Mineralization in Silicic Calderas: Questa, New Mexico and the San Juan Mountains, Colorado, Taos, New Mexico to Lake City, Colorado, July 20-July 25, 1989 PDF
Preview Mineralization in Silicic Calderas: Questa, New Mexico and the San Juan Mountains, Colorado, Taos, New Mexico to Lake City, Colorado, July 20-July 25, 1989
10s0 104° 400 KILOMETERS 25 50 75 100 I I I I PM ~ oDenver Spin[ Glenwood ~ ~t'~ ~/~/ "f~ / t'(C' • ~.., ~", PM PM oPueblo PM -- --.. ---- PM -OTrinidad , ~ -'---"';"'-, ~- //Y'77A Ratono~ ............... PM SAN JUAN BASIN EXPLANATION MI~~~~oAr~~o~t~~~~~:m~;;':~~:~;~~~ng ;;'{;;. S SEDIMENTARYROCKSINRIOGRANDERIFT: MiocenetoRecent PM MIDADndLeEsiTteERtoTIrAhRyoYliVteOLCANICROCKS: INTRUSIVEROCKS:LaramidetolowerMiocene PALEOZOICANDMESOZOICSEDIMENTARY ROCKS:IncludessomelowerTertiaryrocks :.:_~'.'~ PRECAMBRIANROCKS PM TRENDOFCOlORADOMINERALBELTAND OFJEMEZZONE CALDERABOUNDARY * VOLCANICCONEORVENT:LateCenozoic Plate 1, Frontispiece: Index map of southern Rocky Mountains, showing location of volcanic t1elds and calderas in relation to major Laramide and Tertiary intrusions, and late Cenozoic extensional faults (from Lipman, 1983). Mineralization in Silicic Calderas: Questa, New Mexico and the San Juan Mountains, Colorado Taos, New Mexico to Lake City, Colorado July 20-July 25, t 989 Field Trip Guidebook T320 Leaders: w: Philip M. Bethke and Peter Lipman Associate Leaders: Paul B. Barton, Jr., Nora K. Foley, and David A. Sawyer Published 1989 by American Geophysical Union 2000 Florida Ave., N.W., Washington, D.C. 20009 ISBN: 0-87590-654-0 Printed in the United States of America TABLE OF CONTENTS DAY 1 QUESTA CALDERA AND ASSOCIATED PORPHYRY MOLYBDENUM MINERALIZATION, LATIR VOLCANIC FIELD, SANGRE DE CRISTO MOUNTAINS, AND RIO GRANDE RIFT , DAY 2 SAN JUAN MOUNTAINS, SOUTHEAST CALDERA COMPLEX AND ACID-SULFATE EPITHERMAL MINERALIZATION AT SUMMITVILLE 16 DAY 3 CENTRAL SAN JUAN CALDERA CLUSTER AND ADULARIA-SERICITE EPITHERMAL MINERALIZATION AT CREEDE 30 DAY 4 WESTERN SAN JUAN CALDERA COMPLEX AND MULTI- STAGE MINERALIZATION AROUND THE LAKE CITY CALDERA 53 REFERENCES · · ··· · · · · · 68 vii Leaders: Philip M. Bethke U.S. Geological Survey MS 959 National Center Reston, VA 22092 Peter W. Lipman U.S. Geological Survey MS 903 Denver Federal Center Denver, CO 80225 Associate Leaders: Paul B. Barton, Jr. and Nora K. Foley U.S. Geological Survey MS 959 National Center Reston, VA 22092 David A. Sawyer U.S. Geological Survey MS 903 Denver Federal Center Denver, CO 80225 ix Mineralization in Silicic Calderas: Questa, New Mexico and the San Juan Mountains, Colorado IGC FIELD TRIP T 320 MINERALIZATION IN SILICIC CALDERAS: QUESTA, NEW MEXICO, AND THE SAN JUAN MOUNTAINS, COLORADO 1 2 Philip M. Bethke and Peter W. Lipman DAY 1: QUESTA CALDERA AND ASSOCIATED PORPHYRY MOLYBDENUM MINERALIZATION, LATIR VOLCANIC FIELD, SANGRE DE CRISTO MOUNTAINS, AND RIO GRANDE RIFT GEOLOGIC OVERVIEW In many rocks shearing and recrystallization have outlasted the peak of regional metamor The narrow, rugged Sangre de Cristo Moun phism, and the rocks display retrograde tains extend more than 300 km from east of assemblages and cataclastic or mylonitic Santa Fe, New Mexico, to near Salida, Colora textures. These textures are most obvious in do. The range is bounded on the west and the granitic rocks and are best developed in southwest by the Espanola and San Luis Ba a 5- to 10-km-wide NE-trending zone centered sins, that are part of the Rio Grande Rift approximately on Hondo Canyon. This belt of zone (Plate 1, Frontispiece). The Sangre de cataclastic rocks may be a manifestation of Cristo Mountains owe their present relief the Jemez zone, a NE-trending zone of crustal largely to Neogene and Holocene uplift, but weakness of Precambrian ancestry that has also follow closely the east flank of a now been an important control on late Cenozoic largely foundered Laramide (Late Cretaceous volcanism and tectonism (Mayo, 1958; Lipman early Tertiary) uplift (Tweto, 1979). and Mehnert, 1979; Aldrich and Laughlin, The segment of the Sangre de Cristo Moun 1984). tains near Taos, known as the Taos Range, Lead-zircon dates from the Precambrian contains spectacular alpine topography in rocks (Bowring and others, 1984) indicate cludingWheeler Peak (4012 m), the highest that the age of the volcanic rocks is about peak in New Mexico. In the vicinity of the 1.75 Ga, and that all the intrusive rocks Questa caldera, the focus of the Day 1 trip, were emplaced between 1.75 and 1.64 Ga. the Taos Range is composed largely of Precam brian rocks partly mantled by mid-Tertiary Tertiary Rocks volcanic rocks of the Latir volcanic field and invaded by coeval and cogenetic granitic Structural and topographic relief in the rocks related to the caldera (Lipman, 1983; Taos Range provide a remarkable cross section Lipman and others, 1986). Topographic relief through the 26-Ma Questa caldera and cogenet between the highest summits and the floors of ic volcanic and plutonic rocks that make up major canyons where they empty into the San the Latir volcanic field (Lipman, 1983; Luis Valley is more than 1.5 km. Structural 1988). These rocks are the SE margin of a relief on the Precambrian surface between the regional composite volcanic field that blan crest of the range and the deepest part of keted much of the Southern Rocky Mountains in the rift graben near Questa may be as much as mid-Tertiary time (Steven, 1975). The Ter 6-8 km (Lipman and Mehnert, 1979). tiary igneous and sedimentary history of the Latir volcanic field is summarized in Table Pre-Tertiary Rocks 1-1, and representative chemical analyses of the volcanic rocks and associated intrusions Precambrian rocks that form the core of are given in Table 1-2. the Taos Range, are supracrustal rocks of The preca1dera setting in the Latir field volcanic, volcaniclastic, and sedimentary was a cluster of dominantly intermediate origins, and varied mafic, intermediate, and composition stratocones (Fig. 1-1), erupted felsic plutonic rocks, all of which have been at 28.5-27 Ma (Lipman and others, 1986). metamorphosed regionally to amphibolite grade Although largely eroded, remnants of an (Reed, 1984). These rocks have conspicuous ash-flow sheet of silicic alkalic rhyolite foliations, and many have distinct lineations (Amalia Tuff) ,and associated more mafic lavas produced by shearing and recrystallization. are preserved as far as 45 km beyond the source Questa caldera. Within the caldera, the tuff ponded to a thickness of several 1 U.S. Geological Survey, Reston, VA. kilometers and enclosed chaotic megabreccias 2 U.S. Geological Survey, Denver, co. that slumped from the caldera walls. At the T320: 1 Table 1-1: Generalized Tertiary igneous and stratigraphic sequence of the Latir volcanic field. (from Lipman, 1988). [Agesfrom K-Arandfission-track determinations: Lipmanetal. 1986] Unit Age (Ma) Si02content Compositionanddistinctivefeatures Rift-relatedsedimentaryandvolcanicrocks SantaFeGroup Sedimentsfrom Precambriansources 5-15 Silttofanglomerate Sedimentsfromvolcanicsources 16-22 Mostlyconglomerate Basalticlavaflows 15-16 44-51 Basanitetosilicicalkalicbasalt Intrusiverocksofthe Questamagmaticsystem Latemineralisedplutons Intrusionsalongthe RedRiver 22-23 64-77 Mainly aplite and granite porphyry; minor grano diorite,rhyoliteporphyry. Luceropluton 22 76 Medium-grainedcalc-alkalinegranite Resurgentintracalderaintrusions RitodelMediopluton 26 77 Medium-grained calc-alkaline granite; locally miarolitic CanadaPinabetepluton 26 73-77 Medium- to fine-grained calc-alkaline granite; local marginalperalkalinegranite VirginCanyonpluton 26 73.....77 Peralkaline granite at margins; calc-alkaline granite interior CabrestoLakepluton 26* 70-76 Biotite-hornblende calc-alkaline granite; locally more silicicnearmargins Southernbatholithicrocks RioHondopluton 26* 62-76 Porphyritic calc-alkaline granodiorite, locally grading intogranitenearroof AmaliaTuffandcogeneticlavaflows 26·5 76-78 Peralkaline rhyolite tuff and lava; phenocrysts of quartz, irridescentsanidine Precalderavolcanicrocks ComenditeofOrtizPeak about26·5 72 Thick flows and domes; large blocky K-feldspar phenocrysts LatirPeakQuartzLatite 27...:.28 65-66 Thick flows and domes; phenocrysts of plagioclase, biotite, andhornblende Andesiticanddaciticflows 26-28 57-64 Variable in composition and texture; from many clusteredcentralvolcanoes Volcaniclasticsedimentaryrocks 26-28 Mainly mudflow deposits, forming alluvial aprons aroundandesiticvolcanoes RhyolitictuffofTetillaPeakandrelatedlava 28 72-76 Weakly welded lithic-rich ash-flow tuff; associate lava domes;variableinthickness EarlyTertiarysedimentaryrocks Eocene(?) Discontinuous post-Laramide sediments; weakly in durated;locallycharacterisedbygreenishcobblesof quartziteinreddishmatrix *Estimatedage; discordantradiometricagesinterpretedasvariablyresetbylaterigneousevents. Table 1-2: Representative chemical analyses, Latir volcanic field and associated intrusions. (from Lipman, 1988). [Samples8, 12from Johnsonetal. 1986;sample 15from DilletandCzamanske 1987;othermajor-oxide analyses no. 1-2,5,7,9-10, 13-16, 19-20by"single-solution"method(USGSBull. 1401)byF. Brown;othersbyX-rayfluorescencebyJ. WahlbergandJ.Taggart;inweight%, calculatedto 100% volatile-free. U andTh, inppm, bydelayed-neutron radiochemistrybyH. T. Millard, Jr. Otherminorelements, inppm, byX-rayfluoresc~nce (Kevex) byR. Gordon andG. McGimsey] Number 1 2 3 4 5 6 7 8 9 Precalderavolcanicrocks Caldera-relatedvolcanics Tuffof LatirPk. Comendite Grano- Granite, Tetilla Andesite Dacite Quartz of Amalia Rhyolite diorite, Rio Unit Pe'ak flow flow Latite OrtizPeak Tuff flow RioHondo Hondo Sample 78L-144 78L-187 81L-47 81S-148 79L-70 82L-42D 78L-183 Q83J59 78L-126B Si02 74·0 61·6 63·8 64·9 70·3 78·0 76·8 67·6 76·9 Al203 14·2 14·7 16·2 15·5 14·9 11·3 12·1 15·3 12·5 FeO 2·3 5·6 5·3 5·1 3·8 2·0 1·7 3·4 1·2 MgOtot 0·44 3·9 2·4 2·3 <0·10 <0·10 0·13 1·8 0·29 CaO 0·45 4·8 3·4 4·2 0·49 0·15 0·03 3·1 0·65 Na20 3·2 4·1 3·9 3·6 3·6 3·6 4·2 4·5 3·8 K20 5·1 3·6 3·8 3·4 5·9 4·7 4·7 3·6 4·3 TiO., 0·18 0·87 0·71 0·64 0·43 0·11 0·14 0·60 0·20 P20~ 0·06 0·43 0·38 0·28 <0·10 <0·05 <0·05 0·26 0·04 MnO 0·13 .0·06 0·04 0·07 0·04 0·05 0·12 0·04 0·03 LOI* 1·2 1·3 3·2 0·59 0·66 0·64 0·50 0·77 0·71 U, 2·7 3·7 1·8 6·6 Th 6·4 13·9 10·2 24·9 Rb 124 70 80 59 94 124 161 71 156 Sr 341 1070 810 845 18 4 <5 695 138 Y 8 18 17 9 55 63 79 11 <5 Zr 132 174 194 131 835 310 462 130 72 Nb 9 10 11 5 29 37 46 15 14 -, Notdetermined. *,LossonIgnitionforXRFanalyses, orsumofvolatileconstituents(H20+, H20-,CO2)for"single-solution"analyses. T320: 2 time of ash-flow eruption, the volcanic field P°..-__ was being severely deformed along NW-trending JO' faults related to the Miocene Rio Grande rift; the most intense deformation occurred within the caldera. Strata were steeply tilted and locally overturned along presuma bly listric faults (Fig. 1-2). Ash-flow tuffs locally underwent secondary flowage over concurrently developing fault scarps and accumulated within structural basins as rheoignimbritic lava flows. Cogenetic batholithic granitic rocks, exposed over an area of 20 x 35 km, range from mesozonal granodiorite to epizonal porphyritic granite and aplite, with the shallower and more silicic phases most abun dant within the caldera. Compositionally and texturally distinct granitic phases define a highly evolved resurgent intrusion within the caldera, an incomplete ring dike along its southern margin, and a large mass of less fractionated granodiorite south of the cal KILOMETRES dera (Fig. 1-3). 5 10 15 I I I Compositions of the volcanic and plutonic 3360°''--------,----1.-- ---1...- -----6 phases (Table 1-2) changed from early calc alkaline metaluminous rocks to weakly peral Figure 1-1: Generalized area underlain by kaline silicic rhyolite and equivalent ac dominantly intermediate-composition precal mite-arfvedsonite granite at the time of dera rocks of the Latir volcanic field. caldera formation, then back to postcaldera Stipple: volcaniclasitic sedimentary rocks. calc-alkaline granitic rocks (Lipman, 1983; Ruled patterns: near-source lavas and brec Dillet and Czamanske, 1987; Johnson and cias. Solid circles: vent areas. Hachured Lipman, 1988; Johnson and others, 1989). line: topographic wall of Questa caldera Concentrations of alkalis and minor elements (dotted where concealed in Rio Grande rift). such as Rb, Th, Nb, Zr, and Yt reached maxima (from Lipman, 1988). when the caldera formed at 26 Ma, but the Table2(continued) 10 12 13 14 15 16 17 18 19 20 GraniticrocksoftheQuestamagmaticsystem Rift-relatedbasaltflows Granite, Peralk. Granite, Granite, Granite, Granite, Grano- Granite, Silicic Cabresto granite, Rito Sulphur Bear Red diorite, Lucero alkalic Basanite, Lake VirginCan. delMedio Gulch Canyon River RedRiver Peak basalt Amalia 78L-172 Q83J76 78L-114 79L-4 82QC8 78L-134 81L-48 80L-20 79L-93 79L-42 71·6 76·5 78·2 77·0 77·4 73·1 65·5 76·9 50·8 45·4 14·3 12·4 12·0 13·0 12·6 14·3 15·4 12·9 16·6 12·9 2·2 1·4 0·75 0·81 0·61 1·7 4·4 0·83 9·6 10·5 0·59 0·12 0·04 0·15 0·43 2·4 0·20 5·9 8·2 1·4 0·05 0·12 0·52 0·4 1·1 3·6 0·50 7·7 11·8 4·6 4·5 4·0 3·9 3·6 4·5 4·2 3·7 3·0 3·2 4·5 4·8 4·6 4·4 5·1 4·2 3·6 4·6 2·5 1·7 0·40 0·18 0·11 0·13 0·10 0·36 0·59 0·11 2·0 2·7 0·13 <0·05 0·02 0·02 <0·05 0·09 0·29 0·10 0·76 1·9 0·06 0·10 0·06 0·06 <0·02 0·04 0·05 0·04 0·13 0·17 0·68 0·20 0·44 0·31 0·95 0·73 0·70 0·15 2·5 5·3 3·6 3·2 4·7 10·6 13·1 4·9 2·7 10·6 0·93 2·2 12·3 15·6 23·2 34·5 29·1 16·1 10·2 33·8 2·3 6·6 102 130 181 206 222 119 75 188 38 18 299 18 <5 44 36 237 840 52 881 2010 18 54 14 5 8 14 11 3 19 18 197 380 78 71 51 169 122 78 226 211 31 38 42 29 30 31 9 24 25 65 T320: 3 W QUESTA CALDERA MORENO E 15,000 VALLEY RIFT 10,000 5000 o o -10,000 -5 tu -20,0'00 w u.. -30,000 -10 -40,000 -50,000 -15 -60,000 10KILOMETRES I EXPLANATION Sedimentaryrocks, Santa FeGroup Latir Peak Quartz Latite Graniticbatholithicrocks Andesitic lavaflows ~ Rhyolite ash-flowtuff (AmaliaTuff) Pre-Tertiaryrocks,mostlyPrecambrian Figure 1-2: Generalized E-W cross section through the Questa caldera, showing steep, chevron-style tilting of caldera-floor rocks away from caldera walls along listric style faults (from Lipman, 1988). NORTH SOUTH METERS 8000 QUESTA CALDERA 6000 Latir Mesa Tq 4000 2000 SEA LEVEL -2ooo---lo-------------"""":""'"------:S::------,-=-O-----,-S-----2-0-K-IL-O-M-e-r-Re-S---------...l..- I I I I Figure 1-3: N-S cross-section through theQuestacaldera andassociated subvolcanic batholith. Section drawn at 106030' (see Fig. 1-1), parallel to trendofRio Grande rift. Basal Tertiary surface rises to S, probablyowing to uplift overbatholith. Units: Ts, rift sedimentary rocks; Tgy, younger granitic intrusions (22-23 Ma) related to molybdenummineralization; Tgp, peralkaline granite crest andmargins of resurgent intrusions within caldera (chemically identical toAmaliaTuff) ; Tg, low silica granites within caldera, and granite cap ofgranodiorite (Tgd) outside caldera; Tgd, granodiorite at deeper levels ofbatholith; Tt, AmaliaTuff; Tq, precaldera lava dome complex on caldera floor; Ta, precalderaandesite; Pb, Precambrian metamorphic and granitic rocks. (From Lipman, 1988). major molybdenum mineralization of the Questa closely confined to the area of exposed district (Ishihara, 1967; Leonardson and granitic rocks; it also reflects boundaries others, 1983) took place while the late of the Questa caldera (Fig. 1-4). The gravity calc-alkaline granit ic ring intrusions anomaly is interpreted as defining the extent solidified at 22-23 Ma along the south margin of the underlying batholith, emplaced into of the caldera. lower parts of the volcanic sequence and A negative Bouguer gravity anomaly (Cor underlying Precambrian rocks. Paleomagnetic dell, 1978; Cordell, and others, 1986) is determinations also indicate that the T320: 4
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