Geology of the Bohemia mining district, Lane County, Oregon
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Authors Lutton, R. J.
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Link to Item http://hdl.handle.net/10150/565597 GEOLOGY OF THE BOHEMIA MINING DISTRICT,
LANE COUNTY, OREGON
. :■ . ■: -■ by .. . : U".: J Richard JC. Lutton
* - -r" -
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOLOGY
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
c: 1962 THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my direction by Richard J. Lutton entitled "Geology of the Bohemia Mining
District, Lane County, Oregon" be accepted as fulfilling the disserta tion requirement of the degree of Doctor of Philosophy.
jr . Dissertation Director Efcte
After inspection of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:*
9. /% %
^ J 9 & ^
/fto
♦This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examina tion. The inclusion of this sheet bound into the library copy of the dis sertation is evidence of satisfactory performance at the final examina tion. STATEMENT BY AUTHOR : ■ / ' v- . y-j :;. :..a ■■ rvr.;.: AA'nAC'A
This thesis has been submitted in partial fulfillment of require ments for an advanced degree at The University of Arizona and is de posited 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 their judgment 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.
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SIGNED:
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- A XL ■: '1 X. •- 1 ’ LX I x X, - v. ; \ , : - GEOLOGY OF THE BOHEMIA MINING DISTRICT, LANE COUNTY, OREGON .
] ' . ■ by . \ : ... . ■ : ■: - -
; ; ;iin Richard J. Lutton
, ABSTRACT
-y : • v.r - : ' ; y ' : y.v. ? 1 y r n .. '.H The base- and precious-metal quartz veins of the Bohemia ,v '':: . : v-: . r 'y' ' ; y;: . /’ y mining district are genetically related to middle Tertiary granitoid in-
trusives of the same igneous cycle that formed the volcanic country : : " i y ,:i:v ■ C-. vy in . ;.r i'.y n;'; '- : W >■ ' rock.
About two thirds of the volcanics c lo s e d in the immediate
area are pyroclastics, mostly lapilli tuff that was deposited in a fluvial y;>yy y : ' y y ■ y : y ': ' ‘y : . o \yyy y. y :-.y - 1: y : 'O .: -y y:; ':; ,; : ; v environment. The remainder are mostly flows of basalt, andesite,
ry..i ' - :-y' ::y. . •: ' ' y . y - y y ; y y ; .::y . y • >y ' y y ' • dacite, and rhyolite. Local centers fed the flows through hypabyssal
• - yy’ 'y yyyyy dikes and plugs and viscous domes. The basement structural grain
has probably determined the pattern of early intrusives. f.-oyy L ' y y: / ;r ; • ' ' ' y- y y." :r - y ;y : '= yy: : .:" A composite stock of granodiorite and quartz diorite has been
intruded along a north-northeast line parallel to the axis of the Western y .' :-y y y .-y. ;yy.v. y-^y :;i y y yy; yV - ;yy.,y Cascades province. A large percentage of the rocks of the area are propylitized, and in a wide aureole of contact alteration surrounding the stock, the pyrbclastics have at least locally been metamorphosed to the albite-epidote hornfels facies. Somewhat later metasomatism has re placed country rock and the intrusives with quartz, sericite, and tour maline along restricted zones*:: i- i U i . i : t * The granitoid intrusives warped the gently dipping volcanics and developed a system of longitudinal arid.transverse fractures. Dis tention of the fracture system by the intrusives and subsequent normal faulting on individual fractures during the period of mineralization has provided space for deposition of vein minerals. Epidote, specularite,
chlorite, and quartz locally accompany minor sphalerite, chalcopyrite,
galena, and pyrite in an early stage of vein filling. During intermediate
stages, the silica, with or without contemporaneous hematite and sul fides, accumulated in horizontal layers under the influence of gravity.
Such bedding suggests a low specific volume for the hydrothermal fluid,
and therefore, it also suggests a moderate temperature at the prevail
ing shallow depth.
Mechanical washing of siliceous and minor argillaceous debris
from the oxidized zone to just below the water table in the vein has de
veloped secondary vein sediments. Secondary wad, which is inter-
bedded with clastic debris in the oxidized zone, probably results from
hydrolysis of manganese dissolved from carbonate gangue. iii The relatively low economic potential of the Cascades metal- logenetic province is considered to be a consequence of the position near the edge of the continent and of the old age of mineralization with respect to the beginning of andesitic volcanism.
Ore controls in the Bohemia district include: 1) proximity to the center of mineralization, which was probably a cupola; 2) open channels, which resulted from preferential dilation of steep segments of irregular veins; 3) a concentration of subsidiary veinlets; and 4) vein intersections.
• ‘ ’ 1 f ' '■ ( , '
iv TABLE OF CONTENTS
Page
INTRODUCTION ..... ------1
Previous Work . 1...... 2 Procedure of Investigation .'...... 3 Acknowledgments ...... '.....:...... 4
VOLCANIC AND SEDIMENTARY ROCKS...... 6
Lower Unit 7
Massive Pyroclastics 7 Tuffaceous Shale and Sandstone . . . 1 . 8 -J ) Intermediate U nit. i 8
•:?. D om es...... 9 ^ :- Flows ...... 12 Pyroclastics...... 13 Petrography of the Lavas of the Intermediate. Unit 15
Upper U nit...... 17
Basic Lava...... 17 'Vi Bohemia Mountain F lo w s...... la Grouse Mountain F lo w s ...... 19 Basalt of North Fair view Mountain ...... 19 ' Porphyritic Basalt of Champion Creek ...... 19 Porphyritic Basalt of Annie Creek...... 20 -x ' V: ■ i. : ...... v v Intercalated Pyroclastics ...... 20 Dacites and R hyolite...... 21 r; ; -- i ‘Dacite of Grrouse ^4ountam...... , 21 Rhyolite East of Noonday Ridge ...... 21
v Page
. : : Petrography of the Basic Flows of the Upper Unit...... 22
INTRUSIVE ROCKS ...... 27
Porphyritic Dacite ...... 27 Basic Hypabyssal Rocks ...... 28 r1 ... .. : .. : , , . . • ...... * , ^ . ; > J Glen wood S i l l ...... 29 Basalt Near Elephant Mountain...... 30 Porphyritic Basalt Near Musick Mine. 30
Bohemia Stock and Associated Bodies...... 31
Augite Diorite ..; ...... 33 Porphyritic Augite-Quartz Diorite ...... 34 Dacite Porphyry ...... 35 Quartz Phenocryst-B earing Dacite Porphyry . 36 Granodiorite and Porphyritic Granodiorite .... 37 Coarse Inclusions in Porphyritic Granodiorite 40 Granite Porphyry ...... 41 ' Xi ...... STRUCTURE ...... 42
Homocline ______...... ___...... 42 Broad Warp ...... 42 Minor Folds ...... :.. . . r...... 43 Veins and F a u lts...... 43 Joints ...... ;...... 45 Basement Structure ...... '47 GV ALTERATION 49
C arbonatization 50
Calcite Associated With Hypabyssal Rocks 50 Pervasive Carbonatization ...... 51
.H Contact Alteration .. 51 Biotitization...... 54 Tourmalinizatibn ... 54 Sericitization ..... 57 Page
Argillization...... 58 Pyritization...... 59
VEINS ...... 61
Vein Structure...... 62 Champion V ein ...... 65
Primary Vein Deposits ...... 68 Secondary Vein D eposits...... 70
Par agenesis of the Vein F illin g ...... 73
Quartz-Specularite-Epidote Stage...... 74 Sulfide Stage ...... 76
A du laria...... 76 Hematite-Quartz and Hematitic Chert ...... 76 Sulfides ...... 81 Carbonate in the Sulfide Stage ...... 81 Gold ...... o ...... 82
Silica Stage ...... ; 84 Carbonate Stage...... 85
Secondary Environments and Products ...... 87
Possibility of Erosion Surfaces in the Western Cascades ...... 87 Mineralogy of the Oxidized Zone...... 89 Supergene.Deposits...... 90 Secondary Sediments and Bedded Precipitates ...... 90
Particles of Greater Than Colloidal Dimensions .; 91 Bedded Precipitates...... 92
CASCADES DEPOSITS AS PARTS OF A METALLOGENETIC- PETROGENETIC PROVINCE ...... ,95
Distribution of Plutons and Mineralization ...... 95 Characteristics of the Plutons and Associated •...... M ineralization...... 97 vii Page
; Silver Star Mountain Area ...... 97 St. Helens D is tr ic t...... 100 Mount Rainier A rea, ...... 101 Snoqualmie Batholith Area ...... 101
Petrochemistry of the P rovince...... 105
t ■ ■ ■ • .. • , 1 . i I 4 t . I . v 1. . 1 « • •' 'J Inter-Provincial Variations ...... 105 Variations, Within the Province ...... 110
COMPARISON OF THE CASCADE MOUNTAINS PROVINCE_____ WITH SIMILAR PETROGENETIC-METALLOGENETIC PROVINCES - ...... 116
The Green Tuff Province of Northeastern Honshu, Japan . . . 116 The Central Range of Kamchatka, U. S. S. R...... 119 The Aleutian Islands ...... 121 ' ; Southwestern Sumatra, Indonesia ...... 123
INTERPRETATION OF THE GEOLOGICAL HISTORY OF THE BOHEMIA DISTRICT...... 125
Accumulation of Volcanics ...... ,. , 126 Intrusion of the S to ck ...... 127 Structure...... ; ...... 127 A lteration...... 128 Vein Formation ...... 129 Secondary Processes ...... 130 : ; ; ...... : V*-1 INTERPRETATIONS REGARDING ECONOMIC GEOLOGY...... 131
Regional Aspects ...... 131
Position Near the Margin of the Continent...... 131 V, Age of the Deposits in the Volcanic Cycle ...... 132
».Relationships:Among the Districts ...... •,••»,.,«•...., in 134 ;
> • - ‘ » ; • 9. Position Within the Province ...... ----- 134 The Size and Depth of the Plutons ...... 135 Local Factors ...... 136 viii Page
Ore Controls in the Bohemia District. . . ^ . 136 .
The Cupolas ...... ; ...... , 136 Focus of Mineralization ...... V.' 137 Open C hannels...... 138 Dikes ...... ' 138 Vein Intersections ...... 139 Subsidiary Veinlets ...... 139 W allrock...... 140
APPENDIX— TRACE ELEMENTS IN SULFIDES .. .0 ...... 141
BIBLIOGRAPHY ...... 162
V....'; LIST OF PLATES ,, , ,
Plate 7 .. , ...... „ ...... Page
1. Bedrock geology of the Bohemia mining district...... in pocket
2. Isometric diagram of bedrock geology of the Bohemia mining district ...... ’ I... X i % . X M i in pocket
3. Structural geology of the Cascade Mountains metal- logenetic province ..... I...... in pocket
4. Distribution of observed basic dikes of the Bohemia, ; . mining district ...... in pocket
5. Granitoid intrusives of the Bohemia mining district...... in pocket
6. Veins of the Bohemia mining district ...... X i. X ... %. in pocket
7. Joints in the Bohemia mining district ...... '...... in pocket
8. Geology of the Champion mine ...... in pocket
9. Alkalies in some igneous rocks of the western United " " States and adjacent Canada and M exico...... in pocket Plate Page
10. Chemical variations in Cascade Mountain lavas i...... in pocket
11. Five similar petrogenetic provinces...... in pocket
12. Isometric block diagram of west 1050 level stope . . . .v. . . in pocket
13. Photomicrographs of basaltic flow rocks of North Fair view Mountain-type ...... v.v. v.v.v.. 143
14. Photomicrographs of basaltic dike rocks of Elephant Mountain-type ...... 144
15. Photomicrographs of suggested correlative flow and dike rocks on Bohemia Mountain ...... 145
16. imaginary surface of elevations of ridge tops and flats \v . 146
17. Interpretation of fold structure ,...... 147 * ‘ * - ■*, ^ *-.4 — . ^ 18. Detail along 900 drift. Champion m in e...... 148
19. Detail along 1050 drift. Champion m in e...... 149
20. Sketch of a portion of a specimen containing horizontal- * * ' ly(?) bedded hematitic chert with contemporaneous chalcopyrite filling vugs in coarsely crystalline quartz,. 150
21. Sketch of float specimen showing horizontally(?) bedded hematitic chert filling veinlet cavity ...... 151
22. Photomicrographs of vein quartz showing growth in-. * ter vails separated by layers with hematite ...... v...... 152
23. Photomicrographs of detail in specimen illustrated in plate 20 ...... ;...... -...... 153
24. Photomicrographs illustrating contrast of textures in . silica ..'.;...... "...... 154
25. Photograph of a slab surface of layered silica in carbonate gangue...... 155 x Plate Page
26. Sketch of a dump specimen showing encrusted ankerite and bedded ankerite and silica ...... 156
27. Photograph of a slab surface of an oriented sample of a secondary, debris deposit...... 157
28. Sketch of vein debris deposits in the oxidized zone of a carbonate-quartz v ein ...... 158 : - ■ ■ : , - . \ V .. • : V . ' '■ . V;.' \ 29. Silica-variation diagram of middle Tertiary lavas of the Cascade Mountains ,...... 159
30. Silica-variation diagram of middle Tertiary intrusives of the Cascade Mountains ...... ,...... 160
31. Geology of the Sultan Basin d is tr ic t...... 161
LIST OF TABLES '
Table \ ’ • >' Page
1. Silica and potash analyses in some rocks of the ■ Bohemia d istrict...... 142
2. Modal analyses of flows of the Bohemia district (upper unit) ....'...... in pocket
3. Modal analyses of hypabyssal dikes of the Bohemia d istr ic t...... in pocket
4. Modal analyses of granitoid intrusives of the Bohemia district ...... '. in pocket
5. Gold assays in portions of the Champion,and Musick mines ...... in pocket
6. Trace elements in sulfides ...... in pocket
7. Characteristics of deposits of the Cascade Mountains .... in pocket
' '■ " '■ xi ' i;; : ' INTRODUCTION
The Bohemia mining district is situated in southeastern Lane
County, Oregon about 40 m iles southeast of Eugene. Access from
Eugene is by paved highway to Cottage Grove and eastward up the valley
of the Row River. From the valley, forest roads follow Sharps Creek
and Brice Creek in a loop through the center of the mineralized area. -
The district straddles the crest of a ridge, known as the
C alapooya Divide, within the Western Cascades physiographic province.
Drainage to the south is to the Umpqua River. To the north drainage is
to the Willamette River. Elevations range from 5,933 feet above sea
level on Fair view Peak to as low as about 2,100 feet in the bottom of
youthful canyons. An accordance of ridge tops, characteristic of the ..
Western Cascades, is observed locally. ,
Precipitation averages about 70 inches per year. Snow accumu
lates on the peaks in the winter. :
Coniferous forests cover most of the surface though the high
peaks are near timber line. Douglas fir, various true firs, and hem
lock are abundant. Pine is plentiful locally in the Umpqua watershed.
Gold was discovered in 1858 and stimulated extensive prospect
ing and development activity. After depletion of the better oxidized ore,
attention was directed to the primary gold-sulfide mineralization, v; 2
Because of a combination of narrow vein widths, low tenor, complex mineralogy, and high transportation costs the various attempts at min ing primary ores have so far been unsuccessful. Total production value probably slightly exceeds one million dollars.
Previous Work
Considerable information pertaining to the Bohemia district is available in the literature; a list of articles, reports, and theses appear ing previous to 1935 is given by Callaghan and Buddington (1938).
. The bulletin by Callaghan and Buddington has been the most complete geological reference. Though their subject is the mineraliza tion of the entire Cascade Range in Oregon, a sizeable share of the text deals with the Bohemia district. Their map of the district prepared with the aid of H. E. Wheeler was useful as a check to the current work.
It should be noted however, that the arcuate and radial intrusive pattern
(Buddington, 1959) suggested by their map is not borne out by additional work. Another excellent paper by these authors (Buddington and
Callaghan, 1936) treats the characteristics of the Oregon Cascades i plutons and associated, contact alteration. Again the largest part of the information is from the Bohemia district. Taber (1949) has assembled information about mining methods, milling and shipping problems, production, and economic geology. His paper includes maps of the workings on the better veins. .The various 3 properties are listed and briefly described in a compilation of the mines and prospects in the northwest part of the State (The Staff, 1951). Bales
(1951) has made a reconnaissance study of the adjacent area to the north west. ; : - t. if': , .■ v." : -
The latest publication, by Nelson (1959), is a collection of his torical episodes from the colorful past of the district.
Procedure of Investigation
Most of the veins are localized in an area of about 10 square miles. However, it was necessary to map a total of about 25 square miles to include the genetically associated intrusive rocks. The mapping was mostly done at the. scale of the final maps, i. e ., about
3-3/4 inches to the mile. Enlargements of the Fairview Peak quadran gle were utilized in the field. A collection of aerial photographs was provided during the first summer through the kindness of Mr. Robert
Hohenhaas of the County A ssessor's office. Unfortunately, the heavy forest cover minimizes their value.
Approximately eight months were spent in the field. During the summer of 1960 mapping was carried out in the southwest part of the
district from the base camp at the Glen wood cabin, and during the in terval from early May to late October of 1961 the major part of the.
district was mapped with base camp at the Champion bunkhouse. .Activa
tion of the Champion camp boarding house allowed me to make full use 4 of the field time. The last month in the field was mostly devoted to mapping the geology of the accessible workings of the Champion mine.
Examinations of 170 petrographic thin sections and 40 polished ore sections were made in the winters of 1961 and 1962. Optical prop
erties of plagioclase and pyroxene were determined with index oils and with the imiversal stage. A 6-drum integrating stage was used for modal analyses. Chemical analyses and assays are the work of Mr.
L. L. Hougland. Mr. Thomas C. Matthews made the spectrographic
analyses- . ' n,
Acknowledgments ' : ■ ' ■- ' . ' . ‘ ' Vit; ^- It is a pleasure to acknowledge the many courtesies and pleas
ant gestures extended to me during the field work. The Glen wood cabin facilities were placed at my disposal by Mr. Loren L. Stewart and Mr.
Faye Stewart of the Lane Minerals Company, and during the second
summer the Emerald Empire Mining Company provided room and office
space at its Champion camp.
Mr. William Patton of Dorena and my neighbor Mr. D. C.
Bradford were helpful in providing information about many of the aban
doned or outlying properties that are so difficult to relocate. The his
torical anecdotes known to Mr. Ray Nelson were always of interest.
Mr. James C. Miller gave me some vein samples showing free gold
for paragenetic study. 5
Mr. Harold L. Barton of Lane Minerals Company and Mr.
Kenneth O. Watkins of Emerald Empire Mining Company were especial ly generous with their time. In several instances they arranged their assessm ent work to benefit me also. My father, Mr. Francis W. Lutton worked with me in mapping the geology of the Champion mine.
While preparing the manuscript, I used maps and assay infor mation loaned by Mr. John C. Higgins, Dr. William E. Caldwell, and
Mr. Manning W. Cox. Mr. Richard C. Gearhart of the Higgins organi zation endeavored to locate an assay book and other pertinent information of operations at the Champion mine.
All personnel of the Department of Geology and Mineral Indus tries have been pleasantly helpful. Special thanks are due the Director,
Mr. Hollis M. Dole, who suggested the project and arranged the State* s financial support. During visits in the field helpful comments were offered by Mr. Richard G. Bowen, Mr. R. E. Corcoran, Mr. Dole,
Dr. George S. Koch, and Mr. Herbert G. Schlicker of the Department and Dr. Willard C. Lacy of the University of Arizona.
With the help of Dr. Lacy, Dr. Robert L. DuBois, and Dr.
Evans B. Mayo of my dissertation committee, the various phases of the study have been drawn together into what I hope is an interesting ' and valuable contribution. VOLCANIC AND SEDIMENTARY ROCKS
The layered rock units exposed in the district consist of about
4,000 feet of massive to moderately well bedded lapilli tuff and tuff breccia (terminology from Wentworth and Williams, 1932) and flows and domes of basalt, andesite, dacite, and rhyolite. Minor tuff, tuffa- ceous shale, and tuffaceous sandstone are included locally. ■
Three subdivisions of the strata have been identified. However, the restricted extent of individual lava flows and intercalated pyroclastic lenses suggest that the formations will not extend far beyond the im mediate vicinity. In fact, there is apparently an interfingering of the upper and middle units within the district. •
The upper two divisions generally correspond to the two main lithologic divisions of Callaghan and Buddington (1938). Later recon naissance work has adopted the contacts and lithologies of Callaghan and Buddington. Schlicker and Dole (1957) place the section in the upper
Eocene to lower Miocene Mehama volcanics. According to Peck (1960) the strata occur within two of four lithological divisions of the Oligocene to lower Miocene Little Butte volcanic series. This series has a total thickness of 3, 000 to 15,000 feet. The choice between "Mehama vol canics" and "Little Butte volcanic series" is a matter of preference of
1: 6 7 nomenclature.
To the west the continental Mehama or Little Butte volcanics interfinger in part with marine beds of the Eugene Formation. The ; nearest known Eugene Formation outcrop is near Dorena dam 20 miles to the northwest.
Lower Unit
The lower part of the stratigraphic section is composed of mas
sive pyroclastics overlain by well-bedded tuffaceous shale and sand
stone.
Massive Pyroclastics
;, A 1,000-foot thick section of tuff, lapilli tuff, and tuff breccia
is exposed along the Sharps Greek road between Judson Rock Creek in
section 9 and the Glen wood switchback in section 15 (pi. 1). Most of
the formation is characterized by a lack of discernible bedding and by
a variation in size of fragments through the section and locally within
individual outcrops. Faint bedding was noted in one place. Minor
carbonized wood is present. Joints are widely spaced and exfoliation parallel to the erosion surface results in massive, rounded outcrops.
Much of the lapilli tuff and tuff is composed of chloritized frag
ments that presumably were once pumice. Banded lava fragments are
locally conspicuous. ' - 8
The contact with overlying well-bedded elastics on the Sharps
Creek road grades through about 100 feet of interbedded lapilli tuff and tuffaceous sandstone and mudstone. On plate 1 these are included with the massive pyroclastics.
Tuffaceous Shale and Sandstone
Approximately 300 feet of tuffaceous, well-bedded, fine elastics form a conspicuous part of the lower unit from the Glen wood vicinity south to Shane Saddle. Similar deposits crop but in upper Cat Creek, seemingly at about the same horizon.
Most outcrops are composed of shale and mudstone alternating with sandstone or fine sandy lapilli tuff. These commonly have been indurated, probably by subsequent intrusive activity. Individual homo geneous beds range from small fractions of an inch to several inches in thickness, and they extend uniformly for distances of at least tens of feet. No crossbedding was noted. Micro-thrusts and folds are observ able locally.
In cuts along the Shane Saddle road persistent siliceous beds from an eighth to one inch thick have locally been broken up, abraded and incorporated as intraformational conglomerate.
Intermediate Unit
The intermediate unit, composed of andesite to rhyolite domes 9 and flows and intercalated pyroclastics, crops out in a wide northwest trending belt. Three sections illustrate the variability of the formations from place to place: 1) below Jackass Butte four lavas and the inter vening tuff and lapilli tuff total about 1, 500 feet of exposed thickness;
2) along the Sharps Creek road the 1,200 feet of section is mostly lava; and 3) the northwest slope of Cat Mountain reveals about 1,700 feet, largely lapilli tuff.
The strata exposed on both slopes of Champion Creek canyon are lapilli tuff and tuff breccia with considerable inter bedded basic lava.
The area is approximately along the projection of the pyroclastics and dacites to the south, so that in the absence of evidence of intervening faults the sections are considered to be contemporaneous in part.
Domes
Two of the lava masses are believed to overlie their vents as domes (Quellkuppen) on the basis of thickness, internal structure, or detail along the margin.
The complex mass in upper Sharps Creek seems to have had at least two pulses. The lower lava is observable on the road and also at Judson Rock (section 10) where it has overridden a 100-foot thick ness of its own blocky debris. The mega-breccia consists of angular fragments of the faintly banded lava up to 4 feet in diameter. It over- lies sandstone and shale conformably. The minor matrix at the base: 10 is coarse, tuffaceous sandstone. Locally above the lower lava more mega-breccia was observed and it in turn is overlain by crudely bedded tuff breccia, (hi the road an unusual lava crops out with the tuff breccia.
It has lineated vesicles concentrated in 10 or more regularly spaced planes. The rock is altered and its character might be questioned were it not for inclusions of tuff breccia encased in banded lava along the base.
The upper lava, as exposed in Sharps Creek above the Glen- wood cabin, truncates banding in the lower lava along a steep, brecel ated, intrusive contact. Silica and potash analyses of rocks about 200 feet to either side of the contact suggest that the lower lava is basic dacite and the upper is rhyolite (table 1, B-270 and B-272; also see
Callaghan, 1933, table 1, analysis 8, rhyolite).
One can usually identify faint banding in the. rhyolite and a con siderable effort was made to map it in the Sharps Creek mass. The results shown on plate 1 point to the presence of more than one tongue that probably issued from more than one vent. The evidence is not con clusive, but there certainly is more than a simple ’’onion skin” struc ture. A zone of steep foliation trending northwest through the corner between sections 10 and 14 suggests that part of the orifice was a north west-trending dike. . ,
Banding is truncated by the overlying bedded tuff and lapilli tuff along an unconformity, and occasional blocks of a spherulitic facies 11
of the rhyolite are included in the adjacent bedded volcanics.
When compared with individual domes (Williams, 1932) in other
parts of the world, this mass is two or three times larger in diameter
than usual. However, when allowance is made for coalescence over
more than one vent and some lapping out of flows, the comparison is
not unrealistic.
A second dacite to, rhyolite dome, which occupies the top of
Jackass Butte, has a diameter of about 1,500 feet. Banding seem s to
: converge downward and it is on this basis that the lava mass is called
a dome. The lava projecting northeast along a ridge may be a remnant
of an outlapping flow or it may overlie a feeder dike.
Other bodies of dacite in the intermediate unit exhibit features
that suggest domes. The faintly banded lava in Crystal Creek (section
12) has steep attitudes at the south whereas toward the north bands dip
gently. West of the center of section 23 steep banding appears to arc
through 180° of strike.
Porphyritic rhyolite in St. Peter Creek (pi. 1, SE quarter,
section 23) has been considered to be an intrusive (Callaghan and l
Buddington, 1938). A combination of four features suggests that it is
a lava dome rather than a flow or deep intrusive. 1) Adjacent bedded
tuff contains fragments of the porphyr itic rhyolite; and detr ital quartz
is common locally in contrast with the rest of the district. 2) The con
tact projected across St. Peter Creek in section 23 is steep over a 12 vertical distance of 800 feet. No evidence of a fault contact was noted 1 but the contact was not seen. 3) Faint, usually steep banding is devel oped in the mass; a local arch is exposed over a distance of a few hun dred feet along the President mine road. 4) At least three types of andesite or basalt (table 3, B-208 and B-210) intrude the rhyolite.
Numbers (1) and (4) suggest contemporaneity with the lavas and pyroclastics of the intermediate unit. A point in favor of a flow character is that similar porphyritic rhyolite or dacite is common among boulders in Little Rock Creek in section 2, 3 miles to the south, thus indicating an extension to the south into the Little Rock Creek drain age."'"1 1 ' ' ■:r:' ; ^ " '■'iU ;v:7 ■; > •
The porphyritic dacite bodies described under ’’Intrusive Rocks” are believed to be feeders for domes such as have been described.
' Flows of andesite, dacite, and rhyolite up to 300 feet thick are associated with the domes and pyr oclastics. A poorly exposed mass of dacite crops out over a large area north of Weaver Creek (sections 34
and 35). Its banding is usually steep and apparently bears ho relation to the base. On the other hand, the dacite east of the Mustek mine has foliation that parallels faithfully the plane of the flow.
-Below Jackass Butte a dacite flow has an apophysis that seems
to crosscut the prevailing bedding plane of enclosing strata as though it 13 were a dike. Unfortunately exposures of the contact were not found. A similar, steeply foliated dacite crosses’Mohte Rico Ridge three quarters of a mile southwest. These may be feeders for flows or domes.
vl Pyroclastics
’ Light-greenish-gray to tan, coarse tuff, lapilli tuff, tuff breccia, and tuffaceous sandstone surround and locally underlie and overlie lavas of the intermediate unit. The greatest-part of the forma tion is composed of coarsely bedded, pumiceous lapilli tuff, with the fragments averaging about a fourth of an inch across. Attitudes re corded on plate 1 are mostly from such lapilli tuff or interbedded tuffa- ceous sandstone and tuff breccia. Crossbedding is well exposed at
Pilot Rock near the top of the section. Accretionary lapilli are pres ent in a few places along the Utopian road (section 11). Microscopic examination reveals that at least locally, lithic and feldspar crystal fragments are major constituents.
Moderately well bedded tuff and tuff aceous sandstone are ! colored reddish brown with ferric oxide beneath the basic flows of
Elephant and Bohemia Mountains. A 3-foot block of spherulitic rhyo lite has been incorporated in these beds on the south-southeast ridge
of Bohemia Mountain. :This rhyolite appears to be identical with the
rhyolite of the Sharps Creek mass at Bohemia Saddle (N-1/2 sec. 14),
and it is presumed to have been eroded from the dome while the 14 sandstone was being deposited. Unconformities are also inferred be low the upper unit on Grouse Mountain (Callaghan and Buddington,
1938) and at the St. Peter Creek dome.
Rare, accidental fragments of lava as much as two orders of magnitude greater in diameter than enclosing debris were seen at sev- eral places. With increasing percentage of large lava fragments the bedding becomes indistinct and finally is no longer discernible. Such massive tuff breccias were noted on the south slope of Grouse Moun tain where they form steep-faced, rounded outcrops.
Massive lapilli tuff exposed along the Utopian road (west of center, section 11) has a distinct planar element due to tabular, chloritized pumice fragments oriented in a structureless matrix.
Hand specimens of this rock are suggestive of a moderately welded tuff. However, the exposed lower contact is gradational with bedded sandstone. On the northeast ridge from Elephant Mountain the altered pyroclastics also exhibit a foliation suggestive of welded tuff.
A spectacular volcanic conglomerate with subrounded boulders of various types of lava up to 10 feet across crops out in Champion
Creek below the mouth of Weaver Creek. The formation seems to overlie conformably well-bedded tuffaceous shale. Farther down
stream the fragments are smaller and more uniform in size and there
are only a few types of lava. Here the formation resembles the mono- lithologic breccia associated with the basalt of North Fair view Peak. 15
Petrography of the Lavas of the Intermediate Unit
A few simple field criteria were used to distinguish the various lavas. Dacites throughout the district exhibit a dense groundmass, con spicuous plagioclase phenocrysts, faint foliation, and a light color of gray, green, pink, or tan. There is generally a noticeable contrast to andesite and basalt which are visibly crystalline, generally structure less, and medium to dark gray. Rhyolites were distinguished on the basis of the presence of spherulites and to a lesser extent on lighter gray to tan or pink color and the presence of banding.
The validity of these characters as relative criteria was sub stantiated microscopically by optical determinations of the anorthite content of the plagioclase phenocrysts. The mineralogical compositions of the "dacites" were not determined; it is possible that they range well into the andesite field.
Under the microscope the dacites are characterized by a micro crystalline to cryptocrystalline groundmass containing an average of about 18 percent feldspar phenocrysts. The distinguishing features are as follows:
Phenocrysts
1) Primary quartz is lacking.
2) Mafic minerals account for only a small percent; when
present, they have usually been altered to chlorite. and their original character was not determined.
3) Most phenocrysts are plagioclase with dimensions
about two orders of magnitude greater than those
of the groundmass crystals.
4) Oligoclase to albite predominates; however saussurit-
ization has probably resulted in some relative in
crease in soda.
5) Plagioclase has tabular shapes with euhedral to sub-
hedral crystal outlines.
6) Plagioclase is commonly altered. Alteration products
' include sericite and calcite and lesser amounts of
epidote, chlorite, and quartz.
7) Twinning is less complex than in basalt. Carlsbad or
albite twins are most common.
8) Zoning is indistinct.
9) Glomerocrysts and xenoliths are not uncommon.
Groundmass
1) Equidimensional, cryptocrystalline patches with uni
form optical extinction are typical. In the St. Peter
Creek rhyolite similar but coarser patches are com
posed of granophyric growths of quartz in optical
continuity with quartz phenocryst seeds.
2) A common pilotaxitic texture probably characterizes 17
varieties approaching andesite in composition.
3) Both the polycrystalline patches and the pilotaxitic
texture accentuate foliation. c , •
Upper Unit
Andesite and basalt flows of the upper unit dominate the sec tion northeast of Champion Creek. Along Noonday Ridge the strata may be divided into a lower 2, 000 feet, principally of flows and pyro-
clastics, and an upper 1, 000 feet of lava. The lower flows are inter-
layered with lapilli tuff that is thought to be in part contemporaneous
with the dacite lavas and pyroclastics to the south.
On the slopes of Grouse Mountain about 1,000 feet of basic lavas contain some 700 feet of intercalated tuff breccia, lapilli tuff,
tuff, tuffaceous sandstone, dacite, and welded tuff. . i
Basic Lava
The thicker masses of basic lava are each composed of sev
eral flows. A variety of textures and mineralogical compositions have
developed. At least one type of basalt was erupted from a local source,
and the large number of dikes noted in the district seem more than
ample to supply the volume of lava indicated.
Flows of dense basalt; fine-grained porphyritic andesite and
basalt, several varieties of coarsely porphyritic andesite or basalt. 18 and olivine-bearing basalt were distinguished, but in all but well-ex posed areas attempts to trace flows proved futile.
Vesicles ar e hot uncommon but an oxidized, brecciated top was noted in only one place (west of center, section 17).
Bohemia Mountain flows
At least nine flows (Callaghan and Buddington, 1938) with in tercalated tuff make up the 700 or more feet of strata on Bohemia Moun tain. The lower flow exposed on the north slope is a porphyritic basalt that, except for vesicles and conformable upper contact with tuff, is easily mistaken for intrusive dacite porphyry. It does not extend to the south slope, but might be correlated on the basis of megascopic similarity with porphyritic basalt (table 2, B-196 and B-214) along the
Mustek mine road and above the Day cabin (SE-l/4 sec. 11) to the north.
The 40 feet of tuffaceous detritus above includes fragments of the basalt..
The tuff thickens to the east where the basalt pinches out. :
Above the tuff are three 100-foot flows of dense, black basalt separated by thin tuff and breccia lenses. Subhorizontal foliation is ap parent on weathered surfaces.
The upper three flows of the mountain are reddish-gray to
dark-gray, porphyritic basalt which shows alteration products after
olivine. A tuff breccia is intercalated between the second and third flows from the top. Similar olivine-bearing basalt (table.!, B-216) 19 crops out under the Fair view Peak lookout.
Grouse Mountain flows
Four basalt flows total about 150 feet in thickness from the summit down the south face of South Grouse Mountain. The top flow
(table 1, B-262) is vesicular and contains pseudomorphs after olivine.
The lavas overlie bedded tuff. They are correlated on microscopic similarity to the basalt of North Fair view Mountain. .
Basalt of North Fair view Mountain
Some of the lava on the northeast slope of North Fair view
Mountain has a characteristic speckled weathered surface as.a result of a large percentage of small plagiocla.se phenocrysts. The same tex ture was recognized in the lava cropping in the sides of Crystal Basin
and on Elephant and Grouse Mountains,, in the cluster of dike-plugs north of Elephant Mountain, and in the fragments of the monolithologic breccia south of North Fair view Mountain. These: formations are con
sidered to be related as flow, vent, and.debris accumulations of the
same surge of magma. . : ' % ;
Porphyritic basalt of Champion Creek
A 400-foot thick lens of lava can be traced north from, section
13 through section 12. Though some dense lava is present, most of the 20 outcrops are of fine-grained, porphyritic basalt. The rock is of;such a texture and at such as position to be a possible correlative of the lower flow of Bohemia Mountain.
Porphyritic basalt of Annie Creek
Basalt containing coarse phenocrysts of plagioclase in a fine grained groundmass crops out along the Ingham prospect road in Annie
Creek (sections 18 and 19). Similar, conspicuously pitted lava occurs in upper Horse Heaven Creek (south of center, section 8 ) at about the same horizon.
Intercalated Pyroclastics
Characteristics of the lapilli tuff intercalated .with lavas are essentially the same as those of underlying pyroclastics. However, most are massive and the fragment-size range seems to be more uni formly near a quarter of an inch.; Exceptional formations include the
400 feet of well-bedded sandstone and shale in section 36 and the mono- lithologic breccia south of North Fair view Mountain.
Monolithologic breccia cropping out in the W -l/2 sec. 11 is approximately 80 percent angular to subrounded fragments of basalt from a few inches to a foot in diameter. About half of the fragments are vesicular. The matrix is dark-gray, massive, sandy debris which is distinguishable from fragments only on weathered surfaces. To the 21 east above upper Crystal Basin the breccia appears to interfinger with or become diluted by the adjacent tuff. Similarly, bn the south side of -
Fair view Peak the large fragments include a variety of rock types; the matrix makes up a larger part of the rock and is lighter in color, and presumably more acidic.
Dacites and Rhyolite
A few dacitic to rhyolitic lavas intervene among the basic flows of the upper unit in the southern part of the map area. :
Dacite of Grouse Mountain
The best marker bed noted in the district extends 2 miles from section 19 to section 7. It is a 100- to 200-foot thick, tan to gray dacite.
Plagioclase phenocrysts and inclusions rich in magnetite and chlorite are scattered through its dense, faintly banded matrix. Under the microscope the rock is essentially the same as dacite to the southwest except that the cryptocrystalline patches that typify the dacite are iden tifiable as granophyric growths..) - • - . ; : ; i .
Rhyolite east of Noonday Ridge
On the ridge-top road to Johnson Meadows a mass of spherulitic rhyolite is associated with massive lapilli tuff and tuff breccia and a foliated rock that resembles that commonly grouped with welded tuff. 22
The rhyolite body is elongated northward. Banding is moderate to steep and parallel to the axis of the mass suggesting that it may over- lie a dike vent. Contacts were not located., However, a gradational con tact with the welded tuff is suspected.
The welded tuff extends 2 miles southwest from the Johnson
Meadows road to upper Annie Creek. Outcrops at the south are light colored, foliated but massive, and contain abundant feldspar crystals and a few accidental lithic fragments. Microscopically the rock has the diagnostic eutaxitic structure. To the north the formation has few if any lithic fragments and in general has a more lava-like texture and structure.
Above this welded tuff and a thick basalt flow a massive lapilli tuff locally appears to be welded also. The intervening basalt terminates near the spherulitic r,hyolite so that the section in this area consists of at least 700 feet of rhyolitic to dacitic lava and pyroclastics.
Petrography of the Basic Flows of the Upper Unit
Specimens of 16 flows of the upper unit are basalt (table 2) ac cording to some classifications. Callaghan (1933) has found it necessary to distinguish andesite from basalt in the Western Cascades on the basis of the proportion of ferromagnesian to feldspathic constituents, "be cause of the relatively high CaO-content of the plagioclase and the large proportion of SiC2 . " According to Tyrrell (1930) andesite contains 23 more than five-eighths felsic constituents. A large percentage of the flows of the upper unit appear to fall into the class which Callaghan and
Buddington (1938) have called labradorite andesite or basaltic andesite.
However, the reader's attention is called to the two low silica analyses
obtained from lavas of the upper unit (table 1, B-216 arid B-262). If
they are representative of the two flows, they weaken the applicability
of the term andesite iri a broad way. - c i !
A variety of textures is present among the lavas of the upper unit. The following general petrographic characteristics were noted.
Phenocrysts
1) Both augite arid plagioclase are common. ■
2 ) Chlorite pseudomorphs after olivine are locally present.
Hypersthene was seen in orily one section, but probably
some of the chlorite pseudomorphs are after hyper
sthene.
3) The dimensions of the phenocrysts seldom exceed by
an order of magnitude those of the groundmass
■ -' crystals. • -r •.... '1,:' ' ':v;J - 's - --
v;r:'4) Augite has intermediate optical properties. Sixty-seven
— - imcorrected optic angle values from 14 flows and 5
: hypabyssal dikes range from 45° to 56° with one value
of 60°. Thirty of these were 48° to 50° (index of hemi
spheres of the universal stage is 1. 648). Four of 24
six measurements of ny were 1.698. Augite is com-
: monly euhedral, zoned, and twinned.and glomero-
v crysts are abundant. > .
5) Plagioclase ranges from labradorite to bytownite. It
1 . is somewhat more zoned than that in dacite. The
outer layer is apparently a few percent richer in
Ab than the core. The plagioclase is complexly
twinned. :
; Groundmass; .
1 ) Primary quartz is not evident.
2 ) Groundmass plagioclase may be slightly richer in Ab
than are the phenocrysts.. i . :
3) Augite has the same composition as that of phenocrysts.
4) Pigeonite, orthopyroxene, and.olivine were not ob-
• served in the groundmass.
5) A pilotaxitic texture is commonly developed in the
finer grained andesite and basalt.
:. Several tentative correlations were deduced from texture and miner alogical composition. Basalt flows on Elephant Mountain, North
Fairview Mountain, South Grouse Mountain, and in the sides of Crystal
Basin have a characteristic porphyritic texture. The same texture characterizes the dikes and dike-plugs north of Elephant Mountain and the fragments in the monolithologic breccia on North Fairview Mountain 25
(pis. 13 and 14). .
Plagioclase exists as abundant stubby, euhedral phenocrysts along with pyroxene and chlorite pseudomorphs after olivine. The groundmass is composed of granular augite, chlorite, and abundant euhedral plagioclase crystals. The plagioclase crystals of the ground- mass exhibit a range of size which locally may approach that of the phenocrysts. ; - : ; ;
, Mineralogically the rocks contain about 64 percent feldspar
(table 2, B-77, B-140, B-262, B-263, B-264, and B-266). No ortho- clase was recognized in the fine groundmass so that all feldspar was counted as plagioclase. Notably higher plagioclase was recorded in the top flow on South Grouse Mountain summit because of a paucity of py roxene phenocrysts. A corresponding plagioclase-rich dike facies ap pears to exist in the dike-plug extending into the northwest side of
Golden Curry Basin.
Another correlation of lavas with dikes is suggested for the
second and third flows from the top of Bohemia Mountain. Three dikes
on the north flank of the mountain are indistinguishable from the flows
in thin section. The lavas contain subhedral phenocrysts of augite, plagioclase, and chlorite pseudomorphs of olivine in a fine-grained
groundmass of plagioclase laths and augite grains (table 2, B -220 and
B-221; pi. 15). Another group of flows exhibits similar textures except for 26
somewhat finer grained groundmass and the presence of crystallograph-
ically oriented poikilitic inclusions of chlorite and unknown phases in ; _ i _ ' ,
the plagioclase (table 2, B-196, B-217, and B- 8 6 ). These include the porphyritic basalt on the east side of Champion Creek canyon, lava
north of Crystal Basin, the top and bottom flows on Bohemia Mountain,
and basalt composing Kitten Rock. One of the dikes of the Elephant
Mountain swarm is nearly identical texturally to the rock at Kitten Rock.
It is not unlikely that Kitten Rock is an intrusive mass. The basalt body
at Cat Mountain (pi. 4) also may be a plug. . .
Petrographic data of flow rocks of the upper unit are summarized
in ,table 2. Data on the hypabyssal intrusive rocks are summarized in
table 3 INTRUSIVE ROCKS
Intrusive rocks can tentatively be placed in three groups ac cording to age: oldest, porphyritic dacite; intermediate, basic dikes and sills; and youngest, granitoid intrusives. The dacite masses are aligned southwest, probably parallel to the basement structure. The larger granitic bodies trend north-northeast parallel to the belt of gra nitic intrusives of the Western Cascades (pi. 3).
Porphyritic dacite, if it is contemporaneous with the domes of the intermediate unit as suspected, is probably late Oligocene. The granitoid rocks are probably late Oligocene or Miocene. The basic dikes cut the volcanics but are not known to cut any of the granitic in trusives. They are of several ages and in part contemporaneous with the basic flows of the upper unit. ^
Porphyritic Dacite
Four m asses of porphyritic dacite crop out within the inter mediate unit. When grouped with the domes, they form a cluster aligned
S. 40° W. from Champion Creek to the edge of the area near Shane
Saddle. This trend is parallel to the basement structure exposed 30 miles southwest (see "Basement Structure"). An analogous arrangement 28 appears to exist 140 miles southeast in the Macdoel quadrangle (Williams,
1949) where Tertiary rhyolite domes are aligned N. 40° E. parallel to the structural grain of the pre-Tertiary basement exposed 10 miles northwest.
No contacts were seen, but west of the Champion mine an in trusive relation is suspected where the contact climbs 800 feet in eleva tion in a short distance. The possibility remains that the.contact is a fault. ~ ‘ •:: ■ ■_
The megascopic and microscopic appearance is similar to that of the dacites of the flows and domes, except for a lack of banding or foliation and a greater abundance and size of phenocrysts. The feldspar has commonly been sericitized and on weathered surfaces it has been leached with consequent accentuation of the porphyritic texture.
Microscopically the groundmass varies from that composed of patchy crystal growths such as in some dacite lava to microgranular types resembling the later dacite porphyries. Oligoclase phenocrysts are euhedral or subhedral; They are simply twinned and unzoned.
Basic Hyp abyssal Rocks
; In the absence of abundant bedding to provide planes of weak ness for sill emplacement the common manner of hypabyssal intrusion was as dikes. Several types of andesite and basalt are represented
(table 3). These dikes seem to have been intruded along favored 29 directions and possibly in particular belts. The frequency diagram of plate 4 illustrates maxima of strikes at about N. 60° W. and N. 60° E.
Three vague zones of intrusives trending N.. 40° W. are suggested (pi.
4).
Glenwood Sill
Below the Glenwood cabin a thick sill of silicic andesite (table
1, B-274) has penetrated along the bedding of shale and sandstone. At
Glenwood it is a single mass about 50 feet thick. The sill was not fol lowed south, but presumably it is the same one which crops out half a mile southeast. Here it is divided into three or more sheets separated by layers of indurated shale which except for some brecciation along the contact appears to be undisturbed. ;
Placering at Sharps Creek has scoured bedrock and exposed the upper contact. The first eighth of an inch of shale adjacent to the sill contains disc-shaped, epidote-rich knots about half an inch in di ameter. Within the sill there are abundant lineated, flat pores that probably were vesicles. Though uniformly oriented in thin planes parallel to the contact, they change through at least 90° of direction in a thickness of less than a foot. If they are vesicles, there must have been a major change of direction of intrusion or possibly some local anomalous .current.: i I ; Along the Cat Creek road (section 27) the well-bedded tuff has 30 been penetrated by a 1 0 -foot tear-shaped mass of lineated vesicular an desite. The tuff has largely conformed to the shape of the body, sug gesting intrusion while the sediment was still plastic due to its content of connate water.
Basalt Near Elephant Mountain
Three large bodies of basalt crop out in sections 2, 3, and 10.
They average 100 to 300 feet across by several hundred feet long and commonly fray into dikes at the ends. The rock has a.characteristic speckled texture on weathered surfaces which results from abundant small plagioclase phenocrysts. Chlorite-filled vesicles are present locally. \ ■ v: / / y ■ ::: c \ rl1 . v :
Under the microscope coarser, diabasic-textured varieties might be appropriately described as gabbro (pi. 14). However, most are identical with the lavas on North Fairview Mountain and elsewhere.
Modes from several places show little variation (table 3, B-117,. B-80,
B-103, B-104, and B-95). The dikes are believed to have been feeders for major lava eruptions (see discussion under ’’Petrography of the
Basic Flows of the Upper Unit”). - -
Porphyritic Basalt Near Mustek Mine
At least three dikes of a swarm cropping out on the north side of Bohemia Mountain are suggested to have been feeders for the upper 31 flows on the mountain (see "Petrography of the Basic Flows of the Upper
Unit;" table 3, B-159 and B-204; and pi. 15). ; r
Bohemia Stock* and Associated Bodies
Seven varieties of intrusives ranging in composition from diorite to granite were recognized within the district. Their extent has been mapped (pi. 5) and delimited on three sides. The principal - mass trends north-northeast and has steep sides and, at least locally, a flat top. The outcrop pattern suggests a size of about 4 miles by 1 mile. Only a few, persistent dikes extend to the west. To the south a line of small plugs or apophyses trends about S. 40° W. to the edge of the area: 5 Along the east side of the largest mass the volcanics have been complexly penetrated by large apophyses and dikes.; ‘ .
■ : Examination of stream boulders and road cuts in the adjacent areas suggests that the intrusives are mostly confined to the district on the southeast, south, west, and north. Among boulders on Sharps Creek below the mouth of Martin Creek no granitic or porphyry types were recognized. However, one cobble of silicified pyroclastic rock contain ing tourmaline was noted, thus indicating the presence of at least one granitic body in the Martin and Sharps Creek drainage. According to
*In some of the previous work the intrusive complex has been called the Champion stock. 32
Mr. Earl E.; Lillard (personal.communication to H. M. Dole, 1961) granitoid roc^c crops out in the headwaters of Mosby Creek, about 9 m iles west-southwest of Bohemia Mountain. : In Little Rock Creek (sec tion 2 ) to the southeast no granitoid rock was recognized among alluvi um. ;■ V ;
Inspection of cuts along the .Dinner. Ridge and Brice Creek roads to the west of the northern part of the area reveals no granitoid intru- sives except for one small body, of diorite (Bales, 1951) on Brice Creek
7 miles northwest of the stock. Along the road across.the northern side of Sugar loaf Mountain and Holland Point no granitics were observed, and the contact alteration aureole developed in Wyatt Creek does not ex tend this far north.
On the other hand, a traverse along the road crossing Parker
Creek and at present ending near Timbered Knoll revealed that the al teration aureole and small porphyry intrusives extend more than a mile east of the mapped area. Among boulders in Brice Creek in section 29 there is a variety of porphyry and granitoid rocks from sources to the east. It also seems reasonable to expect more granitoid bodies east of the cluster in sections 5 and 6 .
The chronological order is, an early quartz diorite, an inter mediate group including two dacite porphyries.and a granodiorite, and finally very minor aplite dikes. Isolated augite diorite.bodies are pos sibly the earliest of all. The various rock types are considered by 33
Buddington and Callaghan (1936) to be facies formed by "successive in termittent intrusions of the products of differentiation in part along the same general lines of fracturing. " I find little basis for disagreement.
The presence of seven inappable types suggests that the facies' are more independent than are border, core, and dike facies. -
Chemical analyses of Bohemia intrusives and five other Tertiary intrusive rocks of Washington and Oregon form moderately smooth
Barker diagrams (Felts, 1939; pi. 30). The Snoqualmie batholith and
Silver Star Mountain stock of Washington and the Nimrod, Steens Butte
(Wells and Waters, 1935), and Bohemia stocks of Oregon and probably many others are petrochemically similar.
Peck (1960) has obtained evidence that ages vary by several million years, generally within the Oligocene and Miocene^ The Bohemia
stock roofs in the middle of the Oligocene-lower Miocene vblcanics. It is suggested that ther e was about a mile of cover originally. On the basis of this assumption the stock is suggested to have been emplaced near the beginning of the Miocene.
Augite Diorite
; - Two or three small bodies of fine-grained diorite crop out on
the southeast slope of Bohemia Mountain. Unlike most small masses
away from the main cluster, these are surrounded by aureoles of con
tact-altered rock. Contacts of the plugs are sharp and without a chill 34 zone. .... v,
According to Buddirigton and Callaghan the texture is even- granular with interlocking plagioclase (Angg) laths ranging in general from half a millimeter to a millimeter in length. Augite is stubby, euhedral, and locally twinned. There is a little interstitial quartz and sparse orthoclase. Magnetite and ilmenite are disseminated through out. • . ' . . ... V . ' ■ •: r-; • .■•. .
Porphyritic Augite-Quartz Diorite
Quartz diorite occurs independently in several masses and within the granodiorite pluton as remnants in place and as detached xenoliths. There is a gradation from varieties seamed with reticulat ing veinlets of granodiorite to those in which the granodiorite predom inates. Similar xenoliths were also noted in the two varieties of dacite porphyry. The distinguishing features are a dark-gray color and a fine grained porphyritic texture.
Some bodies contain needles of chloritized mafic minerals. At least some of these were originally hornblende. In the small plug in the eastern part of section 14, fresh, thick hornblende individuals and twinned groups measure up to a centimeter across (table 3, B-176). A nearby mass at the Mystery claim contains a few rounded quartz pheno- crysts. Plagioclase phenocrysts appear to vary only slightly from sodic labradorite (Ang^.l - ^ ’ A specimen from the first switchback on the road below the
Champion camp contains sodic bytownite (An^) and no hornblende.
The groundmass is crowded with small plagioclase laths.
In Champion Creek about 500 feet north of Golden Curry Creek the rock is seamed with inconspicuous, irregular lenses of felsic min erals about 2 cm long which locally contain chalcopyrite. Two samples of this rock contained a trace of copper by wet analysis and a trace of nickel and no detectable tin, tungsten, nor molybdenum by spectrograph.
The quartz diorite corresponds to the "porphyritic augite-quartz diorite” of Buddington and Callaghan, as deduced from their petrographic description and source location of the analyzed sample. A silica content of almost 60 percent justifies the classification as quartz diorite.
Dacite Porphyry
Apophyses and plugs of porphyry might be grouped together on the basis of texture. However,, it is possible to divide them further in to porphyry with quartz phenocrysts, and porphyry without quartz pheno- crysts. 1 :
- Large bodies of dacite porphyry (i. e., without quartz pheno crysts) are present among the satellites beside the granodiorite pluton
(pi. 5). The relation to the granodiorite is obscure. The mass in the
SE-l/4 sec. 36 and that on the road to the southwest apparently are remnants that survived the intrusion of granodiorite, but contacts were 36 not discovered. : ;
The plagioclase phenocrysts are, up to a centimeter in length.
They are commonly sericitized and the resultant differentially weathered surface simplifies the mapping of the unit. Under the microscope the plagioclase (oligoclase to andesine) is seen to be euhedral, well zoned, and complexly twinned. The groundmass is microgranular, and in some specimens it appears to contain up to 50 percent quartz with much of the remainder being orthoclase. :
The dacite porphyry is correlated with the au git e - hypers then e granodiorite porphyry of Buddingtoh and Callaghan (1936, table 1 , anal ysis 5) on the basis of the similarity to the specimen illustrated in their figure 4. : •/'
Quartz Phenocryst-Bearing Dacite Porphyry
The quartz phenocryst-bearing dacite porphyry is distinguished from the similar dacite porphyry by its content of as much as 3 percent of partially resorbed quartz phenocrysts.
This porphyry tends to have dike forms. Dikes extend at least
3 miles west of the stock. On the east the rock accounts for only a minor part of the apophyses and plugs. One mass is, however, of par ticular interest; it projects into the large cupola of gr anodiorite in
section 1. The contact was not seen; however, the lack of dikes (other than granite porphyry) cutting the granodiorite suggests that the 37 granodiorite is the youngest major intrusive. In addition, a section cut from an altered specimen of this porphyry shows large percentages of biotite and secondary chlorite and quartz. Presumably the chlorite and secondary quartz, and possibly the biotite, formed during intrusion of the granodiorite. By this reasoning, the mass may be an upper facies of an older, deeper pluton. An alternate possibility is that the body is the last major intrusive, localized in the hood of the earlier cupola.
Four other samples examined in thin section are altered. The groundmass appears to have been largely composed of plagioclase laths arranged in a vaguely oriented fabric which resembles that of the quartz diorite. ;;: . ... :. .
The rock is generally equivalent to the augite dacite porphyry of Buddington and Callaghan (1936, table 1 , analysis 3). A silica anal ysis from a dike south of Shane Saddle supports the classification as dacite (table 1, B -10). . -
Granodiorite and Porphyritic Granodiorite
The stock, cropping out in Champion and Brice Creeks is gen eralized as granodiorite (Gd) on plate 5 on the basis of megascopic criteria. However, a porphyritic granodiorite and an older, medium grained granodiorite can be distinguished locally (see "Coarse Inclusions in Porphyritic Granodiorite"). The satellites and apophyses on all sides are mostly of a somewhat finer grained variety of the porphyritic 38 granodiorite. This variety is shown as granodiorite porphyry (Gdp) on plate 5. : , i.; r v,.-
A few contacts were, found. These combined with the third dimension, which rugged topography provides, give a picture of the shape of the body. It has a western contact near vertical in Champion
Creek. Here its upper contact is flat in conformity with overlying, bedded tuff and probably continues through Noonday Ridge to a similar flat contact exposed on’the north flank. The upper contact appears to dip gently east parallel to country-rock strata in Brice and Wyatt Creek canyons. However, at lower, elevations the east and the west contacts dip steeply. Large, north-northeast longitudinal dike apophyses are evident at the extreme north and above the hypothetical flat roof under
Noonday Ridge.
:v The roof in the northern portion is near, a constant elevation of 3,400 feet above sea level. Peck (1960) has remarked on a similar roofing between 3,000 and 4,000 feet in other Western Cascades plutons.
At the south a large offshoot penetrates at least another 800 feet into the cover. This.cupola or neck is pervaded by abnormally extensive hydrothermal alteration. . .
- The cluster of plugs to the east in sections 5 and 6 probably connects with the main stock at depth, and possibly by way of a single southeast-trending mass directly below.; Additional suggestions of the existence of a cross belt of intrusion are found in the arrangement of 39 alteration zones and vein swarms symmetrically beside it. The texture of the rock in the satellites is similar to that of the main pluton except . for a decrease of crystal size, especially in the groundmass.
Felsic yeinlets and less commonly crystals lining miarolitic cavities locally make up as much as 1 percent of the bulk composition.
Some of these contain tourmaline, bidtite, amphibole, or sulfide along with quartz and feldspar.
Microscopically the porphyritic granodierite contains about 25 percent plagioclase and augite phenocrysts dispersed ih a fine- to medium fine-grained groundmass. The plagioclase phenocrysts (Absq. 5 5 ) are well zoned and complexly twinned; outer layers are more sodic. Au gite has commonly been altered. Optically oriented, branching growths of quartz in the groundmass appear to be a distinguishing feature. It is quickly detected, yet it is not as conspicuous as a micrographic or gr anophyric inter growth. Other groundmass minerals include plagio clase laths, augite, amphibole, and orthoclase (table 4, B-240, B-241,
B-245, B-247, and B-24S). Silica contents range to nearly 6 6 percent.
The presence of an older granodiorite within the Bohemia stock has been established (see following section). The one thin section made from the exposures in Brice Creek is texturally indistinguishable from this older rock. Apparently the Brice Creek portion of the stock is at
least partly of the older facies. In outcrop it is medium grained and
contains the usual dark, fine-grained xenoliths. 40
Under the microscope it is seen to be hypautomorphic-granular.
Large plagioclase subhedra average near andesine (An^g) in composition.
Hornblende appears to replace previous augite and is in turn replaced by chlorite. Quartz and orthoclase fill interstices. Minor biotite is present.
Coarse Inclusions in Porphyritic Granodiorite ! ' . • . ' • : Rounded blocks of a medium-grained granitic rock are common in the porphyritic granodiorite along Champion Creek (section 1). These xenoliths measure up to 5 feet across. The texture is hypautomorphic- gr anular to slightly porphyritic and the overall grain size is coarser than that of the surrounding porphyritic granodiorite. A half an inch selvage of felsic minerals rims each block. Except for the exposures in the well-scoured creek bed, the inclusions would most likely be overlooked.
Uralitic hornblende is the principal mafic mineral. Intricate zoning, complex twinning, and subhedral form of its plagioclase (an desine to oligoclase) along with generally fresh appearance indicate that the xenoliths are not fragments of basement rock.
Nearby outcrops in the creek in the E -1/2 sec. 1 seem distinct from the common porphyritic granodiorite and somewhat similar in tex ture to the large xenoliths. This rock contains some biotite which is a rarity in the district. 41
The implication is that a granodiorite was emplaced prior to the intrusion of the porphyritic granodiorite, but that in the Champion
Creek area only a few xenoliths and remnant masses of the early in trusion remain.
: ! ,, Granite Porphyry ; . ; - ;
Vertical granite porphyry and aplite dikes averaging a foot thick cut the Bohemia stock and its country rock. Their strikes most ly parallel the long axis of the stock. Presumably the aplite dike which
Buddington and Callaghan found intruding a granodiorite dike near
Champion Saddle is of the same period. These authors regard aplite as the last intrusion. It is noteworthy that the volume of this late pulse of magma was very small; all of the dikes observed are plotted on plate 5. '
The rock contains about 50 percent quartz in the form of phenocrysts and as anhedral grains or granophyric clusters in the grouhdmass. Orthoclase is also abundant. Oligoclase is present as phenocrysts in some varieties, and mafic minerals account for less than 5 percent. STRUCTURE
The Tertiary volcanic pile generally dips gently eastward in the Bohemia mining district. However, this homocline is locally warped, and the warp shows a geometrical relation to the pluton which has broken through the strata parallel to the north-northeast intrusive belt of the Western Cascades. H I i ■ ■ .
Homocline
; Flows on Grouse Mountain dip about 22° east (pi. 2). Bedded pyroclastics in Weaver Creek dip 20° east. The tuffaceous shale and sandstone on Sharps Creek average about 10° of dip ;to the northeast..
As with that of Callaghan, and Buddington: (1938), this report did not progress far enough south to verify;DillerTs (1898) convictions that, bedding in the south dips to the south and suggests an anticlinal struc ture.
Broad Warp
. . •" • = ' . • ■ . . ■ - . • • ■ 1 v • • . , • - . . -
. • Though no district-wide marker bed was recognized, several
flows and clastic lenses and locaUy abundant bedding attitudes can be
used to approximate a composite surface representing the homocline 43
: and warps. This is reasonable because the folds are of low amplitude
: and no major angular unconformity is known. The smoothed surface; iiis shown in plate 17. The warp in the homocline; over lies the south end
of the Bohemia stock, i : r . > : - ■
Minor Folds v. : i- ; .a y . aViy ai • f — a- - va
y: Dip reversals suggest a few minor folds. : A gentle syncline
: plunges N. 40°: E. across Monte Rico Ridge. This lines up approximate
ly with the syncline plunging N. 15° E. from section 14 into section 11.
Another, syncline plunges about N. 20° E. across the southeast side of
Jackass Butte. -- a.am. ;a a .a , ;;v .. .a;- : 'r •. v ;«. .-..y ■ a , ;: .
T ; c a • The intervening anticline is of some consequence. Not only
does a swarm of ’’cross veins’! (see explanation of term below) approxi
mately parallel its axial plane, but also it is arched over a cluster of
plutonic dikes and apophyses near the center, of mineralization. Another
anticline trends N. 25° E. across Pilot Rock. - • ; a m a a .
Veins and Faults
y y No fault was Seen that did not manifest some hydrothermal al
teration. Apparently the.veins represent essentially all of the faulting s adjustment that has taken place. They follow four sets. The commonest
veins strike N. 60o-80° W. and are steep. Over 50 percent of these dip
south. A second steep set locally known as "cross veins” strikes N. 44
20o-40o E. In the southern part of the area steep'veins with a strike of about N. 40° W. interfere with those striking more westerly and together they form veins with prominent bends. A fourth, harrow group strikes
N. 80° E. from the Musick vicinity to Grouse Mountain. On plate 6 there is a faint suggestion that these four sets and some veins with in termediate strikes may radially converge on one area, near the Musick mine. This focus, whether fundamental or coincidental, is near the two main mines of the district.
Few veins show displacement. It is common to find the same type of wallrock on both sides of the vein. Additional evidence of little or no displacement is found in the local marker beds mentioned above.
The dacite flow of Grouse Mountain extends nearly 2 m iles, striking across about half of the width of the main group of veins. The Champion and Helena are among the veins that are crossed. No displacement was seen. Among other stratigraphic units that strike across the trend of veins but are apparently not displaced is the shale and sandstone forma tion in Sharps Creek. ^
Similarly, the unbroken, bedded shale and sandstone lens roof ing the Bohemia stock (section 36) serves to eliminate the possibility of any major northeast faults across its trace. This is important because
one of the most suspect places for a fault would have been along the
axis of the intrusive. Three or more faults trending N. 40° W. might be suspected 45
on the basis of topography. Monte Rico Ridge, City Creek, and parts
of:Horseheaven and Champion;Creeks follow conspicuously parallel - northwest trends. A few miles west Mosby Creek and lower Sharps
Creek follow similar trends for about 20 miles. As noted by Schlicker
and Dole.(1957), the course of Mosby and Sharps .Creek is probably
structurally determined. Mapping in the district, however, has failed
to substantiate the existence on any major breaks. . ;
Joints
: Four hundred and fourteen joints.were recorded. They show
two prominent maxima (pi. 7) at about right angles. In the field there
was a tendency to overlook joints with flat dips. The impression gained
is that flat joints are subordinate to those of the two common steep sets.
The two maxima of steep joints are approximately oriented to the Bohemia
stock and the overlying warp as longitudinal and transverse joints.
: Similarly, it is not uncommon to find two well-developed joint
sets at about right angles in individual outcrops. In the few cases
where there are more than three sets evident, they;are usually so :
varied that it is difficult to distinguish individual sets. Such complex
combinations were not recorded. - : ■ -
■ Most joints have no associated hydrothermal alteration, and
thus seem to have formed after the period of mineralization. Never
theless, fractures paralleling the joints have been utilized as channels 46 by certain facies of the stock, indicating that the longitudinal-transverse fracture system began to form early in the period of intrusion. In addi tion, the veins fall largely into the two fracture sets. •
Additional evidence of the close r elation between the two com mon joint sets and the granitic intrusions is revealed by separately plotting all joints within or near the main stock.on a Schmidt net. The result is to strengthen the maxima in comparison to a plot of joints out side the stock vicinity (pi. 7). It should be noted that the transverse and longitudinal joints over the intrusive are not precisely oriented to its apparent axis, whereas those at a distance are. The projections re veal a preference of southerly and westerly dips. They appear to be u - complementary to the eastward to northward dips of flows and pyroclastics and suggest that fractures tend to be normal to bedding.
The fracture pattern has been duplicated in fault model experi- ments described by Beloussov (I960); however, the definite sequence of formation observed in the experiments is not evident in the Bohemia district. - 3 % cr'.: • ':hv . /
‘ Comparing the joint pole diagrams with the strike frequency
diagrams of hypabyssal dikes (pi. 4), one notes that the first maxima
are about at the same orientation. However, the second maxima are
different. •'{ G-:—: x;..: AvV..-v
It seems reasonable-that the hypabyssal dike attitudes repre
sent a part of the fracture pattern of the area previous to major y 47 intrusion. The symmetry of the later plutonic intrusion is oblique to this pattern, but apparently the steep fractures striking N. 60° W. i were suitably oriented for reopening as transverse fractures. The . dikes and joints striking N;, 60° E. were not favorably oriented for use in relieving the later stress and thus are not prominent over the pluton.
The distinction shown by the projections of plate 7 may clarify the nature of the vein set striking about N. 40° W. across the southern part of the district. This group interferes with the transverse set to the north at about the southern limit of the large granitic intrusives.
Thus, though both northwest sets were mineralized contemporaneously, the southern one appears to have followed an old.pattern while the north ern set developed in particular fractures reopened by the underlying pluton. . ■ r : V /:
Basement Structure
On the basis of Peck's (1960) Tertiary stratigraphic thicknesses there is about a 2 -mile thick section of volcanics between the bottom of the upper unit and the pre-Tertiary. The basement is exposed beyond the edge of the Tertiary volcanics 30 miles southwest. Peck has re marked that folds in the Tertiary volcanics reflect control by the struc tural grain of these underlying formations.
In the Grants Pass quadrangle (Wells and others, 1940) the
Triassic formations are dominantly of metasediments that dip steeply 48 southeast. In the Riddle quadrangle (Diller and Kay, 1924) bedding strikes, which are assumed to be represented by the abundant parallel contacts between greenstone, gabbro, and metarhyolite, are about N.
50° E. These Jurassic formations also dip steeply and uniformly south east. To the north in the Roseburg quadrangle (Diller^ 1898), Creta ceous and Tertiary sedimentary formations strike about N. 50° E. : ■
However, in progressing north into younger formations the folds be come gentler. Cretaceous “Myrtle formation" attitudes are somewhat steeper than those of the Eocene Umpqua Formation and Tyee Sandstone.
Two periods of deformation intervened between the accumulation of the
"Myrtle formation" and the Eocene deposits (Baldwin, 1959); but ap parently these and subsequent diastrophism that deformed the Tertiary units were at least partly controlled by pre-existent structural grain.
r: n
• '' V- k ’ ALTERATION
. ■ V: a c Three periods of alteration are evident. 1) Hypabyssal intru sions developed carbonate in adjacent pyroclastics. 2) The Bohemia stock and some of its apophyses are surrounded by aureoles of recrys tallized rock ranging in grade up to the albite-epidote hornfels facies
(Fyfe, Turner, and Verhoogen, 1958). Tourmaline is developed local ly in country rock and also within the granodiorite. 3) Hydrothermal alteration closely followed the contact alteration and is responsible for the argillization and associated vein deposition. V. . . . ' / ' .. % r :: : V A large percentage of the less-altered rocks examined in thin section, especially elastics, contain secondary chlorite with a uniform ly low birefringence and a mean refractive index of about 1 . 61-. This widespread propylitic alteration included some of the changes that were also characteristic of the contact aureole, and the distinctiveness of the contact-altered rock may only be a result Of greater intensity. The granodiorite is also propylitically altered, especially near veins. Thus, it seems that propylite has been developed in two or more different periods. Apparently, at least a moderate degree of propylitic alteration pervades the entire district as an alteration "background. M
Peck has outlined extensive areas of propylitically altered rock
49 50 in the central part of the Western Cascades to the north. He feels that the alteration is related to centers of volcanism. Previous to Peck* s study, Waters (1955) had expressed the opinion that the alteration in the Cascade Tertiary was too widespread to have been related to the intrusives.
Carbonatization
At least two periods of carbonatization are evident. Early. hyp abyssal dikes are locally flanked by carbonated pyroclastics, where as carbonates are concentrated at a distance from the Bohemia stock.
Calcite Associated with Hyp abyssal Rocks
Adjacent to a group of narrow basalt dikes (table 3, B-95) be low Cat Mountain the lapilli tuff has been extensively replaced by calcite
(X-ray identification). The mineral is in the form of chalky white granules a millimeter in diameter that locally coalesce to form the greatest part of the rock. The zone of carbonatization extends about
1 0 0 feet from the nearest dike, as seen in the jeep-road.cut. , vr; Similar, but less intense, alteration is present in faintly bedded tuff about 100 feet above the Glen wood siU along the Shane Saddle road.
The calcite is in the form of pipes about 2 mm in diameter, which pene-~ trate the tuff perpendicular to bedding. On the Cat:Creek road beyond the northwest edge of the map area, andesite emplacement in tuff has 51 resulted in the formation of spherical granules of chalky carbonate.
When these granules weather, they leave pits that give the tuff the ap pearance of a vesicular andesite. ... , ::; ■ :
Pervasive Carbonatization
Though replacement carbonate is not conspicuous in the cen tral part of;the district, local areas at a distance of about 2 miles from the stock contain substantial quantities.. Pyritic tuff breccia along the
Sharps Creek road in section 16 contains veinlets and disseminations of calcite and brown carbonate, and on the east side of Holland Point tuff breccia locally contains a few percent of calcite that is almost black with inclusions of an unknown mineral. :
With the microscope one finds that phenocrysts in dacite are commonly replaced by carbonate, even in the. central part of the district.
Carbonate is also common in hydrothermal veins as a late filling.
Contact Alteration
The Bohemia stock is surrounded by. a halo of contact-altered rock having; a width on the present surface of as much as a mile. Of the
affected rock types the most susceptible has been the tuffaceous shale.
Shale in the bed of Champion Creek over 4,000 feet from the steep west
ern contact of the stock has been recrystallized to a flinty rock contain ing epidote-rich knots. Besides epidote, these knots contain plagioclase, 52 quartz, chlorite, ’and pyrite. n t.
> 'The coarser pyroclastics have been more stable in the contact environment. Their textures have been megascopically altered in ;
Champion Creek to a,distance of 3,500 feet from the steep contact of the stock. ■
Least altered are the andesite and basalt sheets. Contact al teration in these rocks is not. easily distinguished in the field,: however it seems to result in dense, blocky fractured rock, which forms re sistant outcrops. The study of the present topography as presented in plate 16 indicates that all rock types have been made more resistant to erosion in the aureole. . : ; -i ^ y
> . The texture of lapilli tuff may be radically changed. Shape, banding, vesicles, and other internal structure of fragments fade or become conspicuous, depending on whether they are simply recrystal lized or are replaced by epidote-chlorite-quartz-magnetite-plagioclase knots. Exposures are best examined in cuts on the Champion Creek,
Brice Creek, and Dinner Ridge roads. ,
; On the Champion Creek road, according to Buddington;and .
Callaghan (1936): ; : • ; : i : ; r - ^ y : . -
The greater part of the rock retains its original mag- - matic minerals with comparatively little alteration but con tains nodules of epidote and chlorite with centers of mag netite or pyrite. The minerals of the nodules are extremely •y v fine-grained. Some contain orthoclase or albite, possibly r .• some more calcic plagioclase, and quartz. The nodules range from an eighth of an inch to an inch in diameter, but 53
they are predominantly small. 1 _
Presumably this description is of exposures about 2,000 feet from the steep contact of the stock. Very minor chalcopyrite is associated with the magnetite of the knots. In these assemblages containing chalcopyrite, pyrite is rare or absent. Exposures near Weaver Creek at a distance of about 3,000 feet from the stock contain as much as 5 percent of pyrite but no chalcopyrite was noted. This feature appears to be an excellent
example of the affinity of sulfur for copper in preference to iron
(Goldschmidt, 1954, p. 18). Where the partial pressure of sulfur was low, sulfur was largely combined with copper. Where the partial pres
sure of sulfur was high,- pyrite is abundant and masks the sulfides of more chalcophile elements. ,
On the Brice Creek road a thick unit of altered tuff breccia that can be traced diagonally up the side of Noonday Ridge crops out. It
is characterized by heavy chloritization and recrystallization with con
sequent partial eradication of the clastic texture. Epidote is not so
common in this area and pyrite is uncommon. The outcrop is at the
edge of the area mapped so that the extent of the aureole to the east is
unknown. Abundant granitoid boulders in the creek suggest that in
trusive apophyses and contact alteration may extend some distance.
The most extensive outcrops occur along Dinner Ridge road
(section 20). Here a unit of tuff breccia, in places with fragments up
to 2 feet in diameter, has been chloritized and epidotized. Epidote- 54 magnetite-quartz and epidote*calcite assemblages preferentially replace bands, vesicles, and whole fragments and fill fractures. Pyrite is usu ally present but again where specks of chalcopyrite were seen associated with magnetite, pyrite is rare or. absent. A specimen of chalcopyrite- bearing contact rock analyzed 0.10 percent copper by wet method. A trace of nickel was identified but no molybdenum, tungsten, nor tin were detected spectrographically. . v
Biotitization
Biotite was noted in four places in intrusive rocks. In the granodiorite exposed in Brice Creek canyon it occurs as a primary mafic constituent and as coarse books in miarolitic cavities.
A thin section of the altered quartz phenocryst-bearing dacite porphyry in section 1 reveals about 15 percent biotite in a mosaic of secondary quartz. The biotite, in part altered to chlorite, is presum ably a secondary mineral, possibly introduced during intrusion of the adjacent cupola of granodiorite. In a thin section of the altered cupola rock only quartz and sericite are abundant.
Tourmalinization
; Schorlite is locally disseminated and concentrated along frac tures in contact-altered rock, granodiorite, and siliceous replacement lenses. Its presence as rosettes crudely aligned on rehealed fractures 55 in granodiorite dates the tourmaline as early post-magmatic, somewhat later than the contact alteration. It is also found as miarolitic cavity linings in the stock. ; : ; ; % ; . r; v v Samples from the wall of the Mystery vein show veinlets of quartz splitting pre-formed tourmaline spherules. At the adit of the
Diamond crosscut of the Champion mine pyrite replaces tourmaline spherules. The development of tourmaline preceded the base metal vein filling and associated alteration. There appears to be no spatial relation. .■■■-. v ■- ; • uv - : %
The assemblages recognized by Buddingtqn and Callaghan have been modified and supplemented below. Individual minerals are listed from most to least abundant, and the order in which the assemblages are listed represents an apparent decrease in intensity of alteration. ,
Quartz-feldspar-amphibole-epidote-tourmaliner ; . ,
biotite (in miaroUtic cavities)
Quartz-tourmaline-sericite-pyrite i
Quartz-sericite-tourmaline-relic minerals-pyrite
Sericite-relic minerals-tourmaline ^ ,
^ Relic minerals-chlorite-sericite-tourmaline
Tabular masses, essentially of quartz and tourmaline, up to
50 feet across represent the extreme type of replacement during the stage of tourmalinization. Attitudes of these masses are approximately parallel to those of the longitudinal and transverse fractures. They are 56 localized in country rockj in particular lapilli tuff and porphyritic dacite, to the south of the stock.
More widespread tourmalinization has resulted in development of schorlite spherules disseminated and along fractures in pyroclastics, and to a lesser extent in granodiorite and porphyritic dacite. Much of this tourmalinization has developed as vague zones over a wide belt ex tending about 2 miles along the projection of the axis of the Bohemia stock. It seems significant that a large part of this metasomatism has occurred here, near or above the elevation of the flat roof of the stock.
However, in Brice Creek granodiorite exposed 1,000 feet below the roof elevation contains some tourmaline.
The basic lavas are seldom seen to contain tourmaline. Though this may be due mostly to the lack of a color contrast, it seems more likely that the pyroclastics were most favorable to replacement, pos sibly by virtue of their water content and permeability. Commonly, in dividual lapilli have been leached and the cavities are partially filled by tourmaline.
A zone of tourmalinization extends about 5,000 feet west of the
stock into the Weaver Creek drainage (section 2). Its surface dimen
sions and orientation suggest a transverse structure that would also in
clude the porphyry dikes to the west and the cluster of granitic apophyses
to the east: Pyrite is less abundant in this tourmalinized rock arid in
that in Brice Creek (section 29). 57
Intermediate varieties of altered rock are not uncommon. Those containing abundant recrystallized quartz represent more localized al teration. Those containing chlorite appear to be situated as outer zones.
Breccia bordering a small dacite porphyry body in lower
Parker Creek (section 29) shows tourmaline preferentially replacing plagioclase phenocrysts of the porphyry fragments. The tourmaline is not restricted to this border breccia, however. Other local, brecciated areas characterized by quartz-lined interfragment vugs are shown on plate 6. Some of these carry minor sulfides but no tourmaline. The vuggy nature suggests a shattered character rather than development as a gas and debris channelway as has been suggested for many breccia pipes. ■ ■ - - - ; ' , .
Sericitization
Fine-grained sericite is not as easily identified.as the tour maline but is developed as a minor phase in much of the rock. It is present in rhyolite and dacite over large areas, apparently as a result of the availability of potash. It partially replaces plagioclase, and in a
series of spherulitic rhyolite samples across the Mustek vein near
Bohemia Saddle sericite replaces most of the original phases except quartz. Sericite is commonly associated with the tourmaline and is
more widely developed in the same areas. In this respect, sericite
might be considered as characterizing an outer zone of the tourmalinization 58 type of alteration.
Particularly advanced sericitization is evident in breccia near the West Helena claims in upper Champion Creek canyon. Fragments, partly replaced by sericite but preserving the original texture, are iso lated in a matrix of sericite. The restricted outcrop of this body might be interpreted as that of a breccia pipe about 200 feet across.
The cupola of granodiorite east of Champion Creek has been extensively sericitized.
No attempt was made to assess quantitatively the distribution and abundance of sericite in relation to other clay-like minerals. Wide zones of alteration are not regularly distributed about individual veins, however kaolin is apparently more specifically associated with veins.
Argillization
Clay minerals are developed within the narrow alteration zones surrounding and including the individual veins. Sericite is also abun dant in fractured country rock along and in veins. MacDonald (1909) remarked that soft, disintegrated country rock forming a considerable part of the vein matter "is composed essentially of very fine granules of quartz with considerable iron-stained kaolin. At greater depth the same rock contains an abundance of sericite and calcite with very little kaolin. " This suggests that some of the kaolin is a near - surface, sec ondary byproduct of the oxidation of pyrite. 59
X-ray powder photographs of four white to gray vein clays are essentially identical. One clay has replaced the chlorite filling of amyg- dules in basalt at the Elephant No. 4 vein. The others are replacements of fragments in vein quartz. The best match among ASTM cards seems to be with dickite from the Red Mountain mining district, Colorado, sug gesting a hydrothermal origin. However, the comparison is not con clusive. DTA measurements on the Elephant 4 sample only revealed that it is one of the kaolin polymorphs.
Beidellite is reported to be present in country rock in the
Western Cascades mining districts (Callaghan and Buddington, 1938).
Some authorities (Grim, 1953, p. 39-40) question the validity of many early identifications of beidellite; however, the mineral is said to char acterize an alteration zone in the similar Central Kamchatka Range de posits (Vasilevsky, 1960).
Pyritization
Large areas of the district contain minor disseminated pyrite.
As noted by Buddington and Callaghan, pyrite nodules are abundant at the outer edge of the contact-alteration aureole in lower Champion
Creek. On the Dinner Ridge road disseminated pyrite is conspicuous in otherwise only moderately altered rock at the outside of the aureole,
whereas the typical contact rock contains only a little. At Parker
Creek on the Dinner Ridge road (section 21) massive veinlets of pyrite 60 up to half an inch wide are common.
The Sharps Creek road appears to follow about parallel to a wide northwest-trending pyritic zone in section 16. A group of tunnels on the east side of Noonday Ridge (N -l/2 sec. 8) has prospected a pyritic zone of considerable width. Bleached ground on the west side . of the ridge is probably similarly pyritic. : ! ; ;
Most of the other pyritized areas were distinguished on the basis of bleaching and limonite staining. Only the dacite, rhyolite, and pyroclastics show this, and the very much generalized outline of pyriti- zation on plate 5 is based on observations in these rocks mostly. ....
f :
: iV/ ;:.v.
1 '; r VEINS
About 100 veins are known in the district. These are open- space fillings of quartz and minor simple sulfides, hematite, and car bonates. Sporadic shoots of gold ore have yielded the principal produc tion in the past. Widths vary greatly but seldom exceed 5 feet.
Most production has come from an area of 2-1/2 square miles embracing the Mustek (Defiance vein), Champion, Vesuvius, Noonday
(Annie vein), and Helena mines. However, some past producers and a few of the better prospects lie beyond this center in a larger T-shaped area measuring about a m ile by 5 m iles across the top by about a mile by 3 miles in the zone perpendicular to the top. The "T" is symmetrical ly situated across the south end of the Bohemia stock (plate 6).
To the north a small group of northwesterly-trending metal liferous veins is separated from the main group by a mile-wide belt of notably barren veins. This barren belt coincides with the composite transverse zone of tourmalinization, stock apophyses, and the intense pyritization and bleaching on the flanks of Noonday Ridge. . -
Mineralization that can reasonably be ascribed to the Bohemia center is reported in Martin Creek (The Staff, 1951) and east of
Johnson Meadows (D. C. Bradford, personal communication) at 61 62 distances of about 4 miles from the most productive area.
There is little justification for classifying veins into various types. Primary vein filling apparently began with quartz and hematite and ended with barite, though only portions of the sequence are exhibited by individual veins. The quartz-sulfide assemblages and textures are similar from dump to dump.
The areal zoning has been described by Callaghan and Buddington, and consists of a central zone of specularite-quartz-sulfide veins, an in termediate zone of the usual quartz-sulfide veins, and an outer zone of quartz-sulfide veins containing late stibnite. The stibnite-b earing veins are asymmetrically situated in an east-west belt across the southern side of the district. The belt extends from the Western vein (Callaghan and Buddington, 1938) to a prospect near , Puddin Rock (G. Beissner and
K. Watkins, personal communication). Unfortunately, Mr. Beissner and I were unable to find this prospect again. \
Vein Structure
Four sets of steep veins were distinguished. Two of these are oriented N. 60o-80° W. and N. 20°-40° E. such as to qualify as trans verse and longitudinal fractures across the Bohemia stock. As noted previously, the third set trending N. 40° W., though contemporaneous, is probably a reflection of pre-stock fracturing that predominates to the south beyond the stock vicinity. Veins of the fourth set parallel a half a 63 mile-wide zone N. 80° E. across the center of metallization.
The strike of individual veins commonly varies by 30° along drifts. An extreme case is the Mustek vein where strike changes of
55° are encountered between adjacent segments of the vein. On the other hand, in the northern part of the district the strikes of veins are more regular as seen in the Helena, Sunset, Sultana, and Warrener veins. ' ''' '' ^ ' • -
The variation in strike of veins in the southwestern half of the district appears to be largely a result of an interference of the N. 40°
W. fractures with more westerly fractures. Stress localization and consequent vein development have apparently been governed by the pat tern of the intermeshed fracture sets. Mr. M. W. Cox (personal com munication) has noted a tendency for veins to end against other obliquely trending veins. In the Monte Cristo district Spurr (1901) describes similar "vein irregularities consequent upon successive selection of different joints. " He notes a correlation of better ore with more uni form vein strikes and reasons that such straight fractures are important enough to control solution circulation. The presence of relatively large ore shoots on the sinuous Musick and Champion veins suggests that such is not the case in the Bohemia district. It is likely that large irregu larities are somewhat favorable in that they provide for some opening during any minor movement.
No major displacement of veins by other veins has been 64 recognized. It is common for the northeast veins to end against the northwest veins. At the places where the persistent, N. 25° E.
Johnson vein intersects northwest veins, there is usually a short off set in either direction but no drag, gouge, nor consistent slickensides to suggest lateral fault offset. There are enough examples of the re verse relation, such as at the abrupt northwest end of the Ophir vein, to lead one to the conclusion that all veins developed essentially con temporaneously.
The following characteristics support the suggestion that the veins are contemporaneous extension fractures modified by some nor mal movement. >
1) Normal throw on the,Champion vein apparently does
not exceed about 50 feet (note correspondence of op
posite wallrock type on plate 8). ;
2) Some veins cross with only unsystematic displace
ment and no evidence of tectonic offset.
3) Steeper (extension fracture) veinlets penetrate the
adjacent hanging wall (see ’’Champion Vein”).
4) Veins are irregular rather than planar.
5) Open vugs are common.
6) Two prominent sets are symmetrically situated and
oriented with respect to the intrusive as longitudinal
and transverse fractures. 65
7) Joints and veins are essentially parallel (Diller, i
- 1898). V ' ■......
8) Most minerals in the paragenetic sequence are found
on all sets. ' c,
Champion Vein
The Champion vein has been developed on six main levels in addition to workings above the 600 level which are now largely inacces sible (plate 8). The vein has been followed for about 2, 700 feet on the
900 level and has been explored for a.vertical distance of 1,200 feet.
This structure varies both in strike and dip by as much as 30°, partly as a result of intersections with other fractures. There is some doubt that the vein should be regarded as being along only one fault. The structure shown on plate 8 suggests that the vein lies along two or more linked, curved surfaces resulting from an interference of northwest and east-northeast fracture sets. The largest known orebody in the vein was localized across the steeply plunging crest of one of these arcuate surfaces. ' :
Alteration is locally conspicuous, such as east of the east stope on the 1,050 level. Here the back has sloughed to about 12 feet above the floor as a consequence of the loss of strength due to combined py- ritization, argillie alteration, and shearing. Elsewhere,1 particularly where the wallrock has been contact altered, the rock is only moderately 66 argillized. The dense texture and secondary mineral phases of contact- altered rock, with or without tourmaline, seem to have been less sus ceptible to additional pyritic^sericitic/ and argillie alteration.
Quartz is almost always present. It usually forms one- to two- inch stringers penetrating or enclosing altered country rock. Some shearing is commonly evident, but it is riot uncommon to find coarse, single or layered combs of quartz on solid walls of an open cavity along the vein. On the 1,200 level quartz is less abundant^ and carbonate is common in the gangue over, much of the western part of the narrow vein.
Much of the inaccessible, upper part of the vein was stoped, and about half of the vein along the 900 level has been stoped for at least a short distance above the drift. Mining width probably averaged be tween 4 and 5 feet; however, in some stopes much of this was left as backfill. Shoots were as much as 400 feet long bn the level; the some what greater, priricipal dimensions appear to plunge at various angles in the vein. On the 1,050 level, work was started on two small shoots which apparently are the weakened, downward extensions of those above.
On the 1,200 level, the vein has a flatter dip and little or no ore was ex posed along 700 feet of drift. ' : : - : '
• The shoots result either from increased total thickness of quartz-gold-sulfide filling or an increase in the metal value. Some of the shoots were no doubt enriched by residual concentration of gold dur ing leaching of sulfides in the oxidized zone; however, sulfides seldom 67 amount to as much as 25 percent of the mined width of the vein,: and : residual concentration could hardly account for more than a 50 percent enrichment. Probably the important feature that resulted in gold ore shoots in oxidized vein rock was that the leaching left the rock in a state that was amenable to cheap milling methods. . . . : _
Stringers within the vein and subsidiary veinlets in wallrock were mapped on the lower three levels, and the data are presented •
graphically on plate 8. Lines connect subsidiary fracture-plane poles with the poles of planes representing the local attitudes of the main vein.
It is to be noted that subsidiary fractures generally have the orientation
of gash fractures. .This relationship suggests normal-fault movement
during quartz deposition.
On the 1, 200 level where later carbonate is an important gangue mineral, the subsidiary veinlets are of,carbonate. At the west face on the 1, 200 level, l/4-in ch carbonate gash veinlets within the gougy vein
itself are similarly oriented vertically., Normal movement thus appears to have persisted throughout the period of mineralization. .
Another inter esting feature, that was not verified by a system
atic collection of readings, is that there seems to be more subsidiary
veinlets in the hanging wall. There are two ways of explaining this:
first, because the floor (footwall) is covered with debris, more hanging
wall is exposed (assuming that the vein bisects the drift); or second,
during any dilation of a non-vertical fracture the hanging wall is partially 68 unsupported and.thus more susceptible to fracturing. The requisite open dilation would only be expected in near-surface environments. A simi lar explanation has been tendered to explain the hanging-wall bonanza of the Comstock lode (Lindgren, =1928, p. 185). =
The mapping has clarified two factors involved in ore localiza tion. Vein intersections (pi. 12) and steeper segments on the vein are favored places for ore deposition. The evidence of prevailing extensional conditions described above naturally leads to the conclusion that steeper segments of the main vein were more dilated during deposition. Deposi tion was largely in open spaces with little suggestion of replacement of the wallrock, so that mineralization tended to concentrate in steeper, more dilated segments. However, some slopes are on flat segments and many of the steep segments are below grade. ;
, A condition on the 1,200 level suggests that the description
’’steep" is most satisfactorily applied with due regard to the adjacent vein segments. The long crosscut intersected the vein beside a shoot from which a few sets of ore were extracted. The vein dip in this area is about 10° steeper than in adjacent portions, but it is gentler than the average for the whole vein. The reader is referred to a paper by
Newhouse (1942) for a discussion of this type of ore control.
Primary Vein Deposits
Primary vein accumulations are. divisible into three groups in 69 the Champion mine: an early quartz-chlorite-sphalerite assemblage, a later quartz- hematite- chalcopyrite assemblage, and the final carbonate assemblage.
The early quartz is clear to milky, and locally has penetrated along countless fractures into country-rock horses and walls. The en closed fragments have been altered, and many are extensively replaced by silica. Sericite, quartz, and probably clay Minerals have developed leaving chlorite as the only vestige of original mafic minerals. Even a small percentage of finely disseminated chlbrite imparts a characteristic tinge of green to the gaiigue. Where entities have survived this through- going replacement and deposition, thfey are lined with combs of milky to clear quartz, free of chlorite. In the cockade structure sphalerite and minor chalcopyrite and pyrite commonly intervene below the final, coarsely crystalline layer of quartz.
Fractures filled with the conspicuous minerals of the second assemblage generally follow, but locally crosscut, the earlier veinlets.
The filling may be quartz and herdatite, or it may also include sulfides.
The prominent Sulfide is chalcopyrite, however, sphalerite and galena are also present. Hematite may have the form of a micrbcrystalline to earthy, red variety disseminated in bands in quartz, or it may be a purplish, metallic gray, fine specularite smeared along fractures.
In flat veinlets on both the 900 and 1,050 levels the sulfides and hematite are concentrated on the lower side of the veihlet, whereas 70 quartz has grown from both top and bottom at about the same rate.
The carbonates are most abundant along the 1,-200 drift where they cut quartz-hematite-chalcopyrite-sphalerite vein fillings! Flesh- colored anker ite is most common. Calcite fills the center of some ankerite veinlets, and thus was the last hydrothermal mineral to form.
Pyrite is not a major constituent within the veins; it is not un common to find sphalerite or chalcopyrite in greater abundance. Where present in the vein, pyrite is intimately inter grown with the other sul fides. However, it is consistently noted as disseminations in walls and especially in vein-enclosed fragments of the country rock. In this case the mineral was apparently the first of the hydrothermal sulfides.
Secondary Vein Deposits
The combination of open-vein channels and adjacent deeply eroded topography has locally led to the development of irregular, deep oxidized zones. These are characterized by the usual limonite, which may be accompanied in relatively deeper portions by primary and super- gene sulfides. According to Mr. K. O. Watkins (personal communica tion) the large shoot in the east end of the 900 level, at a depth of nearly
900 feet, was largely oxidized with little or no base metal values; in marked contrast to the partially oxidized and primary ores taken from the stope at the west end of the 1,050 level at a depth of about 300 feet.
The only thoroughly oxidized, deep vein rock noted in the 1; 71 accessible portions of the. lower three levels is at the tunnel intersection at coordinates 7970 N. , 11030 El (pi. 8). The vein is about 2 feet wide and consists of fragments of quartz, with or without limonite boxwork, in an unconsolidated aggregate of limonite and gritty vein debris. Anal yses of three samples reveal only minor gold and base metals. :
- Near the west end of the 1,050 level where the Champion vein joins a flatter fault, as illustrated in plate 12, the vein appears to be
: entering the oxidized zone. Secondary copper sulfides coat the primary sulfides and limonite and malachite are common. Directly above on the
900 level the vein and pyritized country rock are conspicuously oxidized at a depth of.less than 100 feet below the surface, a i ..v, Lv:
The coatings of the super gene sulfides, chalcocite and covel- lite, are not of much economic significance, but they serve to identify generally the lower portions of the zone that have experienced at least
some oxidation. It probably would be difficult to distinguish a simple zone of super gene enrichment, since primary, super gene, and oxidized deposits locally occur side by side in the vein. The variability of the water circulatory, system apparently never allowed the slow replacement process to attain uniform and advanced stages. Additional evidence of
the open and dynamic nature of the ground-water system was provided
when, in the course of development low on the vein (at elevation about
4, 500 feet), the spring on the east side of Grouse Mountain that had for
years supplied water to the Noonday bunkhouse (elevation 5, 200 feet)
i 72 suddenly dried up (H. L. Barton, personal communication). !
Accumulations of bedded vein and wallrock debris are exposed on the 900 and 1,050 levels. These deposits are apparently a result of erosion and deposition by downward-moving ground water, but the range in sediment size extends at least to that of colloids as evidenced by the cherty to almost chalcedonic texture of some beds. Bedding is sub horizontal except near the walls of the filled space where it swings part way up the sides.
At coordinates 7929.N.> 11255 E. on the 900 level (pi. 18), a
5-foot width in the back exhibits the bedded debris. It is exposed to 14 feet above the floor along the side of an unfinished stub raise on the vein.
The original vein, as estimated from coarse fragments, was about 6 inches wide and of chloritic quartz with little or no sulfides. No former cavity outline was distinguished. A comb of quartz .on the footwall has apparently been largely disrupted, and it is quite probable that part of the adjacent walls have sloughed in. . Though several fragments of vein quartz up to 8.inches long are included, most of the hard debris is sand to grit size. Some clay gives the accumulation a muddy consistency.-
In some sections, up to 2 feet thick, bedding is obscure, but elsewhere beds of various grit and clay proportions alternate at a frequency of about 5 to 10: per inch. The dense, siliceous layers are commonly of the order of millimeters thick (pi. 25). Microscopically, they have recrystallized to form a mosaic of interpenetrating quartz grains. 73
Locally a layer of finely granular pyrite occurs along the footwall. ;
On the: 1,050 level at 8385 N ., 10170 E. (pi, 19) cherty, sec
ondary accumulations with fine, horizontal bedding fill the central vug
of a 12-inch quartz-hematite-sulfide vein. It may be of significance that both occurrences of bedded debris are probably not more than 200 feet from partially oxidized sections of the vein. ;
Par agenesis of the Vein Filling
Observations from other veins add to the broad picture of vein formation. In addition to the assemblages already mentioned, there ap pears to have been a distinctive specularite-quartz-epidote stage which may be a link to the alteration surrounding the stock. The chlorite-
sphalerite-quartz assemblage observed in the Champion mine was not
distinguished generally. Tentatively, it is included with the hematite,
chalcopyrite, and galena in the "sulfide stage. " Least. clear - cut is the
interrelationship among the sulfides.
The relationships in some assemblages in.veins at the fringe
of the district are probably complicated by areal zoning. The stibnite
in veins to the south presents no great problem because it is clearly
contemporaneous with carbonate and later than galena, sphalerite, and
chalcopyrite; it is placed in the carbonate stage. On the other hand,
some carbonates that seem to be contemporaneous with galena, sphal
erite, pyrite, and chalcopyrite are hard to place in any paragenetic 74
sequence that might be synthesized from relationships in veins near the
center of the district.
Quartz-Specularite-Epidote Stage
In the N. 20o-40° E. cross veins, it is common to observe
that the first layer of the cockade encrustation on fragments is specular
hematite accompanied and;followed by clear crystalline quartz. The quartz is rarely amethystine. This combination is not limited to cross veins, but is apparently restricted in extent to the center of the district,
where cross veins are most common. Usually the quartz coating is no
more than half an inch thick, and later sulfides commonly line the re
maining vugs. The enclosed fragments of country rock are moderately
silicified and the mafic minerals have been chloritized. ; Pyrite, in the
amount of 2 to 5 percent, is notably disseminated through the fragments.
Adjacent to the Johnson mine, near the Mystery vein, a con
spicuous assemblage of minerals has developed in a zone of shattering.
In the surviving open spaces crystals of epidote and tabular crystals, of
specularite up to a centimeter: across have.developed.: Sulfides are
rare. Epidote and hematite are characteristic parts of the contact
aureole of the Bohemia stock, but epidote is rare in veins of the district.
Epidote is locally a major vein gangue mineral at the Amalgamated
mine in the North S anti am district.
Breccia bodies partially filled by crystalline to 75 cryptocrystalline quartz are present at several places near the borders of intrusives in a belt trending north-northeast parallel to the Bohemia stock axis. Some contain epidote and some contain minor sulfides.
Two somewhat similar types of open-space deposits occur on three sides of the center of metallization. On Fair view, North Fair- view, and Elephant Mountains assemblages of well-crystallized, clear, coarse quartz and epidote along with chlorite, calcite, and a zeolite locally line vugs in lava. Some of these cavities are amygdules, so that an analogy to vein deposits is rather tenuous.
On Fair view, Bohemia, and Grouse Mountains sparse, irregular, open-space fillings and planar veinlets along joints are composed of red jasperoid. Float specimens up to 5 inches across were noted, but the veinlets are usually of the order of an eighth of an inch thick. Presum ably the jasperoid is cryptocrystalline silica containing finely dispersed hematite. Except for the distribution of this sparsely developed jasper around the center of mineralization, and except for the chemical simi larity of the jasperoid to the early vein deposits, there is little justifi cation for classing the tw) together. It is of interest that Coats (1940) has noted veinlets of hematitic jasperoid-like material near the Com stock in Nevada. He finds a spatial relation between the jasperoid vein- lets and the famous Comstock propylitic alteration and considers both to be pre-vein. ,v;- / > . !•) '"TNi. 'v1::. ■ f : 76
Sulfide Stage
Most base metal sulfides formed during an intermediate stage of hydrothermal activity beginning during the formation of specularite.
Zoning of minerals in the district probably accounts for the fact that the hydrothermal deposits cannot be fitted to a single paragenetic sequence.
Adularia
Adularia occurs abundantly in Noonday ores and probably is present in many of the other veins. It was deposited with quartz and specular ite following earlier coarse quartz. Quartz partially replaces the adularia. Associated sulfides are sphalerite, pyrite, galena, and minor chalcopyrite, and this assemblage is locally cut by veinlets of quartz and hematite containing chalcopyrite and minor sphalerite. In a northwest-trending vein south of the Peekaboo cabin adularia occurs as the first layer on wallrock fragments and is followed by chloritic quartz.
Hematite-quartz and hematitic chert
Dump samples from the Elephant Mountain, Elephant Lake,
Champion, Musick, Noonday, Alice, and Helena properties contain banded, dark-maroon to light-brown, microcrystalline quartz deposits.
The coloration is due to disseminated, microcrystalline specular ite. 77
The Leroy, Oregon-Colorado, Western, and Alpharetta veins also con tain specular it e-chert gangue but no banding was noted. The specimens illustrated in plates 20 and 21 show conclusively that locally the quartz and hematite settled in cavity basins under the influence of gravity.
Early portions of the hematite are composed of specularite plates whose microscopical arrangement in fans suggests that they are primary crys tals and not detrital products of decomposition from the wallrock. Simi lar, radial clusters of finer grained (microcrystalline) specularite are present in some chert deposits, but small, tabular crystals are evenly dispersed in other chert layers.
A temporal decrease in hematite crystal size provides support for equating the late part of the quartz-specularite-epidote stage to the
early part of the sulfide stage. - In this regard, it should be noted that
10 of the 11 localities known to contain this type of mineralization are
at intermediate distances from the suggested center of mineralization
characterized by megascopic specularite. Possibly the differences of the two stages are largely due to position. In addition, attention is
directed to the correspondence of position of these quartz-hematite vein
accumulations with the jasperoid veinlets described above. '
Galena, sphalerite, pyrite, and especially chalcopyrite ac
cumulated contemporaneously with the quartz and hematite.’" In some
samples no sulfides are present, but in others the better grade of sul
fide ore has a hematitic chert gangue. 78
The hematite-quartz deposits are not necessarily the first fill ing in a vein. This was established in mapping the Champion vein.
Plate 20 shows that a sub-stage of coarse quartz crystal formation y preceded that of bedded hematitic chert at the Noonday mine. This may locally be due to the fact that the hematitic chert preferentially formed in depressional traps, as illustrated in plate 20. Comparing deposits in depressions with equivalent deposits on overhanging walls (the single crystal on left of plate 20), one finds that only a fraction of a millimeter of hematite is locally included in the position corresponding to a 2-mm thick horizontal layer. The second, third, and last layers of hematitic chert have no obvious counterpart in the overhanging wall. Such com plexly bedded hematitic chert appears to be common in both the Annie and the Crystal veins but its relative abundance in other veins was not determined. ; v y -
Another possible explanation for the lack of hematite locally in early vein quartz is suggested by thin sections from several prospects.
Under the microscope quartz overgrowths on quartz crystals contain radially arranged, long inclusions (similar to those in plate 23). These appear to have no distinctive optical properties and possibly are casts of a replaced mineral. They may have been hematite blades. It seems likely, however, that local environmental conditions as well as the district zoning largely determined local variations in the place and time of hematite deposition. As an example, of a local condition that 79 might affect mineral deposition, one might consider the possible effects of local collapse of a ;wall of the vein, with consequent agitation of the - hydrothermal fluid and exposure of new surfaces of minerals that may not be at equilibrium with the prevailing solution. , ; : ; v h
The overgrowths on quartz provide some insight into the con ditions of deposition in the sulfide stage. Under the microscope the well-crystallized quartz is coated by one or more feathery quartz layers
(pi. 22). The individuals of the layers have two or more orientations or are arranged as flamboyant fans, but alLare near the orientation of the underlying crystal. That is, the quartz grains of the layers have a : preferred crystallographic orientation near that of the host. Common ly, the host crystal has been corroded as though a temporary reversal of conditions favored solution of silica. • : ■ :
Late milky white layers are evident on quartz crystals from many prospects. These were identified in Noonday ores as layers of - inclusions within the flamboyant quartz overgrowths (pi. 23). Signifi cantly, these layers of inclusions (minerals, voids, or casts of a re placed mineral) conform to the surface of the underlying host and are cut discordantly by the individual crystals of the enclosing flamboyant overgrowth. / '• r-. -/.;' • . ;■ ; .nr n: sV-v
- : It is suggested that the over growths and hematitic chert beds developed by rapid precipitation of silica in a liquid, aqueous environ ment. Rapid precipitation would quite possibly lead to accumulation of 80 a gelatinous layer or sediment rather than to continued growth on ex isting crystals. Dehydration and crystallization of silica gel could con- ceivably lead to development of the described products.
If a liquid environment actually existed as postulated^ the temperature of the stage can be estimated crudely. At a depth of a mile (tentatively assumed for the adjacent roof of the stock) the pres sure would probably lie between lithostatic and hydrostatic pressure, i. e., between 500 and 200 bars. Pure water would probably be above its critical pressure and a single fluid phase would exist. The specific volume of water at 300 bars and 400°C is about 3.0 cubic centimeters per gram, and it increases to about 10.0 cubic centimeters per gram at 550°C (see tables in Goranson, 1942). By analogy of the vein chert to chemically precipitated sediments deposited in waters of surface environments (specific volume near one), it is suggested that the spe cific volume of water in the vein was also low. An upper limit of 400°C is offered in the absence of observations on the character of sediments in low-density fluids. Silica and other dissolved components add more uncertainties.
Similar overgrowths and chalcedonic to cherty, bedded deposits ‘w i ■ V : ; x X: x-.. . ■ i : - . --x x ■ ■ ■ -x •,; " -x - of the later silica stage are probably a result of a continuation of the
same process. Later deposits commonly show colloform structure. 81
Sulfides• ' 'vH; ch •■jii :;k;; :■ ; ■ ; :'.'j ""n ::U;
-■V: - . \ v , - " viv";;’v t'. - Paragenetic relations among sulfides are not clear. In a few : ■'!-.r.'. .v. 's.:'-' v;: i :.j r'l v'-v instances chalcopyrite was seen to penetrate quartz and other sulfides along fractures. In the Champion and Noonday mines chalcopyrite is V; most common in association with hematite following early quartz and sphalerite deposition. All sphalerite examined in polished sections
(29 sections) contains blebs of chalcopyrite. The constancy of the as- sociation suggests that it is a depositional feature rather than a result of later replacement. Nevertheless, the chalcopyrite appears to be most common in the later part of the sulfide stage.
Galena and sphalerite are more difficult to place. In the
Champion mine some sphalerite is early but hand specimens, slabs, and polished sections from elsewhere reveal relatively late sphalerite.
Microscopic contacts between sphalerite and galena tend to be mutual, and where both are present, they often appear to have been deposited simultaneously.
• - •• ' Vv ^ 1 Ax': ' v':;
Carbonate in the sulfide stage ■;
Although most carbonate is either associated with the final hydrothermal stage or with alteration assemblages in the wallrock, some formed locally as a prominent gangue during sulfide deposition.
Ankerite was identified with galena and sphalerite at the Katmia vein. 82
Carbonate is associated with chalcopyrite, sphalerite, and pyrite in the
Sultana, Peekaboo, and Martha W veins and with stibnite at the P resi dent mine. The stibnite-carbonate assemblage is tentatively grouped with the ”carbonate stage” because it is distinctly later than galena, sphalerite, and chalcopyrite. In the other four veins the carbonate ap parently formed much earlier. The fact that the four are at the periph ery of the district suggests that carbonate gangue in the sulfide stage is characteristic of an outer zone of mineralization.
- The position of gold in the paragenetic sequence also is not clear. It occurs as native gold, usually in the form of fine wires. Ac cording to K. O. Watkins (personal communication), mill tests on pri mary ore have shown that the gold floats with whichever sulfide is floated first, indicating that it is readily freed by mechanical grinding.
: A second characteristic is that it is sporadically distributed through the vein. Table 5 shows two series of assays from the Musick and Champion mines. ./Each contains a single high-grade assay among many intermediate or low-grade values. Durhg operation of the Musick stamp mill in the late 1930’s, mill recovery often exceeded assay values
(K. O. Watkins, personal communication). Mr. Watkins explains that an occasional high-grade pocket raised the overall grade by a substan tial amount. / . 83
Gold was observed to lie as a '’filigree” of wires on the sur face of sphalerite in ores from the Katmia and Sultana veins. A few wires appear to project into the sphalerite. In oxidized ore from the ■ . -,i. y r r . • v ::: •' • ;.: , v : : ■ r/, ")r, . • . Musick mine the gold wires are solidly fixed in limonite boxwork which appears to have been derived from sphalerite. The wires in the box- work are arranged at constant angles with respect to one another, as though they may have replaced the original sulfide along certain crys tallographic lines.
According to MacDonald (1909): "The gold occurred as threads and filaments included in the pyrite. ” However, the ores, at the time of MacDonald1 s visit, were extracted from oxidized portions of the shoots so that the identity of the original host sulfide may have been inferred indirectly.
Gold is said to be associated with carbonate (ankerite) that weathers to a particular shade of light brown at the President mine
(H. L. Barton, personal communication). In the central part of the district carbonates are probably not favorable gangue minerals. The described microscopic features and erratic distribution of gold in quartz-sulfide veins suggest that it is a late replacement mineral formed from agents that were highly selective in regard to mineral- ogical and structural controls. Tentatively, the gold is suggested to have formed late in the sulfide stage in centrally located veins, but in the carbonate stage in the peripheral stibnite zone. 84
Silica Stage '
Additional deposition of silica followed the formation of most sulfides. Accompanying fracturing has led to development of breccias composed of fragments of sulfides, quartz, and vein rock in a cherty matrix. Locally, a second brecciation and filling has occurred. Fine, parallel, planar layers were noted in a few specimens and are assumed to be horizontal bedding.
Several features distinguish this late, hypogene silica precipitate from similar deposits of supergene and earlier hypogene environments.
1) The chert is white to gray; it contains no hematite.
Both the hematitic chert and the gray chert occur
at the Helena mine.
2) The chert does not contain contemporaneous sul
fides.
3) In contrast to the hematitic chert, it is not cut by
crystalline quartz veinlets.
4) Chert in a sample from the Laura dump is over-
lain by ankerite crystals projecting into a vug.
5) A bedded specimen from the Helena dump has
been moderately fractured, and subsequently,
the fractures have been coated by barite crys
tals. 85
■ A set of discontinuous veins along Monte Rico Ridge contains layered chalcedonic quartz that appears to have been deposited conform ably on coarsely crystalline quartz. In plate 24 an intervening over growth on the quartz crystals culminates at a feathery, outer layer.
The succeeding chalcedonic quartz has an abundance of small colloform spheres enclosing patches of submicroscopic inclusions. . The original silica has crystallized to a mosaic of fine-grained quartz.
The late silica, filling vein breccia along the Sultana vein, is almost black with disseminated pyrite. Commonly, the remaining vugs are lined with brilliant crusts of crystalline pyrite (see description under ’’Carbonate Stage").
Carbonate Stage
Varieties of the series ankerite-dolomite and to a somewhat lesser degree calcite were found on most dumps. Ankerite commonly fills interstices and lines vugs in the earlier quartz-sulfide vein mate rial. The last carbonate to be formed was calcite. ; -
The magnesian carbonates range in composition from dolomite to ankerite according to determinations of refractive indices (n0 =
1. 68-1.70) of 13 specimens from eight veins. In most accumulations no consistent change was noted from older portions to younger layers.
However, on thinner encrustations the crystals projecting into vugs are lighter shades of pale brown to tan, and thus appear to be less ferroan 86 than darker,, underlying portions. The change is gradational through material that appears to be crystallographically continuous.
Much more complex carbonate accumulations are present elsewhere. - In the Goolidge (President) vein the ankerite is complexly banded as evident on weathered surfaces (plate 26 illustrates a similar gangue from the prospect in upper Weaver Creek, section 3). The de posits commonly contain interlayer ed quartz. The early portions of this quartz-carbonate gangue formed contemporaneously with the stibnite and probably also the gold at the President mine. By contrast, at the Tall Timber and Old Henery prospects the stibnite is associated with variously textured quartz and no carbonate. Stibnite crystals are commonly replaced by euhedral, granular quartz.
Because of a similarity of the banding of the later portions of the carbonate to that of the three types of siliceous vein sediments, it would seem that locally the carbonate also accumulated at the base of open spaces in the vein under the influence of gravity. :: -~-
A well-stratified carbonate-chert accumulation at the Golden :
Stairs vein may be a secondary deposit, (see "Secondary Sediments and
Bedded Precipitates").
At many places in the district a crust of bright pyrite crystals coats fragments. It is inter layered with silica at the Sultana vein, but usually it intervenes between ankerite and the final comb of calcite.
One of two crystal forms dominate at any given prospect. Samples 87 from the Sultana and Sunset veins show both. The earlier crust is com posed of pyritohedra largely free of other forms, whereas the overlying crust is of cubes modified by the dodecahedron but free of pyritohedral and other forms. Barite and aiikerite are approximately contempora neous with the cubic pyrite crust at the Helena mine.
Secondary Environments and Products
The extent of oxidization within the veins is erratic^ as noted in the Champion mine. That vein,, however, may be somewhat anoma lous because among the many, prospects examined a more orthodox picture prevails. This picture shows a correlation of depth of oxidation with topographic position. As.noted by Callaghan and Buddington (1938), where veins cross ridges they are usually oxidized; where veins cross creeks sulfides are usually unaltered at the surface. This variation of the thickness of the oxidized zone can be correlated with the expected variation in depth of the water table. The possibility of the presence of an old erosion surface in the area has been investigated. Such a surface might help explain the deep oxidation in the Champion and Mustek veins.
Possibility of Erosion Surfaces in the Western Cascades
Various workers in the northern Cascades (Smith, 1903) and
in the Klamath Mountains (Diller, 1902) to the southwest have described
what appear to them to be the remnants of peneplains. When it is 88 realized that in at least three areas in the intervening 450 miles of the
Western Cascades similar possible surfaces or an accordance of ridge tops have been mentioned (Zapffe, 1912; Felts, 1939; Callaghan and
Buddington, 1938), the possibility of erosion surfaces in the Cascades becomes a major geological challenge.
In an endeavor to explore the possibility of a dissected erosion surface in the Bohemia district, I have assigned approximate elevations to ridge tops, flats, and rounded peaks as shown on the Fair view Peak quadrangle topographic map. It was hoped that the resultant surface would simulate any single regional surface that might have been pres
ent. The result (pi. 16) gives no indication of a well-defined regional :: ;■ . o.' • : . . ■ O'' r; ^ : ...... surface. If one was present, it has been modified by the present drain-
age pattern; for indeed there are conspicuous depressions in the con
structed surface across individual drainage basins.. It is possible that
this analysis has been on a scale about one order of magnitude too
great. Contouring elevations of only particular, high topographic forms O'' V.-::. ' c; ■ - ' ■ ' ’ i 1: / r, over several quadrangles might be informative in verifying or refuting
the fundamental nature of the accordance of ridges.
The constructed surface is of interest, however, as a summary
of the gross topography and of its relation to the intrusive masses and
aureole. The high trending northeast across the quadrangle is probably
a reflection of the greater resistance offered to erosion by altered
rocks. 89
Mineralogy of the Oxidized Zone v; :
The two common gangues behave differently in conditions of oxidation. Carbonate is largely dissolved and carried away whereas quartz remains. Iron from the various sulfides is also left behind as limonite. The limonite takes the form of finely layered coatings around fragments of the vein and country rock, and it is also common as box- work after the various sulfides. In the first case some solutional trans port has apparently taken place.
Similarly layered hydrates of iron oxide and also of manganese oxide result from differential solubility among the various cations re leased in the oxidized zone from carbonate gangue. Of particular in- terest here is the horizontally layered wad interbedded with vein debris in the Coolidge vein (pi. 28). It is described below.
Kaolin-type clay was identified with quartz in three veins near the surface, so that it seems safe to assume that it is a common gangue, though a replacement of silicate minerals, not a filling.
MacDonald’s observation (1909) that at depth kaolin becomes subor dinate to sericite suggests that it results from conversion of other aluminous silicates by leaching of alkaline and alkaline-earth cations, all within the secondary, acid-oxidizing environment.
Other minerals that have developed in the oxidized zone include the following observed sulfates: anglesite coatings on galena, gypsum 90 on fractures in country rock, gypsum crystals in secondary vein gangue, chalcanthite, and possibly some barite and iarosite. Carbonates include coatings of malachite and also cerussite crystals with residual wire gold in sphalerite(?) boxwork. Pyromorphite crystals were noted on fractures in oxidized rock on the Utopian, Coolidge, Lead Crystal, and
Western veins or dumps. Stibnite alters to waxy stibiconite and to a lesser extent kermesite. Pitchy, black goethite was noted in one vein sample- . ; . .. y . .
Super gene Deposits
Covellite and sooty chalcocite are the diagnostic minerals for identification of supergene depositions in the Bohemia district. They occur over a rather great vertical range and locally are found beside or within oxidized or hypogene vein assemblages. They are invariably in the form of coatings on the surface and fracture planes of hypogene sulfides. Neither appears to have much economic importance. The hosts in order of decreasing ease of replacement have been sphalerite, galena, chalcopyrite, and pyrite.
Secondary Sediments and Bedded Precipitates
Besides the well-exposed examples of layered vein sediments in the Champion mine similar deposits were noted elsewhere. The bedded wad described from oxidized portions of veins is included in the 91 broad group of secondary horizontally layered deposits. No horizontal layering was noted among the various limonite accumulations, but con sidering the similarity of the geochemistry of manganese and iron, it is likely that comparable beds of limonite may locally be present.
It is notable that in the one example showing deep portions of the vein none of the washing deposits were seen at a depth of more than
2 0 0 feet below the oxidized zone. ; s ■
Other secondary cavity fillings that might be comparable to those in the Bohemia district include detrital cerussite in the Glove mirie^: Arizona (Olson/ 1961), and manganese oxide and sand on the floors of caves in the Golden Cross vein, Waihi, New Zealand (Bell and Fraser, 1912). The cavities in both cases are in oxidized zones in limestone or vein carbonate. In the Golden Cross mine the authors mention no correlation between the vein sediments and the gold values, though they do note that caves up to 50 by 200 feet are developed in the stoped block. The lack of any mention of the tenor of the sediment sug gests that it was not economically important. 1 1 ! k ;
.():■ :v.. Particles of greater than colloidal dimensions ■ f.- . n T ; v V
v The size of fragments varies through a great range. The larger blocks, being at least as great as 8 inches in diameter, cannot Have mbved far arid doubtlessly represent caved vein rock arid sloughed walls from just above. ;The vein fragments commonly show the typical coirib 92 quartz, sometimes with sulfides. Particles with smaller sizes may be single crystals or fragments of crystals of quartz or sulfide. No con centration of the heavy minerals into pockets or layers was observed except for layers of fine pyrite along and near the footwall at the
Champion 900 level occurrence.
Fragments of wallrock contain a preponderance?of light-colored minerals of which clays are conspicuous. This altered rock has tended to disintegrate to form the finer fractions, and the consequent reduction of fragment size appears to lead to a segregation of fine clay from - quartzose rock and mineral chips. Bedding is generally lacking in coarse-debris accumulations, but it becomes more distinct with de crease of particle size.
Carbonate gangue fragments were noted in thin section in chalcedonic silica from the Coolidge vein.; : ; : : ^
Bedded precipitates
The presence of cherty and chalcedonic silica interlayer ed with
sandy sediments suggests that the range of particle size extends at
least to that of colloids. The resultant deposits in the primary part of
veins are mostly of silica; however, a thin section of a bedded deposit
at the Golden Stairs prospect reveals about 50 percent zoned ankerite
rhombohedra in chert. Thick sections of these beds are:more sug
gestive of other secondary deposits than of hypogene-banded carbonates, 93 but the similarity is not conclusive. r / v Secondary silica fills solution cavities in carbonate gangue at the prospect imsection 3 in Weaver Greek (pi. 26; see also pi. 25). The volume of the dump suggests about 250 feet of drift so that the specimen must have been within 200 feet of the oxidized zone. = Some chalcedonic layers have a metallic luster, because of finely disseminated pyrite.
The typical deposits of the oxidized zone believed to have been transported in solution are the limonite and fluffy wad. The mangan- iferous carbonate that is responsible for the wad has been largely dis solved and the resultant space among residual blocks of quartz gangue has been filled by alternating beds of the hydrate and rock debris from above.
Six analyses of vein rock and secondary sediments are given on plate 28. It can be seen that the gold values in the manganiferous sediments are neither high nor low relative to the original vein. The only sample assaying well above the others is from an early vein debris essentially free of wad. This probably reflects only a few in cluded fragments of better-than-average vein. Silver appears to go into solution and to be carried off.
On the other hand, bedded wad is present near the face of the upper Coolidge tunnel about 5 feet from an oxidized vein assay said to run high in gold (H. L. Barton, personal communication). Similarly, ore said to carry 2 ounces of gold per ton (K. O. Watkins, personal 94
communication), at the high point on the Cinderella vein in the Blue
River district, is largely composed of bedded debris and fluffy wad.
The possibility that there has been a placer or solutional con-
centration of gold in the veins is a topic that the mining geologist might
wish to investigate further. . Quantitatively the sediments are a very minor part of the vein filling. \v.. ■. j ■ • : . ’ >.^ .. ■:' : ’:■:■■■ . ■=. \:"u-h . ;:’v r, -r-
' ' ' ■ ' / ■.' * ■ * .. r;:, : y ' : ' / ' ^ ' ' .. ' : . :v;. :
:: '.'V';
. . . V""; '
: :v‘ ;•
. ' i
; ••• . : - - V:,
CASCADES DEPOSITS AS PARTS OF A METALLOCENE TIC - PETROGENETIC PROVINCE
■ - ' ■ : V," • V-: v ■ ■■ ■ ; : •/ :,v , ; ; ’v. ^ The similarities in distribution and type of mineralization among the mining districts of the Cascades of Oregon and Washington were no doubt recognized long ago. Gold-bearing, quartz-base metal sulfide veins predominate, but the districts show a wide variance from deposits containing molybdenite and scheelite to those containing stib- nite. The association of metalliferous veins with phaneritic intrusives is one of the fundamental characteristics of the province.
Chemical variations among the associated Tertiary and Qua ternary lavas are moderate to unrecognizable as traced parallel to the axis of the province, whereas they are appreciable when traced across the province. The Cascades Tertiary volcanics appear to be the low- potash contemporaries of volcanics in the neighboring Basin and Range province.
Distribution of Plutons and Mineralization
Phaneritic dikes and plutons crop out in a 50-mile wide belt extending 425 miles from the Barron mine in southern Oregon to the
Snoqualmie batholith in central Washington. Intrusives believed to be
95 96
Tertiary in age have been mapped farther north (Daly, 1912; Huntting and others, 1961; pi. 3). The known plutons were generally located after mineralization was found in adjacent areas. '
Younger lavas may falsely accentuate the linearity of the in trusive locus by hiding plutons and mineralization that might lie east of the narrow belt in Oregon. To the west a diorite plug that might cor relate with the middle Tertiary intrusives occurs at Steens Butte, Oregon
(Wells arid Waters, 1935). In the Riddle quadrangle Diller and Kay (1924) encountered dacite porphyry which was distinguishable from the quartz diorite composing the Mesozoic batholiths of the area. The narrowness of the belt of plutons should not be taken for granted.
Peck (1960) and Huntting and others (1961) have shown addi tional plutons, scattered through the Western Cascades. On the basis of paleontologically and radiometrically derived dates and a considera tion of the probable sources of the various volcanics. Peck suggests that the north-south belt of volcanic centers migrated eastward during the Tertiary. His conclusion is partly premised on the assumption that intrusive centers such as the Bohemia district were effusive centers.
Conversely, then, the intrusive axis would also have migrated east.
In the northern part of the province the intrusives are markedly greater in size, i. e .th e Snoqualmie batholith, the intrusives near
Mount Rainier, and the two large stocks at the St. Helens district and
Silver Star Mountain (table 7); other batholiths exist farther north. Contact alteration zones and metallization accompany each of the large plutons and elsewhere occur without any associated intrusives besides dikes. The districts in Oregon have been described by
Callaghan and Buddington. In Washington they include the Silver Star
Mountain area, the St. Helens district, and the districts of the
Snoqualmie batholith and around Mount Rainier.
Characteristics of the Plutons and Associated Mineralization
The Snoqualmie batholith, the Silver Star Mountain stock, and the Bohemia stock provide an opportunity to compare features of three
chemically similar plutons of the province. Differences in contact ef fects, mineralization, and internal structure may correlate with volume
of the intrusive and the depth of its exposure (table 7).
Silver Star Mountain Area
Felts (1939) describes the stock as having dimensions of 2 by
10 miles with the long axis trending N. 20° E. ^ Contacts with Oligocene-
Miocene volcanic country rock are steep, but inclusions of a size that
suggest roof pendants are present within the mass. Xenoliths are com
mon. Sparse planar orientation of xenolithic schlieren suggests a
structural dome. A set of steep joints parallels the long axis; aplite
dikes frequently fill these joints. : . ;
The stock is composed of a mass of granodiorite with 98 subordinate amounts of augite diorite and quartz diorite usually devel oped near the periphery. Both light and dark minerals are aligned in a sub-trachitoid structure. The plagioclase is normally zoned outward from a basic to medium andesine core. Orthoclase rims and replaces the plagioclase and occurs in the interstices with quartz. The dark mineral is uralitized pyroxene or, more commonly, hornblende or biotite with varied amounts of chlorite and secondary dusty magnetite.
The early plagioclase of the augite diorite is basic andesine; i
Augite is the principal mafic mineral. Little or no quartz and ortho clase are present in the augite diorite but they occur sparingly in the quartz diorite. : ■. . ■■ ■ ■ , ' .
The aplite dikes are composed of quartz and orthoclase with a small amount of biotite and sulfides. The orthoclase is commonly sericitized. - . v. ■/ . . " . ■ -; :: .V
’ The granodiorite and the augite diorite (Felts, 1939) of the ;
Silver Star stock are chemically similar to the augite-hypersthene granodiorite porphyry and the porphyritic augite-quartz diorite
(Buddington and Callaghan, 1936) of the Bohemia stock. v
i According to Felts the contact alteration and mineralization are "thermal or hydrothermal processes accompanying the intrusion of the stock. " -The granodiorite itself shows indications of local post consolidation "silicification and orthoclasization. ” Near the contact the country rock is recrystallized. Tuffs are extensively replaced by 99 secondary quartz. Beyond this subzone, which varies from a few feet to several hundred, the andesites are propylitized; epidote and chlorite are developed and quartz and pyrite are abundant.
Schaller (1905) identified andalusite, dumortierite, and sericite in a rock specimen reported to have come from the upper Washougal
River. Though the occurrence has never been verified in the literature, there can be little doubt that it is in the contact aureole of the Silver
Star stock. He gave a chemical analysis of the rock along with an anal ysis of the sericite alone. The assemblage is high in alumina. No known lava or pyroclastic has such a composition, so that considerable redistribution of components, including the usually immobile alumina. must have taken place.
Schorlite in veinlets serves as an index of the widespread boron metasomatism which has occurred in the stock (Felts, 1939).
Locally, the granodiorite has been largely replaced by quartz, schorlite, and sericite, especially along large shatter zones.
Later solutions have deposited pyrite, chalcopyrite, bornite. and silica in cavities and veins in the stock and its country rock, and these fillings have locally replaced the wallrock (Howe, 1938). Andesite walls along the veins have been propylitized and other rock types are commonly sericitized. Molybdenite occurs with chalcopyrite, magnet ite, pyrite, quartz, tourmaline, and sericite in veinlets and dissemina tions in the intrusive at the Miner's Queen prospect (Huntting, 1956). 100
St. Helens District
Verhoogen (1937): describes the plutonic rock in the St. : Helens district as a quartz- hornblende - augite diorite, with indications of hydro- thermal alteration. The plagioclase ranges from andesine to oligoclase, and oscillatory zoning is common. Orthoclase commonly replaces the plagioclase. Zapffe (1912), Winchell (1913)^ and Hougland (1935) men tion the presence of more than one facies or composition in a range from diorite to quartz monzonite. Presumably, these are comparable to the distinct facies in other districts. , ' ; ' • . ;
Contact effects in the andesite country rock include decomposi tion of pyroxene and development of chlorite. Near the contact epidote and veinlets of quartz are abundant. "Quartzite" mentioned as occurring northwest of the intrusive (Landes, et a h , 1902) is probably indurated and silicified volcanics within an aureole of contact alteration.
The pluton is cut by dikes of aplite and of a quartz-albite- orthoclase-tourmaline-calcite rock with sulfide mineralization
(Verhoogen, 1937). Elsewhere tourmaline-quartz (Zapffe, 1912) and tourmaline-specularite-magnetite-epidote-quartz (Winchell, •1913) vein- lets are, common. Associated sulfides are pyrite, pyrrhotite, arseno- pyrite, chalcopyrite, bornite, sphalerite, and galena.
- - v v : V V 'v ? . 101
- Mount Rainier Area.' : : h i
A cluster of stocks forms part of the foundation on which the
Mount Rainier volcano is built. The rock is similar to that of the
Snoqualmie batholith, and the stocks have been suggested to be apophyses of an underlying mass connected to the batholith (Coombs, 1936).
Tourmaline and quartz are mentioned as common gangue min
erals at many deposits in the compilation by Huntting (1956). Metal de posits are exclusively veins. They are mostly within granodiorite, to : ' r:o r. '0. r ::! y - :rr o: o . ' - ' : -o o. , ; ..r:' i. - which they presumably are related. However, Patty (1921) notes that the Eagle Peak veins extend into andesite which he considers to be
younger than the intrusive.
Veins on the southeast side of the stock cluster contain molyb
denite, scheelite, pyrrhotite, and arsenopyrite in addition to the usual
chalcopyrite, sphalerite, pyrite, and galena. Linnaeite has been identi
fied at Eagle Peak and wolframite is mentioned at Bumping Lake. Au- r ; " r r. r - r'-r ^rr; ' ; • . r. : , : r : v ■ v r:. tunite and pitchblende have also been reported in the vicinity.
Veinlets in granodiorite in the Carbon River district northwest ■ ; • . : r ' ' rr 'r ■ r • ■ of Mount Rainier are composed of chalcopyrite, bornite, pyrite, and ; : r cc.t ^r"i:v i ■ .vv ' G . ■ ■ g g G, g- -gg arsenopyrite in a quartz gangue that commonly contains tourmaline. : : " : V. ■ G ' ' - ..'GG-'G ' GG .. -G
; vSnoqualmie Batholith Area . : . .
■.G,;-,: ‘ ■ G-. . G ’ ■ ' V .-G'.;: ■ l : UG' i 0 G The Snoqualmie batholith crops out over an area of about 250 102 miles (Knopf, 1955). It consists of a granodiorite with less abundant quartz diorite and biotite granite variants (Smith and Calkins, 1906). -
The predominant constituents are idiomorphic plagioclase, less com mon interstitial orthoclase and quartz, and hornblende and biotite.
Plagioclase is normally zoned from an andesine core to an oligoclase rim. In the quartz diorite plagioclase is locally as basic as calcic labradorite. Only minor augite is present and hornblende commonly predominates over biotite. The biotite granite contains a large per centage of quartz, about an equal amount of microperthitic alkali feld spar, somewhat less oligoclase, and a little biotite. The intrusive is usually altered with sericite, saussurite, kaolin, and pyrite as second ary products.
According to Waters (1955) the batholith is chilled against the
andesitic country rock locally, but elsewhere the andesite is coarsely
recrystallized and intimately penetrated by the intrusive. The presence
of miarolitic cavities suggests solidification under only a thin crust,
and Smith and Calkins (1906) estimate that the original roof elevation
was about 4,000 feet below the surface.
Biotite is commonly developed in country rock. Smith and
Calkins note the general rarity of garnet and andalusite; however, each
is present locally, along with some staurolite and hornblende.
More subtle effects include induration of Tertiary rocks to a
massive, flinty rock by the formation of a secondary, interstitial 103 cement of quartz and sericite. This induration extends as much as 3 miles from the contact. Quartz, hornblende, and other secondary min erals fill amygdules in basalt^ whereas andesites contain secondary quartz, sericite, kaolin, calcite, epidote, and scapolite.
Quartz-tourmaline contact rock is present locally. Limestone lenses of the Tertiary sediments have been replaced by well-crystallized assemblages that include magnetite, garnet, quartz, hornblende, calcite, and pyrite near the contact of the biotite granite facies at Snoqualmie
Mountain. Pre-Tertiary metasediments in the Sultan Basin district are replaced locally near the contact. • New assemblages include the min erals grossularite, quartz, augite, diopside, epidote, prehnite, . : scapolite, vesuviahite/ arsenopyrite, chalcopyrite, and probably hillebrandite (Carithers and Guard, 1945). A m assive pyrite replace ment body in pre-Tertiary schist may be related to the batholith.
Bornite and chalcopyrite occur with quartz crystals locally in pockets in the granodiorite. Molybdenite, pyrite, chalcopyrite, scheel- ite, and siderite occur in prominent steep quartz veins trending north
west transverse to the axis of the batholith in the Devils Canyon area
(Purdy, 1954), and at the nearby Clipper prospect copper-molybdenum
mineralization is associated with a fine-grained, acidic facies of the
intrusive. : ; x-.v. v::; ■■ •l : '.v-r u-j)/: ’-v : ‘
The Miller River deposits (Smith, 1915) contain molybdenite,
tourmaline, apatite, lollingite, and auriferous arsenopyrite, all 104 suggesting high temperatures of formation. Crustified vugs reveal five common stages of mineralization: 1 ) sericitization of country rock, 2 ) quartz-arsenopyrite-pyrite, 3) chalcopyrite-sphalerite, 4) galena- stibnite, and 5) calcite.
To the north at Sultan Basin there is a tendency for molybdenite and magnetite to occur in high-temperature quartz veins separate from those containing galena and ruby silver (Carithers and Guard, 1945). A vuggy breccia mass with sporadic chalcopyrite and molybdenite occupies a position in wallrock near the cupola illustrated on plate 31.
The Sunset mine in the Index district (Weaver, 1912) and the
Mystery-Pride property in the Monte Cristo district (Spurr, 1901) have produced about 4. 6 million dollars in gold and base metals (Wayne S.
Moen, personal communication; Huntting, 1956). This is more than the production from the rest of the province as treated here*. ■ L v : . . ■ ■■ . ■:■=■-:/ ", . ' ' n ' , ■ .i! The age of the Snoqualmie batholith is 18 million years with an uncertainty of 5 percent (Baadsgaard, Folinsbee, and Lipson, 1961).
*Mr. Moen has pointed out that current consensus of opinion favors Tertiary ages for some intrusives to the east. He lists the im portant Holden and Golden King mines of Chelan County, Washington, as probably Tertiary. The linear belt of middle Tertiary to Recent andesitic volcanism with associated plutonism and ore deposition seems to become diffuse north of the Snoqualmie batholith. Therefore, I have not considered the extension of the metallogenetic province to the north and east. " 105
Petrochemistry of the Province
A complete treatment of the petrochemistry of the province and adjacent regions is beyond the scope of this paper. However, a ■ ■ - • - • - •• V. Jt ' . j i w%; .* s'. 4. » « / . .' ■ . ■ . _ / , ■. ■ 1 1 • consideration of the variation in some of the components in the rocks serves to emphasize the close relation of the rocks of the province and their distinction from those of adjacent regions. Attention is called to the facts that: 1 ) alkalies and especially potash in igneous rock series appear to increase away from the Pacific basin; 2) most chemical com- ponents of the Pliocene-Quaternary volcanics gradually change across the Cascade Range; and 3) middle Tertiary volcanics are in marked
chemical contrast to the younger volcanics.
Inter- Provincial Variations
Previous workers have recognized a change in the composition
of plutonic rocks in progressing away from the Pacific basin into the
North American continent. Buddington (1927) found that the Coast
Range batholith in Alaska is principally of quartz diorite near the coast
and of quartz monzonite inland. Lindgren (1933) noted a change in both
the igneous rocks and associated ore deposits across the western United
States. Alkaline rocks and fluorite-telluride-gold deposits are concen
trated on the eastern side of the Colorado Rockies in contrast to grano-
diorite and gold-quartz deposits at the western extreme, in the Mother 106
Lode country.
The latest attempt at identifying regional petrographic zones is that of Moore (1961). He has been able to locate a "quartz diorite line" extending from Alaska to Mexico. West of the line quartz diorite and gabbro predominate among large plutons. East of the line grano- diorite and granite predominate. Moore's method has the advantage of utilizing all accurate petrographic descriptions from the literature.
Another means* of regional analysis is suggested by the com pilation by Larsen and Cross (1956) of chemical-component percentages at "position 15" on Larsen diagrams for various areas. Maps based on this method have the advantage of being more objective. No estimate of relative abundance of rock types in a given area is involved. In addi tion volcanic rocks can also be used. They probably are a better sample of magma because they have cooled quickly without "stewing, " though,
of course, both intrusive and extrusive may have gone through a com plex history before arriving at the places where they have solidified.
Plate 9 shows the result of this treatment. The KgO values of
* Much of the following discussion on the variation of alkalies in igneous rocks across the western United States was an unsuspected duplication of a part of a very interesting and pertinent paper by Moore (1962) that has recently been published. The reader is referred to this paper in which Moore has found an eastward increase of K gO /^O + NagO) in Cenozoic igneous rocks of the region. He finds a correspond ence between the values of this ratio and negative gravity readings, and he reasons that higher potash signifies a thicker or more silicic crust. 107
Larsen and Cross have been supplemented by similar data from other
areas. Alkalies obviously increase eastward. A line can be drawn to
separate a western region usually containing rocks with less than 7.0. percent alkalies from an eastern area of rocks containing more. The
Cascade plutons lie just west of this line. ' The eastern area can be
divided further with high-alkali rocks dominating the belt farthest east.
Similar distribution is noted when K^O is plotted alone. The facts that
the series involved are of various ages from Jurassic to Quaternary,
and are of both extrusives and intrusives, do not seem to effect the
gross pattern. Most of the rocks are Tertiary. The High Cascade
lavas are mostly Pliocene to Quaternary and are discussed separately
below. ^ =:■
The Coast Range lavas and intrusives of western Oregon
(Snavely and "Wagner, 1961) apparently will not fit the gradients. :
The Cascade Mountains are structurally similar to the Basin
Ranges. North-northeast trends predominate. The long northwest
faults that are so common west of the Walker line are secondary in the
Basin and Range province. Therefore, the best place for a comparative
traverse is southeast across the Cascades into southeastern Oregon and
Nevada. ' v. ■ "" The petrochemical changes in such a traverse are accompanied
by changes of the mineralogy of the metalliferous deposits also. In con
trast to the Cascades Tertiary deposits, those of the Basin and Range 108 province are mostly quartz-carbonate-adularia veins containing high gold and silver and low-base metal concentrations. The comparison is weakened, however, by the usual lack of associated phaneritic plutons like those in the Cascades. This feature supports the common conten tion that the deposits were formed at shallow depths.
Bilibin (1950) has taken account of the differences in alkali contents of igneous rocks to distinguish two types of metallogenetic zones in erogenic belts. His interior zone is typified by extensive . basic and ultrabasic magmas and their sodium differentiates. Deposits of platinum, copper, molybdenum, and mercury and the iron group ele ments are characteristic, whereas tin,. tungsten, and bismuth.are in
significant. The ''internal zone" appears to correspond to the Tertiary
of the Basin and Range and westward. , ;■ j . ...
, The external zone lies farther into the continent and contains
abundant potassium granite. Ore deposits include concentrations of
tin, tungsten, bismuth, lead, and zinc. The "external zone" corresponds
approximately to the Rocky Mountain region. ; ^ :
Bilibin emphasized that the two zones are neither contempora
neous nor are they parts of a single geosyncline. ;
, The paper brings put interesting associations that seem to be
corroborated by Cascade geology. Molybdenum, which at first might
be assumed to be only common with the potassium granites of the ex
ternal zone, is said to be one of the most characteristic metals of the 109 internal zone. On the other hand, the tungsten associated with the
Snoqualmie batholith is anomalous in Bilibin's internal zone. Similar- ly, boron is not supposed to be characteristic of internal zone deposits . ■ _ .‘.U.. 1. <).: ' : ‘C ' , : V : 1 ■ though it is in the Cascades.
It is possible that emphasis on orogenesis is misdirected em phasis. Perhaps the differences in the chemistry of the igneous rocks 1 V .; ■'■ -:; : ‘ ; :' and associated ores are related to changes in the character of the earth crust in progressing from the ocean basin into the continent. If assim ilation or anatexis of the upper crust is an important process in magma generation and modification, resultant igneous rocks should be higher in potash in continents. Marginal areas or accreted orogenic belts
should have intermediate potash contents.
Another possibility has been developed by Kuno (1959) to ac
count for the petrographic provinces of Quaternary volcanics in Japan.
The depth of major earthquake epicenters appears to increase system
atically across the trends of the petrographic provinces. The earth-
quakes are considered to accompany partial fusion of the peridotite
mantle. At shallow depths, which are usually near the Pacific basin,
tholeiite is generated. At greater depths to the west, alkaline olivine
basalt is generated. Intermediate types are considered to have ex
perienced some assimilation of sial. 110
. Variations Within the Province
An extensive collection of thorough petrological studies of
Cascade Pliocene-Quaternary volcanism has now accumulated. It pro vides an excellent opportunity to develop a picture of provincial char- acteristics of essentially coeval magmas.; The characteristics noted by the previous researchers include: 1 ) a suggestion of a systematic change in percentage of certain chemical components across the Range;
2) a high (calcic) alkali-lime index throughout the High Cascades; and 3) a change in the degree of differentiation along the Range.
The middle Tertiary igneous rocks with which we are most concerned may be specifically a separate province. Nevertheless, it appears that the Quaternary petrochemistry is pertinent to an under standing of Tertiary igneous and economic geology.
The major chemical components of the volcanoes of the High
Cascades and adjacent areas have been plotted on plate 10. The per centages were determined from Barker diagrams at the 60 percent
SiOg position. This represents an intermediate composition somewhat
more acidic than the andesites and basaltic andesites that predominate
in the province. The datum allows the analyses of flows from Mount
Baker and Mount Rainier to be used even though they are limited in
composition range. The flows of Steens Mountain are Pliocene or
older. Hence, their value in determining chemical variations of mag
mas that have recently been active is questionable. I ll
As noted by Williams (1935), KgO and FegOg percentages are higher to the east, whereas AI2 O3 , CaO, and MgO percentages are less. Plate 10 suggests that the changes are gradual across the prov ince. The alkali-lime indices (Peacock, 1931) are also plotted. High
Cascades lavas are generally calcic in contrast to contemporaneous calc-alkaline lavas to the east at Newberry Crater (Williams, 1935).
Again, the change, even among the High Cascades volcanoes proper,
evidently is a gradual decrease eastward.
Williams notes that Tertiary Western Cascades lavas are calc-
alkaline. He suggests that the structural adjustments in the late Miocene
may somehow account for the contrast with Quaternary lavas. In com paring the Western Cascades lavas with the chemical gradients sug-
gested above for the High Cascade. Range, one finds contrasts in all .
major components except NagO.
The Western Cascades lava Barker diagram (Thayer, 1937) is
based on six analyses, four of which are from the Bohemia district.
They form moderately smooth curves and had been taken as representa
tive of the lavas of the whole Western Cascades. An analysis of middle
Tertiary Keechelus lava from the vicinity of the Snoqualmie batholith
(Smith and Calkins, 1906) is chemically similar to the rocks from the
south (pi. 29). A second Keechelus rock analysis falls on the projec
tion of Thayers curves or can be used to modify them slightly. I have
not done soy however, in the plate. The composition of the hypothetical 112
Tertiary lava at position SiOg = 60 percent compares with the hypothet ical Pliocene-Quaternary lava derived by inspection of the gradient curves nearest the Bohemia district, as follows: . : r . : ;
SiOg KgO Na20 CaO i MgO FegOg TiOg AlgOg Total
Hypothetical High . . . Cascades ■ ■ - '' - - ■ : •:■. • ■ -■ ; lava (west) 60.0 1 . 0 3.9 6 . 3 3.2 5.9 0 . 6 18.0 98.9
Hypothetical Western ■ ■•1 ’■ • v , '• ••• V f n •; V:; ■. i ' Cascades lava 60.0 2.2 3.9 5.3 2.5 8.2 1.3 14.8 98.2 Western ' - ^ ■ .. L:v:- Cascades magma : - ; ■ - -• from Peck’s ' - - ' .. V;' .:. curves (1960) 60.0 1.7 3.7 5.6 3.0 6:7 - 15.8 96.5
- Peck’s recent Harker diagrams (I960), based on 28 analyses of lavas, pyroclastics, and ihtrusives of the Western Cascades of
Oregon, provide an excellent check to the inferred contrasts; The rocks range in age from Oligdcehe to late Miocene and include several that are much younger than the six oh which Thayer’s curves were based. Sig nificantly, the components at SiOg = 60 percent are mostly intermediate between the two hypothetical lavas. ; . , ■ \ -
In summary, there appears to be a gradational change in most I components of contemporaneous lavas across the Cascades province.
These changes are manifested even within the narrow belt of High 113
Cascades volcanoes (i. e. , excluding Medicine Lake and Newberry Cra ter). The exposed middle Tertiary Western Cascades lavas geograph ically flank the younger volcanics but cannot be fitted to the gradient curves. Instead, they are markedly in contrast in all major components except soda.: -..iv':<■ v. . ■ : x : ,"i; . . ^ i v
In the light of the described gradients and contrasts one could hardly resist at least brief speculation. Two such contrasting series might develop by partial fusion of the mantle. Early magmas might be expected to contain appreciable amounts of KgO and iron oxide, com ponents that tend to concentrate in low-melting mixtures. Lavas fol lowing an early period of outpouring would be noticeably impoverished in these components. ( In the center of the region of early eruptions the later lavas would necessarily have formed from partial fusion of the more refractory material left behind. These later lavas might be rel atively enriched in MgO, CaO, and AlgOg. : ■ i e
In addition, the chemical gradients seem explainable by the partial-fusion-of-the-mantle hypothesis. In progressing across the region of middle Tertiary extrusions toward its center, one would ex pect the Pliocene-Quaternary volcanoes to show progressive impover ishment in the components removed in the earlier periods. The gra dients would be a measure of the intensity of volcanism in the peripheral region of middle Tertiary activity. Finally, the eastward migration of the volcanic axis described by'Peck (1960) may not reflect a 114 simultaneous migration of the seismic axis as might be expected. In stead it may be that only on the periphery of the seismic belt was there enough low-melting materia! in the mantle to produce magma.
' An acceptable hypothesis might be tendered from the processes of classical magmatic differentiation such as that which Williams (1935) has suggested to;explain the differences between High Cascade lavas and lavas to the immediate east. According to Williams, the High ’
Cascade lavas with their higher AI2 O3 , CaO, and MgO contents may be in part the products of refusion of crystals which settled in the magma chamber at an earlier stage. By his suggested process, a fundamental difference between High Cascade lavas and their contemporaries to the east is that the High Cascade lavas originated at a deeper level in the magma chamber.
Williams (1935) did not attempt to integrate the petrochemistry of the Pliocene-Quaternary lavas with that of their predecessors of the middle Tertiary. However, it is worth noting that the temporal con trast is compatible with his suggestion. Magmas erupted early in the cycle, before or during crystal differentiation, should be richer in iron oxides and alkalies and poorer in AlgOg,, CaO, and MgO than those that formed at a later time, in part by refusion of the more refractory res idues.
Waters (1955) has suggested that the parent of the Western
Cascades volcanics was tholeiite from a deep source. In passing 115 through the geosynclinal pile of Eocene sediments and older metasedi- ments and metavolcanics, the tholeiite acquired easily removable con- i ;:V. v ! /, C.: A: y stituents such as water, silica, and alkalies. The result is a variety ' ■, /■. Lj.. :V .: - L _T y y ; V y .y ; of lavas from tholeiite to basaltic andesite to dacite.
To explain by this means the gradients and contrasts described above, it seems necessary to modify Waters* suggestions.' First, the- middle Tertiary and Pliocene-Quaternary volcanics do not seem to be of independent cycles. Second, the components added to the middle
Tertiary tholeiite included alkalies but did not include much MgO and
CaO as suggested by Water s. : ■ -
c y y
. r r
"Y, c Y ' Y . : i y :Y
( ..■ : : \ % : Y;.-: Y -V L Y .
■ -
.n 1 * r
: ' '.r . v ■<./r r r:. . L-:: ; COMPARISON OF THE CASCADE MOUNTAINS PROVINCE WITH SIMILAR PETROGENETIC- METALLOGENETIC PROVINCES '•;i •- ; i: , v.., : ... .; u, - r \ , v v .1 x:
The Cascade Mountains province is one of several similar pet- fogenetic provinces! situated at or near the margin of the Pacific basin.
Most of these have middle Tertiary plutons with associated vein de posits. The country rock in each case is largely composed of Tertiary formations rich in andesitic volcanics. In addition^ one or two chains of active volcanoes occur nearby. f ; ; v . v, ;
Differences in the degree of mineralization appear to correlate in part with the ages of the deposits relative to the beginning of andesitic volcanism in the particular regions. The economic importance of base- metal mineralization that has occurred early in a volcanic cycle v
(Sumatra and Honshu) generally exceeds that of mineralization in mid dle or waning stages (Kamchatka and Aleutian Islands). :
The character and degree of mineralization may also be a func tion of the position of the provinces with respect to the margin of the continents. " . K ■ - : ■ ‘ x..V'-O ■
Vi:;';, f : : v :1. ] -; . ■ .u: .X-- The Green Tuff Province of Northeastern Honshu, Japan
A strikingly similar geological setting occurs in northeastern 116 : 117
Honshu, Japan. Ore deposits, commonly with associated intrusives, have formed in the Miocene green tuff volcanics, mainly in Akita and
Yamagata prefectures.
The Cenozoic regional arrangement mirrors, in part, that in the northwestern United States with respect to the margin of the Pacific basin (pi. 11). Following Kawano, Yagi, and Aoki (1961) we find that
.. northern Honshu shows a remarkable arcuate structure, composed of various zones, enumerating from west to east: Japan Sea basin, inner zone, outer zone, and the Pacific Ocean basin. "
In the outer zone pre-Tertiary formations, including meta- morphics and plutons, crop out. In the inner zone the Neogene "green tuffs" and volcanic-rich sediments cover much of the basement.
The Nasu and Chokai volcano arcs extend along the east and west sides, respectively, of the inner zone. Recent volcanic activity has been concentrated on the 800-km long Nasu arc. The chemistry of the two arcs is complicated by eruptions of more than one magma series from individual centers during the Quaternary. However, the
Chokai arc, farthest from the Pacific basin, contains lavas with the highest AlgOg, Na^O, and and the lowest content of CaO, FeO +
FegOg, and MgO. Both arcs are calcic according to Peacock's alkali- lime index. Sugimura (1960) considers the westward increase in al kalies (relative to AlgOg) to prevail throughout northeastern Japan, the culmination to the immediate west being the Circum-Japan Sea alkalic 118 province. Beyond that lies the high-alkali series of northeastern China which is said to be the most alkalic in the world (Tsung-pu, 1956).
The Cenozoic volcanic history began with the accumulation.of the green tuff of the lower Miocene. The series is cdmposed of flows, pyroclastics, welded tuff, and domes of basalt, andesite, dacite, and rhyolite composition. Elsewhere sediments with minor limestone pre- : ’ * • . " i : » •' . , i ■ * t i" dominate. These formations have been folded, complexly faulted .,
(Funayama, 1961),. and intruded by a few stocks. The intrusive rocks range from granodiorite to diorite or granite, and at least some are believed to be Miocene in age. A genetic relation between the ore de posits and the plutons is apparently not always evident.
Ore deposits in the Tertiary include clusters of veins and the economically important replacement bodies. Ore minerals are chalco- pyrite, sphalerite, galena, barite, tetrahedrite, gold, pyrite, and , pyrrhotite. The fine-grained ,,Kuroko,,-type ores replace volcanics at
Hanaoka (Kurushima. 1956) and elsewhere. At the Ohori mine a fresh- water limestone is the favored horizon (Takeuchi, et al., 1961). .
, , A smelting industry has been integrated with the mining opera tions. The base-metal potential of the region appears to be about two orders of magnitude greater than that of the middle Tertiary of the
Cascades: ^ 119
The Central Range of Kamchatka, U. S. S. R. : i ’ '
v:L J Kamchatka is another area having a Cenozoic regional history much like that of the northwestern United States. Two Quaternary vol- cano belts parallel the long dimension of the peninsula. Underlying
Tertiary volcanics and some of the Quaternary have been mineralized near a deep fracture zone.
According to Tomkeieff (1949): "The dominant factor in the
structure of Kamchatka is the Tertiary folding that gave rise to a series • 1 ■ of anticlines and synclines, recumbent folds and faults." Pre-Tertiary
rocks are exposed in the cores of some anticlines. The Tertiary for
mations are volcanics and marine sediments ranging in age from
Paleogene(?) through the Neogene (Vlasov and Vasilevskii, 1958).
Recent mapping reveals that the Tertiary folds have inherited from
older folds a trend that is partly diagonal to the later superimposed
volcano belts (Tikhonov, 1959). This oblique grain has partly deter
mined the detailed arrangement of Quaternary cones. ♦ t -'VP ' « ; 1 : ’ < < ^ ' • •' » ' ' ^ ^ ^ ; .. ' • < : " i ^ ' , ,, The active volcanoes are situated in the Coastal Range along
the Pacific coast. A western belt of dormant volcanoes is superim-
posed on the Central Range, an anticlinorium forming the central part
of the peninsula. Average chemical compositions (Tomkeieff, 1949)
indicate a calcic nature (alkali-lime index of 61) for both belts taken
together. Development of alkaline magma in the hinterland (toward 120
Asia) of the Kamchatka-Kurile arc is negligible (Tomkeieff, 1949).
’’Late Neogene" mineralization is associated with fractures branching from a deep fracture that parallels the axis of the Central
Range, and small diorite intrusives are also present. Recent studies in several altered areas eroded to different levels have led to conclu sions regarding the variation of alteration and metallization with depth
(Vlasov and Vasilevskii, 1958).
' Deep levels are characterized by chalcopyrite-molybdenite mineralization in massive quartz propylite which grade into the enclos ing country rock and which contain actinolite, pyroxene, epidote, bio- tite, pyrite, and metasomatic quartz.
Somewhat higher levels reveal propylite with quartz stock- works containing adularia, epidote, and carbonates. The quartz of the veins is usually as fracture fillings. Associated ore minerals are chalcopyrite, sphalerite, and galena.
At shallower depths one finds that epidote is not present, propylite is less extensive, and new assemblages are developed in the center of altered ground. The shallow-depth minerals include opalite, alunite, sulfur, and abundant clay minerals.
Unfortunately, the evidence supporting a single period of min eralization is not given by Vlasov and Vasilevskii. Mineralization is said to be "Neogene" and "late Neogene" and it affects Quaternary lavas. On the 1955 Geological Map of the U. S. S. R ., the Bystrinskiy 121
Range is shown as composed of Quaternary volcanics. However, ac cording to Vlasov and Vasilevskii the country rocks are Neogene and
Paleogene(?) in the Bystrinskii area. It seems tenuous to correlate the shallow dpalite, sulfur, and alunite deposits in C enozoic volcanics with the copper-molybdenum and copper-lead-zinc mineralization in pre-Tertiary country rock. - ' -
Only general, brief treatments of Kamchatka ore deposits have been noted in recent Soviet literature translations and abstracts. Kish
(1960) and Seltzer (1952) mention no mining industry nor mineral wealth besides coal arid petroleum in Kamchatka. On this basis the potential of the Cenozoic ore deposits is tentatively suggested to be much less than that of northeastern Honshu, though possibly greater than that of the northwestern United States.
The Aleutian Islands
A chain of active volcanoes extends from the Alaskan mainland to Kamchatka. As with the Cascades^ the late Cenozoic cones are built
on a foundation of volcanics and clastic sediments which are intruded by
quartz diorite stocks. The following summary has been largely syn
thesized from the local studies published in U. S. Geological Survey Bull.
1028. ; V-: ' : c:: - ^ - : -
Locally, a transverse variation in the composition of late
Cenozoic lavas has been noted by Byers (1961). The abundance of some 122 components in lavas at the outer (Pacific) side of the volcano arc is sig- nificantly different from that in lavas at the inside. Potash increases as though approaching a culmination in the basaltic-basanite lavas that are sparsely developed in the hinterland (Barth, 1956), especially on the Pribilof Islands.
From the available exposures the Tertiary and older forma- tions appear to be only moderately deformed and slightly metamorphosed.
Paleozoic formations have been identified locally, but much of the foun dation is probably middle Tertiary. Tentative ages of upper Oligocene-
Miocene have been suggested on the peninsula and Kiska Island. On
Amchitka Island fossils in the Banjo Formation suggest an Oligocene-
Miocene age. These units contain basaltic and minor andesitic volcanic debris and some flows. Other units that may be early Tertiary or
Mesozoic are geosynclinal elastics with spilitic volcanics.
The Banjo Formation was intruded by quartz diorite, probably
in the late Miocene. Most of the plutons of the arc have not been dated.
However, they are similar to the Cascades plutons in regard to the
presence of several facies and in regard to chemical composition.
The plutons are commonly surrounded by aureoles of sericitiza-
tion, silicification, and bleaching. Copper mineralization is noted local
ly but no ore has been produced. The lack of exposures beyond the is
lands lessens the value of the comparison. 123
>. i - v v Southwestern Sumatra, Indonesia ;
The active volcanoes of Sumatra and Java are situated along a narrow belt extending the length of both islands. A parallel zone of
Tertiary plutons has intruded the older rocks that form the foundation on which the volcanoes are built.
Rittmann (1953) has shown that the petrochemistry of the vol
canoes is essentially constant along the arc but that there is a marked transverse change. He finds that in progressing away from the Indian
Ocean the lavas change from Pacific type to Atlantic type. In the hinter
land, well to the northeast, the magma is of the Mediterranean (potas-
sic) type.
According to Bemmelen (1949), the largest part of the Cenozoic
sedimentation in southern Sumatra has taken place after the Paleogene.
Paleogene formations to the northwest are marine and are apparently
free of volcanic components. The southern part of the island was prob
ably a positive area at this time. In the lower Miocene, pyroclastics
and flows became important in the column. About 200 m iles southeast
in the Bajah Mountains of Java, Bemmelen mentions andesitic detritus
in Eocene sediments as evidence of the oldest Tertiary andesitic vol-
canism of the region.
Along the southwest side of the volcano arc there is a belt of
middle Miocene intrusives. These plutons range to batholithic 124 dimensions, and are mostly granodiorite in composition. The Bengkunat batholith has diorite marginal facies and a core of biotite granite. The surrounding andesite country rock has been propylitized, and near the contact biotite-garnet hornfels have developed.
Tertiary sulfide and precious-metal veins and replacement bodies occur along the same belt as the intrusives. These include copper, lead-zinc-copper, and iron deposits; however, the most im portant ore bodies are the precious-metal veins. Bemmelen (1949)
states that ”in most occurrences the mid-Tertiary granodidrites of the
Barisah Range are the parental rocks of the epithermal gold-silver
v ein s."
The Lebong district veins are unusual among the interior belt
ore deposits considered here. They are low in copper, zinc, and lead
sulfides and they contain an unusually large amount of selenium. Be
sides propylitization the deposits have also undergone silicification.
sericitization, and some adularization. The five mines of the Lebong
district have produced about three million ounces of gold and about 30
million ounces of silver. In addition, sizeable gold-silver production
1 ) , :."i * j ’ 'V . • : ■ •• ' « ’ % has been recorded in other districts. The base-metal deposits might
be of some importance if they were favorably situated with respect to
sm elters and markets. INTERPRETATION OF THE GEOLOGICAL HISTORY OF THE BOHEMIA DISTRICT , v ———— :———; ------— v ■ - ;
It was not feasible to organize the foregoing portions of the paper to be entirely descriptive, and several interpretations were in terwoven with the descriptive text. First, it was asserted that Several of the large masses of dacite arid rhyolite in the south part of the dis trict are domes (Quellkuppen), and that the four m asses of porphyritic dacite are the hypabyssal facies of other domes or flows that have been removed by erosion. Second, it was suggested that these lavas and as sociated pyroclastics dominate an intermediate horizon on the south, but, in part, interfinger with basic lavas to the north. A third asser tion related a considerable part of the basic flows of the district to the magma that has filled the group of dike-plugs north of Elephant Moun- tain. : ' V.‘ , :■ : ' . "v:.: vn'.
On the basis of the foregoing descriptions, the history of ig neous activity and ore deposition can be outlined. Volcanism localized along the Pacific margin built up a great section of Tertiary volcanic strata. Stocks have intruded the volcahics and released their minor components to form sulfide deposits in adjacent rocks. Finally, ero- sional processes have laid bare the veins arid brought about minor en richment. 125 126
Accumulation of Volcanics
The volcanic formations of the Bohemia district accumulated in the Oligocene and Miocene. They were erupted as parts of the vol canic activity that has prevailed near the edge of the continent through the middle and late Tertiarv and to the Recent. About a third of the eruptions involved andesite lavas and lesser amounts of dacite and rhyolite flows and domes. Local as well as distant sources fed the flows through dikes and plugs. The hypabyssal intrusives were chan neled toward the surface along fractures whose pattern probably re flects the basement structural grain. , - .
, The largest part of the,volcanic accumulation was pyroclastic.
Massive pumice lapilli tuff and tuff breccias probably were deposited from glowing clouds. Other breccia accumulated as mud flows and as blocky surfaces and talus of domes and flows. However, most pyro-
V ■ . ■ ■ . . elastics have undergone some reworking in a fluvial environment. Ac cidental fragments in the tuffs indicate that the domes in the southern part of the district were.high-standing, and that their erosional degrada tion supplemented other sources of pyroclastics. This high area of dacitic eruptions may account for the apparent diminution of the basic lavas of the upper unit toward the south. Small lakes developed and
disappeared in a short time in such dynamic circumstances., 127
Intrusion of the Stock
After several thousand feet of lava and pyroclastics had ac- cumulated, granodiorite invaded the sequence and roofed at a depth of, over 3,000 feet below the surface. The main magma chamber probably did not break through to the surface, however, a cupola on the south rose several hundred feet above the general roof elevation. The stock is composite and involved three or more pulses of magma, possibly with some differentiation in place. That is to say, each pulse may have been differentiated after intrusion by inward and upward migration of low-melting fractions, leaving a diorite to quartz diorite border facies.
The main body of the last pulse probably did not reach the shallow depth now exposed. The quartz-sulfide veins formed from the last residual fractions.
Structure
The basic structure of the region is a homocUne dipping east- northeast. The local, superimposed structure either is symmetrical to the granodiorite pluton or has been inherited from the basement of pre-Tertiary. This is generally true for the entire Cascade petrogenetic- metaUogenetic province.
For example,. in the Silver Star Mountain area, parts of the
Snoqualmie area, and most of the Bohemia district the plutons parallel 128 the middle Tertiary intrusive axis, and veins have developed symmetri
cally in response to the stresses during and after intrusion. In other
areas the basement structural grain has guided the intrusives and thus : ' ' - n':\ :.:"i h-;' ; ’i ' ' . .1 ,, - : -. remotely determined the orientation of dikes, veins, and alteration pat-
terns oblique to the Cascades axis. Excellent examples of this are the
deposits along the cupola- apophysis in Sultan Basin, Washington, and
probably the Blue River deposits of Oregon.
'• ; v' ;: " r n. V Alteration
;: / A portion of the heat energy released to the country rock dur-
ing and after the intrusion of grahodiorite magma was emended in con-
i j' ; %. '' ' - y: - y- -y ; - ; " ' \ y- . - : ;y ' . - y - T - tact alteration. Extensive recrystallization is evident, and changes in
the wallrock suggest that Si, Fe, Cu, S, and O'were m etasomatically
added in an oxidizing environment. The partial pressure of COg and,
to a lesser degree, gaseous sulfur compounds may have increased to a
. : ' f'y y y - " ' : =/ r. " T - :- y . - I'. ?, r - r. . y : . maximum away from the stock. Somevdiat later alteration involved
addition of 13, K, S, and Si and locally A1 along restricted zones.
In other districts in the Cascades, garnet, pyroxene, amphi-
bole, and biotite have developed locally near the contacts with large
plutons. In the Bohemia district the contact alteration did hot exceed
the albite-epidote hornfels facies in "intensity." A much more wide
spread propylitization pervaded the area during the intrusion and sub
sequent vein formation. 129
: Vein Formation ;r
As the final pulse of magma cooled, its original content of r trace elements, that had been partly concentrated into aqueous solu tions, deposited in the fracture channels leading to the surface. The fractures were probably dilated by a district-wide extensive stress caused by pressure of the deep-level magma source, and they probably accommodated intermittent normal displacement by subsidence of hang ing walls throughout most of the period of fissure filling. The vein as semblages are of quartz with lesser amounts of carbonates, sulfides, and gold. On the edge of the principal mineralized area stibnite pre cipitated. Near the center of hydrothermal mineralization specularite is a characteristic early mineral. The specularite and closely associ ated epidote indicate that the oxidizing conditions of contact alteration continued into the hydrothermal stage. Some of the hematite precip itated with silica and settled as horizontal layers in the base of cavities.
The temperature during this stage was probably less than 400°C.
Arsenopyrite, pyrrhotite, molybdenite, and scheelite formed in deposits associated with the large plutons in Washington, but these separate phases are generally lacking in the Bohemia veins. Some sizeable trace amounts of arsenic are present in pyrite, however. 130
Secondary Processes
The moderately heavy precipitation of the region has combined with relatively open channels within the veins to form debris washing deposits. In the oxidized zone some vein minerals have been decom- - " • ' : i ; 1; i-. :! w r,/:- . ^ - im posed and have gone into solution; the exceptions are some of the clays, most quartz, and all of the gold. First iron and then manganese are
oxidized to the high-valence cations and rapidly precipitate as hydroly
sates. The resultant limohite and-wad coat or become interbedded with
debris sloughed from the vein and walls above.
Below the water table a rapid increase in pH is apparently
conducive to hydrolysis of silicon with later formation of chalcedony
and finally quartz. However, most secondary bedded silica was prob
ably transported as a fine silt rather than in solution. Copper also has
precipitated as secondary sulfides near the water table.
- • . ' :: ■ : 5 . ' U-:- ' : v ' - 4 ' - :* v T: ' ; v : : . ■ \ V.r : INTERPRETATIONS REGARDING ECONOMIC GEOLOGY
Four principal factors are considered to determine the poten tial of the veins of the district: 1) the position of the province in the
continent; 2) the position of the district within the province; 3) the age
of mineralization in the volcanic cycle; and 4) the local ore.controls.
Regional Aspects
The potential of the region appears to have been affected by the
position of the metallogenetic-petrogenetic province near the border of
the continent and also by the old age of mineralization with respect to
the beginning of the volcanic cycle: ; %;
Position Near the Margin of the Continent
The westward decrease in alkalies across the Cordilleran re
gion is a characteristic that has its counterpart on;other: continents. It
is probably the most conspicuous of several related variations in the
chemistry of the igneous rocks of the region. Equally conspicuous •
changes occur among the related ore deposits (Lindgren, 1933; Bilibin,
1950). t;' -:v a.-;-. ■
In addition to mineralogical differences there is:a marked 132 contrast in economic potential between the deposits of the Cascades and those to the east. This low potential is riot unique. The similar basalt- andesite provinces flanking the Pacific basin on1 Kamchatka1 and the
Aleutians are apparently also of low economic potential. ; he 'v
Age of the Deposits in the Volcanic Cycle ' ;■ • ' :• ' ' • e-;. in ■ .. : C .. - i . i ;.
Among the generally low-yield, circum-Pacific type provinces there is considerable variation. The province of northeastern Honshu
and a similar one on Sumatra have produced large tonnages from a few
important deposits. The variable potential among the provinces seems
to correlate with the age of the plutons and accompanying mineralization
in the volcanic cycle. v'
i Youthful plutons produce major ore deposits whereas plutonism
in the middle or waning stages of volcanism seems to produce only minor
ore. For instance, volcanism began in the early Miocene in northeast
ern Honshu and southern Sumatra. The period of granodiorite intru
sion and mineralization occurred later in the Miocene after a time •
lapse of about 10 to l4 million years. Late Neogene deposits of
Kamchatka probably formed more than 20 million years after andesitic
volcanism commenced, i The Snoqualmie batholith and its mineral de
posits formed about 25 million years after andesitic volcanism com
menced in the Cascades. The two provinces with predominantly youth
ful mineralization have known major ore deposits. \ v . 133
The chemical gradients and. contrasts among Cenozoic lavas of the Cascades may provide the key to explain the relation between ore and age of this type of province. It was noted that most major chemical components in late Cenozoic lavas change gradually across the Cascade
Range. The chemical gradients projected to the west show a marked contrast to lavas erupted there in the middle Cenozoic. From this it was suggested that the magma source was impoverished in certain major components by early volcanism. It is not necessary to specify whether the source was a magma chamber, a portion of the mantle, or, in part, a portion of the crust.
It follows that certain minor element components would also be preferentially incorporated in magmas formed early in a cycle of regional volcanism. It seems reasonable that these elements would include lead, zinc, gold, silver, and to a lesser extent copper; that is, the elements that do not usually form the relatively refractory silicates or oxides, but instead, might be expected to concentrate in a magma developed by partial fusion or contamination as discussed in the sub section “Petrochemistry of the Province. ” Thus, early magma ar rested in its ascent toward the surface would cool to form plutons ac companied by deposits that are rich in these preferentially included minor elements. Magmas formed later in the cycle of regional vol canism would be relatively impoverished in these elements, and they would yield only minor ore deposits in subvolcanic environments. 134
Relationships Among the Districts
The type of mineralization varies uniformly along the axis of the region. This gradation may be partly due to the differences in size
of the plutons and the depth of exposure. However, less obvious cir
cumstances may have influenced the distribution. One notable feature
is the presence of molybdenum and arsenic sulfides in the districts of
Washington and their rarity in Oregon. -v : v.
Position Within the Province
Using Emitting's (1956) compilations, -one notes that there are
minor occurrences of molybdenum, tungsten, and arsenic minerals in
the Snoqualmie districts and arsenic and molybdenum as far south as
the Silver Star Mountain area. Except for an occurrence of jordisite
(Staples, 1951) in the northern part of Oregon, there have been no -
molybdenum nor tungsten occurrences reported in the Oregon Cascades.
Similarly, arsenic minerals are rare in Oregon. This leads one to
suspect that there was some fundamental difference in the composition
of the source of mineralizing fluids along the axis of the province.
However, all the deposits of Washington are associated with large
plutons and this may explain the difference in mineralization. 135
The Size and Depth of the Plutons
The characteristics of the various districts are summarized on table 7. It is to be noted that the type of mineralization varies with the size of the associated intrusive. The large Snoqualriiie batholith has locally developed biotite, garnet, pyroxene, amphibole, staurolite, and andalusite in adjacent rocks. This is in contrast to the Bohemia contact zone where quartz- epidote-magnetite propylite is typical.
Smith and Calkins (1906) estimate that the Snoqualmie magma penetrated to within 4,000 feet of the surface. It is suggested that the
Bohemia stock originally had no more than a m ile of cover. Therefore, depths were probably shallow throughout the province. Lithostatic pressure was apparently not a major factor in determining differences of alteration and metallization.
On the other hand, the size of the pluton may have been im portant. The area of the contact surface would increase as the square of a linear dimension. The heat energy and volatile content available to act across this surface would be more nearly proportional to the volume, the cube of a linear dimension. - Thus, larger plutons cooling from a given temperature should produce greater alteration in the country rock. !];- Local Factors ; • ; ■
^ •; ^ o : r y-'--; v . :k v , In each of the districts of the province a pluton or group of porphyry dikes is exposed. The mineral deposits mostly accumulated
in fractures geometrically related to these intrusives. The pluton hood
or the country rock overlying a cupola is a favored site for fracturing r. ’ i "' , ,. - * and vein formation. The deposits are excellent examples of the clas-
sical theory of the development of ore deposits from post-magmatic
hydrothermal solutions.
, Ore Controls in the Bohemia District
A number of factors have influenced the deposition of sulfides
and precious metals. As with most ore controls, each was only partly
effective. ' ^ ■ i - : ... ; ■ , ; The Cupolas ; v; :
The cupola that rises above the southeast side of the Bohemia
stock, along with its country rock, has been extensively altered. This
is not unusual in shallow intrusives. It is perhaps surprising that es
sentially no veins are closely associated with the apophysis. The only
veins that can be ascribed to the exposed part of the Bohemia stock
with any degree of confidence are those of the Miller group in the over-
lying country rock on Noonday Ridge. 137
Most veins of the district probably formed above a similar ' cupola lying at greater depth. Some of the porphyry dikes may join with this cupola, but otherwise it is hidden and only inferred from the arrangement of veins. It may have been connected to the exposed por tion of the Bohemia stock, or it may have been of a distinctly later surge of magma. : . .• : '•
An alternate possibility suggested by the distribution of min- - eralization zones is that the exposed cupola was actually the source of the metals, but the conditions favoring deposition of base-metal sulfides reached an optimum at a distance of about a mile. -
Focus of Mineralization : V wVv'i ' - : - - \ :
As noted in the section "Structure, " the veins are preferential ly situated to the south of the exposed part of the Bohemia stock. Pre sumably this focus of fractures reflects a concentration of stress over or adjacent to the final pulse of magma suggested above. The two veins with the most production are situated near the center of the cluster of mineralized fractures, and this indicates that the focus of fractures was also a focus of mineralization. - : • ;
Two factors are suggested to account for the focus of mineral ization. First, it was nearest the source. Second, it was distended by the final intrusion and continued to undergo fault adjustment during the period of mineralization. 138
Because most production has come from a 2-1/2 square mile area, it seems reasonable to expect most additional production to be in or near this area also. -;v : . ^ . •" ■ " . ' I 'o/L; ; •
Open Channels .'■■■ ::v' ,
During each surge of magma the country rock was domed and fractured or reopened. The fractures were basically extensional. How- ever, there was a tendency for normal displacement by slumping of hanging walls. Fractures of several sets locally interfere and form veins with variable dips and strikes. It has been demonstrated in shal low veins elsewhere that steeper portions of normal faults tend to be open, favored sites for ore deposition. However, this guide should be judged with respect to both the adjacent portions and the overall attitude of the vein.
" - - • Dikes
All dikes noted in the district are pre-ore. Locally, veins closely parallel the dikes, which have apparently provided planes of weakness. Veins cutting obliquely across dikes show little if any changes in the character of mineralization. Dikes have apparently not exerted the influence that they have in many other districts. 139
Vein Intersections
The Champion, Musick^ and Vesuvius mines have produced ore from vein intersections. In addition, the Knott shafts on North Grouse
Mountain penetrated a small stockwork of ore at the intersection of two or more veins. ■> •; V...-; ' ■ '• Cr . :
The shoots occur on the intersection or along one branch (pi.
12). There appear to be four factors involved: 1) two veins provide more feeder channels than one; 2) an accompanying change of strike on the main vein favors open sites for ore deposition; 3) one of the faults may have acted as a barrier to ascending solutions; and 4) subsidiary veinlets are common near the intersection.
Subsidiary Veinlets
Subsidiary veinlets in the walls of the vein have assumed im portance when they are so numerous that they form part of an orebody.
Such a situation probably prevailed at the Knott shafts.
Steep subsidiary fractures commonly have developed in the hanging wall as a consequence of extensional or normal displacement on the main fault. It seems possible that the common vein breccias have developed by subsidiary fracturing adjacent to the main fault fol lowed by slumping or normal displacement. 140
Wallrock
Orebodies have formed adjacent to granodiorite porphyry, pyroclastics, rhyolite, basic flows, and contact-altered rocks. No : v" .:-v: i.; " ^ preference was distinguished. However, Mr. M. W. Cox has remarked v v 'iV.- v . ' • u : ■: . i. ' \ • , , % : V ' (personal communication) that the distribution of stopes in the Champion mine suggests that rhyolite is an unfavorable wallrock. '.i ,': ' ':V
■ : ■■■ ' -' . T • ■ .• ■:
■ . : ' 1 : -
>, . . - :r-. : ' 3 •!.•;' ' -.A "
- : V. ^ . v, ■; ^ Ar : - »
■. r; V, , :v.
• - -! V - - = : . ' .:. if r v A x'
- • ■ : ■: f • r V ' . ‘ A : • -- r y: c. /A, A'- . _ -
- ; - . • ' ■ V :"yv■ .y.:; o:" ^ ;.y _':y APPENDIX— TRACE ELEMENTS IN SULFIDES
A small collection of pyrite and sphalerite, separated by hand from veins of the district, has been semiquantitatively analyzed with a spectrograph by Thomas C. Matthews (table 6). The apparatus and procedure apparently do not allow the degree of accuracy that is neces sary for determining spatial and temporal variations in the content of trace elements in sulfides. Repeated deposition of individual sulfides intermittently over a long period in the sulfide stage probably minimizes the usefulness of such studies, unless early, intermediate, and late types can first be distinguished in some other manner.
Notable features are the large amount of As in some pyrite and the expected Cd, Mn, and Fe in sphalerite. The content of Al, Cu, and
Pb may be taken as an approximate index of the contamination of the
■■■■•- ■ ■ , v ;. ; . . ' : ■ V : sample by clays and other sulfides.
: ■ ' . : . -i. "■ '• ' •- 7.' V 7 V
O 7, cr ■■■■'.■■
y - y ' ,, .-.v
141 142
Table 1 SILICA AND POTASH ANALYSES OF SOME ROCKS OF THE BOHEMIA DISTRICT
(Analyses by L. L. Hougland)
S1 O2 K2 O Number Location (percent) (percent) Rock type
B-270 Lower lava of Sharps 62.60 1.70 Dacite Creek mass; at elevation of waterfalls above Glenwood cabin
B-272 A later lava of the Sharps 69.26 2.80 Rhyolite Creek mass; above water falls and placer water- supply tank, at base of 2 5-foot cliff on east side of creek
B-254 In road cut, about 100 feet 49.74 0.69 Tuff breccia in elevation below top of Fairview Peak
B-216 Under Fairview Peak 45.88 0.81 Basalt lookout tower
B-262 Top flow of South 45.58 0.91 Basalt Grouse Mountain
B-274 Glenwood sill, at 59.20 0.26 Andesite Glenwood cabin
B-10 Dike on trail to Canton 62.34 0.78 Quartz Peak, 1/4 mile south of phenocryst- Shane Saddle bearing dacite porphyry PLATE 13
Photomicrographs of basaltic flow rocks of North
Fairview Mountain-type. Light-colored minerals are mostly plagioclase, and dark minerals are mostly augite with some chlorite and magnetite.
Note chlorite psendomorph, probably after an augite phenocryst, in upper right corner of B-77.
B-77, northeast ridge of North Fairview Moun tain; B-264, third flow from top of South Grouse
Mountain. 143
B-77
0. 4 millimeter
B-264 PLATE 14
Photomicrographs of basaltic dike rocks of
Elephant Mountain-type. Light-colored min erals are mostly plagioclase. Dark minerals are mostly augite with some chlorite and magnetite. Note chlorite pseudomorph after augite(?) phenocryst at left of B-117. B-117, dike-plug south of Sweepstakes prospect (sec tion 3); B-472, dike-plug on east flank of ridge northeast of Elephant Mountain. 144
B-117
0. 4 millimeter
B-472 PLATE 15
Photomicrographs of suggested correlative flow and dike rocks on Bohemia Mountain. Augite phenocrysts such as shown at right are common in B-220. B-204 contains only a few phenocrysts
(plagioclase at right). B-220, second or third flow from top of Bohemia Mountain; B-204, dike rock cutting second flow from bottom,
Bohemia Mountain south of Musick mine.
< 145
0. 4 millimeter i------1
B-204 146 IMAGINARY SURFACE OF ELEVATIONS OF RIDGE TOPS & FLATS IN THE FAIRVIEW PEAK QUADRANGLE
PLATE 16 INTERPRETATION OF 147 FOLD STRUCTURE
BASED ON ATTITUDES OF BEDDING
estimated contours at about 5OO-foot in te r v a ls
BOHEMIA STOCK
PLATE 17 148
6" gouge m m - __ _ f 8°u6e along r h y o lite itz veinlet witmcp, si, cc
^horizontally ------‘ a r g illizzed e d and banded silica \ epherulitic silici^ied w ith in ' 'a r h y o lite i15* c i o f Vein two 3" qtz vein* - — ------j lnhorizontally layered ^ ^-nvr-. ~ — — — 5" breccifrted silica fills vug in _ y vein with\qtz, cp, qtz veinlet in - A _____ s i , hm, c v \ c c , fo o tw a ll and mal
26" b r e c c ia w ith py - — — Hi' raise expose! on fragments) post-py, T- t ^ horizontally layei layered debris — — — — \ — — —
anti oxidation; original qtz about 6" thick debris fills 17" caV ity b a s a lt
^30°
_ py smeared on coarse ~ movement grooves 3" non-vuggy qtz-chl and qtz-hm vein py along fractures in sheared, altered walls; chi common on shears
2"-8" qtz-hm-sulfide vein with cp, si, cv, hm-rich gouge — — — — cc, and mal
CHAMPION MINE
DETAIL ALONG 900 DRIFT
Coordinates are in feet.
Broken lines represent vein filling; solid lines are shear planes and zon es. PLATE 18 ___about five qtz 6" qtz-hin-sl-cp on veins in footwall footwall of gougy v e in two 3" qtz-hm veins
about fifteen l" qtz veinlets; seem to approach tangency in — \ ------vicinity of vein
10" qtz-hm -sulfide vein
5" q tz
CHAMPION MINE pyroclastics DETAIL ALONG 1050 DRIFT
Coordinates are in feet.
Broken lines represent vein filling (mostly ----- post-sulfide quartz); solid lines are b r e c c ia shear planes or zones PLATE 19 PLATE 20
Sketch of a portion of a specimen containing horizontally(?) bedded hematitic chert (dotted) with contemporaneous chalcopyrite filling vugs in coarsely crystalline quartz. Datum is the medium fine-grained quartz comb coating all surfaces of an intermediate substage. Earlier deposits are medium-grained quartz and specularite followed by a layer of hematitic chert. Later deposits include three beds of hematitic chert and two of fine comb quartz.
About 5 percent of chlorite accompanies the hematite and chert in the upper layer. See photomicrographs of specimen on plate 23.
Noonday mine dump (Annie vein). I
chftlcopyrlte
CJI coarsely crystalline quarts o PLATE 21
Sketch of float specimen showing horizontally(?) bedded hematitic chert (dotted) filling veinlet cavity. Basal layer contains up to 50 percent fine speculari'te. Note corresponding chert on walls. Three horizontal?) layers of comb quartz correlate with a single comb of quartz on the walls. Elephant Lake prospect (west ern Crystal vein).
PLATE 22
Photomicrographs of vein quartz showing growth intervals separated by layers with hematite.
Early hematite is coarsest, and late quartz is flamboyant. Lower view through crossed polars. Pit on Alice vein. 152 PLATE 23
Photomicrographs of detail in specimen illustrated in plate 20. Upper view shows the overgrowth which coats the large quartz crystal. Nature of inclusions in layers within the overgrowth is un known. Lower view shows an overgrowth on a quartz crystal in the substage of quartz-specu lar ite-chalcopyrite that intervenes between the early, coarse quartz and the first hematitic chert bed. Opaque mineral is chalcopyrite.
The two types of overgrowth may be contempo raneous. Noonday mine dump (Annie vein). I------1 0. 4 millimeter
at i t PLATE 24
Photomicrographs illustrating contrast of tex tures in silica. Lower view shows quartz crys tals with narrow overgrowth and later flamboyant growth of quartz (crossed polars). The flamboyant quartz is intermediate to a milky white silica shown in upper view (1 cm away). Silica of upper view appears to have been deposited in an amor phous form with a crude colloform structure.
Crossed polars reveal that the silica has crys tallized to quartz with the colloform grains acting as nuclei. Narrow vein at saddle on Monte
Rico Ridge near line between sections 22 and 23. 154 PLATE 25
Photograph of a slab surface of layered silica in carbonate gangue. Probably in original orienta tion. "White mineral is ankerite which forms walls and also the unexplained, tabular masses that parallel bedding within the silica deposits.
Various grays are horizontally bedded silica with microcrystalline or finer deposits at the base. Note repetition of sequence on each surface of ankerite. Upper silica layers are microcrystalline and contain fragments of debris including ankerite. President mine, lower adit dump. 5 centimeters PLATE 26
Sketch of a dump specimen showing encrusted
ankerite and bedded ankerite and silica. Early
ankerite deposits consist of at least three crys
talline layers on fragments of vein and wall-
rock. Later, fine-grained, compact, horizontal
ly bedded ankerite contains fragments of early
ankerite. Contact with overlying silica is part
ly conformable. Vein crossing Weaver Creek
(section 3). cockade a n k e rite
• iv u m iiuiiune j.’ ••/irtH.tUmwu** /;,>...... y “! % -- 1 v,tMMiu,ivuir<«*„# ■• C N*>"“..%, \ 1 ...... , X J // XX; fragment *#UI»»V*' ^ - % of cockade nuartz \ I" /(/ X u
2 centimeters 156 PLATE 27
Photograph of a slab surface of an oriented sample of a secondary, debris deposit. Small white fragments are clay. Light-gray beds are recrystallized, microcrystalline quartz. Me dium-gray beds are composed of megascopic clastic debris. Darkest gray patches are rich in pyrite of unknown origin. Coordinates
7929 N ., 11250 E ., 900 level, Champion mine. 157
2 centimeters PLATE 28
Sketch of vein debris deposits in the oxidized zone of a carbonate-quartz vein. Fine manganiferous de bris is well bedded. Assays for gold and silver at several positions are shown. Upper adit, Coolidge vein, at a depth of about 10 feet from the natural surface. 158 1 O.Olt oz. Au, 0.20 oz. Ag leached quartz-carbonate vein; mostly quartz and liir.onite with minor pyromorphihe.
2 tra c e Ku, n i l Ag about 80 percent fine wad at base of horizontally bedded lens; 1/8 inch clayey layers at about one-inch interval*.
3 0.03 oz. Au, tra c e Ag massive to slightly bedded sandy debris; at top of same lens as sample 2
4 0.21 o z . Au, tr a c e Ag massive, unsorted vein debris with much quartz.
leached quartz-c arbonate t 1 fo o t t vein /
i S l rubble of argillized, - wallrock sheared 2 fragm enti countryrock
w allrock w allrock
fragments jbf leactfed qvlartz- carbjcnate vein
crudely bedded u n sorted fragments and sand w ith matrix of wad
Leached quartz-carbonate v ein
5 0.09 oz. Au, trace Ag moderately bedded wallrock debris and wad.
6 0.03 oz. Au, trace Ag bedded 6rind; no wad, no fragments; seven feet below the natural surface. 159
o r< t /
/ cT in so i f / i / I o/ I i "k i© Keechelus /+ 00 i i i =7 1 o' i CM I i / / *1 (f o / i o I CM ID O t
+ -h + o Keechelus J2
—
SILICA-VARIATION DIAGRAM OF MIDDLE TERTIARY LAVAS OF THE CASCADE MOUNTAINS PLATE 29 160
+ o + Nimrod / 4, o d* » + 4-/ 0* / / I Or< I I / / /O fi i \ 1 ; ?° 4 O 4 O 4 1° if) 1 1 1 1 (0 | I * 1 1 t l I • / 1 1 1 1 | / / 1 1 4 0 4 i ° 4 'o 4 o 4 /o ' 4- / o 1 1 l 1 l 1 / 1 1 1 1 1 i +1 1> 4 1 Snoqua Imle 1 J o l 1 1 / 7 o 4 o ,4 0 / '+ <£) 1 1 i i 1 1 1 i 1 1 1 o n i 4 o 4 Silver Star -t> o I U. I I l i I • 1 o f O | o(N ir i O X. _ i O) if) I O I in z 1 I / \ I O * 1 1 \ 4- o\ I P f. 1 f 1 4 04 f' o SiO
mO
SILICA-VARIATION DIAGRAM OF MIDDLE TERTIARY INTRUSIVES OF THE CASCADE MOUNTAINS; FROM BOHEMIA DISTRICT EXCEPT AS SPECIFIED. PLATE 30 161 1 ■ 1 mile--
6 5 , 65-t 65 A >V 40 V
north
8lA l
AX i I-
A Breccia mass \ i !
) v > m Metased I men ts \ \ \ 76 \ \ - \ \ 38
Veins X V \ X \ X I X \ X / GEOLOGY OF THE / / SULTAN BASIN DISTRICT to J V FROM CARITHERS l Bathol i th AND GUAR D ( 1945)
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188. i : ' ' f': / . V-S.. v -
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Moore, J. G. ^ Grants, A., and Blake, M. C., 1961, The quartz diorite
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:: ■ .V , xX : ■ ' ' ‘ -O . - 168
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Washington: Econ. Geology, v. 7, p. 340-350. LUTTON, PLATE I
Structur'd I el ements Lithological types
ARRANGED ACCORDING TO THEIR DOMINANT POSITION IN THE STRATIGRAPHIC SEQUENCE WITH OLDEST BELOW. AGES PROBABLY RANGE FROM LATE OLIGOCENE TO MIOCENE. CONTACTS OBSERVED, PROBABLE, AND POSSIBLE.
GRANITOID INTRUSIVES. ATTITUDES OF BEDDING AND LAVA FOLIATION
LARGE HYPABYSSAL INTRUSIVES IN PART FEEDERS FOR FLOWS.
FLOWS MOSTLY OF ANDESITE. CONTEMPORANEOUS MONOLITHOLOGIC BRECCIA (TRIANGLES), VOLCANIC CONGLOMERATE (CIRCLES), AND PYROCLASTICS SHOWN SEPARATELY. PROBABLY INCLUDES SOME INTRUSIVES.
PYROCLASTICS IN PART CONTEMPORANEOUS WITH ANDESITE FLOWS. WELDED TUFF (wt) DISTINGUISHED LOCALLY.
______PYROCLASTICS IN PART CONTEMPORANEOUS WITH DACITE AND RHYOLITE LAVAS.
FLOWS AND DOMES MOSTLY OF DACITE SPHE RULITIC RHYOLITE (DOTS) AND PYROCLASTICS SHOWN SEPARATELY.
PORPH YRITIC DACITE POSSIBLE FEEDERS OF DOMES AND FLOWS.
TUFFACEOUS SHALE AND SANDSTONE
MASSIVE PYROCLASTICS
no rth
y mile
BEDROCK GEOLOGY
BOHEMIA MINING DISTRICT, LANE CO. OREGON
Richard J. Lutton 1960-62
BASE MAP FROM U. S. G. S. FA I R V IE W PEAK QUADRANGLE E i m LUTTON, PLATE 2
f ( p (
ISOMETRIC DIAGRAM OF BEDROCK GEOLOGY
Bohemia mining district, Oregon
SEE LEGEND ON PLATE I /*oo LUTTON, TABLE 2
MODAL ANALYSES OF FLOWS OF THE BOHEMIA DISTRICT (UPPER UNIT) e m / /f(Z >6/ Number Location Phenocrysts Groundmass composition Bulk composition
1/ 2/ j/ unknown 1/ 2/2/ unknown plagioclase augite chlorite^ magnetite"^ olivine^ % of total plagioclase augite chlorite magnetite olivine or other plagioclase augite chlorite magnetite olivine or other
NE ridge of B-77 — N. F airview litn . 25.0 52 .3 22.7 — 61.7 25 .6 8.1* U.3 — $6.3 29.5 10.5 3.7 4*67-80 4*67-85 B-lliO high cliff near 80 .5 0 18.9 0.6 — ~ 30 5 6 .6 15.6 19.1* 8.1* 6U.$ 10.U 19.3 $.8 — " Golden Stairs prospect, sec .11 4*65-69 B-262 90.8 0 8.1 1.1 'Vf 25 63.3 18 .7 8.1* s .u ” U.2 70.8 13.6 8.3 3-9 0.3 3.1 top flow, summit S. Grouse Mtn. 4*62-80 “ " B-263 second flow from 75.U 0 21*.6 — 30 5U.3 37-0 3.U $ .3 60.5 26.1 9.7 3.7 — “ top S. Grouse Mtn. An8o-85 4*65-77 B-26L third flow from 81*. 8 1 .3 13.9 — “ ~ 25 57.8 30.1* 6 .5 5.3 — 66.1 21.U 8.8 3.7 " — top S. Grouse Mtn. 4*62-68 B-266 fourth flow from 72.9 l*.l* 22.7 — 1*0 51*.1 21*.9 13 .5 7.5 "“ 61.9 16. U 17.3 —— —— top S. Grouse Mtn. u.u 4*68-82 / 4*68-86 B-217 top flo w , 63-7 17.6 1 8 .7 — —— — 29 78.7 12 .2 3.U 3.8 " 1 .9 7U.7 13.6 7-5 2.8 " l.U Bohemia Mtn. ~ 4*70 B-220 second or third flow 1*2.7 31.6 19.7 “ 6.0 21* 78.0 13.0 5 .5 2.1* 1 .1 —— 69.8 17.U 8.8 2.2 1.8 — from Bohemia summit 4*68 4*68 4*68 B-221 third or fourth flow 1*6.6 3 9 .3 11.$ 2.6 ^ 28 70.1 12.6 8 .5 5 .6 3 .1 6U.6 18.9 9.2 U.3 3.0 from Bohemia summit - ^ 7 0 B-200 third flow from base 80.6 1U.3 2.5 2.6 —— — o f Bohemia Mtn. 4*70-8U B-196 lower flow, 66.6 1 6 .9 16.5 — f 27 63.2 31*.2 l.U 1 .2 — — 6U.1 29.5 $.$ 0.9 — “ Bohemia Mtn. 4*71 4*59 B-21k 800 feet north of 62. 1* 15.0 16.7 3 .2 2 .7 18 68.9 18.3 U.2 8 .6 67.7 17.7 6.5 7.6 0.5 " Musick camp 4*61* 4*61* 6.0 B-190 rock point, SE 80.5 2.8 0.2 u*u comer, sec. 11 hypersthene 4*70-91 6.1 amphibole B-86 flow (?), Kitten 71.7 1 3 .1 15.2 — minor 25 6 9 .2 23.1 3 .3 l*.l* —— 69.8 20.7 6.2 3.3 minor Rock 4*79-86 B-35 1000 feet west of 66.7 18 .8 10.1* l* .l —— 35 $2.5 23.8 11*.6 3-2 — 1 .6 57.U 23.8 1U.6 3.2 — 1.0 Shane Saddle quartz 4*68 quartz B-36 west of B-35 61*.1 1 6 .7 27.3 — p resen t 21* 6 6 .5 20.8 10.1* 1 .8 — 0 .5 6U.1 19.9 8.0 l.U 6.2 o.U An67 4*60-67 quartz quartz
1/ includes some secondary quartz and amphibole 2/ probably includes ilmenite 2 / pseudomorphs composed of chlorite and quartz U/ symbol (^ ) indicates "approximately" LUTTON, PLATE 3 lb l
Information froro the g e o l o g i c a l c.ops f Or#-go 19<1 and ;A'a;.h n r to n , 1 /6 Supplementar;. sources 125 ° include ‘ no lecuorJ c of the Uniteu tales, m i vc.aver ( IV 'i7 ) , Henriksen t1 ), Irwin ( l1 60}, an .1 otr.erc mer.1.1 one i in the t ext.
2. MONTE CRISTO
6. ST. HELENS 4. MILLER RIVER
7. SILVER STAR MTN. 5. BUMPING LAKE
8. NORTH SANTIAM
9. QUARTZVtLLE
PLIOCENE-QUATERNARY 10. BLUE RIVER VOLCANICS; CONES SHOWN SEPARATELY.
EARLY TO MIDDLE TERTIARY MOSTLY VOLCANIC; PLUTONS SHOWN SEPARATELY. II. BOHEMIA
PRE-TERTIARY mostly METAMORPHIC ROCKS; BEDDING STRIKES SHOWN
FOLDS AND FAULTS PRE TERTIARY,(HEAVY); TERTIARY,(LIGHT WITH SYNCLINES DOTTED)
VEINS STRIKES OF VEINS, MOSTLY STEEP; NUMBER SHOWN "5" NORTH MARK AT TOP.
AXIS OF MAJOR INTRUSIVE
50 MILES
AXIS OF DISTRICT based ON DISTRIBUTION OF PROSPECTS OR ON STRUCTURAL GEOLOGY TOPOGRAPHY. OF THE CASCADE MOUNTAINS METALLOGENETIC PROVINCE LUTTON, TABLE 3 i f i Z
MODAL ANALYSES OF HYPABYSSAL DIKES OF THE BOHEMIA DISTRICT /'&/
Number L ocation Bulk compost!tiion
l / 2/ 2/ unknown plagioclase augite c h lo r ite m agn etite olivine quartz or oth er
B-117 large mass south of Sweepstakes 63.8 1 9 .L 10.5 2 .3 —" — prospect, sec. 10 ^ 70- 8$ B-80 in roadcut south of 63.6 11. 1* 11*.8 2.2 —— — 8.0 amphibole Sweepstakes prospect ^ 62-70 B-103 on ridge west of Elephant Mtn. 6 3 .9 11.1 20.7 w minor —" **72-88 B-10U on ridge west of Elephant Mtn. 6L.6 16.0 17-3 2.1 minor " —— -4 * 8 2 B-95 w est o f Cat Mtn. 58.5 15.9 22.6 2 .9 — 0.1 —— 4*116-66 B-159 below LOO level dump, 71.5 15.3 8.8 3-3 — 1.1 —— Musick mine ~ 4 n ^
B-20ii on side of Bohemia Mtn., 1000 7 3 .6 1 0 .9 8 . 1* 5-8 — 1 .3 feet south of Musick mine (secondary) 4*7L B-2L in roadcut; Utopian road 6 7 .1 — 25.1 1*.8 —— 1.0 2.0 near Sharps Creek road, sec.10
B-182 south of cliff. Pilot Rock 70.7 8 .3 16.0 1 .7 —— 0 • 3 — (secondary)
B-208 on trail, 300 feet east of 72.2 6 .6 11.3 2 .7 —— 0 .1 7 .1 President mine An66-76 B-210 300 feet north of Martha W 75.2 18.8 0 .1 5 .9 " —— p ro sp ect, s e c . 2li 4*62-76 —— dike (?) in Judson Rock Creek 6 9 .3 22.3 1 .6 6 .8 —— —— — 800 feet upstream from Sharps Creek road, sec. 9
1 / includes some secondary silica and amphibole j/ pseudomorphs composed of chlorite and silica
2/ probably includes minor ilmenite LUTTON, TABLE 4
EWV MODAL ANALYSES OF GRANITOID INTAUSIVES OF THE BOHEMIA DISTRICT iq te / (e> / Number Location Bulk composition Rock type on p la te 5 y 2/ plagioclase a u g ite c h lo r ite m agnetite quartz orthoclase amphibole
75 f e e t from Champion v e in , 5-22*0 in roadcut east of Champion 31*.0 0 .2 13.1* minor 22.1* 25.8 3 .8 granodiorite Saddle porphyry
B -2itl 30 f e e t from Champion v e in , 37-5 26.8 1 .1 1 8 .7 1 5 .9 — granodiorite porphyry
B-2U5 25 f e e t from Champion v e i n ,, 5 2 .1 minor 1 1 .0 — 11.8 1 0 .2 11*.9 granodiorite porphyry
8-21*8 on Champion Creek road near 55.8 5 .2 16.1* 7 .5 1 5 .1 granodiorite Golden Curry Creek **1*5-52 B-2U7 on Champion Creek road near 30.8 —— 8 .0 — 26.5 2 5 .0 9 .7 granodiorite Nelson Creek *^ 5-U 9 B-I76 near ttienzi prospect, ^ 7 3 " 1 1* 2 minor 20 porphyritic s e c . 11* *"30-55 augite-quartz d io r ite
1/ includes some epidote, especially near the vein
2/ uralite, except in B-176 which contains primary hornblende £ ■ ???/
LUTTON, PLATE 4 Ibt
6 s \
9 < y 3'
7' **>%• 85
\ APPROXIMATE* STRIKE FREQUENCY DIAGRAM OF / 106 STEEP, BASIC DIKES
SHADED PORTION IS BASED ON 61 DIKES THAT WERE OBSERVED TO T 22 S HAVE DIPS OVER 70" ; REMAINDER T 23 S ARE PROBABLY OVER 70"; 9 DIKES OBSERVED TO HAVE DIPS LESS THAN 70" WERE NOT PLOTTED.
\ 74X section line
80x v 31 \ / x GLENWOOD SILL \ •/ \ 85 DISTRIBUTION OF 90 OBSERVED & / x 30* \ 4 5 BASIC DIKES 30* \ BOHEMIA MINING DISTRICT, OREGON
60 / DIKE; DISCORDANT CONTACT OR OTHER DIAGNOSTIC CRITERION WAS SEEN. \ PROBABLE DIKE; included IN DIAGRAM ABOVE . DIP AND THICKNESS OF DIKE .
THE TRENDS ARE MOSTLY THE STRIKES BUT IN PART,THE OUTCROP TRACES.
APPROXIMATE STRIKE FREQUENCY DIAGRAM OF 47 MESOZOIC QUARTZ DIOR ITE DIKES IN THE RIDDLE QUADRANGLE ( DILLER AND KAY, I924 ); 50 MILES TO THE SOUTHWEST. LUTTON, TABLE 5
I j b l /&/ GOLD ASSAYS
from a portion of the liusick mine, 700 level
ASSAYS width Au (in.) (oz.)
18 .Ik 38 .08 38 .06 30 .16 10 .12 Ik .Ok 2k .Ik 39 .08 53 .06 k2 .12 k8 .06 k8 .08 51 .10 52 .06 36 .Ok k2 .06 52 .Ik 30 U.98 16 13.1k 20 1.67 32 1.01 U2 • 5U 1.62 30 .Ik $ 2.32 2k .Ok 18 .02 25 .Ok 39 .Ok 58 .06 GOLD ASSAYS 58 .08 from a portion of the Champion 30 .08 mine, 600 winze Ik Ik .8 drift 18 .52 assay map from records of «J. C. Higgins; not verified in current project assay map from records of J. C. Higgins; not verified E m i LUTTON, PLATE 5 'PZ ------1 tfal
v l GRANITE PORPHYRY EPIDOTE NODULES developed \ ' OR APLITE DIKES. AT WEST EDGE OF CONTACT . ALTERATION AUREOLE .
Dpq
GR ANODIORITE (Gd) or G R A NODI 0 RITE PORPHYRY (Gdp). BRECCIA. GR ANODIOR ITE WITH QUARTZ
BIOTI T E LOCALITY
DACITE PORPHYRY (Dp) V . ' \ • . . . . • x WITHOUT QUARTZ PHE NO- ‘ /Gdqr CRYSTS (Dp ); WITH G.dq > / i •' QUARTZ PHE NOCRYSTS (Dfx^). % y z . PORPHYRITIC AUG ITE - M e QUARTZ DIORITE (Qd ). • . .• 7 \ . • / • / E AUGITE DIORITE (Ad). . .>i I . * . ppq-z.v
/ . a v
. z * / ; • 1 45 TOURMALINE disseminated •:/ B AND ON FRACTURES. C; /50- / CONTACT-ALTERED ROCK • / / ■ # * z ...... I I z / / . • • I •I \r. • / / . . I DP . Gdp ' V-
. . ■ ^4 Gdp- • * NORTH ' "l/Gd . Gdp\ \
• • . • Gd . • % . / ^ .1 / f . 1 MILE z /•. • : / i " r ' p ; r i . . &
•i i * • • • . • & v • •i B - .1 • \ n Gdp f ; \ w •V* V' V: Dp • \ Gd • .v.. /
X Dp A ••• 'A*.y %: A Gdp Gdp
\XX • ^ • • • y -t Qd V • • . . Dp x- ( l i) B \Gdp\ /Dpq 1 Xv-X : \ • ' . X \ Gdp 1 \ >>K' i k' -O - _> \ • Gdp
V
Gdp. Gdp
. ; • . • . . . - . f•-= » / •
Gdp Gdp >.4«: k I % . # . • • - Dpq •v>.
- y f e j y .
. •Vt •' • *
* < ^ *• Dp v> Gdp #* • • ; •* • Gdp • . • • • • • • . . . • 'Gdp
/ / Dp ,'F ..Gdp j ! z Z Gd___ ~-> / • ' .QdBfr • / / . -(z fx . • . z Gdp x ^ x r ^ / , Z % Gd •v c^** ------*«£!>-> o
. o \Z dp DP v/ ; •■ -U,r " xO X ' i Dpq ^ J 6'
■W \V
Z O Dpq / T
s X \ Dpq N X
Dpq
% GRANITOID INTRUSIVES
Dpq \ *: Bohemia mining district, Oregon
\ LUTTO.N, TABLE 6
MINOR ELEMENTS IN SULFIDES £ VJt (semiquantitative spectrographic analyses by Thomas C. Matthews) / 6 f A - greater than 10$; B - 10 to 1$; C - 1 to 0.1$; D - 0.1 to 0.01$; E - 0.01 to 0.001$; T - below 0.001$ but present * Sample L ocation Description Fe Cd Mn Cr Co Ni Bi Cu Pb As A1 Others (v ein ) (colors are of fragments about 1 mm acrossj
21 vein crossing Lead honey brown octahedra B B CTE E — DD TE Crystal road near lining vugs sec. line 11-12
22 G rizzly honey brown to B B C+ T TEECC- T E dark brown Elements that were sought but not found (except as - 23 West Helena transparent B- BD- E E C- D+ TE specified) include: Sb> Au, and Sn and the follow 2U Maine honey brown C B— E - ET - DD E T ing elements which are diff icult to identify. As, Ga, - TT 27 Helena canary yellow, later B-BE- D E DD Ge, In, T1 than hematitic chert
28 Salesman greenish yellow B- BD- EE - CCEE
29 P ro fesso r B— BD- E E - C- C-EE
30 vein east of top of yellow to brown B- CD - B E - c - EE Elephant Mountain
31 Autunite claim, east honey brown B- B-E - - E - C— DE E of Salesman, sec. 17
32 P ro fesso r honey brown B— B— E - E E DD C- E E
3U Cripple Creek probably some galena B B— D+ E EE - B— B- E E
35 Hidden Treasure yellow, no other sulfide B B D - E E - D C-TE in the specimen
36 E l D greenish yellow to honey B B- D T D+ E E B+ C- D- T brown with covellite
37 vein crossing Weaver B- - D - EEDCC-ET Creek, sec. 3
38 Albany B- - D- E E - CD T T
39 Faber vein, south zoned with nearly opaque BB D - EE — c C- ET of Maine vein dark layers
LO Salvador pale greenish yellow with B— B- D - B EE c B- D T patches rich in galena
U1 San Marino honey brown to yellow B B C T ED-E c+ B- EE Sn (E) h2 Norway claim , top o f brown CCDE - T — c E E T Noonday Ridge, sec. 36 a V in d icator yellow to brown CC E E E T- c D T T
US Santa Barbara light yellow to brown B c+ D - - T - c+ B- D- T Sb (D), As (D) * - U7 West Helena (west end) honey brown B c+ D E T E c+ C- E E
U8 vein south of Brice brown B c+ D — - T - c D E E Creek, sec. 30
6 (dupl) P lato brown B c+ D — E T - c- C- ET
* Sample L ocation Description As Co Mn Mo Ni Ag V Zn Pb Cu A1 Others
22 Helena late, cubic form - - .05 T T - TT .01 .01 T 2U B lack b ird , N^, s e c . 18 late crystalline crust - T .01 T .05 .01 T - .01 .01 T Ti ? (T)
25 vein on east side of cube and pyritohedron .5 - .01 TT .01 TTT .01 .01 Elephant Mountain forms * Elements that were sought 26 Autunite claim, east pyritohedron and cube •5 - .01 T .01 TT .05 .01 .1 T but not found include Or, of Salesman, sec. 1? forms Sb, Bi, Au, Ti (?), and Sn; In was sought but 28 northwest vein near II II .1 T .01 T T T T .05 .01 T T could not be identified. Teddy v ein As values are probably not very accurate. 9t It 29 Monroe -- .02 T TTT .05 .01 .05 T
30 vein in upper Jumbo -- .01 T .01 -T T .01 T Creek, sec. 31
30b n n pyritohedron and cube .5 - .01 T .01 T T .05 T .05 T forms
32 Ingham late, cubic form .1 - .01 T TT T .05 - TT
33 "cross vein" west of pyritohedron and cube - .01 .02 T TT T .05 .05 .05 T Peekaboo vein forms
36 Norway claim , top o f it ii .5 - .01 T T T T .5 .05 .1 .01 Noonday Ridge, sec. 36
37 Johnson vein near cube and pyritohedron T - .01 T T T T .1 - .01 T Vindicator vein forms t~?79/ LUTTON, PLATE KfU l £ l VEINS
Bohemia mining district, Oregon
VEINS. TRACE ON SURFACE; DIP AS / / * SHOWN; DASHED, PROBABLE, / lee / DOTTED, POSSIBLE OR TAKEN FROM CALLAGHAN AND /#
BUDDI NGTON (1938). .0
z" w rv / contacts / >x PLUTONIC ROCK I \ MOSTLY PROBABLE. a U j / N I Z a i Z BRECCIA. COMMONLY HAS / CRYSTALLINE OR CHERTY QUARTZ LINING OR FILLING z INTERSTICES. E PI DOTE, SULFIDES, * MAGNETITE, OR SPECULARITE PRESENT LOCALLY.
TOURMALINE disseminated and on fractures. — ------# l#
.... "• # 9 0 ^ x TOPOGRAPHIC CONTOURS at: 800-F OOT INTERVALS. 1 ...... KATMIA I
65 55“ X 7 0 7 5 / /> 1 //, , .4
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/; I / Vv tii:. I x \ f ' \ x \ 80 *=■ CT> LEROY A 8 0 ° ' STONEWALL 80/^0' f X '7 x 1 FOUR MONTE 1 : X / ORO X Z x Vvx V X .' BUGLER LUCKY ; ELSIE : 7 5 -. ■ 4 0 0 0 63 WEST H ELEN A \ 524&BA#; HI DDEN^- 8 5 \ •. X VESUVIUS HELENA \ PROFESSOR —XL- \ x < s a : ^ # > " 7 CALIFORNIA X • 8 5 SALESMAN" YUKON •9 d. / * .. • v •■•-••-CHAMPIOr - ^ " x ^ ctLS,OR N a n n i e *.. > s ( NOON DAY L. ^ x vT— : MAGGIES^ >. X X^ X L F I N I SZ \ N >v 85 >. X .8 5 OPHIR < eo,6hT X ^ %• MYRTLE X ;-80\ XZ>Z50>X 60 x O y vz 60 e z I ALPHARETTA ARLINGTONZ Y^RI' V YELLOWJAC KE T ALICE TI PPERARY X: BOON v/' 6 5 9 0 \ 1 / x % PEEKABOO,^/ A60 6 0 65 66 ... . \ Ab PICKE TT’ S MAYFLOWER BULL \ teddy CHARGE so y - > < LEAD .-z. • 67 DEWE Y CRIPPLE ^RATTLESNAKE WESTERN •CREEK \ 75 \ e o / 90 X / / X 8 5 61 X X •• OLD HENERY 8 5 N \ ; V Rl VERSI DE \ MONTE \ . PRESIDENT RICO g^ X DREAM X MONROE 60 X 75 \V OREGON-COLORADO 80 V\ AN Tl MON Y MARTHA W 1••••w. X 70^X 80- X £ ? /? / IfiZ LUTTON, PLATE 1 > u 197 JOINTS IN RECTANGULAR AREA OVER THE STOCK POLES OF JOINTS PROJECTED AND CONTOURED ON A 90 SCHMIDT EQUAL-AREA NET 9 0* (CONTOURS AT 2.5,5, 7.5, 10, AND 12.5 PERCENT PER I PERCENT OF AREA) 216 JOINTS OUTSIDE RECTANGULAR AREA N V 90 75•V 90 4 , V X a - K 7 ~ * ^ \ * - * o , 90 " 8 0 $ V .7 /' 90 90 N^ao % 90 7 90 x 80 ^ 9 0 90*^"" . X ^ s o ^ x 9 0 1 85 e o \ JOINTS STEEP JOINTS V\ x ONLY \ 9 o \ ^ ' n 80x — V * 90 V NORTH APLITIC DIKE ^ . 4 5 9 5 / J r 1 t p C 5 . 90 PLUTON CONTACT i-MILE JOINTS Bohemia mining district, Oregon LUTTON, TABLE 7 1 Contacts Metamorphism Metasomatism Hydrothermal quartz pm/ Intrusives ------vein deposits If is e f f e c t s in s iz e rock in te r n a l name contact wallrock near contacts at a distance f 6/ 32? (sq. ml.) type stru ctu re Blue River Miocene? 1/8 augite diorite sharp no major narrow aureole propylitization ? pyrite, sphalerite, chalcopyrite, galena, dacite porphyry deformation tetrahedrite, adularia, carbonates, barite Nimrod 3$± 10 m. y. biotite granite orthoclasization of stock sto ck Bohemia late Oligocene h granodiorite none sharp minor warping recrystallization epldote-chlorite tourmaline-quartz-sericite pyrite, sphalerite, chalcopyrite, galena, stock or Miocene quartz diorite and silicification -pyrite propylitization tetrahedrite, stibnite, hematite, epidote, johannsenite, adularia, carbonates, barite epidote-albite-quartz propylitization , , sharp no major recrystallization epidote propylitization silicification and molybdenite, chalcopyrite, pyrite, Silver Star post-Keechelus 20 granodiorite local planar deformation and silicification orthoclasization sphalerite, galena, tetrahedrite (?), Mtn. stock (M iocene?) with augite and lin e a r silicification of granodiorite carbonate (?) d io r it e elem ents quartz-andulusite dumortierite pyritization tourmaline-quartz-sericite (partly metasomatic) sharp no major — quartz-epidote propylitization quartz-albite-orthoclase molybdenite, pyrite, arsenopyrite, St. Helens post-Keechelus U0 quartz diorite deformation a n d esite (large range in -tourmaline-calcite pyrrhotite, chalcopyrite, sphalerite, (M iocene?) composition) galena tourmaline-specular!te -magnet!te-epidote-quartz discordant local folding; biotitization silicification tourmaline-quartz molybdenite, scheelite, pyrite, Snoqualmie 18*- m. y. 2$0+ granodiorite to complexly conformable batholith with quartz arsenopyrite, pyrrhotite, lolling!te, penetrating structure in local garnet, diopside, sericitization pyrometasomatic magnetite chalcopyrite, sphalerite, galena, diorite, biotite th e Sultan andalusite, titan!te, replacing limestone ruby silver, stibnite, magnetite, g r a n ite , and d is t r ic t magnetite, and quartz propylitization a p lite apatite, carbonates chalcopyrite-bomite -q u artz CHARACTERISTICS OF DEPOSITS OF THE CASCADE MOUNTAINS /ffZ fZ LUTTON, PLATE 8 / ^ / O: 300 level GEOLOGY OF THE / 40O level o ut c r o P CHAMPION MINE »ppro \ro Bohemia mining district, Oregon mostly ; .... ? stoped a RICHARD J. LUTTON 1961-62 500 level AREAS BEYOND TRIANGLES (A) '.".'AV6V a. a .'Yi. AND ON 600 AND 800 LEVELS a:::..'. / 600 level FROM NOTES BY MANNING W. COX. mostly stope / / BASE MAP BY H 8 H MINES CO. / 700 level STOPE OUTLINES ESTIMATED / / BY KENNETH 0. WATKINS OR FROM CALLAGHAN AND / 800 level BUDDINGTON (1938). 7 partially oxidized partially oxidized oxidized - / (according to K.O Watkins)' :r.v / . 0. / 900 level / partially ox id ized 5'/ a: C i n. 1050 level 5 —I & X 1 *- Longitudinal section SI ^ < • ~ 1200 level I e> II S § > O -H O t> H O tt) d fl> +> to ? to mood r—i 5 2 C I fn 5 £ o I Composite V plan view •H O e> H d O rH •H I to 0) o •H £ 03I 1.6 2.6 ALKALIES IN SOME IGNEOUS ROCKS OF THE WESTERN UNITED STATES 5.2 AND ADJACENT CANADA AND MEXICO. MEASURED AT POSITION ^S.q+KgO-tMgO + FeO + CaO) = 15 13.1 8.4 1:4 9.9 6 S & l°0.l 13.8 7.898 7.0 88 7.2 7.5 I1.8 9 1 7.7 7.8 8.5 9.6 9.6 8.4 9.8 13.2 7.3 7.9 7.9 8.7 8.7 LUTTON, PLATE 10 components measured T CHEMICAL VARIATIONS IN CASCADE MOUNTAINS LAVAS (at position Si02 = 60 percent) f ? 7 f / t f i i /U \ \ X \ MT. GARIBALDI M 3 8 3 5 6 0 2 9 5 9 *0 8 18*8 \ \ \ \ \ \ \ t I \ \ \ \ \\ \ ' 5 7 17*7 WIT. BAKER ... l 1 8 42 h . 5 5 2 9 0*8 <60*1 1 I 1 1 » l \ \ \ I i \ I » \ I 1 \ i \ I I l \ \ I i I / I \ l \ 1 l \ I / i I ; WIT. RAINIER ... I 1 5 3-4 6*1 17-1 -61*5 4 0 2-7 ’5-9 . ^ / 0-9 i \ / V \ » l I ' i WIT. ST. HELENS 1-0 \ 3*9 3 9 . : ~6-lt 3 1 / 5-9 0 6 63 2 I \ l z \ \ • \ l I I // ' I l \ \ \ *. \ i I I I I / \ : \ \ I I : i i \ \ '. \ N. SANTIAM R...... i , / \ I : ) *• i 3 1 5*8 0-8 18*3 • 61*8. (ALDRICH-STRAWBERRY MTN) M l 4 4 4 0 • 6-3/ • • # 19 3*4 I 1*0 17*0 • / / ~ 63*5 NEWBERRY CRATER ...... / / 1-9 II l I , l / Z 2*4 / 8 0 I 16 7 /58* (WESTERN CASCADES! / 'IT I v 4 8 // 2-8 2*2 / 3*9, 1*8 / / / 5 3 , / // 2*5 I \ z /ft ,, i i ' 14*6 58*6 I 7 / // 1 I i 1 I 5*9 -62* | CRATER LAKE ...... w 4 0 3-1 6*3 * 3*2 0*6 I 180 6*1 - 17*1 - 58* (STEENS MTN) ...... I / 2*3 3*5 1*5 I I l 6*8 \ I - 0*2 : i : I I i \ \ \ I I I \ \ WIT. SHASTA v . i x a 7 0 -0*51,2 18*37 2 6 3 ^ 5 MEDICINE L." 1*21 7 4I3'6 6*85*6 ~3 93*2 \ \ t\ : \ \ \ x \V X \ ».Y \ \ \ LASSEN PK...... 3*6 2-6 6 8 ~3'5 5*7 0*6 17*8 63*9 \ AI 2 O 3 A-L index k2o Nq20 Na2C>/L o CaO MgO F<220 3 T lO ; Ef?f/ If i8. LUTTON, PLATE II HI FIVE SIMILAR PETROGENETIC PROVINCES JAVA 100 miles 100 miles, 100 m iles 100 miles 100 m iles HOKKAIDO ALAS KA OREGON Hanaoka K hrebet B ystri nskiy Bohemia Lebong district district KAMCHATKA Pri bi lot Islands SUMATRA WASHINGTON HONSHU KAMCHATKA SOUTHWEST TERTIARY VOLCANISM AND ORE DEPOSITS ALEUTIAN NORTHWEST NORTHEAST SUMATRA ISLANDS UNITED STATES QUATERNARY VOLCANISM HONSHU LUTTON, PLATE I2 £9111 I9M /it CHAMPION MINE ISOMETRIC BLOCK DIAGRAM WEST 1050 LEVEL STORE