Circular 38
GEOLOGY OF THE LUIS LOPEZ MANGANESE
DISTRICT, SOCORRO COUNTY, NEW MEXICO
By ALFRED T. MIESCH
1956
NEW MEXICO BUREAU OF MINES AND MINERAL RESOURCES
NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY
SOCORRO, NEW MEX ICO
THE REGENTS
Members Ex Officio
The Honorable John F. Simms
Mrs. Georgia L. Lusk
Governor of New Mexico
Superintendent of Public Instruction
App o in te d M e mbe rs
Robert W. Botts
Holm O. Bursum, Jr.
Albuque rque
Socorro
Thomas M. Cramer
John N. Mathews, Jr.
Richard A. Matuszeski
Carlsbad
Socorro
Albuque rque
CONTENTS
Page
Abstract ............................................................................................................................... 1
Introduction ...................................................................................................... 1
Acknowledgments .................................................................................... 2
Geology .............................................................................................................................. 3
Geologic Setting ................................................................................ 3
Sequence of Rock Units ......................................................................... 3
Rhyolite Tuff (Tt) ............................................................. 3
Dark Rhyolite (Tdr) ................................................................ 4
Volcanic Breccia (Tvb) ............................................................ 6
Rhyolite (Tbr) ............................................................................... 7
Porphyritic Andesite (Tba) ........................................................ 7
Spherulitic Rhyolite (Tbsr) .......................................................... 8
Tuff (Tts) ................................................................................................... 8
Augite Andesite (Tas) ............................................................... 9
Sandstone (Ts) ........................................................................................ 9
Rhyolite (Tr)................................................................................. 10
Rhyolite (Trs) .............................................................................. 11
Massive Rhyolite (Tmr) ............................................................... 11
Late Rhyolite (TIr) .................................................................... 13
Latite (Tls) ................................................................... 15
Tertiary and Quaternary Fanglomerate (TQf)............................. 15
Quaternary Basalt (Qb) ........................................................ 16
Alluvium (Qal) .................................................................. 16
Structure .................................................................................................... 17
Composition of the V olcanic Rock s .................................................... 18
Interpretation of the Analyses .................................................................. 19
Manganese Deposits ............................................................................... 23
Location .......................................................................................... 23
History and Production ........................................................................ 23
Type of Deposits ................................................................................. 24
Reserves ............................................................................................... 24
Mineralogy ......................................................................................... 25
Origin of the Ores ............................................................................. 27
Bibliography ........................................................................................ 30
ILLUSTRATIONS
Tables
Page
1.
C o m p o s i t i o n o f t h e m a s s i v e r h y o l i t e ( T m r ) . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.
Semiquantitative spectrographic and radiometric (eU)
analyses of se ven vol canic rock s from the Luis Lope z
m a n g a n e s e d i s t ri c t , S o c o rro C o u n t y , N e w M e x i c o . . . . . . . . . . . . . . . . . . 20
3.
Explanation of semiquantitative spectrographic reports in
Table 2, showing ranges with approximate subranges and
midpoints .................................................................................. 21
Figure
1.
Normative orthoclase - albite - anorthite diagram for
vo l c a n i c ro c k s o f th e C h u p a de ra Mo u n ta i ns ............................ 22
Plates
1.
G e o l o g i c m a p a n d s e c t i o n s o f t h e L u i s L o p e z m a n g a n e s e district,
S o c o r r o C o u n t y , N e w M e x i c o . . . . . . . . . . . . . . . . . . . . . . . . . following page 31
2.
M a n g a n e s e - b e a r i n g b r e c c i a f r o m a p o i n t j u s t e a s t o f t h e MCA mine
...................................................................................................... 26
ABSTRACT
T h e L u i s L o p e z m a ng a n e s e d i s t r i c t i s l o c a t e d in t h e C h u p a d e r a M o u n ta i n s , a
north-sou th trending range which lies a few miles southwest of So co rro , in central
New Mexico. The Chupadera Mountains are made up almost wholly of volcanic
ro cks, r an g i n g i n co mp o si ti o n fro m b as alt to rh yo lite , b ut co nsisting ch ie f ly o f rhyo l i t e , t u f f , a n d w e l d e d tu f f o f r h y o l i t i c c o m p o s it i o n .
The co re o f th e ran g e is massi ve rhyo lite , o ve r 1, 000 f e e t thick, co ve r ing
a n a r e a o f a b o u t 1 0 s q u a r e m i l e s . T he o t he r vo l c a n i c u n i t s a r e re s t r i c t e d m a i n l y
to th e n o rthe rn an d so uth e rn p arts of the range. Thoug h similar, the rocks canno t
b e co rre l ate d d i re ctl y fr o m n o rth to so uth; a cc o rd ing ly, the y are d ivid e d into tw o
sequences. The no rthern sequence is made up chiefly of tuff, rhyo lite, and volcanic
breccia with interlayered rhyolite and andesite flows. The southern sequence is
w e l d ed tu ff, rh yo l i te , an d an d e si te . All the se units are o ld e r than the massi ve rhyo lite, which is intruded by a plug and dikes of rhyolite in the north and by latite
dikes in the south.
The Ch u p ad e ra M o u n tai n s are surro und e d b y fang lo me rate s o f p re sum e d
S an ta Fe age . Th ese sed i me n ts are se mico nso lid ate d to co nso lid ated sand, g rave l,
an d so me cl ay. T h e y ar e i n tru d e d b y b asalt sill s, d ike s, and p lug s and are cap p e d
b y b a sa l t f l o w s a t ma n y l o c al i t i e s .
Semiquantitative spectrographic analyses of seven volcanic rocks and a
co mp l e te ch e mi cal an al ysi s o f th e massi ve rhy o lite are p re se nte d .
Wi th i n t h e Ch u p ad e r a M o u nt a in s, f au lt ing an d ti l ti ng o f the o ld e r v o l c ani c
ro cks resu l ted fro m the i n tru si o n o f the massive rhyo lite. Late r f aulting, asso ciate d
with the formation of the Rio Grande trough, is marked on the eastern front of the
mo u n tai n s b y a no rth- so u th tre nd in g e n e che lon f ault syste m.
Small manganese deposits are present throughout the area, but the major
mang ane se d epo si ts d isco ve red so f ar o ccur in b re ccia z one s o r stee ply dip ping
fault zones in the massive rhyolite. Manganese oxide minerals, chiefly psilomelane,
o ccu r a s b an d e d b o tryo i d al ve i n fi l l ing s o r as sto ckw o rks o f ve inle ts ce me nting b re ccia.
Th e o re s are con side re d to be o f h yd ro the rmal (ep ithe rmal) o rig in.
INTRODUCTION
The Luis Lopez mi ning distri ct pro vides one of the best illustrations in New
Mexico of deposits of manganese oxides in breccias and in veins of rhyolite. In
1 95 5, i t w as o ne o f th e mo st p ro ductive d istricts in the U nite d S tate s. The p urpo se
1
of this study is to provide the geologic background for the occurrence of the manganese minerals. The volcanic rocks of the district were studied in detail in order
to contribute to the general knowledge of these rocks. The field work by the writer
in 1951 and 1952 was done prior to the marked expansion of exploration and mining
in 1954 and 1955. In 1955, E. C. Anderson and H. L. Jicha, Jr., of the staff of the
New Mexico Bureau of Mines and Mineral Resources, began a detailed study of the
manganese mines and prospects and have added to the map the locations of new
openings and access roads. Some revisions in the text also have been made to bring
the material up to date.
The Luis Lopez district lies southwest of Socorro in the first range of hills
west of the Rio Grande, about 5 miles west of the villages of Luis Lopez and San
Antonio on the Atchison, Topeka and Santa Fe Railway. A branch line of the latter,
e x t e n d i n g f r o m S o c o r r o t o M a g da l e n a , c r o s s e s t h e n o r t h e n d o f t h e a r e a , a s d o e s
U. S. Highway 60. The El Paso line of the same railroad and U. S. Highway 85
follow the west bank of the Rio Grande, 5 miles to the east. Roads leading to the
pr i n c i p a l m i n e s a n d p r o s p e c t s p r o v i d e r e a d y a c c e s s t o t h e a r e a .
There are only two ranch headquarters in the district. Stock wells supply
m i n i m u m r e qu i r e m e n t s o f w a t e r , n o s p r i n g s o r p e r m a n e n t s t r e a m s b e i n g f o u n d i n t h e
area. Adequate water for the manganese mills is obtained from wells in the valley
f i ll ju s t of f t h e e a s t e d g e o f th e m a p p e d a r e a. M an g an e s e mi ni n g i s th e o nl y no ta b l y
remunerative activity in the area.
T hi s r e por t i s b a s e d o n a t h e si s s u bm it t e d fo r t h e M a s t er o f Sc i enc e d e gr ee a t
Indiana University. The field and laboratory work were performed under the guidance
of Professor Charles J. Vitaliano, of that institution. Work in the field was done on
weekends while the writer was temporarily on the staff of the State Bureau of Mines,
i n So cor ro.
The topographic map of San Antonio quadrangle was not available when the
w ri t er un d er to ok th i s st ud y .
ACKNOWLEDGMENTS
The author is grateful to Dr. Eugene Callaghan, director of the New Mexico
B u r ea u of Min e s a n d Mi ne r a l R e so urc e s, for f aci l iti e s a n d e qu ip m en t u s e d in th i s st u dy .
S pec i al th a nk s a r e a l so du e Dr. R . H. W e b er , of the B ur e a u, fo r m a ny s u g ge s t io n s an d
assistance, particularly in conducting the profile surveys required for the preparation
of the geologic sections.
2
GEOLOGY
Geologic Setting
The Luis Lopez district occupies part of a narrow range of hills extending southward
7 miles from Socorro Canyon and U. S. Highway 60 to Nogal Canyon (see pl 1) The range is
designated Chupadera Mountains on the San Antonio topographic map and is a continuation of
the chain of mountains on the west side of the Rio Grande valley, which includes the
Lemitar Mountains and the Socorro Mountains to the north and unnamed hills south of Nogal
Canyon. The range ends against a westward curve of the Rio Grande valley 8 miles south of
Nogal Canyon. The Chupadera Mountains in the Luis Lopez district are 2-1/2 miles wide,
having a local relief of 1,000 ft and a maximum altitude of 6,300 ft. On the west is the
high Snake Ranch Valley trough underlain by fanglomerate and deeply dissected by stream
courses originating in the Magdalena Range farther to the west. These streams flow eastward
directly through the Chupadera Mountains and cross the much lower dissected fans and alluvial fill
between the Chupadera Mountains and the floodplain of the Rio Grande (altitude 4,550 ft).
The Chupadera Mountains are made up almost wholly of volcanic rocks ranging from
basalt to rhyolite but consisting chiefly of rhyolite or tuff or welded tuff of rhyolitic
composition. The underlying rocks are not exposed but might be expected to be Paleozoic or
Precambrian. Rocks of these ages occur in the Magdalena Range to the wes t , in Socorro
Mountain to the north and in the unnamed hills to the south. Massive rhyolite makes up
most of the core of the mountain range north of Nogal Canyon. For the most part, the
volcanic units dip steeply eastward in contrast to the westward dip of similar units in the
Socorro Mountains north of the district. The range is believed to be bounded on both east and
west sides by faults or fault zones, so that structurally the range is a horst uplifted between relatively depressed areas now covered by fanglomerates (pl 1).
Sequence of Rock Units
Rhyolite Tuff (Tt)
The lowermost unit mapped is a rhyolite-latite tuff (Tt) which occurs in a number of
small outcrops covering about one-quarter square mile between U. S. Highway 60 and Black
Canyon. This tuff rests on massive rhyolite (Tmr) in two localities in Black Canyon, but the
massive rhyolite is interpreted as being intrusive and therefore younger than the tuff. In Black
Canyon, the tuff underlies dark rhyolite (Tdr) and along U. S. Highway 60 it is overlain by
massive volcanic breccia (Tvb). One-half mile southeast of Box Canyon, a large block of the tuff,
over 500 ft thick,
3
d ip s no rthw ard ab ou t 25 de g ree s and is f aulted ag ainst vo lcanic b re ccia. At the
northeast end of Box Canyon, the tuff has been intruded by a dark rhyolite dike.
The tu ff i s ve ry fi ne g rai ne d and is crude ly layered , so that the attitude
o f the bed s i s de te rmi ned e asi l y. O n hig hly weathe red surf ace s, it is dark b row n
to black, but on fresh surfaces it is light pink to gray. Xenolithic fragments are
abundant and usually range from microscopic size to several inches across, though
fragments up to several feet across are present. Although the composition of the
foreign material varies widely, the most prominent fragments are of a porphyritic
rhyo lite wi th a red, glassy gro undmass. Nume ro us angular cavitie s, up to one-half
i n ch a c ro s s, h a ve c l a y l i ke f i l l i n g s a nd co a ti ng s w hi ch t u rn b lue w he n b e nz id i ne
reagent is applied, an indication of the presence of montmorillonite (Mielenz et al,
1950), which probably formed as an alteration product of glass fragments.
Thinsections show the matrix of the tuff to be composed chiefly of sharply
angul ar qu artz and potash feldspar crystal frag ments in a g round mass of almost
completely devitrified glass. The potash feldspar fragments, sanidine and orthoclase,
are up to 2 mm across and form about 30 percent of the matrix. Both these minerals
h ave b e e n kao l i n i ze d and se ri ci ti ze d . Le ss than 10 - 20 p e rce nt o f the g lass is no w
vi t ri c; t he re m ai n d e r h a s ch an g e d to a c ry p t o cr ys t a lli ne ag g re g a te co n t ai ni ng
scattered spherulitic structures. Numerous minute laths of biotite are present; many
are rimmed or almost completely replaced by magnetite. Many of the fresher grains
show wavy extinction as a result of strain. Irregularly distributed magnetite is also
present. S mall epidote grai ns are sp aringly p resent.
Dark Rhyolite (Tdr)
Dark rh yo li te o ve rl ie s th e tuff (T t) and o ccupie s an outcrop are a of slig htly
l e ss th an 1 sq mile . The w ri te r be lie ve s that this rhyolite w as much mo re e xte nsive .
The dark rhyol ite forms the g reater part of the walls o f Box Canyon, where it is o ve r
200 ft thick; i t al so fo rms a dike abo ut 50 ft wide in tuff (TO at the northeast end of
the canyon. It underlies layered volcanic breccia (Tvb) at Blue Canyon, and at Black
Can yo n a si l i ci fi e d p h a se d i p s ste e p ly e astw a rd o f f the m assi ve rhy o lite (T mr ). The
dark rhyoli te is found i n small p atchy outcrops at the west end o f No gal Canyon and
along the southwest margin of the massive rhyolite (Tmr).
Th i s rh yo l i te i s d ark b l u e to lave nd e r and is lo cally p o rp hyriti c, w ith p he no crysts of feldspar that appear as irregular white splotches about 2 mm across. The
groundmass is aphanitic. The rock is mostly massive, though thin flow layering appears
in some places.
The dark rhyolite occurs as numerous inliers in the massive rhyolite at the west
end of Red Canyon. Some of the inliers may be former topographic highs of the dark
rhyo lite surf ace on which the massive rhyolite rests, but othe rs certainly are large
4
xenoliths. Inliers, up to 700 ft across, in the massive rhyolite in the lower parts of the
gulches may be interpreted as former topographic highs. Equally large xenoliths were
observed to be clearly enclosed by the massive rhyolite. Both the
xenoliths and inliers have been penetrated by stringers and dikes of massive rhyolite.
One of the inlier contacts is of special interest. In the southern part of sec. 18, T. 4
S., R. 1 W., very fine-grained massive rhyolite is separated from the dark rhyolite by a zone or
crust of very coarse-grained massive rhyolite about 10 ft thick. The fine-grained rhyolite has
intruded both the coarse-grained crust and the dark rhyolite with numerous dikes, usually 6-8
in. thick, but ranging up to 2 ft thick. The textural variations in the massive rhyolite are distinct
and well defined, with no obvious gradational zone. Both extremes of crystal size are distinctly
different from the normal texture of the rock. It appears that the coarse-grained crust of
massive rhyolite formed along the border of the dark rhyolite prior to the solidification of the
fine-grained phase. A border of the dark rhyolite about 1 ft thick has been altered, with the
formation of abundant epidote and calcite, but the coarse-grained massive rhyolite crust has not
been affected visibly.
Thinsections show the dark rhyolite to be a porphyritic to microporphyritic rock,
consisting chiefly of orthoclase and plagioclase phenocrysts in a groundmass composed of small
interlocking feldspar laths. Some interstitial glass is present locally. Euhedral and subhedral
orthoclase up to 2 mm across is the dominant feldspar. Plagioclase occurs in much smaller lathshaped forms in the groundmass. Biotite occurs prominently as laths up to 1.5 mm in length. Most
of the biotite has been bleached and has magnetite rims, or has been replaced almost completely
by magnetite. A few magnetite pseudomorphs after hornblende were observed. Biotite and
magnetite form about 5 percent of the rock.
The refractive index of glass produced by the fusion of a powder of this rock indicates a
silica content of 64-69 percent (Matthews, 1951; Miesch, 1952). Normative quartz and feldspar
contents were calculated by using this estimation of silica as well as by semiquantitative
spectrographic determination of potassium, sodium, calcium, magnesium, and iron expressed as
oxides (see table 2). Although the spectrographic analyses are described in a later section, it is
necessary to point out here that the oxidation state of the iron is not reported. Therefore, the
upper range of the normative quartz percent was calculated with 69 percent silica, 6.8 percent
Fe 2 O 3 , and 0 percent FeO. The lower quartz range was derived by using 64 percent silica, 0
percent Fe 2 O3, and' 6.8 percent FeO. The probable normative quartz content was derived by
using 66.5 percent silica (midpoint of the estimated range), 2.7 percent Fe 2 O 3 , and 4.1 percent
FeO (the proportions suggested by modal minerals).
Normative Minerals
quartz
18-12 percent (16 percent probable)
orthoclase
albite
anorthite
48 percent
17 percent
10 percent
5
Small anhedral grains of interstitial quartz are sparingly present, but at
several localities abundant secondary quartz has been introduced into the rock.
T h e da r k r h y o l i t e i n B l a c k C a n y o n c o n t a i n s s m a l l a n h e d r a l g r a i n s o f q u a r t z s c a t t e r e d
evenly throughout the rock. These grains, mostly less than 0.2 mm across, show wavy
extinction and, rarely, biaxial interference figures. Some clusters of quartz are
present as vesicle fillings. A dark rhyolite xenolith in volcanic breccia (Tvb) about
5 0 0 f t w e s t o f t h e s a n d s t o n e o u t c r o p n o r th o f B l a c k C a n y o n h a s b e e n p e n e t r a t e d a l o n g
its margin by numerous silica veinlets, which are about 0.3 mm wide and irregular
in length and direction.
C h l o r i t e i s a b u n da n t i n t h e d a r k r h y o l i t e i n a s m a l l a r e a n e a r t h e c o n t a c t w i t h
volcanic breccia, 300 yd north of Black Canyon. The chlorite, along with chalcedony
occurs as a vesicle filling and lining. The chalcedony often forms the core of the
filling. In hand specimen, the chlorite appears as irregular agatelike banded patches
up to 3 mm across. Its green color is very similar to that of malachite; for this reason
a t u n n e l , 3 0 f t l o n g , h a s be e n d r i v e n i n t o t h e a l t e r e d r o c k . S e m i q u a n t i t a t i v e s p e c t r o graphic analysis indicates no unusual quantity of copper.
Volcanic Breccia (Tvb)
Volcanic breccia (Tvb), together with interlayered rhyolite (Tbr), porphyritic
andesite (Tba), and spherulitic rhyolite (Tbsr), has an aggregate outcrop area of nearly
3 sq miles in the northern and northwestern part of the area. The matrix of the breccia
is mainly dark red and aphanitic, the fragments ranging in size from small particles to
bl o c k s s e v e r a l h u n d r e d f e e t a c r o s s . A l t h o u g h t h e f r a g m e n t s r a n g e c o n s i d e r a b l y i n c o l o r ,
grain size, and composition, nearly all are of volcanic origin.
The volcanic breccia is over 400 ft thick and occurs in two strikingly different
phases. The upper part of the unit is very massive, containing practically no flow
structure; the lower 50 ft or more has very well-developed flow layers averaging about
1 ft thick. Typical massive volcanic breccia crops out in the vicinity of Box Canyon,
the layered phase being best represented by the breccia in the west-central part of the
area. The gradation between the two phases may be seen 1 mile south of Box Canyon
and at Blue Canyon.
Massive, highly silicified breccia occurs in Black Canyon and near the west
end of Blue Canyon. Layered volcanic breccia directly overlies dark rhyolite (Tdr);
along U. S. Hi ghway 60, the u pper m as si ve ph as e rest s on tuff (Tt). S and st one (Ts) and
rhyolite (Tr) overlie the massive breccia south of Box Canyon.
The matrix of the volcanic breccia forms about 50 percent of the rock and is
c o m po s e d o f a n g u l a r p a r t i c l e s o f qu a r t z , o r t h o c l a s e , a n d p l a g i o c l a s e i n a n a p h a n i t i c
gr o u n d m a s s t h a t i s c o l o r e d d e e p r e d b y i r o n o x i d e . B i o t i t e a n d m a g n e t i t e a r e p r e s e n t i n
the matrix in small amounts. The crystal fragments in the matrix are mainly from 0.5 to
6
1.0 mm across. The groundmass of the matrix varies in crystallinity from aphanitic
to g l assy and i s p artl y sp he rul i ti c. A f usion te st indicates a silica co ntent o f 62± 2
percent (Matthews, 1951; Miesch, 1952). The rock is probably a latite or quartz
l ati te . Th e co mpo si ti on o f man y of the lithic f rag me nts in the b re ccia d if f e rs g re atly
f r o m t h a t o f t h e m a t r i x , b u t t h e f r a g m e n t s s ho w no a p p r e c i a b le re a c t i o n w i t h t h e
matrix.
Usually, the breccia is only slightly altered. Kaolinite and sericite have
formed from orthoclase, and magnetite has replaced biotite. Notable exceptions,
h ow e ve r, are the in te n se l y si l i ci fi ed zo ne s in the massive p hase of the b re ccia. The se
zones are indicated on the geologic map (pl 1). The silicification has affected all
p arts o f the ro ck ex cep t crystal s an d frag me nts o f quartz.
Rhyolite (Tbr)
Of the three units interlayered with the volcanic breccia (Tvb), porphyritic
re d an d g ray rh yo l i te (T b r) i s th e o ld e st and mo st w id e ly e xpo se d . This rhyo lite has
been intruded and uplifted by massive rhyolite (Tmr) near the west end of Black
C a n y o n . A s m a l l e x p o su r e o f u n d e r l y i n g l a y e re d v o l c a n i c b r e c c i a ( T vb ) i s p r e se n t i n
the bottom of the canyon. At the west end of the canyon, layered breccia overlies
the rhyo li te ; se ve ral h un d red yard s to the no rthwe st, b re ccia ap pe ars to d ip unde r it.
This rhyolite is porphyritic, with a red and gray groundmass and phenocrysts of
q u a r t z , f e l d sp a r , a n d b i o t i t e . B i o t i t e i s e s p e c ia l l y p r o m i n e n t , o c c u r r in g a s s m a l l d a r k
laths and bronze-colored plates scattered throughout the rock. Much of the rhyolite
h as a l a ye re d stru c tu r e , b u t i mme d iate ly no rth o f B lack C anyo n, it is almo st e nti re ly
massi ve an d co n tai n s a b u n d an t ve i nle ts o f o p al.
Th i n se ct i o ns sh o w the r o c k to b e c o m p o se d chi e fl y o f o r th o c l ase , q u a r tz , and
b i o ti te p h e n o crysts i n a d e vi tri fi e d sp he rulitic g ro und mass. T he o rtho clase p he no crys ts
a r e s u b h e d r a l t o e u h e d r a l a n d r a n g e up t o 4 m m a c r o s s . M u c h o f t h e o r t h o c l a s e h a s
b e e n al te re d to kao l i n i t e an d se ri ci te . Q uartz c ryst als, up to 1 mm ac r o ss, are co mmo n
and , fo r th e mo st p art, are d e e p l y e mb aye d b y the g ro und mass. Fre sh b io tite laths, up
to 1 mm i n l e n g th , are ab u n d an t. S tring e rs a n d cluste rs o f anhe d ral q uartz are p ro mine nt
i n the ro ck.
Porphyritic Andesite (Tba)
P o r p h y r i t i c a n d e s i t e o cc u r s o n l y i n t h e n o r t h e r n p a r t o f t he a r e a w i th i n t h e
breccia (Tvb ). East of Box Canyo n, it rests on massive volcanic b reccia (Tvb), ne arb y
b r e c c i a o u t c r o p s c o n t a i n i n g s m a l l a n d e s i te f r ag m e n t s . O n e m i l e e a s t o f B o x C a n y o n ,
a ri d g e o f t h e an d e si te ap p e a r s t o d ip un d e r sp he r ul i ti c rh yo li te (Tb s r ) . The un d e r l yi ng
rock is not exposed, but the presence of layered breccia (Tvb) can be inferred from field
relations.
7
The most stri king fe atu re of the rock is its pitte d weathered surf ace. The
pits are from about one-fourth of an inch to 1 in. across and have been formed by the
w e ath e ri n g o u t o f l arg e p l ag i o cl ase p he no cryst s. The ap hanitic g ro und mass is
b ro w n on weathered surfaces and light brown to lavender on fresh breaks. In fact, it
is very difficult to obtain a specimen in which the plagioclase phenocrysts are not
completely weathered out.
The andesi te is co mposed of andesine phenocrysts up to 10 mm in leng th in a
partly crystalline groundmass with a well-developed trachytic texture. The groundmass
consists chiefly of felted plagioclase and a very small amount of orthoclase. Biotite is
ab u n d an t , u su a l l y b l e a c h e d , an d a t le as t p a r tl y re p l a ce d b y m ag ne t i te . A f e w ho r n b le nde crystal s, l arge l y rep l ace d b y calcite and ep id o te, are scatte re d thro ug ho ut the
ro c k. Th e re f ra c t i ve i n d e x o f a g l a ss p r o d u ce d b y the fu si o n o f t hi s ro c k i nd i c a te s a
si l i ca co n te n t of 63- 67 pe rcen t (M atthew s, 1951; M ie sch, 1952).
Spherulitic Rhyolite (Tbsr)
Th e u ppe rmost flo w in te rlaye red w ith the volcanic b re ccia is a we ll-laye re d
sp h e r u l i t i c r h yo l i te a t l e a st 5 0 f t th i ck , w h i ch is e xp o se d al o ng U . S . Hig hw a y 6 0
and a few hund red yard s to the south of the hig hway. Along the highway, the
rhyo lite dips
to th e we st and p rob ably o ve rl ie s the po rp hyritic ande site (Tb a). S outh of the
highw ay, i t d ip s w estw ard and o ve rlie s laye red vo lcanic b re ccia.
Layering in this rhyolite is particularly striking and locally yields a thin platy
structure. Where the un it i s no t platy, it is massive, dense, and light gray. The platy
zones co nsi st o f ligh t-gray layers abo ut one -fourth of an inch thick that are
separated b y th in ne r pin k b and s. The laye rs are extre mely w avy and co nto rted .
Thinsections reveal about 85 percent of the rock as denitrified glass. Spherulites, about 0.2 mm across, are scattered evenly throughout the sections and
appear as small crosses between crossed nicols. Many vesicles, up to 5 mm across,
are partly fi l led w i th smal l an hed ral g rains o f q uartz. O rig inally ab und ant bio tite
has bee n replaced by magnetite.
Tuff (Tts)
In th e so u th e aste rn p a r t o f the Chup ad e ra M o untains, in the vicini ty o f N o g al
Can yo n , th e ma ssi ve rh yo l i te (Tmr) is o ve rl ain b y tuf f (Its ), aug ite and e site (Tas), and
r h y o l i t e ( T r s ) . N o n e o f t h e s e r o c k s se e m s t o b e e x a c t l y c o m p a r a b l e w i t h r o c k s i n t he
northern part of the area. The tuff may be overlain by either the andesite or the rhyolite.
Th o u g h th i s tu ff (T ts ) i s si mi l a r i n te xture and stru ctu re to the e arlie r tuff (Tt), it co ntai n s ab u n d an t p l ag i o cl ase . In co ntr ast, the e a rlie r tuf f co nt ains ve r y little p lag io clase .
8
Th e l a te r t u f f ( T ts ) i s d e fi n i te l y yo ung e r th a n t he d ar k r hy o l ite (U r ), w he re a s the
e arli e r tuf f (It) is o lde r. So u th o f Nog al Canyo n, sed iments at the b ase o f the late r
tu f f (Tts) l ie on d ark rhyo l i te.
Th e tu ff ( T ts) i s l i g h t g ray to p ink and in mo s t o utc ro p s is w e ld e d . The mat rix
is compo sed of abou t 80 percent denitrif ied glass. Only in a few lo calized places can
crystal s o f f eld sp ar and h o rn ble nd e b e d e te cted meg ascop ically. Xe nolithic f rag me nts,
up to 1 i n . a c ro ss, a re ab u n d an t , a nd l arg e r x e no l it h s a re c o mm o n . A t N o g al C an yo n,
a l aye r o f b l ack pe rl i te ab ou t 2 ft thick is inte rlaye re d w ith the lowe r part o f the tuf f.
East o f th e B i an ch i r an c h i n N o g al Canyo n, the w e ld ed tuf f is und e rlain b y p ink
and cre amy-g ray w ate r-l ai d tu ffs and tuf f ace o us sed ime nts, includ ing co ng lo me rate s,
san d sto n e s, an d cl a ys. B asal ti c an d e site flo w s and asso ci ate d b re ccia o ccur ju st b e lo w
t h e t u f f s a t t h e t o p o f t h e w a t e r- l a i d s e c t i o n . T h e lo w e r p a r t o f t h i s s e q ue n c e i s e x p o sed farthe r to th e sou th we st, outside o f the map ped are a. At No g al Canyo n, the
sedimentary beds are cu t off by a northw ard tre nding fault and by the massive rhyolite
(Tmr). Th e w ate r-l aid mate ri al i s i nclude d w ith the we ld ed tuf f s on the map (pl 1).
Au gi te An de si te (T as)
An andesite flow having an outcrop area of three-eighths of a square mile rests
d i re ctl y o n massive rhyo l i te (T mr) and o n the tuf f (Its). T he f low is abo ut 100 f t thick
a n d , i n m o s t p a r t s , h a s a l a y e r e d o r p l a t y s t r uc t u r e . T h e r o c k i s sl i g ht l y p o r p h y r i t i c ,
with a dark aphanitic groundmass and small phenocrysts of feldspar. A coarser grained
vari e t y o f an d e si te fo rms a d i ke i n the und e rlying tuf f . T his d ike has aug ite crys tal s up
to 2 mm across and f ractu re s l i ned with thin laye rs of op aline silica.
The flow andesite is a distinctly trachytic aphanitic rock composed of minute
c ry s t al s o f p l ag i o c l a se an d m i c ro p he no c r ys t s o f aug ite . P l ag io cl a se an d in te rs t it i al
g l ass fo rm ab o u t 85 p e r ce n t o f th e ro ck. S mall d ark re d - b ro w n laths o f b io tite are
present but, more commonly, the biotite is very intensely bleached and replaced by
m ag n e t i te . O th e r m ag n e ti te i s s c a tte re d t h ro u g ho u t t he ro ck . M in ute ve i nle t s o f
introduced si lica are co mmon . Al though the andesite overlies massive rhyo lite (Tmr),
the andesi te is probabl y the older of the two ro cks, as andesite occurs as xenoliths
in the rhyolite. The writer believes that the rhyolite (Tmr) intruded the andesite,
pushing i t upward and o utward. Dips range up to as much as 30-60 deg rees.
Sandstone (Ts)
Tho u g h o n e - th i rd o f th e T e rtiar y se ctio n at S o c o rro P e ak is e stimate d t o co nsist of sedimentary rocks (Lasky, 1932, p 120), the only Tertiary intravolcanic sediment
found in the Chup adera Mountain s, a few miles to the so uth, is a d ark-red to orange
9
quartz sandstone, which rests on the massive phase of the volcanic breccia (Tvb)
and is overlain by rhyolite (Tr). This sandstone, which has a thickness of about 75
ft, occurs at only two places north of Black Canyon; its total outcrop extent is less
than 40 acres. The sandstone is medium to fine grained, fairly well sorted, and
moderately well bedded. Most of the grains are subround, but near the intrusive
contact with the massive rhyolite (Tmr) many of the
s a n ds t o n e i s m a d e u p c h i e f l y o f q u a r t z , w i t h a f e w
laths, and grains of magnetite, largely altered to
materials. Lithic fragments, especially spherulitic
grains have been shattered. The
grains of orthoclase, some biotite
hematite, all cemented by clay
fragments, are common.
The lithic fragments in the sandstone clearly indicate that the rock is later
t h an co m e vo lca ni c s, bu t i t i s no t cl e a r wh e th e r th e roc k i s ol d e r or y oun ge r th a n th e
volcanic breccia on which it rests. Two sandstone blocks may have been floated on
the lava and brought to the surface from below with the eruption of the volcanic
breccia. The author believes, however, that the sandstone was deposited on top of
the breccia, as there is no evidence to corroborate the hypothesis that the outcrops
are those of floated blocks.
Rhyolite (Tr)
In the northern part of the area, the extrusion of the volcanic breccia sequence
(Tvb) and the depositio n of the sandstone (T s) was follow ed by the extrusion of a rhyo lite
f l o w ( T r) h a v i n g a n outcrop area o f a b o u t o n e - e i g h t h o f a s q u a r e m i l e . I n t h e t w o
exposures of the rhyolite (Tr), four rock types are present. A basal layer of tuff, several
feet thick, is overlain by a dark-red aphanitic rhyolite that is characterized by numerous
gray streaks and lenticles commonly less than 1 in. in length. The uppermost unit is a
dark-red vitrophyre. The entire unit is less than 150 ft thick.
All th e unit s consist es se ntiall y of gl as s, quart z, a nd pota sh fe ld sp ar in var ying
pr o po r t i o n s . T h e v i t r o p h y r e i s a p p r o x i m a t e l y 6 0 p e r c e n t g l a s s , w i t h s o m e s m a l l s p h e r ulites. Fragments and subhedral grains of quartz are a little more abundant than the
potash feldspars, sanidine and orthoclase. No plagioclase is present. Biotite, altered
to chlorite and magnetite, forms about 5 percent of the rock.
The two rhyolites below the vitrophyre are similar in composition and texture
e x c e pt f o r t h e p r e s e n c e o f t h e g r a y s t r e a k s a n d l e n s e s m e n t i o n e d a b o v e i n t h e y o u n g e r
of the two units. The gray areas in thinsections are clearly more crystalline than the
red areas and appear to be a primary feature of the rock. No alteration is associated
with them. The textures range from cryptocrystalline in the red areas to microcrystal line
in the gray areas. The lower gray or orange rhyolite is entirely cryptocrystalline, with
a few small phenocrysts and fragments of quartz and orthoclase.
10
Rhyolite (Trs)
In an outcrop area of about 1 sq mile in the southeastern part of the area,
vo l can i c tu f f (T ts) i s o v e rl ai n b y ab o ut 100 fe et o f mo ttle d re d and g ray rhyo li te (Trs).
T h e u p p e r p a r t o f t h i s u n i t i s a re d v i t r o p h y r e . T h e s i m i l a r i t y o f t h i s u n i t t o t he r h y o lite
(Tr) i n th e n o rth e rn p ar t o f th e Ch up ad e ra M o untains is st riking . The r e d and g ray co lo rs
sh o w th e same te x tu ral vari a ti o n s, and the units are o ve rlain b y d ar k- re d vitro p h y re . The
g l a s se s o f t h e vi t ro p h y r e s a re i d e nt i c al in co lo r , a nd b o t h h a ve sm a ll l e nse s o f
c r y p t o c r y s t a l l i n e m a t e r i a l . T h e p h e n o c r y s t s o f t h e t w o v i t r o p h y r e s a r e e q u a l i n amo unt
and size .
The mineral composition of the rhyolite sequence in the north, however, is
not identi cal to th at in the southe ast. The southeaste rn sequence has well-developed
p h e n o crysts o f o l i g o cl as e i n co n trast to the to t al ab se nce o f o b se rvab le p lag io clase in
the northern sequence. The northern sequence is far mo re abund ant in potash feldsp ars.
B o th se q u e n ce s are ri ch i n q u artz, and b o th co ntain ne arl y e q ual amo unts o f b io tite .
Al t h o u g h th e c o l o r a n d te x t u ra l v ar i a tio ns o f t he tw o se q ue nce s sug g e st a g e ne ti c
re l a ti o n sh i p , fi e l d e vi d e n ce o f s u c h a re l a ti o n s hip is ab se n t .
Massive Rhyolite (Tmr)
The most extensive formation (10 sq miles) and one of the most important
e co n o mi cal l y, co n tai n i n g o ve r 90 pe rce nt o f the mang ane se o re thus f ar d isco ve re d ,
is the massive rhyolite (Tmr). This unit occupies the central core of the Chupadera
Mountains, ending just south of Nogal Canyon. The exposed thickness in canyon
w al l s e x ce e d s 1 , 0 00 f t; th e b a se , ho w e ve r, i s no t e xp o se d , and t he t o p h a s b e e n
eroded.
T h e m a s s i v e r h y o l i t e c h a n g e s ve r y l i t t l e i n a p p e a r a n c e t h r o u g h o u t i ts e x t e n t.
It i s, f o r th e mo st p ar t, h i g h l y p o rp hyritic, w ith p he no crysts o f sanid in e , q uartz , and
s o m e b i o t i t e i n a n a p h an i t i c g r o u n d m a s s t h a t i s l i g h t g r a y t o b l ue o r r e d d i s h. L o c a l l y ,
i t i s o n l y sl i g h tl y p o rp h yri ti c, as fo r e xamp le , ne ar its e aste rn a nd no rt he rn marg ins.
It typically weathers pale reddish brown. Flow structures are almost entirely absent,
and the rock is generally nonvesicular. Prominent joints, especially well developed in
th e R e d Can yo n are a, c u t th e massi ve rhyo lite in se ve ral d ire ctio ns. Lo cally, the jo int
surfaces are covered by films of manganese minerals. Concentrations of xenoliths in
zones that may be as much as 1,000 f t wide give the rock the appearance of a vo lcanic
breccia. The xenoliths are mainly fragments of older volcanic rocks. In small areas,
th e f rag me n ts in clu de pi e ce s of g ranite s, schists, and g neisse s o f p robab le P re camb rian
a g e . T h e x e no l i t h i c f r ag m e n t s i n t h e se zo n e s r a n g e f r o m m i c r o s c o p i c t o se v e r a l f e e t
acro ss: In so me are as, larg e x en ol iths o f ro cks o the r than tho se mentioned abo ve have
b e e n p e ne t r a t e d b y s t r i n g e r s a n d d i k e s o f m a ss i v e r h y o l i te . T he b ig g e s t o f t h e se a r e
the dark rhyolite xenoliths discussed in the section on dark rhyolite (Tdr). North of
Red Canyon, there is a xenolith of black vitrophyre 20 ft across. Coarse-grained
g ran i ti c ro cks and so me hi gh l y schi stose ro cks fo rm xenoliths up to 5 f t in d iame te r in
the western part of sec. 31, T. 4 S., R. 1 W.
11
In thinsection, the massive rhyolite is highly porphyritic, being composed
largely of sharply angular phenocrysts of quartz and sanidine, up to 3 mm across, in
a partly crystalline groundmass that is partially spherulitic. Magnetite and small
di s t o r t e d l a t h s o f b i o t i t e a r e p r e s e n t i n t h e g r o u n d m a s s . E p i d o t e a n d m u s c o v i t e a r e
sparse.
Oligoclase, much of which has been altered, is irregularly distributed through
the rock. Some of the oligoclase laths are twinned according to the carlsbad and
albite laws, with very thin lamellae; others are zoned. Oligoclase is the only mineral
in the massive rhyolite that has been highly altered. Deuteric solutions are believed
to be responsible for the alteration of some oligoclase to quartz and orthoclase, or
to calcite.
Quartz phenocrysts are deeply embayed by the groundmass, and sanidine has
been slightly kaolinized along its nearly invisible cleavage planes. Biotite frequently
has inclusions and rims of magnetite. The chemical analysis, norm, and mode for this
r o c k a r e p r e s e n t e d i n T a bl e 1 .
Abundant silica must have been associated with the final stages of solidification of the massive rhyolite. Nearly all the older volcanics that were disturbed
by its eruption, but chiefly the rhyolite (Tbr), dark rhyolite (Tdr), volcanic breccia
( T v b) , a n d a n d e s i t e ( T a s ) , h a v e b e e n a t l e a s t p a r t l y s i l i c i f i e d . I n a d d i t i o n , t w o l a r g e
masses of white, almost pure, microcrystalline silica, each about 50 ft across, occur
in the massive rhyolite.
T h e e r u pt i v e s o u r c e o f t h e m a s s i v e r h y o l i t e i s , w i t h l i t t l e d o u b t , a t B l a c k C a n y o n ,
where the rock is dense, mildly porphyritic, and somewhat layered parallel to its contact
with older roc ks. In the south ern part of sec. 6 , T. 4 S., R. 1 W., san dstone (T s), volcanic
br e c c i a ( T v b ) , a n d r h y o l i t e f l o w s ( T r ) s t r i k e i n t o a n d a r e c u t o f f b y t h e m a s s i v e r h y o l i t e .
Their contac t with the rhyolite is highly irregular in both plan and cross-section. In
SE¼ sec. 6, th e older rhyolite tuff (Tt), dark rhyolite (Tdr), an d volcanic breccia dip off
the massive rhyolite at angles up to 51 degrees. One mile to the
west, the massive rhyolite is overlain by older rhyolite (Tbr), and in the central part of
sec. 6 volcanic breccia rests directly on massive rhyolite.
Farther south, the massive rhyolite appears to have flowed over volcanic
br e cci a ( T vb ), t uff (T t), a n d d ar k rh yo li t e (T d r). T h e di r ect io n of f low m ay h a v e b e en
co nt ro ll e d b y a st ruc tu r al a n d to po g ra p hic d e p r e s si on, now m a r k e d on l y by t h e i nw ar d
dipping volcanic breccia on the west border of the flow. In the southeast part of the
area, the flow reached an obstruction of andesite (Tas) and tore some of it away to
f o r m x e n o l i t h s . A t pl a c e s , m a s s i v e r h y o l i t e a l s o i n t r u d e d a n d u p l i f t e d t h e a n d e s i t e f lo w .
Toward Nogal Canyon, the flow tongued out, only a few small outcrops occurring south
of the canyon.
12
The fragmental nature of the phenocrysts in the massive rhyolite and the absence of
flow structures suggest the possibility of a pyroclastic origin. The groundmass, however, is not
at all fragmental; rather, it is cryptocrystalline to glassy and spherulitic. This fact, considered
with the field relationships described above, tends to confirm that the rock is not a
pyroclastic. Indeed, it appears that the massive rhyolite was intruded at Black Canyon,
doming the older rocks. South of Black Canyon, the intrusive dome was punctured, and the
massive rhyolite poured out and flowed southward. North of the canyon, the massive rhyolite
spread out under the domed older rocks to produce intrusive forms that may be laccolithic. The
fragmental phenocrysts might be explained by the high viscosity expected in a lava containing 71
percent silica (table 1). The absence of flow structures is probably a result of turbulent, rather
than laminar movement of the lava.
The economic significance of the massive rhyolite lies in the fact that over 90 percent
of the manganese deposits occur in this formation. It appears that the
massive rhyolite was most susceptible to fracturing in a manner which produces breccia zones (in the
tectonic sense). These breccia zones are of extreme importance in the localization of manganese
minerals, as they provide openings for the access of ore-bearing solutions and for the deposition of
the manganese oxides. Some mineralization has occurred also along open joint planes in the
rhyolite.
Late Rhyolite (Tlr)
Highly brecciated, dense, light-gray, intrusive rhyolite (Tlr) forms a small plug and
several northeast trending dikes, as much as 100 ft wide, in the massive rhyolite (Tmr). With
one exception, all these intrusives occur in the area between Black Canyon and Red Canyon.
The plug located in the western part of sec. 18, T. 4 S., R. 1 W., is subcircular in plan and
covers an area of about one-eighth of a square mile. A 10-ft-wide rhyolite dike extends
about 100 yd from the northern edge of the plug. One particularly interesting dike in NW¼
sec. 7, T. 4 S., R. 1 W., covers a large area, but is less than 100 ft thick. The northwest
contact of this steeply dipping dike is exposed for several hundred feet downdip. The dike is
lying against a steep hill of massive rhyolite, the massive rhyolite once present on the
hanging wall side of the dike having apparently been removed.
The late rhyolite is a microporphyritic rock composed of anhedral to subhedral quartz
and potash feldspar phenocrysts up to 0.5 mm across in a partly crystalline groundmass. Many
of the quartz crystals have been deeply embayed by the groundmass. Both orthoclase and
sanidine are present, but no plagioclase feldspar was observed. A striking feature of the rock is
the presence of biotite crystals up to 3 mm across. Some smaller biotite flakes have been
altered almost completely to magnetite. One small crystal (0.2 mm) of garnet was seen.
13
TABLE I. COMPOSITION OF THE MASSIVE RHYOLITE (Tmr)
Chemical Analysis
(By H. B. Wiik, Helsinki, Finland. August 31, 1952)
SiO2 ........................................................................................ 71.09
CaO ............................................ 0.88
TiO2 ......................................................................................... 0.25
Na2O..................................................... 1.89
Al 2 O 3
K2O
6 19
Fe2O3 .................................................... 1.73
P2O5
0 09
FeO ......................................................... 0.32
H2O+
2 20
MnO ........................................ 0.08
H2O-
0 95
MgO ............................................................................... 0.71
CO2
0 00
...........................................................
14.07
100.45%
Norm
Quartz .................................................. 33.5
Magnetite ................................... 0.5
Orthoclase ............................................ 36.7
Hematite ...................................... 1.4
Albite..................................................... 16.2
Ilmenite ....................................... 0.5
Anorthite ................................................ 3.6
Corundum. ............................................ 2.9
Apatite. ............................. 0.3
97.4
H2O+ ........................................... 2.2
Enstatite ............................................... 1.8
H2O- ................................................ 1.0
100.6%
Mode
Aphanitic
groundmass ................................. 63.3
Quartz ..................................................... 5.1
Potash feldspar ..................................... 23.9
14
Oligoclase ..................................... 6.1
Biotite ................................. 1.6
100.0%
Latite (Tls)
Dikes of highly porphyritic, pinkish-gray latite intrude the massive rhyolite
at the west end of Red Canyon. The largest of these dikes occurs just south of the
ca nyon. It tre nds ea st -w est f o r a d istance o f mo re than thre e-q uarters of a mile and
is about 25 ft thick. The total outcrop area of the latite dikes is about one-eighth
o f a square mile .
The latite is highly prophyritic and contains many large phenocrysts of glassy
sanidine, commonly up to one-half inch across. Smaller crystals of plagioclase
f e l d sp a r a r e a l s o a b u n d a n t . Q u a r t z f o r m s l e s s t h a n 5 p e r c e n t o f t h e ro c k . L a t h s o f
b io ti te , co mmon l y up to on e- fo u rth o f an inch acro ss, are scatte red thro ug h the ro ck.
In thinsection, the light-pink groundmass of the latite appears as a micro-
crystalline ma ss of pl ag iocl ase and potash feldspar crystals. The feldsp ars are in
f a i rl y e q ua n t g r ai ns a n d i n ti m a te l y in te rg ro w n . S o me d e vi t ri fie d g l a ss is p re se n t.
Te rti a ry an d Q u ate rn ar y Fan g l o me ra te ( TQ f )
Th e co n so li d ated sand , g rave l, and clay, which f o rms b ro ad , d isse cted alluvial
fans in the Rio Grande Valley and fill in some of the higher intermontane valleys in
New Mexico, has been referred commonly to the Santa Fe formation of late Miocene
a n d e a r l y P l i o ce n e a g e . W a l d r o n ( i n p re p a r a t i o n ) r e p o r t s t h e d i s c o v e ry o f a t o o t h
i n th e se g r a ve l s, w h i ch w a s i d e n ti f ie d b y V. L. V and e rh o o f , f o r me r l y o f S t a nf o rd
University, as belonging to the genus Gigantocamelis, of upper Pliocene and Lower
P l e i sto ce ne ag e . Wal d ro n al so re p o rts tha t rhyo lite is inte rlaye re d w ith the b ase o f
the S anta Fe f o rm ati o n i n at l e ast o ne ne arb y a re a.
Al o n g U . S . Hi g hw ay 60, the S anta Fe (?) co nsi sts p re d o minantl y o f clay and
a p p e a r s t o r e s t o n r h y o l i t e ( Tb s r ) a n d v o l c a n i c b r e c c i a ( T v b ). N o r t h o f t h e h i g hw a y ,
th e cl ay fo r ms a me sa 6 00 ft h i g h , w hich is cap p e d b y a b asalt f lo w . O n the e ast and
w e s t f l an ks o f t h e Ch u p ad e r a M o u n t a in s, the S an t a Fe (? ) fa ng l o me ra t e s a re m o s t ly
covered by a veneer of younger sand and gravel. The Santa Fe (?) is exposed only
at the east end of Nogal Canyon, where it is faulted down against older volcanics,
and i n the so u th w e st part o f th e are a, w he re it fo rms a me s a cap p e d b y a b asalt flo w .
At b o th o f the se lo cal i tie s, i t i s co mpo se d o f conso lid ated to se mico nsolid ated, we ll str ati f i e d san d an d g rav e l . Th e yo unge r g rave ls that co ve r t he S anta F e (?) f o rma tio n
h ave b e e n map p ed w i th i t, u si n g the symb o l TQ f .
It see ms e vid en t th at the S anta Fe (?) sed ime nts co ve red the Chup ad e ra
Mountains almost completely at one time, because where remnants of the erosionalde po si ti on al su rface fo rmed o n the se g ravels have bee n p re se rved b y cap ping s of
15
b asal t, the su rface i s hig he r th an ne arly all the volcanic pe aks. D uring Q uate rnary
time, streams from the Magdalena Range flowed eastward over, these sediments, trave rsin g th e b u ri ed mou n tai n s o n th eir w ay to the Rio G rande. Late r, as the stre ams
began degrading, they were superposed on the exhumed Chupadera Mountains. The
steep-sided canyon s in the resistant volcanic rocks were cut by these superposed
streams.
Quaternary Basalt (Qb)
Q u a te r n a ry b a s al t , cl e a rl y yo ung e r t ha n a t le a st p ar t o f t he Te r ti a r y Quaternary fanglomerates, covers over 2 sq miles in the area. North of U. S.
Highway 60, a basalt flow rests on clays of the Santa Fe (?) formation. These
clays extend southward into the northern part of the Luis Lopez district, where they
have been intruded by numerous basalt dikes. The dikes form prominent long, dark
ri dge s. At the e ast e nd o f N og al Canyo n, b asalt f o rms a sill o ve r 50 ft thick in sand
and gravel of the S anta Fe (?) f ormation. In the southwest part o f the are a, a b asalt
flow cap s fanglo merates, formi ng a mesa that is nearly equal in altitude to the highest
peak of massive rhyolite (Tmr). A plug of basalt, 1 mile southwest of Box Canyon, has
intruded the Santa Fe (?) formation and the older volcanics.
Th e b asal t i s a d ark- b l a ck ho lo cr yst alline g ranu lar ro c k co mp o se d almo st
e n t i re l y o f p l ag i o cl a se , d i o p si d e , a nd o li vi ne i n ne a r l y e q u a nt g r a in s. The p l ag io clase (labradorite) occurs as laths up to 1 mm long and is frequently embedded in
diopside, forming a diabasic texture. Most of the plagioclase and olivine grains are
euhedral, whereas the diopside is subhedral to anhedral. Nearly all the olivine
crystals have rims of iddingsite, some grains showing secondary formation of olivine
aro u n d th e i d d i n g si te rim. M ag n e ti te is ab und ant.
Alluvium (Qal)
The term alluvium is applied to Recent sand and gravel deposited in stream
channels and to talus accumulations of Recent age.
16
S T RU CTURE
The p ri n ci p al stru ctu r al i n f lue nce s o n the Chupad e ra M o untains h ave b e e n
(1 ) th e e mp l ace me n t o f th e massi ve rhyo li te (Tmr) and (2) the f aul ting asso cia te d
w i t h th e f o r m a ti o n o f t h e R i o G r and e t ro ug h a nd t he ad j ace n t p o si t iv e a re as .
Within the Chupadera Mountains, much faulting and tilting of older volcanic
rocks resulted from the intrusion of the massive rhyolite. Several faults extend
radially from the area in Black Canyon believed to be the intrusive center for this
u n i t, an d th e re is a ge ne ral ou tw ard tilt o f the o ld e r vo lcanics aw ay f rom this intrusive
ce n te r, w h i ch i s se e n be st i n th e n o rthe rn p art o f the map p e d are a.
Late r faul ti ng, asso ci ated with the fo rmation o f the Rio G rand e tro ug h, is
marked o n the e aste rn fro n t o f th e Chup ade ra M o untains b y a no rth- south trending
en echelon fault system. The faulting is well exposed at Nogal Canyon, where rhyol i te (Trs) h as b e e n fau l t e d ag ai n st the S anta Fe (?) f o rm atio n. In the no rthe rn p ar t
of the area, the eastern range-front faults are marked by scarps and a parallel
f rac tu re sy ste m i n th e massi ve rh yo lite . It is b e lie ve d that so me f aulti ng also to o k
place on the west side o f the Ch upadera Mountains, but no direct evidence of such
faulting has been found. However, these inferred faults may be covered by the
g r a v e l s w hi c h f i l l t h e so u t h e r n p a r t o f t he S n ak e R a n c h V a l le y t r o u g h ( B ig B a s i n ) .
I n t h e n o r t h e r n p a r t o f t h e r a n g e , a d d i t i o n a l m o v e m e n t p r o b a b l y t o o k p l a c e a lo n g
th e f au l t l i n e s th at w e r e fo rme d d uring the intrusio n o f the massi ve rhyo lite .
Faul ting appears to have controlled in part the localiz ation o f the mineral
de po si ts of the are a. Th e mo ve me nts p rob ab ly caused the f o rmation o f the b re ccia
zones in the massive rhyolite.
17
COMPOSITION OF THE VOLCANIC ROCKS
Al th o u g h o n l y o n e ch e mi cal an alysis w a s o b tain e d , that o f the massive
rhyo lite (Tmr), it was possible, through the courtesy of the Raw Materials Laboratories
o f th e U . S . G e o l o g i c a l S u r ve y , i n D e n ve r, Co lo r ad o , t o o b t ai n se m iq u an t it a t ive
spectrographic analyses and radiometric analyses of a total of seven rock types occurri n g i n th e are a. Al l th e an al yse s a re o f no nf ra g me ntal ro ck s. U sing the se sp e ctro g raphi c an al yse s, i t w as po ssi ble to calculate the app roximate no rmative co mp o sitio n
of the rocks.
It may be helpful to explain the basis on which these analyses are reported.
The percentages of metal oxides determined by spectrographic analyses are reported
i n l o g ari th mi c ran g e s. Th e ran g e i s ind icate d by an " x" p lace d at the de cimal p o sitio r
o f t h e f i r s t s i g n i f i c a n t f i g u r e . T h u s , o . 0 0 x i nd ic a t e s t h a t t h e e x a c t f i g u r e i s i n t he
ran ge 0.001 to 0. 010 perce n t. S i milarly, x. o indicate s a range of 1.0 to 10. 0 pe rcent.
Plus or minu s signs are used to indi cate logarithmic subranges (see table 3). A plus
i n d i c a te s th a t th e v al u e i s i n t h e up p e r p a r t o f the lo g a ri t h mi c r ang e ( e . g . , o . o x +
i n di cate s a val ue be twee n 0. 046 and 0. 100). S imilarly, minus sig ns indicate value s
i n the l owe r p art o f the l og ari th mi c range (e. g. , o. ox- ind icate s a value o f 0.010
to 0.022). When neither a plus nor a minus sign is added, the value is near the midp o i n t o f the l o g a r i t h mi c r an g e (e . g . , o . o x i nd i c a te s a v a lue o f 0. 0 2 2 t o 0. 04 6 ). B y
this method, the results still are given in ranges, rather than specific values, but the
probable values are defi ned more cle arly. The subranges are small eno ugh to make
their midpoints useful for statistical treatment.
The validity of subrange assignments was tested by the spectrographers of the
U . S. Geo l ogi cal S u rve y o n a l arge numbe r of samp le s. The se tests ind icate that
su b ran g e s can b e assi g n e d co rre ctl y in tw o o ut o f thre e case s. T his av e rag e is f o r
al l 5 8 e l e me n ts me a su r e d . H o w e ve r , f o r a n y g i ve n e le me n t, t he ac c u r a c y o f s ub range assignments ranges from 50 to 100 percent.
The equivalent uranium values given are based on measurements of the radioacti vi ty o f th e ro ck samp le s. The value s are de te rmined b y assuming that all the
rad io acti vi ty o f the samp le re su l ts f ro m the d e co mpo sition o f uranium and its d isintegration products, and that no members of the thorium series or other radioactive
elements are present. In addition, it is assumed that uranium and all its disintegration
products are in radioactive equilibrium, i.e., each member of the uranium series
d i si nteg rati ng at ex actl y the same rate as it is be ing fo rme d. Thus, these value s are
not actual measu res of the uraniu m content o f the rock, but rather a method of exp re ss i n g t h e r ad i o a c ti v i t y o f t he ro c k i n f in i te t e r ms . N o si g ni fi c a nt a m o un t s o f
radioactivity were found in any of the rock samples tested.
18
Interpretation of the Analyses
N o r m a t i v e o r t h o c l a s e , a l b i t e , a n d a n o r t h i te w e r e c a l c u l a t e d f o r e a c h o f t he
seven rock types analyzed spectrographically, using the midpoints of the subrange
val u e s re p o rte d fo r p o tash , so d a, and lime , and making the assump tio n that the ro c ks
a r e s a t u r a t e d w i t h s i l i ca a n d a l u m i n a . Th i s a ss u m p t i o n i s r e a s o n a b le , e sp e c i a l l y s i n c e
it was found to be true in the case of the one rock for which a complete chemical
a n a l y s i s i s a v a i l a b l e . T h e no r m a t i v e v a l u e s d e t e r m i n e d f o r t he t h r e e m i n e r a l s w e r e
p lo tte d o n a th re e-p oi n t di ag ram (f ig 1).
19
TABLE 2. SEMIQUANTITATIVE SPECTROGRAPHIC AND
RADIOMETRIC (eU) ANALYSES OF SEVEN VOLCANIC ROCKS
FROM THE LUIS LOPEZ MANGANESE DISTRICT, SOCORRO
COUNTY; NEW MEXICO*
* R ad io me tri c an al yse s b y J. Fenne lly; spe ctrograp hic analyse s b y
R . G. Haven s, U . S . G e o l o g i c al S u r ve y.
Anal yse s made o n beh alf o f the Division o f Raw M ate rials of the Ato mic Ene rg y
Commission.
20
TABLE 3. EXPLANATION OF SEMIQUANTITATIVE SPECTROGRAPHIC REPORTS IN
TABLE 2, SHOWING RANGES WITH APPROXIMATE SUBRANGES AND MIDPOINTS
(U. S. Geological Survey, Raw Materials Laboratory, Denver, Colorado)
21
Figure 1.
N o rmati ve o rth o cl ase - alb ite - ano rthite d iag ra m fo r
vo l cani c ro cks of the Chup ade ra M ountains
S A M P L ES
A
Dark rhyolite (Tdr)
B
Rhyolite (Tr)
C
Massive rhyolite (Tmr)
D
Late rhyo lite (Tlr)
E
Augite andesite (Tas)
F
Rhyolite (Trs)
G
Basalt (Qb)
22
MANGANESE DEPOSITS
Location
Small manganese deposits are present throughout the area examined, but the largest
deposits occur in the area underlain by the massive rhyolite (Tmr). Some deposits of
commercial importance occur at Red Hill and in SE¼ and NE¼ sec. 12,
T. 4 S., R. 2 W. The deposits at Red Hill are being mined by the Manganese Corporation of
Arizona. The Gianera mine is located in SE¼ sec. 12, where a vein of psilomelane occurs. The
vein averages about 11 ft thick and 400-600 ft in length, with a proven stopping depth of about
80 ft. The Tower Mining Company is mining in NE¼ sec. 12, in an extensive zone of
mineralized brecciated massive rhyolite. Most, if not all, of the other properties in the area have
not passed beyond the prospecting or early development stage.
History and Production
Occurrences of manganese in the Luis Lopez manganese district have been known for a
considerable time (Wells, 1918, pp 79-91). The first recorded production took place during World
War I, when about 100 tons of sorted ore containing 35 percent manganese was produced from
the area covered by the present Red Hill and Red Hill Extension claims (Neuschel, 1943). Most of
this ore came from a now inaccessible incline on the Red Hill Extension No. 4 claim.
Major production from the Red Hill claims began in 1944, when the United Mining and
Milling Company constructed a 300-ton mill on the property. Later, the mine and mill were sold
to the Socorro Corporation, and subsequently to Southwest Minerals Industries. Late in 1950, after
a short period of operation by the latter company, the mill was badly damaged by fire, and the
mine was closed down. In 1954, the mine was leased to the Manganese Corporation of Arizona.
Mining was recommenced, and a new, large capacity mill was erected. This company purchased
the property in August 1955.
Complete production figures for the Red Hill area are not available, but Benjovsky
(1946, p 4) quoted the following for the operations of the United Mining and Milling Company
up to November 19, 1946:
1944 ............................. 180 tons of concentrates with 44% Mn
1945 ........................... 3,500 tons of concentrates with 44-45% Mn
1946 .............................. 1,046 tons of concentrates with 45% Mn
Current production is variable. It is reported that more than 2,000 tons of ore per day is
treated to yield a high-grade concentrate.
23
Development of o ther areas in the Luis Lopez manganese district w as stepped
up with the advent of the government purchasing program, in which premium prices are
offered for manganese ores. Many new prospects were uncovered, and attempts were
made to develop them; some of the older prospects were placed again in operation. The
Tow e r and G i ane ra p rope rtie s we re de ve loped unde r this p ro g ram. The T ow e r mine is
cu r re n t l y p ro d u c i n g ab o u t 1, 00 0 to ns o f lo w - g r ad e ( mil li ng ) o re p e r d a y. The G i ane r a
mine is producing a small tonnage of high-grade ore per day. Previously, the only
large-scale mining and milling operation in the area was the work on the Red Hill
cl ai m s. O th e r d e p o si ts w e re be i n g w o rke d o n a small sc ale w ith the o bje ct o f re co ve ring
a product th at easil y co uld be handso rted into high-grade mang anese ore. Although
th i s typ e o f mi ni ng i s sti l l g oi ng o n, the Towe r M ining Co mp any and the M ang ane se
Corporation of Arizona, the two largest producers in the area, are mining low-grade ores
th at are mi lle d to yie ld a h igh -g rade co nce ntrate .
With the veto of the bill that would have extended the stockpiling program in
August 1955, much of the prospecting in the area ceased. Only a few properties other
than the producing mines mentioned are currently active.
Type of Deposits
Al l o f th e majo r mang ane se dep osits d isco ve red so f ar o ccur in b re ccia z o ne s o r
steeply dipping fault zones in the massive rhyolite (Tmr) member of the volcanic sequence.
Most of the faults and breccia zones have a general north to northwest trend, though
some strike northeastward. The amount and direction of fault movement is generally
i n de te rmi n ate . Th e fau lt zo ne s on the e ast side of the vo lcanic mass are no rmal f aults
that are downth rown on the east, following the general patte rn of the f aults borde ring
the Rio Grande depressi on. At many places, the amount of move ment that caused the
b re cci at i o n i s n o t d e fi n e d , th o u g h i t is cle arly no t o f g re at mag nitud e in so me are as.
Large q u anti ti es o f co mme rci al-g rad e (i.e. , milling-g rad e o r be tte r) mang ane se o re are ,
i n mo st ca se s, co n fi n e d to fau l t- b o und e d b lo cks o f b re cciate d rhyo lite , w ith o nly ve ry
low-grade material outside these blocks.
Many small deposits containing manganese minerals in joints and small brecciate d z o n e s are l o cate d th ro u g h o u t the mas sive rhyo lite and in the late rhyo lite (Tlr)
p l u g i n th e n o rth w e ste rn p art o f th e are a. O ne d e p o sit, lo cate d alo ng the co ntact
between massive rhyo lite and earl ier rhyolite (Tbr) in the southeaste rn part of sec. 1,
T. 4 S., R. 2 W., con sists of a vei n appro ximately 2 ft wide exposed vertically fo r a
distance of 20 ft.
Reserves
Reserves of low-grade (milling) ore at the Tower and Manganese Corporation of Arizona properties probably are quite large, but no data for accurate estimates were available.
24
N o e sti mate o f th e re serve s o f th e G iane ra p ro p e rty w as mad e . Re se rve s o f mo st
other prospects are confined to such ore as may be in sight, as exploration has not
p rog re ssed to the ex te nt th at re l i able e stimates can b e made .
Mi nera lo g y
The only prominent ore mineral in the Luis Lopez manganese district is a
h a rd b l a ck m a n g an e se o x i d e i d e n t i f i e d as p s il o me la ne f r o m an X - r a y p o w d e r d i ag r a m
made by Dr. Mi ng-Shan Sun, of the New Mexico Bureau of Mines and Mineral Resources.
T h e m i n e r a l o c c u r s i n m a s s i v e o r c o l l o fo r m l ay e r s o r c o a t i n g s o n f r a g m e n t s o f
m a s s i v e r h y o l i t e i n t h e b r e c c i a t e d zo n e s. T he la y e r s m a y b e a s m u c h a s s e v e r a l
inches thick, but more often the mineral is present only as a thin, paintlike film on
t h e r h y o l i t e f r a g m e n t s . W h e r e t h e b re c c i a f r a g m e n t s a r e c l o s e l y s p ac e d , s e v e r a l o f
t h e c o l l o f o r m l aye rs may co me toge the r to yield a rathe r larg e mass of fairly p ure
mang ane se mine ral . In most cases, the mineral has been deposited in open spaces. The
manganese minerals ap p a re n tl y d o n o t re p l a ce t he c o u n tr y ro c k.
At so me p l ace s, a fel ty b l ack mate rial is p re se nt on the surface o f the hard
black psilomelane. X-ray analysis of the felty material indicates that it also is
co mp o se d ch i e fl y o f p sil o me l an e , i n sp ite o f its marke d re se mb lance to p yro lusite .
B l ack cal ci te i s co m mo n l y p re se nt, b ut at mo st p lace s is no t ne arly so ab un dant as the psilomelane. The vein in the southeastern part of sec. 1, T. 4 S., R. 2 W.,
ho w e ve r, i s mad e u p al mo st e n ti re l y o f b lack c alcite . In e ve ry c ase w he re the p arage ne ti c re l atio nsh ip s of th e tw o mine rals could be de te rmined, the b lack calcite w as
yo u n g e r. Th e c a l ci te o c cu r s i n m as se s o f cr y s t al s th a t a re a s m u ch a s o ne - h a lf in c h
across and intimately intergrown. Dr. Brian Mason, formerly of Indiana University,
made an X-ray analysis of the black residue obtained by leaching the black calcite
with acetic acid. The residue is chiefly psilomelane.
Small irregularly shaped masses of a light-gray mineral with distinctive
internal reflection were observed in polished sections of the black calcite. Although
th e masse s we re to o smal l to b e i de ntified b y etch te sts, so me w e re iso late d and te sted
by mi cro chemical me tho ds. The potassium mercuric thiocyanate test described by Short
(1 94 0, p 1 87 ) s h o w e d t h e p re se n ce o f z in c i n r e p e a te d ap p li c a tio ns . S p e c t ro g r ap h i c
an a l y s i s sh o w e d s ma l l a mo u n ts o f zi n c to b e p re se n t i n the re sid ue o b t ai ne d b y le a c hin g
with acetic acid. It is thought that zinc-containing mineral is a carbonate, because of
th e f act th at af te r th e a ce ti c aci d l e ach it is p re se nt in the re sid ue o nly in small amo un ts.
White, coarsely crystalline calcite is a prominent mineral in the manganese
d e p o s i t s . I t i s p re s e n t c o m m o n l y a s m a s s i v e c r y s t a l g r o w t h s o n m a s s e s o f b l a c k c a l c i t e
a n d a s v e i n l e t s c u t t i ng m a s s e s o f b l a c k c a l c i t e . F r o m t h e se re l a t i o n s , i t i s i n fe r r e d t h a t
the white cal ci te is you n ger than the black calcite and, therefo re, also younger than the
p sil o me l ane .
25
Plate 2
Manganese-bearing breccia from a point just east of the MCA mine. Thin,
bl a c k c o a t i n g s o f m a n ga n e s e o x i d e s s u r r o u n d t h e b r e c c i a f r a g m e n t s . A t l o w e r l e f t ,
later calcite covers the manganese oxides. The gray cement is calcite and some
silica, with scattered small fragments of ground-up rhyolite. At places in the
br e c c i a o r e , t h e m a n g a n e s e c o a t i n g s a r e a s m u c h a s
26
4
in. thick.
Finely crystalline to micro crystalline milky- colo red quartz o ccurs in the
man g an e se d e p o si ts an d i s yo u n g e r th an t he mang an e se mine r als. It s re latio n ship
to the cal ci te s cou ld no t be de te rmined . The q uartz is p resent as fillings o f ope n
sp ace s b e tw e e n frag men ts o f massi ve rhyo lite b re ccia and as a ce me nt b ind ing
frag me nt al p si l o me l an e an d massi ve rhyo lite . It is e vid e nt that so me b re cciatio n o f
the ores took pl ace afte r the deposi tion of the mang anese minerals and before the
i n trod u cti on o f the q u artz.
B ari te al so h as b een obse rved in the d epo sits. It is yo unge r than p silome lane,
b ut i ts re l ati on to o th e r min e ral s w as no t d e te rmine d.
Origin of the Ores
The o re s o f the Lui s Lope z mang ane se district are thought to b e hyd rothe rmal
in origin. The mineralization does not appear to die out at shallow depth (some
mi n in g h as bee n carried on as mu ch as 200 ft be lo w the o utcro p ), and o re o ccurs at
various topographic levels. No systematic relation to the present topography is
apparent. There is no known source from which descending solutions might have
d e ri ve d t h e ma n g an e se n e ce s s a ry f o r the f o r m a tio n o f the se d e p o s it s. I t i s p o s sib le
th a t th e o re - b e a ri n g s o l u ti o n s ma y h ave b e e n w a r m e ma n a tio n s as so c ia te d w i th t he
l a s t st ag e s o f e rup t i v e a c ti vi t y. T he o c c u r re n c e s o f p s il o me l ane d e p o si t s i n a
b r e c c i a t e d p l u g o f l a t e r h y o l i t e a n d a d i k e o f la t e r h y o l i t e a r e c o n s id e r e d t o b e s u g g e sti ve e vi d e n ce i n favo r o f th e asso cia tio n o f the hyd ro the rmal so lut io ns w ith a
l a t e r e r u p t i ve e p o c h. T h e f a c t t h a t s o m e m a n g a n e s e m i n e r a l i z a t i o n o c c u r s i n t h e
gravels of prebasalt and postlate rhyolite age also points to such a conclusion. It is
p o s si b l e th a t th e o re - b e a ri n g so l u tio ns m a y h a ve b e e n a s so ci a te d w i th t he e rup ti o n
o f t h e Q u a t e r n a r y b a s a l t , b u t m a y n o t h a v e b e e n r e l a t e d t o a n y sp e c if i c e r u p t i ve
event.
T h e b r e c c i a t i o n a n d f au l t i n g a s s o c i a t e d w i t h th e o re d e p o s i t s c o n t r o ll e d t h e
l o c a l i z a t i o n o f t h e o r e m i n e r a l s . T h i s b r e c c i a t io n a n d f a u l t i n g m u s t ha v e t a k e n
place after the emplacement of the massive rhyolite, and probably also after the
l ate rh yo l i te , th o u g h so me o f i t may have b e e n as so cia te d w ith The
of the
l a t t e r . M i n o r b r e c c i a t i o n a n d f r a c t u r i n g o f t he o r e s i n d i c a t e s t h a t m o v e m e n t s c o n t i n ue d
i n so me a re as a f te r t h e mi n e r al i z a tio n to o k p l a ce .
P si l o me l an e i s g e n e r al l y re g a rd e d as a s up e rg e ne m ine r al (P a la c he , B e r m an,
and Fro nd e l, 1944). Ho w e ve r, i t i s be lie ve d that the p silo me lane d e p osits in the Luis
L o p e z m a n g a n e se d i s t r i c t a r e h y p o g e ne i n o r ig i n . B o t h t he p s i l o me l a n e a n d t h e h o s t
ro c k a re u nl e a ch e d a n d u n w e a th e re d in m o s t c a se s . T he re i s n o e v id e nce t h at t he
p sil o me l ane rep l ace s any o the r mi ne ral. O n the co ntrary, p silo melane is the e arlie st
deposited mineral in the deposits examined. One concludes, therefore, that although
p sil o me l ane o f te n o ccu rs as a supe rge ne mine ral, in these de po sits it is hypo gene.
27
The deposition of manganese from hot waters of a thermal spring near Delta, Utah, has
been described by Callaghan (1939), who also cites other examples of manganese in thermal
springs. Callaghan does not regard the spring as necessarily having an igneous source, but writes
that such a source of manganese and barium cannot be eliminated.
The manganese deposits of the Talamantes district near Parral, Chihuahua, Mexico
(Wilson and Rocha, 1948), bear a striking resemblance to the deposits in the Luis Lopez
manganese district. There, manganese oxides, including psilomelane, cryptomelane, hollanditecryptomelane, and coronadoite occur in brecciated zones along faults. Trask and Cabo (1948, pp
218-221) indicate that many deposits in Chihuahua and Sonora follow the same general structural
and mineralization pattern. The mode of occurrence in the Talamantes district is described as
follows (Wilson and Rocha, 1948, p 193):
"Most of the manganese deposits of the Talamantes district occur in brecciated
zones along faults. Manganese oxides occur as veinlets, irregular nodular bodies,
and irregular masses surrounding rhyolite fragments of the breccia. About half of
the volume of the breccia zones is composed of fragments of wall rock. The
manganese oxides have filled
open spaces and probably replaced the matrix of the breccia, but the rhyolite
fragments show very little evidence of replacement. In many places narrow
stringers of manganese oxide extend from the brecciated zones into the bordering
wall rock. In some places fairly well defined veins of manganese oxide having
definite walls occur along fissures not accompanied by breccia zones. These veins
are much thinner, however, than the breccia deposits. . . .
The faults along which the veins occur have dips ranging from vertical to
60° E. or W. The average strike is a few degrees west of north, but the strike
ranges to slightly east of north."
The evidence, according to these writers (ibid., p 200), suggests that the deposits may
be hypogene, but is insufficient to justify definite conclusions.
A situation in which manganese minerals are thought to have been deposited directly as
oxides has been described by Park and Cox (1944, pp 316-317) in the Sierra Maestra region in
Cuba:
"The manganese is thought to have been carried in warm waters
emitted during the dying stages of the Sierra Maestro volcanism. . . .
28
Manganese-bearing solu tions rose in channels through the porous
vo l c a n i c ro c k s an d d e p o si te d t he i r mi ne r al lo a d s w he re s t ru c tu r a l
an d c h e mi c al c o n d i t i o n s w e re f a vo r ab le . . . . M a ng ane se mi ne ra l s
were also deposited among permeable conduits, such as fault zones
and fissures. . . . It is likely also, that enough oxygen, and locally
e no ug h sil i ca, we re p rese nt to f o rm oxide s o r silicate s of mang ane se .
Oxygen might well be abundant or at least readily available under.. .
near surface conditions of deposition, such as are postulated for the
f o r m a t i o n o f t he o re s . . . . I t i s c o n c l u d e d t h a t m a n g a n e s e o x id e s
an d s i l i c a te s w e re d e p o si te d d ire c t ly f ro m w a r m w a te rs . . . . "
The mineralog y and vol canic conditions at the Sierra Maestra deposits are
s i m i l a r t o t h e m i n e r a l o g y a n d v o l c a n i c c o n d i t io n s i n t h e L ui s L o p e z m a n g a n e s e
d i stri ct.
The manganese deposits of the Luis Lopez manganese district are thought to
h ave o ri g i n ate d th ro u g h th e ag e ncy o f f airly co o l the rmal so lutio ns asso ci ate d w ith
th e l ast p art o f the vol can i sm i n the Chup adera M o untains. It is be lie ve d that the
man g an e se mi n e r al s w e re d e p o sited d ire ctly as o xid e s, in asso cia ti o n w ith cal cite ,
so me si l i c a, an d b ar i te .
29
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30
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