the approximate extent of thermal metamor phism based on surface exposures and drill core evidence. The map shows that the thermal metamorphic halo is confined to an area about
650 feet from known igneous rocks. In the area northwest of Saginaw
Hill drill core data indicates that within 650 feet of the surface the
Saginaw Mine quartz monzonite porphyry contacts the overlying Amole
Formation metasediments. It should also be noted that a very narrow metamorphic halo borders the intrusive body south, west, and east of
Saginaw Hill, while northward a more extensive halo exists.
The metamorphic effects range from weak to strong, depending on proximity of the sedimentary rocks to the intrusive rocks. In the coarser grained arkoses, graywackes, and sandstones, the quartz grains were recrystallized and the argillic groundmass has been altered to a soft, white clay which appears to be kaolinite. Arkose, graywacke, and sandstone were metamorphosed to meta-arkose, metagraywacke, and quartzite, respectively.
Thermal metamorphic effects are best developed in calcareous shale and limestone beds in the Limy unit and the Graywacke unit.
These grade from weak recrystallization of calcite to the formation of dense, well-crystallized hornfels. For the purpose of field identification thermal metamorphic effects include the development of calcite and quartz-calcite veins developing in the shales and limestone and the development of calc-silicate minerals in the limestones and shales. 65 mm
EXPLANATION
Approximatation of Saginaw stock outcrops
Approximate limits of thermal me ta morphism Figure 24. Approximate Limits of Thermal Metamorphic Effects
No thermal metamorphism is discernible beyond slashed areas. 66 such as diopside and tremolite, which were considered to mark the be ginning of hornfels formation. In most of the hornfelsic rocks, relict bedding appears to be reflected in segregated layers of quartz, diopside, and other metamorphic minerals; other hornfelsic rocks have a relatively homogeneous texture. Thin section analysis and X-ray studies indicate that diopside, tremolite, and quartz form the major minerals while
•plagioclase and grossularite are also present to a lesser degree. The calcareous rocks that abut the intrusive rocks and associated dikes have been metamorphosed to a skarn mineral assemblage.
. It is difficult to tell which of the above assemblages is related to thermal metamorphism and which is a hydrothermal effect. This prob lem will be considered later in this thesis.
Vein Systems
Three distinct mineralogical types of vein systems were noted in the Saginaw area; quartz veins, calcite veins, and quartz-calcite veins. All three vein systems are found cutting the intrusive porphyritic rock and the metasedimentary rocks. The spacing of the veins gets far ther apart away from the center of the stock located near the crest of
Saginaw Hill (Fig. 3). No veins were found in the unaltered and unmeta morphosed sedimentary rocks. The predominant trend of the vein sys tems is N. 60° E. and N. 35° W. with subordinated N. 20° E. and
N. 75° W. trends also present. These trends are the same as those of the joints and faults previously discussed. Near the junction of the
Papago Queen fault and the main mass of the Saginaw Hill quartz latite porphyry, Kinnison (1958) described a series of quartz veins that 67 appeared to flare outward from the dike and parallel to the border of the intrusive body (Fig. 3). Close inspection of these veins shows that they represent the N. 60° E. and N. 35° W. trends already mentioned.
The attitudes of the vein systems are generally quite uniform, but local divergences can be noted. In general, the N. 60° E-trending veins dip from about 75° 8.to 75° N. with the majority of the veins very nearly vertical. N. 35° W. veins are nearly vertical in the intrusive rocks, while in the metasedimentary rocks they parallel the bedding.
The N. 20° E. and N. 75° W. veins are vertical.
The fracture systems followed by the veins both range from a few millimeters to about three inches in width, with the exception of the massive quartz located west of the Papago Queen mine near the crest of Saginaw Hill (Figs. 3). The quartz veins that crop out near the crest of Saginaw Hill, the quartz and quartz-calcite veins near the Saginaw mine, and diamond drill core data provide evidence on the age relation ships of the various vein systems. The evidence suggests that the
N. 35° W.-trending veins have been displaced by the N. 60° E.-trend ing veins. The relationship of the subordinate vein systems could not be conclusively determined. Where displacement is mapped, it is found to be only a few millimeters to a maximum of 18 inches in extent. Most larger veins show three or more generations of quartz that can be recog nized by differences in color, texture, or intersecting relationships
(Fig. 25). Similar relationships exist in the calcite veins. The "ribbon rock" and the massive quartz that is located near the crest of Saginaw
Hill and which will be discussed later in the thesis further substan tiate multiple quartz generations along the veins. 68
Figure 25. Multiple Quartz Veins
Multiple generations of quartz veining filling a fracture in the Saginaw Hill quartz latite porphyry. X I.5; core is 1.5 inches in diam eter. 69
Geographically, the veins fall into several distinct groups:
(1) quartz veins are found in the intrusive rocks south of the Palo Verde fault, especially adjacent to the Papago Queen fault, and are subordi nate elsewhere in the area; (2) calcite veins predominate in the meta sedimentary rocks and in the intrusive rocks north of the Palo Verde fault; and (3) quartz-calcite veins appear to be evenly distributed throughout the Saginaw Hill-Saginaw mine area. It should be noted that the geographic distribution is based on only 373 random surface samples and about an equal number of drill core samples and does not represent a detailed statistical study.
Quartz Veins Quartz veins are most strongly developed near the crest of
Saginaw Hill, adjacent to the Papago Queen fault (Fig. 3). Even though quartz veins are found throught the area, surface outcrops and drill core data indicate strongest quartz vein development near the Papago Queen fault. Two distinct textural types of quartz veins are exposed there and represent the types that are found elsewhere in the area; single or mul tiple generations of comb quartz that line open fractures and completely fill fractures (Figs. 25 and 26); and massivy, milky white, sugary, hypidomorphic quartz that is a composite of numerous smaller quartz veins (Figs. 27 and 28).
Comb quartz is most abundantly developed near the crest of
Saginaw Hill, but it is also found to a lesser extent throughout the min eralized area. It occurs most commonly in fractures that have been completely filled with quartz and sulfide minerals, but some portions of 70
Figure 26 . Single Quartz Veins
Typical comb-structured quartz veins filling fractures in the Saginaw Hill quartz latite porphyry. X I.5; core is 1.5 inches in diam eter. Figure 27. Massive Quartz
Massive quartz cropping out along the Papago Queen fault near the crest of Saginaw Hill. Photograph taken towards the west.
Figure 28. Massive Quartz Showing Master Joints
Master joints trending N. 60° E. (a) and N. 30° W. (b) developed in the massive quartz. Note the relic veins which are parallel to the N. 60° E.-trending joints. 72 the veins have not been filled completely and crystal terminations normal to the vein wall were observed. The individual crystals are small and rarely exceed 4 mm in length.
The comb-structured quartz veins grade into the massive quartz at the crest of Saginaw Hill (Fig. 3). Several interesting relationships are noted as one approaches the massive quartz from the south:
1. About 250 feet south of the massive quartz veins only rare and
scattered N. 60° E.-trending comb-structured quartz veins are
p resen t.
2. Approximately 115 feet from the massive quartz veins the
spacing between the N. 60° E.-trending quartz veins decreases
until they composed about 30 percent, by volume, of the rock.
Subordinate N. 35° W. and N. 75° W. trending quartz veins
are also present.
3. About 50 feet from the massive quartz the veins trend N. 60° E.,
N. 35° W ., and N. 75° W. and comprise up to 45 percent of
the total rock volume. All the veins exhibit comb structures.
4. From 0 to 10 feet from the massive quartz, and grading into it,
only N. 60° E.-trending quartz veins are present. The veins
make up from 70 to 95 percent of the total rock volume. The
veins no longer exhibit typical comb structures but appear to
have a "ribbon rock" type appearance (Fig. 29).
The gradational zone between typical comb-structured veins and
the massive quartz is termed in this thesis "ribbon rock." The ribbon
rock is well exposed on the southern side of the massive quartz and less
well developed on the northern side of the massive quartz. In hand 73
Figure 29. Ribbon Rock
Ribbon rock exhibiting reniform texture with the quartz crystal terminations pointing towards the upper right-hand corner of the photo graph. Individual veins are about one-quarter to one-half inch thick. 74 specimen the individual quartz veins exhibit crude reniform structures with all the quartz terminations pointing in the same direction (Fig. 29).
On the south side of the massive quartz, the terminations point north ward and on the north side they point southward. These relationships can be observed in approximately 50 veins on the south side of the mas sive quartz and only six veins that are exposed on the north side of the massive quartz. Filling the void spaces between each individual quartz vein is a friable, sandy, highly sericitized material. Microscopic examination indicated that about 60 percent of the material is composed of subrounded to angular quartz particles which vary from about 0.01 mm up to 2 mm in size. The remaining 40 percent is sericite with subordi
nate clay minerals and iron oxides. Thin sections were difficult to make because of the friable and finely particulate nature of the infilling mate rial. The immersion grain mounts consequently provided no information
on the grain boundary relationships. Binocular microscope studies indi
cated that the individual grains were cemented together rather than inter-
grown. Some tiny vugs within this infilling zone containing comb quartz
and sericite were noted (Fig. 30).
Quartz
Infilling zone
Figure 30. Expanded Sketch of Infilling Zone
Drill holes TAC-11, TAC-12, and TAC-13 (Fig. 3) all fail to intersect
the ribbon rock at depth, and only the comb-structured quartz veins were
encountered. 75
The ribbon rock grades into the massive sugary quartz dike.
The dike crops out adjacent to the Papago Queen fault near the crest of
Saginaw Hill. It is about 30 to 40 feet wide and 125 feet long (Fig. 27).
Megascopically, the dike has a sugary or granular texture and appears to exhibit relict banding of one-quarter to two inch quartz veins sepa rated by one to two millimeter limonite bands (Fig. 28). It could not be
determined whether these bands were ribbon rock or the comb-structured
quartz veins. Drill holes TAC-11, TAC-12, and TAG-13 also failed to
intersect the massive quartz dike at depth and only intensive comb-
structured quartz veins were encountered. Microscopically, the dike is
composed of 95 percent anhedral quartz ‘grains and 5 percent oxides of
iron and copper plus small amounts of sericite. Figure 31 is a photo
micrograph of the quartz-dike rock.
The comb-structured quartz veins and the composite quartz dike
are most intensely developed, as previously indicated, in the Saginaw
Hill quartz latite porphyry and Saginaw Mine quartz monzonite porphyry
adjacent to the Papago Queen fault. The quartz latite dikes, even where
they are adjacent to the Papago Queen fault, exhibit only weak quartz
veining. The N. 60° E.-trending quartz veins are found only rarely
while the other trends are nonexistent. Drill holes TAC-11, TAC-12,
and TAC-13 also indicated that the intense quartz veining is strongly
associated with the Saginaw Hill quartz latite porphyry and is less ex
tensively developed in the Saginaw Mine quartz monzonite porphyry and
the quartz latite dikes. Many of the quartz veins in the core are cut by
dikes and fingers of Saginaw Mine quartz monzonite porphyry and quartz
la tite . 76
Figure 31. Massive Quartz Dike in Thin Section
Intergrown anhedral to subhedral quartz (Q) with small veinlet of pyrite and iron oxides (P). Crossed nicols; X 17. 77
Calcite Veins
Calcite veins are found cutting all the rocks in the area immed iately adjacent to Saginaw Hill, but they are mostly strongly developed in the altered and thermally metamorphosed sedimentary rocks. The veins vary in thickness from less than 1 mm up to about 8 cm. The abundance of vugs, comb structures, and euhedral calcite and pyrite indicates deposition in open spaces. Although the calcite veins are found throughout the Saginaw Hill-Saginaw mine area, they appear to be most intensely developed north of the Palo Verde fault around the
Saginaw, mine and in the metasedimentary rocks. Numerous examples of calcite veins cutting quartz veins were noted while only one example of a quartz vein cutting a calcite vein was observed.
Quartz-calcite Veins
In the Saginaw area as a whole, quartz-calcite veins are the most common type encountered and are equally distributed throughout the mineralized area. Quartz is usually found filling fractures and form ing a typical comb-structured vein as previously described, and calcite is found coating the quartz crystals or completely filling voids in the vein. ALTERATION AND MINERALIZATION
Two main types of hypogene alteration have been recognized in the rocks of Saginaw Hill: propylitic alteration and quartz-sericite alter ation. Adjacent to some of the quartz veins and near highly fractured zones, weak argillic alteration appears to have occurred. Because of the rare occurrence of this argillic alteration type and the fact that weak argillic and propylitic alteration assemblages denote very similar physi cal-chemical conditions, all recognized occurrences of argillic altera tion are discussed as a phase of propylitic alteration.
Propylitic alteration, as defined by Meyer and Hemley (1967), includes epidote (zoisite or clinozoisite), albite, chlorite, and carbon ate as the main phases. Sericite, pyrite, and montmorillonite may also be present. Creasey (1966) indicates that propylitic alteration can con sist of any of four main mineral assemblages: (1) chlorite-calcite- kaolinite, (2) chlorite-calcite-talc, (3) chlorite-epidote-calcite, and
(4) chlorite-epidote. The different assemblages are formed in response to varying amounts of CO2 in the system. He also reports that quartz, muscovite, albite, leucoxene, and apatite are ubiquitous to this alter ation phase. In the alteration of felsic to intermediate igneous rocks, based on the above discussion, albite, kaolinite, and calcite may replace plagioclase; orthoclase may exhibit minor replacement by kaolinite and montmorillonite; and biotite is replaced by chlorite and leucoxene.
78 79
Quartz-sericite alteration consists of sericite, quartz, and pyrite as the main phases (Meyer and Hemley, 1967). In felsic to inter mediate igneous rocks, sericite replaces plagioclase, biotite, chlorite, and K-feldspar. Introduced quartz and quartz formed as an alteration byproduct are very common.
The igneous rocks have experienced propylitic alteration except in the areas of intense fracturing where quartz-sericite alteration occurs.
The sedimentary rocks have been propylitized except where calcareous rocks abut the igneous rocks resulting the development of skarns.
Mineralization of vein, disseminated, and replacement types were recognized in the Saginaw area. Hypogene minerals consisted of pyrite, chalcopyrite, galena, sphalerite, and tennantite. Molybdenite and gold are reported by Arthur Jacobs (oral communication). Assays of surface and core samples also show molybdenum and gold, although they were not recognized during this study. Secondary copper minerals recognized are tenorite, malachite, azurite, chalcocite, covellite, and chrysocolla. Dr. Sidney W illiams, Douglas, Arizona (personal communi cation) reports that cornetite, devilite, pseudo-malachite, and libethenite are also found at Saginaw Hill. Secondary zinc minerals include smith- sonite and hemimorphite. Anglesite and cerussite were the only secon dary lead minerals recognized. Hematite, goethite, and jarosite(?) were the limonite minerals found. 80
A lteration
Alteration in Igneous Rocks
Surface mapping and diamond drill core studies indicate that all the intrusive igneous rocks in the Saginaw area have been hydrother- ' mally altered to a greater or lesser degree. At least propylitic alteration has affected all the intrusive igneous rocks in the Saginaw area. Plagio- clase is altered to calcite, kaolinite, montmorillonite, and minor sericite; biotite is altered to chlorite and leucoxene. Some argillic alteration was noted adjacent to quartz veins (Fig. 32) and adjacent to intensely sheared zones. Because of the rare occurrence of this altera tion type, it has been included in the propylitic assemblage. Quartz- sericite alteration is found in the Saginaw Hill quartz latite and Saginaw
Mine quartz monzonite porphyries where they are associated with the
Papago Queen fault, the Palo Verde (?) fault, and the Saginaw Mine fault zones. This alteration type has also been found in other zones of strong N. 60° E. fracturing. The quartz latite dikes exhibit little or no quartz-sericite alteration.
The mineralogical composition of the intrusive rocks, as pre viously discussed, is very similar and it appears as if they will respond in a similar fashion to the hydrothermally altering fluids. With this in mind, alteration will be considered on a basis of assemblages rather than individual rock types.
Propylitic Alteration. All three types of intrusive igneous rocks in the Saginaw area have experienced propylitic alteration to a greater or lesser degree. Specimens that appear very fresh in hand 81
Figure 32. Argillic Alteration
Argillic alteration (A) developed adjacent to a mineralized quartz-calcite vein (V) in propylitized (?) Saginaw Mine quartz monzo- nite porphyry. X 1.5; core 1 inch in diameter 82 sample show secondary calcite, kaolinite, montmorillonite, chlorite, and other minor alteration minerals under microscopic examination. In the freshest specimens these alteration products form about 5 to 10 per cent by volume of the rock. At the opposite extreme are altered rocks that exhibit almost total replacement of the original minerals. Between these extremes are the bulk of the rocks showing intermediate degrees of alteration (Figs. 8, 10, and 33). On the average, the intensity of alteration is relatively constant throughout the area except near in tensely fractured zones where quartz-sericite alteration occurs.
In the propylitically altered rocks, the plagioclase crystals have been altered to kaolinite, montmorillonite, calcite, and, rarely, sericite. Albite is often found rimming the plagioclase crystals or along twin planes. Chlorite and leucoxene are found replacing biotite in vary ing degrees. In the composite stock essentially all the biotite has been replaced while in the quartz latite dikes only about 10 to 25 percent of the biotite has been replaced. In general, the dike system is less altered than the composite stock.
Table 5 presents mineral compositions for the three intrusive rock types varying from the least altered to the most altered. The table was compiled from data obtained by microscopic examination of 75 thin sections plus X-ray diffraction studies of the clay minerals. Point counts on the thin sections varied from approximately 500 to 1,500 per slide at a 0.5 mm interval.
An examination of Table 5 shows that a substantial decrease in the amounts of plagioclase and biotite occurs as the rocks become 83
Figure 33 Propylitically Altered Quartz Latite Porphyry Dike in Thin Section
Plagioclase (P) altered to kaolinite and montmorillonite. Rare epidote (E) replaces hornblende and biotite. Crossed nicols; X 17. TABLE 5. Average Modal Analyses of the Intrusive Igneous Rocks Based on the Least Altered and the Most Altered Samples
Saginaw Hill Saginaw Mine Quartz Latite Porphyry Quartz Monzonite Porphyry Quartz Latite Dikes
L east M oderately M ost L east M oderately M ost Least M oderately M ost M ineral Altered Altered Altered Altered Altered Altered Altered Altered Altered
Q uartz 44.2 40.8 42.1 36.3 3 5 .8 32.2 34.6 32.9 35.0 Plagioclase An30-35 27.8 13.7 2.2 30.5 21.0 11.2 22.3 17.9 15.0 O rthoclase 22.6 23.6 21.0 26.6 28.1 21.3 35.6 32.4 33.0 Albite Trace Trace 0 .9 — 0 .3 1.1 — Trace 0 .9 Kaolinite- M ontmorillonite Trace 10.5 21.3 Trace 5.2 19.6 0.3 8.5 8.1 C alcite 4 .1 9 .4 11.3 3.4 4.5 3.3 1.1 1.8 2.1 S ericite Trace Trace Trace Trace Trace Trace Trace 0.1 Trace Biotite 1.1 0 .3 Trace 1.5 0.3 Trace 3 .2 0 .9 Trace C hlorite Trace 0 .8 1.1 — 1.5 1.0 Trace 3 .4 3 .2 Leucoxene Trace 0 .2 0.3 Trace 0 .4 0 .8 Trace 0.3 0 .8 H ornblende — — — 1.0 1.3 0 .6 1.8 0 .9 1.3 Opaque Minerals 0 .8 0.9 0 .9 1.3 2.4 0 .9 1.2 1.4 0 .9 Sphene Trace Trace Trace Trace Trace Trace Trace Trace Trace Apatite Trace Trace Trace Trace Trace Trace Trace Trace Trace Zircon Trace Trace Trace Trace Trace Trace Trace Trace Trace 85 progressively more altered while an increase of their respective altera tion products also occurs. It is also noted that an anomalously high percentage of calcite is present in the strongly altered as well as the lesser altered rocks. This will be discussed later in this thesis.
It can also be seen in Table 5 that the Saginaw Hill quartz latite porphyry is more pervaisely propylitized than the Saginaw Mine quartz monzonite porphyry, which in turn is more altered than the quartz latite dikes. The presence of abundant kaolinite and montmorillonite indi cates that this alteration assemblage is representative of the argillic assemblage. Meyer and Hemley (1967) argue that kaolinite and mont morillonite minerals predominate as alteration products of intermediate to calcic plagioclase in the argillic zone. They also report that meta stable K-feldspar and blot it e from recrystallized chlorite may also be present. However, Creasey (1966) reports that the argillic zone repre sents strong leaching of lime indicated by the absence of epidote and especially carbonate minerals. He also argues that K-feldspar is un stable in the argillic zone although it commonly persists in argillized rocks as a relict, and even may be unaltered in weakly argillized rocks.
Chlorite also is a stable mineral in the argillic zone although Creasey
(1966) reports that it may alter to muscovite. Miscroscopic observations
summarized in Table 5 indicate that (1) plagioclase is altered to kaolinite, montmorillonite, and calcite; (2) chlorite is a stable alteration mineral;
(3) calcite is added rather than leached; (4) K-feldspar is a stable min eral; and (5) potassium has been leached. Because of the stability of
calcite and K-feldspar this alteration assemblage has been assigned to 86
Creasey's propylitic assemblage rather than to Meyer and Hemley's argillic assemblage.
Ouartz-sericite Alteration. Quartz-sericite alteration is well developed in those rocks associated with strong fracturing, especially along the Papago Queen fault, the Palo Verde fault, and the Saginaw
Mine fissure zone. The exceptions are the quartz latite dikes. The only quartz-sericite alteration found in the dikes was located at the crest of Saginaw Hill while all other dike samples exhibited only weak propylitic alteration. Diamond drill core studies indicate quartz-
sericite alteration has developed along the contacts between the Sagi naw Mine quartz monzonite porphyry and Saginaw Hill quartz latite porphyries. However, these zones often are significantly fractured and the alteration probably is related to the fractures and not to the c o n ta c ts.
Figure 34 is a photomicrograph of quartz-sericite alteration in the Saginaw Mine quartz monzonite porphyry. Sericite is found replacing
plagioclase. Sericite also replaces chlorite, after biotite, along cleav
ages. With increased intensity of quartz-sericite alteration, plagioclase
is extensively altered, pseudomorphs of sericite after chloritized biotite
develop, and orthoclase becomes slightly replaced by sericite along
the borders and fractures in the crystals. Alteration of this type is best
developed along the Papago Queen fault near the crest of Saginaw Hill
(Fig. 35). Kinnison (1958) reports that underground studies in the Palo
Verde mine indicate strong quartz-sericite alteration of the Saginaw Hill
quartz latite porphyry adjacent to the Palo Verde fault. His observations 87
Figure 34. Sericitized Saginaw Mine Quartz Monzonite Porphyry in Thin Section
Sericite pseudomorphs after plagioclase (S) and flecking the groundmass. Note large interstitial calcite crystals (C) with inclusions of subhedral quartz phenocrysts (Q) . Crossed nicols; X 17.
Figure 35. Sericitized Saginaw Hill Quartz Latite Porphyry in Thin Section
Shows sericite pseudomorphs (S) after plagioclase. Sericite also replaces plagioclase and very rarely orthoclase in the groundmass. Crossed nicols; X 17. 88 appear to be similar to the alteration effects along the Papago Queen fa u lt.
Summary of Igneous Rock Alteration. The hydrothermally altered igneous intrusive rocks of the Saginaw area consist of the Sagi naw Hill quartz latite porphyry, the Saginaw Mine quartz monzonite porphyry, and rarely the quartz latite dike system. The alteration in these rocks appears to be complex in its surface exposures, yet there is a general consistency of alteration effects when detailed mineralogic studies are considered (Table 5).
. The most extensive and pervasive alteration consists of a pro- pylitic alteration mineral assemblage. The alteration changed a rock composed of quartz, plagioclase, orthoclase, biotite, and a fine-grained groundmass of quartz, orthoclase, and plagioclase into a rock composed of quartz, calcite, montmorillonite, kaolinite, chlorite, and orthoclase.
As previously discussed, the presence of abundant calcite and fresh orthoclase indicates that this is a propylitic zone rather than an argillic zone. Biotite was replaced by chlorite and leucoxene; plagioclase was altered to calcite, montmorillonite-kaolinite, and traces of sericite; orthoclase remained essentially fresh. Quartz-sericite alteration is confined to those rocks associated with strong fracturing, especially along the Papago Queen fault. Sericite is found replacing plagioclase and chlorite and to a lesser extent, orthoclase.
Alteration in Sedimentary and Metasedimentary Rocks
Because the various types of sedimentary rocks have been sub jected to both thermal metamorphism and metasomatism, the description 89 of their transformation to the variety of end products is rather difficult.
The term metasomatism is used to describe material that has been added or diffused through the rocks resulting, at least in part, from the circu
lation of hot waters. In contrast, thermal metamorphism that simply
recrystallized a sedimentary rock without the addition of material is
not considered to be hydrothermal alteration. The term hydrothermal
alteration is used here to indicate not only the introduction of material
from external sources, but also the movement of introduced fluids along
submicroscopic or capillary openings and replacement which probably
occurred with almost simultaneous solution and deposition on an atomic
s c a le .
The original rock types present near Saginaw Hill prior to meta
morphism were arkose, graywacke, sandstone, calcareous shale, and
limestone belonging to the Limy unit and the lower portions of the Gray
wacke unit. As previously discussed, these rocks are now meta-arkose,
metagraywacke, quartzite, and hornfels. Where calcareous shale beds
contact the stock or are associated with strong fracturing skarn minerals
develop. The evidence discussed suggests that only minor composi
tional changes can be attributed to the contact metamorphism associated with the intrusion of the stock.
Subsequent to thermal metamorphism, the metasedimentary rocks
were subjected to propylitic alteration, silicification, and metallization.
No hypogene alteration was observed in the unmetamorphosed sedimen
tary rocks. The extent and nature of silicification is unknown because
of extreme difficulty in distinguishing alteration quartz from recrystal
lized quartz grains. However, silica-rich hornfels and abundant 90 quartz veining indicate that quartz was introduced. Minor to extensive propylitic alteration was observed in the meta-arkose and metagraywacke beds depending on the proximity to the intrusive rocks and associated fracture zones. Montmorillonite, kaolinite, and minor sericite is found replacing plagioclase and groundmass, while chlorite replaced rare bio- tite crystals and appeared to form halos around some of the mineralized veins. Differentiating altered and unaltered hornfels was not possible in the course of this study, although it may be assumed that about 85 percent of the hornfelsic rocks observed experienced some alteration due to the introduction of vein and disseminated sulfides. The skarn zones, which were probably formed by thermal as well as hydrothermal processes, experienced late introduction of vein sulfides which resulted
in alteration effects. Metallization will be discussed later in this thesis.
Mineralization
Vein and disseminated mineralization is found in all the igneous
and thermally metamorphosed rocks in the Saginaw area, while replace
ment-type deposits are confined to the Limy unit and carbonaceous beds within the Graywacke unit.
Disseminated Mineralization
Pyrite and chalcopyrite are the main disseminated sulfides
recognized; hematite and magnetite were found in a few polished sur
faces of Saginaw Hill quartz latite porphyry, and disseminated sphalerite
and galena were rarely noted in both the Saginaw Hill quartz latite and
the Saginaw Mine quartz monzonite porphyries. The sedimentary rocks
generally contain most of the disseminated sulfide mineralization. 91
Sulfide mineralization of the various rock types, in decreasing order of abundance, are as follows: hornfels, calcareous beds, Saginaw Hill quartz latite porphyry, Saginaw Mine quartz monzonite porphyry, arkosic beds, graywacke beds, and the quartz latite dikes. Pyrite is found in all rock types but is less commonly found in the arkosic and graywacke beds. Chalcopyrite is found in the hornfels, calcareous beds, Saginaw
Hill quartz latite porphyry, Saginaw Mine quartz monzonite porphyry, and rarely in the quartz latite dikes.
Meta sedimentary Rocks. In the meta sedimentary rocks pyrite occurs as anhedral to euhedral grains which are usually less than 2.5 mm in diameter. Pyrite is found filling pore spaces in the metasediments, especially in the hornfelsic and calcareous arkosic rocks. Chalcopyrite is found as disseminated anhedral microscopic grains.
Saginaw Hill Quartz Latite Porphyry. In the Saginaw Hill quartz latite porphyry minor euhedral and anhedral pyrite is found close ly associated with altered biotite and plagioclase. The bulk of the pyrite and less abundant chalcopyrite is anhedral and disseminated throughout the igneous rock. At Saginaw Hill, pyrite and chalcopyrite disseminations reach their maximum concentration within 200 feet north and south of the Papago Queen fault. Within this narrow zone, 2.65 percent sulfides by volume were noted, while outside this zone, total volume percent sulfides averages from 0.5 to 1.85 percent. Within this 400-foot zone, copper averages about 0.3 percent, while outside this zone the percentages range from 0.01 to 0.15. In areas where strong N. 60° E. fracturing occurs, the mineralized zone is modified or extended (Fig. 3). 92
Saginaw Mine Quartz Monzonite Porphyry. In the Saginaw
Mine quartz monzonite porphyry, pyrite and chalcopyrite occurrences are similar to their occurrences in the Saginaw Hill quartz latite por phyry. Pyrite and chalcopyrite are commonly associated with altered biotite and plagioclase while the bulk of mineralization is found as discrete disseminated anhedral grains. Total sulfide percentages plus higher copper values are associated with the fractured and sheared z o n es.
Quartz Latite Dikes. Only trace amounts of pyrite and very rare chalcopyrite are disseminated in the quartz latite porphyry dikes.
Minor pyrite and chalcopyrite are associated with altered biotite and feldspars. The remainder of the sulfides are found as anhedral dis
seminated grains.
Vein Mineralization
Hypogene vein-type sulfide mineralization is associated with the quartz, quartz-calcite, and calcite vein systems previously des
cribed. The following pertinent facts were noted about those vein sys tem s:
1. Quartz veins are mostly strongly developed south of the Palo
Verde fault, especially adjacent to the Papago Queen fault.
The primary trend of these veins is N. 60° E. and N. 35° W .,
although N. 70° W. and east-west trends are also present
(Fig. 22). Pyrite and chalcopyrite are the most abundant sul
fides in these veins. 93
2. Calcite veins appear to be more prevalent in the area north of
the Palo Verde fault, in the metasedimentary rocks, and asso
ciated with fracture systems. Pyrite, sphalerite, galena, and tennantite are the most common sulfide minerals noted.
3. Quartz-calcite veins are found equally distributed throughout
the are a.
4. Intensity of all the vein systems decreases as the distance
from the intrusive rocks is increased.
Pyrite is the most abundant hypogene sulfide in the various vein systems and is always associated with the other sulfides: chalco- pyrite, galena, sphalerite, and tennantite. Gold and silver, unknown mineral associations, and molybdenite are reported by Jacobs (oral com munication) and in Bear Creek and Anaconda assay results, but they were not identified during this study. Secondary minerals in the veins are angle site, cerussite, smith sonite, hemimorphite, chrysocolla, tenorite, azurite, malachite, covellite, chalcocite coating pyrite, and various limonites. In addition, Dr. Sidney Williams (personal commun ication) has noted cornetite, pseudo-malachite, libethenite, and devilite in the area.
The major hypogene sulfide minerals tend to be concentrated to a greater or lesser degree in the various vein systems previously described. Pyrite is the most abundant sulfide mineral and is found extensively in all three vein types. Chalcopyrite is most strongly developed in the quartz veins, while galena, sphalerite, and tennan tite are most important in the calcite vein systems. In the 94 quartz-calcite veins, pyrite plus one or all of the other sulfide minerals may be present in varying proportions.
Replacement Mineralization
Replacement-type mineralization is confined to calcareous beds within the Limy and Graywacke units that are associated with N. 60° E.- trending fracture systems and have a close proximity to the intrusive rocks. Kinnison (1958) describes the mineralization in the Palo Verde
mine, where he noted a three-foot limestone bed that was completely
replaced by sphalerite, galena, pyrite, chalcopyrite, and quartz. He
also reports that the replaced limestone bed is adjacent to the Palo
Verde fault near the Saginaw Hill quartz latite porphyry-Amole sediment
fault contact. Mineralization is restricted to zones adjacent to N. 60°
E.-trending fractures. At the present time, the Palo Verde mine is in
accessible and additional data could not be obtained. No information
on the Dakota mine or the Gypsy Queen mine could be obtained. Dump
samples indicate that galena, sphalerite, pyrite, and trace chalcopyrite
replaced a calcareous sediment. The samples collected appeared similar
to those obtained from the Palo Verde dump.
Diamond drill core samples noted considerable sulfide replace
ment of one-inch to two-foot carbonate units within the Limy and Gray
wacke units. Pyrite, sphalerite, galena, tennantite, and chalcopyrite
are the primary sulfides found replacing these units.
Skarn Zones
Skarn zones are developed along intrusive-sediment contacts.
The skarns are only developed in carbonate-rich beds of the Limy and 95
Gray wacko units that abut the intrusive bodies. This selective skarn development results in scattered two to six foot, discontinuous outcrops of mineralization. A series of prospect pits on the west side of Saginaw
Hill and the area adjacent to the Saginaw mine represent the only sur face exposures of skarns found. Drill core samples provided the bulk of the data.
Drill core data suggest that a limestone or calcareous shale that is traced towards the stock becomes progressively metamorphosed until a hornfels is formed. Adjacent to the intrusive the hornfels be comes knotty or nodular, the knots forming around porphyroblasts of garnet and diopside. The skarn minerals include garnet, diopside, epi- dote, calcite, quartz, magnetite, and minor specularite. Sulfides are found in veins cutting the skarn zone and disseminated within the skarn.
Magnetite and specularite predominate as disseminations, while pyrite, chalcopyrite, sphalerite, galena, and tennantite are primarily confined to the veins.
Paragenesis
Figure 36 is the proposed paragenetic sequence of hypogene mineralization in the Saginaw Hill area. The sulfide paragenesis was based on the vein mineralization because the disseminated minerals ex hibited no boundary relationships, except for occasional occurrences of chalcopyrite replacing pyrite (Fig. 37). In the vein systems, pyrite cuts and is cut by all other sulfide minerals. Chalcopyrite is most often found as disseminations (Fig. 38) and associated with quartz veins in the stock and metasediments. In the veins, chalcopyrite is found filling 96
EARLY LATE Quartz
O
7 VllaluOpyi lit: f
Sphalerite 7 - n
G alena 7 7 7
Tennantite ?
Figure 36. Paragenetic Sequence of Hypogene Minerals in the Saginaw Hill Area 97 fractures in pyrite, exsolved from sphalerite (Figs. 39, 40, 43, and 47) and tennantite (Figs. 41 and 45), replaced by and included in galena
(Figs. 42 and 45), replaced by sphalerite (Fig. 43), and replaced by tennantite (Figs. 41 and 45). Galena contains inclusions of and re places pyrite and chalcopyrite (Figs. 42 and 45), replaced by and in cluded in sphalerite (Figs. 39 and 44) and is replaced by tennantite
(Figs. 42 and 45). In many veins, galena also maintains mutual boun daries with sphalerite (Fig. 40) and tennantite. Sphalerite replaces pyrite (Figs. 40 and 47), contains exsolved chalcopyrite (Figs. 39, 40,
43 and 47), replaces galena (Figs. 39 and 44), and is replaced by ten nantite (Figs. 46 and 47).
Zoning
The distribution of major and minor ore minerals in the Saginaw area was considered in light of geochemical results, the distribution of the various veins systems, and hand specimen and polished section studies of the drill core samples. Pyrite is found to be widespread in its distribution and is found in both the metasedimentary rocks associ ated with the intrusive rocks and the intrusive rocks themselves. Mag netite, as previously discussed, is concentrated in the altered carbon ate rocks of the Limy and Graywacke units that abut the intrusive rocks.
The rare magnetite disseminated in the intrusive rocks is not considered in the zoning pattern being described. Copper mineralization, although found throughout the area in trace amounts, is most strongly concentrated near the crest of Saginaw Hill. At this locality, the strongest quartz- chalcopyrite veins and disseminated chalcopyrite are developed. Figure 37. Disseminated Pyrite and Chalcopyrite
Disseminated pyrite (py) replaced by chalcopyrite (cp). Parallel nicols; X60.
Figure 38. Disseminated Chalcopyrite
Chalcopyrite (cp) and minor pyrite (py). Parallel nicols; X 60. 99
Figure 39. Sphalerite-Chalcopyrite-Galena
Galena (gn) is included in and replaced by sphalerite (si). Chalcopyrite (cp) is exsolved from sphalerite. Parallel nicols; X60.
Figure 40. Pyrite-Chalcopyrite-Sphalerite-Galena
Sphalerite (si) contains exsolved chalcopyrite (cp), replaces pyrite (py), and maintains mutual boundaries with galena (gn) . Parallel nicols; X60. 100
Figure 41. Tennantite-Chalcopyrite
Chalcopyrite (cp) is exsolved from and replaced by tennantite (tenn). Parallel nicols; X60 .
Figure 42. Galena-Chalcopyrite-Tennantite Galena (gn) replaces and maintains mutual boundaries with chalcopyrite (cp). Tennantite (tenn) replaces chalcopyrite (cp) and galena (gn). Parallel nicols; X 60 . 101
Figure 43. Pyrite-Sphalerite-Chalcopyrite
Pyrite (py) and chalcopyrite (cp) are disseminated in quartz (q). Sphalerite (si) in vein replaces chalcopyrite (cp) and contains exsolved chalcopyrite (cp) . Parallel nicols; X 150.
Figure 44. Galena-Sphalerite-Chalcopyrite
Galena (gn) included in and replaced by sphalerite (si). Chalcopyrite exsolved from sphalerite (si). Parallel nicols; X 60. 102
Figure 45. Pyrite-Galena-Tennantite-Chalcopyrite Pyrite (py) maintains mutual boundaries with tennantite (tenn) and galena (gn). Tennantite (tenn) replaces galena (gn) and contains ex solved chalcopyrite (cp). Parallel nicols; X 60.
Figure 46. Tennantite-Sphalerite
Tennantite (tenn) replaces sphalerite (si). Parallel nicols; X 60. 103
Figure 47. Sphalerite-Chalcopyrite-Pyrite-Tennantite
Sphalerite (si) contains ex solved chalcopyrite (cp) and replaces pyrite (py). Tennantite (tenn) replaces pyrite (py) and sphalerite (si). Parallel nicols; X 60 . PLEASE DO NOT REMOVE THIS SLIP FROM POCKET RENEWABLE ONLY IF NO OTHER BORROWER HAS ASKED LIBRARY TO HOLD IT FOR HIS USE. 104
Elsewhere chalcopyrite is essentially concentrated in shear zones and fractures associated with lead and zinc with little actual dissemination.
Sphalerite and galena are most strongly developed in the sedi ments that border the intrusive zones as both replacement and skarn bodies. Although veinlets of galena and sphalerite do cut the intrusive bodies, the greatest intensity of this mineralization is found on the periphery of the strongest chalcopyrite mineralization.
In summary, strong copper mineralization occurs near the crest
of Saginaw Hill and in fractures in the Saginaw Mine quartz monzonite
porphyry near the Saginaw mine; magnetite, galena, sphalerite, and
chalcopyrite are found in skarn zones that are located at the intrusive-
sediment contact; galena, sphalerite, and weak copper are found in
satellitic ore zones in the sedimentary rocks peripheral to the copper
zone; pyrite is found in all the zones. Molybdenite was reported in the
drill core assay logs although it was not recognized during this study.
The assay logs suggest that molybdenite is found near the center of the
stock. Gold and silver are also reported in the assays, but their min
eralized relationships are not known. The assays indicate that concen
trations of gold and silver are associated with the peripheral replacement
deposits in the sediments or with galena-sphalerite veins in both the
stock and the metasedimentary rocks. The distribution of manganese
carbonates or oxides is unknown.
Geochemical Analyses
In 1961, the Bear Creek Mining Company conducted an exten
sive copper geochemical surgey of the Saginaw Hill area. The 105 geochemical results plus sample locations were provided to the author by Mr. Arthur Jacobs. The samples were taken on a 100-foot center grid system and plotted on a 1 inch = 400 feet topographic base map. At most of the sample stations both rock and soil samples were taken. Figure 48 is the contoured map constructed by the author.
The geochemical results were contoured into orders of magni tude as follows: 1st order—greater than 0.2 percent (2000 ppm) Cu,
2nd order—0.1 to 0.2 percent (1000-2000 ppm) Cu, 3rd. order—0.05 to
0.1 percent (500-1000 ppm) Cu, 4th order—0.01 to 0.05 percent (100-
500 ppm) Cu, and 5th order—less than 0.01 percent (100 ppm) Cu (Fig.
48). An explanation of the origin of the geochemical anomalies is pro vided when Figure 48 is compared with the geologic map (Fig. 3). The most extensive 1st order anomaly is located near the crest of Saginaw
Hill. The anomaly is related to the mineralization associated with the quartz veins and massive quartz adjacent to the N. 60° E.-trending
Papago Queen fault and to disseminated mineralization that forms a halo around the fault. The broader 2nd order anomaly is also associated with quartz veining plus disseminated mineralization that forms a halo parallel to the main fault. Less intense 1st and 2nd order anomalies are located near drill hole BC-4, south of drill hole BC-5, about 500 feet north of the Papago Queen fault at the contact between the Saginaw Hill quartz latite porphyry and the Graywacke unit and north and east of the Saginaw mine (Fig. 3). At all these localities, strong N. 60° E. fracture systems have developed in metasedimentary and intrusive igneous rocks. The anomalies are elongate along these fracture systems. Near BC-4 and north and south of the Saginaw mine, the 1st, 2nd, and 3rd order 106
EXPLANATION
> 0.2 0 . 1- 0.2 .05-0.1
.01-.05 <.01 PERCENT COPPER AND ORDER OF ANOMALY
SAMPLES ON IOO-FOOT CENTERS o o
8 0 0 FEET
Figure 48. Copper Geochemical Map
Compiled from Bear Creek Mining Company data. 107 anomalies are related to the Saginaw fissure and associated fractures plus subordinate skarn mineralization in the calcareous sediments. Dis
seminated sulfides plus vein sulfides were noted in this zone. The 1st,
2nd, and 3rd order anomalies noted 500 feet north of the Papago Queen
fault at the igneous-metasedimentary rock contact is related to N. 60° E.
fractures and faults. The anomaly located near drill hole BC-5 is asso
ciated with N. 60° E. fractures, but this is not as conclusive as are the other areas.
The Saginaw Hill quartz latite porphyry outcrops are generally
represented by 3rd order anomalies. The anomaly is confined to the
intrusive rocks except in areas where skarn mineralization occurred in
calcareous beds that contact the intrusive rocks. The extensions of 3rd
order anomalies usually are associated with strong N. 60° E. fracturing.
In the area of the Saginaw mine, 3rd order anomalies occur which are
related to the Saginaw fissure and related fractures plus associated
skarn mineralization. Two isolated 3rd order anomalies occur near the
Palo Verde fault. No clear-cut relationships exist that can explain these
anomalies. They may represent mineralization associated with the fault,
or it may represent contamination of the samples which appear to have
been collected in the drainage from the Saginaw mine. The remainder of
the area consists of 4th and 5th order anomalies.
In addition to the geochemical map, complete assay data on all
Bear Creek, Anaconda, Ventures, and Calumet and Arizona diamond core
drilling was made available by Mr. Arthur Jacobs. Drill holes BC-1, BC-2,
BC-4, and BC-10 were collared and bottomed in Limy unit and Graywacke
unit rocks (Fig. 3). No assays were available on drill holes BC-2 or 108
BC-3. In drill hole BC-1, copper assayed from trace amounts to 300 ppm while zinc varied from trace amounts to 500 ppm except in calcareous
zones that were associated with dikes of Saginaw Hill quartz latite por
phyry. In these 40- to 50-foot zones, skarn mineralization occurred,
and zinc values from 1600 ppm to 5200 ppm were noted. No increase of
copper mineralization was indicated in these zones. In drill core BC-10
copper, lead, and zinc averaged 200 ppm, 150 ppm, and 150 ppm, re
spectively. The higher values were associated with calcareous units
and lower values were related to arkosic and graywacke units.
TAC-13 drill hole was also collared in the sediments south of
Saginaw Hill (Fig. 3). The hole was drilled on an angle of 45 degrees
trending N. 40° W. The entire hole averaged about 1200 ppm copper with
varying amounts of lead, zinc, silver, and gold. The metasedimentary
rocks assayed at about 500 ppm copper and trace amounts of lead, zinc,
gold, and silver. From 155 to 165 feet the Saginaw Hill quartz latite por
phyry is intruded by numerous dikes of Saginaw Mine quartz monzonite
porphyry containing fragments of Amole Formation metasedimentary rocks.
In this 10-foot zone, 1.5 percent lead, 0.9 percent zinc, 0.004 percent
molybdenum, and 0.11 percent copper was encountered. The sulfides
occur in quartz and quartz-calcite veins and as disseminations in the
intrusive rocks and altered meta sedimentary rock fragments. A similar
increase in mineralization was encountered under the same controls be
tween 343 and 353 feet and 641 to 673 feet. The next zone of strong
mineralization occurred between 2655 and 2715 feet where skarn miner
alization occurred at the intrusive rock-metasedimentary rock contact.
Copper averaged 2700 ppm and lead and zinc averaged about 15,000 ppm 109 and 5000 ppm, respectively. Trace amounts of silver and gold were re ported. From 2715 to the bottom of the hole at 2740 feet, copper dropped to 600 ppm and only trace amounts of lead, zinc, gold, and silver were n o te d .
Drill hole BC-7 was collared and bottomed in Saginaw Hill quartz latite porphyry. A 9-foot thick dike of Saginaw Mine quartz mon- zonite porphyry and a 16-foot thick quartz latite dike is encountered at
296 feet and 752 feet, respectively. The entire hole averages approxi mately 0.1 percent copper, with only trace amounts of lead and zinc.
Assays for gold and silver were not available, and only two molybdenite assays were made which indicated only about 0.003 percent. Minerali zation is confined to sulfide disseminations and a few quartz and quartz- calcite veins containing chalcopyrite, sphalerite, and galena.
Drill hole BC-8 was collared and bottomed in Saginaw Hill quartz latite porphyry (Fig. 3). The hole ran 850 ppm copper for the first
500 feet, while the remaining 416 feet averaged only about 270 ppm cop per. Zinc and lead assays of 520 ppm and 500 ppm, respectively, were available only from 694 feet to the bottom of the hole. Polished sec tions and megascopic studies indicated only trace amounts of galena and sphalerite in the upper portion of the hole. From 764 to 774 feet, a
few calcite veins containing sphalerite, galena, and chalcopyrite were encountered. In this zone, copper assayed 1500 ppm, while lead and
zinc values were 14,000 ppm and 12,000 ppm, respectively.
Drill hole BC-5 was collared in Saginaw Hill quartz latite por
phyry and bottomed in metasedimentary rocks, which are similar to those
found in the Limy unit. Scattered pods and lenses of Saginaw Mine quartz 110 monzonite porphyry are found intruding the Saginaw Hill quartz latite porphyry, and one 11-foot breccia dike was encountered. From the col lar to 1130 feet, assays indicate about 400 ppm copper and trace amounts of molybdenum, lead, and zinc. Copper values increase to about 1000 ppm in fractured zones and in zones that contain quartz veins. At 856 to 1241 feet, copper, lead, zinc, gold, and silver values increase significantly to 1000 ppm, 1500 ppm, 800 ppm, trace, and 1000 ppm, respectively. In zones where intense fracturing and brecciation occur, 8000 ppm lead, 1000 ppm copper, 57,000 ppm zinc, 50 ppm gold, and 21,000 ppm silver ppm silver have been reported. From 1241 feet to the intrusive rock-metasedimentary rock contact at 2727 feet, copper values ranged between 6000 ppm to 1700 ppm. High values are associ ated with sulfide vein systems and fractures. Lead assays varied from trace amounts up to 3000 ppm and zinc varied from trace amounts up to
8000 ppm. Higher values were related to quartz-calcite veins, calcite veins, and highly fractured zones. Gold and silver appeared only as trace elements. At the intrusive rock-metasedimentary rock contact, at
2727 feet, skarn development and mineralization of calcareous beds plus numerous calcite veins with associated galena, sphalerite, and chalcopyrite mineralization occurred. Within 100 feet of the contact,
0.25 percent copper, 1.85 percent lead, 3.52 percent zinc, and trace amounts of gold and silver were noted. Within five feet of the contact,
1.02 oz/ton silver is reported. From 2827 feet, metal values drop sig nificantly. The copper values vary from 900 ppm to 100 ppm, lead and
zinc are not reported, and only trace amounts of silver and gold were
n o te d . I l l
Only copper assays were available for drill hole BC-9 . The assays varied from trace amounts up to 600 ppm with higher values as sociated with fractured zones and quartz and quartz-calcite veins. The hole was collared and bottomed in Saginaw Hill quartz latite porphyry.
Drill holes TAC-11 and TAG-12 are angle holes that are col lared in Saginaw Hill quartz latite porphyry. TAC-11 plunges at 45 degrees with a trend of N. 33° W ., while TAG-12 plunges 65 degrees with a trend of N. 10° W. TAC-11 penetrated 1483 feet of Saginaw Hill quartz latite porphyry that is completely intruded by dikes and lenses of
Saginaw Mine quartz monzonite porphyry and a few quartz latite dikes.
The hole bottomed at 1759 feet in Limy unit sediments.
In drill hole TAC-11 copper assays averaged about 0.03 per
cent for the entire hole. Gold and silver were present in only trace
amounts. From 1389 to 1394 feet, lead, copper, zinc, and silver values
were 5700 ppm, 600 ppm, 4900 ppm, and 0.40 oz/ton, respectively.
Within this zone a few calcite veins with sphalerite, galena, and ten-
nantite were encountered. In the rest of the hole no significant lead-zinc
assays were noted. Copper values from zero to 392 feet remain relatively
uniform with the exception of strongly fractured zones between 130-150
feet, 244-257 feet, and 287-297 feet. In these zones copper assays
indicate from 4000 to 6000 ppm copper which is related to the fracture
zones and a few quartz-sulfide veins. At 392 feet, a 30-foot dike of
Saginaw Mine quartz monzonite porphyry was encountered which assayed
only 400 ppm copper. This was the only portion of the Saginaw Mine
quartz monzonite porphyry that did not contain copper values similar to
those in the associated Saginaw Hill quartz latite porphyry. From 636 112 to 829 feet, a significant increase in copper values was noted. Assays averaged about 6000 ppm copper. The high copper values are related to quartz-sulfide veins and disseminated sulfide mineralization within this
193-foot zone. Extensive shearing and fracturing is also present. This zone, if projected to the surface, coincides with the quartz veins adja cent to the Papago Queen fault. From 844 to 934 feet, complex interfin gering of Saginaw Mine quartz monzonite porphyry with Saginaw Hill quartz latite porphyry is associated with strong shearing and fracturing.
Within this zone, copper assays range from 0.40 to 1.36 percent. Cop per values drop significantly to about 300 ppm from 1124 to 1483 feet at the bottom of the hole. This zone contains very little fracturing and rare quartz veins.
In drill hole TAC-11, copper assays average 0.19 percent.
Gold, silver, lead, and zinc are present only in trace amounts except when associated with calcite veins in the intrusive bodies or in vein, skarn, or replacement deposits in the metasedimentary rocks. The only anomalous lead and zinc noted in the intrusive rocks occurred in a 6-foot interval at a 558-foot depth. This interval is strongly fractured and con tains calcite and quartz-calcite veins with associated galena and sphal erite. Copper values remain constant, about 800 ppm, in the intersected intrusive rocks, although in zones of intense fracturing and quartz vein-
ing, values may reach 0.5 percent copper. At 1414 feet, the intrusive
rocks are in contact with metasediments which are very similar to Limy
unit metasedimentary rocks. Disseminated sulfides, quartz-calcite and
calcite veins with sulfides, and skarn-type mineralization are present
within 35 feet of the contact. Copper assays were 0.75 percent while 113 lead and zinc were 0.30 percent and 0.65 percent, respectively. Only trace amounts of gold and silver were noted. From 1449 feet to 1595
feet, copper assayed at 500 ppm, and only traces of lead and zinc were
noted. At 1595 feet, an 8-foot, intensely fractured zone with strong cal-
cite veining assayed at 0.16 percent copper, 0.20 percent lead, and
1.0 percent zinc. From 1603 feet to 1745 feet, 500 ppm copper and trace
lead, zinc, silver, and gold were noted. At 1745 feet, a 10-foot thick
hornfels was intersected that was intensely fractured and contained
abundant quartz-calcite and calcite stringers and veins up to 1.25
inches thick. Copper assays were 0.32 percent, while lead and zinc
assayed at 3.4 percent and 5.85 percent, respectively. The last four
feet of the hole contained 500 ppm copper and trace amounts of lead and
zin c.
Drill hole BC-6 was collared in the Graywacke unit; however,
from about 615 feet to the bottom of the hole abundant limestones and
calcareous beds were encountered which may represent Limy unit meta
sediments. No assays are available for the upper 615 feet. Within the
lower portion of the hole copper assays varied from traces to a maximum
of 500 ppm. Within the calcareous beds, lead and zinc assayed 250 ppm
and 2500 ppm, respectively. Mineralization was associated with replace
ment of calcareous beds, in calcite veins, and disseminated in hornfel-
sic ro ck s.
Drill hole BC-4 was collared in the metasedimentary rocks of
the Limy unit. The first 187 feet is composed of Limy unit metasediments
in intrusive contact with dikes and lenses of Saginaw Mine quartz mon-
zonite porphyry and quartz latite porphyry dikes. From 187 feet to 882 114 feet, Saginaw Mine quartz monzonite porphyry is completely intruded by numerous quartz latite dikes. The contact between Saginaw Mine quartz monzonite porphyry and the underlying metasedimentary rocks is at 822 feet. The meta sedimentary rocks continue to the bottom of the hole at
1014 feet. Copper assays average about 900 ppm from 0 to 890 feet.
Below this, no assays are available. Copper values did not vary sig nificantly as the various igneous rock boundaries were crossed nor did they increase at the igneous rock-metasedimentary rock contact. Lead, zinc, silver, gold, and molybdenum assays are available from the sur face to 428 feet. Lead, zinc, gold, and molybdenum are present only in trace amounts while silver assays up to 3000 ppm were noted in strongly fractured zones.
The Calumet and Arizona Mining Company drilled five holes in the highly mineralized area near the crest of Saginaw Hill (Fig. 3). The core and individual assays were not available to the author. However,
Mr. W. H. Loerpabel, in a report supplied by Mr. Jacobs (private report,
American Smelting and Refining Company, 1946) noted the following assay results.
Drill Hole Depth in Feet oz Gold/ton oz Silver/ton % Copper
1 376 0.007 0.16 0 .4 4
2 293 .007 o CD .37 O CD 3 270 .007 0 0
4 290 - - .25
5 271 - - .54
He reports that the holes were sampled in 5-foot sections, a values ranged from 0.10 percent to 1.50 percent copper. No significant 115 change in the assays was reported with depth, the higher values being associated with fractures and quartz veins.
Geochemical assays of mine dump samples and discovery hole samples were also provided to the author by Mr. Jacobs.,The nature of the samples and their method of collection is unknown to the author, and although their representativeness is questionable, they do give an indica tion of the nature of mineralization from their areas of collection. The sample location and assay results are described in Table 6. 116
TABLE 6. Geochemical Assays
Sample Location (width in Au Ag Cu Pb Zn feet) oz/ton oz/ton % % %
1. Palo Verde mine Dump 1 .4 1.0 dump
2. Discovery pit 3 0.01 0 .4 — 0 .7 — 550 ft N of Palo Verde mine
3. Gypsy Queen mine Dump 0.06 3 .6 0.31 3 .1 0 .7
4. Dakota Shaft Shipment 0.20 6 .0 0 .5 1.1 0 .3
5. Discovery pit Dump 0 .0 4 480 ft NE of Gypsy Queen mine
6. Papago Queen mine Dump 0.01 0 .4 0 1.03 — — — —
7. Palo Verde mine
40' Level 4 0.03 1.6 — — 1.2 —
40' Level ? 0 .0 1 3 .8 — 4 .5 7.1
135' Level ? 0.01 0 .7 0.03 — —
135' Level ? 0.02 3 .5 —— 15.8 10.7
135' Level 5 .5 0.02 4 .6 —— 8 .2 6 .5
215' Level 3 0.04 2 .8 — — 0 .3 10.8
215' Level ? 0.04 5.0 — 2 .7 10.2
215' Level 10 0.04 4 .4 —— 1 .4 10.5
215' Level 7.5 0.035 4.1 — — 2 .0 10.1
335' Level 15 0.02 2.0 0.07 0.2 0 .3 5 CONCLUSIONS
Pre-Saginaw Stock Interpretations
The oldest rocks exposed in the Saginaw area consist of about
5,000 feet of interbedded arkose, shale, limestone, graywacke, and sandstone which have been assigned to the Cretaceous Amole Formation by Brown (1939). The Amole Formation, in the Saginaw area, can be subdivided into four mappable units, from oldest to youngest, the Limy unit, the Graywacke unit, the Shale unit, and the Arkose unit. The con tacts between the units appear to be gradational in character. The Amole
Formation sedimentary rocks are bounded by Recent gravels to the north, south, and west of Saginaw Hill and unconformably overlain by the
Tucson Mountain chaos and Cat Mountain Rhyolite to the east. At
Snyder Hill, Permian limestones are exposed, and Bryant (1955) reports that the Amole Formation rests unconformably on them.
Following the deposition of the Amole Formation, but before
Cat Mountain time, the Permian limestones and the Amole Formation were folded into a broad asymmetrically inclined synclinorium that trends ap proximately N. 40° W. and plunges toward the southeast. This folding was a probable response to a major period of northeast-southwest com pression. button (1958) and Wilson (1962) report that northeast-south west compression during the Laramide period in southern Arizona is fairly
common. Brown (1939) reported that the Amole Formation is folded into a
broad, open syncline in the central part of the Tucson Mountains.
Kinnison (1958), Assadi (1964), Bennett (1957), and Colby (1958) infer
117 118 the existence of a major syncline trending about N. 45° W. along the western margin of the Tucson Mountains. Mayo (personal communication) reports that in the Sedimentary Hills area and in the Museum Embayment area the Cretaceous Amole Formation is folded into a broad northwest trending syncline. Robert Metz (personal communication) inferred that the Cretaceous sedimentary rocks in the Sierrita mine area are folded
into a north-northwest-trending syncline. These isolated reports on the
synclinal character of the Amole Formation, of which Saginaw Hill is a
part, may indicate that a major syncline, of regional extent, extends
from the northwestern side of the Tucson Mountains to the southeastern
side of the Sierrita Mountains. Further study is required to substantiate
this proposal.
It has been previously discussed that the major N. 60° E.-
trending faults that cut the Amole Formation are truncated by or rapidly
die out in the Cat Mountain Rhyolite. The exception are those faults
that arc associated with the intrusion of the Saginaw Stock. Bedding
plane faults were also noted in the drill core and prospect pits. The
evidence suggests that in conjunction with the development of the syn-
clinorium major N. 60° E. faulting and fracturing cut the bedding and
N. 35° W. faulting along bedding occurred. The extent or nature of
initial movement along the N. 60° E. faults could not be determined
because of post-stock strike-slip faulting. The post-stock faulting will
be discussed in more detail later in the thesis. The N. 35° W. fractures
possibly represent gravity faults that resulted from tilting. Total dis
placement along these faults also could not be determined. The timing
of this early deformation is fairly straightforward. Deformation followed 119 deposition of the Am ole Formation sediments and preceded deposition of the Cat Mountain Rhyolite. K/Ar dates of 65 to 70 m.y. for the Rhyolite would place deformation slightly prior to the Cretaceous-Tertiary boun
dary. Following deformation, the Tucson Mountain chaos and a thick
series of volcanic rocks, of which the Cat Mountain Rhyolite is the
lower member, was unconformably deposited on the Amole Formation.
Discussion of Volcanism and Emplacement of the Saginaw Stock
The boundary between the Cretaceous and Tertiary periods was
one of intense igneous activity throughout much of southern Arizoha. In
the Saginaw area, the deposition of the Cat Mountain Rhyolite and the
subsequent emplacement of the Saginaw Stock marks this period.
The emplacement of the stock can best be explained by a model
involving a basement shear zone which provided channels for the rising
magmas, superficial folding being the loci for final positioning of the
intrusion. The existence of a synclinorium has been suggested in the
Saginaw area by Kinnison (1958) and in this thesis. The stock, as pre
viously discussed, is located on the steeply dipping western limb of the
synclinorium. Although this synclinorium may be of regional extent, it is
unlikely to extend deep into the crust and is probably restricted to Per
mian and Cretaceous rocks. The basement shear zone is proposed be
cause of the linear trend of the Tucson Mountains and associated Basin
and Range faults. This trend may reflect a major paleostructural trend in
the area. The trend of the basement shear zone would parallel the trend
of the Tucson Mountains. North of the Saginaw area it would trend about
N. 40° W ., while south of the Saginaw area it would trend about S. 20° W. 120
It has been previously noted that West (1970) has proposed a major
N. 60° E.-trending scarp in the Del Bac Hills area which may account for the shift in trend of the Tucson Mountains and the suggested base ment shear zone. The basement shear zone may have allowed magma to reach the shallow portion of the crust where the syncline acted as a locus for emplacement.
Figure 3 shows that the stock appears to be geographically associated with the Limy unit-Graywacke unit contact, which dips at about 40° SE. As previously discussed, the Limy unit contains abun dant evidence for second, third, and fourth order folding, while the overlying Graywacke unit does not exhibit these types of folds. However, it was noted that this may only reflect the fact that the Limy unit con tains excellent stratigraphic marker beds facilitating the recognition of these subordinate folds. Whichever the case, the incompetent Limy unit sediments are intercalated between the competent lower Am ole Formation sediments and the overlying competent Graywacke unit sediments. Dur ing tilting, a zone of weakness may have developed between the Limy and Graywacke units. This zone may have further influenced emplace ment of the stock in the synclinorium.
It is also interesting to note that a projection of the proposed basement shear north from the Saginaw area would pass through or near the mineralized intrusions in the Sedimentary Hills and in the Museum
Embayment area. Southward from Saginaw Hill, considering the proposed
shift in trend of the shear zone, lies the Pima-Mission mining district. 121
Saginaw Stock Implications
Figure 3 shows that the sedimentary rocks contact the intrusive rocks with little apparent disruptions in strike and dip. The contacts are also sharp. Minor divergences in the strikes and dips of the sedi ments on the northwest side of Saginaw Hill and in the vicinity of Sagi naw mine suggest that some deformation of the sedimentary rocks oc curred during intrusion. In these areas, the evidence suggests the sediments have been pushed aside or slightly domed in order to accom modate the stock (Fig. 3). Drill cores show that where the sedimentary rocks and igneous rocks are in intrusive contact, the porphyry contains inclusions of the host rocks. Included angular fragments of sedimentary rocks, exhibiting various degrees of assimilation, range in size from a few millimeters up to 25 feet. Considering the nature of igneous-sedi mentary contacts and the included host rocks, a passive magmatic
sloping emplacement for the stock is proposed. Brown (1939) and
Kinnison (1958) both inferred that magmatic sloping may be the emplace
ment mechanism for the Saginaw stock.
As previously discussed, the Saginaw stock is a composite
intrusion composed of the Saginaw Hill quartz latite porphyry which was
intruded by the Saginaw Mine quartz monzonite porphyry. The stock was
subsequently intruded by a series of quartz latite porphyry dikes. The
contacts between the Saginaw Hill quartz latite porphyry and the Saginaw
Mine quartz monzonite porphyry were found to be gradational in nature
(Fig. 11). The chilled border in the Saginaw Mine quartz monzonite por
phyry contained abundant metasedimentary rock fragments, similar to
Amole Formation type metasedimentary rocks, and rare fragments of 122
Saginaw Hill quartz latite porphyry, all of which exhibit various degrees of assimilation. The Saginaw Mine quartz monzonite porphyry intrudes the Saginaw Hill quartz latite porphyry as irregular pods and lenses.
Occasionally, the Saginaw Mine quartz monzonite porphyry was intruded along fractured and sheared zones. However, usually, no control on the intrusion of the Saginaw Mine quartz monzonite porphyry was noted.
From the limited evidence presented, it may be that intrusion of the
Saginaw Mine quartz monzonite porphyry occurred while the Saginaw Hill quartz latite porphyry was still plastic. However, much additional work is required before this can be substantiated.
The Saginaw stock is about 2,000 feet in diameter and plunges approximately N. 15o-20° W. South of the Palo Verde fault drill holes penetrate sedimentary rocks that are below the intrusive body. From this drill hole information, a plunge of approximately 50 degrees towards the
northwest can be postulated. North of the Palo Verde fault, only drill
hole BC-4 penetrates metasediments beneath the intrusive body. With
only the one drill hole a plunge could not be calculated. The halo of
thermal metamorphism around the Saginaw stock (Fig. 24) and diamond
drilling indicate that the intrusion is somewhat pipelike or sill-like
(Fig. 11). The intrusive body almost parallels the strike of the sediments
(Fig. 3), which further suggests that the emplacement of the stock was
in part controlled by the Limy unit-Graywacke unit contact.
The quartz latite porphyry dike systemcrops out at the surface
in only two locations (Fig. 3). In both locations, the contacts are sharp
and the emplacement of the dikes is controlled by the N. 60° E. frac
tures. Dikes encountered in the drill core also exhibit sharp contact 123 relationships and a tendency to be controlled by fractures or shear zones. It was also previously discussed that the dikes were weakly mineralized, relative to the stock, and cut the quartz, quartz-calcite, and calcite veins. The evidence suggests that the dikes were emplaced in response to tectonic events that occurred after most of the minerali zation was emplaced.
Figure 3 shows that N. 60° E.-trending dikes not associated with the stock are terminated or rapidly die out in the Cat Mountain
Rhyolite. The only strong N. 60° E. fractures found cutting the rhyolite are those that also cut the stock. The evidence suggests that during intrusion of the stock maximum stress occurred in the sediments directly adjacent to the stock. During emplacement of the stock, or shortly after emplacement, the stress was released resulting in reactivation of the
N. 60° E.-trending faults which were directly associated with or adja cent to the stock. Sulfide mineralization that occurs in N. 60° E.-trend ing fractures near the Papago Queen fault and the Palo Verde fault exhibit little or no brecciation. These relationships indicate that reactivation of the faults occurred after intrusion but before or contemporaneous with mineralization.
The large fracture that was filled by the ribbon rock and massive quartz at the crest of Saginaw Hill may represent a large, slowly opening tensional fracture. During intrusion maximum stress was applied to the nose of the stock, which is located near the Papago Queen fault. As re activation of the fault was initiated, the stress was released and the
fracture began opening. Pulsations of the hydrothermal fluids may have resulted in multiple generations of comb-structured quartz veins which 124 were slowly pulled apart giving the ribbon-rock structure now observed.
Between each pulse, debris filled the open space between open fractures.
As the stress was released, the fracture did not open as rapidly, and less debris accumulated between closer spaced quartz veins resulting in the massive banded quartz found adjacent to the fault. The origin of the ribbon rock and the massive quartz is very speculative, and con siderable more work must be done to understand its true relationship and importance to the mineralization and structural history of the Saginaw a re a.
Discussion of the Saginaw Stock Composition
Table 1 shows that all three igneous rock types have essentially the same mineralogical composition and only differ in the phenocryst-to- groundmass ratios. It has also been noted that all the intrusive rocks are closely related in space and time. It therefore seems that the ig neous rocks can be attributed to the same parent magmatic source and differ only in their cooling histories.
It was also pointed out in this thesis that both the Saginaw Hill quartz latite porphyry and the Saginaw Mine quartz monzonite porphyry contain an abnormal amount of calcite. The quartz latite dikes also con tain calcite, but to a much lesser degree (Table 5). Calcite content of fresh samples of the stock vary from about 3 .4 percent up to 4.1 per cent, while in the altered samples calcite is 4.5 percent to 11.3 per cent of the rock. Five sources for the calcite can be proposed: (1) calcite was added to the system during quartz-calcite and calcite vein- ing; (2) calcite was added as a result of the alteration of plagioclase; 125
(3) the original magma was oversaturated with CaO resulting in the crys tallization of magmatic calcite; (4) during intrusion blocks of the Limy unit and possibly underlying Permian limestone were assimilated by magmatic stoping resulting in an enrichment of CaO in the melt and sub sequent crystallization of calcite; or (5) the calcite resulted from an early carbonate phase of propylitic alteration.
It was previously noted that calcite occurs in three principal ways in the igneous rocks; (1) confined to veins with little or no re placement of the host rock; (2) replacing plagioclase and to a lesser degree hornblende; and (3) interstitially and apparently intergrown with groundmass quartz and orthoclase. Only the last occurrence of calcite was considered in computing the volume percents in Table 5.
If the original igneous rocks were essentially free of calcite, the abnormal calcite present may be attributed solely to the alteration of plagioclase. The plagioclase present was found to have a composi tion of An35 and composed of about 25.2 percent of the rock by weight.
The calculated chemical composition ofAng^ plagioclase is:
Oxides Percent
S i0 2 61.4
A12°3 24.7 CaO 6.0
N a20 8.3
If the plagioclase present were 100 percent altered, only 1.5 percent
CaO would be available to form calcite. The maximum alteration of in
dividual plagioclase crystals was only about 70 percent which would 126 yield only about 1.1 percent CaO. It can then be concluded that the alteration of plagioclase would only add small amounts of calcite to the system .
If the calcite is related to the vein some spatial relationships would be observed between calcite in the host and the veins. Careful examination of these veins showed that little if any interstitial calcite is genetically associated with the veins. Even in areas of maximum calcite veining, no appreciable increase in interstitial calcite was ob
served .
If concentrations of CaO were high enough in the primary melt to allow deposition of calcite during crystallization, a felspathoidal- type rock would have formed. Abundant apatite, zircon, and sphene would also reflect a melt with a high initial CaO content. None of these
features was noted in the Saginaw intrusive rocks. The assimilation of
large volumes of country rock by a stoping has also been proposed. The
sediments available for assimilation are the carbonaceous Limy unit, the
carbonaceous beds in the Graywacke unit, and possibly the underlying
Permian limestones. Daly (1910) suggested that assimilation of lime
stone results in the desilication of a granitic magma causing the crys
tallization of lime silicates such as melitite and garnet. He also
deduced that assimilated carbonate rocks resulted in the formation of
igneous alkalic rock series. Experiments by Watkinson and Wyllie (1964)
demonstrate that the assimilation of limestone does cause desilication of
the melt and precipitation of feldspathoids; however desilication is
limited to a narrow temperature and composition range. A temperature
of about 900°C must be maintained and combined with a melt containing 127
15 to 25 weight percent of CaCOg before crystallization of feldspathoids
is allowed to proceed. The evidence suggests that the amount of CaO resulting from assimilation was not high enough to initiate crystalliza tion of carbonate minerals. Consequently, it seems rather unlikely that
assimilation or a melt rich in magmatic CaO is responsible for the cal-
cite now found in the stock.
The best explanation for the calcite appears to be carbonization
of the country rocks as a result of propylitic alteration. It can be seen
from the description on propylitic alteration and Table 5 that the rocks
contain very low concentrations of kaolinite, montmorillonite, epidote,
and chlorite relative to the high concentration of calcite. It is also
noted that the Saginaw Hill quartz latite porphyry, which was intruded
first, contains a greater percentage of calcite than the later emplaced
quartz latite dikes (Table 5). Figure 49 is an AKF diagram showing the
compatibility diagram for high COg concentrations in the propylitic al
teration mineral assemblages.
From the plot of the alteration assemblages in the stock it can
ben inferred that the altering fluids were enriched in CaO resulting in
an excess of calcite being deposited in the rocks. The plot of the quartz
latite dike rock, which has been suggested to have been emplaced later
than major mineralization, indicates a lower CaO concentration. It
therefore can be proposed that the hydrothermally altering fluids that
invaded the country rock evolved from carbonate rich during early min
eralization to carbonate poor after much of the mineralization had occurred. 128
Plot of Igneous Rocks
®SM Soginaw Mine quartz monzonite porphyry
©SH Saginaw Hill quartz latite porphyry
©D Quartz Latite Dike
AI2 O3 - (No20 + K20 + CoO) \ Kaolinite
Chlorite
Chlorite - calcite — talc
Calcite Higher CO CoO MgO + FeO + MnO
Figure 49. Compatibility Diagram for Propylitic Alteration.— After Creasey (1966). 129
Discussion of Mineralization and Alteration
A discussion of the structural and metasomatic processes active in the Saginaw Hill area is difficult to separate from a discussion of the mineralization and alteration phase because they all are so close ly related. The sequence is a continuous one, beginning with the em placement of the stock, passing through a phase of structural deformation, and culminating with the formation of the various alteration assemblages with the associated sulfide mineralization.
The metasomatic phase involves two distinct, but related, effects: the recrystallization of existing minerals and the introduction of fluids which formed new minerals. These activities are rigidly restricted in their occurrence to the rocks that are spatially related to the Saginaw stock. Rocks affected by the metasomatic phase are Amole Formation sediments, the Saginaw Hill quartz latite porphyry, the Saginaw Mine quartz monzonite porphyry, and, to a lesser extent, the quartz latite dikes. As will be discussed later, the restriction of metasomatic effects is largely structural in nature and spatially related to the Saginaw stock.
Propylitic alteration was found to pervade the Saginaw Mine quartz monzonite porphyry, the Saginaw Hill quartz latite porphyry, to a lesser extent the quartz latite dikes, and the metasediments of the
Amole Formation. The metasedimentary rocks have experienced both thermal metamorphism during emplacement of the stock, which resulted in recrystallization of the sediments and possibly skarn development, and subsequent or contemporaneous hydrothermal alteration, which pro- pylitized both the meta sediments and the Saginaw stock. It has been previously noted that the Saginaw Hill quartz latite porphyry, which was 130 emplaced early during the intrusive cycle, is propylitized to a greater extent than the younger quartz latite dikes. It has also been proposed that the altering fluids were rich in CaO early during alteration and poor in CaO by the time the quartz latite dikes were emplaced.
The development of hydrothermal sericite along strongly frac tured zones appears to be related to the emplacement of the quartz-
sulfide veins. Sericite is not present in any great abundance and repre
sents only a minor part of the metasomatic-hydrothermal assemblage. It is significant, however, that hydrothermal sericite is present only in areas of greatest shearing and quartz-sulfide mineralization. It is also
significant that the quartz latite dikes, even when they are in these
sheared zones, show little evidence of sericite development, and usually cute the sericite zone and most of the quartz-sulfide veins. It
should be noted, however, that sericite pervades the sheared host rocks
and rarely halos the quartz veins. Adjacent to the veins argillic and
propylitic alteration is most common.
The evidence suggests that thermal metamorphism of the Amole
Formation sediments occurred during the initial emplacement of the Sagi
naw Hill quartz latite porphyry. During emplacement of the Saginaw Mine
quartz monzonite porphyry propylitic alteration occurred. The altering
fluids varied from CaO rich early during alteration to CaO poor during
the waning stages of alteration. Subsequent to or contemporaneous with
Saginaw Mine quartz monzonite porphyry emplacement, quartz-sericite
alteration occurred along strongly fractured zones. Following quartz-
sericite alteration, the weakly propylitized quartz latite dikes were em placed. 131
A general zoning pattern of hydrothermal sulfide minerals exists in the Saginaw Hill area. For the most part, it is rather crudely devel oped and in some areas seems not to be developed at all. The zoning pattern is largely controlled by the N. 60° E. fracturing and reactive metasedimentary beds within the Am ole Formation. Quartz veining, which was found to be most strongly associated with chalcopyrite, is present to a greater or lesser degree throughout the mineralized area.
The strongest occurrence, however, is in the Saginaw stock, and it is associated with strong N. 60° E. -trending fractures. Seri cite, as pre viously noted, is also restricted to these strongly fractured zones.
Sphalerite, galena, and tennantite, which were contemporaneous or postdate chalcopyrite, are associated with quartz-calcite and calcite vein systems. This later stage of mineralization is confined to fracture zones and the reactive metasedimentary rocks which are spatially related to or abut the stock.
As previously discussed, the multiple generations of quartz
associated with the ribbon rock and the massive quartz and the fact that many of the veins offset each other implies that more than one period of
mineralization occurred in the Saginaw Hill area. The evidence suggests that initial quartz-pyrite-chalcopyrite was emplaced during early pro-
pylitic alteration or contemporaneous with quartz-sericite alteration.
The fact that the quartz latite dikes cut most of the quartz veins indicates
that the main phase of mineralization ended before their emplacement.
The paragenetic sequence of mineralization, previously discussed, in
dicates that sphalerite, galena, and tennantite formed contemporaneously
and later than the quartz-chalcopyrite mineralization and may represent 132 precipitation during the CaO-poor phase of propylitic alteration. This phase of mineralization was also essentially completed before emplace ment of the quartz latite dikes.
The Saginaw Hill area provides an excellent example of struc tural control of mineralization. The main zone of shattering, alteration, and mineralization lies along the N. 60° E.-trending Papago Queen fault,
Palo Verde fault, and the Saginaw fissure. On a more localized basis, mineralization occurs along fractures and veins that reflect local struc tural trends and in reactive host rocks adjacent to fractures and the
Saginaw stock. There is a segregation of sulfide minerals, which is by no means complete, based on rock type. The better grade chalcopyrite mineralization occurs predominantly in the stock, while galena and
sphalerite are more highly developed in the metasedimentary rocks.
It has been pointed out that the spatial and sequential rela tionships between emplacement of the stock and subsequent fracturing,
alteration, and mineralization suggest that the ore deposits of the Sagi
naw Hill area are hydrothermally derived from the Saginaw stock. The
relationships of the ribbon rock and massive quartz to the emplacement
of the stock and mineralization provides some evidence that the intru
sions were emplaced at a rather shallow depth. That mineralization was
accomplished under similar conditions seems likely. It has also been
suggested that emplacement of the intrusions and subsequent minerali
zation occurred shortly after deposition of the Cat Mountain Rhyolite,
suggesting a Late Cretaceous-early Tertiary age. 133
Potential of the Saginaw Area
Drill core data and geochemical studies indicate that minerali
zation of uneconomical grade is confined to the rocks adjacent to the
stock and along N. 60° E.-trending fractures in the stock. Several ex planations are possible for this weak mineralization. The main channel- ways that carried the hydrothermal solutions may have intersected the
Snyder Hill limestones at depth resulting in the deposition of most of the sulfides. Trace amounts of pyrite can be found at Snyder Hill. An
exploration program conducted on the thinly covered pediment between
Saginaw Hill and Snyder Hill may reveal replacement deposits. It has
also been suggested that the Saginaw stock plunges towards the north
indicating that the main portion of the intrusive body lies under Recent
gravels north of the Saginaw mine. Drilling programs were not conducted
in this area to the author's knowledge. It is possible that Saginaw Hill
is a near-surface expression of a larger ore body at depth. It is also
possible that the rocks were not sufficiently fractured and the total
volume of rock affected by hydrothermal fluids was too great, resulting
in only local areas of ore-grade mineralization. Another possible explan
ation is that the mineralization was essentially confined to the Cat Moun
tain Rhyolite, which appears to have covered the stock during ore
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E X P LA NATION
Sedimentary Rocks Igneous Rocks > a: \ Kt me < Qal z 01 — UJ Alluvium t- < (Sand and gravel) _ 3 o
Kao
Arkose Unit Quartz Lot ite D i kes Ktmc (Arkose inter bedded with shale)
Kas LkJ Massive Quartz Q Shale Unit c — s o < (Shale inter bedded with arkose, 01 g ray wacke, and calcareous o Tsmp < _ l beds) E o Saginaw Mine Porphyry u_ \ » \ CD Kog o E Tshp \ Tsmp < 0 35 Gray wacke Unit (Graywacke interbedded with V)3 Saginaw Hill Porphyry arkose, shale, and calcareous y / / \ Pyrite casts and oxide Thsp / Composite dike mineral i zat ion Tshp and Tsmp Strike and dip - h Tqld Anticline showing plunge of axis Massive 'a •'•quartz. Sync line showing plunge of axis Quartz veins Lithologic contacts \ Ktmc Lithologic contacts approximately located Inferred faults /'/ ^ Composite 75 Inferred strike slip faults ■./ dike V showing relative movement ^ Tcr B 13° Shaft; vertical and inclined Tunne I or adit Mine dump (=□ House Ktmc Roods O BC—I APPROXIMATE MEAN Drill-hole location DECLINATION A A' I______I Geologic se ction / TOPOGRAPHIC BASE FROM U. S. GEOLOGICAL SURVEY GEOLOGY BY T. R. FRANK, 1970 SAN XAVIER MISSION QUADRANGLE 900 FEET Contour Interval 50 feet FIGURE 3. GEOLOGY OF THE SAGINAW HILL AREA, AMOLE DISTRICT, PIMA COUNTY, ARIZONA THOMAS RUSSELL FRANK, GEOLOGY THESIS, 1970 EX PLAN ATION FA PAGO A ' QUEEN FAULT PALO VERDE 2800 FAULT IGNEOUS ROCKS SEDIMENTARY ROCKS BC-9 / 2600 Qa I Quartz Lat i t e Dikes Recent Sand and grave I Kaa Quartz Latite Dikes Inter fingering Arkose Unit Saginaw Mine Porphyry Ka s Ka Tsmp f ZTsh P _ Shale U n Undi fferent ioted Saginaw Hill Porphyry Amol e Formation Kag 1000 Tsmp Graywacke Unit Saginaw Mine Porphyry with Meta sedimentary Inclusions Inter- fin g er in g Saginaw Hill Porphyry L\ ■Kal Limy Unit K c r Cat Mountain Rhyolite K tm c HORIZONTAL AND VERTICAL SCALE I " = 500* Tucson Mountain Chaos SECTION A-A' LOOKING NE B' BC-8 BC- 5 Kt m c 2600 - \ x V 2000 JLTAC-II 1000 SECTION B-B‘ LOOKING NORTH D' PA PAGO QUEEN FAULT / BC-4 PALO VERDE 2700 — \ Ka Qal / FAULT C' 2600- r. 2800 2700 t BC-3 BC-2 'J / 2600 • Tsmp 'TAC-13 // 2200 2000 16 00 — SECTION C-C LOOK.NG NW SECTION D-D LOOKING EAST F I G U R E II. GEOLOGIC CROSS SECTIONS FOR FIGURE 3. THOMAS RUSSELL FRANK, GEOLOGY THESIS, 1970