A Thesis
Presented to
t h e F a c u l t y of t h e Depax%ient
of Geoscience
New Mexico I n s t i t u t e o f Mining and Technology
In Partial Fulfillment
of the Requirements for the Degree
. .S c i e n c e
Master of
bY
DonaldBruce
Simon
November, 1973
T A B U OF CONTENTS
PAGE
. . . . . . . . . . . . . . . . . . . . . . . . . . vi
1
......................
Statement of t h e Problem . . . . . . . . . . . . . .
1
Location and A c c e s s i b i l i t y . . . . . . . . . . . . .
P r e v i o uIsn v e s t i g a t i o n s
... . . . . . . . . . . . .
P r e s e nItn v e s t i g a t i o n
............... 7
ABSTRACT
INTRODUCTION
1
..
1
. . . . . . .. . . . . . . . . . . .
STRATIGRAPHY
PETROLOGY . . . . . . . . . . . . . . .
P r e - T e r t i a r y Rocks . . . . . . . . . . . . . . . . .
T e r t i a r y Rocks . . . . . . . . . . . . . . . . . . .
Hells Mesa Formation . . . . . . . . . . . . . . .
PeakFormation . . . . . . . . . . . . . . . .
Massive Member . . . . . . . . . . . . . .
.............
Tuff of La Jencia Creek
Flow-banded Xember . . . . . . . . . . . . . . .
Pumiceous Member . . . . . . . . . . . . . . . .
Andesite Flows . . . . . . . . . . . . . . . . .
of Allen Well . . . . . . . . . . . . . . .
Acknowledgements
AliD
., ...
A-L
'
Gray
Eff
Andesite of Land.avaso Reservoir
.........
...................
Unit of ArroyoMontosa . . . . . . . . . . . . . .
Volcanic
Facies
................
Upper Tuffs
Conglomwrate F a c i e s
La Jara Peak Andesite
..............
...............
8
10
10
11
11
15
16
18
25
28
30
31
31
39
44
45
47
50
-3.1-
Popotosa
Formation
................
Fanglomerate of Dry Lake Canyon
..........
55
55
. . . . . . . . . . 57
Mafic
Dikes
. . . . . . . . . . . . . . . . . . . 51
Monzonite . . . . . . . . . . . . . . . . . . . .
59
Tertiary-Quaternary Deposits . . . . . . . . . . . .
62
Pefiiment Gravels . . . . . . . . . . . . . . . . .
62
Quaternary Deposits . . . . . . . . . . . . . . . . . 6 2
Talus . . . . . . . . . . . . . . . . . . . . . .
62
E o l i a n Sand . . . . . . . . . . . . . . . . . . .
63
Alluvium . . . . . . . . . . . . . . . . . . . . . . . . . .63
STRUCTURZ . . . . . . . . . . . . . . . . . . . . . . . . . 64
Regional
Structure
. . . . . . . . . . . . . . . . . . 64
" L o cSatlr u c t u r e
. . . . . . . . . . . . . . . . . . . 66
Oligocene
Faults
. . . . . . . . . . . . . . . . . . 67
Miocene F a u l t s . . . . . . . . . . . . . . . . . .
70
Folding . . . . . . . . . . . . . . . . . . . . .
75
....................
77
Oligocene
Alteration
. . . . . . . . . . . . . . . . 77
Miocene M i n e r a l i z a t i o n . . . . . . . . . . . . . . .
83
Paragenesis of Vein Minera1S . . . . . . . . . . . . 86
Wall-rock
Alteration
. . . . . . . . . . . . . . . go
Discussion of Vein M i n e r a l i z a t i o n . . . . . . . .
92
Economic P o t e n t i a l . . . . . . . . . . . . . . . . . 92
CONCLUSIONS ......................
95
.....................
98
T e r t i a r y . I n t r u s i v e Rocks
e
e
....
ECONOXIC GBOLOGY
.
BIBLIOGRAPHY
.
..
-111-
LIST OF ILLUSTF~LTIONS
PAGE
PLATE
1.
......
Geologic map and sections of the Silver Hill
area, SocorroCounty, New Kexico
in pocket
FIGUF%
.,
map
of
the
Silver
Hill
.area
......
I.
Location
2.
Generalized stratigraphic column the
of
Tertiary rock 'units exposed in Silver
the
area....
'
. 2
. . . . . . . . . . . . . . . .&ill
. . 12
Modal data on the tuff of La Jencia Creek and
the upper part of the gray massive member from
the Crouch drill hole showing mineralogical
variations.
*.22
.................
4.
Section of core from the Crouch drill hole
showing welded contact between gray massive
member of the A-L Peak Formation and the
overlying tuff of La Jencia Creek
23
.......
Flow-banding developed in the flow-banded
member of the A-L Peak Formation exposed in
large roadcuton U.S. Highway 60 . . . . . . . . 27
Outcrop of partially altered andesite near
Allen Well . . . . . . . . . . . . . . . . . . . 32
Photomicrograph of clinopyroxene phenocryst in
the andesite'of Landavaso Reservoir showing
alteration to vermicular celadonite . . . . . . 38
......
6.
7.
8.
Silicified faultsliver of upper tuffs along
an early Miocene
fault which crosses Hill
"7048".....................
43
9.
Crude 'stratification developed in the
conglomerate facies of the unit
of Arroyo
Montosa
10.
1
1
.
12.
....................
49
Vegetation contrast across fault contact of
fanglomerate of Dry Lake Canyon and the
pumiceousmember of theA-LPeakFormation
56
...
...
Oligocene faults in the Silver Hill .
area
Miocene faults in the Silver Hill area. . . .
68
72
13.
14o
15.
Structural mapof the Magdalena area showing
relationship o f the thesis area
to major
structural components
. . . . . . . . . . . . . 76
Roadcut on
Highway 60'inalteped and
intensely fractured flow-banded member
of the
A-L Peak Formation. . . . . . . . . . . . . . . . 81
Close-up of altered and fractured flow-banded
member of the A-L Peak Formation in roadcut
on
Highway 60 . . . . . . . . . . . . . . .
Rose diagram to 50 vein trends in La Jara Peak
85
Andesite ....................
Boulder of vein material typicalof prospects
in La Jara Peak Andesite
. . . . . . . . . . . . 87
Paragenetic sequence for vein minerals in
the Silver Hill area
. . . . . . . . . . . . . . 89
U.S.
82
U.S.
16e
17
18e
.
-v-
ZIST OF TABLES
TABLE
PAGE
1.
Modal data i n volume p e r c e h t f o r t h e t u f f o f
La Jencia Creek a n d t h e u p p e r p a r t o f ' t h e gray
massive member f r o m t h e Crouch d r i l l hole
e
2.
Modal_ d a t a
. . . 21
from theHale,Wellmonzonitepluton
. 61
..
.
This thesis is accepted on behalf of the faculty of the
Institute by the following conunittee:
.. ...
I
-vi-
.
The
Silver
'
Hill
ABSTRACT
area.is
composed
of generally
eastward
dipping Oligocene and Miocene rocks. Eroded fault blocks
expose tuffs and fiows of the Datil volcanics and,flows
of'
La
Jara
Peak
fanglomerates
of the
Andesite;
Popotosa
Formation fill Miocene.block-faulted basins. The unit
of
,
Arroyo Montosa (early Miocene); consisting
of interbedded
conglomerates .and lava flows, is distinguished from similarof
looking rocksof the Popotosa Formation on the basis
lithologic differences. This newly defined unit lies
stratigraphically
volcanics.
,
altered
between
La
Jara.Peak
L
and the
Andesite
Datil
A late 0ligocene.monzonite pluton intrudes
Datil
volcanics
in
the
. ..
the
southwestern of
portion
area.
The predominant structural features of the Silver Hill
area
are
steeply
dipping
faults
with
diverse
trends.
Faulting is divided into four age periods:
1) middle
'
Oligocene, 2) late Oligocene,3 ) early Miocene, and
4)
I
middle to late Miocene. A shallow graben formed by middle
Oligocene
faults
controlled
3
deposition
of
Lathe oftuff
Jencia Creek. The Mulligan Gulch graben, bordering the.
western
margin
of
the
area,' to
began
form
in
late
Oligocene
and was modified by Miocene faulting. Onset of Basin and
Range deformation maybe extended to very early Miocene
in
the
Magdalena
Late
occur
area.'
Oligocene
argillic
alongthe western
,
.
margin
alteration'and
of
the
Silver
pyritization
Hill
area.
-VAL-
Some
.
may
be related'to supergene
argillization
.
.
oxidation
of
pyrite. Sulfur and mostof the iron in pyrite were probably
introduced
by
hypogene
solutions
which
apparently
were
not
related to the nearby monzonite intrusive, Paragenetic
relationships
on
Miocene
vein
mineralization
suggest
that
some of the assemblage
chrysocolla-malachite-hematite is
hypogene. Excessive depths t o the .Kelly Irimestone,
insufficient
tonnage
extent
of
SLlver
Hill
exploration.
altered
area
of
rocks
vein
mineralization,
collectively
is a not
favorable
and
limited
indicateLne
that
target
for
mineral
INTRODUCTION
Statement of the Problem
purposeof this
The
is
to map
study
and
describe
the
stratigraphic and structural relationships to
and.
investigate.
the
Hill
Socorro'
'area,
Location
and
The
alteration
Silver
County,
New Mexico.
Accessibility
Silver
Hill
New
Mexico
Magdalena,
mineralization
of the
and
.area
in
1.5 miles
located
is
relatively
low
westo f
terrain
northwest
of the Magdalena Mountains. The area
of investigation
covers
approximately22 square
Silver
Hill
and
Arroyo
miles
Landavaso
and
lies
7.5-minute
within . the
topographic
quadrangles. The boundaries are La. ,. . Jencia Creek and Dry
Lake
Canyon
Jencia
Road
on
Creek
on
Canyon
and
the
and
the
north,
a north-south
State
107 on the east, Boxcar Well
Road
south, aand
north-south
Arroyo
Access to the
Landavaso
northern
.on
and
line
southern
part,
ranch
roads
and
Dry
1). (fig.
west
eastern
parts
of the
.-.zesis
area
In addition, there are
woodcutter's
trails
which
the w'estern and eastern portions
of the area. The iarger
arroyos are also driveable.
Previous Investigations
A general
Lake
Highway 60 .:::sects
and souRern
the
boundary of the
can be reached by State Road
107.
numerous
betw.:n
the
area is provided by State Road
52. U.S.
the,
linebetween La
recommissanceof the
Datil
and
Gallinas
connect
-2-
0
35 Mila.
. .
Figure 1
- Location map o f
I
the Silver H i l l area
-3-
Mountains,
northwest
of
Silver
Hill area,was completed
the
by Herrick in l899* In his report of
1900, he concluded
that
the
rocks
were
mostly
In the San I4ateo
and Magdalena
trachyte
and
Mountains,
rhyolite
Lindgren,
intrusives.
Graton
and Gordon,(1910,'~.
239) noted that the first volcanic
rocks
of
the
period
were predominantly
Tertiary
andesite;
later flows iiere mainly of rhyolitic composition, .Winchester
(1920) measured a partial
rocksin the
Tertiary
stratigraphic
'northern
Bear
section
of
Mountains
the
and
crzlled
the entire sequence of andesite, trachyte and rhyolite
flows
and
intrusives,
and
some
associated
conglomerates
sandstones, the Datil Formation. The sedimentary rocks
of
the
basal684 feet of Winchester's
separated
by
section
Wilpolt
and others (1946) and
were
named
later
.the
Baca
Formation.
.
.Mining activity was reported as early
1919as
in the
1920).
North Magdalena district (Pueblo Springs] (Weed,
Meed (1922) noted that the Copper Belt Silver and Copper
Mining
Company,
Silver
Hill,
which
did
a small
owned
13 claims on the east slope
of
amountof assessment
work
consisting
primarily of open cuts and shafjse Two holes were drilled
in 1925 to depths of 820 to 1042 feet (Neale,1926).
Both
holes bottomed out in igneous rocks. Lasky
(1932) also
reviewed
aligned
which
these
claims
and added
alonga series
cut
the
of
that
the'prospects
northwest-trending
predominantly
andesitic
and quartz
quartz
were'
veins
latitic
country rock, The mineralization, which included
and
-4-
argentite,
occurred
chalcocitet.
as
small
covellite,
pockets
and.
chrysocolla
shoots
in
and
the
malachite,
veins
and
structurally controlled by faults and fractures. 'Some gold
was also reported (Neale, 1926). In some concluding
remarks,
Lasky
(1932).
for the.
copper
and'
t o the
postulated
a hidden
silver
magmatic
mineralization
source
possibly
related
intrusives in the Kelly area. The last reported
activity on the property was in 1927 (Howard, p.1967,
201),
although
numerous
small-scale
exploratory
operations
have
been carried out since then.
Loughlin
in
detail
They
that
Magdalena
Koschmann
(1942)
studied,the
ii comprehensive
published
and
noted
the
and
several
Tertiary
Professional
volcanic
extended
to ar.eas
district.
Kelly
area
Paper.
units in
present
north
and
west
outside the district. A geologic summaryof the Magdalena
mining
district
Linchburg
of the
discussion
and
orebody,
located
in
genesisof the
the
southern
portion
of the
1961). Johnson
district, were published by Titley(1959?.
(1955) attempted'to correlate several Tertiary volcanic
units
in
the
Magdalena
district
with
those
that
crop
out
in the Datil and Bear Mountains, On his geologic map, the
rocks
of
shown as
the
eastern
of the
half
Silver
Hill
area
were
Pliocene upper latite.
Detailed
stratigraphicwork on
the
Datil
Formation
was.
conducted by Tonking(1957) in the Bear Mountains. He
subdivided
the
formation
into
a basal
Spears
Ranch
I
a middle
.
.
Hells
Mesa
Member,
and
anL a upper
Jara Peak
Member,
was
Member. The La Jara Peak
Member was later reassigned to
a
series
of
post-Datil
andesites
(1959) and Weber(1963)e
two
investigators
Member
and
basa.lts
by
Millard
In the Silver Hill area, these
correlated La
the
Jara
mistakenly
Peak
witha "lower volcanic group" of Late Cretaceous-.
early Tertiary age. Weber (1971) elevateCthe Datil
Formation to group
would
probably
status
and
necessitate
noted
raising
that
the.
further
mapping
subdivisions
to
formational rank.
'
Weber and Bassett(1963) published a preliminary
report on several K-Ar dates of Tertiary volcanic and
intrusive rocks in west-central
New Mexico, One sample,
near
the
base
of
the
Hells
Mesa
Member,
was
dated
at
30.6 2 1.2 m.y. KottlowsXi, Weber and Willard(1969)
.. ...
dated a number
of
Datil
and
post-Datil
volcanic
rocks
including the Hells Mesa and Spears Ranch Members. Chapin
(1971a)
dated
the
La
Jara
Peak
Andesite
and
discussed
its
significance to mineral exploration. Fission track dates
for the
Potato
A-L Peak
(1973)
Canyon
Formation
were
Rhyolite aand
vitrophyre
determined
by
Smith
the
and'others
'
Elston a n d others (1968, 1970) havethe
within
Datil
framework
volcanic
rocks
relatedto the
into
an
development
attempted
to fit
overall
of
the
volcanic-tectonic
Mogollon
as a largeescale, ring-dike complex. They noted in one
of
their
figures
several
source
cauldrons
for the volcanic
(1973)
rocks.' In the San Mateo Mountains, Deal and Rhodes
.
.
Plateau
-6-
I
have established two additional ash-flow caxldrons. One of
be the
these, the Kt. Withington cauldron, is believed
to
source
for
In
the
another
interpreted
Group
end
'article,
the
volcanic
theA-L Peak
Elston
rocks
consisting 3ofmajor
as
of
ash
f l o w s within
the
last
Basin and Range
episode
faulting
and
of
Formation.
others
the
volcanic
coincident
(1973)
with
about
20 million
or
Formation
Datil
episodes
with
the
years
have
the
beginning.
of-
ago,
'The
r-
development of the Rio Grande rift and its influence .on
structural.
trends
in
the
discussed by Chapin
(1971b),
and
Range
tectonics
and
A relationship
mid-Tertiary
Bear
Mountains
was
between Basin
volcanism
and
plate
postulated
by Elston (1972).
was
Most
have
faulting,
Magdalena
recently,a numbes of theses and dissertations
discussed
in
detail
specific
areas
around
the
pagdalena
\
of the
district. Brown (1972) completed an investigation
southern Bear fiountains. Part of the.area he mapped lies
directly
north
structure
of
and
the
Silver
Hill
area
volcanic
units
present
in
and
some
of
the.southern
the
Bear
Mountains extend into the Silver Hill area. Woodward
(1973) mapped the Lemitar Mountains and described their
stratigraphic
and
structural
relaionships
to
the
Rio
Grande
rift. The origin of the Popotosa Formation was the subject
of a dissertation by Bruning(1973).
central
area
Iilagdalena
and
the
Mountains,
Council
Rock
the
Investigations of the
Cat
district
Mountain-Tres
are
being
completed
I
Krewedl (iri preparation), Wilkinson (in preparation) and
.
.
Montosas
by
'I-
Present
Investiaation
The study of the Silver Hill a.rea began with detailed
geologic
and
mapping
fall
of
which
1972
was
and
carried
sporadically
out
during
through
the
the
summer.
winter,
sprin
and summer of1973. The Arroyo Landavaso'and Silver Hill
7.5-minute topographic quadrangles (scale 1:24000) served
.
as base maps for field work. Aerial photographs of the
GS-VARJ (1963) series were used
to locate outcrops and
to ..
aid in structural interpretation,
..
Petrographic work consisted of .examining
49 thin
smtions from
differentrock units
The LL & E diamond-drill
hole
mapped
located
in
on
the
the
thesis
area.
south of
slope
'Silver Hill(pl. 1) was logged by the author Dr.
andC.E.
Chapin; 21 thin
sections
were
made
and
studied
from
representative samples. For comparison, several thin
sections of core from the Crouch drill hole 7,
(sec.
T. 2 S.,
R. 4 W.) were also examined. In addition, alteration
assemblages were checked
collected
from
3.75 miles
Modal
samples
I
west
ina suite
c
samples ainroadcut
of
thin
onU.S.
sections
Highway 6 0 ,
of'Magdalena,
analyses
fromthe Hale
were
Well
performed J.E.by Bruning
pluton
and
14 samples
on2
from
the
A-L Peak Formation obtained from the. Cro.uch drill Inhole.
ad.dition, the author p,erformed modal analyses
on 12 samples
from many o f the volcanic units aascheck on estimated
2000
phenocryst and groundmass percentages. Approximately
points
per
thin
section
were
obtaineda from
square grid
with l/3-millimeter intervals.All point counting was done
with a Swift
microscope
automatic
point
counter
.
attached
to a.Zeiss
.
of31.25.
usinga magnification
,InJuly 1973? a sample from the volcanic facies of the
unit
of
Arroyo
Montosa
was
submitted
to
Geochron
Laboratories, Inc. for radiometric dating.
Acknowledgements
Appreciation
Mines
and
financdal
Mineral
support
and
for
providing
transportation.for
the
field
author
with
work.
Dr.
Chapin,
supervision,
Resources
of
thanks is given
to thesis.advisor,
. A sincere
Charies
t o the New Mexico Bureau
extended
is
for
help
suggesting
and
the
area
many-discussions
and
for
during
his
the
course
of
is also grateful toD
r. Richard
this thesis, The author
Beane
for
geology,
his
advice,and
andto Dr. A.J.
suggestions
concerning
economic
Budding who aided in the
petrographic work.
Spe2ial thanks are due Dr.
to Charles
Robert
Weber
and
mineral
the
rough
for
their
help
with
identification, to
andmy
drafts
and
final
Walker
X-ray
wife
and
Dr.
diffraction
Sally
who
typed
copy.
to the.
The author also wishes to express his gratitude
I
Louisiana Land and
Exploration
analyses
Company
for
permission
to log
.
core fxom t h e i r diamoxi8-dri.11 bole, and
.
$0
Y z e Ben Donnegan
.
..
...
t
.
3
. .
.
STRATIGWHY AND PETROLOGS
'
Pre-Tertiary
Rocks
No rocks older than Tertiary are exposed within the
study area. However, about4 miles to the east, on the
east sideof Granite
outcrops
of
Mountain
Precambrian
the
geologic
north
of U.S,
and
and
Paleozoic
map
accompanying
shown
on
200 by
Loughlin and Koschmann
(1942).
Highway 6 0 ,
sedimentary
rocks
Professiona1.Paper
They showa
of
Paleozoic section beginning with the kelly Limestone
"ississippiajn
age
and
continuing
'are
through
the
.,
Sandi.a
and
Madera Formations of Pennsylvanian age. However, recent
work indicates that most
of this
stratigraphic
section
is
not
Pennsylvanian but Permian in age (W.T. Siemers,
R.B,
.
..,
Biakestad and C.E. C%apin, oral commun., 1973).
A partial
sequence of the Abo, Yeso (?), Glorieta and San Andres
Formations
is
present
with
Oligocene
age
resting
unconformably
South of U.S.
Highway 60, in
the
the
Spears
Kelly
Formation
on
San
mining
of
Andres
early
Limestone.
district,
the
Sp,ears rest on the
Ab0 Formation. West of the Silver
Hill
-area a&about
(in
2 miles
preparation)
eastof Tres Itontosas, 'rJilkinson
shows
a small
area
of
Paleozoic
rocks
unconfornably overlain by the Spears Formation. The
.
lithology of tizese rocks is similar
to that of the
Ab0
Formation
in
the
exposures
have
area
been
east
of
tentatively
Siemers, oral commun.,1973).
Granite
Mountain
and
Abo as
(W.T.
correlated
Eight miles north
of Silver
Hill in the Puertecito quadrangle (Tonking,
19571, the
the
two
-11-
Spears rests on the Ba-ca Formation
o f Eocene
age
which,
in
L
.
drnP overlies a thick Mesozoic section. Thus, the Silver
Hiil
area
uplift
is
which
located
forms
on
the
the
north
flank
of
'a
Laramide
southern
edge of the Baca basin.
Tertiary Roc&
The
consists
Tertiary
sequence
in
west-central
ofboth sedimentary and volcanic
New'Mexico
rocks
intruded
by numerous dikes and plutons of late OligoceneA age.
thick
sequenceof fanglomerate
interbedded.volcanic
rocks
and
(the
playa
Santa
deposits
Fe
with
Group)
fills
block-faulted basins of Miocene age, Pediment gravels
of
late
Pliocene
Tertiary
and
age
mark
the
end
of
the
Period.
.
The
Quaternary
.. .._
,
sedimentaryand volcanic
rocks
are
divided
into
the Baca Formation (Eocene), the Datil volcanics (Oligocene),
the
La
Jara
Peak
Andesite
(early
Miocene)
and
the
Santa
Group (Miocene-Pliocene). The Baca and Spears Formations
rock
are not exposed within the thesis area. The Tertiary
units mapped in the Silver Hill area,
~f.'.i;hthe exceptionof
intruswe bodies,
are
'
Hells
shown
diagrammatically
Mesa
rhyoiitic and quartz
latitic
in 2.fig.
Formation
The oldest rocks exposed in
,
.
the
Silver
ash-flow
tuffs
Hill
area
belonging
to
the HellsKesa Forma.tion of mid-Oligocene age. Tonking
(1957) named theHells Mesa fora locality of that nameon
the eastedge of the Bear
Moutains; he measureda type
are
F
-12-
l a t e P l l ocene-Quaternmy)
PLIOCENE
enta Fe
Group
,a S a r a Peak Andesite
MIOCENE
volcanic f a c i e s
wroy
unconformitylpper tuffs
mdesite of Landavaso Resorvolr
tuff of N l e n Well
anttesite f l o w s
D a t il
OLIGOCZNI
rolcanic
pumiceous member
A-L Peak Formation
-
flour-bande4 member
v
tuff of L a S e n c i a Creek
g r a y massive member
B e l l s Mesa Formation
-
Flgure 2
G e n e r a l i z e d s t r a t i g r a p h i c column of t h e T e r t i a r y
r o c k u n i t s exposed i n t h e S i l v e r Hill area; i n t r u s i v e r o c k s
excluderf.Figure
shows s t r a t i g r a p h i cp o s i t i o no n l y :
relativethlcknessesnotindicated.
-13-
I
~
i
-
section
Since
and
the. aunit
member of
called
then,
the
Datil
Formation
the
has
Datil
been
Pormation.
to group
elevated
Hells Mesa given formational rank, Using
status and the
the
terminology
by Brovrn (1972), the
adopted
Hells
Mesa
Formation consisted of the tuff of Goat Springs, the tuff of
Bear Springs, some interbedded andesite
f l o w s and the tuff
of Allen Well. Subsequent agreement among the various
geologists
mapping
in
the
Magdalena
of the term Hells Mesa
to the
basal
area
use
restricted
has
crystal-rich,
quartz-
rich, ash-flow cooling unit of Tonking's type section; this
unit
correlates
with
the
tuff
of
Springs
of Brown
Goat
(1972).
The overlying ash-flow units at Tonking's type
section
have
been
to the A-L Peak
assigned
and Rhodes, 19','3; Chapin y d others,
A-L
Peak
correlates
with
vary
30.6 2 1.2 m.y.
from
on
(Deal
preparation)
; the
in
(1972) turf
Brovmls
K-Ar dates
and tuff of Allen Well.
Formation
Formation
of
the
Bear
Hells
Springs
Kesa
in Tonl;ing's type
section to32.4 2 1.5 m.y. in the Joyita Hills (Weber,
1971).
The only outcrops of Hells
Mesa exposed in'thethesis
7
aingroup
occur
.area
of
hills
bordering
Arroyo
PIontosa
in
the north-central portion of the map area 1)(pl.
. The
outcrops
'
'
forma thin
bandbounded o n the
west
by
a north-
trending normal fault and on the east by overlying ash-flour
tuffs.
To the south, the unit disappears beneath talus
cones of the A-L Peak Formationto and
the
north
covered
Jencia
The
by
recent
Hells
Mesa
alluvium
forms
along
small
La
ledges
and
it
is
Creek.
weathers
to
I
massive blocky talus.
fresh
hand
,
The color variesfrompale
specimens
to
very
pale
purple in
orange
in
altered
hand
specimens, Weathered surfaces have grayish tints.
Characteristically,
the
unit
is
moderately
to'
welded and contains approximately
40 to 50,percent
densely
crystals
and crystal fragments. Phenocrysts include sanidine,
plagioclase, quartz and biotite. The average phenocryst
size
from
2 to 3 millimeters but occasional quartz
ranges
I1eyestt
measure
Lithic
as
fragments
present
but
not
much 6as
millimeters
are
abundant
near
in
diameter.
the
base;
pumice
is
conspicuous.
Jn thin section, the texture is porphyritica with
devitrified
groundmass
crowded
with
small
broken
crystal
.fragm'ents. Anhedral and subhedral.. ...sanidine crystals
I
dominate the total phenocryst content and show slight to
moderate degrees of alteration to clay. Quartz
is usually
rounded
and
embayed
and
comprises
10 to 15 volume
percentof
the total rock. Plagioclase grains are often euhedral and
intensely alteredto clays and calcite. Simple'albite
twinning
is
common
but
combined carlsbad-albite twinning
also occur&. The anorthite content of plagioclase'was
determined using the Fouque' method 5.on
crystals;
the
average composition is An3*,
or sodic andesine.. When
,
present, yellow-brown biotite ranges from
1 to 2 percext and
occ&s
as
small
laths,
either
partially
or totally
replaced
by magnetite and hematite. Lithic fragments resembling
dark gray andesites of the
Spears
Formation
contain
altered
plagioclase phenocrysts and have an opaque, hematized
groundmass. Pseudomorphs of limonite after pyrite were
observed
within
several
fragments 1 in
thin section.
A-L Peak
The A-L Peak
Formation
Pormation
is
a composite
ash-flow
sheet
(X.L. Smith, 1960) with widespread occurrence in southwestern
A-L Peak
Socorro County. The formation was named after
the
northern
They
San
described
ash-flow
the
tuffs
Formation
Mountains
by
Deal
'formationa sequence
as
unconformably
overlying
of
and
Rhodes
(1973)
.)
crystal-poor
Hells
Mesa
witha total thickness of
1970 to 2300 feet,
(600
to 700 meters).
Smith
Mateo
in
the
1.7 m.y. fission track date (E.Ie
A 31e8
others,
1973) obtained from the basa.1 vitrophyre
and
of A-L Peak
Formation
to 32.4 m.y. K-Ar dates
is good
in agreement
on
the
with
underlying
the
Hells
30.6
Mesa
Formation.
The
Mt.
Mountains
Withington
is
cauldron
suggested
as
north-south
A-L Peak
flow
tuffs
trends
the
northern
San
Mateo
source
of the A-L Peak.ash
the
flows by Deal and Rhodes
(1973).
in
In the thesis area,
two
determined
indicate
a direction
from
lineated inpumice
consistent
with
the
L
1).
location of the cauldron (pl.
\
Outward
Formation
is
.
from
Mountains,
A-L the
Peak
the
San
Mateo
thinner,
and
andesite
.
flows
and
crystal-rich
ash flows are interstratified. In the southern Bear
Mountains,
theA-L Peak
Formation,
formerly
called
of Bear Springs(Brown, 1972), attains a thickness of
the
tuff
-16-
approximately 1000 fee% and was subdivided Brown
by
into 6
members. Woodward (.1973)recognizes 8 members in the
Nountains,15 miles east of Magdalena, where the
Lemitar
unit
hasa maximum
thickness
14.00 feet.
approaching
In the Silver Hill area, the A-L Peak Formation
is
divided into 6 members
which
are,
in ascending order, the:
1) gray massivemember, 2) tuff of La Jencia Creek,
3) flow-
banded member, 4) pumiceous member,5) interbedded andesite
flow, and6) tuff of Allen Well. An undifferentiated unit
is
mapped
when
a clear
distinction
between
the
gray
massive
member.and the flow-banded member cannot
be made. This unit
is
shown
only
on
the.geologic
nap
and
is
not
described
the text.
Thickness of the
A-L Peak Formation in the thesis
area
difficult
is
to.determine
accurately
because
of
i
fhults
and limited exposures;
a reaqona3le estimate is
700 to 900
feeti
Gray Massive Member. The lowest member of the A-L
Peak
Formation
in
the
Silver
Hill aarea
massive,
is
crystal-poor, rhyolitic ash-flow tuff. Basal andesite flows
observed in surrounding areas
(Brown, 1972;
Krewedl,
in
are
present.
not
pPeparation;
Chapin
and
Woodward,1973;
others,
in
preparation)
.The gray.massive member crops out sporadically
on
'
ridges
and
small
hills
in
the
eastern
and
.
western
portions
of the map area. Upper sections of the member awere
lso
encountered
(see pl. 1).
in
the
Crouch LL
and
& E diamond-drill
holes
The member 'is a'multiple-flow compound cooling
unit and the individual flows forn small
ledgesor, more.
typically, weathered, platy outcrops which comprise the
Usually, the tuff is light gray but
dip slopes ofridges.
densely
brown.
welded
portions
are
darker
and
slightly
reddish-
Altered outcrops have orange-pink to very pale
yellow
hues
The
unit
is
characterized
in both
hand
specimen
and
thin section by
a distinct paucity of crystals. Phenocrysts
rarely exceed 10 percent
of
the
total
volume.of
the
rock
and
commonly are less than
8 percent. Sharply euhedral sanidine
is themost abundant constituent, but quartz, biotite and
occasionally plagioclase alsoocckc.
lithic
fragments
are
Andesitic and rhyolitic
in portions of the unit.
observed
Microscopically, sanidine occurs as subhedral to
euhedral crystals, commonly twinned-.&d ranging in size from
1 ' c o . ' ~millimeters. Sanidine content varies between3 and
-
9 percent of the -total rock volume. The crystals are often
a dusty
brown
color
in
plane
light
to submicroscopic
due
clay particles. Microperthitic texture is frequently
observed.
In thin sections of altered tuff, sanidine.has
b.een largely
the
destroyed
original
dinera1
Anhedral
quartz
leaving
only
an
occasional of
patch
along edges
the of the cavity.
crystals
less
than
1 millimeter
in
diameter are present in most samples.
A few grains show
slight
embayment
and
holes
filled
with
groundmass
crystallites.
Quartz I;.:nerally makes up less 2than
percent of the total
volume of the rock. The small size of the quartz crystals
helps
to
distinguish
upper
tuffs.
Biotite
occurs
*he
A-I; Peak
in
many
tuffs
of
from
the
thin
sections
the
overlying
as
reddish
euhedral lathsrimmed by opaques. In samples from the
Crouch
drill
unaltered,
cleavage
hole,
buta few
traces
the
biotite
laths
are
and
Plagioclasz
is
is
brownish
slightly
and
essentially
hematized
along
fractures.
rare,
but
when
present,
it
is frequently.
altered to calcite or partially etched. The composition of
the
plagioclase
be determined.
not
could
Flattened pumice and
abundant
Ln
some
thin
crystal-rich
clots
to as
ranging
sections,
are
quite
much
as
20
percent of the rock volume. Dark gray andesi.tic lithic
fragments
with
microlites
ai? opaque
plagioclase
in
. .,.
of
groundmass are common, but these fragments are generally
subordinate
in
amount
to pumice and clots.
Devitrified glass shards and cryptocrystalline
to
/
microcrystalline
aggregates
of
potash
feldspar
and
quartz.
usually comprise a large majority of the groundmass. The
glass
in
shards
are
othe+s.an&are
Locally,
feldspar,
gas
conspicuous
frequently
pockets
quartz
Interstitial
more
filled
and
calcite
some
compressed
with
occasionally
and
in
brownish
and
euhedral
thin
sections
deformed.
crystals
of
potash
biotitebe can
seen.
dust
constitute
the
remainder of the groundmass.
Tuff of La Jencia Creek. Welded
to the gray massive.
member
isa crystal-rich
.
ash-flow
tuff
than
designated
the
tuff.
.
.
of La Jencia Creek
(Brown, 1972). The exact position of
this
tuff
within
is not lcnown.
La Jencia
the
lower
of Peak
the Formation
A-L
part
In the southern Bear Mountains, the tuff of
Creek
is
missing
and
the
gray
massive
is member
overlain by the flow-banded member (Brown, 1972).
A similar
sequence
was
observed
by
Woodward
(1973) in
the
Lenitar
Mountains. Where the tuff of La Jencia Creek cropsinout
the thesis area,
no overlying unitscan.be seen. The two
1) that the tuff occurs
most likely alternatives are:
interbedded
tuff
within
the
liesbetween the
gray
gray
o r 2) that the
member,
massive
massive
member
and
the
flow-
.banded member. With the information available, it is
doubtful this problem can
be resolved, Therefore, the tuff
of La
between
Jencia
the
gray
massive
member
‘Thetuff of La
Jencia
Creek
discontinuous
assigneda position
Creek been
has, arb5trarily
ledges
which
I
acd
flow-banded
usually
weather
to platy
members.
cropsas out
slabs
and
occasionally form dip slopes of small hills.
In some
.
.
T. 2 S.,
outcrops, such as ones in 8,
sec.
24, T. 2LS., R. 5
M., the
R. 4
W
. and sec.
welded contact with
the gray
.
massive member canbe seen. At the contact,a pumiceous and
I
relatively crystal-poor zone
at the topof the gray massive
member
grades
into
less
tuff of La JenciaCreek.
pumiceous
crystal-rich
rock
of
Total thicknessof this transition
ranges from2 to 4 feet. Fresh hand specimens from both
typically
densely
welded
and
have
a reddish-purple
zones
are
color.
Weathered samples are pale yellow-brown to light
the
brownish-gray. Data from modal analyses on the upper
'
portion of the
transition
gray
zone,
massive
and
member,
the
welded
of
L a Jencia
tuff
typical
contact,
Creek
the
are
listed in table
1 and shown in fig.
3. A photograph of the.
4.
welded contact is shown in fig.
In thin section, the tuff of La Jencia Creek
a
is
porphyritic
quartz
plagioclase,
groundmass.
percent
'
in
latite
quartz
and
Crystal
the
with
phenocrysts
of
biotite
in a devitrified,
content
generally
aphanitic
from
35 to 40
ranges
zona
to 25 percent
crystal-rich
sanidine,
in
the
transition zone. Otherwise, thetwo zones are mineralogically
identical. Typically, sanidine is more abundant than
.
plagioclase,
and
present
of the thin sections."
many
in
Sanidine
accessory
.
crystals
magnetite
are
and
generally
orthopyroxene
anhedral
and
are
subhedral,
and average from
1 to 2 millimeters in length. Most grains
appear to be quite
fresh
with
only
minor
incipient
calcite
along fractures. Twinning in sanidine is uncommon.
Plagioclase is usually
larger
than
sanidine
and
occurs
psimarilpas subhedral crystals, The degree of alteration'
,
varies
from
crystal
is
moderate
to intense
replaced
by
and
calcite
frequently
and
the
entire
Pine-grained
aggregates
of quartz. Phenocrysts of angular.quartz, occasionally
subrounded,
range
up
to 3 millimeters
in
diameter,
with
the
average being1.5 to 2 millimeters. Scattered laths of
fresh
Very
and
slightly
fine-grained
altered,
yellow-brown
magnetite'is
often
biotite
present
as
also
occur.
borders
,
Table 1
of the
- gray
Modal data in vol&e percent for the oftuff
La
massive member
from the Crouch drill hole.
Phenocryst
Proportions
Total.
Sample
number
Jencia
Creek
and
the
upper
phenocrbts"
Sanidine
Plagioclase
Quartz
Mafics
Pumice
Total
points
and
counted
Opaques
~
Top of tuff of La Jencia
1
"74-4
30034
2
M-74-5
40.73
Creek
16.64
'18.86
19.82
18.26
20.47
23.08
17.70
18e 32
17 30
14 90
'
44.60
36.93
39.21
37.44
37.20
"74-6
3.
4
5
6
7
"74-7
"74-8
M-74-9
"74-10-1
8
"74-11
37.46
9
"74-12
36.91
10
"74-13,
11 27.02
"74-14
'
Ilelded
12
13
14
35.60
contact
"74-15
"74-16
9.47
"74-17.
with
gray
12.44
,
,
5.65
-8e09
'
11.09
massive
9e 8 7
7.38
11.83
11.78
3.95
5.58
9.94
13-74
1.21
10.59
5.06
'6.57
8.90
10.90
10.13.
11.13
11-76
11.82
member
0.46
0,35
0.33,
2.48
'
6.10
5.57
3.08
2.90
0.32
2.50
1.45
0.66
0.48
0.45
1.26
1.66
6.80
4.83
1-47,
3.18
4.46
3.96
3.72
3.09
2.98
3.46
5.40
4.04
3.79
0.90
OQ42
.
2.52
6.20
1.03
10.12
9.21
'2.25
15.76
6,91
0.12
Base o f sequence
~~
*
remainder o f rock
isgroundmass, lithic
fragments
and
crystal
clots,
2176
2131
2301
2234
2135
2179
1000
1938
2075
, 2107
2191
2411
2302
2409
part
-22-
Depth
Sample
a
-
Figure 3 Modal data on the tuff
o f La Jencia Creek and
the upper part
of the gray massive member from the Crouch
drill hole showing mineralogical variations. Point-count
grid measured 0.5 X 0.5 millimeter. Modal data are in
volume percent taken from table
1.
-
Figure 4
Section of c o r e from t h e Crouch d r i l l h o l e
(Sec. 7, T. 2 S., R. 4 W.) showingweldedcontact(dashed.
l i n e ) between gray massive member of t h e A-L Peak Formation
( r i g h t c o r e and lowertwo-thirds
of middlecore) and t h e
o v e r l y i n g t u f f of L a J e n c i a Creek ( l e f t c o r e and t o p onet h i r d of mid&le c o r e ) . Notepumiceouszone
( p ) n e a rt o po f
gray massive member: t h i s zonegrades downward i n t o crystalpoor. .pumice-poor t u f f which i s t y p i c a l of t h e gray massive
member.
-24-
around
biotite.
Pumice
is
abundant.and
highly
compessed
towards
the
basal contact of the tuff. Andesitic lithic fragments. as
much
as1 centimeter
in
diameter
and
moderately
altered
occur ina large majority of the thin sections, The
groundmass is usually .devitrified. Compaction structures
can be seen,
than
in
klthough
other
A true
they
generally
ofA-Lthe
Peak
nelzbers
thickness
are
of
the
less
well-developed
Formation,
tuff
of
La
Jencia.
cannot
Creek
be determined from surface exposures. Partial thicknesses
range
pl. 1).
C-C',
e
from5 feet to approximately 80 feet.
a 206-foot
(cross-section
However, in the Crouch drill hole (see l)*'
pl.
section
of
the
nearly
flat-lying
tuff
was
observed. The upper contact is
a major erosional unconformity
at
thebase of
'sandstone
the
is
La
Jara
Peak
Andesite;
a thin
present
at
the
is weathered to a depth
of
about
6 feet.
Exposures
a crudely
of
the
developed,
tuff
contact
of
La
and
Jencia
northeast-trending
layer
the
wide
and
occur
which
the central portion of the thesis area pl.
(see1).
belt is approximately
2 miles
underlying
Creek
belt
of
tuff
along
transects
The
discontinuous,
being.
To the
paftially obscured by La Jara Peak Andesite.
southwest,
.the
belt
is
truncated
by the
Well fault (pl. 1 and fig. 11, p. 68).
of
the
tuff La
of Jencia
Creek
north-trending
Hale
However, outcrops
continue
outside map
the
area to the northeast
(C.E. Chapin, oral commun.,1973).
The
Creek
limited
geographic
suggests
that
the
distribution
tuff
was
of
the
tuff
of
deposited
in
a paleovalley
La
Jenc
which
is
interpreted
to be fault-controlled
but
may
be
entirely a result of differential erosion. Other examples
of a paleovalley
during
A-L
Peak
time
are
located
in
the
Lemitar Mountains (kroodward,
19731, and ina northwest
extension of Stendel Ridge in the Kelly district (R.B.
Blakestadb oral commun,,1973).
paleovalley
the
was
Lemitar
fault-controlled
Mountains
formed
whereas
by
the
paleovalley
in
erosion,
The flow-banded membes of theA-L
Flow-banded >le-.
Peak
Formation
tuff
witha conspicuous
(fig. 5).
The Stendel Ridge
is
a densely
welded;
streaky
crystal-poor,'
and
banded
ash-flow
appearance
A similar unit is recognized in several localities
in the Magdalena area and in the Lemitar Mountains, Ifiappable
outcrops
of
the
flow-banded
limited to a narrow
ridge
memberin,the
bordering
thesis
Arroyo
area
are
Landavaso
north
of Lardavaso Reservoir. Outcrops of
a pla,ty, densely
welded tuff along La Jencia Creek
(sec. 6, T, 2 S , , R. 4 Ut.)
may
also
flow-banded
but
are shown as
member
undifferentiatedon pl. 1, A flow-banded tuffalso
A-L Peak
occurs
belong
to the
near
the
bottom of theLL & E drill
hole
located
on
!
the so3thwest slope of Silver lIill 1(pl,
).
A measured
section
in
the
southern
Bear
Mountains
(Brown, 1972) shows the flow-banded member overlying and
welded to the gray massive member..
Poor exposures
this
relationship
in
the
Silver
Hill
obscure
area.
The best surface exposure of the flow-banded tuff in
the
map
area
is seen
ina large
roadcuton U.S.
IIighway 60,
3.75 miles west of Magdalena. At the westernend of the
roadcut, the tuff is massive and relatively fresh 51,.
(fig.
whereas to the'east,
it
(see fig. 15, p. 82).
altered
and
severely
fractured
Limonite 'and occasionally
epidote
foundo n fracture
are
is
surfaces
5.n
the
eastern.section
of the
roadcut. The tuff,.where fresh, is light purplish-gray and
contains a few
feldspar
crystals,
lithic
fragments
ana
elongate, dark, lenticular streaks. These strea,ks may
represent
which
low
pressure
volatiles
areas
formed
during
flowage
along
accumulated.
Petrographically,
the
flow-banded
member'varies
rhyolite to latite with phenocrysts
of sanidine,
from
plagioclase
and quartz ina cryptocrystalline groundmass. The
phenocryst
content
is
variable
rangesl t from
o 6
and
.. ...
volume percent. Compressed pumice and crystal clots
constitute 20 to 30 volume-percent. Andesitic lithic
fragments
altered,
are
present
in
minor
euhedral,.plagioclase
amounts
a& typically
laths
in
an
contain
opaque
matrix;
7
lithic fragments of light-colored intrusive
rockare also
present.
1
Sanidine, commonly microperthitic, occurs as subhedral
t o euhed3al
crystals
which
average
less 1than
millimeter
in diameter. Plagioclase is occasionally present in minor
amounts
and
shows
a variety
Scarce
quartz
crystals
of
are
twinning
anhedral
and
and
alteration.
occasionally
slightly embayed. The grounhass is composed primarily
of
microcrystalline
quartz.
and
potash
feldspar
. .
with
locally
-27-
-
Figure 5
Flour-banding developed in the flow-banded
member of theA-L Peak Formationexposed.in large roadcut
on U.S. Highway 60. Note that the streaks bendaround
two lithic fragmentsin upper left corner of the
photograph.
L
conspicuous devitrified glass shards, Small amounts of
calcite
and
submicroscopic
finely
whichbe may
material
divided dust occur interstitially.
I
Pumiceous
Member.,
Peak
Formation
tuff
equivalentto the
The
is
a densely
pumiceous
welded,
upper
tuff
member
of
A-L the
crystal-poor,
of
Bear
ash-flow
Springs
in
the
of the
southern Bear Kountains (Brown, 1972). Outcrops
pumiceous
located
member
in
occur
the
Additional
primarily
northwest
outcrops
are
corner
member
is
of
Allen
the
Well
thesis
which
is
area.
southo f Allen Well
present
2 miles
in sec. 12, T.
2 S., R. 5 If.
pumiceous
near
In the Bear Mountains, the
separated
from
the
member
flowbanded
by andesite flows but these flowsnot
areobserved in the .
study area. The'upper contact of the'pumiceous member is
irregular,
and
apparently
represents
a paleoerosion smface
with'a gently rolling topography. Andesite flows and the tuff
of
Allen
Well
canseen
be
in
depositional
contact
with
the
pumiceous member in outcrops along the lower reaches of
'
Council Bock Arroyo (sec.
1, T. 2 S . , R. 5 VI.).
,
of 280 feet
has
been
measured
for
the
A thickness
member
in
the
southern Bear Hountains (Brown, 1972). Poor exposures
prevent
thickness
an
accurate
determined
measurement
by
Brown
in
(1972)
the
was
Silver
Hill
area;
used
in
cross-
sections A-As and B-B' ( p l . 1).
The unit crops out as ledges and small cliffs which
weather to a g r u s of small platy fragments. Fresh samples
are purplish-gray and contain abundant lineated pumice and.
.
a few phenocrystsof feldspar, On weathered surfaces,
the pumice is frequently etched, Altered samples are
usually
yellowish-brown;
intensely
bleached
samples
are
white. When pumice and phenocrysts are obliterated.by
bleaching, the tuff can
be confused with caliche capping
nearby pediment gravels. Bleached samples characteristically
show hematite staining fractures and cubic outlines
of
pyrite
pseudomorphs
with
a halo
of
limonite.
Microscopically, the pvmiceous member is rhyolitic in
composition.
sections
of fresh
Thin
porphyritic
texture
samples
have
a
with
a cryptocrystalline to
microcrystalline groundmass. Phenocrysts of sanidine,
varying in size from
0.5 to 2 millimeters, constitute less
pumice
than 3 percent of the.tota1 rock volume, Flattened
and
crystal
clots
are
abundant
and
constitute
as 25
much
as
volmne percent in some thin sections. The groundmass is
composed
.
primarily
of.potash
feldspar,
quartz
and
devitrified
glass shards. In thin sections of altered tuff, remnants
of
sanidine
millimeter
and
are
in
intensely
length;
coarse-grained
altered
pumice
and
toandclay
average. 1.
crystal
intergrowths
o f potash
clots
obscured,
are
feldspar
and
quartz compose the groundmass. Limonite pseudomorphs after
pyrite are 'moderately abundant. Near silicified portions of
the tuff, potash feldspar in the groundmass frequently occurs
as
radiating,
fibrous
crystals
which
may
represent
reccystallization. In addition, sanidine contains specks
which
have
high
birefringence,
high
relief
ana
parallel
,-
extinction. These specks are thoughtto be sericite.
Andesite Plows.
Thin andesite flows
and volcaniclastic
sandstones of andesitic
These
andesites
composition
0ccupy.a
cooling
occur.
break
near
Allen
between
Well.
ash
flows
the pumiceous member and the tuff of Allen Well. The
andesites are absent elsewhere in the thesis area. The
andesite flows occur as discontinuous outcrops indicating
that
the
underlying
The
flows
filled
pumiceous
andesites
topographic
l o w s developed
in
the
member.
aze
typically
poorly
exposed
although
3 G 9 T. 1 S,,
good exposures occur along Dry Lake Canyon (sec.
R. 5
W., unsurveyed).
alteration
and
Outcrops show varying degrees of
consequently
form
either
depressions
between
more resistant tuffsor small rounded'ledges. Thin basal
flow. breccias are present locally. Hand specimens
are
medium
gray
brown when
on
fresh
surfaces
and
reddish-
to
yellowish-
altered.
Microscopically; the andesites contain scattered
phenocrysts of oxidized pyroxene about
0.5 millimeter long
in a felty groundmass of plagioclase microlites. The
centers of the pyroxene are totally altered
to calcite and
a fibrous variety of chlorite, possibly pennine. Opaque.
I
iron oxides form the perimeter. Plagioclase microlites
J-
about 0.2 millimeter
long
are
frequently
twinned
and a show
range in composition from An55
to A I I ~ ~ Most
.
of the
to calcite.
plagioclase is moderately altered
Thin layers of volczgiclastic sandstone occur
of
randomly near thetop,ofthe andesite flows. In hand
to medium-grained with
specimen, the sandstones are fine-
colors similarto those of the andesites.' Microscopically,
the sandstones consist primarily of quartz and feldspar
with a fine-grained.cement
Epidote
anda zeolite
of
occur
granular
chert
and
calcite,
small
amount.^.
in
The andesite flows exhibit effects of weak
argillization
Canyon
as
which
can
be seen
irregular
in
discolored
outcrops
patches
along
Dry
.
Lake'
thought
to have
comm.,
formed fro'm channelized solutions (C.E. Chapin, oral.
1973) (fig. 6).
In places, these solutions appear
to have
favored the more permeable sandstone layers, but probably
are
controlled
by
fractures
which
sandstones.
theand
.
transect
both
the
andesites
..
.Tuff of Allen Well. The uppermostmember of the A-5
Peak
Formation
is
a crystal-rich
ash-flow
tuff
named
the
tuff of Allen Well Brown
by
(1972). The member is exposed
in
the
northwest
corner
of
the
thesis
area3 small
and in
12, T. 2 S.;
outcrops 2 miles south of Allen Well (sec.
.
R. 5 W.).
The
and
tuff
of
Allen
weathersto blocky
Well
talus
crops
similar
out
to
as
the
cliffs
Hells
and
ledges
Mesa
Formation which it strongly resembles. Excellent exposures
7 , -
occur
within
Dry :Lake
Council
Rock
Arroyo
and
along
Canyon at the northern boundary
of the
the
north
thesis
area.
An accurate thickness of the:tuff cannot
be obtained
A partial
because the top of the unit is not exposed.
bank
-
Figure 6
Outcrop of partially altered andesite near Allen
Well. ,Tan areas are bleached and exhibit weak argillic
alteration; gray areas are relatively fresh andesite.
Chapneling of solutions along fractures probably produced
the contrasting alteration effects.
-22-
thickness of about 100 feet is indicated
on cross-section
A-A'
tuff
is usually about60 to 70
(pl. 1), although.the
feet thick.
In hand specimen, the tuff of Allen Well: closely
resembles
tuffs
of
groundmass
of
Hells
the-
Hells
Mesa
Mesa
tuffs
Formation. but
.
are
more
the
crowded.
with
sma
crystal fragments. The tuff of Jencia
La
Creek is also
of Allen Well. Bothof these tuffs
similar to the tuff
contain
more
biotite
and
are
slightly
less
crystal-rich
than Hells Mesa tuffs. Distinction between these three
units isnot easily madeexcept where stratigraphic .'
relationships canbe seen. Fresh hand specimens are pale
pale brown.
purple to purplish-gray and weather
altered
tuff
Phenocrysts
-altered
are
are
typically
more
samples
and
mottled
Samples of
brownish-yellow:
.. . ..
conspicuous
in
consist
sanidine,
of
in than
samples
fresh
plagioclase,
quartz and biotite. The latter two minerals increase
slightly
in
abundance
Microscopically,
cryptocrystalline
'Sanidine
is
the
from
the
the tobase
the top of the unit.
tuff
of Allen
quartz
latite
most
abundant
Well
is
a
with
a. porphyritic
phenocryst
15 to 20 volume percentof the rock; it'occurs
and subhedral
crystal
fragments
with
an
texhre.
and
as
comprises
from
anhedral
averageof size
1
millimeter. Minor amounts of clay and sericitic alteration
can be seen
in
most
grains
with
clay
alteration
abundant. Intensely altered crystals appearas cavities
partially
/
rimmed
with
remnants
of original
sanidine.
the
more
,
-34-
.
.
Plagioclase, less abundant and smaller than sanidine,
of the
usually occurs as subhedral crystals, Size
plagioclase crystals is quite variable, averaging less than
1 millimeter
in
some
sections
to about 2 millimeters
thin
in others. Calcite and clay alteration are common in
and,
one thin section of altered tuff, the plagioclase
been has
partially corroded in the center. Quartz occurs as anhedral
.and
subhedral
crystals
which
are
often
rounded
and
partially
resorbed. The crystals range in size from less 1than'
millimeter to 5 millimeters
in
diameter
with
an
average
of
about 2 millimeters. Total volume varies between
3 and.10
percent. Yellow-brown biotite is commonand occurs as
euhedral
laths
which
1 millimeter
about
average
but may be as large 3 as
millimeters.'
altered
tuff,
biotite
has
a ragged
In
in
thin
appearance
length,
sections
of
with.-occasional
/.
inclusions of rutile.A fine-grained mixtureof sericite a h d
livanite often surrounds these grains.
A devitrified
groundmass constitutes60 to 65 percent of the
total
volume
of the rock. Limonite pseudomorphs after pyrite, and patches.
and
bandso f iron
oxide
are
locally
Andesite of Landavaso
The.andesite
of
Landavaso
Landavaso Reservoir(C.E.
the
southwestern
abundant,
Reservoir
Reservoir,
named
for
Chapin, oral commun.,1972) in
of the
part
Silver
Hill
area,
is a highly
variable seriesof porphyritic andesite flows: The andesite
is correlative
with
several
units
Loughlin and Koschmam (1942) in
the
mapped
and
southwestern
described
portion
of
by
the Magdalena mining district, In the Lemitar Mountains,
tioodvlard (1973) recognizes a similar
andesite
which
he
maps
as the basal unit of the Potato Canyon Forplation. The
andesite of Landavaso
Mountain-Tres
Reservoir
Montosas.
area
is.also
(Wilkinson,
observed
in
in
the
Cat
preparation).
In the thesis areap outcrops of the andesite
of
Reservoiroccur predominantly
Landavaso
Highway 60 (pl. 1).
.
south
of
U.S.
To the north, scattered outcrops
border the western margin of the ,thesis area from the
R. 5 VI. to Hill 1t704811
southern boundaryof see. 2 5 , T. 2 S.,
approximately 2 miles to the north (pl1).
.Reservoir,.
the
the A-L Peak
andesite
Formation
overlies
and
is
the
overlain
Near Landavaso
flow-bandedofmember
by
the
upper
tuffs.
Maximum thickness indicated on cross-section
D-Dl (pl. 1) is
.. ._.
800 feet. Woodward
.the
Lemitar
(1973) records a similar thicknessin
Mountains
where
a correlative
andesite
occupies
a paleovalley; outside the paleovalley, the andesite is only
about 20 feet thick. The andesite of,Landavaso Reservoir
has not been
dated
radiometrically
at31.8 2 1.7 m.y (A-L Peak)
dated
(Potato
Canyon)
by
the
fission
but
lies
between
units
and30.3 2 1.6 m.y.
track
method
(Smith
and
others, 1973).
Where well exposed, the andesite
of Landavaso Reservoir
forms hills
with
ledges
and
small.cliffs
which toweather
slopes of blocky talus. Poor exposures are partially
covered
with
by
the
pediment
of a$;rial
use
gravels
and
usuallybe can
found only
photographs.
-36-
The andesite of Landavaso Reservoir
is composed of a
number
of
individual
flows
which
are
highly
variable
in
composition and appearance. Thickness and aerial extent
of
the
flows
cannot
be determined because of limited and
discontinuous outcrops. Basal flow breccias are quite
A dense,
common.
alternating
to
black vitrophyre with
a zonc of
dark
and
light
bands,
ranging1 millimeter
from
2 centimeters in width, occurs on 11692411
Hill in the
extreme southeastern corner of the thesis area. Samples
from
massive
Fresh
portions
surfaces
are
of
flows
commonly
vary
shades
considerably
in
color.
reddish-gray
or gray
of
and weather light brownish-gray. Near faults, colors
frequently become black o r dark .brown. Irregular' bands of
hematite
staining
are
a common
feature
. .. of
most
outcrops.
of the andesite is
Microscopically, the texture
porphyritic
with
arranged
in a felty
phenocrysts
to
pilotaxitic groundmass. Phenocrysts usually comprise from
15 to 40 percent of the total rock volume and consist of
plagioclase, pyroxene, biotite and hornblende. Plagioclase
phenocrysts
comprise
occur
in
all
thin
from10 to 30 volume
Plagioclase
is
commonly
sections
of
the
andesite
and
percentof the rock.
euhedral
and
ranges
in
length
from
less than1 millimeter to 6 millimeters. Zoning with more
calcic
cores
is
frequently
observed
and
is
quite
pronouced
in untwinned crystals. Alterationis present in varying
degrees
buta large
Calcite and clay
number
are
of
typical
the
phenocrysts
alteration
are
products
fresh.
and
occupy
cracks
and
transverse.fractures
through
the
phenocrysts.
Chloritic alteration is rare. The composition of the
plagioclase
was
measured
by
the
Fouque'
method
on
several
unaltered crystals in each thin section. The' anorthite
content
ranged
to An54 with
An41
Trom
most
occurring
in
the
An47 to Anso range.
Pale
thin
green
sections
clinopyroxene
and
is
occurs a in
majority
usually
subordinate
of
the
into amount
plagioclase. The crystals, occasionally glomeropbrphyritic,
1 to 1.5
very from subhedralto euhedral and average from
millimeters'in greatest dimension. Measurements of
extinction angles(ZAC) range from36 to 44 degrees
with
optic an.gles(2V) of 40 to' 50 degrees. These figures are
compatible
with
those
of
augite,
..
,
subcalcic
augite.or
augite (Heinrich, 1965,p. 218).
Alteration products
consist
and
primarily
of
calcite
ferroan
...
hematite
with
minor
amounts
of chlorite and magnetite. Light
brown hornblende and
vermicular celadonite (Loughlin and Koschmann,
1942) are
seen
in2 thin
Biotite
sections
occurs
in
(fig.
7).
fewer
thin
sections
than
and constitutes from
2 to 5 volume percentof the rock.
pyroxene
The
laths average from
1 to 2 millimeters in length. Most
crystals
but
are
fresh
intensely
yellow-brown
altered
to chlorite
biotite,
and
.frequently
magnetite
with
inclusions
of rutile (sagenitic),is occasionally seen..
Hornblende
phenocrysts
are
rare,
occurring
in 1only
thin section. The phenocrysts are euhedral range
and from
.
Figure 7
..
-
Photomicrograph of clinopyroxene phenocrystin
the andesite of Landavaso Reservoir showin alteration to
vermicular celadonite (center of crystal) X-nicols, x80
magnification).
f
2 to 8 millimeters in.length. A l l crystals are completely
altered
to'chlorite
and calcite
in
the
core
and rimmed by
fine-grained magnetite. A few specks of biotite occur
inside some grains.
The
groundmass
microlites
with
Cryptocrystalline
is
interstitial
and
glassy
A fine dust
frequently.
of plagioclase
primarily
composed
pyroxene
and
magnetite.
groundmasses
occur less.
of hematite is.present
in most thin
sections
divide
From the petrographic data, it is possible
to
the
andesiteof Landavaso
of flows
Reservoir
into
4 different
types
based upon 'the major phenocrysts:
1) plagioclase-
'
pyroxene, 2) plagioclase-biotite,3 ) plagioclase-pyroxenebiotite, and 4) plagioclase-hornblende. The. series is
listed in orderof decreasing abundance. Because exposu-es
are lirnited,it could not
be determined where these
f l o w s lie
toone another
in relation
.
crystal-rich
name
and
the
thesis'
area.
Upper Tuffs
The upper tuffs(C.E.
is an informal
in
Chapin, oralcommun,, 1972)
proposed a
for
series of variable
crystal-poor
ash-flow
tuffs
overlying
the
andesite of Landavaso Reservoir. These tuffs may be
correlative to the Potato Ca.nyon Rhyolite described
by.
Deal
and
Rhodes
(1973) which
overlies
the
A-L Peak Formation
in the northern San Mateo Mountains. However, some outcrops
o f upper
tuffs
in
the
Silver
Hill
area
crystals than most crystal-poor tuffs
of the
have
Potato
noticeably
CaEyon
les
Rhyolite. These outcrops maybe equivalent to the Beartrap
(Deal
and
1973) which
Rhodes,
Rhyolite
and
fills
Canyon
Formation
Potato
Canyon
.overlies
moat Mt,
of
the
the
the
Withington cauldron. An exact stratigraphic correlationof
the
upper
Beartrap
tuffs
to either
Canyon
the
Formation
Potato
is
not
Rhyolite
or the
Canyon
possible
at
this
time
owing to discontinuous exposures and ambiguous stratigraphic
relationships. Andesites, similar in stratigraphic position
to
the
andesite
lithologically
of
variable
(Woodward, 1973).
andesite
of
Landavaso
Reservoir,
tuffs
in
the
are
overlain
by
Lemitar.
Mountains
Woodward correlates these units with the
Landavaso
Reservoir
and
the
Potato
Canyon
Rhyolit
respectively. Woodward also recognizesa distinctive
crystal-poor
tuff
witkin
the
Potato
Canyon
Rhyolite
which
ma
be similar to the distinctive crystal-poor tuff within
the
upper tuffs in the Silver Hill area. Rocks similar
to upper
tuffs
are
Magdalena
also
area
(Wilkinson,
in
observed
including
in
the
preparation)
several
Cat
and
other
localities
Mountain-Grey
the
Nagdalena
Hill
in
the
area
Mountains
(Krewedl, in preparation; R.B. Blakestad,
. o r a 3commun., 1973).
The
upper
tuffs
have
a minimum
thicknessof approximately
600 feet, as estimated from cross-section
D-Dl ( p l . 1).
However, maximum thickness may
be much greater. Thickness
of
the
Potato
Canyon
Rhyolite
in
the
northern
San
Mountains is approximately
3280 to 5575 feet (1000 to 1700
meters) (Deal and Rhodes,
1973).
Formation
The Beartrap Canyon
attainsa thickness'of 820 feet (250 meters)
near
Mateo
the same locality (Deal and Rhodes,
1973).
The
upper
tuffs
in
the
Silver
Hill
area been
have.not
dated radiometrically. However, a sample from200 feet
above the base of the Potato Canyon Rhyolite in the type
locality
yieldeda fission
track
of 30.3 2 1.6 m-ydate
(Smith and others,
1973).
The upper tuffs are best exposed in the southwestern
part of the thesis area. They form resistant ledges and
cliffs
to blocks
weather
and
Dip
slopes
are
usually
The
tuffs
vary
from
bounded
by
comprised
of small
joLnt
angular
Arroyo
crystal-rich ofwest
to crystal-poor eastof Arroyo
Landavaso
surfaces.
fragments.
Landavaso
where
they
overlie
the andesite of Landavaso Reservoir. The crystal-rich
varieties
surfaces
Samples
light togray
light
are
and
are
weather
typically
.. brownish-gray
, .
on
pinkish-tan
or light
densely
welded
fresh
grayish-purple.
and
contain
phenocrysts
of colorless and smoky quartz, chatoyant sanidine (moonstone),
plagioclase and copper-colored biotite. . Sanidine is
usually
the
most
abundant
phenocryst
but’some
varieties
contain as much 15
as percent quartz,. The crystal-poor
tuffs
The
canbe divided into a’basalunit and an upper unit.
basal
Reservoir
in
this
purple,
unit
and
unit
and
directly
overlies
attains
a thickness
are
are
poorly
of
welded,
characterized
by
the
andesite
of Landavaso
about
250 feet. Tuffs
to light’
grayish-white
abundant,’
botryoidal
pumice is partially
pumice. On weathered surfaces, the
etched giving the rock
a sponge-like appearance. The basal
..
tuffs grade into moderately
to densely welded, light to
medium gray tuffs. Abundant andesitic lithic fragments are
typical of this unit but small, compressed, botryoidal
.
.
pumice also.occur. The crystal-poor tuffs usually have
phenocrysts of chatoyant blue sanidine, quartz, plagioclase
and
rare
biotite.
Small
outcrops
of
north-trending
fault
lt7O48lt (pl., 1).
The
upper
zone
tuffs
from
outcrops'
also
along
tc Hill
Driveway.
Well
Hale
are
occur
a
exposed
znd in most
poorly
instances reston the andesite of Landavaso Reservoir. One
.
.
isolated
outcrop
is
highly
be a fault sliver (fig.8 ) .
silicified
and
is
interpreted
to
Along the fault zone, both
crystal-rich and crystal-poor varieties are present. Fresh
hana
specimens
are
typically
. light
.. and
gray
weather
bropmish-gray. Phenocrysts are chatoyant blue sanidine,
plagioclase and colorless quartz. Pumice and andesitic
lithic
fragments
Two additional
are
locally
outcrops
at Rabbe Well nearU.S.
abundant
of
ash-flow
are
tuffs
small.
are
Highway 60 and 1-milesouth of
Rabbe Well (sec. 31, T, 2 S., R. 4
similar'in
but
appearance
and
W.).
These tuffs are
located
.
.
lithology
to the upper tuffs
exposed on the west side
of Arroyo Landavaso.
An outcrop of light
is
grayto orangish-pink
present1 mile east of Boxcar
Well
in
of the thesis area (sec.
1, T. 3 S., R. 5
well-indurated
and
stratified
and
air-fall
the
Vi.).
contains
southern
tuff
part
The tuff is
abundant
andesitic
fragments, and tuffaceous ash and lapilli. Pnenocrysts of
sanidine and quartz are scarce.
In
thin
porphyritic
section,
texture
the
upper
tuffs
and
a devitrified
rhyolitic
a
are
with
microcrystalline
6 to 35 volume
groundmass. Phenocryst content varies from
percent. Sanidine is frequently the most abundant .
phenocryst and occurs as anhedral
to euhedral crystals.
Although
commonly
fresh,
a few
crystals
show
minor
clay
alteration along cleavage cracks. Quartz in some
saples is
more abundant than sanidine. The quartz grains are often
rounded
and
range
size 0.from
2 millimeter to 3
in
millimeters. Both sanidine and quartz show effects
of
resorbtion. Plagioclase is minor in amount but frequently
fresh. Two measurements of extinction angles gave
compositions of An35 and An39. ..Biotite
varies from absent
. ..
to 3 percent of the total
rock volume. Typically, it is
'partially o r totally altered to opaques o r chlorite. The
groundmass
is
composed
primarily
of
microcrystalline
aggregates of potash feldspar and quartz. In some thin
sections, spherulitic cha.lcedony makes 60upt o 70 percent
of the groundmass. Chlorite, calcite and iron oxides are
minor constituents.
Unit
ofArroyo Montosa-
Outcrops of interbedded volcanic rocks and conglomerates
occur along the western margin of the thesis area
1). (pl.
These
outcropshave been
tentatively
designated
as
the
unit
of Arroyo Montosa. Originally, this unit was thought to be
a facies of the Popotosa Formation. The
two rock units are
very similar at first glance bnt the conglomerates
of the
Popotosa
Formation
contain
abundant
fragments
of La
Jara
Peak Andesite. Field work by the'author and C.E. Chapix
has shown that: 1) the Arroyo Montosa conglomerates lack
clasts definitely recognizable as La Jara Peak Andesite, . .
and 2)
volcanic
Montosa
lie
rocks
with
interbedded
depositional
in
contact
the ofunit
Arroyo
on
the
eroded
Hale
Well pluton. These observations indicate that the unit
of
Arroyo
Montosa
is
definitely
younger
than
the
monzonite
pluton (approximately 28 m.y.) and predates La Jara Peak
Andesite (23*8m.y:;
1.2 m.y.
Chapin, 1971a). A I<-Ar date of 25.2 2
recently obtained from
a sample of a latite
T e 2 S., R. 5
the unit of Arroyo .Montosa 14,
(sec.
flowin
W.,
unsurveyed) substantiates the above.,. observations.
In
this
investigation,
the
unit
of
Arroyo
Montosa
is
divi3ed into2 mappable, interbedded facies: a volcanic
facies anda conglomerate facies. In cross-section C-C'
(pl. 1), the volcanic facies was arbitrarily chosen as the
lowermost member. At the Hale Well pluton, the volcanic
facies
is
facies
rests
the north, the conglomerate
lowest, to
but
the
on
upper
tuffs.
Volcanic Facies. The volcanic facies consists
flows
varying
from
quartz
latite
to latite
and
of k%va
dacite,
with
of flows present
dacite being dominant. The total number
VJithin the unit
of Arroyo Piontosa cannot
be determined
because of limited exposures. Available outcrops indicate
that at least
2 individual flows occur. Likewise,maximum
.
.
-46-
thickaess of individual
flows
is
not
known;
a minimum
(pl. 1) is 100
C-Ct
thickness obtained from cross-section
feet, but the top of the flow is eroded.
'
The volcanic rocks are generally poorly exposed and
.
.
form slopes and caps
on small hills along the western
boundary of the thesis area. Basal flow breccias are seen
in
some
places
and
contain
fragments
derived
from
the
underlying conglomerates. Hematite is pervasive in the
to the north., Color of
southern outcrops and diminishes
samples
yaries
from
dark
reddish-gray
to light
gray
corresponding to the amountof hematite present. Calcite
veinlets
are
widespread
moderately silicified,
On
and
local
outcrops
are to
slightly
.
Hill11704811,numerous subangular boulders of the
volcanic facies are
found resting on the conglomerate fa.cies,
Some of these
are
boulders
are
believedto represenkthe
as
large6 feet
as
bouldery
in
remains
length
of
and
former
outcrop.
In hand. specimen, the flows contain phenocrysts
of
plagioclase, smoky and colorless quartz, and sanidine.
Som
plagioclase crystals attain dimensions 3..of
75 centimeters
in length and
2 centimeters in wid-th. The large plagioclase
is a distinctive
feature
which
makes
the
flows
easily
recognizable in the field.
In thin section, the volcanic rocks ahave
porphyritic
texture with a microcrystalline
account
-
groundmass. Phenocrysts
'
.
for20 to.30 percent of the total volume of the rock.
-4 I -
Plagioclase
occurs
as
unaltered,
euhedral
crystals
which
are frequently twinned according
to albite, albite-carlsbad
Most phenocrysts have lacy edges
and periclinetwin laws.
owing to minute inclusions of glass. The anorthite
composition
measured
by
the
method from
-ranges
An29
Pouque'
to.An37. Sanidine is present in amounts varying 2from
to
15 volume percent of the rock. The crystals vary from
anhedral
to
euhedral
and
to 1.5 millimeters
average 1from
to
in greatest dimension. Sanidine is commonly altered
calcite and brownish clays. Anhedral and subhedral quartz
1.to 3 percent of the total volume
phenocrysts comprise from
of the rock. .The quartz phenocrysts are typically deeply
e
embayed
and
rimmed
by
fine-grained
aggregates
of
calcite
and
chalcedony. Highly altered biotite and .amphibole occur in
trace amounts in some of the flows. The groundmass consists
predominantly of plagioclase laths, interstitial qv.artz and
potash feldspar, and opaques. In some thin sections, the
opaques, 'vlhich are primarily magnetite, constitute
10
percent of the rockby volume. Pyroxene, calcite and
secondary
quartz
are
also
present
in
the
groundmass.
Conglomerate Facies. The conglomerate facies consists
of
pebble
conglomerates
of
fluvial
origin
with
thin,
interbedded sandstone lenses. A total thicknessof the
facies and the number of individual conglomerate units is
not known.
thickness
On cross-section C-C'
varies
(pl, l), the estimated
500 to 700 feet.
.from
The conglomerate facies crops outsmall
as ledges which
-
-48-
are persistent along the base of ridges and hills. The
outcrops
are
characterized
by
predominantly
westward
dips
of 6 to 14 degrees, buta 10 degree southeastward dip is
25, T. 2 S.,
recorded near the Hale Ranch driveway (sec.
R. 5
The discrepancy is probably
a result of drag from
We).
uplift
ofa major
intra-graben
block
to the
fault
northwest
The conglomerates are poorly sorted, crudely
(pl. 1).
stratified, and well-indurated (fig.9).
Clasts consist. of
fe1si.c and andesitic volcanic rocks.. Imbrication of clasts
indicates a source to the southwest
(J.E. 'Bruning, oral
commun., 1973)
In hand specimen, the color of the matrix is reddishA iarge
brown on both fresh and weathered surfaces.
majority of the clasts are subangular'but vary from angular
to subrounded, The clasts can be as large7 to
as 8 inches
to 2 inches. Welded tuffs
in diameter but commonly 1are
from
the
Potato
Canyon
Rhyoliteto and
a lesser
extent
from
the A-L Peak Formation comprise most of the clasts.. The
remainder of-thefragments
are
andesites
from
various
units
in the Datil volcanics. Clasts of Arroyo Montosa volcanic
rocks occur throughout the conglomerate fac'ies and are
easily
recognized
Some
andesitic
they
are
to have
by
their
clasts
present
originated
in
large
resemble
such
from
La
small
flows
plagioclase
Jara
Peak
volume
inA-L
the
Peak
that
phenocrysts.
Andesite
they
but
are
belie
Formation
which are very similar
t o those in the La Jara Peak,
If
the
Arroyo
Montosa
conglomerates
were
younger
than
La
Jara
-
Figure 9
Crude stratification developed in the conglomerate
facies of the unit of Arroyo Montosa. Note the abundance
of
felsic volcanic detritus and relative scarcity
of darkcolore6 andesitic detritus. !Phis is typical of the facies
Dry Lake
and distinguishes it from the fanglomerate
of
Ctinyon. Note Tebble imbrication which 'indicates transport
was from left S1d) to right.
.Peak
Andesite,
Jara
Peak
the
detritus,
but
La
The
last
represented
should
be crowded withLa
conglomerates
this
Jara
is
Peak
volcanic
event in
bya thick
observed.
Andesite
the
sequence
not
Silver
of
Hill
basaltic
area
is
andesite
flows
with thin beds of intercalated, volcaniclastic sandstone.
?{ember
Tonking (1957) named this unit the La Jara Peak of
the
Datil
Formation
after
La
northof
Jara20 Peak,
miles
Magdalena in the Puertecito quadrangle. The La Jara Peak
Member
was
later
separated
from
the
Datil
rocks
by
Willard
(1959) and is herein considered
t o have formationa.1 status?
Outcrops of La
Jara
portionof the
central
Peak
thesis
Andesite
area
.
occupy ofmuch
the
cover
and approximately 6
. .
square miles. Local outcrops occur along Council Rock.
Arroyo and near Arroyo Landavaso. La Jara Peak Andesite
rests
unconformably
upon
tuffs
of the A-L Peak
Formation,
the andesite of Landavaso Reservoir, and upper tuffs. Along
Council
Rock
Arroyo,
the
fanglomerate
of Dry
Lake
Canyon,
an
andesitic faciesof the Popotosa Formation, apparently
conformably
largely
overliesLa Jara
from
Formation
the'
Jara
Thickness
Andesite
and
was
derived
e
In' the northwestern Bear Mouzztains, the
it
Popotosa
La
Peak
is
Peak
of
interbedded
Andesite
the
La
in
the
of
uppermost
part
(Tonking,
1957; Bruning, 1973)
Jara
Peak
Andesite
in
the
Two.
Hill area is'difficult.to determine aCCU.rately.
diamond-drill
andesite
i
holes
penetrate
(Crouch
and
LL & E) collared
600 feet of
approximately
La
in
the
Jara
Peak
Silver
but
cross
numerous
A minimum
section.
small
faults
whichmay have
(pl. 1).
(1972)
much
the
It
Bear
is
indicate
Tonking (1957) and Brown
greater
maximum
thicknesses
in
feet and 2900 feet, respectively.
2500
Nountains;
doubtful
the
thickness of about 800 feet is estimated
from cross-section C-C'
both
repeated
that
the
La
thickness of that rnag3.itud.ein
Jara
the
Peak
Silver
Andesite
a total
Hill
has
area.,
An accurate radiometric age for the andesite is
difficult to obtain beca-use of the altered and weathered
condition of most samples. However, an unusually fresh
sample
from
the
Bear
yielded
a date of 23.8 2 1,2
Mountains
m.y. (Chapin, 1971a) which places the unit in the , lower
Miocene.
Outcrops of La
Jara
Peak
Andesite
typically
form
of Silver
rounded hills. Occasionally, as on the slopes
Hill
anda few of the
other
hills,
small
ledges becan
seen
but usually they are not continuous. In some places,
individual
flows
can
be distinguished by the presence
of-
autobrecciated tops and bottoms. These flow breccias are
usually oxidized toa reddish-brown color. Individualflows
cannot be traced very far owing
to talus,
soil
cover,
and
lack of ledgey outcrops. However, in2 areasp the basal
zone
impartsa reddish
tintto the
soil
which
can
be
followed for about
150 feet. Hand specimens arem e d i m gray
to dark
gray
on
fresh
surfaces
and
weather
to lighb
grayo r
light grayish-brown. Phenocrysts of hematized pyroxene,
which
are
particular1y.obvious
on.weathered
surfaces,
are
-,c-
characteristic of the unit. Plagioclase phenocrysts are
"
found in some samples. Vesicules and amygdules 'filled with
calcite
and
silica
Flow direction
occur
locally.'
canbe determined in many outcrops that
"
possess elongate, lineated vesicules.F ~ O direction
W
is
1) as a two-directional
shown on the geologic map (pl.
but most
arrow. The lineations havea wide range in trends
indicate a direction
A few
possible
of
feeder
flow
from
fissures
or southeast.
northwest
the
for
the
Jara are
Peak
La
flows'
found in scattered areas. The
rocks which' occupy the
fissures
the
resemble
flows
and
dikes
lack
but
are
observable
mineralogically
chilled
to
identical
margins.
In thin section, the andesites have
a porphyritic
texture
with
an
aphanitic
pilotaxitic
and to
felty
. ,.
groundmass;
ophitic texture is rare. Phenocrysts are usually pyroxene
but
occasionally
may
be plagioclase
and
olivine;
the
groundmass consists of microlites of plagioc3.ase and
pxroxene
The most common phenocryst is clinopyroxene which
comprises from5 to 20 percent of the total rock volume.
Individual
crystals
vary
from
anhedral
to
euhedral
and
average from1 to 2 millimeters in length. Glomeroporphyritic
crystals are seen in some thin sections. Most pyroxenes are
altered to calcite
Fresh
pyroxenes
in
the
center
occasionally
show
with
a rim of hematite.
hourglass
structure,
concentric zoning and have extinction angles
(ZAC) of 43'
to 450.
2
This appears consistent with
a composition between
-22-
diopside.
and
Tonking(1957,
hedenbergitedetermined
as
by
P. 46).
In some thin sections, plagioclase phenocrysts represent
from 5 to 10 volume percent of the rock. They usually
occur
as
euhedral
and
subhedral
ranging
0.5 from
crystals
to 2 millimeters in length. Some are anhedral, partly
. .
resorbed and maybe xenocrysts. Varying degrees o f
alteration to calcite are present. Using the Fouque'.
method,
the
anorthite
composition
on 8
measured
was
phenocrysts that showed the least amount
of alteration. The
range
is
An45
to An51.
Subhedral
in 1 thin
and
section
euhedral
of
olivine
unusually
phenocrysts
fresh
andesite
are
present
and
comprise
20 volume percent of the rock. Optic. angles
(2Vj varying
from 70 to 90 degrees indica.te that the olivine is
Porsterite or ferroan
forsterite
,1965,
p. 145).
(Heinrich,
The crystais average from
1 to 1.5 millimeters
but' some are as large
as
3 millimeters.
fibrous
alteration
borders around
the
product,
probably
phenocrysts
and
.
in
diameter
A pale green,
serpentine,
fills
forms
cracks'through
the
core.
Ragged biotite crystals,
1 to 2.5 millimeters
in
some thin sections. The
length, occur infrequently in
crystals
are
usually
unaltered
and
always
associated
with
calcite. One crystal displays undulatory extinction.
The
groundmass
is
comprised
mainly
of
plagioclase
and
pyroxene microlites with abundant specks of magnetite, The
-54-
I
!
I
!
~
microlites
Calcite
range
in
alteration
size
is
from
0.05 to 0.1 millimeter.
common
with
plagioclase
appearing
more
altered than pyroxene. Cavities filled with
a zeolite may
comprise 15 volume percent in some thin sections. The
zeolite, possibly natrolite, occurs as radiating, needle-like
crystals
with
Thin
positive
elongation
layersof volcaniclastic
and
parallel
sandstone
are
extinction.
present
locally in La Jera.Peak Andesite. Outcrops are discontinuous
and weatherto shallow depressions. Hand samples are ligh-t
gray to light
reddish-gray
on
fresh
surfaces
and
brownish-
gray on weathered surfaces. The sandstones are composed
primarily o f quartz
andesitic
quartz
The
and
ranges
grains
are
and
welded
feldspar
tuff
with
varying
fragments.
. . In thin
amounts
of
section,
from
15 to 40 volume percentof the rock.
typically
subrounded
to rounded
and
average
1 millimeter in diameter. Plagioclase feldspar and
occasionally
potash
feldspar
from
10 to 45
comprise
volume percent. Plagioclase commonly shows very little
alteration
whereas
potash
feldspar
is
frequently
severely
altered to calcite. Biotite is scarce and usually slightly
altered to hematite.. Opaques, primarily magne-kite, are
scattered throughout most thin sections. Andesite and
welded tuff detri-bus constitute
40 volume percent of the
rock
in some instances. The andesite fragments closely resemble
La
Jara
Peak
f l o w s and
are
tJPically
angular
to subrounded.
Most welded tuff clasts are crystal-poor and some show
definite flow-banding with imbricated feldspars. The
-
sandstones
generally
have
a cement
of
sparry
calcite-,
.but
, in
some thin sections silica cement is dominant.
Popotosa
Formation
1940), Niocene.in age, .
The Popotosa Formation (Denny,
.
is the basal unit of the Santa Fe Group. The formation
consists
of
fanglomerates
and
playa
deposits
1973).
interbedded volcanic rocks (Bruning,
rocks
of
the
extending
Popotosa
from
the
Formation
Gallinas
local
The sedimentaxy
formed
a large
in
Range
with
basin
the
west
to beyond
on
the
f o r detritus
Rio Grandeon the east. The major source areas
were the Colorado Plateau and local uplifts
produced by
Basin and Range deformation (Bruning,
1973).
Bruning has
divided
faciesand a
the
Popotosa
Formation ainto
playa
.
fanglomerate facies. Two lithologically unique fanglomerates
are present;one of these, the andesitic fanglomerate
of
Dry
Lake
Canyon,
is
exposed
in.theSilver Hill area.
Fanglomerate of D r y Lake Canyon. The fanglomerateof
Dry
Lake
Canyon
(Brown,' 1972) is
the
youngest.
well-indurated
sedimentary rock present in the thesis area. The
fanglomerate
the
crops
out
discontinuously
southto Allen Well in
the
north
from
and
is
Boxczr inWell
confined
.to %he
down-dropped block west of the Hells Mesa fault. Exposures
of the
fanglomerate
occur
of living vegetation (fig.10).
is
partiallycovered with
rounded
hills typically
as
denuded
The surface of these hills
clasts
which
generally
range
in
diameter froma few inchesto 1 foot. Detritus from La
Jara
-,"-
. .
Peak
predominate
but iocally,
Andesite
6 0 , fragments
which
abundant. In one
R. 5 W.,
clasts
resemble
locality
the
southof U.S. Highway
upper
tuffs
are
more
Allen '(
Well
sec. 3 6 , T. 1 S . ,
near
.
unsurveyed), the fanglomerate. consists primarilyof
derived
from
the
unit of
Arroyo
Montosa
and
the
andesite of Landavaso Reservoir.
Good'exposures
of
massive
fanglomerate seen
are along
Council Rock hrroyo and Dry Lake Canyon. Outcrops are
generally
composed
of
poorly
sorted
sandstone
and
gravel
lenses. Rand specimens contain predominantly subrounded
andesite
fragments
a s much as 10 centimeters
in
diameter
in
to silty matrix. In thin section, %he
a light broia sandy
appearsto be a mixture of chalcedony, calcite and
matrix
ciaye
..
Bruning (1973) has completeda detailed report on the
origin of the
Popotosa
the
which
detritus
Pormztion
makes
up
and
the
discusses
a source
fanglomerate
of
for
Dry
Lake
Canyon. His data indicate that flow directions were mainly
"to northwest, west and southwest from the now down-faulted
north end of the Magdalena Range"
(p. 90).
Tertiary
"
Intrusive
R.ocks
Mafic
Dikes
Two small andesite dikes intrude ash-flow tuffs of the
A-L Peak Formation, One dike cuts the flow-banded
member in
the large roadcut U.S.
on
the tuff of Allen Well in
dike
measures20 feet
in
Highway 60 and the other dike cuts
CouncilRock Arroyo. The roadcut
width
and
is
poorly
exposed
in
'I"-
length for 25
to 30 feet along thetop of the ridge, The
other
dike
is
much
narrower
is for
exposed
only a few
and
feet along strike. A trend of N 3 5 O W to
measured
for
degrees
both
recorded
the
dike
sampleof the
A typical
dipof about 75
dikes awith
westward
for
in
dike
400 W was
the
roadcut.
exposed.
in
the
roadcut
is
on the fresh surface. Weathered
aphanitic and dark gray
fracture
faces
are
specimensof the
Hand
,brown
appearance
weathered
and
brown to pale
pale
other
yellowish-brown.
dike'have
a speckled greenish-
were difficult to obtain
and
fractured
condition
of
dueto the
the
outcrop.
Microscopically, thedikes are similar. Thin sections
show
phenocrystsof pyroxene
and
infrequent
plagioclase a in
pilotaxitic groundmass of plagioclase microlites.. Pyroxene
occurs
as
subhedral
and
from 0.25 to 1 millimeter
Kost
grains
are
totally
euhedral
with
an
crystals
ranging
in
average0 - 5ofmillimeter.
altered
to calcite,
chlorite
and
hematite with rims
o f fine-grained magnetite. Less altered
of
crystals occasionally have relict cleavage. Phenocrysts
plagioclase represent less than
2 perceat of the volume of
the rock. The euhedral crystals are remarkably fresh and
average 0.5 t o 0.75 millimeter in length; Groundmass,
which makes up
90 to 95 percent of the rock, consists of
euhedral
plagioclase
microlites
with
mhor amounts
of
biotite and fresh pyroxene. The composition
of groundmass
plagioclase
was
determined
as
AnGo
by
the
Nichel-LevJ
method. Opaques and interstitial.calcite are abundant 1in
size
thin
section.
Monzonite
. A
Hale
small exposure of monzonite occurs in the vicinity
of
Driveway
Well
area; the intrusive
The
monzonite
one-sixteenth
in
is
crops
the
southwestern
informally
out
ofa square
over
mile
named
an
area
with
an
portion
o f the thesis
the We11
Hale pluton.
slightly
outcrop
smaller
than
configuration
in the shape of
a slightly elongate ellipse oriented
W
. The
approximately N 35O
rounded
mounds
composed
monzonite usually crops out as
of
highly
weathered
angular
fragments.
Where intermittent streams have cut below alluvium, the
intrusive
forms
smoothly
and appears
outcrops
rounded
less
weathered.: Relatively fresh hand specimens are light
.
brownish-red
and
to light
weather
The exposed portion
of
the
.. ..
reddish-gray.
Hale
Well
pluton
is
probably
a border facies of
a much larger buried intrusiye. This
intrusive
Gulch
apparently
has
been
dovrn-faulted
bordering
the
western
Springs
margin
o f the
the
on
thesis
the
area
north
(see
fig. 1, p. 2).
The
Hale
Well
pluton ahas
conspicuous
porphyritic
texture. Plagioclase phenocrysts, some as large as 3
centimeters
in
length,
occura matrix
in
of much
smaller
crystals. The. phenocrysts are euhedral and essentially
fresh but incipient calcite, chlorite
and clay
Mulligan
from
graben,a no&-south structure extending
PIulligan Gulch on the south
ta Abbey
and
into
alteration
can be seen along cracks and cleavage traces.
In some
instances,
.the
plagioclase
appears to be poikilitically
is
speckled
enclosing
with
opaques
and
flakes
of
small
hornblende and biotite. Zoned crystals
sh0w.arange in
composition from An33$ 0 An42. Groundmass constituents are
primarily
potash
feldspar,
plagioclase
and
clinopyroxene
very minor amounts
of
present (table 2).
Opaques are small and numerous. Slender
quartz
and
brown
biotite
are
but
also
laths of apatite occur throlughout the groundmass and are
partially
enclosed
Two thin
within
plagioclase
phenocrysts,
sectionsof the monzonite were previously
by
point counted by J.E. Bruning. The results were checked
the
author
The
and
are
weathered
in 2.table
presented
condition
of
the_intrusive
prevented
collection ofa sampie for radiometric dating. However,
exposures
and
show
the
pluton
tuff La
of Jencia
the
intrvding
Creek
of
the
the
A-L
gray
Peak
massive
member
Formation,
and the upper tuffs.A latite flow in the unit of Arroyo
Nontosa
and
unaltered.
the
in
(C.Z*
Hale
the
nearby
outcrops
of
La
Jara
Peak
Andesite
are
From these observations, it is concluded ti1a-t
Well
pluton
is similar
Kagdalena
axes which
inage .to other
have
been
dated at
intrusives
about
28
m.y.
Chapin, oral commun.,1973).
Roclzs near the Hale Well pluton are argillically
altered and pyritized. The pluton is unfractured, unaltered
and con.tains only minor amounts of hydrous minerals. These
observations
suggest
that
the
pluton
may,not
have
been
the
-61-
Table 2 - Modal data from the Hale Well
monzonite pluton (in volume percent)
..
number*
Sample
Phenocrysts
"83-1
"83-2
48.5
50.0
3401
26.2
I
,
Plagioclase
Matrix
feldspar.
Potash
2.3
Plagioclase
Clinopyroxene
209
..
.
I
5.3
Biotite
0.1
0.5
Quartz
0.0
0,1
12.0
-
10.9
-
99.9
99.9
Opaques
percent
Total
'
*Sample Pi-83-1 was countedor, a 1/2 X 1-2/3 millime-i;er
grid which yielded
1227 counts. .Sample"83-2
was
counted ona l / 3 X 2 millimeter grid which yielded 868
:
counts.
.-",-
source for solutions which created the alteration in the
'
surrounding rocks. Hence, the wall-rock alteration may
pre-date intrusion of thepluton. Effects of contact
metamorphism
relatedto the pluton may
be undistinyishable
from effects of this older period of alteration, although
contact
effects
similarity
in
to be minimal because of the
likely
are
c.omposition
between
the
pluton
and
the.
..
wall-rocks.
Tertiary-Quaternary
Deposits
Pediment
Gravels
Pedinent gravels are the oldest surficial deposits
mapped in the thesis area. The deposits are unconsolidated
and
poorly-sorted
and
contain
volcanic
detritus
ranging
from
pebbles to cobbles in 'size.A caliche' zone, usually somewhat
indurated, caps the pediment gravels. The thickness of the
zone varies from
a few
inchesto
several
feet.
Freiuently,
a layer of brovmish soil covers the caliche zone.
In areas
where this soil layer is extensively developed, the presence
of the pediment surface can
be detected by caliche-covered
pebbles surrounding animal burrows. Cholla and prickly pear
cactus, and assorted' desert grasses are the doninant forms
of
vegetation
cover
over
Quaternary
but
bedrock
juniper
is
occasionally
grows where
pediment
thin.
De-oosits
Talus
Owing to limited topographic. relief extensive Of
areas
talus do not occur in the field area. Zrimarily, the word
talus represents: 1) talus cones derived from mass-wasting
2) colluvium ,where
of upthrown fault blocks, and
stratigraphic
contacts
are
obscured
Eolian
Sand
and
..'
underiying
bedrock
could notbe identified,
Areas
the
thesis
of
windblown
area
and
sand
are
occur
in
the
related
to prevailing
northern
half
southwesterly
on the leeward
winds. The sand accumulations are thickest
to the northeast. Windblown
flanks of large ridges and thin
sand
generally
supports
better
vegetation
than
pediment
deposits; juniper bu.shes and pison pine
a b higher elevations
occur most often.
No soil la,yer is present and ant hills
.
.
and animal burrows are less abundant'than on pediment
.
surfaces.
Alluvium
In this study, allu.vium is confined
to: 1) deposits of
send
an&
gravel
filling
arroyos
and
smaller
intermittent
stream channels, and2) finely divided material filling
small
depressions
and
valleys.
of
STRUCTUIlE
a i o n a l Structure
'West-centralNew Mexico is part of a northern
extension
of the Sonoran-Chihuahua fault system (Eardley,' 1962) which. .
most physiographers
The
extension
central
include
continues
Colorado
and
in
the
Basin
along Rio
theGrande
separates
the
and
Range
depression
Colorado
province.
into
Plateau
on
the
west from the Southern Rocky Mountains and Great on
Plains
.
the east. The present tectonic pattern
of the area largely
reflects 2 major periods of deformation: 1) Laramide
7
(late Cretaceousto' early Tertiary), and2) middle.and
late
Cenozoic.
Lara.mide
forces
northwest-trending
produced
a series of north-
domal
uplifts
separated
and
by
downwarped
basins. The fiagdalena area is situated on the soathern
of
flank of one of these basins,'the Baca basin. Erosion
Mesozoic rocks exposed
produced
clastic
along
debris
the
which
flanks
was
of.the
basin
deposited'in
the
center
of
the basinto form the Baca Pormation of Eocene age. Large
open
folds
and
thrust
belts
are
typical
of
the
Laramide
orogeny (Kelley andWood, 1946; Tonking,1957; Eardley,
1962; King, 1967). Eardley (1962) proposed that vertical
rather
than
compressional
forces
were
mainly
responsible
these features. The theory involves the formation
of
b'asaltic l'megasills't which elevated the overlying crust
into large blisters; hence, the name "blister concept",
King (1967)r.oted that occasionally Laramide stocks were
for
emplaced
transverseto fold axes
vertical
forces
were
which
especially
indicated
active
to
during
him
this
that
time.
of plate tectonics, however,
Recently developed concepts
suggest
that
Laramide
deformation
was
compressional
and
resulted from southwestward driving of the North American
'
plate
away
from
the
opening
gap
between
North
America
and
m o p e (Coney, 197i).
Following
Mexico
the
Laramide
wasbeveled by an
Precambrian
and
Oligocene
erosion
Paleozoic
and Chapin,1973).
orogeny,
much
surface
cores
of
of
west-central
which
New
exposed
Laramide
uplifts
(Epis
.In the Magdalena area, an early
transverse
latitude
of
North
Montosas
downdropped
trending
N 78'
fault
Baldy
in the
the
W from the
Kagdalena
Spears
Piountains Tres
'to
.Pormati.on
,and
older
rocks
1500 to 3000 feet on the south (Krewedl, in preparation;
Chapin and others, in.preparation). Copious ash flow
eruptions, culminating at
32.to 30 m.y., buried the trace of
this
fault
and
formed
the
ignimbrite
plateau
which
caps
the
Datil volcanicpile in the Magdalena area, Northeasttrending
faultsof relatively
drainage
during
small
of 'the
formation
displacement
plateau
and
influenced
may
have
helped localize cauldron development. Immediately following
construction of the
was
broken
these
by
faults
ignimbrite
numerous
pea-tly
plateau,
normal
influenced
the
Nagdalena
area
N loo
faults
ofW trend;
post-volcanic
stock
intrusion
28 m.y. (Loughlin and Koschmann, 1942; Chapin
dated at about
o f basin-and-rangeand others, in preparation). The.onset
-UU-
..
type faulting
Miocene
and
and
related
to the
related
was
rift (Chapin,1971b).
through
the
San
in
Silver
the
'
Hill
of
Formation
greatly
following
Seismic
early
formationof the Rio Grande
Plains
movement
1962; Cnapin, 1971b).
the
in
developed
Miocene
in
late
The cause of rifting' has been attributed
to a northwestward
has
began
Southwestward bifurcation of the rift
Augustin
(Chapin, 1971b).
structures
sedimentation
the
of
affected
area
Colorado
the
rift
the
Plateau
and
related
structural
and bewill
discussed
in
(Eardley,
framework
of
greater
detail
sections.
activity
and
recent
fault
scarps,.
such
as
the
one along the eastern flank of the Magdalena range'about
4
miles east of Nagdalena, suggest that deformation
is still.
continuing
Local
.,
Structure
!?he
Silver
Hill
areaa structurally
is
complex
network
of intersecting, superimposed faults. Faulting occurred
intermittently from middle ( ? ) Oligocene to Recent with
major periods of deformation in late Oligocene, early
Miocene, and latePliocene.
Miocene
periodso f faulting
erosion
surface
The late Oligocene and early
are
separated abyregional
which
was subsequently
covered
by
a thick
sequence of andesite flows; thus many of the Oligocene
faults
areba-ied, and
their
position
and
trend bemust
inferred from cross-secions (pl.
1).
In o r d e r to present the structure
in an
fashion,faultshare
organized
been grouped'inbo 2 age
periods:
..
.
.
Oligocene and Miocene. An overlay for each period, showing
fault
trends,
was
constructed
from
the
reduced to page size (figs..11 and12).
regional
structural
in
thesis
the
map
showing
to. those
area
geologicmap
and
In addition,a
the
elsewhere
relationship
in
the
of
.faults
Magdalena
area
is presented in fig. 13 (p. 76).
Oligocene Faults .
Two stages of faulting are recognized
in the thesis
area during the Oligocene Epoch. The firs* stage is
represented by 2 parallel
formed a graben
transecting
at
the
tima of
Creek
and
A-L Peak
central
emplacement
controlled.the
northeast-trending
the
faults
portion
which
of
the
A paleovalley was present along.the
thesis area (fig.11).
graben
northeast-trending
of
the
tuff.
Formationis compatible
early
in
with
the
La . Jencia
the ' tuff,
A
distributioCof
paleovalley
of
deposition
of the
location
of
the
A-L the
Mt. Withington cauldron, the probable source of
Peak
ash
flows,
and
with
paleovalleys
in
similar-trending
tie Lemitar Piountains (Voodward,
1973) and near Stendel
iiidge (R.Be Blakestad, oral cornu,, 1973).
that
deposition
controlled
banded
of
tuff
of
La
Jencia
Creek
helpsto explain why the gray massive flowand.
membsrs
of
stratigraphically
usually
the
Recognition
the
A-L Peak
below
and
Formation
above
(which
lie
, respectively)
the
tuff
arenot differentiable ou'tside the graben,
Displacement
across
the
bounding
of the
faults
grabenmay
not have been large; cross-section B-B' 1)
(pl.
shows an
was
structu
-68-
i
Figure 11 - Oligocene Taults in the
Si.lver Hill area.
Faults are dotted 7:ihen continuationis unces:tain. Longitude
and latitude are narked along the.borders
o f the thesisarea.
estimated
vertical
offset
of
about
100 feet
on
the
northern
fault.
A second
stage
of
faulting
occurred
in
late
Oligocene
. .
These faults trend fromN 35' W to N 30° E and
(fig. 11).
with 1 exception are'traceable only for short distances.
Half
of
the
faults
are
downthrown
to
the
west
while
the
other half show the opposite sense
of movement. The largest
of the down-to-the-west-type faults is
and most important
(fig, li and p l . 1).
the Kale Well fault
the
thesis
area,cut
paleovalley
and
the
formed
earlier
the
This fault crossed
northeast-trending
boundary
of an
eastern
embryonic
Mulligan Gulch graben. Movement along the Hale Well fault .
may
have
been
initiated
prior
to.deposition
of the
andesi%e
of Landavaso Reservoir,. Distribution, of the andesite
in
the
of the
portion
west-central
suggests
that
the
andesite
thesis
was
area
confined
(see
p l , 1)
within
a nortln-
trending down-faulted area. Movement along the Hale Well
fault
was
renewed
after
emplacement
(cross-section D-Dl, p l . 1).
of
Hale
tiell
fault
may
upper
have
been
and D-D', p l . 1).
instrumental
in controlling
emplacement of the Hale Well pluton,
a monzonite
bodyof
unlnovm shape
exposed
and
the east side
of tne
size,
fault
tuffs
Maximum total displacementis
zpproximately 1000 feet (cross-sectionsB-B'
The
the
which
near
is
partially
Hale
Driveway
along
Well.
North-trending, late Oligocene faults controlled emplacemenb
of stocks
in the Kelly mining district (Chapin, 1971a;
Brown, 1972).
Kiocene
Faults
After Oligocene faulting, the Silver Hill area went
through a period
of
relative
quiescence
during
which
'>
r,
tC I
erosionofupliftedfaultblockssupplieddetritus
3 ..7 4-.*.--*.\
to form the conglomerate facies of the unit of Arroyo
c ...
1
.
-
..
f
Montosa. Lava flowsin the unit, probablyof fissure
?
origin, were erupted during deposition of the conglomerates.
The
unit
of
Arroyo
Montosa
probably
formed
a basin-andin
range-type environment similar
to that present during
deposition of the Popotosa Formation, This suggests that
Basin
and
in
b
e
,
-
Range
very
deformation
early
in
the
Magdalena
area
may
Miocene.
The earliest Miocene fault cuts the unit
Arroyo
of
Montosa (25.2 m.y.), but not La Jara Peak Andesite
(23.8 m.y.)
..
The trace of the fault
is best exposed on the
(fig. 12).
117048'1where the conglomerate facies
of.
south slope of Hill
the
unit
of
juxtaposed
Arroyo
against
Montosa
the
the upper i;ufr's (pl. 1).
has
andesite
been
of
downthrown
Landavaso
and
and
Reservoir
A gray, crystalline calcite vein,
approxipately 8 to 10 feet wide and
30 feet
long,
occupies
the fault zone. Calcite veins are also found within other
fault zones along the Mulligan Gulch graben. Displacement
along
this
fault
may have
been
as
much 800
asfeet,
Following a.n initial stage of fau-lting, the Silver
Hill
area
was
inundated
by
La
Jara
Peak
Andesite
which
. .
filled the middle Oligocene, northeast-trending paleovalley
after it had been exhumed during carving
of the
pre-La
Jara
have
Peak erosion surface. Erosion of the flanks of the
paleovalley
continued
of La
emplacement
during
Jara
Peak
of interbedded
Andesite as evidenced by the presence
.
'
volcaniclastic sandstones. The sandstones are immature,
'
containing
primarily
feldspar rock
and fragments
quartz,
with occasional biotite. Hence, it appears that La Jara
Peak
Andesite
bearing
occupied
a graben
rocks
with
quartz-
.and
feldspar-
accessible
to erosion along the sides.
After emplacement of La Jara Peak flows, the Silver
Hill
area
was
cuta number
by
of
steeply
12).
faults with irregular trends (fig.
central
portions
of
Jara
the
Peak
map
and
transect
La
calcite,
bariteand.a few metalliferous
where
quartz
..
,
some
of
these
faults
veins.
within
the
.
In
the
Nulligan
horst
western
Gulch
minerals
indicated
by
faults
containing
occupy
movement
slickensides
, .
portion
of the thesis area., along the
graben,
causeda reversal
.
is
these
veins
1932)* . Secondary
some of the fault zones (Lasky,
along
normal
In the eastern and
area
Andesite,
dipping
formationa major
of
of
movement
along
12 and pl.. 1).
earliest Niocene fault (fig.
intra-graben
part
of the
On Hill
11704811,.the
conglomerate facies of the unit of Arroyo
Nontosa
Fault
rests
in
fault
sliversof upper
tuff
contact
and
with
andesi.te
La
Jara
of
Peak
Andesite.
Landavaso
Reservoir, wlich were dra.gged up along the fault, also rest
against La Jara Peak Andesite (see 8,
fig.
p. 43).
Near
Kale Driveway l$el12 small outcrops of the conglomerate and
-32-
.
-
-
Figv.re 1 2
Miocene f a u l t s i n t h e S i l v e r
Hill a r e a . Faults
a r e d o t t e d when c o n t i n u a t i o n i s uncertain.Longitudeand
l a t i k d e a r e marked a l o n g t h e b o r d e r s o f . . t h e t h e s i s a r e a .
I
-7 3-
volcanic
faciesof the
overlie upper tuffs,
unit
the
of
gray
Arroyo
Montosa
massive
member
disconformably
of
A-L the
Peak Formation, and the Hale \?ell pluton.' Apparently,
the.
'
fault
forming
the
eastern
boundary
of the
intra-graben
horst is located west of these outcrops
' ( p l . 1).
along
this
fault
may
faciesof the
conglomerate
Nontosafrom.
unitof Arroyo
southwestto southeast
predominantly
ofdips on the
a reversal
caused
have
Movement
near
the
Hale
Ranch
driveway (sec. 25,T. 2 S., R. 5.W.).
Immediately
east
horst, a fault-bounded
of
the
block,
central of
part
the
of the
.consisting
Landavaso Reseriroir andsmall outcrops of the
was
uplifted
This.up1ift
formation
This
..
and
placed
probably
of
the
cannot
be seen
movement
La
along
older
of
tuffs,
Peak
Andesite.
of
contemporaneous
to
the
.horst
in
andesite
upper
Jara
occurred a as
result
intra-graben
relationship
recurrent
against
intra-graben
west
12). (fig.
cross-section
owing
to
faults.
The Hells Hesa fault (Tonking,
1957, p i 38) is a
major
the
north-south
lineament
along
the
SilverHill area (fig. 12). The trace
of this
I
continues
as
far
north
as
Sierra
portion
of
western
Lucero
fault
near
the
nortnern
boundary of the Puertecito quadrangle. Displacement along
the
Hells
Nesa
fault
increases 500
from
to 600 feet
in
southern portion of Puertecito quadrangle (Tonking,
1957)
to about1000 feet in the Bear Mountains (Brown, 1972).
Near Arsoyo Kontosa. in the Silver Hill area, the fanglomerate
of Dry
Lake
Canyon
is
downthrown
and
juxtaposed
against
-74-
undifferentiated
displacement
may
B-B', pl. 1).
A-L
Peak
be
Formation;
hence,
1500
much
asfeet
as
vertical
(cross-section.
.
The Hells Mesa fault
does not transect the
intra-graben horst (fig. 12). A minor fault with identical
trend
and
movement
but
'with
a throw
present instead (cross-sectionC-C'.,.
along
the
bounding
Hells
Mesa
faultsof the
fault
horst
50 feet is
only
of
Movement
p l . 1).
was
apparently
diverted
may
been
which
have
along
th
partially
reactivated. South of the intra-graben horst, the Hells
Mesa fault continues but the vertical offset is greatly
reduced. Near Landavaso Reservoir, displacement of^
approximately 800 feet
is
indicated
from'cross-section
D-D' (pl. 1).
Bifurcation
Augustin
of
graben
north-trending
overprinted
the
Rio
occurred
structural
by
Grande
in
late
grain
rift
along
S a the
Miocene
of
northeast-trending
and
the
the
Magdalena
normal
faults
area
was
which
had
some left-lateral movement (Chapin,,1971b;
Brown, 1972).
The
southern
bounding
fault
of
the
San
Augustin',rift.
.
.
parallels part of State Road
107 which forms the southeastern
,
boundary of the thesis area.
A subsidiary fault trending
'
N TO0 E occurs about2 miles to the northeast along Arroyo
Gato (sec. 25, T. 2 S.,
R. 5
and seco 3 0 9 T. 2 S.,
R* 4 W.). This fault bifurcates into
2 smaller faults
which
cut
the
flow-banded
member
exposed inthe large roadcut onU.S.
intense
frecturing
prevalent
of
the
A-L
Peak
Eighvray 60. The
along
the
be due
road.cut
may
Formation
largely to the influence of these faults (see 14,
fig.
It is not certain if left-lateral.
movement is
p . 81).
associated
dike
in
with
the
roadcut
faults,
although
an
andesite
south
wall of the road-cut ha.s no visible
the
northward continuation. However, this observationcan be
explained
roadcut
with
dorm-faultingon the
dike pinches
the.
if
out
north
side
of
the
upward.
Folding
A short segment ofa flat-bottomed syncline (Brown,
'
1972) extends into the extreme northeastern corner
of the
thesis
area
from
B-B', pl. 1).
the
southern
Bear
Mountains
(cross-section
The axis of the fold trends north-northeast
to 15 degrees. The limbs
and dips on the limbs vary 10
from
of
the
Jara
syncline
Peak
17$ T. 2
syncline
S.,
are
Andesite
R. 4 ti.).
poorly
about
0.5 mile
south
of
Joe Well
byLa
(sec,
No further continuation of the
couldbe located
Information
truncated
exposedareand
in
the
study
concerning
the
origin
area.
of
the
fold
is
limited by the poor exposures present in that part of the
thesis arpa, Better exposures exist
in the southern Bear'
to. previous
Mountains, and therefore, the reader is referred
work by Brown (3972, p. 90-93)
L
-76-
-
Pigure'13 Structural napof the Nagdalena area showing
relationship of the thesis areato major structural
components (after 3rovrn, 1972).
ECONONIC GEOLOGY
The
Magdalena-Tres
Montosas
area
has
Seen
some
form
of mining activity since about
1878, With the area's
variable
lithology
and
.complex
structural
history,
the
occurrence of ore deposits is not surprising. The Kelly
. .
mining district contains the largest and most developed
mines
in
the
area
and
has
a major
been producerof zinc
and
lead with appreciable quantities-of copper and silver.
A'
such
as the Cat Mountain
number of other
mining
districts,
district to the
southwestof Nagdalena
district to the
northwest
of
and
Nagdalena,
the
Council
occur
within
a
Rock
12-mile radius of the Kelly district. The Silver.Hil1 area
is
approximately
equidistant
from
3 districts, .
these
..
Two features
of
the
Silver
Hill
area
warrant
consideration from an economic standpoint. Areas
of
argillic
alterationoccur along
portions.of the
western
. .
margin of the thesis area. The al-teration affects all
volcanic units of Oligocene age. The unit.of Arroyo Montosa(25.2 m.y.) and
La Jara Peak Andesite (23.8 m.y.; Chapin,
1971a) are haltered. Hence, the age of the alteration
to the Oligocene
is assigned as late .Oligocene, Unrelated
alteration, are quartz-calcite-barite Geins .which
CV.~La
o r younger but
Jara Peak Andesite. The veins are Kiocene
no
upper
age
lirLt be
candetermined from exposures in the
thesis area.
Late Oligocene
"
Alteration
-78-
Outcrops
which
exhibit
effects
of
argillic
alteration
and occur in 2
are stippled on the geologic map 1)
(pl.
distinct
western ofhalf
the thesis area.
approximately
2.5 miles apart and are . ,
places
The 2 areas
along
are
the
separated by outcrops which
lack,,orshow very minor,.
hydrothermal alteration. The northern altered area extends
from
Allen
Well
to one-quarter
south
mile
Creek.
Hontosa and east to La Jencia
beyond
Arroyo
The southern altered
area extends from the northern boundary
of sec. 25,T. 2 S,,
R. 5 W., across U.S.
The
Allen
Highway 60 and southto Boxcar Well.
Vell-Arroyo
Montosa
area
contains
the
most
intensely altered rocks in the thesis area. The pumiceous
flows, and the
member of the A-L Peak Formation, andesite
tuff of Allen
Well
show
varying
degrees
of
argillic
alteration. In the pumiceous member, the typical purplishgray color becomes
become severely
only
as
pyrite
altered
rectangular
haloed
staining
bleachedto almost
.
is
by
to
clay
holes
limonite
white,
and
in.thin
is
feldspars
are
frequently
section,
moderately
and
visible
oxidized
abundant..
Hematite
occasionally visible near fractures. Adjacent
veinlets, the pumiceous membe,r appears
to be
to quartz
recrystallized
and
minor
amounts
of
sericite
is
present
locally. In the tuffof Allen Well, bleaching is not as
intense
as
in
the
pumiceous
member,
but
argillic
alteration
and oxidized pyrite are still prevalent. Argillic.
alteration
visible
and
near
bleaching
fractures
in
which
the
andesiteisflows
most
probably'served
as
conduits
for solutions.
Undifferentiated
attacked
by
in a group
A-L
argillic
of
Peak
Formation
alteration
hills
bordering
1 and 12, T. 2 S., R. 5
W.).
also
been
and,
Arroyo.Montosa
near
the
has
arroyo
to the east .(secs.
The most intensely argillized.
rocks exhibit the same features present in the pumiceous
member near Allen Well.
In general, moving northeast from
Arroyo
Montosa
towards
La
Jencia
undifferentiated
unitbecome less
feldspars
lesser
show
Creek,
bleached
degrees’of
the
rocks
the
and
argillic
of
their
alteration;
limonite pseudomorphs after pyrite diminish finally.
and
,
disappear.
In the
alteration
Jencia
Bale
Driveway
affects
Creek
and
the
the
Well-Boxcar
gray
massive
Well
area9
member,
of La
the
tuff
member
of -theA-L Peak
flowbanded
argillic
.
Formation and,to some extent, the upper tuffs. The
andesite
escaped
of
Landavaso
Reservoir
thistype of alteration
.to have
seems
as
the
largely
feldspars
are
exceedingly fresh. Instead, propylitic alteration is
.
dominant, which, in
the context of this study, is
characterized
by
replacement biotite
of
and
chlorite and calcite.
of
La
In the gray ma.ssive member
a n d tuff
Creek, the
Jencia
have been c&anged
weathering
of
to
pyroxeneb g
typical
gray
yellowbrown,
disseminated
pyrite
and
purplish-gray
probably a as
result
under
oxidizing
Hinor amounts of hematite occur near fractures.
In the
large
roadcuton U * S - Highway 60, 3.75 miles
west
of
colors
of
conditions,
I
!
member oftheA-LPeakFormation
Magdalena,theflowbanded
and a mafic dike which
.
cuts
the
member show
weakto moderate'
degrees of argillic alteration (fig. 14). ,However,
chloritic
alteration
of
pumice
and
lithic
fragments
is
frequently observedin-theflow-banded member; hence, these
rocks
maybe in
transition
propylitic
to argillic
from
alteration. Alternatively, the argillic alteration may be
superimposed
Oxidized
banded
on
pyrite
tuff
migrated
propylitized
is
and
inward
disseminated
the
dike, and
from
e
Well,
the
upper
by
supergene
in
ban&
the
processes.
bothflowthe
throughout
hematite
fractures
'onbleached rock (fig.15).
Boxcar
rocks
which
tuff
have
are
superimposed
Near Hale Driveway Well and
tuffs
have
suffered
only
very
weak
argillic alteration. Pervasive hematization occurs locally
.. ...
in both of these areas and pyrite is absent.
Argillic
alteration
to attributeto a single
of
To decipher
formed
formed from supergene
the
process.
is an overprinting
what
in
both
Silver
,Undoubtedly,
hypogene
from
and
area
what
supergene
hypogene
processes,
Hill
processes
requires
more
is'difficul
is
seen
effects.
and
what
information
than is available at present. Almost certainly, the sulfur
to make pyrite are of hypogene origin,
and some of the iron
while
the
probably
bands
of
hematite
related
to
supergene
In the
spatially
Hale
to fractures
adjacent
are
oxidationpyrite,
of
Driveway
Well area.,
relatedto outcrops of the
argillic
Eale
Well
alteration
pluton.
and hydrous
However, the absence of shattering, alteration
minerals
intize exposed
portion
of
the
stock
makes
a genetic
is
-81-
e
-
Figure 1 4
Roadcut on U.S. Highway 60 ( 3 . 1 5 miles west ' .
of Iviagdalena) i n a l t e r e d and intensely fractured flow-banded.
is near
rcember of t h e A-L PeakForreation.Roadcut
i n t e r s e c t i o n o f s e v e r a l f a u l t s and i s about l j 3 - m i l e
s o u t h e a s t o f outcrop o f HaleWell p l u t o n , Dark g r a y r o c k
n e a r v e h i c l e i s r e l a t i v e l y f r e s h ( s e e f i g . !iP p o 27
compared t o t h e b l e a c h e d a n d h e r a a t i t e - s t a i n e d r o c k t o
s i g h t . View i s t o thawest.
correlation doubtful. Mostof the Hale Well pluton has
been
down-faulted
into
the
Nulligan
Gulch
unknownp it
the dimensionsof the stock are
graben.;
because
possible,
but
is
not likely, that the exposed part of the stock
a I1dryft
is
border
Tres
facies
that
Nontosas,
an
resembles
argillic
the
escaped
shattering
exposed
stock,
Hale
Well pluton,
alteration
along
and
some
is
not
alteration.
of
which
spatially
the
western ofedge
the
Near
closely
related
to
Mulligan
Gulch graben (Chapin and others, in preparation). Neither
the
Hale
Well
pluton
nor
beenthe source of the
have
altered
large
the
Tres
14ontosas-stock
hydrothermal
fluids
appear
which
volumes
of rock along the edges
o f the
Mulligan Gulch graben. The Hale Well pluton appears
to
.. ..-
post-date argillic alteration,
Supergene
Peak
both,
erosion
may
effects,
related
to either
the
pre-La
Jara
o r the present erosion surface,
or
surface
have
accounted afor
large
parto f the
argillic
alteration. The abundance of oxidized pyrite in the
altered r o c k s and
associationof argilllic
alteration
with
major fault zones lends credence
to this hypothesis.
t o a hypogene originf o r the
Several features, thoudh, point
argillization,
and
more
work
is
orderin
to decide
necessary
which process, hypogene
or supergene, is dominant.
-
piiocene ( ? ) itheralization
The
central
portion
of
the
thesis
area
is
dotted
'
,
with
shallow prospect pits, shafts and adits. Some
o?? the
workings
date
back
to the 1 9 2 0 t s when
thk
Silver
Hill
area
. .
t
-w-
was the site of small-scale mining activity. Although
copper
and
silver
ores
were
mined,
production
from
the
area
was limited. No records concerning ore grade or tonnage
could
be
located.
The mineralization occurs as veins which are confined
.
primar'ily to outcrops of La
Jara
Peak
Andesite;
a few veins
8re found cutting theA-L Peak Formation. Vein mineralization
was
dependent
zones
and
upon
related
open
space
fracture
a series
created
by of fault
systems
probably
during
Basin
and Range deformation in early Miocene time. In general,
vein
trends
are
northwesterly,
but
varyN 70°
from
W to
N 3 5 O E. The veins are plotted on,the geologic map1)(pl,
and
their
orientation
is
shown
a rose
in diagram (fig.16).
.
Dips on the veins range from
60° to vertical, A few veins
. ..
are
as
wide 6asfeet but usually
/
the
veins
are
8 inches or
less in width. Veinlets and stringers often parallel
or
diverge from larger veins. The veins pinch and swell both
horizontally
and
vertically;
hence,
veins
or vein
systems
are seldom traceable for long distances.
Vein
material
consists
of a variety of gangue
ando r e
minerals, Quartz, calcite, barite and hematite are the
dominant gangue minerals, but minor amounts
of goethite
occur locally. The ore minerals, listed
in order of
approximate decreasing abundance, are chrysocolla, malachite,
chalcocite, covellite, galena, sphalerite, argentite (Laslry,
1 9 3 2 ) and vanadinite. Chrysocolla, and malachite are
widespread
whereas
the
other
ore
minerals
are
.)
observed
in
-8 5-
S
-
Figure 1 6
Xose diagram t o 55 v e i n t r e n d s
Andesite.Trendsweregroupedandcountedwithin
sectors.
i n L a Jar, Peak
5 . d.egree
only a few localities. A common vein assemblage is 'shown
in fig.17.
Paragenesis of Vein
Minerals
.Ina majority of the veins, quartz is the earliest
mineral formed. Calcite, both white and brown varieties,
forms
later,
with
white
calcite
slightly
earlier
than
brown calcite. Initial silicification is absent in some
veins; in such cases, brovm calcite is usually the . first
mineral to form (fig. 17).
.Quartz, in these veins, occurs
as a late-stage mineral. Barite generally forms later than
either broyn o r white
calcite,
although
barite
does'
occur
contemporaneously with brown calcite. Hematite appears
to
be later
thanmost of
principally
Although
the
associated
massive
other
with
hematite
gangue
minerals
malachite.
is
most
.and
frequently
and
is
chrysocolla.
encountered,
specular hematite occurs along cavities in the andesitic
No relationship between
country rocks in some localities.
goethite
and
Malachite
Malachite
other
and
may
have
gangue
o r ore minerals could
be found.
chrysocolla
slightly
are
early
preceeded
ore
minerals.
formation
of
c'hrysocolla, but usually the
two minerals are intimately
associated. I*linor amounts of chalcocite with covellite are
found rimmed by,
intergrown
chrysocolla
malachite ainrecent
and
24, T. 2 .S., R e 5
W e )
with,
and ona few
and
surrounding
prospect(NE1/4 sec,
~ILUII~
of
S
older mines.
Lasky: (1932) reported the presence of "granular orthorhombic
chalcocite'l and associated covelllite from claims
owned by
-87-
Figure 17 - Boulder of vein material typical
of prospects
of
in La Jara Peak Andesite. An intimate association
chrysocolla and hematite occupies the center
of the vein
surrounded by coarsely crystalline,
brown calcite. A
small barite veinlet (left side
of vein) cuts the calcite
and parallels the bo,yndary wi-th chrysocolla and hematite.
Quartz isa common gangue mineral in many.veinsis but
not
abundant here. The prospect is Located
in lJE1/4 sec, 19,
T. 2 S,, R. 4 If-
-uu-
the
Copper
Belt
Companyto the
Silver Copper
and Mining
east of SilTer Hill. Galena and yellow-green sphalerite
may be later
than
the
copper
sulfides
but
are
definitely
to contain both
later than'chrysocolla. The only vein found
R. 4 W.),
galena and sphalerite(SW1/4 sec. 19, T. 2 S.,
also
fragments
of La
contains
Jara
Peak
Andesite
cut
by
veinlets of chrysocolla. Occasionally, where, galena and
sphalerite
perimeter
coexist,
around
galena
is
sphalerite;
observed
'to form a partial
hence,
at
least
some
of
the
galena is definitely earlier than sphalerite. Color and.
X-ray diffraction analyses of the sphalerite show
it an
to be
iron-poor variety. Vanadinite wasnoted in one locality
(SE1/4NEl/4 sec. 18, T. 7. S., R. 4
W e )
as euhedral crystals
encrusting fracture surfaces, Argentite, which was reported
. ,.
by Laslcy (1932), was'
not
seen
by
the
.
author.
The paragenetic sequences observed in the veins is
18. The diagram indicates relative position
shown in fig,
in
tine
Some
and
is
not
interesting
intended
to show
features
mineral
arise
quantities.
from
the
paragenetic
sequence. The copper minerals generally formed earlier
than
and
galena
and
chrysocolla
sphalerite,
preceeding
with
at least
formation
some
of
the
malachite
copper
sulfides,
chalcocite and covellite, Hence, when copper was introduced
into
the
system
epparently
no
sulfur
was
available
to
combine with copper and precipitate as copper sulfides.
Iron;
OCCUTS
representedin the diagram by hematite,
relatively
-
early
in
the
paragenetic
sequence
may.have
and
-uy-
. .
Quartz
White calcite
Brown calcite
Barite
Hematite
Malachite
Chrysocolla
Chalcocite
Covellite
Galena
.. ...
Sphalerite
-
Piwye 18 Paragenetic sequencefor vein mineralsin the
Silver Hill area. Solid lines indicate that the mineral
definitely formed at that tine in the sequence. Dashed
.
lines indicate that
the mineral possibl'y formed at that
tine but lack or' relationships with other minerals Prevents
a definite deterzination.
,
,
been essentially depletedo r absent
in
the
stages
of
later
mineral formation. Evidence which supports this hypothesis
is
basedon the
absence
of
low
'pyrite
iron
content
and
of
sphalerite
chalcopyrite
from
and
the
the
sulfide
assemblages.
Wall-Rock
Weak
alteration
Alteration
commonly
surrounds
veins
which
'cut
In places, the alteration may
be
La Jara Peak Andesite.
hematization,or weak argillization.
silicification,
Generally,
in
veins
with
quartz
as
the
main
gangue
mineral,
silicification is the attendznt alteration. Weak
argillization
is
calcite-dominated
also
associated
veins,
seen or hematization
is
either
the
with
very
veins.:
In
these
little
alteration
is
principaLa1tering
process.
Near some veins, hematite so
ispervasive that phenocrysts
in the andesite are not visible in hand specimen. Bleaching
of wall-rock
adjacentto veins
isnot common,
although
fragments of the andesite entrapped in the veins
oftenare
partially bleached. No pyrite is visible within the
alteration zoneso r the veins; thus, the.bleaching effect
may be due to acidic solutionsof hypogene origin.
Widths
inches
of
alteration
to1 Zoot directly
zones
generally fron
Vary2
proportionalto the
widths
of
the
veins. However, 'In areas of abundant veining, overlapping
alteration zones affect large areas
o f rock. These large
zonss
are
r o c k has
generally
been
located
brecciated,
along
thus
major
faults
where
pexmitting
deep penetration
the
-91-
of alteringfhids into the wall-rock. .
Discussion of Vein
Veins
'in
the
Silver
Mineralization
Hill
area.
show
many
similarities
to epithermal veins described by Liadgren
(1933, p. 444-
513).
Lindgren relates the solutions which formed the
vein
to intrusive
genetically
material
rocks,
but
this
may
not be true f o r veins in the Silver Hill area. .Recently,
Taylor (1973) has proposed that all epithermal vein deposits
may
have
originated
from
heated
meteoric
waters,
provided
the
deposits are: 1) located in continental-eqpted volcanic.
rocks
which
o r inferred
have
been
intensely
faulted,
2) near
intrusiveof approximately
the
an
same
exposed
age
as
the
rocks,3 ) associatei; with intense hydrothermal
volcanic
.
alteration
in
the
volcanic
..,
younger
rocks, 4)and
thano r
correlative in age
to the volcanic rocks
and the intrusive,
With
the
rocks,
of intense
exception
these
conditions
alteration
of
the
volcanic
are
met by vein deposits in the
thesis area. The deposits occur in highly faulted La Jara
Peak
Andesite,and the rhyolitic ilztrusive and
f l o w s at
Magdalena Peak (14.3 m.y.; Weber,
1971) may have been the
necessary heat source.
Silver'
Hill
vein
deposits
show
similarities
in
..
mineral assemblage to a group of vein deposits associated
with
andesitic
volcanic
rocks
in
southern
California
studied
by Beane (1968)- Beane noted that the assemblage
chrysocolla-hematite is typical of the supergene oxidation
zone associated with copper-iron sulfide deposits. However,
-92-
the
presenceof barite
with
the
assemblage
and
the
quantity of early sulfide minerals suggested
to him
small
that.
the vein deposits may have been
of hypogene origin. Some
evidence
suggestsa hypogene
malachite-hematite
Galena
and
origin'for
a chrysocolla-
assemblage
sphalerite
in
the
Silver
to be later
appear
Hill
than
area.
chrysocolla
based on textural evidence. Some chalcocite and covellite
appear to have
although
Barite
formed
ina few
is
later
samples,
common
in
than
the
veins
chrysocolla
sulfides
which
chrysocolla-malachite-hematite and
are
contain
is
and
malachite,
earlier,
the
usually
assemblage
earlier
than
be to
later. The
the.assemblage, but is occasionally found
fact that some sulfides are found earlier than chrysocolla
and
malachite
origin,
in
be construed
may
addition
to the
as
evidence
for supergene
observation
that
chrysocolla
and
malachite are commonlylcnown as supergene minerals, Veins
in
the
Silver
Hill
area
contain
such
minor ofamounts
sulfide minerals that observable relationships,
to chrysocolla
and malachite are scant. However, 'the possibility.that the
chrysocolla-malachite-hematite assemblage
maybe o f hypogene
. .
origin
should
Economic
not
be ruled out.
Potential
-
Three aspects need to be considered in order to .
evaluate the economic potential
of the Silver Hill area.
They axe: 1) b+se-metal replacement depositsin the Kelly
Limestone, 2) vein
and 3 ) porphyry
mineralization
copper
in
mineralization
La
Jara
along
the
Peak
Andesite,
Flulligan
-93-
Gulch
graben.
Much
of
the.early
exploration
in
the
Silver
Hill
area
was directed towards the possibility that the Kelly
Limestone, a host
for
mineralization
in
the
district, underlies volcanic rocks
exposed in
Kelly
the
mining
thesis
area. Present ,information regarding the Paleozoic and
volcanic
stratigraphy
of
the
drilling
depthsto the
Kelly
would probably be at'least
12000
(La
Jara
Peak
Magdalena
Limestone,
8200
feet
area
if
indicat'es
it
were
but be
mayas
that
present,
much
as
Andesite
0-1000 ft., upper tuffs0-600
ft., andesite of Landavaso
Reservoir0-800 ft., A-L Peak
600 ftO9Spears
Formation 0-900 ft., Hells Mesa Formation
7
4 Permian
$3c
,\?2
Formation 2000.ft
2000 ft.,
a
rocks
3000 ft.,
Madera
Limestone
Sandia Formation600 ft.). ..Therefore, within the
Silver Hill area, possible base-metal replacement deposits
in
the
Vein
limestone
would be uneconomic
deposits
in
La
Jara
under
present
Peak
Andesite
conditions.
were
mined
of gold
primarily for copper and silver; small amounts
occurred
asa by-product.
material
collected
'
Geochemical
from
a recent
analyseson vein
sampling
copper, lead
high values ,(>2%/ton) for
showed
and
zinc,
several
with
silver
values about 2 ounces/ton. Analyses were not conducted€or
go2.d.
vein
These values occur erratically and large tonnages of
material
probably
could be
not
sined
economically
from
the deposits.
.In
recent
Magdalena-Tres
years,
Nontosas
exploration
area
has
activity,in
been
renewed
the
in
search
"9"
of possible porphyry copper mineralization and.related .
base-metal deposits. The possibility that such
mineralization
occurs
within
the
Silver
Hill
area
is
extremely unlikely because,except along the western margin,.
the rock units are relatively unaltered and unmineralized.
,
However, there is the possibility athat
buried
lies to the.west
of
the
thesis
area
intrusive
within
the
Mulligan
Gulch graben. Hydrothermal alteration, felsic dikes and
of copper,
epithermal veins containing small quantities
silver,
gold
graben
from
and
the
lead
Cat
occur
along
Mountain
the
district
western
flank
o f the
in
the
south
to the
Council Rock district in the north. Along the eastern flank
of the
graben,
the
alteration
is
more
restricted
and
may
be partially of supergene origin; pre-La Jara Peak
. . ..
mineralization and felsic dikes appear
to be absent.
Purthermore, a partially
Driveway w e l l (Hale
exposed
Well
pluton)
monsonite
is
pluton
unaltered
near
and
appear to be genetically related to hypogene alteration.
Thus,
does
'
explorationfor possible large ore deposits should
be
directed
graben.
Hale
towards
the
western
flenk of .the
Mulligan
Gulch
not
-95-
CONCLUSIONS
Investigation of the
Silver
Hill
area
has
yielded
several important contributions to knowledge of the geologic
framework
of
the
Kagdalena-Tres
Montosas
area.
1) A new stratigraphic unit, named the unit of
Arroyo
Kontosa,
has.been
mapped
along
the
western
boundary
of the thesis area. The unit is comprised'
of 2 facies:
a volcanic facies anda conglomerate facies. Originally,
the conglomerate facies was confused with similar-looking
conglomerates within the Popotosa Formation. However,
based on lack
of
La
Jara
Peals
Andesite
which
a
detritus,
is
major constituent of the Popotosa Formation, the stratigraphic
position
of
the
unit
'oP Arroyo
Montosa
was
deduced beto
below La Jara Peak And'esite.
A K-Ar dateof 25.2 2 1.2 m,y.
.
.
substantiates this conclusion. The unit of Arroyo I<ontosa
probably
formedin a basin-and-range-type
to
present
that
Thus,
the
Magdalena
2)
during
onset
of Basin
area
is
at
deposition
and
Range
Least
of
environment
the
similar
Popotosa
Formation.
deformation
tn the
very
early
Niocene.
The tuff of La Jencia Creek is 1imited:bo
a
narrow, northeast-trending zone' .through the central portion
of the thesis area. Thezone is interpreted to be
a
I
fault-controlled
paleovalley
present
prior
to
deposition
of the tuff. Recognition of this paleovalley reinforces the
existence of a northeast-trending structural gra.in in the
1):agdalena area in mid-Oligocene time. Preservation of La
Jara
Peak
Andesite
in
the
central'part
of Silver
Hill
area
-96-
can
alsobe explained by the presence
of the paleovalley.
3)
graben
Initial stages of formation of the MulLigan Gulch
apparently
occurzed
in
late
Oligocene
with
movement
along the Hale Well fault. The embryonic graben undoubtedly.
attained
greater
deformation
development
which,
unit of Arroyo
as
during
may
and
Range
m.y.,date-on
by 25the
suggested
Nontosa,
Basin
have
begun
in
very
'
the
early
Miocene. Uplift of a major intra-graben horst in Miocene
time
modified
the
Mulligan
Gulch
graben
and
.
accounts
for its
apparent shallow depth west of the thesis area. The graben
was
further
Miocene
.
modified
by
the
Hells
Mesa
fault
later
in
the
Epoch.
4) Argillic alteration along the western margin
of
the
thesis
area
hydrothermal
fluids
Some of the
argillic
does
not beseem
.. related
.
to gene-tically
to
,
derived
from
alteration
the
may
Halepluton.
Well
have
resulted
from
acidic solutions created by supergene oxida.tion
of pyrite.
Howe'ver, hypogene solutions which introduced the sulfur and
possibly
the
iron
to make
pyrite.probably
imparted
some
argillization to the rocks also. Hence, the rocks may have
been
originally
subsequently
by hypogene
altered
overprinted
by
solutions,
alteration
and
related
t o supergene
>
processes.
5) The paragenetic sequence of vein minerals indicates
-that at least some chrysocolla and malachite formed earlier
than
copper
sulfides
and
thus
may
be
of hypogene
However, opposite paragenesis ina few samples
-
origin.
supports.zt
'
-97-
supergene origin. Galena and sFhalerite, though-present
in
small quantities, are late-stage minerals.
.
.
6) The Silver Hill area is one of the less favorable
parts of the
The
Kelly
12000 feet
district
f o r base-metal exploration.
Magdalena
Limestone,
beneath
if
the
present,
surface;
lies
hence,
8200 t o
at ofdepths
exploration
for
possible base-metal replacement deposits is not economically
feasible. Epithermal vein deposits in La Jara Peak Andesite
lack
sufficient
quantities
of
mineralization
-to'.be
economic
under present conditions. Lackof alteration awayfrom the
eastern
drill
the
boundary
of
the.Nu1ligan
detailed
Gulch
hole
data
and
Silver
Hill
areanotisprospective ground for
dfsseminated copper deposits,
geologic
graben,
. ...
mapping,
indicated
from
suggests
that
-98-
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