Volcanic Geology and Mineralization in the Chinati Caldera Complex, Trans-Pecos Texas

Geological Circular 81-2

CalderasandMineralization:VolcanicGeologyandMineralizationintheChiantiCalderaComplex,Trans-PecosTexas

Timothy W. Duex Christopher D. Henry

Bureau of EconomicGeology W. L.Fisher, Director"1981

TheUniversity ofTexas at Austin" Austin, Texas 78712 Contents

INTRODUCTION 1 Lower and middle trachytes 10 STRATIGRAPHY 3 Lower rhyolite 11 Pre-Tertiary rocks 3 Upper trachyte 11 Tertiary volcanic rocks 4 Nonporphyriticdomes and flows 11 Morita Ranch Formation 4 Upper rhyolite 11 Shafter area 4 West Chinati Stock 11 Cienega Mountain area 6 Ring-fracture intrusions 11 Infiernito caldera 6 Geochemistryof the Chinati Precollapsevolcanic rocks 7 Mountains Group 11 Ash-flow tuff 7 MINERALIZATION IN THE CHINATI Postcollapsevolcanic rocks 7 CALDERA COMPLEX 12 Ojo Bonito pluton 9 Shafter area 12 Perdiz Conglomerate 9 Antonio Canyon area 12 Shely Rim area 9 San Group Shely Group Chinati Mountains 12 9 Allen Intrusions 13 Allen Intrusions 9 Infiernito caldera 13 Relations to the Infiernito caldera 10 "Shely cauldron" 10 CONCLUSIONS 13 Chinati Mountains Group 10 ACKNOWLEDGMENTS 13 Mitchell Mesa Rhyolite 10 REFERENCES 14

Figures

1.Index map of Trans-Pecos Texas 1 2.Generalized geologic map of the Chinati caldera complex 2 3. Schematic cross section through Infiernito and Chinati Mountains calderas 3 4.Known and suspected stratigraphic relation between rocks of the Chinati caldera complex 4 5.Stratigraphy of the Morita Ranch Formation . 5 6.Stratigraphy of the Shely and Infiernito Groups in the Infiernito caldera area 6

Table

1.Nomenclature of the Chinati Mountains Group 10

iii Introduction

This report describes preliminary results of an evolution,and evidenceof mineralization inTrans-Pecos ongoing study of the volcanic stratigraphy, caldera to those of the San Juan volcanic field, a major mineral activity, and known and potentialmineralization of the producer,indicates that Trans-PecosTexas also couldbe Chinati Mountains areaof Trans-Pecos Texas.Many ore an important mineralized region. The Chinati caldera deposits arespatiallyassociated with calderas andother complex in Trans-Pecos Texas contains at least two volcanic centers.A geneticrelationshipbetween calderas calderas that have had considerable postsubsidence and base and precious metal mineralization has been activity and that display large areas of hydrothermal proposed by some (Albers and Kleinhampl,1970) and alteration and mineralization. Abundant prospects in deniedby others (McKee, 1976, 1979). Steven andothers Trans-Pecos and numerous producing mines (1974) have demonstrated that calderas provide an immediatelysouth of the Trans-Pecos volcanic field in important setting for mineralization in the San Juan Mexico are additional evidence that ore-grade deposits volcanic field of Colorado.Mineralization is not found in could occur in Texas. all calderas but is apparentlyrestricted tocalderas that The Chinati caldera complex is located in Presidio had complex, postsubsidence igneous activity. A County about 40 km (25 mi) southwest of Marfa, Texas comparisonof volcanic setting, volcanic history,caldera (fig. 1). A simplified geologic map, cross section, and

Figure 1.Index map of Trans-Pecos Texas showing location of generalizedgeologic mapof figure 2.

1 stratigraphiccolumn showingrelativeage relationships the Rio Grande Valley.The Chinati calderacomplex,as are shown in figures 2, 3, and 4. The Chinati caldera defined here, includes at least two calderas that were complex lies in the southern Basin andRange Province eruptive centers for thick accumulations of volcanic and is bounded onthesouthwestbyseveralpostvolcanic strata. Italso includes severalsequencesof volcanic rock normal faults that parallelthe north-northwest trend of for which sources are currentlyunknown.

Figure 2.Generalized geologic map of the Chinati caldera complex, Trans-Pecos Texas (modified from Rix,1953; Amsbury, 1958; Dietrich, 1966; Cepeda, 1977;and Barnes,1979).

2 The ChinatiMountains caldera is thelargestandbest indetail andinterpretedthemasacaldera,buthe didnot documentedcaldera inTrans-Pecos Texas(Cepeda,1977, extendhis map beyondtheChinati caldera ring-fracture 1979). It is the probable source of the Mitchell Mesa zone on the north where it truncates rocks of the Rhyolite, a widespread marker bed throughout Trans- Infiernito caldera.The geologichistory of theInfiernito Pecos, andof at least 1,000 m(3,300 ft) of lava flows and caldera is described in this report. tuffs within thecaldera. Thiscaldera formedbetween 32 Amsbury (1958) named the Shely Group, aseries of and 31m.y.ago. volcanic rocks on the Shely Rim. The Shely Group As defined in this paper, the Infiernitocaldera, the overliesrocks of theInfiernito calderaonits westernedge second caldera of the complex,includes those rock units and isalso truncated bythe Chinati Mountains caldera. exposedbetween theChinatiMountains onthe southand Barnes (1979) extrapolated the Shely units into the theCuesta delBurroonthenorth(fig.2).Itisnamedafter Infiernito area. This report, however, shows that the Cerro Infiernito, a prominent landmark within the Infiernito rocks are distinct from and older than the caldera. The Infiernito caldera isclearly older than the Shely Groupandprobablyformed duringdevelopmentof Chinati Mountains caldera because its southernhalf was a resurgentcaldera. destroyed by the formation of the Chinati caldera; however, no isotopic ages are currently available to document its absolute age. The Chinati caldera complex is entirely within the Stratigraphy Trans-Pecos alkalic igneous province described by Barker (1977). The volcanic rocks of the Chinati Mountains caldera vary systematically with decreasing Pre-Tertiary Rocks age from metaluminous to peralkaline, according to whole-rockchemical analyses(Cepeda, 1977). Complete The oldest rocks exposedin the Chinati Mountains whole-rock chemical data are not available for rocks of areaareUpperPennsylvanianandPermian carbonates. the Infiernito caldera, but they are not peralkaline Theycrop out alongthenorthern, eastern,and southern because many of them contain biotite. borders of the Chinati caldera (Skinner, 1940; Rigby, Rix (1952, 1953) mapped the Chinati Peak 1953; Rix, 1953; Amsbury, 1958). In the northern and Quadrangle and described the Chinati Mountains northwestern parts of the area,the carbonates are silty volcanic series as well as agroup of rocks inthe eastern and sandy, but in the southern parts they contain less part of the area thathe correlatedwith the Buck Hill detritalmaterial. In theShafter area,alongthesouthern volcanic seriesofGoldich andElms(1949).InthePresidio border of the Chinati caldera, theyarethehost rocksfor area, Dietrich (1966) reinterpretedthe latter group of silver and lead-zinc mineralization. Cretaceous marine rocks asadistinct volcanicsequence, which henamedthe carbonates and terrigenouselastics overlie the Permian Morita Ranch Formation. No source areais known for rocks around the edgesof the Chinati caldera and are theserocks. Cepeda(1977)studied theChinati Mountains thickestin theLomaPlata anticline.

Figure 3. Schematic cross section throughInfiernito and ChinatiMountains calderas. Symbols are from figure 2.

3 Tertiary Volcanic Rocks was renamed the Morita Ranch Formation by Dietrich (1966).In the Presidio areahe defined theMorita Ranch Several distinct sequencesof volcanic rocks that were Formationtoincludeallflow andsedimentaryrocks that erupted from various volcanic centers duringTertiary crop out between pre-Tertiary strata and Perdiz time overlie the Permian and Cretaceous sedimentary Conglomerate. In exposures southeast of Shafter, he rocks. Relative age relationships among some volcanic described four members of the formation and equated sequences are not everywherestraightforward,and iso- them with units thatRixmapped.Thefour members are topicage data areincomplete,but therelative agesof dif- Tm2 (unit T2 of Rix), black porphyritic basalt with ferent volcanic strata,from youngesttooldest,are shown abundant large feldspar phenocrysts; Tm3a (unit T3 of schematicallyin figures 3 and 4.Inthe followingsection, Rix), thick sequence of rhyoliticflows and tuffs; Tm3b the majorunits ineach sequencearedescribed,andrela- (lowerpart ofunitT4 ofRix),rhyoliticash-flow tuff with tionships amongseparate sequences are discussed. The abundant feldspar and minor quartz phenocrysts; and volcanicrocks of theInfiernito and Chinati calderas are Tm4 (correlativeat least in partwith the upper part of discussedin relationto mineralizationlater in the text. unit T4 of Rix), medium- to fine-grainedolivine basalt two oldestandpossiblycoeval volcanic sequences (fig. 5). The 1953) layers are exposedadjacent to the Chinati Mountains caldera. Inaddition,Rix (1952, noted of tuff and They are theMorita Ranch Formation and the rocks of olivine basalt below Tm2(his unit T2), which helabeled theInfiernito caldera.Bothsequenceslie disconformably unit Tl. These strata pinchout both north and south of ontheaforementioned pre-Tertiaryrocks andareuncon- Shafter and werenotrecognizedbyDietrich (1966)inthe formablyoverlain atleastin partby thePerdiz Conglom- Presidio area.Inreconnaissance work doneaspartofthis just erate. Both sequences are truncated by the Chinati study, we identified a sequence of rocks east of the caldera ring-fracturezone.However,theyarenowherein boundaryof the Chinati Mountainscaldera andnorth of contact andhave nounits incommon;therefore, therela- Shafter that is at least partlycorrelative with what Rix tive age relationshipbetween the two series cannot be describes andmapsasunitTl(fig.5).Thelowestvolcanic directly established (fig. 4). layerobservedinthisstudyis alithic,moderatelywelded, rhyoliticash-flow tuff thathas been faulted andpossibly folded along with the underlying Permian and Cretaceous sedimentaryrocks. Thisunit, orpart of it,is Morita Ranch Formation most likelywhatRix(1953)refers to asbasaltuff.On top of theash-flow tuff is athick but laterallydiscontinuous Shafter Area porphyriticrhyoliteflow with athickautobreccia zoneat its base.Rix refers to thebase as aflow breccia but maps A sequence of volcanic rocks thatcrop out north and the upper part of thebody as anintrusion. The breccia east of Shafter (fig.2) was initiallyassigned to theBuck and moremassiveupper partareinterpretedhere tobe Hill volcanic series by Rix (1952, 1953). Later, more parts of asingle flow. extensive mapping throughout Trans-Pecos Texas Thenextunitobservedabove the porphyriticrhyolite revealed that the rocks formed a distinct series, which is a thin, poorly exposed sequence of bedded tuffs and

Figure 4. Known and suspectedstratigraphic relations amongrocks of the Chinati caldera complex.

4 minor mud flows.Rix does not mention these beds inhis Insummary,theMoritaRanchFormation is asignifi- descriptionof unit Tl.Thestratigraphicallyhighestpart cant anddistinctive sequence of volcanic rocks that does of unit Tl observed in this areais a thick flow-banded not correlate withthe BuckHillvolcanicseriesto theeast rhyolite that Rix identified and correlated with more or with the rocks of the Infiernito orChinati calderas to massive outcrops alongU.S.Highway67. Rixdescribes the west. The presence of biotite in several units of the olivine basaltin theupperpartof unit Tl,butit was not MoritaRanchFormation servestodistinguishitfromthe observed in the areainvestigatedfor this report.Therest Chinati Mountains Group andfrom other morealkaline of the section (Rix's units T2 through T4 and Dietrich's igneousrocksof Trans-Pecos Texas thathaveno biotite. units Tm2 through Tm4) is similar to what weobserved The presenceof biotite, which also occurs inrocks of the and to what issummarized in figure 5. Infiernito caldera,andthe relative stratigraphicposition

Figure 5. Stratigraphy of the Morita Ranch Formation near Shafter,derived from Rix (1952, 1953), Dietrich (1966), Hardesty (inprogress), and this study. Designations Tl, T2, T3, and T4 are from Rix;designations Tm2, Tm3a, Tm3b, and Tm4 are from Dietrich.

5 of both sequences suggest possible correlation of the numeroussmallintrusions and alargeriebeckite rhyolite Morita Ranch Formation and rocks of the Infiernito intrusive dome that forms the main mass of Cienega caldera.However, the source areafor volcanic rocks of Mountain. Theseunits arenowhereincontact with rocks the Morita Ranch Formation and their relation to the of theChinatiMountains,but theyareprobablyalsoolder Infiernito caldera cannot nowbe preciselyestablished. than the Chinati Mountains Group.Their relationshipto theInfiernito caldera or to the Shely Groupisuncertain. At least a few of the flows originated locally,but the CienegaMountain Area source for most of themisunknown.

Hardesty (in progress) has mapped an area east of Shafter inthe vicinityofCienegaMountain.Heidentified Infiernito Caldera theupper members of the MoritaRanchFormationanda sequence of overlyingmafic to felsic lavaflows and vol- The geologic history of the Infiernito caldera is best caniclastic sediments that we are tentativelyincluding explainedby grouping the rocks into four genetic se- within the Morita Ranch Formation. He also noted quences: precollapse volcanic strata (Tiof figs.2and 6),

Figure 6. Stratigraphy of the Shely andInfiernito Groups in the Infiernito caldera area, northern ChinatiMountains,derived from Amsbury (1958) and this study. Ts designations are from Amsbury (1958).

6 ash-flow tuffsof themaineruptiveevent(T2),postcollapse groundmassthat rarelydisplaysghostsofflattenedshards volcanic units (T3), andintrusionof the OjoBonito resur- in thin sections. The same unit in the stream valleysis gentdome(Tob).Thefirsttwounitshavebeentilted tothe moredistinctly an ash-flow tuff.Eutaxitic structure is north by intrusion of the resurgent dome. Postcollapse moreprominent,and therockcontains abundant (upto50 units dip gentlyand symmetricallyaway fromthe main percent)lithic fragments. ash-flow tuff to the east, west,andnorth. Collapseof the Asmany as four orfive distinct layers arepresenton Chinati caldera truncated the southernhalf of the Infier- the northeastern margin of the ash-flow tuff outcrop, nito caldera and presumablyconcealed it beneath lava where it is in contact with postcollapse volcanic units. flows that fill the Chinati caldera. Therefore, only the These thin ash-flow tuffs havemoderatelywelded bases northernhalfof theInfiernito calderaisexposed.Thecal- with poorlywelded tuffaceous tops and arelaterallycon- dera boundaryfault is buried beneath postcollapsevol- tinuous with the exposures on the massive cliffs. The canic units,but its inferred positionisshowninfigures 2 welding zonation and lateral continuity indicate that and 3. either the ash-flow tuff is asingle coolingunit composed of numerous individual flows or the less-welded layers wereminor ash-flow tuff eruptionsinthe latter stagesof Precollapse Volcanic Rocks caldera developmentthatare seenonlynear the topof the section. Alongits easternside, the ash-flow tuff outcrop The precollapseunits consist of abasal rhyolitelava displays steeply dipping (up to 45°) eutaxitic banding, flow overlain by a slightly porphyritic andesite flow partly caused by resurgence and possibly by a large (fig.6, Ti). They overlie, seeminglyconformably,Creta- amountofdifferential compaction.Theash-flow tuff may ceous and Permian sedimentaryrocks and are overlain havepondedagainstanabutmentorinadepression.This by the main ash-flow tuff. The precollapse units are mayhavebeen aprimaryorsecondarycollapsezoneasso- exposed in low dissected hills and dip 20 to 30° in a ciated withthe eruptionof theash-flowtuff. Theash-flow northerly direction. Exact thicknesses and precisecon- tuff beyondthe twohills inthe center ofthe volcanicpile tacts of theseflowswith theoverlyingunitand witheach is buried beneath youngerrocks of the postcollapseunit. other arenot clear because hydrothermalalterationhas Additional evidence for a caldera wall,revealed by destroyed primary structures and textures over large detailed mapping, includes discontinuous outcrops of areas.At leastsome of therhyoliteisintrusive.Otherout- Permianand Cretaceous rocks throughmore than90° of crops displayexcellent contorted flowbandingandtabu- arc around the northeast side of the proposedcaldera. lar subhorizontal autobreccia zones typicalof lavaflows. These scattered thin outcrops occur at or just inside the Several areas are light- to dark-gray perlite,but most limit of theash-flow tuff outcrop.However,these outcrops exposuresarelight-grayto white,fine-grained,aphyric, couldalsobeinterpretedaslargemegabrecciablocksthat and devitrified rhyolite.The andesite flow overlyingthe slumpedoff an outer calderawalland wereincorporated rhyolite flow is light gray to gray and contains about 3 in the ash-flow tuff, a less favored prospect because no percent white, chalky feldspar phenocrysts in a fine- large blocks of volcanic rocks are visible in the tuff. grainedmatrix.Flowstructures are rare,but the consis- Regardlessof the exact locationof the caldera wall,the tent appearance of the andesite between the rhyolite thick ash-flow tuff issignificantbecause itis theproduct below andtheash-flow tuffaboveindicates thatitisalava of a previouslyunknown major eruption. flow and not an intrusion.

Postcollapse Volcanic Rocks Ash-Flow Tuff A thick sequence of volcanic flows and sediments Eruptionof a thick (up to 300 m, 1,000 ft) lithic ash- unconformablyoverlies theash-flow tuff of theInfiernito flow tuff was responsiblefor the formation of the Infier- caldera and underlies the Perdiz Conglomerateor the nito caldera. Itis best exposedon a steep-sidedhill,re- Shely Group (fig. 6). We have included within this se- ferred to as Cerro Infiernito by local residents, in the quence onlyrocks geneticallyrelated to calderaactivity. centerof thecaldera.An equalthicknessof ash-flow tuff From oldest to youngest these strata are andesite por- is repeatedbynormalfaultingonthehill just tothenorth phyry, tuffaceous mud-flow breccia and agglomerate, (fig. 2). The ash-flow tuff is restricted to the two hills, and a thick sequence of rhyolitic lava flows with minor whereit forms steepcliffs andruggedterrane,andtotwo lenses of volcaniclastic sedimentsandmafic lavas(fig.6). north-trendingstreamvalleysimmediatelywestand east The andesiteporphyrycropsout adjacentto themain of thehills. ash-flow tuff. The generaloutcroppatternindicates that Rock on CerroInfiernito andonthehill to thenorth is it is dominantlya lava flow; however,the andesite por- densely welded but relativelyfeatureless anddifficult to phyry shows several different contact relationshipswith identify as an ash-flow tuff.In places,deep weathering theash-flow tuff.Inareasof low relief,the andesite por- has etched thesurface of some exposuresandbroughtout phyryoverlies theash-flow tuff.Where thecontactis well prominenteutaxitic texturesthatareotherwise toosubtle exposedin streamvalleys,itis commonlysteeplydipping toobserve. Compressedpumice and fine-grainedgrayto (60 to 75°); in places the andesite porphyry even dips red lithic fragmentsconstitute asmuch as 20 percentof underneath the ash-flow tuff.The varietyofrelationships therock.Inaddition,3 to 5percent feldsparphenocrysts suggests that the andesiteporphyryisin partintrusive. arepresentin adark-gray,fine-grained,densely welded Lithologicallysimilar andesite porphyry dikes upto20m

7 (65 ft)wide andmorethan1,500 m(5,000 ft)longcut over- where they aretruncated bythe Chinati caldera. On the lyingrocks. northeasternside of thecaldera theyrest directly onthe The andesiteporphyrycontains 20to 25 percentlarge flow-banded rhyoliteandmafic flow. Onthe easternside (5 to 15 mm) plagioclase phenocrysts in a gray fine- where the brecciapinchesout theylie ontop of andesite grained groundmass. In a few fresh outcrops, trace porphyry.Theflows vary innumber andcharacter indif- amountsof small (1to 2mm)biotite flakes can be seenin ferent partsof the caldera.Ingeneral,thelowest layersare the groundmass; however, the biotite commonly has rhyolites;flowshigherinthesection aremoreintermediate weathered tosmallbrown rustyspots.Noflow bandingor in composition.Inthe easternpartof thearea,atleasttwo flow textures areseen. Exposuresaregenerallymassive and up to five distinct flows occur beneath a cap of the with large rounded boulders that weather along Perdiz Conglomerate.Inthe westernpart of thearea,two fractures. main flow groups arepresent:alower rhyoliticgroupand Tuffaceous breccia and agglomerate overlie the an upper rhyodacitic group. These two flows underlie andesite porphyry (fig. 6). This unitis extremelythick plagioclasetrachytelava flows (Ts2) of the Shely Group (greaterthan 100 m, 330 ft)on the westernsideof thecal- (Amsbury, 1958). The different flow units,includingthe dera, is thinner in the northern part, andpinchesout on trachyte, appear to form a continuous compositionalse- theeasternsideof thecaldera.Itcontains avarietyofrock quenceandmaybegeneticallyrelated.Where outcropsof types.Sandstones and conglomeratesareobviouslywater the Shely units pinch out,the flows arecappedby Perdiz laid because stream channels can be seen in several Conglomerate. places.In other places,distinct sedimentarylayers,con- The lowermost flow inthe westernpartof thecaldera tainingboulders up to 1.5m(5 ft) indiameter, dipup to is amassive rhyoliteup to 100 m(330 ft) thick.It forms 45° off the andesite porphyry.These well-bedded layers largecliffs below the northern Shely Rimbut pinchesout are rare,but consistently dip away from the Ojo Bonito tothe south wheretheShelyGrouplies directlyonthetuf- intrusion. faceousbreccia unit. The steepnessand the abruptaspect The most common lithologiesare mud-flow breccia of the pinch-out suggest high viscosity. Thick layers of and agglomerate.These deposits contain fragments of black,glassyobsidianand gray perliteandminor amounts volcanic rocks thatrangefrom sand andgravelsizeup to (up to 10 percent)of devitrification spherulesare abun- 4.5 m (15 ft) in diameter.Boulders of flow-banded rhyo- dant in thelowest 10 m(33 ft)of the flow.Intheupper 90 lite are abundant and form resistantknobs where they percentof the unit,devitrification is more common, and weatheroutof thesofter yellow-browntuffaceousmatrix. mostof therock consistsoflayersofintergrowndevitrifi- Andesite porphyry and basalt fragments are locally cation spherules withno interstitial glass. The rock con- abundant. Layeringinthebreccia and agglomerateisnot tains minor amounts of biotite and 1to 5 percent white well defined, but theentire unitdips gently awayfrom chalky feldsparphenocrystsinagray-browntored-brown the hillsof ash-flow tuff andbeneathrhyoliticlavaflows. fine-grainedgroundmass.The individual flowbands and Thisrock unitwasformedfrom coarsepyroclasticdebris theoverall attitude of theunitindicate thatit dipsgently shed from the caldera walls and the resurgent central in a west-northwest direction. part of the caldera into amoatlike depressionthat was Overlyingflows arefoundbelow thePerdiz Conglom- deepeston the westernside. eratein thenorthwest part of thecaldera;theypinchout Rhyolitic andbasaltic volcanism was contemporane- to the south,butwhether this isdue to erosionor lack of ous with depositionof the breccia unit. In the northern depositionis not clear. They are generallyabout 50 to part of the caldera,athick moundof flow-banded rhyolite 60m(160 to 190 ft) thick, dipto the west,andconsist of is sandwiched between thin breccia layers.The rhyolite lamellar flow-banded, noncontorted rhyolitic to dacitic consists of contorted flow-banded obsidian, perlite,and lava.The grayto red-gray,fine-grainedgroundmass con- spheruliticallydevitrified rhyolite.Thin,irregularmafic tains minor amounts of biotiteand5 to 10 percentplagio- lava flows arealso found within thebreccia. The largest clase phenocrysts. Glassy layers and devitrification outcropof mafic flow is onthenorthernsideof thecaldera spherules are seen onlynear the base. and stretches for about 3km (1.8 mi).Itrests directlyon The easternpart of thecalderadisplays asequence of the flow-banded rhyolite and is overlain by the breccia volcanic rocks broadlysimilarto thatofthe westernhalf, unit. Smaller flows areperhapsremnantsof oncelarger, but the flows arethinner,less extensive,andmorenumer- more continuousflows that filled inlow spotsaround the ous. Thelowerlayers areflow-banded obsidian and devit- edges of the caldera. The flows consist of massive to rified or crystalline rhyolite.The upper units are more amygdaloidal,fine-grained,reddish-brown basalt. The intermediate and variable in composition,but commonly rhyolite and basalt flows indicate that volcanism was haveminor amountsof biotiteand 5 to 10 percentplagio- relativelycontinuous duringthe erosion andsedimenta- clase phenocrysts in a gray to red-gray, fine-grained tion that occurred after eruption of the ash-flow tuff. groundmass.Locallytheflows are steeply dipping(50° to Such flows may representring-fracturevolcanism. 60°) and intercalated with similarly inclined tuffs and A series of thick rhyolitic lava flows overlies the agglomerates.Numerous faults anddikes arealso appar- breccia-agglomerateunit. The flows extend from mid- ent. All the foregoing evidence points to many local way alongthe westernsideof the exposedpartof thecal- sources for the flows and pyroclastic debris in this area. dera, where they also abut agglomerates that filled in We interpretthepetrographicandstructuralevidence to againstthesouthernedgeoftherhyolites.Fromthere,the indicate thatthe flows result fromring-fracturevolcan- flows continue around to the eastern sideof the caldera ism alongthe edgeof the Infiernito caldera.

8 Ojo Bonito Pluton over much of the caldera cycle. Some of the rhyolitewas clearly emplaced as lava flows and some as intrusions. The largestintrusive bodyin the Infiernito caldera is However,contactrelationshipsarenot clearfor much of theOjoBonitopluton.Theplutoncropsoutonthesouthern- the flow-banded rhyolite, especially where it is exten- most exposedpartof thecaldera whereitabuts andiscut sively hydrothermallyaltered alongthe north-trending by the Chinati Mountains caldera (fig.2). The intrusion streamvalleyeastof CerroInfiernito. Mostof therhyolite mayhave been in the approximatecenterof the caldera in this area is probably intrusive and may have been beforeit was truncatedby the youngercaldera.Itforms a emplacedalongthe north-trendingnormal faults. roughly rectangular outcrop that is surrounded by sedimentary north, Permian rocks on the east,and west. Perdiz Conglomerate Rix(1953, p.131)called thisthe "OjoBonitolaccolith" and described it as "abiotite hornblende alkali granophyre." It consists about percent microcline, percent After igneous activity ceased, therocks of the Infier- of 35 30 product plagioclase,and 20 percent quartz; biotite, hornblende, nito caldera wereunroofed and eroded. The of episode is a conglomerate magnetite, augite, amounts accessory this of denudation boulder that and minor of exposedon Burro mapped minerals compose the remainder of the rock. is theCuesta del andhas been as the Perdiz Conglomerate.The Perdiz Conglomerate Little evidence indicates that the intrusion has the posteruptive of the a Instead, interpreted as a (Ramsey, 1961) records the erosion form of laccolith. it is here (Walton, 1979) and thus is resurgentdome for severalreasons:(1)the Permian and Chinati Mountains caldera Cretaceous sedimentary rocks, the precollapsevolcanic much younger than theInfiernito caldera. However,the conglomerateon delBurrois composeddominantly rocks, and the main ash-flow tuff dip steeply awayfrom Cuesta intrusion;(2) plutons the samerock type of rock fragmentsfrom theInfiernito caldera andcontains the small of cut Group. the ash-flow tuff; (3) petrographically few,ifany,clasts from theChinati Mountains The and similar dikes indicate either that cut the volcanic pileabove andform a patternradiating dominance of Infiernito clasts^could away part plu- the conglomeratein this area is older than the defined from thecentral of the caldera where the was entirely are petrographically Perdiz Conglomerateor thatits source almost ton is located. Most of the dikes conglomerate to porphyry in postcaldera within the Infiernito area. The on Cuesta similar the andesite the mappedas laterallycon- volcanics, afew havebiotite in acoarsergroundmass del Burro is Perdizbecauseit is but with Perdiz outcrops. and more closely resemble the rock of the Ojo Bonito tinuous known intrusion. Boulders from the overlying conglomerate display a range of textures from obvious "granitic"Ojo Shely Rim Area Bonito rock, through those with progressively finer groundmassand lessmegascopicbiotite, to thosethat are Shely Group identical to andesite porphyry specimens.This is interpreted to indicate that the intrusive andesite porphyry Shely Group is a sequence of volcaniclastic sedi- genetically althoughperhaps The anddikes are related to, not ments, rhyoliticash-flow tuffs, andrhyolitic to trachytic exactlycontemporaneouswith, theintrusion anddoming (Amsbury,1958) thatoverliepostcollapse Ojo emplacement. possible lava flows vol- associated withthe Bonito Itis canicunitsofthe Infiernito caldera (fig.6). Themajorout- came asingleparentmagmabut that therocks from were crop of the Shely Group is alonganorth-trendingridge, emplaced over aconsiderable time interval. The postcol- the Shely Rim, alongthe westernedgeof thecaldera (fig. lapsevolcanic unitsare youngerthan theresurgentdome 2). Amsbury eight distinct map but by of the dikes. observed and named are cut some as Tsl, fine- to coarse-grained tuff and rising by units follows: The resurgent dome was broken several conglomerate;Ts2, plagioclasetrachytelava flows;Ts3, normalfaults. One set trends roughlynorth, and asingle light-coloreddensely welded,slightlyporphyriticrhyolite north-trending fault trends due east.The set extendsinto, ash-flow tuff,overlainbydarkercoloredpoorlytodensely but apparentlydisappears within,the pluton,which was welded, fine-grained,slightlyporphyritic, rhyoliteash- probably liquid still at the time. Thefaults aremanifested flow tuff;Ts4, sedimentarybreccia; Tss, light-graytuff in the Permian and Cretaceous rocks as sharp flexures, and tuffaceous conglomeratewith local lenses of perlitic involvingas much asa 70°.changeinstrike.The eastern- flow; Ts6, fine-grained,lithic, densely welded ash- clearlydisplaces lava most fault observedalso lavaflows of the flow tuff; Ts7, very fine grained, slightly porphyritic, precollapsevolcanic unitsandmayhave served tolocalize densely welded, rhyoliteash-flow tuff; and TsB, dense to intrusionof flow-banded rhyoliteand hydrothermalfluids. pumiceous, flow-banded spheruliticrhyolite. Where we The east-trendingfault separatesthe twooutcrops of observed units Ts6, Ts7, and TsB, they consisted of two ash-flow tuff on CerroInfiernito and thehillto thenorth. densely tomoderatelywelded ash-flow tuffs (fig. 6).Also, actual fault has not identified, The trace been but its Tsldoesnotoccur inthe vicinityof theInfiernito caldera; effect on the ash-flow tuff is dramatic. The tuff onboth in this areaTs2 is the basal unit of the Shely Group. hills dips approximately15° north;the topof the unit on Cerro Infiernito is in line with the baseof the tuff onthe northern hill. This suggests about 300 m (1,000 ft) of Allen Intrusions displacement. Flow-banded rhyoliteforms additionalsmallintrusive Amsbury(1958)also mappedagroupof porphyriticto bodies. Petrographicallysimilar rhyolite was emplaced nonporphyriticrhyoliteintrusions,includingvitrophyre,

9 perlite,intrusivebreccia,andminorsedimentarybreccia volcanic and intrusive rocks from the area that encom- thathe named the Allen Intrusive Complex(fig.2). The passesmuch of theInfiernito caldera, the Shely Rim,the Allen Intrusions cropoutjust westof the Shely Rimand AllenIntrusions,andparts of the SierraViejatothe erup- justnorth of theChinaticaldera.Relationsbetweenindi- tion,collapse,andresurgenceof the "Shelycauldron."He vidual intrusions and country rock are complex, but recognizesneither the Infiernito caldera nor the relative Amsbury found at least five distinct rock types. Field timing of events in the volcanic history of the Chinati work during this project has shown thatthe AllenIntru- Mountains area as interpretedin this report.Therefore, sions areat leastinpartcontemporaneouswiththe Shely the conclusions reached in this reportand thoseof Cofer Group. For example,one of the intrusive bodies (Ta4 of (1980) are largelyincompatible. Amsbury, 1958) alongthe easternsideof the complexis laterallycontinuous into a perlitelava flow inunit Tss (unit Tsspof Amsbury)(fig.6).TheAllenIntrusions were Chinati Mountains Group apparently shallow bodiesproducingbothlavaflows and pyroclasticmaterial at thesame timethat theShelyGroup The Chinati Mountains Group is composed of more was erupted. than 1,000 m(3,300 ft)offlows and tuffs withintheChinati Mountains caldera (figs.2, 3).Stratigraphicnomenclature Relations to the Infiernito Caldera of previous works by Rix (1953), Amsbury (1958), and Cepeda(1977) is givenintable 1. This reportsummarizes The Shely Group is a significant accumulation of the stratigraphyas described by Cepeda(1977). volcanic material. However, its source areaand genetic relation toolder and younger volcanicrocksarenot clear. Rhyolitic flows of the postcollapsevolcanic units of the Table 1. Nomenclature of the Infiernito caldera gradeupwardinto moreintermediate Chinati Mountains Group. rocks and appear to form a continuous compositional sequencewith unitTs2,plagioclasetrachyte.Thisassoci- RIX AMSBURY CEPEDA ation suggests that Ts2is geneticallyrelated to the late (1953) (1958) (1977) Infiernito rocks; the complete sequence of rhyolitic to intermediate lavaflows could representsequentialerup- T8 soda rhyolite Tc6 rhyolite upper rhyolite tions from a zoned or evolvingmagma chamber. nonporphyritic The transition from intermediate lavaflows (Ts2) to domes and flows dominantlyrhyolitic sedi- ash-flow tuffs and tuffaceous T7 olivine augite Tcs trachyte upper trachyte TsB) a change style ments(Ts3 to represents marked in of trachyte eruption composition. Considerable local andesine and chemical rhyolite rhyolite rhyolite reliefexistsontheuppersurfaceofTs2,particularlyalong T6 soda Tc4 lower the southernpart of Shely Rim. Ash-flow tuffs of units T5 quartz trachyte Tc3 trachyte middle trachyte Ts3 and possibly Ts6 fill valleys cut into Ts2.The domi- Tc2 trachyte lower trachyte nantlyrhyolitic ash-flow tuffs and tuffaceous sediments Tel conglomerate basal conglomerate of the upper part of the Shely Group may represent renewal of explosivevolcanism inthe Infiernito caldera, initialphasesof pyroclasticactivityintheChinati caldera, Mitchell Mesa Rhyolite or eruption from an as yet unidentified volcanic center. Most evidence indicates that the pyroclasticrocks of the The Mitchell MesaRhyoliteis themost extensiveash- Shely Group arenotderived from theInfiernito caldera. flow tuffof theTrans-Pecosvolcanicfield.Eruptionof the Ash-flow tuffsof theShelyGroup,mostnotablyTs6, both Mitchell Mesa resulted in the formation of the Chinati thicken andincreaseinnumber of flowstoward thesouth Mountains caldera, althoughit is notpart of the Chinati (towardtheChinati caldera)rather thantowardtheInfier- Mountains Group (Cepeda, 1977). The Mitchell Mesais nito calderato the east. Also,conglomeratelenses within notexposedwithinthecaldera;however,athicksectionof Tsscontainabundant coarse clastsof theOjoBonitointru- the tuff may be exposedbeneath the lava flows of the sion. Much time must have elapsed between the initial Chinati Mountains Group(fig. 3).Collapseof thecaldera emplacementof the intrusionand its subsequentunroof- took placealongawell-defined zonethat isexposedalong ing, erosion, and deposition within Tss. Thus the Shely the northern,eastern, and southern sides of thecaldera Groupabove Ts2mayhavebeen depositedlongaftercessa- (fig.2).The calderahas beentruncatedby amajorBasin- tion of activity intheInfiernitocaldera.PossiblytheShely and-Rangenormalfault alongthe westside; the caldera Group and the associated Allen Intrusions representini- boundarythereisburied beneath approximately1,000 m tial local activity of the developingChinati Mountains (3,300 ft) of basin fill. caldera. Absolute ages tosubstantiate hypothesesarenot available,however,and theexactsignificanceof theShely Group remains unclear. Lower and Middle Trachytes

"Shely Cauldron" Both of these units formsmall outcropsnear themar- ginsof thecalderaandtogetherareapproximately250m Cofer (1980) proposedthatalargecalderaexistsinthe (850 ft) thick. They are porphyritic, containingup to 25 areanorthand westof theChinatiMountains.Herelates volume percent plagioclase, anorthoclase, and augite

10 phenocrysts in a fine-grained groundmass of feldspar, 180 m(600 ft)thick.Itcontains approximately10 percent opaques, and clinopyroxene.Magnetite, ilmenite, and resorbed quartz and 1 percent prismatic amphibole apatiteare common accessory minerals. phenocrysts inaddition to the anorthoclase phenocrysts. Aggregates of a dark to bright green, probably sodic, clinopyroxene, and granophyric-like textures are Lower Rhyolite commoninpartsoftheupperrhyolite.Theupperrhyolite is interpreted as a peralkalineash-flow tuff unit that The lower rhyoliteis themost widespreadunit of the accumulated predominantlyinits own collapse zoneand Chinati Mountains Group.Itrangesinthickness from 20 was the last major eruptive activity in the Chinati m(65 ft) at thenorthernmarginof thecaldera toamaxi- Mountains. mumof 415 m (1,350 ft) along the western edgeof the range.Itis characterized bylarge(up to1cm, 0.4 inch,in diameter)anorthoclase phenocrysts, whichconstitute up West Chinati Stock to 15 percentof therock. Quartz is the onlyother phenocryst andmakes up4 to 6 percentof therock.In the east- The WestChinati Stock,the largestintrusive bodyin abruptly by central part, the lowerrhyolite is cut off a theChinati Mountains (60 km2,20mi2,in area),is inter- series of nonporphyritic domes and flows (fig. 2). preted as a major resurgent dome. Two distinct rock Rounded cobbles of lower rhyolitewithin thenonporphy- typescompose the intrusivemass:adark plagioclase-rich ritic flows indicate that the lower rhyolite(andpossibly quartz monzodiorite outer shell at least 250 m (850 ft) the upper trachyte) is older than the nonporphyritic thick and a core of leucocratic porphyritic hornblende domes andflows. granite. Microgranite dikes and plugs up to several meters wide and hundreds of meters long intrude the quartz monzodiorite throughout its outcrop. The West Upper Trachyte Chinati Stock and ring-fracture intrusions along the southern boundary of the caldera are shown as Tic on The upper trachyte so closelyresembles the middle figure 2. trachyte that it is distinguished only where the lower rhyolite is present.Its thickness varies from more than 400m(1,300 ft)near ChinatiPeak to about20m(65 ft)in its easternmost exposure, where it hasbeen thinnedby Ring-Fracture Intrusions erosion. Two distinct textural variations are evident. Lower flows contain from 5 to15 percentfine (1to2mm) Several types of small intrusivebodies occur around plagioclasephenocrysts. Upper flows contain about 25 the marginsof the Chinaticaldera and areinterpretedto percent larger (0.3 to 1cm) phenocrysts. Vitrophyres, berelated toigneous activityalongthering-fracture zone. which are entirelylackingin the middle trachyte,occur Aphyric dikes and plugs with contorted flow banding at the baseof several flows inthelower partof the upper intrude the upper trachyte and the lower rhyoliteinthe trachytenorth of Chinati Peak. The vitrophyrescontain northern part of thecalderabut aretoo small to show on approximately10 percent plagioclaseup to 2mm long, figure 2. The dikes rangeup to 5m(17 ft) thick,and the from 2 to 3 percent augite,2 to 3 percent anorthoclase, plugs are asmuch as100 m(330 ft) wide. A bluecolor is and1to2percentgenerallyequantopaquephenocrystsin imparted to the plugs by small bluish-purple grains of amatrix of light-brownglass. amphibole,which suggests a peralkalineaffinity. Irregularlyshapedsills,dikes,andstocks occur along the southernmarginof theChinati Mountains westof the Nonporphyritic Domes and Flows townof Shafter.Theyoccur between volcanic rocksofthe Chinati Mountains Group and Permian strata, and within the Permian rocks. and hornblende rhyolitic form arcconcave Mica andesite Four flow-banded domes an porphyriesarethe most common rock types. to thesouthintheeasternpartof thecaldera.Flowsextend southfrom thearcof domes and areasmuch as50to60m (165 to 200 ft) thick. Field observations suggest that (1) the arcuate trend of domesprobably representsa ring- Geochemistry of the Chinati Mountains Group fracture zone on the boundaryof a secondary collapse zoneand that (2) collapse of this zone took placeinmore Cepeda(1977) used whole-rock and mineral chemical than one episode. data toshow that crystal fractionation wasan important mechanism in the chemical evolution of the extrusive rocks of the Chinati Mountains caldera.Mixingcalcula- Upper Rhyolite tions show that it is possible toproduce the morediffer- entiated,peralkalinemembersof thevolcanicseriesfrom The upper rhyolite is a porphyritic gray to green themostmafic member by removalof various amountsof lithic-crystal tuff with abundant (20 to25 percent)small phenocryst minerals. For example, calculations show (1to3mm)tabular anorthoclase phenocrysts.Exposures that 100 parts uppertrachyte will yield65 partsmiddle of this rock are largely restricted to a circular area trachyte, 38 parts lower rhyolite, 22 to 25 parts upper approximately5 km (3mi)indiameter inthecentralpart rhyolite,or 23 parts Mitchell Mesa Rhyoliteif variable of themountains,whereit isestimatedto begreater than percentages of plagioclase, olivine, augite, magnetite,

11 and anorthoclase phenocrysts are removed. A parent Shafter Area magma having the composition of the upper trachyte would yield an amount of peralkaline (upper) rhyolite The largest silver-producing area in Texas occurs approximatelyone fifth its originalamount.Actualper- westofShafter at thesouthern edgeof theChinatiMoun- centagesof minerals and mineraland rock composition tains (Ross, 1943). Base metal deposits (Ross, 1943) of arelisted in Cepeda(1977). sphalerite,galena,andrarenativeleadoccur withargen- The MitchellMesa wasprobablyanearlydifferentiate titeinveins andmantosinPermianandCretaceous lime- of the magma that produced the rocks of the Chinati stones along east-west-trendingnormal faults (fig. 2, Mountains Group.Eruptionof thealkali- andsilica-rich mines no.1and2).The ageof mineralizationhas notbeen Mitchell Mesa left amoremafic and alkali-poorresidual established but is mostlikelypost-early-Tertiarybecause magma, probablysimilar to the trachytesincomposition. igneous bodies of probable early Tertiaryage that cut Subsequent differentiation of the residual magma pro- Cretaceous strata show the effects of mineralization duced the lower andupper rhyolites. (Ross, 1943, p. 89). We believe that mineralization Trace-element data given in Cepeda and others (in resultedfrom hydrothermalactivitygeneratedby intru- press)also show consistent trends for volcanicrocksofthe sions alongthesouthernring-fracture zoneof thecaldera. Chinati Mountains.Uranium and thorium(andtoalesser The east-west-trending fractures, which commonly extent,molybdenum,lithium, andberyllium)tend to in- control oredistribution,parallelthe calderamarginand crease systematically from the oldest, least evolved are probablypartof the ring-fracture zone. parentalmaterial to the youngest,mostevolvedproducts Just west of Shafter, disseminated copper- of differentiationof the Chinati Mountains Group. The molybdenum mineralization occurs in intrusive rocks mostclear-cuttrendisfrom uppertrachytethroughflow- that may also be ring-fractureintrusions of thecaldera banded nonporphyritic rhyolite to upper rhyolite and (fig. 2, prospect no. 3) (McAnulty, 1976). The area has peralkaline dikes. Uranium, thorium, molybdenum, recently been explored, and unconfirmed reports lithium,and berylliumall increase; the highest concen- indicate that aminable deposit was discovered. trations areinthe vitrophyreof theupperrhyolite.Lower concentrations in some upper rhyolitesamplesprobably reflect postmagmatic processes. Cepeda and others (in San Antonio Canyon Area press) conclude that uranium concentrations in all the rocks of the Chinati Mountains Group are sufficiently Numerous prospectsand smallabandoned mine sites high to make them adequate source rocks for uranium occur in the SanAntonio Canyon areaalongthe western deposits. side of the Chinati Mountains (fig.2, prospectsno.4 and 5).Mineralization occurs dominantlyinthe West Chinati Stock. Two types of ore deposits arepresent(McAnulty, 1972): (1) lead, zinc, and silver minerals and fluorite in fissure veins,and(2)disseminated coppermineralization in a varietyof rock types around the marginof the West Complex Chinati Stock. Sporadic exploration and mining have Mineralizationin theChiantiCaldera occurred since the 1890's, and potentiallycommercial deposits of lead,zinc, silver, fluorspar,and copperexist. Mineralization in the Chinati Mountains areaisspa- The San Antonio Canyon areais an excellent exampleof tially andprobably geneticallyrelated to caldera struc- alteration andmineralization associated with resurgent tures, development,and activity.Theoretically,economic domingof a caldera. depositscould formin several stages of developmentofa caldera. The ring-fracture,or collapse,zone around the edgeof the calderaprovidesanideal pathwayfor percola- ChinatiMountains Group tion of hydrothermal fluids and subsequent mineral depositioneither in thefracture zoneitselfor inadjacent Fractionalcrystallizationof magma that formed the rocks. Also, ring fractures remain zones of weakness Chinati Mountains Grouphas producedlate-stagediffer- alongwhichlate-stage,trace-element-enriched magmatic entiates, particularlyperalkalinerhyolites,enriched in differentiates can intrude and createeconomicallyexploit- uranium,thorium, molybdenum,beryllium,andlithium able concentrations. A secondpotentialsite of mineraliza- (Cepedaandothers,in press).Devitrification and vapor- tion is in oradjacent to the resurgentdomeinthecentral phase crystallizationof the volatile-rich upper rhyolite partof acaldera. The most likelytypesof depositsinthis have released uranium, molybdenum, beryllium, environment arevein,disseminated porphyry,orcontact lithium,and fluorine,probablyas fluoride complexesin metasomatic deposits.Finally,mobilization andenrich- the vapor phase. Granophyric crystallizationresulting mentof minor elements could occur at several stagesin from slowcoolingof thick caldera-fillupperrhyolitehas caldera development,including (1) differentiation and released thorium inaddition to the other five elements, fractional crystallizationof plutonicigneousbodies;(2)de- probablyalso as fluoride complexesbut inamore hydro- vitrification and vapor-phase crystallizationof volcanic thermal phase.Cepedaandothers (inpress)conclude that rocks; and(3)low-temperatureweatheringor diagenesis hydrothermaluranium depositscould occurintheChinati of anyof theproductsofacaldera.TheChinati Mountains Mountains, particularlyas uraniferous fluorite deposits. areadisplaysexcellentexamplesof thesetypesofdeposits. However,no such depositshave been found.

12 AllenIntrusions have combined tocut deeplyintothe plutonanduncover its mineralized parts. Fracture zonesintheAllenIntrusions (fig.2,prospect Similarly,the ring-fracturezoneof theInfiernito cal- no. 6) contain uranium mineralization consistingof sec- derais not as wellexposedas thatof the Chinati caldera, ondaryuraniumminerals(autunite, metatorbernite,and presumably because it is covered by postcollapse,ring- tyuyamunite) and uraniferous iron-manganese oxyhy- fracture volcanic rocks. The ring-fracture zone of the droxides(Henry andothers,1980).Amsbury(1958)states Chinati caldera places volcanic rock directly against that200 tons of ore,averaging0.34 percent U3OB, were well-exposedPermian and Cretaceous stratathatarethe mined from a trenchalonga widefracture zone; the ore host rocks for vein and manto deposits at Shafter. The was stockpiled nearbybut never processed.Henry and ring-fracturezoneof theInfiernito caldera is not as well others (1980) reportU3OBconcentrations up to1,430 ppm exposed,andonlysmall outcrops of Permian andCreta- for samples from clay gouge alongthe trench,and they ceousrocks are seen. Thesesedimentaryunits doexistat list the Allen Intrusions as afavorable environment for depth,but theyarecoveredby thickrhyoliticandbasaltic the formation of uranium deposits.Theprobableoriginof flows. At onelocalityonthe eastsideof theOjoBonitoplu- the depositsconsistsof (1)release ofuranium fromrocks ton where the CieneguitaFormation of Permian age is of the Allen Intrusions themselves orof theShely Group, fairly well exposed, Rix (1953, p. 134) reports a (2) transportof uraniumingroundwater,and(3)adsorp- pyroxenitesillthat containsmolybdenite,pyrite,galena, tion of uraniumby iron-manganeseoxyhydroxidesalong and sphalerite.The relationshipof this sillto theInfier- the fracture zones. The secondary uranium minerals nito calderaisunknown,butitillustrates thepotentialfor result from supergene enrichment. It is possible that mineralization inandaround theInfiernito. Similarmin- small-scale,low-tonnagedepositsexistintheAllenIntru- eralization elsewhere around the edge of the Infiernito sions, butonthe basisof inspectionof diamonddrillcore, caldera couldbehiddenbeneathvolcanic rocks. Henryandothers(1980)concluded thaturanium concen- Only speculativeconclusions about mineralization in trations decrease with depthalong fractures. the Infiernito caldera are possible because only surface samplinghas been done.However, two environments in which potentiallyeconomic ore deposits could form are Infiernito Caldera (1) inthering-fracturezonearound the edgeof the caldera, where mantosand veins could replacecountryrock,such Abundant areasof hydrothermalalterationandpossi- as the mineralization at Shafter in the Chinati caldera; ble mineralization occur within rocks of the Infiernito and(2)as veinordisseminated depositswithin or adjacent caldera. The most promisingenvironmentsfor potential to the Ojo Bonito resurgent dome, which would be economic depositsare(1)inandaroundthe marginsof the analogousto mineralizationin the San Antonio Canyon OjoBonito plutonandin smallapophysesextendingfrom areaof the Chinati caldera. it(fig.2, prospectsno. 7 and 8); (2)inlargealtered areas within precollapsevolcanic units andflow-banded rhyolite(fig.2, prospectno. 9); and(3) inaltered areasassoci- ated with andesite porphyrybodies. Conclusions The most extensivehydrothermalalteration is associated with intrusionof the OjoBonitoplutonduringresur- The Chinati caldera complexconsists of at least two gentdoming,but additional alteration is associated with resurgentcalderas. The youngestandbestexposedis the the intrusive flow-banded rhyolites,many of which are Chinati Mountains caldera;its extrusiveproductsshowa highly altered. Abundant alteration occurs in the intru- systematicwhole-rock andtrace-element chemical varia- sion and inPermianrocks adjacentto it (fig.2, prospect tion that canbe related to fractional crystallizationof a no. 7). Two altered areasoccur inthe main ash-flow tuff parentmagma.TheInfiernito caldera, anolder and pre- whereit is intrudedbysmallapophysesofOjoBonito(fig. viouslyunidentified volcanic center, occurs north of and 2,prospectno.8).In someplaces,alteredareasareassoci- is cutby theChinati Mountainscaldera. Severalother se- ated withthe andesite porphyryunit thatmaybe geneti- quences of volcanic rocks found in the region cannotbe cally related to resurgence.Pyriteand other currently assigned definitelyto either source and may imply the unidentifiedsulfides are commonlyfound withinaltered presence of additional calderas. Mineralization in the rocks. Mineralization in the Ojo Bonito pluton may be Chinati Mountains areacan begeneticallyrelated to cal- analogous to mineralization in the West Chinati Stock. dera structures and activity.The best targetsfor further Mineralizationintheresurgentdomeof the ChinatiMoun- explorationare(1)inring-fracturezonesaroundtheedges tains is more abundant and well exposed, perhaps of the calderas, and (2) in and around the margins of because postcaldera faulting and subsequent erosion resurgentdomes of the calderas.

Acknowledgements

Much ofthe preliminaryworkonthis project wasdone undersubcontractno.78-215-E fromBendixFieldEngineering Corporation,Grand Junction Operations,under prime contract nos. E(O5-l)-1664 and DE-ACI3-76GJ01664 from the U.S. Departmentof Energy.The latter part of this project was completedunder grant no. G5194019 from the Texas MiningandMineralResources ResearchInstitute.Manythanks aredue thoselandowners intheChinatiMountains area who graciouslygranted access to their land.

13 The authors also gratefully acknowledgeRobert J. Finley, Bureau of Economic Geology, Fred W. McDowell, Departmentof GeologicalSciences, and Gary E.Smith, formerlyof the Bureauof EconomicGeology,whoreviewed the manuscript of this report. JudyP.Culwell designedthis publication;Micheline DavisandDavidRidner,under thedirectionofJamesW.Macon, drafted the illustrations. Typesettingwas byFannie Mac Sellingsloh,under the supervisionof Lucille Harrell.R.Marie Jones edited this report.

References

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