335

The C arudian M ine r alo g istVol. 34, pp.33s-347 (1,996)

PETROLOGY AND MINERAL RESOURCESOF THE WIND MOUNTAIN LAGCOLITH,

CORNUDAS MOUNTAINS. NEW MEXICO AND TEXAS

VIRGINIA T. McLEMORE. VIRGIL W. LT]ETH AND TIM C. PBASE

New Meico Bureau of Mines and Mineral Resources, 801 l-eroy Place, Socorro, New Mertco 87801, U.SA.

JAMES R. GTIILINGER

Addwest Minerals, Inc., 546A Ward Roal, Suite 370, Anada, Colorado 80002, U.S.A.

ABsrRAc"r

The Wind Mountain laccolith is one of many intrusive bodies found in the Cornudas Mountains. These intrusive bodiescomprise the nortlern part of the Trans-Pecos alkaline magmatic province in southem New Mexico and southwestem Texas.Ten laccoliths and stocks, along with numelous dikes, sills, and smaller plqs of varying textures, have infuded Permianlimestone and other sedimentary rocks in the Comudas Mountains axea. Nepheline syenite predominates, although phonolite,trachyte, and syenite are conmon The Wind Mountain laccolith is texturally and mineralogically zoned whereas the otherintrusive bodies in the Comudas Mountains seem homogeneous. Wind Mountain consists of two textural varieties of nephelinesyenite porphyry and four textural varieties of syenite porphyry. The zonation is attributed to crystal fractionation, volatileseparation, and cooling history, not to different pulses ofmagma. Feldspar crystallization under initially hypersolvus conditionscan account for most of the chemical variation in the volumetically lmger nepheline syenite zones. Subsolvus conditions offeldspar crystallization, coupled with the separation of a volatile phase, was responsible for the chemical and mineralogicalvariation within the capping syenite units, which form a rind at the top of the pluton. The Cornudas Mountains have beenexamined for potential economic deposits of gold, silver, beryllium, rnre-eaxth elements, niobium, and uranium, but noproduction has occurred. The nepheline syenite porphyry at Wind Mountain is being considered as raw material for use indark-colored glass, flatglass, and ceramics.

Keywords: Cornudas Mountains, 14'io4 146mtain Laccolith, nepheline syenite, syenite, alkaline rocks, New Mexico, Texas.

Solvlulns

The laccolite de Wind Mountain est un panni plusieurs massifs intrusifs affleurant dans les montagnes Comudas, dI'extr6mit6 nord de la province magmatique alcaline de Trans-Pecos, dans le sud du Nouveau-Mexique et le sud-ouest du Texas.En tout dix laccolites et plutons, ainsi que plusieurs filons, filons-couches et petits massifs a texture variable recoupent lescalcaires et autres roches s6dimentaires d'dge permien de la r6gion. La sydnite n6ph6linique est pr6dominante, quoique la suitecontient aussi phonolite, fachyte et sy6nite. l€ pluton de Wind Mountain, le plus imposant, est zon6 en texture et enmin6ralogie, tandis que les autres massifs intrusifs semblent plutdt homogbnes. Nous ddcrivons deux vari6t6s texturales desy6nite ndphdlinique porphyrique et quatre vari6t6s de sy6nite porphyrique. la zonation semble due d la cristallisationfractionn6e, la sdparation d'une phase volatile, et le taux variable de refroidissement, plut6t qu'i des venues distinctes de magma.I,a crisrallisation de feldspath i des conditions tout d'abord hypersolws peut expliquer la plupart de la variation chimique parmiles diffdrentes venues volumineuses de sy6nite ndph6linique. les conditions propices d une cristallisation subsolws et d las6paration d'une phase volatile rendent compte des variations chimique et min6ralogique parrni les unit€s sup6rieures ale sy6nite,qui constituent une carapace au sommet du massif. La chalne Cornudas a fait I'objet de recherches pour des anomalies en or,axgent, b6ryllium, teres rares, niobium et uranirtm, mais sa:rs d6couvertes dignes d'exploitation. R6cemmenl on a initi6 unprogramme d'exploration et de d6veloppement de la sy6nite n6ph6linique porphyrique pour son potentiel dans la fabrication deverre fonc6, velre en panneaux, et c6ramique; ce mat6riau aurait aussi un potentiel dans la fabrication de granules pour toitureset d abrasifs, et dans I'industrie des pierres tai116es.

(Traduil par la R6daction)

Mots-cl6s: chalne de montagnes Cornudas, laccolite de Wind Mountain, syenite n6phdlinique, sy6nite, roches alcalines,Nouveau-Mexique, Texas.

336 TIIE CANADIAN MINERALOGIST

INTRoDUcnoN

Wind Mountain is one of ten Tertiary intrusivealkaline bodies that form the Cornudas Mountains inthe Otero and Diablo plateaus in soutlern New Mexicoand southwestern Texas (Frg. 1). These intrusive

bodies represent the northern exlent ofthe Trans-Pecosmagmatic province. The Cornudas Mountains wereorigrnally mapped in the early 1940s and 1950s ashomogeneous plutons or laccoliths (Zapp 1941, Timm1941, Clabaugh l94l,Warnet et al. 1959). Subsequentpetrological and geochemical studies along with

Jicarilla Mountains

@@gapitan

a -

MountainsQiena @trtanca

OrograndeI

Une SeparatingAlkalicand Alkali-calcic Fields

a-\o,

Trans-Pecos MountainsTexas Magmatlc Belt

0 50 | 00kml l t t l

$ Big Bend

Flc. l. Trans-Pecos magmatis pl6yince, New Mexico and Texas (modified from Barker 1987, Price et al. 1.987). The inset napshows the Nor0r Arnerican Cordilleran belt of alkaline igneous rocks (Woolley 1987, Mutschler et al. l99l).

TT{E WIND MOT]NTAIN LACCOLITH. NEW ME)(ICO _ TEXAS

Fl.al T@ Fs{!o, qldte f,Mlcdah stldte dlb

Wiid !@lhe sy€e. l@lihM@rh S@lllqDc@e

llf'bgltoe ryld,

SeAdo nsddhecy@no I@&bI\rl@lsh

Cbaifeld d@lile dtllloqtrto

Blar* lo(cryltrc !6!eelhe dllldM sy€do

Wa!fih lo{!fi:trldsnelterh4 81trI'i@io 8:/€do

U|@d qoldtu-n€€dog phStdtr @d!, syedte

337

examination of the Wind Mountain nepheline syeniteby geologists of Addwest Mnerals, Inc. revealed thatthis unit is texfurally and mineralogically zoned(Mclemore & Guilinger 1993, Mclemore et aI. L994,199O.

The purpose of this paper is to 1) summarize thepetrology and mineralogy of the Wind Mountainlaccolith, 2) describe the zones in the laccolith,3) determine the cause of zonation, and 4) describethe mineral resource potential.

REcIoNAL Snrrr,ta

The Cornudas Mountains and the Trans-Pecosmagmatic province form part of the southernportion of the North American Cordilleran alkalineigneous belt. This belt is a diffrrse region ofCenozoic igneous rocks tlat extends along theeastern maryin of the North American Cordillerafrom Alaska and British Columbia southward intoTrans-Pecos Texas and eastern Mexico (Fig. l;Barker 1987, Mutschler et al. L985, 1991, Woolleyr987).

The Trans-Pecos magmatic province is a regionalbelt of alkaline and metalumin6us igneous rocks thatlie within an area defined by the Rio Grande on thewest and south, the Pecos River on the east, and aneast-west line approximately 12 km north of the stateboundary between New Mexico and Texas (Frg. 1).The province contains more than 200 intusive bodieshaving an outcrop area exceeding 1 km2(Barker 1977,1979, L987). The Trans-Pecos magmatic provinceis the eastern limit of Cenozoic magmatic activityin southwestern United States and Mexico. The mag-matism occurred in the region nearly continuouslyfrom 48 to 17 Ma @rjce et aI. 1987). Compositions ofigneous rocks vary from alkaline in the eastern portionsof the province, including the Cornudas Mountains,to calc-alkaline westward into Mexico (Fig. 1; Barkeret al. L977 , Barker & Hodges 1977, Barker 1987, Price& Henry 1984, Cameron & Cameron 1985,Pice et al.1987, Clark 1989).

Early genetic interpretations suggested an analogybetween the Trans-Pecos province and the Kenyaportion of the East African rift (Barker 1977).However, subsequent work has shown that much ofthe Cenozoic faulting in Trans-Pecos Texas, earlierconsidered to be associated with rifting, actually post-dates igneous activity @arker 1987,Henry et al. l99l).Trans-Pecos magmatic activity began at the end of theLaramide compressional tectonic period and may bethked to progressive shallowing of the subduction ofthe Farallon plate beneath the North American platewith time (Coney 7972, Barker t987, Damon et al.1981, Campa & Coney 1983, Henry et al.1989,1991).It is also possible that some Trans-Pecos magmaticactivity is related to back-arc spreading (Barker1987).

GEoLocIcAL Srrrnqc

Wind Mountain is one of ten larger sills, plugs, andlaccoliths (Table 1, Fig. 2) and smaller dikes and plugsin the Cornudas Mountains that intrude relativelyflat-lying limestone and other sedimentary roctrs ofthe Hueco Limestone and Bone Spring Limestone@ermian). Other dikes, sills, and plugs are buried bysedimentary cover, as indicated by subsurface drilling(King & Harder 1985), geophysical surveys, andstuctural anomalies (i.e., anticlines, synclines, faults)in the overlying sedimentary rocks. These intrusivebodies vary in size from less than 700 m to 2.5 km indiameter. Wind Mountain is one of the largest @g. 2),with a reported thickness of 0.25 km, an area of4.3 ffi, and a minimum volume of 1.1 km3 (Barkeret a\.1977).

Barker et al. (1977) divided the fthologies found inthe Trans-Pecos magmatic province into nine qpes onthe basis of mineralogy and texture: 1) nepheline-bearing augite syenite, 2) nepheline-bearing trachyte,3) syenite, 4) nepheline syenite, 5) porphyriticnepheline syenite, 6) phonolite, 7) foliated porphyriticnepheline syenite, 8) quartz-bearing syenite, and9) quartz-bearing trachyte. All nine types axe found inthe Comudas Mountains; however, the predominzfflithology is porphyritic nepheline syenite. The largestlaccoliths consist of dark gray to pink, fine- to coarse-gained equigranular to porphyritic nepheline syenite.

TABII 1. DESRIPNOTT OF IA\TBOUS INIRUSIVE BODIEStN TIIB OORNT'DAS MOUNIAINSI

N@ Preddd R@ ASgIii.loly hlvta

Al@ ph@Ute,btaled dtud@tde€t 36,810.6 B& a d. Agm,}I@t!h pGdrttfrc ls!frslhs 6 dn (K/Ar @ Ad@8lr(1941),9@y

sydc hiodF) d aL (1916.)

O@d6 phgqllmlih 34.6i1J Beds a aI. QYm,M@!to Itotu,E€4ff& (K/Ar@ @(1911),Estdaf

tEdn/to bioee) O98O

Etu a at. (Ym,qab@!!091)

Btu a aL QCm,W@€td. (1959),fr&f@&Cdqq0993)

T t B n ( 1 9 4 1 ) ,BM d el.(lrTD

- Tno094l),8!rtE6td.Qqm

- Berts 4 qL Qgm

- T I B E ( 1 9 4 1 ) ,R8dkaaLorm

36.8i0.6 Da*s et ql, (LgTl),(li/Ar@ C:eb€!8b (1911!IIqytdohe) d4l(rg&i)

De6 *OlEta syrdb plog o la@lh 33.0 t 1.4 Eatu d aL Qgm,Mo@h Gl/Ar@ qabmsh(rg4r, P50),0ltlel?id bto&e E@y 6 aI (lWlrlde)

I Bodi€s 6 fuM h Flg@ 2

338 THE CANADIAN MINERALOGIST

Ftc. 2. Reference mao of Comudas Mountains.

TttB Wno MouNranr Laccomr

Geology and petrology

Wind Mountain is the only pluton mapped in detailin the Cornudas Mountains and is mineralogically andtexturally zoned (Figs. 3, 4; Mctrmore & Guilinger1993). The laccolith consists of six zones (Fig. 3),althougb some of the zones vary only in texture. Therocks are fypically gray- to cream-colored and weatherto darker colors. Accessory minerals form dark-coloredaggregates dispersed throughout the rock. A crude towell-developed trachytic texture defined by fefdspar ispresent in all rock types. In addition, the laccolithdisplays aa igneous foliation that dips steeply awayfrom the center of the intrusive body, roughly parallelto the observed slope surface. Feldspar lineations varyby 90' in the plane of this fohation. These feldsparorientations represent a magmatic lineation that formedsubparallel to the walls of the intrusion.

The outermost zone consists of medium-grainednepheline syenite porphyry GNSPT. The rock iscomprised of alkali feldspar phenocrysts (1,57o byvolume) up to 2 cm long, that occur in a fine- tomedium-grained manix (grain size less than 0.5 cm).The entire rock is composed predominantly 6f alkalifeldspar Q7c/o) and interstitial nepheline (L27o), withminor analcime (2Vo), Other accessory minerals formdark-colored aggregates (less than 0.5 cm long) andinclude aegirine (4Vo), sodic amphibole (2Vo), biotrte(LVo), and magnetite QVo). Nl percentages are basedon modal analysis by optical determination from point

counts (1000 points). At the contact with the HuecoLimestone, a thin zone of diopside hornfels is locallypresent. Within the intrusive rock, schlieren-like maficsegregations and zones of finer-grained textures arepresent in the contact zone for one meter into theintrusive rock. Small xenoliths of hornfels materialalso are present in this zone.

The outer nepheline syenite porphyry (TNSP2)grades into a coarse-grained nepheline syeniteporyhyry (TNSP1), which consists of larger and moreabundant (29Vo of. the rock mass) alkali feldsparphenocrysts (up to 3 cm) in a coarse-grained matrix(0.5 to 3 cm). Modal analysis shoys the rock ispredominallly alkali feldspar QjVo) and interstitialnepheline (lU%o), with minor analcime (8Vo).Accessory minerals in larger dark-colored aggregates(up to 1 cm long) include aegirine (57o), sodicamphibole Qqo), biotfie (zVo), and magnetite (2Vo). Thecontact zone between TNSP, and TNSP1 is marked bya zone of schlieren approximately 10 m wide.

The nepheline syenite porphyry (INSPT) gradesinto syenite porphyry CISPfa), which consists ofK-feldspar phenocrysts (up to 1 cm long) in a fine-gained matrix of K-feldspar and albite. Most of therock consists of K-feldspar (63Vo) nd ilbtto (l9%o),with interstitial analcime (37o). Accessory mineralsform small aggregates Qess than 2 mm long) andinclude aegirine (6Vo), sodrc amphibole (3Vo), clay(2Vo), biotte (l7o), and magnetite (3%). This syeniteporyhyry is typically darker colored than the lightergray or cream-colored nepheline syenite porphyry(TNSPI, TNSPT. The contact zone between TNSPI

toAlamogordo,El Paso

toDellClty

Auoiteb Sye-nite36.8t0.6 Ma

f-.}, \d,iXi5'y'lMountain/ \-/

\ 1 -,/proposed. nepheline,&"f;:,P gxy,.,,,h33.0 c 1.4 Ma -syenite Pit

New MExrco ___A_ffiSanAnton lo \ / \Mounta in , /

Mountain - O -.7

Trxes0 3 lm

F--+0 2m i

WashburnMountain

TIIE WIND MOIINTAIN IACCOLITH. NEW MEXICO _ TEXAS 339

r

ffiH ,",r" and tan deposttg

EIIIe]

Phonollts DlkFPorphyritic

Syonlts Porphyry-llne-graln6d, contalns mlnorInter8tttial analcit€ plus acoessory aegirlnHodaamphlbolFblotils and ma0netlte In a ieldsparrlch groundmaaa.

Syonlto Porphyry-tlns-gralned, mntalns dominantteldspar oncloging aegirlno-soda amphibolFbiotlte.

Syenlte Porphyry-lin€-grainsd, contalns domlnantfsldspar snclosing asgirin+soda amphibolFblotite.The zone ls more matlc thqn TSPtqt.

Sysnlts Porphyry-tlne-grainsd, conlalna moderaiointerotitial analcile plus accessory aegirinFsodaamphlbolFblotite and magnetils In a teldsparrich groundmass.

Nophollns-Syenlto Porphyry-nedlum-gralned alkall-teldspar phsnoorysts In a malrlx ot domlnant loldsparwith moderals Inter8tlt ial n€phgllne and minor analclmsplus acca3sory aogirlnFsoda amphibolFbiolite and minorm agnetlt€.

Mallo zonallon/lanlnatlon

Nephellne-Syonllo Porphyry-medium-graln€d alkall-feldspar is present as subhedral blooky prisms. Intersltlalnsphellne plus accsagory artsdsonllFasglrina and minormagn€llte aro prsssnt in a loldspar rlch groundmass.Tho orisntatlon ol tho tgldspar arystals are vgrlloal ratherthan flat (as In th€ main Intrusive mss), suggsstlng adltter€nt Iniruslve phass.

Nophollns.Sysnlts Dlkss-tlne"grained mssive crystalllngdlke-llke bodles outorop aound the stock In tho horntolslacles ot the llmsslone.

Llmostons-Hueco Fm. and Bono Sprlng Fm. l lmestone un'dlffor€ntlated. Noar lhe coniact wlth the stock, the llmaslonsls attgred lo a horntols racl€8 by contact nsiamorphlsm.

Nsphellns-syenlte plts

M

F r ' \ r r j

M

@

E

EsEuF

giruF

z3EEud

M

fiffil tepnetlns-syontto Porphyry-coaroe-grarnede alkal i - ie ldspar phenocryata in a matr ix ol domlnant

leldgpar wllh modoratg Inter8tltlal nephellns andanalclme plus accassory a€glrlno*9oda-an phiboleand minor magnotitg.

Ftc. 3. Geological map of Wind Mountain (P. Graseah, field mapping, July 1992).

340

and TSPfg is a narrow zone of schlieren betweenI and? m wide.

The fine-grained syenite porphyry (ISPfg) gradesinto a more mafic-looking syenite porphyry GSPfg3).This syenite porphyry is similar in grain size to TSPfgn,except that it contains larger phenocrysts (up to 1 cmlong) of K-feldspar in a fine-grained matrix ofpredominantly K-feldspar and albite. Feldsparproportions in the entire rock are 547o K-feldspar and20Vo albite,. TSPfg3 contains accessory aegSrrne (SVo),sodic amphibole (4Vo), bionte (lEo), clay (L%o) anrdmagnetite (lVo) as discrete grains as well as in mineralaggregates. Analcime is most abundant in this phase ofthe pluton, comprising l4%o of. the rock. The contactbetween TSPfga and TSPfg3 is very gradational and notreadily recognized in the field.

Several thin, discontinuous zones of less mafic,lighter colored, fine-grained syenite porphyry GSPfg)occurs within the syenite poryhyry (TSPfg). Thissyenite porphyry consists of smaller Qess than 2 mm)K-feldspar and albite phenocrysts in a fine-grainedmatrix. The rock consists of K-feldspar (57Vo), albrte(20Vo), analcime (VVo), aegSrtne (87o), sodic amphibole(2Vo), bionte QVo), and magnetite (17a). Also withinthis zone 4p thin and discontinuous dark (in outcrop)dikes that appear more mafic on weathered outcrop butare actually light colored on fresh surfaces. The contactbetween TSPfg2 and TSPfg, is sharp.

The mafic syenite porphyry CISPfgJ grades intothe uppermost cap of syenite poryhyry (TSPfer). Thesyenite porphyry is similar to TSPfg, and consists ofK-feldspar phenocrysts (up to 1 cm long) in a fine-grained maftix. The unit is predominantly K-feldspar(58c/o), albite (2AVo), atalame (8Vo), and accessoryaegirine (4Vo), sodrc amphibole (3Vo), broffie (2Vo),clay (2Vo), and magnetite (3Vo).T\e accessory mineralsform crystal aggregates up to 1 cm long. This phase ofthe pluton contains numerous miarolitic cavities linedwith crystals of feldspar, analcime, occasionallynafrolite, and more rarely, zirconium silicates. Thecontact between TSPfg3 and TSPfgl is gradational andsubtle.

The Wind Mountain laccolith is cut by two dikesof porphyritic phonolile that coalesce into one dike.Eudialyte occurs locally in these dikes (Zapp 1941,Clabaugh 1950, Vy'arner et al. 1959). The dikes are

TIIE CANADIAN MINERALOGIST

Ftc. 4. Schematic cross-section of the Wind Mountain laccolith.

matrix-supported, with large phenocrysts ofanortloclase atrd, more rarely, nepheline. In addition,several thin dikes and sills (less than 1 m) of nephelinesyenite porphyry intrude along bedding planes andfractures in the Permian limestone. Locally, thin zonesof chlorite-epidote hornfels are developed along thecontacts (Mckmore & Guilinger 1993).

Mineralogy

Feldspar-group minerals constitute the mostabundant species, and are represented by two distincflydifferent textures and compositions. Alkali feldspartypically occurs as medium- to coarse-grained micro-perthitic phenocrysts in the nepheline syenite porphyryunits; these phenocrysts are characteristic of hyper-sohus conditions during crystallization. Phenocrystsare commonly turbid and are occasionally altered toanalcime, natrolite, or clay. In the groundmass, alkalifeldspar is typically less altered and turbid than thephenocrysts. The groundmass feldspar also is micro-perthitic. Two primary feldspars are present in thesyenites, representing subsolvus conditions of crystal-lization. Albite and microcline (determined optically,based on twinning and extinction angle) coerist withsmaller laths of albite sunounding the subhedralK-feldspar phenocrysts in the syenite units. Discretegrains of albite and microcline also are present in thematix. Most of the feldspar grains in the syenite unitsare turbid and partially altered to analcime, natroliteo orclay minerals. Feldspar alteration is more pervasive inthe syenite units than in the nepheline syenite. Minoramounts of muscovite werg observed in the syenite asa product of deuteric alteration of the microcline.

Nepheline is found in all zones at Wind Mountain,as well as in several other laccoliths in the ComudasMountains (Tables l, 2). Nepheline is typicallyeuhedral to subhedral, interstitial to the feldspms, andlocally altered to clay minerals and analcime. Largergrains poikilitically enclose bleb-shaped inclusions ofmafic minerals, usually aegirine. Locally in t]tenepheline syenite, the outer edges of the nephelinegrains display extensive alteration to clay.Significantly less nepheline is present in the syeniteunits an4 where presen! is typically corroded andaltered to clay or analcime.

TABI.E Z SEIISIM MNRAIA REPORTED FROM lHE @RNUDAS MOUNTAIN|IAND THEIR, MODB OF GTJRR&{SB

TABLB 3. WHOI.B-RCK CHEI\TC6L @MPOSII]ON OF RBPRSB{TA1IVB

SAMPI.B FROII4 TEB WIND MOUNIAIN I.AC(ITfiE

341

CORN3I@RN33@RN34 lNtlPr lSP{h TSPf82 lsPfga 1EPe4

TTIE WIND MOUIITAIN LACCOLITH. NEW MEXICO _ TEXAS

Mhsal

trrdf

efolxg

olivhg

mdlstyte

cdapldib

Sr6ge0tadb

6gHo!

fr@dtg

dabrrile

rtrlle

f,qlL

O@@

Fd@ nstselfr€, lha vsg$ rodd€ad dlroltfc wttbg

Ell@ ldelip 4d Hd{F1s

tuat aggt€eatB of ftma$Bdmd@k 6d ma$edF

h l8llslb sy@nb

h dilos. d!8, d llroolit8 @rlid dsoltds cavn€

deollia asrld8

d@litlc €etttls

dsoltdr @llts

d@ltb @tles

d@lttt!@id8

dmlld9@ff$

q!|!srh st€tu, Whd !6@1tr

sg8togdes d tu@8gFdE Eh@18

tEEcds

R&@

Bad6A Eod36 OYm, BoEB098r,

Be*d & l{odgs 0qf0

Ba*d& liodgs O97)

BdsASods6 OYt)

Be&HdgB (197),orbndhO9$I Eogg8 O9t5, l9r0

Bog8s 0985)

Bog' O9Bt, loS8r & cseostBogr 0985. 1987,

Bog8 O9E5)

Zatp 0941), Boggs OSs)

BoEE8 O9E5)

ll|l8 !!FGt

fd8 r€Pct

Btu6aL$YA

SPz P!% @,10 59.40 5&mTto: 0.1E 0.18 0.08At"o. w0 1&40 nmF%O:' 5.09 5.64 ilJS}t!O 021 On On!,lro O2A 0.{t 0.19C{O l.16 l8 0:73Na3O 6&! 7:t2 8.04KeO 5.M 533 5.4EPrOo Oo' O09 0.6]]ot tm 2fi 335Totsl t8.6 1@.85 98J0

J9.(Do2A

18'm4330240.46t.457.@

0,4

9t0/

Ba ppB 182 lts 89 106 44 d nO 49G n n 1 3 A 4 7 ! d 3 l K7i 16 184 353 m Pl 216 1!2 ltlG a n z 9 t 2 B S N u uP b l J 3 1 5 5 2 9 ? 2 $ X $m E 3 t n 3 9 2 4 6 3 0 t 6Rb llt 160 N 157 t2l 165 r33 r2Sr 60 4 62 @ Xr} 1@ tlo 135\ u 5 3 1 3 t 6 3 & 6 UIa 6l LA rE4 116 U !d lgl &4C s 1 3 1 E A 2 . 6 B D d l E 1 7h 74 1739 N3 t984 1165 2135 1326 WNb 96 214 360 239 157 250 t?E 126

5&d, 59.@0.r9 026

17.$ 1E.10596 41!o8 0a0u 0J3|'l9 1J0*a 785.3t 'A0.10 0,r22n fig

1018 9&76

t9a0.13rE504:no0.49094165310.ffi

3n1@J0

59J00a

IEJO465o8035l.t96n,

0.111.9a

M€l tu@lse of @ s!€ds dd!ly6: Na.(cr,co)rciez.'ldo,.'Y)zrstooz(oll,o),cdTletlb.NqZrStlOe.2H?O,Ses8edalda:NagdLor.:nlo,psateldt8tteNar?rSLOr.

Analcime locally replaces nepheline and feldsparand also occurs as linings or discrete crystals inmiarolitic cavities in the syenite units (notablyTSPfgl). Where present as a primary phase in thenepheline syenite, analcime is anhedral andintersertal to the feldspars and may represent analteration phase. Analcime is most abundant in thefiner-grained syenite units. In the syenite, analcime isdisseminated throughout the entire rock and variesin texture from euhedral to anhe&al grains intersertalto feldspars.

A variety of mafic and other accessory and traceminerals occur in the Wind Mountain laccolith(Tables 2, 3). Mafic minerals include sodic amphibole,aegirine, biotile, and magnetite (Iable 3). Hematite andclays are common products of alteration that canconstitute tp to l2vo of the sample. The maficaggregates within the nepheline syenite units (n{SPland TNSPT, Fig. 5) allow for easy removal by magneticseparation. Primary magnetite is intimately associatedwith aegirine, commonly observed as inclusions in thepyroxene. Fluorite is reported as breccia filling and6s small rcpl4sement bodies at Wind Mountain @arkeret al. 1977) and may represent a fluorine-richderivative from the laccotth.

A large number of less common minerals arereported to occur in the Cornudas Mountains, andspecifically at Wind Mountain (Table 3). Many ofthese occur within mialetlig cavities, although some

. T@l tiu I Fe?O,. !.16 dar& S@ls @RN 31, 33, @d 34 @ tat{ t@ Eil TNgPt

also are present in the mafrix. Recently, parakeldyshite,a rare Na-Zr silicate, was identified by X-raydffiaction in a heavy-mineral separate of the WindMountain nepheline syenite Cn{SPr. A number ofother Na-Zr silicates have been found in miaroliticcayities in the laccolith. Wind Mountain is the typelocality for the mineral georgechaoite @oggs & Ghose1985).

Geochernisny

Individual samples from each zone of the WindMountain laccolith, except the phonolite dikes, werecrushed and ground following standard practices.Whole-rock and tace-element geochemical analyseswere obtained using a Philips PW 2400 X-ray-fluorescence unit utilizing standard operatingprocedures. Fused glass disls were used for major-element analysis (Hallet & Kyle 1993). Pressedpowder briquet0es were prepared for trace-elementdeterminations (Norrish & Hutton 1969). U.S.Geological Survey rock standards were used tocalibrate the instrument.

Chemical variation among individual map-unitswithin the laccolith cannot be readily discerned byutilizing major-element oxides (Iable 3). Only TiO,values reveal any significant variation between the tworock types. Normative calculations based onwhole-rock compositions reveal a more important

Tr

a

342 THE CANADTAN MINERALOGIST

58 60

Si02

400

(Et zoo

0

II

I

aO 1

discriminating factor: proportion of normativeanorthite. The nepheline syenite units lack normativeanorthite. This is also seen in the lack of modalplagioclase in the petrographic analysis of the rocks.Syenite units contain up to 57o normative anorthite, andalso contain modal albite. No mafic mineral frac-tionation was apparently involved in the transitionfrom syenite to nepheline syenite @g. 5).

Although the different units within the WindMountain laccolith are chemically similar on thebasis of major-element oxides, significant chemicaldifferences in Sr and Ba concentrations are observedbetween the nepheline syenite and syenite units. Thesyenite units generally contain twice as much Sr asthe nepheline syenite @g. 5). Barium concentrationsdisplay even grcater contrast between the two majorrock-types. The variation in Ba and Sr (Fig. 6) is aresult of crystallization and removal of feldspars fromthe melt (Weaver et aI. 1.972). The feldspar-richproducts are considered to form at or migrate towardthe top of the magma chamber, and form a cappingsyenite. The compositional gap in Ba and Sr is furtherevidence of differential cooling, which is alsomanifested by textural variation between the two units.The syenite is consistently finer-grained than the

200

u,100

0

I

I 1I

oD

1

CaO

410

(!t zoo

0

I I

I

o

o

nepheline syenite units.Trace-element variation between the two main rock-

types is not apparent in incompatible elementconcentrations (Ftg. 6). The distribution of Cs, L4 Nb,and Rb as a function of Zr does not indicatepafiitioning of the incompatible elements between themajor rock-units, although the use ofZ as a conservedelement in this system could be problematic sinceZx-nch phases are present in the intrusion (Weaveret al. 1972). The large variation in Z betweenindividual map-units is due to the precipitation ofZr-silicates throughout the laccolith. However, thelinear variation in NbZr, LtZr, and RbZr suggeststhat all the rock units are comagmatic. This is similar totlle case of other alkaline igneous intrusive bodies inTrans-Pecos Texas @arker 1987). The scatter in theCslZr plcrl',less so in thel-alZr plol may be the resultof the development of a volatile phase duringcrystallization (Weaver et al. 1972). The alteration ofthe feldspars and mantling of phenocrysts by albitein syenite units also suggest the possibility of a post-magmatic metasomatic alteration event similar to thatdescribed by Martin & Bowden (1981). The alterationof nepheline to analcime firther indicates Na metaso-matism of the svenites.

0.6

o 0.4EDE

0.2

6256

100 200

KZO Sr

Frc. 5. MgO - SiOr, Sr - CaO, Ba - K2O, and Ba - Sr variation diagrams for the variouszones in the Wind Mountain laccolith. Oxide values are reported as weight percent,trace elements as pprn. Symbols: O: nepheline syenite, I: syenite.

6.05.55.0

TIIE WIND MOIAITAIN LACCOLITH. NEW MEXICO - TEXAS

The high alkalinity of the melt, coupled with thehigh initiat concentrations of Zl and a lack of freesilica, allowed for the formation of a suite of the rarezirconium silicates, which occur throughout theintrusion, without the formation of zircon. The concen-tation of zirconium silicates in the late-stage phonolitedikes (Zapp 1941, Clabaugh 1950, Warner et al. 1959)suggests an overall buildup of Z during crystallization.However, tle presence of significant amounts ofzirconium silicates accompanied by analcime and

natrolite in miarolitic cavities would also indicateconcentration during the separation of a volatile phase.

The formation of the various units in the WindMountain laccolith appears to be the result of fractionalcrystallization and the development of a volatile phase,which separated over time. Initial formation of thenepheline syenite units involved crystallization underhypersolvus conditions. As the magma continuedto crystallize, subsolvus conditions developed inthe upper portions of the magma chamber due to the

20

1 0

oo

200

j roo

0

240

.Gltr' roo

0

204

' r ca

0

400

) zoo

0

400

(5o zoo

0

t l

1000 2000

Zr

II

oaa

1000 2000

Zr3000

a,.d

{000 2000

Zr3000

I

I 1I

aoo

I

3000I

0

1000 2000 3000

Zr1000

Flc. 6. Cs, La" Nb, Rb, Ba, Sr - Zr trace element variation diagrams for the various zonesin the Wind Mountain l,accolith. All values are in ppm. Symbols: O: nephelinesyenite, I: syenite.

3000

Zr

o

I I 'a l

r 'on

o

.taIo

3M TIIE CANADIAN MINERALOGIST

concentration ofvolatiles. and resulted in the formationof syenite units. The progrcssive build-up of residualfluid in the upper zones of the pluton during crystal-lization of the nepheline syenite culminated in theformation ofmineralogical and textural zones, deutericalteration, and minor expulsion offluid, which escapedinto the country rock, forrning the skam and homfelsunits adjacent to the pluton. The various zones at thetop of the laccolith probably reflect vmious zones ofvolatile concentration, which also explains thetendency for these various zones to be separated byschlieren, and not sharp contacts. The syenite unitdisplaying sharp contacts (TSPfg) representsinjections of more fractionated magrna along dikesduring late-stage crystallization. The migration ofresidual fluids also deuterically altered the syeniteunits, and in particular reduced the nepheline toanalcime and clay. Additional mapping of the plutonand more detailed petrography may lead to theidentification of fluid pathways through the earlier-formed unitso similar to the alteration stages describedby Martin & Bowden (1981), and so better constrainthe timing of alteration. Residual fluids trapped at theapex of the laccolith also confributed to the formationof zirconium-silicate-rich miaroles within thenepheline syenite, described by Boggs (1985), duringcrystallization of the syenite units. The two-feldspar,eudialyte-bearing dike that inrudes both the laccolithand the country rocks represents a late-stage magmaformed by the same processes @oggs 1987).

The Wind Mountain laccolith is classified as aWithin-Plate Granite using the classification of Pearce

1000

et al. (L984) (Ftg. 7). In accordance with these types ofintrusions, the magmas probably originated in themantle and acquired a significant enrichment inlithophile-group elements during its movementthrough,continental crust. These data are consistentwiti either continental rift or subduction-relatedback-arc extension settings. Additional studies arerequired to further consfiain the origin of the magmasto a particular tectonic setting.

MINIERAL DEPosrrs

The Cornudas Mountains have been examined in thepast for potential deposits of gold, silver, beryllium,rare-earth elements, niobium, thorium, and uranium(Mclemore & Guilinger 1993, Mclemore et al. 1996),but there has been no production. In the 1950s,prospectors located several areas of anomalously highradioactivity in the Cornudas Mountains and attributedthese to the presence of uranium and thorium. Shallowprospect pits were dug on many claims in the area;however, assay results were very low, and the claimswere later dropped. The potential for economicuranium and thorium deposits in the area seems lowbecause of low assays in areas of anomalously highradioactivity and a depressed uranium marketMclemore & Chenoweth 1989).

Beryllium was first reported from the CornudasMountains during the 1940s. A few samples assayedwere found to contain as much as 0.2Vo BeO (Wameret al. 1959). The limited development of a metasomaticaureole and apparent lack of large-scale metasomatism

1 000

O

ft wPcO

V ORG

{00tlt,2 roo

l 0t 010 100 1000 l0 100

Y Y+NbFIc. 7. Log Nb-Y and log Rb--(Y + Nb) diagrams for the various zones in the Wind

Mountain Laccolith after Pearce et aL (1984). Al1 values are in ppm. Symbols:O: nepheline syenite, l: syenite.

orf o

VAG

THE W]ND MOTJNTAIN I,ACCOLITH. NEW MEXICO - TEXAS 345

on the margins of the pluton would suggest thatWind Mountain may only contain limited berylliumpotential, in comparison to the mineralized laccolithsnear Sierra Blanca- Texas.

A variety of deposits containing gold and silver areassociated with alkaline igneous rocks in New Mexico(Great Plains Margtn deposits; North & Mclemore1986, 1988, Mclemore 1991) and elsewhere along theNorth American Cordilleran alkaline igneous beltGrg. l,Mutschler et al. L991, Thompson 1991,1992).Consequently, numerous companies have examinedthe Cornudas Mountains for similar gold-silverdeposits, but without success (Mclemore & Guilinger1993). The lack of significant evidence for hydro-thermal alleration (i.e., large zones of skarr, complexalteration assemblages, e/c.) associated witl tleemplacement of the Wind Mountain laccolith suggeststhat the potential for precious metal deposits is low(I\dcl,emore & Guilinger 1993).

A few companies have examined the ComudasMountains unsuccessfully for high concentrations ofrare-earth elements, niobium, zirconium, and titanium.U.S. Borax sampled and drilled in the Chess Drawarea, but their assays of mineralized limestone andsyenite dikes were low (up to 0.06Vo total rare-earthoxidesn 10-1400 ppm M, 10-3000 ppm Zr,230-13,000 ppm F). Results of an analysis reported byMclemore et al. (L988U b) contain 1235 ppm Ce,700 ppm LU2i70 ppm Nd and242 ppm Y. Zirconiumsilicates are common in the area (Iable 2) and are welldistributed throughout the pluton. T?u:ee claims forzirconium deposits are located immediately east ofWind Mountain.

Addwest Minerals, Inc. is developing the WindMountain nepheline syenite for use as a constituent inamber-colored beverage containers, ceramics, andflatglass. The nepheline syenite contains high ironcompared to other commercial sources of nephelinesyenite, but, when the Wind Mountain nephelinesyenite is crushed and passed through a specializedrare-earth magnet, the resulting nonmagnetic product issimilar in composition to Grade-B product specified byUdmin Canada Ltd. Specifications and description ofthe industrial applications of the Wind Mountainnepheline syenite are provided by Mcl*more et al.(1996) and Mclrmore & Guilinger (1996). Emissionspectroscopy indicates that the magnetic fractioncontains 20 to 30Eo FEO3, but is low in rare-earthelements (100 ppm La), beryllium (5 ppm), yffium(100 ppm) and zirconium (2000 ppm). The magneticfraction can be sold 4s millite. an iron-rich additiverequired for controlling the color of glass. Several otherconsumers have tesled the nepheline syenite and foundit suitable for use in ceramics, flberglass, and flatglass.The lack of free silica as quartz also enables use ofthe Wind Mountain nepheline syenite as a silica-freeabrasive. Interesting textural variations in the mainmass of the syenite, and the presence of wisps of finer-

gained material in the rock, also make it an attractivebuilding stone.

Suuuanv

The Wind Mountain laccolith is mineralogicallyand lexturally zoned. All six zones in the pluton vary intextureo but are chemically simil6. Nepheline syeniteporyhyry forms the outermost zones of the WindMountain laccolith and may constitute much of themountain (Fig. ), although drilling has not yetreached the center of the laccolith. Thin zones ofsyenite porphyry cap the laccolith. The zonation inthe Wind Mountain laccolith appears to have beencontrolled by crystal fractionation, separation ofvolatiles, and cooling history, rather than differentpulses of magna. Feldspar fractionation can accountfor most of the chemical variation in the differentzones. The syenites crystallized under subsolvusconditions as indicated by feldspar mineralogy and thepresence of metasomatic alteration phases (muscoviteand zeolites). A late-stage hydrothermal event isindicated in all rock types by the alteration ofnephelineto analcime and clay.

Additional mineral resource potential in theComudas Mountains is limited. The nepheline syeniteof Deer and San Antonio Mountains (Fig. 2) may havepotential use for glass or ceramic use. The otherlaccoliths, dikes, plugs, and sills are not suitable forglass or ceramic use because ofhigh iron contents andheterogeneous composition. The lack of large-scalehydrothermal alteration associated with the emplace-ment of the laccoliths in the region suggests that thepotential economic mineralization may be limited.However, the abundant rare minerals at WindMountain suggest that the area has potential fordeposits of rare-earth elements, niobium, andzirconium.

ACK.IOWI.EDGEilMNTS

Special thanks are to P. Gruaseah for his geologicalmap of Wind Mountain. Discussions with M.A.Ouimette and Robert Guilinger are appreciated andenhanced our understanding of the geology of the area.Sample preparation by Daniel Hack and David Brightis acknowledged. Philip Kyle and Chris McKeecarried out the X-ray-fluorescence aoalyses using thePhillips PW 2400 instrument purchased with tundsfrom NSF grant EAR-9316467. Figures were dra.ftedby the NMBMMR Cartography Departrnent. Thiswork is supported by the New Mexico Bureau of Minesand Mineral Resources (Charles E. Chapin, Directorand State Geologist) and Addwest Minerals, Inc.(James R. Guilinger, project supervisor). We thankPete Modreski. Iee Potter. and Robert F. Martin fortheir comments and suggestions conceming an earlierversion of the manuscript.

346 THE CANADIAN MINERALOGIST

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Received September 13, 1994, retised manuscript acceptedNovember 9, 1995,