Redacted for Privacy S Ii

Home , Elkhead Mountains, Hahns Peak

AN ABSTRACT OF THE THESIS OF

ROBERT JOSEPH CASACELI for the degree of MASTER OF SCIENCE

in GEOLOGY presented on December 14, 1983

Title: THE GEOLOGY AND MINERAL POTENTIAL OV INS PEAK

INTRUSIVE PORPHYRY, J?øtYfl C

Abstract approved: Redacted for privacy S ii. ne

Hahns Peak ia composite laccolithic intrusion of latite and

quartz latite porphyry, approximately 12 m.y. old. The country rocks

that were domed by the porphyritic intrusions consist of Paleozoic

siltstones, sandstones, and claystones; Mesozoic sandstones, shales,

and minor limestones; and Cenozoic sandstone and conglomerate. Base-

ment rocks consist of Precambrian gneissic granodiorite. A thin,

cone-shaped body of intrusive breccia (referred to as a breccia cone

sheet) is centered on the intrusive complex and is interpreted to be

relatedto ahidden rhyolitic pluton oflate Tertiaryage. The

brecciaconesheet consistsof monolithicbreccia, that probably formed by boiling of magmatic fluids, anda more fluidized multi-

lithic breccia that probably formed byphreaticexplosionswhen

ground water came into contact with super-heated magmatic fluids.

Multilithic breccia apparently breached the surface on the northwest flank of Hahns Peak as steam-blast eruptions which formed a stratifiedvent complex at thatlocationand a pyroclastic surge deposit approximately one mile to the west. Silicified fragments of porphyry withstockwork veins of silica,

pyrite, and molybdeniteare present in both an early phase of quartz

latite porphyry and in thebreccia cone sheet. These fragments sug-

gest the presence ofa stockwork molybdenite deposit at depth beneath

HahnsPeak. Pyrite,galena,sphalerite,chalcopyrite,and molybdenite occur locally in the matrixof monolithic breccia. Minor

amounts of auriferous pyrite,argentiferous tetrahedrite, and trace

covellite are present locally inthe matrix of multilithic breccia.

Gold that occurs in placerson the perimeter of the peak is thought

to have been derived from theupper-most portion of the breccia cone

sheet. Late-stage sphalerite and galena fillopen spaces locally in

the breccia cone sheet andare thought to have been derived from Pre-

cambrian source rocks thatwere leached by meteoric fluids circulated

above a cooling pluton.

Hydrothermal alteration consists ofan early phase of pervasive

albitization and a later phase ofpropylitic, argillic, phyllic, and

advanced argillicmineral assemblages that are zonedaround the

breccia cone sheet.

Hydrothermal brecciation, mineralization, and alteration at

Hahns Peak is interpretedtobe relatedto a porphyry molybdenum

system. The location of Hahns Peaknear the intersection of deep- seated Precambrian structures witha possible northward extension of the Rio Grande Rift isa favorable tectonic environment for molyb- denurn mineralization. The Geology arid Mineral Potentialof the Hahns Peak Intrusive Porphyry, Routt County, Colorado

Robert Joseph Casaceli

A THESIS

submitted to

Oregon State University

inpartial fulfillment of the requirementsforthe degree of

Masterof Science

December 14, 1983

Cormnencement June, 1984 APPROVED: Redacted for privacy

PTOI tOT t.eoogy 1 rrcnarge & majórJ

Redacted for privacy V artment 0 0 gy

Redacted for privacy

Dean of Gradui School

Date thesis is presented December 14, 1983

Typed by Patricia Brioady for Robert Joseph Casaceli ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to the Anaconda

Minerals Company for providing financial support for thisthesis. In particular,I would like to thank John R. King of Anaconda for intro- ducing the thesis topic to me and for his continued support,encour- agement, and positive criticisms throughout the course of this project.

I am very grateful to Bill Bowes and the staff of W. A. Bowes,

Inc. for their constant help in a variety of scientific and logis- tical matters. In particular I would like to acknowledge the assistance and friendship provided by Steve Aaker and the late Rose Watts.

The geochemical sampling and underground mapping was accomplished through the able assistance of Gail Genasci-Wells,

R. Wade Holder, and Ruth Starkins. I thank Larry Hillesland for his help in the ground radiometrics survey. Drafting assistance was provided by Mickey Edell, Lucy Chronic, Carol Johnston, MoniFox,and

Lynn Schilling.

The extensive editing of Dr. C. W. Field greatly improved the

quality of the manuscript. In addition, I thank John Doucette and

Paul McCarter for critically reading parts of the thesis.

I will be forever grateful to Velma Fisher and the late Leonard

Fisher for their hospitality and friendship and for allowing me to share with them their wonderful little town of Columbine.

Field observations by Dr. W. Atkinson and Dr.J. Kutina proved helpful and are much appreciated. Sincere thanks are due also to Patricia Brioady for typing the final manuscript.

Geo1ogic discussions withmy fellow graduate students Gary

Sidder, Rich Fifarek, and Larry Hilleslandwere beneficial in devel- oping the conceptual models presented herein.

I am deeply indebtedto thelate C. E. uChuckuBeverly for having introduced meto volcanic rocksandthe effectsof hydro-

thermal alteration. His enthusiam for geology was contagious and I dedicate this thesis in hismemory. TABLE OF CONTENTS

Page

INTRODUCTION . 1

Purpose and Method of Study ...... 1 Physiography and Climate ...... 3 History of Mining and Land Use ...... 4 Previous Geologic Investigations ...... 9

REGIONAL GEOLOGIC SETTING ...... 13

PrecarnbrianGeology ...... 13 PaleozoicGeology ...... 15 MesozoicGeology ...... 16 Cenozoic Geology ...... 17 Regional Geophysics ...... 19 Regional Tectonics ...... 22 Precambrian Tectonics ...... 24 Paleozoic Tectonics ...... 25 Mesozoic Tectonics ...... 26 Cenozoic Tectonics ...... 28 Metallogenic - Tectonic Relationships ...... 33

LOCAL GEOLOGIC SETTING ...... 36

Age Determination of Intrusive Rocks ...... 37 Geomorphology ...... 38 GeophysicalSurveys ...... 41 Airborne Magnetics ...... 41 Ground Radiometrics ...... 43 Heat Flow ...... 45

HAHNS PEAK LITHOLOGIES ...... 47

Precambrian Metamorphic Rocks ...... 47 Paleozoic Sedimentary Rocks ...... 48 Mesozoic Sedimentary Rocks ...... 49 JurassicRocks ...... 49 Cretaceous Rocks ...... 50 Cenozoic Sedimentary Rocks ...... 52 Browns Park Formation ...... 52 Cenozoic Volcanic Rocks ...... 53 Vent Complex Pyroclastic Surge ...... 58 Page

Cenozoic IntrusiveRocks 60 Beryl Mountain Porphyry ...... 61 LittleMountain Porphyry ...... 64 Columbine Porphyry ...... 65 SumitPorphyry ...... 70 7DPorphyry ...... 76 Monolithic Breccia ...... 78 Aplite Dikes ...... 80 Multilithic Breccia ...... 82 Late-Stage Porphyry Dikes ...... 86

STRUCTURAL GEOLOGY ...... 88

Evolution of Structural Dome and Normal Faults ...... 89 Reverse Faults ...... 93 Joints...... 95 Structural Evolution of the Breccia Cone Sheet ...... 98 Minor Late Fractures ...... 102

VOLCANIC ACTIVITY AND INTRUSIVE BRECCIATION ...... 104

Intrusion of Laccolith ...... 104 Intrusive Brecciation ...... 106 Pyroclastic Vent Complex ...... 120 PyroclasticSurgeDeposit ...... 125

HYDROTHERMAL ALTERATION ...... 133

Early-Stage of Alteration ...... 134 Main-Stage of Alteration ...... 140 A1tritinn 7nncc ------141 PropyliticZone ...... 141 Argillic Zone ...... 144 MixedPhyllic-ArgillicZone ...... 145 Phyllic Zone ...... 147 Advanced Argillic Zone ...... 148 Alteration Processes ...... 148 Albitic Alteration ...... 151 PropyliticAlteration ...... 152 Argillic, Phyllic, andAdvanced Argillic Alteration ...... 153 Supergene Alteration ...... 159 Page

METALLIZATION ...... 161

Trace Element Geochemistry ...... 163 Sulfide Mineralization ...... 170 Occurrence and Distribution ...... 170 Paragenesis ...... 174 Origin of Hydrothermal Fluids and Controls on Deposition ...... 178 Mineral Potential and Recommendations ...... 192 Molybdenum ...... 193 Silver-Gold ...... 196 Lead-Zinc-Copper ...... 197 Uranium ...... 198 Recommendations ...... 199

SUMMARY...... 201

Geologic History and Interpretations ...... 201

BIBLIOGRAPHY ...... 209 LIST OF FIGURES

Figure Page

1 Index Map of part of northwest Colorado. 2

2 Hahns Peak, view from Farwell Mountain. 5

3 Geologic map of the Hahns Peak Region. 14

4 Portion of Colorado Bouger Gravity Map and Colorado Aeromagnetic Map. 21

5 Tectonic map of the Southern Rocky Mountain Province. 23

6 Total field magnetic map of the Hahns Peak region. 42

7 Generalized section of volcanic vent complex at Hahns Peak. 55

8 Main phase intrusive porphyries at }-Iahns Peak. 67

9 Photomicrograph of Columbine porphyry, massive variety. 69

10 Photomicrograph of Summit Porphyry. 72

11 Silica banding in Summit Porphyry exposed in the 7D Adit. 72

12 Geology of the 7D Adit. 73

13 Geology of the Southern Cross Adit. 74

14 Intrusive breccias: Br-21 monolithic, Br-26 multilithic, Br-23 fluidized multilithic with multiple phases evident. 79

15 Photomicrograph of Monolithic breccia with silica coating later fractures. 79

16 Photomicrograph of coarse Multilithic breccia. 84

17 Contoured distribution of poles-to-plane equal area projection for 500 joints from the igneous intrusive complex and the surrounding sedimentary units at Hahns Peak. 96

18 Formation of joints in the evolution of cone sheets and ring dikes. 100 Figure Page

19 Inclusion from Summit porphyry showing breccia fragments with stockwork veins of silica held in a silica-rich clastic matrix. 108

20 Geologic map showing the relationship of the base surge deposit to the volcanic vent complex and the breccia cone sheet. 126

21 Lithic inclusions from Summit porphyry (Tsp) which show: (A) a porphyry fragment (possibly Beryl Mountain porphyry) with stockwork veinlets of silica containing pyrite and molybdenite; and (B) a Pre- cambrian gneissic rock with well-developed stockwork veins of silica. 137

22 Distribution of total sulfide mineralization at Hahns Peak in relation to the breccia cone sheet. 171

23 Paragenetic sequence of mineralization at Hahns Peak. 175 LIST OF TABLES

Table Page

1 Levels of radioactivity in rock units of the 44 Hahns Peak area.

2 Major oxi de analyses of pri nd pal Hahns Peak rock units 57

3 Typical argillic, phyllic, and advanced argillic alteration reactions between host rock minerals and hydrothermal fluids. 154

4 Trace element concentrations for various lithologies at Hahns Peak 164 LIST OF PLATES

Plate Page

1. Geologic map of Hahns Peak, Routt County, Colorado. in pocket

2 Geologic cross section and legend. in pocket

3 Hahns Peak alteration and geochemistry. in pocket THE GEOLOGY AND MINERAL POTENTIAL OF

THE HAHNS PEAK INTRUSIVE PORPHYRY,

ROUTT COUNTY, COLORADO

INTRODUCTION

Hahns Peak is locatedin Routt County,northwestern Colorado, approximately 27 miles north of the town of Steamboat Springs

(Fig. 1). The thesis study area is contained within the Hahns Peak

7.5 minute topographic sheet (1. 10 N., R. 85 W., Sec. 9).

Access is gained by partially paved road from Steamboat Springs to Hahns Peak village and a graded dirt road the remaining five miles to the town of Columbine. Jeep trails leading from both Hahns Peak

Village and Columbine provide vehicle access to within 300 feet of the summit of Hahns Peak.

The Rio Grande Railroad provides the nearest transport facil- ities with its Denver line that passes through Steamboat Springs.

Purpose and Method of Study

This thesis was designed to evaluate the economic mineral potentialof Hahns Peak. This evaluation was carried out primarily by means of detailed surface and underground geologic mapping and geochemical sampling, core logging, and subsequent petrographic study.

A detailed geologic outcrop map on a scale of 1:2400 (1 in. =

200 ft.) was produced for an area of about two square miles roughly 2

Saambo, SpUi.

D..v.r I

Color ado

Lull. SM.k. Ply., r - - _W9ING ___i410 N COLOR ADO

A 444

010 2 ColumbIft. I .l, - Ha,Psok /;\, ELKMFAD MO(JNTAINS -\F.r.II Zlr*.l'' Mtn I H .ns

VIIlo. / I' Sand Mi,1 Os P ' UI u 0

R1P( yoIi'PO J (

O tO

SCALE (MILES)

Figure 1. Index Map of part of northwest Colorado. 3 centered on Hahns Peak (Plate 1). Field mapping was done on aerial photographs of approximatelythe samescale (1:2400) duringthe summer of 198. A detailed rock chip geochemical sampling program was also carried out at that time with the collection of 208 samples from both surface and underground sites. Underground mapping on a scale of 1:1200 (1 in. = 100 ft.) was done on both the Southern Cross and 70 adits with the aggregate length of workings being approximately 3,600 feet (see Figs. 12 and 13). In addition, approximately

12,000 feet of drill core was logged. All mapping and logging of core androckchip samples involved detai led examinations for the effects of hydrothermal alteration on the host rocks by means of vis- ual estimates in the field and by later petrographic studies of 120 thin sections in the laboratory. In addition, 66 samples were analyzed by X-ray diffraction techniques.

A proper evaluation of the economic mineral potential of Hahns

Peak could not be achieved without a firm understanding of its geologic setting. The development ofa more detailed history of the intrusive and hydrothermal events was therefore an intregal part of this thesis anda definite prerequisite in determining the type of mineral deposit at Hahns Peak, and where future drill sites should be placed to effecti vely eval uate i ts potenti al.

Physiography and Climate

Hahns Peak i s adi sti ncti ye topographi chi gh thatri ses nearly

2,000 feet above its valley floor to a summit elevation of 10,839 4

feet (Fig.2). The peaklies between the western flanks of the

north-trending Park Range, which defines the Conti nental Divide, and

the eastern edge ofthe west-trending Elkhead Mountains (Fig. 1).

Its slopes are precipitous, andare mantled with talus debris usually

near the maximum angle of repose. Much of the peak is above timber-

li ne.

The climate istypically alpine, with long, snowy winters and short, cool summers. The reported average annual precipitation from

Columbine (elev. 8,700 ft.) is 39.7 inches, withan average annual snowfall of184.5 inches(Segerstrom and Young,1972). Thissame source reports that regional mean temperatures are about 160 F for

January and about 60° F for July.

A large portion of the thesisarea, below altitudes of 10,200 feet, isheavily forested withEngelmann spruce, lodgepole pine, alpine fir and aspen. Associated mountain meadows display a unique assortment of wild flowers, grasses and shrubs.

History of Mining and Land Use

Hahns Peak hosted the first settlement in Routt County, owingto the discovery of gold in 1862. In that year, Joseph Hahn, William

Doyle,and Captain George Way found free gold while panningstream sediment from the banks of Willow Creek,near the present site of

Hahns Peak Village. On a return trip in 1865, Doyle and Way climbed the peak which they then named in honor of their leader,Joseph Hahn.

Returning once again in thesummer of 1866, the trio, along with 48 Figure 2. Hahns Peak, view from Farwell Mountain. r;i

other prospectors,organized the first Hahns Peak Mining District

under the wary eye of the Ute Chief, Colorow, and his Top-of-the-

Mountain-People, who were told that the white men's sole interest was

the "yellow metal". Hahn and Doyle remained in camp through a harsh

winter and in the following spring, facing starvation, thetwo

attempted to snowshoe to the mining camp at Empire, Colorado. Hahn,

however, died of exhaustion along the way.

No further mining development was undertaken until 1874 when the

Purdy Mining Company re-established the Hahns Peak Mining District

and began development of the Ways Gulch placer deposit east of the

town of Hahns Peak, now known as Hahns Peak Village.

In 1875 a wealthy Chicagoan, John Farwell, invested a large sum of money into the Hahns Peak placers, but sold out at a loss in 1879 when his shoveland sluice box operations apparently exhausted the

locally rich, yet very narrow, stream bed gravels.

Then, in 1880, Robert Mcintosh developed the Poverty Bar Placer west of Hahns Peak Village using sluice troughs and hydraulic tech- niquesand madea modest fortune. Much later, inabout 1905, a dredging operation on Ways Gulch proved unprofitable and the Poverty

Barand WaysGulch Placers were worked intermittently by various methods until1910,and then again in the 1930's. Placers on the southern flank of the Peak, and in Little Red and Big Red Parks to the north, have seen minor prospecting activity to the present.

Althoughthe district failedto develop intoa major mining camp,it wasthe hubofthe region when Routt County was established in 1877. The town of Hahns Peak (then locally referred to as 7

International Camp and Bugtown)was named the first county seat. It

remained the county seat until 1912.

Reported occurrences of gold andsilver at higher elevationson

theflanks ofHahrts Peak in 1881 eventually ledto the establishment of theRoyalFlushand Tom Thumb mines,as wellasthe town of

Columbine. The Royal Flush is the most extensivemine on the moun-

tain with undergroundworkings totallingover 2,300feet ontwo

levels. Although some work had beenaccomplished earlier, the first

major developmental effortat the Royal Flush mine was initiated in

1906 when H. 0. Granberg, Pat McGill, and J. R. Caron establishedthe

Hahns Peak Mining & MillingCompany. The mine was worked intermit-

tently into the 1920's, andperhaps later, but productionwas negli-

gible. Silver and goldwere found sporadically in fractured and

silicified sedimentary units. The history of this mine involveda

series of promotional playsorchestrated primarily by Henry Granbery.

The Tom Thumb wasa silver mine that produced a totalof 200 tons of galena-rich ore. One reported assay of 18,000 lbs. ofore

contained 52.0 oz/ton Ag, 2.0oz/ton Au, and 51.8 percent Pb (Gale,

1906). The Southern Cross minewas driven in about 1903 by Oeon

Poulson. Production was never reported and the mineralization

appears limited to pervasive pyrite and streaksof galena and tetra-

hedrite confined to a zone of intrusive breccia located near the

portal.

The total value of preciousand base metals mined from Routt

County from 1873 to 1960 didnot exceed $500,000. Over $300,000 of that figure was produced from the Hahns Peak placersduring 1873-1878

(Vanderwilt, 1947; del Rio, 1960).

More recent exploratory work was initiated in 1963 by WilliamA.

Bowes and in 1967 his company wasinvolvedin driving the 7D adit

over 1,700 feet into the mountain. The 70 underground workings were

strictly of an exploratory nature, withno ensuing production.

The Anaconda Company acquireda mining lease on Hahns Peak in

1971 and has conducted various exploratory investigations,including

diamond drilling, from that time to the present.

Although the settlement of the Hahns Peak regionowes its origin

to the mining industry, the Peak hasseen many other uses. In 1908

the U.S.Forest Service constructed a firelookout toweronthe

summit. It was subsequently rebuilt in 1939 and manned until 1950.

The concrete foundation and wooden framework remainintact.

Tourism and stock-raising arethe principalindustries of the region today. Every summer several thousand sheep grazeon federal and privately leased landon and around Hahns Peak. In addition, hikers, horseback riders, hunters,cross country skiers, hang glider enthusiasts, motorcyclists, snowmobilers, and jeepers maintain a year-round recreational use of the peak. Visible from many points within the Mt.ZirkelWilderness area 10 miles to the east, Hahns

Peak is definitely anenvironmentally sensitive area. This widespread and well publicized recreationaluse will be a distinct factor entering into any decisions involving future mining development.

Muchofthe historicalcontentof this section was condensed from the book Historic Hahns Peak, by ThelmaStevenson (1976). Previous Geologic InvestiQations

An abundant and varied amount of geologicwork has been per-

formed on Hahns Peak since the firstdescriptions were recorded in

1872 by S. F.Emmons, whenhe visited the region as part of the

United States Geological Exploration of theFortieth Parallel, super-

vised by Clarence King (Hagueand Emmons, 1877; King, 1878). With

this initialinvestigation, Eninons succeeded in identifyingand cor- relating the rocks of thesedimentary section and described two of the three main igneous intrusivephases making up the Hahns Peak complex. Mt. Zirkel, a distinctive peak of thenearby Park Range, was named for a colleague of Emmons,Prof. Ferdinand Zirkel, apetrog- rapher with the King survey.

In 1906, Hoyt Gale studied thearea, and suggested that Hahns

Peak was a laccolith. He reported that gold in the flanking placer deposits occurred as moderatelycoarse flakes and that it possessed a high alloyed silver content. He further concluded that because the placers are made up of 75-90percent Hahns Peak porphyry, and because virtually all stream gravels surroundingthe peak have anomalous gold values, the placer gold probably originatedfrom an eroded portion of the Peak. He noted that the search for lode depositsof gold on the

Peak had been fruitless, butthat trace amounts of gold were present throughout the rhyolite porphyry. The work of Gale and two other early workers in the area, Marshall Draper and WilliamWeston, was summarized by Arthur Lakes in 1909. 10

George and Crawford (1909) pursued the problem ofprovenance of

the placer gold and suggested that additionalsources, aside from the

porphyries, might be either the basal Dakota conglomerateor the Pre-

cambrian metamorphic rocks.

When it became clear, however, that the Hahns Peak gold placers

were essentially exhausted,and little potentialfor vein deposits

existed on the Peak, the question ofprovenance was ignored and geo-

logic investigations ceased fora time. Much later, Barnwell (1955)

and Hunter (1955) studied the stratigraphy andstructure of the Hahns

Peak area and Buffler (1967) examined the Browns ParkFormation and

its relationship to the late Tertiary geologic historyof the Elkhead

region.

Geologic investigations aimed at determining the economic.potential of the Hahns Peak districtwere renewed in 1963 by William A.

Bowes. He was joined inhiseffortsby personnel ofthe U. S.

Geologic Surveyin 1966, under theauspicesof the Heavy Metals

Program. Surveys that involved geochemistry (soil and rock samples), geophysics (induced polarization and resistivity), and diamondcore drilling were utilized to defineareas of gold, silver, lead, zinc, copper and molybdenite mineralization (Bowes and others, 1968; Bowes,

1969; Young and Segerstrom, 1973).

The gold content of natural waters at Hahns Peakwas studied by

Gosling, Jenne, and Chou (1971), while Kenneth Segerstrom andEdward

Young were engaged in surface mapping and detailed petrographicand mineralogic studies (Bowes, Segerstrom, and Young, 1968; Segerstrom and Young, 1972; Young and Segerstrom, 1973). The base map for the 11 geology shown on Plate1 was drafted from a plane table topographic map done by K. Segerstrom and supplied by W.A. Bowes.

Young and Segerstrom (1973), who reported unpublished microprobe

studies of gold inclusions in pyrite by G.A. Desborough, indicated that gold from the Royal Flush mine on Hahns Peak and from placers in

Little Red Park on the north flank of the peak have similar average

silver contents of 36.3 and 36.8 weight percent, respectively.

Antweiler, Doe,and Delevaux(1972) concluded from Pb isotope data that the gold mineralization of Hahns Peak is related to Tertiary

igenous activity, and that the placer gold originated in the porphyries. Thus, two independent methods of modern research support some of the conclusions of the early workers.

Segerstrom and Kirby (1969) interpreted the bedded outcrop cap- ping Porphyry Mountain to the west of Hahns Peak to be an epiclastic breccia. They suggested that reworked volcanic debris was swept from the slopes of Hahns Peak as lahars and deposited in a standing body of water.

Additional geologic mapping and laboratory investigations were conducted by Frederick Dowsett (1973, 1980) in conjunction with his study ofthe hydrothermalalteration of the I-lahns Peak porphyries, which was a Ph.D. dissertation at Stanford University.

Since their acquisition of the mineral rights on Hahns Peak in

1971, the Anaconda Company has conducted a seriesof mapping and drilling projects, with the most complete previous evaluation being an internal company report by G.M. Park in 1972. In addition to the mapping and rock chipgeochemical samplingdone as part of the 12 present study, Anaconda conducted an airborne magnetometersurvey in

November, 1978. 13

REGIONALGEOLOGIC SETTING

Hahns Peak is located between the west-trending Elkhead

Mountains and thenorth-trending Park Range, withintheRocky Mountains of northwestern Colorado.The Elkhead Mountains consist of a group of extrusive and shallow intrusive volcanic rocks that are late Tertiary in age.The Park Range consists principally of Pre- cambrianmetasedimentaryrocksthatincludegneiss,schist and quartzite. A similar Precambrian metasedimentary basement complex withinthe Hahns Peak quadrangle is overlain by sedimentary rocks ranging from late Paleozoic through Cenozoic inage. These meta- morphicandsedimentary unitsarelocally intrudedby plutonsof Tertiary age.Extrusive rocks of Tertiary age contribute only a very minor part to the geologic column of this area.

Precambri an Geology

The basement rocks consist of fine-grained felsic and biotitic gneiss, mica schist, quartzites and rnetaconglomerates. Relict sedimentary textures such as cross-bedding, graded-bedding and heavy min- eralsegregations are present locally and indicate subaerial deposition of pelitic, arenaceous and conglomeratic sediments during Pre- cambrian time.Lenses of arnphibolite within these rocks are probably indicativeofinterlayeredmaficvolcanicsorintrusivesills. Tectonisrn at about 1.65 to 1.70 b.y. resulted in the regional metamorphism of these sedimentary and interlayered mafic igneous rocks to 14

Qoc çTbjj 1N tte Red JKJ:QOCC'\ ..MorvF4Pork Tbp (j/flJ (v,)

Tp Ip (

H Pecl : ______'>, iMage Tbp Oaç caternory cituvium, co!IuvturTl a Steamboat c ''? and (ands.tde (undivtded) Loe Ip TerflQry porphyry .. Lester - ii Qac Creek Tbp Tert tory Browns Park, ocot Tbp base surqe (wtdtvted) .4 'JKTriossic, Ji.rassic and Cretoceous sedimenthty

rocks undiv1ded) 11 PC Precambrtan metamorphic

\ Fau dashea where interred \

Figure 3. Geologic map of the Hahns Peak Region. Compiled from Dowsett (1980), Segerstrom and Young (1972), and Casaceli (Field mapping 1978-1979). 15

the almandine-amphibolite facies (Segerstrom and Young, 1972). This

aye is similar to that of 1.75 b.y. determined for the Boulder Creek

Granodiorite in the Front Range ofColorado(Petermanand others,

1968), and presumably dates thesame tectonic episode. The resultant

gneisses and schists of the Hahns Peak regionwere isoclinally folded

during this period of metamorphism and tectonism, and they exhibit

foliation and schistosity parallel to the original beddingplanes. A

later period of tectonism createdopen folds striking east-northeast

(Segerstrom and Young, 1972). Both periods of tectonism were accom-

panied by minor intrusions of granite and quartz diorite. Pegmatite

dikesand quartz veins,accompanied by minor amounts of sulfides,

intrudedtheregionally metamorphosed rocks late in the cycle of

Precambrian tectonism. Malachite and azurite are visible in apeg- matite cutting a felsic gneiss-amphibolite complex on Farwell

Mountain, and a rockchip sampletaken bythe authorcontained

1.1 percent Cu, 75.0 ppm Zn, 3.2ppm Ag and 97.0 ppm Mo. Although sulfides are present, the lack of reactive carbonate horizons inthe

Precambriancomplexseverely limits its potentialforthelocali- zation of ore.

Paleozoic Geology

The occurrence of Paleozoic rocksin the Hahns Peak regionis extremely limited. Outcrops of red calcareous siltstone and fine- grained sandstone of the Permian Goose Egg andLower Triassic Red

Peak Formations are exposed at onlya few localities on the Farwell 16

Mountain quadrangle. However, these same units are intersectedin

the lower portion of drillhole DDH-1O1, beneath Hahns Peak, where

they appear tq lie uncomformablyon Precambrian granodiorite.

The relative absence ofPaleozoic rocks is presumably due to

periods of extensive erosion andnon-deposition.

Mesozoic Geology

The upper Triassic rocks ofthe region consist of sandstones,

siltstones, and claystones of theJeirn, Popo Agie, and Nugget Sand-

stone Formations. Textural and compositional evidencesuggests that the Jelm Formation is fluvialin origin; that the Popo Agie Formation is mixed fluvial and lacustrinein origin; and that the Nugget Sand- stone Formation is predominantlytidal flat in origin (Segerstrom and

Young, 1972).

An erosionalunconformity separates the NuggetSandstone Form- ation from the overlying JurassicSundance Formation. The Sundance

Formation consists of sandstoneandshale that was deposited in a shallow water marine environment(Segerstrom and Young, 1972).

Shales and fresh water limestonesof the Late Jurassic Morrison

Formation lie disconformablyabove the Sundance Formation and mdi- cate a mixed fluvial and lacustrine environment of deposition

(Segerstrom and Young, 1972). These varicolored shales are usually poorly exposed, as they readilydisintegrate and are easily eroded.

Previous workers(Segerstrom andYoung, 1972; Dowsett, 1973, 1980) have mappedthe Goose Egg,Red Peak, Jeim,Popo Agie, and 17

Nugget Sandstone Formations togetheras a single unit, and have like-

wise grouped the Sundance and Morrison Formations intoanother single

unit. The same procedure was followed in mapping involved withthis

study.

The Cretaceous rocksof theregionare Dakota Sandstoneand

Mancos Shale. The Dakota Sandstone consists of cross-bedded sand-

stones underlain by a basal conglomerate, and is typicallya resist-

ant ridgeformer. Theseclasticunits are indicative of brief

regionaluplift that had ceased by the time of depositionofthe

overlying Mancos Shale. The Mancos Shale is a calcareous unit prob-

ably deposited during the transgression of an epeiric sea. This

shallow sea soon recededas regional uplift once again put an end to marine deposition when the Rocky Mountain orogeny (Laraniide

Revolution)began in LateCretaceous time at about 65 n.y. ago

(Naiser, 1976).

Cenozoic Geology

Rocks of Paleocene, Eocene and Oligoceneage are missing from this region. This is a consequence of erosion and non-deposition related to continued uplift and deformation ofthe Rocky Mountain region wellinto the Tertiary Period. Deformation was characterized by primarily horizontalcompressionalforces which prevailed until late Eocene time, perhaps until about 40 m.y. ago (Coney and

Reynolds, 1977). Displacement of Precambrian basement rocksthat were thrust westward over Mesozoic sedimentarysequences in the Hahns Peak region probably occurred during the latterpart of this compres-

sional regime. Rock units older than the middle Miocene Browns Park

Formation are locally overturnedadjacent totheregional thrust

fault shown in Figure 3. This suggests that thrusting occurred after

the Late Cretaceous, but before the middleMiocene.

The Browns Park Formation consists ofa coarse basal conglorn-

erate and an overlying tuffaceous sandstone. The basal conglomerate

consists of Precambrian cobbles and pebbles thatwere deposited as

alluvial fans on a monoclinal erosion surfacedipping westward from

the Park Range. This material was subsequently covered by fluviatile

and eolian tuffaceous sands thatare believed to have been derived from the San Juan Mountains-West ElkMountains volcanic complex and from the Mesozoic sedimentary rocks of the Uncompahgre Plateau

(Buffler, 1967). Buffler (1967) interprets the Browns Park Formation to be mid-Miocene in age,and notes that itis older than allthe volcanic rocks in the Elkhead Mountains thateither intrude or over- lie it. Ages of intrusive rocks in the Elkhead Mountainsrange from

11.0 m.y. at Brush Mountain, Colorado (Sec.34, 1. 11 N., R. 88 W.), to 7.6 m.y. at City Mountain, Colorado (Sec.26, T. ii N., R. 86 W.)

(Buffler, 1967).

The majority of the igneous rocks that form flows,dikes, sills, laccoliths, domes and diatremes in the ElkheadMountains are typically quartz latite in composition. However, isolated basalt flows and dikes related to this period of intrusive activityare located in

Little Red Park, three miles north ofHahns Peak, and in another area immediately southwest of Steamboat Lake (Carey, 1955). 19

Sedimentary rocks were locally domed, tilted,and cut by high

anglenormal faults that preceded and accompaniedlate Tertiary

intrusive activity in the region. Hahns Peak occupies the center of

a west-trending horst that is bounded by theKing Solomon fault on

the north and by the GrouseMountain Fault on the south(Fig. 3).

Bowes (1969) has noted that thesefaults follow the generalUinta

trend, and may reflect the eastwardcontinuation ofa Precambrian structure that was reactivated in thelate Tertiary period.

Continued uplift into the Quaternary Period resulted in

I ncreased rates of erosion unti 1 theI ni ti ati on of regi onal gl ad - ation east of Hahns Peakon the western flanks of the Park Range.

The presence of moraines in theeastern part of the Farwell Mountain quadrangle indicate that widespreadglacial activity occurred in that portion of the region (Segerstromand Young, 1972).

Mass wasting has been the dominantgeologic process in Holocene and recent times, as evidenced bylandslide debris and unstable scree slopes that mantle the steeperhillsides around Hahns Peak.

Regional Geophysics

Hahns Peak is located withina broad negative gravity anomaly, defined by a closed -200 milligalBouger contour, that approximately outlines the Middle Rocky Mountainand Southern Rocky Mountain provinces (Hollister 1978; Fenneman, 1946). Thisregional negative

Bouger anomaly reflectsa thick sialic crust (Hollister, 1978) that

. 20

extends to a depth of 50-60 Km beneaththe Southern Rocky Mountains

(Pakiser, 1963).

Larger scale plotsof Bouger gravity and total field magnetic

data from Colorado suggest thepresence of a deep-seated intrusion of

batholithic proportions that is located approximately six miles

northwest of HahnsPeak,within the boundsofthe Rocky Mountain

regional gravity low (Fig.4)(Behrendt and Bajwa, 1974; Zietz and

Kirby,1972). The presence of this plution is inferred from the

nearly coincident positions ofa zone of locally higher Bouger grav-

ity values (up to -200 milligals),and a distinct zone of high mag-

netic intensity (up to 4,300 gammas)(Fig.4). These superimposed highs indicate thepresence of a dense pluton at depth with a nioder- ately highcontentof magnetic minerals. It is likely thatthe pluton hasa composition somewhat more mafic thanquartzlatite, althoughthis cannot be uniquely determined from the geophysical data. The intrusive bodies of latite toquartz latite composition that are exposed at, andnear, Hahns Peak are located on the southeast limb of the proposed batholith, andare interpreted to have been derived from a cupola that projectsfrom it. The more mafic composition that is inferred for the plutonat depth suggests that magmatic differentiation probably occurredto produce the near-surface intrusions.

Regionalreduced heat flow measurements from northernColorado are anomalously high (1.2-3.0 HFU) (Decker and others,1980; Buelow,

1980), and are within therange of values measured in the southern segment of the Rio Grande Rift (1.8-3.4 1-IFU) (Decker and Smithson, 21. O7° WYOMING

Figure 4. Portion of Colorado Bouger GravityMap (Behrendt and Bajwa,1974) and Colorado AeromagneticMap (Zeitz and Kirby, 1972). Scale 1:500,000. 22

1975). On the basis of thisdata, Decker and others(1980) have speculated that the Rio Grande Rift may extend as far north as the

Colorado-Wyoming border.

Thus, thegeophysical evidencesuggests thatthe Miocene-age intrusive rocks of the Hahns Peak region may be associated witha shallow crustal batholith emplaced within the northern extension of the Rio Grande Rift, near its intersectionwith two Precambrian structures (Fig. 5).

Regional Tectonics

The tectonic history ofthe western UnitedStatesfrom Pre- cambrian through Cenozoic time involves complex interactions of lithospheric plates along an evolving continental margin. There is abun- dantevidencethatsubduction-related compressionaltectonicsdom- inated the evolution of the Cordillera from mid-Ordovician to mid-

Tertiary time. Many geologists, however, object to the hypothesis that subduction processes could affecta region up to1,000 miles from the continental margin.

The following is an attempt to synthesize current data, and to introduce some new concepts in the presentation of a generalized history ofthe tectonic evolution ofthe western United States. The emphasis is onthe Southern Rocky Mountain physiographic province

(Fenneman, 1946) in early to late Tertiary time, because it 'is that region and interval of time that are most pertinent to understanding the geology of Hahns Peak. However, tectonic activity throughout the 23

MULLEN CREEK-NASH FORK SHEAR ZONE

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ALBUQUERQUE EXPLANATION Suggested northern extensIon of Rio Grande Rift with approximate lateral SOCORRO. Ø5 extent of nfl system \ I mare by dashed 54 lines (Tweto, 1978) 144 0

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Figure 5.Tectonicmap of theSouthern RockyMountain Province. The molybdenum occurrences at Henderson, Climax, Mount Emmons, Questa, and Hahns Peakare showninrelationship tothe Rio GrandeRiftSystem and older Precambrian structures. Cordillera since Precambrian timehas had important effects on the

Hahns Peak region, and must beconsidered in a comprehensive geologic

interpretation.

Precambri an Tectoni Cs

The crystalline rocks that form the2.7-2.5 b.y. Archean base-

ment complex of the western UnitedStates are exposed northwest of

the Mullen Creek-NashFork shearzone (Fig. 5), andconsistof

gneiss, metagraywacke, greenstone, and localintrusions of mafic and

ultramafic composition (Burchfiel,1979). The Mullen Creek-Nash Fork

shear zone marks a major sutureon the Precambri an conti nental crust

that was formed approximately 2.0 b.y.ago when island arc terrane

collided with the southeast side ofthe Archean craton (Burchfiel,

1979). The island arc-derived rockswere regionally metamorphosed to

amphibolite grade at about 1.75 b.y.ago. By 1.72-1.70 b.y. ago,a

longitudinal wrench fault system that includedboth the Mullen Creek-

Nash Fork shear zone and the ColoradoMineral Belt (Idaho Springs-

Ralston shear zone) (Fig.5), was developed along the accretedcon- tinental margin (Warner, 1978, 1980;Burchfiel, 1979). Warner (1978,

1980) has named this northeast-trendingsystem of en echelon wrench faults the Colorado lineament, andhas suggested that it extends from

Arizona to Minnesota.

Strike-slip deformation of the continentalmargin was followed by intra-cratonic rifting in thewestern part of the mid-Proterozoic continent at 1.4-0.85 b.y.ago. This resulted in the formation of thick sequences oflate Precambrian sedimentary rocksina fault- 25

bounded basin, as exemplified bythe Uinta Mountain Group (Fig. 5)

(Burchfiel and Davis, 1975; King,1977).

Pal eozoi c Tectonics

In late Precambrian to early Paleozoictime (850-470 m.y. ago)

the central-western United Stateswas divided into a cratonic high-

land to the east, and a passivecontinental shelf sedimentary

sequence to the west that developed into the Cordillerangeosyncline.

The boundary betweenthe terrigenous wedge of shelf rocks and the

continentalsource area passes northeasterly through central Utah,

and is known as the Wasatch line(Burchfiel, 1979).

From mid-Ordovician to mid-Triassic,the continental margin of

the western U.S.probably hada Japanese-type setting of marginal seas and intra-oceanic island arcs (Dickinson, 1976; Burchfiel,

1979). In Mississippian time, transgressivecycles that locally deposited thin beds of carbonateson the craton, alternated with epi- sodic continental margin accretionand related tectonism. Cornpres- sion of the island arc terraneat this time resulted in the partial closure ofa marginal basin, and the eastward thrust ofan alloch- thonous block onto the continentalshelf in central Nevada, producing the Antler orogeny (Burchfiel, 1979).

The formation of the AncestralRocky Mountains in Pennsylvanian-

Permian time caused the upliftof Precambrian crystalline basement rocks in a series of northwest-trending highlandsin Colorado, New

Mexico, and Utah. The Paleozoic rocks that originally capped uplands were extensively eroded and deposited into flankingbasins, forming 26

the distinctive arkosic "red beds". The cause of the Ancestral Rocky

Mountain uplift is uncertain, butit may be related to the Quachita

Orogeny that occurred synchronouslyalong the southeast margin of the

craton (Birchfiel, 1979).

Near the end of the Paleozoicera, the Ancestral Rocky Mountains

were strongly eroded, and by mid-Tertiary timeonly isolated high-

lands remained. 8ut whileerosion wassmoothing the continental

landscape inland, tectonic forceswere active once again along the

continental margin to the west, wherea broad belt of deformed eugeo-

synclinalisland arcand ocean floor rocks were accretedonto the

craton. This accretion extended the cratonicmargin 350 miles ocean-

ward (Dickinson, 1976). The collision of islandarc terrane with the

cratonic margin in this phase of latePaleozoic accretion resulted in

blocksof oceanic crust being thrustonto the continent in west-

central Nevada, in whatwas known asthe Sorioma Orogeny (Burchfiel

and Davis, 1972, 1975).

Mesozoic Tectoni Cs

Mesozoic tectonic eventswere dominated by processes related to active subduction along thewestern continental margin (Coney, 1971,

1972; Burchfieland Davis,1972,1975; Dickinson,1976; Burchfiel,

1979). A Japanese-type marginwas prevalent into the early Mesozoic era, but continuedaccretionand compression converted the conti- nentalborder toan Andean-type arc-trench system by mid-Triassic time (Coney, 1972; Dickinson, 1976). The Andean-type setting continued through the Jurassic andinto the Cretaceous, with ophiolitic 27

rocksand continental andarc-derivedclastics developed into an

accretionary wedge to the west. Simultaneously to the east, minor

transgressive and regressive cycles ofan epeiric sea resulted in

alternating shallow marine andnon-marine sedimentary deposits in the

Southern Rocky Mountain Province(Dickinson, 1976).

By late Jurassic time,blocks of pre-Mesozoic eugeosynclinal,

miogeosynclinal and cratonal rockshad been thrust eastward to forma

foreland fold-thrustbelt that extendedthrough western Montana,

eastern Idaho, NW Utah, and SE Nevada. Detritus from the continental

area to the east was deposited ina narrow foreland basin on the edge

of the fold-thrust belt (Dickinson,1976).

The eastward advance of fold-thrust belts and conglomeratic

wedges is evidence that mountainbuilding, associated withcompres-

sive tectonics, progressed continuallyinland from the mid-Jurassic to the early Tertiary (Billingsley and Locke,1941; Gilluly, 1963;

Burchfiel and Davis, 1975; King,1977). The steady eastward progres-

sion of orogenic events duringthis time period suggests that the

entire Cordillera of North Americashould be considered as a single

system that formed bya continuous tectonic process (King, 1977).

This tectonic process probablyinvolved active subduction along the western margin of the craton andthe coincident northwestward rota- tionof North America inresponse to the opening of the Atlantic

Ocean from 180 m.y. to 40m.y. ago (Coney, 1971, 1972).

The eastward progression ofMesozoic tectonism began with deep- seated deformation and plutonismassociated with the Nevadan Orogeny re.]

(170-130 m.y.). This wasfollowed bytheSevierOrogeny (100-

80 m.y.) in which plates of miogeosynclinal rockswere thrust east-

ward toward their foreland, and by the Laramide Orogeny (72-50 rn.y.)

which produced great upfolds and downfolds of basement rock(King,

1977).

Cenozol c Tectoni cs

The Laramide Orogeny of Lake Cretaceous to early Tertiary time

marks the initial uplift of the present-day Rocky Mountains. Naeser

(1976) ha established that this uplift began in the Front Range of

Colorado at about 65 m.y.ago. Despite an abundance of related data,

geologists continue to debate the fundamentalprocesses that have

created the Rocky Mountains. The controversy is primarily keyed to

the question of whetheror not subduction can affect cratonic areas

up to 1,000 miles from the continental margin. Although this question remains unresolved, it is clear that the Rocky Mountains evolved from a compressive tectonic environment in Late Cretaceousto mid-

Tertiary timeto a predominantly extensional environment since the mid-Tertiary (Coney,1971, 1972; Burchfieland Davis, 1972, 1975;

King,1977; Burchfiel, 1979). Any hypothesis on the origin of the

Rocky Mountains must explain thecause of this tectonic transition and how it relates to the generation ofmagma.

Lipman and others (1971, 1972) suggested that Cenozoic volcanism and mountain-building in the western United Stateswere related to an imbricate subduction system. They identified two belts of easterly increasing K20 content in volcanic rocks that presumablyreflect two 29

east-dipping subduction slabs. The easterrmost slab was thoughtto have originated asadecoupled fragment from the low-velocityzone beneath the Rocky Mountains. Gilluly (1971) objected stronglyto

this hypothesis and arguedthat because most known Benioffzones have an average dip of 450, and that caic-alkalinemagmageneration is

unlikely to occur below thelow-velocity zone, it would be unlikely

to evolve magmasmore than 250-300 miles inland from the subduction

trench. He suggested that the upliftof the Rocky Mountains was the

result of up-welling in themantle caused by processes unrelatedto

subducti on.

Structural evidence in supportofa subduction origin for the

Cordillera was introduced byConey (1971,1972). He argued that

beginning 180 m.y.ago, North America was driven overan east-dipping

Benioff zone, and thatstrong resistancefromthe overridden oceanic

platekeptthecontinentalmotion relatively slow and ultimately

caused widespread conipressionalfailure that resulted in the Nevadan,

Sevier, and Laramide orogenies. Coney suggested that the unique cir- cumstance of a continentalplate having forcefully overridden an

active subduction zonecan account for the presence of thrust faults upto 1,000 miles from the continentalmargin of North America, whereas along an Andean-typemargin thrust faults are never developed more than 250 miles inland (Mitchel andGarson, 1976; Richardson and others,1979). If North America had not beendriven against the

Pacific plate in Mesozoicand Early Tertiary time,the Cordillera would more likely have resembledpresent-day west Pacific arcs where 30

active subduction-related tectonic events occur in narrow belts (Coney, 1972).

The steady eastward progression of caic-alkaUne intrusive activity in lhe western United States from late to mid-Tertiarytime has been cited by several authors as evidence of a flatteningBenioff

zone (Lipman and others,1971; Snyder and others,1976; Coney and Reynolds, 1977; Keith, 1978, 1979). Coney and Reynolds (1977)sug-

gest that the apparent steadydecrease in the dip angle of the sub-

duction slab from 55° to 10°in the time period 120 to 40m.y. ago

was the result of direct interactionwith an actively driven North

American plate.

A nullperiod of intrusive activityoccurred at approximately 40 rn.y. ago, and was followed by a westward progression ofintrusions until 15 m.y. ago (Snyderand others, 1976; Coneyand Reynolds,

1977). This westward progressionof igneous activity was interpreted by Cone,y and Reynolds (1977) to be the result of the re-steepeningof a single subduction slab, while Snyderand others (1976) recognized two parallel arcs of magmatismwhich they interpreted to have formed from partially imbricatedBenioff zones.

Thompson and Zoback (1979)interpreted the abrupt narrowing of the belt of arc-related magmatism in the Sierra Nevada andBasin and

Range provinces from 40 to20 rn.y.ago to be the result of a transition tosteep-angle subduction, whichthey suggested was caused by the fragmentation ofa low-angle slab in a strong compressional environment. Their interpretation of seismic,gravity, and heat flow data suggests that the eastern remnant of a shallow-dipping 31

subduction slab was present beneaththe Colorado Plateau 20rn.y. ago.

This hypothesis was corroboratedby Casaceliand Wyss (1978, 1980)

who interpreted azimuthalvariations in teleseismic P-wave residuals

to be indirect evidence ofa fragmental lithospheric slab having been

present beneath Colorado andNew Mexico in mid to late Tertiary time.

If the hypotheticallow-angle subduction slab did fail inan imbricate manner, it islikely that this fragmentation occurredat

approximately 40 m.y. ago, coincident with the relaxation ofCornpres-

sional tectonics (Coney, 1971,1972) and the temporary cessationof

igneous activity in thewestern United States(Snyder and others,

1976; Coney and Reynolds, 1977). The western portion of the failed

slab would correspondto the leading edge ofthe Farallon plate

hypothesized by Atwater (1970). The eastern portion of the failed

slab would have been leftbehind as a remnant plate fragment beneath

the Southern Rocky MountainProvince.

The force couple formed byNorth America being drivenover a

subducting oceanic slab wouldhave been broken upon failure of the slab, and the compressionalstresses would have been relieved in both the slab and the continentalcrust. The two slab fragments would then have pulled apart aseach steepened its dip and continuedto subduct into the mantle underthe force of gravity. This would have permitted mantle material toupwell into the pull-apartzone, and may explain the initiation ofwidespread extensionalfaultingand bi- modal volcanism in theupper crust of the Basin and Range Province

(Casaceli, unpublisheddata). Thepresence ofmetamorphiccore complexes within the BasinRange Province and throughouta continuous 32

zone from Canada to Mexico(Rehrigand Reynolds, 1977; Davisand

Coney, 1979) is likely to bea reflection of the extensional environ-

ment created by the slabfailure. In addition, the apparent exten-

sion of low-velocity material in the upper mantle from the Basinand

Range Province to the RockyMountain front (Archambeau, 1969;

Casaceli and Wyss, 1978, 1980)may be due to upwelled mantle material

that was trapped beneaththe eastern portion of the failed slab.

The westward migration of caic-alkaline intrusions in the

Southern Rocky MountainProvince from 40 to 50m.y. ago (Bookstrom,

1981) may be interpretedas a result of the progressive steepening in

dip of a remnant slab. The Rio Grande Rift,an extensional feature that varies in width from five to one-hundred miles and whichparal-

lels the trend of theRocky Mountainsfrom Mexico to northern Colorado, may be directly relatedtosubduction-generated igneous

activity that occurred inmid to late Tertiary time.(Lamarre and

Hodder, 1978; Bookstrom, 1981;Lipman, 1981). Extensional tectonism was initiated within the southernsegment of the Rio Grande Rift

(El Paso, Texas, toSocorro, New Mexico) at about 32m.y. ago, and progressed steadily northward. Rifting was initiated in the northern segment(Alamosa to Leadville, Colorado) at about 27m.y. ago (Chapin, 1979).

While rift-related tectonismwas evolving inland, parts of the

Andean-type continental marginwere converted to a transform fault by the evolution and migrationof triple junctions. These triple junctions were formed when thecrest of the East Pacific Rise encountered the trench at approximately 29 rn.y. ago (Atwater, 1970). Strike-slip 33 faulting related to this period of continental margin tectonics has not been recognized east of the Great Basin. However, right-lateral

strike-slip tectonism along the Pacific coast continues to the present.

Metallogenic - Tectonic Relationships

Porphyry Cu/Mo deposits, typically associated with caic-alkalic magmatic belts,have been closely linked to subduction processes and several authors have cited their presence throughout the Cordillera of the western United States as evidence for a subduction origin of the Rocky Mountains (Sillitoe, 1972, 1980; Mitchell and Garson, 1976,

Guild, 1978; Lamarre and Hodder, 1978; Keith, 1978, 1979; Westra and

Keith, 1981; White and others,1981; Lipman, 1981). Although some authors argue that mineralized porphyry systems may be developed from isolated mantle upwellings unrelated to subducted lithosphere

(Lowell,1974;Noble,1976), the continuous belt of these deposits from Alaska to Chile (Mitchelland Garson,1976) is strongly suggestive of a comon origin along convergent plate boundaries.

Climax-type stockwork molybdenum deposits are believed to have formed in areas of back-arc rifting, close to the time of cessation of subduction (Lamarre and Hodder,1978; Sillitoe, 1980; Bookstrom,

1981; Westra and Keith, 1981; White and others, 1981; Lipman, 1981).

The Rio Grande Rift (RGR) has been interpreted as a zone of back-arc extension (Lamarre and Hodder, 1978; Lipman, 1981; Bookstrom, 1981), and intersections of the RGR with deep-seated Precambrian structures are important centers for silicic igneous activity and related 34

porphyry molybdenum deposits(Bookstrom,1981; Fig.5). The large

deposits of molybdenum atClimax, Henderson, and Mt. Emons,Colorado

are located at the intersection of theRGR System (Tweto, 1978) with

the Colorado Mineral Belt,a Precambrian shear zone that wasreac-

tivated in late Cenozoic time(Tweto and Sims, 1963; Warner, 1978,

1980). Similarly, the molybdenum deposit at Questa,New Mexico

occurs at the eastern edge of theRGR where it joins with the Red

River Trench, a Precambrianstructure that has undergone extensional

activity in the Tertiary period(Schilling, 1956).

The HahnsPeak molybdenum occurrence (Bowes, 1969), like the

deposits at Climax, Henderson,Mt.Emmons,and Questa, is located

near the intersection of two Precambrianstructures with the probable

northern extension ofthe RGR(Fig. 5). The two Precambrian-Cage

structures are the MullenCreek-Nash Fork shear zone, interpreted by

Warner (1978,1980) to be part ofthe northeast trending Colorado

lineament thatextendsfrom Arizonato Minnesota, andthe Uinta

Mountain Group trend, interpretedby King(1977) to be part of an

ancient intracratonic riftsystem.

Evidencethatthe RGR extendsto the Colorado-Wyoming border comes from severalsources. Tweto (1968,1978) noted that north- trending, en echelon normalfaults in southern Wyoming and northern

Colorado are aligned withthe Rio Grande Rift proper to the south.

In addition, the fluoritedeposits in southern Wyoming (e.g. North- gate-Crystal) and northern Colorado(e.g. Jamestown) are similar to others thatcrop out alongthesouthernportionofthe rift in Colorado, New Mexico, andTexas (Van Alstine, 1976; Lamarre and 35

Hodder, 1978). Similarly, igneous rocks of Oligocene and Miocene age in northern Colorado have been shown to be related to late Cenozoic extensional faulting (Segerstrom and Young, 1972; Steven, 1975;

Blackstone, 1975). Finally,regionalheat flow measurements from northern Colorado (1.2-3.0 HFU) correspond closely to measured values from the southernsegment of the RGR (1.8-3.4HFU) (Decker and others, 1980; Buelow, 1980).

In conclusion,it appearslikely that the felsic magmas that generated the porphyry molybdenum deposits of Colorado and New Mexico were emplaced inan ensialic back-arc rift environment. This rift zone is interpreted to have developed above a fragmented slab that continued to undergo active subductionuntil mid to late Cenozoic time. Hahns Peak, located near the intersection of major Precambrian structures with the probable northern extension of the Rio Grande

Rift, is in the same tectonic setting that has proven to be favorable for some of the world's largest deposits of molybdenum. 36

LOCAL GEOLOGIC SETTING

Hahns Peak is a composite intrusionof quartz latite and latite

porphyry approximately 10-12m.y. old (Segerstrorn and Young, 1972).

The older intrusions of BerylMountain porphyry and Little Mountain

porphyry are latite and quartz latitein composition and sill-like in

their intrusive style. The slightly younger porphyriesare more sil-

iceous in composition andsomewhat more discordant to the wall rocks.

These later intrusions consistof three main phases(the Columbine, Sumit, and 70 porphyries) that comprise the bulk of the HahnsPeak

complex. They form a laccolithic bodythat has domed the older sedi-

mentary units and appears toneck with depth.

A narrow, conical sheet of brecciawas intruded into these igne-

ous units. This breccia cone sheet consistsof an early Monolithic phase and a later Multilithic phase. It is interpreted tohave

formed above a felsic plutonthat is hidden at depth. The breccias were apparently formedby both explosivernagmatic discharges and meteoric steam-blast eruptions. A volcanic vent complex and a pyroclastic surge depositon the west flank of Hahns Peak are believed to be evidence ofthe steam blast eruptions. The breccia cone sheet appears to controlthe hydrothermal alterationand mineralization.

Late-stage dikes and dikeletsthat range in composition from latite to rhyolite are present locally.

The sedimentary unitsthat were domed by the igneous intrusions crop out on the flanks of Hahns Peak. These units consist primarily of siltstone, shale, sandstone, and conglomerate. They are, in 37

chronological order, the Sundance Formation, Morrison Formation,

Dakota Sandstone, Mancos Shale,and Browns Park Formation. Under-

lying the Sundance Formation,and hidden beneath Hahns Peak, is a

Permo-Triassic sequence ofsiltstone andsandstone. Whereit was

intersected in drill hole DDH-101, itwas found to lie unconformably on Precambrian gneissic granodiorite.

Age Determination of Intrusive Rocks

A potassium-argon age determination on biotite from weakly altered quartz latite porphyry located2.5 miles west of Hahns Peak was reported to be 9.5 ± 0.3 ni.y. (McDowell, 1971). In general agree- ment with this determination, Segerstrom and Young (1972)obtained a potassium-argon age of 10.0 ± 0.3m.y. on sanidine from 7D porphyry.

The same authors have provided anotherage determination of 11.5 m.y. based on biotite and sanidine from the prominent late-stageporphyry dike (Plate 1) that clearly cross-cuts the central portionof Summit porphyry. Although the age dates suggest that this dikeis older than the 7D porphyry, the field relationshipsindicate thatit is younger. Segerstrom and Young (1972) suggest thata leakage of rad- iogenic argon, as a consequence of hydrothermal alteration, may account for the younger age obtained from the 7D porphyry. Acccord- ingly, they suggest that approximately 12m.y. may bea reasonable age for the Hahns Peak intrusive complex. 38

Geomorphol ogy

Hahns Peak isa topographic anomaly that dominates the present

landscape of this region. This feature resulted from a combination

of hypabyssal magmatism and accompanying structural doming,as well

as from the inherent resistance of these igneous rocks to erosion.

The Dakota Sandstone unit isalso resistant to erosion and thus it

caps prominent cliffs of Morrison and Sundance shales and sandstones

on the northern flank of the peak.

Although altitudes range from 9,000 to 10,839 feet, Hahns Peak

displays no evidence of glaciation. Typical U-shaped valleys,

abraded and rounded hills (e.g., roches mountonnes), and deposits of glacial till are absent from the immediatearea. Frostaction, which has probably been active since the Pleistocene, is the dominant geomorphologic process that has sculptured Hahns Peak. It has resulted in a thick mantle of unstable talus that blankets the peak

and leaves relatively few exposures of bedrock, despite the lack of a vegetative cover on the upper slopes. There is very little surface runoff because the rain water and snow melt quickly percolate through the interstices of the talus rock.

On the west edge of the peak, a short distance below the lookout house, a downslope lineation was observed in the mantle rock. Here, tabular pieces of rock are aligned in a rille with their long axes oriented vertically. Sharpe (1938) has named this phenomenon "stone stripes" and has described it as being typical of alpine-type mantled and patterned ground. It isa feature that was probably caused by 39

freeze-thaw activity. Much oftheigneousrock,particularly the

Columbine porphyry, is fissile andbroken into platy slabs with frac-

tures oriented parallel to theprimary flow foliation defined by bio-

tites. The large amount of mantle rock islikely caused by the rel-

ative ease by which the hostrock splits along planes of foliation.

The depth ofthe regolith varies froma few inches to several feet and usually resides on steep slopes near the angle ofrepose. The

average slopes on the southwest flank of HahnsPeak are approximately

70 percent. For the most part, the talus slopesare unstable and

they comonly fail causingrock slides thatcover mostexisting roads. A unique wavy pattern of iron-stained and lichen-covered

talus rock is visibleon the south flank of the peak. This feature

probably reflects the generalinstability ofthe slopes. Isolated

landslides are comon,even on the lower slopes covered by grasses,

pine, aspen, and pistol-butted treesattest to the perennial activity

of hillside creep. Debris slides and debris avalanches (Schusterand Krizek, 1978)of bothigneousmaterial andDakota Sandstone are

present on the west flankabove the RoyalFlush Mine (Plate1).

Large slide blocks ofvirtually intact Dakota Sandstone arealso

present here and on the northern flankdirectly beneath the prominent

cliffs. These sandstone blocks containopen fractures that are partially filledwith clusters of crystalline quartz. These are believed to be tension fracturesformed during the process of doming.

These fractures served as planes of weakness that facilitated the pulling apart and downslopemovement ofthe sandstone blocks under 40

the force of gravity when thecap of Dakota Sandstone was undermined

by the erosion of the underlyingMorrison Shale.

A rock glacier, previously undescribedin the literature, forms

a unique geomorphologic feature on the northwest sideof the peak.

This rock glacier is small comparedto others observed by the author

in the Sawatch and San Juanranges of central and southern Colorado,

yet pressure ridges, indicative ofplastic flow, are well developed.

The slope at the toe of this rockglacier was measured at 40°. A

bulldozercut madeduringthe summer of 1968 reportedly exposed

interstitial "black ice"(P. Rogowski, pers.comm., 1979). Compar-

isons of air photos taken in September,1968 with those taken for

this study in July,1978 show that the rock glacier is presently

active and has moved downslopeapproximately 50-60 feet in that ten

year period.

Apparently relatively little erosion hastaken place on Hahns

Peak since its initial formationby doming of the sedimentary rocks

about 12 million yearsago. It is estimated that between 500 and

1,000 feet of overburden has beenremoved. This conclusion is based

on the presence of the tilted strata of the originaldome structure

on the flanks of the peak, the thinveneer of colluvium that exists on its periphery, and the minimal amount of sheet washerosion that has scoured the peak since thePleistocene. Also, the presence of the pyroclastic surge depositand the flanking layered vent complex suggest that relatively littlecover hasbeen removedsince their deposi tion. 41

Geophysical Surveys

Airborne magnetometer, ground scintillometer, and down-holeheat flow surveys ere undertaken in conjunction with thisstudy.

Al rborne MagnetiCs

An airborne magnetometersurvey was conducted over Hahns Peak and the surrounding area in November, 1978 by the AnacondaCompany. The total field magnetic map drawn from data collected atan altitude of 1,400 feet above ground level is given in Figure 6. Hahns Peak is

located on the flankof a magnetic depression that is borderedon threesides byweak magnetic highs. The magnetic high to the northeastis roughly coincident with theTwin Mountain porphyry intrusion. The magnetic high to thesouthwest is over anotherarea of porphyry intrusion, and that to the southeast ofHahrts Peak is 1,250 feet southof theedge of the Anderson Mountain porphyry intrusion. The region of highmagnetic readings east of Hahns Peak consists of upthrust Precambrian basement rocks. Although a reading

taken directlyover Hahns Peak showsa very slight increase over those along the flanks of the magnetic low, theintrusive rocks of Hahns Peak are distinctively less magnetic than surrounding intrusions. This is interpreted tobethe resultof magnetite destruction by hydrothermal fluids. The magnetic signature, therefore, simply verifies that the intrusions at Hahns Peakare more intensely altered than thosein the surrounding area. Although the magnetic data are inconclusive in evaluating the mineral potentialof Hahns Peak, the low magnetic response relative toadjacent areas is 42

LEGNP IOCwfl OOgammas. . orzoAtaCOfltro . . - bosed On ThOtO

VDO gommos SCALE V62,500 Average bird hec3ht.. 400 teet 4000 20002 2000 4000 Feer 20 gacnmos --

.. 5O0 feet 0 gammas. Line spvcng

Figure 6.Total field magnetic map of the Hahns Peakregion. 43

similar to the magnetic signatureover both the Henderson and Mount

Emons porphyry molybdenumdeposits (E. 0. McAlister, pers. comm.,

1981).

Ground Radiometrics

A ground radiometricsurvey using a hand-held Mount Sopris scm-

tillorneter (Model SC-132)was conducted by the author during August,

1978. The results are shown in Table 1and indicate average readings

for a given rock unit. This data shows that the radiation levelsat

Hahns Peak are low.

In August, 1979 the BendixCorporation, under contract with the

Department of Energy, drilledto a depth of 250 feet on the south

flank of Hahns Peak to evaluatethe uranium potential of the Browns

Park Formation. The hole (SWB-27) penetrated 180 feetof Browns Park

Formation and bottomed in MancosShale. Radiation levels were low

(100-120 CPS) with the exceptionof one short zone (40-42 foot depth) that reached300 countsper second (Carter and Wayland, 1981).

The low radiation levelsmeasured at Hahns Peak indicate that little potential exists fora uranium deposit to be present there.

However, the data of Table 1show that slight differences in radiation levels do exist betweenintrusive phases and, more importantly, between different alteration phases. Rocks that display phylhic or mixed phyhhic-argillic alterationconsistently exhibit higher levels of radioactivity. Such effects are believed to have beencaused by an influx of potassium (with a concomitantinflux of radioactive 40K) introduced by hydrothermal fluids. This interpretation is consistent 44

Table 1. Levels of radioactivity in rock units of the Hahns Peak

area.

Rock Uni t Sd nti 11 ometer Readi ng (counts/second)

Beryl Mountain porphyry (propyli tic alteration) 125

Columbine porphyry (argillic alteration) 170

Summit porphyry (phyllic alteration) 225

70 porphyry (mixed phyllic-argillic alteration) 225

7D porphyry (propylitic alteration) 140

Monolithic brecci a (Columbine porphyry, argillic-phyllic alteration) 235

Multilithic breccia 175

Late state prophyry dikes (slight argillic alteration) 185

Volcanic vent complex 140

Base Surge deposit 140

Browns Park roof pendant 140

Browns Park Formation 112

Morrison Formation 170

Dakota Formation 80

Mancos Shale 140 45

with the extremely highcontents of K20 measured in the alteredpor- phyry intrusions of Hahns Peak.

Heat Flow

In conjunction with thepresent study,heat flow measurements

were obtained by E. Decker and K. Buelowof the University of Wyoming

from drill holes DOH-7A (3,173ft.) and DDH-101 (3,571 ft.). The heat flow data consists of down-hole profiles of temperatureversus depth and thermal conductivities measured inthe laboratory from

selected samples of drillcore. The results display a linear thermal

profile for DDH-7A anda similar trend for DDH-101 to a depth of

2,789 ft. (Buelow, 1980). The profile for DDH-101 belowa depth of 2,789 ft. was not linear and was not used in determiningthe cor- rected heat flow value. The fundamental relation for determiningthe heat flow unit value (HFU)is Q = K (where Q = HFU 1 x 10-6 cal/cm2 sec, K = thermal conductivity of the rock, and thermal gradient). The HFU value for DDH-7Awas determined to be 2.48 and the HFU value for the linearportion of DDH-101 was determined to be

2.54 (Buelow, 1980). The steady-state terrain correctionsfor holes

7A and 101 were determinedby Buelow (1980) to be 45 percent and 12.5 percent, respectively. Terrain corrections greater than 20percent are questionable and thus 2.54 HFUis considered to be the best value for heat flux in the HahnsPeak area (Buelow, 1980). This value is considerably higher than theaverage value of 1.5 HFU for the continentalcrust (Royand others, 1968)andis consistent with values measured throughout the Rio Grande Rift(Deckerandothers, 1980;

Decker and Smithson, 1975; see Regional Geophysics). 47

HAHNS PEAK LITHOLOGIES

Precambrian Metamorphic Rocks

Outcrops of Precambrian rock units are not present in theimmed- iatevicinity of HahnsPeak. However, a gneissic granodiorite, undoubtedly of Precambrian age,wasintersectedatthe bottom of drill hole ODH-1O1 (3,477-3,571 ft.). Also, inclusions of gneiss and schist, presumed to be Precambrian in age, have been observedin all

three main phase intrusiveporphyries of Hahns Peak.

Precambrian fragments ofvaried lithologies may also befound

throughout the volcanic vent complex on the westside of the Mountain. These originate inpartfrom explosive transport from depth and in part from fluidization of the roof pendant ofbasal con-

glomerate of the Brown'sPark Formation. Fluidization of the Brown's

Park conglomerate is the result of a diatreme-like breaching of

Multilithic breccia throughthe roof pendant. The roof pendant consists of cobbles and pebbles of Precambrian gneiss, schist,granite, aplite, and quartzite cemented in a calcareous, sandymatrix.

The Precambrian unitintersected at the bottom of DOH-1O1is a gray, fine to medium-grained, equigranular rock of granodioriticcorn- position. It has a faintly developedgneissic texture owing to the foliation and segregation of biotite grains. Petrographic examination of a sample from 3,530 feet in DDH-1O1 shows itto contain a large amount of primary microcline and plagioclase feldsparranging from An30 toAn42 (andesine) in composition. Pyrite is present throughout the granodioritegneiss, and is

usually concentrated along thinfractures in amounts of less thanone percent. Although the rock is relatively unaltered,local sericitiz-

ation of plagioclase and microclineaccompany thin (< one inch) vein-

lets of mineralized intrusivebreccia throughout the interval 3,500-

3,530 feet in DDH-101.

Several weakly slickensided fracturesurfaces are present near

the contact with the overlyingsedimentary units and may owe their

origin to minor movement related to early Tertiary regional thrusting.

Paleozoic Sedimentary Rocks

Paleozoic sedimentary rocks donot crop out within the Hahns

Peak area,and for the purposes of this studythey willbe undif-

ferentiated. However, unitsof the Triassic Popo Agie and Mugget

Sandstone formations, are exposed1.5 miles east of Hahns Peaknear

the contact ofthe thrust fault at the base of FarwellMountain.

Permo-Triassic rocks of the Goose Egg and RedPeak Formations were

intersected at depth in drill holeDDH-101, where they are sandwiched

between apophyses ofa sill of Beryl Mountain porphyry. The base of this unit lies unconformably above the Precambriangneiss, and is fractured and locally granulatedwith someslickensides present.

Displacement appears to have beenminor, and was probably related to early Tertiary thrusting. 49

The Goose Egg and Red Peak Formations havea regional minimum thickness of 285 feet and consist of red calcareoussiltstone and

fi ne-grai ned sandstone. The total thicknessintersected at the bottom of DDH-1O1was 223 feet. These units are predominantlyfinely banded, red to pink calcareous siltstone with interlayered thinbeds of gray brown claystone. Near the base, a pinkishgray, fine-grained sandstone contains traces of disseminated pyrite. A prominent vein of massive pyrite and galena, about two feet thick,cuts pink sandy siltstone two feet from the contact with the sill ofBeryl Mountain Porphyry. The sandy siltstone between the vein and the contactis a lighter shade of pink and displays disseminated cubesof pyrite as

well as thin pyritestringers with bleached selvages.

Mesozoic Sedimentary Rocks

Jurassic Rocks

The oldest rock unit exposed at Hahns PeakistheJurassic Sundance Formation which crops out near the base of theprominent cliff on the north side of the mountain. The Sundance Formationcon- sists of fine to medium-grained, pink to lightgray, locally calcar-

eous sandstone that also containslesser amounts of interbedded shale and siltstone. It is approximately 120feet thick (Segerstroni and Young, 1972). Silty shales that containfurrowed trails of bottom- dwellingorganisms and, occasionally,single pelecypods (possibly Vaugonia conradi according to Segerstrom and Young, 1972) are interlayered with more indurated beds of ripple-marked silty 50

sandstone in the middle portion of the formation. The upper siltstone unit of the Sundance is characterized by havingan abundance of belemnites.

The Morrison Formation, also Jurassic in age, overlies the Sundance Formation. A fine to mediurn-grained, white,cross-bedded, fluviatilesandstone up to75 feet thick marksthe baseof the Morrison. The upper 192 feetof this formation consists of varicolored green, gray, brown, and red shale and claystonewith thin, interbeddedlayers of freshwaterlimestone (Segerstromand Young, 1972). The Morn son Formation i n thi s area i s nan-fossi Ii ferous and,

for the most part, is friable and easily eroded. The outcrop on the northwest flank of Hahns Peak is bleached white andlocally contains finely disseminated crystals of pyrite related to hydrothermalalteration and mineralization.

Cretaceous Rocks

The Dakota Sandstone forms the base of the Cretaceoussection, and is marked by a well-silicified basalpebble conglomerate that

varies from 3 to 20 feetin thickness. This lower unit consists of rounded to subangular fragments of quartzite and chertthat average 0.1 to 0.4 in. in diameter and are cemented withsilica. The conglomerate is overlain bya section of alternating beds of sandstone and black shale,that is in turn capped byan indurated, cross- bedded, fine to mediurn-grained sandstone. The totalthickness of this formation varies from90 to125' feet. Earlier workers have mistakenly referred to theDakota Sandstone as the CloverlyFormation 51

(Barnwell, 1955; Hunter,1955). Tensionalfractures, probably the

result of doming by theoriginal intrusive event,cutthe Dakota

Formation on the north andwest sides of Hahns Peak. These open

fractures are lined with euhedralcrystals of quartz thatare com-

monly hunted by mineralcollectors. Although sulfides have not been observed in association with thisstage ofhydrothermalactivity,

these fillings of quartztestify to the influx of silica-rich hydro-

thermal fluids. The tensional fractures representzones of weakness that have permitted large glide blocks anddebrisslidesof the

Dakota Sandstone tomove downward along the domed flanks of Hahns

Peak. The Dakota Sandstone tends tobe resistant to erosion andcom-

monly forms the uppermostcap-rock to Cliffs of Morrison Formation,

as on the north side of Hahns Peak.

The Mancos Shale was depositedunconformably above the Dakota

Sandstone. It consists of approximately900 feet of light gray to

black, calcareous shale. The shale is oolitic and is predominantly

non-fossiliferous, as deduced fromexposures in a road cut above the

portalof the Royal Flush Mine. However, a fossiliferous zone was

found on the eastern flank ofthe mountain that contains the pele-

cypod Inoceraiiius perplexus(?)as originally identified by Segerstrom

and Young(1972). The presence ofthis fossilsuggests that the

upper portion of the Mancos Shale in thisregion is probably equiv-

alent to the Niobrara Formationthat crops out on the east flanks of

the Front Range. The Mancos Shale may become indurated andslaty near the borders of intrusive rockasa consequeric of hydrothermal additions of silica that migratedfrom the nearby porphyries. 52

Cenozoic Sedimentary Rocks

Browns Park Formation

The Browns Park Formation containsa basalconglomerate that

unconformably overuies Mancos Shale. The conglomerate consistspre-

dominantly of pebbles, cobbles,and boulders of Precambrian gneiss,

amphibolite, quartzite, and schist. These aremoderately well-

cemented in a matrix of carbonateand iron oxide-rich sandy material.

The larger rock fragmentsare usually well-rounded. Sandstone beds

from a few inches to twofeet in thickness may be interlayeredwith the conglomerate. Evidence of fluviatileprocessessuch ascross

bedding,graded bedding,cutand fillstructures, and pebbles are

present throughout, butare more common in the layers of sandstone.

A fine-grained tuffaceoussandstone unit overlies the basalcon-

glomerate of the Browns ParkFormation. This unit has a distinct

reddish brown color andconsists of well-sorted, very fine sand and silt with abundant glass shards. The clay content ishigh, pre-

sumably as a result of thedevitrification of volcanic glass and the

subsequent alteration of feldspar. Where ground waters have been

localized along fractures, thesandstone is yellow in color.

The Browns Park Formationwas drilled at a location one mile due west of Hahns Peak during thesummer of 1979 by 3endix under contract to the UnitedStates Department of Energy.

(SWB-27) penetrated 110 feetof tuffaceous sandstone before passing through 86 feet of basalconglomerate(Carterand Wayland, 1981).

The true stratigraphicthickness of the basal conglomerate, taking 53

the dip of the domed sedimentaryunits into account, is approximately

72 feet at this location.

Thin veinlets ofintrusive breccia up to 1.0 in.in width and

containing fragments of the basal conglomerateset ina fine black

matrix, were observed in outcropsnorthwest of the Royal Flush portal

and in drill core from holeSWB-27.

A pendant of the Browns Park Formation ispreserved within the

intrusive complex on the northwest flankof Hahns Peak. Good expo-

sures of basal conglomerate are present at thislocality along with

poorly exposed traces ofthe overlying tuffaceoussandstone unit.

The true thickness of the exposedbasal conglomerate was determined

to be 73 feet. The complete basal unit is therefore thoughtto be

visible,because ofthe nearly identicalthickness ofthe exposed

beds with those penetrated indrill hole SWB-27.

Cenozoic Volcanic Rocks

Vent Complex

A layered deposit of alternatingtuffaceous andwell-sorted

lapilli-size material is present above the pendantof Browns Park

Formation on the west flank of thepeak. This has been mapped as

Browns Park Formation by all previousworkers, but has been interp-

reted by this author to bea volcanic vent complex related to a dia-

treme-like emplacement of intrusive brecciathrough the Browns Park

pendant. The coarsermaterial consists of fragments of Beryl

Mountain porphyry, Columbine porphyry, Mancos Shale, Morrison 54

Formation, and a mixture of Precambrian lithologies that are

identicalto those contained within thebasal Browns Park conglomerate. The fragments ofporphyry arealwaysthe most abundant, whereas those of Precambrian lithologies have their greatest abundance in the lower-most layers of the vent complex. There are 32

distinct depositional layersexposed in the vent complex. These have been grouped into six major units that are interpreted to bepyro-

clastic surge and air-falldeposits (Fig. 7). The true thickness of

the exposed portion of thecomplex is 140 feet, and in general,there is a gross upward fining with a thick layer of tuffaceousmaterial at the top of the section.

Althoughall of the tuffaceous layersare predominantly fine-

grained, they may be distinguishedby textural variations. Some dis-

play a predominantly fineash (<1/16 mm diarn.) matrix with occasional

lapilli (2 mm to 64 mm diani.)and bomb-size (64 mm to 200mm diam.)

fragments of Beryl Mountainporphyry and Columbine porphyry. Others have a slightly coarser matrix of ash (<2 mn diam.) withmore abundant lapilli-size fragments dispersedthroughout. Within a tuff-

aceous layer, normal graded bedding(fining upward) is common. All

of the tuffaceous layersreact readily with HC1, as will the ground-

mass of the coarser layers. An interruptionindepositionwas

apparent along one horizonnear the middle of layer E (Fig. 7) where mudcracks were exposed inoutcrop.

The coarser, well-sorted beds of predominantly lapilli-size fragments may show eithernormal or reverse grading, with reverse grading the more common. Shapes of the lapilli-size fragmentsare 55

AiR-FALL DEPOSIT:Predominantly air-fall tuft and lopilli tuft with isolated blocks ofporphyry. LopIlistone / at bose displays reverse grading.

PYROCLASTIC SURGE ANDAiR-FALL DEPOSIT:AItnating thin beds of lopiltistone and tuff in lower half. Generallywell sorted. Both - - . . a E reverse and normal grading present.Upper zone is .s. air-fall tuft with isoloted blocks ofporphyryupto '4 six inches in diameter.

PYROCLAST)C SURGE DEPOSIT: Tuffand ...:: D lapiflistone. Some mixed air-fall tuftand lapillitutf. Reverse grading inbasal bed. P!.9

I PYROCLASTIC SURGE DEPOSIT:Lapkstone and o tuff . :...- Reverse and normal eroding present. PYROCLASTIC SURGE DEPOSiT: B luff 20ff - Ri.sy.. 9olftprI$.nf PYROCLASTIC SURGE DEPOSIT:Lopillistone and upper layer of tuft.Individual beds are well ...ea #. ..' sorted. Both reverse and normal gradingpresent. a. .. E a..

Figure 7. Generalized section of volcanic vent complex at (-Iahns Peak. angular to subrounded, and a few are elongate. The maximum

dimensions of the elongate lapilliparallel the bedding. They are

never imbricated. The amount of tuffaceous matrix materialwithin

the coarser layersmay vary, but the ratio of lapilli-size fragments

to tuffaceous matrix is alwayshigh.

Petrographic examination ofa rock sample from a coarse lapilli- rich bed in the middle oflayer E(Fig.7) displays a matrix with feldspar microlitesaligned as in a flow structure. Two of the larger microlites have albitetwinning andtheir compositions are estimated to be An30 (oligoclase-andesine), by the Michel Levy method. Secondary calcite is presentas irregular patches throughout the groundmass and is associatedwith lesser amounts of sericite and cl ay.

Petrographic examination ofa rock sample from the tuffaceous layer F (Fig. 7) indicatesthe presence of an altered clay-rich matrix with lesser amounts of calcite and sericite. Dispersed throughout the matrix are fragments of microlite-rich material similar tothat which is present in the matrixof the sample from layer E.

Whole rock major oxide analysesindicate similar compositions for the tuffaceous and lapilli-richlayers (Table 2). The tuffaceous layer is slightly enriched in Si02(67.3% versus 64.0%), but slightly depleted in GaO (3.0%versus 4.1%) and Na20 (2.8% versus 3.6%). Table 2.Major oxide analyses of principalHahns Peak rock units. Rock Type Sample Fe2Q* 5102 MnO MgO CaO Na20 '°2 2°3 K20 P205 S Total

Beryl Mountain porphyry* BM-1 58.58 0.77 12.59 4.30 0.08 4.48 4.51 Twin Mountain porphyry* 3.81 3.30 0.004 0.03 92.50 TM-I 66.09 0.40 13.75 2.65 0.04 1.26 2.12 4.36 3.54 Little Mountain porphyry LM-1 0.002.0.0j 94.23 69.90 0.10 16.60 2.55 0.06 0.40 0.35 3.00 4.50 0.22 Columbine porphyry CP-1 66.50 0.05 97.73 0.20 16.00 4.00 0.04 1.10 Columbine porphyry 0.55 3.90 5.50 0.27 0.02 CP-2 67.60 0.20 98.08 16.80 1.50 <0.01 0.15 0.10 Summit porphyry 1.00 9.50 0.22 0.14 97.22 SP-.1 69.30 0.10 17.30 1.15 <0.01 0.15 0.15 2.15 6.50 Summit porphyry SP-2 0.19 0.33 97.03 70.00 0.10 16.00 1.45 <0.01 70 porphyry 0.25 0.25 1.45 7.50 0.09 70-I 68.70 0.10 0.11 97.21 17.40 1.65 <0.01 0.20 0.50 Late stage porphyry dike 3.35 4.20 0.30 0.17 96.58 ID-I 67.00 0.30 18.40 1.20 <0.01 0.40 0.45 2.40 5.40 0.36 0.01 96.00 Multilithic Breccia MB-I 70.20 0.30 12.20 3.00 <0.01 0.30 0.15 Multiljthic Breccia 0.20 8.20 0.18 0.56 95.30 145-2 71.40 0.45 14.80 1.00 <0.01 0.40 0.20 0.40 7.40 0.43 Multilithic Breccia MB-3 74.50 0.25 0.06 96.55 12.30 1.15 <0.01 0.25 0.20 0.60 7.90 0.09 0.07 97.32 Volcanic Vent Complex

(lapillistone) VC-1 64.00 0.25 13.10 3.00 0.05 1.20 4.10 Volcanic Vent Comp1e 3.60 3.40 0.28 <0.01 92.99 (fine tuff) VC-2 67.30 0.25 13.30 3.30 0.03 1.50 Browns Park Pendant 3.00 2.80 3.40 0.23 0.01 BP-1 61.60 0.30 14.00 95.12 4.60 0.06 0.90 5.80 3.50 3.20 0.11 0.02 94.10 Base Surge (massive bed) BS-i 73.40 0.20 14.20 0.85 0.02 0.20 Base Surge (plane bed) 1.25 4.30 2.70 0.13 <0.01 BS-2 77.60 0.10 12.70 97.26 1.00 <0.01 0.20 0.10 0.25 5.80 0.05 0.02 97.83 *Anajyses done by Anaconda'sTucson Laboratory.

All other analyses done byBondar-Clegg& Co.,Inc., Vancouver1 B.C.All analyses weredone byX-rayfluorescence spectroscopy. Sample locations plotted onPlate 3,with theFollowing exceptions: 814-1 fro,northern flankof Beryl Mountain; TM-i Twin Mountain summit; 148-3,fromsouthern from flank ofLittleMountain;BS-1,85-2, from Ta iron os Fe203 Porphyry Mountain, see Eig.20.

c-n

1 Pyroclastic Surge

Abedded pyroclasticsurgedeposit on Porphyry Mountain is

approximately 0.75 mi. west ofthe layered vent complex described

above. It is approximately 250 to 300feet thick and contains alter-

nating planar and massive beds. Zones of planar beds, whichmay be

up to six feet in total thickness,are comprised of fine layers of

parallel, silica-rich laminationsthat individually may range in

thickness from 0.1 to 0.6 inches. The rock fragments that comprise

the planar beds range from fineash (<1/16 m) to lapilli size (up to

1 cm), averaging approximately1 mm in diameter. The larger sized

fragments have been identifiedas Beryl Mountain porphyry, Columbine

porphyry, Sumit porphyry,a mixture of Precambrian lithologies common to the Browns Park Formation, Mancos Shale,Dakota Sandstone, and

Morrison Formation shale. The laminated planar beds are locally disrupted by block-sag structures. The angles of incidence of the airborne projectiles that formthese structures, as well as the measured flow directions of the planarbeds, place the surge deposit directly in line with the layered ventcomplex on Hahns Peak. Cross stratification is also present withinplanar beds of the surge deposit,as is both normal and reverse grading. Multiple surge events are indicated by the presence ofa cut and fillstructure on the northeast side of Porphyry Mountain whereearlier laminated beds were scoured by more massive material. Inclusions of laminated planar bedsare contained within the coarser pyroclasticmaterial throughout the scoured zone. Mud cracksarepresentalong some bedding planes, indicative of the high degree ofplasticity that this surge deposit possessed. Step ladder-like veinlets ofsilica probably represent

invasive fillings of mud cracks by multiple surge events. Post-depo-

sitional deformation ofthe planar beds is evidencedby flame struc-

turesand the formation of small-scalenormalfaults. The normal faults were probably caused by the settling of pyroclasticmaterial

along the original depostiona]surface, which was likely to have been a steep-walled gully.

The massive beds displaya mixed assortment of lapilli and bomb-

size fragments commonlyheld in an ash matrix. The ratio of frag- mentsto matrix is usually high and the rock is poorly sortedand nonstratified. Crude reverse gradingis apparent insome layers.

Flame structures anddeflation sags are locallypresent along lower contacts with planar beds. Individual fragments may be subroundedto subarigularand consist of the same lithologiesthat are present within the planar beds. Fragments of porphyry from HahnsPeak are clearly the dominant lithology.

Whole-rock analyses for major oxideconstituents (Table 2)

demonstratethatthe planebed samplecontainsmore S102 (77.6% versus 73.4%) and K20 (5.8% versus 2.7%) and less GaO (0.1%versus 1.25%) and Na20(0.25% versus 4.3%) thanthe massive bedsample. Each represents a channel sample of rock chips collectedacross both bed form types.

Li thi c fragments wi thi n the base surge deposi texhi bi t strong sericitization and argillization,but they contain only trace amounts of sulfide minerals. Cenozoic Intrusive Rocks

The descriptions of intrusiverocks that follow are presented in

the chronological order oftheir emplacement. Some age relationships

are clear, whereas othersare inferred and are so noted. Many of the units described here were first named by Dowsett (1973). Three of his original units have been renamed and seven new units havebeen

introduced. These changes will be noted whereit is appropriate.

Every intrusive rock withinthe Hahns Peak complex hasbeen

hydrothermally altered, althoughthe intensity and style of alteration varies. For this reason, the descriptionsthat follow may in part be based on pseudomorphic textures and relationships. Wherever

possible, feldspar compositionswere determined by maximum extinction

angles using the Michel Levymethod. Despite the overall Intensity

of hydrothermal alteration,many samples contained sufficiently fresh

phenocrysts of plagioclase feldsparto provide reliable compositional

data.

Rock classification isdifficultat Hahns Peak because of the

effects of alteration andbecause the rocks are porphyritic andhave

a groundmass that is too fine-grainedto permit reliable modal analyses. For reasons that willbe explained in the subsequent dis-

cussion of hydrothermal alteration,it is assumed that K was added to non-brecciated porphyritic rocksand that, for the most part, Siwas not. Thus, it would be meaningless to consider K20 contents inthe classification scheme used forHahns Peak porphyries. The classification used in this study is therefore based predominantlyon S102 61

contents and with reference to average compositions of igneous rocks

determined by Nockolds (1954).

Beryl Mountain Poprhyry

Beryl Mountain porphyry is probably the oldest intrusive phase

present at Hahns Peak. This unit, first named by Dowsett(1973),

derives its name from Beryl Mountain, located one mile southeast of

Hahns Peak, where it is exposed. Clear contact relationships exposed

in outcropseastandnortheast of Beryl Mountain demonstrate that

7D porphyrycuts Beryl Mountain porphyry. Contact relationships

exposed elsewhere indicatethatthe 7D porphyryis younger than

Summit porphyry which in turn is younger than Columbine porphyry. A

large inclusion(500 ft.x 600 ft.) of Beryl Mountain porphyry is

found on the southern border of the Summit porphyry intrusion, and

smallerhand-sized inclusionshave beenidentifiedlocally within

Summit porphyry. Direct contact relationships have not been observed

between Beryl Mountain porphyry and either Columbine or Little

Mountain porphyri es.

The Beryl Mountain porphyry is a dark green to black latite por-

phyry with phenocrysts of plagioclase, biotite, quartz, sanidine, and trace amounts of hornblende, apatite, and sphene. Phenocrysts make

up about 25 to 40 percent of the rock, and the groundmass is aphanitic. Phenocrysts of quartz are commonly round, but they may also be square and are usually 1.3 to 5.1 nii in diameter. The remaining phenocrysts are subhedralto euhedreal. Phenocrysts of biotite are fine-grained and those of the feldspars are fine to medium-grained. 62

The relative abundances of the various phenocryst minerals are approximately 40 to 50 percent plagioclase feldspar, 10 to 15 percent sanidine, 15 to 20 percent biotite, and 15 to 20 percent quartz.

The plagioclase composition was determined to be An38 (andes- me), with some normalzoning present. The Si02 content (58.58%) given in Table 2 is the lowest of any of the igneous rocks at Hahns

Peak. The low S102 content of Beryl Mountain porphyry is consistent with the interpretation that this unit is the first to have evolved from a differentiated magma. The Si02 content places this porphyritic phase within the latite (monzonite) range as defined by

Nockolds (1954).

Exposures of Beryl Mountain porphyry are moderately altered by hydrothermal processes to propylitic assemblages. Sericite, calcite, albite, quartz, epidote, and minor clay minerals replace phenocrysts of feldspar, whereas biotiteandhornblendehavebeen alteredto chlorite and magnetite with traces of hematite.

Pyrite, ranging from trace amounts of approximately one percent byvolume, is disseminatedthroughoutthegroundmass. The large inclusion of Beryl Mountain porphyry exposed on the southern flank of

Hahns Peak displays moderate argillic alteration and traces of weak propylitic alteration. Sills of Beryl Mountain porphyry encountered at depth in drill holes are both argillically and phyllically altered.

A small adit on the northwest side of Beryl Mountain shows the contact between porphyry and Mancos Shale to be both sill-like and discordant. TheBeryl Mountainporphyry appearstohave locally 63

domed the surrounding Mancos Shale, which suggests thatit represents

asmall laccolith peripheralto the larger compositelaccolith of Hahns Peak. Dowsett (1973) had reported thatan induced polarization

survey made by Canadian Aero, Inc.from 1966 to 1968 indicated that the intrusions located on Anderson Mountain(1.2 mi.southeast of Hahns Peak) and Little Mountain (1 mile southwest of HahnsPeak) are shallow sills. Textural similarities suchas the shapes and sizes of phenocrysts, and the low phenocryst to groundmass ratiosuggest that they are closely related.

Data from an airbornemagnetometer survey associated with this study (Fig. 6) indicate that the Twin Mountain intrusionmay be plug- like in shape. Geologic mapping by Segerstromand Young (1973) suggests that at least two episodes of intrusive activityare present at

Twin Mountain. An early intrusive phaseis cut by the King Solomon

fault, whereas a laterphase cuts the fault. Although a direct association cannot be made with either of these intrusive phases,it is assumed, again because of textural similarities, that BerylMountain porphyry is also closely related tothe porphyritic intrusions of Twin Mountain. In addition, Columbineporphyry from I-tahns Peak also

exhibits textural features (lowratio of phenocrysts to an aphanitic

groundmass)similar to the Beryl Mountain porphyry and may be slightly younger withrespect to its age of emplacement.

The interpretation of theBeryl Mountain porphyryas being the oldest intrusive phase at Hahns Peak is in completedisagreement with the conclusion of Dowsett(1973, 1980), who believesit to be the 64

youngest. However, my interpretation is clearlysubstantiated by the field evidence.

Little Mountain Porphyrj

Little Mountain porphyry derivesits name from Little Mountain,

located one mile southwest ofHahrs Peak. As described above, the

porphyritic intrusion thereappears to be a shallow sill. The main

exposure of this intrusive phase is centeredon Little Mountain, but

a small portion of it crops out withinthe thesis area on the south-

ern border of the Hahns Peak intrusivecomplex.

Little Mountain porphyry isa light-colored greenish gray quartz

latite that contains phenocrystsof plagiociase feldspar, sanidine, quartz, and biotite. Minute (<1.0 mm diam.) crystals ofapatite, zircon, and sphene are present locally within the groundmass. Pheno-

crysts of quartz are similar tothose found within Beryl Mountain,

Anderson Mountain,and Twin Mountain porphyriesand may be either

round or square, with diametersup to 5.1 mm. The phenocrysts of

quartz present in Little Mountainporphyry may be distinguished from

thoseofthe otherporphyriesby their strongly resorbed nature.

Phenocrysts other than thoseofquartzare commonly subhedral to euhedral. Phertocrysts of biotite are fine-grainedand those of feld-

spar are fine to medium-grained. The relative abundances of thevar-

ious phenocryst mineralsare approximately 35 to 40 percent plagioclase feldspar, 30 to 35percent sanidine, 10 to 15 percent quartz, and 5 to10 percent biotite. The groundmass comprises less than 65

50 percent of the rock and ismorecrowdedthanthat of Beryl

Mountai Ti porphyry.

The composition of plagioclasewas determined to be An30 (oligo-

clase-andesine). The Si02 content is69.9 percent(Table 2)and places this porphyritic phase within the quartz latite(adamellite) range of Nockolds (1954).

LittleMountain porphyrytypicallydisplays weak propylitic alteration. Sericite, clay, silica, andcalcite replace portions of

many feldspar phenocrysts whereaschorite and traces of magnetite replace some biotites. Pyrite is disseminated throughoutthe groundmass in trace amounts.

Dowsett (1973, 1980) didnot differentiate Little Mountainpor- phyry from 7D porphyry. However, petrographic examinationsinvolved with this study indicate that these are very distinct andseparate units. Although no contacts withother porphyritic intrusionswere observed, the Little Mountainporphyry phase appears to have been

truncated by a northeast-trendingnormal fault that preceded emplacement of both the Summit and 7D porphyry units(Plate 1). Little Mountain porphyry may represent a differentiated phase ofthe early intrusive events that formed sills in the Hahns Peakarea. I was probably intruded shortly after emplacement of Beryl Mountainpor- phyry.

Columbine Porphyry

The Columbine porphyry phase as mapped in this studycorresponds closely to the Price Tunnel porphyry unit previouslydelineated by Dowsett (1973, 1980). Itwas renamed because the most prominent

lithology thatis present near the portaland on the dump of the

Price Tunnelis Multilithic breccia. This occurrence of intrusive breccia was not recognized by Dowsett (1973, 1980). Columbine porphyry was named for the beautiful Colorado state flowerthat may be seen near some outcrops of this unit.

Columbineporphyry is a light to medium colored purple-gray

quartz latite porphyry (Fig. 8). It is the oldest of the threemain intrusive phases on Hahns Peak and occupies the northand northwest portions of the complex. A narrow intrusion breccialocated adjacent to the cone sheet intrusive breccia on the ridge due northof the summit house (Plate 1) clearly shows inclusions of Columbineporphyry in a matrix of Summit porphyry and establishesthe relative age of the two phases. Columbine porphyry has not beenobserved in contact with 7D porphyry.

Columbine porphyry containsphenocrysts of plagioclase,sanidine, quartz, biotite, and hornblende with local traces of zirconand apatite. Phenocrysts of feldsparare subhedraland medium grained

and rarely exceed 10.0m in length. Quartz phenocrysts are usually rounded with average diameters of 1.3 to 2.5 mm, butmay occur up to 7.6 mm in diameter. Phenocrysts of biotiteoccur as elongate laths that average 2.5 mm in length. Phenocrysts of hornblendeare subhedral to euhedral with an average length of 1.3 mm. The relative abundances of the various phenocryst minerals are approximately40 to

45 percent plagioclase feldspar,30 to 35 percent sanidine, 10 to 15 percent biotite, and traceto3 percent hornblende. The groundmass 67

Figure 8. Main phase intrusive porphyries at Hahris Peak. 7D-1O =strongly altered 7D porphyry; 7D-7 = weakly altered 7D porphyry;BP-6 = Columbine porphyry (massive variety); SP-8 = Surrriit porphyry. comprises 65 to 75percent of the rock andis distinctively aphan-

itic. Outcrops of Columbine porphyry locatedon the northeast border

of the Hahns Peak intrusivecomplex display a groundmass thatcon-

sists of fine (0.05 to 0.1mm diam.) crystals of quartz and feldspar

that are equigranular and interlocking. This is interpreted to bea

textural variation that resultedfrom a somewhat slower cooling rate

in that portion of theintrusion.

The major portion of the Columbineporphyry phase exhibits well

developed flow banding thatis evidenced by the preferred orientation

of elongate biotite lathswithin diffuse bands of silica. In the

process of weatheringthis rock tends to easily part along the

banding planes, and resultsin talus that consists of thinlyplated

rock debris. The cause of this banding is believedto be induced stress during magma emplacement, withoverlyingsedimentary units

having acted as stressguides. The banding diminishesaway from the

sedimentary contacts, andresultsin more massive texturesnear the

center of the intrusion (Fig.9).

Plagioclase compositionswere determined to be An30 (oligoclase-

andesine), withsome normal zoning present. The Sf02 content varies

from 66.5 percent inthe massive rock to 67.6 percent inthe flow-

banded rock (Table 2). These compositions place the rockwithin the quartz latite (adamellite)range of Nockolds (1954).

Hydrothermal alteration variesfrom weak to intense within this intrusive phase andconsistsof propylitic,argillic, andphyllic mineral assemblages. Weak propylitic alteration is confinedto the extreme northeastportion ofthe intrusion. Here, feldspars are Figure 9. Photomicrograph of Columbine porphyry, massive variety. Note alteration rims aruund biotite and hornblende. Cross polars (X1bO). 70

replaced by sericite, quartz,clay, calcite, and traces of epidote.

Albite rims were observed around some phenocrysts of sanidine. Biotite and hornblende may be partially replaced by chlorite andmag-

netite. Pyrite is disseminated throughout thegroundmass from trace amounts up to five percent.

The major portion of Columbineporphyry is weakly to moderately

argillized, with varyingamounts of clay and sericite that replace

phenocrysts of feldspar. Locally, sericite is well-developed in

zones that display moderate to intensephyllic alteration. Pheno-

crysts of biotite are replacedby sericite. The morestrongly

argillized or sericitized rockis light gray to white in color,in contrast to the distinctive purplehue of less altered Columbinepor- phyry.

Oowsett(1973,1980) interprets the unit that henamed Price

Tunnel porphyry (for themost part equivalent to Columbine porphyry) to be the youngest of the threemain phase intrusions of the Hahns Peak complex. However, field evidence obtained in thisstudy suggests that this conclusion isincorrect. Further, as it is believed that Columbine porphyry andSummit porphyry are differentiates from an essentially common parentmagma, the higher Si02 content of Summit porphyry issupportive of the hypothesisthat itis younger than Columbine porphyry.

Summit Porphyry

The Summit porphyry intrusivephase was first named by Dowsett

(1973). Itis so named because it is theporphyry that outcrops on 71

the summit of Hahns Peak. Exposed contact relationshipsplace it

between Columbine porphyryand 7D porphyry inage.

Summit porphyry isa chalky white to lightgray colored quartz latite that consists of phenocrystsof plagioclase feldspar, sanidine,quartz, and biotite with tracesof hornblende,apatite, and zircon (Fig. 8). Phenocrysts of feldsparare subhedral to euhedral and are medium to coarse-grained. Some sanidines may locallyreach lengths of 4.0 cm. Phenocrysts of quartzare generally rounded with an average diameter of2.5 to 5.0 m and arecommonly strongly resorbed. Phenocrysts of biotiteare subhedral to euhedral, fine to medium-grained, and are commonly seen as elongate lathswith a preferred orientation. The relative abundances ofvarious phenocryst minerals are approximately 40 to 45 percent plagioclasefeldspar, 35 to 40 percent sanidine, 10 to 20 percent quartz, and5 to 10 percent biotite. Phenocrysts comprise approximately50 percent of the rock and the groundmass is distinctively crowded withmany small (0.5 mm) roundedto subhedral crystals of quartz andfeldspar (Fig. 10). Primary flow banding is locally welldeveloped,as evidenced by a strong alignment of biotite laths and well developed bandsof silica. The bands of silica consist of thin(1.3 to 15.0 cm thick),conti nuous streams of si ii ca along wi th di sconti nuous, yet parallel, pods of silica. The orientation ofthe bands may be uniform for distances of a few hundred feet and are best exposedunderground in the the 70 and Southern Cross adits (Fig. 11, Fig.12, Fig. 13). Silica flow banding exposed in the 70 adit dipsapproximately 40°S. and is parallel to the bedding plane orientation of sedimentaryunits on the 72

Figure 10. Photomicrograph of SummitPorphyry. Note biotite alignment and crowded matrix. Cross polars (X150).

Figure 11. Silica banding inSummit Porphyry exposed in the 7P adit.

J., p, i+...I,

SsmviU /

Mt$tcar,0

..4.4 Mcnoithuc 8recco ut..p1 NI 70 Pophyry

tt Sumvut Prp,yry

MoIOlitnC8rscct ///(V 0 100 200

__-i7" I?ththc Scals I :1200 11s4 / \ 70 Porphyry

Figure 13. Geology of the Southern Cross Adit. 7,5

southeast flank of the intrusivecomplex. Near the center of the

intrusion the orientations ofbands of silica are nearly vertical.

The silica flowbanding exposed within the Souther Cross adit

maintains a fairly steep and generallynorthward dip. Silica banding

is best developednear the outer boundaries of Summit porphyry and is

believed to be the result ofinduced stress during magma emplacement,

as wassuggestedforthe flowbandingobserved withinColumbine

porphyry. Border zones of Summit porphyry alsoshow a finer texture

thanrocks nearthe center ofthe intrusion which display larger

phenocrysts of sanidine.

Many of the smaller phenocrystsof feldspar and quartz appear to

be fragmented andmay be aligned with laths of biotite thatare bent

and broken. This is interpretedasan indication that Summit por-

phyry was more forcefully injectedthan either Columbine porphyryor 7D porphyry.

The compositionof plagioclase feldsparwas determined to be

An30 (oligoclase-andesine). Refractive index measurements by Young

and Segerstrom (1973)indicate thatthe apatite present in Summit porphyry is actually fluorapatite. The 5102 content varies from 69.3 to 70.0 percent (Table 2), thehighest values of the three main phase intrusions. These compositionsplace therock within the quartz latite (adamellite)range of Nockolds (1954).

Summit porphyry is the most intenselyaltered of the Hahns Peak intrusive phases. The extreme southwest portion ofthe intrusion displays weak propylitizationwhereasthe remainder of theintrusion displays theeffectsof moderateto strong argillizationand 76

sericitization. The central portion of the intrusion displays well

developed phyllic alteration. Sericite,silica, and clay in this

area replace phenocrysts of feldspar and phenocrysts of biotite are

replaced by sericite alone. Pyrite content throughout the Summit

porphyry phase is consistently one to two percent. The effects of

hydrothermalalteration, together with those of weathering, combine

to give the rock its chalky-white, porous appearance.

Summit porphyry is interpreted by Dowsett (1973, 1980) to be the

oldest of the three main phase intrusions. However, field evidence

described here indicates that this hypothesis is incorrect.

70 Porphyry

This unit was first named by Dowsett (1973) for its occurrence

at the portal of the 70 adit. Where it is in contact with the fine-

grained variety of Summit porphyry inside the portal(Fig.12), it

forms a thin zoneof intrusion breccia that contains fragments of

Summit porphyry in an igneous matrix. The intrusion breccia cross-

cuts the well-developed silica bandingof the Summit porphyry and

establishes the younger age of the 70 porphyry.

This intrusive phase is a light grayto buff-colored quartz

latite porphyry (Fig. 8) that crops out along the southest flank of

Hahns Peak. The outcrop of 70 porphyry visible in the center of the

Summit porphyry phase is interpreted to be an arm that projects from

the main intrusion (Plates1 and 2). The 70 porphyry unit is com-

posed of phenocrysts of plàgioclase, sanidine, quartz, and biotite, with traces of apatite, hornblende, and zircon. Its most distinctive 77

feature is the great abundance and size ofphenocrysts of sanidine. These are typically euhedral and may reachup to 7.5 cm in length. They are conTnonly doubly terminated andmay show Carlsbad and Baveno twins. Phenocrysts of quartz are usually rounded withaverage diameters of 2.5 to 5.0 rwn. Phenocrysts of biotiteare fine to medium- grained and are euhedral to subhedral. The relative abundancesof phenocryst minerals are approximately 40 to 45percent plagioclase feldspar, 35 to 45 percent sanidine, 5 to 10percent quartz, and 5 to 15 percent biotite. The groundmass comprisesabout 50 percent of the rock and is less crowded and slightly more aphaniticthan that of the Summit porphyry. Flow banding andprotoclastic texturesare definitely less pronounced than in the Summit porphyryphase. However, flow-foliated biotite and intrusion brecciawere observed locally in drill core samples of fine-grained, dark-coloredchill border zones of 70 porphyry. It is difficult todistinguish 70 porphyry from the coarser variety of Summit porphyry in hand specimen. The contact where the two meet near the center of the HahnsPeak complex is extremely gradationaL This is probablya result of intrusion of 70 porphyry into the still warm and only partially solidifiedcenter of Summit porphyry.

Compositions of plagioclase feldspar were determined tobe An27 (oligoclase). As with the Summit porphyry phase, the apatitepresent in 70 porphyry is actually fluorapatite (Young andSegerstrom, 1973). The Si02 content of 70 porphyry is 68.7 percent(Table 2),which would place the rock within the quartz latite(adamellite) range of Nockolds (1954). 78

The outer portions of the 70 porphyry unit display weakpropy- litic alterationwith plagioclasefeldspar replacedby sericite,

clay, calcite, albite,and traces of epidote. Biotites may be par-

tially replaced by chloriteand cnagnetite. Pyrite may occur in trace

amounts. Near the center of the HahnsPeak complex the alteration grades into argillic and mixed argillic-phyllic styles. Grus may be

locally developed fromweathered 70 porphyry,allowing the doubly

terminated euhedral sanidinecrystals to be completely exposed.

Monolithic Breccia

Thisis the earliest phase ofintrusive breccia at Hahns Peak and consists of angular to subrounded fragments of hostrock in a light gray matrix of silica with lesser clay and sericite (Fig.14). The rock fragments that comprise the breccia are almost alwaysmono- lithologic with the exception of one locality where the brecciaseems

to havebeen intruded along thecontact of Summitporphyry and

Columbine porphyry (Plate 1). Here, fragments of both lithologies

are present in a silica matrix. Outcrops of Monolithic brecciaoccur

discontinuously inportions of both Summit porphyryand Columbine porphyry. Underground, in the 70 adit, thisbreccia phase was

observed to cut 70porphyry as well as Summit porphyry. where Monolithic breccia cuts Columbine porphyry, the silica matrix corn- prises' 25 percent of the rock and displaysa subtle pink hue. In contrast, Monolithic breccia thatcuts 70 porphyry and Summitpor- phyry in the southeast portion of the complex hasa light gray matrix that comprises 30 to 40percent of the rock. In either case, the Figure 14. Intrusive breccias: Br-21 monolithic, BR-26 multilithic,Br-23 fluidized rnultilithicwith multiple phases evident.

Fure 15. PhotomicroQraph of Mno1ithicbreccia with silica coating laterfractures. Cross polars (X150). is

matrix isaphanitic and consistspredominantly ofholocrystalline silica with very fineclayand some sericite sparsely dispersed throughout. Finely disseminated pyritemay occur locally inthe breccia matrix, comprising up to two volume percent of the rock.The

fragments which compriseMonolithic brecciamay be classified into two groups according to size. The coarser fragments typicallyrange

from 7.5 to 25.5mm in diameter, but may beup to 7.5 cm in diameter. The finer diameter sizes vary from 0.8 mm to 2.5 mm. Some of the

smaller-sized fragmentsmay appear rnegascopically to have crystalline

shapes, thus implyingan igneous origin. However, under microscopic examination,they are seentobe fragmentalpiecesof hostrock. Small(<0.5 to 5.0 mm diam.) open fractures with liningsof silica

occur locally in Monolithic breccia. The presence of these openings suggest a post-depositional episode offracturing anda subsequent influx of silica (Fig. 15).

Exposures of Monolithicbreccia in the 70 aditoccur in zones that may reach over100 feet in width. These zonesare composed of thin (1.3 cm to 1.0 m wide) veinlets of breccia thatmay display branching outlines. The veinlets of brecciabecome thinner and may

be discontinuous towardthe surface. Monolithic breccia was probably

formed by isolated magmaticdischarges due to boiling, it is the oldest of the intrusive phases that comprise the brecciacone sheet.

Aplite Dikes

Light gray to white, fine-graineddikes of aplite occur locally at Hahns Peak. These occurrencesare limited to two surface outcrops 81

(northeast portion of Summit porphyry and southeast portion of

Columbine porphyry,Plate 1), oneunderground exposure in the 70 adit, and seven samples of diamond drill core from DOH-102and DDH- 7A. Due to their limited exposure, relative age relationshipsare difficult to obtain. The aplite dike that cuts Summitporphyry on the surface is locally discontinuous and wispy, suggestingthat the intrusion occurred while the quartz latite wasstillplastic. In contrast, a narrow (<2.5 cm) dike of altered aplite cross-cuts

Monolithic breccia ina sample taken from talus on the west edge of

the sumit ridge. Although relative agesare clearly established in this example, the presence of both altered and unaltered aplitessug- gests that there is more than one generation of these dikes. The main episode of aplitic intrusions, however, is interpretedto have occurred after the emplacement of Monolithic breccia, but priorto the emplacement ofMultilithicbreccia.

Textures vary from fine-grainedto extremely fine-grained and they may be porphyritic. The relative abundances of phenocrystmin- erals ina sample from 1,035 ft.in DDH-102 are approxiamtely 15to

20 percent plagioclase feldspar,40 to 45 percent potassium feldspar,

30 to 35 percent quartz, 2to 3 percent biotite, and 1 to 2 percent apatite. The feldspars are subhedralto euhedral. Crystals of plagioclase are commonly from1.3 to2.5 mm inlength. Crystals of quartz are subhedral to roundedwith average diameters approximately

1.3 mm. The potassium feldsparwas determinedto be microcline. Albi te occurs as microperthite intergrowths withmicrocline. The composition of plagioclase feldspar was determined to be An35 82

(andesine). Myrmekite is present locally wheremicrocline is in con-

tact with plagioclasefeldspar.

Dikes of aplite range in width from one inch to twofeet, and

may display flow banding alongtheir borders. Some of these dikes

may be strongly altered to clayand sericite. However, the dikelet indrill hole DDH-102 is only slightly altered. Pyrite may be present in trace amounts and up to one to two percent. The presence of these dikes on Hahns Peak, although minor, hasnot been reported by any previous authors.

Multi lithic Breccia

Multilithic breccia isa distinctive unit that contains fragments of multiple lithologies held together in a darkgray matrix (Fig. 14). This unit was observedto cross-cut Monolithic breccia at one surface location on thewest edge of the cone sheet,and in the Southern Cross adit 230 feet from the portal. Multilithic breccia, together with Monolithic breccia, forms a continuous ringaround the mountain. This structureappears to be roughly conical in shape and is the result of multiple events of hydrothermal brecciation. It is analogous to an igneouscone sheet. An igneous component, however, has not been observed.

The texture of Multilithic breccia varies from extremelycoarse to extremely fine. The finer-textured brecciais highly fluidized and displays well-rounded, milled fragments of lapillisize suspended in a matrix of rock flour. The ratio of matrix toclasts varies greatly, but averages 3:1. Thestrongly fluidized zonesoccur 83

locally in theupper portions of the breccia cone sheet and ata few

isolated locations outside thisstructure. Exposures of fluidized

breccia adjacent to the volcanicvent complex display such textural

features as flow lamination, sizesorting of suspended particles and

locally crudely developed sizegrading. Multiple intrusive events

within the Multilithic breccia unitare suggested by cross-cutting

phases of breccia (Fig. 14). The rock-flour matrix ofthe fine-

grained variety of Multilithicbreccia consists principally ofcorn- minuted Hahns Peak porphyry andMancos Shale. The matrix is highly

altered and its present mineralogy consists of70 percent quartz,

20 percent sericite, and 10 percent clay, with local traces of alunite. Lithologies that have been recognizedto occur as lapilli- sized fragments are,in decreasing order ofabundance: Hahns Peak porphyries (Beryl Mountain porphyry, Columbine porphyry, Summit porphyry, and ID porphyry), Mancos Shale,Morrison Formation, Dakota

Sandstone, and various Precambrianrock types. These fragments are typically altered to quartz, sericite, and clay.

Coarse-grained Multilithic breccia is more abundantthanthe fine-grained phase. It consists ofassortedlapilli andisolated bomb-size (<6.0 cm diam.) fragments in a dark gray to black tuffaceous matrix (Fig. 16). The shapes ofthe fragments range from angular to rounded. The lithologies present are identicalto those described above for the fine-grainedphase, with Hahns Peak porphyry and Mancos Shale comprising 90 percent of the rock fragments.

Portions of the brecciacone sheet along the eastern margin of the intrusive complex,near the contact with Mancos Shale, display an Figure 16.Pnotomicrograih of' coarse Multilithic breccia.Cross polars (X150). extremely high contribution of black shale. Locally, the rock is

entirely black. Narrow veinlets (1,3 to 2.5cm wide) of fluidized shale and rounded fragments of porphyry emanate from thebreccia cone sheet in this area. The coarse-grained phase ofMultilithic breccia. displays locally strong hydrothermal atleration,withboth lithic fragments and matrix alteredto sericite, silica, clay,and minor alunite. Pyrite, sphalerite, and galena maybe presentin the matrix, and in late open-space fillings, however, most of theexpo-

sures of Multi lithic brecciaare barren of sulfide mineralization.

Multilithic breccia has been the most poorly understood lithology at Hahns Peak. Some earlier workers correctlyidentified this unit as part of a breccia pipe, but did not elaborateon its full extent or origin (Bowesand others, 1968; Bowes,1969). Oowsett

(1973,1980) failed to reporta major portion of its exposure, and that which he did report was interpretedas fault breccia along a discontinuous northeast-trendingnormal fault. Segerstrom and Young (1972), Young and Segerstrom (1973), and Segerstrom and Kirby(1969) interpreted the Multilithicbrecciaunit tobe a circular faUlt formed by a caldera-like down-dropped block along whichhydrothermal brecciation was localized. The relationshipofthe breccia cone sheet to the vent complex and the base surge depositas wellas to the hydrothermal alteration and mineralization at HahnsPeak will be discussed further in chapters that consider Volcanic Activityand

Hydrothermal Mi neral i zation. Late-Stage Porphyry Dikes

A series oflate-stage porphyry dikesintrude Summit porphyry and 70 porphyry on the surface,in drill core,and in the 7D adit (Plate 1, Fig. 12). These dikes vary in width fromtwo to ten feet and tend to occur along, or near, the contacts of 70 porphyrywith

Summit porphyry and withMancos Shale. They are dark gray in color and of quartz latite composition, and have many texturalsimilarities to 70 porphyry. Phenocrysts include plagioclasefeldspar, sanidine,

quartz, biotite, andhornblende. Phenocrysts of feldspar are medium to coarse-grained and subhedral to euhedral. Phenocrysts of sanidine near the center of the dikes may reach up to1.3 cm in length.

Pheriocrystsof biotite are fine tomedium-grained and subhedral to euhedral. Phenocrysts of quartzare commonly rounded, averaging 2.5 to 5.0 mm in diameter. The relative abundances ofphenocryst minerals are approximately 25 to 30 percent plagioclase feldspar,35 to 40 percent sanidine, 10 to 15 percent quartz, 15 to 20percent biotite, and 1 to 2 percent hornblende. The grouridmass comprises 50 to

60 percent of the rockand is aphanitic. Flow banding, in the form of biotite foliation, is strongly developed along thefine-grained margins of the dikes.

The composition of plagioclase feldsparwas determined tobe An28 (oligoclase). Some sanidines display oscillatoryzoning and are rimmed with oligoclase. The Si02 content ofone of these dikes is 67 percent (Table 2), whichplaces the rock within thequartz latite (adamellite) range of Nockolds (1954). The hO2 content is 0.5per- cent and is the highest value of the rocks analysed forthis study. [;pA

Although these dikes are slightly more mafic than 70 porphyry, their major oxidecontents and general texture suggest that theyare closely related to this main phase intrusion. It is possible that a more mafic component was added to the parent magma after emplacement of 70 porphyry.

Late-stage porphyry dikes display markedly less intense alteration than any of the three main phase intrusions. The prominent northwest-trending dike that cuts the central portion of the Summit porphyry intrusion(Plate 1) displays slight to moderate argillic alteration,with plagioclase feldspar partly replaced by clay and lesser sericite and quartz. Phenocrysts of sanidine that are present within this dike display minor clay alteration along borders that are rimmed by oligoclase. Phenocrysts of biotite appear to be nearly fresh. The dike was mapped as 70 porphyry by Dowsett (1973, 1980).

However, its relatively fresh appearance suggests that it was emplaced after the more severely altered 70 porphyry. Late-stage porphyry dikes appear to have been intruded near the end of, and perhaps after, the main phase of hydrothermal alteration. 88

STRUCTURAL GEOLOGY

The most significant structural activity at Hahns Peakoccurred

in late Tertiarytime and will therefore be emphasized in this chapter. The pre-Tertiarystructural historyof the HahnsPeak region has been briefly reviewed in the chapter entitledRegional Geologic Setting.

Hahns Peak islocated in the center ofan east-trending horst that is bounded on the north by the King Solomon faultand on the

south by the Grouse Mountainfault (Fig. 3). This extensional fea-

turepostdatesthe regional thrust faults of Laramideage (late Cretaceous to early Tertiary), and displays two periods ofmovement

along its northern boundary. The first took place in early to mid- Tertiary time, while the second period of movement occurred inlate Terti ary time after deposi ti on of the Browns Park Formation. Both verti cal and hon zontal di spi acements have been deternil ned by Segerstrom and Young (1972) to be approximately 600 feet duringthe second stage of activity. In this later stage, intermittentintru- sions of porphyry within thehorst block were accompanied bycon- tinued displacement alongthe bounding faults. The horst block is probably aresult of upwelled magmaalong an east-west regional trend. Hahns Peakand the Elkhead Mountainsare evidence of the hypabyssal and extrusive volcanicrocks associated with this niagmatic activity. The structural dome and normalfaults related to igneous intrusions in the eastern portion of the horst appear to becentered on Hahns Peak. Evolution of Structural Domeand Normal Faults

The evolution of thestructural dome at Hahns Peakwas initiated with the early intrusion of Beryl Mountain porphyry intohorizontal sedimentary units. Further development of the domewas probably pro-

duced by vertical, piston-likesurges of deep-seated magma located directly beneath the peak. Extensional forces thus imposedupon the

sedimentary units in the formation of this dome resulted in the

development of steeply dipping (80-85°)normal faults. Two of these

pre-main intrusion normal faults (with approximate strikes of

N. 500W. and N. 30-60°E.)appear to intersect at nearly right angles

beneath the center of the intrusive complex (Fig. 3 and Plate 1). A third normal fault strikesapproximately N. 600W. andskirts the northern boundary of the intrusive complex. It also predates the main phase porphyry intrusions, but appears to have offsetthe N. 30-60°E. striking normal fault.

The normal fault thatstrikes N.500W. andpasses beneath the center ofthe intrusive complex (Fig.3 and Plate 1) has not been described by previous workers at Hahns Peak. Nevertheless, I was able to trace slickensided and brecciated outcrops of the oldersedi- rneritary units along the faulttrace for distances of over two miles in either direction from the edge of the porphyry contact. Bedding plane offsets werealso observed locallyalong this trendin the

Browns Park Formation, as well as in the Dakota Sandstone to the northwest and in the Mancos Shale to the southeast. An elongate arm of porphyry southeast ofthe peakis aligned with this structure and go

was apparently injected along it(Fig. 3). This fault also passes

directly beneaththe most strongly mineralized zone on Hahns Peak

(1,400-1,600 ft. into the 70 adit) and through the mineralizedzones

of the Master Key Mine, two miles northwest of the peak, and the Blue

Jay Mine,1.1 miles southeast of the peak. Therefore,this fault

appearstohavehad somecontrol on bothigneousintrusions and

subsequent hydrothermal mineralization.

The normal fault that strikes N. 30-60° E., and which is par-

tially hidden beneath the intrusive complex, has been mapped bypre-

vious workers (Segerstrom and Young,1972; Dowsett, 1973,1980) as

two discontinuous normal faults that postdate the main phase intru-

sions. However, the absence of fault breccia, slickensides, and bed-

rock offsets along the same trend within the intrusive complex sug-

gests that there waslittle or no displacement. The data suggest

that the fault segments abutting the intrusive complex to the

northeast and southwest are portions of one continuous normal fault

that predatesthe main phase intrusions. Theapparentpointof

intersection of the two normal faults near the center of the Hahns

Peak intrusive complex is directly beneath a zone of intense mineralization that is transected by the 70 adit (approx. 1,500 ft.

from the portal). The fault intersection isalso directly beneath

both the apparent geometric center of the breccia cone sheet, and the

exposed arm of 70 porphyry cutting Summit porphyry (Plate 1). Thus,

the area of intersection of the two normal faults is interpreted to

have been a conduit for both magma and later hydrothermal fluids. 1I

Thenormal faultthat strikes N. 600 W.,near thenorthern boundary of the intrusive complex (Plate1), appears to have controlled both the emplacement of igneous intrusions andlater hydrothermal fluids. The presence of this fault ismarked locally by out-

crops of sl-ickensided and brecciatedDakota Sandstone and Morrison

Formation. In addition, severalprospect pits are aligned roughly along its strike.

Drill core intercepts ofMorrison Formation and Permo-Triassic

sedimentary units near the bottoms of DDH-7A and DDH-101, imply a cal dera-1 ike down-droppi ng of these uni ts wi thi n the central portion of the dome. These intercepts of sedimentaryunits are interpreted to be two separate blocks with similar orientations,as determined from bedding attitudes relative to drill coreaxes (Plate 2). None- theless, it is possible that these blocks are fallen roof pendantsof domed sedimentary units engulfed by the porphyry intrusion

(Segerstrom and Young,1972). However,a more random orientation

would be expected forsuch fallen pendants. The preferred interpre-

tation is that the sedimentaryblocks were originally part of the central portion of the structural dome that collapsed priorto intru-

sion of the main phaseporphyries. This interpretation is based, in

part, on the results ofdown-hole directionalsurveys that accurately

defined the orientations ofthe drillholes displayed on Plate 2. The circular fault on Hahns Peak, alongwhichdownward movement

is postulated, isnot exposed onthe surface. However, a fault was intercepted within the sedimentaryunits at the bottom of hole

IS-31 (Plate 1), beneath the outeredge of Columbineporphyry. 92

Interpretation ofthisstructuralfeature suggeststhat the main

phase porphyrieswere intrudedafter caldera subsidence and that their intrusion has masked the surface expression of theinferred circular fault. The cause of the collapse ofthe central portion of the dome is believed to be related to a series ofrnagmatic events. As the parent magma ofthe main phase porphyriesrose to shallow crustal depths, doming of the older sedimentary unitsprobably continued and was likely to have been accompanied by displacementalong

the normal faults previouslydescribed. Withdrawal of magma, perhaps

as a resultof lateralinjections of porphyry such as at Little Mountain, Anderson Mountain, and Twin Mountain, effectivelyremoved

support from beneath thetop of the dome. This sudden removal of support may have caused the central portion of the dometo collapse and form a small (less than4,000 ft.in diameter) caldera-like

structure. The main phase porphyrieswere probably then injected

somewhat in the manner ofa resurgent caldera,and caused further doming and local entrainmentofblocks ofthe sedimentary units.

Continued downward displacementof some of the down-dropped blocks was presumably accomplished by the weight of the overlyingmagma. Injection of the main phase porphyriesinto the sedimentary units appears to have been both sill-like (laccolithic) and discordant. At

some point in its evolution, theporphyritic intrusions caused the

structural dome at Hahns Peakto become asymmetric, with the sedimentary units ofits northern perimeterhaving been elevated more than those on the southernperimeter. 93

The collapse of the centralpart of the structural dome is interpreted by me to have occurred prior to emplacement ofthe main

phase porphyries. Segerstrom and Young (1972) and Young and

Segerstrom(1973) have also suggestedthat caldera-like subsidence occurred at Hahns Peak. However, they interpret this subsidenceto have occurred after emplacement of the main phase porphyries andsug-

gest that the ci rcu)ar pattern of brecci aon Hahns Peak marks the

faulted perimeter of a collapsedblock. I find thishypothesis untenable, because the contact relationships that I have observed

i ndi cate that the breccia i s enti rely i ntrusi ye in nature.

Reverse Faults

The three parallelreverse faults on the northwest edge of the intrusive complex were first mapped by Segerstrom and Young(1972). They were not recognized by Dowsett (1973, 1980). Evidence for reverse displacement has been establishedfrom bedding plane offsets of Morrison shale and DakotaSandstone on the surface and underground in the Royal Flush mine. I was unable to map in the Royal Flushmine because it is presently caved and unpassable. Structural interpretations for this part of the area are, therefore, based inpart on mine maps provided by W.A. Bowes.

Thereverse faults strike approximately N. 300 W. and dip steeply 75-85° NE. They are limited in lateralextent and do not conform with the regional thrust faults of Laramideage(Fig.3).

Therefore, they are believedto be very local in extent and directly related to the intrusions of porphyry. The reverse fault farthestto the northeast contains a small dike-like body of Columbineporphyry that has been intruded along it. Similarly, the middlereverse fault has served as a control for the emplacement ofa thin apoph,ysis of

Multilithic breccia that is probably related tothebreccia cone sheet. Seyerstrom and Young (1972)mapped these faults as cutting the porphyry complex. However, evidence forthe continuationof

these faults into the Columbine porphyry was not observed. Therefore, they are assumed to either predate or to besynchronous

with emplacement of thisunit.

The area containing these reverse faults was probably a bulgeon the northwest flank of the asniimetric dome at Hahns Peak. This bulge

is believed to have been caused by the sill-like injection of

Columbine porphyry beneaththisarea. The northwest trend of the

reverse faults roughly parallels thetrend of the normal fault that strikes N. 500 W. The interpretation is thatColumbine porphyry was intruded upward along this normal fault and injected laterallyin a

southwestdirection, in laccolithic fashion,into the sedimentary units. This upward and outwardthrustofthe intruding porphyry caused segmented blocks of the overlying sedimentary unitsto be

uplifted along the observedreverse faults. As intrusion continued, these faults acted as local conduits to porphyry emplacement. At the same time, some Columbineporphyry was also injected toward the northeast with emplacement controlled in part by the normalfault

bordering the northeast edge ofthe complex. However,for some unknown reason, reverse faults were not developed in thisarea. Elsewhere,other investigatorshave notedthat minorreverse

faults commonly parallel thecontacts between igneous and sedimentary

rocks along the flanks of laccoliths(Irving, 1899; Eckel, 1949; Hunt

and others, 1953; Mackin, 1954). At Granite Mountain, Utah, discon-

tinuous reverse faults both parallel thecontacts between igneous and

sedimentary rocks and strike obliquelyto them (Mackin, 1954). These

oblique reverse faultsare very similar to the ones present at Hahns

Peak.

Joi nts

Determinations of the dip and stike of500 joints within the

Hahns Peak area document thepresence of several discrete joint sets.

These data are summarized in Figure17. This figure is a contoured

diagram of the poles-to-plane projectionof the joints on the lower hemisphere of a Schmidt equal-area net. This illustration is actually a composite diagram of fourdifferent plots that differentiated the joints according to rocktype and location. Although not illus- trated herein, the jointswere plotted on four separate equal area nets defined by location withinquadrants of the intrusive complex.

The trends deduced from thesemore detailed plots will be described where they are pertinent to thisdiscussion.

Ingeneral, themajori ty of the joi nts are steeplydipping (>7Q0) Attitudes of the more distinctive jointsets are as follows:

(a) N. 60° W.and. verttcal;(b) N. 30° W.and verticalto 80° NE.;

Cc) N. 50° E. andnearly vertical; (d)N. 80° W.and verticalto N

6';

0-04%

06-f 1%

.2-28%

2.9-4.1% pet /. area bose< on 500 joint sets

Figure 17. Contoured distribution of poles-to-plane equalarea projection for 500 joints from the igneous intrusive complexand the surrounding sedimentary units at HahnsPeak(see text for explanation and discussion. 97 750 NE.; and (e) nearlyhorizontal. Moreover, a large number of the fractures belong to a possibly separate group that strike in all

azimuthal directions (Fig. 17),but which collectively dip 600to800

toward the center of theintrusive complex.

The joint set that strikes N. 60° W. is presentprimarily in the sedimentary units within the northern half of thearea. This joint set is interpreted to have formed synchronously withthe northwest-. trending normal faults. A similar trend is weaklydeveloped in the Summit and Columbine porphyries and may be relatedto readjustments along older faults after emplacement of the main phaseintrusions. The joint set that strikes N. 30° W. is probablyrelated to the

northwest-trendingreverse fault. Joints that comprise this set have been observed only in outcropsof Morrison FormationandDakota Sandstone within the northwest quadrant of the Hahns Peakarea. The absence of these joints in the Columbine porphyry phasesuggests that

the reverse faults predateemplacement of Columbine porphyry.

The joint set that strikesN.500E.is largely present in the

flanking sedimentary units,and in Beryl Mountain porphyry. It is also weakly developed in the Summit porphyry phase whichmay indicate

latermovement along this fault; perhaps as a result of post- intrusion settling.

The joint set that strikes N. 80° W. is stronglydeveloped in the Summit porphyry phase of the southwest and southeastquadrants of the intrusive complex, as well as underground in the 7D andSouthern Cross adits. It is believed to berelated to the development of the breccia cone sheet. The nearly flat-lying jointset was observed in both the igneous

and sedimentary rocks atHahns Peak. Its presence in the sedimentary

rocks may be due to horizontaltension fractures that resulted from

magma withdrawal after formation of thestructural dome and the sub-

sequent collapse ofthe centralblock. However,the presence of these joints in both the sedimentary and igneous rocks suggeststhat they may have formed after intrusion of the breccia cone sheetwhen

fluid pressure at the topof the cupola had diminished.

The large number of fractures that strike in all azimuthal directions and dip steeply toward the center of the intrusivecomplex

may have formed together with thebreccia cone sheet as described in the section that follows. When plotted in plan view, thesejoints show a concentric pattern centered on Hahns Peak (Plate 1). They are best developed within the intrusive complex, but are also presentin the flanking sedimentaryunits. Although the majority of the joints dip inward toward Hahns Peak,some dip steeply outward; particularly those near the outer boundaryof the intrusive complex.

Structural Evolution of the BrecciaCone Sheet

The high density of steep,inward dipping joints concentrically encircling Hahns Peak is believedto be a significant factor in the formation of the brecciacone sheet. 1nderson (1937) has explained the formation of igneouscone sheetsas being related to intense, vertically oriented magmaticpressures that emanate from the apex of a risingbody of magma. With the magma body actingas a force directed from a point source, tension fractures willdevelop along

di recti ons of pri nci pal stress that are perpendicularto the api cal upper border of the magma body (Fig. 18). The resultant joints

formed in the overlyingrock would taketheshapeof concentric cones, hich have very steeply dipping sides directly above theapex of the magma body and less steep sides outward fromit. Magma that was subsequently injected along these conical joints would resultin a thin cone-shaped intrusive body having a circular surfaceexpres- sion. At Hahns Peak, the termbreccia cone sheet is used ina sense analogous to that of an igneous cone sheet. However, the cone sheet at Hahns Peak is composed entirely of hydrothermally derivedintru- sive breccia without an igneous component. After emplacement of the rnai n phase porphyri es, conti nued magrnati cacti vi ty at depth presum-

ably led to the riseofa cupola composed of highly differentiated

magma. This cupola would contain magmatically derived H20, H2S,CO2, Si02, and dissolved metals. Thus, it might become thesource of a magmatic vapor plume which could then migrate vertically upwardalong the same fault intersections utilizedby the porphyry intrusions. Pressures exerted upon the overlying rock units by therising vapor plume might generate a conical pattern of joints (Fig. 18)to provide channels forthe escape of volatiles. Thesequential explosive release of volatiles from the vapor plume might resultin the formation of multiple intrusivebrecciasalong established fractures. Thus, the final product would be a composite brecciastructure having a roughly conical shape and witha curvilinear surface expression. Similarly, Nrton and Cathles (1973) have suggested that thegeometry / 2\ \iX.'\' 777\/\/ \ / / ti

/ / / / / \ \ \ / / .\\ / \\\\\ / / /1'

I \ / Al

/ N \_ / / / magma border SheOr trocture teton frciure tens,o frc1jrt (defectve P ) (detecnve P ) (oces P forms ring de forms conesl,eets

Figure 18. Formation ofjoints in the evolution of cone sheets and ring dikes (adapted from Anderson, 1937). 101

of a breccia pipe at San Pedro de Cachiyuyo, Chile,was established by the rise of a magmatic vapor plume that createda zone of conical and steeply di,pping sheet fractures in the overlyingrock units.

Emplacement of the breciasat Hahns Peak appears to have been predominantly controlled by the inner, more steeplydipping, conical fractures. However, the isolatedoccurrences of Multilithic breccia

outside thecone sheet structuresuggests that theouter, less steeply dipping tension fractures (Fig. 18)also controlledsome breccia formation. Presumably the forceful andvery fluidized char- acter of the emplacement of the breccias at Hahns Peak causedthem to form narrow, flame-like branches away from the main tensionfracture

as depicted in the geologiccross section (Plate 2). Extending the analogy to Anderson's(1937) theoretical stress field, those joints at Hahns Peak that dip steeplyaway from the cen-

ter of the intrusive complex,as well as those that are nearly horizontal, may be explained by a decrease in verticallyexerted pressures caused by a reduction in volume of the vapor plume. In the model provided by Anderson (1937), both tension and shearfractures develop as a result of reducedpressures caused bya decrease in

magma volume asa consequence of withdrawalor partial crystallization. Tension fractures thatmay parallel the magma chamberare formed when the magmatic pressure is lower than the lithostaticpres- sure. If the drop in magmaticpressure is sufficiently large, shear fractures are formedat approximately 25°to the tension fractures (Fig. 18). Should these shear fracturespenetrate the magma chamber, igneous material mightthen be injected and thus formring dikes. 102

The nearly horizontal jointspresent on Hahns Peak may be analogous to the tension fractures, whereas those joints that dipsteeply away from the center of the intrusive complex may be analogousto the shear fractures (Fig. 18). Because the shear fracturesare not

associated with breccias, theyare inferred to have formed after the brecciation event (i.e. after significant decrease invapor plume

volume). Some of the nearly horizontaljoints in the 7D adit display

thin fillings of fluidizedMultilithic breccia,and may have been

formed after the earlierMonolithic breccia event.

Thus, the joint patternsobserved at Hahns Peakare consistent with those theorized by Anderson (1937) to control thedevelopment of

cone sheets and it is suggested thatthe breccia cone sheetmay have developed in a manner analogous to that of an igneouscone sheet.

Minor Late Fractures

Minor late-stage fractures are evidenced by the presence of a discontinuous northwest-trending fracture zone in the northernexpo-

sures of the Summit porphyry (Plate 1). This fracture zone parallels the normal fault that strikes N. 600 W. along thenorthern boundary of the intrusive complex. An outcrop of Columbine porphyrynear the

east end of this normal faultalso displays a discontinuous fracture

zone that coincides with thepre-intrusion fault. In both cases, these fracture zones display multiple, close-spaced, parallel fractures thatare morestrongly argillized thanthe surrounding rock. It is difficult to determinewhether ornot structural 103 movement occurred along thesefractures. However, to the extent that it did, displacement would have beensmall. These fracturesare interpreted to have been adjustmentsalong, or parallel to, pre- existing faults. They may have been causedby settling of the down- dropped sedimentary blocks due to theweight of the overlying porphyry intrusions. In addition, the weaklydeveloped joint set in the Summit porphyry unit that tendsto parallelthe N.30-60° E. normal fault is believed to have been caused bystresses induced by continued settling of thedown-dropped blocks. 104

VOLCANIC ACTIVITY AND INTRUSIVEBRECCIATION

Extrusive acti vi ty at HahrisPeakappearsto have been minor. The quartz latite porphyry complex isinterpreted asa composite,

near-surface laccolith, althoughthe centralportion of the Summit

porphyry phase probablybreached the surface as a volcanic dome.

Pyroclastic flows and associatedejecta are represented bya layered

vent complex onthe west flankofthe peak,and bya base surge deposit one mile west of the vent. These deposits are related toa

hydrothermal brecci a thatbreached the surface as a steam-bi asterup-

tion (Casaceli and King,1980, 1983).

Intrusion of Laccolith

The Hahns Peak volcaniccomplex was first describedas a lacco- lithby Gale (1906). Segerstrom and Young(1972) recognized the early sills (Tbmp and Tlmp, Plate 1), but interpreted the mainpor-

phyritic intrusions tocomprise a wide, vertically orientedstock.

However, deep diamond drillinghas shown that the Hahns Peak porphy-

ritic intrusionsnarrow with depth. Therefore, the intrusive complex is best-described asa composite laccolith. Itissimilar to the

laccoliths of the HenryMountains, Utah (Gilbert, 1880), inthat it was built up by series of separateintrusions, and that it displays

compositional banding that variesfrom distinct to faint.

The evolution of theHahns Peakvolcaniccomplex was similar to that of a calderathat exhibitedresurgentepisodesof magmatic 105

activity. The initial intrusions of Hahns Peak apparentlyformed a structuraldome, the central part of whichsubsequently collapsed. This caldera-like subsidence is presumed to have been theresult of rapid withdrawal of magma from the upper level of themagma chamber. However, unlike typical calderas, the geologic recordsuggests that

ignimbrites werenot associated with thisevent. Therefore,the removal of magma is interpreted to have been causedby the depletion of the reservoir due to the intrusive emplacementof nearby plutons,

rather than to extrusiveejection.

The main phase porphyritic intrusions were emplaced during a resurgent episode. Magma was presumably injectedupward along the line of intersection of two normal faults directlybeneath the col-

lapsed dome. The intrusionswere confined to a narrow conduit until they reached near-surface levels, where they expandedlaterally along fractures and bedding planes in the sedimentary units. These resurgent injections of magma caused the upper sedimentaryunits of the

down-dropped central blockto be domed upward,and resulted in the entrainment of pendants of Mancos Shale and Browns ParkFormation.

Rocks in the borderzones of the Hahns Peak intrusive complex

are compositionally banded subparallel to the bedding planesof the adjacent sedimentary units. Thesesedimentary units,whichwere domed by the porphyritic intrusions, apparently servedas barriers that guided the emplacement of magma and controlledthe development of primary flow banding in the injectedmagma. However, central portions of both the Columbine porphyry and the Summit porphyrydisplay either faint vertical bands orlack them altogether. These inner 106

zones are texturally similar toportions of the Santiaquito volcanic

dome, Guatemala (Rose, 1972). Therefore, it is possible thatthe

central portion of theHahns Peaklaccolith actually breached the

surface as an extrusive volcanicdome. Nevertheless, the porphyries are primarily intrusive, and their presentexposures have resulted

from the erosion of thedomed sedimentary roof rocks.

Intrusive Brecci ati on

The Hahns Peak intrusivebreccias were formed from hydrothermal

explosions that were triggered by an interaction of ground waterand super-heated magmatic fluids. They display a variety oftextures. Multipleintrusions areinferred fromcrosscuttingrelationships. Field and laboratory data suggest that these breccias controlthe distribution of sulfide mineralization and hydrothermalalteration. Therefore, an understanding of their origin is essentialto an evaluation of the mineral potential at Hahns Peak. Some earlier workers

have interpreted segmentsof these breccias to havebeen formed by tectonic fragmentation alongnormal faults (Segerstrornand Young, 1972; Oowsett, 1973, 1980). However,evidence gathered from the present study i ndi cates that the ri ng of brecci a si tuatedwi thi n the

)-Iahns Peak complex (Plate 1)is intrusive in origin. Its circular shape is interpreted to have been controlled by conicalfractures that channeled the brecciafluids.

The earliest hydrothermalbreccia is known to have been emplaced prior to the intrusion of Summit porphyry,because inclusionsof 107

mineralized breccia were found within the centralportion of this

porphyritic phase. The inclusionsvary from one inch to two feet in diameter and display rock fragments veined bysilica stockworks and

embeddedin a fine clastic matrix ofsilica andminorsericite (Fig. 19). Traces of pyriteare present in both the matrix and the breccia fragments. Early hydrothermal mineralizationis also evidenced by inclusions of an older porphyry phase found withinthe same area. These inclusions ofporphyry display a stockworkof silica veinlets that contain pyrite, molybdenite,and secondary potassium feldspar.

Large-scale hydrothermal brecciation, as indicated by thepresence of the breccia cone sheet, followed the emplacementof porphyries of the main phase. Contact relationships indicatethat the cone sheet is composed of at least two lithologicallydistinct units of intrusive breccia. The oldest of these hasbeen named Monolithic breccia. This breccia unit is characterized by predominantlyangular fragments of host rock contained within a matrix composedof silica and variable amounts of pyrite, sericite, andcomminuted host rock (Fig. 14). Commonly,the rock fragmentsappear to have undergone only minor amounts of rotation and displacement. However, the rotation and the abrasion of fragments are apparentlocally in the upper portions of the Monolithic breccia where well-roundedfragments are contained in a matrixof clay. These zones have been called "pebble dikes" by some workers (Bowes, 1969; Dowsett, 1973)and may have been formed by the boiling of hydrothermal fluidsduring a rapid release of pressure. The Hahns Peak "pebbledikes" are similar to 108

Fiqure 19. Inclusion from Summit porphyry showing breccia fragments with stockwork veins of silica held in a silica- rich clastic matrix. 109

those originally described fromthe Tintic District, Utah (Farmin, 1934), and they may indicate zones where surficialgaseous discharge

occurred (Gilmour, 1977). Nevertheless, the evidence of venting of

the Monolithic breccia islimited,and it is considered to be, for

the most part, a non-explosiveevent.

The hydrothermal effects ofthis phase of brecciation wereper-

vasive argillic and phyllicalteration of wall rocksand variable

sulfide mineralization. Thezones ofhigher sulfide content are

restricted to the "pebble dikes."

The Monolithic breccia isinterpreted to be related to the rise

of a cupola of highly differentiatedmagma. Vapors and liquids asso-

ciated with this cupola presumablyascended along the normal fault

intersection which also controlledthe emplacement of the main phase

porphyritic intrusions. These inagmatically derived fluidswere under

high pressure and probablycontained water, carbon dioxide, hydrogen

sulfide, silica, alkali halides, and metals in solution (White,

1957). The main phase plutons actedas a cap rock to these fluids.

When the pressure at thetop of the cupola exceeded the confining pressure, conical fractures formed in a manner similar to that as suggested by Anderson (1937) forigneous cone sheets. Therefore, the pattern of conical fractures isinterpreted to have been established before fluid injections initiatedthe formation ofintrusive breccias. The enlargement and extensionof these fractures may have been accomplished by hydraulicfracturing as the volatile-richfluids forced their way upward (Henleyand Thornley, 1979). When the fractures had opened to near-surfacelevels, rapid ascent was possible 110

and sudden local releases of pressure caused the fluidsto flash into the steam phase. The metal-bearing fluids wouldhave mixed with ground water, and their subsequent dilution and thermalcooling in response to this mixing may have been the cause of local depositi on of sulfides. The strong sulfide mineralizationwithin the zones of "pebble dikes" suggests that the metalswere transported there primarily in the vapor phase, with only minor dilutionwith ground water (Henley and McNabb, 1978; Henley and Thornley, 1979). Although the

"pebble dikes" may havebeen formed by agaseous fluid, the high content of silica inthe matrix of Monolithicbreccia outside these

zones is suggestive of formationby a viscous liquid (White, 1981). Such a liquid would have been likely to forma breccia which exhibits minor rotation and displacement of its fragments. The precipitation of silica from the breccia-forming fluidsmay have been initiated by the interaction of these fluids with the colder groundwater. This

would have sealed offfractures and broughtan end to the brecciation event,unless there was a later renewal of magrnatic hydrothermal

acti vi ty.

Field relationships suggest that dikes of aplitewere intruded after Monolithic breccia. They may representa siliceous residual fluid that was derived from the magrnatic cupola. A similar asso-

ciation of aplitic dikes andsiliceous breccias hasbeen observed elsewhere including the Andes Mountains of Chile and Peru(Kents, 1964), and at Copper Basin in Arizona (Johnston andLowell, 1961). The emplacement of Monolithic breccia served both to createnew fractures and to fill some that were formed by the initialascent of 111

the magmatic cupola. This breccia is not widespread,It was apparently intruded only into portions of the zone of concentricfrac- tures. The open fractures that remained probably servedas a reservoir for ground water. Continued niagmatic activity atdepth caused additional volatile-rich magmatic fluids to emanate fromthe cupola and to rise along the same fault intersection that controlledearlier hydrothermal events. These fluids were underextremely high pressure. Provided their salinity anddissolved metal contentwere sufficiently high, they would have been capable of sustainingmagmatic temperatures for considerable distances out into the countryrock (Whiteand others, 1971). If such super-heated rnagmaticfluids encountered a ground water reservoir, a phreatic explosion wouldhave occurred as the ground water flashed into steam. The zone of reduced

pressurebehind the explodedcolumn of groundwaterwould have allowedthe super-heated fluids to alsoflash, and to mixwith phreatic steam. Such an event of violenthydrothermal activitymay have formed the Multi lithic breccia phase andgoverned itsdistri- bution as a nearly completeconical sheet.

The Multilithic breccia is characterized by angularto rounded fragments of porphyritic, sedimentary, and granitic rock heldin a matrix of comminuted rock flour, silica, and sericite (Fig.14). The comminution is interpreted to be the result of theautogenous hydrothermal milling of rock fragments suspended in a fluid (gas or liquid, or a mixture of both). The texture of this brecciavaries from coarse-grained (withangular to sub-rounded fragments)in the lower and main parts of the cone sheet tofine-grained (with sub- 112

rounded to well-rounded fragments) in theupper part. Comminution of

the rock fragments is greatest in thefine-grained upper zone where

multilithic pebble brecciasappear. These pebblebrecciasdiffer

from the Monolithic pebble dikes"in that they display a greater

variety of lithic fragments and a lower content of clay in the

matrix. The heterogeneous assemblage of rock fragmentscontained

withinthe Multilithicbreccia is probably due to the explosive

mixing of lithologiesas the breccia-forming fluids moved violently

upwards.

Multilithic breccia is best-exposed and best-developed on the

west side of Hahns Peak, particularly whereit appears along the con-

tact between Columbine porphyry and Summit porphyry. At this loca-

tion the breccia was injected alongfractures that followed the con-

tact of the two intrusive units;a phenomenon that has been described

in other districts (Johnston and Lowell,1961; Sillitoe and Sawkins,

1971). The presence of a layered pyroclasticsequence adjacent to a

zone of pebble breccia on the west flank of the peakserves as pos-

sible evidence that the brecciacone sheet vented at that point. The

layered vent complex displaysan abundance of angular to sub-rounded lapilli-sized fragments of Precambriangraniticcobblesfromthe

Browns Park Formation thatare intermixed with fragments of all other lithologies present in the Multilithicbreccia. A roof pendant of

Browns Park Formationcrops out imediately down-hill from the vent complex. It appears that a focused, gas-rich dischargeof intrusive breccia erupted (diatreme-like) throughthis pendant,and incorporated material from it. The initial flow of gas through the pendant 113

would have caused expanded-bed fluidizationoftheclasticlayers (Reynolds, 195; McCallum and others, 1976). However, whenthe effects of gas streaming and particulate flowwere great enough, the sedimentary beds would have been disrupted by explosiveconiminution (Wolfe, 1980). Multilithic intrusive brecciasthen would have been extruded as pyroclastic material in a steam-blasteruption. The layered vent complex and a base surge deposit one mile to thewest are interpreted to have been the products of this discharge(Casaceli and

King, 1980, 1983). The well-defined beddingof the vent complex1 and the presence of scour and fillstructures within the surge deposit are suggestive of multiple extrusions. Multiple phasesof cross- cutting multilithic pebble breccias, adjacent to the ventcomplex, are also suggestive of a series of high energy events. These pebble

breccias display subtleflow textures suchas weakly developed flow laminations and partial size-sorting of fragnents. These textures furtherindicate that thefluidswhichformed the brecciashad vented. The presence of rounded-pebble fragments,whichresulted from a high degree of abrasion, has previously beendiscussed as evidence of surface discharge (Farmin, 1937; Gilmour, 1977). The Hahns Peakpebble breccias arerestricted tothe upper portionof the brecci a cone sheet because the pressure gradient was hi ghest at that point. Laboratory studies of fluiddischarge from pipes have demonstrated that 80 percent of the pressure drop is inthe last 10 percent of the pipe (McBirney, 1963). Therefore, interactions and subsequent abrasion of fragments would have been greatestnear the point of discharge. 114

The presence of siliceous sinter, secondary orthoclase, andman-

ganosiderite localized alongthe inner wall of the brecciacone sheet

near the proposed ventarea, are evidence that fumarolic activity

accompanied the pyroclasticextrusions. Small pods of opaline sinter that contain tetrahedrite and jarosite are located adjacentto zones of multilithic pebble breccia that contain aluniteand sericite. A similar mineral assemblage is present in rocks borderingactive hot springs at Steamboat Springs, Nevada (White and others, 1964). The rocks within the vent complex at Hahns Peak, and adjacentto it, are anomalous in their manganese content, andmangariosiderite locally replaces plagioclase feldspar within quartz latite porphyry. Sec-

ondary orthoclase isalso presentas euhedral crystals that range from 1.0 to 5.0 niii in diameter withinvugs of the porphyry. Man-

ganese, potassium,and carbon dioxideare transported in the vapor phase at active volcanic fumaroles (White and Waring, 1963),and may have been similarly transported at Fiahns Peak as issuggested by

associated hydrothermalminerals.

The portion of the brecciacone sheet that is located approximately 1,100 feet south of the vent complex, displaysa multilithic breccia composed ofparallel-tabular fragments. The fragments consist of altered Columbine porphyry that are suspended ina matrix of silica, sericite, and finelyfragmented rock of multiple lithologies.

Similar textures have been described in rocks fromseveralbreccia pipes in the Andes Mountains (Sillitoeand Sawkins,1971; Kents, 1964). Kents (1964) referred to theserocks as"burst breccias" and interpreted them to be related to steam-blast eruptions. The 115

hydrothermal fluids that ascended along fractures throughzones of successively lower pressure would,by hisinterpretation,reach a critical stage and flash into steam. Such an explosion would applya lateral stress to the fracturewalls,and the stressed rock would burst inward toward the zone of lower pressure upon the rapidupward discharge of steam. The spalled, tabularfragments thus produced

would be entrained by hydrothermal fluids as theycollectively ascended along the conduit. Such a mechanism providesa reasonable explanation for the parallel tabular breccias at Hahns Peak,and it is consistent with the formational environment of thebreccia cone sheet as proposed herein.

A dike-like body of Multilithic breccia, approximatelyten feet wide, was injected along a reverse fault that cuts Morrisonshale on the northwest edge of the intrusive complex (Plate1). This occurrence is located outside the breccia cone sheet, but it istexturally similar tothe parallel-tabular breccia previously described. In outcrop,this breccia displays a centralzone of coarse,tabular fragments of altered porphyry that areverticallyaligned. The coarse-grained central zone grades laterally into fine-grained margins that may contain fallen blocks of basal Dakotaconglomerate. A similar lateral variation in grain size has been observedfor phenocrysts in igneous dikes and has been interpreted tobe the result of flowage differentiation (Bhattacharjiand Smith, 1964). Komar (1972) has determined that sorting of thistype is a function of both the velocity profile across a dike (fast in center,slow on sides) and the tendency, through mechanical interaction,forthe larger 116

grains to move toward the zone of least shear (centerof dike). He refers to this phenomenon as "grain dispersive pressure." The flow differentiation features that are displayed by this outcrop of breccia suggest that the flow ratewas uniformly high. The breccia is interpreted to be the result ofa steam-blast explosion. The spalled fragments were formed in the manner ofa "burst breccia," and were entrained by a rapidly moving hydrothermal fluid thatvented.

The proximity of thisbreccia to the basesurge deposit sug-gests that it may have also been a source of pyroclasticmaterial. Similar deposits that exhibit characteristicsof flow-differentiation have been postulated to represent gas-rich, pyroclasticvents,and have been referred to in the geologic literatureas tuffisites, agglom- erates, and intrusive ignimbrites (Cloos, 1941; King,1953; Reynolds, 1954).

The Multilithic phasehas been establishedas the most violently emplaced breccia unit on the basis of the followingcharacteristics: (a) the heterogeneous assemblage of lithic fragments; (b)the related pyroclastics that have similar lithic heterogeneities; (c) the most abundant and best-developed pebble breccias;(d) the related fumarolic precipitates; and Ce) the presence of "burstbreccias." Simi- larly, Richard and Courtright (1958) distinguishedan explosive phase of multilithic breccia from a lessviolent monolithic breccia at Toquepala, Peru. They interpreted the multilithicbreccia to be the result of a gas-richhydrothermal explosion.

The explosions that formed the breccias at Hahns Peakare also thought to have been gas-rich. This interpretation is basedon the 117

abundance of pebble breccia and thepresence of a related basesurge deposit. Base surges have been observed to originate fromvapor- rich, phreatoragmatic eruptions. However, the basesurge event at Hahns Peak is believed to have been the result ofa phreatic, rather than a phreatomagmatic, eruption (Williams and McBirney, 1979), because glass shards are lacking in both the intrusivebreccias and the pyroclastic deposits. The absence of glass shardssuggests that ground water was not in contact withmagma. Rather, ground water is interpreted to have reacted with a rnagmatical.ly derived,super-heated hydrothermal fluid.

McBirney (1963) has emphasized the role of groundwater in the formation of breccia pipes and has suggested thatthe rapid escape of magmatic volatiles alone, is unlikely to result inviolent gaseous explosions. However, magmatic gases introduced into geothermal systems may exert a controlling effect on theoccurrence of phreatic explosions. For example, the intensity of the hydrothermalexplo- sionsat the Broadlands, New Zealand, geothermal fieldis directly proportional to the concentration of carbondioxide (presumably magma-derived) in the discharging fluids, and thisdissolved gas may act as a triggering mechanism (Henley and Thornley, 1979). The presence of calcite in the matrix of the near-ventpyroclastic rocks and pebble breccias, and that of manganosideritereplacing feldspars in the hanginywall porphyry adjacent to thevent area, indicate that carbon dioxide was contained in thegaseous discharges at Hahns Peak, and thus could have been a factor in theapparently explosive eruptions that occurred atthis volcano. 118

The most significant stage of sulfide mineralizationat Hahris

Peak accompanied, and closely followed, the emplacement of the breccia cone sheet. The presence of traceamounts of disseminated pyrite in fragments of altered porphyry thatcomprise the intrusive

breccias and pyroclastic deposits,suggest that pervasivesulfide mineralization had not occurred prior to the mainperiod of brecciation. Zones of intense sulfidemineralization are present locally within the matrix of some of the breccias,and also occuras vug fillings within both units of breccia and thealtered porphyry. The sulfide mineralization was probably related to the finalstages of the upward migration of magmatic fluids. Oeposition of silicaas fine, acicular incrustatioris on sulfide mineralsand as vug fillings in and around the breccia cone sheet tookplace at the conclusion of the multilithic stage of hydrothermalbrecciation. Barrington and Kerr(1961) suggested thatthefine, acicular crystals ofquartz associated with the breccias at Cameron, Arizona,were precipitated from a magmatic vapor. It is possible that thisphenomenon occurred at Hahns Peak, as well. Silica isknown to be highly soluble in

super-heatedsteam,where it may betransported as silicic acid (Morey and Hesselgesser, 1951). However, regardless of whether it was precipitated from a liquid or gaseous phase,the lateintro- duction of silica probably helped to seal off thehydrothermal system and terminate brecciation. Multiple events ofhydrothermal brecciation, that are self-sealing because of lateinfluxes of silica, have been documented at the Wairakei and Broadlandsgeothermal areas in New Zealand (Henleyand Thornley, 1979). 119

Field observations indicatethat the breccia cone sheet ofHahns Peak displays many of the features that are characteristicof intrusive breccias From this I conclude that previous interpretations which identified portions ofthe breccia cone sheet as fault breccia

(Segerstromand Young, 1972; Dowsett, 1973, 1980) are erroneous.

This ambiguity, however,attests to the difficulty in distinguishing between differing types ofbreccias. My conclusion that the breccias which comprise thecone sheet structure at Hahns Peakare entirely intrusive in origin is basedon the following criteria:

1. Lackof evidence indicative of relativemovement between

rock on opposite sides ofthe zones of breccia (joint off-

sets, slickensides, etc.);

2. Surface and subsurfaceexposures of the breccias display

intrusive relationships to theirhost rocks and they may

form branching, vein-like,ariastomosing structures;

3. Faults that have beensuggested to account for the breccias

are of moderate normal displacement(less than 200 ft.),

yet lithic clasts identified insurface breccias are also

present in diamond drillcore 2,000 to 3,000 feet below the

surface;

4. Contacts of the brecciasare curvilinear and sharp (except

where "burst brecci as"are developed"), whereas the gra-

dational zones of gougecomon to some fault contacts are

absent (although rock flourmay be present in the breccia

matrix); 120

5. Presence of many large, angular fragments of soft,altered porphyry within the breccia that would be less likelyto be

preserved intact withina fault breccia;

6. Associations of pyroclastic material, "pebble dikes," "burstbreccias," and fumarolicprecipitates are likely common products of steam-blast venting that would be

related to the formationof the breccias; and

7. The conical shape of the brecciastructure is typical of

manyintrusive brecciapipes (Perry, 1961; Norton and

Cathles, 1973; Soregaroli,1975; Sharp, 1979).

Pyroclastic Vent Complex

The steam-blast eruptions that occurred on the westflank of Hahns Peak caused Multilithicbreccia to be intruded into, and exploded through, the pendant of Browns Park Formation. Rock fragments from the pendant were incorporated into the intrusivebreccia, which was extruded as pyroclastic surges and air-fallejecta. Some of the surges travelled only short distances fromthe vent, whereas others carried pyroclastic material to locationsmore than one mile away. The pyroclastic material and minor amounts of localdetritus that were deposited adjacent to the volcanicvent, formed a stratified complex that consists of alternatingbeds of lapillistone, tuff, and lapilli-tuff (Fig. 7). The formation of this complexis interpreted to be. analogous to that of "lag-faiF deposits"which are produced at the vent areas of ignirnbritic eruptions (Wrightand Walker, 1977). Such lag-fail deposits's consist of a residuum oflithic fragments left behind by pyroclastic flows, togetherwith near-vent, air-fall material.

The well-defined beds of the vent complex dip inwardtoward the breccia cone sheet at angles that range from 200 to 450 The lower angleswere measured in the upper portion ofthe complex, and

although faultswere not observed in outcrop,a disruption of the beds is implied by the variation indip. The inward dips of the

disrupted bedsare suggestiveof subsidence, which may have been caused by the continued extrusion of pyroclastic material. Similar subsidence features are cortnon along the upper edges ofdiatremes (McCallum and others, 1976).

The lithic fragments thatconstitute the pyroclastic rocks of the vent are similar to those of the Multilithic brecciaunit. They differ only in that they display a greater abundance andvariety of Precambrian lithologies. The Precambrianlithologiespresent are identical to those that comprise thepebbles, cobbles, and boulders of the Browns Park basal conglomerate. The relative volumetricpro- portions of Precambrian fragments contained within thevent complex varies from 15 to 20 percent in the lower-most beds, to 5percent in the upper layers. Calcite is present throughoutthe matrix of the ventcomplex rocks and the adjacent Multilithic breccia. It is interpreted to have been derivedprincipally from assimilation of the carbonate-rich Browns ParkFormation. Additional carbonate was probably derived from assimilation of Mancos Shale andfrom the introduction of magmatic CO2. 122

Theshapes of lithic fragments in the ventrocks vary from angular to sub-rounded. Glass shardsare absent. The different lithic layers may be well-sorted, although tuffis often intermixed with lapillistone. Bombs (>256 miii)are present in some tuffaceous beds. Grading in the individual beds may be either normal,reverse, or multiple (normal and reverse). Layers of lapillistone generally exhibit reverse graded bedding. These rocks are interpretedto have beendeposited from pyroclastic surges. Reversegrading may be developed as a flow phenomenon in high-densityfluids as a result of either mechanical grain-dispersive pressure(Bagnold,1954; Komar, 1972), or "Bernouli forces& within boundary layers (Fisher and Mattinson,1968). Fluctuationsin eruptive energymay also affect

the size of fragments in either air-fall or pyroclastic surge deposits close to the vent (Lajoie, 1979). As a result, ventcorn- plexes commonly display symmetrical (reverse to normal; normal to reverse), or multiple (normal and reverse) grading(R. V. Fisher, unpublisheddata, 1974). In addition, changes in wind velocity during eruptions maycause reversegrading in air-fall deposits (R. V. Fisher,unpublished data, 1974).

The Hahns Peak vent complex exhibitsan overall upward decrease in grain size (Fig. 7). This is interpreted to havebeen a function of lower eruptive energy during the finalstages of the extrusive cycle. Layers of tuff probably represent air-fall material thatwas ejected near the end of a given eruptivephase. A minimum of six eruptivecycles are apparent fromthe succession of vent rocks exposed at Hahns Peak (Fig. 7). The presence of mudcracksalong the 123

upper horizon of one tuff layer in the central portion of thevent

complex, suggests that therewas a hiatus in volcanic activity after

deposition of this bed.

The mafic-rich Precambrianfragments derived from the pendant of Browns Park Formation are more abundant in pyroclastic rocks ofthe

vent complex than in the basesurge deposit. This is manifest by the higher Fe203 and MgO, and the lower S102 contents of the ventrocks

relative to thesurge deposit (Table 2). This chemical relationship

suggests that the mafic fragments, which have a higher specificgrav-

ity, settled from suspensionmore rapidly than the silicic fragments;

thus, they were concentratednear the vent. The decrease in abundance of ferromagnesian fragments in pyroclastic rocksas a function of distance from source, has been documented in volcanicareas of the

southwest Pacific Ocean (Murrayand Renard, 1884).

Individuallayers of pyroclasticsurge rockin the Hahns Peak

vent area are thinner andmore continuous than those that are developed in the base surge deposit on Porphyry Mountain. The deposit at the latter site displays finely laminated beds that tendto pinch and swell, and coalesce laterally. Similar distinctions have been observed between near-ventand distal pyroclasticsurges at Ubehebe

Craters, California (Croweand Fisher, 1973).

Layered pyroclasticventrocks have been described atother locations within the RockyMountain region. Buffler (1967) reported the occurrence of diatreme-like,layeredvent breccias within the Browns Park Formation at several locations in the ElkheadMountains, Colorado. Each ofthese occurrences is associated with Tertiary 124

volcanic centers, and are located within 30 miles ofHahns Peak. In addition, Parsons (1967)has described a bedded pyroclastic ventcorn- plex and associated distal pyroclastic depositsfrom a volcaniccen- ter in the northern Absaroka Range. He interpreted theserocks to be

related to eruptions of intrusive breccia, and notedthat glass shards were absent from both the breccias andthe pyroclastic units.

He further suggestedthat these deposits were the result ofgaseous explosions related to magmatic exhalations andphreatic steam-blasts. Previous workers at Hahns Peak have mapped the layeredvent corn- plex as partof the Browns Park Formation(Segerstrom and Young, 1972; Young andSegerstrom, 1973; Dowsett, 1973, 1980). However, evidence gathered from the present study indicatesthat this hypothesis is incorrect,and that the deposit is pyroclastic, ratherthan fluvial in origin, the foregoingdiscussion hasemphasized the lithic and textural characteristics of theventcomplex andits extraordinary similarities to other well-documented pyroclastic deposits. The following observations support the premise thatthe vent complex is not a portion of the Browns ParkFormation: 1. The Browns Park Formation hasbeen established as older than the Hahns Peak porphyries (Buffler,1967;Segerstrom and Young, 1972); yet petrographic examinationsrelated to this study have shown that fragments of HahnsPeak porphyry are plentiful in the unit mapped here as the ventcomplex; 2. The complete section ofbasalconglomeratein the Hahns Peak area is present in the pendantoutcrop. Suboutcrops of the upper sandstone unit are alsopresent between the 125

basal conglomerate and thevent complex. Buffler (1967)

has not reporteda clastic unit comparable to the rocks of

the vent complex presentabove the Browns Park sandstone at

any location in the Elkhead Mountains;

3. The basal conglomerate ofthe Browns Park Formation isa fluvial deposit that displays well-roundedclasts,imbri-

cated pebbles, normal gradedbedding, cross-bedding,and

cut-and-fill structures. The vent complex, although

locally conglomeraticin appearance,displays many frag-

ments that are tooangulartohavebeentransportedby

streams for appreciable distances. Moreover, the vent com-

plex doesnot containany ofthe sedimentary structures

common to the Browns Park Formation.

Pyroclastic Surge Deposit

ii Porphyry Mountain, located approximatelyone mile west of the

layered vent complex (Fig. 20),is capped by clastic material that is interpreted to be a base surge deposit related to steam-blasterup-

tions of Multilithic brecciaon the west flank of Hahns Peak.

Subaerial pyroclastic depositshave been classified into three genetic categories: (1) fall, (2) flow, and (3) surge types (Sheridan, 1979). Pyroclastic surges are turbulent,low-concentration density currentsthat forminat leastthree ways. These include: (1) eruption column collapse(ground surge,base surge), (2) elutriation from the top of a movingpyroclasticflow (ash 126

1\'. bins \Coum K

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ieI.rno'y CoIwwIum,Aiuv,umLond,I,d. UnOivi0e1

I BS Bs SurgeD.. Cioeo'o Sonston.

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i..ccioc. s T,,oi.ic Son4sor,. & £ 1Bv Culcos*oh.d vent oulCidi t Un4ivi0e Quarts Lititi POVPPIpCampus ip. Early SIl. Prom,nonti I F.lsic I Broispa Po'Formation luftOcCOuC Sonciton. TB p r Dosted Wh.r, Approeniotey Upper ,Coor.. Congiorn.rota Lo.erUndu vud.d ° j Located,), L)rO , fJOWitfl(OvV1

Figure 20. Geologic map showing the relationship of the base surge deposit to the volcanic vent complex and the brecciacone sheet (from Casaceli and King, 1980). 127

cloud), and (3) a directed volcanic blast without the collapseof a

vertical eruption column (Fisher,1979). Base surges commonly originate from the collapse of phreatomagmatic eruption columns (Waters and Fisher, 1971). Unlike ground surges, whichare associated with hot pyroclastic flows, base surges form cold, vapor-richfragment- laden clouds.

Base surges were first observedas ring-shaped clouds that moved

horizontally outward athurricane velocities from the baseof nuclear

explosion columns (Glasstone,1950). Later workers described similar

phenomena associated with volcanic eruptions (Richards, 1959;Moore, 1967; Waters and Fisher, 1971). The bed forms that characterizebase

surge deposits are similar to thosedeposited by debris-laden streams in upper-flow regimes (Fisher and Waters,1970). However, recent studies have shown that bed forms indicative of lower-flowregimes may be present in some deposits (Stuart and Brenner, 1979). Wohletz and Sheridan (1979) have classifiedpyroclastic surgesinto three

facies according to theprinciple bed forms displayed: (1) sandwave (sandwave and massive beds),(2) massive (planar, massiveand sand-

wave beds), and (3) planar (planarand massive beds). Sandwave beds

typically display dunes,ripples, cross-laminations,and antidunes. Massive beds arepredominantly unstratified, ungraded, and rarely show internal structures, although bedding sags and contortedstrat- ification may be present. Planar beds are commonly inverselygraded.

They may be cross-stratifiedand show internal pinch and swell features that result in a sandwave appearance (Crowe and Fisher,1973; Wohletz and Sheridan, 1979). 128

Near the vent, base surge clouds are fluidized by largevolumes of steam and carbon dioxide, but as theymove outward the volatiles are lost and the system is deflated. However, the deflationmay not

becomplete, and zones ofdense fluidization may persist inthe mobile cloud. Viscous flow prevails withinthese zones, and massive beds result. Planar beds representnon-fluidized zones where iner-

tial flow was dominant (Sheridan and tipdike, 1975; Wohletz and

Sheridan, 1979). Thus, a singlesurge cloud may produce a laminated deposit with alternating planar and massive beds. Separate base surge events may be recognized by large-scale cut-and-fillstructures (Fisher, 1977).

The base surge depositon Porphyry Mountain consists of layered

tuff, lapilli-tuff, and lapillistone. Petrographic examinations indicate that these pyroclastic rocks are composed of thesame lith-

ologies contained withinthe vent complex. However, the ratio of quartz to mafic rock fragments is noticeably greater. Calcite, present in the matrices of vent complex rocks,was not observed in sam- plesfrom Porphyry Mountain. Bothdeposits aredevoid of glass shards.

Exposures of the deposit exhibit alternating planar andmassive bedsthat are representative of the planar facies ofWohletz and

Sheridan (1979). This facies impliesan energy of flow indicative of thelower phaseof theupper-flow regime (Schmincke andothers, 1973). The massive bedsmay exhibit crude stratification and pebble trains. Reverse grading may bepresent locally, but for the most part the individual beds are ungraded and poorly sorted. Deflation 129

has resulted in the contortion of overlying planar bedsaround large clastsof Hahns Peak porphyry. These structures resemblebedding sags; but ths contorted layersareabove,rather than below the clasts. Locally, loadcastsof massive materialprojecting into planar layers have formed by differential settlingand compaction. Planar beds onPorphyry Mountainare much finer-grained than the massive beds. They display multiple, thinlaminae that may becross- stratified. These laniinae commonlyform pinch-and-swell structures. Angled impact sags are present locally, andare presumed to be the result of airborne ejecta. Individualplanar layers may display

either normal or reverse gradedbedding. Someexhibit reverse grading at the base and normal grading at the top. Such layers display a central concentration of coarser fragments. This preferred

distribution may be a result of graindispersive pressuresthat developed within individual laminae(Wohletzand Sheridan, 1979). The stratification observed may have formedfrom layers of different densities that originated within a fluid that underwentlaminar flow (Crowe and Fisher, 1973). High shear stresses wouldhave existed along density interfaces of layers thatmovedrelative to one another. These stresses would havegenerated grain dispersivepres-

sures that might have caused largerfragments to move away from high- shear borders.

At Porphyry Mountain, the planar beds tend to increasein thick-

ness toward the top of the deposit,whereas the fragments within the massive beds tendtobecome smaller and lessmafic in the upper levels. The result isa very subtle,overallupward decrease in 130

grain size, which may be due to gravitational effectswithin a single surge, or to lower eruptive energies of subsequentsurges. The exact number of surgeevents represented at Porphyry Mountain wasnot determined. However, the presence ofa cut-and-fill structure five feet wide, on the north side of the deposit, suggeststhat at least two separate base surges reached this location. Similar large-scale

cut-and-fill structures havebeen reported from basesurge deposits

at other localities (Fisher,1977).

These data collectively suggest that Porphyry Mountain is a

lobate deposit that thins laterally away from the vent area

(Fig. 20), andsuch characteristicsare common to many base surge deposits. The thickened centralportion of the deposit,and the orientations of planar beds shown in Figure 20 are suggestiveof deposition within a pre-existingchannel. It is possible that the base surge moved through a small tributary valley and was depositedwithin the confines of this local topographic feature. Subsequent differential erosion would explain thepresent topographic inversion, with the more resistant pyroclastic deposit forminga cap above the soft upper sandstone unit of the Browns Park Formation. Crenulated planar bedsand microfaults, present within the basesurge deposit, are interpreted to be indications of soft-sediment deformationcaused by gravity slumping prior tolithification. Similar penecontemporaneous deformation has been described in base surges thatwere deposited in pre-existing channels at UbehebeCraters, California (Crowe and Fisher, 1973). 131

Previous workers (Segerstrorn and Kirby, 196)have classified the clastic deposit on Porphyry Mountain asan epiclastic breccia. They suggested that it was formed in lacustrineenvironment turbidity currents that were generated by the introduction of laharsfrom Hahns Peak. This interpretation is based entirely on generaltextural similarities between the Porphyry Mountaindeposit and thevolcani- clastic rock units of the OhanapecoshFprmation, MountRainier National Park, Washington. At Mount Rainier, Fisk andothers (1963) inferred that the tuff-brecciaand interlayered,thinly laminated siltstone and fine-grained, graded sandstone were depositedfrom sub- aqueous volcanic mudflows. However, it is my interpretationthat the

alternating massive andplanar beds on PorphyryMountain have a more

striking resemblance tothe base surge depositon Sugarloaf Mountain, Arizona (Sheridan and Updike, 1975) than to the pyroclasticrocks of the Ohanapecosh Formation. Moreover, evidence is lacking thatwould suggest the presence of a lake prior to deposition of theclastic deposit on Porphyry Mountain. The contact of this depositwith the tuffaceous sandstone unit of the Browns Park Formationwas observed at several localities. Typical products of lake-bottomsedimenta- tion, such as siltstone and claystone, were notobserved at any of these exposures. In addition, intermixedorganic debris, common to pyroclastics of the Ohanapecosh Formation, is absent. Further, the block-sag structures that are present on Porphyry Mountain are angled, and have apparent trajectories that indicatea source in the direction of the proposed vent on Hahns Peak. An angled impact sag is evidence for a subaerial depositionalenvironment. Pyroclastic 132

ejecta such as might fallinto water would settle verticallybecause

of greater viscous dragin the aqueous medium, and thus wouldcome to-rest with . much less impact than direct air-fall deposition

(R. V. Fisher, unpublisheddata, 1974).

Deposition from subaerial lahars isalso an unlikely mechanism

for the origin of the PorphyryMountain clastic unit. Although some

lahars may show subtlestratification (Schmincke,1967), there are none reported in the literature that displaythe planar laminations and cross stratificationpresent in this deposit. In addition, the textural and compositional features that the PorphyryMountain deposit shares with rocksofthe vent complex and the Multilithic breccia unit, indicate that allthree are consanguineous. 133

HYDROTHERMAL ALTERATION

Hydrothermal alterationatHahns Peak is interpreted to be related to two distinct episodes of igneous intrusive activity. The earliest hydrothermal event occurred after the intrusion ofsills of Beryl Mountain porphyry, but prior to the emplacement ofthe Summit porphyry unit. This early stage of hydrothermalalteration is evi-

denced by fragments ofstrongly silicified porphyriticand gneissic

rock held in a host ofquartz latite porphyry. Some of these frag- mentsdisplay a stockwork-texture of quartz veinlets thatcontain traces of pyrite, molybdenite, and secondary potassium feldspar. The

second stage of hydrothermalactivity, referred to as the main-stage

of alteration, is interpretedto have been related to the emplacement, at depth, of a cupola composed of highly differentiatedmagma. This stage of alteration is marked by an early pervasivephase of albitization that was followed by the formation of concentriczones of phyllic, argillic andpropylitic mineral assemblages relatedto a cone sheet of intrusive breccia. Rocks subjected to this stage of alteration locally containanomalous amounts of gold, silver,lead, zinc, and molybdenum. The alteration mineralassemblages resemble those of porphyry copper-molybdenum systems (Lowell andGuilbert,

1970; Gustafson and Hunt, 1975),and are similar, in their apparent mode of emplacement, to thosefound in areas of volcanic hot springs

(Henley and McNabb, 1978). Hahns Peak is interpreted to have hosted hydrothermal mineralizationwithin the uppermost part ofa porphyry molybdenum system. 134

The mineralogy of the alteration assemblages was determinedby:

(a) megascopic examination ofover 800 samples from outcrops under-

ground workings, and diamond drillcore;(b) petrographic examination of 120 of these samples; and, (c) X-ray diffraction analyses of66

samples. X-ray diffraction analyseswere made of selected aggregates of secondary minerals from three surface samples representativeof the advanced argillic,phyllic, and argillicassemblages. These analyseswere performed at OregonState Universityon a Philips XRG 3100 diffractometer. The remaining 63 X-ray diffractionanalyses were performed atthe Anaconda Tucson laboratoryby technical personnel. The analyses were done on crushedwhole-rock samples

obtained from diamond drillcore.

Early-Stage of Alteration

The earliest stage ofhydrothermal alteration at Hahns Peakwas clearly established priorto the intrusion of the Summit porphyry unit, as altered fragmentsof porphyriticandgneissic rock were observed in a host of quartz latite porphyry. Approximately 20 of these altered fragmentswere found within the central portion of the

Summit porphyry unit. Each of theseis intensely silicified,and some have trace amounts of sulfideminerals contained in a stockwork of silica veinlets. A fragment of gneissic rock thatdisplays similar veinlets of silica with traces of sulfideswas observed inan outcrop of intrusive breccia on the northwest side of Hahns Peak. It is presumably representative of the early-stage of alteration. 135

Significant amounts ofsulfide mineralizationmay have accompanied thts stage of hydrothermal activity. However, little is known about the related alteration and mineralization, becausepresent data are

limited to inclusions ofrock fragments.

Samples of 15 lithic inclusions from the Summitporphyry unit

were studied under the petrographicmicroscope. The six largest of these were also analysed for their trace elementcompositions (see Table 4). Petrographic examination ofthe textures of the fragments revealed that five are porphyritic, seven are medium-grainedequi-

granular,and threeare grieissic. The effects ofalteration and veining have obscured the original textures and mineralogiesto the extent that positive identification of the rock units fromwhich the fragments were derived is difficult. Although several of the porphyritic fragments have textures that are similar to theBeryl Mountain porphyry unit, it is possible that they represent anotherintrusive

phase that has not beenexposed on the surface,or in drill core.

The gneissic fragments are presumed to be equivalent to the Precambrian gneissic granodiorite observed indiamond drill hole DDH-1O1. The medium-grained equigranularfragments are unlike any otherrock type yet observed at HahnsPeak. Although some Pre- cambrian rocks of the region display equigranulartextures,itis unli kely that these fragments represent a Precambrian ii thology because: (a) the less intenselyaltered fragments contain remnants of or-thoclase and oligoclase,rather than microcline andandesine which are common minerals in the Precambrian rocks ofthe region; (b) the phenocrysts of primary quartz do not display strong 136

undulatory extinction,which isalso commonto the Precambrian rocks; and (c) the amount ofprimaryzircon inthe specimens exceeds the

average amount presentin thePrecambrianrocks and is more typical of the Tertiary intrusive rocks of Hahns Peak. Therefore, the evidence suggests that these fragments may representa Tertiary equigranular pluton present at depth, but one that hasnot been encoun-

tered either in surfaceexposures or in diamond drill core. The petrographic examination ofthelithic inclusions demon- strates that they are strongly silicified, with 40to 90 percent of the rock composed of quartz. The distribution of stockworkveinlets

varies from dense tosparse (Fig. 21), or veinletsmay be completely

absentinfragments that have beenpervasively silicified. The quartz, present both in the veinletsandasreplacementsinthe groundmass, displays a texture of fine-grained mosaicintergrowths. Finely intergrown crystals of quartz also occuras rims around rem-

nant phenocrysts of potassiumfeldspar.

Numerous extremely smallfluid inclusions are presentinthe veinlets of quartz that cut the lithic fragments. The majority of those large enough to identify are characterized bythe presence of a small vapor-bubble within an elongate fluid inclusion. Similar fluid inclusions described by Gustafson and Hunt (1975) fromthe El Salvador porphyry copper deposit were interpreted bythem to represent fluids of predominantly meteoric origin. Also present within the veinlets of quartz are fluid inclusions that displaya moderately sized vapor-bubble accompanied by crystals of whatare presumed to be haliteand a sulfide mineral. The presenceof sodiumchloride 132

0 2 3 4 cm ;; j'

.t ':'i.. / :;

Figure 21. Lithic inclusions from Summit porphyry (Tsp) which show: (A) a porphyry fragment(possibly Beryl Mountain porphyry) with stockwork veinlets of silica containing pyrite and molybdenite; and (B) a Precambrian qneissic rock with well-developed stockwork veins of silica. 138

indicates that a saline component was present in thehydrothermal

fluids. Athird type of fluidinclusiondisplaysa single large vapor-bubble, and its coexistence with hypersalinefluidinclusions

withinthesame veinletof quartz suggests that the hydrothermal

fluids underwent boiling (Roedder,1979).

The overall content ofsulfides in the rock fragments issmall,

although traces of pyriteare present as fine-grained cubic crystals in the silicaveinletsand as disseminatioris in the groundmass.

Fine-grained molybdenite,in association with pyriteand secondary potassium feldspar, was observed in veirilets of silicafrom one fragment of porphyritic rock (Fig.21). Achemicalanalysisof this sample revealed an NoS2 content of 100ppm. Molybdenite was also noted with pyrite in veinlets of silica that cuta gneissic fragment contained in an outcrop of intrusive breccia on the northwestside of

I-Iahns Peak. The presence ofa lithic inclusion of intrusive breccia in the Summit porphyry unit, in which stockwork-veinedfragments are present in a silica-rich clastic matrix (Fig. 19), isevidence that more than one hydrothermal event was involved in the early-stageof mineralization.

The strongly silicified fragments that represent the earlystage of alteration at Hahns Peak are strikingly similarto rocks from the silicified zones of alterationat the Henderson and Climax molybdenum deposits in Colorado (Wallace and others, 1968; Mackenzie, 1970). At Henderson, the silicified zone occurs both above and below theore body, and locally overlaps the zone of potassium feldsparalteration. Veins of quartz within the silicified zone grade intopervasively 139

silicified rock, and the content of silica varies from 60percent in the veined host to 90 percent in those parts pervasivelysilicified. The quartz has been crystallized to an intricate mosaictexture that has destroyed all but a fewremnantsof the original porphyry (MacKenzie, 1970; Bright and White,1977). The silicified zone at Climax is texturally similar to that at Henderson, although itunder-

lies the ore body at alllocations (Wallace and others, 1968).

The intense silica alterationrecognized in lithic inclusions at HahnsPeak is interpreted to represent a hydrothermaleventthat occurred prior to, but was associated with, theemplacement of the

Columbine, Summit, and 70porphyry units. The extremely large (up to 3.5 in.) phenocrysts of sanidine in these porphyriessuggest early crystallization from a volatile-rich magma (Burnham,1967). Eman- ations from such a magma would be likely to producehydrofracturing that would have led to the formationofthe stockwork texture of silica veinlets present insome lithic inclusions(Fig. 21). The associated hydrothermal alteration and sulfide mineralizationwould have been terminated by the deposition of silica in theveinlets, which would have essentially sealed the system to the flow offluids. Minor brecciation might have resulted locally where thepressure of volatiles at the top of themagma chamber exceeded the strength of thesilicified cap near the end of this stageof hydrothermal activity. 140

Main-Stage of Alteration

The main-stage of hydrothermal alteration at Hahns Peak occurred after theemplacementof the 7D porphyry unit,but priortothe intrusion of the Late-stage porphyry dikes. The alteration is believed to be related to the emplacement, at depth, of a cupola of highly differentiated magma. Amixture of magmatic vapors and liquids enriched in water, carbon dioxide, alkali halides, and hydrogen sulfide presumably formed a plume around the upper part of the cupola, and migrated upward along incipient fractures and mixed pas- sively with minor amounts of ground water. Early chemical reactions between the hydrothermal fluids and the host rock resulted in weakly developed, but pervasive, albitj.zationof thequartzlatitepor- phyries. The continued build-up of volatile pressure at the head of the cupola probably caused conicalfractures to form in the roof rocks. Rapid discharges of super-heated magmatic fluids, together with steam blasts derived by the mixing of cold ground water with these fluids, were presumably channelled along the conical fractures.

The intrusive breccias that formedby thisactivity comprise the breccia cone sheet. The hydrothermal fluids that accompanied this brecciation, together with those fluids that subsequently followed the permeable path through the brecciated rock, formed a zoned assemblage of alteration minerals that was superimposed upon the perva- si vely albi ti zed rock. The di stri buti on patterns of these alteration assemblages generally' coincides with the inferred shape of the breccia cone sheet. 141

Alteration Zones

The assemblages of alteration minerals presentat Hahns Peak consist ofan inner zone of phyllic alteration that grades outward into zones of mixed phyllic-argillic, argillic, and propylitic alteration(Plate 3). Smallzones ofadvancedargillic alterationare locally associated with pebble breccias,and appear to be discontinuous at depth. The effects of hydrothermal alteration are readily apparent inrockoutcrops. The more strongly sericitized central part of the intrusive complex is chalk-white in color, with irregularly distributed patches of yellow and brown. Argillic alteration prevails outward from this central phyllic zone,and the colors of the rocks vary from light to dark gray. Within the outer zone of propylitic alteration, where chlorite is most abundant, the rocks are typically green-gray in color.

The classification of the zones of alteration at Hahns Peak is based on the nomenclature and the descriptive alteration mineralogy presented by Lowell and Guilbert (1970) and Meyer and Hemley (1967).

Propylitic Zone. The characteristic constituents of the propylitic alteration mineralassemblage at Hahns Peak are albite, calcite,montmorillonite, chlorite, andsericite. Minor amounts of quartz,pyrite, magnetite, epidote,and siderite are also present.

Secondary albite occurs as rims around pheriocrysts of both potassium and plagioclase feldspar, and has completely replaced some megacrysts of plagioclase feldspar. Thealbitemay in turn be partially replaced by sericite and calcite, and its outer borders are typically 142

in contact with aggregates of either chloriteor quartz. The albite replacements in this zone are interpreted to be relicsfrom the ear-. her period bfpervasive albitization. Dowsett (1973, 1980)reports that the predominant potassium feldspar containedin the albitized porphyries of Hahas Peak has an orthoclasestructuralstate compo-

sition of approximately 0R90. These feldspars were presumably altered from their original composition ofsanidine (0R67) by a process of alkali exchange.

Phenocrysts of plagioclase feldspar arecommonly altered to aggregates of calcite, niontrnorihlonite,sericite, and quartz, with traces of epidote. Plagioclase feldspar isconsistently more altered than potassium feldspar, although theintensity varies from weakto strong. The interiors of some phenocrysts of potassiumfeldspar have streaks and finespecks of sericite, quartz, calcite, andoccasion- ally epidote. Calcite replaces thegroundmass locally, andmay also be present in thinveinlets. Siderite, in minor amounts,replaces plagioclase feldspar along the inner marginof the propyliticzone, but is more common near the brecciacone sheet in the zones of argillic and mixed phyllic-argillic alteration. Typically, biotite is variably altered to chlorite and sericite,with traces of epidote. Some phenocrysts of biotite contain secondarymagnetite and leucoxene intermixed with chlorite. Crystals of hornblendehave been altered to fine-grained aggregates of chlorite andcalcite. These aggregates also commonlycontain crystals of secondary magnetite. Disseminated crystals ofboth primary and secondary magnetiteare locally Oxi- dized to hematite. Pyrite is present in smallamounts (trace to one 143

percent) as disseminated cubes or as small clots that replacethe ferromagnesian minerals.

The ove-all intensity of propylitic alterationvaries from weak to moderate, and is never strongly developed. Nevertheless, propy-

litically altered rocksform a distinctly arcuatezone for about 1800

alongthe southeastern margin of the intrusivecomplex (Plates I and 3). The effects of propylitic alteration were observed inparts of every major intrusion in the study area, but theyare best developed within the Beryl Mountain porphyry on BerylMountain. Weak to moderate propylitic alteration is also presenton Anderson Mountain (1.1 miles southeast of Hahns Peak), andon Twin Mountain (1.4 miles northeast of Hahns Peak). The zone of propylitic alterationgrades outward into weak deuteric alteration in which pyrite is rare or absent. The outer margin of hydrothermal effects imposedby main- stage alteration encompasses an area with an approximatediameter of two miles centeredon Hahns Peak. From Anderson Mountain toHahns Peak, the intensity of propylitic alterationandsulfide mineralization increases and grades into an argillic assemblage. The con- tinuation of the propylitic zone along the north and northwestmar- gins of the intrusive complex isless distinct because outcrops of igneous rocks are lacking and those of the sedimentaryrocks are low in lime, magnesia, and iron. Hydrothermalalteration isapparent, however, in the bleached shales and si)tstones of the Morrison Formation that are exposed on the northwest flank ofHahns Peak. The alteration minerals are chiefly sericite, clay, and pyrite (Plate3). 144

Argillic Zone. Argillic alteration at Hahns Peak is moderately

to weakly developed, and is typified by montmorillonite, kaolinite,

and sericite,with lesser amounts of chlorite and quartz. Plaglo-

clase feldspar is the most strongly altered mineral, and is commonly

replaced by clay, sericite, and quartz, with traces of siderite. In

contrast, potassium feldspar is usually lightly dusted with secondary

sericite. The intensity of alteration of biotite varies from weak to

strong, and reflects the overall intensity of argillization. Some

phenocrysts of biotite in the outer margin of the argillic zone are

shiny-black and virtually unaffected by hydrothermal alteration.

Many phenocrysts of biotite along the inner margin of the argillic

zone are strongly altered to sericite. However, biotite more com-

monly displays moderate alteration to sericite andchlorite, with

traces of leucoxene and magnetite. Chlorite has also replaced horn-

blende to a variable extent. Disseminated pyrite is present through-

out thi szone in abundances that range from trace amounts to three

percent,and the average is approximately one percent. The total

content of clay minerals typically exceeds that of sericite, and kao-

linite was determined, by X-ray diffraction, to be more abundant than montmoriulonite. It was also determined, by X-ray diffraction analyses of samples of core from diamond drillhole DDH-7A, that the

ratio of kaolinite to rnontmorillonite increases with depth. This

trend is similar to that reported by Segerstrom and Young (1973), and

is evidence that kaolinite was formed primarily by hypogene, rather

than supergene, fluids. 145

Rims of albite were observed around some phenocrysts of potassium feldspar within this zone, and they probably represent remnants of theearlier period of albitization. Thesecondary albite is altered to clay,particularly along the outer edges ofthe rimmed phenocrysts. Dowsett (1973, 1980) reported that two distinct compositional states of potassium feldspar (0r67 and OR9Q) exist in rocks that have undergone intermediate argillic alteration at Hahns Peak.

The Or90 variety predominates with increasingintensity ofalteration.

Theargilliczone is theleast distinctive ofthe alteration zones at Hahns Peak. The principal difficulty isthat many of the rocks appear,in hand specimen, to be unaltered. The argillic zone forms a narrow border around the breccia cone sheet, and grades out- wardly to the propyliticzoneand inwardly tothe zoneof mixed phyllic-argillic alteration (Plate 3).

Mixed Phyllic-Argillic Zone. The zone of mixed phyllic-argillic alteration is characterized by the mineral assemblage sericite, quartz, kaolinite,and moritmorillonite. Sericite predominates over the clay minerals,and kaolinite ismoreabundant than montmorillonite. The totalcontent of clay may approximate 20 percent(by volume) in thiszone, although it is usually under five percent.

A variety of green sericite is present in trace amounts, and displays a greater relative abundance in more intensely sericitized samples.

Phenocrysts of biotite are. commonly replaced by green sericite. The significance of the color is not clear, although Meyer (1979) reports 146

that green sericite at Butte, Montanta has a highermagnesium content than colorless sericite. The breakdown of biotiteat Hahns Peak would have released magnesium, which may be thecause of the green color. Crystals of plagioclase feldsparare usually strongly altered

to sericite, quartz, andlesser amounts of clay. Locally, siderite has replaced phenocrysts of plagioclase feldspar. The occurrences of siderite are normally located near the breccia cone sheet. Distinct geochemical anomalies of manganese were observedfrom rock-chip samples that displayed siderite alteration. The association ofman- ganese with siderite is consistent with the identification ofmangan- osiderite, by Se9erstrom and Young (1972), in diamond drillcore from Hahns Peak. Phenocrysts of potassiumfeldsparmay be slightly speckled with sericite and clay, or strongly replaced bysericite, quartz, and minor clay. Biotite exhibits strong alterationto sericite throughout this zone. Rornblende is commonly replaced by

finely crystalline sericiteor talc(?). Minor amounts of chlorite,

magnetite, and leucoxene alsoreplace some phenocrysts of hornblende.

Disseminatedpyrite is presentthroughoutthiszoneand averages

1.5 percent (by volume). Restricted areas in andnear the breccia

cone sheet may contain pyrite inamounts of three to five percent.

The groundmass of rocks that occur in the mixed phyllic-argilliczone

is moderately to stronglyaltered to silica and sericite.

Thezone of mixed phyllic-argillicalteration is irregularly shaped and bounded approximatelyby the inner margin of the breccia cone sheet (Plate 3). This zone is interpreted tobe gradational between the zones of argillic and phyllic alteration. Small isolated 147

areas of mixed phyllic-argillic alteration also are found along the

northern margin of the intrusive complex (Plate 3).

Phyllic Zone. The characteristic minerals of phyllic alteration

are sericite, quartz, and minor kaolinite. Kaolinite, identified by

X-ray diffraction, is present in amountsup tofivepercent (by

volume). Aggregates of sericite, quartz,and kaolinite completely

replace most phenocrystsof plagioclase feldspar. Pheriocrysts of

potassium feldspar are replaced by a similar assemblage of minerals,

and display an intensity of alteration that varies from moderate to

strong. Dowsett (1973, 1980) reports thatall potassium feldspar

contained in strongly sericitized rock at Hahns Peakis orthoclase

(0r90), that has been been converted froman original composition of

sanidine (0r67) by alkali exchange. Phenocrysts of biotite are com-

pletely altered to sericite, much of which is green in color. Green

sericite is more abundant in this zone of alteration than in others,

and may i ndi cate the addi ti on of magnesi urni nto the lattice structure

ofsericite. Hornblende isnot abundantinthiszone,but where

present itappearstobestrongly sericitized. Disseminated sub-

hedralto euhedral crystals of pyrite constitute one to two percent

(by volume) of the rock from thiszone. Thealteration of the

groundrnass van es from strong sen ci ti zati on wi t fi ne anhedral

aggregates ofsi li ca, to strong si ii ci fi cation with sparse shreds of sen cite.

Phyllic alteration is exposed in a linear trend of isolated outcrops within the zone of mixed phyllic-argillic alteration (Plate 3). 148

Subsurface data suggest a more continuous zone that is closely asso-

ciated with, but not restrictedto, the breccia cone sheet (Plate 3).

Advanced Argillic Zone. The characteristic minerals ofthe zones ofadvanced argillicalterationat Hahns Peak arealunite,

dickite, kaoliriite, andquartz. Opalineand chalcedonicquartz, pyrite, tetrahedrite, and traces of covellite occur locally. Alunite was first tentatively identified at Hahns Peak by Bowes (1969). This

identification was confirmedby X-ray diffraction analyses performed at Oregon State University during the present study. Areas having

anomalous concentrations ofdickite, which in part coincide withthe presently mapped zones of advanced argillic alteration,were delin- eated by Oowsett (1973, 1980).

The distribution of advancedargillic alteration is erratic and irregular, and is confined locally to occurrences of extremelyfine-

grained pebble breccias thatare light-gray to white in color. These

areas of advanced argillicalteration are discontinuous with depth

and probably do not extendmore than 500 feet beneath the surface.

Alteration Processes

The early albitizationwas formed by a process of metasomatisin

in which sodium, probablyas sodium chloride, was introduced along

fractures in the igneousintrusive rocks. The sodium may have been derived from magmatic vapors and liquids that emanated from thetop of a magmatic cupola. The continued emplacement ofmagma from depth

caused the cupola to ascendto higher levels. High pressures imposed both by the forceful emplacement of magma and by supercriticalfluids 149

exsolvedtherefrom would cause fractures to propagate upward in competent roof rocks above the cupola. At some relativelynear- surface locale the expansive pressure of the magma-fluidsystem would have been greater than the confining pressure ofcountry rocks and supercritical fluids would have escapedupward through thenewly- for-med fractures. Monolithic breccia was probablyformed in response to the explosive expansion of these supercritical fluids. The extent

to which meteoric waterwas a component of the supercritical fluids is largely unknown, although probably minor duringthe early, high temperature and saline stages of their evolution. The formation of Monolithic breccia, however, would have caused additionalfracturing of country rocks, thus creating more open spaces thatcould have been

filled with groundwater. When super-heated magmatic fluidscame intocontact withthiscoldground water,steam-blast explosions would have occurred. The cone sheet of Multilithicbreccia was prob-

ably formed by thesephreatic explosions.

The vent complex and base surge deposits of HahnsPeak contain fragments of altered porphyry with moderate amountsof sericite and clay and traces of pyrite. The presence of these mineralssuggests that weak hydrothermal alteration took place prior tothe emplacement of the Multilithic breccia unit from which thepyroclastic rocks have been derived. The hydrothermal fluidsresponsible for this alteration are likely to have been associated with theformation of the

Monolithic brecci a. Although this earlier brecci a is locally enriched in sulfides, theoveralleffects of hydrothermal activity associated with it are weak. More intense alteration and 150

mineralization probably accompanied the emplacement of the Multi-

lithic breccia. The zones of intrusive breccia served as permeable

channelwaysthroughwhich the latervolatile-rich magmaticfluids

were released to mix with ground water. The pressure and temperature

ofthese post-breccia magmaticfluidswould have diminishedfrom

prior conditions, andthusthe interactions with ground water were

probably less violent. The greater involvement of meteoric fluids in

this later stage of alteration resulted in mineral assemblages stable

at lower temperaturesthanthose formed in theearlier stage of

aibitization. However, some of the magmatic fluids may have risen as

vapor plumesto form localzones of higher temperature products of

alteration within and adjacent to the breccia cone sheet. Although

the exact methods oftransport of metalsandalteration fluids is

unclear, the evidence suggests that the brecciated structures served

as passage-ways for the hydrothermal fluids that caused the wall rock

alteration and the sulfide mineralization. The zoned assemblages of

alteration minerals,the distributionsof which mimicthe general shape of the breccia cone sheet, are the result of multiple episodes of brecciation and hydrothermalactivity. The zoning,however, is not the effect of progressivealterationwith time. Rather it resulted from the variable intensity of hydrogen metasomatisni caused by reactionsbetween thewall rocks andthe hydrothermal fluids.

Areas within and adjacent to the breccia cone sheet had the greatest flux of hydrothermal fluids, and the chemical exchanges between those fluids and the wallrocks were therefore intensified. Asa result sericitewas most stronglydeveloped in the inner zones. The 151

distribution of alteration assemblagessuggests a gradational change

outward through the argillicand propylitic zones as hydrogenmeta- somatism weakaned.

Albitic Alteration. The presence of hydrothermal albite,which

replaces phenocrysts ofplagioclase and potassium feldspar at Hahns

Peak was first reported byYoung and Segerstrom (1973). This mineral

was also documented by Dowsett (1973, 1980),who was the first inves-

tigator to realize thatalbitization of feldspars markeda distinctly

early phase of alteration. In addition, he was the first to suggest that the predominant orthoclase structural state (0rg) ofpotassium feldspars is pseudomorphic after primary sanidine(0r67) andthat

this probably formed by alkaliexchange and ordering of sanidine at

the time of albitization. Structural states of the alkali feldspars

were determined by Dowsett (1973, 1980) fromequations relating the a

cell edge and theposition ofthe 201reflections, as measured by

X-ray diffraction,to composition. Petrographic observations from

the present study suggestthat the effects of albitizationare more

widespread than reportedby Dowsett (1973, 1980). For example,

albite was observedas rims enveloping phenocrysts of feldspar from

latite porphyry on BerylMountain that Dowsett (1973, 1980) described as unaltered.

Meyer and Hemley (1967)havesuggestedthatthe process of albitization results from themetasomatic introductionof sodium.

They presented equilibria datafrom which Dowsett (1973, 1980)concluded that albitic alteration at Hahns Peak formed in the 152

temperature range of 400° to 5000C. This estimate is consistentwith the model presented herein of alkali halide-enrichedmagmatic fluids, with minor contamination by ground water, havingproduced the albitic alteration.

Propy)itic Alteration. The formation of the intrusivebreccias and the associated hydrothermal alteration at HahnsPeak caused the previously albitized host rocksto undergo variable intensitiesof

hydrogen rnetasomatism. The effects of hydrogenmetasomatisrn are

least apparent in the marginalpropylitic zone farthestfrom the

breccia cone sheet. The formation ofa typical propylitic assemblage involves several distinct chemical reactions betweenthe wall rocks and the hydrothermal fluids. The predominant chemicalchanges in this zone involve the addition of CO2 duringthe formation of carbon- ates, and the addition of H20 during the formation of hydrousphases suchas chlorite (Hemley and Jones,1964). Weak hydrogen metasomatism can account for the minor amounts ofsericite and montmoril- lonitethat commonly replacephenocrysts of feldspar (Hemley and Jones,1964; Meyer and Hemley, 1967). The potassium necessaryto form sericite after plagioclase feldsparwas probably provided by the alteration of biotite to chlorite, and from theplagioclase feldspar host mineral itself.

The propylitic zone occupies the area farthestfrom the source

of the hydrothermalfluids. The mineralscharacteristic ofthis assemblage suggest that the fluidshad a lower temperature anda higher cation/H+ ratio than those which permeatedzones closer to the 153

breccia cone sheet and formedalteration minerals such as kaolinite

and sericite.

A plot of trace element distributionsat Hahns Peak shows a

relative enrichment of Mn inareas that border the breccia cone sheet

(Plate 3). The propylitically altered rocks generallyhave higher

concentrations of hO2, MnO, MgO,CaO,and Na2O than do those that

display argillic or phyllicalteration (Table 2, Plate 3). The anal-

yses suggest that Ti, Mn, Mg, Ca, andNa were leached from the inner

part of the intrusive complex,where argillic and phyllic alteration

assemblages prevail, andwere transported outward to be localizedas

an enrichment in the propyliticzone. However, this interpretation

is speculative becauseunaltered host rocks are lacking at HahnsPeak

which makes it impossible toquantitatively estimate the chemical

losses and gains fromone zone of alteration to another.

Argillic, Phyllic, and Advanced ArgillicAlteration. The tran-

sitions from argillic to phyllicand to advanced argillic alteration

assemblages represents a progressive increase in theintensity of

hydrogen metasornatism. The wall rocks that were closest tothe brec- dated channelwaysmore readily exchanged base cations for hydrogen ions carried by the hydrothermalfluids than did those more distant.

Typical reactions betweenprimary minerals of the host rocks and the hydrothermal fluids to form the secondary alteration mineralsas suggested by Hemley and Jones (1964)and Meyer and Hemley (1967), are summarized in Table 3. The equations illustrate the conversionof primary minerals, through reactionwith water (reduced to equivalent 154

c alteration:

Andesine Montmorilionite

(1) Na2CaA14Si3024 + 4H MAl2Si4010(OH)2 + 2Na+ Ca

(M = base cations present in amounts chemically equivalent to the charge deficiency, x.)

Albite Na-Montmorillonite Quartz

(2) 1.17 NaAISi3O8 + H4 = 0.5 Na0 33A1, 33S1367010(0ll)2 + 1.67 Si02 + Na

Na-Montniorillonite Kaolinite Ouartz

(3) 3Na033Al233Si36)O10(QH)2 + H4 + 35 H20 3.5 Al7Si20(OH)4 + 4SiO2 + Na4

Andesine Kaolinite Quartz

(4) Na2CaA14Si8O24 + 4H + 21120 2Al2S1205(QH)4 + 4Si02 + 2Na4 + Ca

caltion:

Andesine Sericite Quartz

(5) 0.75 Na,CaA14S18024 + 2H+ K4 KA134 Si3010(011)2 + 1.5 la4 + 0.75Ca 4 35109

K-feldspar Sericite Quartz

(6) 1.5 KA1SI3O8 + H 0.5 KA1,S301Q(0H)2 + K4 + 3S102

Advanced argillic alteration:

Sericite Keolinite

(7) KA13Si3010(OH)2 + H + 1.5 H20 1.5 AI,S1205(011)4 + K4

Sericite Alunit Quartz

(8)KM3S13010(OH)2 + + 2S0 KAI3(SO4)9(oH)6 + 3S102

Table 3. Typicalargillic, phyllic,and aivanced arqillic alteration reactions between host rock minerals and hydrothermalfluids (adapted from Hemley andJones, 1964; and Meyer and Hemley, 1967). 155

H+ ions), to the common alteration products suchas andesine to mont-

morillonite (equation 1), kaolinite (equation 4), andsericite (equa-

tion 5);albite to montmorillonite (equation2); Na-montmorillonite

to kaolinite and quartz (equation 3); K-feldsparto sericite and

quartz (equation 6); sericite to kaolinite (equation 7);and ser-

cite to alunite and quartz (equation 8).

In essence,the extentto which the wallrocks undergo base

leaching is a function of the cation/H ratioin the hydrothermal

fluids with which they react. An increase in the concentration of

hydrogen ions resultsin greater hydrogen metasomatism. The source

of the hydrogen ionsis either from the decomposition ofpure water

(H20 = + OW) or hydrogen sulfide (H2S = HS + HS).

Argillic alteration at Butte,Montana involvedthe step-wise

decomposition of plagioclase feldspar to montmorilloniteand montmo-

rillonite to kaolinite, withpotassium feldspar unaffected(Hemley

and Jones, 1964). A similar process may have occurred at Hahns Peak

in which plagioclase feldsparwas among the first minerals to react

with the hydrothermal fluids to producemontmorilionite (equation 1).

Any albite that was present would have alsoundergone hydrogen meta-

somatism to form sodium-rich montmorillonite (equation 2). An

increase inhydrogenion activity would have converted montmoril-

lonite to kaolinite as in equation 3, andkaolinite might have formed directly from the breakdown of plagioclasefeldspar (equation 4).

The transition from argillic to phyllictypes of alteration is gradational, andis marked by anincreasein the amount of hydrothermal sericite. Phenocrysts of plagioclase feldspar may be 156

strongly sericitized as theresultof reactionssuch as that of equation 5. Fluids having particularly high concentrations of hydrogenions will cause potassium feldspar toalterto sericite (equation 6).

Advanced argillic mineral assemblages may be derived fromeither supergene or hypogene processes (Meyer and Heinley, 1967). At Hahns Peak,the zones of advanced argillic alterationare interpreted to have been formed by hypogene fluids because of theirassociation with pebble breccias. A steady influx of acidvapors (H2S, HC1 )into a ground water systemwould producefluidscharacterized by a low cation/Hf ratio(Hemley and Jones,1964). Under such conditions, sericite would be altered to kaolinite (equation 7). In addition, the oxidation of H2S in near-surface hydrothermalenvironments would

produce H2SO4, and thusprovide sulfate anionsand the appropriate acidic conditions to stabilize alunite (equation 8). The formation of pebble breccias is closely associated withgaseous venting and

collectively they providean excellent environment for the locali-

zation of advanced argillicmineral assemblages.

The alteration assemblageof sericite-kaolinite-montmorillonite-

quartz-pyrite was estimated by Dowsett (1973, 1980) to haveformed in the temperature range of 2500 to 350°C. These temperatures are lower than those suggested for the formation of secondaryalbite, and pre-

sumably reflect a greatercontribution of meteoricwater tothe hydrothermal fluids.

The localized occurrence,withinzones of sericitic-argillic alteration, of small (2.0 to 10.0 rmdiameter) euhedral crystals of 157

orthoclase suggeststhatsome hydrothermalfluidshad considerably

higher temperatures andcation/H+ratios than those responsible for

the formation ofsericite and clay. The crystalsoforthoclase,

identified by X-raydiffraction analysis,occur as secondaryvug

fillings in altered quartz latitie porphyry, adjacent to the breccia

cone sheet on the west side of the mountain. White (1967) reports

that metastable hydrothermal orthoclase may be found associated with

epithermal Ag-Au and Mndeposits. At Hahns Peak, potassium is

believed to have been transported asa chloride (KC1)in plumes of magmatic fluids characterized by high temperatures and high cation/Hf

ratios. These plumes probably consisted of a mixture of vapor and

liquid that ascended through channelways of intrusive breccia within theground water-dominated hydrothermal system. Orthoclase might have been precipitated locally as a metastablephasebecauseof potassium contributed by continued magmatic emanations. The alter-

ation of sanidine(Or67) to orthoclase (Org), by alkaliexchange, has been suggested by Dowsett (1973, 1980) to have taken place in the zones of intense sericitization. This change may have been caused by the influx of magmatically derived potassium, which presumably also reactedwithphenocrysts and groundmassrnicrolites ofplagioclase feldspar to form sericite. Three samples ofstrongly sericitized quartz latite porphyry (CP-2, SP-1, SP-2; Table 2, Plate 3) provide an average K20 composition of 7.5 percent. This value is signif-

icantly above the average K20 value of 4.0 percent for quartz latite determined by LeMaitre (1976). Moreover, it cannot be accounted for bythe observed modal abundance ofpotassium feldspar. Abundant 158

sericite is present in the phenocrysts of plagioclase feldspar, as well as in the groundmass, of these samples. In contrast, the pheno-

crysts of potassium feldsparare only moderately replaced by sericite. Thus, only minor amountsof potassium could have beenliber- ated by the alteration of potassium feldspar. Accordingly, it is

inferred that the majorportion of the potassiumnecessary for the

sericitizatiori of plagioclasefeldspar was probably introducedby the

hydrothermal fluids. The intrusive brecciasare thought to have provided the channeiways for these potassium-enriched fluids. Addition- ally, chemical analyses of threesamples of Multilithic breccia,

which average 7.8 percent K20(Table 2),are consistent with this

i nterpretati on.

The extremely high potassium content of the Hahns Peakpor-

phyriesindicatesthat they arealkali rhyolites (Segerstrom and Young, 1972;Young and Segerstrom,1973). However, evidence presented herein suggests that additional potassium must have been introduced to the system. Thus, any classification of alteredrocks

that utilizes values of K20 cannotprovide a reliable estimate of the

composition of the originalrock. Because all of the intrusive rocks

observed at Hahns Peak displayat least minor alteration, their clas-

sification in this studyis based ona comparison of the measured

S102 content with those ofaverage values for igneous rocks published by Nockolds (1954) and LeMaitre (1976). Although some silica was introduced into the system, it appears to have been contained,for the most part, within the intrusive breccias. Minor micro-veinlets of silica are present inzones of sericite alteration. However, 159

veins of quartz are not abundant at Hahns Peak. Large through-going veins were not observed, althoughamethystine quartz foundon the dump of the Tom Thumb Mine (Plate 1) may representfragments of vein materi al. Petrographi c exami nation ofseveral samples ofsen ci ti zed porphyry revealed finely crystalline secondary quartzas overgrowths

or replacements of primary feldsparphenocrysts and as isolated aggregates within the groundmass. The source of silica requiredto form these secondary growths was presumably releasedby the sericitization of these phenocrysts of feldspar (see Table 3,equations 5 and 6). This interpretation impliesthat the silica content ofser- icitizedrocks, which are devoid of veinlets of quartz, should reflect the original silica content of the rock. Rock samples chosen for chemical analyses in this study were carefully selectedto avoid veinlets of silica.

Supergene Alteration

Limonite stains the surfaces of outcrops throughout most ofthe central part of the Hahns Peak intrusive complex. Surficial weathering has resulted in the oxidation of pyrite and thesubsequent formation of pseudornorphs of limonite composed of mixtures ofgoethite and jarosite (Young and Segerstrom, 1973). Less commonly, the pseudomorphs of limonitehavebeencompletely leached by surface waters leaving a residuum ofiron-stained cubic molds.

X-ray diffractionanalysesindicate that the montmorillonite/ kaolinite ratio is greatest in the upper 600 feet of alteredrock at Hahns Peak (this study, and Young and Segerstrom, 1973). The relative abundance of montmorillonite at near-surface levelsis thought

to be the result of supergene alteration of plagioclase feldspar. In

addition, a non-pleochroic gray-white clay mineral, with an index of refraction above balsam, has replaced sericite in some surface sam-

ples. This is interpreted to be kaolinite which also was formed from the supergene alteration of sericite. The formation of montmoril-

lonite and minor kaolinite by supergene processes, probably preceded the development of open vugs where phenocrysts of feldspar have been completely leached. Such vugs are numerousin surficial exposures and these outcrops display strong clay-sericite alteration and abundant pseudomorphs of limonite after pyrite. Sulfuric acid, produced by the weathering of pyrite, presumably combined with surface waters to provide the lowcation/H+ratios necessary to form supergene montmorillonite and kaolinite.

Variscite (A1PO4.2H20), and lesser wavellite (Al(OH)3(PO4)

5H20), occur as botryoidal crusts and spherules within some cavities

(Young and Segerstrom,1973). These minerals are most abundant in areas of high sulfide concentrations andare probably supergene in origin(Dowsett, 1980). They are likely to have formed from acid solutions that attacked primary fluorapatite and feldspar, thuslib- erating Al203 and P043 (Young and Segerstrom, 1973).

Elsewhere, within areas characterized by propylitic alteration, magnetite ofbothprimary andsecondary origin has been partially oxidized to hematite; presumably by supergene processes. 161

METALLIZATION

Weak to moderate sulfide mineralization was deposited in three pulses of hydrothermal activity at Hahrts Peak. The earliest hydrothermal event deposited minor amounts of pyrite and molybdenite in stockwork veins of silica prior to the intrusion of Summit porphyry.

Two later pulses of hydrothermalactivity depositedsulfidesboth during and after formation of the breccia cone sheet, as part of the main stage of alteration. Pyrite, galena, sphalerite, and chalcopyrite occur locally in the matrix of Monolithic breccia as finely disseminated primary minerals and as disseminated masses and veinlets in adjacent host rock. Finely disseminated molybdenite is also present locally in the matrix of Monolithic breccia. Pyrite,tetrahedrite, andtracecovelliteoccur locally withinthe matrixof

Muitilithic pebble breccia that displaysadvancedargillic alteration. Pyrite, sphalerite, galena, and lesserchalcopyriteand tetrahedrite are present as post-breccia open-space fillings locally throughout the breccia conesheet. The distribution patterns of anomalous lead,silver,and manganese closely match the outline of the brecci a cone sheet and indicate the strong controlthat thi 5 structure had on mineralization at Hahns Peak (Plate 3). Significant sulfidemineralization, however, occurs onlylocally withinthis structure. The major portion of the breccia cone sheetisnon- mineralized, due to its predominantly phreatic origin.

Anomalous amounts of goldand silverare localizedinnear- surface zones of the breccia cone sheet. Erosion of the upper part 162

of the breccia cone sheet probably provided the goldthat is present

I n all uvi al gravels along thepen meter of Hahns Peak. Gal ena and sphalerite are the most common sulfide minerals at thepresent level oferosion. The increase of molybdenumrelativeto lead, zinc, silver,and gold with depth in DDH-1O1 suggests that thevolcanic complex was zoned with gold and silver at thetop, lead and zincpre- dominant in the middle portion, and molybdenum enrichedprimarily at depth. This apparent zonation should be considered conjectural, as exceptions exist and the present density of geochemicalsamplesis

I nsuffi ci ent to presenta defi ni ti ye model.

Molybdenum mineralizationat Hahns Peak is interpretedto have been deposited by fluids that were derived directlyfrom a cupola of

highly differentiatedfelsic magma. Minor amounts of gold, silver, lead, zinc, and copper may also have been precipitateddirectly from magmatic fluids. It is more likely, however,that these elements were leached from mineralized Precambrian basement rocks by bothmag-

matic fluids and bymeteoric fluids thatwere driven in circulation cells by the heat of the intrusion. Some localized areas of mineralization were probably syngenetic with intrusive breccia. In other areas within the breccia cone sheet,mineralization isprobably related to post-breccia fluids that travelled alongpermeable channelways. Hydrothermal brecciation, mineralization, and alterationat Hahns Peak is interpretedto be related to a porphyry molybdenum system. 163

Trace Element Geochemistry

Rock-chip samples, for thepurpose of trace element analysis,

were taken from 180 locationson the surface of Hahns Peak, and from

28 locations within the 70 andSouthern Crossadits. Bedrock was

sampled from an area of approximately50 square feet around each data

site. Individual fragmentswere broken to under one inchin diam-

eter,and approximately five pounds ofrock were taken from each

site. Surface sample siteswere chosen on the basis of a modified

400 foot grid system (Plate3). Deviations from the grid were made

to ensure that only bedrockwas sampled, and to guarantee that each

rock type was adequatelyrepresented.

Chemical analyses were done on 208 samplesby two commercial

laboratories. Bondar-Clegg Inc., Vancouver, BC, Canada,analyzed 116 samples for Cu, Pb, Zn, Mn,Ag, and Mo. The remaining 92 samples were analyzed for the same suite of elementsby Chemical and Mineral

Services(CMS), Salt Lake City,Utah. The rock chip samples were each crushed to a powder, leachedwith hot aqua regia and analyzed by atomic absorption methods. In addition, the F content of 107 samples was extracted by a basic fusionprocess and analyzed by specific ion methods at the Bondar-Clegglaboratory.

The results of thetraceelementanalyses aresummarizedby lithology in Table 4. This table consists of thecombined CMS and

Bondar-Clegy data; The surface distribution of anomalousvalues of

Ag, Pb, Mn, and Mo is shownin Plate 3. The anomalous zones were determined by Raab (1979), basedon the above-threshold values from Table 4. Trace element concentrations forvariouslithologiesat HahnsPeak. Rock Type S/U Cu Pb Zn Mn Ag Mo F n

mm. 3.0 5.0 4.0 14.0 0.2 <1.0 590.0 Columbine porphyry S max 25.0 575.0 330.0 1370.0 5.2 5.0 1000.0 38 18 * 11.0 37.0 28.0 120.0 0.8 1.0 805.0 mm. 3.0 12.0 2.0 6.0 <0.3 <1.0 140.0 Summit porphyry S max. 45.0 940.0 160.0 300.0 58.0 4.0 950.0 52 27 11.0 99.0 15.0 41.0 2.8 1.0 573.0 mm. 4.0 14.0 10.0 10.0 0.4 <1.0 485.0

Li max. 138.0 940.0 410.0 55.0 16.5 29.0 1050.0 12 5 x 25.0 128.0 102.0 20.0 3.4 4.0 761.0 mm. 5.0 5.0 3.0 10.0 0.2 <1.0 455.0 70 porphyry S max. 17.0 68.0 65.0 380.0 3.4 1.0 1300.0 10 7 10.0 21.0 42.0 218.0 0,7 <1.0 741.0 mm. 5.0 20.0 15.0 14.0 0.4 <1.0 715.0 U max.515.0 3200.0 3300.0 40.0 18.0 17.0 1100.0 4 2 x 252.0 1187.0 1275.0 25.0 6.3 5.0 908.0 mm. 3.0 4.0 4.0 14.0 0.3 <1.0 135.0 Monolithic breccia S max. 13.0 840.0 65.0 116.0 9.0 5.0 465.0 9 4 x 7.0 119.0 13.0 38.0 2.8 2.0 333.0 mm. 14.0 20.0 22.0 10.0 0.5 2.0 465.0 U max.j560.O 7700.0 18000.0 44.0 >100.0 2200.0 5300.0 4 3 x 567.0 2602,0 4696.0 26.0 38.0 562.0 3593.0 Table 4. (Continued)

Rock Type S/I) Cu Pb Zn Mn Ag Mo F n

mm. 3.0 5.0 4.0 14.0 0.3 1.0 210.0 Multilithic breccia S max. 55.0 2000.0 3000.0 3400.0 17.2 7,0 1100.0 26 13 x 16.0 265.0 208.0 237.0 3.3 2.0 577.0 mm. 8.0 9.0 6.0 7.0 0.3 < 1.0 850.0 U sax.26.0 90.0 420.0 150.0 90.0 6.0 2200.Q 7 4 ; 55.0 48.0 103.0 32.0 16.0 2.0 1380.0 sin. 3.0 Stockwork-ve2ned 8.0 1.0 18.0 1.3 1.0 79.0 lithic inclusions S max. 14.0 76.0 30.0 45.0 6.3 100.0 320.0 6 5 x 8.0 30.0 5.0 29.0 3.8 22.0 172.0 Beryl Mountain sin. 18.0 10.0 35.0 126.0 porphyry 0.2 1.0 880.0 max. 35.0 35.0 120.0 460.0 0.5 2.0 950.0 4 2 x 26.0 21.0 80.0 424.0 0.3 1.0 915,0 late-stage sin. 8.0 5.0 16.0 26.0 porphyry dike 0.5 2.0 1300.0 S lax. 93.0 46.0 410.0 2600.0 1.9 4.0 1300.00 3 1 x 49.0 27.0 215.0 1195.0 1.0 3.0 1300.0 U - 20.0 15.0 105.0 250.0 0.7 1.0 - 1 little Mountain -

porphyry S - 9.0 39.0 65.0 440.0 0.2 2.0 - Anderson Mountain 1 - porphyry S - 20.0 28.0 53.0 290.0 0.2 2.0 780 Base Surge 1 1

(Plane bed) S - 6.0 58.0 11.0 310.0 0.2 <1.0 1000.0 1 1 0, Table 4. (Continued)

Rock Type S/U Cu Pb Zn Mn Ag Mo F n tin. 4.0 5.0 2.0 15.0 0.2 .0 630.0 Morrison Formation S lax. 30.0 36.0 104.0 1250.0 8.6 2.0 1500.0 9 4 13.0 14.0 30.0 311.0 1.4 < 1.0 1020.0 tin. 1.0 5.0 2.0 34.0 0.4 < 1.0 41.0 Dakota Sandstone S max. 40.0 56.0 35.0 10.0 10.2 4.0 230.0 10 5 x 11.0 17.0 9.0 50.0 3.0 2.0 111.0 tin. 11.0 10.0 5.0 15.0 0.2 < 1.0 590.0 Mancos Shale S max. 488.0 710.0 600.0 330.0 1.5 12.0 1400.0 7 4 x 99.0 125.0 293.0 164.0 0.8 4.0 890.0 tin. 19.0 10.0 43.0 520.0 0.4 2.0 265.0 Browns Park S max. 50.0 14.01 65.0 850.0 1.3 Formation 3.0 265.0 3 350 1 11.0 51.0 563.0 0.8 2.0 265.0 All values are inppm.

- n - number of samples n number of samples forwhich F was analyzed.

S - surface rock-chipsample U - Underground rock-chip sample from either the 70,or Southern Cross adits. Cu, Pb, Zn, Mn, Ag, Mo were analyzed by atomicabsorption method F was analyzed byspecific ion method

I. 167

log probability plots. Separate log probability plotsof data from eachofthe laboratories displayed onlynegligible variations and justified combining the two data sets to determinethreshold values. The distribution patterns of anomalous values ofPb, Ag, and Mn are strongly controlled by the breccia cone sheet (Plate3). Zoning is not recognized for Cu, Zn, or F. The inner zone of high Movalues (Plate 3) is primarily due to a single sample (100.0ppm) from an inclusion of stockwork-veined porphyry. Two samples of Summitpor- phyry within this zone have values of 4.0 ppm Mo, thethreshold value determined by Raab (1979). This was an arbitrarydetermination, as

40 percent of thesamples analyzedwere below the detection limit for Mo.

A single sample of Monolithic pebble breccia located1,400 feet from the portal of the 70 adit(Figure 12)produced the highest values of Cu, Pb, Zn, and Ag that were determinedin this study (Table 4). Veinlets and disseminatedmasses of galena and sphalerite are present in this pebble breccia, as wellas in wallrock of 70 porphyry up to 50 feet from the breccia contact. Anomalous concentrations of base and precious metals occur in 70porphyry only where it is in proximity to mineralized intrusivebreccia. The intrusive

breccias themselves haveboth the highest, andsome of the lowest, trace element contents of the Hahns Peak lithologicunits. This is interpreted to be an indication that: (a) magmatically exsolved metal-rich fluids were diluted by mixing with groundwater in the formation of intrusive breccia; and (b) the intrusivebreccias provided localized channeiways for heated ground water thatcontained dissolved metals that were probably leached from Precambrianbasement rocks.

Molybdenum values at Hahns Peak range from 2,200.0ppm,in a sample of Monolithic pebble breccia in the 70 adit(Table 4), to less than 1.0 ppm (detection limit). With the exception of theinclusion of stockwork-veined porphyry (100 ppm Mo), thehighest value of Mo measured in surface rock was 4.0 ppm, from an outcrop ofSummit porphyry near the center of the intrusive complex. The Mo content of surface rocks is extremely low, with 56percent of the samples from the main phase porphyry units falling below the 1.0ppm detection limit. Fluorine values range from 5,300.0 ppm in the Mo-richpebble breccia from the 7D adit, to 41.0 ppm ina sample of Dakota Sandstone

(Table 4). In general, samples that have a relative enrichmentin Mo also have a relative enrichment in F. The average F value forthe main phase porphyries is 695.0 ppm. This represents a slight enrichment over the suggested average granitic value of 520.0ppm (Turekian and Wedepohi,1961), and presumably reflectsan introduction of F during the main stage of alteration. Apparently, F was not introduced during the early stage of alteration,as indicated by the low (320.0 ppm) content of the stockwork-veinedlithic inclusions. Manganese values range from 3,400.0 ppm ina sample of Multi- lithic breccia, to 6.0 ppm in a sample ofSummit porphyry. The average value of the main phase porphyry units is 78.0ppm. The Beryl Mountain, Little Mountain, and AndersonMountain porphyry unitsare present in the outer zone of propylitic alteration anddisplay a relative enrichmentin manganese, averaging404.0 ppm. However, 169

these rocks are actually depletedin Mn relative to normal granitic

crustal rocks whichaverage 540.0 ppm (Turekian and Wedepohi, 1961).

This widespread depletionof Mn may bea result of early albitic

alteration. Later brecciation and hydrothermal activity brought

about more intense leachingof the inner part of the volcanic complex

and led to theconcentration ofMn infringe areasoutside the breccia cone sheet.

The gross vertical zoning ofMo at depth and precious metals at

the top of the brecciacone sheet is indicated by four additional

samples that were collected andanalyzed by W. A. Bowes in 1979. One ofthese sampleswasfrom Multilithicbreccia onthe west side of Hahns Peak which displayed advanced argillic alteration and

contained 24.0 ppm Au(0.7 oz/ton) and greater than 2,000ppm Ag

(>58.0 oz/ton). The matrix of this brecciawas composed of opaline

and chalcedonic silica, ãlunite,dickite, and disseminated pyrite and

tetrahedrite. Three other sampleswere taken from veinlets of Mono-

lithic breccia within theinterval 3,500-3,550 ft. in diamond drill hole DDH-1O1. These samples average 733.3ppm Mo. Fine masses of molybdenite were observed ina polished section of one of these samples. The Mo analysesare semi-quantitative and were done on a

Jarrel-Ash arc emission spectograph. The Au and Ag analyseswere done by atomic absorptionmethods. All analyses were performed at

Specomp Services, Inc., SteamboatSprings, Colorado. 170

Sulfide Mineralization

The sulfide mineralization at Hahns Peak is confined toan area within and around the breccia cone sheet, averaging 1.5percent total sulfides with localized zones above five percent (Fig.22). Pyrite

is the mostabundant sulfide mineril, with its greatest concen-

trations occurring withinzones ofphyllic alteration. Variable amounts of sphalerite, galena, chalcopyrite, and molybdeniteare also present. Tetrahedrite is present locally inminor amounts, and traces of covellite were also observed in surface rockson the west side of Hahns Peak.

Occurrence and Distribution

Sulfide mineralsmay occur as: (1) disseminations in non-brec-

ciated rock;(2) fine dissecninationswithin the matrix of intrusive breccias; (3) open-space vug fillings; and (4) thin veins adjacentto

zones of intrusive breccia. Individual sulfide grainsmay vary from

subhedral to euhedral, andcommonly range in diameter from 0.2mm to

1.5 cm (average approximately1.0 mm).

Pyrite is most common as disseminatedeuhedral to subhedral grains in both non-brecciatedrockand in the matrix of intrusive breccia. It is also abundantas coatings of open vugs and thin veinlets of pyrite may be present near zones of intrusivebreccia. A two foot thick vein of pyrite, with lesser amounts of galena, was observed to cut hornfelsed Morrison Formationshale in the lower portion of DDH-1O1. Sphaierite may occur throughoutaltered quartz latite porphyryas euhedral to massive disseminationsup to 1.5 cm in S

''5S

5' 5 S ii 1 _ / 'S'J t 4 A

I ,, / TOTAL SULFIDE '.. / ) 1 \i MiNERALIZATION ' 8 1 p .7 PEAK f;5.'

'St \

I i V

% /. ' / I / 8 I / / t\_ + I L.i e n d S S -/

1-2% total sulfide f , --

a S

j4' I S __+ 3-5% total sulfide

feet 0 400 800

Figure 22. Distribution of total sulfide mineralizationat Hahns Peak in relation to the brecciacone sheet (heavy dashed line). 172

diameter and as open-space fillings locally in intrusivebreccia along with quartz, pyrite, and galena. Multilithic breccia present on the Price Tunnel dump contains up to five percentsphalerite as open-space fillings. Minor amounts of sphaleriteare also present in veins containing pyrite, galena, and tetrahedrite. Galena is most common in veins, but may also occur as disseminated masses andas vug fillings. Tetrahedrite is present inminor amountsas fine (0.5- 2.0 mm) euhedralto subhedral grainslocally within the matrixof Multilithic breccia, in cavities in quartz latiteporphyry, andas intergrawthsinveins ofgalena,pyrite,and sphalerite. Chalco- pyrite is rare, but may be intergrown with othersulfides in veins and within the matrix of intrusive breccias. Molybdenite occursas fine (<0.5 mm)flecks both locally in the matrix of Monolithic breccia and in stockwork veinlets of quartz thatare restricted to porphyritic inclusions in Summit porphyry. Covellite is extremely rare, although it may occur as fine blades fillingcavities in quartz latite porphyry adjacent to the vent complexon the west side of the peak.

The only significant gold mineralization detected in thisstudy was from an outcrop of Muitilithic breccia on the inner marginof the breccia cone sheet aproximately 250 feet eastof the vent complex. The Multilithic breccia at this location containseuhedral crystals of pyrite and tetrahedrite (aoproxirnately 1 mm diameter)distributed throughout a matrix of opaline and chalcedonic silica,alunite, and dickite. Although free goldwas not visible, a sample of this rock contained 24.0 ppm (0.7 oz/ton) gold (atomicabsorption analysis) 173

and greaterthan 2,003ppm (58.0oz/ton) silver (spectrographic analysis). The gold is assumed to be present inauriferous pyrite and the silVer is probably contained intetrahedrite. In addition, one 13,000 pound load of galena-richore that averaged 2.0 oz/ton

gold and 52.0oz/ton silver is reported to have been shipped fromthe Tom Thumb Mine (Gale, 1906), located within theMultilithic breccia approximately 800 feet southwest ofthe gold-richareadescribed above. Thesegoldoccurrences within the upperportion of the breccia cone sheet suggest that thisstructure may have beenthe source of the free gold that occurs inplacer gravels surrounding Hahns Peak.

The surface distribution of total sulfide mineralization is shown in Figure 22. The area enclosed by rockwith a total sulfide content greater than one percent strongly resemblesthe area bounded by the outer margin of the brecciacone sheet. A similar pattern is formed by thezone of mixed phyllic-argillic alteration displayed in Plate 3. These two sets of data suggest that thebreccia cone sheet controlled the emplacement of hydrothermal fluidsthat produced the sulfide mineralization. The majority of the sulfide mineralsare interpreted to have been deposited from post-brecciahydrothermal fluids that permeated open-space areas,and only minoramountsof localized sulfides are thought to have formedsyngeaetically with the intrusive breccia. The later influx of hydrothermal fluids,however, failed to permeate the entire breccia cone sheet,with portions of it displaying only traces ofsulfides. 174

Paragenesi S

Theparageneticsequence of mineralization at HahnsPeakis shown in Figure 23. This sequence of mineralizationwas determined primarily by detailed analysis of four polishedsections made from samples of sulfide-bearing rock from diamond drillcore and underground workings. Additionalobservations of parageneticrelation- ships were made from inspection by binocularmicroscope on numerous

samples from both surfaceand subsurfaceareas.

Sulfide mineralization at Hahns Peak occurred intwo distinct episodes that correspond to the Early and Main stagesof alteration. The Early stage of alteration and mineralizationis evidenced bypor- phyritic fragments with stockwork veinletsofsilica, pyrite,and molybdenite that occur aslithic inclusions in Summitporphyry and Multi lithicbreccia. Sulfide mineralizationrelated tothe Main stage of alteration was introduced in three substagesof hydrothermal activity that occurred before, during, and after theemplacement of intrusive breccia. The early albitizationphase of the Main stage of

alteration produced only minor amounts of disseminated pyrite. Following this episode of mineralization, relatively minoramounts of pyrite, niolybdenite, sphalerite, galeria, andchalcopyrite were introduced syngenetically with the emplacement ofintrusive breccia. The final post-breccia phase of the Main stage ofalteration produced the bulk of the sulfide mineralization at Hahns Peak withpyrite, sphalerite, galena, chalcopyrite, tetrahedrite, and covelliteoccurring as open-space fillings in and aroundintrusive breccias. 175 MINERAL TIME >

QUARTZ ______------

PYRITE --- -

MOLYBDENITE ----

SPHALER tIE -- _____ -----

CHALCOPYRITE

GALENA --- -

SIDERITE

GOLD

TETRAHEDRITE

COVELLITE -

VAR ISCITE

MAIN STt3E OF ALTERATION

EARLY PROPYLITIC, ARGILLIC, SERICITIC, STAGE OF SUPERGENE ADVANCED ARGILLIC ALBITIC ALTERATIOIALTERATION ALTERATIO

MONOLITHICMULTILITHIC POST 8RECCLA 8RECCIA BRECCIA

Figure 23. Paragenetic sequence of mineralization at -IahnsPeak. 176

The presence of micron-sizegrainsof pyriteand molybdenite

within the clay-rich matrix ofmonolithic pebble breccias suggests

that these mineralswere precipitated at the time of formation of the

intrusive brecias. It is unlikely that pyrite and molybdenitewere

introduced after the formation of the pebble breccias duetothe

impermeable natureof the clay-rich matrix. In addition, minor

amounts of sphalerite, galena, andchalcopyrite occur as veinlets and

disseminations in wall rock adjacentto pebble breccias. These min-

erals are also interpreted to besyngenetic with formation of Mono-

lithic breccia. The disseminated sulfide minerals thatoccur in wall

rock adjacent to pebble brecciasmay be anhedral to euhedral in habit

and may range in size from 0.5mm to 15.0 mm. Distinctive dodec-

ahedrons of sphalerite,up to 15.0 mm in diameter, were observed to

be disseminated in 7D porphyry1,485 feet inside the portal of the 7D

adit. In the same area, veinlets ofgalena,pyrite,and chalcopyrite, with traces of sphalerite,may reach up to 50 mm in width. A polished sectionof oneof these veinletsdisplayshypautomorphic granular texture with galena replacingchalcopyrite and chalcopyrite replacing pyrite. Isolated, irregularly shaped pockets of pyriteand clay (up to one foot in diameter)are present at various locations beyond 1,300 feet from the portalof the 7D adit. These sulfide-rich pocketsare assumedtobe associated with the Monolithic breccia event, although an exact relationship hasnot been established.

Monolithic breccia displays strong sulfidemineralization only where pebble breccias are present,particularly within the southeast segment of the breccia cone sheet in andnear the 70 adit. Other 177

surface and underground exposures of the Monolithic brecciadisplay only trace amounts of pyrite. This suggests that significantamounts of sulfides occurred only in areas where ventingled to the formation of pebble breccias.

The later Multilithic breccia phase is interpretedto have been primarily a phreatic event in which magma-derivedhydrothermal fluids were mixed with ground water. The explosive nature of thisevent and the dilution effect of ground water caused only minoramounts of very

localized sulfide mineralizationto have precipitated syngeneticafly with the formation of Multilithic breccia. However, in an areaon the west flank of Hahns Peak where focused dischargesappear to have vented, auriferous pyrite and argentiferous tetrahedriteare present as fine (0.5 mm to 2.0 mm) subhedral grains in the matrixof Multi- lithic pebble breccia. These minerals are thoughtto have precipi-

tated from fluids thatformed the pebble breccia. A more passive phase of sulfide mineralizationthat occurred after the last known brecciation event is indicatedby the presence of euhedral sphalerite,galena, and quartz as open-space fillings locally in Multilithic breccia. Examples of this were observedin breccia located on both the northeast and southwestflanks of Hahns Peak. Dump rock of Multilithicbreccia from the Price Tunnel (southwest flank Hahns Peak, Plate 1) contains three to tenvolume percent sulfi des as i ntersti ti al fi lii nys. Sphal en te andgal ena occur as euhedral to subhedral crystals 1.0 to 3.0mm in diameter. Galena is consistently observed to replace sphalerite which isthe dominant mineral by a 2:1 ratio. Althoughthe majority of thesulfide 178

mineralizationrelated to the post-brecciaphase of hydrothermal

activity is restricted to Multilithic breccia, thin veins of sulfides

(1.0 uiiito 10.0 cm wide) and minor amounts of disseminated sulfides

(0.5 to 2.0 mm diameter) occur locally within quartz latite porphyry

adjacent to breccia. The post-breccia phase of mineralization is

probably responsible for the bulk of the lead and zinc at Hahns Peak.

Thebrecciatedrockpresumably acted as a channelway for metal-

bearing epithermal fluids.

The presence of variscite (Al(PO4).2H20) as botryoidal, open-

space coatings of flineralized breccia suggests thatit is a late,

supergerie mineral. Variscite is most abundant in the strongly miner-

alized zones of the 70 adit.

Origin of Hydrothermal Fluids and Controls on Deposition

Hahns Peak is interpreted tobe the very topof a porphyry molybdenum system. This interpretationis based primarily on the presence, within both a pebble breccia and the Summit porphyry intrusive phase, of igneousfragmentsthat display stockwork veins of quartz, pyrite, and molybdenite. The mineralized fragment contained withinpebble breccia is particularly significant in that similar occurrencesare reportedfrom such major molybdenum deposits as:

Henderson and Climax (White and others, 1981); Mt. Emmons and Redwell

Basin (Thomas and Galey, 1982); Pine Grove (D. Munter, personal corn- munication, 1981); and Questa (J. Meyer, personal communication,

1981). The presence of post-mineralpebble breccias is commonin Climax-type (granite)molybdenum systemsand suggests that violent

interactions between meteoricwater and magma (or magmatically- derived fluids) may have occurred in the late stages ofhydrothermal activity. Although meteoric watermay beinvolved in late-stage brecciationand alteration, it is now generallyassumedthatthe hydrothermal fluids responsible for mineralization in typicalstock- work molybdenum deposits are directly derived from highlydiffer- entiated felsic rnagmas (White and others, 1981; Mutschler andothers,

1981; Westra and Keith,1981).

The parent magmas from which the Climax-type molybdenumdeposits of North America were derived are considered by severalauthors to be related tothe subduction of aneast-dipping slab (or slabs) of

oceaniclithospehere in mid to late Tertiary time (Guild, 1978; Sillitoe, 1980; Lipman, 1981; Bookstrom, 1981; Westra and Keith,

1981;see Regional Tectonics). However, the exact relationship of the proposed slab to magma generation remains unclear. It is possible that the primary role of the slab was to provide volatilesthat

initiated partial melting of overlying mantle material (Fyfe and

McBirney, 1975; Burnham,1979). In this case, the less-dense deriv- atives of the partially melted mantle would have risencliapirically into and through the continental crust of North America. If the relative position of the slaband the continent were stationaryfor a period of time, the upwelled magma would have created a riftzone above the slab. The Rio Grande Rift inColorado and New Mexico is interpreted to have formedin this manner, as a zone of back-arc 180

extension within the continental crust (Lipnian, 1981; 800kstrom, 1981).

Climax-type molybdenum deposits tend to be alignedalong the Rio Grande Rift; in particular where the rift intersectsdeep-seated Pre- cambrian shearzones (see Fig. 5). Similarly, Hahns Peak islocated near the intersection of two Precambrian structures (theMullen Creek-Nash Fork shear zone and the Uinta Mt. Group graben)with the suggested northern extension of the Rio Grande Riftsystem (Tweto, 1978; Decker and others, 1980).. The batholith that isindicated to lie northwest of Hahns Peak (see RegionalGeophysics) is interpreted to have been generated by subductionprocesses and is believed to be one of the intrusions of middle to late Tertiary age thatare responsible for the formation of the Rio Grande RiftSystem. tts emplacement in the continental crust is believed to havebeen controlled by one or both of the nearby Precambrian structures. The west-trending Elkhead volcanic field may reflecta seriesof cupolas thatwere emplaced along the trend of the Uinta Mountain Groupgraben on the southeast flank of the proposed batholith. The upward push ofmagma that formed one or more of these cupolasmay have created the King

Solomon-Grouse Mountain horstblock within which HahnsPeak is centered (Fig. 3). In addition, the steeplydipping normal faults that appear to intersect beneath Hahns Peak (see Fig. 3 andStructure) may have been caused by verticalforces related toan upwardly driven body of magma. The continued injectionof this magma along the line of intersection of these two faults probablyled to the emplacement of the Hahns Peak laccolith. This same fault intersectionprobably 181

also controlledthe emplacement of a late-stage felsic stock,the presence of which is inferred from aplite dikes and intrusive breccias.

In the process of magma evolution, melts that producedporphyry molybdenum deposits probably derived H20 andCO2 in unknown proportions from both the subducted plate and themantle wedge. As these melts travelled upward through the mantleand into the lower crust, they would have also carried with them largequantities ofheat.

This heat is likely to have caused partialmelting ofgranulite fades rocks at and near the crust-mantle boundary. Recent studies of Sr, Nd, Pb, and 0 isotopic compositionsof Tertiary-aged felsic

plutons related to porphyry molybdenum deposits in Nevada,Utah, Arizona, and Colorado suggest that these intrusionswere derived primarily by partial melting of lower crustalrocks (Simmons and Hedge, 1978; White and others, 1981; Farmer and DePaulo,1982;Stein and Hannah, 1982). Isotopic analyses of leadfrom feldspar in Hahns Peak porphyry (Pb206/Pb204 17.31; Antweiler and others, 1972) give values that are nearly identical tothose obtained by Stein and Hannah (1982; Pb206/Pb204 = 17.33) from feldspar in the Mt. Eniiions stock. These values indicate that the feldspar leadwas derived from the lower crust at both locations and thatthe assimilation of Pre- cambrian basement rock was importantinthe evolution ofthe two parent magrnas. It is possible that other elements suchas gold, silver, molybdenum, and fluorine were also derived from the lowercrust. The presence of gold and silver in Precambrian-agedveins that cut granite, gneiss, and schist in the Park Rangeten miles east of Hahns 182

Peak (Antweiler and others, 1972) suggests thata basement source for

these elements is present in thisregion. A similar source for

molybdenum and fluorine may be inferred from the presence of molyb-

denite and fluorite in a 1.7 m.y. old pegmatite (K/Ar date,

Segerstrom and Young, 1972) located on Farwell Mountain, five miles

southeast of Hahns Peak. In addition, primary fluorite is present in

the 1.4 m.y. old Mount Ethelpluton located 15 miles southeast of

Hahns Peak(Snyder,1978). The fluorite presentin economic vein

deposits in the North Park area is likely to have been remobilized

from the Mount Ethel pluton in Tertiary time.

The presence of anomalous concentrations of molybdenum in rocks

from Colorado that range in age from 1.7 b.y. to 5.0 rn.y.suggests

that a molybdenum source, probably in the lower crust, has underlain

this region since early Proterozoic time (Bookstrom, 1981; Casaceli,

unpublisheddata). The close correlation of porphyry molybdenum

deposits with areas of thickest crust in North America (as determined by Bouger anomalies) is also suggestive of a lower crustal source for molybdenum (Hollister, 1978). This correlation may be a result of both the composition of basement rocks and the high residence time that a magma would have in rising through a thick crustal sequence..

High residence time would permita high degree of magmatic differentiation and more efficient partitioning oflarge radius elements suchas Mo,Sn, W,and Finto residual aqueous phases rather than into silicate crystal lattices.

Differentiation within a batholith is thought to occur through a process of convection-driven thermogravitational diffusion (Hildreth, 183

1979). Thisprocess involvesionicdiffusion,complexation, and wall-rock exchange andis linked to chemicalgradientsaswellas thermaland gravitational fieldswithin the magma chamber. If differentiation is complete,a volatile-rich, high-silicazone is developed at the top of thebatholith.This upper zone would be enriched in the lithophile elementsLi, Be, F, Cl, Mn, Rb, Y, Nb,Na, Mo, Sn, W, and U and depleted in Sr, Mg, Ba, and Eu (Hildreth,1979).

in the genetic modelproposed by Mutschler and others(1981),

stockwork molybdenite depositsarebelievedtobedeveloped over

cylindrical cupolas thatextend two to three kilometersabove zoned batholiths. Such cupolas are thoughtto have formed primarily from the lithophileelement-enriched, high-silica rhyolitemelt at the top of the batholith.Once emplaced, differentiationis thought to continue within the cupola bythe process of thermogravitationaldif- fusion, further concentratingthe volatiles and lithophile elements in the roof area.The resultant high fluidpressure in the roof zone would lead to faulting inrocks above the cupola, possiblycausing a seriesof ash floweruptions. Typically,Climax-typestockwork molybdenum systems displayevidence of multiple intrusions,many of which vented aseither ash-flow eruptionsor breccia pipes. This multiplicity of intrusionsmay reflect surges of magma thatwere fun- nelled from a batholithicreservoir upward intoa cylindrical cupola. In some cases, such as Pine Grove, Utah, a single tuff eruptionmay be followed by rhyoliteporphyrythatformed a quenched plug (Mutschler and others, 1981).In other cases, suchas the Red Lady stock at Mount Ernrnons,Colorado, upwelledmagma may have failed to 184

develop faults in overlying countryrocks,or failed to encounter pre-existing faults, and venting did notoccur (Thomas and Galey, 1982). In one or more of the intrusive phases in a stockworkmolyb- denum system, a vapor-saturated cap will format the roof zone of the cupola where thermogravitationaldiffusionprocesses haveconcen- trated volatile elements (Mutschler and others, 1981). Such volatiles will exsolve as a true vapor phase whena critical pressure, dependent upon water content, temperature, and bulkcomposition of the magma, is reached (Whitney,1975). When fluid pressures exceed

confining pressures, intense hydrofracturing will occur within country rock above the mineralizing intrusion,and ore fluids will fill the stockwork openings, precipitating quartz, molybdenite, fluorite, and pyrite (Mutschler and others, 1981). Often,as with the Primos intrusion at Henderson, a chilled marginof rhyolite will form before a substantial aqueous phase has a accumulated. The subsequent accumulation of fluidsand build-up of hydraulicoverpres- sures will cause hydrofracturing in the chilled margin of rhyoliteas well as in the confining country rock. The resulting ore shell will then encompass both the upper border zone of the mineralizingintru- sive and the surrounding country rock (White and others,1981). The precipitation ofquartz andrnolybdenite in stockwork veins will decrease the permeability ofthe fractured rock, allowfluid pres- suresto build up again, and permit theprocessto repeatitself

(Mutschlerandothers, 1981). In this way multiple episodes of molybdenite injection may overlap. Such overlap may also beformed bytherelease of aqueous fluids from severalisolatedareasof 185

concentration within a single phase of one intrusion,or by multiple intrusions reaching the same level of emplacement at slightlydif-

ferent times (Whiteand others, 1981).

The abundance of F-bearing mineralsin Climax-type molybdenum deposits may reflect a fundamentalrole ofF in the evolution of

hydrothermal fluids that formedthesesystems. The presence of accessory fluorite (White and others, 1981), high-F topaz(Burt and others, 1982), and F-rich biotite (Gunow and others,1980) in Climax- type systems indicate that the parent rnagmaswere anomalous in F content. It has been suggested that F complexes are essentialin traris-

porting Moin theformation of stockworkdepositsof molybdenite (Lamarre and Hodder, 1978;Mutschler and others,1981). However, recent experimental studies have demonstratedthat the presence of F does not affect the partitioning of Mo betweenmagma and hydrothermal fluids, and although F complexes are capable oftransporting Mo they are of minor importance in the developmentof a stockwork deposit of molybdenite (Candela and Holland,1981, 1982; Tingle and Fenn, 1982). Fluorine does havea fundamental role in the development of molybdenum deposits, however, in that itspresence in a magma increases the amount of water that can be held insolution (Koster Van Groos and Wyllie, 1968), which in turn controlsthe timing of the evolution of the vapor phase (Tingle and Fenn, 1982). Because Mo is more efficiently partitioned into the vapor phase than theaqueous phase(Candelaand Holland, 1982;Tingleand Fenn,1982; Eastoe, 1982), the development of a vapor plume over'a cupola of enriched rhyolite may be essential in the development ofstockwork molybdenum deposits. The initial water content of the parent magma,as dependent upon the initial F content, may then bea primary control ofore deposition. Molybdenum is believed to betransported in thevapor phase by hydroxy-oxides (e.g. M07013(OH)5) anddeposited in stockwork veins above the cupola upon a decrease in temperature andpressure (Henley and McNabb, 1978; Eastoe, 1982). It is likely then that Mo and F occur together in hydrothermal fluidsnot as a result of chemical complexing, but because theyare incompatible elements in the crystal lattices of most major rock-forming minerals. Although F may be present in significant amountsin biotite, muscovite, topaz, garnet, and hornblende, for the most part it ispartitioned into the aqueousphaseinthe process of magma crystallization (Gunowand others, 1980).

In contrast, the transport mechanism of Cu in porphyryCu systems appears to be closely linked to Cl complexes,as the parti- tioningof Cu between magma andhydrothermal fluids isstrongly dependent upon the Cl content ofthe melt(Candela and Holland, 1982). As with F, Cl has little or no affecton the partitioning of Mo. However, both these halogens(Cl in particular)are important

inporphyry Cu/Mo systemsin thatthey willcomplex withK, as evidenced by the pervasive potassic alteration typicallypresent in

root zones (Mutschler andothers, 1981).

The intrusive complex at Hahns Peak resemblesknown porphyry

molybdenum systems in that: (a) a batholithicsourcehas been inferred by geophysical techniques; (b) structural control on emplacement of intrusions have been determined fromfield mapping; 187

(c) field and petrographic data suggest that magmaticdifferentiation occurred; and (d) the alteration and mineralization relatedto hydrothermal activity and brecciation is consistent with thatof typical porphyry molybdenum systems. However, F values at Hahns Peak,

although sporadically highin some breccias(up to 5,300 ppm) and only slightly anomalous in the quartz latite porphyry (average783 ppm), are low compared to values from typicalClimax-type systems (2,000 to 10,000 ppm F; Thomas and Galey, 1982). In addition, there is no evidence to suggest that rhylolitictuffs were extruded at

Hahns Peak. The only venting that isevidenced is that of steam-

blast eruptions relatedto the emplacement of the Multilithicbreccia

phase. The absence of rhylolitictuffs, as well as the relatively low F values are both believed to be related to theextreme depth of

the hidden felsic intrusionthat acted as a source for hydrothermal fluids. The breccia-related ventcomplex and the associatedpyro- clastic flows are believedto be unique in theiroccurrence at Hahns Peak. They are, however, similar to hypothetical deposits thatare thought to have formed above many porphyry molybdenum systems (Mutschler and others, 1981).

Molybdenum deposition atHahns Peak probably occurred in three distinct phases. The earliest phase is evidencedby the presence of molybdenum-bearing stockwork-veinedfragments in both the Summitpor- phyry and Multilithic brecciaintrusive units. The second phase of mineralization is evidenced byanomalous concenrations of molybdenite locally within the matrix ofthe Monolithic breccia. The third phase of molybdenum deposition ispostulated to be present at depth beneath the breccia cone sheet, and to occuras typical stockwork veins of quartz and molybdenite. The presence of preciousand base metal mineralization within andaround the breccia cone sheet reflects both explosive and passive interactions of magmatically-derivedfluids and meteoric water.

The occurrence of inter-breccia felsic dikeletssuggest that the brecciation and hydrothermal activityat Hahns Peak is directly related to a felsic intrusion at depth. Although such a pluton has not been intersected by core drilling, itis assumed to have been emplaced along the intersection of two normalfaults. rt is also

assumed that avolatile-rich carapice containing Si02, H20,CO2, HCI, HF, K, Na, Mo, and other transition metalswas developed along its upper margins. Vertically directedpressure from this rising cyl- inder of magma would have formed conicalfractures in the overlying country rock. Subsequent boiling ofthe volatile-rich, supercritical

fluids in thecap zone would have caused hydrothermal fluids toper- meate through Precambrian basement rocks and ultimatelyform monolithic breccias along some of the conical fractures. Lead,zinc, copper, silver, and minor gold were probably leachedfrom Precambrian rocks by these fluids and deposited locallywithin and around dike-

lets of Monolithicbreccia.

A later more explosive episode of brecciation isevidenced by the Multilithic breccia phase which forms themajor portion of the conesheet. This episode ofbrecciationis interpretedto have formedby steam-blast activity whenthesuper-heated carapice of volatile-rich fluids encountered ground water thatwas held in the 189 previously formed conical fractures. The absence of glassshards throughout the breccia cone sheet suggests that groundwater flashed tosteam upon contact withthe super-heated,rnagrnatically-derived fluid, rather than upon contact with truemagma. However, the presence of glass shards in an isolated sample of Multilithicbreccia taken from aprospect pit outside the northernperimeter ofthe breccia cone sheet (see Plate 1) suggests thatground water may have come in contact withmagma at one location. The Multilithicbreccia phase wasformed primarily by steam explosions of meteoricwater and for the mostpart it is devoid of mineralization. Locally, however, thesudden pressure dropcreated by steam venting ofmeteoric fluids probablypermitted boiling of underlying magmaticfluids.Thus, metals contained inthe volatile- rich carapice above the cupola ofmagma may have been transportedin the vapor phase andlocally introduced intothe breccia cone sheet. Manganosiderite, replacingphenocrysts of feldsparnear the vent complex on the west flank of Hahns Peak,was probably formed by the early discharge of CO2 from maginaticfluidsandthe vapor phase transport of Mn (probably as a chloride complex).Additional CO2may have been added to hydrothermal fluidsby the brecciation ofMancos Shale and Browns Park Formation. A dikelet of Multilithicbreccia that cuts siderite-altered porphyry adjacent to thevent complex contains opaline andchalcedonicsilica,alunite,dickite,pyrite, tetrahedrite,and traces of covellite inthe breccia matrix. A sample of this breccia contained 24 ppm Au and 2,000ppm Ag. Euhe- dral crystals (2.0 to 10.0 nun diameter) oforthoclase occur asvug 190

fillings inaltered porphyry adjacent to this breccia. A vapor-

dominated magmatic discharge isinterpreted to have transported Au,

Ag, and K as chloride complexes. Silica was probably transported in

a coexisting liquid phase.

Solubility experiments have demonstrated the importance of bisulfide complexes in transportingAu (e.g. Au(HS)) in near-

neutral, reducing solutionsupto 300°C(Seward, 1973;Romberger,

1982). However,in low pH,oxidizing solutions above 400°C, with

appropriately elevated CY concentrations,chloride complexes are the

most important mechanism fortransporting Au (Golevaandothers,

1970; Seward, 1973; Romberger,1982). Chloride complexing is particularly prevalent in fluids that are directmagmatic emanations

(White, 1957; Goleva andothers, 1970). Such emanations are inter-

preted to have occurred in thevicinity of the vent complex at Hahns

Peak. The earliest of these fluidsdeposited orthoclase in open vugs, indicatinginitial temperatures above500°C (MacKenzie and Smith, 1961). Subsequent, butcloselyrelated,fluids deposited opaline and chalcedonic silica, alunite, auriferous pyrite, and tetrahedrite. This assemblage suggestsan oxidizing, low pH solution

(Romberger, 1982). The inferred physicaland chemicalcharacter- istics of these two fluid phasesindicate an environment in which chloride complexes would be stable(Romberger, 1982). Bisulfide complexes may have playeda minor role locally in the transport of Au andAginfluids with lower temperatures andhigher HS and S activity, asindicatedby trace occurrences ofcovellite. Such fluids probably deposited auriferous pyrite (2.0oz/tonAu) and 191

argentiferous galena and tetrahedrite (52.0oz/ton Ag) with amethyst-

rich Multilithic breccia at the Tom Thumb Mine located 500 ft.south of the vent complex (see Plate 1;assay values from Gale, 1906). Thismineral assemblage is typical of a Ag-dominatedepithermal deposit with solutiontemperaturesapproximately 200-300°C (White, 1981). The soutions responsible formineralization at the Tom Thumb

are thought to have been magmaticemanations that were less violently

emplaced than those in whichchloride complexes were stable. This less violent emplacement permitted a more complete exchange ofheat 4. and H ions into the walirock, thus creatinga stable environment for

bisulfide complexes.

The rapid upward transportof Au and Ag, primarilyas chloride complexes in the vapor phase, is interpreted to have emplacedthese

elements at surficial levelsin the breccia cone sheet. A crude zonation occurred, with Au atthe top and Ag beneath it. The elements Mo, Cu, Pb, and Zn are likely to have been deposited primarilynear the bottoni of the brecciacone sheet due to the sudden increase in pH and drop in pressureupon the discharge of H2S and CO2 coincident with explosive boiling of the volatile-rich magmatic carapice (Weissberg, 1969). Although the exactphysical-chemical inter- relationships are not known,mineral deposition at any level in the hydrothermal system atHahnsPeakmusthave been a responseto changes in T, P, pH, Eh, andtotal S concentration.

Gold that is thought to havebeen concentrated in theupper part of the breccia cone sheet is a likely source for that whichis contained in placers along the baseof Hahns Peak. The present level of 192

erosion on the peak appears to be coincident withthe Au-Ag transition zone.

Late-stage open space fillings of sphalerite,galena, pyrite, chalcopyrite, and silica occur locally within Multilithicbreccia. This mineralization is interpreted tohave been deposited frompre-

dominantly meteoricfluids thatwere driven in passive circulation

cells above the coolingpluton. Analyses of Pb isotopiccompositions from galena in breccia vug fillingsindicate a strong Precambrian lead component (Antweiler and others, 1972). This implies that the slowly circulating meteoric fluids effectivelyleached metals from

Precambrian basementrocks. In comparison, Pb frominclusions of galena in gold specimens from the RoyalFlush Mine,the Tom Thumb Mine, and the placer deposits is much lessradiogenic, suggesting less contaminationfrom basement rocks. This is consistent withthe hypothesis of gold mineralization havingbeen rapidlyemplaced, allowing less time for hydrothermal fluids to bein contact with the Precambrian source rocks.

Mineral Potential andRecorrnendations

Themineralization at Hahns Peak is thought to have been depositedfromhydrothermal fluids thatwererelated to apor- phyry molybdenum system. An explosive interactionof magmatic and

meteoric fluids led to the formation ofa cone sheet ofintrusive breccia. Gold and silver mineralization is concentratedin the upper portionof this structure,whereas lead, zinc, copper, and 193

molybdenum are distributed sporadically throughout it. In addition, fragments of porphyry that contain stockwork veins ofquartz, pyrite, and molybdenite are present in both the brecciacone sheet and in Summit porphyry. These fragments suggestthe presence of a stockwork molybdenite deposit at depth beneath Hahns Peak. A later episode of stockwork molybdenite mtheral%zation may have formed beneaththe breccia cone sheet. However, the breccia-relatedpyroclastic rocks that are uniquelypreserved at the surfacesuggest asignificant

depth to the hypothethicalstockwork target.

Molybdenum

Molybdenum is considered to be the conoditywith the greatest potential at Hahns Peak. Itis possible that threedistinct deposits of molybdeniteare present. The first is an early stockworkdeposit ofquartz and molybdenite, as evidenced byinclusions in later intrusive rocks. The full extent of this mineralization isnot known, nor has it been determinedhow much of thedeposit was

destroyed by the laterintrusions.

The presence of minute grains of molybdenite locallywithin the matrix of Monolithic breccia is evidence thata second type of molybdenum deposit, a mineralized breccia,may be present at Hahns Peak. This fine-grairied molybdenite is interpretedto have been deposited directly from magmatic fluids and to have formedsyngenetically with the breccja that hosts it. If Monolithic brecciawas actually formed by explosive discharges of CO2 andH25 from magmatic fluids,then the accompanying drop in pressure and elevation in pH of theresidual 194

solution is likely to have causedthe precipitation of sulfide min-

erals near the base of the brecciacone sheet. This breccia-related

deposit is considered to besimilar to the molybdenum brecciapipes located at Redwell Basin, Colorado (Sharp, 1978) andBoss Mountain,

British Columbia (Soregaroli,1975). The Boss Mountain deposit has

particularly striking similaritiesto Hahns Peak in that the plutonic host rocks are of intermediate composition with distinctivelylow

levels of F, and emplacementof the mineralized breccia is controlled

by conical fractures (Soregaroli,1975). Grade and tonnage figures

are not available for Boss Mountain,but Redwell Basin is estimated

to contain 50 million tonsat 0.1 percent Mo (G. Arehart, personal

coninunication, 1983). These figures are probably upper-limitparam-

eters for the hypothetical depositthat may be located at the base of the breccia cone sheetat Hahns Peak. The areasof molybdenite enrichment that are presentlyknown at Hahns Peak are in Monolithic breccia in the 7D adit(2,200 ppm) and in DDH-101(1,000 ppm at

3,535 ft.). These anomalousareas establish that the breccia cone sheet hosts molybdenum mineralization and are the criteria forpostu- lating additional mineralizationat depth. The base of the breccia cone sheet, where breccia-controlledmineralization is most likely to occur, is estimated to be approximately4,000-5,000 ft. beneath the surruiiit of Hahns Peak.

A third potential molybdenumtarget is a large stockwork deposit of molybdenite locatedbeneath, oradjacentto, the breccia cone sheet. Although the existence of sucha deposit is entirely speculative,it is consistent with the modeldeveloped for Climax-type 195

molybdenum systems (Whiteand others,1981). Stockwork molybdenum deposits have been suggested to exist at depth beneath thebreccia pipes at Boss Mountain (Soregaroli, 1975) and RedwellBasin (Sharp, 1978). Such hidden stockworkdeposits would probablycompare in size and grade to Mount Emmons, a 160 million tondeposit that averages 0.25% Mo within an area 1,600 ft. by 2,000 ft.(Thomas and Galey, 1982). Based on estimated dimensions of molybdenum systemsby Sharp (1978), the top of the inferred stockworkdeposit at Hahris Peak probably lies from 4,500 to 6,500 feet beneath thepresent surface.

The Mount Emmorisdeposit displays significantlystronger mm- eralizationthan the youngerRedwell Basin breccia pipe located

approximately 4,000 ft.to the northwest. Thomas and Galey (1982) suggest that the relative lack of intrusive brecciaat Mount Emmons is an indication that the system did not undergosignificant venting,

a situation that may have createda more concentrated zone of mineralization. Thus, although intrusivebreccias are good indicators of volatile-rich felsic magmas, their widespread formationmay result in agreater dispersion of mineralizing fluids and consequentlyless intense mineralization overall. Nevertheless, the hypothetical

stockwork molybdenum depositat }-lahns Peak is consideredto be the primary exploration target due to its potentially largesize.

The presence of intermediate-composition host rocks atHahns Peak may be considered a negative feature, since the parentmagma of Climax-type deposits is typically felsic in composition. However, although a felsic pluton has not been intersected by drillcore or underground workings at HahnsPeak, its presence isinferred by 196

inter-breccia aplite dikes. The low level of gamma radiation throughout the intrusive complex (Table 1) suggestslow concentrations of uranium and may also be considereda negative indication for molybdenum mineralization, because Climax-typedeposits are typically enriched in uranium. The source of uraniumin Climax-type molybdenite deposits is likely to have beenPrecambrian granitic basement rocks,and the elevated concentrations are probablythe result of magmatic differentiation. The low levels ofgamma radiation at Hahns Peak may reflectthe extreme depth of the system, the lackof gra- niticsource rocks, or they may indicatea poorly differentiated magma and a weakly mineralized deposit.

Silver-Gold

Silver and gold mineralization appear to belimited to areas in and around theupper part of the breccia cone sheet. The restriction of precious metal mineralization to thislevel was probablya furic- tion of the dischargerate as well as the temperature, pressure, pH, Eh, and sulfide ion concentration of thehydrothermal fluids. If this apparentzonation is real, then the potential for silverand gold is severelylimited. Indications are that thegold zone has, for the most part,been eroded away. Exploration for goldtargets would therefore befruitless. However, small isolatedareas of predominantly silver mineralization witha gold credit may still exist. These potential areas would be close to thesurface and could be mined by selectivetrenching. Any operation of thissort would have to be on a smallscale, and might consist of a backhoe anda one or 197

two man crew. Because of the probable small size of these types of

occurrences, I find it difficult to reconinend the initiation of a

project of this nature.

Virtually every stream graveldeposit along the periphery of

Hahns Peak contains anomalous amounts of gold. The intensity of this mineralization, however, is weak and placer gold concentrations have not provento be economic (W. A. Bowes, personal coranunication,

1979).

Gale (1906) suggested that the basal conglomerate of the Dakota

Sandstone mayhavebeenthe sourcefor gold in the Hahns Peak placers. Recent exploration efforts in the region by Asarco,and others, have sought detrital gold mineralization in this unit. How- ever, careful inspection of the Dakota Sandstone in the vicinity of

Hahns Peak failed to reveal detrital gold and it is concluded that this is not a worthwhile exploration target in this area.

Lead-Zi nc-Copper

Lead, zinc, and copper mineralization occur primarily within and near the breccia cone sheet. Galena, sphalerite, and minor chalcopyrite occur locally in the matrix of Monolithic breccia and also as veins and disseminations adjacent to dikelets of breccia. In addition,a one foot-thick lacustrine limestone bed encountered in the

Morrison Formation at 3,150 ft.in DDH-101 isreplaced by galena, sphalerite,pyrite, and trace chalcopyrite. Despite these occurrences, however, the amount of lead, zinc, and copper mineralization that is present as either veins or replacements is extremely limited. The possibility of HahnsPeak being the top ofa porphyry copper deposit was considered, but the overall lack ofcopper suggests that this is unlikely.

Late stage open-space fillings in the Multilithic breccia account for the majority of galena and sphalerite presentat Hahns Peak. Samples of this type ofmineralization that were foundon the dump of the Price Tunnel contain up to eight percent combinedlead

andzinc. Although thepresence of high grade mineralization is

encouraging, its occurrence is extremely localized and thebulk tonnage would be negligible. Young and Segerstrom (1973) estimatethat 1.3 million tons of 0.7 percent combined lead and zincare present at Hahns Peak. Such a deposit could not beconsidered economically val-

uable now or atany time in the foreseeable future.

Uranium

In 1979, the Departmentof Energy drilled a holeon the south-

west flank of Hahns Peak (SWB-27)to investigate the uranium potential in the coarse-pebble conglomerate at the base of the BrownsPark

Formation (Carter and Wayland, 1981). The target conceptwas to explore the Browns Park conglomerate throughout the Sand WashBasin for uranium deposits similar to those found near Baggs, Wyoming,and

Maybell, Colorado. The drill hole at Hahns Peakfailed to encounter any si gni fi cant urani urn mineralization.

Measurementsof gaimia radiation throughout the quartz latite porphyries that comprise Hahns Peak indicate extremely low levelsof uranium mineralization (Table1). The indicated levelsare low even 199

for porphyry molybdenum systems which commonly contain minoramounts of uranium mineralization. It is concluded that there isabsolutely no potential for economic-grade vein or sediment-hosted typeuranium deposits at Hahns Peak.

Recornmendati ons

The possibility of a large (50-200 million ton) stockworkmolyb- denum deposit existing at depth beneath Hahns Peakis good. However,

its extreme depth andthe current depressed stateof the molybdenum market make it impossible to recommend deep diamond drillingat this time or in the immediate future. In addition, it is likely to bea low-grade deposit, probably comparable to RedwellBasin, Colorado (0.1% Mo).

Future exploration work at Hahns Peak will definitely be dependent upon the state of the molybdenum market. Trends in this market

are difficult to forecast, however, and economic experts have recently expressed conflicting opinions. Kovisars(1982;see also Eng. and Mining Jour., Oct., 1982,p. 9) has suggested that known reserves should meet demand atleast through 1991,anda price of about $4.50/lb is likely to be maintained until that time. In contrast, Bilhorn (1983) suggests that the need forstrong, weldable, heat-treated steel in energy-related applications will increase, causing the molybdenum market to recover at a faster rate thanthe general economy.

Regardless of economicconsiderations, the area on Hahns Peak with the highest probability of significant molybdenummineralization 200 at depth lies within the perimeter of the breccia cone sheet along the zone of N-S trending pods of phyllic alteration (Plate 3). One or more 5,000. ft. vertical holes collared within this zone would test both the base ofthe breccja cone sheet for a Boss Mountain-type deposit, as well as the potential for a deeper stockwork deposit of molybdenum.

It isremotely possible that the large magnetic low centered approximately 3,500 feet south-southwest ofthe Hahns Peak summit may be indicative of hydrothermalalteration related toa separate porphyry system that failed to vent (see Fig. 6). Such a situation would be analogous to that of Redwell Basin and Mount Emmons (Dowsett and others, 1981; Thomas and Galey, 1982). Deep drilling in the area of the magnetic low near HaThns Peak should be considered a wildcat venture, but a series of shallow holes (under 1,000 ft.) for the pur- poseof fulfilling yearly assessment obligations could be used to search for W and F dispersions possibly indicative of a deep system.

Exploration for shallow Ag-Au in the upper part of the breccia cone sheet is not recommended at this time, as this potential resource would probably occur insmall,isolated pods of marginal economic value. 201

SUMMARY

Hahns Peak is a composite laccolithic intrusionof latite and quartz latite porphyry, approximately 12 m.y. old. A thin, cone- shaped body of intrusive breccia (referredto as a breccia cone sheet) is centered on the intrusive complex and isinterpreted to be related to a rhyolitic pluton at depth. Base and precious metal mm- eralization occurs within andneartheintrusivebreccia and is thought to be part of a porphyry molybdenum system thatformed above the rhyolitic pluton.

Geologic History andInterpretations

Precambrian basement rocksare granodiorite in composition and gneissic in texture. These rockswere formed when island arc terrane was accreted to the Archean craton by collisionprocesses along the Mullen Creek-Nash Fork shear zoneat approximately 2.0 b.y. ago.

These originalisland arc rocks were regionally metamorphosed to

amnphibolite grade at about1.7 b.y. ago.

Extensive erosion andnon-deposition resulted ina paucity of Paleozoic rocks in the Hahns Peak region. Red calcareous siltstone and fine-grained sandstone of the PermianGoose Egg and Lower Triassic Red Peak formations are preserved at depth and lieuncon- formably on Precambrian granodiorite. These sedimentary unitsrepre- sent detritus that filled basins adjacent to isolatedhighlands that were remnants of the AncestralRocky Mountain uplift. 202

Upper Triassicrocks consist of sandstones, siltstones, and claystones of the Jeim, Popo Agie, and Nugget Sandstoneformations. They indicate a mixture of fluvial, lacustrine,and marginal marine depositional environments. These rocks,together with the Permo- Triassic Goose Egg and Red Peak formations,were mapped as a single unit.

Shallow marine sandstone andshale ofthe Jurassic Sundance Formation lie unconformably onthe Triassic rocks. The Sundance Formation is, in turn, overlain disconformably by theLate Jurassic shales and fresh water limestones of theMorrison Formation. The Suridance and Morrison Formations have also been mappedtogether as a single unit.

The Cretaceousrocks ofthe region are Dakota Sandstoneand Mancos Shale. The Dakota Sandstoneconsists of cross-bedded sand- stonesand a basal conglomerate. This unit is typicallya ridge former and is indicative of brief regional uplift. The Mancos Shale isa calcareous unit that was probably deposited during thetrans-

gression of an epeiricsea.

Rocks of Paleocene, Eocene, and Oligoceneage are missing from the Hahns Peak region due to erosion andnon-deposition during the Rocky Mountain uplift. Regional north-trending thrustfaults were developed in the early stages of this orogeny. Depostion of tuff- aceoussandstoneand basal conglomerate of the mid-MioceneBrowns

Park Formation occurredsubsequent to regional thrusting.

The Browns Park Formation is locally cut by igneousintrusions throughout the Elkhead Mountains. These intrusionsare most coriunonly 203

quartz latite porphyry, buttheir composition may range from basalt

to rhyolite. Laccolithic intrusionsare typical, but flows and dikes are also present. The age of the intrusive rocksrange from 7.6 to 12.0 my.

The Hahns Peak laccolith is part of the Elkhead Mountain vol-

canic field. Itis believed to have been derivedfrom a cupola on

the southeast flank ofa regional batholith, the presence of which is

indicated by magnetic andgravity data. Anomalously high heat flow

measurements fromHahns Peak that correspond to valuesmeasured

throughout the Rio Grande Rift,are interpreted as evidence that the

rift extends as far north as the Wyoming border. The magma that formed the Hahris Peak intrusions may have been generatedabove an

imbricated subductionzone. A linear trend of upwelledmagma above

that subduction zone is thoughtto have formed the Rio Grande Riftas

an ensialic back-arc spreading center. At Hahns Peak, that magma was

probably channeled along deep-seated Precambrian structuressuch as

the Uinta Mountain Group grabenand the Mullen Creek-Nash Fork shear

zone. Thislate Tertiary intrusive activityprobably ledto the reactivation ofstructures alongthetrend of the Uinta Mountain

Group graben and resulted inthe formation of a regional horst block within which Hahns Peak is located.

The early intrusion of sillscomprised of Beryl Mountain par- phyry (latite) and Little Mountainporphyry (quartz latite) initiated the doming of the sedimentaryunits at Hahns Peak. The structural dome continued to formas a result of the vertical intrusion ofmagma that was to feed the compositelaccolith. This intrusion apparently 204 pulsed and ebbed, as two radiating and steeply dipping normal faults were formed prior to the collapse of the central portion of the dome.

The main phase of intrusive activity was initiated by emplacement of Columbine porphyry (quartz latite) along the intersection of the two normal faults. The injection of Columbine porphyry caused further doming of the sedimentary units, which resulted in the formation of three parallel reverse faults in the northwestern part of the study area. It also resulted inthe upward displacement and eventual entrainment of the roof pendantofBrowns Park Formation that had been part of the down-thrown central block. Sometime after crystallization of Columbine porphyry, but prior to the emplacement of the Sumit porphyry phase, a period of hydrothermal activity resulted in the local development of intrusive breccias and stockwork veins of quartz, pyrite, and molybdenite. TheSummit porphyry

(quartz latite) phase was then intruded vertically and caused further doming and partial entrainment of earlier down-dropped blocks. The summit porphyry phase had not undergone complete crystallization when the last of the main phase intrusions, 7D porphyry (quartz latite), was injected into it. The alignment of a large mass of 7D porphyry along the southeast portion ofthe northwest-trending normalfault that passes beneath Hahns Peak suggests a separate magma source for this intrusive phase,possibly a perched magma chamber within one mile southeast of the peak. The intrusions of main phase porphyries into the central portion of the collapsed dome resulted in the upward displacement and entrainmentof blocks of BrownsPark Formation,

Mancos Shale, and Beryl Mountain porphyry. Concomitantly, the weight 205

of the overlying magma probably caused further downwarddisplacement ofblocks of Mancos Shale,Dakota Sandstone,Morrison Formation, Beryl Mountain porphyry, and the Permo-Triassicsedimentary units. Continued magmatic activity at depth led tothe intrusion ofa cupola composed of highly differentiatedfelsic magma along portions of the same normal faults that had actedas conduits for the earlier porphyries. The top of this cupola probably consistedof fluids that were enriched in various metals, in addition to Si02,H20, CO2, HCI, HF, Na, and K. The fluids contained in this volatile-richmagrnatic carapace were in a super-critical state due to high ambientpressure. High fluidpressure at the top of the cupola created conicalfrac-

tures in the countryrock. These high pressureswere relieved, however, with the release of fluids. In addition, ambientpressure was lowered as the magma continued to rise to highercrustal levels. At some point, a level of pressure was reached that permittedthe evolu-

tion of a vapor plume that containeda coexisting aqueous phase. This led to further hydro-fracturing of the countryrock and additional fluid emissions. The initial fluiddischarges were relatively passive and causedpervasiveNa+ metasomatism with minordisseminated pyrite. This was followed by the more forcefully emplacedMonolithic breccia phase. The fluids that formed the Monolithic brecciaunder- H+ went metasomatic reactions with the wall rock andlocally precipitated Pb, Zn, Cu, and Mo. The sustained dischargeof these fluids resultedin dilation of the conicalfractures. Thin dikelets of aplite were also intruded at this time, andappear to cut the Mono- lithic breccia locally. 206

A quiescent period in hydrothermal activity permittedground water to collect in the enlarged fractures. A resurgent pulseof magma caused the cupola to continue to rise, whichallowed the super- heated, volatile-rich carapace to come in contact witha reservoir of ground water heldin open fractures. This encounter resultedin phreatic steam-blast explosions. The sudden pressurerelease created by these explosions permitted fluids within themagmatic carapace to flash and form localized violent discharges. The explosive discharges of both ground water and magmatic fluidswere concentrated along the conical fractures and resulted inthe formation ofthe Multilithic breccia phase. It is this intrusive phasethat forms the bulk of the breccia cone sheet. Steam-blast eruptionson the west flank of Hahns Peak formed the volcanicvent complex and thepyroclastic surge deposit.

The major portion of the breccia conesheetwas formedby groundwater explosions and thus does not containsyngenetic sulfide mineralization. However, sulfide mineralizationthat is syngenetic withthe formationof the brecciacone sheet is presentlocally within the matrix of Multilithic brecciawhere magmatic discharges were focused. Within such areas, Au, Ag, and minor Cuare believed to have beentransported in solution as chloride complexes. Associ- ated wall rock alteration adjacentto the breccia cone sheet displays moderate to strong phyllic and localized advanced argillicalteration. Concentric zones of argillic and propylitic mineral assern- blages are displayedoutwardly. 207

Molybdenum is essentially absent from the Multilithic breccia,

but may have been deposited at the base ofthe cone sheet where

sudden physical-chemical changes occurred. Molybdenum was probably t also deposited in stockwork veins of quartz and pyrite at depth.

The occurrence ofanomalous MoandF togetherin Monolithic

breccia suggestsa common origin for these two elements. Although

the transport of Mo by F complexes is one possible genetic link, it

is unlikely that this mechanism is significant in the formation of molybdenite deposits. Molybdenum is interpreted to have been trans-

ported predominantly in the vapor phase by hydroxy-oxide complexes within fluid emanations from the top of the cupola. The degree to which a vapor phase is developed froma magma is directly proportional to the saturation level of H20 within that magma. One way to raise the H20 saturation level of a magma is to increase the Fcon- tent. Thus, the F content of the felsic magma that is interpreted to have been beneath Hahns Peak probably led indirectly to theconcentration of Mo in a vapor plume at the top of the cupola. This relationship, together with the incompatibility of F and Mo for crystal latticesof most major rock-forming silicates,provides anexpla- nation for the common association of these two elements in stockwork niolybdeni te deposi ts.

The final stage of hydrothermal activity at Hahns Peak consists of Pb + Zn ± Ag ± Au mineral i zati on that preci p1 tated as open space vug-fiflings within Multilithic breccia and as minor veins adjacent to to the breccia. This mineralization is interpreted to have precipitated from circulated meteoric fluids that leached the metals from Precambrian source rocks. These fluids were driven by the thermal gradient created by the cooling magmatic cupola. Minor dikes of quartz latite porphyry are the youngest intrusions at Hahns Peak.

Their markedlylowdegree of alterationsuggests thatthey were intruded very near the end of hydrothermal activity.

Freeze-thaw mechanisms and down-slope mass wastingare the dominant geomorphic processesthat havesculptured HahnsPeaksince

Pleistocene time. The presence of active landslides, hillside creep, and a small rock glacier suggest that theseprocesses continue to be acti ye. 209

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