Ridge
fluorite
Sunset
deposit,
FraGristobal
Rango,
Siefta Gourty,NewMexico
byBruceR. VanAllenandJeffreyL. Wilson,
Tenneco
Minerals
Company,
Lakewood,
C0,andJames
C.Hunter,
Colorado
School
of Mines,
Golden,
C0
Introduction
The SunsetRidgefluorite depositwas discovered by Tenneco in 1980during mineral
reconnaissanceof the Fra Cristobal Range,
Sierra and Socorro Counties, New Mex-ico
(Fig. 1). The fluorite fissure-veindeposit occurs within the West Vein at the western margin of the Fra Cristobal Range, midway on
the boundary between protracted secs. 26
a n d 3 5 ,T . 1 0 S . ,R . 3 W . , N M P M , i n t h e P e d r o
Armendaris (Spanish) land grant no. 33,
which remains unsuryeyed in the U.S. rectangular system. The occurrenceis at the
southem end of a 1,000-ft-(300-m-)long ridge,
here named SunsetRidge.The prospectwas
evaluatedin 1981by geologicmapping, geochemicalsampling, and drilling of three angle core holes.
The geologyof the Fra CristobalRangehas
beenmapped and describedin part by Bushnell (1953),Thompson (1961),Cserna (1956),
facobs(1955),McCleary (1960),Wanen(1978),
Lozinsky(1,982),
and Sarkar(in preparation).
The first documentation of fluorite in the Fra
CristobalRangewas by Walker (1914).Peale
(1949)explored the extensivevein systems
in the range and identified many fluorite occurrences.
Sunset Ridge deposit
Sunset Ridge is the topographic expression of the WestVein, a thick, resistant,composite-quartzvein that occupiesthe northtrending West Vein fault (Figs.2 and 3). The
vein dips steeplywestward and is partly exposedalong the fault tracefor a strike length
in excessof 5 mi (8 km). Fissurezonesin the
West Vein fault have been opened repeatedly, mineralized chiefly with quartz, and
brecciated.The planar orientation of mineralsand brecciafragmentsin the fissureand
subsequentweathering are responsiblefor
the well-developedexfoliation joints that give
the vein outcrop a platy appearance.The local true thickness of the West Vein is uncertain, but it is estimated to be at least 130 ft
(40 m) at SunsetRidge. Hanging-wall rocks
remain unknown becausethey are covered
by talus and alluvium and w'ere not interceptedby the drill holes.The footwall is composedof fracturedand brecciatedPrecambrian
granitic rocks (Figs.4 and 5).
Late fissures in the West Vein at Sunset
Ridge are filled with tectonically remobilized, coarse-grained, fluorite-breccia fragmentswith a matrix of fluorite, milky quartz,
and sparse hematite. The most prominent
fluorite-filled fissure, or "main zone," is exposedalong the crestof SunsetRidge and is
associatedwith abundantmilky quartz veinlets. The remainder of the West Vein is mostly
quartz with disseminatediron oxides. The
Pedro
main zone is preferentially eroded relative
A r m e n d o ri s
to the enclosingquartz vein. The zoneranges
l o n dq r o n t r J 5
from 5 to 13 ft (1.5to 4.0 m) in true thickness
and is approximately 400 ft (120 m) in exposed strike length (Fig. 6). Rock-chipsamples collectedfrom acrossthe main zone range
from 37.67oto 62.5%equivalentCaFr,as calculated from specific ion and distillation
analysesfor fluorine. Smallerfluorite zones
arepartly exposedfor hundreds of feet along
the strike of the main zone.
All three of the core holes interceptedthe
FIGURE l-Index map of the Sunset Ridge area, .main zone up to 300ft (90m) downdip from
Fra Cristobal Range, Sierra County, New Mexico. the vein outcrop. The deposit remains open
FIGURE2-View to east of Sunset Ridge, Fra Cristobal Range. The north-trending ridge is the resistantWest Vein. Precambriangranitic rocks and
the Magdalena Group (Pennsylvanian) are exposed in the background.
FIGURE3-View to north of the West Vein at Sunset Ridge, Fra Cristobal Range.
15>
Alsoin thisissue
Geologyandcoalresources
of
CerroPrietoandThe Dyke
quadrangles
p. 6
Ash{lowtuffsof Mogollon-Datil
volcanic
field
o. 10
p. 13
Service/News
p. 19
Statfnotes
Goming
soon
Araeoscelid
reptilefromCutlerFormation
nomenclature
of Upper
Stratigraphic
rocksin SocorroCounty
Cretaceous
at depth and laterally. The known portion of
the main zone averages9 fI (2.7 m) in thicknessand contains approximately MVo equivalent CaFr. Additional fluorite zones were
identified in all core holes.Theseminor zones
range from 1 to 13 ft (0.3 to 4.0 m) in estimated true thickness and occur within both
the West Vein and brecciated granitic footwall rocks.
Structural setting
Many major fluorite districts of the world
arelocatedalong or near continentalrift zones
and lineaments(Van Alstine, 1976).The Rio
Grande rift is one such zone. Many of the
fluorite mines and prospectsof New Mexico,
Colorado, and, possibly, Mexico are distributed along this major north-trending system
of Miocene to Recent grabens. These strucfural basins formed during regional extension and are bounded by faults that have
normal displacementsup to or exceeding
10,000ft (3,000m). The grabens are filled
with thick, semi-consolidated, detrital material and exhibit high heat flow (Reiter and
others, 1975).Theseconditions promote the
convective circulation of geothermal solutions that may be favorable for epithermal
mineralization. As a result, quartz, fluorite,
barite,and base-metalsulfideshavebeen deposited into open spacesand have replaced
favorablehost rocks along major boundary
faults of the rift.
The north-trending West Vein fault is the
most prominent of many subparallel faults
that form the boundary between the Fra
Cristobal uplift to the east and a graben in
New AAexnc@
GEOLOGV
o Scioncs
andSoruice
Volume 6, No. 1, FebruarY 1984
published quarterlY bY
New Mexico Bureau of Mines and Mineral Resources
a division of NewMexicolnstitute of Mining &Technology
FIGURE 4-Simplified
Mexico.
geologic map, Sunset Ridge area, Fra Cristobal Range, Sierra County, New
4,7OO
4 600
FIGURE S-Geologic cross section, Sunset Ridge fluorite deposit, Fra Cristobal Range, Sierra County,
New Mexico. See map legend in Fig. 4 above.
February 79M
Neu Mexico Geology
BOARD OF REGENTS
Ex Officio
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APPointed
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Robst [€e Sanchez,7983-1989,Albuquerque
Steve Tones, 1967-7985, SNofto
New Mexico Instihrte of Mining & Technology
Laurence H Lattman
Prsidffit
New Mexico Bureau of Mines & Mineral Resources
Frank E Kottlowski
Director
George S Austin
Deputy Director
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Address inquiries to Deborah A. Shaw, editor of Nru
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Publishedqs public doruin, thtefore reproducible without
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Circulotion:1,4N
P/intaj University of New Mexico Printing Plant
position
-tassiumfalls within the group of high-poPrecambriangranitic rocksin southcentral New Mexico (Condie and Budding,
t979\.
Frank E. Kottlowski provided a petrographic description of a sample of Fra Cristobal granite (Cserna,1956):
The graniteis lightly gneissicwith a tendenry for alignmentof micaplatesand elongatedquartzcrystals.The rockis mediumhypidiomorphic-granuto coarse-grained,
41%,
orthoclase
lar.Mineralsareqtartz22Vo,
9Vo,muscovite270,
microcline26Vo,-albite
minor biotite, hornblendeand magnetite.
Thequartzis anhedral,1-4mm in diametet
strainedand in part cloudedwith inclulaths,3-8mm
aresubhedral
sions.Feldspars
long,alteredto sericiteandkaolinite.Micas
flakes,lessthan 1 mm long;
arein scattered
nearhornblendecrystals,they areconcentratedin lenses Hornblendeis pleochroic
from aqua to greenishyellow-brown,euhedralto subhedral,lessthan 1 mm long,
alteredin part to chlorite,iron oxide,and
chalcedony
Footwall rocks
The Fra Cristobal granite is typically alThe footwall of the West Vein fault at Sun- tered. Mafic minerals are converted mostly
set Ridge is composedof the Fra Cristobal to chlorite or hematite. If magnetite origipluton, a Precambrian,composite, granitic nally existedat SunsetRidge, it is now oxistock or batholith with inclusions of meta- dized to hematite.A portable magnetometer
sedimentaryrocks. The dominant intrusive failed to reveal significant differencesin magrock. here informallv named the Fra Cris- netic susceptibility among the Fra Cristobal
tobal granite, is of somewhatuncertain age. granite, the West Vein, and alluvial cover.
ln 7982, a K-Ar age of the granite was de- The granite is also variably alteredby potastermined to be 850 + 29 m.v. bv Geochron sium metasomatism,but the Sunset Ridge
The area is not particularly affected. Elsewhere
Laboratories,
Cambridge,Maisachusetts.
age date was obtainedfrom a concentrateof in the Fra CristobalRange,small alteredzones
partly chloritized biotite in granite core from with brick-red microcline and minor musa drill hole in protractedsec. 11, T. 11 S., R. covite occur within the granite. These me3 W Similarrocksin south-centralNew Mex- tasomites are similar to larger bodies in the
ico have Rb-Sr whole-rock isochron and UPrecambrian intrusive rocks of the Caballo
Pb zircon age dates that range from 1.2 to Range(Staatzand others, 1965).
A
The Fra Cristobal pluton contains large inFIGURE6-View to north of main fluorite zone 1.55 b.y. (Condie and Budding, 7979).
at SunsetRidge,FraCristobalRange.Thefluorite chemical analysis of granite from the same clusionsof partly assimilatedchlorite schist,
core hole is provided in Table 1. The com- chlorite-feldspar gneiss, and amphibolite.
zoneis preferentiallyeroded.
These deformed inclusions were not obFissuresin the West Vein fault served as TABLE 1-WsoLE-RocK ANALysEs (from Skvline
servedat SunsetRidge, but they are particthe most significant structural control on Labs, Inc., Wheat Ridge, CO)
ularly abundantin the northern Fra Cristobal
fluorite mineralization.Major fluorite fillings
Range.Abundant, small,Precambrian(?)dikes
Fra Cristobal
Main fluorite
have been brecciated and tectonically reand irregular granitic intrusives that are comgranite core
zone
mobilized by repeatedfault motion. The main
Sec.11, T. 11 S.,
SunsetRidge
posedof simple granite pegmatite,monzonfluorite zone may be controlled by a strike
R. 3 W.
surface
ite, quartz monzonite, and alaskite cut the
(in
(in
percent)
percent)
flexure along the West Vein or, as suggested Composition
Fra Cristobalgranite.The degreeof gneissic
by aerial photographs,by the low-angle in- sio,
36.6
69.0
banding in the late intrusives is the same as,
tersectionof two normal faults. At Sunset Al203
0.77
14.30
or lessthan, that found in the older granite.
Ridge,the alignmentsof joints and foliations FerO3
2.60
0.73
The late dikes are devoid of mafic silicate
in the main fluorite zone fall into two distinct Tio,
0.08
0.33
mineralsand have tracesof specularhema0.010
populationsthat vary in strike.The northern MnO
0.061
tite or limonite pseudomorphsafter pyrite.
Mgo
0.05r
0.74
population of attitudeshas an averagestrike
0.06
3.30
Hanging-wall rocks
of N. 11'W. and a dip of 74" W., and the Naro
4.60
0.35
southernpopulation has an averagestrike of KrO
wall of the West Vein at SunThe
hanging
1..4
N. 10' E. and a dip of 74' tN. A differenceof CaO
set Ridge is concealedby at least 125 ft (38
Ca
3r.88
2L' in strike supports either of the two pos- F
m) of Quaternary talus and alluvium, and it
0.11
30.00
sible structural-genetic models. The enclos- Ba
was not intersectedby exploration drill holes.
0.066
0.310
ing composite-quartzvein containsscattered SOn
<0.10
0.15
The hanging wall may contain the Fra Crisfoliations that averagedue N. in strike and SrO
0.015
0.012
tobal pluton and portions of the approxidip 74" W Another structural consideration, Rbro
0.007
0.036
mately 5,000 ft (1,500 m) of overlying
0.003
0.007
based on drill hobs, is that the main fluorite LirO
Paleozoic,Mesozoic,and Cenozoicsedimen<0.02
0.05
zone may dip more steeply than the enclos- PrOt
tary rocks. In probableorder of abundance,
<0.1
04
ing quartz vein. Fissurescommonly occur Cot
thesestratainclude carbonaterocks, shales,
0.1
0.2
along steepersegmentsof normal faults where Moisture
sandstones,evaporites,and conglomerates.
L.O.r.(7s0'c)
7.6
0.3
extensionis more nearly perpendicular to the
Possible Paleozoic units are the Bliss SandTotal
101.4
98.8
fault plane.
the Rio Grande rift to the west. This fault
strikesN. 15'W.-N. 15'E., dips 53'-84"W.,
and may have more than 5,000ft (1,500m)
of normal separationat Sunset Ridge. Lee
(1907)and Bryan (1938)were first to map the
fault. but thev did not name it. The name
"West Vein fdult" is adopted here after an
report
unpublishedminerals-reconnaissance
on the Fra Cristobal Range (Peale,1949).Later
researchershave called it Hot Springs fault
(Bushnell,1953;Jacobs,1956;McCleary, 1960;
and Thompson, 1,961),Rio Grande fault
(Cserna, 1956),and Walnut Wells fault (Warrcn, L978).
Granitic rock fragments are locally abundant in the West Vein, and drill holes intercepted one major horse of silicified granite
breccia.Numerous lesser-normalfaults that
are subparallel to the master fault occur in
the granitic footwall rocks. These faults are
filled locally with thin qvattz veins that may
contain minor fluorite or base-metal minerals. The faults are associatedspatially with
wide zonesof variably silicified tectonicbreccia. Major west-dipping, northwest- and
northeast-trending normal faults also are
common in the footwall rocks. Fluorite, barite, and manganese-oxidedeposits are localizedalong faults of similar strike elsewhere
in the Fra CristobalRange.Thesestructures
arecut by the systemof north-trending faults
at Sunset Ridge. Minor east-trendingQuaternary(?) normal faults displace the West
Vein, granitic rocks, and perhaps some older
alluvium. These small-displacement,steeplydipping faults are subparallelto one set of
joints in the vein.
Nan Mexico Geology February 1984
glone (Cambrian-Ordovician),El PasoGroup
(Ordovician), Magdalena Group (Pennsyivanian), Abo Formation (Permian),YesoFormation (Permian), Glorieta Sandstone
(Permian),and San Andres Limestone(permian). PossibleMesozoicand Cenozoicrocks
in the hanging wall may include clastic sedimentary units of the Mesaverde Formation
( C r e t a c e o u s ) ,M c R a e F o r m a t i o n ( C r e t a ceouFPaleocene),
and SantaFe Group (Miocene-Pleistocene).Interpretations of ground
and airborne magnetic data indicate [hat local Cenozoicbasalticrocks are not present.
Excellent descriptions and measuied sec-
represent fragmented jasperoid that replaced Paleozoic limestone. No unreplaced
limestone fragments were seen at Sunset
Ridge, but Paleozoic carbonate rocks are
of the MagdalenaGroup, 2.5 mi (4.0 km) to
the north-northeast. The San Andres Limestone is host for brecciated barite-galena
mineralizationnear Hackberry Canyon, 4.4
mi (7.1km) south of SunsetRidge,ind also
hosts minor replacement fluoriie mineralization in the hanging wall of the West Vein
fault, 1..5mi (2.4 km) south of SunsetRidge.
Barite-fluorite replacement mineralizatircn
West Vein mineralization
The West Vein at Sunset Ridge is composed of fine-grained quaftz, coarie-grained
fluorite, and granitic rock fragments. Iron TABLE 2-Wesr VUN urNrneLocy er SurusrrRrocE
oF APPARENT
oxides, barite, and clays occur in minor rN oRDER
ABUNDANCE.
Elements listed
amounts; bracesof pyrite, "psilomelane," cal- with an asterisk (*) are present as constituents of
cite, gypsum, galena, and copper minerals granitic-rock fragments.
also have been identified. The *eathered and
Maior
Minor
Trace
partly covered surface of the West Vein is
elements
elements
mostly quartz with disseminated iron ox- elements
ides. The known mineralsare listed in Table
Microcline*
Pyrite
Quartz
2 accordingto their relative abundances.Vein- Fluorite
Hematite
Psilomelane(?)
Goethite
Barite
Plagioclase'
Chlorite+
Muscovite*
Clays*
quiescence.This late mineralization may have
originated from mixed hypogene and supergene(mesogene)solutions. Some of the
earlier mineralizationwas altered by oxidation and dissolution,and was localiyredistributed in the mesogeneenvironment.
Quartz is the most abundant mineral in
the WestVein. It was depositedin 12or more
distinct episodesof cross-cuttingvein mineralization.Most quartz was depositedprior
to the first fluorite mineralization. The earliest quartz deposit is barren and milky. Subsequentquartz depositscontain disseminated
pyrite, commonly oxidized to limonite. The
lastquartz veinletsemplacedprior to the first
fluorite mineralizationcontain disseminated
red hematite. Continued faulting along the
WestVein opened narrow fissuresthat were
filled with barren, milky-quartz veinlets.
Subsequently,wider fissureswere filled with
coarse-grainedfluorite. Later faulting brecciated and mechanically remobilized the
fluorite. Milky quartz and reddish hematite
were deposited interstitially between fluorite fragments. The resulting fluorite breccia
contains single-crystalfragments up to 1.5
inches(3.8cm) across(Fig. 8). Thesefluorite
fragments exhibit well-developed internal
(111)cleavageand are generallytransparent
riypogene, eprlherrol
I
'
Lxlenstve
Dre.ctotron
Qu o rt z
-
Mossive lo fine. vuggy, white
---
Mossive,groy X
Mossive. reddish
M o s s i v e .o r o n g e t o p i n k
--
,
Hypoqene
i
I
Or,dot on
ond
sJoerqene
Colichificolion
-
_
F i n e t o m e d i u m ,v u g g y , w h i t e
lo colorless
Cholcedony.white
Calcite
Gypsum
Malachite
Chrysocolla(?)
Chalcopyrite
Galena
Chalcocite
Cuprite(?)
Covellite(?)
Azurite(?)
and colorless, purple, or green, if fresh.
Weatheredfluorite is commonly whitish and
opaque. Tracesof coarsely crystalline, beige
to white barite in small brecciafragments are
associatedwith fluorite and also occur elsewhere in the vein without associatedfluorite.
The major period of fluorite mineralization
was followed by two late periods ol quartz
mineralization. Primary base-metal sulfides
were deposited in openings within the
younger of these two quartz phases. Chalcopyrite, pytite, and arsenopyrite(?) crystallizedfirst. Galenacommonly was deposited
as overgrowths on chalcopyrite crystals.
Quartz veins with base-metal sulfides generally occur in the fractured granitic footwall
rocks without attending fluorite.
Minor late-stageminerals filled local voids
in breccias,fractures, joints, and solution
cavities. The late-stageminerals are not fractured and therefore appear to have formed
afterthe cessationof major displacementalong
the West Vein fault. Some fluorite breccia
contains solution vugs that are partly filled
with later fluorite and associatedminerals.
Minor iron and manganeseoxides occur as
colloform coatings on fractures and around
breccia fragments in the granite and vein
quartz. Weathering has altered some pp'ite
to limonite, and chalcopyrite is partly converted to chalcociteand subsequentlyto malachiteand chrysocolla(?).Tracecuprite(?)also
-
F i n e , c r y s t o t t i n e ,c o l o r l e s s
coorSe
-
FI u ori te
fine io medium
tine lo hedium
Borite
Pyrite
-l
Bose- melol
m i n e r o l s ( s e e T o b t e2 )
I
-
Hemotite
Hydroted iron oxides
Mongonese oxides
Colcite
it-
G y p s um
Monfmorillonite(?)
-
J( Vorobly colored by limonite whcre oxidized
Mojor phose of minerolizotion
Mrnor Dhose of
FIGURE 7-Paragenesis of the West Vein at Sunset Ridge, Fra Cristobal Range, Sierra County, New
Mexico
February 1984 New Mexbo Geology
FIGURE 8-Core and surface samples of fluorite
breccia with fluorite, milky quartz, and hematite
matrix from Sunset Ridge, Fia Cristobal Range.
may have formed from chalcopyrite. Latestagebarite crystallizedas tiny laths in open
spaces prior to malachite crystallization,
whereas small, modified cubes of purple
fluorite crystallized on the malachite coatings and in other voids. Theseunbrecciated
fluorite crystals commonly have internal
ghosts of limonite and may have formed by
dissolution and redeposition of the earlier
brecciatedfluorite. Traiesof chalcedonvfinely
crystalline quartz, white poker-chip calcit"e
crystals, gypsum, and calichewere deposited sequentially after the last fluorite crystals.
West Vein geochemistry
Core and surface-rocksamplesfrom Sunset Ridgewere submitted to commerciallaboratories and assayed for fluorine by
quantitative techniques, and 55 elements
above atomic number 21 were analyzed bv
semi-quantitativex-rav fluorescence.Conp, 7.8% Fe. 6.07o
centraiionsof up to ZO.S%
Ba, and}.45%Mn were encountered.All other
detectedelementsshowed concentrationsof
less than 0.2%. A quantitative whole-rock
analysis for the main fluorite zone is provided in Table1 Forty-four stream-sediment
samplesfrom the prospectareawere sieved
to -80 mesh and assayedquantitatively by
commercial laboratories for fluorine, barium, and metals.
Geochemical data indicate that fluorite is
negatively correlatedwith base-metalminerals, barite, and hematite. The greatest
fluorite value, 4,000ppm F,
stream-sediment
was determinedin a sample collectedabout
640 ft (195 m) downstream from the main
fluorite zone. Stream sediments derived
chiefly from silicified footwall rocks contain
the largestbase-metalvalues: up to 72 ppm
C u , 1 1 1p p m P b , 2 2 6 p p m Z n , a n d 3 p p Mo. All sedimentsamplesfrom streamchannels on footwall rocks contain 2,000-15,800
ppm Ba. Similar barium valuesoccur downstreamfrom known barite occurrencesin the
West Vein, but they do not correspond directlywith the main fluoritezone.Hematitic
quartzis spatiallyassociatedwith major fluorite concentrations;however, the greatestiron,
manganese,and titanium values occur in core
samples of fractured footwall rocks.
Age and origin of fluorite mineralization
Fluorite deposits in the western United
Statesare considered to be middle to late
Tertiary in age (Lamarreand Hodder, 1978).
The Sunset Ridge fluorite fills the West Vein
fault, one of the major, graben-boundingfaults
along the Rio Grande rift. Significant regional, intraplateblock faulting occurredbetween 21 m.y. ago and the present (Elston
and Bornhorst, 1979).Therefore, fluorite in
the West Vein is believed to be Miocene or
Pliocenein age.
Lamarreand Hodder (1978)proposedthat
fluorite depositsin the westernUnited States
are related to Tertiary alkalic-igneousrocks.
Hypothetically, fluorine was concentratedin
alkalic magmas that were generated along
subduction zones. They postulated that the
fluorine was carried through the crust as SiFo.
However, Tertiary alkalic-igneousrocks are
conspicuouslyabsent in the Fra Cristobal
Range,and there is no evident relation befween the SunsetRidge depositand Tertiary
volcanic rocks. The nearest Tertiary felsic
volcaniccenter known is the Nogal Canyon
cauldron (Deal and Rhodes, 1976),which is
9 mi (15 km) to the northwest.
We propose that the Fra Cristobal pluton
may have been the fluorine source for the
SunsetRidgedeposit.Fluorine contentof the
main zone at Sunset Ridge is less than 300
times greater than the concentration of fluorine in partly alteredgranite (Table1). Original, unalteredgranite probably had accessory
fluorite or fluorine-bearingbiotite. The extensive alteration of biotite to chlorite and
hematite by hydrothermal solutions associated with the Rio Grande rift would have
liberatedboth fluorine and silicon,two maior
elementsin the West Vein. Thermal, meteoric waters may have leached silicon, fluorine, iron, and other elementsfrom the Fra
Cristobal oluton with the aid of chemical
complexes'. SiFu'z is the prevalent siliconfluorine complex in natural waters at standard temperatureand pressure,but it is unstableif the solution is alkaline or if it has
even a low concentrationof competitive cations such as iron, aluminum, and calcium
(Robersonand Barnes,1978).
The WestVein fault servedas a major conduit for the circulation and possiblemixing
of meteoricwaters.As indicatedby multiple
eoisodesof brecciationand complexmineral
the fault was repeatedlyreacparagenesis,
tivated and sealedwith vein minerals.Fluorite and all other West Vein rnineralsprobably
were depositedunder epithermalconditions
(50-200'C). No high-temperature minerals
or associatedalterationsare evident. Pervasive, periodic, brittle deformation (brecciation and fissurinq) indicates that
mineralizationoccurredat relativelyshallow
depths. The parageneticassociationof the
main-zone fluorite with hematitic quartz
suggeststhat thesemineralswere deposited
in a near-surface,oxidizing environment.
The most likely mechanismsfor fluorite
deposition are: increasingthe pH, cooling,
dilution, or mixing of hydrothermal solutions (Richardsonand Holland, 7979).lxll of
these factors contribute to the instability of
the SiFu' complex. Acidic epithermal solutions may have mixed with cool, near-surfaceground waters. Each successivemotion
of the West Vein fault would have downdropped the possiblelimestonein the hanging wall. This would raise the activity
coefficientof the calciumcation and increase
the pH of the local ground waters. Quartz
and, subsequently, fluorite then would be
deposited from probable acidic epithermal
solutions that circulated or stagnated along
the West Vein fault and mixed with ground
water or simply cooled.Minor, later fluorite
may have been derived from earlier fluorite
by local solution and redistribution.
Summary
The Sunset Ridge fluorite deposit (Miocene or younger), at the easternmargin of
the Rio Grande rift in south-central New
Mexico, is a classicfissure vein. Mineralization is controlled by open spaceswithin
the north-trending, steeply dipping, West
Vein fault. The greaterpart of the West Vein
is composedof a quartz envelope around a
main fluorite zone that averagesalmost 10 ft
(3 m) in true thicknessand rangesfrom 30%
to more than 60% fluorite. Core drilling intersected the main zone and other fluorite
zonesat depths up to 300ft (90m) below the
surface.Footwallrocksare composedmostly
of fractured PrecambrianFra Cristobal granite. Hanging-wallrocks are concealedby surficial debris, but they may contain granitic
or sedimentary rocks. Paleozoiccarbonate
rocksmay occur in the hanging wall and are
potential hosts for significant replacementtype fluorite deposits.Fluorite may have been
depositedfrom fluorine-rich epithermal solutions that resulted from alteration and
leaching of the Fra Cristobal pluton by
hydrothermalfluids that were generatedand
circulatedduring the developmentof the Rio
Grande rift. Quartz and fluorite were deposited into open fissuresas the epithermal
solutions cooled or mixed with alkaline
ground waters that contained calcium derived from local carbonaterocks.
AcrcNowr-eocMENrs-Specialthanks are
extendedto many current and past employeesof TennecoMineralsCompany,including
RalphD. Falconer,RogerD. Gill, FeliciaCottrell and JoleneBiner. Thesepeople assisted
in lithologicallogging, geophysicallogging,
compilationof geochemicaldata, and manuscript preparation.Wolfgang E. Elston, University of New Medco, edited the preliminary
manuscript and provided valuable critical
comments.
References
Bushnell, H. P, 1953,Geology of the McRaeCanyon area,
Siena County, New Mexico:M.S. thesis,University of
New Mexico, 106pp.
Bryan, K., 1938, Geology and ground-water conditions
o{ the Rio Grande depression in Colorado and New
Mexico;ln RegionalPlanning,Part 6, Upper Rio Grande:
Commission,v.
Washington,D.C., NationalResources
7, part 2, section 1, pp. 1.95-225.
Condie, K. C., and Budding, A. J.,7979, Geology and
geochemistryof Precambrianrocks, cenhal and southcentralNew Mexico; New Mexico Bureau of Mines and
Mineral Resources,Memoir 35, 58 pp., 3 pls. and 3
microfiche.
Cserna, E. G., 7955,Structural geology and stratigraphy
of the Fra Cristobal quadrangle, Sierra County, New
Mexico: Ph.D. dissertation, Columbia University, 105
Pp.
Deal, E. G., and Rhodes,R. C., 1,976,Volcano-tectonic
strucfures in the San Mateo Mountains, Socono County,
New Mexico; ln Elston, W. E., and Northrop, S. A.
(eds.), CenozoicVolcanismin SouthwesternNew Mexico: New Mexico GeologicalSociety,SpecialPublication
no. 5, pp. 51-62.
Elston,W. E., and Bomhorst,T.I.,7979, The Rio Grande
rift in context of regional post-4Om.y. volcanic and
tectonicevents; ln Riecker,R. E. (ed.), Rio Grande Rift,
Tectonicsand Magmatism: American Geophysical Union,
Washington,D.C., pp. 41.6-438.
Jacobs,R. C., 1956, Geology of the central ftont of the
Fra Cristobal Mountains, Siena County, New Mexico:
M.S. thesis, University of New Mexico, 47 pp.
'l-2
continued on o.
Ncw Mexico Geology February 1984
ffi
FIGURES 7a, b, c-Lemitar Tuff (samples TS-5,
TS-23,DB-108, all from south and southeastMagdalena Mountains) is a strongly and complexly
zoned unit. The Lemitar Tuff is a moderately crystal rich rhyolite at the base (7a) that grades normally upward into a quartz-Poor quartz latite (7b),
which in turn grades upward into a very phenocryst rich rhyolite (7c).
ff
FIGURES8a, b-South Canyon Tuff (samplesJASC-l, Joyita Hills; 8-11-8, southeast Magdalena
Mountains) is texturally zoned from phenocryst
poor below to moderately phenocryst rich above;
mineralogies are similar in both zones.
Figure 8a illustratesa glassyvitrophyre from the
ba'seof the lower zone. Small gray, irregularly
curved shapesare undeformedglassshards.Note
slight compactionaround phenocrysts.Subequal
amounts of sanidine (s) and quartz (q) make uP
the phenocrysts.
dePosit
fluorite
Ridge
Sunset
a few ft into a phenocryst-rich quartz latite (Fig.
7b).
continued from Page5
Lamarre, A L , and Hodder, R W., 1978' Distribution
and genesisof fluorite depositsin the western United
Statei and their significanceto metallogeny:Geology,
v. 6, pp.236)38.
Lee, C. H., 1907,Water resourcesof the Rio Grande valley
in New Mexico and their development: U S. Geological
ffi
Figure 8b, from the upper interval of this unit,
contains more abundant and larger phenocrysts
of sanidine (s) and quartz (q) with minor biotite,
but little preserved vitroclastic texture.
lnfr
Figure 7b contains abundant sanidine (s), plagioclise (p), biotite and minor quartz, oPaquesand
pyroxene. This middle zone grades upward over
several tens of hundreds of ft into a very phenocryst rich rhyolite (Fig. 7c)
References
Chapin, C. E., and Lowell, G R., 1979, Primary and
seiondary flow structures in ash-flow tuffs of the Gribbles Run-paleovallev, central Colorado; in Chapin, C
E., and Eliton, w. E. (eds.), Ash-flow tuffs: Geological
Mountains, Socono and Catron Counties, New Mexico:
M S. thesis, New Mexico Institute of Mining and Technology, 137 pp.; New Mexico Bureau of Mines and Mineral Resources, Open-file Report 136, 146 pp.
Lopez, D. A., and Bornhorst, T J., 1979, Geologic map
6f th" Dutil area, Catron County, New Mexico: U S
Geological Suruey, Miscellaneous Geologic Investigation Map I-1098, scale 1:50,000.
Osbum, G. R., and Chapin, C. E., 1983a, Nomenclature
volfor Cenozoic rocks of northeast Mogollon-Datil
canic field, New Mexico: New Mexico Bureau of Mines
and Mineral Resources, Stratigraphic Chart 1',7 pp ' 7
sheet
-,1983b,
Ash-flow tuffs and cauldrons in the northsummary: New
volcanic field-a
east Moqollon-Datil
Mexico Geological Society, Guidebook to 34th field conference, pp. 197204.
12
February 1984 New MexicoGeology
Peale,R., 1949,Report on the mineral occunenceson the
Armendaris Grant: Private consultant rePort to Kern
County Land Company, San Francisco,California,25
pp., 4 pls., 16 Photographs.
nJlie.. n .. gawaias, C. t-., Hartmann,H., and Weidman,
C., 1975,Tenestrial heat flow along the Rio Grande rift,
New Mexico and southern Colorado:GeologicalSociety
ofAmerica, Bulletin, v 86, pp 811-818
Richardson,C K, and Holland, H D, 1979,Fluorite
deposition in hydrothermal systems: Geochimica et
Coimochim:ca Acta, v. 43, pp.7,327-1'335
Roberson, C. E., and Bame3,-R. 8 , 7978, Stability of
fluodde complex with silica and its distribution in natural water systems:Chemical Geology, v.27, pp 239256
Sarkar, G., Tungsten-bearing hydrothermal manganese
deposit at the Blackie Mine area, Fra Cristobal Range,
Siena and Socorro Counties, New Mexico: Ph D dissertation, University of New Mexico, in PreParation'
Staatz,M. H., Adams, J. W., and Conklin, N. M., 1965,
Thorium-bearing microcline-rich rocks in the southern
Caballo Mountains, Sierra County, New Mexico: U S'
Suruey.Paper525D' pp. D48-D51'
Geolos,ical
Thomps6n,S., III, i961, Geologyof the southernPart of
the Fra Cristobal Range, Siena Counry, New Mexico:
M.S. thesis (revised),University of New Mexico, 89 pp'
Van Alstine, R. E., 1975,Continental rifts and lineaments
associated with major fluorspar districts: Economic
Geology,
-W v. 77, PP. 977-987
L., 1914, Report of an examination of the
Walker,
Cristobal copper mine, Siena County, New Mexico:
Private consultant report to Victorio Land and Cattle
Company, San Francilco, California, 7 pp.' 1 pl'
Warren, R.-G., 1978,Chancteization of the lower crustupoer mantleof the EngleBasin,Rio Granderift, from
and field geologicstudy of basaltsand
a'petrochemical
their inclusions:M.S. thesis,University of New Mexico,
n
155pp