Geology and Mineral Deposits of the Poncha Springs NE Quadrangle, Chaffee County,Colorado

By RALPH E. VAN ALSTINE

With a section on Fluorspar Mines and Prospects

By RALPH E. VAN ALSTINE and DOAK C. COX

GEOLOGICAL SURVEY PROFESSIONAL PAPER 626

Prepared in cooperation with the Colorado State Mining Industrial Development Board

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1969 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY William T. Pecora, Director

Library of Congress catalog-card: No. 70-602625

For sale by the Superintendent of Docume!lts, U:S. Government Printing Office Washington, "D.C. 20402 o/\ 7 2 3 6 56 3 0 9 -, CONTENTS

Page Page Abstract ______1 Economic geology----- ______------30 Introduction ______2 Fluorspar deposits ______------30 Earlier reports ______2 History of production and ownership _____ -_-_- 30 Field and laboratory work ______2 Localization and structure_- __ -- ___ ------30 Acknowledgments ______4 Mineralogy ______------:n Settlement and climate ______4 Paragenesis ______------_------34 Terrain ______4 Wallrock alteration ______--_---_- 34 Geologic units ______·- 4 Associated thermal · fluoride waters and the solubility of CaF2- ______- _- _____ ------34 Precambrian rocks ______5 Origin ______36 Metamorphic rocks ______5 Grade, size, and resources ______'- ____ _ a7 Banded gneiss ______·______5 Suggestions for prospecting_- ______--_---_- 38 Hornblende gneiss ______5 Origin ______6 Fluorspar mines and prospects, by Ralph E. Van Alstine and Doak C. Cox ______38 Igneous rocks ______6 Colorado-American mine ______-_- 38 Gneissic quartz monzonite ______6 39 Tabular intrusive rocks ______7 Delay adit and Lloyd shaft ____ --_-----_- Tertiary rocks ______Manganese Hill and Chimney Hill mines_- 41 9 Last Chance mine ______-_-_- 42 Rhyodacitic ash, volcanic mudflow deposit, and flow ______Other fluorspar deposits ______------42 9 43 Rhyolitic ash-flow tuff ______Deposits of other materials ______------11 43 Nathrop Volcanics ______Copper prospect ______-_------15 Gold ______44 Browns Canyon Formation ______18 Gravel and sand ______------45 Dry Union Formation ______20 Peat deposit ______------45 Quaternary deposits ______22 Pegmatite minerals ______------45 Pleistocene deposits ______22 Pumice and perlite ______------46 Holocene deposits ______25 Quartzite deposits ______------46 Structural geology ______25 Vermiculite deposit ______------47 Geomorphology ______28 References cit(:ld ______- _------47 Summary of geologic history ______29 IndeX------~------51

ILLUSTRATIONS

[Plates are in pocket] PLATE 1. Geologic map of the Poncha Springs NE quadrangle. 2. Geologic map of the Colorado-American fluorspar mine, Browns Canyon district. 3. Geologic map of underground workings, Colorado-American fluorspar mine. 4. Longitudinal section showing workings along East Vein, Colorado-American fluorspar mine. 5. Geologic maps of the Manganese Hill and Chimney Hill fluorspar deposits. 6. Geologic maps and vertical longitudinal projection of workings, Last Chance fluorspar mine. Page FIGURE 1. Index map of south-central Colorado, showing location of Poncha Springs NE quadrangle______3 2-10. Photographs: 2. Columnar jointing in welded tuff______12 3. ·Pronounced layering in devitrified rhyolitic welded tuff ______-_------12 4. View northwest toward Mount Princeton, showing Pleistocene gravel-covered surfaces __ -_------22 5. View northwest from Arkansas River, showing main fault zone______31 6. Fine-grained layered fluorite ______-----:.----- · 32 7. Nodular fluorite ______----- 32 8. Breccia fragment of welded tuff coated by psilomelane and fluorite ______-_-_-_------33 9. View northwest in open pit at Colorado-American fluorspar mine ______------39 10. Geologic maps and section of the Delay adit and Lloyd shaft ______-_------40 III IV CONTENTS TABLES

Page TABLE 1. Chemical composition of gneissic quartz monzonite, Poncha Springs NE quadrangle______6 2. Norm and mode of gneissic quartz monzonite, Poncha Springs NE quadr-angle______6 3. Chemical compositian and norm of rhyodacite porphyry, Poncha Springs NE quadrangle______10 4. Chemical compositions and norms of black vitrophyric welded tuff and devitrified welded tuffs, Chaffee and Park Counties ______. ______13 5. Sample data for potassium-argon age determinations, black vitrophyric welded tuff, Chaffee County______15 6. Chemical compositions and norms of obsidian pellets and perlite from R·uby Mountain, Chaffee County______16 7. Chemical analysis of spessartite garnet from Ruby Mountain, Chaffee County______17 8. Chemical and semiquantitative spectrographic analyses, gray rhyolite flow, Ruby Mountain, Chaffee County_ 18 9. Sample data for potassium-argon age determinations, Nathrop Volcanic~, Chaffee County______18 10. Pleistocene deposits, Poncha Springs N E quadrangle, Chaffee County______23 11. Semiquantitative spectrographic analysis of manganese oxide, Alderman fluorspar deposit, Browns Canyon district, Chaffee County ______-.- _ _ 33 12. Analyses of alkaline thermal waters from Browns Canyon and Poncha Springs fluorspar districts, Chaffee CountY------~----- 35 13. Analyses of alkaline, cold mine waters from western K~ntucky and St. Lawrence, Newfoundland, fluorspar districts------35 14. Analyses of typical ores of the Browns Canyon fluorspar district______37 15. Estimated fluorspar resources, Browns Canyon district, Chaffee County, expressed as short tons of crude fluorspar------38 16. Semiquantitative spectrographic analysis of aplitic gneiss, brecciated and veined by quartz and calcite ___ ..,____ 44 17. Gravel pits in the Poncha Springs NE quadrangle, Chaffee County______45 GEOLOGY AND MINERAL DEPOSITS OF THE PONCHA SPRINGS NE QUADRANGLE, .CHAFFEE COUNTY, COLORADO

By RALPH E. vAN ALsTINE

ABSTRACT mediately south, a sequence of nine gravel deposits was estab­ lished, five of Wisconsinan Age. and four older. A chevkinite­ The Poncha Springs NE quadrangle, which contains a major bearing rhyolitic volcanic ash in the thkd oldest gravel unit fluorspar district, is in the Arkansas Valley between the Sawatch may be the equivalent of the Pearlette Ash Member of the Sappa Range and the southward extension of the Park Range. Altitudes For:mation of late Kansan· age in Kansas and Nebraska. in the quadrangle lie between approximately 7,290 feet and Holocene deposits consist of alluvium, colluvium, talus, land­ 9,290 feet. slide debris, and peat. 'l'llie peat, determined to be less than 200 The geologic units mapped consist of Precambrian metamor­ years old by radiocarbon dating, contains excellently preserved phic and igneous rocks, Tertia·ry volcanic and sedimentary rock;s, pollen, and bison and horse bones. and unconsolidated Pleistocene and Holocene deposits. Pre­ The quadrangle is on the east :flank of the north-trending Sa­ cambrian hornblende gneiss and banded quartz-feldspar-'bioti.te watch anticline. The geologic ·Structure is widely varied and gneiss are intruded by a large mass of gneissic quartz monzo­ ranges from complex folds ~n the Precambriran rocks to a linear nite. Quartzite and silicated marble are interlayered with the fault trough of 'Tertia·ry age that parallels the Arkansas Valley. gneisses, which, together with other structural and mineralogic ·steeply dipping foliation in the Precambrian metamorphic rocks evidence, suggests ,that the metamorphic rocks were originally generally strikes northeast or east. The foliation in the gneis­ sediments. The gneissic quartz monzonite occupies a 1bout 25 sic quartz monzonite mass commonly is parallel .to that of the square miles of the quadrangle and is cut 'by ta:bular bodies of adjacent gneisses, although locally the mass is slightly d·iscord­ granite,. aplite, pegmatite, ·lamprophy.re, dacite .pol'!phyry, and ant The regional attitudes of several of the Tertiary units re­ d•iabase, and by quartz veins that locally contain orthoclase, . fleet downfaulting to the west, along the linear fault trough. A magnetite, specular hematite, py·rite, chalcopyrite, chlorite, and horst of Precambrian rocks overlain ·bY patches of Miocene silt- trace~S of purple fluorite. . stone extends northwest as a salient into the ·fault trough. The The Tertiary volcanic and sedimentary rocks immediately normal fault bounding the horst on the southwest has localized overlie the Precambrian rocks. Volcanic rocks in the southern major fluorspar deposits in the area. part of the quadrangle locally are more than 600 feet thick. 'r.he landscape is characterized by gently sloping surfaces at The volcanic •rocks consist chiefly of a lower unlit of ush, mud­ ten levels. The highest and earliest surface is a pediment of late flow deposit, and a rhyodacite porphyry flow of Eocene ( ?) age Tertiary age. Three intermediate surfaces are pediments of and an upper unit of ash-flow tuff that ·has !black vitrophyre near eady Pleistocene age, and six lQwer surfaces are on terraces of the base and a grayish-pink to reddish ... brown devitrifted tuff later Pleistocene age, Canyons or valleys bave been cut at in­ above. 'The ash-flow tuff· is early Oligocene in age, based upon tervaLs since the la.te Tertiary pediment formed. The Arkansas potassium-argon dating of the black vitrophyre and upon pollen River is superillllposed from upper Tertiary sediments onto the and spores found in tuff :immediately underlying it. hard rocks of the Browns Oanyon area. The river :flows for 9 A unit consisting of pyroclastic 'rocks, perlite, and a rhyolite miles through this canyon, a narrow gorge cut more than 500 flow that contains garnet and topaz, at the north edge of the feet deep in bedrock. quadrangle, is here named the Nathrop Volcanics. Dating by the· Epithermal deposits in the Browns Canyon district have potassium-argon method indicates that the perlite and rhyolite yielded about $5 million worth of commercial fluorspar. Between flow are late Oligocene in age. 1927 and 1949 about 130,000 tons of fluorspar concentrates was Tuffaceous siltstone forms seven small ma,sses resting di­ recovered from about twice this quantity of ore. Deposits of rectly on Precambrian rocks in the south-"Central pa'rt of the copper, gold, gravel and sand, peat, pegmatite minerals, pumice quadrangle. Thts unit, here called the Browns Oanyon Forma­ and perlite, quartzite, and vermiculite also have. been exploited tion, contains leaves und pollen comparable to the Oreede flora in the quadrangle, but in 1967 only peat was being extracted. and is Miocene in age. The main normal fault zone forming the southwest boundary Unconsolidated beds of clay, ·silt, sand, and gravel of the Dry of a horst is mineralized with fluorspar almost continuously for Union Formation underlie most of the western rpart of the nearly 3,000 feet. This fault zone, striking northwest between quadrangle. These sediments locally contain shards of yolcanic Precambrian rocks and Tertiary volcanic rocks, consists of two glass, bentonite, and rhyolitic tuff layers. Vertebrate fossils in­ nearly parallel branches along much of its length; the east dicate that the formation is Miocene and Pliocene in age. branch has yielded more fluorspar than the west branch. Pleistocene deposits mantle much of the Dry Union Formation · Although the veins consist largely of fluorite and micro­ in the western part of the quadrangle; the deposits formed as crystalline and chalcedonic quartz, some coarser grained quartz, outwash ·:Crom multiple stages of moun.ta.in glaciation in the opal, calcite, barite, pyrite, marcasite, black manganese oxides, Sawatch Range to the west. In this quadrangle and the one im- iron oxides, and clay minerals also are present. The fluorite is 1 2 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

mostly microgranular to fine grained and layered, and commonly EARLIER REPORTS has botryoidat, mammillary, and reniform surfaces and nodular structures. Pyrolusite, manganite, and psilomelane, identified Early geologic inform3Jtion, chiefly concerning the by X-ray and spectrographic studies, were deposited earlier geomorphology and the unconsolidated materials of the than, and contemporaneously with, the fluorite. Silicification upper Arkansas Valley, appears in various accounts of and fluoritization were the major types of wallrock alteration. the Hayden surveys (Hayden, 1869, p. 76-78; 1874, Moderately alkaline fluoride waters, probably ~argely meteoric p. 48-50; and. 1876, p. 51-53). Cross (1886, 1893, 1895) but with volcanic contributions, were found in fluorspar mine workings and nearby thermal. springs. Chemical analyses in­ presented some mineralogic, petrographic, and chemi­ dicated 13-15 parts per million fluoride, which is high for cal data on the Precambrian and Tertiary rocks of the thermal springs closely associated with epithermal mineral Salida area. Campbell ( 1922, p. 90-96 and sheet 3) de­ deposits. scribed the geomorphology of the Denver & Rio Grande The late Tertiary fluorspar deposits formed under near-sur­ Western Railroad route along the Arkansas River in face hot spring conditions from very dilute fluids; primary fluid inclusions in the fluorite are essentially fresh water. Study of Browns Canyon. Powers (1935) discussed the physio­ the inclusions indicates that the temperature of deposition was graphic history of the upper Arkansas Valley and de­ about 119°-168°C. The heat and fluoride are regarded as vol­ scribed terraces that extend into the quadrangle. The canic contributions to the mineralizing fluid. geologic map of Colorado (Burbank and others, 1935) Estimated fluorspar resources of the Browns Canyon district and Lovering and Goddard (1950, pl. 1) showed the total about two million short tons of crude material containing more than 15 percent CaF2. Ore bodies mined were less than· rocks of this area as Precambrian hornblende gneiss 10 feet to nearly 50 feet thick and ranged in grade from l~ss and undivided Tertiary volcanic rocks. than 25 percent to more than 75 percent CaF2. Similar fluorspar Cox (1945, p. 270-277) and Van Alstine (1947, p. resources may be found by exploring the known deposits further 460-463) briefly described some of the fluorspar depos­ and by prospecting along certain other faults that are unex­ its, and Bush (1951, p. 314, 326) gave information on plored or inadequately tested. Geochemical prospecting, as a part of this project, revealed abnormal fluorine concentrations pumice and perlite near Ruby Mountain at the north in residual soil and alluvium near a known vein. edge of the quadrangle. Honea (1955) also reported on the volcanic rocks of the Ruby Mountain area. Informa­ INTRODUCTION tion on the geology and mineral deposits of areas next The Poncha Springs NE quadrangle is at the east to this quadrangle is given in reports by Behre, Osborn, edge of the Colorado mineral belt (Tweto and Sims, and Rainwater (1936), Bhutta (1954), Crawford 1963, fig. 2) in the southern part of Chaffee County, cen­ (1913), DeVoto (1961), Dings and Robinson (1957), tral Colorado. The 71;2-minute quadrangle, about 6 miles Hanley, Heinrich, and Page (1950, p. 23-27), Lindgren north of Salida and 100 miles southwest of Denver, (1908), Russell (1950), and Tweto (1960a, p. 1420- covers nearly 60 square miles ofT. 15 S., Rs. 77 and 78 1422). FIELD AND LABORATORY WORK W., Sixth Principal Mer.idian, and T. 51 N., Rs. 8 and 9 E., New Mexico Principal Meridian (fig. 1). D. C. Cox, assisted at various times by J. 0. Fisher, The quadrangle is in the Southern Rocky Mountains, D. M. Henderson, an<}. T. A. Steven, spent about 10 near the junction of three major elongate ranges, and months from 1943 to ·i945 investigating the fluorspar just east of the Continental Divide. It is north of the · deposits and mapping the adjacent geology on enlarged Sangre de Cristo Range, east of the Sawatch Range, aerial photographs. A preliminary geologic map, cov­ which contains the highest peaks of Colorado, and west ering about 7 square miles of ·this quadrangle, was,com­ of the south-termination of the Mosquito or Park Range. piled on a planimetric base. The study was part of the The area is traversed by the Arkansas River, which flows U.S. Geological Survey's strategic minerals investiga­ south for about 9 miles through Browns Canyon, a nar­ tions and led to publications by Cox (1945) and Van row gorge cut in bedrock. The area west of the Arkansas Alstine (1947). Van Alstine spent about 2 months in the River is readily accessible from U.S. Highway 285, :fluorspar district in 1953 gathering additional data on Colorado State Highway 291, and Chaffee County the fluorspar deposits and reviewing and modifying the roads; east of the river, however, only the margins of earlier unpublished geologic maps. Later he studied the area are accessible by jeep. Elsewhere, service routes more than 100 1thin seotions of rocks from the quadran­ in San Isabel National Forest, east and west of the gle, and in 1958 he began a more comprehensive investi­ Arkansas Valley, provide access in the quadrangle. gation of the geology and mineral deposits and recorded This report describes the Precambrian, Tertiary, and the geology on a new topographic map. The present Pleistocene geology and the mineral deposits that were report combines data acquired by Van Alstine and by investigated. Special attention is given to deposits of Cox and assistants, who did most of the detailed map­ fluorspar, the most economically important mineral ping of the fluorspar mines in 1943-45 when they were product of the area. accessible and in oper3Jtion. INTRODUCTION 3

MIDDLE PARK

COLORADO

MAP LOCATION DENVER

oEagle

cKenosha Pass

oAspen SOUTH PARK

oColorado VOLCANIC Springs Cripple oGuffey oCreek FIELD

oGunnison

SAN JUAN

VOLCANIC

FIELD N LUIS VALLEY oCreede

0 5 10 20 30 MILES

FIGURE 1.-Index map of south-cent.ra.i Colorado, showing location of Poncha Springs NE quadrangle. 4 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO .

.Unless stated otherwise in the text, the rock-forming westward through· Balida, which became the starting minerals were identified with a flat-stage petrographic point for several narrow-gage branch lines. microscope; index oils and compositional curves were The quadrangle is still sparsely inhabited, mainly by employed. ranchers. Nathrop, situated along the Arkansas River ACKNOWLEDGMENTS and U.S. Highway 285 at the north edge of the area, The Colorado State Mining Industrial Development contains about 10 homes. Salida, the seat of Chaffee Board, the Colorado State Geological Survey Board, County, is 6 miles southeast of this area and has about and the Colorado State Metal Mining Fund partici­ 5,000 inhabitants. paJted in the financial support of the geologic work done Temperatures at Salida sinee 1902 have ranged from intermittently in this quadrangle since 1943. 100°F to minus 35°F, with an annual mean of 45.7°F Among the people who facilitated the fieldwork are (Waters, 1958, p. 410-412). The mean annual precipita­ Everett Cole and H. B. Stroup,· Colorado Fluorspar tion is 11 inches at Salida and less than 10 inches in the Corp.; S. F. Wickham, Colorado Fluorspar Mines, Inc.; arid intermontane basin between Salida and Buena S. H. Lloyd, American Fluorspar Corp. ; R. H. Dick­ Vista, 22 miles northwest. At Salida, about 1.5 inches son and W. J. Trepp, General Chemical Division, Allied of monthly precipitation falls in July and in August; . Chemical Corp.; P. L. Bancroft, U.S. Fluorspar & about 1 inch per month in March, April, May, June, Manganese, Inc.; F. A. Mansheim, Commercial Miner­ September, and October; and about 0.5 inch per month als, Inc.; and R. W. Kramer, Kramer Mines, Inc. in January, February, November, and December. The I am indebted to many professional colleagues in the average frost-free growing season is 111 days and U.S. Geological Survey for field discussions of geologic extends from May 30 to September 18. The irrigated, problems, for chemical and spectrographic analyses, fertile, broad benchlands west of the Arkansas River potassium-argon age determinations, for fluid inclusion and south of Nathrop provide the basis for agricultural, investigations, and for X-ray identification of some dairy, and livestock products. As a result of the arid minerals. Collaboration with M. G. Dings and C. T. climate, vegetation elsewhere is meager, except along Wrucke, U.S. Geological Survey, during their geologic the valleys and. north-facing slopes. investigations in the quadrangle bordering on the east, TERRAIN was especially helpful. Several U.S. Geological Survey geologists studied ·and identified the fossil collections~ Maximum relief is 2,000 feet; altitudes range from as follows: vertebrates, G. E. Lewis; plants, R. W. 7,290 feet, where the Arkansas River flows south from Brown and J. A. Wolfe; pollen, E. B. Leopold; and the area, to about 9,290 feet, at the northeast corner and silicified wood, R. A. Scott. I am particularly grateful to east margin of the quadrangle. In most of the area west Dwight R. Crandell, Gerald M. Richmond, and Glenn of the Arkansas River, surfaces on Pleistocene outwash R. Scott, all of the U.S. Geological Survey, for advice, and alluvial deposits rise gradually westward to an in the field, about the Pleistocene feaJtures. Mr. Rich­ altitude of about 8,460 feet at the margin of the quad­ mond determined part of the glacial chronology in the rangle. The Pleistocene deposits and older Tertiary quadrangle; his specific help and that of others are unconsolidated deposits are dissected by Holocene and acknowledged at appropriate places in the text. Pleistocene drainage. In the area east of the river, a The suggestions of R. A. Sheppard and T. A. Steven, highly dissected broad surface, developed chiefly upon U.S. Geological Survey, who reviewed the manuscript, Precambrian rocks, slopes gently westward toward are sincerely appreciated. Browns Canyon. This canyon is 400-600 feet deep throughout most o£ its length. SETTLEM,ENT AND CLIMATE GEOLOGIC UNITS In 1779, Juan Bautista de Anza and a small armed force of Spaniards were the first white men to enter Precambrian metamorphic and igneous rocks and this area (Waters, 1958, p. 714). Zebulon M. Pike. and Tertiary volcanic and sedimentary rocks form the bed­ his men, on an army expedition to the Arkansas Valley, rock exposed in the quadrangle (pl. 1). Locally, uncon­ South Park, and the ·san Luis Valley, spent Christmas solidated Pleistocene and Holocene deposits overlie of 1806 west of Salida. In 1860, placer gold was found these rocks. Paleozoic rocks crop out about 3,000 feet nearby in Chaffee County, and irrigation and farming. east of the quadrangle, and Mesozoic rocks have. been soon started. The Denver & Rio Grande Western Rail­ mapped about 20 miles southwest and northeast of the road in 1880 extended tracks from the Royal Gorge quadrangle (Burbank and others, 1935). . GEOLOGIC UNITS 5

PRECAMBRIAN ROCKS cline is unaltered. Biotite is partlyaltered to chlorite; METAMORPHIC ROCKS hornblende to chlorite, epidote, clinozoisite, and mag­ netite; and magnetite to hematite. Some specimens Precambrian metamorphic rocks occupy about 4 of gneiss contain a few grains of pyrite and microscopic square miles in the southeastern and south-central parts veinlets of oalcite. of the quadrangle and consist of banded quartz-feld­ Gray fine-grained banded m-arble forms thin con­ spar-biotite gneiss and hornblende gneiss. The map cordant layers in handed gneiss in a railroad cut along units shown on plate 1 represent the predominant rock the Arkansas River in SE1JiSW1,4 sec. 14, T. 51 N., R. type u.t any given locality; the two rock types intergrade 8 E. The marble layers are 2-8 inches thick and are in places and commonly are so thinly interlayered that traceable ·along strike for only a few feet. The marble they could not be differentiated at the present rna p scale. is composed of calcite, garnet, scapolite ( mizzonite) , The original thickness of the metamorphic rocks has not diopside, epidote, clinozoisite, and a few grains of micro­ been determined ·but seems to have been several thou­ cline, perthite, qua-vtz, biotite, and sphene. The silicate sands of feet. minerals occur as grains and clusters within and be­ BANDED GNEISS tween calcite grains. Most of the interlocking calcite The banded gneiss is a fine- to medium-grained foli­ grains have twin lamellae that are locally bent. Calcite ated rock that consists chiefly of quartz, plagioclase, also occurs as yery fine grained aggregates and as late­ microcline, and biotite. The color ranges from nearly formed veinlets transecting the si~icate mine-vals. white to almost black, depending upon the relative Skarn, in pendants in a gneissic qua-vtz monzonite amounts of light and dark minerals. Layers of a leu­ body, probably formed from similar calcareous layers. _cocratic or felsic quartz-rich variety are especially At a locality about 2 miles south of the marble layers in resist~ant to e~osion; locally, this variety is clearly a quartzite. the railroad cut described above, the skarn is composed mainly of garnet, epidote, quartz, and calcite; that at Thin sections of banded gneiss have characteristic ranges of mineral composition as follows: Approxi­ another locality, about three quarters of a mile north mately 30 to more than 90 percent quartz, a few percent of the cut, is associated with pegm·atite and is composed to 40 percent plagioclase, 5-25 percent microcline, and of garnet, epidote, quartz, calcite, scapolite (mizzonite), 0-15 percent biotite; hornblende, muscovite, and garnet green fibrous amphibole, and sphene. constitute a few percent of some specimens. Magnetite HORNBLENDE GNEISS apatite, and sphene are the common accessory minerals, and zircon is rare. Quartz occurs chiefly as rounded and The hornblende gneiss is a dark, foliated, and fine- to irregular grains that have wavy extinction; some are medium-grained rock containing 35-65 percent horn­ rimmed by chlorite, and others have sutured margins. blende, 25-35 percent plagioclase, as much as 10 percent The plagioclase ·is chiefly andesine. A dark, mafic each of biotite and strained quartz, and rare microcline, variety of banded gneiss contains a smaii quantity of orthoclase, diopside, and :andalusite; magnetite, apatite, labradorite and hornblende; in the leucocratic variety, and sphene are the abundant accessory minerals. Cor­ oligoclase and albite are the chief plagioclase minerals. dierite is a fairly common bUit easily overlooked constit­ Some of tlie feldspar grains have wavy extinction and uent in some specimens of the hornblende gneiss; it is granulated~ margins. locally twinned, crowded with inclusions, ·and partly The foliation is especially well shown by the arrange­ altered to pinite. Oriented flakes :and sm·all clusters of ment of flakes, clusters, lenses, wispy layers, and larger biotite define the foliation of the gneiss. In places, linea­ bands of the micaceous minerals. In places, lenses and tion is expressed by the parallel orientation of horn­ clots of biotite and muscovite poikilitically enclose blende. The plagioclase commonly is andesine, but lo­ quartz and plagioclase. Lineation locally is evidenced by the elongation of quartz or hornblende within the cally it is as calcic as bytownite. Some of the hornblende plane of foliation. Some of the strongly foliated banded and biotite is altered to chlorite, and the plagioclase is gneiss contains ptygmatic folds and highly crenulated commonly sericitized or saussuritized to aggregates of veinlets of quartz, plagioclase, and .biotite, especially sericite, clinozoisite, epidote, and a little calcite. Vein­ near a large intrusive body. The grains are equidi­ lets of epidote .and chl~rite are conspicu~us in many out­ mensional in some layers of banded gneiss, and the rock crops of hornblende gneiss. At the Ace High and Jack is a granulite. Pot copper prospects, the gneiss has been reorganized The plagiodase in some banded gneiss is partly into a magnesian skarnlike aggregate of silicate altered to sericite and clinozoisite, but adjacent micro- minerals. 6 GEOLOGY AND MINERALS, PONCHA SPRIN,GS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

ORIGIN A chemical analysis ( tahle 1) of the gneissic quartz The banded gneiss probably formed by moderate­ monzonite represents fresh rock from the Pmzle fluor­ to high-grade dynamothermal metamorphism of sedi­ spar mine along the east side of Chaffee County Road mentary rocks. Although nearly all sedimentary textures 60, in NWl/4 sec. 27, T. 51 N., R. 8 E. T3ible 2 gives and structures were destr.oyed during recrystallization,· the norm calculated from the chemical analysis and the the concordance of the thin-marble layers with rthe gneiss mode averaged from three thin sections. As the amounts a.I-e is ~garded as relict from the original sedimentary rocks. of quartz, potassium feldspar, and plagioclaSe nearly The local preponderance of quartz over other minerals, equal, the rock· is a quartz monzonite. The chemical differences in thickness of adjacent layers, and per­ analysis _and mode are strikingly similar to those of the sistence of thin layers along strike for hundreds of feet Browns Pass Quartz Monzonite (Barker and Brock;, 1965, especially sample Bf-20 of-truble 1}, which forms similarly suggest a sedimentary origin. j syntectonic stocks on the Continental Divide about 12 The hornblende gneiss was probably also a sedimen­ miles northwest of, and several thousand feet above, the tary rock, but original textures and structures were . Poncha Springs NE quadrangle. obliterated by metamorphism to the amphibolite facies. Layers commonly are structurally conform3ible with TABLE 1.-Chemical composition, in weight percent, of gneissic 'layering in the handed gneiss; although some l·ayers are quartz monzonite, Poncha Springs N E quadrangle thick ·and persistent, others are less than 6 inches thick [Rapid rock analysis by Paul Elmore, Samuel Botts, ancl Gillison Chloe, employing methods similar to those described by Shapiro and Brannock (1962). BeO and F and lenticular. analyzed by Jesse Warr, Jr. Minor elements determiried by semiquantitative spectrographic analysis, Ivan Barlow, analyst. Lab. No. 159507] IGNEOUB ROCKS MaJor oxides GNEISSIC' QU~TZ MONZONITE

Si02------69.9 NasO------2. 6 PtO&.------0.24 Gneissic quartz monzonite occupies ,a:bout 25 square AbOa------14.5 KtO------4. 9 MnO______.06 co,______.17 miles of the quadrangle and forms the southern part of a FeoFetOa------______1.81.0 HtO-----"------.31 HtO+------.83 F ------. f!1 MgO______• 53 TiOt------.62 large granitic m~ass that e~tends ·about 30 miles north­ cao______2. 3 TotaL _____ 99.83 ward ·along the Arkansas Valley, nearly to Leadville (Tweto and Sims, 1963, pl. 1}. The gneissic quartz mon­ Minor elements 1 Sn ______zonite is typically coarSe grained, gray to pink, foliated, Ag ______<0.00007 DY------<0.003 0. 0003 Ba______.1 Ga______. 0015 Sr ______.02 and porphyroblastic. Crystals of potassium feldspar Be ______.00015 La______;007 v ______.005 Ce______. 02 Nb______.001 y ______Yb ______.007 as much as 2 inches 'long are abundant; in ;a few places Co______. 0005 NL______.003 .0007 Cr ___ ------. 001 Pb_ ------. 007 Zr ______.1 these porphyroblasts are rounded and lenticular and ·cu______. 003 Sc______. 001 form augen. Commonly, the gneissic quartz monzonite 1 Elements looked for but not detected: As, Au, B, Bi, Cd, Er, Eu, Gd, Ge, Hf, weathers into topographic domes and large spheroidal Hg, Ho, In, Li, Lu, Mo, Nd, Pd, Pr, Pt, Re, Sb, Sm, Ta, Tb, Te, Th, Tl, Tm, U, boulders. · W,Zn. NOTE: Powder density by air pycnometer is 2.68. Thin sections indicate that the rock is hypidiomorphic ~ . granular, seriate, and porphyroblastic aild that it is TABLE 2.-Norm and mode of gneissic quartz monzonite, Poncha 1 composed of quartz (15-47 percent), m!crocline (10-40 Springs N E q1J,adrangle percent), orthoclase ( 0-15 percent), plagioclase ( 10-35 Norm (weight percent) percent), biotite ( 0-8 percent), hornblende ( 0-5 per­ Quartz ______29.8 Hypersthene______2. 8 cent) , and the accessory minerals ( 2-4 percent) magne­ Orthoclase _____ ------______28.9 Albite ______22. o ~J~~~::::::::::::::::::::::::: ~ tite, ilmenite, sphene, apatite, zircon, pyrite, and Anorthite ______: ______11.1 Fluorite______t.1 Corundum______. 8 fluorite. Qua!'ltz grains have wavy extinction and some TotaL ______98. 1 sutured margins; quartz· also occurs as blebs in feld­ spars and as rims around them. ·Much of the orthoclase Mode (volume percent) is perthitic, and some is intergrown with quartz, forming Quartz __ ------35 Biotite______5 Microcline-perthite ______30 Magn.etite-Umenite______1 micropegmatite. Microcline poikilitically encloses Andesine (Anu-•o) ------29 quartz, micropegmatite, and plagioclase; Plagioclase, 1 Calculated calcite molecule (0.4 percent) regarded as not forming part of the norm, which is partly sericitized and kaolinized, ~anges from as thin sections indicate calcite is a minor alteration-product. AU CaO was used to albite to andesine; oligoclase and andesine are most com­ calculate normative anorthite and fluorite. mon. Biotite is greenish brown, and hornblende is bluish Radiometric ages have been published for the gneissic green. Colorless garnet and muscovite were observed in quartz monzonite mass in localities· northeast and north several thin sections. Alteration minerals include seri­ of the Poncha Springs NE quadrangle. As reported hy cite, kaolinite, penninite chlorite, epidote, calcite, hema­ Grose and Hutchinson (1960, p. 157), biotite fr.om this tite, and limonite. mass, about 6 miles north of the Poncha Springs NE GEOLOGIC UNITS 7 quadrangle, near Trout Creek Pass along U.S. High­ small quantities of orthoclase, magnetite, specular hem­ ways 24 and 285, was dated by the potassium-argon a6te, martite, pyrite, chalcopyrite, chlorite, and method as being about 1.43 billion years old (Giffin traces of purple fluorite. These veins are especially and Kulp, 1960, p. 220-221). A determination by the abunda;nt in sec. 27 and the north half of sec. 34, T. 51 rubidium-strontium method, however, gave the age of N., R. 8 E., where they generally trend northwest in the same intrusive rock as 1.70 billion years (Hutchin­ the gneissic quartz monzonite ·and adj acep.t meta­ son and Hedge, 1967). Similarly, a r:ubidium-strontium morphic rocks. age determination reported for a whole-rock sample col­ The gneissic· quartz monzonite contains many large lected in the Arkansas Valley, from a roadcut 0. 7 mile and small xenoliths of metasedimentary rocks elon­ north of the intersection of U.S. Highway 24 and Colo­ gated and foliated parallel to the foliation in the igne­ rado Highway 82, near the north end of the quartz mon­ ous rock. Some large. pendants of banded gneiss are zonite mass suggests that it was emplaced albout 1.65 more resistant to erosion than the gneissic quartz mon­ billion years .ago, a time of widespread granitic intru­ zonite and stand out as knobs; two qu3Jrtzite deposits sion J.n the Sawatch Range (Wetherill and Bickford, in such pendants are described later. Inclusions of 1965). Comparison with granitic masses to the north­ black biotite-magnetite rock form knobs in SW·V:t sec. east, in or near the Front Range, indicates that these 20 and SElf:t sec. 19, T. 51 N., R. 9 .E. Thin-section ages are greater than those of the 'Pikes Peak Granite, studies indicate this rock contains about 85 percent bio­ which is about 1 billion years old (Aldrich ·and others, tite, 10 percent magnetite, and •a;bout 5 percent quartz, 1958, p. 1129-1130; Hutchinson, 1960, p. 180), and of sphene, apatite, ·and limonite. At a prospect in SW%, both the Silve~ Plume Granite and the granite near sec. 29, T. 15 S., R. 77 W., biotite in an inclusion of Kenosha Pass, which are 1.3 billion years old (Aldrich hornblende. gneiss is altered to vermiculite. and others, 1958, p. 1130; Hutchinson, 1960, p. 180). Dikes and sills pf granite, ·aplite, pegmatite, lampro­ The rubidium-strontium determinations on the gneissic phyre, dacite porphyry, and diabase, which ·are de­ quartz monzonite agree closely with ·the 1.73 ·billion­ scribed separately below, cut the gneissic quartz mon­ year age of the Boulder Creek Granite, which was de­ zoni•te. These tabul·ar intrusive rocks are considered to termined on zircon by T: W. Stern (U.S. Geological be Precambrian; they do not extend into the Paleozoic Survey, 1964, p. A95). The gneissic quartz monzonite rocks exposed less than a mile beyond the ~ast edge of may represent the widespread crystallization of granitic this quadrangle (C. T. Wrucke, oral commun., Aug. rocks in middle Precambrian time in Wyoming, Colo­ 1965). rado, New Mexico, and Arizona (Aldrich and others, TABULAR INTRPBIVE ROOKS 1957). Grani.te.-Several sm•all dikes of granite cut the The gneissic quartz monzonite is regarded as a syn­ gneissic quartz monzonite and in turn ·are cut by aplite kinematic mass that was emplaced during the period of dikes. The granite dikes ·are exposed at the east edge deformation when the adjacent rocks were folded and of the quadrangle, bet~een Sawmill Gulch •and Spring metamorphosed. The contacts •and foliation of the Gulch. They generally trend east-northeast, dip steeply, quartz monzonite mass comn1only ·are parallel to the and are foliated 'approximately parallel to the contacts. banded structure ·and foliation of the .adjacent gneisses, The dikes ·are ·apophyses of ·a larger body that crops although locally the mass is slightly discordant. The out a few hundred feet east of the· quadrangle edge south contact and foliation of the gneissic quartz mon­ ('C. T. Wrucke, oral commun., Aug. 1965). zonite at the north edge of Stafford Gulch, near the east The granite is medium grained, except for some side of the quadrangle, are parallel to the banding of coarse potassium feldspar crystals •and quartz !aggre­ _the adj•acent gneiss, which dips about 45° N. Near the gates, and consists chiefly of microcline, quartz, and south edge of the quadrangle and northwest of the plagioclase, and minor biotite, apatite, and magnetite. Arkansas River, the foliation dips synformally in The microcline is clear and poikilitically encloses gneissic quartz monzonite 1and in adj·acent septa of qu·artz blebs, plagioclase, 'and smaller microcline crys­ metamorphic rocks. Locally the igneous rock is grada­ tals. Strained quartz occurs as blebs, irregular patches, tional into the metamorphic rocks through a zone of and aggregates of grains with sutured edges. · The migmatite or injected gneiss as much as 10 feet thick. plagioclase is andesine, approxima~ly An35 ; most is Near its margin, the grain size of the gneissic quartz clouded by wisps of sericite and stained with hematite. monzonite commonly is finer and more uniform, biotite Greenish biotite is partly ·altered to chlorite; 'and mag­ content increases, and the foliation is more obvious. netite, to hematite. The rook is strongly foli3Jted ; large 'r.he gneissic qua;rtz monzonite is transected by many microcline crystals, biotite flakes, and plagioclase-rich breccia zones veined hy quartz or chalcedony containing layers especially show the parallel •alinement. Booa~ 8 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

plagioclase is much less abundant than microcline, the tion 1ootween 1.66 aind 1.68 ; Z is g.reenish brown, and rock is granite rather than quartz monzonite. X and Y are pale brown. The garnets weruther out of the Aplite.-Steeply dipping dikes and sills of fine- to gneiss and, accumulate locally at the surface along the medium-grained pink aplite cut the gneissic quartz north-facing slope. Crystals that have well-formed monzonite and generaHy crop out .as more resistant dodecahedral and trapezohedral faces are as much as 5 topographic features. The aplites are 'as much ·as 20 feet inches in diameter and resemble garnets found in thick, are cut by thin pegmatite dikes ·and quartz veins; metamorphic rocks next to pegmatites at two localities and are vaguely foliated parallel to their contacts. 2.75 and 3.75 miles south in the adjacent Poncha Springs Thin-section studies indicate that the 'aplite is com-, SE quadrangle. Garnet from the southernmost locality, posed chiefly of strained quartz (15-60 percent), mi­ at the old Sedalia copper mine, has been chemically crocline ( 15-55 percent) , sericitized plagioclase ( 5-20 analyzed and found to be almandite with a specific percent), biotite (1-5 percent) prurtly altered to pen­ gravity of 4.163 (Pentleld and Sperry, 1886, p. 310-311). ninite chlorite; -and muscovite (1-2 percent). The Lamprophyre.-Many dikes of dark-gray rto green­ plagioclase is albite-oligoClase, a more sodic feldspar ish-gray lamprophyre transect the gneissic quartz mon­ than that generally found in the ·adjacent gneissic zonite; one dike cuts the older banded gneiss unit about quartz monzonite. Minor miner:a;ls .are epidote, green 800 feet from the mass of gneissic quartz monzoni,te. hornblende, brown garnet, n1agnetite, ~patite, sphene, Several dikes have chilled borders, and none show zircon, rutile, calcite, hemaJtite, and limonite. m1arked effoots upon t:he waHrocks. They oommonly are Pegmatite.-Steeply dipping dikes and sills of less than 10 feet thick, but some are as much as 20 foot coarse-grained granitic pegmatite cut the gneissic thick; several dikes are more than 2 miles l01ig. The quartz monzonite ·and adj-acent gneisses. The pegma­ dikes generally strike east or northeast, roughly paralle] tites ·aTe commonly parallel to the foliation in the ad­ ~to the trend of folirution in the wallrock, but commonly jacentt rocks or transect it •at small angles. Pegmatites dip more steeply than the folirution. are ·abundant in the -areas of Precambrian rocks, and The lamprophyres are classed as vogesites or spessar­ only the larger bodies are shown on the map. The tites, as the main constituents are green hornblende and pegmatites range in size from stringers to several bodies or:thoclase or plagioclase, respectively. Porphyritic and more than 50 feet thick north of Cat Gulch at the east panidiomorphic textures are conspicuous; medium­ edge of the quadrangle. Pegmatites locally cut .aplite grained rto fine-grained phenocrysts are set in a fine­ dikes 1and ,are cut by quartz veins. grained to microcrystalline groundmass consisting The pegmatites generally are simple and unzoned; a largely of the same minerals. In most thin sections of few have cores of quartz and outer zones containing vogesite, minor plagioclase (albite to andesine) is small clusters of muscovite. Quartz and pink niicrocline zoned. The zoned plagioclase in the spessartite is ande­ consti1tute about 90 percent of most pegmrutLtes; graphic sine. Biotite and quartz rarely occur as phenocrysts but texture is locally present. Additional minerals are commonly are present in the groundmass. Apatite, albite, oligoclase, muscovite, biotite, magnetite, 'red­ sphene, and magnetite are abundant accessory minerals. brown garnet, black tourmaline, epidote, and very rare Calcite is found as veinlets, irregular patches, grains, green beryl. Much of the magnetite is altered to hema­ and rims around quartz inclusions; it is an alteration tite; a few unaltered octahedrons of magnetite are as product of hornblende and plagioclase. Other altera­ much as 1.5 inches across. Many of the pegmatites in the tion products are kaolinite and sericite formed from northern part of the quadrangle, between Cottonwood orthoclase; sericite, epidote, and clinozoisite, from and Middle Cottonwood Creeks, contain pale-rose plagioclase; chlorite, epidote, clinozoisirte, and magnet­ quartz and black tourmaline, especially in the core ite, from hornblende; chlorite, from bioti,tc; and hema­ zones. Some striated tourmaline crystals are 1 inch in tite, from magnetite. diameter and 4 inches long; many are fractured and cemented by quartz. Inclusions in several lamprophyre dikes consist of rounded quartz derived from the older Precambrian North of Cat Gulch, in .the west-central part of sec. 29, T. 51 N., R. 9 E., an east-trending pegmatite cuts rocks; some are nearly 2 em in diameter. The inclusions banded gneiss near the crest of a divide. North of the consist of grains and aggregates of strained quartz that pegmatite large red-brown garnets occur in a gneissic have embayed and corroded borders. Some of the quartz matrix of cummingtonite, labradorite (approximately grains are surrounded by quartz rims that have the same Anso), biotite, quartz, and minor magnetite, epidote, optic orientation as the grains; other grains are rimmed chlorite, and apatite. The cummingtonite is optically with calcite. In one thin section, a fractured pink garnet positive with a large optic angle and indices of refrac- inclusion is embayed by the groundmass; rimmed with GEOLOGIC UNITS 9 chlovite (penninite), calcite, and epidote; and cut by rocks crops out in sees. 22 and 23, T. 51 N., R. 8 E., in veinlets of chlorite and sericite. the south-central part of the quadrangle. The sequence Dacite porphyry.-At the east edge of the quadrangle, includes, from base to top: Rhyodacitic ash; volcanic SE~ sec. 19, and SW~ sec. 20, T. 51 N., R. 9 E., a mudflow deposit; rhyodacite porphyry flow; and an ash­ vertical dike of gray dacite porphyry trends eastward flow tuff consisting of minor non welded tuff at the base in gneissic quartz monzonite. The dike is about 15 feet overlain in turn by a . black obsidianlike vitrophyre thick and was traced for approximately 1 mile. The and red -brown and pink t ogray devitrified rhyolitic age of the dacite porphyry relative to other dikes in the welded tuff. The rhyodacitic rocks are considered to be Precambrian terrane is not known; it is considered Eocene ( ~), and the rhyolitic tuffs are Oligocene. Precambrian, but in mineral composition it resembles A younger sequence of volcanic rocks exposed in the the granodiorite or quartz monzonite of a Tertiary stock north-central part of the quadrangle, he:re named the exposed about 2 miles east of the quadrangle (Bhutta,, Nathrop Volcanics, consists of pumiceous tuff and brec­ 1954). cia, perlite, and a rhyolite flow. The perlite and flow are The rock consists of medium-grained phenocrysts of late Oligocene. zoned plagioclase, greenish-brown biotite, green horn­ Sedimentary rocks, dominantly- tuffaceous siltstone, blende, quartz, and rare brownish-green augite in a rest directly on Precambrian rocks in several small are-as fine-grained groundmass of these minerals, orthoclase, near the south -central part of the quadrangle~. These aprutite, and magnetite. The plagioclase is oligoclase­ rocks, here named the Browns Canyon Formation, are andesine (approximately An25-4o), as determined by regarded as Mioce1ie. measuring ex1tinction angles, and is slightly sericitized. The southwestern part of the quadrangle is covered Although the hornblende is almost completely altered to by essentially unconsolidated beds of clay, silt, sand, aggregates of chlorite, biotite, magnetite, and calcite, and gravel of the Dry Union Formation (Miocene and the euhedral shapes of original crystals are commonly Pliocene) . These sediments locally contain bentonite, retained. Other alteration products are epidote and shards of volcanic glass, and rhyolitic tuff layers. hemati~te. The qual'!tz grains are rounded and partly re­ sorbed and generally lack undulatory extinction. RHYODACDTIC ASH, VOLCANIC MUDFLOW DEPOSIT, AND FLOW Diabase.-Wi1thin the gneissic quartz monzoni~te Poorly consolidated rhyodacitic ash that typically is NW~ sec. 29, T. 51 N., R. 9 E., at the east edge of the unstratified and weathers to 'a greenish-gray soil covers quadrangle, a vertical black diabase dike trends N. 20° about half a square mile in three separate areas o,f out­ E. The dike is about 20 feet thick and was traced for crop in the south-central part of the quadrangle, west about 500 feet. Its· age is not definitely known; because of the Arkansas River. The ash rests unconformably on it is not markedly foliated and contains relatively un­ Precambrian rocks and almost everywhere is overlain altered augite and plagioclase compared with the other directly by a rhyodacite porphyry flow. Locally, rhyo­ types of dikes, the diabase is regarded as one of the litic tuff overlies the ash, or upper Tertiary sediments youngest dike rocks in the quadrangle. rest unconformably upon it. The ash is about 10 feet Microscopic study indicates that the diabase is thick west of Chaffee County Road 60 in sec. 27, T. 51 subophitic and fine grained, except for some plagioclase N., R. 8 E., 'and is estimated to be more than 100 feet crystals more than 1 mm long. The plagioclase is andes­ thick to the north, in sees. 14, 22, and 23, T. 51 N., R. 8 E. ine (approximately An40 ) , as determined by measuring About 12 feet of distinctly bedded white compact extinction angles. Augite is pinkish brown and not dis­ pumiceous argillized tuff dips 20° SW. in an out­ tinctly pleochroic; some grains are partly altered to crop northwest of the intersection of U.S. Highway chlorite, biotite, and calcite. Magnetite occurs as euhe­ 285 and Fourmile Creek, sec. 33, T. 51 N., R. 8 E. This dral crystals and smaller skeleton crystals within bedded tuff, a local deposit within the rhyodacitic ash, plagioclase and augite; it is accompanied locally by immediately underlies ashy material, lithic tuff contain­ irregular patches of pyrite. Apatite needles are com­ ing fragments of rhyodacite porphyry and devitrified mon, and grains of epidote occur sparingly. perlitic rhyodacite virtrophyre, and a younger rhyo­ dacite porphyry flow. The bedded white tuff is cut by TERTIARY ROCXS veinlets of chalcedonic silica, calcite, and gypsum. Tertiary volcanic and sedimentary rocks immediately Some of the rhyodacitic ash is altered to a waxy, overlie the Precambrian rocks in the Poncha Springs conchoidally fracturing bentonitic clay that disaggre­ NE quadrangle, although Paleozoic and Mesozoic rocks gates and swells rapidly in water. The predominant exposed in adjacent areas probably once covered this clay mineral has indices of refraction of 1.51-1.52 and quadrangle. An older sequence of Tertiary volcanic probably is montmorillonite. Locally the less altered 10 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

ash contains fragments of greenish glass that has an glass having an index of refraction of approximately index of refraction of 1.52 to 1.53, fresh plates of brown 1.52. The phenocrysts ·are biotite, zoned andesine con­ and green biotite, zoned oligoclase-andesine, and rare taining glass blebs, and pleochroic hypersthene that is euhedral quartz. The ash commonly is impregnated partly altered to chlorite and calcite. The accessory with calcite, aragonite, and yellow- to red-weathering minerals are magnetite, apatite, and zircon. Reddish and ankerite. bl;ack vitrophyre was found in the rhyodacitic volcanic A local volcanic mudflow deposit, mapped as part of rocks nearby, to the northeast in sees. 22 and 23. the rhyodacitic ash unit, is 15-20 feet thick, consists Field and thin-section studies first suggested the chiefly of pebbles and cdbbles of rhyodacite porphyry rhyodacite flow is an andesite, but chemical analysis embedded in greenish ash, and lies beneath large boul­ indicated it is more appropriately a rhyodacite ders of gneissic quartz monzonite embedded [n a brown­ (table 3). The analyzed sample was collected in NE% ish matrix in the NE%SE14 sec. 22, T. 51 N., R. 8 E. SE14 sec. 22, T. 51 N., R 8 E. Thin-section study of this Fragments of silicified wood collected at this locality by analyzed rock shows that the mode is approximately 60 D. C. Cox were identified hy R. W. Brown (written percent cryptocrystalline groundmass, 2 percent vesicles, commun., Sept. 22, 1954) as coniferous wood, prob:tbly 30 percent labradorite ( An55_65 ), 5 percent biotite, and Abies sp. Silicified wood, collected from the ash at five 3 percent hypersthene and accessory minerals. other local[ties by the writer, also was examined by R. TABLE 3.-Chemical composition and norm, in weight percent, W. Brown and R. A. Scott (written commun., Oct. 22, of rhyodacite porphyry, Poncha Springs N E quadrangle 1963), but provided no additional information. A sam­ [Rapid rock analysis by Paul Elmore, Samuel Botts, and Gillison Chloe, employing ple of ash was submitted to Estella B. Leopold, who re­ methods similar to those described by Shapiro and Brannock (1962). BeO and F analyzed by Jesse Warr, Jr. Minor elements, determined by semiquantitative ported (written commun., Apr. 27, 1964) that it spectr~graphic analysis, Ivan Barlow, analyst. Lab. No. 159508] contained no pollen or spores; similarly, G. W. Andrews Major oxides (oral commun., Mar. 9, 1965). found no diatoms in sam­ Na2o ______3.6 ples of the ash. Si02------60.5 P206------0. 34 AhOa.------18.9 K20._ ------3. 6 MnO______.06 The ash probably has a chemical composition similar Fe20a------4. 7 H20------1. 2 C02------<.05 FeO. _ ------. 37 H20+------.95 F ------.11 MgO______. 86 Ti02------. 79 to that of the rhyodacite porphyry flow that overlies it CaO. ------4. 4 TotaL _ __ _ 100. 38 in many places; the ash contains abundant pieces of rhyodacite porphyry, and the minerals and indices of Norm refraction of glass in the two units are similar. Evi­ Quartz ______16.1 Corundum______1. 8 Ilmenite. ____ ----- 0. 8 Orthoclase ______21. 1 Enstatite ______c 2.2 Hematite ______4.6 dently the ash is chiefly an early pyroclastic phase of Albite __ ------30.4 Apatite______. 7 Rutile______.4 Anorthite ______19.7 Fluorite______.1 the rhyodacite outburst. Total ______97.9 A rhyodacite porphyry flow, as much as 50 feet thick, is well exposed in the Elh sec. 33 and Wlh sec. 23, T. 51 Minor elements I Ga ______N., R. 8 E., in the south-central part of the quadrangle. Ag __ ------

Northeast of the fluorspar district, pinkish-gray to reddish-brown devitrified welded tuff overlies the black vitrophyre and commonly weathers to pinnacles. This is the thickest and most widespread part of the ash-flow tuff unit; the top is not exposed in the quadrangle, and an unknown thickness has been removed by erosion. Less thanlOO feet of devitrified welded tuff remains west of the Arkansas River in SW% sec. 23, T. 51 N., R. 8 E., but east of the Arkansas River in sees. 25 and 26, the devitrified tuff is more than 500 feet thick. Eutaxitic structure within the devitrified welded tuff is generally obvious in outcrops, except where the rock has been strongly silicified or otherwise altered. This streaky foliate structure, produced by the flattening of pumice fragments, is emphasized by alternating pinkish-gray and reddish-brown lenses of materials that have different degrees of porosity or devitrification. Some lenses 1are more than 2 inches rthick ,and ·about 1 :£oot long. Thin sections show that some lenticul'!llr ~and irregular cavities are lined with vapor-phase minerals and alteration products consisting of sanidine, tri­ dymite plates, quartz that contains chlorite plates, FIGURE 2.-Columnar jointing in black vitrophyric welded tuff, limonite-stained chalcedony, and a central filling of NIDl_4SEl4 sec. 22, T. 51 N., R. 8 E. Scale indicated by 12-foot I euhedral fluorite, barite, 'and 'a montmorillonite-type rod. Photograph by D. C. Cox. clay mineral. Layering resembling flow banding of lava is found less than 1 mm in diameter. Locally, veinlets and small, in a light-pink variety of welded tuff east of the irregular shaped oavirties con-tain calcite, chalcedony, Arkansas River, near the top of the thick section ex­ heulandite, ~ and limonitic day. Inclusions in the black posed on the divide north of Railroad Gulch. The layer­ vitrophyre consist of rhyodacite porphyry, silicified ing is most pronounced (fig. 3) about 50 feet above crystal 'tuff, and crushed pumice with glass shards elon­ and 400-500 feet southwest of an area where a small gated and squeezed between feldspar grains. The inclu­ inlier of Precambrian gneissic quartz monzonite is com­ sions are less than 1 inch in diameter; some have rims pletely rimmed by outward-dipping black vitrophyric of fibrous chalcedony. welded tuff (pl. 1). This layering may have been caused Mineralogical, textural, and chemical differences be­ tween the ash-flow tuffs from the two areas of volcanic rocks that are separated by about 1 mile of upfaulted Precambrian rocks (pl. 1) near rthe south edge of the quadrangle suggest that these ash flows may be different units. The ash-flow tuff in the area southwest of the fluorspar district lacks the persistent basal black vitro­ phyre of the ash-flow tuff to the northeast. An altered and largely devitrified purple porphyritic welded tuff, near the base of a 350-foot section of the southwestern rhyolitic welded tuff and faulted against Precambrian rocks on the 100-foot level of the Colorado-American fluorspar mine, contains unzoned sanidine and alb ire; the black vitrophyre to the northeast contains zoned sanidine and oligoclase-andesine. The feldspars in the purple welded tuff at the fluorspar deposit are partly re­ FIGURE 3.-PJ.'onounced layering in devit:rified J.'hyolitic welded placed by quartz and calcite, which also form many tuff, possibly the >resulit of mass flowage during weld,illlg. veinlets in the rock. NWl_4SWl_4 .sec. 25, T. 51 N., R. 8 E. GEOLOGIC UNITS 13 by mass flowage during welding of the ash flow (Smith, major oxides and minor elements (written communs., 1960, p. 151), which possibly occurred on ~a steep slope M. G. Dings, Apr. 8, 1963, and C. T. Wrucke, Feb. 6, of the buried topography or as a result of movement 1967). The analysis (table 4) is remarkably similar to along .the faults 1about 2,000 feet to the south ~and south­ that of the black vitrophyre of the Poncha Springs NE west. quadrangle, especially if the analysis of the vitrophyre The de vitrified welded tuff is composed chiefly of sani­ is recalculated on a water-free basis. dine, biotite, cristobalite, tridymite, magnetite, apatite, A chemical analysis (table 4) of a sample of devitri­ zircon, sphene, fluorite, and topaz in a microcrystalline fied welded tuff from the Ponoha Springs NE quad­ and locally glassy groundmass that is white, gray, yel­ rangle (SW1,4SW1,4 sec. 27, T. 51 N., R. 8 E.) indicates low, red, or brown in reflected light. Shard outlines are that it is calc-alkalic rhyolite. This analyzed rock is barely recognizable in much of the devitrified ground­ about 62 percent microcrystalline groundmass, 21 per­ mass. Sanidine crystals, some zoned, are as much as 8 cent quartz, 15 percent sanidine, 1 percent biotite, and mm long; some crystals are partly replaced by quartz, 1 percent iron-oxide minerals. The rock is somewhat sericite, chlorite, fluorite, calcite, hematite, and a clay altered in the fluorspar district, as shown by the mineral. Locally near the base of the devitrified welded presence of quartz in veinlets and in aggregates replac- · tuff, a small quantity of oligoclase-andesine accom­ ing some of the sanidine phenocrysts and by the unusu­ panies the sanidine. Biotite is pale brown, reddish ally high K20: N a 20 ratio. brown, and blackish brown and locally contains quartz along the cleavage or is partly replaced by quartz, TABLE 4.-Chemical compositions and norms of black vitrophyric welded tuff and devitrified welded tuffs, Chaffee and Park chlorite, magnetite, or hematite. Inclusions of gray ·Counties silicified tuff, rhyodacite, and Precambrian rocks gen­ . [Weight percent] evally ·rure Iess :than !all inch in di·ameter. The devitrified Black welded tuff commonly is silicified; near faults or veins, sample ______-,__ ~![g~fi~~ Devitrified welded tuff it may ·also be argillized, sericitized, chloritized, car­ Lab. No______159509 14166 159510 bonatized, pyritized, or fluoritized. The devitrified welded tuff and black vitrophyre are Chemical analyses considered different zones of ·a single genetic unit: both Si02------66.3 69. 35 72.7 :rocks have similar plagioclase and chemical composi­ Al203------15. 9 15. 16 13. 6 tions (table 4) ; plagioclase content decreases progres­ Fe20a __ ------_- _ _ _ _ 1. 2 2. 06 1.2 FeO______. 57 . 16 . 28 sively upw.ard within ·the devitri·fied rock; and the MgO ______- _ . 40 . 43 . 32 thin, black vitrophyre beneath the devitrified :rock is re­ CaO ______1. 3 1. 18 . 25 markedly persistent in areas both topographically high ~a20------3.4 3.51 2.0 and low at the time of emplacement. In SW·% sec. 23, K20------~-- __ ------6. 1 6. 14 8. 0 H2o-______. 41 . 46 . 48 T. 51 N., R. 8 E., the devitrified welded tuff is grayer H20+______2. 9 . 54 . 72 near the contact with the underlying black vitrophyric Ti02- _____ ----- ______- __ _ . 52 . 46 . 46 welded tuff; here it contains oligoclase-andesine ( ap­ P205------. 08 . 10 . 08 MnO______. 06 proximately AN25_ 45 ), which is essentially the same . 07 . 02 C02------<. 05 . 01 <. 05 composition as that of plagioclase in the underlying F ______- _-_ - ______- - . 08 . 10 . 04 vitrophyre. Within the devitrified welded tuff, the ratio Cl______. 03 of sanidine to plagioclase is approximately 3: 1 to 5: 1, SubtotaL______99. 76 whereas in the immediately underlying black vitro­ Less 0 ______-·- ______. 05 TotaL______99. 22 99. 71 100. 15 phyre it is approximately 1: 1 to 2: 1. Density t______2. 49 ______2. 62 The analyzed black vitrophyre collected from the center of a column (fig. 2) is about 66 percent glass,. Semiquantitative spectrographic analyses

17 percent sanidine, 14 percent andesine (about An40 ), Ag______< 0. 00007 2 0

TABLE 4.-Chemical compositions and norms of black vitrophyric The ash-flow tuff, including the hasal black vitro­ welded tuff and devitrified welded tuffs, Chaffee and Park phyre, is early Oligocene, based on paleobotanical evi­ Counties-Continued dence ~and potassium-·a:rgon dating. More :than 100 [Weight percent] specimens of dark- ·a:nd light-colored silicified wood were Black collected from unwelded tuff beneath the black vitro­ sample ______;J[3~ffu~ Devitrified welded tuff phyre at various localities in the quadrangle vicinity.

Lab. NO------159509 14166 159510 Some of the first-collected silicified wood specimens were identifjed as coniferous, probably Abies sp., by R. W. Semiquantitative spectrographic analyses Brown (written commun., Sept. 22, 1954). R. A. Scott examined the remainder of the specimens ahd reported Nd ______2 0 . 015 . 01 NL ______2 0 2 0 . 01 (written communs., May 18, 1960 and Oct. 22, 1963) Pb______. 002 . 005 . 001 that they consist of coniferous wood, poorly preserved Sc______. 0007 . 0007 . 0005 Sr ___· _____ :______. 1 . 07 . 05 owing to primary degradation and alteration accom­ panying silicifioation. Scdtt cut sections of the bebter y ______:002 . 003 . 0015 y ______;005/: preserved specimens and tentatively identified one col-· Yb ______. 003 . 002 Zr ______. 0003 . 0005 . 0002 lected near the west edge of sec. 22, T. 15 S., R. 77 W., . 1 • 03 .2 just east of the quadrangle. He stated in his 1960 Norms communication: The observable .features of this wood· are those of the Quartz ______.:______21. 0 22.8 29.5 Podocarpaceae, a family of conifers now chiefly found in the Orthoclase ______~______37. 8 36. 1 47. 3 Albite______29. 3 29.3 16. 8 Southern Hemisphere. The wood is tentatively identified as cf. Anorthite______6. 7 5. 6 1.4 Podocarpomylon. 'Vood of this type is known only from lower Corundum______1. 5 .8 1.1 Tertia·ry oockis in this oonntry. The tentative identifieatiiOn sug­ Enstatite______1. 1 1.0 .8 gests but does not confirm an early 'I'ertiary age for the source Ilmenite______. 9 .5 . 6 beds. Magnetite______. 5 Hematite______1. 0 2. 1 1: 3 Carbonized fossil pl,ant m·aterial also was found in Rutile ___. ______.2 . 2 the unwelded tuff at the base of this ash-flow tuff unit. Fluorite______. 2 .2 A specimen oollected 1at USGS paleobotany locality ' TotaL ______100. 0 98.6 99. 0 D3301, in the SW*SW%, sec. 23, T. 51 N., R. 8 E., along the southwest edge of Chaffee County Road 60, 1 Powder density by air pycnometer. 2· Element was looked for but not detected. Other elements looked for but not contains the following pollen and spores (Estella B. detected in all three rocks: As, Au, B, Bi, Cd, Col Eu, Ge, Hf, Hg, In, Li, Pd, Pr, Pt, Re, Sb, Sm, Sn, Ta, Te, Th, Tl, U, W, Zn. AlSo looked for but not detected in Leopold, written commun., Feb. 1, 1966): 159509 and 159510; Dy, Er, Gd, Ho, J,u, Tb. Tm. 159509. Black vitrophyric welded tuff; Poncha Springs NE quadrangle, Selaginella cf. S. densa Chaffee County; rapid rock analysis by Paul Elmore, Samuel Botts, and Gillison Lycopodium cf. L. complanatum Chloe; F analysis by Jesse Warr, Jr.; semiquantitative spectrographic analysis by Ivan Barlow. cf. L. cernum 14166. Devitrified welded tuff; Cameron .Mountain quadrangle, Park County; standard rock analysis by Ellen Daniels; semiquantitative spectrographic analysis cf. L. selago by J. C. Hamilton. . Pinus 159510. Devitrifled welded tuff; Poncha Springs NE quadrangle, Cha1fee County; rapid rock analysis by Paul Elmore, Samuel Botts, and Gillison Chloe; F analysis Pice a by Jesse Warr, Jr.; semiquantitative spectrographic analysis by Ivan Barlow. Ephedra cf. E. nevadensis cf. E. torreyana Comparison of the minor elements in the rhyodacite Abies cf. A. lasiocarpa porphyry flow (ttable 3) and in :a:sh-flow tuff (table 4) cf. Jumperus from the Poncha Springs NE quadrangle vicinity with Sequoia-Metasequoia group those in similar silicic volcanic rocks elsewhere (Coats, Zelkova-U7hnus 1956, p. 76; Rankama and Sahama, 1950; Lipman and Quercus Betula others, 1966, p. 32-33) indicates that all these local Ostrya-Oarpinus rocks are high in barium, cerium, lranrthanum, stron­ Alnus cf. A. japonioa tium, and zirconium; and the rhyodacitic flow is Juglans unusually high in lead. The local rocks are low in beryl­ Pterocarya lium, boron, and lithium; their fluorine content is Oarya cf. Engelhardtia either below the mean (820 ppm) for 167 samples of cf. Oonzattia silicic volcanic rocks of Cenozoic age in the Western Eucommia United States (Coats and others, 1963), or only slightly Aocording to Miss Leopold, the Sequoia-Metasequoia greater than this value. The maximum fluorine value group 1and iabout 36 percent of rthe identified vascul;a;r is 1,100 ppm for the sample of rhyodacite porphyry. groups are now exotic to the Rocky Mountain region; GEOLOGIC UNITS 15 these include Zelkova-Ulm1lt8 of the elm family, Jug­ locality to a point in the Antero quadrangle about 1 mile lans, Pterooarya, ~and Oarya of .the walnut f,amily, ·and from the northeast corner of .the Poncha Springs NE Euoorrvmia (Eucommiaceae). She stated that in com­ quadrangle. This site is just upslope from the place parison with the Creede flora, which has only 9 percent where the writer collected the silicified wood (p. 14) of its identified genera now exotic to the region (Steven that appeared to be of early Tertiary age. and Ratte, 1965, p. 47), this flora seems older. ·Recent The source of this ash-flow tuff is possibly close to .the potassium-argon data (T. A. Steven, written oommun., Poncha Springs NE quadrangle, where the basal vitro­ Feb. 1, 1966) suggest a late Oligocene or early Miocene pliyre and overlying devitrified welded tuff are thic~­ age for the Creede Formation. On the other hand, the est. Chapin and Epis (1964, pl. 1, p.· 146) regarded th1s flora of the Florissant Formation, which MacGinitie tuff, their ash flow 4, as having conie from the Thirty­ (1953, p. 75) considered early OligOcene, is distinctly nine Mile caldera, about 25 miles east of the Poncha older than the flora from the Poncha Springs NE Springs NE quadrangle. quadrangle. (E. B. Leopold, written commun., F~b. 1, NATHROP VOLCANICS 1966). Aooording to Miss Leopold, these comparisons Pumiceous tuff and breccia, perlite that contains black support an Oligocene age for this plant-bearing tuff in. obsidian pellets, and a rhyolite flow that contains small the Poncha Springs NE quadrangle. crystals of gem-quality ga,rnet and topaz (Pearl, 1958, Potassium-argon age det~rminations on sanidine and p. 70-73) comprise a volcanic sequence more than 500 biotite concentrates from the basal black vitrophyric feet thick along the Arkansas River near Ruby Moun­ welded tuff gave ages of about 35 million and 37 million tain at the north edge of the quadrangle. This lithologic years, respectively. The radiometric data for this unit is sufficiently distinct to warrant a separate rock­ chemically analyzed rock (.table 4 •and fig. 2) ·are given stratigraphic designation and is here called the Nathrop in ta;ble 5. Because of recent refinements in analytical Volcanics after the small community about half a mile techniques used in potassium-argon age determinations, to the west. The volcanic rocks rest directly on Precam­ these 1ages repl,ace the 34-+-3 million-year age previously brian gneissic quartz monzonite and are of late Oligo­ published for this rock (Van Alstine, 1965, p. D60). cene age. They are exposed chiefly in a small readily ac­ The ·rtadiometric 1ages indicate that the vitrophy,re cessible area, about one-quarter of a mile wide and 1.25 formed during early Oligocene according to the time miles long, in the Poncha Springs NE quadrangle and scales of Kulp (1961) and Holmes (1960), or during the Buena Vista quadrangle to the north. The type lo­ the Chadronian "Land-Mammal Age" (32-37 million cality is in ·sees. 11-13, T. 15 .S., R. 78 W. years ago), as proposed by Evernden, Savage, Curtis, These volcanic rocks formerly had a wider distribu­ and James (1964, p. 165, 167). tion but were partly removed by erosion; a much thinner

TABLE 5.-Sample data for potassium-argon age determinations, sequence of the same rocks is exposed i:I?- a narrow graben black vitrophyric welded tuff, Chaffee County in Precambrian gneissic quartz monzonite in NWl!.t, NWJ4 ·sec. 29, T. 15 S., R. 77 W., about 2.75 miles south­ K20 Radiogenic Radiogenic Mineral (weight Ar'o Ar'o Age (millions east of Ruby Mountain. In addition, similar rocks occur percent) of years) TotalAr'O on Bald Mountain about 1.5 miles northeast of Ruby Mountain in the Buena Vista quadrangle (Honea, Sanidlne. ------110.14 0. 00208 0. 92 35. 4=1. 1 Biotite __ ------______7. 94 . 00220 . 87 37. 3=1. 9 1955). . The pumiceous tuff is a white to tan, consolidated, t Average of 10.11 and 10.16. generally indistinctly bedded rock containing angular Decay constants for K'O: >.,=0.585Xlo-to yr-t; >-t~=4.72Xlo-to yr-t. Atomic abundance of K40=l.19X1D-'. Analysts: R. F. Marvin, H. H. Mehnert, Violet Merritt, Paul Elmore, and H. to subrounded unsorted fragments of tuff, glass, 1and, in Smith. U.S. Geol. Survey. some places, Precambrian rocks embedded in a matrix Collection site: NE~SE~ sec. 22, T. 51 N., R. 8 E. of pumice and shards. Some of the glass fragments are The radiometrically dated ash-flow tuff is apparently perlitic spherules and have an index of refraction of the same unit as the Aga.te Creek Formation of De Voto about 1.50; others are not perlitic and have an index of (1964, p. 119-121), which Chapin and Epis (1964, about 1.49. Thin-section studies indicate that some glass p. 148-151) and Dings (U.S. Geological Survey, 1964, fragments are partly devitrified to feldspar and cristo­ p. A97) indicated is older than the Antero Formation of halite and altered to a clay mineral. Fragments of Stark and others (1949, p. 63-68), dated by vertebrate sanidine, oligoclase, and quartz occur in sniall amounts; fossils as early Oligocene. DeVoto (1964, fig. 3) mapped other minerals are magnetite, altered partly to hematite, his Agate Creek Formation, also chiefly reddish devitri­ and a soft black manganese-oxide mineral. The pumice fied welded tuff containing black gl1assy porphy.ritic which has an index of refraction of about 1.50, is partly welded tuff near the base, intermittently from his type altered to feldspar and cristobalite, but cellular and 16 _GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO •. filamentous structures are still recognizable. In some TABLE 6.-Chemical compositions and norms of obsidian pellets and~ specimens the vesicles contain chlorite, calcite, or opal. perlite from Ruby Mountain, Chaffee County 1 · Breccia is present locally within the pumiceous tuff and [Weight percent] is especially conspicuous near the top, where angular Sample______Obsidian Perlite Lab. No______100414 · 100415 blocks of pumice a:s much as 4 feet across are enclosed in pink hematite-stained, partly a.rgillized material; Chemical analyses most of the pumice fragments, however, are less than 2 [Standard rock analyses by E. L. Munson] inches in diameter. Pumiceous breccia is about. 20 feet Si0 ______2 76. 57 74. 34 thick at the east side of Ruby Mountain and overlies AJ203------­ 12. 78 12. 55 about 70 feet of light-colored pumiceous tuff. Fe203------. 35 . 44 FeO ______. 33 . 13 About 110 feet of west-dipping perlite lies above the ~go ______. 05 . 05 pumiceous tuff and breccia along the east-facing slope of Cao ______. 45 . 40 Ruby Mountain. The perlite is a light-gray, brownish­ ~a20------­ 4. 31 3. 97 gray, dark-gray, and black isotropic glass that has K2 0------·------4. 54 4. 72 H2 0 + ___ ------. 22 2. 83 shelly perlitic structure :and flare spherulitic structure. H20- ___ -_------. 05 . 13 Thin-section studies indicate ·that some of the perlite Ti0 ______2 . 08 . 07 contains clay _and hematite along the perlitic structure . 00 . 00 P20s------~n0 ______.and 1oc;ally has chlorite in vesicles. A few phenJOcrysts of . 01 . 01 01------. 07 . 05 quartz, sanidine, and oligoclase were found; the mar­ F ______~------. 18 . 23 gins of the feldspars bear evidence of partial resorption. Subtotal ______10Q 08 100. 01 Much of the perlite is light gray and contains abundant Less 0 ______. 10 .11 hard shiny pellets of black glass. These obsidian pellets TotaL ______.______99. 98 99. 90 are 1-12 mm in diameter and have an index of refraction of about 1.49, in contrast with the 1.50 index of the sur­ Semiquantitative spectrographic analyses rounding perlite. The black glass conta.ins microlites of [Analyst: Harriet Neiman] feldspar, very rare biotite, and sufficient magnetite dust BeBa------~------______0. 1 0. 15 to make rthe pellets slightly magnetic. .Such obsidian . 0001 2 0 CD------~------. 0005 . 0015 pellets, known to collectors as "Apache Tears," are more Cr ______. 0003 . 0005 appropriately called marekanites. Ross and Smith CU-----~------. 0005 . 003 ( 1955) studied marekanites from several other localities Ga ______. 0015 . 0015 ~b ______and found that the obsidian cores surrounded by concen­ 20 . 001 ~i ______. 0003 . 0007 trically cracked perlite contain less than 1 percent water, Pb ______. 002 . 001 that the perlite conta.ins 2-5 percent water, and that . Sc ______20 . 001 the obsidian consistently has a lower index of refrac­ Sr ______. 05 . ot tion than the corresponding perlite. They concluded that v______. 005 . 015 y ______20 . 002 the entire mass was originally obsidian and that the per­ Yb ______20 . 0002 lite formed by hydration after emplacement of the Zr ______. 007 . 007 original glass. Norms Chemical analyses of obsidian pellets and perlite, col­ lected at the north end ·of Ruby Mountain by R. L. Quartz ______------33. 5 33. 6 Orthoclase ______-_---_____ 27. 2 28. 9 Smith and the writer, are given in table 6. Comparisons Albite ______-_____ 36. 2 34. 6 of the chemical analyses with the norms show that the ' Anorthite______1. 5 1.1 Corundum______. 2 .4 change from obsidian to perlite results chiefly in in­ creases in water and K 20, decreases in N3.:!0 and CaO, Ferrosilite ______----- __ -- _ . 1 ~agnetite______. 7 and a change from magnetite to hematite; similar chem­ Hematite ______~_ .5 ical changes in other hydrated ·silicic glasses were re­ Fluorite ______------_ . 4 .5 ported by Lipman (1965) and Truesdell (1966). The TotaL______99. 8 99. 6 fluorine and normative fluorite values for the obsidian pellets and perlite are about twice the maximums of cor­ 1 Locality: Poncha Springs NE quadrangle, north end of Ruby Mountain, 1 responding ~alues for analyzed samples from the older N~:M~:e~ ~~s ~oi'~l~o~··b:t ~~tdetected. Other elements looked for but not detected-are Ag, As, Au, B, Bi, Cd, Ce, Ge, Hf, Hg, In, La, Li, Mo, Pd, Pt, Re, Sb, Tertiary volcanic rocks (tables 3 and 4) . Sn, Ta, Te, Th, Tl, U, W, Zn. GEOLOGIC UNITS 17

, A steeply west-dipping flow of gray rhyolite forms forms recognized are prisms, pyramids, basal pinacoids, the upper pa~t and west face of Ruby Mountain, which macrodomes and brachydomes. Sinkankas ( 1959, p. rises about 400 feet above the Arkansas River. The 102) stated 'that this topaz has a specific gravity of rhyolite overlies the perlite and is about 300 feet thick. 3.567 and that some of the largest crystals can be cut The rock is pinkish when wet and locally is stained with into small brilliant gems. manganese oxide; generally it has very conspicuous Very small crystals of glassy sanidine are especially flow .J,ayering :and 1a par·allel pl,aty structure. Commonly, abundant in the lithophysal cavities. The .Y index of the rhyolite is spherulitic, lithophysal, or vesicular. The refraction is about 1.525, which ~s thrut of 'a ·soda sanidine spherulites 'rtange f·rom less than 0.1 inch to 3 inches in containing about 30 percent Ab (Troger, 1952, p. 96). diameter; the largest, near the top of Ruby Mountain, Silica minerals, magnetite, and hematite also are consist chiefly of quartz and feldspar. Lithophysae, as common in the cavities of the rhyolite. Quartz forms much as 2 inches in diameter, are composed mainly of slender prisms, both singly and doubly terminated, that sanidine shells coated with quartz. Elliptical and irregu­ rest on garnet or are intergrown with it. According to lar vesicles generally are less than 1 inch in longest Rogers and Cahn (1937), minute prismatic quartz dimension. crystals from Ruby Mountain have prominent pina­ Minerals in the vesicles and lithophysae of the rhyo­ coidal faces, one of the very few authentic occurrences lite include garnet, rtopaz, sanidine, quartz, tridymite, of the pinacoid on quartz. Other silica minerals found opal, calcite, magnetite, and hematite. The presence of in the rhyolite are pla:ty tridymite; hard, bluish, iso­ garnet and topaz was first reported by Smith ( 1883, p. tropic opal; and fibrous chalcedony. Magnetite occurs 32) . The garnet occurs as transparent, glassy, red-brown as tiny octa,hedrons, and hematite as six-sided flat to cinnamon-brown crystals, generally less than 0.25 rhombs. inch in diameter, and it evidently was mistaken for the The rhyolite flow consists of sparse phenocrysts of ruby variety of corundum when Ruby Mountain was quartz, euhedral and partly resorbed feldspar tablets, first named. Accordingly to Sinkankas ( 1959, p. 284), and very rare tiny crystals of brown biotite, in a dense some of the garnets at the Ruby Mountain locality are as microcrystalline groundmass. A modal analysis of this much as half an· inch in diameter and are large enough rock gives a composition of 85.9 percent crystalline to cut as gem stones. A few crystals are simple trapezo­ groundmass, 6.6 percent sanidine, 3.1 percent plagio­ hedron:s, burt most 1a·re 1trapezohedrons modified by dodec­ clase, 4.4 percent quartz, and a trace of iron oxides ahed:vons. The garnet has 'a ~Specific gvavirty of 4.23 :at (Carmichael, 1963, p. 105). Some of the quartz crystals 18°0 (Cross, 1886, p. 435) and an index of refraction of are doubly terminated, and most are smoky, especially 1.820 ( J a:ffe, 1951, p. 136). The following published in the central part of the crystals. In reflected light chemical analysis (table 7) of the garnet indicates that many of the feldspar crystals are chatoyant. M·rucKen­ it is the spessartite variety. Spectrographic analysis zie and Smith (1956, p. 411, 413, 418) found that the (Jaffe, 1951, p. 134) of garnet from this locality indi­ sanidine phenocrysts have an optic axial angle of about mutes Si, Mn, AI, and Fe as major constituents; Ca as a 31° and are cryptoperthitic with pericline twinning of minor constituent; traces of Mg, Zn, Ti, N a, Y, Sc, and Ga· and the absence ofF, Li, Sr, Nb, Or, and V. Large the sodium feldspar phase; chemical analysis gave dodecahedral almandite garnets found near Salida at their composition as Or60.4Ab 31. 5An2.1. The plagioclase the inactive Sedalia copper mine have been incorrectly was determined optically by the writer to be oligoclase, attributed to this Ruby Mountain locality (Ford, 1932, about An15• Partial chemical analysis indicates that the p. 596) 0 plagioclase phenocrysts are Or1. 8Abs1.2Anu (MacKen­ zie and Smith, 1956, p. 426). The groundmass, where TABLE 7.-Chemical analysis of spessartite garnet from Ruby Mountain, Chaffee County not silicified or partly altered to clay, consists of quartz, [From Cross (1886, p. 435)] feldspar (apparently untwinned and largely sanidine), Weight Weight tridymite, chlorite, topaz, magnetite, and very rare Constituent percent Constituent percent fluorite. Locally, quartz and feldspar are intergrown, Si 0 ______35. 66 CaO______1. 15 2 Al20 ____ _ :.. ___ - _- _-- 18. 55 K 0 __ - _------_--- . 27 and quartz poikilitically encloses feldspar crystals; 3 Na2 0______. 21 Fe20a------. 32 2 alternating layers of these two minerals emphasize the FeO______1~ 25 H 20 ______- _---- _ _ __ . 44 MnO_ ------29. 48 --- flow layering · in the rhyolite. Tridymite occurs in ' TotaL ______100. 33 characteristic pseudohexagonal plates with wedge Clear transparent, yellowish to colorless, prismatic twinning. Crystals and clusters of topaz are fairly crystals' of topaz one-quarter of an inch or less In. common ·in the groundmass. Cubes of fluorite were diameter are much less abundant than garnet. Crystal found in one thin section. 18 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO. A chemical analysis of the groundmass of the rhyolite distinctive rhyolite from the Nathrop occur within the and a spectrographic analysis of the whole rock a-re Dry Union along U.S. Highway 285 near the inter­ given in table 8. Comparisons of these analyses with an section with Chaffee County Road 60. The most com­ early whole-rock analysis (Cross, 1886, p. 438) of this mon volcanic rocks forming these cobbles are a silicified flow and with the analyses (table 6) of the immediately spherulitic rhyolite that contains phenocrysts of argil­ underlying obsidian and perlite, suggest that the 69.89 lized feldspar and smoky quartz, and a purplish-gray percent Si02 in the analysis reported by Cross is too low rhyolite that has flow layering and lithophysae con­ and. that the 17.94 percen.t Al20a reported by Cross is· taining brown garnet crystals and clay. too high. Potassiilm-argon age determinations indicate that the Nathrop Volcanics were erupted in late Oligoce.ne time, TABLE B.-Chemical and semiquantitative spectrographic analyses in weight percent, gray rhyolite flow, Ruby Mountain, Chaffee about 28 million to 29 million years ago. The determina­ County tions were made on obsidi·an pellets from perlite th31t Chemical analyses from Carmichael (1963, p. 109). Spectrographic analyses from underlies the 'rhyolite flow near the north end of Ruby D. R. Shawe (written commun., May 10, 1963); analyst, J. C. Hamilton; lot 0060] Mountain and on sanidine concentrated from samples Chemical analysis or ground mass of a phenocryst-rich part of the rhyolite flow, collected SIQ, ______-- _-- _ 77. 1 MnQ______0.07 P10a------Ntl TiOs------.07 MgO______.04 H20+------0.44 . ·about 0.67 mile northwest of Ruby Mountain in itJhe AhOa------12.4 CaQ______.43 H,o-______.09 Fe,Oa------. 35 Na,o______4.5 Buena Vista quadrangle. Table 9 gives additional in­ FeO ____ ------. 25· KsO.______4.4 TotaL ____ 100. 2· formation •about the •samples (R. F. M·arvin, written Speetrogra phlc analysis or whole rock I . commun., July 29, 1966). Si-Major constituent Ga ______0. 003 AL ______7 TLK------______5.05 Fe.______. 3 Mn ______.05 PbNb------______.005 TABLE 9.-Sample data for potassium-argon age determinations, Mg ______.05 Ba ______.002 Ca ______• 2 Be ______.0005 ZrYb------______..00015 005 Nathrop Volcanics, Chaffee· County Na ______3 Cu ______.0005

I Elements looked for but not found: Ag, As, Au, B, Bi, Cd, Ce, Co, Cr Ge Hf, K20 Radiogenic Radiogenic Age Hg, In, La, Li, Mo, Ni, P, Pd, Pt, Re, Sb, Sc, So, Sr, Ta, Te, Th, Tl, U, V, W, Y, Material (weight .Af40 .Af40 (millions Zn. percent) of years) K40 Total .Af40

Two possible sources of the Nathrop Volcanics are in Obsidian 1______4.56 0. 00172 0.91 29.3±1. 5 Sanidine 2______410.25 .. 00171 .98.' 29.1± . 9 the Buena Vista quadrangle immediately to the north. Sanidine a______6 9. 90 . 00165 .98 28.0± .8 Near the north end of Sugarloaf Mountain, less than a 1 Pellets from perlite collected by R. E. Van Alstine, Ruby Mountain, NEUNW~ mile north of Ruby Mountain, the rhyolite is separated sec. 13 T. 15 S., R. 78 W., Poncha Springs NE quadrangle; pellets treated with HJ:" to rem'ove perlite coating. from Precambrian gneissic quartz monzonite by a nar­ 2 Concentrated from porphyritic rp)_:olite collected by R. F. Marvin, near north­ west end of Sugarloaf Mountain, SE~NEU sec. 11, T. 15 S., R. 78 W., Buena Vista row gulch containing talus of the two rocks but no quadrangle. . · . a Concentrated from porphyritic rhyolite collected by R. F. Marvm, from small evidence of :faults .. Flow layering of the ·rhyolite dips quarry on west side of Arkansas River, SWUSEU sec. 11, T. 15 S., R. 78 W., Buena Vista quadrangle. very steeply here and is highly contorted, whereas near • Average of 10.26 and 10.24. the south end of Sugarloaf Mountain the layering dips 5 A v:erage of 9.88 and 9.93. · Decay constants for K•o: :>.,=0.585X1Q-IO yrl; :>.p=4.72X1Q-IO yr-1. less than 40° SW. The steep gulch trends southeast, and Atomic abundance of K•D=l.19X1Q-4. the trend was followed about 1,000 feet southeast to a Analysts: R. F. Marvin, H. H. Mehnert, Wayne Mountjoy, and Violet Merritt. lamprophyre dike, striking N. 55°-80° W. and dipping BROWNS CANYON FORMATION 65°-75° N., in gneissic quartz monzonite. The dike ex­ Seven small residual masses of Miocene rocks, con­ tends southeast for a total length of 4.25 miles and taining a flora similar to that at Creede, Colo. (about into the Poncha Springs NE quadrangle, where it was 70 miles southwest of the Poncha Springs NE quad­ mapped for 2.5 miles. Possibly, the structure in the Pre­ rangle) , 1and consisting predomin3Jntly of thin-bedded cambri:Mlll'ocks thrut looalized the lamprophyre dike was tuff:aceous siltJStone, ·rest unconformably on P·rec:ambrian reactivated in Tertiary time and· at Sugarloaf Mountain gneissic quartz monzonite at altitudes ranging from became the outlet for an eruption of rhyolite. The sec­ 7,700 to 7,860 feet in the south-central part of the ond possible source of these volcanic rocks is Bald quadrangle. The unit is not definitely oorrelated with Mountain, about 1.5 miles northeast of Ruby Mountain, -named formations, for it cannot be continuously traced which Hayden (1869, p. 78) regarded as an old volcano. into any type area of Tertiary rocks. As it is mappable According to Honea (1955), it is a composite cone that and forms a significant part of the Tertiary record, the has an upper alkalic rhyolite flow, a basal quartz latite unit is here named the Browns Canyon Formation, after flow, and intervening pyroclastic rocks dipping away the ne~rby feature to the east in the Poncha Springs from a central conduit on all sides. NE quadrangle. No type section of this thin unit is The Nathrop Volcanics are older than the Dry Union · described, as it has patchy distribution because of fault­ Formation of Miocene and Plioce.ne age; cobbles of the ing and erosion, and the lithology varies considerably GEOLOGIC UNITS . 19 from remnant to remnant. The type locality is in sees. calcite; ( 2) the siltstone has a different lithology and 22, 27, and 28, T. 51 N., R. 8 E. locally is mineralized with fluorite, and nearby Dry The thickness of the formation ranges from a few Union strata are not; ( 3) ·in its northeasternmost ex­ feet .to about 50 foot. The thickest section caps a hill east. posure, the tuffaceous siltstone dips steeply· on the up­ of the Puzzle fluorspar deposit in sec. 27, T. 51 N., R. 8 thrown side of a normal fault, and nearly horizontal E., where the beds are faulted, folded, and cut by Dry Union gravels form the hanging wall; ( 4) the fos­ fluorite veinlets. sils of the two formations are quite different, for fissile The formation consists of gray and brown to yellow­ beds of the tuffaceous· siltstone contain plant fossils re., brown tuffaceous siltstone, locally finely ripple marked garded as l\iiocene, in contrast to Pliocene vertebrate and interbedded with calcareous siltstone, sandy silt­ fossils of the nearby unconsolidated strata (Van Alstine stone, ferruginous claystone, and lithic and crystal tuff. and Lewis, 1960). Diatoms, sometimes helpful in dating '11he 'rocks gen~ally ;are moder.rutely well consolidated; Tertiary deposits, were searched for in both the tuf­ looally they .'are dense ;and flinty and have 'a oonchoidal faceous siltstone and Dry Union Formation but none fracture. A basal conglomeratic arkose contains angular were found (G. W. Andrews, written commun., Mar.14, to subrounded fragments mainly of the underlying 1962, and oral commun., Feb. 25, 1965). quartz monzonite. The tuffaceous siltstone is composed Plant fossils collected from fissile tuffaceous siltstone chiefly of angular quartz and feldspar grains in a rut two localities along Chaffee County Road 60 were locally silicified silty matrix in which shard structure studied by R. W. Brown and J. A. Wolfe of the U.S and pumice fragments are evident. Many of the quartz Geological Survey. Specimens from USGS paleobotany grains are strained, and the feldspars are orthoelase, loca:lity D4265 (SE%NW%, sec. 27, T. 51 N., R. 8 E.) microcline, perthite, sanidine, and andesine. Other con­ contain Abies rigida Knowlton and Pinus arossii stituents. of the tuffaceous siltstone are frayed and Knowlron (Wolfe, written oommun., Marr. 10, 1962). euhedral biotite, magnetite, zircon, .pink garnet, and From USGS paleobotany locality 9497, -about 150 feet ·va,rer gr:ains of ~apatite, epidote, sillim·anite, and musco­ northwest of Chaffee County Road 60 in SW% vite. Silt particles of some of these minerals · and SE% sec. 22, T. 51 N., R. 8 E., Brown reported Pinus chlorite, limonite, sericite, chalcedonic quartz, and a sp., Sequoia sp., ~Sorbus sp., Abies sp., Ohamaeayparis clay mineral, proba;bly kaolinite, locally form 50-90 sp., and Alnus sp. ( wri,tten communs., Oct. 1, 1954, percent of the rock. The ferruginous claystone is made June 4, 1957, Md Nov. 19, 1958). According to Brown up chiefly of very fine grained kaolinite, limonite, and the leaf called ~Sorbus sp. is probably the same species hematite. Interbedded lithic tuff near the Manganese as that found in Tertia.ry strata at Creede, Colo., and Hill fluorspar deposit (No. 6 on pl. 1) contains frag­ the presence of pine, sequoia, 1a;nd firr •also points rtow31rd ments of pyroclastic rocks, volcanic glass, some of which comparison with the Creede flora described by Knowl­ is. perlirtic, and flow-layered, spherulitic, vesicular ton (1923). Wolfe found Pinus. arossii Knowlton and rhyolite like that of the Nathrop Volcanics. Abies (probably Abies rigida) in the writer's speci­ The sediments, derived ·chiefly from Precambrian mens from USGS paleobotany locality 9497 and g·ave rocks and Tertiary volcanic rocks, evidently were de­ the •age 1as probrubly late Ter-tiary, Miocene •rut the oldest posited under lacustrine or flood-plain conditions in (w,ritten conimuns., Mar. 10, 1962 and Sept. 27, 1963). rather quiet and shallow water. The basin of deposition More rooenJtly, Wolfe (oval commun., M~ar. 18, 1965) is now localized on a horst structure, which is still topo­ stated thaJt cumulative evidence indicates .this flora in graphically as well as structurally high. Oligocene the Poncha Springs NE quadrangle, 'and in the M;Uirshall rhyolitic welded tuff and younger Tertiary strata are P.ass 'area ,to tihe southwest, is Miocene. This Miocene •age nearby, both to the northeast and southwest (pl. 1); to assignment, based upon the fossil plants, is supported the southeast, a high ridge of Precambrian metamorphic · by :a pollen· study and by the previously mentioned rocks rises 400-500 feet above the siltstone remnants. Tuffaceous siltstone may underlie upper Tertiary sedi­ lithologic and structur·al evidence that indic-ate the ments in a trough between here and the Sawatch Range Browns Canyon Formation is older than· the Dry Union to the west. Formation (Miocene ·and Pliocene) ; both formations The Browns Canyon Formation is mapped as an contain clasts of the distinctive rhyolite flow (the older unit and separate from the Dry Union Formation youngest unit of the upper Oligocene N:athrop for the following reasons : ( 1) The siltstone is more Volca;nics) . consolidated, indurated, folded, and faulted than the A processed sample of weakly calcareous but well in·­ Dry Union sediments, which are unconsolidated in the durated tuffaceous siltstone, bearing fossil fir and pine quadrangle except where :;t few beds are cemented with at USGS paleobotany locality D4265, contains about 20 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO. 100 rather wel'l preserved pollen grains (E. B. Leopold, is more probable (T. A. Steven, written commun., Feb. written communs., Feb. 25 and Mar. 7, 1969) : 1, 1966).

Pinus (primarily non-white pines)------~------84 Similar tuffaceous fine clastics in a continental sedi­ Picea ------4 mentary ~uence are reported from South Park, about Chenopodiaceae ------1 13 miles northeast of the Poncha Springs NE quad­ ~onocot. undet______1 Compositae, Ambrosia type______1 rangle. A thick section of conglomerate, arkosic sand­ Oompositae, lohg-spined type______1 stone, and mudstone, which are locally tuffaceous and Quercus? ------.------1 dip as much as 28°, rests with angular unconformity on 1Undet. non... tree pollen______3 the Antero Formation of Stark and others (1949, p. Miss Leopold reported : 63-68), dated as Oligocene by vertebrate fossils (De The sample was scrubbed with wire brush, soap, and water Voto, 1964, p. 123-124); DeVoto considers these South before preparation. • • • ·Both a large pollen (similar to pon­ Park clastic rocks as Oligocene-Miocene, although fos­ derosa) and a very small pollen (similar to closed cone pines sils were not found. such as pinyon) of Diploxylon pine forms ·are present, but mostly the latter. The assemblage is extremely simple and, DRY UNION FORMATION from pollen alone, contains no Tertiary relict genera. • • • Poorly consolidated sediments of the Dry Union For­ We can suppose f·rom the presence of long-spined and also Ambrosia-type Composttae that the sample is of ~iocene or mation of Miocene and ·Pliocene age unconformably younger age. overlie deformed and eroded earlier Tertiary volcanic Similar tuffaceous siltstones, interbedded with limy rocks and Precambrian rocks, and in .turn· are uncon­ siltstone and ash and containing a flora like that of the formably overlain by Pleistocene pediment gravels and Browns Canyon and Creede Formations, are at the glacial outwash. The type locality of the Dry Union north edge of a Tertiary volcanic field, adjacent to Pre­ Formation is in the Leadville area, about 30 miles nonth of the Poncha Springs NE quadrangle (Tweto, 1961). cam~rian ·and Paleozoic rocks, about 22 miles southwest of the plant fossil localities in the Poncha Springs NE Tweto and the writer together examined exposures of quadrangle, between this quadrangle and Creede, .Colo. the formation in the intervening area in the Arkansas The flora was collected 1-3 miles west of Marshall Pass Valley to ensure that this name is justified for the beds at the west edge of the 31djacent Bonanza quadrangle; in the Poncha Springs NE quadrangle. The beds in the there, the writer found six sHtstone exposures along Poncha Springs area were first regarded as Pliocene the road that follows the abandoned railroad route over lake deposits by H1ayden (1869, p. 77; 1874, p. 48)., who Marshall Pass. The two best fossil localities are south called them the Arkansas marI. of the former Shavano siding (Knowlton, 1923, p. 184; The Dry Union Formation occupies much of the "Loc. Cross. 1914, No. 516 ·[6889]"), in the NE14 sec. western part of the quadrangle and consists chiefly of 26, 'and the NW~ sec. 34, T. 48 N., R. 6 E., where the interlayered white, gray, and tan clays, silts, sands, and writer collected pl,ants that Wolfe (written communs., gravels. Areas underlain by this formation commonly Dec. 22, 1959, and Mar: 10, 1962) identified as Abies have a badland topography and are covered with peb­ rigida Knowlton, Pinus crossii Knowlton, Mahonia bles and cobbles that concentrruted at the surface as the marginata (Lesquereux) Arnold, and Oercocarpus hol­ finer materials were washed away. Individual beds gen­ mesi Axelrod. According to the written communica­ erally are more than 10 feet thick and have little sort­ tions from Wolfe, this assemblage is characteristic of ing and internal stratification. Some of the sands and the Creede flora (Knowlton, 1923, p. 184; Wolfe, 1964, · gravels are finely bedded, however, and locally are cross­ bedded; bedding dips gently in various directions. p. N25). These detrital sediments are poorly consolidated except The presence of a distinctive flora in tuffaceous silt­ where cemented by porous caliche or coarse calcite. The stones in these areas and in the Creede Formation sug­ finer sediments are slightly calcareous. Sands and silts gests that the rocks may be about the same age, though locally contain nodules of calcite and black manganese deposited in separate basins. The Creede Formation was oxide and tubular concretions of calcite and chlorite. deposited in a structural trough, about 15 miles in Some of the clays and silts are bentonites, formed by . diameter, around the domed core of a caldera in the cen­ alter·ation of volcanic ash. A sample of silty bentonite, tral 8a:n J wan Mountains (Steven ~and Ratte, 1964, p. collected in the roadcut of U.S. Highway 285 at the D62). Although a recently published estimate of its south edge of the quadrangle and studied by X-ray age ranges from late Miocene through middle Pliocene methods, consists of montmorillonite, quartz, sanidine, (Steven and Ratte, 1965, p. 47), later potassium-argon and muscovite; the overlying bentonitic clay is about 99 data suggest that a late Oligocene or early Miocene age percent montmorillonite (Helen Mark, U.S. Geol. Sur- GEOLOGIC UNITS 21 vey, written commun., Jan. 31, 1962). A sample of clay Arkansas River drainage in the Leadville area (Tweto, from USGS vertebrate fossil locality D298 (SE* sec. 1960b, p. 73). 16, T 51 N., R. 8 E.), about 1,200 feet east of the high­ Lack of a continuous section or of widespread hori­ way, .is c?mposed of these minerals and illite; the clay zon markers, variable direction of dip, and local fault­ fractiOn IS about equally divided between montmoril­ ing of unknown displacement prevent determination of lonite and illite. The clays at both localities feel greasy the thickness of the Dry Union Formation; further­ and readily swell and disaggregate when immersed in more, the top of the formation is eroded. About 500 feet water. of the formation is exposed in .the Ponoha Springs SE The sands are locally tuffaceous and contain shards in quadrangle, along U.S. Highway 285 and west of the addition to quartz, sanidine, orthoclase, microcline, Arkansas River, where the bed~ extend to an unknown plagiocl~, biotite, muscovite, blue-green hornblende, depth in a fault trough. magnetite, epidote, sphene, zircon, and garnet. The The Miocene and Pliocene age of the Dry Union shards are isotropic and have an index of refraction of Formation is based upon vertebrate fossils. A molar about 1.50. found in the Poncha Springs SE quadrangle at a local­ The gravels contain angular and subrounded pebbles ity south of Salida was identified by A. S. Romer as and cobbles of predominantly Precambrian rocks and that of the Pliocene equid Plioldppus leidyanus Osborn Tertiary volcanic rocks like those exposed in the east (Powers, 1935, p. 189) ; correspondence with W. E. half of the quadrangle. The following also are abundant Powers (Jan. 27, 1960) suggests that this fossil locality as clasts within these gravels: The Mount Princeton is in the SWl)! sec. 1, T. 49 N., R. 9 E. Van Alstine and Quartz Monzonite {Tertiary), exposed in the Sawatch Lewis (1960) reported that Pliocene vertebrate fossils Range to the west; tan chert, purplish quartzite, green­ from localities in the Poncha Springs SE quadrangle ish siltstone, and gray limestone from Paleozoic beds; are comparable to the fauna of the Ogallala Formation and various quartz-feldspar-biotite porphyries, proba­ of Nebraska. G. E. Lewis reexamined all specimens of bly of Tertiary age. Tertiary vertebrate fossils that the writer collected since 1958 from the two localities (Pl. 1) in the Poncha Coarse gravel of the Dry Union Formation, exposed Springs NE quadrangle and reported as follows (writ­ in sec. 35, T. 51 N., R. 8 E., near the south edge of the ten commun., Feb. 27, 1969) : quadrangle, rests on a widespread pediment that slopes 2 o -4 o westward across chiefly Precambrian rocks and USGS fossil vertebrate locality D296 ( SW% sec. 34, T. 51 N., R. volcanic rocks of Oligocene age. (See pl. 1.) The gravel 8 E.). locally is more than 100 foot thick and contains sub­ mastodont, genus and species indeterminate, enamel frag­ ments of cheek tooth. rounded and rounded boulders and cobbles of these ?Neohipparion sp., proximal phalanx from l·ateral toe of nearby Precambrian and Tertiary rocks, and fragments . 3-toed horse. of chert, quartzite, and other Paleozoic rocks that crop camelid, genus and species indeterminate, fragments. of out about 5 miles to the east. The gravel is exposed east distal end of metapodial. of and about 750 feet above the Arkansas River and merycodont, genus and species indeterminate, proximal about 400 feet above a deposiJt of early Pleistocene articular fragment of radius. gravel. Approximately 2 miles southeast of and just USGS fossil vertebrate locality D298 (center of the S% sec. 16, beyond the quadrangle, this coarse gravel is cut by faults T. 51 N., R. 8 E.). and displaced about 200 feet vertically. Noohipparion sp., cheek teeth and fragments thereof, upper The Dry Union Formation consists chiefly of flood­ and lower, deciduous and permanent; distal fragments of metapodials, one proximal phalanx and two medial plain and alluvial-fan deposits. Some of the Dry Union phalanges of digit 3. sediments were derived from the nol'lth; gravels nearr very large camelid, genus and species indeterminate, two the south edge of the quadrangle contain fragments of distal fragments of metapodial. spherulitic rhyolite and garnet-bearing lithophysal merycodont, genus and species indeterminate, astragalus. rhyolite found in .the N~athrop Voloanics ·at Ruby Lewis stated : Mountain at the p.orth edge of the quadrangle. Addi­ tional support for deposition of the Dry Union Forma­ The .specimens identified from these two localities are of Plio­ tion in part from a through-flowing stream from the cene age, but of themselves they are not diagnostic of any particular ·subdivision of the Pliocene. We do know, however, north is the recognition (Ogden Tweto, oral commun., that I have identified upper Miocene and upper Pliocene ver­ July 1963) 131long Chaffee County Road 60 of pebbles of tebl'lates from the Dry Union Formation in the adjoining south­ the distinctive Pando Porphyry of early Tertiary age ern half of the Poncha Springs quadrangle. There, M erychippus (Pearson and others, 1962). This porphyry is a dense cf. M. oolamarius (Cope) occurs, and ?Megatylopus sp. and white qu~rtz latite that forms plugs and sills along the ?Yumaceras figginsi Frick occur together with Neohipparian. 22 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

I.t may be assumed, therefore, that the age of the Dry Union Formation in the northeast quarter of the Poncha Springs quad­ rangle is not restricted to only a short part of Pliocene :time. Earlier, Lewis (written commun., Dec. 17, 1968) had suggested that the Miocene and Pliocene formations of the Santa Fe Group (Spiegel and Baldwin, 1963, p. 38-63) to the south in the Rio Grande trough pos­ sibly once extended continuously northward into the upper Arkansas Valley. Van Alstine (1968) has shown that the Arkansas Valley and San Luis Valley, the · northward continuation of the Rio Grande trough, are ' connected by a structural trough that contains upper

Tertiary sediments, rather than being separated by a FIGURE 4.-View northwest toward Mount Princeton, showing barrier of Precambrian rocks as previously believed. Pleistocene ,gravel-covered surfaces on Dry Union Formation Evidence of pl~ant Efe from the Dry Union Forma­ of Miocene and Pliocene age. From NE:tA, sec. 27, T. 51 N., R. tion in the quadrangle consists of fragments of uniden­ 8 E. Photograph by D. C. Cox. tifiable silicified wood (R. A. Scott, written commun., Oct. 22, 1963) mrd pollen of chenopods, pine, Artemisia, terraces along the Arkansas VaJley and regarded the and the family Compositae (E. B. Leopold, written two highest terraces as older than any glacial stages of commun., July 23, 1959), from SE%SE% sec. 28, T. the region. In his figure 10, Powers showed three mo­ 51 N., R. 8 E. Samples of clay 'and silt collected from raines along Chrulk Creek, beyond the northwest corner this formation at two localities >vere disaggregated, of the Poncha Springs NE quadrangle. The youngest concentrated, and searched for diatoms without success moraine was cited as being Wisconsinan in age and was (G. \V. Andrews, oral commun., Feb. 25, 1965). correlated (1935, p. 197) with his second terrace; the first pre-Wisconsinan moraine was correlated with his The Dry Union Formation in the quadrangle may be third terrace; and the earliest moraine was correlated partly equivalent to the lithologically similar Wagon­ with his fifth terrace. Ray (1940, p. 1902-1903) believed tongue Formation of Stark and others (1949, p. 68-69) that all three moraines shown by Powers should be to the northeast, in South Park. C. L. Gazin, U.S. assigned to rtlhe Wisconsin Gl1acirution. National Museum, identified a tooth-bearing jaw from The present interpretation of the Pleistocene deposits, this formation as that of a fossil horse of late Miocene summarized in table 10, is based chiefly upon: (1) the or early Pliocene age. According to Lewis (Van Alstine topographic posi,tions of three older units on high-level and Lewis, 1960), the Neohipparion from the Poncha erosion surfaces regarded as pediments, and of six units Springs NE quadrangle possibly is comparable in age in outwash plains and terraces graded to former levels to this horse from the Wagontongue Formation. of the Arkansas River; (2) the tracing of all but the QUATERNARY DEPOSITS second earliest unit upslope into terminal moraines of :PLEISTOCENE DE:POSITS former glaciers that descended to altitudes as low as 8,200 feet and ended less than 3 miles west of the Poncha A sequence of Pleistocene deposits, which is remark­ Springs NE and SE quadrangles; ( 3) the field deter­ ably complete when compared with sequences estab­ mination of the youngest outwash gravels as deposits of lished elsewhere in the Rocky Mountains (Richmond, various stades of the Wisconsin Glaciation (G. M. Rich­ 1965), locally mantles the easily eroded Dry Union mond, written commun., Mar. 25, 1964); (4) the dis­ sediments in the western part of the quadrangle (fig. 4). covery of a chevkinite-be31ring rhyolitic volc31llic a~ These deposits formed in multiple stages, chiefly as within the third oldest gravel unit, which may be the outwash from mountain glaciers in the Sawatch Range equivalent of the Pearlette Ash Member of the Sappa t.o the west. In this quadrangle and the one imrrieditakely Fonnation of late Kansan age in Kansas and Nebraska; south, a sequence of nine gravel deposits was estab­ and ( 5) the presence of more strongly developed soils lished, five of Wisconsinan Age and four older. on the older gravels, which also commonly contain more The Pleistocene deposits in this part of the Arkansas highly weathered rock fragments. The writer deter­ V:alley have ·been interpreted sever.al ways in the geo­ mined the heights (table 10) and distribution of the logic liter.atu;re. Successive levels of deposits were first various Pleistocene deposits in the Poncha Springs NE mentioned by Hayden (1869, p. 77; 1874, p. 48), who and SE quadrangles from measurements in the field, believed they formed in a glacial lake basin. Powers from aerial photographs, and from special topographic (1935, p. 188) recorded seven boulder-capped alluvial maps of the Ar~ansas River, Colo. (U.S. Geologi0al GEOLOGIC UNITS 23

Survey, 1955). Heights at which the east-sloping gravels posits that occur there. Many pieces of rotten rock, lie above rtJhe presenrt dTiainage system ~are not given especially coarse-grained granitic types, were thor­ in table 10, for the heights vary with horizontal dis­ oughly weathered in pl1ace; fragments of boulders can be tan<~e from the Arkansas River. broken off and crumbled in the hands. Commonly, the clasts and sandy and silty matrix are stained red-brown TADLE 10.-Pleistocene deposits, Poncha Springs NE quadrangle, with iron oxides. The gravels are moderately well to Chaffee County poorly sorted ·and locally stratified. In some outcrops, Height! Gravel unit (feet) Probable age a few feet of locally cros8bedded sandy and silty allu­ Wisconsinan: vium occurs at the top· of the gravel units. 9------g ______<10 Pinedale III 30 Pinedale II Soils on the surfaces of the four pre-Wisconsinan 1------20 Pinedale I 6 ______50 Bull Lake II gravels loc·ally are relict and have been forming since 5------40 Bull Lake I deposition of the gravels. In general, these ·soils are 4------70 Illinoian much thicker and more strongly developed than those 3------2 ______80 Kansan 70 Nebraskan on the younger Pleistocene gravels; they also are 1------100 Nebraskan greatly enriched in. calcium carbonate, possibly the I Approximate height of upper surface of gravel above surface on next younger gravel. . result of a long period of development under arid to semiarid conditions. The dark-brown B-horizons, Some idea of the thickness of the various Pleistocene locally calcareous, are 1:..._2 feet thick and in some places gravels exposed in the Poncha Springs NE and SE are underlain by as much as 19 feet of oxidized, iron­ quadrangles can be obtained from table 10. The thickest stained m·aterial. The thickest Cca-horizon (lime-en­ and most widespread is gravel 1, which is more than riched material) that was observed beneath the B­ 100 feet thick about 1.5 miles west of the northwest horizon is in the quadrangle to the south, in a roadcut corner of the quadrangle; the deposit consists of out­ along U.S. Highway 285 about 2 miles north of Poncha wash gravel overlain by alluvial-fan gravel, in which Springs; there it forms 25 feet of a 28-foot thickness G. M. Richmond (written commun., Mar. 25, 1964) of gravel 3. In the four pre-Wisconsinan gravel units, recognized buried soils in a roadcut in NW.14 sec. 19, calcite forms caliche crusts several inches thick on the T. 15 S., R. 78 W. undersides of boulders; the crusts commonly are thick­ Gravels 1, 2, and 3 are typically exposed on high est in the oldest units. benches west of the Arkansas River, and they cap divides between tributary streams that flow eastward A narrow east-trending deposit of gray gravel that from the Sawatch Range. In the Poncha Springs NE may be a channel fill in gravel 2 was mapped as part quadrangle and the one immediately south, the gravels of that unit. This coarse gray gravel, which contains rest with angular unconformity upon faulted, tilted, well-rounded boulders and cobbles, is best exposed in and bevelled Dry lJilion sediments ; angular differences a recent roadcut of U.S. Highway 285, in sees. 34 and are as much as 12° between the Dry Union beds and the ~5, T. 15 S., R. 78 W., ·and in the gravel pit to the east; overlying eastward-sloping pediment veneers. Locally, It extends eastward to a hill of gravel1 and westward springs mark the unconformities below the gravels. The to a Holocene deposit of peat. unconformity beneath gravel 2, where cut by U.S. In gravel 2, tan silty volcanic ash occurs as a white­ Highway 285 between sees. 34 and 35, T. 15 S., R. 78 W., weathering layer about 1 foot. thick on the west side was so saturated with water that perforated pipe was of a roadcut ~along U.S. Highway 285 in SW~SW1Jt installed to drain the roadbed. . sec. 9, T. 51 N., R. 8 E., where it is overlain by about 6 The four pre-Wisconsinan gravels consist chiefly of feet of silt. Panned samples of the ash contained outwash gravel that contains tightly packed, rounded magnetite, ~brown biotite, blue-green hornblende, sani­ cobbles and boulders-some of them striated-and dine, epidote, pink garnet, and small rounded fragments alluvial fan gravel that contains more angular rock of intrusive rocks; probably m(jst of these constituents fragments. Most of the clasts are various Precambrian are of detrital origin. R. E. Wilcox (written commun., intrusive and metamorphic rocks and Tertiary granitic May 17, 1966) determined the predominant index of re­ and porphyritic intrusive rocks that crop out in the fraction of glass shards within the ash to be 1.496- Sawatch Range (Dings and Robinson, 1957). A boulder 1.497. During -a preliminary study of the heavy fraction of Mount Antero Granite from gravel 1, found in of a sample of this ash, Wilcox did not find chevkinite ·SE.~SE~. sec. 17, T. 51 N., R. 8 E., contains aqua­ or other types of glass-bearing phenocrysts that are marine. This boulder undoubtedly was derived from the present in volcanic ·ash in the younger gravel 3; he con­ Mount Antero region, about 7 miles due west· Adams cluded that the sa.mple probably represents a different · . ' (1953) described the aquamarine-bearing beryllium de- ash f-all. Similarly, volcanic .ash has been reported from 24 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO • . the La Sal Mountains, Utah, inNebraskan allu~ral sand 1963·a, p. 16-18), :and near rthe Gunnison River, ·about and gravel that unconformably underlies Kansan 75 miles west (Dickinson, 1964) ·of the quadrangle. This alluvium •and colluvium that contains chevkinite-bear­ !ash .type }s ~also present in .the La Sal Mountains, Utah, ing ·aSh (Richmond, 1962, p. 34, 35, 94). in Kansan alluvium and colluvium (Richmond, 1962, Two pre-Kansan Pleistocene alluvial deposits are p. 34, 35, 95). known from other .localities in Colorado.- East of the The above data do not provide conclusive evidence Front Range, between Pueblo and Plainview (north that ·the ash from gravel 3 at the two localities in the of Golden), two fluvi·rutile deposits, which rest on pedi­ Poncha Springs quadrangles is the Pearlette Ash Mem­ ments 'ahout 100 feet ~apart vertically, h:ave been re­ ber of the Sappa Formation, although Wilcox (written garded ~as N ebraslmn or earliest Pleistocene (Scott, comlhun., May 17, 1966) pointed out their similarities 1963a, p. C52-C53; Scott, 1963b) . in mineral content and index of refraction of the shards. Gravel 3, probably of Kansan age, contains 3 inches The distribution of the Pearlette and Pearlette-like to 1 foot of white-weathering volcanic ash in colluvium ash localities and problems concerning sources and above alluvium along the west side of a roadcut o£ U.S. correlation of the ash beds are considered in a publica;. Highway 285, on the boundary between sees. 34 and 35, tion by Wilcox ( 1965, p. 811-812). T. 15 S., R. 78 W.; unidentifiable bone fragments and Gravel 4, probably of Illinoian age, forms a terrace half a lower cheek tooth of fossil Equus sp. (horse) of above gravel 5, n.orth of and below the volcanic ash­ Pleistocene age (G. E. Lewis, written commun., Nov. bearing gravel 3 locality along U.S. Highway 285 in 2, 1967) were found in the oolluvium. Magnetite, brown the northwestern part of the quadrangle. The clasts in biotite, blue-green hornblende, sanidine, 'and epidote the well-developed Cca-soil horizon are thickly coated are the chief mineral constituents of panned concen­ with caliche where the gravel is exposed in an irriga­ trates of •the ash. Aocording to a preliminary study ( R. tion ditch north of Gas Creek in NWl,iNE% sec. 34, E. Wilcox, written commun., May 17, 1966), glass-bear­ T. 15 S., R. 78 W. The thickness of this soil horizon ing phenocrysts in the heavy fraction of an ash sample could not be measured in the mapped area; but in the include clinopyroxene (nX 1.728-1.730), chevkinirte, Poncha Springs SE quadrangle where deposits of magnetite, colorless zircon, and ilmenite ( ~) ; frosted gravel4 are more extensive, the Cca-horizon is approxi­ and slightly milky shards that have a predominant in­ mately 14 feet thick in a roadcut along U.S. Highway dex of refraction of 1.499-1.502 are also present. 285, about a mile north of Poncha Springs. Similar ash was found :in gravel 3 in the Poncha Deposits of gravel 5 are found in both the PQncha Springs SE quadrangle in the SE:%NE% sec. 17, T. Springs NE and SE quadrangles, although they were 50 N., R. 8 E. It is as mu~h as 3 feet thick and is ex­ largely eroded from the area when younger deposits posed in :an east-facing cliff •rut an :altitude of '3Jbout 7,820 formed. Some deposits of gravel 5 are traceable west­ feet. The ash dips gently northeast within silt that un­ ward as outwash from early Bull Lake moraines. derlies 'about 20 feet of gravel and overlies about 8 feet Typically, the gravels occur in channels cut chiefly in of gravel resting unconformably on the Dry Union older Pleistocene deposits, or they form a terrace below Formation (Miocene and Pliocene). A preliminary pre-Wisconsinan gravel and above a sequence of ter­ study of this ash (R. E. Wilcox, written commun., May races of other Bull Lake deposits and Pinedale de­ 17, 1966) indicates that the predominant index of re­ posits. The soil on gravel 5 is· thinner and less well fraJction of the glass shards is 1.499-1.501, ·and that the developed than the soils on the older and higher gravel glass-bearing phenocrysts in the heavy fraction include deposits. It consists of about 1 foot of a brown-clay clinopyroxene (nX 1.727-1.731), chevkinite, magnetite, B-horizon and nearly 9 feet of Cca-horizon, in which colorless zircon, and ilmenite ( ~). Chevkinite, a titano­ caliche crusts are about 1 inch thick on the undersides silicate of cerium earths char3Jcterized by extreme ple­ of boulders. ochroism •and high indices of ·ref·r·action, occurs in Gravel 6 was examined in the field by G. M. Rich­ rhyolitic volcanic 'ash of Pleistocene •age in several mid­ mond, who interpreted it (written commun., Mar. 25, continental •and western States (Swineford, 1963; 1964) as outwash from the upper Bull Lake Glaciation. Young iand Powers, 1960) . Ohev kinite-bearing ·ash, pet­ This deposit forms an extensive outwash plain, sloping rographically similar to the Pearlette Ash Member of east at about 140 feet per mile, south of Chalk Creek the Sappa Formation of late K·ansan age in Kansas in the northwestern part of the quadrangle. The gravel and Nebraska, has been found elsewhere in Colorado in is of unknown thickness and extends upslope 1.5-2.5 deposits regarded as Kansan in age; these deposits ·are miles west of the quadrangle into terminal moraines in at various localities in the Denver area, 65-100 miles the valleys of Chalk Creek and Browns Creek. The northeast of the Poncha Springs quadrangle ( Soott, unit consists chiefly of gravel with sandy lenses; the STRUCTURAL GEOLOGY 25

rock fragments are like those found in the older Pleisto­ Pleistocene in age, for huge angular blocks of ~the Pre­ cene gravels, but thoroughly weathered boulders are not cambri•an 1110Cks 'are ~abundant downstream in gmvel 7, so common~~ The Cca-horizon of the soil developed on south of the mouth of Browns Canyon. The second l'and­ gravel6 is as much as 7 feet thick and contains boulders slide is across the river and about a mile southeast; it with caliche crusts less than half an inch thick on the is composed l·argely of blocks of Tertiary welded tuff undersides. that came from the east from a cliff above nonwelded Gravels 7, 8, and 9 are coarse outwash gravels repre­ tuff. senting three stades of the Pinedale Glaciation (G. M. A Holocene deposit of pea~t, about 3 miles long, 100-­ Richmond, written commun., Mar. 25, 1964). These ·1,500 foot wide, and 1-10 feet thick, follows the course deposits locally occupy terraces at successively lower of Gas Creek in the northwestern part of the quad­ levels along Chalk Creek and the A.rkansas River. (See rangle (pl. 1). The peat is being excavated for market table 10.) Gravel 7 is the most extensive and forms a about. 1 mile east of U.S. Highway 285, ~along the prominent terrace. In Browns Canyon, only terraces drainage near the south edge of the outwash ·plain of containing gravel 7 are shown· on the geologic map; gravel 6; here the peat rests chiefly on sandy ma:terial local remnants of the two younger terraces, too small of this Pleistocene unit. Much of the 'area underlain to be mapped, were found at the south edge of sec. 23, . by the peat is swampy and covered with ht~mmocks of T. 51 N., R 8 E. Similarly, only a few remnants of the grass. terrace and gravel 8 along Chalk Creek are· large Vertebrate remains, pollen, and ·a sam pie for a radio­ enough to map. Deposits of gravel 9 are small and are carbon age deter-mination were collected from the peat not shown as features on the topographic m·ap; these (USGS paleobotany loc. D1373) near the southeast are included with Holocene flood-plain gravels on the corner of sec. 27, T. 15 S., R. 78 W., where the Colorado geologic map (pl. 1). State Highway Department cut about 10 feet of peat In the valley of Chalk Creek, outwash-gravel units along Gas Creek in constructing a new section of U.S. 7, 8, and 9 extend 1.5-2.75 miles west of the quadrangle Highway 285 in 1958. The writer examined many into corresponding terminal moraines, according to cubic yards of this peat when it was spread upon a field Richmond. Similarly, gravel 7 along Browns Creek, rubout 2,000 feet southward and above the east edge of about 4 miles south, ends upstre·am at a terminal mo­ the highway cut, and found the proximal end of 'a tibia raine a:bout 2 miles beyond the west edge of the quad­ from E quus sp. indet., of late Pleistocene or Holocene r.angle. Rock fragments in the three Pinedale gravels age, and a metatarsa;l from Bison sp. indet., probably are like those in the older gravels at higher levels but Bison bison Linnaeus of Holocene age, according to generally are less weathered. The surface of gravel 7 G. E. Lewis, U.S. Geological Survey, and C. B. Schultz slopes about 100 feet per mile throughout much of its and L. G. Tanner, University of Nebraska State Mu­ extent along Chalk Creek and steepens to approxi­ seum and Geology Department (G. E. Lewis, written mately 140 feet per mile near the moraine. commun., June 1, 1959). The mud matrix associa;ted Soils formed on gravels 7, 8, and 9 are poorly de­ with the vertebrate collections was ex-amined ·by E. B. veloped ·and are thinner ·and grayer than those on the Leopold, who reported (written commun., Jan. 26, older gravels. The slightly oxidized B horizon, com­ 1959) many excellently preserved pollen grains of pine, posed of several inches of clay, does not effervesce with spruce, oak, Juncus, grass, Cyperaceae, Erioganu1n, dilute hydrodhloric acid. Similarly, the underlying C Artemisia, Compositae, Chenopodiaceae, and undet. horizon is not oalca:roous, except foT a thin layer of monocots, dicots, and spores. She concluded that the cal,iche on the underside of some boulders. pollen could be Pleistocene or Holocene. A sample (USGS lab. No. W-989) of fire-charred wood from the HOLOCENE DEPOSITS peat at this locality, however, was determined to be Only a few Holocene deposits are extensive or thick less than 200 years old (Meyer Rubin, written commun., enough to be shown on the geologic map ; these consist July 18, 1961). chiefly of silt, sand, 1and gravel comprising rthe ·alluvium STRUCTURAL GEOLOGY and colluvium of the Arkansas River valley and tribu­ taries. Most mountainous slopes in the eastern part of The complex geologic structures within the quad­ the area are covered with coarse talus or finer colluvium. rangle are dominated by folds in the Precambrian rooks Two l-andslides 1are shown near the south-central edge and by northwest-trending and north-trending faults, of the quadrangle. The landslide on the west side of · chiefly of Tertiary age, that are apparently related to Arkansas River consists of blocks of Precambrian ig­ a structural trough along the Arkansas Valley. De­ neous and metamorphic rocks; it may be partly late furmations that produced the structural features prob- 26 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

ably occurred during Precambrian, Laramide, and late ·The regional geology indicates the rocks of the quad­ Tertiary times. rangle also were deformed during the Laramide orogeny The earliest recorded structural event was the Pre­ of Late Cretaceous to early Tertiary age. The quad­ cambrian folding that accompanied metamorphism rangle lies on the east flank of the north-trending . and emplacement of the syntectonic body of gneissic Sawatch anticline. Thousands of feet of once-overlying quartz monzonite. In the large pendant of gneisses in Paleozoic and Mesozoic sedimentary rocks have been the south-central part of the quadrangle, the foliation er9ded, and the ·Core of Precambrian crystalline rocks and lithologic layering strike northeast and dip steeply is exposed high·in the Sawatch Range to the southwest northwest and southeast, parallel to the foliation of the (Dings and Robinson, 1957, pl. 1). East-dipping lower adjacent gneissic quartz monzonite. These relationships Paleozoic rocks crop out about 3,000 feet east of the suggest thrut !the mooamorphic rocks here .lie in large northeast edge of the quadrangle. During the Laramide isoclinal folds with steep axial planes. The plunges of orogeny in early Tertiary time, a batholith and ·several a few small drag folds and of the lineation of horn­ stocks were emplaced in the Sawatch Range beyond the blende crystals in hornblende gn~iss range from about west edge of the quadrangle (Dings and Robinson, 25 o to 60° NE. In the metamorphic rocks south of the 1957, p. 33-34). About 2 miles east of the quandrange, igneous body near the southeast corner of the quad­ a Laramide stock invaded the Paleozoic rocks and was rangle, the foliation and lithologic layering were folded eroded and partly covered by Tertiary volcanic rocks into northwest-trending synform and antiform struc­ ( Behre and others, 1936). The quadrangle lies between tures. The gneisses here, as those elsewhere in the quad­ these two centers of Laramide intrusion and probably rangle, were completely recrystallized, and original was involved in similar processes. textures and structures of the metasedimentary rocks The attitudes of many of the Tertiary units in the were obliterated. For these reasons and because of a Poncha Springs NE quadrangle reflect downfaulting lack of stratigraphic markers, additional complexities to the west, associated with a trough along the Arkansas of folding in the gneisses were not determined. Valley. West of the river the rhyodacitic volcanics of Foliation in the main b~dy of gneissic quartz mon­ early Tertiary age dip westward beneath Dry Union zonite chiefly trends northeast to east and dips steeply sediments. Although moderate dips, generally less than north or south. ~n ~he central part of the quad~angle, 45°, in Oligocene ash-flow tuff are largely the result however, the fohatwn dips gently north, and in other of folding near faults, the ash-flow tuff in the southeast­ places the ~attitude of the folioation is highly vari·able. ern part of the quadrangle slopes gently west toward A less conspicuous foliation in most of the dike rocks the trough. The Nathrop Volcanics. of later Oligocene cutting the gneissic quartz monzonite is similar to that age also dip west, at moderate to steep angles, into the of the host rock, and it indicates that these dikes were adjacent trough. The tuffaceous siltstones of the intruded before the close of the regional metamorphism Browns Canyon Formation of Miocene age generally that affected the intrusive qody and adjacent rocks. have low dips; near faults, however, they dip as much as 55 ° and are thrown into small folds .. Directions of No fa~lts of unquestioned. Precambrian age were dip in the younger Dry Union Formation are highly obsery,ed 1n the ~U:31drMl.g1e. The persistent east-trending 1amprophyre -d1kes :and some of the other Prooambrian variable and are believed to reflect initial dips, as well dikes, however, .may htave been intruded talong :£aults as tilting of the beds near faults. but eviden~e for ·movement :along such predike 'Struc~ In the southwestern part of the quadrangle, several tUJres was not found in tJhe intrusive rock ·th31t .they north-trending faults cut Dry Union beds but not the transeot. Prob31bly some of rthe f.aults shown on the overlying Pleistocene gravels. Their position on plate 1 is based chiefly upon lineaments on aerial photographs. geologic map (pl. 1) formed originally in Precambrian The faults extend southward through the Poncha time and were reactivated much later, for nearly all dis­ Springs SE quadrangle, where they cut late Tertiary pla?e Oligocene volcanic rocks or younger Tertiary sediments and Precambrian bedrock near the south sedunents. The quartz veins that contain magnetite, margin of the Arkansas Valley. specularite, pyrite, chalcopyrite, chlorite, and traces of Many of the faults shown on the geologic map ap­ purple fluorite may be of Precambrian age. Many of parently are related to the east side of the basin and these quartz veins trend northwest along faults and range type structural depression that parallels the joints, across the grain of the Precambrian host rocks north-trending Arkansas Valley. This rift or graben and roughly parallel to normal faults along the structure ,(Gabelman, 1952, p. 1608-1609) has been margins of a horst in the south-central part of the referred to (Tweto, 1964, p. 19-20; Karig, 1965) as a quadrangle. northward extension of the Rio Grande trough, a long STRUCTURAL GEOLOGY 27 narrow depression broken by many en echelon horsts fault zones the feldspar pQrphyroblasts and gneissic and grabens that trends south through the San Luis quartz monzonite :are reduced to lenticular augen in a Valley into New Mexico and Mexico (Burbank and s~histose m·atrix. Elsewhere, these faults are obscured Goddard, 1937, p. 933; Kelley, 1952, 1956; Joesting and by soil, alluvium, or colluvium and are difficult to lo­ others, 1961). The total thickness and nature of the cate in the field. Phdtogeologic interpr~tation was help­ rocks beneath the Dry Union sediments in the trough ful because many of the fault traces are recognized on in this quadrangle are unknown ; on the horst (sec. 27, aerial photographs a;s lineaments that follow strike T. 51 N., R. 8 E.) which extends northwest into the valleys, alined saddles or sags, and scarps or other topo­ depression, the Precambrian rocks are overlain by graphic breaks. Ground checks failed to confirm fault­ lower Tertiary volcanic rocks, Miocene tuffaceous silt­ ing ,along several linear ferutures that were noted on stones, and Dry Union sediments. the aerial photographs near, and parallel to, the mapped The main fault near the east margin of the trough north-trending faults near the east edge of the quad­ along the Arkansas Valley is probably buried beneath rangle. These features may be the result of soil, vegeta­ Dry Union sediments. According to James E. Case tion, or topographic differences not necessarily related (written commun., Apr. 23, 1965), preliminary results to underlying faults. of a geophysical survey indicate a steep gravity gradi­ A horst of Precambrian rocks with patches of over­ ent, suggestive of a steep fault downthrown onthe west lying lower Tertiary volcanic rocks and Miocene silt­ side, along a trend extending south from sec. 14, T. 15 stone forms a salient extending northwest from the east S., R. 78 W., between tJhe Ark!ansas River 1and U.S. side of the Arkansas Valley at the south-central edge of Highway 285. In ~the Buena Vista quadrangle, approxi­ of the quadrangle. The horst is bounded on the north­ mately a third of a mile north along this trend, the axes east and southwest by normal faul1ts that bring Pre­ of small folds-possibly drag folds near a concealed cambrian rocks ·abruptly in oontact with Oligocene vol­ fault-in the Nathrop Volcanics similarly strike N. Cianic ·rocks ·that dip gently ~away f.rom the structure. 10° W. at the west edge of the Arkansas River. This The fault that bounds the horst on the southwest lodtl­ hypothetical fault, not shown on plate 1, is parallel to ized major fluorsprur deposits of the Browns Canyon the north-south axis of the Sawatch anticline. district; this fault is described in detail in the section Several other north-trending faults, west and east on fluorspar deposits. A graben and a narrower horst of the Arkansas River, are east of the buried fault and with volcanic rocks lie immedi31tely southwest of the probably are related to it. Displacement of Precam­ fault. The fault bounding the major horst on the north­ brian aplite and lamprophyre dikes and Oligocene vol­ east dips 65°-75° NE., and places Oligocene volcanic canic rocks in the quadrangle and to the south, indicates rocks on the northeast down against Preca.mbrian rocks. that the west sides are dropped down-possibly as The Miocene and Pliocene Dry Union Formation simi­ much as 1,000 feet-along the steeply west-dipping to ~arly is dropped down against a small patch of Miocene vertical normal faults. At the Ace High and Jack Pot tuffaceous siltstones resting on Precambrian gneissic claims, in the southeast corner of the quadrangle, a quartz monzonite in the SW%SE% sec. 22, T. 51 N., quartz-chalcopyrite vein that dips 75° W. and a vertical R. 8 E. To the southeast this fa:ult has offset, for sev­ pegmatite dike along a north-trending fault zone may eral hundred feet, the: pediment on whi~h the Dry Union indicate that movement along some of the faults of this sediments were deposited (pl. 1) ; the northeast side system started as early as Precambrian time. Extensive erosion of Paleozoic and Mesozoic rocks and ·earlier moved down. F-aults and joints containing northeast­ movements along the north-trending faults took place and northwest-trending fluorspar veins and northwest­ before .the Oligocene welded tuffs were emplaced upon trending quartz-specularite veins transect the horst. the Precambrian rocks. Movement along some of these This 'horst may extend northwest across the Arkansas north-trending faults evidently stopped before late Vtalley trough for 1ahourt 5 miles from the contact of Tertiary time because tfie Dry Union Formation con­ these late Tertiary -and Precambrian rocks in .the SE% ceals the fault in Wlh, sec. 11, T. 51 N., R. 8 E., and the sec. 21, T. 51 N., R. 8 E., because a preliminary geo­ erosion surface on which this formation was deposited physical survey indioates the axis of a ~tmnsverse gravity truncates a north-trending fault farther east. (See high lies along this trend (James E. Oase, written pl. 1.) commun., Apr. 23, 1965). Similarly, faults rel3Jted to Locally, the north-trending faults are marked by the horst may extend about 6 miles southeast of Salida zones of schistose, slickensided, brecciated, or altered to Wellsville, where northwest-trending faults could rock containing vienlets of epidote, chlorite, and quartz not be dated more closely than post-Permi~an and pre­ and grains of chalcopyrite; furthermore, in some of the Pleistocene (Litsey, 1958, pl. 1, p. 1164, 1171). 28 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

Near the east edge of the quadrangle and south of 9 E., and in the adjoining section to the west, where its Green Gulch, a steep east-trending fault cuts the gneis­ calculated slope is about 7° WSW. Possibly, later ero­ sic quartz monzonite. The fault surface and the inter­ sion that formed the late Tertiary pediment surface section of the f·ault with the long north-south fault were partly exhumed this segment of the older surface. not observed, but the trace of the east-trending fault is A younger and better exposed pediment of late Ter­ marked by abundant chlorite, epidote, and quartz. To tiary age, east of the Arkansas River, slopes west­ the west, the fault has displaced an aplite dike down­ southwest across the present course of the river and ward on the south side; similarly, east of the quadran- disappears beneath the Dry Union Formation. (See 1 gle this fault has dropped Paleozoic rocks downward pl. 1.) Remnants of Dry Union alluvial gravel deposited on the south side (C. T. Wrucke, oral commun., Aug. upon this surface were mapped near the south-central 1964). This fiault may belong to a syste~ of l:ate Ter­ edge of the quadrangle ·and in the adjoining quadrangle tiary east-trending faults that have preserved the to the south. The pediment is regarded as the erosion Nathrop Volcanics in a narrow graben structure to the surface from which the Dry Union sediments were north (NWi;4NWV! sec. 29, T. 15 S., R. 77 W.) and partly derived and across which they were transported that displace Dry Union sediments just beyond the southward and westward to a structural trough east of south edge of the quadrangle. the Sawatch Range. In the quadrangle this surface Several small normal faults t:P.at cut the Dry Union slopes 2-4 o west, from an altitude of about 8,800 feet Formation in Wlh sec. 21 and near the center of sec. 22, to about 7,700 feet. Although the pediment slopes less T. 51 N., R. 8 E., may have resulted from renewed than 1 o from north to south, it has more than twice movement along major concealed faults. At the first the present gr_adient of 33 feet per mile for the Arkansas of these localities, a roadcut along an older section of River from Chalk Creek to the mouth of Browns U.S. Highway 285, four faults occur within a distance Canyon, about 9 miles south. This erosion surface is of 15 feet; the faults strike within 30° of N. 80° E., faulted near the south edge of Railroad Gulch (pl. 1) dip 50°-75° S., and apparently have very small dis­ and also in Longs Gulch, about a mile south of the placement. This exposure is approximately at the in­ quadrangle. At both localities the surface is covered tersection of projected northwest trends of several· with Dry Union gravel along the south sides of these faults that are related to the major horst structure. An east-trending faults. east-trending :fault at the locality in section 22 dips Some· areas of resistant Precambrian rocks rise sev­ 75° N., has a displacement of at least 15 feet, and may eral hundred feet above this late Tertiary pediment be related to movement along the buried north-south and form remnants east and west of the Arkansas River. fault that marks the east margin of the trough along Canyons cut the pediment surface to depths greater the Arkansas Valley. than 600 feet; the deepest is Browns Canyon along the Arkansas River. Probably the course of the river was ;GEOMORPHOLOGY entrenched in the hard rocks of Browns Canyon after The landscape of this part of the Arkansas Valley is superimposition from the Dry Union Formation characterized by gently sloping surfaces at 10 levels. (Miocene and Pliocene), which rests on the pediment. The earliest and highest late Tertiary pediment slopes West of the Arkansas River, three east-sloping pedi­ west-southwest and was cut on Precambrian rocks and ments covered with outwash and alluvium of gravels Oligocene volcanic rocks, mainly east of the Arkansas 1, 2, and 3 of early Pleistocene age (table 10) lie at River. Three intermediate pediments of early Pleisto-, successively lower elevations and are graded to former cene age slope east and were cut chiefly on Dry Union levels of the south-flowing river. The pediment beneath sediments between the Arkansas River and the Sawatch gravel 1 is the most widespread and extends for miles Range. Six lower, later Pleistocene terraces locally along the mountain front; it slopes eastward about 35 border the Arkansas River or its main tributary feet per mile near the river and steepens to about 300 streams. :feet per mile near the front of the Sawatch Range. The An older Tertiary surface, buried beneath the rem­ difference in height between the pediments beneath nants of Oligocene volcanic rocks, was too extensiv:ely gravels 1 and 2 is about 100 feet, measured approxi­ faulted and .folded for its origin to be determined. In mately 3 miles south of Nathrop and near the Arkansas places, however, the basal non welded tuff and overlying River. A similar difference exists between two Pleisto­ black vitrophyre oftheash-flow tuff unit were deposited cene pediments, also of pre-Kansan age (Scott, 1963a, upon a gently sloping or rolling surface. The most p. 18, 52, 53), at various localities along the east flank extensive remnant is exposed for nearly a mile along of the Front Range, which is about 65 miles east of the the feature named "The Reef" in sec. 30, T. 51 N., R. Poncha Springs NE quadrangle. SU~RY OF GEOLOGIC EITSTORY 29

The three pediments, which bevel faulted and tilted r.ated and later folded, recrystallized, and foliated to Dry Union beds, probably formed when streams over­ form banded gneiss, marble, and hornblende gneiss, loaded with glacial outwash and ·alluvial-fan material probably during a period of metamorphism accompany­ eroded laterally. The changes in base level of the major ing emplacement of a body of gneissic quartz mon­ drainage that produced pediments and associated de­ zonite about 1.7 billion years ago. The gneissic quartz posits at the various levels are attributed largely to monzonite was cut by dikes of granite, aplite, pegma­ Pleistocene climatic changes. tite, lamprophyre, dacite porphyry, -and diabase, and by Canyons or valleys have been cut at intervals by the quartz veins locally containing magnetite, specularite, Arkansas River and many of its tributaries since the late pyrite, chalcopyrite, chlorite, and traces of purple Tertiary pediment formed. The accelerated erosion may fluorite. be partly attributed to widespread uplift of the Rocky Records of Paleozoic and Mesozoic history are miss­ Mountain region. Abandoned canyons cut in Precam­ ing; any rocks of these eras that remain in the quad­ brian rocks at different Pleistocene levels are found in rangle are buried in the Arkansas Valley trough. several places along the Arkansas River through Evidence from adjacent quadrangles indicat('s that Pre­ Browns Canyon. A canyon, now abandoned, also was cambrian rocks were eroded extensively to a surface of cut in Oligocene volcanic rocks in SWJASW!lJ.t, sec. 27, very low relief before the area received about 1,000 feet T. 51 N., R. 8 E., probably in Wisconsinan time when of sediments from shallow Paleozoic seas; deposition Threemile Creek was at levels below a deposit of gravel was interrupted by several periods of uplift and ero­ 4 about a mile downstream, to the southeast just beyond sion. A thickness of 3,000-5,000 feet of nonmarine and the quadrangle. Evidence for repeated valley formiation marine sediments was laid down later in the Paleozoic consists of abandoned valleys, valleys that contain misfit era ( Gabelman, 1952, p. 1582, fig. 3), and an additional drainage in the earlier Pleistocene pediments and de­ 5,000 feet· or so of Mesozoic marine and nonmarine sedi­ posits, and the succession of terraces south of Chalk ments probably covered the area (Stark and others, Creek. 1949, p. 34--45; Tweto, 1964, p. 15-16). Late Mesozoic The landforms and drainage that developed on the time was marked by withdrawal of the Cretaceous- sea, hard rocks east of the Arkansas River contrast strongly the last sea that invaded Colorado, and also by the with those on the soft sediments in the western part start of the Laramide orogeny, a major mountain­ of the quadrangle. Erosion ·east of the river produced building event and period of •igneous activity here and a west-sloping pediment of late Tertiary age, narrow elsewhere . in the Rocky Mountains. During :this canyons and fault-controlled valleys, domes of granitic orogeny, which continued well into the Tertiary period, rock, and spires and cliffs of welded tuff. Streams east the Sawatch and Park Ranges (near the west and east of the river are intermittent, commonly enter the river edges of the quadrangle, respectively) were uplifted, from valleys with greatly increased gradients near the folded, and faulted. The quadrangle is on the east flank river, and evidently never were fed by glacial melt of the faulted Sawatch anticline that also formed at waters. West of the river, the terrain ischaracterized this time. by: Badland topography developed on the Dry Union After ·an extensive period of erosion, the empl·ace­ Formation; three east-sloping pediments beneath ment of the volcanic rocks was initiated by extrusion Pleistocene gravels; rather broad, straight, east­ of rhyodacitic ash and lava upon the Precambrian rocks trending valleys with gravel-capped interfluves; and in early Tertiary time. An unconformity developed, terraces. ~he higher levels provide grazing land in inasmuch as these volcanics locally were entirely eroded much of the area, and the lower terraces yield most of before the extrusion of ash flows that for the most part the crops. Several evenly graded perennial streams rise formed the rhyolitic welded tuffs in Olligocene time, high in the Sawatch Range west of terminal morf,iines. about 35 million years ago. Faulting and small-scale Apparently they have been well supplied with water folding evidently accompanied and followed the em­ since Pleistocene time and now furnish water for irriga­ placement of ash-flow tuff. North-trending block faults tion ditches. that formed the framework for the Arkansas V·alley trough may have been active at this time and during SUMMARY OF GEOLOGIC HISTORY late Oligocene time, when pumiceous tuff, breccia, per­ The local geologic record began in Precambrian time lite, and a rhyolite flow of the Nathrop Volcanics were with deposition from marine or continental waters of emplaced. A:£ter 1a period of erosion, ·the basal oonglom­ thick arkosic quartz-rich sands, a few thin lenses of erate ·and tuffaceous siltstones of the Browns Canyon limestone, and thicker layers that possibly were dolo­ Formation (Miocene) were laid down, chiefly as local mitic or ·argillaceous sediments. These units were indu- flood-plain or lake deposits. These Miocene rocks were 351-967 0--69----3 30 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO. then faulted and. folded. Fluorspar mineralization reoorded shipments were made in 1927. An a,nalysis of followed. shipment figures . furnished by some of the mine and In later Miocene and Pliocene time thick clays, mill operators and of those recorded by the U.S. Bureau silts, sands, gravels, and bentonitic ash formed the Dry of Mines indicates that approximately 130,000 tons of Union Formation. Deposition was accompanied by re- fluorspar concentrates was recovered from about twice current faulting and qeepening of the Arkansas Valley that quantity of ore from the district. Concentrates were trough, in which most ·of these sediments accumulated. shipped until1949, about 80 percent of them in the last Detritus for the Dry lTnion Formation was transported 8 years of operation. Approximately 59 percent of the westward and southward across an erosion surface, concentrates was metallurgical grade, containing about well-formed in the eastern part of the quadrangle. The 85 percent CaF2 ; 39 percent was ceramic grade, con­ last clearly dated tectonic actJivity in the quadrangle taining about 92-97 percent CaF2 ; and 2 percent was occurred next; the Dry Union Formation was faulted acid grade, containing about 98 percent CaF2. Produc­ and tilted in various directions, and the raising of a tion of acid-grade fluorspar by flotation and metallurgi­ horst extending northwest into the Arkansas Valley cal-grade fluorspar by jigging was difficult because the trough was completed. The Rocky Mountain region was ore commonly consists of intimate intergrowths of very upli:fited in }ater Pliocene or early Quaternary time, fine grained fluorite and silica minerals. Much of the and the ancestral Arkansas River was superimposed metallurgical-grade and some of the ceramic-grade from the Dry Union Formation and cut downward into. products resulted from handpicking of selectively the hard rocks of the Browns Canyon area. Erosion mined ore. and canyon cutting were dominant thereafter in the Most.of the production has come from claims formerly eastern part of the quadrangle. held by Colo~a;do Fluorspar Mines, Inc. (successor to During Pleistocene time the Sawatch Range, beyond Colorado Fluorspar Corp. in 1945), and American the west edge of the quadrangle, was glaciated exten- Fluorspar Corp. AHied Chemical Corp., General Ohem­ sively; terminal moraines ended less than 3 miles from ical Division, acquired the holdings of both organiza­ the Poncha Springs NE and SE quadrangles. Outwash tions, the former in 1948 and the latter in 1946. Other and alluvial-fan gravels, representing at least eight mining centered on claims of United States Fluorspar & stages of glaciation, were de{X:>sited on a series of three Manganese, Inc. (formerly United States Fluorspar, pediments and six lower terraces that developed on the · Inc., successor to Chaffee County Fluorspar Corp.) and Dry Union Formation. The deposits at various heights .Commercial Minerals, Inc. (formerly Universal Mines, are related to former levels of the Arkansas River and successor to Mansheim-Salida Fluorspar Co.); some probably to Pleistocene climatic changes.

HISTORY OF P:J;tODUCTION AND OWNERSHIP LOCALIZATION AND STRUCTURE The Browns Oanyon di·strict is one of the major Most of the fluorspar production has come from a fluorspar districts of the Unitted States boo3Juse of its normal 'fault zone that strikes N. 30°-50° W. and dips substant'Ji:al production -and resources .. Fluorspar W1aS 65°-85° SW.; several other mineralized faults shown discovered in the distriot 11:bout 1923, 1and the earliest on plate 1 have yielded ore. The main fault zone (fig. 5), ECONOMIC GEOLOGY 31

MINERALOGY The veins consist almost entirely of fluorite and micro­ crysballirre ·and chialoodonic qua·rtz. Minor quantities of coarser grained quartz, opal, calcite, brurite, pyrite, ma,rc;asite, black manganese oxides, iron oxides, and clay minerals .also occur. The fluorite ·ranges from miorogranular to oorurse grained; most is microgranular to fine grained. Locally, crystals of medium-grained fluorite loosely interlock .and form the so-called ''suga;r spar." Coarsely crysbal­ line fluoriote generally is glassy •and has mdial-fibrous or columnar structure; a piece is very •rarely cleavable into large :£mgments. Much of the fluorite is botryoidal, mammillary, or reniform. The surf.ace of some fine­ grallined fluorite is rippled in .a manner thiaJt suggests FIGURE 5.-View nor,t:hwest :Jlrom Arkansas River, showi111g the slight downwa •rd flow of gelaJtinous material. Chocolate­ main fault wne between hills of Tertiary rhyolitic welded brown, purple, red, pink, green, yellow, gray, white, tuff (left) and Precambri•an rocks (right) and the Colorado­ and colorless varieties of fluorite have been observed; American fluorspar mine. the colors fade upon weathering of the fluorite. All varieties were tested with an ultraviolet lamp, and none which forms the southwest boundary of the horst de­ fluoresces. scribed previously, is mineralized with fluorspar almost The fluorite is commonly made up of layers (fig. 6) continuously for nearly 3,000 feet in sees. 34 and 27, ranging in thickness from less than 1 mm to more than T. 51 N., R. 8 E. Along much of its length this fault 2 feet. Material consisting of layers of fluorite of differ­ zone consists of two nearly parallel branches, best ex­ ent colors and grain sizes is known locally as "ribbon posed in the Colorado-American deposit. The west wall spar" or "bacon spa,r." Some of the l-ayered fluorite of the west branch is composed of approximately 350 forms nodular ore (fig. 7) composed of nearly concen­ feet of welded ash-flow tuff of Oligocene age overlying tric layers of fluorite around breccia fragments of P•reaambr~an rrocks; P·rooamhrian rocks form the east country rrock, fluorite, or chalcedony. An origin for wall or footwall. The more productive east branch has the formation of such nodular fluorspar ore by alter­ Precambrian rocks on both walls. Various Precambrian nate rotation of breccia fragments and deposition of units here are offset by left-lateral as well as vertical dis­ fluorite has been suggested (Van Alstine, 1944, p. 122~ placement. Some of the strike-slip movement took place 123). after the fluorspar mineralization because slickensided Well-formed crystals of fluorite are not very common fault surfaces, bearing gently raking striae and grooves, and are generally less than a millimeter on edge. Hand­ cut the fluorspar •and miorocrystallin.e quartz that lens and microscopic examination of specimens, how­ were deposited .towllird the close of the period of ever, revealed the following crystal habits: cube; octa­ mineralization. hedron; dodecahedron; octahedron modified by cube, Many of the veins nearly fill single fissures between dodecahedron, trapezohedron, trisoctahedron, tetrahex­ highly slickensided abrupt walls. Vugs and water ahedron, and hexoctruhedron; dodecahedron modified by courses, some containing wad and clay, are •as much as cube; and trapezohedron modified by cube and octa­ 4 by 8 feet in horizontal cross section and 30 feet high hedron. Cubes are most abundant, especially as a coat­ (Cox, 1945, p. 273). The walls of the deposit are indefi­ ing of the botryoidal, mammillary, and reniform sur­ nite in other places where the mineralized fault zone is faces. Some octahedra are built of smaller octahedra in more complex, consisting of .replaced breccia :£rngments parallel growth. and parllillel and en echelon lenses. The footwall gen­ Although octahedral and dodecahedral fluorite have erally is better defined than the hanging wall. been considered higher temperaJture or earlier forms Precambrian gneissic qua,vtz monzoni.te •and Tertirury than cubic fluorite (Kalb and Koch, 1929; Drugman, rhyolitic welded tuff were the most competent rocks 1932; Ingerson, 1955, p. 394), in specimens from the during faulting and maintained openings suitable for Browns Canyon district tJhese three forms were found 1Jhe movement of ore-forming solutions. 'llhroughout the together and evidently were deposited simultaneously. district, the walls of oreshoots generally are one or both Cubes and dodecahedra occur together in one specimen, of these rock types. and dodecahedra and octahedra modified by cubes in an- 32 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

FIGURE 7.-Nodular fiuOII"ite OOillSisting of brecciated, si-licified rhyolitic welded tuff, chalcedony, and early fluorite sur­ rounded by thin layers of gray and white fluodte; Colorado­ American deposit. ·Scale shown in centimeters. Photograph .by J. A. Denson.

line quartz against part of tJhe hanging wall is as much FIGURE 6.-Fine-grained layered ·fluorite, Ilast Chance fluor­ as 14 inches thick; the quartz replaced the wallrock spar deposit. Scale shown in centimeters. Photograph ·by slightly, but it !has 'a slickensided and polished contact J. A. Denson. with the fluorite. Locally it contains grains of pyrite and is cut by veinlets of fluorite and pyrite. other. In a specimen from the lower level of the Chim­ Minor quantities of more coarsely crystalline quartz ney Hill deposit, fluorite crystals less than 0.04 mm in are present in microcrystalline quartz and fine-grained diameter, extracted from a crack at the contact of layers fluorite. Colorless and pinkish doubly terminated quartz of fluorite and microcrystalline quartz, have the follow­ crystals locally line vugs in fluorite and, in turn, are ing four habits, listed in decreasing order of abundance : coated with botryoidal fluorite. cube and dodecahedron, cube, octahedron and cube, and Opal with ring structures is interlayered w~th fluo­ dodecahedron. rite, microcrystalline and chalcedonic quartz, mont­ MO.oroorystalline 1to chalcedonic quartz is the other morillonite, and iron oxides. Opaline silica, coating major constituent of the fluorspar deposits; it is white, botryoidal fluorite at the Alderman deposit and lining gray, or pinkish and is commonly interlayered and inter­ a cavity in the fluorspar vein at the Colorado-American grown with fine-grained and microgranular fluorite. In­ deposit, represents the latest phase of mineralization in tergrowths of fluorite and silica minerals range from these places. X-ray examination of this opaline material soft, nearly pure fluorite to hard, nearly pure siliceous by J. C. Hathaway (written commun., Jan. 28, 1958) material. Chalcedony with typical fibrous or aggregate and J. W. Hosterman (written commun., June 10, 1965) structure is not nearly so abundant as microcrystalline revealed approximately 50 percent low-temperature quartz. The microcrystalline quartz occurs as tiny cristobalite, 25 percent flfluorite, 15 percent montmoril­ wedge-shaped . crystals, some twinned, in fluorite; as lonite, and 10 percent quartz. microscopic films along the cleavage of fluorite; as rims Calcite occurs chiefly as veinlets in greenish fluorite, around fluorite grains and linings of vugs in fine-grained as tiny grains in soft red fluorite with iron oxides and fluorite; ·and laS mosaics within and between }ayers of clay, and as brecciated fragments with fine gray fluorite fluorite. The grain size of much of the quartz in fluorite in microcrystalline quartz. More calcite was seen on the is less than 0.01 mm. Quartz also forms veins and re­ lower level of the Chimney Hill deposit than elsewhere places fault gouge and breccia fragments. In the Colo­ in the district; some of 'the calcite there is lameller and rado-American fluorspar deposit a vein of microcrystal- is partly replaced by quartz. ECONOMIC GEOLOGY 33

Barite chiefly forms irregular grains in fine-grained fluorite; one specimen of layered fluorspar contains nearly 20 percent barite. Tiny barite crystals locally corut botryoidal fluorite in vugs in the veins. Pyrite is rare in the fluorspar deposits but does occur in some small pawhes and veins of microcrystalline quartz, in fluoritized breccia fragments, and in a few specimens of layered fluorite with marcasite, hematite, and limonite. A polished section of the sulfide minerals shows that marcasite fills cracks in pyrite and is partly replaced by fluorite. Manganese-oxide minerals were observed in all the productive fluorspar deposits, chiefly as layers and vein­ lets formed before and during the deposition of the fluorite. Postfluorite manganese oxide, associated with barite and montmorillonite, coats fluorite and quartz crystals in a few vugs in the Colorado-American fluor­ FIGURE 8.-Breccia fragment of rhyolitic welded tuff coated spar deposit. Material from small pockets in the Man­ by psilomelane (dark layer) and fluorite; specimen from ganese Hill fluorspar deposit contains about 20 percent dump at Alderman fluorspar deposit. Phot<;>graph by J. A. manganese. No sizable bodies of minable manganese ore, Denson. however, have been found in the Browns Canyon district. eral forms a uniform coating over the breccia frag­ Three manganese-oxide minerals-pyrolusite, man­ ment and presents a microcolloform surface to the gan:i,te, and psilomelane-are constituents of fluorspar fluorite. The fluorite was deposited in the open space veins in or near the Browns Canyon district. X-ray around the manganese mineral and in shrinkage cracks study of these minerals was made by Edward Chao within the layer of manganese oxide. In other specimens (written commun., June 13, 1958). Pyrolusite, a soft the sequence is fluorite, psilomelane, fluorite, and psilo­ radiating acicular black metallic mineral with black melane (Hewett, 1964, p. 1452). Manganese-oxide min­ streak, occurs along a fault 'between rhyolitic welded erals are found also in the Last Chance and Snowflake tuff of Oligocene age and Precambrian rocks east of fluorspar deposits, as described later. Epithermal veins the Arkansas River (SE!4 sec. 2, T. 50 N., R. 8 E.), of fluorite and manganese-oxide minerals that contain just beyond the south edge of the quadrangle. The almost 1 percent tungsten, here and elsewhere in south­ pyrdlusite is associated with chalcedony, pinkish fluo­ western United States, have been placed in a single late rite, and barite, and is clearly earlier than the fluorite. Tertiary metallogenetic epoch of short duration (Hew­ Manganite occurs as a moderately hard massive brown ett and Fleischer, 1960; Hewett and others, 1963; mineral with brown streak, in the Manganese Hill Hewett, 1964) . fluorspar mine of U.S. Fluorspar & Manganese, Inc., TABLE 11.-Semiquantitative spectrographic analysis, in weight NEl,l,t sec. 28, T. 51 N., R. 8 E. The manganite is accom­ percent, of manganese oxide, Alderman fluorspar deposit, Browns panied by quartz, pyrite, fluorite, and barite, in a de­ Canyon district, Chaffee County posit that occupies a fault in Precambrian gneissic [From Hewett (1964, p. 1444); Jab. No. 302037] quartz monzonite and tuffaceous siltstone of the Browns SL ______Major Mn ______Major Nb ______0. 003 AL ______1. 5 Ba ______3. 0 Pb______. 007 Canyon Formation. The manganite formed earlier than, Fe ______. 15 Be______. 003 Sr______. 07 and contemporaneously with, the fluorite. Tungsten­ Mg ______. 3 Cr______. 0007 V ______. 003 Ca ______3. 0 Cu______. 007 W ______. 7 bearing (table 11) psilomelane, a hard grayish-black Na ______. 07 Ga ______. 007 Zr______. 015 metallic mineral with black streak, coats fragments of Ti_------. 003 silicified and brecciated rhyolitic welded tuff at the NoTE: Elements looked for but not detected : Ag, As, B, Co, Ge, K, La, Mo, Nd, Ni, P, Sb, Sc, Tl, Y, Zn. Alderman fluorspar deposit (SEl,l,t sec. 34, T. 51 N., R. 8 E.), where it preceded (fig. 8), and is interlayered Small quantities of iron oxides occur as rims of with, botryoidal yellowish and_colorless fluorite. A limonite and hematite around fluorite grains; as blebs, thin section of a fault-breccia fragment of the volcanic patches, and veinlets within fluorite; as films between rock and its outer layers of minerals was examined by layers of fluorite; wri.tJh mi•rorocrystJalline quG~:rltz in E. S. Larsen, 3d, and S. K. Neuschel, who noted (writ­ breccia frngments cemented by fluori.te; as coatings on ten commun., Aug. 25, 1958) that the manganese min- fluorite crystals in V'l.lgs; and with m

TABLE 12.-Analyses of alkaline thermal waters from Browns closely ~assoeiaJted with epithermal mineral deposits Canyon and Poncha Springs jf.uorspar districts, Chaffee County (White and others, 1963, p. F20). Fluoride concentra· [Data in parts per million; n.d., not determined] tions of 10 ppm or more are rare in natural waters ~and Sample (Hem, 1959, p. 113), waters high in fluorine gen­ 267284 4712 4713. 4714 erally are high in sodium, low in calcium, and have a high pH. Lindgren ( 1933, p. ·69-73) pointed out the Si02------45 42 38 83 Fe (total) _____ ------______n.d. .17 .04 .05 ;association of fluoride with sodium bicarbonate spring ca ___ ------7 7. 6 7.9 17 water in rolcanirc regions 'and cited evidence for the dep­ Mg------­ <(5) . 7 1. 8 .9 Na------­ 156 158 151 194 osition of fluorite from thermwl waters rut· Wagon K------­ 5 6.8 4.8 8 COs------­ 0 4.9 4.9 0 Whe~ Gap, Odlo.; Ojo Caliente, N. Mex.; Oa:rlsbad HCOa------126 128 127 215 so. ______~ _ 146 141 142 204 Springs, Germ'any; Thplitz, Bohemia; and Plom­ CL------­ 55 55 53 51 F------­ 15 15 13 12 bieres, France. The ,association of such thermal waters NOs------·------­ n.d. .0 .0 .1 BOs------n.d. .3 . 3 • 2 Dissolved solids_------______n.d. 489 474 669 with fluorite deposits suggests, but does not prove, a 8~cific conductance (micromhos at 25°C) __ 754 761 739 959 genetic relation. p ------8. 5 8. 2 8. 2 7.9 Fluorite is one of the least soluble fluorides; its solu­ 267284. Analyzed by W. D. Goss and I. C. Frost, U.S. Geological Survey; other analyses by C. S. Howard, U.S. Geological Survey. bility is 1i mg/1 fluorine at ooc, 13 mg/1 fluorine at 267284 and 4712. Water from spring issuing from fissure in rhyolitic welded tuff, NE~NW~ sec. 34, T. 51 N., R. 8 E., Browns Canyonfiuorspardistrict. Temperature 10°C, 15 mg/1 fluorine at 20°0, and 30 mg/l fluorine at 22°C; fiow esti~ated by D. C. Cox to be 5 gallons per mintue in November 1945. 1 Sample 267284. Collected by D. H. Richter, J. J. Remley, and R. E. Van Alstine 100° C (Kazalrov and Solro1ova, 1950). Far water of in August 1958. Sample 4712. Collected by D. C. Cox in November 1945. 4713. Water from fissure in mineralized breccia composed of gneiss, pegmatite unit density, these mlue'S ~orrespond applioxim~ately to microcrystalline quartz, and fluorite on 100-level of Colorado-American fluorspar mine, Browns Canyon district (pl. 3). Temperature 18.5°C; fiow estimated by 23 ppm, 27 ppm, 31 ppm, and 62 ppm of OaF 2, respec­ D. C. Cox to be 2 gallons per minute in November 1945. 4714. Water from central sump at Poncha Hot Springs, Poncha Springs fluorspar tively. The overaN effect of the oonstirtmen'ts of the district, about 9 miles south of Browns Canyon district. Water issued from shear 2 zones in Precambrian gneiss and schist. Temperature 67~C in November 1945 Browns Canyon thermrul waters on the sO'lubiEty of OaF measured by D. C. Cox. Rate offiow for the many springs has been variously esti: mated at 100 gallons per minute (George and others, 1920, p. 229), 500 gallons per is not known. The solubility 10f OaF 2 incre:ases in the minute (Stearns and others, 1937, p. 133), and 50 g3llons per minute (White and others, 1963, p. F51). A similar analysis on a sample of this water collected in 1958 presence of 002 (Rankama and Staihama, 1950, p. 763). has in additionAl, 0.53; Mn, 0.10; Sr, 0.4; Ba, 0.1; and P04, 0.02 ppm (White and others, 1963, p. F50). Experiments hy l{iazakov and Sowolova (19'50), how- 36 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

ever, indicate thrut oolcium salts decrease the solubility percent HF, and incrustations around the vents con­

of OaF2 because of \the oommon-ion effec;t and that tained ~as muc;h as 15 percent fluorine as silicofluorides

Na2S04 dloes not have any noticeruble effect on the solu­ ( Zies,-1929).

bi'l~ty of OaF2 • At 10°C, incr~asing the NaCl content to Regardless of whether the ore fluids that formed the 100 gil Tesulted in a gmdua!l increase in the solubHity of Browns Canyon deposits had a high halogen-acid con­ OaF2 to a maximum of 18 mg/1 fluorine; · th~ fluorite tent at a magmatic source, there is little evidence that sdlu:bility decroosed witJh fuTit'her increase of tJhe N aCl the solutions were acid at the site of fluorite deposition. content. Effects of argillic alteration, probably by acid solutions ORIGIN (Lovering, 1950), were noted In some of the wallrocks, Evidence cited below indicates the Browns Canyon but there is little evidence for highly acid alteration fluorspar deposits are epithermal; such deposits form effects· such as those at the Valley of Ten Thousand at rather shallow depth, under moderate pressures and Smokes, Alaska. In Alaska, rhyolitic ash next to a temperatures between 50° and 200°C+ (Lindgren, fumarole that emitted HCl and HF was altered chiefly 1933, p. 212). According to Schmitt (1950, p. 209-210) to magnetillte, hematite, goethite, hydromica, opal, mont­ few deposits of this type exceed a depth of 3,000 feet, morillonite, and kaolinite, and the. inner and outer zones and most originated in a fumarolic-hot springs -en­ of alteration were leached of silica by the halogen gases vironment. The following are especially indicative of and halogen-acid solutions, respectively (Lovering, the epithermal origin of the Browns Canyon deposits: 1957). Silicification and fluoritization were ,the major (1) their occurrence near thermal springs; (2) labora­ types of alteration in the Browns Canyon district, and tory evidence for deposition of the fluorite from very the presence of fluorite interlayere.d with late opal and dilute solutions aJt 1aJbourt 119°-168°C; (3) the ·abun­ intergrown with calcite suggests that the solutions were dance of nodul,ar ore 'associated with brecci!a f·ragments; alkaline, at least during the late stages of mineraliza­ ( 4) the presence of vugs lined with fluorite and other tion. Stilbite, a zeolite in rhyolitic welded tuff next to · minerals that have botryoidal and colloform struc­ the Chimney Hill deposit, may have formed from an tures; and ( 5) the association of fine-grained, fine­ alkaline type of water similar to that found in the layered fluorite with barite, microcrystalline and nearby thermal springs. White( 1957, p. 1649-1651) chalcedonic quartz, opal, manganese· oxides, and clay recorded the formation of zeolites . from sodium­ minerals. According to Hewett ( 1964) such mineral bicarbonate waters and proposed the genesis of stich associations are typical of epithermal veins throughout thermal waters of volcanic origin, some of which are southwesteTn Uruited .States. White (1955, p. 100, 104, high in fluorine and low in calcium, by several possible 121), in reviewing the relation of thermal spl'1ings to mechanisms, depending upon the depth of penetration ore deposits, found that opal seldom forms at a tem­ of meteoric water and density of the gas phase. The perature much greater than 100°C and below a depth of open fractures and breccia along the mineralized faults 100 feet in sinter and veins at Yellowstone Park, Wyo.; of the Browns Canyon district suggest that circulat­ Steamboat Springs, Nev.; and Wairakei, New Zealand. ing meteoric water may have been a significant com­ Similarly, chalcedony and microcrystalline quartz are ponent of the mineralizing fluid that formed the phases that generally form under conditions present at near-surface fluorspar deposits. Dilution of hydrother­ shallow depths. mal fluids by meteoric water would decrease the Fluorine is a common constituent of igneous rocks temperature and salinity and may have caused precipi­ (Rankama and Sahama, 1950, p. 757) and is about the .tat~on of the vein minemls. fifteenth most abundant element in the earth's crust. Based upon the composition of the vein minerals, the Estimates of the quantity of fluorine in the crust range heated mineralizing fluid contained calcium fluorine from 260 to 900 parts per million (Fleischer, 1953, silicon, manganese, aluminum, iron, barium, hydrogen: p. 4) ; according to Fleischer and Robinson ( 1963, oxygen, ,a;nd sulfur. Analyses of hrines from primary p. 67) the average fluorine content of the continental fluid inclusions in coarse fluorite and associated miner.als rocks of ~he. erurth's crust is 650 ppm. Fluorine, :the from other localities have shown that sodium; chlorine most reactive element known, nearly always occurs in potassium,. and magnesium also may have been impor-' intrusive and volcanic rocks as a constituent of fluorite ta-nt elements of the ore-forming fluids (GruShkin and apatite, micas, amphiboles, tourmaline, topaz and' Prikhid'ko, 1952; Ames, 1958; Roedder, 1963; Hall and . other minerals. Fumarolic exhalations of steam' -and Friedman, 1963) ; however, the freezing temperatures acid gases in the Valley of Ten Thousand Smokes, of primary fluid inclusions in fluorite from the Colo­ Alaska, annually yielded hundreds of thousands of tons rado-American mine and workings to the northwest in of fluorine-containing gases in concentrations of 0.032 the Browns Oanyon district indicrute .that the fluid is es- ECONOMIC GEOLOGY 37 senti·ally :£resh water (Edwin Roedder, U.S. Geologioa;l with fluorspar veinlets at an altitude of about 7,560 feet Survey, oral commun., Mar. 1966). Roedder ( 1963, p. in NW~ sec. 2, T. 50N., R. 8 E. 171, 177, 179) also reported low salinities in inclusions GRADE, SIZE, AND RESOURCES in fluorite from similar Tertiary, near-surface, hot spring-type deposits at the Wagon Wheel Gap mine, The grade of the fluorspar deposits depends chiefly Mineral County, Colo., and the Cougar Spar mine, upon the relative quantities of fluorite, microcrystalline Beaver County, Utah. · quartz, and brecciated wallrock in the mineralized fault The fluorite in the Browns Canyon district evidently zones. Channel samples more than 5 feet long range from was deposited at about 119 6-168°0, the temperatures 93 percent CaF2 and 5 percent Si02 to less than 1 per­ at which primary fluid inclusions homogenized in five cent OaF2 and more than 80 percent Si02• The grade samples, including both gray ·and octahedral, early of mined ore bodies ranges 'from less than 25 percent green types. of fluorite (Edwin Roedder, written OaF2 to more than 75 percent OaF2 ; some of the largest commun., Aug. 11, 1966) ; two of the samples were ore bodies contain 50-55 percent OaF2 • During World collected from the Colorado-American mine ·and three War II, when four mills were active in the Browns Can­ from areas to the northwest-from the Delay adit, at yon district, mill heads ranged approximately from· 15 a cut about 800 feet northwest of the Delay adit, and to 75 percent CaF2 • Published analyses (Batty and at .a shaft .about 400 feet east-southeast of the Chimney others, 1947, p.12) of ore-dressing samples from several Hill mine. (See pl. 1 for locations.) No correlation veins in the district are given in table 14. exists between these two color varieties of fluorite and TABLE 14.-Analyses of typical ores of the Browns Canyon fluor­ the temperatures at which the vapor phase in the inclu­ spar district sions disappears uponheating. Inclusions in extremely [From Batty, Havens, and Wells (1947, p. 12)) fine-grained and fine-layered fluorite, the most common Fe MgO AIJOa R20a type in the Browns Canyon district, are too small for such temperature investigations. Temperature correc­ 67.8 20.4 0.45 0. 7 0.3 1.95 2.95 42.1 36.0 .09 1.35 .3 6. 0 7.9 tions for the pressure that existed when the fluorite was 21.1 50.2 1.1 2. 0 ------11.0 13.8 deposited apparently would be very small, as the fluorite NOTE: In each analysis the balance of the constituents should probably be attributed formed essentially in a hot spring environment. The chiefly to CaO, K20, and N a20 of feldspars and other minerals of the wallrocks. above temperature range is slightly greater than the Analyses of ores indicate that fluorspar from the 100°-150°0 range that fluid-inclusion study suggests Colorado-American mine generally contains less micro­ as the temperatures at which similar fluorite in the crystalline quartz, clay, and iron oxides and fewer brec­ N orthgate district, Jackson County, Colo., formed cia fragments of the wallrocks than are found in fluor­ (Steven, 1960, p. 409-411). spar from other parts of the district. The greatest con­

· The heat and fluoride, although regarded as volcanic tent of CaC03 , about 4 percent, was found in reddish contributions to the mineralizing fluid, cannot be at­ microcrystalline fluorite from .the Last Chance deposit tributed to any known igneous body within the area. (Norman Davidson, analyst, Feb. 9, 1943). Generally, The NaJthrop Voloanios of l'ate 01igocene ·age, remlllants ·the BaS04 oonrtent of district ores is less tihan a few of which are exposed 6-7 miles north of the fluorspar tenths of a percent, and the sulfur content is less than deposits, were emplaced close to the period of fluorspar a few hundredths of a percent. Nine spectrographic mineralization; the rhyolite flow contains· the fluorine analyses (COL-51-1 to 9) of ore, concentrates, and tail­ minerals, topaz, and rare fluorite, and the underlying ings from the Colorado-American deposit have been perlitic glass contains 0.23 percent fluorine, twice as published (Kaiser and others, 1954, p. 40-41). The much as that in any of the analyzed older Tertiary vol­ analyses show as much as 0.0005 percent BeO and 0.0005 canic rocks of the area. As described later, however, peroent NiO in the flotation mill heads, which contained fluorspar mineralization at several. deposits in the about 45 percent OaF2 ; 0.0003 percent BeO and 0 per­ Browns Canyon district is younger than the Browns cent NiO in the flotation concentrates that contained about 95 percent OaF2 ; and 0.0005 percent BeO and Canyon Formation of Miocene age. The principal fault­ 0.004 percent NiO in the tailings, which contained about . ing and mineralization preceded deposition of the Dry 10 percent CaF2 • Union Formation of Miocene and Pliocene age and are The Browns Canyon fluorspar deposits are large and definitely older than gravels 3 and 4, of early to middle have extensive mine workings. As shown on plate 1, one Pleistocene age. Gravel 3 lies upon the major fluorspar of the fault zones that coh,tains fluorspar ore bodies is vein near the north boundary of sec. 34, T. 51 N., R. 8 E., more than 3.5 miles long; fluorspar ore is almost con­ and gravel 4 contains boulders of rhyolitic welded tuff tinuous for about 3,000 feet· along the east branch of 38 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

this major fault zone at the Colorado-American deposit p. 415; Hewett, 1964, p. 1460-1463) ·have suggested that ~and has been mined for more than 1,600 foot. About 40 such low-sulfide, epithermal fluorspar deposits may feet of ore was exposed underground here at the inter­ contain valuable metal sulfides or precious metals at section of a small fault with the west branch of the greater depths. main fault; the average thickness in the workings, how­ Geochemical-prospecting investigations in the dis­ ever, is less than 10 feet. The veins commonly are lower trict (Van Alstine, 1965) revealed abnormal fluorine grade and contain more breccia fragments of wallrock concentrations in residual soil directly above, and down­ where they are narrow. The greatest vertical distance slope'from, the principal fluorspar vein, and in alluvium through which any of the veins in the district has been downstream from the vein. The largest anomalies are exposed is about 450 feet, at the Colorado-American appro~imately 6 to 20 times the background in the soil deposit. Ore shoots elsewhere in the district range from and 3 times the background in the ~aHuv~um. Fluorine lenses several tens of feet long and a few feet thick to values in soil near a concealed fault are too low t9 sug­ sheeted zones more than 1,000 feet long and about 25 gest the presence of an underlying fluorspar deposit. The feet thick. The sheeted ore body mined in an open cut study of the geochemical anomalies suggests that · at the Last Chance deposit had a maximum thickness of further investigation of the fluorine content of soil and

about 25 feet and averaged about 30 percent CaF2 • alluvium in this district might be profitable in the Estimated fluorspar resources of the Browns Canyon search for additional fluorspar deposits. district, totaling about 2 million short tons of crude FLUORSPAR MINES AND PROSPECTS fluorspar, are given· in table 15. Resources containing BY RALPH E. VAN ALSTINE and DOAK C. Cox more 35 percent CaF 2 , estimated to a depth of 100 feet

below the workings, average about 50 percent OaF2 • Re­ The following descriptions of the fluorspar mines

sources oontJaining 15-35 percent OaF2 !aver.age about 20 and prospects are based largely upon data acquired percent OaF2· when they were accessible between 1943 and 1945. Written permission for publication was obtained from TABLE 15.-Estimated fluorspar resources, Browns Canyon dis­ trict, Chaffee County, expressed as short tons of crude fluorspar Colwado Fluol'ISpar M1ines, Inc., American Fluorspar Corp., K~r,amer Mines, Inc., and United States Fluor­ >35 _per- 15-35_per- spar & Manganese, Inc. cent CaF2 cent CaF2 Total COLORADO-AMERICAN MINE Measured and indicated___ 250, 000 . 150, 000 400, 000 Inferred______150, 000 1, 500, 000 1, 650, 000 About 90 percent of the fluorspar concentr·ates shipped from the Browns Canyon district came from the TotaL______400,000 1, 650,000 2, 050,000 Colorado-American mine (No. 2 on pl. 1) , about half a mile east of the former route of Colorado State High­ SUGGESTIONS FOR PROSPECTING way 291. Amerioan Fluorspar ·Corp. ·acquired th~ north­ Bodies of fluorspar similar to those already discovered western part of the deposit from Stephen Trefone and probably would be found by e~ploring: ( 1) the known associates in 1930 and was a steady_producer from 1939 deposits at greater depths; (2) northwest and southeast through 1945. The southeastern part of the deposit was extensions of the east and west branches of the main worked by Oolomdo Fluorspar Co. in 1934 ~and 1935, fault zone that localized· the Colorado-American de­ by Colomdo Fluorspar Corp. £,rom 1936 throug:h 1944, posit; ( 3) the main fault zone that displaces the and by Colorado Fiuorspar Mines, Inc., N-om 1945 to Tertiary volcanic and Precambrian rocks between the 1948 ; these organizations accounted for about 60 per·· Chimney Hill and Manganese Hill deposits; ( 4) the cent of the production from the Colorado-American fault extending northwest from the Alderman deposit; deposit. Development work by Allied Chemical Corp., ( 5) shorter faults in the gneissic quartz monzonite, General Chemical Division, which acquired the 1nine similar to the fault that localized the Last Chance de­ from American F~luorspa;r Corp. in 1946 'and Colopado posit; and ( 6) the fault that bounds a major horst on F1uor·spar Mines, Inc;, in 1948, was looalized chiefly in the northeast and extends southeast from sec. 22, T. 51 .tJhe northwestern part of the mine. N., R. 8 E., as this fault also may be mineralized locally The geology and workings of the Colorado-American with fluorspar, similar to the productive deposits along mine are shown on plates 2, 3, and 4. Mine workings the fault bounding the horst on the southwest. (See pl. 1 and surface·cuts accessible in 1945 exposed the fluorsp·ar for all mine and fault locations.) deposit from 'an altitude of about 1,210 to about 1,120 Although no changes in the mineralogy have been feet, ·a vertical range of 450 foot. Since most of the map­ detected in the veins to a mining depth of about 450 ping in 1'945, the open pit was enla.rged ·and deepened feet in this district, other investigators (Steven, 1960, approximately as shown on plate 2; the A·merican shaft, ECONOMIC GEOLOGY 39 shown on plate 1 along the boundary between sees. 27 The West vein is explored underground in the Mill and 34, was sunk and a level was driven on the vein adit (pl. 3) near the intersection with the Branch vein, southeast to connect with the Trefone adit (pls. 2, 3) where a drift exposes a 150-foot segment of high-grade and also northwest for about 125 feet. Plate 3 shows ore 6-8 foot thick and a 70-foot segment of low-grade about 4,700 feet of drifts and 1,200 feet of crosscuts in breccia to the southeast. The fault that localized the the southern part of the Colorado-American deposit. West vein is well exposed (fig. 9) at the northeast edge Most of the shrinkage stopes shown on plate 4 are on of the large open pit shown on plate 2. the thicker and richer parts of the veins. Although the The Branch vein, which was worked mainly in the southeastern p!lirt of the deposit is largely stoped above open pit, contains the thickest fluorspar body in the the Colorado 100-level, ore is exposed below ·and in most Browns Canyon district; in the open pit between the of the working faces at the ends of the levels. Mill adit and the original surface the ore body is 40-50 The mine workings expose three main veins-the East feet thick. The ore body in the open pit consists largely vein, 'the West vein, and the Branch or Collins vein. The of shells of fluorite coating rounded blocks of rhyolitic East and West veins strike about N. 40° "'\iV., 'and di'P welded tuff that are discarded in mining. In 1948 flota­ 65°-85° SW.; the BTanch vein strikes west 'and dips tion-mill feed from the open pit contained 35-40 per­ rubowt 45° S. The East and West veins "

EXPLANATION

~~ Contact <(<( :JZ } oo:: 85 Alluvium w Fault, showing dip Dashed where approx­ >- imately located j{ 0:: Rhyolitic welded tuff <( 1- Vertical fault 0:: w Topography and geology by D. C. Cox, August 1943. 1- Modified by R. E." Van Alstine, 1953 Strike and dip of Surface: pace and compass Rhyodacitic ash Underground: tape and compass foliation

z <( Fluorspar 0:: Banded quartz-feldspar­ !D biotite gneiss ::E <( Microcrystalline quartz u w and chalcedony 0:: a.. Hornblende gneiss Opencut or trench

....,\I I I 1_

Dump

11!1 Shaft

IJI Shaft extending through level

Iii Bottom of shaft

FEET A 7700 100-LEVEL \ L .l " 7680

55-LEVEL "7 L

7660

.J '1 7640

APPROXIMATE MEAN 7620 DECLINATION. 1969 LLOYD

7600

100 0 100 PLANS AND SECTION, LLOYD SHAFT

CONTOUR INTERVAL 20 FEET 40 0 40 80 FEET DATUM IS MEAN SEA LEVEL

FIGURE 10.-Geologic maps ·and section of the Delay adit and Lloyd shaft, Browns Canyon fluorspar diStrict, Chaffee County, Colo. ECONO~C GEOLOGY 41 ash commonly is silicified and argillized, euhedral hi­ The grade of the mineralized zone 'averages about 20. otite plates are still recognizable. percent CaF2, which is also the grade of a 15-foot channel sample taken in a bulldozer cut in 1943. Analy­ MANGANESE HILL AND CHIMNEY HILL MINES ses of individual lenses of fluorspar in the incline give Several thousand tons of fluorspar have been ex­ as much as 56 percent CaF2 for a 5-foot channel sample. tracted from the various shafts, winzes, inclines, adits, Some of the higher grade ore was hand picked and pits, and trenches (pl. 5) at the Mangane~e Hill ( N ?· 6, shipped •as metallurgioal-gr:ade fluorsprur containing 'at pl. 1) ®nd Chimney Hill (No. 5, pl. 1) mines of Un1ted least 85 percent CaF2 • The lower grade ore was proc­ States Fluorspar & Manganese, Inc. The mines -are at essed chiefly into ceramic-grade concentrates in the mill ,an ,altitude of ~rubout 7,600 feet, and rure :about 2,000 feet on the property. Some acid-grade concentrates from from U.S. Highway 285 by way of Chaffee County Road Manganese Hill ore were made in the mill at the Last 60 and a company access road to the north. The deposits, Chance mine. known originally as the Morgan Ranch deposits, were ·The workings at the Chimney Hill mine, about 650 first worked about 1936. In 1939 Chaffee County Fluor­ feet south of the Manganese Hill mine, 'are shown on spar Corp. acquired the deposits and shifted most of plate 5. The main adit is 34 feet below the collar of a the mining activity from the Chimney Hill mine to the two-compartment shaft, and the lower level is 51.5 feet Manganese Hill mine. United States Fluorspar, Inc., below the main adit at this shaft. In 1953 the lower superseded this company in 1945 and completed a flota­ workings were flooded, and the water level in the shaft tion mill on the property. In 1950 United States Fluor­ was 10 feet below the main adit. The workings are spar & Manganese, Inc., acquired the property and almost entirely in silicified and chloritized rhyolitic expanded the capacity of the mill to about 100 tons a welded tuff; locally, sanidine phenocrysts ·are altered day. . . . . to quartz fluorite, and calcite. Breccia -and gouge, cut The deposit at the Manganese Hill mine IS mam1 y by veinlets' of microcrystalline quartz, fluorite,. an d in gneissic quartz monzonite of Precambrian ag~; t?e barite, were exposed at the north end of the lower -level. rock is highly weathered in this part of the district Thin sections of this breccia and gouge show angular and contains much hematite and clay and traces of f,vagments of gneissic quartz monzonite :and Trhyolitic mrunganese oxide :as stains, especi•a:lly in the upper pa;rt welded tuff in ,a matrix of quartz, sericite, green biotite, of the workings. Locally the wallrock is the Browns chlorite, -a clay mineral, pyrite, limonite, and a man­ Canyon Formation, which here consists of tuffaceous ganese-oxide mineral. siltstone, claystone, lithic tuff, and generally a basal Fibrous and massive fluorite, microcrystalline quartz, conglomeratic arkose. At the southeast edge of the open :and calcite occupy the mineralized zone that strikes N. pit (pl. 5) brown siltstone is composed of angular or 60°-75° W. and dips steeply south. Most of the calcite subrounded grains of quartz, microcline, sanidine, and is coarsely crystalline, -and some is lamellar and partly biotite in a matrix of finer quartz, chlorite, sericite, replaced by quartz. Examination of the vein minerals kaolinite, magnetite, hematite, limonite, barite, and in the workings and in specimens indicates that some fluorite. FJuorspar, whioh forms discontinuous lenses of the oalcite is earlier vhan quartz :and fluorite. and veinlets more than 6 inches thick in sheeted zones as much as 12 feet thick, was exposed in 1945 in workings The Chimney Hill workings expose at least four through a strike length of more than 250 feet and a bodies of fluorspar, some more than 50 feet long, through a strike length of 350 feet along the mineralized depth of ,about 100 fe~t. The mineralized fault zone strikes about N. 70° E., and dips 75°-80° N. zone. The main zone splits west of the shafts, where the Three inclines were driven ·eastward from the same content of microcrystalline quartz appears to be higher. portal :at angles of about 10°, 20°, and 30° from the The greatest thickness of fluorspar, about 15 feet, was horizontal. The third or lowest incline (pl. 5) slopes observed in ·the pit on the southernmost split. The about 30° in the westernmost 100 feet of workings and larger bodies contain about 30 percent CaF2 ; most of the is nearly level eastward to the face, which is nearly 100 remaining content is silica, and calcite forms less than feet below the east edge of the open pit. The workings 3 percent of the exposed fluorspar bodies. Although expose several types of fluorite-soft, fibrous, fine­ examination of ore specimens under the microscope in­ layered greenish to purplish fluorite, and dense brown dicates the fluorite contains minute quantities of quartz, fluorite, which is partly earthy and partly hard and chalcedony, and iron-oxide and clay minerals, both granular. The brown fluorite is later than the other m~aH'Urgiml-grade and cwamic-grrude fluorspar con­ types, contains manganese-oxide minerals as small centrates have been made from the small quantity of pockets and coatings, and forms the main body of fluor­ ore mined at the Chimney Hill deposit. spar exposed in the lowest incline. Diamond drilling of the Manganese Hill and Chim- 42 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

ney Hill deposits reportedly (Engineering and Mining of the ore bodies is about 25 feet, which is .the thickneSs Journal, 1953) revealed more than 1 million tons of of the' mineralized sheeted zone mined in the large open fluorite-bearing material, which contains about 20 per­ pit west of Chaffee Oounty Road 60. The width of the cent CaF2. The adjacent northwest-trending fault zone stoped sections ranges from 2 to 12 feet, and the average between tihe Tertiary and Precambrian rocks might width of ore in the stopes is nearly 8 feet. profit.rubly be explored. where i·t is ooncealed. {.See pL ·5.) The vein minerals at the Last Chance mine are fluorite, microcryst~lline quartz, . chalcedony, pyrite, LAST CHANCE MINE calcite, hematite, and manganese oxides. Most of the·: The Last Chance fluorspar mine (No. 10 on pl. 1) was fluorite is very fine grained and white to red; some is the source of nearly 1,000 tons of metallurgical-grade colorless, brownish," or greenish. The silica minerals concentrates before· 1942 and about 10,000 tons of flota­ commonly are finely intergrown with fluorite in the tion concentrates between 1942 and 1944. This mine, veins and ·also replace fault breccia, gouge, and the wall­ known as the Kramer mine during the World War II rock. Pyrite occurs as rare disseminations in the silici­ period of mining activity, is at an altitude of about fied wallrock. The black manganese-oxide minerals 7,800 feet. The workings extend beneath Chaffee County locally line vugs in the fluorspar ore and form a narrow Road 60 and are almost entirely in claims of Commer­ vein cut by fluorite veinlets in the hanging wall of the cial Minerals, Inc. (formerly Universal Mines, Inc., and eastern ore body. . . Mansheim.,Salida Fluorspar Co.), that were leased to The average grade of the ore mined and treated in Kramer Mines, Inc., from 1942 to 1944. Kramer Mines, the flotation mill between 1942 and 1944 was about 30 Inc., completed a flotation mill on rthe property in Octo­ ~rcent CaF2: A 4-foot channel sample cut at the face ber 1942, which yielded ceramic-grade and a small of the west dl'1ift on the Lower Adit level contained 55.9 quantity of acid -grade concentrates before it burned in percent CaF2, and a 4.3-foot channel sample cut in the July 1944. The flotation concentrates contained 93-98 top of the stOpe mapped on the 90-level contained 41.3 percent CaF and 1-5 percent Si0 ; an earlier hand­ 2 2 percent CaF2 ; the remaining content of these two picked metallurgical-grade product contained 80-95 samples is mainly silica. A specimen of red ore con­ percent CaF2 and about 2.5-15 percent Si02. tained 59.2 percent CaF2, 24.6 percent Fe20a, 9.8 per­ The geology -and exrtent of the Last Chance workings cent Si02, 3.8 percent CaCOa, and 1.7 percent H20 as of August 1943 are shown on plate 6. Shortly there­ (Norman Davidson, analyst, written commun., Feb. 3, after, the 90-level reportedly was extended eastward an 1943); m'OSt ore from .the Last Chance mine, however, additional 210 feet from the winze and exposed a 50- contains less than 1 percent. CaCOa. foot segment of· ore 6 feet thick; the stope above the 90-level was 90 feet long, and the lower level was ex­ OTHER FLUORSPAR DEPOSITS tended more than 150 feet west and showed mineralized A small quantity of metallurgical-grade fluorspar segments 2-3 feet thick. None of these newer workings was mined and shipped in 1944-1945 from the Alder­ were mapped, however, as most of the mine is inaccessi­ m31n deposi1t (No. 1 on pl. 1) by John Aldermm and ble because of caving and flooding. associates of Denver. This mine, owned then by James The Last Chance fluorspar deposit is localized along Bopp, is at an altitude of about 7,400 feet, northeast a fault that strikes about N. 65' 0 W. and dips about 60° of Colorado State Highway 291 and southwest of the NE. The fault cuts gneissic quartz monzonite of Pre­ Arkansas River. In 1945 a crosscut. adit, now caved, ~·amhrian •ruge and ·a few sm1a;l1l lenses of the o'lder extended east-southeast about 210 feet to a 90-foot drift metamorphic rocks; locally, the faulrt consists of a along a steep mineralized zone that strikes about N. 85 ° sheeted zone containing many veins of fluorspar. E. in rhyolitic welded tuff. Many steep northwest­ Fluorite and silica partly replace the wallrock and trending faults in the crosscut transect altered volcanic fault breccia, and in places fluorspar veins as much as ash. One fault zone, about 145 feet from the portal, 6 inches .thick extend into the hanging wall. Fluorspar separates the ash from the rhyolitic volcanic rocks on is exposed intermittently along the fault through a the northeast. Some of these faults are mineralized strike length of about 1,500 feet and a depth of about locally with calcite and a small quantity of fluorite. 300 feet. The ore body in the drift consists of a partly mineral­ Two main ore bodies were found along the fault. ized fissure containing loose blocks of rhyolrl.tic welded The eastern ore body, which was developed by open tuff several feet in diameter that are cemented by pits, Adit No. 1, Ad it No. 2, the Lower Adit, and the fluorite, chalcedonic quartz, and black manganese oxide. 90-~evel, is ·rubout 460 feet 1ong in the .adits. The west­ Vugs between the blocks are as much as 3 feet wide and ern ore body, worked from open 'pits and the Lower are olined wi.th botryoidal ·and columnaT white to yellow­ Adit, is about 450 feet long. The maximum thickness ish fluorite less than an inch to several inches thick. ECONO~C GEOLOGY 43

The mineralized zone is about 10 feet thick and less The Snowflake deposit (No.3 on pl. 1) was the source than 50 feet long and contains an estimated 30 percent of about 400 ~tons of fluorspar between 1933 and 1943. CaF2. This deposit, owned by E. Lionelle and ,J. H. Lionelle, is Botryoidal fluorite is abundant on the dump at the an eaJst-trending vertioa:l vein in rhydlitic we'1ded tuff. Alderman deposit; with the aid of a binocular micro­ Several lenses of fluorspar, 2-5 feet thick and estimated scope, various combinrutions of seven isometric crystal to contain 40-70 percent CaF2, are exposed in a short forms can be seen on the fluorite. Many specimens of adit and in a lower incline about 15 feet below the portal this botryoidal material consist of alternating layers of of the adit. The incline slopes we~tward about 15 o and fluorite and psilomelane; a spectrographic analysis of is about 220 feet long. Fluorite veinlets cut l-inch vein­ tJhe psilome}ane ~s given in table 11. lets of manganese-oxide minerals at the end of the The White King, Blue Stone, and Puzzle deposits incline. along Chaffee County Road 60 in the NW% sec. 27, DEPOSITS OF OTHER MATERIALS T. 51 N., R. 8 E., yielded a total of about 1,700 tons of COPPER PROSPECT fluorspar as a hand-picked metallurgical-grade product A magnesian skarnlike aggregate of silicate minerals and ceramic-grade flotation concentrates. Nearly half locally contains copper minerals near the southeast of this production was in 1929-1934 and 1943-1944 from corner of the quadrangle and about 700 feet above Rail­ the White King deposits (No. 7 on pl. 1), which road Gulch (NW% sec. 32, T. 51 N., R. 9 E.). The Allied Chemical Corp. later acquired from American inactive prospect can be reached by an access road F~luorspa;r Corp. Several caved workings, consisting branching from the road to Turret, which is half a mile mainly of a 220-foot adit and a shallow shaft, are local­ east of the quadrangle; it is known as the Ace High ized along steep northeast-trending mineralized faults and Jack Pot prospect, according to a claim notice by in the gneissic quartz monzonite. Several hundred feet Glen R. Lemberg and' Son of Salida, Colo. The deposit southwest of the adit, the mineralized faults cut the does not crop out but is probably a lenticular mass in Browns Canyon Formation, which overlies the gneissic metasomatized hornblende gneiss between two north­ quartz monzonite. The ore bodies, 6 inches to 5 feet trending faults that localized a narrow chalcopyrite­ thick, consist of a network of veins averaging about 30 qua:rtz vein dipping 85° W. 1and a ver.tioal pegm'atite . percent OaF2 and some higher grade lenses containing dike (pl. 1). The foliation of adjacent unaltered fine­ about 70 percent OaF2 • The fluorite is chiefly white, pale grained hornibllerrde gneiss, ·Consisting of hornb1ende, green, or pink and sugary, 'and the gangue is fine­ by!townite, oordieri:te, ·and sm-a'll quantities of biotite, grained quar.tz. quruvtz, epidote, and apati~te, dips gently towa·rd the Southeast of the White King deposits, the adjacent east. The fuUowing inrormation is based liargely upon Blue Stone deposits (No. 8 on pl. 1) of Commercial 'a study of specimens oolllooted from the dump next to an .Minerals, Inc., were the source of about 600 tons of iillruccessible ~shaft that ihrus . underground workings of fluorspar in 1943-1944. A shallow shaft, an adit, and unknown extent, and from an incline driven north a:long several cuts form the workings, which are on irregular the chalcopyrite-quartz vein. vein zones trending northeast and northwest in the The hornblende gneiss is reorganized into a skarn­ gneissic quartz monzonite. Mineralized zones are locally like, coarse-grained, locally schistose aggregate com­ more than 5 feet thick and contain 25-40 percent OaF2· posed chiefly of hydrous magnesium, calcium, iron, and East of Chaffee County Road 60, the Puzzle deposit aluminum silicates, which possibly formed by retro­ (No.9 on pl. 1), owned by E. Lionelle and J. H. Lion­ grade metamorphism. Specimens consist of layers con­ elle, yielded about 400 tons of metallurgical-grade fluor­ taining one or more of the following minerals, which spar between 1937 and 1949. Workings from a 135-foot were identified by microscopic study of crushed frag­ shaft expose about 1: feet of fluorspar along an east~ ments in index oils: actinolite, anthophyllite, apatite, trending vein in the gneissic quartz monzonite. The biotite, calcite, chlorite, cummingtonite, gahnite, phlogo­ overlying tuffaceous siltstone of the Browns Canyon pite, quartz, sphene, talc, tremolite, and zoisite. Formation exposed on the south side of a hill east of Prisms of tremolite and anthophyllite ( nZ = 1.650 the Puzzle deposit also is locally mineralized with fluor­ approx.) form radial growths; fibrous tremolite is cut ite, which constitutes about 50 percent of one thin section by veinlets of phlogopite ·and talc. Magnetite and of the rock. Fluorite is associated with barite and a chalcopyrite locally impregnate the rock-forming sili­ brownish-yellow iron -oxide mineral in veinlets and cates as tiny grains or fracture fillings. Magnetite is blebs less than 0.05 mm in diameter. Some of the fluorite almost completely altered to limonite ·and hematite. forms pale-purple cubes that have edges truncated by Chalcopyrite is altered chiefly to fibrous and botryoidal dodecahedrons. · malachite and to small quantities of chalcocite, azurite, 44 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

chrysocolla, brochantite, and chalcanthite. The presence percent of the total production of placer gold in Chaffee of malachite, brochantite, ·azurite, and chalcanthite was County was made (Vanderwilt, 1947, p. 42). The fine­ confirmed by X-ray investigation (Mary Mrose, oral ness of some placer gold recovered from the Browns commun., Feb. 18, 1966). Specimens from the dump Canyon area averaged 0.836 gold and 0.158 silver (U.S. show that chalcopyrite-quartz veinlets were brecciated, Bureau of Mines, 1934, pt.l, p. 521-522). cut by malachite veinlets, and then coated by botryoidal According to Vanderwilt ( 1947, p. 43) , the Browns clusters of calcite and opa1. A psilomelane-type black Creek placer deposit is 2-3 miles south of Nathrop, but manganese oxide mineral contains small patches of bar­ the writer observed an old sluice, screens, shovels, and ite and tiny grains of calcite. worked gravels about 4 miles south of Nathrop ; this Gahnite, the zinc spinel, occurs in tremolite-talc rock locality is south of the mouth of Browns Creek where and in quartz veinlets as blue-green subhedral to euhe­ T. 15 S. and T. 51 N. join. The worked gravels, con­ dral octahedra as large as 0.5 em; it forms nearly 10 baining some very large boullders of gneis8ic quartz percent of some specimens. The index of refraction of monzonite, came from a narrow, low Pleistocene terrace

this gahnite is slightly less than 1.780, and a0 is 8.12 A along the west bank of the Arkansas River. (D. B. Stewart, oral commun., Dec. 10, 1964). Some Much more extensive gravels are in the older Pleisto­ grains contain microscopic inclusions of apatite and cene pediment, alluvial-fan, and outwash deposits at sphene, and others are slightly magnetic. A qualitative levels above the Browns Creek placer, west of the Ar­ X-ray fluorescence investigation of the gahnite indi­ kansas River and east of the large Tertiary intrusive cated zinc and lesser amounts of iron as major constitu­ bodies and associated base and precious metal deposits ents, traces of calcium and titanium, and no cobalt in the Sawatch Range (Dings and Robinson, 1957). It (F. J. Flanagan and H. J. Rose, Jr., written commun., it not known whether these gravels and the immediately Jan. 29, 1962). underlying thick section of Dry Union sediments have · This deposit ~ibll~ other Prooambrian smarn'like been tested for metals. assemblages in Colorado, especially some gahnite-bear­ Mine workings, consisting of several short adits and ing copper-zinc deposits once worked in adjacent areas, a; shaft, possibly explored for gold or tungsten between which L~ndgren (1908) regarded as magmatic segre­ Stafford Gulch and The Reef (sec. 30, T. 51 N., R. 9 E.). gates in gabbro· dikes regionally metamorphosed to The workings are in north -dipping banded gneiss and amphibolite. More recently, however, one of these de­ hornblende gneiss, south of the large body of gneissic posits, the Cotopaxi, about 20 miles southeast of Salida, quartz monzonite and the overlying Tertiary rhyolitic has been described as a pyrometasomatic skarn deposit welded tuff. Specimens from the dumps are chiefly the in an amphibolite ~layer between biotite gneiss and silli­ two types of gneiss; some are cut by pegmatite, and manite gneiss (Heinrich and Salotti, 1959); galena others are aplitic, brecciated, cemented and veined by from the Cotopaxi deposit is probably not much younger quartz and gray calcite, and stained yellow brown with than 1,300 +100 million years old (Phair and Mela, limonite. Fluorescence tests on ·the specimens, using an 1956, p. 428). In the northern part of the Front Range, ultraviolet light, are negative for scheelite. The semi­ a copper-zinc deposit in layered anthophyllite-cum­ quantitative spectrographic analysis in table 16 indi­ mingtonite-actinolite skarn probably formed by metaso­ cates that brecciated and veined aplitic gneiss from the matism of regionally metamorphosed sedimentary rocks '£lldit n~rurest Stafford Gu'lch contains smaU quantities of (Sims and others, 1958, p. 184-185). Similarly, layered metals. X-ray examination of this· material found but unmineralized anthophyllite-cordierite rocks asso­ orthoclase, quartz, and calcite as the major minerals and dated with amphibolite 'and p3!mgneisses in the Central a possible trace of hematite (Marie L. Lindberg, written City area may have formed from metasomatism of commun., Nov. 4, 1966). metasedimentary rocks (Sims and Gable, 1963) . TABLE 16.-Semiquantitative spectrographic analysis, in weight GOLD percent, of aplitic gneiss, brecciated and veined ·by quartz and calcite At the Browns Creek placer deposit, gravels along the [J. L. Harris, analyst; lab. No.·W-167073] Arkansas River in Browns Canyon yielded a small SL ____ _ >10 Mn ___ _ 0.07 Nb ____ _ 0. 002 AL ___ _ 3 Ag ____ _ <. 0001 NL ___ _ . 003 quantity of gold from sluicing, rocker, and panning Fe ____ _ 3 Ba ____ _ :01 Sc ____ _ . 002 Mg ___ _ Co _____ · Sr ______operations intermittently during 1906-17 and 1931-39 . 07 . 0015 y ______. 003 Ca ____ _ >10 Cr_· ___ _ . 01 . 007 (U.S. Geological Survey, 1907-17; U.S. Bureau of Na ____ _ . 05 Cu ____ _ . 0007 y ______. 003 K ______7 Ga ____ _ . 0007 Yb ____ _ . 0003 Mines, 1934; U.S. Bureau of Mines, 1933-40; Vander­ TL ___ _ Mo ___ _ Zr ____ _ wilt, 194 7, p. 43) . Possibly gold was recoverred from this . 15 . 001 . 003 NOTE: Elements looked for but not detected: As, Au, B, Be, Bi, Cd, Ce, Eu, Ge, deposit also between 1859 and 1906, when about 90 Hf, Hg, In, La, Li, I', Pb, Pd, Pt, Re, Sb, Sn, Ta, Te, Th, Tl, U, W, Zn. ECONO~C GEOLOGY 45

GRAVEL AND BAND here were favorable for the accumulation of slightly West of the Arkansas River, extensive deposits of humified vegetation in the poorly drained depression. gravel and sand are worked intermittently, and on a The ground, which is being worked on a small scale, is small scale, as sources of material for road construc­ said to be on the property of Joseph N achtrieb, who tion by the State and County Highway Departments. leased this part of the deposit for mining. Six roadside pits have been excavated in rec~nt years The limits of the area underlain by pea.t (pl. 1) were in the quadrangle, chiefly in four older Pleistocene de­ determined from trenches and by examination of aerial posits, in which the ratio of gravel to sand is very photographs, on which the boggy terrain is darker gray high. Younger Pleistocene outwash and Holocene allu­ than the adjacent area. The peat deposit covers about vium also are large potential sources of gravel and half a square mile and extends about 3 miles along the sand in the area. The Dry Union sediments, however, creek, approximately from the west edge of sec. 34 to contain thinner and less desirable gravel and sand de­ the middle of sec. 25, T. 15 S., R. 78 W.; the peat lies posits that are generally mixed with silty material, between altitudes of 8,050 and 7,600 feet. The thickness bentonitic clay, or volcanic ash. of the peat ranges from less than l foot at the east edge The four Pleistocene gravels, worked as bank deposits of the deposit to about 10 feet where it is being mined at the edges of topographic benches, are largely pedi­ and to the west where it was excavated along U.S. High­ ment and terrace deposits that consist of fragments of way 285; the average thickness seems to be about 5 feet. Precambrian and Tertiary granitic rocks, fine-grained The peat is readily extracted by power shovel from the and porphyritic Tertiary intrusive and volcanic rocks, open, grassy and swampy area. Other factors favoring Precambrian metamorphic rocks, and Paleozoic sedi­ low production costs are the scarcity of interbedded sedi­ mentary rocks in a sandy and silty matrix. Many of the ments, roots, and large parts of trees. Furthermore, the abundant boulders of granitic rocks in these older deposit is readily accessible and close to major highways Pleistocene deposits are highly weathered and crumble and a railroad serving the la-rger cities of Colorado. The readily ; these weathered boulders are not regarded as peat is ground, screened, and used chiefly as a soil con­ objectionable in gravel for use as road-building mate­ ditioner; peat adds vegetable matter to soil, increases riaJ, whereas excessively large and hard boulders of soil porosity, and 1ightens heavy clay soils. similar rocks in younger Pleistocene deposits are avoided. The locations of six gravel pits and the Pleis­ PEGMATITE MINERALS tocene units represented are given in table 17; another Precambrian pegma.tites within the Poncha Springs pit in gravel1 is about 500 feet west of the quadrangle NE quadrangle have had no appreciable production of at the southwest edge of sec. 28, T. 15 S., R. 78 W. The minerals. In contrast, similar pegmatites in the quad­ four gravel units are exposed over a:ri area of about 7 rangle to the south and in the Turret district (Hanley square miles in the northwestern part of the quadrangle and others, 1950) immediately to the east have been the (pl. 1) ; these units represent a resource of several hun­ source of potash and soda feldspars, muscovite, bery I, dred million cubic yards of gravel and sand, even and columbite-tantalite, chiefly during World War II though an average depth of only 25 feet is assumed for and at intervals thereafter. The quarry at the Home­ workable deposits. stake mine, 2.5 miles east of the quadrangle, operated on

TABLE 17 .-Gravel pits in the Poncha Springs N E quadrangle, a large scale between 1950 and 1963 as a source of soda Chaffee County spar processed for the glass industry, and for several Pleistocene unit Location represented 1 years it was the State's largest feldspa.r producer (U.S. SW~j sec. 27, T. 15 S., R. 78 w ______Gravel 5 Bureau of Mines, 1951-64). The maximum annual pro­ NW7'4; sec. 35, T. 15 S., R. 78 W ______Gravel 3 NW~; ~ec. 35, T. 15 S., R. 78 W______Gravel2 duction figure published (Baillie, 1962, p. 9) is for 1955, NE~; sec. 34, T. 15 S., R. 78 w______Gravel 2 when 29,650 tons was quarried there. SW~~i sec. 9, T. 51 N., R. 8 E _____ :. ______Gravel 2 NE~~; sec. 15, T. 15 S., R. 78 W ______Gravell The poorly zoned pegmatite dikes 'and sills in the I Stratigraphic position and possible correlation given in table 10. Poncha Springs NE quadrangle occur chiefly as tabular PEAT DE.POSIT bodies, generally 2-20 feet thick. Some of those in the A Holocene peat deposit, previously described in this northeastern and eastern parts of the quadrangle have report, has been excavated for several years along Gas been explored by excavating shallow pits or by strip­ Creek, about a mile east of Centerville and U.S. High­ ping adjacent to the outcrops. These exploratory work­ way 285. The peat rests on glacial outwash of early ings yielded much of the information on pegmatites Wisconsinan Age immediately north of terraces com­ previously presented in this report under the descrip­ posed of older Pleistocene gravels. Climatic conditions tion of the Precambrian rocks. 46 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

The pegmatites probably are of greatest value as gray vesicular porphyritic rhyolite; all dip approxi­ potential sources of potash feldspar ( microcline). The mately 60° N. These volcanic rocks were traced for soda feldspar, muscovite, beryl, and columbite-tantalite about 200 feet along the fault zone, in outcrops and in are either of poor quality or too scarce in the pegmatites pits. examined to be of commercial importance. Most of the Small quantities of pumiceous and perlitic materials muscovite is in small flakes, clusters, or books, generally were removed'from the pits at Ruby Mountain and the less than 1 inch in diameter; some books are as much as loqality to the southeast ; it is not known if any tests 3 ·inches in diameter but are hard to split and have were performed on these rocks. The deposits seem •to be wedge structure, A-structure, inclusions, or iron-oxide moderate to small in size, and additional exploration stains. Weathering of the pegmatites and wallrocks and testing of samples would be needed to estimate ex­ locally yielded surface concentrations of resistant mag­ ploitable tonnages of material ~uitable for lightweight netite and red-brown garnet th3it form small accumula­ aggregate. Such work would indicate whether impuri­ tions on the slopes below and in adjacent stream beds; ties in the rocks would make them unsuitable for this the commercial value of these deposits has not been use. Impurities in the pumiceous rocks in different demonstrated. places consist of opal, chlorite, and calcite in vesicles ; PUMICE AND PERLITE debris from the underlying Precambrian rocks; frag­ Pumiceous volcanic rocks and perlite at the north ments of phenocrysts and silicified tuff; spherulites; edge of the Poncha Springs NE quadrange are pos­ and devitrified and ;al-tered glass. The perlite contains sible sources· of lightweight aggregate. The pumice and mineral fragments _and hard pellets of obsidian with perlite are thicker and somewhat more extensive areally feldspar microlites and magnetite dust, which locally than previously recognized (Bush, 1951, p. 314-315, might prove to be objectiooohle if the rock were heated 326-327). The southernmost extensions of these rocks and expanded for use as a lightweight material. are found on Ruby Mountain and in .a smaller area QUARTZITE DEPOSITS about 2.75 miles southeast. (See pl. 1.) Information on Two quartzite deposits in pendants within the large lithology, chemical· composition (table 6), and struc­ body of gneissic quartz monzonite east of the Arkansas ture was presented under the description of theNathrop River were quarried on a small scale in 1961-62. The Volcanics of late -Oligocene age. The Ruby Mountain quartziro was used chiefly for .aggregate, roofing gran- . deposits .are less than a mile east of U.S. Highway 285 ules, and decorative material in gardens and landscap­ and the Denver & Rio Grande Western Railroad in the ing. Both deposits are accessible from Salida over Arkansas Valley ; these deposits are .accessible by road. Chaffee County Road 180 (Ute Trail) in the adj-acent .The Ruby Mountain perlite locality has been assigned Cameron Mountain quadrangle and a recently bull- incorrectly in various publications to Fremont County . dozed route extending north and west from Turret, an (Vanderwilt, 1947, p. 253; Jaster, 1956, p. 384; Sharps, old mining camp in Cat Gulch ·about half a mile beyond 1961, p. 6). the east margin of the quadrangle. Whi·te to tan pumiceous tuff ·rubout 70 fe·et thick, over­ A quartzite deposit south of Green Gulch is exposed lain by about 20 feet of pink pumiceous breccia and 110 along strike for nearly 500 feet on a conspicuous white feet of black and gray perlite containing small black knob at an altitude of about 8,500 feet along the bound­ pellets of obsidian, rests on Precambrian gneissic quartz ary between sees. 18 and 19, T. 51 N., R. 9 E. Layers of monzonite along the east side of Ruby Mountain banded gneiss in a pendant within the gneissic quartz (NW-14 sec. 13, T. 15 S., R. 78 W.). The lower pyro­ monzonite strike N. 40° E. ·and dip 50°-70°· SE; two clastic unit dips about 30° W.; the dip gradually in­ ' principal sets of joints are oriented N. 10°· W., 70° W., creases in the overlying units and is 50°-70° W. in a and N. 70° E., 85° S. Near the south end of the knob the rhyolite flow above the perlite. Although they are locally gneissic quartz monzonite cuts the banded gneiss and covered by talus, the pumiceous and perlitic rocks were contains smaH inolusions of it. The pendant of leuoo­ traced for about 1,500 feet from the northeast end of cratic quartz-feldspar-mica gneiss includes a few small Ruby Mountain to the southeast end. pegmatites and contains 40-60 feet of coarse, gray to Southeast of Ruby Mountain (NW-!4NWI4 sec. 29, white, granulose quartzite. Impurities within the quart­ T. 15 S., R. 77 W.) at an altitude of about 8,900 feet, zite eonsist of minor microcline, albite, biotite, mus­ an east-trending narrow graben in the gneissic quartz covite, and magnetite, and a thin discontinuous chlori­ monzonite has preserved the same, but much thinner, tized zone. The quartz grains are medium to coarse and sequence of volcanic rocks. From south to north, ex­ elongruted and have serrated, interlocking margins. posures show about 4 feet of gray pumicoous tuff and Quartzite was quarried and trucked to a plant in Salida, breccia, 8 ·of gray-black perlite, and 10 feet of 'light- where it was crushed and sized for shipment. REFERENCES CITED 47

The second quartzite deposit,· on a hill and at anal­ p. 10-14) are considered to have :formed by the aotion titude of about 8,880 feet near the east edge of the ,of hydrothermal solutions from granitic or pegmatitic quadrangle ·and ·south of Sawmi'l[ Gu'lch ('south-central intrusives upon rocks bearing hornblende, biotite, phlo­ part of sec. 8, T. 51 N., R. 9 E.), similarly is associated gopite, or serpentine. with thin pegm·ati.tes and handed quartz-miorocline gneiss within the gneissic quartz monzonite. The units REFERENCES CITED strike N. 0°-50°· E. and dip steeply- westward. In a .A!dlalms, J. W., 1953, Becyllium. deposiltJs of ltlhe Mount Antero bulldozer cut near the crest of the hill, the quartzite is region, Chaffee County, Colorado: U.S. Geol. Survey Bull. about 150 feet thick, and in a lower cut to the north 982-D, p. 95-118. Aldrich, L. T., Wetherill, G. W., and Davis, G. L., 1957, Occur­ nearer Sawmill Gulch, it is about 65 feet thick. The rence of 1350 million-year-old granitic rocks in Western quartzite is gray to white and gra.nulose and contains United States: Gool Soc. America Bull., v. 68, p. 655-656. rare tiny grains of albite and thin books of muscovite Aldrich, L. T., Wetherill, G. W., Davis, G. L., and Tilton, G. R., less than one-fourth inch in diameter; it is cut by north­ 1958~ Radioactive ·ages of micas from granitic rocks by trending shears that dip about 80° W. An unknown but Rb-S.r and K-Ar methods: Am. Grophys. UniQn T.tans., v. 39, p. 1124-1134. small quantity of quartzite was mined from this deposit. Ames, L. L., Jr., 1958, Chemical analyses of the fluid inclusions VERMICULITE DEPOSI·T in a groUJp of New Me:Xliclo mliln€Tials: Econ. Grol·ogy, v. 53, p. 473-480~ A vermiculite deposit of unknown dimensions is lo­ Baillie, W. N ., 1962, Feldspar ~urrences in Colorado : Colorado calized in a small pendant of biotitic hornblende gneiss School Mines Mineral Industries Bull., v. 5, no. 4, 12 p. within the large body of gn~issic quartz monzonite, Barker, Fred, and Brock, M. R., 1965, Denny Creek Grano­ about 3.3 miles southeast of Nathrop. The foliation in diorite, Browns Pass Quartz Monzonite, and Kroenke Oramodi'Orlltle, Ml()unJt Haa-vall"d Qwadrfa'lllgle, Oolor.adO, in both rocks strikes approximately N. 50° W. and dips Cohee, G. V., and West, W. S., Changes in stratigraphic 50° NE. The deposit has not been worked extensively, nomenclature by the U:S. Geological Survey, 1964: U.S. partly because of its grade and location in an area of Geol. Survey Bull. 122~A, p. A23-A26. difficult accessibility. The workings are at an altitude Batty, J. V .. ,.-Havens, Richard, and Wells, R. R., 1947, Conc_en­ of about 8,130 feet along the south edge of a gulch in tration of Colorado fluorite ores: U.S. Bur. Mines Rept. Inv. 4139, 28 p. SWl4SWl4 sec. 29, T. 15 S., R. 77 W. This prospect Behre, C. H., Jr., OsiJorn, E. F., nnd RlaiJD.watetr, E. H., 1986, has been called erroneously the Tung Ash deposit (Bush, Contact ore deposition at the Calumet iron mine, Colorado: 1951, p. 33·3), •a ve·r.miculite deposit southeast of Tur­ Econ. Geology, v. 31, p. 781-804. ret in the Cameron Mountain quadrangle to the ea.St, Bhutta, M. A.', 1954, Geology of the Salida area, Chaffee· and where a je:fferisite (Henahen, 1914, p.135-139) or hydro­ Fremont Counties, Colorado : Golden, Colorado School Mines, unpub. Ph. D. thesis, 173 p. biotite (Sterrett, 1923, p. 50) variety was produced. Burbank, W. S., and Goddard, E. N., 1937. Thrusting in Huer­ A shaft, inclined about· 65 ° S. and filled with water fu:rro Prurk, Oolorado, and ·rel.ated problems of orogeny in to about 20 feet below the collar, exposes the gneissic the Sangre de Cristo Mountains : Geol. 'Soc. America Bull., quartz monzonite and two layers of vermiculite-bearing •v. 48, p. 931-976. hornblende gneiss. About. 5 feet of gneissic quartz mon­ Burbank, W. S., Lovering, T .•s., Goddard, E. N., and Eckel, E. B., 1935, Geologic map of Golorado: U.S. Geol. Survey, zonite separates a 2-foot layer of vermiculite-bearing scale 1 : 500,000. gneiss from a mass of similar rock at least 12 feet thick. Bush, A. L., 1951, Sources of lightweight aggregates in Colo­ The gne1ssic quartz monzonite clearly cuts the horn­ rado: Colorado Sci. Soc. Proc., v. 15, no. 8, p. 305--368. blende gneiss and contains inclusions of the older rock; Campbell, M. R., 1922, Guidebook of the Western United States; loC)al[y the quruvtz monzonite is tpegm:atitic. Pt. E, The Denver & Rio Grande Western route: U.S. Geol. Survey Bull. 707, 266 p. Thin, flexible and inelastic plates of pale-green and Carmichael, I. S. E., 1963, The crystallization of feldspar in yellowsh-brown vermiculite :form more than 50 per­ volcanic acid liquids: GeoL Soc. London Quart. .Jour., v. 119, cent o:f the crumbly hornblende gneiss; quartz, mi­ p. 95--131. croeline, green 'hornblende, sphene, 'and la·patite 'are the Chapin, C. E., and Epis, R. C., 1964, Some strat~graphic and impurities. 'I'he vermiculite exfoliates readily into tiny structural features of the Thirtynine Mile volcanic field, central Colorado: Mtn. Geologist, v. 1, no. 3, p. 145-160. silver- to bronze-colored books when he31ted in the flame · Coats, R. R., 1956, Uranium and certain other trace elements of a match. It is optically negative and has a 2V of less in felsic volcanic rocks of Cenozoic age in Western United than 10°.; nZ is between 1.595 and 1.600. Some of the States, in Page, L. R., Stocking, H. E., and Smith, H. B., basal plates observed under the microscope have the compilers, Contributions to the geology of uranium and hexagonal outline of biotite, from which much of the thorium by the United States Geological Survey and Atomic Energy Commission for .the United Nations International vermiculite probably was derived. Vermiculite deposits Conference on Peaceful Uses of Atomic Energy, Geneva, in Precambrian rocks o:f Colorado (Goldstein, 1946, p. Switzerland, 1955: U.S. Geol. ·Survey Prof. Paper 300, p. 17; Bush, 1951, p. 332) and Wyoming (Hagner, 1944, 75-:-78. 48 GEOLOGY AND MINERALS, PONCHA SPRINGS NE QUADRANGLE, CHAFFEE COUNTY, COLO.

Coats, R. R., Goss, W. D., and Rader, L. F., 1963, Distribution Grose, L. T., and Hutchinson, R. M., 1960, Leadville to Trout of fluorine in unaltered silicic volcanic rocks of the western IOI"eeek, in \V'e'imer, R. J., and Haun, J. D., eds., Gulide tJo the conterminous United States: Econ. Geology, v. 58, p. 941- geology of Colorado: Denver, Geol. Soc. America, Rocky 951. .Mtn. Assoc. Geologists, and Oolorado Sci. Soc., p. 155--157. Cox, D. C., 194.5, General features of Colorado fluorspar de­ Grushkin, G. G., and Prikhid'lro, P. L., 1952, [Ohemical composi­ posits: Colorado Sd. Soc. Proc., v. 14, no. 6, p. 263--285. tion, concentration, and pH of liquid inclusions in fluorite] : Crawford, R. D., 1913, Geology and ore deposits o! the Monarch Zwpiski Vsesoyuz, Minm-alag. Obshehestw, v. 81, p. 120-126 and Tomichi distr!cts, Colorado : Colorado Geol. Survey (Ohern. Abs., v. 46, col.10055, 1952.) Bull. 4, 317 p. Hagner, A. F., 1944, Wyoming vermiculite deposits·: Wyoming Cross, C. W., 1886, On the occurrence ·of topaz and garnet in Geol. Survey Bull. 34,47 p. 'litJhopihsnsies of rhyQlli.te: Am. Jloor. ·Sci., 3d •ser., v. 31, Hall, W. E., rand ·Friedman, Irving, 1963, Composition of fluid p.43~8. inclusions, Cav~in-Rock fluorite district, Illinois, and ---1893, Itinerary-Nathrop to ·Salida: Internat. Geol. Cong., Upper Mississippi Valley zinc-lead district: Econ. Geology, 5'th, Wa:shiin'gtWn, 1891, ·COimlptle rendu, p. 423-424. v. 58, .p. ~911. ---1895, On a series of peculiar schists near Salida, Colo­ Hanley, J. B., Heinrich, E. W., and Page, L. R., 1950, Pegmatite rado: Oolorado Sci. Soc. ·Proc., v. 4, p. 286--293. investigations in 'Colorado, Wyoming, and Utah, 1942-1944: Cross, C. W., and Emmons, ·s. F., 1883, On hypersthene andesite U.:S. Geol. Survey Prof. Paper 227, 126 p. and on triclinic pyroxene in augitic rocks [with a geologic Hayl(}oo, F. V., 1'869, PlrelimriJn!alry field ,report, [Third] Annual sketch of Buffa'lo Peaks, Colorado] : U.S. Geol. 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{Italic pa.,g-e numbers indicate majol' l'eferences]

A 'Page Fluorspar deposits-Continued Page Page Browns Creek placer deposit. ___ . __ •.•• _. __ _ _ 44 resources. ______• _____ . _____ .•• ______38 Abbott, G. A., analyst______35 Buffalo Peaks volcanic center------10 size.-.-----..• ---.------37 Accessibility of area ______------______2 Bull Lake glaciation._.------_ 24 temperature at db position ___ ••• _. __ •• _. __ 37 Acknowledgments •••. ------4 vein mineralogy ______..•• ______••• _. __ 91 c wallrock alteration. ____ • ___ •.. _. ____ • ___ _ Agate Creek Formation·------15 Calcite •••..• ______.______32 94 Age, Browns Canyon Formation ______19,20 Folding, Precambrian. ____ .• _____ • ___ . ______t6 deformations in region______26 Caliche._.------__ ------. 23 Canyons, abandoned ______. __ ._ •• ___ .___ 29 Foliation, aplite dikes and sills.------_ 8 gneissic quartz monzonite______6 banded gneiss. ____ ••• _. ___ . __ •. ______. 5 Holocene peat. __ .•• __ ._ ..•. ______25 Cat Gulch, pegmatitesnear______8 gneissic quartz monzonite_ •• _. __ ... ______Chalcedony __ . _____ •. _. ______._._.__ 11 7 Nathrop VolcaniCS------18 granite dikes .• __ .• __ -•. ---.----_._------. 7 Pleistocene gravels.----- __ ---- __ ------23 veins •. ------__ ----- 7 Precambrian gneisses, relation to struc- rhyodacitic volcanic rocks._ •• __ ..• _._____ 10 Chevkinite------24 · ture •• ______. __ .• ______• __ • __ • _ 26 rhyolitic ash-flow tufi______15 Chloe, Gilllson,analyst. ______~-- 6, 10,13 Fossils, plant, Browns Canyon Formation_ •• _ 19, 20 Tertiary rocks.------9 Clay minerals in the fluorspar deposits. __ •. __ 34 plant, Dry Union Formation______22 Alteration. dacite porphyry______9 Climate •• ·- ______. ______. __ . ______.• __ • 4 Holocene. __ .______25 lamproph··res. ___ • ------..... ______8 Collins vein, Colorado-American mine______39 rhyodacitic ash. ______•• __ •• __ • __ ._ 10 rhyodacitic ash______9 Columnar structure•• __ • __ . ______. ___ . _. __ _ 11 rhyolitic ash-flow tufi ______11,14 rhyolitic ash-flow tufl'______11 Contact relation, Browns Canyon Formation. 18 vertebrate, Dry Union Formation_ ••• _.__ 21 wallrock, fluorspar deposits______94 Dry Union Formation______20 Holocene. ______..• ______._____ 25 Analyses, chemical, cold mine waters from gneissic quartz monzonite. __ • ___ •• ___ •• _. 7 Pleistocene. ______. _____ . 24 Nathrop Volcanics______15 Kentucky and Newfoundland Frost, I. C., analyst .•••• ------:. 35 fluorspar districts •• ______._ 35 Pleistocene gravels. _____ ------23 rhyodacitic ash. _____ . ______.______9 chemical, gneissic quartz monzonite...... 6 G rhyodacite porphyry______10 Copper•. __ ------_._. __ ---.-----.. ------.- 49 Gahnite. ______• __ -- _. --. _------. _. 44 spessartite garnet ••. ____ ..•. ______... 17 Creede Formation______15 Garnet, gem-quality __ ------15,17 thermal waters from Chaffee County Cummingtonite ••. _____ .. __ • __ . _-- _.... _. ___ • 8 red-brown. __ -- ____ --- _____ -- •• __ ----_____ s fluorspar districts •.•. __ . __ .______35 Gas Creek, peat______25 obsidian pellets ••• ------16 D Gel)chel!lical prospectiilg, fluorspar. __ .______38 perlite ••. ------______.. ----- 16 Dacite porphyry, dike.------9 Geologic history, summary_._._. ____ .• ____ ••• S9 potassium-argon, vitrophyric welded tuft. 15 Daniels, Ellen, analyst______13 Geomorphology of area______~ rhyolite flow on Ruby Mountain______18 Deformation, relation to emplacement of igne- Glaciation in area. __ ------22,30 semiquantitative spectrographic, aplitic ous rocks. ___ ------7 Glass ______------11 ages. ______..•. _-- ___ .. _. ___ --______26 gneiss •• _.------____ ------44 Gneiss, hornblende, lineation______26 manganese oxide______33 Diabase, dike ______._ •. ___ • ___ . ______.__ 9 Precambrian ••• ____ •. _------.___ 6 typical fluorspar ores______37 Dikes, Precambrian______7 Gold. __ .----______------.-.------44 welded tufis. _____ .... ______13 relative age •. ------__ ----- _____ ------26 Goss, W. D., analyst------35 Antero Formation •••. ------11,15 Dry Union Formation ______9,110 Granite dikes ______----_ •. _--_.----•• --_ 7 Apache Tears. __ • ______._. _____ .... ______16 badland topographY------29 Gravel ••• _-- ______--.------46 Pleistocene. __ .. ___ . ______._. ______.____ 2S Aplite, dikes and sills------8 beveled by pediments______t8 Arkansas Valley, downfaulting______26 Tertiary ___ . ______• ____ . ______.___ 21 Ash, rhyodacitic______9 E,F volcanic, in Pleistocene gravel unit. ___ 22, 23, 24 H Ash-flow tuff, age______15 East vein, Colorado-American mine______39 Elmore, Paul, analyst. ______6, 10, 13,15 Hamilton, J. c., analyst------13,18 rhyolitic .•• ______.... _... ___ •• ____ •. __ . 11 Herring Park, analyzed tufis______13 Faults, evidence for______27 minor elements______14 Holocene deposits______t6 source.----...•.. ______•. ______• 15 late Tertiary ___ ------______------26 Hornblendb gneiss______6 Precambrian, reactivation______26 Howard, c. S., analyst______35 B relation to Browns Canyon fluorspar Bacon spar_ ... ______•.....• ______•.• __ .__ 31 deposits. ______. 27 Intusive rocks, tabular •• ------7 Badland topographY------29 relation to fluorspar localization.______SO L Bald Mountain______18 Fieldwork·------2 Lamprophyre, dikes ••••. ------8 volcanic sequence·------15 Florissant Formation______15 Landslides. ______. ___ •... ___ .. __ ... _ 25 Banded gneiss·------5 Flows, rhyodacite porphyry. ______------_ 10 Laramide orogeny ______------__ ------26 Barite. ___ . ______------______33 Fluorine, abundance in earth______36 Layering, fluidal. _____ ------____ 11 Barlow, Ivan, analyst______6, 10,13 Fluorite, solubility ______.______35 in welded tuff of ash-flow tuft unit______12 Bentonite, swelling______9, 20 Fluorspar deposits, associated thermal springs. S4 Leopold, E. B., quoted______20 Botts, Samuel, analyst.------6, 10,13 grade •. ____ ------______37 Lewis, G. E., quoted______21 Boulder Creek Granite______7 history in Browns Canyon district______30 Location of study area______2 Branch vein, Colorado-American mine______39 localization •• __ •.. ______. ____ •. _ SO Lohr, E. W., analyst______35 Breccia, Nathrop Volcanics.------16 mines and prospects______98 Breccia zones, in gneissic quartz monzonite... 7 origin ______------_ 96 M Browns Canyon, fluorspar district __ •. _____ . _ SO ownership ______._. _____ .• ______30 Magnetite ••• ______------.. ------8, 33 geomorphology, previous study_ •.• _____ . _ 2 paragenesis ______------______34 Marble. __ • __ . ____ . ____ . ______-- ____ .. __ -- __ • 5 superimposition._------____ ------. 28 previous studies.------2 Marekanites. ___ • ____ •.... ___ ----... _•..• ---.. 16 Browns Canyon Formation______18 problems in concentration •• _••. _••••••• _. 30 Marleer bed, black vitrophyre •• __ . _..... __ .. _ 11, 12 age. ______------______------9 production. ______.. ------______30 Marvin, R. F., analyst.------15,18 depositional environment._------29 prospecting suggestions •••.•..•• ______• 38 Mehnert, H. H., analyst.·------15,18 51 52 INDEX

Page Page Page Merritt, Violet, analyst. ______15,18 Pleistocene deposits.-_. ______.______SS Smith, H., analyst.______15 Mineralogy of fluorspar veins. ______.__ 91 Poncha Hot Springs______· 34 Soils, relict..------__ ------23,25 Mines and prospects, Ace High prospect______43 Poncha Springs fluorspar district______35 Spessartites .• _---- __ ------__ ------__ _ 8 Alderman deposit______42 Precambrian rocks, depositional environment. 29 Springs.-_------__ ------23 Blue Stone deposits______43 igneous .. ______.---______6 Structural geology. ____ -- ______----_ t6 metamorphic .•• ______6 Brown Creek placer deposits______44 Structure, eutaxitic. ------12 Chimney Hill mine ______. 41 Precambrian structural events.______S6 Sugar spar ____ ------31 Colorado-American mine______98 Previous investigations______2 Sugarloaf Mountain.______18 Delay adiL. ______.... ______99 Psilomelane. ______--- ______33 Gas Creek peat deposit______45 Pumice. ______.. ______.__ 46 T Terraces, Pleistocene .• ______22, 28 Homestake feldspar mine______45 previous study_. ______-·--______2 previous study __ .______2 Jack Pot prospect.______43 Pyrite•. ____ .____ • ____ •• ___ --- ______•• ____ ••• 33 Kramer mine ..• ___ -_._.-.------4S Pyrolusite______33 Terrain in area .• __ ------_------4 Tertiary rocks. ______----___ 9 Last Chance mine·------~------34, 4S Lloyd shaft. ____ ------.------.______99 Q Thirtynine Mile caldera.- ______---______15 Manganese Hill mine •• ---.----•••• -----.- 41 Thirtynine Mile volcanic field.______10 Morgan Ranch deposits ••• ------41 Quartz.______32 ::I;opaz, gem-quality-·-.------15, 17 near Stafford Gulch______44 Tourmaline, black, striated______8 pale-rose. ___ ------8 Tuff, at Buffalo Peaks.______11 Puzzle deposit.-_------34,43 veins ••• ------7, 8 Snowflake deposit..______43 Quartz monzonite, gneissic______6 bedded.______9 Tung Ash deposit______47 gneissic, competence during faulting______31 opalized. ____ ------__ ------______11 White King deposits.------43 relationship of emplacement to struc- pumiceous, Nathrop Volcanics.______15 Moraines, terminaL ______• ______24, 30 purple porphyritic welded. ______-;____ 12 ture._------26 Mountjoy, Wayne, analyst______18 Quartzite.______5, 46 rhyolitic ash-flow ______•. 11 Mudflow, volcanic •.. ______----_____ 10 rhyolitic welded, competence during Munson, E. L., analyst ______.______16 R faulting __ ------31 Turret district, pegmatite minerals. ______" 45 N Railroad Gulch, layering in welded tuft near. 12 v thick rhyolitic ash-flow tuft near______11 Vermiculite ••• _. ______••• ______47 Nathrop Volcanics ••••• ______16 Reef, The._------28 Vitrophyre, in ash-flow tuff______11 age.------_ 9,18 References cited______47 Vogesites. --- __ ------_------8 source.---- ______---- ______••• ______18 Rhyodacite porphyry flow ____ ------10 minor elements •• ______----______14 Neiman, Harriet, analyst._-~------16 w Rhyolite flow, Ruby Mountain______17 Wagon tongue Formation.______22 O,P Ribbon spar ______•• ______31 Warr, Jesse, Jr., analyst ______6, 10,13 Rio Grande trough ______22, 26,27 Weathering, devitrified welded tuff in as~flow Obsidian .• ------16 Ruby Mountain, pumiceous and perlitic rocks, tuff unit______12 Opal.--.------11,32 use.------___ 46 Dry Union Formation______20 rhyolite flow.------17 gneissic quartz monzonite______6 Pando PorphyrY------21 volcanic rocks near, previous study______2 pegmatltes. _•• ______--· ___ --- ___ _ 46 Paragenesis in fluorspar deposits- -. ______34 volcanic sequence near______15 rhyodacitic ash.______9 Pearlette Ash Member, Sappa Formation____ 22 Peat._--______-= __ • ______•• 25,46 West vein, Colorado-American mine______39 s Wisconsin Glaciation.______22 Pediments. _____ ------____ ------.--- ______22, S8 Sand._------46 Wood, fossil, in rhyodacitic ash·------·· 10 Pegmatite, dikes and sills._.------8 Santa Fe GroUP------22 fossil, in rhyolitic ash-flow tuff ______11,14 minerals. ______• ____ •• _____ •• -. __ ---.- 46 Sawatch anticline------26 potential value._----______46 Sawatch Range, mountain glaciers______22 z Perlite ••• _-- ____ ---.------_.--.------46 Scope ofreport------2 Zoning, feldspars ______11,12 Nathrop Volcanics. __ ------_ 16 Scott, R. A., quoted______14 pegmatites •••• ______------"------8 previous studies------2 Settlement of area______4 plagioclase •.•. ______~- __ -~_____ 8, 9 Pikes Peak Granite______7 Sills, Precambrian. ___ ------______------7 rhyolitic ash-flow tuff .. ------11, 13 Pinedale Glaciation.------.------25 Silver Plume Granite------7 san! dine ••. ______-.--_ 13 0

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