geochemistry and structure of tertiary...
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Geochemistry and structure of tertiary volcanic rocksin the southwestern Monte Cristo Range, Nevada
Item Type text; Thesis-Reproduction (electronic); maps
Authors Hambrick, Dixie Ann
Publisher The University of Arizona.
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Link to Item http://hdl.handle.net/10150/557990
GEOCHEMISTRY AND STRUCTURE OF TERTIARY VOLCANIC ROCKS IN
THE SOUTHWESTERN MONTE CRISTO RANGE, NEVADA
by
Dixie Ann Hambrick
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements For the Degree of
MASTER OF SCIENCE '
In the Graduate College
THE UNIVERSITY OF ARIZONA
:i 9 8 4
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
Datbs Jr "
P. E. DamonProfessor of Geosciences
ACKNOWLEDGMENTS
The development and completion of this thesis was realized
through the help and guidance of many people. I would like to thank
all members of the Tucson office of U.S. Borax for their support during
the course of this study. Particular gratitude is extended to Barry
Watson, who helped formulate this thesis topic and provided guidance,
assistance and much pertinent information during this project. The
financial assistance provided by this office helped make the chemical,
isotopic and radiometric work done in this study possible.
My gratitude also goes to Dr. P. E. Damon whose knowledge of
geochemistry and volcanic systems helped improve this project. Funding
for the analyses performed in this study was partially provided by the
Laboratory of Isotope Geochemistry. Appreciation goes to all members
of this laboratory for their assistance. A special thanks goes to
Drs. M. Shafiquallah and D. Lynch, who taught me the tedious labora
tory procedures of isotope work.
I would like to extend my appreciation to Dr. J. Stewart of the
U.S. Geological Survey for his interest in this study, and for sharing
with me his knowledge of the geology of the Monte Cristo Range.
iii
TABLE OF CONTENTS
LIST OF ILLUSTRATIONS . . . . . . . . . .
LIST OF TABLES ................. .. . . .
ABSTRACT o @ o o o @ e o @ p o @ @ o @ @
1o INTRODUCTION . . . . . . . . . . . . .
Purpose and Location of Study . . Methods of Study ..............
20 REGIONAL GEOLOGY AND TECTONIC SETTING
e o o o o Vllo e o o e I X
Page
30 ROCK UNITS
Pre-Tertiary Basement . . . . . . . . . .Tertiary Volcanic and Sedimentary Rocks
Castle Peak Volcanic Sequence . . . Castle Peak Tuff1 o T Cp o o o e o o e e e o e2 o T Cp o o o o o o o o o o o3 o ■ T Cp o e o o o o o o o o o 4,o T Cp 0 o o o o o o o o o o oBanded Rhyolite Intrusion p » .1O Tri O O O O O O O O O O O O
Coaldale Volcanic Sequence 0 . . . .. Sediments and Tuffs e . . »1O Tstl o O O 0 0 - 0 9 o o o o2 o Tstu O O O O O O O O O O OHornblende Andesites 0 » » o »1o That O O O O O O O O O O O 2 o Thai O O O O O O O O O O O 3o Thai O O O O O O O O O O O. 4 O Tha O O O O O O O O O O O OFine-Grained Andesites 0 » » o1o Tfai o o o o o o o o o o oPyroxene Andesites 0 » o « » »1 o Tp 3-t O O O O O O O O O O O2 o Tpai o o o o o o o o o o o3 o Tpa O O O O O O O O O O O O
O O O O o o o
O O O O O o o
o o o o o O o
111212121414161921212122222223232628313434343536 39
iv
V
TABLE OF CONTENTS— Continued
Page
Intrusive Rhyolite . . . . . . . . .1 Ti-*-e J.O. o o o e o o o o o o e e . o o o
Blair Junction Volcanic Sequence ... .. . .1 T c-*-e J.o-/ 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0
2 o T C a b o o o o e 0 0 o o o o 0 o 9
3 o T e a 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
a T-fa• o. 1 J- CL 0 0 0 g 0 0 0 0 o o 0.0 o 0 , 0
Younger Sedimentary and Basaltic Sequence1 Tp .J.O A w Q 0 o O O O O O O 0 O O O O O
2 Th— ' o ls p o 0 0 0 o e o o .o 0 0 0 0 0
Quaternary Deposits . . . . . . . . . . . . .lo Q . . . • e e o e e o o o o o o o2 e QSp 0 0 . e . . . . a . . .
40404242424445464647 47 47 47
4. AGE AND CORRELATION OF TERTIARY UNITS
Castle Peak Volcanic Sequence . . . . . . . . . . . . . . 50Coaldale Volcanic Sequence . . . . . . . . . . . . . . . 54
' Blair Junction Volcanic Sequence . . . . . . . . . . . . 57Younger Sedimentary and Basaltic Sequence . . . . . . . . 57
5. GEOCHEMISTRY . . . . . . 59
Whole Rock Chemistry . . . . . . . . . . . . . . . . . . 59Isotope Geochemistry . . . . . . . . . . . . .......... 71
6. STRUCTURAL GEOLOGY . . . . . . . . . . . . . . . . .......... 74
F o l d S e o ft e e e e o e 0 . . .Faults . . . . . . . . . . . .Hornblende and Pyroxene Dikes . Discussion o’. o e e . o e o ft
74
7. ECONOMIC GEOLOGY . . . . . . . . . . . . . . . . . . . . . . 86
Formation of Borate Deposits . . . » . . . . . . . . . . 87Distribution of B in the Southwestern Monte Cristo Range . . . . . . . . . . . . . . . . . . . 90Cause of B Anomalies in the SouthwesternMonte Cristo Range . . . . . . . . . . . . . . . . . . . 104
Original Magmatic Composition . . . . . . . . . . . . 105Secondary Hydrothermal Alteration . . . . . . . . . . 106
Age of Alteration in the Southwestern Monte Cristo Range . . . . . . . . . . . . . . . . . . . 109
viTABLE OF CONTENTS— Continued
8. TERTIARY GEOLOGIC HISTORY . . . . . . .
9. CONCLUSIONS . . . . . . . ............
APPENDIX A: SAMPLE DESCRIPTIONS . . . . . .
Page
112115
119
REFERENCES 136
LIST OF ILLUSTRATIONS
Figure Page
1 e Index Map of a Part of Southwestern. Nevada . „ » , * .
CM
20 Geology of the Southwestern Monte CristoRange 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 in pocket
30 Structural Features within the Walker Laneof Nevada and California 0 0 0 0 0 0 0 0 0 . 0 0 0
00
4. Texture in Unwelded Member of the CastleP eak Tuff © o . © © © © . © © © © © © © © © © © © © © © 15
5© Multiple Cooling Units of the Castle Peak Tuff in the Northwestern Portion of the Study Area © e © © © © © © © © © © © . © © © © © © © 00
6© Exotic Block of Oxidized Feldspar-Rich Rhyolite in Unwelded to Partially Welded CastleP©ak Tuff © © o o o o e o o o o o o o o e o o o o
OCM
7© Series of Photographs that Depict Textures inthe Hornblende Andesite Tuff Breccia © © © © © © 0 © o o 25
8© Silicified Lower Portion of the HornblendeAndesite Lahar near Blue Mountain © © © © © © © * CM
9 © Blue Mountain Intrusion © © © © © © © © © © © © © © © © . . . 30
10© West Flank of Coaldale Ridge © © © ,© © © © © © © © © © „ . . 32
11© Pyroxene Andesite Tuff Breccia nearBlue Mountain © o © © © © © © © © © © © © © © © © . . . 37
12© Small Pink Rhyolite Plug in the SoutheasternPortion of the Study Area © © © © © © © © © © © © o o . 41
13© Blair Junction Volcanic Sequence © © © © © © © © © © © . © . 43
14© Structural and Geochemical Overlay © © © © © © © © © © in pocket
15© Geologic Sections Across the SouthwesternMonte Cristo Range © © © © © , © © © © © © © © © © ©
vii
in pocket
viii
LIST OF ILLUSTRATIONS— Continued
Figure Page
17. IUGS Classification of Chemical Analyses,Based on Normative Mineralogy ................. ' . . . . 64
18. Variation Diagrams of Chemical Analyses,Differentiated by Volcanic Sequence . . . . . . . . . . 65
19. Triangular Variation Diagrams of Chemical Analyses,Differentiated by Volcanic Sequence . . . . . . . . . . 66
20. Oxide-Silica Variation Diagrams of Coaldale andBlair Junction Chemical Analyses, Differentiated by Rock Type . . . . . . . . . . . . . . . . . . 68
21. Oxide-DI Variation Diagrams of Coaldale and BlairJunction Chemical Analyses, Differentiatedby Rock Type . . . . . . . . . . . . . . . . . . . . . 69
22. Equal Area Stereonet Plots of Strike Data . . . . . . . . . 75
23. Equal Area Stereonet Plots and Frequency Diagramsof Fault Data . .. . . . . . . . . . . . . . @ .. . . 78
24. Equal Area Stereonet .Plot of Trend and Plunge ofFault Slickensides . . . . . . . . . . . . . . . . . . 7 9
25. Equal Area Stereonet Plots and Frequency Diagramsof Dike Data . . . . . . . . o . . . . . . . . . . . . 83
26. Caved Shafts on the Western Flank of CoaldaleRidge o e e o . ' e . o o o t t o o o o o o o o e . e e o-o 88
27. B-Sr Variation Diagram of Average Values . . . . . . . . . . 100
28. B-Li Variation Diagram of Average Values . . . . . . . . . . 101
29. B-Sr-Li Triangular Variation Diagram of AverageValues . . . . . . . . . . . . . . o o . 0 . 0 0 . . . 102
30. Sericitized Hornblende Andesite Flow(?) on theWestern Flank of Coaldale Ridge . . . . . . . . . . . . 108
LIST OF TABLES
Table Page
lo Average Thin Section Modal Percents ofTertiary Units o o o o o . o o . o o o o o o o e o o o o o 13
20 Analytical Data for Age Determinations ofTertiary Rocks „ <> o <, o « » o » «> <> «»«<,«, o <, e « «, » 51
30 Chemical Analyses (in weight percent) ofTertiary Units , . o , „ . . „ „ „ . „ , „ » . . . * 60
4o Barth Normative Mineralogy (in weight percent)of Tertiary Units „ . . „ * „ , „ , * , 61
5» IUGS Classification of Chemical Analyses ............... .. . 62
60 Analytical Data for Initial Sr Isotopes ofTertiary Rocks @ o o o o o @ e o @ o @ o @ o o o e » @ 72
7e Sx9 and Li Geochemistry of Rock Chip andSoil/Fluff Samples o o o o o o o o o o o o o o o o o o 91
8o Average B5 Sx9 and Li Geochemistry......................... 97
9o Comparison of Average Fresh Rock B5 Sr5 and Li Geochemistry to the Average Composition of Similar Rock Types and Clarke Values ofTurekian (1972) » o'© © © © © © © © © © © © © © © © © © 103
ix
ABSTRACT
The chronological, geochemical and structural history of
Oligocene to Miocene rhyolites, latites, trachytes and sediments in
the southwestern.Monte Cristo Range exemplifies the complexity of a
small, discrete volcanic center. Geologic, isotopic and geochemical
constraints deny a genetic relationship between the four volcanic
sequences in the area. Prominent north-trending, high-angle faults with
minor right-lateral displacements were caused by east-west Basin and
Range extension. Secondary conjugate shear stress formed moderate displacements on east-trending, left-lateral faults. These deformations
are compatible with the regional Walker Lane and Warm Springs discon
tinuities.
Widespread boron anomalies in the area were caused by a
Pliocene(?) hydrothermal event. A sericitized alteration center is
surrounded by clay alteration, an opaline vein system and weak B
enrichment in the permeable volcanic rocks. Mobilization and/or
leaching have caused localized, B-rich surficial accumulations.
x
CHAPTER 1
INTRODUCTION
Purpose and Location of Study
The southwestern portion of the Monte Cristo Range, Esmeralda
County, Nevada, is composed of Tertiary volcanic rocks and lacustrine
sedimentSo Initial reconnaissance sampling by UoS* Borax has shown that the volcanic as well as the sedimentary units are at least super
ficially enriched in B, Sr and Li (Bo Watson, pers, conmio, 1982), This
anomalous condition is unusual in an interstratified volcanic sequence;
B enrichment in lacustrine sediments is not uncommon and many major
borate deposits are known to occur in similar sequences (Bates, 1960)«
The structural and depositional history of the Esmeralda Formation
sediments in the Blair Junction area of the Southern Monte Cristo Range
has been described by Moore (1981), although no detailed study of the
adjacent volcanic rocks has been done. This study was undertaken to
provide the basic geologic and geochemical evidence necessary to attempt
to explain the cause of the anomalous B enrichment of the volcanic
sequence in the southwestern Monte Cristo Range.
The project area is located on the southwestern flank of the
Monte Cristo Range approximately 60 miles west of Tonopah in south
western Nevada. Figure 1 is an index map of this part of Nevada and
shows the geologic map area (Figure 2). The project area lies,within
the Coaldale and Blair Junction 7.5 minute quadrangles. Two predominant1
118*15' 118*00' 117*45 ’ 117*15'
Figure 1. Index Map of a Part of Southwestern Nevada.Geologic map (Figure 2) and localities referred to in the text shown.
38*30'
r 38*15'
- 38*00'
-37*45 '
3
volcanic landforms9 Coaldale Ridge and Blue Mountain, are informally
named to aid in description of localities mentioned in the text and
are shown on Figure 2.
The topography and geology of the study area are conspicuously
dominated by a large, central pre-Tertiary basement high flanked on ,
either side by thick volcanic accumulations (Figure 2). As the volcanic
rocks are the major emphasis, the pre-Tertiary basement is only
evaluated in this study for ways in which it could have affected the
volcanic or Tertiary history of the area. To distinguish the primary
or secondary nature of the B anomalies in the area the aims .of this
study are threefold: first, to determine the chronological history;
second, to define the structural history; and third, to examine the
geochemistry of the Tertiary rocks in the southwestern Monte Gristo
Range.
Methods of Study
The methods used to accomplish these aims involved both field
and laboratory studies. The majority of the field work was performed
during the period from August through October, 1982. Minor follow-up
work took place in the latter part of April and early May, 1983. The
bulk of the laboratory work was accomplished from November, 1982 to
March, 1983, although several analyses were not completed until November,
1983.
Detailed geologic mapping at a 1:12,000 scale of approximately
10 square miles covering the Tertiary exposures was done using air
photographs as a base. Regional reconnaissance of volcanic rocks in
4
the nearby Silver Peak Range and Candelaria Hills was performed to com
pare compositions and gross structural style of similar units. More
detailed reconnaissance of the remainder of the Monte Cristo Range was
completed, partially in conjunction with J. Stewart of the U.S. Geo
logical Survey, to evaluate the geology and correlative possibilities
in the immediate surroundings of the project area.
Several laboratory analyses were also necessary to further
refine the field work and the chronological and geochemical history of
the project area. Thin section petrography of 41 samples was used to
improve descriptions and clarify differences between the field units.
Detailed sampling of all units throughout the project area was done
during the latter part of the field work. A total of 75. samples was
collected; 70 unduplicated samples were analyzed for their B, Sr and Li
contents to establish the amount, stratigraphic preference and geographic
distribution of the geochemical anomalies in the study area. Nine of
these samples were also analyzed for their whole rock geochemistry to
determine the normative composition and variation of the major volcanic
units. Three of these geochemistry samples were further isotopically
analyzed. Four K-Ar mineral age dates were performed on these three
samples to define the duration of volcanic activity and to give minimum
age constraints to the structural deformation in the study area., Finally, 8 7 g r
initial ---- ratios were determined for these three samples to identify86Sr
possible magmatic source materials for the volcanic sequences. The
relative freshness of the geochemical and isotopic samples was estab
lished prior to their analysis by thin section inspection.
5
The B, Sr and Li analyses were performed by atomic absorption
in the U.S. Borax Research Laboratory located in Anaheim, California.
The whole rock geochemistry determinations and isotopic analyses were
done in the Analytical Research Laboratory and the Laboratory of Isotope
Geochemistry, respectively, at the University of Arizona in Tucson.
Initial sample preparations for both these analyses were done by the
author. These preparations included: crushing, homogenization and
cleaning; mineral separation for K-Ar determinations, utilizing standard
vibration, magnetic and heavy liquid techniques; K analyses by atomic
absorption; Sr determinations by X-ray fluorescence; and Sr extractions
by calibrated ion exchange resin columns. R. Butcher performed the Ar
isotope analyses and D. Lynch the Sr isotope analyses on 6" sector
Nier-type mass spectrometers. Normative and variation calculations of
the whole rock geochemistry were performed through a computer program
by D. Lynch.
CHAPTER 2
REGIONAL GEOLOGY AND TECTONIC SETTING
The Monte Gristo Range is characterized by complex geology in
both its pre-Tertiary and Tertiary unitso It was originally mapped by
Ferguson and others in 1953 at a regional scale of-1:125,000. The
dominant pre-Tertiary unit in the Monte Cristo Range, the Ordovician
Palmetto Formation, is considered allocthonous and forms the leading
edge in the Roberts Mountain thrust of the Antler Orogeny (Stewart,
1980)o Although this orogeny is believed to have caused most of the
complex pre-Tertiary structure seen, isolated Triassic Excelsior
Formation exposures are part of the Golcanda thrust and evidence the
effects of the Sonoma Orogeny6 These major thrusting episodes have
given the Monte Cristo Range a structurally complex and brittle base
ment framework for Tertiary deformations.
The Tertiary volcanic rocks in the Monte Cristo Range have been
grouped into three units: basalts, the Gilbert Andesite, and a lower
rhyolitic breccia unit in the Esmeralda Formation (Ferguson et al.,
1953; Albers and Stewart, 1972). A recent study by Moore (1981) in the
Blair Junction area has informally named the latter rhyolite the Castle
Peak Tuff, and has firmly dissociated it from the overlying sedimentary
Esmeralda Formation for its obvious ash-flow features. The entire Monte
Cristo Range is currently being studied in detail by J. Stewart and
others as part of the U.S. Geological Survey Tonopah 2° sheet project.
6
7
Recently published geologic quadrangle maps nearby the project
area include areas in the Silver Peak Range by Robinson and others
(1976) and the Miller Mountain region by Stewart (1979). Detailed work
by Moore (1981) in the Blair Junction area and by Speed and Cogbill
(1979a-c) in the Candelaria Hills provide a basis for comparison with
some of the units and structures in the project area.
More regionally, the project area lies along the western margin
of the Basin arid Range Province, a region dominated by north-south
mountain ranges and intervening linear valleys (Eaton, 1979). Steep
normal faults control this physiography arid formed as a rigid crustal
response to regional extensional stress operative in this region from
approximately 17 m.y. ago until the present time (McKee, 1974; Stewart,
1978, 1980). The study area occurs in an area affected by, and near the
intersection of, two regional structural features in this Province, the
Walker Lane and the Warm Springs lineament.
Although originally defined as a physiographic lineament by .
Locke and others (1940), many workers have shown that the Walker Lane
can also be considered a structural province (Shawe, 1965; Albers, 1967;
Stewart, 1967; Ekren et al., 1976; Speed and Cogbill, 1979a). This broad
zone, shown in Figure 3, is believed by.most to be a large-scale deforma-
tional feature characterized by right-lateral displacements accommodated
by both strike-slip faults and pervasive folds (Stewart, 1980). The
timing and amount of offset in this zone is still a matter of great
debate, but minimum generalizations can be made.
i.Reference
Bell and Slemrnons, 1980Offset 10 mi
AgeMiocene -*■ Recent2. Bonham, 1967 20 mi Miocene(?)3. Stewart, 1980 I 10 mi Miocene Recent4. Ekren et al., 1979 Z 20-30 mi Early Miocene5. Albers, 1967 12 mi Miocene (?)6. Gilbert and Reynolds, 1973 ? Oligocene -*• Recent7. Speed and Cogbill, 1979a ? Oligocene -*■ Recent8. Moore, 1981 Z <2 mi Miocene -» Recent9. Stewart, 1980 60-80 mi Miocene10. Albers, 1967 10-12 mi Miocene
11. Stewart, 1968 25-40 mi Late Miocene
Figure 3. Structural Features within the Walker Lane of Nevada and California. East- northeast lineaments (Ekren et al., 1976): YR = Yerrington-Rawhide; PR = Pancake Range; WS = Warm Springs.
9
South of Tonopah (Figure 3) the Walker Lane is dominated by
large right-lateral strike-slip faults. These have at least 50 to 80
miles of offset and tend to die northwards into large arcuate mountain
ranges, or 'oroflexes' as defined by Albers (1967). The northern
section of the Walker Lane is characterized by en echelon sets of
smaller right-lateral strike-slip faults which have offsets that range
from less than one mile to greater than 20 miles (Bonham, 1969; Ekren
et al., 1980; Stewart, 1980). This change in deformational style in
the Walker Lane occurs over a narrow zone that includes the current
project area. Although a Jurassic age for some of the deformation in
this zone is advocated (Speed, 1978), most workers believe that the
offsets are of Miocene to Recent age. Many of the northern faults have
Quaternary and historic displacements whereas most of the southern ones
may be older (Bell and Slemmons, 1980).
Recent work in the northern Walker Lane has identified three
east-northeast zones of left-lateral strike-slip faults (Ekren et al.,
1976). Each has a strong geomagnetic expression and is characterized
by short en echelon strike-slip faults, similar to the style in the
northern Walker Lane (Stewart et al., 1977). Offsets along individual
faults are usually small but total displacements across fault zones up
to 10 miles have been documented (Stewart, 1980; Speed and Cogbill, 1979a;
Ekren et al., 1979). The east-northeast Warm Springs lineament of Ekren
and others (1976) projects into the current project area. The age of the
deformation in these zones is believed equivalent to that of the Walker
Lane deformation.
10Tectonically* many have likened the Walker Lane to a
continental version of the San Andreas fault (see Stewart* 1980)» The
east-northeast lineaments are equally analogous to the Garlock fault»
Both these features are thought to be the continental result of late Tertiary to Recent regional conjugate shear stress across the western
Great Basin.
I
CHAPTER 3
ROCK UNITS
Pre-Tertiary Basement
The pre-Tertiary (pT) rocks in the study are composed
dominantly of the Ordovician Palmetto Formation. They consist of
interbedded shales, siltstones, tan argillaceous limestones and cherts
Interlayered limestone and bedded black chert units predominate in
the central portion of the project area whereas shale and siltstone
units predominate in the northeast (Figure 2). The color of the
shales and siltstone is varied and includes greenish gray, brown,
reddish purple, gray and black. Alternation of dark and light units
is common. Tan weathering argillaceous limestones are medium gray■' - -
when fresh. Limestones" usually occur interlayered with bedded cherts
but also exist as thin interbeds within the shale and siltstone units.
Minor exposures of the Triassic Excelsior Formation have also
been mapped in and near the study area (Albers and Stewart, 1972).
Consisting of greenstone and greenstone breccias, this unit occurs in
the northernmost portion of the project area. It has also been
suggested that Devonian strata may exist as structural interleaves
within the Palmetto Formation in the Monte Cristo Range, as they do
northward near Miller Mountain (Stanley et al., 1977; Stewart, 1980).
Neither of these age strata were noted within the study area during
this project.
i i ■
12Tertiary Volcanic and Sedimentary Rocks
Four sequences of Tertiary volcanic and sedimentary rocks have
been identified in the project area* These are informally named (from
bottom to top): the Castle Peak volcanic sequence5 the Coaldale
volcanic sequence, the Blair Junction volcanic sequence, and the
younger sedimentary and basaltic sequence* Each sequence has distinct
mappable units. The following discussion describes the distribution,
contact relations, lithology and petrography of the Tertiary map units
in the study area. Table 1 summarizes the average thin section compo
sition of the volcanic units.
Castle Peak Volcanic Sequence
The Castle Peak volcanic sequence has been subdivided into two
related rock types. The first, the Castle Peak Tuff (Tcp), occurs
throughout the project area whereas the second, a banded rhyolite intru
sion (Tri), occurs only in the south central portion (Figure 2).
Castle Peak Tuff. The Castle Peak Tuff was first named and
identified by S. Moore (1981) in the Blair Junction area of the southern
Monte Cristo Range east of, and adjacent to, the current study area. He
divided the Castle Peak Tuff into three informal members comprising
different degrees of welding in an ash-flow (Smith, 1960a,b). These
members have been extrapolated westward into the area of this report as:
Tcpj, the unwelded member; Tcp2, the partially welded member; and Tcp^,
the densely welded member. In addition to these members, however, the
tuff in the southwestern Monte Cristo Range contains large mappable
if
TABLE 1
Average Thin Section Modal Percents of Tertiary Units Southwestern Monte Cristo Range
Sample Number of Thin Sections Name
Phenocryst Composition (%) Groundmass Composition (*)
Quartz Sanidine Plagioclase(series) Hornblende Augite Hypersthene Total Microlitic
Plagioclase Opaques Pyroxene Alt.
Tb 1 Bas. Andes. _ - - - - 5 95 65 2 10 -Ti 2 Rhyo. Int. 20 10 <1 (ab) 2% biotite - 68 cryptofelsic textureTpa 3 Px Andes. (tr in one
thin sect.) - 25 (olig) - 5-7 - -65 10 5 10 -Tpai 1 Px. Int. - . - 30 tr 10 - 60 < 5 5 2 -
1 Px. Dike - - 20 - 5 - 75 20 2 5 -Tpat 1 Px. TXiff bx. - (tr?) 35 - 5 - 60 50% clasts FeOxTfai S Fg Andes. Int. - - 8 (olig) 8 8 (10 in one
thin sect.) 76 32 5 2 ChiTha 3 Hb Andes - - 10 (olig) 10 2-5 -75 35 3 tr -Thai 2 Hb Int. - - 20 18 - - 62 tr-20 2 - -
1 Hb Dike - - 20 (olig) 7 - - 73 10 2 Chl/CarboThat 2 Hb Tuff Bx. - 5 25 5-10 - - -65 30% to 50% clasts FeOxThai 2 Hb lahar - - 25 20 - - 55 tr 3 - -Tstu 1 Lithic tuff 20 25 tr 5% biotite - 50 55% clasts;
unwelded texture Chl/FeOxTstu 1 Air-fall tuff - 5 5 tr biotite - 90 extremely fine-grained -Tea 2 Cg Andes. (tr?) - 15 (olig) 10 5 - 70 25 3 - ChiTcab 1 Cg Andes. Bx. - - 25 10 tr - 65 50% clasts FeOxTfa 2 Fg Andes. - - 7 5 3 - 85 35 3 - ChiTri 2 Banded Rhyo. Int. 2/tr(?) 5/7 5/25 - - - 88/58 alternating light/dark bands FeOxTcp S Castle Peak Tuff 8 13 10 2% biotite - 67 5% clasts; eutaxitic texture FeOx
TcPe 3 Exotic Blocks - variable. see text - —
14blocks of exotic material, Tcpe, not noted by Moore in the Blair
Junction area0
1„ Tcpj o The thickest and most extensive subdivision of the
Castle Peak Tuff is the unwelded member* Its thickness is variable and .
ranges from less than 251 in the central south to greater than 300? in
the northeast. Where observable, the unwelded portion of the Castle
Peak Tuff overlies the Palmetto Formation with angular conformity. An
irregular erosional surface is indicated by the undulatory contact
between these units in the southwestern exterior of the Monte Cristo
Range. A linear exposure of the tuff's lower contact in the north
central portion of the study area suggests structural control on its
deposition may have also existed.
Lithologically, the unwelded member is distinctive in its gray
to white color. Little to no flattening of pumice occurs although
devitrification and vapor phase alteration of the groundmass are common
(Figure 4). It usually contains up to 5% lithic clasts but up to 10%
is not uncommon near its base. The basal contact zone differs only in
that it is a thin (6n) pink to orange chilled margin; only one occur
rence of a true basal vitrophyre occurs in the study area. • Distinctly
bedded lapilli tuff beds in the unwelded member seen in the Blair
Junction area (Moore, 1981) are not observed in the current study area.
2. Tcp2 o The partially welded member of the Castle. Peak Tuff
is distinguished by its light pink to purple color, moderate eutaxitic
texture and substantial vapor phase alteration of both its phenocrysts
15
- W - .
. * ■
. ■• V ;V
"
Figure 4. Texture in Unwelded Member of the Castle Peak Tuff.
■■■■
16
and groundmasso Flattening ratios (long axis to thickness) of
pumices range from 2:1 to 10:10 This more resistant unit commonly
forms the slopes and ledges above the lower unwelded member whereas
more gradational upper contacts are seen with the overlying welded
membere This member contains slightly fewer lithic clasts, 3% to
5%, throughout its variable thickness0
3e Tcpg0 The upper densely welded member of the Castle Peak
Tuff is dark reddish purple to brown and exhibits a- strong eutaxitic
texture, with flattening ratios reaching a maximum of 30:1* Extremely
resistant, this member commonly caps ridges in the west and central
portions of the study area*. The densely welded member is very sili-
cified and completely devitrified but does not show the vapor phase
alteration prevalent in the partially welded tuff. Maximum welding to
a black vitrophyre with a pronounced eutaxitic texture is exposed only
in the south central portion of the study area* The lower contact of
the densely welded member is variable and usually abrupt, with the"
upper member frequently forming ledges or cliffs 10* to 50* .thick..
Lithologically, this member is similar in composition to the partially
welded member.
Thin section inspection reveals that all three members of the
Castle Peak Tuff are compositionally similar. Quite crystal-rich, the
phenocrysts in the tuff compose approximately 35% of the rock and con
sist of quartz, sanidine, plagioclase and minor biotite (Table 1). The
quartz and sanidine phenocrysts are very clean, neither embayed nor
altered, and average 0.5 mm to 1 mm in size. Fine-grained biotite
■ 17is also fresh and has only thin magnetite resorbtion rims. Plagioclase
crystals exhibit only minor clay alteration in the unwelded and densely
welded members; maximum alteration of these grains occurs in the
partially welded member with intense vapor phase alteration. Lithic
clasts are composed of Paleozoic rock types and minor andesite.
Although locally more abundant, they average 5% of the total composition.
Lunate and cuspate glass shards dominate the groundmass, commonly very
Fe-stained in the welded subunits. Large pumices, usually 1 mm to 6 mm
in length, and interstitial opaques are also ubiquitously present.
Devitrification of the shards and pumices to axiolitic intergrowths
of microcrystalline quartz and feldspar increases with welding in the
tuff. Microscopic'alignment and flattening of shards, pumices, biotite. • . ' .. ■ . ■
and plagioclase laths evidence the increase in welding in the tuff.
At least two cooling units in the Castle Peak Tuff have been
identified in the northwest portion of the study area. Here, two
densely welded zones are separated by a partially welded unit that con
tains a distinctive 10' thick baked rubble zone. Lithic clasts in this
zone are up to 2s in size and composed of a rhyolitic(?) vitrophyre and
pre-Tertiary sediments (Figure 5a). This rubble zone marks the bottom
of the upper cooling unit (Smith, 1960a,b). No compositional differ
ences occur between the partially welded tuffs on either side of the
rubble zone. The upper cooling unit has a much thicker (201 to 50*) and
more resistant densely welded zone than the lower (81 to 10*). The
upper is more siliceous and the lower has slightly higher flattening
ratios. Both make ridge-forming dip slopes (Figure 5b), with the rubble
Figure 5. Multiple Cooling Units of the Castle Peak Tuff in the Northwestern Portion of the Study Area.
a. Rubble zone between the two cooling units of the Castle Peak Tuff shown in Figure 5b.Clasts are composed of welded and vitrophyric tuff (red and black), as well as Paleozoic limestones (light gray).
b. Multiple cooling units of the Castle PeakTuff are indicated by two densely welded zones that form dip-slope ridges (center of photograph) . Age date sample CVD 7 was taken in the upper cooling unit on the ridge to the left. Looking north, Columbus Marsh occurs to the far left and the Candelaria Hills in the distance. _
18
Figure 5a.
Figure 5b.
19
bed near the center of the intervening valley. The majority of the
Castle Peak Tuff exposures in the remainder of the project area only
indicate one cooling unit.
4. Tcpe. The unwelded member of the Castle Peak Tuff contains
large exotic blocks of different compositions. These blocks have been
identified elsewhere in the Monte Cristo-Range (Stewart, pers. comm.,
1982) and indicate proximity to the source area of the tuff. Apparently
unrooted, the blocks are oxidized, Fe-stained and commonly have slicken-
sided, silicified rinds. The tuff surrounding them is unaltered and
has only a slight darkening of color. Three compositionally distinct
blocks have been identified in the study area. The largest, at least
200* x 400* x 200* large, occurs in the northeast and is composed of a
partially vesiculated, feldsparr-rich rhyolite (Figure 6). Heavily
Fe-stained, banding and pumice alignment in the block is northerly and
almost perpendicular to the attitude of the surrounding tuff (Figure
2). Two other large blocks have been identified in the southern portion
of the study area; each is compositionally distinct and heavily Fe-
stained. The one in the south central portion is composed of a lithic-
and plagioclase-rich andesite tuff and is clearly surrounded by the
Castle Peak Tuff. The others in the west central portion do not exhibit
as clear contact relations as the ones discussed above and consist of a
crystal-poor, lithic-rich rhyolitic tuff. Each block has attitudes
highly discordant to the orientation of the surrounding Castle Peak
Tuff.
20
Figure 6. Exotic Block of Oxidized Feldspar-RichRhyolite in Unwelded to Partially Welded Castle Peak Tuff. Exposed in the northeastern portion of the study area, the smaller blocks to the right are 101 to 20' wide.
21Banded Rhyolite Intrusion*1. Tri. The banded rhyolite intrusion crops out only in the
central exterior of the southwestern Monte Cristo Range (Figure 2)„
Characterized by its alternating light gray and reddish purple bands
Oo5 mm to 2 cm wide, the rhyolite forms three large, east-west trending
dikes that intrude the Castle Peak Tuffe Nearly vertical, the rhyo
litic mass is fault-bounded on its northern edge but has a thin
vesiculated chilled zone at its southern margin. No pumice or eutaxitic
textures are noted in the interior of the dikes or at the other Castle
Peak Tuff contacts. The tuff at these contacts, however, is highly. 1 .altered and consists almost totally of clay.
In thin section, the alternation of the light and dark bands
is very pronounced. The dark bands are composed dominantly of a very
Fe-stained aphanitic groundmass and have only 5% to 1% resorbed and
altered plagioclase phenocrysts. The light bands are much more
plagioclase-rich and contain minor quartz; these unaltered phenocrysts
occur in a slightly devitrified, aphanitic groundmass (Table 1). In
thin section the bands have irregular contacts, occasionally pinched and
feathered, although they are sharp and continuous in hand sample. Rare
inclusions of the Fe-rich material also occur in the light bands.
Coaldale Volcanic Sequence
This volcanic sequence dominates the Tertiary exposures in the
study area. It is informally named and grouped for its local exposure
and lack of continuity throughout the rest of the Monte Cristo Range
and other nearby volcanic centers. The Coaldale volcanic sequence is
22composed of five major subdivisions, three of which can be broken down
further into, mappable units. In increasing age, these subdivisions are:
sediments and tuffs (Tstl and Tstu), hornblende andesites (That, Thai,
Thai, and Tha), fine-grained andesites (Tfai), pyroxene andesites
(Tpat, Tpai, and Tpa), and an intrusive rhyolite (Tij.
Sediments and Tuffs. The sediments and tuffs at the base of
this volcanic sequence can be divided into upper and lower members.
The majority of the exposures of both members is in' the southern
portion of the study area. The unit is exposed best just east of, and
on the east flank of, Coaldale Ridge (Figure 2).
1. Tstl. The lower member of this unit is composed of poorly
resistant, highly weathered tuffs with clasts of a feldspar-rich
andesite up to 21 in size. These more resistant clasts form a
pronounced "bpuldery" weathering characteristic in the lower member:
unaltered clasts weather out of, and sit atop, a clay-altered tuf-
faceous matrix. The matrix has white phenocrysts in a greenish gray
groundmass; both are now totally altered to clays. This member is at
least 50* thick and its lower contact is never observed.
2. Tstu. The upper member of this unit is more variable in
composition. Upsection, it consists of a lithic- and pumich-rich pink
ash-floW tuff, thin-bedded air-fall tuffs, reworked tuffs as white
paper shales, and thin quartz-rich rhyolitic tuffs. The pinkish brown
lithic tuff.is quite distinct from the Castle Peak Tuff; unwelded and
friable, it is composed of up to 50% clasts of variable volcanic
23
compositions in an Fe-stained quartz- and plagioclase-rich matrix. No
pre-Tertiary clasts, common in the Castle Peak Tuff, were noted in this
tuff. It is only observed directly east of Coaldale Ridge; nothing
similar was seen further east or west although the air-fall tuffs and
quartz-rich tuffs also seen in this area are seemingly continuous.
These latter units are relatively unweathered and thin-bedded. The
entire sequence is of variable thickness, and at least 150* thick where
both upper and lower contacts are observable. In.general, the upper
contact is with Quaternary alluvium and the lower contact is grada
tional but usually covered with the lower member of the sediment and
tuff unit.
Hornblende Andesites. The hornblende andesites are the most
common volcanic group in the Coaldale volcanic sequence. Except for
the northeastern and north central portions, these andesites occur .
throughout the study area (Figure 2). They consist of four mappable
units: a hornblende andesite tuff breccia (That), a hornblende
andesite lahar (Thai), hornblende andesite intrusives (Thai, including
dikes, sills and plugs), and hornblende andesite flows (Tha).
1. That. This unit is the oldest of the widespread hornblende
andesites and occurs throughout the southern portion of the project
area (Figure 2). Like the sediment and tuff unit, it is best exposed
on the east flank of Coaldale Ridge. This unit is of variable thick
ness; over 300' thick in the Coaldale Ridge area, the tuff thins to
approximately 25' thick in the southeast, the tuff breccia conform
ably overlies the sediment and tuff unit described above. Its upper
24
contact is usually quite sharp with the overlying hornblende andesite
flows.
The hornblende andesite tuff is composed of andesite clasts,
comprising up to 50% of the unit, in a hornblende- and plagioclase-rich
tuffaceous matrix (Figures 7a and 7b). The predominance of clasts in
this unit predicates the term "tuff breccia" as its descriptor. Clasts
in the lower portion of this unit are totally composed of a feldspar
porphyry andesite. Upsection the clasts include a hornblende-rich
andesite, sometimes exclusively, as well as the feldspar porphyry. The
matrix also becomes slightly more hornblende-rich upsection. The color
of the tuff breccia varies from reddish purple to bluish gray in
fresher samples. In thin section the matrix hornblendes are present
in a 5% abundance and average 3 mm in length. These grains are rela
tively clean, only slightly fractured and have only thin magnetite
resorption rims. Plagioclase phenocrysts dominate the matrix, average
0.5 mm in size and are very altered to clays. Sanidine is present in
minor amounts. The phenocrysts occur in a heavily Fe-stained, glassy
groundmass (Table 1). The clasts range in size from less than 0.5"
to greater than 2* large. In thin section these clasts are all rimmed
by extremely heavy Fe-staining and opaques.
On the east flank of Coaldale Ridge the vertical succession in
this unit includes a distinct 10* to 20' thick flow unit; its clasts
are monolithologic and composed of vitrophyric hornblende andesite.
This marker unit is not seen elsewhere in the study area. Other flow
units are delineated by the presence of several 6" yellowish gray ash
Figure 7. Series of Photographs that Depict Textures inthe Hornblende Andesite Tuff Breccia.
a. Hornblende-rich clasts enclosed in a bleached tuffaceous matrix.
b. Embayed contact between two flow units of the hornblende andesite tuff breccia.
c. „ Multiple flow units in the hornblendeandesite tuff breccia. Looking west- southwest, pyroxene andesite flows and Columbus Marsh occur.in the background.
(Photographs a and b are textures exhibited near the contact shown in c.)
25
Figure 7a. Figure 7b.
Figure 7c.
26layers that are interspersed throughout its thickness. Multiple flow
units are also inducated by chilled contacts between flows in the
northwest (Figure 7c); the embayed, deformed and bleached upper contact
of the lower flow in this figure suggests little time elapsed between
flow eruptions.
2. Thai. The hornblende andesite lahar occurs only on the
flanks of Blue Mountain (Figure 2). Its true thickness in indeter
minate, but exposures over 2501 thick occur on the southwestern flank
of Blue Mountain. Its upper and lower contacts with the hornblende
andesite tuff and intrusion are usually obscured by float.
This unit consists of a monolithologic hornblende andesite
breccia; the clast composition is indistinguishable from the matrix
composition except for degree of silicification. The matrix is
softer and tuffaceous whereas the clasts are very silicified and quite
hard. As Blue Mountain is approached, the matrix of this unit becomes
more and more silicified until only weathering accents the clast
outline (Figure 8). Multiple flow units are indicated by irregular
bedding in the lahar; beds 10' to 50' thick can be identified only in
its upper, more tuffaceous portions. Where basal contacts are chilled,,
ledgy exposures result. A true "laharic" origin of this unit is
doubtful; the term is used here to distinguish it from the hornblende
tuff breccia, its slightly tuffaceous matrix and bedded nature suggest
a pyroclastic origin on the flanks of a major andesite volcanic center
and are representative of laharic features.
27
Figure 8. Silicified Lower Portion of the Hornblende Andesite Lahar near Blue Mountain.
28Compositionally this unit is much more hornblende-rich than
the tuff breccia. Clasts and matrix in the lahar contain up to 20%
altered oxyhornblende phenocrysts with thick magnetite resorption rims.
Plagioclase phenocrysts are fine-grained and gradational with micro- litic plagioclase in the groundmass. These grains are only moderately
altered to clays. Light gray to bluish gray, the groundmass is very
vitrophyric and unaltered (Table 1).
3. Thai. The hornblende andesite intrusions occur as dikes
(+H+M+H-), widespread throughout the southern and western portion of the
project area, sills (Thais), localized in the south central portion, and
large plugs (Thai), prominent in the west central portion (Figure 2).
The dikes range from 2' to IS' wide, but an extremely wide, multiple
dike intrusion occurs near the largest plug. Only two sills have been
identified in the area, each at least 25* thick. The largest intrusive
plug occurs at Blue Mountain and another, only slightly smaller, to
the west. Two smaller plugs have been tentatively identified in the
extreme northwest.
The dikes and sills are of similar appearance and composition
(excluding the large dike complexes near Blue Mountain). With a
greenish gray matrix, both are very porphyritic with hornblende
phenocrysts that average 8 mm in length but can reach 2 cm in size.
These grains have thick, well-defined magnetite resorption.rims and
are very fractured. Abundant plagioclase phenocrysts are much finer-
grained, with 1 mm as an average size, and are gradational to microlitic
groundmass plagioclase. The crystals are very altered to clays and
29calcite in their cores and on their rims. The groundmass is very dirty
and usually altered to a fine-grained mixture of chlorite, calcite and
clays. Calcite also commonly fills the fracture openings in the horn
blende phenocrysts (Table 1).
The dikes and sills have well-defined chilled margins 6" to 3’
thick when the contacts are exposed. Usually instrusive into the
unwelded Castle Peak Tuff, the tuff shows only minor alteration by a
color change from white to purple within 10' of the contact and chilled
zones less than 2' wide. When intrusive into other volcanic types, the
chilled margins are much thinner and no contact alteration is evident
in the intruded units.
The large intrusive plugs in the study area occur close to the
contact between pre-Tertiary and young volcanic rocks. The Blue
Mountain plug is the largest of the intrusions and has a conical shape.
Intrusive into the hornblende lahar, it has steep slopes that are
parallel to the joints and flow foliation in the andesite (Figures 9a
and 9b). Very large dike complexes trend northwest-southeast on either
side of this intrusion. These dikes do not exhibit the chloritic
alteration common in the smaller dikes throughout the area; rather, they
are much more similar in composition to the Blue Mountain intrusion.
Semi-parallel chilled rinds occur on either side of the central plane
in a few of the dikes present in. this complex; mirror image repetition
of the chilled margins indicates a multiple intrusive history in these
rocks. ,
Figure 9. Blue Mountain Intrusion,
a. Looking northeast, Light unit on the flanks of the plug is the hornblende ; andesite lahar; dark units in the foreground are hornblende and pyroxene andesite tuff breccias intruded by dikes,
b. Looking northwest, Ledgy units to the left are hornblende andesite lahar flows(?).
Figure 9b.
31The large intrusion to the northwest of Blue Mountain is
elongate rather than conical in shape (Figure 2). Flow foliations are
variable in this body but generally dip to the south. A small, dike
like plug with characteristic chloritic alteration continues to the
south of this intrusion.
Two additional, but much smaller, plugs occur in the north
western portion of the area. Contact relationships in these units are
not as clear-cut as in the intrusions discussed above. These smaller
plugs and the large intrusion northwest of Blue Mountain are in contact
with the Castle Peak Tuff rather than the hornblende lahar.
The intrusions and large dikes are much more hornblende-rich
than the related dikes and sills. The former contains 15% to 20%
unaltered aligned hornblende phenocrysts which average 3 mm in length
and have only thin magnetite resorption rims. Abundant plagioclase
phenocrysts are fine-grained, average 0.5 mm in size and are generally
unaltered to clays. Aligned microlitic plagioclase is absent in the
intrusive plugs but well-defined in the large dike complexes. The
plugs* groundmasses are extremely glassy and aphanitic with no evidence
of alteration; the large dikes show only slight chloritic alteration of
a less glassy groundmass. Both contain up to 2% unoxidized interstitial
opaques (Table 1).
4. Tha. Hornblende andesite flows are the most pronounced
unit in this andesitic series. Occurring throughout the southern
portion of the study area, this resistant unit commonly caps ridges
(Figure 10). The thickest and best exposures of the hornblende flows
32
Figure 10. West Flank of Coaldale Ridge. Most ledges and ridge caps in the background are hornblende andesite flows. Lower reddish brown ledge is a pyroxene andesite flow; CVA 29 was taken in the outcrop to the left. Small greenish gray outcrop on the lower left is a sericitized hornblende andesite flow(?) where CVA 25 was taken (see Figure 30). All unresistant units are the hornblende andesite tuff breccia with thick salt accumulations just beneath the surface.
33are on. the west flank of Coaldale Ridge (Figure 2), Two major flows,
each greater than 50? thick, occur in the south central portion of the
study area whereas only one flow is evident near the Blue Mountain
plug. The two flows in the south are very siliceous horizons separ
ated by a thin, less siliceous unit of similar composition. Multiple
flow units near Blue Mountain are suggested by the thickness of the
unit in this area, 100• to 300*, but are not defined because of their
proximity to the intrusive source. Hornblende andesite flows usually
conformably overlie the tuff breccia.
Minor Paleozoic sedimentary rocks and Castle Peak Tuff clasts
occur as lithic inclusions at the very base of those flows farthest
from the intrusions. Vertical cooling joints and gently dipping flow
foliation parallel joints are pronounced in the siliceous center(s)
of the flow(s). Less siliceous, more weathered, hornblende andesite
also occurs as interlayers between flows and as individual units to the
southeast of Blue Mountain. Although highly fractured, these do not
exhibit the pronounced foliation parallel joints seen in the siliceous
portions of the flows.
The hornblende andesite flows differ from the other hornblende
units by containing 2% to 5% fine-grained augite(7) phenocrysts.
Averaging 0.5 mm in size, these crystals are fractured but unaltered
although thin rims of pigeonitef?) are common. The flows also differ
by having finer-grained, 2 mm long, aligned oxyhornblende phenocrysts
in less abundance. Thick magnetite resorption rims surround the
oxyhornblende grains. Fine-grained plagioclase phenocrysts, 10% of
34the mineralogy, have clay-altered cores and rims, Microlitic
plagioclase shows a well-defined flow foliation and is generally
unaltered. The groundmass is very glassy and has only a slight
chloritic alteration. Pyroxene exists as interstitial granules within
the groundmass. The less siliceous equivalents of this unit have
totally altered oxyhornblende phenocrysts and a heavily Fe-stained
groundmass (Table 1)..
Fine-Grained Andesites.,
1. Tfai. Only one phase of the fine-grained andesite has been
identified in the study area. The majority of the exposures of this
intrusive unit occurs in the south central project area, although minor
outcrops in and near the northern edge of Coaldale also exist (Figure
2). Always intrusive into the hornblende lithic tuff, the fine-grained
andesite exhibits only thinly chilled margins. Minor baking of the tuff
along the contact is displayed by a slight darkening of its color.
Closely spaced joints, 0.5" to 2" apart, parallel to the flow foliation
are prominent in all exposures of the fine-grained andesite. Typically
a deep reddish brown on weathered surfaces, the andesite is gray when
fresh. No younger volcanic rocks were seen to intrude or overlie this
unit.
Compositionally the fine-grained andesite is related to both
the hornblende and pyroxene andesites and contains equal proportions of
hornblende, pyroxene and plagioclase fine-grained phenocrysts. The
hornblende crystals average 0.6 mm in length and have thick magnetite
resorption rims with only moderate flow alignment. Augite(?)
35phenocrysts are less than 0.4 mm in size and are unaltered although
commonly rimmed by pigeonitef?). Plagioclase phenocrysts are slightly
coarser-grained and. average 0.8 mm in size. The majority of the
grains have cores altered to clays. Microlitic plagioclase in the
groundmass is unaltered and exhibits a discontinuous grain alignment.
The groundmass is glassy, with interstitial pyroxene, and typically
altered to clays, chlorite and carbonate(?). In only one occurrence in
the south central portion of the study area has hypersthene been
recognized in this unit.
Pyroxene Andesites. This volcanic group overlies and intrudes
the older hornblende andesites and is prominently exposed along the
western exterior of the Monte Cristo Range (Figure 2). Like the horn
blende andesites these andesites exist as both intrusive and extrusive
phases and can be subdivided into three mappable units: a pyroxene
andesite tuff breccia (Tpat), pyroxene andesite intrusions (Tpai,
including dikes, sills and plugs), and pyroxene andesite flows (Tpa). 1
1. Tpat. This unit is analogous to the hornblende tuff breccia
and, except for a pyroxene composition, is very similar to it in
character. Although the best exposures are on the southern flank of
Blue Mountain, the pyroxene tuff occurs throughout the northwestern
portion of the study area. The tuff is not as widespread or as thick
as its corresponding hornblende unit. It irregularly overlies the
hornblende tuff breccia and is overlain or intruded by the pyroxene
flows and sills. Although its true thickness is unknown, exposures of
the tuff vary between 10' and 80' in thickness (Figure 2).
36
Commonly reddish purple on weathered surfaces, fresh pyroxene
tuff breccias are pinkish gray or green, Compositionally it is almost
monolithologic, with coarse-grained siliceous pyroxene clasts in a
tuffaceous pyroxene-rich matrix. Clasts contribute up to 50% of the
total rock, again predicating the use of the term "tuff breccia." The
clasts average 4" in size although blocks greater than 21 are not
uncommon (Figure 11)„ Only 10% of the clasts are composed of a
feldspar-rich porphyry rather than the pyroxene andesite. The matrix
typically consists of 5% augite(?) and 35% plagioclase phenocrysts,
Augite(?) grains are highly fractured, sometimes with clays filling
the interstices, and commonly occur as large cummulates, The plagio
clase grains are highly altered to clays. The groundmass is heavily
Fe-stained and has minor chloritic alteration. Fine-grained pyroxene
and opaque granules are interstitial in a slightly devitrified
groundmass (Table 1), The pyroxene clasts are similar in composition
with a more glassy and less devitrified groundmass, As in the
hornblende tuff breccia, the clasts are always rimmed by heavy Fe-
staining and numerous opaques,
2, Tpai, The pyroxene intrusions are represented in three
phases. Dikes (—» ■>-) are the most abundant and widespread of the
intrusions and occur throughout the northwestern portion of the study
area. Several dike-fed sills (Tpais) have also been tentatively
identified near Blue Mountain, Intrusive plugs are much less numerous
and much smaller than their hornblende analogs and occur intermittently
in the northwestern arid central portions of the project area (Figure 2),
t
37
Figure 11. Pyroxene Andesite Tuff Breccia near Blue Mountain. Monolithologic siliceous pyroxene-rich clasts occur in a tuffaceous matrix. Most clasts are less than 4”, although blocks up to 2' are not uncommon (top left).
38
All phases of the intrusions weather a dark reddish brown but are
greenish gray when fresh.
Pyroxene dikes are concentrated on the southern flank of Blue
Mountain and the northwestern edge of Coaldale Ridge. Intruding the
hornblende tuff breccia in the latter area, they have strongly altered
the surrounding tuff and produced a widespread Fe-staining. In the
Blue Mountain area they intrude the pyroxene tuff breccia as well as
the hornblende unit and are again associated with a widespread
Fe-staining<, The unresistant dikes are 5 1 to 158 wide and sometimes
trap more resistant wedges of hornblende andesite flows between
closely spaced intrusions. In thin section the dikes have a con
spicuous, very glassy groundmass with only minor chloritic alteration.
Phenocrysts of augite(?) and plagioclase are less abundant than in the
other pyroxene phases. Plagioclase crystals exhibit excellent flow
foliation by alignment of laths (Table 1).
A few of the dikes in the Blue Mountain area can be followed
upsection to overlying pyroxene sills or flows (Figure 2). The intru
sive or extrusive nature of the latter units cannot be determined as
they lack overlying exposures. They are typically very coarse
grained and exhibit only thinly chilled lower contacts; on this basis,
and for petrographic dissimilarities (see below), they have been
tentatively identified as sills rather than flows.
The other intrusive phase of this unit is very small pyroxene
plugs, none of which have areas greater than 300 square feet. In this
phase augite(?) phenocrysts are more abundant than in the other phases
39
and have a distinct bimodal size distribution. Most are fine-grained
and average 0.5 mm to 1 mm in size, but at least 3% are coarse-grained
and average 3 mm to 8 mm in size. Minor cummulate grains up to 1.5 cm
large also occur. Abundant plagioclase phenocrysts, 1 mm in size, have
only moderate clay-altered cores. Similar to the other pyroxene
phases, the groundmass includes interstitial pyroxene and opaque
granules. However, the groundmass is slightly coarser-grained and
shows greater chloritic alteration than in the other phases (Table 1).
3. Tpa. The pyroxene flows are best exposed at the extreme
western edge of the study area and range in thickness from 101 to 50'.
Like the hornblende flows, these resistant units commonly form dip
slopes and ridge caps. Their lower contacts have moderately chilled
margins usually 2" to.5" wide. The pyroxene flows usually overlie
unaltered hornblende or pyroxene tuff breccias, and occasionally,
hornblende andesite flows. At least one angular unconformity occurs
between this unit and underlying volcanic rocks on the east flank of
Coaldale Ridge (Figure 2). .
These flows are composed of augite(?) and lath plagioclase
phenocrysts which average 2 mm and 1 mm, respectively. The augite(?)
grains are unaltered but fractured with groundmass and clays filling
the interstices. Plagioclase laths exhibit fairly good alignment and
have clay-altered cores. Quartz was tentatively identified in trace
amounts in one thin section. In addition to these phenocrysts, an
altered equigranular mineral is ubiquitously present and contributes-
up to 15% of the total mineralogy. Fine-grained, usually 0.5 mm in
40
in size, it has a strong, nonpleochroic reddish brown color. It has
been tentatively identified as hydrobiotite, an original alteration
product of a fine-grained, unstable pyroxene„ This mineral is not seen
at all in the pyroxene sills discussed above, supporting an intrusive
rather than extrusive origin for these questionable outcrops near Blue
Mountain. The groundmass of the pyroxene flows consists of aligned
microlitic plagioclase and interstitial opaques in a chloritically
altered glass (Table 1).
Intrusive Rhyolite.
1. Ti. Only one exposure of the intrusive rhyolite has been
identified and it occurs in the southeastern portion of the study area.
Quite small, this plug does not intrude or contact any other rock type
and seems localized along a major fault (Figures 2 and 12). It is a
late volcanic phase and tentatively identified with the Coaldale
volcanic sequence.
This rhyolite is remarkably fresh when compared to the Castle
Peak Tuff and related banded rhyolite dikes. Very light pink, it has
a dense finely crystalline texture in its interior. Toward its
exterior, however, lithic clasts composed of feldspar- and hornblende-
rich andesites and vesiculated pumice become prominent in a light
brown chilled margin. The pumices show a variable attitude around the
plug's exterior and circumscribe its contact. This unit is crystal-rich
with abundant unaltered quartz, sanidine and minor biotite phenocrysts.
Glass shards are prevalent near its outer margin but become more indis
tinct in the fine-grained felsitic groundmass in its interior.
41
Figure 12. Small Pink Rhyolite Plug in the Southeastern Portion of the Study Area. Age date sample CVD 2 was taken on the far side of the plug (center of photograph). Prominent hills just behind the plug are fine-grained andesite intrusions. To the right of the rhyolite plug are Esmeralda Formation sediments (CVA 8 sample locality), as are the light rocks in the distance.
42
Blair Junction Volcanic Sequence
This volcanic sequence only occurs in the northeastern and
north central portions of the study area (Figure 2). The units are
represented throughout the remainder of the Monte Cristo Range and are
especially prominent in the Blair Junction area; they are herein
informally named for this exposure. This sequence overlies the Castle
Peak Tuff in all cases and can be subdivided into four mappable units:
sediments (Ts), a coarse-grained andesite breccia (Tcab)/ coarse-grained
andesite flows (Tea), and fine-grained andesites (Tfa),
1, Ts, These sediments overlie the Castle Peak Tuff and are
preserved under an andesite cap along the northeast ridge in the north
eastern portion of the project area. They are usually 25r to 508 thick
and a distinct bluish gray color. Very tuffaceous and volcaniclastic,
they consist of interbedded mudstones, siltstones and coarse-grained
pebbly sandstones. The fine-grained units are laminated and thin-
bedded, 2n to 6f? thick, whereas the coarse-grained units are more
moderately bedded, 6?f to 8fT thick. Occasional massive, unsorted
pebble sandstones also occur (Figure 13a). Clasts in the sandstones
are composed of pumice and tuff chips.
2. Tcab. The coarse-grained andesite breccia invariably under
lies the coarse-grained andesite flows. Of variable thickness, usually
ranging from 5 8 to 50*, it weathers to a reddish purple color but is
gray when fresh. This basal breccia is monolithologic and consists"
of 50% to 70% angular clasts of coarse-grained andesite which range
Figure 13. Blair Junction Volcanic Sequence.
a. Volcaniclastic. sediments underlying the coarse-grained andesite flows in the northeastern portion of the study area.
b. A typical volcanic section in the northeastern portion of the study area. Greenish gray ridge caps are coarsegrained andesite flows. CVC 12 was taken on the far ridge. These flows overlie a reddish purple monolithologic breccia. Basal unit is the unwelded member of the Castle Peak Tuff.
43
Figure 13a.
Figure 13b.
44in size from less than 0.5" to greater than 2 *„ These siliceous clasts
are encased in a matrix of comminuted andesite (Figure 13b).
Thin section inspection shows the clasts contain more, coarser-
grained, and less altered oxyhornblende phenocrysts than the surrounding
matrix. Plagioclase crystals are present in each in equal amounts and
have prominent clay-altered cores. The groundmass in the matrix is
less dense and less Fe-stained than in the clasts. As in the other
breccias in the project area, each clast is rimmed by an extremely
Fe-stained and opaque-rich groundmass. Only less than 5% of the clasts
are of a slightly different composition: these resemble crystal-rich
cummulates with less than 20% evident groundmass and equal proportions
of oxyhornblende arid plagioclase laths (Table 1).
3. Tea. The coarse-grained andesite caps the prominent ridge
in the northeastern portion of the study area (Figures 2 and 13b).
It abruptly overlies the breccia described above and varies in thick
ness from 25* to greater than 500* north of the current study area. The
color of the coarse-grained andesite ranges from light gray to, more
typically, a grayish green. Discrete exposures of this andesite alorig
the northeast ridge have pronounced rosette-type columnar jointing.
Marked with an asterisk on Figure 2, these are tentatively identified
as source vents for the local exposures of this widespread unit.
This unit is characterized by fresh, very coarse-grained laths
of plagioclase that average 2 mm in size but commonly reach up to 1 cm
large. Oxyhornblende is the only other dominant phenocryst and
exhibits a bimodal size distribution. Most average only 1 mm in size
45and are totally altered to Fe-oxides and chlorite. At least 2%,
however, average 5 mm to 8 mm.in length and are less altered. The
finer-grained oxyhornblende grains have thin, very distinctive ghost
outlines of fine-grained clays. Minor augite(7) also is present as
fine-grained phenocrysts and are 0.5 mm to 1 mm in size. Although,
fractured, these grains are unaltered. Biotite and quartz (?) occur
only in trace amounts. The groundmass consists of oriented plagioclase
microlites and interstitial opaques in a chloritically altered glass
(Table 1).
Tfa. The fine-grained andesite is compositionally similar to,
but texturally different from, the coarse-grained andesite. It occurs
as discontinuous dikes and small plug in the northeastern and north
central portions of the project area (Figure 2). In all cases it
intrudes the Castle Peak Tuff without large chill margins or major
alteration in the surrounding tuff. It does not overlie or intrude
the characteristic breccia associated with the coarse-grained flows.
The fine-grained andesite lacks the large plagioclase laths
characteristic of the coarse-grained andesite; rather, it contains
slightly less abundant plagioclase phenocrysts. that average 0.5 mm to
1 mm in size. Fine-grained oxyhornblende and pyroxene phenocrysts are
present in minor abundance and are both equal in size to the plagioclase
grains. Like the finer-grained oxyhornblende grains in the coarse
grained flows, these crystals in the fine-grained andesite have thin
ghost outlines of clays and are totally altered to Fe-Oxides and chlorite. The groundmass is aphanitic and chloritically altered; it
46
consists of aligned plagioclase microlites and interstitial opaques in a glassy matrix (Table 1) «,
Younger Sedimentary and Basaltic Sequence
The younger units in the study are Esmeralda Formation sediments
(Te) and basalt (Tb)» These units have limited exposures in the study
area although both are regionally distributed.
1. Te. The Miocene age Esmeralda Formation is only represented
in the southeastern portion of the project area (Figure 2). One
definite outcrop, and two questionable occurrences of this formation
exist although it is extensively exposed in the adjacent Blair Junction
area to the east. vSe Moore concentrated his work on this sedimentary
sequence and the reader is referred to his analysis (Moore, 1981) for
a complete description of the Esmeralda Formation in the southern Monte
Cristo Range.
The one definite exposure of this formation is composed of
interbedded tuffaceous mudstones, siltstones, reworked air-fall tuffs
as paper shales, and lignitic shales. This occurrence represents less
than 300? of Moore’s lowest member of the Esmeralda Formation. Mud
stones and reworked tuffs dominate the observable section, each
present in intervals greater than 50’ thick. The other units are
thinner and interbedded with mudstones and reworked tuffs. This outcrop
is in fault contact at its southern extent with hornblende andesites
and the lower member of the sediment and tuff unit. Its northern
contact is sloughed and of an ambiguous nature. The lack of fault
47
indicators in this zone suggests that the sediments may be in deposi-
tional contact with the underlying lower member of the sediment and
tuff unit. A depositional contact between these units is also
suggested by the questionable outcrops further south (Figure 2).
2. Tb. The basalts occur as small isolated outcrops in the
northwestern portion of the study area (Figure 2)„ These are presum
ably thin remnants of the voluminous basalt flows seen in the northern
Monte Cristo Range.. The basalt overlies the Castle Peak Tuff; contact
exposures are poor and cannot refute an intrusive origin for this unit.
The dark gray to black basalt is very aphanitic. The only
phenocrysts present are hypersthenef?) which average 1 mm in size and
suggest a basaltic andesite composition. These crystals occur in a
pilotaxitic groundmass consisting of extremely abundant microlitic
plagioclase with interstitial opaques, pyroxene granules and glass
(Table 1).
Quaternary Deposits
1. Q-. Quaternary alluvium occurs throughout the project area
and has not been subdivided into older and younger units.
2. Qsp. Two spring deposits with geochemical significance are
given special designation in this study. One modern spring occurs in
the northeastern portion of the project area (Figure 2). Currently
water-producing, this spring is surrounded by calcite crusts and thick
white salt- and gypsum-rich accumulations in the nearby soils.
48
The other spring deposit is one tentatively identified in the
northwestern portion of the study area (Figure 2). Here, thin-bedded
sediments overlie the Castle Peak Tuff and hornblende tuff breccia in
angular unconformity. These gently dipping sediments consist of a v
basal chert conglomerate and upper thin-bedded sandy limestones.
Although the majority of these sediments resemble older alluvium or
even the Esmeralda Formation, distinct local limestone breccia beds
with layers of open-space filling chalcedony and travertine occur within
the sequence. The layered silica and calcite suggest a spring may have
functioned during their deposition.
CHAPTER 4
AGE AND CORRELATION OF TERTIARY UNITS
Many correlations of the rock units in the Monte Cristo
Range have been made without radiometric age control. The variety
of andesites alone in this large mountain range makes correlation
based only on petrographic similarities difficult. When originally
mapped by Ferguson and others in 1953, all andesites in the Monte
Cristo Range were grouped and collectively called the "Gilbert Andesite."
K-Ar ages obtained from the "Gilbert Andesite" have been reported at
15.5 ±0.5 m.y. (Silberman et al., 1975) and 15.5±0.6 m.y. (Albers
and Stewart, 1972). Subsequent correlations did not account for the
number and variety of andesites included in the "Gilbert Andesite,"
and thus many porphyritic andesites in the Monte Cristo Range have
been assumed to be 15 m.y. old. The age relationships between the
many tuffs, andesites and sediments in the Monte Cristo Range are
currently being studied by J. Stewart and others of the U.S. Geological
Survey. His detailed mapping has revealed many subdivisions of the
"Gilbert Andesite" and many age relationships not previously identified.
Although contact relationships can. be used for relative age deductions
within a local area, radiometric age data are needed for more regional
correlations— even within the same mountain range if it has had a
complex volcanic history.
49
50
Radiometric K-Ar age determinations for three key units in the
southwestern Monte Cristo Range are presented in this report. They
include two mineral dates on the Castle Peak Tuff, and one mineral date
each on the Coaldale rhyolite and hornblende andesite intrusions.Table 2 summarizes the age data obtained and descriptions for all
samples are given in Appendix A.. Figure 14 is an overlay to the
geologic map and shows the sample localities in the study area. Four
geologic cross sections across the study area that illustrate most of
the contact relationships between the units are given in Figure 15.
With these age controls and crosscutting relationships a complete,
correlatable stratigraphic sequence for the study area can be devised.
Regional correlations with the new data are tentative but, in light of
Stewart’s current work, more accurate than previously possible.
Figure 16 presents a correlation chart that references thel *Tertiary units in the project area to those in the nearby Candelaria
Hills and Silver Peak Range. The regional Tertiary section by Gilbert
and Reynolds (1973) for the western margin of the Basin and Range Pro
vince is given as an overall comparison. Asterisks mark positions of
currently available age dates, while those in parentheses are units
that have age dating in progress. The time scale used in this chart
is that being used by the U.S. Geological Survey in its Decade of North
American Geology project.
Castle Peak Volcanic Sequence
The Castle Peak Tuff in the study area is correlative to many
late Oligocene and early Miocene rhyolite ash-flows in the Basin and
TABLE 2Analytical Data for Age Determinations of Tertiary Rocks
Southwestern Monte Cristo Range
SampleNo.
LithologicUnit
MineralDated IK v°
(xl0~12 moles/gm) %AratmAge m.y.
(± one std. dev.) Location
CVD 2 Ti sanidine 5.882 190.7 3.8 18.6 ± 0.4small plug in 38*02.51' N
SE
CVD 5 Thai hornblende 0.769 29.85 25.4 22.2 ±0.5
117*50.36' W
SE flank Blue 38*04.35' N
Mtn.
CVD 7 TcP3 sanidine 6.252 260.9 4.2 23.9 ±0.6
117*51.40' W
Upper cooling unit
CVD 7 Tcp3 biotite 7.195 306.8 12.8 24.4 ± 0.6
in NW38*05.24' N
117*52.54' W
Constants used: BX
40K/K
4.963 * 10'10 yr"1 0.581 xlO"10 yr"1 5.544 x10~10 yr"1 1.167x10 atom/atom
All analyses done at the University of Arizona Isotope Geochemistry Laboratory. Ar determinations done with a Nier-type 60° sector 6" mass spectrometer. K determinations by atomic absorption.
Figure 16. Regional Correlation of Oligocene to Pleistocene Rocks in Selected Areas of Southwestern Nevada. Vertical lines indicate strata absent. Local unconformities are not shown. Asterisks mark available age dates, while those in parentheses indicate age dating in progress. • In southern Monte Cristo Range (column 4): T e = Esmeralda Fm.; Tai= intrusive andesite;Tri = intrusive rhyolite. See text for correlation of undated units in this column with age dated volcanic rocks of this report (column 6). Definition and correlation of units throughout the Monte Cristo Range (column 5) are still in progress.
®*y• b*p•
Epoch
Early
10-
18-
Early
Ollgocene
Early
Generalized Western Margin of the Basin and Range Province(Gilbert I Reynolds,
1973)
Morgan Ranch Fn.
Basaltflows
CoalValley Fn.
Aldrich Station Fn.
RhyoliteIntr.
Andesite flows I dikes
Older Andesite
RhyoliteIgninbrltes
CandelariaMills
(Speed | Cogblll, 1979b)
PlioceneBasalt
MioceneSedlnentary
Rocks
]]]]]]]]:MioceneAndesite
Upper Ollgocene ■Hiffs
North Fish Lake Valley
Stiver Peak Range (Robinson I Chowder,
1973)
JJJ/
ITltlTlTSilver Peak Volcanics
Esneralda Pn. •
Basaltic Andesite
TIT)"oznrJJJJ Tel 1U^>er Trl
Andesite
Hornblende Andesite
Rhyoliteniffs
Southern Monte Crlsto Range
(Moore, 1981)
Esneralda Fo nation
OHIO ICO :Coarse-grained Andes.
icon icon:Fine-grained Andes.
Castle Peak Tuff
Monte CrlstoRange
(Stewart,pers. conn., 1983)
_ _ I"'Basalt jsed. Rocks
|Rhyo. Intr.*
Esneralda Fo nation
icon icon:Gilbert Andesite
McLeans Fn. (diaton.)
§ Upper Mb. ^ 1 Coarsegrain.andes.
Junct;
latior | \55 Lower Mb.
1Fines
1 1• - sedlhehts “.i i u n sitrain.andes.
JULLCastle Peak Tuff
Southwestern Monte Crlsto Range
(this report)
Esneralda Fo nation
Rliyo. Intr.
PyroxeneAndesiteOne-grainHornblendeAndesite^
sedsjtuff
Banded r l iy o ^ /
Castle Peak Tuff
LnK)
53
Range (Figure 1.6). It has been dated at 23.9 ±0.6 and 24.4 ±0.6 m.y.
in the northwestern portion of the project area (Table 2; Figure 14).
This unit clearly corresponds to the late Oligocene tuffs in the
Candelaria Hills (Speed and CogbilL, 1979a) and rhyolite tuffs at the
base of the Silver Peak volcanic sequence (Robinson et al., 1968).
Current work by Stewart (pers. comm., 1983) in the Monte Cristo
Range suggests the Castle Peak Tuff, mapped by Moore and myself as one
unit in the south, may be composed of two tuff units. He places the
Castle Peak Tuff (proper) below the informally named tuff of Blair
Junction (Figure 16). Stewart feels that the age date reported here
may be on the upper ash-flow. This would make the "true" Castle Peak
Tuff of Stewart equivalent to the older tuffs of Speed and Cogbill
(1979a). Supporting this hypothesis is an older and regionally exten
sive series of ash-flow tuffs in the Gabbs Valley and the Gillis Ranges,
northwest of the Monte Cristo Range (Ekren et al., 1980). These range
in age from 24 to, 28 m.y.
As mentioned above, however, my work did not find any appreci
able petrographic differences in the Castle Peak Tuff as mapped in the
current study area. Thin section inspection of the upper and lower
cooling units present where the age date sample was taken (Figure 14)
showed no irregular phenocryst or lithic assemblages. On this basis,
and supported by the similarity of andesites as discussed below, the
Castle Peak Tuff in the current study area is taken to be one unit.
Although two tuffs may be present elsewhere in the Monte Cristo Range,
only one is believed present on its southwestern flank. Further
54
radiometric age control and a detailed study of the tuffs should
clarify the possibility of multiple ash-flows in this region.
The flow banded rhyolite dikes clearly intrude the Castle Peak
Tuff in the central portion of the study are (C-C, Figure 15). Their
compositional similarity to the Castle Peak Tuff suggests an age only
slightly younger than the 24.2 m.y. old tuff.
Coaldale Volcanic Sequence
The sediment and tuff unit of this sequence clearly underlies
all andesitic units and overlies the Castle Peak Tuff; it is the
oldest of the Coaldale volcanic sequence (Figure 14). This unit has
no known correlatives outside the Monte Cristo Range (Figure 16).
Intruded and overlain by the pyroxene and fine-grained andesites,
the hornblende-rich units are the oldest andesitic volcanic rocks in
the Coaldale sequence.• All the hornblende units are related closely in
age; the Blue Mountain plug has been K-Ar dated at 22.2 ± 0.5 m.y. (Table
2). Within this series the flows are the youngest and overlie the tuff
breccia. Although the tuff breccia and lahar may be time equivalent,
one near source and the other more distal, their contact exposure
suggests the tuff breccia, at least in part, overlies the lahar (B-B',
Figure 15). The relative age of the plugs is more difficult to ascer
tain. Although the Blue Mountain plug intrudes the lahar, it could
have been partially time equivalent to lahar and flow eruptions.
Studies of modern andesitic centers (MacDonald, 1972) suggest intrusion
occurs throughout the volcanic activity, with a larger pulse towards its
end. Such a scenario is implied in the Blue Mountain area. The
55chloritically altered dikes and sills of the hornblende andesite intrude
almost all units in the study area. Their intrusion presumably climaxed
during the maximum phase of hornblende andesite activity but definitely
continued through most of the Coaldale volcanism (C-C, Figure 15).
The pyroxene andesites show variable contact relationships.
They unconformably overlie and intrude both the hornblende-rich units
and the sediment and tuff unit (Figures 2 and 15).. This suggests at
least a moderate angular deformational and/or erosional period occurred
prior to their extrusion. Like the hornblende-rich units, the pyroxene
andesites are all closely related in age and probably just slightly
younger than 22.2 m.y. old Blue Mountain plug. The pyroxene tuff
breccia is clearly 'the oldest unit, and the flows, dikes and sills the
youngest. The small, plugs probably did not span as great a length of
intrusion as the hornblende plugs, and are most likely the youngest in
this series.
Although the fine-grained andesite is younger than the horn
blende units (C-C, Figure 15), its upper age limit is not constrained.
As it is compositionally intermediate between the widespread hornblende
and pyroxene volcanic rocks, the age of the fine-grained andesites is
believed to be restricted between these two events. Without good age
control, however, a younger age cannot be refuted.
The youngest unit of the Coaldale volcanic sequence is the
small rhyolite plug in the southeastern portion of the study area. With
its lack of contacts with any of the other units, the age of this plug
was unknown. It has been K-Ar dated at 16.8 ± 0.4 m.y. (Table 2),
56slightly younger than the andesitic units that dominate the southwestern Monte Cristo Range,
Although difficult to correlate regionally, the Coaldale
volcanic sequence is easy to equate with other volcanic rocks within
the Monte Cristo Range, Regionally, this sequence may correspond with
the undated andesitic volcanic rocks in the Silver Peak Range (Figure
16). The lower hornblende unit is especially similar in composition
and texture to a hornblende andesite in the Silver Peak Range. Within
the Monte Cristo.Range, the Coaldale sequence is considered equivalent
to the Blair Junction Formation of Stewart (Figure 16). The lower
member of the Blair Junction Formation is fine-grained and hornblende-
rich and the upper member coarse-grained and plagioclase- and
pyroxene-rich, strikingly similar to the Coaldale units. Again, no
intervening ash-flow tuff occurs between the andesites in the study
area as in Stewart's section.
The Coaldale sequence corresponds to the coarse- and fine
grained andesites of Moore (1981) in the Blair Junction area. Without
age control, Moore considered these equivalent to the 15 m.y. old
"Gilbert Andesite" because of their petrographic similarity. Recent
work by Stewart (pers. comm., 1983) suggests the dated "Gilbert
Andesite" is equivalent to Moore's younger basaltic andesite (Figure
16). The young rhyolite plug in this study is the same as Moore's
Tri unit, and much older than he suspected. His Tai unit is the same
as the chloritically altered, coarse-grained hornblende dikes in the
current project area. Moore assigned a younger age to both these units
57
(Figure 16) because he thought they intruded the Esmeralda Formation.
A re-evaluation of these contacts makes this interpretation suspect.
The interior joint planes, and when observable the exterior, of the
dikes are silicified and slickensided, suggesting a fault-controlled
contact of the two units.
Blair Junction Volcanic Sequence
The Blair Junction andesites were not radiometrically dated in
this study; they are compositiona11 y and stratigraphically similar to
the Coaldale volcanic sequence. On this basis, they are assigned an
early Miocene age and correlate to the coarse- and fine-grained units
of Moore (1981) and the Blair Junction Formation of Stewart (pers.
comm., 1983) as shown in Figure 16.
The relative ages of the units in this sequence are quite clear.
The sediments overlying the Castle Peak Tuff are the oldest and roughly
correlate to the sediment and tuff unit at the base of the Coaldale
sequence to the southwest. The volcanic breccia and andesite flows are
probably time equivalent, with the basal breccia being formed during the
transport of the flows. The fine-grained andesites cannot be assigned
a definitive time bracket, but most likely postdate the sediments and
are equivalent to the flow units.' - • ’ , :
Younger Sedimentary and Basaltic Sequence
The Esmeralda Formation sediments in the study area are just a
small part of this regionally widespread unit (Figure 16). They are a
portion of the lowest member of the Blair Junction sequence of the
58
Esmeralda Formation of middle Miocene age (Moore, 1981). Excellent
exposures of the Coaldale sequence of this formation are seen just
south of Coaldale at the northern end of the Silver Peak Range
(Robinson et al„, 1968).
The local outcrops of the basalt in the northwestern portion of
the study area are believed the youngest volcanic rocks in the south
western Monte Cristo Range„ Extremely fine-grained and hypersthene-rich,
they are similar to the voluminous undated basalts in the northwestern
Monte Cristo Range. Young basalts of Pliocene age occur throughout
this region (Figure 16). Although the regional basalts are olivine-
rich, a tentative correlation can be drawn with the basalts of the
Candelaria Hills, with isotopic ages of 3.0 ±0.1 and 4.0 ± 0.4 m.y.
(Marvin et al., 1977), and in the Silver Peak Range, with an isotopic
age of 4.9 ±0.6 m.y. (Robinson et al., 1968). As the basalts in the
study area lack age data, they could possibly correlate to Moore's
basaltic andesite and Stewart's "Gilbert Andesite" although they lack
the porphyritic texture distinctive in both.
CHAPTER 5
GEOCHEMISTRY
Whole Rock Chemistry
Whole rock geochemistry was done on nine representative
volcanic units in the study area. In addition to the three samples
radiometrically dated, the Castle Peak Tuff, the hornblende intrusion
and rhyolite plug, the six units chemically analyzed were: the fine
grained andesite, the chloritically altered hornblende sill, the
banded rhyolite dikes, and flow units of the hornblende, pyroxene,
and coarse-grained andesites. The chemical analyses and normative
mineralogy are presented in Tables 3 and 4, respectively. Sample
descriptions are included as Appendix A and sample locations are on an
overlay to the geologic map (Figure 14). The analysis of the hornblende
sill sample falls well below a 100% oxide composition; although not
acceptable data, its oxide composition is similar to the other horn
blende units, and it is included in the following discussions.
The terms applied to the units thus far in this report were
field descriptions; with the chemical data one can apply a stricter
rock classification to the units. The IUGS classification (Streckeisen,
1979), based on the normative mineralogy calculated from the chemical
analyses, is used here and presented in Table 5.
According to this classification, all the Coaldale andesitic
units should be considered quartz latites and the coarse-grained Blair
59
TABLE 3
Chemical Analysis (in weight percent) of Tertiary Units Southwestern Monte Cristo Range
Oxides CVC 3 CVC 4 CVD 2 CVC 6 CVC 8 CVD 5 CVC 10 CVD 7 CVC 12Tfai Thais Ti Tri Tha Thai Tpa TcP3 Tea
sio2 58.40 58.60 77.90 67.20 . 57.90 58.30 59.00 70.90 61.10Ti02 1.04 0.97 0.34 0.74 0.86 0.79 1.26 0.37 0.88Al2*3 18.50 16.60 11.80 16.30 18.10 17.40 17.00 14.10 16.30
1.97 2.03 0.29 0.89 2.04 2.09 2.51 0.54 - 1.62FeO 3.61 3.71 0.63 1.95 3.79 3.89 4.53 1.15 3.13MnO 0.11 0.07 0.03 0.44 0.10 0.11 0.09 0.02 0.04MgO 2.08 2.43 0.15 0.30 2.14 2.67 2.48 0.53 1.86CaO 5.07 4.72 0.22 1.30 5.84 4.77 5.86 1.86 4.05Na20 4.36 4.12 1.87 4.22 4.57 4.54 3.89 3.54 4.09K2° 1.96 2.09 4.70 4.53 2.21 2.23 1.90 3.96 2.91P2°5 •— — -- -- — . — ■ — —CO' 0.04 0.16 0.02 0.07 0.03 0.04 0,11 0.03 0.19
H2° 0.51 — 0.67 1,17 0.68 1.37 1.23 0.94 1.59Total 98.05 95.92 98.69 98.92 98.69 98.63 100.37 98.06 98.11Sum of the Alkalies 6.32 6.21 6.57 . . 8.75 . 6.78 ... ..5.76 . . 5.79 7.50 7.00
All analyses done at the University of Arizona Analytical Research Laboratory.
OnO
TABLE 4
Barth Normative Mineralogy (in weight percent) of Tertiary Units Southwestern Monte Cristo Range
Normative CVC 3 CVC 4 CVD 2 CVC 6 CVC 8 CVD 5 CVC 10 CVD 7 CVC 12Mineralogy Tfai Thais Ti Tri Tha Thai Tpa TcP3 TeaSialic Minerals 82.28 81.05 98.14 94.29 81.15 80.42 78,79 94.47 84.63
Quartz 12.13 13.55 46.92 21.01 8.01 9.32 13.38 29.75 15.18Orthoclase 11.90 12.94 29.13 27.48 13.32 13.54 11.46 24.31 17.89Albite 40.22 38.76 17.61 38.90 41.87 41.88 35.65 33.03 38.21Anorthite 12.84 11.94 0.53 3.17 14.72 12.08 14.62 4.73 10.07Corundum 5.19 3.87 3.95 3.73 3.24 3.60 3.68 2.64 3.27
Femic Minerals 17.72 18.95 1.86 5.71 18.85 19.58 21.21 5.53 15.37Enstatite 5.90 7.03 0.43 0.85 6.03 7.57 6.99 1.52 5.34Wollastdnite 5.14 4.78 0.21 1.27 5.89 4.83 5.85 1.89 4.03Ferrosilicate 3.02 3.24 0.37 1.47 3.48 3.74 3.72 0.95 2.65Magnetite 2.11 2.23 0.32 0.95 2.18 2.24 2.63 0.58 1.77Ilmen it e . 1.49 1.42 0.50 1.06 1.22 1.13 1.79 0.54 1.28Calcite 0.06 0.26 0.03 0.11 0.05 0.06 0.18 0.05 0.31
Plagioclase oligoclase oligoclase albite albite oligoclase oligoclase oligoclase oligoclase oligoclaseComposition Ab76-An24 Ab76-An24 Ab97-An3 Ab92~An8 Ab74~An26 Ab78™An22 Ab7r An29 Ab87-An13 Ab79~An21
Differentiation Index 64.25 65.24 93.66 87.39 63.20 64.74 60.49 87.10 71.29
os
62
TABLE 5
IUGS Classification of Chemical Analyses Southwestern Monte Cristo Range
Sample Lithologic Unit Classification
CVC 3 Tfai Quartz Latite
CVC 4 Thaig Quartz Latite
CVD 2 . Ti Alkali (feldspar) Rhyolite
CVC 6 Tri Alkali (feldspar) Rhyolite
CVC 8 Tha Quartz Latite
CVD 5 Thai Quartz Latite
CVC 10 Tpa Quartz Latite
.CVD 5 Tcp Rhyolite
CVC 12 Tea Quartz Trachyte
Classification based on normative mineralogy, from Streckeisen (1979).
63
Junction andesite a quartz trachyte. As most of the "andesites" in
these sequences lacked modal quartz (Table 1), a latitic and trachytic
IUGS classification can be applied. Both intrusive rhyolites in the
study area are classified as alkali (feldspar) rhyolites by their
chemical and modal compositions. The Castle Peak Tuff is of rhyolitic
composition. Figure 17 shows the placement of these units in the IUGS
classification scheme.
Oxide variation diagrams have been used in many volcanic centers
to help determine the genetic relationship of variable rock types
(Krauskopf, 1979; Carmichael et al., 1974; Cox et al., 1979; Keith,
1977). The weight percent oxide compositions are commonly plotted
against percent SiO^. Also useful are plots of percent oxides versus
a differentiation index parameter, defined as the sum of normative
quartz + orthoclase + albite + nepheline + leucite + kalsite (Thorton
and Tuttle,.1960). Both these variables serve to measure the change
in a magma's composition with increasing differentiation. Triangular
plots, usually of alkali, iron and another variable, are also useful
and show the simultaneous variation of three components.
Figure 18 is a plot of the sum of the alkalies, Na20 and K^Q,
versus percent SiC^ for all the analyses, and a plot of percent SiO^
versus the differentiation index (DI). Two triangular plots of the
analyses are given in Figure 19 and show the variation of alkalies-
iron-magnesium.(AFM) and sodium-potassium-calcium (Na-K-Ca) oxides.
The calc-alkalic— tholeiitic compositional division is plotted on the
alkali versus SiC^ plot and the general calc-alkaline differentiation
64
Q
alkali (-feldspar) rhyolite
rh yo li te dacite
quartz-alkali(-feldspar)
trachyte
quartzlatite
alkali( fe ld s p a r )
trachytebasaltquartz
trachyteandesite
latitetrachyte andesite
Figure 17. IUGS Classification of Chemical Analyses, Basedon Normative Mineralogy. Q = quartz; P = anorthitic plagioclase; A = alkali feldspar.
Figure 18* Variation Diagrams of Chemical Analyses, Differentiated by Volcanic Sequencee Calc-alkalic— tholeiitic division on upper plot from Carmichael and others (1974)0 Differentiation index (DI) in lower plot defined as the.sum of normative quartz + orthoclase + albite + nepheline + leucite + kalsite (Thorton and Tuttle, 1960)e
O • Castle Peak Sequence
0 Blair Junction Sequence
O Coaldale Sequence
WT.
% S
i02
Sum
of
Alk
alie
s65
10-i
 4 th o le i l t ic /
ca lc - alkaline
H h 60 70 80 90
WT. % S i 0 2
80-|
70- ©©
60-• v
Q
50-
H h60 70 80 90 100
D I
Figure 18
Figure 19. Triangular Variation Diagrams of Chemical Analyses, Differentiated by Volcanic Sequence. Coordinates of lower plot defined as: A = Na20 + K2 O; F =FeO-j.; M = MgO. Calc-alkaline trend shown from Carmichael and others (1974).
© Castle Peak Sequence
E3 Blair Junction Sequence
O Coaldale Sequence
66
k2o
Na20
F
ca lc -a lka l ine trend
Figure 19.
67
trend on the AFM diagram. Both these reference lines are from
Carmichael and others (1974). In these figures each volcanic sequence is differentiated by symbol.
As readily apparent from these figures, the analyses straddle
the calc-alkalic^-fholeiitic composition line but also conform well
with a calc-alkalic differentiation suite. These apparent relation
ships, particularly emphasized by the linear trend on the SiO^ versus differentiation index plot (Figure 18), are misleading. There are no
geologic indicators that these units have a common magmatic parent;
certainly no caldera features (other than the exotic blocks) were noted
in the study area that suggested an immediate source of the Castle Peak
Tuff, nor does the 'relative position of the Blair Junction trachyte
indicate a common magmatic source with the Coaldale latites. What
these plots do suggest by their linear trends is that the individual
magmas may have had similar compositions with similar differentiation
paths.
Gradational petrographic compositions and the local distribution
of the older hornblende-rich to pyroxene-rich Coaldale volcanic rocks
suggest a single zoned magmatic system was succesively tapped to ever
deeper levels through time (Figure 2; Table 1). The variation within
one system can be tested by using only the Coaldale latites in oxide
variation plots. Figure 20 presents plots of the weight percent oxides
versus SiC^ composition, and Figure 21 gives the oxides versus the
differentiation index for the Coaldale latites. The Blair Junction
trachyte is included for comparison. Most of these diagrams do hot
CaO
A
I2O
3 N
a20
08
2.0-
0.5-
6 A
6 ©
4' 0
56 ASIO,
Coaldale L a tltea
• hornblende O fine-grained A pyroxene
B lair Junction Trachyte
B coarse-grained
Figure 20 Oxide-Silica Variation Diagrams of Coaldale and Blair Junction Chemical Analyses, Differentiated by Rock Type.
69
6.0-1
18-1
nOCM18-
17- A
©
ie-J--r66 60 65B
70D I
7-nA
K*'O£ 6-
□5 5 6 0 6 5 7o
D I
Coaldale Latltea
• hornblende © fine-grained A pyroxene
Blair Junction Trachyte
□ coarae-grained
Figure 21. Oxide-DI Variation Diagrams of Coaldale and Blair Junction Chemical Analyses, Differentiated by Rock Type.
70
show pronounced patterns because the units do not have greatly varying
compositions. The Coaldale rhyolite plug is not included in these
plots as a consequence of its extremely SiC^-rich composition and the
lack of intermediate dacitic or rhyodacitic units in the Coaldale
sequence. This lack makes the assumption of a common magmatic source
for the Coaldale latites and rhyolite plug uncertain.
A few of the oxide plots in Figures 20 and 21 do show trends
within the Coaldale latites. Both the flow units are considerably
more CaO-rich and SiO^-poor than the intrusive units, a function of
the predominance of microlitic plagioclase in each (Table 1). The
pyroxene flow is much more Fe-rich and slightly less K^O- and Na20-rich
than the hornblende units. A more complete analysis of the Coaldale
units is not possible; the tuffaceous and other intrusive units of
this sequence (Tpai, Tpat, That, Thai, and Tst) are generally too
weathered to yield reliable chemical results'.
The inclusion of the Blair Junction trachyte in these plots
allows a comparison of the relative position of magmatic differentiation
at the time of the andesitic eruptions. The high SiC^ and K^O and low
CaO, MgO, and FeO content of the Blair Junction trachyte suggests a
more differentiated parent magma than the Coaldale latites (Figures 18
and 19). In fact, the overall trends of the oxide versus differentia
tion index plots (Figure 21) show a surprising linearity considering
they are of different source magmas, and further supports a similar
differentiation path in each.
71
Isotope Geochemistry87Sr
Initial ---- isotope ratios were determined on the three age86Sr
dated samples, the Castle Peak Tuff, the hornblende intrusion of Blue
Mountain, and the small rhyolite plug in the southeast portion of the
study area. As mentioned above, all sample locations are plotted on
an overlay to the geologic map (Figure 14), and all sample descriptions
are included as Appendix A„ Initial Sr isotopes have long been used as
geologic indicators of commonality of parental magmas and the type
and/or contamination of the source material from which these magmas
were derived. That is, magmas derived from sources with low Rb/Sr87Sr
ratios (e,g„ the mantle) should have low initial — ratios, whereas ■ , 8bSrthose derived from, or contaminated with, sources with high Rb/Sr
ratios (e.g, the upper continental crust) should have higher initial 87cr— — ratios (Cox et al„, 1979; Faure and Powell, 1972; Faure, 1977b),8 6 SrA summary of the isotope data is presented in Table 6,
Both the rhyolitic and latitic initial ratios are in the
range of other determinations of equivalent rock types. At 0,7086 and
0,7089 the rhyolitic values are slightly more radiogenic than the
0,7054 average of ash-flow tuffs in the San Juan caldera complex
(Lipman et al„, 1978)* In contrast, they are less radiogenic than other
rhyolites in southwestern Nevada which have ratio values that range from
0.710 to 0.715 (Noble and Hedge, 1969). The hornblende latite initial
ratio of 0.7054 is also slightly more radiogenic than the average 0.704
value of andesites in continental margins (Carmichael et al., 1974).
TABLE 6Analytical Data for Initial Sr Isotopes of Tertiary Rocks
Southwestern Monte Cristo Range
SampleNo. LithologicUnit K-ArAge to(PPm)
Sr(PPm) 87to/86Sr ■ Measured87Sr/86Sr Initial87Sr/86Sr Location
CVD 2 Ti 18.6 212 26.8 22.92 0.71474 ±0.00007 0.7086 small plug in SE 38°02.51' N 117°50.36' W
CVD 5 Thai 22.2 70 1398 0.145 0.70542 ±0.00006 0.7054 SE flank of Blue Mtn.38°04.35' N 117°51.40' W
CVD 7 Tcpj 24.2 155 450 0.997 0.70921 ±0.00007 0.7089 Upper cooling unit in NW 38°05.24' N 117°52.54' W
decay constant: X = 1.42 ICT1 yr~* All analyses done at the University of ArizonaIsotope Geochemistry Laboratory. All isotopeuncertainty: 87Sr/86Sr ± one std. deviation determinations done with a Nier-type 60° sectorD. q meaf" ine 6” mass spectrometer. Elemental determinationsmeas. -5 t0 1U'6 done with X-ray fluorescence.
73
Several implications of commonality and type of magma sources
of the volcanic rocks in the study area are apparent in these initial 8 7 S r— — determinations. The hornblende intrusion, with a low value, ti6Srclearly was derived from a less contaminated or less radiogenic source
material than the rhyolites. The higher values of the Castle Peak Tuff
and Coaldale rhyolite plug indicate derivation from similar radiogenic
sources. The discrepancy between the initial ratios of the Coaldale
rhyolite plug and latite intrusion deny the possibility of a common
magmatic source, unless the magma was contaminated by crustal material
after the andesitic phase. Although the Castle Peak Tuff and the
Coaldale rhyolitic plug were probably derived from magmas with similar
source materials, a single parental magma is not supported by their
age difference and absence of geologic indicators (see above). When
taken in concert with the implications of the oxide variation diagrams,87g
the difference in initial ---- ratios suggests that the Castle Peak86Sr
Tuff, Coaldale rhyolite and Coaldale latite each had discretely
different magmatic sources that followed similar differentiation paths.
Further, the latitic magmas had less crustal contamination than the
rhyolitic magmas.
CHAPTER 6
STRUCTURAL GEOLOGY
The project area lies near the intersection of two highly-
faulted, large structural discontinuities, the northwest-southeast
Walker Lane shear zone and the east-west Warm Springs lineament
(Figure 3; Albers, 1967; Albers and Stewart, 1972; Ekren et al., 1976;
Speed and Cogbill, 1979b)„ It is not surprising, then, that its
dominant structural feature is a series of steeply dipping faults that
trend east-west and northeast to northwest. These faults crosscut a
broadly arched volcanic homocline that dips to the south and flanks a
complexly deformed pre-Tertiary basement. Four structural domains
have been defined to aid in identifying changes in structural style
throughout the project area. The northeast (NE), southeast (SE),
central (C), and northwest (NW) domains, with the fault traces in each
are located on an overlay to the geologic map (Figure 14). Figure 15
contains four geologic cross-sections that illustrate the structural
and stratigraphic relationships within each domain.
Folds
The attitudes of the volcanic units within the study area vary
only slightly; their average attitude, from a contoured plot of the
poles to bedding or foliation, strikes 32°W and gently dips021- S
(Figure 22). Evaluation of the attitudes in the NW, C, and SE domains
75
N
All Domains n* 169
avg. 32°W/21eS
NE Domain n>39
avg. 80oE/22°S
SE DomainC Domainn*55
Figure 22. Equal Area Stereonet Plots of Strike Data. All plots with 5%-10%-(15%-20?o) contour intervals.
76
individually» however, shows a gradual change in strike and dip across
the area that defines a broadly arched, south dipping homocline (Figure
22)„ The average attitude in the NE domain is similar to the east-
northeast trend in the SE domain but cannot be considered part of the
large fold structure because of the wide intervening expanse of highly
deformed pre-Tertiary rocks in the central portion of the study area
(Figure 2; B-B’, Figure 15).
The large arched homocline is ill-defined but is the only
recognizable fold structure in the volcanic rocks of the study area.
Unconformable contacts between the pyroxene latites and other strati-
graphically lower units (C domain. Figure 2) imply that periods of
uplift and/or erosion occurred during volcanic activity. The broad
arch that dominates the southwestern portion of the project area may be
controlled in part by the original depositional attitudes of the
volcanic units, but has also been accented by the tilted uplift of the
Monte Cfisto Range during Basin and Range faulting.
Faults
A pronounced faulted structural fabric dominates the project
area (Figure 2). Most faults are readily apparent on both aerial
photographs and in the field. Slickensided surfaces, silicified
breccias, clay gouge zones and minor silica or calcite veining are the
most common features associated with the faults and are very pronounced
in the wider fault zones. The high competency of these units makes
local drag structures along fault planes rare. Although not studied
77quantitatively, most faults have 1' to 25’ wide envelopes of closely
spaced joints 0.5" to 2 ’ apart.
The study area has two discrete sets of steeply dipping faults
that trend east-west and northeast to northwest. The orientations of
the faults in the area are included in Figure 23a. This includes a plot
of all trends on a polar coordinate frequency diagram and two equal area
stereOnet plots, one of the poles to the fault planes and another with
5% and 10% contoured intervals of the same data. Dips of 75° were
assumed for faults without observable data; measured dips range from
60° to 90° and average close to 75°. Dip direction was chosen by the
observed displacement of units. As can be seen in these diagrams,
fault trends vary almost continuously throughout the study area. When
the frequency distributions are examined by domain (Figure 23b), a
gradational change in fault orientations occurs across the southwestern
portion of the study area. The prominent trends vary from strongly
northeast-southwest directions in the SB domain to more variable north-
south orientations in the C domain to, finally, northwest-southeast
trends in the NW domain. Both the NW and C domains are also character
ized by an east-west fault trend (Figure 23b); this secondary trend is
almost as strong in the NW domain as the predominant northwest trend.
Measurable normal separation along the majority of these faults
is rare because they lack known marker horizons and the thickness
variations of the units offset. Using regular thickness changes in
the units, the normal separations shown on the geologic cross-sections
(Figure 15) are approximate but reveal offsets from 10’ to 300’. The
Figure 23, Equal Area Stereonet Plots and Frequency Diagrams of Fault Data,
a. Fault data of all domains. Equal area stereonet plots of poles to fault planes and a contoured plot of the same data (5%-10% interval), Frequency diagram of all fault traces in polar coordinates*
b. Frequency diagrams of fault traces in polar coordinates, differentiated by domain.
X NW Domain
O C Domain
A SE Domain
□ NE Domain
°o t>n
N
Figure 23a.
NI
S X 1 O 1n
NW Domain
N
T i
C Domain
N
i i iNE Domain
I
* 1 1 0 1 2 3n
SE Domain
Figure 23b
79
N
n = 10
□ — NE Domain Q - C Domain X ” NW Domain
Figure 24. Equal Area Stereonet Plot of Trend and Plunge of Fault Slickensides.
80
west flank of Coaldale Ridge (D-D', Figure 15) has a 500' separation
of the base of the hornblende flow units across four major faults that
strike north and dip steeply west. Lateral separations of the units
along the faults are more easily documented. Most north-northwest to
north-northeast trending faults show right-lateral separations of 20'' /
to 100*, whereas most east-west trending faults show separations of
507 to greater than 5007 (Figure 2)0
True slip directions were determined on 25% of the faults in
the project area* The trend and plunge of the measured slickensides
are shown in Figure 24* These lineations show both steep and shallow 'plunges, the latter with northwest to southeast orientations. Com
bining these directional indicators with known separations along the
faults indicates that although both sets are oblique-slip faults, the
north-trending ones are dominantly dip-slip faults with only minor
right-lateral components and the east-west faults primarily left-
lateral strike-slip with minor normal components. This can be
generalized to categorize most of the normal and left separation faults
throughout the project area.
Although most of the faults offset all volcanic units, there
are several indications -of fault activity concurrent with volcanism.
Localization of the small rhyolite plug (CVD 2, Table 2) along one of
the faults in the SE domain (Figure 2), suggests that faulting occurred
as early as 18 m.y. B.P. Unconformable volcanic contacts indicate
periods of uplife and/or erosion during volcanic activity; active
81
faulting in modern volcanic complexes is common (MacDonald, 1972) and implies faulting helped control volcanic deposition..
As the prominent north and east trending faults in the project
area displace all Tertiary volcanic rocks, most of the fault deformation
is younger than 18 m.y. B.P. Mutually crosscutting fault relationships
in the study area indicate overlapping movement histories. On Coaldale
Ridge the east-west faults offset the north trending faults, but in the
NW domain at least one northwest trending fault offsets an east-west
fault (Figure .2), Variable displacements along several faults suggests
multiple movement histories and reactivation by segments. The post
18 m.y. faulting may have reactivated some of the earlier faults active
during vplcanism.
One pre-Tertiary fault that controlled the deposition of the
Castle Peak Tuff in the interior of the central Paleozoic high in the
study area deserves special mention (Figure 2). Breccias, slickensides
and complex drag folding occur on either side of this structure.
Although only three segments could be documented, it is believed to
be a single (or at least en echelon) fault structure that strikes north
west and dips moderately to steeply northeast. Nowhere was the tuff
offset by this structure, rather, the base of the tuff was occasionally
underlain by severely baked soil horizons. This northwest structure is
not only older than 24 m.y. but it also had a strong topographic
expression that caused the ponding of the Castle Peak Tuff. Several
other isolated outcrops of the tuff in the Paleozoic high also occur and
are most likely expressions of older fault-controlled topography (Figure
2).
82
Hornblende and Pyroxene Dikes
These dikes occur throughout the southwestern portion of the
study area in the NW, C and SE domains. Dikes are tensional features
and their orientations commonly indicate the minimum stress direction
that operated during volcanic activity, unless emplaced in a strongly
oriented prevolcahic basement fabric.
The hornblende and pyroxene dikes are largely very linear
features with easily measured attitudes; only in a few cases are the
dikes more arcuate than straight and contact attitudes or flow folia
tions unobservable (Figure 2). The orientation of the dikes throughout
the project area is shown in Figure 25a. As with the fault data, this
figure includes a frequency diagram as well as two equal area stereonet
plots of their poles and 5% and 10% contour intervals of the data. The
plots,show a strong northwest trend with steep, but variable, dips for
the overall area. A secondary northeast trend with southward dips also
occurs. A northwest trend is apparent in each domain (Figure 25b), but
accented by the strong alignment and concentration of the dikes in the
C domain. The northwest trending dikes in this domain occur on the
southern flank of Blue Mountain and dip toward this intrusion (D-D',
Figure 15); The dikes form a half-arc, or partially "concentric,"
intrusive pattern around their source, as is common in other andesitic
volcanic centers (MacDonald, 1972). The northeast trending dikes occur
outboard of Blue Mountain and form a perpendicular to radial pattern
away from this plug. Like the northwest dikes, the northeast trending
dikes are found in all domains (Figure 25b).
Figure 25« Equal Area Stereonet Plots and Frequency Diagrams of Dike Data,
a. Dike data of all domains. Equal area stereonet plot of poles to dike orientations
and a contoured plot of the same data (5%- 10% interval),
b. Frequency diagrams of dike orientations in polar coordinates, differentiated by domain.
X NW Domain
O C Domain
A NE Domain
S3
N N
All Domains
n»49
i 2 a 5 ! e 7 el r I iAll Domains
Figure 25a.
N NI I
s 2 r 1 2 3
SE DomainNW Domain
C Domain
e ; ; i i $ j
Figure 25b
84
Excellent flow foliation, lack of xenolithic material and
only thinly chilled margins along the dike contacts indicate non-
obstructed dike emplacement. Most dikes do not show much^ if any,
relative movement of their walls.
Discussion
The folds, faults and dike's are all structural elements that
can be used to interpret the forces and differential stresses that
caused the observed deformation in the study area. The broad arching
of the volcanic rocks indicates uplift in the central domain, perhaps
near the Blue Mountain intrusion, and a relative downdropping of the
surrounding volcanic pile especially in the southeast. Localization
of the small rhyolite plug along a northeast trending fault in the SE
domain (Figure 2) suggests that tensile stress was operative during
volcanism, and may have been a determining factor in the eruptive
history of the area. The strong northwest orientation of the dikes on
the flank of Blue Mountain implies a N40-50°E directed tensional stress
near their intrusive source. A more northeast and northwest rectilinear
dike pattern elsewhere in the area suggests that a much less directed
stress regime also operated during the volcanic activity. Their
patterns may also be controlled in part by an older fracture pattern
in the pre-Tertiary basement.
The post-volcanic fault system has two elements. First, the
northeast to northwest trending, dominantly dip-slip faults indicate a
general east-west extensional stress. Its orientation grades from more
northwest-southeast in the SE domain to northeast-southwest in the NW
85
domain„ These largely dip-slip faults belong to the north trending,
high-angle Basin and Range style normal faults predominant in this:,
physiographic province. Their right-lateral oblique movements are
consistent with the right-handed shear direction documented in the
Walker Lane deformational zone (Albers, 1967; Albers and Stewart, 1972;
Stewart, 1980). The east-west trending, dominantly left-lateral strike-
slip faults create a secondary fabric across the study area. These
irregularly present faults form a gross conjugate shear system with
the northwest to northeast trending right-lateral dip-slip faults.
This conjugate system is similar to fault trends in adjacent
areas. Left-lateral movements along the eastrwest faults in the
Candelaria Hills are well-documented by Speed and Cogbill (1979), and
similar structural trends are suggested north of the Volcanic Hills
(Stewart, 1979) and in the northern Silver Peak Range (Robinson et al.,
1976). Although oblique-siip, the east-west faults in the study area
probably formed in response to the same regional forces that caused
the fault deformation in adjacent areas and indicate conjugate shear
as well as extensional stress existed. The secondary east-west faults
are analogous to the left-lateral oblique-slip faults documented
throughout the Warm Springs lineament suggested by Ekren and others
(Figure 3; 1976).
CHAPTER 7
ECONOMIC GEOLOGY
Previous exploration and mining in and near the Monte Christo
Range has included coal, industrial minerals, and both base and precious
metals. Lignitic coal was discovered in the Esmeralda Formation south
of the study area near Coaldale, and much of the Monte Cristo Range
was included in a Nevada, coal withdrawal area in the early 1900's.
The Gilbert and South Gilbert districts, northeast of the study area
in the center of the Monte Cristo Range, have produced gold, silver,
antimony and mercury. Molybdenum, with minor gold and antimony, was
mined just east of Gilbert in the mid-1900's (Albers and Stewart, 1972).
Precious metal exploration continues sporadically throughout the Monte
Cristo Range and recent drillsites, collared in jasper- and barite-rich
pre-Tertiary rocks, were found in the southern portion of the project
area.
Extensive borate exploration and production has occurred in
this portion of Esmeralda and Mineral Counties. Total borate production,
occurring in the late 1800's, may have reached $900,000.00. Most of
this production was from cottonball ulexite (NaCaB^Og ° SH^O) mined from
marsh, or playa, deposits. The earliest production was from Columbus
Marsh just west of the study area. Fish Lake Valley (west of the Silver
Peak Range), Teels Marsh (west of the Candelaria Hills), and Rhodes
Marsh (west of the Pilot Range) are other mined ulexite deposits. Minor86
87
borate exploration and production occurred in 1939 and concentrated
in young Tertiary lacustrine sediments in the Silver Peak Range (Albers and Stewart, 1972).
Fallen timbers mark two caved shafts on the western flank of
Coaldale Ridge in the study area (Figure 26). Surrounded by extremely
B-rich soils over the volcanic tuffs in this area, these structures
indicate that active borate exploration also took place in the current
project area, probably in the late 1800?s.
Formation of Borate Deposits
The two major types of borate deposits are the playa concen
trations, such as Columbus Marsh, and the bedded borates in young
lacustrine sequences, such as the Kramer deposit near Boron, California.
In both, the borates are associated with anomalous Li and (sometimes)
Sr. The major borate minerals found in the lacustrine deposits are
borax (Na^B^Oy "lO^O), kernite (Na^B^Oy ° 42^9), ulexite (NaCaB^Og- SH^O)
and colemanite (Ca^B^O^^ <> SH^O). Ulexite and borax are the major
borate minerals in playa deposits (Kistler and Smith, 1975; Muessig,
1959).
Although these deposit-types have somewhat different origins,
both result from one determining geochemical property of B, Its. extreme
solubility. As large ion lithophile (LIL) elements, B and Li are
preferentially enriched in late stage magmatic, and metamorphic fluids
(Foldvair-Volg, 1978; Krauskopf, 1979). The cause of the association
of Sr in some of these deposits is less clear. Sr is a dispersed rare
element that geochemically follows Ca. Only rarely does it form
88
Figure 26. Caved Shafts on the Western Flank of Coaldale Ridge. The timbers are surrounded by fluff- rich soils overlying the upper member of the sediment and tuff unit. Sample CVA 26 was taken adjacent to the timbers to the right.
89independent carbonates and sulfates, usually found in hydrothermal
deposits (Faure and Powell, 1972; Faure, 1977a)«
The solubility of B allows its concentration in the fluids
responsible for the formation of borate deposits. The bedded borate
deposits in lacustrine sequences are believed the result of borate
precipitation from B-rich brines in a lacustrine basin (Bates, 1960;
Kistler and Smith, 1975). Searles Lake in California can be considered
a Recent analog of the Tertiary deposits. The downward migration of
B-rich saline brines has caused the deposition of borax in the late
Quaternary substratum of the lake (Freidman et al., 1982; Smith, 1979).
All major bedded borate deposits have many indicators of
contemporaneous biitiodal voleanism. Air-fall tuffs and/or basalts are
present in or near the deposits. This association has led most workers
to believe that the source of B in the brines is by volcanic exhala
tions (Bates, 1960) of B-rich primary magmatic fluids.
The playa deposits are also a result of the high B solubility.
These form from the continuing concentration of B in saline evaporative
waters. Migration and evaporation of such waters finally leads to
solution saturation, and the deposition and growth of the high hydrate
borates in the playa muds (Muessig, 1959). The B in these waters is
believed caused by the leaching of the surrounding units, which are
likely to be of sedimentary origin.
Evapo-transpiration, or the migration and evaporation of ground
water, operates in arid climates and may cause B enrichment in soils.
Concentration of B in eluvium is the result of highly sorptive minerals.
90
such as kaolinite, in weathering profiles (Shcherbov, 1982). These
clays provide sites for the solution transported B. This process of
continuing concentration leads to the surface accumulations of B that
commonly occur as white salt crusts, coatings or "fluffs." Fluffs are
thick carbonate- and sulfate-rich concentrations just above soil
profiles that sometimes contain observable cottonball ulexite.
Bedded borate deposits almost always have such surface expressions,
although surface accumulations do riot necessarily indicate a borate
deposit at depth. Springs in B-rich environments, or that carry B-rich
waters, are commonly surrounded by these crusts, fluffs, or even
carbonate sinters.
Distribution of B in the Southwestern Monte Cristo Range
To evaluate the extent and distribution of strong B anomalies
in volcanic and volcaniclastic rocks, as indicated in previous recon
naissance sampling by U.S. Borax (B. Watson, pers. comm., 1982),
sixty-three rock chip and seven soil and/or fluff samples were
collected throughout the study area. These were analyzed by atomic
absorption for their B, Sr and Li contents. The assay results are
given in Table 7; asterisks indicate mostly to wholly fluff and/or soil
samples, whereas asterisks in parentheses mark samples that include
only minor fluff and/or soil. Sample locations are plotted on an over
lay to the geologic map (Figure 14) and complete sample descriptions
are included in Appendix A.
TABLE 7
Southwestern Monte Cristo Range
B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples
SampleNo.
AlternateDesignation Lithology Domain B Sr
(PPm)Li
CVA 1 Tha SE 10 523 12CVA 2 That * SE 1300 346 67CVA 3 That SE 21 317 14CVA 4 CVC 3 Tfai SE 4 312 14CVA 6 CVC 4 Thais SE 4 245 82CVA 7 That (*) SE 165 307 53CVA 8 Te (*) SE 98 211 34CVA 9 CVD 2 Ti SE 4 14 14CVA 10 Tstl * SE 186 115 24CVA 11 Tstu (*) SE 299 154 24CVA 12 Tfai SE 7 322 10CVA 13 Tstl (*) SE 286 379 43CVA 14 -H+H- SE 7 289 . 43CVA 16 TcPe SE 28 53 14CVA 16A CVC 6 Tri C 31 38 10CVA 17 Tcpj (*) C 37 509 14
TABLE 7— continued
Southwestern Monte Cristo Range
B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples
SampleNo.
AlternateDesignation Lithology Domain ‘ B Sr
(ppm)LI .
CVA 18 Tcpi (*) C 38. 29 <5CVA 19 Thais C 12 293 14CVA 20 • Tha C . 5 226 19CVA 21 Tpai C 4 302 24CVA 22 Tpa * C 879 274 67CVA 23 Tstu * C 265 , . 192 19CVA 24 Tpa C 29. 442 19CVA 25 Tha * C 19 605 19CVA 26 Tstu (*) c 791 235 34CVA 28 Tha c 15 398 10CVA 29 That 0) c 70 451 10CVA 30 CVC 8 Tha C 12 389 10CVA 31 Tstl (*) c 196 302 34CVA 32 That c 58 235 34CVA 33 opaline veins c 33 182 <5CVA 34 c 12 293 10
TABLE 7— continued
Southwestern Monte Cristo Range
B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples
SampleNo.
AlternateDesignation Lithology Domain , B Sr
(ppm)Li
CVA 35 That C 15 278 14CVA 36 Tstu C 26 240 19CVA 37 Tpat C 16 374 24CVA 38 C 58 394 29CVA 39 Thai C 11 250 24CVA 40 CVD 5 Thai C 9 379 ■ 10CVA 41 TCP! C 8 264 10CVA 42 H+Hf C 10 408 24CVA 43 That C 35 221 31CVA 44 Tpat C 9 415 26CVA 45 TcP2/3 C 7 <10 14CVA 46 Thai C 6 206 10CVA 47 Tcpj C 150 . 149 10CVA 48 ■ Tfai C 9 293 24CVA 49 Tfai C 7 187 10CVA 51 That C 21 158 <5
TABLE 7— continued
Southwestern Monte Cristo Range
B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples
SampleNo.
AlternateDesignation Lithology Domain B Sr
Cppni)Li
CVA.52 Tha C 18 302 10CVA 53 CVC 10 Tpa NW 11 326 10CVA 54 That '(*) NW 7 206 14CVA 55 Tstu * NW 268 235 24CVA 56 Thai NW 20 192 48CVA 57 PT NW 14 115 <5CVA 58 Tcp3 NW 13 72 14CVA 59 That NW 34 67 38CVA 61 Tstu (*) C 106 43 29CVA 62 Tcp2 c 20 240 10CVA 63 CVD 7 Tcp3 NW 9 110 10CVA 64 Tb NW 16 456 19CVA 65 Qsp (OA?) NW 12 235 13CVA 66 Tcpe NE 11 34 10CVA 67 Tfa NE 8 322 14CVA 68 Qsp * NE 349 293 58
TABLE 7— continued
Southwestern Monte Cristo Range
B, Sr, and Li Geochemistry of Rock Chipand Soil/Fluff Samples
Sample No.
AlternateDesignation Lithology Domain B Sr
(ppm)Li
CVA 69 - Ts NE 66 326 34CVA 70 CVC 12 Tea NE 6 1.54 53CVA 71 Tcab NE 4 125 10CVA 72 Tcpi NE 71 394 10CVA 73 PT • NE 11 38 29CVA 74 Te SE 10 240 14
* highly weathered, contains significant fluff f * 1v weathered, contains minor fluff
96
Assay values range from 4 to 1300 ppm B, and average 76 ppm*
Cottonball ulexit e was observed only in two fluff-.rich samples, CVA 2
and CVA 22. Sr and Li values range from <10 to 605 ppm and from <5 to.
82 ppm, respectively. Sr analyses average 224 ppm and Li 21 ppm.
Differing ,?whole earth1’ models cause a range in the clarke of B from
8 to 12 ppm (Foldvair-Vogl, 1978); using an average 10 ppm for this
useful parameter, the average B concentration in the study area is over
seven times more enriched than the clarke.
To determine the geochemical distribution of B, Sr and Li
concentrations within the Tertiary section, their averages were
calculated by rock type and differentiated by freshness of the sample., / • ' •Table 8 presents the average concentrations of each rock type; "altered"
samples are those that showed chloritic or sericitic alteration, or
most commonly, extreme- weathering. Asterisks indicate averages from
fluff- and/or soil-rich samples, whereas those in parentheses mark
averages from weathered samples with minor fluff and/or soil contents.
. Clearly apparent in this table are that the altered/weathered samples
are much more enriched in B than the fresh samples. The only exceptions
to this are the chloritically altered hornblende and pyroxene dikes
which have concentrations more like the fresh volcanic units. The
altered/weathered samples average close to 260 ppm B when the dike
concentrations are excluded, or 26 times the clarke concentration of .
B. The fresher -units are much less B-rich, only slightly greater than
the clarke, and dominate the samples taken in the study area. Largely
representative, the fresh samples indicate the average volcanic and
TABLE 8
Average B, Sr, and Li Geochemistry Southwestern Monte Cristo Range
LithologicUnits
Number of Samples
B
Fresh(PPm)
Altered (* with fluff) (ppm)
Fresh Altered Sr Li B Sr Li
Opaline 1 33 182 5VeinsQsp 1 1 12 235 13 349 293 58 *
Tb 1 — — 16 456 19 -- — —Te 1 1 10 240 14 98 211 34 (*)
Ti 1 -- 4 14 14 — --Tpa 2 1 20 . 384 15 879 274 67 *Tpai 1 -- 4 302 24 — --
— - . 2 -- 35 344 20 'Tpat 2 -- 13 395 25 — --
■--
Tfai . 4 — 7 288 15 -- --Tha 5 12 . 368 12 mm mm mm mm mm
Thai*-1-1 5 10 263 33^) — mm — mmmm
-H-H-f- — 2 —- -- 9 346 34
TABLE 8— continued
LithologicUnits
Number of Samples
Fresh(ppm)
Altered (* with fluff) (ppm)
Fresh Altered B Sr Li B Sr Li
That 6 4 26 236 18 389 427 37 (*)Thai 2 -- 23 159 31 — —Tstu 1 5 26 240 19 128 172 26 (*)Tstl — 3 — — 223 265 34 (*)
Tcu 1 — 6 154 53 — — - - -Tcab 1 -- 4 125 10 — —Tfa 1 -- 8 322 14 — : —Ts 1 66 326 34 — - — -
Tri 1 31 38 10 -- — — --Tcp ' 7 2 17 176 11 110 272 10 *Tcpg 2 — 19 44 12 -- — --
pT 2 -- 13 77 17 — --
Avg. 49 21 17 244 13 211(3) 289 27
Includes analyses from Thais„ f 21 •'20 ppm without one very anomalpus sample CVA 6i ^^256 ppm without chloritically altered dikes.
99
sedimentary rocks in the study area are only slightly anomalous with
respect to B.
B versus Sr and B versus Li variation diagrams for the average
fresh and altered values of each rock type are shown in Figures 27 and
28, respectively. Included in each plot are the modern spring crusts
and pre-Tertiary rock compositions for comparison. Both plots indi
cate low B concentrations for fresh samples and higher values for the
altered samples. Tie lines connect fresh and altered average concentra
tions of equivalent samples. The Sr versus B plot (Figure 27) shows a
wide range in Sr values in fresh samples and both Sr enrichment as well
as Sr depletion in the altered/weathered samples (excluding the dike
compositions). The Li versus B plot (Figure 28) shows that most of
the fresh rocks cluster between 10 and 20 ppm Li, with a few anomalies
near 50 ppm. Most samples show a strong increase in Li in the altered/
weathered samples. A triangular B-Sr-Li variation diagram (Figure 29)
illustrates the relative dominance of Sr in the fresh rock types and
the higher relative B concentrations in the altered/weathered samples.
In all the plots, the altered/weathered sample averages fall well below
the B content of modern spring crusts in the area and above the pre-
Tertiary average.
Table 9 compares the average concentrations of B, Sr and Li in
the fresh samples of the study, area by major rock type to the reported
averages of equivalent rock types and the clarke values of Turekian
(1972). It indicates that the B contents of the volcanic rocks in the
study area are slightly, but consistently, greater than the volcanic
B (p
pm)
4 0 0 - i
• - fresh
300-
* - pTor Qsp
200-
100-
Sr (ppm)
Figure 27. B-Sr Variation Diagram of Average Values. Tie lines connect fresh and altered samples of the same rock type. Qsp and pT values given as reference.
100
102
fresh
altered
pT or Qsp
Figure 29. B-Sr-Li Triangular Variation Diagram of AverageValues. Fresh and altered samples differentiated; Qsp and pT values given as reference.
TABLE 9
Comparison of Average Fresh Rock B, Srp and Li Geochemistry to the Average Composition of Similar Rock Types and
Clarke Values of Turekian (1972)
LithologicUnit
This Study TurekianType B Sr Li B Sr Li
Basalt Tb 16 456 19 5 465 17AndesitesW All 13 314 22
Blair Junction Tcap Tcab, Tfa 6 200 26 9 440 24Coaldale Tpa, Tpai, Tpat, Tfai
Tha, Thai, That, Thai 15 329 23
RhyolitesC2) All 17 101 11 •tuffs Tcp 17 176 11 10 100 40intrusives Tri, Ti 18 26 12
Sediments All 25 180 20
PT PT 13 77 17 52 310 29^T Ts, Te. 38 283 24
Clarke(3) All 17 224 18 8 375 24
^High Ca rocks from Turekian (1972) e ^ L o w Ca rocks from Turekian (1972) „
Average from model A and model B from Turekian (1972)0 Includes sandstonesP shales and carbonates.
103
104
rock averages and over twice the clarke values. Sr and Li concentra
tions in the fresh units are generally much closer to the norms,
although the Sr values show a large variation. The sedimentary rock
averages are much lower than the Turekian values. Subdivisions of the
andesitic, rhyolitic and sedimentary analyses are given in Table 9 to
show the variation within one rock type. Rhyolitic rocks vary only
in their Sr contents whereas both andesitic and sedimentary rocks
differ in all three values.
The assay results allow some basic generalizations about the
relative B anomalies in the study area. First, the fresh volcanic and
sedimentary samples are representative of the overall area and are only
slightly anomalous in B, with generally low Li and variable Sr. Second,
weathered units are consistently much more enriched in B and Li than
the fresh samples. Third, fluff and/or soil over the weathered units
can contain very anomalous amounts of B and Li, Fourth and last,
modern springs in the area are surrounded by B-rich crusts and fluff
and are themselves presumably anomalous in B,
Cause of B Anomalies in the Southwestern Monte Cristo Range
Some of the geochemical variations present in the fresh samples
(Table 9) are easily explained, while others are more ambiguous. The
general depletion of B, Sr and Li in the pre-Tertiary rocks, for
example, is probably caused by dehydration during metamorphism. On
the other hand, the Tertiary sediments have a more average composition,
although they are also low in B. This may be caused by the inclusion
105of marine shales which have high B contents (100 ppm) in Turekian‘s
sedimentary rock average. The Tertiary sandstones and lacustrine
mudstones are more equivalent to the Turekian sandstone average of
35 ppm B, 20 ppm Sr and 15 ppm Li■(Turekian, 1972). The high Sr con
centrations in the Tertiary sediments in this comparison are caused by
the large calcic-rich volcanic component in these tuffaceous sediments.
The cause of the overall B and Li enrichment, although slight,
and the wide Sr variations in the fresh samples (Table 9) is less
clear and needs further consideration. The most probable causes of
the enrichment and variations are (1) original magmatic compositions
and (2) secondary hydrothermal alteration. The first can be examined
in light of the whole rock geochemistry performed on all major units,
and the latter in respect to the physical properties of the units and
the distribution of the anomalies in the study area.
Original Magmatic Compositions
The variable Sr contents in the fresh volcanic units is caused
by original magmatic compositions. Sr is well known to follow Ca in
magmatic differentiation (Faure, 1972), and the lower CaO concentration
in the Blair Junction trachytes (Figure 20) explains their lower Sr
content. In thin section, this is seen by a greater plagioclase
abundance in the Coaldale latites (Table 1).
The greater B and Li values in the Coaldale latites, however,
are not so strongly suggestive of a primary magmatic origin. As both
are LIL elements, they should have at least slightly greater concen
trations in the more differentiated Blair Junction trachytes (Figure
106
21). The higher B and Li contents in the Coaldale units require their
magmas to be originally more B- and Li-rich than the Blair Junction
magmas. Although this possibility cannot be refuted, several other
lines of evidence lead to an epigenetic hydrothermal cause of the
enrichment of B and Li in the study area. •
Secondary Hydrothermal Alteration
The enrichment of B and Li in the fresh volcanic rock samples
by a superimposed hydrothermal event is more difficult to document than
the cause of the Sr variations. Comparing the geochemistry of competent
and relatively impermeable hornblende flow and intrusion samples in
Table 8 to the less competent, more permeable hornblende tuff and
lahar samples reveals strikingly different averages. The competent
units have an average of.11.3 ppm B, very close to the expected
Turekian value. The more permeable units, however, have an average
of 24.3 ppm and are definitely enriched with respect to B. The perme
able units also show higher Li values, if one extremely Li-rich sample
(CVA 6) is excluded. If one anomalous pyroxene sample is also
ignored (CVA 24, see below), the difference between competent and
permeable units is further substantiated in the Coaldale latites.
The direct relationship between enriched B and Li and permeable,
fresh volcanic units strongly suggests an epigenetic enrichment of
these elements in the study area. Hydrothermal alteration by a late
B-rich fluid can be inferred by three relationships: first, the pre
sence of small, but significant, B anomalies in fresh units and large
anomalies in weathered units near secondary, crosscutting opaline veins;
107
second, the extreme weathering and thick fluff accumulations near
such veins and present around a sericitic alteration center; and third,
the presence of modern B-rich springs in the area.
Three instances of crosscutting opaline veins occur in the study
area. On the east flank of Coaldale Ridge, several north trending, 1"
opaline and chalcedonic veins crosscut an unweathered hornblende tuff
breccia (Figures 2 and 14). Separate analyses of the vein material
and surrounding tuff reveal small B anomalies in each material. These
assayed 33 and 58 ppm B, respectively (Table 7, CVA 32 and 33).
Another opaline vein system occurs on the southwestern exterior of the
Blue Mountain complex (Figures 2 and 14, CVA 47). Here the veins are
locally surrounded by extremely weathered and fluff-rich soils, but
also crosscut less altered Castle Peak Tuff. A sample of the latter
contains 110 ppm B (Table 7).
The third locality of the secondary opaline veins is in aysericitically altered outcrop of the hornblende latite (flow?) on the
western flank of Coaldale Ridge (Figures 2, 14, and 30). This occur
rence is perhaps the most convincing evidence of a hydrothermal
enrichment of B and Li in the study area. The sericitized latite is
not itself very anomalous in B (19 ppm, CVA 25), but it is surrounded
by an extensive area of extremely weathered units, commonly completely
altered to clays, and very thick (6" to 12") surface fluff accumula
tions. A fluff-contaminated sample of the clay-altered upper sediment
and tuff unit surrounding this locality assays 791 ppm B (CVA 26).
Also adjacent to this sericitized outcrop are hornblende tuffs and
/ ■ .
Figure 30. Sericitized Hornblende Andesite Flow(?) on the Western Flank of Coaldale Ridge. Irregular "ribbed” weathering is caused by thin resistant opaline veins. This outcrop is the location of sample CVA 25. The photograph is a detail of the area shown in Figure 10 and just east of that shown in Figure 26.
109
pyroxene latites that contain 70 and 29 ppm B, respectively (CVA 29 and
24). It was this area that was explored for its borate potential in
the late 1800's.
Some fracture and fault control of B in this area is suggested
by salt coatings oh fractured, fresh hornblende flows adjacent to the
fault east of the sericitic outcrop (Figure 2). An early sample of
this flow assayed 75 ppm B and helped to provoke the original interest
in this area (B. Watson, pers. comm., 1982). An equivalent sample was
taken 200' east of this fault where no fracture coatings occur (Figures
2 and 14, CVA 28). It assayed only 15 ppm B and suggests at least
partial fault and/or fracture control of the geochemistry in the area.
Other fresh samples in faulted or fractured areas, outside of this
alteration center (CVA 4, 34 and 35, for example) are not significantly
enriched in B.
Age of Alteration in the Southwestern Monte Cristo Range
The timing of the hydrothermal event responsible for the B
and Li enrichment in the project area is uncertain. Definitely later
than the volcanism of the Blue Mountain, the presence of B-rich springs
suggests that it was fairly recent. Bimodal volcanism, commonly
associated with bedded borate deposits (Bates, 1960; Kistler and Smith,
1975), occurred in the Monte Cristo Range as early as 7 m.y. ago and
perhaps lasted a considerable length of time (Albers and Stewart, 1972;
Stewart, pers. comm., 1983). Even more recent bimodal volcanism is
indicated in the Silver Peak Range and Candelaria Hills where both
110
Pliocene and Pleistocene volcanic rocks occur (Figure 16; Robinson
and Chowder, 1973; Speed and Cogbill, 1979b), The presence of young,
very B-rich sediments in Fish Lake Valley also supports a young altera
tion age. Robinson (1972) has been able to constrain the age of these
lacustrine sediments between 4.8 and 6.0 m.y. B.P. All these factors
support a latest Miocene to Pleistocene age for the alteration in the
project area.
The localized surface accumulations of B-rich fluffs in the
area are only indirectly related to the main alteration event. Evapo-
transpiration of B during weathering resulted in the formation of
locally significant, B-rich surface concentrations. The widespread
distribution of these concentrations— some quite distant from the
sericitized center or opaline veins--deserves further consideration.
It can be argued that the occurrence of B-rich fluffs through
out the project area was caused by a downward leaching of an overlying,
B-rich sedimentary sequence. At the beginning of this study, the
Esmeralda Formation was believed to be a likely candidate for this
hypothesis. Despite the strong surficial B enrichment in these
lacustrine sediments in the Blair Junction area, drilling has shown
that the anomalies do not continue to depth (B. Watson, pers. comm.,
1982). Thus, the only other support for this hypothesis is _if a
younger sedimentary sequence, contemporaneous with alteration, was once
present in the study area. The obvious analog for these would be the
young B-rich beds in Fish Lake Valley. Perhaps the Qsp (Oa?) sediments
)
Ill
in the northwestern portion of the area are an erosional remnant of
such a (hypothetical) sedimentary sequence.
A more straightforward cause of the widespread fluff accumula
tions exists. Modern and/or paleo-groundwater mobilization of the
earlier concentrated hydrothermal anomalies would afford the widespread
distribution of the B-rich fluff localities, as well as the weak
anomalies in all the permeable units in the study area. The presence
of modern B-rich springs suggests this recycling continues unto the
present time.
CHAPTER 8
TERTIARY GEOLOGIC HISTORY
The earliest volcanism throughout western Esmeralda County was
the extrusion of rhyolitic ash-flow tuffs. In the Monte Cristo Range
this episode involved the 24.2 m.y. old Castle Peak Tuff which was
deposited on an irregular pre-Tertiary erosional surface. Presumably
of caldera origin, the source region for this tuff was to the northeast
in the center of the Monte Cristo Range. The Castle Peak Tuff was
accompanied in the project area by local rhyolitic diking; both were
derived from magmas with substantial upper crustal contamination.
In early Miocene time, independent andesitic volcanic centers
formed in the southwestern Monte Cristo Range. These centers produced
the petrologically and geochemically distinct latitic Coaldale and
trachytic Blair Junction volcanic sequences, each derived from magmas
with only minor crustal contamination* The Blair Junction trachytes
were erupted throughout the northern Monte Cristo Range and formed in
part from localized vents in the northeastern portion of the study
area. The Coaldale units were localized in the southwestern exterior
of the Monte Cristo Range. Volcanism in both areas began after the
deposition of a volcaniclastic sedimentary package that included thin
air-fall tuffs and ash-flows in the southwest.
More varied than the Blair Junction sequence, the Coaldale
latites were erupted from the Blue Mountain intrusion, a discrete
1 1 2
113
volcanic center surrounded by a half-arc of "concentric" dikes and
thick volcanic lahars. This 22.2 m.y. old sequence includes three
distinct latites, two of which have extrusive tuff breccias and flow
units as well as intrusive expressions. Discrete yet gradational
petrographic signatures of each latite suggest a single zoned magmatic
system was tapped to ever deepening levels through time. Intermittent
faulting during this period of volcanism caused periods of uplift
and/or erosion to occur, and controlled the deposition of units to
form a broadly arched, south dipping volcanic homodine. N40-50°E
extension produced northwest orientations of both the large intrusions
and smaller dikes. Volcanism in the Coaldale region continued until
18.6 m.y. B.P., when a rhyolitic plug intruded the southeast portion
of the study area, perhaps localized by early extensional stress.
Geochemically distinct, this unit was derived from magmas with a major
upper crustal component, apparently unrelated to earlier andesitic
Coaldale volcanism.
Quiescence dominated in at least the southeastern portion of
the study area and in the adjacent Blair Junction area during the
middle Miocene. Deposition of the tuffaceous Esmeralda Formation
13 to 6 m.y. ago was accompanied by concurrent volcanism in the region,
although no active volcanism took place in the nearby Blue Mountain
center.
Strong east-west directed extension formed north trending,
primarily dip-slip faults in the latest Miocene to Pliocene. Secondary
conjugate shear caused right-lateral displacements on the north-south.
114
dominantly dip-slip faults and left-lateral displacements on east-west,
dominantly strike-slip faults. Multiple movement and reactivation of
faults occurred during this lengthy period of extensiohal and shear stress.
Late-stage B-rich hydrothermal fluids altered the Coaldale
volcanic rocks and caused localized, minor B and Li enrichment in the
permeable units of the study area. Major faults and highly fractured
areas partially served as conduits for these fluids. Sericitizing the
latites in the immediate source area, the fluids also produced more
widespread opaline stringer veins and minor clay alteration of the
permeable volcanic units throughout the area. This secondary event
may have coincided'with Pliocene bimodal volcanism in the Monte Cristo
Range and adjacent areas, although the latter is only locally repre
sented by basalts in the northwest portion of the study area. Modern
B-rich springs in the area attest to the fairly young age of this
alteration event. Modern and/or paleo-groundwater systems, and/or
the downward leaching of young lacustrine sediments contemporaneous
with alteration, produced the widespread B enrichment in the soils and
fluffs overlying the volcanic and sedimentary rocks in the southwestern
Monte Cristo Range.
CHAPTER 9
CONCLUSIONS
The volcanic, geochemical and structural record in the south
western Monte Cristo Range exemplifies the complexity of even a small,
discrete volcanic center and allows-comparison of its units to more
widespread, regional volcanic sequences. Of the four major Tertiary
rock types in the area, only the isolated, distinctive Coaldale
volcanic sequence has local distribution and is easily associated with
a specific source vent. The others, including the Castle Peak volcanic
sequence, the Blair Junction volcanic sequence, and the younger sedimen
tary and basaltic sequence, are composed of units with regional extent.
In the Monte Cristo Range, the rhyolitic Castle Peak Tuff is
24.4 m.y. old and correlates well with many other late Oligocene ash-
flows in adjacent areas. The younger sedimentary and volcanic rocks
are composed of middle to late Miocene lacustrine sediments of the
Esmeralda Formation and Pliocene (?) basalts. Both are widespread and
correlate to equivalent units throughout this region. The Blair
Junction volcanic rocks are present through'much of the Monte Cristo
Range and consist of a thick, homogeneous sequence of trachytic flows
overlying a basal monolithic breccia. The 22.2 m.y. old Coaldale latites
are probably correlative with this widespread- sequence, although
neither of these units have definite correlatives outside the Monte
115
116
Cristo Range. Both volcanic units overlie a volcaniclastic sedimentary
sequence that includes air-fall tuffs and ash-flows in the southwest.
The petrographically and geochemically distinct Coaldale
volcanic sequence is quite variable and consists of three latitic
compositions: older hornblende-rich tuff breccias, lahars, flows and
intrusions; intermediate age(?) fine-grained hornblende- and pyroxene-
rich intrusions; and younger pyroxene-rich tuff breccias, flows and
intrusions. These discrete units are petrographically gradational
suggesting a single magmatic source was successively tapped to
increasing depths through time. Thick volcanic lahars and a half^arc
of "concentric" dikes, as well as thickness variations of the volcanic
rocks, identify the Blue Mountain intrusion as the source vent for
this varied sequence.S .7 srLow initial —--- ratios in the Coaldale latites and oxide8 6 Sr
variations in both of the volcanic sequences imply that each were
derived from magmas with little upper crustal contamination which
followed similar differentiation paths. The 18.6 m.y. old rhyolitic
plug in the southeast portion of the project area, considered part of
the Coaldale volcanic sequence, is geochemically similar to the 24.287Srm.y. old Castle Peak Tuff and rhyolitic dikes. High initial — —86Sr
ratios in both suggest derivation from magmas with a significant
crustal component. The lack of intermediate volcanic compositions and
a geochemical dissimilarity deny the genetic relationship between the
Coaldale latites and this rhyolite plug.
Most of the structural complexity in the project area is related
to Basin and Range extensional tectonics, although minor deformation
during volcanism is also indicated. Prominent northeast to northwest
orientations of faults with largely dip-slip movements formed in
response to west-northwest directed extension in the southeast to east-
northeast directed extension in the northwest. Secondary conjugate
shear stress, with orientations compatible with northwest trending,
right-lateral Walker Lane shear zone and the east trending left-
lateral Warm Springs lineament, caused the right oblique-slip on
north-south, dominantly normal-slip faults and the left oblique-slip
on east-west, primarily strike-slip faults. Simultaneous or over
lapping extensional" and shear Stress deformations and reactivation
of faults are indicated by mutual crosscutting relationships and
variable offsets.
The widespread B anomalies in the Coaldale volcanic sequence
were caused by a late-stage hydrothermal event. Anomalous in B and Li,
the fluids produced localized enrichment in the permeable units of the
study area. Presumably controlled by temperature, pressure and composi
tional restraints, the fluids severely sericitized a local source area
without any major B deposition. Outward from this center, however, more
widespread opaline stringer veins and clay alteration were associated
with weak B enrichment. Faults and highly fractured zones sometimes
served as conduits for these fluids. Modern B-rich springs indicate a
young age of this alteration, perhaps coincident with Pliocene bimo.dal
volcanism elsewhere in the Monte Cristo Range and adjacent areas. The
117
118occurrence of 4.8 to 6.0 m.y. old, B-rich lacustrine sediments in Fish
Lake Valley strongly suggests an equivalent age for the alteration in
the proj ect area.
It is doubtful that an economic borate deposit ever formed as a
result of this hydrothermal event. Modern or paleo-groundwater mobiliza
tion of the original, weak and localized anomalies, and/or the downward
leaching of a more B-rich sedimentary sequence contemporaneous with
alteration, have caused the locally significant surface B enrichment
in modern soils and fluffs in the study area.
APPENDIX A
SAMPLE DESCRIPTIONS
The appendix below describes the location and petrology of all samples analyzed in this study. Sample locations are given by structural domain, and are plotted on an overlay to the geologic map (Figure 14). Sample designations are as follows: (1) CVD— Coaldalevolcanic rocks date, (2) CVC— Coaldale volcanic rocks chemistry, (3) CVA— Coaldale volcanic rocks assay, and (4) Coaldale volcanic rocks thin section. UAKA and UARS are University of Arizona Laboratory of Isotope Geochemistry identifications.
Isotope Analyses
Sample no.: CVD 2 ,Alternate designations: CVC 5, CVA 9, CV 1, UAKA 83-06, UARS 83-06.Location: SE; lat. 38°02.51' N, long. 117*50.26' WDescription: Ti; light pink to brown, partially vesiculated crystal-
lithic rhyolitic intrusion. Lithics (5%) are composed of andesite and Paleozoic chert, but are not included in the isotopically or chemically analyzed sample. Quartz (20%) is the major phenocryst and sanadine grains are minor (5%) but very fresh. The groundmass is very glassy, with well-defined shards, and only partially devitrified. Vesiculated outer edge of outcrop is not included in analyses.
Sample no.: CVD 5Alternate designations: CVC 9, CVA 40, CV 12, UAKA 83-07, UARS 83-07Location: C; southeast flank of Blue Mountain; lat. 38*04.35* N,
long. 117*51.40* WDescription: Thai; light blue-gray hornblende andesite intrusion.
Hornblende (20%) and plagioclase (10%) phenocrysts are extremely fresh in a slightly devitrified, glassy groundmass. Less than 2 % of the hornblende crystals have oxidized cores and rims.Only slight chloritic alteration of the glassy groundmass is observed in thin section.
Sample no.: CVD 7Alternate designations: CVC 11, CVA 63, CV 30, UAKA 83-08, UARS 83-08Location: NW; lat. 38*05.24 N, long. 117*52.54* WDescription: Tcp?/^; light tan to gray, crystal-lithic, partially and
densely welded tuff. The sample was taken in the upper flow119
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unit of the tuff. Sanadine (10%) and tiiotite (2%) phenocrysts are very fresh and unfractured. Eutaxitic pumices (5%) and portions of the. very glassy groundmass (75%) show vapor-phase alteration and minor devitrification. Paleozoic chert and limestone lithics (<3%) are not included in the analyzed sample.
Chemical Analyses
Sample no.: CVC 3Alternate designations: CVA 4, CV 38, CV 11Location: SEDescription: Tfai; greenish-gray fine-grained andesite intrusion. The
hornblende phenocrysts (1 0 %) are altered to chlorite and clays, with magnetite resorption rims prominent. Augite phenocrysts (10%) are unaltered. The plagioclase crystals (7%) and cumulate grains (<2%) are altered to clays. The groundmass is composed of unaltered plagioclase microlites, pyroxene granules and glass. The sample was taken toward the center of the intrusion with closely spaced (0.5" to 2") flow fractures.
Sample no.: CVC 4Alternate designations: CVA 6 , CV 14Location: SE xDescription: Thais; slightly weathered, green-gray hornblende andesite
sill. Hornblende grains (20%) are moderately fractured, with only a few ( 5%) that have cores altered to chlorite and clays. Plagioclase crystals (20%) are generally more weathered; all large phenocrysts, both individual as well as cumulate grains, are converted to clays and calcite. Approximately 10% of the groundmass consists of clays, chlorite and carbonate. Although weathered, this represents the freshest sample of the hornblende dikes or sills.
Sample no.: CVC 6
Alternate designations: CVA 16A, CV 21Location: CDescription: Tri; reddish purple banded rhyolite intrusive dike. The
sample includes interior banded rhyolite (90%) as well as vitrophyric outcrop edge (10%) Interior of dikes consists of alternating reddish purple and gray bands, 0.5" to 1" wide.The dark bands are 90% to 95% hematite-stained, very finegrained groundmass with minor phenocrysts of unaltered plagioclase (5%). The light bands are plagioclase-rich (35%), in a fresher groundmass composed of glass shards and plagioclase.The vitrophyric phase of the sample shows minor vesiculated pumice and much less pronounced banding.
121Sample no.: CVC 8 ,Alternate designations: CVA 30, CV 10Location: C; southern end of Coaldale RidgeDescription: Tha; bluish gray hornblende andesite flow. The hornblende
phenocrysts (1 0 %) are rimmed by magnetite, but relatively fresh. The augite grains (5%) are much finer-grained and unaltered. Large plagioclase crystals (10%) almost always have cores altered to clays. The groundmass is composed of unaltered plagioclase microlites (30%) and glass (45%). This sample is very representative of its rock type.
Sample no.: CVC 10Alternate designations: CVA 40, CV 4Location: C; north of Coaldale RidgeDescription: Tpa; greenish gray pyroxene andesite flow. Augite
phenocrysts (7%) are relatively fresh with minor magnetite inclusions. Hydrobiotite(?) is present (10%) as pseudomorphs of an extremely fine-grained pyroxene. Plagioclase (30%) occurs as unaltered, medium- to fine-grained phenocrysts (0.5 mm). The groundmass (50%) is an intergrowth of pyroxene granules, chlorite and glass.
Sample no.: CVC 12Alternate designations: CVA 70, CV 23Location: NE -Description: Tea; greenish gray coarse-grained andesite flow. Horn-
belnde phenocrysts (1 0 %) are very altered; 2 % retain hornblende cores within a magnetite rim, whereas 8 % are almost completely replaced by magnetite and clay. All have ghost outlines of the crystal laths composed of clays. The augite phenocrysts (5%) are much finer-grained and only altered to clays along fractures. Biotite crystals (2%) are fairly fresh. Plagioclase phenocrysts (15%) are altered to carbonate and clays along their rims. The groundmass (60%) is composed of fine-grained opaques and unaltered plagioclase microlites in a chloritically altered glass.
Assay Analyses
Sample no.: CVA 1Alternate designations: noneLocation: SEDescription: Tha; bluish gray hornblende andesite flow. The hornblende
grains are slightly altered to chlorite and clays, whereas the plagioclase crystals are moderately altered to clays. The sample locality has a low fracture density (6 " to 1 '), and occurs 50* below the top of 1251 thick flow.
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Sample no.: CVA 2Alternate designations: noneLocation: SBDescription: That; weathered hornblende andesite tuff breccia. The
sample consists of mixed gypsum-rich fluff (50%) and fresher grayish green weathered tuff (50%). The fluff occurs as a 6 " thick layer under a crusty soil. The tuff breccia is completely weathered to clays with only relict clast textures preserved.
Sample no.: CVA 3Alternate designations: honeLocation: SBDescription: That; reddish brown hornblende andesite tuff breccia.
The hornblende grains are totally altered to biotite, and the plagioclase crystals to clays. Clasts of a feldspar porphyry occur in a tuffaceous feldspar- and hornblende-rich matrix.The sample is slightly weathered but representative of this rock type. The sample locality occurs 20' below capping finegrained intrusion. -
Sample no.: CVA 7Alternate designations: noneLocation: SBDescription: That; light reddish brown hornblende andesite tuff
breccia. Fine-grained hornblende- and feldspar-rich porphyry clasts (35%) occur in a tuffaceous matrix of similar composition. The sample is slightly weathered to clays, but no fluff is included in it.
Sample no.: CVA 8
Alternate designations: noneLocation: SBDescription: Te; Esmeralda Formation lacustrine sediments. The
sample represents a 2 0 ' section of interbedded tuffaceous gray clay, a gray pumice-rich sandstone, reworked air-fall tuff paper shales, lignitic shale with plant debris, and grayish green tuffaceous shales. Only minor gypsum-rich fluffs are present, although salts along fractures are common. Overall, the sample is only moderately fresh, but representative of this unit.
Sample no.: CVA 10Alternate designations: noneLocation: SBDescription: Tstl; weathered lower member of sediments and tuffs.
The sample is from a tuff unit with large boulders of a hornblende- and pyroxene-rich andesite weathering out of a tuffaceous matrix of similar composition, each of which is equally represented. Remnant clay clasts are apparent in the soil, and gypsum-rich fluff (1 " thick) occurs just beneath
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the surface and was included in the sample (30%)„ The sample -. locality is less than 1 0 0 * away from a major fault trace.
Sample no.: CVA 11Alternate designations: noneLocation: SEDescription: Tstu; weathered upper member of interbedded sediments and
tuffs. . The sample consists of interbedded fine-grained quartz- rich tuffs and gray, biotite-rich claystones. Gypsum-rich fluff (3" thick) occurs in the soil above the latter, and minor contamination of the sample by this fluff is probable.
Sample no.: CVA 12Alternate designations: CV 37Location: SEDescription: Tfai; greenish gray fine-grained andesite intrusion.
Hornblende, pyroxene and plagioclase phenocrysts are relatively fresh. Hornblende crystals (7%) have thin magnetite resorption rims and have a thin ghost outline of clay and chlorite. The pyroxene grains (7%) have only minor clay alteration in their cores. Both rims and cores of the plagioclase phenocrysts (10%) are altered to clays. The groundmass consists of an intergrowth of glass, pyroxene granules and slightly altered plagioclase microlites. In general, the sample is very fresh and representative of its rock type. The sample locality has closely spaced (0.5" to 3") flow fractures..
Sample no.: CVA 13Alternate designations: noneLocation: SEDescription: Tstlj weathered lower jnember of the sediment and tuff
unit. Clay-altered pyroxene- and hornblende-rich andesite clasts occur in a tuffaceous hornblende-rich andesite matrix. Both phases are equally represented in the sample. The matrix is highly weathered, greenish gray in color and almost totally altered to clays. Thick fluff occurs just beneath the surface and minor contamination is probable.
Sample no.: CVA 14Alternate designations: noneLocation: SEDescription: -h +h -; greenish gray hornblende andesite dike. Very
coarse-grained hornblende phenocrysts occur in a chloritically altered groundmass. Minor carbonate is also present. The sample dike has moderately spaced (6 " to 1 ') flow fractures, which increase in density toward the dike's contacts.
Sample no.: CVA 16Alternate designations: CV 33Location: SE
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Description: Tcpe; light purple exotic block in Castle Peak Tuff.The sample consists of a fine-grained, plagioclase-rich porphyritic tuff with discordant attitudes and well-defined contacts with the Tcp. The plagioclase is altered to clays but the groundmass is relatively fresh. The block has moderately spaced fractures (6 H to 1 *) which increase at its contacts.
Sample no.: CVA 17Alternate designations: noneLocation: CDescription: Tcp^; light pinkish, unwelded Castle Peak Tuff. Quartz-
and sanadine-rich, the tuff occurs between two large Tri dikes. The sample includes clay-altered pink tuff and siliceous light purple tuff at the dike contacts, as well as fresher white tuff in the interior of the exposure. Minor fluff occurs above the clay-altered tuff but was not included in the sample.
Sample no.: CVA 18Alternate designations: CV 29Location: CDescription: Tcp^light purple, unwelded Castle Peak Tuff. Baked
and siliceous, the tuff is at the contact between a large Tri dike and the Tcp. This baked zone is the largest in the study area, and is 50* to 70' wide. The tuff is generally unfractured and fresh. Sanadine phenocrysts (10%) have thin clay-altered rims. Quartz (5-7%) crystals are unembayed and unfractured. Plagioclase grains (7-10%) are more altered to clays. The groundmass is dense and glassy, and only partly devitrified.
Sample no.: CVA 19Alternate Designations: noneLocation: CDescription: Thais; grayish green hornblende andesite sill. The
hornblende phenocrysts are fine-grained and relatively fresh. Plagioclase grains are largely altered to clays. The sample includes material from a 3’ thick chilled margin above the Tcp. The outcrop has moderately to closely spaced (3" to 1') fractures.
Sample no.: CVA 20Alternate Designations: noneLocation: CDescription: Tha; light bluish purple hornblende andesite flow. The
hornblende phenocrysts are completely altered to chlorite and biotite, and the plagioclase to clays. The groundmass is unsiliceous and altered to clays. The sample was taken from in-place float, as no outcrop of this unsiliceous phase of the Tha.
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Sample no.: CVA 21Alternate designations: CV 6
Location: CDescription: Tpai; light green pyroxene andesite plug. Augite
phenocrysts (10%) are unaltered and unfractured. Unlike the Tpa, this sample has no fine-grained hydrobiotite phenocrysts. The plagioclase grains (30%) have clay-altered cores. The groundmass (60%) is an intergrowth of glass, chlorite and clays, with interstitial pyroxene and magnetite grains. This sample is fresh and representative of the rock type.
Sample no.: CVA 22Alternate designations: noneLocation: CDescription: Tpa; soil and fluff sample above the basal contact of a
pyroxene andesite flow. The base of this flow is very weathered and consists of red-brown (above) to green-gray (below) clay with few remnant Tpa clasts preserved. The soil is fluff- rich and has gypsum and cottonball ulexite as distinct layers and crosscutting veins.
Sample no.: CVA 23Alternate designations: noneLocation: C; eastern flank of Coaldale Ridge.Description: Tstu; weathered upper member of the sediment and tuff
unit. The sample consists of equal proportions of fluff, soil and weathered tuff. The tuff is a white, thinly bedded, quartz-rich rhyolite with most of its feldspars and groundmass altered to clays. This exposure has large clasts of a feldspar porphyry weathering out of a clay-altered matrix at its base, and is capped by the rhyolite tuff. Thick fluff (6 " to 8 ") occurs in the soil above the former unit. The sampled interval is 50' thick.
Sample no.: CVA 24Alternate designations: noneLocation: C; western flank of Coaldale RidgeDescription: Tpa; light reddish purple pyroxene andesite flow. The
augite grains are unaltered, but the plagioclase crystals all show pronounced clay alteration. The groundmass is porous and unsiliceous. The sample was taken 3* above the base of a flow, very near major fault traces, and immediately adjacent to the sericitized alteration center (CVA 25).
Sample no.: CVA 25Alternate designations: CV 41Location: C; western flank of Coaldale RidgeDescription: Tha; light tan sericitized hornblende andesite flow(?).
Almost all the plagioclase crystals (30%) are altered to
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sericite with minor calcite. Most of the large hornblende grains (1 2 %) are altered to chlorite and have thick magnetite resorption rims. The groundmass is a fine-grained intergrowth of calcite, sericite and minor chlorite. The weathered surface of the outcrop has irregular ribs of resistant opaline veins. The sample locality forms, a small alteration center surrounded by clay-altered and fluff-rich tuffs, and occurs near a large ridge-bounding fault (Figure 30).
Sample no.: CVA 26Alternate designations: noneLocation: C; western flank of Coaldale RidgeDescription: Tstu; altered/weathered upper member of the sediment and
tuff unit. The sample consists of clay-altered, highly fractured (0.5".chips) light gray quartz-rich tuff. The chips have clay pseudomorphs of feldspar crystals and salt coatings on most fracture surfaces. Gypsum-rich fluff (6 " thick) occurs just beneath the surface above the tuff, and is included in the sample (10%). This locality is just west of the sericitized alteration center (CVA 25), and within 50‘ of the westernmost ridge-bounding fault.
Sample no.: CVA 28Alternate designations: CV 9Location: C; western flank of Coaldale RidgeDescription: Tha; light bluish gray hornblende andesite flow. The
hornblende phenocrysts (5%) ubiquitously show strong magnetite resorption, with over half of these having cores altered to clays and chlorite. Augite crystals (5%) are unaltered and fine-grained. The plagioclase grains (25%) are largely altered to clays on their rims and cores. The groundmass consists of plagioclase microlites, glass and minor magnetite. The sample was taken 2 0 0 ' further east of a similar rock chip sample taken by B. Watson. The latter sample was anomalous in B (75 to 100 ppm) and near a large fault trace. CVA 28 is less fractured and further away from this fault than the earlier sample.
Sample no.: CVA 29Alternate designations: noneLocation: C; western flank of Coaldale Ridge . .Designation: That; light bluish gray hornblende andesite tuff breccia.
Spheroidally weathering clasts of hornblende andesite occur in a tuffaceous hornblende-rich matrix. The hornblende grains in each phase are totally altered to chlorite and clays as are the plagioclase crystals. The groundmass is slightly altered to clays. The matrix material has salt crusts beneath the weathered surfaces. The sample includes both phases, and overall, is fairly fresh for this rock type.
127\
Sample no.: CVA 31Alternate designations: noneLocation: C; southeastern end of Coaldale RidgeDescription: Tstl; weathered, greenish gray lower member of the
sediment and tuff unit. The sample was taken primarily from the weathered matrix of a clast-rich tuff. The piagioclase crystals are altered to clays. Gypsum-rich fluff under the soil surface and salt coatings on fractures slightly contaminate the sample..
Sample no.: CVA 32Alternate designations: CV 16Location: C; eastern flank of Coaldale RidgeDescription: That; gray hornblende andesite tuff breccia. The sample
consists of hornblende-rich clasts that occur in a tuffaceous matrix of similar composition. It also includes material from several 3” to 6 " thick ash layers that form thin interbeds in the tuff. The sample area is crosscut by several opaline and chalcedonic veins, 0.5" to 2" wide. The sample is relatively fresh, with no salt crusts or fluffs, in the immediate area; The plagioclase crystals are only slightly altered to clays, and the hornblende to chlorite and bibtite. The groundmass, however, has plagioclase microlites almost completely altered to clays, and pumices and glass shards strongly altered to clays, chlorite and carbonate.
Sample no.: CVA 33Alternate designations: noneLocation: C; the same as CVA 32Description: Opaline and chalcedonic veins, 0.5" to 2" Wide, that
crosscut the tuff described in CVA S2. The veins are white to gray, banded opal and chalcedony, with open-space filling textures.
Sample no.: CVA 34Alternate designations: CV 7Location: C; eastern flank of Coaldale RidgeDescription: > > ■>-> ; green pyroxene andesite dike, the sample
consists of a chloritically altered pyroxene dike that crosscuts the hornblende andesite tuff breccia. Augite phenocrysts (5%) are fresh and the glassy groundmass is chloritically altered. Plagioclase crystals (20%) are only slightly altered to clays. The dike is highly fractured, especially near its contacts.
Sample no.: CVA 35Alternate designations: noneLocation: C; the same as CVA 34Description: That; dark purple to green hornblende andesite tuff
breccia. Baked tuff breccia at its contact with the pyroxene
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dike described in CVA 34. The clasts are a very dark hornblende-rich feldspar porphyry that occur in a light green clay-altered tuffaceous matrix. The sample includes both matrix and clasts in the altered tuff.
Sample no.: CVA 36Alternate designations: CV 18, CV 19Location: C; west of Coaldale RidgeDescription: Tstu; fresh upper member of the sediment and tuff unit.
The sample is of the best exposure of this member. It includes an upper air-fall tuff (5% unaltered plagioclase phenocrysts, 95% glass shards), thin-bedded reworked tuff paper shales, and a lower tan quartz-rich ash-flow tuff. The lower tuff is relatively fresh, with quartz (20%) and biotite (5%) phenocrysts unaltered. Plagioclase (20%) and sanidine (20%) phenocrysts have only thin rims of chlorite and clay. The groundmass is slightly devitrified.
Sample no.: CVA 37Alternate designations: CV 8
Location: C; southern flank of Blue MountainDescription: Tpat;, light purplish gray pyroxene andesite tuff breccia.
The tuff consists of dark pyroxene-rich, siliceous clasts (50%) in a light purplish gray tuffaceous matrix of similar composition. The plagioclase crystals are clay-altered. White salt crusts occur on the ground surface nearby the sample locality, but are not included in the sample.
Sample no.: CVA 38Alternate designations: noneLocation: C; southern flank of Blue MountainDescription: >■> > > ; greenish gray pyroxene andesite dike. A highly
fractured, but relatively fresh chip sample of this rock type. The augite phenocrysts are fractured but unaltered, whereas the plagioclase grains are mostly altered to clays. It is more fractured than any other Tpa sample.
Sample no.: CVA 39Alternate designations: CV 17Location: C; southern flank of Blue MountainDescription: Thai; light bluish gray hornblende andesite lahar. The
sample consists of both clasts and matrix of this rock type.The clasts (40%) are very siliceous and the matrix less siliceous, slightly more weathered hornblende andesite. The hornblende phenocrysts (2 0 %) are very fresh, with only thin magnetite resorption rims. The plagioclase grains (25%) are only slightly altered to clays in the clasts, and moderately altered in the matrix. The groundmass is very glassy, with little to no chioritic alteration.
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Sample no.: CVA 41Alternate designations: CV 35Location: C; southwestern flank of Blue MountainDescription: Tcpj; white to gray unwelded Castle Peak Tuff. A fresh
sample of this quartz-rich unit. Lithie clasts in the tuff consist of Paleozoic chert and limestone. The sample is from relatively fresh tuff near its contact with Thai.
Sample no.: CVA 42Alternate designations:, noneLocation: C; southeastern flank of Blue MountainDescription: w h -; greenish gray hornblende andesite dike. The horn
blende phenocrysts are fresh, but the plagioclase grains and groundmass are weathered to clays, chlorite and carbonate. Moderately spaced fractures (lu to 6 ") are parallel to flow foliation and increase in density toward the dike's outer contacts.
Sample no.: CVA 43Alternate designations: none ■Location: C; southwest of Blue MountainDescription: That;, grayish purple hornblende andesite tuff breccia.
The sample includes both the clasts (35%) and matrix (65%) of the breccia. The clasts are composed of a light red hornblende- and feldspar-rich porphyry, and the matrix is a tuffaceous material of the same composition. The hornblende grains are slightly altered to chlorite and clays. The plagioclase phenocrysts and groundmass are both altered to clays. The sample was taken 10' above the Tcp-That contact. Minor silicification of the That occurs at this contact.
Sample no.: CVA 44Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Tpat; purple pyroxene andesite tuff breccia. Moderately
weathered, the sample includes both siliceous clasts (45%) and tuffaceous matrix (55%) of the tuff breccia. Both are composed of augite and plagioclase phenocrysts in a glassy, chloritically altered groundmass. The sample was taken at the contact of this unit with a pyroxene dike.
Sample no.: CVA 45Alternate designations: noneLocation: C; northeast of Coaldale RidgeDescription: TCP2 /3 ; pink to orange, partially to densely welded
Castle Peak Tuff. Unweathered, the sample is fresh and unaltered. Quartz and sanidine crystals show no alteration.The groundmass is devitrified and slightly clay-altered.Lithic clasts in the tuff consist of Paleozoic chert and a coarse-grained feldspar porphyry.
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Sample no.: CVA 46Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Thai; light bluish gray hornblende andesite intrusion.
Very fresh sample of a large plug of a hornblende andesite. Hornblende and plagioclase phenocrysts are only slightly altered in a glassy groundmass.
Sample no.: CVA 47Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Tcp^; light purple to white, unwelded Castle Peak Tuff.
The sample locality is 10* away from a fluff-rich soil above extremely clay-altered tuff. The sample area is crosscut by opaline veins as in CVA 32. The chip sample taken is relatively fresh, with only slight clay alteration of the groundmass and feldspars. No fluff is included.
Sample no.: CVA 48Alternate designations: noneLocation: C; southwest of Blue MountainDescription: Tfai; light green to purple fine-grained andesite
intrusion. This fresh sample of andesite was taken 2 1 above its contact with That, The plagioclase crystals and groundmass are only slightly altered to clays, with minor chloritic alteration occurring at the contact. The hornblende phenocrysts are relatively fresh. The sample locality is characterized by closely spaced (0.5" to 2") flow foliation fractures.
Sample no.: CVA 49Alternate designations: CV 40Location: C; northern end of Coaldale RidgeDescription: Tfai; dark grayish green fine-grained andesite intrusion.
The sample was taken in a 150* wide fault zone that has three 2" to 6 " wide quartz-cemented fault breccias. The sample includes only the relatively fresh andesite adjacent to each of the breccias, but not the fault breccias. The plagioclase phenocrysts are only slightly weathered to clays, and the augite and (minor) hornblende grains are very fresh. The andesite is moderately to highly fractured, with the joints 1 " to 6 " apart.
Sample no.: CVA 51Alternate designations: noneLocation: C; north of Coaldale RidgeDescription: That; light pinkish purple hornblende andesite tuff
breccia. The sample consists of both the clasts (35%) 'and matrix (65%) of this relatively fresh tuff. Of similar composition, the matrix is more tuffaceous and slightly more
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hornblende-rich than the siliceous clasts. Only slight clay alteration of the plagioclase grains and groundmass is apparent„
Sample no.: CVA 52Alternate designations: noneLocation: C; northern end of Coaldale RidgeDescription: Tha; bluish gray hornblende andesite flow. The horn
blende phenocrysts are very fresh, whereas the plagioclase grains are moderately altered to clays. The groundmass is very glassy, with only slight chioritic alteration. Moderately spaced (6 " to 1 ') flow foliation fractures are pronounced in the sample locality.
Sample no.: CVA 54Alternate designations: noneLocation: . NWDescription: That; gray hornblende andesite tuff breccia. The sample
includes bleached tuff breccia at the intersection of pyroxene and hornblende andesite dikes, and was taken within 2 ' of both contacts. The hornblende and plagioclase phenocrysts are slightly clay-altered. The tuffaceous matrix is only slightly more weathered/altered, and shows minor chloritic alteration. Closely spaced fractures (0.5" to 3") are parallel to the weathered surface and dike contacts at this locality.
Sample no.: CVA 55Alternate description: noneLocation: NWDescription: Tstu; fluff sample in soil above the upper member of the
sediment and tuff unit. Thick gypsum^rich fluff (6 " to 8 ") occurs above an extremely clay-altered quartz-rich tuff. The sample does not include any fresh material, nor identifiable cottoriball ulexite.
Sample no.: CVA 56Alternate designations: noneLocation: NWDescription: Thai; bluish gray hornblende andesite intrusion. The .
hornblende phenocrysts are altered to chlorite, and the plagioclase to clays. The groundmass is an intergrowth of clays, chlorite and carbonate. In the sample locality, a rectilinear pattern of calcite veins (0.5" to 1") crosscut highly jointed andesite. The intrusion is a large dike-like plug that occurs near the contact between the young volcanic rocks and the Paleozoic limestones. The sample was taken less than 500* away from this contact.
132
Sample no.: CVA 57Alternate designations: noneLocation: NWDescription: pT; Palmetto Formation (?) sediments . This sample
represents the geochemistry of the basement to the Tertiary volcanic rocks. It includes dark gray, laminated argillaceous limestones and thin-bedded phyllitic shales. The sample was. taken less than 500’ northwest of the contact between the pre- Tertiary rocks and volcanic plug described above (CVA 56).
Sample no.: CVA 58Alternate designations: noneLocation: NWDescription: Tcp^; reddish brown densely welded Castle Peak Tuff. The
sample was taken at the contact of the tuff with a Thai plug. Highly fractured and baked, the tuff has feldspar crystals altered to clays, and biotite laths converted the chlorite.The groundmass is extremely rich in Fe-oxides and is slightly clay-altered.
Sample no.: CVA 59Alternate designations: noneLocation: NWDescription: That; hornblende andesite tuff breccia. The sample
consists of both clasts (40%) and matrix (60%) of this unit. Bluish gray to pink, the clasts have relatively fresh hornblende laths and clay-altered plagioclase crystals. Although most of the clasts are of the same composition as the matrix, minor clasts of a baked, quartz-rich tuff (Tcp?) also occur. The less siliceous, light gray matrix has fresh hornblende crystals and altered plagioclase. The sample was taken 5 * below the baked base of an overlying flow unit of the same tuff breccia. The lower, sampled flow unit of the tuff breccia shows soft- sediment deformational features; the overlying tuff has an embayed contact with the lower tuff (Figure 7).
Sample no.: CVA 61Alternate designations: noneLocation: C; west of Coaldale RidgeDescription: Tstu; weathered upper member of the sediment and tuff
unit. The sample consists of a highly fractured (1" chips), quartz- and biotite-rich clay-altered tuff. The groundmass is completely altered to a deep red clay, and the feldspar pheno- crysts to a white clay. The biotite and hornblende (?) laths are altered to clays and chlorite. Minor fluff occurs in the soil above the tuff and a salt coating on the chip's fracture surfaces is common. The fluff was not included in the sample, but minor contamination is probable.
133
Sample no.: CVA 62Alternate designations: noneLocation; NW-Description: Tcpg; greenish gray partially welded Castle Peak Tuff.
The sample was taken at the southern extent of two northwest trending faults. In the 20 square foot sample area, three small-scale left separation faults occur in a highly fractured, silicified and chloritically altered tuff. Silicification decreases as clay alteration increases with distance away from each small fault. The Castle Peak Tuff in this locality is very biotite-rich (now chlorite) and is part of the upper cooling unit defined in this area.
Sample no.: CVA 64Alternate designation: CV 2Location: NWDescription: Tb; black basalt flow(?). This sample was taken from one
of the few Tb outcrops in the area. It consists of fresh hypersthene crystals (15%) in an extremely fine-grained pilotaxitic groundmass. The lack of olivine indicates this rock type may be better classified as a "basaltic andesite."
Sample no.: CVA 65Alternate designation: noneLocation: NWDescription: Qsp(?); bedded spring deposits in alluvium(?). The
sample includes a basal limy chert conglomerate, and upper alternating beds of chalcedony, limestone breccia, opal and travertine. Open-space filling textures are common in these upper bands. These gently dipping sediments unconformably overlie both the Tcp and That. They are most likely older alluvium, but could be a remnant Tertiary sequence or even Te.
Sample no.: CVA 6 6
Alternate designations: CV 34Location: NEDescription: Tcpe; exotic rhyolite block in Castle Peak Tuff. The
sample is of the largest exotic block within the Tcp in the study area. The block is heavily Fe-stained and oxidized. It is partially vesiculated, and has pronounced eutaxitic pumices in portions of the outcrop. The attitudes do not vary in this exposure and are almost perpendicular to the surrounding Tcp. The pumice, feldspars and groundmass are altered to clays, but quartz grains are fresh. Biotite laths are altered to chlorite. Pronounced silicification of the groundmass also occurs.
134
Sample no.: CVA 67Alternate designations: CV 25Location: NEDescription: Tfa; gray fine-grained andesite intrusion(?). The
hornblende grains (5%) are altered to chlorite and most are totally resorbed by magnetite with ghost envelopes of clays and carbonate. The augite phenocrysts (2%) are fresh. The plagioclase phenocrysts (8 %) are only slightly altered.to clays. The groundmass consists of microlitic plagioclase, glass and pyroxene granules. Overall, this sample is very fresh and representative of this rock type.
Sample no.: CVA 6 8
Alternate designations: noneLocation: NEDescription: Qsp; fluffs and crusts surrounding a modern spring. The
sample includes white gypsum-rich fluff (6 " to 8 ") that occurs under a crusty, limy soil surrounding a modern spring. The area affected by the spring is at least 500 square feet.
Sample no.: CVA 69Alternate designations: noneLocation:/ NEDescription: Ts; gray thin-bedded tuffaceous sediments. The sample
consists of alternating beds of sandy siltstones, silty mudstones and pebble conglomerates. All units are extremely volcaniclastic, with clasts of pumice quite pronounced in the coarser units. The fine-grained sediments are very biotite- rich and tuffaceous. The sample is fresh, with no evidence of salts or fluff anywhere nearby.
Sample no.: CVA 71 ’Alternate designations: CV 26Location: NEDescription: Tcab; reddish purple coarse-grained andesite breccia.
The sample contains both clasts (50%) and matrix (50%) of the breccia. Dark red clasts are very hornblende-rich (25%) with minor plagioclase phenocrysts (5%). The groundmass of the clasts is composed of glass and microlitic plagioclase and is heavily Fe-stained. The matrix has less and finer-grained hornblende phenocrysts (1 0 %) in a plagioclase-rich glassy groundmass with little to no Fe-staining. In both the clasts and matrix, the hornblende grains are resorbed by magnetite. The plagioclase crystals are slightly altered to clays. In general, the sample is fresh for this rock type.
Sample no.: CVA 72Alternate descriptions: noneLocation: NE
i
135
Description: Tcp^; white unwelded Castle Peak Tuff. Very fresh, thetuff in this locality is lithic-rich with clasts of a feldspar andesite and Paleozoic chert, limestone and phyllitic shales. The groundmass and sanidine grains are only slightly altered to clays.
Sample no.: CVA 73Alternate designations: noneLocation: NEDescription: pT; Palmetto Formation (?). The sample represents the
pT basement geochemistry in the northeast. It consists of light tan to gray phyllitic shales and minor thin-bedded limestones.
Sample no.: CVA 74Alternate designations: noneLocation: SEDescription: Te(?); Esmeralda Formation lacustrine sediments. This
sample is representative of fresh, fairly coarse-grained Esmeralda Formation sediments. The sample consists of an upper gray sandstone, minor tuffaceous siltstones, and lower limy pebble conglomerates. The sample locality includes 25* of section.
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Y -
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Ekren, E. B., Byers, F. M., Hardyman, R. F., Marvin, R. F. andSilberman, M. L., 1980, Stratigraphy, preliminary petrology, and some structural features of Tertiary volcanic rocks in the Gabbs Valley and Gillis Ranges, Mineral County, Nevada:U.S. Geol. Sur. Bull. 1464, 54pp.
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136
137
Faure, G., 1977b, Isotope Geology of Strontium, in Principles of Isotope Geology: John Wiley and .Sons, inc„, pp. 75-95.
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Foldvair-Vogl, M., 1978, Theory and Practice of Regional Geochemical Exploration: Akademiai Kiado, publisher, 272pp.
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Keith, W. J., 1977, Geology of the Red Mountain mining district,Esmeralda County, Nevada: U.S. Geol. Sur. Bull. 1423, 45pp.
Kistler, R. B., and Smith, W. C., 1975, Boron and borates, inLefrond, S. J., ed.. Industrial Rocks and Minerals., 4th edition: American Inst, of Mining, Metallurgical and Petroleum Engineers, publisher, pp. 473-496,
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Locke, A., Billingsley, F. R. and Mayo, E. B., 1940, Sierra Nevadatectonic patterns: Geol. Soc. Am. Bull., v. 51, pp. 513-540.
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Marvin, R. F., Mahnert, H. H., Speed, R. C. and Cogbill, A, H„, 1977,K-Ar ages of Tertiary igneous and sedimentary rocks of the Mina-Candelaria region: Isochron/west, no. 18, pp. 9-12.
McKee, E. H., 1974, Timing of late Cenozoic crustal extension in the western United States: Geol. Soc. Am. Abst. with Prog.,Cordilleran Section, p. 218.
138
Moore, S. W., 1981, Geology of a part of the southern Monte CristoRange, Esmeralda County, Nevada: U.S„ Geol. Survey Open-FileReport 81-710, 160pp.
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Robinson, P. T., and Crowder, D. F., 1973, Geologic map of the Davis Mountain Quadrangle, Esmeralda and Mineral Counties, Nevada, and Mono County, California: U.S. Geol. Survey GeologicQuadrangle Map GQ-1078, scale 1:62,500.
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139
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140
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;
LEGEND:FAULT, BAR AND BALL INDICATES DOWNTHROWN SIDE
c v a COALDALE VOLCANIC ASSAY SAMPLE
cvc COALDALE VOLCANIC CHEMISTRY SAMPLE
c v d COALDALE VOLCANIC DATE SAMPLE
c v a 23 SAMPLE DESIGNATION 12 BORON VALUE IN PPM
---------DOMAIN BOUNDARIES
STRUCTURAL AND GEOCHEMICAL OVERLAYSOUTHWESTERN MONTE CRISTO RANGE
X
FIGURE 14
DIXIE A. HAMBRICK, MS THESIS, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, 1984
/' ~ V Z * ' z
3/ ^ ^ '
/ z x. \
Geologic Sections Across The Southwestern Monte Cristo RangeEsmeralda County, Nevada
6000'n r 6000Tcp3 Tcp3
Thai Cp Tcp2 Tcpg TcP2x | / f ! / / / \ _
5 0 0 0 - -5 0 0 0
400 0 4000
7000'n r 7000TcabTcab A Tcab
BLUEMTN
Tcp _ —
6000 '- -6 0 0 0Thai (?) Tcp
5000 - -5 0 0 0
4000 ' 4000
LITHOLOGIC UNITS
r6 0 0 0ROAD 7 TCP (?)Tfai /That TCP That
5000 - 5000
Tfai I '
4 0 0 0 4 0 0 0
6000 ' -i COALDALERIDGE r 6000
Tstl
5 0 0 0 - -5 0 0 0Tha HTpoi
TstuTha That VlTstl _ -n / Tstu-V- — - Tstu 4
y// Tcp4000 4 0 0 0
QUATERNARY DEPOSITS
YOUNGER SEDIMENTARY AND BASALTIC SEQUENCE
COALDALE VOLCANIC SEQUENCE
|Tpa| |Tpdis|
| Tpat|
|fpai X
IHIH M 1 Thai 1
| Thai |
| Tstu|
| That|
lTs"l
BLAIR JUNCTION VOLCANIC SEQUENCE
1 Tcal
|Tcob|
PH
SCALEl: 12,000
2QQQ __ 1000 Q_________1000 2000______3000
(F E E T )
CASTLE PEAK VOLCANIC SEQUENCE
500t—
(M E T E R S )
500 1000 — |
C A STLE PEAK T U F F
|Tcpe| | Tcp11 |Tcp2|
NO VERTICAL EXAGGERATIONPRE-TERTIARY BASEMENT
FIGURE 15
DIXIE A. HAMBRICK, MS THESIS, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, 1984
w
Geology of the Southwestern Monte Cristo RangeEsmeralda County, Nevada
EXPLANATION OF MAP UNITS
QUATERNARY DEPOSITS
QUATERNARY ALLUVIUM Qsp QUATERNARY (?) SPRING DEPOSITS- S A L T AND C A R B O N A T E - RICH CRUSTS
AND F L U F F S ; BEDDED C H AL C ED O N IC AND T R A V E R T IN E DEPOSITS
YOUNGER SEDIMENTARY AND BASALTIC SEQUENCE
BASALT FLOWS (?)
ESMERALDA FORMATIONS- T A N TO GREEN TUFFACEOUS M UDSTO NES,
S IL T S T O N E AND SANDSTONES
COALDALE VOLCANIC SEQUENCE
INTRUSIVE RHYOLITE- A L K A L I (F E L D S P A R ) R H Y O L ITE
PYROXENE ANDESITE FLOWS
- L A T I T E 1-
Tpais PYROXENE ANDESITE SILLS Tpoi PYROXENE
ANDESITE INTRUSIONS / PYROXENE ANDESITE DIKES
Tpat
Tfoi
PYROXENE ANDESITE TUFF BRECCIA
FINE-GRAINED ANDESITE INTRUSIONS- L A T I T E f
HORNBLENDE ANDESITE FLOWS
Thai, HORNBLENDE ANDESITE SILLS Thai
- L A T I T E 1
Thai
- L A T I T E 1-
HORNBLENDE ANDESITE LAHAR
HORNBLENDE ANDESITE INTRUSIONS
- L A T I T E */ HORNBLENDE
ANDESITE DIKES
That HORNBLENDE ANDESITE TUFF BRECCIA
Tstu
Tstl
UPPER MEMBER OF SEDIMENT AND TUFF UNIT- I N T E R B E D D E D A I R - F A L L T U F F S , PAPER S H A L E S
AND Q U A R T Z - R I C H ASH FLOWS
LOWER MEMBER OF SEDIMENT AND TUFF UNIT- H I G H L Y W E A T H E R E D , L IT H IC - RICH T U F F S
BLAIR JUNCTION VOLCANIC SEQUENCE
Tea* COARSE-GRAINED ANDESITE FLOWS Tfa( A S T E R IK S MARK POSSIBLE V E N T AREAS)
FINE-GRAINED ANDESITE INTRUSIONS
- T R A C H Y T E t
Tcab COARSE-GRAINED ANDESITE BRECCIA
SEDIMENTARY ROCKS- G R A Y V O L C A N IC L A S T IC PEBBLY CONGLOMERATES,
SANDSTO NES AND S ILTSTONES
CASTLE PEAK VOLCANIC SEQUENCE
h-CL
fiT^-u ' 1 Q Tri BANDED RHYOLITE INTRUSIONS - A L K A L I (F E L D S P A R ) R H Y O L IT E 1-
CASTLE PEAK TUFF- R H Y O L I T E 1"
( 4 members )
EXOTICBLOCK MEMBER
Tcp, DENSELY WELDED MEMBER
Tcpg PARTIALLY WELDED MEMBER
TCP3 UNWELDEDMEMBER
PRE-TERTIARY BASEMENT
PRE-TERTIARY ROCKS- M O S T LY T H E ORDOVICIAN P A L M E T T O FO R M A T IO N , ' IN T E R B E D D E D
L I M E S T O N E S , B L A C K C H E R T S , AND P H Y L L IT IC S L A T E S
t IUGS CLASSIFICATION
CORRELATION OF MAP UNITS
QUATERNARY
4
IOOO
FAULTS
(M E T E R S )
CONTOUR INTERVAL - 20 FEETBASE FROM US GEOLOGICAL SURVEY
COALDALE AND BLAIR JUNCTION QUADRANGLES
MAP SYMBOLSS T R IK E AND DIP OF BEDS - DASHED WHERE A P P R O X IM A T E
S T R IK E AND DIP OF FLOW BANDS OR OTHER P L A N A R FLO W FE A T U R E S - DASHED WHERE A P P R O X IM A TE
S T R IK E AND DIP OF O P A L IN E V E IN
D E P O S IT IO N A L OR IN T R U S IV E CONTACT, SHOWING DIP W HERE KNOWN - DASHED WHERE A P P R O X IM A T E , D O TT E D WHERE IN FE R R E D
82 R I G H T - L A T E R A L FA U L T , SHOWING DIP WHERE AND PLUNGE 0 F A n Y L IN E A T IO N St 5*35 d a s h e d W HERE A P P R O X IM A TE . BAR AND B A L L ON DOWNTHROWN SIDE
PLIOCENE
UPPERANDMIDDLEMIOCENE
UNCONFORMITY
IB.6 m. y.
Tpa Tpoi Tpais < < <% T p o t ^ ^
Tfoi Tea
2 2 . 2 m.y. Thai' T h a i s
III H + + - ,
Tha — —
hat Thai Tcab
Tstu"Tstl"
LOWERMIOCENE
UNCONFORMITY
2 4 .2 m.y. Tcpe
UPPEROLIGOCENE
TERTIARY
t35vT9
DIKES
kfsTHicLAi£6L« rrowKsSoLri.r"85" ANVHORNBLENDE AN DESITE D I K E , SHOWING DIP W HERE KNOWN .
PYROXENE A N D E S ITE D IK E , SHOWING DIP WHERE KNOWN
UNCONFORMITYORDOVICIAN PRE-TERTIARY
FIGURE 2
DIXIE A. HAMBRICK, MS THESIS, DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF ARIZONA, 1984
\4 9 f %
\ \