Deformation and age of the Red Mountain intrusive system...

Deformation and age of the Red Mountain intrusive system(Urad-Henderson molybdenum deposits), Colorado:Evidence from paleomagnetic and 40Ar/39Ar data

JOHN W. GEISSMAN Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116LAWRENCE W. SNEE U.S. Geological Survey, MS 963, Box 25046, Denver Federal Center, Denver, Colorado 80225GARRETT W. GRAASKAMP Dunn Geoscience Corp., 5 Northway Lane, North, Latham, New York 12110RICHARD B. GARTEN U.S. Geological Survey, APO New York, New York 09697-7002ENNIS P. GERAGHTY Stillwater Mining Company, Box 365, Nye, Montana 59061

Colorado Geological Survey,Earthquake Reference Collection

ABSTRACT

Paleomagnetic and 4()Ar/39Ar age-spec-trum data from most stocks of the Red Moun-tain intrusive system, in the northwest Colo-rado mineral belt, provide an improvedunderstanding of the structural and coolinghistory of the suite of intrusions host to aworld-class molybdenum deposit. Paleomag-netic data from five stocks at the surface andeight younger stocks exposed in the subsur-face Henderson Mine support field observa-tions (for example, dike and vein orientations,stock geometries, and distribution of zones ofmineralization) that imply moderate tilting(15°-25° down to the east-southeast) sincelatest Oligocene time after cooling and miner-alization. Surface stocks contain magnetiza-tions carried by both magnetite and hematite.The Red Mountain stock is the youngest sur-face intrusion and contains mostly normal po-larity magnetizations (for example, D = 321°,I = 59°, a95 = 19°, k = 9, N = 6 samples, siteRM9), whereas older East Knob and RubbleRock breccia intrusions contain a nearly an-tipodal, well-characterized magnetization(East Knob stock: declination = 161°, inclina-tion = -47°, 095 = 13°, k = 23, average of fivesite means). Polarity changed from reverse tonormal during emplacement and cooling ofthe Red Mountain intrusions exposed at thesurface. 40Ar/3'Ar age-spectrum data on bio-tite and orthoclase from the Red Mountainstock and stocks of the Henderson Mine indi-cate the reversal to be older than 30 Ma. AllHenderson Mine stocks have normal polaritymagnetizations (Primos stock: D = 333°, I =51°, 095 = 5°, k = 44, average of six sitemeans) which, on the basis of 40Ar/39Ar age

spectra from orthoclase and biotite, wereblocked between 28.7 and 27.6 Ma. Magnet-ite and maghemite are the major carriers ofmagnetization in these rocks.

On the basis of an ̂ Ar/^Ar thermochrono-logic study of the Red Mountain intrusivesystem, thermal activity started at or just be-fore 29.9 + 0.3 Ma and ended at 26.95 ± 0.08Ma. The age-spectrum data are interpreted toindicate that the porphyry of Red Mountain,one of the oldest stocks, was emplaced before29.9 ± 0.3 Ma (possibly before 30.38 ± 0.09Ma). Nearby lamprophyre dikes were em-placed at 29.8 ±0 .1 Ma; rhyolite dikes in-truded at 29.4 ± 0.2 Ma. The Urad andSeriate stocks intruded after 29.8 Ma but be-fore emplacement of the Vasquez stock at28.71 ± 0.08 Ma. The system core cooledbelow 280 ± 40 °C (the argon closure temper-ature of biotite) at 27.59 ± 0.03 Ma. The lastperiod of thermal activity involved pulses ofmagnetite-sericite alteration around the Se-riate stock between 27.51 ± 0.03 and 26.95 ±0.08 Ma; this activity did not thermally over-print unaltered parts of the intrusive system.

Tilting of the Red Mountain area is impliedby a comparison between a grand mean (onthe basis of 10 stock means, D = 333°, I = 49°,«95 = 5°, k = 78) and a mid-Tertiary referencefield. The Red Mountain intrusive system andhost Precambrian rocks probably were de-formed along a nearly north-south horizontalaxis in response to northwest-side down,strike-slip faulting with displacement largelyalong the Woods Creek fault zone. Late Ter-tiary deformation of Precambrian-cored partsof the Front Range, host to numerous mineraldeposits, was more complicated than simple,near-vertical uplift of the crust.

INTRODUCTION

Paleomagnetic methods are useful for quanti-fying crusta! deformation within regions ofgreatly varied scale. For batholiths and stocks,paleomagnetic analysis is one of the few meth-ods by which total deformation may be inferred(Beck, 1980; Geissman and others, 1980;Geissman and others, 1982; Shaver and McWil-liams, 1987; Faulds and others, 1992). The lackof conventional reference to the paleohorizontaldatum, however, at the time during which mag-netization was acquired has occasionally led tocontroversy over the interpretation of paleo-magnetic data from large, batholithic terranes(Beck, 1980; Butler and others, 1991; Marquisand Irving, 1990; Irving and Thorkelson, 1990).Other methods may allow estimation of the pa-leohorizontal datum. Whole-rock and mineralchemistry, isotope data, and field studies al-lowed Barnes and others (1986a) and Barnesand others (1986b) to infer postcrystallizationtilting of Mesozoic plutons in the KlamathMountains. Petrologic studies of the JurassicYerington batholith, west-central Nevada, ledDilles (1987) to corroborate structural (Proffett,1977) and paleomagnetic (Geissman and others,1982) investigations that indicated about 70°-90° tilting of the intrusions and wall rocks.

Structural setting is important in the explora-tion for and resource evaluation of mineral de-posits in intrusive rocks. Nonetheless, becauseof complications resulting from complete orpartial thermal and/or chemical remagnetiza-tion, few paleomagnetic studies have focused oncomplex, multiple-intrusion and hydrothermallymineralized systems. Particular concerns are thetime at which magnetization was acquired rela-tive to the time of system intrusion and tempera-

Geological Society of America Bulletin, v. 104, p. 1031-1047, 15 figs., 5 tables, August 1992.

1031

1032 GEISSMAN AND OTHERS

ture changes during intrusion and alteration.These may be answered by 40Ar/39Ar age-spectrum data on different mineral phases withdifferent argon closure temperatures.

At the Red Mountain intrusive system, in theFront Range in Colorado (Fig. 1), late Oligo-cene stocks host the Urad (surface) and Hender-son (underground) porphyry molybdenum de-posits (Wallace and others, 1978; Garten andothers, 1988). Several field relationships implythat after mineralization the Red Mountain sys-tem may have been tilted in latest Oligocene andyounger time. These include the asymmetrical

orientation of intrusive contacts, radial dikes,and veins as well as the asymmetrical distribu-tion of intrusive textures, alteration facies andore zones (Wallace and others, 1978; Gartenand others, 1988; Geraghty and others, 1988).The system intrudes Middle Proterozoic SilverPlume Granite and, therefore, the paleohorizon-tal plane at the time of intrusion cannot be refer-enced. The nearest outcrops of Phanerozoicstrata are 18 km to the northwest in the FraserBasin south of Granby (Fig. 1). Differentiatingasymmetries caused by deformation from thoserelated to intrusion bears on continued explora-

Figure 1. Location of Red Mountain (RM) with respect to the regional tectonic setting of thenorthern Rio Grande Rift and associated fault system (after Tweto, 1975, 1979). Inset showsmajor faults near Red Mountain; cross section A-A' is shown in Figure 14. Uplift boundariesdefined by the extent of Proterozoic rock (stippled). Fault traces (heavy lines) generalized afterLevering and Goddard (1950), Theobald (1965), Wallace and others (1978), and Theobald andothers (1983).

tion and development of the deeper levels of theHenderson deposit as well as the Neogene struc-tural setting of the Front Range.

Paleomagnetic and 40Ar/39Ar age-spectrumdata obtained from most surface and subsurfaceintrusions provide an improved understandingof the structural and cooling history of the intru-sive system; many of our observations may beapplicable to other porphyry systems. The pa-leomagnetic data support geologic evidence for15°-25° of down to the east-southeast tilting ofthe system in latest Oligocene time. Both fieldstructural and paleomagnetic data have simul-taneously documented a paleomagneticallymeasureable amount of local tilting of a series ofshallow stocks. The presence of dual polaritymagnetizations indicates that stocks cooled dur-ing at least one geomagnetic field polarity rever-sal (from reverse to normal polarity) beforeabout 30 Ma and that later thermochemical ac-tivity during one or more normal polarity chronsdid not unblock the reverse polarity magnetiza-tion. 40Ar/39Ar age-spectrum data identify theepisodic nature of intrusion and alteration; theyare interpreted to indicate that cooling belowbiotite argon-closure temperature of the entireintrusive system after emplacement of the un-derground stocks occurred in about 2 x 106

years. Paleomagnetic and 40Ar/39Ar age-spec-trum methods used in this study are summarizedin the Appendix.

The 40Ar/39Ar age-spectrum data presentedbelow are summarized from an ongoing study(L. W. Snee and R. B. Garten, 1990, U.S. Geo-logical Survey, unpub. data) on the Red Moun-tains intrusive system and related Urad-Hender-son mineral deposit. All argon data and theirinterpretation are included in a forthcomingpaper to focus on the age and thermal history ofplutons and alteration within the intrusive sys-tem as well as the origin of the Urad-Hendersondeposit. Because of the importance of high-resolution cooling ages for interpretation of thepaleomagnetic results presented here, however,we include interpreted cooling ages, compositeage-spectrum diagrams, and an interpretedthermal history derived from these unpublishedargon data. We summarize all existing geochron-ologic data from previous published and un-published studies for completeness. The ^Ar/39Ar data are favored in the interpretationspresented below because of higher precision, insome cases as good as 0.1% or 30,000 years.

GENERAL GEOLOGY

The Red Mountain intrusive system lieswithin the 1.4-Ga Silver Plume Granite batho-lith (Fig. 2). The northeast-striking BerthoudPass and north-striking Vasquez Pass faults are

Geological Society of America Bulletin, August 1992

RED MOUNTAIN INTRUSIVE SYSTEM, COLORADO 1033

Figure 2. Generalized surface geology ofRed Mountain; modified from Wallace andothers (1978) and Geraghty and others(1988). Solid circles are numbered paleomag-netic sampling sites. Medium heavy lines aredikes and small intrusive bodies. D, U =down, up.

continuous for several tens of kilometers in theFront Range in Colorado and pass within 2 kmof the system (Figs. 1 and 2). The intrusive sys-tem is bounded by the Woods Creek fault, anortheast-striking strand of the Berthoud Passfault, the Vasquez Pass fault, and a suspectedeast-striking fault in the valley of the West Forkof Clear Creek (Theobald and others, 1983;Figs. 1 and 2). The specific geology of the Uradand Henderson deposits and the Red Mountainarea in general has been discussed by Wallaceand others (1978), White and others (1981),Garten and others (1988), Levering and God-dard (1950), Tweto and Sims (1963), andTheobald (1965).

The Red Mountain intrusive system is madeup of 15 major stocks and 4 igneous brecciazones. Of these 19 stocks and breccia zones, 13were sampled for paleomagnetic study; fromoldest to youngest they are as follows: breccia ofEast Knob, breccia of Rubble Rock, Red Moun-

Silver Plume Granite ;

Silver Plume Granite

Figure 3. Location ofnumbered subsurface pa-leomagnetic sampling sites(solid circles) with respectto stocks in the Hen-derson Mine, projectedto the 7,500-ft (2,330-m)level. Geology modifiedfrom Wallace and others(1978). Lettered stocksare as follows: H, Hen-derson; P, Primos; S,Seriate; B, Berthoud; A,Arapaho; R, Ruby; N,Nystrom. Dashed lines re-fer to fact that the Ara-paho and Berthoud stocksare not exposed at the7,500-ft level. East/northcoordinates are specific tothe Henderson Mine.



tain border, porphyry of Red Mountain, igneo-fragmental breccia (all exposed at the surface),Urad, Berthoud, Henderson, Primes, Arapaho,Seriate, Ruby, and Nystrom (Fig. 3). All stocksand fragments in intrusion breccia are similar inchemistry and mineralogy and consist of quartz,alkali feldspar, albitic plagioclase, and minorbiotite. The surface rocks are generally porphy-ritic and locally exhibit clastic and/or fragmen-tal textures consistent with subvolcanic em-placement. Sample sites on the surface of RedMountain lie within a general zone of phylliticquartz-sericite-pyrite alteration and a more dis-tant outer zone of argillic alteration (MacKenzie,1970). These rocks contain abundant hematite,presumably formed during alteration of pyrite,biotite, and/or magnetite. The paleomagneticdata discussed below suggest, but do not conclu-sively prove, that oxidation occurred during hy-drothermal activity, not during recent, near-surface weathering. At most subsurface sites(Henderson Mine, -1.3 km below the surface ofRed Mountain), intrusions are porphyritic-aplitic to equigranular in texture. With respectto our collection for paleomagnetic investiga-tion, the type and intensity of hydrothermal al-teration and degree of molybdenum mineraliza-tion vary among individual samples from a siteand among specimens from a given sample.Typically, secondary sericite and kaolinite(Walker, 1984) partially replace feldspar atmany sites.

Field relations suggest that the intrusive sys-tem was not emplaced in its present, off-verticalorientation. First, mineralized rocks and high-silica alteration halos delineated undergroundare best developed southeast of the apical partsof mineralizing stocks (Garten and others,1988). Second, primary igneous textures withinindividual stocks (Shannon and others, 1982)and geometries of stock contacts imply thatstructurally higher levels of stocks presently areat lower elevations on their southeast sides thanon their northwest sides. Third, radial dikes ex-posed on the surface and radial vein sets exposedunderground in the Henderson Mine are marked-ly asymmetric in orientation. The dike and veinorientations imply either a nonvertical principalstress direction during dike emplacement or tilt-ing (rotation about a near-horizontal axis) of thesystem after intrusion, hydrothermal alteration,and mineralization (Geraghty and others, 1988).

PALEOMAGNETIC RESULTS

Red Mountain Surface Stocks

Magnetizations from samples of the five intru-sions (nine sites) on Red Mountain are reason-ably well defined and suggest acquisition over a

2 ml

NRM

RMT1D13,East Knob

RMT1G2a,East Knob

660 \ RMT3D1a,East Knob

Figure 4. Orthogonal progressive demagnetization diagrams (Zijderveld, 1967) of represen-tative response by Red Mountain surface rocks. The endpoint of the magnetization vector issimultaneously projected onto two orthogonal planes (horizontal: E-W/N-S plane, solid sym-bols; vertical, E-W or N-S/U-D plane, open symbols). Peak demagnetizing inductions inmillitesla or temperatures in degrees Celsius (°C) are given adjacent to data points on thevertical projections. Data are plotted in geographic coordinates. Specimen identifier is givenbefore a brief description of the rock. Magnetization directions isolated in progressive demag-netization and determined using principal component analysis are as follows: (A) 159°/-34°(declination/inclination), maximum angular deviation (MAD) = 3.4°, 300-560 °C; (B)137V-440, MAD = 3.8°, 25-80 mT; (C) 100°/-59°, MAD = 4.0°, 23-80 mT; (D) 143°/-48°,640-660 °C; (E) 134°/-52°, MAD = 8.0°, 300-627 °C.

time period of reverse and then normal polarity.Median destructive inductions (MDI) in alter-nating field (AF) demagnetization (the peak in-duction required to reduce the natural remanentmagnetization [NRM] intensity to half the origi-nal value) commonly exceed 15 millitesla (mT).For MDI's exceeding 15 mT, the NRM is car-ried predominantly by fine (pseudo-single- orsingle-domain) magnetite, maghemite, and (or)hematite. Magnetizations carried by hematite(with unblocking temperatures greater than

-580 °C and MDI's > 100 mT) are usually sim-ilar in direction to those residing in magnetite,unblocked at lower temperatures.

The two oldest intrusions are breccias (EastKnob and Rubble Rock); they contain magneti-zations of southeast declination and moderateupward (negative) inclination (Figs. 4 and 5A).We interpret this magnetization as a reverse-polarity-thermoremanent magnetization (TRM)characteristic of the rocks. In single-componentmagnetizations, the reverse-polarity characteris-


Figure 5. Paleomagneticdata from the Red Moun-tain intrusive system.(A) Stocks exposed atthe surface of RedMountain (Table 1).(B) Henderson Minestocks (Table 2). Site-mean directions of mag-netizations isolated inprogressive demagneti-zation with associatedprojected cones of 95%confidence. Equal areaprojections with closed(open) symbols represent-ing lower (upper) hemi-sphere projections.

RED MOUNTAIN INTRUSIVE SYSTEM, COLORADO

N

1035

tic magnetization usually resides in magnetite(see, Figs. 4A and 4B), but commonly much ofthe NRM is carried in hematite, as revealedin thermal demagnetization. Although short(-10,000 yr) reversed-polarity subchrons oc-curred during the Brunhes normal polaritychron (ca. 730 Ka to present) (Champion andothers, 1987), magnetizations of reverse polaritycarried by hematite were probably not acquiredduring recent weathering. The relationship ismore complicated where behavior is multicom-ponent, such as site 3 in porphyry at East Knob.The reverse-polarity remanence in site 3 samplesis carried by both hematite (see Fig. 4D) andmagnetite (see Fig. 4C). When magnetite carriesthe reverse-polarity magnetization, higher un-blocking temperature magnetizations residing inhematite are of positive inclination yet aredispersed in declination.

The border and porphyry phases of the RedMountain stock and the igneo-fragmental brec-cia cut the East Knob and Rubble Rock brecciastocks. These younger intrusions contain mag-netizations of mixed polarity, but the magnetiza-tions are predominantly of northwest declina-tion and moderate positive inclination (normalpolarity). The remanence is carried by magnetiteand hematite. Using the paleomagnetic datasummarized in Table 1, we infer that intrusionof stocks exposed on Red Mountain spanned atleast one field reversal.

Magnetizations carried by hematite in theRed Mountain intrusions may have been ac-quired over a prolonged period of oxidation thatspanned several polarity chrons. In some rocks,more than 80% of the NRM is carried by hema-tite. Because the directions of magnetizationscarried by hematite generally parallel those re-siding in magnetite, we suggest that the magneti-

zations in hematite record an ambient field; themagnetizations could have been acquired duringthe same polarity chron as when a TRM wasblocked in magnetite. We assume that the use ofmagnetizations residing in hematite, as well asthose in magnetite, is reasonable for structuralinterpretations.

Henderson Mine Stocks

All eight intrusions (30 sites) sampled in theHenderson Mine are characterized by a normalpolarity magnetization, in contrast to the surfacestocks which are principally of reverse polarity(Table 2 and Table 3). The magnetization iso-lated in all intrusions has north-northwest decli-nation and moderate positive inclination (Fig.6), although the amount of within-site dispersion

varies considerably (Fig. 5B). Probable contrib-utors to the within-site dispersion of directionsare discussed below. In specimens with multi-component magnetizations, the first-removedcomponent usually has a more northward decli-nation and steeper positive inclination (such asspecimen HEN19Bla, Fig. 6E). In total, NRMdirections are somewhat dispersed, but a well-defined, reverse polarity secondary magnetiza-tion was not defined for any site or individualintrusion. Unlike the surface rocks, magnetiteand possibly maghemite are the only magnetiza-tion carriers, because the intensity of magnetiza-tions remaining above ~580 °C never exceededa few percent of the original NRM intensity.MDI's for most rocks containing magnetite asthe principal magnetic phase are < 15 mT, sug-

TABLE I PALEOMAGNETIC DATA FROM SURFACE STOCKS OF RED MOUNTAIN

Slock Sue-number

East Knob IEasl Knob 2Easl Knob 3East Knob 4Easl Knob 7

Mean

Rubble Rock, 6breccia

Red Mountain 5border

Red Mountain. 9porphyry

Iffneo-fragmen. 8breccia

In situt

decl.

149209153175163168

161

155

321

162

incl

-44-48-44-46-40-46

-47

-36

59

-52

R°95

122014122013

14

10

19

23

It"

165

12II6

23

21

22

9

8

n/n0/Ntt

9/9/77/11/78/10/7

12/12/88/10/75 siles

5/6/5

8/10/7

6/7/6

5/6/3

•All sites arc shown in Figure 2.fy/r silu declination (decl.), in decrees cast of north; inclination (incl.), positive downward of the site or stuck mean.^Semi-angle of the cone of 95% confidence around the calculated mean direction."Best estimate of the precision of the distribution of the n magnetization vectors (Fisher, 1953)TtThc ratio of progressively demagnetized specimens used for statistical calculations (n) to the total analyzed <n 0 ) from tbe number of independent samples

collected (N).



TABLE 2. PALEOMAGNETIC DATA FROM HENDERSON MINE SUBSURFACE STOCKSOF RED MOUNTAIN

Stock

Urad

Mean

BerthoudHenderson

Mean

Primos

Mean

ArapahoSeriate

Mean

RubyNystrom

Mean

Site'

147

172123

20235

1624

61525262728

1989

1011121314

18222930

In siiu*

decl. incl.

326 69316 57322 25292 61

2 45343 56328 54

348 57326 48323 51313 40310 37332 46320 45

342 39332 59311 49327 43339 59352 54333 51

348 54323 53331 44328 46335 46334 33328 29299 59327 48

335 44332 38331 51328 54330 47

"95

910

t

2020IS14

1914198

1287

119

211512228.6

10311216II1426158

23121298.6

* n/n0/N

41 4/6/432 6/8/5- 2/4/3

10 4/5/513 3/5/422 4/5/415 6 sites

13 4/4/412 8/9/510 5/6/428 10/10/545 3/5/357 5/6/582 5 sites

26 6/8/450 5/6/514 3/5/414 5/6/629 5/5/5

7 6/6/644.1 6 sites

34 5/7/56 4/5/4

20 6/8/712 6/7/622 8/10/713 7/7/66 5/6/4

14 6/6/639 7 sites

6 6/6/624 6/6/626 5/5/544 6/7/688.6 3 sites

*AII sites are shown in Figure 3.*The number of samples 'was insufficient to calculate statistical parameters.

gesting that multidomain grains carry much ofthe NRM.

MAGNETIC MINERALOGY ANDROCK MAGNETISM

The history of the Red Mountain intrusivesystem includes a complex series of igneous aswell as hydrothermal mineralization and altera-tion events (White and others, 1981; Garten andothers, 1988). Observations on the magnetic

phases that carry geologically important magne-tizations may limit the age of magnetization ac-quisition relative to subsolidus cooling, altera-tion, and later uplift and local deformation.

Red Mountain Surface Stocks

Sample response to progressive demagnetiza-tion, rock magnetic tests, and petrographic ex-amination all indicate that the principal magne-tic phases in the surface intrusions are magnetite,

TABLE 3. SUMMARY OF PALEOMAGNETIC DATA. RED MOUNTAIN INTRUSIVE SYSTEM.AND EXPECTED FIELD DIRECTIONS

Determination

StocksSites

Expected directions

Irving and Irving (1982)(30 Ma)Dichl and others 1 1983)(22-38 Ma)

N/N0 Decl..

10/13 33334/39 332

14 355

105 351

Mean Pole position

Incl. a9j k Lat. Long. K* ag5*

49 5.0 77.5 66 149 62.5 5.649 4.3 31.2 65 151 22.9 5.0

58 86 154 144 3

57 83 148 13 4

S§

1.712.5

0.3

38.8

•Best estimate of the precision of the N site virtual geomagnetic pole positions.'Semi-angle of the cone of 95%-contidence defining the location of the true mean pole position at the 95^-confidence level.^Angular standard deviation of the vinual geomagnetic poles.

hematite, and goethite (Fig. 7). Textural rela-tions suggest that hematite and goethite havereplaced pyrite and magnetite. Pyrite precipi-tated during hydrothermal mineralization andalteration; magnetite is of magmatic origin. In-tensities of the NRM for the Red Mountain sur-face intrusions are low; mean values for eachintrusion range from 140 to 2 milliamperes/meter (mA/m). Such low NRM intensities at-test to a very low concentration of magneticphases, especially of magnetite.

Isothermal remanent magnetization (IRM)acquisition curves usually increase gradually inmagnetization to as much as 1.2 T, but neverreach saturation. The lack of abrupt increase inIRM in inductions less than 0.3 T implies thathigh coercivity phases predominate in the sur-face intrusions. Where enough magnetite couldbe separated from these surface rocks, poorlydefined saturation magnetization versus temper-ature curves indicate Curie temperatures be-tween 550 and 585 °C, indicative of low-Timagnetite as the saturated phase.

The magnetic properties of samples from in-dividual stocks, as well as from individual sites,are varied which leads to some between-site var-iations in dispersion of magnetization data. Asmentioned above, demagnetization results fromsite RMT3 (Fig. 8) in the East Knob stock ex-emplify such variations. In thermal demagneti-zation, specimens from samples G and I exhibitmagnetization unblocking temperatures below580 °C; those of specimens from samples D andF are above 650 °C. In AF demagnetization,behavior of duplicate specimens indicated dom-inance by magnetite (samples G and I) andhematite (samples D and F).

Henderson Mine Stocks

Magnetization carriers in the Henderson Minestocks are magnetite and probably maghemite.IRM acquisition curves show nearly completesaturation by about 0.3 T which indicates domi-nance by moderate- to low-coercivity cubicphases. Hematite was not an equilibrium min-eral phase during molybdenite mineralization(Drabek, 1982; Garten and others, 1988), andsubsequent oxidation of magnetite and pyrite tohematite has been fairly minor in the under-ground stocks. NRM intensities for the under-ground rocks are also low; mean values for eachintrusion range from 3.5 to 40 mA/m. Again,such low NRM intensities imply only smallamounts of cubic magnetic phases. If fine (mag-netically single domain) grains carry the mag-netization, then a TMR of 5 mA/m could residein grains whose volume fraction is less than -2.6x 10-6(Sugiura, 1979).

Magnetite grains are predominantly inclu-


Figure 6. Orthogonal progressivedemagnetization diagrams (Zijder-veld, 1967) of representative re-sponse by Henderson Mine rocks.The endpoint of the magnetizationvector is simultaneously projectedonto two orthogonal planes (hori-zontal: E-W/N-S plane, solid sym-bols; vertical, E-W or N-S/U-Dplane, open symbols). Peak demag-netizing inductions (in mT) ortemperatures (°C) are given adja-cent to data points on the verticalprojections. Data are plotted ingeographic coordinates. Specimenidentifier is given before a brief de-scription of the rock. Magnetiza-tions isolated in progressive demag-netization and determined usingprincipal component analysis are asfollows: (A)334°/50°,MAD = 7.7°,42-mT origin. (B) A comparison ofresponse to thermal and AF de-magnetization; thermal: 344°/57°,MAD = 2.3°, 300-500 °C, AF; highmedian destructive inductions ofsamples from this site preclude iso-lation of a well-defined componentof magnetization. (C) 330°/38°,MAD = 3.5°, 25-100 mT. (D)338°/39°, MAD = 15.9°, 18-95mT. (E) 323°/42°, MAD = 2.2°,25-100 mT. (F) 330°/58°, MAD =1.8°, 8-50 mT.


HEN2E1b,Hend.porphyry

2 mTQ - -4mA/m

NRM Dn

500 HEN9C2b,Ser. border

- -1 mA/m

8 286C2a M^f

NRM[E10C$300

80^NRM

•'NRM

2 - -0.05 mA/m

Dn

0NRM0.5 mA/m- -

HEN19B1aZ porphyry

\ry\

HEN9C2b\50

Dn E %2mTNRM

10 mA/m

N

100

Dn

X porphyry

- -100 mA/m

2 mT

sions in feldspar crystals and are usually euhed-ral, unaltered, and homogeneous. We foundlittle petrographic evidence for hydrothermallyintroduced magnetite that may contain chemicalremanent magnetizations (CRM) acquired atsubsolidus temperatures. All underground siteswere outside the zone of magnetite-topaz veinsdescribed by Wallace and others (1978) andSeedorf(1987).

Fine-grained maghemite in some of theHenderson Mine rocks is inferred from meas-urements of thermal demagnetization and Curietemperature (Fig. 9). Specimens from a total of38 samples were thermally demagnetized as partof pilot demagnetization. Of these, 25 had lineartrajectories in demagnetization. Response wascharacterized by distributed unblocking temper-atures with complete unblocking below 580 °C.On the basis of monitoring low-induction sus-ceptibility between heating steps, no changes in

the magnetic mineralogy with temperature wereobserved. The other 13 specimens, containingquartz-sericite-pyrite veins, behaved erraticallybetween each heating step, especially at temper-atures >350 °C. At temperatures greater than-300 °C, pyrite oxidizes to magnetite and (or)hematite under atmospheric conditions, hamper-ing thermal demagnetization of pyrite-bearingrocks (Schwarz and Laverdure, 1982; Geissmanand others, 1983). In most of the 13 specimens,low-induction susceptibilities increased severalorders of magnitude when the specimens wereheated above 300 °C.

In 10 of the 25 specimens for which demag-netization trajectories were well defined, themagnetization was dominated by titanium-poormagnetite. More than 99% of the magnetizationwas unblocked by 580 °C; in some specimensthe range of unblocking temperatures is less thana few tens of degrees. Curie temperatures be-

tween 550 °C and 580 °C are typical of theserocks, and the heating and cooling curves aregenerally reversible. For the other 15 specimens,>90% of their magnetization was unblockedover a narrow yet low temperature range be-tween 300 °C and 390 °C. Unlike rocks forwhich magnetization resided largely in magnet-ite, these samples had MDI's exceeding 50 mT.Results from site HENS in porphyry of theHenderson stock illustrate this behavior (Fig.10). In Curie temperature analysis, the satura-tion magnetization of rocks that have low un-blocking temperatures usually decreases uponcooling (Fig. 9), which probably reflects the in-version of a metastable maghemite phase tohematite and Curie temperatures of about 600°C. The low unblocking temperatures of theserocks also may represent the inversion of mag-hemite to hematite, a temperature- and time-dependent process (Reefer and Shive, 1980;

LGeological Society of America Bulletin, August 1992

t .

|;;;|v.

I,

S s -

S « oC (1 «-•= a i-« -3 £a S 201 *•

.s? a s3 3 Jj

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1038 Geological Society of America Bulletin, August 1992


oCTJN

D)(0

HEN 5A3a, Porphyry ,Henderson Mine

HEN20D13, Porphyry,Bethoud

100 200 300 400 500 600 700

Temperature, °C

Figure 9. Saturation magnetization versustemperature heating and cooling curves formagnetic separates from Henderson Minerocks (samples HEN5A3a and HEN20Dla).Experiments performed in air with an induc-tion of -0.18 T and heating/cooling rates of10 °C/min.

Ozdemir and Banerjee, 1984). Keefer and Shive(1980) reported inversion temperatures of 500°C for pure maghemite and 360 °C for mag-hemite of oxidation parameter z = 0.5. Ozde-mir and Banerjee (1984) found synthetic,single-domain maghemite to remain stablebelow -510 °C and probably to higher tempera-tures. The discrepancy between experimentaland observed inversion temperatures may lie inthe fact that Ozdemir and Banerjee (1984)heated samples at a rate of 10-20 °C/min. Inthermal demagnetization, samples are heatedat a fixed temperature for at least 30 min. To ac-count for the observed high MDI's in mag-hemite, the magnetization must be containedlargely in grains less than a few microns in di-ameter (Dankers, 1979). Coarse, multidomainmagnetite is present in small quantities at thesesites.

The low unblocking-temperature magnetiza-tion could not reside in high-Ti titanomaghemitegrains for which Curie temperatures are <400°C, because all magnetite in the Henderson sys-tem contains < 1.0 wt% TiO2 (Seedorf, 1987). Ifsamples are heated as Curie and laboratory un-blocking temperatures are measured, then satu-ration magnetization and NRM intensity do notincrease. Single-domain, low-Ti magnetitegrains near the superparamagnetic thresholdhave low unblocking temperatures (Dunlop,

1.0

.8

.4

.2

O T h e r m a l

D AF

20 402

603

804

1005

mT6 7 x100 C

Treatment

Figure 10. Normalized intensity (J/J0) decay curves for specimens from porphyry samplesfrom the Henderson Mine, site HENS. Open squares indicate results for AF demagnetization,°pen circles for thermal treatment. Capital letters refer to discrete samples from site HENS.

1973), but Curie temperatures >600 °C wouldnot be expected. Lastly, fine-grained pyrrhotiteis also not likely to carry the low unblockingtemperature magnetization. This phase is spa-tially restricted within the Henderson system,and these regions were not sampled in the pres-ent investigation.

We conclude that low-Ti maghemite carriesthe low unblocking temperature and high-MDImagnetization in some of the Henderson stocks,and that the maghemite formed during low-temperature oxidation of fine magnetite attemperatures <400 °C during alteration andthermal decay of the hydrothermal system. Thepresence of maghemite, although pervasive on asite level, is not specifically related to an indi-vidual pluton. Rather, it appears related to theintensity of quartz-sericite-pyrite alteration.

Is the maghemite-dominated magnetization inthese rocks an accurate recorder of the field atthe time of acquisition? Experiments producingfine-grained, low-Ti maghemite from single-domain magnetite particles (Johnson and Mer-rill, 1974; Heider and Dunlop, 1987) haveshown that the initial remanence carried bymagnetite is preserved in single-phase oxidationto a cation-deficient spinel, regardless of the di-rection of the field during oxidation. If realisticfor natural conditions, the experimental dataimply that an original TRM acquired by magnet-ite during blocking below 580 °C could still beretained in the most extreme conditions of per-vasive maghemitization of Henderson stocks.

40Ar/39Ar RESULTS

Fission-track and K/Ar isotopic dates fromearlier studies (Table 4; Naeser and others,1973; Shannon, 1982) poorly define the age ofthe Red Mountain intrusive system between23.8 ± 2.3 and 34.4 ± 1.7 Ma. This wide rangein dates possibly reflects thermal complexitiesthat resulted from the emplacement of manyclosely spaced intrusions, the effect of subse-quent alteration of dated phases, and subsequentcooling of dated phases through correspondingisotopic closure temperatures. In contrast, the"OAr/^Ar age-spectrum dates (Table 4; L. W.Snee and R. B. Garten, U.S. Geological Survey,unpub. data) range between 26.95 ± 0.08 and29.9 ± 0.3 Ma; all 40Ar/39Ar dates are reason-able when compared to geologic relationships.These dates were obtained on biotite, orthoclase,and muscovite; isotopic closure temperatures areabout 280 ± 40, 350 ± 50, and 325 ± 25 °C,respectively, assuming moderately high (>25°C/m.y.) rates of cooling (Snee, 1982; McDou-gall and Harrison, 1988; Snee and others, 1988).



TABLE 4. SUMMARY OF K-Ar ISOTOPIC. Âr/Âr ISOTOPIC AGE SPECTRUMAND FISSION-TRACK (F-T) AGE DATA FROM RED MOUNTAIN INTRUSIVE SYSTEM

Number

Red Mountain

1

2

3456

7

8

9

10

Henderson Mi

II

12

13

Ii l4

i 15

£E 1 16

<. 17(

18

i

!1 ";J : 20/;t: 2'; - ji; j 22

fi 2iI \ 24i; 'i

* ; 25

f ik . 27

V5' ' 28t-'; *

B ' 29

; ' so

f ' 31:•

? 32)- '

Stock

Surface Slocks

Easl Knob.porphyry

East Knob,porphyry

Square quartzSquare quartzRadial rhy.Red Mtn..

porphyryRed Mtn..

porphyry •Red Mtn..

porphyryLamprophyre

Rhyolite dike

ne Stocks

Urad Mine.porphyry

Urad Mine.porphyry

Urad Mine,porphyry

Urad Mine.porphyry

Urad Mine.porphyry

Urad Mine.porphyry

Urad Mine.porphyry

Henderson,pegmalite

Henderson.pegmatite

Seriate,pegmatite

Seriate,pegmatite

Senate

Seriate,granite por.

Senate,pegmatite

Vasquez,pegmatite

Vasquez,pegmatite

Seriate, mag.-sericite alt.

Seriate, mag.-sericite all.

Vasquez, por.Vasquez, por.

Ule border

Ute border

Age (Ma)

28.5 * 2.2

27.2 » 2.6

26.9 r 0.928.5 t 2.234.4 t 1.728.7 f 2.9

28. t 3

29.9 t 0.3

29.81 t 0.10

29 4 t 0.2

33.7 t 2.5

28.5 t 1.2

26.2 t 2.5

23.8 t 2.3

28.0 ± .09

28.6 t 0.3

28.3 t 1.4

28.1 i- .2

28.0 t 0.2

27.1 t 0.2

27.6 tO.l

28.2 t 0.2

27.6 t 0.2

27.6 ± 0.2

28.71 t 0.08

27.6 i 0.3

26.95 1 0.08

27.51 10.03

27.6 t O.I27.83 ±0.10

28.4 i 0.3

27.6 ± 0.2

Mineral dated laboratory

Method V

Zircon. F-T

Zircon. F-T

Sencite. K-ArZircon. F-TZircon, F-TZircon, F-T

Orihoclase, K-Ar

Onhoclase.«Ar/»Ar

Biotile. 40Ar/39Ar4 Dis

Orthoclase.4flAr/39Ar

Biotile. K-Ar

Orthoclase, K-Ar

Zircon. F-T

Zircon. F-T

Orthoclase.ÂrX-Âr

Biolite,*°Ar/39Ar

Biolile. "ArP'Ar

Onhoclase,«Ar/-"Ar

Biotile, "Ar/Âr

Alt. onhoclase."Âr/Âr

Biolite, '"WÂr

Onhoclase.

Biolite. *Ar/39Ar

Biotite, ̂ Ar/Âr

Onhoclase,

•"Ar/3»ArBi«,te.'1<W9Ar

Muscovite,«Ar/3'Ar

Muscovite,

*>/Ar/39ArBiotite, *Ar/ÂrOrthoclase.

40Ar/39ArOrthoclase,

*>Ar/39ArBiome, 40Ar/39Ar

ear

I

1

211I

3

4

urbed.

4

5

5

I

1

4

4

4

4

4

4

4

4

4

4

4

4

4

4

44

4

4

Character ofage spectrum*

Disturbed,preferred age

preferred ageDisturbed,

excess argon



Plateau, smallsample

Excess Ar, lossto 28.1 Ma

Plateau

Plateau

Plateau

Plateau

Plateau

Plateau


Plateau

Plateau

Plateau

PlateauPlateau

Plateau

Plateau

All K-Ar isotopic age determinations have been recalculated using revised decay constants. Sample numbers refer to those in Figure 15. Laboratory, year: I. U.S.Geological Survey (Naescr and others, 1973); 2. Oeochron Labs, summarized in Shannon (1982); 3. U.S. Geological Survey, summarized in Shannon (1982); 4. U.S.Geological Survey (1990. L W. Snee and R. B. Garten, unpub. data); 5. Geochron Labs, summarized in Shannon (1982).

•Plateaus are calculated according u> the method described in Snee and others (1988); "disturbed" refers to an age spectrum that does not exhibit a "plateau"; a"preferred age" is calculated for a disturbed spectrum by weight-averaging the apparent ages of temperature steps lhal are statistically identical within three standarddeviations of the weighted average.

In most cases, the age spectra are well defined; ina few (best exhibited in Figs. 11A and 11B),excess argon or argon loss are recorded in initialrelease of 39ArK.

On the basis of the 40Ar/39Ar age spectra(Fig. 11), the emplacement and cooling historyof the Red Mountain intrusive system, summa-rized in Table 5, is as follows:

(1) We interpret the age spectrum for Ortho-clase from the porphyry of Red Mountain (Fig.11 A) as an indication that the intrusion cooled

below -350 °C (orthoclase closure tempera-ture) following its emplacement before —29.9 ±0.3 Ma (a "preferred" age based on an averageof last 60% of the age spectrum) and possiblybefore 30.38 ± 0.09 Ma (apparent age of the lasttemperature step). The age spectrum is disturbedand exhibits minor excess argon in the lowertemperature steps and evidence for a thermaldisturbance at ~26.9 Ma.

(2) Lamprophyre dikes were emplaced at-29.8 ± 0.1 Ma and rhyolite dikes were em-


placed at about 29.4 ± 0.2 Ma. These emplace-ment ages are based on data for biotite from alamprophyre dike (age spectrum not shown inFig. 11) and orthoclase from the rhyolite dike(Fig. 11 A).

(3) The Urad and Seriate stocks were em-placed before 28.71 ± 0.08 Ma, which is theinterpreted age of emplacement of the Vasquezstock. The Ute stock was emplaced between28.7 and 28.4 Ma. These interpretations are de-rived from five orthoclase and one biotite agespectra shown in Figure 1 IB.

(4) The central part, or core, of the intrusivesystem cooled below 280 °C at 27.59 ± 0.03Ma. Age spectra for six biotites from theHenderson, Ute, Vasquez, and Seriate stockswithin the core of the system (Fig. 11C) arestatistically indistinguishable in age and repre-sent the time during which the system cooledbelow the argon closure temperature of biotite(-280 ± 40 °C).

(5) The magnetite-sericite alteration zonearound the Seriate stock formed between 27.51± 0.03 and 26.95 ± 0.08 Ma based on'"'Ar/^'Ar age spectra for three muscovites (Fig.1 ID) crystallized during this event. One of thesemuscovites is statistically older than the twoyounger muscovites; this suggests that magnet-ite-sericite alteration affected different parts ofthe alteration zone at different times. An alteredorthoclase from the Seriate stock has a 40Ar/39Ar date (age spectrum not shown in Fig. 11)of 27.13 ± 0.17 Ma and was probably affectedduring the magnetite-sericite alteration event. Ingeneral, however, the lack of thermal overprinton unaltered parts of the Red Mountain intru-sive system indicates that the thermal effect fromthe magnetite-sericite alteration was limited.

The interpreted 40Ar/39Ar age-spectrum dataare consistent with observed cross-cutting rela-tions and define the emplacement, cooling, andalteration history of the system (Fig. 12).

DISCUSSION

Overall, paleomagnetic data from stocks ofthe Red Mountain intrusive system are wellgrouped on both between-site and between-stock levels. East Knob and Rubble Rock brec-cia stocks on the surface of Red Mountaincontain characteristic magnetizations of reversepolarity. All younger surface stocks containmagnetizations of mixed polarity. HendersonMine stocks contain only normal polarity mag-netizations antipodal, at a 95%-confidence level(McFadden and Lowes, 1981), to those of re-verse polarity. Magnetizations defined in pro-gressive demagnetization do not cluster aboutpresent-day or time-averaged Quaternary fielddirections. We suggest that the paleomagneticdata adequately represent a late Oligocene field


40

30

2040

CO5

30

c20£40IDa#30

2040

30

20

Porphyry of Red Mountain

Rhyolite dike

Urad, Seriate, Henderson, Ute,and Vasquez stocks

B

System core, cooling below 280°C

Magnetite-sericite alteration

Figure 11. Composite Âr/Âr age-spectrum diagram for ortho-clase, muscovite, and biotite from the Red Mountain intrusive system.(One sigma analytical errors for data for most statistically significanttemperature steps are between 0.06 and 0.10 Ma.) (A) Age-spectrumfor orthoclase from porphyry of Red Mountain and potassium feld-spar from a rhyolite dike emplaced into Precambrian host rocks.(B) Six (five orthoclase, one biotite) age spectra for minerals from theUrad, Seriate, Henderson, Ute, and Vasquez stocks. (C) Six age spec-tra for biotites from Henderson, Ute, Vasquez, and Seriate stockswithin the central part of the Red Mountain intrusive system. (D)Three age spectra for muscovite from the magnetite-sericite alterationzone around the Seriate stock.

Percent

50

"ArK released100

Red Mountain

Figure 12. Schematic block diagram of theRed Mountain intrusive system showing rela-tive ages of stocks and cooling of emplace-ment (see Table 5); all ages in millions ofyears. Dashed lines represent approximatecontacts between intrusions. Stippled regionrepresents approximately the part of the sys-tem that cooled below -280 °C at 27.59 ±0.03 Ma on the basis of six biotite 40Ar/39Ardates. Patterned region represents area of•nagnetite-sericite alteration formed between27.51 ± 0.03 and 26.95 ± 0.08 Ma at the endof system cooling.

and that the results may be used to assess struc-tural deformation of the intrusive system andhost Precambrian rocks. Before discussion of thestructural history, we address the dispersion ofpaleomagnetic data.

Dispersion of Magnetization Data

Potential contributions to within-site disper-sion of data from the intrusive system includesampling and data reduction errors; irregular,prolonged thermal histories of parts of stocks;and acquisition of CRM's in hematite andmaghemite during alteration of primary mag-netic or precipitation of secondary magneticphases.

For both surface and subsurface sites, within-site dispersion is affected by occasional inaccu-racy in sample orientation and/or collection ofin-situ material. Red Mountain is rugged, steep,and largely covered with talus. Although we at-tempted to collect samples broadly over undis-turbed outcrops, parts of outcrops may beslumped. In subsurface, the fragmentary rocksand mirior, local, magnetic field disturbances,for which we could not fully account, may haveincreased dispersion. As much as 5° of angulardispersion may be due to errors in sampling. Wewere not able to apply principal componentsanalysis to results from the entire collection be-cause of an unfortunate accident destroying allrecords of most demagnetization data after vec-tor subtraction was applied. Where possible,comparison of results of both methods yieldsvector differences as large as 5°. Differences arenot systematic, however, and the contribution tothe total dispersion as a result of using the vectorsubtraction technique is less than this value.

TABLE 5. SUMMARY OF INTERPRETED '•"Ar/Âr THERMOCHRONOLOGY.MAGNETIZATION ACQUISITION AND STRUCTURAL EVENTS,

RED MOUNTAIN INTRUSIVE SYSTEM

Age (Ma)

posi-26

26.95 ± 0.08 1027.51 3- 0.03

27.59 * 0.03

>28.4 i 03

-28.71 ~ 0.08

--28.71 * 0.08

-•28.71 ± 0.08

29 4 and 29.8

> 29.85 ± 0.34.possibly ;• 30.3X t 0.09

Pan of system

Entire sysiem

Seriate stock

Core of system

Ute stock

Vasquez stock

Seriate andHenderson stocks

Urad porphyr>

Lamprophyre andrhyolne dikes

Porphyry of RedMountain and oldersurface stocks

Geologic activity(i.e., thermal event)

Tilling.ESE-side down

Magnetite-seriatealteration

Cooled below 280 °C

Emplacement

Emplacement

Em place men l

Emplacement

Emplacement in Silver PlumeGranite

Emplacement inSilver Plume Granite

Effect of eventon magnetization

Normal polarity of Henderson Minestocks completely blocked

Reverse to normal polarity

Geological Society of America Bulletin, Augusl 1992


Some dispersion of data from a single site orstock must be attributed to processes affectingvolumes as small as individual specimens. De-magnetization of at least six specimens per sam-ple permitted estimation of within-sample disper-sion for several randomly chosen samples. As anexample, six specimens from sample 8C (siteHENS in the Seriate stock) give a mean direc-tion with declination of 358° and inclination of+57° (a95 = 12.9°, k = 27.8). Although between-specimen dispersion is usually less than that be-tween samples (within site), implying sample-orientation errors, between-specimen dispersionmay be considerable. Magnetizations in the RedMountain stocks, each with a cross section onthe order of 0.04 to 0.09 km2 at the elevation ofthe Henderson Mine, were acquired over severalthousand years, if not considerably longer, dur-ing blocking below 580 °C and alteration. Forthe surface rocks with abundant hematite, atleast part of the dispersion may arise from altera-tion, because Heider and Dunlop (1987)showed that secondary magnetizations acquiredduring the oxidation of magnetite to hematitemay have no record of either a pre-existingmagnetization or the ambient field attendingalteration.

Within-site dispersion of all Henderson stocksmay also reflect past secular variation, and thisdispersion may be compared with predicted an-gular variance values on the basis of models offield variations. Although data from young lavaflows compare favorably with model predic-tions, we are not sure whether data from slowlycooled intrusions can be compared with valuesprovided by such models. For a site latitude of39.8°, McFadden and McElhinny's (1984) pa-leosecular variation model for Cenozoic virtualgeomagnetic pole (VGP) data predicts a VGPangular dispersion of 16.3°. McFadden and oth-ers' (1988) model G, on the basis of data fromlavas <5 Ma in age, predicts a value of 16.1°.Individual sites and individual stocks of the RedMountain intrusive system have angular disper-sions that usually exceed 15°, suggesting thatmagnetizations were acquired over extendedtime periods.

Deformation of the Red MountainIntrusive System

Wallace and others (1978) recognized theasymmetry of the Henderson deposit and sug-gested that parts of the deposit might have beendeformed since stock emplacement and mineral-ization. Field evidence described by Geraghtyand others (1988) suggests that Red Mountain

has been tilted in an east-southeast-side-downfashion.

Stocks of the intrusive system give magnetiza-tions of northwest declination and moderatepositive inclination (and antipodes); they do notresemble mid-Cenozoic reference directions.The tight grouping of pluton mean directions,which themselves are moderately dispersed, andthe presence of dual polarities suggest that thelate OJigocene field has been time averaged bythe ensemble of all stocks.

Grand mean directions for the intrusive sys-tem (Fig. 13) were determined in two ways(Table 3). We first used mean directions fromthe 10 stocks for which 0:95 values were <15°(Tables 1 and 2). The mean directions from theremaining three stocks are statistically identicalto the overall mean for the complex at a 95%-probability level (McFadden and Lowes, 1981),yet they are determined at a poor level of preci-sion. Alternatively, we assumed that each site,rather than each stock, provided an independentrecord of an ambient field, resulting from acomplicated cooling and alteration history. Allsite means that had a95 values of <20° (34 of 39sites) were included. The mean determined bythis method is statistically indistinguishable, at a99%-probability level (McFadden and Lowes,1981), from that determined using stock means.The pluton means yield a VGP angular varianceof 1.7°, whereas the site means yield a VGPangular variance of 12.5°, values less than those

1+182

Figure 13. Partial equal-area projection ofin situ stock (10 stocks accepted) mean andsite (34 sites accepted) mean directions (solidcircles) and associated projected cones of 95%confidence. Solid squares represent mid-Oligocene expected directions determinedfrom paleomagnetic poles of Irving andIrving (1982) (I+I) and Diehl and others(1983) (D).

for individual samples, sites, and stocks. The12.5° variance is closer to that expected for asmall number of independent field observations.We interpret the small angular-dispersion valuederived from stock means to imply an averagingof the late Oligocene field by each stock.

Expected time-averaged geomagnetic field di-rections for central Colorado (Fig. 13) havebeen calculated from paleomagnetic poles com-piled by Irving and Irving (1982) (30 Ma mean)and Diehl and others (1983, 1988) (38-22 Maresults). In-situ magnetizations from the intru-sive system are discordant in both declination(westward) and inclination (shallower) fromeither of the above reference directions. The dis-crepancy between observed and expected direc-tions may be explained by rotation about onestructural axis or a combination of several axesof differing orientations. Given the absence oflate Oligocene and younger strata near RedMountain, we evaluate the discrepancy in termsof a single event that uniformly deformed theentire Red Mountain area. This approach mayreveal that a particular bounding fault or set offaults controlled structural adjustment. Defor-mation in all likelihood consisted of a morecomplicated sequence of movements involvingdifferential adjustments on several boundingfaults.

Because the discrepancy in paleomagnetic in-clination is less than that in declination, single-step deformations are of two types. The first istrue counterclockwise rotation about near-vertical axes in response to shear along a singlefault or sets of faults bounding Red Mountain.This mechanism is unlikely given the requiredamount of rotation for all orientations of rota-tional axes that could be related to known struc-tures. On both local and regional scales, there isno evidence to suggest an amount of strike-slipalong faults in the Front Range of Colorado inlate Oligocene and younger time (Taylor, 1975;Tweto, 1975; Theobald, 1965) large enough tocause ~30° of vertical axis rotation (Mackenzieand Jackson, 1983).

The second type of deformation—moderatetilting about a near-horizontal axis—can ac-count for the discrepancy. From 15° to 25° ofwest-side-down tilting about an axis orientedN15°E is required to "correct" the observeddata into agreement with expected directions.This axis is perpendicular to the strike of theWest Fork of the Clear Creek fault and is within10° and 30° of strikes of the Vasquez Pass andBerthoud Pass fault zones (Figs. 1 and 2), re-spectively. Deformation involving dip-slip alongthe West Fork of the Clear Creek fault (Fig. 14)



Clear Creek fault/Vasquez

\ / Pass /\

;;':?:: Porphyry o f : ;

Figure 14. Cross section A-A' (Fig. 1) showing geometry of Cenozoic fault displacement in the Red Mountain area. H = Henderson stock; P =Primes stock. On the basis of crosscutting relations, the most recent fault displacement is progressively younger to the northwest from theWoods Creek fault. We estimate a minimum of 830 m of Neogene dip-slip displacement for the Woods Creek fault, 390 m for Vasquez Passfault, and 1,100 m for Clear Creek fault Modified from Geraghty and others (1988).

may explain such tilting. With a hinge in themiddle of the Red Mountain block, about 540m of dip-slip offset along the combined WoodsCreek and Berthoud Pass faults (southeast ofRed Mountain and the Vasquez Pass fault to thenorthwest) (Fig. 14) would result in the inferredtilt. This slip estimate is in accord with belt's(1975) suggested upper limit of about 1 km forNeogene displacement along normal faults innorth-central Colorado. We prefer a style of de-formation involving variable dip-slip with offsetalong at least two major structures to simpledip-slip along either the Vasquez Pass or Ber-thoud Pass-Woods Creek faults. Dip-slip dis-placement along these northeast-trending struc-tures and attending tilt would give a morenorthward declination for the observed RedMountain mean direction.

Assuming that the data from the Red Moun-tain intrusive system record local tilting from15° to 25° about an approximately NNE, hori-zontal axis, then the pre-tilt orientation ofmineralizing stocks can be determined. Plungesof axes of the cylindrical Urad, Henderson, andSeriate stocks define a great circle of attitudeN57°E, 66°NW. After restoring the great circleto its pre-tilt position, the orientation is N52°E,80°NW. This trend parallels the Woods Creekstrand of the Berthoud Pass fault zone, suggest-ing that structure controlled the emplacement of

the stocks. Correcting for east-side-down tiltingresults in a more vertical orientation of stocksand their contacts.

Implications of 40Ar/39Ar Age-SpectrumData for Intrusion Emplacementand Acquisition of Magnetizations

The age-spectrum and magnetic polarity dataprovide an improved assessment of the age ofintrusion, subsolidus cooling, alteration, andstructural history of the Red Mountain intrusivesystem (Table 5), just as 40Ar/39Ar age-spectrum data on individual mineral phases ofestimated isotopic closure temperatures havebeen used to define the subsolidus tectonother-mal history of intrusive igneous rocks and asso-ciated hydrothermal deposits and regionallymetamorphosed rocks in many settings (Sutlerand others, 1985; Snee and others, 1988; Fosterand others, 1990).

In the following discussion, we compare theRed Mountain magnetic polarity and ^Ar/39AT isotopic age-spectrum data with publishedgeomagnetic polarity time scales. Considerablecontroversy still exists about the accuracy ofthese polarity time scale calibrations and somecalibrations are currently being revised substan-tially (W. A. Berggren, 1989, personal com-mun.). As a result, we are not able to confidently

correlate magnetization polarity in the intrusivesystem with particular polarity chrons. The du-rations of individual magnetozones, however,are far less controversial and have direct bearingon the interpretation of the magnetization acqui-sition history of the Red Mountain intrusivesystem.

Magnetization blocking spanned at least onefield reversal following intrusion of the reverselypolarized East Knob pluton and Rubble Rockstocks, possibly during cooling of the porphyryof Red Mountain. 40Ar/39Ar age-spectrum dataon orthoclase from this stock indicate coolingbelow 350 °C possibly as early as 30.38 ± 0.09Ma (Fig. 11A). Lamprophyre and rhyolite dikeson the surface yield 40Ar/39Ar age-spectrumdata of 29.81 ± 0.10 (biotite) and 29.4 ± 0.9 Ma(orthoclase). Although we were not able to ob-tain 40Ar/39Ar age-spectrum data for the re-versely magnetized porphyry of the East Knobpluton, the age spectrum data from the porphyryof Red Mountain provide a basis for the inter-pretation that the polarity reversed to normalbefore ca. 30 Ma. The time scale of Harland andothers (1982) gives the boundary between chron9r and chron 9 at 29.94 Ma (Fig. 15). With theexception of short polarity chrons, the imme-diately older and younger reverse to normaltransitions are placed at 31.2 and 28.3 Ma (Har-land and others, 1982) and do not compare fa-



JVj

1

-23

-24 -

-25 -

-26

-27

-28

-29

-30

-31

-32

-33

23- •

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03

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28- •

29- •

30 - -

31 - •

32- •

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fo

<f *-l ) - CDi TJ

Figure 15. Geomagnetic polarity time scales for late Eocene and Oligocene time compared with zircon fission-track (F-T) dates, K/Ar isotopicand 40Ar/39Ar age-spectrum determinations for the Red Mountain intrusive system. Numbers adjacent to fission track dates and isotopic agedeterminations correspond to sample numbers in Table 4. Vertical lines indicate error bars; for the *°A.r/39\r age-spectrum data, O, B, and Mrefer to orthoclase, biotite, and muscovite (sericite), respectively.

vorably with the Âr/Âr data. More recentgeomagnetic polarity time scales systematicallyassign polarity chrons to younger ages duringthis time period (Fig. 15) and would requirecooling of the porphyry of Red Mountain duringchron 11 (Montanari and others, 1988; Swisherand Prothero, 1990; Mclntosh and others,1992), possibilities which are not excluded bythe argon data. According to Mclntosh and oth-ers' (1992) internal calibration based on high-precision, sanidine Âr/^'Ar age-spectrumdata from volcanic rocks, the reverse to normaltransition during emplacement of the RedMountain surface rocks would be the 11R/11chron boundary at ca. 29.5 Ma.

Rocks at the Henderson Mine levels cooledlater than those exposed at the surface. The

normal polarity porphyry of Urad Mine, thedeep-seated equivalent of the porphyry of RedMountain (Garten and others, 1988), yields dis-turbed orthoclase and biotite age spectra of 28.0± 0.09 and 28.6 ± 0.3 Ma, respectively. TheHenderson, Seriate, and Primes stocks, whichintrude porphyry of the Urad Mine, also ac-quired characteristic magnetizations of normalpolarity. 40Ar/39Ar isotopic age determinationsfrom the Seriate, Vasquez, and Ute stocks indi-cate that orthoclase isotopic blocking occurredbetween 28.5 and 28.7 Ma. Age-spectrum dataon biotite (average of 27.59 ± 0.03 Ma; Table 5;Fig. 11C) specify when the central mineralizedzone cooled below -280 °C. Muscovite (seri-cite) determinations (26.95 ± 0.08 and 27.51 ±0.03 Ma) are younger than those of biotite, be-

cause sericite formed after the system cooledbelow biotite-blocking temperatures.

Whereas reverse polarity magnetizations oflow coercivity and (or) low-unblocking temper-ature spectra are present in some samples fromthe subsurface rocks, we interpret the absence ofreverse polarity magnetizations of high coerciv-ity and (or) unblocking temperature to suggestthat magnetization blocking occurred duringone normal polarity chron, after intrusion of theUrad and Red Mountain stocks. Unblocking-temperature data support the likelihood thatblocking occurred at temperatures from muchhigher than 500 °C to as low as 300 °C. The40Ar/39Ar orthoclase and biotite age spectragive estimates for a lower bound of magnetiza-tion blocking. The time between 28.5 and 28.7



Ma is principally one of reversed polarity ac-cording to the time scales of Harland and others(1982) (chron 8R), Montanari and others(1988) (chron 9R), Harland and others (1989)(chron 9R), Mclntosh and others (1992) (chron10R), and Swisher and Prothero (1990) (chron10R), and of normal polarity (chron 9) accord-ing to Berggren and others (1985). Although theBerggren and others' (1985) time scale seems tomatch the orthoclase age-spectrum and polaritydata more appropriately, we note that reversepolarity chron 8R lies between the time of ortho-clase and biotite closure in this time scale in theHenderson Mine rocks. The position of normalpolarity chron 8 in the time scales of Harlandand others (1982) and Berggren and others(1985), chron 9 in the time scales of Montanariand others (1988) and Harland and others(1989), and chron 10 in the time scales of Mcln-tosh and others (1992) and Swisher andProthero (1990) is consistent with the dates forbiotite closure. The general absence of reversepolarity magnetizations in the Henderson Minestocks can be explained by emplacement beforeabout 28.5 Ma (Table 5). The entire systemcooled rapidly and magnetizations blocked totemperatures below 280 °C during a single,normal polarity chron, or, less likely, a time be-fore 27.6 Ma dominated by normal polarity.Chron 8, of ~0.8 m.y. (Harland and others,1982) or 0.7 m.y. (Berggren and others, 1985)duration; chron 9, of 0.7 m.y. (Montanari andothers, 1988) to 1.0 m.y. duration (Harland andothers, 1989); or chron 10, —0.6 m.y. duration(Mclntosh and others, 1992; Swisher andProthero, 1990) may be the likely candidates forthe intervals of normal polarity remanence ac-quisition. Conclusions on the cooling andmagnetization-acquisition history for the Hen-derson Mine rocks cannot be applied to theVasquez and Ute stocks; these stocks were notsampled for paleomagnetism. Similarities inorthoclase and biotite cooling ages for these andsampled stocks lead us to predict that the Vas-quez and Ute stocks also have normal polaritymagnetizations.

CONCLUSIONS AND REGIONALTECTONIC IMPLICATIONS

Paleomagnetic and 40Ar/39Ar age-spectrumdata from most stocks of the Red Mountain in-trusive system provide an improved understand-ing of the structural and cooling history of thesuite of intrusions host to a major porphyrymolybdenum deposit. Characteristic magnetiza-tions, acquired during reverse polarity before ca.30 Ma and normal polarity between 28.7 and

27.6 Ma, are discordant with expected direc-tions for late Oligocene time. The discrepancyimplies about 15°-25° of east-side-down tiltingof the Red Mountain area about a north-northeast-trending axis and is consistent withobserved field relations.

The 40Ar/39Ar age-spectrum data are inter-preted to indicate emplacement of surface stocksof Red Mountain before 29.85 ± 0.34 Ma (pos-sibly before 30.38 ± 0.09 Ma). Intrusion of theUrad porphyry and the Seriate, Henderson,Vasquez, and Ute stocks, all exposed in the sub-surface, occurred between ca. 28.4 and 28.7 Ma.Biotite age-spectrum data tightly define coolingof the core of the mineralized system below-280 °C at about 27.6 Ma. Magnetite-sericitealteration, in particular of the Seriate stock, oc-curred between ca. 26.9 and 27.6 Ma. Magneti-zations carried by maghemite in some of theunderground sites may have been acquired overthis time period and appear to have accuratelyrecorded the ambient field. Comparison of theRed Mountain age-spectrum and polarity datawith published geomagnetic polarity time scalesfails to identify unequivocally chrons in whichthe observed magnetizations were acquired. Thelikely candidates for intervals of normal polarityremanence acquisition are either Chrons 9 or 10.

Styles of local, basement-involved deforma-tion similar to that affecting the Red Mountainarea have been described by Kellogg (1973) andHoblitt and Larson (1975) for the easternmost,east-tilted flank of the Front Range. The absenceof layered rocks near Red Mountain precludesdirectly relating tilting of area with post-middleOligocene tectonic events in much of centralColorado and New Mexico. Early uplift and de-formation along the margins of Precambrian-cored blocks in the southern Rocky Mountainsof Colorado and New Mexico have been as-cribed to Laramide events of latest Cretaceous toearliest Cenozoic age (Eaton, 1986; Oppen-heimer and Geissman, 1989). Our interpreta-tion of Red Mountain data indicates that blockswithin the uplifted ranges were tilted at leastlocally in late Oligocene and younger time.Other Precambrian-cored uplifts of the southernRocky Mountains may have been similarly de-formed; the Red Mountain data should not beinterpreted to indicate uniform magnitude andsense tilting of fault blocks in this part of theFront Range. In the Jamestown area (Fig. 1),about 15 km west of the eastern edge of theFront Range, Sheldon and Geissman (1983)concluded that early to mid-Tertiary stocks havebeen tilted only slightly during uplift. Evidencefor regional, east-side-down tilting of the entireFront Range would be manifested by differential

structural relief, systematic changes in metamor-phic facies of Precambrian rocks, and variationsin uplift ages of rocks at a constant topographiclevel. Variations in uplift ages were not docu-mented by Bryant and Naeser (1980) in theirfission track study in the Front Range ofColorado. Deformation in other Precambrianbasement cored uplifts may be similarly compli-cated. Steidtman and others (1989) demon-strated late Oligocene to mid-Miocene reactiva-tion and tilting of parts of the core of thesouthern Wind River Mountains. Although tilt-ing of the Red Mountain area may be in re-sponse to extension along the northern terminusof the Rio Grande Rift, the absence of layeredstrata precludes relating the timing of deforma-tion to early (late Oligocene-early Miocene) orlate (late Miocene and Pliocene) phases ofnorthern Rio Grande Rift extension (Morganand Golombeck, 1984). Elsewhere in northernColorado, there is geologic evidence for differen-tial uplift and block faulting in Miocene andyounger time (Izett, 1975; Tweto, 1980; Larsonand others, 1975).

ACKNOWLEDGMENTS

Financial and logistical support from theClimax Molybdenum Company are most ap-preciated. Financial support was also providedby Sigma Xi to G. W. Graaskamp. Mark Hud-son and Ken Shonk aided in field sampling, andMark Hudson also measured Curie temperaturesat the U.S. Geological Survey, Denver. Com-ments by G. Brent Dalrymple, Steve Harlan,Richard Reynolds, and Deke Schnaebel im-proved the manuscript. We thank the Bulletinreviewers, Mike McWilliams and an anony-mous colleague; Associate Editor Bob Butler; andEditor Art Sylvester for their comments. Discus-sions with W. A. Berggren about problems withcurrent geomagnetic polarity time scales wereenlightening. Dag Lopez prepared some of theillustrations.

APPENDIX: PALEOMAGNETIC AND40Ar/39Ar AGE-SPECTRUM METHODS

At 39 sites in the intrusive system (Figs. 2 and 3),215 samples were collected for paleomagnetic study.There were 60 samples collected from 5 intrusions at 9sites on the surface; 155 samples were collected from 8stocks exposed in underground workings of theHenderson Mine. The underground sites are distrib-uted over 180 vertical meters on the 81,00-, 8,050-,7,755-, 7,625-, and 7,500-ft levels (Fig. 3).

All samples were collected as independently ori-ented blocks using a magnetic compass. The intensityof NRM of most samples is <0.1 A/m. Samples onthe surface of Red Mountain were collected from the



base of the least prominent outcrops to avoid the ef-fects of lightning strikes on the rocks. During under-ground sampling, we took azimuthal backsights toavoid effects of anomalous inductions due to ferro-magnetic construction and mining equipment. Becauseof the intensely fractured nature of the rocks, we couldnot always collect seven or more samples per site.

Sites were chosen with two principal goals: (1) tosample as many separate intrusions as possible and(2) to minimize the possibility of complete thermalresetting of TRM's and isotopic ages in older stocks byprogressively younger stocks. An Oligocene field di-rection is best averaged by sampling many small, rap-idly cooled plutons, each of which might provide arecord of the field during a geomagnetically short pe-riod of time. We attempted to selectively sample pe-rimeters of stocks most distant from contacts withyounger stocks rather than interiors. In some places,sites in older stocks were intentionally sampled closeto younger stocks for "contact" tests.

Block samples were drilled in the laboratory to ob-tain at least three standard (11 cm3) cylindrical speci-mens and were measured using a computer-interfacedSchonstedt SSM-1A spinner magnetometer. At leastone specimen per sample was subjected to progressiveAF demagnetization using a Schonstedt GSD-1 single-axis system. For information on the distribution oflaboratory unblocking temperatures (T|ub's) of mag-netizations, specimens from at least two samples persite were subjected to progressive thermal demagneti-zation using a Schonstedt TSD-1 furnace. In somesamples, especially for those from surface rocks, AFdemagnetization to 100 mT was not enough to re-move >50% of the NRM (typically, much more than5% of the NRM remained). Continued thermal treat-ment was used to more completely isolate the rema-nence. In general, demagnetization yielded consistentbehavior characterized by linear segments, and one ormore of the magnetization components could be iden-tified using orthogonal demagnetization diagrams(Zijderveld, 1967). Directions, relative intensities, andgeneral coercivity/T|ub spectra of all components weredetermined using demagnetization diagrams, plots ofnormalized intensity versus demagnetization, and vec-tor subtraction (Hoffman and Day, 1978) methods. Asa check on the results of vector subtraction, principalcomponent analysis (Kirschvink, 1980) was applied tothe demagnetization data from about 5% of thesamples.

Rock magnetic tests were made on representativesamples from all intrusions and consist of acquisitionand backfield demagnetization of IRM, determinationof Curie temperatures, and measurement of low fieldsusceptibility after each thermal treatment. An elec-tromagnet capable of 1.2 T inductions supplied DCinductions for IRM tests. Curie temperatures weremeasured using both a vertical balance (at the U.S.Geol. Survey laboratory, Denver) and a horizontalbalance (at the Univ. of New Mexico laboratory), insaturating inductions of -0.2 T. Samples of all intru-sions were inspected with reflected light petrographyto identify parageneses of visible magnetic phases.

High-precision 40Ar/39Ar age-spectrum data wereobtained to define the thermal history of the intrusivesystem with the principal intent of quantifying thenumber and duration of mineralizing events. Fromstratigraphically known sections of drill core, 21 mus-covite, biotite, and potassium feldspar samples used forargon thermochronology were carefully selected. Pure

mineral separates could be extracted directly fromsome cores; from other cores, standard separationtechniques were used to obtain pure samples. X-raydiffraction patterns of all potassium feldspars showedthem to be orthoclase. All mineral separates were un-altered except one potassium feldspar from the Seriatestock that included -10% illite formed during a later(the magnetite-sericite) alteration event.

Samples for argon analysis were irradiated in twoseparate groups: one in the U.S. Geological SurveyTRIGA reactor and one in the University of MichiganPhoenix reactor, using normal encapsulation proce-dures described in Snee and others (1988). The iso-topic composition of argon was measured at the U.S.Geological Survey, Denver, Colorado, using a MassAnalyzer Products, Limited, 215 series, rare-gas massspectrometer. (Trade, product, or firm names are usedfor descriptive purposes only and do not imply anendorsement by the United States government.) Iso-topic abundances were corrected for mass discrimina-tion. The neutron flux monitor used in this study ishornblende MMhb-1, the age of which is 520.4 Ma(Alexander and others, 1978; Samson and Alexander,1987); an error of 0.25% (1 sigma) was determinedexperimentally by calculating the reproducibility ofseveral aliquants of argon for all monitors. Samplesirradiated at the University of Michigan were cor-rected for irradiation-produced, interfering isotopes ofargon by measuring production ratios for those iso-topes in pure ^SC^ and CaF2 irradiated simultane-ously with the samples. For samples irradiated at theU.S. Geological Survey, production ratios determinedfor this reactor by Dalrymple and others (1981) wereused; the assumptions made in their study have sincebeen shown to be adequate based on production ratiosdetermined for many irradiations. Corrections weremade for additional interfering isotopes of argon pro-duced from irradiation of chlorine (Roddick, 1983).Quantities of 39Ar and 37Ar were corrected for ra-dioactive decay. Constants used in age calculations arethose of Steiger and Ja'ger (1977). Error estimates forapparent ages of individual temperature steps wereassigned by using the equations of Dalrymple and oth-ers (1981). The equations were modified to allow theoption of choosing the larger of two separately derivederrors in the 40ArR/39ArK ratio, the calculated errorfrom differential equations or the experimentally de-termined error derived from the reproducibility ofanalyses of multiple aliquants of argon from themonitors. Dates are reported with 1-sigma errors. Ageplateaus were determined by comparing contiguousgas fractions using the critical value test described byDalrymple and Lanphere (1969).

REFERENCES CITED

Alexander. E. C. Michelson. G. M.. and Lanphere, M. A.. 1978. MMhb-l: Anew *°Af/39Ar dating standard, in Zarlman, R. E., ed., Shon papers ofthe Fourth International Congress. Geochronology, Cosmochronology.and Isotope Geology: U.S. Geological Survey Open-File Report 78-701. p. 6-8.

Bailey, M.. and Hale, C. J.. Anomalous magnetic directions recorded bylaboratory-induced chemical remanent magnetization: Nature, v. 294,p. 739-740.

Barnes, C. G, Allen, C. M., and Saleeby, J. 8., 19861. Open- and closed-systemcharacteristics of a tilted pluton system, Klamalh Mountains, California:Journal of Geophysical Research, v. 91. p. 6073-6090.

Barnes. C. G.. Rice. J. M., and Gnbble, R. F_ I986b. Tilted plutons in theKlamath Mountains of California and Oregon: Journal of GeophysicalResearch, v. 91, p. 6059-6071.

Beck, M. E., Jr.. 1980, Paleomagnetic record of plate-margin tectonic processesalong the western edge of North America: Journal of GeophysicalResearch, v. 88. p. 7115-7131.

Berggren, W. A.. Kent, D. V.. Flynn, J. J., and Van Couvenng, J. A, 1985.Cenozoic chronology: Geological Society of America Bulletin, v. 96.p. 1047-1418.

Bryant. &., and Naeser. C. W., 1980, The significance of fission track ages ofapatite in relation to the tectonic history of the Front and SawatchRanges. Colorado: Geological Society of America Bulletin, v. 91,p. 156-164.

Butler, R. F. Dickinson. W. R., and Gehrds, G. E.. 1991. Pileomagnetism ofcoastal California and Baja California: Alternatives to large-scalenorthward transport: Tectonics, v. 10. p. 561-576.

Carten, R. B., Geraghty, E. P.. Walker. B. M., and Shannon. J. R.. 1988. Cyclicdevelopment of igneous features and their relationship to high-temperature hydrolhermal features in the Henderson porphyry molyb-denum deposit, Colorado: Economic Geology, v. 83, p. 266-2%.

Champion, D.. Lanphere, M. A., and Kuntz, M. A., 1988, Evidence for a newgeomagnetic polarity reversal from lava flows in Idaho: Discussion ofshort polarity reversals in the Brunhes and tale Matuyama polaritychrons: Journal of Geophysical Research, v. 93. p. 11.667-11.680.

Dalrymple. G. B.. and Lanphere, M. A, 1969, Potassium-argon dating: SanFrancisco, California, W. H. Freeman, 258 p.

Dalrymple. G. B.. Alexander. E. C, Jr, Lanphere. M. A, and Kraker. G. P.,1981, Irradiation of samples for *°Ar/wAr dating using the GeologicalSurvey TRIGA reactor. U.S. Geological Survey Professional Paper1176. 56 p.

Dankers, P.H.M.. 1979, Magnetic properties of dispersed natural iron-oxides ofknown grain size [Ph.D. dissert.}: Utrecht, The Netherlands, UtrechtUniversity, 142 p.

Diehl. J. F., Beck. M. E.. Beske-Diehl, S, Jacobson. D, and Heam, B. C. Jr.1983, Paleomagnelism of the Laic Cretaceous-Early Tertiary north-central Montana alkalic province: Journal of Geophysical Research,v. 88. p. 10593-10609.

Diehl, J. F., McClannahan. K. M.. and Bomhorst. T. 1, 1988. Pakomagneticresults from the Mogollon-Daul volcanic field, southwestern New Mex-ico, and a refined mid-Tertiary reference pok for North America: Jour-nal of Geophysical Research, v. 93. p. 4869-1879.

Dilles. J. H., 1987, Petrology of the Yerington Batholith. Nevada: Evidence forevolution of porphyry copper ore fluids: Economic Geology, v. 82.p. 1750-1789.

Drabek, M., 1982, The system Fe-Mo-S-O and its geologic application: Eco-nomic Oology, v. 77. p. 1053-1056.

Dunlop, D. J., 1973, TRM in sub-microscopic magnetite: Journal of Geophysi-cal Research, v. 78. p. 7602-7613.

Eaton, G. P., 1986, A tectonic redefinition of the southern Rocky Mountains:Tectonophysics, v. 132, p. 163-193.

Epis, R. C., and Chapin, C. E., 1975, Geomorphic and tectonic implications ofthe post-Laramide, Late Eocene erosion surface in the Southern RockyMountains, in Curtis. B., ed., Cenozoic history of the southern RockyMountains: Geological Society of America Memoir 144, p. 45-74.

Fluids. J. E., Ceissman, J. W.. and ShaGqullah. M.. 1992, Implications ofpaleomagnetic data on Miocene extension along a major accommoda-tion zone in the Basin and Range province, northwestern Arizona andsouthern Nevada: Tectonics, v. 1 1 , p. 204-227.

Fisher, R. A, 1953. Dispersion on a sphere: Royal Society of London Proceed-ings. Series A. v. 217, p. 295-305.

Foster, D. A.. Harrison. T. M, Miller. C. F.. and Howard, K. A, 1990. The'"Ar/^Ar ihermochronology of the eastern Mojave Desert, California,and adjacent western Arizona with implications for the evolution ofmeumorphic core complexes: Journal of Geophysical Research, v. 95,p. 20005-20024.

Geissman. J. W.. Van der Voo. R. Kelly, W. C, and BnmK.ll. G.. Jr. 1980,Paleomagnelism, rock magnetism, and aspects of structural deformationof the Butte Mining district. Butte, Montana: Journal of Geology, v. 88,p. 129-159.

Geissman, J. W.. Van der Voo. R.. and Howard. K. L. Jr.. 1982. A paleomag-netic study of the structural deformation in the Yerington district,Nevada 2. Mesozoic "basement" units and their total and pre-Oligocenetectonism: American Journal of Science, v. 282, p. 1080-1109.

Geissman. J. W., Strangway. D. W., TasiUo-Hirt, A. M, and Jensen, L S..1983, Paleomagnetism of late Archean metavolcanics and metasedi-menu, Abilibi Orogen, Canada: tholeiites of the Kinojevis Group:Canadian Journal of Earth Sciences, v. 20. p. 436-461.

Geissman, J. W., Gillian. J. T. OUow, J. S.. and Humphries, S. E., 1984.Paleomagnetic assessment of oroflexural deformation in west-centralNevada and significance for emplacement of allochlhonous assem-blages: Tectonics, v. 3, p. 179-200.

Ceraghty. E. P.. Carten. R. B. and Walker. B., 1988, Tilling of Urad-Hcnderson and Climax porphyry molybdenum systems, central Colo-rado, as related to northern Rio Grande rift tectonics: GeologicalSociety of America Bulletin, v. 100. p. 1780-1786.

Harland, W. B., Cox. A. V.. Llewellyn. P. G.. Picklon. C.A.G.. and Smith,A. G., 1982. A geologic time scale: Cambridge, England, CambridgeUniversity Press, 131 p.

Hariand, W. B, Armstrong, W. B.. Cox, A. V, Craig. L E, Smith. A. G, andSmith. D. G., 1989, A geologic time scale 1989: Cambridge, England,Cambridge University Press, 263 p.

Heider, F., and Dunlop, D. J., 1987, Two types of chemical remanent magneti-zation during the oxidation of magnetite: Physics of the Earth andPlanetary Interiors, v. 46. p. 24-45.

HobliR, R., and Larson. E., 1975. Paleomagnetic and gcochronologic databearing on the structural evolution of the northeastern margin of theFront Range, Colorado: Geological Society of America Bulletin, v. 86,p. 237-242.

Hoffman. K. A., and Day, R.. 1978. Separation of multicomponenl NRM: ageneral method: Earth and Planetary Science Letters, v. 40, p. 433-438.

Irving, E., and Irving, G. A.. 1982, Apparent polar wander paths: Carbomfer-


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