i
GEOLOGY OF THE
MONTOSAS-CATMOUNTALiJ AREA,
TRES
.
A Thesis
Presented to
the Faculty of
,
the Department of Geoscience
New Mexico Institute
of Mining and Technology
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
by
William Holbrook Wilkinson, Jr.
March, 1376
.
-eThis thesisis accepted on behalf
of the facultyof the
Institute by the following committee:
*
i
CONTENTS
PAGE
..............................................
viii
INTRODUCTION.........................................
1
ABSTRACT
....................
Location and Accessibility .....................
Previous Investigations
........................
Methods of Investigation .......................
Geography ......................................
Acknowledgements ...............................
STRATIGRAPHY AND PETROLOGY..........................
Prevolcanic Rocks
..............................
. .
Sandia Formation ...........................
Ab0 Formation..............................
Sedimentary Rocks of Unknown Age...........
Purpose
of
the
Investigation
Tertiary Volcanic Rocks........................
................
Upper Spears ...........................
Tuff of Granite Mountain...............
Hells Mesa Tuff............................
..............................
Andesite of Landavaso Reservoir............
A-L Peak Tuff
2
7
.8
9
10
10
10
11
21
24
Lower Spears ...........................
Mountain
2
24
Spears Formation............................
Tuff of Nipple
1 .
25
.
27
31
35
39
44
50
Tuff of Gray Hill
..........................
52
Potato Canyon Tuff.........................
56
.......................
Latite Dikes................................
Tres Montosas Stock........................
Tertiary Intrusive Rocks
.................................
Monica Canyon Pluton.......................
Post-Oligocene Rocks ...........................
Piedmont Gravels ............................
Basalt of Council Rock
.....................
Quaternary Deposits........................
Talus and Colluvium....................
.
.
Eolian Sand ............................
Facies
Alluviurn ...............................
...........................................
Regional Structure .............................
Local Structure ................................
Paleozoic Faults...........................
STRUCTURE
Oligocene Tectonics ........................
58
58
63
69
79
81
81
81
88
88
89
90
91
91
93
96
96
Late Eocene-Early Oligocene Transverse
Faults .................................
96
.
Development of the
Mt Withington
Cauldron ...............................
I
..............
Faults..................
97
Northeast-Trending Faults
100
North-Trending
101
iv
ILLUSTRATIObTS
PLATE
I.
PAGE
Geologic map and sections
of the Tres
Montosas-Cat Mountain area, Socorro County,
in pocket
New
Mexico
...............................
11.
111.
Tres Montosas quadrangle, residual magnetic
intensity
in
pocket
'
................................
Detailed
prospect
geologic
map
of
the
Sixty
..................................
copper
in
pocket
FIGURE
1.
Location
Mountain
mapof the
Tres
Montosas-Cat
.3
area......................."^....
2.
Generalized stratigraphic column of rocks
the Tres Montosas-Cat Mountainarea..........
3.
Clasts from the
Ab0 Formation
overlying Spears Formation
4.
Outcrop ofa limestone pebble conglomerate
unit in the Ab0 Formation ....................
18
Photomicrograph of contact metasomatic
minerals in theAb0 Formation..
19
Photomicrograph of contact metasomatic
minerals in the Ab0 Formation......,.
19
5-A.
5-B
enclosed
in
...................
the
14
...............
-
6.
7.
8.
........
Cross-sectional view of the flow-banded
member of the A-L Peak Tuff on Hill....
Gray
in
13
.
47
........
49
View of Gray Hill showing
a major paleovalley filled withA-L Peak Tuff.....
Residual magnetic intensity map
of the Tres
Montosas quadrangle and part of the Gallinas
62
Peak quadrangle
..............................
9.
Outcrop showing the contact between the
monzonite facies of the Tres Montosas stock and.
.......... 65
hornfelsed tuff of Granite Mountain
V
10.
11
-
Exposure of the contact ketween the monzonite
facies of the Tres Montosas stock and andesites
of the Spears Formation showing a thin, chilled
67
.........................
margin of the stock.
Outcrop of andesites in the. Spears Formation
intruded bya granit'e .dike from
the Tres
68
Montosas Stock
...............................
12.
Inferred distribution of the principal
types in the Tres Montosas stock..
13.
Photomicrograph of granophyric texture devethe Tres Montosas stock.74
loped in granite from
14.
Classification of the basalt of
after the method
of Church (1975)
15.
16.
rock
70
............
Council
Rock
............
87 .
Variations of the Plagioclase compositions
in the basalt of Council
Rock...
.............
Major structural features in
region with respect to the
87
the Magdalena
94
thesis
area.......
...
17.
18.
-
'19
Overall view of the
of Tres Montosas..
thesis
area
from
...........................
the
top
95
Relationship of faults in the Cat Mountain
99 .
district to the Mt. Withingtcn
cauldron......
Diagrammatic cross section illustrating the
development of the intrusive monocline along
105
the eastern margin
of the Gallinas uplift
....
20.
Overall viewof the
open cut at the
112
.....
21.
Paragenetic
minerals..
118
22.
Dhotograph showing the occurrence
of
chrysocolla in the silicified tuff
of Nipple
Mountain.. ...................................
119
Photograph showing a wave of hematite
banding moving away from a fracture...
120
23
-
sequence
mineralization in the
Sixty prospect
................
for
vein
.......
.
vi
Paragenetic sequence for mineralization at
the Sixty prospect
121
25.
Cross section
123
26.
Intense fracturing of the tuff of Nipple
Mountain in a roadcut a l o n g Highway 60..
24.
...........................
through the Sixty prospect .....
.- ..,. .
-140
v ii
0
TABLES
TABLE
1.
Representative modal analyses of principa.1
rock types from the. Tres Montosas stock. 75
2.
Modal analyses
tuff of Granite
3.
4.
a
PAGE
.....
Chemical
Rock
of fresh
Mountain
and
hornfelsed
.....................
78
analysesof the basalt of Council
.........................................
Normative compositions
Council Rock
of
the
85
basalt
of
.................................
86
5.
Radiometric dates which bracket the of
age .
the Oligocene north-trending faults
.......... 103
6.
Geochemical
inclusions
7.
8.
data
obtained
from
fluid
...................................
Lead
isotopes
from
the
Council
Rock
131
district.
131
Comparison of Pb/K rations. from barite
.fluorite.....................................
and
135
viii
e
ABSTRACT
Tha Tres Montosas-CatMountain
relatively deeply eroded horst
a r e a i s s i t u a t e d on a
between t h e G a l l i n a s a n d
San Mateo Mountains.Thishorst
i s c a l l e dt h e
are r e p r e s e n t e d by
u p l i f t .P a l e o z o i cs e d i m e n t a r yr o c k s
small exposures of the
Ab0
Gallinas
Formationandbyrocks
in the
Banner d r i i l hole which may b e p a r t o f ' t h e S a n d i a
Formation.Bedrock
c o n s i s t sd o m i n a n t l yo fv o l c a n i c l a s t i c
sedimentaryrocks,weldedash-flow
inter-
t u f f s , andminor
bedded l a v a f l o w s o f t h e S p e a r s F o r m a t i o n ( e a r l y ' t o m i d d l e
Oligocene).
Remnantsof
ash-flow t u f f s of t h e Hells Mesa
and A-L Peak Formations overlie the Spears Formation,
e s p e c i a l l yi nt h es o u t h e r np o r t i o no f
.
.
s t r a t i g r a p h i cu n i t ,t h et u f f
overlies the
A-L Peak Tuffand
the district.
A new
of Gray H i l l , i s d e s c r i b e d .
c o n s i s t s of a c r y s t a l - p o o r ,
moderately to densely welded, multiple-flow
compound c o o l i n g
u n i t o fa s h - f l o wt u f f s .N o r t h e a s t - t r e n d i n gp a l e o v a l l e y s
were i m p o r t a n t i n c o n t r o l l i n g t h e d i s t r i b u t i o n . o f
many of
theash-flowtuffs.Maficlavaflows,ofprobable
late
Pliocene age, cap piedmont gravels in grabens.formed during
b a s i n andrangefaulting.Chemicalanalysesoftheseflows
( b a s a l t ofCouncil
Rock) i n d i c a t e t h a t t h e y a r e . h i g h
s i l i c a and a l k a l i e s and a r e a l k a l i - t r a c h y t e s t o t r a c h y andesites.
It
in
ix
Major
structural
features
of
the
map are:
area
1.) the Mt. Withington cauldron margin;
2.) high-angle
faults with a dominant north-northwest trend;3 . ) a northtrending
monoclinal
fold
the
the
eastern of
flank
the
northeast-trending faults related
Gallinas uplift; and4.)
to
forming
lineament the'
and San Augustin
Morenci-Magdalena
arm of the Rio Grande rift.A subsidary graben extends
northward
from
do
and
dikes
the
edge
of the
gold-bearing
Mt.
Withington
veins
of
The monoclinal flexure and some
of the
.angle
faults
Oligocene
are
the
batholith
result
of
the
Cat
as
Mountain
north-trending,
the
approximately
cauldron
district.
high-
intrusion
of an elongate
twenty
miles
long.
Intrusives from this"batho1ith rose
to within 2000 feet of
the surface. Two intrusives are exposed in the map area and
a third 'is inferred from felsic.dike
quartz
veins,
hydrothermal
swarms,
epithennal
alteration, aand
weak
aeromagnetic
low. Late Cenozoic block faulting uplifted the
San Mateo
and
Gallinashorsts;
the
Tres
Montosas-Cat
Mountain
area
occupies a subdued area where the San Augustin arm of
the
Rio
Grande
Hydrothermal
rift
cuts
the
alteration,
San
Mateo-Gallinas
to the
related
axis.
movementof
fluids along faults, is widespread in the
map area. Analyses
of
fluid
inclusions
ore-forming
fluids
from
were
vein
saline
fluorite
indicate
that
solutions
at temperatures
the
X
0
n e a r 18OoC.
The solutionscontainedlead,copper,
and z i n c .
A small oxide copper deposit, the Sixty prospect, of probable
s u p e r g e n eo r i g i n
i s exposedsouthof
Highway 60.Corefrom
a deep d r i l l h o l e a t t h e S i x t y p r o s p e c t
t o a depth of
shows a l t e r e d r o c k s
1622 f e e t w i t h c h a l c o p y r i t e , s p h a l e r i t e , a n d
the bottom.
g a l e n ap r e s e n ti nv e i n l e t sn e a r
The a l t e r a t i o n
and m i n e r a l i z a t i o n i n t h e d r i l l h o l e s u g g e s t t h a t ' t h e . h o l e
i s near a b u r i e di n t r u s i v e .
The Tres Montosas-CatMountain
area i s one of the most favorable parts
area for base metal exploration.
o f the Magda.lena
INTRODUCTION
Purpose of the Investigation
The objectivesof this investigation are to determine
the stratigraphic relationships, structural
trends,-digtribution of intrusive rocks, and
distribution
of
the
mineralization and hydrothermal alteration in the Tres
Montosas-Cat Mountain area, Socorro County, New Mexico.
This
thesis
was
project of the
Bureau'
of
as' of
part
the
undertaken
Magdalena
Xines
and
area
extensive
conducted
Mineral
by
the
mapping
New
Mexico
Resources.
The objectives listed above are important for the
following
reasons:
1. The stratigraphic relationships provide further data
for correlating rock types between the Bear, Magdalena,
Gallinas,
and
extension'
"
L.
of
San
geologic
of
the
An
understanding
used
to
Mateo
Datil-Mogollon
locate
of
Mountains
work
westward
volcanic
the
..
the
allow
into
the
province.
structural
additional
and
trends b?may
bodies
with
intrusive
ti12
possibility of associated porphyry- or replacemerlttype deposits.
3.
The overall relationships between stratigraphy, .
structure,
mineralization
and
the
'distribution
of
intrusive rocks are important in evaluating the
economic
potential
of
the
map
area.
heart
*
Location
and
The
ten
Accessibility
Tres
miles
Montosas-Cat
west
of
Mountain
Magdalena,
New
area
is
Mexico,
located
in
the
approximately
northeast
1).
corner of the Datil-Mogollon volcanic province (see Fig.
The
area
mapped
covers
a
nearly
rectangularof area
about
forty square miles. The northern part
is
inthe Cibola
National Forest. The area is approximately bounded 37'by 7 ' - N
latitude on the north, 3a0
0 0 ' N' latitude on the south,
107'
2 3 ' 07" W longitude
on the
on the east, and
107'
28'
30" W longitude
west.
Easy
accessto the
areais provided by several routes.
U. S. Highway 60 crosses the north-centralpart.and.NewMexico
Highway 5 2 provides access to the southwestern part. The
(I,
,
eastern and southeastern parts can be reached by the Cat
Mountain
Ranch
road
and
Tres
Montosas
can
be
reached
by
the
North Lake road. Numerous unimproved ranch roads and woodcutters
.trails
Only
in
wheel
provide
easy
sandy
areas
drive
vehicle
access
to the remainder of the area.
along
the
northern
boundary
is
a
four-
investigation
in
the
required.
Previous Investigations
The
earliest
recorded
geologic
(1900) as
Montosas area was conducted by Herrick
reconnaissance
surveyof western
Socorro
and
Tres
partof a
Valencia
Counties.
He observed that the Datil, Gallinas, and Bear Mountains were
composed of trachyte and rhyolite intrusives. Lindgren,
Graton, and Gordon(1910) presented a general discussion
of
3
NEW
MEXICO
COUNTY
t
Figure 1.
Location map of t h eT r e s
Hontosss-Ca .t Mountain area.
4
geology
the
and
ore
Rosedale
deposits
mining
of
the
district
Magdalena
in
the
San
mining
Mate0
district
and
Mountains.
Winchester (1920) observed volcanic rocks overlying sedimentary
rocks of Cretaceous age along Alamosa Creek (now the Rio Salado)
and named these the Datil Formation.
He described a type
section consistingof tuffs, rhyolites, conglomerates, and
sandstones
in
the
northern
Bear
Mountains.
Lasky (1932) catalogued and described the base metal
deposits
of
the
of
Socorro
gold
mines
County
near
which
Cat
included
Mountain
and
brief
descriptions
of the
the
mines
Council Rock district just north of the map area. Laughlin
and Koschman (1942) published a detailed investigation of
*
the
geology
They
and
reported
observed
ore
that
there
deposits
of the
several
extendedto the
of
Magdalena
the
north
mining
Tertiary
and
district.
volcanic
northwest
units
outside
the district. Thurmond (1951) prepared a private report
6 , T. 3
at the Sixty propsect ,in Sec.
R. 5 W.
S.,
Slawson
and Austin (1962) investigated lead isotopes from mines in
the Council Rock district..
Detailed
stratigraphic
work
on
the
Datil
Formation
was
done by Tonking (1957) and Givens (1957) in the Puerticito
and Dog Springs Quadrangles, respectively. Tonking
divided the DatiI Formation into three members: Spears,
Hells Mesa, andLa Jara Peak; Givens further. subdivided
.
the Hells Mesa Member into seven informal units. Equivalents
e
of
theLa
Jara
Peak
.
Member
have
not
been
recognized
on
the
e
5
Gallinas uplift. Willard (1959) tentatively correlated
the
La
Jara
Peak
Member
with
the
Manqas
basalt,
a
post-Datil
sequence. Weber (1963) formally excluded the La Jara Peak :
Member
from
the
Datil
Formation
and
in
1971
he
elevated
the
Datil Formation to group status.. Chapin (1971-a) elevated
the
Spears,
Hells
Mesa,
La
and Jara
Peak
Members
to foma-
tional status. Brown (1972) subdivided the Hells Mesa
of Goat Springs
Formation into two informal units: the tuff
and the tuffof Bear Springs. Deal and Rhodes (in press)
subsequently
renamed
Tuff
type
for
a
the
tuff
section
on
of
Bear
Springs
A-L the
Peak
A-L
Peak
in
the San
northern
Mateo
Mountains. Chapin (1974-b) restricted the Hells Mesa Forma- .
tion to the
quartz-rich,
crystal-rich
ash-flow
tuffs
formerly called the tuff of Goat Springs.
Recent work by Elston and others (1968,
1970, 1973) has
attempted to fit the
Datil-Moqollon
Volcanic
Province
into
an overall volcano-tectonic framework. Chapin (1971-b)
discussed
effects
modifications
upon
the
of
the
structural
Rio
trends
Grande
of
the
rift
and
Magdalena
its
area
and Chapin and others (1974) proposed an exploration framework for the Maqdalena-Tres Montosas area. Deal .(1974) and
Deal and Rhodes (in press) investigated the formation and
'development
of
end
San
of
the
the
Mt.
Mateo
Withington
Mountains
cauldron
in
the
which
they.proposed as the
northern
6
source
of
the
A-L
Peak
Tuff
and
the
Potato
Canyon
Tuff.
Most recently, several theses and dissertations have
studied the volcanic stratigraphy, structure, and mineralization in the Magdalena area. Brown( 1 9 7 2 ) completed an
investigation of the
to the
Bear
understanding
Group. Simon
of
Mountains
the
and
contributed.greatly
stratigraphy
of the
Datil
( 1 9 7 3 ) recently finished an investigation of
the stratigraphy, structure, and mineralization
of the
Silver Hill area. Siemers ( 1 9 7 3 ) completed a studyof the
Paleozoic'stratigraphy-of
origin'of the
Popotosa
the
Magdalena
Formation
dissertation by Bruning ( 1 9 7 3 ) .
the
Lemitar
Mountains
and
structural
and
Mountains
was
the
andthe
subjectof a
Woodward ( 1 9 7 3 ) mapped
discussed
relationship the
to Rio
their
Grande
stratigraphic
rift.
Chamberlin ( 1 9 7 4 ) conducted a detailed investigation
of
the
Council
Rock
area
immediately
north
of the Tres Montosas
area and Krewedl( 1 9 7 4 ) completed a dissertation in the
central Magdalena Mountains. Blakestad ( 1 9 7 6 ) is remapping
the
Kelly
mining
Numerous
district.
radiometric
dates
have
been
published
on the
rocks of the Datil-Mogollon province.
.Weber and Bassett
( 1 9 6 3 ) reported
Canyon
stocks
K-Ar
in
age
the
dates
Kelly
for
mining
the
Nitt
and
Anchor
district
and
for
the
of the Hells Mesa Member at Tonking's type section. Burke
.*
and others ( 1 9 6 3 ) dated a latite boulder from
the
Spears
Member
and
two
the base of
unwelded of
tuffs
the Hells
Mesa
base
7
e
Member. Kottlowski', Weber and.Willard (1969) published
49
radiometric
in
the
New
previously
dates
of Cretaceous
Mexico
and
Tertiary
igneous
rocks
region
and Weber (1971) reported
five
unpublished
dates
for
Tertiary
rocks
in
igneous
central New Mexico. Chapin (1971-a) reported a K-Ar age
date
for
the
La
Sara
Peak
Formation
and
discussed
its pos-
sible significance to mineral exploration.
.Recently,
Simon
(1973)
Montosa,
presented
which
a
new
date
on the
consists
of interbedded
unit
of
Arroyo
fanglomerates
and
dacite lava flows. Smith and others (in press) dated the
A-L
Peak
Tuff
Mountains
and
using
the
the
CanyoninTuff
the San Mateo
Potato
fission
track
method.
Methods of Investigation
Detailed
1:24000
geologic
using
the
published by theU.
detailed
mapping
mapping
Tres
S.
of
a base prepared byS.
was
conducted,ata scale of
Montosas
7.5-minute
quadrangle
Geological Survey as a base map.The
the
S.
Sixty
copper
prospect
was don: on
Thurmond in 1951 (Brunton compass
1" = 200'.
and chain).at a scale of
Aerial photographs
of 1:31680 and
of the GS-VARJ series, 3-20-63, at a scale
of
the
GS-VMA
to aid in
the
series,
location
i956,
of
scale
at a of 1:23480 were used
outcrops
and
in
structural
inter-
pretations. Mapping was done during the summer and
fall of
1971 and the summer of 1975. A topographic base
map on
mylar was used'
e
for
the
preparation
Two hundredthirty-fivethinsections
of finaLgeologic
the
map.
were made andexamined
from samples collected over the area. Thesewere used to
,
'
8
correlate rock units, to study hydrothermal alteration,
and,to interpret the paragenesis of vein minerals.
A
diamond-drill hole located in Sec.
6 , T. 3 S., R. 5 W. was
logged byC. E. Chapin and forty-eight thin sections were
of ten thin sections
made from the core. Modal analyses
from
the
Tres
using a Zeiss
Montosas'stock
microscope
J. by
E. Bruning
prepared
were
equipped
with
a Swift automatic
point counter. Five of these samples were collected by
R. M. Chamberlin from the north border of the stock. Several
samples o f fluorite
collected
f o r fluid
were
inclusion
analyses. Four basalt samples were
a l s o prepared.for X-ray
fluorescence
andAA analyses.
Geography
.
.
Physiographically
comprises
high
the
which
the
southern
connects
Tres
Montosas-Cat
end a of
north-trending
the
San
Mateo
Hountain
area
topographic
Mountains
with
the
Gallinas Mountains. The present topography is strongly
controlled
to
the
by
west
block
and
faulting
the
with San
the Augustin
Mulligan
Gulch
graben
graben
to
the
east.
Relief over most of the area is relatively not
low,
exceeding 700 feet.; however, Tres Montosas
in the northwest
corner
isa prominent landmark with relief
of 1300 feet
above the surrounding area. The southern boundary approximately
coincides
with
the
northern
margin
Withington cauldron as defined by Deal (1973).
area is near
the
boundary
between
the
of
the
The map
Datil-Mogollon
Mt.
e
volcanic province
9
and t h e ColoradoPlateauprovince
( F i g . 1).
Acknowledgements
F i n a n c i a ls u p p o r t ,c h e m i c a la n a l y s e s ,
and. f i e l d t r a n s - .
by t h e New Mexico Bureauof
p o r t a t i o n were provided
and Mineral Resources, to
whom t h e a u t h o r
Appreciation i s e x t e n d e d t o P e t e
i s .grateful.
Evans of t h e P e t e
Ranch and t o Bob Dunlap oftheCatMountain
Mines
Evans
Xanch f o r
their hospitality in allowing access to their lands.
S . Thurmond and W. Brunson providedinformationconcerning
theSixtycopperprospect
all.owed'accesstocore
and t h e Banner Mining Company
from a h o l e t h e y d r i l l e d
at this
praspect.
D r . C . T . Smithand
D r . K. C . Condie served on t h e t h e s i s
committee and p r o v i d e d t h e a u t h o r w i t h h e l p f u l i d e a s
d i s c u s s i o n sb o t hi nt h e
f i e l d and i n t h e o f f i c e . R i c h a r d
Chamberlin a l s o p r o v i d e d i d e a s
and h e l p e d i n . t h e f i e l d .
The author would l i k e t o e x t e n d t h a n k s t o
Chapin.
and
A s t h e s i sa d v i s o r
D r . Charles E .
i t was he who f i r s t suggested
t h e problemandthenaidedgreatlyinthefield
o f f i c ed i s c u s s i o n s .S p e c i a lt h a n l c s . a l s o
L i n d a ,f o rt y p i n gt h em a n u s c r i p t
and i n
go t o my w i f e ,
10
STUTIGRAPHY AND PETROLOGY
Prevolcan'ic 'Rocks
Prevolcanicrocks
s m a l la r e a .
(Ab0 Formation)cropout
To t h en o r t h ,
theSpearsFormationwhich
i n only one
Givens (1957) mapped the b a s e of
rests comformably on t h e Eac'a
Formationbut,tothesouth,thebaseoftheSpears
Forma-
t i o n rests unconformably on t h e Ab0 Formationandsouth
of
Highway 60 i t r e s t s uncomformably on c a r b o n a c e o u s s i l t s t o n e s
of unknown age.
A s i m i l a rr e l a t i o n s h i p
BearMountainswhere
the base
i s present in the
of t h e v o l c a n i c p i l e
r e s t s on
the Kelly mining
progressively older rocks southward into
district.
areas are situatt?cl
The Tres MontosasandBearMountain
on t h e f l a n k s
of a Laramide u p l i f t which w a s t r u n c a t e d by
1975).
the l a t e Eocene erosionsurface(EpisandChapin,
The r e s u l t a n t d e t r i t u s
t ot h en o r t h .
formed t h e Baca Formation i n a b a s i n
The t r a n s i t i o n fromconformable
t o unconforn-
able contact relationships at the base.of the volcanic pile
i s n o t exposed a t t h e s u r f a c e .
SandiaFormation
A s i n g l e e x p o s u r e of q u a r t z i t e , interpreted t o b e p a r t
of the Sandia Formation of Pennsylvanian age.occurs over an
areaaboutthesizeof
p a r t o f t h eT r e s
a football field
i n the s o u t h - c e n t r a l
Montosas s t o c k .T h i sb l o c k
be a x e n o l i t h c a u g h t
is b e l i e v e dt o
up a s t h e s t o c k r o s e t h r o u g h ' t h e
P a l e o z o i cs e d i m e n t a r ys e c t i o n .
Chapin (oral commun.,1 9 7 5 ) has found similar
quartzite
blocks
in
the
upper
portions
of
the
Magdalena
composite pluton. In the deeper portions of this pluton
(the
Nitt
and
Anchor
Canyon
stocks}
xenoliths.of
limestone
and Pre-Cambrian rocks are exposed. He has interpreted these
to
be
the
result
of
selective
flotation
of
stoped
blocks:
quartzites have lower densities than limestones, thus they
would
The
be
selectively
elevation
of
floated
the
while
limestones
would
quartzite
xenolith,in.theTres
sink.
Montosas
2000 feet,
stock above its parent strata is at least
the
same figure Chapin(op. cit.) has calculatedfor the quartzite
xenoliths
The
in
the
quartzite
Magdalena
is
a
composite
medium
pluton.
grained,
moderately
sorted,
pure quartz sandstone. The Sandia Formation
in the
Magdalena
Mountains
consists
of
gray
to
black,
sandy
carbonaceous shales and siltstones with thin beds of gray,
medium-grained, crinoidal limestones and greenish-gray to
1973).
brown, medium to coarse-grained quartzites (Siemers,
Abo
The
of about
only
other
Formation'
pre-Tertiary
three-quarters
of
a
rocks
square
exposed
cover
an
area
mile
on hill" 7 4 8 4 " in
the northeast corner of the map area. These rocks are a
series of interbedded quartzites, quartzite breccias, limestone pebble to boulder conglomerates, and siltstones. They
form
a
high,
rounded
hill
with
exposures
only
where
streams
cut through the cover. Siemers ( 1 9 7 3 ) measured a partial
150 as
feet, but
section and obtained a thickness of as much
12
..
e.
cross sections suggest a minimum thickness of
feet.
450
The section examined in this report (see 2Fig.
) is
consistent
of
that
measured
presence
of more
the
silty
with
beds
near
the
by
limestone
top
of
Siemers
with
conglomerates
the
section
the
and
that
exception
thin
were
observed
during subsequent mapping. Siemers' petrographic descriptions
have
been
used
to
supplement
contact
of
the
samples bytaken
the
author.
The
lower
faulting;
the
upper
contact
Abo
is
Formation
is obscured
a
by
with
contact
depositional
the Spears Formation. The lower Spears through the
tuff o f
Nipple
upper
Mountainis
part
of
in
the
fault
Spears
contact
clastic
with
and
the
Abo
andesitic
but
rocks
in
the
are
depositional contact with the
Abo.. Figure 3 is a photograph
the
at
the
overlying
The
inches
Abo
thick
Abo-Spears
Spears
is
showing
clasts
Ab0 of
in
Forma.tion.
thinly
and
contact
bedded
conglomerate
with
and
shalytwo.to
beds six
breccia
beds
as
as much
'
eight feet thick. Quartzite.andquartzite brecciaswith'
thin
interbedded
silty
layers
predominate
at the
base
of
the exposed section. Limestone conglomerates become abundant
upward, grading back into quartzites and shales. Above this
interval, limestone conglomerates, quartzite breccias, and
quartzites are approximately equal in abundance.
,Theupper
a
part
of
quartzite
the
section
breccias.
consists
predominantly
of
quartzites
and
13
I.
!
i
I
!
Figure 2 . G e n e r a l i z e ds t r a t i g r a p h i c column of rocks in ?lie .
Tres Montosas-CatNountiinarea.T.hicknesses.indicated
are
inaximams estimatdd from s t r u c t u r e s e c t i o n s .
. .
..
"
15
The
quartzites
are
calcareous,
fine-grained
rocks
with
some appearing almost aphanitic. The color varies from
gray in
very-fine-grained
rocks
to
shades
of brown
olive-green in coarser grained rocks.
Much
c o l o r is
theresult,of the oxidation
On weathered
surfaces
these
of
rocks
are
of
and
the
lighter
disseminated
all
shades
pyrite.
of
grays
and browns. Fine laminations and cross laminations
are'
readily visible in some beds. Several
of the beds show
grading of the sand particles over several feet.
Petrographically,
the
quartzites
are
very-fine-grained
(0.2 m
m
) to medium-grained (0.7 mm), moderately to wellsorted, angular to subround, almost pure quartz sandstones
a
with some carbonate cement. Feldspar accounts for less
than
one
percent
in
several
slides'and ther,eis a trace of
mica in the others. Point contacts are dominant over line
and concave-convex contacts. All slidesshow some evidence
of alteration. Disseminated pyrite, oxidized to hematite
or limonite, and epidote in disseminated patches and veinlets are most common. Garnet, wollastonite, secondary
quartz veinlets, and chlorite veinlets are also present
in
some thin sections. In most thin sections, calcite replaces
the quartz grains to varying degrees.
In some sections the
matrix
or
a
is
the
The
very-fine-grained
result
of
quartzite
quartz
hydrothermal
breccias
are
which
may be diagenetic
alteration.
composed
of poorly sorted,
angular to subround, reddish to gray quartzite clasts
in a
e
e
16
carbonate cement. The clasts are usually small, one to
two inches in di.ameter, but may reach nine inches in diameter.
In some of the breccia beds, and in some of the clasts.
themselves, thin laminations are 'visible.
The quartzite
clasts are very similar to the quartzites described above
suggesting
that
previously
deposited
Ab0
is
being
eroded
and redeposited as quartzite breccias. C.T. Smith (oral
commun., 1975) has seen similar features
in the Ab0
tion
in
The
theLos Pinos
quartzite
Forma-
Mountains.
breccias
consist
of
fragments
of quartzite
in a carbonate cement. In some, the cement is coarse,
crystalline
sparwith scattered
sand
grains
and
in others
the carbonate is very fine grained.
The silty beds are as much as six Caches thick, partly
calcareous, fine siltstones. They exhibita poorly
developed
pits
shaly
where
they
parting
have
and
not
visible in
only
prospect
are
been
subjected
to
weathering.
Their color.is the same as that of the quartzites.
The
limestone
pebble
to
boulder
conglomerates
form
beds from one to eight feet thick. They contain poorly
sorted, round to subround, light-gray to dark-bluish-gray
Iimesrone clasts in
a coarsely crystalline spar cement.
The
size
varies
from
less
than
one
inch
to
as
much
as
feet in diameter. The clasts are uniformly nonfossiliferous
with only a few showing some fossil fragments.
The only
sedimentary
structure
observed
is
the
imbrication
of some
of the larger clasts. The average strike and dip of
four
e
0
17
e
imbricated clasts at one locality Nwas
8 5 O W, 27' E (flow
direction from east to west). (See Fig.4).
Several samples of the clasts sent for fossil
identification
were
identified
as
wackestonesto packstones,
peloid
molluscan-brachiopod-
brachiopod-ostracod
mudstones, and micropelletoid, possibly algal wackestones
to
packstones (Armstrong, written commun., 1971). PossiLs are
. generally
fragmented
and
poorly
preserved to
due recrystal-'
lization.
One
noteworthy
breccias
there
limestone
e
are
observation
very
conglomerates
few
there
is
that
in the
quartzite
limestone
clasts
are
few
very
and
the in
quartzite.clasts.
Several hand specimens and thin sections showed
evidence
the
for
most
varying
intense
degrees
of contact
alteration
metasomatism
developed
in
the
with
carbonate
rocks.
Almost all of the thin sections showed scattered,
small
patches
the
of
most
calcite
definite
and
epidote
contact
replacing
metasomatic
quartz
grains
but
assemblage
was
in a thin section of a limestone conglomerate. Epidote,
as
distinct crystals and crystal aggregates; garnet
as individual
euhedral
crystals
in
crystals;
radial
Figs. 5-A and 5 - B ) .
and
aggregates
wollastonite
'as
elongate
pervade .thin
the section .(see
Chamberlin (1374) and the author have
observed garnets in some thin sectionsthe
of Spears
Forma-
tion around the eastern edge
of the Tres Montosas stock.
These
contact
metasomatic
effects
are
undoubtedly
the result
observed
.
.
I
Figure 4. Outcrop 01 a umestone pebble conglomerate unit
in the Ab0 Formation. Note the imbrication of sone of the
larger limestone clasts. A thin quartzite bedis visible
just above the hammer. The outcrop is located in a large
arroyo in the eastern part
of the Ab0 outcrops.
, 0 . 5 m.
,
Figure 5-A.
Photomicrograph of contactmetasomatic minerals i n
a limestone'pebbleconglomeratefromthe
Abo F.ormation. Abundant
e u h e d r a l g a r n e t . (G) and small e p i d o t e (Ep) . c r y s t a l s p e m a d e t h e
(Q = q u a r t z ,X - n i c o l s ,
x60 m a g n i f i c a t i o n . )
rock.
;0.5
mm.
I '
Figure 5-Bo Photomicrograph of contactnletasomaticmineralsin
a limestone pebble conglomerate from the
Abo Formationshowing a
r a d i a l group of w o l l a s t o n i t e Cdo) c r y s t a l s i n q u a r t z
(Q), e p i d o t e
Bothsamples a r e f r o m the northern margin
(Ep) and g a r n e t (G)
of the Abo outcrops. (X-nicols x60 m a g n i f i c a t i o n . )
,
,
metasomatism
The
may
increase
existenceof this
with
depth.
sedimentary
section
has
been
known for many years but its exact stratigraphic position
is
still
not
definitely
known
due
to
poor
exposures
no
and
visible upper or lower pre-Tertiary contacts.
It is
necessary
to
know
the
stratigraphic
position
to
predict
drilling depths to favorable limestone units. These
sedimentary
Montosas
rocks
stock
are
which
close
creates
to
and
the
affected
by
the
Tres
possibility
of replacement-
type deposits in favorable limestones at depth.
Kottlowski (1960) mentioned these sedimentary rocks and
stated, "No fossils were'found, but the beds are similar to
parts
of
the
Pennsylvanian
in the
Magdalena
Mountains."
Slawson and Austin (1962) mentioned the "brecciated limestones"
but
made
no attempt
to
work
out
the
stratigraphy.
Siemers (1973) measured a section, conducted petrographic
studies, and compared these sedimentary rocks with the
Pennsylvanian and Permian Formations.He concluded that
they were
too
correlative
with
the
dissimilar
but
Ab0
that
a
Formation,
to
the
Sandia
reasonable
t o be
quartzites
correlation
could
be
made
o n lithologies and sedimentary
based
structures. Loughlin and Koschmann ( 1 9 4 2 ) , Kottlowski and
others (1956), and Jaworski (1973) all noted limestone
pebble
conglomerates
Limestone
clasts
in
the
basal
containing
parts
of the
fossil
Ab0
material
Formation.
were
sent
to the U.S. Geological Survey for identification. BernardL.
. Mamet
(written commun., 1975) recognized the following three
21
e
generaofforaminifera:BISERIELLAsp.,EOLASIODISCUS
sp. , and ORTHOVERTELLA sp. He placedan age of Middle
Pennsylvanian or slightly younger (Madera Limestone) on
at least some of the limestone clasts; therefore,
the
conglomerates
and
quartzites
must
be
younger
than
Middle
Pennsylvanian.
The identification
several
independent
of
lines
this
of
section
was based upon
evidence,
none
of
which
are
diagnostic alone. When taken together, however, they
strongly
suggest
that
these
rocks
are of
part
the
Ab0
Formation, probablyin the basal part.
Sedimentary
e
Thelast
Rocks
of
Unknown
Age
289 feetoftheBannerdrillholepenetrated
a sequence of carbonaceous, calcareous siltstones and
sandstones. Because no pre-Tertiary contactswere encountered,
the age of these rocks
is unknown.
The Banner Mining Company
interpreted these rocks as Mesozoic sedim.entary rocks and
stopped drilling. The data presented here suggests that
these
rocks
In hand
may
be
specimen,
part
of the
these
Sandia
rocks
Formation.
are
black
to dark
gray
to
medium-gray brown, calcareous siltstones
to fine-grained sandstones. They are often laminated and contain plant fragments
along carbonaceous partings. The only other fossils observed
were sparse foraminifera.
e
In thin section, these rocks are very fine-grained,
calcareous sandstones to siltstones. They are composed of
22
e
e
grains
p o o r l y -t ow e l l - s o r t e d ,a n g u l a rt os u b r o u n dq u a r t z
which v a r y from 0 . 0 1 t o 0 . 4 mm i nd i a m e t e r .
the p r i n c i p a l cementsand
calciumcarbonateare
grains are slightly
S i l i c a and
embayed by t h e c a r b o n a t e
the quartz
cement.
1973) examined the t h i n s e c t i o n s
Siemers ( w r i t t e nc o m u n . ,
was a l o w ,
andsuggestedthatthedepositionalenvironment
t o moderateenergy
marine environment..Alteration,
form of disseminated pyrite
.
i n the
and q u a r t z - c a l c i t e and p y r i t e
v e i n l e t s ,o c c u r st h r o u g h o u tt h es e c t i o n .
This s e c t i o n of rocks i s s i m i l a r t o t h e m i d d l e S a n d i a
Formation of Pennsylvanian age
i n t h e MagdalenaMauntains
.
.
where i t c o n s i s t s of c a r b o n a c e o u ss h a l e s ,s i l t s t o n e s ,a n d
frorn t h e
f i n e , .q u a r t z i t i cs a n d s t o n e s ,a l t h o u g ht h er o c k s
d r i l l h o l ea r ef i n e r - g r a i n e d .S i e m e r s( o r a l
commam.,
.
1975)
hasinterpretedthePennsylvanianrockseastoftheRio
Grande a s a d e l t a i c s e q u e n c e g r a d i n g i n t o
ment t ot h es o u t h w e s t .T h e r e
a marine environ-
would be a progressivedeepening
of t h e m a r i n e b a s i n t o w a r d t h e T r e s
Montosas a r e a a n d . f a c i e s
changes would c a u s e t h e c l a s t i c s e d i m e n t a r y r o c k s t o b e
finer grained.
'
,
Mesozoic c l a s t i cr o c k sa r eg e n e r a l l yf e l d s p a t h i c ,
sandstones(Kottlowski,oral
e
d r i l l hole.
a r ec l e a n ,q u a r t zs a n d s
and s i l t s i n which f e l d s p a r i s
The r o c k si nt h ec o r e
a l m o s te n t i r e l ya b s e n t .T h i ss u g g e s t st h a tt h e s er o c k s
the Mesozoic s e c t i o n b u t b e l o n g
thePaleozoicsection.
;
commun., 1975) which c o n t r a s t s
w i t ht h es e c t i o ni nt h e
not belong in
cherty
do
somewhere i n
23
e
As discussed on page 92, the thesis area is
o n the
northern
flank
of
a
Laramide
uplift
which
has-
been
beveled
by the Eocene erosion surface. This erosion surface has
caused
rocks
of
progressively
older to
ages
be exposed
southward. Where Mesozoic rocksare-preservedin the
Gallinas
Mountains,
the
Baca
Formation
of Eocene age always
overlies them. To the south, the Baca Formation apparently'
disappears and the Ab0 Formation (Permian) is exposed two
miles north of the Banner drill hole. This suggested progression indicates that the siltstones and sandstones
in the
drill
hole
could
be
the
Sandia
Formation.
Several samples of the core containing fossils'were
e
sent to theU.S. Geological Survey.and
the
University
of
Pennsylvania for identification. Armstrong (written commun.,
1971) identified two foraminifera (TURBERITINA sp. and
GLOBIOVALVULINA sp.) and stated,
" *
the total fauna sugge'sts
I !
to me an Atoka (Pennsylvanian-Permian) or slightly younger
age.( 1Kosanke (written commun.,
1974) stated, "Because
of this (poor preservation) it has not been possible to make
any identifications with certainty. Noneof the spores or
pollen grains resemble Paleozoic taxa.
Bob Tschudy examined
several slides and it is our guess that of
a the
few
.specimens
suggest
a
similarity
to
Mesozoic
taxa."
Pfefferkorn (written commun.,1974) stated, "Seeds and the
e
leaf fragment indicate only that the material is Upper
so badthatthe
Devonianoryounger.Thepreservationis
material
from
the
could
come
Tertiary."
from
any
of
the
Mesozoic
systems
or even,
24
a
There i s no d i r e c t e v i d e n c e
r o c k sw i t h
at t h i s t i m e t o p l a c e these
any c e r t a i n t y i n t h e s t r a t i g r a p h i c , s e c t i o n .
l i t h o l o g i e s and s t r u c t u r a l s e t t i n g s u g g e s t
The
a Paleozoic age,
probably the SandiaFormation.Althoughthepaleontological
i t does n o t eliminate the p o s s i b i l i t y
d a t a i s notdiagnostic,
thattheserocks
may betheSandiaFormation.
T e r t i a r y V o l c a n i c Rocks
Volcanic rocks comprise the great majority of the
exposures i n t h e map a r e a .
flows predominate
Welded a s h - f l o wt u f f sa n dl a v a
with i n t e r b e d d e d v o l c a n i c l a s t i c s e d i m e n t a r y .
2 , a maximmi t o t a lt h i c k n e s s
rockssubordinate.From-Figure
. of 6300 f e e t canbe
e
Formation
e s t i m a t e df o rt h eO l i g o c e n er o c k s .
Spears
The e a r l i e s t v o l c a n i c r o c k s
exposed i n the TresMontosas
a r e a are a t h i c k s e q u e n c e o f v o l c a n i c l a s t i c s e d i m e n t a r y r o c k s
lava f l o w s c a l l e d t h e '
and interbeddedash-flowtuffsand
SpearsFormation.
The Spears t y p e s e c t i o n was described
by Tonking(1957)and
Formation.
named t h e S p e a r s Member of t h e Datil
Weber (1971) r a i s e d the D a t i l Formation t o Group
s t a t u s andChapin(1971-a)
status.
r a i s e dt h eS p e a r st oF o r m a t i o n a l
A latitetuffbrecciaintheJoyita
H i l l s (45 miles
n o r t h e a s t of Tres Montosas), which has been correlated with
the Spears Formation, yielded
a K-Ar a g e d a t e of 3 7 , l m.y.
(Weber, 1963,1971).
e
The SpearsFormationcomprisesthelargestoutcroparea
and e x h i b i t s t h e p o o r e s t o u t c r o p c h a r a c t e r i s t i c s o f
t h er o c ku n i t s
mapped.
It occursas
anyof
l o w , rounded h i l l s and
.
.
e
e
e
25
slopes covered with distinctive float.Most of the central
portion
of
the
well-exposed
map
only
area
is
rarely
in
underlain
roadcuts
by
and
Spears
which
arroyo
is
bottoms.
Brown (1972) and Chamberlin (1974) subdivided the
Spears Formation
into
two
members:
a
lower
'
member
composed
of latitic, epiclastic, volcanic sandstones and conglomerates
and
'
minor
autobrecciated
latite
flows
and
an member.
upper
composed of latitic ash-flow tuffs, andesite-latite flows,
and minor volcanic breccias and sedimentary rocks. The break
between
of
a
lower
"turkey
Nipple,
e
the
and
track"
Mountain,
an
upper
members
andesite
occurs
at the appearance
which
excellent
underlies
the tuff of
stratigraphic
marker
within
the Spears. This conven?:ion will be followed in this report.
The
Spears
Forma'tion
in
into three mappable units:Ts
the
-
map
area
has
been
subdivided
undivided Spears which
includes. the lower and the upper volcaniclastic rocks and
andesite
flows;
- the tuff of Nipple Mountain; and.Tsg Tsn
the tuff of Granite Mountain, a distinctive latitic ash-flow
tuff
at
the
of the
top
Spears
Formation.
Lower Spears. The lower Spears Formationis very poorly
exposed and much mapping was done from float.
An excellent
st.ratigraphic section of the lower Spears
has been
obtained
from
Mining
core
from
in Sec.. 6 , T. 3
I*
tuff
of
of
the
S.,
Nipple
lower
a
hole
R.
5
drilled
W.
Mountain
Spears
by
the
Banner
Company
The hole collared in the
and
drilled
Formation
to
through
its
almost
contact
with
1100
pre-
feet.
.
e
26
0 .
Thelower
the
Spears
rocks
ofconsist
predominantly
much as two f e e t
of l a t i t i c c o n g l o m e r a t e s w i t h c l a s t s a s
i nd i a m e t e r ,t h i nv o l c a n i c l a s t i cs a n d s t o n e s ,a n d . m i n o r
When f r e s h these r'ocks are
a u t o b r e c c i a t e dl a t i t ef l o w s .
typically maroon,' purple, or red-brown, but pervasive
.them's greenish t,o gray
propylitic alteration has given
color.
A f e w samples &om t h e d r i l l c o r e
fresh, but
most of t h e c o r e
andmost
outcrops are a l t e r e d .
from green to gray to red-brown.
The c l a s t s v a r y i n c o l o r
They, too,havebeenaffected
by t h e a l t e r a t i o n a n d , i n
where the c o n t a c t
many c a s e s , i t i s d i f f i c u l t t o t e l l
between clast and m a t r i x . i s .
e
are . r e l a t i v e l y .
The g r a i n s i z e
v a r i e s from s i l t t o b r o k e n f e l d s p a r c r y s t a l s a 5
6 m i nl e n g t h .P l a g i o c l a s e
m i n e r a l s i n b o t hm a t r i x
much a s
and s a n i d i n ea r et h ep r e d o m i n a n t
and c l a s t s with minorhornblende
and b i o t i t e .C l a s t sa r et y p i c a l l yw e l l - r o u n d e d
indiameterfrom
of t h e matrix
6 nun t o 6 1 cm.
.
and v a r y
The c l a s t s a r e h e t e r o l i t h i c ,
comprisedofvariousandesiticand
latiticrocksand,near
, t h eb a s e ,c l a s t so ft h eu n d e r l y i n gs e d i m e n t s
The p r o p y l i t i c a l t e r a t i o n h a s
(see F i g . 3 ) .
made i d e n t i f i c a t i o n and
In t h e matrix, p l a g i o c l a s e
p e r c e n t a g ee s t i m a t e sd i f f i c u l t .
(0.3. t o 2 . 5 mm) and s a n i d i n e ( 0 . 1 t o 0.9 mm i n diameter)
are the predominant minerals
mostabundant
w i t h plagioclase usually the
(10 t o 30 p e r c e n t ) .O c c a s i o n a l l ys a n i d i n e
i t u s u a l l y varies from
more abundantthanplagioclase,but
0
5 t o 25 p e r c e not h
tfr eo c k
is
volume.
..
The f e l d s p a r s are
'
e
e
27
weremeasuredon
sixty-
a l t e r eivd
na r y i ndge g r e etcosa l c i t ce l, a a
yn
,mdi n o r
c h l o r i t e .P l a g i o c l a s ec o m p o s i t i o n s
s i x g r a i n su s i n gt h e
ranged from
Fouque method; thecompositions
,
An6 t o An39 w i t h an a r i t h m e t i c mean of An22
a similar range. f r o m Anlo to'An30
Tonking(1957)found
Hornblendeand
biotite are present in
amounts up t o
The hornblende i s always a l t e r e d t o
seven percent.
m a g n e t i t e ,c a l c i t e ,
and e p i d o t ew i t ht h em a g n e t i t eo u t l i n i n g
o r i g i n a lg r a i nb o u n d a r i e s
and f r a c t u r e s .
A p a t i t e and
quartzareveryminorconstituents..
The c l a s t s , many of which show a t r a c h y t i c a l i g n m e n t
of feldspar
a
and hornblende in
a c r y p t o c r y s t a l l i n e groundmass,
a r e a l s o c h a r a c t e r i z e d by p l a g i o c l a s e ( o l i g o c l a s e ) a n d
sanidirie as t h e dominant p h e n o c r y s t s .A l t e r a t i o nh a s
the i d e n t i f i c a t i o n of minerals
p e r c e n t a g e sd i f f i c u l t .
and t h e e s t i m a t i o n o f r e l a t i v e
b i o t i t e are minor
Hornblendeand
were observed:
c o n s t i t u e n t s .T h r e et y p e so fc l a s t s
1.
a n d e s i t i ct ol a t i t i cr o c k s ;
2.
t u f f a c e o u rso c k s
c r y s t a l - p o o r with d e v i t r i f i e d groundmass;and
sedimentaryrocks
- limestone and s i l t s t o n e .
Paleozoic sedimentary rocks occur only near
the section
made
3.
-
Paleozoic
The c l a s t s of
the bottom of
. .
and are derived from the underlying formations.'
TuffofNippleMountain.
i s aninformal
The tuffofNippleMountain
name used by Brown (1972)andChapin
(1974-b)
f o r a pink,moderatelytodenselywelded,crystal-poor,
ash-flow tuff
which s e p a r a t e s t h e u p p e r
andlower
members of
e
a
e
the
Spears
Formation.
It i s a
en
x c e l l em
n ta r kuenr i t
Tres Montosas,,
within the Spears, not only in the vicinity of
b u tw i t h i nt h ee n t i r e
it: has
Magdalena area.Accordingly,
been mapped a s a s e p a r a t e u n i t .
t u f f ofNippleMountain
a n d e s i t ef l o w s
28
Chamberlinredefinedthe
to include the "turkey track"
commonly foundbelow
it.
H i s r e a s o n sf o r
t h e change were " t h e o c c u r r e n c e o f a n d e s i t i c ( a n d l a t i t i c )
l a v a s which a r e i n t e r b e d d e d
i n the ash f l o w t u f f s
that
form t h e main body of the u n i t and t h e common s t r a t i g r a p h i c
and s p a t i a l a s s o c i a t ' i o n
tuffs"(Chamberlin,
w i t h these
of the turkey track lavas
1974, p. 16).
S i m i l a r observations,have
been made i n t h e Tres Montosas a r e a and Chamberlin's defi-
a
by the author.
nition has been adopted
.
.
t u f f of Nipple
The o n l y a v a i l a b l e s e c t i o n ' o f t h e
Mountain i n t h e T r e s
Montosas a r e a i s f r o m t h e
Banner d r i l l
h o l e ment.ioned e a r l i e r .
The h o l e c o l l a r e d i n t h e t u f f o f
N i p p l e Mountainandgave
a minimum thickness of
71 feet plus
21 feet for a basal "turkey track" andesite. (total
t h i c k n e s s 92 f e e t ) .
a
However, c r o s ss e c t i o n si n d i c a t e
'minimum t h i c k n e s s of 500 f e e t t h r o u g h
The t u f f o f N i p p l e
minimum
several s e c t i o n s .
Mountain i s a m u l t i p l e f l o w ,
compound
c o o l i n g u n i t with t h e c o o l i n g b r e a k s o c c u p i e d ' b y t h i n
a n d e s i t i ct o
l a t i t i c lavaflows.There
i s an apparent
gradationfromfine-grained,extremelycrystal-poorash-flow
tuffsnearthebottomtodenselywelded,crystal-poor,ash-
@
flow
tuffs
a t t hteospi m i l attroh o sdee s c r i b e d . b y
Brown
~
i
29
(1972).
drill
The “turkey track” andesite is observed only
in the
hole
and
in
a
small
exposure
in the
eastern
part
of
Section 15, T. 3 S, R. 6 W.
The tuff of Nipple Mountain has an outcrop of
area
’
approximately
one
exposures
limited
are
square
mile;
to
a
the
‘great
north-trending
majority
belt
.
of
along
.
the
the
eastern edge of the Gallinas uplift. The
tuff is everywhere
altered
and
is
silicifiedand
inaround
intensely
the
Cobb prospect. Where silicification is less intense, the
tuff
of
Nipple
Mountain
crops
out
poorly
and
is
often
visible only as float-covered slopes. But where silicification is intense, the outcrops
.areprominent and in
e
some
cases cliff-formers. The light color
of the bleached and
highly
stlicified
aerial
photographs.
In
is
a
fresh
outcrops
hand
characteristically
specimens in the
leads
to
specimen,
pink,
Tres
identification
on
the of
tuff
Nipple
but
Montosas
easy
Mountain
because
of alteration,
area
a wide variety of
show
light colors. It is a fine- to medium-grained; unwelded
to
The
densely
welded
dominant
with
3 to 10 percent
rock
phenocrysts
are
sanidine
phenocrysts..
and
plagioclase.
Dark, latitic lithic fragments are present
in amounts
ranging from 0 to 5 percent by volume.
in
e
the
medium-grained
In thin
porphyritic
sections,’
with
a
ash-flow
the
totally
tuff
They are most common .
tuffs.
of
Nipple
devitrified
Mountain
and/or
altered
is
30
(silicified).groundmass. Glass shards are well preserved
in
some
sections
and'
generally
show
some
degree
of
compaction.
However, in a few sections they are undistorted. The groundmass in'most thin sections is devitrified and intensely
.
silicified
with
veinlets
Outlines of glass
axiolites
and
shards
and
are
spherulites
patches
of'silica common.
preserved
in some
sections
with
developed
in some.
Sanidine is the most abundant phenocryst
(2 to 10 percent)
in
some
thin
scetions
while
plagioclase
(not
mentioned
by
Brown, 1972) is most abundant in others
(2 to 8 percent).
The
high
degree
distinguish
e
of
between
alteration
sanidine
makes
i-tdifficult to
and
plagioclase
in many
thin
sections. The sanidine occurs in euhedral
to subhedral.:
crystals' 0.2 to 2.0 irun in
diameter
which
are
normally
show microaltered to clay and calcite. Some grains
perthitic unmixing. Plagioclase is typically altered to
clay
to
mineralsalso, but
obtain
111123 to
The
fairly
enough
consistent
fresh
grains
are
composition
available
measurements
of
An35 (Fouque method,
15 grains, average of
An27).
plagioclase
occurs
as
euhedral
to
subhedral
crystals
0.2 to 2.0 nun in length. Euhedral quartz grainsare very
minor constituents.
The "turkey track" andesite
is a porphyritic, vesicular
lava flow with abundant, large plagioclase crystals.
It is
e
gray
to
color.
black
when
fresh
and
weathers
to a brownish-gray
A small prospect pit was dug in the "turkey track"
31
1 5 , T . 3 S , R. 6 W, a p p a r e n t l ya f t e r
a n d e s i t ei nS e c t i o n
was i d e n t i f i e d as c e l a d o n i t e and
a green mineral which
f i l l s v e s i c l e s and h o l e s l e f t by w e a t h e r i n g f e l d s p a r .
The t h i c k n e s s of t h e " t u r k e y t r a c k " a n d e s i t e i n t h e
Banner
d r i l l h o l e i s 2 1 f e e t .I ni h i ns e c t i o n ,t h e" t u r k e yt r a c k "
20 percenteuhedral
andesitecontains15to
10.7 mm i n l e n g t h w i t h a
plagioclasephenocrysts0.75to
composition of
t o subhedral
An
2 7 . 5 (Rittmannzonemethod,
.3 grains).
Minor anhedralpyroxene, 0.1 t o 0.75 nun i n diameter i s the
The groundmass i s composed of
o n l yo t h e r . p h e n o c r y s t .
t r a c h y t i c a l l y aligned p l a g i o c l a s e m i c r o l i t e s , m a g n e t i t e ,
pyroxene, andminor
Upper Spears.
apatite.
the SpearsFormation
The u p p e rp a r to f
. .
i s composed p r e d o m i n a n t l y - o f a n d e s i t i c and l a t i t i c flows
with minor, poorly bedded volcaniclastic sedimentary rocks.
Exposures are poor
,and b e c a u s e t h e u p p e r v o l c a n i c l a s t i c
rocks are difficult to distinguish
clasticrocks,theupper
from t h e l o w e r v o l c a n i -
andlowerSpearsFormationhas.
been, mapped as a s i n g l e map & i t ,
Ts.
The number o fi n d i v i d u a la n d e s i t eo r
t h e i rt h i c l i n e s s e sa r en o t
i s much g r e a t e r t h a n t h e
(1974)immediately
l a t i t e flowsand
known; however, t h e i r t h i c k n e s s
200 f e e t r e p o r t e d byChamberlin
to the north
i n t h e C o u n c i l Rock area.
A minimum t h i c k n e s s of 600 f e e t h a s b e e n e s t i m a t e d i n t h e
Cat Mountainarea.
to almost black rocks
The a n d e s i t e sa r eg e n e r a l l yd e n s e ,g r a y
which u s u a l l y f o r m low, rounded h i l l s .
,
.
32
.a
They atryep i c a lpl o
y r p h y r i twi cipt lha g i o c l a s e 'tahse
dominant phenocryst.
However, s e v e r a l flows have l a r g e
much a s 6 . 0 mm in diameter.
phenocrysts of pyroxene as
.
I n some o ft h e s ef l o w s ,t h ep y r o x e n e' g r a i n sa r ep a r t l y
In p l a c e s ,t h ed e n s e
a l t e r e dt o. r e d d i s hi r o no x i d e s .
and fractures
flowsexhibithematitebandingalongjoints
l i t h i c fragment i s observed
andanoccasionaldark-brown
i n some.
The weatheredsurfaces
of brown.Near
are u s u a l l y some shade
Montosas s t o c k , t h e a n d e s i t e s a r e
t h eT r e s
extremely dense due to baking
by t h e s t o c k
and e x h i b i t a
waxy l u s t e r on f r e s h s u r f a c e s .
Petrographically,theandesites
a
w i t h a felty to trachytic alignment of
Many of t h e t h i n s e c t i o n s
are porphyriticrocks
groundmass m i c r o l i t e s .
show g l o m e r o p o r p h y r i t i c c l o t s
of
p l a g i o c l a ' s ea n d / o rp y r o x e n e .E u h e d r a lp l a g i o c l a s eg r a i n s ,
0 . 1 t o 4 . 0 . m in length, are the
dominant phenocryst
accounting for 1-0 t o 30 volume p e r c e n t of t h e r o c k .
A l t e r a t i o n i n some s l i d e s , e s p e c i a l l y t h o s e
from near the
Tres Montosas s t o c k , i s i n t e n s e and t h e p l a g i o c l a s e may b e
completelyalteredtocalcite,epidote
Pyroxene, i n e u h e d r a l g r a i n s
0 t o 15 p e r c e n to ft h er o c k .
and c h l o r i t e .
up t o 6 . 6 mm, a c c o u n t sf o r
The pyroxene i s wellzoned
and i s p r o b a b l yp i g e o n i t e
(2V approx. 30" t o 4 5 " ) .
Hornblende and sanidine are minor constituents varying
e-
0 t o 4 p e r c e n t byvolume.
areextremelylarge,as
Some o ft h eh o r n b l e n d eg r a i n s
much as 6 . 6 mm i n l e n g t h . . They
from
33
are generally altered to
~.
i d e n t i f i c a t i o nb e i n g
opaque i r o n . o x i d e s w i t h
made from g r a i no u t l i n e s .
The
and small
groundmass i s composed o f p l a g i o c l a s e m i c r o l i t e s
I n i n t e n s e l ya l t e r e d
pyroxenegrains.
t h i n s e c t i o n s ,t h e
groun'dmass is completely obscured.
Measurements of p l a g i o c l a s e c o m p o s i t i o n s i n d i c a t e t h a t
there are
fTows.
some v a r i a t i o n s w i t h i n t h e a n d e s i t e
Specifically,they
may begroupedcompositionallyinto
t h r e ec a t e g o r i e s :
An30-32, t h r e ef l o w s ;
flows;and
An55, one flow.
An41-48, f i v e
A s i m i l a rm i n e r a l o g i cv a r i a t i o n
.
i s manifested.
.
Those flows with compositionsranging'from
An41-55 are pyroxene-bearing rocks
sodic plagioclase are hornblende-
while f l o w s w i t h more
and b i o t i t e - . b e a r i n g r o c k s .
is
How t h e s e v a r i a t i o n s a r e r e l a t e d s t r a t i g r a p h i c a l l y
a complete
unknown because of t h e l a c k o f e x p o s u r e s o f
s e c t i o n of the upper Spears Formation.
Thereappeartobe
flows.
two d i f f e r e n t t y p e s of l a t i t i c
The f i r s t has a d a r kp u r p l i s h . g r a y
matrix w i t h l a r g e
(up t o 1 2 . 0 mm) c h a l k yf e l d s p a rp h e n o c r y s t s .S e v e r a lh a n d
specimensexhibit
f e l d s p a rc r y s t a l s .P y r o x e n e ,b i o t i t e ,
the other visible minerals.
toshadesofgreen
andbrown.
',
andmagnetite
f i n e rg r a i n e da p p e a r a n c e .
whichweather
are
These rocks usually
weather
The secondtype
i s lighter
incolor,graytolightpurplishgray,with
rich rocks
of the
a faintalignmentofthelongaxes
a more crowded,
They a r ef e l d s p a r t o a light-gray color.
and b i o t i t e -
34
TEe latites are porphyritic rocks. some of which
exhibit
a
poorly
developed
alignment
of
biotite
and
feldspar
phenocrysts. The principle phenocrysts are euhedralto
subhedral
grains
of
ranging 0.1
fromto 4.0 mm
plagioclase
in length. Compositions range from An22 to An with an
31
average o'f An (Fouque method,,.5 grains). Sanidine accounts
27
for as much as 15 percent
by volume of the rock
iiz grains
as much as 1.9
nun in diameter. In rocks.that are altered,
'it is especially difficult to distinguish sanidine from
plagioclase. Biotite, in.grains 0.1 to 1.5 mm long, accounts
for 3 to 5 percent
by.volume, and is usually altered to
reddish iron oxide and opaques. Apatite(0.1 nun) is a
minor
accessory
The'upper
mineral.
.
Spears
volcaniclastic
rocks
are
.
similar
to
those in the lower part of the section, but thin sections
of
rocks
from
near
outcrops
of
the
Ab0
Formation
contain
lithic fragments of quartzite. Similar clasts are visible
3)
in outcrop (see Fig.
Rocks
to
the
of
the
stock
.
upper
have
been
Spears
Formation
affected
in
in
close
varying
proximity
degrees
by
thermal
metamorphism, The dense andesites show few effects beyond
.
some secondary quartz in the groundmass but several thin
sections
of
the
clastic
rocks
show
pervasive
metasomatism
by minerals indicative of contact metamorphism: calcite,
a
'epidote, quartz, and euhedral garnet grains.
No wollastonite,
as observed
in
the
Ab0
Formation,
was
observed
here.
Chamberlin (1974) observed similar mineral assemblages
in the
L
Ia
35
metamorphosed " t u r k et rya c ka "n d e s ibt ee l otw
ht ue f f
NippleMountainimmediztely
of the
of
to the north of the outcrops
Ab0 Formation.
Tuffof%r'anCte
Mounfa'in.
Mountain i s an informal
The t u f f ofGranite
name proposed b y Chapin(1974-b)
t u f f s near the
f o rt h ec r y s t a l - r i c h ,q u a r t z - p o o ra s h - f l o w
i n t h ed e g r e e
topoftheSpearsFormation.Variat'ions
.welding and the
of
'amounts of pumice i n d i c a t e that these
t u f f s form a multiple-flow,
compound c o o l i n gu n i t .
H e l l s Mesa Tuff i s s i m i l a r , b u t t h e r e a r e
The
some r e c o g n i z a b l e
differences:
1. The t u f f ofGraniteMountaincontains
1 percent
o r less q u a r t z a s opposed t o 5 - 15 p e r c e n t q u a r t z
i n t h e Hells Mesa Tuff.
2 . They b o t hh a v es i m i l a ro u t c r o pc h a r a c t e r i s t i c s ,
b u t where the t u f f of G r a n i t e Mountain i s , o v e r l a i n
by t h e Hells Mesa T u f f , t h e r e i s a s l i g h t b r e a k i n
slope at the contact
w i t h the g e n t l e r s l o p e o n t h e
t u f f of Granite Mountain.
The t u f f o f G r a n i t e
Mountainhas
approximately one square mile
map a r e a .
butalongfractures
narrowzonesof
observed.
w i t h the principal outcrops
Gray H i l l , and i n t h e s o u t h - c e n t r a l
on Tresliontosas,
p o r t i o no ft h e
an o u t c r o p a r e a ' o f
Outcrops a r e r e l a t i v e l y f r e s h ,
and n e a r t h e u p p e r a n d l o w e r c o n t a c t s
more i n t e n s e p r o p y l i t i c a l t e r a t i o n
To t h en o r t hi nt h eC o u n c i l
(1974) observed a maximum thicknessof
on Tres ilontosas and
are
Rock a r e a , Chamberlin
300 f e e t .
However,
Gray H i l l t h e r e i s a maximum of 600
f e e t of t h e t u f f o f G r a n i t e M o u n t a i n , a n a b r u p t t h i c k e n i n g
to the south.
36
4
The
lower
'contact
tuff
of
Granite
Tres
Montosas
is exposedonGrayHillwherethe
Mountain
where
it
rests
rests
on
on
Spears
a
andesites
on and
thin,
poorly ash-welded
flow tuff. The upper contact is not as easily established.
In the area northeast of Magdalena, Brown
(1972) described
a hematite-stained conglomerate,
0 to 40 feet thick at the
top of the Spears Formation. Chapin (oraX,couunun., 1975)
has
mapped
this
conglomerate
in
the
Granite
Mountain
area
where it thickens to200 feet. However, in the Tres Montosas
area,
'
the
Mesa
Tuff
rests
directly
of
the
on
tuff
Granite Mountain and the contact is placed at the first
appearance
e
Hells
of
observable in
abundant
hand
quartz,
specimen
and
a
feature
occurs
which
over
a
is
readily
very
narrow
range. This change 'in quartz content coincides with a
slight
On
breakin
slope
which
is
GrayHill, the tuff of Granite
visible
on aerial
photographs.
forms
a prominent
Mountain
ridge. On Tres Montosas, however, talus from the overlying
Hells
Mesa
obscures
much
of
the of.Granite
tuff
Mountain.
Where it does outcrop, the lower part forms blocky,.massive
outcrops similar to much
of the Hells Mesa. A s the upper
contact is.approached,the
tuffof Granite
Mountain
becomes
lighter in color and the outcrops become pxaty.
In fresh
*
specimens,
from 1ight-pinkish-purple
tbe
to
tuff
of Granite
Mountain
dark-purple
with the
varies
weathered
surfaces showing darker, browner colors. When altered, it
is a light-pcnkish-gray. The tuff of Granite.Mountain is
a
medium-grained,
moderately
to
densely
welded,
'
crystal-rich
l a t i t i c a s h - f l o wt u f f .
are
The predominantphenocrysts
p l a g i o c l a s e ,s a n i d i n e ,
andcoppery
biotite.
Minor
clinopyroxene and q u a r t za r eo c c a s i o n a l l yp r e s e n t .L i t h i c
f r a g m e n t sa r ep r e s e n tt h r o u g h o u tt h eu n i t .D a r k ,a n d e s i t i c
fragments are most common, b u t l e s s e r
amounts o ft u f f a c e o u s
'
fragmentsandlight-colored,equigranularigneousrock
Pumice i s p r e s e n ti n
f r a g m e n t sa r ea l s op r e s e n t .
amounts
0 t o 5 volume p e r c e n t .
varying from
i s p o r p h y r i t i c , seriate with
In thin section the rock
40 t o 45' perc.ent by volume.
phenocrysts varying from
P l a g i o c l a s e ,v a r y i n g
abundantphenocryst.
from 10 t o 20 p e r c e n t , i s the most
It o c c u r si ne u h e d r a l
,
t o anhedral,
i r r e g u l a r l y embayed g r a i n s 0.2 t o 2 . 4 mm i n l e n g t h .
Alteration to calcite
f r o mi n c i p i e n t
with minor e p i d o t e and c l a y s r a n g e s
t o complete.Plagioclasecompositionsrange
'
from An31 t o An36 with an average from six measurements
i s t h e most common
An34 (Fouque method).Albitetwinning
withsubordinateCarlsbadtwins.
of
Normai zoning i s well
deve1ope.d i n some phenocrysts.
Sanidinevariesfrom
e u h e d r a l ,i r r e g u l a r l y
diameter.
10 t o 15 p e r c e n t as s u b h e d r a l t o
embayed g r a i n s 0 . 2 t o ' 2 . 0
mm i n
,
Some g r a i n s show i n c i p i e n t a l t e r a t i o n t o c a l c i t e ,
e p i d o t e , and c l a y s w h i l e o t h e r g r a i n s
are completely
a l t e r e dt ot h e s ep r o d u c t s .B i o t i t e ,v a r y i n gf r o m
volume p e r c e n t ,o c c u r s
4 to 7
as e u h e d r a l t o s u b h e d r a l , p l e o c h r o i c
g r a i n s 0.3 t o 1 . 5 mm i nl e n g t h .
Some c r y s t a l s are f r e s h
b u t most shorc a t l e a s t i n c i p i e n t a l t e r a t i o n t o d a r k r e d
38
and opaque iron oxides. Opaque grains, probably magnetite,
account for 1 to 3 percent of the rock. Minor quartz and
clinopyroxene may be present. Quartz occurs as large,
embayed,
anhedral
crystals
0 - 7 to 1.4 mm in diameter and
clinopyroxene as euhedral crystals
0.3 to 0.4 m . ' i ndiameter
altered to calcite and iron oxides. The groundmass consists
of
partiallyto completely
devitrified
glass
stained
dark-
reddish-brown by finely disseminated hematite dust. Outlines
of
extremely
crysts
are
The
compacted
indicative
tuff
of
glass
of
shards
dense
Granite
wrapping
around
pheno-
welding.
Mountain
around
the
Tres
Montosas
stock has been greatly affected by the intrusion.
It is'
difficult to determine in hand
rock is
part
of
the
specimen
stock
or part
of
if
the
a
particular
tuff
of Granite
Mountain. Only upon close examination are features of ashflow tuffs, such as lithic fragments, pumice cavLties,
and
faint
of
and
eutaxtic
Granite
a
textures
Mountain
waxy
Recrystallization
of
derived
tuff
the
metamorphosed
'
has
appearance
from
observed.
The
a
on
the
darker
a
freshly
groundmass
of
color
Granite
than
broken
has
tuff
the
fresh
rock
surface.
given
Xountain
a faint
the
hornfels
sugary
texture.
.Petrographically, the effects are more apparent.
baking
has
groundmass
resulted
in
a
distinct
.into
0.4 to 0.8 mm feldspar
The
'
'
recrystallization
the of
grains
and
the
intro-
duction of abundant, small pyroxene grains has given the
by
rock its darker color. The increase in temperature caused
e
e'
a
39
the intrusion and then slow cooling has caused exsolution
of potassium feldspar from the plagioclase. Graphic intergrowths of quartz
and
feldspar
were
observed
in
two
thin
sections. Despite the effects of metamorphism, the rock
still retains its pyroclastic texture.
Hells
Mesa
Tuff
The Hells Mesa Tuff was restricted by Chapin
(1974-b)
to include only a quartz-rich, densely welded, multiple-flow
simple
cooling
unit
tuffs
occurring
of
quartz
widely
latite
to rhyolite ash-flow
throughout
the
Magdalena
district,
the San Mateo, Gallinas and Lemitar Mountains, andof east
the Rio Grande. It is the equivalent of the lower, quartz-
e
rich portion of Tonking's (1957) Hells Mesa Member of the
Datil
.Formation
(1972) tuff
and Brown's.
.to
of.
Goat
Springs
in the Hells Mesa Formation.
A K-Ar age date of 30.6
f 2.8
m.y. (Weber
&
Bassett, 1963) was obtained for the base of
.
this formation. Dates of.32.1 m.y. and 32.4 m.y. (Burke and
others, 1963) for crystal-rich tuffs in the Gallinas
Mountains
and
Mesa
Tuff
(the
Spears
The
the
Joyita
Hills
in
better
agreement
are
Formation
Hells
map
the
area
Mesa
and
Tuff
and
is most
the
correlated
A-L
with
Peak
covers
about
prominent
in
with
other
Hells
dated
units
Tuff)
three
the
the
square
miles
northwest
where
it forms the high part of Tres Montosas. Because of poor
exposures,
a
a
good
.stratigraphic
However, from cross sections
varies
from
as
section
is notavailable.
it appears that the thickness
great
as 800 feet on
little as150 feet on Gray Hill.
Tres
Montosas
to
as
of
40
Just south of Tres Montosas, ain
roadcut along Highway60,
a
carmel-colored
vitrophyre
of
Hells Mesa is exposed,
but
the outcrops cannot be traced away from the roadcut. The
lower
contact
with
the
tuff
of
Granite
is present
Mountain
on Gray Hill but is obscured by talus. The upper contact
with
theA-L Peak
Eill
where
the
Tuff
two
is
well-exposed
in
a
formations separated.by
are
roadcut
on Gray
a
one- to
two-foot-thich, gray-green, friable bentonite layer. Brown
(1972) reports a "turkey track" andesite occurring
at this
break
in
On
the
Tres
massive
Bear
Mountains.
Montosas,
cliffs
which
the
Hells
extend
Mesa
forms
prominent,
to west
the as dip slopes
and
project under the sand cover. Elsewhere,' the outcrops
typically form low, rounded, talus-covered hills with little
relief. Several cliff-forming outcropsare exposed in the
central portion of the map area. The Hells Mesa typically
weathers
it
into
may
also
In
fresh
large
weather
hand
rounded
as
blocks
flat,
specimen,
bounded
by joints,
platy
the
but
outcrops.
Hells is
Mesa
pink to
brownish-purple, although most of the outcrops
in the
central area are bleached to
a pink-cream color. Weathered
surfaces yield buff to dark purple colors.
The Hells Mess
is
a
medium-grained,
densely
welded with
rock crystalsand
crystal fragments accounting for
40 to 55 percent by volume.
The major phenocrysts are sanidine, plagioclase, quartz, and
coppery-colored' biotite. Lithic fragments typically
e
account for0 to 4 percent of the rock, but one part of the
section containsas much as 20 percent dark, 'andesitic
41
lithic fragments. Flattened pumice fragments are uncommon
a
but do occur
In
thin
continuous
in
some
sections.
section,
gradation
the
of
samples
are
particles
porphyritic
with'
a
from
0.1 to 3.0 mm giving
a seriate texture. The'groundmass is composed
of small crystal
fragments, glass shards, fine magnetite dust, and devitrification products. The dense weldingis evidenced by highly
compacted
glass
Sanidine
shards
is
the
and
most
pumice
preserved
abundant
in
phenocryst
some
and
samples.
varies
from 10 to 25 percent of the rock volume.
It occurs in
subhedral to euhedral, irregularly embayed grains ranging
from 0.1 to 3 . 3 mm in diameter. Some grains show microperthitic
unmixing
with
the
development
of
patchy
exsolved
albite-twinned plagioclase. Most samples show incipient
* .
alteration
along
cracks
and
cleavage
planes
to chlorite,
sericite, calcite, and hematite, and in some
the.sanidine is
completely
altered.
Plagioclase
varies
50 to 20 percent
from
ofthe rock
as subhedral to euhedral, irregularly embayed, lath-shaped
crystals 0.1 to 3.0 mm in diameter. Alteration to clays
and
calcite
varies
The
plagioclase
from
incipient
to complete
compositions
were
measured
An
method; the composition varied from
26
replacement.
by
the
Fouque
to An29 with an
average from I grains of An (olgioclase). Albite twinning
27
is
*
the
most
common
twin
form
in
plagioclase
but
Carlsbad
twins are also present. Both normal and reverse zoning are
well developed in some phenocrysts. In some, the Ca-rich
cores reach a composition of An4* (andesine).
42
Quartz accounts for
5 to 15 percent of the rock as large,
'
rounded grains varying from 0.1 to 3.3 mm in diameter. Grains
as much as 6.5 mm in diameter were observed.Host grains are
embayed,
some
very
deeply,
and
filled
with
groundmass
glass
and small crystal fragmen.ts. Bronzy biotite.occurs as
euhedral
phenocrysts
from0.2 to 1.6 mm
long
and
varies
from
2 to 6 percent of the rock volume. The grains are pleochroic
with
colors
brown
In
and
some
ranging
are
thin
Hornblende
from
partially
sections,
occurs
light-brownish-green
to dark-reddishto
completely
the
asa minor
biotite
altered
to iron oxides..
laths
constituent,
are
(as
bent
and
broken.
much
as 1 percent)
in euhedral grains0.2 to 1.2 mm in diameter. .Incipient
. .
alteration
e
planes
to
and
magnetite
around
is
the
developed
borders
of
along
the
some
crystals
cleavage
minor
pyroxene
and apatite.are also present. 'In several thin sections,
cubic-shaped
the
holes
alteration
The
with
of
with
limonite
have left
beenby
pyrite.
groundmass
spherulitic
filled
is
brownish-gray
and. normally
aggregates
of
cristobalite
devitrified
.and
feldspar
common. The spherulites areso abundant in some thin sections
that they obscure the groundmass texture. Outlines
of glass
shards
are
observed
Fine
*
well
wrapped
iron
Lithic
oxide
preserved
around
dust
fragments
in
phenocrysts
makes
vary
some
up
the
thin
and
sections be
and
lithic
can
fragments.
remainder
the groundmass.
of
from
0 to 4 percent in the
majority
of the formation. Near the base there iszone
a
containing
43
as much as20 percent lithic fragments. The fragments are
typically dark-gray-brown, porphyritic, andesitic fragments
probably derived from the Spears.In thin section, they
consist of an opaque, hematized groundmass containing
altered plagioclase phenocrysts. Lithic fragments range
in size from5 mm to 5 cm. Where the lithic fragments
weather
out
of
the
lithic-rich
zone,
they
give
the
false
impression of Spears float. Pumice is uncommon but
in
some
thin
sections
it
is
observed
compacted
around
phenocrysts.
On
Gray
Hill,
a
paleovalley
has
been
cut
through
the
Hells Mesa. The Hells Mesa outcrop ends abruptly against
the you.nger A-L Peak Tuff. Within the paleovalley, the
A-L
Peak
Tuff
is
in
depositional
contact
with
the
of,tuff
Granite Mountain. The vitrophyre at the base
of the
A-L
Peak
Tuffis continuous around Gray Hill to the north.
Where the.Hells Mesa
Formationis in contact
with
the Tres Montosas stock, there is only slight recrystallization of the groundmass. The phenocrysts are not
affected as they are in the tuff
of Granite Mountain. In
hand
specimen,
luster on fresh
the
Hells Mesa
surfaces.
does
exhibit
a dull,
waxy
44
A-L Peak Tuff
The A-L Peak Rhyolite i s t h e name proposedbyDealand
.
Rhodes ( i n press)for"gray-to-purple,crystal-poorash-flow
tuffs that are widely distributed in southwestern Socorro
County.''
Thisformation
was f o r m e r l yc a l l e d - t h et u f f
of
,
'
Bear S p r i n g s by Brown (19721, who d e s c r i b e d it as a member of.
t h e H e l l s Mesa Formation.
Brown (1972)and
Simon (1973) w e r e
able t o s u b d i v i d e t h e A-L Peak i n t o s i x mappable u n i t s : o n l y
one u n i t was observed i n . t h e Tres Montosas a r e a .
track age date of
5 1.7 m.y.was
31.8
A fission
obtained by Smith
and
o t h e r s ( i n p r e s s )f o rt h ef o r m a t i o n .
The A-L PeakTuff
.
i s a multiple-flow,
compound c o o l i n g
.
u n i t which, i n the map a r e a , c o n s i s t s o n l y o f t h e d e n s e l y
welded,flow-banded
unit with
brown v i t r o p h y r e a t the b a s e ,
banded member i s p u r p l e t o p i n k
chalky sanidine phenocrysts
a two- t o t h r e e - f o o t - t h i c k ,
The lowerportionof
i n color with fresh or slightly
and highly'compacted pumice
which g i v e s a s t r e a k y o r bandedappearance.
oftheformationcontainsabundantpyrite
which upon o x i d a t i o n h a s g i v e n
The upper p a r t
(1 t o 3 p e r c e n t )
the rock a b u f f t o w h i t e
and h a sc o m p l e t e l ya l t e r e dt h es a n i d i n et oc l a y
therock
the flow .
color
and given
a chalky appearance.
Except for
two s m a l l e x p o s u r e s i n t h e n o r t h e a s t c o r n e r
o f t h e map a r e a , t h e
A-L Peak i s r e s t r i c t e d t o the extreme
'
45
e
southwestcornerwherethetotaloutcroparea
onesquaremile.
It t y p i c a l l y forms low, r o u n d e d . h i l l s
coveredbyangularfragmentswith
Gray H i l l , t h e A-L Peakforms
The A-L Peak Tuff
Mesa Tuff onGray
onlyone
is less than
fewgood
outcrops.
On
platy, ruljble-covered slopes,
rests d i r e c t l y on ' t h e u n d e r l y i n g H e l l s
H i l l , b u t on t h e small h i l l i n S e c t i o n 14,
m i l e away, a t h i n " t u r k e y t r a c k " a n d e s i t e s e p a r a t e s
..
thetwo.tuffs.
The uppercontact
i s drawn a t t h e base o f a
t h i n sequence of volcaniclastic sedimentary rocks
s e p a r a t e t h e A-L Peak Tufffrom
Hill.
A-L
0
which
the overlying tuff of
Gray
On Gray H i l l , where b o t hc o n t a c t s a x ee x p o s e d ,t h e
Peak v a r i e s from 300 t o 1000 f e e t i n t h i c k n e s s .
I n handspecimen,
t h e A-L.Peak Tuff varies from
thevitrophyretopurpleinfreshsamples-
Tuff i s c r y s t a l - p o o r w i t h
s a n i d i n e and q u a r t z .
When a l t e r e d b y
it h a s a b u f f c o l o r .
theoxidationofpyrite
brown i n
The A-L Peak
5 t o 10 percent phenocrysts of
The s a n i d i n e i s g e n e r a l l ye u h e d r a l ,
v a r y i n g from c l e a r t o chalky-white when altered; the q u a r t z
formssmallroundedgrains.Scattered,
crystals of
t i t a n i t e a r e v i s i b l e i n handspecimen,although
none w e r e observed i n t h i n s e c t i o n s .
the tuff contains
e
pumicewhich
'
small honey-colored
The lower p o r t i o n of
as much a s 20 p e r c e n t h i g h l y
d e c r e a s e s i n abundancetoward
compacted
the t o p .
The
pumice i s dark-brown t o g r a y and c o m p l e t e l y d e v i t r i f i e d w i t h
46
II!
the deyree of compaction decreasing toward the top. Extreme
examples of compaction around lithic fragments a.nd phenocrysts
6).
are observed near the base (see Fig.
!
Tuff
contains
as
much 10
as percent
The A-L Peak
andesitic
and
tuffaceous
lithic fragments.
Petrographically, tuffs of the
A-L.Peak
are
fine-grained,
porphyritic rocks containing euhedral
to anhedral, partially
resorbed phenocrysts of sanidine, quartz, plagioclase, and
biotite. Sanidine is the dominant constituent and varies
from 3 to 5 percent of the rock volume.
It typically forms
euhedral
grains0.1 to 3.3 mm in
diameter
and
is
conkonly
microperthitic. The sanidine is usually altered to clays.
a
Quartz, in partially resorbed, rounded grains as asmuch
2.0 nun in
diameter, .accounts for
2 to 3 percent
ofthe
rock. Plagioclase and biotite are minor constituents
occurring in very small grains, each accounting
for less
of the
than 1 percent of the rock volume.. The composition
plagioclase, as determined on two grains using the Fouque
method, was An
29.
The groundmass
is
completely
devitrified
with
some
well-preserved outlines of compacted glass shards. The
devitrified groundmass consists of fine-grained quartz and
feldspar, oftenas axiolites and spherulites. Finely
disseminated
brown color.
iron
oxide
gives
the
groundmass
a
reddish-
Figure 6. Cross-sectional view of the flow-banded member of
the A-L Peak T u f f in a roadcut on Gray Hill. Note the fluidal
banding caused by extreme compaction a n d . e l o n g a t i o n of pumice
during primary welding. A well-developed lineation (not
visible in photograph) i s present on the f o l i a t i o n planes.
48
e
Thedistributionandthicknessofthe
A-L PeakTuffis
controlled by a large, northeast-trending paleovalley which
was
cut
through
the
Hells
Mesa
Tuff
and
of Granite
the
tuff
Mountain prior to eruption of the
A-L Peak (see Fig. 7).
This
accounts
less
than
for
the
dramatic
one-quarter
of
a
thickening.of
the
unit
within
nile.
A pronounced feature of the paleovalley
is the tendency
of
the
foliation
attitudes
change
to
steepen
from
those
as
the
valley'
wall.
is
approach
shown
on Plate I to nearly
vertical within short distances. This line of vertical
foliation is interpreted
to
be
N 30" E to
varies in trend fron
e
the
N
paleovalley
55O E.
wall
and
Measurements of
lineated pumice (which should parallel the direction of flow)
vary from N 16" E to N 50' E.
the
wall
of
the
Secondary flow folds along
paleovalley
were
0bserved.i.n
and the axisof one such fold had aN 3 8 " E trend.
there
is
a
marked
parallelism
of
vertical
the
outcrops
So
flow
foliation,
pumice lineations, and secondary fold axes. Lowell and
Chapin (1972) and Chapin and Lowell (1975) have described
similar
of
structures
divergent
slumping
down
in
paleovalleys
foliation,
the
interpreted
valley
wall
and
in
as
Colorado.
. Evidence
masses
hotof
tuff
becoming'welded
to
subsequently deposited tuff, is common in the outcrops.
Deal and Rhodes (in press) suggested that the
a
Mt.
Withington
cauldron
was
the
source
of
the
A-L
Peak
Tuff.
..
.-. ......
1
.
,
.
'.
I
"
Figuxe 7. View of Gray Hill from the northwest showing
a
major paleovalley filled with the
A-L Peak Tuff. From right
to left, theA-L Peak restsdn progressively older units'
towards the deepest part of the valley. (Thm
= Hell Mesa
Tuff; Tsg = tuff of Granite Mountain; Tsp = volcaniclastic .
line.= outline of m e
rocks of the Spears Formation; solid
paLeovalley.)
e
This
source
on Gray
0
is
Hill
consistent
and
with
with
the
the
50
of
trend
the
azimuths
of
paleovalley
lineation
.measured
Brown (1972),. Deal (1973), Simon (19731, Chamberlin (19741,
Blakestad
(1976),
and
the
measured 1800 to' 2100 feet
author.
of
the
Deal
A-L
and
Rhodes,
- &.)(op.
Peak
Tuff within
the
cauldron. .
Andesite
The
andesite
of
of
Landavaso
Landavaso
Reservoir
Reservoir
is the
informal
to a highly variable series
name given by Chapin (1974-b)
of
porphyritic
basaltic-andesiteflows cropping out about
four miles east of the study area. Charnberlin (1974)
e
mapped a local vent for these lavas in the Council Rock
area to the north. The occurrence of these andesites
east of the range-bounding fa.ult north
of Cat
and
in
of
Cat
juxtaposition
Mountain,
with
the
provides
a clue
Spears
to
Mountain,
Formation
southwest
minimum
displacement
the
along the range-bounding fault.
Exposures of the andesite
o€ Landavaso Reservoir
consist
of
platy
to
massive
lavas
'
which an
cover
area of
less than one-quarter square mile. The maximum thickness
is
unknown
but
cross
due
to poor
sections
and
widely
indicate
a
scattered
minimum
outcrops,
thickness
of about
400 feet. The lower contact is not exposed
in themap
e
by
51
e
area. On Cat Mountain, the andesite of Landavaso Reservoir
is overlain
by
correlated
in
of younger
sequence
a
this
report
ash-flow
tuffs
with Potato.Canyon
the
Tuff.
In hand specimen, the andesite of Landavaso Reservoir
is a dense; gray to reddish-gray, porphyritic rock with
phenocrysts of plagioclase, pyroxene, and biotite. Flow
banding is
well
developed
in
some
areas
and
secondary
hematite banding is a common feature of the unit.
In
most samples, there are distinct hematite alteration
aureoles
around
the
ferromagnesian
minerals.
Petrographically, the lavas are porphyritic with a
e
felty
to
pilotaxitic
glomeroporphyritic
Plagioclase
is
the
groundmass
clotsof pyroxene
dominant
and
occassionally
and
phenocryst
contain
plagioclase.
and
comprises
15 to 20 percent of the rock volume. It occurs in
euhedral to subhedral grains 0.4 to nun
4.5
in length.
Normal and oscillatory zoning are commonly observed.
The
plagioclase
phenocrysts
are
typically.
fresh
but
show incipient alteration to clays and calcite.
The
composition of the plagioclase ranges from An to AnsO
44
with an average of An (Rittmann Zone Method,25 grains),
54
suggesting that the lavas are basaltic-andesites.
e
many
52
e
Pale
green
clinopyroxene
comprises1 to 5 percent
and
was
observed
in
the
rock
euhedral
of
as
all
thin
section
to
subhedral grains0.4 to 1.9 mm in diameter. It is typically
altered
in
varying
Biotite
2.5
nun in
volume.
occurs
length
degrees
to
in
thin
all
hematite
and
sections
in
comprises
1 to 4 percent
and
magnetite..
grains
0.3 to
ofthe rock
It is almost always altered to hematite but two
thin sections contain fresh, brown, plechroic grains.
Apatite,
in
grains
much 0.
as
4 nun in
as
1-ength,
and
magnetite
..
are
common
accessory
The groundmass
e
pilotaxitic
is
typically
arrangement
interstitial
was
minerals.
pyroxene
observed
in
several
tuff
for a thick
rich,
Tuff
of
sequence
rhyolitic
and
areas,
the
this
thin
of
Potato
break
is
is
tuffs
Canyon
groundmass
Hill
the:
crystal-poor
ash-flow
glassy
with
sections.
Gray
Hill
felty.to
microlites
magnetiteSome
of
Gray
plagioclase
-
and
Tuff
The
of
composed
of a
to
on
by
proposed
moderately
occurring
Tuff
occupied
name
e
Hill
of Gray
where
Hill
the
by
a lense
A-L
of
Peak
Tuff
is
volcaniclastic
crystal-
andesite
other
of
Reservoir. The lower contact is exposed ina roadcut on
Gray
this
between
A-L the
Peak
Gray In
Hill.
most
the
in
Landavaso
.
separated
the tuff
from
sedimentary
rocks-
rep
53
i n the tuff of
These sedimentary rocks are included
The contact of
Gray R i l l .
the t u f f of Gray H i l l w i t h the o v e r l y i n g
P o t a t o Canyon Tuff i s e x p o s e d . i n t h e s o u t h w e s t c o r n e r o f t h e '
map a r e a .
The t u f f o f
Gray H i l l i s p r o b a b l y c o r r e l a t i v e
withSimon's
(1973) c r y s t a l - p o o r u p p e r t u f f s
t h ea n d e s i t e
of LandavasoReservoir.
Gray H i l l i s c l o s e l y b r a c k e t e d b y
t r a c k d a t e on t h e u n d e r l y i n g
1.6 m.y.
fission track date
Tuff(Smithand
wkiich o v e r l i e
. .
The a g e o f t h e t u f f o f
a 31.8
3 1.6 m.y.
A-L PeakTuffand
a 30.3
on t h e o v e r l y i n g P o t a t o
others, i n press)
fission
2
Canyon
.
The t u f f of Gray H i l l h a s a minimum t h i c k n e s s i n e x c e s s
of 1000 f e e t as e s t i m a t e d from c r o s ss e c t i o n s .
i s s e p a r a t e d from t h e main p a r t
exposure of the upper contact
o f the formation by
a f a u l t . w i t h unknown displacement, s o t h e
maximum t h i c k n e s s i s n o t known.
Gray H i l l are r e s t r i c t e d t o t h e
ofthe
The o n l y
Exposures of Yne t u f f o f
extreme southwestern,corner
map area where t h e y form h i g h , rounded h i l l s .
tuffs weather
The
t o rounded b l o c k s ; h i l l s l o p e s are covered w i t h
small,angularfragments.
Simon's(1973)conventionofdividing
tuffs into
f o l l o w e di n
two p a r t s , a b a s a l u n i t
this r e p o r t .
and a n u p p e r u n i t ,
The b a s a l u n i t
to-purple, chalky, moderately
the c r y s t a l - p o o r
is
i s composed ofgray-
t o poorly welded tuffs
54
characterized by small, botryoidal pumice.A similar unit
containing
as
.the
botryoidal
Luis
Lopez
comun., 1975).
pumice
has
district
been
as far
observed
away
(Chapin,
oral
of
south
Socorro
Some of these tuffs contain
(15 to 20 percent)
small, andesitic and tuffaceous lithic fragments.' The basal
unit
the
erodes
more
The
as
low
densely
upper
valleys,
welded.upper
unit
is
volcaniclastic
sorted
of
quartz,
Tuff
from
unit.
separated
medium-grained
grains
separating
A-L the
Peak
from
sandstone
sanidine,
thebylower
a thin,
composed
of moderately-,
and
plagioclase
a finein
grained siliceous matrix. The upper unit consistsof gray
to
purplish-gray,
characterized
by
moderately
small,
Petrographically,
to
banded
the
densely
welded
rhyolitic
tuffs
are
tuffs
lithic
fragments.
porphyritic awith
devitrified, microcrystalline groundmass. The phenocryst
content varies from
5 to 25 volume percent. Sanidine,
occasionally
exhibiting
usually the most
abundant
microperthitic
phenocryst
intergrowths,
and
is
accounts 2for
to
15 percent of the rock. It is often completely altered
to
clays and.calcite. Quartz is rarely more abundant than
sanidine and accounts for
2 to 13 volume percent.It occurs
as
rounded,
slightly
embayed
grains
as
mm in
much
3.0 as
diameter. Plagioclase is almost absent in the basal unit
but
increases
to
as
much
as
five
percent
the top
near of
.the
55
upper unit. Compositions range from An to An with an
32
28
7 grains). Biotite,
average of An (Fouque method,
30
usually altered to opaques or hematite, comprises about
one
percent
of
the
rock
volume
in
the
upper
part
of
the
section.
The
groundmassis completely
crystalline
aggregatesof potash
devitrified
feldspar
to
and
micro-
quartz
with
spherulitic and axiolites textures developed
in some thin
sections.
Outlines
preserved in
Lithic'
a
Grav
-
the
of
glass
upper
fragments
shards
are
typic?lly
well-
unit.
are
common
throughout
the
tuff
of
Hill, accounting for 0 to 30. volume percent. They are
generally small, rangirly from 6 mm to 5 cm in diameter.
Several distinct types were observed:1.) gray to black,
fine-grained andesitic fragments;
2.) brown to gray
tuffaceous fragments: 3.) crystal-rich Hells Mesa-type
fragments; -and 4. ) pinkish-gray,
banded
rhyolitic
fragments.
The outcrop of distribution, thickness, and local extent
of the
the
tuff
of
Gray
paleovalley
Hill
previously
suqgest it
thatwas
filled
by
deposited
in
thePeak'Tuff.
A-L
The sedimentary rocks between the two formations, and local
uncomformities
foliation
was
in
between
the
tuff
of Gray
the
A-L
Peak
Tuff,
re-entrenched
and
then
filled
Hill
and
the
suggest
that
the
by
tuff
of
the
vertical
paleovalley
Gray
Hill.
a
a
Sucha
56
of the a n d e s i t e
process could also explain the absence
ofLandavasoReservoirbetweenthe
of i t s p r e s e n c e ' j u s t t o t h e
two f o r m a t i o n s i n s p i t e
w e s t of the paleovalley wall.
P o t a t o Canyon Tu'ff
The P o t a t o Canyon R h y o l i t e i s the name proposed by
and Rhodes ( i n p r e s s ) f o r
Deal
a t h i c k section ofmoderately
crystal-rich ash-flow tuffs
i n t h e M t . Withington cauldron.
Subsequent mapping (Woodward, 1973; S p r a d l i n , 1975) h a s shown
that the Potato
Mountainsand
the J o y i t a H i l l s .
obtained a f i s s i o n t r a c k d a t e o f
a
as the L e m i t a r
Canyon T u f f o c c u r s a s f a r e a s t
Smithand
o t h e r s( i np r e s s )
+ 1.6 m.y.
30.3 -.
area
sample n e a r t h e b a s e o f t h e f o r m a t i o n . I n t h e s t u d y
exposures of the Potato
Mountainand
Canyon Tuff a r e r e s t r i c t e d t o C a t
t o t h e mesa i n the southwest corner.
Deal(1973)divided
i n f o r m a lu n i t s :
the P o t a t o Canyon T u f f i n t o
tuff"unit.
The exposures i n t h e
southwest corner of the study area correlate
unit.
Here, t h eP o t a t o
consisting of
two
a lower u n i t o f g r a y , d e n s e l y w e l d e d t u f f s
andanupper"inoonstone
w i t h h i s lower
Canyon Tuff i s v e r y t h i n
a gray, densely
p e r c e n tc h a t o y a n ts a n i d i n e
e
from a
(100 f e e t ) ,
welded t u f f w i t h 15 t o 2 0
and smokey q u a r t z .
depositional contact w i t h the tuff of
Its base i s a
Gray H i l l and t h e u p p e r
c o n t a c t i s an e r o s i o n a l s u r f a c e on which the b a s a l t o f
58
a
i n a reddish-browngroundmass.
L i t h i cf r a g m e n t sa c c o u n tf o r
0 t o 25 volume percentwiththefollowingtypesobserved:
1.) small brown aphaniticfragments;
I
2.)
g r a yt o
brown
.
and 3 . ) b l a c ka n d e s i t i cf r a g m e n t s .
tuffaceousfragments;
Pumice v a r i e s from 10 t o 30 percent of .the rock
In thin section, the tuffs are porphyritic
volume.
with a
d e v i t r i f i e d groundmass i n which s p h e r u l i t i c t e x t u r e s a r e
cccasionallydeveloped.
The p h e n o c r y s tc o n t e n tv a r i e sf r o m
5 t o 30 volume percent.Sanidine
abundantphenocryst
i s t y p i c a l l yt h e
and comprises 2 t o 15 percent of
volume as e u h e d r a lt os u b h e d r a lg r a i n s .
m i c r o p e r t h i t i c unmixingof
a
most
the rock
Some g r a i n se x h i b i t
p l a g i o c l a s e .P l a g i o c l a s ea c c o u n t s
f o r 1 t o 10 perc'ent and i s o c c a s i o n a l l y more abundant .than
sanidine.
Cornposit-ions
were
measured
(Fouque method)and
of An25.
on s e vge rna i n s
t o Anz7 with anaverage
22
0 t o 15 p e r c e n t as rounded,
ranged from An
Quartzaccountsfor
s l i g h t l y embayed g r a i n s as much as 3 . 0 mm i n d i a m e t e r .
Biotite, typically altered to
opaquesand
hematite, v a r i e s f r o m
0 t o 2 volume p e r c e n t .
Tertiary Intrusive
Rocks
L a t i t e Dikes
Latitedikesarepresentin
map area:aroundthepsriphery
a n di nt h e
e.
Cat Mountainmining
two d i s t i n c t parts o f t h e
of t h e Tres Montosas s t o c k
district.
Thqse d i k e sn e a r
the s t o c k are r a d i a l l y d i s t r i b u t e d a b o u t t h e s t o c k f o r
d i s t a n c e of as much as two m i l e s .T h e r ea r e
a
two v a r i e t i e s
.
'
5.7
Council Rock was deposited. Outside the study area,
the
Potato
the
Canyon
andesite
The
of
Tuff
of
Landavaso
section
densely
normally
exposed
welded,
overliesA-L
the
Peak
Tuffor
Reservoir.
on
Cat
crystal-rich,
Mountain
is
quartz-rich
composed
tuffs
mainly
with
some
unwelded pumiceous tuffs and fluidal-banded, crystal-poor
tuffs which correlate with Deal's
(1973) uppermost
"moonstone
tuff" unit. This section is very similar to the section
exposed in Monica Canyon,
north.of Mt. Withington. Stratigraphic
it
boundaries
appears
that
are
amkiguous
the
Potato
due
Canyon
to
poor
Tuff
on
exposures,
but
. .
Cat
Mountain
over
the andesite of Landavaso Reservoir.It. is unconformably
basaltof Council Rock. These exposures may
overlain by the
be correlative with Simon's
(1973) crystal-rich upper tuffs,
but
poor
and
discontinuous
exposures
in
both
areas
prohibit
any certain correlation. The Potato Canyon Tuff on Cat
Mountain
The
hasa minimum
Potato
thicknessof 700 feet.
Canyon
Tuff
is
gray
to
on fresh
pink
surfaces
and weathers to gray to dark reddish-brown. The tuffs vary
from
crystal-poor
to
crystal-rich
and
are
typically
densely
welded, although several samples are only slightly welded.
The
dominant
phenocrysts
are
sanidine
(which
may
be
chatoyant)
colorless to smokey quartz, plagioclase, and biotite. of
Two
the
crystal-rich
tuffs
have
distinct
white,
flattened
pumice
e
e,
59
of dikes, both latites. The first is a dark greenish-black,
porphyritic rock with a fine-grained, aphanitic to phaneritic
I
matrix.Thisvarietyoccursasadikesix
wide
and'2100
feet
long
to eightfeet
with
a
distinct
vertical
alignment
of plagioclase phenocrysts. It also occurs as several short
dikes cutting the wall rocks of the stock.
The dikes
weather to a reddish-brown color.
In thin section, the dike rock is porphyritic-phaneritic
with a pilotaxitic groundmass. Plagioclase phenocrysts as
much
as 1.8 cm in length comprise
20 to 25 volume percent.
Compositions range from An to An42 (Rittmann zone method,
40
3 grains) with an average of41'An Clinopyroxene and
apatite are minor accessories accounting 1
for
and 2 percent,
respectively. Tne groundmass consists of altered plagioclase
microlites
with
interstitial
clinopyroxene
and
ortho-
clase. This rock is very similar to the border facies of
the stock, the only difference being a slightly finer
grained
matrix
in
The
second
type
feet
It
wide
and
as
the
of
with
fewer
latite
much
is*a yellowish-brown
matrix
dikes.
and
as
rock
one
occurs
and
with
smaller
a
as
three
dikes
eight
quarters
fine-grained,
plagioclase
to
mile
with
a
trachytic
phenocrysts
than
groundmass
of plagioclase
microlites and interstitial orthoclase.
'Most of the phenocrysts
were
plucked
out
during
the of
thin
cutting
long.
phaneritic
the first type. In thin section,it is porphyriticphaneritic
ten
section,
but plagioclase was probably the only kind present. The
60
a
compositions
of
the
dikes
and t h e ipr r o x i m i t tytohset o c k
the s t o c k .
a geneticrelationshipto
s u g g e s t st h a tt h e yh a v e
I n thesoutheasternpartof
the map area, i n t h e C a t
Mountain'mining d i s t r i c t , a d i k e swarm c r o p s o u t o v e r a n
area oneand
one-half miles wide and two miles long.
dikes trend
NNW t o NNE with steep d i p s f l u c t u a t i n g t o
eithersideofvertical.
f e e t wideandtwo
Where observed, they have
n oc o n t a c t
are as much as t e n
Thesedikes
can be t r a c e d f o r
The
one mile o r more.
no c h i l l e d m a r g i n s
and t h e r e are
metamorphic e f f e c t s on t h ec o u n t r yr o c k s .
Some
fresh i n c l u s i o n s of the wall rocks.
of t h e d i k e s c o n t a i n
F r e s h dike rocks are d a r k g r e e n i s h - b l a c k , P o r p h y r i t i c
a
with a p h a n i t i ct of i n e - g r a i n e dp h a n e r i t i c
groundmass.
They
as l i n e s of d i s -
formpooroutcrops,oftentraceableonly
Most of the d i k e s are altered due t o t h e
t i n c t i v ef l o a t .
oxidation of contained pyrite
and t h i s i m p a r t s
b u f fc o l o rt o ' t h eo u t c r o p s .
a brown o r
On some w e a t h e r e ds u r f a c e s ,
f e l d s p a r c r y s t a l s as much as 2.0 cm long stand out in relief.
l a t i t e s are p o r p h y r i t i c w i t h
Inthinsection,the
and o r t h o c l a s e i n a p i l o t a x i t i c
phenocrystsofplagioclase
groundmassof
plagioclasemicrolites..Euhedralplagioclase
phenocrysts, as much as 6.2 mm i n d i a m e t e r ,
5 t o 20 volume p e r c e n t .
dominant phenocryst accounting for
Compositionsrangefrom
An33 (Rittmanzonemethod,
An
27
are the
t o An
45
with anaverage.of
14 g r a i n s ) .O r t h o c l a s e
phenocrysts, as much as 2 . 2 mm i n diameter, comprise 2 t o
,4 volume p e r c e n t .
Euhedral a p a t i t e g r a i n s , as much as
e
.,
*
61
2 t o 4 pasrcent and clinopyroxene
1.4 mm l o n g , a c c o u n t f o r
may o c c u ri n
amounts up t o 2 p e r c e n t .
composed o f p l a g i o c l a s e m i c r o l i t e s
is
Thegroundmass
with i n t e r s t i t i a l
o r t h o c l a s e and a p a t i t e .
The occurrenceof t h i s d i k e swarm, ' i n i n a s s o c i a t i o nwith
extensive quartz veining
and hydrothermal a l t e r a t i o n , may
indicate the presence of
a pluton buried beneath the
district.
Mountainmining
S e v e r a lw o r k e r si n
Cat
the Magdalena
d i s t r i c t (Brown, 1972; Krewedl, 1 9 7 4 ; Blakestad, 1976; and
Chapin,pers.
comnun.,
1 9 7 5 )h a v en o t e dt h ec o r r e l a t i o n
of
mafic and p o r p h y r i t i c f e l s i c ' d i k e s with known i n t r u s i v e s .
They showed t h a t t h e d i k e s
within a two-mile r a d i u s o f
zoneandextendonly
e
Chamberlin(1974)
are c o n c e n t r a t e d i n t h e r o o f
and the a u t h o r n o t e d
-
the s t o c k s .
a similar c o r r e l a t i o n
.
of l a t i t e d i k e s with t h e T r e s
Montosas stock.Witkind
and
o t h e r s (1970) r e p o r t e d t h e c o r r e l a t i o n o f f e l s i c d i k e s a n d '
p l u g s with a g r a v i t y o u t l i n e d s t o c k i n
Mountains,Central
presence of
Montana.
the L i t t l e B e l t
Therefore, it appears that the
a dike swarm i s a v a l i d g e o l o g i c i n d i c a t o r
of a
b u r i e di n t r u s i v e .
Furtherevidence
i n support of
intrusive i s theexistence
.
the presence of
of a magnetic low on the low-level
.aeromagnetic map (USGS, Residual .Magnetic I n t e n s i t y Map,
Area 4-3N, SW New Mexico) centered over
!
*
a buried
swarm (see F i g . 3 and P l a t e 11).
the area of the d i k e
Figure 8
.
Z e s i d u a lm a g n e t i ci n t e n s i t y
quadrangle and p a r t o f t h e G a l l i n a s
map of t h e Tres Xontosas
the
U'.S.C,.S. Open
Peakquadrangleshowing
anamoly centered over the Tres Nontosas stock (from
F i l e ? t a p , Area 4-3 X, SXJ Met7 I[exico).
63
e
Tres Montosas Stock
Two s t o c k sa r e .e x p o s e di nt h e
of t h e s e o c c u r s
map a r e a .
Tlie l a r g e s t
as a r o u g h l y c i r c u l a r body approximately
two, miles i n d i a m e t e r , l o c a t e d a l o n g
the northern boundary
It i s a composite body c o n s i s t i n g o f
of t h es t u d ya r e a .
l a t i t e t o quare2
t h r e ef a c i e s :a n d e s i t et og r a n o d i o r i t e ,
monzonite and g r a n i t e .
Chamberlin ( 1 9 7 4 ) named. it the
T r e s Montosas s t o c k f o r
it p r o x i m i t y t o Tres Montosas,
three prominent peaks between
Mountains.
the SanMateoand
Gallinas
The m a j o r i t y of the Tres Montosas s t o c k i s .
mantled by blow sand and gravel; but the outcrop
topographic expression,
pattern,
and a corresponding aeromagnetic
8 ) i n d i c a t e that it i s a continuous,
high(seeFig.
c u p o l a - l i k e body b e n e a t h t h e s u r f a c e .
. .
The Tres. Montosasst.ock
from the lower Spears Formation
thushas
t o t h e A-L Peak Tuff, and
a maximum age of 31.8 m.y.
Smith and o t h e r s , i n
(fission track date
press, on t h e A-L Peak T u f f ) .
age i s i n good agreementwith
Anchor Canyon s t o c k s o f t h e
1.4 m.y. and 28.3 -E
- 1.4 m.y.,
o f a similar age.Chamberlin
3.
r e s p e c t i v e l y , Weber and
Montosas s t o c k is
( 1 9 7 4 ) c o l l e c t e da . s a m p l e
A whole-rock
a n a l y s i s by Geochron'Laboratories Inc., yielded
anage
This
Magdalena D i s t r i c t (28.0
Big John mine f o r K-Ar d a t i n g .
33.9 m.y.,
by
K-Ar ages of the N i t t and
B a s s e t t , 1 9 6 3 ) and s u g g e s t s t h a t t h e T r e s .
f r o mt h e
i n age
intrudes rocks ranging
a d a t e of
greaterthantheyoungestrockintruded
by the stock.Subsequent.petrographicexamination
of the
64
sample revealed thatit contained as much as
10 percent
pyroxene, which according to Damon (1968,
p. 14) commonly
yields high ages due to excess argon. Based on the above
data,
the
author'assumes
an
age
similar
to
the
Nitt
and
Anchor Canyon stocks for the Tres Montosas stock.
Most outcrops of the stock occur along the stockcountry
rock
contacts,
but
there
are
a
few
isolated
outcrops
in the central part of the stock. Exposures in the central
core formlow, rounded hills completely surrounded by blow
sand 'and gravel. Better outcrops occur around the periphery
where the border facies are "held up" by the more resistant
. .
metamorphossc2 country rocks.
e
The stock-country rock contact is quite sharp (see
. .
Fig. 9) and is exposed
in two places; elsewhere
it is
covered by soil and colluvium. Chamberlin (1974) reported
that
the
contact
between
quartz
monzonite
and
the
Hells
Mesa
Tuff was exposed along the west margin. Be estimated a dip
of
about30 degrees to the
this figure
to
calculate
westat'this location
andused.
that
the
the
roof
of
stock
was
approximately 1500 feet above the present erosion level.
However,
along
monzonite
and
and dips 60° N.
data
a
points
the
the
south
tuff
margin
of
the
Granite
contact
Mountain
between
strikes
N 63' W
Any prediction basedon only these two
would
of
necessity
be
tentative,
but
the
shape
of the aeromagnetic anomaly indicates that the stock has a
moderately shallow, outward dip. Therefore, Chamberlin's
,
(9.
g . )estimate of thedepth,of erosion is reasonable.
Local
variations
in
the
dip
of
the
contact
are
to
be
expected
.
F i g u r e 9. Outcropshowing
t h ec o n t a c t . b e t w e e nt h e . m o n z o n i t e .
~. '.
facies' o f , t h e Tres Montosas stock,. (Tm) ' a n d h o r n f e l s derived
from t h et u f f
of G r a n i t e Mountain ( T s g )
T h e o u t c r o pd i p s
. .~
, . ,
60' toward t h es t o c k( i n t ot h ep i c t u r e ) .
The o u t c r o p , i sl o c a t e d
along
the
southwestera
margin
of t h se t o c k ,
,
.
,
.
,
.
:
..
. .
.. .
66
0
Chamberlin (1974) s u g g e s t e d t h a t t h e
s t o c k was f o r c e f u l l y i n t r u d e d i n t o
d a t a obtainedfromsubsequent
Tres Montosas
the country rocks but
passive
mapping i n d i c a t e d t h a t
i n t r a s i o n was probably .the most important mechanism,
i s l i t t l e or no
There
doming o f , t h e c o u n t r y r o c k s e x c e p t a l o n g
northwesternmarginofthestock.
the
The s t e e pe a s t w a r dd i p s
along the e a s t e r n margin are the result of a monoclinal
s t r u c t u r ed e s c r i b e d
by Chamberlin (op. c i t . ) .
"
The occurrence
piecemeal
of q u a r t z i t e x e n o l i t h s i n t h e s t o c k i n d i c a t e s t h a t
s t o p i n g was o p e r a t i v e a t some t i m s d u r i n gi n t r u s i o n .F i g u r e
10 shows x e n o l i t h s o f S p e a r s a n d e s i t e i n
the c h i i l e d m a r g i n
of the monzonire f a c i e s . F u r t h e r e v i d e n c e . .
where a g r a n i t e d i k e
observed along the southern margin
. desites (see Fig.
intrudes the Spears . an
ofthedikehave
begun t o b l o c k o u t
of t h ea n d e s i t e .C o n c e n t r i cf a u l t i n g
theperipherywiththesense
c e n t e r of the s t o c k .
of s t o p i n g was
11).. Small "fingers."
and i n c o r p o r a t e f r a g m s n t s
i s well-developedaround
of movementdown
toward the
The f a c t t h a t the d i p s on t h em o n o c l i n a l
structuredecreasetowardthestock
and the s c a r c i t y of
c o n c e n t r i c and r a d i a l dikes suggesq
t h a t the c o n c e n t r i c
faulting represents collapse over
the s t o c k d u e t o s u b s i d e n c e
of magma p r e s s u r e , . If f o r c e f u l i n t r u s i o n ' p l a y e d
r o l e , doming shouldbe
concentric faults
a dominant
more a p p a r e n t and the r a d i a l and
would be e x p e c t e d t o c o n t a i n
more d i k e s .
The' above d a t a do n o t e x c l u d e f o r c e f u l i n t r u s i o n , b u t
rl);
suggest that p a s s i v e i n t r u s i o n
was the dominantmechanism.
Figure 10. Exposure of the contact between.the monzonite
facies of the Tres Montosas stock(Tilt) and andesites of the ;
Spears Formation (TS) showing a thin, chilled margin
of the
stock (Tmc). The outlined, dark fragmentsin the- chilled
zone are xenoliths of the Spears andesites. The.outcrop is
located along the southern margin of the stock.
Figure 11. Outcrop of andesites in the Spears Formation
Tres Montosasstock.
intruded by a granite dike from the
Note the thin "finaers"
of the dike surroundinq and
Reconstruction o f the stratigraphic section indicates
that Che Tres Montosas stock was intruded to within
2000
feet of the Oligocene ground surface.
Facies. The Tres Montosas stock is composed of three
principal facies: andesite to granodiorite, latite to quartz
monzonite, and granite. The inferred distributionof,facies
12 (modified after Chamberlin,
is illustrated in Figure
1 9 7 4 , p. 59); lack
. makes
of exposures along the eastern margin
that part of the illustration highly speculative.
Sample
locations
of
the
stock
rocks
are
also
piotted
on
the
figure. The undifferentiated mafic rocks shown by Chamberlin.
e
(op. cit.)
along
the
southern
border
have
been
shown
by
subsequent mapping to be metamorphosed hornfelsed Spears
Formation.
The
andesite
and
granodiorite
were.not observed
in
the
map area, but Chamberlin
(1974) mapped rocks of this type as
the
outermost
"Andesites
facies
form
a
along
chilled
the stock at the northwest
the
northern
sheath
about
. . . . margin.
margin
the
of
outer
the
stock.
surface
of
Near the Big John
mine, andesite porphryis apparently gradational into fineto
medium-grained
Latites
along
the
contact
and
quartz
western
with
granodiorite."
(%.,- tit., p. 60).
the
and
Hells
monzonites
southern
Mesa
form
margins
and
Spears
the
border
where
facies
they
in are
Formations,
respectively.
Along the southern margin there are several latite dikes
cutting
the
wall
rocks
in
a
radial
pattern
and
Chamberlin
70
.
MINOR FAULT
L A T I T € / MONZONISE
"
.
'.
* ' ' INFERRED
"
0-
CONTACT
INFERRED STOCK
PERIMETER
COUXTRY ROC!(STOCK CONTACT
0UTCROP.BOUNDARY
(STOCK ROCKS OXLY)
0
,.:.:.; :
'ANDESITE/GRANODIORITE
H.
METASOMATIZED ROCK
x
TM-IZ
SAMPLE
LOCATION
.
.
F i g u r e 12. I n f e r r e d d i s t r i b u t i o n of t h e i r i n c i p a l . rock ' t y p e s
intheTres:lontosasstock.Ifetasomatized
rocks are c h i e f l y
h e d e n b e r g i t es y e n i t e s .( ? 4 o d i f i e da f t e rC h a n b e r l i n ,
1974.)
. . .
.
-
71
a
(1974) r e p o r t e d t h a t l a t i t e i n t r u d e s g r a n o d i o r i t e n e a r t h e
Big John mine.
The l a t i t e s are d i s t i n c t l y p o r p h y r i t i c w i t h l a r g e
plagioclase phenocrysts as
- . t o - g r a y ,a p h a n i t i c
much as 1.9 cm long i n , a b l a c k -
groundmass.
They t y p i c a l l y formpoor
outcropsalongfloat-coveredslopes.'Zoningofthelarge
i s v i s i b l e i n handspecimen.
p l a g i o c l a s ep h e n o c r y s t s
In
t h i n section,thelatitesconsistoflarge,euhedralplagioclase phenocrysts in
a p i l o t a x i t i c groundmass o f ' p l a g i o c l a s e
. .
m i c r o l i t e sw i t hi n t e r s t i t i a lo r t h o c l a s e .P l a g i o c l a s e
An4o t o An45 w i t h an average of
compositions range from
0
An42 (Rittmanzonemethod,
3 g r a i n s ) .M a g n e t i t e
e a c ha c c o u n tf o rl ' p e r c e n to ft h er o c k
m i n e r a l sc o n s i s to fp l a g i o c l a s e
(30 p e r c e n t ) ,m a g n e t i t e
and a p a t i t e
volume.Thegroundmass
(30 p e r c e n t ) , o r t h o c l a s e
(10 p e r c e n t ) ,p y r o x e n e
(5 p e r c e n t ) ,
and a p a t i t e ( 2 p e r c e n t ) .
The quartz monzonite
i s a porphyritic,fine-grained
phaneritic rock with phenocrysts of plagioclase
The p l a g i o c l a s e , a c c o u n t i n g f o r
glomeroporphyritic clumpsand
2.5 cm l o n g .A l t e r a t i o n
6 volume p e r c e n t , o c c u r s a s
as single crystals as
was t o o i n t e n s e
plagioclasecompositionsbutChamberlin
e
that compositions for
and b i o t i t e .
much a s
t o determine
(1974) r e p o r t e d
the quartz monzonite average
An37
e
72
Biotite accounts for 4 volume percent. The groundmass
.
.
consists dominantlyof granophric intergrowth of quartz
and orthoclase (20 percent and
42 percent, respectively)
and approximately 20 percent plagioclase laths. Biotite,
apatite,
and
magnetite
are
minor
accessory . minerals.
.
Chamberlin (1974) and the author observed orthoclase
mantling
high
temperature
plagioclase
in
the
above
border facies rocks.
Granite
o f the
and
granite
porphry
Tres Nontosasstock:
occupy
the
central
portion
Granite was observed cutting
andesites of the Spears Formation (see Fig.
11) 'and
according to.Figure 12 it presumably cuts the latite'.
monzonite border facies along the southern margin. Most
of
the granite is distinctly porphyritic with large, pinkish
orthoclase phenocrysts as much 3.8
as cm
coarse
crystallice
matrix
of
quartz
long
and
in
a very
orthoclase,,
but
one sample is equigranular. The granite is usually altered
with
most
samples
containing
a
few
percent
of
oxidized
pyrite.
.In thin section the granite
is typically a coarsegrained
rock
with
textures
allotriomorphic-granular and
varying
from
from
hypidiomorphic-
porphyritic-phaneritic
to equigranular. The dominant constituents are orthoclase
( 5 2 to 60 percent), quartz (16 to 38 percent), and
to
e
e
73
p l a g i o c l a s e (0.4 t o 20 p e r c e n t )
. O r t h o c l a stey p i c a l loyc c u r s
a sc l o u d e d ,a n h e d r a lt os u b h e d r a lg r a i n sa s
much a s 2.5
indiameter.Granophyricintergrowthsoforthoclase
quartz are
common i n a l l samplesof
(1974) reported
and
the g r a n i t e and a r e
i n some (see Fig. 1 3 )
especiallywell-developed
cm
.
Chamberlin
t h a t the a l k a l i - f e l d s p a r of one'sample
(1-14) was s a n i d i n e i n s t e a d o f o r t h o c l a s e .
Quartzoccursin
resorbed grains as
two forms: . as rounded, p a r t i a l l y
much as 4.8 mm- i n d i a m e t e r and as grano-
phyricintergrowthsaveragingabout
a
0.4 mm i nd i a m e t e r .
t h i n s e c t i o n shows e a r l y q u a r t z g r a i n s b e i n g r e p l a c e d
One
by
orthoca
l algo
sb
renaogiunn d a r i e s .
P l a g i o c l a s e i s more 'abundant i n t h e g r a n i t i c d i k e s n e a r
where it o c c u r s a s e u h e d r a l t o
the margin of the stock
s u b h e d r a lg r a i n sa s
t h e s ed i k e s ,
.
much a s 2 - 0 c m long (see Table 1)
it i s t h eo n l yp h e n o c r y s t
much a s 20 volume p e r c e n t .
and a c c o u n t sf o r
the c e n t e r o f . t h e s t o c k , p l a g i o c l a s e a c c o u n t s f o r
.,
volume.Compositions
from the dikes ranged from
(Rittmanzonemethod,
central granite ranged
as
from
3 percent
o fp l a g i o c l a s e
An32 t o An47 andaveraged
1 0 g r a i n s ) ,c o m p o s i t i o n s
An39
Zrom the
from An28 t o An36 '(Rittmanzonemethod,
6 grains)(Chamberlin,1974).
.
H e r e i t - i s mantledbyorthoclase
( C h a m b e r l i n ,1 9 7 4 ) .I nt h ec o a r s e rg r a i n e dg r a n i t e s
o r less of t h er o c k
In
'
.
, 0.5
mm. ,
i g u r e 13. Photomicrographofgranophyrictexturedeveloped
in
g r a n i t e from t h e c e n t r a l p o x t i o n of t h e Tres Montosas stock.
Note , t h ed i f f e r e n t zones of granophyricintergrowths.(X-nicols,
x50 magnification, Q = q u a r t z , K = p o t a s s i u m f e l d s p a r . )
-1-80" no.
75
Table 1. Representative modal analysesof principal rock types
from the Tres Xontosas stock.
Rock type
Quartz
Monzonite'
Sample
27.7
plagioclase
P2n36
X-feldspar
48.7'
38.2
quartz
5.46
0.35
hornblende
3.67
_""
biotite
opaques
2.36
"_"
groundmass
x
Granite
Dike
s-1
TN-22
0 -39
18.7
An29-36
An3 6
59.1
52.6
14.7
pyroxene
totals
Granite
.
.
100.0
collected by R.M. Cnamberlin
holcanic (high temperature)
( l o w temperature)
'plutonic
0.79
""0.08
tr
_""
100.0
_""
"_"
2.26
_""
100.0.
76
Pyroxene i s g e n e r a l l y a v e r y m i n o r c o n s t . i t u e n t , b u t
may a c c o u n t f o r a s
much a s 5 volume p e r c e n t as- s m a l l ( 0 . 1
mm) rounded grains.Hornblende,:magnetite
and r u t i l e a r e
o t h e r minor c o n s t i t u e n t s .
Chamberlin(1974)described
from near the Big John
a hedenbeigitesyenite
mine, on the nor'thwestern margin,
.
,
which h e a t t r i b u t e d t o f e r r o - p o t a s s i c
granodiorite.
metasomatismof
H e describesmagnetitecomprising
volume p e r c e n t o f t h e r o c k
,
,
much asone
t u f f of
Granite
Mountain
occur i n an
mile i n w i d t h around t h e s t o c k .
the c a r b o n a t e r o c k s i n
t ow o l l a s t o n i t e ,g a r n e t ,
p. 73) d e s c r i b e d t h e
The
and a n d e s i t e so tf h eS p e a r s
Formation w e r e baked and r e c r y s t a l l i z e d .
i s bakedand
5 t o 25
i n v e i n l e t s as much as 5 c m wide.
Contact metamorphismandmetasomatism
a u r e o l ea s
the
the
The
Ab0
Ab0
Formation
m a y be a l t e r e d
and epidote.Chamberlin(1974,
metasomatismof
a n d e s i t ei nt h eS p e a r sF o r m a t i o n .
a "turkeytrack"
The r e a d e r i s r e f e r r e d
to the descriptions of these specific formations for
more
details.
Several features observed during
examinations provide clues concerning
*.
t h es t o c k .
mappingand
petrographic
the p e t r o g e n e s i s o f
A s h a l l o wd e p t ho fi n t r u s i o n . i si n d i c a t e db y
reconstruction of the stratigraphic section
s e v e r a lp e t r o l o g i cf e a t u r e s .G r a n o p h y r i c
and born out by
textures,
77
". . .
interpreted by Barker (1970) as resulting from
relatively rapid simultaneous growth of alkali feldspar and
quartz
well
from
a
melt,
developedin the
vapor,
or
granites'
devitrifying
and
some
of the
"
glass.
are
border
facies rocks. The occurrence of high-temperature plagioclase
in
the
border
indicates
'
that
facies
the
and
stock
sanidine
was
in
emplaced
the
granite
near
the
also
surface
and
cooled too rapidly for conversion from high temperature
to
low temperature feldspars.
The
extreme
paucity
of alteration thatcan
that it
was very
a
of
be
dry
hydroxyl
minerals
attributed
magmaat the
to
time
the
of
and
the
stock
lack
suggests
intrusion.
Holland (1972) presents data that suggests siliceous and
intermediate
require
2 to 4 weight
magmas
percent
water
before
biotite and hornblende begin to crystallize. The lack of
these
the
hydroxyl
Tres
minerals
Montosas
indicates
a
low
water
content
in
magma.
Chamberlin (1974) invoked the process of alkali
metasomatism
account
for
the
formation
of
the
granitic
core.
It is evident that such a process occurred, but the
water
for
source at
*
to
the
some
metasomatism
time
after
must
the
have
come
'from
initial'intrusion
of
another
the
magma. Several observations support alkali metasomatism:
1.) granophyric orthoclase replacing early quartz and
78
Table 2. Modal analyses of fresh and metamorphosed tuff of
Granite Mountain showing the chemical changes accompanying
metasomatism.
Hornfels
Fresh
sample no.
average
TM- 27
CR-1-55*
plagioclase
10
-
29-00
00.20
An3 4
An32
20
An2 6
10
K-feldspar
-
0.30
4 - 7
tr
"_
1 - 3
tr
-"
tr
22.00
31.00
tr
biotite
magnetite
* collected
by R.M.
44.00
42.00
5.00
quartz
clinopyroxene
15
Chamberlin
e
79
d i f f e r e n t m i n e r a l o g i e s between g r a n i t i c
plagioclase, 2.)
c o r e and g r a n i t i c d i k e s ,
and 3 . ) t h ee n r i c h m e n to fa l k a l i s
i n t h e metamorphosed t u f f of Granite
Because o f t h e l a c k o f e v i d e n c e f o r
as proposedbyChamberlin
that the granite
(1974),
Mountain . ( t a b l e 2)
.
a high water content
the author does not fhink
is the result of in-situ alkali.metasoma-
tism b u t of d i f f e r e n t i a t i o no f
the p a r e n t magma.
observations support differentiation
1.) t h e e x i s t e n c e o f b a s i c t o
S'everal
from t h e p a r e n t
s i l i c i c f a c i e s , 2.)
magma:
cross-
c u t t i n g r e l a t i o n s h i p s which s u g g e s t s e p a r a t e i n t r u s i o n s o f
'
e
,
eachof
the f a c i e s , and 3 . ) p l a g i o c l a s e becomesmore
toward t h e c e n t e r
--
g r a n i t e An
More data, both chemical
26-36'
along with better
l a t i t e An40-45,
sample c o n t r o l , a r e
monzonite An
sodic
and
37.'
and p e t r o l o g i c ,
needed b e f o r e a
r e l i a b l e p e t r o g e n e t i c model can be proposed for the
Tres
Montosasstock.
Monica Canyon Pluton
The Monica Canyon.pluton i s a small i n t r u s i v e body
located in the extreme southwestern part of the
where it forms a s m a l l , c o n i c a l h i l l a b o u t
diameter.
.a
map a r e a
800 f e e t i n
I t i s probably i n i n t r u s i v ec o n t a c t 'w i t ht h e
basaltofCouncil
Rock, the P o t a t o Canyon Tuff, and t h e
t u f f of Gray H i l l b u t the a c t u a l c o n t a c t i s obscured by
80
I
e
piedmont gravels and talus. The pluton
in
a
N 85'
north-south
direction
.
is intensely sheeted
anda lesser
to
extent
along
a
W direction, The Monica Canyon pluton is significant
h e Gallinas
in that it is the third exposed intrusive t along
uplift
and
beneath
The
may
the
be
an
piedmont
Monica
apophysis afrom
larger
stock
occurring
gray,
fresh,
gravels.
Canyon
pluton
is
a
dark
porphyritic rock of monzonitic composition. It contains
'large plagioclase and fresh, black pyroxene phenocrysts
in a pinkish-gray, sugary matrix.It is a dark, brownishgray
color
In thin
'e
on
weathered
section
the
surfaces.
Monica
Canyon
pluton
is a porphyritic-
phaneritic, hypidiomorphic-granular.rock with
glomeroporphyritic
clumps of plagioclase and clinopyroxine. Plagioclase
as subhedral to
accounts for 15 percent of the rock volume
'euhedral grains 0.14 to 6.0 mm long. They are typically
fresh
and
most
are
rimmed
by
thin
rinds
of potassium
feldspar. The plagioclase compositions range fromAn39 to
An4l with an average
of An44 (Fouque method,3 grains).
Zoning
is
well
developed
with
oscillatory
and
normal
zoning
the most common types. Clinopyroxene accounts for
as anhedra1,to
approximately 5 percent of the rock volume
euhedral grains 0.3 to 1.5 nun in diameter. I t is usually
fresh
but
may
show
incipient
The groundmass is dominantly
plagioclase
and
potassium
to iron
alteration
composed
feldspar
of
grains
oxides.
very
which
small
comprise
81
*
34 and 2 0 percent of the rock volume, respectively.
Clinopyroxene
and
magnetite
each
account
for approximately
10 volume percent. Quartz and apatite occur as minor
accessory
minerals.
Post-Oligocene
Rocks
Piedmont
Piedmont
the
San
Gulch
gravels,
Augustin
graben,
sloping
graben
cover
Gravels
westward
southward
into
eastward the
into
Mulligan
and
about
and
one-third
of the
map
area
and
constitute the oldest surficial deposits.
The maximum
thickness is
.
not
known
but
one
drill
hole one
within
mile
of the eastern exposures of bedrock was still in basinfill sediments whenit bottomed at 1000 feet.
gravels
are
sorted
with
poorly
to moderately
particle
sizes
indurated
ranging
the
Datil
volcanics
and
poorly
from tosand
boulders
as much as four feet in diameter.
The
of
The piedmont
clasts
exposedth.e
inmap
are
dominantly
area,
both
fresh
and altered, but in the northeast corner, clasts
of granite
and the Permian sedimentary rocks are also present. The
gravels
amount
are
of
presently
dissection
being
dissected
sometimes
in
by
streams
with the
excess
of 100 feet.
Because the piedmont gravels represent filling
of grabens
*.
formed
age
by
Basin
and
Range
faulting,
a
'
Miocene
or younger
isinferred.
Basalt of Council
The
youngest
volcanic
rocks
Rock
in Tres
the Montosas
area
are
82
e
a series of thin lava flows capping piedmont gravels to
the
east
and
of the
tuffs
ash-flow
Potato
Canyon
Tuff
on
Cat Mountain. The outcrop areais les's than one-half
square mile within the map area. The lava flows attain
their
maximum
four
individual
The
outcrop
but
this
is
thickness
flows
patterns
due
to the
on
with
Cat
a
suggest
Mountain
total
a
attitudes
thickness
much
of
where
greater
the
there
of
75
are
feet.
thickness
flows to
and
slumping along the edges. A Pliocene age is inferred for
these lava flows.
In hand specimen the lava flows are dense, fine-
e
grained, porphyritic, gray-to-black, vesicular rocks with
plagioclase,olivine,andpyroxenephenocrysts.
The
alteration of olivine to iddingsite is readily visible in
hand specimen. Some of the vesicles are flattened and
drawn
out
and
some
are
filled
with
calcite
and
drusy
coatings of quartz. The rock weathers to a brodnish-gray
color.
Petrographically, the lava flows are qlomeroporphyritic
rocks with plagioclase, olivine and pyroxene as the
only phenocrysts. Plagioclase grains 0.5 to 3.0 nun in
length comprise 10 percent of the rock. Their compositions
range
fromAnq4 to
AnG0
with
an
average
of
An5*
(Rittmann
zone method, 18 grains). Normal zoning is common with some
0.
very small compositional changes (An47
to And4;
core
to rim)..
83
e
In several well-zoned crystals, there was no measurable
compositional
change
from
the
corethe
to rim..
Olivine
constitutes 3 to 6 percent of the rocks as grains
0.1 to
0.6 nun in diameter.
All
of the olivine showssome degree
.
of alteration to iddingsite. The alteration occurs
predominantly
there
is
a
along
grain
gradation
from
boundaries
and
frac.tures
to fresh
iddingsite
olivine
where
near
the
centers of the grains. Several ofthe olivine phenocrysts
are zoned.
Two types of pyroxenes occur in the lava flows,
clinopyroxene and hypersthene. Clinopyroxene'is the most
0
abundant, 1 to 4 percent, occurring as neutral, nonpleochroic grains 0..3 to 3.0 k~
in diameter. Chamberlin
(1974) observed this clinopyroxene to be endiopside
(2Vz = 5 0 '
+
2O,
5 AC = 3 8 ' ) .
1 to 2 percent of the
rock
Hypersthene
as
accountsfor
euhedral
to subhedral
grains
0.3 to 3.0 mm in diameter. It occurs as neutral, slightly
plechroic
grains
often
associated
with
the
clinopyroxene.
The groundmass exhibits a trachytic-intergranular texture
and is composed of microlites
of,plagioclase (75 percent),
clinopyroxene (10 percent), magnetite (10 percent) and
minor apatite (1 percent) and iddingsite (5 percent) .
*
The
groundmass plagioclase has a composition
of An47 (Fouque
method, 2 grains).
84
Samples
were
collected
from
the
four
flows
on
Cat
Mountain for chemical analysis (X-ray fluorescence and
3
atomic absorption). The results are "listed in Table
and
the
normative
compositions
are
listed
in
Table
4.
On traditional plots, such as Kuno's (1960)
A1 0 vs.
2 3
Na 0 + X 0, the lavas plot as alkali basalts. However,
2
2
on Church's (1975) triaxial variation plot, they fall
on
the line of intersection of the basalt and ande.site fields
(see Fig.10) indicating that they are not true basalts
or andesites. Based on the classification scheme of
Rittmann (1952), these lava flows are classified.as
a
alkali-trachyte
to
trachy-andesites.
The values of the solkdification index [Kuno, 19G0,
+
+ FeO + Fe
.O + Na 0 + K O ) ]
2 32
2
are low (21 to 24) indicating a high degree of differentiation.
defined as (SI
An
additional
MgO) x 100,"gO
indication
of
differentiation
is illustrated
in Figure 15. Mere.the average plagioclase phenocryst
compositions
section.
is
A
are
plotted
for
each
flow
vs. heighth in
the
distinct trend toward more calcic plagioclase
indicated
by
the
first
three
flows
with
the
fourth
falling off the trend line. The trend of the first three
samples
is
the
of a
reverse
normal
differentiation
trend.
and can be explained in two ways: tapping a differentiated
e-
magma
chamber
in
the
middle
and
draining
upward
or
by
85
Table 3 . Chemical analyses of the basalt of Council Rock in
weight ,L percent. * indicates samples analyzed by atomic absorption, :"indicates samples analyzed by weight loss i n a furnace,
and the remainder were analvzed by X-ray fluorescence.
TMC-1-1 TMC-1-3TMC-1-2
Si02
54.43
53.46
54.41
54.91
Ti02
1.45
1.46
1.43
1.44
A.1203
14.48'
15.51
15.63
15.48
*FeO
3.02
3.09
2.73
2.33
*
Fe203
5.76
5.99
6.36
6.22
MgO
5.07
4.93
5.17
4.62
CaO
6.76
7.28
6.83
6.76
0.12
0.12
0.10
0.13.
K20
2.06
2.12
2.11
2.32
Na20
4.91
5.93
0.20
0.94
0.09
0.78
0.47
0.14
,o.
MnO
5;28
TMC-1-4
'
#H20+
'H20-
totals
0.65
99.75
20
100.011 0 1 . 4 8
100.38
3
3.90
86
T a b l e 4 . N o r m a t i v ec o m p o s i t i o n s
of t h e b a s a l t of C o u n c i l
Rock c a l c u l a t e d a c c o r d i n g t o t h e
C.I.P.W..
system f r o m t h e
i n T a b l e3 .V a l u e sa r e
i n w e i s h tp e r c e n t .
c h e m i c a la n a l y s e s
12.79
Normative
Mineral
TMC-1-1
ilmenite
2.74 2.74 2.74
TMC-1-2
TMC-1-3
TMC-1-4
13.34
8.90
17.28
15.98
19.22
2 -74
1 2 . 2 o3 r t h o c l a s e
42.71
albite
6.67
6.03
1.44
anorthite
magnetite
hematite
2 0 . 9 5d i o p s i d e
olivine
2.10
3.01
3.85
1.89
nepheline
4.12
0.11
0.31
4.29
100.00
100.04
98.67 98.99 totals
'
87
WEIGHT PERCENT
-
....
. .
.~
..
Figure 14. Classification of the basaltof Council Roc!: after
the method of Church (1975). The solid circles represent the
chemfcal compositions of the basalt of Council Xock.
I-
o
e
8G-
7°C-1-4
W
W
LL
Y
W
cn
60"
8
TMC-1-3
ILI
>
0
m
U
w
40"
TMC-1-2
8
4
i-
tn
I
L)
W
G
z
-X
8
20"
TMC-1-1
0
[y.
a
a
4
*
045
I
I
1
I
50
55
60
65
I
PLAGIOCLASE
COMPOSlTfON
Figure 15. Variations OF t'7.s nlep5mclase compositions in .the
basalt of Council Xock.
88
progressive partial meltingof the crust. Because the
solidification
index
indicates
a
high
degree
of
differ-
entiation, the first explanation is probably more
reasonable.
Lipman (1970) suggested that the Miocene-Pliocene
period
of
basaltic
eruptions
was
approximately
coincident
in time with the formation of the Rio Grande rift. However,
Chapin and Seager (1975) showed that Lipman's model was
too
simple
and
calc-alkalic
that
transition
volcanismto basaltic
a 10 to 15 million
a
the
year
interval
from
middle
Cenozoic
volcanism
occurred
over
beginning
approximately
30 m.y. ago. Chapin (1971-b) proposed a bifurcation of
the Rio
Grande
rift
southwestward
along San
theAugustin
lineament into Arizona. These ideas suggest that the
basalt of Council Rock
is a product of crustal extension
along
the
the
parent
San
Augustin
magma
of
lineament not
anda
the
Datil
product
volcanic
field
of
as
implied
Chamberlin (1974). The compositional differences between
the
Niocene-Pliocene
and
Oligocene
andesites
and
basaltic-
andesites support this interpretation.
Quarternary
Deposits
Talus and Colluvium'. In several areas the slopes are
steep enough to allow significant accumulations
of angular
debris derived from mass wasting
of.resistant units overlying
by
89
-
less resistant, slope-forming units Extensive talus
deposits
Hells
are
Mesa
Mountain
formed
Tuff
where
on
Tres
overlies
a
Montosas
the
resistant
Spears
where
the
resistant
Formation Cat
and
basaltic-andesite
caps
on
older
by
ash-flow tuffs. The talus slopes have been stabilized
the
growth
being
of
eroded
underlying
by
debris
by
on
small but
treesinplaces
gullies
which
expose
they
are
patches
of the
units.
is
mapped
thin,
where
contacts
deposits
of angular
are
rock
slopes.
An extensive
S a .
stratigraphic
unconsolidated
gentler
Eolian
and
small
rock
Colluvium
obscured
grasses
veneer of windblown sand
surrounds Tres Montosas and the Tres Montosas stock. Along
the
northeastern
boundary
sand
depressions. , T h e yellowish
eastward
by
prevailing
to
fills
beige
southwest
arroyos
and
sand
was carried
winds
low
north-
blowing.
. .
from
the
San Augustin Plains. A large pluvial lake occupied these
plains
from
in
the
Wisconsin
times
desication
and
Areas of eolian
photographs
abundant
by
sand
their
growths
of
and
much
deflation
of the
are
light
easily
of
lake
and
basin.
distinguished
on aerial
colorbyand
the
juniper
the
wassand
derived
pinon
much
more
treeson than
the
piedmont surfaces. Where there are thick deposits
of sand
filling
arroyos,
small
stands
of
large
Ponderosa
pine
grow. Evidently the sand can retain enough moisture to
trees'
90
support these large trees.
Alluvium.
recent
stream
In this study alluvium is used to indicate
deposits
in
the
larger
drainages
and
deposits
of deeply weathered gravels which occur
as valley fill. The
stream deposits consist dominantly of sand and gravel. When
they occur together, alluvium generally fills channels cut
into
the
during
eolian
dry
sand,
but some
periods.
sand
blows
into
drainages
91
STRUCTURE
R e q i o n a lS t r u c t u r e
A detailed regional structural analysis
New Mexico h a s y e t t o
be undertaken.
physiographic study, included the
MexicanHighland
of west-central
. H u n t (1974); i n h i s
Magdalena a r e a i n t h e
section of the Basin
and Range province.
The Tres Montosas a r e a i s s i t u a t e d on a north-northwesttrending uplifted block
Gulchgrabenand
boundedon
on t h e w e s t b y t h e
area has been affected by
the east by the Mulligan
North Lakegraben.
The
two major periods of deformation:
Laramide (Late Cretaceous and e a r l y T e r t i a r y )
and middle t o
l a t e Cenozoic.
DuringLaramide
l i f t e d anddeformed
t i m e , theprevolcanicrocks
i n t o a s e r i e s of l a r g e open f o l d s and
t h r u s t b e l t s (Kelly and Wood, 1946;Tonking,
investigators attributed the
sive forces,
1957).
Early
Laramide s t r u c t u r e s t o compres-
b u t Eardley(1962)attributed
expansion of acolumn
were up-
them t o t h e
of magma due t o p a r t i a l m e l t i n g .
Coney (1971) viewed t h e d e f o r m a t i o n a s t h e r e s u l t o f
com-
p r e s s i v e stresses due t o "massivewestwardcratonic
u n d e r t h r u s t i n g " andChapin(1974-a)
r e p o r t e d t h a t Laramide
s t r u c t u r e s were c h a r a c t e r i z e d b y e a s t - n o r t h e a s t d i r e c t e d
compression.
I
92
a
time, t h e Laramide u p l i f t s were beveled
GuringEocene
by a ne r o s i o ns u r f a c e( E p i s
The Magdalena area i s s i t u a t e d
t h ec o r e so ft h eu p l i f t s .
on t h e n o r t h e r n f l a n k
the north into the
older rocks exposed
andChapin,1975)whichexposed
of s u c ha nu p l i f t .E r o s i o n
Baca b a s i n (Eocene) w i t h p r o g r e s s i v e l y .
t o the south.
During l a t e Eocene-earlyOligocene
trending faults
(Chapinand
was to'.
time, west-northwest
were a c t i v e a l o n g t h e C a p i t a n l i n e a m e n t
o t h e r s ,1 9 7 4 ) .T h i s
zone hasbeen
documented
i n t h e Magdalena Range (Krewedl, 1 9 7 4 ; Blakestad,1976),
i n t h e map a r e a , and i n t h e Datil Mountains'(Lopez,1975).
I
D u r ienagr l y
t o middle
Oligocene
e p i c l a s t i c apron of
l a t i t i c d e t r i t u s andvoluminousasharea t o d e p t h s of 4 0 0 0
flowtufferuptionscoveredthe
t o 5000 f e e t .
time, a t h i c k
Some of t h ea s h
trending paleovalleys which
flows f i l l e d n o r t h e a s t -
were s t r u c t u r a l l y c o n t r o l l e d
by t h e Morenci-Magdalena lineament(Chapinand
1974).
T h i si g n i m b r i t ep l a t e a u
was thenbroken
broad,north-trendingextensionalfault
others,
by a
zonewhichcon-
t r o l l e d t h e emplacement of numerous s t o c k s a n d d i k e s
( 2 8 m.y.,
Weber and Bassett, 1963)and"ushered
Basinand
Range epoch"(Chapin,
1974-a).
Late Cenozoic,north-trendingfaultsrelated
i
developmentofthe
in the
to the
Rio Grande r i f t (Chapin, 1971-a) were
. .
93
superposed on t h e e a r l i e r developed s t :ructur:a1 p a t t e r n s .
Southwestbifurcationofthe
Kio Grande r i f t a l o n g t h e
I
c i t . ) developed i n t h e l a t e
San AugustinPlains(op.
"
Miocene.
The c a u s e o f r i f t i n g
Eardly ( 1 9 6 2 ) andChapin(op.
northwestpulling
has b e e n ' i n t e r p r e t e d by
c i t . ) as t h e r e s u l t
"
of t h e
away of the Colorado Plateau.
L o c a lS t r u c t u r e
map area are p a r t of a complex
Structures in the
systemofseveralperiodsoffaulting,andfolding.
Figure 1 6 i l l u s t r a t e s t h e r e l a t i b n s h i p
ofthesestructural
t r e n d s t o major s t r u c t u r e s of t h e Magdalena r e g i o n .
e f f e c t s of t h e M t . Withington cauldron near
are r e a d i l y a p p a r e n t a s
trending fault zones
C a t Mountain
are t h e major north- and northeast-
which p r o b a b l y r e p r e s e n t older
basement s t r u c t u r e s t h a t h a v e
oneofthe
The
been r e a c t i v a t e d d u r i n g
several p e r i o d s of deformation.
The major
periodsoffaultingoccurredduringtheOligoceneand
Miocene, b u t t h e r e
i s i n d i r e c t . e v i d e n c e for f a u l t i n g as
o l d as t h e Permian.
several
A structural interpretation must.explain
anomolous f e a t u r e s of t h e map area:
S.) t h e
r e l i e f compared t o t h e h i g h s t r u c t u r a l
2.)
theabrupt
l o w topographic
relief of the
area:
eastward d i p s o f t h e . O l i g o c e n e v o l c a n i c r o c k s
towardtheMulliganGulchgraben;and
3 . ) thehightopographic
r e l i e f of t h e Tres Montosaspeaks.Figure
1 7 shows t h e low;
94
.
'C
e
F i g u r e 16. Generalizedsketch map o f t h e m a j o r s t r u c t u r a l f e a t u r e s
i n t h e Magdalena r e g i o nw i t hr e s p e c tt ot h et h e s i s
area. Exposed
o r i n f e r r e ds t o c k sw i t h i nt h et h e s i s
area: 1. Tres Montosas;
2. Cat Mountain; 3. Monica Canyon.
(Modified
from
NMB"R Open
F i l e R e p o r t No. 4 7 ) .
t
"
. .
.* .*
-
.~
.
.
.~
~
.~
. . ~
~~
:I
:
. . . .. .
. . .
Figure 17. Overall view of the thesis area from the top o
Tres Montosas. Note the low, undulating topography between
Tres Montosas and Gray Hill
(GH) and Cat Mountain ( ' 3 ) .
The Magdalena Mountains are in the left background and
San Mateo Mountains are in the right background. (Dashed
line = outline of.the thasis area,
CRF = southern.extensionof
the Council Rock fault.)
e
e
undulating
and
Cat
96
topography
Mountain
between
and
Gray on
Hill
the
Paleozoic
The
in
theAb0 Formation
accompanied by faulting
The
the
Madera
north
south.
conglomerates
suggests
occurred
Limestone
existenceof large
Montosas
on the
Faults
occurrenceof limestone
breccias
with
Tres
at
that
the
exposed
on the
limestone
and'quartzite
uplift
possibly
time
of Ab0 deposition
uplifted
boulders
b s much
block.
as
four
Ab0 indicates a nearby source. A
feet in diameter) in the
single
e
azimuth
N 8 5 O W with
measured
on inbricated
the
imbrication
cobbles
indicating
yielded
transport
from
east'
to west.. The elongationof the uplift was probably northsouth,
perhaps
outlined
by an
older
fault
system
which
influenced the late Cenozoic north-trending faults.
Oligocene
Tectonics
Late Eocene-Early Oliqocene Transverse Faults.A
N SO0 W-trending
fault
zone
along
the
Capitan
lineament
was
-active during the late Eocene-early Oligocene.This zone
was mapped by Krewedl (1974) and Blakestad (1976) in the
Magdalena
Range
as
a
down-to-the-south
fault
zone
with
about
1400 feet of displacement. The fault displaces the Spears
II
Formation
The
but
not
evidence
the
for
Hells
this
Mesa
zone
in
Tuff.
the
map
area
is:
its
97
0
expression
on
the
aeromagnetic
map inand
stratigraphic
relationships from surface and drill hole data.
In the
Banner
drill
carbonaceous
hole,
only
two
and
the
upper
of
so movement
indicates
Nipple
lower
siltstones
but
The 'tuff
the
miles
Nipple
Formation onrests
thought be
to the
to
Spears
Spears
the
north
Formation
Mountain
Formation
the
tuff Mountain.
of Nipple
rest
does
Sandia
Ab0the
Formation.
on
not
zone
thicken the
across
must have predated deposition
of thetuff.
that
at
Mountain,
the
the
time
of deposition of the
displacement
across
This
tuffof
the
Capitan
lineament was approximately 1300 feet down-to-the-south.
a
If
the
sediments
in
age,
then
in
there
the
must
drill
be
hole
are
approximately
assumed
be Mesozoic
to
5000
of downfeet
to-the-'south displacement, an unrealistic figure
in terms
of the available data.
The
been
westward
documented
Along
its
in
and
the
Datil
projection
he mapped
of north-trending
ships
continuation
faults
of
the
Capitan
lineament
has
-
Mountains
by Lopez (1975)
an
together
direction
of displacement
abrupt
westward-curving
with
similar
along
faults
age
relat'ion-
paralleling
this zone.
Development of the Mt. Withinqton Cauldron. The
development of the
Mt.
Withington
cauldron
has been
discussed
by Deal (1974) andDeal and Rhodes (in press). The tectonic
98
effects of the
cauldron
small but the
southern
on
the
part
of
Magdalena
area
a whole
as
the
Montosas-Cat
Tres
are
Mountain
area has been gxeatly affected. The approximate position of
the cauldron marginis shown on Plate I and in
The
.
initial
doming
of the
cauldron
resulted
Figure18
in
the
.
develop-
ment of radial and concentric faults which are well-developed
in the Cat Mountain mining district. Most of the faults
appear
to
be
radial
faults
Doming
beginning
e
approximately
have
was
with
been
contemporaneous
but
some
of
the
displaced
by concentric'faults.
followed
by the
A-L Peak
the
eruptionof ash-flow
Tuff
at
31m.y.
and
tuffs
ending
with
the Potato Canyon Tuff at m.y.
30 (Smith and others; in press).
Deposition of the A-L Peak
Potato
Canyon
Tuff
was
Tuff,
tuff
controlled
of
in
Gray
part
Hill,
by
and
northeast-
trending paleovalleys (Woodward, 1973; Simon, 1973; Blakestad,
1976; this
paper)
.following
the
Morenci-Magdalena
lineament
(Chapin and others, 1974), an assumed northeast-trending
basement structure. Chamberlin (1974) described a northtrending
of the
fault
Hells
After
*
in
the
which
Mesa
and
deposition
south-central
the
tuff
Tuff
and
of
Granite
preserved
was
of
contemporaneous
lower
A-L Peak
the
part
Mountain,
them
as
a
the
the
deposition
Tuffs.
ash-flow
of
with
map
Hells
tuffs,
a graben
area
Mesa
formed
which
downdropped
Tuff,
and
A-L Peak
north-trending
outcrop
belt.
99
. .
1/2
0
I
MILES
EXPLANATION
-"---
.........-.
Cauldron
'
,
Martrin
1All.r DID! ond Rt#&,
ID$rei.)
Radial Foulls
Ccncenlric FOUIIS
Transverse Faults
100
The
graben
probably
formed
collapse
of the
during
cauldron,
similar to the formation of grabens around
the Creede
caldera (Steven and Ratte,1960).
A l s o associated with
cauldron coll.apse are a series of transverse faults (Scbwartz,
1968) which
cut
the
angles (see Fig. 18).
collapse
Potato
radial
Tuff;
concentric
faults
at high
Deal (1974) believes that catastrophic
didnot occur
Canyon
and
until
this
after
the
eruption
of the
interpretation
agrees
with the
timing cited above. Dike intrusion and mineralization
in
tine Cat
Mountain
district
followed
and
utilized
radial
and
transverse faults as passageways.
Northest-Trending Faults. Late Oligocene faults with
a
a
northeast
trend
Highway 60 where
are
the
especially
Hells
evident
Mesa
T u f f has
just
been
south
of
preserved
in
a northeast-trending graben less than
one.mile wide. Northin attitude
trending faults crossing this graben show changes
from
N 30'
W south
(1973) mapped
projection
of
a
of
the
zone
to N 20" E north of it. Simon
northeast-trending
'this
structure
graben
which
along
the eastern
controlled
deposition of
the tuff of La Jencia Creek. Differences
in the level of
erosion between these two areas accounts
for the
different
rock associations. Chapin (oral commun.,.1975) suggested
that parts of the Spears Formation (the tuffs of Nipple
*
Mountain
and
Granite
trending
paleovalleys
Mountain)
in
the
were
controlled
Magdalena
area
by
and
northeastChamberlin
101
( 1 9 7 4 ) mapped n o r t h e a s t - t r e n d i n g f a u l t s
of Oligoceneage
in
t h e Council Rock area.
same system,
If a l l of the above faults represent the
then the northeast faulting
SpearsFormation
( 3 7 m-y. ),
began d u r i n g d e p o s i t i o n o f t h e
.
The grabensouthof
i s o f f s e t by n o r t h - t r e n d i n g f a u l t s
Highway 60
which i n t u r n are o f f s e t
by s l i g h t l yy o u n g e r ,n o r t h e a s tf a u l t s .V e i n s ,w h i c h
a p p a r e n t l yp o s t d a t et h ei n t r u s i v e s
a l o n gt h i s
( 2 8 m.y.1
are n o t o f f s e t
were probably
z o n e .T h e r e f o r e ,t h e s ef a u l t s
activeduringdepositionoftheOligocenevolcanicrocks
b u t ceased
a f t ej ur s t
2 8 m.y.
This northeast trend
..
may b e related t o n o r t h e a s t -
t r e n d i n gb a s e m e n ts t r u c t u r e s .' T h eP r e c a m b r i a ni nt h e
'Magdalena a r e a (Woodward, 1 9 7 3 ; Chamberlin, o r a l commun.,
1 9 7 5 ) and i n t h e S o u t h w e s t ' a s
a whole, has a predominant
were probably
n o r t h e a s t e r l yg r a i n .T h e s es t r u c t u r e s
reactivatedintheOligocene,perhaps
from t h e i n t r u s i o n o f b a t h o l i t h s
by i n c r e a s i n g p r e s s u r e
which must have preceeded
t h e Oligocene volcanic activity.
North-TrendingFaults.During
t h e l a t e Oligocene,
N l o a W high-angle faulting dominated the beginning of
r e g i o n a le s t e n s i o n .
Chapin ( 1 9 7 4 - a )
t o changesfromconvergent
west edgeofthe
a s c r i b e st h i sf e a t u r e
t o transform motion along the
American p l a t e .T h i sb r o a d
zone ofnorth-
trending faults broke the ignimbrite plateau and controlled
102
the emplacement of numerous stocks and dikes.
The zone
extends
and
from
from
northward
Tres
Ryan
to
Montosas
Hill
eastward
Canyon
Riley,
an
in
area
to
the
the
South
about
20 miles
Ladron
Baldy
Mountains
quadrangle
wideby 40 miles
long. The timing of this period
of faultingis well-bracketed
by radiometric
dates
on
prior to faulting and
on
the
youngest
several
ash-flow
intrusives
tuff
emplaced
deposited
along
the faults (see Table5 ) .
Chamberlin (1974) discussed thestructurerelated
to
the intrusion of the Tres Montosas stock.
T'ne main faults
cutting
e
the
Tres
Montosas
peaks
were
first
activated
as
radial faults during intrusion of the stock. Several other
radial
are
and
not
concentric
as
well
faults
expressed
occur
as
in
around
the
the but
stock
they.
Council
Rock
area.
Foldinq. Along the eastern boundary
of the map area,
a narrow
bandof steep
rocks, as much
as
one
eastward-dipping
mile
wide,
Oligocene
extends
from
through Cat Mountain,a distance of about20 miles:
simiiar
occurrence
eastern
edge
of
of
the
dipping
San
volcanic'rocks
Mateo
volcanic
Abbey
Springs
A
along
the
Mountains
be amay
continuation
of eastof this zone. Chamberlin (1974) defined this zone
ward dips as the anticlinal crest
of an
e
east-sloping
foldandassignedalateOligoceneagetoit.
its
formation
to
the
widespread
intrusion
monoclinal
He attributed
of
magma
along
Table 5.
R a d i o m e t r i cd a t e sw h i c hb r a c k e tt h e
Unit Dated
age oftheOligocenenorth-trending
Age
faults.
Source
N i t t stock
26.0 + 1 . 4 m. y.
Monzonite d i k e n o r t h e a s t
of Riley
26.1
+
1 . 2 m. y.
Chapinand
Anchor Canyon s t o c k
26.3
+
-
1 . 4 m.
Weber and Bassett, 1963 (E;-Ar)
Water Canyon s t o c k
30.5 + 1 . 2 m. y.
Chapin
and
Lucero Mesa l a c c o l i t h
27.1 + 1.0 m. y.Chapin(unpublisheddate
y.
PERIOD OF NORTH-TRENDING
P o t a t o Canyon Rhyolite
(youngest ash-flow
t u f f involved in i n t r u s i o n and f a u l t i n g )
30.3
+
Weber and Bassett, 1963 ( K - A r )
1 . 6 m t y.
o t h e r s , 1974 (I<-Ar)
o t h e r s , 1974, (K-Ar)
) (K-Ar)
FAULTS
Smith
and
others,
( i n press)
(Fission track)
0
104
north-trending basement fractures (see Fig.
19).
Chapin
and Seager (1975) suggested that this monoclinal fold forms
the
western
boundary
of the
basin
into
which
the
Peak Andesite and Popotosa Formation
were deposited.
why the
explain
'would
observed
on
The
the
La
Jara
Gallinas
conspicuous
Peak
La
Jara
This
Andesite not
has
been
uplift.
north-south
alignment
of intrusives
(see Fig.16) supports the idea
of formation of the
monocline
by magma intrusion. Three plutons are exposed and three
more
are
inferred
over
the
distance
from
Abbey
Springs
to
Cat Mountain. The greatest intensity
of alteration in the
a
study'
is
also
area
and.
in
the
concentrated
Basin
Council
along
and
the
Range
Rock
area
(Chamberlin,
1974)
'
moncline.
Faults
Lonqitudinal Faults. If the formation of fanglomerate
deposits
is
a
valid
criterion
for
evaluating
onset.of
the.
Basin and Range faulting, then the first such evidence
in
the
Magdalena
25 m.y.
to
the
area
is
the
unit
of Arroyo
Montosa
dated
(Simon, 1973). Faulting of this period is related
formation
of
the
Rio
development of north-trending,
Grande
rift
block-faulted
and
resulted
in
the
horsts
and
grabens which dominate the present topography. The
e
'.
at
Magdalena-Bear Mountains and Gallinas Ranges were uplifted
105
A
.
. WEST
M I D D L E OLIGOCEN
.
EAST
.
./
.
I
GALLINAS
.
SPRINGS
FAULT
MIDDLE MlOCENE
R
VIEST
I
Figure 19. Diagrammatic
approximate latitude of
rangle) illustrating tke
cline along the eastern
Cnamberlin, 1974).
'
.
EAST
-i
east-west cross .section
at the-.
Callinas Canyon (Gallinas Peak quaddevelopment of the intrusive nonoedgeof the Callinas uplift( f r o a
e
and t h e La Jencia, Mulligan Gulch,
andNorth
Lake grabens
were downdropped d u r i n g t h i s p e r i o d .
The major f a u l t o f t h e B a s i n
and Range p e r i o d i s t h e
southern extension of the Council
1974)whichforms
uplift.
Rock f a u l t (Chamberlin,
t h e e a s t e r n boundary of the Gallinas
w i t h l o w topographic
I t i s marked by a d i s t i n c t s c a r p
r e l i e f .T h i sf a u l te x t e n d st h ee n t i r el e n g t ho f
.the map
a r e a and probably forms the eastern margin of the
SanMateo
Mountains ( s e eF i g . 1 6 ) .
Along mostof
older rocks
e
the Council
Rock f a u l t , O l i g o c e n e
crop out a g a i n s t piedmont g r a v e l s b u t a t
Mountain,Oligocenerocksareexposed
and
Cat
on b o t hs i d e s .
Here,
rocks of the upper Spears Formation are juxtaposed against
the andesite of
Tuff: this gives
feet.
LandavasoReservoirand
the P o t a t o Canyon
a m i n i m u m displacement i.n
A drill holejusteastofthe
excess of 2000
map a r e a and southof
Highway 60 was s t i l l i n g r a v e l s when it bottomed a t 1000
feet.
Chamberlin(1974)
feet in the
e s t i m a t e d a throw .of 800 t o 1500
Coun'cil Rock a r e a andChapin
r e p o r t s a small exposure of the
miles southeast of Council
A-L
( o r a l commun.,
Peak Tuff about
1975)
two
Rock, n e a r the c e n t e r o f t h e
graben.Theseobservationssuggest
e
.
t h a t t h ed e p t ho f
the
Mulligan Gulch g r a b e n v a r i e s c o n s i d e r a b l y .
Along the western boundary of the
map area, numerous
107
s h o r t e r segmentsof
a similarnorth-trendingfaultdrop
down t o t h e w e s t i n t o t h e
blocksofthevolcanicrocks
North Lake graben.
The g r e a tt o p o g r a p h i c
T r e s Montosaspeaks
i s due t o r e a c t i v a t i o n of l a t e Oligocene
and t o westward t i l t i n g o f g r e a t e r t h a n
north-trending faults
.normal thicknesses
relief o f the
of t h e H e l l s Mesa Tuff.
The North Lake g r a b e n e x e r t s
a greater influence
d r a i n a g e and s t r u c t u r e of t h e G a l l i n a s u p l i f t t h a n
on t h e
the
This i s i l l u s t r a t e d i n s e v e r a lp l a c e s
Mulligan G u l c h graben.
where the westward d r a i n a g e p e n e t r a t e s t o w i t h i n o n e m i l e of
theMulliganGulchgraben.IntheCouncil
0
from t hien f l u e n coteh
fe
.
M t . Withington
cauldron,
normal
.
f a u l t s w i t h their west sides
Lake.graben.
down toward the North Lake
two miles o f t h e M u l l i q a n
graben extend to within
g r a b e n .L i t t l e
Rock a r e a , away
i s known aboutthedepthof
Gulch
f i l l inthe
North
A completesectionoftheOligocenevolcanic
rocks was encountered i n t h e Sun No. 1 d r i l l h o l e d r i l l e d
ona
s t r u c t u r a l s a d d l e between the North Lake and San Augustin
grabens.
A minimum displacementof
s a d d l e and t h e G a l l i n a s u p l i f t
3700 f e e t b e t w e e n t h i s
i s i n d i c a t e d , so displacement
i s probably much g r e a t e r i n t h e d e e p e r p a r t s
The occurrence of
e
cutting the alluvium
05
the graben.
young n o r t h - t r e n d i n g f a u l t s c a r p s
on t h e La J e n c i a P l a i n i n d i c a t e s t h a t
thisnorth-trendingBasin
and Range f a u l t i n g i s s t i l l a c t i v e .
e
0
a
The b a s a l t of Council
108
Rock i s a l s o d i s p l a c e d by n o r t h - t r e n d i n g
f a u l t s i n the Mulligan
Gulch graben.
T r a n s v e r s eF a u l t s .L a t eC e n o z o i cf a u l t s
t r e n d were observed by
w i t h a northeast
Brown (1972) andChapinand.
others
(1974) i n t h e Magdalena a r e a where t h e y form a g r a b e n f o u r .
to six miles
wide w i t h a v e r t i c a l o f f s e t a p p r o a c h i n g
0.8 miles.
f e e t and l e f t - l a t e r a l o f f s e t o f
2000
Chapin (1971-a)
..
interpreted this
Rio
Grande
zone a s a s o u t h w e s t b i f u r c a t i o n
r i f t alongechelongrabensextendingthrough
t h e San AugustinPlains
*
of the
and i n t o t h e Blue Range of Arizona.
There i s l i t t l e expression of t h i s senseof-movement i n t h e
map a r e a .
Chamberlin(1974),however,
f a u l t s i n the Council
mapped t r a n s v e r s e
Rock a r e a w h i c h w e r e e x p r e s s e d b y
downfaulted piedmont gravels and
a northeast-trending
b a s a l t i c d i k e along the n o r t h e r n h a l f o f t h e
stock.
The grabendefinedby
Brown (1972)andChapinand
w e s t and'passesto
others(1974)widensconsiderablytothe
the north
and south of the Council
areas respectively,
andon
Rock and T r e s Montosas
i n t o the San A u g u s t i n P l a i n s .
The south bounding fault passes through
t h e San Mateo Mountainsand
Tres Montosas
E s t a l i n e Canyon i n
the north-bounding fault passes
through Deep Well Canyon a b o u t t h r e e m i l e s n o r t h o f C o u n c i l
e
Rock (see Fig. 16).
Chapinand
i
others(1974)inferred
a maximum a g e o f l a t e s t
.
109
Miocene f o r t h e developmentof
Rhyoliteflows,dated
t h e SanAugustin
a t 1 4 . 3 m.y.(Weber',
from t h e dome a t Magdalena Peakandflowed
If the graben
hadformed
bifurcation.
1971), , e r u p t e d '
to the south.
e a r l i e r , t h e r e wouldhavebeen
depression t o t h e n o r t h i n t o
a
which t h e s e l a v a s h o u l d h a v e
flowed.
The p o s i t i o n s o f t h e g r a b e n b o u n d i n g f a u l t s
with other structural
elements h e l p t o e x p l a i n why the
Gallinas uplift has such
topographic relief
t h e i n i t i a l Basinand
high andformed
a high structural
r e l i e f b u t low
wnen compared t o t h e Magdalena Range
and Bear Mountains.
e
combined
The G a l l i n a su p l i f tw a s ' f o r m e dd u r i n g
Range f a u l t i n g .
It remained s t r u c t u r a l l y
the western margin of the-basin into. which
t h e L a Jara Peak Andesite and Popotosa Formation
d e p o s i t e d .C o n s i l e r a b l ed e b r i s
from t h eG a l l i n a su p l i f t .
i n thePopotosa
were
was d e r i v e d
The Nagdalena Range underwent
further uplift during deposition
of the Popotosa Formation
and again during the breakup of the Popotosa Basin
when
tine range was u p l i f t e d 3 0 0 0 t o 5 0 0 0 f e e t w h i l e t h e G a l l i n a s
u p l i f t remained r e l a t i v e l y s t a b l e .
The G a l l i n a su p l i f t
was a l r e a d y a s t r u c t u r a l l y complex b l o c k ( f o r m a t i o n o f t h e
Mt.
Withingtoncauldron,
l a t e Oligocene f a u l t s , and s t o c k
i n t r u s i o n s ) w h i c hf a v o r e dr e l a t i v e l yr a p i de r o s i o n .
e
Superimposing t h e SanAugustin
added f u r t h e r t o the complexity
relief.
I
'
bifurcation.on the uplift
and t o t h e low t o p o g r a p h i c
110
ECONOMIC GEOLOGY
map a r e a i s r e s t r i c t e d t o a zone
Mineralization in the
aboutone
mile widealongtheeasternmargin.
zation has been divided into
The m i n e r a l i -
two s e p a r a t e c a t e g o r i e s t o
f a c i l i t a t ed i s c u s s i o n :v e i nm i n e r a l i z a t i o ni nt h eC a t
Mountainmining
d i s t r i c t and i n t h e Abo Formationand
seminated mineralization
d i s t r i c t is a small district
1900 f o rg o l d .
I t c o n s i s t s of two i n t e r -
secting vein trends with the
most e x t e n s i v e w o r k i n g s l o c a t e d .
thsaverage
58 'samples was $6.85 p e r t o n ,
of t h e a s s a y s o f
but he collected samples
assayed 0.06 oz.gold
.
a t t h e S i x t y copper prospect.
The CatMountain.mining
operatedaround
dis-
from under the stamp
and 0.46
02.
m i l l which
s i l v e rp e rt o n .
A 20-
stamp m i l l w a s e r e c t e d i n 1 9 0 2 b u t was s h u t down i n 1903.
J o n e s( 1 9 0 4 )r e p o r t e dt h a tt h eo r e
was mainly a r e f r a c t o r y
o r e and t h a t t h e o p e r a t i o n c e a s e d
r e c o v e rt h em e t a l s .
The v e i n s i n
N o p r o d u c t i o nf i g u r e s
andaround
worked a t a b o u t t h e
the
district indicate that
(Lasky,1932).
Ab0
are a v a i l a b l e .
Formation w e r e presumably
same t i m e a s t h e C o u n c i l
t h r e e miles t ot h en o r t h .R e p o r t s
e
due t o the i n a b i l i t y t o
Rock d i s t r i c t
from t h eC o u n c i l
Rock.
some of the ore averaged $250 per ton
The w o r k i n g sa r er e p r e s e n t e db yt h r e es h a f t s ,
111
a short adit, and several prospects. No production figures
are available.
The. Sixty prospect
is located in Sec.6, T. 3
just south of Highway
60.
with
mineralization
S.,
R.
5 W.,
It is a small oxide copper body
disseminated
in
highly
silicified
and
fractured tuff of Nipple Mountain.The predominant minerals
are malachite, chrysocolla, and mottramite; minor chalcocite
and azurite .have been observed. The workings consist
of a
shaft
and
several
open
cuts,
one
of
which
is
about
100
yards
long. Figure 2 0 shows the mineralization in one of these
open cuts. Total production was356 tons which averaged
.e
3.01 oz. silver per ton and
0.81 percent copper. As much
as 1.33 percent
lead
and
a
trace
of
gold
have
been.
reported
(Thurmond, 1951).
Mineralogy
Veins
Although the veins are located in two separate districts,
they
are
similar
mineralogically
and
will
be
described.
two groups:
together. Two differences do exist between the
.
veins
and
in
were
the
Cat
worked
Mountain
for
gold,
district
whereas
are
the
more
other
.
quartzose
veins
were
worked for lead and silver.
Two major trends are readily apparent: north-south
to
Figure 20. Overall view of the mineralization in the open cut
at the Sixty prospect. Hematite is.the predominate mineral,
but copper silicates and oxides are common. View is looking
south.
..
.
113
X 30'
i-7 and
The veins dip very steeply with dips.
N 5 0 ' E.
rarely less than60'.
They vary in width from veinlets a'
few millimeters across to veins eight feet across. Pinching
and
swelling
of
the
veins
is
common
which to
leads
much
discontinuity. Several veins, however, were followed for more
than a mile. The pinching and swelling probably also
occurs in vertical section; few shafts exceed
100 feet
in
depthso either
Textures
of
the
the
cease
or the
values
vein
minerals
veins
pinch.
indicate the
thatveins
filled open spaces. Most contain evidence of brecciation or
slickensiding of the wall rocks
so they apparently filled
spaces opened due to faulting. One vien is a composite of
small quartz veinlets filling a narrow shear zone.
.Veins
are typically slightly sinuous in plan.
Tile veins
are
associated
nap area: the Spears and
Ab0
with
only
two
rock in
units
the
Formations. Chamberlin
(1974)
shows a similar association in the Council Rock district, but
whether there is a real stratigraphic control
or it is
result of the
The
veins
level
are
of
erosion
predominantly
is
not
quartz
the
known.
and
black
calcite;
one or the other is always present. The quartz
is generally
or
white occurring as massive veins of intergrown crystals
cementing brecciated wall rocks. Occasionally amethystine
quartz is present. Late-stage quartz fills cavities throughout
I14
e
the veins asa drusy coating of clear crystals. Black calcite
forms
massesof intergrown
crystals
typically
occurring
as
alternating bands with quartz. The contacts between
quartz
and calcite bands are generally very sharp.
The black
is
due
to
arborescent
and iron oxides:when
and
these
reniform
color
inclusions
of manganese
inclusions
are
not present, the
calcite is clear. In many districts throughout the Southwest,
the manganese oxides
in calcite are argentiferous (Hewett
and Radtke, 1957).
for
manganese
A sample
of black calcite
was analyzed
by emission
silver
and
spectrophotometry;
manganese was present, but silver was not.
e
rule
out
the
be contained
possibility
in
the
that
manganese
some
This doesnot
of
the
silver
mayvalues
oxides,
but most is present
in
galena (assay). Late-stage calcite is clear and formssmall,
platy'
crystals
Barite
and
filling
vugs
fluorite
are
scattered
throughout
the
most
next
the
abundant
veinsgangue
minerals. Barite occurs as large, white, bladed crystals
filling
cavitiesoin
the
black
calcite
and
occasionally.in
the quartz. It may occur as scattered crystals or as solid,
vein-like masses. Fluorite occurs as purple, green, or
clear
octahedral
and
cubic
crystals
filling invugs
quartz
and calcite. When barite and fluorite occur together,
a
fluorite
covers
pods of barite.
barite
crystals
or fills
open
spaces
within
115
e
The
veins
contain.
minor
amounts
of minerals
character-.
istic of the oxidized zone. Cerussite, mimetite, wulfenite,
and
anglesite
Cerussite
was
of suspected
have
been
identified
from
identified
by microchemical
mimetite
was
analyzed
several
tests
for
veins.
and
arsenic
a
sample
with
positive
results. These minerals typically occur as microscopic,
crystals filling cavities. Black manganese oxides and brown
iron oxide are also present, generally as inclusions in
calcite and as stains. In one sample manganese oxide
was
observed
filling
Near
0
small
the
quartz
fractures
southern
vein
a
much
larger
The
only
scale
barite.
margin
of.the Tres
was
crystals. Chamberlin
in
found
with
Montosas
stock
a
disseminated
magnetite
(1974) described magnetite veiningon
around
sulfides
the
ovserved
northern of
margin
the
in
the
veins
stock.
were
pyrite
and
galena. Pyrite occurs as small cubes within the veins and
in
the
wall
rocks,
where
it
is
generally
oxidized'to
hemati
and limonite. Galena occurs sparingly throughout the veins
as isolated crystals and as shiny, spongy masses coated
with
cerussite.
as
tiny
In the calcite and fluorite, galena ofter occurs
blebs
contained
within
the
crystals
and
aligned
along cleavage planes. Galena is usually fresh b.ut may
a
show
oxidation
rinds
to
anglesite.
116
Figure 21 represents
the
paragenetic
sequence
observed
in the vein minerals.It represents only the relative times
of deposition, not the amounts deposited.
Disseminated
The
prospect
Sixty
prospect
occurring
in
Deposits
is
a
highly
small,
disseminated
silicified
and
copper
fractured
of tuff
Nipple Mountain. The important.minerals,in decreasing order
of abundance, are: chrysocolla, mottramite, malachite,
tenorite, cuprite, azurite, and chalcocite. Chrysocolla,
mottramite,
a
and
malachite
account 98
forpercent of the
mineralization, with cuprite, azurite, and chalcocite making
up the rest. Chalcopyrite, as relict cores within chalcocite,
was
reportedby Thurmond (1.951) but
was
not
observed
by the
author.
The chrysocolla, malachite, and chalcocite occur
disseminated
throughout
the
rock
as
tiny
blebs,
as
pseudo-
morphs after feldspar phenocrysts, and as fracture coatings
(see Fig.2 2 ) .
replacing
A similar occurrenceof copper mineralization
feldspar
phenocrysts
was
by Clement
described
(1968) in the Copper Canyon intrusive in Nevada. Mottramite,
tenorite, and azurite occur strictly as fracture coatings
a
or filling vugs. Cuprite occurs as distorted octahedral
crystals associated with tenorite. The most intense
m
117
Late
Early
Quartz
Black
calcite
White calcite
Barite
Fluorite
I
'
I
Pyrite
Galena
I
t--/
Cerussite
H Mimetite
Wulfenite
Manganese
Iron
oxide
oxide '
Figure 21. Paragenetic sequence for veinminerals in the
Tres Montosas-Cat Mountain area. The diagram represents
of deposition, not the amounts
only the relative times
deposited.
.
."
1
I
. .. - - - .
.
1.
~
. .
.
..
I
..
Figure 22. Photograph showing the occurrence of chrysocolla
as fracture fillings, veinlets, and disseminated blebs in the
silicified tuff of Nipple Mountain. Note the hematite
banding along one of the veinlets. Outcrop is in the open
cut at the Sixty prospect.
119
a
mineralization
occurs
minerals
apparently
have
along
fractures
where
been'remobilized
the
and
copper
redeposited
by circulating groundwater. Copper mineralization isnot
readily
apparenton weathered surfaces.but
beneath
the
surface
Hematite
is
a
the
only
a few
mineralization
common
alteration
is
inches
present.
mineral
at
the
Sixty
prospect as fracture fillings. Wavesof hematite staining
are
of-ten
decreasing
visible
moving
intensity
along
away
from
the
fractures
with
micro-fractures
or around
grain
boundaries (see Fig. 2 3 ) .
Mottramite
e
discussion.
is
an
unusual
occurrence
and
some
It occurs as yellow, dark green,
or black
..
It.was apparently
botryoidal fracture fillings (see Fig.23).
the
last
locally
mineral
deposited
remobilized
since
it coats
copper
minerals
values
in
The silver
and
lead
tained
the
mottramite
so it is a
in
Figure 24 represents
at the
Sixty
the
the
tuff
of
the
hematite,
may
coat
assays
are
.
paragenetic
probably
con-
.
sequence
of mineralization
Nipple
is
restricted.to
Mountain,
although
boundariesofmineralizationarefairlywellknown.
and
mottramite.
potentialore mineral.
one
east
sides
are
outcrop
only of
similarly
fractured exposures of the tuff exist close
by.
south
although
prospect.
The mineralization
*
deserves
altered
The
The
bounded
by faults.. The eastern
and
I
~~
.
,.
~
Mountain. The boundary of the hematite band with unaltt
rock is sharp except alongmicrofractures. The outcrop
in the open cutat the Sixty prospect. The olive to d a ~
A
e
a
121
Early
Late
Silicification
'c"-----l
c-l
Pyrite
H
Chalcocite
H
Chalcopyrite
Malachite
Azurite
U
Chrysocolla
H
Tenorite
H
Cuprite
Hematite
Mottramite
Figure 24. Paragenetic sequence for mineralization at the
Sixty copper prospect. The diagram represents only the
relative times of deposition, not the amounts deposited.
122
fault
is
upthrown
to
the so
east
the
tuff
of
Nipple
Mountain
is repeated but no copper mineralization is present. Part
of
the
west
side
is
bounded
by
a
fault
although
a
drill
h
west of this fault is mineralized. The northern extension
and soil cover. The
is indefinite due to poor exposures
vertical
holes.
extent
The
is
a
the
well
known
from
several
drill
PlateI1 penetrated the mineralization. Figure
cross
Banner
mineralized
a
moderately
A northeast-southwest line of five
drillholes along
line A-13 on
25
is
section
of the
drill
hole
just
south
of this
zone
along
line
throughout
Mountain.
mineralized
about 800 feet
line
A-B.
collaredin
rockbut containdlittle mineralization
tuffof Nipple
The
mineralized
zone
is
an
longby 5 0 0 feet
irregularly
wide.. The
shaped
bottom
of
blanket
the
zone
on
closely follows the contour of the surface. The relief
the top probably represents areas of intense fracturing from
'
which the copper has been leached. Along the fault ofeast
the
ore
lain by a
body,
the
small
repeated
slice
of the
tuff
of
"turkey
Nipple
track"
Mountain
is
under-
andesite.
Brunson (oral commun.,1975) reported that assays of this
andesite
I
high
silver
values,
but the
typeof minerali-
known.
not
zation
is
In
I
yielded
drill
1968,the Banner
hole
to
1622
Mining
feet
Company
which
put
bottomed
down
in
a
what
diamond
they
interpr
7400
7300
i
7400
.I
7300
a
TUFF OF NIPPLE MOUNTAIN
a
LOWER SPEARS FORMATION
200
0
FEET.
.
MINERALIZED
ZONE
F i g u r e 25. Cross sectionthroughtheSixtycopperprospectshowingthe
shape of the mineralized
zone (based on d r i l l c u t t i n g s f u r n i s h e d by
Brunson, hfagdalena, New Mexico)
.
, .
W.
124
a s Mesozoicsedimentaryrocks.Subsequent
work i n d i c a t e d
thatthesecarbonaceoussiltstonesareprobablytheSandia
F o r m a t i o n .P r o p y l i t i ca l t e r a t i o n
t h eh o l ew i t ha b u n d a n tf r e s h
was pervasivethroughout
and o x i d i z e d p y r i t e .
pyriteoccursasdisseminatedcubes,veinlets,
calcite veinlets in
amounts rangingfrom
The
and i n q u a r t z 0 t o 4 percent.
Below about 1330 f e e t q u a r t z - c a l c i t e - e p i d o t e v e i n l e t s
but five
more abundant.Normallytheycontainonlypyrite,
samplesfrom
the last
small amounts of
100 f e e t c o n t a i n
p y r i t ep l u sc h a l c o p y r i t e ,s i i n a l e r i t e ,
become
and g a l e n a .
They
occur as discrete grains in the veinlets with, occasional
e
exsolvedblebsofchalcopyrite
i n sphalerite.
One such
v e i n l e t was approximately one inch wide.
Alteration
A l t e r a t i o n of v a r i o u s t y p e s
i s widespread within the
a r e a and a l o n g t h e G a l l i n a s u p l i f t .
t i o n s h i p s were observed but
bution of the alteration
No o v e r a l l z o n a l r e l a -
it does appear
that t h e d i s t r i -
is controlled in part by faults
and i n p a r t b y s t r a t i g r a p h y .
concentrated near faults
Most o f t h e a l t e r a t i o n
is
w i t h the lateral extent controlled
b yt h ep e r m e a b i l i t yo ft h ew a l l . r o c k s .C e r t a i nv o l c a n i c
0-
u n i t st,h teu f f
.
of Nipple
Mountain
and
H e l l s Mesa T u f f s ,a r e
map
the A-L Peak
and
more p e r m e a b l ea n dc o n t a i na l t e r a t i o n
e
e
125
'e
over a wider area than others. No pervasive alteration
pattern
stock
related
was
Five
to
the
exposed
portion
of the
Tres
Montosas
observed.
different
types
of alteration
were
recognized
during mapping and petrographic examinations: propylitic,
pyritic, argillic, silicic, and lime silicate. Each will
be discussed
may
be
separately,
present
in
the
although
same
Propylitic
Propylitic
e
map
types
of alteration
rock.
Alteration
alteration
alteration in the
several
is
area
the
and
in
most
the
widespread ofform
Magdalena
area
ti
as
whole (Chapin and others,1974). It is characterized by the .
alteration of ferromagnesian minerals to carbonate, chlorite,
and epidote. The conglomerates of the Spears Formation show
the most pronounced effects
of propylitization. They are
characteristically
purple
to
reddish-brown
when
fresh,
but
the alteration changes the color to greenish-gray. This type
of
alteration
Mountain
occurs
through
outcrops of the
Ab0
the
along
the
Council
Sixty
prospect
Rock
and
Formation but.appears
fault
Cat from
theof area
the
to
diminish
north-
ward in the Council Rock district. The propylitic alteration
is
0
probably
Rock fault.
related
to
solutions
moving
along
the
Council
126
Pyritic
The areas
with
of
Alteration
pyritic
thoseof propylitic
alteration
alteration
approximately
and
may
.
coincide
in be
part
.
related to it. There are areas where pyrite,
with its
attendant bleaching, is the only alteration observed.
Pyrite
occurs
fairly
extensively
It is
especially
Nipple
Mountain
as
disseminated
conspicuous
and
the
in
A-L
cubes
of the
outcrops
Peak
and
and
veinlets.
tuffof
Hells
Mesa
Tuffs
where
they are bleached toa much lighter color than normal.
Some
of
these
pyritized
and
bleached
areas
are
visible
on aerial
photographs. This type of alteration also appears be
to
a
fault controlled with the lateral extent controlled
by the
.
permeability of the wall rocks. Pyritization is relatively
continuous
drill
at
depth
at
least
1622 feet
in
the
Banner
hole.
Oxidation of the
water
to
which
was
pyrite
then
released
redeposited
as
iron
into
the
ground-
limonite
or hematite
along joints and fractures. This stainingis especially
visible
Sixty
in
the
tuff
of
Mountain
in
and the
around
prospect
Silicic
a
Nipple
Alteration
Silicic alteration occurs by the addition
of Si0 -as
2
microcrystalline
quartz
to
the
groundmass
and
as
quartz
veinlets. Silicification is dominantly restricted to the
t u f f ofNippleMountain
and i n a n d e s i t e s a d j a c e n t t o q u a r t z -
theSpearsFormation
c a l c i t ev e i n s .
b u t some occurs i n c l a s t i c r o c k s o f
The coppermineralization
attheSixty
prospect i s contained entire1y.i.n silicified tuff of Nipple
Mountain.
However, s e v e r a lo t h e ra r e a so fi n t e n s e l y
f i e d t u f f of Nipple Mountain
silici-
e x i s t w i t h no copper mineralization.
Argillic Alteration
A r g i l l i c a l t e r a t i o n is. n o t so widespread and i s g e n e r a l l y
restricted t o o u t c r o p s of t h e t u f f o f N i p p l e
Mountain.
occursasthereplacementoffeldsparbyclays
e
o fc l a y si nt h e
groundmass.
It
and a s p a t c h e s
A r g i l l i ca l t e r a t i o no ff e l d s p a r
provided s i t e s f ocro p p edre p o s i t i oatnh
tSei x tpyr o s p e c t .
Lime S i l i c a t e A l t e r a t i o n
The occurrence of
alteration in the
Ab0
section(p.17).Small
a l i m i t e d amount of lime s i l i c a t e
Formation was d i s c u s s e d i n
a previous
amounts o fg a r n e t ,w o l l a s t o n i t e ,
e p i d o t e , and q u a r t z were observed replacing calcareous
p o r t i o n so f
the
Ab0
Formation.Becauseofpoorexposures,
no zonation was o b s e r v e d b u t t h e a l t e r a t i o n o c c u r s m o s t l y
a l o n gt h ew e s t e r n
and northernedgesoftheoutcrops.
g a r n e t s were also observed
a
Some
i n a thin section of the Spears
Formation along the northern edge of the
Ab0
outcrops.
128
*
Chamberlin (1974) described .a similar assemblage from
the Council Rock area in the "turkey track" andesite' at the
base
of
the
tuff
of
Nipple
Mountain.
He noted'the
presence
of andradite, wollastonite, ferrosalite, diopside, magnetite,
and
biotite
and
suggested
that
recrystallization of hematitic.
they
were
calcite
theofresult
the
amygdules
,common
in
the andesites. Both of these areas are near the margin
of
the
Tres
thermal
Montosas
stock
metamorphism
Discussion
of
and
during
probably
its
reflect
the of
effects
intrusion..
Mineralization
Veins
The overall.
Montosas-Cat
and
other
geologic
Mountain
mining
Mineralogically
setting
of
area
is
districts
and
very
veins
structurally
the
in
the
similar
to,thatof
in San
theJuan
the epithermal deposits discussed
.
the
Tres
Creede
Regionof Colorado.
veins
are
to
similar
by 'Lindgren (1933).
He
.
believed
the
hydrothermal
solutions
were derived
from
intrusive rocks. Utilizing oxygen and hydrogen isotope data,
several
tance
researchers
of
meteoric
have
ground
been
able
water
in
to
demonstrate
some
the
hydrothermal
impor-
systems.
Taylor (1973, 1974) has shown that meteoric ground waters
predominate in some Tertiary, epithermal deposits in volcanic
terranes (the San Juan Mountains, Colorado: western.
the
129
*
Cascade Range, Oregon: and three gold-silver districts in
western Nevada). Shepard and Taylor (1974) examined the
Butte,
Montana
ore
deposits
and
found
that
meteoric
ground
water was the dominant component.
O'Niel and Silberman
(1974) examined twenty Tertiary, epitherma1,gold-silver
deposits
inthe Great
". . . the
water in
Basin,
Nevada,
and
demonstrated
dominance and probable exciusivity 'of meteoric
the
hydrothermal
Although
this
data
fluids
of such
appears
deposits."
overwhelming
for
Tertiary
epithermal veins in volcanic terranes,
it does not eliminate
the involvementof magmatic hydrothermal solutions. In
fact,
some
magmatic
hydrothermal
components
were
observed
by the above authors (Pasto Bueno and Casapalca, Peru, and
probably Providencia, Mexico). Oxygen and hydrogen isotope
data
are
needed
before
any
single
deposit
be interpreted.
can
It appears reasonable, however, that Tertiary, epithermal,
precious
United
thermal
metal
States
veins
in
volcanic
terranes
in.thesouthwestern
can
be interpreted to have formed
solutions
with
dominant
from
hydro-
meteoric
ground
water
data
available
components.
A
small
amountof geochemical
is
several tentative conclusions can
be deduced from it. A
sample of fluorite
from
a
vein
in the
Ab0
outcrops
was
prepared for fluid inclusion analysis. Homogenization
and
130
e
temperatures
formation
d to ge
:t the
measure
were
and
the
freezing
temperatureof
temperature
was
measured
to
calculate the salinity in NaCl equivalents.
A sample of
the fluid from the primary inclusions (procedure described
in Appendix I) was prepared for atomic absorption analysis.
The
results
are
Table 7 is
listed
a
in
6.
Table
compilation
of
lead
isotope
data
obtained
Iiock district.
by Slawson and Austin(1962) from the Council
The
first
sample
is
from
a
vein
in
brecciated
volcanics
the second from brecciated limestone. The sample from the
brecciated
limestone
may
be
from
the
outcrops
of
Ab0 the
Formation. J-ness is the sum of 206Pb plus 207Pb plus
208Pb
divided
by
204Pb
and
provides
a
measure
of the mixing
of normal and radiogenic leads. 73.5 is the valueat
which the lead age is zero. Higher values represent
anamolous
or
J-type
leads
and
those
lower
represent
normal
leads.
The
the
fluid
veins
solutions
inclusion
and
are
epithermal
which
contained
lead
and
isotope
were
sufficient
data
deposited
verify
from
quantities
that
saline
of to
lead
deposit galena. The lead isotope values are near
the
boundary between normal and J-type leads and indicate
either
mixing
of
primeval
and
radiogenic
leads,
or a
primary isochron.
Based
deposits
on
in
oxygen
other
and
hydrogen
regions,
the
isotope
author
data
from
believes
similar
that
the
and
e
Table6
.
Sample
Temperature
no.
Geochemical d a t ao b t a i n e d
from f l u i d i n c l u s i o n a n a l y s e s i n f l u o r i t e
K
Homogenization
Freezing
Salinity
NaCl
PPm
equivalents
Temp.
(press. c o r r . )
-11.4"
P-50
fluorite
186" C
226" P-50
calcite
C
P-49
fluorite
181' C
Table 7 .
Lead isotopesfrom
Sample
204Pb
c
3.07 m
Na
K
PPm
Na
47775
42435
t h e Council Rock d i s t r i c t ( a f t e r
and c a l c i t e .
Pb
PPm
Ba
PPm
6200
7865
SlawsonandAustin,1962).
20 6pb
207pb
208Pb
J-ness
b1r e. 3c4
c4
iated
volcanics
%
21.10 %
15.70
52.25 %
73.4
1.000
25.30 %
18.83
38.88
brecciated
limestone
1.336 %
1.000
25.33 %
18.96
21.08 %
15.77
52.25 %
39.11
73.8
132
*
hydrothermal
solutions
thesis
were
area
involved
dominantly
in
forming
heated
the
meteoric
veins
in the
ground
water
driven by intrusives. Geological and geophysical evidence
.
indicate the presence of an intrusive'beneath Cat
the
(1974) postulated a buried
Mountain district and Chamberlin
intrusive
beneath
intrusions
plus
the
energy
the
the
Council
Tres
necessary
district; two
these
Rock
Montosas
to
heat
stock
the
would
ground
have
provided
water upand
set
the hydrologic flow system.
,
Beane (1974) and Allmendinger (1974) conducted a much
more
e
in
detailed
eastern
district,
study
of the
Co.,
which includes.
Socorro
based
barite-fluorite-galena
upon
the
same
the
deposits
Hansonburg
'kinds
of geologic
and
geochemical
data presented in this paper. They concluded that the
hydrothermal
solutions
were
2 to 3 molal
temperatures between140° and 200°
sills
were
ground
to
postulated
water;
the
*
have
been
derived
The
K/Na
ratio
albite,a reaction
takes Na'
is
the
from
of
authors
in
to
have
constituents
Permian
observed
the
arkosic
Tres
which
lead
X+ into
releases
heated
were
Montosas
the
thought
sedimentary
albitization.
of potassium
out of solution.
release
these
solutions
at
Tertiary dikes and
C.
mineral-forming
can be explained by the
to
by
NaCl
rocks.
sample
feldspar
solution
and
A second effectof albitization
and
barium
which
commonly
substitute
133
in the potassium feldspar'lattice. The volcanic rocks
contaic
potassium
feldspar
but
Precambrian
granites
and
arkosic sedimentary rocks (i.e.Ab0 and Baca Formations)
within
the
ground
water
flow
pattern
would
provide
much
more feldspar. A s more potassium feldsparis albitized,
the
concentration
The K/Na
those
ratio
of
from
observed
by
lead
the
Beane
and
Tres
and
barium
in
solution
is higher
sample
Montosas
Allmendinger
in the
increases.
than
Hansonburg
district.
The
closer
and
lead
to
lead
isotopes
normal
is
from
leads
slightly
the
than
more
Council
those
from
Rock
the
district
are
Hansonburg
concentratedthe
influid
district
inclusions
of nonradiogenic
from the Tres Montosas veins. The leaching
lead
requires
higher
temperatures
than
of
leaching
the
radiogenic lead. Non radiogenic lead is held the
in lattice
of feldspar, whereas radiogenic lead occurs as interstitial
fillings in the rocks. The interstitial lead is easily
leached
by
warm
water,
whereas
the in
lead
the
lattice
requires
higher temperatures to be leached. Since the isotopic
ratios
.lea2
of
samples
than
those
from
from
Council
Rock
Hansonburg,
contain
more
nonradiogenic
higher
heat ora given
heat over a longer period of time is implied.
The occurrence
of
numerous
heat
flow
stocks
that
in
the
Magdalena
area
indicates
lasted
for
a
time
than
longer
a
that
with the smaller dikes and sills at Hansonburg. Thus,
more
lead
and
more
hydrothermal
districts.
normal
of lead were available in the
mixtures
solutionsin the Tres P4ontosas-Council Rock
higher
associated
134
Walker (unpub. data, 1975) analyzed barite irom veins.
in
the
Tres
Montosas-Cat
IJlountain. d-istrict for trace elements
and found moderately high concentrations of lead, zinc, and
copper. He did not know, however, whether the values were
in the crystal latticeor in fluid inclusions. Table 8
is a comparison of Pb/R rations of his trace element analyses
with the concentrations in fluid inclusions listed in 6Table
. . .
Although
the
analyses
are two
on different
minerals,
barite
and fluorite, these minerals are approximately contemporaneous
in time and, to a first approximation, the solutions are
assumed to be similar. The Pb/K ratio in sample
P-50 is
from fluid inclusions; if the solutions
in the two minerals
a
are similar, then the Pb/K ratios should be similar. The
slightly higher ratio from barite suggests that most Pb
occurs
in
the
inclusions
but
some
must
be
present
in
the
'lattice. This is compatible,with the
openess of the barite
crystal structure compared to fluorite. Although data are
not available, it is assumed the distributions of copper and
zinc are similar to that of lead. Regardless
of where the
metals are in the barite, the analyses indicate the presence
of
copper
observed
and
in
Several
the
data
1.
.-*
zinc
the
even
though
no such
minerals
were
veins.
conclusions
can
be
deduced
about
the
veins
presented:
The veins are epithermal veins deposited by saline
hydrothermal
solutions.
from
135
Table 3 . Comparisonof
Pb/K r a t i o s from b a r i t e ( a f t e r Walker,
unpub, d a t a , 1 9 7 5 ) .and f l u o r i t e( f r o mT a b l e
6 , t h i sp a p e r ) .
18 a n a l y s e s .
The b a r i t e r a t i o i s the average of
Sample
Pb/K
Materi'al
Analytic
Mineral
Method
ofAverage
Fluorescence
samples
Walker's
P- 50
0.269
Barite
X-ray
Analyzed
Whole
mineral
0.146
Fluorite
Atomic
Fluid
A b s o r p tiinocnl u s i o n s
..
136
2.
The s o l u t i o n s wereprobablydominatedbyheated
meteoric ground water.
3.
The h e a t was p r o b a b l ys u p p l i e db yi n f e r r e d
and
exposed s t o c k s .
4.
The h i g h K/Na
r a t i o and t h el e a di s o g o p ed a t a
from
t h e T r e s Montosas a r e a s u g g e s t s t h a t t h e a l b i t i z a t i o n
of p o t a s s i u m f e l d s p a r may have been
f o rt h el e a d
and barium.
eliminate contributions
5.
a possible source
However, t h e s e d a t a
do n o t
from magmatic sources.
N o similar d a t aa r ea v a i l a b l ef o rc o p p e r ,z i n c ,
s i l v e r , and gold.
The copper,zinc,andsilvercould
havebeenderivedfromred-bedtypedeposits;
or a l l .
f o u r may have come from t h e i n t r u s i v e s ;
a combination
of sources i s a l s o a p o s s i b i l i t y .
. .
The i d e a s p r e s e n t e d
d a t a and a r e b y
above a r e b a s e d
no means conclusive.
on scanty geochemical
The data doesnot
preclude the contribution of hydrothermal constituents from
.
the intrusives but
.
it does f i t t h e model ofheatedground
water carrying mineral constituents derived
from t h e
s u r r o u n d i n gs e d i m e n t a r yr o c k s .F u r t h e rd a t af r o md i f f e r e n t
partsoftheveinsystem
and f o r d i f f e r e n t e l e m e n t s
require a different interpretation.
may'
a
a
Disseminated
The
137
Sixty
Mineralization
prospect
is
interpreted
as
an
epigenetic,
supergene deposit with copper, lead,
silver,.and vanadium
transported by ground water at low temperatures. The
alteration
in
hydrothermal
the
tuff
of Nipple
processes
and
Mountain
served
as
is
the
ground
result
of
preparation
for
the solutions. The fracturing and argillic alferation.
provided
sites
slightly
reducing
cite.
'
The
for
deposition
and
environment
mineralization
for
was
pyritization
the
localized
provided
a
deposition
of chalcoprobably
by a combin-
of a major
ation of two things:1.) the area being part
e
2.)
ground water channelway, and
The
copper-bearing
faulting.
solutions
came
in
contact the
with
pyrite and deposited chalcocite and chalcopyrite(?). The
oxidation of chalcocite
resulted
in
the
remaining
copper
minerals: malachite, cuprite, tenorite, and chrysocolla.
This
mineral
assemblage
is
similar
to
those
observed
'in
-
sedimentary copper deposits (red-bed deposits)The reported
silver values maybe in chalcocite (Lindgren,1933),
mottramite, or a separate, unrecognized phase.
The
associationof copper
and
silver
suggests
a
super-
.
7
gene origin. Copper and silver are not closely associated
0
.ina typical hydrothermal system: copper occurs in mesothennal
deposits and silver occurs in the lower temperature epithermal
..
13.8
deposits. When they do occur together, it is generally
in
supergene deposits formed
by oxidation, leaching, migration,
and redeposition. The shape
of
the
deposit
(see
F.ig. 25 )
is also indicativeof a supergene origin. It is an irregular
blanket-shaped body with small dimensions. The bottom
of
the
mineralized
which
zone
suggests
The
deposition
originof
the
parallels
above
the
mineralization
the
ground
water
table.
and
its
surface
age
are
No datable structures, veins,or dikes cut the
uncertain.
deposit
closely
which
suggests
that
it is
only
late
Oligocene
or
younger. One possible sourceof the mineralization is
a
leaching
from
significant
nearby
veins,
amountsof copper
although
at
the
none
of these
present
contain
level
of.
erosion. Copper-bearing veins are presentin the Silver
Hill area,
about
be present
beneath
graben.
four
A similar
miles
the
alluvial
mineralogy
reported by Simon (1973).
age of
at
the
Silver
deposit
Hill
cut
to
in
the
east,
fill
of the
the
and
others
Mulligan
Silver'
Hill
might
Gulch
veins
was
If this is .the source, then the
must
be
Miocene
the
La
Jara
or
Peak
younger
since
the
ve
Andesite
(24 m.y.,
Chapin, 1971-a). Another possible origin is through lateral-
*
migration of copper
ized
intrusive
now
from
supergene
buried
beneath
alteration
of a
the
mineral-
Mulligan
Gulch
graben.
139
e
The
mineralization
in
the
Banner
drill
hole
is
different
and apparently unrelated to the Sixty prospect.
A general
zonal
relationship
has
been
observed
pyritepyrite
plus
quartz-calcite
~
quartz-calcite
Pyrite
is
veinlets
relatively
with
increasing
vein1ets")pyrite
plus
depth;
plus
chalcopyrite-sphalerite-galena.
consistent
chalcopyrite-galena-sphalerite
throughout
occur
only
the
hole,
but
within
the
lower
100 feet. The quantities are small,but the economic
implication may be significant. The zonal.re1ationshi.p
suggests
*
stock
at
the
proximity
depth,
of
perhaps
the
the
rocks
in
northern
the
drill ahole
perimeter
of the
to
infer-
red Cat Mountain stock.If the sedimentary rocks beneath
the Spears Formation are the Sandia Formation, then the
Kelly
Limestone
replacement-type
may
be in
deposits
contact
with
similar
to
the
those
'stock
in
the
and
Kelly
distric
may be present. If so, possible mineralization in the Kelly
Limestone
The
wouldbe within 2000 feet
Sixty
prospect
occurs
of
a-t
the
the
surface.
intersection
of
major structural trends: 1.) the northwest-trending Capitan
lineament: 2.) the north-trending monoclinal flexure and
fault zone: and3 . ) the northeast-trending Morenci-Magdalena
*.
lineament which controlled major paleovalleys. This favorable structural setting
has caused the rocks, especially
the
tuffof Nipple
Mountain
along
Highway
60 (see Fig.261,
three
. .
Figure 2 6 .
Photograph showing the intense fracturing
of the
tuff of Nipple Mountain in a roadcut along Righway
60.. The
fracturing is the result of the location
of the highly silicified
tuff at the intersection of three major structural.trends:
1. ) the northwest-trending Capitan lineament;.
2. ) the norththe
trending monoclinal flexure and fault zone;
and 3 , )
northeast-trending Morenci-Magdalena lineament. Photograph
was taken lookingnortheast.
141'
e
to be intensely fractured. The location
of the. Sixty
prospect
..
and
the
mineralization
in
the
Banner
drillat hole
this structural intersection is probably not coincidental
and
shouldbe considered
Tarqet
in
any
exploration
plan.
Areas
Based
upon
the
geological
and
geochemical
evidence,
three target possibilities are outlined:
1.
Replacement deposits in the Kelly Limestone.
The Kelly Limestone is probably present within
a.
2000 feet of.the surface
Deepening
a
the
Banner
stratigraphic
test
at
the
Sixty
prospect.
drill
hole
would
and
test
of
a
provide
both
possible
mineralization.
b.
Beneath the Cat Mountain district, the Kelly
Limestone
the
c.
is
surface
The
Tres
Ab0
probably
and
near
present
2600 feet of
within
the
of
top
the
inferred
stock.
Formation is near the margin
of the
Montosas
stock
and
is
affected
by contact
metasomatism. The Kelly should
be present within
2600 feet of the
However,
at
a
if
low
surface
the
angle,
the 'Ab0 Formation
Tres
the
and
may
be mineralized.
Montosas
stock
sedimentary
maybe stoped
out.
dips:
rocks
outward
underlying
e
2.
142
V e i n m i n e r a l i z a t i o na td e p t h .
A small possibility exists for increased minerali-
z a t i o n i n t h e t u f f of NippleMountain
t h e CatMountain
andesitesacted
district.If
the overlyingSpears
as a r e l a t i v e l y impermeablecapping,
then the hydrothermal solutions
ward i n t o t h e
may have spread out-
more permeable t u f f s .A l s o ,t h e
siliceous composition
and r e l a t i v e l y b r i t t l e
character of the tuff of Nipple
favorable to better vein
Mountain may be
'developmentand
values.
3.
a t depth in
higher gold
-
E x t e n s i o no ft h eC a tM o u n t a i nd i s t r i c tt ot h es o u t h .
I f the piedmont g r a v e l s s o u t h o f
the CatMountain
d i s t r i c t form a t h i n c o v e r , t h e n m i n e r a l i z a t i o n
from t h e . d i s t r i c t may extend southward under the
cover.
The p r e s e n c eo ft h e
pluton suggests that stocks
surface.
'exposedMonica
Canyon
may e x i s t c l o s e t o t h e
143
CONCLUSIONS
Several contributions to the geology of the
area have resulted
Magdalena
from the i n v e s t i g a t i o n o f t h e
Tres Montosas-
Cat Mountain area.
1.) The only exposures of Paleozoic. sedimentary rocks
o u t s i d e t h e Magdalena. and San Mateo Mountains crop
w i t h i nt h e
map a r e a .
The presenceof
that the Gallinas uplift
these rocksand
the SpearsFormation
theextensiveoutcropsof
out
indicates
is a relatively deeply eroded,
structurally high block.
2.)
A sequenceofveryfine-grained,calcareoussand-
stones to siltstones
hole.
was p e n e t r a t e d b y t h e
Because no p r e - T e r t i a r y
t h e ageoftheserocks
suggests that they
Banner d r i l l
marker bedswereencountered,
i s unknown.
The evidencepresented
may b e p a r t o f t h e P e n n s y l v a n i a n
SandiaFormation.Lithologicallyandpetrographically
..
theserocks
are s i m i l a r t o t h e m i d d l e S a n d i a F o r m a t i o n
i n the MagdalenaMountains.
They c o n t r a s t w i t h Mesozoic
clastic rocks by being clean, quartz sands and
silts
which a r ea l m o s te n t i r e l yf r e eo ff e l d s p a r .M e s o z o i c
c l a s t i cr o c k sa r eg e n e r a l l yf e l d s p a t h i c ,c h e r t ys a n d -
area on t h e n o r t h e r n
stones.
The l o c a t i o n o f t h e t h e s i s
flank of
a Laramide u p l i f t which exposes progressively
m
144
olderrockstothesouthplacesthesesedimentaryrocks
i n a reasonable structural setting to
SandiaFormation..
The exposuresofthe
be p a r t o f
Ab0
Formation
just to the north of the drill hole strengthens
i n t e r p r e t a t i o n .P a l e o n t o l a q i c a l
the
this
and p a l y n o l o g i c a ld a t a
are not definitive, but they
do n o t e l i m i n a t e t h e
possibility that these rocks
may be p a r t o f t h e S a n d i a
Formation.
3.) The rocks of
the
map a r e a a r e 'composed dominately
of volcaniclastic sedimentary rocks overlain by
welded
a s h - f l o wt u f f sw i t hm i n o ri n t e r b e d d e dl a v af l o w s .
The
stratigraphicsectionofCenozoicrocks
is s i m i l a r t o
. .
khatdescribedbyotherworkers
i n t h e Maqdalena area .
(Brown, 1972; Simon, 1973;Chamberlin,1974;Blalcestad,
1976).
A new s t r a t i g r a p h i c u n i t , i n f o r m a l l y
tuff of
Gray H i l l , has been
p a r to ft h es t u d ya r e a .
mapped i n t h e s o u t h w e s t e r n
I t i s a crystal-poor,moderately-
t o densely-welded,multipleflow,
of ash-flow
named t h e
compound c o o l i n g u n i t
tuffs occurring between the andesite of
LandavasoReservoirandthePotato.CanyonTuff.
tuff of
The
Gray H i l l f i l l s a p a l e o v a l l e y i n c i s e d t h r o u g h
t h ea n d e s i t eo f
LandavasoReservoir.Withinthepaleo-
valley the tuff
of Gray H i l l rests d i r e c t l y on the
A-L
Peak T u f f .
The tuffof
Gray H i l l i s probably
I
145
m
correlative with Simon's (1973) crystal-poor upper tuff
unit.
4.) The A-L Peak Tuff fills a north-northeast-trending
paleovalley on Gray Hill. Thicknesses ofthe.tuff
..
range from 300 to 1000 feet within the valley. This
paleovalley
was
later
reentrenched
and
filled
by
the
tuff of Gray Hill. The paleovalley is probably a tributary of a larger paleovalley which trends northeastward
from Tres Montosasto the Granite Mountain area. Simon
(1973) described a paieovalley in the Silver Hill area
which
controlled
distribution
of
the
tuff
of
La
Jencia
Creek. Older paleovalleys would account for the unusual
thickness
and
of
These
of
the
the
tuff
paleovalleys
tuff
of
of
Granite
Nipple
are
Mountain
on Tres
Mountain
probably
near
fault
the
Montosas
Sixty
controlled
prospect.
by
the
,Morenci-Magdalena lineament.
5.) Chemical analyses of the basalt of Council Rock have
led to its classification as an alkali-trachyte to trachyandesite. The average of4 analyses showit to be high
in Si02 (54.30 percent) and alkalis (Na
0
2
and IC20
-
2.15
Plagioclase
with
a
percent)
with
compositions
classification
alkali-trachyte
to
of
normative
vary
the
about
-
5.49 percent
nepheline.
AnLiO
of Council
basalt
trachy-andesite.
and
Rock
are
as
consistent
an
146
6 . ) The occurrence of limestonepebbleconglomerates
i n the outcrops of the
and q u a r t z i t e b r e c c i a s
Formation suggest that faulting
Abo
may havebeen
active
d u r i n g Permian time.
7 . ) The developmentof
approximately 32 m.y.
t h e M t . Withington cauldron
agoformed
o fr a d i a l ,c o n c e n t r i c ,
a complicated series
and t r a n s v e r s e f a u l t s a l o n g
cauldronmargin.These
the
f a u l t s w e r e l a t e r u t i l i z e d by
gold-bearing quartz veins
i n the C a t Mountain d i s t r i c t .
Emplacement of a small p l u t o n , t h e
Monica Canyon s t o c k ,
may have been controlled
i n p a r t b y the s t r u c t u r a l
marginof
A north-trendinggraben
thecauldron.
r a d i a t i n g from thecauldronmarginforms
s t r u c t u r ee a s to f
Gray H i l l .
a prominent
These s t r u c t u r a l and
mineralogical observations suggesban analogy of the
CatMountain
district with the
Creede mining
district
i n Colorado.
8.)
N o r t h - t r e n d i n gf a u l t s ,
which c o n t r o l l e d the emplace-
ment of numerous i n t r u s i v e s ,b r o k et h ei g n i m b r i t e
plateauapproximately
utilized by basin
30 m . y . a g o .
This same t r e n d was
and range faulting which began
Magdalena area approximately
25 m.y.
agoand
i n a series ofnorth-trendinggrabensandhorsts.
MulliganGulchgraben
.
in the
resulted
The
and G a l l i n a s u p l i f t w e r e formed
147
*
at this
time.
rangefaulting
Although t h e dominant t r e n d o fb a s i n
was t o t h e n o r t h ,
and
a s e r i e s of, n o r t h e a s t -
trending faults related to the bifurcation of the
Rio
Grande r i f t i s a l s o a p p a r e n t .
9 . ) Three known and t h r e e i n f e r r e d i n t r u s i v e s o c c u r
H i l l on t h e s o u t h
along a t w e n t y m i l e i n t e r v a l
fromGray
Abbey. S p r i n g s on the n o r t h .
They a r e p r o b a b l y r e l a t e d
t o an e l o n g a t e b a t h o l i t h b e n e a t h t h e a r e a
the high-angle, Oligocene faulting
formation of
to
which initiated
and r e s u l t e d i n .the
a monoclinalflexurealongthewestern
marginoftheMulliganGulchgraben.Stocksextending
from t h i s b a t h o l i t h r o s e t o w i t h i n
2000 f e e t o f
Oligocenesurface.
exposed i n t h e
area.
W o s t o c k sa r e
A t h i r ds t o c k ,l o c a t e d
i s i n f e r r e d from f e l s i c d i k e
veins,hydrothermalalteration,
i n t h eC a t
the
map
Mountain d i s t r i c t ,
swarms, e p i t h e r m a l q u a r t z
anda
weak aeromagnetic
low.
1 0 . ) Hydrothermal a l t e r a t i o n i s widespread i n t h e map
area.
There i s no a p p a r e n tr e l a t i o n s h i p
thermallyalteredareas
of thehydro-
t o exposed stocks; concentration
of alteration along faults suggests that the fluids.
were d e r i v e d a t d e p t h
and moved upward a l o n g t h e f a u l t s .
ll.)'Fluid inclusion analyses
fromsamplesofvein
m i n e r a l i z a t i o n showed t h a t the ore-forming solutions
were 3-molal-NaC1-equivalent
solutions, at temperatures
around 180OC.
The s o l u t i o n sc o n t a i n e ds u f f i c i e n t
(6200 ppm) t o p r e c i p a t e . g a l e n a . A n a l y s e s o f
from t h e same veinsbyWalker
lead
barite
(unpub. d a t a ) showed
t h a tt h es o l u t i o n sa l s oc o n t a i n e dc o p p e r ' ( 4 4 3
ppm) and
'
z i n c (330 ppm).
1 2 . ) A small oxide copper deposit,
i s l o c a t e ds o u t ho f
Highway 60.
o fc h r y s o c o l l a ,m a l a c h i t e ,
c h a l c o c i t e whichoccurs
d i s s e m i n a t e db l e b s .
the S i x t y p r o s p e c t ,
M i n e r a l i z a t i o nc o n s i s t s
and o t h e ro x i d e sw i t hm i n o r
as f r a c t u r e f i l l i n g s
and
Based on t h e t y p e o f m i n e r a l i z a t i o n
and i t s occurrence, the p r e s e n c e o f s i l v e r
copper, and t h e b l a n k e t
w i t h the
sha,?e of t h e d e p o s i t ,
the S i x t y
p r o s p e c t i s i n t e r p r e t e d t o be of supergene origin.
1 3 .) A deep diamond d r i l l h o l e a t the S i x t y p r o s p e c t
penetrated the volcanic rocks
andbottomed
The c o r e w a s p r o p y l i t i c a l l y
c a r b o n a c e o u ss i l t s t o n e s .
altered with disseminated pyrite, plus pyrite
q u a r t z - c a l c i t ev e i n l e t st o
C h a l c o p y r i t e ,g a l e n a ,
veinlets i n t h el a s t
in dark,
a depthof
and
1622 f e e t . .
.
and s p h a l e r i t e w e r e observed i n
1 0 0 f e e t o fc o r e .T h e . c o n s i s t e n t
p r o p y l i t i c a l t e r a t i o n and i n c r e a s i n g i n t e n s i t y of
mineralization w i t h depth suggests that the
drill h o l e
is near a buried intrusive.
14.) The occurrence of the Sixty prospect.
a t the
'
149
intersection of three
possibility
that
the
major
structural
Kelly
trends,
the
Limestone
is within 2000
feet of the surface, and the exposed and inferred
intrusives
one
of
for
base
make
the
the
most
metal
Tres
Montosas-Cat
favorable
exploration.
parts
of
Mountain
the
area
Magdalena
area
15 0
APPENDIX
ProceduresforObtaininq
Absorption Analysis
F l u i d Inclusionsfor.Atomic
The following procedure
w a s followed i n o r d e r t o o b t a i n
unmntaminated fluid inclusions
from f l u o r i t e f o r a t o m i c
absorption analysis.
1.) Hand pick 1 0 t o 50 grams ofbrolien
assure that the
fluorite to
sample i s pure.
2 .) Break up t h e sample i n a t e f l o n m o r t a r ( t o p r e v e n t
sodium contamination) to .about
.
'
8 mesh (most breaks w i l l
occuralongsecondaryplanesoffluidinclusions;this
the wash s o l u t i o n s ) .
exposesthesecondaryinclusionsto
3 ; ) Rinsethe'samplethoroughlyat
least t h r e e o r f o u r
t i m e s with deionized water.
4.)
Wash t h e sample w i t h h o t
(70°
HN03 p l u s HCl i n a t e f l o n b e a k e r
t o 8OoC) , c o n c e n t r a t e d
'
andp1ac.e
beaker with
sample i n an u l t r a s o n i c c l e a n e r f o r t e n m i n u t e s .
5.)
Repeat s t e p 3.
6.)
Crush t h e washed sample t o 2 2 0 0 mesh i n a t e f l o n
mortar i n 2 5 t o 30 m l . of an 0.1 N I-ICl' p l u s d e i o n i z e d
w a t e rs o l u t i o n .
sodiumand
7.)
Avoid contamination of t h e sample w i t h
potassium by using plastic gloves.
F i l t e r t h e sampleandsave
analysis.
Keep t h e s o l u t i o n i n
the solution for
AA
a plastic bottle.
151
8 .) Use an extra sample o f p l a i n w a s h w a t e r frop. s t e p . 6
a s a blank.
I52
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.
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73 p.
A
?
107"2 2
' 30"
R. 5 W.
34' 07' 3 0'
34" 07' '
0
3
c
T. 2 S.
T. 2 S
i
5'
T. 3 S
17
2' 304
2' 30'
19
20
"'I 97
I.
i
"SO
I
34" 00'
107' 30'
27/30''
I .O Mile (SI
might
Line
Spacing
R.. 5 W.
GEOLOGICAL SURVEY
OPEN FILE' MAP
AREA
4 3 N
10000 Feet A S L
Flight
Altitude
"-
V.i
U. S.
52,164 -79 Gammas
Datum
R. 6
ENLARGED FROM
I O . 0 Gammas
Contour Interval
Flown and
25'
SW
1973
Compiled
Outline of
Thesis Area
SCALE
I:24000
NEW MEXlCO
prepared in cooperation with the New Mexico
of Mines and Mineral Resources.
Bureau
t
.
.
. .
. .
.
. . .
.
I
I
.
J
. .
..... ..
PLATE I11
DETAI.~EDGEOLOGK
MAP OF
THE /SIXTY' COPPER PROSPECT
I
.
By; WILLIAM H.
Base mad compiled
1
from S.G. S. Tres Mmtasus 7 + 5 minute quadrangle published in f9G4.
I
WILKINSON,
JR,,
1975
.